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
1995-05-19
2001-02-06
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...
C427S385500, C427S386000, C427S388200, C428S413000, C428S418000, C428S458000, C428S480000, C525S437000, C525S444000, C525S448000, C525S440030, C525S934000
Reexamination Certificate
active
06184311
ABSTRACT:
This invention relates to powder coating compositions based on a thermosetting resin which can be applied to a substrate, for example by electrostatic spray, and can be heated on the substrate to cause the resin to melt and flow and to cure the resin to a hard tough film. Powder coatings are known to provide a number of advantages over their liquid counterparts. Powder is completely free of solvent which has to be removed after paint application and is applied in one coat, with any powder not reaching the substrate being collected and re-used, and the coatings provide excellent corrosion resistance and outdoor-durability.
Powder coatings based on carboxylic acid group-containing amorphous polyesters with glass transition temperature (Tg) in the range 30-100° C., preferably 40-75° C., and a curing agent such as triglycidyl isocyanurate are widely used. They are stable during storage and form coating films of excellent corrosion resistance and outdoor-durability. There is, however, a danger with such powder coatings of onset of the cure reaction before the polyester is in a sufficiently molten and mobile state. This can cause surface appearance defects such as “orange peel” texture. There is a need for thermosetting powder coatings which will melt and flow on the substrate more readily at the temperatures used for curing powder coatings, which are typically in the range 140-225° C.
Powder compositions designed to offer high outdoor-durability coatings often contain amorphous polyesters which are resistant to ester hydrolysis. This is achieved to a large extent when isophthalic acid is used on its own or in combination with other diacids to form the required polyesters. A problem often found when such polyesters are incorporated into powder compositions, however, is that the resultant coatings are very intolerant of mechanical deformation, with direct impact in particular often causing tearing or even delamination of the coating. Moreover, the curing time of these powders is often longer than that required for powders not based on polyesters with a high isophthalic acid content. There is a need, therefore, for powder compositions based on isophthalic acid-rich polyesters which offer faster thermal cure and provide coatings which have both high outdoor-durability and increased resistance to mechanical deformation.
The so-called “hybrid” powder coatings, i.e. those based on carboxylic acid group-containing, amorphous polyesters and epoxy resins, provide coatings having excellent corrosion resistance, flexibility, hardness and solvent resistance. There is a need, however, for hybrid powders which can be thermoset over a wide range of temperatures (e.g. 160° C. to 200° C.), affording thin film (for example 40-50 microns) coatings with consistency both in their appearance and in mechanical performance.
EP-A-322827 describes a thermosetting powder coating composition comprising a co-reactable particulate mixture of: (a) a carboxylic acid group-containing acrylic polymer having a Tg in the range of 35° C. to 100° C.; (b) a crystalline carboxylic acid group-containing polyester having a number average molecular weight from 300 to 1,500 and an acid equivalent weight of 150 to 750 (acid value about 375 to 75 mg KOH per gram); and (c) a beta-hydroxyalkylamide, the ratio of beta-hydroxyalkylamide equivalents to carboxylic acid equivalents being within the range 0.6 to 1.6:1.
EP-A-322807 describes a thermosetting powder coating composition comprising a co-reactable particulate mixture of: (a) a carboxyl group-containing acrylic or polyester polymer having a Tg in the range 35° C. to 100° C.; (b) a polyepoxide, and (c) a beta-hydroxyalkylamide. The composition can also contain a carboxyl group-containing crystalline polyester with a number average molecular weight of from 300 to 1500 and an acid equivalent weight of from 150 to 750 (acid value 375 to 75). The crystalline polyester if used is present at 0 to 25 per cent by weight. It is asserted that amounts greater than 25 per cent by weight based on total resin solids are undesirable because of powder stability problems.
U.S. Pat. No. 4,217,426 describes melt blends of semi-crystalline polyesters with certain polyethylene to provide material that is readily grindable by cryogenic grinding techniques for use in powder coating compositions. The semi-crystalline polyesters have an inherent viscosity ranging from about 0.4 to 1.2, a melting point of about 80 to 155° C. and an apparent heat of fusion of no more than 10 calories/gram and contain from 100 to 60 mole % terephthalic acid, 0 to 40 mole % isophthalic acid, 100 to 35 mole % 1,6-hexanediol and 0 to 65 mole % 1,4-butanediol.
WO-A-89/05320 describes a polyurethane powder coating composition comprising (a) 10-80 weight % of at least one amorphous polyester containing free hydroxyl groups and having a Tg of greater than 40° C., a hydroxyl number of 20,200, and an inherent viscosity of 0.1-0.5; (b) 10-80 weight % of at least one semi-crystalline polyester containing free hydroxyl groups and having a Tg of −10 to 50 ° C., a number average molecular weight of 500-10,000, a heat of fusion of greater than 5 calories per gram, a hydroxyl number of 20-200, and an inherent viscosity of 0.1-0.5; and (c) 5-30 weight % of a polyisocyanate crosslinking agent. In order to bestow easier and safer handling properties, such a curing agent is adducted with caprolactam which volatilises from the applied powder at about 160° C. and allows the curing reaction to commence. The emission of volatile organic species such as caprolactam, however, is seen as undesirable by many powder applicators and limits sales of polyisocyanate-cured powders in many countries. Crosslinking agents designed for use in powders containing carboxylic acid group-functional polyesters generally do not involve release of noxious materials during stoving.
A thermosetting powder coating composition according to the present invention comprises as binder a co-reactable particulate mixture of a carboxylic acid-functional polyester component and a curing agent having groups reactive with carboxylic acid groups, characterised in that the carboxylic acid-functional component comprises at least 5% by weight of a semi-crystalline polyester having generally an acid value of from 10 to 70 mg KOH per gram.
More especially the carboxylic acid-functional component comprises:
(A) 5-100% by weight of at least one semi-crystalline polyester having an acid value of from 10 to 70 mg KOH per gram and a hydroxy value of no more than 11 mg KOH per gram, and
(B) 0-95% by weight of at least one amorphous polyester having a Tg of at least 30° C. and an acid value of 15 to 90, preferably 15 to 70, mg KOH per gram.
The semi-crystalline polyester is generally characterised by having one or more Tg values no more than 55° C. and a sharp melting point which is preferably in the range 50° C. to 200° C.
When an amorphous polyester is used the amount of amorphous polyester is preferably 10-95%, especially 40-90%, for example 60-90%, by weight and the amount of semi-crystalline polyester used is preferably 5-90%, especially 10-50%, for example 10-40%, by weight.
Powder coatings are usually formulated with amorphous polyesters which have a Tg greater than 30° C., preferably at least 45° C., to allow good storage stability of the powders. Coatings based on amorphous polyesters with a Tg lower than 45° C. tend to agglomerate when stored at ambient temperature. However, the flow of the polymer when molten tends to vary with its Tg in that polymers with a high Tg (greater than 70° C., for example) can have poor flow and afford coatings with an “orange peel” texture.
We have found that carboxylic acid group-containing, semi-crystalline polyesters which have a Tg less than 45° C. can be used according to the invention in storage-stable powders. These semi-crystalline polyesters also provide coatings free of “orange peel” and with excellent overall appearance. The semi-crystalline polyesters proposed by this invention can be used in place of, or as a supplement to those carb
Cameron Colin
Nixon Steven Alistair
O'Keeffe Luke James
Penman Alan Keith
Courtaulds Coatings (Holdings) Limited
Nixon & Vanderhye
Sellers Robert E. L.
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