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BACKGROUND OF THE INVENTION
A coating system gaining wide acceptance, particularly in the automotive
industry, is one which is known as "color plus clear". In this system the
substrate is coated with one or more applications of a pigmented
basecoating composition, which is in turn coated with one or more
applications of a generally clear topcoating composition.
However, there are several difficulties in employing "color plus clear"
coating systems especially as attempts are made to employ coating
compositions having high solids contents and also as metallic flake
pigments are used to provide a special two tone appearance to the coated
substrate as it is viewed from different angles to a direction normal to
the surface of the substrate. For example, it is important in a "color
plus clear" coating system that the applied basecoat not be attacked by
components of the topcoating composition, particularly solvents, at the
interface of the two, a phenomenon often referred to as strike-in.
Strike-in adversely affects the final appearance properties of the coated
product. Strike-in is an especially serious problem when metallic-flake
pigments are employed in the basecoating composition. Strike-in, among
other things, can destroy the desired metallic-flake orientation in the
basecoat.
Additionally, irrespective of the problems associated with strike-in, it is
important to prevent sagging during curing of the coating composition
after application to a nonhorizontal substrate. Also, especially where
metallic-flake pigments are employed, it is important to achieve and
maintain proper pigment orientation in the pigmented basecoating
composition during the curing or drying operation. Moreover, where a
material is incorporated in the topcoating composition to prevent sagging
of the topcoating composition during cure, it is particularly desirable
that such material not seriously affect the clarity of the cured topcoat,
for example, by imparting to the topcoat a cloudy or milky appearance.
One attempt to address some of these problems has been to incorporate in
the basecoating composition as part of the organic polymer system present,
a proportion of organic, insoluble polymer microparticles as described for
example in U.S. Pat. No. 4,220,679 to Backhouse. Another attempt to
address at least some of the problems of achieving proper metallic-flake
orientation in a high solids basecoat has been to substantially increase
the amount of metallic-flake pigment in the basecoating composition as
described in U.S. Pat. No. 4,359,504 to Troy.
It has now been found that the incorporation of substantially inorganic
microparticles in the basecoating composition permits the basecoating
composition to be formulated for example at high solids content and
alleviates the problems of strike-in, the problems of achieving excellent
metallic-pattern control where metallic-flake pigments are employed, and
the problem of sagging of the coating composition on a nonhorizontal
substrate during curing or drying. It has also been found that the
incorporation of substantially inorganic microparticles, for example based
on silica, in the topcoating composition, not only alleviates sagging of
the topcoating composition during cure but also does not seriously affect
the clarity of the transparent topcoat.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a method for coating a substrate comprising
the steps of: (A) coating the substrate with one or more applications of a
basecoating composition comprising (1) an organic film-forming resin, and
where the film-forming resin can be crosslinked, optionally a crosslinking
agent for the film-forming resin, (2) substantially colorless,
substantially inorganic microparticles stably dispersed in the basecoating
composition, (3) a solvent system for the film-forming resin in which the
inorganic microparticles do not dissolve, and (4) pigment particles, to
form a basecoat; and (B) coating the basecoat with one or more
applications of a topcoating composition comprising (1) an organic
film-forming resin, which may be the same or different from the
film-forming resin of the basecoating composition, and where the
film-forming resin of the topcoating composition can be crosslinked,
optionally a crosslinking agent for the film-forming resin of the
topcoating composition, and (2) a solvent system for the organic
film-forming resin of the topcoating composition, to form a transparent
topcoat.
The present invention also provides a method for coating a substrate
comprising the steps of: (A) coating the substrate with one or more
applications of a basecoating composition comprising: (1) an organic
film-forming resin, and where the film-forming resin can be crosslinked,
optionally a crosslinking agent for the film-forming resin, (2) a solvent
system for the film-forming resin of the basecoating composition, (3)
organic polymeric microparticles which are insoluble in the solvent system
of the basecoating composition and which have a diameter in the range of
from about 0.01 to about 40 microns, and (4) pigment particles, to form a
basecoat; and (B) coating the basecoat with one or more applications of a
topcoating composition comprising: (1) an organic film-forming resin which
may be the same or different from the film-forming resin of the
basecoating composition, and where the film-forming resin of the
topcoating composition can be crosslinked, optionally a crosslinking agent
for the film-forming resin of the topcoating composition, (2)
substantially colorless, substantially inorganic microparticles stably
dispersed in the topcoating composition ranging in size from about 1 to
about 150 nanometers, and (3) a solvent system for the organic
film-forming resin of the topcoating composition to form a transparent
topcoat.
DETAILED DESCRIPTION OF THE INVENTION
The film-forming resin of the basecoating composition may be any of the
film-forming resins useful for coating compositions. The film-forming
resins of the basecoating composition can be film-forming thermoplastic
resins and/or thermosetting resins. Examples of such film-forming
thermoplastic resins and/or thermosetting resins include the generally
known cellulosics, acrylics, aminoplasts, urethanes, polyesters, epoxies,
and polyamides. These resins, when desired, may also contain functional
groups characteristic of more than one class, as for example, polyester
amides, uralkyds, urethane acrylates, urethane amide acrylates, etc. As
indicated above, the film-forming resin may be thermoplastic or it may be
thermosetting. As used herein, the term thermosetting is intended to
include not only those resins capable of being crosslinked upon
application of heat but also those resins which are capable of being
crosslinked without the application of heat.
Cellulosics refer to the generally known thermoplastic polymers which are
derivatives of cellulose, examples of which include: nitrocellulose;
organic esters and mixed esters of cellulose such as cellulose acetate,
cellulose propionate, cellulose butyrate, and cellulose acetate butyrate;
and organic ethers of cellulose such as ethyl cellulose.
Acrylic resins refer to the generally known addition polymers and
copolymers of acrylic and methacrylic acids and their ester derivatives,
acrylamide and methacrylamide, and acrylonitrile and methacrylonitrile.
Examples of ester derivatives of acrylic and methacrylic acids include
such alkyl acrylates and alkyl methacrylates as ethyl, methyl, propyl,
butyl, hexyl, ethylhexyl and lauryl acrylates and methacrylates, as well
as similar esters, having up to about 20 carbon atoms in the alkyl group.
Also, hydroxyalkyl esters can readily be employed. Examples of such
hydroxyalkyl esters include 2-hydroxyethyl acrylate, 2-hydroxypropyl
acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate,
3-hydroxypropyl-4-hydroxybutyl methacrylate, and mixtures of such esters
having up to about 5 carbon atoms in the alkyl group. In some instances,
corresponding esters of other unsaturated acids, for example, ethacrylic
acid, crotonic acid, and other similar acids having up to about 6 carbon
atoms can be employed. Where desired, various other ethylenically
unsaturated monomers can be utilized in the preparation of acrylic resins
examples of which include: vinyl aromatic hydrocarbons optionally bearing
halo substituents such as styrene, alpha-methyl styrene, vinyl toluene,
alpha-chlorostyrene, alpha-bromostyrene, and para-fluorostyrene;
nonaromatic monoolefinic and diolefinic hydrocarbons optionally bearing
halo substituents such as isobutylene, 2,3-dimethyl-1-hexene,
1,3-butadiene, chloroethylene, chlorobutadiene and the like; and esters of
organic and inorganic acids such as vinyl acetate, vinyl propionate,
isopropenyl acetate, vinyl chloride, allyl chloride, vinyl
alpha-chloroacetate, dimethyl maleate and the like.
The above polymerizable monomers are mentioned as representative of the
CH.sub.2 .dbd.C< containing monomers which may be employed; but
essentially any copolymerizable monomer can be used.
Aminoplast resins refer to the generally known condensation products of an
aldehyde with an amino- or amido-group containing substance examples of
which include the reaction products of formaldehyde, acetaldehyde,
crotonaldehyde, benzaldehyde and mixtures thereof with urea, melamine, or
benzoguanimine. Preferred aminoplast resins include the etherified (i.e.,
alkylated) products obtained from the reaction of alcohols and
formaldehyde with urea, melamine, or benzoguanimine. Examples of suitable
alcohols for preparing these etherified products include: methanol,
ethanol, propanol, butanol, hexanol, benzylalcohol, cyclohexanol,
3-chloropropanol, and ethoxyethanol.
Urethane resins refer to the generally known thermosetting or thermoplastic
urethane resins prepared from organic polyisocyanates and organic
compounds containing active hydrogen atoms as found for example in
hydroxyl, and amino moieties. Some examples of urethane resins typically
utilized in one-pack coating compositions include: the isocyanate-modified
alkyd resins sometimes referred to as "uralkyds"; the isocyanate-modified
drying oils commonly referred to as "urethane oils" which cure with a
drier in the presence of oxygen in air; and isocyanate-terminated
prepolymers typically prepared from an excess of one or more organic
polyisocyanates and one or more polyols including, for example, simple
diols, triols and higher alcohols, polyester polyols and polyether
polyols. Some examples of systems based on urethane resins typically
utilized as two-pack coating compositions include an organic
polyisocyanate or isocyanate-terminated prepolymer (first pack) in
combination with a substance (second pack) containing active hydrogen as
in hydroxyl or amino groups along with a catalyst (e.g., an organotin salt
such as dibutyltin dilaurate or an organic amine such as triethylamine or
1,4-diazobicyclo-(2:2:2) octane). The active hydrogen-containing substance
in the second pack typically is a polyester polyol, a polyether polyol, or
an acrylic polyol known for use in such two-pack urethane resin systems.
Many coating compositions based on urethanes (and their preparation) are
described extensively in Chapter X Coatings, pages 453-607 of
Polyurethanes: Chemistry and Technology, Part II by H. Saunders and K. C.
Frisch, Interscience Publishers (N.Y., 1964).
Polyester resins are generally known and are prepared by conventional
techniques utilizing polyhydric alcohols and polycarboxylic acids.
Examples of suitable polyhydric alcohols include: ethylene glycol;
propylene glycol; diethylene glycol; dipropylene glycol; butylene glycol;
glycerol; trimethylolpropane; pentaerythritol; sorbitol; 1,6-hexanediol;
1,4-cyclohexanediol; 1,4-cyclohexanedimethanol;
1,2-bis(hydroxyethyl)cyclohexane; and
2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate. Examples of
suitable polycarboxylic acids include: phthalic acid; isophthalic acid;
terephthalic acid; trimellitic acid; tetrahydrophthalic acid;
hexahydrophthalic acid; tetrachlorophthalic acid; adipic acid; azelaic
acid; sebacic acid; succinic acid; maleic acid; glutaric acid; malonic
acid; pimelic acid; suberic acid; 2-2-dimethylsuccinic acid;
3,3-dimethylglutaric acid; 2,2-dimethylglutaric acid; maleic acid; fumaric
acid; and itaconic acid. Anhydrides of the above acids, where they exist,
can also be employed and are encompassed by the term "polycarboxylic
acid." In addition, certain substances which react in a manner similar to
acids to form polyesters are also useful. Such substances include lactones
such as caprolactone, propylolactone and methyl caprolactone, and hydroxy
acids such as hydroxy caproic acid and dimethylol propionic acid. If a
triol or higher hydric alcohol is used, a monocarboxylic acid, such as
acetic acid and benzoic acid may be used in the preparation of the
polyester resin. Moreover, polyesters are intended to include polyesters
modified with fatty acids or glyceride oils of fatty acids (i.e.,
conventional alkyd resins). Alkyd resins typically are produced by
reacting the polyhydric alcohols, polycarboxylic acids, and fatty acids
derived from drying, semi-drying, and non-drying oils in various
proportions in the presence of a catalyst such as litharge, sulfuric acid,
or a sulfonic acid to effect esterification. Examples of suitable fatty
acids include saturated and unsaturated acids such as stearic acid, oleic
acid, ricinoleic acid, palmitic acid, linoleic acid, linolenic acid,
licanic acid, elaeostearic acid, and clupanodonic acid.
Epoxy resins, often referred to simply as "epoxies", are generally known
and refer to compounds or mixtures of compounds containing more than one
1,2-epoxy group of the formula
##STR1##
i.e., polyepoxides. The polyepoxides may be saturated or unsaturated,
aliphatic, cycloaliphatic, aromatic or heterocyclic. Examples of suitable
polyepoxides include the generally known polyglycidyl ethers of
polyphenols and/or polyepoxides which are acrylic resins containing
pendant and/or terminal 1,2-epoxy groups. Polyglycidyl ethers of
polyphenols may be prepared, for example, by etherification of a
polyphenol with epichlorohydrin or dichlorohydrin in the presence of an
alkali. Examples of suitable polyphenols include:
1,1-bis(4-hydroxyphenyl)ethane; 2,2-bis(4-hydroxyphenyl)propane;
1,1-bis(4-hydroxyphenyl)isobutane;
2,2-bis(4-hydroxytertiarybutylphenyl)propane;
bis(2-hydroxynaphthyl)methane; 1,5-dihydroxynaphthalene;
1,1-bis(4-hydroxy-3-allylphenyl)ethane; and the hydrogenated derivatives
thereof. The polyglycidyl ethers of polyphenols of various molecular
weights may be produced, for example, by varying the mole ratio of
epichlorohydrin to polyphenol in known manner.
Epoxy resins also include the polyglycidyl ethers of mononuclear polyhydric
phenols such as the polyglycidyl ethers of resorcinol, pyrogallol,
hydroquinone, and pyrocatechol.
Epoxy resins also include the polyglycidyl ethers of polyhydric alcohols
such as the reaction products of epichlorohydrin or dichlorohydrin with
aliphatic and cycloaliphatic compounds containing from two to four
hydroxyl groups including, for example, ethylene glycol, diethylene
glycol, triethylene glycol, dipropylene glycol, tripropylene glycol,
propane diols, butane diols, pentane diols, glycerol, 1,2,6-hexanetriol,
pentaerythritol, and 2,2-bis(4-hydroxycyclohexyl)propane.
Epoxy resins additionally include polyglycidyl esters of polycarboxylic
acids such as the generally known polyglycidyl esters of adipic acid,
phthalic acid, and the like.
Addition polymerized resins containing epoxy groups may also be employed.
These polyepoxides may be produced by the addition polymerization of epoxy
functional monomers such as glycidyl acrylate, glycidyl methacrylate and
allyl glycidyl ether optionally in combination with ethylenically
unsaturated monomers such as styrene, alpha-methyl styrene, alpha-ethyl
styrene, vinyl toluene, t-butyl styrene, acrylamide, methacrylamide,
acrylonitrile, methacrylonitrile, ethacrylonitrile, ethyl methacrylate,
methyl methacrylate, isopropyl methacrylate, isobutyl methacrylate, and
isobornyl methacrylate.
Many additional examples of epoxy resins are described in the Handbook of
Epoxy Resins, Henry Lee and Kris Neville, 1967, McGraw Hill Book Company.
When desired, generally known crosslinking agents may be utilized in the
method of the invention particu[arly when thermosetting resins containing
active hydrogen atoms are employed in the coating compositions.
As will be appreciated by one skilled in the art, the choice of
crosslinking agent depends on various factors such as compatibility with
the film-forming resin, the particular type of functional groups on the
film-forming resin and the like. The crosslinking agent may be used to
crosslink the film-forming resin either by condensation or addition or
both. When the thermosetting reactants include monomers having
complementary groups capable of entering into crosslinking reactions, the
crosslinking agent may be omitted if desired.
Representative examples of crosslinking agents include blocked and/or
unblocked diisocyanates, diepoxides, aminoplasts and phenoplasts. When
aminoplast resins are employed as crosslinking agents, particularly
suitable are the melamine-formaldehyde condensates in which a substantial
proportion of the methylol groups have been etherified by reaction with a
monohydric alcohol such as those set forth previously in the description
of aminoplast resins suitable for use as film-forming resins in
compositions of the invention.
The term "solvent system" as used herein, for example in the phrase
"solvent system for the film-forming resin", is employed in a broad sense
and is intended to include true solvents as well as liquid diluents for
the film-forming resin which are not true solvents for the film-forming
resin. The solvent system may be organic or aqueous. It may be a single
compound of a mixture of compounds. When the solvent system comprises both
water and an organic portion, the components are usually miscible in the
proportions employed. The relationship between the solvent system and the
film-forming resin depends upon the absolute and relative natures of these
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