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

Homopolymeric polyacrylonitrile and fibre-forming copolymers containin g either vinyl acetate or methyl acrylate comonomer have been studied by thermal analysis (DSC, TGA and DTG) at various heating rates (10-10 0 K min-1) and under air and nitrogen. Three well-defined pyrolysis st ages have been observed which occur over the temperature ranges 250-35 0-degrees-C, 350-550-degrees-C and above 550-degrees-C. Each stage inv olves a competition between volatilisation and cyclisation or char-for ming reactions which depends on heating rate and the presence or absen ce of oxygen. The well-established dominance of cyclisation in the 250 -350-degrees-C temperature range obtained during carbon fibre producti on from acrylic precursors occurs only at low heating rates. At high h eating rates, volatilisation dominates and this explains why acrylic p olymers have high flammabilities when heating rapidly. The full pyroly sis 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.