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CROSS-REFERENCE TO RELATED APPLICATION
Reference is made to application Ser. No. 767,019, filed on Aug. 19, 1985.
This application claims liquid crosslinkable compositions which are useful
in the process of the present invention.
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a process for preparing multi-layered
coated articles comprising a pigmented or colored base coat and a
transparent or clear top coat, and to the coated articles themselves.
2. Brief Description of the Prior Art:
Color-plus-clear coating systems 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 systems
have outstanding gloss and distinctness of image, and the clear coat is
particularly important for these properties. Two-pack clear coat
compositions comprising polyols such as polyester polyols, polyurethane
polyols and acrylic polyols and polyisocyanate curing agents give
outstanding gloss and distinctness of image. However, the polyisocyanates
are difficult to handle being sensitive to moisture and require cumbersome
safety precautions.
It is an object of the present invention to provide a color-plus-clear
coating system which avoids the problems of polyisocyanate curing agents
but which provides a finish which has outstanding gloss and distinctness
of image so that the coating is useful as an original finish for
automobiles.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process for applying a
composite coating to a substrate is provided. The process comprises
applying to the substrate a film-forming composition to form a base coat
and applying to the base coat a film-forming composition to form a
transparent top coat over the base coat. Either the base coat, the
transparent top coat or both the base and top coats are derived from a
crosslinkable composition comprising a polyepoxide and a polyacid curing
agent which comprises a half-ester formed from reacting an acid anhydride
with a polyol. The curing agent can be tailor made with specific
anhydrides and polyols without forming high molecular weight materials
enabling the formulation of high solids coating compositions which provide
cured coatings with excellent gloss and distinctness of image. The
invention also provides for the resultantly coated article.
DETAILED DESCRIPTION
The film-forming composition of the base coat can be any of the
compositions useful in coatings applications, particularly automotive
applications. The film-forming composition comprises a resinous binder and
a pigment to act as the colorant. Particularly useful resinous binders are
acrylic polymers, polyesters, including alkyds, and polyurethanes.
The acrylic polymers are copolymers of one or more alkyl esters of acrylic
acid or methacrylic acid optionally together with one or more other
polymerizable ethylenically unsaturated monomers. These polymers may be
either of the thermoplastic type or thermosetting crosslinking type.
Suitable alkyl esters of acrylic acid or methacrylic acid include methyl
methacrylate, ethyl methacrylate, butyl methacrylate, ethyl acrylate,
butyl acrylate and 2-ethylhexyl acrylate. Suitable other copolymerizable
ethylenically unsaturated monomers include 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.
Where the polymer is of the crosslinking type, suitable functional monomers
may be used in addition to the other acrylic monomers mentioned above and
include, for example, acrylic acid, methacrylic acid, hydroxyethyl
acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, and
hydroxypropyl methacrylate. The coating composition in such cases contains
a crosslinking agent such as a condensate of an amine or an amide with
formaldehyde such as urea, melamine, or benzoguanamine reacted with
formaldehyde or a lower alkyl ether of such condensate in which the alkyl
groups contain from 1 to 4 carbon atoms. Other crosslinking agents such as
polyisocyanates including blocked polyisocyanates may also be used. Also,
the acrylic polymer can be prepared with N-(alkoxymethyl)-acrylamides and
N-(alkoxymethyl)methacrylamides which result in self-crosslinking acrylic
polymers.
The acrylic polymer may be prepared by solution polymerization techniques
in the presence of suitable catalysts such as organic peroxides or azo
compounds, for example, benzoyl peroxide or N,N'-azobis(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 and ketones such as methyl amyl ketone. Alternately,
the acrylic polymer may be prepared by aqueous emulsion or dispersion
polymerization techniques.
Besides acrylic polymers, the resinous binder for the base coat composition
may be an alkyd resin or a polyester. Such polymers may be prepared in a
known manner by condensation of polyhydric alcohols and polycarboxylic
acids. Suitable polyhydric alcohols include ethylene glycol, propylene
glycol, butylene glycol, 1,6-hexylene glycol, neopentyl glycol, diethylene
glycol, glycerol, trimethylolpropane, and pentaerythritol.
Suitable polycarboxylic acids include succinic acid, adipic acid, azelaic
acid, sebacic acid, maleic acid, fumaric acid, phthalic acid,
tetrahydrophthalic acid, hexahydrophthalic acid, and trimellitic acid.
Besides the polycarboxylic acids mentioned above, functional equivalents
of the polycarboxylic acids such as anhydrides where they exist or lower
alkyl esters of the polycarboxylic acids such as the methyl esters may be
used.
Where it is desired to produce air-drying alkyd resins, suitable drying oil
fatty acids may be used and include those derived from linseed oil, soya
bean oil, tall oil, dehydrated castor oil or tung oil.
The polyesters and preferably the alkyd resins contain a portion of free
hydroxyl and/or carboxyl groups which are available for further
crosslinking reaction. Suitable crosslinking agents are the amine or
amide-aldehyde condensates or the polyisocyanate curing agents as
mentioned above.
Polyurethanes can also be used as the resinous binder of the base coat.
Among the polyurethanes which can be used are polymeric polyols which are
prepared by reacting the polyester polyols or acrylic polyols such as
those mentioned above with a polyisocyanate such that the OH/NCO
equivalent ratio is greater than 1:1 so that free hydroxyl groups are
present in the product.
The organic polyisocyanate which is used to prepare the polyurethane polyol
can be an aliphatic or an aromatic polyisocyanate or a mixture of the two.
Diisocyanates are preferred, although higher polyisocyanates can be used
in place of or in combination with diisocyanates.
Examples of suitable aromatic diisocyanates are 4,4'-diphenylmethane
diisocyanate and toluene diisocyanate. Examples of suitable aliphatic
diisocyanates are straight chain aliphatic diisocyanates such as
1,6-hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates can be
employed. Examples include isophorone diisocyanate and
4,4'-methylene-bis-(cyclohexyl isocyanate). Examples of suitable higher
polyisocyanates are 1,2,4-benzene triisocyanate and polymethylene
polyphenyl isocyanate.
Most of the polymers prepared as described above are organic solvent-based
polymers, although acrylic polymers can be prepared via aqueous emulsion
polymerization techniques and used as aqueous-based base coat
compositions. Water-based base coats in color-plus-clear compositions are
disclosed in U.S. Pat. No. 4,403,003, and the resinous compositions used
in preparing these base coats can be used in the practice of this
invention. Also, water-based polyurethanes such as those prepared in
accordance with U.S. Pat. No. 4,147,679 can be used as the resinous binder
in the base coat.
The resinous binder for the base coat can also be the resinous binder used
in formulating the clear film-forming composition described below.
The base coat composition also contains pigments to give it color.
Compositions containing metallic flake pigmentation are useful for the
production of so-called "glamour metallic" finishes chiefly upon the
surface of automobile bodies. Proper orientation of the metallic pigments
results in a lustrous shiny appearance with excellent flop, distinctness
of image and high gloss. By flop is meant the visual change in brightness
or lightness of the metallic coating with a change in viewing angle, that
is, a change from 90.degree. to 180.degree.. The greater the change, that
is, from light to dark appearance, the better the flop. Flop is important
because it accentuates the lines of a curved surface such as on an
automobile body. Suitable metallic pigments include in particular aluminum
flake, copper bronze flake and mica.
Besides the metallic pigments, the base coating compositions of the present
invention may contain non-metallic color pigments conventionally used in
the surface coating compositions including inorganic pigments such as
titanium dioxide, iron oxide, chromium oxide, lead chromate and carbon
black, and organic pigments such as phthalocyanine blue and phthalocyanine
green. In general, the pigment is incorporated into the coating
composition in amounts of about 1 to 80 percent by weight based on weight
of coating solids. The metallic pigment is employed in amounts of about
0.5 to 25 percent by weight of the aforesaid aggregate weight.
If desired, the base coat composition may additionally contain other
materials well known in the art of formulated surface coatings. These
would include surfactants, flow control agents, thixotropic agents,
fillers, anti-gassing agents, organic co-solvents, catalysts and other
customary auxiliaries. These materials can constitute up to 40 percent by
weight of the total weight of the coating composition.
The base coating compositions can be applied to various substrates to which
they adhere. The compositions can be applied by conventional means
including brushing, dipping, flow coating, spraying and the like, but they
are most often applied by spraying. The usual spray techniques and
equipment for air spraying and electrostatic spraying and either manual or
automatic methods can be used.
Coatings of the present invention can be applied over virtually any
substrate including wood, metals, glass, cloth, plastic, foam, including
elastomeric substrates, and the like. They are particularly useful in
applying over metal and elastomeric substrates that are found on motor
vehicles.
During application of the base coat composition to the substrate, a film of
the base coat is formed on the substrate. Typically, the base coat
thickness will be about 0.01 to 5, preferably 0.1 to 2 mils in thickness.
After application to the substrate of the base coat composition, a film is
formed on the surface of the substrate. This is achieved by driving
solvent, i.e., | | |
