Heat-curable coating materials

Details Heat-curable coating materials Heat-curable coating materials

1999-02-10

2001-02-20

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C525S438000

Reexamination Certificate

active

06191195

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to heat-curable coating compositions, also called powder coating materials, consisting of a carboxyl-functional polyester, and an epoxy resin and a special stabilizer.
2. Description of the Prior Art
Powder coating materials whose binders comprise carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin, so-called hybrid powder coating materials, have already been known for a relatively long time; see e.g. T. A. Misev: Powder Coatings (Chemistry and Technology), 1991 and David A. Bates: The Sciences of Powder Coatings, 1990.
Powder coating materials based on carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin possess outstanding mechanical properties but only little weathering stability and are therefore unsuitable for use in the exterior sector.
A disadvantage of powder coating materials whose binders comprise carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin is, furthermore, the low yellowing resistance in directly gas- and/or oil-heated ovens in comparison to electrically heated ovens. In directly gas- and oil-heated ovens the coated parts are in direct contact with the combustion gases of the gas- and/or oil-heated burners during the curing operation. In powder coating materials whose binders comprise carboxyl-function polyesters and epoxy resins based on bisphenol A and epichlorohydrin this leads to relatively severe yellowing of the cured powder coating film in comparison to powder coating films cured in electrically heated ovens.
SUMMARY OF THE INVENTION
The aim of the present invention is to eliminate this disadvantage of the lower stability of powder coating materials whose binders comprise carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin in directly gas- and/or oil-heated ovens and to propose a corresponding powder coating material and a process for its preparation.
This object is achieved by the features of claim
1
in respect of a powder coating material and by the features of claim
5
in respect of the process. The subclaims indicate advantageous developments.
It has surprisingly been found that by the addition of compounds of the formula (I)-(X) this disadvantage of the lower yellowing stability in directly gas- and/or oil-heated ovens can be eliminated.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the invention, the compounds of the general formula I-X are employed for this purpose. The coating material consists accordingly of: (A) a carboxyl-functional polyester prepared by a condensation reaction of aliphatic and/or cycloaliphatic polyols with aliphatic and/or cycloaliphatic and/or aromatic polycarboxylic acid and anhydrides, where the polyester has a Tg in the range from 30° C. to 80° C. and an acid number of from about 20 to 100 [mg of KOH/g]; (B) an epoxy resin based on bisphenol A and epichlorohydrin, where the equivalents ratio of epoxy equivalents to carboxylic acid equivalents lies within the range from 0.6 to 1.2; and (C) a compound of the formulae (I)-(X)
I: with R
1
=OH, R
2
=OR
7
, R
3
, R
4
, R
5
, R
6
, R
7
=H or C
1
-C
10
-alkyl linear and/or branched
II: with R
1
=OH, R
4
=OR
7
, R
2
, R
3
R
5
, R
6
, R
7
=H or C
1
-C
10
-alkyl linear and/or branched
III: with R
1
=OH, R
2
=OR
9
, R
3
, R
4
, R
5
, R
6
, R
7
, R
8
, R
9
=H or C
1
-C
10
-alkyl linear and/or branched
IV: with R
1
=OH, R
4
=OR
9
, R
2
, R
3
, R
5
, R
6
, R
7
, R
8
, R
9
=H or C
1
-C
10
-alkyl linear and/or branched
V: with R
1
=OH, R
8
=OR
9
, R
2
, R
3
, R
4
, R
5
, R
6
, R
7
, R
9
=H or C
1
-C
10
-alkyl linear and/or branched
VI: with R
2
=OH, R
6
=OR
9
, R
2
, R
3
, R
4
, R
5
, R
7
, R
8
, R
9
=H or C
1
-C
10
-alkyl linear and/or branched
VII: with R
1
=OH, R
4
=OR
11
, R
2
, R
3
, R
5
, R
6
, R
7
, R
8
, R
9
, R
10
, R
11
=H or C
1
-C
10
-alkyl linear and/or branched
VIII: with R
2
=OH, R
6
=OR
11
, R
1
, R
3
, R
4
, R
5
, R
7
, R
8
, R
10
, R
11
=H or C
1
-C
10
-alkyl linear and/or branched
IX: with R
9
=OH, R
10
=OR
11
, R
1
, R
2
, R
3
, R
4
, R
5
, R
6
, R
7
, R
8
, R
11
=H or C
1
-C
10
-alkyl linear and/or branched
X: with R
9
—OH, R
10
=OR
11
, R
1
, R
2
, R
3
, R
4
, R
5
, R
6
, R
7
, R
8
, R
11
=H or C
1
-C
10
-alkyl linear and/or branched
In the context of the compounds I-X, particular preference is given to hydroquinone, toluhydroquinone and mono-tert-butylhydroquinone.
Suitable carboxyl-functional polyesters can be prepared in accordance with the prior art by the condensation processes known for polyesters (esterification and/or transesterification). It is also possible if desired to use suitable catalysts, such as, for example, dibutyltin oxide or titanium tetrabutylate.
As acid components, carboxyl-functional copolyesters include primarily aromatic polybasic carboxylic acids, such as terephthalic acid, isophthalic acid, phthalic acid, pyromellitic acid, trimellitic acid, 3,6-dichlorophthalic acid, tetrachlorophthalic acid and—where available—their anhydride, chloride or esters. In most cases they contain at least 50 mol % of terephthalic acid and/or isophthalic acid, preferably 80 mol %. The remainder of the acids (the difference from 100 mol %) consists of aliphatic and/or cycloaliphatic polybasic acids, such as 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroendomethyleneterephthalic acid, hexachloro-phthalic acid, azelaic acid, sebacic acid, decane-dicarboxylic acid, adipic acid, dodecane-dicarboxylic acid, succinic acid, maleic acid, or dimeric fatty acids. Hydroxycarboxylic acids and/or lactones, such as 12-hydroxystearic acid, epsilon-caprolactone or hydroxypivalic esters of neopentyl glycol, can likewise be used.
Also employed in minor amounts are monocarboxylic acids, such as benzoic acid, tertiary-butylbenzoic acid, hexahydrobenzoic acid and saturated aliphatic monocarboxylic acids.
Suitable alcohol components that may be mentioned are aliphatic diols, such as ethylene glycol, 1,3-propanediol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,2-dimethyl-1,3-propanediol, (neopentyl glycol), 2,5-hexanediol, 1,6-hexanediol, 2,2-[bis(4-hydroxycyclohexyl)]propane, 1,4-dimethylolcyclohexane, diethylene glycol, di-propylene glycol and 2,2-bis[4-(2-hydroxy)]-phenylpropane. Also employed in relatively minor amounts are polyols, such as glycerol, hexanetriol, pentaeryltritol [sic], sorbitol, trimethylolethane, trimethylolpropane and tris(2-hydroxy) isocyanurate [sic].
In addition, epoxy compounds are used instead of diols or polyols. Preferably, the proportion of neopentyl glycol and/or propylene glycol in the alcohol component is at least 50 mol %, based on total acids.
The epoxy resins based on bisphenol A and epichlorohydrin that are used are, for example, diglycidyl ethers of bisphenol A and its higher adducts. Various commercially available epoxy resins are suitable, such as Grilonit-Harze® from EMS-Chemie AG, Epikote-Harze® from Shell, Araldit-Harze® from Ciba and DER-Harze® from Dow Chem. Corp. In general, these resins have an average molecular weight of between 300 and 4000 and an epoxy equivalent weight of between 150 and 2000 (g/eq.).
The equivalents ratio of epoxy equivalents to carboxylic acid equivalents lies within the range of 0.6 to 1.2, preferably from 0.8 to 1.2.
The proportion of the compounds according to the general formulae I-X lies, based on the overall material, between 0.001% by weight and 10% by weight, preferably between 0.01 and 5% by weight.
The invention relates additionally to a process for preparing powder coating materials.
For the preparation of the powder coating materials in this case it is possible to use the customary pigments and/or fillers and/or additives.
These are additives from the group

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