Process for the production of polyisocyanates with a biuret structure

In the production of a polyurethane resin wherein a polyisocyanate is reacted with an active hydrogen containing material, the improvement wherein said polyisocyanate has a biuret structure and a maximum viscosity of 50,000 cP at 20.degree.C. and is produced by a process comprising:

• A. reacting (1) excess quantities of an organic diisocyanate having aliphatically bound isocyanate groups with (2) a biuretizing agent selected from the group consisting of water and organic compounds which convert organic isocyanates into the corresponding biurets at elevated temperatures, the substituents of said organic compounds being inert in the biuretization process and not forming a constituent of the biuret, at a temperature of from 60.degree. to 250.degree.C, and
• B. removing excess unreacted diisocyanate, said process characterized in that the organic diisocyanate and biuretizing agent are used in proportions corresponding to a molar ratio of diisocyanate to monofunctional biuretizing agent of at least 11:1.