GB2424382A - Antireflective coatings - Google Patents

Abstract

An antireflective coating on a substrate comprises forming a porous silica layer on the substrate by a sol-gel process and treating the coating so as to prevent or minimise the conversion of residual silanol groups to siloxane linkages and stabilise the antireflective properties.. The coating may be baked until substantially all residual groups are removed. Alternatively a water-retentive agent such as a hydrophilic polymer or oligomer such as polyethylene glycol (PEG), an ether group containing polymer or a hygroscopic salt such as an alkaline earth metal or alkali metal salt e.g. magnesium chloride or calcium chloride may be incorporated in the silica sol before formation of the mesoporous silica layer. Still further the porous layer is treated with an agent which converts silanol groups to a substituent that does not form siloxane linkages, which agent may be liquid or vapour such as a silylating agent e.g. hexamethyldisilazane. Finally an agent such as methyltriethoxysilane which converts silanol groups to a substituent that does not form siloxane linkages may be incorporated in the silica sol before formation of the porous silica layer.

Description

ANTI-REFLECTIVE COATINGS

FIELD OF INVENTION

The present invention relates to improvements in the preparation of antireflective coatings that can be used on video screens and flat panels to provide very low reflectance and high transparency. Further, because of the hardness of such films, they can be successfully used on the windscreens of automobiles and in window glazing.

BACKGROUIND OF THE INVENTION

Polyethylene terephthalate (PET) films with a typical refractive index of about 1.55 reflect 4-5% of incident radiation in the visible range from the film surface. As described in WO 03/052003, this can be reduced to 00.5% reflectance in the visible region by coating the PET film with a single layer of mesoporous silica (pore dimensions of 2 - 50 nm) via the highly cost-effective sol-gel method.

Unfortunately, the reflectivity of the silica anti-reflective coating decays with the passage of time and this makes it commercially less attractive. The cause of this deterioration in properties has often been attributed to the absorption of water in the pores and disruption of the mesoporous framework (P. Nostell, A. Roos and B. Karisson, Thin solid films, 351 (1999) 170-175; M.L.P. da Silva, I.H. Tan, A.P.

Nascimento Filho, E. Galeazzo and D.P. Jesus, Sensors and Actuators B, 91(2003) 362-369.

Further, it has also been found that silica films derived by the sol-gel method, continue to evolve over a long period of time. This involves a slow, dynamic reversible conversion of silanol groups to siloxane groups (G.L.G. Goring and J.D.

Brennan, .J. Mater. Chem., 12 (2002) 3400-3406; R.A. Dunbar, J.D. Jordan and F.B.

Bright, Anal. Chem., 68 (1996) 604).

The present invention is based on the reasoning that these structural changes cause decay in the anti-reflective properties of the film, leading to the finding that impeding the conversion of silanol to siloxane stabilises the structure of mesoporous silica and hence reduces the decay of the anti-reflective properties of the film.

SUMMARY OF THE INVENTION

In its broadest aspect, the present invention provides a method of forming an anti- reflective coating on a substrate, which comprises forming a porous silica layer on the substrate by a sol-gel process, and treating the coating so as to prevent or minimise the conversion of residual silanol groups to siloxane linkages.

In one aspect of the invention, the coating of porous silica is baked until substantially all residual silanol groups are removed.

In another aspect of the invention, a water-retentive agent is incorporated in the silica sol before formation of the mesoporous silica layer. The water-retentive agent retains water in a system. It is considered that the conversion of silanol to siloxane is impeded by the eater-retentive agent, because the conversion is followed by dehydration.

The water-retentive agent may suitably be a hydrophilic polymer or oligomer, such as polyethylene glycol (PEG), an ether group containing polymer, or a hygroscopic salt, especially an alkaline earth metal salt such as magnesium chloride or calcium chloride or an alkali metal salt.

In a further aspect of the invention, the porous silica layer is treated with an agent which converts silanol groups to a substituent that does not form siloxane linkages, for example by conversion of silanol groups to silyl groups, especially to Si-O-SiR3, and more preferably Si-O-Si(CH3) 3. A silylating agent such as (CH3)3SiC 1, (CH3)SiNHSi(CH)3, (CH3)3SiNH-CO-NHSi(CH3)3, (CH3)3SiO-CF3C-NSi(CI-13)3, (CH3)3SiOSO2CF3, (C2H5)3SiC 1, t-(C4F19)(CH3)2SiC1, or Cl(i-C3H8)2SiOSi(i- C3H8)2C1 can be used as the agent.

A suitable agent for this aspect of the invention is hexamethyldisilazane (HMDS) which may be contacted with the porous silica layer as a liquid or vapour.

In a still further aspect of the invention, an agent which converts silanol groups to a substituent that does not form siloxane linkages is incorporated in the silica sol before formation of the porous silica layer. A suitable agent for this aspect of the invention is an alkyl alkoxy silane, such as methyltriethoxysilane (MTES).

Yet another aspect of the invention is a transparent substrate having a anti-reflective coating comprising porous silica formed by a so 1-gel process, in which the coating contains one or more agents which prevent or minimise the conversion of residual silanol groups to siloxane linkages.

The agent incorporated in the coating may suitably be a hydrophilic polymer or oligomer, such as polyethylene glycol (PEG), an ether group containing polymer, or a hygroscopic salt, especially an alkaline earth metal salt such as magnesium chloride or calcium chloride or an alkaline metal salt.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure la is a graph showing reflectivity of the films after various water treatments compared with the untreated films.

Figure lb is a graph showing reflectivity of the films after treatments with CO2 compared with the untreated films.

Figure ic is a graph showing reflectivity of the films at different temperatures compared with the untreated films.

Figure 2 is a graph showing effect of water on the stability of reflectivity compared to untreated films.

Figure 3 is a graph showing effect of PEG on the stability of reflectivity compared to untreated films.

Figure 4 is a graph showing effect of HMDS on the stability of the films compared to untreated films.

DETAILED DESCRIPTION OF THE INVENTION

The reflection loss of an optical surface is related to the difference between the refractive indices of the optical material and its environment. The optical properties of a porous silica layer on the optical surface, when the pore size is much smaller than all optical wavelengths, are such that reflection loss from the surface is markedly reduced. Therefore the porous layer is typically prepared as a nanoporous layer (pore dimensions of 1 - 100 nm, and preferably as a mesoporous layer (pore dimensions of 2-SOnm).

Mesoporous silica coatings may be formed as anti-reflective coatings on optical surfaces using the sol-gel process by (i) forming a so! of colloidal silica in the presence of structure directing agents, (ii) forming a layer of the sol on the intended surface, (iii) polymerising the colloidal silica particles to form a gel, (iv) removing the structure directing agents to create pores within the polymerised network, and (v) baking the gel to form a solid mesoporous silica coating on the surface.

In the present invention the so! is formed by hydrolysis of suitable silica precursors to create silanols, which condense to form colloidal silica as a sol, and then can be cured to form silica networks. In the sol stage, coatings may be formed on the intended substrate by conventional coating methods, such as dip-coating, spin- coating, bar- coating.

Preferably the sol is formed by acid or base catalysed hydrolysis of alkoxysilanes, such as tetraethoxysilane or tetramethoxysilane. Suitable structure directing agents for incorporation into the sol include cationic, non-ionic and anionic surfactants, which can be removed by an aqueous or alcoholic wash. A typical anionic surfactant suitable for this procedure is cetyltrimethylammonium bromide (CTAB).

Cetyltrimethylammonium chloride (CTAC) and cetylpyridininium chloride (CPC1) may also be mentioned. Suitable non-ionic surfactants include polyethylene oxide- polypropylene oxide-polyethylene oxide, such as the commercially available Pluronic P103, P104, P105 and P123 surfactants. As anionic surfactants there may be mentioned long chain fatty acid suiphates and suiphonates.

Over time, any residual silanol groups in the final silica coating revert to siloxane groups, which the present inventors have assessed as contributing to loss of anti- reflective properties of the silica coating.

The present inventors have found experimentally that keeping silica films in water helped stabilise the films over a longer time compared to the dry films. This is not itself a practical means for preserving antireflective properties. However it was recognised that the conversion of silanol to siloxane would be minimised in the presence of moisture, and as a result the anti-reflective properties would be stabilised.

In one aspect of the present invention, this is achieved by the incorporation of a hygroscopic material like PEG, MgCl2, Na2SO4 or CaCI2 into the silica material, which retains moisture and reduces the conversion of silanol to siloxane.

An effective treatment with PEG may be achieved by an aqueous or an ethanolic solution (0.05-0.3% w/v of PEG) using PEG polymers having molecular weight between 200-2000.

The same results are achieved in another aspect of the invention by curing the films for a longer time e.g. for at least 24 hours, as this converts all the silanol groups to siloxane in the films leaving little room for further conversion and any property changes thereafter.

In a further aspect of the invention, treatment of the film with, for example, an alkyl silazane, such as HMDS or hexamethylmethylsilazane, or more generally N1-12Si(CH2+1)3)2, will convert Si-OH to, for example, SiO-SiR3, and lock the structure without any further conversion to Si-O-Si networks.

An effective HMDS treatment of a film may be achieved through the vapour absorption of HMDS for a period of 10, 20, 30 mm etc. or by dipping in liquid HMDS for 30 mm or lh.

In another aspect of the invention, the addition of a proportionate amount of an alkyl alkoxysilane, such as MTES, to the alkoxysilane silica precursor will convert SiUH to Si-CH3 and lock the structure without any further conversion to Si-O-SiR3.

An effective modification of the film using MTES may be achieved by addition of MTES to TEOS at aS - 50% weight/weight ratio.

After the stabilisation treatments of the invention, the films retain their transparency and mechanical hardness along with the anti-reflective properties. The reflectivity of the films is less than 0.5%, and hardness passes the H level in the pencil scratch test, and these properties remain constant over time after the stabilisation treatment. The hardness is measured in accordance with the JIS S400-8-4.

The procedures of the invention may be used to form anti-reflective coatings on plastics substrates, for example polyester films such as PET (polyethylene terephthalate), TAC (triacetylcellulose) that can be used on video screens and flat panels to provide very low reflectance and high transparency. However, because of the hardness of such films, they can also be successfully used on glass substrates, such as the windscreens of automobiles and in window glazing.

The practice and effects of the invention are illustrated in the following experimental

examples.

EXPERIMENTAL

Anti-reflective coatings of mesoporous silica were prepared on a PET substrate by the sol-gel method using a K-bar coater. A two step synthesis method was adopted. In the first step TEOS was hydrolysed with 0.IM HCI for 3h at room temperature. The hydrolysed TEOS was added to an alcoholic solution of CTAB surfactant in different proportions to maintain different weight ratios of surfactant (eg CTAB) to silica, expressed here as P/Si ratio, then mixed for 2h at 40 C and aged for 1 h at RT. The different sols [(1): P/Si = 1.6, (2) : P/Si = 1.2, (3) : P/Si = 1.11 were then coated on a PET substrate using the K-bar coater and the films were cured at 120 C for 10 mm.

The cured films were cooled down to room temperature (RT) and extracted in ethanol for 20 mill to remove the surfactant and give the mesoporous silica. The films were dried at RT and cured for 2 mm at 120 C and the anti-reflective (AR) properties studied.

FIG. 1 shows a comparison of the reflectivity of the AR films under various treatments. The horizontal axes in FIG. 1 (a, b and c) represent the different treatments and the vertical axis represents the reflectivity measured as a percentage.

All treated films have been compared with the untreated films aged for the same length of time. The results in FIG. 1 (a and b) prove, in contrast to the conventional belief, that the presence of water or CO2 does not increase the reflectivity of the films.

A dry and shielded atmosphere on the other hand of the fridge, the incubator or the empty box increases the reflectivity and the percentage of increase is more than the untreated films.

The films prepared using sols (1), (2) and (3) were then stored in water and the reflectivity studied over a length of time and compared with the untreated films.

Films are constantly stored under water and reflectivities measured after 15 mm of drying. FIG. 2 shows the change in reflectivity of the films stored under water [curves Water (I) - P/Si = 1.6, Water (2) - P/Si = 1.2, Water (3) - P/Si = 1.1] compared with the untreated samples [curves Exptl - P/Si = 1.6, Expt2 - P/Si = 1.1].

The untreated samples showed a constant increase in reflectivity whereas the water treated samples either showed a constant reflectivity or a reflectivity that decreased with time. It was therefore seen that, contrary to the conventional belief, water did not increase the reflectivity but helped in stabilising it. The reflectivity however increased gradually when the films were taken out of water and kept under the normal laboratory atmosphere and the decay in the AR property however was much more gradual compared to the untreated films.

As leaving the films submerged in water is not a viable solution towards stabilising the reflectivity, hygroscopic materials such as PEG, CaC12 or MgC12 were incorporated into the films. This would help in retaining the moisture and maintaining the stability of the films. Films (P/Si = 1.2) were dipped in either ethanolic or aqueous solutions of PEG (0.05% to 0. 3%) of molecular weights between and 2000. FIG 3 shows the change in reflectivity of the films dipped in PEG solution (0.2% w/v of molecular weight 400) in water and ethanol compared to the untreated film [curve (1) untreated film; (2); 0.2% w/v of PEG in ethanol; (3) 0.2% w/v of PEG in water]. The films were dipped in the PEG solution for 24h and dried at room temperature prior to measuring the reflectivity. Following PEG treatment, the films remained stable for longer time compared to the untreated films.

The films (P/Si = 1.2) were also treated with HMDS both in the liquid form and also by vapour at RT for different intervals of time. For the liquid treatment the cured films were dipped into HMDS for 10, 30 and 60 mm followed by washing in ethanol for 2 mm and curing at 120 C for 2 mm. FIG 4 shows the change in reflectivity of the films dipped in liquid HMDS compared to the untreated films. As shown in the Figure, the treatment with HMDS stabilises the films for at least 1 month keeping the reflectivity between 0.4% and 0.6% whereas the reflectivity of the untreated samples increases from 0.4% to 1.8%. The I h HMDS treatment however gave the best results.

Similar behaviour was observed with films treated with HMDS vapour.

Claims (27)

1. A method of forming an anti-reflective coating on a substrate, which comprises forming a porous silica layer on the substrate by a sot-ge! process, and treating the coating so as to prevent or minimise the conversion of residual silanol groups to siloxane linkages.

2. A method according to claim 1 in which a water-retentive agent is incorporated in the silica so! after formation of the porous silica layer.

3. A method according to claim 2 in which the water-retentive agent is a hydrophilic polymer or oligomer.

4. A method according to claim 2 in which the water-retentive agent is an ether group containing polymer.

5. A method according to claim 3 in which the water-retentive agent is polyethylene glycol.

6. A method according to claim 2 in which the water-retentive agent is a hygroscopic salt.

7. A method according to claim 2 in which the water-retentive agent is an alkaline earth metal salt or an alkali metal salt.

8. A method according to claim 7 in which the water-retentive agent is magnesium chloride or calcium chloride.

9. A method according to claim 2 in which the porous silica layer is treated with an agent which converts silanol groups to a substituent that does not form siloxane !inkages.

10. A method according to claim 9 in which the porous silica layer is treated with an agent which converts -Si-OH groups to -Si-O-SiR3 groups.

11. A method according to claim 9 in which the porous silica layer is contacted with a liquid agent.

12. A method according to claim 9 in which the porous silica layer is exposed to a vapour agent.

13. A method according to claim 11 or 12 in which the vapour agent or the liquid agent is a silylating agent.

14. A method according to claim 11 or 12 in which the agent is hexamethyldisilazane.

15. A method according to claim 1 in which an agent which converts silanol groups to a substituent that does not form siloxane linkages is incorporated in the silica so! before formation of the porous silica layer.

16. A method according to claim 15 in which the agent is an alkyl alkoxy silane.

17. A method according to claim 15 in which the agent is methyltriethoxysilane.

18. A method according to any one of claims ito 17 in which the antireflective coating is a mesoporous silica layer.

19. A transparent substrate having an anti-reflective coating comprising porous silica formed by a sol-gel process, in which the coating contains one or more agents which prevent or minimise the conversion of residual silanol groups to siloxane linkages.

20. A substrate according to claim 19 in which the water-retentive agent is incorporated in the coating.

21. A substrate according to claim 20 in which the water-retentive agent is a hydrophilic polymer or oligomer.

22. A substrate according to claim 21 in which the water-retentive agent is an ether group containing polymer.

23. A substrate according to claim 20 in which the water-retentive agent is polyethylene glycol.

24. A substrate according to claim 20 in which the water-retentive agent is a hygroscopic salt.

25. A substrate according to claim 20 in which the water-retentive agent is an alkaline earth metal salt or an alkali metal salt.

26. A substrate according to claim 25 in which the water-retentive agent is magnesium chloride or calcium chloride.

27. A substrate according to any one of claims 20 to 26 in which the antireflective coating is a mesoporous silica layer.