II. STRUCTURE–PROPERTY RELATIONSHIPS FOR GENERAL-PURPOSE ELASTOMERS USED IN TIRE APPLICATIONS

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 performance is the glass transition temperature, Tg. Aggarwal et al. (2) showed that

the tangent delta at 60jC of filled rubber vulcanizates made from ‘‘conventional rubbers’’ correlated with tire rolling resistance and then determined

that the tangent delta values were approximately a linear function of the

compound’s Tg 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

Tg 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 Tg). 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 temperature 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 polybutadiene, and a medium-vinyl SBR at the same Tg were compared and shown to be



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Figure 1 Effect of Tg on traction of (x) solution polymers and (n) emulsion polymers. (From Ref. 7.)



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 2 Effect of Tg on wear of (x) solution polymers and (n) emulsion polymers.

(From Ref. 7.)



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Figure 3 Damping curve of ESBR 1500 tread compound. (From Ref. 8.)



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



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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 relationship 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.



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