The stress relaxation measurements were made with four typical crosslinked natural rubber vulcanizates which were cured with (1) sulfur-dibenzothiazyl disulfide, (2) sulfur-tetramethyl thiuram disulfide (low sulfur content) (3) tetramethyl thiuram disulfide (nonsulfur) and (4) dicumyl peroxide.
The results were obtained as follows :
(1) The stress relaxation behaviors of each unextracted sample showed apparent resistance to oxidation of each cured system. The vulcanizates cured with sulfur and sulfur containing acceleraters relaxed faster, whereas the vulcanizates cured with dicumyl peroxide, and tetramethyl thiuram disulfide had excellent resistance to oxidation.
(2) The resistance to oxidation of the vulcanizates extracted with hot acetone was poor, especially sulfur cured system and non-sulfur tetramethyl thiuram disulfide cured system showed very poor resistance to oxidation after extraction. From these results it was found that the by-products of crosslinking reaction, for instance ZnDC (Zinc dithio carbamate), and natural antioxidant contained in rubber were removed by extraction.
(3) From kinetic analysis of polymer network on these results, it seems that in peroxide cured system which had c-c bond network, scission reaction chiefly occurred at main chain, in other systems crosslinks and main chain scission occurred at the same time.
(4) Comparison of the continous stress relaxation curve with the intermittent one was made to determine the extraction effect.
It was found that the extraction procedure was important to know the true behavior of oxidation and network construction.

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In the cleavage reaction of crosslinked polymer, it is assumed that (i) random cleavage of main chain, (ii) cleavage at crosslink, and (iii) cleavage near crosslink will occur.
In the scission reaction of natural rubber vulcanizates, it is assumed from the behaviours of the stress relaation of the vulcanizates in the previous report that (i) and (ii) will occur at the same time and (iii) will be contained in (i).
The following equation is derived :
f (t) /f (o) = {1-4kx/k+2x·Q (t) /M₀} e-2x²/k+2x·Q (t) /M₀+2k/k+2x·Q (t) /M₀e-4x³/k+2x·Q (t) /M₀
Q (t) =q_m (t) +Q_c (t)
Where;
Q (t) : the total number of cleavages of network for cubic centimeter.
q_m (t) : the number of moles of cleavages of effective chains per cubic centineter.
Q_c (t) : the number of moles of cleavages at crosslink per cuic centimeter.
x : the total number of moles of unit molecules, of which one effective chain consists.
k : the ratio of the probability of random cleavages of main chains to the one of cleavages at crosslinks.
The scission reaction of natural rubber vulcanizates was presumed by calculating k from the assumption that the number of moles of cleavages of main chains q_m (t) of sulfur cure system would be the same as that of the peroxide cure system, because of their having same polymer structure, only random scission of main chains occurring in the peroxide cured system.

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Dielectric properties, i.e., dielectric constant ε′, loss factor ε″ and loss tangent tan δ of the vulcanizates obtained from the same grafted and blended polymers as those used in our previous papers respectively were determined here. Since variations of these properties as function of frequency for the grafted polymers were similar to those of the backbone polymers, it was thought that the former maintained some specific properties of the latter. On the other hand, variations of these properties as function of frequency in the blended polymers depended mainly on the acrylonitrile contents. Determination of dielectric properties also indicated that grafted polymers produced by the procedure (i) had more prominent grafting effect than those by (ii), and that grafted polymers from SBR had more prominent effect than those from NR. These results coincided with those obtained by determination of mechanical properties of the same vulcanizates reported in our previous papers. Dispersion state of the vulcanizates of grafted and blended polymers was also examined under a phasecontrast microscope.

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Butadiene grafted polyethylene (GPE), prepared by γ-irradiation of PE in the presence of butadiene, was blended in SBR Rubber. Compounding and Vulcanizing characteristics of the blend rubber were studied, and several physical properties of the vulcanizates were compared with those of original PE blended SBR vulcanizates.
The following points were observed with the GPE blend :
1) GPE facilitates the open mill mixing and compounding processes.
2) Mooney viscosities of the compounds were lowered but the vulcanization rates were increased.
3) No phase separation and bleeding of GPE from the compound was observed.
4) Hardness, tensile stress, tear-strength, abrasion resistance of GPE blended vulcanizates were better than those of control PE blended vulcanizates.
5) The tensile hysteresis loss of the vulcanizates of GPE and PE blends were of much the same order.

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In the case of reinforcement of styrene-butadiene rubber with thermosetting resins, remarkable difference in the extent of reinforcing effect appears according to the mixing method, even if same resin is used. Phenolformaldehyde resin substituted with C₉ Hydrocarbon as thermosetting resin was applied in this paper to study the relationship between dispersion state and reinforcing effect in SBR.
The mixing methods are as follows :
1) Blend in SBR latex
2) Blend in solvent
3) Mixture on mill
The results show that resin blend in SBR latex is superior to the other two methods in dispersion state and reinforceing effect on SBR. But thermosetting resins having good reinforcement and dispersion in SBR are desirable even if mixture on mill in practice. It was found that aminoplast resins modified with m-xylene formaldehyde resin showing good solvency for SBR had high reinforcing effect on SBR compound.

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