Process for the preparation of biuret polyisocyanates, the compounds obtained by this process and their use

A urethane-modified biuret polyisocyanate which is prepared by a process which comprises reacting

• (a) 1,6-diisocyanatohexane with subequivalent quantities of
• (b) a biuretizing agent comprising water and/or an organic compound which reacts with isocyanate groups to form biuret groups,
• to form biuret groups and subsequently removing unreacted excess, 1,6-diisocyanatohexane and any other volatile constituents present, the process additionally comprising adding prior to said removing step
• (c) about 1 to 50% by weight, based on the weight of said 1,6-diisocyanatohexane, of at least one diol containing ester groups and having an average molecular weight of 350 to 950
• to the reaction mixture to form urethane groups, provided that when said biuretizing agent comprises a cycloaliphatic or aliphatic diamine and temperatures of greater than 250.degree. C. are employed during the biuretizing reaction, the molecular weight of said polyester diol is above 500, the process being conducted such that
• (i) the reaction mixture contains at least 10% by weight, based on the weight of the reaction mixture excluding any inert solvents used, of unreacted 1,6-diisocyanatohexane after the reaction has been terminated and prior to said removing step and
• (ii) the molar ratio of biuret groups to urethane groups in the product is about 20:1 to 0.2:1.