Supercritical Spray Application of Paints and Coatings

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Each year, American industry spray applies more than 500 million gallons of paints and other coatings. For every gallon of paint sprayed, an average of four and one half pounds of solvents are released into the atmosphere. These solvents are mixtures of volatile organic compounds (VOCs) that are chosen for their ability to dissolve the coating's materials and to reduce their viscosity. A new process has been developed for the spray application of paints and other coatings. It was conceived by the PI and is being commercialized by the Union Carbide Corporation and other companies. The process, which improves the quality of sprayed coatings for automobiles, furniture, and other commercial products and reduces pollution, is based on the use of supercritical carbon dioxide as a replacement for organic solvents in spray-coatings' formulations. The result is a more uniform and improved coating and a reduction of the emissions of VOCs by up to 80%. Though the process is in commercial use, there are problems that must be overcome before there will be widespread adoption of this technology, and hence large reductions in VOC emissions from spray-coating operations in the U.S. The major impediment is that each paint must be reformulated before it can be used in this process. Since there are hundreds of different paint formulations, each with dozens of different components, this will be a major undertaking because relatively little is known about the phase behavior of polymer-solvent-supercritical fluid mixtures, particularly when solid phases can form. Hence, the reformulation must be done by trial and error changing many different variables. Also, the reformulation process will be done separately, and secretly, by every paint manufacturer. The PI plans to study the thermodynamic and rheological properties of polymer-solvent-carbon dioxide mixtures. The experimental part of the project will involve measurements to determine phase diagrams and solution viscosities for these mixtures. The theoretical portion will involve the development of models for the effects of hydrogen bonding on the liquid-liquid phase behavior and on modelling the effects of solids precipitation on the overall phase diagram. This should facilitate the implementation of this technology for a wider variety of paint formulations and should help eliminate process shut-downs that result when solids form in these systems.