Theory and Application of the Parallel Plate Plastometer

A method has been established for the measurement of the viscosity of high polymers at low rates of shear in the range 104 to 109 poises using a parallel plate plastometer. This is based on a mathematical criterion for separating the viscous portion of the deformation from the ``elastic'' and ``delayed elastic'' components. Experimentally, the plate separation is measured at a given temperature as a function of time. The theory furnishes a relation, which is also the criterion for predominantly viscous deformation, between viscosity, plate separation, applied load, and time. This relation, a modified form of Stefan's equation, is used for calculating the viscosity from the experimentally observed quantities. The method has been applied to polyethylene and vinyl chloride-acetate resin compounds. The viscosity-temperature behavior of these materials is shown to be simple over the temperature range studied; that is, log viscosity varies linearly with the reciprocal of the absolute temperature. Data are presented which show that polyethylene resins and polyethylene resin-paraffin wax mixtures follow Flory's relation; that is, log viscosity varies linearly with the square root of the weight average molecular weight. Accordingly, the parallel plate plastometer offers promising possibilities for the empirical determination of the weight average molecular weight of these materials. Data are also presented on plasticized vinyl chloride-acetate resin systems which point to a close parallel between the effects of increasing temperature and increasing plasticizer concentration.