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 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
ii 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.
iii 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.
iv 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.
v 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
vi 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
vii 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
viii 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
ix 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
x 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
xi 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
xii 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
xiii 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
xiv 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 115 compounds………………………………………………………………...
xv 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
xvi 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
xvii 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
xviii 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
1 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.
2 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
3 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
O O O heat 2 R C OH R C O C RH+ 2O (1)
R = polymer chain
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
4 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
O O O - + + - 2 R C OH + H2N R NH2 R C O H3N R NH3 O C R O O + - (2) R C O H3N R NH C R O O
R C NH R NH C R + 2 H2O
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 5 O
2 R C OH + H2C CH R CH CH2 O O (3)
O O
R C O CH2 CH R CH CH2 O C R
OH OH
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
O O
NR3 + CH2 CH R3N CH2 CH
O O
R C OH + R3N CH2 CH (4)
O OH
R C O CH2 CH +NR3
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). 6 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.
O
2 R C OH + OOC N R N C
O O O O
R C O C NH R NH C O C R (5)
O O
R C NH R NH C R + 2 CO2
R = polymer chain
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).
7 O Radiation R C OH R + CO2 + H
R + CH CH CH CH CH CH2 2 2 n C OOH
Coupling CH CH2 CH2 CH CH CH2 n C R OOH (6) OOH C R
CH CH2 CH2 CH CH CH2 n
CH CH2 CH2 CH CH CH2 n C R OOH
R + R RR
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.
8 Stoichiometrically, curing reaction of carboxylic rubbers by divalent metal oxide
can be written as equation 7;
O O O
2 R C OH + MO R C O MOC RH+ 2O (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
9 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
10
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
11 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
28 a similar effect. 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 11 masterbatch of medium acrylonitrile NBR and technical grade ZnO2)
12 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
13 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:
⎛ 0.806 ⎞ s = d ⎜ − 1⎟ (8) ⎝ ν1 3 ⎠
14 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:
3 ⎡⎛ 2 t ⎞ ⎤ ν ⎢⎜1 + ⎟ − 1⎥ ⎢⎝ d ⎠ ⎥ ν = ⎣ ⎦ (9) t 1 − ν
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-
15 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
16 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
2 3 ⎛ ρ N ⎞ ⎜ A ⎟ TS = ⎜ ⎟ Fc (10) ⎝ 3Mc ⎠
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. 17
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.
18
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 –
19 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
20 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
3 vp rm = (11) Sch
and the number (n0) of ion pairs in the multiplet can be calculated from
vm Sm n 0 = = (12) vp Sch
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
3 2 ethylene-methacrylic acid copolymer, vp is about 12 Å and Sch is about 12 Å , yielding
3 rm ~ 3 Å, and vm ~ 100 Å . 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
21 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
3 2 2 2 3 ρ N ⎡ 4l2 h M k' 1 e2 ⎛ n M ⎞ ⎤ n = A ⎢ c + 2 ⎜ 0 c ⎟ ⎥ (13) M ⎢3k T 2 M K 4πε r ⎜ ρ N ⎟ ⎥ c ⎣ c h 0 0 0 ⎝ A ⎠ ⎦
and the distance (R) between clusters is given by
1 3 ⎛ n M c ⎞ R = ⎜ ⎟ (14) ⎝ ρ N A ⎠
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
2 2 temperature, h is the mean square end-to-end distance of the free chain, h 0 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 22 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 23 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
24 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.
25 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
Tensile Ultimate Polymer and Treatment Strengtha Elongationa (psi) (%) Raw polymer, 0.12 ephrb of < 100 > 1,600 carboxyl group Treated with 0.12 ephr of aqueous 1,700 900 NaOH 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
26 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 27 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
28 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
29 79, 80 Ibarra and Alzorriz 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
30 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
31 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, 44 18 22 30 23 29 29 55 37 kg/cm2 Tensile strength, 389 157 132 128 190 220 241 394 249 kg/cm2 Relative 850 800 760 740 890 835 660 770 675 elongation, % Residual 22 10 22 14 23 15 2 28 18 elongation, %
Table 2.3 Active and inactive metal compounds25
Active Metal Compounds Inactive Metal Compounds
MgO ZnO MgCO3 SiO2 Mg(OH)2 ZnO2 Al2O3 Al(OH)3 Ca(OH)2 CdO TiO2 ZnS SrO HgO FeO Fe2O3 BaO PbO NiO CuO Ba(OH)2 Pb3O4 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.
32 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
33
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).
34
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 35
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.
36 Table 2.4 Influence of HAF carbon black loading on mechanical properties of ZnO-cured XNBR85
Ingredients A B C
Krynac 7.50 XNBR 100.0 100.0 100.0 Zinc oxide 12.0 12.0 12.0 Stearic acid 1.0 1.0 1.0 HAF carbon black 0.0 20.0 30.0 Properties Modulus at 100 % elongation (MPa) 1.8 4.2 6.6 Modulus at 300 % elongation (MPa) 3.2 11.6 18.0 Tensile strength (MPa) 33 31 28 Elongation at break (%) 1150 1000 900 Tear strength (kN/m) 32 45 48 Hardness (IRHD) 56 70 75 Tension set at 100 % elongation (%) 9 11 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
37 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
38 Table 2.5 Effect of silica types on tensile properties of XNBR vulcanizates87
Ingredients A B C
Chemigum NX775 XNBR 100.0 100.0 100.0 Harwick DSC-18 0.3 0.3 0.3 Stearic acid 2.0 2.0 2.0 Dibutylphthalate 5.0 5.0 5.0 Wingstay 29 1.0 1.0 1.0 Sulfur, Spider 0.5 0.5 0.5 TMTD 2.0 2.0 2.0 Pasco 558T ZnO 5.0 5.0 5.0 Hi-Sil 233, precipitated silica (0.022 μm) 30.0 - - Hi-Sil BP, precipitated silica (0.04 μm) - 30.0 - Cab-O-Sil MS-7SD, fumed silica (0.014 μm) - - 30.0
Cure properties at 163 oC Minimum torque, (N.m) 1.0 0.8 1.6 Maximum torque, (N.m) 8.8 8.9 10.4 tS2, (min) 3.2 1.8 4.2 tc90, (min) 10.8 4.8 12.7 tc95, (min) 16.5 6.5 16.5 Tensile properties 100 % Modulus, (MPa) 3.7 3.2 4.4 200 % Modulus, (MPa) 7.2 5.4 7.8 300 % Modulus, (MPa) 11.2 8.5 12.5 Tensile strength, (MPa) 26.6 19.1 26.4 Elongation at break, (%) 480 490 490 Hardness, (Shore A) 82 77 85 Aging properties; 70 hr at 121 oC in air oven Tensile strength, (MPa) 23.1 20.0 25.9 % Change -13 5 -2 Elongation at break, (%) 360 380 370 % Change -25 -22 -24 Hardness, (Shore A) 87 84 91 Point change 5 7 6 Tear strength, die C, (kN/m) 56.0 45.5 62.0 Tear strength, die C, 100 oC, (kN/m) 18.9 15.3 21.5 Compression set, 72 hr at 100 oC, (%) 41 25 51 Pico abrasion index 348 249 417 Pico abrasion index, aged 1 1hr at 149 oC 541 314 767
39 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 100 100 100 100 100 100 100 ZnO 12 12 12 12 12 12 12 Stearic acid 1 1 1 1 1 1 1 Calcium carbonate - 10 20 30 - - - Clay - - - - 10 20 30 Properties 100 % Modulus, (MPa) 1.85 1.95 2.20 2.90 2.19 2.45 3.11 300 % Modulus, (MPa) 3.11 3.95 4.25 5.30 3.64 3.95 4.70 Tensile strength, (MPa) 32 28 27 24 30 28 27 Elongation at break, (%) 1100 1050 1020 900 1040 1030 990 Tear strength, (N/cm) 32 32 36 38 32 37 39 Hardness, (IRHD) 56 63 64 66 61 63 65 Tension set, (%) 9 11 11 12 11 12 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 40
Figure 2.14 Effect of silica loading on storage modulus (E’) and tan δ of ZnO- 90 vulcanized XNBR (Z0 = 0 phr, Z10 = 10 phr, Z20 = 20 phr, Z30 = 30 phr).
41 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.
42 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
43 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
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR MgA0.5 MgA1.0 MgA1.5 MgA2.0 MgA3.0 MgA4.0 MgA5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) Elastomag 0.0 0.75 1.5 2.25 3.0 4.5 6.0 7.5 100 (98.0%) (0.0) (0.77) (1.53) (2.30) (3.06) (4.59) (6.12) (7.65)
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR MgB0.5 MgB1.0 MgB1.5 MgB2.0 MgB3.0 MgB4.0 MgB5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) Magchem 50 0.0 0.75 1.5 2.25 3.0 4.5 6.0 7.5 (98.0%) (0.0) (0.77) (1.53) (2.30) (3.06) (4.59) (6.12) (7.65)
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR MgC0.5 MgC1.0 MgC1.5 MgC2.0 MgC3.0 MgC4.0 MgC5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) Magchem 40 0.0 0.75 1.5 2.25 3.0 4.5 6.0 7.5 (98.0%) (0.0) (0.77) (1.53) (2.30) (3.06) (4.59) (6.12) (7.65)
44 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.
45 Table 3.3 XNBR-DCP formulations
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR P0.25 P0.50 P0.75 P1.0 P1.5 P2.0 P3.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) Dicumyl - 0.25 peroxide 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.
46 Table 3.4 XNBR-CaO compositions
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR Ca0.5 Ca1.0 Ca1.5 Ca2.0 Ca3.0 Ca4.0 Ca5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR )
CaO UN1910 - 1.05 2.10 3.15 4.20 6.30 8.40 10.5 (98.0 %) (1.07) (2.14) (3.21) (4.29) (6.43) (8.57) (10.7)
Table 3.5 XNBR-Ca(OH)2 compositions
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR Ch0.5 Ch1.0 Ch1.5 Ch2.0 Ch3.0 Ch4.0 Ch5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) Ca(OH) 1.39 2.78 4.17 5.56 8.34 11.1 13.9 2 - (97.0 %) (1.43) (2.87) (4.30) (5.73) (8.60) (11.5) (14.3)
Table 3.6 XNBR-BaO compositions
XN- XN- XN- XN- XN- XN- XN- Ingredients XNBR Ba0.5 Ba1.0 Ba1.5 Ba2.0 Ba3.0 Ba4.0 Ba5.0
Nipol 1072 100 100 100 100 100 100 100 100 (XNBR ) BaO - 2.88 5.75 8.63 11.5 17.3 23.0 28.8 (99.99%)
Table 3.7 Designation and stoichiometric amount of metal oxides or compounds
Type of Metal Oxides Stoichiometric Amount Aa or Compounds (phr) Mg MgO 1.50 Ca CaO 2.10
Ch Ca(OH)2 2.87 Ba BaO 5.75 47 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.
48 Table 3.8 Cure times for compositions
Cure Time Compounds (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.
49 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
σ 2 C 2 = 2 C1 + (15) ⎡ 1 ⎤ λ λ − ⎢ 2 ⎥ ⎣ λ ⎦
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:
2C ν = 1 (16) RT
where R is the gas constant (8.314 J/mol.K), and T is the absolute temperature.
50 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
2 ⎛ 1 ⎞ [ln (1 − Vr ) + Vr + χ Vr ] ν = ⎜− ⎟ (17) ⎝ 2Vs ⎠ ⎛ 1 3 Vr ⎞ ⎜Vr − ⎟ ⎝ 2 ⎠
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
51 Vrubber Vr = (18) Vrubber + Vchloroform
Vchloroform and Vrubber can be calculated using equations 19 and 20, respectively.
M gel − Mi Vchloroform = (19) ρchloroform
M ⎛ f ⎞ dry ⎜ MO ⎟ Vrubber = Vdry − VMO = − Mi ⎜ ⎟ (20) ρdry ⎝ ρMO ⎠
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
3 (Mliq) was determined. The weight when immersed in ethanol (ρ =0.785 g/cm ) 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
ρliq M air ρdry = (21) M air − M liq
52 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.
53 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
54 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.
55 80 Testing temperature: 165 oC
70 XN-MgA4.0 XN-MgA5.0
XN-MgA3.0 60 XN-MgA2.0
50 XN-MgA1.5
40 Torque (dN.m) 30 XN-MgA1.0
20 XNBR
10 XN-MgA0.5
0 0 50 100 150 200 250 Time (min)
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)
56 100 o 90 Testing temperature: 165 C 80 XN-MgA5.0 XN-MgA4.0 70 60 XN-MgA3.0 50 XN-MgA2.0 40 XN-MgA1.5
30 XN-MgA1.0
20
Torque (dN.m) XNBR
XN-MgA0.5
10 9 8 7 6 5 1 10 100 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)
57 Testing temperature: 165 oC 70 XN-MgB5.0
XN-MgB4.0 60 XN-MgB3.0
50 XN-MgB2.0
40 XN-MgB1.5
Torque (dN.m) 30 XN-MgB1.0
20 XNBR
10 XN-MgB0.5
0 0 50 100 150 200 250 Time (min)
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)
58 100 90 Testing temperature: 165 oC 80 70 XN-MgB5.0
60 XN-MgB4.0
50 XN-MgB3.0
40 XN-MgB2.0
30 XN-MgB1.5
20
Torque (dN.m) Torque XN-MgB1.0 XNBR
10 9 XN-MgB0.5 8 7 6 5 1 10 100 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)
59 Testing temperature: 165 oC 70 XN-MgC5.0 XN-MgC4.0 60
XN-MgC3.0 50
XN-MgC2.0 40
XN-MgC1.5
Torque (dN.m) 30
XN-MgC1.0 20 XNBR XN-MgC0.5 10
0 0 50 100 150 200 250 Time (min)
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)
60 100 90 Testing temperature: 165 oC 80 70 XN-MgC5.0 60 XN-MgC4.0 50 XN-MgC3.0
40 XN-MgC2.0
30
20 Torque (dN.m)
10 9 8 XN-MgC1.5 7 XN-MgC1.0 6 XNBR XN-MgC0.5 5 110100 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)
61 70 Testing temperature 165 oC 60 A = 140 m2/g B = 65 m2/g 50 XN-MgA2.0 C = 45 m2/g 40 XN-MgB2.0
30 XN-MgC2.0
20
Torque (dN.m) XNBR
10 9 8 7 6
5 1 10 100 Time (min)
Figure 4.7 ODR curves of the XNBR cured with 2.0x stoichiometric amounts of different magnesium oxides at 165 oC.
62 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.
63 Testing temperature: 165 oC 100 XN-P3.0
XN-P2.0
80 XN-P1.5
60 XN-P1.0
Torque (dN.m) Torque XN-P0.75 40
XN-P0.50
20 XN-P0.25
XNBR
0 0 102030405060 Time (min)
Figure 4.8 ODR curves of XNBR cured with dicumyl peroxide at 165 oC
64 100 Extrapolated line from lower concentration of peroxide
80
Experiment
60 (dN.m) L
- M H 40 M ΔΜ =
20
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Amount of peroxide (phr)
Figure 4.9 Delta torque, ΔM = MH – ML, as a function of dicumyl peroxide content.
65 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.
66 45 ODR curves of XN-CaO compounds at 165 oC
40 XN-Ca5.0 XN-Ca3.0 XN-Ca4.0 35 XNBR
30 XN-Ca2.0 XN-Ca1.5 25 XN-Ca1.0 Torque (min) Torque 20 XN-Ca0.5
15
10
5 0 200 400 600 800 1000 Time (min)
Figure 4.10 ODR curves of XNBR cured with calcium oxide at 165 oC. (linear scale)
67 50 o ODR curves of XN-CaO compounds at 165 C 40
30
XN-Ca2.0
20 XN-Ca5.0
XN-Ca4.0
XNBR Torque (dN.m) Torque XN-Ca1.0 XN-Ca1.5 10 XN-Ca0.5
XN-Ca3.0
1 10 100 1000 Time (min)
Figure 4.11 ODR curves of XNBR cured with calcium oxide at 165 oC. (log-log scale)
68 XN-Ca(OH) compounds : 165 oC 2 100 XN-Ch4.0 XN-Ch3.0 XN-Ch5.0
80 XN-Ch2.0
XN-Ch1.5 60 Torque (dN.m) Torque
40
XN-Ch1.0 XNBR
20 XN-Ch0.5
0 50 100 150 200 250 Time (min)
Figure 4.12 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (linear scale)
69 XN-Ca(OH) compounds : 165 oC 2
100 XN-Ch5.0 90 XN-Ch4.0 80 XN-Ch3.0 70 60
50 XN-Ch1.5 40 XN-Ch2.0
30 XN-Ch1.0 Torque (dN.m) Torque 20 XNBR XN-Ch0.5
10 9 8 7 6 1 10 100 Time (min)
Figure 4.13 ODR curves of XNBR cured with calcium hydroxide at 165 oC. (log-log scale)
70 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.
71 90 Testing temperature : 165 oC
80 XN-Ba5.0
70 XN-Ba4.0
60 XN-Ba3.0
50 XN-Ba2.0
Torque (dN.m) Torque 40 XN-Ba1.5
30 XN-Ba1.0 XNBR
20 XN-Ba0.5
10
0 50 100 150 200 250 Time (min)
Figure 4.14 ODR curves of XNBR cured with barium oxide at 165 oC. (linear scale)
72 100 o 90 Testing temperature : 165 C 80 XN-Ba5.0 70 XN-Ba4.0 60 XN-Ba3.0
50 XN-Ba2.0
40 XN-Ba1.5
XN-Ba1.0 30
XNBR
Torque (dN.m) 20
XN-Ba0.5 10 9 8 7 110100 Time (min)
Figure 4.15 ODR curves of XNBR cured with barium oxide at 165 oC. (log-log scale)
73 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
0.0147 0.0389 0.0711 0.0827 0.0962 V r ± 0.0017 ± 0.0007 ± 0.0007 ± 0.0002 ± 0.0025
71.1 42.6 22.8 16.3 11.8 Sol content (%) ± 1.7 ± 0.1 ± 0.2 ± 0.1 ± 0.6
ν 0.47 0.69 3.63 4.45 6.38 (x 105 mol/cm3)* ± 0.05 ± 0.07 ± 0.04 ± 0.15 ± 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
O O O
2 R C OH R C O C RH+ 2O (25)
R = polymer chain
74 0.12 80 Raw XNBR thermally cured at 165 oC
70 0.10 V r 60
0.08
50
0.06 r
V 40 Sol content (%) 0.04 30
20 0.02 Sol content
10
0.00 0 200 400 600 800 1000 1200 Cure time (min)
Figure 4.16 Vr and sol content of thermally cured XNBR as a function of cure time.
75 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.
76 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)
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgA0.5 MgA1.0 MgA1.5 MgA2.0 MgA3.0 MgA4.0 MgA5.0
0.0389 0.0275 0.0512 0.0758 0.1028 0.1130 0.1136 0.1223 Vr ± ± ± ± ± ± ± ± 0.0007 0.0005 0.0003 0.0019 0.0002 0.0010 0.0014 0.0008
Sol content 42.6 64.5 39.6 8.8 6.4 5.6 5.2 5.2 (%) ± 0.1 ± 0.3 ± 2.0 ± 0.3 ± 0.1 ± 0.1 ± 0.2 ± 0.1
ν (x 105 0.69 1.46 4.31 6.91 11.5 15.9 18.6 20.0 mol/cm3)* ± 0.07 ± 0.36 ± 0.21 ± 0.30 ± 0.5 ± 0.6 ± 0.5 ± 0.8
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgB0.5 MgB1.0 MgB1.5 MgB2.0 MgB3.0 MgB4.0 MgB5.0
0.0389 0.0231 0.0389 0.0849 0.1098 0.1105 0.1104 0.1134 Vr ± ± ± ± ± ± ± ± 0.0007 0.0014 0.0005 0.0040 0.0007 0.0053 0.0027 0.0015
Sol content 42.6 73.0 52.9 7.2 6.4 5.6 4.8 4.6 (%) ± 0.1 ± 1.7 ± 0.5 ± 0.7 ± 0.1 ± 0.2 ± 0.1 ± 0.1
ν (x 105 0.69 0.57 1.50 2.81 7.39 9.00 15.7 18.1 mol/cm3)* ± 0.07 ± 0.06 ± 0.17 ± 0.59 ± 0.24 ± 0.86 ± 0.7 ± 0.5
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgC0.5 MgC1.0 MgC1.5 MgC2.0 MgC3.0 MgC4.0 MgC5.0
0.0389 0.0193 0.0319 0.0828 0.1011 0.0920 0.0900 0.1081 Vr ± ± ± ± ± ± ± ± 0.0007 0.0005 0.0007 0.0019 0.0047 0.0082 0.0018 0.0007
Sol content 42.6 72.8 63.3 9.5 7.7 7.1 6.4 6.3 (%) ± 0.1 ± 0.2 ± 1.0 ± 0.3 ± 0.3 ± 0.1 ± 0.1 ± 0.3
ν (x 105 0.69 0.23 0.97 2.65 4.44 8.57 15.1 19.5 mol/cm3)* ± 0.07 ± 0.02 ± 0.19 ± 0.40 ± 0.33 ± 0.32 ± 0.34 ± 0.25 * Determined by near equilibrium stress-strain measurement
77 0.14 90 XNBR-MgO compounds cured 120 min at 165 oC
80
0.12 Vr 70
0.10 60
50 0.08 XN-MgA
r XN-MgB V XN-MgC 40 opened is V 0.06 r Sol (%) content closed is sol content 30
0.04 20
Sol content 10 0.02
0 0123456 Amount of MgO/ Stoichiometric Amount
Figure 4.17 Vr and sol content of MgO-cured XNBR as a function of MgO concentration.
78 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
XN- XN- XN- XN- XN- XN- XN- Property XNBR P0.25 P0.5 P0.75 P1.0 P1.5 P2.0 P3.0
0.0147 0.0410 0.0713 0.0905 0.1098 0.1370 0.1552 0.1869 Vr ± ± ± ± ± ± ± ± 0.0017 0.0010 0.0015 0.0007 0.0008 0.0007 0.0005 0.0003
Sol content 71.1 34.7 19.6 13.9 10.4 7.7 6.0 5.2 (%) ± 1.7 ± 0.1 ± 0.3 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.2
ν(x 105 0.47 1.83 3.38 4.81 5.85 12.4 13.1 23.4 mol/cm3)* ± 0.05 ± 0.17 ± 0.24 ± 0.40 ± 0.47 ± 0.5 ± 0.4 ± 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.
79 80 XNBR-Peroxide compounds cured 60 min at 165 oC 0.20 70
Vr 60 0.16
50
0.12 40 r
V
30
0.08 (%) Sol content
20
0.04 10 Sol content
0.00 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Amount of peroxide (phr)
Figure 4.18 Vr and sol content of XNBR-Peroxide vulcanizates as a function of peroxide concentration.
80 Table 4.4 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium oxide 1000 min at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ca0.5 Ca1.0 Ca1.5 Ca2.0 Ca3.0 Ca4.0 Ca5.0
0.0962 0.0883 0.0806 0.0822 0.0849 0.0833 0.0869 0.0803 Vr ± ± ± ± ± ± ± ± 0.0025 0.0011 0.0011 0.0062 0.0021 0.0007 0.0004 0.0004
Sol content 11.8 12.9 14.6 15.1 15.1 13.5 13.8 14.9 (%) ± 0.6 ± 0.1 ± 0.3 ± 2.4 ± 1.3 ± 0.5 ± 0.1 ± 0.4
ν (x 105 6.38 6.33 5.13 4.20 5.94 4.37 4.47 4.21 mol/cm3)* ± 0.10 ± 0.17 ± 0.08 ± 0.12 ± 0.17 ± 0.26 ± 0.36 ± 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.
81 0.20 25 XN-Ca compounds cured 1000 min at 165 oC
0.16 20
0.12 Sol content 15 r V Vr 0.08 10 Sol content (%)
0.04 5
0.00 0 0123456 Amount of CaO/Stoichiometric Amount
Figure 4.19 Vr and sol content of XNBR-CaO vulcanizates as a function of CaO concentration.
82 Table 4.5 Volume fraction (Vr) of rubber, sol content, and crosslink density of XNBR cured with calcium hydroxide 240 min at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ch0.5 Ch1.0 Ch1.5 Ch2.0 Ch3.0 Ch4.0 Ch5.0
0.0711 0.0193 0.0316 0.0588 0.0717 0.0777 0.0880 0.0963 Vr ± ± ± ± ± ± ± ± 0.0007 0.0039 0.0011 0.0012 0.0008 0.0015 0.0004 0.0004
Sol content 22.8 66.4 41.7 12.8 9.4 7.2 6.3 5.4 (%) ± 0.2 ± 1.4 ± 0.3 ± 0.9 ± 0.2 ± 0.6 ± 0.2 ± 0.4
ν (x 105 3.63 5.34 14.7 29.1 30.6 32.1 32.5 26.7 mol/cm3)* ± 0.04 ± 0.08 ± 0.9 ± 0.8 ± 1.2 ± 0.8 ± 0.1 ± 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.
83 0.12 80 o XN-Ca(OH)2 compounds cured 240 min at 165 C
70 0.10
60 V r 0.08 50
0.06 40 r V
30 0.04 (%) Sol content
20
0.02 Sol content 10
0.00 0 0123456 Amount of Ca(OH) /Stoichiometric Amount 2
Figure 4.20 Vr and sol content of XNBR-Ca(OH)2 vulcanizates as a function of Ca(OH)2 concentration.
84 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
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ba0.5 Ba1.0 Ba1.5 Ba2.0 Ba3.0 Ba4.0 Ba5.0
0.0711 0.0344 0.0441 0.0491 0.0552 0.0588 0.0649 0.0684 Vr ± ± ± ± ± ± ± ± 0.0007 0.0015 0.0005 0.0005 0.0087 0.0002 0.0017 0.0010
Sol content 22.8 38.1 23.1 17.4 13.7 9.9 6.2 4.2 (%) ± 0.2 ± 2.5 ± 0.2 ± 0.1 ± 0.1 ± 0.1 ± 0.2 ± 0.8
ν (x 105 3.63 2.12 3.09 6.17 8.59 9.29 11.0 11.5 mol/cm3)* ± 0.04 ± 0.21 ± 0.47 ± 0.04 ± 0.31 ± 0.65 ± 0.7 ± 0.8 * Determined by near equilibrium stress-strain measurement
85 0.09 50 XNBR-BaO compounds cured 240 min at 165 oC
0.08 40
0.07
30 0.06 r V 0.05 20 Sol content (%)
0.04
10 0.03
0.02 0 0123456 Amount of BaO/Stoichiometric Amount
Figure 4.21 Vr and sol content of XNBR-BaO vulcanizates as a function of BaO concentration.
86 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.
87 0.14
0.12 XN-MgO
XN-Ca(OH) 0.10 2
XN-CaO 0.08 r
V XN-BaO
0.06
0.04
0.02
0123456 Amount of metal compounds/Stoichiometric amount
Figure 4.22 Vr of XNBR vulcanized by various metal compounds as a function of concentration.
88 80 XN-CaO XN-Ca(OH) 2 70 XN-BaO XN-MgO
60
50
40
30 Sol content (%) Sol content
20
10
0 0123456 Amount of metal compounds/Stoichiometric amount
Figure 4.23 Sol content of XNBR vulcanized by various metal compounds as a function of concentration.
89 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
Coordination Ionic Radii Type of Ion Number (Å)
4 0.71 Mg++ 6 0.86 8 1.03 6 1.14 Ca++ 8 1.26 6 1.49 Ba++ 8 1.56
90 The Coulombic force, F, between an anion of charge, q-, and a cation of charge, q+, is given by
q + q − F = (26) 2 4πε0 ε r
where r is the distance between the centers of the ions, ε is the dielectric constant of the
-12 2 2 medium, and ε0 is the permittivity of free space (8.8542 x 10 C /N.m ). 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.
91 Thermally cured raw XNBR at 165 oC 10
1000 min x 500 min 8 x x240 min
120 min x 6
Stress (MPa)
4
2
0 0 200 400 600 800 1000 1200 1400 1600 Strain (%)
Figure 4.24 Stress-strain curves of thermally cured XNBR.
92 11 1600 Raw XNBR thermally cured at 165 oC
10
9 Tensile strength 1400
8
7 1200
6 Elongation at break
1000 300% Modulus 1 Elongation at break (%) break Elongation at
800 300% Modulus or Tensile strength (MPa) strength Tensile Modulus or 300%
0 600 0 200 400 600 800 1000 Cure time (min)
Figure 4.25 Tensile properties of thermally cured XNBR as a function of cure time.
93 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.
94 60 XN-MgA vulcanizates cured 120 min at 165 oC
XN-MgA5.0 xx 50 x
XN-MgA4.0 x
40 XN-MgA3.0
x XN-MgA1.0
30 Stress (MPa) XN-MgA2.0 20
XN-MgA1.5 x XN-MgA0.5
10 XNBR x
0 0 400 800 1200 1600 Strain (%)
Figure 4.26 Stress-strain curves of XN-MgA vulcanizates (cured 120 min at 165 oC).
95 60 XN-MgB vulcanizates cured 120 min at 165 oC
x 50 x x
x XN-MgB2.0 40 x XN-MgB1.5
30
Stress (MPa) x XN-MgB1.0 XN-MgB5.0 20 XN-MgB4.0
XN-MgB3.0 XN-MgB0.5 x 10 XNBR x
0 0 400 800 1200 1600 Strain (%)
Figure 4.27 Stress-strain curves of XN-MgB vulcanizates (cured 120 min at 165 oC).
96 60 XN-MgC vulcanizates cured 120 min at 165 oC
50 x x x
x XN-MgC2.0 40 x XN-MgC1.5 XN-MgC4.0 30 x
Stress (MPa) XN-MgC1.0
20 XN-MgC5.0
XN-MgC3.0
10 XN-MgC0.5 x XNBR x
0 0 400 800 1200 1600 Strain (%)
Figure 4.28 Stress-strain curves of XN-MgC vulcanizates (cured 120 min at 165 oC).
97 100 1600 XNBR-MgO vulcanizates cured 120 min at 165 oC XN-MgA Tensile strength
1400 XN-MgC XN-MgB
1200 XN-MgA
10 1000 XN-MgB XN-MgC 300% Modulus
800 Elongation at break (%) at break Elongation
Elongation at break 600
300% Modulus or Tensile strength (MPa) strength 300% Tensile Modulus or 1 XN-MgC XN-MgA XN-MgB
400 0123456 Amount of MgO/Stiochiometric Amount
Figure 4.29 Tensile properties of XNBR cured with different magnesium oxides (cured 120 min at 165 oC).
98 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.
99 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.
100 10 XN-Peroxide vulcanizates cured 60 min at 165 oC
XN-P0.25 XN-P0.50 x 8 x
XN-P0.75 6 x
XN-P1.0 XNBR x not break Stress (MPa) Stress 4 XN-P1.5 x
XN-P2.0 XN-P3.0 x x 2
0 0 200 400 600 800 1000 1200 1400 1600 1800 Strain (%)
Figure 4.30 Stress-strain curves of XN-peroxide vulcanizates (cured 60 min at 165 oC).
101 10 1600 XN-Peroxide compounds cured 60 min at 165 oC 9 1400
8 1200 7
1000 6
Tensile strength 5 800
4 Elongation at break 600 3 Elongation at break (%) break at Elongation 400 2
300% Modulus or Tensile strength (MPa) strength Tensile or Modulus 300% 200 1 300%Modulus
0 0 0.00.51.01.52.02.53.03.5 Amount of dicumyl peroxide (phr)
Figure 4.31 Tensile properties of XN-peroxide vulcanizates (cured 60 min at 165 oC).
102 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
53, 56, 71 modulus of XNBR cured with Ca(OH)2. 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.
103 10 XN-Ca vulcanizates cured 1000 min at 165 oC XNCa1.5 x
XNCa3.0 x 8 x x x x XNCa2.0 x x
6 XNCa1.0
XN-Ca0.5 Stress (MPa) 4 XNBR XNCa4.0
2 XNCa5.0
0 0 100 200 300 400 500 600 700 800 Strain (%)
Figure 4.32 Stress-strain curves of XN-Ca vulcanizates (cured 1000 min at 165 oC).
104 10 1000 XN-CaO compounds cured 1000 min at 165 oC 9
8 900 Tensile strength 7
6 800 Elongation at break
5
4 700
3 Elongation at break (%) at break Elongation
2 600 300% Modulus 300% Modulus or Tensile strength (MPa) strength 300% Tensile Modulus or 1
0 500 0123456 Amount of CaO/Stoichiometric Amount
Figure 4.33 Tensile properties of XN-Ca vulcanizates (cured 1000 min at 165 oC).
105 60 XN-Ca(OH) compounds cured 240 min at 165 oC 2
50 x xxx x x 40 XN-Ch1.0
30 Stress (MPa) XN-Ch5.0 20 XN-Ch4.0 XN-Ch0.5 XN-Ch3.0 x
10 XNBR XN-Ch2.0 x XN-Ch1.5
0 0 200 400 600 800 1000 Strain (%)
Figure 4.34 Stress-strain curves of XN-Ch vulcanizates (cured 240 min at 165 oC).
106 100 1000 XN-Ca(OH) compounds cured 240 min at 165 oC 2
Tensile strength 900
300% Modulus 800
10 700
600 Elongation at break (%) Elongation at break
Elongation at break 500
300% Modulus or Tensile strength (MPa) strength Tensile or 300% Modulus 1
400 0123456 Amount of Ca(OH) /Stoichiometric Amount 2
Figure 4.35Tensile properties of XN-Ch vulcanizates (cured 240 min at 165 oC).
107 4.3.6 XNBR-BaO Vulcanizates
Figure 4.36 shows stress-strain curves of XNBR-BaO vulcanizates cured for 240
o min at 165 C. 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.
108 XN-BaO vulcanizates cured 240 min at 165 oC 30
XN-Ba4.0 x x x XN-Ba3.0 25
XN-Ba5.0 x XN-Ba2.0 20 x XN-Ba1.5
15 Stress (MPa) xXN-Ba1.0
10 XN-Ba0.5 XNBR x x
5
0 0 200 400 600 800 1000 Strain (%)
Figure 4.36 Stress-strain curves of XN-Ba vulcanizates (cured 240 min at 165 oC).
109 35 1100 XN-BaO vulcanizates cured 240 min at 165 oC
1000 30
Tensile strength 900 25
800 20
700 300% Modulus 15 600
10 Elongation (%) at break 500
5 300% (MPa) Modulus or Tensile strength 400 Elongation at break
0 300 0123456 Amount of BaO/Stoichiometric Amount
Figure 4.37 Tensile properties of XN-Ba vulcanizates (cured 240 min at 165 oC).
110 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
111 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.
112 25 Cured at 165 oC
XNBR-Ca(OH) 20 2
XNBR-BaO 15
XNBR-MgO 10 300% Modulus (MPa) 300% Modulus
5
XNBR-CaO
0 0123456 Amount of metal compounds/Stoichiometric amount
Figure 4.38 300% Modulus of the XNBR vulcanized by various metal compounds.
113 Cured at 165 oC 60
XNBR-MgO 50
XNBR-Ca(OH) 2
40
30 XNBR-BaO
20 Tensile strength (MPa) strength Tensile
10 XNBR-CaO
0 0123456 Amount of metal compounds/Stoichiometric amount
Figure 4.39 Tensile strength of the XNBR vulcanized by various metal compounds.
114 1600 Cured at 165 oC
1400
1200
1000
800 XNBR-CaO
600 Elongation at break (%) break at Elongation XNBR-MgO
400 XNBR-BaO
XNBR-Ca(OH) 2
200 0123456 Amount of metal compounds/Stoichiometric amount
Figure 4.40 Elongation at break of the XNBR vulcanized by various metal compounds.
115 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,
116 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
117 24 XN-MgA XN-MgB XN-MgC 20 XN-P XN-Ch XN-Ca XN-Ba 16 Ionic crosslinks Heated XN
12
8 300% Modulus (MPa) Modulus 300%
4 Covalent crosslinks
0 0.00 0.04 0.08 0.12 0.16 V r
Figure 4.41 300% Modulus of the XNBR vulcanized by different curing agents as a function of Vr.
118 80 70 XN-MgA 60 Ionic crosslinks XN-MgB 50 XN-MgC 40 XN-P XN-Ch 30 XN-Ca XN-Ba 20 Heated XN
10 9 8 7 6 Covalent crosslinks 5 Tensile strength (MPa) strength Tensile 4
3
2
0.00 0.04 0.08 0.12 0.16 0.20 V r
Figure 4.42 Tensile strength of the XNBR vulcanized by different curing agents as a function of Vr.
119 1600 XN-MgA XN-MgB 1400 XN-MgC XN-P XN-Ch XN-Ca 1200 XN-Ba Heated XN
1000 Covalent crosslinks
800
600 Elongation at break (%) at break Elongation
400 Ionic crosslinks
200 0.00 0.04 0.08 0.12 0.16 V r
Figure 4.43 Elongation at break of the XNBR vulcanized by different curing agents as a function of Vr.
120 920, 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.
121 CH CH2 O O CH2 CH2 O O 2 H C C C C C CH (26) H3C C C OH + HO C C CH3 3 3 O CH CH CH2 CH2 2 2
Isobutyric anhydride
+ 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 Assignment (cm-1) Out-of-plane vibration of the methylene hydrogen atom of 920 the vinyl group Out-of-plane vibration of the hydrogen atom of the 1,4-trans 965 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
122
Figure 4.44 ATR-IR spectra of uncured and thermally cured XNBR in the range 800 to 4000 cm-1.
123 0.6 Thermally cured raw XNBR at 165 oC uncured O H O cured 120 min C C 0.5 cured 240 min O H O cured 500 min 1697 cured 1000 min
0.4
H O C 0.3 O 1730 Absorbance 1670
0.2 1640
Anhydride crosslink 0.1 1803
0.0 1850 1800 1750 1700 1650 1600 1550 Wave number (cm-1)
Figure 4.45 ATR-IR spectra of uncured and thermally cured XNBR in the range 1550 to 1850 cm-1.
124 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
125 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 Assignment (cm-1) Out-of-plane vibration of the methylene hydrogen atom of 920 the vinyl group Out-of-plane vibration of the hydrogen atom of the 1,4-trans 965 component 1440 In-plane deformation of methylene group 1612 Carbonyl stretching of magnesium hydroxycarboxylate salt Asymmetric carbonyl stretching of magnesium carboxylate 1587 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
126 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.
127
Figure 4.46 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 800 to 4000 cm-1.
128 1.6 Uncured XN-MgA compounds O H O C C O O 1440 1.4 H 1697 H O O C C 1.2 O 1730 O MgOH 1612
1.0 XNBR
0.8 XN-MgA0.5 Absorbance
0.6 XN-MgA1.0
0.4 XN-MgA2.0
0.2 XN-MgA5.0
0.0 2000 1800 1600 1400 1200 Wave number (cm-1)
Figure 4.47 ATR-IR spectra of the uncured neat XNBR and XNBR-MgO compounds in the range 1200 to 2000 cm-1.
129
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).
130 1.6 XN-MgO vulcanizates cured 120 min at 165 o C O H O C C 1.4 O H O 1697 O C 1440
H O O MgOH C 1612 1.2 O O O + + 1730 C Mg C O O 1587 1.0 XNBR
0.8 XN-MgA0.5 Absorbance
0.6 XN-MgA1.0
0.4 XN-MgA2.0
0.2 XN-MgA5.0
0.0 2000 1800 1600 1400 1200 Wave number (cm-1)
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).
131 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.
132
Figure 4.50 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO compounds in the range 800 to 4000 cm-1.
133
0.6 Uncured XN-CaO compounds XNBR O H O XN-Ca0.5 C C 0.5 XN-Ca1.0 O H O XN-Ca2.0 1697 XN-Ca5.0
0.4
H O C O 0.3 1730 Absorbance
0.2
0.1
0.0 1850 1800 1750 1700 1650 1600 1550 Wave number (cm-1)
Figure 4.51 ATR-IR spectra of the uncured neat XNBR and XNBR-CaO compounds in the range 1550 to 1850 cm-1.
134
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).
135 o XN-CaO vulcanizates cured 1000 min at 165 C XNBR 0.5 O H O XN-Ca0.5 C C XN-Ca1.0 O H O XN-Ca2.0 1697 0.4 XN-Ca5.0
H O C 0.3 O 1732 Absorbance
0.2
Anhydride crosslink 0.1
0.0 1850 1800 1750 1700 1650 1600 1550 Wave number (cm-1)
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).
136
Cured 1000 min at 165 oC 0.5 O H O C C O Uncured XNBR O H 1697 Cured XNBR Uncured XN-Ca2.0 0.4 Cured XN-Ca2.0
H O C 0.3 O 1730 Absorbance
0.2
Anhydride crosslink 0.1
0.0 1850 1800 1750 1700 1650 1600 1550 Wave number (cm-1)
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). 137 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
-1 3642 cm , 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.
138 65, 66, 110, 111, 113 Table 4.10 Characteristic group frequencies of XNBR-Ca(OH)2 samples
Wave number Assignment (cm-1) Out-of-plane vibration of the methylene hydrogen atom of 920 the vinyl group Out-of-plane vibration of the hydrogen atom of the 1,4-trans 965 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
-1 carboxyl groups, evidenced by disappearance of the Ca(OH)2 peak at 3642 cm . 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.
139
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.
140 1.8 Uncured XN-Ca(OH) compounds 2 O H O 1.6 C C O H O 1440 H O 1697 1.4 C O 1730
1.2
XNBR 1.0
XN-Ch0.5 Absorbance 0.8
XN-Ch1.0 0.6
XN-Ch2.0 0.4
XN-Ch5.0 0.2
2000 1800 1600 1400 1200 Wave number (cm-1)
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.
141
Figure 4.57 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 800 to 4000 cm-1.
142 1.8 o XN-Ca(OH) vulcanizates cured 240 min at 165 C 2 O H O 1.6 C C O H O H O 1697 1440 C O O + + C 1.4 O C Ca 1730 O O 1410 1560 1.2
XNBR 1.0
Absorbance XN-Ch0.5 0.8
XN-Ch1.0 0.6
XN-Ch2.0 0.4
XN-Ch5.0 0.2
2000 1800 1600 1400 1200 Wave number (cm-1)
Figure 4.58 ATR-IR spectra of the neat XNBR and XNBR-Ca(OH)2 vulcanizates in the range 1200 to 2000 cm-1.
143 4.4.5 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
144 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 Assignment (cm-1) Out-of-plane vibration of the methylene hydrogen atom of 920 the vinyl group Out-of-plane vibration of the hydrogen atom of the 1,4-trans 965 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
145
Figure 4.59 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 800 to 4000 cm-1.
146 1.8 Uncured XN-Ba compounds O H O 1.6 C C O H O 1440 H O 1697 1.4 C O 1730
1.2
XNBR 1.0
XN-Ba0.5 Absorbance 0.8
XN-Ba1.0 0.6
XN-Ba2.0 0.4
XN-Ba5.0 0.2
2000 1800 1600 1400 1200 Wave number (cm-1)
Figure 4.60 ATR-IR spectra of the uncured neat XNBR and XNBR-BaO compounds in the range 1200 to 2000 cm-1.
147
Figure 4.61 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 800 to 4000 cm-1.
148
1.8 XN-BaO vulcanizates cured 240 min at 165 oC
O H O 1.6 C C 1440 O H O H O 1697 C O O 1.4 O C + Ba + C 1405 1730 O O 1546 1.2
XNBR 1.0
XN-Ba0.5 Absorbance 0.8
XN-Ba1.0 0.6
XN-Ba2.0 0.4
XN-Ba5.0 0.2
2000 1800 1600 1400 1200 Wave number (cm-1)
Figure 4.62 ATR-IR spectra of the neat XNBR and XNBR-BaO vulcanizates in the range 1200 to 2000 cm-1.
149 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
1 f 1 f (m + m ) ν′ = = 1 2 (26) 2πc μ 2πc m1 m 2 where f is the force constant of the bond (dyne/cm or g/s2), c is the velocity of light
10 110, 115 (2.998 x 10 cm/s), and m1, m2 are the masses (g) of atoms 1 and 2, respectively.
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
150 0.7
XNBR O O XN-MgA2.0 C + Ca + C 0.6 XN-Ca2.0 O O O O XN-Ch2.0 1560 C + Ba + C XN-Ba2.0 O O O H O 0.5 1546 C C O O O H O 1697 C + Mg + C 0.4 O O 1587 H O C O 0.3 1730 Absorbance
0.2 Anhydride crosslink 0.1
0.0
1850 1800 1750 1700 1650 1600 1550 1500 Wave number (cm-1)
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.
151 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
152 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
153 XN-MgA vulcanizates cured 120 min at 165 oC
109
108 E' (Pa)
XN-MgA1.0 XN-MgA2.0 107 XN-MgA3.0
106 XN-P1.0
-100 -50 0 50 100 150 Temperature (oC)
Figure 4.64 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz.
154
XN-MgA vulcanizates cured 120 min at 165 oC
108
107 E" (Pa)
XN-MgA1.0 XN-MgA2.0 XN-MgA3.0 106
XN-P1.0
105 -100 -50 0 50 100 150 Temperature (oC)
Figure 4.65 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz. 155
Figure 4.66 Schematic drawing of ion hopping mechanisms (opened and closed circles represent ion pairs).121
156 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.
157 XN-MgA vulcanizates cured 120 min at 165 oC 1 0.9 0.8 0.7 0.6 0.5 XN-MgA2.0 0.4 XN-MgA3.0 0.3 XN-MgA1.0
δ 0.2 tan
0.1 0.09 0.08 0.07 0.06 XN-P1.0 0.05 0.04
0.03
0.02 -100 -50 0 50 100 150 Temperature (oC)
Figure 4.67 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-MgA vulcanizates at frequency 1.0 Hz.
158 Table 4.12 Molecular transition temperatures of XN-P1.0, and XN-MgA vulcanizates at a frequency of 1.0 Hz
Ionic transition temperature Vulcanizates T (oC)* g 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
159 Frequency 1.0 Hz XN-MgA1.0 XN-MgC1.0 109 XN-MgA2.0 XN-MgC2.0 XN-P1.0 A = 140 m2/g C = 45 m2/g
108 E' (Pa)
XN-MgA2.0
XN-MgC2.0 107 XN-MgA1.0
XN-P1.0 106 XN-MgC1.0 -100 -50 0 50 100 150 Temperature (oC)
Figure 4.68 Effect of specific surface area on dynamic storage modulus (E′) of XN-Mg vulcanizates at frequency 1.0 Hz.
160 Frequency 1.0 Hz XN-MgA1.0 XN-MgC1.0 108 XN-MgA2.0 XN-MgC2.0 XN-P1.0 A = 140 m2/g C = 45 m2/g
107 E" (Pa)
XN-MgA2.0
XN-MgC2.0 106
XN-MgC1.0 XN-MgA1.0 XN-P1.0
105 -100 -50 0 50 100 150 Temperature (oC)
Figure 4.69 Effect of specific surface area on dynamic loss modulus (E″) of XN-Mg vulcanizates at frequency 1.0 Hz.
161 Frequency 1.0 Hz XN-MgA1.0 XN-MgC1.0 XN-MgA2.0 1 XN-MgC2.0 XN-P1.0 A = 140 m2/g C = 45 m2/g XN-MgA1.0 XN-MgC1.0 δ tan
XN-MgC2.0 0.1 XN-MgA2.0
XN-P1.0
-100 -50 0 50 100 150 Temperature (oC)
Figure 4.70 Effect of specific surface area on loss tangent (tan δ) of XN-Mg vulcanizates at frequency 1.0 Hz.
162 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. 163 XN-CaO vulcanizates cured 1000 min at 165 oC
109
108 E' (Pa)
107
XN-P1.0 XN-Ca3.0
106 XN-Ca1.0 XN-Ca2.0
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.71 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz.
164 XN-CaO vulcanizates cured 1000 min at 165 oC
108
107 E" (Pa)
106 XN-Ca2.0 XN-Ca3.0
5 XN-P1.0 10 XN-Ca1.0
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.72 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz.
165 XN-CaO vulcanizates cured 1000 min at 165 oC
XN-P1.0 1 XN-Ca1.0 XN-Ca2.0 XN-Ca3.0 δ tan
0.1
-100 -50 0 50 100 150 200 o Temperature ( C)
Figure 4.73 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ca vulcanizates at frequency 1.0 Hz
166 Table 4.13 Molecular transition temperatures of XN-P1.0, and XN-CaO vulcanizates at frequency 1.0 Hz
Ionic transition temperature Vulcanizates T (oC) g 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
167 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.
168 o XN-Ca(OH)2 vulcanizates cured 240 min at 165 C
109
108
XN-Ch3.0 E' (Pa) XN-Ch2.0
107
XN-Ch1.0
XN-P1.0
106
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.74 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
169 o XN-Ca(OH)2 vulcanizates cured 240 min at 165 C
108
107
XN-Ch3.0 E" (Pa)
XN-Ch2.0
106 XN-Ch1.0
XN-P1.0 105
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.75 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
170 o XN-Ca(OH)2 vulcanizates cured 240 min at 165 C
1
XN-Ch1.0 δ
tan XN-Ch2.0
XN-Ch3.0 0.1 XN-P1.0
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.76 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ch vulcanizates at frequency 1.0 Hz.
171 Table 4.14 Molecular transition temperatures of XN-P1.0 and XN-Ca(OH)2 vulcanizates at frequency 1.0 Hz
Ionic transition temperature Vulcanizates T (oC) g 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
172 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
Ionic transition temperature Vulcanizates T (oC) g 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
173 XN-BaO vulcanizates cured 240 min at 165 oC
109
108 E' (Pa) XN-Ba3.0
XN-Ba2.0 107
XN-Ba1.0
106 XN-P1.0
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.77 Temperature dependence of dynamic storage modulus (E′) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz.
174 XN-BaO vulcanizates cured 240 min at 165 oC
108
107 E" (Pa) XN-Ba3.0 XN-Ba2.0
6 10 XN-Ba1.0
XN-P1.0 105
-100 -50 0 50 100 150 200 Temperature (oC)
Figure 4.78 Temperature dependence of dynamic loss modulus (E″) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz
175 XN-BaO vulcanizates cured 240 min at 165 oC
1
XN-Ba2.0
XN-Ba1.0 δ tan XN-Ba3.0
0.1 XN-P1.0
-100 -50 0 50 100 150 200 o Temperature ( C)
Figure 4.79 Temperature dependence of loss tangent (tan δ) of XN-P1.0 and XN-Ba vulcanizates at frequency 1.0 Hz.
176 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.
177 Frequency 1.0 Hz
1
XN-MgA2.0 XN-Ba2.0 δ
tan XN-Ca2.0
0.1 XN-P1.0
XN-Ch2.0
-100 -50 0 50 100 150 200 Temperature (oC)
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.
178 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.
179 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.
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189
APPENDICES
190 APPENDIX A
CURE PROPERTIES
Table A.1 Cure properties of XNBR cured with different magnesium oxides at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgA0.5 MgA1.0 MgA1.5 MgA2.0 MgA3.0 MgA4.0 MgA5.0
ML (dN.m) 7.3 6.6 7.6 8.4 8.5 9.9 10.2 11.6
MH (dN.m) 14.5 13.2 28.6 47.2 57.1 61.2 64.9 67.1
tS2 (min) 6.0 28.6 4.0 57.1 3.5 2.5 2.5 2.0
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgB0.5 MgB1.0 MgB1.5 MgB2.0 MgB3.0 MgB4.0 MgB5.0
ML (dN.m) 7.3 6.6 6.9 7.0 7.0 7.7 8.8 10.5
MH (dN.m) 14.5 13.0 24.9 38.9 47.0 56.1 62.3 66.9
tS2 (min) 6.0 50.0 22.0 14.0 11.0 8.0 3.5 2.5
XN- XN- XN- XN- XN- XN- XN- Property XNBR MgC0.5 MgC1.0 MgC1.5 MgC2.0 MgC3.0 MgC4.0 MgC5.0
ML (dN.m) 7.3 6.6 6.5 6.5 6.7 6.6 7.9 8.6
MH (dN.m) 14.5 10.7 19.9 33.9 42.2 53.2 61.4 65.3
tS2 (min) 6.0 120.0 50.0 25.0 18.0 10.0 5 3
191 Table A.2 Cure properties of XNBR cured by dicumyl peroxide at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR P0.25 P0.5 P0.75 P1.0 P1.5 P2.0 P3.0
ML (dN.m) 7.3 9.1 8.8 8.4 8.5 8.0 8.8 9.1
MH (dN.m) 14.5 19.0 31.1 41.3 52.6 71.8 88.0 98.8
MH - ML (dN.m) 3.5 9.9 22.3 32.9 44.1 63.8 79.2 89.7
tS2 (min) 6.0 6.0 3.0 2.5 2.0 1.8 1.5 1.3
Table A.3 Cure properties of XNBR cured with calcium oxide at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ca0.5 Ca1.0 Ca1.5 Ca2.0 Ca3.0 Ca4.0 Ca5.0
ML (dN.m) 10.8 9.1 9.6 8.5 10.5 9.2 9.1 8.8
MH (dN.m) 38.2 32.6 36.0 36.1 37.4 36.0 38.5 39.8
tS2 (min) 6 65 45 55 40 55 55 53
Table A.4 Cure properties of XNBR cured with calcium hydroxide at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ch0.5 Ch1.0 Ch1.5 Ch2.0 Ch3.0 Ch4.0 Ch5.0
ML (dN.m) 10.8 10.1 8.9 10.0 10.2 8.7 9.5 11.0
MH (dN.m) 24.5 18.7 29.6 74.0 76.5 87.8 91.9 94.9
tS2 (min) 6 14 13 10 10.5 6.5 2.5 3.0
Table A.5 Cure properties of XNBR cured with barium oxide at 165 oC
XN- XN- XN- XN- XN- XN- XN- Property XNBR Ba0.5 Ba1.0 Ba1.5 Ba2.0 Ba3.0 Ba4.0 Ba5.0
ML (dN.m) 10.8 8.8 9.9 10.9 9.9 11.0 9.6 11.6
MH (dN.m) 24.5 18.8 27.4 34.7 44.8 54.3 65.1 75.2
tS2 (min) 6 9 15 20 7 8 5 6
192 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 > 4.20 TS (MPa) 6.46 ± 0.15 7.79 ± 0.13 8.19 ± 0.18 9.08 ± 0.14 ± 0.56 > 1655 EB (%) 1420 ± 14 939 ± 13 827 ± 3 751 ± 2 ± 52
193 Table B.2 Tensile properties at room temperature (~25 oC) of XN-MgA vulcanizates (cure time 30 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR MgA0.5 MgA1.0 MgA1.5 MgA2.0 MgA3.0 MgA4.0 MgA5.0
25 % Mod. 0.29 0.83 0.86 1.40 1.40 1.43 1.52 1.61 (MPa) ± 0.01 ± 0.03 ± 0.02 ± 0.12 ± 0.03 ± 0.06 ± 0.05 ± 0.03
50 % Mod. 0.41 1.10 1.24 2.08 2.20 2.28 2.41 2.52 (MPa) ± 0.01 ± 0.04 ± 0.02 ± 0.16 ± 0.04 ± 0.05 ± 0.05 ± 0.04
75 % Mod. 0.46 1.25 1.48 2.55 2.80 2.94 3.09 3.29 (MPa) ± 0.01 ± 0.05 ± 0.02 ± 0.19 ± 0.04 ± 0.05 ± 0.05 ± 0.06
100 % Mod. 0.48 1.34 1.66 2.95 3.33 3.56 3.72 3.97 (MPa) ± 0.01 ± 0.05 ± 0.02 ± 0.22 ± 0.05 ± 0.07 ± 0.06 ± 0.08
150 % Mod. 0.50 1.43 1.98 3.55 4.38 4.83 5.04 5.41 (MPa) ± 0.01 ± 0.05 ± 0.02 ± 0.30 ± 0.09 ± 0.12 ± 0.10 ± 0.14
200 % Mod. 0.50 1.50 2.31 4.60 5.60 6.32 6.53 7.01 (MPa) ± 0.02 ± 0.05 ± 0.02 ± 0.30 ± 0.12 ± 0.15 ± 0.16 ± 0.20
250 % Mod. 0.50 1.57 2.72 5.77 7.19 8.10 8.35 8.94 (MPa) ± 0.02 ± 0.04 ± 0.04 ± 0.42 ± 0.17 ± 0.26 ± 0.21 ± 0.28
300 % Mod. 0.50 1.64 3.24 7.34 9.44 10.4 10.8 11.5 (MPa) ± 0.02 ± 0.04 ± 0.07 ± 0.59 ± 0.28 ± 0.4 ± 0.3 ± 0.4
350 % Mod. 0.50 1.73 3.94 9.67 12.7 13.8 13.6 14.9 (MPa) ± 0.02 ± 0.05 ± 0.12 ± 0.86 ± 0.4 ± 0.6 ± 1.5 ± 0.7
400 % Mod. 0.50 1.86 4.93 13.0 17.6 18.3 19.0 19.4 (MPa) ± 0.02 ± 0.04 ± 0.19 ± 1.3 ± 0.6 ± 1.0 ± 0.7 ± 1.1
450 % Mod. 0.51 2.03 6.53 17.8 24.1 24.6 25.1 25.3 (MPa) ± 0.03 ± 0.05 ± 0.36 ± 1.9 ± 0.8 ± 1.5 ± 1.0 ± 1.3
500 % Mod. 0.52 2.27 9.13 24.2 32.5 32.5 32.8 32.7 (MPa) ± 0.03 ± 0.06 ± 0.55 ± 2.6 ± 1.2 ± 2.1 ± 1.0 ± 1.8 Not TS 13.5 28.5 43.5 46.5 48.4 48.6 52.8 break ± 0.5 ± 0.9 ± 2.4 ± 1.7 ± 0.5 ± 2.4 ± 2.0 (MPa) >3.60 Not EB 830 674 614 572 584 583 611 break ± 12 ± 13 ± 15 ± 11 ± 12 ± 15 ± 14 (%) >1600
194 Table B.3 Tensile properties at room temperature (~25 oC) of XN-MgA vulcanizates (cure time 120 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR MgA0.5 MgA1.0 MgA1.5 MgA2.0 MgA3.0 MgA4.0 MgA5.0
25 % Mod. 0.34 0.57 0.95 1.25 1.43 1.72 1.79 1.72 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.03 ± 0.01 ± 0.05 ± 0.01 ± 0.02
50 % Mod. 0.46 0.76 1.40 1.85 2.07 2.42 2.51 2.59 (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.03 ± 0.01 ± 0.07 ± 0.01 ± 0.01
75 % Mod. 0.51 0.86 1.70 2.20 2.68 3.13 3.25 3.22 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.03 ± 0.01 ± 0.09 ± 0.03 ± 0.03
100 % Mod. 0.54 0.93 1.90 2.51 3.14 3.66 3.79 3.95 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.05 ± 0.02 ± 0.11 ± 0.03 ± 0.04
150 % Mod. 0.56 1.02 2.39 3.28 4.14 4.98 5.17 5.27 (MPa) ± 0.01 ± 0.03 ± 0.04 ± 0.07 ± 0.02 ± 0.16 ± 0.05 ± 0.08
200 % Mod. 0.58 1.08 2.88 4.21 5.46 6.67 6.79 6.89 (MPa) ± 0.01 ± 0.03 ± 0.06 ± 0.12 ± 0.05 ± 0.22 ± 0.08 ± 0.12
250 % Mod. 0.59 1.15 3.48 5.51 7.21 8.74 8.94 8.97 (MPa) ± 0.01 ± 0.03 ± 0.07 ± 0.20 ± 0.05 ± 0.27 ± 0.15 ± 0.35
300 % Mod. 0.61 1.23 4.30 7.30 9.67 11.6 11.7 11.7 (MPa) ± 0.01 ± 0.03 ± 0.09 ± 0.31 ± 0.10 ± 0.3 ± 0.3 ± 0.3
350 % Mod. 0.63 1.33 5.51 10.4 13.0 15.9 15.6 15.8 (MPa) ± 0.01 ± 0.04 ± 0.13 ± 0.5 ± 0.2 ± 0.5 ± 0.3 ± 0.5
400 % Mod. 0.65 1.46 7.30 15.6 18.1 21.8 21.2 21.1 (MPa) ± 0.01 ± 0.04 ± 0.16 ± 0.9 ± 0.3 ± 0.6 ± 0.5 ± 0.7
450 % Mod. 0.68 1.63 10.3 23.1 25.6 30.2 29.4 29.2 (MPa) ± 0.01 ± 0.06 ± 0.3 ± 1.5 ± 0.3 ± 1.0 ± 0.8 ± 0.8
500 % Mod. 0.71 1.85 15.0 33.2 35.3 40.0 39.1 38.0 (MPa) ± 0.01 ± 0.06 ± 0.4 ± 2.2 ± 0.3 ± 1.4 ± 1.0 ± 1.0
TS 6.46 15.9 34.8 44.7 48.3 51.5 51.4 51.5 (MPa) ± 0.15 ± 0.5 ± 1.2 ± 1.0 ± 1.1 ± 1.7 ± 1.2 ± 1.0
EB 1420 872 635 549 553 548 552 561 (%) ± 14 ± 7 ± 5 ± 3 ± 16 ± 8 ± 7 ± 20
195 Table B.4 Tensile properties at room temperature (~25 oC) of XN-MgB vulcanizates (cure time 30 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR MgB0.5 MgB1.0 MgB1.5 MgB2.0 MgB3.0 MgB4.0 MgB5.0
25 % Mod. 0.29 0.44 0.70 0.94 1.15 1.49 1.63 1.58 (MPa) ± 0.01 ± 0.02 ± 0.02 ± 0.01 ± 0.02 ± 0.04 ± 0.02 ± 0.05
50 % Mod. 0.41 0.62 1.01 1.35 1.71 2.35 2.50 2.57 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.01 ± 0.05 ± 0.03 ± 0.06 ± 0.03
75 % Mod. 0.46 0.70 1.19 1.62 2.12 3.00 3.18 3.36 (MPa) ± 0.01 ± 0.02 ± 0.01 ± 0.03 ± 0.06 ± 0.05 ± 0.06 ± 0.02
100 % Mod. 0.48 0.76 1.32 1.83 2.47 3.58 3.83 4.09 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.04 ± 0.0 ± 0.07 ± 0.07 ± 0.03
150 % Mod. 0.50 0.83 1.53 2.22 3.15 4.72 5.04 5.49 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.05 ± 0.09 ± 0.09 ± 0.07 ± 0.05
200 % Mod. 0.50 0.90 1.78 2.65 3.91 5.96 6.40 6.99 (MPa) ± 0.02 ± 0.02 ± 0.05 ± 0.08 ± 0.12 ± 0.13 ± 0.10 ± 0.05
250 % Mod. 0.50 0.96 2.01 3.17 4.84 7.53 8.02 8.73 (MPa) ± 0.02 ± 0.02 ± 0.02 ± 0.11 ± 0.16 ± 0.18 ± 0.18 ± 0.05
300 % Mod. 0.50 1.03 2.35 3.86 6.09 9.63 10.2 11.0 (MPa) ± 0.02 ± 0.02 ± 0.03 ± 0.13 ± 0.26 ± 0.29 ± 0.3 ± 0.1
350 % Mod. 0.50 1.13 2.82 4.79 7.90 12.6 13.3 14.2 (MPa) ± 0.02 ± 0.03 ± 0.05 ± 0.21 ± 0.39 ± 0.4 ± 0.4 ± 0.2
400 % Mod. 0.50 1.26 3.47 6.17 10.7 17.0 17.8 18.8 (MPa) ± 0.02 ± 0.04 ± 0.08 ± 0.32 ± 0.6 ± 0.8 ± 0.7 ± 0.3
450 % Mod. 0.51 1.45 4.49 8.36 14.7 22.9 23.8 25.1 (MPa) ± 0.03 ± 0.05 ± 0.15 ± 0.52 ± 0.9 ± 1.5 ± 1.1 ± 0.4
500 % Mod. 0.52 1.70 6.21 11.7 20.4 30.6 31.6 33.2 (MPa) ± 0.03 ± 0.07 ± 0.24 ± 0.8 ± 1.3 ± 1.6 ± 1.4 ± 0.5 Not TS 5.96 17.2 26.0 33.3 40.1 45.8 46.0 break ± 0.24 ± 0.5 ± 1.3 ± 1.5 ± 0.5 ± 1.6 ± 2.1 (MPa) >3.60 Not EB 724 642 619 586 553 575 567 break ± 12 ± 12 ± 11 ± 11 ± 11 ± 15 ± 14 (%) >1600
196 Table B.5 Tensile properties at room temperature (~25 oC) of XN-MgB vulcanizates (cure time 120 min)
XN- XN- XN- XN- XN- XN- XN- XNBR Properties MgB0.5 MgB1.0 MgB1.5 MgB2.0 MgB3.0 MgB4.0 MgB5.0
25 % Mod. 0.34 0.62 0.97 1.24 1.41 1.61 1.71 1.71 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.04 ± 0.03 ± 0.05 ± 0.09 ± 0.07
50 % Mod. 0.46 0.87 1.42 1.88 2.20 2.62 2.68 2.87 (MPa) ± 0.01 ± 0.02 ± 0.01 ± 0.05 ± 0.06 ± 0.12 ± 0.12 ± 0.08
75 % Mod. 0.51 1.01 1.71 2.35 2.80 3.49 3.48 3.74 (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.06 ± 0.07 ± 0.17 ± 0.12 ± 0.08
100 % Mod. 0.54 1.10 1.95 2.76 3.32 4.28 4.20 4.60 (MPa) ± 0.01 ± 0.01 ± 0.02 ± 0.07 ± 0.07 ± 0.22 ± 0.12 ± 0.08
150 % Mod. 0.56 1.25 2.39 3.54 4.33 5.76 5.61 6.19 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.10 ± 0.08 ± 0.32 ± 0.16 ± 0.10
200 % Mod. 0.58 1.38 2.87 4.42 5.48 7.37 7.13 7.85 (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.16 ± 0.11 ± 0.46 ± 0.22 ± 0.12
250 % Mod. 0.59 1.54 3.47 5.53 6.93 9.32 9.01 9.82 (MPa) ± 0.01 ± 0.03 ± 0.04 ± 0.27 ± 0.14 ± 0.68 ± 0.34 ± 0.17
300 % Mod. 0.61 1.74 4.27 7.09 8.85 11.9 11.5 12.3 (MPa) ± 0.01 ± 0.05 ± 0.06 ± 0.42 ± 0.23 ± 0.9 ± 0.5 ± 0.3
350 % Mod. 0.63 2.01 5.43 9.42 11.7 15.5 15.1 15.8 (MPa) ± 0.01 ± 0.05 ± 0.07 ± 0.65 ± 0.4 ± 1.5 ± 0.7 ± 0.5
400 % Mod. 0.65 2.38 7.21 13.0 15.8 20.5 20.2 20.7 (MPa) ± 0.01 ± 0.08 ± 0.09 ± 0.9 ± 0.7 ± 2.2 ± 1.0 ± 0.8
450 % Mod. 0.68 2.92 10.1 18.1 21.5 27.1 27.0 27.2 (MPa) ± 0.01 ± 0.15 ± 0.2 ± 1.4 ± 1.1 ± 3.3 ± 1.5 ± 1.2
500 % Mod. 0.71 3.77 14.3 24.9 28.9 35.4 35.7 35.5 (MPa) ± 0.01 ± 0.24 ± 0.4 ± 2.1 ± 1.2 ± 4.4 ± 1.9 ± 1.6
TS 6.46 13.6 25.2 38.7 42.3 47.9 49.3 51.3 (MPa) ± 0.15 ± 0.5 ± 0.9 ± 2.8 ± 2.7 ± 1.7 ± 1.9 ± 1.3
EB 1420 687 588 579 571 563 568 579 (%) ± 14 ± 12 ± 9 ± 9 ± 14 ± 28 ± 13 ± 15
197 Table B.6 Tensile properties at room temperature (~25 oC) of XN-MgC vulcanizates (cure time 30 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR MgC0.5 MgC1.0 MgC1.5 MgC2.0 MgC3.0 MgC4.0 MgC5.0
25 % Mod. 0.29 0.32 0.46 0.83 0.98 1.34 1.34 1.56 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.04 ± 0.02 ± 0.03
50 % Mod. 0.41 0.44 0.62 1.14 1.45 2.05 1.91 2.28 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.03 ± 0.03 ± 0.04
75 % Mod. 0.46 0.49 0.71 1.34 1.75 2.57 2.36 2.85 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.04 ± 0.03 ± 0.04
100 % Mod. 0.48 0.52 0.76 1.49 1.99 3.04 2.88 3.38 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.04 ± 0.03 ± 0.03
150 % Mod. 0.50 0.53 0.81 1.73 2.45 3.99 3.80 4.54 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.03 ± 0.03 ± 0.06 ± 0.03 ± 0.03
200 % Mod. 0.50 0.53 0.85 2.00 2.96 5.05 4.85 5.72 (MPa) ± 0.02 ± 0.01 ± 0.01 ± 0.03 ± 0.03 ± 0.09 ± 0.02 ± 0.03
250 % Mod. 0.50 0.53 0.90 2.31 3.58 6.36 6.19 7.43 (MPa) ± 0.02 ± 0.01 ± 0.02 ± 0.04 ± 0.04 ± 0.14 ± 0.02 ± 0.04
300 % Mod. 0.50 0.53 0.95 2.71 4.39 8.07 7.99 9.55 (MPa) ± 0.02 ± 0.01 ± 0.02 ± 0.05 ± 0.07 ± 0.22 ± 0.03 ± 0.09
350 % Mod. 0.50 0.53 1.02 3.24 5.49 10.5 10.8 12.6 (MPa) ± 0.02 ± 0.01 ± 0.02 ± 0.07 ± 0.13 ± 0.4 ± 0.1 ± 0.1
400 % Mod. 0.50 0.54 1.12 3.96 7.12 14.0 15.0 17.3 (MPa) ± 0.02 ± 0.01 ± 0.03 ± 0.10 ± 0.26 ± 0.7 ± 0.1 ± 0.2
450 % Mod. 0.51 0.54 1.24 5.04 9.64 19.1 21.6 23.7 (MPa) ± 0.03 ± 0.01 ± 0.04 ± 0.15 ± 0.44 ± 1.1 ± 0.2 ± 0.2
500 % Mod. 0.52 0.54 1.41 6.77 13.4 25.6 30.6 32.2 (MPa) ± 0.03 ± 0.01 ± 0.05 ± 0.29 ± 0.7 ± 1.6 ± 0.5 ± 0.2 Not Not TS 6.3 18.9 28.9 40.4 37.3 46.4 break break ± 0.2 ± 2.0 ± 1.4 ± 1.3 ± 1.2 ± 1.4 (MPa) >3.60 >1.70 Not Not EB 834 651 624 588 537 570 break break ± 14 ± 14 ± 13 ± 17 ± 7 ± 6 (%) >1600 >1600
198 Table B.7 Tensile properties at room temperature (~25 oC) of XN-MgC vulcanizates (cure time 120 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR MgC0.5 MgC1.0 MgC1.5 MgC2.0 MgC3.0 MgC4.0 MgC5.0
25 % Mod. 0.34 0.46 0.89 1.20 1.33 1.54 1.46 1.51 (MPa) ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.03 ± 0.03 ± 0.06 ± 0.03
50 % Mod. 0.46 0.63 1.30 1.78 2.06 2.43 2.16 2.32 (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.02 ± 0.03 ± 0.04 ± 0.05 ± 0.02
75 % Mod. 0.51 0.73 1.56 2.17 2.61 3.14 2.72 2.96 (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.02 ± 0.03 ± 0.07 ± 0.05 ± 0.02
100 % Mod. 0.54 0.78 1.75 2.51 3.10 3.78 3.23 3.60 (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.02 ± 0.04 ± 0.08 ± 0.05 ± 0.02
150 % Mod. 0.56 0.84 2.10 3.17 4.08 5.05 4.33 4.84 (MPa) ± 0.01 ± 0.02 ± 0.02 ± 0.03 ± 0.06 ± 0.12 ± 0.05 ± 0.03
200 % Mod. 0.58 0.89 2.49 3.91 5.18 6.42 5.58 6.19 (MPa) ± 0.01 ± 0.02 ± 0.04 ± 0.06 ± 0.09 ± 0.17 ± 0.07 ± 0.05
250 % Mod. 0.59 0.94 2.97 4.81 6.54 8.09 7.06 7.70 (MPa) ± 0.01 ± 0.02 ± 0.05 ± 0.09 ± 0.13 ± 0.26 ± 0.10 ± 0.08
300 % Mod. 0.61 1.00 3.60 5.98 8.36 10.3 9.04 9.79 (MPa) ± 0.01 ± 0.02 ± 0.07 ± 0.14 ± 0.22 ± 0.5 ± 0.13 ± 0.13
350 % Mod. 0.63 1.06 4.46 7.72 11.0 13.4 11.1 12.7 (MPa) ± 0.01 ± 0.03 ± 0.12 ± 0.21 ± 0.4 ± 0.8 ± 0.1 ± 0.2
400 % Mod. 0.65 1.16 5.77 10.3 14.8 17.6 16.4 16.6 (MPa) ± 0.01 ± 0.03 ± 0.20 ± 0.4 ± 0.6 ± 1.1 ± 0.2 ± 0.4
450 % Mod. 0.68 1.29 7.88 14.2 20.1 23.3 21.1 22.2 (MPa) ± 0.01 ± 0.04 ± 0.34 ± 0.6 ± 1.03 ± 1.6 ± 0.3 ± 0.6
500 % Mod. 0.71 1.45 11.1 19.7 26.9 30.7 30.2 29.2 (MPa) ± 0.01 ± 0.06 ± 0.5 ± 0.9 ± 1.5 ± 2.2 ± 0.3 ± 0.7
TS 6.46 7.5 26.6 36.2 43.3 49.6 47.4 48.5 (MPa) ± 0.15 ± 1.4 ± 1.2 ± 2.0 ± 2.2 ± 2.0 ± 0.2 ± 0.9
EB 1420 815 637 605 592 600 581 607 (%) ± 14 ± 14 ± 5 ± 15 ± 12 ± 13 ± 3 ± 6
199 Table B.8 Tensile properties at room temperature (~25 oC) of XNBR-peroxide vulcanizates (cure time 60 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR P0.25 P0.50 P0.75 P1.0 P1.5 P2.0 P3.0
25 % Mod. 0.28 0.35 0.38 0.37 0.38 0.43 0.48 0.57 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01
50 % Mod. 0.37 0.48 0.52 0.53 0.58 0.67 0.75 0.93 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02
75 % Mod. 0.42 0.55 0.60 0.63 0.69 0.82 0.92 1.22 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02
100 % Mod. 0.44 0.59 0.66 0.71 0.77 0.94 1.07 1.48 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01 ± 0.02 ± 0.04
150 % Mod. 0.46 0.64 0.74 0.82 0.91 1.16 1.34 2.04 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.02 ± 0.01 ± 0.06
200 % Mod. 0.48 0.68 0.82 0.94 1.04 1.41 1.64 - (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02 ± 0.02 ± 0.01
250 % Mod. 0.49 0.72 0.90 1.08 1.20 1.70 2.04 - (MPa) ± 0.01 ± 0.01 ± 0.02 ± 0.04 ± 0.04 ± 0.02 ± 0.03
300 % Mod. 0.50 0.76 1.00 1.24 1.42 2.06 - - (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.05 ± 0.04 ± 0.04
350 % Mod. 0.51 0.82 1.11 1.43 1.66 2.52 - - (MPa) ± 0.01 ± 0.02 ± 0.03 ± 0.05 ± 0.04 ± 0.06
400 % Mod. 0.52 0.86 1.23 1.63 1.96 3.18 - - (MPa) ± 0.01 ± 0.01 ± 0.03 ± 0.05 ± 0.04 ± 0.10
450 % Mod. 0.54 0.92 1.36 1.86 2.33 - - - (MPa) ± 0.02 ± 0.02 ± 0.04 ± 0.05 ± 0.02
500 % Mod. 0.55 0.98 1.51 2.11 2.78 - - - (MPa) ± 0.02 ± 0.02 ± 0.04 ± 0.05 ± 0.03
TS > 4.20 8.33 8.06 5.66 4.59 3.58 2.51 2.35 (MPa) ± 0.56 ± 0.25 ± 0.22 ± 0.93 ± 0.26 ± 0.42 ± 0.09 ± 0.09
EB > 1655 1424 1010 769 611 421 292 173 (%) ± 52 ± 12 ± 4 ± 32 ± 12 ± 20 ± 9 ± 10
200 Table B.9 Tensile properties at room temperature (~25 oC) of XNBR-CaO vulcanizates (cure time 1000 min)
XN- XN- XN- XN- XN- XN- XN- XNBR Properties Ca0.5 Ca1.0 Ca1.5 Ca2.0 Ca3.0 Ca4.0 Ca5.0
25 % Mod. 0.35 0.41 0.38 0.43 0.43 0.42 0.44 0.44 (MPa) ± 0.01 ± 0.02 ± 0.02 ± 0.01 ± 0.02 ± 0.01 ± 0.02 ± 0.01
50 % Mod. 0.52 0.58 0.55 0.59 0.60 0.60 0.63 0.65 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01
75 % Mod. 0.62 0.68 0.65 0.69 0.70 0.71 0.73 0.77 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01
100 % Mod. 0.69 0.75 0.71 0.75 0.76 0.78 0.80 0.84 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01
150 % Mod. 0.78 0.87 0.81 0.83 0.86 0.87 0.90 0.94 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01
200 % Mod. 0.86 0.98 0.89 0.91 0.95 0.96 0.98 1.04 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.01
250 % Mod. 0.94 1.10 0.98 0.98 1.03 1.05 1.07 1.13 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01
300 % Mod. 1.03 1.22 1.07 1.06 1.13 1.14 1.18 1.23 (MPa) ± 0.02 ± 0.01 ± 0.01 ± 0.02 ± 0.01 ± 0.01 ± 0.02 ± 0.01
350 % Mod. 1.14 1.35 1.18 1.15 1.22 1.24 1.28 1.35 (MPa) ± 0.03 ± 0.01 ± 0.01 ± 0.02 ± 0.01 ± 0.01 ± 0.03 ± 0.02
400 % Mod. 1.27 1.51 1.31 1.26 1.34 1.35 1.39 1.47 (MPa) ± 0.04 ± 0.01 ± 0.01 ± 0.03 ± 0.02 ± 0.02 ± 0.01 ± 0.02
450 % Mod. 1.42 ± 1.70 1.46 1.39 1.47 1.49 1.52 1.60 (MPa) 0.06 ± 0.01 ± 0.01 ± 0.04 ± 0.02 ± 0.02 ± 0.03 ± 0.03
500 % Mod. 1.63 1.98 1.64 1.55 1.63 1.66 1.70 1.77 (MPa) ± 0.08 ± 0.02 ± 0.01 ± 0.05 ± 0.02 ± 0.03 ± 0.03 ± 0.03
TS 9.08 8.21 7.71 7.95 7.52 6.96 7.16 7.59 (MPa) ± 0.14 ± 0.18 ± 0.15 ± 0.20 ± 0.60 ± 0.33 ± 0.11 ± 0.09
EB 751 692 746 769 764 772 766 785 (%) ± 2 ± 10 ± 3 ± 15 ± 19 ± 8 ± 7 ± 5
201 o Table B.10 Tensile properties at room temperature (~25 C) of XNBR-Ca(OH)2 vulcanizates (cure time 240 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR Ch0.5 Ch1.0 Ch1.5 Ch2.0 Ch3.0 Ch4.0 Ch5.0
25 % Mod. 0.31 0.53 1.67 2.39 2.28 2.37 2.47 2.61 (MPa) ± 0.01 ± 0.01 ± 0.08 ± 0.01 ± 0.02 ± 0.08 ± 0.09 ± 0.05 50 % Mod. 0.47 0.73 2.67 4.14 4.20 4.44 4.69 5.05 (MPa) ± 0.01 ± 0.01 ± 0.06 ± 0.02 ± 0.02 ± 0.04 ± 0.04 ± 0.12 75 % Mod. 0.55 0.86 3.36 5.54 5.64 6.13 6.50 6.97 (MPa) ± 0.01 ± 0.02 ± 0.06 ± 0.05 ± 0.01 ± 0.04 ± 0.06 ± 0.09 100 % Mod. 0.61 0.96 3.93 6.68 6.92 7.49 7.94 8.46 (MPa) ± 0.01 ± 0.02 ± 0.08 ± 0.03 ± 0.03 ± 0.5 ± 0.04 ± 0.09 150 % Mod. 0.66 1.12 5.01 8.69 9.17 9.76 10.2 10.8 (MPa) ± 0.01 ± 0.03 ± 0.10 ± 0.03 ± 0.11 ± 0.07 ± 0.1 ± 0.1 200 % Mod. 0.71 1.26 6.23 10.8 11.2 11.9 12.4 13.0 (MPa) ± 0.02 ± 0.04 ± 0.12 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ±0.2 250 % Mod. 0.76 1.43 7.87 13.3 13.8 14.6 15.0 15.7 (MPa) ± 0.02 ± 0.05 ± 0.17 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.2 300 % Mod. 0.81 1.63 10.3 16.9 17.3 18.2 18.5 19.3 (MPa) ± 0.03 ± 0.05 ± 0.3 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.3 350 % Mod. 0.87 1.88 14.1 22.0 22.2 23.2 23.1 24.2 (MPa) ± 0.03 ± 0.06 ± 0.4 ± 0.1 ± 0.1 ± 0.2 ± 0.2 ±0.5 400 % Mod. 0.94 2.20 19.9 29.0 28.7 29.7 29.4 30.3 (MPa) ± 0.03 ± 0.07 ± 0.5 ± 0.3 ± 0.1 ± 0.2 ± 0.2 ± 0.7 450 % Mod. 1.02 2.64 27.8 37.6 36.9 37.8 36.8 37.8 (MPa) ± 0.04 ± 0.08 ± 0.7 ± 0.5 ± 0.4 ± 0.5 ± 0.3 ± 0.7 500 % Mod. 1.11 3.31 38.0 46.3 46.9 45.1 46.1 - (MPa) ± 0.05 ± 0.12 ± 0.9 ± 0.4 ± 0.3 ± 0.5 ± 0.8 TS 7.79 14.9 41.8 45.1 50.9 48.2 47.5 48.1 (MPa) ± 0.13 ± 0.8 ± 0.8 ± 1.7 ± 1.0 ± 0.7 ± 0.6 ± 0.2 EB 939 712 516 486 522 507 514 511 (%) ± 13 ± 12 ± 3 ± 11 ± 7 ± 4 ± 2 ± 3
202 Table B.11 Tensile properties at room temperature (~25 oC) of XNBR-BaO vulcanizates (cure time 240 min)
XN- XN- XN- XN- XN- XN- XN- Properties XNBR Ba0.5 Ba1.0 Ba1.5 Ba2.0 Ba3.0 Ba4.0 Ba5.0
25 % Mod. 0.31 0.45 0.64 0.84 0.96 1.29 2.03 2.38 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.04 ± 0.06 ± 0.01
50 % Mod. 0.47 0.66 0.99 1.32 1.58 2.21 3.19 3.76 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.04 ± 0.04 ± 0.06 ± 0.05
75 % Mod. 0.55 0.77 1.18 1.62 1.99 2.83 4.06 4.85 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.03 ± 0.03 ± 0.06 ± 0.06 ± 0.07
100 % Mod. 0.61 0.84 1.32 1.85 2.29 3.34 4.84 5.80 (MPa) ± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.05 ± 0.07 ± 0.04 ± 0.09
150 % Mod. 0.66 0.96 1.57 2.28 2.87 4.29 6.34 7.60 (MPa) ± 0.01 ± 0.02 ± 0.01 ± 0.03 ± 0.07 ± 0.09 ± 0.08 ± 0.12
200 % Mod. 0.71 1.06 1.83 2.73 3.50 5.34 8.02 9.58 (MPa) ± 0.02 ± 0.02 ± 0.01 ± 0.04 ± 0.08 ± 0.13 ± 0.10 ± 0.18
250 % Mod. 0.76 1.17 2.13 3.29 4.28 6.71 10.2 12.1 (MPa) ± 0.02 ± 0.03 ± 0.01 ± 0.07 ± 0.11 ± 0.16 ± 0.1 ± 0.2
300 % Mod. 0.81 1.30 2.54 4.04 5.34 8.58 13.1 15.4 (MPa) ± 0.03 ± 0.04 ± 0.01 ± 0.10 ± 0.13 ± 0.23 ± 0.1 ± 0.2
350 % Mod. 0.87 1.47 3.08 5.06 6.86 11.3 17.1 19.7 (MPa) ± 0.03 ± 0.05 ± 0.02 ± 0.18 ± 0.22 ± 0.3 ± 0.1 ± 0.3
400 % Mod. 0.94 1.69 3.84 6.58 9.26 15.3 22.5 25.3 (MPa) ± 0.03 ± 0.04 ± 0.03 ± 0.24 ± 0.32 ± 0.4 ± 0.4 ± 0.5
450 % Mod. 1.02 1.98 5.05 9.09 12.9 20.9 - - (MPa) ± 0.04 ± 0.05 ± 0.07 ± 0.37 ± 0.3 ± 0.2
500 % Mod. 1.11 2.38 7.10 13.1 18.3 - - - (MPa) ± 0.05 ± 0.06 ± 0.19 ± 0.5 ± 0.3
TS 7.79 7.52 12.8 19.2 21.7 26.6 25.6 26.0 (MPa) ± 0.13 ± 0.20 ± 0.6 ± 0.5 ± 1.6 ± 0.7 ± 0.3 ± 0.3
EB 939 684 580 554 526 491 424 407 (%) ± 13 ± 5 ± 9 ± 4 ± 8 ± 3 ± 5 ± 3
203 APPENDIX C
MOLECULAR TRANSITION TEMPERATURE
Table C.1 Molecular transition temperatures of XN-MgB vulcanizates at frequency 1.0 Hz
Ionic transition temperature Vulcanizates T (oC) g 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
Ionic transition temperature Vulcanizates T (oC) g range (oC)
XN-MgC1.0 -21 15 - 85 XN-MgC2.0 -17 20 - 100 XN-MgC3.0 -17 25 - 105
204