EP2335096A1

EP2335096A1 - Antireflection coatings including scattered objects having two separate ranges with separate refraction indices

Info

Publication number: EP2335096A1
Application number: EP09748431A
Authority: EP
Inventors: Mélanie LOUARN; Vincent Gaud; Fabien Rouge
Original assignee: Polyrise SAS
Current assignee: Polyrise SAS
Priority date: 2008-09-23
Filing date: 2009-09-23
Publication date: 2011-06-22
Also published as: JP2012502874A; FR2936321B1; WO2010034936A1; CN102209915A; CN102209915B; FR2936321A1; US20120038989A1

Abstract

The present invention relates to a surface treatment method making it possible to confer upon the surface of a substrate antireflection properties with respect to electromagnetic radiation. A coating that is transparent with respect to said electromagnetic radiation is deposited on said surface and contains, in the scattered state within said layer, objects of less than 5 microns in size that include: - a core having a first refraction index nc, and a so-called skin layer, which surrounds the core and has a second refraction index nE separate from the refraction index nc of the core, where the ratio of the dimensions of the core to the dimensions of the core and the skin together is 1:1.5 to 1:5. The invention also relates to the coated substrates obtained according to said method.

Description

Anti-reflective coatings comprising dispersed objects having two separate domains having distinct refractive indices

The present invention relates to a method for modifying the optical properties of the surface of a material, giving the surface anti-reflective properties. The invention relates in particular to anti-reflection treatments of this type applied to transparent substrates, in particular glass or polycarbonate, which make it possible to improve the light transmission through these transparent substrates. The invention also relates to substrates coated with an anti-reflection coating which are obtained in this context, which can typically be transparent optical devices (optical lenses for example) with optimized optical transmission.

By "surface treatment conferring antireflection properties" is meant here a modification of the surface of a solid substrate making it possible to reduce the reflection properties of at least some electromagnetic waves in the range from ultraviolet to infrared (typically having a wavelength between 150 nm and 2500 nm) on the modified surface of the solid substrate. More specifically, within the meaning of the present invention, the concept of "anti-reflection" treatment designates a treatment of this type which, when applied to a transparent substrate, inhibits the reflection of at least some of the electromagnetic waves for which the material is transparent, increasing the transmission of these waves through said transparent substrate. Anti-reflective properties of this type are particularly sought after in many industrial fields, especially in optics (for laser-type devices, for example, it is particularly advantageous to have lenses with the most optimized transmission properties possible).

Various surface treatments for imparting an antireflection effect of the aforementioned type have been described, in particular to confer an anti-reflection effect on glass or polycarbonate substrates.

The surface treatment methods developed in this context generally consist in depositing several successive layers having distinct refractive indices, typically an alternation of at least three layers (usually a refractive index layer h, a refractive index layer i ₂ with i ₂ <ii, then a refractive index layer i ₃ with i ₃ > i ₂ ) - For more details about multilayer coatings of this type, one can particularly refer to Advances in nanomaterials and processing, pts 1 and 2; Solid State America, Vol 124-126, p. 559-562, Solar Energy Materials and Solar CeIIs, Vol. Nov. 90, 2006; or else to patent applications EP 1433809 and US 2004/71889.

The antireflection coatings of the aforementioned type, used in particular to provide an anti-reflection effect on spectacle lenses, have the disadvantage of being cumbersome to implement, especially insofar as they involve the deposition of several successive layers. which translates into both cost and high production time. In addition, each of the successive deposits is generally carried out according to processes carried out in vacuums, the implementation of which further increases the preparation costs. In addition, the need for a deposit of several successive layers on the treated substrate leads to a final deposition of relatively high thickness, which can adversely affect the transmission properties (a portion of the waves being capable of being absorbed at the level of the coating multilayer). Although these absorption phenomena are relatively unimportant on spectacle lenses, they have a much more significant implication in optical devices such as lasers, where even a very small decrease in transmission has a very significant impact on the final efficiency. of the device.

An object of the present invention is to provide a new method of anti-reflective treatment which is at least as effective, and preferably more effective, than the aforementioned multilayer coating method, and which allows to change more easily and less expensive transparent substrates, in particular based on glass or polycarbonate, to give them a particularly high light transmission, preferably equivalent to or even greater than that obtained with the method by the aforementioned multilayer coating techniques. For this purpose, according to a first aspect, the subject of the present invention is a method of treating a surface of a substrate making it possible to impart antireflection properties to this surface with respect to electromagnetic radiation. which is deposited on said surface a transparent coating vis-à-vis said electromagnetic radiation, which contains, in the dispersed state within said layer, objects of dimensions less than 5 microns, preferably less than 2 microns, said objects comprising at least two zones consisting of two different substrates, transparent to said electromagnetic radiation and having distinct refractive indices, namely: a core having a first refractive index n _c , and

. a layer surrounding the heart (hereinafter referred to as "bark"), having a second refractive index n _E , distinct from the refractive index n _c of the heart, where the ratio of the dimensions of the heart to the dimensions of the whole heart / bark is between 1: 1, 5 and 1: 5.

The coating deposited according to the method of the present invention, and the objects it contains, are transparent at least with respect to certain electromagnetic waves in the range from ultraviolet to infrared and are in particular transparent to waves for which the anti-reflection effect is sought. They can typically be visually transparent (ie transparent for all or part of the visible light). Alternatively, they may be only optically transparent, ie transparent only for certain non-visible radiation (UV and / or infrared).

According to a particular embodiment, the substrate whose surface is modified according to the method of the present invention is a transparent substrate. This is for example a substrate of glass or polycarbonate

For the purposes of the present description, a coating, an object, a material or a substrate is said to be "transparent", for an electromagnetic radiation of wavelength λ given, when it is allowed to pass through a flux of said electromagnetic radiation, of preferably without absorbing this flow or absorbing only a minority part of this flow. A material or a substrate said to be transparent at a wavelength λ preferably has an absorption coefficient the molar absorption coefficient (also called the molar "extinction" coefficient) being preferably less than or equal to 200 L. mol- ¹ .cm- ¹ , and more preferably less than or equal to 100 L. mol ^-1 cm ^-1 , at the wavelength considered.

The core and the bark of the objects which are present in the dispersed state in the coating deposited according to the method of the present invention consist of substrates which are transparent especially with respect to the electromagnetic radiation for which the anti-corrosive effect -reflet is searched. The refractive indices of the substrates constituting the heart and bark of these scattered objects referred to in the present description, namely respectively the first refractive index n _c of the core and the second refractive index n _E of the layer surrounding the core, denote the refractive indices of the substrates at the wavelength (or wavelengths) of the electromagnetic radiation for which the antireflection treatment is sought.

The transparent antireflection coating deposited according to the method of the present invention is preferably a monolayer coating, resulting from the deposition of a single layer on the surface to be modified. Typically, the deposited coating has a thickness between 10 nm and 10 microns, more preferably between 50 nm and 5 microns. Anti-reflective properties according to the invention are obtained both for thin films of a few tens to a few hundred nanometers (for example between 10 and 900 nm, and in particular between 50 and 500 nm) than for micrometer thickness (for example from 1 to 10 microns, especially from 1 to 5 microns).

Especially for antireflection applications aimed at increasing the transmission (surface treatment of a lens typically), and in particular for anti-reflective applications of this type concerning electromagnetic radiation in the UV field and / or of the visible, it is generally preferable (especially to limit any phenomenon of parasitic absorption by the deposited layer) that the deposited transparent coating has a thickness of less than 1 micrometer, preferably less than 800 nm, and more preferably still less at 500 nm, this thickness being advantageously between 10 and 600 nm, in particular between 50 and 500 nm; by example between 100 and 400 nm. For antireflection applications concerning electromagnetic radiation in the infrared range, higher thicknesses (up to a few microns) may be desirable. It should be noted in this regard that it is preferable that the sizes of the objects present in the coating deposited according to the invention is even higher than the wavelength of the radiation for which it is sought to obtain the anti-radiation effect. -reflet is large (typically, for an electromagnetic radiation of wavelength λ given it is preferable that the heart of objects has dimensions greater than λ / 4 and that the thickness deposited on the core is also greater than λ / 4 ). Micron layers (ie thicknesses greater than or equal to 1 micron) are also recommended when it is desired to impart anti-reflective properties to the surface of a non-transparent material.

The objects that are dispersed within the transparent coating are isotropic or anisotropic objects preferably having dimensions less than 2 microns, these dimensions typically being between 2 nm and 1 micron. In particular so that they have zones of heart and bark of clearly distinct refractive indices, these dispersed objects preferably have dimensions equal to at least 3 nm, and more preferably at least 5 nm ( these dimensions being advantageously greater than or equal to 10 nm, or even 20 nm, for example at least 50 nm). Typically, the objects which are dispersed within the transparent coating according to the invention have dimensions of between 10 nm and 800 nm, for example between 20 and 600 nm. These dimensions are to be chosen even lower than the thickness sought for the transparent coating is low (the thickness of the transparent coating is generally at least equal to the size of the dispersed objects it contains). Thus, in order to obtain thin coating layers that are well adapted to increase the transmission of UV or visible radiation within a transparent material, it will typically be advantageous to deposit a layer where the dispersed objects have dimensions smaller than 400 nm, for example less than 300 nm, more preferably less than 200 nm, or even less than 100 nm. The objects that are dispersed within the transparent coating deposited in the context of the method of the present invention are generally constituted by a core of the aforementioned type having the first refractive index n _c surrounded by the bark having the second refractive index n _E Alternatively, however, the scattered objects may include at least one additional coating layer around the entire heart-bark. Where appropriate, each of the additional coating layer is constituted by a material that is transparent to the electromagnetic radiation for which the anti-reflection effect is desired and, preferably, each of the additional coating layer has an index different refraction of the layer (s) with which it is in contact.

In the objects present in the dispersed state within the transparent coating deposited in the context of the present invention, the bark which surrounds the core is constituted by a substrate of organic and / or inorganic nature, as well as the (or the possible coating layer (s). Most often, the heart of the objects present in the dispersed state within the transparent coating is also constituted by an inorganic and / or organic substrate. According to another embodiment, more particularly, the core may be empty (according to this specific mode, the dispersed objects are hollow-particle type and the refractive index _nc of the core is then substantially equal to 1).

In the objects that are dispersed within the transparent coating deposited in the context of the method of the present invention, the average thickness of the bark surrounding the heart has dimensions of the same order of magnitude as those of the heart and with a ratio dimensions of the heart to the dimensions of the heart / bark assembly constituted by the core having the first refractive index n _c surrounded by the bark having the second refractive index n _E is between 1: 1.5 and 1: 5 this ratio being advantageously of the order of 1: 2.5. For the purposes of the present description, the expression "ratio of the dimensions of the heart to the dimensions of the core / bark assembly" designates the ratio of the characteristic dimension of the heart to the characteristic dimension of the core / bark assembly in the case of isotropic particles, or the ratio of the characteristic dimensions of the core and dimensions characteristic of the whole heart / bark in the context of anisotropic particles. Thus, in the context of isotropic particles, this ratio can be defined as the ratio of the mean diameter of the heart to the average diameter of the core / bark assembly. Typically, a dispersed object according to the invention may for example be in the form of a core of isotropic morphology (substantially spherical for example) and forming, with the layer having the second refractive index n _E surrounding it, a heart / bark set of isotropic morphology (substantially spherical for example) having an average dimension dc + E of between 2 nm and 1 micron, for example between 5 nm and 800 nm, in particular between 10 nm and 500 nm, with a dc / ratio dc + E advantageously between 1: 1, 5 and 1: 5, for example between 1: 1, 8 and 1: 4, and typically of the order of 1: 2.5.

The dimensions of scattered objects referred to in the present description refer to dimensions as measured by light scattering, in particular by dynamic light scattering, for example using Malvern type equipment (Zetasizer).

Typically, the dimensions measured by light scattering are determined on objects in the dispersed state. To do this, if necessary, it is possible to disperse the objects whose size is sought to be determined in a suitable solvent (water, ethanol, a water / ethanol mixture, tetrahydrofuran, or dimethylsulfoxide, for example), at a concentration typically of

0.1 mg / ml to 20 mg / ml. The sample to be analyzed, which contains the objects in the scattered state, is placed in the incident beam of a laser and the diffusion is measured at a 90 ° angle. The dimensions measured according to this light scattering method have a high resolution (typically the measurement is made with an accuracy within +/- 0.4 nm).

These measurements made in light scattering are corroborated by measurement methods using electron microscopy, which also makes it possible to access the dimensions of the constituent parts (heart and bark in particular) of objects whose more global dimension is determined by diffusion of light. the light. Analysis methods particularly adapted to access The dimensions of the objects in the dispersed state and their constituent parts are SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) electron microscopy techniques, the principles of which are described in particular in ASTM standards, Digital library, chapter 72, JG Sheehan (1995).

In the context of the present invention, the inventors have now shown that, unexpectedly, when transparent substrates comprising micron or submicron objects of the aforementioned type, namely having a core, are deposited on the surface of a substrate. and a coating layer having refractive indices n _c and n _E , the deposition of this single layer results in an anti-reflection effect on the surface thus treated.

Without wishing to be bound by a particular theory, in the light of the work done by the inventors in the context of the invention, it seems possible to be argued that this anti-reflection effect is explained, at least in part, by the fact that each of the scattered objects would behave as a kind of "nanodomain" having, at its local level, a multilayer structure that would make it locally suitable for providing an effect similar to that observed with the usual (more macroscopic) multilayer deposits. addition of these local effects giving the material overall anti-glare properties of particular interest.

From the experiments carried out by the inventors, it appears that the dispersion of the objects having this "local multilayer structure" within the deposited coating provides a similar, if not improved, antireflection effect with respect to the effect observed with the usual multilayer deposits. The antireflection coatings prepared according to the invention also have transparency qualities at least similar to those of multilayer deposits known from the state of the art. In certain cases, this transparency is even greater (indeed, insofar as they only require a single deposit in the form of a monolayer, the antireflection deposits according to the invention are likely to be more transparent than multilayer coatings. thicker where radiation absorption is more likely to occur produce). These transparency qualities, obtained without having to use complicated techniques, make anti-reflective deposits of the invention an alternative solution of choice to multilayer coatings, which allow simple access to transparent materials having a high transmission. electromagnetic waves from the UV to the infrared.

Furthermore, insofar as it only requires the deposition of a single layer to obtain the desired antireflection effect, the method of the invention is less expensive and less time consuming than the deposition method. multilayer, which is yet another of its advantages. Although a monolayer deposit is sufficient to obtain an anti-reflection effect of the type sought according to the invention, it is possible, according to a particular embodiment of the method of the invention, to carry out several successive antireflection deposits (for example at least 2 or at least 3) on the surface of the treated substrate, wherein at least one of the antireflection deposits contains, in the dispersed state, objects having the aforementioned heart-shell structure. Multilayer deposits of this type can in particular be used to confer particularly pronounced antireflection properties and / or to provide an anti-reflective effect.

The aforementioned effects generally prove even more pronounced than the refractive indices n _c n _E and the heart and the coating layer (cortex) surrounding are different. In this context, it is generally advantageous for the wavelength (or wavelengths) of the electromagnetic radiation for which the anti-reflection effect is desired to be the difference (n _c - n _E ). between the refractive indices of the core and the surrounding coating layer is, in absolute value, greater than 0.01, this difference being more advantageously at least 0.1, and more advantageously still at least 0.2. Differences of 0.3 or more lead to even more interesting results.

The transparent coating which acts as a vehicle of the objects dispersed in the method of the present invention may be any type of removable coating in the form of a layer of dimension less than 10 microns, more preferably less than 5 microns and more advantageously still less than 1 micron. It may be for example a varnish or a polymer layer.

According to a particularly interesting embodiment, this coating is a sol / gel coating. The sol / gel coatings are well-known type coatings, which are obtained by hydrolyzing inorganic alkoxides such as silicon, titanium or zinc alkoxides, which leads to a reaction similar to a polymerization of the mineral species, leading in a first step in the formation of a soil of inorganic oxide particles (silica, UO2 or ZrO2 for example) and then to a progressive gelation of the medium, ultimately leading to cross-linking of all the species mineral in the form of a rigid structure similar to a glass. The so-called "sol / gel" deposits are deposits made by depositing on a substrate a layer of a reaction medium of this type, in the state of ungelled or partially gelled sol, and then allowing the gelation to continue until get a hardening of the layer. The deposition can be carried out by any suitable conventional method, in particular by the so-called "dip-coating" technique or spin-coating, which are well-known techniques, especially those of process. Engineering Analysis in Semiconductor Device Manufacturing, S. Middlemann & A. Hochberg, Mcgraw-Hill College, p. 313 (1993), or from application EP 1 712 296.

The deposition of an anti-reflection coating according to the invention employing the aforementioned sol / gel technique advantageously comprises a heat treatment step (drying) at the end of the gelation, which makes it possible to optimize the curing of the soil layer. gel / deposited, and thereby give good cohesion to the coating obtained in fine. This heat treatment can be carried out using hot air as well as infrared radiation. This treatment is preferably carried out by placing the substrate provided with the anti-reflection coating in formation in an oven at a temperature between 20 and 200 ⁰ C, more preferably between 50 and 150 ⁰ C. According to an interesting embodiment, the treatment thermal is carried out by progressively raising the temperature of the deposition temperature of the sol / gel coating on the substrate (typically between 10 and 25 ° C.) at the heat treatment temperature (typically at least 50 ^° C.), with a temperature increase rate typically between + 0.5 ° C / minute and + 5 ° C / minute.

When the method of the invention implements transparent coating deposited by the aforementioned sol / gel technique, the inorganic alkoxide which is used is advantageously a tetraalkoxysilane, preferably tetramethoxysilane.

(Compound corresponding to the formula Si (OCH ₄ ) ₄ , generally designated by its abbreviation TMOS, and sometimes referred to as tetramethylorthosilicate) or alternatively tetraethoxysilane (or TEOS, of formula (Si (OC ₂ H ₅ ) ₄ ). The inorganic alkoxide used is TMOS tetramethoxysilane, alternatively the inorganic alkoxide which is used can be a titanium alkoxide.

(eg titanate isopropoxide) or a zinc alkoxide (such as zinc isopropoxide).

According to a particular embodiment, which proves to be generally of interest, the transparent coating which acts as a vehicle of the objects dispersed in the method of the present invention may advantageously be a particular sol / gel type coating, obtained from a mixture initially comprising (i) at least one mineral alkoxide, preferably of the aforementioned type; and (ii) at least one monomer crosslinkable under UV or under the effect of heat treatment (typically in the presence of a source of free radicals). In this case, a coating having a particularly high cohesion is generally obtained, since the synthesis of the coating then comprises a double degree of hardening, namely:

a first hardening is obtained by hydrolysis and condensation of the mineral alkoxide according to the sol / gel technique; and - jointly and / or in addition, additional hardening is effected by crosslinking the (or) crosslinkable monomer (s), typically under the effect of UV irradiation and / or heating in depending on the exact nature of the monomers to be crosslinked.

The crosslinkable monomers that can be used according to this specific variant of the process of the invention may be unpolymerized monomeric species carrying functions that make them crosslinkable under UV or thermally. Alternatively, they may be macromolecular species such oligomers or polymers carrying functions capable of making them crosslinkable under UV or thermally. The UV-crosslinked or thermally crosslinkable monomers used according to this embodiment are typically compounds carrying methacrylate, epoxy acrylate or vinyl ether groups. Alternatively, it is possible to use mixtures of two types of monomers carrying complementary functions which react with one another when they are in contact to lead to a condensation crosslinking (in the context of this variant, it is possible, for example, to use the following reactive functional groups: epoxy / amine, acrylate / amine, isocyanate / alcohol, thiol / ene, or epoxy / isocyanate.

In general, when the transparent coating used in the method of the present invention is of the sol / gel type, it is preferable that this sol / gel coating is synthesized in the presence of at least one surfactant, in particular of the type described in SoI -GeI Science: SoI-GeI: The Physics and Chemistry of Soi Gel Processing, Jeffrey Brinker C. and George W. Scherer, Academy Press (1990) or in the Journal of Colloids and Interface Science, Vol. 274, Issue 2, 355-361. The use of this type of surfactant makes it possible to limit the size of the particles in the soil obtained by hydrolysis of the alkoxide and thus makes it possible to control the thickness of the coating layer obtained in fine. As a suitable surfactant in this context, there may be mentioned polyoxyethylenated surfactants (polyoxyethylenated esters in particular), such as TWEEN 85, for example.

Another means for controlling the particle size formed in the soil by hydrolysis of the inorganic alkoxide employed in the sol / gel technique is to employ a mixture of alkoxides comprising alkoxides having 4 hydrolyzable groups and alkoxides having plus 3 hydrolyzable groups (for example two or even one). In this context, the sol / gel coating can typically be synthesized using, as inorganic alkoxide, a mixture of alkoxides comprising: at least one silane having 4 hydrolyzable groups (such as TMOS tetramethoxysilane, or TEOS tetraethoxysilane) ; and at least one silane having less than 4 hydrolyzable groups, this silane corresponding preferably to the formula R _n SiX _4-11 , in which:

n is an integer equal to 1, 2 or 3;

each of the groups R, identical or different, denotes a non-hydrolyzable organic group, optionally functional, and

- X a hydrolyzable group (typically a halogenated alkoxy group, for example a trimethoxysilane, thethoxysilane, γ-propylthmethoxysilane, γ-propyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropylethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyl- triethoxysilane, γ- (meth-

acryloylpropryltrimethoxysilane, γ- (meth-) acryloylpropryltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylethoxysilane, di-methoxysilane, diethoxysilane, polydimethylsiloxane α-ω-disilanol, or polydiethylsiloxane α-ω-disilanol; or a halogen group, such as - Cl or Br.

According to this embodiment, it is possible to use, for example, silanes carrying a single hydrolyzable group, or precursor compounds of such monofunctional silanes, for example compounds producing a monofunctional silane following a hydrolysis reaction, for example, for example 1,1,1,3,3,3-hexamethyldisilazane (HMDS) or chlorosilanes such as trimethylchlorosilane.

On the other hand, when a sol / gel process is used to effect a coating according to the invention, the synthesis medium of this coating comprises water, optionally in combination with one or more water-miscible solvents. (for example ethanol). The water is then preferably present in an amount equivalent to half of the hydrolysable silane functions present in the sol / gel formulation.

Whatever the nature of the transparent coating that acts as a vehicle of the objects scattered in the method of the present invention, the objects It contains advantageously the preferred characteristics of one of the 3 variants defined below.

According to a first variant of the invention, the heart of the objects present within the transparent coating deposited on the substrate to be treated according to the invention is organic in nature.

In the context of this first variant of the invention, the core may for example comprise or consist of:

at least one hydrocarbon polymer, linear or (advantageously) branched, the chains of which optionally carry heteroatoms; or

at least one component or a mixture of components having a low molecular weight, typically less than 250 g / mol, for example solvents or oily substances. In the context of the first variant defined above, the layer (bark) surrounding the organic core is typically a polymer layer, which can typically be formed around the organic core by emulsion polymerization, dispersion, mini-polymerization techniques. emulsion or spontaneous emulsion. These techniques and their mode of implementation are known to those skilled in the art. For more details about them, we can for example refer to Soft Matter, vol. 2, pp. 940-949 (2006) or Chem Phys Chem. Flight. 6, pp. 209-215 (2006).

The core / shell structure objects obtained according to this first variant of the invention are typically capsules (most often, but not necessarily spheroidal), which comprise a polymeric shell, constituting the bark, trapping an organic material of heart, preferably of the aforementioned type (polymer distinct from the polymer of the bark or non-polymeric organic compounds, for example). Whatever their exact structure, these objects typically have dimensions between 50 nm and 2 microns, these dimensions being preferably less than 1 micron, and more preferably less than 800 nm, or even less than 500 nm.

Objects having a core / shell structure according to the first variant of the invention may for example be capsules comprising a polyurethane or polyamide shell surrounding a hexadecane core.

Other objects with a core / bark structure of interest according to the first variant of the invention comprise two polymers of the same type as a core polymer and a bark polymer (for example two (meth) acrylates) with one of the carrier polymers. specific groups not carried by the other polymer (fluorine groups -F, for example). In this case, the core-shell structure is typically obtained by carrying out the polymerization of the corresponding monomers, initially starting from a polymerization medium containing only the monomers leading to the formation of the core polymer (for example, not carrying the specific groups) , then enriching the polymerization medium with the monomers leading to the formation of the polymer of the bark (for example carriers of specific groups). Objects having a core / bark structure which can be used according to the first embodiment can for example be of the type of butyl acrylate and trifluoroethylmethyl methacrylate copolymers described in Macromolecules, Vol. 30, 123-129 (1997). Other objects with a core / bark structure that can be used according to the first variant of the invention are the self-assemblies of diblock structure-based block polymers comprising a first block having an affinity for a given solvent bonded to a second block having a lower affinity. and preferably no affinity for said solvent. When these polymers are introduced into the solvent, they self-assemble in the form of bark-core objects (the blocks having a strong affinity for the solvent forming an outer layer surrounding an inner core where the blocks containing a lower affinity for the solvent). Examples of block copolymers leading to this type of self-association in a solvent medium have in particular been described in Langmuir, vol. 22, pp. 4534-4540 (2006) (block of poly (ethylene oxide) -block of (N, N-diethylaminoethyl methacrylate) or Adv Funct Mater., Vol 16, pp. 1506-1514 (2006) (block copolymers type diblocks (block of poly (ethylene oxide)) - (poly (ε-caprolactone) block). The block polymers described in these documents assemble when placed in a solvent medium to form objects comprising a core based on one of the block polymers and a shell based on the other block polymer.

According to a second variant of the invention, the core of the objects present within the transparent coating deposited on the substrate to be treated according to the invention is of inorganic nature.

In the context of this second variant of the invention, the core may for example comprise or consist of one or more of the following materials:

a mineral oxide, in particular silica or a metal oxide

- a metal sulphide

- a metal nitride

- a metal halide - a metal.

More preferably, the inorganic core objects according to the second variant consist of silica, metal oxides, metal sulfides and / or metals, more preferably silica, metal oxides (TiO2 or alumina, in particular) or metals (gold, silver, for example). In the context of this second variant, according to a first embodiment, the layer (bark) surrounding the inorganic core is a polymer layer, this polymer bark can then be prepared according to two major access routes, namely:

(1) So-called "grafting onto" methods According to this first approach, one starts from pre-existing inorganic cores (typically inorganic colloidal particles) and immobilizes pre-existing polymer chains (or pre-existing grafts) on the surface of these inorganic cores . To do this, the polymer chains or grafts that are to be immobilized are generally carrying chemical functions capable of creating a covalent or electrostatic bond with the surface of the inorganic cores or with a group present on the surface of the cores.

For example, colloidal gold particles may be derived from and grafted to them polymer chains carrying a thiol end, for example according to the method which has been described for example in J. Am. Chem. Soc., Vol. 120, 12696 (1998), where α-methoxy-ω-mercapto-poly (ethylene glycol) is grafted onto gold particles.

The so-called "grafting from" methods

According to this second approach, polymer chains are grown from functionalized core particles carrying organic groups.

A widely used method in this context is to initiate a polymerization from inorganic cores (colloidal particles preferably) modi fied in suface by groups initiating the polymerization Advantageously the functional groups introduced on the surface of inorganic cores are control agents allowing a controlled radical polymerization reaction of the ATRP type. For example, gold particles functionalized with thio groups can be used. Brominated polymerization initiators can be grafted by the ligand exchange method, and the polymerization can be initiated in the presence of monomers such as (meth) acrylic monomers (methyl methacrylate, ethyl methacrylate, ethyl acrylate). ....) according to the method described for example in Angew. Chem. Int.

Ed., 40, 4016 (2001) or even in Macromol. Chem. Phys., 1941-1946 (2005). The objects obtained according to the method of this second article (gold coated with poly (N-isopropylacrylamide) are particularly well suited to confer according to the invention an anti-reflection effect in the field of infrared on the surface of a substrate.

The synthesis of polymeric bark by ATRP can also be used on inorganic inorganic oxide cores, in particular inorganic cores of silica or titanium oxide (in the form of colloidal particles in particular), for example by processes of the type described in Materials Letters, Vol. 62, Issue 8-9, (2008), or in Composites Science and Technology, Vol. 66, Issue 9, July 2006. It is also possible to graft polymer chains onto the surface of preformed inorganic objects advantageously carrying -OH functions and / or -SH (thio-functionalized gold particles for example) from polycondensation reactions (for example between a dithiol and a di ester), advantageously by contacting preformed inorganic objects carrying -OH and / or -SH functions with:

monomers containing reactive groups, including:

(i) at least one group including an unsaturated α-β carbonyl group C = C-C = O (for example an acrylic, methacrylic or acrylamide group) and / or an unsaturated α-β thiocarbonyl group C = C-C = S; and or

(ii) at least one heterocyclic group comprising from 3 to 5 members (preferably 3 or 4 members), chosen from cyclic ethers, cyclic thioethers and aziridine rings, this heterocyclic group preferably being at least one epoxy group, thioepoxy group; or aziridine, and more preferably at least one epoxy or thioepoxy group; and or

(iii) at least one group chosen from isocyanate groups -N = C = O or thioisocyanate -N = C = S, and trivalent groups of formula> C = CZ-, where Z is an electron-withdrawing group (for example a group 4- nitrophenyl, cyano or -C = N-);

and

a catalyst (C) bearing at least one conjugated guanidine function, preferably carrying a conjugated bis-guanidine function corresponding to the following formula (I): where each of the groups R1 to R7 represents, independently of the other groups:

a hydrogen atom; or - a cyano group -CN; or

a hydrocarbon chain, saturated or unsaturated, linear or branched, optionally cyclized in whole or in part, optionally substituted, and optionally interrupted by one or more heteroatoms (O, S, N, P, or Si, for example) and / or by groups carrying heteroatoms such as carboxy, amide, or carbamate (for example by divalent groups -C (= O) O-, -OC (= O) -, -OC (= O) -O-,> NC (= O) -, -C (= O) -N <,> NC (= O) -O-, -OC (= O) -N <, - C = N-, -N = C-, this chain being typically:

a linear or branched alkyl, alkenyl or alkynyl group advantageously comprising from 1 to 12 carbon atoms, for example from 1 to 6 carbon atoms, this alkyl, alkenyl or alkynyl group being optionally substituted, for example by an alkoxy group;

a cycloalkyl group advantageously comprising from 6 to 18 carbon atoms, optionally substituted for example by at least one alkyl or alkoxy group;

an aryl group advantageously comprising from 6 to 18 carbon atoms, optionally substituted, for example by at least one alkyl or alkoxy group; a heterocycle, optionally aromatic, comprising one or more atoms chosen from S, O or N; an alkylaryl or arylalkyl group advantageously comprising from 8 to 18 carbon atoms, wherein the aryl part is optionally substituted, in particular by an alkyl or alkoxy group;

an ester, amide or carbamate group; or

a polymer chain, optionally carrying other guanidine groups (preferably conjugated guanidine groups, if appropriate).

The catalyst employed preferably corresponds to the formula below:

The inorganic cores based on metal oxide, metal sulphide, metal nitride, metal halide or metal may also be covered (encapsulated) with a polymer shell by any usual method of emulsion or dispersion synthesis, in particular according to radical synthesis methods in emulsion or dispersion.

More generally, it is also possible to create an emulsion bark in particular by metathesis polymerization by any other suitable encapsulation method, for example according to the method described in Soft Matter, vol. 2, pp. 940-949 (2006).

According to another interesting embodiment of the second variant using cores of inorganic nature, the layer (bark) surrounding the inorganic core is made of an inorganic material distinct from that present in the heart, this material constituting the bark including then typically an oxide or a sulfide. In this case, it is preferable that the core consists of metal oxide, metal sulfide or metal.

Commercial products having a metal oxide core and bark are marketed, for example by the company Ibu-Tech (Germany), which proposes, for example, ZnO / SiO2 compositions whose overall size is 40 nm under the reference NA403.

By way of example of particles that may be used according to the invention, mention may be made of the gold-core and silica-bark particles obtained in the article published in the Journal of Nanoparticle Research, 8, 1083-1087 2008), by a reverse emulsion technique involving the formation of micelles of NH4AuCl4 coated with a protective layer of silica obtained and reduction of a gold salt within the micelles.

According to a third variant of the invention, which is more specific, the core of the objects present within the transparent coating deposited on the substrate to be treated according to the invention is a hollow cavity, typically filled with air, having a substantially equal refractive index. at 1, this cavity having dimensions advantageously less than 1 micron, and preferably greater than 20 nm, for example between 50 and 500 nm.

According to this third variant, the layer (bark) surrounding the core is typically constituted by an inorganic material. Most often, the objects present within the transparent coating are typically hollow mineral particles, for example hollow particles of silica or mineral oxide called "hollow sphere" type (that is to say "hollow spheres"). ), for example obtained by microemulsion or precipitation of colloidal particles around texturing agents (called "templates" in English), in particular according to the methods described in Materials Chemistry and Physics, Vol 111, Issue 1, (2008) or Materials Letters Flight. 62, Issue 24, (2008).

In another aspect, the present invention relates to substrates having a surface having anti-reflective properties as obtained according to the method of the present invention. In this context, the invention is particularly object transparent substrates having a surface having anti-reflective properties according to the invention, which have particularly interesting transmission properties.

Substrates whose surface properties are modified to give them an antireflection effect according to the process of the invention can vary to a very large extent. It is advantageously transparent materials, but, according to a particular embodiment, it may also be non-transparent substrates. As examples of substrates, transparent or not, whose surface can be modified according to the process of the invention, there may be mentioned, in a nonlimiting manner:

supports made of organic materials, for example plastic materials, advantageously transparent, for example polycarbonate;

the supports made of inorganic materials, for example:

glass supports or more generally based on inorganic oxides such as, for example, silica and its derivatives, quartz, indium and tin oxide, etc.); or

- metal supports (such as titanium supports)

- the silicon supports.

Furthermore, it should be noted that the surface of the substrate modified according to the invention does not have to be flat so that the deposit can be deposited therein effectively: in fact, soil / gel deposition techniques of the type described higher in the present description allow homogeneous and effective deposits on almost any surface geometry. Thus, the substrate whose surface is modified according to the method of the invention may be in the form of a mass material whose shape does not matter. It can for example be a plate, a lens, a molded part

The transparent surface-modified substrates obtained according to the method of the present invention find many applications, in particular in the field of optics or ophthalmia (eyeglass lenses, etc.), or even in the construction of optical systems. display (LCD screens), of solar cells, for the elements of external architectures (storefront for example), According to a possible embodiment, the modified substrates according to the invention may comprise other layers than the transparent coating providing the anti-reflection effect. In particular, the substrate may for example be coated with one or more underlayer (s) of the hard-coat type, according to means known per se before performing the coating according to the invention.

Various aspects and advantages of the invention will become apparent from the following examples, in which silica-polyester hybrid particles, which have a silica core with a diameter of 80 nm covered by a polymer bark of 200 nm.

These hybrid particles, hereinafter referred to as "hybrid material HR1", used in all of Examples 1 to 4 below, were prepared according to the protocol described below:

Synthesis of hybrid material HR1

Under vigorous stirring, an aqueous dispersion of 20% by mass of silica particles of size equal to 15 nm (Sigma Aldrich) was formed.

Maintaining the stirring, a silane (tetramethoxysilane TMOS) was introduced into the dispersion thus produced at 40 ^° C. in a proportion of 50% by weight relative to the mass of the silica present in the reaction medium, in the presence of a basic catalyst (ammonia), then 0.6 molar equivalents (relative to the amount of TMOS introduced) of a dihydroxy precursor dissolved in ethanol (intended to improve the "grip" of the polymer layer on the core of silica).

The dihydroxy precursor employed was prepared by carrying out an equimolar mixture of isocyanatopropylthethoxysilane and diethanolamine, in the presence of dibutyltin dilaurate, at a temperature of 50 ^° C. Trimethylolpropane (TMP) was then added to this reaction medium and dimethylsuccinate (DMS), each at 8 molar equivalents relative to the TMOS. The medium was allowed to evolve for a few minutes, then the solvents present (water and ethanol) were evaporated under vacuum at 95 ° C. There is thus obtained silica particles of size substantially equal to 80 nm.

Then introduced into the medium, at 40 ⁰ C and under high vacuum (-1 bar), a bisguanidine catalyst corresponding to the following formula:

The introduction of this catalyst induced a polycondensation of the TMP and DMS present in the reaction medium, whereby a polymer bark (polyester type) was created around the silica particles.

The bark-like structure thus obtained was then modified to make it dispersible (in water or monomers). For this purpose, functionalization of the surface of the objects obtained by methacrylate functions was carried out, adding to the obtained particles of methyl methacrylate, in a proportion of 1, 2 molar equivalents with respect to TMP (at 40 ^° C. and under vacuum). -1 bar).

At the end of these different treatments, the hybrid material was obtained

HR1, in the form of a powder comprising silica-polyester hybrid particles, having a silica core of diameter equal to 80 nm, covered by a 200 nm polymer shell.

Example 1

In a flask, at room temperature (25 ° C.), 0.340 g of distilled water, 6.053 g of ethanol and 30 mg of hydrochloric acid (37%), marketed by Sigma Aldrich under the reference 310331, were mixed and then a mixture of 1.446 g of TMOS (99% purity tetramethylorthosilicate) was added. sold by Sigma Aldrich under the reference 218472) and 0.076 g of MPTS (3- (methacryloxy) propyltrimethoxysilane, purity equal to 97% marketed by ABCR under the reference AB117674).

The flask was sealed and the mixture was allowed to react at room temperature (25 ° C.) and with stirring for 4 h.

A solution containing 0.152 g of the hybrid material silica-polyester HR1 above, dissolved in a mixture of 0.038 g of distilled water and 0.673 g of absolute ethanol, was then added to the reaction medium.

The medium thus obtained was stirred at room temperature (25 ° C.) for 1 h and then kept at ambient temperature for 20 h.

70 μL of the composition obtained at the end of these various steps (partially gelled sol) were deposited on a surface of a transparent polycarbonate flat sheet of 2.5 cm × 2.5 cm and of equal thickness. at 0.4 cm. The polycarbonate that has been used in this context is a polycarbonate treated anti-UV brand Makrolon, marketed by Bayer.

The deposit of the composition on the plate was carried out according to the spin-coating technique, by rotating the plate at a speed of 2000 rpm for 10 seconds, immediately after having deposited the soil on this plate. by which a continuous, homogeneous and transparent coating has been obtained on the surface of the plate.

The polycarbonate plate provided with the deposit thus produced was then placed in an oven and underwent the following heat treatment:

1 h at 30 ° C

1 h at 50 ° C for 1 h at 70 ° C.

Thus, a coating layer having 290 nm was deposited on the polycarbonate surface.

This coating of the polycarbonate plate decreases the reflection properties of the plate (antireflection coating), which is demonstrated by measuring the light transmission through the plate before and after treatment. In in this example, the treatment of the plate induces an increase of + 2.9% in the transmission of a radiation of wavelength equal to 550 nm across the plate.

Example 2 In a flask, at room temperature (25 ° C), 0.340 g of distilled water and 6.053 g of ethanol were mixed and then a mixture of 1.446 g of TMOS, 0.076 g of MPTS and 0 was added. 152 g of tetrahydrofurfuryl methacrylate (sold by Sartomer Europe under the reference SR203).

A solution containing 0.152 g of the hybrid material silica-polyester HR1 above, dissolved in a mixture of 0.038 g of distilled water and 0.673 g of absolute ethanol, as well as 0.009 g of Irgacure 184 (photoinitiator), was then added to the reaction medium. The medium thus obtained was stirred at room temperature.

(25 ° C) for 1 h, then stored at room temperature for 2 h

70 μl of the composition obtained at the end of these different steps were then deposited on the surface of a flat transparent polycarbonate plate under the same conditions as in Example 1 (deposition by the spin coating technique. ), then the polycarbonate plate provided with the deposit thus produced was placed in an oven and underwent the following heat treatment:

1 h at 30 ° C

1 h at 50 ° C for 1 h at 70 ° C.

The polycarbonate substrate coated with the cured film from the heat treatment was then irradiated with a Fusion F300S lamp equipped with a bulb H, passing the substrate under the lamp at a speed of 3m / min (this which corresponds to an energy of 1.7 J / cm ² in UV-A (320-390nm) and 1.3J / cm ² in UV-V (395-445nm)) This resulted in a hardened, crosslinked coating layer having a thickness of 290 nm deposited on the polycarbonate surface.

This coating is an antireflection treatment, with an increase of + 2.9% in the transmission of radiation of wavelength equal to 570 nm through the plate.

Example 3

In a flask, at room temperature (25 ° C.), 0.340 g of distilled water containing 2% by weight of Tween 85 and 6.053 g of ethanol were mixed and then a mixture of 1.446 g of TMOS, 0.076 g was added. g of MPTS, 0.009 g of Irgacure 184 and 0.152 g of tetrahydrofurfuryl methacrylate (sold by Sartomer Europe under the reference SR 203. The flask was sealed and the mixture was allowed to react at ambient temperature (25 ° C.) and stirring for 4h.

The medium thus obtained was stirred at room temperature (25 ° C.) for 1 h, then stored at room temperature for 20 hours.

70 μL of the composition obtained at the end of these different steps were then deposited on a surface of a transparent polycarbonate flat plate under the same conditions as in Example 1 (deposition by the spin coating technique), then the polycarbonate plate provided with the deposit thus produced was placed in an oven and underwent the following heat treatment:

1 h at 30 ° C 1 h at 50 ° C

1 h at 70 ° C. 1 h at 70 ° C.

The polycarbonate substrate coated with the cured film from the heat treatment was then irradiated with a Fusion F300S lamp equipped with a bulb H, passing the substrate under the lamp at a speed of 3m / min. In this way, a coating layer having a thickness of 310 nm was deposited on the bicarbonate surface.

This coating provides anti-reflection treatment, with a + 3.2% increase in the transmission of 620 nm wavelength radiation across the plate.

Example 4

In a flask, at room temperature (25 ° C.), 0.340 g of distilled water and 6.053 g of ethanol were mixed and then a mixture of 1.446 g of TMOS, 0.076 g of MPTS and 0.009 g of Irgacide was added. 184 and 0.152 g of tetrahydrofurfuryl methacrylate (marketed by Sartomer Europe under the reference.

The flask was sealed and the mixture was allowed to react at room temperature (25 ° C) with stirring for 2h.

0.046 g of HMDS (1,1,1,3,3,3-hexamethyldisilazane of 99% purity marketed by ABCR under the reference AB109172) was then added to the medium and the mixture was again allowed to react at room temperature (25%). ° C) and stirring for 2h.

The medium thus obtained was stirred at room temperature (25 ° C.) for 1 h, then stored at room temperature for 20 h.

1hà30 ° C

1h at 50 ° C 1h to 70 ° C. 1 h at 70 ° C.

The polycarbonate substrate coated with the cured film from the heat treatment was then irradiated with a Fusion F300S lamp equipped with a bulb H, passing the substrate under the lamp at a speed of 3m / min. This coating provides an anti-reflective effect, with an increase in light transmittance through the polycarbonate plate of + 3% at 690 nm and + 2.6% at 435 nm.

EXAMPLE 5 In this example, an antireflection coating was produced on a transparent polycarbonate flat plate, similarly to the coating made in Example 3, except that the polycarbonate substrate was previously coated with a coating of polycarbonate. type "hard coat" deposited on the plate by the dip coating technique. This hard coat was made using a polysiloxane commercial varnish, marketed by the Korean company Gaema Tech under the trade name Mexmer TE 0801 P

The polycarbonate plate was dipped in the hard coat varnish for 5 seconds, at 20 ⁰ C, and then removed at a speed of 5mm / s. Thermal drying was then applied in an oven at 120 ^° C. for 1 hour.

Once this hardcoat deposited and heat-treated, the coating was carried out on an anti-reflective coating by immersing for 5 seconds the hard-coat-coated plate in the composition C3 of Example 3, stabilized at 20 °. The substrate was then removed from the composition at a speed of 0.5 mm / s, then the plate provided with the deposit thus produced was placed in an oven and underwent the following heat treatment:

1 h at 30 ° C, then

1 h at 50 ^° C., then 1 h at 70 ° C. The polycarbonate substrate thus coated with the hardcoat and the anti-reflection coating was irradiated with a Fusion F300S lamp equipped with a bulb H, passing the substrate under the lamp at a speed of 3 m / h. min.

This coating provides an anti-reflective effect, with an increase in light transmittance through the + 5% polycarbonate plate at wavelengths of 470 to 800nm.

Comparative example

For comparison purposes, a PMMA (polyl methacrylate) deposit was made on a transparent flat polycarbonate plate as used in the previous examples.

PMMA deposition was performed as follows.

A PMMA varnish was prepared by dissolving 1.3 g of a PMA polymer with molecular weights Mn = 77000 g / mol and Mw = 10,000 g / mol (sold by Interchim) in toluene (pure grade). , Xilab). The dissolution was carried out with magnetic stirring for 10 minutes. (Obtaining a solution obtained transparent).

. 100 μl of the PMMA varnish thus obtained was then deposited on the polycarbonate flat surface under the same conditions as in Example 1 (deposit by the spin coating technique), then the polycarbonate plate provided with the deposit was left at room temperature. room temperature (25 ° C) for 2 hours.

The homogeneous coating of PMMA thus deposited provides an anti-reflective effect, with an increase in light transmission through the polycarbonate plate of + 2.4% at 800 nm and +1.6% at 540 nm. This anti-reflection effect in the visible range, due to the refractive index of the

PMMA (1, 49) is less pronounced than the antireflection properties obtained in the context of Examples 1 to 4. This example clearly highlights the specific antireflection effect obtained according to the invention and makes it particularly clear that the effect obtained according to the invention is not due to the overall refractive index of the deposited layer (1, 47 in the context of Example 3 which is close to the value of 1.49 of the present example) but to the "local multilayer structure" specifically made in the context of the invention.

Claims

1. A method of treating a surface of a substrate for imparting to this surface anti-reflective properties vis-à-vis electromagnetic radiation, in which is deposited on said surface a transparent coating vis-à-vis screw of said electromagnetic radiation, which contains, in the dispersed state within said layer, objects of dimensions less than 5 microns, preferably less than 2 microns, said objects comprising at least two zones consisting of transparent substrates vis-à-vis screws are electromagnetic radiation and have distinct refractive indices, namely:

a core having a first refractive index n _c , and

. a layer, called a bark, surrounding the core, having a second refractive index n _E distinct from the refractive index n _c of the heart, where the ratio of the dimensions of the core to the dimensions of the core bark assembly is between 1: 1, 5 and 1: 5.

2. The method of claim 1, wherein the substrate whose surface is modified is a transparent substrate, for example a substrate of glass or polycarbonate.

3. The method of claim 1 or 2, wherein the deposited clear coating is a monolayer coating, preferably having a thickness of 10 nm and 10 microns.

4. Method according to one of claims 1 to 3, wherein in the scattered objects:

the core has dimensions of between 1 nm and 800 nm; and the heart / bark assembly constituted by the core having the first refractive index n _c surrounded by the layer surrounding the core having the second refractive index n _E has dimensions of between 2 nm and 1 micron; and the ratio of the dimensions of the heart to the dimensions of the core / bark assembly being between 1: 1, 5 and 1: 5.

5. Method according to one of claims 1 to 4, wherein the difference (n _c - n _E ) between the refractive indices of the core and the coating layer which surrounds it is, in absolute value, at least 0.01, preferably at least 0.1.

6. Method according to one of claims 1 to 5, wherein the transparent coating is a varnish or a polymer layer.

7. Method according to one of claims 1 to 5, wherein the transparent coating is a sol / gel coating, obtained by hydrolysis of a mineral alkoxide.

The method of claim 7, wherein the sol / gel coating is obtained from a mixture initially comprising (i) at least one inorganic alkoxide; and (ii) at least one monomer crosslinkable under UV or under the effect of a heat treatment.

9. The method of claim 7 or 8, wherein the sol / gel coating is synthesized in the presence of at least one surfactant.

10. Method according to one of claims 7 to 9, wherein the sol / gel coating can typically be synthesized using, as inorganic alkoxide, a mixture of alkoxides comprising: - at least one silane having 4 hydrolyzable groups; and

at least one silane having less than 4 hydrolyzable groups, this silane corresponding preferably to the formula R _n SiX _4-11 , where:

n is an integer equal to 1, 2 or 3; each of the groups R, identical or different, denotes a non-hydrolysable organic group, and - X a hydrolyzable group.

11. Method according to one of claims 1 to 10, wherein the heart of the objects present in the transparent coating deposited on the substrate to be treated according to the invention is organic in nature, and wherein the layer (bark) surrounding said organic core. is a polymer layer.

12. Method according to one of claims 1 to 10, wherein the core of the objects present in the transparent coating deposited on the substrate to be treated according to the invention is of inorganic nature, and wherein the layer (bark) surrounding said organic core. is a polymer layer.

13. Method according to one of claims 1 to 10, wherein the heart of the objects present in the transparent coating deposited on the substrate to be treated according to the invention is of inorganic nature, and wherein the layer (bark) surrounding said organic core. consists of an inorganic material distinct from that present in the heart.

14. Method according to one of claims 1 to 10, wherein the heart of the objects present in the transparent coating deposited on the substrate to be treated according to the invention is a hollow cavity and wherein the layer (bark) surrounding said organic core is made of an inorganic material.

15. Substrate having a surface having anti-reflective properties, obtainable according to the method of one of the preceding claims.