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? LIBRARIES
o ‘. MICHIGAN STATE UNIVERSITY
it * " EAST LANSING. MICH 48824-1048

This is to certify that the
thesis entitled

SUSTAINABLE COMPOSITE MATERIALS FROM
RENEWABLE RESOURCES FOR AUTOMOTIVE
APPLICATIONS

presented by

ARIEF CAHYO WIBOWO

has been accepted towards fulfillment

of the requirements for the
Master of degree in Material Science and
Science Engineering

 

 

 

 

 

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2/05 EHRC/Dateouelnddms

SUSTAINABLE COMPOSITE MATERIALS FROM RENEWABLE
RESOURCES FOR AUTOMOTIVE APPLICATIONS

By

Arief Cahyo Wibowo

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Chemical Engineering and Material Science

2004

ABSTRACT

SUSTAINABLE COMPOSITE MATERIALS FROM RENEWABLE
RESOURCES FOR AUTOMOTIVE APPLICATIONS

By

Arief Cahyo Wibowo

As part of an ongoing program on the development of biocomposites from
renewable resources, e.g. cellulosic fibers and cellulosic plastics, to produce more eco-
friendly/green automotive parts, this thesis deals with the development of a cellulose
acetate (CA) biopolymer. Through plasticization, CA was found to be processable at 170-
180°C, approximately 50°C below the melting point of neat CA. Eco-friendly
green/biocomposites were fabricated from chopped hemp fiber and cellulose ester
biodegradable plastic through two process engineering approaches: powder impregnation
through compression molding (Process I) and extrusion followed by injection molding
(Process II). Cellulose acetate plasticized with 30% citrate plasticizer proved to be a
better matrix compared to polypropylene (PP) for hemp fiber reinforcements in terms of
flexural and damping properties. In addition to plasticized CA, we used cellulose acetate
butyrate plastic (CAB) as matrix in developing the biocomposites. Both matrices had
good interaction (hydrogen bonds) with hemp fibers with CAB having higher strength.
The addition of a small amount of compatibilizer (maleic anhydride functionalized CAB)
into the system led to formation of ester linkages (covalent bonds) between hemp fibers
and the matrix to improve the adhesion and to enhance their thermomcchanical

properties.

In dedication to GOD, my mother, father, wife, daughter, and all of my
family back home in Indonesia. Your love and support is unquestionably
cherished. Thank you.

iii

ACKNOWLEDGMENTS

First and foremost, the author would like to thank GOD for his blessings and
helps throughout his life. The author would like to wish a sincere thank to three of his
supervisors, Dr. Lawrence T. Drzal, Dr. Amar K. Mohanty, and Dr. M. Misra for their
notable guidance throughout his master degree. The author also would like to thank all
staff and students of Composite Materials and Structures Center of Michigan State
University for their support and assistance. The author thanks Eastman Chemical
Company, Morflex, Inc., and Hempline for providing cellulose esters samples, citrate
plasticizer, and hemp fibers, respectively. The author is exceptionally grateful of NSF-

EPA-TSE (Award # DMI-012478) for financial support.

iv

TABLE OF CONTENTS

LIST OF TABLES ........................................................................ viii
LIST OF FIGURES ........................................................................ ix
INTRODUCTION ........................................................................ 1
LITERATURE REVIEW ............................................................... 2
l. Cellulose Fibers for Reinforcement .................................... 2
1.1. Utilization of cellulose fibers: opportunities and limitations 3
1.2. Chemical modification of cellulose fibers .................. 5
1.3. Factors influencing the performance of short fiber composites 8
2. Cellulose Ester ............................................................... 10
2.1. What is Cellulose Ester (CE)? .................................... 11
2.2. Cellulose ester and its applications ........................... 14
2.2.1. Modern coatings applications ........................... 15
2.2.2. CE in plastics applications ........................... 20
2.2.3. Biodegradation of CE .................................... 23
2.2.4. CE in composites and laminates ........................... 27
3. Conclusion ............................................................... 33
4. References ............................................................... 35

1. Development of Renewable Resource Based Cellulose Acetate Bioplastic:

Effect of Process Engineering on the Performance of Cellulosic Plastics 4O
1. Abstract ............................................................... 40

2. Introduction ............................................................... 41

3. Experimental ............................................................... 43
3.1. Materials ............................................................... 43

3.2. Processing ...................................................... 44

3.3. Analysis and Testing ............................................. 45

4. Results and Discussion ............................................. 47

4.1. Comparison of Preperties of Plasticized Cellulose Acetates

- - Compression Molding vs. Extrusion followed by

Compression Molding ............................................. 48
4.2. Comparison of Properties of Plasticized Cellulose Acetates

- - Extrusion Followed by Compression Molding vs.

Extrusion Followed by Injection Molding ........................... 50
4.3. Effect of Plasticizer Content on Performance of Cellulose
Acetate Plastic ...................................................... 52
4.4. Thermal and Thermo—mechanical Behaviors of Plasticized
Cellulose Acetates ............................................. 52
5. Conclusion ............................................................... 55

6. Acknowledgments ...................................................... 56
7. References ............................................................... 57

II. Effect of Process Engineering on the Performance of Natural Fiber

Reinforced Cellulose Acetate Biocomposites ........................... 59
1. Abstract ............................................................... 59
2. Introduction ............................................................... 60
3. Experimental ............................................................... 62

3.1. Materials ............................................................... 62

3.2. Hemp Fiber Density and Tensile Modulus Measurements 62
3.3. Spectral, Thermal, and Morphological Analysis of Hemp

Fibers ............................................................... 63

3.4. Composite Fabrication Processing ........................... 63

3.5. Analysis and Testing ............................................. 64

4. Results and Discussion ............................................. 66

5. Conclusion ............................................................... 75

6. Acknowledgments ...................................................... 76

7. References ............................................................... 77

III. Chopped Industrial Hemp Fiber Reinforced Cellulosic Plastic

Biocomposites: Thermomechanical and Morphological Properties 79
1. Abstract ............................................................... 79
2. Introduction ............................................................... 80
3. Experimental ............................................................... 83

3.1. Process 1: Powder Impregnation Process followed by
Compression Molding ............................................. 84
3.2. Process H: Extrusion followed by Injection Molding ......... 85
3.3. Analysis and Testing ............................................. 86
4. Results and Discussion ............................................. 88
4.1. Processing Approaches Vs. Performance of Biocomposites 88
4.2. Fiber Contents Vs. Performance of Green/Biocomposites 90
4.3. Hemp Fiber Reinforced Cellulose Acetate Vs. Hemp Fiber
Reinforced Polypropylene Composites ........................... 92
4.4. Hemp - CAP based Biocomposites Vs. Hemp - CABP based
Biocomposites ...................................................... 93
5. Conclusion ............................................................... 94
6. Acknowledgments ...................................................... 96
7. References ............................................................... 97

IV. A Solvent free Graft Copolymen'zation of Maleic Anhydride onto

Cellulose Acetate Butyrate Bioplastic by Reactive Extrusion ......... 98
1. Abstract ............................................................... 98
2. Introduction ............................................................... 98
3. Experimental ............................................................... 100

vi

99‘1“?

3.1. Materials ...................................................... 100

3.2. Instrumentation ...................................................... 100
3.3. Synthesis of maleic anhydride grafted CAB (CAB-g-MA) 100
3.4. Characterization of CAB-g-MA by FTIR (qualitative) 102
3.5. Characterization of CAB-g-MA by titration (quantitative) 102
Results and Discussion ............................................. 103
Conclusion ............................................................... l 10
Acknowledgments ...................................................... l 1 1
References ............................................................... 1 12

V. Effect of Compatibilizer on Thermomechanical and Morphological

Properties of Hemp Fiber Reinforced Cellulose Ester Biocomposites 113

1. Abstract ............................................................... 113

2. Introduction ............................................................... 114

3. Experimental ............................................................... 115

3.1. Materials ............................................................... 115

3.2. Biocomposites processing .................................... 116

3.3. Plastics Fabrication using Micro Compounder .................. 116

3.4. Characterization and analysis .................................... 118

4. Results and Discussion ............................................. 120

5. Conclusion ...................................................... 130

6. Acknowledgments ...................................................... 132

7. References ............................................................... 133
CONCLUSION and FUTURE WORK ............................................. 134
APPENDIX 1 ........................................................................ 137
APPENDIX 2 ........................................................................ 138

vii

LIST OF TABLES

1. Development of Renewable Resource Based Cellulose Acetate
Bioplastic: Effect of Process Engineering on the Performance of
Cellulosic Plastics

Table 1a. Effect of optimized processing conditions on the tensile strength
and the tensile modulus of plasticized (30% TEC) cellulose acetate ..... p. 51

Table 1b. Effect of temperature variation for extruded - injection molding
CA plastics (30% TEC) ...................................................... p. 51

Table 1c. Effect of varying rpm of extruder for extruded - injection molding
CA plastics (30% TEC) ............................................. p. 51

IV. A Solvent free Graft Copolymerization of Maleic Anhydride
onto Cellulose Acetate Butyrate Bi0plastic by Reactive Extrusion

Table 1. Effect of initiator concentration (CAB-g-MAl, 2, and 3) and
effect of MA concentration (CAB-g-MAZ, 4, and 5) on acid
number (AN) and % MA grafting .................................... p. 108

Table 2. Acid Number and Maleic Anhydride percentage (% MA)
comparison between samples obtained from Process I
(DSM micro extruder) and Process H (ZSK-30 large extruder). p. 109

V. Effect of Compatibilizer on Thermomechanical and
Morphological Properties of Hemp Fiber Reinforced
Cellulose Esters Biocomposites

Table 1. OH content of cellulose acetate butyrate (381-20),
dioctyl adipate (DOA), cellulose acetate (CA 398-30),
and triethyl citrate (TEC). ............................................. p. 123

APPENDIX 2

Table 1. Calculated Hildebrand Solubility Parameter (6) based upon the
chemical structure. ...................................................... p. 138

viii

LIST OF FIGURES

LITERATURE REVIEW

1. Cellulose Fibers for Reinforcement

Figure 1. Schematic representation of cellulose fiber (one cell wall) --

http://www.bath.ac.uklmechenglbiomimetics/HVO1.2df p. 2

Figure 2. Typical CTE curve of biobased plastic and biocomposites
(plasticized cellulose acetate - CAP and its biocomposites
— HP30CAP) ............................................................... p. 5

Figure 3. Relationship between fiber length (1), critical fiber length
(1c), and ultimate tensile strength of the fiber (om). .................. p. 9

Figure 4. Structure of cellulose comprised of cellobiose repeat unit
with alternate anhydroglucose containing three active OH
groups at position 2, 3, and 6 as marked ........................... p. 12

Figure 5. Structure of Cellulose esters (Cellulosic Plastics):
R = H (Cellulose), acetyl (Cellulose acetate), acetyl and
propionyl (Cellulose acetate propionate), or butyryl
(Cellulose acetate butyrate). .................................... p. 13

Figure 6. Simplified processing scheme of esterification of
cellulose (adopted from Eastman Chemical Company). ...... ,- . .p. 14

Figure 7. Simplified biodegradation/biodestructability mechanism
of cellulose ester ...................................................... p. 23

1. Development of Renewable Resource Based Cellulose Acetate
Bioplastic: Effect of Process Engineering on the Performance of
Cellulosic Plastics

Figure 1. Structure of Cellulose esters (Cellulosic Plastics):
R = H (Cellulose), acetyl (Cellulose acetate), acetyl
and propionyl (Cellulose acetate propionate), or
butyryl (Cellulose acetate butyrate). ........................... p. 41

Figure 2. Effect of processing conditions on the tensile strength (TS)

and tensile modulus (Ten. Mod.) of plasticized (30% TEC)
cellulose acetate, A = compression molded (CM)

ix

at 190 0C, B = CM at 180 °C, C = CM at 170 ”C,
D = extruded at 180°C (100 rpm) and CM at 180 °C. ......... p. 46

Figure 3. Effect of processing conditions on the impact strength (IS)
and elongation at break (EB) of plasticized (30% TEC)
cellulose acetate, A = compression molded (CM) at 190 0C,
B = CM at 180 °C, C = CM at 170 °C, D = extruded at 180°C
(100 rpm) then CM at 180 0C. .................................... p. 46

Figure 4. ESEM images of: (a) pure cellulose acetate (CA) powder with
particle size of 5-10 mm (500x, scale bar 100 mm), (b) extruded
followed by compression molded CA plastic with 30%
Triethyl citrate plasticizer (1500X, scale bar 30 mm) ......... p. 47

Figure 5. Effect of plasticizer content on the tensile strength (TS)
and tensile modulus (Ten. Mod.) of extruded followed
by compression molded cellulose acetate plastics. ......... p. 48

Figure 6. Effect of plasticizer content on the impact strength (IS)
and elongation at break (EB) of extruded followed by
compression molded cellulose acetate plastics. .................. p. 48

Figure 7. Stress Strain plots for varied amount of plasticizer
of extruded followed by compression molded cellulose
acetate plastics. ...................................................... p. 49

Figure 8. TGA results of weight loss versus temperature
with varying amounts of plasticizer of extruded
followed by compression molded cellulose acetate plastics. ......... p. 51

Figure 9. TMA results of the coefficient of thermal expansion
for varied amounts of plasticizer of extruded followed
by compression molded cellulose acetate plastics. .................. p. 53

Figure 10. DSC curves for varied amounts of plasticizer
of extruded followed by compression molded cellulose
acetate plastics. ...................................................... p. 54

II. Effect of Process Engineering on the Performance
of Natural Fiber Reinforced Cellulose Acetate Biocomposites

Figure 1. Structure of Cellulose esters (Cellulosic Plastics):
R = H (Cellulose), acetyl (Cellulose acetate), acetyl and
propionyl (Cellulose acetate propionate), or butyryl
(Cellulose acetate butyrate). ..................................... p. 61

Figure 2. ESEM micrograph of raw hemp fiber (100 um scale bar, 600X). p. 65

Figure 3. Flexural properties of powder processing (Process I) and
extrusion followed by injection molding (Process H):
A = CA Plastics (CAP) of Process I, B = CA Plastic
Biocomposites (30wt% hemp) of Process I, C = CA Plastic
of Process H, D: CA Plastic Biocomposites (30wt% hemp)
of Process H. ...................................................... p. 65

Figure 4. ESEM micrographs of impact fractured samples: A = CA Plastic
Biocomposites (30wt% hemp) of Process I «150 um scale bar
(300K), B = CA Plastic Biocomposites (30wt% hemp) of
Process H -- 200 um scale bar (250X). ........................... p. 66

Figure 5. Flexural properties of CAP Bio—composites of Process H:
A = CAP (0wt% hemp), B = 15wt% hemp, C = 30wt% hemp
content. ............................................................... p. 68

Figure 6. Stress-strain plot of: A = virgin cellulose acetate plastics
(CAP) and B = CAP biocomposites (30wt% hemp). .................. p. 69

Figure 7. Comparison between tensile modulus of Rule of Mixture
(ROM), modified ROM, and experimental Tensile Modulus
with density of raw hemp (p): 1.29 g/cc, Tensile Modulus
of hemp fiber (Bf) = 42 GPa, E... = 2.1 GPa. .................. p. 70

Figure 8. Heat Deflection Temperature (HDT) data comparison of
A = CAP (0wt% hemp), B = 15wt% hemp, C = 30wt%
hemp content. ...................................................... p. 71

Figure 9. Coefficient of Thermal Expansion (CTE) data comparison
of A = virgin cellulose acetate plastics (CAP) and
B = CAP bio-composites (30wt% hemp). .................. p. 72

Figure 10. Flexural properties comparison of: A = CAP, B =
CAP -30wt% hemp composites, C = PP, D = PP-30wt%
hemp composites. ...................................................... p. 73

IH. Chopped Industrial Hemp Fiber Reinforced Cellulosic Plastic
Biocomposites: Thermomechanical and Morphological Properties

Figure 1. Structure of Cellulose esters (Cellulosic Plastics):
R = H (Cellulose), acetyl (Cellulose acetate), acetyl and
propionyl (Cellulose acetate propionate), or acetyl and
butyryl (Cellulose acetate butyrate) ........................... p. 80

xi

Figure 2. Possible hydrogen bonds can be formed which leads to
increase in adhesion between fibers and cellulose ester matrix.

Figure 3. Fabrication procedure of hemp fiber reinforced cellulose acetate
biocomposites using Process I: powder impregnation process

followed by compression molding ........................

p. 81

...p. 84

Figure 4. Cross sectional image of compression molded hemp reinforced cellulose

acetate based biocomposites (50X magnification, 1mm scale bar)

Figure 5. Flexural properties of powder processing (Process I) and
extruded followed by injection molding (Process H): A =
CA Plastics (CAP) of Process 1, B = CA Plastic Biocomposites
(30wt% hemp) of Process 1, C = CA Plastic of Process H,
D: CA Plastic Biocomposites (30wt% hemp) of Process H.

Figure 6. Impact properties of powder processing (Process I) and
extruded followed by injection molding (Process H):
A = CA Plastics (CAP) of Process 1, B = CA Plastic
Biocomposites (30wt% hemp) of Process 1, C = CA Plastic
of Process H, D: CA Plastic Biocomposites (30wt% hemp)

of Process H. ...................................................

Figure 7. ESEM micrograph of impact fractured samples:
A = CA Plastics Biocomposite (30wt% hemp) of Process I
with 100X and 450 um scale bar, B = CA Plastics Biocomposite
(30wt% hemp) of Process H with 95X and 450m scale bar

Figure 8. Comparison between rule of mixtures (ROM), modified ROM, and
experimental tensile modulus of CAP Biocomposites.

Figure 9. Comparison between rule of mixtures (ROM), modified ROM, and
experimental tensile modulus of CABP Biocomposites

Figure 10. Flexural properties comparison of: CAP, CAP Biocomposites
(30wt% hemp), PP, and PP-30wt% hemp composites

Figure 11. Damping properties comparison between CAP, CAP-Hemp
Biocomposites (30wt% hemp), PP, and PP-Hemp composites

(30wt% hemp). ...................................................

Figure 12. Stress-strain comparison between CAP, CAP Biocomposites
(30wt% hemp), CABP, and CABP Biocomposites (30wt% hemp)

xii

p. 85

p. 137

...p. 87

p. 88

p. 89

p. 92

p. 93

p. 94

Figure 13. ESEM micrograph of tensile fractured samples of CAP
Biocomposites (Process H): A = 200x with 250 um scale bar,

B = 1000X with 45 um scale bar. .................................... p. 95

Figure 14. ESEM micrograph of tensile fractured samples of CABP
Biocomposites (Process H): A = 200x with 250 um scale bar,

B = 1500X with 30 um scale bar. .................................... p. 95

IV. A Solvent free Graft Copolymerization of Maleic
Anhydride onto Cellulose Acetate Butyrate Bioplastic
by Reactive Extrusion

Figure 1. FTIR curve showing unreacted maleic anhydride (MA) peak
before putting in vacuum oven (A) and after putting in
vacuum oven at 80-90°C for overnight (B), and (C) is neat CAB.

Figure 2. FTIR analysis of OH protection on cellulose acetate butyrate
(CAB) using different materials: NaOH, NaH, and MA with

and without initiator. ............................................. p.
Figure 3. Proposed mechanism of maleic anhydride grafted cellulose

acetate butyrate (CAB-g-MA). .................................... p.
Figure 4. FTIR analysis of varying initiator (1) concentration

(0, 0.5, 0.9, and l.4wt%) with 5wt% maleic anhydride

(MA) content kept constant. .................................... p.

Figure 5. FTIR analysis of varying maleic anhydride (MA) concentration
(0, 5, 7.5, and 10wt%) with 0.9wt% initiator content kept constant

V. Effect of Compatibilizer on Thermomechanical and
Morphological Properties of Hemp Fiber Reinforced
Cellulose Esters Biocomposites

Figure 1. Effect of addition of different compatibilizer: C1
(acid number ~33), C2 (acid number ~18), on tensile properties
of plasticized cellulose acetate butyrate (T enite Butyrate, TEB)

— hemp biocomposites. ............................................. p.
Figure 2. Effect of addition of different compatibilizer: Cl

(acid number ~33), C2 (acid number ~18), on flexural properties

of plasticized cellulose acetate butyrate (T enite Butyrate, TEB)

-— hemp biocomposites. ............................................. p.

Figure 3. Effect of addition of different compatibilizer: C1

xiii

. 101

105

106

107

. 109

117

118

(acid number ~33), C2 (acid number ~18), on impact properties
of plasticized cellulose acetate butyrate (T enite Butyrate, TEB)
— hemp biocomposites. ............................................. p. 118

Figure 4. Effect of addition of different compatibilizer: C1
(acid number ~33), C2 (acid number ~18), on storage moduli
of plasticized cellulose acetate butyrate (Tenite Butyrate, TEB)
- hemp biocomposites. ............................................. p. 119

Figure 5. Comparison between rule of mixtures (ROM), experimental
tensile modulus data of plasticized cellulose acetate butyrate
(Tenite Butyrate, TEB) — hemp biocomposites with
and without compatibilizer: C1 (acid number ~33), and
modified ROM. ...................................................... p. 120

Figure 6. ESEM micrographs of compatibilized TEB-hemp
biocomposites using compatibilizer: C1 (acid number ~33).
A = 450nm, 105x, B = lOOpm, 490x ........................... p. 121

Figure 7. ESEM micrographs of compatibilized TEB-hemp
biocomposites using compatibilizer: C2 (acid number ~18).
A = 450nm, 105x, B = lOOum, 500x ........................... p. 121

Figure 8. ESEM micrographs of non compatibilized TEB-hemp
biocomposites. A = 450nm, 105X, B = 100nm, 500x ......... p. 122

Scheme 1. Proposed mechanism of how grafted maleic anhydride
(CAB-g-MA) interact with biofibers (alcoholysis reaction). p. 122

Figure 9. Chemical structure of cellulose ester (R = acetyl is cellulose acetate
- CA, R = acetyl and butyryl is cellulose acetate butyrate - CAB),
— triethyl citrate (TEC) plasticizer, dioctyl adipate (DOA)
- plasticizer. ...................................................... p. 124

Figure 10. Effect of addition of different compatibilizer: C1
(acid number ~33), C2 (acid number ~18), on tensile properties
of plasticized cellulose acetate (CAP) - hemp biocomposites. p. 125

Figure 11. Effect of addition of different compatibilizer: Cl
(acid number ~33), C2 (acid number ~18), on flexural properties
of plasticized cellulose acetate (CAP) - hemp biocomposites. p. 126

Figure 12. Effect of addition of different compatibilizer: Cl

(acid number ~33), C2 (acid number ~18), on impact properties
of plasticized cellulose acetate (CAP) — hemp biocomposites. p. 126

xiv

Figure 13. Effect of addition of different compatibilizer: C1
(acid number ~33), C2 (acid number ~18), on storage moduli
of plasticized cellulose acetate (CAP) - hemp biocomposites
at 40 0C. ............................................................... p. 127

Figure 14. Effect of addition of different compatibilizer: Cl
(acid number ~33), C2 (acid number ~18), on storage moduli
of plasticized cellulose acetate (CAP) - hemp biocomposites
at 100 °C. ............................................................... p. 128

Figure 15. Effect of addition of different compatibilizer: Cl
(acid number ~33), C2 (acid number ~18), on glass transition
temperature (peak of tan delta curve) of plasticized cellulose
acetate (CAP) — hemp biocomposites. ........................... p. 128

Figure 16. Effect of addition of different compatibilizer: C1
(acid number ~33), C2 (acid number ~18), on heat deflection
temperature (HDT) of plasticized cellulose acetate
(CAP) - hemp biocomposites. ........................... p. 129

Figure 17. Effect of inclusion of CAB or C1 into TEB system on
storage modulus: A = neat TEB, B = A + 10wt% neat CAB,
C=A+10wt%C1. ............................................. p.130

Figure 18. Effect of inclusion of CAB or C1 into CAP system
on storage modulus: A = neat CAP, B = A + 10wt% neat CAB,
C=A+10wt%Cl. ............................................. p.131

Figure 19. Effect of inclusion of CAB or C1 into CAP system on

glass transition temperature (peak of tan delta curve): A = neat CAP,
B=A+10wt% neatCAB,C=A+10wt% Cl. .................. p. 131

XV

INTRODUCTION

Sustainability, industrial ecology, coo-efficiency and green chemistry are guiding
the development of the next generation of advanced materials, products and processes.
Biodegradable polymers and bio-based polymer products based on annually renewable
agricultural and biomass feedstock can form the basis for a portfolio of sustainable, eco-
efficient products. These products can capture markets currently dominated by products
based exclusively on petroleum feedstock l's. Cellulose is attracting interest as a
substitute for petroleum feedstock in making plastics (cellulosic plastic - - cellulose
esters) for the consumer market '. Natural cellulose fibers are one of the most abundant
renewable resources throughout the world with high potential as reinforcing element in
structural composite application. In this project, we are investigating various issues that
are components of the structure-property relationships of cellulose esters-short natural
fiber biocomposite materials in order to identify potential ‘green’ applications in
automotive parts. The results of this research have been drafted into research papers
which have been submitted for publication to leading journals in the field. They make up
the main content of this thesis.

The first paper describes biomatrix development and characterization, i.e.
plasticization of cellulose acetate with environmentally friendly plasticizer, using
different processing methods necessary to make them suitable for biocomposites
fabrication. The second paper focuses on the fabrication and characterization of the
biocomposites utilizing the developed matrix with the main emphasis on optimization of

the method of processing of the biocomposites. The third paper compares the mechanical

and damping properties of different cellulose esters (CE) as matrices for biocomposites,
including their comparison to petroleum based polypropylene (PP).

The fourth and the fifth papers describe the synthesis and characterization of a
compatibilizer suitable to improve adhesion between cellulose ester matrix and fibers and
fabrication as well as characterization of compatibilized biocomposites utilizing our
synthetic compatibilizer, respectively. Finally, this thesis addresses issues and challenges
that need to be faced in the future in order to implement the biocomposites in SMC (sheet

molding compound).

LITERATURE REVIEW

1. Cellulose Fibers for Reinforcement

 

Microfibrillar

Angle (o) "— Microfibrils

 

 

Cell

Wall
Micro I...-

 

 

Hemicellulose
and pectin

Hemicellulose joining two microfibrils

Figure 1. Schematic representation of cellulose fiber (one cell
wall) -- http://www.bath.ac.uk/mechenglbiomimetics/HV01.pdf

Cellulose fibers were used as reinforcement material since ancient times when
mud bricks were reinforced by grass and straw 9. The automotive industry had
extensively used coconut fiber/natural rubber latex ‘0 in a few applications , however, the
usage, was decreased during 1970’s and 1980’s due to the innovation of synthetic fibers
with better performance. In the past few years the interest of using natural fiber has been
renewed due to increasing cost of plastics and environmental concerns regarding non-

degradable, petroleum based plastics and fibers 1'.

1.1. Utilization of cellulose fibers: opportunities and limitations
Natural cellulose fibers are abundant renewable resources found throughout the
world with the potential to function as reinforcements in structural composite
applications. Depending upon the plant source from which cellulose fibers are obtained,
they can be classified as:
1. Grasses and reeds (from the stem): such as bamboo and sugar cane 9.
2. Leaf fibers (from the leaves, lengthwise): sisal, henequen, abaca, and esparto, are
most commonly used as reinforcing material 9.
3. Bast fibers (from inner bark of the stem): jute, hemp, kenaf 9' ‘2.
4. Seed and fruit hairs (from seed-hair and flosses): cotton and coconut ‘0‘ '2.
5. Wood fibers (from hardwood and softwood trees): maple, yellow poplar, and
spruce.

The above fibers can be extracted using different techniques such as: rotting, scrapping

and pulping '2.

Cellulose fiber generally is built from lignocellulosic containing helically wound
cellulose microfibrils (run along the length of the fiber) held together by an amorphous
matrix of lignin and hemicellulose (Figure 1). Different chemical compositions and
variations of internal fiber structure among natural fibers are dependent on their location
in the plant.'°' '3. In general, the best natural fiber used as a reinforcing element is the one
with a high cellulose content (more than 60%) and low microfibrillar angle (between 7-
12°) ‘3.

Natural fibers have several advantages including: their abundance, their
renewability 14' '5, low cost, relatively high specific properties due to low density 9' '5’ 16‘
'7‘ '8‘ '9, their flexibility and toughness i.e. will not fracture when processed over sharp
curves 1', non abrasive (thus permits high volume fraction and prevents machine wear
problem) as compared to glass and ceramic. They are non-toxic whereas glass fibers can
cause skin irritation and respiratory disease when fibrous dust is inhaled 20. Their surfaces
can be modified to increase the interfacial adhesion with the matrix '4, they require
processing with a lower expenditure of energy 9‘ '4, and biodegradable 9' '5' 2' are other
advantages of these natural fibers.

There are, however, some drawbacks of cellulose fibers when using as
reinforcement in composites including thermal stability is limited to 200°C 9' '“6- ‘9' 22.
Agglomeration of fibers can occur in the resulting composites due to strong hydrogen
bonding between fibers '4‘ '8. The inherent polarity and hydrophilicity of natural fibers
results in lower compatibility with non-polar, hydrophobic olefin thennoplastics. High
moisture absorption causes swelling of the fiber and the resulting composites, which

11.16

leads to a reduction in dimensional stability and mechanical properties. Another

weakness of natural fibers is the limitation of service life when exposed to outdoor
environment. This is due to the hydrophilic nature and the fact that they are highly
biodegradable. Another factor that may restrain the use of natural fibers in plastic based
composites is the difficulty in getting natural fibers with high and uniform quality due to

collection and storage methods are not yet mechanized and standardized.

1.2. Chemical modification of cellulose fibers
Processing cellulose fibers at high temperature (200°C and up) for long periods of

time will result in degradation indicated by the formation of a tar-like product and

 

 

 

 

80
60 - CAP-Biocomposites /
E 401
a
g, i
C
E
o 201
_8
a
S
..E.
C) 0..
-20‘
i
40 v r V I ' l ' I ' T ‘ I W
0 20 40 60 80 100 120 140

 

Temperature (‘6)

Figure 2. Typical coefficient of thermal expansion (CTE) curve of
cellulose acetate plastics (CAP) and CAP biocomposites (30wt% hemp
fiber content)

pyrolysis acid that could damage processing equipment and the resulting composite
pr0perties 23. High moisture absorption of cellulose fibers, due to formation of hydrogen

bonding between water and hydroxyl groups of cellulose cell wall, will cause swelling of

the fibers and the resulting composite (fiber-matrix interface) 9‘ '1. The swelling affects
the dimensional stability of the cellulose based composites. The coefficient of thermal
expansion (CTE) is defined as the change in dimension when a material is subjected to a
temperature increment with constant pressure and can be measured using
therrnomechanical analyzer (TMA). Shrinkage of bio-based material (hydrophilic
material) starting from about 80°C during CTE measurement using the TMA is explained
by the reversible and irreversible swelling of the composites 9‘ 24‘ 25. Increasing
temperature releases moistures inside the composites and hence shrinkage of the tested
composites is observed (Figure 2).

Swelling of the composites also affects the fiber matrix adhesion and thus the
mechanical properties as well. Lee et a1 24 found the reduction of mechanical properties
after weathering of jute-polyester composites. Surface crazing and debonding between
resin and jute fiber occurred due to adsorption and desorption of moisture by the
composites, which resulted in high shrinkage. The hydrophilic nature of cellulose fibers
affects the composite processing, namely formation of voids during compounding 22. The
voids would cause stress concentration, which could lead to early failure of the composite
during loading.

Acetylation of cellulose fibers was reported to overcome these drawbacks of
natural fibers. Rana et al reported that acetylated jute fibers had higher thermal stability
compared to those of untreated jute fibers 26. Replacing hydroxyl groups on the cellulose

fibers dramatically reduced moisture absorption of both fibers and resulting composites.

9. ll. 13 10

Acetylation, or grafting a vinyl monomer such as acrylonitrile , were some

examples of fiber surface treatments suitable for handling the hydrophilic nature of
cellulose fibers.

Agglomeration of cellulose fibers when incorporated inside synthetic polymer is
often due to differences in polarity and strong hydrogen bonding between the fibers.
These clumps of fibers will be the center of stress concentration, which will initiate
cracks during loading and hence will lead to mediocre mechanical properties. Fibers
coated by the matrix 27 and the addition of a dispersion aid (stearic acid and mineral oil)
28 were reported to improve the separation of individual fibers inside the matrix and fiber
wetting, thus enhancement of mechanical properties was observed.

Non-compatibility between polar, hydrophilic cellulose fibers and non polar,
hydrophobic polyolefins leads to the formation of porosity, and poor interfacial adhesion.
The use of coupling agents or compatibilizer and fiber surface treatment are widely used
to improve the wetting of this system. Isocyanates and silanes are coupling agents that
work as a bridge forming a chemical bond between fiber and matrix 29, which results in
an improvement of interfacial adhesion suggested by increase in mechanical properties.
Gatenholm et a1 observed an improvement in mechanical properties of polypropylene
based composites with cellulose fibers treated with maleic anhydride grafted
polypropylene (MAPP). The improvement observed was due to enhanced fiber wetting
(formation of covalent and hydrogen bonds across the interface — proved by FTIR
analysis), and fiber dispersion as observed in SEM micrographs 30.

Lignocellulosic material can be easily degraded when exposed to nature in

different ways: biological, thermal, aqueous, photochemical, and mechanical means of

degradation. Retardation of biodegradability, by acetylation, is required to obtain fiber-

based composites with long service life 25' 3 '.

1.3. Factors influencing the performance of short fiber composites
Properties of short fiber-reinforced composites depend on the following factors 23'

27):

1. Fiber dispersion
Good fiber dispersion means fibers are individually separated homogeneously and
surrounded by the matrix, such that stress transfer from matrix to fiber is efficient. Lack
of fiber dispersion leads to resin rich areas (weak area) and fiber rich areas (vulnerable to
microcracking), which results in lower mechanical properties. The extent of fiber
dispersion depends on two major factors, fiber to fiber interactions (such as hydrogen
bonds), and fiber length 32. A critical length of the fiber is a very crucial factor for
homogeneous fiber dispersion. If fiber is too long then it may get entangled with other
fibers, whereas less effective reinforcement effect will be observed if the fiber length is
too small. Effective fiber (nylon, rayon, polyester fiber) dispersion can be achieved by
using 0.4 mm fiber length. The fiber dispersion problem can be solved by several
methods including fiber surface modification such as acetylation, use of dispersing agent
such as compatibilizer, stearic acid and mineral oil 28, and increase of shear force and
mixing time 9’ 11.21.32.

2. Fiber-matrix adhesion

In short fiber based composites system, effective stress/loading transfer from matrix (low

modulus) to fiber (high modulus) requires good adhesion at the fiber-matrix interface.

This good interfacial adhesion can be achieved by the use of coupling agents, polymer
coating materials, fiber surface treatments, and chemical grafting. Selke et al 32 reported a

promising improvement in fiber-matrix adhesion reflected by the increase in mechanical

Ultimate Tensile Stress (O'fu)

I
A /\ / \

l<lC i=1c l>lc

 

 

Figure 3. Relationship between fiber length (1), critical fiber length (1,), and ultimate
tensile strength of the fiber (Ofu).

properties of wood fiber/high density polyethylene composite with addition of MAPP,
and ionomer modified polyethylene.

3. Fiber aspect ratio
The critical length to diameter ratio of a fiber or ‘aspect ratio’ is inversely proportional to

interfacial shear stress (based on shear slip analysis 33):

 

a), = 21',m(s) (1)
_ 0n
" ' 2m(s) ‘2)

where: Cf“ = ultimate tensile strength of fiber

1 = interfacial shear stress

s = aspect ratio (=l/d)
m = slippage length
Composites should have a higher fiber aspect ratio, i.e. fiber should be longer than its
critical fiber length to achieve its ultimate fiber strength — Figure 3, such that interfacial
shear stress is low and thus maximum stress transfer to fiber can be accomplished.
Altering the interfacial shear stress of the fiber can be achieved by modifying the surface
of the fiber by chemicals such as coating agents. High performance short fiber reinforced
composites can be obtained if the fiber aspect ratio after processing can be maintained in
the range of 100-200.
4. Fiber orientation
In a short fiber system, the mechanical properties also depend upon the fiber orientation
with respect to the loading axis 3". Higher mechanical properties resulted from
composites with fibers oriented parallel to the test direction 35‘ 3°. The polymer melt
experiences elongational, extensional and shear flow, which helps in fiber alignment
during processing.
5. Fiber volume fraction

The rule of mixtures can predict the mechanical properties of a composite with various
fiber contents. At low fiber volume fraction, dilution of the matrix and flaws at the fiber
ends can lead to a high stress concentration, which causes interfacial failure between fiber
and matrix 37. At higher fiber content, the stress is more evenly distributed, i.e.

reinforcement effect compensates for the dilution effect.

10

2. Cellulose Ester

In the middle of 19th century, a cellulose derivative namely nitrocellulose, was
first employed as a synthetic plastic for applications such as film and cast objects. It was,
however, flammable, hence cellulose acetate (CA) took over in early 20th century due to
lower flammability and it is easy formability. This ester derivative of cellulose, in
addition to cellulose ether, has persisted in their intended application of coatings and
films since their development about half century ago. Photographic film, in particular, is
an example of the long product life cycle of cellulose triacetate. However, the innovation
of synthetic, petroleum-based polymers in the middle of 20th century has substituted CA
due to relatively low cost and good properties.

Cellulose ester plastics, despite requiring larger equipment per amount produced
and higher capital cost, continued to satisfy the market and to attract more interest. Mixed
acetyl and longer acyl side chain polymers were introduced in the 1930’s and 1940’s as
cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB). They are
tougher versions of CA and thermally processable with little or no plasticizer. General
features of cellulosic plastics are stiffness, moderate heat resistance, high moisture vapor
transmission, grease resistance, clarity and appearance, and moderate impact resistance.
Some unfavorable properties include the need of plasticizer to widen the window of
processing due to relative lability of the polysaccharide backbone at high temperature,
tendency to creep under load, inelasticity because of the rigid rod nature of cellulose
backbone. Cellulosic plastics are gradually reclaiming interest as a result of their

biodegradability.

11

2.1. What is Cellulose Ester (CE)?

Cellulose ester (CE) is derived from cellulose and is considered to be a semi-synthetic

material due to the fact that some of its

 

 

CHzOH OH basrc characterlstlcs (upper molecular
0 weight, polydispersity, etc) are inherent
O
\O. OH 01/ from its base polymer cellulose, which
predominantly depend upon its tree
— OH CHon _ n

species (source). Cellulose, as depicted
Figure 4. Structure of cellulose comprised

of cellobiose repeat unit with alternate in Figure 4, is a linear polysaccharide
anhydroglucose containing three active OH
groups at position 2, 3, and 6. composed of cellobiose repeat units

linked at the B-1-> 4 positions 38. A cellobiose consists of two anhydroglucose repeat
units with opposite orientation (in terms of hydroxyl groups’ locations) from each other.
These B—l—+ 4 linkages lead to materials with relatively low water solubility and low
thermal stability. Excessive heat during processing will break this linkage hence chain
degradation occurs. In general, cellulose extracted from natural resources is a high
molecular weight non-branching polysaccharide and has a conformation with a great
tendency to form intermolecular hydrogen bonding. Extensive hydrogen bonding
formation produces cellulose fibers which are highly insoluble and relatively
impermeable to liquids 39. The inherent rigidity of cellulose originates from hydrogen
bonding of the free hydroxyl groups in the anhydroglucose repeating unit within and

between chains. The stability of the structure, thus mainly depends upon the ability to

12

form hydrogen bonds. As a consequence, cellulose is not a thermoplastic but degrades on
heating before the theoretical melting point is reached. Breaking the hydrogen bonds by
solubilizing and reacting the free hydroxyl groups yields a thermoplastic, such as
cellulose ethers and cellulose esters.

A convenient basis for nomenclature is the degree of polymerization (DP) and the
degree of substitution (D3) of the anhydroglucose unit, C6HmOs, the building block of
cellulose. DP can be defined as the average number of anhydroglucose repeat unit per

molecule. In natural form, cellulose has

 

 

“20Fl R DP ranging from 4,000 — 20,000 for
O
OR different species of plants 39. During the
\C R O/’
O extraction of cellulose and during
0R CH20R manufacturing of cellulose ester, DP loss

— —n

Figure 5. Structure of Cellulose esters due to chain degradation will inevitably

(Cellulosic Plastics): R = H (Cellulose), . .
acetyl (Cellulose acetate), acetyl and occur. Typrcally, DP falls 1“ the range 0f

propionyl (Cellulose acetate propionate), _ .
or butyryl (Cellulose acetate butyrate). SOD-1,500 for the commercrally avarlable

CE.

In one anhydroglucose unit, there are three hydroxyl groups which, in theory, can
be esterified. The location of these hydroxyl groups (2, 3, and 6 according to IUPAC
nomenclature) is schematically represented in Figure 4. If all three are replaced by acyl
groups (acetyl, propionyl, butyryl, etc), then the degree of substitution (DS) is three.
Figure 5 represents the general chemical structure of commercially available esterified

cellulose including mixed CE (cellulose acetate propionate-CAP, and cellulose acetate

butyrate-CAB).

13

Figure 6 shows a simplified processing scheme of esterification of cellulose
(adopted from Eastman Chemical Company). In general, 4.78 pounds of wood chips are

required to produce 1 pound of
ACETIC ANHYDRIDE

   
  

cellulose. Esterification of extracted

cellulose (high quality, high-alpha

  

cellulose, part of a cellulosic material

WOOD PULP
DRYING & that is insoluble in a 17.5% solution
STORAGE OF . ".'
CELLULOSE a . . .
ACETATE ‘— of sodlum hydroxrde at 20° C) rs
POWDER A
PLASTIC PRECIPITATION & achieved using reactive organic

WASHING

. . , _ anhydrides in the presence of
Figure 6. Srmplrfied processrng scheme of

esterification of cellulose (adopted from

. catalysts such as sulfuric acid. As
Eastman Chermcal Company).

much as 0.59 pound of cellulose is needed to obtain 1 pound of cellulose ester. Careful
derivatization is done on relatively high molecular weight (MW) cellulose focusing on
controlling the inevitable MW breakdown (DP loss) that occurs during industrial scale
processing. To date, an economically feasible technique to make a non-randomized
substitution pattern of CE similar to the stereoregular polyoleflns has not been

discovered.

2.2. Cellulose ester and its applications

Research and development on esterified cellulose during the past 20 years is
reviewed in this part. Polysaccharide cellulose is altered to its organic esters for two main
reasons:

1. Cellulose is not very soluble in common solvents and is not melt processable
hence it decomposes before it undergoes melt flow.
2. This conversion to cellulose ester can greatly change the physical properties of the
parent polysaccharide, which greatly expands its application.
This part of paper contains the review on understanding the structure-property
relationship of cellulose ester (CE) and the performance of cellulose ester in modern
coatings, plastics (biodegradable plastics), composites and laminates. The fundamental
understanding of how esterification of cellulose impacts the overall properties, which

then leads to performance vs. application requirements, is discussed 40.

2.2.1. Modern coatings applications

Cellulose esters, such as cellulose acetate (CA), cellulose acetate propionate
(CAP), and cellulose acetate butyrate (CAB), have played crucial roles in solvent borne
coatings for over 50 years. By using CE in coatings applications, one can obtain:
improved flow and leveling, reduced cratering, reduced dry time, stable carrier for
metallic pigments, polishability, UV stability, resistance to yellowing, improved
sprayability, cold-crack resistance, solvent-craze resistance, reduced plasticizer
migration, viscosity control, pigment-dispersion medium, and redissolve resistance. One
can control structural, physical, and chemical characteristics by manipulating the polymer

structure (CAB for example) mainly by the degree of substitution (DS), hydroxyl content,

15

.1

 

and chain length 4°. By increasing the butyryl content, flexibility, solubility, hydrocarbon
tolerance, and compatibility increase. Increasing DS leads to a decrease in chemical and
grease resistance as well as hardness. Increasing chain length or ester molecular weight
decreases the compatibility and solubility. Toughness and melting point increase with
increasing chain length. Moisture resistance and toughness both decrease with increasing
hydroxyl content.

CE improves metal flake orientation and is used as basecoat for metallic
appearance in automotive coatings. The high viscosity of CE restricts the mobility and
orientation of metal flakes so that they are aligned parallel to the metal surface by film
shrinkage during drying. By adding CE to the basecoat formulation, the dissolution of the
basecoat will be prevented by the relatively high molecular weight.

In the mid 1960’s, the US. government regulations reduced the amount of VOC
released during coating applications. Solvent borne high solid coatings, waterborne
coatings, and radiation curable coatings are examples of new coating technology with
less VOC which can be achieved by modifying the traditional CE to improve the

compatibility with resins and solvents.

Modern coatings technologies

To minimize the amount of VOC release, volatile organic solvents have been
replaced by exempt solvents such as water (waterborne coatings). In addition, radiation
curable coating is another method adopted for modern coatings using CE. In general,
radiation curable coatings typically use non volatile monomers as reactive solvents. Upon

exposure to UV radiation, a hard coating is formed due to crosslinking of monomers and

16

associated resins with essentially no VOC emission. A common strategy to impart water
dispersibility and radiation curability is accomplished by grafting of alternative functional

groups onto CE 40.

A. Waterbome coatings

Several methods were adopted to implement easy dispersion in water:

1) Dispersion of conventional CE in water.
CE with higher hydroxyl content imparts greater hydrophilicity and thus leads to
more stable dispersion than that of conventional CE. Das and coworker 4‘
developed an aqueous dispersion of conventional CE with at least two vinyl
monomers using a high shear technique. In this method shear and cavitation
provided by a microfluidizer enabled micro dispersion. This basic composition,
CE with at least two vinyl monomers, is the aqueous dispersion medium for
metallic pigments for automotive coatings. Aqueous coating compositions are
claimed with good flow and leveling, a high resistance to mottling, acceptable
humidity resistance, good appearance, improved adhesion, and chip resistance
when used in low bake repair processes.

2) Introduction of carboxylate functionality to CE.
The methods included:

a) The radical-initiated grafting of acrylate monomers.

The grafted products contained a copolymer of cellulosic backbone and carboxyl-

containing branches. The anionic aqueous dispersion can be achieved by

17

b)

C)

d)

neutralizing the carboxylic functionality with tertiary amine, ammonia, or
ammonium hydroxide 40.

The esterification with cyclic anhydrides of dicarboxylic acids.

Examples of esterification with cyclic anhydrides of dicarboxylic acids were
cellulose acetate phthalate (CAP) and cellulose acetate butyrate succinate (CAB-
SU). Both result in improved water dispersibility. CAP was used in leather
coating and pharmaceutical enteric coating applications. CAB-SU improved the
metallic appearance by playing an important role in aluminum flake orientation 4°.
Preparing oxidation products of CE (XAB/XAP)

Treatment of CAB with ozone resulted in carboxylic acid containing CE and
pendant peroxide groups. Grafting other functional groups was possible through
the formation of a radical due to the decomposition of formed peroxide. The
hydroxyl groups could be the crosslinking site with melamine formaldehyde, urea
formaldehyde, or isocyanate resins. The product led to improved alcohol tolerance
and resin compatibility, rheology control agents (XAB and XAP), and improved
metal flake orientation. In addition, with proper thixotropy agent, the lower MW
XAB allowed an increase in the solid content from 16.7% to about 31% in
solventbome coating method at constant spray viscosity 40.

Esterification of carboxymethyl CE (CM-CAB).

Allen et a1 ‘2 prepared carboxyalkyl CE by esterification of carboxyalkyl
celluloses with the following benefits: good metallic flake orientation, good face

brightness, excellent substrate wetting and good adhesion. The carboxyl group of

CM-CAB is more hydrolytically stable than that of CAB-SU since the carboxyl

18

group is attached through an ether linkage whereas the latter is attached through
an ester linkage.

e) Cellulose acetoacetate esters: Edgar et al 43 treated cellulose or CE with diketene.
It was readily crosslinkable through enamine formation, Michael addition, and
reaction with melamine or isocyanates. Improved hardness and solvent resistance

were observed as well as water resistance for the product (up to DS = 0.61).

B. Radiation curable coatings

Introduction of pendant branches, readily crosslinkable upon radiation curing or
sometimes with addition of crosslinking agent and/or photoinitiator, into CE formed a
hard, scratch resistant, and solvent resistant surface after UV curing. The presence of CE
gave advantages such as a quick drying as well as workable lacquer. Most of these
pendant-carboxyl containing branches were esterified form of or, B unsaturated
compounds, readily crosslinkable upon exposure of UV radiation. Some examples were:
cellulose carboxylate/glycidyl methacrylate derivatives, cellulose
ester/urethane/methacrylates. Some benefits included enhancement in cure speed,
adhesion, and impact strength (J aylink®). Wood coating application was observed for
cellulose ester urethane. Improved crosslinking efficiency with the reduction of the
reactive diluents needed, improved stain resistance, scratch resistance, and adhesion were
the properties of CE urethane acrylates. Greater improvements in hardness and solvent
resistance as well as pot life were observed for CE modified with both methacrylate and
isocyanate. Maleated CE was prepared by UV radiation of alkali-catalyzed maleation of 1

CAP in the presence of vinyl crosslinking agent and photoinitiator. CAP-maleate can be

19

used in waterborne and solventbome UV curable coatings, which provided coatings with

desired level of resistance and hardness as well as excellent pot life 40.

C. Modification of conventional CBS for solventbome coatings.

Transesterification of mono or di-ester of maleic or fumaric acid led to formation of
maleate and fumarate derivatives of mixed CE. These compounds were then
copolymerized with or, B-ethylenieally unsaturated monomers to give increased viscosity

due to poor compatibility with resins 40.

2.2.2. CE in plastics applications

Mixed acetyl and longer acyl side chains than acetyl were introduced in the
1930’s and 1940’s as cellulose acetate propionate (CAP) and cellulose acetate butyrate
(CAB). They are tougher versions of CA and are thermally processable with little or no
plasticizer. General features of cellulosic plastics are stiffness, moderate heat resistance,
high moisture vapor transmission, grease resistance, clarity and appearance, and
moderate impact resistance. Some unfavorable properties include the need of plasticizer
to widen the window of processing due to relative lability of polysaccharide backbone at
high temperature, tendency to creep under load, inelasticity because of the rigid rod
nature of cellulose backbone. Cellulosic plastics have been proven to be biodegradable,

opening up new interest to a societal need for better plastic waste management 40.

A. Long Chain Ester of Cellulose (LCCEs)

20

The main objective to synthesize cellulose esters with longer chain acids than
those of CA, CAP, and CAB is to avoid the use of external plasticizer. Plasticizer leads to
volatilization, which causes change in material properties 40. There are two combinations
successfully used as solvents in making LCCEs. First is N, N-dimethylacetamide and
lithium chloride (DMAC/LiCl). A homogeneous reaction of cellulose with acid chlorides
or long chain anhydrides in DMAC/LiCl with acid or base catalyst results in CBS with
any desired degree of substitution (DS) and very long chain length (Cu-C20). In general,
glass transition temperature, Tg decreases as ester chain length increases. DMAC/LiCl
combination, however, has inherent disadvantage in recycling the expensive lithium salt.
Thus, as the second combination, the use of DMAC as solvent with titanate catalyst
(T i/DMAC method) in reacting cellulose with carboxylic acid anhydrides produces
partially substituted LCCEs. These are soluble in common solvents and are thermally
processable. A comparison between externally plasticized CE with LCCEs shows that
both have similar mechanical properties with the latter having lower viscosity evaluated
at the same temperature and thus having lower processing temperature. In addition, Tg
and flexural modulus vary linearly as a function of solubility parameter, hence the values

can be predicted.

B. Blends of cellulose esters with other polymers

Blending of cellulose esters with other polymers has been done to compensate for
detrimental properties. For cellulose esters, in particular, blending is one strategy to
improve toughness in both low and high temperature applications, to avoid fugitive

plasticizer and property degradation issue, to improve biodegradability, elasticity and

21

heat deflection temperature, as well as to reduce cost. In general, an immiscible blend
with a well dispersed rubbery phase with diameter of about 10m will significantly
improve the toughness of CE blend. An example of improvement in toughness and
elasticity is the work done by Light et a1 46 where they used a copolymer of methyl
acrylate and butyl acrylate to blend with CAB (DS Butyryl = 1.76, Acetyl = 1.03). An
immiscible blend of 25% copolymer (50:50) resulted in 260% enhancement in impact
resistance. Lee et al 47 blended CAB with a low percentage of polyestercarbonate (PEC)
and reported it to be biodegradable. Another polymeric plasticization was done by
Alberts and coworkers 48 using a series of graft polymers containing ethylene/vinyl
acetate and vinyl monomers to improve softening temperature and impact strength.

In most cases, among different compositions adopted, partially miscible blends
can be achieved when blending CE with other polymers due to the presence of hydroxyl
and carbonyl groups which form hydrogen bonds. In addition, similarity in solubility
parameter, 8 will lead to smaller enthalpy (AI-I) value since

AH = x,x2(5l —§2)2 (1)
where X; and x2 are volume fraction of component 1 and 2, respectively (detailed
example of the use of solubility parameter is shown in APPENDIX 2). This will give
negative Gibbs free energy of mixing, AGm which is the main requirement of miscibility.
49’ 50' 5' The entropy, AS is generally small and positive 52:

A0,, = AH - TAS (2)
One exception is blending CA with poly(vinylphenol) (PVP). All composition ranges

were found to be miscible judging from the clarity of film, DSC and DMA analysis.

22

Landry et al 53 explained that it is due to strong interaction potential between the acidic

phenolic hydroxyl group and hydroxyl/carbonyl groups of CA.

2.2.3. Biodegradation of CE

Biodegradation or mineralization is the microbial catalyzed conversion of a
polymeric substrate in a biologically active environment to biogas (C02 under aerobic
conditions), biomass, and other biological by products. Biodestructability, in contrast, is
40

measured by the weight loss, film or fiber disintegration, or loss in physical properties

(Both definitions are depicted in Figure 7). Cellulose acetate (CA) with a degree of

b - N
.( monomers
(glucose) Biodegradation

A= Enzyme 0 > or

(Cellulase) Deacylation Mineralization
._. co2 2
Hydrolysis or
°( Oxidation Weight Loss
Biodestmctab'l'
Change In my

8 = Substrate. Properties
Polymer (Cellulose
Ester)

 

Figure 7. Simplified biodegradation/biodestructability mechanism of cellulose ester.
substitution (DS) less than 2.5 is inherently biodegradable. In general, as the DS

decreases the rate of biodegradation increases. CA with a DS below 2.1 has a degradation
rate similar or in excess of other known biodegradable polymers. In this part, discussion
about the mineralization and/or biodestructability of CE under aerobic, and composting
conditions was presented. Effect of blending CE with other polymers or plasticizers on

biodegradation rate was also investigated using the composting method.

23

,.
J.
I I ‘-

A. Aerobic biodegradation

Buchanan et al 54 used enriched culture obtained from activated sludge inoculum
to determine the biodestructability of CA film and fibers with DS 1.7 and 2.5,
respectively. Mineralization of CA was also performed by a biologically mediated
pathway using an enrichment assay where the carbonyl carbon of the acetyl substituent of
CA was labeled carbon-14. Deacetylation of CA resulted in a conversion of acetate to
C02; as much as 68% conversion for CA with D8 of 1.85.

In general, as the DS and the acyl chain length increased, the degradation rate
decreased. CA needs to have a DS less than 2.5 to be considered biodegradable.
According to Komarek et a1 results 55, CA with a D8 of 1.85 released more than 80%
C02 in 14 days, whereas CA with DS 2.07 to 2.57 yielded only 60% of C02 in 14-31

days period.

B. Composting

A method to conserve metabolic heat by putting organic matter into piles or
heaps, which will accelerate the natural degradation of a material, is known as
composting 40. It produces humus, heat, biomass, C02, and water. There were two
methods of bench scale composting utilized in this review paper. Method 1 utilized a
compost bioreactor with natural dynamic progression common to municipal composting.
Method 2 used controlled composting conditions, typically at 53°C and 60% moisture.

In method 1, CA with a DS 2.97 and 2.52 underwent minimal weight loss and the

film remained intact. 38% weight loss was observed for CA with D8 of 2.21 and

24

complete disintegration was found for CA with D8 of 2.01. Method 2 had a
biodestruction rate higher than Method 1. CA with DS 2.5 only needed 18 days for film
disintegration. Again, the disappearance of the film was due to a biologically mediated
process. Mineralization of CA required about 24 days and 60 days for D8 of 1.7 and 2.5,
respectively.

In general, biodegradation of CA involved an attack by cellulase enzymes
(microorganisms) on the unsubstituted residues (free OH groups) in the polymer

backbone as depicted in Figure 7 4°.

C. Effect of plasticizer on biodegradation of CA

Using composting Method 1, plasticized CA (CA with DS = 2.05 + triethyl
citrate) films were broken up into chips after 12 days (Buchanan et a1 56). They found that
the remaining chips underwent possible internal chain cleavage. As the amount of citrate
plasticizer increase, the biodegradation rate was increased for plasticized CA (with CA of
DS = 2.5) exposed to Method 2 (Ghiya et al). Hence, a slower degrading CA can be
altered into a faster degrading CA with addition of triethyl citrate plasticizer. The
accelerated degradation is due to synergistic effect of plasticizer on CA. Rapid

mineralization was observed as well after addition of plasticizer into CA 57.

D. Efi’ect of polyester on biodegradation of cellulose ester
Buchanan and coworkers 58 blended cellulose acetate propionate (CAP) with
poly(ethylene glutarate) (PEG), poly(tetramethylene glutarate) (PTG), poly(hydroxyl

butyrate) (PHB), and poly(lactic acid) (PLA) which formed, in general, a stable, optically

25

clear, amorphous blend. They used composting method 1 and found a general trend such
that when the amount of PEG and PT G increased and as DS of CAP decreased, the
weight loss increased. When PEG was the continuous phase and CAP DS was lower than
2.15, maximum weight loss was observed. CAP/PHB and CAP/PLA blend were more
resistant to microorganisms as compared to CAP/PEG and CAP/PT G blends. The authors
mentioned that the PI-IB degradation mechanism was similar to that of cellulose where it
required the attachment of an extracellular enzyme, whereas PEG, PTG and PLA
underwent hydrolytic degradation to form monomer or short chain oligomers prior to
being disintegrated by microorganisms. In addition, at fixed D8 of CAP, the rate of
composting and weight loss increased as the PEG amount increased. Initial degradation
was due to chemical hydrolysis of PEG which depended upon the temperature of
composting and D8 of CAP. Blend of CA (DS = 2.5)lpoly(ethylene succinate) (PBS)
with CA being the continuous phase revealed the degradation rate similar to CA and thus

the blend’s (75% CA and 25% PES) degradation rate was dominated by CA.

E. Effect of polysaccharide on biodegradation of cellulose ester

Mayer et a] 59 observed that blending starch into a CA matrix with propylene
glycol as a plasticizer decreased its degradation rate due to starch dispersion inside the
slower degrading CA. A synergistic effect on biodegradation was found when making
nonwoven fabrics containing cotton and CA fibers. The addition of CA with DS of 2.0
into CA with D8 of 2.5 was proven to increase the biodestructability of the latter in
composting method 1. The rate was further increased by addition of PEG plasticizer in

the formulation (Buchanan et a1 60).

26

F. Future work of biodegradation of CE

It would be desirable to develop or find plasticizers or polymers which would
accelerate the biodegradation rate of the highest volume current commercial products yet
slow the degradation of CA (DS = 2.5) to an acceptable level, lower the cost, and
enhance the properties. The mechanism of biodegradation and biodestruction of cellulose
ester should be thoroughly investigated and defined. One way is to isolate and

characterize the enzymes responsible for the disintegration of cellulose ester.

2.2.4. CE in composites and laminates

Good adhesion can be observed between cellulose esters and natural fillers due to
their similar structure. Several applications in composites and laminates include paper
applications, polymer-polymer laminates for packaging material and food contents, ,

structural and utility applications 40.

A. Cellulose ester in laminate and composite paper applications
A low level of CA (1-5%) can be incorporated into paper to increase the tensile
strength of the paper, and the ability for print inclusion. A higher level of CA tends to

decrease tensile strength.
B. Polymer-polymer laminates containing cellulose esters

The residual hydroxyl groups on cellulose esters help to compatibilize it with

other polymers such as polyurethane and PVC. CA sandwiched between PVC resulted in

27

a light-transmitting, insulating panel for potential building and construction applications
(Rankle et al 6'). CA can be used as a protective layer to prevent migration of different
materials into each other, mainly due to the inherent grease resistance of CA. Lu et al 62
of Mobil Corporation made a microwavable laminate food container with an inner
protective layer of CAP which prevents direct contact between the food and the bulk

container plastic.

C. Cellulose ester as structural and utility applications

Duckett et a1 63 described the use of 25-7 5% cellulose acetate (DS 2.5) with cotton
fibers to make a nonwoven composite fabric. They pretreated them with acetone vapor so
that the cotton fiber and CA can be thermally bonded at lower temperature (90-1400C) as
compared to non-solvent assisted bonding (176-1900C). The resulting fabrics were of
higher tensile strength and were proven to be biodegradable.

Warth and coworkers 6" worked on compatibilization of cellulose/cellulose ester
composites. They emphasized reactive extrusion with hydroxyl containing plasticizer and
fillers (starch and lignin) in the presence of a lactone for grafting reactions on available
hydroxyl groups.

Glasser et al 65 utilized cellulose acetate butyrate (CAB) as a matrix for
continuous cellulose fibers (yellow poplar wood fiber and regenerated cellulose - Lyocell
fibers) reinforced material. In their first paper, they investigated the effect of different
methods of processing on the performance of the composites.

Lyocel fiber is regenerated from cellulosic solutions of N-methyl morpholine-N-

oxide/water. Cellulose acetate butyrate (CAB) is particularly promising as a matrix

28

material. Its thermoplastic flow and fiber wetting characteristics have resulted in
composites with high fiber content (>60%) and high modulus (>20GPa). An unusual
occurrence was observed for Lyocel fiber/CAB which had brittle matrix failure at strain
considerably below the value of lyocel fiber strain. Generally, composites fail when their
fibers have reached their ultimate tensile strain, since matrix strain is greater than that of
fiber.

In their research, they used CAB 381-20 and they encountered problems such as
uniformity of matrix and fiber distribution that were inherent in the prepregging
technique. The solvent removal technique was responsible for extensive void formation
in the matrix, and difficulties with fiber wetting by liquid matrix were found, especially
in the film stacking technique.

Depositing powdered matrix from turbulent air assisted by the electrostatic charge
different between fiber and particle led to uniform surface coverage and adequate
adhesion. Usually smaller matrix with particle size about 600nm (<10mm) was good for
fiber reinforcement. Oversized matrix particles led to insufficient (gap filling) melt flow,
and resulted in void formation.

The film stacking method had a non-uniform fiber distribution problem. They
attempted to remove excess matrix by the use of pressure and temperature during
consolidation and the results were a material with extensive discoloration.

After all methods had been investigated, they concluded that solution prepregging
provided the most uniform matrix application to fiber surfaces and was most repeatable.
It utilized most common solvents for cellulose: acetone and methyl ethyl ketone (MEK)

to coat the fibers with the matrix and to tailor fiber volume content to desired level. Note

29

here that modulus did not depend upon the method of manufacturing or void content.
Tensile strength could not be predicted by rule of mixtures since it depended upon fiber-
matrix stress transfer and fiber-matrix orientation.

Lyocel fiber reinforced composites failed at 3 to 4% strain due to matrix failure.
The finer fibers, however, continued to carry on load and remained intact. This indicated
interfacial delarnination problems were present. In SEM images, plenty of fiber pullouts
were observed indicating lack of adhesion between fiber and matrix. Note here that fibers
might have suffered molecular or macroscopic damage during processing which resulted
in decreased tensile strength. Cellulose fiber mechanical property measurements
depended upon moisture content. With low moisture content, the strength could be
unrealistically high. Lower strain values of composites compared to that of pure matrix
and pure fiber were due to inadequate fiber-matrix wetting or adhesion.

The second part of Glasser et al paper discussed the fiber surface modification
and consolidation conditions of CAB based biocomposites 66. Acetylated lignocellulosic
fibers had considerably stronger adhesion to a thermoplastic cellulose ester compared to
the unmodified fiber. Continuous Lyocel fiber reinforced composites, however, did not
show a significant increase in tensile strength and modulus. SEM showed no significant
decrease in interfacial failure (fiber pull outs).

Theory of Agarwal and Broutman relating aspect ratio and optimum stress
transfer from matrix to fiber for short fiber composites (similar to that of shear slip
analysis):

I o

E 22'

3.1

 

Where 1,; = fiber critical length, by definition it is a minimum fiber length at which the
maximum fiber stress (om) occurs at the breaking strength of the fiber itself or in
between the fiber and matrix; (1 = diameter of fiber, and 1: = fiber matrix interface. The
theory suggests that increasing the amount of stress transfer between the matrix and fiber
lowers the fiber critical length to an optimum length. As a result, it will increase the
efficiency of short fiber reinforcement due to increased interfacial adhesion, which leads
to increased tensile strength and modulus. If the fiber is too long, however, the amount of
stress transferred from matrix to fiber will remain constant at the breaking stress of the
fiber, thus changes in interfacial stress transfer are undetected. They did not observe any
increase in strength proving that interfacial stress transfer was not a limitation in this
system.

Melt consolidation studies showed that two methods seemed to produce the
lowest void content. The conditions were: 200°C, 81.4 kPa for 13 minutes and 200°C,
163 kPa for 8 minutes. Maximum strength was achieved with above consolidation
conditions: 246 MPa in tensile strength and about 21.6 GPa for tensile modulus.

SEM images indicated that fiber pullouts were present in relatively small
amounts. Cohesive failure of the matrix was observed in that bundle of fibers with
extensive amounts of fractured matrix particles adhering to the fiber surface, i.e. matrix
spread over the fiber and adhered to the surface.

Matrix embrittlement sometimes results from matrix transcrystallization on fiber
surfaces indicating the comparison between sized and unsized fibers are important. In
plasticization, void content is also recognized to influence ultimate strength but not

stiffness. By plasticization, the material will have superior melt characteristics. The effect

31

of fiber sizing will occur in composites interfacial properties (adhesion fiber to matrix).
Acetylation has failed to produce significant strength gains in lyocel/cellulose ester.

Composites made from unsized fibers gave higher ply thickness, and hence gave
double void contents, which led to lower ultimate tensile strength. Sized fibers were easy
to handle and reduced per-ply-thickness, and produced composites with superior strength
and low void content.

The third part of Glasser et a1 discussed the effect of fiber type on the properties
of CAB based composites. Rayon, for example is less organized and less costly 67. It has
fewer filaments in a yarn and thus adheres to the matrix more uniformly. Rayon
composites gave lower void content indicating better consolidation behavior. Alternating
unidirectional plies gave lower properties than that of unidirectional. In fiber stacking,
they observed lower strength and stiffness due to reduced interply adhesion and thicker
composites with the possibility of more defects. Rayon composites exhibited the usual
strain failure mode (fiber<matrix) and thus it gave greater composite failure at strains
than lyocel composites.

Gatenholm et al investigated the effect of furfuryl alcohol on the performance of
CAB biocomposites 68. Furfuryl alcohol (FA) is a product of the acid-thermal
transformation of biomass-based pentoses, such as xylan whose traditional applications
include foundry cores and molds, polymer concretes, coating for corrosion resistance,
wood adhesives and binders, materials with low flammability and smoke release, and
graphitic electrodes from carbonaceous products. FA polymerizes under acid conditions
and heat and ultimately yields crosslinked resins. Cellulose acetate butyrate used here is

CAB-553-0.4 supplied by Eastman Chemical Co.

32

0"
5

 

Flax reinforced CAB/FA was prepared by mixing the required amount of FA with
CAB while stirring to achieve a homogeneous clear solution or paste. Composites with
flax fiber content of 50wt% were prepared by the addition of CAB/FA mixtures, catalyst,
and acetone (evaporated afterward under vacuum) to improve the impregnation of the
flax mat. The composite was prepolymerized in an oven followed by curing in
compression molding with applied pressure.

The melt viscosity of CAB is dramatically decreased after introducing FA, which
acted as a CAB solvent and facilitated the impregnation of the flax fiber mats. Modulus
and maximum fiber stress (strength) of flax composites was observed to increase linearly
with decreased toughness with addition of FA into CAB. DMA showed that
CAB/poly(furfuryl alcohol) (PFA) matrices were miscible because the glass transition
temperature (T g) in the resulting blend occurred between the Tg of homopolymers. This
indicates a good combination between thermoplastic and therrnoset because of the
miscibility and affinity. Increasing the FA content, resulted in the improvement of storage
modulus. The flexural strength and modulus of the composites were about 70 MPa and 6

GPa with addition of 25wt% FA into CAB/Flax composites.

3. Conclusion

Cellulose esters (CEs), as have been illustrated above, have much potential in
many applications. In this particular project, we examined the potential of CEs reinforced
by natural fibers to replace/substitute petroleum based glass reinforced polypropylene for
automotive parts. The structure and property relationships of cellulose esters, namely

cellulose acetate (CA) and cellulose acetate butyrate (CAB), starting from matrix

33

development for biocomposites until the development of the “engineered biocomposites”

for “greener” automotive and structural applications were discussed.

34

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39

1. Development of Renewable Resource Based Cellulose Acetate
Bioplastic: Effect of Process Engineering on the Performance of
Cellulosic Plastics

A. K. Mohanty“, A. Wibowo, M. Misra, L. T. Drzal

Composite Materials and Structures Center

Department of Chemical Engineering and Materials Science
2100 Engineering Building, Michigan State University

East Lansing, MI 48824

* Corresponding author: Tel: +1-517-353-5466; Fax: +1-517- 432-1634; E-mail address:
mohantya@egr.msu.edu

1. Abstract

As part of an ongoing program on the development of biocomposites from
renewable resources, e.g. cellulosic fibers and cellulosic plastics, to produce more eco-
friendly/green automotive parts, this paper deals with the development of a cellulose
acetate biopolymer. Plasticization of this biopolymer under varying processing conditions
so as to make it a suitable matrix polymer for biocomposite applications was studied. In
particular, cellulose acetate was plasticized with varying concentrations of an eco-
friendly triethyl citrate (TEC) plasticizer, versus a conventional, petroleum derived
phthalate plasticizer. Three types of processing were used to fabricate plasticized
cellulose acetate parts: compression molding, extrusion followed by compression
molding, and extrusion followed by injection molding. It was found that processing
mode affected the physico-mechanical and thermal properties of this cellulosic plastic.
Compression molded samples exhibited the highest impact strength tending towards the
impact strength of a thermoplastic olefin (TPO), while samples that were extruded then
injection molded exhibited the highest tensile strength and modulus values. Increasing

the plasticizer content in the cellulosic plastic formulation improved the impact strength

40

and strain to failure while decreasing the tensile strength and modulus values. The
coefficient of thermal expansion (CTE) of the cellulose acetate increased with increasing
amounts of plasticizer. Through plasticization, this cellulose acetate was found to be
processable at 170-1800C, approximately 50°C much below the melting point of neat
cellulose acetate.

Keywords: Cellulose acetate, plasticization, citrate plasticizer, compression molding,

extrusion and injection molding, thermo-mechanical pmperties.

2. Introduction

Sustainability, industrial ecology, eco-efficiency and green chemistry are guiding
the development of the next generation of advanced materials, products and processes.
Biodegradable polymers and biobased polymer products based on annually renewable

agricultural and biomass feedstock

 

Cell lg t
u as as can form the basis for a portfolio of
on _ c.4203 05 CH20R sustainable, eco-efficient products
0 0 OH

on ‘ OR OR ‘0 on that can capture markets currently

0 o _
01420.; OR CHzon on dorrrrnated by products based
— '— " exclusively on petroleum feedstock

 

Figure 1. Structure of Cellulose esters 1-3
(Cellulosic Plastics): R = H (Cellulose), acetyl '
(Cellulose acetate), acetyl and propionyl
(Cellulose acetate propionate), or butyryl
(Cellulose acetate butyrate).

Cellulose from trees is
attracting interest as a substitute for
petroleum feedstock in making
plastics (cellulosic plastic - - cellulose esters) for the consumer market 1.

Cellulosic plastics (Figure 1) like cellulose acetate (CA), cellulose acetate

41

propionate (CAP), and cellulose acetate butyrate (CAB) are thermoplastic materials
produced through the esterification of cellulose. A variety of raw materials such as:
cotton, recycled paper, wood cellulose and sugarcane are used in making cellulose ester
biopolymers in powder form. Such cellulose ester powders combined with plasticizers
and additives are extruded to produce various grades of commercial cellulosic plastics in
pelletized form. The phthalate based plasticizers, currently used in commercial cellulose
ester plastics, is under environmental scrutiny and perhaps poses a health threat, raising
concerns about their long-tenn use. One of our objectives is to replace phthalate
plasticizers with an eco-friendly plasticizer based on citrate and blends of citrate and
derivatized vegetable oil when designing sustainable cellulosic plastiCs for biocomposite
applications. Besides material selection, processing plays a vital role in the design of new
biobased polymers. Most commercial cellulose acetate products are clear, strong and stiff
and have a broad range of applications. Some of the applications of cellulose ester
biopolymers are film substrates for photography, toothbrush handles, selective filtration
membranes in medicine, and automotive coatings 9. In general, those cellulose acetates,
which have an acetyl substitution number of 2.2 or less, are biodegradable in soil and
marine environments as well as being suitable for composting. Higher substitution
numbers ranging from 2.2 to 3.0 are less biodegradable. The main drawback of cellulose
acetate is that its melt processing temperature is very close to its decomposition
temperature, as determined by the structure of its parent cellulose ‘0. This requires that
cellulose acetates should be plasticized if they are to be used in thermoplastic processing
applications. The plasticization of cellulose acetate, although reported by Ghiya et al. '1

has not been studied in terms of its effects on processing-properties in detail.

42

Citric acid esters are non-toxic and have been approved as plasticizers for many
applications such as additives in medical plastics, personal care, and food contact and are
useful in a variety of polymers. Since citrate esters are a derivative of natural compounds,
it is of interest to determine their effect on plasticization with special reference to
processing parameters. The effects of TEC plasticizer on the physico-meehanical
properties and thermal behavior of the resulting cellulosic plastics with varied
processing conditions were studied.

The development of cellulose plastic as a matrix polymer in biocomposites
requires that properties such as: percent elongation, flexibility and impact behavior can
be tailored to meet the needs of the application. Our recent research results '2'” have
demonstrated that natural fiber reinforced polypropylene (PP) composites have potential
to replace glass fiber-PP composites. With the goal to replace or substitute Polypropylene
with cellulosic plastic when designing more coo-friendly and sustainable biocomposite
materials, our fundamental studies focus on structure-processing-property relationships.
This paper gives a detailed analysis of the effect of material selection and processing

conditions on the performance of cellulose ester bioplastics.

3. Experimental
3.1. Materials

Powdered cellulose acetate (CA) free from any plasticizer or additives was
supplied by Eastman Chemical Co., (Kingsport, TN) for the present study. The degree of
substitution of the cellulose acetate was approximately 2.5. The triethyl citrate (TEC)

plasticizer was provided by Morflex Inc., (Greensboro, North Carolina). The CA powder

43

was dried overnight in vacuum oven at 80°C before processing.

3.2. Processing

Three types of processing commonly used for thermoplastics were used to make
test specimens of plasticized cellulose acetate so as to study the effect of processing
conditions on the physico-mechanical properties of the corresponding cellulosic plastics.

I. Compression Molding: A specific amount of liquid plasticizer was added
drop-wise onto dried cellulose acetate powder and the mixture stirred mechanically in a
kitchen mixer for 30 minutes. The mixture was compression molded using a three-piece
picture-frame mold in a Carver Press SP-F 6030 at three different temperatures 170, 180
and 190°C. During compression molding at a specific temperature, for the first ten
minutes the materials were kept at a constant pressure of 1.1 MPa and then held at 2.67
MPa for last five rrrinutes. The samples were cooled while maintaining the pressure.

H. Extrusion Followed by Compression Molding: In this process, the cellulose
acetate powder and TEC plasticizer in the desired pr0portions were pro-mixed
mechanically and then extruded as therrnoforrnable sheets. The resulting therrnoforrnable
sheets were then compression molded to make the test specimens. In this case, the
cellulose acetate with TEC plasticizer was fed into a twin-screw extruder ZSK 30
(W emer-Pfleiderer). The temperature profile of extruder from Zone 1 through Zone 6
was kept between 170 to 180°C and the exit die temperature was 190°C. The extruder
was operated at two different rpm screw speeds (100 rpm and 200 rpm). The resultant
thermoforrnable sheets were compression molded at 180°C and 1.1 MPa for 10 minutes

and at 2.67 MPa for 5 minutes and then cooled down while maintaining the pressure.

HI. Extrusion Followed by Injection Molding: The samples were extruded as
described above (subsection H) and instead of collecting thermoforrnable extruded sheets,
through proper die arrangement, the thin strands of plasticized cellulose acetate were
pelletized and stored for future injection molding. The injection-molding machine used
was an 85-ton Cincinnati-Millacron press. The molding conditions were as follows:
temperatures on zones 1 to 3 as well as the die were held between 180 to 190 0C with 45
seconds cooling time, the fill, hold and pack pressures were maintained at 8.2, 4.8 and 5.5
MPa, respectively. In addition, the optimum processing temperature for injection molding
was determined to be between 180 and 190 0C. The resulting dumbbell shaped products

were used appropriately for mechanical properties based on ASTM standards.

3.3. Analysis and Testing

Differential scanning calorimetry (2920 Modulated DSC, TA instruments,
USA) and thermogravimetric analysis (Hi-Res TGA 2950, TA instruments, USA) were
used for the thermal analysis of cellulose acetate plastics. In DSC measurements, the pure
and plasticized cellulose acetate samples (10 mg) were sealed in aluminum pans and
heated from 25 to 200°C at a rate of 10°C/min (cpm), held at 200°C for 5 min, then
cooled down to -60°C before a second heating scan at 10 cpm from -60 to 300°C. A
nitrogen purge (50 ml/min.) was maintained throughout the test. Dimensional stability of
the materials as a function of temperature were measured as per ASTM D 696 using
Therrno-mechanical Analyzer (TMA) 2940 from TA instruments, USA. The heating rate
was 4 cpm over a temperature range of 25 to 100°C. The coefficient of thermal expansion

(CTE) at 800 C was determined with respect to 25°C as the reference temperature from

45

 

 

 

 

30 2.0
+Strength

1; 25 ~- o Modulus I if
“- c:
E 20 - . T;
8 e e 2
r: 3
0 15 ~- -~ 1.5 o
a E
o 10 r f g
.2 5 -- i i ..

0 4 i i 1.0

A B c D

Figure 2. Effect of processing
conditions on the tensile strength and
tensile modulus of plasticized (30%
TEC) cellulose acetate, A
compression molded (CM) at 190 0C, B
= CM at 180 °C. C = CM at 170 °C. D
= extruded at 180°C (100 rpm) and CM
at 180 °C.

the samples for impact measurements
was maintained at around 3.15 mm for
compression molded test specimens and
3.3 mm for injection molded test
specimens. Samples were notched using
a TNH Notchin g Cutter, model TM] 22-
05, and were left for one day at ambient
temperature to equilibrate prior to
testing. A pendulum capable of

delivering impact energy of 6.8 J was

used for these experiments.

Impact Strength (Jlm)

the resulting plot in units of urn/moC.
Tensile strength and elongation at
break (EB) were determined using a
United Calibration Corporation test
frame, model SFM-20 as per ASTM D
638. Test conditions were: a crosshead
speed of 5.1 mm/min, 0.9 kg preload and
a 454 kg capacity load cell. The impact
resistance testing was carried out with an
Impact tester from Testing Machines Inc.,

model 43-0A-01, according to IZOD

method (ASTM D 256). The thickness of

 

 

 

 

750
i ‘* .. i"
X
500 __ 27 g
l e s
l:
—- 18 .3
250 -— a
1:
.Impact Strength i -- g 2
oElongation ru
0 l i i 0
A B C D

Figure 3. Effect of processing conditions
on the impact strength and elongation at
break of plasticized (30% TEC) cellulose
acetate, A = compression molded (CM) at
190 °C. B = CM at 180 °C. C = CM at 170
°C. D = extruded at 180°C (100 rpm) then
CM at 180 °C.

46

Environmental scanning electron microscope (ESEM) images were taken using a

Phillips Electroscan 2020. The specimens were fractured after cooling in liquid nitrogen

 

Figure 4. ESEM images of: (a) pure
cellulose acetate (CA) powder with
particle size of 5-10 mm (500x, scale bar
100 um), (b) extruded followed by
compression molded CA plastic with
30% Triethyl citrate plasticizer (1500X,
scale bar 30 um)

47

I Recently,

and were gold sputtered before taking the

images.

4. Results and Discussions

Cellulose acetate (CA), a

renewable resource based plastic, has

excellent potential to replace

polypropylene as a matrix polymer in
composite formulations to design eco-
friendly biocomposite materials ‘5'”.
several of world’s leading
chemical companies have announced

major new businesses based on

bioresources instead of petrochemicals 18'

'9 the

as biopolymers move into
. 20. 21 -

marnstream . Bropolymers were not

developed by nature to be plastics, but to

act as cellular components to build the

structure of an organism to survive in the

environment. Biopolymers need to be

modified so as to make them suitable

matrix polymers for commercial composite applications. The main drawback of cellulose

acetate plastic is that its melt processing

 

 

 

 

60 8
1? lStrength 1; temperature is close to its decomposition
g i oModulus 6 g
g 40 - E temperature. Cellulosic plastics may be
c - '5
g I 4 E developed either by blending with
.2 20 “L . 3
'2 . ” 2 'g another suitable polymer or being
.2 e .2
0 . . O plasticized with a miscible plasticizer for
20 30 4O
Pleetlclzer Content (“11%) specific applications. Cellulose ester

Figure 5. Effect of plasticizer content on polymers are blended with different
the tensile strength and tensile modulus

of extruded followed by compression polymers for film and molding

molded cellulose acetate plastics.

applications 22'”. Plasticizers are widely

used in plastics industry to increase the processibility, flexibility, and ductility of

polymeric materials.

4.1. Comparison of Properties of
Plasticized Cellulose Acetates - -
Compression Molding vs.
Extrusion followed by
Compression Molding: The
effect of the above processing
conditions, on the tensile strength,
modulus and impact strength of

plasticized cellulose acetates

 

 

 

 

270 100
Ilmpact Strength A
E oElongatIon ¥ __ 80 g:
” i
V 180 m
S
‘8 I ‘" 6° ‘1”-
I
5 6 -~ 40 5
O
i f 2° .8
- ru
0 i i 0
20 30 40
Plasticizer Content (wt%)

Figure 6. Effect of plasticizer content on the
impact strength and elongation at break of
extruded followed by compression molded
cellulose acetate plastics.

48

containing a fixed amount of TEC plasticizer (30 wt.%) is illustrated in Figures 2 and 3.
The samples A, B and C are obtained through compression molding (CM) of the mixture

of pure cellulose acetate powder and

 

20w1% TEC plasticizer at temperatures of 190,

180 and 170°C respectively. As can be
3mm

 

Miss

observed, the sample processed at 170°C

Stress (MPa)
§
58

(“C”) exhibited superior tensile and

 

 

O

o 10 20 30 40 50 impact strengths over the samples

Str 1 as
a M ) processed at higher temperatures. The

Figure 7. Stress Strain plots for varied
amount of plasticizer of extruded followed plasticized cellulose acetate as obtained

by compression molded cellulose acetate
plastics. through extrusion followed by

compression molding (“D”) exhibited better tensile properties versus compression-
molded samples. All of the plasticized cellulose acetates had elongation at break (Figure
3) in the range of 24 to 34%. If we compare the tensile properties of the compression-
molded sample (“B”) versus the extruded followed by compression-molding sample
(“D”) we find a 48 and 13% enhancement of the tensile strength and modulus
respectively of the latter material. The superior strength of the extruded material is
attributed to the better mixing of cellulose acetate and TEC plasticizer during high shear
extrusion processing. The lower impact strength of the extruded followed by
compression molding material versus the compression molding material suggests that
cross-linking of cellulose acetate with TEC may have occurred during the melt extrusion
processing, which exposes the material to shear stresses during processing. This

observation is supported by the environmental scanning electron microscope images in

49

Figure 4. As can be seen, homogeneity of the blended mixture was observed indicating
that it will serve as a good polymer matrix. Similar reduction in the elongation at break
for sample “D” is also observed as shown in Figure 3. The plasticized cellulose acetate
obtained through compression molding exhibits superior impact properties as contrasted
with samples obtained through extrusion followed by compression molding. The main
motivation of this research is to develop cellulose ester plastics to replace polypropylene
in biocomposite applications. Although PP has adequate tensile and flexural properties,
its low impact strength, necessitated the invention of thermoplastic olefins (TPO) - -
known as extended polypropylene which is industrially produced through incorporation
of an elastomeric component into the virgin polypropylene. The impact strength of
plasticized cellulose acetate under various processing conditions ranges from 173 Mm to
419 J/m (Figure 3), which approaches to impact strength value of TPO 25. Such high-
impact plasticized cellulosic acetate can be used as a suitable matrix for biocomposites in

automotive applications that require superior impact strength.

4.2. Comparison of Properties of Plasticized Cellulose Acetates - - Extrusion
Followed by Compression Molding vs. Extrusion Followed by Injection Molding:
Plasticized cellulose acetate fabricated through extrusion followed by injection molding
showed better tensile properties and lower impact properties over its counterpart obtained
through extrusion followed by compression molding. As can be seen in Table 1a, when
using injection molding versus compression molding, we get a significant increase in
tensile strength and modulus by about 40 and 60%, respectively. The additional shear

force applied during injection molding may have enhanced the cross-linking between the

50

120 cellulose acetate and TEC

 

 

 

 

 

10° ‘ plasticizer, thereby increasing
a °°*
‘2' 60 . the stiffness of the injection-
0
'5
B 40 . molded material. As expected,
20 - . .
the increased stiffness
o . . . .
30 130 230 330 430 530

sacrifices the impact strength
Temperature (°C)

. , of the injection-molded
Frgure 8. TGA results of werght loss versus

temperature with varying amounts of material (Table 1a).

 

 

plasticizer of extruded followed by
compression molded cellulose acetate
plastics.

Table la“. Effect of optimized processing conditions on the tensile strength and the
tensile modulus of plasticized (30% TEC) cellulose acetate

 

 

 

Processing Tensile Tensile Elongation at IZOD Impact
Strength (MPa) Modulus (GPa) Break (%) (J/m)
E—CM at 180°C 26 i 1.1 1.3 :l: 0.1 24 i 0.0 173 i 35
EM at 190C 36 i 0.4 2.11: 0.2 13 :t 1.7 68 2t 15

 

 

 

 

 

Table 1b*. Effect of temperature variation for extruded - injection molding CA

 

 

 

 

 

 

 

 

 

 

 

plastics (30% TEC)
Processing Tensile Tensile Elongation at IZOD Impact
Strength (MPa) Modulus (GPa) Break (%) (J/m)
E-IM at 180°C 27 :l: 3.2 2.1 :I: 0.0 8.3 :l: 3.2 241 :I: 0.0
E-IM at 190°C 36 :I: 0.4 2.1 i 0.2 13 :t 1.7 68 i 15
Table lc*. Effect of varying rpm of extruder for extruded - injection molding CA
plastics (30% TEC)
Processing Tensile Tensile Elongation at IZOD Impact
Strength (MPa) Modulus (GPa) Break (%) (J/m)
E-IM at 190°C, 36 i 0.4 2.1 :I: 0.2 13 :l: 1.7 68 i 15
100 rpm
E-IM at 32:1:3.8 2.1i0.l 14i 1.2 88:21
190°C, 200
rpm

 

 

 

 

 

 

*E-CM = Extruded followed by Compression Molding;

E-IM = Extruded followed by Injection Molding.

51

 

 

 

Note: Reported are mean values along with its standard deviation (i)

Different processing temperatures in the injection molding experiments were used
to study the effect of temperature on the properties of the resulting cellulosic plastics
(Table 1b). Increasing the injection molding temperature from 180 to 190°C enhanced the
tensile strength by about 32%, whereas the impact strength decreased by about 75%.
Hence, a 190°C processing temperature was utilized. To study the effect of extruder
screw rpm on properties, we used the plasticized cellulosic acetates fabricated through
extrusion followed by injection molding processing for comparison. We found that
material processed at 100-rpm exhibited better mechanical properties than that processed
at ZOO-rpm (Table 1c). This means that the longer resident time (lower rpm) gave better
mixing between the CA powder and the TEC plasticizer, which led to a stiffer product as
cross-linking was enhanced. In all of our previous descriptions on extrusion, we had

processed the materials at 100 rpm.

4.3. Effect of Plasticizer Content on Performance of Cellulose Acetate Plastic: In our
above-mentioned discussions on processing-property evaluations of plasticized cellulose
acetate, we had kept the plasticizer content at 30wt.%. In order to investigate the effect of
varying the amount of TEC plasticizer on the performance of cellulosic plastics (Figs. 5-
7) we have prepared samples fabricated through extrusion followed by compression
molding. It was not possible to process the cellulose acetate with plasticizer content of 10
and 15 wt.%. With an increase of plasticizer content from 20 to 40wt.%, the tensile
properties decreased while impact strength and percent elongation increased (Figs. 5 and

6). The plasticized cellulose acetate containing 30 wt.% plasticizer shows tensile strength

52

about 45% less and impact strength about 86% more over plasticized cellulose acetate
with 20 wt.% plasticizer. In order to obtain cellulosic plastic with an optimum balance of
strength and stiffness, 30 wt.% plasticizer content cellulose ester is quite appropriate for
biocomposite applications. Again, it is easier to process cellulose ester with 30 wt.%
plasticizer over 20 wt.% plasticizer. Although 40 wt.% plasticized cellulose ester would

be processed easily, the tensile

 

 

180 strength of such plastic would be too
8 Q
°E 160 i low for any structural applications,
3
‘3 “OI es iall in com site area As
E 4 Q pec Y P0 -
O 120

100 r . much as 21 and 63% improvements

 

 

20 30 40

“and”, Content (wt%) in rmpact strength and elongatlon at

break respectively were found in 40
Figure 9. TMA results of the coefficient of

thermal expansion for varied amounts 0f wt% plasticizer content cellulose
plasticizer of extruded followed by

compression molded cellulose acetate ester plastic as compared to 30 wt%
plastics.

plasticizer content cellulose ester

plastic (Figure 6). In stress-strain plot (Figure 7), 40 wt% plasticizer content cellulosic

plastic sample covers a larger area under the curve indicating that it is a ductile material,

and hence it has the highest impact strength and elongation at break as shown in Fig.6.

4.4. Thermal and Thermomechanical Behaviors of Plasticized Cellulose Acetates:
The plasticized cellulose acetates fabricated through extrusion followed by compression
moldings were studied via thermal analysis. The TGA curves of pure and plasticized

cellulose acetates (with 20, 30 and 40% TEC plasticizer) show overall addition of

53

plasticizer shifts the maximum degradation temperature towards the higher temperature
range (Figure 8). Pure cellulose acetate shows a smooth weight loss curve as contrasted
with plasticized cellulose esters. We observe that as the plasticizer content increases, the
maximum decomposition temperature of CA plastics also increased as can be observed in
Figure 8. Additionally, the thermal stability of the materials shows that as the plasticizer
content increases, the coefficient of thermal expansion (CTE) increased as well (Figure
9). It is well known that the CTE of the low molecular weight TEC plasticizer is greater
than the cellulose acetate polymeric matrix and thus with increase of plasticizer content
in the formulations, the CTE of the resulting plasticized cellulose acetate increases, as
shown in Figure 9. The presence of plasticizer limits the crystallinity in the matrix and
increases the free volume present in the system. Thus, increasing the amount of

plasticizer in the formulation

 

 

 

 

‘ 40mm gives a more flexible material.
30m . .
I The increase of CTE wrth
20w
g was decrease of rigidity of
g . polymeric materials also has
i , been observed by Tyan et a1. 26.
I
The sharp crystalline
-60 0 so 120 180 240

melting peak of pure cellulose
Temperature (“6)

Figure 10. DSC curves for varied amounts of acetate also dlsappears upon

plasticizer of extruded followed by compression

molded cellulose acetate plastics. plastlmzatlon as observed from

54

the DSC curves (Figure 10) confirming the shift of properties of the CA from rigid to
ductile. Similar observations from DSC have also been noted by Ghiya et al. 1', which

indicates that the resulting plasticized CA is an amorphous material.

5. Conclusions

Cellulose acetate, a renewable resource based bioplastic has excellent potential to
replace petroleum based polyolefins as matrix polymers when designing green, eco-
friendly biocomposites for a new millennium. It is encouraging to see that with control
of plasticizer content the cellulosic plastic can exhibit the stiffness and toughness
properties comparable to either PP or TPO. Through plasticization of cellulose acetate by
an environment friendly citrate plasticizer, the cellulose acetates are processable at 170-
180°C much below the melting point of cellulose acetate (233°C). Extrusion allows better
mixing of cellulose acetate and plasticizer due to the high shear of extrusion processing
as shown by ESEM images. The material processed at lower extruder screw rpm (100
rpm) exhibited better mechanical properties over its counterpart processed at higher
extruder rpm (200 rpm). The extruded sample showed lower elongation at break, thus it
is a stiffer material and showed least impact strength where as the compression molded
sample exhibited highest elongation at break, thus was a tougher material and exhibited
the highest impact strength. Extrusion followed by injection molding processed materials
exhibited better properties as compared to extrusion followed by compression molding as
additional shear forces applied during injection molding resulted in stiffer product.
Cellulosic plastics fabricated through injection molding at higher temperature (190°C)
exhibited better tensile properties over their counterpart that was injected molded at a

comparatively lower temperature (180°C). Thus, processing parameters played a vital

55

role in designing cellulosic plastics with the desired properties for biocomposite

applications.

6. Acknowledgements

Financial support from the NSF/EPA (Award Number DMI-0124789) under the
2001 Technology for a Sustainable Environment (T SE) program is gratefully
acknowledged. We are thankful to Eastman Chemical Company (Kingsport, TN),
Johnson Controls (Milwaukee, Wisconsin) and Flaxcraft, Inc. (Cresskill, NJ) for their
collaborations. The authors also thank Dr. Brian D. Seiler, Group Leader of Cellulose
Ester Technology, Eastman Chemical Company for the cellulose acetate samples and his
valuable discussions during the course of this research work. The authors also thank

Morflex Inc. (Greensboro, North Carolina) for supplying citrate plasticizer.

56

7. References
1. S. L. Wilkinson, Chem. Eng. News, January 22, 61 (2001).

2. “Polymers from Renewable Resources”, C. Scholz and R. A. Gross Edn.; ACS
Symposium Series 764, American Chemical Society, Washington, DC. 2001.

3. A. K. Mohanty, M. Misra, G. Hinrichsen, Macromol. Mater. Eng., 276, 1 (2001).

4. Plant/Crop-based Renewable Resources 2020, URL:
http://www.oit.doe.gov/pgiiculture/pdfsA/ision2020.pdf

5. The Technology Roadmap for Plant/Crop-based Renewable Resources 2020, URL:
http://www.oit.doe.goL/agriculture/pdfflgZ5942pdf

6. B. Berenberg, Composites Technology, Nov/Dec, 13 (2001)
7. A. Scott, Chemical Week, September 13, 73 (2000).

8. “Biodegradables: Polymers gain credibility in waste collection systems”, Modern
Plastic, November, 28 (2000).

9. K. J. Edgar, C. M. Buchanan, J. S. Debenham, P. A. Rundquist, B. D. Seiler, M. S.
Shelton, D. Tindal, Prog. Polym. Sci. 26, 1607 (2001).

10. K. J. Edgar, T. J. Pecorini, W. G. Glasser,. in Cellulose Derivatives — Modificartiong
Characterization and Microstructures. Thomas J. Heinze and Wolfgang G. Glasser;
Eds.; ACS Symposium Series 688, 38 (1998).

11. V. P. Ghiya, V. Dave, R. A. Gross and S. P. McCarthy, ANTEC, p.3726—3730 (1995).

12. A. K. Mohanty, M. Misra and L. T. Drzal, Compos. Interf. 8(5), 313 (2001).

13. A. K. Mohanty, L. T. Drzal and M. Misra, J. Adh. Sci. Technol. 16 (8), 999 (2002).

14. L. T. Drzal, A. K. Mohanty and M. Misra, Polym. Preprint — Polym. Chem. Div.,
American Chemical Society, 42(2), 31 (2001).

15. A. K. Mohanty, A. Wibowo, M. Misra and L. T. Drzal, Composites Part A
(Submitted 2002).

16. A. K. Mohanty, L. T. Drzal and M. Misra, Polym. Mater. Sci. Eng, 85, 594 (2001).
17. A. K. Mohanty, D. Hokens, M. Misra and L. T. Drzal, “Biocomposites from Biofibers

and Biodegradable Polymers” in Proc. American Comppsite Society. 17 Tech. Conf.
Edn.; M. W. Hyer, A. C. Loos, Year 2001.

57

18. A. M. Thayer, Chem. Eng. News. November 23, 17 (1998).
19. A. M. Thayer, Chem. Eng. News. February 8, 17 (1999).

20. “Biopolymers from Renewable Resources, Macromolecular Systems - Materials
Approach”, Edn.: D. L. Kaplan, Springler — Verlag, Berlin Heidlerberg, 1998.

21. R. D. Leaversuch and M. DeFosse, Modern Plastics, November, 28 (2000).

22. C. M. Buchanan, R. M. Gardner, A.W. White and M. D. Wood, World Patent W0
9428062 (1994).

23. C. J. Landry, D. M. Teegarden, K. J. Edgar and S. S. Kelley, EP 580123 (1994).
24. M. Pizzoli, S. M. Maria, G. Ceccorulli, Macromolecules, 27 (17), 4755 (1994).

25. M. Misra, A. K. Mohanty, and L. T. Drzal, “Natural Fiber Reinforced Polyolefin
Composites: Opportunities and Challenges in the Materials World” in: Proc. 0f
Global Plastic and Environmental Conference, Society of Plastics Eng'neers, Detroit,
February 13 and 14, 2002.

26. H. L. Tyan, C. Y. Wu, and K. H. Wei, J. Appl. Polym. Sci., 81, 1742 (2001).

58

II. Effect of Process Engineering on the Performance of Natural Fiber
Reinforced Cellulose Acetate Biocomposites

A. K. Mohanty, A. Wibowo, M. Misra, L. T. Drzal“
Department of Chemical Engineering and Materials Science
Composite Materials and Structures Center

2100 Engineering Building, East Lansing, MI 48824, USA

* Corresponding author: Tel: +1-517-353-5466; Fax: +1-517- 432-1634; E-mail address:
drzal@egp.msu.edu

1. Abstract

Eco-friendly green/biocomposites were fabricated from chopped hemp fiber and
cellulose ester biodegradable plastic through two process engineering approaches:
powder impregnation through compression molding (Process 1) and extrusion followed
by injection molding (Process H). Cellulose ester e.g. cellulose acetate (CA) plasticized
with 30 wt. % citrate plasticizer (CAP) was used as the matrix polymer for biocomposite
fabrication. Intimate mixing due to shear forces experienced in Process H produced
superior strength biocomposites over their counterparts made using Process 1.
Biocomposite fabricated through Process H containing 30 wt.% hemp natural fiber
showed an improvement of storage modulus by 150% over the virgin matrix polymer.
The coefficient of thermal expansion (CTE) of the said biocomposite decreased from the
CAP polymer by 60% whereas the heat deflection temperature improved by 30% versus
the virgin bioplastic, indicating superior thermal behavior of the biocomposite.
Plasticized cellulose acetate is proved to be much better matrix than non-polar
polypropylene (PP) for hemp fiber (HF) reinforcements because of the better interaction
of polar cellulose ester with the polar natural fiber. Fabricated through Process H and

with same content of hemp (30 wt.%) the CAP-HF based biocomposite exhibited flexural

59

strength of 78 MPa and modulus of elasticity (MOE) of 5.6 GPa as contrast to 55 MPa
and 3.7 GPa for the corresponding PP-HF based composite. The experimental findings of
tensile modulus of the biocomposites are compared with the theoretical modulus using
the rule of mixture (ROM). The fiber-matrix adhesion is evaluated through
Environmental Scanning Electron Microscopy (ESEM) studies.

Keywords: A. Cellulose Acetate; A. Natural hemp fiber; E. Powder impregnation; E.
Extrusion/Injection/Compression Molding; B. Physico-mechanical and thermal

characteristics.

2. Introduction

There is a growing urgency to develop and commercialize biobased composite
materials those can reduce the widespread dependence on petroleum and the same time
can enhance the environment and economy. The manufacture, use and removal of
traditional composite structures, usually made of glass, carbon or aramid fibers
reinforcing nonbiodegrable polymers like epoxies, unsaturated polyester resins,
polyurethanes, phenolics etc. are considered critically '. Eco-friendly green composites
from plant derived fiber and crop-derived plastics are novel materials for the 21m century
and can be of great importance to the materials world 2. Biocomposites are emerging as a
viable alternative to glass fiber reinforced composites especially in automotive
applications 3. While they can deliver the same performance for lower weight, they can
also be 25-30 percent stronger for the same weight 4. Moreover, they exhibit a favorable
non-brittle fracture on impact, which is another important requirement in the passenger

compartment. Interior parts from natural fiber — polypropylene and exterior parts from

60

natural fiber — polyester resins are already in use 5. Johnson Controls, Inc. has started
production 6 of door-trim panels from natural fiber and polypropylene.

Advantages of natural fibers over man-made glass fiber are: low cost, low density,
competitive specific mechanical properties, reduced energy consumption, carbon
sequestration and biodegradability. Auto companies are seeking materials with sound

abatement capability as well as reduced weight for fuel efficiency. Natural fibers possess

mm excellent sound absorbing efficiency
.. _ and are more shatter resistant and have
on cnzon on CHzofi
H 0 o 0 0H better energy management
OR _ on OR \ OR
o o 0 . .
charactenstrcs than glass fiber
cnzon on CHZOR on

 

 

" " reinforced composites. In automotive
Figure 1. Structure of Cellulose esters
(Cellulosic Plastics): R = H (Cellulose),
acetyl (Cellulose acetate), acetyl and
propionyl (Cellulose acetate propionate), or

butyryl (Cellulose acetate butyrate). fibers reduce the mass 7 of the

parts, compared to glass composites,

the composites made from natural

component and can lower the energy needed for production by 80%. In Europe,
renewable materials are of increasing interest 8 because of a directive of the European
Parliament that an automotive material recycling rate should be greater than 80% by the
year 2006. This directive is an opportunity to introduce materials from renewable
resources.

Cellulose from trees and cotton is used in place of petroleum as a feedstock to
make cellulosic plastics 9. Cellulose esters are considered as potentially useful
biodegradable polymers. The structures of cellulose esters include cellulose acetate (CA),

cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB), are illustrated in

61

Figure 1. The production of cellulose esters from recycled paper and sugarcane has also
been demonstrated 10. Some of the applications of cellulose ester biopolymers are film
substrates for photography, toothbrush handles, selective filtration in the medical field,

automotive coatings, etc. ”.

In an ongoing project, we are investigating various
processing issues to study the structure-property relationship of cellulose esters-natural
fiber biocomposite materials to find their applications in green automotive parts. As a

part of our current investigation, this communication describes the effect of process

engineering on performance of hemp fiber-cellulose ester biocomposites.

3. Experimental
3.1. Materials

Powder Cellulose Acetate (CA 398-30) free from any plasticizer and additives
was supplied by Eastman Chemical Co., (Kingsport, TN). The degree of substitution of
the cellulose acetate used was 2.5. The triethyl citrate (TEC) plasticizer was procured
from Morflex Inc., Greensboro, North Carolina. The chopped raw hemp fiber (1/4 inch)

was kindly supplied by Hempline, Canada.

3.2. Hemp Fiber Density and Tensile Modulus Measurements

Pro-dried raw hemp was used for both the measurements. The density of raw
hemp was measured using Archimedes method with ethanol using ASTM D 3800. The
average density (20 measurements) of hemp fiber was found to be 1.29 gram/cm3. The
tensile modulus of hemp fiber was measured using United Calibration Corp. SFM-20
with load cell of 20 lbs and speed rate of 0.05-in/sec. Single fiber tests were conducted

with 20 trials. The average tensile modulus of hemp was found to be 42 GPa.

62

3.3. Spectral, Thermal, and Morphological Analysis of Hemp Fibers

The Fourier Transform Infrared (FI‘IR) spectrum of raw chopped hemp in KBr
pellets was measured using a Perkin-Elmer Spectrum® 2000 FT IR spectrophotometer.
Thermogravimetric analysis (T GA) was performed under nitrogen atmosphere using Hi-
Res. TGA 2950 Thermogravimetric Analyzer at heating rate of 10°C/min to 600 0C.
Environmental scanning electron microscopy (ESEM) was carried out at 20 kV and

chamber pressure of 3 Torr using a Phillips Electroscan 2020.

3.4. Composite Fabrication Processing

Two different types of processing are adopted in fabricating the biocomposites:
Process 1: Novel Powder Impregnation Processing (compression molding of the mixture
of chopped hemp natural fiber and powder cellulose acetate along with liquid plasticizer)

A premeasured amount of triethyl citrate plasticizer was added drop wise on to
pro-dried cellulose acetate (CA) powder and the mixture was stirred mechanically in a
kitchen mixer for 30 minutes. The investigated cellulosic plastic composition contained
30 wt. % of TEC plasticizer. The vacuum dried chopped hemp was added according in
the desired weight percentage to the CA + TEC mixture. The mixture was then
mechanically mixed for another 20 minutes. The resulting mixture was compression
molded using a picture-frame mold in a Carver Press SP-F 6030 at temperature 195°C
under pressure. During compression molding at specified temperature, for the first 12
minutes the materials were kept at contact pressure of 1.1 MPa and then held at a
pressure of 2.67 MPa for three minutes followed by cooling under pressure. The

fabricated biocomposites were then cut into the required shapes and were kept sealed

63

inside zip lock bags for 2 days (at room temperature) prior to physico-mechanical
property evaluation.
Process 11: Extrusion followed by injection molding

Two-step extrusion was carried out in this process. In the 1St extrusion step,
cellulose acetate plastic (CAP) pellets were made by pre-rnixing cellulose acetate powder
with 30 wt. % TEC plasticizer. In the 2‘“l step, CAP pellets as obtained through 1St
process; were fed into the twin-screw extruder (ZSK-30 from Wemer-Pfleiderer with UD
= 30 and six temperature controlled zones) while feeding the chopped hemp (~ 1/4 inch)
into the last zone of the extruder. The temperature profiles along the barrel were 190°C
with die temperature held at 195°C. A screw speed of ISO-rpm was used with ~ 40%
torque reading, the resulting output was 65 to 70 grams/minute. The thin strands of
composites were pelletized and stored for further injection molding. The injection
molding (85-ton Cincinnati-Milacron press) conditions used were: temperatures in zones
1 through 3 were ~1950C, mold at 60°C, 40 sec. cooling time, 25 sec. extruder delay, fill
pressure of 1500 psi, pack pressure and hold pressures were 1500 and 1200 psi,
respectively. The thermo-mechanical properties were determined following ASTM
standards. The test specimens for the thermo-mechanical properties were cut in the
desired shapes from the injection molded tensile coupons and were kept for two days

inside zip lock bags (at room temperature) prior to properties evaluations.

3.5. Analysis and Testing
The flexural strength and modulus of elasticity (MOE) of virgin cellulose acetate

plastics and their biocomposites as fabricated through above mentioned two processing

approaches were determined using a United Calibration Corp. SFM-20 as per ASTM D

790. Similarly, the tensile strength and modulus were measured using the same machine

 

Figure 2. ESEM micrograph of raw
hemp fiber (100 um scale bar, 600X).

between hemp fiber and cellulose acetate

plastic (CAP) matrix. The fractured
impact specimens were gold sputtered
prior to examination.

Dimensional stability
measurements were made following
ASTM D 696 using a therrno-
mechanical analyzer (TMA) 2940 from
TA instruments, USA. The heating rate
was 4°C/minutes (cpm) starting at room

temperature to 120 0C. The coefficient of

thermal expansion (CTE) was calculated

following ASTM D 638. The notched Izod
Impact strength was carried out with an
Impact tester from Testing Machines Inc.
(model 43-OA-01) according to ASTM D 256.
Environmental scanning electron microscope
(ESEM) photo micrographs of impact
fractured samples were taken at 20 kV and

chamber pressure of 3 Torr using a Phillips

Electroscan 2020 to evaluate the adhesion

8‘
O)

 

    
   

   

\\

FA

25 -~

Flexural Strength (MPa)
Modulus Of Elasticity (GPa)

 

 

fl/

.\

Figure 3. Flexural properties of powder
processing (Process I) and extrusion
followed by injection molding (Process
H): A = CA Plastics (CAP) of Process
1, B = CA Plastic Biocomposites
(30wt% hemp) of Process 1, C = CA
Plastic of Process H, D: CA Plastic
Biocomposites (30wt% hemp) of
Process H.

65

from the slope of the curve of dimension change vs. temperature. The heat deflection

temperature (HDT) and storage modulus of virgin CAP bioplastic matrix and

biocomposites were determined using a TA dynamic mechanical analyzer (DMA) 2980

at a heating rate of 2 cpm for HDT measurements and 4 cpm for storage modulus

measurements. The material was heated from room temperature to 100°C. A modified

ASTM D 648 was used for I-IDT measurement

I since the DMA could only handle a smaller

size of specimen as compared to that of

ASTM D 648.

4. Results and Discussion

In an on-going project to develop eco-
friendly biocomposites from various natural
fibers like hemp, kenaf, jute, flax etc. and
cellulose ester bioplastics to design and
engineer greener automotive parts, the present
investigations report the effect of two

processing approaches on the physico-

, , mechanical and therrno-mechanical properties

Figure 4. ESEM micrographs of
impact fractured samples: A = CA
Plastic Biocomposites (30wt% hemp)
of Process I --150 um scale bar
(300X), B = CA Plastic
Biocomposites (30wt% hemp) of
Process H -- 200 um scale bar
(250X).

 

of the resulting hemp fiber based
biocomposites. Besides fiber surface
modification, matrix modification and the use

of compatibilizer during composite fabrication

66

to get optimum fiber-matrix adhesion, different processing approaches also affect the
over all properties of the resulting composite 2. Powdered cellulose esters are obtained
through a proprietary (Eastman Chemical Co.) acetylation process. The cellulose ester
powder in combination with different additives and plasticizer is extruded into cellulose
ester pellets for commercialization. In cellulose acetate plastic formulation, the major
plasticizer used in the industry now is dioctyl phthalate (DOP), which is under
environmental scrutiny. In our investigations we are using an coo-friendly citrate
plasticizer. We obtained cellulose ester powder from the Eastman Chemical Company
and combined the plastic with the citrate plasticizer for biocomposite applications. We
are targeting to fabricate the biocomposite pellets through one-step extrusion process by
adding powder cellulose ester, liquid plasticizer and chopped natural fiber followed by
injection molding to the desired shape.

Hemp is an important ligno-cellulosic natural fiber and has attracted much
attention of auto-parts manufacturer in Europe and North America. Hemp fiber is a bast
fiber, meaning it is attained from the stem of the plant. As reported in the literature I
hemp contains (by weight) 70.2-70.4% cellulose, 3.7-5.7% lignin, 17.9%-22.4%
hemicellulose, 0.9% pectin, 0.3% wax and 10.8 % moisture. The hemp fiber used under
the present studies is characterized through FTIR, ESEM and TGA analysis. The broad
absorption band in the region around 3500 cm'1 is the characteristic of hydrogen bonded
O-H stretching. The characteristic carbonyl peak at 1740 cm'1 of the fiber is due to lignin
and hemicellulose components, which exists in the hemp fiber. The hemicellulose
contains groups that absorb in the carbonyl region. The ESEM micrograph (Figure 2) of

raw hemp fiber shows the presence of natural waxy substances on the fiber surface. Such

67

waxy substances contribute to ineffective fiber-matrix bonding and poor surface wet-out.
Although the present investigations deal with raw hemp fiber based composites we are
also investigating the effect of various surface treatments of hemp fiber to improve fiber-
matrix adhesion of the resulting biocomposites for superior performance. The results of
such investigations will be reported in our subsequent communications. The low
temperature resistance of natural fibers does not allow an arbitrary choice of
thermoplastic as matrix material. For the manufacture of biocomposites with
thermoplastic matrix systems, it is important to know about the degradation of
mechanical properties when the fibers are exposed to manufacturing temperature for a
certain time. The TGA analysis (Figure is not shown) indicates that the raw hemp fiber
starts to decompose at 300°C and its maximum rate of thermal decomposition as
indicated by the derivative of the weight curve vs. temperature was found to be 396°C. In

the present studies, during composite

180 7
DFS QMOE

O

 

fabrication we used a maximum
processing temperature of ~195°C, well
below the decomposition temperature of

the hemp fiber.

Flexural Strength (MPa)
Modulus of Eleetlclty (GPa)

 

 

The main objective of the present

 

investigation is to study the effect of two
Figure 5. Flexural properties of CAP
Biocomposites of Process H: A = CAP different process engineering approaches
(0wt% hemp), B = 15wt% hemp, C =
30wt% hemp content. of biocomposite fabrication on the
performance of the resulting biocomposites. In Process 1, the powder impregnation

processing approach through compression molding is followed and in Process H

68

extrusion followed by injection molding is adopted in fabricating the hemp-cellulose
ester plastic based biocomposites. Recently, powder impregnation processing through
compression molding for biocomposite fabrications from chopped henequen natural fiber
and powder polypropylene has been reported '2. Most work on natural fiber-
thermoplastic composites is based on melt mixing of short length natural fiber and
polymer pellets through extrusion followed by injection molding processing. Such
processing exposes the natural fibers to high shear forces, which can break or otherwise
damage the fibers, decreasing their aspect ratio. It is well known that the lower the aspect
ratio the lower stress transfers across the length of the fiber. However, one of the main
advantages of extrusion-injection molding processing is the intimate mixing of fiber and
matrix, which plays a vital role in giving the requisite performance. In comparison to the
powder impregnation process, although the fibers are free from high shear, thus avoiding
fiber damage unlike the extrusion-injection molding technique, the lack of intimate

mixing sometimes adversely affected the

 

 

 

 

 

:1 B over-all properties of the resulting
E 40 1 A biocomposites.
E 30 I i— The flexural properties of
:. biocomposites fabricated through the above-
0 ' ' ' mentioned processes are presented in Figure.
0 5 10 15
gamma) 3. The flexural strengths (FS) of cellulose

Figure 6 Stress-strain plot of: A = acetate plastic (CAP) and its biocomposites

zggffiug’ s: égafigzssgzsfi e s show superior performance when fabricated

(30wt% hemp). through process H as contrasted to Process 1.

69

The PS of CAP matrix polymer made through process H is 33% more than CAP as made
through Process 1. The corresponding biocomposites (30 wt.% hemp) exhibit a FS
variation of ~ 30%, processing H thus showing superior strength over processing 1. As
expected; the impact strengths of both CA plastic (CAP) and CAP based biocomposites
fabricated through Process H showed lower values when compared to their counterparts

12 made through Process 1 (Data is not

 

—o—nou(e1)
A Ex '

permntal shown). The variation of properties is
. . . . -NbdliledFDM(k=O.35)

..e
o

attributed to the fact that in Process 1, we

to

adopt powder processing and lack of

shear force is applied whereas in process

 

H the extrusion and injection molding

Modulus of Composites (GPa)
a:

processing exert sufficient shear forces

 

 

 

0.00 0.05 0.10 0.15 0.20 0.25

Fiber Volume Frectlon

Figure 7. Comparison between tensile
modulus of Rule of Mixture (ROM),

for the intimate mixing of powder
polymer, liquid plasticizer, and raw hemp

fibers. This observation is supported by

modified ROM, and experimental Tensile
Modulus with density of raw hemp (p):
1.29 glcc, Tensile Modulus of hemp fiber
(Er) = 42 GPa, Em = 2.1 GPa.

Environmental Scanning Electron
Microscope (ESEM) images of fractured
impact sample taken for Process I (Figure 4 A) and Process H (Figure 4 B). Process H
shows a lesser amount of fiber pull out indicating better adhesion between hemp fiber and
CAP matrix as compared to that of Process 1. In contrast, plenty of longer fiber pull-outs
are observed in Process 1 and thus they can be easily distinguished from CAP matrix, thus
indicating poor fiber-matrix adhesion. Therefore, non-uniformity of fiber distribution

could be found in some parts of the sample. As a result, modulus of elasticity (MOE) of

70

the biocomposite of Process I has a lower value than its counterpart made through
Process H. Much better performance of biocomposites made through powder processing
approach is targeted through better consolidation processing by the use of a new coupling
agent. The use of the right coupling agent is expected to improve the wetting and fiber-
matrix adhesion, since the fiber length remains intact the resulting biocomposites are
poised to show much improved performances. Such investigations are in progress and
will be reported in our future communications.

In order to study the effect of fiber content on the performance of these
composites, the usual extrusion and injection molding processing (Process H) was used in
making the biocomposite samples. Both the flexural strength and MOE increase with

increasing of fiber content from 15 to 30

 

 

 

 

90
wt.% (Figure 5). We target to further
851
80 . increase the fiber content to 40 and 50 wt.
8
f: 75 . % in our biocomposite system; however
0
7° ‘ . such 1nvest1 gatlons are beyond the scope
65 ‘ \\\ .: \\\\\
m g5 . 5; .5 of the present study. Flexural strength and
so a ‘ ‘ ‘

 

MOE of virgin cellulose acetate plastic

Figure 8. Heat Deflection Temperature (CAP) enhance by 75 and 200%
(HDT) data comparison of A = CAP

(OWI% hemp), B = ISWI% hemp, C = respectively on reinforcements with 30
30wt% hemp content.

wt% hemp fibers. The tensile stress vs.

strain curves of cellulose acetate plastic and hemp-CAP based biocomposite (30 wt. %

hemp) are showed in Figure 6. It is well observed that natural fiber reinforcement

improved the stiffness and decreased the elongation of the virgin CAP matrix.

71

Although a number of models have been developed to study the mechanical
properties of a fiber-reinforced polymer, the simplest and reasonable equation for
prediction of these is given by the rule of mixtures (ROM). The modified ROM '3'15 for
determining the tensile modulus of a composite is shown in equation 1. A comparison
between the theoretical modulus value from Rule of Mixtures, modified Rule of Mixtures
and experimentally deterrrrined values is shown in Figure 7. The Rule of Mixtures
estimates the composites stiffness by a weighted mean of the moduli of the two
components -- matrix and fiber -- depending upon the volume fractions of the
components '6. This equation is valid for long fibers, assuming they have equal strain.
Since short fibers (1/4 inch long) with a random orientation were used in this experiment,
a constant k '3’” is used in fiber part of ROM (modified ROM) to compensate non-
equality of fiber strain that causes the inferiority of tensile modulus of experimental

composites as compared to tensile modulus of ROM. The k value can be calculated from

 

 

 

modified ROM as:
250 EC = kaEf +VMEM
uMachine .Transverse
200 — Where: Ec = Modulus of resulting
3:;
g 150 - composites
g 100 - k = “Empirical factor” representing the
o 50 _ reinforcing efficiency dependent on the
o orientation of the fibers and the stress

 

 

 

transfer between the matrix and the fibers.

Figure 9. Coefficient of Thermal
Expansion (CTE) data comparison of A =
virgin cellulose acetate plastics (CAP) 3
and B = CAP biocomposites (30wt% 1-29 gin/cm )
hemp).

vi = Fiber Volume Fraction (p of hemp =

72

E; = Modulus of fiber (E; = 42 GPa)

Vm = Matrix volume fraction

E... = Matrix modulus (2.1GPa)

In this experiment, k=0.40 (k1) is obtained for ~10vol% hemp fiber-CAP biocomposite

whereas in ~20vol% hemp fiber-CAP

 

 

 

 

 

 

 

 

 

 

120 7
DFS OM05 E biocomposites k = 0.3 (k2). Curve fitting
1:? 0 t5
0- -- ._ v
E. 90 5 .E’ method is done among the experimental
4% 2
Q . . _ .

g 60 __ 5 data, hence it yields k — 0.35, or it
E T _ é simply the average between k) and k2 in
:1 . 2
6 3° “ = . .
I E thrs partrcular case.

0 Thermomechanical properties

A B C

Fl 10. Flexural properties namely storage modulus, heat deflection

comparison of: A = CAP, B = CAP -
30wt% hemp composites, C = PP, D =
PP-30wt% hemp composites.

temperature (HDT) and coefficient of
thermal expansion (CTE) of hemp fiber
reinforced CAP biocomposites (fabricated through Process H) were evaluated. The
storage modulus represents the ratio in-phase stress to the applied strain, which further is
related to the mechanical energy stored per cycle of deformation '7. The storage modulus
(at 30°C) of virgin CAP bioplastic was enhanced by ~ 150% when reinforced with 30 wt.
% of hemp fiber (Data not shown). The heat deflection temperature (HDT) is widely used
in automotive applications and represents the temperature at which the material deflects
by 0.25 mm at an applied force (three point bending arrangement) of 66 psi (ASTM D
648). After incorporating the hemp fiber into the CAP matrix, the stiffness of the

resulting biocomposite was enhanced thereby improving the HDT (Figure 8). The HDT

73

of virgin the CAP matrix improved by 13 and 30% respectively at reinforcement loadings
of 15 and 30wt% of hemp fibers. Enhancement of stiffness means reduction in free
volume presented in the system. This fact will not only affect storage modulus and HDT,
but also it will reduce the coefficient of thermal expansion (CTE) of the material. The
CTE is defined as the dimension change with respect to initial dimension per degree of
temperature change (normally measured below or above the glass transition temperature
of material). Two sampling methods were adopted when measuring the CTE, based upon
flow direction during processing; they are machine (flow direction -- 0°) and transverse
direction (perpendicular to flow directio -- 90°). In this paper we evaluate CTE from
room temperature up to the CAP glass transition temperature (~80 0C). Figure 9 shows a
dramatic reduction of CTE for both directions (64% reduction for machine and 54%
reduction for transverse direction) of CAP-hemp based biocomposites in respect to virgin
CAP. The above explanation clearly demonstrates that the thermomechanical properties
of cellulose ester plastic improve upon reinforcement with hemp fibers.

We are targeting to replace/substitute PP with the cellulose ester bioplastic and
synthetic glass fiber with natural fibers to design coo-friendly biocomposites for greener
automotive parts. The comparative reinforcement effect of hemp fiber on both the
matrices e.g. cellulose acetate plastic (30 % citrate plasticizer) and commercial PP as
regards to flexural properties of 30wt% fiber based composites fabricated through
Process 11 is represented in Figure 10. The flexural strength and MOE of hemp-CAP
based biocomposite are superior by about 40 and 50% respectively over the hemp-PP

based composite counterpart. Natural fiber and cellulose ester both being polar; binds to

74

each other more effectively in the composite structure unlike PP, which is a non-polar

polymer.

5. Conclusions

Two types of processing e.g. powder impregnation processing through
compression molding (Process 1) and conventional extrusion followed by injection
molding (Process 11) were successfully applied in making the biocomposites from
chopped hemp fiber and cellulose acetate plastics. Biocomposites fabricated through
Process II showed better strength over their counterparts made through Process 1. This is
due to sufficient shear forces for the intimate mixing of powdered polymer, liquid
plasticizer and fibers. The ESEM examination of impact fractured surfaces proved that
biocomposites made through Process I show poor adhesion and lack of fiber dispersion as
compared to Process 11. In thermomechanical properties, enhancements of 150% and
30% are observed for the storage modulus (at 30°C) and heat deflection temperature,
respectively, of CAP based biocomposite (30wt% hemp content fabricated through
Process 11) as compared to its base line data (0wt% hemp content). The coefficients of
thermal expansions (CTE) of CAP based biocomposites are decreased as much as 64%
and 54% for machine and transverse direction respectively. Between non-polar
polypropylene (PP) matrix and 30% plasticized polar cellulose acetate plastic (CAP) the
latter proved to be a much better matrix for hemp fiber (polar in nature) and thus
exhibited improved physico-mechanical properties. Fabricated by the same processing
method (Process 11, extrusion and injection molding) with the same content of hemp fiber

(30wt%) the CAP-hemp based biocomposite exhibited a FS of 78.3 MPa and MOE of 5.6

75

GPa as contrasted to 55.3 MPa and 3.7 GPa for the corresponding PP-hemp based
composite. The incorporation of a high content of natural fiber say up to 50wt% or even
more in the biocomposite system along with suitable surface treatments of natural fibers
along with the use of coupling agents during composite fabrications, which are currently

under investigations are poised to generate superior performance biocomposites.

6. Acknowledgements

Support for this research was provided by NSF/EPA Award # DMI-0124789.
The authors wish to thank the Eastman Chemical Co. (cellulose ester samples) and Dr.
Brian D. Seiler for many fruitful discussions, as well as Mr. Hugh McKee (Flaxcraft,
NY) and Hempline, Canada, for natural fibers. Authors also thank to Morflex Inc for

citrate plasticizer.

76

7. References

l.

A. K. Mohanty, M. Misra, G. Hinrichsen “Biofibers, biodegradable polymers and
biocomposites: An overview”, Macromol Mater Eng 2000; 276/277:1-24.

A. K. Mohanty, M Misra, L. T. Drzal. “Sustainable BioComposites From Renewable
Resources: Opportunities and Challenges in the Green Materials World”, J. Polymers
and the Environment 2002; 10(1): 19-26.

P. Mapleston,. Auto makers see strong promise in natural fiber reinforcements,

Modern Plastics, April 1999. p.73-74.

M Misra, A. K. Mohanty, L. T. Drzal. “Natural/Biofiber Reinforced Polyolefin
Composites: Biobased Opportunities and Challenges in the Materials World”,
Proceedings of 8‘h Annual Global Plastics Environmental Conference (GPEC 2002) —
Plastics Impact on the Environment, February 13 & 14, 2002, Detroit, MI, Society of
Plastics Engineers, Environmental Division, Full paper published in the proceeding,
2002. p. 383-392.

Anonymous. Grown to Fit the Pa_rt, DaimlerChrysler High Tech. Report 1999. p. 82-
85.

Anonymous. “Green Door-Trim Panels Use PP and Natural Fibers”, Plastic
Technology, November 2000. p. 27.

Broge JL. “Natural fibers in automotive components”, Automotive Engineering
Internationals, October 2000. p. 120.

A Nick, U Becker, W Thoma, “Improved Acoustic Behavior of Interior Parts of

Renewable Resources in the Automotive Industry”. J. Polymers and the Environment
2002, 10(3): 115-118.

S. L. Wilkinson,. “Nature’s Pantry is open for business”. Chem. & Eng. News,
January 22, 2001. p.61 - 62.

10. G. R. Elion, 1993, US Patent 5244 945.

11. K. J. Edgar et al. Prog Polym. Sci 2001; 26:1607-1688.

12. A. K. Mohanty, L. T. Drzal, M Misra. “Engineered natural fiber reinforced

polypropylene composites: influence of surface modifications and novel powder
impregnation processing”. J. Adh. Sci. Technol 2002; 16(8): 999-1015.

13. R. G. Raj, B. V. Kokta, Polym. Eng. & Sci. 1991; 31(18): 1358.

14. A. R. Sanadi et al. J. Reinf. Plast. Compos. 1994; 13: 54.

77

15. R. T. Woodhams, G Thomas, D. K. Rodgers, Polym. Eng. Sci 1984; 24(15): 1166-
1171.

16. D Hull, T. W. Clyne, An Introduction to Composite Materials, 2nd Edition,
Cambridge Solid State Science, 1996.

17. A. T. Edith, TherrLalChflterization of Polymeric Materials, 2““ Edition, 1997.

78

III. Chopped Industrial Hemp Fiber Reinforced Cellulosic Plastic
Biocomposites: Thermomechanical and Morphological Properties

Arief C. Wibowo’, Amar K. Mohantyz, Manjusri Misra], Lawrence T. Drzal].
1Composite Materials and Structures Center, Department of Chemical Engineering and
Materials Science, 2100 Engineering Building, Michigan State University East Lansing,
MI 48824-1226

2The School of Packaging, 130 Packaging Building, Michigan State University East
Lansing, MI 48824

*Corresponding author: Tel: +1-517-353-5466; Fax: +l-517- 432-1634; E-mail address:
drzal@egr.n;§u.edu

1. Abstract

Biocomposites, i.e. biopolymers reinforced with natural fibers offer an
environmentally benign alternative structural material for automotive applications.
Cellulose esters (bioplastic made from cellulose) are potentially useful biosourced
polymer. By embedding inexpensive plant-based cellulosic fibers (chopped hemp fiber)
into a biopolymeric matrix (cellulose ester) novel biocomposites have been made
utilizing two different processing approaches: (Process I) powder impregnation and
(Process 11) extrusion followed by injection molding. The resulting biocomposites have
been evaluated for their physico-mechanical and thermomechanical properties. Cellulose
acetate plasticized with 30% citrate plasticizer proved to be a better matrix compared to
polypropylene (PP) for hemp fiber reinforcements in terms of flexural and damping
properties. Biocomposites with 30 wt% of industrial hemp fiber processed through
extrusion and injection molding exhibited a flexural strength of ~78 MPa and modulus of
elasticity of ~5.6 GPa. Cellulose acetate butyrate plastic (CABP) proved to be a better

matrix than plasticized cellulose acetate (CAP) for biocomposite applications. The fiber-

79

matrix adhesions are evaluated through Environmental Scanning Microscopy (ESEM)
analysis.

Keywords: cellulose ester, natural fiber, biocomposites, therrno-mechanical properties.

2. Introduction

Eco-friendly biocomposites from plant derived fiber (natural/biofiber) and crop-
derived plastics (bioplastic) are novel materials for the 21St century and can be of great
importance to the materials world, not only as a solution to growing environmental
threats but also as a solution to alleviating the uncertainty of the petroleum supply 1’2.
There is a growing urgency to develop coo-friendly biobased polymers and composites
and other innovative technologies that can reduce the widespread dependence on fossil

fuel. Natural/biofiber composites (biocomposites) are emerging as a viable alternative to

Cellulose esters

 

pa lCHZOR '05 CHZOR
O 0 OH
HO\ OR @0 OR OR
“o ‘o/
—O l O I
CHZOR OR CH20R on
— —n

 

Figure 1. Structure of Cellulose esters (Cellulosic Plastics): R = H
(Cellulose), acetyl (Cellulose acetate), acetyl and propionyl (Cellulose
acetate propionate), or acetyl and butyryl (Cellulose acetate butyrate)

glass fiber reinforced composites especially in automotive applications 3. While they can
deliver the same performance for lower weight, they can also be 25-30 percent stronger

for the same weight 4. Moreover, they exhibit a favorable non-brittle fracture on impact,

80

which is another important requirement in the passenger compartment. Interior parts from
natural fiber—polypropylene and exterior parts from natural fiber—polyester resins are
already in use 5. For example, Johnson Controls, Inc; has started production 6 of door-
trim panels from natural fiber and polypropylene.

Studies of the American market identify the potential impact and opportunities for
natural fiber composites 2‘7. It is estimated that ~75% of a vehicle’s energy consumption
is directly related to factors associated with a vehicle’s weight. To reduce vehicle

weight; a shift away from steel alloys towards aluminum, plastics and composites has
Possible Hydrogen Bonding

H- O—-* Biofibers
..BLQQMJ _

H OR CHZO- c— CH3 OR CHZOR

0 R0 0 O OH
OR . OR
0 O

CH20R O_ C- CH2“. CH2— CH3

 

 

 

 

 

 

O
:0 mm

   
 

mu m

 

 

\

_O ...m mer

 

Possible Hydrogen Bondings
Biofi bers

 

 

 

Figure 2. Possible hydrogen bonds can be formed which leads to increase in
adhesion between fibers and cellulose ester matrix.

predicted that in near future polymer and polymer composites will comprise ~15% of a
car weight 8. Weight reduction is critical to producing safe and cost-effective light-
weight vehicles. Auto companies are also seeking materials with sound abatement

capability as well as reduced weight for fuel efficiency. Natural fibers possess excellent

81

sound absorbing efficiency and are more shatter resistant and have better energy
management characteristics than glass fiber based composites. In automotive parts,
compared to glass fiber composites, biocomposites made from natural fibers reduce the
mass 9 of the component and can lower the total energy consumed in producing this
material by 80%. The main motivation for using natural/biofibers like kenaf and hemp to
replace glass fibers is the low cost (~1/3 of E-glass fiber), low density (~ V2), acceptable
specific mechanical properties, enhanced energy recovery, CO; sequesterization, and
biodegradability 2.

Plant-based materials for transportation are the wave of the future. Using natural
fibers with polymers (plastics) based on renewable resources will allow many
environmental issues to be solved. Natural fiber—PP (thermoplastic base) or natural
fiber—polyester (therrnoset base) composites are not sufficiently eco-friendly due to the
petroleum-based source as well as non-biodegradable nature of the polymer matrix. By
embedding biofibers like hemp, kenaf, pineapple leaf fiber, henequen, corn straw fibers
and grasses with renewable resource based biopolymers such as cellulosic plastic, com-
based plastic, starch plastic and soy-based plastic, green biocomposites with acceptable
mechanical properties are being developed at Michigan State University. This paper
relates to the fabrication as well as thermo-mechanical property evaluation of
biocomposites from natural fiber and cellulose ester biopolymers for the 21“ century
‘green’ car.

Cellulose esters are considered as potentially useful biosourced polymers for the
future”. As shown in Figure l, R of cellulose esters is replaced by acyl groups (RCO-).

The hydroxyl and carbonyl groups thus have good potential to form hydrogen bonding

82

with hydroxyl groups of cellulose fibers as represented in Figure 2. We are focusing on
two kinds of cellulose ester, they are cellulose acetate (CA), where acetyl groups are
replacing the R groups in the chemical structure, and cellulose acetate butyrate plastic
(CABP) in which not only acetyl groups are present but also a moderate content of
butyryl groups (37%) in the chain. The CABP polymer processes easier without the need
for a plasticizer and has higher moisture resistance compared to CA. CABP possesses
excellent surface hardness and good strength in comparison to CA besides lower
viscosity at 195 0C due to the presence of these higher functional groups, which can act
as an internal plasticizer. In contrast, CA needs an external plasticizer to be added in
order to enhance its flow and allow processing below its degradation temperature (230

0C), which is near the lower portion of its typical melting range.

3. Experimental

Powdered cellulose acetate (CA 398-30) and cellulose acetate butyrate plastic
(CABP = CAB 381-20) free from any plasticizer and additives were supplied by Eastman
Chemical Co., Kingsport, TN. The degree of substitution of acetyl groups in the CA is
approximately 2.45. CABP 381-20 has a butyryl content of 37%, acetyl content of 13.5%,
hydroxyl content of 1.8%, with melting and glass transition temperatures of 195°C and
141°C, respectively. The triethyl citrate (TEC) plasticizer was obtained from Morflex
Inc., Greensboro, North Carolina. Chopped industrial hemp fiber of length ~1/4 inch
(hence forward, chopped industrial hemp fiber will be written as only hemp fiber after

this section) was supplied by Hempline, Ontario, Canada.

83

Two different types of processing methods were employed in fabricating the

green/biocomposites:

3.1. Process 1: Powder Impregnation Process followed by Compression Molding
A specific amount of liquid plasticizer (triethyl citrate, TEC) was added drop wise
to pre-dried CA powder and the mixture was stirred mechanically in a kitchen mixer for

30 minutes. Vacuum dried, chopped hemp fiber was added in the required weight

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Steel Plate_.

Teflon Sheet 1 L‘

Picture Frame

Mold ’ ~. . Steel Plate Hemp Fibers Teflon Sheet
Hemp Fibers + //

Matrix / aim

 

 

 

 

Top View
Figure 3. Fabrication procedure of hemp fiber reinforced cellulose acetate

biocomposites using Process I: powder impregnation process followed by
compression molding

percentage into the CA powder premixed with 30 wt% of TEC plasticizer (plasticized CA
— CAP). The mixture (total of 60 grams) was mechanically mixed for another 20
minutes. The resulting mixture was compression molded using a picture-frame mold, as
pictured in Figure 3, in a Carver Press SP-F 6030 at temperature 195 0C under pressure.
A bed lay up of the mixture of 30 wt% hemp fibers + 70 wt% matrix (CAP) with total of
60 grams was fitted into picture frame mold with dimension of 180m (length) * 14cm
(width) * 0.15cm (thickness). This amount surpasses the volume defined by picture frame

mold and the platens so that hydrostatic pressure is applicable. The resulting compression

84

molded plaque has thickness of 0181-0003 cm. An image of cross section of the
compression molded sample was taken using optical microscope showing a compacted

sample is achieved (Figure 4).

During compression molding at the specified temperature, the materials were kept
at contact pressure of 1.1 MPa for 12 minutes and then compacted at a pressure of 2.67
MPa for three minutes followed by cooling under pressure. The fabricated samples were

cut to desired shapes for physico-mechanical properties evaluations.

3.2. Process II: Extrusion followed by Injection Molding

A two-step extrusion was
carried out in making CAP-hemp
biocomposites in contrast to a one
step extrusion step in making the
CABP-hemp biocomposites. The
first step was to produce CA

plastic granules from CA powder

 

and 30 wt% TEC plasticizer. In the

Figure 4. Cross sectional image of compression
molded hemp reinforced cellulose acetate based 2"d step, CA plastic granules were
biocomposites (50X magnification, 1mm scale
bar) fed into a twin-screw extruder

(ZSK-30 Werner Pfleiderer, co-rotating, intermeshing, with IJD = 30) while feeding the
chopped hemp (~ 1/4 inch) in the 2“" to last port of the extruder. The temperature profiles
were ~190°C with a die temperature ~195°C. A ISO-rpm screw speed was used with ~

40% machine torque reading and a resulting extrudate output of about 65—70

85

grams/minute. For CABP-hemp biocomposite pellet production, both CABP powder and
chopped hemp fibers in the requisite amounts were fed into the extruder under similar
conditions for extrusion. The thin strands of composites were granulated and stored for
further injection molding. The injection-molding machine used was an 85-ton
Cincinnati-Milacron press. The injection molding conditions used were: temperatures on
zone 1 to 3 were at 195°C, die at 60°C with 40 seconds cooling time, 25 seconds extruder
delay, fill pressure was 1500 psi, pack pressure and hold pressure were 1500 and 1200
psi, respectively. All of the physico-mechanical properties were determined through

ASTM standards.

3.3. Analysis and Testing

Flexural strength and modulus of elasticity (MOE) were determined using United
Calibration Corp. SFM-20 as per ASTM D 790. Similarly, tensile strength and modulus
properties were measured using the same machine following ASTM D 638. The notched
Izod Impact strength was carried out with an Impact tester from Testing Machines Inc.
43-OA-01 according to ASTM D 256. Environmental scanning electron microscope
(ESEM) micrographs of impact and tensile fractured samples were taken using Phillips
Electroscan 2020 (at 20 kV and chamber pressure of 3 Torr) to evaluate the adhesion
between hemp fiber and CA matrix. The fractured specimens were gold sputtered before

taking the images.

86

Storage, loss modulus, and damping (tan delta) of a material can be determined

using DMA. In DMA, a sample is exposed to a sinusoidal strain input and the output

 

 

 

 

 

 

 

450

E

3

£300-

C

2

iii

13150-4

O.

E Fl

0 I I I
A B C D

Figure 6. Impact properties of powder processing
(Process 1) and extruded followed by injection molding
(Process H): A = CA Plastics (CAP) of Process 1, B =
CA Plastic Biocomposites (30wt% hemp) of Process 1,
C = CA Plastic of Process II, D: CA Plastic
Biocomposites (30wt% hemp) of Process 11.

 

stress is measured. The
portion of this output stress
represents energy stored
(Storage Modulus, E’) by the
material or elastic
component of the material,
which is in phase with the
applied sinusoidal strain
input. The viscous
component or the remainder
of the output stress that is out
of phase represents the

amount of energy dissipated

by the material (Loss Modulus, E”). The relationship between the two is given by:

E* = E’+E”

where E* is the complex modulus. The ratio between E” and E’ is the phase lag or tan

delta or damping factor. The peak of loss modulus (E”) where the storage modulus (E’)

decreases rapidly is generally the glass transition (Ts) of a material.

87

Besides storage, loss modulus and tan delta, one can get damping characteristics

 

Figure 7. ESEM micrograph of impact fractured samples: A = CAP Biocomposite
(30wt% hemp) Process I (100X and 450 um scale bar), B = CAP Biocomposite
(30wt% hemp) of Process II (95X and 450nm scale bar)

using DMA. The temperature step / frequency sweep method using a single cantilever

clamp in DMA (TA dynamic mechanical analyzer, DMA model 2980) was used in
determining the damping factor (tan delta) of the materials at temperatures around the
glass transition (Ts) of CAP with frequency ranges from 0.01 to 100 Hz. Sample
dimensions for damping characteristics are 17.18mm long, 12.76mm wide, and 3.18mm

thick.

4. Results and Discussion

4.1. Processing Approaches Vs. Performance of Biocomposites

88

_L
O)

 

+ROM k =1
0 Experimental

Modified ROM k=0.35

.s
N
r

 

 

 

Modulus of Composites (GPa)

 

 

O l I l l
0.00 0.05 0.10 0.15 0.20 0.25

Fiber Volume Fraction

Figure 8. Comparison between rule of mixtures
(ROM), modified ROM, and experimental tensile
modulus of CAP Biocomposites.

The physico-
mechanical properties of
the biocomposites
fabricated from cellulose
acetate plastic and
chopped hemp fiber as
fabricated through the
above-mentioned two
processing approaches are
represented in Figures 5

(Appendix 1) and 6. As

can be observed from Figure 5; the flexural strengths (FS) and modulus of CA plastics

and their biocomposites show superior performance when fabricated through process 11

as contrast to Process I. The PS of CAP made through process 11 is 33% greater than

CAP plastic made through Process 1. Similarly the corresponding biocomposites (30 wt%

hemp) exhibit a FS variation of ~ 31% - - process 11 thus show superior strength over

process I. As expected, the impact strengths of both of CA plastic (CAP) and CAP based

biocomposites fabricated through Process II, showed a lower value as compared to their

counterparts as made through Process 1. The variation of properties is attributed to the

fact that in Process 1; we adopt powder processing and lack of shear force is applied

whereas in process 11 the extrusion and injection molding processing exert sufficient

shear forces for the intimate mixing of powder polymer, hemp and liquid plasticizer.

89

This phenomenon is supported by Environmental Scanning Electron Microscope (ESEM)

images of fractured impact sample taken for Process I (Figure 7 a) and 11 (Figure 7 b).

Process 11 shows a smaller amount of fiber pull out indicating better adhesion between

hemp fiber and CA matrix as compared to that of Process 1. In addition, it is difficult to

distinguish between fiber and CA matrix in Process 11 as the fiber is chopped to smaller

size during pelletizing, which might improve adhesion with CA matrix, and give better

 

+ ROM k=1
9 Experimental

 

Modified ROM k =0.45

 

Modulus of Composites (GPa)
o to a: to KS 5: as

 

l

0.0 0.1

T

0.2

Fiber volume fraction

 

0.3

Figure 9. Comparison between rule of mixtures
(ROM), modified ROM, and experimental tensile

modulus of CABP Biocomposites

fiber dispersion. In
contrast, the long fiber in
Process I can be easily
distinguished from CA
matrix -- indicating poor
adhesion -- as it does not
undergo sufficient shear
force and chopping during
the processing. Therefore,
non-uniformity of fiber
distribution could be found

in some parts of the

sample. As a result, the modulus of elasticity (MOE) of CAP-based biocomposite of

Process I has a slightly lower value than its counterpart made through Process 11.

4.2. Fiber Contents Vs. Performance of Green/Biocomposites

In order to study the effect of the variation of fiber content on the performance of
the composites, the usual extrusion and injection molding processing (Process H) was
utilized. Comparison between the theoretical modulus values from Rule of Mixtures
(ROM), modified ROM and experimental tensile modulus is shown in Figures 8 and 9.
A least squares fit of the experimental values in Figures 8 and 9 with respect to rule of
mixture equation yields a constant k value of 0.35 and 0.45 for CAP biocomposites and

CABP biocomposites, respectively, in the following equation:

EC = kaE, +VmEm

Where: E,: = modulus of resulting composites
k = constant value (fiber orientation parameter) used to fit experimental tensile modulus
data with theoretical modified ROM
Vf = fiber’s volume fraction (using p of hemp = 1.29 gm/cm") 1'
Ef = modulus of hemp = 42 GPa 1'
Vm = matrix’s volume fraction
Em = modulus of the matrix

The variation of theoretical and experimental modulus (Figures 8 and 9) value
seems reasonable since the fiber orientation is random throughout the samples, and there
is no modification on the surface of the fiber or no compatibilizer is used to make natural

fiber and matrix become more compatible (improvement in adhesion).

91

4.3. Hemp Fiber Reinforced Cellulose Acetate Vs. Hemp Fiber Reinforced

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Polypropylene Composites
100 8
7; Strength 9 Modulus
“- 80 --
5 60 gas;
3 .. if??? — r 4
9 40 i"
a 5.2; 5 SE 3:323:333513331
h ........
a $25.3. 35.31;: _.. 2
9 2° “ ’ o 22:22::
“- .
0 1r iii-3': JI : 0
CAP CAP- PP PP-

Hemp Hemp

Figure 10. Flexural properties: CAP, CAP
Biocomposites, PP, and PP-30wt% composites

Flexural Modulus (GPa)

Our ultimate goal is to
replace existing synthetic
glass fibers with natural fibers
as reinforcements and also
petroleum-based polymers
with renewable resource-
based biopolymers as matrices
in designing and engineering
of biocomposite materials.
With this in mind, the flexural

properties of un-sized hemp-

polypropylene (PP-Hemp) composites were compared with CAP-Hemp based

biocomposites. Both samples contain 30 wt% fibers and are extruded and injection

molded (Process H). Figure 10 shows a higher flexural strength -- by 42% -- for CAP-

Hemp based biocomposites than that of PP-Hemp based biocomposites.

Besides mass, stiffness, and dimensional stability, NVH (noise, vibration, and

harshness) is another crucial factor in automotive world for passenger comfort.

Preliminary studies have shown that certain natural fiber reinforced thermoplastics gain

similar or improved damping characteristics as compared to petroleum-based, glass

fibers-reinforced thermoplastics. Damping characteristics of these biocomposites were

92

 

 
   
   
 

 

 

 

 

0.4-
o 0.35
3’ 03 ——)(——PP ---O---PP-Hemp
1, ' +CAP - -l- - CAP-Hemp
8 0.25
’5 0.2
g 015
0
a .

I

g 0.1 “-.-"..l---.e------n
'—

0.05 ......... o.. . . .

0| 1 T I i l
0 20 40 60 80 100
quuencymz)

Figure 11. Damping properties: CAP, CAP-Hemp
Biocomposites, PP, and PP-Hemp composites

measured using the
temperature step /
frequencies sweep
method (DMA) and
were evaluated at the
temperature interest of
80°C, since both CAP
and PP are in their
rubbery states. As can
be seen in Figure 11,

both CAP and its

biocomposites have better damping characteristics (higher tan delta values for all

frequencies applied) and thus better sound absorption and less vibration as compared to

PP and its biocomposites. Thus, CAP can be a green alternative for replacing PP in

plastic composites for automotive applications.

4.4. Hemp - CAP based Biocomposites Vs. Hemp - CABP based Biocomposites

Besides CAP based biocomposites, a 2"d type of promising cellulose ester matrix,

cellulose acetate butyrate plastic, CABP is also evaluated for the biocomposites

applications. One main advantage of CABP as compared to CAP is that CABP does not

need plasticizer in order to be processed. The longer acyl substituted groups present in

the CABP chain may act as internal plasticizers so that its processing window is wider as

93

compared to CAP. By

adopting the 2“‘1 processing

 

 

 

 

 

50 .
CAP-Hemp approach e. g. extrusron
.5 we. CABP .
0. followed by injection
a /
3 30 7 molding (Process H); we
0 V\
.2; CAP . .
CD 20 evaluated therr stress-strain
10 behavior and
, , 0 , r , , morphological properties.
-10 -5 0 5 10 15 I F' 12 th
n r ure , e
Extension (%) g

inherent higher energy
Figure 12. Stress-strain comparison between CAP,

CAP Biocomposites, CABP, and CABP absorption capacity and
Biocomposites
higher strength of CABP
biocomposites compared to CAP biocomposites was observed as the larger area under the
stress-strain curve was obtained for the former. Further, a rougher tensile fracture surface
is produced with CABP biocomposites (Figure 14A). Good interaction between hemp
fiber and cellulose ester matrix (both fiber and matrix are polar) can be observed in the
tensile fractured samples as depicted in Figure 13 and 14. Fiber pull out micrographs,
especially for CABP biocomposites (Figure 148), show that individual fibers are covered

by the matrix as a result of hydrogen bonds formed between matrix and fibers.

5. Conclusions

Eco-friendly green composites from plant derived fibers and plant derived plastics

94

are new, emerging “green” materials for automotive applications. Cellulose acetate,

 

Figure 13. ESEM micrograph of tenSile fractured samples of CAP Biocomposites
(Process H): A = 200x with 250 um scale bar, B = 1000X with 45 um scale bar.

plasticized with eco-friendly citrate plasticizer, and cellulose acetate butyrate plastic,

 

Figure 14. ESEM micrograph of tensile fractured samples of CABP Biocomposites
(Process H): A = 200x with 250 um scale bar, B = 1500X with 30 um scale bar.
CABP (without additives or plasticizers) can be successfully used as matrix polymers in

95

fabricating the green composites. Comparing the plasticized (30%) cellulose acetate
(CAP) with neat CABP, CABP is a better matrix as far as composite processing and
physico-mechanical properties are concerned. Comparing the non-polar polypropylene
(PP) matrix and the 30% plasticized polar cellulose acetate plastic (CAP), the CAP
proved to be a much better matrix for hemp fiber (polar in nature) and thus exhibited
improved flexural and damping properties. Both CABP and CAP matrix have good
interaction with hemp fibers as shown by fiber pull out micrographs, with the former

being the better, and thus it leads to higher strength.

6. Acknowledgments

Financial support from the NSF/EPA (Award Number DMI-0124789) under the
2001 Technology for a Sustainable Environment (TSE) program is gratefully
acknowledged. The authors also thank Eastman Chemical Company, Kingsport, TN for
the cellulose ester samples. The authors also thank Flaxcraft, Inc. (Cresskill, NJ),
Hempline, Ontario, Canada and Morflex Inc. (Greensboro, North Carolina) for supplying

natru'al fibers and citrate plasticizer respectively.

96

7. References

1.

A. K. Mohanty; M. Misra; L. T. Drzal, “Sustainable Biocomposites From Renewable
Resources: Opportunities and Challenges in the Green Materials World”, Journal of
Polymers and the Environment, 2002,10(1).

A. K. Mohanty; M. Misra; L. T. Drzal, “Surface modifications of natural fibers and
performance of the resulting biocomposites: An Overview”. Composite Interface.
2001, 8, 313.

P. Mapleston, Automakers see strong promise in natural fiber reinforcements,
Modern Plastics, April 1999, 73.

M. Misra; A. K. Mohanty; L. T. Drzal, “Natural/Biofiber Reinforced Polyolefin
Composites: Biobased Opportunities and Challenges in the Materials World”,
Proceedings of 8th Annual Global Plastics Environmental Conference (GPEC 2002) —
Plastics Impact on the Environment. February 13 & 14, 2002, Detroit, MI, Society of

Plastics Engineers, Environmental Division, Full paper published in the proceeding,
2002, p. 383-392.

Grown to Fit the Part. minderChrvsler High Tech. Report. 1999, 82.

“Green Door-Trim Panels Use PP and Natural Fibers”, Plastic Technology,
November 2000, 27. '

Carl Eckert (Kline & Co, NJ), “Opportunities of Natural Fibers in Plastic
Composites”, 3rd International Ag Fiber Technology Showcase, October 4-6, 2000,
Memphis, TN, USA.

R. Stauber, Proc. Kuntststoffe im Automobilbau, Mannheim/1999, VDI-Verlag,
Dusseldorf, 1999.

J. L. Broge, “Natural fibers in automotive components”, Automotive Engineering
Internationals, October 2000, 120.

10. S. L. Wilkinson, “Nature’s pantry is open for business”, Chemical & Engineering

News, Jan 22, 2001, 61.

11. A. K. Mohanty; A. Wibowo; L. T. Drzal; M. Misra, “Effect of Process Engineering

on the Performance of the Natural Fiber Reinforced Cellulose Acetate

Biocomposites”, Composite Part A: Applied Science and Manufacturing, 2004, 35,
363.

97

IV. A Solvent free Graft Copolymerization of Maleic Anhydride onto
Cellulose Acetate Butyrate Bioplastic by Reactive Extrusion

Arief c. Wibowol, Shrojal M. Desai‘, Manjusri Misra‘, Lawrence T. Drzall and
Amar K. Mohanty"
1Department of Chemical Engineering and Materials Science, Composite Materials and
Structures Center, 2100 Engineering Building
Michigan State University, East Lansing, MI, USA, 48824
7’The School of Packaging, 13o Packaging Building
Michigan State University, East Lansing, MI, USA, 48824, Fax: 01- 517-353-8999;
Phone: 01-517-355-3603;
'E-mail: mohantya@msu.edu
1. Abstract

Interfacial adhesion between fibers and matrix is crucial factor for effective stress
transfer from matrix to fiber to enhance composite properties especially in short fiber
composite system. The use of a chemical compatibilizer is a way to achieve such
adhesion. We have synthesized maleic anhydride grafted cellulose acetate butyrate
(CAB-g-MA), which can be used as compatibilizer in biocomposite fabrication. The
research was directed at maleation of cellulose acetate butyrate (CAB) to study the effect
of initiator, monomer — maleic anhydride (MA) concentration and processing conditions
on grafting efficiency. CAB-g-MA was characterized by FTIR and titration methods.
This compatibilizer was also produced under optimized conditions on a large scale
extruder. The unique feature of this process is its solvent-free approach to grafting of

maleic anhydride onto CAB, without hydroxyl group protection.

Keywords: Functionalization of biopolymers, extrusion, FT-R.

2. Introduction

98

With the growing importance of environmental concern, there is increasing
development of coo-friendly materials. Sustainable biocomposites can be fabricated using
biopolymers derived from biomass as matrix and natural plant fibers (PF) as reinforcing
materials. A potentially useful biodegradable, biobased polymer that is compatible with
polar plant fibers (PF) is cellulose ester (CE). Our initial work has shown that cellulose
esters particularly plasticized cellulose acetate (CAP) and cellulose acetate butyrate
(CAB) have better mechanical and damping properties as compared to those of non-
polar, petroleum based polypropylene matrix in composites prepared from industrial
hemp (a bast natural fiber) as reinforcing material 1'2. The polar-polar interaction between
the PF and CE produces good interfacial adhesion and thus better stress transfer from
matrix to fibers, resulting in high performance bio-composites.

In order to compete with petroleum based glass-reinforced polypropylene for
structural applications, the interfacial adhesion between PF and CE has to be improved
further. To improve adhesion the addition of a suitable coupling agent would be an
economically feasible option as an alternative to surface treatment of fibers. Significant
enhancement of mechanical properties was achieved for polypropylene based composites
after addition of maleated polypropylene (PP-g-MA) 3'4. Currently, a number of maleated
polypropylenes with varying acid numbers and molecular weights are commercially
available 5. No report is yet available to the literature on grafting of MA onto cellulose
ester bioplastics. The mechanism of maleation onto polar bioplastic is quite different to
that of nonpolar plastic; the main motivation of the present investigations. In this study,
maleic anhydride grafted CAB (CAB-g-MA) has been synthesized as a compatibilizer by

reactive extrusion and characterized with FTHt and acid base titration. Initiator and

99

monomer concentrations were optimized based on the FI‘ IR and titration results using a
microextruder. Reaction conditions (temperature, rpm) in a twin-screw extruder were
optimized to obtain similar acid number and percentage MA grafting values as obtained

using the microextruder.

3. Experimental
3.1. Materials

Cellulose acetate butyrate (CAB 381-20) free from any plasticizer and additives,
was supplied by Eastman Chemical Co., Kingsport, TN, with a butyryl content of 37 %,
acetyl content of 13.5 %, hydroxyl content of 1.8 % and has melting and glass transition
temperatures of 195 °C and 141 °C, respectively. Maleic anhydride (MA), acetone, and
0.1011 N methanolic KOH were purchased from Aldrich Chemical Co and liquid
initiator: 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane (Luperox 101) was obtained from
Akzo Nobel, USA. Sodium hydride (NaH), sodium hydroxide (NaOH) were purchased

from Aldrich Chemical Co as well.

3.2. Instrumentation
The FTIR spectra of neat CAB and CAB-g-MAs were recorded in absorbance
mode from compression molded thin films using a Perkin-Elmer Spectrum 2000 FTIR

spectrophotometer.

3.3. Synthesis of maleic anhydride grafted CAB (CAB-g-MA).

Two processing techniques were adopted for the synthesis of CAB-g-MA:

100

Process I: Small-scale extrusion using DSM micro extruder. Pre-measured amounts of

CAB, MA (all in powder form) were mixed in small beaker. Liquid initiator was added

 

,/"

Absorbance

_ ./ .

1850 1825 1800 1775‘

 

 

 

 

CHI-1

 

Figure l. FI'Hl curve showing unreacted maleic anhydride
(MA) peak before putting in vacuum oven (A) and after putting
in vacuum oven at 80-90 C for overnight (B), and (C) is neat
CAB.

dropwise using a syringe while stirring. The resultant mixture (about 12 grams total) was
fed into a DSM Research 15cc Micro Extruder, DSM research, Netherlands, under the
following conditions: temperatures in zone 1, 2, and 3 were kept between 195 to 205°C,
screw speed was 100 rpm, cycle time was 3 minutes. Thin strand (extrudate) was
collected and pelletized into granules.

Process II: Large-scale extrusion using ZSK-30 twin-screw extruder. Premeasured CAB,
MA (all in powder form) were mixed in kitchen mixer. Liquid initiator was added drop

wise while stirring. The resulting mixture (about 1500 grams total) was hand fed into

101

ZSK-30 twin screw extruder with the following optimized conditions: temperatures in
zone 1 to 6 were kept between 190 to 195 °C with die temperature of 200 0C, 150 rpm

screw speed and 50 % torque reading. Extrudate was collected and pelletized.

3.4. Characterization of CAB-g-MA by FTIR (qualitative)

The pellets as obtained by Process I and H were vacuum dried overnight at 80 to
90 °C to remove unreacted maleic anhydride. Disappearance of unreacted maleic
anhydride (MA) after granulated samples were put inside vacuum oven at 80-90°C for
overnight is shown in Figure 1. A relatively small peak at 1850cm-1 was initially present
as unreacted MA. Once the sample was heated at 80-90°C (as melting points of MA is
about 53°C) in vacuum condition, this unreacted MA was emitted. Hence the
corresponding peak was disappeared.

Some of the vacuum dried CAB-g-MA was compression molded (in a Carver
Press SP-F 6030) into thin films at 197 °C for FI‘Hi measurements. Here, anhydride
groups belonging to MA were assigned to a peak value of 1786 cm'1 whereas carbonyl
groups of neat CAB were designated by 1778 cm'1 peak. Similarly, Silverstein et al.
reported that grafting MA onto the ester showed band at l788cm—l corresponding to the

symmetric C=O stretch of a saturated cyclic five membered anhydride 6.

3.5. Characterization of CAB-g-MA by titration (quantitative)
A part of purified CAB-g-MA was used to determine acid number (AN) and
percentage MA grafting (% MA) using titration method. Following procedure was

adopted in this quantitative measurement: 0.2 g of Maleic Anhydride grafted CAB was

102

dissolved in hot acetone (~200 ml) using magnetic stirrer. The hot solution was then
titrated against 0.1011 N methanolic KOH adding 5 drops of thymol blue in DMF (1 %)
as an indicator. Appearance of stable yellow color was considered as the visible end
point. Acid number (AN) and grafting percentage (% MA) were calculated using the

following equation:

 

_ VKOHNK0H( EW) (_ mg KOH )
_ weight Of sample —g of sample
%MA =VK0H NK0H( EW)( g of sample) ( 100%)

MW
number of H+ or OH' donor

where EW =

 

where, V, N, EW are volume, normality, and equivalent weight, respectively. Blank
titration was done as well on neat CAB. However, the details of this procedure are

available elsewhere 7.

4. Results and Discussion

Cellulose is a linear polysaccharide comprises of cellobiose repeat units linked at
B-l—e 4 position 8. A cellobiose consists of two anhydroglucose repeat units with
opposite orientation (in terms of hydroxyl groups’ locations) from each other. This B—l—>
4 linkages lead to material with relatively low water solubility. In general, cellulose
extracted from natural resources are high molecular weight non-branching
polysaccharides and are having conformational with great tendency to form
intermolecular hydrogen bonding. Extensive hydrogen bonding formations produce

cellulose fibers with high insolubility and relatively impermeable to liquids 9. The

103

inherent rigidity of cellulose originates from hydrogen bonding of the free hydroxyl
groups in the anhydroglucose repeating unit within and between chains. The stability of
the structure, thus mainly depends upon the ability to form hydrogen bonds. As a
consequence, cellulose is not a thermoplastic but degrades on heating before the
theoretical melting point is reached. Breaking the hydrogen bonds by solubilizing and
reacting the free hydroxyl groups yields a thermoplastic, such as cellulose ethers and
cellulose esters.

Free hydroxyl groups are normally expected to react with anhydrides under
suitable reaction conditions leading to anhydride ring opening reactions. Thus, grafting of
maleic anhydride onto cellulose esters would require hydroxyl group protection in order
to avoid the reaction with anhydride. We allowed these OH groups in cellulose acetate
butyrate (CAB) to react with NaI-I, NaOH, and maleic anhydride to determine their
reactivity with the following procedures:

1. Procedure of NaH reaction:

 

NaH + R-OH > R-ONa + H2
dry acetone

We added NaH into dissolved CAB according to the following calculation:

1 mole equivalent 0H (CAB) = 1 mole equivalent NaH

1—'8—(O°-—l) = 0.0105 mole equivalentOH

17
= 0.0105(23 +1) = 0.254gm minimum of NaH needed

The mixture was heated while stirring until it dissolved. Obtaining CAB-ONa was done
by precipitation using methanol. The precipitate was vacuum dried for overnight at 80°C
prior to FTIR analysis.

2. NaOH reaction procedure:

104

R-OH + NaOH > R-ONa + H2O
Heat

 

Into DI water (T = 60-700C), we added CAB such that free OH groups ready to react, i.e.

if CAB+MA+hitiator A
a... m

 

 

 

l

 

Absorbanee

 

 

3600 3400 3200 3000 2800

cm"

 

 

 

Figure 2. FTIR analysis of OH protection on cellulose acetate
butyrate (CAB) using different materials: NaOH, N aH, and MA
with and without initiator.
effective swelling agent (water) is needed to encounter the strong hydrogen bonds

between cellulose chains, which creates hindrance during reaction 10. Aqueous NaOH
(20wt%) was added into it while stining. The mixture was left for stirring for 6 hours.
Collecting CAB-ONa powder was done by Buchner filtration. The recovered powder was
vacuum dried for overnight at 80°C prior to FTIR analysis.

3. Extruded CAB+MA and CAB+MA+Initiator

105

These experiments were conducted also to investigate whether MA reacts with the free

I. Initiation
A
R—O—O— R—r 211—0 .

II. Propagation +Terrnimtion

 

 

 

R-O . + 0R C H2011
H0 0 on
R 0R
0 i010
0112011-wa n on
Hydn gen Abstrmtion
1 Hydrogen Abstraction

9°H.°\:7’/—-

‘CTOR -0R ———r-- H -OR
H
0 0

0 o o o
(FinalProduct)

Figure 3. Proposed mechanism of maleic anhydride grafted cellulose
acetate butyrate (CAB-g-MA).

OH groups or not. Pre-measured amounts of CAB, MA (all in powder form) were mixed
in small beaker. Liquid initiator, for the second experiment, was added dropwise using a
syringe while stirring. The resultant mixture (about 12 grams total) was fed into a DSM
Research 15cc Micro Extruder, DSM research, Netherlands, under the following

conditions: temperatures in zone 1, 2, and 3 were kept between 195 to 205°C, screw

106

speed was 100 rpm, cycle time was 3 minutes. Thin strand (extrudate) was collected and
pelletized into granules. All samples were put in vacuum oven for overnight at 80-90°C
to eliminate unreacted MA at ~1850cm'1 peak. This vacuum dried samples were
compression molded (in a Carver Press SP-F 6030) into thin films at 197 °C for FTIR
measurements. A

FI‘HI spectra (Figure 2) of these reacted CAB did not show any changes in the

intensity of OH peak, indicating that almost all reactive (primary and secondary)

 

1

1.4wt°/cl A
0.9wt% l A ‘ v

-.:::"° ya”?

 

 

1 800 1 795 1 790 1 785 1 780 1775

cm '1

 

 

 

Figure 4. FTIR analysis of varying initiator (I) concentration (0,
0.5, 0.9, and l.4wt%) with 5wt% MA content kept constant.

hydroxyl groups are esterified in CAB. The remaining free hydroxyl groups are either
hindered or docile and do not react with maleic anhydride. Thus, free radical grafting of
MA onto CAB could be carried out without OH group protection. The proposed
mechanism of CAB maleation is schematically represented in Figure 3. The process

involves free radical generation on the CAB backbone using a peroxy radical initiator.

107

The free radicals on the polymer chains react with the MA to give CAB-g-MA (Note:

Table 1. Effect of initiator concentration (CAB-g-MAI, 2, and 3) and effect of
MA concentration (CAB-g-MAZ, 4, and 5) on acid number (AN) and % MA

grafting.

 

Material Description AN StDev °/o MA StDev

 

CAB, neat 100% CAB powder 381 -20 - - - .

0.5wt% Initiator + 5wt% MA 4-
CAB-g-MA1 94.5wt% CAB, neat 12.8 0.0 0.6 0.0

0.9wt% Initiator + 5wt% MA +
CAB-g-MA2 94.1wt% CAB, neat 19.7 1.1 0.8 0.2

1.4wt°/c Initiator + 5wt% MA 4-
CAB-g-MA3 93.1wt% C AB, neat 18.2 0.0 0.8 0.0

0.9wt% Initiator + 7.5wt% MA «1-
CAB-g-MA4 91-6M% CAB, neat 31.7 1.5 1.8 0.4

0.9wt% Initiator + 10wt% MA +

CAB-g-MAS 89.1wt% CAB, neat 24.2 0.0 1.2 0.0

hydrogen abstraction is from the CAB backbone). It was found that the residence time of

 

 

 

 

 

 

 

 

 

 

 

 

 

the polymer melt in the extruder did not affect the grafting reaction significantly.
Optimized compositions of process I yielding the best results of grafting were chosen for
process H (with optimized temperature and rpm) in order to get similar acid number and
percentage grafting.

The initiator concentration was varied as follows: 0.5, 0.9, and 1.4 wt% while
keeping 5 wt% MA constant with the following nomenclature: CAB-g-MA 1, 2, and 3,
respectively (Figure 4 and Table 1). The increase in the anhydride peak intensity with
initiator concentration is apparent in the FTIR spectra. As initiator amount increased, acid
number and % MA increased up to 0.9 wt % initiator and then leveled off due to the lack
of free radicals on the CAB backbone. From above results, 0.9wt% was set as optimum
amount for initiator for this system.

The initiator amount of 0.9 wt % was kept constant while varying MA

concentration from 5 to 10 wt %. Figure 5 and Table 1 shows that acid number and %

108

MA increases with the amount of MA in the reaction mixture. Maximum grafting was

 

 

f 177

1 786cm1

10wt%MA
~ 7.5wt°/cMA A4fi“
1 5wt%MA ’ l‘
< -0wt%MA ’6'“
— ' ‘/

a ‘

 

 

I

I

I

1

1800

1 795

1 790

cm“1

1 785

1 780

1775

 

 

Figure 5. FTIR analysis of varying maleic anhydride (MA)
concentration (0, 5, 7.5, and 10wt%) with 0.9wt% initiator content
kept constant

observed for 7.5 wt% of MA in the reaction mixture and reached steady state due to

limited free radicals on the CAB backbone. This result is supported by the increase in the

Table 2: Acid Number and Maleic Anhydride percentage (% MA) comparison
between samples obtained from Process 1 (DSM micro extruder) and Process H (ZSK-

 

 

 

 

 

 

 

30 large extruder).
Process l (DSM extruder) Process ll (ZSK-30 extruder)
Mm“ Wm" N533“ StDev 94. MA StDev Nflgfl StDev 91. MA StDev
CAB-g-MAZ ghmgnéfifgngzmm* 19.7 1.1 0.8 0.2 18.8 1.7 0.8 0.1
CAB-g-MA4 imflmfiwm 31.7 15 1.3 0.4 33.2 1.4 1.5 0.1

 

 

 

 

 

 

 

 

intensity of the anhydride peak in CAB-g—MA samples. Based upon FT Ht, AN, and %

MA results, compositions for CAB-g-MA 2 and CAB-g-MA 4 (Table 1) were chosen for

higher quantity scale up.

109

 

One of the aims of this research was to develop a commercially and industrially
feasible process for the production of CAB-g-MA with predetemrined acid number and
% MA content. This was achieved by optimizing the reaction conditions of Process H on
the basis of Process I while keeping compositions (CAB-g-MA 2 and 4) constant. With
such conditions described in experimental part for Process H, we have successfully
developed products as shown in Table 2 in which equivalent values of acid number and
% MA (considering the overlapping error bars) were observed for Process H as compared

to those of Process 1.

5. Conclusions

We have successfully synthesized maleated cellulose acetate butyrate (CAB-g-
MA) with controlled acid number by a solvent free process in order for it to be used as
compatibilizer in cellulose ester based biocomposites. OH group protection could be
avoided since the reactivity of the hydroxyl groups on the CAB backbone was low. The
effects of initiator and maleic anhydride (MA) concentration on the synthesis of CAB-g-
MA were studied by Process I (DSM Micro Extruder — small scale). Acid numbers of the
maleated CAB increased with the MA and the initiator concentrations. However, the
effect of residence time on the product was negligible. Using optimized reaction
conditions and reagent compositions, the grafting reaction was reproduced by Process H
in a large-scale fabrication facility using ZSK-30 Twin Screw Extruder - to achieve
similar acid numbers and percentage grafting to those of Process 1. The effect of CAB-g-
MA compatibilizer on adhesion of cellulose fiber based biocomposites will be presented

in future publications.

110

6. Acknowledgements

Financial support from the N SF/EPA (Award Number DMI-0124789) under the
2001 “Technology for a Sustainable Environment (T SE)” program, and NSF-NER 2002
Award #BES- 0210681 under “Nanoscale Science and Engineering (NSE); Nanoscale
Exploratory Research (NER) program” and NSF 2002 Award # DMR-0216865, under
"Instrumentation for Materials Research (Ht/IR”) program for providing financial support.
The authors also thank Eastman Chemical Company, Kingsport, TN for the cellulose

ester samples.

111

7. References

1. A. K. Mohanty, A Wibowo, L. T. Drzal, M. Misra. Composites Part A: Applied
Science and Manufacturing 2004; 35 (3): 363.

2. A Wibowo, A. K. Mohanty, M. Misra, L. T. Drzal. Ind. Eng. Chem. Res. (Submitted,
December 2003).

3. Li Haijun, Sain M Mohini. Polymer-Plastic Tech. Eng. 2003; 42(5): 853.
4. A. K. Mohanty, L. T. Drzal, M. Misra. J. Adh. Sci. Technol 2002; 16(8): 999.

5. T. J. Keener, R. K. Stuart, T. K. Brown. Composites Part A: Applied Science and
Manufacturing 2004; 35 (3): 357.

6. R. M. Silverstein. Swtrometric Identification of Organic Communds; Wiley: New
York, 1981.

7. D. Carlson, L. Nie, R. Narayan, P. Dubois. J. Appl. Polym. Sci. 1999; 72: 477.

8. R. D. Gilbert, J. F. Kadla. in Biomlmers from Renewable Resources, D. L. Kaplan
(ed.), Springer-Verlag, NY, 1998, chap. 3.

9. J. F. Robyt. in Essentials of CgrbohydrateChemistrv. Springer-Verlag, NY, 1998.

10. K. C. Gupta et al. Cellulose. 2001; 8: 223.

112

V. Effect of Compatibilizer on Thermomechanical and Morphological
Properties of Hemp Fiber Reinforced Cellulose Ester Biocomposites

A. Wibowo], A. K. Mohantyz, L. T. Drzall, M. Misra“

I Composite Materials and Structures Center and Department of Chemical
Engineering and Material Science, 2100 Engineering Building,Michigan State
University, East Lansing, MI,USA,48824, Fax: 01 517432 1634; Phone: 01 517353
5466; E-mail: misramalz@egr.m§u.edu

2 The School of Packa ing, I30 Packaging Building, Michigan State University,
East Lansing, MI, US ,4 824

1. Abstract

Biocomposites are emerging as a viable alternative to glass fiber reinforced
composites especially in structural and automotive applications. By embedding
inexpensive natural cellulosic fibers into biopolymeric matrices, 100% biobased
composite materials (biocomposites) can be made. Cellulose esters e.g. cellulose acetate
(CA) and cellulose acetate butyrate (CAB) plasticized with 25-30 wt.% plasticizer have
been successfully reinforced with chopped hemp fiber using an extrusion—injection
molding process to produce a potential alternative to glass fiber/polyolefin composites for
automotive applications. The addition of a small amount of suitable compatibilizer
(CAB-g-MA) into these systems had been shown to improve the adhesion of the hemp
fiber to the biopolymer matrix as well as their thermomechanical properties.
Keywords: composites, biofibers, adhesion, extrusion-injection molding,

thermomechanical properties, morphological properties.

2. Introduction

113

Cellulose ester is one biobased polymer that has been produced for decades
(Eastman Chemical Company). This environmentally benign plastic has outstanding
clarity, high surface gloss, toughness and durability, good chemical resistance, ease of
processing and molding, and colorability. As a result it is commonly used in such
common everyday items such as hair brushes, tooth brushes, sunglasses frames, tool
handles, toys, packaging applications, automobile trim, and so forth. The objective of this
research is to utilize cellulose ester as a biomatrix and reinforce it with inexpensive
natural fibers, with a goal of improving the interfacial adhesion between matrix and fiber;
in order to provide a biobased alternative to glass fiber reinforced polypropylene for
automotive applications. Previously completed research has shown that cellulose esters
(particularly plasticized CA (CAP) and CAB) are much better matrices in terms of
mechanical and damping properties compared to non-polar polypropylene matrix in using
natural fiber, such as hemp, as a reinforcing material 1'2. The natural polar-polar
interactions between these constituents produce good interfacial adhesion and thus better
stress transfer from the low modulus matrix to the high modulus natural fibers resulting
in high performance biocomposites.

To compete with petroleum based glass filled polypropylene for structural
applications, however, the interfacial adhesion between natural fiber and cellulose esters
matrix has to be improved further. Grafting a polar group (maleic anhydride) onto
polymer matrix, for example maleic anhydride functionalized polypropylene (PP-g-MA),
is widely used as one method to improve compatibility in polymer blends, to improve
adhesion of glass and carbon fibers, and is used as processing aid for recycling plastic

waste 3‘4’5. Enhancement of tensile strength and impact strength has been observed after

114

‘r

adding PP-g-MA into cellulose-polypropylene (PP) composites 6‘7‘8. Gatenholm et al have
proposed that the formation of entanglements at brushlike interfaces in cellulose-PP
composites is responsible for the improvement of adhesion between PP-g-MA treated
cellulose fibers and PP.

Based on this approach, CAB-g-MA has been produced 9 by reactive extrusion as
a compatibilizer for cellulose esters based biocomposites and their thermomechanical,

and morphological properties have been evaluated .

3. Experimental
3.1. Materials

Powdered cellulose acetate (CA 398-30) free from any plasticizer and additives
and plasticized cellulose acetate butyrate (Tenite Butyrate 485E3720016) were supplied
by the Eastman Chemical Co., Kingsport, TN. The degree of substitution of the acetyl
groups in the CA was approximately 2.45. The supplied Tenite Butyrate (TEB) was
composed of about 72% cellulose acetate butyrate (CAB), ~25% bis(2-ethylhexyl)
adipate (DOA) plasticizer, and ~7% additives. Triethyl citrate (TEC) plasticizer was
obtained from Morflex Inc., Greensboro, North Carolina. Chopped industrial hemp fiber
(1/4 inch) was supplied by Hempline, Delaware, Ontario, Canada. CAB-g-MA granules
(with acid number of ~33 and ~18) were made following the procedure described in
earlier paper 9. CAB-g-MA with acid number ~33 and ~18 have been used in this
research and are subsequently identified as compatibilizer 1 (C1) and compatibilizer 2

(C2), respectively.

115

3.2. Biocomposites Processing

A two-step extrusion process was utilized in making the plasticized CA-hemp
biocomposites in contrast to a one step extrusion step in making the TEB-hemp
biocomposites. All biocomposites produced were of 30 wt% fiber content. Note that CA,
TEB, compatibilizer, and hemp were vacuum dried overnight at 80 0C prior to
processing. The first step was to produce CA plastic granules from CA powder and 30
wt% TEC plasticizer. In the second step, CA plastic granules with or without
compatibilizer were fed into a twin-screw extruder (ZSK-30 Werner Pfleiderer, co-
rotating, intenneshing, with IJD = 30) while feeding the chopped hemp (~ 1/4 inch) in
the next to last port of the extruder. The temperature profiles were ~190 °C with a die
temperature of ~195 0C. A ISO-rpm screw speed was used with ~ 40% machine torque
reading and a resulting extrudate output of about 65—70 grams/minute was obtained. TEB
pellets with or without compatibilizer and chopped hemp fibers in the requisite amounts
were fed into the extruder under similar conditions of extrusion to produce the
biocomposites. The thin strands of composites were granulated and stored for subsequent
injection molding. The injection-molding machine used was an 85-ton Cincinnati-
Milacron press. The injection molding conditions used were: temperatures on zone 1 to 3
were at 195 0C, die at 60 0C with 40 seconds cooling time, 25 seconds extruder delay, fill
pressure was 1500 psi, pack pressure and hold pressure were 1500 and 1200 psi,
respectively. All of the physico-mechanical properties were determined through ASTM

standards.

3.3. Plastics Fabrication using Micro Compounder

116

Pre-measured quantities of TEB with CAB or compatibilizer were mixed in small
beaker. Similarly, CAP granules (made on the ZSK-30) with CAB or compatibilizer were
mixed in a small beaker. The above compounds (about 10 grams per batch) were fed

separately into a DSM Micro 15 cc compounder (DSM research, Netherlands) under the

 

  

following conditions:
50
Strength

A . A
g 40 __ Modul g
E 9
5 30 -~ :3
8’ a
2 o
.7, 20 ~- *1 s

0
é’ a
g 10 a g
1- e: I-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TB 0% 5%C1 10%C1 10%C2
Figure 1. Effect of addition C1 (acid number ~33), C2
(acid number ~18), on tensile properties of plasticized

cellulose acetate butyrate (Tenite Butyrate, TEB) —
hemp biocomposites.

temperatures in zone 1, 2, and 3 were kept between 195 to 205 0C, screw speed was 100
rpm, and cycle time was 3 nrinutes. This extruder is equipped with a screw of length 150
mm, IJD of 18, and net capacity of 15 cm3. In order to obtain the desired specimen
samples required for various measurements and analysis, the molten plastics samples
were transferred after extrusion in a preheated cylinder to the mini injection molder,
which is pre-set with desired temperature (mold temperature of 50 0C), pressure (~100 psi)

and cooling.

117

 

75 10

 
   

El Strength

e Modulus
a 60“ . -8 a
a ..... a
E e
‘3, 45 - _ 6 3
c :l
g 30— _4 8
m , 2
X x
0
fi 15 - __ 2 E

o q mt: H 0

 

 

TB 0% 5%C1 10%C110%C2

Figure 2. Effect of addition of C1 (acid number ~33), C2 (acid
number ~18), on flexural properties of TEB hemp
biocomposites.

3.4. Characterization and analysis

 

 
   

 

 

250 I 30
llnpacteEB A
2 20°" {- e
‘ € » «~20:
3 150 -- g
a m
n. .-
g wo- “I
o 1,. +10 =5”
1; ‘50- j m

0 1 1 1 1 0

 

 

 

 

TEB 0% 5%C1 10%C1 10%(2

Figure 3. Effect of addition of C1 and C2 on impact properties
of TEB hemp biocomposites.

118

 

aat4OC
aat80C

1

1

 

 

1

Storage Modulus (MPa)
.e N 8
. a a i 5 a

 

 

 

 

 

 

TB 0% 10%C1 10%C2
Figure 4. Effect of addition of C1 (acid number ~33),

C2 (acid number ~18), on storage moduli of TEB hemp
biocomposites.

The tensile and flexural properties of the composite specimen were measured with
a United Testing System (SFM-20) testing machine according to ASTM D638 and
ASTM D790, respectively. The Notched Izod impact strength was measured with a
Testing Machines Inc. 43-02-01 Monitor/Impact machine according to ASTM D256.
The storage modulus and tan delta of the composite specimen were measured as function
of temperature (from room temperature to 140°C) using a TA instruments 2980 Dynamic
Mechanical Analyzer (DMA) with the frequency of 1 Hz at a heating constant rate of 4°C
Imin (cpm). The heat deflection temperature (HDT) of the samples was determined using
the same machine at a heating rate of 2 cpm. HDT is widely used in automotive
applications and represents the temperature at which the material deflects by 0.25 mm at
an applied force (three point bending arrangement) of 66 psi (ASTM D 648). A modified
ASTM D 648 was used for HDT measurement since the DMA could only handle a

smaller size of specimen as compared to that of ASTM D 648. Environmental scanning

119

electron microscope (ESEM) micrographs of tensile fractured samples were taken using

Phillips Electroscan 2020 (at 20 kV and chamber pressure of 3 Torr) to evaluate the

adhesion between hemp fiber and matrix. The fractured specimens were gold sputtered

before taking the images.

16

 

—6— ROM k=1

A Experimental
——- Nbdiiied ROM k=0.3
--—— Modified ROM k=0.42

12-

 

Modulus of Composites (G Pa)

 

 

o I I I I I
0.00 0.05 0.10 0.15 0.20

Fiber Volume Fraction

 

0.25

Figure 5. Comparison between rule of mixtures (ROM),
experimental tensile modulus data of TEB — hemp
biocomposites with and without compatibilizer: C1 (acid

number ~33), and modified ROM.

4. Results and Discussion

Interfacial adhesion between hemp fibers and cellulose ester (CE) matrix was

investigated in this study by the addition of a small amount compatibilizer (C1 or C2)

made by grafting maleic anhydride onto cellulose acetate butyrate (CAB-g-MA) 9. In

optimizing the amount of compatibilizer needed, we have found (Figure 1) that between 5

120

 

Figure 6. ESEM micrographs of compatibilized TEB-hemp biocomposites using
compatibilizer: C 1 (acid number ~33). A = 450nm, 105X, B = lOOum, 490x

 

Figure 7. ESEM micrographs of compatibilized TEB-hemp biocomposites using
compatibilizer: C2 (acid number ~18). A = 450nm, 105x, B = 100nm, 500X

and 10 wt% of C1, the latter amount is sufficient to get considerable improvement of

adhesion between hemp fibers and TEB matrix.
As can be seen in Figure 1, the addition of 10 wt% of C1 into the TEB-hemp
system improved the tensile strength about 20% and tensile modulus about 45%. Similar

trends were observed when adding 10 wt% of C2 into the system, i.e. a 25% increase in

121

 

Figure 8. ESEM micrographs of non compatibilized TEB—hemp biocomposites. A =
450nm, 105X, B = 100p.m, 500x

flexural strength (Figure 2). Correspondingly, 20 % increase in storage modulus at 40 °C

and 80 °C were also obtained for compatibilized TEB-hemp biocomposites as compared
to the non-compatibilized one (Figure 4). There was, however, no improvement in impact
properties (Figure 3), glass transition temperature and heat deflection temperature (HDT)

of the biocomposites.

@ —.®

H _OR PIG—U Iii—0R biofiber

biofiber HO—C C—
0 0 o 3 ll

Scheme 1. Proposed mechanism of how grafted maleic anhydride
(CAB-g-MA) interacts with biofibers (alcoholysis reaction).

A number of models have been developed to predict the mechanical properties of
a fiber-reinforced polymer composite. A useful relationship for predicting the

mechanical properties of unidirectional composites is the rule of mixtures (ROM). The

122

modified ROM 7"°’" for determining the tensile modulus of a short fiber composite is

Table 1. OH content of cellulose acetate butyrate (381-20), dioctyl adipate (DOA),
cellulose acetate (CA 398-30). and triethvl citrate (TEC).

 

 

 

 

 

 

 

 

Matrix System Description Material OH Content
CAB Cellulose Acetate Butyrate CAB 381-20 1.8wt%‘
.
TEB Plasticized CAB CAB 381'” 1'8”“
DOA (Dioctyi Adipate) 0wt%'
' 8
CAP Plasticized Cellulose Acetate CA 39330 35““
TEC (Triethyl Citrate) Plenty

 

 

 

 

‘ Eastman Chemical Company (www.castman.com)

shown in equation 1. A comparison between the theoretical modulus value fi'om Rule of
Mixtures, modified Rule of Mixtures and experimentally determined values is shown in
Figure 5. The Rule of Mixtures estimates the composites stiffness by a weighted mean of
the moduli of the two components - matrix and fiber -- depending upon the volume
fiactions of the components 12. This equation is valid for long and parallel aligned fibers.
Since short fibers (1/4 inch long) with a random orientation were used in this experiment,
a constant k 7"°’" is used in the fiber part of the ROM (hence named modified ROM) to
compensate for the non-equality of fiber strain that causes the reduction of tensile
modulus of the experimental composites as compared to the tensile modulus predicted by
the ROM. The k value, representing the reinforcing efliciency dependent on the
orientation of the fibers and the stress transfer between the matrix and the fibers, can be

calculated from modified ROM as:
_ (1)
E, _ kaE, + VmEm
Where: Ec = Modulus of resulting composites

V: = Fiber Volume Fraction (p of hemp = 1.29 gm/cm3)‘

E; = Modulus of fiber (E2 = 42 GPa)‘

123

Vm = Matrix volume fraction

Em = Modulus of matrix.

3Value is taken from earlier paper].

In this experiment, k=0.3 is obtained for non compatibilized TEB-hemp biocomposite
whereas in compatibilized TEB-hemp biocomposite k=0.42. The increase of reinforcing
efficiency suggests an improvement in fiber dispersion resulting in improvement of
stiffness of the biocomposites.

Such major improvements in tensile and storage modulus properties are due to the

 

 

 

CHz—COO—CH2CH3 _ R _
HO-C—COO_CH2CH3
OR ,
CH2—COO—CH2CH3 o’
O
Triethyl Citrate (TEC) OR CH20R
— - n
Cellulose Ester (CE)

CH2CH3 p p CH2CH3
CH3(CH2)3CHCH2O—C—CH2CH2CH2CH2—C—OCH2CH(CH2)3CH3

Dioctyl Adipate (DOA)

Figure 9. Chemical structure of cellulose ester, triethyl citrate (TEC) plasticizer,
dioctyl adipate (DOA) plasticizer.

improved dispersion and adhesion between hemp fibers and TEB matrix. As presented in
Figures 6 and 7, smaller fiber pull out lengths, more fiber fibrillation and individual fibers
covered with TEB matrix, are indications of an improvement in adhesion after adding C1
or C2. This improvement is believed to be due to alcoholysis reaction between grafted

maleic anhydride (C=O groups) with OH groups of hemp fibers forming esters as

124

100 8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

r- .
w ‘F '- v
V. -
- -
.
. ~1-
.
..
. n.
..
.. .-
. .-
. l-
A
. ..
w -
. . ,
u-l— N
v . v
.-
. .-
.C ’ .
. w
.-
9" 'r 3
.w
D "+‘
a ..- :I
C --
-- *' '0
..
2 V .l
.
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.
I 'v E
.
a. .
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.-t
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. .. .
. .
r. . ..
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- ,,- - -
. . ..
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. ,. wen-.4 -
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1.... ..--
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di- r I. 00‘-
F r- --.
' ' .".
. -...-
’ I' --."
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1 . ----
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, . ..-.
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- '9' r
e- ..
l " r
,H _,
I i r " O

 

 

 

 

 

 

 

 

 

 

CAP 0% 5%(21096Q15%C210%C1 trtl-P

Figure 10. Effect of addition of C1 and C2 on tensile
properties of CAP - hemp biocomposites.

proposed in Scheme 1. In contrast, lower quality adhesion was observed for non-
compatibilized TEB-hemp biocomposites such as: longer fiber pull outs and wider gap
between hemp fibers and TEB matrix after tensile fracture (Figure 8).

A lower content of OH groups on TEB matrix (Table l and Figure 9) makes it
easier for grafted maleic anhydride to be reacted with OH groups of fibers without
significant competition from OH groups of TEB matrix during reactive extrusion. This
nucleophilic substitution leads to ester linkage formation on the fibers resulting in better
adhesion between matrix and fibers. In addition, the similar CAB backbone used in the
compatibilizer and matrix may give good entanglement necessary to improve interfacial
adhesion.

In the plasticized cellulose acetate (CAP)-hemp biocomposites system, after
adding compatibilizer there was no significant improvement in either tensile, flexural or

impact properties (Figures 10,11,12, respectively). A possible reason for this result is the

125

 

9 Strength 0 Nbdulus

Tensile Strength (MPa)
8
J;

Tensile Modulus (G Pa)

    
    

 

 

20»

—-2
10-~
0- ,0

CAP 0% 5%C2 10%Q 15%Q 10%C1 trtHP
Figure 11. Effect of addition of C1 (acid number ~33), C2

(acid number ~18), on flexural properties of plasticized
cellulose acetate (CAP) — hemp biocomposites.

existence of available OH groups on the CAP matrix. As depicted from their chemical

 

40

Impact Strength (Jlm)
0
Elena. at Break (%)

20

 

 

CAP 0% 5%02 10%C215%0210%01 trtl-P
Figure 12. Effect of addition of C1 and C2, on impact

properties of plasticized cellulose acetate (CAP) —
hemp biocomposites.

structure (Figure 9), CA and TEC have plenty of OH groups (Table 1), which can

compete with OH groups of fibers in reacting with the grafted maleic anhydride during

126

Storage Modulus (MPa)

 

CAP 0% 5%C2 10%02 15%0‘2 10%C1 trti-P

Figure 13. Effect of addition of Cl and C2 on storage moduli of
CAP — hemp biocomposites at 40 °C.

reactive extrusion. Hydrogen bonds on the matrix and on the fibers are broken due to heat
'3 during reactive extrusion making OH groups free to react.

Grafting the compatibilizer (C1) onto the fibers by dissolving C1 into hot acetone
followed by immersing the fibers for 15 minutes was utilized to avoid competition among
OH groups of CAP matrix and fiber. The resulting biocomposites which utilized the
treated hemp fibers were named ‘trt-HP’. Gelation of the solution, however, was a
problem during the reaction forcing the use of a very dilute solution (1wt%) compared to
the optimum concentration (~7wt% of solution). Therefore, low C1 grafting and low
distribution of C1 on the fiber might be responsible for the zero change of mechanical
properties observed when using the treated fiber (Figures 10, 11, 12). In the future, the
gelation problem can be solved by using higher miscible solvents.

The storage moduli of CAP biocomposites at 40 °C do not improve after addition

of the compatibilizer (Figure 13). This result retaliates with the modulus data obtained

127

2500

 

at 1000

2000 -

1 500 _ EI;~..-.-.-E:

10001

Storage Modulus (MPa)

    

 

 

   

CAP 0% 5%C2 10%Q 15%02 10%C1 trtHP

Figure 14. Effect of addition of different compatibilizer: Cl (acid
number ~33), C2 (acid number ~18), on storage moduli of plasticized
cellulose acetate (CAP) - hemp biocomposites at 100 C.

from the flexural test. There was significant improvement, however, in storage modulus

0.7

 

 

0.6 -
0.5 —
0.4 —

0.3 -

Tan Delta

0.2 4

 

 

 

 

Tem perature (°C)

Figure 15. Effect of addition of Cl and C2 on glass
transition temperature (peak of tan delta curve) of
plasticized cellulose acetate (CAP) —- hemp biocomposites.

especially at high temperature (100 °C). An improvement of storage modulus of about

128

120

 

100-

80-

60~

40-

HDT (’C) at 66 psi

20—

 

 

 

CAP

 

Figure 16. Effect of addition of C1 (acid number ~33), C2 (acid

number ~18), on heat deflection temperature (HDT) of

plasticized cellulose acetate (CAP) ~ hemp biocomposites.
260% was obtained at 100 0C (Fig.14). This fact is probably due to inclusion of more
thermally stable compatibilizer into the CAP-hemp biocomposites system. An
enhancement of ~20 0C in glass transition temperature (from tan delta peak results —
Figure 15) and heat deflection temperature (HDT — Figure 16) was observed to support
the fact that inclusion of more thermally stable compatibilizer into the system led to
reduction in polymer segmental motion inside CAP backbone.

A thorough investigation to find out whether CAB-g-MA functioned as filler or as
compatibilizer in TEB or CAP based biocomposites system was conducted using DMA
analysis. As described in experimental section, compatibilizer (C1) was not only added
into both systems (TEB and CAP) but also neat CAB was included to investigate the real

cause of improvement in each case (addition of 10wt% of CAB or C1). Figure 17 showed

that there was no improvement in storage modulus after addition of either compatibilizer

129

or neat CAB into TEB from room temperature to about 100 0C. This proved that CAB-g-

MA acted as compatibilizer

 

 

 

 

 

 

 

1600 enhancing interfacial adhesion
E A 8 between TEB matrix and hemp
E 12001 c
5 fiber as well as improving the
a 800 . .
2° hemp fiber dlsperSlon and
0
g 400 - fibrillation as suggested in the
earlier section. The CAP based
0 I If I
30 50 70 90 system, in contrast, showed an
Temperature (°C)

increase of storage modulus at

Figure 17. Effect on storage modulus: A = TEB, B

= A + 10wt% neat CAB, C = A + 10wt% c1, “18" temperature (starting from

about 70-80°C: CAP’s glass
transition temperature, as depicted in tan delta curves - Figure 19) after addition of both
CAB and Cl (Figure 18). This supported the assumption about inclusion of more
thermally stable materials (CAB based) into CAP based systems (for both plastic and

biocomposites systems).

5. Conclusions

Interfacial adhesion between hemp fibers and cellulose ester matrices can be
enhanced by the addition of small amount of a suitable compatibilizer. By the addition of
~10 wt% compatibilizer (CAB-g-MA) into plasticized cellulose acetate butyrate (TEB)-
Hemp system we found an improvement in tensile strength of about 20%, a 45%

enhancement in tensile modulus, and a 25% increase in flexural strength. A 20% increase

130

in storage modulus at 40

 

and 80 °C was also
obtained for compatibilized
TEB-hemp biocomposites
as compared to non-

compatibilized ones.

Storage Modulus (MPa)

 

 

Comparison with ROM

 

 

and DMA analysis showed
Temperature (°C)
that these improvements

Figure 18. Effect of inclusion of CAB or C1 into CAP
system on storage modulus: A = neat CAP, B = A +
10wt% neat CAR. C = A +10wt% C1.

were predominantly due to
the effectiveness of the

compatibilizer in improving

 

the fiber dispersion and

fibrillation as well as

 

 

 

 

increasing the interfacial g
adhesion between TEB E
matrix and hemp fibers.
Enhancement in CAP based B
biocomposites, in contrast, ‘ ' ‘
30 60 90 120 150

has been shown to be due to Temperature (°C)

msertron 0f the more Figure 19. Effect of CAB or C1 into CAP system on

Tg (peak of tan delta curve): A = neat CAP, B = A +
10wt% neat CAB, C = A + 10wt% Cl.

into the CAP based system. This filler improved storage modulus of CAP biocomposites

thermally stable CAB-g-MA

131

by about 260 % at 100 °C. A ~20 °C increase in glass transition temperature and heat

deflection temperature was observed to support the ‘inclusion effect’ statement.

6. Acknowledgements

Financial support from the NSF/EPA (Award Number DlVH-0124789) under the
2001 Technology for a Sustainable Environment (TSE) program and NSF 2002 Award #
DMR-0216865, under “Instrumentation for Materials Research (IMR) Program" are
gratefully acknowledged. The authors thank Eastman Chemical Company, Kingsport, TN
for the cellulose ester samples. The authors also thank Flaxcraft, Inc. (Cresskill, NJ),
Hempline, Ontario, Canada and Morflex Inc. (Greensboro, North Carolina) for supplying

natural fibers and citrate plasticizer respectively.

132

7. References

1. A. K. Mohanty, A. Wibowo, L. T. Drzal, M. Misra, Composite Part A: Applied
Science and Manufacturing, 2004, 35(3), 363.

2. A. Wibowo, A. K. Mohanty, M. Misra, L. T. Drzal, Ind. Eng. Chem. Res. (accepted
April 30, 2004).

3. R. J. Ehrig, Plastic Recycling, Products and Processes, Carl Hanser Verlag, 1992.
4. Haijun Li, Mohini M. Sain, Polymer-Plastic Tech. Eng. 2003, 42(5), 853.

5. A. K. Mohanty, L. T. Drzal, M. Misra, J. Adh. Sci. Technol 2002, 16(8), 999.

6. H. Dalvag, C. Klason, and H. —E. Stromvall, Int. J. Polym. Mater., 1985, ll, 9.

7. R. T. Woodhams, G. Thomas, and D. K. Rogers, Polym. Eng. Sci., 1984, 24, 1160.
8. J. M. Felix, P. Gatenholm, J. Appl. Polym. Sci., 1993, 50, 699.

9. A. Wibowo, A. K. Mohanty, M.Misra, and L. T. Drzal, Polymer Preprint, Spring
2004 (accepted), 227th ACS National Meeting, Anaheim, CA, 2004,March 25.

10. RC. Raj, B.V. Kokta, Polym. Eng. Sci., 1991, 31(18), 1358.

11. A. R. Sanadi, R.A.Young, C. Clemsons, R.M. Rowell, J. Reinf. Plast. Compos., 1994,
13, 54.

12. D Hull, T. W. Clyne, An Introduction to Commsite Materials, 2'” Edition,
Cambridge Solid State Science, 1996.

13. R. T. Morrison, R. N. Boyd, Organic Chemisg, 6'” Edition, Prentice Hall, New
Jersey, 1992.

133

 

CONCLUSION and FUTURE WORK

Sustainability, industrial ecology, and green chemistry are challenging today’s
society. Rapid growth in technology can result in over dependence on petroleum
feedstocks, increased energy consumption and increased carbon dioxide emission. Issues
such as global warming, shortage of petroleum feedstock, and limited landfill space are
becoming more important considerations and causing scientists and engineers to consider
the increased use of environmentally benign materials for many applications. Renewable
resources made primarily from carbon dioxide, can be a potential solution to achieve
equilibrium between consumption and emission of carbon dioxide. Recyclability and
biodegradability are common features of these renewable resources and their products. .

Many plants therefore are grown to collect their fibrous materials such as hemp,
jute, kenaf (from stem), henequen, pineapple (from leaf), and cotton (from seed). On the
other hand, agriculture based plants, which also lignocellulosics, are mostly getting
wasted and solely used as source of energy by burning them. The cellulose fibrils in
many lignocellulosics are potential biobased reinforcements. Extraction of these
nanofibrils can be achieved by chemical/mechanical processes. These bio-sourced nano
reinforcements would be compatible with a biomatrix (matrix derived from renewable
resources such as cellulose esters, poly(hydroxy alkanoates)) and thus will result in a
biobased composite with superior properties. The modification of the nanofibrils either
by adding or subtracting functional group/s can tune the interfacial properties as desired.

For example, hemp fibers interact with a cellulose ester matrix through hydrogen
bonding. Stronger interaction can improve the resulting composite properties. Formation

of covalent bonds through ester linkages between maleic anhydride functionalized

134

cellulose acetate butyrate (CAB-g-MA) and hemp fibers was accomplished to improve
adhesion and fiber distribution resulting in improvement of thermomechanical and
morphological properties. [Refer to: A. Wibowo, A. K. Mohanty, M. Misra, L. T. Drzal,
“Effect of Compatibilizer on the Thermomechanical and Morphological Properties of
Hemp Fiber Reinforced Cellulose Ester Biocomposites”, Ind. Eng. Chem. Res. (to be
published)]. When using plasticized cellulose acetate (30wt% plasticizer content) as the
matrix in the biocomposites formulation, the addition of CAB-g-MA produces a matrix
with better thermal properties than the CAP matrix. Advantage of the reaction of
hydroxyl groups on the biofober surface with the carbonyl groups of CAB-g-MA can be
gained by increasing the surface area of the hemp fibers (by for example doing alkali
treatment) such that more OH groups are exposed on the surface for easier reaction.

In attempt to replace/substitute petroleum based glass fiber reinforced
polypropylene composites by natural fiber reinforced cellulose ester biocomposites,
several issues still need to be addressed including improvement in impact strength,
reduction of moisture absorption and better fiber distribution when using sheet molding
compound (SMC) in fabricating the biocomposites. Impact strength can be improved by
addition of rubberized materials, which induce phase separation, which leads to
improvement in impact fracture. Inclusion of leaf fibers in addition to hemp (bast) fibers
can improve the impact strength as well. Moisture absorption can be reduced by blocking
most of the functional groups that can form hydrogen bonds with water. Such protection
can be done by, for example silane grafting or blending with hydrophobic polymer,
which is miscible or partially miscible with cellulose ester. Finally better fiber dispersion

for any process with lacking of applied shear force can be achieved by optimizing the

135

 

fiber length so as to have minimum fiber entanglement with optimum fiber ‘aspect ratio’.
Another crucial parameter is to alter the fiber surface tension to be higher than the surface
tension of the matrix to get good wetting by addition of compatibilizer or by suitable

surface treatment.

136

 

APPENDIX 1

 

    

 

 

      
   

 

 

 

 

 

 

 

 

 

 

10
3 3 a
____..g m
~5- 3 9
5 2 re
O —----6 2
r: g; :I
g e E g
.1; s —4 E
a E
a E 25
.2 O E ” ti
IL E E
1-—- -0
C D

Figure 5. Flexural properties of powder processing
(Process I) and extruded followed by injection
molding (Process H): A = CA Plastics (CAP) of
Process 1, B = CA Plastic Biocomposites (30wt%
hemp) of Process I,- C = CA Plastic of Process H, D=
CA Plastic Biocomposites (30wt% hemp) of Process
H.

137

APPENDIX 2

In this project, we utilized cellulose acetate (CA) as one of the matrix in
developing biocomposites for future automotive parts. CA needs to be plasticized in
order to increase its flowing capability due to its thermal decomposition temperature is
close to its melting temperature. Plasticizer selection was conducted using equation 1
(Introduction section page 22) to get similar value of solubility parameter for a miscible
blend. Hildebrand solubility parameters (8) of several plasticizer and CA with DS of 3.0
and 2.5 were calculated based upon the knowledge of their chemical structures (Barton,
Allan F. M, CRC Handbook of Solgbility Pmeters m Other Cohesion PJar_ameters,

Boca Rotan, CRC Press, 1991) using the following equation:

,2 U
6 = —
Z V
Where 6 = Hildebrand solubility parameter Me”)
U = group contribution to the molar vaporization energy (J/mol)
V = group contribution to the molar volume at 25 °C (cc/mol)
Table 1 shows that CA (DS = 3.0 and 2.5), Triethyl Citrate (TEC) plasticizer, Diethyl
Phthalate (DEP) plasticizer have 5

Table 1. Calculated Hildebrand Solubility
Parameter (6) based upon the chemical structure values close among one another.

 

 

 

 

 

 

 

Material 5 (MPa°~") s cal/cc ° .
Cellulose acetate 03 = 3.0 22.8 11.1 ““31 TEC can “:le DEF 1“ CA
Cellulose acetate 08 = 2.5 24.0 11.7 , , _

Triethyl citrate 22.5 11.0 bioplastic formulation for more
Glycerol 37.0 1 8.1

Diethyl phthalate 21.6 10.5 environmentally benign

 

 

 

 

plasticizer. Detailed discussion on the effect of the TEC plasticizer and process

engineering in developing CA bioplastic suitable for biocomposite application is

138

described on part I of this thesis.

139

   

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