Reflector for vehicle rear view mirror, with hydrophilic- and attenuating properties, includes reflective layer with metal oxidized or nitrided sub-stoichiometrically

Hydrophilic reflective article

The present invention relates to a reflective article, in particular for automobile rear-view mirrors, having hydrophilic properties and an attenuated reflection rate. The present invention also relates to the method of manufacturing such an article.

Mirrors are known comprising a metallic layer (generally in silver, aluminum or chrome) applied either on the rear face of a transparent substrate, that is to say on the face opposite the observer or on the front face. of the substrate, therefore the face directed towards the observer. With a chromium metal layer of the order of 40 to 60 nm thick, a light reflection of about 65% is obtained, which is perfectly suitable for use as a rear-view mirror. With greater light reflections, the rearview mirror, on the other hand, has the drawback of causing the driver to dazzle.

Mirrors are also known, the surface of which is made hydrophilic (see for example EP 689 962, EP 1 022 588 or JP 2001033607).

The hydrophilicity of a surface increases its surface energy, which allows water drops to spread out in a film instead of beading. On a non-hydrophilic mirror, rain forms droplets which impede visibility. On a mirror with a hydrophilic surface, spreading the water in a film allows better visibility. Different materials are known for their intrinsic hydrophilic properties, in particular titanium oxide, and silicon oxide.

In addition to its hydrophilic properties, titanium oxide, in particular when it is crystallized in the anatase form, is also well known for its intrinsic photocatalytic properties, that is to say in particular, that it is suitable for degrade organic matter when it has been excited by light or UV irradiation.

Patent applications EP 978 494 and EP 1 099 671 describe anti-fog mirrors comprising a metallic reflective film respectively on the rear face or on the front face and a TiO stack.₂/ SiO₂ on the front side.

The TiO layer₂ having a high refractive index (n = 2.4), the reflection rate of the stack in the visible is high, of the order of 80% for a stack of neutral coloration. To decrease the glare, the layer thicknesses should be chosen such that the wavelength of the reflected light has a peak between 400 and 510 nm, which gives a blue color in reflection and a light reflection of the light. around 60%. EP 1 099 671 provides that a reflection adjustment layer can be added between the reflective film and the TiO layer.₂ to avoid excessive reduction of light reflection.

Stacks of alternating high and low refractive index layers are commonly used to increase light reflection. In documents EP 456 488, and EP 1 040 963 are described mirrors with high light reflection (> 70%) using a metallic layer as reflective layer, and a succession of low index layers (SiO₂) and high index (TiO₂) to increase reflection.

There is a need to provide a reflective article having a photocatalytic and hydrophilic action allowing good visibility in the event of rain, while keeping a moderate reflection rate to reduce glare. Such a moderately reflective article should be able to be supplied in a simple manner in neutral reflective tones as well as in colored tones, for example in the blue range.

The object of the present invention is to remedy the drawbacks described above. In particular, an object of the present invention is to provide a reflective article which has hydrophilic and photocatalytic properties and whose reflection of light is maintained at a non-excessive level of reflection even with neutral coloring.

The present invention relates to a composite article comprising a substrate, a reflective layer (generally called reflector) comprising a sub-stoichiometrically oxidized or nitrided metal, in particular CrOx, optionally surmounted by a barrier layer, then a layer with photocatalytic properties, based on titanium dioxide, then optionally a thin hydrophilic porous layer comprising in particular silicon oxide. This surface layer can be discontinuous.

According to an advantageous embodiment shown in FIG. 1, the layers are arranged on the same side of the substrate. However, it is also possible to place the reflecting layer on the rear face, that is to say on the face opposite the observer, and the photocatalytic layer, optionally surmounted by the hydrophilic layer, on the front face as shown in FIG. . 2.

The thickness of the photocatalytic layer can be between 20 and 120 nm and preferably between 40 and 75 nm. The thickness of the surface layer is for its part generally between 2 and 10 nm and preferably between 3 and 8 nm. This last layer makes it possible to retain the hydrophilic character of the surface longer after stopping the light irradiation. The very thin thickness of this overlayer makes it possible to preserve to a certain extent the photocatalytic effect of the TiO layer.₂,

When a barrier layer is placed between the reflecting layer and the photocatalytic layer, this barrier layer advantageously comprises silicon oxide. Its thickness can be between 10 and 80 nm and preferably between 20 and 60 nm. This barrier layer makes it possible to reduce or avoid the migration of alkalis from the glass, in particular the Na ions.⁺, towards the titanium oxide layer and also makes it possible to isolate the titanium oxide layer from the reflector.

The metal of the reflective layer can be selected from titanium, chromium, aluminum, silicon, zirconium and alloys of these metals. Advantageously, the reflecting layer comprises partially oxidized or nitrided chromium oxide. Its thickness can be between 20 and 100 nm, preferably between 30 and 60 nm.

It is advantageous that the reflecting article described above has a light reflection (integrated over the whole of the visible) of between 40 and 75% and preferably between 45 and 70% of the incident visible light.

When the color in reflection of the article according to the invention is neutral (that is to say that the coefficients a * and b * of the Lab system are between -5 and 5), it is advantageous that the rate of reflection is between 55 and 75%, preferably between 60 and 70%, and when the color in reflection is in the blue range (i.e. a * is between -10 and 0 and b * is less than -10), it is advantageous for the reflection rate to be between 40 and 55%, preferably between 40 and 50%.

The coefficients a * and b * are measured with the illuminant D65, at an angle of incidence of 2 °.

The light transmission of the article should be very low and preferably less than 3%, or even less than 2%.

A subject of the present invention is also a method of manufacturing a reflective and hydrophilic article which comprises the following steps:

- deposition, on the front or rear face, of a metal layer slightly oxidized or nitrided by magnetron cathodic sputtering in a reactive atmosphere controlled with a metal target;

- possibly deposition of an SiO barrier layer₂ by magnetron sputtering in a reactive atmosphere with an Si target, on the front face of the support;

- deposit on the front face of a TiO layer₂ by magnetron cathode sputtering, for example in a reactive atmosphere with a Ti target;

- heat treatment at a temperature between 300 and 500 ° C, in particular 350 to 450 ° C, for a period which may vary from 15 min. at 6 hours, in particular 30 min. at 4 hours, which allows the crystallization of TiO₂ in anatase form while avoiding the formation of TiO cracks₂ and the resulting haze;

In particular, the method according to the invention also comprises a step of depositing a thin surface layer of SiO₂, by magnetron sputtering in a reactive atmosphere with Si target;

When the reflective layer is placed on the rear face, it is advantageously deposited first. The barrier layers, photocatalytic, and the surface layer are then deposited on the opposite face. The entire coated substrate can then be subjected to the heat treatment.

The present invention is described below by non-limiting exemplary embodiments.

Example 1:

Glass / CrO stacking_x/ SiO₂ / TiO₂/ SiO₂ of neutral coloring, as shown in FIG. 1 is produced on a clear soda-lime glass (10) 2 mm thick by magnetron sputtering.

The conditions for depositing the different layers constituting the stack are as follows:

A first layer (20) of lightly oxidized chromium is deposited on the substrate 5 (10) from a metallic chromium target in an atmosphere of 80 wt% argon and 20 wt% oxygen. The thickness of the layer is of the order of 45 nm.

A barrier layer (30) of SiO₂ is deposited on the first layer from a metal target of Si in an atmosphere of 75 wt% argon and 25 wt% oxygen. The thickness of the layer is of the order of 40 nm.

A layer of TiO₂ (40) is deposited on the barrier layer from a target of oxidized titanium in an atmosphere containing 75% by mass of argon and 25% by mass of oxygen. The thickness of the layer is of the order of 60 nm.

A last very thin layer of SiO₂ (50) is then deposited on the stack. The deposition is carried out starting from a metallic target of Si in an atmosphere containing 75% by mass of argon and 25% by mass of oxygen. The thickness of the layer is of the order of 5 nm.

The coated substrate is then subjected to a heat treatment for 1 hour at 400 ° C. The temperature rise is carried out rapidly but the cooling is carried out very gradually (approximately 3 ° C./min).

The light reflection rate (RL), integrated over the entire visible spectrum, is measured according to the SAE J 964 standard with an integrating sphere. The substrate covered according to Example 1 has an RL of 65% while the same SiO stack₂/ TiO₂/ SiO₂ on a layer of metallic chrome of the same thickness would have given an RL of 80% and would therefore have been too dazzling for an application as a rear-view mirror (see fig. 3).

The color of the stack in reflection is determined by the colorimetric coordinates L *, a *, b * according to the illuminant D65 with an angle of incidence of 2 °.

The values ​​obtained are shown in the table below. The very low values ​​of a * and b * show that the stack does not exhibit any significant color in reflection.

The light transmission (TL) integrated on the visible is 0.9%.

Example 2:

Glass / CrN stacking_x/ SiO₂/ TiO₂/ SiO₂ , as also shown in fig.l, blue in reflection is produced on a clear soda-caicic glass (10) 2 mm thick by magnetron sputtering.

The conditions for depositing the different layers constituting the stack are as follows:

A first layer (20) of lightly nitrided chromium is deposited on the substrate from a metallic chromium target in an atmosphere of 50 wt% argon and 50 wt% nitrogen. The thickness of the layer is of the order of 45 nm.

A barrier layer (30) of SiO₂ of the order of 25 nm thick then a layer of TiO₂ (40) of the order of 40 nm thick, and then a last layer of SiO₂(50) of the order of 5 nm thick are successively deposited under the same conditions as those described in Example 1.

The coated substrate is then subjected to a heat treatment under the same conditions as in Example 1.

The light reflection rate (RL), integrated over the entire visible spectrum, is measured according to the SAE J 964 standard with an integrating sphere. The substrate covered according to Example 2 has an RL of 43% while the same SiO stack₂ / TiO₂ / SiO₂ on a layer of metallic chromium of the same thickness would have given an RL of 56% (see fig. 4).

The color of the stacking in reflection is determined by the colorimetric coordinates L *, a *, b * according to the illuminant D65. The values ​​obtained are shown in the table below. The negative values ​​of b * and very slightly negative of a * show that the stack has a slightly greenish blue color in reflection.

The light transmission (TL) integrated on the visible is 1.5%.

Table 1:

RL L * at* b * Example 1 65 85.8 -3.8 -1.4 Example 2 43 75.1 -6.4 -16.4