Oxidative Stabilization of Acrylic Fibers. III. Stabilization Dynamics.
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF MATERIALS SCIENCE AND ENGINEERING
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As acrylic fibers are heated in air to induce the stabilization reactions, the tension developed when stabilized at constant length and the instantaneous velocity of stabilizing fibers undergoing continuous processing both depend on the chemical composition, diameter, and orientation of the precursor fiber. An oriented fiber will tend to shrink when heated to the range of 130-160C, and hence will develop tension if restrained at constant length. Although this process has no direct relation to the stabilization process, it will influence the instantaneous velocity of the fiber during the later stages of continuous processing. As a fiber held at constant length is heated above 160C the tension developed by entropic relaxation decreases and the fiber starts to undergo the prefatory and sequent reactions of stabilization described in the first paper in this series. If the prefatory reactions are rapid, a rigid structure is quickly established in the fiber and tension again increases rapidly. However, if the prefatory reactions are slow, select portions of the fiber react preferentially and the unreacted portions tend to relax to maintain a temporary quasi-equilibrium tension level. In both cases the fibers shrink at the later stages of stabilization because of chemical reactions. The shape of the tension-time curve is similar to the oxygen-uptake curves the diffusion-limited mechanism of stabilization produces parabolic curves whereas the reaction-limited mechanism produces linear curves. Because each element of a fiber undergoing continuous processing is subjected to the same tension at all times, previously oriented fibers first shrink, then stretch, and finally shrink again.