Aqueous dispersions of polyvinylbutyral having low content of hydrosolubles

A process for preparing an aqueous dispersion of polyvinylbutyral having a particle size of from about 0.01.mu. to 5.mu. and containing, dissolved therein, less than about 2.5% by weight of hydrosoluble compounds relative to the weight of polyvinylbutyral, comprising the steps of

• (a) bringing an aqueous dispersion of the polyvinylbutyral, formed by reacting polyvinyl alcohol with butyraldehyde in an aqueous medium, in an aqueous solution containing, dissolved therein, an amount of no more than 2.5% by weight of hydrosoluble compounds relative to the weight of polyvinylbutyral, into contact with a semipermeable ultrafiltration membrane in such a manner that a portion of the aqueous dispersion comprising the polyvinylbutyral particles is retained and a sufficient portion of the aqueous solution is filtrated through the membrane in order to remove such an amount of hydrosoluble compounds that the amount of hydrosoluble compounds within the retained portion of the dispersion is reduced to less than about 2.5% by weight relative to the weight of the polyvinylbutyral and
• (b) in the course of the ultrafiltration, adding to the retained portion of the dispersion a sufficient amount of deionized water in order to replace at least a major part of the filtrated off portion of the aqueous solution.