Sol-Gel Nir-Reflective Multilayer Coatings On Glass Through Uvpolymerizable Ceramic Nanoparticles
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Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
Experimental Sol synthesis
High refractive index sol To synthesize the high-refractive-index sol, 4 g of tetraisopropyl orthotitanate was slowly dropped into a mixture of 40 g isopropanol with 1.5 g methacrylic acid and 0.83 g water under stirring at 30 'C. The solution was stirred for 24 hours at 25 "C.A part of the isopropanol (8 g) was removed from the sol by vacuum distillation at 5 mbar and 25 'C and 6 g of 2isopropoxyethanol was added. As a photoinitiator 0.02 g Irgacure 184 was added. Low-refractive-index sol The SiO 2 nanoparticles were synthesized by base-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) mixed with isopropanol. 65 g TEOS were mixed with 100 g alcohol, 34 g ethanol and 10 g methacrylic acid to give a homogeneous solution. 4 g H20 were added under vigorous stirring at 35 °C. The mixture was heated under reflux at 45 'C with stirring for two days. This silica sol was concentrated by vacuum distillation to 4 wt% of SiO 2 in isopropanol. After adding 2-propanol, the sol was concentrated by vacuum distillation up to 2 wt% of SiO 2 and water was removed to 0.3 wt% (Karl Fischer titration). As a photoinitiator 0.015 g Irgacure 184 was added. Coating preparation
Float glass samples of 120 mm x 100 mm x 2 mm were cleaned in a water based washing line with ultrasound (Hamo GmbH) and dried at 40 °C. The coatings were prepared by ADDC with a constant angle of 40 to the vertical using withdrawal speeds of 3 to 5 mm/s. Each layer was cured by UV irradiation with 2.1 J/cm 2, before the next one was deposited. Finally, the coating stack was heated up to 450 °C within 1 h, held for 30 min. and cooled in air to room temperature. Characterization
The reflectance spectra were measured with an angle of 70 in a Cary 5 spectrometer and the transmission curves were recorded in a UV-VIS-NIR spectrometer (Omega 30, Bruins Instruments). The abrasion resistance was tested with a Taber Abraser test which was carried out with a Taber Industries 5150 Abraser (load 540 g/wheel, rubber CS10F, ASTM D 335978) and the damage of the coating and substrate material was measured with a Hazeguard Plus (BYK Gardner GmbH). The adhesion of the coatings was tested by cross cut tests [15] and rubber test [16]. In the saline-test, the coated substrates were stored in water containing 0.7 wt % of NaCI at 100°C for several days. The coated substrates were cleaned with ethanol and a qualitative analysis of the films was performed every 24 h with a microscope to detect film delamination or grazing. The sun-test consisted of irradiating the antireflective coated float glass with 760 W/m 2 (simulation of the sun irradiation with Suntest Atlas, xenon lamp without IR filter) and characterized by the determination of the yellowing (Ag) [17] of the coating measured by UV-VIS spectroscopy. Results Application of the ADDC technique The ratio between the coating thickness on the top and the bottom side of the glass substrate is independent of the type of the
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