Preparation of biuret-containing polyisocyanates

A process for the preparation of a polyisocyanate which contains one or more biuret groups by reacting

• a) an aliphatic or cycloaliphatic isocyanate containing two or more isocyanate groups (isocyanate (a)) with
• b) 0.5 to 20 mol % based on the isocyanate groups in (a) of a tertiary alcohol or a mixture of water and a tertiary alcohol (biuretizing agent (b)) at from 100 to 250° C., which comprises carrying out the reaction in the presence
• c) from 0.01 to 2.0 mol % based on the isocyanate groups in (a) of a stabilizer (c) selected from the group consisting of urea, ammonia, biuret, ethylene urea, a urea derivative of the formula I
• in which R1, R2, R3 and R4 are hydrogen, C1 to C10 alkyl or C6 to C10 aryl, or a carboxamide of the formula II
• in which R5 is C1 to C12 alkyl which is unsubstituted or in which 1, 2 or 3 hydrogen atoms are replaced by a radical