Anomalous Thickness Increase in Crosslinked Closed Cell Polyolefin Foams During Heat Treatments

摘要

When crosslinked closed cell polyolefin foams are under a temperature above the melting point of the base polymer, a reduction of their size is expected due to the gas diffusion out of the foam. However, some kinds of crosslinked closed cell polyolefin foams present one direction (thickness direction) in which the foam size increases during the first minutes fo the thermal treatment. The thickness fo the foams after the thermal treatment can be higher than the thickness of the original foams. An experimental study is presented on teh thickness increase, as well as on the changes in te dimensions and the properties of foams with different densities, which were obtained from different foaming processes and made of different base polymers, as a function of the treatment temperature and the treatment time. This investigation sought to discover the physics mechanisms that control te anomalous thickness increase. The experimental results show that the thickness increase of these materials is related to the anisotropic cellular structure of the original foams.