Process for the Preparation of a Dry Powder of a Polymeric Additive

A process for the preparation of a dry powder of a polymeric additive, which process comprises the steps of

• (a) providing in a vessel a polymeric additive optionally dissolved in a solvent,
• (b) adjusting the temperature of the polymeric additive to a range between 40° C. to 200° C.,
• (c) pressurizing a highly compressible fluid at a pressure between 50 and 1000 bar and heating the highly compressible fluid to a temperature between 100° C. and 200° C.,
• (d) mixing the heated polymer additive and the heated highly compressible fluid to reach a gas to polymer ratio (GTP) of between 2 to 8,
• (e) expanding the mixture obtained in d) by means of an expanding device into a spraying tower,
• (f) removing from the stream of expanded highly compressible fluid the dry polymeric additive particles which have been formed,
• wherein the polymer additive is a polymeric pigment dispersant which is
• a) a polyurethane dispersant dissolved in a solvent and whereby the polyurethane dispersant is the reaction product of
• A. one or more polyisocyanates having an average functionality of from 2.0 to 5, with
• B. a mixture of
• B1. at least one monohydroxyl compound selected from the group consisting of monohydroxyl compounds with an aliphatic hydrocarbon, monohydroxyl compounds with an aliphatic hydrocarbon substituted with aryl groups, monohydroxyl compounds with an aliphatic hydrocarbon substituted with halogen groups, monohydroxyl compounds with a cycloaliphatic hydrocarbon, monohydroxyl compounds with a cycloaliphatic hydrocarbon substituted with aryl groups, and monohydroxyl compounds with a cycloaliphatic hydrocarbon substituted with halogen groups, and
• B2. at least one monohydroxy-monocarboxylic acid compound, the monohydroxy-monocarboxylic acid compound obtained by esterification of hydrocarboxylic acids or by reaction of a diol with at least one dicarboxylic acid or anhydride compound, to form an intermediate, followed by reacting the intermediate with
• C. a compound containing at least one basic ring nitrogen and a group which is capable of reacting with an isocyanate group;
• or
• b) an acrylic block copolymer dispersant prepared from an acrylate or methacrylate and at least one cationic monomer.