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Matting Additives For Powder Coatings
14

Jan

Matting additives (or additive matting agents) are defined by Bate as products, which have been designed to reduce the gloss of a powder coatings, without interfering with the systems (which involve chiometry. That means, neither matting hardeners, not chemical matting systems (which involve mixing of binders or powders with different reactivity) belong to this group of additives.

These matting additives area able to reduce gloss down to 60°values of about 50 to 60%. For 60° gloss value lower than 50%, materials (and/or methods) are needed, which cannot be regarded as "additives".

Two main types of matting additives are available:

l Waxes (either incompatible and (non)-melting waxes, or wax/accelerator masterbatches);

l Silicas

The use of both types is restricted to maximum levels of 5% on total powder, as higher percentages lead to detrimental powder coating characteristics.

Gloss reduction by waxes takes place because during stoving the molten wax migrates and concentrates near the film surface, forming a thin surface layer, enriched with wax.

This floating or "bloom" effect can be aided by the incompatibility of the molten wax with the molten coating.

Various wax types are used, often in micronized form. So polypropylene powder can produce satin finishes at addition levels of 2 to 5% (on total formulation). Polypropylene wax and combination(based on total binder), providing 60°gloss values of 55 to 65%.

A product, more or less based on this patent, is available as "Ciba Matting Agent DT125-2". Silica matting agents are prepared by a precipitation process, in which sodium silicate and sulphuric acid react together to give a slurry of precipitated silica. After filtration, washing, drying and milling, an untreated silica is formed, consisting of silicon dioxide(SiO2) in the form of a hydrated amorphous "synthetic" silica.

Suitable precipitated silicas have an average particle size of 3 to 10 μm. They should be added at the premix stage and can produce semi-gloss and satin finishes at addition levels of 3 to 4% (on total formulation). In contract with the mentioned wax types, precipitated silicas show a greater tendency to affect film flow in a negative way.

Apart from waxes and silicas some other matting additives should be mentioned, Loar and Verle describes small particle sized Zinc-ionomers (prepared by partial neutralization of an ethylene-acrylic acid copolymer with zinc), which by replacing up to 5% of the polyester, give 60°gloss values of 55 to 75% (in "Primid"-cured polyester powder coatings) depending on the amount of Zn-ionomer.

As many texturzing polymeric additives also cause a visible reduction of gloss. A survey of "waxes for powder coatings", with all their various functions (not only matting), is given by Heiling(Heiling, G., xxvi. Fatipec Congress, Dresden 2002, Macromol, Symposia 187,437)

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