| Abstract
| - The process of bubble dissolution in reactive coatings and the role of benzoin as a degassingagent have been investigated. The gas bubble shrinkage was monitored using a light microscope equippedwith a hot stage. In the absence of benzoin, the air bubbles started to shrink very slowly as a result ofa diffusion-controlled process. Because of the continuing cross-linking reaction and increase in the viscosity,the bubble shrinkage halted at elevated temperatures. Quite remarkably, we observed that in the presenceof benzoin the process of bubble shrinkage was accelerated to such an extent that most air bubblesdissolved before any significant increase in the viscosity occurred. This suggests that benzoin functionsby accelerating the rate of bubble shrinkage. To elucidate the mechanism of the action of benzoin indetail, we studied the coating formulations using various techniques. X-ray diffraction in combinationwith deuterium NMR on labeled benzoin indicated that benzoin is distributed on a molecular level inpolyester resin and becomes mobile above the glass transition of the matrix. Mass spectroscopy experimentsrevealed that benzoin, in its oxidized form (benzil), starts to evaporate above 100 °C. As can be expected,the conversion of benzoin to benzil is halted when experiments are carried out under nitrogen. Wepostulated that the action of benzoin as a degassing agent is related to its ability to oxidize in situ. Thisclaim was substantiated by the results of bubble dissolution experiments using different gases such asoxygen and nitrogen. It was found that in the presence of benzoin oxygen bubbles shrink much fasterthan air bubbles. On the other hand, the shrinkage of nitrogen bubbles is not affected by benzoin. On thebasis of the above results, a two-step mechanism is proposed for the action of benzoin as a degassingagent.
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