Oxidative stabilization of acrylic fibres - Part 5 The decolouration reaction

When acrylic fibres are heat treated for various times at 220 to 250° C, they form dark, insoluble structures of uncertain chemical character which are inert to many strong oxidizing and reducing agents. The heat-treated fibres are, however, rapidly decoloured by warm alkaline hypochlorite solutions. When fibres which have undergone short-time heat treatment are subjected to the hypochlorite, incubation periods are observed before decolouration is noted; and a swollen acrylic network remains after decolouration is complete. The acrylic network is primarily unreacted precursor units save for a small amount of hydrolyzed material. The decoloured reaction is zero order, indicating a reaction at the surface. The rate of the decolouration reaction also increases with increasing duration of the stabilization heat treatment. In fibres which have undergone partial diffusion-controlled stabilization, a dark mantle surrounds a lightly coloured core. The rate of decolouration is unaffected as the decolouration interface passes from the mantle to the core, indicating that the decolouration reaction is not influenced by the occurance of any sequent reactions. The existence of the acrylic residue indicates that the prefatory reactions are continuing in both mantle and core during the course of stabilization.¹³C-NMR spectra of the acrylic residue show the same triad methine peak areas as those obtained on the untreated fibre; hence the stereoregularity of the nitrile groups has no influence on the rate of nitrile polymerization. The mechanisms of nitrile initiation and of decolouration are discussed. The residue obtained by sulphuric acid etch is different from that obtained by hypochlorite treatment. These results suggest that during the early-to-intermediate stages of stabilization, the fibre consists of interpenetrating networks of original material, i.e., fibre which has undergone only the prefatory reactions and fibre which has undergone the sequent reactions.