Polypropylene filaments drawn at 98°C were subjected, under tension or tensionless state, to dry heat-treatment or to wet heat-treatment in polyethyleneglycohol of lower molecular weight, at 100_??_140°C. Shrinking behaviour in heat-treatment and properties of heat-treated filaments, such as density, birefringence, Young's modulus, disperse-dye absorption and shrinkage in boiling water, were observed. The results obtained were as follows.
i) Heat-treatment under tensionless state: Filaments shrink more in wet heat-treatment than in dry heat-treatment, and thus birefringence decreases more by the former treatment than by the latter. Density increases by heat-treatment, especially greatly by wet heat-treatment; this is supposed to be caused by the increase of the degree of crystallinity. By wet heat-treatment, however, Young's modulus seems to rather decrease and dye absorption increases remarkably. Dimensional stability to boiling water is better for filaments wet-heat-treated than for those dry-heat-treated.
ii) Heat-treatment under tension: The effect of heat-setting is more remarkable in wet heattreatment than in dry heat-treatment. The increase of the tension in the treatment increases the density and Young's modulus.
iii) The above tendencies are increased with rising temperature of treatment, especially greatly above 120°C. We may conclude, therefore, that the optimum temperature of heat-treatment for polypropylene fibers is 120_??_130°C. Furthermore changes in fiber structure by heat-treatment were discussed, in connection with the role of heating medium for loosing the packing of intermolecular structure in the amorphous regions.
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