Technical Development of the Supercritical Fluid Spray Process for the Application of Coatings

Technical development and commercial implementation of the supercritical fluid spray process have produced the surprising result that coating appearance and performance can actually be improved by removing 60 to 80 percent of the organic solvent from solvent-borne coating formulations. Unlike normal diluent solvents and water, the decompressive release of supercritical carbon dioxide during spray formation produces a new type of spray that can have superior atomization characteristics compared to air, HVLP, air-assisted airless, and conventional airless spray methods. Not only can this improve coating quality, it can reduce material and operating costs by eliminating application steps and reducing the amount of coating solids sprayed per part. Furthermore, air toxic solvents such as l,l,l-trichlor~etha~e c a ~ he eliminated from the reformulated coating. The process is compatible with most types of polymer systems and is being applied to UV coatings and two-package reactive systems. It is undergoing commercial development and implementation in a variety of applications including wood furniture, plastics, automotive topcoats and components, general industrial, release surfaces, and adhesives. With continued polymer development, the process can be used as a liquid analog of powder coatings but have appearance and application advantages. @ 1993 Union Carbide All Rights Reserved TECHNICAL DEVELOPMENT OF THE SUPERCRITICAL FLUID SPRAY PROCESS FOR THE APPLICATION OF COATINGS INTRODUCTION The supercritical fluid spray process (1,2) was introduced in 1990 as a new pollution prevention technology for the spray application of coatings. Since then the process has undergone considerable technical development and evaluation as it has undergone commercial development and implementation. This has generated the surprising result that the process can actually improve coating appearance and application performance in addition to substantially preventing pollution. Indeed, commercial implementation activity in Europe is due primarily to appearance and performance improvements instead of pollution reduction. The process uses supercritical carbon dioxide to replace volatile organic solvents in conventional and high solids coating formulations. Volatile organic compound (VOC) emissions have been reduced up to 80% and air toxic solvents have been totally eliminated for many coatings. More importantly, the decompressive release of carbon dioxide can produce superior atomization by a new atomization mechanism (3). This new type of spray has been shown to produce coatings having equal or improved quality to those applied by conventional spray methods with highsolvent formulations. Furthermore, improved application performance has been demonstrated by reducing application steps, reducing the amount of coating solids sprayed, or by enabling thicker coating films to be applied without sag. This paper discloses how coatings are reformulated and sprayed using supercritical carbon dioxide and reports coating application results obtained during commercial implementation. COATING REFORMULATION Conventional and high solids solvent-borne coating formulations can often be reformulated for spray application with supercritical carbon dioxide by adjusting the solvent level and solvent blend, with little or no change being made to the polymer system and pigments. Generally, the polymer system or pigments are adjusted only if too much medium or slow evaporating solvents must be removed from the coating formulation, in addition to the fast evaporating diluent solvents that are replaced by carbon dioxide, in order to meet emission requirements. COATING SYSTEMS The process has been found to be broadly applicable to most types of coating systems. It has been demonstrated using air-dry lacquers, thermoplastic polymers, thermosetting polymers ultraviolet light cured polymers; and two-component polymer systems. A variety of clear, pigmented, and metallic coatings have been applied. Polymers that have been used in single-package systems include acrylics, air-dry alkyds, baking alkyds, polyesters, melamines, melamine formaldehydes, nitrocellulose, silicones, vinyls, epoxies, phenolics, ureas, urethanes, and waxes. Acrylates have been used in ultraviolet light cured coatings. Polymer molecular weights have ranged from very low to very high.