SOME RESULTS OF SOLVING THE NON-STATIONARY PROBLEM OF GAS DIFFUSION IN POLYETHYLENE

Abstract

The problems of diffusion of molecular oxygen and ethylene in amorphous-crystalline polyethylene are considered. The temperature of the medium is assumed to be constant, and the diffusion process is described by the classical Fick partial differential equations. It is assumed that the material obtained from the polyethylene granules using the thermomechanical extrusion process is located in a limited sealed container. The impermeability of the container shell leads to the need to take into account the parameters of the gas in the free volume. The problem is nonlinear and non-stationary, and it is assumed that the rate of interphase processes at the gas-polyethylene interface significantly exceeds the rate of diffusion. Using the finite element method, numerical results are obtained describing the process of oxygen evacuation from polyethylene and the process of ethylene diffusion into the material, at different values of the external pressure of the penetrant gas. The main characteristics of these processes are determined. An effective variant of double vacuuming is considered, which makes it possible to obtain a significantly lower concentration of oxygen in polyethylene. The results are of practical significance for evaluating the parameters of the production of high-quality heat-shrinkable polyethylene pipes produced by the technology of gamma-ray radiation exposure in the protective environment of an inert gas.