Highly efficient UV-absorbing thin-film coatings for protection of organic materials against photodegradation

Abstract

UV-protective coatings have been prepared by the sol–gel method, to reduce the destructive effects of UV radiation on easily photodegradable devices, i.e. those containing organic compounds, such as dyes and pigments or plastic materials to be used in outdoor applications. A benzophenone derivative (2,2-dihydroxy, 4-methoxybenzophenone), showing high photostability and strong absorption in the UV range, was embedded in an ormosil matrix. The usage of an organically modified silica matrix enhances the solubility of the UV absorber in the matrix allowing the preparation of highly loaded coatings. The protective coatings, prepared at room temperature, require no thermal treatment after deposition, allowing therefore their application on temperature sensitive materials. The resulting films have a strong absorption in the UV range with a thickness of only 1.0 µm. In addition, the UV-absorbing coatings are transparent, colourless, and exhibit high optical quality. The UV-protective coatings offer an easy method to prevent the photodegradation of organic materials without altering their optical properties in the visible region.

Fluorescent rhodamine dye-doped thin ormosil films were coated with a UV-protective layer in order to study their effectiveness in the reduction of the photodegradation of the dye upon irradiation with UV light. The degradation of 20% of the molecules in coated samples was 14 times slower than that of the uncoated samples. The effective temperature range of the UV-protective coatings was established by measuring the photodegradation of the samples at different temperatures.

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Introduction

The need of protecting organic matter against solar irradiation in outdoor applications has motivated numerous research works about photodegradation and its mechanisms.^1,2 The damaging UV radiation is responsible for the discoloration of dyes and pigments, weathering, yellowing of plastics, loss of gloss and mechanical properties (cracking), sunburnt skin and other problems associated to UV light. Paints, plastics, wood and cosmetic manufacturers have a great interest in offering products that remain unaltered for long periods of time under the worst light exposure conditions.^3–6

UV absorbers have been used in order to reduce these damaging effects and achieve an adequate conservation of the properties of the materials. In order to offer an effective protection against UV irradiation, one of the requirements for the UV absorber molecules is the ability of transforming the absorbed radiation energy into less damaging thermal energy via a photophysical process.^1,10–15

UV absorbers with an intramolecular hydrogen bond (e.g, o-hydroxybenzophenones, 2-(2-hydroxyaryl)-benzotriazoles and 2-(2-hydroxyaryl)-1,3,5-triazines) are used for protection against photodegradation owing to their exceptional photostabilities and small quantum yields of photodecomposition (10⁻⁷ to 10⁻⁶).^7,8 These compounds possess an efficient radiationless mechanism of energy dissipation. The molecule in the first excited state (S₁) undergoes an excited state intramolecular proton transfer (ESIPT) to create another species in its first excited singlet state (S′₁). This excited proton–transferred species loses its energy by a non-radiative decay process as thermal energy (S′₀). It should be noted that the contribution of this energy to the thermal degradation of the material is negligible compared to the much stronger thermal energy reaching the sample from the solar radiation⁹ The fundamental form of the UV absorber (S₀) is regenerated by a reverse proton transfer mechanism,^10–15 illustrated in Fig. 1. Klöpffer¹² reported that certain derivatives of benzophenone may undergo a variety of mechanistic pathways of energy dissipation, following the excitation to S₁, including the formation of triplet states (T₁ and T′₁). The ESIPT mechanism is responsible for the exceptionally high photostabilities of these molecules. The interruption of the intramolecular hydrogen bond inhibits the progress of the ESIPT mechanism resulting in the loss of the photostability of the UV absorber. The environment around the stabilizer is a critical factor for the efficiency of the mechanism of energy dissipation.¹ H-acceptor moieties are capable of disrupting the internal hydrogen bond of these compounds increasing their photoreactivity and, hence, their photodegradation. The absorbed energy in these systems is mainly released by radiative processes.

Fig. 1 Mechanism of dissipation of energy for an UV absorber with an intramolecular hydrogen bond.

UV absorbers embedded in organic coatings have been extensively used to inhibit the photodegradation of polymers and other articles preventing the damaging UV radiation from reaching the substrate.^16,17 However, the use of polymeric matrices is limited by the low stability of the polymer upon UV irradiation. The free radicals generated in the photo-decomposition of the organic matrix can destroy the active form of the stabilizer, reducing the effectiveness of the protective coating.¹

The sol–gel method allows the preparation of transparent, solid and porous inorganic matrices at low temperature, and the incorporation of organic molecules in its porosity.^18,19 The method involves reactions of hydrolysis and condensation of silicon alkoxides to produce a 3-D, amorphous, porous and stable silica network. Organically modified alkoxide precursors are an excellent modifier of the structural properties of these materials’ porous surface, as they allow controlling the polarity and size of the pores according to the nature and amount of the non-hydrolysable organic group used.^20,21 In addition, these matrices allow the introduction of large amounts of organic molecules, especially those with large organic modifiers.

The sol–gel method has been used to prepare coatings and nanoparticles with UV-absorbing properties. These materials have been applied to cosmetic products,^22,23 textile industry,^24,25 wood²⁶ and polymers²⁷ to reduce the destructive effects of UV radiation. UV-absorbing films based on metal oxides were deposited on glass to reduce the UV light transmission of windows.^28,29 These coatings can not be used to protect heat-sensitive materials against UV radiation as they must be cured at relatively high temperatures. Glass coatings containing organic UV-absorbers were prepared by the sol–gel method,³⁰ consisting of SiO₂ and TiO₂ films doped with relatively low amounts of UV-absorbing molecules. These films, however, are not fully transparent in the visible region and therefore affect the optical properties of the substrate.

The aim of this work is to prepare a UV-protective coating, with excellent optical properties and high transparency in the visible range, by the incorporation of 2,2-dihydroxy- 4-methoxy-benzophenone in a sol–gel ormosil matrix. Large organic modifiers in the matrix (Ph groups) will be used to screen the strongly polar silanol groups of the surface of the pores and avoid the disruption of the intramolecular hydrogen bond of the UV-absorber³¹ allowing the ESIPT energy dissipation process to take place.

In order to determine the effectiveness of the UV-protective coating, fluorescent rhodamine dye-doped films will be deposited on glass substrates. The protective UV-absorbing film will be deposited as a second layer on top of the fluorescent film. The photodegradation of the fluorescent dye, in samples coated and uncoated with the UV-absorbing films, will be carried out at different temperatures in order to study the effect of temperature on the photostability of the dye and the UV absorber in the ormosil matrix, and establish the effective temperature range of the protective coatings.

Experimental

Materials

Phenyltriethoxysilane (PhTES) was purchased from ABCR. Tetraethoxysilane (TEOS) and 2,2-dihydroxy-4-methoxy-benzophenone were purchased from Aldrich and absolute ethanol from Merck. The Rhodamine-101 perchlorate dye was from Lambdachrome. The chemical structure of the dye is shown in Fig. 2. Doubly-distilled water has been used for all preparations. Nitric acid, used as a catalyst, was from Panreac Quimica.

Fig. 2 Chemical structure of the Rhodamine-101 dye.

Preparation of the protective coatings

Mixtures of the corresponding amounts of TEOS and PhTES were dissolved in ethanol. Water, in a hydrolysable group ∶ H₂O molar ratio of 2 ∶ 2, and nitric acid, in a Si ∶ H⁺ molar ratio of 2 ∶ 0.17, were added to the solution. The sol was allowed to hydrolyse under stirring for 24 h at room temperature, prior to the addition of an ethanolic solution of the UV-absorber. The deposition of the films was carried out using the spin-coating technique, with the sample holder spinning at 2000 rpm on glass substrates or as a second layer on Rhodamine-101 doped ormosil coatings. The samples were dried at room temperature for 2 d followed by 2 h at 50 °C. Samples were prepared, using different TEOS ∶ PhTES ∶ UV-absorber molar ratios in order to find a compromise between the UV-absorber loading in the matrix and the optical properties of the films. A TEOS∶PhTES∶UV-absorber molar ratio of 1 ∶ 1 ∶ 0.4 was chosen for the preparation of the protective coatings described in this work.

In order to test the efficiency of the UV-protective coatings a fluorescent Rhodamine-101 dye was used to measure its degradation with and without the protective coating. This dye was chosen due to its high sensitivity to UV light, to allow carrying out the experiments in a reasonable period of time.

Preparation of the Rhodamine-101 doped ormosil coating

The Rhodamine dye was embedded in an organically modified silica matrix to allow the preparation of highly concentrated coatings. The precursor solutions used for the deposition of the thin films consist of mixtures of tetraethoxysilane (TEOS), phenyltriethoxysilane (PhTES) and ethanol in a 0.7 ∶ 0.3 ∶ 1 molar ratio. Water and HNO₃ were added to allow the acid catalysed hydrolysis and condensation of the precursor alkoxides. The water and the acid were added in a hydrolysable group ∶ H₂O molar ratio of 2 ∶ 1, and a Si ∶ H⁺ molar ratio of 2 ∶ 0.17, respectively. The sol was allowed to hydrolyse at room temperature for 24 h. An ethanolic solution of the Rhodamine-101 dye was added to the alkoxide solution after the hydrolysis to obtain a dye ∶ Si molar ratio of 1 ∶ 1000. The films were deposited on glass substrates using the spin-coating technique, with the sample holder spinning at 2000 rpm. The films were dried for 2 d at room temperature followed by 24 h at 100 °C

Irradiation of the samples

The samples were irradiated with a 200 W Xe–Hg lamp for several hours. A water cell was used to filter out the IR radiation in order to avoid the heating of the sample during irradiation. The UV light was conducted through a fibre optics to the irradiation cell. The measured light power reaching the sample was 0.4 mW cm⁻² in the UV-B range, mainly responsible for the degradation of the sample. This light power is more than 5 times the average UV-B component of the sun at midday in Madrid (calculated by Modtran 3.7 program). The sample was placed in a thermostatic holder (cell) to allow measuring the degradation of the coated and uncoated samples at constant temperatures during the irradiation.

Characterization

The absorption spectra of the resulting films were measured in a Varian Cary 50 Bio UV-visible spectrophotometer in the 275–800 nm range. The photodegradation kinetics were carried out monitoring the absorption of the samples at the wavelength of the maximum absorption of the samples (580 nm) vs time of irradiation with UV light, using an Instruments Systems 320 optical spectrum analyzer. The kinetic constants of photodegradation were calculated using a bi-exponential decay equation. The thickness of the films was measured with a Mitutoyo SV-3000 rugosimeter.

Results and discussion

The absorption spectrum of the UV-protective coating on a glass substrate is shown in Fig. 3. The film presents very strong absorption in the 275–400 nm range and very high optical quality. In addition, the coating shows a very high transparency in the visible having only a very small absorption tail between 400–420 nm, responsible for its slight yellowish colouration. The resulting films have a thickness in the range of 0.9–1.1 µm. The drying process of the UV protective films was carried out at 50 °C allowing the application of these coatings on heat sensitive materials.

Fig. 3 Absorption spectrum of 2,2-hydroxy-4-methoxybenzophone in an ormosil film with a TEOS/PhTES/BP molar ratio of 1/1/0.4.

In order to establish the efficiency of the UV-protective coating a dye-doped ormosil film and the same layer coated with a UV-absorbing layer were exposed to a high power UV lamp. A very sensitive fluorescent dye (Rh-101) was chosen to allow the degradation of the dye in a reasonable period of time. The Rh-101 dye-doped ormosil films were deposited by the spin-coating technique on glass substrates. The resulting films have a thickness of 0.8–0.9 µm. The absorption spectra of the dye in the ormosil matrix, given in Fig. 4, show a shoulder around 540 nm and a strong band around 580 nm in the wet sample, which is progressively shifted to 580 nm as the drying process takes place. The emission spectra of the Xe(Hg) arc-lamp are shown in Fig. 5.

Fig. 4 Absorption spectrum of Rhodamine-101 in ormosil film with a molar ratio PhTES ∶ TEOS ∶ Rh-101 of 0.3 ∶ 0.7 ∶ 0.001, coated and uncoated with the protective film.

Fig. 5 Emission spectra of the Xe(Hg) arc-lamp (after IR filter).

When the protective coating is deposited on top of the Rhodamine-101 dye-doped film, the same effect is observed as in the wet Rh-101 doped films. This effect is due to the solvents and uncondensed silanol groups of the protective coating sol, which penetrates into the porosity of the Rh-101 doped film, affecting the environment of the fluorescent dye.

The UV-visible absorption spectra of Rh-101 films, uncoated and coated with a UV-protective film are given in Fig. 4. The protective coating has no significant absorption in the visible range of the spectrum and therefore does not affect the absorption band of the fluorescent dye. On the other hand, the very strong absorption of the UV-absorber reduces drastically the UV light reaching the fluorescent dye, which is responsible for its photodegradation.

In order to determine the effectiveness of the UV-protective coating, the coated and uncoated samples were exposed to intense UV radiation, monitoring the intensity of the absorption maximum as a function of the irradiation time. The photodegradation of the dye molecules in the coated samples was much slower compared to the uncoated samples at 25 °C, as shown in Fig. 6. The degradation of 20% of the dye molecules is 14 times slower in coated samples. In order to establish the contribution of the matrix and the UV absorbing molecules of the coating on its protective properties, a fluorescent Rh-101 film was coated with an ormosil film with the same composition of the protective coating without the UV absorber molecules. The degradation curve of this sample, given in Fig. 6 was slightly slower than that of the uncoated Rh-101 film, showing that the UV absorbing molecules are mainly responsible for the absorption in the UV range. Since the ormosil matrix has a very low absorption in the near UV range (280–400 nm), the effect can be due to the reflection of light on the surface of the protective coating, reducing slightly the intensity of the UV irradiation on the sample.

Fig. 6 Photodegradation of the Rh-101 films: (a) uncoated; (b) coated with an ormosil coating without UV absorber; (c) coated with a UV protective ormosil coating.

The photodegradation of the fluorescent dye in the ormosil matrices upon exposure to UV-radiation was measured at different temperatures: 10, 25, 50 and 65 °C. A faster degradation was observed as the temperature of the sample is increased, as shown in Fig. 7. Even at 65 °C the coating of the sample with a UV-absorbing layer shows an effective protection of the dye molecules against photodegradation.

Fig. 7 Photodegradation of uncoated fluorescent Rh-101 samples at 10, 25, 50 and 65 °C.

The kinetics of the photodegradation process was found to follow a bi-exponential equation based on the correlation coefficients and χ² (eqn (1)).

This behaviour is probably due to the presence of two different chemical environments on the porosity of the ormosil matrix (pores rich in OH groups and pores rich in phenyl groups), where the molecules are located, that can affect the photodegradation process. The existence of two different pore environments in ormosil matrices, one of them rich in the organic modifying groups was pointed out in an early work,³¹ when photochromic-dye doped ormosil coatings were prepared. The kinetic constants calculated from the decay curves of the coated and uncoated samples are shown in Table 1 and eqn (1) (bi-exponential equation used to adjust the kinetics of photodegradation of the dye).

Table 1 Kinetic constants of the photodegradation process of coated and uncoated samples at different temperatures (kinetic constants 10⁻⁵ s⁻¹)

Temp./°C	10		25		50		65
	k 1	k 2	k 1	k 2	k 1	k 2	k 1	k 2
Sample
Uncoated	3.67	14.00	6.04	30.2	9.88	38.5	10.22	70.31
Coated	0.04	3.86	0.16	3.88	0.61	4.73	1.15	8.42

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A substantial decrease of both kinetic constants is observed by the application of the protective coating, which accounts for the efficient protection of the coating against photodegradation. It is interesting to observe that the protective coating has a much larger effect on the fast kinetic constant (k₁), and diminished as temperature is raised. This issue, probably related to the interaction of the dye molecules with the pore surface in the matrix will be the subject of future studies.

Conclusions

A UV absorber has been incorporated into thin ormosil film matrices prepared by the sol–gel method, resulting in highly efficient UV-absorbing films. The usage of an organically modified alkoxide precursor allowed the incorporation of large amounts of UV absorber molecules in the matrix, resulting in a strong absorption in the UV range. The protective coating does not affect the optical properties of the substrate as it shows no significant absorption in the visible range. The mild drying process needed for the preparation of the protective films allows them to be used on heat sensitive materials.

Fluorescent dye-doped ormosil films have been prepared and coated with the UV-absorbing film, to study the effectiveness of the coating for the protection of the fluorescent dye against photodegradation. A substantial decrease in the photodegradation of the dye, upon irradiation with UV-light, was observed in the samples coated with the protective layer. At 25 °C, the photodegradation of the coated samples was 14 times slower than in uncoated samples. The ability to increase the durability of outdoor products that can withstand uncoated the solar radiation for months or years, by a factor of 14 makes the protective coatings very attractive for usage in commercial applications.

The protective coatings were tested for operating at different temperatures, showing efficient performance even at 65 °C, which entitles the protective coatings to be used in outdoor applications.

Due to their high transparency in the visible range and its low temperature processing, the UV-absorbing coatings can be used in a wide range of applications, especially for the protection of optical devices.

Acknowledgements

This work was supported by a research project from Comunidad de Madrid (GR/MAT/0445/2004), and from MEC (NAN2004-09317-C04-02 and MAT2005-05131-C02-01). Pilar García-Parejo is grateful to Calvo Rodés (INTA) for a doctoral scholarship.

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