Stock material or miscellaneous articles – Composite – Of polyester
Patent
1995-07-20
1997-04-15
Acquah, Samuel A.
Stock material or miscellaneous articles
Composite
Of polyester
528272, 528274, 528296, 528300, 528302, 528308, 5283086, 525437, 525440, 525441, 524 81, 524413, 524539, 428458, B32B 2736, C08F 2000
Patent
active
056208014
DESCRIPTION:
BRIEF SUMMARY
This Application is 371 of PCT/AU94/00783 filed Dec. 21, 1994.
1. Field of the Invention
The present invention relates to a thermosetting coating composition and more particularly to a crosslinkable polyester resin suitable for coating metal sheets and coils.
2. Background of the Invention
Desirably, coating compositions for metal sheets and coils should be resistant to sunlight, UV radiation, hydrolysis, adverse weather conditions, solvents and should also display good gloss and hardness retention and rapid cure.
The requirements for resistance to colour change and for good gloss retention when exposed to weather are difficult to satisfy and present a particular problem because data obtained by means of a UV-A accelerated weatherometer does not correlate well with data from a UV-B accelerometer and both correlate poorly with actual weather exposure. Sample resins which "pass" in the weatherometer sometimes "fail" on exposure to weather and sample resins which fail in the weatherometer are frequently rejected without further testing.
Thermosetting coatings are generally used as decorative coatings for metal sheet coils. These coatings are typically produced by reacting at elevated temperatures a blend of hydroxyfunctional resin and a crosslinking agent such as a derivative of melamine formaldehyde, benzoguanamine formaldehyde, glycouril formaldehyde, urea formaldehyde or a blocked isocyanate resin. Hydroxyfunctional resins such as polyester, acrylic and alkyd are generally used as the major component of the decorative coatings and of those resins, polyesters are preferred.
Hydroxyfunctional polyester resins are generally manufactured by reacting glycols with a mixture of dicarboxylic acid components together with a trifunctional branching component to form a branched polyester. The mixture of dicarboxylic acid components comprises an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid component. The aromatic dicarboxylic acid component may be, for example, orthophthalic acid, terephthalic acid and/or a corresponding anhydride and the aliphatic dicarboxylic acid component may, for example, be sebacic or succinic acid. At least 20 mole % and as much as 80 mole % of the total acid component is an aliphatic dicarboxylic acid which is employed to prevent crystallization of the product due to inherent crystallinity of aromatic acids and because aliphatic systems tend to be less UV absorbing.
Although many such resins are known there remains a need for resins for use in coating compositions with improved gloss retention, and resistance to colour change on extensive exposure and which are suitable for coating onto metal coil. The latter entails the ability to be applied uniformly in a film thickness of 15 to 25 microns, a capacity for rapid thermal film cure, satisfactory film durability, flexibility, hardness, peel resistance, impact shock resistance and the like.
Accordingly, it is an object of the present invention to provide an improved thermosetting coating composition for metal sheets and coils.
SUMMARY OF THE INVENTION
According to one aspect, the invention consists in a polyester for use in a thermosetting coating composition, said polyester comprising an acid component, an alcohol component and a trifunctional branching agent; wherein the acid component consists of from 90 to 100 mole % of an aromatic acid or aromatic anhydride or mixture thereof and from 0 to 10 mole % of a polycarboxylic acid or polycarboxylic anhydride or mixture thereof; and wherein the alcohol component consists of at least 60 mole % of one or more glycols selected from the group consisting of CH.sub.3 --CH.sub.2 --C(CH.sub.2 OH).sub.2 --R.sub.1 wherein R.sub.1 is an alkyl and derivatives of said glycols, from 0 to 20 mole % of neopentylglycol, and of up to 20 mole % of a trifunctional branching agent.
For preference R.sub.1 is methyl, ethyl or butyl. Desirably, the glycol comprises at least 80 mole % of the total acid component.
According to a second aspect the invention consists in a thermosetting coating composition c
REFERENCES:
Derwent Abstract Accession No. 80-304980/42.
Derwent Abstract Accession No. 88-348835/49
Patent Abstract of Japan, C-279, p. 60, JP,A, 59-230022.
Patent Abstract of Japan, C-248, p. 44, JP,A, 59-113026.
Binns Malcolm R.
Maliphant Jenny A.
Viatos Jim
Acquah Samuel A.
BHP Steel (JLA) Pty. Ltd.
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