Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2002-08-27
2004-04-13
Short, Patricia A. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S413000, C428S458000, C428S482000, C427S180000, C427S185000, C427S385500, C427S388200, C427S386000, C427S461000, C427S485000, C427S486000, C525S438000, C525S448000, C525S934000
Reexamination Certificate
active
06720079
ABSTRACT:
The present invention relates to novel polyesters containing tertiary carboxyl groups, to a process for their preparation and to thermosetting powder compositions containing, as binder, the said polyesters and a crosslinking agent capable of reacting with their carboxyl groups.
The invention also relates to the use of these compositions to prepare powdered paints and varnishes for making coatings, as well as to the coatings thus obtained.
Thermosetting powder compositions are well known in the state of the art and are widely used as paints and varnishes for coating a very varied range of objects. These powders have many advantages: on the one hand, the problems associated with the use of solvents is completely eliminated, on the other hand, the powders are used at 100%, given that only the powder in direct contact with the substrate is retained by that substrate, the excess of powder being, in principle, entirely recoverable and reusable. Consequently, these powdered compositions are preferred over coating compositions in the form of solutions in an organic solvent.
Thermosetting powder compositions have already found a large opening in the coating of household appliances, accessories for the motor vehicle industry, etc. They generally contain thermosetting organic compounds constituting the binder for the paint, fillers, pigments, catalysts and various additives to adapt their behaviour to their use.
Besides the thermosetting powder compositions, thermoplastic compositions in powder form also exist. These compositions comprise polymers which do not contain any reactive chemical groups. When this type of powder is heated on a substrate, the particles melt,
The present invention relates to novel polyesters containing tertiary carboxyl groups, to a process for their preparation and to thermosetting power compositions containing, as binder, the said polyesters and a crosslinking agent capable of reacting with their carboxyl groups.
The invention also relates to the use of these compositions to prepare powered paints and varnishes for making coatings, as well as to the coatings thus obtained.
Thermosetting powder compositions are well known in the state of the art and are widely used as paints and varnishes for coating a varied range of objects. These powders have many advantages: on the one hand, the problems associated with the use of solvents is completely eliminated, on the other hand, the powders are used at 100%, given that only the powder in direct contact with the substrate is retained by that substrate, the excess of powder being, in principle, entirely recoverable and reusable. Consequently, these powdered compositions are preferred over coating compositions in the form of solutions in an organic solvent.
Thermosetting powder compositions have already found a large opening in the coating of household appliances, accessories for the motor vehicle industry, etc. They generally contain thermosetting organic compounds constituting the binder for the paint, fillers, pigments, catalysts and various additives to adapt their behaviour to their use.
Besides the thermosetting powder compositions, thermoplastic compositions in powder form also exist. These compositions comprise polymers which do not contain any reactive chemical groups. When this type of powder is heated on a substrate, the particles melt, coalesce and spread over the substrate to form a smooth and uniform coating.
In contrast, the polymers used in the thermosetting powder compositions contain a reactive chemical group, for example a carboxyl, hydroxyl or epoxide group. In this type of powder, a crosslinking agent is added which contains chemical groups which react with those of the polymers when the powder is fired on the substrate. This results in harder coatings which are more resistant to chemical agents, which adhere better to metal surfaces and which do not soften when they are exposed to high temperatures. However, the crosslinking of thermosetting powders creates a new series of problems when compared with thermoplastic powders. Firstly, the crosslinking can give rise to a lack of surface uniformity, which gives it an orange-peel appearance. This is due to the fact that, during its firing, the powder crosslinks before the powder particles have had the time to spread out to make a very uniform coat. Another consequence of this premature crosslinking is that air bubbles can remain trapped in the coating, which adversely affects its surface appearance, its adhesion to the substrate and its protective effect. Another problem encountered with thermosetting powder compositions is that of giving coatings which lack flexibility. When a coating is deposited on metal sheets intended to manufacture articles which will need to be folded at various angles, it is essential for the coating not to crack at the location of the fold. In addition, it is essential for the powder to be stable in storage. In other words, it must remain in a finely pulverulent state for a sufficiently long period, and must not reagglomerate, which implies that the polymer used as binder for the powder should have a sufficiently high glass transition temperature. Lastly, when the thermosetting powder composition is intended to make coatings on articles forming part of architectural constructions, it is very important for these coatings to withstand bad weather. In the latter case, it is common practice to use amorphous polyesters, bearing carboxyl groups, containing a majority of isophthalic acid as acidic constituent, as a mixture with triglycidyl isocyanurate (TGIC) as crosslinking agent. This does indeed give the coatings good resistance to photodegradation and to hydrolysis of the ester groups, but the coatings thus obtained do not have good impact strength. In addition, TGIC is tending to be used less than previously, since it exhibits health and safety problems. Specifically, TGIC is relatively toxic, its LD50 lethal dose being 0.4 g/kg orally in rats, and it is mutagenic according to the Ames mutagenicity test; it is also thought to give rise to allergic reactions, reflected by eczema and asthma.
Attempts have thus been made to replace TGIC with other crosslinking agents for polyesters bearing carboxyl groups. Among these, acrylic copolymers bearing epoxide groups have been used, as in patent EP 38635. However, binders containing these two types of compound give coatings whose impact strength and flexibility are not entirely satisfactory. Attempts have also been made to use beta-hydroxyalkylamides as crosslinking agents for polyesters bearing carboxyl groups. The hydroxyl group located in the beta position relative to the amide group is highly reactive in the esterification of the carboxyl group in the polyesters, which leads to problems as regards the rate of crosslinking of compositions containing this type of crosslinking agent. The reason for this is that, since this rate is high, the coating does not have enough time to spread out correctly when it melts, which leads to surface defects such as the formation of an orange-peel skin. In addition, this esterification is accompanied by a release of water, which does not have time to escape from the coating as it hardens, which also leads to surface defects.
Patent EP 322,834, for example, describes thermosetting powder compositions essentially containing a polyester bearing carboxyl groups and a beta-hydroxyalkylamide, which is applied to a substrate and is then crosslinked at a temperature of 160 to 200° C. Despite the presence of benzoin in these compositions, which is added as degassing agent, the bubbles of water and air remain trapped in the hardened coating after it has melted and crosslinked, especially if the coating is relatively thick. In addition, the flow of the powder when it melts is not optimal.
Patent application WO 91/14745 describes thermosetting powder compositions containing an amorphous polyester containing carboxyl groups, a semi-crystalline polyester containing carboxyl groups and a crosslinking agent. 10 to 40% by weight of the semi-crystalline polyester is preferab
Buysens Kris
Maetens Daniel
Moens Luc
Short Patricia A.
UCB S.A.
Wenderoth , Lind & Ponack, L.L.P.
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