Stabilisation of low density, closed cell polyethylene foam

Low density, closed cell polyethylene foams show severe post-extrusion collapse when isobutane or carbon dioxide are used as physical blowing agents. This dimensional instability is caused by the fact that the blowing agent permeates faster out of the foam than the air permeates into the foam. Permeation experiments on polyethylene films showed that this is an intrinsic property of polyethylene: for both isobutane and carbon dioxide the permeability is 12 to 15 times larger than the air permeability. However, it was observed that the addition of small amounts (<2 wt.%) of certain additives, such as stearyl stearamide or glycerol monostearate, could improve the dimensional stability of isobutane blown foams considerably. The working mechanism of these additives was found to be a surface effect. The additive forms a (partially) crystalline surface layer, which decreases the isobutane permeability to a larger extent than the air permeability. It reduces the isobutane to air selectivity from about 15 to 4. The presence of this layer on the polyethylene films was confirmed with electron microscopy and infrared spectroscopy. The crystalline character of this layer was revealed by X-ray diffraction.