Low viscosity polyuretidione polyurethanes and their use as curatives for solvent and water borne coatings

A process for preparing a polyuretidione polyurethane composition that is free of carbonate groups and carboxylic acid ester groups, said composition comprising, based on the weight of the polyuretidione polyurethane:

• (a) from 5% to 20%, by weight, of uretidione groups, calculated as(NCO) .sub.2 having a molecular weight of 84,
• (b) from 10 to 30%, by weight, of urethane groups, calculated as NHCO.sub.2 having a molecular weight of 59,
• (c) less than 2%, by weight, of free isocyanate groups, calculated as NCO having a molecular weight of 42, and
• (d) less than 7%, by weight, of isocyanurate groups, calculated as (NCO) .sub.3 having a molecular weight of 126,
• said process comprising reacting, at an equivalent ratio of isocyanate groups to hydroxyl groups of 1.0:1 to 0.7:1, a reaction mixture comprising:
• (A) a uretidione group-containing polyisocyanate having an average isocyanate functionality of between 2.0 and 2.4,
• (B) a diol, or a mixture of diols, that is free of carbonate groups and free of carboxylic acid ester groups, and has an average molecular weight of more than 76 but less than 676, with the proviso that, of the hydroxyl groups present, more than 40% of them are secondary hydroxyl groups or are in a position alpha to an alkyl chain branch point, and
• (C) not more than 40% by weight of mono-hydroxyl compound, based on the total weight of components (B) and (C) in said reaction mixture.