Removal of polymer coating with supercritical carbon dioxide

Abstract

This work investigates the use of supercritical fluids, and carbon dioxide in particular, for the removal of polymer coatings. Research into new supercritical fluid applications is nearly always based on a trial and error approach, and frequently requires evaluating each operation on a case-by-case basis. A significant improvement in this approach is accomplished with the development of a framework in which polymer-CO2 interactions can be evaluated and the number of experimental trials reduced. The basic model developed is built upon the three-component solubility parameter (HSP) concept, which is widely used in the coatings industry to aid in the selection of solvents. Temperature and pressure dependent HSP values have been develop for supercritical CO2, using a methodology extendable to other supercritical fluids. Equations were also developed to calculate HSP's for cosolvents and polymers. With the solvent, cosolvent, and polymer thus fully characterized in terms of the HSP values, the systems are then analyzed in terms of the like and unlike (solvent/polymer, cosolvent/polymer, and solvent/cosolvent) binary pairs. In addition to this study, consideration of specific interactions, such as Lewis acid/base interactions between the solvent and polymer or between the cosolvent and polymer are examined for their role in determining a favorable (polymer coating removal) result. The model was tested on two real-world applications: involving poly(methyl methacrylate) (PMMA) and polycarbonate (PC) coatings. Several organic liquids were evaluated as cosolvents, including at least one example of a non-polar fluid, a Lewis acid, and a Lewis base. Results of this study found the following interactions, listed in order of importance in the removal of polymer coatings, to be (1) specific interactions between the solvent and polymer, in the case of PM M A and CO2 , or specific interactions between the cosolvent and polymer, in the case of PC and CO2, (2) weaken polymer/polymer interactions as a result of polymer swelling and subsequent lowering of the polymer HSP values, (3) specific interactions between the solvent and cosolvent are not necessary and in the case of specific interactions between the cosolvent and polymer, may be undesirable.