Synthetic elastomers have supplemented or replaced natural rubber to a great extent in the fabrication of tires and other rubber products. Synthetic rubbers have demonstrated physical properties which are superior to those of natural rubber in some respects. However, a major deficiency of many synthetic elastomers, including cis-1,4-polyisoprene, is a lack of green strength. This has been a particularly difficult problem in the fabrication of large rubber articles, such as tires. The abatement of this deficiency in many synthetic rubbers has long been sought by the industry and various means of improving the green strength of synthetic elastomers are known throughout the industry and described in the prior art.
The term "green strength" is commonly employed and generally understood by persons skilled in the rubber art, nevertheless it is a difficult term to precisely define. Green strength is basically the mechanical strength of an elastomer prior to vulcanization which allows it to be built into multiple component articles with little or no relative movement of the assembled components in such articles subsequent to assembly, but prior to being vulcanized. In other words, green strength is the mechanical strength required for fabricating multiple component rubber articles prior to the rubber in the article being cured. Pneumatic rubber tires are almost always composed of multiple rubber layers. It is necessary for the rubbers used in building such tires to possess adequate green strength in order for the tire being built to maintain its structural integrity during the building and vulcanizing procedures which must be carried out.
In elastomers which possess poor green strength the yield stress which the unvulcanized elastomer exhibits during deformation is low and the stress drops off quite rapidly as the deformation continues. Unvulcanized strips or other forms of such elastomers often pull apart in a taffy-like manner during building operations. Green strength is typically quantified in terms of the stress/strain curves of the unvulcanized elastomer. Usually, the performance of a green compound (unvulcanized) is based upon two points of the stress/strain curve, namely the first peak or yield point and the ultimate or breaking tensile point. Improvement in either of these stress/strain properties indicates improved green strength.
Numerous additives have been utilized in association with various elastomers in order to improve their green strength. However, the utilization of such additives commonly causes unwanted results, such as a rise in the hysteresis of the elastomer. For various reasons the incorporation of additives into synthetic elastomers in order to improve green strength has generally not proven to be satisfactory. Electron beam precure (microwave precure) is a technique which has gained wide commercial acceptance as a means of improving the green strength of synthetic elastomers. For instance, electron beam precure is widely used in the tire industry to improve the green strength of synthetic elastomers utilized in building tires. However, electron beam precure techniques are costly. Nevertheless, electron beam precure often represents the only acceptable means for providing adequate green strength to maintain dimensional stability during tire building and curing procedures.