Comparative study of the impact of additives against oxygen inhibition on pendulum hardness and abrasion resistance for UV-curable wood finishes

The multiple advantages of UV-curable coatings, such as high curing speeds and cross-linking densities, low VOC and energy consumption led to a widespread interest for a wide range of applications. Free-radical photopolymerization constitutes one of the most used methods of polymerization. However, oxygen present in the atmosphere is able to inhibit radical formation and propagation, which is considered a major drawback. Among all the strategies developed for reducing the oxygen inhibition, the addition of reactive additives is often preferred. This method does not require major changes in the production lines of the industries or in the formulations compositions. The efficiency of reactive additives against oxygen inhibition has been investigated in multiple studies, but in different experimental conditions. The addition of the reactive additives can also influence coatings hardness and abrasion resistance, which are considered important mechanical properties. The aim of this study is to evaluate additives overall performances. Eleven of the chosen additives were previously described in the literature. The four remaining are commercial products never used for that application, such as a polyhedral oligomeric silsesquioxane bearing a thiol function, which later showed some of the best results. Reduction of oxygen inhibition was analyzed by photo differential scanning calorimetry (Photo-DSC) and Confocal Raman Micro-spectroscopy (CRM). Pendulum hardness and abrasion resistance measurements were used to determine the influence of the additives on the mechanical properties of the coatings. The results obtained show that most of the additives tested had reducing effect on the pendulum hardness, but helped in improving the abrasion resistance