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
2002-02-01
2004-03-23
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S454000, C525S457000, C106S287250, C106S287260, C428S423100
Reexamination Certificate
active
06710137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resin composition for a powder coating, which is useful for applications such as building materials, metal products, and parts for automobiles, and also relates to a powder coating and an article coated therewith.
2. Description of Related Art
The demand for thermosetting powder coatings as coatings for the protection and decoration of household electric appliances, furniture made of steel, building materials, and parts for automobiles has rapidly increased because thermosetting powder coatings are less polluting and free from organic solvents, and are also superior in the physical properties of the coating film and are inexpensive compared with conventional solvent type coatings.
As powder coatings which are being practically used, for example, polyester resin powder coatings, epoxy resin powder coatings, and acrylic resin powder coatings are generally known. Among these powder coatings, polyester powder coatings are widely used because coated films of such polyester powder coatings have superior physical properties and are inexpensive.
When using this polyester resin, terephthalic acid is generally used as the main polycarboxylic acid component because the glass transition temperature must be raised in view of the storage stability. However, this polyester resin has had the problem that the resulting coating film has poor weatherability, for example, the coating film undergoes whitening, choking or severe gloss reduction in a very short period.
As a means for solving this problem, Japanese Unexamined Patent Application, First Publication No. Hei 9-71738 proposes a polyester resin for powder coating comprising isophthalic acid as an acid component. However, there is the problem that this powder coating can be used only for specific purposes which do not require physical properties such as flexibility of the coating film because the physical properties are adversely affected, although the weatherability of the coating film is markedly improved.
To solve the problems described above, (1) a method of using terephthalic acid in combination with isophthalic acid, and (2) a method of combining weatherability and flexibility of a coating film by polycondensation of isophthalic acid with an aliphatic dicarboxylic acid and an aliphatic glycol which are used in combination with isophthalic acid have been proposed. However, the method (1) has the problem that the physical properties such as the flexibility are degraded as the amount of the isophthalic acid relative to the terephthalic acid increases and, if the amount of isophthalic acid is small, good weatherability, which is a feature due to isophthalic acid is not exhibited, while the method (2) has the problem that a powder coating having a sufficient storage stability to be practical cannot be obtained, although the weatherability and the flexibility of the coating film are improved. Therefore, it has been difficult to obtain a polyester resin powder coating comprising isophthalic acid as a main acid component which produces a coating film having good flexibility and good weatherability.
Japanese Unexamined Patent Application, First Publication No. Hei 7-247448 does not actually disclose a composition for powder coating wherein a crystalline polyurethane resin and a polyester resin are used in combination. Although a crystalline polyester polyurethane resin containing isophthalic acid as a carboxylic acid component is disclosed, a polyester resin prepared from isophthalic acid as a raw material is not disclosed. Assuming that only a crystalline polyuretane resin prepared from isophthalic acid as a raw material is used, a coating film having good balanced performance cannot be obtained.
BRIEF SUMMARY OF THE INVENTION
Objects of the present invention are to provide a resin composition for a powder coating, which can form a coating film having superior appearance, superior flexibility and superior weatherability while maintaining the storage stability of the coating, and to provide a powder coating and an article coated therewith.
To overcome various problems, the present inventors have carried out intensive research and found that a composition for a powder coating, comprising a polyester resin, a carboxylic acid component constituting the polyester resin containing a specific amount of isophthalic acid as a component, a crystalline polyester polyurethane resin, and a curing agent which is reactive with a hydroxyl group, can form a coating film having superior appearance, superior flexibility and superior weatherability. Thus, the present invention was completed.
The present invention provides a resin composition for powder coating, comprising a polyester resin (A) having a hydroxyl group or having a hydroxyl group and an acid group, isophthalic acid accounting for 50 mol % or more of the polycarboxylic acid units constituting the polyester resin; a crystalline polyurethane resin (B); and a curing agent (C-1) made of a compound having a functional group reactive with the hydroxyl group or a combination of a curing agent (C-1) made of a compound having a functional group reactive with the hydroxyl group and a curing agent (C-2) made of a compound having a functional group reactive with the acid group. The present invention also provides a coated article obtained by coating a substrate with the powder coating and curing the coating film with heat.
The resin composition for the powder coating and the powder coating of the present invention are useful for applications such as automobile bodies, building materials, plastic products, and woodworking products because they are superior in appearance, flexibility and weatherability of the coating film while maintaining the storage stability of the coating film. The coated article of the present invention is superior in appearance, weatherability, flexibility, water resistance, moisture resistance, boiling water resistance, and corrosion resistance.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail.
First, the polyester resin (A) having a hydroxyl group or having a hydroxyl group and an acid group used in the present invention will be described.
The polyester resin (A) having a hydroxyl group or having a hydroxyl group and an acid group (hereinafter referred to as a polyester resin (A)) is in the form of a solid at normal temperature, and the polycarboxylic acid units constituting the resin contain isophthalic acid as an essential component.
The hydroxyl value of the polyester resin (A) is preferably within a range from 10 to 100 in view of the curability.
In view of the reactivity with the curing agent, primary hydroxyl groups preferably account for the half or more of the hydroxyl groups. In the case of secondary or tertiary hydroxyl groups, the physical properties of the cured coating film are less likely to be exhibited.
The acid value of the polyester resin (A) is not specifically limited, and is preferably from 10 to 100 because the crosslink density is enhanced, thereby to enhance the mechanical strength and to enhance the storage stability.
As used herein, the term “hydroxyl value” refers to the number of mg of potassium hydroxide required to neutralize the carboxylic acid which can be bonded to hydroxyl groups in 1 g of the resin by acetylation.
As used herein, the term “acid value” refers to the number of mg of potassium hydroxide required to neutralize the carboxylic acid groups in 1 g of the resin.
The glass transition temperature (hereinafter referred to as Tg) of the polyester resin (A) is not specifically limited, but is preferably 50° C. or higher, and particularly preferably 60° C. or higher.
Tg is based on the numerical value determined from a differential scanning calorimetry (DSC) curve obtained according to the conditions of JIS K7122.
The softening point of the polyester resin (A) is not specifically limited, but is preferably within a range from 80 to 150° C., and more preferably from 100 to 120° C., in view of the blocking resistance and smooth
Agawa Tetsuro
Matsumoto Akira
Bissett Melanie
Dainippon Ink and Chemicals Inc.
Seidleck James J.
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