CROSS-LINKED SURFACE LAYER OF POLYVINYL ALCOHOL FIBER FORMED BY HEATING IN AIR

Polyvinyl alcohol filaments heated in air at 180°C for 5 or 20 hours were immersed in 80wt% ethylenediamine aqueous solution on a slide glass, and their insoluble surface layers were observed under a microscope. Insoluble surface layers like these were not observed even when the filaments were heated for 20 hours in the atmosphere of nitrogen or under vacuum. Thickness of the insoluble layer was 4_??_5 and 20_??_30μm for the filaments heated in air for 5 and 20 hours, respectively. After a few minutes immersion of the filament in ethylenediamine solution, the insoluble layer of the filament heated for 5 hours swelled, expanding up to 5_??_6 times in diameter, while contracted to a half in length. This insoluble layer separated eventually from the body of the filament. After about one hour, the body itself dissolved away. For the filament heated for 20 hours, the insoluble layer swelled less than that of the filament heated for 5 hours, and did not separate from the body of the filament. But the body dissolved gradually from the surface of the cross sections leaving the insoluble layer. Both filaments heated for 5 and 20 hours were treated in water at 130_??_140°C in an autoclave and were washed by acetone to remove the water. Both residual insoluble surface layers looked like hollow fiber in shape. These surface layers were also examined by a scanning electron microscope, and network pattern cleavages on the surface were observed. These results may show that the formation of the insoluble layer was due to cross linkages introduced into polyvinyl alcohol chains by thermooxydative reactions, consequently leading to decrease in strength and elongation of the fiber heated in air.