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MAY BE RECALLED with earlier due date if requested.

 

 

DATE DUE

DATE DUE

DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6/01 cJClRCIDateDuepss-sz

 

CHARACTERIZATION OF POLYMERIC COMPOSITES WITH LOW CTE
CERAMIC PARTICULATE FILLERS

By

PRADEEP U. SONJE

 

A THESIS
Submitted to
Michigan State University
In partial fulﬁllment of the requirements
For the degree of
MASTER OF SCIENCE

Department of Chemical Engineering and Materials Science

2001

 

ABSTRACT

CHARACTERIZATION OF POLYMERIC COMPOSITES WITH LOW CTE
CERAMIC PARTICULATE FILLERS
By

PRADEEP U. SONJE

Use of particulate ﬁllers is a potential approach to develop isotropic low
coefﬁcient of thermal expansion polymeric composite materials for electronic circuit
board applications. Low coefﬁcient of thermal expansion ceramic particulate ﬁllers such
as Cordierite, Beta quartz and BS-SO were incorporated into three different epoxy resins.
The effect of different particulate ﬁllers on the coefﬁcient of thermal expansion of
resultant composites was studied. In order to promote the adhesion with epoxy resin
matrix, particulate ﬁllers were treated with surface-active agents. Fracture study and
microhardness testing of the composites in addition to coefﬁcient of thermal expansion
measurements revealed the dispersion of particulate ﬁllers, nature of interfacial bonding
and its effect on reducing coefﬁcient of thermal expansion of the composites. The
dependence of coefﬁcient of thermal expansion of the composites on the properties of the
epoxy resins as well as on the properties of the particulate ﬁllers along with other factors

is discussed.

suppor

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ACKNOWLEDGEMENT

I would like to express my gratitude for my advisor Dr. K.N. Subramanian for his
support, encouragement and help throughout the project. I would like to thank my co-
advisor Dr. A.Y. Lee for his help, guidance and encouragement. I would like to thank Dr.
J.P. Lucas for his guidance and help. I thank H.T. Chen and S.L.Choi for their help in
getting acquainted with various equipments required for this project.

I would also like to thank Composite Material and Structures Center, Michigan
State University and Semiconductor Research Corporation for funding this project. I
would like to thank Corning Inc. (New York) for providing Beta quartz and Cordierite. I
thank LoTEC Inc. (Utah) for providing samples of BS-SO required in this project. I thank
Resolution Performance Products for providing samples of epoxy resins and curing
agents. I thank Dow Coming for providing samples of silane coupling agents.

Finally, I would like to thank Department of Materials Science at Michigan State

University for providing me golden opportunity of graduate studies at this university.

iii

TABLE OF CONTENTS

LIST OF TABLES ................................................................................. vii
LIST OF FIGURES ................................................................................. x

1. INTRODUCTION
1.1 Necessity of polymer matrix composites with low coefﬁcient of

thermal expansion ...................................................................... 1
1.2 Particulate reinforced polymer matrix composites ................................ 3
1.3 CTE ...................................................................................... 7
1.4 Stresses in particulate reinforced epoxy resin matrix
composites .............................................................................. 8
1.5 Effect of volume of particulate ﬁllers on CTE of composites .................. 10
1.6 Effect of adhesion of particulate ﬁllers with the matrix on CTE
of composites ......................................................................... 1 1
1.6.1 Non—adherent particulate ﬁllers ........................................ 12
1.6.2 Perfectly adherent particulate ﬁllers ................................... 12
1.6.3 Adhesion mechanisms ................................................... 13
1.7 Effect of moduli of elasticity of particulate ﬁllers and epoxy resin
matrix on composite properties ..................................................... 14
1.8 Effect of arrangement of particulate ﬁllers in the composites .................. 15
1.9 Reduction of stresses developed in particulate reinforced composites ........ 15
1.10 Effect of shape and size of the particulate ﬁllers ................................. 16
1.11 Effect of wettability of particulate ﬁllers .......................................... 20
1.12 Effect of chemistry of the particulate ﬁllers ...................................... 23
1.13 Viscosity of the mixture of particulate ﬁllers and epoxy resins ............... 23
1.14 Settling of particulate ﬁllers in epoxy resin matrix .............................. 24
1.15 Curing of the composites ............................................................ 24
1.16 Density of composites ............................................................... 26
1.16.1 Porosities in composites .................................................. 28
1.17 Role of silane coupling agents ...................................................... 29
1.18 Fracture study of particulate reinforced composites ............................. 31
1.19 Weight fraction to volume fraction conversion of particulate ﬁller
addition ................................................................................ 32
1.20 Mathematical models to predict CTE of particulate reinforced composites..33
1.20.1 Turner’s model ........................................................... 35
1.20.2 Kemer’s model ........................................................... 37
1.21 Aim of this study ...................................................................... 39
2. EXPERIMENTAL PROCEDURE ........................................................ 40
2.1 Properties of materials used in this study ...................................... 40
2.1.1 Epoxy resins ............................................................... 40
2.1.2 Curing agents ............................................................. 40

3.1113

2.1.3 Cured epoxy resins ....................................................... 45

2.1.4 Particulate ﬁllers ......................................................... 45
2.1.5 Silane coupling agents ................................................... 53
2.2 Specimen preparation ............................................................. 53
2.2.1 Effect of process variables ............................................... 57
2.2.2 Treating particulate ﬁllers with silane coupling agents .............. 57
2.3 Testing of composites ............................................................ 59
2.3.1 Measurements of density of composites .............................. 59
2.3.2 Measurements of CTE of composites using TMA .................. 60
2.3.3 Effect of testing parameters on CTE of composites .................. 62
2.3.4 Microhardness measurements of composites ......................... 62
2.3.5 Microscopy for observing composite microstructures ............... 65
2.3.6 Fracture study of composites ........................................... 65
3. RESULTS AND DISCUSSION ............................................................ 67
3.1 CTE of composites ................................................................... 67
3.1.1 Effect of volume fraction of particulate ﬁllers ........................ 66
3.1.2 Effect of properties of matrix ............................................ 73
3.1.3 Effect of size and shape of particulate ﬁllers .......................... 74
3.1.4 Effect of wettability of particulate ﬁllers .............................. 82
3.1.5 Effect of aggregates of particulate ﬁllers ............................... 83
3.1.6 Effect of settling of particulate ﬁllers .................................. 83
3.1.7 Density measurement of the composites ............................... 88
3.1.8 Pores in composites ....................................................... 88
3.1.9 Microhardness measurements of composites .......................... 96
3.1.10 Effect of additives in epoxy resins ..................................... 107
3.1.11 Effect of varying curing agent quantity .............................. 108
3.1.12 Nature of interfaces of composites .................................... 110
3.2 Effect of treating particulate ﬁllers with silane coupling agents ............. 115
3.2.1 Effects of silane coupling agent treatment on CT E
of composites ............................................................ 115
3.2.2 Effects of silane coupling agent treatment of particulate ﬁllers
on dispersion in epoxy resins and density of composites. . . . . .....1 15
3.2.3 Effect of silane coupling agent treatment on fracture
behavior of composites ................................................. 132
3.2.4 Reasons for ineffectiveness of excess silane coupling
agent treatment of particulate ﬁllers on CTE and
fracture toughness of the composites ................................. 139
3.2.5 Effect of excess silane coupling agents on CTE of

epoxy resins ............................................................. 140

3.2.6 Recommendations for improving the silane coupling agent
treatment of particulate ﬁllers ......................................... 141

3.3 Comparison of experimental CTE values of composites with those
predicted by mathematical models ............................................. 143
3.4 Conclusions ........................................................................ 159

3.5 Recommendations for future work ............................................. 160

REFERENCES .................................................................................... 163

vi

Table

Table

Table

Table

Table

Table

Table
Table
Table

Table

Table
Table
Table

Table

Table

Table

Table

LIST OF TABLES

Table l. CTE of materials commonly used in printed circuit boards ......................... 2

Table 2. Maximum quantity of particulate ﬁllers added to DGBBA epoxy resin
cured with MPDA and CTE values achieved in the resultant composites... . . . . .4

Table 3. Effect of particulate ﬁllers on the properties of the epoxy resin matrix
Composites .................................................................................. 6

Table 4. Surface energies for various particulate ﬁllers and plastics ........................ 22

Table 5. Effect of particulate ﬁllers on viscosity of DGBBA, cured with
primary amine ............................................................................. 25

Table 6. Effect of particulate ﬁllers on peak exothermic temperature rise in DGEBA

cured with primary amine ............................................................... 27

Table 7. Properties of epoxy resins used as matrix in this study .............................. 41

Table 8. Properties of curing agents used in this study ........................................ 43

Table 9. Typical properties of epoxy resin DGBBA cured with DETA ..................... 46
Table 10. Measured values of properties of cured epoxy resins, used in this study.

No particulate reinforcements .......................................................... 47

Table 11. Typical composition of BS-50 ......................................................... 48

Table 12. Properties of the particulate ﬁllers used in this study .............................. 49

Table 13. Properties of silane coupling agents used in the study ............................. 54

Table 14. CTE of composites reinforced with Beta quartz particles. Matrix-EPON
Resin 8280 cured with EPI-CURE 3223 ............................................. 68

Table 15. CTE of composites reinforced with Beta quartz particles. Matrix-EPON
Resin 8281 cured with EPI-CURE 3223 ............................................. 68

Table 16. CTE of composites reinforced with Beta quartz particles. Matrix-EPON
Resin 9405 cured with Ancamine 9470 ............................................. 68

Table 17. CTE of composites reinforced with Cordierite particles. Matrix-EPON
Resin 8280 cured with EPI—CURE 3223 ............................................. 69

vii

 

 

Ta

Ta

Ta

Table 18. CTE of composites reinforced with Cordierite particles. Matrix-EPON

Resin 8281 cured with EPI-CURE 3223 ............................................. 69
Table 19. CTE of composites reinforced with BS-50 particles. Matrix- EPON Resin

8280 cured with EPI-CURE 3223 ..................................................... 70
Table 20. CTE of composites reinforced with BS-50 particles. Matrix- EPON Resin

8281 cured with EPI—CURE 3223 ..................................................... 70
Table 21. Pr0perties and behavior of particulate ﬁllers used in this study .................. 75

Table 22. Observations about distribution of particulate ﬁllers in
various epoxy resins ..................................................................... 86

Table 23. Effect of treating Beta quartz particles with silane coupling agent, on the
CTE of composites. Matrix-EPON Resin 8280
cured with EPI-CURE 3223 .......................................................... 116

Table 24. Effect of treating Beta quartz particles with silane coupling agent, on the
CTE of composites. Matrix-EPON Resin 8281
cured with EPI-CURE 3223 .......................................................... 116

Table 25. Effect of treating Cordierite particles with silane coupling agent, on the
CTE of composites. Matrix-EPON Resin 8280 cured with
EPI-CURE 3223 ........................................................................ 116

Table 26. Effect of treating Cordierite particles with silane coupling agent, on the
CTE of composites. Matrix-EPON Resin 8281 cured with
EPI-CURE 3223 ........................................................................ 117

Table 27. Effect of treating BS-50 particles with silane coupling agent, on the
CTE of composites. Matrix- EPON Resin 8280 cured with
EPI-CURE 3223 ........................................................................ 117

Table 28. Effect of treating BS-50 particles with silane coupling agent, on the
CTE of composites. Matrix- EPON Resin 8281 cured with
EPI-CURE 3223 ........................................................................ 117

Table 29. Observations about the effects of silane coupling agent treatment on
dispersion of particulate ﬁllers in epoxy resin matrices .......................... 121

Table 30. Observations about the effects of silane coupling agents on the density of
The composites. Matrix-EPON Resin 8280 cured with EPI-CURE 3223. . 128

viii

Table 31. Observations about best ﬁt mathematical models for predicting CTE
of the composites, developed by reinforcing three types of epoxy resins
with three types of particulate fillers, in this study ................................. 151

Table 32. Elastic constants for various ﬁllers and matrix materials ........................ 154

ix

LIST OF FIGURES

Figure 1. Typical shapes of particulate ﬁllers ................................................. 18
Figure 2. Schematic representation of effect of particle shape and occluded polymer... 19
Figure 3. Effect of reducing inter-particle distance. I. Inter-particle are more

at low volume fractions of particulate ﬁllers.II. Inter-particle distances are less
at high volume fraction of particulate ﬁllers, where the overlapping of shells

of adsorbed epoxy resin matrix takes place ...................................................... 21
Figure 4. Structure of DGBBA .................................................................... 42
Figure 5. Micrograph of Beta quartz particles ................................................... 50
Figure 6. Micrograph of Cordierite particles .................................................... 51
Figure 7. Micrograph of BS-SO particles. These particles tend to agglomerate. . . .....52

Figure 8. Flow-chart for preparing composite reinforced with Beta quartz. Matrix-EPON
Resin 8280 cured with EPI-CURE3223 .......................................................... 56

Figure 9. Important steps in treating particulate ﬁllers with silane coupling
agent Z6040 .......................................................................................... 58

Figure 10. Micrograph showing the diagonals of the indenter of a microhardness
tester. Diagonals are spread along the epoxy resin matrix as well as particulate
ﬁllers, hence the microhardness value represents hardness of composite ................... 64

Figure 11. Micrograph of composite reinforced with 2.35 volume % of
Beta quartz. Matrix- EPON Resin 9405 cured with Ancamine 9470. Interparticle
distances are high due to low volume fraction of the particulate ﬁllers ...................... 71

Figure 12. Micrograph of composite reinforced with 25.76 volume % of Beta

quartz particles. Matrix- EPON Resin 9405 cured with Ancamine 9470.

Interparticle distances are very less due to high volume fraction of the

particulate ﬁller ...................................................................................... 72

Figure 13. Micrograph of composite reinforced with 13.54 volume % of Beta
quartz. Matrix- EPON Resin 8280 cured with EPI-CURE 3223 .............................. 76

Figure 14. Micrograph of composite reinforced with 13.16 volume % of Beta
quartz. Matrix- EPON Resin 8281 cured with EPI-CURE 3223. It shows the
mechanical interlocking of the particles with the matrix ....................................... 77

the
Fig

M;
on:

for

Figure 15. Micrograph of composite reinforced with 13.64 volume % of Cordierite.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ....................................... 78

Figure 16. Micrograph of composite reinforced with 13.54 volume % of
Cordierite. Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ......................... 79

Figure 17. Micrograph of composite reinforced with 10.16 volume % of BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ....................................... 80

Figure 18. Micrograph of composite reinforced with 10.47 volume % of
BS-50. Matrix- EPON Resin 8281 cured with EPI-CURE 3223 .............................. 81

Figure 19. Micrograph of fracture surface of composite reinforced with 13.52
volume % of Beta quartz. Matrix- EPON Resin 8280 cured with EPI-CURE 3223.
Few pores are present as indicated ................................................................ 84

Figure 20. Micrograph of fracture surface of composite reinforced with 13.54
volume % of Cordierite in EPON Resin 8280 cured with EPI-CURE 3223. Few pores
and an agglomerate are present indicated ........................................................ 85

Figure 21. Micrograph of fracture surface of composite reinforced with 10.16
volume % of BS-50. Matrix- EPON Resin 8280 cured with EPI-CURE 3223.
Composite contains aggregates of particles as well as pores .................................. 86

Figure 22. Micrograph of composite reinforced with 10.16 volume % of BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223. Fracture surface shows
the aggregates as indicated .......................................................................... 87

Figure 23. Density variation in composites reinforced with Beta quartz.
Matrix- EPON Resin 9405 cured with Ancamine 9470 Note: Data obtained from
one specimen for each volume % of Beta quartz particulates ................................. 89

Figure 24. Density variation in composites reinforced with Beta quartz.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223. Note: Error bars not visible
for certain volume % of Beta quartz in the scale presented ..................................... 90

Figure 25. Density variation in composites reinforced with Beta quartz.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223. Note: Error bars not
visible for certain volume % of Beta quartz in the scale presented ........................... 91

Figure 26. Density variation in composites reinforced with Codierite.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223. Note: Error bars not visible

for certain volume % of Cordierite in the scale presented ...................................... 92

Figure 27. Density variation in composites reinforced with Cordierite.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223. Note: Error bars not

xi

Fil
M

Fie
M.

Fig

visible for certain volume % of Cordierite in the scale presented ............................. 93

Figure 28. Density variation in composites reinforced with BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ....................................... 94

Figure 29. Density variation in composites reinforced with BS-50.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223. Note: Error bars not
visible for certain volume % of BS-50 in the scale presented ................................. 95

Figure 30. Hardness variation in composites due to Beta quartz reinforcements.
Matrix- EPON Resin 9405 cured with Ancamine 9470 ........................................ 97

Figure 31. Hardness variation in composites due to particulate ﬁller reinforcements.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ....................................... 98

Figure 32. Hardness variation in composites due to particulate ﬁller reinforcements.
Matrix- EPON Resin 8281 cured with EPI—CURE 3223 ....................................... 99

Figure 33. Hardness variation in composites reinforced with Beta quartz.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 100

Figure 34. Hardness variation in composites reinforced with Beta quartz.
Matrix - EPON Resin 8281 cured with EPI-CURE 3223 ..................................... 101

Figure 35. Hardness variation in composites reinforced with Beta quartz.
Matrix- EPON Resin 9405 cured with Ancamine 9470 ....................................... 102

Figure 36. Hardness variation in composites reinforced with Cordierite.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 103

Figure 37. Hardness variation in composites reinforced with Cordierite.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ...................................... 104

Figure 38. Hardness variation in composites reinforced with BS-SO.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 105

Figure 39. Hardness variation in composites reinforced with BS-50.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ..................................... 106

Figure 40. CTE variation in epxoy resin matrices and composites with varying
quantity of curing agent. Matrix- EPON Resin 82820 cured with
EPI-CURE 3223. Composites are reinforced with 12.29 volume % Beta quartz ......... 109

Figure 41. Micrograph of fracture surface of composite reinforced with 13.52
volume % of Beta quartz. Matrix- EPON Resin 8280 cured with EPI-CURE 3223......111

xii

 

Figure 42. Micrograph of fracture surface of composite reinforced with 13.64
volume % of Cordierite. Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ....... 112

Figure 43. Micrograph of composite reinforced with 10.16 volume % of BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223. Fracture surface
of the composite indicates effect of aggregates diverting the cracks ........................ 113

Figure 44. CTE variations in composites reinforced with 13.5 volume % Beta
quartz with and without silane coupling agent treatment. Matrix- EPON
Resin 8280 cured with EPI-CURE 32 ........................................................... 118

Figure 45. CTE variations in composites reinforced with 13.6 volume % Cordierite
with and without silane coupling agent treatment. Matrix- EPON Resin 8280
cured with EPI-CURE 3223 ...................................................................... 119

Figure 46. CTE variations in composites reinforced with 10.2 volume % BS-50
with and without silane coupling agent treatment. Matrix- EPON Resin 8280
cured with EPI-CURE 3223 ....................................................................... 120

Figure 47. Micrograph of fracture surface of composite reinforced with 13.52
volume % of Beta quartz treated with Z6020. Matrix- EPON Resin 8280 cured
with EPI—CURE 3223. No porosities and no aggregates are observed ..................... 122

Figure 48. Micrograph of fracture surface of composite reinforced with 13.52
volume % of Beta quartz particles treated with Z6040. Matrix- EPON Resin 8280
cured with EPI-CURE 3223.Few pores are present as indicated ............................. 123

Figure 49. Micrograph of fracture surface of composite reinforced with 13.64
volume % of Cordierite treated with Z6020. Matrix- EPON Resin 8280 cured with
EPI-CURE 3223. Few pores are present as indicated .......................................... 124

Figure 50. Micrograph of fracture surface of composite reinforced with 13.64
volume % of Cordierite with Z6040. Few pores are present as indicated.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 125

Figure 51. Micrograph of fracture surface of composite reinforced with 10.16
volume % of BS-50 treated with Z6020. Matrix- EPON Resin 8280 cured with
EPI-CURE 3223. Fracture surface shows pores and aggregates as indicated .............. 126

Figure 52. Micrograph of fracture surface of composite reinforced with 10.16

volume % of BS-50 treated with Z6040. Matrix- EPON Resin 8280 cured

with EPI—CURE 3223. Composite shows fewer pores and smaller aggregates

compared to the composite containing untreated BS-50 particles, shown

in Figure 22 ......................................................................................... 127

Figure 53. Effect of silane coupling agents on density of composites reinforced

xiii

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with Beta quartz. Matrix- EPON Resin 8280 cured with EPI-CURE 3223. . . . . . . . . .. 129

Figure 54. Effect of silane coupling agents on density of composites reinforced
with Cordierite. Matrix- EPON Resin 8280 cured with EPI-CURE 3223. . . . . . . . . 130

Figure 55. Effect of silane coupling agents on density of composites reinforced
with BS-50. Matrix- EPON 8280 cured with EPI-CURE 3223 .............................. 131

Figure 56. Variation in fracture toughness of matrix and composites reinforced
with 13.5 volume % of Beta quartz with and without silane coupling agent
treatment. Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ......................... 133

Figure 57. Variation in fracture toughness of composites reinforced with 13.6
volume % of Cordierite with and without silane coupling agent treatment.
Matrix-EPON Resin 8280 cured with EPI-CURE 3223 ...................................... 134

Figure 58. Variation in fracture toughness of composites reinforced with 10.2
volume % of BS-50 with and without silane coupling agent treatment.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 135

Figure 59. Micrograph of fracture surface of composite reinforced with 13.52
volume % of Beta quartz treated with Z6020. Matrix- EPON Resin 8280 cured
with EPI-CURE 3223 ............................................................................. 136

Figure 60. Micrograph of fracture surface of composite reinforced with 13.6
volume % of Cordierite particles treated with Z6020. Matrix- EPON Resin 8280
cured with EPI-CURE 3223 ...................................................................... 137

Figure 61. Micrograph of fracture surface of composite reinforced with 10.16
volume % of BS-SO treated with 26020. Matrix- EPON Resin 8280 cured
with EPI-CURE 3223 ............................................................................. 138

Figure 62. CTE variations cured epoxy resins by addition of excess silane coupling
agent into epoxy resin matrices. N o particulate ﬁllers are added ............................ 142

Figure 63. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement- Beta
quartz. Matrix- EPON Resin 9405 cured with Ancamine 9470 ............................. 144

Figure 64. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement- Beta
quartz. Matrix- EPON Resin 8280 cured with EPI—CURE 3223 ............................. 145

Figure 65. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement- Beta
quartz. Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ............................. 146

xiv

 

 

  
  
   
   
  
 
   

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Figure 66. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement-
Cordierite. Matrix- EPON Resin 8280 cured with EPI-CURE 3223 ........................ 147

Figure 67. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement-
Cordierite. Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ....................... 148

Figure 68. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement- BS-50.
Matrix— EPON Resin 8280 cured with EPI-CURE 3223 ..................................... 149

Figure 69. Comparision of experimentally observed CTE values of composites
with CTE values predicted by various mathemal models. Reinforcement- BS-50.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223 ..................................... 150

XV

 

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1. INTRODUCTION

1.1 Necessity of polymer matrix composites with low coefficient of

thermal expansion

Use of particulate ﬁllers is a potential approach to develop isotropic low
coefﬁcient of thermal expansion (CTE) polymeric matrix composite materials for
electronic circuit boards. One of the most common materials used for making the
laminates for electronic circuit boards is the epoxy resin reinforced with E-glass ﬁbers.
The CTE of circuit boards ranges from 12 to 16 ppm/°C in the in-plane direction of ﬁbers
(x, y direction) while it takes value ranging from 40 to 120 ppml°C in the out-of-plane
direction (2 direction). The CTE of leadless ceramic chip carriers that are mounted on the
circuit board have the CTE values ranging from 5.9 to 7.4 ppm/°C. Table 1 lists CTE of
various materials commonly used in printed circuit boards indicating the wide differences
in the CTE of those materials. The in-plane CTE mismatch due to differences in CTE of
various components of circuit board, affects the solder joint reliability. The out-of-plane
CTE of circuit boards has much higher value than the CTE of copper plated through
holes. This can lead to failure of copper in plated through holes [1]. Thus it is necessary
to minimize the out-of-plane CTE of the circuit board materials along with the reduction
of the in-plane CTE. Low CTE composites have wide range of applications, especially in
electronic industry. One approach is to develop low CTE particulate reinforced epoxy
resin matrix composites, which will have isotropic low-CTE value. Silica particles are
often incorporated in epoxies for encapsulating integrated circuit devices [2]. In addition,
it has been pointed out in the literature that negative CTE particulate ﬁllers are more

effective as additives to lower CTE of epoxy resin matrix composites at higher volume

Table l. CTE of materials commonly used in printed circuit boards [3].

 

 

 

 

 

 

in prli’rlitetflrgrl'sct‘riistegoard CTE (ppm/°C)
COPper 17
Lead-tin solder (6OSn-40Pb) 24
Silicon 2. 5
E glass ﬁbers 50
Epoxy resin 81-117

 

 

 

 

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Pa
CO

Dr

fractions of particulate ﬁllers [4,5]. Table 2 compares the values of CTE achieved in
composites by adding maximum quantities of different types of particulate ﬁllers into the
epoxy resin matrices. The data listed are for epoxy resin DGEBA cured with MPDA
(Meta phenylene diamine). DGEBA stands for Di glycidyl ether of bispenol A. DGEBA is

epoxy resin monomer equivalent to the epoxy resin monomers used in this study.

1.2 Particulate reinforced polymer matrix composites

Particulate reinforced polymer matrix composites is the class of materials, which
consists of the particulate ﬁller phase embedded in the polymer matrix phase. This
encompasses a wide range of therrnosets and thermoplastic polymers along with organic
and inorganic particulate ﬁllers.

Epoxy resins are the most common therrnosets used as matrices for ﬁber
reinforced composites for electronic applications. The epoxy resins are preferred in the
electrical and electronics applications due to various advantages like low cost, optimum
combination of various properties, superior adhesion, permeability, purity, superior
corrosion resistance and stress resistance. In addition, the properties of epoxy resins can
be altered by using various combinations of epoxy resin types, curing agents, modiﬁers
and particulate ﬁllers [6]. Introducing particulate ﬁllers in the epoxy resin matrix
modiﬁes its rnicrostructure and the state of residual stresses, and thus alters various
resultant properties. The properties of epoxy resins that are affected by adding the
particulate ﬁllers include stiffness, strength, toughness, hardness, density, CTE,
conductivity, machinability, electrical properties, flame retardation, rheology, adhesive

properties and the cost [6,7,8]. The particulate ﬁllers act as heat sink for the exothermic

Table 2. Maximum quantity of particulate ﬁllers added to DGEBA epoxy resin cured
with MPDA and CTE values achieved in the resultant composites [9].

 

 

 

 

 

 

 

 

 

 

Particulate filler Maximum particulate Value of CTE achieved
filler by weight % in composites (ppm/°C)
Silica 68.6 26.1
Mica 37 37.8
Aluminum powder 63.6 33.5
Zirconium silicate 68 28
Lithium silicate 72.3 19
Iron powder 72.3 33
Copper powder 72.3 36
Hydrated alumina 30.7 41
Alumina 56.5 30

 

 

 

 

 

Note- CTE of DGEBA can vary between 81-117 ppm/°C

 

 

 

i:

(J

curing reactions of epoxy resins, decrease shrinkage during curing and add opacity or

color to the resulting composites [10]. Particulate ﬁllers can be the cheapest

reinforcements that can produce isotropic properties. They may be reactive or inert.

Various types of particulate ﬁllers and their roles in enhancing the properties of epoxy

resin are tabulated in Table 3 [11,12,13]. The particulate ﬁllers do not improve

mechanical properties as much as do the continuous ﬁbers, but they allow the processing
of epoxies in paste form, which is a very important criterion in electrical and electronics

applications. [14,15].

Properties of the particulate reinforced composites are governed by following
factors [6,8,15,16,17] -:

1. Type of epoxy resin -: The most signiﬁcant properties of epoxy resin that affect
processing and the resultant properties of composite include viscosity, chemical
composition, density, elastic modulus, and tensile strength,

2. Type of curing agent,

3. Type of particulate ﬁller -: The most signiﬁcant properties of the particulate ﬁllers that
affect processing and resultant properties of composite are chemical composition, the
internal geometry i.e. shape and size (average size as well as distribution), nature of
surface, adsorption characteristics, density and elastic modulus,

4. Volume fraction of the particulate ﬁllers,

5. Processing parameters -: Important parameters are amount of shear force applied

during processing, interaction between matrix and particulate ﬁllers, and type of
dispersion achieved,

6. Presence of porosities in composite,

Table 3. Effect of particulate ﬁllers on the properties

of the epoxy resin matrix composites [11,12,13].

 

 

 

Particulate ﬁller Property improved
Silica, calcium carbonate Cost
Glass micro balloons Density
Impact strength

 

Fibrous glass, chopped nylon and glass

 

Asbestos, ﬂuorocarbon micro ﬁbers
and other ﬁbers

Tensile strength

 

Aluminum and copper powder,
calcium carbonate and silicate

Flexural strength
and machinability

 

Carbon black, asbestos

Heat resistance

 

Silica, metal powders

Dimensional stability

 

Metallic powders and ﬁbers,
silica, alumina and beryllium oxide

Thermal conductivity

 

Lithium aluminum silicate

CTE

 

 

Inorganic particulate ﬁllers

Fire retardance
and burning rate

 

Asbestos, powdered coal

Chemical resistance

 

Mica, silica, powdered coal

Moisture resistance

 

Silver or aluminum powder, carbon, graphite

Electrical conductivity

 

Mica, asbestos, silica, powdered glass

Electrical resistivity

 

Graphite, mica, molybdenum disulphide

Lubricity

 

Alumina, ﬂint powder, carborundum, silica

Abrasion resistance

 

Barium titanate

Dielectric constant

 

 

 

7. Presence of additives, modiﬁers and diluents if any, and

8. Type of coupling agents used if any.

1.3 CTE

Most of the solid materials expand upon heating and contract on cooling. The
change in length of solid material with temperature is expressed by-
All 1 = orAT ,
where,
A1 = the change in length of the solid material,
a = linear CTE of the solid material, expressed in parts per million i.e. ppm/°C or
um/rn/°C, and
AT = the change in temperature.
We can also write,
AV/ V= 'yAT,
where,
AV = the change in volume of solid material, and
y = cubical or volumetric CTE of the isotropic solid material.
For isotropic materials,
753a,
There is an universal relationship between linear and cubical CT E for all solid materials
and it is expressed as,
'YCubical = 06x + Oty + 014,.

where,

Youbica] = Cubical CTE of the anisotropic solid material, and

(1,, Qty , 0t, are linear CTE of the solid material in X,Y and Z directions respectively [18].
CTE of the composites is governed by properties of epoxy resin, curing agent,

properties of particulate ﬁllers, additives and processing parameters [19].

1.4 Stresses in particulate reinforced epoxy resin matrix composites

There are two types of shrinkages in the epoxy resins. One is due to the reaction
and compact rearrangements of the molecules that occurs during curing process. During
curing of epoxy resin, the secondary bonds or van der Waals bonds between monomer
molecules or monomer chains, with separation distance of about 3-5 A°, get converted
into primary covalent bonds with separation distances of about 1.5 A°. This causes
volume reduction or shrinkage [20, 21]. The other shrinkage is thermal shrinkage caused
by cooling of the epoxy resin from high processing temperature to room temperature.
Epoxy resins show 3-6.5% shrinkage during polymerization. Due to the shrinkage that
occurs in polymerization of epoxy resins, stresses are developed, and they can be large
enough to crack the component or the bond (when epoxy resin is used as adhesive). The
maximum value of the stresses that develop depends upon the curing temperature, curing
agent, type of catalyst and most importantly degree of cure. Values as high as 1500 Psi
(0.01035 GPa) for this stress, at room temperature are reported in the literature [22].

There are two sources of the stresses in the particulate reinforced epoxy resin
matrix composites. First source is stresses due to epoxy resin shrinkage. Overall
shrinkage of epoxy resin reduces as the quantity of particulate ﬁllers that are added into

epoxy resin increases [23, 24]. Though the overall CTE and shrinkage of epoxy resin

reduce on addition of particulate ﬁllers, the composite is not stress free. The epoxy resin
shrinkage during curing process in presence of particulate ﬁllers sets up stresses at the
interface of epoxy resin and particulate ﬁller. So with addition of particulate ﬁllers, the
stresses are merely transferred to microscopic level arising at the interface of the
particulate ﬁllers and the epoxy resin matrix [25,26].

Second source of stresses in particulate reinforced composites is thermal stresses
caused by high difference in the CTE of the epoxy resin matrix and the particulate ﬁllers.
Contraction of composites occurs during manufacture. Heating and the associated
expansion of composites, takes place in service. In composites, particulate ﬁllers and the
epoxy resin are in intimate contact. Therefore there are constraints at the interface for
expansion or contraction of epoxy resin as well as that of the particulate ﬁllers. This
creates geometric mismatch or shear stresses at the interface of the particulate ﬁllers and
the epoxy resin matrix. This mismatch created at the interface will be positive or negative
depending upon the CTE of the particulate ﬁllers and that of the matrix [27]. Warming
the composite reverses the direction of the stresses. The stresses developed in composites
affect the mechanical behavior and thermal characteristics of the composites to a great
extent [28]. The magnitude of thermal stresses developed in composites depends upon
CTE of the matrix, CTE of the particulate ﬁllers, volumetric concentration of the
particulate ﬁllers, bulk modulii and elastic modulii of epoxy resins as well as particulate

ﬁllers, service temperatures, and glass transition temperature of the epoxy resin [29].

1.5 Effect of volume of particulate fillers on CTE of composites

Particulate ﬁllers affect the properties of composites by partially occupying the
volume of epoxy resin by rigid, immobile masses. This leads to disturbance of the matrix.
The extent to which the state of stress of matrix is affected varies based upon the volume
fraction of particulate ﬁllers [30]. At low volume fractions of particulate ﬁllers, the epoxy
resin dominates the continuum and hence governs the CTE of the composite, and effect
of CTE of particulate ﬁllers on the CTE of the composites is negligible [31]. At low
volume fractions of particulate ﬁllers, the inter particle distances are very large. So the
interaction between the particles is negligible [32]. Only at low volume fractions of
particulate ﬁllers, the interface of the particulates particulate ﬁllers and epoxy resin
matrix is in the state of compression due to polymerization shrinkage [33]. When the
composite is loaded with high volume fraction of particulate ﬁllers, the resin is conﬁned
to small areas between the particles. The interparticle distances are small. When curing
process begins, the particles come even closer due to contraction of the matrix. The
particles get packed closely, and several particles push against each other. Close packing
of particles forces epoxy resin in interparticle spaces to complete the curing process at
constant volume, setting up tensile stresses at the interface of epoxy resin and particles as
the curing progresses to completion [21]. Decrease in interparticle spacing caused by
increase in volume fraction of particulate ﬁllers results in collisions, turbulence and
rotation of the particulate ﬁllers that produces a complex stress pattern in the matrix [32].
Cumulative effect is that, at higher volume fractions of particulate ﬁllers, CTE of the
particulate ﬁllers has pronounced effect on the CTE of the composite [34]. At high

volume fractions of particulate ﬁllers, the interparticle spacing is such that, if there is

10

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Rain
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1.6

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depei
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the n
the p

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Penal

perfect adhesion at the particle-epoxy resin interface, the interface and epoxy resin are in
a state of hydrostatic tension. If the interfacial adhesion is poor or insufﬁcient, void
formation and separation can occur due to epoxy resin shrinkage away from the
particulate ﬁller surface. The extent to which the porosities form is a function of adhesive

strength, type of particulate ﬁller and their volume fraction [13,32].

1.6 Effect of adhesion of particulate fillers with the matrix on CTE of

composites

Effectiveness of particulate ﬁllers as reinforcing agents in epoxy resin matrix
depends upon the extent of adhesion between the particulate ﬁllers and the epoxy resin
matrix. If thermal stresses can be transferred across the interface of particulate ﬁllers and
the epoxy resin matrix, then overall CTE of composite reduces compared to the CTE of
the matrix. Hence CTE value of composites can reveal the nature of adhesion between
the particulate ﬁllers and the epoxy resin matrix. Adhesion is created by the [pressure
increase arising from the shrinkage during polymerization of epoxy resins [35,36]

It is difﬁcult to have perfect adhesion between the particulate ﬁllers and epoxy
resin matrix by any binding media due to the large differences between the CTE of the
two materials. If one of the materials is thin enough to permit relief of stresses by
bending or warping, then the perfect adhesion may be achieved. Otherwise stable and
perfect adhesion is possible only if CTE of the two phases are matched. The adhesion
between two phases of a composite depends upon following factors -

1. The dimensions of the phases,
2. The modulii of elasticity of the phases,

3. Changes in dimensions with changes in moisture content for all phases,

1]

4. Temperature changes encountered in the service,
5. Value of CTE of the phases, and
6. The pressure of application during the manufacture.

There can be two extremes of adhesion, either absent of any adhesion between the
particulate ﬁllers and the epoxy resin matrix or there would be perfect adhesion between
the epoxy resin and the particulate ﬁllers. Both these are very uncommon situations and
the most common situation is intermediate between the two. But discussing these two
extremes can be useful to understand the effect of intermediate adhesion level on the CTE

of composites.

1.6.1 Non-adherent particulate ﬁllers

If there is no adhesion between the particulate ﬁllers and the epoxy resin matrix
in the composite, then there will be no residual stresses across the interface. There will be
no thermal stresses because on application of heat, the matrix will expand away from the
particulate ﬁllers without any resistance from particles to the expansion of the matrix. If
there is no adhesion, no stress will be transferred across the interface of particulate ﬁllers
and epoxy resin. The particulate ﬁllers will share no stresses. Hence the CTE of the
composite will be independent of the volume of particulate ﬁllers. Then the CTE of the

composite will be same as that of the epoxy resin matrix [27,37].

1.6.2 Perfectly adherent particulate fillers

If perfect adhesion exists between the particulate ﬁllers and the matrix, thermal

stresses will be completely transferred across the interface of particulate ﬁllers and epoxy

12

resin. Hence perfect adhesion of the particulate ﬁllers and the epoxy resin matrix will
resist thermal stresses and the CTE of the composite will be substantially lowered with
addition of even small quantity of particulate ﬁllers with low CTE. The most important
factor to achieve perfect adhesion is that the particulate ﬁllers should be well dispersed in

the epoxy resin matrix without formation of weak aggregates [37].

1.6.3 Adhesion mechanisms

The degree of adhesion between the particulate ﬁllers and epoxy resin matrix
depends upon the interaction between them. There can be three types of interactions
between the particulate ﬁller and the epoxy resin, namely mechanical, physical and
chemical. The surface chemistry of the particulate ﬁllers and the chemistry of the epoxy
resin decide the type of interaction between them. Mechanical interlocking of the
particulate surfaces with the matrix can result in a strong adhesion between the
particulate ﬁllers and the matrix. In the absence of chemical interaction between the
particulate ﬁllers and the matrix, the mechanical adhesion plays a key role. The
particulate ﬁllers can remain physically attached to the matrix just due to compressive
forces without interlocking. When the particulate ﬁllers are chemically reactive with the
matrix, a chemical bond may be created. Chemical bonds can also be created by use of
coupling agent between the matrix and the particulate ﬁllers. The particulate ﬁllers that
react chemically and become integral part of the cured system are the most effective
reinforcements. Few examples of composites that have chemical bond between the

particulate ﬁllers and the polymer matrix are carbon black ﬁller in rubber matrix, Mica in

13

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matt

1.7

mat

unia
the r.

thea

effee
panic
Stress
the st
is as
COncc

telati

Where

 

polymeric matrix. Talc can also pickup water during storage and react with the polymer

matrix [35,38,39,40].

1.7 Effect of modulii of elasticity of particulate ﬁllers and epoxy resin
matrix on composite properties

Modulus of elasticity is deﬁned as the ratio of stress to strain for a material under
uniaxial tension or compression. The modulus of elasticity is a measure of the stiffness of
the material. The greater the elastic modulus, the smaller the elastic strain resulting from
the application of given stress.

The modulii of elasticity of the matrix and particulate ﬁllers indicate the
effectiveness of stress-strain transfer across the interface of the epoxy resin matrix and
particulate ﬁllers. The materials with lower elastic modulii deform more easily under the
stresses [41]. If the modulus of elasticity of any one of the two adjacent phases is zero,
the stresses set up between then will be zero [42]. If the modulus of the particulate ﬁllers
is as high as 100 times more than that of the matrix, then it will cause high stress
concentration at the interface, which can result in breaking of any interfacial bond. A
relationship applicable to many solids and composites is [43]-

Eur2 = 150 dynes/cm2° K2,
where,
E: Elastic modulus of the material.

Above equation supports the close relation between elastic modulus and CTE.

14

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31(

wi

1.!

C0.

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9, De

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1.8 Effect of arrangement of particulate ﬁllers in the composites

The arrangement of particulate ﬁllers in the epoxy resin matrix plays important
role in affecting the properties of the composite. For spherical particles, the higher the
packing factor, lower will be the hindrance for the shrinkage of the epoxy resin and so the
magnitude of stresses developed in composites is lower. It is possible to compare various
arrangements of the particles in the epoxy resin matrix with that of the arrangements of
atoms in crystal systems like HCP, BCC, and FCC. Similarly the shape of the particles

will affect the magnitude of the stresses that can develop [21].

1.9 Reduction of stresses developed in particulate reinforced
composites

From the ongoing discussion, it can be summarized that, the stresses set up in the
particulate ﬁlled composite are function of the following variables:
1. Strength of epoxy resin,
2. Volume fraction of the particulate ﬁllers,
3. Geometrical arrangement of particles,
4. Ratio of elastic modulii of epoxy resin and that of particulate ﬁller,
5. Poisson’s ratio of the epoxy resin as well as particulate ﬁllers,
6. Shape of particulates,
7. Adhesion of particulate ﬁllers to the epoxy resin matrix,
8. Percentage of inherent shrinkage of the epoxy resin,
9. Degree of polymerization in cured epoxy resin, and
10.Volume fraction of porosities.

Following are alternatives for reducing the residual stresses in composites -:

15

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1.11

mecl
prep,-
Panic
are de
1- Orj.

1. The lower the cure temperature, lesser will be the stresses that develop in composites.
Hence epoxy resins with low glass transition temperatures are better as matrix for
composites [44]. The more ﬂexible epoxy resins will develop lesser stresses. Matrix

resin can be selected accordingly.

2. Stresses developed in composites can be reduced by matching the CTE of the two
phases of composites. (This is contradictory to the aim of this project.)

3. Addition of 1-3 % photosensitive ﬁllers like cinnamic acid resins and cinnamal ketones
in epoxy matrices, followed by exposure to strong ultraviolet rays relieves the internal
stresses in cured epoxy resin system [45].

4. Annealing the composite slightly above its glass transition temperature can minimize
thermal stresses. The cooling after the annealing process generates minimal thermal
stresses [46].

5. It is possible to add carefully measured quantities of chemical agents that expand

during curing of epoxy resin to nullify the effect of polymerization shrinkage [21].

1.10 Effect of shape and size of the particulate fillch

The shape and size of the particulate ﬁllers affect the powder ﬂow, viscosity,
mechanical properties (elastic modulus, tensile strength, impact strength etc.), thermal
properties and optical properties of the composites [47]. The effective surface area of the
particulate ﬁllers is decided by their shape and size. Sizes and shapes of particulate ﬁllers
are decided by the following factors -

1. Origin of the particulate ﬁllers,

2. Processing method, and

16

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dis

ePC
Sha
efbe
and

dese

3. Chemical composition

Common shapes of particulate ﬁllers are spherical, platy, acicular, blocky and
irregular as indicated in Figure 1 [47]. Particulate ﬁllers can occur in various forms in the
composite, such as single or primary particles, agglomerates (weakly bonded particle
collections) or aggregates (strongly bonded particle collections) or fragments [48].
Aggregates need maximum amount of energy for the breakdown and dispersion in the
epoxy resin matrix. Fragments require the least amount of dispersion energy for
dispersion in the matrix.

The shape of the particulate ﬁllers is important because it decides the amount of
epoxy resin that can be shielded and thus the amount of epoxy resin that need not deform.
Shape of particulate ﬁllers also increases the effective ﬁller volume. Figure 2 shows the
effect of particle shape in shielding the polymer. The aspect ratio is the ratio of the largest
and the smallest dimensions of the particle. Aspect ratio is important quantity for
describing the shape of particulate ﬁller.

The ﬁneness of the particles, the tendency to form ﬂocks after addition of
particles in liquid epoxy resin, and strength of the bond between the particulates in
aggregates, affects the dispersion process of particulate ﬁllers in the matrix during
synthesis of composites. Intensive mechanical breakdown, ultrasonic mixing or use of
dispersants, are the various alternatives to obtain better dispersion of the particulate ﬁllers
in the epoxy resin matrix [48]. Any powder sample consists of a range of particle sizes.

Hence the average particle size as well as the distribution play important role in affecting

17

O A

, Acicular
Spherrcal Irregular

 

Plate-like

Porous aggregates

Figure 1. Typical shapes of particulate ﬁllers [49].

18

  
  

Effective volume
of particulate ﬁller

occluded epoxy resin,
which is shielded from
deformation

 

idealized
structure of
particle

Figure 2. Schematic representation of effect of particle shape and occluded polymer
[49].

19

the composite properties [50]. Particle size decides the interparticle spacing. The
interparticle spacing affects the composite properties by two ways. When the interparticle
distances become small, the epoxy resin chains have less freedom of movement
compared to the freedom they have in the bulk polymer. The surface of the particles may
also participate in the growth of the polymers [17]. But it is difﬁcult to measure these
effects directly. Secondly, if there are shells of the epoxy resin adsorbed on the particles,
then at some limiting distances, these shells will overlap as indicated in Figure 3 and

affect the properties of the composite.

1.11 Effect of wettability of particulate ﬁllers

The nature of interaction between the epoxy resin and particulate ﬁller depends
signiﬁcantly upon the wettability of the particulate ﬁllers. Wettability can also affect the
nature of adhesion between the particulate ﬁllers and the matrix [51]. Surface energy is
the free energy increase in a system on creating a unit area of new surface at constant
temperature, pressure and composition. The surface energies of particulate ﬁllers and
polymers control the wettability and hence the compatibility between the two. Table 4
lists the surface energies of different particulate ﬁllers. Wettability directly affects the
dispersion process resulting into evenly distributed particulate ﬁllers. Particulate ﬁllers
increase the viscosity of the epoxy resin systems. In most of the cases, equal weights of
ﬁner particulates increase the viscosity more than the coarse particulates due to the
presence of higher wettable surface area [52]. High wettability of the particulate ﬁllers
can give rise to low viscosity of epoxy resin and particulate ﬁller mixture, which

facilitates the processing of the composite. Thus good wettability in turn allows

20

‘— shell of adsorbed
. epoxy resin

particulate ﬁller

‘

O O particulate ﬁller

overlapping shells
of adsorbed epoxy
resin matrix

 

Figure 3. Effect of reducing inter-particle distance [17].
I. Inter-particle are more at low volume fractions of particulate ﬁllers.
II. Inter-particle distances are less at high volume fraction of particulate ﬁllers,

where the overlapping of shells of adsorbed epoxy resin matrix takes place.

21

Table 4. Surface energies for various particulate ﬁllers and plastics [39].

 

 

 

 

 

 

 

 

 

 

 

 

Material Surface energy (mJ/mz)
Diamond 10000
Glass 1200
Titanium dioxide 650
Kaolin 500-600
Calcium carbonate 65-70
Stearate coated calcium carbonate 28
Talc 65—70
Polymers 15-60
Polypropylene 3 1

 

22

 

maximum loading of particulate ﬁllers. Higher the wettability, the chances of void
formation in composite are less. Good processability, higher loading of particulate ﬁllers

and fewer porosities render better mechanical properties to the composites [53,54].

1.12 Effect of chemistry of the particulate fillers

If there is hydroxyl group present on the surface of the particulate ﬁllers, it may
affect the curing reaction. If the particulate ﬁller is sensitive to pH change, then there
may be deﬂocculation and viscosity increase under alkaline conditions such as in
presence of amine curing agents [52]. If the particulate ﬁllers are reactive, then the
stoichiometric ratio of curing agent needs to be adjusted depending upon the effect of

particulate ﬁllers on the rate of curing reaction [55].

1.13 Viscosity of the mixture of particulate fillers and epoxy resins

Viscosity is an important parameter in composite manufacturing process. The
particulate ﬁllers increase viscosity of the epoxy resins the least while the ﬁbrous
particulate ﬁllers increase it the most. Some particulate ﬁllers are also sensitive to the
shear rate in modifying the viscosity [56].

Low viscosity of the epoxy resin can facilitate the dispersion of the ﬁller in the
system [57]. The viscosity can be reduced by heating the resin or adding diluents (like
styrene, butyl glycidyl ether, phenyl glycidyl ether and cycloaliphatic resins). Since, the
use of diluents affects electrical properties of resulting composites, moderate heating of
the epoxy resin is a better way of reducing viscosity up to a limited extent [9]. The extent

to which resin viscosity is affected by ﬁller addition is a function of particle size and

23

shape, chemical reactions, wettability etc.Viscosity is such an important processing
criteria that certain particulate ﬁllers are added only up to certain level of viscosity,
instead of adding to certain volume percent. [58,59]. Table 5 shows effect of various
particulate ﬁllers on the viscosity of DGEBA cured with primary amine. The type and
volume of the particulate ﬁllers signiﬁcantly affect the viscosity of the mixture of

particulate ﬁllers and the epoxy resin.

1.14 Settling of particulate ﬁllers in epoxy resin matrix

Centrifuging of the particulate ﬁller- epoxy resin mixture is often necessary prior
to curing in order to get rid of air bubbles. The particulate ﬁllers can show settling
tendency during centrifuging of the mixture. Settling of the particulate ﬁllers depends on
the speciﬁc gravity of the particulate ﬁllers, the size of particulate ﬁllers, speciﬁc gravity
of the epoxy resin and the viscosity of mixture of epoxy resin and particulate ﬁllers. The
ﬁner the particulate ﬁller, lesser the tendency to settle. To achieve balance, sometimes
some ﬁne particulate ﬁllers are added to the coarse particulate ﬁllers to reduce the
settling [60]. Bulk density of the particulate ﬁllers is another important property that

affects settling, mixing and distribution of the particulate ﬁllers in the epoxy resin matrix.

1.15 Curing of the composites

Epoxy resins are thermosetting polymers, which are converted into permanent
shape by irreversible, exothermic chemical reaction called curing. They can be softened,

but cannot be reformed [16]. Curing of epoxy resins is a highly exothermic reaction. The

24

Table 5. Effect of particulate ﬁllers on viscosity of DGEBA,

cured with primary amine [24].

 

 

 

 

 

 

Viscosity at 25° C (Poise)
Particulate filler
Volume %
of particulate
fillers 20% 35 % 50 % 65%
No ﬁller ll 11 11 11
Quartz 21 37.5 131.5 1000
Mica 21 48 545 90‘
available

 

 

 

 

 

25

 

epoxy resins have a low thermal conductivity and moderate heat capacity. Hence, in the
polymerization reaction, the heat, if liberated during a short course of time, results in a

substantial increase in temperature of the reactive mixture. This temperature increase is
called as curing exotherm and measured using thermocouple. The order of curing
reaction is independent of the type of particulate ﬁller and ﬁller content. But the rate
constant of reaction or the monomer consumption decreases depending upon the type of
particulate ﬁller and content of particulate ﬁller. Particulate ﬁllers reduce the effective
quantity of the epoxy resin in a given volume, thus reducing heat generated per unit
volume of the mixture or in other words they lower peak exothermic temperatures
encountered during curing. In addition, particulate ﬁllers, due to higher thermal
conductivity, act as heat sink and aid to reduce the maximum temperature reached during
the curing reaction. Here the particulate ﬁllers act as diluents. Table 6 lists the reduction
in peak exotherm temperature of the epoxy resins due to addition of particulate ﬁllers.
The particulate ﬁllers also alter the curing pattern of the epoxy resins. The important
ﬁller properties that affect curing process are the density and the speciﬁc heat, and
presence of any speciﬁc chemical reaction between the particulate ﬁllers and the epoxy
resin [61,57]. If the curing agents are adsorbed on the particulate ﬁllers, then it is possible
to get higher cross-link density in the vicinity of the particulates surfaces, but this effect

is difﬁcult to measure [62].

1.16 Density of composites

Density of composites is an important property, that can be measured easily.

Density indicates the packing of constituent phases [19]. If the value of density of

26

Table 6. Effect of particulate ﬁllers on peak exothermic temperature rise

in DGEBA cured with primary amine [24].

 

Peak exothermic temperature rise (°C)

 

 

 

 

 

Particulate ﬁlle
Volume % of
particulate ﬁllers 20% 35% 50% 65%
No ﬁller 223 223 223 223
Quartz 178 113 51 34
. Not
Mrca 159 111 51 available

 

 

 

 

 

27

 

composite is less than expected, it indicates presence of air bubbles and formation of
aggregates in the composites. Density value can also indicate the level of stresses in the
composites.Higher than theoretically expected value of density of composite indicates
error in the assumed values of densities. If the expected density and the observed density
of the composite are exactly the same then one can assume that the packing of particulate

ﬁllers is perfect and the distribution of the particulate ﬁllers is uniform [27].

1.16. 1 Porosity in composites

Pores of various shapes and sizes can form in the particulate reinforced
composites depending upon the nature of the epoxy resin, nature of the particulate ﬁllers,
the size of the particulate ﬁllers, volume fraction of the ﬁller, strength of adhesive bond
between the particulate ﬁllers and the epoxy matrix, and most importantly the processing
method followed. The pores act as the inclusions with zero stiffness at room
temperatures. At high temperatures, pores may exert pressure on the composite. Pores
should not affect the CTE of the composite, but they can reduce the strength and elastic
modulus of composites sharply. If the porosity is present on the composite surface, they
can act as stress raisers. If porosities are present just below the surface of test specimen,
they might affect CTE measurement. Volume of porosities can be estimated by observing
polished surfaces or by Archimedes principle of speciﬁc gravity measurement or by using
ultrasonic scanning. For calculating volume of porosities from speciﬁc gravity method,
the exact values of densities and volume fractions of particulate ﬁllers as well as those of
epoxy resin matrix must be known. Agglomerates and aggregates contain porosities so

their apparent volume is much greater than the real volume [63].

28

1.17 Role of silane coupling agents

Surface chemistry of particulate ﬁllers is important for adhesion of the particulate
ﬁllers with the matrix. Better the adhesion, stronger the interface and better the
mechanical properties of the composite [64]. One way to achieve best adhesion of
particulate reinforcements with the polymer matrix is by use of coupling agents. The
coupling agents are bifunctional additives that form strong covalent bonds to the ﬁllers
and then bond to a polymer by variety of mechanisms. The most common coupling
agents are organosilanes and organotitanates. Silane coupling agents are the
organosilanes that are commonly used for silica type ﬁllers. Silane coupling agents have

two different types of reactive functional groups as indicated below: -

OR
XmSl£OFl
\OR

a
where,

‘X’ represents the functional group that reacts with organic materials like synthetic resins
and may be selected from the following types of functional groups: vinyl, epoxy, amino,
methacryl, acryl, isocyanato, mercapto. OR represents the alkoxy functional group that
may be one of the following: methoxy , ethoxy , acetoxy . These alkoxy groups react with
hydroxyl group or water commonly by hydrolysis mechanism. Water for this hydrolysis
is deliberately added water or water absorbed from environment. The organometallic

hydroxide that forms then, condenses with the hydroxyl group present on the surface of

29

inorganic ﬁllers like glass, metals, silica etc. The particulate ﬁllers treated with coupling
agents are then mixed with polymer matrix.

Silane coupling agents are commonly used in ﬁber-reinforced composites to
achieve better adhesion between ﬁbers and the matrix. Silane coupling agents are
commonly used in particulate ﬁlled composites for better adhesion between inorganic
particulate ﬁllers and epoxy resin matrix that can result in better mechanical strength and
improved chemical resistance in the composite. Treating surfaces of particulate ﬁllers
with silane coupling agents can make interfaces of composites so strong that the failure
mechanisms of composites may change from interfacial failures to transparticle failures
or to matrix failures. Curing process of the epoxies can be affected strongly by the glass
and mineral particulate ﬁllers, but can be corrected by longer curing times. It is observed
by other researchers that the silane treatments of particulate ﬁllers can overcome cure
inhibition in the epoxies. Particulate ﬁllers treated with silane coupling agents show
improved wettability and are much easier to disperse in epoxy resin matrix because the
water layer on the surface of particulate ﬁllers is replaced by organofunctional silanes
after treating with silane coupling agent. lrnproved wettablity of particulate ﬁllers can
avoid aggregate formation and will result in uniform distribution of particulate ﬁllers in
epoxy resin matrix. Due to uniform distribution of more wettable particulate ﬁllers,
density of composite will be closer to the expected ideal density. Treating particulate
ﬁllers with silane coupling agents also affects the viscosity and ﬂow of the mixture of
epoxy resin and particulate ﬁllers [65]. Thus, treating the particulate ﬁllers with silane

coupling agents can improve dispersion of particulate ﬁllers, adhesion between phases,

3O

curing characteristics, tensile strength, ﬂexural strength and electrical properties of

composites [66].

1.18 Fracture study of particulate reinforced composites

Fracture study of particulate reinforced composites reveals the nature of adhesion
at the interfaces of reinforcements and the epoxy resin matrix. Epoxies are brittle due to
their highly cross-linked structure. But at the same time they have highest fracture energy
amongst cross-linked glassy polymers [67]. Pure epoxy resins have less fracture
toughness due to shrinkage stresses. Toughness can be increased by addition of
particulate ﬁllers without marginal decrease in tensile or ﬂexural strength [68]. When the
failure of particulate reinforced composites begins, with the applied load the crack travels
between the particulate ﬁllers, deﬂects into matrix and gets arrested near the particulate
ﬁllers. Thus particulate ﬁllers increase fracture toughness of epoxy resin matrix by
arresting the cracks more often, which results in higher fracture toughness in the
composites. The important considerations are the size of particulate ﬁllers, the volume
fraction of particulate ﬁllers and the adhesion between the particulate ﬁllers and epoxy
resin matrix [69]. Crack propagation in the composite can take place by trans-particle
fracture, by interfacial failure or by failure of the matrix. The trans-particle fracture can
take place if the particles are weaker compared to the interface. If the particulate ﬁllers
are strong enough, then the crack propagates around the particle [70]. One of the
mechanisms for explaining the increase in toughness is crack pinning mechanism. This
mechanism assumes that the rigid particulate ﬁllers act as obstacles for crack propagation

in epoxy matrix and force the cracks to bow. The increase in fracture energy is given by-

31

 

Yeomposire = Yrcsin + 1720.
where,
Yeomposim = fracture energy of the composite,
7min = fracture energy of resin,
2c: interparticle distance, and
T: 2r/3('ymin), where 2r is the particle diameter.

Another mechanism to explain increase in the toughness of particulate ﬁlled
composites is by decohesion of the particles from the matrix in front of the crack causing
blunting of crack tip [70]. Fracture toughness values of composites varies with the
particle size. Detailed discussion of the effect of particle size on the fracture toughness of

particulate reinforced epoxy resin matrix composites are available literature [71].

1.19 Weight fraction to volume fraction conversion of particulate filler
addition

It is convenient to use weight fraction of particulate ﬁller addition for
manufacturing the composites, but most of the composite properties are correlated with
the volume fractions of the particulate ﬁllers. Also most of the mathematical models
about composite properties are based on the volume fractions of the particulate ﬁllers.
The relation for converting the weight fraction to volume fractions of the phases is as

follows [72]: -
Va = We /Pa]/[wa [pa +W13 I 913], and

Wu =[Vapa]/ [Vapor + VBPB],

where,

32

 

Va = volume fraction of (1,
Wu: weight fraction of or,

Pa = density of or,

VB: volume fraction of [3,

W3 = weight fraction of B, and

pg: density of B [72].

1.20 Mathematical models to predict CTE of particulate reinforced

composites

Characterization of particulate reinforced composites is complex. For ﬁber
reinforced composites, slab model is used most often to predict the properties of the
composites. Slab model is used to analyze the behavior of aligned, long ﬁber reinforced
composites based on the reasoning that the composite can be treated as if it were
composed of parallel slabs of two constituent bonded together with relative thickness in
proportion to the volume fractions of matrix and ﬁber. In the particulate reinforced
composites, since changes occur from point to point, it is not possible to apply the slab
model for predicting the composite behavior. Several other mathematic models are
developed to predict the CTE of the composites. These models allow the design of
composite with particular CTE by selecting the CTE of the particulate ﬁllers and the
volume fraction of particulate ﬁllers. These models make various assumptions about
shape and size of the particles to simplify the treatment. They predict the CTE of the
composites based on the elastic behavior of the constituents involved. Hence these

predictions may not be valid at very high temperatures when the amount of internal

33

 

stresses becomes very high, and the matrix may undergo creep or plastic ﬂow, in turn
altering the dimensions of the composite [73].

The most basic model to predict CTE of the composites is called rule of mixture.
Rule of mixtures is the ﬁrst order approximation for calculating CTE of the composites
[74]. In a well-annealed composite, the rule of mixture is applicable unless the particulate
ﬁllers are strongly adhered to the matrix [4]. For a two-phase system, rule of mixture for
predicting CTE of composites is described as-

“composite = armuixvmatrix + aﬁttchﬁttcr ,
where,
damn: = CTE of the composite,
(Xmmix = CTE of the matrix,
me-x = volume fraction of the matrix,
(Inner = CTE of the particulate ﬁllers, and
Van“: volume fraction of the particulate ﬁllers.

Most of the experiments show, that the CTE of composites do not simply follow
the rule of mixtures. Stresses developed in the composites, variations in processing
parameters and variations in the degree of adhesion produced between the particulate
ﬁllers and epoxy matrix are some of the reasons due to which most of the composites do
not follow rule of mixtures. Hence various other models for predicting the CTE of the
particulate reinforced composites are developed and are discussed in literature [27,36,75].
The two commonly used mathematical models, namely Turner’s model and Kemer’s
model, were selected for comparing experimental CTE values of composites in this study

and are discussed here.

34

 

 

tea)

1.20.1 Turner’s model

The oldest mathematical model to predict the CTE of the composites is Turner’s
equation [42]. This is more like a lower bound for the CTE values of the particulate ﬁlled
composites. Turner’s model is based on the fact that the CTE of the composites will
depend on the relative compressibilities of the matrix and particulate ﬁllers, CTE of the
particulate ﬁllers and proportions of the two phases in the composite.

Assumptions made in deriving Turner’s equation -

1. The sum of the internal forces in composite can be equated to zero because the internal
stresses of the system are such that stresses are not sufﬁcient enough to disrupt the
composite.

2. Each component in the composite is constrained to change dimensions with
temperature changes at the same rate as the aggregate and the shear deformation is
negligible.

3. The composite is homogeneous. N 0 cracks develop in the composites. All the
microstresses are hydrostatic in nature.

Turner’s equation is as follows-

a'tPth + 02P2K2

 

 

 

. = d1 d2
“WW" PiKr+P2K2 ’
d1 d2

where,
(11 = linear CTE of phase lin composite,
or; = linear CTE. Of phase 2 in composite,

P1 = fraction or percent by weight of phase 1,

35

P2 = fraction or percent by weight of phase 2,
d1 = density of phase 1,

d2 = density of phase 2,

K; = bulk modulus of phase 1, and

K2 = bulk modulus of phase 2.

According to above equation of Turner’s model, CTE of the composite is related
to the volume fractions of the phases, if the constituent phases of the composite have
same bulk modulii. This equation had been applied to various metallic systems.

If the constituent phases have nearly equal values of Poisson’s ratios, the bulk
modulii are proportional to the corresponding Young’s modulii. Also in many cases the
bulk modulii of the constituents of composite are not available. Hence substituting
Young’s modulii in place of bulk modulii, above equation is transformed into a
simpliﬁed form written as follows -

arPrEt + a2P2E2

 

 

 

. _ d l d 2
“WW" PIE! + P252
(1 2 d 2

where,
E, = Young’s modulus of phase 1, and
E2 = Young’s modulus of phase 2.

Rest of the terms, are same as explained in previous equation of Turner’s model.
Simpliﬁed equation of Turner’s model is successfully applied to composites containing
aluminum oxide in polystyrene matrix. It is also applied to composite containing glass

ﬁbers in phenol formaldehyde matrix.

36

In the case of polymer matrix composites, the size and shape of the particulate
ﬁllers have an effect on the resultant CTE of the composites. Above equations do not
account for the effect of particle size and shape. Hence above equations can be further
modiﬁed by substituting an empirically determined constant C in place of KM for each
material. The constant C is not the ratio of K/d, but is proportional to Kid. The
proportionality factor is also dependent on the shape and size of the particles and the
distribution of the material in the matrix. It is also assumed that the constant C for each
speciﬁc ﬁller and each plastic material is independent of the other components of a
composite if the ingredients are evenly distributed. The details of this modiﬁed equation

are patented and are not available in literature [42].

1.20.2 Kerner’s model

Another mathematical model for predicting CTE of particulate reinforced composites,
which is commonly used and shows good agreement with the experimental results, is
Kerner’s model [76]. Kemer’s model takes into account the effect of shear stresses at
interfaces. The assumptions made in applying the Kemer’s equation to thermosets are as
follows [18] —:

1. Composite is macroscopically isotropic and homogeneous

2. Particles are suspended in and bonded to matrix.

3. Particles are distributed spatially at random and they are spherical.

4. The thermosets are stable during the measurement.

37

 

5. The thermosets are fully cured and no posturing occurs, no transitions occur, no
enthalpy or volume relaxations occur and no weight changes occur during CTE
measurement.
6. The thermosets are below glass transition temperature during measurements.

Kemer’s equation that is applied to particulate reinforced, thermoset matrix
composites, is as follows -:

Yeomposite = Ymauix Vmauix+ Yﬁnchmtcr ‘(Ymauix 'Yﬁller) Vmatn'xvﬁller 9 .

where,

_ (1/K......-.)-(1/Kﬁu..)
(th. / Km...) + (Vﬁuﬂ / Km.) + (3 / 40......) ’

 

where,

Kmam = bulk modulus of the matrix,

Kﬁller = bulk modulus of the particulate ﬁller,
Vm = volume fraction of the matrix,

V511,, = volume fraction of the particulate ﬁller,
Gmatrix = shear modulus of the matrix,

’Ymauix = volumetric CTE of the matrix , and

7mm = volumetric CTE of the matrix.

38

1.21 Aim of this study

Following were the objectives of this study -:

1. To study the effect of low and negative CTE particulate ﬁllers such as Beta quartz,
Cordierite and BS-50 on reducing the CTE of epoxy resin matrices.

2. To develop isotropic low CTE composites, that have wide range of applications,
especially in electronic industry. Application of the above-mentioned particulate ﬁllers
in epoxy resin matrix to reduce its CTE is not documented in the published literature.

3. To evaluate the role of surface treatments of particulate ﬁllers with silane coupling
agents on the dispersion of the above ceramic particulate ﬁllers in the epoxy resin
matrices and to study their inﬂuence on the CI‘E of the resultant composites.

4. To conduct density measurements, microhardness testing and microstructure studies to
evaluate the quality of processing, and to analyze distribution and adhesion of the
particulate ﬁllers with the epoxy resin matrix.

5. To compare the experimentally observed values of CTE of the composites with those

predicted by theoretical models.

39

 

.. "—55—... _._._ _ —ﬁ_

2. EXPERIMENTAL PROCEDURE

2.1 Properties of materials used in this study

2.1.1 Epoxy resins

The composites used in this study were prepared by using three types of epoxy
resins. The epoxy resins with brand names EPON Resin 8280, EPON Resin 8281 and
EPON Resin 9405 were obtained from Resolution Performance Products, formerly
known as the Resins and Versatics Business of Shell Group of Companies. The properties
of these epoxy resins are listed in Table 7. The epoxy monomer for all these epoxy resins
is DGEBA (Diglycidyl ether of bispenol A). The structure of DGEBA is in given

Figure 4.

2.1.2 Curing agents

For curing of EPON Resin 8280 and EPON Resin 8281, curing agent EPI-CURE
3223 was selected. For curing of EPON Resin 9405, curing agent Ancamine 9470 was
selected as a curing agent based on manufacturer’s recommendations. Both curing agents
were also obtained from Resolution Performance Products. The properties of these curing
agents are listed in Table 8. Ancamine 9470 contains 80% Diethyltoluenediamine and
20% proprietary diluent. Ancamine 9470 is commonly used as curing agent for
manufacturing composite laminates. EPI-CURE 3223 is commonly known as DETA or
diethyl triamine and has chemical formula HzN- (CH2); - NH - (CH2); -NH2 . DETA is a
general purpose-curing agent useful for making small castings, rigid laminates, medium

strength adhesives and baking-type solution castings [77]. Most commonly used

40

 

 

 

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42

Table 8. Properties of curing agents used in this study [source — product literature].

 

EPI-CURE 3223 (used

 

 

 

 

 

 

 

f . Ancamine 9470 (used for
”WW EPON: climatic (1 “ring
win an .
EPON Resin 8281) EPON Rm“ 9405)
D' h l . . Diethyltoluenediarrﬁne
Formula let y enetnamine (80%) proprietary diluent
[DETA](100 %) ’
(20%)
Speciﬁc gravity 0.95 Not available
Density (g/cc) Not available 1.03
Viscosity (Poise) 10 x 10'2 1.2 x 10'2 --1 .7x 10'2
Formula molecular .
Weight 103 Not available
Stoichiometric ratio (phr) ll 28

 

 

 

 

43

q

 

‘I‘ulm X-n_

stoichiometric ratio of DETA is 11phri.e. parts by weight of amine curing agent per 100
parts resin. Sometimes, stoichiometric ratio of curing agent is reduced so as to reduce
curing exotherm and permit castings of larger masses. Lowering the quantity of curing
agent also reduces the deﬂection temperature of the epoxy resin. Deﬂection temperature
or heat distortion temperature is the temperature range over which the polymer begins to
soften and get deformed under the inﬂuence of load. During curing of DGEBA epoxy
resins, with amine curing agents, reactivity is very high, resulting in quick liberation of
heat and high exotherms [78]. The stoichiometric ratio of curing agents to epoxy resins is

calculated as explained below [55].

phr of curing agent = AHEW x 100/ EEW,
where,
phr = parts by weight of curing agent per hundred parts resin,

A HEW = Molecular weight of the arrnne 1n the cunng agent

 

number of active amine hydrogen sin the curing agent
AHEW = amine hydrogen equivalent weight, and
EEW = epoxide equivalent weight of the resin.
EEW is deﬁned as the weight of resin in grams, which contains one-gram equivalent of
epoxy. EPI-CURE 3223 is used for curing EPON Resin 8280. In this case, the quantity of
EPI-CURE 3223 is calculated as follows.
EPI—CURE 3223 (i.e. DETA) has AHEW = 103/5 =20.6, and
EEW of EPON Resin 8280 = 185,
Hence, stoichiometric ratio of EPI—CURE 3223 required for curing of EPON Resin 8280

is equal to 11 phr, ( 20.6 x 100/185).

 

2.1.3 Cured epoxy resins

Various properties of DGEBA cured with DETA adapted from literature are listed
in Table 9. Table 10 lists important properties of cured epoxy resins that were used in this
study. These properties were measured for cured epoxy resins without any particulate

ﬁller reinforcements.

2.1.4 Particulate fillers

Three particulate ﬁllers that were used in this study are Beta quartz, Cordierite
and BS-50. Beta Quartz has a negative CTE of —l ppml° C, which makes it very useful
in applications requiring thermal stability. Cordierite ceramics are important due to their
low CTE and good thermal shock resistance. CTE of Cordierite can vary with the
processing method and usually in the range of 0.6 to 2.3 ppm /°C. BS-50 belongs to the
class of NZP ceramics i.e. alkali and alkaline earth zirconium phosphates. NZP ceramics
have very low and tailorable CTE and high thermal shock resistance. NZP materials have
lesser tendency towards microcracking, high melting temperature (in excess of 1800°C),
excellent high temperature stability and strength retention, and low thermal
conductivity. Typical composition of BS-50 is indicated in Table 11.Typical properties of
the particulate ﬁllers used in this study are listed in Table 12. Figures 5 through Figure 7
show size and shape of these particulate ﬁllers under SEM (scanning electron
microscope). Cordierite particles do not agglomerate. Beta quartz particles seem to form

card-pack type agglomerates. BS-50 particles form card-house type agglomerates.

45

in

' W n".""

Table 9. Typical properties of epoxy resin DGEBA cured with DETA [79].

 

 

 

 

 

 

 

 

 

 

 

Property Typical value of property
Shrinkage 4.5%
Elastic modulus (GPa) 3.5
Hardness (Rockwell L) 118
Fracture toughness (MPa‘Jm) 0.6
Density (g/cc) 1.11-1.40
Dielectric constant 3 8
(100 Hz- 10 MHz) '
Thermal conductivity (W/mK) 0.19
Poisson’s ratio 0.34

 

 

46

Table 10. Measured values of properties of cured epoxy resins, used in this study.

No particulate reinforcements.

 

 

 

 

 

 

 

 

 

 

 

 

EPON Resin 8280 EPON Resin 8281 EPON Resin 9407
Property cured with cured with cured with
EPI-CURE 3223 EPI-CURE 3223 Ancamine 9470
CTE (Ppm/°C) 82.3 81 72.5
Density (g/cc) 1.188 1.19 1.165
Modulus (GPa) 3.8 3.8 3.6
Hardness (GPa) 0.244 0.244 0.232
Estimated bulk
modulus (GPa) 3.96 3.96 3.75
Estimated shear
modulus GPa 1.418 1.418 1.343
Glass transition
temperature [Tg] ~125 ~125 ~175
( ° C)

 

47

 

Table 11. Typical composition of BS-50 [80].

 

 

 

 

Theoretical weight %
Ba Zr P Si 0
18.1 32.1 13.6 2.5 33.7

 

 

 

 

 

48

 

Table 12. Properties of the particulate ﬁllers used in this study [81,82,83,84,85,97].

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Property Beta quartz Cordierite BS-50
. $102 and small 2Mg0.2A1203.58i02 Bat-$343530“
Chemical . . . . Barium
F quantities of ZnO- Magnesrum alutruno . .
orrnula . . le‘COIllum
A1203 Silicate
phosphate
Crystal
structure Hexagonal Hexagonal Hexagonal
Density (g/cc) 2.533 2.508 3'ffaflﬂ23‘c’ﬁfgrfy
-1 2.3 -2.5
o Negative along c Positive along a axis Positive along c
CTE (PPm/ C) axis, constant along and negative along axis and negative
a axis. c-axis. along a-axis.
Modulus of
elasticity (GPa) 100.6 117.3 74.5
Bulk modulus
(GPa) 56.5 129 44.34
Shear modulus
(GPa) 41.8 54 30.53
Hardness
(GPa) ~8.2 ~8.2 ~ 4.5
Poisson’s ratio 0.203 0.31 0.22
Dielectric ~4.0 [100 Hz- 1 K11 K11 ~7 [IOOOKHz -
Constant MHz] 5 [1 -10 ] 3GHz]
Thermal
conductivity ~1.4 ~3.222 ~l
(W/mK)

 

49

 

particles

 

Figure 5. Micrograph of Beta quartz particles

50

 

inf- ,_ . I .. g ’31.,-
Cordlcrlié'paNICICS > , ‘ -,5,,’10um

n! " .,.

Figure 6. Micrograph of Cordierite particles.

51

 

Figure 7. Micrograph of BS-SO panicles. These particles tend to agglomerate.

52

2.1.5 Silane coupling agents

In this study, silane coupling agents with trade names Z6020 and 26040 procured
from Dow Corning Corporation were used. The properties of silane coupling agents used

in this study are tabulated in Table 13.

2.2 Specimen preparation

Following specimen preparation method was followed to prepare the composite
specimens.

Glass tubes of 10mm diameter and 70mm length were coated with release agent
Miller-Stephenson MS.122N/C02 few hours in advance. The epoxy resin was preheated
at 50l60° C for 2 hours to lower its viscosity and to melt the crystals formed during
storage of epoxy resin. The epoxy resin was subjected to vacuum for 15 minutes to get
rid of air bubbles. Globules present in the particulate ﬁller sample were broken and the
powder samples were sieved, whenever required. Drying the particulate ﬁllers helps to
remove water or moisture in particulate ﬁllers. Heating under vacuum is the most
satisfactory procedure. Adding hot particulate ﬁllers in epoxy resin also aids the
dispersion of the particles [55]. Hence the particulate ﬁllers were dried at 150 ° C for 4
hours. The powders were added into the resin in required proportion as soon as they were
taken out of the oven. The particulate ﬁllers were mixed in hot epoxy resins by applying
shear force using wooden spatula to achieve complete wetting of the particulate ﬁllers
and to distribute particulate ﬁllers intimately without agglomeration [54]. The overall
process of dispersion of particulate ﬁllers can be divided into various steps -

l. Wetting of the particle surfaces by the epoxy resin,

53

 

 

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«35.—eh

5.5.3.5

 

 

A2382: 8:85 I 0833 3:5 05 5 com: 9:03 3:950 0527. Mo 8:385 .m— 033—.

54

2. Rupture of the aggregates, and
3. Separation and distribution of the aggregates so that they do not agglomerate again.

After the particulate ﬁllers were distributed uniformly, curing agent was added in the
required stoichiometric ratio or as per the recommendation of the manufacturer. After
uniform mixing of curing agent, the mixture was poured in glass tubes, treated with
release agent. Centrifuging was conducted for 2 minutes at 2000 rpm to remove air
bubbles. The composites were cured as per the recommendations of the manufacturer.
Curing cycle for EPON Resin 8280 and 8281 cured with EPI-CURE 3223 was overnight
at room temperature followed by cure at 100° C for 2 hours. The cured composites were
taken out of glass tubes and they were available in the form of rods. The surfaces of the
rods were polished to remove any surface discontinuities and residues of release agent.
Then the composite rods were machined and polished to required sizes and shapes.
Composite samples were annealed for 2 hours at a temperature 20°C above their glass
transition temperature, and cooled to room temperature in air to reduce the residual
stresses. Figure 8 shows the ﬂow-chart of preparation of composite specimens.

For some batches of composites, air release agent A500 from BYK Chemie was
added to the mixture of particulate fillers and epoxy resins. Air release agents are
additives with low surface tension and demonstrate insolubility in the medium to be
defoamed. They have positive entering coefﬁcient and positive spreading coefﬁcient. Air
release agent draws the neighboring foam bubbles within the resin together, enlarges size
of bubble and increases the rising speed of the foam bubbles from mixture of particulate

ﬁllers and resin to the surface.

55

“II-1.- 1" A *‘wn‘

 

Dry Beta quartz particles at 150° C for 4
hours.

Heat EPON Resin 8280 up to 50° C and
maintain for 2 hours. Vacuum treat the epoxy
resin. Coat glass tubes with release agent.

47

Add dried Beta quartz particles in the epoxy
resin and mix well.

3

Add llphr of EPI-CURE 3223.

L

Cure composite at room temperature
overnight, followed by 2 hours at 100° C.

4

Cut composite rod to required sizes. Machine
and polish them. Anneal 20° C above the glass
transition temperature for 2 hours. Conduct
testing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 8. Flow-chart for preparing composite reinforced with Beta quartz. Matrix -
EPON Resin 8280 cured with EPI-CURE 3223.

56

 

2.2.1 Effect of process variables

It was noted during preparation of composite specimens that the process
variations inﬂuence the properties of the composites signiﬁcantly. The most important of
them are,

1. The temperature to which epoxy resin is preheated prior to addition of particulate
ﬁllers,
2. Temperature and mechanism used for drying the particulate ﬁllers, and
3. The mixing efﬁciency of particulate ﬁllers in epoxy resin matrix.
These factors were maintained constant during the preparation of composite

specimens.

2.2.2 Treating particulate fillers with silane coupling agents

Particulate ﬁllers were treated with silane coupling agents Z6020 and Z6040 prior
to addition into the epoxy resin matrix. Z6020 has diamino group for organic reactivity
and trimethoxysilyl group for inorganic reactivity. 26040 has epoxy group for organic
reactivity and trimethoxysilyl group for inorganic reactivity. Epoxy reactivity allows
Z6040 to undergo ring-opening reactions with acids, amines, alcohols and epoxides.
Trimethoxysilyl group is subjected to hydrolysis in water or water/alcohol solutions. The
initial product of hydrolysis is silanetriol. Silanetriols undergo condensation with the
hydroxyl groups at the surface of particulate ﬁllers. After condensation, the remaining
silanol groups are capable of hydrogen bonding or condensing with adjacent silanol
groups. Thus the coupling agent is bonded to the particulate surface by the combination

of hydrogen bonding and covalent bonding. Figure 9 shows important steps in treatments

57

 

Step I. Hydrolysis reaction
R Si (OCH3)3 + 3H20 -) R Si (OH)3 + 3CH30H

Silane coupling Silanetriol
agent Z6040

Step 11. Reaction of Silanetriol with hydroxyl group present on the mineral

surface.
OH OH
---OH + H0 — Sli - R -) ---O -Si — R + H20
2... 5H
particulate particulate
ﬁller ﬁller

Step III. The particulate ﬁller reacts with epoxy resin matrix.

Figure 9. Important steps in treating particulate ﬁllers with silane
coupling agent Z6040.

58

of particulate ﬁllers with silane coupling agents. Silane coupling agents may form
monolayer or multilayers on the surface of particulate ﬁllers.

The process of treating particulate ﬁllers with silane coupling agents was
recommended by the manufacturer of the coupling agents and it was carried out in the
following manner -:

For prehydrolysis, 40 parts of silane coupling agent, 55 parts of solvent,
(methanol, in this case), and 5 parts of water were used. Solubility of silane coupling
agents was enhanced by alcohol. The pH of water was adjusted to 4.5-5.5pH to produce
rapid prehydrolysis and relatively stable silanols. Methanol was mixed with pH adjusted
water. After stirring the solution, to form a vortex, silane coupling agent was slowly
added into the solution. It was allowed to complete hydrolysis overnight. 2.5 ml of this
solution was added to 50ml methanol, to achieve 2% concentration of silane coupling
agent. 30 gm particulate ﬁllers were added to this solution and mixed well for 2 minutes.
Time of two hours was allowed for the condensation reaction to complete with surface of
particulate ﬁllers. The excess methanol was ﬁltered using ﬁlter paper, and the particulate
ﬁllers were dried at 100° C for 30 nrinutes. Dried particulate ﬁllers were used

immediately to prepare composite specimens.

2.3 Testing of composites

2.3.1 Measurements of density of composites

Density measurement of composites was conducted to ensure proper dispersion of
the particulate ﬁllers and to detect the level of porosity. For density measurements,

guidelines in ASTM Standard D792-98 were followed. This standard describes

59

 

displacement method for measuring density and speciﬁc gravity of plastics. Samples of

various sizes were cut from the composites specimens. These composite pieces were

weighed in air, then immersed in water using a thin wire and weighed in water. The

speciﬁc gravity of composite pieces was determined using following formula:

Speciﬁc gravity of composite at 23/23° C: a/ (a+w-b),

where,

a = apparent mass of specimen in air without wire,

b = apparent mass of specimen completely immersed in water and the wire partially
immersed in water,

w= apparent mass of wire partially immersed in water, and

23/23°C is the ASTM standard designation to indicate that mass of specimen in air is

measured at 23°C and weight in water is also measured when water is at 23° C.

Density of composite in kg/m3 at 23° C = speciﬁc gravity at 23/23°C x 997.6.

2.3.2 Measurements of CTE of composites using TMA

CTE measurement was the most important step in characterizing composites in
this project. TMA (thermo mechanical analyzer) was used for measuring CTE of
materials i.e. linear changes in specimen dimensions with rise in temperature under non-
oscillatory load were measured. TMA can also be used to determine the glass transition
temperature of epoxy resins. For CTE measurement, ASTM standard E83l-93 which
describes standard test method of measuring linear thermal expansion of solid materials
by thermomechanica] analysis, was followed. CTE measurement was conducted using

TMA 2940 of Thermal Analysis Instruments.

60

TMA was calibrated at the beginning of CTE measurement experiments as well as
after testingIO specimens. TMA has a provision for calibrating -
1. temperatures,
2. variations in the length the probe due to heating, and
3. loads, if load is to be applied during CTE measurement of certain materials.

Aluminum specimen of known CTE provided by the TMA manufacturer was
used for the calibration purposes. TMA has inbuilt softwares to monitor calibration
process.

The specimens used for CTE measurements in this study were cylindrical rods up
to 12 mm length and 4-7 mm diameter. The specimens were machined and polished to
tolerance +/- 25m. The specimens were annealed at a temperature approximately 20 °C
above the glass transition temperatures of the epoxy resins just prior to CT E
measurements. The CTE measurement process was carried out as explained below.

The specimen was placed inside the furnace of TMA on a flat support. Furnace of
TMA is a small unit with electric heating coils, which heats up the specimen at a uniform,
predeﬁned heating rate. A thermocouple is ﬁtted close to the specimen support. For low
temperature CTE measurement experiments, liquid nitrogen circulation accessories are
available in TMA. A probe rests on the top of the specimen, which measures the changes
in the specimen dimension by means of a transducer. The probe as well as the base
support for the specimen are generally made from materials like quartz or silica, which
have low and precisely known CTE. Various conﬁgurations of the probe can be used
with or without application of compressive loads. Flat tipped standard probe without load

is used to measure the CTE of the composites. In this study, specimen were heated at the

61

. '." A,

rate of 4° C/min, which is neither too fast nor too slow. The range used for measuring the
CTE of composites was from 40°C to 100° C, which was well below their glass transition
temperatures of the resins. The specimen deforms on heating. The changes in specimen
dimension are converted into the upward movement of the probe resting on the top of the
specimen. A plot of the changes in length of specimen versus the temperature change, in
similar unit systems, is generated. The slope of this plot gives the value of linear CTE for
the composites [86]. The CTE was measured for three different specimens of each type of

composite and an average value was obtained.

2.3.3 Effect of testing parameters on CTE of composites
It was noted during testing and analysis of particulate reinforced composites that
certain testing parameters affect the measurement of CTE of composites within a few
ppm/°C. These parameters are-
1. Heating rate during CTE measurement experiment,
2. The temperature range over, which CTE measurements are carried out,
3. Surface ﬁnish of the specimen, and
4. Length of the specimen.
Hence all these parameters were maintained constant for all the CTE

measurement experiments to avoid any discrepancies.

2.3.4 Microhardness measurements of composites

Microhardness testing of materials is a very common tool for rrricromechanical

characterization of materials. It is described in details in literature along with its use for

62

the characterization of epoxies. Microhardness testing applied to ﬁber reinforced epoxy
resin matrix and polyarnide matrix composites, is also discussed in literature [87,88,89].

Microhardness testing was conducted to study the reinforcing effect of various
particulate ﬁllers on the epoxy matrices. Microhardness testing was conducted using
microhardness tester of Leco Corporation M-400-Gl. These measurements were carried
out at room temperature using a diamond square pyramid indenter with included angle of
136° at the tip. A load of SOgm was applied for 15 seconds from the time of contact till
the load was removed. The length of the diagonals formed by the pyramid indenter was
measured immediately after removing the load. Microhardness of the composites was
calculated using the equation,

H = (23in 68°) (P/D2)= 1354* (P/Dz),
where,

P = the applied load, and

D = the average diagonal of the indentation.

The average microhardness values were calculated from 15 indents on
representative samples. Numerous readings at different locations of the sample surface
ensured that the hardness value obtained is true representation of the average value of the
composite hardness. Figure 10 shows the micrograph of composite, which has
indentation marks from microhardness testing. This Figure shows that the microhardness
values are true representation of composite hardness because the indentation diagonals

cover the area composed of particles as well as matrix.

63

.. 5"}

: indenter

diagonals I

 

Figure 10. Micrograph showing the diagonals of the indenter of a microhardness tester.
Diagonals are spread along the epoxy resin matrix as well as particulate ﬁllers, hence the
microhardness value represents hardness of composite.

2.3.5 Microscopy for observing composite microstructuros

Microstructure analysis is a qualitative technique for observing the dispersion of
particulate ﬁllers in the polymer matrices as well as for studying the nature of interfaces
in composites. Low magniﬁcation examination is useful to obtain the features such as
porosities, overall distribution of particles etc. SEM was used for studying fracture
surface because the specimens can be prepared easily and thick samples can be used
without any modiﬁcation. In addition, SEM gives high resolution and high depth of ﬁeld.
SEM model Hitachi S-2500C was used for studying microstructures. The composites in
the current study were electrically non-conductive. In composites reinforced with high
volumes of particulate ﬁllers, the particle pullout is a problem during observation of the
composite surface. For these two reasons, surfaces of composites were gold coated. The
evaporation or sputtering technique of gold coating involves the erosion of gold atoms by
energized gas plasma of argon via the production of glow discharge. This produces a
continuous gold coating layer on the specimen surface [90]. All samples were coated with
~14nm thick gold coating layer. Sides of the specimens were covered with multiple
layers of carbon tape to achieve better electrical conductivity. An accelerating voltage of

13 KeV and the working distance of 13mm were used to have optimum resolution.

2.3.6 Fracture study of composites

Fracture of the composites was studied to evaluate the adhesion between phases
and the effect of silane coupling agents on the adhesion of particulate ﬁllers with
matrices. Fracture toughness testing was carried as per the guidelines in ASTM standard

E1304-97. This standard describes guidelines for measuring plane strain fracture

65

toughness of metallic materials. Fracture testing was conducted at room temperature.
Short rod type of specimens with chevron notch, were prepared. The specimen geometry
is discussed along with drawings in the abovementioned standard. All specimen
dimensions were measured carefully prior to testing. During fracture testing of
composites, a pair of grips was ﬁtted inside the specimen in a groove machined on the
specimen and an outwardly directed force was applied causing a crack to initiate at the tip
of chevron notch. A plot of load versus mouth opening displacement of the specimen was
plotted. Epoxy resins showed crack jump behavior. Crack jump behavior means that the
crack advanced through rapid jumps rather than smooth crack growth. Each crack jump
was accompanied by a decrease in applied loading force. Between the jumps, as the
loading continued, the crack was nearly stationary until the load was increased to a level,
which caused the next jump. For each type of composite, three specimens were tested.

Fracture toughness of composites was calculated using following formula -:
KICSR = ArCc(F -1/2 AFH)/B3/2 ,

where,
Km“: plane-strain critical stress intensity factor,
A,-—- compliance constant,
Cc: correction factor,
F: load,
AF“: hysterisis in load which was ignored for all specimens, and
B = diameter of the specimen.
The fractured specimens were preserved in a desiccator, subsequently gold coated

and studied using SEM.

66

3. RESULTS AND DISCUSSION

3.1 CTE of composites

The CTE of various epoxy resin matrix composites with three types of particulate
ﬁller reinforcements prepared in this study, are tabulated in Tables 14, through 20. As
can be observed from these tables, the CTE of the composites decrease as the volume
fraction of the particulate ﬁllers in the epoxy resin matrices increases. The ordering of
intrinsic CTE values of the particulate ﬁllers used in this study is BS-50 (-2.5 ppm/°C) <
Beta quartz (-1 ppm/°C) < Cordierite (2.3 ppm/°C). As the results indicate, equal volume
fractions of these particulate ﬁllers do not decrease the CTE of composites in the same
order in which the CTE of particulate ﬁllers varies. Comparison of Tables 15 and 18
shows that Cordierite is more effective as reinforcement than Beta quartz in EPON Resin
8281, which means that processing parameters play signiﬁcant roles in inﬂuencing CTE
of the composites, apart from the properties of matrix. The variables that affect the CTE

of the composites are discussed along with the various experimental results.

3.1.1 Effect of volume fraction of particulate fillers

Figures 11 and 12 show that as the volume fraction of particulate ﬁllers in the epoxy resin
matrices increases, the inter-particle distances decrease. Hence increased volume fraction

of particulate ﬁllers, inﬂuences the CTE of composites signiﬁcantly.

67

Table 14. CTE of composites reinforced with Beta quartz particles.
Matrix-EPON Resin 8280 cured with EPI-CURE 3223.

 

 

 

 

 

 

(1:213: 2323;; CTE (32:17:13? rtes % Reduction in CTE
0 82.3 0
12.29 69.1 16.04
13.52 67.7 17.74
19.32 62.8 23.69

 

 

 

Table 15. CTE of composites reinforced with Beta quartz particles.
Matrix-EPON Resin 8281 cured with EPI—CURE 3223.

 

 

 

 

 

Volume % of Beta quartz CTE of composites . .
in composites (pp ml° C) % Reduction 1n CTE
0 81 0
13.17 67 17.28
15.77 59 27.16

 

 

 

Table 16. CTE of composites reinforced with Beta quartz particles.
Matrix-EPON Resin 9405 cured with Ancamine 9470.

 

 

 

 

 

 

 

“m“: ﬁgﬁfgmﬂz ”13;;leng % Reduction in CTE
o 72.5 o
2.36 71.75 1.03
4.86 71.6 1.20
12.08 65.33 9.90
25.76 46.96 35.23

 

 

 

68

 

 

 

Table 17. CTE of composites reinforced with Cordierite particles.
Matrix-EPON Resin 8280 cured with EPI—CURE 3223.

 

 

 

 

 

Volume % of Cordierite CTE of composites % Reduction in CTE
1n composrtes (ppm/°C)
0 82.3 0
13.64 67.4 18.10
23.25 58.6 28.80

 

 

 

Table 18. CTE of composites reinforced with Cordierite particles.
Matrix-EPON Resin 8281 cured with EPI-CURE 3223.

 

 

 

 

 

Volume % of Cordierite CTE of composites . .
in composites (ppm/°C) % Reduction in CTE
0 81 0
13.16 61.4 24.20
13.54 59.64 26.37

 

 

 

69

 

 

Table 19. CTE of composites reinforced with BS-50 particles. Matrix- EPON Resin

8280 cured with EPI-CURE 3223.

 

 

 

 

 

Volume % of BS-SO in CTE of composites . .
commites (ppm/°C) % Reduction in CTE
0 82.3 0
10.16 72.6 11_79
17.22 67.6 17.86

 

 

 

Table 20. CTE of composites reinforced with BS-50 particles. Matrix- EPON Resin

8281 cured with EPI-CURE 3223.

 

 

 

 

 

Volume % of.BS-50 1n CTE of composrtes % Reduction in CTE
conjugates (ppm/°C)
0 81 0
6.9 70.8 11.35
10.47 67.4 16.80

 

 

 

70

 

 

.’ ,‘B'eta qUartz'
:_. ’1.

(If ' '
partlcles f

 

Figure 11. Micrograph of composite reinforced with 2.35 volume % of Beta quartz.
Matrix- EPON Resin 9405 cured with Ancamine 9470. Interparticle distances are high
due to low volume fraction of the particulate ﬁllers.

71

361% quartz

3 paritiéles
, f 5.

 

Figure 12. Micrograph of composite reinforced with 25.76 volume % of Beta quartz
particles. Matrix- EPON Resin 9405 cured with Ancamine 9470. Interparticle distances
are very less due to high volume fraction of the particulate ﬁller.

72

3.1.2 Effect of properties of matrix

It can be seen from Table 16 that, at low volume fractions of Beta quartz, in a
matrix of EPON Resin 9405 cured with Ancamine 9470, the composites have CI'E values
same as the CTE of the matrix. Particulate ﬁllers do not exhibit any effect on the
properties of the composites. Hence the matrix simply expands according to its original
value of CTE without any resistance from particulate ﬁllers. At higher volumes of Beta
quartz, the trend changes. It can also be observed from table 16 that, even 12.1 volume %
reinforcement with Beta quartz in matrix EPON Resin 9405 reduces CTE of the
composite only by 9.9%. This CTE reduction is less compared to the reduction achieved
by Beta quartz in two other epoxy resin matrices. Poor performance of EPON Resin 9405
with Beta quartz reinforcements in terms of CTE reduction, must be attributed to its low
viscosity. EPON Resin 9405 has one tenth of the viscosity of EPON Resin 8280 and
EPON Resin 8281, as listed in Table7. EPON Resin 9405 is modiﬁed EPON Resin 8280.
It contains more than 7% styrene added to reduce its viscosity. Hence particulate ﬁllers
can be mixed easily with least shear force in EPON Resin 9405 than in the other two
epoxy resins. The polished surfaces of composites made using EPON Resin 9405 do not
show any air bubbles. But due to low viscosity of resin, Beta quartz particulates show
signiﬁcant settling in EPON Resin 9405. This results in higher CTE of composites than
expected.

Due to dilution, EPON Resin 9405 has density 1.165 g/cc after curing, as against
1.19 g/cc in EPON Resin 8280 and 8281 cured with EPI-CURE 3223. The lower density
of EPON Resin 9503 may be causing poorer packing of the Beta quartz particles in the

matrix compared to that in other two epoxy resins (EPON Resin 8280 and EPON Resin

73

8281). Hence particulate ﬁllers share no load on application of stress at low concentration
of the particulate ﬁllers. Since the particulate ﬁllers do not form direct bonds with the
epoxy resins, the density of the epoxy resin is may play an important role. As the
particulate ﬁller volume increases, as can be seen in Figure 12, decreased inter-particle
distances lead to increased packing of particles, increased interaction between the
particles, increased interaction with the epoxy resin matrix and reduced settling. Higher
packing density results into improved stress transfer across the interface of particulate
ﬁllers and the epoxy resin matrices. Hence at higher volume fractions, the CTE of
composites is strongly affected by CTE of Beta quartz particles. EPON Resin 9405 cured
with Ancamine 9470 has much higher glass transition temperature (~175°C) compared to
cured EPON Resin 8280 and EPON Resin 8281 (~125°C), this may be causing more
thermal stresses in composites with EPON Resin 9405 as matrix, than in the other two

epoxy resins.

3.1.3 Effect of size and shape of particulate f'rllers

Table 21 lists observations about shape, size and behavior of particulate ﬁllers in epoxy
resins and the effects of particulates on the composite processing. Figures 13, through 18
show the micrographs of various composites with 3 types of particulate ﬁllers. It can be
observed that shape and surface area of Beta quartz particles allow the best mechanical
interlocking with the epoxy resin matrices. BS-SO particles have the poorest mechanical
adhesion with the epoxy resin matrices due to small particle size and least surface area.

Figures 17 and 18 also show that the BS-50 particles form aggregates.

74

Table 21. Properties and behavior of particulate ﬁllers used in this study.

 

 

 

 

 

 

 

 

 

 

 

Property Beta quartz Cordierite BS-50
Elongated, Polygonal,
Particle shape polygonal, low aspect ratio, Round
high aspect ratio
Pam” 8‘“ ~03 to 10 ~05 to 5 ~ 0.3-0.6
(llm)
Nature of card-pack type no agglomerates card-house type
particles agglomerates agglomerates
Surface of Partly smooth,
particles Rough partly rough Smooth
Observed Intermediate
effect on Least increase increase in Increases viscosity
viscosity of in viscosity. viscosity amongst 3 signiﬁcantly.
epoxy resins ﬁllers.
. . High wettability, Poor wettabilty, takes
Observed mflhxzsetetgllrty, mixes well high shear force to
wettability . y. after applying break the
1n all epoxy resrns.
shear force. agglomerates.

 

75

 

. 4, Qparticle

‘4‘

 

Figure 13. Micrograph of composite reinforced with 13.54 volume % of Beta quartz.
Matrix- EPON Resin 8280 cured with EPI—CURE 3223.

76

 

Figure 14. Micrograph of composite reinforced with 13. 16 volume % of Beta quartz.
Matrix- EPON Resin 8281 cured with EPI—CURE 3223. It shows the mechanical
interlocking of the particles with the matrix.

77

 

Figure 15. Micrograph of composite reinforced with 13.64 volume % of Cordierite.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223.

78

- Cordierite» ‘
. . particle

 

Figure 16. Micrograph of composite reinforced with 13.54 volume % of Cordierite.
Matrix- EPON Resin 8281 cured with EPI—CURE 3223.

79

 

Figure 17. Micrograph of composite reinforced with 10.16 volume % of BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223.

80

 

Figure 18. Micrograph of composite reinforced with 10.47 volume % of BS-50.
Matrix- EPON Resin 8281 cured with EPI-CURE 3223.

81

Since the three particulate ﬁllers belonged to quite different ranges of the particle
sizes and had different shapes, this project does not compare the relative inﬂuences of
particle sizes on CTE of composites. It appears that particle size to achieve optimum CTE
values for composites should exist for each type of particulate ﬁller and each resin
system. This argument is supported by the fact that, for many materials including
composites, product of CTE and elastic modulus is a constant. Alter et al. have shown
that elastic modulus of particulate reinforced composites is inversely proportional to the
particle size [91]. These two results suggest that selecting particular range of particle size
will have the optimal inﬂuence on the CTE of the composites. It is not clear on the basis
of this study whether the crystal structure and composition of particulate ﬁllers may have

any inﬂuence on the CTE of the composites.

3.1.4 Effect of wettability of particulate fillers

It was observed during preparation of composites that Beta quartz particles show
the best wettability with all three epoxy resins. Cordierite has intermediate wettability
and BS-50 has poorest wettability amongst three particulate ﬁllers. BS-SO particles form
small agglomerates, which do not rrrix well with the epoxy resins and also require higher
shear force and longer times for thorough mixing with the epoxy resins. Good wettability
results in good mechanical interlocking of particulates with the matrices, which in turn
makes it possible to achieve higher elastic modulus for the composites compared to the
elastic modulus achieved in composites containing particles with lower wettability.
Hence it can be predicted that, higher elastic modulus will be achieved with Beta quartz

reinforcements than with two other particulate ﬁllers.

82

3.1.5 Effect of aggregates of particulate fillers

Table 22 enlists the nature of distribution of various particulate ﬁllers in different
epoxy resins. Figures 19, through 21 show the distribution of particulate ﬁllers in EPON
Resin 8280 and pores in composites. Figure 22 shows that BS-50 particles tend to form
signiﬁcant number of aggregates, even though the particulates were dried before adding
them to the epoxy resin. Aggregates may be formed due to van der Waals forces and
charges present on the surface of particulates. Cordierite particulates don’t aggregate, but
do exhibit limited aggregation when added to the epoxy resins. Since, Beta quartz
particles ﬂow freely both prior to and after adding to the epoxy resins and do not form
aggregates. Hence it can be seen from Figures 19 through 21, that the most uniform
distribution in the epoxy resins is achieved with Beta quartz particles. The aggregates
increase the hardness of composite locally, since aggregates are harder compared to

individual particles.

3.1.6 Effect of settling of particulate fillers

Settling of particulate ﬁllers in epoxy resin matrix was not observed during the
course of this study. This was conﬁrmed by measurement of CTE of specimens obtained
from different regions of a composite rod. No difference in CTE values with respect to
the locations in the specimen was observed. All the composites studied had good

antisettling resistance.

83

Beta quartz . .__-—>
Particles \\ '-

.t

 

Figure 19. Micrograph of fracture surface of composite reinforced with 13.52 volume
% of Beta quartz. Matrix- EPON Resin 8280 cured with EPI—CURE 3223. Few pores
are present as indicated.

84

Cordierite / 1
particles

 

Figure 20. Micrograph of fracture surface of composite reinforced with 13.54 volume %
of Cordierite in EPON Resin 8280 cured with EPI-CURE 3223. Few pores and an
agglomerate are present indicated.

85

"aggregates of 3

-.4""" 138-50
_- . »~ , 10.011m
,. . particles-- '

Figure 21. Micrograph of fracture surface of composite reinforced with 10.16 volume

% of BS-50. Matrix- EPON Resin 8280 cured with EPI—CURE 3223. Composite
contains aggregates of particles as well as pores.

 

86

,BJSESO

aggregate

 

Figure 22. Micrograph of composite reinforced with 10.16 volume % of BS-50.
Matrix- EPON Resin 8280 cured with EPI-CURE 3223. Fracture surface shows the
aggregates as indicated.

87

 

3.1.7 Density measurement of the composites

Plots provided in Figures 23 through 29, show density variations due to
particulate additions in various composites. It is clear from these ﬁgures that the density
of composites increases as the volume fraction of the particulate ﬁllers increases, due to
higher density of particulate ﬁllers as compared to the density of epoxy resins. Density
of all the composites was found to be close to the expected value or slightly lower, which
indicated the lack of signiﬁcant pores and absence of generation of water as byeoproduct.
Comparison of Figures 28 and 29 indicates that, BS-50 reinforced in EPON Resin 8280
matrix composites show wider range of density values. Such an observation points out
that distribution of BS-50 particles in EPON Resin 8280 is less uniform as compared to

that in EPON Resin 8281.

3.1.8 Pores in composites

Figures 19 through 21, show the pores in composites reinforced with the three
particulate ﬁllers in epoxy resin matrices. BS-SO and Cordierite reinforced composites
show larger pores formed near the aggregates. Composites reinforced with Beta quartz
show no speciﬁc locations of pores within the specimen. Hardness measured in regions
adjacent to the pores is low. It was about one-half the value of hardness measured on
specimen surface in a region devoid of pores. Though pores do not affect the CTE of
composites directly, they could be used as a measure to indicate the quality of the

composite processing and the uniformity of distribution of the particulate ﬁllers [73].

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3.1.9 Microhardness measurements of composites

Plots provided in Figures 30 through 39, indicate the variations in microhardness
values of composites as well as the average microhardness values for various composites.
Addition of particulate ﬁllers increased the microhardness of the composites. Most of the
composites showed uniform scatter in microhardness values. It is important to note that
the composites reinforced with 10.16 volume % BS-SO in EPON Resin 8280 (Figure 38),
showed wide scatter in hardness values indicating non-uniform distribution of particles
and presence of porosity.

Increase in hardness of composites with BS-SO reinforcements is less than that
with Beta quartz and Cordierite. This is so because, BS-SO has inherently low hardness
(~4.5 GPa) compared to hardness of Cordierite and Beta quartz. (~ 8.2 GPa). BS-SO
particles are small in size and are rounded in shape with poor wettability with epoxy
resins. Hence BS—SO particles have poor adhesion with the epoxy resin matrices and are
less effective in reinforcing them. BS-SO particles also form aggregates in the epoxy
resins. Since these particles tend to aggregate, they do not distribute uniformly in the
epoxy resins. The non-uniform distribution of BS-SO is clear from Figure 21, as well as
from the wide scatter in microhardness values and density values in composites
reinforced with BS-SO. It is interesting to note that the trend in improvement in hardness
is similar to the trend in decrease of the CTE values.

Generally higher the volume fraction of particulate ﬁllers, lesser the inter-particle
distances and higher the hardness of the composites. Hardness value improvement
indirectly reﬂects the improvement in mechanical properties of epoxy resins resulting

from the presence of particulate ﬁllers. Measured hardness values can indicate presence

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of aggregates and pores because aggregates have excess hardness values while regions
containing pores will have lower hardness values.

The composites can exhibit much higher hardness values when the adhesion
between the particulate ﬁllers and the epoxy resin matrices is perfect. In this study, the
particulates do not have perfect adhesion with the matrix. Hence on application of
indentation load to the composite, the load is not fully transferred across the interface and
it is not shared between the particulate ﬁllers and the matrix. In addition, the distribution

of the particles is uneven in some composites.

3.1.10 Effect of additives in epoxy resins

It can be seen from Tables 14 and 15 that, 19.3 volume % reinforcement of Beta
quartz in EPON Resin 8280 reduced the CTE of the composite just by 23.7% while 15.8
volume % reinforcement of Beta quartz in EPON Resin 8281 reduced CTE of the
composites by 27.16%. Tables 17 and 18 show that, 13.6 volume % reinforcement of
Cordierite in EPON Resin 8280 reduced its CTE by 18.1%, while 13.2 volume %
reinforcement of Cordierite in EPON Resin 8281 achieved 24.2% reduction in CTE of
composite. Tables 19 and 20 show that, 10 volume % of BS-SO reinforcement in EPON
Resin 8280 achieved 11.8% reduction in composite CTE, while 10.5 volume %
reinforcement of BS-SO in EPON Resin 8281 achieved 16.8 % reduction in CTE. Thus it
is observed that the performance of all three particulate ﬁllers in EPON Resin 8281 is
better for all the three particulate ﬁllers as compared to those in EPON Resin 8280.

Similar trend can be noted from microhardness values of composites as well.

107

 

EPON Resin 8281, has a very small quantity of additive (~ 0.15%, the details of
additive are proprietary) different from EPON Resin 8280. This additive is specially used
to avoid settling of silica particles. The speciﬁed viscosity values of resins EPON Resin
8280 and EPON Resin 8281 are almost same. The densities of both epoxy resins after
curing are almost same. Therefore, improved reduction in CTE of composites with EPON
Resin 8281 as matrix, has to be attributed to some other factors. The additive in EPON
Resin 8281 probably causes uniform distribution of particulate ﬁllers in it. It also
provides better interlocking of particulates ﬁllers with the epoxy resin matrix. BS-SO
particulates show more uniform distribution in EPON Resin 8281 than in EPON Resin
8280, which is clear from comparison of Figures 17 and 18, and also from the density
variation and hardness variations as discussed previously. The particles of Beta quartz
show better interlocking with EPON Resin 8281 than with EPON Resin 8280 as can be
seen from Figure 13 and 14. It is also likely that the additive in EPON Resin 8281, has
some stress relieving effects, which make particulate fillers more effective in reducing
the CTE. Thus, it can be concluded, that the small quantity of additive in EPON Resin

8281 resin seems to aid in reducing the CTE of the composite.

3.1.11 Effect of varying curing agent quantity

Quantity of curing agents may affect the CTE of composites, hence the results of
the experiment in which curing agent quantity was varied in composites, are shown in
Figure 40. In particulate reinforced composites, curing agent may be adsorbed on the
surface of particulates and the effective quantity of curing agent participating in curing

process may be reduced. It is also possible that if curing agent is added more than the

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stoichiometric ratio, the cross-link density of epoxy reins may increase, resulting into
decreased CTE of the composites. Above two effects were evaluated by increasing the
quantity of curing agent by 20% than its stoichiometric ratio. The particulate ﬁllers may

have hydroxyl group adhered on their surfaces, which may accelerate the curing reaction.
Hence rate of curing reaction can be controlled, by reducing the quantity of curing agent.
This effect was evaluated by reducing curing agent to 20% less than its stoichiometric
ratio. These experiments were carried out for EPON Resin 8280 cured with EPI-CURE
3223 with no particulate ﬁllers and with 12.29 volume % of Beta quartz. As seen from
Figure 40, reducing the quantity of curing agents in epoxy resins did not change the CTE
of the CTE of composites. This result conﬁrmed that Beta quartz particulate ﬁllers do not
accelerate the curing reaction in a way that will affect the CTE of the composites. This
leads one to conclude that Beta quartz particles are inert in curing reaction of epoxy
resins. Increase in the quantity of curing agent exhibited no effect on the CTE of the
composites as well. This result conﬁrmed that curing agent was not consumed by the
Beta quartz particulates through adsorption on the surface. The CTE of epoxy resins did
not change signiﬁcantly by reduction or addition of curing agent quantity, which meant
that curing of epoxy resins was almost complete by using stoichiometric ratio of the

curing agent.

3.1.12 Nature of interfaces of composites

Figures 41, through 43 show fracture surfaces of various composites. As observed
in these ﬁgures, failures occurred at the interfaces of particulate ﬁllers and the epoxy

resin matrix for composites with all three particulate ﬁllers. It proved that the interface of

110

crack propagation ,3...
around particle '

5
z‘
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Beta quartz ‘ '~

particle

 

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Figure 41. Micrograph of fracture surface of composite reinforced with 13.52 volume %
of Beta quartz. Matrix- EPON Resin 8280 cured with EPI—CURE 3223.

111

Cordierite
particle

L”

Cordierite
crack --> ~/

particle

05*

 

Figure 42. Micrograph of fracture surface of composite reinforced with 13.64 volume %
of Cordierite. Matrix— EPON Resin 8280 cured with EPI—CURE 3223.

112

”crack propagation
around aggregate of BS-}

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Figure 43. Micrograph of composite reinforced with 10.16 volume % of BS-SO. Matrix-
EPON Resin 8280 cured with EPI—CURE 3223. Fracture surface of the composite
indicates effect of aggregates diverting the cracks.

113

epoxy resins and particulate ﬁllers is weak due to high stress concentration present at the
interfaces. From the measured CTE values of the composites, it can be concluded that the
adhesion between the particulate ﬁllers and the epoxy resins is not perfect because CTE
reduction achieved is minimal even after adding large volumes of particulate ﬁllers.
Perfect adhesion at interfaces would have resulted in trans-particle and matrix failures,
and much less CTE of composites. Composites reinforced with Beta quartz and
Cordierite particles show crack propagation around the particle-matrix interfaces. In
composites reinforced with BS-SO, with presence of aggregates, stress and strain
concentration occurs at the interface of the aggregates and matrix. Thus aggregate
interfaces are weak spots for mechanical failures. Figure 43 shows that crack got diverted
around the interfaces of aggregates and matrix, indicating presence of high stress
concentration at the interface of aggregates and the matrix. Aggregate-matrix interfaces
can also be weak spots for initiating mechanical or chemical failure.

It can be seen from Figures 13 through 18, and Figures 41 through 43, that the
main mechanism for adhesion between the particulate ﬁllers and the epoxy resins seems
to be mechanical interlocking of the particulate ﬁllers with the matrices. Hence particles
of Beta quartz and Cordierite with rougher surfaces and high wettability seem to have
better adhesion with epoxy resins as compared to those with BS-SO particulate ﬁllers.

The main reasons for weak interface are, lack of chemical bonding between the
matrix and the particulate ﬁllers and high difference in CTE values of the phases in
composites. The particulate ﬁllers were treated with silane coupling agents to create
chemical bonds with the epoxy resin matrices and the results are discussed in following

sections.

114

 

3.2 Effect of treating particulate fillers with silane coupling agents

CTE of various composites made with particulate ﬁllers treated with silane
coupling agents are given in Tables 23 through 28. CTE values provided are based on
CTE of three specimens of each type of composite. Treatment of particulate ﬁllers with

silane coupling agents has multifold effects.

3.2.1 Effects of silane coupling agent treatment on CTE of composites

Figures 44 through 46 show that, treating particulate ﬁllers with silane coupling
agents did not result in any decrease of the CTE of composites. CTE of some of the
composites have actually increased slightly after treating particulate ﬁllers with silane

coupling agents, instead of resulting into any reduction.

3.2.2 Effects of silane coupling agent treatment of particulate fillers on
dispersion in epoxy resins and density of composites

Table 29 lists effects of treating particulate ﬁllers with silane coupling agents on
the dispersion of particulate ﬁllers in epoxy resin matrices. It can be observed in Figures
47 through 52, that treatment of particulate ﬁllers with silane coupling agents increased
the wettability of the surfaces and reduced aggregation. Treating particulate ﬁllers with
silane coupling agents has also reduced the pores in some composites such as Beta quartz
treated with Z6020 and BS-SO treated with Z6040. Hence the density of composites
containing particulate ﬁllers treated with silane coupling agents is closer to the expected
density of the composites. Table 30 lists effects of treating particulate ﬁllers with silane

coupling agents on the density of the composites. Figures 53 through 55 show the effect

115

 

 

Table 23. Effect of treating Beta quartz particles with silane coupling agent, on the CT E
of composites. Matrix-EPON Resin 8280 cured with EPI—CURE 3223.

 

 

 

 

 

 

V0] % 9f Beta CTE of composites % Reduction in
q“ “ m (p ml°C) CTE
composites p
0 82.3 0
13.52 67.7 17.74
13.52, treated with
Z6020 71.27 13.40
13.52, treated with
Z6040 70.53 14.30

 

 

 

Table 24. Effect of treating Beta quartz particles with silane coupling agent, on the CTE
of composites. Matrix-EPON Resin 8281 cured with EPI-CURE 3223.

 

 

 

 

 

Volunureazizqueta CTE of composites % Reduction in
q . (ppm/°C) CTE
composrtes
0 81 0
13.17 67 17.28
13.20, treated with
Z6020 68.25 15.74

 

 

 

Table 25. Effect of treating Cordierite particles with silane coupling agent, on the CTE
of composites. Matrix-EPON Resin 8280 cured with EPI-CURE 3223.

 

 

 

 

 

 

222313;: 3: CTE of composites % Reduction in
0
composites (pp ml 0 CTE
o 82.3 0
13.64 67.4 18. 10
136453;? With 7033 14.54
1164;:ng W“ 71.93 12.60

 

 

 

116

 

 

 

 

Table 26. Effect of treating Cordierite particles with silane coupling agent, on the CTE
of composites. Matrix-EPON Resin 8281 cured with EPI-CURE 3223.

 

 

 

 

 

‘Clzilaiii‘: 3f CTE of composites % Reduction in
. (ppm/°C) CTE
composrtes
0 81 0
13.16 61.4 24.20
12.35 3:51;? wrth 68.9 14.93

 

 

 

 

Table 27. Effect of treating BS-SO particles with silane coupling agent, on the CI‘E of
composites. Matrix- EPON Resin 8280 cured with EPI—CURE 3223.

 

 

 

 

 

 

“buglfpgﬁ‘so i“ "Egg/‘35?“ % Reduction in CTE
o 82.3 0
10.16 72.6 11.79
10.16 treated with Z6020 75.7 8.02
10.16, treated with Z6040 75.1 8.75

 

 

 

Table 28. Effect of treating BS-SO particles with silane coupling agent, on the CTE of
composites. Matrix- EPON Resin 8281 cured with EPI—CURE 3223.

 

 

 

 

 

 

V°'““;::P:::ess'50 i“ “Egg?“ % Reduction in CTE
o 81 o
6.9 70.8 11.35
10.47 67.4 16.80
9.22 treated with Z6020 70.8 12.60

 

 

 

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120

Table 29.

Observations about the effects of silane coupling agent treatment on

dispersion of particulate fillers in epoxy resin matrices.

 

 

 

 

 

 

Silane coupling
Matrix agent used Beta quartz Cordierite BS-50
for treating
particulate filllers
EPON Resin Some pores, Pores and a Pores, and
8280 cured no few large
with EPI— aggregates. aggregates. aggregates.
CURE 3223. (Figure 19). (Figure 20). (Figure 21).
EPON Resin Z6020 No ores no Pores and a Pores
833313;? agg’iegar’es. a 521.6. aggregates.
CURE 3223‘ (Figure 47). (giggufe 493 (Figure 51).
EPON Resin Z6040 Some pores, No aggregates Pores, but
8280 cured no few pores ’ smaller
with EPI- aggregates. (Fi re 50$ aggregates.
CURE 3223. (Figure 48). g“ ' (Figure 52).

 

 

 

 

 

121

 

B

. Beta quartziipafticrets /
~ (treated. with Z6020>i

.
/

 

Figure 47. Micrograph of fracture surface of composite reinforced with 13.52 volume
% of Beta quartz treated with Z6020. Matrix- EPON Resin 8280 cured with
EPI-CURE 3223. No porosities and no aggregates are observed.

122

 

Beta quartz particles __’
treated with 260465“

 

Figure 48. Micrograph of fracture surface of composite reinforced with 13.52 volume %
of Beta quartz particles treated with 26040. Matrix- EPON Resin 8280 cured with EPI-
CURE 3223.Few pores are present as indicated.

123

,

.Cordierite, particles /
treated with Z602?“ .

 

Figure 49. Micrograph of fracture surface of composite reinforced with 13.64 volume %
of Cordierite treated with Z6020. Matrix- EPON Resin 8280 cured with EPI—CURE 3223.
Few pores are present as indicated.

124

Cordierite particles
treated with 20040

 

Figure 50. Micrograph of fracture surface of composite reinforced with 13.64 volume %
of Cordierite with Z6040. Few pores are present as indicated. Matrix- EPON Resin 8280
cured with EPI—CURE 3223.

125

/

BS—SO particles
treated with 26020

' ¢

. "aggregate of
A/ . . 3350,; ‘

.f . ,
‘ particles

 

Figure 51. Micrograph of fracture surface of composite reinforced with 10.16 volume %
of BS—SO treated with Z6020. Matrix— EPON Resin 8280 cured with EPI-CURE 3223.
Fracture surface shows pores and aggregates as indicated.

126

BS-SO particles/
treated with Z6040 L

I

a??? .

/ “i"

g v
, .

‘

aygg‘reaga‘te

'- ¢ . I

 

Figure 52. Micrograph of fracture surface of composite reinforced with 10.16 volume %
of BS-50 treated with Z6040. Matrix- EPON Resin 8280 cured with EPI-CURE 3223.
Composite shows fewer pores and smaller aggregates compared to the composite
containing untreated BS-50 particles, shown in Figure 22.

127

Table 30. Observations about the effects of silane coupling agents on the density of the

composites. Matrix-EPON Resin 8280 cured with EPI-CURE 3223.

 

 

 

 

 

No coupling agent Particulate fillers Particulate fillers
Particulate filler treatment to treated with 2% treated with 2%
particulate fillers Z6020 Z6040
Measured density
1 3.5 volume % Measured density Measured density more than expected,
Beta quartz less than expected. close to expected. may be due to
experimental error.
13. 6 volume % Measured density Measured densrty Measured densuy
Cordierite less than expected very close to very close to
' expected. expected.
10.2 volume % Measured density Measured density Mifrgrglo‘sistigty
BS-SO less than expected. close to expected. expected.

 

 

 

 

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131

of particulate ﬁllers on the density of composites. The reason for increased wettability of
particulate ﬁllers after treating with silane coupling agents may be that, the water layer on
the surface of particulate ﬁllers is replaced by organofunctional silanes after treating with
silane coupling agent. Comparison of Figures 47 and 48, shows that treating Beta quartz
with Z6020 results in better dispersion in epoxy resins. Comparison of Figures 49
through 52 shows that treating Cordierite and BS-SO particles with Z6040, results in

better dispersion of these ﬁllers in epoxy resin.

3.2.3 Effect of silane coupling agent treatment on fracture behavior of
composites

Figures 56 through 58 show the effects of treating particulate ﬁllers with silane
coupling agents on fracture toughness of composites. Fracture toughness of composites
did not increase substantially after treating the particulate fillers with silane coupling
agents as would be expected. In some composites, the fracture toughness has rather
decreased. Figures 59 through 61 show the representative microstructures of fracture
surfaces of composites treated with silane coupling agent Z6020. Even after treating the
particulate fillers with silane coupling agents, fracture in all the composites took place at
interfaces of particulate ﬁllers with epoxy resins, which means the treatment of
particulate ﬁllers with silane coupling agents did not create strong enough bonds at

interfaces to result in trans-particle failures.

132

 

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135

7 4_——— Beta‘ quartz
. -0. ‘6. "
' partlcles tr‘éh‘ted

with 26020

O

 

Figure 59. Micrograph of fracture surface of composite reinforced with 13.52 volume %
of Beta quartz treated with Z6020. Matrix- EPON Resin 8280 cured with
EPI-CURE 3223.

136

at?"

Fﬁordiente particles
.0 _ ' treated with ZQQZO

K.

( 1’ Q g
f , ~_._.; 2!

4.9 ‘ ﬂ

 

Figure 60. Micrograph of fracture surface of composite reinforced with 13.6 volume %
of Cordierite particles treated with 26020. Matrix- EPON Resin 8280 cured with
EPI—CURE 3223.

137

BS—SO particles
treated with Z6020

 

Figure 61. Micrograph of fracture surface of composite reinforced with 10.16 volume %
of BS-SO treated with 26020. Matrix- EPON Resin 8280 cured with EPI—CURE 3223.

138

3.2.4 Reasons for ineffectiveness of silane coupling agent treatment of

particulate fillers on CTE and fracture toughness of the composites

There are various possible reasons for the ineffectiveness of silane coupling agent
treatment of particulate ﬁllers on CTE and fracture toughness of composites. They are as
follows-

1. The concentration of silane coupling agents may have been too high while treating the
particulate ﬁllers. This can result in the excessive adsorption of silane coupling agents
instead of monolayer, on the surface of particulate ﬁllers [92]. The excess adsorbed layer
of silane coupling agent supposedly diffuses into the epoxy resin. It can create blend of
epoxy resin and silane coupling agent, near the surface of particulate fillers, and this
blend presumably has inferior properties than the epoxy resin matrix. Graf et al. have
shown that the decrease in strength of composites is directly proportional to the increase
in concentration of silane coupling agent for ﬁbrous ﬁllers [92]. If the particulates ﬁllers
with known particle size distribution or known surface area are used, it is possible to
calculate exact quantity of the silane coupling agent required to form monolayer on the
surface of particulate ﬁllers. An alternate approach is to carry out several trials with
varying quantities of silane coupling agents.

2. The silane coupling agent treatment of particulate ﬁllers is conducted in two steps,
hydrolysis of the mixture of silane coupling agent, water and methanol, to generate
Silanetriols followed by condensation of Silanetriols with the hydroxyl group present on
the surface of the particulate ﬁllers. One may hypothesize, since the hydrolysis time was
12 hours, it is likely that the condensation reaction has completed, even before adding the

particulate ﬁllers into the solution of silane coupling agent, water and methanol.

139

3. The differences in the CT E of the matrices and the particulate ﬁllers are very high.
The glass transition temperatures of the epoxy resins used in this study are high. Hence
high thermal stresses are generated at the interface. One may hypothesize that, the
strength of bond formed due to silane coupling agents is not enough to withstand the
thermal stresses developed at the interfaces.

4. Exact nature of functional groups present on the surface of particulate ﬁllers is not
known. It is difﬁcult to predict the chemical products formed and pH changes that
occurred on treating the particulate ﬁllers with silane coupling agents.

One more possibility is that the silane coupling agent treatment is successful, but
there are no effects on the CTE of composites. The elastic modulus and CTE of
composites are related. It has been observed that silane coupling agent treatment of
particulate ﬁllers does not alter the elastic modulus of composites, although it reduces the
degradation of elastic modulus when exposed to water [93]. Such anomalies may explain
why there is no noticeable change in CTE of composites even if the silane coupling

agents might have formed bond between the epoxy resins and the particulate fillers.

3.2.5 Effect of excess silane coupling agents on CTE of epoxy resins

There is a possibility that excess silane coupling agent present on the particulate
ﬁllers may have blended with epoxy resin to form complex phase, which decreased the
CTE of the epoxy resins and so effectively decreased the CTE of composites. The effect
of blending excess coupling agent with the epoxy resin on the CTE of epoxy resin was
tested. 1% of Z6020 and 1% of Z6040 silane coupling agents by weight were deliberately

added in the epoxy resins, containing no particulate fillers and the CTE of the cured

140

epoxy resins were measured. Figure 62 shows the effect of deliberate addition of excess
silane coupling agents to epoxy resin matrices. It is clear that the excess silane coupling
agents added to the epoxy resin matrices did not signiﬁcantly affect the CTE of epoxy

resin matrices used in this study.

3.2.6 Recommendations for improving the silane coupling agent
treatment of particulate fillers

Recommendations to improve performance of silane coupling agents:

1. Z6020 and Z6040 silane coupling agents were selected based on the manufacturer’s
recommendation, because these coupling agents were found to be successful with silica
ﬁllers by other users, in past. It is likely that other coupling agents with different
functionalities may prove to be more compatible with the particulate ﬁllers used in this
study.

2. Various ways to improve the treatments of particulate ﬁllers with silane coupling
agents can be explored. One way is not to prehydrolyze the silane coupling agents and
reduce the silane coupling agent addition to 0.5 % from 2%. Another way is to
prehydrolyze silane coupling agents, but reduce the amount of silane coupling agent
added.

3. Condensation reaction might complete in the aqueous solution itself if hydrolysis time
is prolonged. Hence it is important to treat particulate ﬁllers with hydrolyzed solution

of silane coupling agents when the solution is most reactive.

141

 

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3.3 Comparison of experimental CTE values of composites with those

predicted by mathematical models
Figures 63 through 69 compare experimental values of CTE of composites

containing three types of particulate ﬁllers in three epoxy resin matrices, with those
predicted by various mathematical models. These observations are compiled in Table 31.
For EPON Resin 9405, at low volume fractions of Beta quartz reinforcement, CTE of
composites is same as the CTE of matrix. CTE of these composites vary as per Kemer’s
model at high volume fractions of Beta quartz. CTE of the composites reinforced with
Beta quartz and Cordierite particulate ﬁllers in EPON Resin 8280, vary according to the
CTE values predicted by Kemer’s model. CTE of composites reinforced with BS-SO
particulate ﬁllers in EPON Resin 8280, vary according to the rule of mixtures. CTE of
the composites with EPON Resin 8281 as matrix are lower than those predicted by
Kemer’s model for all particulate ﬁllers. Equation of Turner’s model and simpliﬁed
equation of Turner’s model yield same value of CTE for the composites when the
particulate ﬁllers and the matrix have nearly same values for the Poisson’s ratio. This
happens in the case of Cordierite, which has Poisson’s ratio of 0.34, a value same as that
for epoxy resin used.

Tummala et al. have shown that CTE of particulate reinforced composites
closely agree with the CTE values predicted by Kemer’s model. [18,94]. The
mathematical models made assumptions regarding the size, shape and surface area,
distribution of the particulate ﬁllers to facilitate the computations. But presence of
aggregates i.e. non-homogeneous composites, and insufﬁcient adhesion between the

particulate ﬁllers and matrices, result in higher CTE values of composites than those

143

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Table 31. Observations about best ﬁt mathematical models for predicting CI‘E of the

composites, developed by reinforcing three types of epoxy resins with three types of

particulate ﬁllers, in this study.

 

 

 

 

 

 

Matrix Beta quartz Cordierite BS-50
EPON Resin 8280
cured with EPI- Kemer’s model Kemer’s model. Rule of mixtures.
CURE 3223
Lower Lower
EPON Resin 8281 Low“ “Penmcmal “Penmcmal experimental CTE
. CTE CTE values,
cured wrth EPI— . . values, than
Values,than predrcted than predicted .
CURE 3223 . . Predlcted by
by Kemer 5 model by Kemer s ,
Kemer s model.
model
At low volume fractions
of Beta quartz, CTE of
EPON Resin 9405 composites is same as
cured with the CI'E of matrix. CTE
Ancamine 9470 varies as per Kcmer’s

 

model at high volume
fractions of Beta quartz.

 

 

151

 

 

 

 

predicted by mathematical models. Kemer’s mode] serves as upper bound and Turner’s
model serves as the lower bound for predicting the CTE of the particulate reinforced
composites. It is desirable to achieve CTE values as low as predicted by Turner’s model
in particulate reinforced epoxy resin matrix composites. There have been ﬁndings that
Turner’s equation may be used to predict CTE of various composite systems, and
predicted values in the systems cited closely matched with the experimental values of
CTE of the composites [42,75]. CTE of metal-metal composites, ceramic-metal
composites and polymer composites reinforced with ﬁbers, show good agreement with
Turner’s equation. All these cases are discussed on an individual basis. The differences
between the properties of phases of composites used in this study, and those of
composites for which Turner’s model has been successfully applied, are pointed out.
According to Turner’s model -

mPtKl + a'szKz

 

 

 

. _ d1 d2
momm— P1K1+P2K2
d1 d2
where,

on] = linear CTE of phase lin composite,
a2 = linear CTE. Of phase 2 in composite,
P1 = percent by weight of phase 1,

P2 = percent by weight of phase 2,

d1 = density of phase 1,

d2 = density of phase 2,

K1 = bulk modulus of phase 1, and

K; = bulk modulus of phase 2.

152

 

 

~10

 

As indicated in introduction, bulk modulii can be replaced by elastic modulii
provided the Poisson’s ratio of both the phases present in the composite are comparable.
The bulk modulus is the ratio of the hydrostatic pressure to the dilation, that it produces,
and it is indicated by the letter K. Density is the mass per unit volume, and it is indicated
by letter ‘d’. K/d or E/d ratio for the particulate ﬁller and that of the matrix are the
signiﬁcant terms in Turner’s equation for predicting the CTE of composite. The elastic
constants and Kid ratios and E/d ratios for various particulate ﬁllers and composite
matrices discussed here, are listed in Table 32.

Turner’s equation was used to predict CTE of the composites prepared by mixing
magnesia and tungsten in various volumetric pr0portions [95]. Turner’s equation was also
used to predict CTE of composites prepared by mixing silica glass and aluminum in
various volumetric proportions. These composites were prepared by pressing the mixed
powders followed by sintering. The important points to be noted in these composites are,
the difference in the CTE values of the two phases i.e. magnesia and tungsten as well as
that between aluminum and silica are much smaller compared to the differences in the
CTE of epoxy resin and particulate ﬁllers, used in this study. Smaller CI‘E difference
between phases of composites results in lesser thermal stresses in composites.
Additionally, in magnesia-tungsten composites as well as aluminum-silica composites,
there is high probability that bonding will be generated between the phases. The K/d ratio
of magnesia and aluminum are high. It can be veriﬁed from Turner’s equation that the
least CTE of the composites will be obtained when both phases have equal bulk modulii.
When the difference between CTE of the two phases of composites is small, CTE of the

composite is low even with when difference between K/d ratio of the phases is high.

153

 

 

 

Table 32. Elastic constants for various ﬁllers and matrix materials [3,43,47,82,97].

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Density Elastic Poisson’ Bulk CTE
Material ‘d’ modulus s ratio modulus Eld K/d (ppml°
(glee) ‘E’ (GPa) ‘K’ (GPa) C)
Beta quartz 2.533 100.6 0.203 56.5 39.7 22.3 -1
Cordierite 2.508 117.3 0.31 129 46.8 51.4 2.3
BS-SO 3.50 74.5 0.22 ~29.76 21.3 8.5 -25
59°.“ 1.19 3.8 0.34 3.96 3.2 3.33 81
1'83“]
MgO 3.58 207 0.169 154.1 57.8 43.0 13
w 19.3 355.35 0.287 308.1 18.4 15.9 4
Silica glass 2.2 69.7 0.17 35.20 31.7 16.0 0
Aluminum 2.702 70.8 0.348 77.5 26.2 28.7 23.6
Antimony 6.7 80.3 0.207 45.6 12 6.81 11
Lead 1 1.34 28.1 0.39 42.7 2.5 3.8 29
Beryllium 1.8 307.7 0.1 17 133.9 171 74.4 12
Phenol
formal- 1.3 6.9 - - 5.3 59
dehyde
Glass 2 58 72 5 - - 28 1 7 44
ﬁbers ' ° ' '
P°'y‘ 1.05 2.75 0.33 2.69 2.6 2.56 70
styrene
Aluminum 3.99 380 0.23 252 95.2 63.2 8.7
Ox1de
calm“ 2.65 179 ~o.19 ~96.24 67.6 36.3 10
carbonate
Anhydlws 2.58 138 ~O.28 ~104.55 53.5 40.5 8
kaolln

 

 

 

 

 

 

 

 

154

 

 

Table 32. (cont’d)

 

 

 

 

 

 

 

 

 

 

 

 

 

Density Elastic Poisson’ 5 Bulk CTE
Material ‘d’ modulus ratio modulus Eld K/d (ppm/°
(g/cc) ‘E’ (GPa) ‘K’ (GPa) C)
Talc 2.8 138 _ _ 49.3 _ 8
Mica 2.79 100.8 0.227 61.5 61.5 22.6 8
WOW 2.9 207 - - 71.4 - 6.5
stonlte
Lithium
Aluminum 2.3 68 ~0.28 51.52 29.6 22.4 0.5
Silicate

 

Note- Units for K/d and Eld are GPa/g/cc

155

 

Turner’s equation uses weight percent of the phases. Lower the value of density
of any of the phases in composite, higher will be the volume of that phase in composite
and CTE of the composite will be lower as can be seen from the Turner’s equation.

If one of the phases of composite has high K value, then the stresses will be
transferred easily across the interface of two phases, resulting into minimal CTE for the
composite. Suppose particulates with high bulk modulus are surrounded by matrix with
low bulk modulus and bonded to the matrix. During thermal expansion of such a
composite, particulates with higher bulk modulus will restrain the matrix from expansion,
resulting in low CTE composite.

Turner’s equation was successfully applied to many alloy systems. In the original
paper it is applied to lead-antimony and beryllium-aluminum composites [42]. It can be
noted that the difference in the CTE of antimony and lead is very small. Difference in
CTE of beryllium and aluminum is also small. The CTE values of these composites, as
predicted by rule of mixture, by Turner’s model and by Kemer’s model, are very close. In
beryllium-aluminum system, K/d is high for beryllium. Most importantly, since both
lead-antimony and beryllium-aluminum form solid solutions, the bonding between the
phases present will be very strong.

The observed values of CTE of glass ﬁbers reinforced phenol formaldehyde
matrix composites were very close to those predicted by Turner’s equation [42]. In this
composite, the difference in CTE of the phenol formaldehyde and ﬁbers is high, but still
not as high as the difference in the CTE of ceramic particulate ﬁllers and epoxy resins
used in present study. The lower difference in CTE of phases of composite, generate

smaller thermal stresses. The E/d ratio of the ﬁbers is higher compared to the phenol

156

formaldehyde, this will allow transfer of stresses across the interface of ﬁbers and phenol
formaldehyde. Density of phenol formaldehyde is 1.3g/cc, which is higher, as compared
to density of epoxy resins (1.19 g/cc) used in this study. High density of phenol
formaldehyde results in better packing of ﬁbers in the matrix. Unlike particulate ﬁllers,
the uniform arrangement of continuous ﬁbers constrains the matrix most [96]. This
results in signiﬁcant improvement in the mechanical properties of the composites due to
maximum transfer of stresses across the interfaces.

Turner’s equation was also used to predict the CTE of composites reinforced with
aluminum oxide particles in polystyrene matrix. [42]. The difference between the CTE of
these two phases is ~60 ppm/°C, while the difference between the CT E of particulate
ﬁllers and epoxy resins used in this study is ~80 ppm/°C. Smaller difference in CTE of
the phases in a composite generates smaller thermal stresses. Additionally, aluminum
oxide has very high K/d ratio compared to that of Beta quartz, Cordierite and BS-50.

From the ongoing discussion, one can conclude that the most important
differences between the phases of composites used in this study and those of the
composites for which Turner’s equation was successfully applied are-

1. Difference in the CTE of the particulate ﬁllers and the epoxy resins
used in this study is the highest, compared to the CTE differences in
phases of all the composites discussed so far. High CTE differences
cause maximum thermal stresses.

2. No bonds are formed between the particulate ﬁllers and epoxy resins,
used in this study. Particulate ﬁllers are adhered to the epoxy resin

matrix mainly through mechanical interlocking.

157

 

 

 

The density of the epoxy resins is lower as compared that of phenol
formaldehyde.

The K/d ratio of ceramic particulate ﬁllers is smaller compared to that
of aluminum oxide. Mica, Wollastonite which are commercially
popular particulate ﬁllers for reducing CTE of polymer matrices, have
higher K/d ratio, as compared to ceramic particulate ﬁllers used in this
study, as shown in Table 32. Lithium aluminum silicate, is also
commonly used for reducing the CTE of the polymer matrix
composites. K/d ratio for Lithium aluminum silicate and Beta quartz are
very close. K/d ratio of Cordierite is higher than Beta quartz. BS-50 has
very small value of KM ratio compared to Beta quartz, Cordierite and
other particulate ﬁllers. During the course of this investigation, it was
found that the CTE reduction is least with BS-50 reinforcements as
compared to that with other two particulate ﬁllers. One of the factors

responsible for poor performance of BS-SO may be its low K/d ratio.

158

 

3.4 Conclusions

1.

Amongst three epoxy resins used in this study, cured EPON Resin 9405 shows the
best distribution of particulate ﬁllers with no agglomeration along with minimal
porosity. However, the low viscosity monomer, results in lower density of cured
epoxy resin and it does not provide the lowest CTE composite.

EPON Resin 8281, which has uses high viscosity monomer and has high density
after curing, gives minimal CTE for composites reinforced with any of the three
particulate ﬁllers.

Amongst three particulate ﬁllers used in this study, Beta quartz particulates show
maximum wettability, and least agglomeration with the epoxy resins. BS-50
particulate form maximum aggregates as well as maximum pores in the
composite. The CTE reductions achieved with Cordierite and Beta quartz are
comparable. BS-SO particles give minimal CTE reduction in composites.

Treating the particulate ﬁllers Beta quartz, Cordierite and BS-50 with silane

coupling agents Z6020 and Z6040, did not reduce CTE of the composites.

. However, treating the Beta quartz particulates with silane coupling agent Z6020,

and treating Cordierite and BS-SO particulates with silane coupling agent Z6040

improved the distribution of these particulate ﬁllers in the epoxy resin matrix.

159

3.5 Recommendations for future work

1.

It is observed in this study, that uniform distribution of particulate ﬁllers is
essential to achieve maximum possible reduction in CTE values. To achieve this
uniform distribution of particulate ﬁllers, mechanical mixing or ultrasonic mixing
should be employed in the preparation of the composites.

Thermal strain at the interfaces of epoxies and particulate ﬁllers is proportional to
the difference in CTE of the two phases and to the temperature difference
between temperature of stress free state and room temperature. Hence it is
important to use epoxy monomer and curing agent combination, which will result
in minimum thermal stresses at the interface region. These thermal stresses can
cause interfacial debonding and make the reinforcements ineffective. Selecting
epoxy resins with low glass transition temperature may also help in reducing
thermal stresses.

The most effective mechanism of adhesion between particulate ﬁllers and the
polymer matrices is chemical bonding. If this cannot be achieved, use particulate
ﬁllers with rough surfaces can facilitate mechanical interlocking.

Wettability of particulate ﬁllers plays key role in effective processing of
composites. Wettability is difﬁcult to measure. Polymers have lower surface
energies compared to ceramic particulate ﬁllers. It has been pointed out, that
higher the difference in surface energies of particulate ﬁllers and the polymer,
higher will be the wettability. For solid materials, generally, higher the hardness,

higher is their surface energy. BS-SO particles had least hardness and also least

160

 

wettability. Hence particulate ﬁllers with high hardness values should be selected
for making composites.

. Higher the K/d ratio of the particulate ﬁller, lower is the CT E of particulate
reinforced polymer matrix composites. Hence is important to select particulate
ﬁllers with high K/d values.

. If the concentration of silane coupling agents used for the treatment of particulate
ﬁllers is excess then there may be deteriorating effects on the composite
properties. If the concentration of silane coupling agents is less, then sufﬁcient
bonding cannot be produced at the interfaces of particulates and the epoxy resins.
Yarnaguchi et al. have demonstrated the use of formula for calculating the exact
concentration of silane coupling agents required for the monolayer coverage of
particulate ﬁllers [98]. This formula or similar method should be used to calculate
exact concentration of silane coupling agents, when the size of particulate ﬁllers
is known.

. Low viscosity of epoxy monomer helps in easy distribution of particulates, while
high viscosity avoids settling of particulate ﬁllers. Hence it is important to use
epoxy monomer of optimum viscosity. The viscosity can be tailored by
controlling temperature and by adding diluents.

. Size of particulate ﬁllers plays key role in affecting CTE of composites. Fine
particles have signiﬁcant tendency to agglomerate. But ﬁne particles show least
settling tendency. Hence it is important to establish optimum size of particulate

ﬁllers for the particular type of epoxy resin.

161

 

 

 

 

9.

10.

ll.

12.

Drying of particulate ﬁllers at high temperatures and use of vacuum for drying
can be more effective to reduce absorbed water and it may also reduce
agglomeration of particulate ﬁllers.

Both silane coupling agents used in this study had methoxy group for reacting
with particulate ﬁllers. Silane coupling angents with different reactive groups or
other type of coupling agents should be checked for their effectiveness.

Presence of pores can affect the CTE of composites in various ways. If the pores
contain air, that air may expand and may result in compressive stresses in matrix.
Other possibility, as indicated by Hatta et al., is that presence of small volume
percent of pores can cause additional reduction in CTE of composites [99]. The
rationale for this is as follows: in particulate reinforced composites, the particles
are in tension while the matrix is in compression. Pores have zero stiffness. Hence
they help reduce the compressive stresses of the matrix, and the presence of such
pores reduces the overall CTE of composites.

The exact effect of pores on CTE of composites needs to be understood. If the
effect of pores is found to be detrimental, it is possible to control pressure and
temperature of curing process in order to control the amount of porosity. It is also
possible to add air release agents in order to reduce the pores in composites.

In order to reduce stresses in composites, photosensitive ﬁllers such as cinnamic
acid resins and cinnamal ketones can be added up to l-3%, to epoxy resins and
then these epoxies can be exposed to strong ultraviolet rays to relieve internal

stresses [45]. This might reduce the CTE of composites further.

162

 

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