The Arrested Diffusion Phenomenon in Dense Polyethylene Foams

Abstract

The diffusion of gases from a polymeric matrix containing a dispersed and non-interconnected gaseous phase at a pressure higher than the ambient pressure is studied. Specifically, a dense polyethylene foam, manufactured from LDPE which is expanded using high pressure nitrogen, is characterised in terms of its mass and density changes as a function of time and its behaviour at triaxial (hydrostatic) compression. This material was found to be degassing when stored at ambient conditions of temperature and pressure but this process occurs, apparently, at a slow rate and is accompanied by a volume contraction of the material leading to an increase of its apparent density. At the same time, a decrease in the compressibility of the foam was observed with lapsed storage time, indicating a decrease in the void volume it contained. A model was used to simulate the mass transport process of the gas and the volume relaxation of the solid. The model, although a first order approximation of the phenomena taking place, provides results which are qualitatively consistent with the experimental data and predicts the mass loss rate of the foam reasonably well. The model indicates that introduction of voids in a solid reduces the apparent diffusion coefficient of a gas in the solid. This is due to a pneumatic “buffer action” of its voids which becomes more pronounced as the volume of the latter increases. An extension of the model is used to quantitatively rationalise the triaxial compressibility of the same foams as a function of their history.