Effects of using different polymers in post-synthesis treatments of zeolite 4a coatings

Zeolites are crystalline microporous aluminosilicates which may be used in diverse applications related to catalysis, adsorption/separation and ion exchange. Adsorption heat pumps may provide heating and cooling effects and possess some advantages, such as being environmentally friendly and employing solar energy or waste heat sources. Zeolites are very suitable for adsorption heat pump applications, owing to their high water adsorption capacities as well as their suitable shapes of adsorption isotherms. Significant advances have been obtained in the performances of adsorption heat pumps by the commencement of using zeolite coatings on metal substrates. Zeolite coatings directly crystallized on metal surfaces may reduce the heat and mass transfer limitations to a great extent. The good contact between the metal surface of the heat exchanger and the zeolite may improve the quality of heat transfer across the interface, and the mass transfer limitations may be eliminated by adjusting the coating thickness. Quite thick layers of zeolite on metal surfaces have already been obtained. The nature of the coating also has an important effect on the performance of the heat pump system. The thick zeolite coatings should not be compact for obtaining relatively high diffusion coefficients. Another critical point for providing the commercial use of zeolite coatings in adsorption heat pumps is related to their mechanical and thermal stabilities. Zeolite coatings may be prepared by using various methods. Relatively thick and open coatings may be obtained especially by using the substrate heating method. In this method, a resistance directly heats the metal support in the reaction mixture and a water bath keeps the reaction mixture at a lower temperature. In this manner, the reaction in the bulk is suppressed while that on the metal support is promoted. In previous studies, it was observed that the substrate heating method might allow the preparation of quite thick zeolite A and X coatings with open and stable nature on stainless steel surfaces. However, covering the zeolite coatings further with a thin polyacrylic acid layer was also proposed in order to enhance the long-term mechanical and thermal stabilities of the coatings for allowing their use in commercial applications. The purpose of this study was to increase the stabilities of zeolite 4A coatings by coating them further with different polymers such as, polyvinyl acetate, polyvinyl alcohol and cellulose acetate and to investigate the effects created by the different polymers used. Materials prepared by substrate heating and conventional synthesis methods were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) while their thermal and mechanical stabilities were determined by applying rapid heating-cooling cycles and ultrasonic treatments, respectively. The results were also compared to previous results obtained when polyacrylic acid polymer was used for covering zeolite coatings with the same purpose. According to the results obtained, it was observed that the polymer-coated zeolite samples prepared under various conditions exhibited quite different desorption behaviors. When the open zeolite coatings prepared by using the substrate heating method were covered by polymer, they desorbed somewhat higher amounts of water, when compared to the pure zeolite 4A coatings. The amounts of water desorbed from all the pure polymers investigated were much less than those desorbed from pure zeolite A, which signified lower theoretical desorption values for the composite coatings. The sample using cellulose actetate performed best regarding water desorption. The favorable effect started to lose its significance as the amount/thickness of polymer increased in the composite sample. This may be related to the contact area between zeolite and polymer not increasing as much as the polymer mass in the sample for these samples. On the other hand, different results were obtained for the more compact zeolite coatings prepared by repeated conventional syntheses. In these cases, the polymer-covered coatings desorbed less amount of water when compared to the pure zeolite A coatings, as also expected from the theoretical calculations. This seemed to originate from the lower amount of interaction between zeolite and polymer in the compact coatings, when compared to the open coatings, in which the polymer could penetrate more deeper into the zeolite layer. The stabilities were improved when the zeolite coatings prepared by using both the substrate heating and repeated conventional syntheses methods were covered by the additional polymer layer. As a result of the consecutive ultrasonic treatments and rapid heating-cooling cycles applied, it was observed that the use of the polyvinyl acetate, polyvinyl alcohol and cellulose acetate polymers all enhanced the stabilities of the zeolite 4A coatings. The highest mass loss was observed for the use of polyvinyl acetate, but, in general, negligible amounts of decrease took place in the mass of the composite coatings. The glass transition temperature, which is lowest for polyvinyl acetate among the polymers investigated in this study, might have played some role in the stabilities of the composite coatings. It was previously determined that using polyacrylic acid for covering zeolite 4A coatings also improved the coating stability while the desorption capacity increased by up to 24% for the coatings prepared by using the substrate heating method. In this study, cellulose acetate provided the highest amount of increase in the desorption capacity of zeolite 4A coatings prepared by using the substrate heating method. The increase was limited to about 12% but this may also be related to the differences in the thicknesses of the zeolite coatings obtained in different studies. It may be concluded that the polymer types and contents used to cover the open zeolite coatings prepared by using the substrate heating method affected the water desorption properties of the zeolite-polymer composite coatings. The stabilities of the composite samples, the natures of the zeolite coatings, the interactions between polymer and zeolite, the diffusion coefficients of water in these materials, the changes occurring in the adsorption/desorption capacities and the thickness of the polymer should also be taken into account for the preparation of polymer-zeolite composite coatings suitable for adsorption heat pumps and other applications.