Solar sealants

Nonlinear optics, lithography, conductors, semiconductors, piezoelectronic, pyroelectronic, solar energy conversion, electrodes, computer chip circuitry UV absorption, smart materials, nanocomposites, laser, sealants, paints, caulks, lubricants, gaskets... 

Fig. 7.9. Efficiency data for two test cells (active area 4 cm2) exposed to outdoor conditions for 1 year. The efficiencies of the cells were measured from time to time with an indoor solar simulator. The dye used is Ru(NCS)2(2,2 -bipyridyl-4,4 -dicarboxylate)2 (N719). 0.6 M hexylmethylimidazolium iodide (HMII), 0.1 M Lil, 0.05 M I2, 0.5 M tert-butylpyridin (TBP) in propionitril was used as the electrolyte. Surlyn 1702 (Dupont) was used as the sealant. The cells were placed under a window to protect them from rain. Test location INAP, Gelsenkirchen, Germany...

Elastomeric materials are used in thermal solar collectors as gasket and caulking compounds. In addition to these sealant applications, polymeric materials are also widely employed as thermal insulation and occasionally as glazing and frame components. This paper provides a supplement and a continuation of a previously reported study( 1) of the endurances of several commercially available sealants to the harsh environment of the collector. 

Until very recently almost no new materials were developed for the solar collector industry, which is essentially in its infancy. This investigation was performed to help Identify deficiencies of the available sealant materials with the expectation, which we believe is beginning to be realized, that this would lead to development of superior materials for solar collector applications, ... 

The flat-plate solar service environment is likely to involve not only intermittent exposure to high moisture levels (from high atmospheric humidities, from dew or from rain), but also concurrent elevated temperatures as the collector unit warms up immediately after a period of exposure to high moisture levels. Therefore, we are concerned with the hydrolytic stability of the sealant. 

Three principal criteria were initially employed to select materials for evaluation. The first consisted of expectations, based upon an extensive computerized literature survey and manufacturers recommendations, that the materials would offer superior resistance to the environment of the collector. The next factor, that the sealant should not exhibit an extremely high cost, eliminated the very expensive high temperature specialty compounds. Thirdly, certain materials were examined because of their current extensive use in solar collectors although we did not expect them to display high thermal stability or good endurance characteristics. 

The above criteria were employed to select several commercially supplied Class PS elastomers for laboratory screening by employing selected tests taken from National Bureau of Standards NBSIR 77-1437(j4) and ANSI/ASTM D-3667-78 specifications for "Rubber Seals Used in Flat-Plate Solar Collectors". Four silicone, three EPDM, two fluorocarbon, three epichlorohydrin, one ethylene-acrylic, one polyacrylic, one chlorosulfonated polyethylene, one bromobutyl and two butyl rubbers were studied in these screening tests. These materials are identified in Table I and those compositions which were revealed by their manufacturers are shown in Table II. Undoubtedly some materials which should have been included were omitted however, we hope that this sampling will provide an indication of the applicability of a wide range of materials for use as sealants in thermal solar collectors. 

The compression set data, which we had obtained, reflect far poorer performances than we had anticipated on the basis of conversations with manufacturers of solar collector units and suppliers of sealants. A summary of thse data are presented in Table VIII. All three silicones, G, I, and J, tested exhibited compression sets of the order of 100% after only one day of aging at either 250°C or 225°C. (Compression set values greater than 100% are attributed to essentially complete loss of resilience by the specimen combined with an additional reduction in sample thickness due to shrinkage associated with thermal degradation). For example, the best of the three silicones, J, showed compression set values of 94% after one day at 225 C and 89% after 28 days at 175°C. We feel that a sealant should not exceed 50% in compression set if it is to retain good sealing ability. 

Among the materials evaluated the one displaying the best overall properties for use as a sealant in a thermal solar collector is the fluorocarbon, Viton. However, it has the dlsadvanatages of high cost and high compression set at temperatures of -10°C and lower which could present problems in northern climate. 

For example, the reflectivity of silvered mirror heliostats can deteriorate with time in the presence of moisture The mechanisms of silver deterioration are currently an area of conjecture (3). In flat-plate solar collectors high temperatures, humidity, ozone and ultraviolet (UV) radiation all contribute to aging processes which limit the lifetime of sealants, adhesives and gasket materials (4). 

Mendelsohn, M. A., Luch, R. M., Yoemon, F. A., Navish, F. W., "Sealants of Solar Collectors," Westinghouse R D Center, LASL Contract. 

Encapsulants are necessary for electrical Isolation of the photovoltaic circuit. They also provide mechanical protection for the solar cell wafers and corrosion protection for the metal contacts and circuit interconnect system over the 20-year design life of a photovoltaic array. The required components Include the solar cell circuit, the rigid or structural member, the pottant, and the outer cover/insulator. Surface modifications may be needed to develop strong, stable bonds at the Interfaces in the composite. If the module is to be framed, edge sealants may also be required. The functions of the Individual components and the performance requirements as they are now known are described. Costs are ccmipared where possible and candidate materials identified. 

Filled liquid neoprene sealants are cured at room temperature by the addition of lead oxide, tertiary amines or epoxy resins. Neoprene sealants covered by a top coat of white chlorosulfonated polyethlyene are being used on solar collectors.21 Mixtures of carboxyl terminated acrylonitrile elastomers (CTBN) and liquid epoxy resins cure readily at room temperature in the presence of bis phenol A and amines.22 Coal tar-epoxy resin, two component epoxy resin, and epoxypolyester resin systems have been used as sealants.22... 

Usable in Hot Climates. Most organic sealants perform satisfactorily when temperatures are warm. But silicones remain effective in such hot locations as the sandwich panel of a solar collector or around heat ducts or hot pipes. However, the designer must choose the right silicone, since some perform up to 150 C, some to 200 C and 250 C. These temperatures can be contrasted to the 70 C to 120 C operating maximum of nonsilicones. 

Bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate stabilizer, light sealants Benzophenone-12 stabilizer, light solar film 4-(2-Acryloyloxyethoxy)-2-hydroxybenzophenone polymer stabilizer, light storage tanks Benzophenone-12 stabilizer, light styrenics 2-(2 -Hydroxy-3,5 -di-t-amylphenyl) benzotriazole stabilizer, light substrates 2-(2 -Hydroxy-3,5 -di-t-amylphenyl) benzotriazole... 

This review emphasizes new materials for adhesives and sealants. Thus, we discuss both new adhesives and new forms of adhesives. As a result of socio-economical demands, energy-reduction and pollution-control are two main reasons for the development of radiation-curable, hot-melt, film, and waterborne adhesives. We shall examine some of these new forms of adhesives in detail. The applications of new adhesive materials to aerospace and solar-energy industries will also be briefly mentioned. 

In the following sections/ we shall discuss adhesion chemistry/ adhesion physics / radiation-curable adhesiveS/ high-temperature adhesiveS/ anaerobic and structural adhesiveS/ hot-melt adhesives/ film adhesiyes/ waterborne adhesives/ aerospace structural adhesiveS/ conventional sealants/ advanced aerospace sealants/ and adhesives and sealants for solar collectors. 

Besides durability, premium sealants are judged by special properties as shown in Table 4. The ability to take on greater elongation and compression is measured by movement capability in terms of joint width. The stability to UV exposure is important for those glazing and insulation compounds used in modern high-rise structures. Thermal stability is in demand for solar collectors, or for other structural materials. On the basis of these evaluations, we can foresee future trends of sealants as shown in Table 4. Silicones appear to out-perform others. In the meantime, technical advances will provide low-modulus polysulfides, and better movement ability for both polysulfides and polyurethanes. Their cure time will be decreased and the UV stability will be improved to match or compete with silicones. All three will be developed for better adhesion under the un-primed conditions. 

Adhesives and sealants used for the aerospace structures must endure severe environments. Another family of adhesive materials has also been developed for solar collectors, presumably based on the same ability to endure severe environments. Though there are different kinds of severe environments, conventional polymers generally do not survive in the solar collector environment. We shall discuss those adhesive materials used for solar applications in the following section. 

RECENT DEVELOPMENTS ADHESIVE AND SEALANT CHEMISTRY XIII. SOLAR COLLECTOR ADHESIVES AND SEALANTS... 

The sealant materials used for the solar collectors must be durable under harsh environments (132). They must... 

Navish, Jr., "Sealants for Solar Collectors," Org. Coatings and Plastics Chem., 232 (1980). 

R. M. Luck and M. A. Mendelsohn, "The Degradation and Outgassing of Polymeric Sealants and Plastics and Their Effects on Solar Collector Efficiency," Polymer Preprints, 7, No. 1, 235 (1982). 

Typical Use High vacuum. Industrial and space applications requiring an adhesive-sealant with low emission of volatiles. (Bonding of solar ... 

Aerospace applications have always been extensive and are still growing, due to the outstanding resistance of these sealants to extremes of temperature and various forms of radiation. Sealants designed to emit virtually no volatile components in the high-vacuum environment of deep space are used to fasten solar panels in place and to perform other sealing functions in delicate satellite assemblies where stray condensable contaminants must be avoided near sensitive optical and electronic devices. Other sealants are used to fasten space shuttle tiles in place and for other applications where the maintenance of elastomeric properties is essential over a wide range of temperatures. 

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