Oxygen permeability of polyethylene-extrusion-coated greaseproof paper

Polyethylene-extrusion-coated greaseproof paper exhibits significantly lower oxygen permeability than predicted from the oxygen permeability of polyethylene and that of the greaseproof paper itself. Greaseproof paper has no barrier whatsoever against oxygen because it is a porous material. Two mechanisms have been proposed for the Superior oxygen permeability of polyethylene-extrusion-coated greaseproof paper: 1) the formation of a transcrystalline or crystalline layer oil the polyethylene and 2) the dense top layer of the greaseproof paper forms a laminate with the polyethylene. The cellulose in that top layer contributes to the low oxygen permeability. Those mechanisms have been examined. The material for the examination of those mechanisms was produced by the polyethylene-extrusion coating of two greaseproof papers oil a pilot scale. The oxygen transmission rate of the polyethylene layer and of the extrusion-coated papers was measured at 0%, 50% and 90% relative humidity, the Surface porosity of the greaseproof papers was also measured. A simple model for transmission of oxygen through the polyethylene-extrusion-coated paper is proposed and used to analyse the oxygen permeability. The results showed no support for the contribution to the oxygen permeability from the formation of a transcrystalline or crystalline Surface layer of the polyethylene layer. This mechanism is therefore most likely not responsible for the superior permeability. It is more likely that the formation of a laminate between the extremely dense surface layer of the greaseproof paper and the polyethylene contributes to the oxygen permeability. Support for that scenario was obtained from the fact that only the oxygen permeability of the extrusion-coated paper was influenced by the relative humidity while the oxygen permeability of the polyethylene layer was not affected. According to this mechanism, the oxygen permeability of the extrusion-coated paper is governed by the area fraction where polyethylene is in contact with the paper as well as the oxygen permeability of the polyethylene laminate and the surface layer of greaseproof paper. The thickness of the paper surface layer that contributes to the oxygen permeability of the extrusion-coated paper was estimated to be within the 2-8 mu m range.