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Description  |
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FIELD OF THE INVENTION
The present invention concerns an epoxy-based film-forming composition. In
particular, it concerns a stable and etch-resistant film-forming
composition which is particularly useful in color-plus-clear coating
systems.
BACKGROUND OF THE INVENTION
Color-plus-clear coating systems involving the application of the colored
or pigmented basecoat to a substrate followed by the application of a
transparent or clear topcoat to the basecoat are becoming increasingly
popular as original finishes for automobiles. The color-plus-clear systems
have outstanding gloss and distinctness of image, and the clear topcoat is
particularly important for these properties.
Two-component clearcoat compositions comprising polyisocyanate curing
agents and polyols as polyester polyols, polyurethane polyols and acrylic
polyols give outstanding gloss and distinctness of image. However, the
polyisocyanates are difficult to handle because they are sensitive to
moisture and require cumbersome safety precautions because of their
toxicity. U.S. Pat. Nos. 4,650,718, 4,681,811, 4,703,101, and 4,764,430
disclose color-plus-clear coatings employing polyepoxides and polyacid
curing agents which overcome many problems of polyisocyanate curing
agents, but which are still useful as original finishes for automobiles.
A recognized limitation for commercial polyepoxide-based coatings is that
commercial embodiments of such coatings are typically two-component
systems. Because the polyepoxides and polyacids are typically highly
reactive to provide a thorough cure, these components cannot be mixed for
a long period of time prior to application. Otherwise, unacceptable
increases in viscosity are encountered. Two-component systems require two
reservoirs for the different components, as well as separate feed lines
and mixing apparatus.
Single-component aminoplast-cured polyol coatings are well-known and
provide many excellent properties. However, it is widely recognized that
such coatings have poor resistance to etching by acid. Because many
geographic areas encounter acid precipitation, these coatings are not
highly effective for providing protection for acid resin.
The present invention provides a film-forming composition, which is
particularly useful in color-plus-clear coating systems, which has
improved stability and improved etch resistance properties. The
composition has outstanding gloss and distinctness of image so that the
coating is useful as an original finish for automobiles.
SUMMARY OF THE INVENTION
The present invention is directed toward a one package stable
etch-resistant film-forming composition which includes a polyepoxide
having an epoxy equivalent weight on resin solids of less than about 600
and a polyacid curing agent having an average acid functionality of
greater than 2. The composition is further characterized in that it is
substantially free of basic esterification catalyst and it has a cured
softening point of greater than about 20.degree. C.
The present invention further includes a process for applying a composite
coating to a substrate which includes applying an acid-catalyzed
thermosetting film-forming composition to a substrate to form a basecoat
followed by applying a stable etch-resistant film-forming composition to
the basecoat. The topcoat includes a polyepoxide having an epoxy
equivalent weight on resin solids of less than about 600 and a polyacid
curing agent having an average acid functionality of greater than 2. The
topcoat is further characterized in that it is substantially free of basic
esterification catalyst and it has a cured softening point of greater than
about 20.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a one package stable etch-resistant film-forming
composition. The composition includes a high functionality polyepoxide and
a polyacid curing agent having high acid functionality. The composition is
further characterized in that it is substantially free of basic
esterification catalyst, it has a cured softening point of above about
20.degree. C. and it is stable for use as a single-component coating
composition. This composition is particularly useful as a clear topcoat in
a color-plus-clear system. Further, the present composition provides
excellent etch resistance in terms of acid etching and water spotting. In
addition, the present composition is particularly useful in conjunction
with a high solids acid-catalyzed basecoat because the lack of basic
esterification catalyst in the topcoat allows for the use of a strong acid
catalyst in the basecoat without cure inhibition. Since a strong acid
catalyst can be used in the basecoat, it is possible to use a high solids
basecoat and still get acceptable cure.
The polyepoxide of the present composition has a high epoxy functionality
(corresponds to low epoxide equivalent weight). This aspect of the
polyepoxide component of the present invention is important to obtain good
cure and acceptable etch resistance. More specifically, the polyepoxide of
the present invention has an epoxide equivalent weight on resin solids of
less than about 600, more preferably less than about 400, and most
preferably less than about 300.
The polyepoxide of the present invention also preferably has a relatively
low molecular weight. This aspect of the polyepoxide component of the
present invention is useful in achieving acceptable stability and high
solids content. More specifically, the polyepoxide of the present
invention has a weight average molecular weight of less than about 20,000,
more preferably less than about 10,000, and most preferably less than
about 5,000.
The polyepoxide of the present invention is further characterized as
providing the film-forming composition with a high cured softening point.
The term "cured softening point" refers to the softening point of a cured
material of about 1 to 2 mils in thickness as measured by the following
procedure. The material in which the cured softening point is to be
measured is applied in two coats with a ninety second 75.degree. F. flash
between coats to a steel substrate coated with corrosion resistant primer
and basecoat as described below in the Examples or other similarly treated
substrate. The coating is allowed to air flash at 75.degree. F. for
fifteen minutes before baking at 275.degree. F. for 30 minutes to cure.
The coated substrate is heated with a thermal mechanical analyzer, such as
a Perkin-Elmer TMS-2, from -25.degree. C. to 150.degree. C. at a heating
rate of 10.degree. C./min. A penetration probe having a hemispherical tip
with a diameter of about 0.089 cm. and a net load of 5 grams is applied. A
cured softening point temperature is the mean value of at least three
separately determined temperatures at which there is a deflection from the
baseline in a plot of indentation versus temperature. The cured softening
point of the film-forming composition is at least above about 20.degree.
C., more preferably above about 50.degree. C. and most preferably above
about 60.degree. C.
The polyepoxide of the present invention is further characterized as having
a high calculated glass transition temperature (Tg). Tg can be calculated
as described by Fox in Bull. Amer. Physic. Soc., 1, 3, page 123 (1956).
The calculated Tg of the polyepoxide is sufficiently high such that, in
conjunction with other components of the composition, a cured softening
point of the film-forming composition as described above is achieved. It
is recognized that softening point temperatures are related to glass
transition temperatures and that softening occurs during glass transition.
More specifically, the polyepoxide of the present invention typically has
a calculated Tg of greater than about 20.degree. C., more preferably
greater than about 50.degree. C., and most preferably greater than about
60.degree. C. It should be noted that of the various specific
epoxy-functional acrylic resins prepared as discussed below,
epoxy-functional acrylic resins prepared using methacrylates, styrenes and
mixtures thereof have particularly high Tg values.
Among the polyepoxides which can be used are epoxy-containing acrylic
polymers, epoxy condensation polymers such as polyglycidyl ethers of
alcohols and phenols, polyglycidyl esters of polycarboxylic acids, certain
polyepoxide monomers and oligomers and mixtures of the foregoing.
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 acrylates and
methacrylates include methyl methacrylate, ethyl methacrylate, butyl
methacrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate.
Examples of other copolymerizable ethylenically unsaturated monomers are
vinyl aromatic compounds such as styrene and vinyl toluene; nitriles such
as acrylonitrile and methacrylonitrile; vinyl and vinylidene halides such
as vinyl chloride and vinylidene fluoride and vinyl esters such as vinyl
acetate.
The epoxy group-containing ethylenically unsaturated monomer is preferably
used in amounts of from about 20 to 90, more preferably from 30 to 70
percent by weight of the total monomers used in preparing the
epoxy-containing acrylic polymer. Of the remaining polymerizable
ethylenically unsaturated monomers, preferably from 10 to 80 percent, more
preferably from 30 to 70 percent by weight of the total monomers are the
alkyl esters of acrylic and methacrylic acid.
The acrylic polymer may be prepared by solution polymerization techniques
in the presence of suitable catalysts such as organic peroxides, such as
t-butyl perbenzoate, t-amyl peracetate or ethyl-3,3-di(t-amylperoxy)
butyrate or azo compounds, such as benzoyl peroxide, N,N'-azobis
(isobutyronitrile) or alpha, alpha-dimethylazobis(isobutyronitrile). The
polymerization can be carried out in an organic solution in which the
monomers are soluble. Suitable solvents are aromatic solvents such as
xylene and toluene, ketones such as methyl amyl ketone or ester solvents
such as ethyl 3-ethoxypropionate. Alternately, the acrylic polymer may be
prepared by aqueous emulsion or dispersion polymerization techniques.
The epoxy condensation polymers which are used are polyepoxides, that is,
those having a 1,2-epoxy equivalency greater than 1, preferably greater
than 1 and up to about 5.0. A useful example of such epoxides are
polyglycidyl esters from the reaction of polycarboxylic acids with
epihalohydrin such as epichlorohydrin. The polycarboxylic acid can be
formed by any method known in the art and in particular, by the reaction
of aliphatic alcohols with an anhydride, and in particular, diols and
higher functionality alcohols. For example, trimethylol propane or
pentaerythritol can be reacted with hexahydrophthalic anhydride to produce
a polycarboxylic acid which is then reacted with epichlorohydrin to
produce a polyglycidyl ester. Such compounds are particularly useful
because they are low molecular weight. Accordingly, they have low
viscosity and therefore, high solids coatings compositions can be prepared
with them. Additionally, the polycarboxylic acid can be an acid-functional
acrylic polymer.
Further 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) and 1,1-bis(4-hydroxyphenyl)ethane. Examples of suitable
aliphatic alcohols are ethylene glycol, diethylene glycol,
pentaerythritol, trimethylol propane, 1,2-propylene glycol and
1,4-butylene glycol. Also, cycloaliphatic polyols such as
1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,4 cyclohexane dimethanol,
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 of such low molecular weight polyepoxides are
3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate and
bis(3,4-epoxycyclohexylmethyl) adipate. These materials are aliphatic
polyepoxides as are the epoxy-containing acrylic polymers. As mentioned
above, the epoxy-containing acrylic polymers are preferred because they
result in products which have the best combination of coating properties,
i.e., smoothness, gloss, durability and solvent resistance. Such polymers
have been found to be particularly good in the formulation of clear coats
for color-plus-clear applications.
The polyepoxide is present in the film-forming composition in amounts of
about 10 percent by weight to 90 percent by weight, preferably from 20
percent by weight to 80 percent by weight and more preferably from 40
percent by weight to 70 percent by weight based on total weight of resin
solids.
The composition of the present invention further includes a polyacid
component having a high average acid functionality. More specifically, the
polyacid curing agent of the present invention on average contains greater
than two acid groups per molecule, more preferably three or more and most
preferably, four or more, such acid groups being reactive with the
polyepoxide to form a crosslinked coating as indicated by its resistance
to organic solvent. The parameter of greater than two acid groups per
molecule is intended to encompass mixtures of polyacid curing agents in
which di-functional curing agents are mixed with tri- or higher
functionality polyacid curing agents. Polyacid curing agent mixtures
including up to about 50 percent of a di-functional curing agent with a
tri-functional curing agent are suitable. Higher percentages of
di-functional material can be useful with the remainder of the curing
agent mixtures being higher than tri-functional or if the polyepoxide
component is highly functional. The acid functionality is preferably
carboxylic acid, although acids such as phosphorus-based acid may be used.
Preferably, the polyacid curing agent is a carboxylic acid terminated
material having, an average, greater than two carboxylic acid groups per
molecule. Among the polyacid curing agents which may be used include
carboxylic acid group-containing polymers such as acrylic polymers,
polyesters, and polyurethanes; oligomers such as ester group-containing
oligomers and monomers.
The preferred polyacid curing agents are ester group-containing oligomers.
Examples include half-esters formed from reacting polyols and 1,2-acid
anhydrides or acid functional polyesters derived from polyols and
polyacids or anhydrides. The half-esters are preferred because they are of
relatively low molecular weight and are quite reactive with epoxy
functionality enabling the formulation of high solids fluid compositions
while maintaining outstanding properties such as gloss and distinctness of
image.
The half-ester is obtained by reaction between a polyol and a 1,2-acid
anhydride under conditions sufficient to ring open the anhydride forming
the half-ester with substantially no polyesterification occurring. Such
reaction products are of relatively low molecular weight with narrow
molecular weight distributions and low viscosity 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 means that the carboxyl groups formed by the
reaction of the anhydride are not further esterified by the polyol in a
recurring manner. By this is meant that less than 10, preferably less than
5 percent by weight high molecular 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 the polyol has been reacted
with a 1,2-dicarboxylic acid anhydride, R is an organic moiety associated
with the anhydride and A is equal to at least 2.
Subsequently, carboxylic acid 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 a
polycondensation reaction resulting in products with higher molecular
weights.
To achieve the desired reaction, the 1,2-acid anhydride and polyol are
contacted together usually by mixing the two ingredients together in a
reaction vessel. Preferably, reaction is conducted in the presence of an
inert atmosphere such as 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 as other organic solvents such as dimethyl formamide and
N-methyl-pyrrolidone.
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
elimination 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.
The equivalent ratio of anhydride to hydroxyl on the polyol is preferably
at least about 0.8:1 (the anhydride being considered monofunctional) 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 lower
functionality half-esters.
Among the anhydrides which can be used in formation of the desired
polyesters are those which, exclusive of the carbon atoms and the
anhydride moiety, contain from about 2 to 30 carbon atoms. Examples
include aliphatic, including cycloaliphatic, olefinic and cycloolefinic
anhydrides and aromatic anhydrides. Substituted aliphatic 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. Examples of anhydrides
include succinic anhydride, methylsuccinic anhydride, dodecenyl succinic
anhydride, octadecenylsuccinic anhydride, phthalic anhydride,
tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride,
hexahydrophthalic anhydride, alkyl hexahydrophthalic anhydrides such as
methylhexahydrophthalic anhydride, tetrachlorophthalic anhydride,
endomethylene tetrahydrophthalic anhydride, chlorendic anhydride, itaconic
anhydride, citraconic anhydride and maleic anhydride.
Among the polyols which can be used are simple polyols, that is, those
containing from about 2 to 20 carbon atoms, as well as oligomeric polyols
and polymeric polyols such as polyester polyols, polyurethane polyols and
acrylic polyols.
Among the simple polyols are diols, triols, tetrols and mixtures thereof.
Examples of the polyols are preferably those containing from 2 to 10
carbon atoms such as aliphatic polyols. Specific examples include but are
not limited to the following compositions: di-trimethylol propane
(bis(2,2-dimethylol)dibutylether); pentaerythritol; 1,2,3,4-butanetetrol;
sorbitol; trimethylol propane; trimethylol ethane; 1,2,6-hexanetriol;
glycerine; trishydroxyethyl isocyanurate; dimethylol propionic acid;
1,2,4-butanetriol; TMP/epsilon-caprolactone triols; ethylene glycol;
1,2-propanediol; 1,3-propanediol; 1,4-butanediol; 1,5-pentanediol;
1,6-hexanediol; neopentyl glycol; diethylene glycol; dipropylene glycol;
1,4-cyclohexanedimethanol and 2,2,4-trimethylpentane-1,3 diol.
With regard to oligomeric polyols, suitable polyols are polyols made from
reaction of diacids with triols, such as trimethylol propane/cyclohexane
diacid and trimethylol propane/adipic acid.
With regard to polymeric polyols, the polyester polyols are prepared by
esterification of an organic polycarboxylic acid or anhydride thereof with
organic polyols and/or an epoxide. Usually, the polycarboxylic acids and
polyols are aliphatic or aromatic dibasic acids or acid anhydrides and
diols.
The polyols which are usually employed in making the polyester include
trimethylol propane, di-trimethylol propane, alkylene glycols such as
ethylene glycol, neopentyl glycol and other glycols such as hydrogenated
bisphenol A, cyclohexanediol, cyclohexanedimethanol, the reaction products
of lactones and diols, for example, the reaction product of
epsilon-caprolactone and ethylene glycol, hydroxy-alkylated bisphenols,
polyester glycols, for example, poly(oxytetramethylene)glycol and the
like.
The acid component of the polyester consists primarily of monomeric
carboxylic acids or anhydrides having 2 to 18 carbon atoms per molecule.
Among the acids which are useful are phthalic acid, isophthalic acid,
terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid,
methylhexahydrophthalic acid, adipic acid, azelaic acid, sebacic acid,
maleic acid, glutaric acid, chlorendic acid, tetrachlorophthalic acid and
other dicarboxylic acids of varying types. Also, there may be employed
higher polycarboxylic acids such as trimellitic acid and tricarballylic
acid. However, the use of these higher functionality polycarboxylic acids
are not preferred because of resultant high viscosities.
Besides the polyester polyols formed from polybasic acids and polyols,
polylactone-type polyesters can also be employed. These products are
formed from the reaction of a lactone such as epsilon-caprolactone and a
polyol such as ethylene glycol, diethylene glycol and trimethylolpropane.
Besides polyester polyols, polyurethane polyols such as polyester-urethane
polyols which are formed from reacting an organic polyisocyanate with a
polyester polyol such as those described above can be used. The organic
polyisocyanate is reacted with a polyol so that the OH/NCO equivalent
ratio is greater than 1:1 so that the resultant product contains free
hydroxyl groups. The organic polyisocyanate which is used in preparing the
polyurethane polyols can be an aliphatic or aromatic polyisocyanate or a
mixture. Diisocyanates are preferred, although higher polyisocyanates such
as triisocyanates can be used, but they do result in higher viscosities.
Examples of suitable diisocyanates are 4,4'-diphenylmethane diisocyanate,
1,4-tetramethylene diisocyanate, isophorone diisocyanate and
4,4'-methylenebis(cyclohexyl isocyanate). Examples of suitable higher
functionality polyisocyanates are polymethylene polyphenol isocyanates.
It is also possible to use acid-functional acrylic crosslinkers made from
copolymerizing methacrylic acid and/or acrylic acid monomers with other
ethylenically unsaturated copolymerizable monomers as the polyacid curing
agent. Alternatively, acid-functional acrylics can be prepared from
hydroxy-functional acrylics reacted with cyclic anhydrides.
The polyacid curing agent is present in the crosslinkable composition in
amounts of about 10 to 90, preferably 25 to 75 percent by weight based on
total weight of resin solids.
The present film-forming composition is also substantially free of basic
esterification catalyst. Although the absence of catalyst has a negative
effect on cure of the composition, it is beneficial because it provides
for a stable composition and is also beneficial in reducing or eliminating
cure inhibition between layers in a color-plus-clear formulation when the
base coat contains an acid-catalyzed resinous binder. Also the high
functionality associated with the polyepoxide and polyacid provide for
sufficient cure response. More specifically, in a preferred embodiment,
the composition of the present invention has no or only small amounts of
basic esterification catalyst such that the composition is stable for a
time sufficient to allow formulation of the composition as a
single-component composition. Stability of the present composition is
discussed more fully below.
A number of basic esterification catalysts are known in the art. The
present invention is substantially free of all such catalysts. Such
catalysts include, secondary amine catalysts such as piperidine; tertiary
amine catalysts such as N,N-dimethyldodecylamine, pyridine and
N,N-dimethylaniline; ammonium compounds, including tetrabutylammonium
bromide, tetrabutylammonium hydroxide, and tetrabutylammonium acetate;
phosphonium compounds, including ethyltriphenylphosphonium acetate and
tetrabutyl-phosphonium bromide; and other ammonium and phosphonium salts.
In addition to the foregoing identified basic esterification catalysts, it
is recognized that other common ingredients in coating formulations can
have incidental catalytic properties even though the ingredients are added
for other purposes. For example, coating compositions frequently include
hindered-amine light stabilizers, such as TINUVIN 292. It is recognized
that such compositions have small catalytic effect. It is also expressly
recognized that compositions of the present invention which are
substantially free from basic esterification catalyst can include such
compounds provided that the composition otherwise is within parameters of
the present invention relating to stability of the composition and absence
of cure inhibition. In particular, as shown in a comparison of Examples 2
and 7 below, a standard basic esterification catalyst causes a viscosity
increase of over 200 seconds in a #4 Ford cup test after 16 hours at
140.degree. F. as compared with a high of a 12.4 second increase of all
compositions which are substantially free of basic esterification
catalyst.
The present invention further optionally includes an aminoplast resin for
improved resistance to water spotting. As discussed more fully below, the
term "etch resistance" refers to a composition's resistance to acid
etching and to water spotting. While aminoplast resins improve resistance
to water spotting, it is recognized that high levels of aninoplast resins
can degrade the acid etch resistant properties of the composition.
Typically, when present, the aminoplast resin of the present invention is
present in the composition in amounts up to about 30 percent by weight,
more preferably up to about 20 percent by weight, and most preferably up
to about 15 percent by weight.
Aminoplast resins are condensation products of amines or amides with
aldehydes. Examples of suitable amine or amides are melamine,
benzoguanamine, urea and similar compounds. Generally, the aldehyde
employed is formaldehyde, although products can be made from other
aldehydes such as acetaldehyde and furfural. The condensation products
contain methylol groups or similar alkylol groups depending on the
particular aldehyde employed. Preferably, these methylol groups are
etherified by reaction with an alcohol. Various alcohols employed include
monohydric alcohols containing from 1 to 4 carbon atoms such as methanol,
ethanol, isopropanol and n-butanol, with methanol being preferred.
Aminoplast resins are commercially available from American Cyanamid Co.
under the trademark CYMEL and from Monsanto Chemical Co. under the
trademark RESIMENE. The preferred aminoplast resin is methylated
melamine-formaldehyde condensate.
The present composition can also include other optional ingredients, such
as plasticizers, anti-oxidants, UV light absorbers and epoxy crosslinkers.
A particularly useful class of crosslinkers in the present composition are
copolymers of alpha-olefins and olefinically unsaturated anhydrides, such
as a 1-octene and maleic anhydride copolymer. In such a crosslinker, the
anhydride is preferably opened with ethanol to form an ester and an acid
to maintain stability. Examples of these materials and amounts are further
described in U.S. Pat. No. 4,927,868.
As discussed above, the present film-forming composition is stable. In
particular, it is sufficiently stable to be useful as a single-component
system in which the polyepoxide component and the polyacid component are
combined substantially prior to application without gelation of the
composition and without unacceptable increases in viscosity before use.
Once a composition gels, it is no longer possible to use it as a coating
composition. If the viscosity of a composition increases to the point of
encountering sprayability problems, but the composition is not gelled,
additional solvent can be added to reduce viscosity to acceptable levels.
Stability can be measured as an increase in viscosity over time at a given
temperature. Various standard tests for measuring viscosity can be used.
For example, the Ford cup test is a recognized measure of viscosity. This
test measures the amount of time it takes a given volume of a composition
to flow out through the orifice of a standard cup.
The stability of a composition for use as a single-component composition
can be evaluated by comparison of a Ford cup value of a fresh composition
against the same composition after it has been subjected to heat over
time. The composition of the present invention is formulated such that
with an initial #4 Ford cup viscosity of about 20-30 seconds, after 16
hours at 140.degree. F., the composition has less than about a 25 second
gain in #4 Ford cup viscosity, more preferably less than about a 15 second
gain, and most preferably less than about a 10 second gain. Alternatively,
a composition is considered stable for use as a single-component
composition if after about 28 days and more preferably after about 42 days
at ambient temperature the composition has less than about a 25 second
gain in #4 Ford cup viscosity, more preferably less than about a 15 second
gain and most preferably less than about a 10 second gain. Ambient
temperature is considered to be less than about 90.degree. F. and more
typically at about 70.degree. F. or less.
The composition of the present invention is also suitable for use in
formulating high solids compositions. Because the present composition is
highly stable, it can be formulated as a high solids composition without
the danger of premature gelation. More particularly, the present
composition can have a resin solids content of greater than about 40
percent, more preferably greater than about 50 percent and most preferably
greater than 55 percent. The resin solids content can be determined by
heating 0.3-0.4 grams of the resinous ingredients in the composition in an
aluminum weighing dish at 230.degree. F. for 60 minutes.
The composition of the present invention is further characterized in that
it has excellent etch resistance properties. As used herein, the term
"etch resistance" refers to the ability of a cured composition to resist
etching by acids and water spotting. Etch resistance is typically
evaluated by visual examination of coated substrates after actual or
simulated weathering. It should be noted that simulated weathering, such
as that described in Table 4 below, typically, but not always, corresponds
to actual weathering. Moreover, it should be noted that cured compositions
may have different etch resistance properties when subjected to actual
weathering at different geographic sites. An etch resistant composition,
as discussed herein, refers to a composition which has etch resistant
properties under actual weathering in at least one geographic site or
which has etch resistant properties under simulated weathering conditions.
The etch resistant properties of the present film-forming composition are
attained by a combination of parameters of the composition. It should be
recognized that acceptable etch resistance can be achieved by compositions
having different specific combinations of such parameters in which certain
variables may be outside of specific numerical ranges provided herein.
Provided that acceptable etch resistance is attained, such compositions
are within the scope of this invention. As identified above, the primary
factors affecting etch resistance are high epoxy functionality, high acid
functionality, and high Tg characteristics of the film-forming
composition.
A further advantage of the present stable etch-resistant composition is its
usefulness in combination with a basecoat in a color-plus-clear system.
The present composition can be used in conjunction with a wide variety of
basecoats and still provide excellent stability and etch resistance.
Moreover, the present invention is particularly useful in conjunction with
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