Effects of slow heating rates on products of polyethylene pyrolysis

Abstract

Polymeric materials, although they are not volatile themselves, may be analysed by gas chromatography (GC) through the use of analytical pyrolysis. Because the fragmentation pattern produced by heating is reproducible, the pyrolysates provide much information about the original macromolecule of which they were a part. Analytical pyrolysis, coupled with GC, mass spectrometry (MS), GC-MS and Fourier transform infrared (FTIR) spectroscopy has provided a wealth of information concerning polymer microstructure, branching, defect structure and degradation mechanisms. Traditionally, the pyrolyses have been carried out at very fast heating rates (for example, heating to 800 °C for 5 s at a rate of 10 000 °C s–1). Recently, however, interest has been shown in how polymers behave when heated to pyrolysis temperatures at slow rates (°C min–1) for long periods of time. Many polymers show considerable shifts in the kinds and relative amounts of pyrolysates produced at these slower rates. The polyolefins are good examples of polymers for which the pyrolysate distribution is sensitive to both the final temperature and the rate of heating. Examples are given indicating these differences for polyethylene in a series of pyrolyses in which the temperature is varied and the heating rate held constant, and for pyrolyses at a variety of heating rates to the same final temperature.