Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-12-29
2003-03-11
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S430000, C524S437000, C524S492000, C523S216000
Reexamination Certificate
active
06531524
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to powder coating compositions and to their use.
Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed. Charging of the powder in the spray gun is effected by means of an applied voltage or by the use of friction (tribo-charging). The applied composition is then heated to melt and fuse the particles and to cure the coating. The powder coating particles which do not adhere to the substrate can be recovered for reuse so that powder coatings are economical in use of ingredients. Also, powder coating compositions are generally free of added solvents and, in particular, do not use organic solvents and are accordingly non-polluting.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more coloring agents such as pigments, and optionally also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer).
The compositions are generally prepared by intimately mixing the ingredients (including coloring agents and performance additives) for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding to the desired particle sizes (“micronizing”).
The particle size distribution required for most commercial electrostatic spray apparatus is up to 120 microns, often between 10 and 120 microns, with a mean particle size within the range of 15 to 75 microns, preferably 25 to 50 microns, more especially 20 to 45 microns.
BACKGROUND OF THE INVENTION
Currently, powder coating manufacturing methods allow the manufacturer to offer commercially a range of full gloss coatings in a variety of colors. The range of products available in different finishes is, however, limited.
Reduction of gloss to some other lower level, for example satin gloss (55-65% gloss) or matte (<30% gloss), is achieved by creating a surface which is rough on a microscopic scale. This surface roughness must be sufficient to cause a reduction in the specular reflection from the film by scattering the incident light; if it is visible, however, a texturing effect is achieved in the film.
In liquid paints this gloss reduction is usually achieved by the use of pigment and/or filler particles at high volume concentrations. However, this technique cannot be used as the sole route to gloss reduction in powder coatings as a high content of filler particles would unacceptably lead to too reduced a surface flow during curing.
Gloss reduction and/or texture in powder coatings may be achieved by using incompatible components or components that generate incompatibility. For example, an acrylic component and a polyester, epoxy, polyester-epoxy or polyurethane component are incompatible, and cannot be blended to form a single (stable) phase. Incompatibility during film formation can also be achieved by using components that are initially miscible (compatible) but that become incompatible during curing. Thus, for example, two systems of similar chemistry and approximately the same gel time are compatible, but components with different gel times are initially compatible but become incompatible as curing (and molecular weight build-up) proceeds. Materials that are incompatible during film-formation can separate into different phase domains which can give rise to incompatibility effects such as matting. Aside from this, the presence of two materials of different surface tension at the surface of the film and in discrete areas/domains can lead to surface disruption (texturing).
In practice, for gloss reduction, the procedure used is to set up reactions within the curing film such that two different gelation rates are set up within the curing matrix. With acid-functional polyesters, for example, a fast gelling (reacting) powder and a slow gelling powder may be manufactured separately using polyesters of different functionality, and mixed after the micronizing stage or, more usually, the components are mixed prior to micronizing. The components should have the same color and particle size. The faster gelling domains form particles which disrupt the surface flow of the slower gelling portion of the matrix. Production of small batches of reduced-gloss coating composition, however, is uneconomic. For mixing after the micronizing stage, a product called 'gloss killer' is often used, available from the company Tiger. However, this product, a clear powder coating of a conventional particle size, can be added only in limited amounts to a conventional colored powder coating before the presence of the gloss killer can be detected from the sparkle generated by the particles of the clear gloss killer powder in the film. Therefore, the product is limited to adjusting gloss by a few percentage points. Similarly, waxes and various extenders can be used to produce satin finishes, but do not provide a matte finish. Gloss-reducing agents also include a second catalyst which will give a much faster gel time than the principal catalyst used to cure the film, e.g., for polyesters the Ciba-Geigy products “XG 125” (zinc N-ethyl-N-phenyl-dithiocarbamate) and “XB 3329” (the zinc salt plus a matting wax that further reduces the gloss level, possibly through incompatibility), and, for pure epoxy and epoxy-polyester (hybrid) systems, a catalyst which has a chemical structure that causes two rates of gelation on curing, e.g., the products described in GB 1,545,780, and including the Hüls products B55 and B68 which are adducts of pyromellitic acid and 2-phenylimidazaline.
For texture, a number of agents can be used to achieve different surface effects, acting by disrupting the flow of the polymer film. For a stippled finish, micronized PTFE (polytetrafluoroethylene) is used. For a wrinkle finish, cellulose acetate butyrate resin (CAB) or acrylate homo- and co-polymers may be used. In addition, texturing agents based on high molecular weight thermoplastics are commonly added to thermosetting powder coatings, giving rise to uniformly poor flow across the surface, which manifests itself as a texture. PTFE is preferably added to the premix before extrusion and is fully incorporated into the extrudate. However, the micronized PTFE has to be prepared in a particular fashion to give batch-to-batch consistency. CAB may be added to the finished powder or, preferably, is also added at the premix stage before extrusion; it does not melt in the extruder, and the inhomogeneous dispersion thereby produced gives rise to the texture. Some texturing agents, however, for example Acronal™ 4F, have to be added post-extrusion since the extrusion process would render them ineffective by mixing them intimately with the continuous phase of the coating system. However, powders containing post-additives show inconsistency in the texturing effect on application of the powder. A further post-extrusion texturing additive is the ester-modified polyether product Powdermate 508TEX sold by the company Troy. However, this product has to be added to a powder whose original gloss is dictated by the formulation used.
The production of metallic effect powder coatings is normally achieved by bonding the metallic pigment to the powder coating. If the pigment is incorporated into the powder coating composition with the other ingredients in the extruder the luster effect is likely to be destroyed by the strong shear forces found in the extruders and in the subsequent micronizing step. If, however, the metallic flake is simply incorporated by dry-blending, the loose metallic flakes in such processes can cause unwanted electrical discharge within the spray
Crapper Gareth Dale
Kittle Kevin Jeffrey
Ring John
Cain Edward J.
Esq. Kristin Neuman
International Coatings Limited
Proskauer Rose LLP
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