Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-04-14
2003-01-14
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S403000, C523S404000, C523S427000, C523S434000, C524S501000, C524S503000, C525S058000, C525S386000, C525S404000, C525S407000, C525S408000
Reexamination Certificate
active
06506820
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for manufacture of particles for powder coating, particles for powder coating, a method for formation of a multilayer film and a multilayer film obtained therefrom.
RELATED ART
In recent years, powder coatings which do not release organic solvents to an atmosphere have been widely noted as eco-friendly coatings. Among powder coatings, thermosetting powder coatings are advantageously used for their excellent weather resistance. However, the conventional thermosetting powder coatings providing a good appearance of a coating film suffer from the deficiency in blocking resistance, which is one of the properties associated with the storage stability. The increase in Tg of a component resin, while generally known to be effective to improve blocking resistance, results in the reduced surface smoothness of a coating film. This actually has prohibited their application to automotive bodies which demand a high-quality appearance.
Also, with the recent change of the circumstances surrounding us, a need has arisen for coating films, particularly automobile clear topcoats, which can endure an acid rain, i.e., exhibit improved acid resistance.
In order to achieve the improvement in acid resistance of powder coatings, the use of various curing systems has been attempted. However, the resulting coating films have low levels of molecular weight between cross-linkages to result in the unsuccessful improvement in acid resistance thereof.
In a parallel development, a wet process has been proposed which produces resin particles in an aqueous medium. For example, in U.S. Pat. No. 5,610,269, , use of the wet process is disclosed by which resin particles having a narrow size distribution are produced. Yet, if this technique was applied for producing particles for powder coating, it has been insufficient to achieve desired property improvements, such as in blocking resistance of particles for powder coating and in surface smoothness of a coating film prepared therefrom.
SUMMARY OF THE INVENTION
The present invention provides resin particles for powder coating which have improved blocking resistance and can be rendered into a film having a sufficient acid resistance, and a method for manufacture of such particles.
More specifically, the present invention provides a method for manufacture of spherical resin particles for powder coating in which the particles are prepared from a suspension obtained from a mixture of a thermosetting resin solution and an aqueous solution containing a water-soluble polymer. The thermosetting resin solution contains A resin, B resin and an organic solvent. The A and B resins are associated with each other such that
(a) 0.5≦{(SP value of the A resin)−(SP value of the B resin)}≦1.5,
(b) {(Tg of the A resin)−(Tg of the B resin)}≧10° C.,
(C) 40° C.≦(Tg of the A resin)≦100° C., and, 20° C.≦(Tg of the B resin)≦50° C., and
(d) 5/95≦(a ratio in solids weight of the A resin to B resin)≦50/50.
The term “SP value” stands for solubility parameter value.
The present invention also provides spherical resin particles for thermosetting powder coating. The particles contain A and B resins which are associated with each other such that
(a) 0.5≦{(SP value of the A resin)−(SP value of the B resin)}≦1.5,
(b) {(Tg of the A resin)−(Tg of the B resin)}≧10° C.,
(C) 40° C.≦(Tg of the A resin)≦100° C., and, 20° C.≦(Tg of the B resin)≦50° C., and
(d) 5/95≦(a ratio in solids weight of the A resin to B resin)≦50/50.
The present invention further provides a method for formation of a multilayer film which includes the steps of applying a basecoat on an undercoated or further intercoated substrate, applying a layer of the above-specified resin particles for powder coating on the basecoat overlying the substrate, and heating the substrate carrying thereon the layer of particles and the basecoat. The present invention further provides a multilayer film obtained from the practice of the above-described method.
DETAILED DESCRIPTION OF THE INVENTION
Method for Manufacture of Particles for Powder Coating
In a method for manufacture of particles for powder coating in accordance with the present invention, the particles are prepared from a suspension obtained from a mixture of a thermosetting resin solution and an aqueous solution containing a water-soluble polymer.
In the present method, the resin particles are prepared in an aqueous medium using two types of resins respectively having specified characteristic values. This results that the prepared resin particles exhibit excellent blocking resistance. In addition, a coating film prepared therefrom has an excellent surface smoothness.
This is considered due to the specific structure of the resin particles obtained by the present method, as will be described below. That is, within a shell portion of the particle surrounded by water as a dispersing medium, the A resin is considered as being present in an increased concentration relative to the B resin because the A resin has a higher SP value than the B resin. This is considered to result in the increased Tg in the shell portion than in an inner portion of the particle, leading to the improved blocking resistance thereof. On the other hand, Tg of the particle as a whole is lower than Tg of the particle shell portion, which is believed to insure a desired level of surface smoothness. Also, the use of resin having an increased molecular weight relative to those for use in conventional powder coatings results in the formation of coating film excellent in acid resistance.
The spherical thermosetting resin particles of the present invention for powder coating are manufactured in an aqueous medium. During manufacture, this technique subjects raw materials for powder coating to a reduced degree of heat, relative to a melt kneading technique well-known in the art, resulting in the production of resin particles having improved resistance to solid-phase reaction. Also, the thermosetting resin particles of the present invention for powder coating have a uniform shape of a sphere and exhibit a narrow particle size distribution. A marked reduction in amount of fine and coarse particles results in the improved workability such as transportability and transfer efficiency. In addition, the particles of the present invention, when formed into a thin film, provide a good appearance and, when recovered, can be reused like a fresh supply.
Also in the present method for manufacture of particles for powder coating, an eco-friendly recycle system can be established by the reuse of recovered particles as a fresh source. When in use, those recovered particles are dissolved into an organic solvent. This operation allows the reuse of fine particles previously regarded as a solid waste and, when combined with a filtering operation, enables the effective removal of dusts from the recovered particles.
The method for formation of a multilayer film, in accordance with the present invention, utilizes a powder coating, so that the use amount of an organic solvent can be reduced. This effect becomes remarkable particularly when waterborne coatings are used as base coatings.
Also, the multilayer film formed in accordance with the method of the present invention provides an excellent surface smoothness suitable for application to automotive bodies which demand a high-quality appearance.
A: Components for use in the Present Method
A-1: Water-soluble Polymer
One of the components for use in the present method of manufacture of particles for powder coating is a water-soluble polymer. This water-soluble polymer is classified into two types; a water-soluble polymer which does not exhibit a cloud point and a water-soluble polymer which exhibits a cloud point within the range of 30-90° C.
Examples of the water-soluble polymers which do not exhibit a cloud point include completely-saponified polyvinyl alcohol, partially-saponified polyvinyl alcohol having 85% or higher saponific
Harada Yutaka
Satoh Haruhiko
Yamada Atsushi
Nippon Paint Co. Ltd.
Reddick Judy M.
Townsend & Banta
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