LIFETIME PREDICTION OF POLYMERS

Natural and artificially accelerated weathering trials were carried out on low-density polyethylene samples used for agricultural and disposable purposes. Modifications in polymer characteristics were studied using Fourier transform infrared (FTIR) spectroscopy, different scanning calorimetry (DSC), gel permeation chromatography (GPC), scanning electron microscopy (SEM), and mechanical properties testing. The natural exposure trials were conducted at five geographical locations representing varying ambient temperature and humidity conditions and receiving different doses of solar radiation. The artificially accelerated exposure trials were carried out in an Atlas weatherometer for 5,000 hours, whereas the natural exposure was for 24 months. Rates of photo-oxidative and thermal degradation have been determined by measuring the formation of non-volatile carbonyl oxidation products which absorb in the infrared region with a maximum absorbance level at 1710 cm(-1) Thermal 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 trend with exposure time. The crystalline melting temperature (T-m) remains almost steady for both exposure tests. The drop in mechanical properties and the modification in polymer chain length were also monitored. The GPC analysis indicated the change in molecular weight distribution (MWD), which shows the formation of lower molecular-weight species. Surface modifications were revealed in terms of abrasion effects apparent from SEM micrographs. A correlation between natural and artificial weathering was considered for lifetime prediction in a short exposure time. It was found that the confidence level of predicting lifetime an the basis of artificially accelerated exposure trials is dependent on many parameters, which include time, material, equipment, etc. (C) 1995 John Wiley & Sons, Inc.