Thermosetting powder coating composition

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...

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Details

525107, 525384, 5253272, 525408, 525117, C08F 1626, C08F 2032, C08F 2400, C08F11616

Patent

active

059454873

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a novel thermosetting powder coating composition which is excellent in the finished appearance of coating film and in the blocking resistance.


BACKGROUND ART

In recent years, thermosetting powder coating compositions which are usable without a solvent have been increasingly used from the viewpoints of prevention of environmental pollution and conservation of environment.
However, conventional thermosetting powder coating compositions generally have a drawback of giving a coating film which is unsatisfactory in the finished appearance.
To produce powder coating compositions which are free of such drawback and excellent in the finished appearance of coating film, the molecular weight of a vinyl copolymer used as the resin component of the composition has been lowered to reduce the viscosity of the coating composition thermally fused. Yet, the powder coating composition obtained in this way has posed a problem of being likely to cause blocking phenomenon that particles in the composition during storage weld together, thereby forming agglomerates.


DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a novel thermosetting powder coating composition free of the foregoing prior art drawback.
Another object of the invention is to provide a novel thermosetting powder coating composition which is superior in any of the finished appearance of coating film and the blocking resistance.
Other objects and features of the invention will become apparent from the following description.
According to the present invention, there is provided a thermosetting powder coating composition comprising: 6,000, a molecular weight distribution of 1.0 to 1.5 as represented by a weight average molecular weight/a number average molecular weight and a glass transition temperature of 40 to 100.degree. C., the copolymer containing at least one species of functional group selected from the class consisting of epoxy group, carboxyl group and hydroxyl group, and
The present inventors conducted extensive research to overcome the foregoing prior art drawback and found the following. A vinyl copolymer used as the resin component of the composition has generally a molecular weight distribution (represented by a weight average molecular weight/a number average molecular weight) of at least 2, namely too broad of a distribution. Therefore, when the molecular weight of a vinyl copolymer is lowered, the blocking phenomenon would tend to occur in the composition due to the low-molecular-weight compound. The inventors further researched based on the finding and discovered the following. When a copolymer as specified above in number average molecular weight, narrow molecular weight distribution and glass transition temperature and containing the specific functional groups is used as the resin component in a powder coating composition, the resulting powder coating composition is excellent in any of the finished appearance of coating film and the blocking resistance.
The present invention has been completed based on these novel findings.
The vinyl copolymer (A) used as the resin component in the composition of the present invention is 1,500 to 6,000 in number average molecular weight, 1.0 to 1.5, preferably 1.0 to 1.2, in molecular weight distribution as represented by a weight average molecular weight/a number average molecular weight and 40 to 100.degree. C. in glass transition temperature, and contains at least one species of functional group selected from the class consisting of epoxy group, carboxyl group and hydroxyl group.
A copolymer having a number average molecular weight of less than 1,500 tends to lower the blocking resistance of the composition and to impair the film properties such as mar resistance, weatherability, etc., whereas a copolymer higher than 6,000 in number average molecular weight tends to degrade the finished appearance of coating film. Hence the number average molecular weight outside said range is undesirable.
If the molecular weight distribut

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