Thermal and Oxidative Degradation of Polymers

In recent decades synthetic polymeric materials, because of their unique physical properties, have rapidly replaced more traditional materials such as steel and nonferrous metals, as well as natural polymeric materials such as wood, cotton, and natural rubber. However, one weak aspect of synthetic polymeric materials compared with steel and other metals is that these materials are combustible under certain conditions. Thus the majority of polymer-containing end products (for example, cables, TV sets, electric appliances, carpets, furniture) must pass some type of regulatory test to help ensure public safety from fire. However, traditional pass/fail tests have not provided any information regarding the relationship between flammability properties and the physical and chemical characteristics of polymeric materials. Such information is needed to develop more fire-safe materials, a need that has accelerated because of European environmental concerns about the use of halogenated flame retardants (because of potential formation of dioxins in the incineration of spent end products). The paper Effects of Weak Linkages on the Thermal and Oxidative Degradation of Poly(methyl methacrylate) [1] is one of a series published on this topic by the members of the Materials Fire Research Group at NBS/NIST from 1985 to 1994 [2-11]. These papers report a new approach that studies the effects of molecular-level structure of polymers on their thermal stability and flammability properties instead of a traditional global thermal-balance approach. This series of studies is built upon the pioneering work on thermal degradation of polymers conducted at NBS from the late 1940s to early 1970s [12]. Providing the technical basis for industrial clients to design less flammable materials requires unfolding the structural features that determine thermal stability. This paper [1] reports a study of the thermal and oxidative degradation mechanisms of an acrylic polymer in atmospheres of nitrogen and air by measuring the change in the sample mass while various specially polymerized samples were heated from 80 C to 480 C. Thermal degradation of the acrylic polymer, polymerized using a free-radical method, proceeds in three steps of mass loss: the first and easiest (scheme 1 in Fig. 1) is initiated by scissions of head-to-head linkages at about 160 C (representing one type of defect at the polymer backbone); the second (scheme 2) by scissions at the chain-end initiation from vinylidene ends at around 270 C; and the last (scheme 3) by random scission within the polymer chain (at the weakest bonds).