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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to curable compositions comprising
polyepoxides and polyacid agents. More specifically, the present invention
relates to composite coatings which are color plus clear coatings wherein
the clear top coats are powder coatings. The invention also relates to the
process for preparing color plus clear coated articles and to the coated
articles themselves.
2. Brief Description of the Prior Art
Color plus clear coatings involving the application of a colored or
pigmented base coat to a substrate followed by the application of a
transparent or clear top coat to the base coat are becoming increasingly
popular as original finishes for automobiles. The color plus clear
coatings have outstanding appearance, manifesting in gloss and
distinctness of image (DOI). The nature of the clear coat is particularly
important for these properties.
Also, the nature of the base coats can be important for these properties.
Generally, base coats containing the likes of melamines that produce
volatile emissions during cure can adversely affect the appearance of the
color plus clear coatings. In the instance of powder coatings, the
emissions and associated problems can become more pronounced. This is
because powder coatings are generally baked to relatively high
temperatures at which volatile emissions are more readily produced.
It is an object of the present invention to provide the means for avoiding
problems of appearance in color plus clear coatings, particularly those
involving powder coatings.
SUMMARY OF THE INVENTION
In accordance with the foregoing, the present invention encompasses:
an improved composite multi-layered coating composed of a pigmented base
coat over which is applied to a clear powder top coat, in which the
improvement is a base coat which comprises a liquid crosslinkable
composition comprising a polyepoxide and a polyacid curing agent, and
optionally comprising an auxilliary curing agent.
The present invention also encompasses substrates having the composite
multi-layered coatings on their surfaces. The term "composite" is intended
to denote that there is a combination of the essential characteristics of
the individual coatings, i.e., the base and top coats to produce the final
coating of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The base coat is a crosslinkable composition comprising a polyepoxide and a
polyacid which comprises a half ester formed from reacting an acid
anhydride with a polyol.
Among the polyepoxides which can be used are epoxy-containing acrylic
polymers, which are preferred, epoxy condensation polymers such as
polyglycidyl ethers of alcohols and phenols and certain polyepoxide
monomers and oligomers.
The epoxy-containing acrylic polymer is a copolymer of an ethylenically
unsaturated monomer having at least one epoxy group and at least one
polymerizable ethylenically unsaturated monomer which is free of epoxy
groups. Examples of ethylenically unsaturated monomers containing epoxy
groups are those containing 1,2-epoxy groups and include glycidyl
acrylate, glycidyl methacrylate, and allyl glycidyl ether.
Examples of ethylenically unsaturated monomers which do not contain epoxy
groups are alkyl esters of acrylic and methacrylic acid containing from 1
to 20 atoms in the alkyl group. Specific examples of these are acrylates
and methacrylates such as methyl methacrylate, ethyl methacrylate, butyl
methacrylate, ethyl acrylate, butyl acrylate, and 2-ethylexyl acrylate.
Examples of other polymerizable ethylenically unsaturated monomers are
vinyl aromatic compounds, nitriles, vinyl and vinylidene halides, and
vinyl esters such as vinyl acetate. Acid group-containing copolymerizable
ethylenically unsaturated monomers such as acrylic and methacrylic acid
are preferably not used because of the possible reactivity of the epoxy
and acid group.
The epoxy group-containing ethylenically unsaturated monomer is preferably
used in amounts of from about 5 to 60, more preferably from 20 to 50
percent by weight of the total monomers used in preparing the
epoxy-containing acrylic polymer. Of the remaining polymerizable
ethylenically unsaturated monomers, preferably from 40 to 95 percent, more
preferably from 50 to 80 percent by weight of the total monomers, are the
alkyl esters of acrylic and methacrylic acid.
In preparing the epoxy-containing acrylic polymer, the epoxide functional
monomers and the other ethylenically unsaturated monomers can be mixed and
reacted by conventional free radical initiated solution polymerization as
generally described above.
The epoxy-containing acrylic polymer typically has a number average
molecular weight between about 500 and 20,000, preferably 1,000 to 10,000,
and more preferably from 1,000 to 5,000. The molecular weight is
determined by gel permeation chromatography using a polystyrene standard.
In determining molecular weights in this fashion, what is measured is not
the actual molecular weight but an indication of the molecular weight as
compared to a standard such as polystyrene. The values which are obtained
are commonly referred to as polystyrene numbers. However, for the purposes
of this invention, they are referred to as molecular weight.
The epoxy condensation polymers which are used are polyepoxides, having a
1,2-epoxy equivalency greater than 1, preferably greater than 1 and up to
about 3.0. Examples of such epoxides are polyglycidyl ethers of polyhydric
phenols and of aliphatic alcohols. These polyepoxides can be produced by
etherification of the polyhydric phenol or aliphatic alcohol with an
epihalohydrin such as epichlorohydrin in the presence of alkali.
Examples of suitable polyphenols are 2,2-bis(4-hydroxyphenyl)propane
(bisphenol A), 1,1-bis(4-hydroxyphenyl)ethane, and
2-methyl-1,1-bis(4-hydroxyphenyl)propane. Examples of suitable aliphatic
alcohols are ethylene glycol, diethylene glycol, 1,2-propylene glycol, and
1,4-butylene glycol. Also, cycloaliphatic polyols, such as
1,2-cyclohexanediol, 1,4-cyclohexanediol,
1,2-bis(hydroxymethyl)cyclohexane, and hydrogenated bisphenol A, can also
be used.
Besides the epoxy-containing polymers described above, certain polyepoxide
monomers and oligomers can also be used. Examples of these materials are
described in U.S. Pat. No. 4,102,942 in column 3, lines 1-16. Specific
examples thereof which are low molecular weight polyepoxides are
3,4-epoxycyclohexylmethyl, 3,4-epoxycyclohexanecarboxylate, and
bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate. These materials are
aliphatic polyepoxides as are the epoxy-containing acrylic polymers.
The polyepoxide is present in the crosslinkable composition in amounts of
about 10 to 90, preferably from 25 to 75 percent by weight based on total
weight of resin solids.
The polyacid contains two or more acid groups per molecule which are
reactive with the polyepoxide to form a crosslinked coating as indicated
by the resistance of the coating to organic solvent. The polyacid
comprises a half ester formed from reacting an acid anhydride with a
polyol. The acid functionality is preferably a carboxylic acid although
other acids, such as sulfonic acid, may be used but their use is not
preferred. The half esters are relatively low in molecular weight and
quite reactive with epoxies enabling the formulation of high solids fluid
compositions.
The half ester is obtained by reacting a polyol and a 1,2-acid anhydride
under conditions sufficient to ring open the anhydride and to form the
half ester with substantially no polyesterification occurring. The
reaction products are of relatively low molecular weight with narrow
molecular weight distributions and provide lower volatile organic contents
in the coating composition while still providing for excellent properties
in the resultant coating. By "substantially no polyesterification
occurring" is meant that the carboxyl groups of the anhydride are not
esterified by the polyol in a recurring manner. By this is meant that less
than 10, preferably less than 5 percent by weight polyester is formed.
Two reactions may occur in combining the anhydride and the polyol together
under suitable reaction conditions. The desired reaction mode involves
ring opening the anhydride ring with hydroxyl, i.e.,
##STR1##
where X is the residue of the polyol after reaction with the
1,2-dicarboxylic acid anhydride, R is an organic moiety associated with
the anhydride, and A is equal to at least 2.
Subsequently, carboxyl groups formed by opening of the anhydride ring may
react with hydroxyl groups to give off water via a condensation reaction.
This latter reaction is not desired since it can lead to polycondensation
reaction resulting in products with higher molecular weights.
For the desired ring opening reaction and half ester formation, a
1,2-dicarboxylic anhydride is used. Reaction of a polyol with a carboxylic
acid instead of an anhydride would require esterification by condensation
eliminating water which would have to be removed by distillation. Under
these conditions, this would promote undesired polyesterification. Also,
the reaction temperature is preferably low, that is, no greater than
135.degree. C., preferably less than 120.degree. C., and usually within
the range of 70.degree.-135.degree. C., preferably 90.degree.-120.degree.
C. Temperatures greater than 135.degree. C. are undesirable because they
promote polyesterification, whereas temperatures less than 70.degree. C.
are undesirable because of sluggish reaction. The time of reaction can
vary somewhat depending principally upon the temperature of reaction.
Usually the reaction time will be from as low as 10 minutes to as high as
24 hours.
To achieve the desired reaction, the 1,2-acid anhydride and polyol are
contacted usually by mixing the two ingredients together in a reaction
vessel. Preferably, the reaction is conducted in an inert atmosphere of,
say, nitrogen and in the presence of a solvent to dissolve the solid
ingredients and/or to lower the viscosity of the reaction mixture.
Examples of suitable solvents are high boiling materials and include, for
example, ketones such as methyl amyl ketone, diisobutyl ketone, methyl
isobutyl ketone; aromatic hydrocarbons such as toluene and xylene; as well
other organic solvents such as dimethyl formamide and
N-methyl-pyrrolidone.
The equivalent ratio of anhydride to hydroxy on the polyol is preferably at
least about 0.8:1 (the anhydride being considered monofunctional) in order
to obtain maximum conversion to the desired half ester. Ratios less than
0.8:1 can be used but such ratios result in increased formation of less
preferred half esters.
Among the anhydrides which can be used in the formation of the desired half
esters are those which contain from about 2 to 30 carbon atoms, exclusive
of the carbon atoms of the anhydride moiety. Examples thereof are
aliphatic, including cycloaliphatic, olefinic and cycloolefinic and
aromatic anhydrides. Substituted aliphatic and aromatic anhydrides are
also included within the definition of aliphatic and aromatic provided the
substituents do not adversely affect the reactivity of the anhydride or
the properties of the resultant polyester. Examples of substituents would
be chloro, alkyl, and alkoxy. Specific examples of anhydrides include
succinic anhydride, methylsuccinic anhydride, dodecenyl succinic
anhydride, octadecenylsuccinic anhydride, phthalic anhydride,
tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride,
hexahydrophthalic anhydride, and alkyl hexahydrophthalic anhydrides such
as methylhexahydrophthalic anhydride, tetrachlorophthalic anhydride,
endomethylene tetrahydrophthalic anhydride, chlorendic anhydride, itconic
anhydride, citraconic anhydride, and maleic anhydride.
Among polyols which can be used in the formation of the desired half esters
are those which contain from about 2 to 20 carbon atoms. Preferred are
diols, triols, and mixtures thereof. Specific examples include aliphatic
polyols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol,
1,4-butanediol, 1,5-pentanediol, glycerol, 1,2,3-butanetriol,
1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol,
1,4-cyclohexanedimethanol, trimethylolpropane,
2,2,4-trimethylpentane-1,3-diol, pentaerythritol and
1,2,3,4-butane-tetrol. Aromatic polyols such as bisphenol A and
bis(hydroxymethyl)xylene can also be used.
The half ester is present in the coating composition in amounts of about 10
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