솔-겔법에 의한 하트코팅 silica film 제조를 위한

precursor 용액의 합성 및 특성연구

김선일, 김구열, 임형미, 나재운*, 이봉우**

조선대학교 화학공학과, 순천대학교 고분자공학과*, 건설화학공업(주)**

Synthesis and Characterization of the Precursor Solution for

a Hard Coating Silica Film prepared by Sol-Gel method

Sun-Il Kim, Gu-Yeol Kim, Hyung-Mi Lim, Jae-Woon Nah*, Bong-Woo Lee**

Dept. of Chem. Eng., Chosun Univ., Kwangju 501-759, Korea

*Dept. of Polymer Sci. and Eng. Sunchon Nat'l Univ., Chonnam 540-742, Korea

**Technical Institute of Kun-sul Chem. Kyungki-do 435-030, Korea

Introduction

Plastics and polymer have been widely used as materials for optics, electronics, construction, and car parts due to its characteristics of light weight, easy processability, and non-corroding behavior.1 Those materials often need post-processing such as applying hard coating on the surface, to increase the chemical and mechanical durability or to introduce specific functionality. There are three different types of functional coating materials. One is organic coating material as melamine, acryl, and urethane. Another is inorganic material as silicone compounds. The other is organic-inorganic hybrid composite, which have shown many interesting mechanical, optical, and thermal properties. One of the simplest system in the hybrid materials, methyl-modified silicate materials were used to obtain a transparent, crack-free, and characterized hard coating thin films. Sol-gel process in the preparation of the coating films are known to be relatively easy to prepare. In addition, it provides uniform surface throughout the substrate, and improves the surface property by changing mechanical or chemical properties and thereby providing optic, electromagnetic, or catalytic qualities. Also it was found easy to produce ceramic thin films ranging in characteristics dense to highly porous layers.2 The control of porosity has a special interest on the preparation of chemically enforced materials3,4 or selective chemical sensors5,6 or membrane7,8. On the other hand, the density and hardness is required property for the purpose of protection as a hard coating material. Here we focused on finding the synthesis condition of precursor solution to produce the transparent crack-free hard coating film starting from the two types of silanes, tetraethoxysilane and methylmethoxysilane, and examined physical properties of the precusor solution and the final hard coating film.

Experimental

To the mixture of Tetraethoxysilane (TEOS), methyltrimethoxysilane (MTMS), methanol (MeOH), and  isopropylalcohol (IPA), distilled water and acetic acid was slowly added, using a dropping funnel, for 60 min under vigorous stirring at ice bath. The molar ratio of TEOS + MTMS : IPA + MeOH : acetic acid : distilled water was 1 : 0.278 : 0.077 : x, where x = 2.92, 3.95, and 4.74. The molar ratio of TEOS : MTMS was 1 : 8 and IPA : MeOH was 1 : 0.82. The mixture solution after addition of acid catalyst was removed from ice bath and stirred at room temperature for 60 min after the temperature was equilibrated to room temperature, which took about 30 min. The solution was heated slowly to 60 ℃ for 30 min and reflux at 60℃ for 60 min, and cooled to room temperature. Each step was carried out under nitrogen gas flux. A catalyst, dibutyltin dilaurate (DBTDL) was added to the precursor solution by 0.2 wt % and stirred for 30 min at room temperature vigorously, just before deposition to the glass plates. Glass plate was used as a substrate for the coating. The glass plate was ultrasonically cleaned with THF, benzene, and water for 5min, respectively. Coating films on the glass plates were prepared by a flow coating or dip coating method after filtering with a syringe filter (DAE HA MEDICAL Corp., 0.45□). The coating films on glass plate were typically dried at room temperature for 20 min and heated at 180 ℃ for 30 min. After heating the films at 180 ℃, they were cooled to room temperature slowly.

Infrared (IR) absortion spectra of the precursor solution at various reaction time and the coating films were measured using an FT-IR spectrophotometer. The OH group in the intermediate mixture solution at overall reaction time 120 min was capped by addition of the mixture of trimetylchlorosilane (TMCS) and pyridine (1:1). Then, precipitates were separated by centrifuge. The distribution of molecular weight for reaction intermediates was studied by gel permeation chromatography (GPC). Flux rate and concentration of a sample was 1.0 ml/min and 8.34 mg/ml of tetrahydrofuran (THF). The surface morphology of coating films were examined by a scanning electron microscope (SEM). The thickness of the coating films was evaluated by observing the cross section of the films on glass substrate using SEM. Thermal analysis were performed by thermo gravimetric  analyzer (TGA) and differential scanning calroimeter (DSC) to investigate the thermal stability of coating films.

Results and Discussion

The precursor solution were studied by infrared spectra in the process of hydrolysis and condensation. The selected spectra at various reaction time have been plotted in Figure 1. The first spectrum, Figure 1(a) was taken for the mixture solution of TEOS, MTMS, MeOH, and IPA, just before the addition of acid and water mixture. In the successive spectrum, characteristic peaks for hydrolysis and condensation changed with time. The broad absorption band about 3000 - 3700 cm-1 corresponds to the fundamental stretching vibrations of isolated and H-bonded SiO-H and hydrogen-bonded molecular water. The sharp peak at 2975 cm-1 is assigned to stretching vibrations for aliphatic C-H bonds. Absorption bands at 1270 and 1040-1090 cm-1 are assigned to Si-O-Si asymmetric modes. The absorption band at 790 cm-1 is associated with symmetric Si-O-Si stretching or vibrational modes of ring structures. The peak appears about 20  min after acid addition at 905 cm-1 and it may be assigned to Si-OH stretching vibration. After all, infrared spectrum of the coating film after drying and heat treatment is shown in Figure 1(g), where the greatest Si-O-Si peaks tells that the condensation process was further progressed after a deposition on the glass.

The surface of coating film depends on the coating process, such as aging time and heat treatment, as well as the starting materials. The water content in the synthesis of precursor solution was found optimized when the molar ratio of water was 3.95, among three different ratios tried in this experiment. The crack appeared in the case of lower water content and the clearness was lower in the case of higher water content, due to fast hydrolysis3. Without the heat treatment, the pores in (a) are formed larger than those in (b) after heating at 180 ℃ for 30 min. After ageing a precursor solution for a week at 5 ℃, the surface of coating film, (d) did not show any pores or any protuberances. Therefore, we could assume the densification of the micro structure during the ageing process. The film (c) prepared by dip coating revealed larger pores and greater thickness than the film (b) by flow coating. The pores were expected to be larger in the thinner film but the opposite trend was  observed in Figure 2(b) and 2(c), which indicate that there were not only substrate effect2 of preventing shrinkage of the precursor solution but also the more effective force evolving a pore in the thicker films while condensation progress.

The OH group in the precursor solution at the overall reaction time 120 min was capped by trimethoxysilane group and capping was checked by IR spectra. The characteristic SiO-H peak disappeared after the capping procedure. Weight average

molecular weight for the capping solutions were found to be 383 and 391 g/mol

(a)	(b)               (c)               (d)

Figure 2. SEM images of the coating films prepared in

various ageing, drying, heating conditions.

for samples at the overall reaction time 120 min and 210 min.  TGA and DSC curves of the coating film showed a large exotherm and weight loss between 460 - 600 ℃ about 22.8 %. The coating film reveals thermal stability up to 460 ℃. We attribute the large exotherm above 460 ℃ to the decomposition of  alkyl groups of organosilane.

The degree of condenation was analyzed by 29Si-NMR spectroscopy which revealed five different types of 29Si peaks, T1, T2, T3, Q2, and Q3, where Tn denotes for CH3-Si-(OSi)nO3-n and Qn denotes for Si-(OSi)nO4-n from MTMS and TEOS origin, respectively. It can be concluded that the degree of condensation of TEOS was greater than that of MTMS from the distribution of Tn and Qn peaks.

Acknowledgement

We are greatly indebted to the Korea Research Fundation for the financial support made for this work in the program year of 1998.

References

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