Low bake melamine cured coating compositions
The present invention provides reactive aqueous or solvent borne compositions comprising one or more low-temperature curing alkylether functional aminoplast resins having an average degree of polymerization of from 1.0 to 2.0, preferably alkylether group containing melamines, one or more hydroxyl group containing resins or polymers, such as acrylics and polyesters, one or more unblocked acid catalysts each having a pKa of 0.25 to 1.0, such as organic sulfonic acids, and one or more volatile amines, e.g. N-dipropylamine, that have a relative evaporation rate (RER) of 50-500, wherein, RER equals (p)(M)/11.6, with p being the vapor pressure of the amine in mm Hg and M being the molecular weight of the amine. The compositions cure at from 60° C. to 135° C., e.g. below 105° C., to form durable coatings suitable for plastic substrates, such as automotive plastics. The present invention also provides methods of forming the compositions comprising mixing the said aminoplast resins, hydroxyl group containing resins or polymers, unblocked acids and volatile amines to form a mixture, wherein the acids and the amines are added separately into the mixture.
1. An aqueous or solvent borne composition comprising one or more low-temperature curing alkylether functional aminoplast resins, one or more hydroxyl group containing resins or polymers, one or more unblocked acid catalysts, each having a pKa of 0.25 to 1.0, and one or more volatile amines having a relative evaporation rate (RER) of 50-500,
wherein, RER equals (p)(M)/11.6, with p being the vapor pressure of the amine in mm Hg and M being the molecular weight of the amine, and wherein the mole ratio of the said one or more volatile amines to the said one or more unblocked acid catalysts is from about 0.02 to 0.09.
2. A composition as claimed in claim 1 , wherein the said one or more low-temperature curing alkylether functional aminoplast resins comprises alkylether functional polyamino nuclei, wherein the said nuclei are chosen from melamine, acetoguanamine, adipoguanamine, and benzoguanamine.
3. A composition as claimed in claim 2 , wherein the said one or more low-temperature curing alkylether functional aminoplast resins comprise one or more alkylether group containing melamines having from 4 to 6 alkylether groups per melamine ring and an average degree of polymerization of from 1.0 to 2.0.
4. A composition as claimed in claim 3 , wherein the said average degree of polymerization is from 1.0 to 1.5.
5. A composition as claimed in claim 1 , wherein the said one or more hydroxyl group containing resins or polymers are chosen from acrylic polymers having an hydroxyl number of from 20 to 140 mg KOH/g, polyesters having an hydroxyl number of from 100 to 300 mg KOH/g and mixtures thereof.
6. A composition as claimed in claim 5 , wherein the said one or more hydroxyl group containing resins or polymers comprise acrylic polymers having a primary hydroxyl functionality, defined as number average molecular weight/primary hydroxyl equivalent weight, of from 2 to 10.
7. A composition as claimed in claim 1 , wherein the said one or more unblocked acid catalysts comprise a sulfonic acid.
8. A composition as claimed in claim 1 , wherein the said one or more volatile amines comprise a secondary amine.
9. A composition as claimed in claim 3 , wherein the said one or more alkylether group containing melamines comprise mixtures of methoxymethyl and butoxymethyl groups, such that the weight ratio of methoxymethyl to butoxymethyl groups ranges from 80:20 to 95:5.
10. A method of making the composition as claimed in claim 1 comprising mixing the one or more alkylether functional aminoplast resins, the one or more hydroxyl group containing resins or polymers, the one or more unblocked acids and the one or more volatile amines to form a mixture, such that the said one or more acids and the said one or more amines are added into the said mixture separately from one another.