Improvements in/or relating to a process for preparing a polyvinyl acetal resin and the polyvinyl acetal resin resulting from the said process

摘要

A polyvinyl acetal resin is prepared by reacting a sulphuric acid hydrolysed polyvinyl ester with an aldehyde in the presence of a mixture of sulphuric acid and an alkanol containing from 1 to 8 carbon atoms until the hydroxy content of the product is substantially constant and thereafter maintaining the reaction conditions till the non-extractable sulphur content of the resin does not exceed 0.025 per cent by weight calculated as SO4, not over 1.3 per cent of SO4 groups calculated as H2SO4¦te being present based on the total charge and 250 to 1000 parts of the alkanol being present for every 100 parts of the hydrolysed polyvinyl ester. The acetalization is usually complete in 1 to 3 hours, using heating and stirring, these conditions being continued for 3 to 5 hours to obtain the required minimum of non-extractable sulphur. The polyvinyl esters specified are polyvinyl acetate, propionate and butyrate, while as aldehydes butyraldehyde, formaldehyde, acetaldehyde, propionaldehyde or valeraldehyde may be used. Methanol, ethanol, propanol, butanol, hexanol and octanol are specified as alkanols. The acetal resin may be heat stabilized by treating with an alkali metal hydroxide or a salt thereof with a weak acid. U.S.A. Specifications 2,258,410 and 2,282,057 are referred to. The Specification as open to inspection under Sect. 91 comprises also using cyclohexanone in place of an aldehyde and using octadecanol, glycols, glycerine, pentacrythritol, sorbitol, mannitol and benzyl alcohol in place of the specified alkanols. This subject-matter does not appear in the Specification as accepted.