Process for producing barium sulfate with reactive surface

Claim 1

A process for producing a barium sulfate pigment having a chemically reactive halogen, hydroxyl, amino, imino, mercapto or terminal olefinic functional group bonded to its surface, said functional group capable of effecting a covalent interaction between said pigment and an organic resin, which comprises the steps of: (a) coprecipitating barium sulfate and a poorly soluble barium salt of an additional anion by providing barium ions in the presence of sulfate ions in an aqueous medium containing said additional anion in the form of a water-soluble organic salt or compound selected form the group consisting of alkali metal alkyl or aryl sulfonates, alkali metal alkyl or aryl sulfates, and alkyl or aryl phosphoric acid esters, wherein said alkyl or aryl groups are unsubstituted or substituted by halogen, hydroxyl, amino, imino or mercapto, or contain a terminal olefinic functional group, said additional anion being corprecipitable with barium ions to form said poorly water-soluble barium salt; (b) recovering the resulting corpreciate from the aqueous medium; and (c) drying the coprecipitate and afterwards treating the corprecipitate with a silane or siloxane.