Replacement of Organic Solvents by Carbon Dioxide for Forming Aerosols in Coating ProcessesEPA Grant Number: R824728
Title: Replacement of Organic Solvents by Carbon Dioxide for Forming Aerosols in Coating Processes
Investigators: Sievers, R. E. , Watkins, Barbara A. , Xu, Chongying
Current Investigators: Sievers, R. E. , Miles, Barbara A.
Institution: University of Colorado at Boulder
EPA Project Officer: Karn, Barbara
Project Period: October 1, 1995 through September 30, 1997
Project Amount: $200,000
RFA: Technology for a Sustainable Environment (1995) Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Description:Spinel films, which are composed of cobalt-iron-chromium oxides, are of considerable interest for passive solar shading because of their combined favorable optical, physical, and chemical properties. A preferred oxide film is approximately 400 Å thick with the metallic constituents in the ratio of 70% cobalt, 18% iron, and 12% chromium. Presently in industry, spinels are coated on glass pyrolytically by spraying aerosol powders directly onto a glass float line at approximately 600°C. The aerosol powders have historically been prepared by spray-drying solutions of metal acetylacetonates dissolved in methylene chloride (CH2Cl2). The emission of this halogenated organic solvent causes environmental and worker safety problems, so the development of a process that avoids volatile organic compound (VOC) emissions is desired.
The purpose of this project, involving collaborative work with Ford Glass Division, is to develop an improved method to deposit spinel films that eliminates the need for toxic organic solvents and reduces the emission of halogenated and oxidant-forming organic compounds. The method to be developed employs two environmentally acceptable solvents, water and carbon dioxide (CO2). CO2 in its supercritical state (SF-CO2) can facilitate aerosol formation for thin film deposition. Metal coating precursors are dissolved directly in SF-CO2 or are dissolved in water and the resulting solution is mixed with the SF-CO2 forming an emulsion. The solution or emulsion passes through a restrictor, decompresses, and forms a vapor or a very fine aerosol that contains the precursor compounds. As the aerosol or vapor contacts a heated substrate, surface reactions take place, and coatings are deposited. This more environmentally benign process significantly broadens the scope of compounds that can be used as precursors for thin film deposition. Unlike the conventional chemical vapor deposition (CVD) process, this new SF-CO2-facilitated approach enables the use of non-volatile precursors, such as water-soluble metal acetates and salts. These compounds are significantly less costly and less toxic.
In this project, special attention will be paid to the deposition of spinel films on glass substrates. In our preliminary studies, reflective, transparent, and durable spinel thin films were deposited on glass or quartz substrates at about 600°C by spraying aqueous solutions containing metal acetate or metal nitrate precursor compounds. Research will include optimization of the process to produce high quality spinel films. The effect of the type of precursor(s) used will be studied, as well as the composition and concentration of the precursor solution, the diameter of the aerosol particles formed, the temperature of the substrate, the angle of projection of the spray, the by-products, etc. The continuing research will provide data to evaluate the practicability of the SF CO2-facilitated method for coating spinel thin films in industry. Deposition of other materials will also be studied.