Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-11-27
2002-07-23
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S537000
Reexamination Certificate
active
06423771
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 stone-chip resistance.
2. Description of the Related Art
DE-A 39 18 510 describes conventional baking surfacers based on polyesters and blocked isocyanates (dissolved in organic solvents). There was a need to produce equivalent coatings using aqueous systems.
EP-A 0 249 727 describes aqueous surfacer coating compositions based on phosphoric-acid-modified epoxy resins, water-dilutable polyesters as known from AT-B 328 587, and water-dilutable melamine resins. DE-A 40 00 748 relates to aqueous surfacer coating compositions of water-dilutable hydroxyl-containing polyurethane resins, which may if desired include other, admixed binders, and aqueous amine resins as curing agents. Mixtures of water-dilutable polyurethane resins, water-dilutable, epoxy-resin-modified polyesters and optionally water-dilutable amino resins are known from DE-A 38 13 866. Other aqueous surfacer coating compositions based on carboxyl-containing polyesters or acrylic copolymers and water-soluble blocked isocyanate prepolymers, alone or in combination with water-dilutable amine resins, are described in DE-A 38 05 629. EP 0 594 685 describes the use of condensation products of carboxyl-containing polyurethane resins and hydroxyl-containing polyester resins, with or without urethane modification, together with water-insoluble blocked iso-cyanates 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 are still in need of improvement, especially as regards gloss and stone-chip resistance (especially at low temperatures).
SUMMARY OF THE INVENTION
It has now been found that by adding a water-insoluble, low molar mass polyester which is rich in hydroxyl groups to condensation products of hydroxyl group-containing and carboxyl group-containing resins, and combining this mixture with curing agents which become active on heating, it is possible to obtain coating compositions which in relation to the known systems possess, after baking, better gloss and improved stone-chip resistance even at low temperatures.
The invention accordingly provides a coating composition comprising
a condensation product A of a carboxyl group-containing resin A1 and a hydroxyl group-containing resin A2, 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,
a low molar mass, water-insoluble polyester B having a hydroxyl number of from 100 to 450 mg/g and a Staudinger Index (“limiting viscosity number”) as determined on a solution in dimethylformamide of from 2.5 to 6 cm
3
/g, obtainable by condensing aliphatic polyols B1 and aliphatic, cycloaliphatic or aromatic polycarboxylic acids B2, the average functionality (average number of the hydroxyl groups and/or acid groups per molecule, as applicable) of component B
1
preferably being greater by at least 0.2, preferably by at least 0.3, than that of component B2, and
a curing agent C which becomes active only at an elevated temperature of at least 80° C., with the proviso that at least 20% of the mass of the curing agent C is insoluble in water.
“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.
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 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 meaning 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 “d1/g”.
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
The condensation product A 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 in the condensation process in a mass ratio of preferably from 10:90 to 80:20, in particular from 15:85 to 40:60. 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 acrylate resins A15. 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 All may be prepared in a conventional manner from polyols A11 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, cyclohexane dicarboxylic acid, phthalic acid, isophthalic and terephthalic acids, trimellitic acid and trimesic acid, and benzophenone tetracarboxylic 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 rea
Dworak Gert
Staritzbichler Wrner
Michl Paul R.
ProPat L.L.C.
Solutia Austria GmbH
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