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
2000-09-05
2003-01-21
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...
C525S934000, C526S202000, C526S207000, C526S218100, C526S273000
Reexamination Certificate
active
06509420
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an acrylic resin for a powder coating composition and its preparation method.
BACKGROUND ART
Examples of coating compositions include a solvent type coating composition in which a pigment and a polymeric substance are dispersed in an organic solvent, a water-soluble type resin coating composition in which a water-soluble resin that can be insolubilized by heating is used, an emulsion coating composition in which a resin is dispersed in water, and a powder coating composition in which a resin is ground into powder, applied and then heated to form a coating film, but the solvent type coating composition has been mainly used from the viewpoints of the beauty of an obtained coating film, durability and the like.
In recent years, however, regulations for organic solvents become severe in view of problems such as air pollution and the danger of a fire, and much attention has been paid to solventless coating compositions. Above all, the powder coating compositions have been in the limelight from the viewpoint of environmental protection, and in particular, acrylic powder coating compositions are characterized by being excellent in a coating film performance such as weathering resistance. For this reason, it has been noticed to use these acrylic powder coating compositions for building materials and car parts, particularly for clear coatings for cars.
With regard to the powder coating composition, in order to form a smooth coating film, it is necessary to lower the molecular weight of a selected resin. For the sake of this realization, as preparation methods of the resin for the powder coating composition, there have been suggested bulk polymerization, solution polymerization, suspension polymerization and the like, but in fact, the solution polymerization method in which the molecular weight can be easily lowered is often employed (Japanese Patent Application Laid-Open No. 138437/1978).
However, in the solution polymerization method, a step of removing a used solvent is required, and for the removal of the solvent, the conditions of a high temperature and a reduced pressure are usually necessary, which is inconvenient in point of energy. In addition, a production process is prolonged, which inconveniently results in poor productivity. Furthermore, in the solution polymerization method, the resin is obtained in the state of masses, and in order to obtain the coating composition, a grinding step of the resin is necessary. However, when the resin is ground, fine powder is easily generated, and such fine powder is liable to fly about during handling, which easily makes an operating efficiency poor. Moreover, if the insufficiently ground resin is present, the mixing of the resin and a crosslinking agent is insufficient during the preparation of the powder coating composition, which causes a problem that the performance of the obtained coating film deteriorates. Furthermore, a method for preparing an acrylic resin for the powder coating composition by the bulk polymerization under solventless and catalyst-free conditions has been suggested in Japanese Patent Application Laid-Open No. 140395/1978, but also in this bulk polymerization, the grinding step of the resin is necessary, so that the bulk polymerization has the same problem as in the case of the solution polymerization.
On the other hand, the suspension polymerization has an advantage that the solvent removing step and the resin grinding step are unnecessary, but there is a tendency that the molecular weight of the obtained resin is very high. In consequence, the gloss and the smoothness of the formed coating film are inconveniently easily impaired. Thus, a mercaptan has heretofore been used as a chain transfer agent to lower the molecular weight. However, a powder coating composition comprising the resin obtained by the use of the mercaptan as the chain transfer agent is so odorous as to be impractical, and a formed coating film is apt to be poor in weathering resistance.
On the contrary, in Japanese Patent Application Laid-Open No. 158682/1975, it has been disclosed that for the purpose of obtaining a low-molecular weight vinyl polymer which is suitable for the powder coating composition and is less odorous by the suspension polymerization, &agr;-methylstyrene and/or its dimer is used as the chain transfer agent and as an essential component.
However, a coating film formed from the vinyl polymer obtained in Japanese Patent Application Laid-Open No. 158682/1975 is still poor in coating film performance such as smoothness. Particularly, since a large amount of the chain transfer agent is used, a polymerization rate deteriorates, so that a polymerization step is prolonged. In consequence, the productivity of the resin is low, and when the resin is molten and kneaded at a high temperature in the preparation of the powder coating composition, an odor is given off owing the increased amount of the remaining monomer. In addition, during the formation of the coating film, vaporization of the remaining monomer occasionally causes the formation of pinholes in the coating film.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a resin for a powder coating composition which is excellent in workability, productivity and coating film performances such as gloss and smoothness and which is free from the problems of an odor and pinholes.
Another object of the present invention is to provide a method for efficiently preparing the resin for the powder coating composition.
The present inventors have intensively investigated with the intention of achieving the above-mentioned objects, and as a result, it has been found that an epoxy group-containing acrylic resin having a specific particle shape and specific physical properties can achieve the above-mentioned objects, and in consequence, the present invention has been completed.
That is to say, the present invention is directed to an epoxy group-containing acrylic resin for a powder coating composition which is a copolymer, having a weight-average molecular weight of 3000 to 20000, of a vinyl monomer mixture comprising 10 to 60% by weight of alkyl ester(s) of (meth)acrylic acid, 10 to 60% by weight of an epoxy group-containing vinyl monomer(s) and other vinyl monomer(s), said epoxy group-containing acrylic resin
having a mean particle diameter in the range of 80 to 800 &mgr;m,
including 95% by weight or more of particles satisfying the relation of 1≦R/r≦1.2 wherein R is a long diameter and r is a short diameter of each particle,
the amount of the particles having long diameters R of less than 62 &mgr;m being 5% by weight or less, that of the particles having short diameters r of more than 1000 &mgr;m being 5% by weight or less, and
including 1000 ppm or less of volatile components.
In the present invention, it is required that an epoxy group-containing acrylic resin, which can be used as a resin for a powder coating composition of the present invention, has a mean particle diameter in the range of 80 to 800 &mgr;m and includes 95% by weight or more of particles satisfying the relation of 1≦R/r≦1.2 wherein R is a long diameter and r is a short diameter of each particle, and the amount of the particles having long diameters R of less than 62 &mgr;m is 5% by weight or less and that of the particles having short diameters r of more than 1000 &mgr;m is 5% by weight or less.
The reason why the mean particle diameter of the epoxy group-containing acrylic resin is 80 &mgr;m or more and the particles having long diameters R of less than 62 &mgr;m are present in an amount of 5% by weight or less is that fine powder does not fly about during the preparation of a coating composition, and the deterioration of a working environment and the decline in a yield of the coating composition can be prevented. Furthermore, the reason why the mean particle diameter is 800 &mgr;m or less and the particles having short diameters r of more than 1000 &mgr;m are present in an amount of 5% by weight or less is that the resin and a crosslinking
Harada Yoko
Inagaki Motoshi
Shimizu Koji
Tajiri Noriyuki
Knobbe Martens Olson & Bear LLP
Mitsubishi Rayon Co. Ltd.
Sellers Robert E. L.
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