Heat-treated and unheated polyvinyl alcohol fibers are acetalized with terephthaldehyde.
The acetalization is peculiarly influenced by concentration of surfuric acid used as catalyst. As the concentration is increased, the initial rate of reaction becomes greater, but the subsequent rate is more rapidly decreased, and the apparent equilibrium degree of reaction seems to be lower.
The wet-heat resistance of acetalized fiber is chiefly determined by extent of reaction rather than by conditions of reaction, differing from the case of acetalization with glyoxal. When the heattreated fiber is acetalized to several, ten or more, or about twenty mole per cents fiber can resist to water of 100°. 120°_??_130°. or 140°C respectively. Where the percentage is computed from weight increase due to the reaction by assuming that all bound terephthaldehydes are cross-linked between molecules of polyvinyl alcohol. When even the unheated fiber is acetalized to twenty or more mole per cents, it resists to water of 100_??_140°C. These results show that terephthaldehyde cross-links much more readily than glyoxal.
The tensile recovery of the acetalized fiber is very high, but the tenacity. the breaking extension and particularly the knot strength generally tend to decrease as compared with formalized fiber, though the extents of these decreases depend on the concentration of sulfuric acid in the acetalizing bath. The unusual effects of the concentration of sulfuric acid on the rate of reaction and on the tenacity and extension seem to be due to differences of distribution of the crosslinks within fiber.
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