Stock material or miscellaneous articles – Composite – Of polyester
Reexamination Certificate
2000-08-09
2003-12-09
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Composite
Of polyester
C428S480000, C525S165000, C525S173000, C525S174000, C528S297000, C528S302000, C528S303000, C528S305000, C528S307000, C528S308000, C528S308600, C427S189000, C427S195000
Reexamination Certificate
active
06660398
ABSTRACT:
This application is a national stage filing of International Application No. PCT/BE98/00200, filed Dec. 16, 1998, the content of which is incorporated herein by reference. This application claims benefit of priority under 35 U.S.C. §119 to Belgian patent application no. 9701039, filed on Dec. 18, 1997, the content of which is also incorporated by reference herein.
DESCRIPTION
The present invention relates to thermosetting powder compositions comprising, as binder, a blend of an amorphous polyester, of a semicrystalline polyester and of a crosslinking agent, which compositions provide, by curing, low-gloss coatings, particularly matt coatings.
The invention also relates to the use of these compositions for the preparation of powder paints and varnishes which provide low-gloss coatings, as well as to the low-gloss coatings obtained from these compositions.
At the present time, thermosetting powder coating compositions are widely employed as paints and varnishes in order to form durable protective coatings on the most varied of objects. Powder coating compositions have many advantages over coating compositions that are in the form of solutions in an organic solvent; on the one hand, the safety and environmental problems caused by solvents are completely eliminated and, on the other hand, whereas solvent-based coating compositions have the drawback of only being able to be used partially—in certain types of application, only 60% or less of the coating composition applied comes into contact with the substrate and that part which does not come into contact with the substrate cannot be recovered—power coating compositions are used 100%, given that only the powder in direct contact with the substrate is retained by the latter, the excess powder being, in principle, entirely recoverable and reusable. This is why these powder compositions are preferred over coating compositions that are in the form of solutions in an organic solvent.
Powder coating compositions generally contain a thermosetting organic binder and, optionally, fillers, pigments, catalysts and various additives for tailoring their behaviour to their use.
These thermosetting powders are prepared in the following manner. The polyester or polyesters, the crosslinking agent, the optional catalyst, the pigments, the fillers and other additives are dry blended at ambient temperature in the required proportions in order to obtain a powder paint or varnish. The blend thus obtained is put into an extruder in order to produce melt homogenization therein at a temperature generally between 80 and 150° C. The blend leaving the extruder is left to cool and then ground and screened in order to obtain a powder having the desired particle size of between 10 and 150 micrometres.
The powder paints and varnishes thus obtained are applied in a manner known per se by means of an electrostatic or triboelectric spray gun or using the technique of fluidized-bed deposition on the object to be coated. The object thus coated is then heated in an oven where the melting and crosslinking of the binder is carried out at high temperature.
The cured coatings obtained from thermosetting powder coating compositions must have a smooth, uniform and defect-free appearance and must especially be free of any “orange peel”; they must have good mechanical and chemical properties and good weatherability.
Furthermore, it is essential that the powder coating compositions remain in the form of freely-flowing powders for a long enough period after their manufacture and their packaging, without reagglomerating during transportation and storage.
The thermosetting powder coating compositions commonly used and commercially available contain, as binder, a blend of an amorphous polyester containing carboxyl or hydroxyl groups having a glass transition temperature (T
g
) of between 45 and 80° C. with a cross-linking agent having functional groups capable of reacting with the carboxyl or hydroxyl groups of the polyester.
The amorphous polyesters used in these compositions are those obtained from aromatic dicarboxylic acids, such as terephthalic acid or isophthalic acid, and optionally from aliphatic dicarboxylic acids, such as adipic acid, and from various polyols, such as neopentylglycol, ethylene glycol, trimethylolpropane, etc.
These amorphous polyester-based compositions are storage-stable and produce coatings of excellent quality, inter alia a surface without any visible defects, and good mechanical properties. Among these compositions, amorphous polyester-based compositions rich in isophthalic acid provide coatings which are highly valued for their excellent performance in outdoor exposure.
However, after curing, these amorphous polyester-based compositions provide coatings with a very high gloss. The gloss, measured at an angle of 60°, according to the ASTM D 523 standard, is very often greater than 90%.
Powder coating compositions whose binder contains a semicrystalline polyester have also already been proposed.
Thus, in European Patent 521,992, the binder proposed consists of a blend of at least one semi-crystalline polyester having an acid number of 10 to 70 mg of KOH per gramme and of at least one amorphous polyester having a glass transition temperature (T
g
) of at least 30° C. and an acid number of 15 to 90 mg of KOH per gramme, with a crosslinking agent which may be an epoxidized compound, a compound containing activated thiol or hydroxyl groups or an oxazoline.
Semicrystalline polyesters are characterized by one or more glass transition temperatures (T
g
) not exceeding 55° C. and a sharp melting point of 50 to 200° C.
According to that patent, semicrystalline polyesters differ from amorphous polyesters by the fact that semicrystalline polyesters have a heterogeneous morphology (they contain a mixture of phases), are opaque and white in colour at ambient temperature, have a low melt viscosity, are more insoluble in organic solvents and have a very high structural regularity. The semicrystalline polyesters described and used in the illustrative embodiments of that patent are those obtained by polycondensation from dicarboxylic acids containing an aromatic or aliphatic ring, such as terephthalic acid and 1,4-cyclohexanedicarboxylic acid, and from saturated aliphatic diols with a linear chain, such as 1,6-hexanediol and 1,10-decanediol; further-more, aliphatic dicarboxylic acids with a linear chain, such as adipic acid, succinic acid or 1,12-dodecanedioic acid, are added thereto. The amorphous polyesters used are polyesters containing carboxyl groups usually employed in powder paints and varnishes; these polyesters may be rich in isophthalic acid for the purpose of obtaining outdoor coatings having good weatherability.
According to that patent, the presence of the semicrystalline polyester in the binder gives a coating having an excellent overall appearance, free of “orange peel”, and improved mechanical properties.
However, as shown in Table I at the end of the description of that patent, the coatings obtained from these compositions all have a very high gloss; the gloss, measured at an angle of 60°, according to the ASTM D 523 standard, may vary between 82 and 87%.
U.S. Pat. No. 5,373,084 proposes thermo-setting powder coating compositions which comprise, as binder, a blend of a particular semicrystalline polyester, of an amorphous polyester and of a cross-linking agent. The particular semicrystalline polyesters proposed in that patent are those obtained by the esterification of a saturated aliphatic dicarboxylic acid having a linear chain, mainly 1,12-dodecanedioic acid, with a saturated aliphatic diol having a linear chain, mainly 1,6-hexanediol, and by optionally incorporating a trifunctional polyol, such as trimethylolpropane or glycerol, or a trifunctional polycarboxylic acid, such as trimellitic acid, in order to obtain branched polyesters. These semicrystalline polyesters have an acid number or hydroxyl number of approximately 20 to 120, preferably approximately 30 to 80 mg of KOH per gramme and a melting point of 40 to 200° C., preferably 60 to 150° C.
Buysens Kris
Maetens Daniel
Moens Luc
Finnegan Henderson Farabow Garrett & Dunner LLP
UCB S.A.
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