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
2000-09-18
2003-10-21
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...
C523S461000, C524S513000, C524S604000, C525S173000, C525S174000, C525S176000, C525S444000, C427S385500, C427S386000
Reexamination Certificate
active
06635721
ABSTRACT:
DESCRIPTION
The present invention relates to thermosetting powder compositions comprising as binder a mixture of an amorphous polyester containing carboxyl groups, of a semicrystalline polyester containing carboxyl groups and of a crosslinking agent having functional groups capable of reacting with the carboxyl groups of these polyesters, and in particular to thermosetting powder compositions which, by curing, give coatings which have very good weatherability and have good mechanical properties.
The invention also relates to the use of these compositions for the preparation of powder paints and varnishes allowing these coatings to be produced, as well as to the coatings thus obtained.
Thermosetting powder compositions are well known in the art and are widely used a paints and varnishes for the coating of the most varied of articles. These powders have many advantages; on the one hand, the problem of solvents is completely eliminated, and on the other hand the powders are 100% used, given that only the powder in direct contact with the substrate is retained by the latter, any excess powder being, in principle, completely recoverable and reusable. This is why these powder compositions are preferred to coating compositions in the form of solutions in an organic solvent.
Thermosetting powder compositions have already been widely used in the coating of domestic electrical appliances, accessories for the motor-vehicle industry etc. They generally contain thermosetting organic compounds constituting the binder of the pain, fillers, pigments, catalysts and various additives in order to match their properties to their use.
Various types of thermosetting powder compositions exist. The most widely known compositions contain as binder either a mixture of polymers containing carboxyl groups, such as a carboxyl-group containing polyester or a carboxyl-group-containing polyacrylate, and of epoxy compounds such as triglycidyl isocyanutate or acrylic copolymers containing glycidyl groups or &bgr;-hydroxyalkylamides, or a mixture of polymers containing hydroxyl groups, usually a hydroxyl-group-containing polyester, with blocked or unblocked isocyanates, glycolouril or melamine resins, polycarboxylic acid anhydrides, etc.
Polyesters containing carboxyl groups or hydroxyl groups which can be used for the preparation of powder varnishes and paints have already formed the subject of numerous publications. These polyesters are usually prepared from aromatic polycarboxylic acids, mainly terephthalic acid and isophthalic acid and optionally a minor proportion of aliphatic or cycloaliphatic dicarboxylic acids, and from various aliphatic polyols such as, for example, ethylene glycol, neopentyl glycol, 1,4-butanediol, trimethylolpropane, etc. These polyesters based on aromatic dicarboxylic acids, when they are used with a suitable crosslinking agent, provide thermosetting compositions which give paint and varnish coatings having good properties both with regard to their appearance and with regard to their mechanical properties (impact strength, flexibility, etc.). Some of these polyesters and the powders prepared from them are also commonly employed because of their outstanding weatherability properties.
This latter type of polyester generally contains, as acid, mostly isophthalic acid. However, although the coatings obtained from these polyesters have good weatherability, they do not on the other hand have good mechanical strength.
Currently, the great majority of polyesters used in the thermosetting powder compositions are amorphous polyesters. However, when the polyester is amorphous it is difficult to prepare perfect pulverulent thermosetting compositions, because they must satisfy often contradictory criteria. Thus, these powders must not reagglomerate while they are being handled, transported and stored, which means that the amorphous polyester must have a sufficiently high glass transition temperature (T
g
). On the other hand, for the powder particles to be able to coalesce and form a perfectly homogenous and uniform coating, it is necessary for its viscosity at the curing temperature, and therefore the glass transition temperature (T
g
) of the polyester, to be sufficiently low to ensure good wetting of the pigments and other solid materials accompanying the polyester in the formulation of the said thermosetting powder compositions. In addition, the powder must be capable of melting at the curing temperature in order to form an even film before the crosslinking reaction leading to final curing commences. In order to obtain good spreading of the molten film on the surface of the substrate, it is therefore necessary for the melt viscosity of the polyester to be sufficiently low. This is because a very high melt viscosity prevents good spreading of the molten film and results in a loss of evenness and of gloss in the coating. Finally, the rate of the crosslinking reaction of the composition, at a given temperature, may be controlled only by varying the quantity and/or nature of the crosslinking agent and of the optionally used crosslinking catalyst.
These various problems are solved in powder compositions of which the binder is formed by a semicrystalline polyester containing carboxyl groups or hydroxyl groups, this semicrystalline polyester being used alone or as a mixture with an amorphous polyester containing carboxyl or hydroxyl groups. This is because the properties of semicrystalline polyesters make it possible to avoid, to a large extent, the drawbacks described previously which arise from controlling the glass transition temperature (T
g
), the melt viscosity and the reactivity of amorphous polyesters.
Firstly, semicrystalline polyesters have a high melting point and, as the case may be, a low glass transition temperature (T
g
). Consequently, the melt viscosity of semicrystalline polyesters is much lower than that of the amorphous polyesters of comparable molecular weight which are commonly used in thermosetting powder compositions. This means that powder coating compositions based on semicrystalline polyesters exhibit better fluidity of the coating film in the molten state, giving final coatings free of orange-peel effects.
Furthermore, by virtue of the crystallinity of the polyesters, the powders formulated with a suitable crosslinking agent have very good storage stability. In addition, compared to amorphous polyesters, semicrystalline polyesters provide coatings having outstanding mechanical properties.
Thermosetting compositions containing semicrystalline polyesters have already formed the subject of a number of publications in the form of articles and patents. Thus, in PCT International Patent Application WO 91/14754, thermosetting powder coating compositions are described which comprise as binder a mixture of a crosslinking agent and of a polyester containing carboxyl groups, which mixture is composed of:
(a) from 5 to 100% by weight of a semicrystalline polyester containing carboxyl groups, having an acid number of from 10 to 70 mg of KOH/g and a hydroxyl number of less than 11 mg of KOH/g, and
(b) from 0 to 95% by weight of an amorphous polyester containing carboxyl groups, having an acid number of from 15 to 90 mg of KOH/g.
The crosslinking agent may be a polyepoxy compound or a bis(&bgr;-hydroxyalkylamide).
According to the illustrative embodiments, these semicrystalline polyesters are prepared in one or two steps, from a major proportion of terephthalic acid or 1,4-cyclohexanedicarboxylic acid and a minor proportion of, on the one hand, adipic, succinic or 1,12-dodecane-dioc acid and, on the other hand, an aliphatic diol such as 1,6-hexanediol or 1,10-decanediol.
These semicrystalline polyesters possess a number-average molecular weight of between 1,600 and 12,000, one or more glass transition temperatures (T
g
) below 55° C., a melting point of from 50° to 200° C. and a melt viscosity of from 100 to 7,000 mPa.s at 200° C. and from 4,000 to 20,000 mPa.s at 160° C. The constituents of the amorphous polyesters are the usual constituents well known in the art, for example terepht
Loosen Patrick
Loutz Jean-Marie
Maetens Daniel
Moens Luc
Sellers Robert E. L.
UCB S.A.
Wenderoth , Lind & Ponack, L.L.P.
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