Chapter 37 - Polyethylene: Overview

Abstract

This chapter discusses the results of weathering exposure of polyethylene. Polyolefin homopolymers are made from ethylene, propylene, butylene, and methyl pentene. Other olefin monomers such as pentene and hexene are used to make copolymers. The principal resins of the polyolefin family are polyethylene and polypropylene, and polyolefin copolymers such as ethylene-vinyl acetate, ionomer, polybutylene, and polymethyl pentene. The crucial role of temperature on the weathering of polyethylenes was illustrated in a study on desert exposure of polyethylene films. Two sets of polyethylene film samples, one maintained at 25◦C at all times in an air-cooled, UV-transparent enclosure, and the other left under the much higher ambient temperature, were exposed to sunlight outdoors. The air temperature varied in the range 26◦C–36◦C during the period of exposure. However, the surface temperature of plastics exposed to sunlight was much higher (by as much as 60◦C for common plastics depending on the color and the thickness) than that of the surrounding air due to heat buildup. Samples kept at the lower temperature deteriorated much slower than those at ambient temperature although both were exposed to the same dose of solar UV radiation. It is the synergistic effect of high temperature and solar UV radiation that is responsible for the rapid degradation of polyethylene films under these conditions. The findings are consistent with the observation that weathering rates of common plastics are very much slower when exposed floating in sea water, in marine environments, compared to those exposed on land. Water acting as a heat sink is able to maintain low sample temperatures, retarding deterioration.