SUPERCRITICAL CARBON DIOXIDE SPRAY SYSTEMS

摘要

Supercritical carbon dioxide as a viscosity reduction solvent was first commercialized in 1990. The process is designed to overcome many of the issues facing the coatings industry today. Initially, reduction of air pollution was the sole goal, but as the technology develops, a myriad of other benefits are being recognized. These benefits include waste reduction, cost reduction, quality improvements, less dirt on the painted parts, and less booth cleaning. The desire to improve the environment has spawned a variety of new technologies that have had some success, but their inherent drawbacks have limited their acceptance. Waterborne coatings have become widely accepted in the architectural field where they are generally brush applied; however, for spray application in the industrial market there are problems. The inherent high latent heat of vaporization of water and the humidity dependence on drying are issues in booth and oven design. The difficulty of spraying waterborne systems electrostatically increases overspray and increases costs and pollution even where systems have been installed. Powder coatings create the problem of transporting a powder that wants to stick together. This material may have to be stored for extended periods. The powder then has to be conveyed to the gun where it is sprayed. This powder, which at one stage in its processing life was already a liquid, is then turned back into a liquid by heating before being crosslinked back into a solid coating. After all this the goal is to get coatings with acceptable appearance and economics for the manufacturers as well as the applicators. All this is done with a material that could presumably have been applied as a liquid if a suitable delivery system were available. Ultraviolet (UV) coatings have had limited success primarily because of the need for the coating material to be "seen" by the energy source required for curing. For coating flat stock they can be very effective. Supercritical carbon dioxide overcomes many of the deficiencies stated above. With possible material viscosity in excess of 10,000 centipoise, ultrahigh solids systems can be developed. The carbon dioxide vaporizes almost explosively as it exits the gun without overcooling the coating and, in fact, reduces the dependency of the system on atmospheric conditions. Exotic curing mechanisms are no longer necessary. Many existing coatings can even be reformulated by solvent changes and sprayed with the supercritical carbon dioxide system.