DENSITY CHANGES OF ACRYLIC FIBERS BY THERMAL STABILIZATION

Abstract

The density changes of acrylic fibers by isothermal stabilization at temperatures from 221 to 281°C have been investigated. The density increased with increasing heat treatment time and reached independently of heat treatment temperature a value of about 1.58_??_1.60g/cm3 at longer heat treatment times. The molecular weight and molecular volume per repeat unit of polymer of the original and heat-treated fibers were calculated from elemental constitutions and density by assuming that _??_ repeat unit contains three carbon atoms in average. By the heat treatment in air the molecular weight increased up to about 11%, and the molecular volume decreased up to about 17%. The deference in densities between the acrylonitrile copolymer fibers heat-treated in air and in a silicone oil bath was small at initial periods of the heat treatments. This indicates that with the copolymer fibers the density change at initial periods of the heat treatment in air is mainly due to a volumetric shrinkage accompanied with the cyclization of nitrile side groups from initiation centers already present in the fibers. Polyacrylonitrile homopolymer fibers showed a distinctive retardation in the relation between density and heat treatment time. This retardation is attributed to the fact that the formation of initiation centers of the cyclization in homopolymer fibers is rate-determined by the diffusion of oxygen from the atmosphere into the fibers.