Dental Materials Journal 2011; 30(2): 170–175

INTRODUCTION

There is a great demand for resin composite materials because esthetic restoration materials are used in a wide range of applications in dental care. Currently, a veneering resin composite is being used as an esthetic prosthetic material to reproduce the natural tooth crown color. However, due to physico-chemical deterioration, discoloration may occur and replacement may be necessary1-3). Ceramics have proven to be superior in chemical and color stability and to be adequately biocompatible4,5). An additional advantage is that ceramics accumulate less plaque6). Therefore, coating the surfaces of veneering resin composite with a ceramic film may eliminate its inherent disadvantages.

Coatings produced by depositing silica are widely used to create gas barriers, chemical corrosion resistance, electrical isolation, mechanical hardness, and various optical features5,7-9). Silica coating of steel and plastic materials with silicon alkoxides, such as tetraethylorthosilicate (TEOS: Si(OC2H5)4), has been shown to improve corrosion-resistance and heat-resistance of the base materials10,11). To obtain compact silica films with TEOS, a temperature exceeding 600˚C is required. In dentistry, the Silicoater method (Silicoater, Kulzer, Hanau, Germany) and PyrosilPen-Technology (PyrosilPen, SurA Instruments, Jena, Germany) are used as silica coating methods in the application of TEOS.

The silica precursor perhydropolysilazane (PHPS: AZ Electronic Materials Co. Ltd, Tokyo, Japan) converts to silica film at 300–350˚C in the presence of catalysts12,13). For silica deposition on the surface of materials with low heat-resistance, silica films can be formed at temperatures below 100˚C using PHPS with catalysts to convert to silica by exposure to water vapor10). However,

this method requires an impractically long reaction time of approximately one hour.

The purpose of the present study was to establish a low-temperature method to convert PHPS, applied to the surface of veneering resin composites having low heat-resistance, to a silica film in a dental laboratory, thereby preventing discoloration of the veneering resin composite.

MATERIALS AND METHODS

Coating materialDilute PHPS solution was used as the silica coating material. PHPS: (-SiH2-NH-)n, an inorganic macromolecule, is composed of a framework of Si-N, terminal groups of Si-H and N-H, a chain component, and a ring structure consisting of 6- and 8-membered rings9,14,15). We used two solutions of PHPS: NP-110, which contained 10 wt% PHPS and an amine catalyst in Xylene (NP, Lot. 06020156), and NL-120, which contained 5 wt% PHPS and a palladium catalyst in dibutyl ether (NL, Lot. 06051054). PHPS is converted to amorphous silica through reaction with oxygen or water. In this conversion process, PHPS releases nitrogen and hydrogen as ammonia and takes up oxygen into its network as it converts to silica. Reactions (1) and (2) show the conversion of PHPS to silica16).

(-SiH2-NH-)+O2 → SiO2+NH3 (1)(-SiH2-NH-)+2H2O → SiO2+NH3+2H2 (2)

Silicon wafer and veneering resin compositeSince the single-crystal silicon wafer (100) is transparent to infrared radiation17), sections (15×15 mm) of silicon wafers (Lot. 078, Sumitomo Metal Industries, Ltd, Osaka, Japan) were used to study the conversion process to silica. The veneering resin composite Prossimo (shade

Silica film coating method for veneering resin compositeTakahiro TANAKA1, Koji HANAOKA1, Masuji YAMAGUCHI1, Toyohiko SHINDO2, Karl-Heinz KUNZELMANN3 and Toshio TERANAKA1

1Division of Restorative Dentistry, Department of Oral Medicine, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka, Kanagawa 238-8580, Japan 2Contamination Control Services, 529-3 Shimokuzawa, Sagamihara-shi, Kanagawa 229-1134, Japan3Department of Restorative Dentistry, Dental School of the Ludwig-Maximilians-University, Goethestr. 70 80336 Munich, GermanyCorresponding author, Toshio TERANAKA; E-mail: teranaka@kdcnet.ac.jp

The preceramic polymer perhydropolysilazane (PHPS) is an attractive candidate as a coating material to prevent discoloration of veneering resin composites. At the present time, however, a practical method to apply this material is not available. The purpose of this study was to establish a low-temperature method for applying a silica film coating to a veneering resin composite. Two types of PHPS, NP and NL, were coated onto a veneering resin composite. The specimens were exposed to hydrogen peroxide vapor at 97˚C, and the state of the conversion process was evaluated using FT-IR. With exposure to the hydrogen peroxide vapor, a 0.5-µm-thick silica film similar to that produced by baking was formed on the surface of the NP samples in 10 min, while a 0.2-µm-thick film was formed on the NL in 15 min. The silica coating method described in this study may mitigate the discoloration of veneering resin composite.

Keywords: Perhydropolysilazane, Silica coating, Veneering resin composite, Hydrogen peroxide vapor

Received Jul 22, 2010: Accepted Oct 27, 2010doi:10.4012/dmj.2010-113 JOI JST.JSTAGE/dmj/2010-113

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E1, Lot. 061204A, GC Corp., Tokyo, Japan) was packed in a polyethylene mold (inner diameter, 16 mm; depth, 3 mm) and cured with Solidilite (Shofu Inc., Kyoto, Japan) for 5 min on each side. The material was polished using a water-proof abrasive paper (#2000) and 0.3 µm finishing film (263X, 3M, St. Paul, USA).

Silica film formationThe coating solution was applied by the flow method and dried for 5 min. Either 100 mL of distilled water or a 3% hydrogen peroxide solution (Lot. TSQ0628, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was poured into a 1,000-mL beaker and boiled on a 170˚C hotplate (Fig. 1). The temperature during conversion from PHPS to silica was monitored with K-thermocouples (Toa Denki Co., Ltd, Tokyo, Japan). After twenty minutes the vapor within the beaker reached a constant temperature of 97˚C. A PHPS-coated specimen was placed on a stainless-steel stage and after 10 min the specimen reached 97˚C.

Fourier transform-infrared spectroscopy (FT-IR) analysisThe PHPS-coated silicon wafers were divided into two groups, one for exposure to water vapor and one for exposure to the hydrogen peroxide vapor. Periods of exposure to the vapor were set to 5, 10, 15 and 60 min. The PHPS-coated specimens were maintained in dry air for 1 hour at 97˚C and used as the negative control. In addition, silica films formed from NP and NL coats by baking 1 hour at 300˚C and 500˚C, respectively, were used as the positive control. Infrared absorption spectra

of the films formed on silicon wafers were recorded with a spectrophotometer (IR-Prestige 21, Shimadzu Corporation, Kyoto, Japan) in the wave number range from 650 to 4,600 cm−1. The IR spectrum of the film was obtained by subtracting the spectrum of the uncoated wafer from that of the coated wafer. As in the case of the silicon wafers, either NP or NL was applied to each polished sample of veneering resin composite. Conversion to silica was analyzed with FT-IR using the attenuated total reflection method.

Film thicknessHalf of each veneering resin composite specimen was coated with either NP or NL. These coatings were converted in the same manner as described above. The film thickness for each specimen was measured with a confocal laser scanning microscope (OLS1100, Olympus Corporation, Tokyo, Japan) at 10 randomly selected locations.

RESULTS

FT-IR spectraFig. 2 and Fig. 3 show the FT-IR spectra from 700 cm−1 to 1,200 cm−1 of the silicon wafers to which NP and NL, respectively, had been applied. The FT-IR spectrum of an “as-coated” NP silicon wafer (the NP coating is fresh and unconverted) showed the Si-N peak at 830 cm−1, indicating unreacted PHPS (Fig. 2-A-a). Five min after exposure to hydrogen peroxide vapor, the Si-N peak had diminished, but the Si-H peak at 880 cm−1, also indicating

Fig. 1 Schematic diagram of PHPS-to-silica conversion method. Schematic diagram of the environment of conversion from PHPS to silica and the methods of measurement of temperature of the vapor within the beaker and the surface of the specimen.

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unreacted PHPS, remained (Fig. 2-A-b). Ten min after exposure, the Si-N and Si-H peaks disappeared, and Si-O peaks at 1,060 cm−1 and 800 cm−1 could be discerned (Fig. 2-A-c). The spectrum at ten min was similar to that of the positive control (Fig. 2-A-g). The negative control showed a peak around 1,060 cm−1, but the Si-N and Si-H peaks were still seen, showing the presence of residual unreacted PHPS (Fig. 2-A-f). In NP exposed to water vapor for 10 min, Si-O peaks were observed along with a peak of Si-H (Fig. 2-B-c). Even after exposure for 1 hour, the Si-N peak was greatly reduced, however, the Si-H

peak still remained (Fig. 2-B-e). In the NL exposed to hydrogen peroxide vapor for 15

min, the Si-N and Si-H peaks almost disappeared, while a peak of Si-O was clearly recognized (Fig. 3-A-d), and the spectrum equivalent to that of silica film baked at 500˚C for 1 hour in air was obtained (Fig. 3-A-g). On the other hand, when NL was exposed to water vapor, residual Si-N and Si-H peaks were still observed, even after exposure for 1 hour (Fig. 3-B-e). Spectra of PHPS that had been exposed to hydrogen peroxide or water vapor showed a peak at 940 cm−1 assigned Si-OH. Spectra

Fig. 2 FT-IR spectra of the as-coated NP film and the films exposed to the vapors from H2O2 or H2O. FT-IR spectra of NP-applied silicon wafer during exposure to hydrogen peroxide vapor (A) and water vapor (B). Peaks of PHPS are shown by Si-N at 830 cm−1 and Si-H at 880 cm−1. Conversion to silica is represented by the indicators of Si-O peaks at 1,060 cm−1 and 800 cm−1. From the bottom upwards, as-coated of PHPS (a), exposure to vapor for 5 min (b), 10 min (c), 15 min (d) and 1 hour (e), 1 hour heating at 97˚C in the air as the negative control (f), and 1 hour baking at 300˚C in the air as the positive control (g).

Fig. 3 FT-IR spectra of the as-coated NL film and the films exposed to the vapors from H2O2 or H2O. FT-IR spectra of NL-applied silicon wafer during exposure to hydrogen peroxide vapor (A) and water vapor (B). Peaks of PHPS and silica are same as Figure 2. From the bottom upwards, as-coated (a), exposure to vapor for 5 min (b), 10 min (c), 15 min (d) and 1 hour (e), 1 hour heating at 97˚C in the air as the negative control (f), and 1 hour baking at 500˚C in the air as the positive control (g).

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of silica films baked in dry air did not show this peak.Since the hydrogen peroxide vapor exposure method

is a more effective and efficient conversion method than water vapor exposure, the following experiment was carried out with hydrogen peroxide vapor. Fig. 4 shows the FT-IR spectra of the veneering resin composite and the PHPS-coated veneering resin composite before and after conversion to silica. In the spectrum of the veneering resin composite, a peak at 1,726 cm−1 derived from the carbonyl group (-C=O) in the methacryloyl group of the base resin and a peak at 1,100 cm−1 derived from Si-O bonds in the filler were detected. The spectrum of the NP as-coated, had peaks that showed unreacted PHPS was present. In the spectra of NP and NL coats, the Si-O peaks can be seen after being exposed to hydrogen peroxide vapor for 10 and 15 min, respectively, which were adequate conversion times to silica. Under the same conditions the Si-N and the Si-H peaks disappeared.

Film thicknessIn the cross-sectional profiles created by confocal laser scanning microscopy, a ridge was observed around the periphery of the coating. The surface leveled off at about 450 µm from the edge of the coat. We determined the film thickness of NP to be 0.5 µm and that of NL to be 0.2 µm in the leveled-off regions (Fig. 5).

DISCUSSION

Samples of both the NP and the NL coats showed the Si-N peak at 830 cm−1 and the Si-H peak at 880 cm−1, representing residual unreacted PHPS, even after being exposed to air at 97˚C for 1 hour. When exposed to water vapor at 97˚C, conversion time needed 1 hour or more. In contrast, when exposed to hydrogen peroxide vapor, a silica film similar to that baked at high temperature was formed at 97˚C in periods shorter than in the case of exposure to water vapor, specifically, 10 and 15 min for NP and NL coats, respectively. These results indicate that, for conversion of PHPS to silica at temperatures below 100˚C in a short time, hydrogen peroxide vapor, which provides both oxidation by oxygen molecular having high energy of molecular motion and hydrolysis by the water vapor in the atmosphere, is effective12).

Fig. 4 FT-IR spectra of resin composite and each of the PHPS-to-silica conversion. FT-IR spectra of NP- and NL-applied veneering resin composite after conversion to silica by exposure to hydrogen peroxide vapor. The bottom spectrum of composite rein shows a peak at 1,100 cm−1 derived from Si-O of the filler and that at 1726 cm−1 derived from -C=O of the base resin (a). In the spectrum of NP-applied veneering resin composite (NP as-coated), no peaks of veneering resin composite were found, while peaks at 830 cm−1 derived from Si-N was present (b). In the spectrum of NP 10 min after exposure to hydrogen peroxide vapor (c) and NL 15 min after exposure to hydrogen peroxide vapor (d) peaks at 1,060 and 800 cm−1 derived from Si-O were observed.

Fig. 5 Confocal laser scanning microscope images of film thickness. Birds-eye view of confocal laser scanning microscopy of the films. The film thicknesses converted from 10% NP (A) and 5% NL (B) were 0.5 µm and 0.2 µm respectively.

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With regards to the mechanism of conversion to silica by baking at high temperature, Shimizu et al.8) and Iwamoto et al.13) have reported that PHPS releases nitrogen and hydrogen as ammonia, and takes in oxygen, the film converts to ceramic via conversion of the bond between Si-N and Si-H to a siloxane bond. Based on FT-IR analysis, when the PHPS coated silicon wafer was baked, the peak derived from unreacted PHPS was undetected at from 200 to 300˚C of NP and at from 400 to 500˚C of NL. For the conversion of PHPS to amorphous silica glass at low temperature, the addition of amine or palladium catalysts to PHPS is required, along with water vapor and a molecular oxygen source. Since these catalysts promote the oxidation and hydrolysis of PHPS, conversion to silica becomes possible at temperatures below 100˚C. As shown in the present study, however, treatment for 1 hour is necessary, which is longer than the time needed using hydrogen peroxide vapor.

The active molecules of water in the vapor state react with PHPS and induce hydrolysis. The conversion to silica was found to be possible at temperatures below 100˚C by this reaction. In addition, in the environment of hydrogen peroxide vapor, oxygen molecules emerge in a large quantity along with actively moving water molecules18). Accordingly, the conversion to silica is assumed to be effective through hydrolysis by water molecules and oxidation by oxygen molecules at temperatures below 100˚C. The catalysts, such as amine and palladium, added in PHPS can convert to silica at low temperature19). In addition, in the method of conversion to silica by exposure to the vapor, the formed film shows a spectral feature corresponding to Si-OH at 940 cm−1, and the silica film formed by this method is more hydrophilic than that formed by baking in air.

The FT-IR spectrum showed that a PHPS coating on veneering resin composite was adequately converted to silica by exposing it to hydrogen peroxide vapor in the same way that it forms on a silicon wafer. Although the glass-transition temperature of veneering resin composites varies with the type of monomer and the degree of conversion, for most materials it is more than 120˚C20,21). In the temperature environment of the present study, formation of silica from PHPS was possible at temperatures below that described above. Simple and quick conversion of NP and NL coats by exposure to hydrogen peroxide vapor for a short time is a novel method and a promising method for the silica coating of such macromolecules as veneering resin composites in a dental laboratory. Further modifications seem possible that allow this method to be used at even lower temperatures with materials that should not be heat-treated.

In the present study, PHPS was applied by the flow method, and thicknesses of 0.5 µm and 0.2 µm were obtained for NP and NL coats, respectively. In order to guarantee crack-free film formation, film thickness must be less than 2.0 µm for vapor-oxidized film7,14). The film thickness can be controlled using the method of coating of PHPS, the number of coats and the concentration of the solution, in accordance with the properties of the

surface of the object to be coated and the purpose of the coating14).

The silica films examined in the present study may be applicable not only to veneering resin composite but also to various restorative materials22-24). Dental metal alloy is not chemically stable, and, due to corrosion and discoloration, its esthetic quality degrades, it physically deteriorates, and biological stimuli are increased25). Allergic contact dermatitis and oral lichen planus have been reported26,27). The silica coating method presented in the present study may mitigate these problems in many dental materials28).

ACKNOWLEDGMENTS

This work was partially supported by Grants-in-Aid for scientific research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), Grant-in-Aid for Scientific Research (B) #21390511 and Grant-in-Aid for Young Scientists (B) #22791850.

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