On Mooney and Mooney stress relaxation. II. A nonlinear viscoelastic description: Experimental and numerical results

It has been investigated whether the stress build-up and the stress relaxat ion involved in a Mooney test, with subsequent Mooney stress relaxation, ca n be described by nonlinear viscoelastic theory, more particularly the Wagn er constitutive model. For this purpose, the viscoelastic behavior of three nonvulcanized EPDM materials, with similar Mooney viscosity but varying el asticity, has been studied. Relaxation time spectra were obtained from dyna mic mechanical experiments, from which the step-strain stress-relaxation mo dulus was calculated. Stress build-up experiments were performed with a con e and plate system in order to obtain the so-called damping function (a mea sure for the deformation sensitivity) of the materials. Using these materia l functions, the Mooney test was successfully described with the Wagner con stitutive model. Experimental and theoretical Mooney stress-relaxation rate s are in close agreement. The predicted Mooney viscosity is up to 25% lower than the measured value. This may be due to nonideal conditions during the Mooney test, such as inhomogeneous heating and secondary flows, and to ina ccuracy of the damping function. The model calculations confirm the strong experimental dependence of Mooney measurements on small variations in instr umental conditions such as geometry, rotation speed, and so forth. (C) 1999 John Wiley & Sons, Inc.