1. Selecting Hydroxyl Polyesters

    The main chemical reaction by which polyurethane powder coatings cure into films is the reaction between hydroxyl groups (—OH) in the polyester resin and blocked isocyanate groups (—NCO). The degradation rate of a polyurethane powder-coating film is mainly governed by decomposition of the hydroxyl polyester backbone. Selecting a suitable hydroxyl polyester is therefore crucial to the coating's chemical resistance. Polyurethane powder coatings made with hydroxyl polyesters from different manufacturers can differ considerably in their physical and chemical properties. Comparative testing is used to select a resin with high gloss and good aging resistance as the final film-forming resin for the polyurethane powder coating.

    2. Selecting Curing Agents

    Curing agents have a major influence on the physical and chemical properties of polyurethane powder coatings. If the selected curing agent does not crosslink well with the resin, these properties will be significantly affected. Powder-coating films are tested for aging resistance, gloss, cupping performance and various other physical and chemical properties. One curing agent currently performing well in the market is Evonik Degussa B1530. This product is a VESTANAT IPDI adduct used as a crosslinker for hydroxyl-containing saturated polyesters in weather-resistant polyurethane powder coatings. Caprolactam keeps the NCO groups blocked below approximately 170°C. The product is granular and has excellent weather resistance. Its NCO content is 14.8–15.7% by weight, and its typical glass-transition temperature is 48°C. Select a product with superior performance as the final curing agent for the polyurethane powder coating.

    3. Selecting Additives

    Suitable additives must be selected to improve the coating film's aging resistance. The catalyst used for polyurethane powder coatings is a free-flowing dibutyltin dilaurate catalyst. Its addition level also has a crucial influence on the film's aging resistance and impact resistance.