146
292
THS

 

 

100‘? LIBRARY
Michigan State

University

 

 

 

This is to certify that the
thesis entitled

A STUDY OF MASS TRANSFER OF MOISTURE IN
BIODEGRADABLE SHEETS AND RESINS

presented by

MAHESH KHURANA

has been accepted towards fulfillment
of the requirements for the

MS. degree in Packaging

 

 

 

“W Major Wure
87 v14 —- 09

Date

 

MSU is an Affirmative Action/Equal Opportunity Employer

..-.-.—.--------—---.-------—.‘.--.-.-.-o--u--.--n-.-.-.-.-.—

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5108 K:IProj/Aoc&PrelelRC/DateDue.indd

A STUDY OF MASS TRANSFER OF MOISTURE IN BIODEGRADABLE SHEETS
AND RESIN S

By

Mahesh Khurana

A THESIS

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

MASTER OF SCIENCE
Packaging

2009

ABSTRACT

A STUDY OF MASS TRANSFER OF MOISTURE IN BIODEGRADABLE SHEETS
AND RESINS

By
Mahesh Khurana

Through this research the moisture sorption of two biopolymers, Biomax® and
Sorona®, was investigated. DuPont Biomax® is a hydro-biodegradable modified
polyester. Sorona®, Poly(trimethylene terephthalate), is a thermoplastic polyester
produced by condensation of terephthalic acid and 1,3-propanediol (derived form
renewable sources). The moisture sorption profile of both polymers was assessed at 23
and 40°C using gravimetric technique.

The moisture sorption isotherms of both polymers showed a positive temperature
dependence, i.e. more moisture was sorbed at higher temperature. The solubility,
diffusion and permeability coefficients of both polymers were also determined. The
impact of processing was evaluated by comparing the sorption of moisture on resin and
sheet of both polymers. The moisture sorption in resins was significantly lower than in
sheets at either temperature, which is due the morphological changes during the
extrusion.

The effect of moisture on physical and mechanical properties of both polymers
was evaluated. Moisture sorption in Sorona® has little or no impact on the tensile
strength, Tg and crystallinity. In Biomax®, the water acts as a plasticizer as shown by a
drop in glass transition temperature from 40 to 30°C. Also, the mechanical properties of

®

Biomax were drastically reduced after the moisture sorption.

5a my fiamily and [cicada

iii

ACKNOWLEDGEMENTS

I am really thankful to Dr. Maria Rubino, my major advisor for her unlimited and
invaluable support, guidance, and mentoring through out the course of my Master’s
degree. She was always there for the support and guidance, whenever I needed. I would
also, like to thank my other committee members, Dr. Rafael Auras and Dr. Laurent
Matuana, for their support and suggestions which helped me a lot in my research.

I also thank the Center for Food & Pharmaceutical Packaging Research (CFPPR)
for the financial support to this project.

Next I would like to thank my family, especially my brother, without their support
I wouldn’t have been able to come to US for my Masters. Also the family and fi'iends
back in India who prays for my well being.

I am also thankful to the faculty and staff of The School of Packaging and a
special thank to the friends at school and also outside the school for their help. I would
really like to add here my personal appreciation to Dr. A.F. Venero and Marcelo
Man'cheno from VTI Corporation for their suggestions and support.

Last but not least, I would like to thank GOD for everything.
Thank you all

Mahesh Khurana

iv

Table of Contents

List of Tables .................................................................................................................... vii
List of Figures ................................................................................................................. viii
Chapter 1: Introduction ................................................................................................. 1
1. Background ................................................................................................................... 1
2. Scope of the Study ........................................................................................................ 4
3. References ..................................................................................................................... 5
Chapter 2: Background and Methodology ................................................................... 7
1. Introduction ................................................................................................................... 7
2. Biodegradable Polymers ............................................................................................... 7
2.1. Classification of Biodegradable Polymers ............................................................. 7
2.1.1. Renewable Source-based Polymers ............................................................ 8
2.1.2. Petroleum-based Biodegradable Polymers ................................................. 8
2.1.3. Polymers from Mixed Petroleum and Renewable Sources ........................ 9
2.2. DuPont Biomax® ................................................................................................... 9
2.3. DuPont Sorona® ................................................................................................... 11
3. Mass Transfer Phenomenon ........................................................................................ 12
3.1. Sorption ................................................................................................................ 13
3.2. Diffusion .............................................................................................................. 14
3.3. Permeability.......... ............................................................................................... 15
3.4. F ickian and Non-Fickian Kinetics ....................................................................... 16
3.5. Parameters Affecting Permeability ...................................................................... 17
4. Methods for Studying the Sorption in Polymeric Materials ....................................... 19
4.1. Gravimetric Method ............................................................................................. 20
5. Symmetrical Gravimetric Analyzer (SGA-lOO) ......................................................... 22
6. Noise Quantification ................................................................................................... 24
7. Consistency Test ......................................................................................................... 26
8. References ................................................................................................................... 27
Chapter 3: A Study of Mass Transfer of Moisture in Biodegradable Sheets
and Resins .................................................................................................. 32
1. Introduction ................................................................................................................. 32
2. Theoretical Background .............................................................................................. 35
2.1. Sorption by Polymer Film ................................................................................... 35
2.2. Consistency Test .................................................................................................. 36
3. Experimental ............................................................................................................... 37
3.1. Materials .............................................................................................................. 37
3.2. Differential Scanning Calorimeter ....................................................................... 38
3.3. Instrument SGA-lOO ............................................................................................ 39
3.4. Noise Quantification ............................................................................................ 4O

3.5. Sorption Experiment ............................................................................................ 4O

4. Results and Discussion .............................................................................................. 41
4.1. Physical Properties .............................................................................................. 41
4.2. Instrument Noise and Drift ................................................................................. 41
4.3. Equilibrium Criterion Determination .................................................................. 45
4.4. Moisture Uptake in Sheets .................................................................................. 45
4.5. Consistency Test and Correction ........................................................................ 48
4.6. Sheet Moisture Sorption Results ......................................................................... 51
4.7. Effect of Temperature on Permeability, Diffusion, and Solubility ..................... 56
4.8. Resin Moisture Sorption Results ......................................................................... 57
5. Conclusions ................................................................................................................ 58
6. References .................................................................................................................. 59
Chapter 4: The Effect of Moisture Sorption on the Physical and Mechanical
Properties of Biomax® and Sorona® ....................................................... 62
1. Introduction ................................................................................................................ 62
2. Methodology .............................................................................................................. 63
2.1. Polymer Processing ............................................................................................. 63
2.2. Conditioning of the Samples ............................................................................... 64
2.3. Physical Properties .............................................................................................. 64
2.3.1. Differential Scanning Calorimeter (DSC) ................................................ 64
2.3.2. X-Ray Diffraction (XRD) ......................................................................... 65
2.3.3. Dynamic Mechanical Analyzer (DMA) ................................................... 65
2.4. Mechanical Properties ......................................................................................... 66
2.5. FT-InfraRed ......................................................................................................... 66
3. Results and Discussion ............................................................................................... 67
3.1. Physical Properties .............................................................................................. 67
3.2. Mechanical Properties ......................................................................................... 70
3.3. FT-IR Results ...................................................................................................... 72
4. Conclusions ................................................................................................................ 73
5. References .................................................................................................................. 74
Chapter 5: Conclusions ................................................................................................ 76
1. Conclusions ................................................................................................................ 76
2. Future Work Recommendations ................................................................................. 78

vi

List of Tables

Chapter 2:
Table 2.1: Physical and mechanical properties of Biomax® 4026 .................................... 10
Table 2.2: Physical and mechanical properties of Sorona®, PTT ..................................... 11
Chapter 3:
Table 3.1: Processing condition for Biomax® and Sorona® ............................................. 38
Table 3.2: Regression analysis of the equipment’s noise and drift ................................... 44
Table 3.3: Average Solubility, Diffusion and Permeability coefficients for Biomax®
and Sorona® sheets at 23°C and 40°C ............................................................. 55
Table 3.4: Enthalpy of sorption and activation energy of Biomax® and Sorona® sheets
(average of 3 runs 1 SE.) ................................................................................ 56
Table 3.5: Moisture uptake in Biomax® and Sorona® resin after 1400 minutes .............. 57
Chapter 4:
Table 4.1: Physical properties of dried and sorbed Biomax® and Sorona® samples ........ 68

vii

List of Figures

Chapter 2:
Figure 2.1: Schematic of Symmetrical Gravimetric Analyzer (SGA-100) ....................... 23
Chapter 3:
Figure 3.1: Schematic of Symmetrical Gravimetric Analyzer (SGA-lOO) ....................... 39

Figure 3.2: Percent weight change vs. time and linear regression line for a 50 mg
standard weight at 40°C, 90% RH ................................................................. 42

Figure 3.3: Residual output vs. time for 50 mg standard weight at 40°C, 90% RH ......... 43

Figure 3.4: Average percent weight gain (3 runs) vs. time (min) for Biomax® sheets
at 23°C and 40°C ............................................................................................ 46

Figure 3.5: Average percent weight gain (3 runs) vs. time (min) for Sorona® sheets
at 23°C and 40°C ............................................................................................ 48

Figure 3.6: Experimental Mt/Moo (before correction) and theoretical Mt/MOo vs. square
root of time (secondm) for a Biomax® sheet at 23°C and 30% RH ............... 47

Figure 3.7: Average of K values of different relative humidities for Biomax® and
Sorona® sheets before and afier correction at 23°C and 40°C ....................... 49

Figure 3.8: Experimental Mt/M00 after correctiong and theoretical Mt/M00 vs. square
root of time (secondl/ ) for a Biomax sheet at 23°C and 30% RH .............. 50

Figure 3.9: Moisture sorption isotherms of Biomax® and Sorona® sheets at 23°C

and 40°C ......................................................................................................... 52
Chapter 4:
Figure 4.1: DMA results for Biomax® dried and sorbed samples .................................... 68
Figure 4.2: Refractogram from XRD analysis for Biomax® dried sample ....................... 69

®

Figure 4.3: Stress-strain curves for Biomax® and Sorona samples ................................ 71

®

Figure 4.4: Mechanical properties of dried and sorbed Biomax® and Sorona samples. 71

Figure 4.5: IR Spectrum of Biomax® sheet before and after moisture sorption ............... 72

viii

Chapter 1

Introduction

1. Background

The US packaging industry uses around one-third of the total plastics produced in
the country.1 Plastic is the largest segment of the packaging industry, accounting for
approximately 50% of the total materials used in industry.2 Most of the plastics used
currently are derived from the petroleum sources and do not degrade easily after disposal.
In 2006, only 10% of the total packaging plastic was recycled; the remainder landed in
landfills, accounting for 6% of the total Municipal Solid Waste (MSW).3 Dwindling
petroleum sources and landfill sites have prompted the development of alternative and
environmentally friendly polymers. In the past few years, many new alternative polymers
have been developed that are either derived from non-petroleum sources or are
biodegradable. A few examples of such polymers are Poly(lactic acid) (PLA), Poly(s-
caprolactone)(PCL), and Polyhydroxylalkanoates (PHA).4 The global production of
biodegradable polymers in 2006 has increased 20-folds since 1995,5 but it is still just a
small fraction of the total polymer production. The major obstacle to commercialization
of these ‘novel’ polymers in the packaging industry is their cost and the limited
knowledge about their performance properties. Especially the barrier properties of such
polymers to different gases and vapours dictate the use of polymer in food and
pharmaceutical industry.

Polymeric packaging materials are permeable to small molecules like water

vapour, organic vapours, and gases. Because these permeants are often responsible for

product deterioration, their permeation through the polymer determines the shelf life of
the product. Water is one of the permeants responsible for the deterioration of most food
and pharmaceutical products. Therefore, a thorough understanding of water vapour
permeation through the new alternative biodegradable polymers is of great commercial
importance.

Permeation process in polymers is characterized by the sorption of compounds by
the polymer, their diffusion through the polymer, and finally desorption of the permeant
from the polymer to product or environment. Sorption is the uptake of the permeant,
water vapour, gases, or liquid, by the polymer membrane. Sorption of the permeant
molecule depends on the affinity between permeant and polymer.6 After sorption, the
permeant diffuses through the polymer’s free volume from the high concentration to low
concentration (or chemical potential) side and polymer characteristics also play an
important role. Thus permeability of gases through common packaging polymers can be
defined as:

P = D * S (1)
where P is the permeability coefficient, D is the Fickian diffusion coefficient and S is the
solubility coefficient.

Extensive research had been conducted for the evaluation of mass transfer
properties of polymers using various methods. While isostatic and quasi-isostatic
methods are generally used to evaluate the permeation of the permeant through the
polymer membrane;7'” gravimetric methods, using a spring balance, an electrobalance,
or a quartz crystal microbalance, evaluate the permeant sorbed in the polymer.”'16 The

gravimetric sorption method has the advantage that it can record real time mass

(moisture) uptake in a controlled environment at both the steady state as well as during
the transient state. The moisture content at steady state at different water activities can be
used to generate the moisture sorption isotherm, defined as the relation between the
moisture and polymer membrane at a constant temperature and pressure. The moisture
content at the steady state is used to determine the solubility coefficient (S) by the

following relation:

Moo
5 = (2)

v.p

 

where M00 is mass at steady state, v is the volume of the polymer and p is the permeant

force in unit of concentration or pressure. The transient state and the steady state data can
be used in determination of diffusion coefficient (D).17
As discussed earlier, several alternative polymers have recently been developed.

Biomax® and Sorona® are two such alternative polymers developed by DuPont. Biomax®

l8, 19

is an aliphatic-aromatic hydro-biodegradable polyester. It is a co-polymer of

polyethylene terephthalate (PET), produced by adding up to three different monomers to

® can be used for oriented

make this polymer degradable through hydrolysis.19 Biomax
films, blown films, molded plastic articles, or coating in disposable products such as
bowls, plates, cups, and sandwich wraps.20 To the best of the author’s knowledge no
research has been conducted to study the moisture uptake of Biomax®. Sorona®,
Poly(trimethylene terephthalate) (PTT), derived from renewable corn sugar, is a
thermoplastic polyester produced by condensation of 1,3-propanediol and terephthalic

acid.” PTT was developed several years ago but was not commercialized due to high

costof production. However recent developments have reduced the cost significantly.

Sorona® has several unique characteristics such as high elasticity and recovery as
compared to PET or nylon and better chemical properties as compared to PET,22 hence it
is gaining a lot of attention in packaging and textile industry. Research had been
conducted to tailor the physical and mechanical properties of Sorona® through blending it
with other polymer or nano-composites,2]‘ 23 but fewer researchers have studied the
moisture uptake of Sorona®.
2. Scope of the Study

The main goal of this study was to generate a moisture sorption profile for
Biomax® and Sorona® sheets and resins using a gravimetric method and to generate the
moisture sorption isotherms of both polymers sheets at 23 and 40°C. The effect of
processing, from resin to polymer, on the moisture sorption was evaluated. Lastly, the

effect of moisture sorption on physical and mechanical properties of both polymers was

investigated.

3. References:

l.

10. '

11.

12.

13.

Szabo, F. Int Polym Sci Technol 2001, 28, 11, T/1-9.

Tapuriah, V. The Future of Plastics in Packaging; Frost & Sullivan: 6 May, 2003.

Municipal Solid Waste Generation, Recycling, and Disposal in the United States:
Facts and Figures for 2006; United States Environmental Protection Agency:
Washington, DC, 2007.

Gross, R. A.; Kalra, B. Science 2002, 297, 5582, 803—807.

The Future of Global Markets for Biodegradable Packaging; Pira International
Ltd: 2006.

Hernandez, R. J.; Selke, S. E. M.; Culter, J. D., In Plastics Packaging: Properties,
Processing, Applications and Regulations. First ed.; Hanser Gardner Publications,
Inc.: 2000.

Gavara, R.; Catala, R.; Hemandez-Munoz, P.; Hernandez, R. J. Packag Technol
and Sci 1996, 9, 215-224.

Duncan, B.; Urquhart, J .; Roberts, S. Review of Measurement and Modelling of
Permeation and Diffusion in Polymers; National Physical Laboratory: 2005.

Yasuda, H.; Rosengren, K. J Appl Polym Sci 1970, 14, 11, 2839-2877.

Apostolopoulos, D.; Winters, N. Packag Technol and Sci 1991, 4, 131-138.

Barr, C. D.; Giacin, J. R.; Hernandez, R. J. Packag Technol and Sci 2000, 13, 4,
157-167.

Hernandez, R. J .; Gavara, R. J Polym Sci, Part B: Polym Phys 1994, 32, 2367-
2374.

Hernandez, R. J.; Giacin, J. R.; Grulke, E. A. J Membr Sci 1992, 65, 1-2, 187-
199.

14.

15.

16. '

17.

18.

19.

20.

21.

22.

23.

Wong, 8.; Zhang, Z.; Handa, Y. P. J Polym Sci, Part B: Polym Phys 1998, 36,
2025-2032.

Wong, C. H.; Bhethanabotla, V. R. Fluid Phase Equilib 1997, 139(371).

Oliverira, N. S.; Oliverira, J .; Gomes, T.; F erreira, A.; Dorgan, J .; Marrucho, I. M.
Fluid Phase Equilib 2004, 222-223, 317-324.

Crank, J., In The Mathematics of Diffusion. Second ed.; Oxford University: 1975.

Nagarajan, V.; Singh, M.; Kane, H.; Khalili, M.; Bramucci, M. J Polym Environ
2006, 14, 3, 281-287.

DuPont Launches Biodegradable Polyester - Biomax®, Press Release.
http://www.(mont.com.tw/020604-e.htm (April, 2008)

Leaversuch, R., Biodegradable Polyesters: Packaging Goes Green. Plastics
Technology 2002.

Liu, W.; Mohanty, A. K.; Drzal, L. T .; Misra, M.; Kurian, J. V.; Miller, R. W.;
Strickland, N. Ind Eng Chem Res 2005, 44, 4, 857-862.

Kurian, J. V. J Polym Environ 2005, 13, 2, 159-167.

Khonakdar, H. A.; Jafari, S. H.; Asadinezhad, A. Iran Polym J 2008, 17, 1, 19-38.

Chapter 2

Background and Methodology

1. Introduction

The first part of this chapter presents an overview of biodegradable polymers and
their classification based on raw materials. The second part focuses on the mass transfer
phenomenon, followed by sorption and diffusion kinetics. The last part includes the
details of the gravimetric equipment used for this study and noise quantification.
2. Biodegradable Polymers

Biodegradable polymers can be define as polymers whose chemical structure
changes significantly under specific environment condition as the action of natural
occurring micro-organisms. Bio-based polymers can be defined as polymers produced
from renewable agricultural or biomass feedstock. Biodegradable or bio-based polymers
can make the base for environment friendly and eco-efficient products that can compete
and capture current market dominated by petroleum-based polymers.
2. 1 Classification of Biodegradable Polymers

Different approaches have been used to formulate biodegradable polymers.
Depending on the sources of raw materials, biodegradable polymers can be divided in
three categories:" 2 a) Renewable source-based biodegradable polymers b) Petroleum-

based biodegradable polymers 0) Polymers from mixed petroleum and renewable sources

2. 1.] Renewable Source-based Polymers

Poly(lactic acid) (PLA) obtained from a renewable agricultural source (corn) is
synthesized by condensation polymerization of lactic acid or ring opening polymerization
of lactide.”4 PLA is currently used in packaging as thermoformed containers for fresh
produce and short shelf life products.3’ 5 Poly hydroxyalkanoates (PHA) is a class of
biodegradable polymers produced from renewable resources by microbial fermentation.
PHA alone is very brittle and difficult to process, therefore various copolymers have been
biosynthesized.6
2.1.2 Petroleum-based Biodegradable Polymers

Petroleum-based biodegradable polymers can be divided into two main
categories: aliphatic biodegradable polyesters and aromatic-aliphatic co-polyesters.

Aliphatic Polyesters: Poly(e-caprolactone) (PCL), is a biodegradable
thermoplastic obtained by ring opening polymerization of e-caprolactone (obtained form
crude oil) using a catalyst.7 PCL has good barrier properties and is easy to process, but
due to the high cost it is generally blended or copolymerized.5’ 8 Poly(butylene succinate)
(PBS) is a petroleum-based polymer made by condensation polymerization of 1,4-
butanediol with aliphatic dicarboxilic acids.

Aliphatic-Aromatic Co-polyesters: These are generally based on terephthalic acid.
Poly(butylenes adipate-co-terephthalate) (PBAT) is a thermoplastic co-polyester
produced by condensation of 1-4-butanediol, terephthalic acid and adipic acid. PBAT has
barrier and mechanical properties similar to LDPE and has been approved for food
contact applications.9 Easter Bio® from Eastman Chemicals and Biomax® from DuPont, a

hydro-biodegradable polyester, also belong to petroleum-based polymer category.lo

Other petroleum based polymers are Polyesteramides (PEA), made by
copolycondensation of polyamide and adipic acid.” Poly(vinyl alcohol) (PVOH) is
manufactured by hydrolysis of poly(vinyl acetate) in alcohol and has been reported to be
biodegradable. I 2
2. 1.3 Polymers from Mixed Petroleum and Renewable Sources

Poly(trimethylene therephathalate) (PTT), tradenarne Sorona® from Dupont, is a
3-carbon glycol terephthalate (3GT) synthesized by condensation of 1,3-propanediol and
terephthalic acid.” Propanediol can be produced from renewable sources (corn). PTT has
mechanical and barrier properties similar to PET.l4 Other polymers in this class are
Polyurethanes (PU) manufactured from polyols (vegetable oil based) and isocyanate.7
Various biodegradable blends have been studied, such as starch based blends, PLA based
blends or PHA based blends.

In this study we focused on two of the alternative polymers——Biomax® and Sorona®.
2.2 DuPont"! Biomax®

Biomax® (4026) is a hydro-biodegradable modified polyester, developed by E.I.
DuPontTM de Nemours & Co. (Wilmington, DE). The material is based on polyethylene
terephthalate (PET) and is synthesized by the polymerization of terephthalic acid,
ethylene glycol, glutaric acid, and sulfonic acid saltm’ '5 Properties of Biomax® are

shown in Table 2.1.

Table 2.1 : Physical and mechanical properties 0fBi0max® 4026*

 

 

Properties

Density, g/cm3 1.35
Glass Transition Temperature (Tg), °C 30
Melting Temperature (Tm), °C 195
Decomposition Temperature, °C 340
Flexural Modulus, GPa 3
Tensile Strength at Break, MPa 30

 

* Published on Matweb16

Biomax® has been awarded a ‘COMPOSTABLE’ logo by The Biodegradable
Products Institute (BPI),l7 because it meets ASTM D 6400-99 “Standard Specification of
Compostable Plastics”.'8 The polymer can be incorporated with up to three aliphatic
monomers according to the end-use. These monomers create weak spots, thus making
Biomax® degradable through hydrolysis. The moisture cleaves the polymer’s larger
molecules into smaller molecules, which are then consumed by natural micro-organisms.
Although Biomax® is intended mainly for composting, it can also be recycled,
incinerated, or landfill.19

Biomax® has been blended with soy proteins and polycaprolactone (PCL), and
with cotton (Bionature by Kurabo, Japan).2°’ 2' It is easy to process and can be processed
in standard PET equipment with minor changes. It can be used as blown film, oriented
film, or molded articles. Its major applications can be single-use products, such as waste
bags, blister packs, geotextiles, seed mats and coating for disposable eating utensils

(Cups, bowls or plates)”: 22

10

2.3 DuPontm Sorona®

. Sorona® is the trade name for Poly(trimethylene terephthalate) (PTT)
manufactured by E.I. DuPontTM de Nemours & Co. (Wilmington, DE). PTT is a
condensation polymer of 1,3-propanediol (PDO) and terephthalic acid (TPA). DuPont has
developed a technique to produce PDO from renewable resources (corn) or via biological
processes.[3 Sorona®, a three carbon glycol terephthalate (3GT), can also be produced
using only petroleum resources. Both type of polymers, petroleum-based and bio-based,
are expected to have similar properties as they have the same chemical structure.

Table 2.2: Physical and mechanical properties of Sorona®, PTT” ’4

 

 

Properties

Density, g/cm3 1.33
Glass Transition Temperature (Tg), °C 45-55
Melting Temperature (Tm), °C 228
Flexural Modulus, GPa 3.76
Tensile Strength, MPa 67.6

 

Sorona®, 3GT is member of the thermoplastic aromatic polyester family, which
includes polyethylene terephthalate (PET)-2GT and polybutylenes terephthalate (PBT)-
4GT.'3‘ 23 Sorona® glass transition temperature is in the range of 45-55°C, which is lower
than that of PET (80°C), but higher than that of PET (25°C).l4 Sorona® has a semi-
crystalline molecular structure with a zigzag shape that gives it excellent stretch recovery
and tensile and compressive properties. Sorona® has comparable strength, stiffness, and
heat resistance as of PET. As expected from linear aromatic polyesters, Sorona® has a

good barrier to moisture, is dimensionally stable, and has good weather and chemical

11

resistance. Sorona® can also be used as engineering therrnoplastics because of its good

thermal and mechanical properties.”’ 24

Sorona® can be cast into films at a setting comparable to polypropylene. Sorona®
has been blended with other polymers to offset its relatively high cost and tailor to
specific perfonnance-cost profile.14 Sorona® has also been used as Polymer/Clay
nanocomposites (PCNs), produced by dispersing nanoscale composites in a polymer
matrix. PCN possesses the excellent combination of barrier and mechanical properties,
which may eliminate the need for multilayer polymers for packaging application
(reviewed by Khonakdar, et al.).14
3. Mass Transfer Phenomenon

Historically, the main purpose of a package was containment (cellophane used for
bread in 1930’s), but today packaging plays more active role in protecting the product,
providing utility and communication. One of the major roles of packaging is to provide
shelf life to the product. Currently, plastic is the leader in packaging industry because of
its benefits like light weight, appearance, aesthetics, economy, and many others. Plastics
allow permeability of small molecules to through them. Polymer allows moisture, aroma,
and gases to pass through them, which can modify or deteriorate the product contained by
the package. Although this mass transfer can also be used to improve shelf life of some
products, for example modified-atmosphere packaging (MAP).25 Mass transfer or
permeation processes in polymers is characterized by the sorption of compounds by the

polymer, their diffusion through the polymer, and finally their desorption from the

polymer to product or environment.

12

3.1 Sorption

Sorption is the uptake of the permeant, such as moisture, flavor, aroma or colorant
by the polymeric packaging material. Sorption of the permeant molecule depends on the
affinity between permeant and polymer. When a polymer comes in contact of a liquid or
vapour phase, sorbates in both phases are exchanged until their potentials become equal.
The equilibrium of every sorbate can be established by a partition coefficient. At low
concentration levels, as in case in packaging, the mixing of permeant with polymers
behaves as ideal solution. The relationship between sorbate concentration and partial

pressure follows Henry’s law of linear sorption isotherm?"
Ci 2 S . Pi (1)

where C ,- is concentration of the permeant
S (Solubility Coefficient) is Henry’s Law proportionality constant and
p,- is the equilibrium vapor pressure of the permeant.

Henry’s law holds only at low concentration or when there is no interaction

between the polymer and permeant, as in case of Oz and N2 at low pressures or moisture

26. 27

sorption in hydrophobic polymer. The solubility coefficient (S) is an equilibrium

partition coefficient for distribution of the penetrant between polymer phase and vapor

phase and is given as:

C
_ p
S_ Cv (2)

where Cp is the concentration of penetrant in the polymer and CV is the concentration of

penetrant in the vapor phase at the steady state.

13

3.2 Diffusion

Diffusion can be defined as the movement of substance within itself or another
substance. Diffusion, for polymeric packaging materials, can be defined as the process
through which the matter (permeant) transport from high concentration side to low
concentration side as a result of random molecular motions. Diffusion coefficient (D) is
the quantitative measurement of the rate at which a penetrant diffuses. D is defined as the
rate of transfer of the penetrant across unit area of a section, divided by the space gradient
of concentration. This definition holds good only for the one dimensional diffusion and
when diffusion is normal to the section. The basic equations describing above definition

are Fick’s first law:28

F=— 99

6x (3)

where F is the rate of transfer,
D is the diffusion coefficient,
C is the concentration of penetrant,
x is space coordinate
. The negative sign in Eq. 3 indicates that the permeant molecules flow from high

concentration side to low concentration side. For unsteady or transient state, the rate of

flow of permeant can be described by Fick’s second law which can be written as

2
a z Di;
6: ax (4)

where t is the time. To obtain the flux (F) or D from above equations, initial and

boundary conditions associated with the experimental method are needed. The solution of

14

Eq. 4 is given analytically and a power-series of solution is derived. Crank28 presented
simplified equations related to the first approximation of the power series. Thus the
values of D obtain through these equations are only approximate values. Mass sorbed on
the polymer film as a function of time provides the necessary information to calculate the
diffusion coefficient of the permeant in the polymer.
3.3 Permeability

Permeability can be defined as the transfer of gases, vapors, liquid through a
homogeneous packaging material. It is the mass transfer of the permeants from higher
concentration side to the lower concentration side through the polymer. Mass transfer of
permeant through cracks, perforation or other defect is not considered as permeability.
Permeability is the overall process of sorption of permeant in the polymer, its diffusion
through the polymer and finally desorption on other side. Permeability of simple gases
through common packaging polymer at steady state is given by following relation:

P = D . s (5)

where P is the permeability coefficient, D is the F ickian diffusion coefficient and S is the
solubility coefficient. The above equation holds at low concentrations of the permeant
and when there is no interaction between permeant and polymeric material.

The permeability diffusion and solubility coefficients of a material are dependent
on the temperature and sometimes the environment relative humidity (RH). The

dependence of P, D and S can be described by a Van’t Hoff-Arrhenius equation:

S(T)=Soexp(—AHS /RT) (6)
D(T)=Doexp(-ED/RT) (7)
P(T)=PoeXP(-Ep/RT) (8)

15

where AHS = enthalpy of sorption
ED = activation energy of diffusion
Ep = activation energy of permeation

R = Gas constant and,
T = Temperature
Enthalpy of sorption (AHS) can also be considered as sum of two terms
AHS = AHC + AH," (9)
where AHC is the enthalpy of condensation of pure gaseous penetrant to the liquid phase
and AH," is the partial molar enthalpy of mixing the condensed penetrant with polymer
segments. As the solubility depends on the heat of sorption and unlike activation energy
of diffusion, it can take positive or negative values.
3.4 F ickian and Non-Fickian Kinetics
The diffusion behavior of polymer films can be classified in three classes on the
basis of relative rates of diffiision and polymer relaxation:29
a) Case I or Fickian diffusion in which diffusion is a function of concentration,
diffusion is much less than relaxation.
b) Case 11 diffusion, where the diffusion is very rapid as compared to relaxation.
c) Non-Fickian or anomalous diffusion, where the rate of polymer relaxation is
almost same as rate of diffusion.
According to Fick’s law, distribution of diffusant during sorption is governed by
the one-dimensional differential equation for diffusion, given by Eq. 3. Assumption for

Eq. 3 is that when the ambient pressure changes from a initial to a final value,

concentration at the film surface also changes to final value instantaneously. A sorption

16

curve is a plot of the amount of vapor absorbed or desorbed as a function of square root
of time. Sorption curve which follows the above-mentioned assumption of D and surface
concentration is known as Fickian or normal type sorption.

The fickian sorption kinetics was also explained on the basis of sorption and
desorption curves by Crank.28 If diffusion is only dependent on the concentration, then in
the initial stage the curves are linear; for absorption this is true up to 60% or more of Moo.

/ .
'2 axrs and

Above the linear portion the sorption curves become concave towards t
steadily approaches the final steady state value.

Diffusion in some systems is found to be dependent on other factors such as time,
polymer relaxation, etc. Generally, this is the case in glassy polymer systems and is
called anomalous or non-Fickian diffusion. In such cases the diffusion rate is on the order
of the polymer relaxation time scale. A simple deduction of diffusion coefficients from
experiment showing this behavior is difficult and often explained by Flory Huggins30 and
clustering effect“ wherein the first mechanism accounts for increase in diffusion
coefficient, the other explains decrease in diffusion coefficient.

3.5 Parameters Affecting Permeability

There are many factors that affect the permeability, sorption and diffusion, of
packaging materials towards moisture, gases, and organic vapors; these factors can be
environmental (temperature, relative humidity), polymer (chemical structure and
morphology), permeant (molecular size, concentration).

Permeability or sorption in polymers has been found to be different in the regions

below and above T8 of polymer. As also explained above in case of glassy polymer,

below T8, diffusion is generally found to anomalous or non-Fickian. Glassy polymers are

17

not in a state of true thermodynamic equilibrium and have very long relaxation times.
These long relaxation times let the polymer to homogenize with the penetrant
environment. Generally, to define the sorption and diffusion in this state a dual mode

sorption model theory is used.32 Normally the diffusion at temperature above T8 is rather
simple and follows Fickian kinetics. At temperature above T8, the polymer is in rubbery
state and its molecular chains are free to move. The micro-Brownian motion of polymers
molecules at temperature above Tg enable the polymer to reach the equilibrium rapidly
and diffusion is not time dependent. Diffusion in semi-crystalline polymers and polymers
with‘rigid chain conformations is usually found to non-Fickian even above the Tg.

The barrier properties of a polymer also depend on its chemical structure and
morphology. The barrier properties of polymer will increase with the increase in the
crystallinity and orientation. It has been proved that the permeation of molecules take
place only through the amorphous region of the polymer, thus increasing the crystalline
region will increase the barrier properties.33 Orientation of polymer brings the chains
closer and the molecular mobility in oriented region decreases, thus decreasing the
permeability.

Generally with the increase in temperature the segmental motion of polymer
chains increases and thus increasing the free volume of the polymer and the diffusion of
penetrant through the polymer. Also with the increase in temperature the permeant
molecules have the required energy to diffiise through a polymer. The effect of
temperature is best defined by the Arrhenius-type relationships given by Eq. 6-8.
Generally barrier properties of the polymer decreases with increase in temperature. But

few polymers have been reported which do not follow this trend. Polystyrene and

18

 

Polylactide have been reported to have negative activation energy.3 Temperature also
affects the partition coefficient, thus affecting solubility. With the increase in temperature
the solubility decreases. There are few exception to this rule also, e. g., the partition
coefficient of SARAN and ethyl acetate, n-hexanal and d-limonene were found to
decrease at 0.2 vapor activity.34

The diffusion of a penetrant through any polymer will be governed by the
molecular size and vapor pressure of the penetrant. As the energy required by a smaller
and lighter molecule to move is low, therefore they can easily diffuse through polymers.26
Diffusion of permeant through polymer is also affected by the vapor pressure of the
permeant, which defines the mobility of permeant at particular temperature. The sorption
of a molecule in a polymer will be affected by the affinity of the penetrant and polymer.
4. Methods for Studying the Sorption in Polymeric Materials

The first study of gas permeation through a polymer was conducted by Thomas
Graham in 1829.35 Since then extensive research had been conducted for the evaluation
of mass transfer properties of polymers using various methods. While isostatic and quasi-
isostatic methods are generally used to evaluate the permeation of the permeant through

the polymer membrane?“10

gravimetric methods, using a spring balance, or an
electrobalance, or a quartz crystal microbalance, evaluate the permeant sorbed in the

polymer.4045 For this research the moisture sorption in two polymers was evaluated using

a gravimetric method.

19

4.1 Gravimetric Method

Gravimetric technique is considered as a type of isostatic technique. In this
technique the polymer sample is exposed to the permeant and weight change is recorded
as the function of time. The gravimetric sorption method has the advantage that it can

record real time mass (moisture) uptake in a controlled environment at both the steady
state (M00) as well as during the transient state (Mt). Thus it is possible to obtain both

sorption isotherms (from steady state uptake) and the diffusion coefficient (fiom transient
state sorption). The gravimetric sorption method has been adopted by many researchers

to measure the sorption of moisture or organic vapors in polymer.

Different types of balance for this technique have been reported, such as Cahn

40, 41, 43, 46, 47

. - 4 ,4 .4
suspensmn electrobalance, quartz crystal microbalance, 5 8 9 Rubotherm

magnetic suspension balance,50 or spring balance.5 "53
The appropriate solution of the diffusion equation (Fick’s laws) for a gravimetric
technique, given by Crank,28 can be written as:

M __8_ °° — D(2n +1)27r2t

Moo — ”2 n=0( (2n+1)2 exp [2 (11)

 

 

where l is thickness of the sheet,
M, is the amount of penetrant sorbed by polymer sheet sample at time t,
‘ M00 is steady state sorption after theoretical infinite time

Assumptions for Fick’s equations and Eq. (11) are: temperature and pressure
remains constant, polymer sheet has a constant thickness, during the experiment,

moisture uptake follows fickian behavior (D is independent of concentration), vapor

20

concentration on both sides of the sheet is constant, and sheet is initially free of penetrant

26’ 28. The Eq. 11 can be solved for Mthoo =0.5 with approximate error of 0.001 %:

2
D = 0.049 [— (12)
t 1 / 2
where t1/2, half time, is the time when Mt/Mw =0.5 and can be determined experimentally
by plotting Mt/M00 vs. square root of time.
The solubility coefficient (S) is defined as the equilibrium partition coefficient for

the distribution of permeant between the polymer phase (Cp) and vapor phase (CV) and is

expressed as the mass of permeant sorbed at steady state by a unit volume of polymer per

unit of driving force (partial pressure).

5 = Ea = £49.
CV v.P (13)

where M00 is the mass of the vapor absorbed by the polymer at equilibrium, v is
the volume of the polymer, p is the permeant force in units of concentration or pressure. S
is calculated from the sorption experiments by dividing the steady state sorption (kg of
sorbate / kg of polymer) by density of polymer and vapor pressure. Once the solubility
and diffusion coefficient are determined, the permeability coefficient can be calculated

using Eq. 1.

21

For this study a symmetrical gravimetric analyzer (SGA-lOO) from VTI Corp was
utilized.
5. Symmetrical Gravimetric Analyzer (S GA-I 00)

A Symmetrical Gravimetric Analyzer (SGA-100, from VTI Corp., Hialeah, FL)
equipped with a Cahn Electrobalance Model D-200 (Cahn Instruments Co., Cerritos, CA)
was used to measure moisture sorption in the polymer sheets and resins. The SGA-lOO

40’ 54 and is capable of generating

uses a continuous flow method for the sorption studies,
relative humidity from 2 to 98% with variability of i1.0% RH. The operational
temperature range of the equipment is from 5° to 60°C with variability of i0.1°C. The

main components of the SGA-lOO are shown in Fig. 2.1. The equipment is divided into

three different sections each of which is maintained at different, constant temperature.

22

 

Electrobalance

 

 

 

 

 

 

 

 

 

 

 

 

Chamber
Thermal Zone 1
Sample
Water Chamber
Bath
Thermal Zone 2
Vaporizer . . . L? / Dew Point Analyzer
1 l Vapor
Mass Flow ,_ ; Generation
Controllers M 1. Chamber
L : . t ' Thermal Zone3

 

 

 

Figure 2.1 : Schematic of Symmetrical Gravimetric Analyzer (SGA-I 00)

The lower section is the vapor generation chamber, which includes a vaporizer, a
chilled mirror Dew Point Analyzer (DPA) and the mass flow controllers. The temperature
of this section is maintained at 15°C above the experiment temperature, to prevent vapor
condensation in the tubing. The water vapor activity is monitored by a chilled mirror dew
point analyzer (DPA), which measures the dew point of the vapors before it enter into the
sample chamber and send a signal to the software. Software then send the signals to the

mass flow controllers, which generates the target relative humidity by mixing wet and dry

23

streams of nitrogen gas. In case of the organic vapors, the vapor activity is monitor by
measuring the sample chamber temperature and the vaporizer temperature and the
Wagner Equation is used to adjust specific vapor activity.50’ 55

The middle section contains double walled aluminum block which includes the
sample and reference chamber. Both chambers are insulated and of identical size (10.8 x
3.8 x 3.8 cm). The aluminum block is maintained at constant temperature by a circulating
water bath. Film samples can be suspended from the arm of the microbalance, while resin
or powder samples can be placed in a glass pan suspended form the arm of the
microbalance. Both sample and reference chambers have a platinum thermometer to
measure the chamber temperature as well as the incoming vapor stream temperature.

The uppermost chamber encloses the CAI-TN electrobalance, with maximum
capacity of 100 mg and resolution of 0.1 pg. This section is also thermally insulated and
is maintained at 40°C. The electrobalance-charnber is purged with a dry nitrogen stream
to prevent moisture or organic vapors from condensation. The weight change of the
sample is recorded by the Flow System Software (developed by VTI Corporation) on a
computer connected to the system. The software is also use to define the different
experiment parameter like temperature, relative humidity, etc.

6. Noise Quantification

Every analytical or instrumental measurement is made up of two component—
signal and noise. Noise can be defined as the unwanted, extraneous signals.” 57 Noise
can originate from small fluctuations of various sources like in the power provided to the

instrument or from external source such as other instruments nearby or building

vibration. Noise is random in nature; it can be positive or negative therefore it is treated

24

statistically. Ideally when no analyte is present there should be no signal, but in practice
recorded signal include some random signals. There are several types of noise — white
noise, drift or flicker noise, and line noise. White noise can be due to the thermal motion
of charge carrier (thermal noise) or when electron crosses a junction (shot noise). Drift or
flicker noise has a magnitude that is inversely proportional to the frequency of the signal
being measured. It is still not understood what causes the drift. Drift becomes significant
at low frequencies. Line or environmental noise can be due to the power lines, conductors
in the instrument or building vibrations.” 57

Therefore, to get reliable and robust results from an analytical instrument, its
noise should be quantified. The amplitude of noise and the drift of gravimetric equipment
were quantified using standard stainless steel weights. Ideally, the stainless steel should
not absorb any moisture, the weight change observed is the noise associated with the
instrument. A least square method for regression was used to evaluate the amplitude of
noise and drift. The assumptions for the hypothesis testing of regression analysis are: (i)
the relationship between x and y is linear, (ii) the errors are independent and (iii)
normally distributed and (iv) variances of the errors are constant across the observations.

The linear model for experimental observation can be written as:58

yi=fl0+fl1xi+gi i=112!”°n (14)
where x and y represent the time and weight change respectively and ,6} is the slope of the
regression line. The drift is given by the slope of the regression line and the amplitude of

noise of the equipment was calculated as 35, where s is the square root of means squared

error (MSE). Based on the empirical rule, y,- i 38 contains 99.7% of the distribution.

25

 

7. Consistency Test
To determine the experimental data fit to the model described by Eq. 11 certain
assumptions are made:28

0 The polymer is free of any vapor at the start of the sorption experiment.

0 The vapor concentration at each side of the polymer sheets at time=0 is the same

as the vapor concentration at equilibrium.

e The vapor concentration remains constant through out the experiment.

0 The thickness of the polymer sheet is constant.

However, any significance variation or departure from any of these assumptions or
conditions will affect the calculated value of the diffusion and permeability coefficients.
Thus a consistency test was established by Hernandez et a1. 59 For the consistency test,
Eq. 11 was solved by keeping Mthco equal to 0.25, 0.50, and 0.75 to obtain t1/4, tl/z, and
t3/4, respectively. The basis of the test was that the ratio between t1/4, t1 ,2, and t3/4 remains

fixed, irrespective of the penetrant-polymer system:59

I
Kl zflzozso K2 =M=0.103 K, =tfl=a413

tl/2 t3/4 t3/4

9 ,

To determine the validity of the data obtained, experimental K1, K2 and K3 were

compared to the theoretical values.

26

 

8. References:

1.

10.
11.

12.
13.

14. .

Biodegradable Polymers in North America & Europe; MarTech, New York: July
1998.

Mohanty, A. K.; Misra, M.; Hinrichsen, G. Macromol Mater Eng 2000, 276/277,
1-24.

Auras, R.; Harte, B.; Selke, S. Macromol Biosci 2004, 4, 835-864.

Auras, R.; Harte, B.; Selke, S. J Appl Polym Sci 2004, 92, 1790-1803.

Gross, R. A.; Kalra, B. Science 2002, 297, 5582, 803-807.

Pool, R. Science 1989, 245, 1187.

Okada, M. Prog Polym Sci 2002, 27, 87-133.

Bastioli, C. Polym Degrad Stab 1998, 59, 263-272.

Yarnarnoto, M.; Witt, U.; Skupin, G.; Beimbom, D.; Muller, R., In Biopolymers,
Polyesters 111: Applications and Commercial Products. Wiley-VCH: Weinheirn,
2002.

Nagarajan, V.; Singh, M.; Kane, H.; Khalili, M.; Bramucci, M. J Polym Environ
2006, 14, 3, 281-287.

Grigat, E.; Koch, R.; Timmermann, R. Polym Degrad Stab 1998, 59, 1-3, 223-
226.

Chiellini, E.; Corti, A.; Solaro, R. Polym Degrad Stab 1999, 64, 2, 305-312.

Kurian, J. V. J Polym Environ 2005, 13, 2, 159-167.

Khonakdar, H. A.; Jafari, S. H.; Asadinezhad, A. Iran Polym J 2008, 17, 1, 19-38.

27

 

 

15.

16.

17.

18.

19. °

20.

21.

22.

23.

24.

25.

26.

27.

Akiko, M.; Mureo, K.; Makoto, M. Biodegradable Oriented Aromatic Polyester
Film and Method of Manufacture. W00110928, 2001.

Matweb DuPont Biomax® 4026.
http://www.matweb.com/search/datasheet.aspx?matid=33539&n=1 (April, 2008)

“Compostable Logo” of the Biodegradable Products Institute gains momentum
with approval DuPontTM Biomax® resin.
htmzl/wwwbpiworld.m/Files/PressRelease/PRY U WigUndf (April, 2007)

Standard Specification for Compostable Plastics. In ASTM D6400-O4, ASTM
Book of Standards, 2000; Vol. 08.03.

DuPont Launches Biodegradable Polyester - Biomax®, Press Release.
http://www.dupont.com.tw/020604-e.htm (April, 2008)

Mungara, P.; Chang, T.; Zhu, J.; Jane, J. J Polym Environ 2002, 10, 1/2, 31-37.

Editor of textileinfo, I. C. L. A forecast on the textile processing industry in 21C.
http://textileinfo.com/en/tech/2lc/02.html (April, 2008)

Mohanty, A. K.; Misra, M.; Drzal, L. T. J Polym Environ 2002, 1-2, 10, 19-26.

Liu, W.; Mohanty, A. K.; Drzal, L. T.; Misra, M.; Kurian, J. V.; Miller, R. W.;
Strickland, N. Ind Eng Chem Res 2005, 44, 4, 857-862.

Drown, E. K.; Mohanty, A. K.; Parulekar, Y.; Hasija, D.; Harte, B. R.; Misra, M.;
Kurian, J. V. Composi Sci Technol 2007, 67, 3168-3175.

Farber, J. M. J Food Prot 1991, 54, 1, 58-70.

Hernandez, R. J .; Selke, S. E. M.; Culter, J. D., In Plastics Packaging: Properties,
Processing, Applications and Regulations. First ed.; Hanser Gardner Publications,
Inc.: 2000.

Gerlowski, L. E. In Water Transport Through Polymer, American Chemical
Society, Dallas, Texas, 1989; Koros, W. J ., Ed. American Chemical Society:
Dallas, Texas, 1989; pp 177-191.

28

28.

29.

30. °

31.

32.

33.

34.

35.

36. ‘

37.

38.

39.

40.

41.

Crank, J ., In The Mathematics of Diffusion. Second ed.; Oxford University: 1975.

Alfrey, A.; Gumee, E. F .; Lloyd, W. G. Journal of Polymer Science 1966, C12,
249.

Flory, P. J ., In Principle of Polymer Chemistry. Cornell University Press: Ithaca,
NY, 1953.

Freeman, 3.; Yampolskii, Y.; Pinnau, I., In Materials Science of Membranes for
Gas and Vapor Separation. Wiley & Sons, Ltd.: 2006.

Barrer, R. M.; Barrie, J. A.; Slatter, J. Journal of Polymer Science 1958, 27, 177-
197.

Meares, P. J Appl Polym Sci 1965, 9, 917-932.

Baner, A. L.; Kalyankar, V.; Shound, L. H. J Food Sci 1991, 56, 4, 4.

Stannett, V. J Membr Sci 1978, 3, 97-115.

Gavara, R.; Catala, R.; Hemandez-Munoz, P.; Hernandez, R. J. Packag Technol
and Sci 1996, 9, 215-224.

Duncan, B.; Urquhart, J .; Roberts, S. Review of Measurement and Modelling of
Permeation and Diffusion in Polymers; National Physical Laboratory: 2005.

Yasuda, H.; Rosengren, K. J Appl Polym Sci 1970, 14, 11, 2839-2877.

Apostolopoulos, D.; Winters, N. Packag Technol and Sci 1991, 4, 131-138.

Barr, C. D.; Giacin, J. R.; Hernandez, R. J. Packag Technol and Sci 2000, 13, 4,
157-167.

Hernandez, R. J .; Gavara, R. J Polym Sci, Part B: Polym Phys 1994, 32, 2367-
2374.

29

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

Hernandez, R. J .; Giacin, J. R.; Grulke, E. A. J Membr Sci 1992, 65, 1-2, 187-
199.

Wong, B.; Zhang, Z.; Handa, Y. P. J Polym Sci, Part B: Polym Phys 1998, 36,
2025-2032.

Wong, C. H.; Bhethanabotla, V. R. Fluid Phase Equilib 1997, 139, 371-389.

Oliverira, N. S.; Oliverira, J .; Gomes, T.; Ferreira, A.; Dorgan, J.; Marrucho, I. M.
Fluid Phase Equilib 2004, 222-223, 317-324.

Karniya, Y.; Hirose, T.; Mizoguchi, K.; Naito, Y. J Polym Sci, Part B: Polym
Phys 1986, 24, 1525-1539.

Hernandez, R. J .; Giacin, J. R.; Lawrence, B. A. J Plast Fihn Sheet 1986, 2, 187-
211.

Moylan, C. R.; Best, M. E.; Ree, M. J Polym Sci, Part B: Polym Phys 1991, 29,
87-92.

Smith, A. L.; Mulligan, S. R.; Shirazi, H. M. J Polym Sci, Part B: Polym Phys
2004, 3906.

Qin, Y. Impact of Polymer Processing on Sorption of Benzaldehyde Vapor in
Rubbery Polypropylene. MS. Thesis, Michigan State University, East Lansing,
MI, 2006.

C. C. McDowell, D. T. C., and B. D. Freeman. Rev Sci Instrum 1998, 69, 6,
2510-2513.

Dhoot, S. N.; Freeman, B. D.; Stewart, M. E.; Hill, A. J. J Polym Sci, Part B:
Polym Phys 2001, 39, 1160-1172.

Palamara, J. E.; Mulcahy, K. A.; Jones, A. T.; Danner, J. R.; Duda, J. R. Ind Eng
Chem Res 2005, 44, 26, 9943-9950.

Nielsent, T. J .; Giacin, J. R. Packag Technol and Sci 1994, 7, 247-258.

30

55.

56.
57. '
58.

59.

Golding, P. D.; Machin, W. D. J Chem Soc 1992, 88, 14, 1985-1991.

Skoog, D. A.; Holler, F. J .; Crouch, S. R., In Principles of Instrumental Analysis.
Sixth ed.; Thomson Brooks/Cole: 2007.

Robinson, J. W.; Frame, E. M. 8.; Frame 11, G. M., In Undergraduate Instrumental
Analysis. Sixth ed.; Marcel Dekker: 2005.

Lyman, O. R.; Longnecker, M., In An Introduction to Statistical Methods and
Data Analysis. 5th ed.; Duxbury Press, A Division of Wadsworth, Inc.: 2000.

Hemandez-Munoz, P.; Gavara, R.; Hernandez, R. J. J Membr Sci 1998, 3993, 1-
10.

31

Chapter 3

A Study of Mass Transfer of Moisture in Biodegradable Sheets and Resins

1. Introduction

Dwindling petroleum sources and landfill sites have prompted the development of
alternative and environmentally fi'iendly polymers for different applications. In the past
few years, many new alternative polymers have been developed that are either derived
from non-petroleum sources or are biodegradable. A few examples of such polymers are
Poly(lactide) (PLA), Poly(s-caprolactone) (PCL), and Polyhydroxylalkanoates (PHA).l
The global production of biodegradable polymers in 2006 has increased 20-folds since
1995,2 but this represent just a small fi'action of the total polymer production. The major
obstacle to commercialization of these ‘novel’ polymers in the packaging industry is their
cost and the limited knowledge about their performance properties.

The US packaging industry uses around one-third of the total plastics produced in
the country.3 Plastic is the largest segment of the packaging industry, accounting for
approximately 50% of the total materials used in industry.4 The barrier properties of
polymer to different gases and vapors dictate the application of polymer as a package to
be used in the food and pharmaceutical industries. Therefore, it is important to measure
the barrier properties of these alternative polymers for their application in packaging
industry.

Unlike glass or metal packaging, plastic packaging materials are permeable to
small molecules like water, volatiles and gases. Extensive research had been conducted

for the evaluation of mass transfer properties of polymers using various methods. While

32

isostatic and quasi-isostatic methods are generally used to evaluate the permeation of the
permeant through the polymer membrane?”9 gravimetric methods, using a spring balance,
an electrobalance, or a quartz crystal microbalance, evaluate the permeant sorbed in the
polymer.""'4 The gravimetric sorption method has the advantage that it can record real

time mass (moisture) uptake in a controlled environment at both the steady state (M00) as
well as during the transient state (Mt). The M, and M.,o values can be use to determine the

solubility (S) and diffusion (D) coefficients experimentally and hence calculate the
permeability (P) coefficient using the following relation:
P=D*S (D
The above equation holds at low concentrations of the permeant and when there is
no interaction between permeant and polymeric material. In the above equation D follows

d

Fickian kinetics (is independent of concentration) and S follows Henry’s law. In this
research moisture sorption of two environment friendly polymers, Biomax® and Sorona®,
was studied using the gravimetric method.

Biomax® (4026) is a hydro-biodegradable modified polyester, developed by
DuPont. The material is based on polyethylene terephthalate (PET) and is synthesized by
the polymerization of terephthalic acid, ethylene glycol, glutaric acid, and sulfonic acid
salt.15‘ 1" Biomax® has been awarded a ‘COMPOSTABLE’ logo by The Biodegradable
Products Institute (BPI).17 Biomax® can be used for oriented films, blown films, moulded
plastic articles, or coating in disposable products such as bowls, plates, cups, and
sandwich wraps.18 Because it is modified polyester, it can be easily processed using
standard PET equipment. Biomax® has been blended with soy proteins and

polycaprolactone (PCL), and with cotton (Bionature by Kurabo, J apan).'9' 2°

33

Sorona®, Poly(trimethylene terephthalate) (PTT) is a thermoplastic polyester
produced by condensation of 1,3-propanediol and terephthalic acid.21 The propanediol
(PDO) monomer can be derived from corn (a renewable resource). PTT was developed
several years ago but was not commercialized due to the high cost of production of PDQ.
However, recent developments have reduced the cost significantly. Sorona® belongs to
the thermoplastic aromatic polyester family and due to three carbon glycol in its
backbone it has better mechanical properties as compared to the other two members of
this family (PET and PBT). Sorona® has high elasticity and recovery as compared to PET
or nylon and better chemical properties than PET;22 hence it is gaining much attention in
the packaging industry. Research had been conducted to tailor the physical and
mechanical properties of Sorona® through blending it with other polymer or nano-

composites,” 23 but few researchers have studied the moisture uptake of Sorona®.

This study aims to generate a moisture sorption profile of Biomax® and Sorona®

sheets and resins using a gravimetric method. The solubility and diffusion coefficients of

both polymer sheets were determined through the sorption data.

34

2. Theoretical Background
2.1 Sorption by Polymer Film

In a gravimetric method for moisture sorption, weight gain of polymer sample is
recorded under specific relative humidity and temperature till the steady state is reached.
The weight gain data is used to obtain the moisture sorption isotherm and the diffusion
coefficient. Sorption during the transient state can be used to estimate the diffusion

coefficient (D) using the following equation 24

 

 

M 8 ° D—2n+127r2t
1_(___22 ( )

= 2
Moo 71' n=0 (Mi-1 l (2)

where l is thickness of the sheet, M, is the amount of penetrant sorbed by the polymer
sheet sample at time t, and M00 is the steady state sorption after theoretical infinite time.
The‘fit of the experimental data model described by Eq. 2 is based on the following
assumptions” 25

o The polymer sheet has a constant thickness

0 Temperature and pressure remain constant during the experiment

0 Moisture uptake follows Fickian behavior (D is independent of concentration)

0 Vapor concentration on both sides of the sheet is constant and the change in

concentration from initial concentration to final is instantaneous

o The sheet is initially free of penetrant

The Eq. 2 can be solved for Mt/Moo =0.5 with approximate error of 0.001 %:

[2

tl/Z

where t1/2, half time, is the time when MI/M00 =0.5 determined experimentally.

35

The solubility coefficient (S) is defined as the equilibrium partition coefficient for
the distribution of permeant between the polymer phase (Cp) and vapor phase (CV) and is
expressed as the mass of permeant sorbed at steady state by a unit volume of polymer per

unit of driving force (partial pressure).

 

S=B=Mw
C v.P (4)

v
where M00 is the mass of the vapor absorbed by the polymer at equilibrium, v is the
volume of the polymer, p is the permeant force in units of concentration or pressure. The
common unit of S is kg/m3Pa and is calculated by dividing the steady state sorption (kg
sorbate / kg polymer) by polymer density and vapor pressure. Once the solubility and
diffusion coefficient are determined, the permeability coefficient can be calculated using
Eq. 1.
2.2 Consistency Test

The assumptions in determining D through Eq. 2 are listed above. However, any
significance variation or departure from any of these assumptions or conditions will
affect the calculated value of the diffusion and permeability coefficients form
experimental data. Thus to determine the fit of experimental data to the Eq 2, a

126, was utilized.

consistency test for gravimetric method, established by Hernandez et 3
Eq. (2) was solved for Mt/M,o equal to 0.25, 0.50, and 0.75 to obtain t1/4, III/2, and t3/4
respectively. The basis of the test was that the ratio between t1 /4, t1 /2, and t3/4 remains
fixed, irrespective of the penetrant-polymer system 26:

t
K1=tfl =0.250 K2 = 5'11- =0.103 K3 =fl =0.413

t1/2 t3/4 t3/4 (5)

9 9.

36

3. Experimental
3.1 Materials

i Biomax® (4026) and Sorona® resin pellets were obtained from E.I. DuPontTM de
Nemours & Co. (Wilmington, DE). Biomax® pellets were odourless and opaque white in
appearance with density of 1344.3 kg/m3 (1.34 g/cc). Sorona® pallets were odourless and
translucent in appearance with density of 1334.7 kg/m3 (1.33 g/cc). Both polymer types
were processed using a DSM twin screw Micro-Extruder and Injection Molder (TS/I-02,
DSM, Netherlands), to obtain small discs of approximately 25.4 mm diameter and 2 mm
thick. A compression molding press (Carvar Hydraulic Press, Wabash, IN) was used to
process the discs into sheets form, for the sorption experiments. The sheets samples were
cooled quickly in the compression press by rushing cold water through the platens. The
processing conditions for both polymers are given in Table 3.1. The thickness of both

polymer sheets was maintained at 4 to 6 mils.

37

Table 3.1: Processing condition for Biomax® and Sorona®

 

 

Biomax® Sorona®
Micro-Extruder and Injection Molder
Barrel Temperature, °C 200 250
Melt Temperature, °C 190 240
Screw Speed, rpm 100 150
Process Cycle Time, min 2 3
Transfer Cylinder Temperature, °C 190 240
Mold Temperature, °C 60 85
Pressure, psi 100 120
Compression Press
Platen Temperature, °C 200 250
Compression Pressure, psi 10,000 12,000
Cooling Time, min 5 7

 

3.2 Differential Scanning Calorimeter (DSC)

A differential scanning calorimeter (TA Instruments, New Castle, DE) was used
to determine the percent crystallinity of both polymers’ sheets and resins using one
heating cycle followed by one cooling cycle. Biomax® samples were heated from 10°C to
210°C at a rate of 10°C/min and then cooled down to 10°C at a rate of 5°C/min. Sorona®
samples were heated from 10°C to 265°C at a rate of 10°C/min and then cooled to 10°C
at a rate of 5°C/min. The data were analyzed with Universal Analysis software (TA

Instruments).

38

3.3 Instrument SGA-I00

A Symmetrical Gravimetric Analyzer - SGA-lOO (VTI Corp., Hialeah, FL)
equipped with a Cahn Electrobalance Model D-200 (Cahn Instruments Co., Cerritos, CA)
was ‘used to measure the sorption of moisture in the polymer sheets and resins. It is
capable of generating relative humidity from 2 to 98% with variability of dz] .0% RH. The
instrument is divided into three thermally isolated sections: a vapor generation chamber,
sample chamber, and electrobalance chamber. The aluminum sample chamber (10.8 x 3.8
x 3.80m) can be maintained at a constant temperature with variability of 101°C. The
relative humidity inside the sample chamber is obtained by mixing wet and dry streams
of nitrogen using mass flow controllers. The electrobalance has a maximum capacity of

100 mg with resolution of 0.1 pg. The main components of the SGA-lOO are shown in

 

 

 

 

 

 

 

Fig. 3.1.
‘ Electrobalance
Chamber
Thermal
Zone 1
, Sample
Water. -. . Chamber
Bath i ’ ' ‘ i w: Thermal
Zone2 t
Vaporiz ' Dew Point
er ‘ ;———-/ Analyzer
L— Vapor
Mass .. . Generation
Flow M Chamber
Controllers Thermal
Zone3

 

 

 

Figure 3.1: Schematic of Symmetrical Gravimetric Analyzer (SGA-I 00)

39

3.4 Noise Quantification

Every measurements carried out by an analytical instrument is made up of two
components - signal and noise. Noise can be defined as the unwanted, extraneous signals
which can make the actual signals indiscemible.27’ 28 A protocol was implemented to
quantify the noise level associated with the SGA-100 by measuring the amplitude of the
noise and the drift. To quantify the noise level, three standard stainless steel weights of
20, 50, and 100mg were exposed to 5%, 50%, and 90% at three different temperatures.
Weight change of the standard stainless steel weight was recorded every 10 minutes for
72 hours.
3. 5 Sorption Experiment

The sheet and resin samples were stored in a desiccator to remove the moisture
from the samples at 23°C. To minimize the variation in thickness within the sample,
small pieces of the sheets (around 1 cm2 and 30 mg) were used. The samples were
suspended on the arm of the electrobalance. After conditioning at 60°C and 0% RH for
1h, the sheet samples were exposed to a specific relative humidity (10, 30, 50, 70 and
90%) at a constant temperature (23 and 40°C). The weight gain (moisture uptake) was
recorded, from i=0 to the time at which no more weight gain was observed (steady state),
at 2 min interval thus providing a detailed information about the transient state. In this
sorption study, the steady state was determined by a specific equilibrium criterion. The
resin samples of both polymers were exposed (after conditioning) to specific relative
humidity of 50 and 90 % RH at two different temperatures (23° and 40°C). All the
sorption experiments of sheets and resins of both polymers were done in triplicate and

standard error was calculated.

40

4. Results and Discussion
4.1 Physical Properties

The thickness of the processed Biomax® sheets was measured as 0.14 i 0.03 mm
(5.5 at 1 mil) and for Sorona® as 0.11:!:0.03 mm (4.53:1 mil). The density of Biomax® and
Sorona® sheets was measured at 23°C as 1344.3 kg/m3 (1.34 g/cc) and 1334.7 kg/m3
(1.33 g/cc), respectively ( in accordance with ASTM D792-00). The percent crystallinity

of polymer sheet and resin was calculated using the following formula:

AH
=100x ’"
1‘ AH

 

(6)

where AB,“ is enthalpy of fusion and AH0 is the heat of melting of purely crystalline

® resin was found out to be around 18.3 %

polymer. The percent crystallinity of Biomax
and after processing the percent crystallinity of Biomax® sheets dropped to 9.5 %. The
percent crystallinity of Sorona® polymer was found to be 42 % for resin and 26 % for
sheets. The drop in percent crystallinity after processing is due to the quenching of the
polymer samples during sheet making in the compression press.

4.2 Instrument Noise and Drift:

To evaluate the amplitude of noise and drift of the equipment, a least squares
method for regression was adopted.28 The assumptions for the hypothesis testing of
regression analysis are: (i) the relationship between x and y is linear, (ii) the errors are
independent and (iii) normally distributed and (iv) variances of the errors are constant
across the observations. The linear model for experimental observation can be written

9
3822

yi:fl0+fl1xi+8i i=1!2’°”n (7)

41

where x and y represent the time and weight change respectively and ,8] is the slope of the
regression line.

The percent weight change vs. time graph of 50 mg standard weights at 40°C and
90% RH is shown in Fig. 3.2. The drift of the equipment is given by the slope of this

regression line. The noise residual output vs. time plot for the same experiment is shown

 

in Fig. 3.3.
0.025
I
0.020 .
in 0.015 1 J I"
2 1 .1
o u '
3" 0.010 E
g y =2*10'O6x + 0.0102
0.005
1
0.000 +— 1 . - —r——
0 1000 2000 3000 4000

Elap Time (min)

Figure 3.2: Percent weight change vs. time and linear regression line for a 50 mg

standard weight at 40°C, 90% RH

42

0.005 1

 

     

 

 

Amplitude

0.003 m A of Noise
.21. 0.001 '
cc
:1 __ _
E
8 -0.001 -:
05 1

-0.003 j

-0.005 i

Elap Time (min)
Figure 3.3: Residual output vs. time for 50 mg standard weight at 40°C, 90% RH

The results from linear regression statistical analysis are given in Table 3.2. The
significant F values of all the experiments (not shown in table) were found to be very
close to 0, thus null hypothesis H0 (,81 = 0) was rejected, concluding that the weight

change (y) and time (x) are not independent (01 = 0.05). A small drift ranging from 10'06
to 10'07 was observed in the equipment at different experimental conditions. Both positive
and negative drift was observed during the experiments. The drift of the equipment was
not found to be dependent on either weight or experimental conditions.

The amplitude of the noise was calculated as the difference between the
experimental values and the predicted value (std. metal weight’s actual weight). The
amplitude of noise of the equipment can be given as 33, where s is the square root of
means squared error (MSE) from the residual analysis. Based on the empirical rule, yi i
35 contains 99.7% of the distribution. The amplitude of noise under different conditions

is given in Table 3.2. The noise level was found to be very low as compared to the signal.

43

Although the noise amplitude increased with weight, the Signal to Noise ratio (S/N) was
improved. The noise was independent of humidity and temperature, except for 20 mg

weight, where an increase in noise was observed with increase in temperature.

Table 3.2: Regression analysis of the equipment ’s noise and drift

 

 

Standard Amplitude of

Weight Condition Regression Line MSE (sz) noise (3s) % S/N

20 mg 10°C 90% y = 1*10'06x + 0.0183 1.10*10'°6 0.003145 6359.30
23°C 90% y = -4*10*’°x + 0.0269 1.56*10'°° 0.003744 5341.88
40°C 90% y = -6*10*’6x + 0.0185 2.10110:06 0.004351 4596.64

50 mg 10°C 5% y = -4*10’06x — 0.0062 5.02810“6 0.006723 7437.16
10°C 50% y = -2*10*’7x - 0.0083 2.371104)6 0.004615 10834.24
10°C 90% y = -5*10'06x + 0.0107 3.85*10'°° 0.005890 8488.96
23°C 5% y = -3*10*’6x + 0.0091 4.67:“10"6 0.006481 7714.86
23°C 50% y = -3*10'06x + 0.0064 3.62110“6 0.005710 8756.57
23°C 90% y = -6*10*’6x - 0.0093 6.84*10'°° 0.007847 6371.86
40°C 5% y = -4*10*’7x - 0.0026 3.61*10'°° 0.005697 8776.55
40°C 50% y = -1*10*’°x + 0.0007 5.3111006 0.006916 7229.61
40°C 90% y = 2*10'°°x + 0.0102 3.56*10’°° 0.005661 8832.36

100 mg 23°C 90% y = -7*10*’6x - 0.0063 1.58*10*’5 0.011923 8387.15

 

44

4.3 Equilibrium Criterion Determination

The fundamental driving force that promotes the diffusion of a molecule through
a polymer is the difference between the chemical potential of the fluid and the polymer
phase. When the chemical potential of permeant in the fluid and polymer phase becomes
equal, an equilibrium or steady state is reached.25 In the gravimetric method, the steady
state is said to be reached when the weight gain of the sample is zero or negligible.

In this study, the steady state is controlled and determined by an equilibrium
criterion, provided in the flow system software of the SGA-100. The equilibrium criterion
for both polymer types was set as ‘weight change of 0.005% in 30 min’. It means if the
weight change of the polymer is less than 0.005 % for continuous 30 minutes, the steady
state or equilibrium is considered to be reached.

4.4 Moisture uptake in Sheets

The moisture sorption in both polymer sheets followed F ickian kinetics. The
average percent moisture uptake for the triplicate runs vs. time (min) for Biomax® sheets
at 23°C and 40°C for different water activities is shown in Fig. 3.4. The time taken to
reach the steady state increased with relative humidity. The time to reach the steady state
at 40°C was less than that at 23°C, due to higher saturated vapor pressure at higher

temperatures.

45

 

 
  
 

 

 

 

 

 

 

 

2.5
++++++++ ++++++++++H~H++++++H+H+1+I
20 ++++ +
$43 23°C
.5 3
an 1.5 ++
fl
-= f
.2.” 4*
g 10 f xxxxxxxxxxxxxxxxxxmmmxxml
- +
+
°\° r
05 _: ““.‘mammmx
... III-II-I I-III- I
O 0 QOOOOOOOOOOOOOOO. z
0 200 400 600 800 1000
Time (min)
2.5
#+WW+++++++++ + +-H—H—H+H++HHH- I
++
2.0 $++ 40°C
+
.5 f
2.” 1.5 ,_ +
0 +
3 1.0 ~ +fm
e\° +x
3"“. A an“:
0.5 4K;
‘ f'lll III I I I- a:
00 ‘ Q .0 0 «one. =
0 50 100 150 200 250 300

Time (min)

Figure 3.4: Average percent weight gain (3 runs) vs. time (min) for Biomax® sheets at

23 °C and 40°C, S. E. bars are shown only for the lost data point.

0 10% RH I 30%RH A 50%RH X 70% RH + 90%RH

46

The average percent moisture uptake vs. time for Sorona® sheets at 23°C and 40°C at
different relative humidities is shown in Fig. 3.5. Moisture uptake increased with the
increased water activity and temperature. The moisture uptake in Sorona sheets (0.03-
0.5%) was much lower than in Biomax sheets (0.1-2.5%) due to lower affinity to water of

Sorona than Biomax® to water.

 

0.60
0.50 ' 23°C
W
5 0.40 ”new
DD ++++
a ++++
'50 030 +£ppmxxxxWI
'5
3
3

 

 

 

I 1 f

0 100 200 300 400 500 600 700 800 900

 

 

 

 

 

 

 

1000
Time (min)
0.60
+++++++++-H--H++++ =
0.50 +++++++++++
++
+
++
5 0.40 ff 40°C
in 41- xxxxxxxoxxxxmxmntm:
'50 0 30 1%?”
'3
3 0.20 if ‘ u ...aaaanusaaaaaum -
A
‘ A
...IIIIIIIIIIIIIIIII I
0.10 .-
.«mum I
0.00 I ‘ . ‘
0 50 100 150 200 250 300 350 400 450
Time (min)

Figure 3.5: Average percent weight gain (3 runs) vs. time (min) for Sorona® sheets at

23 °C and 4 0°C, S. E. bars are shown only for the last data point.

910%RH

I 30%RH A 50%RH X 70% RH + 90%RH

47

4.5 Consistency Test and Correction

To determine the fit of the experimental data to the model defined by Eq. 2,

theoretical Mt/Moo values were calculated using Eq. 2. The experimental Mt/M,o and

theoretical Mt/M00 vs. square root of time (secondm) are shown in Fig. 3.6.

1.20 j
1.00

0.80 4:

  

l— Theoretical l

Mt/Moo
O
8

   

 

040 . Emmmmm
:tT—EEEEQQQH
0.20 >
000 _.e_ .————_ .__fi___._"__.x._,z_____fi__
0.00 50.00 100.00 150.00 200.00
Time (sec)“2

Figure 3. 6: Experimental M/Moo (before correction) and theoretical M,/M,,o vs. square

root of time (second/0) for a Biomax® sheet at 23 °C and 3 0% RH

The initial part of the curve was concave and a large difference was observed

between experimental and theoretical values. A similar difference was found for all the
sorption experiments for both polymers. Ki parameters, for consistency test, of each

sorption experiment were calculated for both polymers sorption runs (Fig. 3.7). As seen,

48

K3 Values

K2 Values

 

 

6 .

 

 

C

Temperature

 

 

 

  

 

 

 

 

 

 

 

 

 

 
 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ci After correction

49

Temperature

theoretical K,- value.

for both polymers experimental the K1 and K2 values differed from the theoretical values

by more than 100%.

' K1 Values

Biomax®

 

 

 

 

 

 

 

 

 

00000000

 

 

mm m m va ow
%_ .. w. W m: C

o m mm smsxss w
W9 W.; 5T

w . m. w c

.. mm. 8 .. w

Temperature

Figure 3. 7: Average of K,- values of different relative humidities for Biomax® and

Sorona® sheets before and after correction at 23 °C and 4 0°C, dotted line represents the

Before correction

This deviation of experimental data from the theoretical model could be related to
the time taken by the vapors inside the sample chamber to reach the target experimental
vapor activity, as one of the assumptions for Eq. 2 is that the sorbate concentration
around polymer sample changes stepwise from the initial zero concentration to the target
experimental concentration. However, this is hardly possible in actual experiments.
Therefore, to compensate for the time taken by vapors to reach the target constant relative
humidity a correction in experimental data was proposed. The linear part of the
experimental Mt/M,O curve was extrapolated to the x-axis (Fig. 3.6). The time obtained on
x-axis by extrapolation was then considered as the zero time for the sorption experiments.
After the correction, the experimental Mt/M,o and theoretical Mt/Mm (from Eq. 2) values
were obtained. A very good fit was observed between the time-corrected experimental

and the theoretical values for the same Biomax® sheet (Fig. 3.8).

  
   
 

 

 

1.20 5
1.00 .
|
0.80 «.-
8
E 0.60 -
2 I Corrected
0.40 ~ Experimental
—Th oretical
0.20 - e
0.004 ~w—«--—~-’- . ————
O 50 100 150 200

Time (sec)m

Figure 3. 8: Experimental M/M,o (after correction) and theoretical M/M.no vs. square root

of time (secondw) for a Biomax® sheet at 23 °C and 3 0% RH

50

The K, parameters were also calculated using the new corrected tm, tm, and t3/4.
As seen in Fig. 3.7, the difference between the experimental and theoretical K, values was
reduced to 0-10%, hence verifying the validity of corrected experimental data. A similar
correction was applied to all sorption experiments of both types of polymer sheets. The
corrected data were used to calculate the solubility and diffusion coefficients.
4. 6 Sheet Moisture Sorption Results

The moisture sorption of the polymer sheet at the steady state was used to
generate the moisture sorption isotherms for Biomax®(a) and Sorona®(b) sheets at 23°C
and 40°C (Fig. 3.9). The polyester Sorona® sheets had low moisture sorption (0.5% at
40°C, 90%RH) as compared to Biomax® (2.5% at 40°C, 90%RH), which is a hydro-
biodegradable polymer. Moisture sorption in both polymers was dependent on the RH.
For Sorona® sheet the moisture sorption increased significantly with increase in

temperature, except at 10% RH. For Biomax®

moisture sorption was found to be slightly
dependent on temperature only in the middle range of the relative humidities.

a) Biomax® Isotherm

    
    

 

 

 

2.5 7
+23°C
+4 0
2.0 7 O C
.E. l
(6
SE 1.5 -
.1:
.29
Q)
3 1.0
c,\"
0.5 r
0.0 ——__"T_-—"— '1— ".T' H ‘*—' '1
0 20 40 60 80 100
% Relative Humidity

51

b) Sorona® Isotherm

.9
ON
1

+ 23°C
+ 40°C

.9
LII
1

 

.9
4:.

% Weight Gain
o
DJ

 

 

0.2
0.1
0.0 “T '_—"_“—“ 1 i 1 1
0 20 40 60 80 100
% Relative Humidity

Figure 3. 9: Moisture sorption isotherm of Biomax®(a) and Sorona®(b) sheets at 23 °C
and 40°C, error bars are SE. for 3 runs

Most food and biomaterials are characterized by negative heat of sorption, which
means equilibrium moisture sorption isotherm corresponding to lower temperature is
situated above to that corresponding to higher temperature.30 But both Biomax® and
Sorona® did not followed this pattern, the moisture sorption at 40°C was more as
compared to 23°C. Though not common but similar isotherms with such a positive
temperature dependence have been reported previously.3 "33 For Biomax® sheets this
behavior may be due to two reasons: (1) it is a hydro-biodegradable polymer and 40°C is
above the glass transition temperature; thus, increasing the movements within the chain
and allowing more moisture to be sorbed; and (2) the moisture sorption is driven by the

hydrolysis of the ester linkages, as also seen in the case of PLA films.32

52

A sum of square technique was applied to the determine the best estimated
diffusion from the experimental data.9 The best estimated diffusion coefficient can be
defined as diffusion coefficient for which the difference between the experimental and
the theoretical curve (Eq. 2) is least. A representative plot of the sum of squares values as
function of diffusion coefficient values (calculated around ill/2) was used, from which the
best estimated diffusion coefficient was determined from minimum sum of square value
(plot not shown).

Table 3.3 presents the best estimated diffusion, solubility, and permeability
coefficients of Biomax® and Sorona® sheets at 23°C and 40°C. As the temperature
increases the mobility of the chain and free volume increases, so more permeant can pass
through the sheet easily. Hence, diffusion coefficient of both polymer sheets increased
with increase in temperature. For Biomax® sheet, the diffusion coefficient drops at 90%
RH which can be related to the condensation of vapors on the sheet at high water activity.
The solubility coefficient of both polymers increased with relative humidity. A statistical
analysis, Least Square Means Difference (LSMD),29 was conducted to determine whether
the solubility and diffusion coefficients of both polymers differed as a function of relative
humidity. The solubility coefficient of Biomax® was found to be slightly dependent on
the RH and temperature. The S increased significantly with increased in RH from 10% to
50% and from 50% to 90%. The diffusion coefficient of Biomax® slightly increased with
increase in RH except at 90% RH at both temperatures, where a drop in D was observed
which could be related to the condensation of vapors on the sheet at high humidity. The
changes in diffusion coefficient of Sorona® sheets were not statistically significant

(LSMD results) because of the unequal residual variance at different RH. The solubility

53

coefficient of Sorona® sheets at 23°C increased significantly with increase in RH from
10% to 50% and 50% to 90% and at 40°C a significant increased was observed from

10% to 70% and higher RH.

54

 

 

 

 

 

 

 

 

vd H 5d a vd H EN 0 mod H so; 5d H dmd e md H Ah.— 0 cod H wNN om
ed H 91. m wd H mud on 5d H mdd ad H 3...4 m ed H mm a 3d H mm; on
d._ H $5 a m._ H 346 a 8d H dad v; H 36 m md H mod n mod H x: cm
N; H on...” a n; H Pd 9 cod H and md H mod n Nd H mm; a 3d H em; cm
_d H Sam m N; H ovd a :d H dmd md H add a vd H NN.N a mod H cm; 3
ae— : 2.25 Z: J _ 2.2 f. 25...; 1: am 5.
o<ZO¢Om
md H dod— m 3 H owdm 0 mod H 2.6 wd H mmé a dd H ohm u Rd H 2.2 dd
N; H do.m_ o mé H dvdv a dmd H mod m; H Ed 0 Wm H end 0 3d H dad on
N; H mod n We H dmdm 3 3d H dim o._ H dad on. m; H 3.x on «Nd H mad cm
a; H ems e dd H dddm 9 Ed H KN md H 3mm one _d H $6 an mmd H wmd om
hd H mmxc. m ed H 3.2 a Ed H mm; md H em; as md H add a Ed H 2:2 2
8. . .. a... _ 2...
, ex<EoE 4 5.
AmmoommfiEdmv Aoom\~Ev Ana macho—V AmmoomNEEmv—v Aoom\~Ev Ana macho—v
med—u.— n_-d_xn .42 um end—x.— n_-d—un era—um

 

 

. «mddfla encumkfi b23352: 8: mac .0th 8:3 .3 hegemoKueeeé 32.43 QEMN 5.2: $33 m\e mm. H»

Dada use Do mm 3 Some}. esteem. nae mgoa LoXnEmBfieoe bNNSemExek .38 =SS§Q 52350.0 mmoxmg. "min. uEeh

55

4. 7 Effect of Temperature on Permeability, Diffusion, and Solubility
The temperature dependence of transport parameters are usually described by Arrhenius-
type relationships, and are given as

S(T)=S0exp(-AHS /RT)

(8)
D(T) = Do eXIX-ED /RT) (9)
ND = P0 eXP(-Ep/RT) (10)

where AHS is the enthalpy of sorption, ED is the activation energy of diffusion, Ep is the
activation energy of permeation.

The AHS, E D, and E p for polymeric sheets were determined and are reported in Table 3.4.
The enthalpy of sorption was determined from the experiment runs at 23 and 40°C and
was averaged over the whole range of RH. The enthalpy of sorption of Biomax® is lower
than. Sorona® suggesting that there is a higher interaction between the moisture and
Biomax. While the ED of Sorona was significantly lower than that of Biomax indicating
that diffusion of moisture through Biomax is more temperature dependant. The enthalpy
of mixing (AHM) was determined as 16 and 7.8 kJ/mol for Biomax and Sorona
respectively confirming the different interaction with moisture.

Table 3. 4: Enthalpy of sorption and activation energy of Biomax®

and Sorona® sheets (average of 3 runs :1: SE.)

 

AHs AHM ED EP
kJ/mol kJ/mol kJ/mol kJ/mol

 

Biomax® -38.03 2 2.1 16.0 70.23 :1: 3.3 32.92 2 1.8

Sorona® -3420 :1: 1.6 7.8 37.48 2 6.5 2.93 2 6.9

 

56

4.8 Resin Moisture Sorption Results

To determine the effect of processing on the moisture sorption, the sorption
experiments were also done using the resin pellets of both polymers. For both polymers,
moisture uptake by resin did not follow fickian kinetics and did not reach steady state
within the allowable time range of the equipment. To compare the sorption of moisture in
polymer sheets and resin, moisture uptake after 1400 minutes was recorded for the resin
(Table 3.5). Moisture sorption in resin was observed to be very low as comparable to
sheets for both polymers, at both temperatures. When the polymer is processed into
sheets, it undergoes some morphological changes depending on processing conditions
such as temperature, cooling rate, orientation, and pressure. During the processing the
polymer is heated above its melting temperature and then cooled down. The cooling time,
during which the polymer chains rearrange themselves, affects the crystallinity of the
processed polymer, hence affecting the sorption properties.

®

Table 3. 5: Moisture uptake in Biomax® and Sorona resin after 1400 minutes,

Average of 3 runs :1: SE.

 

Moisture uptake at 1400 min

(kg sorbate/kg polymer)
RH Biomax® Sorona®
(%) 23°C 40°C 23°C 40°C

 

50 0.19 i 0.01 0.53 i 0.03 0.07 :l: 0.01 0.13 :1: 0.01

90 0.68 :l: 0.03 1.86 d: 0.04 0.15 i 0.00 0.29 :l: 0.01

 

As seen from the DSC results, the crystallinity decreased after the processing of

polymer from resin to sheet, hence moisture sorption in sheets was observed to be more

57

than that in the resin. For Biomax®, the moisture uptake in resin was observed as 70%
and 21% less than that in sheets at 23°C and 40°C, respectively. For Sorona®, moisture
uptake was 64% lower at 23°C and 43% at 40°C as compared to sheets at respective
relative humidities.

5. Conclusions

A protocol for noise quantification of gravimetric equipment was proposed and
amplitude of noise and drift associated with symmetrical gravimetric analyzer was
quantified. A complete sorption profile of DuPont Biomax® and Sorona® sheets was
generated at two different temperatures (23°C and 40°C) and five different water
activities by gravimetric method. A time correction, supported by assumptions and
consistency test, was done for all the sorption experiments of both polymers. The
moisture uptake, at steady state, by Biomax® sheets was approximately five times as that
of the Sorona® sheets. Both Biomax® and Sorona® sheets isotherm showed a positive
temperature dependence i.e. more moisture was sorbed at higher temperature. The
solubility coefficients for both polymer sheets increased with water activity and
decreased with increase in temperature. The data generated would be usefiil in the
application of these new polymers in packaging industry.

Moisture sorption in both types of resins did not follow Fickian kinetics. For
Biomax® the moisture uptake in resins at 1400 min. was lower by 70% at 23°C and 21%
at 40°C than the steady state moisture uptake of the sheets, whereas for Sorona® resins it
was 64% lower at 23°C and 43% at 40°C as compare to steady state moisture uptake of

the sheets.

58

6. References:

10.

11.

12.

13.

Gross, R. A.; Kalra, B. Science 2002, 297, 5582, 803-807.

The Future of Global Markets for Biodegradable Packaging; Pira International
Ltd: 2006.

Szabo, F. Int Polym Sci Technol 2001, 28, 11, T/1-9.

Tapuriah, V. The Future of Plastics in Packaging; Frost & Sullivan: 6 May, 2003.

Gavara, R.; Catala, R.; Hemandez-Munoz, P.; Hernandez, R. J. Packag Technol
and Sci 1996, 9, 215-224.

Duncan, 8.; Urquhart, J .; Roberts, S. Review of Measurement and Modelling of
Permeation and Diffusion in Polymers; National Physical Laboratory: 2005.

Yasuda, H.; Rosengren, K. J Appl Polym Sci 1970, 14, 11, 2839-2877.

Apostolopoulos, D.; Winters, N. Packag Technol and Sci 1991, 4, 131-138.

Barr, C. D.; Giacin, J. R.; Hernandez, R. J. Packag Technol and Sci 2000, 13, 4,
157-167.

Hernandez, R. J. ; Gavara, R. J Polym Sci, Part B: Polym Phys 1994, 32, 2367-
2374.

Hernandez, R. J.; Giacin, J. R.; Grulke, E. A. J Membr Sci 1992, 65, 1-2, 187-
199.

Wong, B.; Zhang, Z.; Handa, Y. P. J Polym Sci, Part B: Polym Phys 1998, 36,
2025-2032.

Wong, C. H.; Bhethanabotla, V. R. Fluid Phase Equilib 1997, 139(371).

59

14. '
15.
16.

17.

18.

19. '

20.
21.

22.
23.
24.

25.

26.

Oliverira, N. S.; Oliverira, J .; Gomes, T.; Ferreira, A.; Dorgan, J .; Marrucho, I. M.
Fluid Phase Equilib 2004, 222-223, 317-324.

Akiko, M.; Mureo, K.; Makoto, M. Biodegradable Oriented Aromatic Polyester
Fihn and Method of Manufacture. W00110928, 2001.

Nagarajan, V.; Singh, M.; Kane, H.; Khalili, M.; Bramucci, M. J Polym Environ
2006, 14, 3, 281-287.

“Compostable Logo” of the Biodegradable Products Institute gains momentum
with approval DuPontTM Biomax® resin.
http://www.bpiworld.chg/Files/PressRelease/PRYUWi2U.Ddf (April, 2007)

Leaversuch, R., Biodegradable Polyesters: Packaging Goes Green. Plastics
Technology 2002.

Mungara, P.; Chang, T.; Zhu, J .; Jane, J. J Polym Environ 2002, 10, 1/2, 31-37.

Editor of textileinfo, I. C. L. A forecast on the textile processing industry in 21C.
http://textileinfo.com/en/tech/21c/02.html (April, 2008)

Liu, W.; Mohanty, A. K.; Drzal, L. T.; Misra, M.; Kurian, J. V.; Miller, R. W.;
Strickland, N. Ind Eng Chem Res 2005, 44, 4, 857-862.

Kurian, J. V. J Polym Environ 2005, 13, 2, 159-167.

Khonakdar, H. A.; Jafari, S. H.; Asadinezhad, A. Iran Polym J 2008, 17, 1, l9-38.

Crank, J ., In The Mathematics of Diffusion. Second ed.; Oxford University: 1975.

Hernandez, R. J .; Selke, S. E. M.; Culter, J. D., In Plastics Packaging: Properties,
Processing, Applications and Regulations. First ed.; Hanser Gardner Publications,
Inc.: 2000.

Hemandez-Munoz, P.; Gavara, R.; Hernandez, R. J. J Membr Sci 1998, 3993, 1-
10.

60

27.

28.

29.

Skoog, D. A.; Holler, F. J .; Crouch, S. R., In Principles of Instrumental Analysis.
Sixth ed.; Thomson Brooks/Cole: 2007.

Robinson, J. W.; F rarne, E. M. 8.; Frame H, G. M., In Undergraduate Instrumental
Analysis. Sixth ed.; Marcel Dekker: 2005.

Lyman, O. R.; Longnecker, M., In An Introduction to Statistical Methods and
Data Analysis. 5th ed.; Duxbury Press, A Division of Wadsworth, Inc.: 2000.

61

CHAPTER 4
The Effect of Moisture Sorption on the Physical and Mechanical Properties of

Biomax‘1° and Sorona®

1. Introduction

DuPont Sorona®, PTT, Poly(trimethylene terephthalate) is a thermoplastic
polyester produced by condensation of 1,3-propanediol (derived from renewable corn
sugar) and terephthalic acid (derived from fossil fuel).1 Sorona® belongs to the
thermoplastic aromatic polyester family and due to the three carbon glycol in its
backbone it has better mechanical properties as compared to the other two members of
this family (PET and PBT). Sorona® has a semi-crystalline molecular structure with a
zigzag shape that gives it excellent stretch recovery and tensile and compressive
properties.2 Sorona® has a good barrier to moisture, is dimensionally stable, and has good

weather and chemical resistance. Sorona®

can also be used as an engineering
thermoplastic because of its good thermal and mechanical properties.2’ 3 Research had
been conducted to tailor the physical and mechanical properties of Sorona® through
blending it with other polymers or nano-composites," 4 but few researchers have studied
the effect of moisture on the physical and mechanical properties of Sorona®.

DuPont Biomax®, polyethylene terephthalate succinatef is a hydro-biodegradable
modified polyester. It is a modified polyethylene terephthalate (PET) and up to three
aliphatic monomers can be added to the polyester, which creates weak spots that promote

degradation through hydrolysis.6 Biomax® is one of the few polymers that have received

the “COMPOSTABLE” logo from The Biodegradable Products Institute (BPI).7 Due to

62

its lower Tm as compared to PET it is easy to process into blown film, oriented film, or
molded articles. Its major applications can be single-use products, such as waste bags,
blister packs, geotextiles, seed mats and coating for disposable eating utensils (cups,
bowls or plates).6’ 8

In the previous chapter, the moisture sorption profile of Biomax® and Sorona®
was evaluated. This study aims to characterize the effect of moisture sorption on the
physical (T8 and crystallinity) and mechanical properties (modulus of elasticity and
tensile strength) of Biomax® and Sorona®.
2. Methodology
2. 1 Polymer processing

Biomax® (4026) and Sorona® resin pellets were obtained from El. DuPontTM de
Nemours & Co. (Wilmington, DE). The resin was processed using a DSM twin screw
Micro-Extruder and Injection Molder (TS/I-02, DSM, Netherlands) to obtain small discs
(25.4 mm dia. and 2 mm thick) and ‘dumbbell’ shaped samples. The processing
temperature of the barrel of DSM micro extruder was kept at 200°C with a screw speed
of 100 rpm for Biomax®. For Sorona®, the barrel temperature was 250°C and the screw
speed was 150 rpm. A compression molding press (Carvar Hydraulic Press, Wabash, IN)
was used to process the discs into sheets. The temperature of the hydraulic press platen
was kept at 200°C with 12,000 psi pressure for Biomax® and 250°C with 10,000 psi
pressure for Sorona®. The sheets samples were cooled quickly in the compression press

by rushing cold water through the platens. The thickness of both polymers sheets was

maintained at 4 to 6 mils. Details of processing are given in the previous paper.

63

2.2 Conditioning of the samples

Dried and moisture-sorbed sheet and dumbbell samples were used for this study.
To remove the residual moisture, the polymer sheets and dumbbell shaped samples were
preconditioned in an airtight jar with desiccant at 23°C until no further weight loss was
observed; these samples had been referred as dried samples throughout this chapter. A
saturated salt solution of sodium chloride (Aldrich Chemical Co., Milwaukee, WI) was
prepared in an airtight jar in accordance with ASTM E104-02(2007) to obtain 75:1:1 %
relative humidity at 23:1:1°C. Both polymer sheets and dumbbell shaped samples were
stored at 75% RH at 23°C until they had reached the steady state. These samples had
been referred as sorbed samples throughout this chapter.

2.3 Physical Properties
2.3.1 Differential Scanning Calorimeter (DSC)

A differential scanning calorimeter (TA Instruments, New Castle, Delaware) was
used to determine the glass transition temperature (Tg), melting temperature (Tm), and
percent crystallinity in accordance with ASTM D3418-03.9 For both polymer samples
one heating cycle followed by one cooling cycle was used. Aluminum pans were used for
the DSC experiments. Biomax® sheet samples were heated fi'om 10°C to 210°C at a rate
of 10°C/min and then cooled down to 10°C at a rate of 5°C/min. Sorona® sheets were
heated from 10°C to 265°C at a rate of 10°C/min and then cooled to 10°C at a rate of
5°C/min. The data were analyzed with Universal Analysis software (TA Instruments).
For ‘both polymers, three measurements were performed on dried and sorbed sheet

samples.

64

2.3.2 X-Ray Diffraction (XRD)

The percent crystallinity of Sorona® and Biomax® was also determined at 23°C
using the X-Ray diffi'action (XRD) method,10 using a Rigaku RU-200B equipped with a
CuKa radiation source (1.= 0.1541 nm). The XRD tube power and current used were 45
kW and 100 mA respectively. Both dried and sorbed samples were used for XRD
analysis. The measurements were done in triplicate using an angle of exposure of 10° to
30° and scan speed of 1° per min. Results consisted of a refractogram in which peaks
(crystallinity peaks) corresponds to rings shown on the diffraction image. Fractional
crystallinity was determined as the ratio of the area under the crystallinity peaks and the
total area (sum of crystallinity peaks and amorphous area).lo Intensity was plotted against
angle (20) and area under curve was calculated using Jade-7 software (Materials Data
Inc., Livermore, CA).

2.3.3 Dynamic Mechanical Analyzer (DMA)

For Biomax®, due to ambiguous results of T8 from DSC, the T8 was determined
using a dynamic mechanical analyzer (DMA) (TA Instruments, DE). The center part of
the dumbbell shaped samples was cut (30 x 5 x 1.4 mm) and used for determination of T g
throngh single cantilever mode. Three samples of both dried and sorbed samples were
tested at a frequency of 1 Hz and 50 um amplitude over the temperature range of 0° to
80°C at a ramp rate of 2°C/min. The T8 had been determined from the peak of the loss

modulus.

65

2.4 Mechanical Properties

The mechanical properties, Young’s modulus of elasticity and tensile strength,
were measured on a tensile instrument model SFM-20 made by United Testing Systems
(Huntington Beach, CA). To study the effect of moisture on mechanical properties, the
dried and sorbed dumbbell shaped samples of both polymers were tested in accordance
with ASTM D638-03.ll A sample width of approx. 1.4 mm and gage length of 25.4 mm
(1 in) with crosshead speed of 1.27 mm/min (0.05 in/min) was used. Five runs of each
type of sample (both polymer- dried and sorbed) were examined and the average and the
standard error had been reported.
2.5 F T -InfraRed

Dried and sorbed Biomax® sheet samples were analyzed using a Shimadzu IR
Prestige—21 Spectrophotometer (Shimadzu Scientific Instruments, Columbia, MD) to
study the interaction of moisture with the polymer. The experiment was performed by
first taking the background scan of air and then taking the transmission spectrum of a
Biomax® sheet samples at room temperature (around 23°C). The spectrum was recorded
in the wavenumber range from 600 to 4000 cm], by taking 40 scans per run to get a
better resolution. The sheet was then kept in a jar with water at 23°C, to let the sheet
absorb the moisture. Same sheet was then again scanned to study the interaction of

moisture with the polymer.

66

3. Results and discussion
3. 1 Physical properties

The amount of moisture sorbed by Biomax® and Sorona® at 23°C and 75% RH
was determined using the moisture sorption isotherms as 1.3 g/ 100 g of polymer and 0.3
g/ 100 g of polymer, respectively.

The melting temperature (T...) of Biomax® and Sorona® sheets was determined as

198°C and 228°C respectively; no significant changewas observed in Tm before and after
moisture sorption for both polymers (data not shown). The results of T8 and crystallinity
from DSC, XRD and DMA analysis are given in Table 4.1. As the moisture uptake in
Sorona® was very low, no significant difference in the physical properties, T8 and
crystallinity, was observed among the dried and sorbed samples. The Tg of Sorona®

sheets was determined as 57°C for the dried samples and 546°C for the sorbed samples.
A student’s t-test showed that the change in Tg was not significant (01 = 0.05).12

For Biomax®, the DSC results of T8 were ambiguous and had significant
variability within different runs (data not shown). Thus the DMA single cantilever mode
was used to determine the T3, identified as the peak of loss modulus (Fig. 4.1). The T8 of
Biomax® dropped by 10°C (from 40°C to 30°C) after moisture sorption, which shows the
plasticizing effect of water. Water acts as plasticizer by reducing the inter- and intra-
macromolecular forces also seen in PET.13 These results are also supported by the fact
Biomax® is a hydro-biodegradable polymer; moisture lowers the T3 and thus helps in

degradation of the polymer by micro-organisms.

67

Table 4.1: Physical properties of dried and sorbed Biomax® and Sorona® samples

 

 

Average of 3 runs :t SE
.m T (c0) Crystallinity (%) Crystallinity (%)
ample g DSC XRD
Sorona®
Dried Samples 57.0 1: 0.4 ’1‘ 26.4 2 0.9 22.4 2 0.1
Sorbed Sample * 54.6 2 0.3 i 28.5 .2 0.5 22.4 2 1.0
Biomax®
Dried Samples 39.9 d: 0.7 A 9.5 d: 0.7 12.9 :I: 0.9
Sorbed Sample * 29.7 :1: 0.5 A 3.6 d: 1.1 19.4 d: 0.6

 

* Samples sorbed with moisture at 75% RH at 23°C
A For Biomax® Tg reported is from DMA results
# For SoronaQ Tg reported is from DSC results

 

 

 

 

 

1000;
1003 "““
1; i
9..
a .
m .
.2
8 3 -------- 29.7
2 .
H
O ____________
A ‘‘‘‘‘
1‘:
1 _________
0.1-eT ....T..-...-
-20 O 20 40 60 80
Temperature (°C)

Figure 4.1: DMA results for Biomax® dried and sorbed samples,

solid line- Biomax® dried and dotted line- Biomax® sorbed

68

The percent crystallinity determined by DSC and XRD was found to be different
for both polymers. This can be due to the fact that during a DSC run, the sample is heated

to its melting point, which causes the sample to loose moisture. In DSC the percent

crystallinity (70;) was calculated by using the following equation:

yc=100AHm

 

(1)

where AHm is enthalpy of fusion and AH0 is the heat of melting of purely crystalline
polymer. Poly(ethylene terephthalate) AHo (140 J/g) was used for calculating the percent
crystallinity as AH0 value for both polymers was not available. The percent crystallinity
was determined using the area under the crystallinity peaks and the total area (sum of

crystallinity peaks and amorphous area) (Fig. 4.2).10

AreaCrystaI/ine

 

)(C = 100 . (2)
AreaCrystalline + Area/Morpheus

 

Crystalline Area 1

 

 

 

 

lzvv‘v'vvmv'V"V‘Vi'gvvttfi'18""rr'aé'V'V'V'hif'vvvvvidv 26 28

 

Figure 4.2: Refiactogram from XRD analysis for Biomax® dried sample

69

For Sorona®, the percent crystallinity determined by DSC was close to the results
from XRD. No significant effect of moisture on the crystallinity of Sorona® sheets was
observed through either technique.

3.2 Mechanical Properties

The mechanical properties of the polymers were evaluated by the stress and strain
characteristics under tensile deformation (Fig. 4.3). The modulus of elasticity is given by
the slope of the initial linear portion of the curve and tensile strength is given as the
highest point of the curve. Modulus of elasticity and tensile strength of Sorona® dried
samples were determined as 2.2 GPa and 60 MPa, respectively (Fig. 4.4).

Tensile strength and modulus were found to be very close to the values reported
in the literature.3’ 4 The effect of moisture on the mechanical properties in case of Sorona®
was ‘not significant. Modulus of elasticity and tensile strength of Biomax® dried samples
was determined as 2.2 GPa and 52 MPa, respectively. Mechanical properties of Biomax®
samples were reduced drastically after exposure to moisture, which could be the result of
cleaving of the larger polymer molecules into smaller molecules by moisture.‘5 The

modulus of elasticity and tensile strength of Sorona® samples and dried Biomax® samples

was found to be close to that of PET.” '5

70

  
 

 

 

53 ~ a
E 40 t1~1+++++ ++ +++ ++ +
H b
g 30
m
20
1 0
O 1 1 'fi
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Strain
Figure 4.3: Stress-strain curves for Biomax® and Sorona® samples;

a) Sorona® dried, b) Sorona® sorbed, c) Biomax® dried, and d) Biomax® sorbed

 

 

 

 

 

 

 

 

 

 

Modulus of Elasticity Tensile Strength

2-50 7 2.18 2-19 2.13 60:53 57.49 i 7"

+. . ' —. ” 60

r 50
r 40 ‘3
~ 30 E

r 20

r 10

® ® ® ® 0
Biomax Sorona Biomax Sorona
Figure 4. 4: Mechanical properties of dried and sorbed Biomax® and Sorona® samples,

Average of 5 runs :t SE Dried samples DSorbed samples

71

3.3 F T-IR Results

The transmission spectrum of a dried Biomax® sheet is shown in Figure 4.5. Also shown
is the scan of the same sheet when exposed to moisture for 1710 min and 4080 min. As
seen in the spectrum an increase in transmission was observed around 3300 cm'l. The
broad peak in this 3200 to 3600 cm-1 is attributed to the stretching of OH bonds. The
center peak of this region was around 3400 cm-1, which corresponds to the
intermolecular hydrogen bonding.'6 The F TIR results confirm that the water is interacting

with the polymer and is bonded with the polymer.

 

 

 

 

 

 

 

 

 

 

 

 

 

4 7 f“
3 .4
a»
'5‘: 1800 1600
:12 e
*2
._. C
b "
.1 g
i l
., 1
O 1 1 fil 1 1 1 fl

 

 

4000 3500 3000 2500 2000 -1 1500 1000 500
Wavenumber (cm )

Figure 4. 5: IR Spectrum of Biomax® sheet before and after moisture sorption; a) dried

sample, b) exposed to moisture for 1 710 min, and c) exposed to moisture for 4 080 min.

72

4. Conclusions

The mechanical and physical properties of Biomax® and Sorona® were evaluated
using DSC, XRD, DMA and UTS. Furthermore, the effect of moisture sorption on the
physical and mechanical properties of both polymers was evaluated. Moisture sorption
had little or no impact on the mechanical and physical properties of Sorona®. These
results indicate that Sorona® would be usefirl as a packaging polymer or an engineering
thermoplastic.

In Biomax®, the water acts as a plasticizer as shown by a drop in glass transition
temperature after moisture sorption. A drastic drop in mechanical properties of Biomax®

samples was observed after exposure to moisture, possibly due to cleaving of the larger

molecules into smaller molecules and reduction in molecular weight.

73

5. References

1. Liu, W.; Mohanty, A. K.; Drzal, L. T.; Misra, M.; Kurian, J. V.; Miller, R. W.;
Strickland, N. Ind Eng Chem Res 2005, 44, 4, 857-862.

2. Kurian, J. V. J Polym Environ 2005, 13, 2, 159-167.

3. Drown, E. K.; Mohanty, A. K.; Parulekar, Y.; Hasija, D.; Harte, B. R.; Misra, M.;
Kurian, J. V. Composi Sci Techn012007, 67, 3168-3175.

4. - Khonakdar, H. A.; Jafari, S. H.; Asadinezhad, A. Iran Polym J 2008, 17, 1, 19-38.

5. Japan BioPlastics Association - GreenPla Positive List (PL) - Biodegradable
plastics. http://www.jbpaweb.net/english/positive_list/A.htm

6. DuPont Launches Biodegradable Polyester - Biomax®, Press Release.
http://www.dupont.com.tw/020604-e.htm (April, 2008)

7. “Compostable Logo” of the Biodegradable Products Institute gains momentum
with approval DuPontTM Biomax® resin.
http://www.bpiworld.org/Files/PressRelease/PRY U W LgUpdf (April, 2007)

8. Mohanty, A. K.; Misra, M.; Drzal, L. T. J Polym Environ 2002, 1-2, 10, 19-26.

9. ASTM, Standard Test Method for Transition Temperatures and Enthalpies of
Fusion and Crystallization of Polymers by Differential Scanning Calorimetry. In ASTM
D34'18-03, ASTM Book of Standards, 2003; Vol. 08.02.

10. Alexander, L. E., In X-Ray Diffiaction Methods in Polymer Science. Robert E.
Krineger Publishing Company: 1979.

11. ASTM, Standard Test Method for Tensile Properties of Plastics. In ASTM D63 8-
03, ASTM Book of Standards, 2004; Vol. 08.01.

12. Lyman, O. R.; Longnecker, M., In An Introduction to Statistical Methods and
Data Analysis. 5th ed.; Duxbury Press, A Division of Wadsworth, Inc.: 2000.

13. Launay, A.; Thominette, F.; Verdu, J. J Appl Polym Sci 1999, 73, 1131-1137.

74

14. Torres, N.; Robin, J. J.; Boutevin, B. Eur Polym J 2000, 36, 2075-3080.

15. . Hernandez, R. J .; Selke, S. E. M.; Culter, J. D., In Plastics Packaging: Properties,
Processing, Applications and Regulations. First ed.; Hanser Gardner Publications, Inc.:

2000.

16. Robinson, J. W.; Frame, E. M. 8.; Frame 11, G. M., In Undergraduate Instrumental
Analysis. Sixth ed.; Marcel Dekker: 2005.

75

Chapter 5
Conclusions

1. Conclusions

DuPont Biomax® and Sorona® are two of many newly developed biopolymers
which are gaining attention in the packaging industry. Through this research the moisture
sorption in these polymers was studied using a gravimetric technique. Furthermore, the
effect of moisture on mechanical and physical properties of these two polymers was also
assessed. The main conclusions of this study can be summarized as follow:

V A protocol for the noise quantification of gravimetric equipment — Symmetrical

I Gravimetric Analyzer (SGA-100) was proposed. The drift of the instrument was
observed in the range of -7 * 10*)6 to 2 * 10'06 within the working range. The
amplitude of noise was found to be in the range of 0.0031 to 0.0119 %.

V A complete sorption profile of DuPont Biomax® and Sorona® sheets was
generated at two different temperatures (23°C and 40°C) and five different water
activities by gravimetric method.

V The moisture sorption data obtained did not fit the theoretical data. Hence a time
correction of the data, supported by the assumptions and a consistency test, was
done for all the sorption experiments of both polymers. The time taken by the
vapors to reach the experimental relative humidity was subtracted from the
experimental time.

V Moisture sorption isotherms of both polymers sheets were generated at 23°C and
40°C. Both Biomax® and Sorona® sheet isotherms showed a positive temperature

dependence, i.e., more moisture was sorbed at higher temperature.

76

V The moisture uptake, at steady state, by Biomax® sheets was approximately five
times that of Sorona® sheets. The moisture sorption in both polymer sheets
followed Fickian kinetic.

V Solubility coefficients were determined form the steady state moisture uptake.
The solubility coefficients for both polymer sheets increased with water activity
and decreased with increase in temperature.

V A sum of square technique was used to determine the best estimated diffusion
coefficients from the transient state sorption data. The diffusion coefficient of
Biomax® slightly increased with increase in RH except at 90% RH at both
temperatures. For Sorona® sheets the diffusion coefficient was found to be
independent of relative humidity.

V Moisture sorption of both polymer resins was evaluated at two temperatures and
two water activities. Moisture sorption in resin did not follow Fickian kinetics.
For Biomax® the moisture uptake in resins at 1400 min was lower by 70% at
23°C and 21% at 40°C than the steady state moisture uptake of the sheets,
whereas for Sorona® resins it was 64% lower at 23°C and 43% at 40°C as
compared to steady state moisture uptake of the sheets.

V Moisture sorption had little or no impact on the mechanical and physical
properties of Sorona®.

V In Biomax®, the water acts as a plasticizer as shown by a drop in glass transition
temperature after moisture sorption. A drastic drop in mechanical properties of

®

Biomax samples was observed after exposure to moisture, possibly due to

77

cleaving of the larger molecules into smaller molecules and reduction in
molecular weight.
The moisture sorption and its impact on properties studied in this research would be

useful in application of these novel polymers in packaging industry.

2. Future Work Recommendations

This work studied moisture sorption and its effect on the properties of the
Biomax® and found that moisture is interacting with polymer. However, the exact
interaction of moisture with polymer on molecular level could not be evaluated due to the
lack‘of information about the chemical structure of Biomax®. A study can be conducted
to eXamine this interaction on the molecular level and the structure of polymer in more
details.

Due to the lack of the information about the interaction parameters of both
polymers, the experimental moisture sorption isotherms were not fitted to any sorption
model. To fit the sorption models such as BET or Flory Huggins, a study could be

conducted to determine the interaction parameters experimentally.

78

"'11

IGAN STATE UNIVERSITY LIBRARIES

1111111111111 11111111

1293 3062 9129