PURPOSE: To produce a polyvinyl acetal in which the content of 1μ or layer foreign substance is low, by reacting in the presence of an acid catalyst a PVA and an aldehyde in an aqueous phase in the presence of a rhodanate and a lithium salt.
CONSTITUTION: In precipitating a polyvinyl acetal by reacting a polyvinyl alco hol with an aldehyde in the presence of an acid catalyst in an aqueous phase, a rhodanate or a lithium salt is added to the reaction system t produce the polyvinyl acetal. It is preferred to add 0.5pt.wt. or more, based on 100pts.wt. polyvinyl alcohol, the rhodanate or the lithium salt, because if the amount is too small, the effect is too low to suppress the occurrence of foreign substances, whereas if the amount is too large, the effect dose not increase proportionally, which is uneconomical. As the polyvinyl alcohol to be used, one having an average degree of polymerization of 300W1,500 and a degree of saponification of 80mol% or more is preferred.
SAWADA SATOSHI
MAEJIMA KAZUO
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