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
2001-12-13
2003-09-02
Short, Patricia A. (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...
C525S438000, C525S440030, C525S934000, C428S413000, C428S423100
Reexamination Certificate
active
06613840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel and useful resin composition for a powder coating, to a powder coating, and to a coated article therewith. More specifically, the present invention relates to a resin composition for a powder coating which is highly suitable for practical use, comprising a non-crystalline polyester resin having carboxyl groups, a crystalline polyester polyurethane resin, and a curing agent thereof as an essential film-forming component, the resin composition yielding a powder coating which has superior storage stability (blocking resistance) and forms a coating film having superior smoothness and superior curability at low temperatures as well as having characteristics such as impact resistance (hereinafter referred to as mechanical properties) and processability and weatherability at the same time, and relates to a powder coating and to a coated article therewith.
2. Description of Related Art
In view of environmental problems such as air pollution, the powder coatings have recently become more common coatings containing no organic solvent because the powder coatings are less polluting. Although the powder coating has been widely used for various purposes, including coating of metal products in the fields of household electric appliances and building materials, the use as materials having low heat resistance such as coating agents for plastics and lumber has been studied because of the characteristic of the powder coating compositions that the solvents do not substantially evaporate during the application.
As a powder coating which is now being put to practical use, for example, polyester, epoxy and acrylic powder coatings have generally been known. Among these powder coatings, a polyester powder coating, especially a polyester powder coating comprising a combination of a non-crystalline polyester resin having a hydroxyl group or a carboxyl group and a curing agent has been widely used for the purposes which require such characteristics, for example, household electric appliances and building materials, because such a polyester powder coating has good balanced performance as a coating film and imparts good appearance and pliability to the coating film.
However, the above polyester powder coating comprising the non-crystalline polyester and the curing agent is suited for use in the field of household electric appliances which require good appearance and pliability of the coating film because of comparatively high molecular weight and high viscosity in a melted state, but is not suited for use in plastics, coating agents for lumber and vehicles which require smoothness of the coating film and curability at low temperatures because they are inferior in these performances.
Therefore, as a method of improving the smoothness of the surface of the coating film of the polyester powder coating, there have been proposed a method of lowering the viscosity in a melted state by reducing the average molecular weight of a polyester resin as a main component of the coating and a method of lowering the glass transition temperature by copolymerizing a component capable of imparting the flexibility.
However, these methods have a problem in that the storage stability (blocking resistance) of the coating is reduced, although the smoothness of the surface of the coating film is improved by reducing the softening point or glass transition temperature of the polyester resin.
Methods of improving curability at low temperatures includes a method of adding a curing catalyst. In this case, the smoothness of the coating film is inferior, although the curability at low temperatures is improved.
Although Japanese Unexamined Patent Application, First Publication No. 59-25841 discloses a technique about a composition for a powder coating, which suggests the use of a polyester polyurethane resin in combination with a polyester resin, a powder coating, using them in combination is not actually described. Moreover, the polyester polyurethane resin used alone is non-crystalline and, even if the polyester polyurethane is used in combination with the polyester resin, the resulting composition for powder coating, is inferior in storage stability.
BRIEF SUMMARY OF THE INVENTION
Objects of the present invention is to provide a resin composition for a powder coating which is highly suitable for practical use, the resin composition yielding a powder coating which has superior storage stability (blocking resistance) and forms a coating film having superior curability at low temperatures and superior smoothness, and to provide a powder coating and a coated article therewith.
To overcome various drawbacks and problems in the conventional art described above, the present inventors have intensively researched in view of the objects to be achieved by the present invention, and the present inventors have found that a resin composition for a powder coating, comprising a non-crystalline polyester resin having a carboxyl group, a crystalline polyester polyurethane resin, and a curing agent as an essential film-forming component has excellent smoothness of the coating film under curing conditions at low temperatures and has excellent storage stability (blocking resistance) of the coating and yields a powder coating which is highly suitable for practical use. Thus, the present invention has been completed.
The present invention provides a resin composition for powder coating, comprising a non-crystalline polyester resin (A) having a carboxyl group, a crystalline polyester polyurethane resin (B) and a curing agent (C) made of a compound having a functional group reactable with the carboxyl group as an essential component. The present invention also provides a powder coating comprising the resin composition for powder coating and a surface modifier. The present inventions 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 powder coating and the powder coating of the present invention are useful for the purposes such as automobile bodies, building materials, plastic products, and woodworking products because they are superior in storage stability (blocking resistance) of the coating and mechanical properties of the coating film while maintaining the weatherability of the coating film and are also superior in performances of the coating film, such as appearance (smoothness) of the coating film, boiling water resistance, moisture resistance, and corrosion resistance. The coated article of the present invention is superior in weatherability, mechanical properties, appearance (smoothness), boiling water resistance, moisture resistance, and corrosion resistance.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be described in detail.
The non-crystalline polyester resin (A) having a carboxyl group used in the present invention (hereinafter referred to as a non-crystalline polyester resin (A)) is not specifically limited as long as it has a carboxyl group in a molecule and is non-crystalline.
The term “non-crystalline” refers to the fact that a crystallization peak temperature is not exhibited in a differential scanning calorimetry (DSC) curve obtained in accordance with the conditions defined in JIS K7121.
Examples of the polyhydric carboxylic acid unit constituting the non-crystalline polyester resin (A) include terephthalic acid, isophthalic acid (including isophthalic acid wherein a hydrogen atom of the aromatic ring is substituted with an alkyl group, such as 4-methylisophthalic acid or 4-tertiary butylisophthalic acid) and an anhydride thereof, adipic acid, sebacic acid, succinic acid, maleic acid, fumaric acid, tetrahydrophthalic acid, hexahydrophthalaic acid and anhydrides thereof.
Among these polyhydric carboxylic acid units, isophthalic acid accounts for 50 mol % or more, and preferably for 70 mol % or more of the polyhydric carboxylic unit, because the coating film having excellent weatherability and smoothness can be obtained.
In th
Agawa Tetsuro
Itou Hiroyuki
Armstrong Westerman & Hattori, LLP
Dainippon Ink and Chemicals Inc.
Short Patricia A.
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