Tech Briefs

Long-Life, Hydrophilic, Antimicrobial Coating for Condensing Heat Exchangers

New coating uses a modified structure intended to inhibit diffusion, slow hydrolysis, and lengthen the coating life.

Future manned spacecraft and lunar or Mars outposts will need a condensing heat exchanger (CHX) to control humidity in the cabin atmosphere. Condensing surfaces must be hydrophilic to control condensate flow and ensure efficient operation in zero gravity, and biocidal to prevent growth of microbes and formation of biofilms on condensing surfaces. Coatings must be extremely stable, adhere to the condensing surface, and maintain hydro philic and biocidal properties for many years.

In a previous development of this invention, an innovative zeolite coating was developed that adheres extremely well to heat exchanger materials and structures, is extremely hydrophilic, and contains silver ions to prevent microbe growth. However, results of long-term exposure testing show that the coating’s life may be limited in prototypical environments.

In this work, a new coating chemistry has been developed with the goal of remaining stable for many years. Gradual dissolution of the original zeolite coating was due to rapid diffusion of water molecules and hydrolysis products through the zeolite matrix. The new coating approach uses a modified structure intended to inhibit diffusion, slow hydrolysis, and lengthen the coating life. The coating is applied to solid substrates by crystallization from a liquid synthesis solution. By forcing the synthesis solution to flow through the condensing flow passages in a CHX, thin crystalline coatings can be grown directly on the condensing surfaces after the CHX has been fabricated and assembled. Silver ions are infused into the coating after deposition by exposing the coated surfaces to silver nitrite solution. If the coating becomes contaminated due to constituents in the spacecraft cabin atmosphere, wetting properties can be restored with mild cleansing agents.

The feasibility of the approach was investigated through the development of chemical models for coating stability, the production of trial coatings based on those models, and the demonstration of the improved coating. Two different candidate coatings were created and used on CHX surfaces. The new coatings were hydrophilic, highly biocidal, and showed decreased rates of diffusion in simulated crew condensate, in agreement with theoretical predictions.

This work was done by Michael G. Izenson and Michael D. Jaeger of Creare, Inc. for Johnson Space Center. MSC-24952-1/5230-1