2 General-Purpose Elastomers Howard Colvin Riba-Fairfield, Decatur, Illinois, U.S.A. I. INTRODUCTION General-purpose elastomers played a critical role in the history of the last half of the 20th century. In 1942 the Rubber Reserve program developed both the basic technology and manufacturing capability to make emulsion styrene butadiene rubber (SBR) just a few years after World War II had interrupted natural rubber supplies. Historians have noted that the scientific contribution to that effort is comparable to the nuclear research program at Los Alamos that occurred at the same time (1). After the petroleum shortages of the 1970s, fuel economy became a primary driving force in the automotive industry, and the tire industry was challenged to develop new products that would improve gas mileage. New elastomers based on solution SBR technology proved to be part of the answer. Today the tire industry is challenged to meet new environmental standards while maintaining or improving the vehicle handling, ride, and durability that has already been achieved. To meet this challenge, the rubber technologist must have a thorough understanding of how general-purpose elastomers (i.e., polybutadiene, styrene/butadiene, and styrene/butadiene/ isoprene) affect compound processability, tire rolling resistance, tire traction, tire treadwear, and overall cost of tire components. Use of these elastomers outside of the tire industry requires the same type of understanding of fundamental polymer characteristics and how they affect the final applica- tion. This review will describe the basic structure–property relationships between general-purpose elastomers and end-use properties, with a focus on 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 51 Copyright © 2004 by Taylor & Francis the tire industry. The processes used to make the general-purpose elastomers will be described with an emphasis on how the polymerization variables (mechanism, catalyst, process) affect the macrostructure and microstructure of the polymer. It is polymer microstructure and macrostructure that determine whether a polymer is suitable for a particular application, not the type of process or catalyst used to produce the polymer. Some important terms used in this chapter are defined in Table 1. II. STRUCTURE–PROPERTY RELATIONSHIPS FOR GENERAL-PURPOSE ELASTOMERS USED IN TIRE APPLICATIONS A. Laboratory Testing Methods Prediction of tire properties based on laboratory properties has met with various degrees of success, depending on which property was being predicted. There is a good correlation between the rolling resistance of tires and the tread compound tangent delta at 60jC and 40 Hz (2). There is a reasonable Table 1 Definitions Polymer microstructure Monomers incorporated into the polymer and the stereo- chemistry of enchainment (i.e., cis, trans, vinyl). Polymer macrostructure Polymer molecular weight and molecular weight distribu- tion, molecular geometry (linear, branched, comb), and the order in which mono- mers are incorporated (block, tapered block, or random). Number-average molecular weight (M n ) Summation of the number of polymer chains (N) with a given molecular weight (m) times the molecular weight of each chain divided by the total number of polymer chains: Sm i N i /SN i . Weight-average molecular weight (M w ) Summation of the number of polymer chains (N) with a given molecular weight (m) times the square of the molecular weight of each polymer chain divided by the total number of polymer chains times the molecular weight of each chain: Sm i 2 N i /Sm i N i . Molecular weight distribution M w /M n . Glass transition temperature (T g ) Temperature at which local molecular motion in a polymer chain virtually ceases. General-purpose elastomers behave like a glass below this temperature. Weight-average T g Average T g of a compound: X  wt: polymer X n total polymer wt:  ðT g polymer X n Þ  4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 52 Copyright © 2004 by Taylor & Francis correlation between tire traction and tangent delta of the tread compound at 0jC and 40 Hz (2). Tire wear is more difficult to predict, with one researcher observing, ‘‘Despite more than 50 years of effort to devise laboratory abraders that give a good prediction of the wear resistance in real-world situations, no abrasion device currently exists that does an acceptable job’’ (3). Typically, DIN abrasion or some type of blade abrader is used as a general indicator, however. Rubber processability has been defined in a number of ways (4) but is usually determined by what type of equipment will be used to process the rubber. Mooney stress relaxation time to 80% decay (MSR t-80) is a rapid, effective processability test that works well with both emulsion (5) and solution SBR (6). Other more sophisticated instruments such as the rubber processability analyzer (RPA) or capillary rheometer are now becoming more popular. B. Glass Transition Temperature The most important elastomer variable in determining overall tire perform- ance is the glass transition temperature, T g . Aggarwal et al. (2) showed that the tangent delta at 60jC of filled rubber vulcanizates made from ‘‘conven- tional rubbers’’ correlated with tire rolling resistance and then determined that the tangent delta values were approximately a linear function of the compound’s T g value. This was true whether the polymers were made by a solution process or an emulsion process. They did not compare solution and emulsion polymers at the same glass transition temperature. Oberster et al. (7) showed that traction and wear properties were not dependent on the way the polymer was manufactured but were functions of the overall glass transition temperature of the compound, as shown in Figures 1 and 2. In actual tire tests, results are more complicated. The weight-average T g of the tread compound is still a major variable, but it is not as dominant as in laboratory tests. A comprehensive study of tire wear under a variety of environmental and road conditions showed that tire wear improves linearly as the ratio of BR to SBR is increased in BR–SBR tread compounds (lower weight-average T g ). The wear behavior was more complex in BR–NR blends with low carbon black levels and was shown to be a function of ambient test temperature (3). Nordsiek (8) expanded the concept of using the glass transition tem- perature to using the entire damping curve to predict tire performance. He divided the damping curve into regions that influenced various tire properties (Fig. 3). The damping curves for an emulsion SBR, a high-vinyl polybutadi- ene, and a medium-vinyl SBR at the same T g were compared and shown to be 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 53 Copyright © 2004 by Taylor & Francis different at temperatures of 20–100jC. This led to the proposal of an ‘‘ integral rubber’’ that would have a compilation of damping curves from a number of polymers and would incorporate damping behavior that would lead to the ‘‘ideal’’ elastomer for tread compounds. It was implied that this elastomer consisted of segmented blocks of different elastomers with different glass transition temperatures. An ‘‘integral rubber’’ was prepared and compared to Figure 1 Effect of T g on traction of (x) solution polymers and (n) emulsion poly- mers. (From Ref. 7.) Figure 2 Effect of T g on wear of (x) solution polymers and (n) emulsion polymers. (From Ref. 7.) 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 54 Copyright © 2004 by Taylor & Francis natural rubber and SBR 1500 controls in a laboratory compounding study. The ‘‘integral rubber’’ had a hot rebound within one point of the natural rubber control and was three points higher than the SBR 1500 control. Abrasion resistance was better than that of the natural rubber control but slightly worse than that of the SBR 1500. The 0jC rebound was lower than that of either control. C. Molecular Weight and Molecular Weight Distribution The molecular weight aspect of polymer macrostructure affects the rolling resistance (via hysteresis) and processability of the tread compound. As the molecular weight is increased, the total number of free chain ends in a rubber sample is reduced, and energy loss of the cured compound is reduced. This leads to improved rolling resistance, but at the expense of processability. Caution should be used in extrapolating lab data on high molecular weight rubbers to factory-mixed stocks, because filler dispersion is not as efficient with large-scale equipment. Thus, low hysteresis in lab compounds may not translate into low hysteresis in commercial tire compounds. There is an optimum balance between molecular weight and processability that is defined Figure 3 Damping curve of ESBR 1500 tread compound. (From Ref. 8.) 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 55 Copyright © 2004 by Taylor & Francis by the type of mixing equipment used. Increasing the molecular weight distribution at equivalent molecular weight by branching produces more free chain ends and more hysteresis but at moderate levels can improve other properties. Saito (9) showed that in silicon-branched solution SBR the effect on hysteresis could be minimized and ultimate tensile strength could be improved because of better carbon black dispersion. In emulsion polymers, the branching is uncontrolled and the polymers have poorer hysteresis than the corresponding solution polymer (10). From a practical standpoint, some branching in tire polymers is necessary to prevent cold flow and ensure that the elastomer bales will retain their dimensions on storage. Polymer scientists have worked hard to take advantage of the relation- ship between free chain ends and hysteresis. In one case, an attempt was made to eliminate chain ends completely by preparing cyclic polymers. Hall (11) polymerized butadiene with a cyclic initiator and claimed to have made a mixture of linear and cyclic polybutadiene. Cyclic structure was inferred from a comparison of the viscous modulus of the cyclic polymer to that of a linear control. All of the cyclic polymers had a lower viscous modulus than the controls. No compounding data were reported, however. A more popular method of reducing the effective number of free chain ends is to functionalize the end of the polymer chain with a polar group. Functional end groups can enhance the probability of cross-linking near the chain end and interact directly with the filler, thus reducing end effects. Ideally, difunctional low molecular weight polymers would be mixed with filler and then chemically react with the filler during vulcanization to give a network with no free chain ends. This ideal can be approached, depending on how effectively the polymer chains are functionalized and the strength of the interaction of the functional group with the filler. This will be discussed further in the section on anionic polymerization and anionic polymers (Section IV). D. Sequence Distribution in Solution SBR Day and Futamura (12) compared different 35% styrene solution SBRs at equivalent molecular weights and found that hysteresis is a linear function of the block styrene content. The effect of the polystyrene block length on hysteresis is shown in Figure 4. Sakakibara et al. (13) made block polymers of polybutadiene and SBR with anionic polymerization and compared them to an SBR with the same overall microstructure. They found that the block polymers had broader glass transition temperatures that resulted in better wet skid resistance and lower rolling resistance than the corresponding random SBRs. They also found that blocky styrene in the SBR block was detrimental to overall performance. 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 56 Copyright © 2004 by Taylor & Francis III. EMULSION POLYMERIZATION AND EMULSION POLYMERS The copolymerization of styrene and butadiene is accomplished by dispersing the monomers in water in the presence of a surfactant, an initiator, and a chain transfer agent. The process offers limited control over polymer micro- structure, and the polymers are branched. Emulsion SBR, however, has played and continues to play an important role in tire compounds. A. Polymerization The best way to consider the overall emulsion process is to examine the original recipe used to produce GR-S rubber at the beginning of World War II (14) (Table 2). It is important that the polymerization be done in the absence of oxygen. Oxygen is removed from the water by bubbling nitrogen through it prior to the polymerization, and the polymerization is conducted under a nitrogen atmosphere. When the ingredients are mixed, the monomers are partitioned between the water, micelles, and monomer droplets. The water solubility of styrene and butadiene is very low, so there is little of either in the water phase. Micelles are aggregates of surfactant (fatty acid soap) with the polar carbox- ylic group on the outside oriented toward the polar water and the nonpolar hydrocarbon tail oriented toward the inside of the micelle. The nonpolar styrene and butadiene are ‘‘soluble’’ inside the nonpolar environment of the micelle. Still, only a small portion of the monomer is located in micelles. There Figure 4 Effect of block styrene on hysteresis in SBR. (From Ref. 12.) 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 57 Copyright © 2004 by Taylor & Francis are approximately 10 17 –10 18 micelles per milliliter of emulsion (15). Most of the monomer is contained in monomer droplets, which are in lower concen- tration (10 10 –10 11 monomer droplets per milliliter emulsion) and much larger than the micelles (15). When the mixture is heated to 50jC, the potassium persulfate decomposes into radicals in the aqueous phase. Because the surface area of the micelles is much greater than that of monomer droplets, the radicals are more likely to inoculate the micelles to begin the polymerization. A representation of this is shown in Figure 5. As the polymerization proceeds, monomer migrates from the monomer droplets to the micelles until the monomer droplets are gone. Chain transfer to the mercaptan controls polymer molecular weight. Conversion is stopped at approximately 70% by addition of a radical trap such as the salt of a dithiocarbamate or hydroquinone. The latex is stabilized, then coagulated to give crumb rubber. A major improvement in this process was the development of the redox initiation system shortly after World War II (16) (Table 3). With this recipe, the polymerization could be conducted at 5jC by changing the initiator system from potassium persulfate to cumene hydroperoxide. The iron(II) salt lowers the activation energy for the decomposition of the cumene hydroper- oxide and is oxidized to iron(III) during the process. The dextrose is present to reduce the iron(III) back to iron(II) so more peroxide can be decomposed. The importance of the lower polymerization temperature is shown in Figure 6. As the polymerization temperature is decreased, the ultimate tensile strength of cured rubber increases dramatically (17). This is because there is less low molecular weight material and less branching at the lower polymer- ization temperature (18). There is little control over butadiene polymer microstructure in the emulsion process. It remains fairly constant at 12–18% cis, 72–65% trans, and 16–17% vinyl as the polymerization temperature is increased from 5jCto Table 2 GR-S Recipe for Emulsion SBR a Component Parts by weight Styrene 25 Butadiene 75 Water (deoxygenated) 180 Fatty acid soap 5 Dodecyl mercaptan 0.5 Potassium persulfate 0.3 a Polymerization conducted at 50jC. Source: Ref. 15. 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 58 Copyright © 2004 by Taylor & Francis 50jC. Butadiene microstructure does not vary significantly as the styrene content is changed (19). The glass transition temperature of emulsion SBR is controlled by the amount of styrene in the polymer. B. Functional Emulsion Polymers It is easy to incorporate a functional monomer into an emulsion polymer as long as there is some water solubility. Emulsion butadiene or styrene Figure 5 Species present during emulsion polymerization. (From Ref. 15. Re- printed by permission.) 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 59 Copyright © 2004 by Taylor & Francis Table 3 ‘‘Custom’’ Recipe for Emulsion SBR Component Parts by weight Styrene 28 Butadiene 72 Water 180 Potassium soap of rosin acid 4.7 Mixed tertiary mercaptans 0.24 Cumene hydroperoxide 0.1 Dextrose 1.0 Iron(II) sulfate heptahydrate 0.14 Potassium pyrophosphate 0.177 Potassium chloride 0.5 Potassium hydroxide 0.1 Source: Ref. 16. Figure 6 Effect of polymerization temperature on mechanical properties of ESBR. (From Ref. 18. Reproduced with permission.) 4871-9_Rodgers_Ch02_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 60 Copyright © 2004 by Taylor & Francis [...]... control polymer A partial list of the amide initiators studied and their solubilities is given in Table 5 Interestingly, although almost all of the amide initiators effectively initiated polymerization, not all of the resulting polymers showed reduced hysteresis on compounding N-Lithiohexamethyleneimine 3 and N-lithio-1 ,3, 3-trimethyl-6-azabicyclo [3. 2.1]octane 4 were studied further They were both shown to... Table 14 Characterization and Stress Relaxation of SBR Prepared by Selected Initiators Sample Type Mw Mn M w/ Mn ML4 Tg (jC) Styrene (%) Vinyl (%) T80 n-BuLi/ SMTa n-BuLi/ SDBSb Bu2Mg/ SMTc 5.20E+05 2.42E+05 2.15 58 À14 26 53 0. 033 3. 29E+05 1.92E+05 1.71 54 À 13 27 51 0.016 2.89E+05 1.82E+05 1.59 57 À12 26 58 0.0 13 4 n-BuLi/ SMT 3. 78E+05 1.74E+05 2.17 70 À70 18 15 0.020 5 Dist feed 3. 15E+05 1.98E+05 1.59... 2 3 a n-Butyllithium/sodium t-amylate n-Butyllithium/sodium dodecylbenzene sulfonate c Dibutylmagnesium/sodium t-amylate Source: Ref 6 b Copyright © 2004 by Taylor & Francis Figure 26 Root-mean-square radius versus molar mass of solution SBRs (x) 1, (n) 2, and (E) 3 (From Ref 6.) Runs 4 and 5 (Fig 27) illustrate the microstructural differences between two low-vinyl SBRs In run 4, the styrene is randomized... 0 18 25 35 35 Vinyl % 12.0 9.8 9.0 7.8 7.8 Tg (jC) À96 À75 À65 À 53 À45 MWD 1.75 2.05 1.85 1.8 1.8 ML 1+4 (100jC) 55 100 110 148 148 Mn 133 ,000 200,000 200,000 225,000 225,000 Compounded properties Rheometer at 150jC TS2 8.0 9.5 10.5 10 .3 9.0 T50 15.8 16.0 17.0 17.6 16.7 21 .3 22.5 25.0 24.8 25.0 T90 30 0% Modulus (MPa) 6.99 9 .30 8.27 10.68 11.02 Elongation (%) 515 515 550 535 510 Die C (kN/m) 36 .9 41.1... imides, N-alkyl-substituted oxazolydinones, pyridyl-substituted ketones, lactams, diesters, xanthogens, dithio acids, phosphoryl chlorides, silanes, alkoxysilanes, and carbonates (57), Amine- and tin-containing electrophiles provide the greatest interaction with carbon black Epoxy compounds and alkoxysilanes are most beneficial for silica-filled compounds The early work focused on termination with amine-containing... Is, isoprene; MSt, a-methylstyrene; H, hydrogen; Sn, tin (From Ref 63. ) Copyright © 2004 by Taylor & Francis delta values at 80jC than runs 1 3 (uncoupled polymer terminated with tributyltin chloride) For silica compounds, different functional groups are required for polymer–filler interaction Alkoxysilanes such as 3- triethoxysilylpropyl chloride, chlorodimethylsilane, and bis- ( 3- triethoxysilypropyl)... compared two solution SBRs and a high-vinyl polybutadiene with the same glass transition temperature and similar macrostructures Cure time was longer, and tensile strength, elongation, and tear strength were poorer as the vinyl content of the polymers increased (Table 13) (Fig 25) Finally, all rubber technologists should appreciate that solution SBRs with similar Tg values and styrene and vinyl contents do... 1,2-Dipiperidinoethane Source: Refs 50, 51 Copyright © 2004 by Taylor & Francis Modifier/Li 30 jC 50jC 70jC 270 12 96 5 85 1.14 1 10 37 22 36 44 73 76 99 99 33 16 26 25 49 61 68 95 25 14 23 20 46 46 31 84 Figure 16 Effect of potassium butoxide/lithium ratio on polybutadiene microstructure (n) Percent trans; (E) percent vinyl (From Ref 53. ) C Termination Termination is easily accomplished by reaction of the living... butadiene–styrene block, and a styrene block (52) Addition of polar compounds will randomize the styrene and increase the rate of polymerization Choice of modifier is critical to get the proper degree of randomization and control the vinyl content Modifiers such as potassium tert-butyl alkoxide (t-BuOK) are used to randomize the styrene without significantly increasing the vinyl content At a ratio of t-BuOK/nBuLi... Diethyladipate Silicon Tetrachloride Tin tetrachloride ML-1+4 (100jC) Compounded Tensile strength Elongation Tan y Tan y ML-1+4 (MPa) at break at 50jC at 0jC (100jC) 54 51 47 57 93 70 74 89 22 .3 22.5 21.6 23. 5 400 400 410 400 0.121 0.125 0.126 0.126 0. 235 0.241 0. 237 0.240 57 76 25.0 400 0.096 0. 239 a Formulation (phr): Polymer 100, HAF black 50, zinc oxide 3, stearic acid 2, antioxidant 1.8, accelerator 1.8, . the resulting polymers showed reduced hysteresis on compounding. N-Lithiohexamethyleneimine 3 and N-lithio-1 ,3, 3-trimethyl-6-azabicy- clo [3. 2.1]octane 4 were studied further. They were both shown. n-butyl- lithium are more associated than the secondary organolithium compounds and thus are less reactive (31 ,32 ). Functional organolithium reagents are used to make functional poly- mers (33 ) tire properties (Fig. 3) . The damping curves for an emulsion SBR, a high-vinyl polybutadi- ene, and a medium-vinyl SBR at the same T g were compared and shown to be 487 1-9 _Rodgers_Ch02_R2_052404 MD: