Polyvinyl alcohol cross-linked with two aldehydes

A method of producing a cross-linked polyvinyl alcohol element which comprises the steps of:

• providing an aqueous admixture comprising
• a film-forming polyvinyl alcohol resin,
• a dialdehyde cross-linking agent capable of cross-linking the polyvinyl alcohol resin, said cross-linking agent being present in an amount of from 1 to 20% by weight of the theoretical amount required to react with all of the hydroxyl groups of the polyvinyl alcohol resin, and
• a water soluble acid-aldehyde containing a reactive aldehyde group capable of reacting with hydroxyl groups in the polyvinl alcohol resin and containing an ionizable acid hydrogen atom, said acid-aldehyde being present in an amount of from 1 to 50% by weight of the theoretical amount required to react with all of the hydroxyl groups of the polyvinyl alcohol resin and sufficient to reduce the pH of the aqueous admixture to not more than 5;
• forming the admixture into a desired physical shape;
• and
• heating the shaped admixture to form a self-supporting polymeric article and to simultaneously effect a cross-linking reaction between the dialdehyde cross-linking agent and the polyvinyl alcohol resin to produce a cross-linked polyvinyl alcohol element.