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YU SUN

VULCANIZATION AND DEVULCANIZATION OF RUBBER

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirement for the Degree

Doctor of Philosophy

YU SUN

August, 2020

VULCANIZATION AND DEVULCANIZATION OF RUBBER

YU SUN Dissertation

Approved: Accepted:

______Advisor Department Chair Dr. Li Jia Dr. Tianbo Liu

______Committee Chair Interim Dean of the College Dr. Toshikazu Miyoshi Dr. Ali Dhinojwala

______Committee Member Acting Dean of the Graduate School Dr. Gary R. Hamed Dr. Marnie Saunders

______Committee Member Date Dr. Junpeng Wang

______Committee Member Dr. Kevin Cavicchi

ABSTRACT

The research in this dissertation includes two parts: vulcanization of isobutylene- isoprene rubber (IIR) containing geminal vinylidene-acrylate groups and surface devulcanization of ground rubber particles (GRPs) for rubber recycling.

In Chapter 2, EIIR was synthesized by grafting ethyl propiolate to IIR via EtAlCl2- catalyzed Alder-ene reaction. Methods for crosslinking EIIR using dicumyl peroxide

(DCP) with m-Phenylene-N,N’-bismaleimide (BMI) as a coagent were investigated. The strength, extensibility, and overall toughness of the peroxide-cured EIIR significantly exceed those of previously reported peroxide-cured IIR derivatives. A wide range of stiffness can be realized without sacrificing the strength. The crosslinking density achieved using such curatives is indexed as a function of DCP/BMI ratio, and the curing efficiency of peroxide as a function of DCP/BMI ratio and DCP loading. The role of the geminal vinylidene-acrylate moiety in curing was studied by model reactions, which suggest that the geminal vinylidene-acrylate moiety undergoes both radical addition and hydrogen abstraction. Coagent trimethylolpropane trimethacrylate (TMPTMA) was studied to replace BMI due to high toxicity of BMI. Low degrees of crosslinking (~50%) were found in all vulcanizates cured by DCP and TMPTMA, likely because of a mismatch between the reactivities of DCP and TMPMTA. Benzoyl peroxide (BPO) was then used in conjunction with TMPTMA. EIIR was successfully cured with BPO and TMPTMA to give

iii high stress and strain at break, and toughness exceeding those achieved with DCP and BMI as the curing agents. Atomic force microscopy and transmission electron microscopy indicate that phase-separated nanodomains presumably of polymers or oligomers of

TMPTMA are present in the continuous rubber phase. These domains are likely also responsible for achieving the observed superior mechanical properties. The combination of BPO and BMI are also capable to sufficiently cure EIIR, but the resultant vulcanizates have tensile properties somewhat worse than those cured by DCP and BMI.

In Chapter 3, recycling of GRPs was studied. Poor interfacial bonding and interfacial modulus contrast were identified as two inherent causes for the inferior mechanical properties of vulcanizates containing GRPs in comparison to vulcanizates of fresh rubber. The poor interfacial bonding is caused by limited depth of mixing and molecular contact between the polymer chains in the GRPs and fresh rubber. The interfacial modulus contrast is caused by diffusion of curatives from the fresh rubber to the

GRPs. Surface devulcanization of GRPs gives rise to strong interfacial bonding equal to the cohesive strength of the rubber but only improves the tensile properties of vulcanizates containing GRPs to a limited extent. When the interfacial modulus contrast is erased, the tensile strength of vulcanizates containing GRPs become equal to that of the fresh rubber vulcanizate. This study also shows that oxidative aging harms the interfacial bonding more than it does the bulk properties. When the surface devulcanization method was applied to

GRPs, the vulcanizates containing aged GRPs display similar tensile properties to those of the vulcanizates containing GRPs without aging.

ACKNOWLEDGEMENT

I would like to give my sincere and deep gratitude to my advisor, Dr. Li Jia, for his guidance and support throughout the duration of my stay in the research group. It is a great opportunity for me to work with Dr. Jia. He is a true scientist. I have learnt lots of things from him in research and the way of thinking.

Besides of my advisor, I would like to thank Dr. Gary Hamed for helping me with the research and providing me many papers related to rubber science and technology. I would also like to thank Dr. Toshikazu Miyoshi, Dr. Junpeng Wang and Dr. Kevin

Cavicchi for being on my committees and giving me valuable comments and suggestions.

My sincere thanks also goes to Chengkai Fan, Junyi Chen, Xuesong Yan, Yihong

Zhao, Xin Tan, Honghe Liang, Nathan Schmitz, Yiwei Dai, Mengsha Qian and all the other group members for their help and support. I must thank Dr. Jiansheng Feng for helping me on AFM and nano-indenter. I also want to thank Dr. Crittenden Ohlemacher and Yaohong

Pang for their help during my research. Furthermore, I must thank Dr. Georg Bohm for helping me with the research.

Finally, I must thank my family and Ye Xiao for their love and support. They always stand by me through the good times and bad. In the end, I want to say, “I came; I saw; I conquered”.

TABLE OF CONTENTS

Page

LIST OF TABLES ...... ix

LIST OF FIGURES ...... xi

LIST OF SCHEMES...... xv

CHAPTER

I. GENERAL INTRODUCTION AND BACKGROUND ...... 1

1.1 Introduction of Rubbers ...... 1

1.1.1 Butyl Rubber and Halogenated Butyl Ruber ...... 2

1.1.2 Polybutadiene Rubber ...... 4

1.2 Vulcanization of Rubbers ...... 5

1.2.1 Vulcanization by Sulfur and Accelerators ...... 7

1.2.2 Vulcanization by Phenolic Compounds ...... 12

1.2.3 Vulcanization by Peroxide-coagent System ...... 18

1.3 Recycling of Rubbers ...... 29

1.3.1 Reclaiming of Vulcanizates by Physical Reclaiming Processes ...... 33

1.3.2 Reclaiming of Vulcanizates by Chemical Reclaiming Processes ...... 35

II. VULCANIZATION OF BUTYL RUBBER CONTAINING GEMINAL VINYLIDENE-ACRYLATE GROUPS ...... 51

2.1 Introduction ...... 51

2.2 Experimental ...... 53

2.2.1 Material ...... 53

2.2.2 Chemical Structure Characterization ...... 54

2.2.3 Synthesis of Ethyl Propiolate Grafted Butyl Rubber (EIIR) ...... 55

2.2.4 Synthesis of Ethyl 4,5-dimethyl-(E-2),5-hexdienoate (EDHEX) ...... 55

2.2.5 Rubber Compounding ...... 56

2.2.6 Compositions of EIIR Compounds ...... 56

2.2.7 Vulcanization Kinetics ...... 59

2.2.8 Vulcanization ...... 59

2.2.9 Stress Relaxation ...... 59

2.2.10 Tensile Test ...... 59

2.2.11 Swelling Test for Crosslinking Density ...... 60

2.2.12 Reactions of EDHEX with DCP and N-Phenylmaleimide (PMI)...... 62

2.2.13 Microscopic Characterization ...... 62

2.3 Results and Discussion ...... 63

2.3.1 EIIR Compounds Cured by DCP-BMI ...... 63

2.3.2 EIIR Compounds Containing TMPTMA ...... 87

2.4 Conclusion ...... 107

III. SURFACE DEVULCANIZATION OF GROUND RUBBER PARTICLES FOR RUBBER RECYCLING ...... 109

3.1 Introduction ...... 109

3.2 Experimental ...... 110

3.2.1 Materials...... 110

3.2.2 Formation of Rubber Compounds Containing GRPs ...... 111

vii

3.2.3 Vulcanization Kinetics ...... 112

3.2.4 Vulcanization ...... 112

3.2.5 Preparation of Sheets with Cord Backing ...... 113

3.2.6 Preparation of Laminates for Physical Adhesion Test ...... 113

3.2.7 Preparation of Co-cured Laminates for Adhesion Test ...... 114

3.2.8 Tensile Test ...... 115

3.2.9 Adhesion Test ...... 115

3.2.10 Indentation Study ...... 116

3.2.11 Modification of Cured Strips ...... 116

3.2.12 Modification of GRPs ...... 118

3.3 Result and Discussion ...... 119

3.3.1 Selection of System to Study ...... 119

3.3.2 Interfacial Bonding between Pre-cured and Virgin Rubber ...... 123

3.3.3 Modulus Contrast at Interface ...... 130

3.3.4 Erasing Modulus Contrast across the Interface...... 137

3.3.5 Aging by Oxidation ...... 139

3.4 Conclusion ...... 140

IV. CONCLUSION...... 142

REFERENCES ...... 144

APPENDICES ...... 160

APPENDIX A ...... 161

APPENDIX B ...... 166

viii

LIST OF TABLES

Table Page

Table 1. 1. The bond energy of crosslinks.40 ...... 13

Table 1. 2. U.S. scrap tire market summary (2009-2017).106...... 30

Table 1. 3. Chemicals used to cleave crosslinks.107 ...... 42

Table 2. 1. Recipes of EIIR compounds containing DCP and BMI...... 57

Table 2. 2. Recipes of EIIR compounds containing TMPTMA...... 58

Table 2. 3. Recipes of EIIR compounds containing BPO and BMI...... 58

Table 2. 4. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds cured by DCP and BMI...... 67

Table 2. 5. Summary of formulations of model reactions...... 71

Table 2. 6. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds cured by DCP-BMI-EDHEX...... 79

Table 2. 7.Mechanical properties of EIIR compounds.a ...... 83

Table 2. 8. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds cured by BPO and TMPTMA...... 92

Table 2. 9.Mechanical properties of EIIR compounds cured by BPO and TMTPMTA.a ...... 98

Table 2. 10. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds cured by BPO and BMI...... 102

Table 2. 11. Mechanical properties of EIIR compounds cured by BPO and BMI...... 106

Table 3. 1. Rubber compound compositions...... 111 ix

Table 3. 2. Compositions of GRP-containing rubber compounds...... 112

Table 3. 3. Elemental analysis results of nonvolatile residuals in devulcanization solution...... 126

Table 3. 4. Summary of tensile properties of vulcanizates in this study...... 128

LIST OF FIGURES

Figure Page

Figure 1. 1. Structure of butyl rubber...... 3

Figure 1. 2. Structure of polybutadiene rubber...... 4

Figure 1. 3. Vulcanization...... 5

Figure 1. 4. Vulcanizate properties as a function of crosslinking density.2 ...... 6

Figure 1. 5. Development of the accelerated-sulfur vulcanization in natural rubber.2 ...... 8

Figure 1. 6. The possible structures of HIIR...... 11

Figure 1. 7. The percent of volume increase of sulfur-cured IIR as a function of curing time at different temperature.39 ...... 13

Figure 1. 8. The possible structure of phenol-formaldehyde resin...... 15

Figure 1. 9. Vulcanization of IIR by sulfur-accelerators and phenolic compounds.44 ...... 17

Figure 1. 10. The structures and ten-hour half-life temperature of several common peroxides for rubber vulcanization.51 ...... 19

Figure 1. 11. The structures of IIR derivatives containing reactive functional groups.74 ...... 26

Figure 1. 12. Synthesis of IIR derivatives with pendant carboxylic acid.88...... 27

Figure 1. 13. The structures of common coagents used in peroxide vulcanization.51 ...... 28

Figure 1. 14. Schematic of the devulcanization reactor.129 ...... 35

Figure 1. 15. The devulcanization process by triphenylphosphine.132 ...... 36

Figure 1. 16. The devulcanization process by sodium di-n-butyl phosphite.131 ...... 37

Figure 1. 17. The reaction of diallyl sulfide with methyl iodide.135 ...... 38

Figure 1. 18. The devulcanization process by lithium aluminum hydride.136...... 39

Figure 1. 19. The devulcanization process by isopropylthiol-piperidine.135 ...... 39

Figure 1. 20. The devulcanization process by hexane-1-thiol and piperidine.139 ...... 40

Figure 1. 21. Reactions of disulfides with butane-1-thiol in piperidine solution.139 ...... 41

Figure 1. 22. The structures of models in Figure 1. 21...... 41

Figure 1. 23. The devulcanization process by NaOH under phase transfer catalyst.140 ...... 43

Figure 1. 24. The devulcanization process by sodium under hydrogen atmosphere.141 ...... 43

Figure 1. 25. Functionalizing agents used for devulcanization.145, 146 ...... 45

Figure 1. 26. The devucanization of vulcanized polyisoprene rubber with tetrabenzylthiuram disulfide as the functionalizing agent.145 ...... 45

Figure 1. 27. The devulcanization mechanism of GRPs with TMTD.147 ...... 47

Figure 1. 28. The process for preparation of devulcanized SBR.149 ...... 48

1 Figure 2. 1. H spectrum of EIIR in CDCl3...... 64

Figure 2. 2. The curing kinetic curves of EIIR compounds cured by DCP-BMI at 160 ºC...... 66

Figure 2. 3. Correlation between M and crosslinking density (ρcx) of EIIR compounds...... 68

Figure 2. 4. (A) Correlation between ρcx and DCP and BMI loadings. d and m are DCP and BMI loadings in phr, respectively. The term “-1” is added so that when d = 0, ρcx is zero. (B) Correlation between peroxide efficiency (Ecx) and DCP and BMI loadings...... 70

Figure 2. 5. 1H NMR spectra of the reactions corresponding to (A) entry 1 and (B) entry 3 in Table 2. 5...... 73 xii

Figure 2. 6. 1H NMR spectra of the reactions corresponding to (A) entry 4 and (B) entry 5 in Table 2. 5...... 74

Figure 2. 7. ESI-MS spectra and identities of the observed species in the model reactions corresponding to (A) entry 1 and (B) entry 4 in Table 2. 5...... 75

Figure 2. 8. The curing kinetic curves of compounds EIIR cured by DCP-BMI- EDHEX at 160 oC...... 78

Figure 2. 9. Stress relaxation curves of EIIR compounds...... 80

Figure 2. 10. (A) Correlation between ρcx and terminal moduli (Eeq). (B) Comparison of ρcx obtained from swelling and equilibrium moduli...... 81

Figure 2. 11. Correlation of tensile properties with ρcx (B) stress at break (b) vs ρcx (C) toughness vs ρcx ...... 85

Figure 2. 12. Stress-strain curves of representative EIIR compounds...... 87

Figure 2. 13. (A) The curing kinetic curves of EIIR compounds cured by DCP- TMPTMA and (B) Comparison of ΔM of EIIR compounds cured by DCP- TMPTMA and DCP-BMI...... 89

Figure 2. 14. The gel fraction of EIIR compounds cured by DCP-TMPTMA...... 90

Figure 2. 15. The curing kinetic curves of EIIR compounds cured by BPO- TMPTMA at 110 oC...... 91

Figure 2. 16. Correlation between M and ρcx of EIIR compounds...... 93

Figure 2. 17. Stress relaxation curves of EIIR compounds cured by BPO- TMPTMA...... 95

Figure 2. 18. (A) Correlation between ρcx and terminal moduli (Eeq). (B) Comparison of ρcx obtained from swelling and equilibrium moduli...... 96

Figure 2. 19. Stress-strain curves of EIIR compounds cured by BPO-TMPTMA...... 97

Figure 2. 20. (A) AFM-QNM modulus image of compound B2T10 and (B) AFM phase image of compound B2T10...... 99

Figure 2. 21. TEM image of compound B2T10...... 100

Figure 2. 22. Curing kinetic curves of EIIR compounds cured by BPO-BMI at 110 oC...... 101

Figure 2. 23. Correlation between M and ρcx of EIIR compounds...... 103

xiii

Figure 2. 24. Stress relaxation curves of EIIR compounds cured by BPO-BMI...... 104

Figure 2. 25. (A) Correlation between ρcx and terminal moduli (Eeq). (B) Comparison of ρcx obtained from swelling and equilibrium moduli...... 105

Figure 2. 26. Stress-strain curves of EIIR compounds cured by BPO-BMI...... 106

Figure 3. 1. Geometry of adhesion test...... 115

Figure 3. 2. Procedure for surface modification of single cured strip...... 118

Figure 3. 3. Comparison of stress at break (B) and strain at break (B) of vulcanizates of blends of virgin rubber compound and particles of various size, history of usage, and composition against the virgin rubber compound (VRC)...... 120

Figure 3. 4. Representative optical images and size histogram of 30-60 mesh GRPs...... 122

Figure 3. 5. Adhesion of laminates...... 124

Figure 3. 6. 1H spectrum of nonvolatile residual in a 1:10 mixture of nBuSH and piperidine...... 127

Figure 3. 7. Stress-strain curves of compounds containing GRPs...... 129

Figure 3. 8. The modulus map of the laminate L0 around the interface...... 131

Figure 3. 9. The modulus map of the laminate L1 around the interface...... 132

Figure 3. 10. The modulus maps of the laminate L2 around the interface by (A) micro-indenter and (B) nano-indenter...... 134

Figure 3. 11. The modulus maps of single cured strip with (A) carbon black and (B) without carbon black...... 135

Figure 3. 12. The image of failed interface of L2...... 136

Figure 3. 13. The modulus maps of the laminate L3 around the interface by (A) micro-indenter and (B) nano-indenter...... 138

Figure 3. 14. Stress-strain curves of compounds containing aged and unaged GRPs...... 140

xiv

LIST OF SCHEMES

Scheme Page

Scheme 1. 1. The possible reaction path of accelerated-sulfur vulcanization.2 ...... 9

Scheme 1. 2. The possible intermediate in accelerated-sulfur vulcanization.2 ...... 9

Scheme 1. 3. The allyl hydrogen mechanism of resin cure.43 ...... 15

Scheme 1. 4. The chroman mechanism of resin cure.44 ...... 16

Scheme 1. 5. The cationic reaction mechanism of resin cure.48 ...... 16

Scheme 1. 6. The general peroxide-induced vulcanization.56 ...... 21

Scheme 1. 7. The energy levels of radicals involved in peroxide vulcanization.51 ...... 22

Scheme 1. 8. Chain scission and crosslinking of IIR by peroxide. 64 ...... 23

Scheme 1. 9. Synthesis of IIR derivatives by BIIR esterification.79 ...... 25

Scheme 2. 1. Synthesis of EIIR...... 63

Scheme 2. 2. Synthesis of EDHEX...... 71

CHAPTER I

GENERAL INTRODUCTION AND BACKGROUND

1.1 Introduction of Rubbers

Elastomer is a polymer with viscoelasticity. It is often used interchangeably with the term rubber.1 In this dissertation, I will use the two terms interchangeably. In most cases, un-vulcanized rubber would tend to flow at room temperature. Most useful rubber products are vulcanized rubbers and used above their glass transition temperature. Rubbers are relatively soft with low Young’s modulus ranges from 1 to 10 MPa and stress at break between 10 and 50 MPa. Vulcanized rubber can undergo large deformation and recover to its original shape after removal of external force.2, 3

Rubber used in ancient Mesoamerica was almost 3500 years ahead of Charles

Goodyear’s vulcanized rubber. The ancient Mayan people used latex to make rubber balls.

Till 1839, rubber goods produced from un-vulcanized natural rubber did not show good properties. It became sticky during hot weather and brittle during cold weather.4 The vulcanization process Charles Goodyear discovered gave rubber products superior properties.5 With the development of times, Williams in 1859 demonstrated that natural rubber was composed of isoprene. Bouchardat in 1879 was able to make rubberlike material from isoprene, which was from rubber pyrolysis. Tilden in 1884 repeated the

1 process but used isoprene from pyrolysis of turpentine. At that time, the transformation of simple molecules of a diene into rubber was far beyond the comprehension of chemical science. At the beginning of 20th century, the commercial production of synthetic rubber was already established in Germany and Russia before Staudinger proposed his macromolecular hypothesis during 1920s.4, 6 It is probably the invention of synthetic rubber leads to the understanding of macromolecular chemistry, which was a new filed of organic chemistry. Macromolecular chemistry developed rapidly during 1930s and 1940s and pointed the way to synthesis of vast types of new polymeric materials.2 Synthetic rubbers were developed quickly during World War II due to the uncertainty of the supply of natural rubber.7

Rubber is an important material in modern society. It is used in various products like tires, electric insulation cables, rubber bands, conveying belts and hoses. Industrial rubber market is driven principally by the automotive industry. Industrial rubber is primarily used for making tires. According to a new report by Reports and Data, the rubber market is expected to reach USD 38.31 billion by 2026. The synthetic rubber accounts for a large share of 63.8% in 2018.8 Here, two types of synthetic rubbers, butyl rubber and polybutadiene rubber, are talked in this dissertation.

1.1.1 Butyl Rubber and Halogenated Butyl Ruber

Butyl rubber was first synthesized in 1937 and commercial in 1943. It is the copolymer of isobutylene with a small amount of isoprene that is used as crosslinking site, and hence is also commonly known as isobutylene-isoprene rubber (IIR).9 The structure of butyl rubber is shown in Figure 1. 1. In polymerization of butyl rubber process, the

2 utilization of simple olefin rather than a diolefin as the raw material shows the economic advantage. In addition, the initial unsaturation of butyl rubber is very small, and this low unsaturation is greatly reduced after curing, it gives butyl rubber excellent chemical stability.10 Since a large amount of isobutylene units exist in butyl rubber, butyl rubber also has excellent gas impermeability and so it is widely used for inner tubes and inner liners of pneumatic tires. The heat resistance of butyl rubber renders its use for tire curing bags and bladders. Its resistance to ozone, weather, and moisture renders its use for roofing, reservoir membranes, electrical insulation, and automotive components.11

Figure 1. 1. Structure of butyl rubber. where x is an average number from 10 to 150; and n is an integer from about 100 to 10,000.

However, low unsaturation of butyl rubber results in the slow vulcanization, which limits the application of butyl rubber. Halogenated butyl rubber was introduced to solve this problem. Brominated butyl rubber was first synthesized at Goodrich and commercialized in 1954.12 Chlorinated butyl rubber was developed at ExxonMobil and commercialized in 1961.12, 13 Halogenated butyl rubber provides much higher vulcanization rates and improves the compatibility with highly unsaturated elastomers, 3 such as natural rubber (NR) and styrene-butadiene rubber (SBR).9 It becomes the most successful butyl rubber derivative and extends greatly the usefulness of butyl rubber.

1.1.2 Polybutadiene Rubber

Polybutadiene rubber (BR) is a polymer formed from polymerization of monomer

1,3-butadiene. The structure of BR is shown in Figure 1. 2. There are three types of connectivity in polymer backbone. The cis- and trans- microstructures form by 1,4- addition, which connects monomer butadiene end-to-end. Vinyl- microstructures form by

1,2-addition, which reacts one double bond of monomer butadiene. Cis- microstructures give BR high elasticity and low glass transition temperature. Trans- microstructures could form microcrystalline in polybutadiene rubber. Vinyl- microstructures could provide crosslinking sites.14-16 High cis- BR has high abrasion resistant, high elasticity and exceptional rolling resistance.17

About 70% of BR is used in manufacture of tires. BR is also used in golf balls and used as additive to improve the toughness of plastics such as polystyrene.18

Figure 1. 2. Structure of polybutadiene rubber.

where reflects the backbone of polymer, x, m and n are the number of cis-, trans- and vinyl- repeating units, respectively. 4

1.2 Vulcanization of Rubbers

Vulcanization is indispensable to produce most useful rubber products like tires.

Un-vulcanized rubber is usually not strong. It could flow under the external force at the room temperature. It cannot recover its original shape after a large deformation. The application of un-vulcanized rubber is limited. Vulcanization works by the formation of a crosslinked molecular network, where crosslinks formed between polymer chains. The vulcanization process is shown in Figure 1. 3. In an un-vulcanized rubber (above its melting point), only molecular chain entanglements act as crosslinks. Vulcanization inserts more and much stronger crosslinks between polymer chains. These crosslinks tie polymer chains together to form network.2 Vulcanized rubbers are elastic solid.

Figure 1. 3. Vulcanization.

The relationship between vulcanizate properties and crosslinking density is shown in Figure 1. 4.2 Hysteresis is the ratio of viscous component to the elastic component, and is reduced greatly by increasing crosslinking density. It shows the deformation energy, which is converted to heat. Properties such as tear strength, fatigue life, toughness and stress at break are related to the breaking energy. The breaking energy is increased by 5 increasing crosslinking density and hysteresis. Since hysteresis is decreased by increasing crosslinking density, it results in a trade-off between crosslinking density and hysteresis.

There is optimum crosslinking density for properties related to the breaking energy.2

Figure 1. 4. Vulcanizate properties as a function of crosslinking density.2

Vulcanizate properties are also affected by curing agents, more specifically, the types of crosslinks. Generally, there are three types of curing agents used for vulcanization.19 The first is sulfur and accelerator, which are the most widely used in industry. It introduces mono-, di- and poly-sulfide crosslinks into vulcanizate. The second is the phenolic compounds, which are usually di-substituted by -CH2-X groups, where X is OH group or a halogen atom. The third is peroxide. The latter two curing agents are

6 always used to introduce carbon-carbon crosslinks into vulcanizate. Vulcanization of butyl rubber and its derivatives by these three types of curing agents are talked below.

1.2.1 Vulcanization by Sulfur and Accelerators

Rubber products were not useful until Charles Goodyear found sulfur and heat could enhance the properties of natural rubber in 1841.20 Even this discovery is quite simple and accidental, Goodyear had lots of headaches in getting his process into action not only from technical part, but also from the business angle. People were disgusted with rubber products at that time, he had to overcome the prejudice and antagonism. For vulcanization by sulfur alone, 5-8 parts of sulfur were mixed into 100 phr natural rubber, and mixture was heated at 141 oC for about 3-4 hours. As amount of sulfur was increased to 14-18 parts, the vulcanizates have lower stress at break. When sulfur loading was increased further to

30-50 parts, the vulcanizates become hard, and it is called hard rubber or ebonite.20-22

In 1906, Oenslager discovered that aniline could accelerate sulfur vulcanization.

Since aniline is toxic, less toxic product thiocarbanilide was introduced in 1907.23 After that, guanidine and dithiocarbamates were also used as accelerators. With the development of 2-mercaptobenzothiazole (MBT) and 2-benzothiazole disulfide (MBTS), the delayed- action accelerators were introduced in 1925.24 The delayed action is required for processing and shaping before the formation of vulcanized network. The development of accelerator toward faster vulcanization with better scorch resistance (Figure 1. 5).2, 25

Figure 1. 5. Development of the accelerated-sulfur vulcanization in natural rubber.2

It is well known that the chemistry of accelerated-sulfur vulcanization is very complex and still not clearly understood. The possible reaction path of accelerated-sulfur vulcanization is shown in Scheme 1. 1.2 Accelerator reacts with sulfur to give transition complex of Ac-Sx-Ac, where Ac is an organic radical derived from accelerator (e.g., benzothiazyl-). Then it interacts with rubber to form structure of rubber-Sx-Ac. Based on the results from model reaction26, 27, where sulfur is attached to the rubber hydrocarbon almost exclusively at allylic positions, a six-membered ring intermediate in Scheme 1. 2

2 probably forms during vulcanization process. Finally, rubber-Sx-Ac react either directly or through an intermediate to give crosslinks, rubber-Sx-rubber.

Scheme 1. 1. The possible reaction path of accelerated-sulfur vulcanization.2

Scheme 1. 2. The possible intermediate in accelerated-sulfur vulcanization.2

Sulfur and accelerators react with isoprene units in IIR. Compared to other highly unsaturated elastomers such as NR and SBR, vulcanization of IIR is slow or requires high temperature due to its low unsaturation. This disadvantage makes IIR curing incompatible with curing of highly unsaturated elastomers.12, 26-28 The vulcanization of IIR is usually used by an efficient vulcanization or semi-efficient vulcanization systems, which usually contain lower amount of sulfur and a higher amount of accelerators. It caused the crosslinks in vulcanizates IIR are mainly mono-sulfidic crosslinks.26, 29, 30 Accelerators used in IIR include fast accelerators (thiazoles, sulfenamides), very fast accelerators (thiurams), ultra- accelerators (dithiocarbamates) and their combinations.31 Halogenated butyl rubbers (HIIR) are developed to overcome the slow vulcanization of IIR. The possible structures of HIIR are labeled by II, III and IV in Figure 1. 632, where structure III is the dominant structure.32-

36 After halogenation of the isoprene units, the vulcanization rate of HIIR is significantly improved. It renders HIIR better compatibility with highly unsaturated rubbers without weakening basic properties of butyl rubber, and makes HIIR much more attractive. As the result, HIIR is the common choice for inner liner in tubeless tire.12, 35, 37

Figure 1. 6. The possible structures of HIIR.

where reflects the backbone of polymer, n and m are number of units in the polymer backbone.32

After looking back at the history of sulfur-accelerator curing system, I want to cite what Oenslager wrote down at the beginning of his paper to end this part.23 “During my career as research chemist I have worked on many problems the solution of which was required for economic reasons, and, as is no doubt the experience of research men generally, at times my efforts were successful but more often, I fear, were productive of little that was of practical value. Whatever the results, however, it has been my practice when my studies of a problem were completed to dismiss it from my mind and give my attention to new lines of work; and so it was something of a surprise to me to learn a few weeks ago that one particular line of research work which occupied my attention during my early days in the rubber industry had been considered by my fellow chemists to be worthy of special commendation—the work recognized tonight in the award of the Perkin Medal.”

1.2.2 Vulcanization by Phenolic Compounds

Sulfur and accelerator are the most popular way to vulcanizate IIR in industry.

However, sulfur-cured vulcanizates show poor high temperature properties. These materials tend to soften when exposed to elevated temperatures of 350-400 oF for an extended period.38, 39 As shown in Figure 1. 7, the value of volume increase at Y axis was determined by equilibrium volume swelling. The lower value indicates higher crosslinking density in compound. The curves of three compounds at low temperature 250-300 oF reached to the limiting value of 500%. However, there was reversion in compounds at high temperature 350 oF and 400 oF. It indicates poor high temperature resistance of sulfur-cured

IIR.38, 39 This is due to the low dissociation energy of sulfidic crosslinks. As shown in Table

1. 1, the bond energy of carbon-carbon crosslinks (-C-C-) is higher than sulfidic

12 crosslinks.40 Much more stable vulcanizate could be prepared by carbon-carbon crosslinks instead of sulfidic crosslinks. Then phenolic compounds are used as curing agents to introduce carbon-carbon crosslinks into compounds.40-42

Figure 1. 7. The percent of volume increase of sulfur-cured IIR as a function of curing

time at different temperature.39

Table 1. 1. The bond energy of crosslinks.40

Bond Energy Type of crosslinks (kJ/mol)

-C-C- 351

-C-S-C- 285

-C-S-S-C- 267

-C-Sx-C- < 267

The vulcanization of natural rubber with 2,6-dimethylol-4-hydrocarbyl-phenols and their condensation polymers was tried as early as 1936.43 Phenolic compounds were then used to cure other diene rubbers, such as IIR and BR.44-46 The possible structure of phenol-formaldehyde resin is shown in Figure 1. 8.44 The resin usually has di-substituted -

CH2-X groups, where X is an -OH group or a halogen atom. These groups are used for crosslinking. The crosslinking mechanism by phenolic compounds are studied. Van der

Meer proposed the allyl hydrogen mechanism43 (Scheme 1. 3). The first step is to form o- methylene quinone intermediate, then it reacts with double bond via a six-centered “ene” intermediate. The new formed keto intermediate would rearrange to the phenol. Then it forms the new o-methylene quinone intermediate and reacts with another double bond. It gives the crosslinked product. This process retains the unsaturation of rubber. However,

Van der Meer did not show the evidence to confirm his mechanism. Later Lattimer studied the mechanism of resin cure by model reaction, and proposed the chroman mechanism.44

For the chroman mechanism in Scheme 1. 4, the first step is also to form o-methylene quinone intermediate, then it combines with double bond via 1,4-cycloaddition reaction to form product with the chroman structure. Subsequent dehydration and a second addition of alkene gives the crosslinked product. This process requires bisphenol and destroys the unsaturation of rubber during forming crosslinks. Martin Van Duin used 2-ethylidene norbornane and 2-hydroxymethylphenol as model to study the mechanism.47, 48 The results showed evidence for the existence of the methylene bridged structures. The reaction product also contained unsaturation. He proposed the cationic reaction mechanism and thought benzylic cations as intermediate rather than o-methylene quinone intermediate.

Figure 1. 8. The possible structure of phenol-formaldehyde resin. where R is an alkyl or hydrocarbyl group, and n may vary from 0 to 5 or 6.44

Scheme 1. 3. The allyl hydrogen mechanism of resin cure.43

Scheme 1. 4. The chroman mechanism of resin cure.44

Scheme 1. 5. The cationic reaction mechanism of resin cure.48

Although the vulcanization mechanism of resin cure is not clear, the studies of IIR with phenolic compounds show those vulcanizates are much more stable than sulfur-cured

IIR.44 45, 49 As shown in Figure 1. 944, sulfur-cured compound S-1 reached the maximum stress at 200% elongation in approximately 1 hour at 322 oF. Then reversion happened by continued heating. But resin-cured compound R-1 showed extreme thermal stability. There is no reversion even after 16 hours at 322 oF. It is noteworthy that resin-cured compound

R-1 shows slow curing behavior, the curing was still going on even after 8 hours.44

Figure 1. 9. Vulcanization of IIR by sulfur-accelerators and phenolic compounds.44

1.2.3 Vulcanization by Peroxide-coagent System

Since the curing rate of rubbers by phenolic compounds is slow, and it also needs unsaturation of rubbers, peroxide is another way to introduce carbon-carbon crosslinks into vulcanizates. Peroxide can be used to cure most rubbers, especially saturated rubbers, which are unsuitable for sulfur and resin vulcanization.50 Vulcanizates cured by peroxide have good aging and low compression set properties in comparison to vulcanizates cured by sulfur-accelerator. Peroxide-based formulations are also simple and generate little byproducts.51-55

There are several requirements for selection of a suitable peroxide. It should be stable and safe during preparation, processing and storage of rubber compounds. It could decompose fast at vulcanization temperature and give efficient crosslinking of rubber matrix.52, 56 Depending on the structures of peroxides, peroxides are generally classified into six groups: 1) dialkyl peroxides, R-O-O-R; R, alkyl group; 2) alkyl-aralkyl peroxides,

R-O-O-R’; R, alkyl group; R’, aryl group; 3) diaralkyl peroxides, R’-O-O-R’; R’, aryl group; 4) diacyl peroxides, R-C(O)-O-O-(O)C-R’; R, R’, alkyl and/or aryl groups; 5) peroxyketals, R-O-O-(R)C(R’)-O-O-R’; R, R’, alkyl and/or aryl groups; 6) peroxyesters,

R-C(O)-O-O-R’; R, R’, alkyl and/or aryl groups.52 The stability of peroxide is estimated by ten-hour half-life temperature (10-h HL), which is defined as the temperature required to decompose one half of the peroxide in ten hours.51 There is a trade-off between scorch minimization and process speed. Peroxide with higher 10-h HL requires higher vulcanization temperatures. But It is less likely to cause scorch. Peroxide with lower 10-h

HL is prone to scorch and provides a lower temperature cure. The structures and 10-h HL of several common peroxides used for rubber vulcanization are shown in Figure 1. 10.51

Figure 1. 10. The structures and ten-hour half-life temperature of several common

peroxides for rubber vulcanization.51

The general peroxide-induced vulcanization is shown in Scheme 1. 6.56 The initial step is peroxide decomposes to form radicals (equation A in Scheme 1. 6). Then radicals could abstract hydrogen atom from nearby polymer chain and radicals are transferred to polymer backbone. This process is called hydrogen abstraction (equation B in Scheme 1.

6), which is an important step in peroxide curing reaction. A crosslink forms by combining two formed polymer radicals (equation D in Scheme 1. 6). It is worth mentioning that radicals generated by different types of peroxide have different energy levels. Hydrogen abstraction proceeds only if the energy level of radical is reduced in the process. The energy levels of several radicals are shown in Scheme 1. 7.51 For example, an alkoxy radical can abstract hydrogen from allylic carbon, but it cannot abstract from a benzene molecule, which has higher energy level than alkoxy radical. Besides of hydrogen abstraction, radical addition is the other important step (equation C in Scheme 1. 6). Radical adds to one of the carbon atoms involved in a double bond and is transferred to polymer backbone. After that radical is able to form additional bonds with another double bonds. The crosslinks form during this process. One radical can generate several crosslinks in radical addition process.

It is different from hydrogen abstraction process, where two radicals only form one crosslink. In some cases, radical from peroxide also could combine with polymer radical to terminate the reaction without forming crosslink (equation E in Scheme 1. 6).

Scheme 1. 6. The general peroxide-induced vulcanization.56

where reflects the backbone of polymer.

Polymer structure is an important determinant in the competition between hydrogen abstraction and radical addition. For example, hydrogen abstraction is predominant in natural rubber cured by dicumyl peroxide. Radical addition is prevalent in SBR and BR cured by dicumyl peroxide, although hydrogen abstraction also exits in this case.57, 58 In addition, the type of peroxide also plays an important role. Acyloxy radicals from diacyl peroxides and peroxyesters have more tendency to undergo radical addition than alkoxy radical does. This might be due to less steric hinderance and the higher energy level of the acyloxy radical.51, 56 Bulky alkoxy radicals would not be efficient in radical addition.51, 59-

Scheme 1. 7. The energy levels of radicals involved in peroxide vulcanization.51

Peroxide is always used to cure highly saturated hydrocarbon polymers, such as polyethylene, ethylene-propylene rubber and ethylene-propylene-diene terpolymer

(EPDM). Unfortunately, curing IIR and HIIR with peroxides is problematic because the quaternary carbons in the polyisobutylene chain make chain scission competitive against crosslinking..62-64 The chain scission and crosslinking reactions of IIR by dicumyl peroxide are shown in Scheme 1. 8.64

Scheme 1. 8. Chain scission and crosslinking of IIR by peroxide. 64

where reflects the backbone of polymer.

Many people are trying to prepare peroxide-curable IIR, which could extend the application of IIR.44, 57, 58, 65-67 Three approaches have been explored to enhance peroxide curability of polyisobutylene-based polymers. The first is to increase the isoprene content in IIR. Loan found chain scission of IIR was decreased by increasing unsaturation of IIR, where IIR could be crosslinked by peroxide when isoprene content was above 3 mol%.64

Early studies show that isoprene acts as a chain transfer agent during the cationic copolymerization of isobutylene and isoprene, resulting in IIR with unacceptable molecular weights.68 Recent reports indicate that the problem can be surmounted to give high molecular weight IIR with high isoprene contents (up to 6 mol%).69-71 Another method is to increase the reactivity of the unsaturations in IIR. Oxley synthesized a terpolymer of isobutene, isoprene and divinylbenzene, which can be crosslinked by peroxide. But crosslinking also happened during polymerization of terpolymer and gave high gel content in the product. It caused poor processabilities.72

Post-polymerization modification of IIR or HIIR is another way to make peroxide- curable IIR. Parent group used HIIR as raw materials to synthesize different types of IIR derivatives, owing to the high reactivity of allylic halogen functionality in HIIR.73-84

Ether76, ester73, ammonium74 or phosphonium84 functionalities are introduced into HIIR by nucleophilic substitution reaction. The synthesis of IIR derivatives by BIIR esterification is shown in Scheme 1. 9.79 These IIR derivatives contain pendant polymerizable functional groups such as styrenic74, acrylic73, 76, maleimidic74 and vinylic73 functional groups, which act as crosslinking sites. The network could form due to high reactivity of these functional groups. The structures of some of IIR derivatives containing reactive functional groups are

24 shown in Figure 1. 11.74 However, these systems are limited by the allylic bromide content of BIIR and still have issues with chain scission.74, 82, 84-86

Scheme 1. 9. Synthesis of IIR derivatives by BIIR esterification.79

Figure 1. 11. The structures of IIR derivatives containing reactive functional groups.74

Another peroxide-curable IIR derivative was synthesized via Suzuki-Miyaura coupling reaction of HIIR with 4-vinylphenylboronic acid and phenylboronic acid.87 The crosslinking behavior of resulting polymer could be controlled by changing ratio of 4- vinylphenylboronic acid to phenylboronic acid, since only vinyl groups from 4- vinylphenylboronic acid act as effective crosslinking sites. However, it is also limited by the allylic bromide content of BIIR, and material has to be purified for synthesis, since the acidic residue contained in BIIR spoils the catalyst, it results in the low coupling efficiency.

In another study, the epoxidized butyl rubber was synthesized by m-chloroperoxybenzoic acid with IIR.88 The ring-opening/elimination of epoxidized butyl rubber provides another

26 way to prepare multifunctional graft copolymers.88-90 The synthesis of IIR derivatives with pendant carboxylic acid is shown in Figure 1. 12.88

Figure 1. 12. Synthesis of IIR derivatives with pendant carboxylic acid.88

In parallel to the above approaches of chemical modification, coagents that contain multiple allylic, acrylic53, maleimide55, 91-93, or other unsaturated groups94 have been investigated to promote crosslinking and suppress chain scission in peroxide cure.95 The above approaches are often combined to obtain optimal mechanical properties, such as higher stress at break, hardness and modulus, etc..95, 96 The structures of several common coagents used in peroxide vulcanization are shown in Figure 1. 13.51 There are two types of coagents used in peroxide vulcanization. The type I coagents in Figure 1. 13 include acrylates, methacrylates and maleimides. These coagents tend to increase the cure rate and may lead to scorch. Most of them do not have allylic hydrogens, so it can be expected that

27 radical reactions involve these coagents, especially acrylates and methacrylates, via radical addition rather than hydrogen abstraction. It is noteworthy that maleimides can undergo both radical addition and abstraction.77 The type II coagents in Figure 1. 13 include polybutadiene, triallyl isocyanurate and diallyl phthalate, etc. These coagents contain readily accessible vinyl unsaturation sites and allylic hydrogens for radical addition and hydrogen abstraction. Unlike the type I coagents, these coagents improve crosslinking efficiency without increasing the cure rate or adding scorch.51, 96-99

Figure 1. 13. The structures of common coagents used in peroxide vulcanization.51

Different coagents have different mechanisms to improve crosslinking efficiency.99-104 In general, coagent reacts with polymer radicals. It is grafted on polymer and forms a new radical, which can do radical addition or react with another polymer radical to form crosslinking. Most coagents are relatively polar and are not easily miscible with nonpolar rubber compound. A significant portion of coagent probably favors phase separation in rubber compound and forms distinct domains where it can homopolymerize.

The homopolymerized region can be covalently bonded to rubber matrix. But it also can just consume peroxide without bonding to rubber matrix and result in inefficient vulcanization.51

When coagent is used in IIR, severe chain scission still happens.53, 54 In HIIR, bismaleimide is often used to prevent chain scission.55, 91-93 Polybutadiene (PB) and ethylene-propylene-diene monomer rubber (EPDM) are also tried to mix into BIIR as coagent to make peroxide-cured BIIR.94 105 In some cases, the reactive group in the IIR derivative is specifically designed to be used with a certain cogent. For example, vinyl ether side chains provide good crosslinking yields due to their reactivity with N- arylmaleimides.76, 77 Although significant progress has been made to cure IIR and its derivatives with peroxide-based curing systems. However, these systems still fail to produce vulcanizates having mechanical properties comparable to those of sulfur-cured

IIRs.76, 92, 105

1.3 Recycling of Rubbers

Disposal of waste rubbers is a serious environmental problem. As shown in Table

1. 2, over 4 million tons of scrap tires were generated in USA in 2017.106 In addition, the

29 management of other waste rubbers has become a growing problem in industry since over

150,000 tons or more rubber are scrapped from production of non-tire goods.2 These waste rubbers need very long time to degrade in nature due to the crosslinking structure of rubber and presence of stabilizers and other agents.107

Table 1. 2. U.S. scrap tire market summary (2009-2017).106

Landfill is one of way to dispose waste rubbers.111, 112 But this process is no longer feasible due to the decreasing of available sites and corresponding cost explosion. Waste tires in stockpiles also provide good breeding conditions for mosquitoes, snakes and other pests, and cause several fire related issues. In addition, landfilling of waste rubbers wastes the valuable rubber and deteriorates soil properties because of leaching small molecular weight additives from rubbers to environment.108, 109 The development of suitable technology to reuse waste rubbers is important. A number of methods have been applied 30 in an attempt to solve the problem and find more efficient way to reuse of waste rubbers.

For example, most of waste rubbers are used as fuel, it is one of the most popular way to recycle waste rubbers. However, it is a low value recovery process of waste rubbers.110

Coal gas, oil and charcoal black can also be made by thermal decomposition of waste rubbers. But the cost of this process is quite high.111

Waste rubbers are also turned into ground rubber particles (GRPs). GRPs are prepared by mechanical grinding process, where waste rubbers are placed in an open two- roll mill, and milling is performed to reduce particle size.112, 113 The grinding process varies according to requirement of final product. Depending on grinding condition, the process is divided as cryogenic, ambient and wet grinding.108 Cryogenically GRPs are obtained by cooling scrap tires below their glass transition temperature and pulverizing them in the mill. Cryogenically GRPs have much finer particle size varying from 30 to 100 meshes and less degradation inside of rubber, since little heat is generated during the process. But the ambient grinding process is often used due to the high cost of cryogenically grinding process. It produces relatively large particles (10-30 meshes). Since the process generates significant amount of heat, it can degrade the rubbers. Compared to ambiently GRPs, cryogenically GRPs have smoother surface. However, the smoother surface of cryogenically GRPs shows limited physical binding with matrix material and deteriorates the mechanical properties of composites.114 Wet or solution grinding process is developed from ambient grinding process. It involves putting coarse ground rubber particles (10-30 mesh) into a liquid medium and grinding between two closely spaced wheels, where 400 mesh particles can even be produced.111

The utilization of GRPs has been attractive in relation to environmental protection and cost of materials. GRPs are not just considered as a cheap filler but as a valuable component of sustainable rubber composites, which are used in molded/extruded products

(wheels, gasket, sole), playgrounds, mulch, animal bedding, artificial sports surfacing and automotive industries.115 For example, GRPs are used in thermoplastic vulcanizates

(TPVs), which consist of thermoplastic matrix and a vulcanized rubber phase, where the rubber phase is vulcanized under dynamic shear while maintaining the thermoplastic of blend during process.116 The novel TPVs with unique properties have been studied by many studies.117-119 GRPs as filler in a thermoplastic matrix offers an opportunity to obtain materials which are similar with TPVs.120, 121 However, TPVs obtained with GRPs differ from conventional TPVs, since GRPs filled TPVs is based on the processing of thermoplastic matrix with already crosslinked rubber particles. Compatibility of GRPs with matrix has to be considered. Sonnier used γ irradiation to increase adhesion between GRPs and thermoplastic matrix.122 Kim used ultraviolet and photo-initiator to graft acrylamide on GRPs surface. Maleic anhydride-grafted polypropylene was then added as a compatibilizer to improve the adhesion between modified GRPs and HDPE.123 The coupling agent silane A-174, ethylene propylene diene monomer rubber (EPDM), Styrene– ethylene–butylene–styrene (SEBS) and maleic anhydride-grafted SEBS were also used as compatibilizer to improve interfacial adhesion between GRPs and thermoplastic matrix.108,

124 Besides of TPVs, GRPs are also blended with virgin rubber to make new vulcanized products.115

Unfortunately, incorporation of GRPs into polymer blends is often not suitable due to the network in particles, which leads to the weak interface and deteriorates the properties

32 of final product.108 The loading of particles into formulation is usually limited. In order to solve the poor adhesion between components of mixture, reclaiming is usually carried out.

It is a procedure in which vulcanized rubber waste is converted, using mechanical and thermal energy and chemicals, into a state in which it can be mixed, processed and vulcanized again.2 The principle of this process is devulcanization, which means breakdown of three-dimensional network through the cleavage of crosslinks or chain backbone. Physical reclaiming processes and chemical reclaiming processes are generally used.

1.3.1 Reclaiming of Vulcanizates by Physical Reclaiming Processes

Physical reclaiming processes include microwave and ultrasonic processes.108 For microwave devulcanization, electromagnetic energy is used to break sulfur-sulfur and carbon-sulfur bonds in rubber. In this method, rubber is exposed to the specific amount of microwave energy at the specific frequency in order to break crosslinks. Generally, microwave is feasible for rubbers with polar groups. For non-polar rubbers such as styrene- butadiene rubber (SBR), microwave can be achieved by using conducting fillers in rubber compounds.125 The microwave processing can be rapid and uniform. The electromagnetic energy is converted into heat. The temperature around 260-350 oC can be achieved.

Therefore, the fast heating rates encountered using microwave energy can lead to reduced processing time and energy conservation.126 The particle size of GRPs is an important factor in microwave devulcanization. For larger particles, such as 0.5 cm in diameter, hot spots appeared in the particles, which caused a part of sample degraded, the other part may still remain vulcanized.127

In ultrasonic devulcanization, ultrasonic energy is used for devulcanization of vulcanizates. In this process, rubbers such as tires were immersed into the liquid and then ultrasonic energy was put into system. It caused rubber article to disintegrate and suspend or dissolve into liquid.128 Continuous ultrasonic devulcanization of waste rubbers was studied by using coaxial designed extruder, which is shown in Figure 1. 14.129 The effect of processing parameters (amplitude of ultrasound, flow rate, dimension of treatment zone, temperature, pressure and ultrasonic power) on properties of waste rubbers were investigated. In the study, the overtreated sample was found. It is probably related to the significant degradation of macromolecular backbone.129 They also reported that the devulcanized rubber is re-vulcanized by same recipe as the virgin rubber. Bimodal network forms during the process of re-vulcanization of ultrasonically devulcanized rubber.130

Figure 1. 14. Schematic of the devulcanization reactor.129

1.3.2 Reclaiming of Vulcanizates by Chemical Reclaiming Processes

The chemical reclaiming processes are always used in the recycling of waste rubbers. A large number of chemical reclaiming agents are used to devulcanize crosslinks of natural and synthetic rubbers.

Moore studied the structural features of sulfur crosslinks in vulcanizate by triphenylphosphine and sodium di-n-butyl phosphite.131, 132 The reaction of triphenylphosphine with mono-, di- and poly-sulfides indicates that sulfur in mono-sulfides cannot be removed by triphenylphosphine. The number of sulfur atoms removed from poly- sulfides (R-Sx-R’, where R, R’ are alkyl, aralkyl or alkenyl) depends on the nature of the

R and R’ groups. When R and R’ are alkyl or benzyl group, x is reduced to 2. When R and

R’ are alkenyl (such as CH2=CH-CH2-, MeCH=CH-CH2-, Me2C=CH-CH2-, Me2C=CH-

CHMe-, where Me is methyl group) then x is reduced to 1. For alkyl alkenyl disulfide

(RS2R’, where R is ethyl group and R’ is Me2C=CH-CHMe-), x is also reduced to 1. The devulcanization of vulcanizate with triphenylphosphine is similar with the reaction of triphenylphosphine with elemental sulfur, where the nucleophilic displacement mechanism was proposed.133 The devulcanization process for dialkyl and dibenzyl poly-sulfides is represented in Figure 1. 15132 (equation 1). In some cases, the devulcanization of dialkenyl disulfides and the alkyl alkenyl disulfides follows the allylic rearrangement of one of the alkenyl groups (equation 2 in Figure 1. 15). The partial devulcanization of unaccelerated

NR-sulfur networks by triphenylphosphine indicates that poly-sulfidic links are conversed into either mono- or di-sulfidic links.

Figure 1. 15. The devulcanization process by triphenylphosphine.132

Sodium di-n-butyl phosphite is used to break di- and poly-sulfide crosslinks without destroying carbon-carbon and mono-sulfide crosslinks in compound.131, 134 It reacts readily with dialkyl disulfides at room temperature (equation 1 in Figure 1. 16), where R is alkyl group.131 The reaction with dialkenyl trisulfides and disulfides at room temperature is shown in equations 2-4 (Figure 1. 16), where R’ is alkenyl group. Unfortunately, sodium di-n-butyl phosphite is hydrolyzed readily to sodium hydroxide. It requires rigorous drying of solvent and rubber samples before treatment to prevent hydrolysis of sodium di-n-butyl phosphite.

Figure 1. 16. The devulcanization process by sodium di-n-butyl phosphite.131

In another study, methyl iodide could be used to break mono-sulfidic crosslinks in vulcanizate.135 It is easily swollen into the rubber and can be removed under vacuum situation. The reaction of di-n-propyl sulfide with methyl iodide is low, but the reaction can be catalyzed by mercuric iodide. Chemicals, like diallyl sulfides, cyclic mono-sulfides, alkenyl t-alkyl sulfides and ditertiary alkyl sulfides, show the good reactivity with methyl iodide. It is likely because mono-sulfides where the two groups attached to sulfur atom are capable to exist as carbonium ions. The reaction of diallyl sulfide with methyl iodide is shown in Figure 1. 17.135

Figure 1. 17. The reaction of diallyl sulfide with methyl iodide.135

Porter used lithium aluminum hydride (LAH) to break di-sulfidic and poly-sulfidic crosslinks in the network.136 LAH cleaves the di-sulfidic crosslinks to give two molecules of thiol (equation 1 in Figure 1. 18).136 When organic polysulfides are treated with LAH, the terminal sulfur atoms are liberated as thiols and the interior sulfur atoms are transferred to hydrogen sulfide (equation 2 in Figure 1. 18).

Figure 1. 18. The devulcanization process by lithium aluminum hydride.136

The isopropylthiol-piperidine system was applied to cleave poly-sulfidic bonds

(Figure 1. 19)135 in natural rubber network while having insignificant action on corresponding mono-sulfidic, di-sulfidic and carbon-carbon bonds.135, 137, 138 The thiol- piperidine combination gives an associate, possibly piperidinium propane-2-thiolate ion pair, where the nucleophilic properties of sulfur atom are enhanced.

Figure 1. 19. The devulcanization process by isopropylthiol-piperidine.135

The more reactive thiol reagent system (hexane-1-thiol in piperidine solution) is used to cleave di-sulfidic and poly-sulfidic crosslinks according to equations 1 and 2 in

Figure 1. 20.139 Piperidine swells natural rubber vulcanizates to approximately the same extent as benzene does. Thiol is easy to access the crosslinks and sufficiently basic to break the sulfur-sulfur bonds at a reasonably rapid rate. The reaction of several model disulfides

(0.1M) with butanethiol (1M) in piperidine solution at 25 oC is shown in Figure 1. 21. The structures of models used are shown in Figure 1. 22.139 The model reaction is likely

39 different from the situation in rubber network. For rubber network, the ratio of thiol to chemical crosslinks within swollen gel is generally greater than the model reaction. In addition, the di-alkyl disulfide formed during reaction with network equilibrates with the bulk of the solution, so reducing its effective concentration in the swollen gel, which pushes the equilibrium further to complete cleavage. Based on these arguments, there would be an insignificant number of disulfide crosslinks remaining after the treatment of rubber network.

Figure 1. 20. The devulcanization process by hexane-1-thiol and piperidine.139

Figure 1. 21. Reactions of disulfides with butane-1-thiol in piperidine solution.139

Figure 1. 22. The structures of models in Figure 1. 21.

The common chemicals used to cleave crosslinks of vulcanizates are summarized in Table 1. 3.107 Besides of chemicals described above, more specific examples, devulcanization of GRPs, are talked below.

Table 1. 3. Chemicals used to cleave crosslinks.107

Chemicals Types of crosslinks which can be broken

Triphenylphosphine Poly-sulfide crosslinks

Sodium di-n-butyl phosphite Di- and poly-sulfide crosslinks

Propane-thiol/piperidine Poly-sulfide crosslinks

Hexane-1-thiol/piperidine Di- and poly-sulfide crosslinks

Dithiothreitol Di-sulfide crosslinks

Lithium aluminium hydride Di- and poly-sulfide crosslinks

Phenyl lithium in benzene Di- and poly-sulfide crosslinks

Methyl iodide Mono-sulfide crosslinks

Aqueous sodium hydroxide (NaOH) is first used to reclaim vulcanizates of natural rubber at ~190 oC. A phase transfer catalyzed devulcanization process was then invented by a phase transfer catalyst, which allows the transport of OH- ions into rubber vulcanizate particles at low temperature (below 150 oC) to selectively cleave di- and poly-sulfide crosslinks (Figure 1. 23).140 The solution of phase transfer catalyst (Aliquat 336) in benzene was mixed with GRPs, and the mixture was stirred slowly while heating to reflux. Aqueous sodium hydroxide solution was then added, and the mixture was stirred at reflux for two

42 hours. The mixture was cooled and washed by water until pH of washing water was about

7-8. The treated particles have substantially lower crosslinking density.140

Figure 1. 23. The devulcanization process by NaOH under phase transfer catalyst.140

Note that Rp represents the polymer chains.

Alkali metal, like sodium, is also used to devulcanize GRPs.141 The GRPs were dispersed in the solvent which swells the GRPs. The alkali metal was then added to the

GRPs dispersion. The reaction was conducted under hydrogen atmosphere without oxygen.

The temperature was controlled at ~250 oC, which is below the decomposition temperature of rubber. The process is required several minutes to devulcanize rubber. The devulcanization may follow the reaction in Figure 1. 24, where [(R)n-Sx-(R’)m]y represents the vulcanizate.

Figure 1. 24. The devulcanization process by sodium under hydrogen atmosphere.141

In another study, GRPs were mixed with plasticizer (dipentene, tall oil pitch) first, followed by adding phenyl hydrazine (0.2-1 wt%) or diphenyl guanidine (0.2-0.8 wt%).

Then an oxidizable iron metal chloride (ferrous chloride) was added. The reaction happened in the solid phase at most 100 oC by agitation for about 30 min. The reaction requires the addition of oxygen from air in the mixer. In this process, the phenyl hydrazine or diphenyl guanidine breaks a few percent of double bonds on polymer chains of GRPs.142

Sverdrup invented a devulcanization method by heating GRPs comprising the thiophene ring, such as thiophene thiol, dithiophene disulfide, di-tertiary-butyl thiophene, etc..143 The mercaptans (xylyl mercaptan, thiophenol, benzyl mercaptan, amyl mercaptan and butyl mercaptan, etc.) and sulfide (diphenyl sulfide, diphenyl disulfide, benzyl disulfide, amyl disulfide and butyl disulfide, etc.) are also used as the reclaiming agents to devulcanize GRPs.144 Jasiunas also patented a method to prepare chemically functionalized

GRPs, which performs as an uncured elastomer rather than merely serving as a filler.145

The GRPs were blended with the processing aid and functionalizing agents, which are shown in Figure 1. 25 (1)-(3), then the blended mixture was processed under high shear and low temperature, followed by adding a stabilizer to the reacted mixture to produce the renewed rubbers. Another functionalizing agent comprising urea or urea derivative

(structure 4 in Figure 1. 25) is also selected to devulcanize GRPs. It is used with dicarboxylic acid, wherein the molar ratio of the urea or urea derivative to the dicarboxylic acid ranges from about 0.5:1 to about 2.5:1.146 The devulcanization of polyisoprene rubber with functionalizing agent tetra-benzyl thiuram disulfide was also studied and is shown in

Figure 1. 26.145 The sulfur-sulfur bonds in GRPs are broken by functionalizing agents with only a minimal number of double bonds in the backbone of the polymer being broken.

Figure 1. 25. Functionalizing agents used for devulcanization.145, 146

Figure 1. 26. The devucanization of vulcanized polyisoprene rubber with

tetrabenzylthiuram disulfide as the functionalizing agent.145

De describes a mechanical reclaiming process of GRPs by tetramethyl thiuram disulfide (TMTD), which is used as devulcanization agent during reclaiming process.147

TMTD also acts as curing agent during re-vulcanization process. The possible devulcanization mechanism is proposed in Figure 1. 27. Mechanical shearing leads to breakdown of polymer chain or crosslinking bonds in polymer, and breaks TMTD into radicals (a and b in Figure 1. 27). Radicals from TMTD combine with broken polymer radicals and prevent recombination of polymer radicals (c in Figure 1. 27). Zedler also added accelerator TMTD to GRPs.148 It results in the worse mechanical properties of vulcanizate, where the stress and strain at break decreased about 30% and 70%, respectively. This is because of high crosslinking density in sample. High shear mechanical process also could deteriorate physical properties of materials due to main chain scission.

Figure 1. 27. The devulcanization mechanism of GRPs with TMTD.147

The coupling agent bis(3-triethoxysilyl propyl) tetrasulfide (TESPT) was also used as a devulcanizing agent in GRPs.149 The process to prepare devulcanized rubber is shown in Figure 1. 28. The effect of milling time and amount of TESPT are characterized by sol- gel content and crosslinking density in devulcanized SBR. The dispersion of silica in devulcanized SBR was also studied by SEM, which shows devulcanized SBR by TESPT facilitates the silica dispersion.149

Figure 1. 28. The process for preparation of devulcanized SBR.149

The treatment of GRPs by sulfuric acid was used to improve adhesion with other

polymers. Hydrogen atom of C-H bond on polymer chain is removed and replaced by SO3 molecular, which is hydrogenated to form sulfonic acid. Ammonium was used to neutralize sulfonic acid in order to form more stable structure.150 Compared to untreated GRPs, modified GRPs in HDPE was found to improve interaction between GRPs and HDPE. It is because micropores and cavities formed by acid treatment and it helped to improve the interfacial contact between matrix and particles.151 However, the exist of cavities can also introduce defects into compound, which decreases the properties.108 In another study,

GRPs was modified by nitric acid and 30% hydrogen peroxide solution. This treatment introduced some functional groups on the surface of GRPs. Modified GRPs was incorporated in natural rubber alone or in combination with carbon black. Compared to untreated GRPs, the stress at break and aging resistant of vulcanizates containing treated

GRPs were improved.152

The numerous attempts of recycling vulcanizates are tried, since devulcanized rubbers still have low cost in comparison to virgin rubber. The reuse of devulcanized rubber also makes contribution to reducing carbon emission. In addition, it has some improvement in processing. For example, compounds containing devulcanized rubbers have much more uniform, lower die swell and calendar shrinkage. It is less thermoplastic than raw rubber compounds, due to some gel contents in compound.111 However, although processability is generally achievable via devulcanization140-144, 153-156, these devulcanized rubbers still have inferior mechanical properties, because it is structurally and compositionally different from its virgin counterpart.147-149 Alternatively, GRPs is blended with virgin rubber to make new vulcanized products, it also significantly lower the mechanical properties even if only

49 present at a small fraction in the blend with virgin rubber.115, 145, 146, 152, 157-159 The application of devulcanized rubbers in tire industry is limited, as radial tires requires high tensile properties. Utilization of devulcanized rubbers in large quantity is only possible in some applications of low performance requirement, such as mats, hose, conveyer belt and road paving, etc.108, 160 Significant advances are necessary for recycling vulcanizates into high-value products such as tires. However, little attention has been given to the underlying scientific and engineering challenges despite of the numerous attempts of recycling rubbers.

CHAPTER II

VULCANIZATION OF BUTYL RUBBER CONTAINING GEMINAL VINYLIDENE-

ACRYLATE GROUPS

2.1 Introduction

Polyisobutylene is unique among rubber materials for its gas impermeability at temperatures well above its glass transition temperature at ~-55 °C. With the incorporation of a small amount of isoprene,10 the copolymer, known as isobutylene-isoprene rubber

(IIR), retains gas impermeability and becomes vulcanizable. Vulcanization of IIR with a low degree of unsaturation is slower than those of highly unsaturated general-purpose rubbers such as styrene-butadiene rubber (SBR) and natural rubber (NR) under the same conditions.26 Halogenation of the isoprene units in IIR results in halogenated isobutylene- isoprene rubber (HIIR) that displays a much improved vulcanization rate that allows co- vulcanization with SBR and NR.12

Various vulcanization methods have been developed for IIR and HIIR,26 with accelerated sulfur cure25 and phenolic resin cure44 being the most common. Sulfur cure of

HIIR is usually adopted when co-curing with other rubbers are required (for example, in tire manufacture), but sulfur-cured vulcanizates shows poor thermal aging resistance and high set due to the low dissociation energy of sulfidic crosslinks.38 Resin cure forms

51 crosslinks through the much stronger C-C bonds and compensates the drawbacks of sulfur cure. With the appropriate choice of curing method, IIR and HIIR satisfy the needs of a wide range of applications that require gas impermeability, oxidative stability, and chemical resistance. These applications include inner liners of tires, curing bladders, roofing, reservoir membranes, electrical insulation and automotive components.161

Curing IIR and HIIR with peroxides is problematic because the quaternary carbons in the polyisobutylene chain make chain scission competitive against crosslinking.63, 64

Nevertheless, there have been sustained efforts to overcome the limitation of peroxide cure.

One motivation comes from healthcare-related applications, as leachable curatives and their residuals increasingly become a concern.53-55 Compared to sulfur cure and resin cure, peroxide-based formulations are simple and generate little byproducts.51, 52

Three approaches have been explored to enhance peroxide curability of polyisobutylene-based polymers. The first is to increase the isoprene content in IIR. Early studies show that isoprene acts as a chain transfer agent during the cationic copolymerization of isobutylene and isoprene, resulting in IIR with unacceptable molecular weights.68 Recent reports indicate that the problem can be surmounted to give high molecular weight IIR with high isoprene contents (up to 6 mol%).70, 71, 162 Another method is to increase the reactivity of the unsaturations in IIR. Since the early failed attempt of curing isobutylene-isoprene-divinylbenzene terpolymers, which suffered from poor processabilities caused by their high gel contents,72 post-polymerization modification of

IIR88-90 or HIIR73, 74, 84-87 to install functional groups with enhanced reactivities for peroxide-initiated crosslinking has been the main approach. In parallel to the above approaches of chemical modification, coagents that contain multiple allylic, acrylic53,

52 maleimide55, 91-93, or other unsaturated groups94 have been investigated to promote crosslinking and suppress chain scission in peroxide cure.95 The above approaches are often combined to obtain optimal mechanical properties. In some cases, the reactive group in the

IIR derivative is specifically designed to be used with a certain cogent. For example, vinyl ether side chains provide good crosslinking yields due to their reactivity with N- arylmaleimides.76, 77

Significant progress has been made to cure IIR and its derivatives with peroxide- based curing systems. However, these systems still fail to produce vulcanizates having mechanical properties comparable to those of sulfur cured IIRs.76, 92, 105 We report here a highly efficient method to produce activated IIR and the development of peroxide-based curing systems for the activated IIR. The mechanical properties of the peroxide-cured activated IIR reported herein exceeds the previously reported peroxide-cured IIR and its derivatives and of approaches those of sulfur-cured IIR.

2.2 Experimental

2.2.1 Material

m-Phenylene-N,N’-bismaleimide (BMI), trimethylolpropane trimethacrylate

(TMPTMA) and benzoyl peroxide (BPO) were purchased from Sigma-Aldrich and used as received except 2-Methyl-2-butene, which was dried over Na/K, distilled, and stored under nitrogen. Exxon Butyl 268S and Bromobutyl rubber 2222 were donated by Goodyear

Chemical Corporation. Dicumyl peroxide (DCP), stearic acid, zinc oxide, sulfur, tetramethylthiuram disulfide (TMTD) and 2-mercaptobenzothiazole (MBT) were obtained from AkroChem Corporation. 53

2.2.2 Chemical Structure Characterization

Nuclear magnetic resonance (NMR) spectra were obtained on a Varian Avance 500

MHz NMR spectrometer. 1H chemical shifts were determined using solvent peaks as the references.

Gel permission chromatography (GPC) was performed at 40 °C with THF as the eluent using a Tosoh HLC8320GPC with two TSK-GEL SuperH3000 columns and one

TSKGEL SuperH5000 column equipped with a refractive index detector. The flow rate was 0.350 mL/min. THF solutions of polymers (3 mg/mL) were prepared and filtered through 0.45 µm nylon microfilters before injection. The molecular weight was determined relative to polystyrene standards.

Mass spectrometry (MS) experiments were performed on a HCT Ultra II quadrupole ion trap mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with electrospray ionization (ESI) source. Samples were dissolved in chloroform and then diluted by methanol to the concentration of 3 μg/mL. All solvents used during sample preparation were purchased from Fisher (Fair Lawn, NJ, USA). The sample solutions were injected into the ESI source by direct infusion, using a syringe pump, at a flow rate of 3

µL/min. The tip of the ESI needle was grounded, and the entrance of the capillary, through which ions enter into the vacuum system of the mass spectrometer, was held at 3.5 kV. The pressure of the nebulizing gas (nitrogen) was set at 10 psi, and the flow rate and temperature of the drying gas (nitrogen) at 8 L/min and 300 °C, respectively. Data collection was performed on positive mode. The ESI-MS data was analyzed by Bruker Daltonik’s

DataAnalysis v4.0 software.

2.2.3 Synthesis of Ethyl Propiolate Grafted Butyl Rubber (EIIR)

IIR (Mw = 425,000 g/mol, PDI = 3.72; 150 g, 40.2 mmol C=C) was dissolved in anhydrous hexane (1.5 L) agitated by a mechanical stir under a nitrogen atmosphere. Ethyl propiolate (9.68 g, 98.7 mmol) was added into the flask. EtAlCl2 in hexane (86 mL, 1 M in hexane) was added into the solution via a syringe. After the reaction was stirred at 50 ºC for 12 h, the solution was poured in ethanol (3 L) stirred with a mechanical stirrer to precipitate the rubber. The bulk of the solvent in the rubber was removed with a paper towel. The remaining solvent in the rubber was removed under vacuum. The yield of the product, EIIR, was 149 g (99% yield). Mw of EIIR was 391,000 g/mol, PDI = 3.86. The

1H NMR spectrum of the product is shown in Figure 2. 1.

2.2.4 Synthesis of Ethyl 4,5-dimethyl-(E-2),5-hexdienoate (EDHEX)

2-Methyl-2-butene (6.62g, 94.4 mmol) was dissolved in 50 mL anhydrous hexane under a nitrogen atmosphere. Ethyl propiolate (2.36 g, 24.1 mmol) was added into flask.

Then, EtAlCl2 (16 mL, 1.0 M in hexane) was added into the solution via a syringe. The solution was stirred for 10 min at room temperature and for 10 h at 50 °C in an oil bath.

Water (20 mL) was added into the flask to quench the reaction. The organic phase and aqueous phase were separated, and the volatile components in the organic phase was removed on a rotovap. A clear colorless liquid was obtained (2.14 g, 53% yield). 1H NMR

(CDCl3): δ (ppm) 6.91 (dd, J = 15.7, 7.2 Hz, 1H), 5.81 (d, J = 17.0 Hz, 1H), 4.79 (d, J =

10.6 Hz, 2H), 4.18 (q, J = 7.1 Hz, 2H), 2.97 (p, J = 7.8, 7.3 Hz, 1H), 1.29 (t, J = 7.1 Hz,

+ 3H), 1.18 (d, J = 6.9 Hz, 3H). ESI-MS: m/z calcd for C10H16O2 169.1 (M + H) , 191.1 (M

+ Na)+, found 169.1 and 191.0.

2.2.5 Rubber Compounding

Method 1. This method was used when all additives in a compound were solid. The additives and rubber are mixed on a two-roll mill (Reliable Mill Supply Co., Ukiah, CA).

The distance between the two rolls was set as ~0.5 mm, and the roll temperature was set at

50 °C. All additives were physically blended together first and then added to the rubber on the two-roll mill. After 30 passes, the rubber sheet was collected in the sealed bag and stored at -20 °C.

Method 2. If a compound includes a liquid additive, the additives and rubber were first dissolved in chloroform (60 mL per 10 g of rubber). Chloroform was removed under high vacuum at room temperature. The resultant blend was passed through the two-roll mill 30 times as in Method 1. The rubber sheet was then collected in the sealed bag and stored at -20 °C.

2.2.6 Compositions of EIIR Compounds

The recipes of EIIR compounds containing DCP and BMI are given in Table 2. 1.

The recipes of EIIR compounds containing TMPTMA are given in Table 2. 2. The recipes of EIIR compounds containing BPO and BMI are given in Table 2. 3.

Table 2. 1. Recipes of EIIR compounds containing DCP and BMI.

Compound DCP BMI EDHEX

D0.1M3 0.1 g 3 g -

D0.1M7 0.1 g 7 g -

D0.1M14 0.1 g 14 g -

D0.2M3.5 0.2 g 3.5 g -

D0.2M5.6 0.2 g 5.6 g -

D0.2M11 0.2 g 11 g -

D0.2M14 0.2 g 14 g -

D0.3M3.5 0.3 g 3.5 g -

D0.3M7 0.3 g 7 g -

D0.3M14 0.3 g 14 g -

D0.5M3.5 0.5 g 3.5 g -

D0.5M7 0.5 g 7 g -

D0.5M14 0.5 g 14 g -

D0.3M1.7E0.6 0.3 g 1.7 g 0.6 g

D0.3M1.7E0.9 0.3 g 1.7 g 0.9 g

D0.3M3.5E0.9 0.3 g 3.5 g 0.9 g

D0.3M3.5E1.7 0.3 g 3.5 g 1.7 g

* The unit is part per hundred part of rubber, phr. All compounds contain 100 phr of EIIR.

Table 2. 2. Recipes of EIIR compounds containing TMPTMA.

Compound DCP BPO TMPTMA

D0.2T10 0.2 g - 10 g

D0.2T15 0.2 g - 15 g

D0.3T10 0.3 g - 10 g

D0.3T15 0.3 g - 15 g

B1T10 - 1 g 10 g

B1T15 - 1 g 15 g

B2T10 - 2 g 10 g

B2T15 - 2 g 15 g

B3T10 - 3 g 10 g

B3T15 - 3 g 15 g

* The unit is part per hundred part of rubber, phr. All compounds contain 100 phr of EIIR.

Table 2. 3. Recipes of EIIR compounds containing BPO and BMI.

Compound BPO BMI

B1M7 1 g 7 g

B2M7 2 g 7 g

B4M7 4 g 7 g

* The unit is part per hundred part of rubber, phr. All compounds contain 100 phr of EIIR.

2.2.7 Vulcanization Kinetics

A Montech 2000 moving die rheometer (MDR) was used to monitor the vulcanization process. A sample of ~6 g was placed between two heated moving dies at a selected temperature. The dies were closed. Sinusoidal shear was applied at the frequency of 1.66 Hz and deformation angle of 0.5º. The torque was recorded as a function of time.

2.2.8 Vulcanization

Vulcanization was carried out on a Dake Hydraulic Mold 4419 hot press. A sample of ~5 g was placed in a steel window mold (80 × 80 × 0.5 mm). Teflon films and two

o aluminum plates were placed on each side. The sheets were cured at 160 C for (T90 + 10) min in Dake hydraulic press under a load of 25 tons. The pressure was released, and the cured compound was quenched in water.

2.2.9 Stress Relaxation

Stress relaxation test was performed on an RSA3 dynamic mechanical analyzer. A compression-molded sheet was manually cut into strips of 15 mm × 8 mm. The clamp distance was set at 5 mm. The samples were deformed to 10% strain, and the engineering modulus was recorded as a function of time at room temperature.

2.2.10 Tensile Test

Tensile test was performed on an Instron Model 5567 equipped with a 1000 N load cell. Samples were cut into pieces of with an ASTM D638 type V dumbbell die. Then

59 samples were held in two clamps with 50 mm gap and extended at a rate of 500 mm/min at room temperature until failure. Strain was measured with an extensometer.

2.2.11 Swelling Test for Crosslinking Density

A sample of ~0.6 g was weighed to give an accurate Wo and put into a 20 mL vial.

Hexane (15 mL) was added into the vial. The liquid phase in the vial was poured out after

3 days, and fresh hexane (15 mL) was added into the vial. This was repeated after another

3 days. After 7 days in total, the swollen sample was taken out of the vial. The excess solvent on the surface of the sample was dabbed away with Kimwipes. The swollen sample was weighed to give Wg,s. After that, the swollen sample was put into a vacuum oven at 50

ºC to remove the remaining solvent until a constant weight was reached. The weight of the dry sample was recorded as Wg,d.

The gel fraction is calculated as

Wg,d Fgel = × 100% Equation 2. 1 Wo

163 The volume fraction of rubber in the gel φr is calculated as

Wg,d−W0fcoagent ρr φr = Equation 2. 2 Wg,d−W0fcoagent Wg,s−Wg,d + ρr ρs where fcoagent is the weight fraction of coagent in formulation, defined as fcoagent = (weight

3 164 3 of coagent)/(total weight); ρr = 0.92 g/cm is the density of the rubber; ρs = 0.65 g/cm is the density of hexane.

Crosslinking density, ρcx is calculated from the Flory-Rehner equation based on the affine network model:165

2φr In (1-φr) + φr + χφr2 = - 2ρ Vs(φr1/3 - ) Equation 2. 3 cx f

where φr is the volume fraction of the rubber; χ = 0.516 is the polymer-solvent interaction

166 3 parameter; Vs =130.8 cm /mol is the molar volume of the hexane solvent; and f = 4 is the functionality of the crosslinks. The molecular weight between crosslinks of the gel fraction is approximately calculated as 167

ρ Mc = r Equation 2. 4 2ρcx where the terminal strands in the network are neglected.

Peroxide efficiency Ecx, defined as the number of crosslinks generated by each peroxide, is calculated according to the following formula:

ρcx mr mcoagent mperoxide Ecx = × ( + + ) × Fgel Equation 2. 5 nperoxide ρr ρcoagent ρperoxide

where nperoxide is micromoles of peroxide in the compound; mr is the weight of rubber in

3 gram; ρr is the density of IIR and equals 0.92 g/cm ; mcoagent is the weight of coagent in the compound in grram; ρ coagent is the density of the coagent; mperoxide is the weight of peroxide, and ρperoxide is density of the peroxide.

2.2.12 Reactions of EDHEX with DCP and N-Phenylmaleimide (PMI).

The reactions were studied as model reactions for vulcanization. The general procedure is described below. EDHEX, along with the solvent and PMI if needed, was added into a 25 mL flask. DCP was added. The mixture was stirred at room temperature for 10 min and then placed in an oil bath at 110 oC. The reaction was stirred for 24 h and cooled to room temperature. The solvent was removed by distillation, and the nonvolatile residual was characterized by 1H NMR and MS.

2.2.13 Microscopic Characterization

The cured rubber sample was microtomed at -100 oC using a Leica EM UC7 ultracryomicrotome. The surface of microtomed sample was cleaned with compressed air before imaging to remove any particles sticking to the surface. Atomic Force Microscope

(AFM) measurements were performed using a Veeco Dimension Icon AFM. Property maps of DMT modulus were simultaneously acquired using Peak Force Quantitative

Nanomechanical Mapping (QNM). Probe model RTESPA-150-30 was used for QNM mode. The AFM phase image was obtained by using the tapping mode. Bruker probe

PFQNE-AL was used for the tapping mode. Particle analysis was performed using the

“Particle Analysis” feature in the “NanoScope Analysis v2” software.

The compression-molded bulk sample was microtomed at −100 °C using a Leica

EM UC7 ultracryomicrotome. The thickness of the slices was about 80 nm. The thin film was stained by HYDAZINE vapor followed by osmium tetroxide. A JEOL JEM-1230 electron microscope operated at 120 kV was used for imaging.

2.3 Results and Discussion

2.3.1 EIIR Compounds Cured by DCP-BMI

2.3.1.1 Synthesis of EIIR

In our previous study of polyisobutylene thermoplastic elastomers,168 we discovered that pentafluorophenyl propiolate can be grafted to the isoprene units of IIR nearly quantitatively at room temperature via Lewis acid-catalyzed Alder-ene reaction.169,

170 It occurred to us that the resultant geminal vinylidene-acrylate motif appears highly reactive via a number of reaction mechanisms including radical mechanisms. We hence attempted to extend the method to an inexpensive commercially available propiolate substrate. The reaction of ethyl propiolate Scheme 2. 1 was slower than that of its pentafluorophenyl analog and did not reach completion in weeks.

Scheme 2. 1. Synthesis of EIIR.

50000000

45000000 PIB CDCl3 40000000 35000000

30000000

PIB B 25000000

f 20000000 15000000

e 10000000 a g d 1,2 5000000 c b * 0 7 6 5 4 3 2 1 Chemical shift (ppm)

1 Figure 2. 1. H spectrum of EIIR in CDCl3.

Note that the peak with an asterisk is from the unreacted trans-isoprene units.

Increasing the reaction temperature to 50 ºC resulted in ~95% conversion of the isoprene units overnight without causing any side reactions. Essentially all cis-isoprene units reacted. The remaining unreacted isoprene units were trans. The molecular weight of the resultant ethyl propiolate-grafted IIR (EIIR) is slightly lower than that of the IIR starting material (Mw = 391,000 g/mol, PDI = 3.86 vs 425,000 g/mol, PDI = 3.72). It is known that Brønsted acids, but not Lewis acids, causes chain scission of IIR. The use an anhydrous solvent is therefore critical for the reaction to avoid generation of Brønsted acids. EtAlCl2 as the Lewis acid catalyst also reduces the loss of molecular weight in

64 comparison to AlCl3 likely because the ethyl group acts as a proton scavenger to some extent.

2.3.1.2 Vulcanization Studies

Dicumyl peroxide (DCP) was used in this work because it is known to be compatible with reinforcing particulate fillers including carbon black and silica,51, 56 which are often required for many applications.171, 172 It was apparent at the onset of the investigation that EIIR remained uncurable by DCP alone despite of the presence of the highly reactive geminal vinylidene-acrylate moiety. m-Phenylene-N,N’-bismaleimide

(BMI) was chosen as the coagents because it undergoes both radical addition and abstraction,77 as the geminal vinylidene-acrylate moiety does.

The curing kinetics of compounds with a wide range of DCP/BMI loadings were investigated using a moving die rheometer. The resistant torque generated by the compound provided the first indication of degree of cure.173 For the compounds containing

0.1 phr DCP, a substantive rise of torque requires a relatively high amount of BMI in Figure

2. 2. Increasing the DCP loading increases the degree of cure in general as long as accompanied by an increase in BMI loading. Reversion is observed when the BMI loading

64 is insufficient, most obviously in D0.5M3.5. This is apparently attributable to chain scission after consumption of the majority of the coagent. It should be noted that the lack of reversion does not imply the absence of chain scission. In fact, it is a reasonable assumption that crosslink and chain scission compete at all times. Marching is observable when the

DCP/BMI ratio is too low, most obviously in D0.2M14. In these cases, BMI likely is able to sustain the radical chain reaction even after decomposition of DCP is complete.

7 D0.1M3 D0.1M7 D0.1M14

D0.2M3.5 D0.2M5.6 D0.2M11 D0.2M14 6 D0.3M3.5 D0.3M7 D0.3M14 D M D M D M 5 0.5 3.5 0.5 7 0.5 14

3 Torque(dNm)

0 0 10 20 30 40 50 Time (min)

Figure 2. 2. The curing kinetic curves of EIIR compounds cured by DCP-BMI at 160 ºC.

Note that the curing characteristics of those compounds are summarized in Appendix A,

Table A. 1.

The degree of cure is further evaluated by measuring the gel and sol contents of the vulcanizate and the equilibrium-swelling ratio of the gel insoluble in hexane, which is shown in Table 2. 4. Gel contents in the vulcanizates cured by 0.1 phr DCP, even in the presence of 14 phr of BMI, are noticeably lower than those in the vulcanizates cured by >

0.2 phr DCP. The approximately 90% gel afforded by DCP cure of EIIR is about the same as a sulfur-based curing package usually afford for the parent IIR.

Table 2. 4. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds

cured by DCP and BMI.

Gel fraction Crosslinking Peroxide Compound (%) density (mol/m3) efficiency

D0.1M3 72% ± 1.8% 0.7 ± 0.3 0.20 ± 0.08

D0.1M7 82% ± 0.3% 5.8 ± 0.1 1.77 ± 0.03

D0.1M14 78% ± 0.5% 6.2 ± 0.7 1.97 ± 0.22

D0.2M3.5 88% ± 0.3% 9.9 ± 0.7 1.49 ± 0.11

D0.2M5.6 89% ± 0.1% 16.4 ± 0.4 2.49 ± 0.06

D0.2M11 88% ± 0.5% 28.3 ± 0.1 4.42 ± 0.07

D0.2M14 87% ± 0.3% 28.0 ± 0.7 4.45 ± 0.24

D0.3M3.5 89% ± 0.6% 14.0 ± 0.3 1.40 ± 0.03

D0.3M7 89% ± 0.3% 22.3 ± 0.7 2.27 ± 0.07

D0.3M14 89% ± 1.0% 48.4 ± 3.9 5.49 ± 0.41

D0.5M3.5 88% ± 0.6% 13.6 ± 0.3 0.82 ± 0.02

D0.5M7 89% ± 0.1% 26.8 ± 0.4 1.64 ± 0.02

D0.5M14 92% ± 0.2% 75.8 ± 0.6 4.81 ± 0.04

Crosslinking density of the gel fraction is determined using Flory-Rehner equation with nonelastic BMI component taken into consideration using the method of Welding,163 where the volume occupied by EIIR in swollen state is corrected by removing the insoluble

BMI. Only crosslinks that result in elastically effective strands are counted. As shown in

Figure 2. 3, the crosslink density correlates reasonably well with increase in torque recorded from the curing kinetic study. This suggests that both reasonably reflect the degree of cure as will be further corroborated by additional data (vide infra).

4 Linear regression with R2 = 0.88

M (dNm) M 2 Δ

D0.1M3, D0.1M7, D0.1M14 1 D0.2M3.5, D0.2M5.6, D0.2M11, D0.2M14 D0.3M3.5, D0.3M7, D0.3M14

D0.5M3.5, D0.5M7, D0.5M14

0 0 10 20 30 40 50 60 70 80 3 ρcx (mol/m )

Figure 2. 3. Correlation between M and crosslinking density (ρcx) of EIIR compounds.

Note that M is the torque increase from its minimum to the value at T90+10 min, which is the actual curing time.

It is interesting to notice that the crosslinking density does not correlate in an straightforward manner with the DCP/BMI ratio but rather approximately follows a power function of the DCP/BMI ratio as shown in Figure 2. 4 (A), where d is the DCP loading, and m is BMI loading in phr.

Similarly, peroxide efficiency, defined as number of crosslinks generated by each peroxide, is also not a simple function of the DCP/BMI ratio. Instead, the ratio powered by a DPC loading function appears to approximately index peroxide efficiency as shown in

Figure 2. 4 (B). The correlations in Figure 2. 4 thus provide practical guides for curing efficiency although they may not necessarily have any definitive physical basis.

90 D 0.1M3, D0.1M7, D0.1M14 D 0.2M3.5, D0.2M5.6, D0.2M11, D0.2M14 75 D 0.3M3.5, D0.3M7, D0.3M14 D M , D M , D M

) 0.5 3.5 0.5 7 0.5 14 3

mol/m (

cx ρ

30 A 15

Linear regression with R2 = 0.81 0 0.0 0.5 1.0 1.5 2.0 02.5 3.0 d m -1

6 D M , D M , D M 0.1 3 0.1 7 0.1 14 0 D 0.2M3.5, D0.2M5.6, D0.2M11, D0.2M14 5 D 0.3M3.5, D0.3M7, D0.3M14

D 0.5M3.5, D0.5M7 D M 4 0.5 14

cx E 3

2 B 1

Linear regression with R2 = 0.82 0 4 6 8 10 12 14 (m/d)^(d^0.4)

Figure 2. 4. (A) Correlation between ρcx and DCP and BMI loadings. d and m are DCP and BMI loadings in phr, respectively. The term “-1” is added so that when d = 0, ρcx is

zero. (B) Correlation between peroxide efficiency (Ecx) and DCP and BMI loadings.

2.3.1.3 Crosslinking Mechanism

The crosslinking mechanism was probed using small molecule models for the functionalities present in EIIR. EDHEX synthesized again by Lewis acid-catalyzed Alder ene reaction in Scheme 2. 2 was used as a surrogate of the geminal vinylidene-acrylate moiety. N-Phenylmaleiimide (NMI) was used as a model for BMI. Octane or hexane was used as a surrogate for the saturated hydrocarbon backbone of EIIR. The reagents used in the model reactions are summarized in Table 2. 5.

Scheme 2. 2. Synthesis of EDHEX.

Table 2. 5. Summary of formulations of model reactions.

Entry EDHEX NMI DCP Chlorobenzene Hexane Octane

1 0.05 g - 3.5 mg - - 1.3 mL

2 0.05 g - 3.5 mg - - -

3 0.05 g - 0.2 g - - 0.1 mL

4 0.05 g 0.05 g 3.5 mg 1.0 mL 2 mL -

5 0.05 g 0.05 g 0.2 g 1.3 mL - -

The reaction between EDHEX and DCP in octane was investigated first. When the concentrations of EDHEX and DCP were comparable to those of the geminal vinylidene- acrylate functional group and DCP in the actual EIIR compounds (entry 1, Table 2. 5), only a very limited extent of reaction took place according to 1H NMR (Figure 2. 5 (A)). The low conversion of the reaction prevented the isolation and full characterization of the reaction products. ESI-MS analysis174 of the reaction mixture provided useful insights about the transformations that took place. The most prominent peak at m/z = 335.1 in

Figure 2. 7 (A) is attributable to protonated a, which is due to coupling of two (E-H) radicals resulting from hydrogen abstraction of EDHEX. Product b attributable to Me radical addition to EDHEX followed by combination with radical (E-H) is observed as both b+H+ and b+Na+. Other species, c, d and e, are all attributable to a similar reaction sequence comprising addition of a radical to EDHEX followed by combination of the resultant radical with another radicals or hydrogen. Noteworthy is that octane is not involved in the observed products.

Figure 2. 5. 1H NMR spectra of the reactions corresponding to (A) entry 1 and (B) entry 3

in Table 2. 5.

Note that all samples were dissolved in CDCl3. The peak with an asterisk was from product of reaction.

Figure 2. 6. 1H NMR spectra of the reactions corresponding to (A) entry 4 and (B) entry 5

in Table 2. 5.

Note that all samples were dissolved in CDCl3. The peak with an asterisk was from product of reaction.

Figure 2. 7. ESI-MS spectra and identities of the observed species in the model reactions

corresponding to (A) entry 1 and (B) entry 4 in Table 2. 5. 75

When octane was removed (entry 2, Table 2. 5), the neat mixture of EDHEX and

DCP also only reacts to a limited extent and give similar products with e being the most abundant species observed (Figure A. 1, Appendix A). When a large amount of DCP was used (entry 3, Table 2. 5), EDHEX was completely consumed to give what appears to be a mixture of its higher oligomers according to 1H NMR (Figure 2. 5 (B)). The species observable by ESI-MS (Figure A. 2, Appendix A) remain qualitatively the same as those observed with low DCP loadings (entries 1 and 2, Table 2. 5). The difference in the extent of reaction between the model reactions and the actual EIIR compounds is possibly due to the difference in radical diffusion rates in the two circumstances. Radical combinations likely happen more readily between small molecular radicals than macromolecular radicals and in the low-viscosity solution phase than in the high-viscosity polymeric medium, resulting in vastly decreased initiator efficiency in the model reaction.

The reactions of NMI as a model for coagents in peroxide cure were studied by

Parent and coworkers.77 Among the important findings is that alkanes promote the oligomerization of NMI. This observation was confirmed in our laboratory. The reaction of EDHEX and NMI at the concentrations that simulate compound D0.3M7 was thus studied in the presence of hexane (entry 4, Table 2. 5). A small amount of chlorobenzene was used to completely dissolve NMI. Again, 1H NMR (Figure 2. 6 (A)) again showed a limited extent of reaction, and ESI-MS (Figure 2. 7 (B)) revealed considerable insight. Co- oligomers of NMI and EDHEX initiated by methyl (p), hexyl (i, j, o, and q), and E-H (k-n) radicals are observed in addition to the homo-oligomers of EDHEX (a, f and h). The fact that all observed species contain at least one EDHEX-derived unit without exception underpins the importance of the germinal vinylidene-acrylate motif in the curing process

76 of EIIR. The species containing multiple EDHEX-derived units attest that EDHEX not only undergoes hydrogen abstraction but also radical addition to its C=C bond. Again, when a large amount of DCP is used, EDHEX and NMI completely react to give oligomers or co-oligomers that display broad 1H NMR signals (Figure 2. 6 (B)).

To further elucidate the role of the geminal vinylidene-acrylate group during vulcanization, we added EDHEX in the EIIR compound as a coagent along with BMI

(Figure 2. 8). The curing kinetic curves of EIIR compounds containing DCP, BMI, and

EDHEX are shown in Figure 2. 8. Comparison between D0.3M3.5 and D0.3M3.5E0.9 indicates that the presence of EDHEX prevents reversion and, by inference, chain scission during the entire curing process. However, EDHEX cannot replace BMI as D0.3M1.7E0.9 is significantly less cured than D0.3M3.5. Further, too much EDHEX reduces the degree of cure as shown by D0.3M3.5E1.7 presumably via biradical combination and therefore terminating the radical chain without generating a crosslink. The crosslinking density and peroxide efficiency obtained by swelling measurements support the above assessments

(Table 2. 6). It should be mentioned that if the additional concentration of geminal vinylidene-acrylate moiety were grafted to the backbone, biradical combination would result in a crosslink. Therefore, a high concentration of such functional groups grafted to the backbone can be expected to reduce chain scission and afford crosslinks at the same time.

2.5

2.0

1.5

1.0

D0.3M3.5 Torque (dNm)

D0.3M1.7E0.6 0.5 D0.3M1.7E0.9 D0.3M3.5E0.9

D0.3M3.5E1.7 0.0 0 10 20 30 40 50 Time (min)

Figure 2. 8. The curing kinetic curves of compounds EIIR cured by DCP-BMI-EDHEX

at 160 oC.

Note that the curing characteristics of those compounds are summarized in Appendix A,

Table A. 1.

Table 2. 6. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds

cured by DCP-BMI-EDHEX.

Gel fraction Crosslinking Peroxide Compound (%) density (mol/m3) efficiency

D0.3M3.5 89% ± 0.6% 14.0 ± 0.3 1.40 ± 0.03

D0.3M1.7E0.6 83% ± 0.2% 3.6 ± 0.1 0.36 ± 0.01

D0.3M1.7E0.9 82% ± 0.4% 2.9 ± 0.3 0.29 ± 0.03

D0.3M3.5E0.9 89% ± 0.5% 14.3 ± 0.5 1.44 ± 0.05

D0.3M3.5E1.7 87% ± 0.2% 10.2 ± 0.2 1.03 ± 0.02

2.3.1.4 Mechanical Properties

Stress relaxation was carried out to unambiguously ascertain whether transition

from viscous liquid to elastic solid was realized for the EIIR vulcanizates. Except D0.1M3 all other vulcanizates display a finite modulus after one week at room temperature under

10% strain (Figure 2. 9). The time scale for D0.1M3 to relax is consistent with the time scale

175 of polyisobutylene terminal relaxation at 25 ºC. D0.1M3 was therefore inadequately crosslinked and remained an elastic liquid. The other vulcanizates are viscoelastic solids.176

The terminal moduli, which can be approximated as the equilibrium moduli, show a good linear correlation with crosslinking densities determined from the swelling ratios using

Flory-Rehner equation (Figure 2. 10 (A)). The absolute values of crosslinking density calculated using the ideal elastic equation combined with Guth-Gold equation, E =

3ρ RT r (1 + 2.5φ), where φ is the volume fraction of BMI in the rubber compound, are Mc

79 higher to various extents than the values determined from the swelling ratio (Figure 2. 10

(B)). The difference is likely caused by the permanent entanglements trapped by the crosslinks.

D0.1M3

D0.1M7 D0.3M3.5

Modulus(MPa) D M D M 0.1 0.1 14 0.3 7 D M D M 0.2 3.5 0.3 14 D0.3M1.7E0.6 D M D M 0.2 5.6 0.5 3.5 D0.3M1.7E0.9 D M D M 0.2 11 0.5 7 D0.3M3.5E0.9 D M D M 0.2 14 0.5 14 D0.3M3.5E1.7 0.01 10-2 10-1 100 101 102 103 104 105 Time (s)

Figure 2. 9. Stress relaxation curves of EIIR compounds.

1.0 Linear regression with R2 = 0.92 0.8 A

0.6 (MPa) 0

eq D 0.1M3, D0.1M7, D0.1M14 E 0.4 D 0.2M3.5, D0.2M5.6, D M , D M 0 0.2 11 0.2 14 D 0.3M3.5, D0.3M7, D0.3M14 0.2 D 0.5M3.5, D0.5M7, D0.5M14 D M E , D M E 0 0.3 1.7 0.6 0.3 1.7 0.9 D0.3M3.5E0.9, D0.3M3.5E1.7 0.0 0 10 20 30 40 50 60 70 80 3 ρcx (mol/m )

90 From stress relaxation From swelling 75.8

) 3 60 B 53.2

mol/m 48.4 (

45 42.4 cx ρ 31.7 29.4 30 0 27.5 28.3 28 27.6 26.8 22.3 19.5 18.7 19.2 20.5 16.7 16.4 16.3 16 15 14 13.6 14.3 11.5 9.9 9.8 10.2 5.8 6.2 5.3 3.6 2.9 0.8 0.7 0 0 M 3 M 7 M 14 M 3.5 M 5.6 M 11 M 14 M 3.5 M 7 M 14 M 3.5 M 7 M 14 E 0.6 E 0.9 E 0.9 E 1.7 D 0.1 D 0.1 D 0.1 D 0.2 D 0.2 D 0.2 D 0.2 D 0.3 D 0.3 D 0.3 D 0.5 D 0.5 D 0.5 M 1.7 M 1.7 M 3.5 M 3.5 D 0.3 D 0.3 D 0.3 D 0.3 0 Figure 2. 10. (A) Correlation between ρcx and terminal moduli (Eeq). (B) Comparison of

ρcx obtained from swelling and equilibrium moduli.

Tensile test was carried out. The tensile parameters of all samples are summarized in Table 2. 7. A wide range of extensibility can be achieved. As does the equilibrium modulus (Figure 2. 10 (A)), the strain at break also correlate with the crosslinking density and shows a remarkable linear relationship with the square root of molecular weight

81 between crosslinks (Figure 2. 11 (A)) as ideal elastomer chain statistics predict.177 The stress at break of all samples fall within a fairly narrow range and do not shown the expected maximum at an optimal crosslinking density (Figure 2. 11 (B)).19 The different degrees of chain scission in the individual samples undoubtedly contribute to obscuring the anticipated correlation. Those with a high number of terminal strands would show stress at break inferior to the expected at any given crosslinking density. The toughness, which is also expected to exhibit a maximum value at an optimal crosslinking density, apparently is dominated by the strain at break (Figure 2. 11 (C)).

Table 2. 7.Mechanical properties of EIIR compounds.a

b σ100% σ200% εb σb Toughness Compound (MPa) (MPa) (%) (MPa) (106 J/m3)

D0.2M3.5 0.83 ± 0.07 1.35 ± 0.07 429 ± 34 3.2 ± 0.4 6.6 ± 0.9

D0.2M5.6 1.02 ± 0.05 1.46 ± 0.10 347 ± 28 3.5 ± 0.4 5.5 ± 0.8

D0.2M11 1.04 ± 0.07 2.19 ± 0.11 299 ± 31 4.0 ± 0.6 5.5 ± 1.2

D0.2M14 1.10 ± 0.01 2.41 ± 0.03 303 ± 5 3.6 ± 0.1 5.3 ± 0.2

D0.3M3.5 0.80 ± 0.07 1.23 ± 0.12 426 ± 34 3.2 ± 0.3 6.3 ± 0.8

D0.3M7 0.79 ± 0.10 1.55 ± 0.14 324 ± 6 3.9 ± 0.4 5.0 ± 0.6

D0.3M14 1.61 ± 0.05 - 196 ± 18 3.8 ± 0.3 3.6 ± 0.6

D0.5M3.5 0.64 ± 0.04 1.11 ± 0.09 370 ± 18 2.9 ± 0.5 4.7 ± 0.6

D0.5M7 0.92 ± 0.12 1.97 ± 0.36 261 ± 14 3.4 ± 0.4 3.7 ± 0.5

D0.5M14 2.31 ± 0.14 - 149 ± 10 4.0 ± 0.4 2.8 ± 0.4

D0.3M1.7E0.6 0.89 ± 0.04 1.09 ± 0.05 641 ± 56 4.0 ± 0.3 12.3 ± 0.9

D0.3M1.7E0.9 0.79 ± 0.07 0.95 ± 0.05 616 ± 49 3.4 ± 0.4 9.4 ± 1.3

D0.3M3.5E0.9 0.87 ± 0.07 1.40 ± 0.05 379 ± 9 3.9 ± 0.2 6.0 ± 0.2

D0.3M3.5E1.7 0.77 ± 0.07 1.15 ± 0.05 440 ± 29 3.6 ± 0.3 7.0 ± 0.9 a σ100% and σ200% are stress at 100% and 200% strain, respectively. εb is strain at break. σb is stress at break. barea under the stress-strain curve.

700 700 Linear regression A 600 with R2 = 0.93 600

500 (%)

0 b

500 400 Ɛ 300

(%) 400 0

b 200

Ɛ 100 300 0 100 200 300 400 Mc^0.5 200 D 0.2M3.5, D0.2M5.6

D0.2M11, D0.2M14 100 D M , D M , D M 0.3 3.5 0.3 7 0.3 14 D 0.3M1.7E0.6, D0.3M1.7E0.9 D 0.5M3.5, D0.5M7, D0.5M14 D 0.3M3.5E0.9, D0.3M3.5E1.7 0 0 10 20 30 40 50 60 70 80 ρ (mol/m3) cx

Figure 2. 11. Correlation of tensile properties with ρcx (A) strain at break (b) vs ρcx The

inset shows a linear relationship between b and the square root of molecular weight

between crosslinks (Mc).

5 B 4 0

(MPa) b

σ 0 2 D 0.2M3.5, D0.2M5.6, D0.2M11, D0.2M14 0 D 0.3M3.5, D0.3M7, D0.3M14 1 D 0.5M3.5, D0.5M7, D0.5M14 D 0.3M1.7E0.6, D0.3M1.7E0.9 D 0.3M3.5E0.9, D0.3M3.5E1.7 0 0 10 20 30 40 50 60 70 80 ρ (mol/m3) cx

14 C D 0.2M3.5, D0.2M5.6, D0.2M11, D0.2M14 D M , D M , D M

12 0.3 3.5 0.3 7 0.3 14 )

3 D 0.5M3.5, D0.5M7, D0.5M14

0 D 0.3M1.7E0.6, D0.3M1.7E0.9 J/m 6 10 D 0.3M3.5E0.9, D0.3M3.5E1.7

8 0

6 Toughness (10 0

2 0 10 20 30 40 50 60 70 80 ρ 3 cx (mol/m )

Figure 2. 11. Correlation of tensile properties with ρcx (B) stress at break (b) vs ρcx (C)

toughness vs ρcx

The stress-strain curves of D0.5M14 and D0.3M1.7E0.6 are shown in Figure 2. 12. Both display a stress at break of ~4.0 MPa. This value is about the same as the best stress at break achievable by sulfur cure in the absence of filler reinforcement.167 The stress at break of previously reported peroxide-cure IIR derivatives are below 2.5 MPa in the absence of filler reinforcement.76, 92, 105 We have optimized the stress at break for BIIR cured by DCP and BMI based on the study of Nah92 and have arrived at approximately the same value as reported by Nah. D0.5M14 represents the highest stiffness achieved in the present study using σ100% as a practical indicator. The 2.3 MPa of σ100% is several times higher than what is previously achieved for peroxide-cured IIR derivatives.76, 92, 105 Note that unlike in sulfur cure, stiffness cannot be increased easily by increasing the curative amount due to competition of chain scission. D0.3M1.7E0.6 exemplifies the highest extensibility achieved in the present study. The ~600% strain at break is substantially better the extensibility of previously reported peroxide-cured IIR derivatives (~500% achieved using IIR with a high degree of unsaturation)76 but remains significantly lower than the extensibility achievable

167 by sulfur cure (~1000%). D0.3M1.7E0.6 also displays the highest overall toughness. In fact, all samples that include EDHEX as a coagent along with BMI display relatively high toughnesses, attesting to the importance of the geminal vinylidene-acrylate group in vulcanization.

4 D M E D0.5M14 0.3 1.7 0.6

2 Tensile stress (MPa) 1

0 0 100 200 300 400 500 600 700 Strain (%)

Figure 2. 12. Stress-strain curves of representative EIIR compounds.

2.3.2 EIIR Compounds Containing TMPTMA

2.3.2.1 TMPTMA as Coagent

While the above discussed curing methods for EIIR afford mechanical properties unprecedented for peroxide-cured IIR derivatives, the use of BMI is undesirable because a major incentive for development of peroxide cure is for health-related applications but BMI is highly toxic according the safety data sheet of major chemical vendors, for example,

Sigma Aldrich and TCI178. TMPTMA is highly desirable from the viewpoint of health- related application due to its low toxicity identified by major chemical vendors.179

2.3.2.2 Vulcanization Studies

The combination of DCP and TMPTMA was probed. Curing kinetics were studied as the first indication of the curing effectiveness. As shown in Figure 2. 13 (A), the torque of compounds containing 0.2 phr DCP is increased by increasing amount of TMPTMA.

Reversion is observed when 0.3 phr DCP is used. This is apparently attributable to chain scission.64 As shown in Figure 2. 13 (B), although large amount of TMPTMA was used,

ΔM of these compounds are lower than compounds cured by DCP-BMI. It indicates DCP-

TMPTMA is less efficient than DCP-BMI.

2.0 A

D T 1.5 0.2 15

0 D0.3T10

D0.2T10 1.0

0 D0.3T15 Torque (dNm)

0.5 0

0.0 0 10 20 30 40 50 Time (min)

4 3.66 B 3

0 2.4 2.2

M (dNm) 2

Δ 1.82 0 1.68 1.22 1.16 1 0.93 0.7 0.74 0.530

0 T 10 T 15 T 10 T 15 M 3.5 M 5.6 M 11 M 14 M 3.5 M 7 M 14 D 0.2 D 0.2 D 0.3 D 0.3 D 0.2 D 0.2 D 0.2 D 0.2 D 0.3 D 0.3 D 0.3

Figure 2. 13. (A) The curing kinetic curves of EIIR compounds cured by DCP-TMPTMA

and (B) Comparison of ΔM of EIIR compounds cured by DCP-TMPTMA and DCP-

BMI.

Note that the curing characteristics of EIIR compounds cured by DCP-TMPTMA are summarized in Appendix A, Table A. 2.

The degree of cure is further evaluated by measuring the gel and sol contents of the vulcanizate, which is shown in Figure 2. 14. Gel contents (~50%) in all vulcanizates are noticeably lower than vulcanizates cured by DCP-BMI, where the approximately 90% gel is achieved.

It is possible that there exists a mismatch between the reactivities of DCP and

TMPTMA. The cumyloxy radical is primarily a radical abstractor56 but does not readily undergo radical addition, while TMPTMA primarily undergoes radical addition96 but not radical abstraction. Under such a hypothesis, then BPO became a natural choice of peroxide51 to be used in conjunction with TMPTMA.

100

60 53 52 53 45

40 Gel fraction (%) fraction Gel

0 D T D T D T D T 0.2 10 0.2 15 0.3 10 0.3 15

Figure 2. 14. The gel fraction of EIIR compounds cured by DCP-TMPTMA.

As shown in Figure 2. 15, when 10 or 15 phr TMPTMA was used, the torque is increased by increasing BPO loading to 2 phr. But reversion is observed when BPO loading is increased further to 3 phr. This is due to chain scission as described before.64 In general, the higher torque is achieved in compounds containing 15 phr TMPTMA.

B2T15 B1T15

B2T10 2 B3T15 B1T10

B3T10 Torque (dNm) 1

0 0 5 10 15 20 25 30 Time (min)

Figure 2. 15. The curing kinetic curves of EIIR compounds cured by BPO-TMPTMA at

110 oC.

Note that the curing characteristics of these compounds are summarized in Appendix A,

Table A. 2.

The degree of cure is also evaluated by measuring the gel and sol contents of the vulcanizate and the equilibrium-swelling ratio of the gel insoluble in hexane, which is shown in Table 2. 8. Gel contents in the compounds cured by BPO-TMPTMA are higher than compounds cured by DCP-TMPTMA in Figure 2. 14. It supports the hypothesis that

BPO is the better choice to be used in conjunction with TMPTMA. The low gel fraction of vulcanizate B1T10 is because of small amount of BPO used in compound. Compound B3T10 also has low gel fraction, it is probably caused by chain scission in compound. Other compounds have similar gel fraction ~80%, which approaches compounds cured by DCP-

BMI.

Table 2. 8. Gel fraction, crosslinking density and peroxide efficiency of EIIR compounds

cured by BPO and TMPTMA.

Gel fraction Crosslinking Peroxide Compound (%) density (mol/m3) efficiency

B1T10 66% ± 1.4% 0.7 ± 0.2 0.02 ± 0

B2T10 79% ± 0.8% 6.3 ± 1.3 0.09 ± 0.02

B3T10 67% ± 2.8% 1.3 ± 0.4 0.01 ± 0

B1T15 79% ± 1.6% 6.4 ± 0.3 0.18 ± 0.01

B2T15 78% ± 0.5% 7.4 ± 0.1 0.11 ± 0

B3T15 81% ± 0.7% 7.2 ± 0.3 0.08 ± 0.01

Crosslinking density of gel fraction is determined using Flory-Rehner equation with nonelastic TMPTMA component taken into consideration using the method of Welding,163 where the volume occupied by EIIR in swollen state is corrected by removing the insoluble

TMPTMA. Only crosslinks that result in elastically effective strands are counted. As shown in Figure 2. 16, the crosslink density correlates reasonably well with increase in torque recorded from the curing kinetic study. It is noteworthy that compounds cured by

BPO-TMPTAM generally have low crosslinking density and peroxide efficiency in comparison to compounds cured by DCP-BMI. It is likely because acyloxy radical from

BPO favors to combine with another radical without forming crosslinking due to its less steric hinderance.56 The other possibility is TMPTMA forms oligomers without covalently bonding to rubber matrix.51

2.0

1.5

M (dNm) M 1.0 Δ

B T , B T , B T 0.5 1 10 2 10 3 10 B1T15, B2T15, B3T15 Linear regression with R2 = 0.94 0.0 0 2 4 6 8 10 ρ (mol/m3) cx

Figure 2. 16. Correlation between M and ρcx of EIIR compounds.

2.3.2.3 Mechanical Properties

Stress relaxation was carried out to ascertain whether transition from viscous liquid to elastic solid was realized for the EIIR vulcanizates. All vulcanizates display a finite modulus after one week at room temperature under 10% strain in Figure 2. 17. Vulcanizates are viscoelastic solids. The terminal moduli, which can be approximated as the equilibrium moduli, show a good linear correlation with crosslinking densities determined from the swelling ratios using Flory-Rehner equation (Figure 2. 10 (A)). The absolute values of crosslinking density calculated using the ideal elastic equation combined with Guth-Gold

3ρ RT equation, E = r (1 + 2.5φ), where φ is the volume fraction of TMPTMA in the rubber Mc compound, are higher to various extents than the values determined from the swelling ratio

(Figure 2. 10 (B)). The difference is likely caused by the permanent entanglements trapped by the crosslinks.

B1T10

B2T10

Modulus(MPa) 0.1 B3T10

B1T15

B2T15

B3T15 0.01 10-2 10-1 100 101 102 103 104 105 Time (s)

Figure 2. 17. Stress relaxation curves of EIIR compounds cured by BPO-TMPTMA.

0.3 A

0.2 0

(MPa) eq E 0 0.1 B T , B T and B T 0 1 10 2 10 3 10 B1T15, B2T15 and B3T15 Linear regression with R2 = 0.88 0.0 0 2 4 6 8 ρ 3 cx (mol/m )

15 From stress relaxation From swelling

11.3 11.5 10.6 ) B 10.3 3 10 8.6 7.7 7.4 mol/m 7.2

( 0

6.3 6.4

cx ρ 5 0

1.3 0 0.7 0 T 10 T 10 T 10 T 15 T 15 T 15 0 B 1 B 2 B 3 B 1 B 2 B 3

Figure 2. 18. (A) Correlation between ρ and terminal moduli (E ). (B) Comparison of 0 cx eq ρcx obtained from swelling and equilibrium moduli.

96 0

0 The stress-strain curves of EIIR cured by BPO-TMPTMA are shown in Figure 2.

19. All compounds except B1T10 and B3T10 display higher stress at break in comparison to those compounds cured by DCP-BMI. These compounds, especially compounds containing 10 phr TMPTMA, also show high strain at break, which are substantially better than previously reported peroxide-cured IIR derivatives (~500% achieved using IIR with

76 a high degree of unsaturation and ~600% of D0.3M1.7E0.6). All compounds display a few times higher toughness in Table 2. 9 in comparison to compounds cured by DCP-BMI. The superior mechanical properties of compounds cured by BPO-TMPTMA, especially B2T10, are probably because TMPTMA forms oligomers, which reinforces the compounds. The study of oligomers in compound was then characterized by atomic force microscopy

(AFM) and transmission electron microscopy (TEM).

B2T10

5 B2T15

B3T15 B1T15 4

B T 3 3 10 B1T10

2 Tensilestress (MPa) 1

0 0 200 400 600 800 1000 Strain (%)

Figure 2. 19. Stress-strain curves of EIIR compounds cured by BPO-TMPTMA.

Table 2. 9.Mechanical properties of EIIR compounds cured by BPO and TMTPMTA.a

b σ100% σ200% εb σb Toughness Compound (MPa) (MPa) (%) (MPa) (106 J/m3)

B1T10 0.83 ± 0.04 1.12 ± 0.07 762 ± 78 2.9 ± 0.4 16.9 ± 4.3

B2T10 1.08 ± 0.16 1.59 ± 0.20 730 ± 60 5.5 ± 0.1 19.5 ± 0.1

B3T10 0.89 ± 0.11 1.28 ± 0.08 879 ± 69 3.1 ± 0.3 16.3 ± 0.3

B1T15 1.14 ± 0.15 1.60 ± 0.21 740 ± 82 4.3 ± 0.9 17.6 ± 0.6

B2T15 1.75 ± 0.10 2.40 ± 0.09 543 ± 65 4.3 ± 0.3 15.1 ± 1.6

B3T15 1.11 ± 0.11 1.67 ± 0.17 631 ± 68 4.4 ± 0.5 15.4 ± 0.3 a σ100% and σ200% are stress at 100% and 200% strain, respectively. εb is strain at break. σb is stress at break. barea under the stress-strain curve.

2.3.2.4 Microphase Morphology

The morphology of compound B2T10 was studied by AFM and TEM. The quantitative nanomechanical mapping mode (QNM) was used to reveal nanometer-sized features on sample B2T10. As shown in Figure 2. 20 (A), bright regions (above 1 GPa) are much stiffer than matrix (50-400 MPa), which has similar modulus with sulfur-cured vulcanizate in the absence of filler reinforcement (50-300 MPa). The long dimensions of bright regions vary from 15 nm to 60 nm. The tapping mode was also used. AFM phase image is shown in Figure 2. 20 (B). The long dimensions of bright regions range from 15 nm to 70 nm. Combined with Figure 2. 20 (A), nanoparticles with high modulus (above 1

GPa) exist in the vulcanizate. These nanoparticles probably are oligomers of TMPTMA.180-

182

Figure 2. 20. (A) AFM-QNM modulus image of compound B2T10 and (B) AFM phase

image of compound B2T10.

TEM imaging was also used to characterize compound B2T10. Nanoparticles in

Figure 2. 21 were identified by arrows. The diameters of nanoparticles range from 12 nm to 35 nm. It is consistent with the size observed by AFM. Nanoparticles were observed in compound B2T10 by AFM and TEM. These nanoparticles probably reinforced compounds and improves mechanical properties of compounds.

Figure 2. 21. TEM image of compound B2T10. where nanoparticles were identified by arrows.

2.3.2.5 Combination of BPO and BMI

Since TMPTMA works with BPO but not DCP, the combination of BPO and BMI was explored out of curiosity. Curing kinetic curves in Figure 2. 22 show that a substantive rise of torque requires a relatively high amount of BPO (> 1 phr). No reversion is observed in these compounds.

100

4 B1M7

B2M7

B4M7

3 Torque (dNm) 2

1 0 10 20 30 40 50 60 70 Time (min)

Figure 2. 22. Curing kinetic curves of EIIR compounds cured by BPO-BMI at 110 oC.

Note that the curing characteristics of those compounds are summarized in Appendix A,

Table A. 3.

The degree of cure is evaluated by measuring the gel and sol contents of the vulcanizate and the equilibrium-swelling ratio of the gel insoluble in hexane, which is shown in Table 2. 10. Compounds except B1M7 have ~90% gel fraction, which is same as a sulfur-based curing package usually afford for the parent IIR.

101

Table 2. 10. Gel fraction, crosslinking density and peroxide efficiency of EIIR

compounds cured by BPO and BMI.

Gel fraction Crosslinking Peroxide Compound (%) density (mol/m3) efficiency

B1M7 78% ± 0.1% 2.3 ± 0.2 0.06 ± 0

B2M7 91% ± 0.2% 16.8 ± 0.3 0.23 ± 0

B4M7 91% ± 0.2% 46.1 ± 0.9 0.32 ± 0.01

D0.1M7 72% ± 1.8% 5.8 ± 0.1 1.77 ± 0.03

D0.3M7 89% ± 0.3% 22.3 ± 0.7 2.27 ± 0.07

D0.5M7 89% ± 0.1% 26.8 ± 0.4 1.64 ± 0.02

Crosslinking density of the gel fraction is determined by method described before, where the volume occupied by EIIR in swollen state is corrected by removing the insoluble

BMI. Only crosslinks that result in elastically effective strands are counted. As shown in

Figure 2. 23, the crosslink density correlates reasonably well with increase in torque recorded from the curing kinetic study. The similar cure level of compound is achieved by

BPO-BMI in comparison to those cured by DCP-BMI in Table 2. 10. It is noteworthy that vulcanizate B4M7 shows the highest crosslinking density without reversion observed in curing kinetic curves. However, the peroxide efficiency of BPO is still low as it does with coagent TMPTMA.

102

2.5

2.0

1.5

M (dNm) Δ 1.0

0.5 B1M7, B2M7, B4M7 Linear regression with R2 = 0.99 0.0 0 10 20 30 40 50 3 ρcx (mol/m )

Figure 2. 23. Correlation between M and ρcx of EIIR compounds.

Stress relaxation was carried out to unambiguously ascertain whether transition from viscous liquid to elastic solid was realized for the EIIR vulcanizates. All vulcanizates display a finite modulus after one week at room temperature under 10% strain in Figure 2.

24. The vulcanizates are viscoelastic solids.176 The terminal moduli, which can be approximated as the equilibrium moduli, show a good linear correlation with crosslinking densities determined from the swelling ratios using Flory-Rehner equation (Figure 2. 25

(A)). The absolute values of crosslinking density calculated using the ideal elastic equation

3ρ RT combined with Guth-Gold equation, E = r (1 + 2.5φ), where φ is the volume fraction Mc of BMI in the rubber compound, are higher to various extents than the values determined

103 from the swelling ratio (Figure 2. 25 (B)). The difference is likely caused by the permanent entanglements trapped by the crosslinks.

1 Modulus(MPa) 0.1 B1M7

B2M7

B4M7 0.01 10-2 10-1 100 101 102 103 104 105 Time (s)

Figure 2. 24. Stress relaxation curves of EIIR compounds cured by BPO-BMI.

104

1.0 A 0.8 0

0.6

(MPa) eq

E 0 0.4 0 0.2 B1M7, B2M7, B4M7 Linear regression with R2 = 0.99 0.0 0 10 20 30 40 50 3 ρcx (mol/m )

60 From stress relaxation From swelling 50 48.3 45.4

) B 3

40 mol/m

( 0 30

cx 24.1 ρ

20 0 17.4

9.2 10 0 2.3 0 B1M7 B2M7 B4M7

Figure 2. 25. (A) Correlation between ρcx and terminal moduli (Eeq). (B) Comparison of

ρcx obtained from swelling and equilibrium moduli.

105

The stress-strain curves of B2M7 and B4M7 are shown in Figure 2. 12. Both display low stress and strain at break, and overall toughness (Table 2. 11) in comparison to compounds D0.3M7 and D0.5M7, which were cured by DCP-BMI. It is noteworthy that the crosslinking density of B2M7 is similar to D0.3M7. It is not clear what causes worse mechanical properties of compounds B2M7 and B4M7. One possibility is clusters of crosslinks form in compound via radical addition, and introduce local stress concentration, which causes worse mechanical properties.

Table 2. 11. Mechanical properties of EIIR compounds cured by BPO and BMI.

σ σ ε σ Toughness Compound 100% 200% b b (MPa) (MPa) (%) (MPa) (106 J/m3) B2M7 1.03 ± 0.04 1.31 ± 0.05 289 ± 39 2.15 ± 0.32 3.8 ± 0.9 B4M7 1.17± 0.11 - 185 ± 15 2.10± 0.30 2.1± 0.5

2.5

B2M7 2 B4M7

1.5

1 Tensile stress (MPa) 0.5

0 0 100 200 300 Strain (%)

Figure 2. 26. Stress-strain curves of EIIR compounds cured by BPO-BMI.

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2.4 Conclusion

EIIR, an IIR derivatives containing geminal vinylidene-acrylate groups, was synthesized by grafting ethyl propiolate to IIR via EtAlCl2-catalyzed Alder-ene reaction.

Methods to cure EIIR with DCP were investigated with the assistance of BMI as the coagent. The crosslinking density achieved using such curatives can be indexed as a function of DCP/BMI ratio, and the curing efficiency of peroxide can be indexed as a function of DCP/BMI ratio and DCP loading. The role of the germinal vinylidene-acrylate moiety in curing was studied by model reactions using small molecular surrogates for the functionalities present in EIIR. Evidence primarily from MS spectrometry suggests that the germinal vinylidene-acrylate moiety undergoes both radical addition and hydrogen abstraction. Its importance is further demonstrated by the fact that its small molecule surrogates EDHEX reduces chain scission when used as a coagent in conjunction with

BMI. A wide range of mechanical properties can be achieved using combinations of DCP,

BMI, and EDHEX to cure EIIR. The EIIR vulcanizates significantly exceed the previously reported IIR derivatives cured by peroxides in stiffness, stress at break, strain at break, and overall toughness. The stress at break of the EIIR is on par with that of sulfur-cured IIR, but the strain at break remains inferior. It should be noted that the degree of unsaturation of IIR and consequently its EIIR derivative are moderate. One can expect that further improved properties are achievable using IIRs with high degree of unsaturations.

Coagent TMPTMA was used to replace BMI due to high toxicity of BMI. The low gel fraction (~50%) were found in all vulcanizates cured by DCP and TMPTMA. It is possible due to a mismatch between the reactivities of DCP and TMPTMA. The cumyloxy radical is primarily a radical abstractor but does not readily undergo radical addition, while

107

TMPTMA primarily undergoes radical addition but not radical abstraction. BPO was then used in conjunction with TMPTMA. EIIR was successfully cured with BPO and TMPTMA to give high stress and strain at break and toughness exceeding those achieved with DCP and BMI as the curing agents. This is probably due to the reinforcement of oligomers of

TMPTMA in vulcanizate, where nanoparticles were observed by AFM and TEM. Since

TMPTMA works with BPO but not DCP, the combination of BPO and BMI was also explored. The combination of BPO and BMI are also capable to sufficiently cure EIIR, but the resultant vulcanizates have tensile properties somewhat worse than those cured by DCP and BMI.

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CHAPTER III

SURFACE DEVULCANIZATION OF GROUND RUBBER PARTICLES FOR

RUBBER RECYCLING

3.1 Introduction

Millions of tons of vulcanized rubber items are produced each year globally. Most of them are vehicle tires.106 Recycling of vulcanized rubber presents a unique challenge different from recycling of thermoplastics, as the former cannot be easily re-processed without breaking the covalent crosslinks of the rubber network.183 Efforts to break the covalent crosslinks has a history almost as long as the history of vulcanization.107, 184

Although processability is generally achievable via devulcanization140-144, 153-156, the resultant rubber, which is termed reclaimed rubber, affords inferior mechanical properties upon vulcanization because it is structurally and compositionally different from its virgin counterpart.147-149 The reclaimed rubbers are therefore only suitable for downcycling for applications less demanding than tires are, for example, for road paving and making ebonite.108 An alternative widely explored route of rubber recycling is to grind the vulcanized rubber to small particles112, 113 and blend the ground rubber particles (GRPs) with pristine rubber for making new vulcanized products.115 Although they account for the majority of recycled rubbers, GRPs also significantly lower the mechanical properties even

109 if only present at a small fraction in the blend with virgin rubber.145, 146, 152, 157-159 They are also downcycled for making items such as floor mats and sport turfs.108, 160

Significant advances are necessary before either routes can become suitable for recycling vulcanization rubbers into high-value products such as tires. In particular, little attention has been given to the underlying scientific and engineering challenges despite of the numerous attempts of recycling GRPs. As the result, the problems encompassed by such processes are not clearly understood, and the solutions are out of the question. In the present study, we first articulate the difficulties involved in recycling GRPs and then provide the initial strategies to overcome the difficulties.

3.2 Experimental

3.2.1 Materials

All solvents and chemical reagents were purchased from Sigma-Aldrich and used as received. Masterbatches of butadiene rubber compounds with the compositions summarized in Table 3. 1, with and without curing agents (sulfur and TBBS), were provided by Akron Rubber Development Laboratory, INC. The cured rubber was cryogenically ground at the facility of Advanced Cyrogenics, Akron, Ohio. Ground rubber particles (GRPs) collected between a 30- and a 60-mesh screens were used in this study.

110

Table 3. 1. Rubber compound compositions.

Ingredients Amount (phr)*

Butadiene Rubber 100 (CB PBD 1220)

Carbon Black N330 40.0

Stearic Acid 2.00

Zinc oxide 2.00

Sulfur 1.25

TBBS 0.83

* part per hundred parts of rubber.

3.2.2 Formation of Rubber Compounds Containing GRPs

The compositions of the GRP-containing rubber compounds are summarized in

Table 3. 2. An 80-cc Brabender mixer was preheated to 50 °C, and the rotor speed was set at 60 rpm. A measured amount of rubber compound from the masterbatch without curing agents was fed into the mixer. After 2 min, GRPs were added into the mixer. After another

2 min (i.e., at the 4th minute), the curing agents were added, and the entire compound was mixed for another 3 min. The compound was then dumped with the dumping temperature being ~90 °C. The rubber compound was then passed through a two-roll mill (Reliable Mill

Supply Co., Ukiah, CA) with 1 mm distance between rolls 15 times.

For compound containing additional piperidine, 2% piperidine by weight was added into compound during mixing compound on the two-roll mill.

111

Table 3. 2. Compositions of GRP-containing rubber compounds.

Ingredients Amount (g)

Masterbatch without curatives 55.09

GRPs 10.72

Sulfur 0.47

TBBS 0.32

3.2.3 Vulcanization Kinetics

A Montech 2000 moving die rheometer (MDR) was used. A sample of ~6 g was taken from the compound of interest and placed between two heated moving dies at 150

°C. Sinusoidal shear was applied with a frequency of 1.66 Hz and a deformation angle of

0.5°. The torque was recorded. The curing curves of compounds are shown in the Appendix

B (Figure B. 1 and Table B. 1).

3.2.4 Vulcanization

Vulcanization was carried out on a Dake Hydraulic Mold 4419 hot press. A sample of ~30 g was placed in a steel window mold (160 × 160 × 0.5 mm). Mylar films and two aluminum plates were placed on each side. The sheets were cured at 150 oC for (T90 + 3) min in Dake hydraulic press under a load of 35 tons. The pressure was released, and the cured compound was quenched in water.

112

3.2.5 Preparation of Sheets with Cord Backing

60 g masterbatch of a butadiene rubber compound with curing agents was placed in the DAKE HYDRAULIC MOD 4419 compress machine and compressed at 90 oC for 5 min with the load of 0.5 tons to form the sheet with the dimension of 15 cm × 15 cm × 0.20 cm. After that, the sheet was laminated with a cord-reinforced rubber sheet with the dimension of 15 cm × 15 cm × 0.10 cm and kept at 90 oC for 10 min with the load of 0.5 tons. The formed sheet with cord backing was still uncured and stored at 2~5 oC for further use.

Some of rubber sheets with cord backing were cured at 150 ºC. The curing time was 25 minutes. The cured sheets were stored at 2~5 oC for further use.

3.2.6 Preparation of Laminates for Physical Adhesion Test

The uncured and cured sheets with the dimension of 15 cm × 15 cm × 0.30 cm were cut into strips with the dimension of 1.27 cm × 7.62 cm × 0.30 cm. Dremel 200 grinding wheel was used to buff the surface of cured strips. The surface of strips was grinded back and forth three times. The buffed surface was cleaned with a jet of compressed air until no visible rubber powders were left on the surface.

A buffed cured strip was placed into an aluminum mold with a window of the same dimension as the strip. An uncured strip with the same dimension was placed on the top of the cured strip. A -shaped mylar film with the outer dimensions of 1.27 cm × 7.40 cm and inner opening dimensions of 0.63 cm × 5.05 cm was placed between cured strip and uncured strip to make them separable after compression. The mold was then placed in the

DAKE HYDRAULIC MOD 4419 compress machine. The laminate was compressed at 113 room temperature with the load of 0.5 tons. The pressure gradually decreased somewhat due to the deformation of uncured strips. After 24 hours, the pressure was removed, and laminates were stored overnight at room temperature before testing.

3.2.7 Preparation of Co-cured Laminates for Adhesion Test

A buffed cured strip, uncured strip and -shaped mylar film were placed into an aluminum mold following the same procedure in physical adhesion test. After that, the mold was placed into the compress machine and compressed at 150 °C with the load of 0.5 tons. After 25 min, the pressure was removed, and co-cured laminates were quenched in water and stored overnight at room temperature before testing.

To prepare the cured strip perfused with piperidine, a cured strip and a vial containing 1 mL piperidine were put into a sealed flask. The flask was put into liquid nitrogen and evacuated. After that, flask was removed from liquid nitrogen and warmed up to room temperature and kept in a 50 oC oil bath overnight. This resulted in even absorption of an excess amount of piperidine by the cured strip. The strip was taken out from flask and put into a fume hood to allow evaporation of piperidine. The strip was weighed until the weight reached to the expected mass. A pre-cured strip perfused by piperidine was pressed against an uncured strip into the compress machine as described before and compressed at 70 °C with the load of 0.5 tons for 20 min. Then laminate was cured at 150

°C for 25 min.

114

3.2.8 Tensile Test

ASTM D412 Type C dumbbells were cut from cured sheets. Tensile properties were measured on an Instron Model 5567 equipped with a 1000 N load cell. The width of the narrow section was 6.35 mm, and the thickness was 0.6-0.8 mm. Crosshead speed was 500 mm/min with an initial gap of 65 mm. Strain was measured with an extensometer.

3.2.9 Adhesion Test

Adhesion test was used in an Instron Model 5567 tensile tester equipped with 1000

N load cell, using sample shown in Figure 3. 1; 180-degree peel geometry was used. The crosshead speed was set as 50.8 mm/min (2 in/min). Laminate was stretched until two strips were completely separated. The adhesion of laminate was calculated by the average force divided by the width of sample.185

Figure 3. 1. Geometry of adhesion test.

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3.2.10 Indentation Study

The cross-sectional surface of co-cured laminate was protected by a scotch tape, and other sides of laminates were imbedded in 635 thin epoxy resin. The 635 thin epoxy resin was allowed to cure for four days at room temperature. After that, the tape was removed from laminate, and the cross-sectional surface of co-cured laminate was cleaned by a jet of compressed air. Then the laminate was stored in the sealed bag for the test.

Microindentation. Microindentation experiments were carried out at Akron rubber

Development Laboratories, Akron, Ohio. An instrument previously described186, 187 for profiling the Young’s modulus of polymers was used to map the cross-sectional surface of the cured laminates. The instrument gives a lateral resolution of about 100 µm.

Nanoindentation. A Hysitron TI Premier Nanoindenter (Bruker) with a TI-0039 probe was used to map the modulus of the cross-sectional surface.188 The probe gives a spatial resolution up to ~1 µm. Measurements of at least three different loci at the same distance to the interface were made, and the average value was taken.

3.2.11 Modification of Cured Strips

3.2.11.1 Modification of Cured Strips by Mercapo-n-butane/Piperidine Solution

Two 1.27 cm × 7.62 cm × 0.30 cm buffed cured strips were immersed into a 30 mL solution of mercapo-n-butane (nBuSH) and piperidine (1:10 volume ratio) for 10 seconds at room temperature. Strips absorbed the solution and swelled during this process. Then strips were taken out from solution, residual solution on surface of strips was wiped down by paper tower. The strips were packed with foil and put into sealed flask for 1 hour. After that, strips were washed by 100 mL toluene for 1 min before drying in vacuum oven at 116 room temperature for at least 12 h to a constant weight. The strips were then collected and stored in a sealed bag.

3.2.11.2 Modification of Cured Strips by nBuSH /Piperidine in Acetone

Two 1.27 cm × 7.62 cm × 0.30 cm buffed cured strips were immersed into a solution of nBuSH and piperidine in acetone with a volume ratio of 1:10:100 (50 mL) for 1 hour at room temperature. No visible swelling was observed. Then strips were taken out from solution and washed by 150 mL acetone for 3 hours. After that, strips were pumped down under vacuum oven at room temperature for at least 12 h to a constant weight. The strips were then collected and stored in a sealed bag.

3.2.11.3 Surface Modification of Single Cured Strips by nBuSH /Piperidine in Acetone

The single cured strips with and without carbon black were used in this experiment.

The surface of the strip was not buffed. The procedure is shown in Figure 3. 2. One side of the strip was taped with a scotch tape. The other sides of strip are embedded in epoxy resin contained in a stainless-steel ring (Step 1). After allowing the epoxy resin to cure at room temperature for four days, the epoxy resin block embedded with the strip was removed from the steel ring, and the tape was also removed. The strip embedded in the epoxy block was treated by the solution of nBuSH and piperidine in acetone with a volume ratio of

1:10:100 (50 mL) for different time at room temperature (Step 2). After that, the strip was removed from epoxy resin and pumped down at room temperature to remove residual solvent. The surface of strip was cleaned by compressed air. The strip was cross-sectioned for nano-indentation test (Step 3). 117

Figure 3. 2. Procedure for surface modification of single cured strip.

3.2.12 Modification of GRPs

3.2.12.1 Modification of GRPs by nBuSH /Piperidine Solution

15 g of GRPs was added into a Buchner funnel, followed by the addition of a neat solution (100 mL) of nBuSH and piperidine in 1:10 volume ratio. The liquid was allowed to dwell in the funnel for 10 seconds before being removed by suction. The funnel containing the wet GRPs was wrapped with aluminum foil and allowed to sit at room temperature for 1 h and the GRPs were then washed by 300 mL toluene. After the bulk toluene was removed by suction filtration, the residual toluene was removed under vacuum at room temperature until a constant weight was reached.

3.2.12.2 Modification of GRPs by Mercapo-n-butane/Piperidine in Acetone

15 g of GRPs was added into a solution (100 mL) of nBuSH and piperidine in acetone with a volume ratio of 1:10:100 and stirred for 1 h at room temperature.

The solvent was removed by suction filtration. The GRPs was stirred in fresh acetone (300 118 mL) for 3 h. After the bulk acetone was removed by suction filtration, the residual acetone was removed under vacuum at room temperature until a constant weight was reached.

3.3 Result and Discussion

3.3.1 Selection of System to Study

We started the investigation by comparing the tensile properties of carbon-black reinforced polybutadiene compounds containing 16% by weight of cryogenically ground rubber particles (GRPs) or glass beads against the tensile properties of the virgin rubber compound (VRC) without any micrometer-sized particles. The GRPs are of various sizes, compositions, and history of usage, and the amount of the GRPs in the blend was 16% by weight. The glass beads were 250 m in diameter (~60 mesh). As summarized in Figure 3.

3, all vulcanizates containing GRPs or glass beads show a significant decrease in both stress and strain at break in comparison to VRC without any particles. The size of the particles clearly has an effect on the extent of deterioration in the ultimate properties. This is commonly observed and explained by considering the particles as defects in the continuous rubber phase. Tearing energy is a function of the cut size in the rubber. The intrinsic defect has an effective cut size less than 100 m.189 Any extrinsic defect larger than the intrinsic defects in the effective cut size gives rise to a faster crack propagation and eventually rapid catastrophic failure above a critical size.

119

Stress at break 18 VRC 500 Strain at break

GRPs made GRPs from TBR Tires 15 from VRC Glass-Spheres 400 (250 m) 12 300 9 200

Strain at break(%) at Strain Stress at breakStress at (MPa) 100 3

0 0

Control 30 mesh 3.1 wt%8.7 wt%15 wt% 60 mesh 270 mesh120 mesh80 mesh40 mesh

Figure 3. 3. Comparison of stress at break (B) and strain at break (B) of vulcanizates of

blends of virgin rubber compound and particles of various size, history of usage, and

composition against the virgin rubber compound (VRC).

The particles are obviously more complex than simple cuts because if they were, the effect of the particles of the same size, either GRPs or glass beads, would be the same.

Composition and history of usage can be immediately pointed to as an additional factor that influences the failure properties. Nevertheless, when GRPs freshly made from vulcanized VRC was blended with uncured VRC and cured, the resultant vulcanizates are also substantially inferior to VRC in stress and strain at break. The complexity of the problem at hand prompted us to choose a simple system with a reduced number of variables to study. From here on, only the GRPs made from the VRC will be used so that bulk 120 composition is excluded as a variable. Only 30-60 mesh GRPs will be used so that the particle-size variable is excluded. The actual size of the 30-60 mesh GRPs was characterized by optical microscope (Figure 3. 4). The area of the two-dimensional images of the GRPs were analyzed by Image J. The number average area per particle is 95618

µm2. If the particles were assumed to be spherical, the average diameters of the GRPs are

349 µm. As shown in the histogram of such normalized diameters (Figure 3. 4), ~75% of the GRPs have a diameter between 100 to 500 m. With the size and composition being held invariant, the interface between the GRPs and the host VRC became a natural focus of this study. The effect of aging will be briefly explored at the end of this study.

121

0.4

0.329 0.3 0.285

0.2

0.132 0.130 0.124

0.1 Number fraction of GRPsNumber of fraction

0.0

> 500 μm350-500 μm250-350 μm100-250 μm< 100 μm

Figure 3. 4. Representative optical images and size histogram of 30-60 mesh GRPs.

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3.3.2 Interfacial Bonding between Pre-cured and Virgin Rubber

To begin to understand the interfacial bonding between GRPs and its host virgin rubber compound, we studied the bulk interface of a pre-cured strip and an uncured strip of VRC. First, T-peel test was carried out to measure the physical adhesion between the two strips. The laminate of the two strips, L0, were subjected to high pressure for 24 h to allow sufficient time for interpenetration equilibrium of the rubber chains to be established at the new interface.190 The adhesion is minimal in comparison to that between two uncured strips of VRC (Figure 3. 5), not surprisingly as penetration of the PBD chains on the uncured side into the network on the cured side results in restriction of the conformational freedom of uncured chains and incurs an entropy penalty. The contact at the molecular level must be shallow and not include entanglements in substantial numbers. Curing L0 resulted in an increase in the adhesion energy, indicating that some covalent bonds across the interface. However, the adhesion energy of cured L0 is only a small fraction of the cohesion energy, measured by peeling cured laminate of two uncured VRC strips. The low level of molecular contacts is directly or indirectly responsible for the poor adhesion.

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20 Physical adhesion Adhesion of cured laminate

14.99 15 14.71

11.83 11.58

10 Adhesion (N/mm) 5

1.48 1.01 1.01 0.78 0.38 0 VRC L0 L1 L2 L3

Figure 3. 5. Adhesion of laminates.

To overcome this problem, de-vulcanization or partially de-vulcanization near the surface of the pre-cured rubber is desirable. Campbell reported in the 1960s the cleavage of S-S bonds at room temperature by mercaptans dissolved in piperidine.191 The latter serves as the catalyst for the S-S/H-S metathesis reaction. Although the method does not cleave mono-sulfidic crosslinks, it is potentially useful for our purpose. Upon briefly exposing a cured strip of VRC in a 1:10 mixture of nBuSH and piperidine and allowing the reaction to proceed at room temperature for 1 hour, a tacky surface was generated. The treated, cured strip was then pressed against an uncured strip to form laminate L1. Physical adhesion between the strips in L1 in Figure 3. 5 was comparable to or even slightly better

124 than that of two uncured strips of VRC. After curing L1, the adhesion energy between the two strips approached the level of the cohesion energy of VRC in Figure 3. 5.

Unfortunately, when applied to GRPs, the 1:10 mixture of nBuSH and piperidine proved to penetrate and swell GRPs too rapidly. Despite of the very short dwell time (10 seconds) in the above liquid, the GRPs lost 40% of its weight upon washing with toluene after the reaction. The nonvolatile substance dissolved in toluene contained very little carbon black, as assessed by its elemental compositions in Table 3. 3, and is primarily PBD according to 1H NMR in Figure 3. 6. The vulcanizate V1, which contains 16 wt% of GRPs treated by the 1:10 mixture of nBuSH and piperidine, showed even worse tensile properties than V0, which contains the same amount of the GRPs without any treatment (Figure 3. 7 and Table 3. 4).

125

Table 3. 3. Elemental analysis results of nonvolatile residuals in devulcanization solution.

Entry C (%) H (%) N (%) S (%)

VRC* 89.53 7.83 0.07 1.00

VRC minus carbon black* 85.62 10.76 0.09 1.38

Nonvolatile residuals 85.67 11.25 0.24 1.19 in 1:10 mixture of nBuSH and piperidine

Nonvolatile residuals 66.20 11.08 4.12 4.60 in mixture of nBuSH, piperidine and acetone

Note that the elemental analysis results of samples with asterisk are calculated from formulation in Table 3. 1. Other samples were carried out by Micro-Analysis INC.,

Delaware.

126

CDCl3 *

8 7 6 5 4 3 2 1 0

Chemical shift (ppm)

Figure 3. 6. 1H spectrum of nonvolatile residual in a 1:10 mixture of nBuSH and

piperidine.

Note that the peak with an asterisk was from water. The peaks between 1 and 1.5 ppm originated from nBuSH, curatives, or their remnants.

127

Table 3. 4. Summary of tensile properties of vulcanizates in this study.

σ100% σ300% εb σb Toughness Entry (MPa) (MPa) (%) (MPa) (106 J/m3)

VRC 1.97 ± 0.13 8.21 ± 0.42 492 ± 17 16 ± 0.8 35 ± 3

V0 1.57±0.17 7.23 ± 0.95 353 ± 27 9.2 ± 0.3 13 ± 1

V1 0.93 ± 0.04 3.47 ± 0.20 331 ± 5 4.1 ± 0.2 6 ± 0.3

V2 1.64 ± 0.04 7.59 ± 0.20 355 ± 15 10.0 ± 0.6 14 ± 1.5

V3 2.69 ± 0.22 12.69 ± 0.85 336 ± 15 14.9 ± 1.3 21 ± 3

V4 2.33 ± 0.31 10.91 ± 1.30 305 ± 20 10.9 ± 1.7 14 ± 3

V0-a 1.24 ± 0.06 4.83 ± 0.29 318 ± 19 5.3 ± 0.5 7 ± 1

V0-aa 1.46 ± 0.13 5.89 ± 0.23 314 ± 24 7.0 ± 0.3 9 ± 0.9

V3-a 2.31 ± 0.11 11.08 ± 0.44 352 ± 7 14.5 ± 0.8 20 ± 1.1

V3-aa 2.66 ± 0.07 12.98 ± 0.41 321 ± 7 14.5 ± 0.4 19 ± 0.6

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16 V3 VRC 14 12 V4 10 V2 8 V0

6 Tensile Stress (MPa) 4 V1 2 0 0 100 200 300 400 500 Strain (%)

Figure 3. 7. Stress-strain curves of compounds containing GRPs.

To limit the depth of devulcanization and hence avoid severely damaging the interior of the GRPs, we sought to use a solution of nBuSH and piperidine, which does not swell vulcanized the rubber network, to treat the GRPs. Vulcanized strips of VRC were used to screen the solvents and reaction conditions. The reaction proved to take place in polar nonprotic solvents such as acetone and dimethylformamide but not in protic solvents such as alcohols (Figure B. 2, Appendix B). Acetone was chosen as the solvent because it is volatile and easy to remove, and the reaction conditions were established as detailed in

Experimental Section. The strength of physical adhesion between a cured strip treated by this method and an uncured strip of VRC rose to the level on a par with that between two uncured strips in Figure 3. 5. After the laminate of these two strips, L2, was cured, the resultant adhesion strength again became equal to the cohesion strength in Figure 3. 5. The 129 acetone solution was then used to treat the GRPs. Negligible weight loses of the GRPs were detected after the treatment. The solid residuals in the acetone solution are composed of a high level of N and S elements shown in Table 3. 3, indicating that they originate from nBuSH, piperidine, curatives, or their remnants. Further washing the treated GRPs with toluene does not give additional extractable contents. The above evidence suggests that the modification method using the acetone solution allows sufficient surface devulcanization without extensively damaging the interior network of the rubber.

The GRPs modified with the acetone solution of nBuSH and piperidine was blended with fresh rubber and vulcanized to give V2. Unfortunately, only small improvement was achieved in comparison to V0 in Figure 3. 7. V2 remains significantly weaker and less extensible than VRC. The difference between the bulk adhesion strength between the two strips and the mechanical strengths of the vulcanized blend suggests that strength of interfacial bonding is not the only parameter that has to be optimized.

3.3.3 Modulus Contrast at Interface

To find other factors that significantly influence the interface between the GRPs and the VRC, we resorted to micro-indentation study of the cured laminates L0-L2 described above.

For L0 in Figure 3. 8, a substantial modulus difference exists on the two side of the interface. Since the bulk modulus away from the interface is approximately 2.0 - 2.5 MPa, the low modulus on the “uncured” side account for a significant portion of the difference although the modulus near the interface on the “pre-cured” side is also somewhat higher than normal. We attribute the difference in modulus to diffusion of curatives192 from the

130

“uncured” side to the “cured” side during vulcanization of the laminate. The lower than normal concentrations of curatives resulted in a region of low crosslink density on the

“uncured side”, and the curatives that migrated into the “cured” side gave rise to additional crosslinks.

4.0 Interface

3.5 "Cured" Side "Uncured" Side

3.0

2.5 Modulus away from interface 2.0

1.5 Modulus(MPa) 1.0

0.5

0.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Distance (mm)

Figure 3. 8. The modulus map of the laminate L0 around the interface.

131

For L1 in Figure 3. 9, a substantial modulus difference also exists on the two side of the interface. However, the rise of modulus on the cured side appears more prominent than in L0. While the aforementioned diffusion-related additional cure presumably contribute, one can reasonably argue that the loss of the soft PBD component and hence an increase in carbon black content near the interface is another likely reason for the sharp rise of modulus. The width severely high modulus exceeds the diameter of the GRPs, consistent with the observed loss of weight when the GRPs are were treated with this devulcanization method.

4.0 Interface 3.5 "Cured" Side "Uncured" Side 3.0

2.5

2.0

Modulus(MPa) 1.5

1.0

0.5

0.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Distance (mm)

Figure 3. 9. The modulus map of the laminate L1 around the interface.

132

For L2 in Figure 3. 10 (A), the low-modulus region on the uncured side exists very much like L0, and the high-modulus region on the cured side is barely discernable again like L0 and in contrast to L1. Its existence of the high-modulus region is confirmed by a small probe for the indentation study and shown in Figure 3. 10 (B). It should be noted that the micro-indention186 and nano-indentation188 studies were carried out using two different instruments (see Experimental Section) following different algorithms to produce the

Young’s modulus. The micro-indentor was specially developed for studying polymers and has been demonstrated to give moduli consistent with the values measured using conventional methods.186 The nano-indentor is a general instrument, which is known to produce moduli higher than values obtained from conventional methods for polymeric materials.186

Nano-indentation study of a single cured strip with and without carbon black was carried out to understand the surface devulcanization process. As show in Figure 3. 11 (A), after the surface treatment, a high modulus region near the surface was observed. This is unexpected result. We attribute the modulus increase to carbon black flocculation. In support to the assessment, a cured single strip without carbon black was also studied. the anticipated decrease of modulus was observed. Both experiments indicate that the depth of devulcanization is on the order of 100 m.

133

4.0 Interface 3.5 "Cured" Side "Uncured" Side 3.0

2.5

2.0

Modulus (MPa) 1.5

1.0 0.5 A 0.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0 Distance (mm)

0 10 Interface

"Cured" Side "Uncured" Side 9 0

Modulus (MPa) 6

5 B 4 -300 -200 -100 0 100 200 300 0 Distance (μm) Figure 3. 10. The modulus maps of the laminate L2 around the interface by (A) micro- 0 indenter and (B) nano-indenter. Note that the penetration depths0 of the nano-indenter and micro-indenter are different. The absolute modulus values from the two technical cannot be directly compared.

134

10 Surface Without treatment 20 μm 30min-treatment 70 μm 8 1h-treatment 190 μm 3h-treatment

6 Modulus (MPa) 4 A

2 0 200 400 600 0 800 1000 Distance (μm)

6 Surface 0 75 μm B 5 120 μm 230 μm 4 0

3 0 Modulus (MPa) 2 Without treatment 30min-treatment0 1 1h-treatment 3h-treatment 0 0 200 400 600 800 1000 Distance (μm)

Figure 3. 11. The modulus maps of single cured strip with (A) carbon black and (B)

without carbon black.

135

The above study reveals that a sudden change in modulus across the interface. Since a modulus contrast raises the stress concentration,193 this interfacial modulus contrast is possible to be responsible for the lower than expected tensile properties of V1 and V2. The contrast has little or no effect to the T-peel study of L1 and L2 because the force and overall direction of failure is orthogonal to the modulus contrast. In a blend of GRPs and VCR, the force and fracture are not stipulated in any given direction. This possibly allows the detrimental effect of the modulus contrast to be fully manifested. A close inspection of the failed interface of L1 and L2 (Figure 3. 12) revealed features consistent the above assessment. The fracture line invariantly runs on the pre-cured side and intermittently penetrates the entire depth of the pre-cured strip. Upon reaching the cord-reinforced backing sheet, it rebounds back, propagates through the pre-cured strip again, becomes arrested vertically once it reaches the interface, and propagates along the interface. The back-and-forth repeats a number of times until the complete separation of the two strips.

The modulus contrast appears to encourage the crack to propagate in the direction of the contrast.

Figure 3. 12. The image of failed interface of L2.

136

3.3.4 Erasing Modulus Contrast across the Interface

To confirm the conjecture that the modulus contrast was the problem and was caused by diffusion of curatives across the interface, a method that prevents the diffusion would be needed. Stopping diffusion is difficult in a rubber especially at vulcanization temperatures. It occurred to us that perhaps sulfur diffusion can be outcompeted. If sulfur on the uncured side were consumed quickly for crosslinking the uncured rubber before it had time to diffuse into the cured side, the modulus contrast would be eliminated or at least reduced. Following this logic, a small amount of piperidine was allowed to perfuse a pre- cured strip of VRC after the strip was treated with the acetone solution of nBuSH and piperidine. The strip was pressed against an uncured strip to give laminate L3. The piperidine in the pre-cured strip is expected to diffuse quickly into the uncured side before sulfur can diffuse and catalyze the sulfur vulcanization once L3 is subjected to vulcanization conditions. The adhesion energy between the strips in L3 was confirmed to approach the cohesion energy of VRC in Figure 3. 5. Indentation study of L3 in Figure 3.

13 reviewed that the low modulus region on the uncured side was eliminated. A slow rise of modulus seemed exist from the interface into the previously “uncured” side apparently because the degree of crosslink is increased due to the catalytic action of piperidine.

137

4 Interface

2 Modulus (MPa)

1 "Cured" Side "Uncured" Side A 0 -1.5 0 -1.0 -0.5 0.0 0.5 Distance (mm) 0

8 0 Interface 7 "Cured" Side "Uncured" Side

5 Modulus(MPa) 4

3 B 2 -100 0 -50 0 50 100 150 200 Distance (μm) Figure 3. 13. The modulus0 maps of the laminate L3 around the interface by (A) micro-indenter and (B) nano-indenter. 0

138

The method was applied to treat GRPs. The vulcanizate V3, containing 16% by weight of GRPs treated by the acetone solution of nBuSH and piperidine and perfused with

2 % by weight of piperidine, displayed a remarkably improved stress at break in comparison to V0 and V2 and comparable to VRC. V3 remains less extensible than VRC because of the aforementioned increase in crosslinking density due to the actions of piperidine (the stress-strain curve of VRC perfused with 2 wt% piperidine is shown in

Figure B. 3, Appendix B). It should be emphasized that interfacial bonding remains a key to achieve such strength. Vulcanizate V4, which contains unmodified GRPs perfused with piperidine, is significantly weaker and less extensible than V3 in Figure 3. 7.

3.3.5 Aging by Oxidation

Aging in air has an obvious effect on the performance of GRPs. The vulcanizate

V0-a containing GRPs aged in air for 2 years showed further decreased stress and strain at break in comparison to V0 in Figure 3. 14, which contains unaged and unmodified GRPs.

The effect of oxidative aging on GRPs can be approximately simulated by heating fresh

GRPs in an autoclave filled with air for 20 min at 150 oC. The vulcanizate containing the

GRPs subjected to the accelerated aging process, V0-aa, shows tensile properties similar to V0-a (Figure 3. 14). Interestingly, after treatment with the acetone solution of nBuSH and piperidine followed by perfusion with piperidine, the GRPs subjected to natural aging and accelerated aging both perform nearly the same as the fresh GRPs in Figure 3. 14. The tensile properties of V3-a and V3-aa are only slightly worse than as V3. These appear to suggest that at least within the scope of the present study, aging harms the interfacial bonding much more significantly than it does the bulk properties. The present surface

139 devulcanization method can effectively overcome the detrimental effects of aging at the interface surface.

18 16 V3 V3-aa 14 V3-a 12 10 V0 8 V0-aa 6

Tensile stress (MPa) V0-a 4 2 0 0 100 200 300 400 500

Strain (%)

Figure 3. 14. Stress-strain curves of compounds containing aged and unaged GRPs.

3.4 Conclusion

We have identified poor interfacial bonding and interfacial modulus contrast as two inherent causes for the inferior mechanical properties of vulcanizates containing GRPs in comparison to vulcanizates of fresh rubber. The poor interfacial bonding is caused by limited depth of mixing and molecular contact between the polymer chains in the GRPs and fresh rubber. The interfacial modulus contrast is caused by diffusion of curatives from the fresh rubber to the GRPs and consequently over-cure on the GRP side and undercure on the fresh rubber side. Surface devulcanization of GRPs gives rise to strong interfacial 140 bonding equal to the cohesive strength of the rubber but only improves the tensile properties of vulcanizates containing GRPs to a limited extent. When the interfacial modulus contrast is erased, the stress at break of vulcanizates containing GRPs become equal to that of the fresh rubber vulcanizate. Finally, it appears from this study that oxidative aging harms the interfacial bonding more than it does the bulk properties.

141

CHAPTER IV

CONCLUSION

Vulcanization and devulcanization of rubber are discussed in this dissertation. In

Chapter 2, EIIR, an IIR derivatives containing geminal vinylidene-acrylate groups, was synthesized by grafting ethyl propiolate to IIR via EtAlCl2-catalyzed Alder-ene reaction.

Methods to cure EIIR with DCP and BMI were investigated. The crosslinking density achieved using such curatives can be indexed as a function of DCP/BMI ratio, and the curing efficiency of peroxide can be indexed as a function of DCP/BMI ratio and DCP loading. The role of the germinal vinylidene-acrylate moiety in curing was studied by model reactions using EDHEX. Evidence primarily from MS spectrometry suggests that the germinal vinylidene-acrylate moiety undergoes both radical addition and hydrogen abstraction. Its importance is further demonstrated by the fact that its small molecule surrogates EDHEX reduces chain scission when used as a coagent in conjunction with

BMI. A wide range of tensile properties of EIIR can be achieved using combinations of

DCP, BMI, and EDHEX. The EIIR vulcanizates significantly exceed the previously reported IIR derivatives cured by peroxides in stiffness, stress and strain at break, and overall toughness. The stress at break of the EIIR is on par with that of sulfur-cured IIR.

Coagent TMPTMA was used to replace BMI due to high toxicity of BMI. The low gel fraction (~50%) were found in all vulcanizates cured by DCP and TMPTMA. It is 142 possible due to a mismatch between the reactivities of DCP and TMPTMA. The cumyloxy radical is primarily a radical abstractor but does not readily undergo radical addition, while

TMPTMA in vulcanizate, where nanoparticles were observed by AFM and TEM. Since

TMPTMA works with BPO but not DCP, the combination of BPO and BMI was also explored out of curiosity. The combination of BPO and BMI are also capable to sufficiently cure EIIR, but the resultant vulcanizates have tensile properties somewhat worse than those cured by DCP and BMI.

In Chapter 3, we have identified poor interfacial bonding and interfacial modulus contrast as two inherent causes for the inferior mechanical properties of vulcanizates containing GRPs in comparison to virgin rubber compounds (VRC). The poor interfacial bonding is caused by limited depth of mixing and molecular contact between the polymer chains in the GRPs and fresh rubber. The interfacial modulus contrast is caused by diffusion of curatives from the fresh rubber to the GRPs and consequently over-cure on the

GRP side and under-cure on the fresh rubber side. Surface devulcanization of GRPs gives rise to strong interfacial bonding equal to the cohesive strength of the rubber but only improves the tensile properties of vulcanizates containing GRPs to a limited extent. When the interfacial modulus contrast is erased, the tensile strength of vulcanizates containing

GRPs become equal to that of VRC. Finally, it appears from this study that oxidative aging harms the interfacial bonding more than it does the bulk properties.

143

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159

APPENDICES

160

APPENDIX A

SUPPORTING INFORMATION OF CHAPTER 2

Contents Page

Figure A. 1. ESI-MS spectra and identities of the observed species in the model reaction corresponding to entry 2...... 164

Figure A. 2. ESI-MS spectra and identities of the observed species in the model reaction corresponding to entry 3...... 164

Figure A. 3. ESI-MS spectra and identities of the observed species in the model reaction corresponding to entry 5...... 165

Table A. 1. The curing characteristics of EIIR compounds cured by DCP and BMI...... 162

Table A. 2. The curing characteristics of EIIR compounds cured by DCP- TMPTMA and BPO-TMPTMA...... 163

Table A. 3. The curing characteristics of EIIR compounds cured by BPO and BMI...... 163

161

Table A. 1. The curing characteristics of EIIR compounds cured by DCP and BMI.

t10 t90 ML MH ΔM Compound (min) (min) (dNm) (dNm) (dNm)

D0.1M3 4.40 40.38 0.88 1.05 0.17

D0.1M7 4.44 36.29 0.97 1.64 0.67

D0.1M14 4.53 35.68 1.08 2.43 1.35

D0.2M3.5 3.99 25.90 0.89 2.16 1.27

D0.2M5.6 3.85 28.47 0.92 2.67 1.75

D0.2M11 3.81 31.32 1.06 3.37 2.31

D0.2M14 4.12 33.08 1.10 3.60 2.5

D0.3M3.5 2.45 10.52 0.84 2.03 1.19

D0.3M7 2.64 13.88 0.89 2.72 1.83

D0.3M14 3.09 24.71 1.06 4.91 3.85

D0.5M3.5 1.83 6.44 0.82 1.95 1.13

D0.5M7 2.57 15.94 0.82 2.69 1.87

D0.5M14 3.06 21.85 0.98 4.93 3.95

D0.3M1.7E0.6 2.35 13.66 0.73 1.45 0.72

D0.3M1.7E0.9 3.16 22.04 0.77 1.46 0.69

D0.3M3.5E0.9 3.20 22.86 0.88 2.28 1.40

D0.3M3.5E1.7 3.47 28.27 0.83 2.00 1.17

162

Table A. 2. The curing characteristics of EIIR compounds cured by DCP-TMPTMA and

BPO-TMPTMA.

T10 T90 ML MH ΔM Compound (min) (min) (dNm) (dNm) (dNm)

D0.2T10 0.88 2.54 0.67 1.20 0.53

D0.2T15 0.92 3.67 0.57 1.50 0.93

D0.3T10 0.88 2.88 0.64 1.34 0.70

D0.3T15 0.81 1.97 0.50 1.24 0.74

B1T10 1.17 5.62 0.92 1.73 0.81

B2T10 0.75 6.36 0.98 2.21 1.23

B3T10 0.08 1.55 0.98 1.77 0.79

B1T15 0.81 7.93 0.89 2.37 1.48

B2T15 0.68 8.00 1.02 2.51 1.49

B3T15 0.76 4.52 0.90 2.30 1.40

Table A. 3. The curing characteristics of EIIR compounds cured by BPO and BMI.

t t M M ΔM Compound 10 90 L H (min) (min) (dNm) (dNm) (dNm) B1M7 6.36 30.64 1.48 1.77 0.29 B2M7 4.91 40.19 1.49 2.39 0.90 B4M7 3.94 41.20 1.32 3.70 2.38

163

Figure A. 1. ESI-MS spectra and identities of the observed species in the model reaction

corresponding to entry 2.

Figure A. 2. ESI-MS spectra and identities of the observed species in the model reaction

corresponding to entry 3.

164

Figure A. 3. ESI-MS spectra and identities of the observed species in the model reaction

corresponding to entry 5.

165

APPENDIX B

SUPPORTING INFORMATION OF CHAPTER 3

Contents Page

Figure B. 1. The curing curves of vulcanizates containing GRPs at 150 oC...... 167

Figure B. 2. Adhesion of co-cured laminates...... 169

Figure B. 3. Stress-strain curves of vulcanizates...... 170

Table B. 1. The curing characteristics of compounds containing GRPs...... 168

Table B. 2. Comparison of tensile properties of vulcanizates...... 170

166

Torque (dNm) 6 VRC V2 V0 V3 V0-a V3-a 3 V0-aa V3-aa V1 V4 0 0 10 20 30 40 50 Time (min)

Figure B. 1. The curing curves of vulcanizates containing GRPs at 150 oC.

167

Table B. 1. The curing characteristics of compounds containing GRPs.

ts2 t90 ML MH ΔM Entry (min) (min) (dNm) (dNm) (dNm)

VRC 11.5 26.0 1.8 12.5 10.7

V0 10.7 25.1 2.6 10.5 7.8

V0-a 10.0 21.4 2.6 9.8 7.2

V0-aa 8.00 18.9 2.5 9.8 7.2

V1 3.1 6.8 2.2 7.8 5.6

V2 8.8 19.7 2.6 10.3 8.5

V3 0.7 3.7 2.2 14.8 12.8

V3-a 0.8 5.0 2.1 14.1 12.0

V3-aa 1.00 3.6 2.3 13.9 11.6

V4 0.6 4.5 2.24 15.68 13.4

168

14.71

10.75

10 Adhesion(N/mm) 5 4.3

1.48 0 VRC L0 UC-DMF UC-Et

Figure B. 2. Adhesion of co-cured laminates.

Note that laminate UC-DMF containing pre-cured strip treated by a solution of nBuSH and piperidine in dimethylformamide with a volume ratio of 1:10:100. Laminate UC-Et containing pre-cured strip treated by a solution of nBuSH and piperidine in ethanol with a

volume ratio of 1:10:100.

169

18 16 VRC-Pip VRC V3 14 12 10 8

6 Tensile Stress (MPa) 4 2 0 0 100 200 300 400 500 Strain (%)

Figure B. 3. Stress-strain curves of vulcanizates.

Note that vulcanizate VRC-Pip is VRC perfused with 2 wt% piperidine.

Table B. 2. Comparison of tensile properties of vulcanizates.

σ100% σ300% εb σb Toughness Entry (MPa) (MPa) (%) (MPa) (106 J/m3)

VRC 1.97 ± 0.13 8.21 ± 0.42 492 ± 17 16.0 ± 0.8 35 ± 3

VRC-Pip 2.78 ± 0.08 13.06 ± 0.56 312 ± 32 14.0 ± 1.8 18 ± 4

V3 2.69 ± 0.22 12.69 ± 0.85 336 ± 15 14.9 ± 1.3 21 ± 3

170