Hydrophobic coating process for aeronautical glazing

Abstract

The invention relates to a method for hydrophobicizing a glass substrate, preferably aircraft cockpit glazing, comprising the following successive steps: formation of a primary layer, preferably of silica, on one of the main surfaces of the glass substrate; grafting a layer of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane onto the primary layer; and optionally, a polishing step with a water / alcohol mixture. It also relates to a hydrophobic glass substrate obtained by this method, comprising a transparent glass substrate covered, on one of its main faces, with a primary silica layer, said primary silica layer being grafted with (C3-C8 alkyl)silyl groups.

Description

Title of the invention: Method for hydrophobizing aeronautical glazing

[0001] The present invention relates to a glass substrate, in particular aircraft cockpit glazing, the surface of which has been made hydrophobic by grafting non-fluorinated alkylsilanes and a method of manufacturing such a substrate.

[0002] Hydrophobic properties are sought for windshields in the field of transport, in particular for aircraft such as airplanes and helicopters, for which it is required by the certification authorities (aeronautical regulations CS25 and FAR25) to provide, in the event of rain, sufficiently large and transparent glazing areas to guarantee sufficient vision without pilot intervention. These transparent vision areas can be created by sweeping the rain away using windshield wipers. The flow of raindrops is encouraged by hydrophobic properties of the surface of the glazing.

[0003] Several types of aeronautical glazing are used, but it is generally a laminated assembly consisting of several sheets of structural materials bonded together by adhesives or so-called "interlayer" sheets. The outermost sheet of structural material of the glazing is generally made of glass, a material which intrinsically has hydrophilic properties. Limiting vision disturbances due to rain then requires the implementation of a hydrophobization process for the outer face of this glass sheet.

[0004] The hydrophobization process must both allow effective evacuation of rainwater and preserve the optical quality of the glazing. High contact angles (0) of a water droplet are sought which, on the one hand, reduce the wetted glazing surface and, on the other hand, increase the aerodynamic forces exerted on the drops, thus allowing them to flow during flight. Furthermore, hydrophobic treatments reduce the adhesion forces of the drops and thus promote their flow either under the effect of gravity or under the effect of aerodynamic forces. Hydrophobic treatments thus prevent the formation of continuous water films which correspond to the worst optical degradations.

[0005] Two parameters can be used to characterize the hydrophobic nature of the surface of a glazing: - the contact angle (0) of a drop of water deposited on a horizontal substrate, and - the detachment volume of a drop of water deposited on an inclined surface.

[0006] The hydrophobicity of the glazing must of course resist abrasion for as long as possible, in particular by dust present in the atmosphere, and by in influences from the natural environment (water causing hydrolysis of deposits, UV radiation) and industrial (cleaning products). However, known hydrophobic treatments do not guarantee sufficient hydrophobicity throughout the lifetime of the glazing.

[0007] Thus, visibility in the event of rain generally becomes insufficient when the contact angles of the drops are less than 50°.

[0008] The processes for hydrophobizing glazing must be able to be implemented in situ, that is to say on glazing mounted on the aircraft, so as to allow simple and rapid maintenance. We will then speak of regeneration of the hydrophobic function. As a result, a good hydrophobic treatment, in addition to being durable, must require limited equipment (including safety equipment), a limited number of steps and a short implementation time.

[0009] It is known to increase the hydrophobicity of vehicle glazing by grafting perfluorinated alkylsilanes onto the external surface of the glass sheet, generally after prior creation of a silica primer layer by a sol-gel process, by magnetron sputtering or by chemical vapor deposition (CVD).

[0010] Perfluorinated compounds are, however, considered harmful to the environment and human health. The persistent nature of these compounds in the environment and in the human body, combined with adverse health effects, has recently increased regulatory pressure regarding their use, particularly through the European REACH and POP regulations. This could lead to a massive ban on these compounds by 2025 or, at the latest, 2030.

[0011] The present invention aims to propose a process for hydrophobizing glass substrates intended for the aeronautical industry, which makes it possible to obtain glazing having a satisfactory, persistent and easily regenerable hydrophobic character on the aircraft without the use of perfluorinated compounds.

[0012] As part of its research aimed at finding non-toxic molecules capable of replacing the perfluorinated alkylsilanes used until now for the hydrophobization of glass substrates, the Applicant tested a large number of hydrophobic functional silanes with a small non-hydrolyzable hydrocarbon chain, i.e. comprising from 1 to 8 carbon atoms (Ci to C8).

[0013] All the alkylsilanes tested made it possible, after deposition, to obtain satisfactory contact angles of a water droplet (0), i.e. greater than 50°, or even greater than 95°, and even for some greater than 100°, as well as detachment volumes little different from those obtained after grafting of 1H,1H,2H,2H-perfluoroalkyltriethoxysilane (SiF5E). However, a degradation of the hydrophobic performances following exposure to water and UV was observed.

[0014] The present invention is based on the discovery that fatty chain alkylsilanes C3 to C8 alkyl chains provided satisfactory durability to both hydrolysis and UV. Alkyl chains smaller than C3 provided insufficient resistance to hydrolysis, and longer alkyl chains (>C8) resulted in deposits that were too sensitive to UV radiation.

[0015] The present application thus relates to a method for hydrophobizing a glass substrate comprising the following successive steps: - formation of a primary layer, preferably silica, on one of the main surfaces of the glass substrate, - grafting of a layer of trialkoxy-(C3-C8 alkyl)-silanes and / or trichloro-(C3-C8 alkyl)-silanes onto the primary layer.

[0016] Another technical problem encountered during the tests was the milky appearance of the alkylsilane deposits obtained, observed more particularly in the case of application by wiping. The treated surface of the glazing was slightly diffusive, thus unacceptably reducing the transparency and brightness of the glazing. This problem was overcome by a polishing step, carried out after the step of grafting the layer of trialkoxy-(C3-C8 alkyl)-silanes and / or trichloro-(C3-C8 alkyl)-silanes. This polishing step is advantageously carried out by rubbing the surface using a cloth soaked in a solvent, advantageously a mixture of water and alcohol, until specular reflection of the light is restored by the entire treated surface. The water / alcohol mixture is preferably a water / isopropanol mixture and contains from 30 to 90% by weight of water, preferably from 50 to 80% by weight of water.

[0017] The method for hydrophobizing a glass substrate of the present invention therefore further comprises, after the grafting step, an additional polishing step.

[0018] The present application also relates to a glass substrate obtained by the above process. This hydrophobic glass substrate comprises a transparent glass substrate, covered, on one of its main faces, with a primary layer, preferably made of silica, said primary layer being grafted with (C3-8 alkyl)silyl groups.

[0019] The glass substrate obtained by the method of the invention is preferably a cockpit glazing of an aircraft, in particular an aircraft windshield.

[0020] In a preferred embodiment, the trialkoxy(C3-C8 alkyl)-silane is a trialkoxy-alkylsilane of formula CnH2n+iSiR3 where n = 3 - 8, preferably n = 5 - 8, and each R represents a C1-3 alkoxy group, preferably a methoxy or ethoxy group.

[0021] The trichloro(C3-C8 alkyl)-silanes are preferably trichloro-alkylsilanes of formula CnH2n+iSiR3 where n = 3 - 8, preferably n = 5 - 8, and each R represents a chlorine atom.

[0022] The best performances are obtained with a trialkoxysilane or trichlorosilane of formula CnH2n+iSiR3 where n = 8.

[0023] To obtain good durability of the hydrophobic coating on the glass substrate, it is essential to first form a primer layer, preferably made of silica, on the surface to be treated.

[0024] For this primer layer to be effective in anchoring the hydrophobic coating of alkyl-trialkoxy-silane and / or alkyl-trichlorosilane on the substrate, it is important that it contains as many active silanol (Si-OH) groups as possible, i.e. groups capable of reacting with the silanol groups released by the hydrolysis of the alkoxy groups or chlorine atoms.

[0025] Furthermore, it is recommended to precede the step of forming the primary layer with a step of polishing the substrate, for example by means of a felt impregnated with an aqueous suspension of fine abrasive particles, followed by rinsing with water and drying. In the case of regeneration of the hydrophobic function, this step is even essential to completely eliminate the pre-existing degraded layer.

[0026] The formation of the primary silica layer is carried out by a sol-gel step in which a tetrachlorosilane and / or a tetraalkoxysilane, preferably tetraethoxysilane, is hydrolyzed in an acidic hydro-alcoholic medium.

[0027] To carry out this step, an appropriate amount of tetrachlorosilane and / or tetraalkoxysilane is dissolved in a water / alcohol mixture, typically an acidic water / isopropanol mixture, containing 3 to 50% by weight, preferably 5 to 15% by weight of water whose pH has previously been adjusted to a value between 0 and 3. The tetrachlorosilane and / or tetraalkoxysilane content of this hydroalcoholic solution is advantageously between 0.1 and 1% by weight, preferably between 0.15 and 0.8% by weight.

[0028] It is preferable not to apply the hydroalcoholic solution immediately after dissolving the tetrachlorosilane / tetraalkoxysilane, but to allow the hydrolysis to take place at room temperature (20 - 25 °C) for a period of between 2 minutes and 2 hours, preferably between 3 minutes and 1 hour, in particular between 5 minutes and 30 minutes.

[0029] The acidic hydroalcoholic solution of tetrachlorosilane / tetraalkoxysilane can be applied in principle by any technique allowing the formation of a thin liquid film on the surface of the substrate. Examples include application by immersion, by liquid curtain, by spraying or by wiping. Wiping is most suitable for regenerations on aircraft. Preferably, spraying, or nebulization, and application using a cloth soaked in the hydroalcoholic solution of tetrachlorosilane / tetraalkoxysilane will be used.

[0030] After application to the surface of the glass substrate, the liquid film formed is allowed to dry for a period of at least 2 minutes, preferably at least 15 minutes.

[0031] After drying of the first deposit, it is possible to repeat the application of the hydro-alcoholic tetrachlorosilane / tetraalkoxysilane solution until the desired thickness of silica primer layer is obtained.

[0032] The silica primer layer advantageously has a thickness of between 5 nm and 250 nm, preferably between 10 nm and 100 nm, in particular between 15 nm and 75 nm.

[0033] The grafting of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane is carried out after at least 2 minutes of drying of the silica primer layer at room temperature.

[0034] The step of grafting the alkyltrialkoxysilane or alkyltrichlorosilane is carried out, like the formation of the silica primary layer, by a sol-gel step in which the alkyltrialkoxysilane and / or the alkyltrichlorosilane is hydrolyzed in an acidic hydro-alcoholic medium. It can be carried out in a similar manner to that used for the formation of the primary layer.

[0035] The grafting composition is an acidic hydroalcoholic (water / isopropanol) solution of (C3-C8 alkyl)-trialkoxysilane and / or (C3-C8 alkyl)-trichlorosilane. The content of (C3-C8 alkyl)-trialkoxysilane and / or (C3-C8 alkyl)-trichlorosilane in the acidic hydroalcoholic solution is preferably between 0.1% and 4% by weight and in particular between 1 and 3.5% by weight.

[0036] The water / alcohol mixture in which the alkyltrialkoxysilane and / or the alkyltrichlorosilane is dissolved contains 5 to 70% by mass, preferably between 10 and 50% by mass of water.

[0037] The hydroalcoholic solution of (C3-C8 alkyl)-trialkoxysilane and / or (C3-C8 alkyl)-trichlorosilane is advantageously left at room temperature for a period of approximately 10 minutes to approximately 3 hours, to allow hydrolysis, before being applied by spraying, watering or wiping onto the glass substrate covered on one of its main faces with the primary silica layer.

[0038] To evaluate the hydrophobicity of deposits, procedures known in the technical field of hydrophobic coatings are used:

[0039] Contact angle measurement:

[0040] The measurement of the contact angle of a water drop (0) at equilibrium is carried out using a Krüss MSA (Mobile Surface Analyzer) goniometer on a 2 μl drop. The observation of the drop is done using a fast camera which takes the photos on which the contact angle is then recorded. The higher the value of the contact angle, the more hydrophobic the surface of the sample tested is.

[0041] Measurement of the stall volume:

[0042] The measurement of the dropout volume is carried out for an inclination of the glazing of 45 °. A series of drops of increasing calibrated volume are positioned by means of a micropipette on the surface of an inclined sheet of glass. Beyond a certain volume, the drop will flow by gravity on the surface of the inclined glazing. The dropout volume is the smallest volume of drop allowing its flow under its own weight. This measurement is carried out with an accuracy of IpL.

[0043] We generally seek to obtain the lowest possible stall volume. Resistance to hydrolysis:

[0044] The resistance to hydrolysis is determined by immersing a sheet of glass bearing the hydrophobic coating to be tested for 7 days in a 35g / L saline solution of pH=9 (adjusted with NaOH) at 35°C. After this treatment, the contact angle is measured again. UV resistance:

[0045] Resistance to UV radiation is determined using a device called QUV, of the QUV-B type equipped with fluorescent lamps having a maximum irradiance at 313nm (under the conditions of the ASTM G-154 standard).

[0046] UV exposure tests alternate phases: - irradiation (fluorescent lamps) at a temperature of 70°C for 16 hours - dark with exposure to humid heat at a temperature of 40°C with generation of condensation on the surface of the samples for 8 hours

[0047] The total exposure time for this test is 150 hours. Examples

[0048] Samples of glass substrates are functionalized with different alkyltrial-koxysilanes by proceeding as follows:

[0049] A 10 cm x 10 cm sample is cleaned by polishing using a felt impregnated with an aqueous suspension containing 20% ​​cerium oxide particles. The polished sample is rinsed with distilled water and dried with a dry cloth.

[0050] For the formation of the silica primer layer, a solution of 0.3% by mass of tetraethoxysilane (TEOS) is prepared in a mixture of 10% acidic water (0.3N HCl) and 90% isopropanol.

[0051] After 30 minutes of reaction at room temperature, this solution is applied using a soaked cloth to the surface of the glass substrate. It is left to dry for approximately 2 minutes.

[0052] Immediately after, a 1.6% alkyltriethoxysilane solution in a mixture of 10 % acidic water (0.3N HCl) and 90% isopropanol.

[0053] Leave to dry for 15 minutes at room temperature and remove the whitish traces by polishing with a cloth soaked in a water / alcohol mixture containing 70% by mass of water and 30% by mass of isopropanol until the diffusing residues on the surface of the glazing disappear.

[0054] Table 1 shows the contact angle of a water droplet (0) after deposition of the hydrophobic coating, the detachment volume on a 45° surface (V45=) after deposition and the contact angles after hydrolysis and UV exposure of all the samples thus prepared, in comparison with a sample according to the state of the art using a perfluoroalkyltrialkoxysilane.

[0055] [Table 1] [Tables 1] Alkyl chain length 0 after deposition v45 after deposition 0 after salt hydrolysis 0 after UV exposure Ch 80° 25 pL 48° 68° -c3 90 40 pL 84° 67° -c5 99° 40 pL 95° 60° -c8 105 ° 38 pL 94° 53° 1H,1H,2H,2H-perfluorooctyl-triethoxysilane 107° 32 pL 87° 99°

[0056] It is found that substrates treated with long alkyl chain alkyltriethoxysilanes (C5 and C8) are less sensitive to hydrolysis but more sensitive to UV. The hydrolytic stability of a methyltriethoxysilane-based coating is insufficient (contact angle less than 50°C). By extrapolating the degradations observed after exposure to UV, it appears that alkyl chains comprising more than 8 carbons would provide insufficient durability with respect to this environmental factor.

[0057] In conclusion, to obtain satisfactory durability with regard to sensitivity to hydrolysis and UV, alkylsilanes with C38 chains prove to be the best.

Claims

Claims

1. A method for hydrophobizing a glass substrate, preferably an aircraft cockpit glazing, comprising the following successive steps: - forming a primer layer, preferably made of silica, on one of the main surfaces of the glass substrate, - grafting a layer of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane onto the primer layer in which the trialkoxy-(C3-C8 alkyl)-silane is a trialkoxy-al-kylsilane of formula CnH2n+iSiR3 where n = 3 - 8, each R representing a C[ 3 alkoxy group, preferably a methoxy or ethoxy group and in which the trichloro-(C3-C8 alkyl)-silane is a trichloro-al-kylsilane of formula CnH2n+iSiR3 where n = 3 - 8, each R representing a chlorine atom.

2. Method according to claim 1, characterized in that it further comprises, after the grafting step, a polishing step using a fabric soaked in a water / alcohol mixture, preferably water / isopropanol, containing from 30% to 90% by weight of water, preferably from 50% to 80% by weight of water.

3. A method according to any one of the preceding claims, characterized in that the trialkoxy(C3-C8 alkyl)-silane is a trialkoxy-alkylsilane of formula CnH2n+iSiR3 where n = 5 - 8, and each R represents a C1-3 alkoxy group, preferably a methoxy or ethoxy group.

4. Process according to any one of claims 1 or 2, characterized in that the trichloro(C3-C8 alkyl)-silane is a trichloro-al-kylsilane of formula CnH2n+iSiR3 where n = 5 - 8, and each R represents a chlorine atom.

5.

6. Method according to claim 3 or 4, characterized in that n = 8. Method according to any one of the preceding claims, characterized in that the primary layer is made of silica and that the formation of the primary silica layer is carried out by a sol-gel step in which a tetrachlorosilane and / or a tetraalkoxysilane, preferably tetraethoxysilane, is hydrolyzed in an acidic hydro-alcoholic medium.

7. Method according to claim 6, characterized in that the sol-gel step for the formation of the primary silica layer is carried out with an acidic hydroalcoholic solution containing from 0.1% to 1% by weight of tetrachlorosilane and / or tetraalkoxysilane in a water / alcohol mixture containing 3 to 50% by mass, preferably 5 to 15% by mass of water whose pH has been previously adjusted to a value between 0 and 3.

8. Method according to claim 7, characterized in that the hydro-alcoholic solution of tetrachlorosilane and / or tetraalkoxysilane is left at room temperature for a period of between 2 minutes and 2 hours, preferably between 3 minutes and 1 hour, in particular between 5 and 30 minutes, before being applied to the glass substrate by immersion, liquid curtain, spraying or wiping.

9. Process according to any one of the preceding claims, characterized in that the step of grafting trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane is carried out by a sol-gel step in which the trialkoxy-(C3-C8 alkyl)-silane and / or the trichloro-(C3-C8 alkyl)-silane is hydrolyzed in an acidic hydro-alcoholic medium.

10. Process according to claim 9, characterized in that the step of grafting trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane is carried out with an acidic hydroalcoholic solution containing from 0.1% to 4% by weight, preferably from 1 to 3.5% by weight of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane in a water / alcohol mixture containing 5 to 70% by weight, preferably 10 to 50% by weight of water.

11. Method according to claim 10, in which the hydro-alcoholic solution of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane is left at room temperature for a period of about 10 minutes to about 3 hours, before being applied by spraying or wiping onto the glass substrate covered on one of its main faces with the primary layer of silica.

12. Method according to any one of the preceding claims, characterized in that the grafting of trialkoxy-(C3-C8 alkyl)-silane and / or trichloro-(C3-C8 alkyl)-silane is carried out after at least 2 minutes of drying of the primary silica layer at room temperature.

13. Hydrophobic glass substrate obtained by the method according to any one of the preceding claims, comprising a transparent glass substrate, covered, on one of its main faces, with a primary silica layer, said primary silica layer being grafted with (C3-C8 alkyl)silyl groups, said alkyl group corresponding to the formula (CnH2n+i) where n = 3 - 8.

14. Hydrophobic glass substrate according to claim 13, characterized in that the silica primary layer has a thickness of between 5 nm and 250 nm, preferably between 10 nm and 100 nm, in particular between 15 nm and 75 nm.

15. Hydrophobic glass substrate according to claim 13 or 14, characterized in that it is aircraft cockpit glazing.