The fin surfaces of the condensing heat exchangers onboard manned spacecraft require a hydrophilic surface for effective gravity independent water separation, as well as an antimicrobial function to prevent bacterial growth. We have previously shown that silver exchanged zeolite A coating is highly hydrophilic and antimicrobial [1], and easily formed by a low temperature in-situ crystallization process. In order to be a suitable replacement for the current technology, the coating must be highly durable under the expected working conditions.

Here, we show that the coating remains highly hydrophilic and antimicrobial after long term leaching in water [2], and after many repeated bacterial inoculations. However, the silver exchanged zeolite A coating showed considerable degradation in mildly acidic and basic solutions. The acid/base stability issues are resolved by using zeolite A/ZSM-5 hybrid coating. In order to achieve all desired properties, we prepared a coating that consisted of zeolite A seeds imbedded within a ZSM-5 matrix. ZSM-5 is a high silica zeolite which can be produced by an in-situ crystallization process using TPAOH as a template. ZSM-5 shows superior acid/base resistance, as well as corrosion resistance [3], and acts to protect the zeolite A imbedded within the coating while the zeolite A seeds provided the hydrophilic and antimicrobial function. Data was obtained for hybrid coatings formed on top of a ZSM-5 pure phase membrane, as well as for hybrid coatings formed directly on the substrate using a template free ZSM-5 synthesis solution. All coatings were shown to be highly hydrophilic and antimicrobial after silver exchange.

References:

1. McDonnel AMP, Beving D, Wang A, Chen W, Yan YS. Hydrophilic and antimicrobial zeolite coatings for gravity-independent water separation. Advanced Functional Materials. 2005;15:336-340.

2. O'Neill C, Beving D, Chen W, Yan YS. Durability of hydrophilic and antimicrobial zeolite coatings under water immersion. AIChe Journal. 2006;52:1157-1161.

3. Cheng XL, Wang ZB, Yan YS. Corrosion-resistant zeolite coatings by in situ crystallization. Electrochemical And Solid State Letters. 2001;4:B23-26.