| 摘要 |
A polyvinyl acetal resin is purified by adding a non-solvent liquid to a solution of an impure polyvinyl acetal resin, the amount and temperature of the non-solvent being such that a gel of said resin and liquor, containing solvent and non-solvent, are produced and then separating the liquor from the gel. Specified acetals are those produced by the reaction of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde or valeraldehyde with polyvinyl alcohol or polyvinyl esters in the presence of acidic catalysts, e.g. sulphuric acid, in a liquid medium. Specified solvents are methanol, ethanol, propanol and acetic acid while water or water-solvent mixtures are used as the non-solvent. The separated gel may be further dissolved in a polyvinyl acetal solvent and the purification repeated. The amount and temperature of the non-solvent may be such that there are dissolved in the liquor the major proportion of polymers of such molecular weight as to be soluble in methanol-water mixtures of specific gravity 0.84 or alternatively at least 0.93. The separated gel may be heated to liquefy it and/or sprayed in particle form into a zone where the solvent and non-solvent are removed therefrom, e.g. by evaporation. |
