Studies on Carbon Black. III. Theory of Bound Rubber
Abstract Theories of reinforcement may be grouped into two general classes, mechanical and chemical. The mechanical type of theory attempts to explain reinforcement by alteration of direction of tear or by mechanieal entrainment. The chemical type of theory invokes the formation of bonds between the filler and rubber. Because of its implication with respect to the latter, Fielding of Goodyear developed a “bound rubber” test. The amount of rubber bound to carbon black was defined as that unextractable from the raw masterbatch by benzene. Some ten years ago, Baker and Walker reported an insolubilization of GR-S, on mixing with carbon black, over and above the amount of naturally occurring gel. The amount of insolubilized polymer increases with increasing molecular weight of the GR-S, and a selective adsorption of the high molecular weight material was found. Since this phenomenon was obtained also in polymers where they believed chemical gelation to be impossible, the conclusion was drawn by them that it is purely physical—this notwithstanding the fact that they found that extractions at higher temperatures failed to remove the insolubilized polymer. Because the method of analysis for insolubilized polymer used by Baker and Walker was essentially a bound-rubber analysis, interest in the latter was revived and it became desirable to set up a hypothesis to explain the mechanism of bound-rubber formation. (Let us understand the term “rubber” as applying in its more general sense as synonymous with “elastomer”.) The present paper reports a theory developed by the writer about ten years ago to explain various observations on the hypothesis that bound rubber is a gel of carbon black particles, the bonding agent of which consists of the longer polymer molecules. The theory interprets the observed linear dependence of bound rubber on loading in terms of an elemental area associated with the segmental adsorption of elastomer molecules, the molecular weight of these segments, and the functionality of the carbon black particles.