Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-07-28
2002-08-13
Dawson, Robert (Department: 1712)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S485000, C523S462000, C523S465000, C525S194000, C525S199000, C525S208000, C525S209000, C528S408000, C528S418000
Reexamination Certificate
active
06432488
ABSTRACT:
The present invention relates to powdered thermosetting compositions comprising a glycidyl group containing acrylic copolymer, and particularly, to powdered thermosetting compositions which produce a uniform matte texture after application and cure onto the surface of a substrate. The invention also relates to the use of said compositions for the preparation of powdered paints which produce non-yellowing uniform matte texture coatings and also to the coatings obtained by the use of these compositions.
BACKGROUND OF THE INVENTION
Powdered thermosetting compositions are widely utilized for coating numerous objects. Such compositions have gained considerable popularity in the surface coatings industry for various reasons. For one, since they are virtually free of the potentially harmful fugitive organic solvents which are normally present in liquid coatings, they are safer to handle and apply. Further, their use results in less damage which might be caused to the surrounding environment by the release of these potentially harmful solvents.
Powder coatings generally contain one or more organic thermosetting binders, fillers, pigments, catalysts and various other additives to modify their properties to suit their intended use. There are two principal types of powdered thermosetting compositions, the first of which consists of a mixture of carboxyl group-containing polymers, such polymers comprising polyesters or polyacrylates, and epoxy compounds, such as cyanurates, and the second of which consists of a mixture of hydroxyl group-containing polymers, such polymers being polyesters, and isocyanates blocked with phenol or caprolactam.
These powder coatings are generally prepared as follows. The polymer, cross-linking agent, catalyst, pigments, fillers and other additives, if any, are first dry blended. The resulting mixture is then conveyed to an extruder, heated to between 80° and 120° C. while being forced through the extruder head in order to homogenize the various ingredients of the powder coating. The homogenized mixture is then cooled and ground into a powder having a particle size of between 20 and 150 microns. The fine powder thus obtained is then applied onto a pre-heated substrate by conventional means, such as by an electrostatic spray gun. The coated substrate is then heated in an oven for a time sufficient to cross-link the binder and achieve cure of the powder coating.
Powdered coating compositions in general give coatings which have good adhesion and exhibit excellent weatherability. However, the majority of these compositions result in coatings having high gloss after fusion and coating. Gloss is measured at an angle of 60°, in accordance with the test method set forth in ASTM D523. High gloss finishes typically have reflection values which exceed 90%.
With an ever increasing demand for powdered coatings which provide matte texture finishes, many attempts have been made to achieve this objective. However, it has proven difficult under conventional methods of extrusion and curing to produce powdered coatings capable of producing a matte texture in a consistent, uniform and reliable way. One proposed solution involved the inclusion of matting agents, such as silica, talcum, chalk or metal salts, in the powder coating formulation. However, this approach proved unacceptable because of poor adhesion and an inconsistent, or mottled, surface appearance. Another approach, as set forth in EP patent 165207 is to incorporate waxes, such as a polyolefin wax, and metal salts, such as 2-benzothiazolethiolate, into powder coating compositions based on carboxyl group-terminated polyesters and epoxy compounds. However, the waxes migrate readily to the surface of the final coating, causing unacceptable variations in the degree of matt texture as it ages. Similarly, U.S. Pat. No. 4,242,253 discloses the use of calcium carbonate and finely divided polypropylene particles as additives, to provide low gloss coatings. A disadvantage of this system is that the inorganic fillers, which are often required in significant amounts in order to reduce gloss, can damage the extruders and impair the quality of the finished coating by exhibiting a rough and irregular surface finish. Furthermore, and most fundamentally, the need to include additional fillers adds to the cost of the coating.
One attempt at obtaining a matte texture provided for the dry-blending of two powdered thermosetting compositions subsequent to each having been separately extruded. This process is exemplified in U.S. Pat. No. 3,842,035. As described herein, one composition is slow curing (long gel time) and the other is fast curing (short gel time). The use of this system results in a coating that has a matte texture, without the need to use a specific matte producing agent. The problem with this system, however, is the requirement of having to dry blend large amounts of formulated powders. This is not an easy task, especially on an industrial scale. Moreover, since this formulation cannot be produced on a continuous basis but only in individual batches of powder, each batch of powder will vary ever so slightly from any other, thus resulting in variations in the appearance of the matte finish. Further, because dry-blending is less efficient than extrusion to commingle all of the formulation ingredients, powder which is recovered after spraying and recycled for re-application may not have the same formulation as it did when first sprayed, thus giving rise to more variations in the matte appearance.
There are other systems which are intended to make matte texture coatings wherein two polymers of different types or rates of reaction are employed, together with one or more cross-linking agents, such that two distinct cross-linking mechanisms or two different reaction rates are induced. In such systems, the blending of the various powder components may be carried out in a continuous extrusion process. For example, JP 154771/88 discloses a resin composition for a matte texture powder coating which comprises a mixture of a branched hydroxyl group containing polyester having a high hydroxyl number with another hydroxyl group containing polyester having a lower hydroxyl number, together with a blocked isocyanate as the cross linking agent.
EP 366608 A describes matte texture producing powdered paints obtained by a single extrusion process which include two cross-linking agents. These formulations contain an epoxy resin, in particular, bisphenol A diglycidyl ether, a polycarboxylic acid, such as 2,2,5,5-tetra (&bgr;-carboxyethyl) cyclopentanone, as the first cross-linking agent, and a saturated carboxyl group-terminated polyester, tolylbiguanide or dicyandiamide as the second cross-linking agent.
A further approach to providing matte texture coatings is disclosed in EP 104424 A. Herein, the powder coating composition contains both a hydroxyl group containing polyester resin, and a polyepoxy compound, such as triglycidyl isocyanurate, as the binder. It teaches the use of a particular cross-linking agent which contains both carboxyl groups (to react with the epoxy compound) and blocked isocyanate groups (to react with the hydroxyl group containing resin). Total control over the extrusion temperature and shear rates is required in order to achieve and/or maintain uniform blending of the formulation ingredients. Otherwise, an inconsistent matte texture finish coating results.
STATEMENT OF THE INVENTION
It is therefore an object of the present invention to provide a powder coating which results in a uniform non-yellowing matte texture finish exhibiting good UV and moisture resistance versus the vinyl laminate coating systems which have been used for years in the coating of kitchen cabinet doors and the like. The present invention provides a powder coating consisting of a glycidyl group containing acrylic copolymer resin, a dicarboxylic acid cross-linking agent, a select catalyst and a matte, texturing agent. The powder coating of the invention produces a matte texture having a substantially uniform appearance.
DETAILED DESCRIPTION
The present i
Daly Andrew T.
Haley Richard P.
Nicholl Edward G.
Aylward D.
Dawson Robert
Rohm and Haas Company
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