Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-04-23
2003-11-25
Sellers, Robert E. L. (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S410000, C525S186000, C525S449000
Reexamination Certificate
active
06652917
ABSTRACT:
The present invention relates to a novel thermosetting powder coating composition and a method for forming a topcoat using the same.
Topcoats for coating substrates such as automotive exterior panels are required to have excellent finished appearance such as surface smoothness and excellent coating properties such as weather resistance, solvent resistance, acid resistance, etc., in view of uses. In recent years, the required levels have been getting even higher.
Thermosetting powder coating compositions are conventionally used as clear coating compositions for forming topcoats by 2-coat 1-bake systems, etc. It is desired that the thermosetting aqueous coating compositions be used as base coating compositions, from the viewpoints of environmental protection and energy saving.
However, the thermosetting powder coating compositions have the following problems. When surface smoothness of the coating film is improved, other properties such as weather resistance, solvent resistance and acid resistance are impaired. On the other hand, when the other properties are improved, surface smoothness is reduced. It has been difficult to achieve a proper balance therebetween. When conventional thermosetting powder clear coating compositions are applied to surfaces of aqueous base coats, such problems develop to a serious level, and it has been difficult to form a topcoat that excels in both surface smoothness and the above-mentioned other coating properties.
For example, a 2-coat 1-bake coating method for forming a topcoat is known from U.S. Pat. No. 5,585,146. This method comprises the steps of applying to a substrate a thermosetting aqueous base coating composition comprising a hydroxyl- and carboxyl-containing resin and an alkyletherified methylol melamine resin, applying to the uncured surface of the base coat a thermosetting powder clear coating composition comprising an epoxy-containing acrylic resin, and a dibasic acid such as dodecanoic diacid or acid anhydride thereof as a curing agent, and curing the two coats by heating.
However, in the above 2-coat 1-bake coating method, when the coat of the aqueous base coating composition and the coat of the powder clear coating composition are heated for curing at the same time, a reaction between the hydroxyl group and the alkylether group in the aqueous coat produces an alcohol as a byproduct. When the alcohol evaporates out through the thermally fused powder coat, it causes cratering due to popping on the surface of powder coat, thereby impairing surface smoothness, distinctness-of-image gloss, and physical properties of the coating film. When the aqueous coat comprises a flaky metallic pigment, orientation of the pigment flakes is altered, thus impairing metallic effects, etc.
In this case, if curing initiation time is delayed so as to fully melt and flow the powder coat particles by heating with the purpose of improving coating surface smoothness, etc., an insufficiently crosslinked powder coat will result, thus impairing coating properties such as weather resistance, solvent resistance and acid resistance.
An object of the present invention is to provide a novel thermosetting powder coating composition that excels in finished appearance such as surface smoothness and also in other coating properties such as weather resistance, solvent resistance and acid resistance.
Another object of the invention is to provide a method for forming a topcoat using the above-mentioned thermosetting powder coating composition.
Other objects and features of the invention will become apparent from the following description.
The present inventors carried out intensive research to overcome the prior art defects and found that the above objects can be achieved with a thermosetting powder coating composition comprising an epoxy-containing resin and a curing agent for the resin containing a specific acid anhydride as an essential ingredient. More specifically, the present inventors found that the above-mentioned powder coating composition is excellent in curability, and when it is used as a clear coating composition in a 2-coat 1-bake coating method using an aqueous base coating composition, a fully crosslinked powder coat will be formed even with the curing initiation time being delayed, thus preventing popping and providing a topcoat with excellent finished appearance such as smoothness and excellent coating properties such as weather resistance, solvent resistance and acid resistance. The present invention has been accomplished based on this novel finding.
The present invention provides a thermosetting powder coating composition comprising:
(A) an epoxy-containing resin; and
(B) a curing agent containing acid anhydride (a) and at least one species selected from the group consisting of acid anhydride (b) and dibasic acid (c),
the acid anhydride (a) being represented by formula
X—COO—[CO—(CH
2
)
m
—COO]
n
—CO—X (1)
wherein the two X's are the same or different and independently represent a C
4-12
alkyl group having a branched structure, R
1
—O—CO—R
2
—, R
1
—CO—R
2
— or R
1
—CO—NH—R
2
— in which R
1
is a C
1-10
monovalent saturated hydrocarbon group; R
2
is a C
1-10
bivalent saturated hydrocarbon group; m is an integer of 6 to 18; and n is an integer of 8 to 50,
the acid anhydride (b) being represented by the formula
HO—[CO—(CH
2
)
p
—COO]
q
—H (2)
wherein p is an integer of 6 to 18 and q is an integer of 2 to 18, and having carboxyl groups at both ends,
the dibasic acid (c) being represented by the formula
HOOC—(CH
2
)
t
—COOH (3)
wherein t is an integer of 6 to 18,
the molar ratio of carboxyl groups to acid anhydride groups in the curing agent being in the range between 0.15:1 and 2:1, and
the molar ratio of the total of carboxyl groups and acid anhydride groups in the curing agent (B) to epoxy groups in the epoxy-containing resin (A) being in the range between 0.7:1 and 1.2:1.
The present invention further provides a method for forming a topcoat according to a 2-coat 1-bake system comprising the steps of applying to a substrate a thermosetting aqueous colored base coating composition, applying to the uncured surface of the base coat a clear coating composition, and curing the two coats by heating, wherein the clear coating composition is the above-mentioned thermosetting powder coating composition.
The epoxy-containing resin (A) of the thermosetting powder coating composition according to the invention may be any conventionally known resin that is solid and contains at least one epoxy group, preferably at least two epoxy groups, on average per molecule.
Examples of resins usable as the epoxy-containing resin (A) are bisphenol epoxy resins such as “EPIKOTE 1001” (with an epoxy equivalent of 450 to 500, product of Shell Chemical Co., Ltd., trade name), “EPIKOTE 1004” (with an epoxy equivalent of 875 to 975, product of Shell Chemical Co., Ltd., trade name), and “EPIKOTE 1007” (with an epoxy equivalent of 1,750 to 2,200, Shell Chemical Co., Ltd., trade name); and epoxy-containing copolymers each prepared by copolymerizing an epoxy-containing vinyl monomer and at least one other polymerizable vinyl monomer.
Examples of epoxy-containing vinyl monomers used in the epoxy-containing copolymers include glycidyl (meth)acrylate, allyl glycidyl ether, 3,4-epoxycyclohexyl (meth)acrylate and &bgr;-methylglycidyl (meth)acrylate. The monomers can be used singly or in combination of two or more. Particularly preferred are glycidyl (meth)acrylate and &bgr;-methylglycidyl (meth)acrylate.
Examples of other polymerizable vinyl monomers are vinyl aromatic compounds such as styrene, &agr;-methylstyrene, vinyltoluene and &agr;-chlorostyrene; and C
1-24
alkyl esters or cycloalkyl esters of acrylic or methacrylic acids, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl
Adachi Takato
Kawamoto Yugen
Yoshihara Ichiro
Armstrong Westerman & Hattori, LLP
Kansai Paint Co. Ltd.
Sellers Robert E. L.
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