This study was undertaken to elucidate the effect of primary molecular weight and molecular weight distribution on M_C of vulcanizates. A sample of cis-polybutadiene was fractionated using the column extraction method and the fractions thus obtained were characterized mainly by the method of dilute solution measurements. The results showed that the fractions had narrow molecular weight distribution and no appreciable amount of branched molecules. A suitable condition for vulcanization of rubber was determined using the nonfractionated original sample. All fractionated samples were vulcanized under under the same condition. M_C of these vulcanizates including that of original were measured by means of Flory-Rehner's swelling method. A relationship between M_C and M_N has been established : samples having M_N10⁵ were hard to vulcanize and above this M_N, M_C decreased gradually from 2.5 × 10⁴ to 1.5 × 10⁴ with an increasing M_N. Corresponding to the relationship of Ma and M_C to the molecular weight distribution curve of the original sample, the tendency of the distribution of M_C in its vulcanizate was estimated.

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NR and BR vulcanizates of sulfur accelerated with DPG or MBT, of TMTD and of Di-Cup were prepared. Peroxide vulcanization was carried out for comparison with sulfur-containing crosslinks. Effect of the nature of crosslinks on the radiation deterioration of vulcanizates was investigated by determining the cross-linking densities before and after irradiation. The results obtained are summarized : (1) sulfurcontaining crosslink has higher resistance to irradiation than C-C crosslink and (2) polysulfide link is more resistant than monosulfide one.

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Fifteen active filler-loaded unvulcanized rubber samples were prepared. The polymers used were the fractionated and the unfractionated cis-polybutadiene (Cis 4) and both ISAF (Vulcan 6) and HMF (Sterling L) types were used as fillers.
Tensile stress relaxation data were then obtained for those samples at temperatures from -30° to 80°C. The amount of strain applied ranged from 0.1 to 3.0.
Application of the blending law previously proposed to the experimental data appeared to be essentially successful, though the viscoelastic properties of the polymeric matrix were found to be progressively modified by the addition of fillers. The effective volume fraction of fillers estimated therefrom seemed to be practically the same as the real one in general.
It was then shown that the change in the viscoelastic properties of the polymeric matrix caused by the presence of fillers could reasonably be described by taking into account of the fact that each filler particle acts as a multifunctional cross-linking agent in and after mixing processes.

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The gel quantity of rubber loaded with calcium carbonate differs according to the kind of calcium carbonate as well as the kind of rubber.
When the rubber is loaded with carbon black, the gel quantity corresponds to the specific surface area. Similar tendency can be seen in the rubber loaded with calcium carbonate, but its gel developing mechanism and its properties are still unknown and seem to be quite different from carbon-gel.
For example, the gel quantity of calcium carbonate formed with the straight chain of ultra fine primary particles is not influenced by the degree of milling, even though its structure may extremely resemble carbon black.
Chain combinations of ultra fine calcium carbonates are very weak and easily broken to primary particles when mixed in rubber. These calcium carbonates improve reinforcement of vulcanized rubber because of their dispersed fine particles.
And, properties of gel gained from the rubber compounded with calcium carbonate or FT black vary depending on the presence of accelerator or coagent.

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The first asymptotic probability function of the smallest values proposed by Kase is not satisfactorily applicable to the distributions of tensile data for all types of vulcanized rubbers.
From a statistical point of view, another distribution function, the third asymptotic probability function was introduced in the present paper. By using the past tensile data of Gorcorn et al., the applicability of this function to the distributions was verified to be reasonable.
In general, the third asymptote contains three parameters, the characteristic value V, the lower limit ε, and the scale parameter1/α, which has no dimension. The values of these parameters were found to be systematically variable with the type of rubber used.

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As a result of a statistical analysis of Pico abrasion data, the observed distribution is skewed to the right side with a long tail, as compared with what would have been expected from Gaussian distribution. The third asymptotic probability function of the largest value is satisfactorily applied to analyze the distribution of abrasion data of rubber vulcanizates. In general, abrasion of rubber vulcanizates is due to mechanical failure under the repeatedly tractive stresses produced frictionally by the asperities of the track. These facts suggest that abrasion of rubber vulcanizates may be explicable to be extremal phenomena. The influences of cure state and dynamic fatigue on the shape of the distribution function are investigated, too.
The third asymptotic probability function contains three parameters, the characteristic value ν, the upper limit ω₀, and the shape parameter 1/α which has no dimension.

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