Blocked polyisocyanates with improved thermal stability

A blocked polyisocyanate containing at least one isocyanate group which is reversibly blocked with a monofunctional blocking agent for isocyanate groups and at least one isocyanate group in the form of a urea group, which may be converted to a thermally stable hydantoin group, wherein the blocked polyisocyanate corresponds to the formula ##STR7## wherein X represents an organic group which has a valency of n+1 and is inert towards isocyanate groups at a temperature of 100.degree. C. or less,

• R represents the residue obtained by removing the isocyanate groups from a polyisocyanate having a functionality of m+p,
• R.sub.1 and R.sub.2 may be the same or different and represent optionally substituted hydrocarbon radicals,
• R.sub.3 and R.sub.4 may be identical or different and represent hydrogen or organic groups which are inert towards isocyanate groups at a temperature of 100.degree. C. or less,
• Z represents --OH, --COOH, a polymer backbone or a group corresponding to the formula ##STR8## BI represents a reversible, monofunctional blocking agent for isocyanate groups,
• m has a value of 1 to 5,
• n has a value of 1 to 3,
• p has a value of 1 to 5 and
• m+p is 2 to 6.