One class of multiphase elastomers are those capable of undergoing strain-induced crystallization, as was discussed separately in chapter 12. In this case, the second phase is made up of the crystallites thus generated, which provide considerable reinforcement. Such reinforcement is only temporary, however, in that it may disappear upon removal of the strain, addition of a plasticizer, or increase in temperature. For this reason, many elastomers (particularly those which cannot undergo strain-induced crystallization) are generally compounded with a permanent reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers, and the addition of silica to siloxane rubbers. In fact, the reinforcement of natural rubber and related materials is one of the most important processes in elastomer technology. It leads to increases in modulus at a given strain, and improvements of various technologically important properties, such as tear and abrasion resistance, resilience, extensibility, and tensile strength. There are also disadvantages, however, including increases in hysteresis (and thus of heat buildup) and compression set (permanent deformation). Another problem in this area is the absence of a reliable molecular theory for filler reinforcement, in general, and even simple molecular pictures of the origin of the reinforcement are lacking. The subject is not even discussed in what has long been the standard reference book on rubberlike elasticity! On the other hand, there is an incredible amount of relevant experimental data available, with most of these data relating to reinforcement of natural rubber by carbon black. Recently, however, other polymers such as poly(dimethylsiloxane), and other fillers, such as precipitated silica, metallic particles, and even glassy polymers, have become of interest. These studies have shown that materials which act as fillers can vary substantially with respect to the chemical nature of their surfaces, and probably most solid, finely divided materials may advantageously be incorporated into an elastomer. In fact, this is one of the ways the crystallites discussed in chapter 12 improve the mechanical properties of an elastomer. Experimental evidence indicates that the extent of the reinforcement depends strongly on particle size.
"Carbon Black Reinforcement of Rubbers" No.6: Strain-Induced Crystallization and Self-Reinforcement of NR (1) What has been made clear concerning the phenomenon?
