Modified aminoplast crosslinkers and powder coating compositions containing such crosslinkers

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

• (1) combining the following reactants:
• (A) at least one aminoplast resin;
• (B) at least one polyhydric aromatic compound;
• 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):
• 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):
• 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 paint of at least 80° C.,
• such that the ratio of moles of aminoplast resin (A) to the total combined moles of (B) and (C) ranges from 1.5 to 5.0:1, to form a reaction admixture;
• (2) heating the reaction admixture formed in step (1) to a temperature ranging from 90° to 135° C.;
• and
• (3) maintaining the temperature achieved in step (2) for a time sufficient to produce an ungelled reaction product having a glass transition temperature of at least 10° C. which is essentially free of hydroxyl functionality as determined by infrared spectroscopy.

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

• (1) a polymer having functional groups reactive with aminoplast and/or benzoxazine groups, said polymer having a glass transition temperature of at least 30° C.;
• and
• (2) a crosslinking agent having reactive benzoxazine groups comprising an ungelled reaction product of the following reactants:
• (A) at least one aminoplast resin;
• (B) at least one polyhydric aromatic compound;
• 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):
• 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):
• 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).

A multilayer composite coating composition comprising a base coat deposited from a base coating composition and a top coat deposited from a transparent powder top coating composition, the powder top coating composition comprising a solid particulate, film-forming mixture of the following components:

• (1) a polymer having functional groups reactive with aminoplast resin and/or benzoxazine groups, said polymer having a glass transition temperature of at least 30° C.;
• and
• (2) a crosslinking agent having reactive benzoxazine groups comprising an ungelled reaction product of the following reactants:
• (A) at least one aminoplast resin;
• (B) at least one polyhydric aromatic compound;
• 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):
• 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 (H) or (III):
• 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).