A curable, film-forming composition is provided which includes: (a) a hydroxyl-containing acrylic solution polymer, and (b) a polyisocyanate crosslinking agent. The hydroxyl-containing acrylic solution polymer is present in an amount ranging from between about 60 to about 90 weight percent; and, the polyisocyanate crosslinking agent is present in an amount ranging from between about 10 to about 40 weight percent. The acrylic solution polymer is the reaction product of: (a) a cycloaliphatic and/or aromatic ester of (meth)acrylic acid having at least 6 carbon atoms in the cycloaliphatic and/or aromatic group, and (b) a hydroxyl functional acrylic monomer component. The hydroxyl functional acrylic monomer component is present in an amount such that the hydroxyl number of the resulting acrylic solution polymer is less than 60. This invention also provides a color-plus-clear coating system. This system includes: (a) applying a film-forming base coat onto a substrate, and (b) applying a clear, film-forming topcoat over the base coat. The clear topcoat includes the curable, film-forming composition described above.

FIELD OF THE INVENTION 
The present invention relates to polyisocyanate-curable, film-forming 
compositions, and processes for preparing multi-layered coated articles 
comprising a pigmented or colored base coat and a clear topcoat. 
BACKGROUND OF THE INVENTION 
Color-plus-clear coating systems have become conventional as original 
finishes for automobiles. These systems typically involve the application 
of a pigmented base coat to a substrate. This is followed by the 
application of a clear topcoat over the base coat. These color-plus-clear 
systems have outstanding gloss and distinctness of image, most of which is 
provided by the clear topcoat. 
In addition to having a high gloss, it is often desirable for the clear 
topcoat in this color-plus-clear system to be ultradurable and acid etch 
resistant. As used herein, the term "ultradurable" refers to the ability 
of the topcoat to retain a large percentage of its original gloss when 
exposed to environmental conditions such as rain, sun, heat, freezing 
temperatures, and the like. 
Coating systems which are known to be durable are described in U.S. Pat. 
No. 4,565,730. That Patent describes coating compositions which contain as 
binders a combination of two specific hydroxyl-containing acrylate resins 
and an aliphatic and/or cycloaliphatic polyisocyanate or a mixture of such 
polyisocyanates. The hydroxyl number of the first of the two specific 
acrylic polymers ranges from between 80 to 180 and is present in an amount 
ranging from between 50 to 90% by weight, based on total acrylic polymers. 
The hydroxyl number of the second of the two specific acrylic polymers 
ranges from between 40 to 120 and is present in an amount ranging from 
between 10 to 50% by weight, based on total acrylic polymers. 
In the past, it was believed that, in order to make coatings ultradurable, 
it is necessary for the polymers to contain fluoroine. However, while 
coatings based on polyvinylidene difluroide are ultradurable, they do not 
have a high gloss. 
Notwithstanding the above, the fluoropolymers described in U.S. Pat. No. 
4,345,057. have a high gloss and are ultradurable. The fluoropolymers 
described in that Patent are based on chlorotrifluoroethylene, vinyl 
ethers and hydroxy functionality to facilitate curing with 
polyisocyanates. However, these fluoropolymers are very expensive. 
Moreover, there is special equipment required to work with these gaseous 
monomers. 
Processes for applying a double-layered covering lacquer on the surface of 
a substrate are disclosed in International (PCT) Application WO 94/22969. 
This PCT Application discloses processes which use a transparent covering 
lacquer containing a hydroxyl group-containing polyacrylate resin produced 
by polymerizing: (a) 10 to 51% by weight, preferably 25 to 41% by weight, 
of 4-hydroxy-n-butylacrylate or 4-hydroxy-n-butylmethacrylate, or a 
mixture thereof, and (b) 28 to 85% by weight, preferably 40 to 70% by 
weight, of an aliphatic or cycloaliphatic ester of methacrylic acid with 
at least 4 carbon atoms in the alcohol radical which is different from 
(a), above. The resulting polyacrylate resin has a hydroxyl number ranging 
from between 60 to 200, preferably, from between 100 to 160. 
Commonly-owned U.S. Pat. No. 5,445,850 is entitled "Aminoplast Cured Acid 
Etch Resistant Coating with Good Durability." That Patent discloses 
coating compositions and their use in color-plus-clear coating systems. 
These coating compositions include acrylic polymers prepared with high 
levels of cycloaliphatic or aromatic ester of acrylic and methacrylic acid 
and hydroxypropyl- or hydroxybutyl acrylates and methacrylates and 
aminoplast crosslinking agents. 
SUMMARY OF THE INVENTION 
It is one object of this invention to provide a novel high gloss clear 
coating composition which is ultradurable and resists acid etching. 
Another object of this invention is to provide a color-plus-clear coating 
system which includes a novel clear, high gloss topcoat which is 
ultradurable and resists acid etching. 
These and other objects are achieved by the discovery of novel coating 
compositions which can be used as a clear top coat of a color-plus-clear 
coating system. In accordance with the present invention, a curable 
film-forming composition is provided which includes: (a) a 
hydroxyl-containing acrylic solution polymer, and (b) a polyisocyanate 
crosslinking agent. The hydroxyl-containing acrylic solution polymer is 
present in an amount ranging from between about 60 to about 90 weight 
percent; and, the polyisocyanate crosslinking agent is present in an 
amount ranging from between about 10 to about 40 weight percent. These 
weight percentages are based upon the total resin solids of the 
hydroxyl-containing acrylic solution polymer and the polyisocyanate 
crosslinking agent, respectively, in the film forming composition. 
The acrylic solution polymer is the reaction product of: (a) a 
cycloaliphatic and/or aromatic ester of acrylic acid or methacrylic acid 
(hereinafter referred to as "(meth)acrylic acid") having at least 6 carbon 
atoms in the cycloaliphatic and/or aromatic group, and (b) a hydroxyl 
functional acrylic monomer component. The cycloaliphatic and/or aromatic 
ester component is typically employed in an amount ranging from between 
about 45 to about 95 weight percent. Moreover, the hydroxyl functional 
acrylic monomer component is present in an amount such that the hydroxyl 
number of the resulting acrylic solution polymer is less than 60. It has 
been discovered that the coating compositions made in accordance with the 
present invention are not only ultradurable, but also acid etch resistant. 
This invention also provides a process for applying a composite coating 
onto a substrate. This process includes: (a) applying a film-forming base 
coat onto a substrate, (b) at least partially curing the base coat, and 
(c) applying a clear, film-forming top coat over the at least partially 
cured base coat. The clear top coat being applied over the base coat 
includes the curable, film-forming composition described above. 
These and other aspects and advantages of this invention will become 
apparent to those skilled in the art upon reading the following Detailed 
Description. 
DETAILED DESCRIPTION 
The present invention pertains to the development of a novel, crosslinkable 
film-forming composition which has a high gloss, is ultradurable and 
resists acid etching. This film-forming composition includes: (a) a 
hydroxyl-containing acrylic solution polymer, and (b) a polyisocyanate 
crosslinking agent. 
As used herein, the term "acrylic solution polymer" means that the acrylic 
polymer is prepared by solution polymerization techniques while in the 
presence of suitable initiators such as organic peroxides or azo compounds 
(e.g., benzoyl peroxide or 2,2'-azobis(2-methylbutanenitrile)). This 
polymerization can be carried out in an organic solvent in a conventional 
manner in which the monomers and polymers produced are soluble. 
When practicing this invention, the acrylic solution polymer is typically 
present in the film-forming composition in an amount ranging from between 
about 60 to about 90 weight percent, and preferably, from between about 70 
to about 85 weight percent. These weight percentages are based upon the 
total resin solids weight of a hydroxyl-containing acrylic solution 
polymer in the film-forming composition. On the other hand, the 
polyisocyanate crosslinking agent is typically present in the film-forming 
composition in an amount ranging from between about 10 to about 40 weight 
percent, and preferably, from between about 15 to about 30 weight percent. 
These weight percentages are based upon the total resin solids weight of 
an polyisocyanate crosslinking agent in the film-forming composition. 
The acrylic solution polymer is the reaction product of: (a) a 
cycloaliphatic and/or aromatic ester of (meth)acrylic acid having at least 
6 carbon atoms in the cycloaliphatic and/or aromatic group, and (b) a 
hydroxyl functional acrylic monomer component. The hydroxyl functional 
acrylic monomer component is present in an amount such that the hydroxyl 
number of the resulting acrylic solution polymer is less than 60. The 
cycloaliphatic or aromatic ester component is typically employed in an 
amount ranging from between about 45 to about 95 weight percent, and 
preferably, from between about 60 to about 90 weight percent. These weight 
percentages are based upon total weight of monomers used in preparing the 
acrylic solution polymer. Moreover, although the cycloaliphatic or 
aromatic ester component has at least 6 carbon atoms in the cycloaliphatic 
or aromatic group, typically, it has from between 6 to 12 carbon atoms. 
Examples of such compounds which can be used when practicing this 
invention include: benzyl methacrylate, phenyl methacrylate, 
t-butyl-cyclohexyl methacrylate and cyclohexyl methacrylate or mixtures 
thereof. Cyclohexyl methacrylate is preferred. 
On the other hand, the hydroxyl functional acrylic monomer component is 
typically employed in an amount such that the hydroxyl number of the 
resulting acrylic solution polymer is less than 60. Preferably, the amount 
of the hydroxyl functional acrylic monomer component employed is such that 
the acrylic solution polymer's hydroxyl number is in the range from 
between about 25 to about 55, and more preferably, from between about 30 
to about 55. Any suitable hydroxyl functional monomer, or combination 
thereof, can be used when practicing this invention. Examples of such 
suitable monomers include: hydroxyethyl acrylate, hydroxyethyl 
methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 2- 
and 4-hydroxybutyl acrylate, and 2- and 4-hydroxybutyl methacrylate or 
mixtures thereof. The hydroxyl functional monomers most preferred for 
providing acid etch resistance are 2-hydroxybutyl acrylate and 
2-hydroxypropyl acrylate. 
The acrylic solution polymer may further include up to 45 weight percent of 
a monomer(s) such as vinyl aromatic compounds and alkyl acrylates and 
methacrylates which contain from 1 to 8 carbon atoms in the alkyl group. 
This weight percent is based upon total weight of monomers used in 
preparing the acrylic solution polymer. Suitable vinyl aromatic compounds 
which can be used include: styrene and vinyl toluene. Styrene is 
preferred. Suitable alkyl acrylates and methacrylates which can be used 
include: methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 
ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate. A mixture of 
n-butyl acrylate and methyl methacrylate is preferred. 
The acrylic solution polymer typically has a number average molecular 
weight ranging from between about 1,000 to about 30,000, and preferably, 
from -between about 1,500 to 20,000. These molecular weights are 
determined by gel permeation chromatography using polystyrene as standard. 
The crosslinking agent can be any one or more polyisocyanate(s) which can 
be used in organic solvents. Examples of polyisocyanates which can be used 
when practicing this invention include: toluene diisocyanates, 
isocyanurates of toluene diisocyanate, diphenylmethane 4,4-'diisocyanate, 
isocyanurates of 4,4'-diisocyanate, 1,4-cyclohexane diisocyanate, 
p-phenylene diisocyanate, triphenylmethane 4,4',4"-triiocyanate, 
metaxylene diisocyanate and polyisocyanates, 1,6-hexamethylene 
diisocyanate, isocyanurates of 1,6-hexamethylene diisocyanate, isophorone 
diisocyanate, isocyanurates of isophorone diisocyanate, methylene 
bis-4,4'-isocyanatocyclohexane, and 1,3-bis-(2-isocyanate 
propyl-2-)benzene (TMXDI), as well as adducts of these polyisocyanates on 
polyols, especially low molecular weight polyols (e.g., trimethylol 
propane) and isocyanurate group-containing and/or biuret group-containing 
polyisocyanates derived therefrom. As polyisocyanates, preferred use is 
made of 1,6-hexamethylene diisocyanate and isophorone diisocyanate, 
isocyanurate- and/or biuret-containing polyisocyanates which are derived 
from the latter and which preferably contain more than 2 isocyanate groups 
in the molecule. 
The polyisocyanates used when practicing this invention can be both in free 
and in blocked form. If the polyisocyanate being employed is in a blocked 
form, any blocking agent can be used, provided that the agent has a 
sufficiently low deblocking temperature. 
Groups suitable for use as the blocker portion of a blocked isocyanate are 
also well-known in the art. Examples of such suitable groups include: 
alcohols, lactams, oximes, malonic esters, alkyl acetoacetates, triazoles, 
phenols and amines. Of these, oximes (e.g., acetone oxime, methyl ethyl 
ketoxime, methyl amyl ketoxime, diisobutyl ketoxime, formaldehyde oxime) 
are preferred. 
In a preferred embodiment of the present invention, the film-forming 
composition contains a catalyst to accelerate the cure of the 
polyisocyanate and other -crosslinkable groups. Examples of useful 
catalysts include: metal acetonyl acetates, quaternary ammonium salts, 
zinc N-ethyl-N-phenyl dithiocarbamate, pentamethyl-diethylenetriamine 
benzoate, cyclohexylamine acetate, n,n-dimethyl cyclohexylamine acetate, 
ketimines, N-methyl morpholine, tin octoate, stannic chloride, butyl tin 
trichloride, dibutyl tin diacetate, dibutyl tin dilaurate, 
bis(2-ethylhexyl) tin oxide, 1,3-diacetoxy tetrabutyl stannoxate, dibutyl 
dibutoxy tin, lead naphthenate, bismuth trichloride, bismuth octoate, 
tetrabis (2-ethylhexyl)titanate, tetrabutoxy titanium, stannous octoate, 
manganese, zirconium, cobalt, lead, bismuth stannate, lead stannate, 
zirconium octoate, tin, dibutyl tin maleate, stannous oxalate, stannous 
stearate, barium nitrate, zinc nitrate, dibutyltin dilauryl mercaptide, 
bismuth stearate, lead stearate, dimethyltin dichloride, stannous 
naphthate, dibutyltin bis-O-phenylphenate, dibutyltin 
S,S-dibutyldithio-carbonate, and triphenylantimony dichloride. 
Organometallic catalysts having tin as the metal are preferred. Of these, 
dibutyltin dilaurate is most preferred. 
If employed, the catalyst is usually present in an amount ranging from 
between about 0.1 to about 5 weight percent, and preferably, from between 
about 0.5 to about 2 weight percent. These weight percentages are based 
upon the total weight of the resin solids in the coating composition. 
In addition to the above, optional ingredients, such as plasticizers, flow 
controllers, anti-oxidants, UV light absorbers and other similar additives 
known in the art, can also be included in the coating composition. If 
employed, the cumulative weight of these optional ingredients are 
typically present at up to about 25 weight percent. This weight percentage 
is based upon the total weight of the resin solids in the coating 
composition. 
The crosslinkable film-forming coating composition of the present invention 
can be used as the clear top coat layer in a "color-plus-clear" coating 
system. The base coat in a color-plus-clear system encompassed by the 
present invention can be any of the compositions useful in coatings 
applications, particularly those in automotive and general industrial 
applications. The film-forming composition of the base coat typically 
includes a resinous binder and a pigment. Examples of particularly useful 
resinous binders include acrylic polymers, polyesters (including alkyds) 
and polyurethanes. 
The base coat compositions may be solvent-borne or water-borne. Examples of 
water-borne base coats in a color-plus-clear system encompassed by the 
present invention are disclosed in U.S. Pat. No. 4,403,003. Moreover, 
water-borne polyurethanes, such as those disclosed in U.S. Pat. No. 
4,147,679, can also be used as the resinous binder in the base coat to 
prepare a coating-plus-clear system which is encompassed by the present 
invention. Further, water-borne coatings, such as those described in U.S. 
Pat. No. 5,071,904, can also be used as a base coat. 
Base coats also often contain pigments in order to give them the desired 
color. Moreover, base coat compositions containing metallic flake 
pigmentation are often used in the production of so-called "glamour 
metallic" finishes. These types of finishes are mainly used in the 
automotive industry. Examples of metallic flakes which can be used in a 
base coat of a color-plus-clear system encompassed by this invention 
include: aluminum flake, copper bronze flake and metal oxide coated mica. 
Besides the metallic pigments, the base coat compositions may also contain 
non-metallic color pigments conventionally used in surface coatings. 
Examples of such pigments include: 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, non-metallic pigments are typically incorporated into the base 
coat composition in an amount ranging from between about 1 to about 80 
percent by weight based on the total weight of coating solids. On the 
other hand, metallic pigments are typically employed in an amount ranging 
from between about 0.5 to 25 percent by weight based on the total weight 
of coating solids. 
If desired, the base coat composition may contain additional materials well 
known in the art of formulated surface coatings. Examples of such 
additional materials include: surfactants, flow control agents, 
thixotropic agents, fillers, anti-gassing agents, organic co-solvents, 
catalysts, and other customary auxiliaries. If employed, the cumulative 
weight of these materials can constitute up to 40 percent by weight of the 
total weight of the coating composition. 
The base coat compositions of a color-plus-clear system encompassed by the 
present invention can be applied to various substrates such as metals, 
plastics, wood, glass, woven fibers, non-woven fibers, foams, or a 
combination thereof. They are, however, particularly useful when applied 
over metals and elastomeric substrates that are found on motor vehicles. 
Moreover, the base coat compositions can be applied over such substrates by 
conventional means including brushing, dipping, flow coating, spraying and 
the like. Most often, the base coat composition is applied by spraying. 
Conventional spray techniques and equipment for air spraying and 
electrostatic spraying and either manual or automatic methods can be used. 
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 ranges from between about 0.01 to about 10 mils, and preferably, 
from between about 0.05 to about 5 mils, and even more preferably, from 
between about 0.1 to about 2 mils. 
After application of the base coat to the substrate, a film is formed on 
the surface of the substrate by driving solvent out of the base coat by 
heating or by an air drying period. Preferably, the drying period is only 
for that time which is sufficient to ensure that the clear top coat can be 
applied over the base coat without the top coat dissolving the base coat. 
Suitable drying conditions will depend, in part, upon the particular base 
coat composition, the ambient humidity, and the like. In general, a drying 
time of from about 1 to about 5 minutes, at a temperature of about 
68.degree.-250.degree. F. (20.degree.-121.degree. C.), will be adequate to 
ensure that mixing or "soak-in" of the two coats is minimized. At the same 
time, however, the base coat film is adequately wetted by the clear top 
coat composition so that satisfactory intercoat adhesion is obtained. 
More than one base coat and/or top coat may be applied to develop the 
optimum appearance of a color-plus-clear system encompassed by the present 
invention. Usually, the previously applied coat is flashed (i.e., exposed 
to ambient conditions for about 1 to 20 minutes) between coats. 
The clear top coat composition can be applied over the base coat film by 
any conventional coating technique. Examples of such conventional 
techniques include: brushing, dipping, flow coating, spraying and the 
like. Most often, the top coat is applied by spraying. Conventional spray 
techniques and equipment for air spraying and electrostatic spraying and 
either manual or automatic methods can be used. 
After application of the top coat over the base coat, the coated substrate 
is heated to cure the coating layers. In the curing operation, solvents 
are driven off and the film-forming materials of the clear coat and the 
base coat are each crosslinked. The heating or curing operation is usually 
carried out at a temperature of about 160.degree.-350.degree. F. 
(71.degree.-177.degree. C.). If needed, however, lower or higher 
temperatures may be used as necessary to activate crosslinking mechanisms. 
The thickness of the clear coat layer usually ranges from between about 
0.1 to about 10 mils, preferably, from between about 0.5 to about 7 mils, 
and even more preferably, from between about 1 to about 5 mils.