Aqueous coating composition

Coating processes – With post-treatment of coating or coating material – Heating or drying

Reexamination Certificate

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C523S404000, C525S165000, C525S172000, C525S418000, C525S419000, C528S271000, C528S297000, C528S405000, C528S421000

Reexamination Certificate

active

06406753

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an aqueous coating composition suitable in particular for producing coatings on automobile parts. The parts thus coated, especially those where the coating composition described here is used as surfacer coat, are notable for high hardness and freedom from defects of the coating film and for high stone-chip resistance.
2. Description of the Related Art
EP-A 0 594 685 relates to the use of condensation products of carboxyl group-containing polyurethane resins and hydroxyl group-containing polyester resins, with or without urethane modification, together with water-insoluble blocked isocyanates for producing stoving enamels. An improvement of this formulation with reduced sensitivity to so-called overbaking is known from EP-A 0 548 873, the improvement being achieved by adding a water-soluble amine resin as crosslinker.
All of these known systems must be cured by heating to temperatures at which either the blocked isocyanate curing agent is at least partly deblocked and so becomes active or the amino resin curing agents exhibit sufficient (crosslinking) reactivity. The object was therefore to provide aqueous coating compositions which are suitable, inter alia, for producing surfacer coats in automotive finishing and which cure even at room temperature or only slightly elevated temperature (up to 120° C., preferably up to 100° C., and in particular up to not more than 90° C.) to give paint films having properties at least equal to those of the prior art.
SUMMARY OF THE INVENTION
It has now been found that by using a combination of water-insoluble unblocked isocyanates and water-soluble or water-dispersible, partly etherified amino resins as curing agents together with anionically stabilized, hydroxyl group-containing resins, especially condensation products of hydroxyl group-containing and carboxyl group-containing resins, it is possible to obtain coating compositions which, compared to the known systems, exhibit, after curing, defect-free films, a higher film hardness and good stone-chip resistance even at low temperatures.
The invention therefore provides a coating composition comprising
an anionically stabilized, hydroxyl group-containing resin A,
and
a curing agent C comprising a water-insoluble unblocked isocyanate C1 and a hydrophilic partly etherified amino resin C2.
The term “anionically stabilized” here is intended to denote that the resin in question has acid groups in an amount sufficient such that, with at least partial neutralization of the acid groups by addition of alkali in a mixture with water, a solution (single-phase mixture) or a dispersion (multiphase mixture) is formed which does not undergo separation spontaneously or on storage at room temperature for at least 7 days.
“Water-insoluble” is a term used to refer to those compounds for which, following the achievement of equilibrium at 20° C. with an amount of water the mass of which is ten times that of the compound in question, less than 5% of the mass of the compound that is used is present in solution in the aqueous phase.
Further objects, features and advantages of the invention will become apparent from the detailed description of the preferred embodiments that follows.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferably, the anionically stabilized, hydroxyl group-containing resin A is a condensation product Ak of a resin A1 containing acid groups and a resin A2 containing hydroxyl groups, A1 preferably having an acid number of from 100 to 230 mg/g, in particular from 120 to 160 mg/g, and A2 preferably having a hydroxyl number of from 50 to 500 mg/g, in particular from 60 to 350 mg/g.
The acid number is defined in accordance with DIN 53 402 as the ratio of the mass m
KOH
of potassium hydroxide required to neutralize the sample under analysis to the mass m
B
of this sample (mass of the solids in the sample in the case of solutions or dispersions); its customary unit is “mg/g”. The hydroxyl number is defined in accordance with DIN 53 240 as the ratio of that mass m
KOH
of potassium hydroxide which has exactly the same number of hydroxyl groups as the sample under analysis to the mass m
B
of this sample (mass of the solids in the sample in the case of solutions or dispersions); its customary unit is “mg/g”.
The condensation product Ak preferably has an acid number of from 25 to 75 mg/g, in particular from 30 to 50 mg/g. Its Staudinger Index (“limiting viscosity number”) is usually from 10 to 20 cm
3
/g, in particular from 12 to 19 cm
3
/g, and with particular preference from 13 to 18 cm
3
/g. It is prepared using components A1 and A2 preferably in a mass ratio of from 10:90 to 80:20, in particular from 15:85 to 40:60.
The formerly so-called “limiting viscosity number”, called “Staudinger Index” J
g
in accordance with DIN 1342, Part 2.4, is the limiting value of the Staudinger function J
v
at decreasing concentration and shear stress, J
v
being the relative change in viscosity based on the mass concentration &bgr;
B
=m
B
/V of the dissolved substance B (with the mass m
B
of the substance in the volume V of the solution); i.e., J
v
=(&eegr;
r
−1)/&bgr;
B
. Here, &eegr;
r
−1 is the relative change in viscosity, in accordance with &eegr;
r
−1=(&eegr;−&eegr;
s
)/&eegr;
s
. The relative viscosity &eegr;
r
is the ratio of the viscosity &eegr; of the solution under analysis and the viscosity &eegr;
s
of the pure solvent. (The physical definition of the Staudinger Index is that of a specific hydrodynamic volume of the solvated polymer coil at infinite dilution and in the state of rest.) The unit commonly used for J is “cm
3
/g”; formerly often “dl/g”.
The resins A1 containing carboxyl groups are preferably selected from polyester resins A11, polyurethane resins A12, the so-called maleate oils A13, the graft products A14 of fatty acids and fatty acid mixtures grafted with unsaturated carboxylic acids, and the acrylate resins A15. Instead of or in a mixture with resins containing carboxyl groups, it is also possible to use epoxy resins modified with phosphoric acid and/or phosphonic acids, or similarly modified reaction products of epoxy resins with fatty acids, referred to comprehensively as A16.
Preferably, the acid number of the resins A1 is from 100 to 230 mg/g, in particular from 70 to 160 mg/g. Its Staudinger Index, measured in dimethylformamide as solvent at 20° C., is generally from about 6.5 to 12 cm
3
/g, preferably from 8 to 11 cm
3
/g.
Suitable polyester resins A11 may be prepared in a conventional manner from polyols A111 and polycarboxylic acids A112, where also some—preferably up to 25% of the amount of substance—of the polyols and polycarboxylic acids can be replaced by hydroxycarboxylic acids A113. By appropriate choice of the nature and amount of the starting materials A111 and A112 it is ensured that the resulting polyester has a sufficient number of acid groups, in accordance with the acid number indicated above. The polyols A111 are preferably selected from aliphatic and cycloaliphatic alcohols having 2 to 10 carbon atoms and on average at least two hydroxyl groups per molecule; glycol, 1,2- and 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, di- and triethylene glycol, di- and tripropylene glycol, glycerol, trimethylolpropane and trimethylolethane are particularly suitable. Suitable polycarboxylic acids A112 are aliphatic, cycloaliphatic and aromatic polycarboxylic acids such as adipic acid, succinic acid, cyclohexanedicarboxylic acid, phthalic acid, isophthalic and terephthalic acids, trimellitic acid, trimesic acid and benzophenonetetracarboxylic acid. It is also possible to use compounds having both carboxylic acid groups and sulfonic acid groups, such as sulfoisophthalic acid, for example.
Suitable polyurethane resins A12 may be prepared by reacting aliphatic polyols A121, as defined under A111, hydroxyalkanecarboxylic acids A122 having at least one, preferably two, hydroxyl groups and a carboxyl group which under esterification cond

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