akron 1158067153
TRANSCRIPT
CURE AND MECHANICAL PROPERTIES OF CARBOXYLATED NITRILE
RUBBER (XNBR) VULCANIZED BY ALKALINE EARTH
METAL COMPOUNDS
A Dissertation
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Tulyapong Tulyapitak
December, 2006
ii
CURE AND MECHANICAL PROPERTIES OF CARBOXYLATED NITRILE
RUBBER (XNBR) VULCANIZED BY ALKALINE EARTH
METAL COMPOUNDS
Tulyapong Tulyapitak
Dissertation
Approved: Accepted: ______________________________ ______________________________ Advisor Department Chair Dr. Gary R. Hamed Dr. Mark D. Foster ______________________________ ______________________________ Co-Advisor Dean of the College Dr. Frank N. Kelley Dr. Frank N. Kelley ______________________________ ______________________________ Committee Chair Dean of the Graduate School Dr. Darrell H. Reneker Dr. George R. Newkome _____________________________ ______________________________ Committee Member Date Dr. Alexei P. Sokolov ______________________________ Committee Member Dr. Ali Dhinojwala ______________________________ Committee Member Dr. Avraam I. Isayev
iii
ABSTRACT
Compounds of carboxylated nitrile rubber (XNBR) with alkaline metal oxides and
hydroxide were prepared, and their cure and mechanical properties were investigated.
Magnesium oxide (MgO) with different specific surface areas (45, 65, and 140 m2/g) was
used. Increased specific surface area and concentration of MgO resulted in higher cure
rate. Optimum stiffness, tensile strength, and ultimate strain required an equimolar
amount of acidity and MgO. The effect of specific surface area on tensile properties was
not significant. Crosslink density of XNBR-MgO vulcanizates increased with increased
amounts of MgO. ATR-IR spectroscopy showed that neutralization occurs in two steps:
(1) During mixing and storage, MgO reacts with carboxyl groups (RCOOH) to give
RCOOMgOH. (2) Upon curing, these react bimolecularly to form RCOOMgOOCR and
Mg(OH)2. Dynamic mechanical thermal analysis revealed an ionic transition at higher
temperature, in addition to the glass transition. The ionic transition shifts to higher
temperature with increasing MgO concentration. Like MgO-XNBR systems, cure rates of
XNBR-calcium hydroxide (Ca(OH)2) and XNBR-barium oxide (BaO) compounds
increased with increased content of curing agents. Curing by these two agents resulted in
ionic crosslinks. To ensure optimum tensile properties, equimolar amounts of carboxyl
groups and curing agents were required.
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Dynamic mechanical analysis revealed the ionic transition in these two systems. It shifted
to higher temperature with increased amounts of curing agents. In contrast to MgO,
Ca(OH)2, and BaO, calcium oxide (CaO) gave results similar to those for thermally cured
samples. No ionic transition was observed in XNBR-CaO systems. Tensile strength of
XNBR depended on the strength of ionic crosslinks, which was dependent on the size of
the alkaline metal ions.
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ACKNOWLEDGEMENTS
I really appreciate the support and guidance provided by my advisor, Dr. Gary R.
Hamed. Your enthusiasm, helpful advises, and attention to detail have motivated, and
inspired me to become a better scientist. I would like to place my special thanks to my
co-advisor Dr. Frank N. Kelley, who helps me through a tough situation.
I would like to thank all my committees, Dr. Darrell H. Reneker, Dr. Alexei P.
Sokolov, Dr. Ali Dhinojwala, and Dr. Avraam I. Isayev for useful comments.
I wish many thanks to Dr. Alan N. Gent for his helpful suggestion, and comments.
I would like to extend my sincere thanks to Mr. Robert Seiple, Dr. Critt Olemacher, and
Mr. John Page for your friendly and unconditional help with instrumental analysis. I am
grateful to my group members for their support and friendships.
Most of all I would like to thank my family members, especially my mother, who
has never given up on me. Without you, I will not be a person I am today; Kia, my
beloved wife, who sacrifices her career for taking care of me. I have learned from the first
day we met that you will never leave me behind, and so will I.
Finally, I would like to thank Royal Thai Government for all kinds of support, and
opportunity.
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TABLE OF CONTENTS
Page
LIST OF TABLES.............................................................................................................. x
LIST OF FIGURES .......................................................................................................... xii
CHAPTER I INTRODUCTION........................................................................................................ 1
II HISTORICAL REVIEW.............................................................................................. 3
2.1 Vulcanization of Carboxylic Rubbers....................................................................... 3
2.1.1 Sulfur and Peroxide Vulcanization .................................................................... 4
2.1.2 Vulcanization via Reactions of Carboxyl Groups ............................................. 4
2.1.3 Cure Behavior of Carboxylic Rubbers............................................................. 10
2.2 Rubber Reinforcement ............................................................................................ 13
2.2.1 Reinforcement by Particulate Fillers ............................................................... 13
2.2.2 Reinforcement by Thermodynamic Phase Separation..................................... 15
2.2.3 Reinforcement by Reaction-Induced Phase Separation................................... 16
2.3 Tensile Strength of Rubbers.................................................................................... 16
2.4 Ionic Aggregation ................................................................................................... 20
2.4.1 Theory .............................................................................................................. 20
2.4.2 Experimental Evidence .................................................................................... 23
vii
2.4.3 Ionic Aggregation Models ............................................................................... 25
2.5 Mechanical Properties of Carboxylated Rubbers ................................................... 26
2.5.1 Effect of Carboxyl Content.............................................................................. 28
2.5.2 Influence of Types of Metal Oxides or Salts ................................................... 30
2.5.3 Effect of Metal Oxide Level ............................................................................ 33
2.5.4 Effect of Specific Surface Area ....................................................................... 35
2.5.5 Effect of Filler.................................................................................................. 36
2.5.6 Effect of Plasticizers ........................................................................................ 40
III EXPERIMENTAL..................................................................................................... 42
3.1 Materials ................................................................................................................. 42
3.1.1 Carboxylated Nitrile Rubber (XNBR) ............................................................. 42
3.1.2 Curing Agents .................................................................................................. 42
3.1.3 Solvents............................................................................................................ 43
3.2 Equipments ............................................................................................................. 43
3.3 Compound Preparation ........................................................................................... 43
3.3.1 XNBR-Magnesium Oxide Compounds ........................................................... 43
3.3.2 XNBR-Peroxide Compounds........................................................................... 45
3.3.3 Compounds of XNBR and Other Metal Oxides or Compounds...................... 46
3.4 Cure Behaviors and Molding.................................................................................. 48
3.5 Molding................................................................................................................... 48
3.6 Tensile Testing........................................................................................................ 49
3.7 Crosslink Density Measurements ........................................................................... 50
3.7.1 Near Equilibrium Stress-Strain measurement.................................................. 50
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3.7.2 Volume Fraction (Vr) of Rubber by Equilibrium Swelling ............................. 51
3.8 Dynamic mechanical properties.............................................................................. 53
3.9 Infrared spectral analysis ........................................................................................ 53
IV RESULTS AND DISCUSSION................................................................................ 54
4.1 Cure Behaviors........................................................................................................ 54
4.1.1 XNBR-MgO Compositions ............................................................................. 54
4.1.2 XNBR-Dicumyl Peroxide Compositions......................................................... 63
4.1.3 XNBR-CaO Compositions............................................................................... 66
4.1.4 XNBR-Ca(OH)2 Compositions........................................................................ 66
4.1.5 XNBR-BaO Compositions............................................................................... 71
4.2 Crosslink Density Measurements ........................................................................... 71
4.2.1 Thermally-Cured XNBR.................................................................................. 71
4.2.2 XNBR-MgO Vulcanizates ............................................................................... 76
4.2.4 XNBR-CaO Vulcanizates ................................................................................ 79
4.2.5 XNBR-Ca(OH)2 Vulcanizates ......................................................................... 81
4.2.6 XNBR-BaO Vulcanizates ................................................................................ 85
4.2.7 Comparison among Metal Compounds ........................................................... 87
4.3 Tensile Properties.................................................................................................... 91
4.3.1 Thermally Cured XNBR.................................................................................. 91
4.3.2 XNBR-MgO Vulcanizates ............................................................................... 94
4.3.3 XNBR-Peroxide Vulcanizates ....................................................................... 100
4.3.4 XNBR-CaO Vulcanizates .............................................................................. 100
4.3.5 XNBR-Ca(OH)2 Vulcanizates ....................................................................... 103
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4.3.6 XNBR-BaO Vulcanizates .............................................................................. 108
4.3.7 Comparison of Tensile Properties among Metal Compounds ....................... 108
4.3.8 Comparison between Ionic and Covalent Crosslinks .................................... 116
4.4 ATR-IR Spectroscopy........................................................................................... 117
4.4.1 Thermally Cured XNBR................................................................................ 117
4.4.2 XNBR-MgO Compositions ........................................................................... 125
4.4.3 XNBR-CaO Compositions............................................................................. 132
4.4.4 XNBR-Ca(OH)2 Compositions...................................................................... 138
4.4.5 XNBR-BaO Compositions............................................................................. 144
4.4.6 Comparison among Metal Compounds ......................................................... 145
4.5 Dynamic Mechanical Properties ........................................................................... 152
4.5.1 XNBR-MgO Vulcanizates ............................................................................. 152
4.5.2 XNBR-CaO Vulcanizates .............................................................................. 163
4.5.3 XNBR-Ca(OH)2 Vulcanizates ....................................................................... 167
4.5.4 XNBR-BaO Vulcanizates .............................................................................. 172
4.5.5 Comparison among Metal Compounds ......................................................... 177
V CONCLUSIONS..................................................................................................... 179
REFERENCES ............................................................................................................... 181
APPENDICES ................................................................................................................ 190
APPENDIX A CURE PROPERTIES......................................................................... 191
APPENDIX B TENSILE PROPERTIES ................................................................... 193
APPENDIX C MOLECULAR TRANSITION TEMPERATURE ............................ 204
x
LIST OF TABLES
Table Page
2.1 Influence of salt formation on tensile properties of butadiene- methacrylic acid copolymer containing carboxyl group of 0.12 ephr (equivalent per a hundred part of rubber)…………………………………
26
2.2 Tensile properties of gum XSBR vulcanized by 10 phr of divalent metal oxides and hydroxides……………………………………………………..
32
2.3 Active and inactive metal compounds……………………………………
32
2.4 Influence of HAF carbon black loading on mechanical properties of ZnO-cured XNBR………………………………………………………………
37
2.5 Effect of silica types on tensile properties of XNBR vulcanizates………..
39
2.6 Effect of clay and calcium carbonate on tensile properties of ZnO-vulcanized XNBR vulcanizates…………………………………………...
40
3.1 Formulations of XNBR-MgO compounds………………………………...
44
3.2 Mixing method……………………………………………………………
45
3.3 XNBR-DCP formulations…………………………………………………
46
3.4 XNBR-CaO compositions…………………………………………………
47
3.5 XNBR-Ca(OH)2 compositions……………………………………………
47
3.6 XNBR-BaO compositions…………………………………………………
47
3.7 Designation and stoichiometric amount of metal oxides or compounds….
47
3.8 Cure times for compositions……………………………………………… 49
xi
4.1 Volume fraction of rubber (Vr), sol content, and crosslink density of the raw XNBR cured at 165 oC………………………………………………
74
4.2 Volume fraction (Vr) of rubber, sol content, and crosslink density (ν) of XNBR cured with different magnesium oxides (120 min at 165 oC)……
77
4.3 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with dicumyl peroxide 60 min at 165 oC…………………...
79
4.4 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium oxide 1000 min at 165 oC……………………
81
4.5 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium hydroxide 240 min at 165 oC…………………
83
4.6 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with barium oxide 240 min at 165 oC………………………
85
4.7 The effective ionic radii of Mg++, Ca++, and Ba++ ions with various coordination numbers……………………………………………………...
90
4.8 Characteristic group frequencies of the raw XNBR………………………
122
4.9 Characteristic group frequencies of XNBR-MgO compositions……….....
126
4.10 Characteristic group frequencies of XNBR-Ca(OH)2 samples……………
139
4.11 Characteristic group frequencies of XNBR-BaO samples………………...
145
4.12 Molecular transition temperatures of XN-P1.0, and XN-MgA vulcanizates at a frequency of 1.0 Hz……………………………………..
159
4.13 Molecular transition temperatures of XN-P1.0, and XN-CaO vulcanizates at frequency 1.0 Hz…………………………………………
167
4.14 Molecular transition temperatures of XN-P1.0 and XN-Ca(OH)2 vulcanizates at frequency 1.0 Hz…………………………………………
172
4.15 Molecular transition temperatures of XN-P1.0 and XN-CaO vulcanizates at frequency 1.0 Hz…………………………………………
173
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LIST OF FIGURES
Figure Page 2.1 Cure rheometry of XNBR containing ZnO of different surface area at
twice stoichiometry (S = 35 m2/g, M = 3.5 m2/g, and L = 0.5 m2/g)……...
11
2.2 (a) Cure rheometry, and (b) bin stability of the ZnO-XNBR compounds along with those of the ZnO2-XNBR compound. (KRYNAC PA-50 is a 50/50 masterbatch of medium acrylonitrile NBR and technical grade ZnO2)………………………………………………………………………
12
2.3 Two dimensional schematic of spherical particles of diameter d arranged on a three dimensional square lattice. (s is particle-particle spacing, and t is the thickness of restricted mobility layer of rubber chains)……………………………………………………………………..
14
2.4 Schematic of an ideal rubber network…………………………………….
18
2.5 Tensile strength of gum NR vulcanizates as a function of 1/Mc for various vulcanization systems. ○ accelerated sulfur; × TMT sulfurless; ● peroxide; ∆ high energy radiation……………………………………….
19
2.6 SAXS profiles of a) low density polyethylene, b) a copolymer of ethylene-methacrylic acid, and c) a sodium salt (90% neutralization) of the copolymer……………………………………………………………...
24
2.7 Tensile properties of carboxylic nitrile rubber (0.099 ephr of COOH) cured by various curing systems. A) 0.2 ephr of ZnO, B) sulfur + zinc dimethyldithiocarbamate (0.003 ephr of unavailable zinc), C) 0.2 ephr of zinc + sulfur + zinc dimethyldithiocarbamate (0.003 ephr of unavailable zinc)………………………………………………………………………..
27
2.8 Tensile strength as a function of carboxyl content in butadiene-methacrylic acid copolymers treated with an excess amount of ZnO (twice stoichiometry)……………………………………………………...
28
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2.9 Modulus-temperature behavior of butadiene-methacrylic acid copolymers and their lithium salts. (---) RA1 4.7% acid, (− −) RA2 7.7 % acid, (×) RA3 11.6 % acid, (○) RA1 Li 4.7 % salt, (□) RA2 Li 7.7 % salt, (∆) RA3 Li 11.6 % salt……………………………………………………………...
29
2.10 Tensile properties of carboxylic nitrile rubber (butadiene: acrylonitrile: methacrylic acid; 55:35:10) containing 0.1 ephr of carboxyl groups vulcanized by various metal salts and oxides……………………………..
31
2.11 Effect of ZnO level on tensile properties of the carboxylic nitrile rubber (butadiene: acrylonitrile: methacrylic acid; 55:35:10) containing 0.099 ephr of carboxyl content…………………………………………………..
34
2.12 Effect of ZnO levels on temperature-dependent loss tangent (tan δ) of XSBR……………………………………………………………………..
35
2.13 Influence of specific surface areas and levels of ZnO on abrasion resistance of carboxylated nitrile rubber. (□ 3.0 m2/g, + 4.3 m2/g, ◊ 10.0 m2/g)……………………………………………………………………….
36
2.14 Effect of silica loading on storage modulus (E’) and tan δ of ZnO- vulcanized XNBR (Z0 = 0 phr, Z10 = 10 phr, Z20 = 20 phr, Z30 = 30 phr)...
41
4.1 ODR curves of XNBR cured with type A magnesium oxide (specific surface area of 140 m2/g) at 165 oC. (linear scale)………………………..
56
4.2 ODR curves of XNBR cured with type A magnesium oxide (specific surface area of 140 m2/g) at 165 oC. (log-log scale)………………………
57
4.3 ODR curves of XNBR cured with type B magnesium oxide (specific surface area of 65 m2/g) at 165 oC. (linear scale)………………………….
58
4.4 ODR curves of XNBR cured with type B magnesium oxide (specific surface area of 65 m2/g) at 165 oC. (log-log scale)………………………..
59
4.5 ODR curves of XNBR cured with type C magnesium oxide (specific surface area of 45 m2/g) at 165 oC. (linear scale)………………………….
60
4.6 ODR curves of XNBR cured with type C magnesium oxide (specific surface area of 45 m2/g) at 165 oC. (log-log scale)………………………..
61
4.7 ODR curves of the XNBR cured with 2.0x stoichiometric amounts of different magnesium oxides at 165 oC…………………………………….
62
xiv
4.8 ODR curves of XNBR cured with dicumyl peroxide at 165 oC…………..
64
4.9 Delta torque, ΔM = MH – ML, as a function of dicumyl peroxide content……………………………………………………………………..
65
4.10 ODR curves of XNBR cured with calcium oxide at 165 oC. (linear scale)………………………………………………………………………
67
4.11 ODR curves of XNBR cured with calcium oxide at 165 oC. (log-log scale)………………………………………………………………………
68
4.12 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (linear scale)………………………………………………………………………
69
4.13 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (log-log scale)………………………………………………………………………
70
4.14 ODR curves of XNBR cured with barium oxide at 165 oC. (linear scale)………………………………………………………………………
72
4.15 ODR curves of XNBR cured with barium oxide at 165 oC. (log-log scale)………………………………………………………………………
73
4.16 Vr and sol content of thermally cured XNBR as a function of cure time…
75
4.17 Vr and sol content of MgO-cured XNBR as a function of MgO concentration………………………………………………………………
78
4.18 Vr and sol content of XNBR-Peroxide vulcanizates as a function of peroxide concentration…………………………………………………….
80
4.19 Vr and sol content of XNBR-CaO vulcanizates as a function of CaO concentration…………………………………………………………
82
4.20 Vr and sol content of XNBR-Ca(OH)2 vulcanizates as a function of Ca(OH)2 concentration……………………………………………………
84
4.21 Vr and sol content of XNBR-BaO vulcanizates as a function of BaO concentration…………………………………………………………
86
4.22 Vr of XNBR vulcanized by various metal compounds as a function of concentration………………………………………………………………
88
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4.23 Sol content of XNBR vulcanized by various metal compounds as a function of concentration………………………………………………….
89
4.24 Stress-strain curves of thermally cured XNBR……………………………
92
4.25 Tensile properties of thermally cured XNBR as a function of cure time…
93
4.26 Stress-strain curves of XN-MgA vulcanizates (cured 120 min at 165 oC)………………………………………………………………….
95
4.27 Stress-strain curves of XN-MgB vulcanizates (cured 120 min at 165 oC)………………………………………………………………….
96
4.28 Stress-strain curves of XN-MgC vulcanizates (cured 120 min at 165 oC)………………………………………………………………….
97
4.29 Tensile properties of XNBR cured with different magnesium oxides (cured 120 min at 165 oC)…………………………………………………
98
4.30 Stress-strain curves of XN-peroxide vulcanizates (cured 60 min at 165 oC)………………………………………………………………….
101
4.31 Tensile properties of XN-peroxide vulcanizates (cured 60 min at 165 oC)…………………………………………………………………….
102
4.32 Stress-strain curves of XN-Ca vulcanizates (cured 1000 min at 165 oC)…………………………………………………………………….
104
4.33 Tensile properties of XN-Ca vulcanizates (cured 1000 min at 165 oC)......
105
4.34 Stress-strain curves of XN-Ch vulcanizates (cured 240 min at 165 oC)…..
106
4.35 Tensile properties of XN-Ch vulcanizates (cured 240 min at 165 oC)……
107
4.36 Stress-strain curves of XN-Ba vulcanizates (cured 240 min at 165 oC)…..
109
4.37 Tensile properties of XN-Ba vulcanizates (cured 240 min at 165 oC)…….
110
4.38 300% Modulus of the XNBR vulcanized by various metal compounds….
113
4.39 Tensile strength of the XNBR vulcanized by various metal compounds…
114
4.40 Elongation at break of the XNBR vulcanized by various metal compounds………………………………………………………………...
115
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4.41 4.41 300% Modulus of the XNBR vulcanized by different curing agents as a function of Vr…………………………………………………
118
4.42 Tensile strength of the XNBR vulcanized by different curing agents as a function of Vr…………………………………………………………
119
4.43 Elongation at break of the XNBR vulcanized by different curing agents as a function of Vr………………………………………………………....
120
4.44 ATR-IR spectra of uncured and thermally cured XNBR in the range 800 to 4000 cm-1…………………………………………………………..
123
4.45 ATR-IR spectra of uncured and thermally cured XNBR in the range 1550 to 1850 cm-1…………………………………………………………
124
4.46 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 800 to 4000 cm-1………………………………...
128
4.47 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 1200 to 2000 cm-1…………………………….....
129
4.48 ATR-IR spectra of the neat XNBR and XNBR-MgO vulcanizates in the range 800 to 4000 cm-1 (cured 120 min at 165 oC)…………………
130
4.49 ATR-IR spectra of the neat XNBR and XNBR-MgO vulcanizates in the range 1200 to 2000 cm-1 (cured 120 min at 165 oC)………………..
131
4.50 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO Compounds in the range 800 to 4000 cm-1………………………………..
133
4.51 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO compounds in the range 1550 to 1850 cm-1……………………………….
134
4.52 ATR-IR spectra of the neat XNBR and XNBR-CaO vulcanizates in the range 800 to 4000 cm-1 (cured 1000 min at 165 oC)………………..
135
4.53 ATR-IR spectra of the neat XNBR and XNBR-CaO vulcanizates in the range 1550 to 1850 cm-1 (cured 1000 min at 165 oC)………………
136
4.54 ATR-IR spectra of the uncured neat XNBR and XN-Ca2.0 compounds, and the neat XNBR and XN-Ca2.0 vulcanizates in the range 1550 to 1850 cm-1 (cured 1000 min at 165 oC)……………………………………. 137
4.55 ATR-IR spectra of the uncured neat XNBR and XNBR-Ca(OH)2 compounds in the range 800 to 4000 cm-1………………………………... 140
xvii
4.56 ATR-IR spectra of the uncured neat XNBR and XNBR-Ca(OH)2 compounds in the range 1200 to 2000 cm-1……………………………….
141
4.57 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 800 to 4000 cm-1……………………………………………...
142
4.58 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 1200 to 2000 cm-1…………………………………………….
143
4.59 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 800 to 4000 cm-1………………………………...
146
4.60 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 1200 to 2000 cm-1…………………………….....
147
4.61 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 800 to 4000 cm-1………………………………………………...
148
4.62 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 1200 to 2000 cm-1……………………………………………….
149
4.63 ATR-IR spectra of the neat XNBR, XN-MgA2.0, XN-Ca2.0, XN-Ch2.0, and XN-Ba2.0 vulcanizates in the range 1475 to 1850 cm-1…. 151
4.64 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz…………………………..
154
4.65 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz…………………………..
155
4.66 Schematic drawing of ion hopping mechanisms (opened and closed circles represent ion pairs)………………………………………………...
156
4.67 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz……………………………….
158
4.68 Effect of specific surface area on dynamic storage modulus (E′) of XN-Mg vulcanizates at frequency 1.0 Hz…………………………………
160
4.69 Effect of specific surface area on dynamic loss modulus (E″) of XN-Mg vulcanizates at frequency 1.0 Hz…………………………………
161
4.70 Effect of specific surface area on loss tangent (tan δ) of XN-Mg vulcanizates at frequency 1.0 Hz……………………………..................... 162
xviii
4.71 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz……………………………..
164
4.72 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz……………………………..
165
4.73 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz………………………………….
166
4.74 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz……………………………..
169
4.75 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz……………………………..
170
4.76 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz………………………………….
171
4.77 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz……………………………..
174
4.78 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz……………………………..
175
4.79 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz……………………………….....
176
4.80 Temperature dependence of loss tangent (tan δ) of XNBR cured with 2.0x stoichiometry of various metal compounds at frequency 1.0 Hz…….
178
1
CHAPTER I
INTRODUCTION
Elastomers are generally characterized by relatively weak interchain interactions
and lack of symmetry or order within molecules. Altering physical characteristics or
designing rubber molecules with specific functions can be made by introducing
functional monomers into conventional rubbers. Incorporation of carboxyl bearing
monomers into polymer chains increases intra- and intermolecular interactions, resulting
in increased tensile strength with inevitably some loss of extension and recovery
properties. Not only are carboxyl groups regarded as polar functional groups, but also
they can be employed to crosslink rubber molecules or attach them to other molecules or
surfaces.
Carboxylated nitrile rubbers (XNBR) are terpolymers of acrylonitrile, butadiene,
and monomers containing carboxyl groups, such as acrylic and methacrylic acids.
Pendant carboxyl groups provide additional curing sites, and make possible using curing
agents that can react with carboxyl groups. XNBRs exhibit self-reinforcement when
vulcanized by divalent metal oxides. This results from ionic crosslinks that aggregate and
form nanometer size domains that phase-separate from the rubber matrix. These domains
are thought to act as multifunctional crosslinks and fillers, thereby producing high
reinforcement. To obtain optimum tensile properties, about twice the stoichiometric
2
amount of ZnO is needed. However, recent ATR-IR studies have shown that
neutralization is essentially complete at about the stoichiometric amount of ZnO.
The main purpose of this research was to probe this paradox by studying the
XNBR/MgO systems. The effect of the specific surface area on cure and mechanical
properties was investigated. The systems of XNBR/CaO, XNBR/BaO, and
XNBR/Ca(OH)2 were also studied.
3
CHAPTER II
HISTORICAL REVIEW
A copolymer of butadiene and acrylic acid was first recognized in a French patent
awarded to I.G. Farbenindustrie in 1933.1 In 1946, a carboxylic nitrile rubber was first
recorded in a patent.2 The incorporation of carboxyl functional groups into polymer
chains aimed to alter rubber properties. Because of high polarity of carboxyl groups, the
resulting polymers were regarded as polar rubbers. Brown realized the importance of
carboxyl groups as crosslink sites to achieve non-sulfur vulcanizations.3 Carboxylic
nitrile rubbers have been reviewed in greater detail in extensive publications.4-13
2.1 Vulcanization of Carboxylic Rubbers
Unvulcanized raw rubbers are high molecular weight viscoelastic liquids, which
are inelastic, weak, and completely dissolve in solvents. They cannot be useful unless
vulcanized. Vulcanization is a process in which rubber molecules are linked together to
form a three dimensional infinite network, therefore viscoelastic liquids are converted to
viscoelastic solids. Carboxylic rubbers can be vulcanized using curing agents that react
with carboxyl groups, and also by sulfur and peroxide vulcanizations.4-13
4
2.1.1 Sulfur and Peroxide Vulcanization
Carboxylic elastomers can be cured using sulfur or peroxide vulcanization recipes
commonly employed in analogous non-carboxylic rubbers.4-8 Carboxyl groups have little
effect on peroxide vulcanization of carboxylic rubbers. A peroxide-cured carboxylic
nitrile rubber containing 40 phr of FEF black had similar properties to those of an
analogous non-carboxylic one.6 Frank, Kraus, and Haefner14 reported that mercaptan-
modified butadiene methacrylic copolymers cured with cumene hydroperoxide have high
bond strength with steel. Unmodified copolymers underwent self-curing due to residual
peroxide left in the polymers.
2.1.2 Vulcanization via Reactions of Carboxyl Groups
a) Anhydride Formation
Carboxylic rubbers can be cured by utilizing reactions of carboxyl groups. Small
amounts of anhydride linkage may be formed via coupling of carboxyl groups (eq. 1)
when heated under rather severe conditions.8
R C
O
C
O
OH2 C
O
OR R H2O (1)
R = polymer chain
+heat
b) Vulcanization by Amines
Diamines, such as ethylene diamine and hexamethylene diamine, have been used
to vulcanize carboxylic elastomers. Crosslink structures range from ionic to covalent
5
bonds, depending on heat history. At low heat history, rubber vulcanizates possessed high
tensile strength and compression set. A decrease in tensile strength and compression set
resulted with increasing heat history. This was interpreted as a result from conversion of
ammonium salt crosslinks to amide crosslinks as shown in equation 2.8, 15
R C
O
OH R NH2H2N
R C
O
O R NHH3N
+
+-
R C
O
O R NH3H3N-
C
O
O R2++ -
C
O
R
R C
O
R NHNH C
O
R + 2 H2O
(2)
R = polymer chain
Cooper reported that copolymers of butadiene and acrylic acid when vulcanized with
N,N,N′,N′-tetramethyl ethylenediamine gave vulcanizates with properties similar to those
vulcanized with sulfur.15 Hexamethylenetetramine and hexamethylenediamine were used
to vulcanize carboxylic elastomers prepared from scrap tires, and vulcanizates with high
hardness and low elongation resulted.16
c) Vulcanization by Epoxy Compounds
Carboxylic elastomers can also be cured by epoxy compounds (eq. 3).5, 7 1,2,3,4-
diepoxybutane was found useful for carboxylic polyacrylates. EP201 resin or 3,4-epoxy-
6-methylcyclohexamethyl-3,4-epoxy-6-methylcyclohexane carboxylated proved to be
suitable for Hycar 1072, a carboxylic nitrile rubber.6
6
R C
O
OH2 + R CHH2C CH2CH
O O
R C
O
O O C
O
RR CHCH2 CH2CH
OH OH
(3)
R = polymer chain
Mika reported that epoxy resins were highly effective in curing a carboxylic
rubber, Hycar 1571 latex, and that tertiary amines activated the curing, as shown in
equation 4.17
CH2 CH
O
+NR3 R3N CH2 CH
O
R3N CH2 CH
O
R C
O
OH +
R C
O
O CH2 CH
OH
+ NR3
(4)
Chakraborty and De18 found that 7.5 phr of bisphenol A diglycidylether resin
gave a good compromise in processing and technical properties of XNBR containing 40
phr of FEF black. However, plasticizing effect was observed at higher resin content (20
phr).
7
d) Vulcanization by Diisocyanate Compounds
Diisocyanate compounds, such as p-tolyl diisocyanate, p-phenylene diisocynate,
and hexamethylene diisocyanate, can be employed to vulcanize carboxylic nitrile
rubbers.6, 8 However, they were difficult to handle due to scorchiness problems. Tensile
properties of vulcanizates were similar to those obtained from sulfur vulcanization
without metal oxide. Carbon dioxide, a by product of reaction (eq. 5), may cause blowing
of vulcanizates.
R C
O
OH2 +
(5)
R = polymer chain
O OC CR NN
R C
O
O C
O
C
O
O C
O
R NHNH R
R C
O
C
O
R NHNH R + 2 CO2
f) Vulcanization by Radiation
Mladenov and coworkers reported vulcanization of a series of carboxylic styrene
butadiene rubbers using gamma radiation.19 Crosslinkages increased linearly with
carboxyl content at small doses. Curing mechanisms were proposed to involve
decarboxylation to form polymeric radicals, which may attack other molecules at double
bonds followed by coupling to form crosslinks, or recombine with other polymeric
radicals (eq. 6).
8
R C
O
OHRadiation
R + + HCO2
R + CH
C
O OH
CH2CH2 CH CH CH2 n
CH
C
O OH
CH2CH2 CH CH CH2 nR
CH
C
O OH
CH2CH2 CH CH CH2n
R
Coupling
CH
C
O OH
CH2CH2 CH CH CH2 nR
R R+ R R
(6)
g) Vulcanization by Metal Oxide and Salts
Brown and Duke4 pointed out that carboxylic nitrile rubbers can be vulcanized by
neutralizing carboxyl groups with oxides and salts of polyvalent metals, such as Zn, Pb,
Cd, Mg, and Ca. Vulcanizates with high gum strength can be obtained by using only
ZnO as a curing agent. Tensile properties depend on the levels of ZnO and carboxyl
groups.
9
Stoichiometrically, curing reaction of carboxylic rubbers by divalent metal oxide
can be written as equation 7;
R C
O
OH2 MO+ R C
O
O C
O
M O R H2O+ (7)
R = polymer chain
However, Brown and Gibbs5 found that the amounts of zinc bound to carboxyl groups are
chemically equal to the carboxyl content of the polymer. Brown8 suggested that ZnO may
react with carboxyl groups to form the basic salt, – COOZnOH, and also react with
carboxyl groups from the same chain as well as those from different chains.
Monovalent metal can impart a degree of crosslinking. A butadiene methacrylic
copolymer when cured by sodium hydroxide possessed better tensile properties than the
cured neat one.6 Dolgoplosk and coworkers20 obtained similar results on studies of a
terpolymer of butadiene (73.2%), styrene (25.5%), and methacrylic acid (1.5%)
vulcanized by sodium hydroxide.
f) Vulcanization by Combination of Metal Oxides and Sulfur or Peroxide
Generally, sulfur type recipes always contain zinc oxide. In TMTD-accelerated
sulfur vulcanization, reactions between zinc oxide and carboxyl groups are very fast and
dominate at short curing cycles, resulting in a vulcanizate with high tensile strength but
poor compression set and stress relaxation-dependent properties. At longer cure times,
slower sulfur vulcanization takes over, resulting in a vulcanizate with improved
10
compression set but lower tensile strength. Similar observations were made in
ZnO/Sulfur/TMTM, ZnO/Sulfur/ZDMC, ZnO/Sulfur/MBTS, and ZnO/Sulfur/CBS
systems.4, 6-8
Chakraborty21 reached the same conclusion for XNBR cured by dual curatives of
sulfur and zinc peroxide. Beekman and Hastbacka22 reported that when half of the ZnO in
ZnO-activated sulfur vulcanization is replaced by magnesium oxide or magnesium
hydroxide, the increased surface activity of MgO resulted in the increased cure rates. The
mixed vulcanization systems of zinc peroxide/ sulfur/ peroxide were studied in XNBR.21
For the XNBR cured by both DCP and metal oxides, peroxide and metal oxide curing
proceeded independently.6, 21
2.1.3 Cure Behavior of Carboxylic Rubbers
Surface area and the amount of zinc oxide play an important role in governing the
cure behavior of carboxylic rubbers vulcanized only by ZnO23, 24 or by both peroxide and
ZnO.25 Cure rate increases with increasing surface area and concentration of ZnO.24
Figure 2.1 shows the role of specific surface area of ZnO on the cure behavior of ZnO-
XNBR compounds. However, carboxylic rubbers vulcanized by zinc oxide suffer from
scorchiness because zinc oxide is very reactive towards carboxyl groups.6, 8, 26
Compounds even cure during milling or storage. Humidity seriously affects Mooney
scorch time of carboxylic and non-carboxylic rubbers.9, 27 Three approaches employed to
solve scorch problems are: i) the use of scorch controllers,6, 8 ii) the use of coated ZnO,28
and iii) the use of zinc peroxide.9, 11, 29
11
Figure 2.1 Cure rheometry of XNBR containing ZnO of different surface area at twice stoichiometry (S = 35 m2/g, M = 3.5 m2/g, and L = 0.5 m2/g).24
Organic acids, organic acid anhydrides, silica, boric acid, amines and basic
organic reagents were used as cure retarders and controllers for metal oxide-vulcanized
carboxylic rubbers. Phthalic anhydride, stearic acid, sebacic acid, and succinic anhydride
12
were the most effective.6, 8 According to Zakharov and Shadricheva,30 maleic anhydride
was effective for carboxylic styrene butadiene rubbers. These substances not only
reduced scorchiness but also improved tensile and flow properties of compounds.
Hallenbeck28 found that the use of zinc sulfide- and zinc phosphate-coated ZnO instead
of ZnO can improve scorch safety and bin stability of the carboxylated NBR and BR
compounds without affecting final physical properties. The use of metal alkoxides, such
as aluminium isopropoxide, and aluminium ethoxide, along with standard ZnO also gave
a similar effect.28 Zinc peroxide (ZnO2) has been reported to improve scorchiness and
shelf life of carboxylic rubber compounds.9, 11, 27, 29 Cure rheometry and bin stability of
the ZnO-XNBR compound compared to those of the ZnO2-XNBR compound are shown
in Figure 2.2. Zinc peroxide gives much better scorch safety and bin stability than ZnO.
Figure 2.2 (a) Cure rheometry, and (b) bin stability of the ZnO-XNBR compounds along with those of the ZnO2-XNBR compound. (KRYNAC PA-50 is a 50/50 masterbatch of medium acrylonitrile NBR and technical grade ZnO2)11
13
2.2 Rubber Reinforcement
Uncrosslinked rubbers are highly entangled molecular chains and viscoelastic.
They can creep and flow under applied forces. They become stiff and more elastic when
chemically crosslinked. However, they have little strength. To be useful, reinforcement is
required. Reinforcement refers to the stiffness and strength imparted to a rubber
vulcanizate by incorporating small hard domains. This can be achieved by many
approaches, for example, by addition of particulate fillers,31, 32 by thermodynamic phase
separation,33 or by reaction-induced phase separation.34, 35
2.2.1 Reinforcement by Particulate Fillers
Reinforcement of rubbers by particulate fillers has been the most popular method
for decades. The most commonly used particulate fillers are carbon black and silica. The
extent of reinforcement depends on many parameters, such as particle size or surface area,
structure, and surface chemistry of the filler particle.36 The key parameter is particle size
or surface area. To give substantial reinforcement, particle size of fillers must be less than
1 μm.32, 36 With increasing surface area (smaller particles), modulus, strength and
abrasion resistance generally increase. Structure, the term used to describe morphology of
fillers, is another important parameter. High structure fillers increase strength and
stiffness.32 Surface chemistry influences physical and chemical interactions between filler
particles and the rubber matrix. Although chemical interactions at the filler-rubber
interface enhance reinforcement, they are not necessary. Physical interactions seem to be
more important.32, 36
14
Hamed and Hatfield37 simply modeled how particle size can affect particle-
particle spacing in a particulate-filled rubber. They assumed a volume fraction ν of a
spherical filler of diameter d is dispersed on a three dimensional square lattice of a
continuum rubber matrix (Figure 2.3).
Figure 2.3 Two dimensional schematic of spherical particles of diameter d arranged on a three dimensional square lattice. (s is particle-particle spacing, and t is the thickness of restricted mobility layer of rubber chains)37
According to the model, the nearest neighboring particle spacing is given by:
⎟⎠⎞
⎜⎝⎛ −
ν= 1806.0ds 31 (8)
15
which is valid for ν ≤ 0.524. Assuming that each particle is surrounded by a restricted
mobility rubber layer of thickness t, the volume fraction νt of the rubber phase within t
can be calculated by:
ν−⎥⎥⎦
⎤
⎢⎢⎣
⎡−⎟
⎠⎞
⎜⎝⎛ +ν
=ν1
1dt21
3
t (9)
Equation 9 is valid for t < s/2, because at t = s/2 the restricted rubber from adjacent
particles begins to overlap. By taking ν = 0.25, and t = 2.8 nm, a micron-sized particle
will result in a small volume of restricted mobility rubber (νt = 0.0056). However, for a
20 nm particle, at the same ν and t, νt = 0.3657. Clearly, the amount of rubber with
restricted mobility is greater in composites containing smaller particle fillers.
Mobility of the rubber phase in a composite with micron-sized particles is very
much like that in bulk unfilled rubber. However, for a composite containing very fine
particles, the rubber matrix may behave differently from unfilled rubber. Restricted
rubber chains increase energy dissipation and may result in crack splitting, which reduces
local stress concentration and inhibits catastrophic growth of the crack.38
2.2.2 Reinforcement by Thermodynamic Phase Separation
Another approach to create hard domains uniformly dispersed throughout the
rubber matrix is by thermodynamic phase separation. Styrenic thermoplastic elastomers,
such as poly (styrene-b-butadiene-b-styrene) or SBS, and poly (styrene-b-isoprene-b-
16
styrene) or SIS, form two phases; rigid domains of polystyrene dispersed throughout a
polybutadiene (or polyisoprene) matrix.33, 39 The domain radius of SBS and SIS triblock
copolymers containing polystyrene with molecular weight of about 10,000 g/mole is less
than 20 nm. These rigid domains function both as multiple crosslinks and as filler
particles. An SBS with 27.5 % styrene content was reported to have a tensile strength of
27.1 MPa with elongation at break of 860%,33 comparable to those of a 50 phr N330-
reinforced SBR (23.5 % bound styrene), which has a tensile strength of 28.7 MPa with
ultimate elongation of about 300 %.40
2.2.3 Reinforcement by Reaction-Induced Phase Separation
Substantial reinforcement can also be achieved by blending a rubber with a
compound which can self-react and phase separate to form hard domains. Hydrogenated
acrylonitrile butadiene rubber (HNBR) vulcanized by peroxide and coagent zinc
dimethacrylate (ZDMA) is an example. Upon curing, very fine particles (about 2 nm) of
poly (zinc dimethacrylate) are formed as an in-situ filler, which phase separates from the
HNBR matrix. These primary particles are covalently linked to form secondary ionic
clusters of 20 to 30 nm in size.34, 41 Maximum tensile strength of such a system was
reported to be about 55 MPa with about 500 % ultimate strain.34, 41
2.3 Tensile Strength of Rubbers
Rupture of rubber can occur under a variety of imposed mechanical conditions
such as on stretching to break, during abrasion, or deformations under small cyclic
loading. A corresponding measure of resistance to failure or strength is created for each
17
type of rupture. The simplest method is the tensile test, in which the rubber sample is
subjected to a uniform uniaxial tension. Tensile strength and breaking strain are two
important properties used to establish the influence of the nature of rubber and test
conditions.42
When the rubber sample is subjected to simple extension, only a small number of
rubber molecules crossing the fracture plane actually undergo rupture, while most of
rubber molecules remain unaffected. When a crack grows, those molecules will break
successively.43 Consider an ideal network consisting of network chains of molecular
weight Mc between crosslinks arranged in space as shown in Figure 2.4. Upon stretching,
assuming that the load must be carried only by rubber molecules parallel to the direction
of extension, Bueche44 showed that tensile strength (TS) of the ideal network is given by
c
32
c
A FM3NTS ⎟⎟
⎠
⎞⎜⎜⎝
⎛ ρ= (10)
where ρ is the density of the rubber, NA is the Avogadro’s number, and Fc is the
maximum load that each molecule can hold. If reasonable values of ρ, Mc and bond
energy (to determine Fc) are taken, the tensile strength calculated from equation 10 is
always greater than the observed value. The deviation arises from neglecting many
important factors, such as, effect of chain ends, distribution of network chain length,
molecular flaws, crystallinity, and viscoelastic effects.44 Many theories45-47 have been
made to explain the tensile strength of rubbers by taking some of these factors into
consideration.
18
Figure 2.4 Schematic of an ideal rubber network44
The tensile strength of rubber is influenced by both crosslink types and crosslink
density (Figure 2.5).42 The tensile strength passes through a maximum as the crosslink
density is increased. Flory48 explained that the increased tensile strength of gum NR
vulcanizates with increasing degree of crosslinking before a maximum is attributed to
crystallization of rubber chains upon stretching. At high crosslink density tensile strength
is low because the breaking point is reached before crystallization can occur.
Taylor and Darin46 found similar behavior in gum SBR vulcanizates, which are
not strain-crystallizing. They proposed that chain orientation is a critical factor in
determining tensile strength.
19
Figure 2.5 Tensile strength of gum NR vulcanizates as a function of 1/Mc for various vulcanization systems. ○ accelerated sulfur; × TMT sulfurless; ● peroxide; ∆ high energy radiation42
Epstein and Smith49 found that the maximum is greatly dependent on the rate of
extension, and is not shown in swollen samples. Smith and Chu50 studied Viton polymers
and found that the maximum diminishes and finally disappears with increasing
temperature. They concluded that changes in tensile strength with degree of crosslinking
are primarily due to viscoelastic effects, especially energy dissipation.
The tensile strength of rubbers also depends on crosslink structure (Figure 2.5).
Tensile strength decreases in order of increasing strength of crosslink;51
– COO-M+ > – C – S>2 – C – > – C – S2 – C – > – C – S – C – > – C – C –
20
Bateman and colleagues51 explained that if crosslinks are weaker than bonds in the main
chain, they will slip and interchange with neighbors. This relieving mechanism will allow
the load to be shared over neighboring chains, and thereby permitting the whole network
to bear higher stress. Tobolsky and Lyons52 studied stress relaxation of rubbers
crosslinked by weak and strong linkages and found no evidence of mechanical lability of
weak crosslinks. They proposed that high tensile strength of rubber crosslinked with
weak bonds is a result of an internally relaxed network, formed at vulcanization
temperatures due to thermal lability of the crosslinks, rather than to relaxation at the
temperature of tensile testing.
2.4 Ionic Aggregation
2.4.1 Theory
The concept of ionic aggregation in metal oxide-vulcanized carboxylic rubbers
was first introduced to explain high tensile strength of vulcanizates by Tobolsky and
coworkers.53 This concept had been proposed by many researchers to account for the
unique behavior of sodium salts of ethylene-methacrylic acid copolymers.54, 55 The first
attempt to treat ionic aggregation theoretically was by Eisenberg.56 He assumed that in a
polymer of low dielectric constant, ionic species would exist fundamentally as contact
ion pairs. This assumption is quite reasonable because the work required to separate ion
pairs is nearly two orders of magnitude greater than the available thermal energy. An
even higher form of ionic aggregates, “multiplet”, would exist in such a system
depending on i) the dimension of the polymer chain and of ion pairs, ii) the tension on the
21
chains resulting from ionic aggregation when adjacent ion pairs incorporated into
different multiplets, iii) the electrostatic energy released upon multiplet formation.
The theory assumes that the multiplet is a spherical drop containing only ions and
that polymer chain segments are confined only at the surface of the multiplet. To simplify
calculation, multiplets are assumed to be distributed on a body-centered cubic lattice.
Eisenberg showed that the multiplet radius (rm) is given by
ch
pm S
v3r = (11)
and the number (n0) of ion pairs in the multiplet can be calculated from
ch
m
p
m0 S
Svv
n == (12)
where vp is the volume of an ion pair, vm is the volume of the multiplet, Sm is the surface
area of the multiplet, and Sch is the contact surface of a chain. For a sodium salt of an
ethylene-methacrylic acid copolymer, vp is about 12 Å3 and Sch is about 12 Å2, yielding
rm ~ 3 Å, and vm ~ 100 Å3. For perfect volume occupation, the maximum number of ion
pairs is therefore eight.
Eisenberg also postulated that these multiplets will join together to form larger
aggregates, which he termed “clusters”. Many factors can affect cluster formation, which
are
22
i) the work done to stretch the polymer chain upon clustering of multiplets,
ii) the electrostatic energy released on cluster formation, which depends on the
geometry of clustering, and the dielectric constants of the media,
iii) the critical temperature (Tc) at which electrostatic and chain extension
energies are balanced, and
iv) a half of adjacent ion pairs are assumed to be incorporated into the same
cluster.
It is shown that the number of ion pairs per cluster is given by
2332
A
c02
00
c20
2
c
2
c
A
NMn
2r
e4
1K
'kMM
h
hTk3l4
MN
n⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
+επ
ρ= (13)
and the distance (R) between clusters is given by
31
A
c
NMn
R ⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
= (14)
where n is the number of ion pairs per cluster, ρ is the density of the polymer, NA is
Avogadro’s number, Mc is the molecular weight of the polymer chain between pendant
ionic groups, l is C – C bond length, k is Boltzmann’s constant, Tc is the critical
temperature, h 2 is the mean square end-to-end distance of the free chain, h 02 is the mean
square end-to-end distance of the freely-jointed chain, M0 is the molecular weight of the
repeat unit, k′ is a parameter related to the particular cluster geometry, K is the dielectric
23
constant of the polymer, ε0 is the permittivity of free space, e is the electronic charge, and
r is the distance between the centers of positive and negative charges. Calculations of
intercluster distance were made for a butadiene-sodium methacrylate copolymer
assuming various models. The calculated values are in the range of 44 to 95 Å.
2.4.2 Experimental Evidence
Existence of ionic clusters has been confirmed by many experimental methods,
such as small angle X-ray scattering (SAXS),54, 57, 58 transmission electron microscopy
(TEM),59-61 and dynamic mechanical analysis.59, 62, 63 Figure 2.6 shows the SAXS profiles
of low density polyethylene, a copolymer of ethylene-methacrylic acid, and a sodium salt
(90% neutralization) of the copolymer.54, 57 The peak at low angle (2θ = 4.5o),
corresponding to a spacing of 2 nm, in the profile of the ionomer suggested the presence
of ionic clusters. The peak was observed with all cations, including monovalent and
divalent metals, also ammonium and quaternary ammonium ions.54
Electron microscopy has shown nanometer-sized ionic aggregates.59-61 Marx and
coworkers60 reported ionic aggregates of 1.3 to 2.6 nm in size for butadiene-sodium
methacrylate copolymers. For ZnO-vulcanized XSBR, Sato59 found ionic domains of
about 5 nm uniformly dispersed in the rubber matrix. STEM images of Zn-neutralized
ethylene-methacrylic acid copolymers revealed nearly spherical ionic aggregates of 2.5 to
2.8 nm randomly distributed throughout the polymer matrix.61
Dynamic mechanical studies of ZnO-activated sulfur vulcanization of XSBR
showed, other than the glass transition, a second transition at temperatures 45 to 60 oC in
the temperature-tan δ plot.59 This transition did not appear in the sulfur cured sample
24
without ZnO. It is attributed to ionic aggregates. Sato and Blackshaw64 investigated
dynamic mechanical properties of XNBR cured by various metal oxides, and found the
second transition at temperatures 60 to 70 oC.
Figure 2.6 SAXS profiles of a) low density polyethylene, b) a copolymer of ethylene- methacrylic acid, and c) a sodium salt (90% neutralization) of the copolymer54
25
Fourier transform infrared spectroscopy (FTIR) has also been employed to study
the morphology of ionomers.65, 66
2.4.3 Ionic Aggregation Models
Many models have been proposed to explain the morphology of ionic aggregates,
such as a hard sphere model,67 a modified hard sphere model,68 and a core-shell model.69
These models are based on interpretation of SAXS profiles of the systems investigated.
Although these models well-explained the SAXS profiles, they were not successful in
describing mechanical properties, especially the appearance of two transitions, the glass
and the ionic transitions, in those ionomers that showed SAXS peaks. The existence of
two transitions indicates that the materials behave like a two-phase system. The
dimensions of the phase-separated region are at least 50 to 100 Å, while the calculated
interspacing between scattering entities from the proposed models is in the order of 30 Å.
This casts a doubt on how to pack 50 to 100 Å particles into a 30 Å lattice.70 The model
that better explains the morphology of ionomers is the Eisenberg-Hird-Moore (EHM)
model.71 This model is based on multiplet formation. The important feature of the model
is that chain mobility in the vicinity of the multiplet is greatly restricted, and the thickness
of the restricted mobility layer is expected not to exceed the persistent length of the
polymer. An individual multiplet effectively acts as a large multifunctional crosslink and
raises the Tg of the polymers, but the restricted layer around the individual multiplet is
not large enough to exhibit its own Tg. The cluster is formed when a number of the
restricted regions overlap in relatively large region (50 to 100 Å), which exhibit its own
Tg, which is significantly higher than that of the unclustered component.
26
2.5 Mechanical Properties of Carboxylated Rubbers
Carboxylic rubbers vulcanized by metal oxides or salts exhibit substantial
reinforcement. These vulcanizates have much greater tensile strength and modulus than
those vulcanized by peroxide or sulfur (without ZnO in the recipe), but are poorer in
compression set and properties related to stress relaxation.4-6 Table 2.1 shows influence
of salt formation on tensile properties of copolymer of butadiene and methacrylic acid.6
Table 2.1 Influence of salt formation on tensile properties of butadiene- methacrylic acid copolymer containing carboxyl group of 0.12 ephr (equivalent per a hundred part of rubber).6
Polymer and Treatment Tensile
Strengtha (psi)
Ultimate Elongationa
(%) Raw polymer, 0.12 ephrb of carboxyl group < 100 > 1,600
Treated with 0.12 ephr of aqueous NaOH 1,700 900
Treated with 0.12 ephr of ZnO 6,000 400 Gum sulfur vulcanizate < 500
a Cured 20 min at 132 oC
b ephr = equivalent part per hundred part of rubber
Cooper72-74 proposed that ionic crosslinks interchange under mechanical stress,
and this mechanism will relieve localized stress concentration, resulting in high tensile
strength. Halpin and Bueche75 studied fracture of sulfur- and ZnO-vulcanized carboxylic
nitrile rubbers and suggested that tensile rupture is a viscoelastic effect and unique
properties of ZnO-cured rubber are natural reflections of a sparse crosslink density.
However, according to Tobolsky and coworkers53, the high strength of carboxylic rubbers
27
vulcanized by metal oxides resulted from the presence of ionic clusters, which give rise
to a two-phase, reinforced structure.
Although metal oxide crosslinking of carboxylic rubbers enhances tensile strength,
and stiffness, poor compression set and loss of strength at high temperatures are the main
disadvantages. Compromise properties can be achieved by a combination with covalent
crosslink systems; for example, metal oxide combined with sulfur vulcanization (Figure
2.7).6, 8
Figure 2.7 Tensile properties of carboxylic nitrile rubber (0.099 ephr of COOH) cured by various curing systems. A) 0.2 ephr of ZnO, B) sulfur + zinc dimethyldithiocarbamate (0.003 ephr of unavailable zinc), C) 0.2 ephr of zinc + sulfur + zinc dimethyldithiocarbamate (0.003 ephr of unavailable zinc)8
Bhowmick and De76 reported that in XNBR vulcanizates with a mix of sulfur and
metal carboxylate crosslinks, the technical properties are little affected by variations in
sulfur/accelerator ratios. Chakraborty and coworkers77 found that properties of XNBR
vulcanizates formed with mixed sulfur and metal carboxylate crosslinks are guided more
by the ionic crosslinks, especially at long cure times, where they claimed destruction of
28
sulfur crosslinks is counterbalanced by formation of ionic crosslinks. In metal oxide-
cured carboxylic rubbers, many important factors can affect the final properties of rubber
vulcanizates, as discussed next.
2.5.1 Effect of Carboxyl Content
For carboxylic rubbers cured with an excess amount of divalent metal oxides, i.e.
twice the stoichiometric amount of ZnO, the tensile strength increases with the increased
carboxyl content as shown in Figure 2.8.8
Figure 2.8 Tensile strength as a function of carboxyl content in butadiene-methacrylic acid copolymers treated with an excess amount of ZnO (twice stoichiometry).8
29
Studies by Otocka and Eirich78 have shown that ionic crosslinks enhance the
rubbery modulus in lithium salts of butadiene-methacrylic acid copolymers, and the
degree of enhancement increases with the increased content of carboxylate groups
(Figure 2.9). However, carboxylate links are thermally labile over the entire rubbery zone.
Figure 2.9 Modulus-temperature behavior of butadiene-methacrylic acid copolymers and their lithium salts. (---) RA1 4.7% acid, (− −) RA2 7.7 % acid, (×) RA3 11.6 % acid, (○) RA1 Li 4.7 % salt, (□) RA2 Li 7.7 % salt, (∆) RA3 Li 11.6 % salt
30
Ibarra and Alzorriz79, 80 studied ZnO2-XNBR systems and found that crosslink
density and physical properties increase with carboxyl content and curing time.
2.5.2 Influence of Types of Metal Oxides or Salts
Brown6, 8 reported that carboxylic rubbers can be vulcanized using monovalent,
divalent, and multivalent metal compounds. A butadiene-methacrylic acid copolymer
with carboxyl group of 0.12 ephr (equivalent per a hundred part of rubber), when treated
with 0.12 ephr of sodium hydroxide and cured 20 min at 132 oC, showed an improvement
in tensile properties compared to those of the raw polymer (Table 2.1). Theoretical
amounts of sodium carbonate, potassium carbonate, and lithium hydroxide gave similar
results. The terpolymer of 73.2 % butadiene, 25.5 % styrene, and 1.5 % methacrylic acid
treated with sodium hydroxide was reported to have a tensile strength of 6.1 MPa, but it
fell to zero on raising the temperature from 70 to 100 oC.20 Zakharov81 found that rubber
solutions of butadiene-styrene-methacrylic acid terpolymers in isopropylbenzene when
treated with sodium and potassium hydroxides completely gel within 3 and 24 hr,
respectively.
Salts and oxides of multivalent metals can be used to crosslink carboxylic rubbers.
Tensile properties of a carboxylic nitrile rubber vulcanized by various metal oxides and
salts are shown in Figure 2.10.6, 8 ZnO and PbO give vulcanizates with the highest
properties. Dolgoplosk and coworkers82 studied cure and mechanical properties of
carboxylic styrene butadiene rubber (XSBR) vulcanized by oxides and hydroxides of
divalent metals. The results were shown in Table 2.2. Cure rate and mechanical
properties depend considerably on the nature of the metal oxides and hydroxides. The
31
best mechanical properties were obtained when cured with magnesium oxide and calcium
hydroxide.
Figure 2.10 Tensile properties of carboxylic nitrile rubber (butadiene: acrylonitrile: methacrylic acid; 55:35:10) containing 0.1 ephr of carboxyl groups vulcanized by various metal salts and oxides
Starmer25 evaluated effectiveness of various metal oxides and hydroxides using a
peroxide curing recipe, and found that these materials fell into two categories, active and
inactive types (Table 2.3). The active materials behaved similarly to ZnO in that they
increased hardness, modulus, and abrasion resistance. The oxides and hydroxides of the
IIA alkaline earth metals, and IIB together with lead appeared to be in this class.
However, calcium oxide, which was expected to be active, gave conflicting results. No
improvement in properties was observed in the case of the inactive ones. The difference
32
between the active and inactive materials was that the former had basic groups on the
particle surface, while the latter did not.
Table 2.2 Tensile properties of gum XSBR vulcanized by 10 phr of divalent metal oxides and hydroxides82
Property MgO ZnO CaO PbO CdO Mg(OH)2 Zn(OH)2 Ca(OH)2 Ba(OH)2
Cure time, min 20 10 100 30 120 20 10 80 60
300% Modulus, kg/cm2 44 18 22 30 23 29 29 55 37
Tensile strength, kg/cm2 389 157 132 128 190 220 241 394 249
Relative elongation, % 850 800 760 740 890 835 660 770 675
Residual elongation, % 22 10 22 14 23 15 2 28 18
Table 2.3 Active and inactive metal compounds25
Active Metal Compounds Inactive Metal Compounds
MgO ZnO Mg(OH)2 ZnO2 Ca(OH)2 CdO SrO HgO BaO PbO Ba(OH)2 Pb3O4
MgCO3 SiO2 Al2O3 Al(OH)3 TiO2 ZnS FeO Fe2O3 NiO CuO SnO Sb2O3
Ibarra and Alzorriz83 reported that the cure and tensile properties of CaO-cured
carboxylated nitrile rubbers increase with increased CaO content to reach an optimum,
then dropped with excessive amounts.
33
Metal peroxides, such as, zinc peroxide (ZnO2), magnesium peroxide (MgO2),
and calcium peroxide (CaO2), gave vulcanizates with tensile properties comparable to
those of ZnO-cured samples, while scorch safety was much improved. The use of mixed
metal peroxides is also possible.29
Tant and coworkers84 studied the structure and properties of carboxy-terminated
polyisoprene neutralized by various metals. They found that mechanical properties
depend strongly on the neutralizing cations. Elements of groups IA (Na, and K) and IIA
(Mg, Ca, and Ba) formed highly ionic complexes, and the strength of ionic association
increased with decreasing cation size and with increasing cation charge within each group.
2.5.3 Effect of Metal Oxide Level
Mechanical and physical properties of carboxylic rubbers are greatly dependent
on the degree of neutralization. The amount of metal oxides or salts required for complete
neutralization depends upon the carboxyl content of the polymer. For a carboxylic nitrile
rubber, Brown6, 8 found that optimum tensile properties can be achieved by using twice
the stoichiometric amount of ZnO, assuming that each Zn++ ion reacts with two carboxyl
groups. Figure 2.11 shows the dependence of tensile properties of the carboxylic nitrile
rubber on ZnO concentrations. He suggested that in addition to the zinc carboxylate salt,
– COOZnOOC – , the zinc hydroxycarboxylate salt, – COOZnOH, may also form, the
same suggestion made by Dolgoplosk and coworkers.20
34
Figure 2.11 Effect of ZnO level on tensile properties of the carboxylic nitrile rubber (butadiene: acrylonitrile: methacrylic acid; 55:35:10) containing 0.099 ephr of carboxyl content.8
Dolkoplosk and coworkers82 obtained similar results in MgO-cured carboxylic
SBR. Sato59 also found the same behavior for carboxylated SBR vulcanized by ZnO.
Based on evidence from dynamic mechanical analysis, which suggested that ionic
crosslinks exist as ionic aggregates, he therefore proposed that the basic salt would
contribute to ionic crosslinks. Furthermore, the increase in the amount of ZnO up to twice
the stoichiometric amount shifted the position of the ionic transition to higher
temperatures (as much as 15 oC), with little change when using greater amounts of ZnO
(Figure 2.12).
35
Figure 2.12 Effect of ZnO levels on temperature-dependent loss tangent (tan δ) of XSBR.59
2.5.4 Effect of Specific Surface Area
Starmer25 studied the effect of specific surface area of ZnO on the mechanical
properties of carboxylic nitrile rubbers, and found that Pico abrasion resistance
significantly improves with increasing specific surface areas (Figure 2.13). Beekman and
Hastbacka22 observed similar behavior when replacing a half amount of ZnO with MgO
of different specific surface area. Starmer25 also recognized that specific surface areas
have little effect on tensile strength and ultimate elongation, as did Beekman and
Hastbacka,22 and Hua.23
36
Figure 2.13 Influence of specific surface areas and levels of ZnO on abrasion resistance of carboxylated nitrile rubber. (□ 3.0 m2/g, + 4.3 m2/g, ◊ 10.0 m2/g)
2.5.5 Effect of Filler
Reinforcing fillers, such as carbon black and silica are always used to improve the
mechanical properties of non-carboxylic rubbers. In the case of ZnO-vulcanized
carboxylic nitrile elastomers, Brown and Gibbs5 reported that stress-strain properties of
EPC- and whiting-filled rubber vulcanizates are essentially the same. Thus, they
concluded that EPC black does not increase tensile strength, but acts more like a load
extender. Influence of HAF carbon black on the mechanical properties of ZnO-
vulcanized XNBR is shown in Table 2.485 With carbon black, modulus, hardness, and
tear strength increased, but tensile strength changed little.
37
Table 2.4 Influence of HAF carbon black loading on mechanical properties of ZnO-cured XNBR85
Ingredients A B C
Krynac 7.50 XNBR Zinc oxide Stearic acid HAF carbon black
100.0 12.0 1.0 0.0
100.0 12.0 1.0 20.0
100.0 12.0 1.0 30.0
Properties Modulus at 100 % elongation (MPa) Modulus at 300 % elongation (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN/m) Hardness (IRHD) Tension set at 100 % elongation (%)
1.8 3.2 33
1150 32 56 9
4.2 11.6 31
1000 45 70 11
6.6 18.0 28 900 48 75 11
Weir and Burkey86 reported that an excellent balance of compound viscosity and
vulcanizate properties, such as hardness, tensile strength, abrasion and flex cut growth
resistance, can be achieved by using semi-reinforcing carbon blacks (N550, N600, and
N774). Types of carbon black had little effect on hardness and tensile strength. Therefore,
they concluded that highly reinforcing carbon blacks are not necessary in the XNBR
formulations. Sato59 reached a similar conclusion in the case of ZnO-cured XSBR filled
with N660. He found that 300 % modulus very much increases, while tensile strength
undergoes little change. Tensile properties strongly depended on the amounts of both zinc
oxide and carbon black.
Shaheen and Grimm87 studied the effect of silica type on the properties of sulfur-
cured XNBR using the recipes shown in Table 2.5, and found that fumed silica gave
vulcanizates with higher modulus, tear strength, and abrasion resistance than those
38
obtained from precipitated silica, but poorer compression set. For precipitated silica,
increasing particle size resulted in shorter scorch and cure times, and lower modulus,
tensile strength, and abrasion resistance. Chakraborty and De88 reported that silica and
clay enhance the properties of XNBR vulcanized by a mixed crosslinking system
(sulfur/zinc peroxide); however, silica is more reinforcing. Because carboxyl groups of
polymer chains can react with silanol groups on silica surface, yielding better filler-
polymer interaction, therefore coupling agents were not necessary.
Mandal and Tripathy89 studied the influence of clay and calcium carbonate on the
physical properties of ZnO-cured XNBR (Table 2.6), and found that with increasing filler
contents, modulus, hardness, and tear strength increase at the expense of tensile strength.
In addition, fillers also affect dynamic mechanical properties of XNBR
vulcanizates. Carbon black,59, 62 silica,90 clay, and calcium carbonate89 have been reported
to shift the ionic transition temperature to a higher temperature with increased loadings.
Figure 2.14 shows the effect of silica loading on storage modulus (E′) and tan δ of ZnO-
vulcanized XNBR.90
39
Table 2.5 Effect of silica types on tensile properties of XNBR vulcanizates87
Ingredients A B C
Chemigum NX775 XNBR Harwick DSC-18 Stearic acid Dibutylphthalate Wingstay 29 Sulfur, Spider TMTD Pasco 558T ZnO Hi-Sil 233, precipitated silica (0.022 μm) Hi-Sil BP, precipitated silica (0.04 μm) Cab-O-Sil MS-7SD, fumed silica (0.014 μm)
100.0 0.3 2.0 5.0 1.0 0.5 2.0 5.0 30.0
- -
100.0 0.3 2.0 5.0 1.0 0.5 2.0 5.0 -
30.0 -
100.0 0.3 2.0 5.0 1.0 0.5 2.0 5.0 - -
30.0
Cure properties at 163 oC Minimum torque, (N.m) Maximum torque, (N.m) tS2, (min) tc90, (min) tc95, (min) Tensile properties 100 % Modulus, (MPa) 200 % Modulus, (MPa) 300 % Modulus, (MPa) Tensile strength, (MPa) Elongation at break, (%) Hardness, (Shore A) Aging properties; 70 hr at 121 oC in air oven Tensile strength, (MPa) % Change Elongation at break, (%) % Change Hardness, (Shore A) Point change Tear strength, die C, (kN/m) Tear strength, die C, 100 oC, (kN/m) Compression set, 72 hr at 100 oC, (%) Pico abrasion index Pico abrasion index, aged 1 1hr at 149 oC
1.0 8.8 3.2 10.8 16.5
3.7 7.2 11.2 26.6 480 82
23.1 -13 360 -25 87 5
56.0 18.9 41 348 541
0.8 8.9 1.8 4.8 6.5
3.2 5.4 8.5 19.1 490 77
20.0 5
380 -22 84 7
45.5 15.3 25 249 314
1.6 10.4 4.2 12.7 16.5
4.4 7.8 12.5 26.4 490 85
25.9 -2
370 -24 91 6
62.0 21.5 51 417 767
40
Table 2.6 Effect of clay and calcium carbonate on tensile properties of ZnO-vulcanized XNBR vulcanizates89
Ingredients A B C D E F G
XNBR ZnO Stearic acid Calcium carbonate Clay
100 12 1 - -
100 12 1 10 -
100 12 1 20 -
100 12 1 30 -
100 12 1 -
10
100 12 1 -
20
100 12 1 -
30 Properties 100 % Modulus, (MPa) 300 % Modulus, (MPa) Tensile strength, (MPa) Elongation at break, (%) Tear strength, (N/cm) Hardness, (IRHD) Tension set, (%)
1.85 3.11 32
1100 32 56 9
1.95 3.95 28
1050 32 63 11
2.20 4.25 27
1020 36 64 11
2.90 5.30 24 900 38 66 12
2.19 3.64 30
1040 32 61 11
2.45 3.95 28
1030 37 63 12
3.11 4.70 27 990 39 65 12
2.5.6 Effect of Plasticizers
Plasticizers greatly affect the mechanical properties of ionomers. Because of this
biphasic nature, the ionic aggregates and hydrocarbon chains can be plasticized
independently by using high and low polarity plasticizers, respectively.91 Dual phase
plasticization is also possible. Makowski and Lundberg92 studied plasticization of metal
sulfonated EPDM with derivatives of stearic acid, and reported that fatty acids, especially
zinc stearate, not only reduced melt rheology of the polymer, but also helped improve the
mechanical properties.
Mandel, Tripathy, and De93 examined the plasticizing effect of ammonia on
properties of gum and filled XNBR vulcanized by ZnO, and found that ammonia
treatment results in a reduction in modulus and tensile strength. Furthermore, a smaller
peak of the ionic transition was observed in ammonia-treated samples. The plasticization
of ZnO-vulcanized XNBR by zinc stearate was also studied.94
41
Figure 2.14 Effect of silica loading on storage modulus (E’) and tan δ of ZnO- vulcanized XNBR (Z0 = 0 phr, Z10 = 10 phr, Z20 = 20 phr, Z30 = 30 phr).90
42
CHAPTER III
EXPERIMENTAL
3.1 Materials
3.1.1 Carboxylated Nitrile Rubber (XNBR)
Nipol 1072, bound acrylonitrile content (%), 27 ± 1, carboxyl content (ephr), 0.075
± 0.005, Zeon Chemicals L.P..
3.1.2 Curing Agents
a) Dicumyl peroxide: Di-Cup R, GEO Specialty Chemicals.
b) Magnesium oxide:
(i) Elastomag 100, specific surface area of 140 m2/g, Akrochem Corporation.
(ii) Magchem 50, specific surface area of 65 m2/g, Martin Marietta Magnesia
Specialties Inc.
(iii) Magchem 40, specific surface area of 45 m2/g, Martin Marietta Magnesia
Specialties Inc.
c) Calcium oxide, CaO UN1190, Fisher Scientific.
d) Calcium hydroxide, Ca(OH)2 C97-500, Fisher Scientific.
e) Barium oxide, 99.99% Purity, Sigma-Aldrich.
43
3.1.3 Solvents
a) Trichloromethane, Reagent grade, EMD Chemicals Inc.
b) Ethyl alcohol, Reagent grade, Fisher Scientific.
3.2 Equipments
a) A 50 cc laboratory internal mixer, Rheocord System 40, a product of Haake
Buchler.
b) Farrel laboratory mill with a roll diameter of 15 cm and length of 30 cm
c) Monsanto ODR R-100
d) Dake press with platen size of 12.5 in x 12.5 in
e) Instron 5567
f) Nicolet 4700 FT-IR spectrometer equipped with SensIR Durascope to utilize
attenuated total reflectance (ATR) measurement
g) DMTA V, Rheometric Scientific
h) Dumbbell die type V according to ASTM D63895
i) Window mold with dimension of 1.0 mm x 93 mm x 118 mm
3.3 Compound Preparation
3.3.1 XNBR-Magnesium Oxide Compounds
Compound formulations are shown in Table 3.1. Three grades of magnesium
oxide (MgO) were used; Elastomag 100 (140 m2/g), Magchem 50 (65 m2/g), and
Magchem 40 (45 m2/g), assigned as “A”, “B”, and “C”, respectively. Each grade of MgO
contained certain amounts of impurities, thus, all recipes were adjusted accounting for the
44
impurity content. Numbers in parentheses are the amounts of materials added; the other
numbers are actual phr of MgO. The neat XNBR is designated as XNBR. Compositions
are designated XN-MgL_, where XN represents XNBR, Mg is for MgO, the letter L can
be “A”, “B”, or “C”, indicating a type of MgO, and the suffix is the amount relative to
stoichiometry, assuming that each Mg++ ion can neutralize two carboxyl groups. The
stoichiometric amount of MgO is 1.5 phr.
Table 3.1 Formulations of XNBR-MgO compounds
Ingredients XNBR XN-MgA0.5
XN-MgA1.0
XN-MgA1.5
XN-MgA2.0
XN-MgA3.0
XN-MgA4.0
XN-MgA5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
Elastomag 100 (98.0%)
0.0 (0.0)
0.75 (0.77)
1.5 (1.53)
2.25 (2.30)
3.0 (3.06)
4.5 (4.59)
6.0 (6.12)
7.5 (7.65)
Ingredients XNBR XN-MgB0.5
XN-MgB1.0
XN-MgB1.5
XN-MgB2.0
XN-MgB3.0
XN-MgB4.0
XN-MgB5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
Magchem 50 (98.0%)
0.0 (0.0)
0.75 (0.77)
1.5 (1.53)
2.25 (2.30)
3.0 (3.06)
4.5 (4.59)
6.0 (6.12)
7.5 (7.65)
Ingredients XNBR XN-MgC0.5
XN-MgC1.0
XN-MgC1.5
XN-MgC2.0
XN-MgC3.0
XN-MgC4.0
XN-MgC5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
Magchem 40 (98.0%)
0.0 (0.0)
0.75 (0.77)
1.5 (1.53)
2.25 (2.30)
3.0 (3.06)
4.5 (4.59)
6.0 (6.12)
7.5 (7.65)
45
Compounds were prepared (Table 3.2) in a 50 cc internal mixer (Rheocord 40,
Haake Buchler) using a fill factor of 0.85, and rotor speed of 40 rpm. Final mix
temperature was 85 oC to 95 oC. Compounds were milled and sheeted to about 1 mm
thick on an open mill (friction ratio of 1:1.21), and stored in sealed plastic bags in the
dark until further used.
Table 3.2 Mixing method
Time (min) Procedure
0 - 1 Add rubber
1 – 3 Masticate rubber
3 - 4.5 Add MgO
4.5 – 7 Mix rubber with MgO
7 Dump
3.3.2 XNBR-Peroxide Compounds
XNBR-Peroxide compounds were prepared to compare their properties to those of
metal oxide-cured compounds. Peroxide curing gives covalent crosslinks, while metal
oxide curing gives ionic linkages. Formulations of XNBR-peroxide are shown in Table
3.3. Compositions are named as followed; XN-P_, where letters XN are for XNBR, P is
for dicumyl peroxide (DCP), and the suffix indicates phr of DCP used. All compounds
were prepared in the same way as XNBR-MgO compositions, both mixing (Table 3.2)
and milling methods.
46
Table 3.3 XNBR-DCP formulations
Ingredients XNBR XN-P0.25
XN-P0.50
XN-P0.75
XN-P1.0
XN-P1.5
XN-P2.0
XN-P3.0
Nipol 1072 (XNBR )
100 100 100 100 100 100 100 100
Dicumyl peroxide - 0.25 0.50 0.75 1.0 1.5 2.0 3.0
3.3.3 Compounds of XNBR and Other Metal Oxides or Compounds
In addition to MgO, calcium oxide (CaO), calcium hydroxide (Ca(OH)2), and
barium oxide (BaO) were also used as curing agents for the XNBR. All these are
compounds of alkaline earth metals (group IIA in periodic table), with a valency of 2.
Compositions of XNBR-CaO, XNBR-Ca(OH)2, and XNBR-BaO are shown in Tables 3.4,
3.5, and 3.6, respectively. In each case, two levels of metal oxides or compounds are
shown. Numbers in parentheses are added amounts; others are actual content, taking into
account purity. Compositions are designated as XN-Aa_, where XN stands for XNBR,
Aa indicates a type (Table 3.7) of metal oxides or compounds, and the suffix is the
amount relative to stoichiometry, assuming that one mole of metal ion (M++) reacts with
two moles of COOH groups. The stoichiometric amount of each metal compound is
shown in Table 3.7. All compositions were mixed and milled using the same procedure
used in preparing XNBR-MgO and XNBR-peroxide compounds.
47
Table 3.4 XNBR-CaO compositions
Ingredients XNBR XN-Ca0.5
XN-Ca1.0
XN-Ca1.5
XN-Ca2.0
XN-Ca3.0
XN-Ca4.0
XN-Ca5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
CaO UN1910 (98.0 %) - 1.05
(1.07) 2.10
(2.14) 3.15
(3.21) 4.20
(4.29) 6.30
(6.43) 8.40
(8.57) 10.5
(10.7)
Table 3.5 XNBR-Ca(OH)2 compositions
Ingredients XNBR XN-Ch0.5
XN-Ch1.0
XN-Ch1.5
XN-Ch2.0
XN-Ch3.0
XN-Ch4.0
XN-Ch5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
Ca(OH)2 (97.0 %) - 1.39
(1.43) 2.78
(2.87) 4.17
(4.30) 5.56
(5.73) 8.34
(8.60) 11.1
(11.5) 13.9
(14.3)
Table 3.6 XNBR-BaO compositions
Ingredients XNBR XN-Ba0.5
XN-Ba1.0
XN-Ba1.5
XN-Ba2.0
XN-Ba3.0
XN-Ba4.0
XN-Ba5.0
Nipol 1072 (XNBR ) 100 100 100 100 100 100 100 100
BaO (99.99%) - 2.88 5.75 8.63 11.5 17.3 23.0 28.8
Table 3.7 Designation and stoichiometric amount of metal oxides or compounds
Aa Type of Metal Oxides or Compounds
Stoichiometric Amount (phr)
Mg MgO 1.50 Ca CaO 2.10 Ch Ca(OH)2 2.87 Ba BaO 5.75
48
3.4 Cure Behaviors and Molding
Cure behaviors of all compounds were determined according to ASTM D208496
using Monsanto ODR R100. A specimen of each composition weighing 8 to 9 g was put
into an electrically heated chamber, maintained at 165 oC (329 oF). When the chamber
was closed, the internal biconical disk rotor was oscillated at 3o arc within the rubber
sample. A rheometry curve was recorded and cure parameters such as minimum torque
(ML), maximum torque (MH), and scorch time (ts2) were determined from the curve. MH
is the highest torque attained at the specified time when no plateau or maximum torque
was obtained, ts2 is the time at which the torque rises above ML by 2.0 dN.m.
3.5 Molding
Tensile sheets were prepared by compression molding in a window mold (1 mm x
93 mm x 118 mm) at 165 oC. A rectangular sheet (13 to 14 g) of a compound was placed
between two Mylar sheets, and was placed into the mold; Teflon sheets were used if
rubber compounds stuck to Mylar sheets. The mold was placed between the upper and
lower platens of the press. The applied pressure was 25 tons. Cure times of compositions
are specified in Table 3.8. In the case of thermally cured of XNBR, cure times were 60,
120, 240, 500, and 1,000 minutes. After being removed from the press, the mold was
allowed to cool down at room temperature for 15 to 20 min. The tensile sheet was then
taken out of the mold and kept in a sealed plastic bag for 24 to 48 hr before tensile testing.
49
Table 3.8 Cure times for compositions
Compounds Cure Time (min)
XNBR-MgO 120 XNBR-Peroxide 60 XNBR-CaO 1,000 XNBR-Ca(OH)2 240 XNBR-BaO 240
3.6 Tensile Testing
Tensile properties at room temperature (25 ± 2 oC) of all vulcanizates were
determined using the Instron machine model 5567 equipped with an extensometer.
Dumbbell specimens were prepared using a type V die according to ASTM D638.95 The
distance between upper and lower clamps was set at 40 mm and the crosshead speed was
100 mm/min, producing a strain rate of 2.5 min-1 (0.042 s-1). The thickness of each
specimen was taken as an average value of three different positions on the narrow section
of the specimen, which was 3.18 mm wide. Two marks 10.0 mm apart were placed on the
narrow section. When the specimen was stretched, the change in the separation of the two
marks was followed by the extensometer. The tensile properties of the XNBR
vulcanizates, such as stress at specified strain, elongation at break, and tensile strength
were calculated from the measured quantities.
50
3.7 Crosslink Density Measurements
3.7.1 Near Equilibrium Stress-Strain measurement
Crosslink densities of vulcanizates were determined by equilibrium stress-strain
measurements. A dumbbell specimen about 1.0 mm thick, with two marks 10.0 mm apart,
was clamped at both ends using metal clips. One end was fastened to a steel bar, and the
other end was hung from a small weight. After 30 minutes, the extension was measured
using a cathetometer. Then an additional weight was added, and again the sample was left
for 30 minutes before the extended length was measured. These processes were repeated
until the extension ratio (λ) of the sample was greater than 3.5. The crosslink density of
the specimen was then determined by using the Mooney-Rivlin equation:97-99
λ+=
⎥⎦⎤
⎢⎣⎡
λ−λ
σ 21
2
C2C2
1 (15)
where λ is the extension ratio, σ is the engineering stress, and C1 and C2 are constants. By
plotting the quantity in the left side of equation 15 versus 1/λ, the intercept 2C1 is
obtained. It is related to crosslink density (ν) through equation 16:
RTC2 1=ν (16)
where R is the gas constant (8.314 J/mol.K), and T is the absolute temperature.
51
3.7.2 Volume Fraction (Vr) of Rubber by Equilibrium Swelling
Generally, crosslink density of rubber vulcanizates can also be determined by
equilibrium swelling using the well-known Flory-Rehner equation:100
⎟⎠⎞
⎜⎝⎛ −
χ++−⎟⎟⎠
⎞⎜⎜⎝
⎛−=ν
2VV
]VV)V1(ln[V21
r31r
2rrr
s (17)
where ν is the number of moles of tetrafunctional crosslinks per unit volume, Vr is the
volume fraction of rubber in the swollen gel, Vs is the molar volume of the solvent, and χ
is the rubber-solvent interaction parameter. However, ionic crosslinks exist as ionic
aggregates; therefore, it is better to use Vr as a meassure of crosslink density for the
XNBR vulcanizates.
Chloroform was used as a solvent, because its solubility parameter (δ = 9.30) is
similar to that of 75/25 butadiene/acrylonitrile copolymer (δ = 9.38).101
Specimens with dimensions about 1 mm x 2 mm x 25 mm were cut from cured
sheets and weighed on an analytical balance with an accuracy of 0.01 mg (Mi). These
specimens were put into vials (25 mm in diameter and 95 mm in length), and allowed to
swell in 30.0 mL of chloroform for 7 days in the dark. Then, they were taken out of the
solvent, blotted on paper and quickly weighed (Mgel). After drying for 24 hr at room
temperature, the specimens were dried at 70 oC in a vacuum oven for 24 hr. The dry
weight (Mdry) was measured. The volume fraction of rubber can be determined from
52
chloroformrubber
rubberr VV
VV
+= (18)
Vchloroform and Vrubber can be calculated using equations 19 and 20, respectively.
chloroform
igelchloroform
MMV
ρ
−= (19)
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
−ρ
=−=MO
MOi
dry
dryMOdryrubber
fM
MVVV (20)
VMO is the volume of the metal compound. ρdry is the density of the dry rubber compound.
fMO is the weight fraction of the metal compound. ρMO is the density of the metal
compound; the densities of MgO, CaO, Ca(OH)2, and BaO are 3.20, 3.30, 2.24, and 5.75
g/cm3, respectively. Chloroform has a density of 1.473 g/cm3 at 25 oC, and the neat
XNBR has a density of 0.98 g/cm3.
To measure the density of the dry rubber (ρdry), a specimen with dimension 1 mm
x 3 mm x 25 mm was cut from a cured sheet, and its weight in air (Mair) and ethanol
(Mliq) was determined. The weight when immersed in ethanol (ρ =0.785 g/cm3) is less
than that in air by the weight of ethanol displaced. The volume of ethanol displaced is
equal to that of the specimen. The density of dry rubber compound can be calculated
from
liqair
airliqdry MM
M−
ρ=ρ (21)
53
3.8 Dynamic mechanical properties
Dynamic mechanical properties of all XNBR vulcanizates were determined using
the Rheometric Scientific DMTA V. The tension mode and strain amplitude of 0.05%
were employed. A frequency of 1.0 Hz was used. A rectangular specimen (25 mm x 6.35
mm x 1.0 mm) was cut from a cured sheet, and then put into a temperature-controlled
chamber. The specimen was cooled down to -80 oC and then held between two clamps
with a gap between them of 10.0 mm. The sample was maintained at -80 oC for 30 min,
and heated from -80 oC to 180 oC at a rate of 2 oC/min. Dynamic mechanical properties,
such as E’, E” and tan δ, were recorded by computer.
3.9 Infrared spectral analysis
ATR-FTIR spectroscopy was employed to study the neutralization of XNBR
compounds. Two FT-IR spectrophotometers, Nicolet 4700 FT-IR and Nicolet 5SXC FT-
IR, were employed. They were equipped with a SensIR Durascope utilizing attenuated
total reflectance (ATR). The former was used to study XNBR-MgO, and XNBR-peroxide
systems; the latter was employed for the rest of the compounds. A specimen about 1.0
mm thick was scanned with a resolution of 4 cm-1 and 215 times, and the final result was
the average of 215 spectra. The cured samples were kept about 2 weeks in a dark
container at room temperature before testing.
For uncured compounds (aged about 2 weeks), specimens were compression-
molded to a thickness of about 1.0 mm.
54
CHAPTER IV
RESULTS AND DISCUSSION
4.1 Cure Behaviors
4.1.1 XNBR-MgO Compositions
Cure curves at 165 oC of the neat XNBR and compounds with different
magnesium oxides are shown in Figures 4.1 to 4.6. The neat XNBR stiffens slightly upon
heating, possibly due to self-coupling of carboxyl groups to form anhydride bridges (eq.
1). Cure rheometry of XN-MgA compounds is shown in Figure 4.1. The same results on
log-log scales are given in Figure 4.2, which shows that rheometric torques at early stage
(less than 10 min) of all compounds, except for XN-MgA0.5, are higher than that of the
neat XNBR. Maximum torque of XN-MgA0.5 is slightly lower than that of the neat
XNBR. With the increased amounts of MgO, cure rate increases, while scorch time
decreases. Maximum torques enormously increase with increasing MgO concentrations
up to twice stoichiometry, and little increase thereafter with excess amounts. Therefore,
optimum cure requires at least 2.0x stoichiometric amounts.
ODR curves of the neat XNBR and XN-MgB compounds are given in Figures 4.3
(linear scale), and 4.4 (log-log scale). Similar to XN-MgA compounds, cure rates
increase with the increased MgO contents, while scorch time decreases. However, XN-
MgB compounds cured slower than did XN-MgA compounds. At an early stage of
55
heating, compositions containing up to 2.0x stoichiometry have lower torques than does
the neat XNBR. The torque of XN-MgB0.5 remains lower than that of the neat XNBR
for the entire heating time.
Cure rheometry of XN-MgC compounds is shown in Figure 4.5. Torque of XN-
MgC0.5 is lower than that of the neat XNBR for the whole heating time. With increasing
MgO concentration, cure rate and maximum torque increase, while scorch time decreases.
However, cure rates of XN-MgC compounds are lower than those of XN-MgB, and XN-
MgA compositions. The log-log plot of cure rheometry of XN-MgC compounds (Figure
4.6) shows that at early stages of heating the torque of the neat XNBR is higher than that
of XN-MgC compounds containing MgO up to 3.0x stoichiometry, except for XN-
MgC4.0 and XN-MgC5.0. Thermal crosslinking of the XNBR is retarded by the presence
of large surface area MgO. Apparently, some carboxyl groups have reacted with MgO.
Based on ATR-IR results which will be discussed later, neutralization appears to be
involved in two steps, equations 22 and 23, respectively.102 MgO first reacts with
carboxyl groups, resulting in the magnesium hydroxycarboxylate salt, (– COOMgOH),
followed by coupling of hydroxycarboxylate salts to form the magnesium carboxylate
salt, (– COOMgOOC –). The overall reaction is in accord with equation 24, indicating
that equimolar amounts of acidity and MgO are required. The first neutralization does not
yield an elastically effective rubber network, but it decreases the concentration of
carboxyl groups available for thermal crosslinking. At longer time, torques of compounds
rise due to the increase in intermolecular links produced by the second neutralization, or
aggregation of salt products from both neutralization steps.
56
0 50 100 150 200 2500
10
20
30
40
50
60
70
80
XN-MgA4.0 XN-MgA5.0
XN-MgA3.0
XN-MgA2.0
XN-MgA1.5
XN-MgA1.0
XN-MgA0.5
XNBR
Tor
que
(dN
.m)
Time (min)
Testing temperature: 165 oC
Figure 4.1 ODR curves of XNBR cured with type A magnesium oxide (specific surface area of 140 m2/g) at 165 oC. (linear scale)
57
1 10 1005
6789
1010
20
30
40
50
60708090
100100Testing temperature: 165 oC
XN-MgA5.0XN-MgA4.0
XN-MgA3.0
XN-MgA2.0
XN-MgA1.5
XN-MgA1.0
XN-MgA0.5
XNBRTor
que
(dN
.m)
Time (min)
Figure 4.2 ODR curves of XNBR cured with type A magnesium oxide (specific surface area of 140 m2/g) at 165 oC. (log-log scale)
58
0 50 100 150 200 2500
10
20
30
40
50
60
70XN-MgB5.0
XN-MgB4.0
XN-MgB3.0
XN-MgB2.0
XN-MgB1.5
XN-MgB1.0
XN-MgB0.5
XNBR
Tor
que
(dN
.m)
Time (min)
Testing temperature: 165 oC
Figure 4.3 ODR curves of XNBR cured with type B magnesium oxide (specific surface area of 65 m2/g) at 165 oC. (linear scale)
59
1 10 1005
6789
1010
20
30
40
50
60708090
100100Testing temperature: 165 oC
XN-MgB3.0
XN-MgB5.0
XN-MgB4.0
XN-MgB2.0
XN-MgB1.5
XN-MgB1.0
XNBR
XN-MgB0.5
Tor
que
(dN
.m)
Time (min)
Figure 4.4 ODR curves of XNBR cured with type B magnesium oxide (specific surface area of 65 m2/g) at 165 oC. (log-log scale)
60
0 50 100 150 200 2500
10
20
30
40
50
60
70XN-MgC5.0
XN-MgC4.0
XN-MgC3.0
XN-MgC2.0
XN-MgC1.5
XN-MgC1.0
XNBR
XN-MgC0.5
Tor
que
(dN
.m)
Time (min)
Testing temperature: 165 oC
Figure 4.5 ODR curves of XNBR cured with type C magnesium oxide (specific surface area of 45 m2/g) at 165 oC. (linear scale)
61
1 10 1005
6789
1010
20
30
40
50
60708090
100100Testing temperature: 165 oC
XN-MgC5.0
XN-MgC4.0
XN-MgC3.0
XN-MgC2.0
XN-MgC1.5
XN-MgC1.0XNBR
XN-MgC0.5
Tor
que
(dN
.m)
Time (min)
Figure 4.6 ODR curves of XNBR cured with type C magnesium oxide (specific surface area of 45 m2/g) at 165 oC. (log-log scale)
62
1 10 1005
6
789
10
20
30
40
50
60
70
XNBR
XN-MgC2.0
XN-MgB2.0
Tor
que
(dN
.m)
Time (min)
XN-MgA2.0
Testing temperature 165 oCA = 140 m2/gB = 65 m2/gC = 45 m2/g
Figure 4.7 ODR curves of the XNBR cured with 2.0x stoichiometric amounts of different magnesium oxides at 165 oC.
63
RCOOH + MgO RCOOMgOH (22)
2 RCOOMgOH RCOOMgOOCR + Mg(OH)2 (23)
2 RCOOH + 2 MgO RCOOMgOOCR + Mg(OH)2 (24)
R = polymer chain
Evidently, cure reactions of MgO-vulcanized XNBR are dependent on both the
specific surface area and the concentration of MgO. Compounds cure quickly with
increasing specific surface area and concentration. Figure 4.7 compares XNBR
compositions containing different magnesium oxides at twice stoichiometry. Clearly,
large surface area MgO results in faster curing. Dependence of cure reaction on both
surface area and concentration indicates that it is a diffusion-controlled reaction.23, 24
4.1.2 XNBR-Dicumyl Peroxide Compositions
ODR curves of XN-P compounds are given in Figure 4.8. Minimum torque is
little affected by increased amounts of peroxide. With increasing peroxide content, cure
rates and maximum torques increase, while scorch time decreases. The increase in
maximum torque or stiffness is due to increased crosslink density. Figure 4.9 shows the
dependence of ΔM, MH – ML, on peroxide content.
64
0 10 20 30 40 50 600
20
40
60
80
100
Testing temperature: 165 oC
XN-P3.0
XN-P2.0
XN-P1.5
XN-P1.0
XN-P0.75
XN-P0.50
XN-P0.25
Tor
que
(dN
.m)
Time (min)
XNBR
Figure 4.8 ODR curves of XNBR cured with dicumyl peroxide at 165 oC
65
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
20
40
60
80
100
ΔΜ
= M
H -
ML (d
N.m
)
Amount of peroxide (phr)
Extrapolated line from lower concentration of peroxide
Experiment
Figure 4.9 Delta torque, ΔM = MH – ML, as a function of dicumyl peroxide content.
66
The increase in ΔM suggests that crosslink density increases with increasing peroxide
concentration. However, at higher concentrations the slope declines suggesting that the
efficiency of peroxide crosslinking decreases. This may be due to some acidic nature of
the rubber matrix. Acids induce heterolytic or ionic decomposition of the peroxide, in
which the peroxide is consumed without radical formation.103, 104
4.1.3 XNBR-CaO Compositions
Cure rheometry of XNBR cured with calcium oxide is shown in Figure 4.10. The
same results using log-log scales are given in Figure 4.11. The minimum torque is highest
for the neat XNBR. This suggests solubilization and plasticization by calcium oxide.
Thermal crosslinking is initially retarded by presence of CaO particles. Maximum torque
of the raw XNBR is comparable to those of XNBR-CaO compounds, suggesting that
similar levels of cure are reached
4.1.4 XNBR-Ca(OH)2 Compositions
Cure curves of XNBR-Ca(OH)2 compounds are shown in Figure 4.12. A log-log
plot of the same results is given in Figure 4.13. At early stages of heating, torque of the
raw XNBR is higher than those of XNBR-Ca(OH)2 compounds, except for XN-Ch5.0.
Similar to XNBR-MgO and XNBR-CaO systems, thermal crosslinking is retarded by
presence of Ca(OH)2. Torque of XN-Ch0.5 remains lower than that of the neat XNBR for
the entire heating time. Cure behaviors of XNBR-Ca(OH)2 compounds are quite different
from those of XNBR-CaO systems.
67
0 200 400 600 800 10005
10
15
20
25
30
35
40
45
Tor
que
(min
)
Time (min)
ODR curves of XN-CaO compounds at 165 oC
XNBR
XN-Ca3.0
XN-Ca4.0
XN-Ca5.0
XN-Ca0.5
XN-Ca1.0
XN-Ca1.5
XN-Ca2.0
Figure 4.10 ODR curves of XNBR cured with calcium oxide at 165 oC. (linear scale)
68
1 10 100 1000
10
20
30
40
50ODR curves of XN-CaO compounds at 165 oC
XN-Ca1.0
XN-Ca1.5
XN-Ca0.5
XN-Ca3.0
XN-Ca2.0
XN-Ca5.0
XN-Ca4.0
Tor
que
(dN
.m)
Time (min)
XNBR
Figure 4.11 ODR curves of XNBR cured with calcium oxide at 165 oC. (log-log scale)
69
0 50 100 150 200 250
20
40
60
80
100XN-Ch5.0XN-Ch4.0
XN-Ch3.0
XN-Ch2.0
XN-Ch1.5
XN-Ch0.5
XN-Ch1.0
Tor
que
(dN
.m)
Time (min)
XN-Ca(OH)2 compounds : 165 oC
XNBR
Figure 4.12 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (linear scale)
70
1 10 1006789
10
20
30
40
50
60708090
100T
orqu
e (d
N.m
)
Time (min)
XN-Ca(OH)2 compounds : 165 oC
XN-Ch3.0XN-Ch4.0
XN-Ch5.0
XN-Ch2.0
XN-Ch1.5
XN-Ch1.0
XNBR
XN-Ch0.5
Figure 4.13 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (log-log scale)
71
There is an induction period, which is dependent on the concentration of Ca(OH)2.
Scorch time reduces, while cure rate increases with increased amounts of Ca(OH)2. A
sharp increase in torque indicates that cure reactions are fast. Maximum torque increases
essentially with increasing Ca(OH)2 concentrations up 1.5x stoichiometry, with little
change thereafter, with excess amounts (2.0x – 5.0x stoichiometric amount).
4.1.5 XNBR-BaO Compositions
ODR curves of XNBR-BaO compounds at 165 oC are shown in Figures 4.14
(linear) and 4.15 (log-log), respectively. At short times, torque of the raw XNBR is
higher than that of compounds containing BaO up to 1.5x stoichiometry. Torque of XN-
Ba0.5 remains lower than that of the neat XNBR for the entire heating time. Similar to
other systems, thermal crosslinking is inhibited by the presence of curatives. The increase
in torque for XN-Ba0.5 with heating time suggests the formation of salts, but these salts
do not yield strong crosslinks, resulting in lower torque than the neat XNBR. It is
important to note that BaO particles are not easily dissolved in the XNBR matrix, and
still remain visible as large particles in all green compounds. Cure rate and stiffness
increase with increasing BaO concentration.
4.2 Crosslink Density Measurements
4.2.1 Thermally-Cured XNBR
Volume fraction (Vr) of the rubber in the swollen gel, sol content, and crosslink
density of thermally cured XNBR at different cure times are shown in Table 4.1, and the
plot of Vr and sol content as a function of cure time is given in Figure 4.16.
72
0 50 100 150 200 250
10
20
30
40
50
60
70
80
90Testing temperature : 165 oC
XNBR
XN-Ba5.0
XN-Ba4.0
XN-Ba3.0
XN-Ba2.0
XN-Ba1.5
XN-Ba1.0
Tor
que
(dN
.m)
Time (min)
XN-Ba0.5
Figure 4.14 ODR curves of XNBR cured with barium oxide at 165 oC. (linear scale)
73
1 10 100789
10
20
30
40
50
60
708090
100Testing temperature : 165 oC
Tor
que
(dN
.m)
Time (min)
XN-Ba5.0
XN-Ba4.0
XN-Ba3.0
XN-Ba2.0
XN-Ba1.5
XN-Ba1.0
XNBR
XN-Ba0.5
Figure 4.15 ODR curves of XNBR cured with barium oxide at 165 oC. (log-log scale)
74
Table 4.1 Volume fraction of rubber (Vr), sol content, and crosslink density of the raw XNBR cured at 165 oC
Cure time (min) Property
60 120 240 500 1000
Vr 0.0147
± 0.0017 0.0389
± 0.0007 0.0711
± 0.0007 0.0827
± 0.0002 0.0962
± 0.0025
Sol content (%) 71.1 ± 1.7
42.6 ± 0.1
22.8 ± 0.2
16.3 ± 0.1
11.8 ± 0.6
ν (x 105 mol/cm3)*
0.47 ± 0.05
0.69 ± 0.07
3.63 ± 0.04
4.45 ± 0.15
6.38 ± 0.10
* Determined by near equilibrium stress-strain measurement
Vr and crosslink density increase with the increased cure times, while sol content
decreases, suggesting self-crosslinking of the raw XNBR, possibly by anhydride
formation. Brown6, 8 suggested that carboxylic rubbers are capable of self-crosslinking to
form anhydride linkages. However, rather severe conditions were required. Lee and
coworkers105 reported that copolymers of ethylene and methacrylic acid containing
various acid contents can form anhydride structures when heated above 140 oC. However,
Vr increases markedly with increasing cure times from 60 to 240 min, and slightly
thereafter. This may be due to existence of equilibrium (eq. 25).106
R C
O
C
O
OH2 C
O
OR R H2O (25)
R = polymer chain
+
75
0 200 400 600 800 1000 12000.00
0.02
0.04
0.06
0.08
0.10
0.12
10
20
30
40
50
60
70
80Raw XNBR thermally cured at 165 oC
Vr
Cure time (min)
Vr
Sol content
Sol
con
tent
(%)
Figure 4.16 Vr and sol content of thermally cured XNBR as a function of cure time.
76
4.2.2 XNBR-MgO Vulcanizates
Results from equilibrium swelling and stress-strain measurements of XNBR cured
with different magnesium oxides are given in Table 4.2. Figure 4.17 shows the plot of Vr
and sol content against MgO concentration. For vulcanizates containing MgO, Vr
increases with increased concentration of MgO. However, vulcanizates with 0.5x
stoichiometric amount of MgO have lower Vr than the neat XNBR. The swollen gels of
these samples were soft and fragile, and did not maintain their original shape. They lay
flat on the surface of aluminum pans, suggesting weak crosslinking. This is consistent
with ODR results in that the torque of the neat XNBR is higher than those of XN-
MgA0.5, XN-MgB0.5, and XN-MgC0.5. In these vulcanizates, a basic magnesium
hydroxycarboxylate salt is largely formed, and is not expected to give efficient crosslinks.
This salt is a product of the first neutralization step (equation 22), which is an intrachain
reaction, and is expected to be very mobile at curing temperature. Swelling levels of these
samples are much higher than that of the thermally cured XNBR sample. Sol contents of
vulcanizates containing 0.5x stoichiometric amount of MgO are much higher than that of
the neat XNBR, indicating lesser amounts of rubber molecules bound to the networks. Vr
increases markedly with the increased amounts of MgO up to 2.0x stoichiometry, and
changes little thereafter. Apparently, strong rubber networks are obtained when the MgO
concentration is at least 2.0x stoichiometric amounts. Sol content decreases enormously
with the increased concentration of MgO up to 1.5x stoichiometry, and changes little
thereafter, suggesting that at this level of MgO large amounts of rubber are bound to the
networks. High surface area MgO give a slightly higher Vr than do low surface area ones,
but the values are comparable.
77
Table 4.2 Volume fraction (Vr) of rubber, sol content, and crosslink density (ν) of XNBR cured with different magnesium oxides (120 min at 165 oC)
Property XNBR XN-MgA0.5
XN-MgA1.0
XN-MgA1.5
XN-MgA2.0
XN-MgA3.0
XN-MgA4.0
XN-MgA5.0
Vr 0.0389
± 0.0007
0.0275±
0.0005
0.0512 ±
0.0003
0.0758 ±
0.0019
0.1028 ±
0.0002
0.1130 ±
0.0010
0.1136 ±
0.0014
0.1223 ±
0.0008
Sol content (%)
42.6 ± 0.1
64.5 ± 0.3
39.6 ± 2.0
8.8 ± 0.3
6.4 ± 0.1
5.6 ± 0.1
5.2 ± 0.2
5.2 ± 0.1
ν (x 105 mol/cm3)*
0.69 ± 0.07
1.46 ± 0.36
4.31 ± 0.21
6.91 ± 0.30
11.5 ± 0.5
15.9 ± 0.6
18.6 ± 0.5
20.0 ± 0.8
Property XNBR XN-MgB0.5
XN-MgB1.0
XN-MgB1.5
XN-MgB2.0
XN-MgB3.0
XN-MgB4.0
XN-MgB5.0
Vr 0.0389
± 0.0007
0.0231 ±
0.0014
0.0389 ±
0.0005
0.0849 ±
0.0040
0.1098 ±
0.0007
0.1105 ±
0.0053
0.1104 ±
0.0027
0.1134 ±
0.0015
Sol content (%)
42.6 ± 0.1
73.0 ± 1.7
52.9 ± 0.5
7.2 ± 0.7
6.4 ± 0.1
5.6 ± 0.2
4.8 ± 0.1
4.6 ± 0.1
ν (x 105 mol/cm3)*
0.69 ± 0.07
0.57 ± 0.06
1.50 ± 0.17
2.81 ± 0.59
7.39 ± 0.24
9.00 ± 0.86
15.7 ± 0.7
18.1 ± 0.5
Property XNBR XN-MgC0.5
XN-MgC1.0
XN-MgC1.5
XN-MgC2.0
XN-MgC3.0
XN-MgC4.0
XN-MgC5.0
Vr 0.0389
± 0.0007
0.0193 ±
0.0005
0.0319 ±
0.0007
0.0828 ±
0.0019
0.1011 ±
0.0047
0.0920 ±
0.0082
0.0900 ±
0.0018
0.1081 ±
0.0007
Sol content (%)
42.6 ± 0.1
72.8 ± 0.2
63.3 ± 1.0
9.5 ± 0.3
7.7 ± 0.3
7.1 ± 0.1
6.4 ± 0.1
6.3 ± 0.3
ν (x 105 mol/cm3)*
0.69 ± 0.07
0.23 ± 0.02
0.97 ± 0.19
2.65 ± 0.40
4.44 ± 0.33
8.57 ± 0.32
15.1 ± 0.34
19.5 ± 0.25
* Determined by near equilibrium stress-strain measurement
78
0 1 2 3 4 5 6
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0
10
20
30
40
50
60
70
80
90
XN-MgA XN-MgB XN-MgC
opened is Vr
closed is sol content
Vr
Amount of MgO/ Stoichiometric Amount
XNBR-MgO compounds cured 120 min at 165 oC
Vr
Sol content
Sol
con
tent
(%)
Figure 4.17 Vr and sol content of MgO-cured XNBR as a function of MgO concentration.
79
4.2.3 XNBR-Peroxide Vulcanizates
Vr, sol content, and crosslink density of XNBR-peroxide vulcanizates are given in
Table 4.3. The plot of Vr and sol content as a function of peroxide content is shown in
Figure 4.18. As expected, Vr and crosslink density increase, while the sol content
decreases with increased peroxide content. The shape of the plot between Vr and peroxide
amount is very similar to Figure 4.9, indicating a decline of crosslink efficiency.
Table 4.3 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with dicumyl peroxide 60 min at 165 oC
Property XNBR XN-P0.25
XN-P0.5
XN-P0.75
XN-P1.0
XN-P1.5
XN-P2.0
XN-P3.0
Vr 0.0147
± 0.0017
0.0410±
0.0010
0.0713 ±
0.0015
0.0905 ±
0.0007
0.1098 ±
0.0008
0.1370 ±
0.0007
0.1552 ±
0.0005
0.1869 ±
0.0003
Sol content (%)
71.1 ± 1.7
34.7 ± 0.1
19.6 ± 0.3
13.9 ± 0.1
10.4 ± 0.1
7.7 ± 0.1
6.0 ± 0.1
5.2 ± 0.2
ν(x 105 mol/cm3)*
0.47 ± 0.05
1.83 ± 0.17
3.38 ± 0.24
4.81 ± 0.40
5.85 ± 0.47
12.4 ± 0.5
13.1 ± 0.4
23.4 ± 0.3
* Determined by near equilibrium stress-strain measurement
4.2.4 XNBR-CaO Vulcanizates
Results from equilibrium swelling and stress-strain measurements are given in
Table 4.4, and the plot of Vr and sol content against CaO content is shown in Figure 4.19.
Vr and crosslink density of the neat XNBR is slightly higher than for XN-Ca vulcanizates,
while the sol content is slightly less. However, Vr and the sol content of all compounds
are comparable. Apparently, curing in all the compounds is very similar; that is thermal
crosslinking.
80
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.00
0.04
0.08
0.12
0.16
0.20
0
10
20
30
40
50
60
70
80
Vr
Amount of peroxide (phr)
Vr
XNBR-Peroxide compounds cured 60 min at 165 oC
Sol content
Sol
con
tent
(%)
Figure 4.18 Vr and sol content of XNBR-Peroxide vulcanizates as a function of peroxide concentration.
81
Table 4.4 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium oxide 1000 min at 165 oC
Property XNBR XN-Ca0.5
XN-Ca1.0
XN-Ca1.5
XN-Ca2.0
XN-Ca3.0
XN-Ca4.0
XN-Ca5.0
Vr 0.0962
± 0.0025
0.0883±
0.0011
0.0806 ±
0.0011
0.0822 ±
0.0062
0.0849 ±
0.0021
0.0833 ±
0.0007
0.0869 ±
0.0004
0.0803 ±
0.0004
Sol content (%)
11.8 ± 0.6
12.9 ± 0.1
14.6 ± 0.3
15.1 ± 2.4
15.1 ± 1.3
13.5 ± 0.5
13.8 ± 0.1
14.9 ± 0.4
ν (x 105 mol/cm3)*
6.38 ± 0.10
6.33 ± 0.17
5.13 ± 0.08
4.20 ± 0.12
5.94 ± 0.17
4.37 ± 0.26
4.47 ± 0.36
4.21 ± 0.14
* Determined by near equilibrium stress-strain measurement
A lower Vr in XN-Ca vulcanizates is probably because self-crosslinking is
prohibited by the presence of CaO. At first CaO is expected to have a similar impact on
properties of the XNBR as ZnO. However, CaO curing does not lead to ionic crosslinks,
which give high tensile properties. Starmer25 classified CaO as an inactive material,
having a conflicting effect on the properties of the XNBR when compared to ZnO, an
active one.
4.2.5 XNBR-Ca(OH)2 Vulcanizates
Vr, sol content, and crosslink density of XNBR-Ca(OH)2 vulcanizates are given in
Table 4.5. The plot of Vr and sol content versus Ca(OH)2 content is shown in Figure 4.20.
82
0 1 2 3 4 5 60.00
0.04
0.08
0.12
0.16
0.20
0
5
10
15
20
25
Vr
Amount of CaO/Stoichiometric Amount
XN-Ca compounds cured 1000 min at 165 oC
Sol content
Vr
Sol
con
tent
(%)
Figure 4.19 Vr and sol content of XNBR-CaO vulcanizates as a function of CaO concentration.
83
Table 4.5 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium hydroxide 240 min at 165 oC
Property XNBR XN-Ch0.5
XN-Ch1.0
XN-Ch1.5
XN-Ch2.0
XN-Ch3.0
XN-Ch4.0
XN-Ch5.0
Vr 0.0711
± 0.0007
0.0193±
0.0039
0.0316 ±
0.0011
0.0588 ±
0.0012
0.0717 ±
0.0008
0.0777 ±
0.0015
0.0880 ±
0.0004
0.0963 ±
0.0004
Sol content (%)
22.8 ± 0.2
66.4 ± 1.4
41.7 ± 0.3
12.8 ± 0.9
9.4 ± 0.2
7.2 ± 0.6
6.3 ± 0.2
5.4 ± 0.4
ν (x 105 mol/cm3)*
3.63 ± 0.04
5.34 ± 0.08
14.7 ± 0.9
29.1 ± 0.8
30.6 ± 1.2
32.1 ± 0.8
32.5 ± 0.1
26.7 ± 2.1
* Determined by near equilibrium stress-strain measurement
Vr and crosslink density increase with increased Ca(OH)2 content, while the sol
content decreases. Obviously, Vr of the neat XNBR is higher than those of vulcanizates
containing Ca(OH)2 up to 1.5x stoichiometry, but similar to that of the XN-Ch2.0
vulcanizate. The swollen gels of XN-Ch0.5 and XN-Ch1.0 were soft and fragile, and did
not maintain their shape, and lay flat on the aluminum pan surface, suggesting weak
crosslinking. Vulcanizates containing Ca(OH)2 3.0x to 5.0x stoichiometry have higher Vr
than the neat XNBR, indicating strong crosslinking. The sol content decreases sharply
with increasing amount of Ca(OH)2 up to 1.5x stoichiometry, and then changes slightly
with a great excess of Ca(OH)2, suggesting that most of the rubber molecules are bound
to the rubber network. Apparently, strong crosslinking results when the amount of
Ca(OH)2 is at least twice stoichiometry.
84
0 1 2 3 4 5 60.00
0.02
0.04
0.06
0.08
0.10
0.12
0
10
20
30
40
50
60
70
80
Vr
Amount of Ca(OH)2/Stoichiometric Amount
XN-Ca(OH)2 compounds cured 240 min at 165 oC
Vr
Sol content
Sol
con
tent
(%)
Figure 4.20 Vr and sol content of XNBR-Ca(OH)2 vulcanizates as a function of Ca(OH)2 concentration.
85
4.2.6 XNBR-BaO Vulcanizates
Results from equilibrium swelling and stress-strain testes are given in Table 4.6.
Figure 4.21 shows the influence of the amounts of BaO on Vr and the sol content of
XNBR-BaO vulcanizates. As in the cases of MgO and Ca(OH)2, Vr and crosslink density
increase with increasing BaO content, with a corresponding decrease in sol content. Vr
increases greatly with the increased BaO content up 2.0x stoichiometry. The sol content
decreases with increasing BaO amounts up to 2.0x stoichiometry. It is interesting to note
that for all the XNBR-BaO vulcanizates, Vr is less than that of the neat XNBR. However,
crosslink density of XNBR containing BaO at least 1.5x stoichiometry, determined by
near equilibrium stress-strain measurement, is higher than that of the neat XNBR. This
suggests that solvent resistance of ionic crosslinks formed is worse than covalent
crosslinks.
Table 4.6 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with barium oxide 240 min at 165 oC
Property XNBR XN-Ba0.5
XN-Ba1.0
XN-Ba1.5
XN-Ba2.0
XN-Ba3.0
XN-Ba4.0
XN-Ba5.0
Vr 0.0711
± 0.0007
0.0344±
0.0015
0.0441 ±
0.0005
0.0491 ±
0.0005
0.0552 ±
0.0087
0.0588 ±
0.0002
0.0649 ±
0.0017
0.0684 ±
0.0010
Sol content (%)
22.8 ± 0.2
38.1 ± 2.5
23.1 ± 0.2
17.4 ± 0.1
13.7 ± 0.1
9.9 ± 0.1
6.2 ± 0.2
4.2 ± 0.8
ν (x 105 mol/cm3)*
3.63 ± 0.04
2.12 ± 0.21
3.09 ± 0.47
6.17 ± 0.04
8.59 ± 0.31
9.29 ± 0.65
11.0 ± 0.7
11.5 ± 0.8
* Determined by near equilibrium stress-strain measurement
86
0 1 2 3 4 5 60.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0
10
20
30
40
50XNBR-BaO compounds cured 240 min at 165 oC
Vr
Amount of BaO/Stoichiometric Amount S
ol c
onte
nt (%
)
Figure 4.21 Vr and sol content of XNBR-BaO vulcanizates as a function of BaO concentration.
87
4.2.7 Comparison among Metal Compounds
Vr and sol content of XNBR vulcanized by various metal compounds are shown
as a function of metal compound concentration in Figures 4.22 and 4.23, respectively. Vr
and sol content of XNBR-CaO vulcanizates (cured for 1000 min at 165 oC) show little
change with increasing CaO contents, and are comparable to those of the neat XNBR. In
these vulcanizates, curing is very similar, and occurs by thermal coupling of carboxyl
groups to from covalent anhydride bridges. Presence of CaO particles may somehow
inhibit thermal crosslinking. Therefore, Vr of XNBR-CaO samples is slightly lower than
that of the neat XNBR.
XN-Ba0.5 has a lower sol content and slightly higher Vr than those of XN-Ch0.5
and XN-MgA0.5. This may be due to the effect of particle size. As mentioned before, all
XN-Ba vulcanizate sheets contained visible particles. These may lead to larger amounts
of carboxyl groups that have not reacted with BaO. These carboxyl groups can undergo
self-coupling to form anhydride crosslinks. Another possibility is that ionic salts formed
in these samples do not give efficient crosslinking, therefore anhydride links will be more
important. For samples containing 1.0x stoichiometry, MgO gives higher Vr than BaO
and Ca(OH)2. We will show later that in XN-MgA1.0 all carboxyl groups are essentially
neutralized, and both magnesium hydroxycarboxylate (inefficient crosslink) and
magnesium carboxylate (efficient crosslink) are formed, while in XN-Ch1.0 and XN-
Ba1.0 there are certain amounts of carboxyl groups, which have not reacted with metal
compounds.
88
0 1 2 3 4 5 6
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Vr
Amount of metal compounds/Stoichiometric amount
XN-MgO
XN-Ca(OH)2
XN-CaO
XN-BaO
Figure 4.22 Vr of XNBR vulcanized by various metal compounds as a function of concentration.
89
0 1 2 3 4 5 60
10
20
30
40
50
60
70
80
Amount of metal compounds/Stoichiometric amount
XN-CaO XN-Ca(OH)2
XN-BaO XN-MgO
Sol c
onte
nt (%
)
Figure 4.23 Sol content of XNBR vulcanized by various metal compounds as a function of concentration.
90
With increasing amounts of metal compounds up to 1.5x stoichiometry, sol
contents decrease, but change little thereafter. It is interesting to note that sol contents are
similar among XN-Ba, XN-Ch, and XN-MgA systems above this concentration. This
suggests that the amount of rubber molecules bound into a network is very similar in
these systems. Vr markedly increase with increased concentration of metal compounds up
to 2.0x stoichiometry, and slightly increases with further great excess. Vr and sol content
obviously indicate that the amount of rubber bound into a network is similar in these
three systems, but they differ in the degree of swelling. This suggests a difference in the
strength of the crosslinks in each system, and a possible relation between the strength of
crosslinks and the size of metal ions. The strength of ionic crosslinks is in the following
order: XNBR-MgO > XNBR-Ca(OH)2 > XNBR-BaO. Table 4.7 shows the effective
ionic radii of Mg++, Ca++, and Ba++ with various coordination numbers.107 At the same
coordination number, ionic radii are ranked as followed: Ba++ > Ca++ > Mg++.
Table 4.7 The effective ionic radii of Mg++, Ca++, and Ba++ ions with various coordination numbers107
Type of Ion Coordination Number
Ionic Radii (Å)
Mg++ 4 6 8
0.71 0.86 1.03
Ca++ 6 8
1.14 1.26
Ba++ 6 8
1.49 1.56
91
The Coulombic force, F, between an anion of charge, q-, and a cation of charge,
q+, is given by
20 r4qqFεεπ
=−+
(26)
where r is the distance between the centers of the ions, ε is the dielectric constant of the
medium, and ε0 is the permittivity of free space (8.8542 x 10-12 C2/N.m2). The attractive
force is directly proportional to the product of ionic charges, and varies inversely with the
square of the distance between them. Clearly, swelling behaviors of XNBR-MgO,
XNBR-Ca(OH)2, and XNBR-BaO systems can be well explained by the classical
Coulomb law. Tant and coworkers84 made similar observations in IA and IIA metal-
neutralized carboxylate telechelic polyisoprene. They also suggested that this simple rule
holds only within the particular group of the periodic table, but cannot be applied across
groups. Bagrodia and Wilkes108 commented that not only does the nature of the cation
play a role in determining ionomer properties, but also the electronic configuration of the
cation, which governs its covalent characteristics.
4.3 Tensile Properties
4.3.1 Thermally Cured XNBR
Stress-strain curves of thermally cured XNBR are shown in Figure 4.24, and
tensile properties, 300% modulus, tensile strength, and breaking strain, are plotted as a
function of cure time in Figure 4.25.
92
0 200 400 600 800 1000 1200 1400 16000
2
4
6
8
10
120 min
240 min
500 min
x
x
x
Stre
ss (M
Pa)
Strain (%)
Thermally cured raw XNBR at 165 oC
x
1000 min
Figure 4.24 Stress-strain curves of thermally cured XNBR.
93
0 200 400 600 800 10000
1
6
7
8
9
10
11
600
800
1000
1200
1400
1600
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Cure time (min)
Raw XNBR thermally cured at 165 oC
Tensile strength
Elongation at break
300% Modulus
Elo
ngat
ion
at b
reak
(%)
Figure 4.25 Tensile properties of thermally cured XNBR as a function of cure time.
94
Modulus and tensile strength increase rapidly, while the ultimate strain decreases
with increased cure time from 60 to 240 min. This is due to the formation of anhydride
crosslinks upon heating. Further increase in cure times (500 and 1000 min) results in little
improvement in tensile properties. This may be due to reaching an equilibrium in
anhydride formation (equation 25),106 which will limit the amount of anhydride crosslinks
formed.
4.3.2 XNBR-MgO Vulcanizates
Stress-strain curves of XN-MgA, XN-MgB, and XN-MgC vulcanizates (cured
120 min at 165 oC) are given in Figures 4.26, 4.27, and 4.28, respectively. Similar
behaviors are observed in all three systems. Tensile modulus and strength are very much
improved, while the breaking strain decreases with the increased MgO content up to 1.5x
to 2.0x stoichiometry, with little change for a great excess of MgO. Tensile results are
consistent with those from ODR and swelling measurements.
Figure 4.29 shows tensile properties of XN-MgA, XN-MgB, and XN-MgC
vulcanizates as a function of MgO contents. 300% Modulus and tensile strength increase
with increasing MgO content up to 1.5x to 2.0x stoichiometry, and slightly change
thereafter. Brown5, 6 obtained similar results on ZnO-cured XNBR, and so did Sato on
ZnO-cured XSBR.64 Breaking strain markedly decreases with increasing MgO content up
to 1.0x to 1.5x stoichiometry, and changes little after that. A tensile strength of about 48
to 52 MPa with an ultimate strain of 500 to 600% is exceptional high, suggesting that
MgO-cured XNBR is a self-reinforced system. However, optimum properties require an
MgO content of at least twice stoichiometry.
95
0 400 800 1200 16000
10
20
30
40
50
60
xxx
x
x
x
x
XN-MgA5.0
XN-MgA4.0
XN-MgA3.0
XN-MgA2.0
XN-MgA1.5
XN-MgA1.0
XN-MgA0.5
Stre
ss (M
Pa)
Strain (%)
XNBR
XN-MgA vulcanizates cured 120 min at 165 oC
x
Figure 4.26 Stress-strain curves of XN-MgA vulcanizates (cured 120 min at 165 oC).
96
0 400 800 1200 16000
10
20
30
40
50
60
xxx
x
x
x
x
XN-MgB5.0
XN-MgB4.0
XN-MgB3.0
XN-MgB2.0
XN-MgB1.5
XN-MgB1.0
XN-MgB0.5
XNBR
Stre
ss (M
Pa)
Strain (%)
XN-MgB vulcanizates cured 120 min at 165 oC
x
Figure 4.27 Stress-strain curves of XN-MgB vulcanizates (cured 120 min at 165 oC).
97
0 400 800 1200 16000
10
20
30
40
50
60
XN-MgC4.0
XN-MgC5.0
XN-MgC3.0
XN-MgC2.0
XN-MgC1.5
XN-MgC1.0
XNBRXN-MgC0.5
xx
x
x
x
x
x
Stre
ss (M
Pa)
Strain (%)
x
XN-MgC vulcanizates cured 120 min at 165 oC
Figure 4.28 Stress-strain curves of XN-MgC vulcanizates (cured 120 min at 165 oC).
98
0 1 2 3 4 5 6
1
10
100
400
600
800
1000
1200
1400
1600
Elongation at break
XN-MgB
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Amount of MgO/Stiochiometric Amount
XNBR-MgO vulcanizates cured 120 min at 165 oCXN-MgA Tensile strength
XN-MgC
XN-MgA
XN-MgB XN-MgC
300% Modulus
XN-MgA XN-MgB XN-MgC
Elo
ngat
ion
at b
reak
(%)
Figure 4.29 Tensile properties of XNBR cured with different magnesium oxides (cured 120 min at 165 oC).
99
It may be deduced from the tensile results that carboxyl groups are completely
neutralized at about 2.0x stoichiometry. High tensile strength and modulus is due to
aggregation of ionic crosslinks to form a biphasic reinforced structure as suggested by
Tobolsky and coworkers.53 Ionic aggregates may function both as multifunctional
crosslinks and as reinforcing filler particles. Additionally, Cooper72-74 suggested that high
tensile strength may be due to an interchange between ionic crosslinks under mechanical
stress. This mechanism will prevent the development of local stress concentration which
can lead to catastrophic failure. The required amount (at least 2.0x stoichiometry) of
MgO to gain optimum properties proves that the classical neutralization (eq. 7) for
divalent metals, in which one mole of metal oxide neutralizes two moles of carboxyl
groups, is incorrect and we will show later that it also cannot hold for other metal
compounds studied. Many researchers have suggested that a basic salt, – COOMOH, may
form in carboxylic rubbers neutralized by divalent metal oxides, and that its polar nature
can lead to strong intermolecular interactions.5-8, 59, 72-74, 81, 82
Apparently, the amount of MgO is a major factor governing the tensile properties
of vulcanizates up to the point (less than 2.0x stoichiometry) that all carboxyl groups are
completely neutralized and a strong neutral salt, – COOMOOC –, is formed. After that,
the effect of concentration is not significant. Specific surface area is not an important
factor in determining the tensile properties of vulcanizates (Figure 4.29), although high
specific surface area gives slightly better tensile properties. It appears that specific
surface area has a great impact only on the cure behavior.
100
4.3.3 XNBR-Peroxide Vulcanizates
Stress-strain curves of peroxide-cured XNBR are given in Figure 4.30, and the
plot of tensile properties against peroxide content is shown in Figure 4.31. The neat
XNBR heated for 60 min at 165 oC has an elongation greater than 1600 % with tensile
stress greater than 4.2 MPa. A maximum tensile strength of 8.33 MPa with breaking
strain of about 1400 % is obtained in a vulcanizate containing small amount of dicumyl
peroxide (0.25 phr). With increasing curative amounts the tensile strength drops. The
ultimate strain decreases, while modulus increases linearly with the amount of peroxide.
In the rupture of rubber vulcanizates, a portion of the input energy is stored elastically
and released upon crack propagation. The rest of the energy is lost in internal dissipative
processes, such as chain motion. At high crosslink levels, chain motions are restricted,
and not much energy is dissipated. This results in brittle fracture at low strain.109
4.3.4 XNBR-CaO Vulcanizates
Figure 4.32 shows the stress-strain curves of XN-Ca vulcanizates (cured 1000 min
at 165 oC). They are very similar to that of the neat XNBR. The effect of CaO content on
the tensile properties of XN-Ca vulcanizates is given in Figure 4.33. Tensile strengths (7
to 8 MPa) of XN-Ca samples are slightly less than that of the neat XNBR (about 9 MPa),
but not by much. The 300% modulus and elongation at break of XN-Ca vulcanizates are
approximately the same as those of the raw, thermally cured XNBR. This is indirect
evidence that curing of the raw XNBR and CaO-containing XNBR are similar, and that
salt formation does not occur.
101
0 200 400 600 800 1000 1200 1400 1600 18000
2
4
6
8
10
x x
x
x
x
x
XNBRnot breakSt
ress
(MPa
)
Strain (%)
XN-Peroxide vulcanizates cured 60 min at 165 oC
XN-P0.50XN-P0.25
XN-P0.75
XN-P1.0
XN-P1.5
XN-P2.0XN-P3.0
x
Figure 4.30 Stress-strain curves of XN-peroxide vulcanizates (cured 60 min at 165 oC).
102
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
1
2
3
4
5
6
7
8
9
10
0
200
400
600
800
1000
1200
1400
1600
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Amount of dicumyl peroxide (phr)
XN-Peroxide compounds cured 60 min at 165 oC
Tensile strength
Elongation at break
300%Modulus
Elo
ngat
ion
at b
reak
(%)
Figure 4.31 Tensile properties of XN-peroxide vulcanizates (cured 60 min at 165 oC).
103
These XNBR-CaO systems do not exhibit self-reinforcement. It appears that their
tensile properties are largely determined by covalent crosslinks rather than by ionic
crosslinks. In fact, Starmer25 classified CaO as an inactive material for XNBR. It is not
yet understood why CaO does not provide ionic crosslinks in XNBR.
4.3.5 XNBR-Ca(OH)2 Vulcanizates
Stress-strain curves of XNBR cured with Ca(OH)2 are given in Figure 4.34, and
the dependence of tensile properties on the concentration of curatives is shown in Figure
4.35. Tensile strength and 300% modulus increase markedly with increased amounts of
Ca(OH)2 up to 1.5x stoichiometry, with little change thereafter. Elongation at rupture
decreases with increasing Ca(OH)2 contents up to 1.0x to 1.5x stoichiometry, and then
saturates at about 500%. The maximum tensile strength of about 50 MPa is much greater
than that of the raw XNBR (about 8 MPa), indicating that Ca(OH)2-cured XNBR exhibit
self-reinforcement like MgO-vulcanized XNBRs. Clearly, ionic crosslinks are formed in
these vulcanizates, and aggregate to form hard domains which act as multifunctional
crosslinks and reinforcing structures. This accounted for the high tensile strength and
modulus of XNBR cured with Ca(OH)2.53, 56, 71 The ability for crosslinks to interchange
under mechanical stress will also prevent local stress concentration. These mechanisms
are reasons for high tensile modulus and strength of XNBR-Ca(OH)2 vulcanizates.
104
0 100 200 300 400 500 600 700 8000
2
4
6
8
10
XNCa3.0
XNCa4.0
XNCa5.0
XNCa1.5
XNCa2.0
XNCa1.0
XN-Ca0.5
xxx
xx
x
x
Stre
ss (M
Pa)
Strain (%)
x
XNBR
XN-Ca vulcanizates cured 1000 min at 165 oC
Figure 4.32 Stress-strain curves of XN-Ca vulcanizates (cured 1000 min at 165 oC).
105
0 1 2 3 4 5 60
1
2
3
4
5
6
7
8
9
10
500
600
700
800
900
1000
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Amount of CaO/Stoichiometric Amount
XN-CaO compounds cured 1000 min at 165 oC
Tensile strength
Elongation at break
300% Modulus
Elo
ngat
ion
at b
reak
(%)
Figure 4.33 Tensile properties of XN-Ca vulcanizates (cured 1000 min at 165 oC).
106
0 200 400 600 800 10000
10
20
30
40
50
60
XN-Ch5.0
XN-Ch4.0
XN-Ch3.0
XN-Ch2.0XN-Ch1.5
XN-Ch1.0
xxx
x
x
x
xXN-Ch0.5
Stre
ss (M
Pa)
Strain (%)
XNBRx
XN-Ca(OH)2 compounds cured 240 min at 165 oC
Figure 4.34 Stress-strain curves of XN-Ch vulcanizates (cured 240 min at 165 oC).
107
0 1 2 3 4 5 6
1
10
100
400
500
600
700
800
900
1000
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Amount of Ca(OH)2/Stoichiometric Amount
XN-Ca(OH)2 compounds cured 240 min at 165 oC
Tensile strength
300% Modulus
Elongation at break
Elo
ngat
ion
at b
reak
(%)
Figure 4.35Tensile properties of XN-Ch vulcanizates (cured 240 min at 165 oC).
108
4.3.6 XNBR-BaO Vulcanizates
Figure 4.36 shows stress-strain curves of XNBR-BaO vulcanizates cured for 240
min at 165 oC. Similar to MgO- and Ca(OH)2-cured samples, the tensile moduli increase,
while the breaking strain falls with increased amount of BaO. Dependence of tensile
properties on BaO concentrations is given in Figure 4.37. The 300% Modulus
monotonically increases, while the tensile strength greatly increases with increased BaO
concentration up to twice stoichiometry. The breaking strain decreases with the BaO
content up to 1.0x - 1.5x stoichiometric amounts, and slightly decreases thereafter. The
maximum tensile strength of about 27 MPa at about 500 % breaking strain suggests that
BaO-cured XNBR is also self-reinforcement, like MgO- and Ca(OH)2-cured XNBR.
However, the degree of reinforcement in BaO-cured samples is less than that in MgO-
and Ca(OH)2-vulcanized XNBRs. BaO is not well-dissolved in the XNBR. When the
vulcanized sheets were prepared, BaO particles remained large, visible, and not well-
dispersed. These large particles will act as stress-raisers which magnify applied stresses
and reduce the tensile strength.43
4.3.7 Comparison of Tensile Properties among Metal Compounds
Figure 4.38 shows 300% moduli of XNBR vulcanized with different metal
compounds. The 300% moduli of XNBR-CaO vulcanizates are approximately the same
as that of the raw, thermally cured XNBR. In these systems no ionic crosslinks are
formed, and tensile properties are governed by covalent crosslinks. For the rest of metal
compounds, Ca(OH)2 gives the highest modulus. MgO gives higher modulus than BaO
when the amounts of curatives present are equal or less than 4.0x stoichiometry.
109
0 200 400 600 800 10000
5
10
15
20
25
30XN-BaO vulcanizates cured 240 min at 165 oC
XN-Ba5.0
XN-Ba4.0XN-Ba3.0
XN-Ba2.0
XN-Ba1.5
XNBR
XN-Ba1.0
xx
x
x
x
xx
Stre
ss (M
Pa)
Strain (%)
x
XN-Ba0.5
Figure 4.36 Stress-strain curves of XN-Ba vulcanizates (cured 240 min at 165 oC).
110
0 1 2 3 4 5 60
5
10
15
20
25
30
35
300
400
500
600
700
800
900
1000
1100
300%
Mod
ulus
or
Ten
sile
stre
ngth
(MPa
)
Amount of BaO/Stoichiometric Amount
XN-BaO vulcanizates cured 240 min at 165 oC
Tensile strength
300% Modulus
Elongation at break
Elo
ngat
ion
at b
reak
(%)
Figure 4.37 Tensile properties of XN-Ba vulcanizates (cured 240 min at 165 oC).
111
For XNBR-Ca(OH)2 vulcanizates, strong ionic salts are formed even at a stoichiometric
amounts of Ca(OH)2 (discussed later in the ATR-IR section). Therefore, strong ionic
domains are expected, and the number of effective network chains is high, resulting in
high modulus. A slight increase with excess amounts of Ca(OH)2 may be due to a
hydrodynamic effect. In the case of MgO-cured samples, strong ionic salts are not formed
until the amount of MgO is equal or greater than 2.0x stoichiometry. At low
concentrations (0.5x to 1.0x stoichiometry), the magnesium hydroxycarboxylate salt,
– COOMgOH, is the main product. This type of salt will not give efficient crosslinks,
therefore, the number of effective network chains is low, resulting in a low modulus
when compared to Ca(OH)2-cured vulcanizates. For XNBR cured with BaO, due to
incomplete solubility, neutralization is not complete until 2.0x stoichiometry of BaO is
present; therefore the amount of salts formed is low. The number of effective network
chains is expected to be lower than in MgO- and Ca(OH)2-vulcanized samples, with a
lower modulus.
Tensile strengths of XNBR vulcanized by various metal compounds are compared
in Figures 4.39. The effect of metal compounds on tensile strength is as followed: MgO >
Ca(OH)2 > BaO > CaO. The first three metal compounds result in ionic crosslinks, while
no salt is formed in XNBR cured with CaO. Substantial reinforcement is obtained with
MgO and Ca(OH)2, while BaO gives a moderate effect. As discussed before, high tensile
strength is attributed to aggregation of ionic crosslinks to form reinforcing hard domains
which also function as multifunctional crosslinks.52, 56, 71 Apparently, such reinforcing
domains are obtained when at least 2.0x stoichiometric amounts of metal compounds are
present. Furthermore, interchange between ionic crosslinks will prevent locally high
112
stress concentration, and allow all of the whole network chains to bear mechanical load.
However, ionic crosslinks should be strong enough to yield the characteristic high tensile
strength.72-74 From the swelling results, the strength of ionic crosslinks are in the
following order: MgO > Ca(OH)2 > BaO (Figure 4.22). Clearly, the tensile results are in
good agreement with these swelling measurements.
Figure 4.40 shows the ultimate strains of XNBR cured with different metal
compounds. Breaking strains of XNBR-CaO vulcanizates are approximately the same as
for the neat XNBR. As discussed earlier, curing of these systems is similar to that of
thermally cured XNBR. Covalent crosslinks are the main products. The effect of the
other three metal compounds on breaking strains, MgO, Ca(OH)2, and BaO, are very
similar. The ultimate strains decrease with the increased amounts of metal compounds up
to 1.0x to 1.5x stoichiometry, and change little thereafter. In fact, at a stoichiometric
amount of metal compounds, salts are already formed, but apparently, they do not yield
efficient crosslinks, evidenced by low tensile strength. However, association of these
ionic salts can change the topology of rubber networks, causing highly entangled
networks which result in rupture of the rubber at low strains.
It appears that the breaking strains reach a maximum level at lower concentrations
(1.0x to 1.5x stoichiometry) of metal compounds than for tensile strengths (1.5x to 2.0x
stoichiometry). This is indirect evidence that additional network chains are not formed
until 1.5x to 2.0x stoichiometric amounts of metal compounds are present.
113
0 1 2 3 4 5 60
5
10
15
20
25
300%
Mod
ulus
(MPa
)
Amount of metal compounds/Stoichiometric amount
XNBR-Ca(OH)2
XNBR-BaO
XNBR-MgO
XNBR-CaO
Cured at 165 oC
Figure 4.38 300% Modulus of the XNBR vulcanized by various metal compounds.
114
0 1 2 3 4 5 60
10
20
30
40
50
60
Amount of metal compounds/Stoichiometric amount
Ten
sile
stre
ngth
(MPa
)Cured at 165 oC
XNBR-MgO
XNBR-Ca(OH)2
XNBR-BaO
XNBR-CaO
Figure 4.39 Tensile strength of the XNBR vulcanized by various metal compounds.
115
0 1 2 3 4 5 6200
400
600
800
1000
1200
1400
1600
Amount of metal compounds/Stoichiometric amount
Elo
ngat
ion
at b
reak
(%)
Cured at 165 oC
XNBR-CaO
XNBR-MgO
XNBR-BaO
XNBR-Ca(OH)2
Figure 4.40 Elongation at break of the XNBR vulcanized by various metal compounds.
116
4.3.8 Comparison between Ionic and Covalent Crosslinks
In this section, the effect of crosslink types on the tensile properties of XNBR is
discussed. Figure 4.41 shows 300% modulus of XNBR vulcanized by various curing
agents, which yield different types of crosslinks. The 300% Moduli of ionically
crosslinked samples (cured by MgO, Ca(OH)2, and BaO) increase with Vr more than for
covalently crosslinked ones (cured by dicumyl peroxide, CaO, and thermal energy). This
is attributed to the aggregation of ionic crosslinks to form hard domains. However, the
effect varies among systems, probably due to a difference in the number of effective
network chains formed. In the case of covalently crosslinked samples, the 300% modulus
increases linearly with Vr, and appears to be independent of the crosslink structures, i.e.,
carbon-carbon crosslinks in XNBR-peroxide systems or anhydride crosslinks in XNBR-
CaO systems and in thermally cured XNBRs.
Tensile strengths of XNBR cured by different agents are given in Figure 4.42 as a
function of Vr. Apparently, the tensile strength of ionically crosslinked rubbers increase
with increased Vr until all the carboxyl groups are completely neutralized, and changes
slightly thereafter. For covalently crosslinked rubbers, the tensile strength decreases with
increased crosslink density. In comparison with covalent crosslinks, ionic crosslinks give
vulcanizates with much higher tensile strength. This is a result of association of ionic
crosslinks to form hard reinforcing domains.53, 56, 71 These ionic crosslinks can
interchange under mechanical stress. This relieving mechanism will prevent locally high
stress concentrations.72-74 In other words, the high tensile strength in ionically crosslinked
rubbers is primarily due to the ability to relax stress. However, ionic crosslinks should be
sufficiently strong to give the characteristic high tensile strength. As mention before,
117
much of the input energy is expended in dissipative processes, i.e., molecular motions.
For covalently crosslinked rubbers, the networks will be more elastic with increasing
crosslink densiy. Not much energy is dissipated. This leads to rupture at low strains.
Furthermore, covalent crosslinks cannot relax by breaking and reforming. When the
network chains break, molecular flaws will be created. If stresses around the molecular
flaws are magnified, catastrophic failure results.38 Therefore, the difference between
tensile strengths of ionically and covalently crosslinked rubbers is mainly due to the
ability to relax stress in the former case.
Figure 4.43 shows the breaking strains of XNBR cured by various curing agents.
Ultimate strains of covalently crosslinked rubbers decrease continuously with increased
crosslink density. For ionically crosslinked rubbers, breaking strains decrease at first, and
then seem to saturate. However, in the range of crosslink densities studied breaking
strains of ionically crosslinked rubbers are lower than those of covalently crosslinked
samples. This is due to a different topology of the networks. Aggregation of ionic
crosslinks will result in highly entangled networks, which will be broken at low strain.
4.4 ATR-IR Spectroscopy
4.4.1 Thermally Cured XNBR
ATR-IR spectra of uncured and thermally cured XNBR are shown in Figure 4.44.
A portion (1550 to 1850 cm-1) of these spectra is given in Figure 4.45. Characteristic of
the neat XNBR are peaks at 1697 cm-1 (carbonyl of acid dimer), 1730 cm-1 (carbonyl of
acid monomer), and 2235 cm-1 (triple bond of nitrile).110-113 The other designated peaks at
118
0.00 0.04 0.08 0.12 0.160
4
8
12
16
20
24 XN-MgA XN-MgB XN-MgC XN-P XN-Ch XN-Ca XN-Ba Heated XN
300%
Mod
ulus
(MPa
)
Vr
Ionic crosslinks
Covalent crosslinks
Figure 4.41 300% Modulus of the XNBR vulcanized by different curing agents as a function of Vr.
119
0.00 0.04 0.08 0.12 0.16 0.20
2
3
4
56789
10
20
30
40
50607080
Ten
sile
stre
ngth
(MPa
)
Vr
Ionic crosslinks XN-MgA XN-MgB XN-MgC XN-P XN-Ch XN-Ca XN-Ba Heated XN
Covalent crosslinks
Figure 4.42 Tensile strength of the XNBR vulcanized by different curing agents as a function of Vr.
120
0.00 0.04 0.08 0.12 0.16200
400
600
800
1000
1200
1400
1600 XN-MgA XN-MgB XN-MgC XN-P XN-Ch XN-Ca XN-Ba Heated XN
Elo
ngat
ion
at b
reak
(%)
Vr
Covalent crosslinks
Ionic crosslinks
Figure 4.43 Elongation at break of the XNBR vulcanized by different curing agents as a function of Vr.
121
920, 965, 1440, 1640, 1670, 2845, and 2920 cm-1 are contributed by hydrocarbon parts of
the elastomer backbone (Table 4.8). Evidently, most of carboxyl groups exist as the
hydrogen-bonded acid dimer. The corresponding O – H stretching frequency of the acid
dimer appears as a broad band at about 3200 cm-1 lying under the sharp stretching band
of C – H groups.105 Upon heating, the positions of the characteristic peaks do not change.
Qualitatively, the spectra of the cured samples are approximately the same as that of
unheated XNBR. However, in all cured samples the appearance of a small shoulder at
1750 to 1775 cm-1 is observed. Its origin is possibly due to the presence of anhydride
structure. Because carboxyl groups are randomly incorporated into the polymer backbone,
a butyric anhydride structure is more favorable than a cyclic structure (eq. 26). Grant and
Grassie106 studied thermal decomposition of poly(methacrylic acid) and reported that
characteristic butyric anhydride structures are shown by twin peaks at 1743 cm-1 and
1803 cm-1. Lee and coworkers105 studied the effect of temperature on anhydride
formation in poly(ethylene-co-methacrylic acid) and assigned characteristic frequencies
to butyric anhydride at 1735 cm-1 and 1780 cm-1. However, these bands appear as
shoulders in the spectra, not as the twin peaks observed by Grant and Grassie, probably
due to a difference in the content of methacrylic acid of studied polymers.
122
CH2
CH3C
CH2
CO
O
C
O
C CH3
CH2
CH2
Isobutyric anhydride
CH2
CH3C
CH2
C OH
O
C
O
C CH3
CH2
CH2
HO+ (26)
+H2O
Absorbance of the small shoulder in heated XNBR changes little with increased
heating time. This may be due an equilibrium which limits the amount of the anhydride
structure formed.106
Table 4.8 Characteristic group frequencies of the raw XNBR110-113
Wave number (cm-1) Assignment
920 Out-of-plane vibration of the methylene hydrogen atom of the vinyl group
965 Out-of-plane vibration of the hydrogen atom of the 1,4-trans component
1440 In-plane deformation of methylene group 1640 – 1670 Stretching of C = C
1697 Carbonyl stretching of hydrogen-bonded acid dimer 1730 Carbonyl stretching of monocarboxylic acid 2235 Stretching of nitrile triple bonds 2845 Symmetric stretching of methylene group 2920 Asymmetric stretching of methylene group
123
Figure 4.44 ATR-IR spectra of uncured and thermally cured XNBR in the range 800 to 4000 cm-1.
124
1850 1800 1750 1700 1650 1600 15500.0
0.1
0.2
0.3
0.4
0.5
0.6
1670
1640
1803
1730
Abs
orba
nce
Wave number (cm-1)
uncured cured 120 min cured 240 min cured 500 min cured 1000 min
Thermally cured raw XNBR at 165 oC
C
O
OH
C
O
O H
C
O
OH
1697
Anhydride crosslink
Figure 4.45 ATR-IR spectra of uncured and thermally cured XNBR in the range 1550 to 1850 cm-1.
125
4.4.2 XNBR-MgO Compositions
Figure 4.46 gives the ATR-IR spectra in the range 800 to 4000 cm-1 for uncured
neat XNBR and for uncured compositions containing high surface area MgO at 0.5x to
5.0x stoichiometry, assuming that one mole of MgO reacts with two moles of carboxyl
groups. Results in the range 1200 to 2000 cm-1 are shown in Figure 4.47. The peaks at
1697 cm-1 and 1730 cm-1 are assigned to carbonyl stretching in hydrogen-bonded and free
carboxylic acids, respectively. With the addition of MgO, intensities of these peaks
decrease, accompanied by a new peak at 1612 cm-1, together with the appearance of a
broad band centered at 3420 cm-1, which is assigned to vibration of OH groups (Table
4.9). Therefore, a peak at 1612 cm-1 is assigned to the magnesium hydroxycarboxylate,
– COOMgOH. A similar structure, zinc hydroxycarboxylate (– COOZnOH), which is a
product of reaction between carboxyl terminated polyester and ZnO, was assigned in the
same way.114 Carboxyl groups are essentially neutralized at stoichiometry of MgO, and
disappear when an equimolar amount of MgO was added. These results suggest that
neutralization occurs during mixing and continues during storage (samples were about 2
weeks old before collecting spectra).
The ATR-IR spectra of cured samples in the range 800 to 4000 cm-1 are given in
Figure 4.48. A portion (1200 to 2000 cm-1) of these spectra is shown in Figure 4.49. The
IR spectrum of cured XN-MgA0.5 is approximately the same as that of the uncured
sample. At stoichiometry (XN-MgA1.0), most of carboxyl groups are essentially
neutralized, and the salt peak becomes broader. With increased amounts (2.0x and 5.0x
stoichiometry) of MgO, a peak at 1587 cm-1 appears. This peak is attributed to
asymmetric carbonyl stretching of magnesium carboxylate salt (Table 4.9).66, 111 In the
126
XN-MgA0.5 vulcanizate, the magnesium hydroxycarboxylate (the peak at 1612 cm-1) is
the main product from neutralization as in the uncured rubber. This salt will not yield
efficient crosslinks and effective network chains. Although the cured XN-MgA0.5 has
higher tensile strength than the cured neat XNBR, it has lower Vr in the swollen gel and
higher sol content. A mix of ionic salts is obtained for the XN-MgA1.0 sample.
Apparently, a large amount of the magnesium carboxylate salt is obtained when at least
2.0x stoichiometry of MgO is present. This salt will give efficient crosslinks and effective
network chains. ATR-IR results are in good agreement with ODR, swelling and tensile
results, which show that equimolar amounts of MgO and carboxyl groups are needed to
give optimum properties. Therefore, the assumption that one mole of MgO reacts with
two carboxyl groups is incorrect.
Table 4.9 Characteristic group frequencies of XNBR-MgO compositions110-113
Wave number (cm-1) Assignment
920 Out-of-plane vibration of the methylene hydrogen atom of the vinyl group
965 Out-of-plane vibration of the hydrogen atom of the 1,4-trans component
1440 In-plane deformation of methylene group 1612 Carbonyl stretching of magnesium hydroxycarboxylate salt
1587 Asymmetric carbonyl stretching of magnesium carboxylate salt
1640 – 1670 Stretching of C = C 1697 Carbonyl stretching of hydrogen-bonded acid dimer 1730 Carbonyl stretching of monocarboxylic acid 2235 Stretching of nitrile triple bonds 2845 Symmetric stretching of methylene group 2920 Asymmetric stretching of methylene group 3420 Stretching of O – H
127
The proposed neutralizations (equations 22 to 24) are reasonably well-explained
by the behavior of the XNBR-MgO systems. Assume that all carboxyl groups have the
same reactivity. When MgO is added to the rubber, all carboxyl groups have equal
opportunity to react with MgO. But not all carboxylic acid groups are expected to react at
the same time, because of several reasons, for example: i) Because reactions occur in
solid state, and MgO particles are not dissolved in the rubber matrix, reactions are
expected to occur first at the surface of the MgO particles. The issues of surface area (or
particle size) and concentrations will then become important. That is cure reaction will be
controlled by the diffusion of the curing sites, ii) The diffusion of carboxyl groups can be
limited by the topology of the rubber matrix, i.e., by highly entangled rubber chains.
However, many carboxylic acid groups will react with MgO at the same time. The second
carboxyl group will not wait until the first one has reacted to form the magnesium
hydroxycarboxylate salt. Therefore, most of carboxylic acid groups will form the
magnesium hydroxycarboxylate salt. This type of salt does not give efficient crosslinks
and effective network chains, which is evidenced by a high degree of swelling, and a high
sol content. It is the coupling of magnesium hydroxycarboxylate salts to form magnesium
carboxylate that yields efficient crosslinks and effective network chains.
128
Figure 4.46 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 800 to 4000 cm-1.
129
2000 1800 1600 1400 12000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Abs
orba
nce
Wave number (cm-1)
Uncured XN-MgA compounds C
O
O H
C
O
OH
1697
C
O
OH
1730C
O
O MgOH
1612
1440
XNBR
XN-MgA0.5
XN-MgA1.0
XN-MgA2.0
XN-MgA5.0
Figure 4.47 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 1200 to 2000 cm-1.
130
Figure 4.48 ATR-IR spectra of the neat XNBR and XNBR-MgO vulcanizates in the range 800 to 4000 cm-1 (cured 120 min at 165 oC).
131
2000 1800 1600 1400 12000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C
O
O
C
O
O+ Mg +
C
O
O MgOH
C
O
OH
C
O
O H
C
O
OH
XN-MgO vulcanizates cured 120 min at 165 o CA
bsor
banc
e
Wave number (cm-1)
XNBR
XN-MgA0.5
XN-MgA1.0
XN-MgA2.0
XN-MgA5.0
1730
1697
1612
1587
1440
Figure 4.49 ATR-IR spectra of the neat XNBR and XNBR-MgO vulcanizates in the range 1200 to 2000 cm-1 (cured 120 min at 165 oC).
132
4.4.3 XNBR-CaO Compositions
ATR-IR spectra of uncured raw XNBR and XNBR-CaO compounds are given in
Figures 4.50. A portion (1550 to 1850 cm-1) of these spectra is shown in Figure 4.51. The
spectra of XN-Ca compounds are very similar to that of the neat XNBR. Unlike the
uncured XN-Mg compounds, no salt formation is observed in uncured XN-Ca samples.
They are the same as that of the neat XNBR (Table 4.8).
Figure 4.52 gives ATR-FTIR spectra of the neat XNBR and XNBR-CaO
vulcanizates. Spectra of XN-Ca vulcanizates are similar to that of the cured neat XNBR,
indicating similar cure mechanisms. Figure 4.53 shows spectra in the region 1550 to 1850
cm-1. A small shoulder in the range 1750 to 1775 cm-1 appears in all cured samples. The
difference between uncured and cured samples in the region 1550 to 1850 cm-1 is given
in Figure 4.54. The small shoulder is absent in all uncured compounds and appears in all
the cured samples. It may be attributed to anhydride structure. However, the absorption
frequencies of anhydride structures reported in the literature vary dependent on the
polymer.105, 106 Weak absorption in XN-Ca compositions may be due to an equilibrium in
condensate anhydride formation.106 As discussed earlier, swelling and tensile behavior of
XN-Ca vulcanizates are similar to those of the cured neat XNBR. This is strongly
supported by the ATR-IR results. Why these particular XNBR-CaO compositions do not
form ionic crosslinks is not clearly understood at this point.
133
Figure 4.50 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO compounds in the range 800 to 4000 cm-1.
134
1850 1800 1750 1700 1650 1600 15500.0
0.1
0.2
0.3
0.4
0.5
0.6A
bsor
banc
e
Wave number (cm-1)
Uncured XN-CaO compounds XNBR XN-Ca0.5 XN-Ca1.0 XN-Ca2.0 XN-Ca5.0
C
O
O H
C
O
OH
1697
C
O
OH
1730
Figure 4.51 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO compounds in the range 1550 to 1850 cm-1.
135
Figure 4.52 ATR-IR spectra of the neat XNBR and XNBR-CaO vulcanizates in the range 800 to 4000 cm-1 (cured 1000 min at 165 oC).
136
1850 1800 1750 1700 1650 1600 15500.0
0.1
0.2
0.3
0.4
0.5A
bsor
banc
e
Wave number (cm-1)
XN-CaO vulcanizates cured 1000 min at 165 oC
XNBR XN-Ca0.5 XN-Ca1.0 XN-Ca2.0 XN-Ca5.0
C
O
O H
C
O
OH
1697
C
O
OH
1732
Anhydridecrosslink
Figure 4.53 ATR-IR spectra of the neat XNBR and XNBR-CaO vulcanizates in the range 1550 to 1850 cm-1 (cured 1000 min at 165 oC).
137
1850 1800 1750 1700 1650 1600 15500.0
0.1
0.2
0.3
0.4
0.5A
bsor
banc
e
Wave number (cm-1)
C
O
O H
C
O
OH
1697
C
O
OH
1730
Uncured XNBR Cured XNBR Uncured XN-Ca2.0 Cured XN-Ca2.0
Anhydride crosslink
Cured 1000 min at 165 oC
Figure 4.54 ATR-IR spectra of the uncured neat XNBR and XN-Ca2.0 compounds, and the neat XNBR and XN-Ca2.0 vulcanizates in the range 1550 to 1850 cm-1 (cured 1000 min at 165 oC).
138
4.4.4 XNBR-Ca(OH)2 Compositions
ATR-FTIR spectra of the uncured neat XNBR and XNBR-Ca(OH)2 compounds
are given in Figure 4.55. Absorption of these samples in the range 1200 to 2000 cm-1 is
shown in Figure 4.56. The spectra are characterized by the peaks at 1697 cm-1 (acid
dimers), 1730 cm-1 (a free acid), and 2235 cm-1 (nitrile). The difference between spectra
of XNBR-Ca(OH)2 compounds from that of the neat XNBR is the presence of a peak at
3642 cm-1, which becomes prominent with increased amounts of Ca(OH)2 (Table 4.10).
This peak is contributed to vibration of Ca(OH)2. No peaks are observed in the frequency
region (1500 to 1600 cm-1) of asymmetric stretching of carboxylate anion, suggesting that
neutralization does not occur before curing.
Upon curing, the spectrum of the neat XNBR remains largely the same as that of
the uncured sample, except for the appearance of a small shoulder in the range 1750 to
1775 cm-1, which may be attributed to the anhydride structure type. However, large
amounts of carboxylic acid groups remain unreacted, evidenced by strong intensities of
the peaks at 1697 cm-1 (acid dimer), and 1730 (acid monomer), respectively. For the
compounds containing Ca(OH)2, the intensities of the acid peaks decrease, accompanied
by a new peak at 1560 cm-1 (Figures 4.57 and 4.58). This peak is assigned to asymmetric
carbonyl stretching of calcium carboxylate.65, 66, 111, 115 In a XN-Ch0.5 vulcanizate, there
is a certain amount of unreacted carboxyl groups left in the sample, and the salt peak is
broad. Carboxyl groups are essentially neutralized at a stoichiometric amount of Ca(OH)2.
Complete neutralization is obtained when at least 2.0x stoichiometry of Ca(OH)2 is
present, and the salt peak becomes narrower.
139
Table 4.10 Characteristic group frequencies of XNBR-Ca(OH)2 samples65, 66, 110, 111, 113
Wave number (cm-1) Assignment
920 Out-of-plane vibration of the methylene hydrogen atom of the vinyl group
965 Out-of-plane vibration of the hydrogen atom of the 1,4-trans component
1410 Symmetric carbonyl stretching of calcium carboxylate 1440 In-plane deformation of methylene group 1560 Asymmetric carbonyl stretching of calcium carboxylate salt
1640 – 1670 Stretching of C = C 1697 Carbonyl stretching of hydrogen-bonded acid dimer 1730 Carbonyl stretching of monocarboxylic acid 2235 Stretching of nitrile triple bonds 2845 Symmetric stretching of methylene group 2920 Asymmetric stretching of methylene group 3400 Stretching of O – H 3642 Ca(OH)2
It seems that the structure of the ionic salts formed in XN-Ch1.0, XN-Ch2.0, and
XN-Ch5.0 is very similar, but the amount of salt formed is different. In fact, these three
samples show a substantial increase in tensile strength compared to the neat XNBR, 42
MPa for XN-Ch1.0, 51 MPa for XN-Ch2.0, and 48 MPa for XN-Ch5.0. In compositions
containing up to 2.0x stoichiometry, Ca(OH)2 is completely used in neutralization of
carboxyl groups, evidenced by disappearance of the Ca(OH)2 peak at 3642 cm-1. In the
XN-Ch5.0 vulcanizate, however, unreacted Ca(OH)2 is observed. Apparently, the excess
amount of Ca(OH)2 has a little effect on the tensile properties of the vulcanizates.
140
Figure 4.55 ATR-IR spectra of the uncured neat XNBR and XNBR-Ca(OH)2 compounds in the range 800 to 4000 cm-1.
141
2000 1800 1600 1400 1200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Abs
orba
nce
Wave number (cm-1)
Uncured XN-Ca(OH)2 compounds
C
O
O H
C
O
OH
C
O
OH
1730
1697 1440
XNBR
XN-Ch0.5
XN-Ch1.0
XN-Ch2.0
XN-Ch5.0
Figure 4.56 ATR-IR spectra of the uncured neat XNBR and XNBR-Ca(OH)2 compounds in the range 1200 to 2000 cm-1.
142
Figure 4.57 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 800 to 4000 cm-1.
143
2000 1800 1600 1400 1200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1410
XN-Ca(OH)2 vulcanizates cured 240 min at 165 oC
1440
Abs
orba
nce
Wave number (cm-1)
C
O
O H
C
O
OH
1697C
O
OH
17301560
C
O
O
C
O
O+ Ca +
XNBR
XN-Ch0.5
XN-Ch1.0
XN-Ch2.0
XN-Ch5.0
Figure 4.58 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 1200 to 2000 cm-1.
144
4.4.5 XNBR-BaO Compositions
ATR-FTIR spectra of the uncured raw XNBR and XNBR-BaO compounds are
shown in Figures 4.59 (in the range 800 to 4000 cm-1) and 4.60 (in the range 1200 to
2000 cm-1), respectively. Characteristic group frequencies of these spectra are listed in
Table 4.11. Apparently, ATR-IR spectra of uncured XN-Ba compounds are
approximately the same as that of the uncured neat XNBR, characterized by the peaks at
1697 cm-1 (acid dimer), 1730 cm-1 (acid monomer), and 2235 cm-1 (nitrile). No
absorption in the region 1500 to 1600 cm-1 is observed, indicating that no salt structure
has developed before curing.
Figure 4.61 shows the ATR-IR spectra in the region 800 to 4000 cm-1 of the neat
XNBR and XNBR-BaO vulcanizates. A portion (1200 to 2000 cm-1) of these spectra is
given in Figure 4.62. Frequencies of characteristic functional groups are assigned in
Table 4.11, and some of them are marked in the Figures. As discussed earlier, the neat
XNBR undergoes self-crosslinking upon curing. However, large amounts of carboxyl
groups are expected to remain, which is evidenced by strong absorption of the dimeric
acid (1697 cm-1), and free acid (1730 cm-1), respectively.
Upon heating XN-Ba compounds, carboxyl groups are neutralized to form ionic
salts. This is evidenced by the decrease in intensities of the acid peaks (1697 and 1730
cm-1) accompanied by appearance of the peak at 1546 cm-1, assigned to asymmetric
carbonyl stretching of barium carboxylate salts. Degree of neutralization increases with
the increased amounts of BaO. Although carboxyl groups are mainy neutralized at 2.0x
stoichiometry of BaO, there are still unreacted carboxyl groups. This is due to BaO that is
not dissolved and well-dispersed in the XNBR matrix. The poor dispersion and large
145
particle size also account for the inferior tensile properties of XN-Ba vulcanizates
compared to XNBR-MgO and XNBR-Ca(OH)2 vulcanizates. However, complete
neutralization is observed in the XN-Ba5.0 vulcanizate.
Table 4.11 Characteristic group frequencies of XNBR-BaO samples
Wave number (cm-1) Assignment
920 Out-of-plane vibration of the methylene hydrogen atom of the vinyl group
965 Out-of-plane vibration of the hydrogen atom of the 1,4-trans component
1405 Symmetric carbonyl stretching of barium carboxylate 1440 In-plane deformation of methylene group 1546 Asymmetric carbonyl stretching of calcium carboxylate salt
1640 – 1670 Stretching of C = C 1697 Carbonyl stretching of hydrogen-bonded acid dimer 1730 Carbonyl stretching of monocarboxylic acid 2235 Stretching of nitrile triple bonds 2845 Symmetric stretching of methylene group 2920 Asymmetric stretching of methylene group 3400 Stretching of O – H
4.4.6 Comparison among Metal Compounds
Figure 4.63 shows ATR-IR spectra in the range 1475 to 1850 cm-1 for the cured
neat XNBR and XNBR vulcanized by 2.0x stoichiometry of metal compounds. The
spectra of the neat XNBR and XN-Ca2.0 are essentially the same. No peaks are observed
in the frequency region (1500 to 1600 cm-1) for asymmetric carbonyl stretching of
carboxylate groups. Curing mechanisms of these two samples involve coupling
146
Figure 4.59 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 800 to 4000 cm-1.
147
2000 1800 1600 1400 1200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Abs
orba
nce
Wave number (cm-1)
Uncured XN-Ba compounds
C
O
O H
C
O
OH
1697C
O
OH
1730
1440
XNBR
XN-Ba0.5
XN-Ba1.0
XN-Ba2.0
XN-Ba5.0
Figure 4.60 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 1200 to 2000 cm-1.
148
Figure 4.61 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 800 to 4000 cm-1.
149
2000 1800 1600 1400 1200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8XN-BaO vulcanizates cured 240 min at 165 oC
Abs
orba
nce
Wave number (cm-1)
C
O
O H
C
O
OH
C
O
O
C
O
O+ Ba +
C
O
OH 1697
17301546
1440
1405
XNBR
XN-Ba0.5
XN-Ba1.0
XN-Ba2.0
XN-Ba5.0
Figure 4.62 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 1200 to 2000 cm-1.
150
of carboxyl groups to form anhydride crosslinks. In contrast to the neat XNBR and XN-
Ca2.0 vulcanizates, ionic crosslinks are formed in XN-MgA2.0, XN-Ch2.0, and XN-
Ba2.0 vulcanizates, as evidenced by the disappearance of the acid peaks (1697 and 1730
cm-1) accompanied by new peaks corresponding to asymmetric carbonyl stretching of
carboxylate groups in the range 1500 to 1600 cm-1. The difference among ATR-IR
spectra of XN-MgA2.0, XN-Ch2.0, and XN-Ba2.0 vulcanizates is in the frequency region
of asymmetric carbonyl stretching. Apparently, these frequencies decrease in a
predictable manner with increasing cation mass and size. Similar results were obtained by
Brozoski and coworkers,66 and Han and Williams,115 who utilized infrared spectroscopy
to study local structures of copolymers of ethylene-methacrylic acid neutralized by alkali,
alkaline earth, and transition metals.
In the case of vibration of a simple harmonic oscillator, the relationship between
the wave number (ν′) of the absorption peak and the vibration frequency of bonds in the
molecule is given by
21
21
mm)mm(f
c21f
c21 +
π=
μπ=ν′ (26)
where f is the force constant of the bond (dyne/cm or g/s2), c is the velocity of light
(2.998 x 1010 cm/s), and m1, m2 are the masses (g) of atoms 1 and 2, respectively.110, 115
Assume that this principle can be applied in our case, and that metal ion types influence
the asymmetric stretching of carboxylate anions. Because Mg++ ion has the lowest mass
and size, and according to swelling results, Mg++ ion forms the strongest ionic bond with
the carboxylate anion when compared with Ca++ and Ba++ ions, therefore, the absorption
151
1850 1800 1750 1700 1650 1600 1550 1500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
C
O
OH
C
O
O H
C
O
OH
C
O
O
C
O
O+ Ba +
C
O
O
C
O
O+ Ca +
C
O
O
C
O
O+ Mg +
Abs
orba
nce
Wave number (cm-1)
XNBR XN-MgA2.0 XN-Ca2.0 XN-Ch2.0 XN-Ba2.0
1697
1730
1587
1560
1546
Anhydridecrosslink
Figure 4.63 ATR-IR spectra of the neat XNBR, XN-MgA2.0, XN-Ca2.0, XN-Ch2.0, and XN-Ba2.0 vulcanizates in the range 1475 to 1850 cm-1.
152
by asymmetric stretching by the magnesium carboxylate is expected to be at a higher
wave number than for Ca++ and Ba++ ions. Although this assumption is oversimplified, it
explains the shift in absorption frequency of asymmetric carboxylate stretching with the
increasing mass and size of alkaline earth metal ions.
4.5 Dynamic Mechanical Properties
4.5.1 XNBR-MgO Vulcanizates
Substantial reinforcement of XNBR vulcanized by MgO is attributed to
aggregation of ionic crosslinks to form hard ionic domains which act as multifunctional
crosslinks and reinforcing structures.53, 56, 71 Dynamic mechanical analysis has commonly
been employed to study these ionic aggregates.59, 62, 64, 89-91, 93, 94, 116, 117 The storage
moduli (E′) of XN-MgA and XN-peroxide vulcanizates as a function of temperature at a
frequency 1.0 Hz are shown in Figure 4.64. Glassy moduli of all the vulcanizates are
similar. With increasing temperature passing through the transition zone, the moduli of
all samples drop sharply by about three orders of magnitude. This is the well-known
glass-rubber transition which arises from segmental relaxation of polymer chains. In the
rubbery zone, the storage moduli of XN-P1.0 and XN-MgA are quite different. The
modulus of XN-P1.0, which is covalently crosslinked, is lower than those of ionically
crosslinked XN-MgA vulcanizates, but it does not change with increasing temperature,
suggesting stability of crosslinks towards heat. XN-MgA vulcanizates have greater
moduli due to aggregation of ionic species to form hard domains. The rubbery modulus
increases with increased amount of MgO from 1.0x (XN-MgA1.0) to 2.0x (XN-MgA2.0)
stoichiometry, and slightly thereafter (XN-MgA3.0 sample). This is due to an increase in
153
the number of effective ionic crosslinks, from the magnesium carboxylate salt. Because
of the physical nature of ionic crosslinks, the rubbery modulus decreases with increasing
temperature. This is not surprising because in the XN-MgA1.0 sample most of the ionic
salt is magnesium hydroxycarboxylate (eq. 22), which is formed intramolecularly.
Because of the polar nature of this salt, it yields ineffective ionic crosslinks. The ionic
crosslinks are expected to be thermally labile, as shown by the decrease in rubbery
modulus with increasing temperature. In the vulcanizates containing MgO 2.0x
stoichiometry or more, the magnesium carboxylate salt (eq. 23) is the main product
which links two polymer chains together. This salt gives effective crosslinks, which are
expected to be less thermally labile. Therefore, the rubbery modulus is more stable
towards heat.
The dynamic loss moduli (E″) of XN-P and XN-MgA vulcanizates as a function
of temperature are given in Figure 4.65. In the glassy zone, the loss moduli of all
vulcanizates are similar, suggesting that input energy is dissipated in similar processes.
However, the behavior of XN-P1.0 and XN-MgA vulcanizates in the rubbery region is
different. The covalently crosslinked XN-P1.0 has the lowest dynamic loss modulus,
suggest that energy dissipation is much smaller than for XN-MgA vulcanizates.
Apparently, there are other loss processes at high temperatures (20 to 100 oC).
Surprisingly, the loss modulus becomes larger with increased amounts of MgO, although
the elastic modulus rises (Figure 4.64). The dissipative processes have been suggested to
involve ion hopping processes or migration of ion pairs attached to a particular polymer
chain segment from one ionic aggregate to another (Figure 4.66).118-120 This concept is
154
-100 -50 0 50 100 150
106
107
108
109
XN-MgA vulcanizates cured 120 min at 165 oC
XN-P1.0
XN-MgA2.0 XN-MgA3.0
E' (
Pa)
Temperature (oC)
XN-MgA1.0
Figure 4.64 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz.
155
-100 -50 0 50 100 150
105
106
107
108
XN-MgA vulcanizates cured 120 min at 165 oC
XN-P1.0
XN-MgA2.0XN-MgA3.0
E"
(Pa)
Temperature (oC)
XN-MgA1.0
Figure 4.65 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz.
156
Figure 4.66 Schematic drawing of ion hopping mechanisms (opened and closed circles represent ion pairs).121
157
closely related to bond interchange which has been used to explain the strength of these
elastomers.51
The temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-MgA
vulcanizates is shown in Figure 4.67. Molecular transitions corresponding to peaks
appearing in the plot are listed in Table 4.12. In the XN-P1.0 sample only the glass
transition is observed. For the XN-MgA vulcanizates, there is another broad transition at
a high temperature other than the glass transition of the rubber matrix. This transition is
not observed in the XN-P1.0 sample, where the rubber molecules are covalently
crosslinked. Therefore, this transition must involve the ionic aggregates formed in the
XN-MgA vulcanizates. Its origin has been suggested to arise from relaxation processes
by exchange of ion pairs between ionic aggregates (Figure 4.66).91, 119 However, the
precise position of the ionic transition is difficult to determine due to lack of a clear
maximum. For XNBR vulcanized by zinc oxide, a similar peak in the range 50 to 80 oC
has been reported, depending on the testing frequency.62, 64, 89, 90, 116 With increasing MgO
content, the transition shifts to higher temperatures. Similar observations has been
reported for the ZnO-cured XNBR90 and ZnO-activated sulfur-cured XSBR.59
The effect of the specific surface area of MgO on the storage modulus of XNBR
vulcanizates is shown in Figure 4.68. The glassy moduli of all vulcanizates are similar.
However the high surface area (type A) MgO gives vulcanizates with a slightly higher
rubber modulus than does the low surface area MgO (type C). This is due to the
difference in the amount of effective ionic crosslinks formed (Table 4.2). As in the case
of XN-MgA, the rubbery moduli of XN-MgC increase with increased concentration of C-
type MgO, and the rubbery modulus becomes more thermally stable.
158
-100 -50 0 50 100 150
0.02
0.03
0.040.050.060.070.080.090.1
0.2
0.3
0.40.50.60.70.80.9
1
XN-MgA vulcanizates cured 120 min at 165 oC
XN-MgA3.0
XN-MgA2.0
tan
δ
Temperature (oC)
XN-P1.0
XN-MgA1.0
Figure 4.67 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz.
159
Table 4.12 Molecular transition temperatures of XN-P1.0, and XN-MgA vulcanizates at a frequency of 1.0 Hz
Vulcanizates Tg (oC)* Ionic transition temperature range (oC)
XN-P1.0 -24 Not observed
XN-MgA1.0 -22 10 - 100
XN-MgA2.0 -20 20 - 110
XN-MgA3.0 -18 25 - 115 * Taken from the temperature at a tan δ maximum
Figure 4.69 shows the dependence of the dynamic loss modulus of XN-Mg
vulcanizates on the specific surface area of MgO. The loss moduli in the glassy zone of
all vulcanizates are similar. As in the case of XN-MgA vulcanizates, a small hump in the
rubbery region appears in XN-MgC vulcanizates, and becomes more pronounced with
increasing amounts of MgO. As discussed earlier, the origin of the small hump, i.e. more
energy lost, is believed to be due to the migration of ion pairs attached to a particular
polymer chain from one ionic aggregate to another.91, 118-120 At the same concentration,
high surface area MgO (type A) results in a slightly higher loss modulus than for the low
surface area MgO (type C). This is probably due to a difference in the amount of ionic
crosslinks formed.
The effect of specific surface area of MgO on the loss tangent (tan δ) of XN-Mg
vulcanizates is illustrated in Figure 4.70. Two transitions are observed, as for XN-MgA.
The transition at low temperature is the glass transition of the rubber matrix. Another
transition at high temperature is associated with ionic species, because it is absent in
160
-100 -50 0 50 100 150
106
107
108
109
XN-MgC2.0
XN-MgA1.0
XN-MgC1.0
XN-MgA2.0
Frequency 1.0 Hz XN-MgA1.0 XN-MgC1.0 XN-MgA2.0 XN-MgC2.0 XN-P1.0
A = 140 m2/gC = 45 m2/g
E' (
Pa)
Temperature (oC)
XN-P1.0
Figure 4.68 Effect of specific surface area on dynamic storage modulus (E′) of XN-Mg vulcanizates at frequency 1.0 Hz.
161
-100 -50 0 50 100 150
105
106
107
108
XN-MgA1.0 XN-MgC1.0 XN-MgA2.0 XN-MgC2.0 XN-P1.0
A = 140 m2/gC = 45 m2/g
E"
(Pa)
Temperature (oC)
Frequency 1.0 Hz
XN-MgA2.0
XN-MgC2.0
XN-MgA1.0
XN-MgC1.0
XN-P1.0
Figure 4.69 Effect of specific surface area on dynamic loss modulus (E″) of XN-Mg vulcanizates at frequency 1.0 Hz.
162
-100 -50 0 50 100 150
0.1
1
XN-MgA1.0 XN-MgC1.0 XN-MgA2.0 XN-MgC2.0 XN-P1.0
A = 140 m2/gC = 45 m2/g
tan
δ
Temperature (oC)
Frequency 1.0 Hz
XN-MgA2.0
XN-MgC2.0
XN-MgA1.0XN-MgC1.0
XN-P1.0
Figure 4.70 Effect of specific surface area on loss tangent (tan δ) of XN-Mg vulcanizates at frequency 1.0 Hz.
163
covalently cured XNBR. The ionic transition is dependent on both the concentration and
the surface area of MgO. It shifts to higher temperatures with increase in both
concentration and surface area.
4.5.2 XNBR-CaO Vulcanizates
The temperature dependence of the dynamic storage modulus (E′) of XN-P1.0
and XNBR-CaO vulcanizates is given in Figure 4.71. All vulcanizates have similar glassy
and rubbery moduli. XN-P1.0 has a slightly higher rubbery modulus relative to CaO-
cured specimens. This is because curing in XNBR-CaO systems occurs via coupling
reaction of carboxyl groups to form anhydride crosslinks, which are limited by the
amount of carboxyl groups. However, all vulcanizates behave similarly. The rubbery
zone in XN-Ca vulcanizates is almost a plateau, indicating the stability of crosslinks
towards heat. This is indirect evidence that most of crosslinks in XN-Ca vulcanizates are
covalent.
The effect of temperature on the dynamic loss modulus (E″) of XN-P1.0 and
XNBR-CaO samples is also very similar (Figure 4.72). XN-Ca vulcanizates have slightly
higher loss modulus than XN-P1.0 in the rubbery region. No peak from ionic aggregates
is observed.
Figure 4.73 shows a plot of tan δ of XN-P1.0 and XN-Ca vulcanizates against
temperature. Only one peak at a temperature of about -22 to -24 oC is observed, which is
the glass transition temperature of the XNBR matrix (Table 4.13). In contrast to XN-Mg
vulcanizates, no ionic transition is observed. No ionic aggregates are formed in these
systems.
164
-100 -50 0 50 100 150 200
106
107
108
109
XN-Ca3.0
XN-Ca2.0
E' (
Pa)
Temperature (oC)
XN-P1.0
XN-Ca1.0
XN-CaO vulcanizates cured 1000 min at 165 oC
Figure 4.71 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz.
165
-100 -50 0 50 100 150 200
105
106
107
108
E"
(Pa)
Temperature (oC)
XN-CaO vulcanizates cured 1000 min at 165 oC
XN-P1.0
XN-Ca3.0XN-Ca2.0
XN-Ca1.0
Figure 4.72 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz.
166
-100 -50 0 50 100 150 200
0.1
1 XN-P1.0 XN-Ca1.0 XN-Ca2.0 XN-Ca3.0
tan
δ
Temperature (oC)
XN-CaO vulcanizates cured 1000 min at 165 oC
Figure 4.73 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz
167
Table 4.13 Molecular transition temperatures of XN-P1.0, and XN-CaO vulcanizates at frequency 1.0 Hz
Vulcanizates Tg (oC) Ionic transition temperature range (oC)
XN-P1.0 -24 Not observed
XN-Ca1.0 -24 Not observed
XN-Ca2.0 -22 Not observed
XN-Ca3.0 -22 Not observed
4.5.3 XNBR-Ca(OH)2 Vulcanizates
The dynamic storage modulus, loss modulus, and tan δ as a function of
temperature of XN-P1.0 and XNBR-Ca(OH)2 are given in Figures 4.74, 4.75, and 4.76,
respectively. No significant difference in the storage moduli in the glassy region is
observed. The glass transition slightly shifts to higher temperatures with increased
amount of Ca(OH)2. However, the behavior of covalently crosslinked (XN-P1.0) and
ionically crosslinked (XN-Ch) rubbers are quite different. The rubbery modulus of XN-
P1.0 is the lowest, but the most thermally stable, suggesting that the crosslinks are
permanent. At the beginning of the rubbery region, XN-Ch1.0 has a higher modulus than
XN-P1.0, but the modulus decreases with increased temperature, suggesting that the
crosslinks are unstable towards heat. At temperatures above 150 oC, the rubbery modulus
of XN-Ch1.0 becomes less than that of XN-P1.0. Although XN-Ch1.0 has high tensile
strength (~ 40 MPa) due to the ionic salt, calcium carboxylate, that is formed, ATR-IR
results show that not all carboxyl groups are neutralized. The decrease in the rubbery
168
modulus with temperature suggests that large ionic aggregates may not form at this
concentration of Ca(OH)2. With increasing Ca(OH)2 concentration, the rubbery modulus
increases greatly and is almost an order of magnitude higher than that of XN-P1.0. The
rubbery zone becomes more of a plateau due to the contribution from ionic aggregates.
At above 150 oC the storage modulus begins to drop, indicating thermal instability of the
crosslinks.
The loss moduli in the glassy region of all cured specimens are similar. However,
in the rubbery zone, the loss moduli of XN-Ch vulcanizates are greater than for XN-P1.0,
although XN-Ch vulcanizates have higher storage moduli. This is contributed to ionic
aggregates.
The plot (Figure 4.76) of tan δ against temperature of XN-Ch samples reveals two
transitions, the glass transition of the rubber matrix at low temperatures and the ionic
transition at high temperatures (Table 4.14). However, at high concentrations of Ca(OH)2
it is difficult to determine the precise position of the ionic transition because there is no
clear maximum. The mechanism of the ionic transition has been suggested to arise from
the interchange of ion pairs between ionic aggregates (Figure 4.66).91, 119-121 As in the
case of XN-Mg samples, the ionic transition of XN-Ch vulcanizates shifts to higher
temperatures. In contrast to XN-Ch vulcanizates, XN-P1.0 sample has only one transition,
corresponding to the glass transition of the rubber matrix.
169
-100 -50 0 50 100 150 200
106
107
108
109
XN-Ca(OH)2 vulcanizates cured 240 min at 165 oC
XN-Ch3.0XN-Ch2.0
XN-P1.0
E' (
Pa)
Temperature (oC)
XN-Ch1.0
Figure 4.74 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
170
-100 -50 0 50 100 150 200
105
106
107
108
XN-Ca(OH)2 vulcanizates cured 240 min at 165 oC
E"
(Pa)
Temperature (oC)
XN-Ch2.0
XN-Ch3.0
XN-Ch1.0
XN-P1.0
Figure 4.75 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
171
-100 -50 0 50 100 150 200
0.1
1
tan
δ
Temperature (oC)
XN-Ca(OH)2 vulcanizates cured 240 min at 165 oC
XN-Ch3.0
XN-Ch2.0
XN-Ch1.0
XN-P1.0
Figure 4.76 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
172
Table 4.14 Molecular transition temperatures of XN-P1.0 and XN-Ca(OH)2 vulcanizates at frequency 1.0 Hz
Vulcanizates Tg (oC) Ionic transition temperature range (oC)
XN-P1.0 -24 Not observed
XN-Ch1.0 -18 20 - 145
XN-Ch2.0 -15 Not clear
XN-Ch3.0 -16 Not clear
4.5.4 XNBR-BaO Vulcanizates
The effect of temperature on dynamic storage modulus, loss modulus, and tan δ of
XN-P1.0 and XN-Ba vulcanizates is shown in Figures 4.77, 4.78, and 4.79, respectively.
As for XN-Mg and XN-Ch vulcanizates, the rubbery moduli of cured XN-Ba samples are
higher than for XN-P1.0 at the beginning of the rubbery zone, and increase with
increasing BaO concentrations due to the increase in number of ionic crosslinks. Because
of the physical nature of ionic crosslinks, the rubbery moduli of XN-Ba vulcanizates
decrease with temperature, and are finally lower than for XN-P1.0, indicating that the
ionic crosslinks are unstable towards heat. In the case of covalently crosslinked XN-P1.0,
the rubbery modulus remains constant over the entire range of temperature because
crosslinks are permanent. In the ionically crosslinked rubbers, the glass transition shifts
slightly to higher temperatures than for the covalently crosslinked rubber.
The dynamic loss moduli in the glassy zone are similar for all the vulcanizates
(Figure 4.78). The XN-Ba vulcanizates have a higher loss modulus than XN-P1.0
173
samples in the rubbery zone, and the loss moduli increase with the increase in BaO
concentrations. The increase in loss modulus is probably due to energy dissipated in
interchange of ion pairs.91, 119-121
The effect of temperature on tan δ of cured XN-P1.0 and XB-Ba samples is
shown in Figure 4.79. As in the case of XN-Mg and XN-Ch vulcanizates, two transitions
are observed in the cured XN-Ba specimens, the glass transition of the rubber matrix at a
low temperature, and the ionic transition at a high temperature (Table 4.15). However, it
is difficult to determine the precise position of the ionic transition due to there is no clear
maximum. For the covalently crosslinked sample only the glass transition is observed.
The glass transition shifts to higher temperature in the ionically crosslinked rubbers.
Table 4.15 Molecular transition temperatures of XN-P1.0 and XN-CaO vulcanizates at frequency 1.0 Hz
Vulcanizates Tg (oC) Ionic transition temperature range (oC)
XN-P1.0 -24 Not observed
XN-Ba1.0 -18 Not clear
XN-Ba2.0 -15 25 – 140
XN-Ba3.0 -18 25 – 140
174
-100 -50 0 50 100 150 200
106
107
108
109
XN-Ba3.0
XN-Ba2.0
XN-Ba1.0
E' (
Pa)
Temperature (oC)
XN-BaO vulcanizates cured 240 min at 165 oC
XN-P1.0
Figure 4.77 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz.
175
-100 -50 0 50 100 150 200
105
106
107
108
XN-BaO vulcanizates cured 240 min at 165 oC
E"
(Pa)
Temperature (oC)
XN-Ba3.0
XN-Ba2.0
XN-Ba1.0
XN-P1.0
Figure 4.78 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz
176
-100 -50 0 50 100 150 200
0.1
1
XN-BaO vulcanizates cured 240 min at 165 oCta
n δ
Temperature (oC)
XN-Ba3.0
XN-Ba2.0
XN-Ba1.0
XN-P1.0
Figure 4.79 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz.
177
4.5.5 Comparison among Metal Compounds
Figure 4.80 shows tan δ as a function of temperature for peroxide-cured XNBR
and XNBR cured with 2.0x stoichiometry of various metal compounds. Clearly, two
transitions are observed in XN-MgA2.0, XN-Ch2.0 and XN-Ba2.0 vulcanizates. One is
the glass-rubber transition of the rubber matrix; the other, a high temperature is the ionic
transition. The ionic transition does not appear in XN-P1.0 and XN-Ca2.0 where the
rubber chains are covalently crosslinked. It appears only in the vulcanizates that are
crosslinked ionically, and may be associated with the exchange of ion pairs between ionic
aggregates. It seems to shift to higher temperature with increasing cation size.
178
-100 -50 0 50 100 150 200
0.1
1
tan
δ
Temperature (oC)
Frequency 1.0 Hz
XN-Ch2.0
XN-MgA2.0XN-Ba2.0
XN-P1.0
XN-Ca2.0
Figure 4.80 Temperature dependence of loss tangent (tan δ) of XNBR cured with 2.0x stoichiometry of various metal compounds at frequency 1.0 Hz.
179
CHAPTER V
CONCLUSIONS
1. The cure behavior of XNBR vulcanized by MgO depends greatly on both
concentration and specific surface area of MgO. Cure rate increases with increasing
both specific surface area and concentration.
2. Tensile properties of XNBR-MgO vulcanizates improve greatly with increased
amounts of MgO to the point where all carboxyl groups are completely neutralized,
and slightly change thereafter. The effect of surface area is not significant.
3. For XNBR-MgO compounds, neutralization occurs during mixing and continues
during storage.
4. ODR, tensile, swelling and ATR-IR results suggest that neutralization of XNBR by
MgO requires an equimolar amount of acidity and MgO. The proposed mechanisms
are 1) MgO reacts with carboxyl groups (RCOOH) to give the magnesium
hydroxycarboxylate salt, RCOOMgOH, 2) This salt reacts bimolecularly to form the
magnesium carboxylate salt, RCOOMgOOCR and Mg(OH)2.
5. Ca(OH)2 and BaO give similar effect to MgO on cure and mechanical properties of
XNBR compounds, while CaO gives similar results to thermally cured XNBR.
6. Crosslink density increases with increasing amounts of crosslinking agents, except for
the case of CaO.
180
7. The temperature-tan δ plot reveals an additional peak at a higher temperature in
addition to the glass-rubber transition in all ionically crosslinked systems, but not in
covalently crosslinked vulcanizates. The peak shifts to higher temperatures with
increasing concentration of curing agents.
8. The strength of ionic crosslinks increases in a predictable manner with the decrease in
size of the cations as followed: Mg++ > Ca++ > Ba++ ions.
9. The wave number of asymmetric carbonyl stretching of the carboxylate anion shifts
to lower values with increasing cation mass and size.
10. The ionic transition seems to shift to higher temperatures with increasing cation mass
and size.
181
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190
APPENDICES
191
APPENDIX A
CURE PROPERTIES
Table A.1 Cure properties of XNBR cured with different magnesium oxides at 165 oC
Property XNBR XN-MgA0.5
XN-MgA1.0
XN-MgA1.5
XN-MgA2.0
XN-MgA3.0
XN-MgA4.0
XN-MgA5.0
ML (dN.m) MH (dN.m) tS2 (min)
7.3 14.5 6.0
6.6 13.2 28.6
7.6 28.6 4.0
8.4 47.2 57.1
8.5 57.1 3.5
9.9 61.2 2.5
10.2 64.9 2.5
11.6 67.1 2.0
Property XNBR XN-MgB0.5
XN-MgB1.0
XN-MgB1.5
XN-MgB2.0
XN-MgB3.0
XN-MgB4.0
XN-MgB5.0
ML (dN.m) MH (dN.m) tS2 (min)
7.3 14.5 6.0
6.6 13.0 50.0
6.9 24.9 22.0
7.0 38.9 14.0
7.0 47.0 11.0
7.7 56.1 8.0
8.8 62.3 3.5
10.5 66.9 2.5
Property XNBR XN-MgC0.5
XN-MgC1.0
XN-MgC1.5
XN-MgC2.0
XN-MgC3.0
XN-MgC4.0
XN-MgC5.0
ML (dN.m) MH (dN.m) tS2 (min)
7.3 14.5 6.0
6.6 10.7
120.0
6.5 19.9 50.0
6.5 33.9 25.0
6.7 42.2 18.0
6.6 53.2 10.0
7.9 61.4
5
8.6 65.3
3
192
Table A.2 Cure properties of XNBR cured by dicumyl peroxide at 165 oC
Property XNBR XN-P0.25
XN-P0.5
XN-P0.75
XN-P1.0
XN-P1.5
XN-P2.0
XN-P3.0
ML (dN.m) MH (dN.m) MH - ML (dN.m) tS2 (min)
7.3 14.5 3.5 6.0
9.1 19.0 9.9 6.0
8.8 31.1 22.3 3.0
8.4 41.3 32.9 2.5
8.5 52.6 44.1 2.0
8.0 71.8 63.8 1.8
8.8 88.0 79.2 1.5
9.1 98.8 89.7 1.3
Table A.3 Cure properties of XNBR cured with calcium oxide at 165 oC
Property XNBR XN-Ca0.5
XN-Ca1.0
XN-Ca1.5
XN-Ca2.0
XN-Ca3.0
XN-Ca4.0
XN-Ca5.0
ML (dN.m) MH (dN.m) tS2 (min)
10.8 38.2
6
9.1 32.6 65
9.6 36.0 45
8.5 36.1 55
10.5 37.4 40
9.2 36.0 55
9.1 38.5 55
8.8 39.8 53
Table A.4 Cure properties of XNBR cured with calcium hydroxide at 165 oC
Property XNBR XN-Ch0.5
XN-Ch1.0
XN-Ch1.5
XN-Ch2.0
XN-Ch3.0
XN-Ch4.0
XN-Ch5.0
ML (dN.m) MH (dN.m) tS2 (min)
10.8 24.5
6
10.1 18.7 14
8.9 29.6 13
10.0 74.0 10
10.2 76.5 10.5
8.7 87.8 6.5
9.5 91.9 2.5
11.0 94.9 3.0
Table A.5 Cure properties of XNBR cured with barium oxide at 165 oC
Property XNBR XN-Ba0.5
XN-Ba1.0
XN-Ba1.5
XN-Ba2.0
XN-Ba3.0
XN-Ba4.0
XN-Ba5.0
ML (dN.m) MH (dN.m) tS2 (min)
10.8 24.5
6
8.8 18.8
9
9.9 27.4 15
10.9 34.7 20
9.9 44.8
7
11.0 54.3
8
9.6 65.1
5
11.6 75.2
6
193
APPENDIX B
TENSILE PROPERTIES
Table B.1 Tensile properties at room temperature (~25 oC) of thermally cured XNBR
Cure time Properties
60 min 120 min 240 min 500 min 1000 min
25 % Mod. (MPa) 0.28 ± 0.01 0.34 ± 0.01 0.31 ± 0.01 0.34 ± 0.01 0.35 ± 0.01
50 % Mod. (MPa) 0.37 ± 0.01 0.46 ± 0.01 0.47 ± 0.01 0.50 ± 0.01 0.52 ± 0.01
75 % Mod. (MPa) 0.42 ± 0.01 0.51 ± 0.01 0.55 ± 0.01 0.59 ± 0.01 0.62 ± 0.01
100 % Mod. (MPa) 0.44 ± 0.01 0.54 ± 0.01 0.61 ± 0.01 0.64 ± 0.01 0.69 ± 0.01
150 % Mod. (MPa) 0.46 ± 0.01 0.56 ± 0.01 0.66 ± 0.01 0.72 ± 0.01 0.78 ± 0.01
200 % Mod. (MPa) 0.48 ± 0.01 0.58 ± 0.01 0.71 ± 0.02 0.79 ± 0.01 0.86 ± 0.01
250 % Mod. (MPa) 0.49 ± 0.01 0.59 ± 0.01 0.76 ± 0.02 0.85 ± 0.01 0.94 ± 0.01
300 % Mod. (MPa) 0.50 ± 0.01 0.61 ± 0.01 0.81 ± 0.03 0.92 ± 0.02 1.03 ± 0.02
350 % Mod.(MPa) 0.51 ± 0.01 0.63 ± 0.01 0.87 ± 0.03 1.01 ± 0.03 1.14 ± 0.03
400 % Mod. (MPa) 0.52 ± 0.01 0.65 ± 0.01 0.94 ± 0.03 1.10 ± 0.04 1.27 ± 0.04
450 % Mod. (MPa) 0.54 ± 0.02 0.68 ± 0.01 1.02 ± 0.04 1.22 ± 0.05 1.42 ± 0.06
500 % Mod. (MPa) 0.55 ± 0.02 0.71 ± 0.01 1.11 ± 0.05 1.36 ± 0.06 1.63 ± 0.08
TS (MPa) > 4.20 ± 0.56 6.46 ± 0.15 7.79 ± 0.13 8.19 ± 0.18 9.08 ± 0.14
EB (%) > 1655 ± 52 1420 ± 14 939 ± 13 827 ± 3 751 ± 2
194
Table B.2 Tensile properties at room temperature (~25 oC) of XN-MgA vulcanizates (cure time 30 min)
Properties XNBR XN-
MgA0.5 XN-
MgA1.0 XN-
MgA1.5 XN-
MgA2.0 XN-
MgA3.0 XN-
MgA4.0 XN-
MgA5.0
25 % Mod. (MPa)
0.29 ± 0.01
0.83 ± 0.03
0.86 ± 0.02
1.40 ± 0.12
1.40 ± 0.03
1.43 ± 0.06
1.52 ± 0.05
1.61 ± 0.03
50 % Mod. (MPa)
0.41 ± 0.01
1.10 ± 0.04
1.24 ± 0.02
2.08 ± 0.16
2.20 ± 0.04
2.28 ± 0.05
2.41 ± 0.05
2.52 ± 0.04
75 % Mod. (MPa)
0.46 ± 0.01
1.25 ± 0.05
1.48 ± 0.02
2.55 ± 0.19
2.80 ± 0.04
2.94 ± 0.05
3.09 ± 0.05
3.29 ± 0.06
100 % Mod. (MPa)
0.48 ± 0.01
1.34 ± 0.05
1.66 ± 0.02
2.95 ± 0.22
3.33 ± 0.05
3.56 ± 0.07
3.72 ± 0.06
3.97 ± 0.08
150 % Mod. (MPa)
0.50 ± 0.01
1.43 ± 0.05
1.98 ± 0.02
3.55 ± 0.30
4.38 ± 0.09
4.83 ± 0.12
5.04 ± 0.10
5.41 ± 0.14
200 % Mod. (MPa)
0.50 ± 0.02
1.50 ± 0.05
2.31 ± 0.02
4.60 ± 0.30
5.60 ± 0.12
6.32 ± 0.15
6.53 ± 0.16
7.01 ± 0.20
250 % Mod. (MPa)
0.50 ± 0.02
1.57 ± 0.04
2.72 ± 0.04
5.77 ± 0.42
7.19 ± 0.17
8.10 ± 0.26
8.35 ± 0.21
8.94 ± 0.28
300 % Mod. (MPa)
0.50 ± 0.02
1.64 ± 0.04
3.24 ± 0.07
7.34 ± 0.59
9.44 ± 0.28
10.4 ± 0.4
10.8 ± 0.3
11.5 ± 0.4
350 % Mod. (MPa)
0.50 ± 0.02
1.73 ± 0.05
3.94 ± 0.12
9.67 ± 0.86
12.7 ± 0.4
13.8 ± 0.6
13.6 ± 1.5
14.9 ± 0.7
400 % Mod. (MPa)
0.50 ± 0.02
1.86 ± 0.04
4.93 ± 0.19
13.0 ± 1.3
17.6 ± 0.6
18.3 ± 1.0
19.0 ± 0.7
19.4 ± 1.1
450 % Mod. (MPa)
0.51 ± 0.03
2.03 ± 0.05
6.53 ± 0.36
17.8 ± 1.9
24.1 ± 0.8
24.6 ± 1.5
25.1 ± 1.0
25.3 ± 1.3
500 % Mod. (MPa)
0.52 ± 0.03
2.27 ± 0.06
9.13 ± 0.55
24.2 ± 2.6
32.5 ± 1.2
32.5 ± 2.1
32.8 ± 1.0
32.7 ± 1.8
TS (MPa)
Not break >3.60
13.5 ± 0.5
28.5 ± 0.9
43.5 ± 2.4
46.5 ± 1.7
48.4 ± 0.5
48.6 ± 2.4
52.8 ± 2.0
EB (%)
Not break >1600
830 ± 12
674 ± 13
614 ± 15
572 ± 11
584 ± 12
583 ± 15
611 ± 14
195
Table B.3 Tensile properties at room temperature (~25 oC) of XN-MgA vulcanizates (cure time 120 min)
Properties XNBR XN-MgA0.5
XN-MgA1.0
XN-MgA1.5
XN-MgA2.0
XN-MgA3.0
XN-MgA4.0
XN-MgA5.0
25 % Mod. (MPa)
0.34 ± 0.01
0.57 ± 0.01
0.95 ± 0.01
1.25 ± 0.03
1.43 ± 0.01
1.72 ± 0.05
1.79 ± 0.01
1.72 ± 0.02
50 % Mod. (MPa)
0.46 ± 0.01
0.76 ± 0.01
1.40 ± 0.03
1.85 ± 0.03
2.07 ± 0.01
2.42 ± 0.07
2.51 ± 0.01
2.59 ± 0.01
75 % Mod. (MPa)
0.51 ± 0.01
0.86 ± 0.02
1.70 ± 0.03
2.20 ± 0.03
2.68 ± 0.01
3.13 ± 0.09
3.25 ± 0.03
3.22 ± 0.03
100 % Mod. (MPa)
0.54 ± 0.01
0.93 ± 0.02
1.90 ± 0.03
2.51 ± 0.05
3.14 ± 0.02
3.66 ± 0.11
3.79 ± 0.03
3.95 ± 0.04
150 % Mod. (MPa)
0.56 ± 0.01
1.02 ± 0.03
2.39 ± 0.04
3.28 ± 0.07
4.14 ± 0.02
4.98 ± 0.16
5.17 ± 0.05
5.27 ± 0.08
200 % Mod. (MPa)
0.58 ± 0.01
1.08 ± 0.03
2.88 ± 0.06
4.21 ± 0.12
5.46 ± 0.05
6.67 ± 0.22
6.79 ± 0.08
6.89 ± 0.12
250 % Mod. (MPa)
0.59 ± 0.01
1.15 ± 0.03
3.48 ± 0.07
5.51 ± 0.20
7.21 ± 0.05
8.74 ± 0.27
8.94 ± 0.15
8.97 ± 0.35
300 % Mod. (MPa)
0.61 ± 0.01
1.23 ± 0.03
4.30 ± 0.09
7.30 ± 0.31
9.67 ± 0.10
11.6 ± 0.3
11.7 ± 0.3
11.7 ± 0.3
350 % Mod. (MPa)
0.63 ± 0.01
1.33 ± 0.04
5.51 ± 0.13
10.4 ± 0.5
13.0 ± 0.2
15.9 ± 0.5
15.6 ± 0.3
15.8 ± 0.5
400 % Mod. (MPa)
0.65 ± 0.01
1.46 ± 0.04
7.30 ± 0.16
15.6 ± 0.9
18.1 ± 0.3
21.8 ± 0.6
21.2 ± 0.5
21.1 ± 0.7
450 % Mod. (MPa)
0.68 ± 0.01
1.63 ± 0.06
10.3 ± 0.3
23.1 ± 1.5
25.6 ± 0.3
30.2 ± 1.0
29.4 ± 0.8
29.2 ± 0.8
500 % Mod. (MPa)
0.71 ± 0.01
1.85 ± 0.06
15.0 ± 0.4
33.2 ± 2.2
35.3 ± 0.3
40.0 ± 1.4
39.1 ± 1.0
38.0 ± 1.0
TS (MPa)
6.46 ± 0.15
15.9 ± 0.5
34.8 ± 1.2
44.7 ± 1.0
48.3 ± 1.1
51.5 ± 1.7
51.4 ± 1.2
51.5 ± 1.0
EB (%)
1420 ± 14
872 ± 7
635 ± 5
549 ± 3
553 ± 16
548 ± 8
552 ± 7
561 ± 20
196
Table B.4 Tensile properties at room temperature (~25 oC) of XN-MgB vulcanizates (cure time 30 min)
Properties XNBR XN-
MgB0.5 XN-
MgB1.0 XN-
MgB1.5 XN-
MgB2.0 XN-
MgB3.0 XN-
MgB4.0 XN-
MgB5.0
25 % Mod. (MPa)
0.29 ± 0.01
0.44 ± 0.02
0.70 ± 0.02
0.94 ± 0.01
1.15 ± 0.02
1.49 ± 0.04
1.63 ± 0.02
1.58 ± 0.05
50 % Mod. (MPa)
0.41 ± 0.01
0.62 ± 0.02
1.01 ± 0.03
1.35 ± 0.01
1.71 ± 0.05
2.35 ± 0.03
2.50 ± 0.06
2.57 ± 0.03
75 % Mod. (MPa)
0.46 ± 0.01
0.70 ± 0.02
1.19 ± 0.01
1.62 ± 0.03
2.12 ± 0.06
3.00 ± 0.05
3.18 ± 0.06
3.36 ± 0.02
100 % Mod. (MPa)
0.48 ± 0.01
0.76 ± 0.01
1.32 ± 0.01
1.83 ± 0.04
2.47 ± 0.0
3.58 ± 0.07
3.83 ± 0.07
4.09 ± 0.03
150 % Mod. (MPa)
0.50 ± 0.01
0.83 ± 0.01
1.53 ± 0.01
2.22 ± 0.05
3.15 ± 0.09
4.72 ± 0.09
5.04 ± 0.07
5.49 ± 0.05
200 % Mod. (MPa)
0.50 ± 0.02
0.90 ± 0.02
1.78 ± 0.05
2.65 ± 0.08
3.91 ± 0.12
5.96 ± 0.13
6.40 ± 0.10
6.99 ± 0.05
250 % Mod. (MPa)
0.50 ± 0.02
0.96 ± 0.02
2.01 ± 0.02
3.17 ± 0.11
4.84 ± 0.16
7.53 ± 0.18
8.02 ± 0.18
8.73 ± 0.05
300 % Mod. (MPa)
0.50 ± 0.02
1.03 ± 0.02
2.35 ± 0.03
3.86 ± 0.13
6.09 ± 0.26
9.63 ± 0.29
10.2 ± 0.3
11.0 ± 0.1
350 % Mod. (MPa)
0.50 ± 0.02
1.13 ± 0.03
2.82 ± 0.05
4.79 ± 0.21
7.90 ± 0.39
12.6 ± 0.4
13.3 ± 0.4
14.2 ± 0.2
400 % Mod. (MPa)
0.50 ± 0.02
1.26 ± 0.04
3.47 ± 0.08
6.17 ± 0.32
10.7 ± 0.6
17.0 ± 0.8
17.8 ± 0.7
18.8 ± 0.3
450 % Mod. (MPa)
0.51 ± 0.03
1.45 ± 0.05
4.49 ± 0.15
8.36 ± 0.52
14.7 ± 0.9
22.9 ± 1.5
23.8 ± 1.1
25.1 ± 0.4
500 % Mod. (MPa)
0.52 ± 0.03
1.70 ± 0.07
6.21 ± 0.24
11.7 ± 0.8
20.4 ± 1.3
30.6 ± 1.6
31.6 ± 1.4
33.2 ± 0.5
TS (MPa)
Not break >3.60
5.96 ± 0.24
17.2 ± 0.5
26.0 ± 1.3
33.3 ± 1.5
40.1 ± 0.5
45.8 ± 1.6
46.0 ± 2.1
EB (%)
Not break >1600
724 ± 12
642 ± 12
619 ± 11
586 ± 11
553 ± 11
575 ± 15
567 ± 14
197
Table B.5 Tensile properties at room temperature (~25 oC) of XN-MgB vulcanizates (cure time 120 min)
Properties XNBR XN-MgB0.5
XN-MgB1.0
XN-MgB1.5
XN-MgB2.0
XN-MgB3.0
XN-MgB4.0
XN-MgB5.0
25 % Mod. (MPa)
0.34 ± 0.01
0.62 ± 0.02
0.97 ± 0.03
1.24 ± 0.04
1.41 ± 0.03
1.61 ± 0.05
1.71 ± 0.09
1.71 ± 0.07
50 % Mod. (MPa)
0.46 ± 0.01
0.87 ± 0.02
1.42 ± 0.01
1.88 ± 0.05
2.20 ± 0.06
2.62 ± 0.12
2.68 ± 0.12
2.87 ± 0.08
75 % Mod. (MPa)
0.51 ± 0.01
1.01 ± 0.01
1.71 ± 0.03
2.35 ± 0.06
2.80 ± 0.07
3.49 ± 0.17
3.48 ± 0.12
3.74 ± 0.08
100 % Mod. (MPa)
0.54 ± 0.01
1.10 ± 0.01
1.95 ± 0.02
2.76 ± 0.07
3.32 ± 0.07
4.28 ± 0.22
4.20 ± 0.12
4.60 ± 0.08
150 % Mod. (MPa)
0.56 ± 0.01
1.25 ± 0.02
2.39 ± 0.03
3.54 ± 0.10
4.33 ± 0.08
5.76 ± 0.32
5.61 ± 0.16
6.19 ± 0.10
200 % Mod. (MPa)
0.58 ± 0.01
1.38 ± 0.02
2.87 ± 0.03
4.42 ± 0.16
5.48 ± 0.11
7.37 ± 0.46
7.13 ± 0.22
7.85 ± 0.12
250 % Mod. (MPa)
0.59 ± 0.01
1.54 ± 0.03
3.47 ± 0.04
5.53 ± 0.27
6.93 ± 0.14
9.32 ± 0.68
9.01 ± 0.34
9.82 ± 0.17
300 % Mod. (MPa)
0.61 ± 0.01
1.74 ± 0.05
4.27 ± 0.06
7.09 ± 0.42
8.85 ± 0.23
11.9 ± 0.9
11.5 ± 0.5
12.3 ± 0.3
350 % Mod. (MPa)
0.63 ± 0.01
2.01 ± 0.05
5.43 ± 0.07
9.42 ± 0.65
11.7 ± 0.4
15.5 ± 1.5
15.1 ± 0.7
15.8 ± 0.5
400 % Mod. (MPa)
0.65 ± 0.01
2.38 ± 0.08
7.21 ± 0.09
13.0 ± 0.9
15.8 ± 0.7
20.5 ± 2.2
20.2 ± 1.0
20.7 ± 0.8
450 % Mod. (MPa)
0.68 ± 0.01
2.92 ± 0.15
10.1 ± 0.2
18.1 ± 1.4
21.5 ± 1.1
27.1 ± 3.3
27.0 ± 1.5
27.2 ± 1.2
500 % Mod. (MPa)
0.71 ± 0.01
3.77 ± 0.24
14.3 ± 0.4
24.9 ± 2.1
28.9 ± 1.2
35.4 ± 4.4
35.7 ± 1.9
35.5 ± 1.6
TS (MPa)
6.46 ± 0.15
13.6 ± 0.5
25.2 ± 0.9
38.7 ± 2.8
42.3 ± 2.7
47.9 ± 1.7
49.3 ± 1.9
51.3 ± 1.3
EB (%)
1420 ± 14
687 ± 12
588 ± 9
579 ± 9
571 ± 14
563 ± 28
568 ± 13
579 ± 15
198
Table B.6 Tensile properties at room temperature (~25 oC) of XN-MgC vulcanizates (cure time 30 min)
Properties XNBR XN-
MgC0.5 XN-
MgC1.0 XN-
MgC1.5 XN-
MgC2.0 XN-
MgC3.0 XN-
MgC4.0 XN-
MgC5.0
25 % Mod. (MPa)
0.29 ± 0.01
0.32 ± 0.01
0.46 ± 0.01
0.83 ± 0.02
0.98 ± 0.02
1.34 ± 0.04
1.34 ± 0.02
1.56 ± 0.03
50 % Mod. (MPa)
0.41 ± 0.01
0.44 ± 0.01
0.62 ± 0.01
1.14 ± 0.02
1.45 ± 0.02
2.05 ± 0.03
1.91 ± 0.03
2.28 ± 0.04
75 % Mod. (MPa)
0.46 ± 0.01
0.49 ± 0.01
0.71 ± 0.01
1.34 ± 0.02
1.75 ± 0.02
2.57 ± 0.04
2.36 ± 0.03
2.85 ± 0.04
100 % Mod. (MPa)
0.48 ± 0.01
0.52 ± 0.01
0.76 ± 0.01
1.49 ± 0.02
1.99 ± 0.02
3.04 ± 0.04
2.88 ± 0.03
3.38 ± 0.03
150 % Mod. (MPa)
0.50 ± 0.01
0.53 ± 0.01
0.81 ± 0.01
1.73 ± 0.03
2.45 ± 0.03
3.99 ± 0.06
3.80 ± 0.03
4.54 ± 0.03
200 % Mod. (MPa)
0.50 ± 0.02
0.53 ± 0.01
0.85 ± 0.01
2.00 ± 0.03
2.96 ± 0.03
5.05 ± 0.09
4.85 ± 0.02
5.72 ± 0.03
250 % Mod. (MPa)
0.50 ± 0.02
0.53 ± 0.01
0.90 ± 0.02
2.31 ± 0.04
3.58 ± 0.04
6.36 ± 0.14
6.19 ± 0.02
7.43 ± 0.04
300 % Mod. (MPa)
0.50 ± 0.02
0.53 ± 0.01
0.95 ± 0.02
2.71 ± 0.05
4.39 ± 0.07
8.07 ± 0.22
7.99 ± 0.03
9.55 ± 0.09
350 % Mod. (MPa)
0.50 ± 0.02
0.53 ± 0.01
1.02 ± 0.02
3.24 ± 0.07
5.49 ± 0.13
10.5 ± 0.4
10.8 ± 0.1
12.6 ± 0.1
400 % Mod. (MPa)
0.50 ± 0.02
0.54 ± 0.01
1.12 ± 0.03
3.96 ± 0.10
7.12 ± 0.26
14.0 ± 0.7
15.0 ± 0.1
17.3 ± 0.2
450 % Mod. (MPa)
0.51 ± 0.03
0.54 ± 0.01
1.24 ± 0.04
5.04 ± 0.15
9.64 ± 0.44
19.1 ± 1.1
21.6 ± 0.2
23.7 ± 0.2
500 % Mod. (MPa)
0.52 ± 0.03
0.54 ± 0.01
1.41 ± 0.05
6.77 ± 0.29
13.4 ± 0.7
25.6 ± 1.6
30.6 ± 0.5
32.2 ± 0.2
TS (MPa)
Not break >3.60
Not break >1.70
6.3 ± 0.2
18.9 ± 2.0
28.9 ± 1.4
40.4 ± 1.3
37.3 ± 1.2
46.4 ± 1.4
EB (%)
Not break >1600
Not break >1600
834 ± 14
651 ± 14
624 ± 13
588 ± 17
537 ± 7
570 ± 6
199
Table B.7 Tensile properties at room temperature (~25 oC) of XN-MgC vulcanizates (cure time 120 min)
Properties XNBR XN-MgC0.5
XN-MgC1.0
XN-MgC1.5
XN-MgC2.0
XN-MgC3.0
XN-MgC4.0
XN-MgC5.0
25 % Mod. (MPa)
0.34 ± 0.01
0.46 ± 0.01
0.89 ± 0.02
1.20 ± 0.02
1.33 ± 0.03
1.54 ± 0.03
1.46 ± 0.06
1.51 ± 0.03
50 % Mod. (MPa)
0.46 ± 0.01
0.63 ± 0.01
1.30 ± 0.03
1.78 ± 0.02
2.06 ± 0.03
2.43 ± 0.04
2.16 ± 0.05
2.32 ± 0.02
75 % Mod. (MPa)
0.51 ± 0.01
0.73 ± 0.01
1.56 ± 0.03
2.17 ± 0.02
2.61 ± 0.03
3.14 ± 0.07
2.72 ± 0.05
2.96 ± 0.02
100 % Mod. (MPa)
0.54 ± 0.01
0.78 ± 0.01
1.75 ± 0.03
2.51 ± 0.02
3.10 ± 0.04
3.78 ± 0.08
3.23 ± 0.05
3.60 ± 0.02
150 % Mod. (MPa)
0.56 ± 0.01
0.84 ± 0.02
2.10 ± 0.02
3.17 ± 0.03
4.08 ± 0.06
5.05 ± 0.12
4.33 ± 0.05
4.84 ± 0.03
200 % Mod. (MPa)
0.58 ± 0.01
0.89 ± 0.02
2.49 ± 0.04
3.91 ± 0.06
5.18 ± 0.09
6.42 ± 0.17
5.58 ± 0.07
6.19 ± 0.05
250 % Mod. (MPa)
0.59 ± 0.01
0.94 ± 0.02
2.97 ± 0.05
4.81 ± 0.09
6.54 ± 0.13
8.09 ± 0.26
7.06 ± 0.10
7.70 ± 0.08
300 % Mod. (MPa)
0.61 ± 0.01
1.00 ± 0.02
3.60 ± 0.07
5.98 ± 0.14
8.36 ± 0.22
10.3 ± 0.5
9.04 ± 0.13
9.79 ± 0.13
350 % Mod. (MPa)
0.63 ± 0.01
1.06 ± 0.03
4.46 ± 0.12
7.72 ± 0.21
11.0 ± 0.4
13.4 ± 0.8
11.1 ± 0.1
12.7 ± 0.2
400 % Mod. (MPa)
0.65 ± 0.01
1.16 ± 0.03
5.77 ± 0.20
10.3 ± 0.4
14.8 ± 0.6
17.6 ± 1.1
16.4 ± 0.2
16.6 ± 0.4
450 % Mod. (MPa)
0.68 ± 0.01
1.29 ± 0.04
7.88 ± 0.34
14.2 ± 0.6
20.1 ± 1.03
23.3 ± 1.6
21.1 ± 0.3
22.2 ± 0.6
500 % Mod. (MPa)
0.71 ± 0.01
1.45 ± 0.06
11.1 ± 0.5
19.7 ± 0.9
26.9 ± 1.5
30.7 ± 2.2
30.2 ± 0.3
29.2 ± 0.7
TS (MPa)
6.46 ± 0.15
7.5 ± 1.4
26.6 ± 1.2
36.2 ± 2.0
43.3 ± 2.2
49.6 ± 2.0
47.4 ± 0.2
48.5 ± 0.9
EB (%)
1420 ± 14
815 ± 14
637 ± 5
605 ± 15
592 ± 12
600 ± 13
581 ± 3
607 ± 6
200
Table B.8 Tensile properties at room temperature (~25 oC) of XNBR-peroxide vulcanizates (cure time 60 min)
Properties XNBR XN-
P0.25 XN-
P0.50 XN-
P0.75 XN-P1.0
XN-P1.5
XN-P2.0
XN-P3.0
25 % Mod. (MPa)
0.28 ± 0.01
0.35 ± 0.01
0.38 ± 0.01
0.37 ± 0.01
0.38 ± 0.01
0.43 ± 0.01
0.48 ± 0.02
0.57 ± 0.01
50 % Mod. (MPa)
0.37 ± 0.01
0.48 ± 0.01
0.52 ± 0.01
0.53 ± 0.01
0.58 ± 0.01
0.67 ± 0.01
0.75 ± 0.02
0.93 ± 0.02
75 % Mod. (MPa)
0.42 ± 0.01
0.55 ± 0.01
0.60 ± 0.01
0.63 ± 0.01
0.69 ± 0.01
0.82 ± 0.01
0.92 ± 0.02
1.22 ± 0.02
100 % Mod. (MPa)
0.44 ± 0.01
0.59 ± 0.01
0.66 ± 0.01
0.71 ± 0.01
0.77 ± 0.02
0.94 ± 0.01
1.07 ± 0.02
1.48 ± 0.04
150 % Mod. (MPa)
0.46 ± 0.01
0.64 ± 0.01
0.74 ± 0.01
0.82 ± 0.02
0.91 ± 0.02
1.16 ± 0.02
1.34 ± 0.01
2.04 ± 0.06
200 % Mod. (MPa)
0.48 ± 0.01
0.68 ± 0.01
0.82 ± 0.01
0.94 ± 0.02
1.04 ± 0.02
1.41 ± 0.02
1.64 ± 0.01 -
250 % Mod. (MPa)
0.49 ± 0.01
0.72 ± 0.01
0.90 ± 0.02
1.08 ± 0.04
1.20 ± 0.04
1.70 ± 0.02
2.04 ± 0.03 -
300 % Mod. (MPa)
0.50 ± 0.01
0.76 ± 0.02
1.00 ± 0.03
1.24 ± 0.05
1.42 ± 0.04
2.06 ± 0.04 - -
350 % Mod. (MPa)
0.51 ± 0.01
0.82 ± 0.02
1.11 ± 0.03
1.43 ± 0.05
1.66 ± 0.04
2.52 ± 0.06 - -
400 % Mod. (MPa)
0.52 ± 0.01
0.86 ± 0.01
1.23 ± 0.03
1.63 ± 0.05
1.96 ± 0.04
3.18 ± 0.10 - -
450 % Mod. (MPa)
0.54 ± 0.02
0.92 ± 0.02
1.36 ± 0.04
1.86 ± 0.05
2.33 ± 0.02 - - -
500 % Mod. (MPa)
0.55 ± 0.02
0.98 ± 0.02
1.51 ± 0.04
2.11 ± 0.05
2.78 ± 0.03 - - -
TS (MPa)
> 4.20 ± 0.56
8.33 ± 0.25
8.06 ± 0.22
5.66 ± 0.93
4.59 ± 0.26
3.58 ± 0.42
2.51 ± 0.09
2.35 ± 0.09
EB (%)
> 1655 ± 52
1424 ± 12
1010 ± 4
769 ± 32
611 ± 12
421 ± 20
292 ± 9
173 ± 10
201
Table B.9 Tensile properties at room temperature (~25 oC) of XNBR-CaO vulcanizates (cure time 1000 min)
Properties XNBR XN-Ca0.5
XN-Ca1.0
XN-Ca1.5
XN-Ca2.0
XN-Ca3.0
XN-Ca4.0
XN-Ca5.0
25 % Mod. (MPa)
0.35 ± 0.01
0.41 ± 0.02
0.38 ± 0.02
0.43 ± 0.01
0.43 ± 0.02
0.42 ± 0.01
0.44 ± 0.02
0.44 ± 0.01
50 % Mod. (MPa)
0.52 ± 0.01
0.58 ± 0.01
0.55 ± 0.01
0.59 ± 0.01
0.60 ± 0.01
0.60 ± 0.01
0.63 ± 0.02
0.65 ± 0.01
75 % Mod. (MPa)
0.62 ± 0.01
0.68 ± 0.01
0.65 ± 0.01
0.69 ± 0.01
0.70 ± 0.01
0.71 ± 0.01
0.73 ± 0.01
0.77 ± 0.01
100 % Mod. (MPa)
0.69 ± 0.01
0.75 ± 0.01
0.71 ± 0.01
0.75 ± 0.01
0.76 ± 0.01
0.78 ± 0.01
0.80 ± 0.02
0.84 ± 0.01
150 % Mod. (MPa)
0.78 ± 0.01
0.87 ± 0.01
0.81 ± 0.01
0.83 ± 0.01
0.86 ± 0.01
0.87 ± 0.01
0.90 ± 0.02
0.94 ± 0.01
200 % Mod. (MPa)
0.86 ± 0.01
0.98 ± 0.01
0.89 ± 0.01
0.91 ± 0.01
0.95 ± 0.01
0.96 ± 0.01
0.98 ± 0.02
1.04 ± 0.01
250 % Mod. (MPa)
0.94 ± 0.01
1.10 ± 0.01
0.98 ± 0.01
0.98 ± 0.01
1.03 ± 0.01
1.05 ± 0.01
1.07 ± 0.01
1.13 ± 0.01
300 % Mod. (MPa)
1.03 ± 0.02
1.22 ± 0.01
1.07 ± 0.01
1.06 ± 0.02
1.13 ± 0.01
1.14 ± 0.01
1.18 ± 0.02
1.23 ± 0.01
350 % Mod. (MPa)
1.14 ± 0.03
1.35 ± 0.01
1.18 ± 0.01
1.15 ± 0.02
1.22 ± 0.01
1.24 ± 0.01
1.28 ± 0.03
1.35 ± 0.02
400 % Mod. (MPa)
1.27 ± 0.04
1.51 ± 0.01
1.31 ± 0.01
1.26 ± 0.03
1.34 ± 0.02
1.35 ± 0.02
1.39 ± 0.01
1.47 ± 0.02
450 % Mod. (MPa)
1.42 ± 0.06
1.70 ± 0.01
1.46 ± 0.01
1.39 ± 0.04
1.47 ± 0.02
1.49 ± 0.02
1.52 ± 0.03
1.60 ± 0.03
500 % Mod. (MPa)
1.63 ± 0.08
1.98 ± 0.02
1.64 ± 0.01
1.55 ± 0.05
1.63 ± 0.02
1.66 ± 0.03
1.70 ± 0.03
1.77 ± 0.03
TS (MPa)
9.08 ± 0.14
8.21 ± 0.18
7.71 ± 0.15
7.95 ± 0.20
7.52 ± 0.60
6.96 ± 0.33
7.16 ± 0.11
7.59 ± 0.09
EB (%)
751 ± 2
692 ± 10
746 ± 3
769 ± 15
764 ± 19
772 ± 8
766 ± 7
785 ± 5
202
Table B.10 Tensile properties at room temperature (~25 oC) of XNBR-Ca(OH)2 vulcanizates (cure time 240 min)
Properties XNBR XN-Ch0.5
XN-Ch1.0
XN-Ch1.5
XN-Ch2.0
XN-Ch3.0
XN-Ch4.0
XN-Ch5.0
25 % Mod.
(MPa)
0.31 ± 0.01
0.53 ± 0.01
1.67 ± 0.08
2.39 ± 0.01
2.28 ± 0.02
2.37 ± 0.08
2.47 ± 0.09
2.61 ± 0.05
50 % Mod.
(MPa)
0.47 ± 0.01
0.73 ± 0.01
2.67 ± 0.06
4.14 ± 0.02
4.20 ± 0.02
4.44 ± 0.04
4.69 ± 0.04
5.05 ± 0.12
75 % Mod.
(MPa)
0.55 ± 0.01
0.86 ± 0.02
3.36 ± 0.06
5.54 ± 0.05
5.64 ± 0.01
6.13 ± 0.04
6.50 ± 0.06
6.97 ± 0.09
100 % Mod.
(MPa)
0.61 ± 0.01
0.96 ± 0.02
3.93 ± 0.08
6.68 ± 0.03
6.92 ± 0.03
7.49 ± 0.5
7.94 ± 0.04
8.46 ± 0.09
150 % Mod.
(MPa)
0.66 ± 0.01
1.12 ± 0.03
5.01 ± 0.10
8.69 ± 0.03
9.17 ± 0.11
9.76 ± 0.07
10.2 ± 0.1
10.8 ± 0.1
200 % Mod.
(MPa)
0.71 ± 0.02
1.26 ± 0.04
6.23 ± 0.12
10.8 ± 0.1
11.2 ± 0.1
11.9 ± 0.1
12.4 ± 0.1
13.0 ±0.2
250 % Mod.
(MPa)
0.76 ± 0.02
1.43 ± 0.05
7.87 ± 0.17
13.3 ± 0.1
13.8 ± 0.1
14.6 ± 0.1
15.0 ± 0.1
15.7 ± 0.2
300 % Mod.
(MPa)
0.81 ± 0.03
1.63 ± 0.05
10.3 ± 0.3
16.9 ± 0.1
17.3 ± 0.1
18.2 ± 0.1
18.5 ± 0.1
19.3 ± 0.3
350 % Mod.
(MPa)
0.87 ± 0.03
1.88 ± 0.06
14.1 ± 0.4
22.0 ± 0.1
22.2 ± 0.1
23.2 ± 0.2
23.1 ± 0.2
24.2 ±0.5
400 % Mod.
(MPa)
0.94 ± 0.03
2.20 ± 0.07
19.9 ± 0.5
29.0 ± 0.3
28.7 ± 0.1
29.7 ± 0.2
29.4 ± 0.2
30.3 ± 0.7
450 % Mod.
(MPa)
1.02 ± 0.04
2.64 ± 0.08
27.8 ± 0.7
37.6 ± 0.5
36.9 ± 0.4
37.8 ± 0.5
36.8 ± 0.3
37.8 ± 0.7
500 % Mod.
(MPa)
1.11 ± 0.05
3.31 ± 0.12
38.0 ± 0.9 - 46.3
± 0.4 46.9 ± 0.3
45.1 ± 0.5
46.1 ± 0.8
TS
(MPa)
7.79 ± 0.13
14.9 ± 0.8
41.8 ± 0.8
45.1 ± 1.7
50.9 ± 1.0
48.2 ± 0.7
47.5 ± 0.6
48.1 ± 0.2
EB
(%)
939 ± 13
712 ± 12
516 ± 3
486 ± 11
522 ± 7
507 ± 4
514 ± 2
511 ± 3
203
Table B.11 Tensile properties at room temperature (~25 oC) of XNBR-BaO vulcanizates (cure time 240 min)
Properties XNBR XN-Ba0.5
XN-Ba1.0
XN-Ba1.5
XN-Ba2.0
XN-Ba3.0
XN-Ba4.0
XN-Ba5.0
25 % Mod. (MPa)
0.31 ± 0.01
0.45 ± 0.01
0.64 ± 0.01
0.84 ± 0.01
0.96 ± 0.02
1.29 ± 0.04
2.03 ± 0.06
2.38 ± 0.01
50 % Mod. (MPa)
0.47 ± 0.01
0.66 ± 0.01
0.99 ± 0.01
1.32 ± 0.01
1.58 ± 0.04
2.21 ± 0.04
3.19 ± 0.06
3.76 ± 0.05
75 % Mod. (MPa)
0.55 ± 0.01
0.77 ± 0.01
1.18 ± 0.01
1.62 ± 0.03
1.99 ± 0.03
2.83 ± 0.06
4.06 ± 0.06
4.85 ± 0.07
100 % Mod. (MPa)
0.61 ± 0.01
0.84 ± 0.01
1.32 ± 0.01
1.85 ± 0.02
2.29 ± 0.05
3.34 ± 0.07
4.84 ± 0.04
5.80 ± 0.09
150 % Mod. (MPa)
0.66 ± 0.01
0.96 ± 0.02
1.57 ± 0.01
2.28 ± 0.03
2.87 ± 0.07
4.29 ± 0.09
6.34 ± 0.08
7.60 ± 0.12
200 % Mod. (MPa)
0.71 ± 0.02
1.06 ± 0.02
1.83 ± 0.01
2.73 ± 0.04
3.50 ± 0.08
5.34 ± 0.13
8.02 ± 0.10
9.58 ± 0.18
250 % Mod. (MPa)
0.76 ± 0.02
1.17 ± 0.03
2.13 ± 0.01
3.29 ± 0.07
4.28 ± 0.11
6.71 ± 0.16
10.2 ± 0.1
12.1 ± 0.2
300 % Mod. (MPa)
0.81 ± 0.03
1.30 ± 0.04
2.54 ± 0.01
4.04 ± 0.10
5.34 ± 0.13
8.58 ± 0.23
13.1 ± 0.1
15.4 ± 0.2
350 % Mod. (MPa)
0.87 ± 0.03
1.47 ± 0.05
3.08 ± 0.02
5.06 ± 0.18
6.86 ± 0.22
11.3 ± 0.3
17.1 ± 0.1
19.7 ± 0.3
400 % Mod. (MPa)
0.94 ± 0.03
1.69 ± 0.04
3.84 ± 0.03
6.58 ± 0.24
9.26 ± 0.32
15.3 ± 0.4
22.5 ± 0.4
25.3 ± 0.5
450 % Mod. (MPa)
1.02 ± 0.04
1.98 ± 0.05
5.05 ± 0.07
9.09 ± 0.37
12.9 ± 0.3
20.9 ± 0.2 - -
500 % Mod. (MPa)
1.11 ± 0.05
2.38 ± 0.06
7.10 ± 0.19
13.1 ± 0.5
18.3 ± 0.3 - - -
TS (MPa)
7.79 ± 0.13
7.52 ± 0.20
12.8 ± 0.6
19.2 ± 0.5
21.7 ± 1.6
26.6 ± 0.7
25.6 ± 0.3
26.0 ± 0.3
EB (%)
939 ± 13
684 ± 5
580 ± 9
554 ± 4
526 ± 8
491 ± 3
424 ± 5
407 ± 3
204
APPENDIX C
MOLECULAR TRANSITION TEMPERATURE
Table C.1 Molecular transition temperatures of XN-MgB vulcanizates at frequency 1.0 Hz
Vulcanizates Tg (oC) Ionic transition temperature range (oC)
XN-MgB1.0 -20 10 - 85
XN-MgB2.0 -19 15 - 105
XN-MgB3.0 -18 30 - 110
Table C.2 Molecular transition temperatures of XN-MgC vulcanizates at frequency 1.0 Hz
Vulcanizates Tg (oC) Ionic transition temperature range (oC)
XN-MgC1.0 -21 15 - 85
XN-MgC2.0 -17 20 - 100
XN-MgC3.0 -17 25 - 105