Ultraviolet ray absorbing coatings on glass for automobiles

Transcription

1 Thin Solid Films 351 (1999) 61±65 Ultraviolet ray absorbing coatings on glass for automobiles Takeshi Morimoto*, Hiroyuki Tomonaga, Akemi Mitani Research Center, Asahi Glass Company, 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi , Japan Abstract Ultraviolet ray (UV) absorbing coating consisting of a complex oxide of cerium and titanium was deposited on glass via the sol±gel process. Addition of TiO 2 to CeO 2 changed the crystallinity and coordination state of CeO 2 and improved UV absorption. Formation of a double-layered coating comprised of outer CeO 2 ±TiO 2 layer and inner TiO 2 ±SiO 2 layer diminished the interference coloring and improved the adhesion of the coating to the glass substrate. The double layer coated glass shows high UV-shielding properties without visible light absorption and excellent durability against chemical and physical attacks. Therefore, it has been widely used in automobile windows. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sol±gel process; Ultraviolet absorption; Optical coating; Spin-coating 1. Introduction Excessive exposure to ultraviolet rays (UV) in sunlight (6%) induces coloring and deterioration in paper, fabrics and resins. It also causes harmful damage to the human body such as premature aging of skin, skin cancer and cataract. Recent destruction of the ozone layer around the earth increases UV irradiation and its harmful effect on the human body [1]. Shielding from UV irradiation is of interest in many elds. For automobile applications, it is necessary to prevent the interior materials and the skin of passengers from damaging by UV. For this purpose, we have developed an UV-shielding coating comprised of two metal oxides. It is well known that glass containing mixed oxides of cerium and titanium absorb UV effectively and exhibit yellow color [2]. It is also reported [3,4] that CeO 2 ±TiO 2 coatings deposited on glass by the sol±gel process show UV absorption, high re ectance and interference coloring. However, its UV absorption is not suf cient because of its composition of CeO 2 =TiO 2 1:0. Moreover, high re ectance and interference coloring are not preferable for automobile windows. The present research was performed to solve the abovementioned problems. An attempt was made to (1) maintain CeO 2 /TiO 2 ratio above 1.0 to increase UV absorption, and (2) form a double-layered coating on glass to diminish re ectance and coloring. This paper describes the materials, mechanisms and properties of the double layer coated glass. 2. Experimental 2.1. Coating solution for outer UV absorbing layer Cerium nitrate hexahydrate (Daiichi Kigenso Co.) and titanium n-butoxide (TBT, Matsumoto Kosho Co.) were used as metal precursors of metal oxides. TBT was dissolved in ethanol and the solution was stirred for 1 h at 258C. An aqueous solution of 6 wt.% HCl was added to the solution of TBT. The molar ratio of H 2 O/TBT was 8.5. Concentration of this solution was 0.8 M as Ti (Solution A). Cerium nitrate hexahydrate was dissolved in ethanol. Then, polyethyleneglycol (PEG, average molecular weight ˆ 200) and acetylacetone (AA) were added as multidentate ligands. The molar ratio of Ce/PEG/AA was adjusted to 1:1:1. The obtained solution was heated up to 708C for 2 h and cooled to room temperature (Solution B). Concentration of solution B was also 0.8 M as Ce. Solutions A and B were mixed in various ratios and aged at 358C for 1 day. * Corresponding author. Tel.: ; fax: address: morimoto@agc.co.jp (T. Morimoto) Fig. 1. UV transmittance of CeO 2 ±TiO 2 coatings with various composition /99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S (98)

2 62 T. Morimoto et al. / Thin Solid Films 351 (1999) 61±65 Fig. 3. XRD pro le of CeO 2 ±TiO 2 coatings treated at 7008C for 3 and 60 min. Fig. 2. XRD pro le of CeO 2 ±TiO 2 coatings CeO 2 /TiO 2 molar ratio: (a)100:0, (b)80:20, (c) 70:30, (d) 60: Coating solution for inner layer Titanium isopropoxide (TPT, Mitsubishi Gas Chemical Co.) and tetraethyl-orthosilicate (TEOS, Tama Chemical Co.) were used as precursors for metal oxides. Various ratios of TPT and TEOS were dissolved in a solution of ethanol and acetylacetone (90:10 in weight ratio) and stirred for 1 h at 258C. Then 1 wt.% HCl aqueous solution was added to the solution under stirring to the molar ratio of H 2 O/(TPT 1 TEOS) ˆ 2. Concentration of this solution was 0.4 M as Ti1Si. Then the solution was aged at 358C for 1 day Coating procedure and analysis Polished and cleaned soda-lime silicate glass ( :5 mm) was used as the substrate. Solution was applied to the substrate using the spin-coating process. After drying at 1208C for 5 min, the coated glass was heated up to 7008C and red for 3 min. The lm thickness was determined with a pro lometer (Sloan, Dektak 3030). The microstructure of the coatings was observed with a scanning electron microscope (Hitachi, S-900SEM). Crystal phases were determined with an X-ray diffractometer (Rigaku, RINT-2000). Transmittance and re ectance of the coatings were measured with a spectrophotometer (Hitachi, U-3500). State analysis was carried out by ESCA (Phi, type5500) and uorescent EXAFS (Rigaku, R-EXAFS2000). 3. Results 3.1. CeO 2 ±TiO 2 UV absorbing coating Fig. 1 shows the UV transmittance of glasses coated with various CeO 2 ±TiO 2 binary compositions. Film thickness was 160 nm for all coatings. The addition of TiO 2 to CeO 2 lowers UV transmittance in the composition range of CeO 2 / TiO 2 $1 and shows its minimum at around 40 mol%. Although it was reported [4] that CeO 2 precipitates in a solution of the molar ratio of CeO 2 =TiO 2 1, no precipitation occurred in any ratios of CeO 2 /TiO 2 solutions used in this experiment. Acetylacetone and polyethyleneglycol added to Fig. 4. SEM surface photographs of CeO 2 ±TiO 2 coatings treated at 7008C for (a) 3 and (b) 60 min.

3 T. Morimoto et al. / Thin Solid Films 351 (1999) 61±65 63 Table 1 EXAFS results Sample Coordination number of Ce distance to neighboring O 22 (nm) CeO (standard) ^ CeO 2 TiO ^ Fig. 5. Transmittance spectra of CeO 2 ±TiO 2 coatings treated at 7008C for 3 and 60 min. the solution seem to play important roles in stabilizing Ce ions in the solution. Fig. 2 shows XRD pro les of various compositions of CeO 2 ±TiO 2 coatings on glass. The coating containing only CeO 2 shows several peaks which have been indexed to the cerianite, CeO 2, crystalline structure. However, addition of TiO 2 to CeO 2 coating decreases the crystallinity of cerianite and forms a complete amorphous phase at the composition of TiO 2 /CeO 2 ˆ 40:60. Moreover, the peaks indexed anatase or rutile structure of TiO 2 do not appear by the addition of TiO 2 up to 40mol%. This means that addition of TiO 2 to CeO 2 increases disorder in the crystalline structure of cerianite. Figs. 3 and 4 compare X-ray spectra and SEM surface photograph of the CeO 2 /TiO 2 ˆ 60:40 coatings treated at the temperature of 7008C for 3 min. and 7008C for 60 min. The X-ray spectra and SEM surface photograph of the coating treated at the temperature of 7008C for 60 min show the peak indexed to cerianite, CeO 2 and the precipitates below 50 nm, respectively. The precipitates are expected tobe composed of cerianite. Fig. 5 compares UV transmittance spectra of the coatings. The coating treated at the temperature of 7008C for 3 min absorbs UV effectively, whereas the coating treated at 7008C for 60 min does not absorb UV effectively. These facts suggest that the amorphous phases composed of TiO 2 /CeO 2 play an important role in absorbing UV. As mentioned above, addition of TiO 2 to CeO 2 changes crystallinity. It is supposed that these changes are caused by some differences in the valence or coordination state of Ce ion. ESR measurements at 6 K (liquid Helium temperature) shown in Fig. 6 indicate that trivalent Ce 31 ions are not present in both CeO 2 100% and CeO 2 /TiO 2 ˆ 60:40 coatings. This means that only tetravalent Ce 41 ions are present in both coatings. Table 1 shows the results of EXAFS analysis on Ce±L orbital in the CeO 2 and CeO 2 ±TiO 2. Although the addition of TiO 2 to CeO 2 does not change the distance to neighboring O 22, it reduces the coordination number of Ce ion to nearly 6. Considering that CeO 2 takes uorite structure of which the coordination number of Ce is 8 and TiO 2 takes the anatase or rutile structure of which the coordination number of Ti is 6, it is expected that the structure of CeO 2 changes to anatase or rutile structure gradually by the addition of TiO 2 to CeO Inner layer coating The refractive index of UV absorbing layer is about 2.10 and higher than that of substrate glass (1.52). Therefore, the glass coated by UV absorbing layer shows high re ectance and re ection coloring owing to optical interference. The inner layer was applied between the glass substrate and UV absorbing layer in order to reduce the re ectance and re ection coloring. Refractive index and optical thickness required for the inner layer coating were calculated to be 1.7±1.8 and 110±150 nm, respectively. Sol±gel derived SiO 2 ±TiO 2 binary coatings were used as the inner layer. Fig. 6. ESR spectra of Ce 31 standard and CeO 2 ±TiO 2 coatings. Fig. 7. Refractive index of TiO 2 ±SiO 2 coatings with various compositions.

4 64 T. Morimoto et al. / Thin Solid Films 351 (1999) 61±65 Fig. 8. Re ection spectra of UV absorbing layer coated glass. Fig. 7 shows the refractive indices of various compositions of TiO 2 ±SiO 2 coatings. The coating with molar ratio of TiO 2 /SiO 2 ˆ 33:67 had the refractive index of Fig. 8 shows the re ection spectra of the glass coated with UV absorbing layer with and without an inner layer of which the refractive index and the optical thickness are 1.76 and 125 nm, respectively. The inner layer reduces re ectance and prevents the glass from exhibiting a re ection coloring. Moreover, this inner layer plays a very important role for strengthening the adhesion between the coating and the substrate as shown in Fig. 9. The insertion of the inner layer increases the number of surface active hydroxyl groups [5] which can bond with the hydroxyl groups in the UV absorbing layer. Fig. 10 shows the cross-sectional SEM image of the double layer coated UV-shielding glass. 4. Application of UV-shielding glass to automobile windows 4.1. Manufacturing procedure Double-layered UV-shielding coatings have been applied to automobile windows. Several factors related to the coating solutions (e.g. solvents, ligands and additives) and the coating method have to be modi ed in order to apply the coating to large area glass substrate (max.,2 m 2 ). The Fig. 10. Cross-sectional SEM image of UV-shielding coating. coated glasses are manufactured using following method. Polished and dried at glass substrates are coated with inner layer and outer UV absorbing layer. After drying, the coated glasses are cut into the desired shape. Then the coatings are red and hardened in the bending and tempering process of glass at around 7008C for a few minutes Performances of UV-shielding glass Fig. 11 compares transmission spectrum of the UVshielding glass with substrate glass. The UV-shielding Fig. 9. Abrasion resistance of UV absorbing layer coated glass. Fig. 11. Transmission spectra of W shielding glass compared with substrate glass.

5 T. Morimoto et al. / Thin Solid Films 351 (1999) 61±65 65 Table 2 Various stability test results of the UV-shielding glass Stability test Test condition Results Abrasion Taber abrasion 1000 cycles a Haze ˆ 2.6 ^ 0.5% Acid Immersion in 0.1 N H 2 O h No damage Alkali Immersion in 0. 1 N NaOH 240 h No damage Humidity 558C 95% RH 240 h No damage Heating 3508C 240 h No damage Weathering SWOM h No damage a Abrasion wheel: CS-1OF. glass absorbs UV effectively without signi cant absorption of visible light. The UV-shielding glass has the SPF (sunprotect factor) value of about 70, which is higher than that of commercial UV protect cosmetic materials. Results of the stability tests are shown in Table 2. As the coated glass has excellent stability against various chemical and physical attacks, they can be handled in the same way as normal glass. The UV-shielding coated glass has been practically used in side and rear windows of industrially produced automobiles for more than four years without any damage. 5. Conclusions Ultraviolet rays shielding CeO 2 ±TiO 2 coatings on glass were prepared by the sol±gel process. UV absorption of CeO 2 was improved with the changes of crystallinity and coordination state by adding TiO 2. The inner layer composed of TiO 2 ±SiO 2 applied between the substrate and CeO 2 ±TiO 2 coating prevented the glass from exhibiting interference coloring and improved the adhesion of coatings to the substrate. This double-layered UV-shielding coating on glass is useful for protecting human bodies and materials from the harmful effects of UV and shows excellent durability. The coated glass is widely used in the automobile industry. References [1] Executive Summary of the Environmental Assessment of Ozone Depletion, [2] C.A. Hampel, Glass Ind. 41 (2) (1960) 82. [3] A. Makishima, H. Kubo, K. Wada, Y. Kitami, T. Shimohira, J. Am. Ceram. Soc. 69 (6) (1986) C127. [4] M.A. Sainz, A. DuraÂn, J.M. FernaÂndez Navarro, J. Non-Cryst. Solids 121 (1990) 315. [5] F. Gunji, T. Yoneda, T. Morimoto, New Glass 11 (4) (1996) 49.