Examination of a low density polyethylene (LDPE) film after 15 years of service as an air and water vapour barrier

摘要

A low density polyethylene (LDPE) film used as an air and water vapour barrier for 15 years in an exterior wooden wall con- struction has been examined regarding ageing properties. The wall in question was part of a small house used for building material testing purposes only. During its use, the major part of the film was firmly pressed between plywood boards made of spruce and plasterboards. By pure chance, however, parts of the film had been hanging loose over the plasterboard into the room and thus exposed to room temperature air as opposed to the rest of the film. Different parts of the same LDPE film had thus been exposed to natural ageing in different environments allowing for interesting comparisons to be made. Tensile testing, size exclusion chroma- tography (SEC), oxygen induction temperature measurements, UV and FTIR spectroscopy, and MALDI mass spectrometry have been used to examine the differently aged parts of the LDPE film. It was found that the film contained the antioxidant β-(3,5-di- tert-butyl-4-hydroxyphenyl)-propionic octadecyl ester (trade name Irganox 1076). Despite the fact that the part o f the film found inside the wall had lost about 75/100 of its initial antioxidant concentration as compared to the part exposed to the room's environ- ment, the latter had a substantially lower stabilising power as indicated by a considerably lower oxygen induction temperature. Most likely, the antioxidant has been “deactivated” through its protective action in preventing oxidative degradation of the polymer of the “room” film. The loss of antioxidant in the “皐all” film can most likely be attributed to migration and loss to the surrounding wall construction materials. Despite substantial losses of active antioxidant for both parts of the film, through different mechanisms, both parts have retained their tensile properties as indicated by normal values of elongation- and stress-at-break. Furthermore, the SEC chromatograms indicate no degradation of the polymer in neither parts of the film. Accordingly, the polymer itself seems not yet to have been affected by ageing.