Effect of Reinforcing Pigments on Rubber Hydrocarbon

Several investigators have established that sulfur vulcanization of rubber involves chemical combination of sulfur and rubber hydrocarbon. A definite decrease in unsaturation of the rubber hydrocarbon as vulcanization progresses has been generally noted. While it has often been concluded that a double bond is saturated for each atomic equivalent of combined sulfur, recent work by Brown and Hauserdemonstrates that this conclusion cannot be applied in all cases. In certain compounds they found the loss in unsaturation with extent of vulcanization to be considerably less than anticipated on the basis above. Their results indicate that stocks reaching optimum cure with the least loss of unsaturation possess the greatest tensile strength. Although a considerable amount of work has been done on this problem, we have not found a published account of similar investigations performed on stocks containing significant loadings of reinforcing fillers. Since, as pointed out below, the nature of the bonding between such fillers and the rubber molecules has not been clearly defined, one is not justified in applying previous results obtained on stocks containing no reinforcing fillers to those bearing appreciable loadings of such substances. Accordingly one portion of the present investigation was concerned with determining the effect of various fillers on the course of sulfur vulcanization, as judged from combined sulfur and unsaturation values. As pointed out by Gehman and Field it is undoubtedly true that the black particle in a carbon-black-reinforced rubber stock is firmly attached to the rubber molecule. The nature of the bonding between the black and rubber has not been clearly defined. Some investigators maintain that the association is physical and involves definite forces of adhesion or adsorption; others have suggested the formation of primary valence linkages with rubber hydrocarbon. The opinion of the present authors is that if such linkages are formed, the ethylenic bonds of the rubber molecule would probably be involved. If this latter view is correct, then a specific loss in unsaturation of rubber hydrocarbon, due to the reinforcing filler, should occur. Thus the second objective of the present study was to determine whether it was possible by chemical means to detect such a linkage. If measurable, this effect, together with the surface area determinations reported previously, would be particularly valuable in estimating the reinforcing value of various fillers.