An XPS Investigation of Thermal Degradation and Charring Processes for Polymers in the Condensed Phase. I

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Abstract
Crosslinking and charring in the condensed phase of polymers during heating, processing, and/or burning are of central importance in their flame retardance and smoke suppression and in formulating polymeric compos ites. Combining with thermal analysis techniques (TGA, DSC), ethylene-vinyl acetate (EVA) copolymers and polyethylenes (HDPE, LDPE) with/without inor ganic phosphorus-containing additives, were selected as model systems for XPS studies of the charring process. A new parameter, Tgr, the transition tempera ture from mesophase to graphitic structure (graphite-like structure, GLS) of the char has been well-defined and determined by combination of core levels and valence bands in XPS. The reason for Tgr to be selected as a likely supplemen tary parameter for characterizing polymer charring in condensed phase is two fold : 1) a clear-cut phase transition; 2) at which temperature a highly cross linked network results. A special emphasis in the paper has been given to the roles of the oxygen atoms either introduced from an adventitious atmosphere or inherent Ac group itself. Along this line some features in both thermal degradation and flame retardance of polymers have been rationalized in terms of transition temperature Tgr.

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