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To assess the photothermal stability of candidate polymeric absorber materials, mechanical properties (tensile modulus and strength, and percent strain at break) were measured as a function of time of Ci5000 exposure to UV light at 60°C. 3. Results and Accomplishments The major results and accomplishments of the project during FY 2005 include the following: • New glazing samples were provided by major polymer manufacturers and are under test. (09/05) • UV resistance of candidate absorber materials was quantified in terms of loss in percent strain at break as a function of UV weathering. (09/05) 3.1 Glazing Materials Based on accelerated screening tests, we have found that fluoropolymers and acrylic are UV weatherable, and all other polymeric glazing materials tested lose transmittance and yellow. However, fluoropolymers are relatively expensive and realistically would be limited to use as thin-film glazings. Acrylic tends to be too brittle and exhibits thermal sag, which is a concern for a glazing candidate for solar collector applications. We have also found that polycarbonate (PC) laminated to an acrylic UV-screening film (Korad®, a product of Polymer Extruded Products) can also be UV weatherable. The most promising construction uses a UV-screening film that is adhesively laminated to a PC sheet. Without the additional UV-screening layer, PC products exhibit 3%–5% loss in solar-weighted hemispherical transmittance after about 2–3 years’ equivalent exposure. In addition, severe visual yellowing (an aesthetic concern) occurs in the same timeframe. With the addition of a UV-screening film, significant loss in hemispherical transmittance does not begin until between 10–15 years’ equivalent outdoor exposure at elevated operating temperatures. Based on these results, we interacted with industrial vendors to obtain samples in which the UV screening film was thermally bonded to a PC sheet in a manner more representative of commercial manufacturing processes. These sheets were subsequently thermoformed into collector glazings, and samples of the final construction were subjected to accelerated testing. Glazing constructions in which the UV-screen is thermally bonded to the PC substrate exhibit yellowing much earlier than when adhesives are Solar Thermal R&D Solar Heating and Lighting used. The high temperature of the extruded PC sheet during thermal bonding thins and severely degrades the UV screening layer, resulting in loss of transmittance. Another concern has been whether mechanical degradation accompanies optical yellowing. During UV exposure, greater crosslinking can occur, which can result in increased stiffness, embrittlement, and consequent lower impact strength. We have used an ARES rheometer to perform dynamic mechanical analysis (DMA) of PC glazing samples. The effect of UV exposure is clearly evident (Fig. 1); 7502 h exposure in an Atlas XR-260 WeatherOmeter (providing 1.9 years equivalent outdoor exposure) results in an increased modulus, implying increased stiffness and consequent loss in impact strength. Measurements were made at a tortional frequency of 10 rad/s over a wide temperature range (–25° to 75°C). 1.4 1.3 1.2 1.1 1.0 0.9 0.8 -30 -20 -10 0 10 20 30 40 50 60 70 80 Temperature (°C) Further interactions have been held with material suppliers to identify several new types of samples that have the potential to preclude the thermal bonding problem. GE Plastics, a major supplier of PC sheet, is interacting with Polymer Extruded Products to provide thermally bonded Korad®/PC samples. The UV screening layer is thicker than previously used (0.08–0.15 mm versus 0.05 mm thick), and enhanced screening films (which provide twice as much UV absorption as previously tested) are incorporated into their Weathered Unweathered 136 Fig. 1. Shear modulus as a function of temperature for accelerated weathered (1.9 years equivalent outdoor exposure) versus unweathered thermal-bonded Korad®/PC. G' (GPa)PDF Image | DOE Solar Energy Technologies Program
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