Thermal-Oxidative Stability and Degradation of Polymers
This chapter discusses thermal-oxidative stability and degradation of polymers. Thermal stability refers to the ability of a material to maintain desirable mechanical properties such as strength, toughness or elasticity at a given temperature. At the other extreme, thermal degradation can be defined functionally as the deterioration of those properties of polymers which make them useful commercially as rubbers, plastics and fibers. The structure-property relationships of polymers are provided. The oxidative degradation of polymers involves free-radical chain reactions. For example, degradation of polyolefins such as polyethylene (PE) is commonly initiated by hydroperoxide impurities incorporated during synthesis and processing. The polymers selected for discussion are polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinylchloride) (PVC), poly(acrylonitrile) (PAN), poly(tetrafluoroethylene) (PTFE), polyamides (PAs), and heat-resistant polymers. Enhanced stability has been achieved by the use of additives which are frequently called antioxidants or heat stabilizers. Polymer degradation can be monitored by measurement of molecular weight using viscometry, osmometry, light scattering, ultracentrifuge, and gel-permeation chromatography (GPC). The GPC (more generally called size-exclusion chromatography) can be used in estimating the effect of degradation on molecular-weight distribution (MWD). Spectroscopic probes of thermal degradation include ultraviolet (UV) spectroscopy, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, electron-spin resonance spectroscopy (ESR, EPR), and mass spectrometry (MS). Multiple internal reflectance infrared spectroscopy (MIRS) allows a very thin surface layer to be examined. Thermal-stability data extracted from a variety of sources in the literature on selected familiar and commercial polymers are tabulated.