Mastication. IX. Shear-Dependence of Degradation on Hot Mastication

The rate of degradation of rubber during mastication is minimal at about 115° C, degradation increasing progressively on lowering or raising the temperature as far below or above this temperature as practicable. Designation of the degradation processes with the negative and positive temperature coefficients as “cold” and “hot mastication”, respectively, is supported by differences in their chemical mechanisms. The essential degradation step of cold mastication is rupture of rubber molecules by the imposed deforming forces to radicals which are converted to the degraded molecules after reaction with oxygen or other radical acceptor present in the rubber. Hot mastication is less well understood; scission appears to be by an oxidative reaction, with the implication that mastication serves in the main to expose fresh surfaces for absorption of oxygen. The mechanicodegradation of cold mastication produces an unusually rapid decrease in viscosity with molecular weight, logarithmic plotting of these two quantities giving an apparent α, according to the relation [η]=KMα, of greater than unity. The value of α for fractionated rubber is 0.67. The apparent high α is most readily explicable by a decrease in K on degradation, while α has its normal value of 0.67 for natural rubber. A decrease in K would occur if the molecular weight distribution around the mean became sharper. This distribution change has been considered to be caused by rupture only of molecules above a certain molecular weight, and this in the central sections of the molecules. For the oxidation of thin films and latex or any other chemical process whereby scission conceivably occurs with equal probability at every monomeric unit, the chain-length distribution would tend to a random one. In particular, if the rubber had initially a random distribution, viscosity-molecular weight decrease would yield an α equal to that for fractionated rubber and a K which is Γ (α + 2) times higher. Viscosity-molecular weight data on hot mastication have not been previously published. If a normal chemical degradation takes place, a difference in the viscosity-molecular weight relationship from that on cold mastication should be detectable. Differences in molecular weight distribution should also be reflected in differences in other properties of masticated rubber and, to a lesser extent, of vulcanized products. A comparison of viscosity-molecular weight relationships and other properties of rubbers masticated in air forms a starting point for answering the complex query of the relative merits of cold and hot mastication. Interaction of rubber with fillers and vulcanizing ingredients would then have to be systematically investigated.