LIFETIME PREDICTION OF POLYMERS

Natural and artificially accelerated weathering trials were carried ou t on low-density polyethylene samples used for agricultural and dispos able purposes. Modifications in polymer characteristics were studied u sing Fourier transform infrared (FTIR) spectroscopy, different scannin g calorimetry (DSC), gel permeation chromatography (GPC), scanning ele ctron microscopy (SEM), and mechanical properties testing. The natural exposure trials were conducted at five geographical locations represe nting varying ambient temperature and humidity conditions and receivin g different doses of solar radiation. The artificially accelerated exp osure trials were carried out in an Atlas weatherometer for 5,000 hour s, whereas the natural exposure was for 24 months. Rates of photo-oxid ative and thermal degradation have been determined by measuring the fo rmation of non-volatile carbonyl oxidation products which absorb in th e infrared region with a maximum absorbance level at 1710 cm(-1) Therm al characteristics were noticed to vary in terms of the shape and size of melting peak for both the natural and artificial weathering. This implies a change in crystallinity, which has indicated an increasing t rend with exposure time. The crystalline melting temperature (T-m) rem ains almost steady for both exposure tests. The drop in mechanical pro perties and the modification in polymer chain length were also monitor ed. The GPC analysis indicated the change in molecular weight distribu tion (MWD), which shows the formation of lower molecular-weight specie s. Surface modifications were revealed in terms of abrasion effects ap parent from SEM micrographs. A correlation between natural and artific ial weathering was considered for lifetime prediction in a short expos ure time. It was found that the confidence level of predicting lifetim e an the basis of artificially accelerated exposure trials is dependen t on many parameters, which include time, material, equipment, etc. (C ) 1995 John Wiley & Sons, Inc.