Transient behaviour of the mechanical loss factor of high density polyethylene during creep and relaxation

The mechanical loss factor of high density polyethylene has been measured during creep and stress relaxation experiments. The application of a constant stress (creep) or strain (relaxation) resulted in an instantaneous increase in tan above the value obtained in absence of the creep or relaxation process. During the flow the tan-value decreased and approached an apparent equilibrium value slightly above the value obtained without the static load or deformation. This behaviour was observed both at 1 and 10 Hz and at 23 and 60 °C. It is suggested that this time dependence of the mechanical loss factor is associated with the basic mechanisms responsible for the creep or relaxation process itself.