PURPOSE: To obtain the titled polymer, useful as a coating material, etc., and having good transparency and solubility industrially and advantageously, by reacting PVA with an aldehyde in the aqueous phase in the presence of an acidic catalyst, at a specific temperature, and further adding an aqueous solution of PVA to the reaction system during the time just before to just after the deposition of precipitates.
CONSTITUTION: In preparing polyvinyl acetal by condensing PVA with an aldehyde, e.g. n-butylaldehyde, in the aqueous phase in the presence of an acidic catalyst, e.g. hydrochloric acid, the condensing reaction is carried out in a reaction system at a temperature of 20°C or above, and an aqueous solution of PVA is added to the reaction system during the time just before to just after the deposition of polyvinyl acetal precipitates. Thus, the reaction is continued to give the aimed polymer. The concentration of the aqueous solution of PVA is 3W15wt%, and the amount of the solution is preferably 0.5W20wt% based on the total weight.
NOMURA SHIGERU
MIYAGAWA MASASHI
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