In this application, carbon dioxide (CO2) gas is made supercritical and transformed into a liquid under high pressure which then dissolves the liquid organic coating material much like an organic solvent. Liquefied CO2 reaches a critical temperature of 31.3 oC and a critical pressure of 7.4 Mpa in this supercritical phase. In its supercritical phase, CO2 has solvent characteristics that are similar to those of aromatic hydrocarbons. Hence, it is possible to switch a portion of the solvent in the coating with CO2. For this purpose, specially designed airless spray guns can be used. As the coating + CO2 mixture leaves the gun, CO2 transforms into a gas as a result of an intensive boiling which is then dispersed into the atmosphere. However, since CO2 gas is not a VOC, it does not result in atmospheric pollution. On the other hand, CO2 that is homogeneously distributed inside the atomized coating droplet at the outlet of the spray gun meets the atmospheric pressure for the first time right at that point. Following the decrease in pressure, CO2 rapidly expands and is transformed into the gas phase thus disintegrating the droplet from within thereby attaining high quality atomization. Therefore, even though the application is made using an airless gun, the acquired surface quality is at the level obtained with air spray guns. The fact that airless spray gun is used for the application also eliminates the rebound effect observed in applications with air spray guns, thus ensuring that a transfer efficiency close to those obtained for airless applications is obtained. Moreover, since CO2 rapidly leaves the coating, the viscosity of the coating that reaches the surface from the gun increases thereby minimizing risks of observing coating defects such as sagging.
In conclusion, it is possible in airless spray applications performed using liquefied CO2 to bring together all the positive aspects of various spray applications. The problem here is the cost of liquefied CO2 and the application device.