Aminoplast-based crosslinkers and powder coating compositions containing such crosslinkers

A method for preparing a powder crosslinking agent comprising the following steps:

• (1) reacting the following reactants:
• (a) a polyester polyol, and
• (b) a mono-isocyanate
• in a molar ratio of reactant (a) to reactant (b) ranging from 1:1.8 to 2.0 to form a urethane group-containing reaction product;
• (2) combining the reaction product of step (1) with an aminotriazine compound having one or less non-alkylated NH bond per triazine ring in a molar ratio of urethane group-containing reaction product to aminotriazine compound ranging from 1:1.5 to 3.2 to form a reaction admixture;
• and
• (3) heating the reaction admixture of step (2) to a temperature ranging from 95° C. to 135° C. for a time sufficient to form a powder crosslinking agent having a glass transition temperature of at least 25° C. which is essentially free of urethane NH functionality as determined by infrared spectroscopy.

A curable powder coating composition comprising a solid particulate film-forming mixture of the following components:

• (A) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30° C.;
• and
• (B) a crosslinking agent having functional groups reactive with the functional groups of (A),
• said crosslinking agent comprising the ungelled reaction product of the following:
• (1) a reactive urethane group-containing reaction product of the following reactants:
• (a) a polyester polyol, and
• (b) a mono-isocyanate;
• and
• (2) an aminotriazine compound having one or less non-alkylated NH bond per triazine ring, wherein said crosslinking agent is essentially free of urethane N—H functionality and has a glass transition temperature of at least 10° C.