STUDIES ON THE CARBONIZATION OF POLEACRYLNITRILE FIBER

(II) CHANGES IN MOISTURE SORPTION AND STRUCTURE WITH HEAT-TREATMENT

Abstract

Moisture sorption isotherms were determined at 20°C on flame-proofed fabrics heat-treated for 40min. at various temperatures in the range of 400_??_1000°C. The heat-treating condition is the same as described in the preceding paper I. Results are discussed in conjunction with the changes in composition and mechanical properties (reported in the paper I). Following explanation may be given to structural changes by heat-treatment.
1. Flame-proofed fiber (prepared at 300°C in air)
The flame-proof fiber seems to have a pretty irregular structure with some crosslinks and a number of hydrophilic functional groups.
2. Flame-proofed fiber heat-treated below 500°C
The structure is tightened by introduction of more crosslinks, but the chemical composition is not much changed except for decrease in hydrogen content. These structural changes are considered to bring about both decrease in breaking elongation and increase in abrasion strength.
3. Flame-proofed fiber heat-treated above 500°C
Generation of capillaries is observed in the heat-treated fibers. The number of capollaries tends to increase with elevation of heating temperature up to 1000°C. Heteroatoms are eliminated markedly, resulting in increase of carbon content as the heating temperature is raised. Rapid increase in Young's modulus by raising the heating temperature may therefore have a connection to increase the number of the C-C condensed rings. The sorption curves also indicate that the fibers heat-treated above 900°C have a carbon-type structure which will shift to a more regular one at higher temperatures.