Mechanism of action of benzoin as a degassing agent in powder coatings

The mechanism of action of benzoin as a degassing agent in powder coatings has been analyzed. The gas bubble shrinkage was monitored using a light microscope equipped with a hot stage. In the absence of benzoin, the air bubbles start to shrink very slowly as a result of a diffusion-controlled process. Because of the continuing cross-linking reaction and increase in the viscosity, the bubble shrinkage halts at elevated temperatures. Quite remarkably, we observed that in the presence of benzoin the process of bubble shrinkage is accelerated to such an extent that most air bubbles disappear before any significant increase in the viscosity occurs. This suggests that benzoin functions by accelerating the rate of bubble shrinkage. To analyze the mechanism of action of benzoin in detail, we studied the coating formulations using various techniques. X-ray diffraction in combination with deuterium NMR using labeled benzoin indicated that benzoin dissolves on a molecular level in polyester resin and becomes mobile above the glass transition temperature of the matrix. Mass spectroscopy experiments revealed that benzoin, in its oxidized form (benzil), starts to evaporate above 100 °C. At 200 °C more than 90% of the initial concentration of benzoin has evaporated from the coating. As was to be expected, the conversion of benzoin to benzil halted when the experiments were carried out in nitrogen. We postulated that the action of benzoin as a degassing agent is somehow related to its ability to oxidize in situ. This claim is substantiated by the results of bubble dissolution experiments using different gases such as oxygen and nitrogen. It is shown that in the presence of benzoin oxygen bubbles shrink much faster than air bubbles. On the other hand, the shrinkage of nitrogen bubbles is not affected by benzoin. Based on the above results, a two-step mechanism is proposed for the action of benzoin as a degassing agent. This mechanism has been successfully used as a guideline to identify alternative degassing agents with higher efficiency and fewer side-effects, such as less severe yellowing.