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UV powder coatings are making strong inroads into the powder coatings market, especially for temperature-sensitive substrates. The existing UV-curing powder coatings systems generally build on two binders. This paper presents an option to the existing systems and consists of using unsaturated amorphous or amorphouscrystalline polyesters comprising maleic/fumaric moieties as reactive double bonds in combination with crystalline crosslinkers containing unsaturated reactive groups. The study further presents a comparison of the crosslinking performance for Zn diacrylate, hexahydro-1,3,5-tris(l-oxo-2-propenyl)-1,3,5-triazine (1,3,5-triacryloil-hexahydro-triazine) Tradename M-1530) and unsaturated derivatives from 3,9-bis(2-hydroxy-l,l-dimethyl)2,4,8,10-tetraoxispiro[5,5]undecane (penta-spiroglycol) suitable for this application.
The latest development in powder coatings is the combination of powder coating technology and UV technology. This approach shows some very important benefits. The benefits due to powder coating technology versus liquid coatings are:
* Elimination or reduction of intermediate sanding steps;
* High performance at high utilization rate, reclaim leading to 95% powder usage;
* No VOC during the curing process; and
* Easy recycling and no waste treatment.
New benefits due to the combined technology of UV and powder coating are:
* Split between melting by IR or convection and the curing process;
* Application of complete board in one step in vertical line;
* Better and undisturbed levelling by separation of the flow and cure phases;
* No cooling of the coated object required;
* Good storage stability below 50 °C;
* Good adhesion on different substrates; and
* Design with good ODD.
State of the Art
Although there are a few limitations, UV-curable powder coatings have made progress, and several options are now available in the marketplace. These include:
* Maleic/fumaric polyester combined with a vinyl ester;
* Acryl/methacryl functional polyesters;
* Unsaturated polyester combined with acrylic functional polyurethane;
* Acrylic functional polyacrylic combined with acrylic functional polyurethane; and
* Maleic/fumaric polyester combined with allyl functional polyesters.
All these UV-curing systems have limited applications. Some of them have a very narrow range of the ratio between components.
Crosslinkers for UV Powder Coatings
The most suitable approach for a coating formulation is the use of a major binder and a crosslinker. The crosslinker may control the network density for the coating, while the binder determines properties of the coating such as discoloration, outdoor stability, mechanical properties, etc. Furthermore, this approach will lead to...