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Matt and Semi-matt Powder Coatings
14

Jan

The use of matt and semi-matt powder coatings account for more than 40% of all powders for interior and exterior applications. Such low gloss coatings are demanded for both decorative and architectural purposes. Undesired light reflectance can be reduced , when matt or semi-matt coatings are used. In addition, surface defects such as scratches and imperfections in the substrate are less visible on matt surfaces.

Reduction in gloss of a powder coating down to 60°gloss values of less than 50% is brought about by the development of an irregular and/or micro-textured film surface during the curing reaction.

Various methods have been developed to produce irregular surfaces with their consequent light scattering effect, leading to low gloss readings.

At present there are four main matting methods in use:

Addition of pigments and/or fillers above the critical pigment volume concentration (CPVC) can form irregularities in the film and as a result gloss reduction. This method has limited use, since the high levels of fillers and/or pigments result in poor film flow and in reduction of mechanical properties of the cured film and the overall matting effect is not always appreciated.

Addition of incompatible waxes or plasticizers is a method of creating surface inhomogeneity, because the waxes migrate to the surface during film curing, forming "micro-droplets", which provide a fine and rough surface with a reduced gloss effect. However, there is a tendency of the wax to leach out of (or to "exude") from the film at those levels of addition (>5%) where they are likely to be effective; an effect that limits the application to light colors (where the whitening effect of the micro-droplets is less noticeable) and to 60° gloss values higher than 50%.

Dry blending of two different powders, each having a different reactivity, or consisting of two incompatible types (based on the "contamination phenomenon"). For improved mixing dry blending is often replaced in practice by "co-grinding" of the extrudates of the two powders. The oldest known examples of this method are decribed in patents by Wagemakers' Lakfabrieken and Shell. They obtain low gloss values, either by dry blending two epoxy/DICY powders with different reactivities, or by dry blending an epoxy powder (without hardener) with an epoxy /DICY powder or with DICY lone. A well known example of dry blending two polyester powder coatings with different reactivities is the combination of high gloss powders, based on polyester/TGIC 90:10 and polyester/TGIC 96:4 or on polyester/HAA "Primid" 90:10 and 96:4 (or 96.5:3.5).

The greater the difference in reactivities, the lower the gloss of the coating in this way 60° gloss value of 25 to 35% are achievable.

An alternative is the dry blending of two powders, based on different chemistries with different melt viscosities, surface tension or miscibility properties, leading to film disturbances (often also caused by contamination ) and to gloss reduction. Examples are: dry blend mixtures of high gloss hybrid and polyester/TGIC powders, hybrid and pure epoxy powders, polyurethane and polyester/TGIC powders, etc. Some combinations can lead to 60° gloss values of 35 to 40%. Gloss reduction, caused by contaminations (or incompatibility) of two powders often gives powders with a damask-like semi-matt finish.

The suitability of two powders to form a semi-matt effect after dry blending, can be estimated by the ISO 8130-12 method "Determination of compatibility".

The dry blending manufacturing is time-consuming (and thus costly), as always two powders should be produced. Moreover, it is often difficult to obtain a uniform (semi-)matt aspect and reproducible gloss values.

Producing (semi-)matt powder coatings by a "one-shot" process and in this way get around the uneconomic dry blending technique. This " chemically induced incompatibility" process involves the use of binders, by which at least two chemical reactions with different speeds take place during stoving. It is based on the principle of the "double (or multiple) curing" mechanism, which can consist of premixing either one resin with a dual (or bi-) functional curing agent, or one resin with (a physical mixture of) two curing agents, or two different resins with a single curing agent, or two different resins with two or three curing agents.

Here too counts: the greater the differences in reaction speeds, the lower the gloss of the film.

The oldest example of bi-functional curing agents are homogeneous and well-defined mono-salts of certain aromatic polycarboxylic acids (f.e. Pyromellitic acid) and special cyclic amidines (f.e. 2-phenyl-2-imidazoline or 2-methyl-2-imidazoline),. These salts are the well-known"Vestagon B 55" and "B 68", used as "chemical" matting hardeners for epoxy and hybrid powder coatings.

During stoving the salts decompose into their original products, namely the highly reactive amidine and the much less reactive polycarboxylic acid. Here too, the matting effect is based on differences in reactivity during stoving.

Matt powder coatings, containing "B 55" or "B 68", are distinguished by good film flow and good resistance to hot water and detergents. The absence of any visible surface texture, despite a markedly unform matt effect, is surprising. The mechanical properties (particularly the impact resistance) are poor, when the film thickness exceed 50 μm. Both Vestagons" are sensitive to yellowing on overbake, especially with white and light colors. Stoving temperature must be at least 180℃, which can be seen as a disadvantage for some applications.

Trials have not failed to improve the mentioned negative properties and simultaneously maintaining the benefits of the "Vestagons".

Ciba uses a polyolefine wax in combination with the zinc salt of an organic anion (preferably a mercaptobenzotriazole), which has the ability to accelerate the curing reaction of epoxy/polyester (hybrid) powder coatings. The matting effect is probably due to its poor "solubility" in the binder system, causing localized acceleration in the curing reaction and as a result an irregular film formation, giving a micro-textured surface. Therefore, this system can also be classified as a double )or dual)-curing system.

This product is commercially available as Ciba "Matting Agent DT 3329-1". It compliments the matting effect of the already cited "DT 125", as it allows 60° gloss levels of 35 to 40% to be realized, by adding 3 to 6% on binder. The matting efficiency of "DT 3329-1" is dependent on the choice of the polyester resin, the degree of homogenization and the cure temperature.

As DT3329 contains a waxy component, it is best suited to semi-matting light and not to dark colors, in order to prevent visible leaching-out of the wax. So does our matting agent M-21.

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