Rubber compounds experimental observations

With either pure, unfilled elastomers or slightly filled rubber compounds (typically filler volume fraction lower than 10 %) however, a Unear viscoelastic region is observable within the experimental window of most dynamic rheometers providing the strain amplitude does not exceed 10—20 %. 

In principle, the time-temperature superposition principle applies only to materials that are said thermo-rheologically simple and therefore its use with filled rubber compounds should give poor results. As seen in Fig. 10, this is not the case and, in the author s experience, it is common observation that good mastercurves are obtained with many complex polymer materials through time-temperature superposition, providing experimental data are of quality. In this respect, closed-cavity rheometers offer obvious advantages over open-gap instruments. 

The dynamic-mechanical properties of elastomers have been studied extensively by rubber physicists and technologists for about 50 years. The principal objective in much of this work has been to relate the experimental observations to the known composition and structure of the materials. At first sight it appears that elastomers exhibit extremely complex behaviour, having time-, temperature- and strain-history-dependent hyperelastic properties. This is because elastomers are compounded for practical use and are mixtures of a hyperelastic material (the polymer) with materials exhibiting only short-range elasticity (the filler). 

An electrode with a plastic membrane containing valinomycin as the active carrier is now predominantly used in clinical analyzers. Nearly four decades of experience with this sensor have proven that it fulfils all demands concerning sensitivity, selectivity and lifetime. An anionic interference that can be observed during measurements in undiluted urine may be eliminated by the use of silicone rubber instead of polyvinyl chloride in the membrane or by pre-dilution of urine. Despite some experimental trials, no other ionophore has replaced valinomycin as the active compound in potassium ISEs. This is basically due to the better stability and lipophilicity of this compound in comparison to the others proposed. 

Following the speculations raised by the above results, it was considered important to devise experiments which would make it possible to observe stress softening in vulcanizates as a result of swelling by solvents. This phenomenon had not been observed before. The experimental scheme was based on the observation that stress softening apparently recovers faster the higher is its temperature (see Fig. 5) and that presumably, at temperatures approaching Tg no recovery should take place. Rigbi75) showed that at 0 °C, recovery is extremely slow even in compounds for which Tg is of the order of -40°. However, when the rubber is swollen in a solvent,... 

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