TiO2 Coatings Via Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition for Enhancing the UV-Resistant Properties of Transparent Plastics.

Herein, TiO2 coatings were deposited on photodegradable polymers for protection from UV irradiation using the atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique. Polymethylmethacrylate (PMMA) and polycarbonate (PC) substrates were coated with titanium tetraisopropoxide as the precursor in an open-air atmospheric-pressure nonequilibrium argon plasma jet. The AP-PECVD-derived TiO2 coatings exhibited good adhesion to PMMA and PC. The TiO2 coatings could shield more than 99% of UV light in the wavelength range of 200-300 nm, without affecting the transmittance of visible light. UV irradiation tests on polymer films demonstrated that the degradation rates of PMMA and PC were significantly reduced by one-tenth after they were coated with TiO2 films.

Introduction

Transparent plastics, such as polymethylmethacrylate (PMMA) and polycarbonate (PC), are being used to replace glass for both industrial and domestic purposes. However, long-term exposure of PMMA and PC to UV irradiation causes severe degradation of these polymers due to UV-induced photochemical oxidation.[1] Although PMMA and PC have outstanding optical clarity and mechanical properties, their photosensitive nature renders them unsuitable for application in outdoor conditions, for example, car windows, greenhouse, and electronics. Hence, transparent UV-protective coatings are required to protect these polymers. Aklalouch et al. successfully covered PMMA sheets with a hybrid organic–inorganic sol–gel coating that comprised organically modified silicate and CeO2 nanoparticles. This hybrid coating showed efficient UV protection in the outdoor weathering aging test for the application of concentrated photovoltaic panels.[2] Anma et al. coated ZnO thin films on PC plates via the plasma-enhanced chemical vapor deposition (PECVD) method. The obtained ZnO-coated PC was observed to exhibit outstanding characteristics of UV protection in the xenon arc weatherability test, wherein it remained smooth and uncolored even after 1000 h of UV irradiation.[3] Weber et al. fabricated vacuum-deposited organic–inorganic hybrid coatings embedded with hydroxyphenyltriazine and hydroxybenzotriazoles for UV protection of PC. The fabricated hybrid coatings exhibited high stability during UV irradiation experiments.[4]

Currently, there are several UV absorbers available for UV protection, including titanium dioxide (TiO2),[5] cerium oxide,[6] zinc oxide,[7] 2,2-dihydroxy-4-methoxy-benzophenone,[8] MIL-125(Ti)-NH2, and MIL-68(In)-NH2.[9] TiO2 is one of the most widely used UV absorbers because it exhibits nontoxicity, good physical and chemical stabilities, cost effectiveness, good visible light transparency, and outstanding UV-blocking properties.[10−12] TiO2 has also been used as a UV-protective coating on polymers. Cui et al. studied the coating of a transparent TiO2/mixed oxide UV-protective thin film on heat-sensitive substrates using a low-temperature sol–gel method.[13] Hwang et al. also fabricated a TiO2 UV-protective hard coating and deposited it via spray coating on PC.[14] However, the use of solvents in the aforementioned methods renders the fabrication process complex and leads to a waste of chemicals.

Atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) is a single-step solvent-free technique,[15,16] which has recently drawn considerable attention for the fabrication of oxide coatings on polymers. Unlike the low-pressure plasma-enhanced chemical vapor deposition, AP-PECVD can be operated without a vacuum system; this eliminates the required limits with regard to the shape and size of the coated materials.[17,18] Moreover, the use of plasma has enabled fabrication at low temperatures, wherein the film formation reaction proceeds by the collision of monomers with charged particles (electrons, ions) in the plasma and not by thermal decomposition.[19,20] Therefore, the low processing temperature of AP-PECVD can prevent the polymers from melting. In addition, an improvement in the adhesion of oxide coatings to polymers using AP-PECVD is possible owing to the surface modification of the polymer using plasma in the initial stage of deposition.[21] Scopece et al. studied the deposition of SiO on polypropylene for gas permeation by an atmospheric-pressure plasma jet using hexamethyldisiloxane as the precursor.[22] Watanabe et al. showed that ZnO transparent thin films can be deposited onto polymer substrates by atmospheric plasma in ambient air at room temperature using diethylzinc as the precursor.[23] Therefore, comparing with the reported UV-protective film coating methods, including hydrothermal method and sol–gel method, the AP-PECVD technique could overcome the drawbacks like a complex process, high processing temperature, and use of a solvent due to its single-step, solvent-free, low-temperature, and vacuum-free fabrication process.

In our previous studies,[24,25] we successfully applied AP-PECVD for the fabrication of TiO2 thin films at room temperature, which can exclude the use of solvents during fabrication as compared to the sol–gel method and the spray coating technique. Furthermore, we described the film formation mechanism that was dependent on the process parameters. We reported that the morphologies of the films, which determine their optical properties, are extremely sensitive to the deposition temperature and precursor concentration. Moreover, we demonstrated that TiO2 films fabricated at optimized conditions demonstrate an excellent absorption of UV light and visible light transparency. Accordingly, AP-PECVD has proven to have potential advantages for the fabrication of TiO2 coatings onto polymers, which can improve the stability of polymers against UV irradiation. However, our previous studies were primarily performed using silicon and glass as the substrates; this was done to ensure that the substrate is stable under plasma exposure and the film formation mechanism could be properly discussed. To our knowledge, there are limited reports on the fabrication of TiO2 coatings on polymers via AP-PECVD, and hence the effect of AP-PECVD-derived TiO2 coatings on the improvement of the stability of polymers against UV irradiation has merely been reported.

In the present work, therefore, we investigated AP-PECVD of TiO2 coatings onto transparent plastics and their capability to prevent the UV-induced photodegradation of polymers. In the first part of the present work, we performed the deposition of TiO2 coatings using PMMA and PC as the polymer substrates. The UV-shielding properties of the TiO2 coatings were investigated using ultraviolet–visible (UV–vis) spectroscopy. We also confirmed that the structures of the polymer substrates were unchanged after AP-PECVD, and that the obtained TiO2 coatings had high adhesion, proving that AP-PECVD of TiO2 is applicable for the polymer substrates. Subsequently, in the second part of the present work, we demonstrated the UV-absorbing capability of TiO2 coatings formed on the polymer substrates and revealed their ability to prevent the UV-induced photodegradation of polymer substrates. To this end, we conducted UV-irradiation tests on the polymer films with and without TiO2 coatings and tracked the change in Fourier transform infrared (FTIR) spectra with UV irradiation time and evaluated the degree of photodegradation of polymer substrates.

Results and Discussion

Characterization of the TiO2 Layer Deposited on Polymer Substrates

Figure shows the SEM images of TiO2 coatings deposited on silicon, PMMA, and PC via AP-PECVD. As we optimized the deposition conditions (Table ) based on our previous studies[24,25] that were performed using a Si wafer as the substrate, the purpose of SEM observation is to confirm the structural similarity of the coatings formed on different substrates. The TiO2 films had a granular morphology, and the grain size increased as the deposition time increased. The cross-sectional SEM images indicate that the thickness of the coated TiO2 film fabricated over a deposition time of 3 min was around 400 nm. A dense layer was formed at the bottom of the TiO2 films. It is expected that this dense layer will prevent incoming UV rays from reaching the coated surface and protect the polymers from damage. SEM observations showed no obvious differences in the morphology of the TiO2 coatings on silicon, PMMA, and PC, although it is well known that the morphology of coatings is influenced by the substrate properties. The possible reason may be because the morphological difference caused by substrates was only pronounced at the initial stage but diminished gradually with an increase in deposition time since the substrate was gradually covered by TiO2.

Figure 1

SEM images of TiO2-coated silicon, PMMA, and PC via AP-PECVD.

Table 1

Deposition Conditions of TiO2 Coating on Polymer Substrates

plasma working gas	Ar, 5 L min–1
precursor carrier gas	Ar, 0.5 L min–1
bubbler temperature	25 °C
TTIP vapor pressure[47]	6.0 Pa
substrate temperature	50 °C
distance between the nozzle and the substrate	5 mm

The Fourier transform infrared (FTIR) absorbance spectra of PMMA and PC before and after coating with TiO2 are shown in Figure . It should be noted that the thicknesses of PMMA and PC formed on the silicon wafer were approximately 3 and 1.5 μm, respectively. The FTIR peaks in Figure a at 2951, 1732, 1448, and 1149 cm–1 assigned to the bands of C–H, C=O, O–CH3, and C–O–C,[26] respectively, were observed for both samples. A new peak at 400–800 cm–1, which can be assigned as the Ti–O bond from the amorphous nature of TiO2,[27] was detected for the TiO2-coated samples. This peak was also confirmed for the TiO2 film coated on silicon (Figure S1 in the Supporting Information). Therefore, the result indicates that TiO2 was indeed coated on PMMA by AP-PECVD. Similarly, as shown in Figure b, the FTIR peaks at 2969, 1775, 1503, and 1229 cm–1 correspond to C–H, C=O, C=C, and C–O from PC,[28] respectively. Whereas, a peak in the region of 400–800 cm–1 corresponds to Ti–O from TiO2.[27] This shows that TiO2-coated PC was fabricated. Moreover, for TiO2 coatings supported by silicon, a broad peak in the wavenumber range of 400–800 cm–1 can also be observed. Furthermore, it is worth noting that there was no difference in the peak intensity of the polymers with and without TiO2 coatings, and this suggests that the polymers were not damaged during the AP-PECVD process.

Figure 2

FTIR spectra of PMMA (a) and PC (b) before and after TiO2 coating fabricated over a deposition time of 3 min.

UV–vis spectra of TiO2-coated PMMA and PC fabricated by AP-PECVD are presented in Figure a,b. The spectra of a quartz sheet were subtracted from the spectra of the samples. The TiO2 coatings fabricated over a deposition time of 3 min almost completely shielded the UV light at wavelengths less than 315 nm (UVB and UVC). For PMMA, the transmittance at 250 nm in the UV light region decreased from 62% to nearly 0%, and the transmittance at 550 nm in the visible light region was maintained at a high level of 95%, even after coating with TiO2. Similar results were observed using PC as the substrate. The transmittance at 250 nm decreased from 26% to nearly 0%, and the transmittance at 550 nm was maintained at 95%. These results demonstrate that TiO2 coatings fabricated via AP-PECVD can significantly block UVC and UVB light, and the coatings cause only a slight decrease in the transparency of the polymers.

Figure 3

UV–vis spectra of TiO2-coated PMMA and PC fabricated using AP-PECVD (a, b) over a deposition time of 3 min, and UV–vis spectra of TiO2-coated PMMA and PC fabricated by spin-coating (c, d) with the following fabrication parameters: TiO2 sol (STS-01), a concentration obtained via 7-fold dilution with ethanol, spin-coating speed of 1500 rpm, and spin-coating time of 10 s.

The adhesion strength of the TiO2 coatings to polymers was evaluated by comparing the difference in the UV–vis spectra before and after 1 h of ultrasonication in water and then wiping. The UV–vis spectra of TiO2-coated polymers by AP-PECVD before and after ultrasonication and wiping overlapped for both the TiO2-coated PMMA and the TiO2-coated PC, as shown in Figure a,b. This indicates that the TiO2 coatings did not peel off from the polymer substrates. Therefore, the TiO2 coatings fabricated via AP-PECVD had good adhesion to PMMA and PC, and the coatings were also resistant to ultrasonication and wiping.

In addition, to buttress our conclusion, the adhesion of TiO2 coatings fabricated via sol–gel spin-coating to different polymers was also measured using the same process. TiO2 coatings were prepared on PMMA and PC via spin-coating using the following fabrication parameters:[29,30] TiO2 sol (STS-01, Ishihara Sangyo, Japan, ∼7 nm diameter) was used under a concentration obtained via 7-fold dilution with ethanol at the spin-coating speed of 1500 rpm and the spin-coating time of 10 s; the prepared coating was then dried in air overnight. The UV–vis spectra of PMMA and PC coated with TiO2 via spin-coating are presented in Figure c,d. The transmittance of PMMA in the UV light region decreased dramatically after it was coated with TiO2, while no significant change was observed in the transmittance of PC even after coating with TiO2. This observation shows that the sol–gel-derived TiO2 layer was easily coated on PMMA than on PC. The adhesion of TiO2 to PC is not as strong as that of PMMA due to the difference in the number of C=O and C–O groups present in them.[31] This also explains the better hydrophobic properties of PC than that of PMMA obtained from the water contact angle test. Moreover, the UV–vis spectrum of the PMMA coated with TiO2 via spin-coating changed after ultrasonication in water for 1 h and wiping; the transmittance in the UV light region increased due to particle detachment of the TiO2 coating from PMMA as a result of poor adhesion. A comparison of the change in transmittance at 250 nm after ultrasonication and wiping (Figure ) also shows enhanced adhesion of the TiO2 coating to the polymer substrate fabricated by AP-PECVD than the one fabricated by spin-coating.

Figure 4

Change in transmittance at 250 nm after ultrasonication and wiping of TiO2-coated PMMA and PC prepared by AP-PECVD and the spin-coating method.

These results indicate that the TiO2 coatings fabricated via AP-PECVD show good adhesion regardless of the substrate. We have also confirmed that the TiO2 coating on PC maintained the UV-absorbing performance even after folding and flattening, indicating good adhesion between TiO2 coating and PC (Figure S2 in the Supporting Information). This is likely due to the plasma exposure of the polymer substrate in the initial stage of deposition before a uniform coating of TiO2 covered the substrate.[21] This can lead to the formation of polar groups that can improve the hydrogen and covalent bonds as well as the van der Waals interactions. It can also intensify the creation of chain ends that can enhance interdiffusion and induce a change in the surface topography, which, in turn, could improve the mechanical interlocking between the coatings and polymers.[32−34]

Light Absorption Property of the AP-PECVD-Derived TiO2 Layer

The transmittance values at 250 and 550 nm (Trans.250 and Trans.550), which correspond to the UV and visible light regions, respectively, of TiO2 coating fabricated over different deposition times on different substrates, including quartz, PMMA, and PC, are plotted in Figure . In Figure , the transmittance of only the TiO2 coatings was compared by subtracting the absorption of the corresponding substrates. Trans.250 of the TiO2 coatings decreased from approximately 8% to nearly 0% when the deposition time increased from 1 to 5 min. This observation is due to the increase in the film thickness that followed the Beer–Lambert law, as discussed in our previous study.[24,25] Trans.550 decreased from approximately 95 to 90% as the deposition time increased. This is because of the increase in surface roughness.[25] TiO2 coatings with smaller Trans.250 and larger Trans.550 are preferable because they show a stronger ability to block UV light and higher transparency in the visible light region. It is worthy to note that the Trans.250 value of the TiO2 coatings fabricated for 3 min is as small as the coatings prepared for 5 min, and these coatings also show a Trans.550 value that is as large as that of the coatings fabricated for 1 min. Therefore, the TiO2 coatings fabricated over a 3 min deposition time were suitable for improving the polymer stability against UV irradiation without decreasing the transparency of the polymers. Moreover, it is worth noting that TiO2 coatings supported by different substrates of quartz, PMMA, and PC had similar Trans.250 and Trans.550 values. This indicates that the TiO2 films coated on different substrates had similar optical properties, although it is well known that the coating properties are influenced by the substrate properties. As discussed in Section 2.1, it is possible that the difference in coating properties caused by substrates probably became evident at the initial stage but gradually diminished as the deposition time increased since the substrate was gradually covered by TiO2.

Figure 5

Transmittance at 250 and 550 nm of TiO2 films supported by different substrates of quartz, PMMA, and PC (absorption values of the substrates were subtracted) over a deposition time of 0–5 min.

Stability Improvement against UV Irradiation

PMMA and PC with and without TiO2 coatings were irradiated at different UV irradiation times in the range of 0–5 h, and the FTIR spectra of the samples were obtained, as shown in Figure . For PMMA and PC without TiO2 coatings, there was a dramatic decrease in the peak intensity with an increase in the UV irradiation time. This indicates that the degradation of PMMA and PC without TiO2 coatings occurred during UV irradiation. We have also confirmed by laser microscopy that there was an obvious decrease in the thickness for PMMA films after UV irradiation, as shown in Figure S3 in the Supporting Information. However, for the polymers with TiO2 coatings, no significant change in the FTIR peak intensity of PMMA and PC was observed after UV irradiation. This suggests that the AP-PECVD-derived TiO2 coatings can effectively protect PMMA and PC from UV degradation. The strongest peaks in the FTIR spectra of PMMA and PC are 1732 and 1229 cm–1, respectively. To evaluate the degree of UV protection offered by the TiO2 coatings, we calculated the ratio of FTIR absorbance intensity at a UV irradiation time t (A) to that at t = 0 (A0) of TiO2-coated PMMA and PC at 1732 and 1229 cm–1, respectively. The calculated results are presented in Figure .

Figure 6

FTIR spectra of PMMA (a), TiO2-coated PMMA (b), PC (c), and TiO2-coated PC (d) at different UV irradiation times ranging from 0 to 5 h at the TiO2 deposition time of 3 min.

Figure 7

Ratio of FTIR absorbance intensity at UV irradiation time t (A) to that at t = 0 (A0) for TiO2-coated PMMA at 1732 cm–1 (a) and TiO2-coated PC at 1229 cm–1 (b) in the TiO2 deposition time range of 0–3 min.

As shown in Figure , A/A0 of PMMA and PC without TiO2 coatings (TiO2 deposition time was 0 min) decreased dramatically as the UV irradiation time increased to 5 h, which indicates that PMMA and PC were degraded rapidly under UV irradiation. It is interesting to note that the degradation rate of PC was slower than that of PMMA under the same UV irradiation process. This observation is in line with other reports, and it is ascribed to a photo-Fries rearrangement in the degradation of PC.[35−37] After coating the films with TiO2 over a 0.5 min deposition time, both the A/A0 values of PMMA and PC remained high, indicating that the degradation by UV irradiation was markedly reduced. When the deposition time of coating the TiO2 layer was increased to 3 min, the values of A/A0 after the UV irradiation time of 5 h were improved from 0.18 to 0.90 for PMMA, and from 0.36 to 0.86 for PC, compared to those for polymers without TiO2 coatings, respectively.

Generally, the UV-induced degradation rates of PMMA and PC are first-order,[38,39] that is, d(A/A0)/dt = −k·(A/A0), therefore, the first-order rate plot for the change in absorbance at 1732 and 1229 cm–1 in the respective FTIR spectra of PMMA and PC are shown in Figure , and the degradation rate constants, k, were calculated from these results.[38] The degradation rate constants of PMMA decreased from 0.36 to 0.018 h–1 after it was coated with TiO2 films. Similarly, for PC, the degradation rate constant decreased from 0.23 to 0.029 h–1, which suggests that the TiO2 coatings fabricated via AP-PECVD effectively protected the polymers of PMMA and PC from degradation, and it significantly improved the stability of these polymers against UV irradiation.

Figure 8

First-order rate plot for the change in the absorbance at 1732 cm–1 in the FTIR spectra of PMMA and TiO2-coated PMMA with a deposition time of 3 min (a), and absorbance at 1229 cm–1 of PC and TiO2-coated PC with a deposition time of 3 min (b) under a UV irradiation time ranging from 0 to 5 h.

Conclusions

The stabilities of PMMA and PC against UV irradiation were significantly improved by coating them with TiO2 films via AP-PECVD. TiO2 films with a granular morphology were coated on the polymers of PMMA and PC, and the Ti–O peak in the FTIR spectra indicated the successful coating of TiO2 films on the polymers. The transmittance in the UV light region of PMMA and PC decreased dramatically after it was coated with TiO2 films; however, it remained at a high level in the visible light region. TiO2 films coated on different substrates of quartz, PMMA, and PC had similar optical properties. Moreover, the overlapped UV–vis spectra and the nearly unchanged transmittance values at 250 and 550 nm of TiO2-coated PMMA and PC before and after ultrasonication and wiping indicate that the AP-PECVD-derived TiO2 coatings exhibited good adhesion to the polymers. Finally, the values of A/A0 for PMMA and PC after the UV irradiation time of 5 h were improved from 0.18 to 0.90, and from 0.36 to 0.86, respectively, after they were coated with TiO2 for a deposition time of 3 min. The degradation rate constants of PMMA and PC under UV irradiation decreased from 0.36 to 0.018 h–1 and from 0.23 to 0.029 h–1, respectively, after they were coated with TiO2. This indicates that the TiO2 coatings fabricated via AP-PECVD effectively protected the polymers of PMMA and PC from degradation and significantly improved the stability of these polymers against UV irradiation.

Experimental Section

Materials

The polymer substrates used in this study were obtained by spin-coating PMMA and PC onto a silicon wafer (thickness 280 ± 20 μm, orientation P ⟨100⟩) and a quartz sheet (thickness 1 mm, fused). The polymers spin-coated on silicon wafers were used for Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) measurements, while the polymers spin-coated on quartz sheets were used for UV–vis spectroscopic analysis. PMMA powder and PC pellets were purchased from Tokyo Chemical Industry and Acros Organics, respectively. Toluene (99.5%) and chloroform (99%), which were used as the solvents for spin-coating of PMMA and PC, respectively, were purchased from Sigma-Aldrich and used without further purification. The titanium(IV) isopropoxide (TTIP, 97%) precursor for coating TiO2[40−42] was purchased from Sigma-Aldrich.

Preparation of PMMA and PC Substrates

For the spin-coating of PMMA, a solution with a concentration of 10 wt % was prepared by dissolving PMMA powder in toluene. The spin-coating speed was 3000 rpm and spin-coating time was 60 s.[43,44] Similarly, for the spin-coating of PC, a solution with a concentration of 6 wt % was prepared by dissolving PC pellets in chloroform. The spin-coating speed was 2500 rpm and spin-coating time was 20 s.[45,46] The spin-coated PMMA and PC were cured at room temperature overnight and the samples were further placed in an oven for 2 h at 80 and 60 °C, respectively.

AP-PECVD Coating of TiO2 on Polymers

The TiO2 coating was conducted using an AP-PECVD system using an atmospheric-pressure plasma jet generator (damage-free multi-gas plasma jet, Plasma Factory Corporation, Japan). The details of the film preparation procedure are described in one of our previous studies.[25] The deposition conditions used in this study are presented in Table . The substrate temperature was set to 50 °C to avoid thermal degradation of the polymer substrate. To eliminate the effect of plasma stability in the initial stage, rotatable shielding was used to stop the deposition of TiO2 film during the first 1 min of the AP-PECVD process.

Characterization of TiO2-Coated Polymers

Scanning electron microscopy (SEM, Hitachi S-4800) was used to determine the morphologies of the TiO2-coated polymers. Fourier transform infrared (FTIR, Jasco FT/IR-4100) spectroscopy was used to confirm the chemical structure of the polymers and TiO2-coated polymers. UV–vis transmission spectra were measured using a UV–vis–NIR spectrophotometer (Shimadzu UV-3600 plus). The level of adhesion of TiO2 on the surface of the polymers was evaluated by comparing the difference in the UV–vis spectra before and after ultrasonication in water using an ultrasonic cleaner (100 kHz, AS ONE VS-100 III),[48−50] and wiping using a paper for experimental use (S-200, Nippon Paper Crecia Corporation). The stabilities of polymers with and without TiO2 coatings against UV irradiation were measured using the changes in the FTIR absorbances of the polymers under UV irradiation from a UV lamp. The UV lamp was purchased from Sun Energy Corporation (DGM2501A-01, Japan). The samples were placed below the UV lamp at a distance of 5 cm, and the average spectral irradiance density was 1.6 mW cm–2 nm–1 in the UV light region ranging from 200 to 400 nm.