Anomalous thickness increase in crosslinked closed cell polyolefin foams during heat treatments

When crosslinked closed cell polyolefin foams are under a temperature above the melting point of the base polymer, a reduction of their size is expect ed due to the gas diffusion out of the foam. However, some kinds of crossli nked closed cell polyolefin foams present one direction (thickness directio n) in which the foam size increases during the first minutes of the thermal treatment. The thickness of the foams after the thermal treatment can be h igher than the thickness of the original foams. An experimental study is pr esented on the thickness increase, as well as on the changes in the dimensi ons and the properties of foams with different densities, which were obtain ed from different foaming processes and made of different base polymers, as a function of the treatment temperature and the treatment time. This inves tigation sought to discover the physics mechanisms that control the anomalo us thickness increase. The experimental results show that the thickness inc rease of these materials is related to the anisotropic cellular structure o f the original foams. (C) 1999 John Wiley & Sons, Inc.