Metallic coating using two coat one bake method

There is provided a two-coat, one-bake metallic coating method which comprises applying, first, (a) a base coat composition containing an acrylic polyol oligomer having an average molecular weight of 500 to 2000 and a non-yellowing polyisocyanate compound in a NCO/OH ratio of 0.5/1 through 1/0.5 and further containing an appropriate amount of a metallic pigment and, then, on top thereof (b) a clear top coat composition containing said acrylic polyol oligomer and non-yellowing polyisocyanate compound in a NCO/OH ratio of 0.5/1 through 1/0.5 and causing the resulting coats to cure simultaneously. There are also provided coating compositions to be used in the above coating method.

FIELD OF THE INVENTION 
This invention relates to a two-coat, one-bake metallic coating method and 
coating compositions used therein. 
The coating materials commonly used today for metallic coating of 
automobiles are based on acrylic-melamine resins, i.e. acrylic resins 
cured with a melamine resin. This is because these resins provide stable 
coating materials and, also, coats which are resistant to weathering. 
Further, to prevent deterioration of the gloss and other aethetic 
appearance of the coat upon prolonged weathering, it is common practice to 
use a wet-on-wet coating procedure of the two-coat, one-bake type, wherein 
a clear coat is superimposed on a pigmented base coat and the two coats 
are simultaneously baked to cure. 
In metallic coating, aluminum pigment is incorporated in the coating matrix 
to impart a metallic luster to the coat but if the fluidity of the coating 
material is too high at the time of application there will be a migration 
of aluminum particles causing an uneven color or gloss and a loss of the 
metallic tone desired in the finished coat. 
In a coating regimen such as the two-coat, one-bake system wherein uncured 
coating materials are applied in successive layers, the base coat 
composition must be so designed that the aluminum pigment particles will 
not drift therein and that the pigment particles will not be dislocated on 
account of the compatibility of the solvent and resin contained in the 
clear top coating. For this reason, attempts have been made to decrease 
the compatibility of the base coat resin and clear coat resin to some 
extent, to increase the molecular weight of the base coat resin to reduce 
the influence of the clear top coating, or to use a low-boiling solvent to 
reduce the amount of solvent present at the time of application. However, 
none of these known methods is able to prevent migration of aluminum 
particles. 
Recently, from the standpoint of economizing natural resources and energy 
and because of the requirements for pollution control, much research has 
been done for increasing the nonvolatile contents of coating materials. 
However, if an acrylic polyol whose molecular weight has been reduced for 
increased nonvolatile contents is cured with a melamine resin, the coating 
film will have only a drastically reduced service life and if such a 
system is utilized for two-coat, one-bake metallic coating, the 
disadvantages referred to hereinbefore arise of necessity. 
This invention overcomes the above-mentioned disadvantages. Thus, in 
accordance with this invention, a non-yellowing polyisocyanate is employed 
as a curing or cross-linking component. This feature coupled with other 
features described hereinafter, of this invention ensures the following 
advantages. In accordance with this invention, a low molecular weight 
acrylic polyol can be used to increase the nonvolatile content of the 
coating composition. Moreover, the resulting decrease in the required 
amount of the solvent plus the thickening effect due to the rapid curing 
reaction of the acrylic polyol with the polyisocyanate compound helps 
prevent migration of metallic particles to thereby ensure a metallic coat 
having improved gloss and other aesthetic qualities and improved weather 
resistance. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to a two-coat, one-bake metallic coating method 
wherein a base coat composition containing a first film-forming binder, a 
metallic pigment and a solvent for said first film-forming binder is 
applied, a clear top coat composition containing a second film-forming 
binder and a solvent thereof is applied wet-on-wet onto the base coat, and 
the resulting multicoat is cured simultaneously. The improvement therein 
comprises decreasing the solvent required in said method and producing a 
rapidly curable coat, thereby ensuring a metallic coat with good aesthetic 
and weathering properties by preventing migration of metal particles from 
the metal pigment into the top coat, by employing as said first 
film-forming binder one consisting essentially of (a) a hydroxyl 
group-containing acrylic oligomer having an average molecular weight of 
500 to 2000, (b) a non-yellowing polyisocyanate in an NCO/OH ratio of 
0.5/1 to 1/0.5 relative to said acrylic oligomer, or a combination thereof 
with up to 30 percent by weight of the polyol (as non-volatile solids) of 
a melamine resin; and (c) a film-forming cellulose derivative, and as said 
second film-forming binder one consisting essentially of an acrylic 
oligomer as defined hereinabove and a polyisocyanate as defined 
hereinabove, in an NCO/OH ratio of 0.5/1 to 1/0.5 relative to said acrylic 
oligomer, or a combination thereof with up to 30 percent by weight of the 
polyol (as non-volatile solids) of a melamine resin. 
The base coat composition mentioned above preferably contains a cellulose 
derivative in a proportion of 5 to 30 percent based on the nonvolatile 
content thereof. 
The above-mentioned base coat composition and/or top coat composition may 
optionally contain an ultraviolet absorber and an antioxidant. 
The acrylic polyol oligomer as used according to this invention includes, 
for example, copolymers of OH-containing unsaturated monomers such as 
hydroxyalkyl acrylates (e.g. .beta.-hydroxyethyl acrylate, 
.beta.-hydroxypropyl acrylate, etc.), and hydroxyalkyl methacrylate (e.g. 
.beta.-hydroxyethyl methacrylates, etc.), with other unsaturated monomers 
such as acrylic acid and its esters (e.g. ethyl acrylate, propyl acrylate, 
butyl acrylate, etc.) and methacrylic acid esters. These acrylic polyol 
oligomers should have OH numbers in the range of 50 to 300, preferably 80 
to 200, acid numbers in the range of 5 to 50, preferably 10 to 30, and 
number average molecular weights in the range of 500 to 2000, preferably 
800 to 1400. If the number average molecular weight of the acrylic polyol 
oligomer is less than 500, the mechanical and other physical properties of 
the resulting film will be unsatisfactory, while a number average 
molecular weight in excess of 2000 will lead to only a low nonvolatile 
content at the time of coating and, therefore, the desired high 
nonvolatile contents will not be attained. 
The polymerization method that can be used in the production of said 
acrylic polyol oligomer is optional and may for example be a process using 
a large quantity of a radical polymerization initiator or a process 
employing a large amount of a chain transfer agent such as an organic 
thiol compound. 
The non-yellowing polyisocyanate compound includes, among others, such 
polyisocyanates as aliphatic or alicyclic diisocyanates such as 
hexamethylene diisocyanate (HMDI), isophoron diisocyanate (IPDI), 
hydrogenated xylylene diisocyanate (hydrogenated XDI), hydrogenated 
tolylene diisocyanate (hydrogenated TDI), etc., adducts of such 
polyisocyanates to polyols such as ethylene glycol, propylene glycol, 
trimethylolpropane, etc., and blocked polyisocyanates. Trimers of 
aliphatic or alicyclic diisocyanate such as HMDI or IPDI having an 
isocyanurate structure are preferable. 
The proportion of said polyisocyanate, in terms of NCO/OH ratio, is within 
the range of 0.5/1 through 1/0.5. If the proportion is less than 0.5/1, 
the performance of the film will be adversely affected resulting in a too 
flexible, soft coat, whereas the use of polyisocyanates in an amount over 
1/0.5 will increase the cost of the coating material and result in a hard, 
non-flexible, unserviceable coat. 
While these polyisocyanate compounds may be employed singly, several 
different polyisocyanate compounds may be used in combination, or in 
conjunction with melamine resins for the purpose of securing a balance 
between the curing property of the composition and the physical 
performance of the coat. When a melamine resin is used in combination with 
the polyisocyanate compound, the former is used in a proportion of 5 to 30 
percent (as nonvolatile solids) based on every 100 weight parts of the 
acrylic polyol oligomer. If the amount of such melamine resin exceeds the 
above range, the durability, especially the water resistance, of the coat 
will be sacrificed. 
The following is a partial list of commercially available polyisocyanate 
compounds that can be employed in the practice of this invention. Coronate 
EH (Japan Polyurethane) and Desmodur KL-2444 (Bayer A. G.) in the HMDI 
series containing isocyanurate rings; Sumidur N, Desmodur L-2291 (Bayer A. 
G.) and Duranate 24A-90CX (Asahi Kasei) in the HMDI series containing 
biuret groups; Desmodur Z-4370 (Bayer A. G.) and T-1890 (Hultz in the IPDI 
series containing isocyanurate rings; and Takenate D-120N (Takeda Chemical 
Industries, Ltd.) in the hydrogenated XDI series. 
The cellulose derivatives which may be employed in accordance with this 
invention include cellulose acetate butyrate (CAB), nitrocellulose (NC), 
etc. 
The ultraviolet absorber that can be employed in accordance with this 
invention may be any of salicylic acid esters, benzophenones, 
benzotriazoles, benzoates and substituted acrylonitrile compounds, or a 
mixture thereof. 
The antioxidant that can be employed in accordance with this invention may 
be any of, or a mixture of, imidazoles, phenols, amines, quinones, 
thiopropionic acid esters and organophosphites. Moreover, the base coat 
and/or the top coat composition according to this invention may contain a 
curing catalyst. The curing catalyst may be a known catalyst, and from the 
standpoints of reactivity, weather resistance and odor, is preferably an 
organotin compound. 
The metallic pigment of the base coat composition may be aluminum pigment, 
copper pigment, mica powder or the like and its proportion may range from 
2 to 20 weight parts per 100 weight parts of resin. The coloring pigment 
may be a conventional inorganic or organic pigment, and may be used in a 
proportion of 5 to 200 weight parts based on 100 weight parts of resin. 
In the coating procedure, the base coat composition is first applied and 
after a setting time of 1 to 10 minutes at room temperature, the top coat 
composition is applied, followed by another setting time of 5 to 20 
minutes at room temperature. The coats so applied are then cured. The 
curing conditions vary with different curing agents. In the case of a 
two-component system using a polyisocyanate, the curing temperature is 
room temperature to 120.degree. C. When a blocked isocyanate or/and a 
melamine resin is employed, the curing temperature is 100.degree. to 
200.degree. C.

The following examples are intended to illustrate this invention in further 
detail. In these examples, all parts and percents are by weight. 
PRODUCTION EXAMPLE 1 
[Production of acrylic polyol (A)] 
A one-liter flask equipped with a stirrer, thermometer, nitrogen gas inlet 
pipe, cooler and drip funnel was charged with 170 g of an aromatic solvent 
(Solvesso 100, Esso) and the solvent was heated to 155.degree. C. The 
flask was further charged with 116 g of n-butyl acrylate, 80 g of 
2-hydroxyethyl acrylate, 4 g of acrylic acid and a mixture of 14 g of 
tert-butylperoxy-2-ethylhexanoate and 30 g of the aromatic solvent 
(Solvesso 100) dropwise at 155.degree. C. over a period of 3 hours. After 
the dropwise addition had been completed, the mixture was maintained at 
155.degree. C. for a further 90 minutes. Then, the reaction mixture was 
cooled to 100.degree. C. and the solvent was removed under a vacuum at 5 
mmHg to give an acrylic copolymer. The characteristics of this acrylic 
copolymer resin are shown in Table 1. 
PRODUCTION EXAMPLE 2 
[Production of acrylic polyol (B)] 
The procedure of Production Example 1 was repeated except that the 
following conditions were used instead. The characteristics of the 
resulting acrylic copolymer are set forth in Table 1. 
______________________________________ 
Reaction temperature 
130.degree. C. 
Initial charge 
Xylol 170 g 
Monomers n-Butyl acrylate 116 g 
2-Hydroxyethyl acrylate 
80 g 
Acrylic acid 4 g 
Lauryl mercaptan 12 g 
Initiator solution 
2,2'-Azobisisobutyronitrile 
6 g 
Butyl acetate 120 g 
______________________________________ 
PRODUCTION EXAMPLE 3 
[Production of acrylic polyol (C)] 
The procedure of Production Example 1 was repeated except that the 
following conditions were used instead. The characteristics of the 
resulting acrylic copolymer are set forth in Table 1. 
______________________________________ 
Reaction temperature 
130.degree. C. 
Initial charge 
Xylol 170 g 
Monomers n-Butyl acrylate 128.4 g 
2-Hydroxyethyl acrylate 
62 g 
n-Butyl methacrylate 
5.6 g 
Acrylic acid 4 g 
2-Mercaptoethanol 8 g 
Initiator solution 
2,2'-Azobisisobutyronitrile 
6 g 
Butyl acetate 120 g 
______________________________________ 
TABLE 1 
______________________________________ 
Characteristics of Resins 
Product 
Acrylic Acrylic 
Specification 
Acrylic polyol A 
polyol B polyol C 
______________________________________ 
Nonvolatile (%) 
99 98 99 
OH number 193 150 193 
Acid number 16 16 16 
Tg (.degree.C.) 
-40 -40 -40 
Mn 900 950 1100 
.alpha. = M-.sub.w/M.sbsb.n 
2.0 2.0 2.0 
______________________________________ 
EXAMPLE 1 
On a tempered steel sheet pretreated with zinc phosphate was formed an 
electro-deposition film and, then, a primer surfacer. Then, a base coat 
composition of the following formulation A and a top coat composition of 
the following formulation B were successively applied in a wet-on-wet 
relation. The coats were then baked at 100.degree. C. for 30 minutes. 
The coating viscosities were adjusted to 20 seconds (Ford Cup #4/25.degree. 
C.) for the base coat and 25 seconds (Ford Cup #4/25.degree. C.) for the 
top coat. The diluents for these adjustments were a 50:50 mixture of butyl 
acetate and xylol for the base coat and an aromatic hydrocarbon solvent 
mixture (Solvesso 100 and Solvesso 150, Esso) for the top coat. 
The coating was performed at 25.degree. C., and an interval of about 3 
minutes was provided between the applications of the base and top coat 
compositions. After application of the top coat composition, the work was 
allowed to stand for 10 minutes and, then, oven-baked to cure. 
The performance characteristics of the resulting coating film are shown in 
Table 2. 
______________________________________ 
(Formulation A - Base coat) 
Acrylic polyol 100 parts 
Aluminum pigment A 30.4 parts 
(Asahi Kasei, Alpaste 51-231) 
Cellulose derivative solution 
64 parts 
(Eastman Kodak, 20% CAB in butyl acetate) 
Isocyanate prepolymer A 87 parts 
(Bayer A. G., Desmodur Z-4370) 
Isocyanate prepolymer B 21 parts 
(Japan Polyurethane, Coronate EH) 
Solvent (xylol) 10 parts 
Total 312.4 parts 
(Formulation B - Top coat) 
Acrylic polyol A 100 parts 
Isocyanate prepolymer A 87 parts 
Isocyanate prepolymer B 21 parts 
Surface modifier 0.5 part 
(Monsant, Modaflow solution) 
Ultraviolet absorber 3.6 parts 
(Ciba-Geigy, Tinuvin LS-770) 
Solvent (xylol) 10 parts 
Total 222.1 parts 
______________________________________ 
EXAMPLE 2 
A coating film was produced by the same procedure as Example 1 except that 
Formulation C was used for the base coat and Formulation D for the top 
coat. The performance characteristics of the coating film are shown in 
Table 2. 
______________________________________ 
(Formulation C - Base coat) 
Acrylic polyol C 100 parts 
Aluminum pigment A 30.4 parts 
Cellulose derivative solution 
64 parts 
Isocyanate prepolymer A 
125.4 parts 
Solvent (xylol) 10 parts 
Total 329.8 parts 
(Formulation D - Top coat) 
Acrylic polyol C 100 parts 
Isocyanate prepolymer A 
125.4 parts 
Surface modifier 0.5 part 
Solvent (xylol) 10 parts 
Total 235.9 parts 
______________________________________ 
EXAMPLE 3 
A coating film was produced by the same procedure as Example 1 except that 
Formulation E and Formulation F were used. The performance characteristics 
of the coating film are shown in Table 2. 
______________________________________ 
(Formulation E - Base coat) 
Acrylic polyol B 100 parts 
Aluminum pigment B 30 parts 
(Toyo Ink, Alpaste 1109MA) 
Cellulose derivative solution 
85 parts 
Isocyanate prepolymer A 
96 parts 
Surface modifier 0.5 part 
Solvent (xylol) 10 parts 
Total 321.5 parts 
(Formulation F - Top coat) 
Acrylic polyol B 100 parts 
Isocyanate prepolymer A 
96 parts 
Surface modifier 0.5 part 
Ultraviolet absorber A 1.0 part 
Antioxidant 2.0 parts 
(Ciba-Geigy, Irganox 1010) 
Solvent (xylol) 10 parts 
Total 209.5 parts 
______________________________________ 
EXAMPLE 4 
A coating film was produced by the same procedure as Example 1 except that 
0.2 part of dibutyltin dilaurate was added to Formulation A for the base 
coat. The performance characteristics of the film are shown in Table 2. 
EXAMPLE 5 
A coating film was produced by the same procedure as Example 2 except that 
the cellulose derivative solution was omitted from Formulation C for the 
base coat. The performance characteristics of the film are shown in Table 
2. 
EXAMPLE 6 
A commercial acrylate-melamine coating material of melamine-curing type, 
which is commonly employed, and the coating composition of this invention 
were used as the base coat composition and the top coat composition, 
respectively. The base coat was applied by diluting the coating material 
to 15"/#4FC/25.degree. C. with a solvent mixture of butyl acetate, ethyl 
acetate and xylene (0.25/0.25/0.5 wt. part) and after an interval of 3 
minutes, the top coat composition of this invention was applied. The work 
was baked at 140.degree. C. for 30 minutes. The dilution of the top coat 
composition was carried out in the same manner as Example 1. The 
performance characteristics of the resulting film are shown in Table 2. 
______________________________________ 
(Formulation G - Base coat) 
Acrylic polyol 100 parts 
(Dainippon Ink and Chemicals Inc., 
Acrydic 47-712) 
Butoxylated melamine 21 parts 
(Dainippon Ink and Chemicals Inc., 
Super Bekkamin L-117-60) 
Aluminum pigment B 8.5 parts 
Solvent (n-butanol/xylol) 
7.1 parts/16 
parts 
Total 152.6 parts 
(Formulation H - Top coat) 
Acrylic polyol A 100 parts 
Isocyanate prepolymer A 87 parts 
Butoxylated melamine 15 parts 
(Mitsui Toatsu Co., Ltd., Uvan 20SE-60) 
Surface modifier 0.5 part 
Ultraviolet absorber A 2.0 parts 
Solvent (xylol) 10 parts 
Total 214.5 parts 
______________________________________ 
CONTROL EXAMPLE 1 
The coating system used commonly today was used for control purposes. The 
base coat composition was diluted in the same manner as Example 6 and the 
top coat was applied at a coating viscosity of 20 seconds/#4FC/25.degree. 
C. A coating film was produced under otherwise the same conditions as 
described in Example 6. The performance characteristics of the resulting 
film are shown in Table 2. 
(Formulation I--Base Coat) 
Same as Formulation G of Example 6. 
______________________________________ 
(Formulation J - Top coat) 
Acrylic polyol (Dainippon Ink and Chemicals 
100 parts 
Inc., Acrydic 44-179) 
Butoxylated melamine (Dainippon Ink 
27.2 parts 
and Chemicals Inc., Super Bekkamin L-117-60) 
Surface modifier 0.4 part 
Solvent (xylol) 17.3 parts 
Ultraviolet absorber A 1.2 parts 
Total 146.1 parts 
______________________________________ 
CONTROL EXAMPLE 2 
Formulation K and Formulation L, both shown below, were used for the base 
coat and the top coat, respectively, and the baking was performed at 
140.degree. C. for 30 minutes. Otherwise the same procedure as Example 1 
was followed to produce a coating film. The performance characteristics of 
the film are shown in Table 2. 
______________________________________ 
(Formulation K - Base coat) 
Acrylic polyol A 100 parts 
Butoxylated melamine 71 parts 
(Mitsui Toatsu, Uvan 20SE-60) 
Aluminum pigment A 24 parts 
Cellulose derivative solution 
73 parts 
Solvent (xylol) 10 parts 
Total 278 parts 
(Formulation L - Top coat) 
Acrylic polyol A 100 parts 
Butoxylated melamine 71 parts 
(Mitsui Toatsu, Uvan 20SE-60) 
Surface modifier 0.4 part 
Ultraviolet absorber A 1.5 parts 
Solvent (xylol) 10 parts 
Total 182.9 parts 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Performance Characteristics of Coating Films 
Control 
Control 
Example 
Example 
Example 
Example 
Example 
Example 
Example 
Example 
Test parameter 
1 2 3 4 5 6 1 2 
__________________________________________________________________________ 
Nonvolatile matter (%) 
at spraying 
Base coat 52 47 53 52 62 23 23 40 
Top coat 64 59 63 64 65 59 40 53 
Appearance Very Good Very Very Fair Good Good Fair 
good good good 
Gloss 94 93 94 93 92 95 95 93 
Hardness 2H 2H 2H 2H 2H 2H 2H 5B 
Weather 
resistance 
% Gloss retention 
99 99 99 99 99 99 99 -- 
.DELTA.E 0.90 1.24 1.20 1.10 1.10 1.20 1.00 -- 
Solvent resistance 
Good Good Good Good Good Good Good Not 
Good 
__________________________________________________________________________ 
The testing methods are as follows. 
(1) Nonvolatile matter at spraying: For the base coat composition, the 
nonvolatile matter at a Ford Cup viscosity of 20 seconds (25.degree. C.) 
at spraying was determined. As to the top coat composition, the 
nonvolatile matter at a Ford Cup viscosity of 30 seconds (25.degree. C.) 
at spraying was determined. 
(2) Appearance: The metallic feeling and sharpness of the film were 
visually evaluated. 
(3) Gloss: The gloss at 60.degree. was measured. 
(4) Hardness: The Mitsubishi-Uni pencil was used. 
(5) Weather resistance: QUV (UV 4 hrs., moisture resistance 4 hrs., 
60.degree. C.): Each specimen was subjected to 8 cycles. 
(6) Solvent resistance: The xylol rubbing test was performed.