The effects of excimer light irradiation on polysilazane coatings formed on PET films with vacuum-evaporated SiO 2 coatings and the effects of these coatings on gas barrier characteristics have been investigated. The temperature during light irradiation has a large effect on the coating’s molecular structure and gas barrier characteristics. When irradiation was performed at 100 ℃, the polysilazane coating transformed into a silica coating, and a compact silica coating at a much lower temperature than with heat treatment alone was produced. Surface irregularities in the vapor-deposited silica coating were smoothed out by the formation of a polysilazane coating, which was transformed into a compact silica coating when irradiated with light, resulting in a significant improvement in the gas barrier characteristics. The water vapor permeability of the thin coating irradiated with excimer light at 100 ℃ showed only 0.04 g/m 2 •day (40 ℃, 90% RH). According to the results of investigation of temperature variation of water-vapor permeability, it is inferred that the developed film has an excellent gas barrier value, namely, 4.90 × 10 –4 g/m 2 •day at 25 ℃. This gas barrier coated PET film is transparent and flexible, and can be used in the fabrication of flexible electronics. Also, the proposed fabrication method effectively provides a simple low-cost and low-temperature fabrication technique without the need for high vacuum facility.
For the next generation of electronic products, attention has recently turned to products with flexible properties such as wearable devices, flexible batteries, electronic paper and flexible display devices that are thin, light and flexible [
The most common material for organic films is polyethylene terephthalate (PET). Attempts have been made to improve the gas barrier characteristics of this material by applying inorganic coatings. The heat tolerance of PET is about 100˚C, and the most popular way of forming an SiO2 coating is to use large-scale high vacuum deposition facility to implement deposition methods such as vacuum deposition or sputtering with a reduced level of substrate heating [
Another method that has been studied recently involves applying polysilazane coatings to PET films, which are then exposed to excimer light irradiation or ultraviolet ray irradiation [
As a precursor for forming thin coatings, a solution of polysilazane in dibutyl ether (NL-110, 20 wt%: AZ Electronic Materials Co.) was used. A 5 wt% dibutyl ether solution of this polysilazane solution was spin-coated (2000 rpm, 60 sec) onto a PET film (10 cm × 10 cm × thickness 100 µm: sp51, Nitto Denko Co.) on which SiO2 had been formed by vapor deposition. This film was exposed to light from an excimer lamp (M.D.COM Inc.) in an N2 atmosphere (commercial grade: O2 < 20 ppm). The film was irradiated at 42 mW/cm2 for 2 minutes. This irradiation was performed at three different substrate temperatures: room temperature (R.T.), 80˚C and 100˚C. The coating was applied to one side of the film. Samples were prepared with a single-layer and two-layer polysilazane coatings. To find out how the molecular structure of these coatings changes during heat treatment, samples heat-treated at 80˚C, 100˚C, 200˚C and 300˚C were prepared for infrared spectrum measurements.
The changes in the molecular structure of the coatings based on their infrared absorption spectra were observed. The absorption and reflection spectra of the coatings were measured by a visible/ultraviolet spectrophotometer (Shimadzu UV-245). The reflectivity was measured by determining the 5˚ specular reflectance after blackening the opposite side of the PET film. The surface of the coating was examined with a scanning electron microscope (SEM: JEOL JSM-7610) and an atomic force microscope (AFM: 5100N, Hitachi Hightech Science Co.). The cross-sectional structure of the coating with a transmission electron microscope (TEM: H-9000NAR, Hitachi Ltd.) was observed. The TEM samples were prepared by FIB (focused ion beam) processing. The elemental analysis (EDX) of the observed cross-sections was performed. To assess the gas barrier characteristics, the water vapor permeability (40˚C, 90% RH: Mocon Permatran-W) was measured. Water vapor transmission rates of 1 g/m2・day or above were measured using the dish method. The water vapor permeability was measured at a humidity of 90% RH and at temperatures of 40˚C, 45˚C, 50˚C and 55˚C. The adhesion strength of the coating on the PET film was evaluated by performing tape peeling tests according to the JIS standard (JIS K5600).
The gas barrier coatings were produced by light irradiation at 100˚C or less, considering the heat tolerance of the PET film. Light irradiation conditions were as follows, light irradiation wavelength: 172 nm, irradiation intensity: 42 mW/cm2, irradiation time: 2 minutes, irradiation area: 10 cm × 10 cm, respectively.
The coating exhibited high adhesion strength to the PET film, achieving a value of 100/100 (remaining number of sheets/Number of cuts) in tape peeling tests according to the JIS standard.
temperatures during irradiation with the spectra of materials produced only by heat treatment.
As for the heat-treated samples, the absorption peaks corresponding to NH (~3300 cm−1), SiH (~2200 cm−1) and SiN (~800 cm−1) were observed directly after the coating had been formed. This suggests that the coating has a structure that includes Si-N bonds due to polysilazane. These absorption peaks decreased as the heat treatment temperature was increased over the range 80˚C → 100˚C → 200˚C → 300˚C, during which there was in increase in the absorption peak near 1100 cm−1 associated with siloxane bonds (-O-Si-O-). When heat treatment is performed at 300°C, a strong siloxane bond absorption peak is observed, showing that the coating was transformed into SiO2. At 200˚C, there is still an absorption peak corresponding to polysilazane, showing that the transformation into SiO2 is not yet complete. On the other hand, in the coatings that were subjected to simultaneous heat treatment and light irradiation, the absorption peaks corresponding to NH (~3300 cm−1), SiH (~2200 cm−1) and SiN (~800 cm−1) that were observed directly after the coating had been formed decreased greatly as the temperature during light exposure was increased (r.t. → 80˚C → 100˚C), whereas the siloxane (-O-Si-O-) peak around 1100 cm−1 increased. The absorption peaks due to polysilazane were observed in the coating exposed to light at room temperature, but in the coating exposed at 80˚C, these peaks were greatly reduced and the absorption peak near 1100 cm−1 was much larger. In the coating exposed at 100˚C, hardly any polysilazane absorption peak could be observed, indicating that the polysilazane coating had almost completely transformed into SiO2. This shows that simultaneous light exposure and heat treatment can produce silica coatings at much lower temperatures than heat treatment alone.
The coated surfaces were very smooth, and no differences could be seen in SEM images taken at magnification factors of 10,000 - 20,000×. Therefore SEM observations at 100,000× magnification and AFM observations of the surface were performed. The results are shown in
forming a polysilazane coating at low temperature effectively helps to make the surface more compact, resulting in improvement of gas barrier properties of the film.
therefore assumed that this is the reason why it did not make a large improvement to the gas barrier characteristics. The application of a polysilazane solution on top of this SiO2 is thought to have covered up defects in the SiO2 vapor-de- posited PET coating, resulting in better gas barrier property. The two-layer coated films produced only with heat treatment at 80°C and 100˚C showed almost the same values as the SiO2 vapor deposited PET, and hardly exhibited any reduction in the gas barrier property.
Temperature dependence of water vapor transmission rate of the double- coating film by photo irradiation at 100˚C was measured. A natural-log plot of measured water-vapor transmission rate under certain conditions (temperatures of 40˚C, 45˚C, 50˚C, and 55˚C and relative humidity of 90%) against the reciprocal plot of temperature is shown in
Polysilazane has a strong absorption peak in the ultraviolet region below 260 nm, and absorbs vacuum ultraviolet light at the excimer lamp’s emission
wavelength of 172 nm. The energy level of light at the excimer lamp’s emission wavelength (172 nm) is 166 kcal/mol. On the other hand, the Si-N, Si-H and N-H bond energies of polysilazane are 105, 71 and 92 kcal/mol, respectively. Since the energy of the ultraviolet light is greater than these bond energies, it has enough energy to break the polysilazane bonds. Broken bonds are instantly oxidized and transformed into SiO2 by singlet oxygen (O(1D)) or ozone (O3) generated when ultraviolet light is absorbed by oxygen present in trace quantities inside the light irradiation chamber or in the inert nitrogen gas. A higher temperature during light irradiation is thought to increase the diffusion of active oxygen species into the coating where it further promotes the oxidation of Si. This can also be seen from the fact that the water vapor permeability and changes in the molecular structure of the polysilazane coating in the infrared absorption spectrum are strongly dependent on the light irradiation temperature.
In the infrared spectra, as the temperature during light irradiation increases, the N-H, Si-H and Si-N absorption peaks originating from polysilazane decrease while the peak corresponding to Si-O-Si bonds originating from SiO2 increases. This confirms that the polysilazane is converted into SiO2 more efficiently as the temperature increases. In the coatings subjected to light irradiation, the transformation into an SiO2 coating has already occurred at 100˚C. Considering that the transformation to SiO2 occurs at 300˚C with heat treatment alone, this demonstrates the effectiveness of gas barrier films made according to the technique proposed here for the formation of gas barrier coatings at low temperature.
Also, from the elemental analysis results of TEM cross-sectional structure observations, no N in the coating was found. The coating is thought to consist of SiO2 or SiOx-based silica. In the future, we plan to use XPS to perform a precise analysis of the composition ratio of the silica coating.
The gas barrier property of a PET film was improved by the vacuum vapor deposition of silica, but a major improvement was not observed. The formation of a polysilazane-derived SiO2 coating on this vapor-deposited coating promotes increased density and evens out the surface irregularities, resulting in a significant improvement in the gas barrier property. Coatings formed by ordinary vapor deposition or sputtering processes grow by depositing atoms on convex parts of the substrate surface, and are thus liable to form column-shaped structures with surface irregularities. Since gases diffuse more readily along the grain boundaries of these column-shaped structures, this is thought to have an adverse effect on the gas barrier characteristics. The polysilazane coating fills in the surface irregularities of the SiO2 vapor-deposited PET coating with a solution that has a certain molecular weight when the polysilazane solution is applied, and when it hardens, the solvent evaporates away and shrinks in the thickness direction to leave a uniform layer of amorphous silica. Therefore, it is thought that the surface of the vacuum-evaporated SiO2 layer is smoothed out, and that the surface defects are filled in, resulting in better gas barrier property.
The gas-barrier property depends on the formation conditions and thickness of the thin film. It is possible that the gas-barrier property is greatly enhanced by formation of the lamination layers when the substrate temperature is raised during photo-irradiation. The double-coated thin film photo-irradiated at 100˚C showed a water-vapor transmission rate (0.04 g/m2・day) near the measurement limit of the measurement apparatus (0.02 g/m2・day). The temperature dependence of water vapor permeability shows a linear relation with the reciprocal of temperature in accordance with the Arrhenius equation. The value of water-vapor permeability at 25˚C calculated from the Arrhenius plot is an extremely low value (4.90 × 10−4 g/m2・day) and the activation energy is 236 kJ/mol. The same linear dependence of water vapor permeability was also obtained in PET film. PET film shows the activation energy of 42 kJ/mol and the value of water-vapor permeability of 3.10 g/m2・day at 25˚C, respectively. The developed film shows much larger activation energy than that of PET, indicating excellent gas barrier characteristics.
The gas barrier coatings used in this study were formed on one side only of a PET film. It is expected that a significant improvement in gas barrier performance can be achieved by forming gas barrier coatings on both sides of the film. The polysilazane-derived SiO2 film formed on both side of a PET film (no vacuum-evaporated SiO2) by low-pressure mercury lamp showed the WVTR value of 0.03 g/m2・day (40˚C, 90% RH) [
The effects of excimer light irradiation on polysilazane coatings formed on PET films with vacuum-evaporated SiO2 coatings and the effects of these coatings on gas barrier property have been investigated. The temperature during light irradiation has a large effect on the coating’s molecular structure and gas barrier property. When irradiation was performed at 100˚C, the polysilazane coating transformed into a silica coating, and a compact silica coating at a much lower temperature than with heat treatment alone was produced. Surface irregularities in the vapor-deposited silica coating were smoothed out by the formation of a polysilazane coating, which was transformed into a compact silica coating when irradiated with light, resulting in a significant improvement in the gas barrier characteristics. The water vapor permeability of a thin coating formed at 100˚C while irradiating with excimer light was found to be only 0.04 g/m2 per day (40˚C, 90% RH), which is a good result. According to the results of investigation of temperature variation of water-vapor permeability, it is inferred that the developed film has an excellent gas barrier value, namely, 4.90 × 10−4 g/m2・day at 25˚C. This gas barrier coated PET film is transparent and flexible, and can be used in the fabrication of flexible electronics. Also, since it can be formed by light irradiation of a polysilazane coating without the need for high vacuum facility, the proposed fabrication method effectively provides a simple low-cost and low-temperature fabrication technique.
This work was supported by JSPS KAKENHI Grant Number 26420711.
Ohishi, T. and Yamazaki, Y. (2017) Formation and Gas Barrier Characteristics of Polysilazane-De- rived Silica Coatings Formed by Excimer Light Irradiation on PET Films with Vacuum Evaporated Silica Coatings. Materials Sciences and Applications, 8, 1-14. http://dx.doi.org/10.4236/msa.2017.81001