Rubber softening effect

Stress-Softening of Silicone Rubbers (Mullins Effect)... 

A difficulty in comparing stress-softening data of various authors is the different degree of emphasis given by them to the extent of recovery of the sample before determination of the softening effect. When the sample is swelled in the vapor of a good solvent, elastic recovery should be very nearly complete and, indeed, there is very little permanent set, even under conditions of severe pre-strain. Under these conditions amorphous gum rubbers show no detectable softening, but black rubbers do. 

Resins with solubility close to that of rubber (I, 14), the terpene, and olefin polymers have a softening effect on rubber. Indene polymers... 

Beside the characteristic stress softening up to large strains (Mullins effect) as shown in Fig. 36.13, the model also considers the hysteresis behavior of reinforced rubbers (Payne effect). Obviously, since the sum in Eq. (36.10) is taken over stretching directions with ds/dt > 0, only, up-and down cycles are described differently. An example considering a fit of the hysteresis cycles of silica filled EPDM rubber in the medium strain regime up to 50% is shown in Fig. 36.14. For these adaptations an alternative form of the cluster size distribution has been assumed, which allows for an analytical solution of the integrals in Eqs. (36.9) and (36.10) ... 

Rapid decrease of E was observed with strain increase. Pure NR and composites with a low CNT content (1 wt%) showed a moderate increase of stress up to about 75% strain, a sort of plateau up to 300% strain and a final more evident increase above 300%. Said increase could be due to rubber crystallization and to CNT alignment. The stress-softening effect, known as Mullins effect, was observed at large strain and attributed to detachment of rubber molecules from the surface of filler particles. The presence of CNT bundles (at 5, 7, 10 wt%) was commented to bring about a decrease of the stress." ... 

Hardness (see Fig. 3). Although there were slight differences among the individual oils used, presumably caused by differences in oil viscosity, there is little effect of class of oil at 10 phr. At 20 phr the paraffinic oils had a slightly greater softening effect upon the rubber compound than the other two types (see Table 5). There does not seem to be a relationship between Saybolt viscosity and hardness. 

It is observed that the values do not reach its initial position within the relaxation time of the experiment, but a recovery of the E values have been attained (Fig. 18). This behaviour of a rubber can be explained by the stress softening effect during the dynamic strain. Nevertheless, a high extent of recovery in the reverse amplitude sweep indicates that a good filler-filler network has been re-established at a low loading of tubes in the S-SBR-BR matrix. So, at least it can be said that rather than damage or permanent break of the tubes, the amplitude sweep disrupted the filler-filler network in the rubber matrix. It is noted that the absolute values of E at small amplitudes are somewhat differed from each other as compared with the value obtained from the phr CNT-filled compound. The difference may be developed from ageing of the samples. 

Harwood JAC, Mullins L, Payne AR (1966) Stress softening in natural rubber vulcanizates. Part II. Stress softening effects in pure gum and filler loaded rubbers. Rubber Chem Technol 39 814-22... 

Table 5.14 [86]. Reaction conditions are the same as described for the system natural rubber-methyl methacrylate. The effect of monomer concentration and temperature on styrene polymerization [88] is illustrated in Fig. 5.18. The mastication reaction is slower than in vibromilling. It is useful to underline that in this last case the tests were run at temperatures well below the glass transition temperature of the resins and the system viscosities are nearly the same. The initial polymerization rate is influenced predominantly by the softening effect of added monomer. Subsequently, the reaction is strictly dependent on the properties of the plastomer product [86]. For extensive grafting, a step procedure is suggested to avoid a major initial softening effect [86].

A number of experimental data have shown that a great variety of filled rubber vulcanizates exhibit the dynamic strain softening effect schematically illustrated in Figure 5.42, at given frequency and temperature. The limiting... 

In the rubber industry hydrocarbon oils are often used to reduce the softness and facilitate the processing of hydrocarbon rubbers. These appear to have a small interaction with the polymer but spacing effects predominate. Such materials are generally referred to as softeners. The rubber industry, like the plastics industry, commonly uses the term plasticisers to describe the phthalates, phosphates and sebacates which are more commonly used with the more polar rubbers. 

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