The reliability of plasticity data on rubber is affected by (1) variance in precision of measurement by the instrument used and (2) heterogeneity of the rubber. While the relative precision of the instrument and observer error must be considered, the problem is somewhat complicated by the fact that each sample is used only once. Thus a single measurement is made on n like things rather than n like measurements on a single thing. Methods commonly used are the Williams parallel-plate, Dillon and Johnston extrusion, and Mooney shear. Rather meager data have been published on plasticity measurements of large numbers of samples (200–300) of elastomers. Riebl and Martin, Davey and Baker have reported no dispersion values, only averages of D30 plasticity values of smoked sheet and crepe being shown, using a parallel-plate plastometer. Aside from Mooney plasticity values of GR-S on 45–50 samples given in recent papers by Heide and Vila and Gross no statistical data for GR-S plasticity are available in the literature. It is the purpose of this paper to present a simple statistical analysis of the three types of plasticity measurements for GR-S and natural rubber (Hevea smoked sheet) based on 256 specimens.
Within the past ten years the general trends of theory and experimental evidence suggest that aging or degradation is quite like polymerization in nature. These reactions are chain mechanisms; the physical changes of which are the net sum of several simultaneous processes: (a) cross-linking, cyclization, and continued polymerization which harden and stiffen the rubbers, and (b) scission which produces tackiness in the rubbers and results in loss of tensile strength. The relative degree to which different rubbers are susceptible to these mechanisms is discussed.
Several physical methods are available for isolating and measuring the relative rate of one or both of the cross-linking and scission reactions. Among these the use of viscosity, sol-gel relationships, stress-relaxation, creep and permanent set methods for measuring the oxidation effects in various rubbers are described and interpreted.
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