MECHANICAL-BEHAVIOR OF POLYACETAL AND THERMOPLASTIC POLYURETHANE ELASTOMER TOUGHENED POLYACETAL

In view of the high potential of polyacetal (polyoxymethylene, POM)/thermoplastic polyurethane (TPU) elastomer blends in engineering applications, greater emphasis is placed on long-term properties of these blends. Though the creep behavior of pure viscoelastic polymers has been extensively studied and the temperature effect within the linear viscoelastic range has been explained on the basis of time-temperature superposition principle, only a little information is available about these blends. This work was carried out in pursuit of workable theories for actual engineering applications with their applicability defined, and secondly, to seek comprehension of the physical mechanisms which control the manifestation of nonlinear viscoelastic behavior in these blends. Validity of an equation of the type epsilon = epsilon-degrees + epsilon + t(n) has been analyzed. The blends creep more than POM and the tendency to creep increases as the TPU content increases. It is found that n is a constant for a given blend and is independent of stress over the stress range considered. For all the blends, epsilon-degrees is approximately a linear function of stress, whereas epsilon + is a nonlinear function of stress irrespective of the composition of the blend. Monotonic tension tests have been carried out at three different strain rates and both POM and blends are found to be rate sensitive. Activation volume of POM and its blends with TPU has been evaluated and is found to increase as stress increases, in both cases.