This thesis covers the investigation of the weathering behaviour of mixed waste plastics. It includes a review of the literature on polymer degradation mechanisms with particular emphasis on environmental degradation from ultra violet radiation. Products manufactured from recycled polyolefin blends were subjected to accelerated weathering conditions and subsequent tests. Field–aged products were tested also. Samples were analyzed for changes in mechanical, rheological and chemical properties. Scanning Electron Microscopy and Electron Probe Microanalysis were used to examine samples for both impurities and signs of degradation. Data obtained in this study, by means of tensile and melt flow tests, with GPC, gloss and colour analyses carried out by Rapra1, showed that the effect of UV exposure, whether in the field or artificial environments, was not significant as far as the mechanical properties of the materials were concerned. There was some change in the surface characteristics but such surface effects would not compromise the mechanical integrity of the product when recycled. During reprocessing of the materials, it is likely that stabilizer is brought to the surface of any new moulding, thus renewing the surface characteristics. Hence damaged or end-oflife products need not be discounted from recycling. However, surface appearance may be an issue. SEM EDX Analysis was useful in identifying impurities in the recyclate, but there was no conclusive evidence of degradation as a direct result of exposure to UV radiation. The variation in field-aged materials and the artificially aged materials showed that it is likely that the difference in results for field aged and artificially aged material was due to variability in the source materials and showed that in practice, it is difficult to control the composition of the mix coming from the waste suppliers. This was more significant for the 1 properties of the materials than any UV exposure. This increase could be due to degradative cross-linking. However, it was more likely to be caused by variability in the batches of raw material. Chain scission in polypropylene and cross–linking in polyethylene in mixed blends could have a compensating effect on each other and lead to non–degradative effects although evidence for this was inconclusive. Overall, the study has shown that all the materials tested have generally maintained their mechanical properties with little change under accelerated UV exposure. This has positive implications for the use of plastic recyclate of this nature in manufacturing processes.
|Date of Award||14 Aug 2007|