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
2000-08-03
2002-02-26
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...
Reexamination Certificate
active
06350809
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to water-borne base coats which are suitable for preparing water-borne base coat/clear coat-two-layer coatings. The invention also relates to the use of water-borne base coats in processes for preparing water-borne base coat/clear coat-two-layer coatings.
BACKGROUND OF THE INVENTION
Top coatings in the automotive sector are currently almost universally applied as two-layer coatings of a color- and/or effect-imparting base coat layer and a transparent clear coat layer. In order to rationalise procedures there is a desire to apply such two-layer coatings essentially only onto outer surfaces (surfaces of a motor vehicle which are directly visible to the observer) and to apply onto inner surfaces (visible surfaces in the vehicle body interior, for example in the motor space, the passenger space or the trunk) solely base coat, i.e. to dispense with the clear coat layer in areas where less importance is given to optical appearance. The base coat layers not protected by a clear coat have to be tack-free (non-tacky) after stoving, first in order not to hinder the subsequent assembly steps and secondly in considerance of the practical value of the vehicle for the customer.
Due to environmental issues, water-borne base coats are preferably used for the preparation of color- and/or effect-imparting base coat/clear coat-two-layer coatings. It is thereby particularly environmentally acceptable to apply clear coat layers of powder clear coat.
In the preparation of water-borne base coat/powder clear coat-two-layer coatings an undesirable stoving-yellowing of the powder clear coat layer often occurs. By stoving-yellowing is meant an undesirable deviation in color tone of the stoved two-layer coating from the desired value and it is particularly noticeable if a base coat layer of a water-borne base coat of a light color shade is applied. The problem of stoving-yellowing of the powder clear coat is naturally enhanced due to the fact that the layer thickness of the powder clear coat layers is normally high.
An object of the invention is to provide water-borne base coats capable of being widely used for the preparation of water-borne base coat/clear coat-two-layer coatings. They should in particular be suitable for the preparation of water-borne base coat/powder clear coat-two-layer coatings which are desirably little prone to a stoving-yellowing of the powder clear coat layer. The base coat layers applied from water-borne base coats which are not coated over with clear coats, should be tack-free after stoving.
SUMMARY OF THE INVENTION
Experiments of the applicant have resulted in the object being achieved by suitably choosing the resin solid composition of the water-borne base coats.
The invention consists of providing water-borne base coats, the resin solid of which contains
40 to 90 wt-% of a water-dilutable polyester binder,
0 to 50 wt-% of one or more additional binders,
0 to 20 wt-% of one or more paste resins and
10 to 40 wt-% of one or more cross-linking agents,
wherein the water-dilutable polyester binder has a weight average molecular mass of 5000 to 50000, an acid value of 10 to 50 mg KOH/g, a hydroxyl value of 30 to 100 mg KOH/g and an average hydroxyl-functionality of 2 to 3 and consists of 5 to 25 wt-% of structural units—O—R—O— derived from one or more macrodiols having a hydroxyl value of 25 to 120 mg KOH/g, wherein R represents residues located between the terminal hydroxyl groups of the one or more macrodiols.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The resin solid of the water-borne base coats according to the invention contains 40 to 90 wt-%, preferably 50 to 80 wt-% of a water-dilutable polyester binder as precedingly defined. By “water-dilutable” we mean that the polyester binder is soluble in water or is dispersable in water after neutralization. The polyester can be prepared by conventional melt condensation of at least one polycarboxylic acid (or anhydride) and at least one polyol as well as at least one, preferably only one, macrodiol at temperatures of 160 to 240° C. These are processes which are conventionally used for preparing water-dilutable polyesters known to the person skilled in the art.
The polycarboxylic acids suitable for being used in the synthesis of the polyester are polycarboxylic acids having 4 to 36 carbon atoms. Examples are dimeric fatty acids as well as polycarboxylic acids, having a molecular weight ranging from 116 to 230, for example maleic acid, phthalic acid, isophthalic acid, trimellitic acid, dodecane dicarboxylic acid, tetrahydrophthalic acid, hexahydrophthalic acid, cyclohexane-1,2- and -1,4-dicarboxylic acid, sebacic acid, adipic acid. Where they exist, the corresponding acid anhydrides may also be used.
The polyols suitable for being used in the synthesis of the polyester are polyols with 2 to 36 carbon atoms. Examples are dimeric fatty alcohols as well as (cyclo)aliphatic polyols, which have a molecular weight ranging from 60 to 204, for example ethylene glycol, butanediol, hexanediol, decanediol, neopentylglycol, trimethylolpropane, trimethylolethane, glycerin, pentaerythrite, cyclohexanediol, cyclohexanedimethanol.
The macrodiols suitable for use in the synthesis of the polyester have the formula HO—R—OH, wherein R represents residues located between the terminal hydroxyl groups of the one or more macrodiols. The macrodiols have a hydroxyl value of 25 to 120, preferably of 45 to 120 mg KOH/g. Examples are the corresponding telechelic poly(meth)acrylate diols; polyurethane diols; polyester diols; polycarbonate diols; polyether diols, which can for example also be derived from epoxy resins; polybutadiene diols, which may be non-hydrogenated, partially or completely hydrogenated; alpha-omega-hydroxymethylpolysiloxanes. Polytetrahydrofurane diols are preferably used as the macrodiols in the synthesis of the polyester.
Polycarboxylic acids, polyols and macrodiol(s) are used in the preparation of the water-dilutable polyester in quantity proportions, such that a polyester with a weight average molecular mass of 5000 to 50000, preferably 10000 to 30000, an acid value of 10 to 50, preferably 20 to 30 mg KOH/g, a hydroxyl value of 30 to 100, preferably 40 to 90 mg KOH/g, and an average hydroxyl-functionality of 2 to 3, preferably 2.3 to 2.8 is obtained, wherein the polyester consists of 5 to 25, preferably 8 to 20 wt-% of structural units—O—R—O—derived from one or more macrodiols having a hydroxyl value of 25 to 120 mg KOH/g.
The polyester can be used as an organic solution or as an aqueous dispersion for the preparation of the water-borne base coats according to the invention. For example, the polyester can be dissolved in an organic solvent or in a mixture of organic solvents after the synthesis has been completed, and be converted into the aqueous phase after full or partial neutralisation with a base. A polyester solution at 50 to 80 wt-% can for example be prepared and be converted into an aqueous polyester dispersion by addition of a base and water.
Examples of suitable solvents are those which are at least partially water-miscible, such as mono- or polyhydric alcohols, for example propanol, butanol, hexanol; glycol ethers, for example diethylene glycol dialkyl ether, dipropylene glycol dialkyl ether, each having alkyl residues comprising one to six carbon atoms, ethoxypropanol, butyl glycol; N-alkylpyrrolidones, such as, for example N-methylpyrrolidone. These may be the same solvents used in the water-borne basecoats. Examples of suitable bases for neutralizing the polyester are ammonia, organic amines such as triethylamine, N-methylmorpholine or aminoalcohols such as dimethylisopropanolamine, dimethylethanolamine and 2-amino-2-methylpropanol-1. Before and/or after the addition of water, the organic solvent can be removed until the desired content is reached, for example by distilling off under reduced pressure. The aqueous polyester dispersions have a solid content of for example 35 to 65 wt-%.
Apart from the water-dilutable polyester, the resin solid of the water-borne base coats can
Goebel Armin
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Vogt-Birnbrich Bettina
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
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