POLYMERIC TRANSPARENT INSULATION MATERIALS The Dependence of Performance Properties on Time, Temperature and Environment

POLYMERIC TRANSPARENT INSULATION MATERIALS The Dependence of Performance Properties on Time, Temperature and Environment G. Wallner and R. W. Lang Institute of Materials Science and Testing of Plastics, University of Leoben Institute of Polymer Technology, JOANNEUM RESEARCH Franz-Josef-Str. 18, Leoben, A-8700, Austria, +433842402583, +433842402525, wallner@unileoben.ac.at Abstract At the Institute of Polymer Technology (JOANNEUM RESEARCH Forschungsges.m.b.H., Leoben, Austria) a research project to develop and optimize a transparent insulation (TI) structure based on commercially available films was carried out. It was found, that in addition to poly(carbonate) (PC) and poly(methylmethacrylate) (PMMA) other amorphous and semicrystalline polymers such as cellulose triacetate (CTA) and poly(ethyleneterephtalate) (PET) exhibit favorable property profiles for application in low temperature systems (e.g. TI wall systems). Investigations were carried out in order to study the influence of time, temperature, humidity and solar radiation on the solar and infrared performance properties of various polymer films. Whereas the infrared properties are only slightly influenced after exposure to elevated temperatures, humidity and solar radiation, in the solar range especially extinction due to scattering increases. Detailed results are presented in the paper for four polymer film types and for TI structures made thereof. Furthermore, the results are interpreted in terms of both the molecular and supermolecular structure of the polymer films. Besides PC and PMMA films, PET and CTA films show an interesting long-term performance, being an interesting alternative to the more expensive PC and PMMA polymers. 1. INTRODUCTION At present for solar wall applications mainly small-celled capillary or rectangular honeycomb transparent insulation (TI) structures are available on the market. The structures are produced by an extrusion and cutting process. The types of plastics used are the thermoplastics, poly(carbonate) (PC) and poly(methylmethacrylate) (PMMA). Due to the applied extrusion and cutting processes, the commercially available TI structures contain bulk and surface imperfections which adversely affect solar. At the Institute of Polymer Technology (JOANNEUM RESEARCH Forschungsges.m.b.H., Leoben, Austria) a research project to develop and optimize a TI structure was carried out in close cooperation with the Institute of Materials Science and Testing of Plastics (University of Leoben) and the Fraunhofer Institute for Solar Energy Systems (Freiburg, Germany). It was found, that absorber-perpendicular, lamellar TI structures based on commercially available polymer films with appropriate film thicknesses from 30 to 50 µm have a lot of advantages compared to extruded structures (Wallner, 1998). On the one hand, polymer films can be manufactured in a better surface quality, which helps to reduce scattering losses in TI structures. On the other hand, polymer films allow a higher flexibility according to material selection and geometry variations. In addition to the absorber-perpendicular, lamellar structure developed within the project, a square-celled TI structure based on polymer films was developed by O. Kehl in Bielefeld, Germany. Polymeric TI materials for solar wall applications with black absorbers have to be selective in the solar and infrared wavelength range. Whereas in the solar range a high is required, in the infrared range the absorptance should be high. Regarding the functional groups with high absorptance in the range of about 10 µm, carbon-oxygen single bonds (C-O) are of great importance for commodity and technical plastics with service temperatures between 80 and 120 C. The higher the density of C-O bonds in the molecular structure, the better is the performance of the polymer (Wallner, 1999). In addition to PMMA and PC films there are commercially available other polymer films with service temperatures mentioned above and with C-O bonds in the molecular structure; e.g. poly(ethyleneterephtalate) (PET) and cellulose triacetate (CTA). Transparent PET and CTA films are significantly cheaper than PC and PMMA films (i.e. PET film: 5-7,5 Euro/kg, CTA film: 10-12,5 Euro/kg, PC and PMMA films: 20-25 Euro/kg). Furthermore, these films have a higher density of C-O bonds in the molecular structure. The performance properties of plastics are dependent on time, temperature and environmental variables like solar radiation or humidity. However, so far no investigations have been reported which systematically consider the effects of environmental variables on the performance properties of polymeric TI materials. Thus, different commercially available polymer films were investigated as to the dependence of the solar and infrared optical properties on different accelerated weathering conditions. The purpose of this paper is to describe and discuss the

results of the aging studies (artificial weathering), in particular for polymers with C-O bonds in the molecular structure and service temperatures between 80 and 120 C. 2. METHODOLOGY AND PROCEDURE The main performance properties of TI structures are the heat conductance, Λ, and the total energy, g h. Theoretical models show that the main performance properties are correlated to optical and thermal material properties, the cell geometry, ambient conditions and various system parameters. Using a non-spectral theoretical model, the optical and thermal properties are the refractive index and the extinction coefficients in the solar and infrared region and the heat conductivity. In this paper the influence of weathering on the relevant TI material properties as well as on the TI performance properties will be described and discussed. The paper is confined to plastics for solar wall applications. Therefore, appropriate films with thicknesses of approximately 50 µm and service temperatures ranging from 80 to 120 C were selected. The films were aged under artificial weathering conditions, in hot air and in distilled water. The unaged and aged films were characterized as to their relevant optical material properties. From spectroscopically determined /reflectance data integral, non-spectral values were calculated for the solar and infrared range. The refractive index and the heat conductivity were assumed to be constant and were taken from data sheets and from the literature. For defined small-celled, glazed TI structures numerical calculations were performed using a model of Platzer (program GWERT (Platzer, 1994)) for the determination of Λ and g h as a function of aging time. The changes due to artificial weathering for the different materials investigated will be discussed and compared. Furthermore, the results are interpreted in terms of both the molecular and the supermolecular structure of various polymer film types. 3. EXPERIMENTAL AND CALCULATION PROCEDURES 3.1 Polymer film types The commercial film types investigated and discussed in this paper are listed below along with informations as to the film grade, the manufacturer, the polymer nomenclature, the long time service temperature, the index of refraction, the film thickness, the film production process, the morphology and additives added. Triphan N882 GL, Lonza-Folien GmbH, Cellulose triacetate (CTA), 100 C, 1.49, 50 µm, casted, amorphous, plasticizer and no UV-Absorber added. Melinex 400, Pütz GmbH-Co. Folien KG and Deutsche ICI GmbH, Poly(ethyleneterephtalate) (PET), 100 C, 1.67, 50 µm, extruded, semicrystalline, no UV-Absorber added. Plexiglas 99845, Röhm GmbH, Poly(methylmethacrylate)copolymer (PMMA), 80 C, 1.49, 60 µm, extruded, amorphous, UV- Absorber added. Makrolon 3103 FBL, Lipp-Terler GmbH and Bayer AG, Poly(carbonate) (PC), 120 C, 1.59, 50 µm, extruded, amorphous, UV-Absorber added. 3.2 Aging conditions Film samples were artificially weathered in a Xenotest Beta weathering device utilising three 2.8 kw Xenon arc lamps and a Xenochrome 300 filter (irradiation between 300 and 400 nm: 60 W/m 2 ; wavelength > 300 nm; normal incidence) and operating under TI wall relevant conditions (black panel temperature: 100-110 C; air temperature: 65 C; rel. humidity: 80 %). Furthermore, samples were heat aged in hot air at 80 and 120 C and exposed to distilled water at 80 C. 3.3 Spectroscopical characterization and calculation of optical material data The optical data of the polymer films were determined using two experimental techniques. Over the medium IR range from 400 to 4000 cm -1 (2500 to 25000 nm) the collimated-collimated spectra at normal incidence were obtained with a Perkin Elmer PE1600 spectrometer (Perkin Elmer; Überlingen, D). According to Rubin (Rubin, 1982) the infrared absorption coefficient κ IR was calculated under the assumption of a constant index of refraction. As a good approximation the solar index of refraction was used for the polymer films investigated (Platzer, 1988; Wallner, 1998). Over the solar spectrum from 300 to 2500 nm the collimated-hemispherical and collimated-diffuse values at normal incidence and reflectance values at near-normal incidence were determined using a Perkin Elmer Lambda 9 Ulbricht globe spectrophotometer (Perkin Elmer; Überlingen, D). The hemispherical and diffuse values allowed the calculation of both, the extinction coefficient due to absorption (integral solar absorption coefficient, κ S ) and due to scattering (integral solar scattering coefficient, σ S ). Therefore, the spectral and reflectance values were averaged by the AM1.5 Global Irradiance source function. Using the 4-flux model (a generalization of the Maheu model (Maheu, 1984)), the experimental spectral averages (or integral data) were adapted by optimizing the model parameters κ S, σ S and the forward scattering ratio ξ. As input for the TI model calculations (GWERT) an effective integral solar scattering coefficient (σ S eff) was determined by applying the delta- M method of Wiscombe (Wiscombe, 1977).

3.4 Calculation of TI structure properties The g h - and Λ-values of glazed, square-celled TI structures were calculated using the GWERT simulation program of Platzer (Platzer, 1994). The volumetric material fraction was chosen as 2.6 and 3.0 V% for 50 and 60 µm thick films, respectively. The structure thickness was 10 cm, the aspect ratio was 26. As system values an average temperature of 10 C and solar irradiance of 400 W/m 2 were defined. As glazing material on both sides low-iron glass was assumed. 4. RESULTS AND DISCUSSION In the following the results will be described and discussed considering both analytical aspects and technological aspects. 4.1 Analytical Spectra Discussion In Fig. 1 to 8 infrared and solar spectra are shown for the four polymer types and different aging conditions. In general, artificial weathering had the greatest impact on the spectra. After 480 hours of accelerated, artificial weathering in the Xenotest device the PC, PET and CTA films were brittle showing retained mechanical strain-to-break values less than 5 % of the initial value. In this regard the performance of the impact-modified and UV-stabilized PMMA-film was remarkable as the strain-to-break value after 480 hours of weathering decreased to only approximately 75 % of the initial value. Due to these findings, the infrared and solar spectra after 480 hours of artificial weathering are shown and will be compared and discussed. Furthermore, selected results after exposure in hot water or air are shown in Fig. 1 to 8. At this point it should be mentioned, that the accelerated weathering conditions investigated do not represent real service conditions in TI systems. Concerning the UV light intensity, the angle of incidence and the temperature the worst conditions were selected. In practice, TI structures are installed behind a partly UV-absorbing glass pane, the angle of incidence is not perpendicular to the film and the temperatures in the UV absorbing outer zone are lower. Thus, the results shown cannot directly used for life time predictions of polymeric TI wall systems. The infrared spectra of the films in Fig. 1 to 4 show an intensive absorption in the range between 1000 and 1500 cm -1. These absorptions are mainly related to C-O bonds. The lowest values were determined for CTA, the polymer with the highest density of C-O bonds in the molecular structure. In the range below 1000 cm -1 and above 1500 cm -1 the absorption is weaker and the bands can be resolved spectrally. Important absorption bands are occurring at 1600, 1750, 3000 and 3500 cm -1, which can be attributed to C=C, C=O, C-H and O-H bonds, respectively. Below 1000 cm -1 induced motions in larger groups or segments of polymeric molecules are responsible for absorption bands. For CTA the band around 750 cm -1 can be assigned to molecular motions of the plasticizer phosphoric acid triphenyl ester. Fig. 1. Collimated-collimated infrared spectra for the (initial state) and 480 h artificially weathered PC film PMMA 60µm Fig. 2. Collimated-collimated infrared spectra for the (initial state) and 480 h artificially weathered PMMA film Although the PC, PET and CTA samples became brittle after 480 hours of weathering, the effect on the infrared spectrum is moderate. The strongest effects can be observed for PET and PC films. These films show a decrease in infrared collimated-collimated over the whole range (400 to 4000 cm -1 ), which may be attributed to enhanced scattering. Important are the changes at 1600 cm -1 for C=C bonds in PC and PET. For both, PC and PET, the changes are related to the yellowing observed in the visible. Like PC, PET and CTA the main changes of the PMMA infrared spectrum happen at wavenumbers around 3500 cm -1. The changes can be assigned to an increase in intermolecular hydrogen link bonds, which are enhanced because of the formation of low molecular degradation

products (e.g. hydroperoxides, acids, aldehydes, ketones, ester). In Fig. 4 the spectrum of CTA after exposure in water at 80 C for 960 hours is shown. The spectrum exhibits a reduced absorption at selected wavenumbers (e.g. 689, 758, 780, 964, 1485, 1587 cm -1 ), which indicates an extraction of a significant amount of plasticizer by the presence of water at 80 C. Whereas the same phenomenon was oberved after exposure in hot air at 120 C, no changes were detected after weathering or exposure in hot air at 80 C. PET 50µm Fig. 3. Collimated-collimated infrared spectra for the (initial state) and 480 h artificially weathered PET film collimated beam. These errors will be discussed in the next section. The spectra of the unaged () films show a slight decrease at shorter wavelengths, which can be attributed to dispersion effects. In the near UV range (300 to 400 nm), PC and PMMA exhibit a pronounced absorption due to the presence of UV light stabilizers. No UV stabilizer is added to the PET film. The absorption at wavelengths smaller than 325 nm is related to the molecular structure (benzene rings). The unaged CTA film exhibits no absorption bands in the plotted range (300 to 800 nm). For all the unaged films, the amorphous PC, the impact-modified PMMA, the semicrystalline PET and the plasticized CTA, the τ cd values are in the range of 1 over the whole spectral range. CTA 50µm water 960h Fig. 5. Collimated-hemispherical and collimated-diffuse solar ( ) spectra for the (initial state) and 480 h artificially weathered PC film Fig. 4. Collimated-collimated infrared spectra for the (initial state), 480 h artificially weathered and 960 h in water exposed CTA film PMMA 60µm water 80 C 960h The collimated-hemispherical ( ) and collimated-diffuse (τ cd ) spectra of differently aged films of PC, PMMA, PET and CTA are shown in Fig. 5 to 8. In order to detect spectral changes the plot of the spectra is limited to the range between 300 and 800 nm. Complete spectra for different polymer film types are shown and discussed e.g. in (Wallner, 1999). The effect of aging on the collimated-hemispherical reflectance is small. Thus, these spectra are not plotted in Fig. 5 to 8. Furthermore, the error of the diffuse reflectance measurements on aged films was high due to imperfect specular reflection of the Fig. 6. Collimated-hemispherical and collimated-diffuse solar ( ) spectra for the (initial state), 480 h artificially weathered and 960 h in water exposed PMMA film The strongest effect of artificial weathering on the solar collimated-hemispherical spectrum was observed for PC. A pronounced decrease of between 320 and 500 nm could also be detected for PET. The

values of PMMA and CTA are nearly unchanged after exposure in the Xenotest weathering device for 480 hours. The decrease of values of the weathered CTA film in the range smaller 400 nm is due to absorption. The results correlate with visually determined color changes of the weathered films. Whereas for the aged PC film an intensive yellowing could be observed, PET shows minor yellowing. For PMMA and the embritteled CTA no color changes could be detected visually. The collimated-diffuse values after weathering are slightly changed from initially 1 to 4 and 5 for PC, PMMA and CTA. The strongest effect of artificial weathering on the solar collimateddiffuse was observed for PET; the integral τ cd value changed from 1 to 0.12. Furthermore an increase of τ cd with decreasing wavelength can be seen for the aged films. PET 50µm air 120 C, 960h 960 hours in air at 120 C and of PMMA exposed 960 hours in water at 80 C are plotted in Fig. 6 and 7, respectively. For PET a decrease in and a strong increase in τ cd is monitored over the whole range. However, aging of PET films in air at 120 C for 960 hours does not lead to molecular changes observed after weathering. Furthermore, supermolecular structures at the surface or in the volume of the film develop, giving the PET film a milky appearance. In the same way, supermolecular structures emerge in the impact-modified PMMA films, probably due to phase separation in the volume, which results in intensive scattering after exposition in water at 80 C. 3.2 Technological Values Discussion Assuming that extinction in the infrared range consists only of absorption and that the index of refraction remains constant, the infrared absorption coefficients (κ IR ) were calculated. The obtained values for four different films before and after 480 hours of artificial weathering are plotted in Fig. 9. It can be seen, that CTA shows an outstanding infrared absorption compared to PET, PC and PMMA. Due to artificial weathering the κ IR values are increased. Whereas for CTA and PMMA small changes were derived, the changes for PC and PET are markedly increased by 14 and 23 %, respectively. 350 300 303 316 Fig. 7. Collimated-hemispherical and collimated-diffuse solar ( ) spectra for the (initial state), 480 h artificially weathered and 960 h in hot air (120 C) exposed PET film κ IR [cm -1 ] 250 200 150 100 50 160 183 120 131 175 215 CTA 50µm Fig. 8. Collimated-hemispherical and collimated-diffuse solar ( ) spectra for the (initial state) and 480 h artificially weathered CTA film In addition to the solar spectra for the and weathered film, the spectra of PET exposed 0 ref. wea. ref. wea. ref. wea. ref. wea. PMMA 60µm PET 50µm CTA 50µm Fig. 9. Infrared absorption coefficient (κ IR ) as a function of polymer film type and artificial weathering (ref., wea. 480 h artificially weathered) The solar optical values (solar absorption coefficient (κ S ), solar scattering coefficient (σ S ), forward scattering ratio (ξ)) for transparent polymer films before and after weathering are summarized in table 1. The values of PC and PMMA were calculated considering the effect of multiple film interaction (Wallner, 1999), which usually occurs when an incident ray passes the TI structure. As a consequence of multiple film interaction, for the calculation of averaged solar values ranges of strong absorption (e.g. in the near UV due to light stabilizers) have to be neglected. In the case of PC and PMMA collimated-hemispherical s smaller 0.5 were neglected in the averaging procedure. Table 1

shows that scattering contributes mainly to the solar extinction. For the unaged films the solar absorption coefficients are smaller than 1 cm -1. The solar scattering coefficients of unaged films are in the range between 3 and 4 cm -1. The forward scattering ratio is equal 0.5, which means isotropic scattering. Weathering results in a significant increase in the solar scattering coefficient. Simultaneously, the forward scattering ratio increases to values between 0.70 and 9. Significant changes in the solar absorption coefficient were determined for the yellowed and embritteled PC film. Tab. 1. Solar absorption and scattering coefficient (κ S, σ S ) and forward scattering ratio ξ as a function of polymer film type and artificial weathering (ref., wea. 480 h artificially weathered) κ S [cm -1 ] σ S [cm -1 ] ξ [ - ] PC ref. < 1 3.20 0.50 PC wea. 3.84 12.83 0.71 PMMA ref. < 1 3.64 0.50 PMMA wea. 7 10.34 0.70 PET ref. < 1 3.28 0.50 PET wea. < 1 32.61 9 CTA ref. < 1 3.64 0.50 CTA wea. < 1 13.24 0.78 In Fig. 10 and 11 the heat conductance (Λ) and the total energy (g h ) for square-celled, glazed TI structures (structure thickness: 10 cm, aspect ratio: 26) are shown as a function of polymer type and artificial weathering. In accordance with the infrared absorption coefficients, plotted in Fig. 9, the heat conductance values of the CTA structure are significantly lower than Λ values of PET, PC or PMMA structures. Furthermore, weathering results in a moderate decrease of Λ, whereas the greatest changes were obtained for PET. Λ [W/(m 2 K)] 0.9 0.7 0.95 0.92 0.93 8 ref. wea. 0.91 6 0.79 0.78 ref. wea. ref. wea. ref. wea. PMMA 60µm PET 50µm CTA 50µm Fig. 10. Heat conductance (Λ) for square-celled, glazed TI structures (thickness: 10 cm, aspect ratio: 26) as a function of polymer type and artificial weathering (ref., wea. 480 h artificially weathered) From Fig. 11 it can be seen, that the initial total energy values of glazed, square-celled structures made from commercial transparent polymer films with thicknesses between 50 and 60 µm are in the range from 0.52 to 0.55; PC and PET structures exhibit the best values. Assuming a uniform degradation of the polymer film in the TI structure, g h drops after 480 hours of artificial weathering to values ranging from 1 to 7. These values can be interpreted as lower limits. Due to the strong yellowing of PC, a weathered structure will reveal a lower degree of total energy as for example structures made from PET or CTA. Finally, it should be mentioned, that PMMA is rather stable to weathering compared to PC, PET or CTA. g h [ - ] 0.5 0.3 0.55 1 ref. wea. 0.52 4 0.55 6 0.54 7 ref. wea. ref. wea. ref. wea. PMMA 60µm PET 50µm CTA 50µm Fig. 11. Total energy (g h ) for square-celled, glazed TI structures (thickness: 10 cm, aspect ratio: 26) as a function of polymer type and artificial weathering (ref., wea. 480 h artificially weathered) CONCLUSIONS In addition to the extruded TI structures made of PC and PMMA, recently novel polymer film based TI structures were developed. These allow a higher flexibility as to material selection and geometry variations. Furthermore, a better surface quality which helps to reduce scattering losses can be realized. Besides PC and PMMA films, the cheaper polymer film types PET and CTA exhibit an interesting performance profile for TI wall applications. As typical for polymers, the long-term properties are dependent on time, temperature and environmental variables like solar radiation or humidity. Aging and weathering studies on four commercially available polymer films with thicknesses between 50 and 60 µm show, that the long-term behavior of PET and CTA can compete with the long-term performance of PC. An outstanding weathering stability was confirmed for an impact-modified, UV-stabilized PMMA film. In contrast to PC, especially CTA does not degrade associated with intensive yellowing. Whereas the infrared properties are only slightly influenced after exposure to elevated temperatures,

humidity and solar irradiation, in the solar range especially extinction due to scattering increases. The TI performance property heat conductance is less affected by aging. The total energy exhibits a significant decrease after artificial weathering, which is related to an increase in scattering and to yellowing effects. Regarding the structure and morphology of the polymers, care has to be taken for the impact-modified PMMA and for the semicrystalline PET. After exposure to water impact-modified PMMA presumably shows phase separation and the transparency diminishes. As a consequence of aging of PET films in air at 120 C supermolecular structures at the surface or in the volume of the film develop, which give the PET film a milky appearance. The studies confirmed, that plasticizer loss of CTA films after exposure to hot water at 80 C or in hot air at 120 C has no significant impact on the transparency of the film. Summarizing the results it can be concluded, that CTA and PET films are an interesting alternative to the more expensive PC or PMMA polymers. ACKNOWLEDGEMENTS The authors wish to express their acknowledgements to the Fraunhofer Institute for Solar Energy Systems (Freiburg, Germany) for the cooperation in the course of this study, especially for making the modelling software and some of the laboratory equipment available for investigations. REFERENCES Maheu B., Letoulouzan J.N., and Gouesbet G. (1984). Four-flux models to solve the scattering transfer equation in terms of Lorenz-Mie parameters. Applied Optics, Vol. 23, No. 19, 3353-3362. Platzer W.J. (1988). Solare Transmission und Wärmetransportmechanismen bei transparenten Wärmedämmaterialien, PhD Thesis, University of Freiburg. Platzer W.J. (1994). TSET 2.0 User Manual, Freiburg. Rubin M. (1982). Infrared properties of polyethyleneterephtalate films. Solar Energy Materials 6, 375-380. Wallner G., Schobermayr H., Lang R.W., Platzer W.J. (1998). Development and optimization of a novel polymer based transparent insulation wall heating system. In Proceedings of EuroSun 98 Congress, 14-17 September, Portoroz, Slovenia, Vol. 2, VI.9-1 VI.9-7. Wallner G., Schobermayr H., Lang R.W., Platzer W.J. (1999). Solar optical and infrared radiative properties of transparent polymer films. In Proceedings of ISES Solar World Congress, 4-9 July, Jerusalem, Israel. Wiscombe W.J. (1977). The delta-m method: Rapid yet accurate radiative flux calculations for strongly asymmetric phase functions. Journal of the Atmospheric Sciences, Vol. 34, 1408-1422.