Modified aminoplast crosslinkers and powder coating compositions containing such crosslinkers

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

• (I) combining the following reactants:
• (a) at least one aminoplast resin;
• (b) at least one polyfunctional polymer comprising functional groups reactive with aminoplast resin (a) and having a glass transition temperature of at least 15° C.;
• and
• (c) at least one compound different from (b) having active hydrogen groups reactive with aminoplast resin (a), said compound selected from at least one of:
• (i) compounds having the following structure (I):
• 13
• wherein X is aromatic;
• R1, R2, and R3 can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R1, R2, and R3 represents an active hydrogen-containing group which is reactive with the aminoplast resin (a);
• (ii) compounds having the following structure (II) or (III):
• 14
• where R′ and R″ are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms;
• an d
• (iii) compounds different from (i) and (ii) and having a melting point of at least 80° C., to form a reaction admixture;
• (II) heating the reaction admixture formed in step (1) to a temperature ranging from 90° C. to 135° C.;
• and
• (III) maintaining the temperature achieved in (II) for a time sufficient to obtain an ungelled reaction product having a glass transition temperature of at least 15° C. and which is essentially free of active hydrogen-containing groups as determined by infrared spectroscopy.

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

• (1) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30° C.;
• and
• (2) a crosslinking agent having functional groups reactive with the functional groups of component (1),
• said crosslinking agent comprising an ungelled reaction product of the following reactants:
• (a) at least one aminoplast resin;
• (b) at least one polyfunctional polymer comprising functional groups reactive with aminoplast resin (a) and having a glass transition temperature of at least 15° C.;
• and
• (c) at least one compound different from (b) having active hydrogen-containing groups reactive with aminoplast resin (a), said compound selected from at least one of:
• (i) compounds having the following structure (I):
• 15
• wherein X is aromatic;
• R1, R2, and R3 can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R1, R2, and R3 represents an active hydrogen-containing group which is reactive with the aminoplast resin (a);
• (ii) compounds having the following structure (II) or (III):
• 16
• where R′ and R″ are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms;
• and
• (iii) compounds different from (i) and (ii) and having a melting point of at least 80° C.,
• wherein said crosslinking agent has a glass transition temperature of at least 10° C. and is essentially free of functional groups which are reactive with aminoplast resin.

A multilayer composite coating composition comprising a base coat deposited from a film-forming coating base coating composition and a top coat over at least a portion of the base coat deposited from a curable powder top coating composition comprising a solid particulate film-forming mixture of the following components:

• (1) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30° C.;
• and
• (2) a crosslinking agent having functional groups reactive with the functional groups of the polymer (1), said crosslinking agent comprising an ungelled reaction product of the following reactants:
• (a) at least one aminoplast resin;
• (b) at least one polyfunctional polymer comprising functional groups reactive with aminoplast resin (a) and having a glass transition temperature of at least 15° C.;
• and
• (c) at least one compound different from (b) having active hydrogen-containing groups reactive with aminoplast resin (a), said compound selected from
• (i) compounds having the following structure (I):
• 17
• wherein X is aromatic;
• R1, R2, and R3 can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R1, R2, and R3 represents an active hydrogen-containing group which is reactive with the aminoplast resin (a);
• (ii) compounds having the following structure (II) or (III):
• 18
• where R′ and R″ are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms;
• and
• (iii) compounds different from (i) and (ii) and having a melting point of at least 80° C., and mixtures thereof,
• wherein said crosslinking agent has a glass transition temperature of at least 10° C. and is essentially free of functional groups which are reactive with aminoplast resin.