FLAMMABILITY OF POLYACRYLONITRILE AND ITS COPOLYMERS .2. THERMAL-BEHAVIOR AND MECHANISM OF DEGRADATION

Homopolymeric polyacrylonitrile and fibre-forming copolymers containing either vinyl acetate or methyl acrylate comonomer have been studied by thermal analysis (DSC, TGA and DTG) at various heating rates (10-100 K min-1) and under air and nitrogen. Three well-defined pyrolysis stages have been observed which occur over the temperature ranges 250-350-degrees-C, 350-550-degrees-C and above 550-degrees-C. Each stage involves a competition between volatilisation and cyclisation or char-forming reactions which depends on heating rate and the presence or absence of oxygen. The well-established dominance of cyclisation in the 250-350-degrees-C temperature range obtained during carbon fibre production from acrylic precursors occurs only at low heating rates. At high heating rates, volatilisation dominates and this explains why acrylic polymers have high flammabilities when heating rapidly. The full pyrolysis mechanism has been semi-quantitatively analysed and the role that comonomers play discussed. This has enabled a fuller understanding of the potential burning behaviour of these polymers to be developed.