Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-02-27
2002-06-11
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S449000
Reexamination Certificate
active
06403715
ABSTRACT:
The present invention relates to a 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. From the viewpoints of environmental protection and energy saving, it is desired that the thermosetting aqueous coating compositions be used as base coating compositions.
However, the thermosetting powder coating compositions have the following problems. When surface smoothness of the coating film is improved, other coating 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, followed by 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 the above method, 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 invention provides the following thermosetting powder coating composition and method for forming a topcoat using the composition:
1. A thermosetting powder coating composition comprising:
(A) an epoxy-containing resin; and
(B) a curing agent containing, as an essential ingredient, an acid anhydride (a) represented by the formula
HOOC—(CH
2
)
m
—[COO—CO—(CH
2
)
m
]
n
—COOR
1
(1)
wherein R
1
represents a C
1-10
monovalent saturated hydrocarbon group, m is an integer of 6 to 18, and n is a number of 1 to 20 on average, the molar ratio of carboxyl groups to acid anhydride groups in the curing agent being in the range from 0.15:1 to 2.0:1, 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 from 0.7:1 to 1.2:1.
2. The powder coating composition according to item 1 wherein the epoxy-containing resin (A) is an epoxy-containing copolymer prepared by copolymerizing an epoxy-containing vinyl monomer and at least one other polymerizable vinyl monomer.
3. The powder coating composition according to item 2 wherein the epoxy-containing resin (A) comprises 20 to 70% by weight of an epoxy-containing vinyl monomer as a monomer component of the epoxy-containing copolymer.
4. The powder coating composition according to item 1 wherein the epoxy-containing resin (A) has a glass transition temperature of 40° C. to 80° C.
5. The powder coating composition according to item 1 wherein the epoxy-containing resin (A) has a number average molecular weight of 1,000 to 10,000.
6. The powder coating composition according to item 1 wherein the curing agent (B) contains acid anhydride (a) and at least one species selected from the group consisting of acid anhydride (b) and dibasic acid (c), the acid anhydride (b) being represented by the formula
R
2
OOC—(CH
2
)
p
—[COO—CO—(CH
2
)
p
]
q
—COOR
2
(3)
wherein R
2
represents a C
1-10
monovalent saturated hydrocarbon group; p is an integer of 6 to 18; and q is a number of 1 to 20 on average, and the dibasic acid (c) being represented by the formula
HOOC—(CH
2
)
r
—[COO—CO—(CH
2
)
r
]
s
—COOH (3)
wherein r is an integer of 6 to 18; and s is a number of 0 to 20 on average.
7. The powder coating composition according to item 1 or 6 wherein the molar ratio of COOR
1
groups or the total of COOR
1
groups and COOR
2
groups in the curing agent (B) to the total of carboxyl groups and acid anhydride groups in the curing agent (B) being in the range from 0.01:1 to 0.1:1.
8. The powder coating composition according to item 1 wherein the acid anhydride (a) in the curing agent (B) has a melting point of 60° C. or higher.
9. The powder coating composition according to item 6 wherein the acid anhydride (b) and dibasic acid (c) in the curing agent (B) have melting points of 60° C. or higher.
10. The powder coating composition according to item 6 wherein the mixture of acid anhydride (a) with acid anhydride (b) and/or dibasic acid (c) has a melting point of 60° C. or higher.
11. 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 thermosetting powder coating composition according to item 1.
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 which comprises, as a curing agent for an epoxy-containing resin, a curing agent essentially containing the above specified acid anhydride (a). Stated more specifically, the present inventors found that the above 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 epoxy-containing resi
Adachi Takato
Harakawa Hiromi
Kawamoto Yugen
Kansai Paint Co. Ltd.
Sellers Robert E. L.
LandOfFree
Powder coating of epoxy resin and COOH- and ester-terminated... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Powder coating of epoxy resin and COOH- and ester-terminated..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Powder coating of epoxy resin and COOH- and ester-terminated... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2926970