Polyurethane dispersions (PUDs) consist of polyurethane particles dispersed in water with the aid of an organic solvent or a dispersing agent. These dispersions have good film formation and provide superior appearance and resistance properties.1 They are also free of isocyanate residues and can be formulated at low VOC levels, both of which make them safer to use than their solvent-based counterparts. While these advantages are desired by formulators, the usage of PUDs has been limited by their cost, which is sometimes higher than conventional acrylic emulsions. To offset the cost, PUDs can be combined with acrylic emulsions through a physical blend2 or a chemical reaction, resulting in a PUD-acrylic hybrid.3 In this study, we have evaluated the performance of a PUD-acrylic hybrid, Joncryl® HYB 6340, as a high-performance wood coating using added crosslinkers to improve the crosslink density in the film. The objective of the study is to evaluate how crosslinkers affect properties and which crosslinker would provide the best performance in a PUD-based wood coating. In addition, the mechanism of crosslinking will be investigated.4
Crosslinking is the process of chemically joining two or more polymer chains by covalent bonds and is usually initiated by heat, pressure or change in pH. Increasing the crosslink density of a coating forms an impenetrable barrier that prevents water and chemicals from reaching the substrate. This results in improved chemical resistance. Crosslinking also augments adhesion and scratch resistance, thereby providing a tougher coating with overall better performance properties.5 Crosslinking is usually done by using external crosslinkers such as isocyanates, aziridines, melamines, polycarbodiimide (PCDI), etc. Table 1 shows the different functional groups on a polymer backbone that could react with a crosslinker.