EP1778598A2

EP1778598A2 - Abrasion-resistant and scratch-resistant coatings having a low index of refraction on a substrate

Info

Publication number: EP1778598A2
Application number: EP05752553A
Authority: EP
Inventors: Heike Arndt; Mohammad Jilavi; Martin Mennig; Peter William Oliveira; Helmut Schmidt
Original assignee: Leibniz Institut fuer Neue Materialien Gemeinnuetzige GmbH
Current assignee: LEIBNIZ-INSTITUT fur NEUE MATERIALIEN GEMEINNUETZ
Priority date: 2004-06-08
Filing date: 2005-06-07
Publication date: 2007-05-02
Also published as: JP2008501557A; US20140017399A1; US20080261053A1; DE102004027842A1; WO2005120154A3; WO2005120154A2

Abstract

Disclosed is a substrate with an abrasion-resistant and scratch-resistant coating that has a low index of refraction and comprises magnesium fluoride and at least one metal oxide or metalloid oxide. Said coating can be obtained by applying a coating composition containing magnesium fluoride or a precursor thereof and at least one metal oxide, metalloid oxide, or a precursor thereof onto a substrate and then subjecting the same to a thermal treatment. The inventive coating is suitable for optical layers particularly on translucent substrates. Examples of adequate uses include antireflective layers and packets of interference layer.

Description

ABRASION AND SCRATCH-RESISTANT COATINGS WITH LOW REFRIGERATION ON A SUBSTRATE

The invention relates to a substrate with an abrasion and scratch-resistant coating with a low refractive index, comprising magnesium fluoride and at least one metal or semimetal oxide, a process for its production and its use and the coating composition used for the process and its production.

Many fluorides of alkaline earth metals, in particular magnesium fluoride (MgF ₂ ), are distinguished by a low refractive index and are therefore of great interest as materials for dielectric multiple coatings and especially for anti-reflective coatings. These materials are used both as components in multilayer systems and as single layers. Magnesium fluoride is used in particular as the λ / 4 anti-reflective single layer.

In general, thin MgF ₂ layers are produced using complex and expensive PVD and CVD processes or sputtering. The disadvantage of this method is that the coating of large substrates becomes very tedious and expensive and curved substrates cannot be coated homogeneously. In addition, good abrasion resistance cannot be achieved. Another disadvantage of the layers produced in this way is that they are generally very dense and therefore approximately have a refractive index (n = 1.38) that corresponds to the bulk material. This refractive index is low enough to be used in multi-layer systems. However, in order to achieve optimal anti-reflective properties on common substrates with a refractive index of approx. 1.5 (eg glass), an anti-reflective single layer ideally has a refractive index of n = 1.22

(n _S c _h , c _h t ~ -jn _substrate ^■ n _air = Jϊ $ = 1, 22).

One way to lower the refractive index of a layer is to increase its porosity. This can be achieved by applying the MgF ₂ layers wet-chemically. EP-A-0641739 describes the synthesis of sodium magnesium fluoride sols (NaF-MgF ₂ ). Aqueous sodium fluoride and magnesium salt solutions are mixed and the by-product salts formed are then separated off by complex filtration processes. The aggregates of the colloidal particles obtained are finally wet-milled. In order to obtain anti-reflective coating materials, the NaF-MgF ₂ sols are mixed with film formers after a solvent exchange and applied to glass. However, the transmission of the coated glasses is only 94.05% (550 nm) compared to 91.61% (550 nm) for the uncoated glass. In addition to the complex synthesis of the coating material, the inadequate antireflection effect of these layers is a disadvantage.

Similarly, JP-A-2026824 produces magnesium fluoride brine. Here too, aqueous magnesium salt solutions are mixed with aqueous fluoride solutions and heated. Here too, by-product salts have to be removed by means of ultrafiltration.

According to EP-A-1 315 005, thin layers with a refractive index of n = 1.16 (193 nm) on optical substrates lead to a transmission loss of less than 0.5%. The layers are produced by applying an MgF ₂ sol. The MgF ₂ sol is obtained by reacting magnesium acetate with hydrofluoric acid in methanol and then autoclaved.

IM Thomas, Applied Optics 27 (1988) 3356-3358, also describes the production of magnesium fluoride sols by reacting magnesium acetate tetrahydrate or magnesium methoxide with hydrofluoric acid in dry methanol. Quartz glass coated with these sols shows a transmission of approximately 100% (- 350 nm), ie the refractive index of these layers is approximately 1.2. Thus, anti-reflective coatings with the desired optical properties are accessible, but an immense disadvantage of this method is the use of hydrofluoric acid (HF), since hydrofluoric acid is highly toxic. A sol-gel route for the production of magnesium fluoride with non-toxic starting materials is described in EP-A-071348. For example, magnesium is dissolved in an anhydrous solvent and converted to the fluoroalkoxide with fluorinated alcohols. After filtering the solution, the Mg alkoxides are hydrolysed. Although this process has the advantage that non-toxic, harmless starting materials are assumed, starting materials such as anhydrous solvents and fluorinated alcohols are expensive. Furthermore, this patent application does not contain any information about the optical or mechanical properties of layers which can be obtained by immersion application, for example on glass of the brine described above.

Another method for producing magnesium fluoride layers, which is based on non-toxic starting materials, is described in US Pat. No. 4,4932,721. Magnesium fluoride layers are produced there by the thermal disproportionation of fluorine-containing magnesium compounds such as magnesium trifluoroacetate, magnesium tπ ^' fluoroacetylacetonate or magnesium hexafluoroacetylacetonate. The compounds mentioned are dissolved in organic solvents such as butyl acetate or ethylene glycol monoethyl ether, applied to substrates (glass, quartz glass) by spinning, spraying or dipping and cured at at least 300 ° C. for at least 1 min. The layers obtained in this way have a refractive index of 1.36 to 1.38 and are therefore in the range of the bulk material. Glass substrates coated in this way nevertheless show a residual reflection of 0.5%.

Magnesium fluoride layers are produced in a similar manner according to S. Fujihara et al., Journal of Sol-Gel-Science and Technology 19 (2000) 311-314. One route includes the conversion of magnesium acetate with trifluoroacetic acid (TFA) and water in 2-propanol. The application of this brine to quartz glass by spinning and subsequent hardening of the layers at 400 ° C to 500 ° C result in layers whose refractive index is in the desired range (n = 1.2). However, our own studies have shown that serious wetting problems occur when the brine produced according to this regulation is applied. If, on the other hand, magnesium ethanolate (Mg (OEt) ₂ ) is reacted with trifluoroacetic acid (TFA) in 2-propanol to magnesium trifluoroacetate (S. Fujihara et al., Thin Solid Films 304 (1997) 252-255), it has been shown in the application that it has been applied rarely wetting problems. The brine produced in this way was applied by spinning onto quartz glass and cured at temperatures from 300 ° C. to 600 ° C. for 10 minutes. The substrates coated in this way have a relatively low transmission of a maximum of approximately 96.6%. Information on the refractive index of these layers is missing.

It is known that sols of metal or semi-metal oxides for layers made of metal or semi-metal oxides, such as ZrO ₂ , Al ₂ O 3, TiO ₂ , Ta ₂ O ₅ or SiO ₂ layers, give coatings with good optical quality can, but their refractive index is significantly higher (1.46 to 2.3) than that of MgF ₂ layers.

The object of the invention was to provide a wet-chemical synthesis route for low-refractive optical layers using non-toxic or only slightly toxic starting materials, which are characterized by good optical quality and in particular a low refractive index. In addition, these layers should have an abrasion resistance that goes beyond the state of the art.

Surprisingly, the object was achieved by a coating composition comprising magnesium fluoride or a precursor thereof and at least one metal or semimetal oxide or a precursor thereof. The coating composition according to the invention can easily be applied wet-chemically to a substrate and hardened or compacted by heat treatment. The invention thus also provides a substrate with an abrasion and scratch resistant coating with a low refractive index, comprising magnesium fluoride and at least one metal or semimetal oxide.

Surprisingly, it was found that no significant increase in the refractive index of the magnesium fluoride metal / metal oxide layers compared to pure magnesium fluoride layers, but their scratch resistance increases significantly.

The coating composition comprises magnesium fluoride or a precursor thereof and at least one metal or semimetal oxide or a precursor thereof. Preferably the at least one metal or semimetal oxide or a precursor thereof is present in the coating composition as a sol, i.e. the coating composition is preferably a coating sol. The magnesium fluoride or a precursor thereof can be in the form of a sol or as a solution. The coating composition is preferably prepared by mixing a sol or a solution of magnesium fluoride or a precursor thereof and a sol of at least one metal or semimetal oxide or a precursor thereof.

The sol or solution of magnesium fluoride or a precursor thereof can be prepared in any manner known in the art, some of which have been listed above. The sol or solution is preferably obtained from the reaction of a magnesium compound, preferably a hydrolyzable magnesium compound, with a fluorinated organic compound, the reaction usually being carried out in an organic solvent. Hydrolyzable here also means the hydration ability of the Mg compound.

A preliminary stage here means, in particular, compounds of magnesium which can be converted into MgF ₂ , in particular under the conditions for producing the substrate according to the invention, such as in the heat treatment. For example, magnesium compounds or complexes of fluorinated organic compounds can be converted into magnesium fluoride by a thermal disproportionation reaction. If necessary, disproportionation reactions or the conversion into MgF _{2 take place} already at room temperature, so that MgF ₂ can already be contained in the sol or the solution. Of course, if necessary, the mixture of magnesium compound and fluorinated compound can also be heated, for example in order to promote the conversion into MgF ₂ in the sol or the solution. All compounds which can be reacted with a fluorinated organic compound, in particular hydrolyzable magnesium compounds, are suitable as the magnesium compound. Examples are magnesium alkoxides. The alkoxy group of the magnesium alkoxide preferably has 1 to 12 carbon atoms, with magnesium methoxide, magnesium ethoxide, magnesium propoxide and magnesium butoxide being preferred. The most preferred compound is magnesium ethanolate (Mg (OEt) ₂ ). The alkoxide can be linear or branched, for example n-propanolate or isopropanolate.

An organic compound having a CF ₃ group is preferably used as the fluorinated organic compound. Organic compounds which are preferably used are ketones, in particular β-diketones, and carboxylic acids. Examples are trifluoroacetyl acetone, hexafluoroacetylacetone and trifluoroacetic acid, with trifluoroacetic acid being particularly preferred.

Any suitable solvent may be used as the solvent, e.g. one of those listed below for the manufacture of the metal or semimetal oxides. For example, Alcohols. Examples are ethanol, n-propanol, 2-propanol or butanol.

A preferred production route for the sol or the solution with magnesium fluoride or a precursor thereof can be described as follows. A hydrolyzable magnesium compound, preferably magnesium alcoholate, particularly preferably magnesium ethylate, is dispersed in an organic solvent, preferably an alcohol, particularly preferably 2-propanol, and then reacted with an organic compound which contains at least one CF ₃ group. For this purpose, ketones and carboxylic acid, in particular trifluoroacetic acid, containing CF ₃ groups are preferably used. Any undissolved constituents that may be present are then filtered off.

All oxides of metals or semimetals (hereinafter also abbreviated as M) can be used as metal or semimetal oxides. In particular, oxides of metals or semimetals from main groups III to VI, in particular in particular the main groups III and IV, and / or the subgroups, preferably the subgroups II to V, of the periodic table of the elements and also lanthanides and actinides or mixed oxides thereof. Preferred metals or semimetals M for the metal or semimetal oxides are, for example, B, Al, Ga, In, Si, Ge, Sn, Pb, Y, Ti, Zr, V, Nb, Ta, Mo, W, Fe, Cu, Ag , Zn, Cd, Ce and La or mixed oxides thereof. A type of oxide or a mixture of oxides can be used.

Examples of oxides which can optionally be hydrated are ZnO, CdO, SiO ₂ , GeO ₂ , TiO ₂ , ZrO ₂ , CeO ₂ , SnO ₂ , Al ₂ O ₃ (boehmite, AIO (OH), also as aluminum hydroxide), B ₂ O _3l ln ₂ O ₃ , La ₂ O ₃ , Fe ₂ O ₃ , Fe ₃ O, Cu ₂ O, Ta ₂ O ₅ , Nb ₂ O ₅ , V ₂ O ₅ , M0O3 or WO ₃ . Also silicates, zirconates, aluminates, stannates of metals or semimetals, and mixed oxides, such as indium tin oxide (ITO), antimony tin oxide (ATO), fluorine-doped tin oxide (FTO), luminous pigments with Y or Eu- containing compounds, spinels, ferrites or mixed oxides with a perovskite structure such as BaTiθ ₃ and PbTiO ₃ can be used.

Preference is given to semimetal or metal oxides, which are optionally hydrated (oxide hydrate), of Si, Ge, Al, B, Zn, Cd, Ti, Zr, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V, Mo or W. Particularly preferred are SiO ₂ , Al ₂ O ₃ , Ta ₂ Os, ZrO ₂ and TiO ₂ , with ZrO _{2 being} the most preferred.

The sol of at least one semimetal or metal oxide can be produced by dispersing produced particles, in particular nanoscale particles, in a solvent or in situ. The particles can usually be made in various ways, e.g. through flame pyrolysis, plasma processes, colloid techniques, sol-gel processes, controlled germination and growth processes, MOCVD processes and emulsion processes. These methods are described in detail in the literature.

The sol of at least one semimetal or metal oxide is preferably produced by a sol-gel process. In the sol-gel process, hydrolyzable compounds are usually hydrolyzed with water, if appropriate with acidic or basic catalysis, and, if appropriate, at least partially condensed. The Hydrolysis and / or condensation reactions lead to the formation of compounds or condensates with hydroxyl, oxo groups and / or oxo bridges, which serve as precursors. The sol containing the oxides or precursors can be obtained by suitably setting the parameters, for example degree of condensation, solvent, temperature, water concentration, duration or pH. The precursors of the oxides mean in particular the condensation products mentioned. Further details of the sol-gel process are available, for example, from CJ Brinker, GW Scherer: "Sol-Gel Science - The Physics and Chemistry of Sol-Gel-Processiπg", Academic Press, Boston, San Diego, New York, Sydney (1990) described.

The hydrolysis and condensation can be carried out in a solvent, but they can also be carried out without a solvent, with the hydrolysis being able to form solvents or other liquid constituents.

Suitable solvents are both water and organic solvents or mixtures. These are the usual solvents used in the field of coating. Examples of suitable organic solvents are alcohols, preferably lower aliphatic alcohols (Ci-Cs alcohols), such as methanol, ethanol, 1-propanol, isopropanol and 1-butanol, ketones, preferably lower dialkyl ketones, such as acetone and methyl isobutyl ketone, ethers, preferably lower Dialkyl ethers, such as diethyl ether, or diol monoethers, amides, such as dimethylformamide, tetrahydrofuran, dioxane, sulfoxides, sulfones or butyl glycol and mixtures thereof. Alcohols are preferably used. High-boiling solvents can also be used. In the sol-gel process, the solvent can optionally be an alcohol formed from the alcoholate compounds during the hydrolysis.

In principle, all hydrolyzable metal or semimetal compounds, for example the metals and semimetals M listed above, are suitable as hydrolyzable compounds. One or more hydrolyzable compounds can be used. The hydrolyzable metal or semimetal compound is preferably a compound of the general formula MX _n (1), in which M is the metal or semimetal defined above, X is a hydrolyzable group which can be identical or different, two groups X being by a bidentate hydrolyzable group or an oxo group can be replaced or three groups X can be replaced by a tridentate hydrolyzable group, and n corresponds to the valence of the element when X has a charge of 1 and is often 3 or 4. If appropriate, the hydrolyzable compound can also have non-hydrolyzable groups which partially replace the hydrolyzable groups.

Examples of the hydrolyzable groups X, which can be identical or different, are hydrogen, halogen (F, Cl, Br or I, in particular Cl or Br), alkoxy (for example C _1-6 alkoxy, for example methoxy, ethoxy, n-propoxy, i-propoxy and n-, i-, sec.- or tert-butoxy), aryloxy (preferably Cβ-io-aryloxy, such as phenoxy), alkaryloxy, e.g. benzoyloxy, acyloxy (e.g. Cι _-6 - Acyloxy, preferably -C _-4 -acyloxy, such as acetoxy or propionyloxy), amino and alkylcarbonyl (eg C _2-7 alkylcarbonyl such as acetyl) or complexing agents such as ß-dicarbonyls (eg acetyl acetonato). The groups mentioned can optionally contain substituents, such as halogen or alkoxy. Preferred hydrolyzable radicals X are halogen, alkoxy groups and acyloxy groups, with alcoholates being particularly preferred. The compounds can also be stabilized with additional complexing compounds.

Examples of titanium compounds of the formula 1X ₄ are TiCl ₄ , Ti (OCH ₃ ) ₄ , Ti (OC ₂ H ₅ ) ₄ , Ti (pentoxy) ₄ , Ti (hexoxy), Ti (2-ethylhexoxy), Ti (n-OC ₃ H ₇ ) or Ti (i-OC ₃ H ₇ ) ₄ . Further examples of usable hydrolyzable compounds of elements M are AI (OCH ₃ ) ₃ , AI (OC ₂ H ₅ ) ₃ , AI (OnC ₃ H ₇ ) ₃ , AI (OiC ₃ H ₇ ) ₃ , AI (OnC ₄ H ₉ ) ₃ , AI (O-sec.-C ₄ H ₉ ) ₃ , AICI ₃ , AICI (OH) ₂ , AI (OC ₂ H ₄ OC ₄ H ₉ ) ₃ , ZrCI ₄ , Zr (OC ₂ H ₅ ) ₄ , Zr (OnC ₃ H ₇ ) ₄ , Zr (0-iC ₃ H ₇ ) ₄ , Zr (OC Hg), ZrOCI, Zr (pentoxy) ₄ , Zr (hexoxy) ₄ , Zr (2-ethylhexoxy) ₄ , and Zr compounds that have complexing residues, such as, for example, β-diketone and (meth) acrylic residues, boric acid, BCI ₃ , B (OCH ₃ ) ₃ , B (OC ₂ H ₅ ) ₃ , SnCl ₄ , Sn ( OCH ₃ ) ₄ , Sn (OC ₂ H ₅ ) ₄ , VOCI ₃ and VO (OCH ₃ ) ₃ . Examples of silanes of the formula SiX ₄ are Si (OCH ₃ ), Si (OC ₂ H ₅ ) 4, Si (0-n- or -iC ₃ H ₇ ) ₄ , Si (OC ₄ H ₉ ) ₄ , SiCl ₄ , HSiCI ₃ , Si (OOCCH ₃ ) ₄ . Of these Silanes are preferred tetraalkoxysilanes, with those having CC alkoxy being particularly preferred, in particular tetramethoxysilane and tetraethoxysilane (TEOS).

If appropriate, the semimetal or metal oxides can also be prepared in the presence of a complexing agent. Examples of suitable complexing agents are e.g. unsaturated carboxylic acids and ß-dicarbonyl compounds, e.g. (Meth) acrylic acid, acetylacetone and ethyl acetoacetate.

Furthermore, if necessary, an adhesion promoter can be used, which usually interacts or is bound or complexed with the particle of semimetal or metal oxide or the precursor thereof and thereby surface-modifies the particle and thereby promotes adhesion to the substrate. In addition to the group for the attachment to the semimetal or metal oxide or the precursor thereof, the adhesion promoter preferably has another functional group. Complexing agents can also be suitable as adhesion promoters.

Examples of an adhesion promoter are unsaturated carboxylic acids such as (meth) acrylic acid and a hydrolyzable silane with at least one non-hydrolyzable group, the silane being particularly suitable for sols of SiO ₂ .

Examples of hydrolyzable silanes with at least one non-hydrolyzable group as adhesion promoters are compounds of the formula RSiX ₃ (II), in which X is as defined in formula (I). The non-hydrolyzable radical R can be non-hydrolyzable radicals R without a functional group or preferably with a functional group.

The non-hydrolyzable radical R is, for example, alkyl (preferably ds-alkyl), alkenyl (preferably C _2-6 alkenyl), alkynyl (preferably C _2-6 alkynyl) and aryl (preferably C _6- io-aryl). The radicals R and X may optionally have one or more customary substituents, such as halogen or alkoxy. Specific

Examples of the functional groups of the radical R are the epoxy, hydroxy, ether, amino, monoalkylamino, dialkylamino, amide, carboxy, vinyl, acryloxy, Methacryloxy, cyano, halogen, aldehyde, alkylcarbonyl, and phosphoric acid groups. These functional groups are bonded to the silicon atom via alkylene, alkenylene or arylene bridge groups, which can be interrupted by oxygen or -NH groups. The bridge groups mentioned are derived, for example, from the alkyl, alkenyl or aryl radicals mentioned above. The radicals R having a functional group preferably contain 1 to 18, in particular 1 to 8, carbon atoms.

Examples of silanes of the formula (II) are hydrolyzable silanes with a glycidyloxy group, amino group or (meth) acryloxy groups, such as γ-glycidyloxypropyltrimethoxysilane, γ-glycidyloxypropyltriethoxysilane, 3- (meth) acryloxypropyltri (m) ethoxysilane, 3- (meth ) acryloxypropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-aminoethyl-3-aminopropyltrimethoxysilane, trimethoxysilylpropyldiethylenetriamine. (Meth) acrylic stands for methacrylic or acrylic. Further specific examples of hydrolyzable silanes with non-hydrolyzable groups can e.g. can be found in EP-A-195493. The surface modification of nanoscale particles is a known method, as described by the applicant e.g. in WO 93/21127 (DE 4212633) or WO 96/31572.

The semimetal or metal oxide sol is thus preferably synthesized from the corresponding hydrolyzable compound, preferably from the metal alkoxide, by hydrolysis, optionally in the presence of a catalyst and / or complexing agent. ZrO ₂ sols are preferably used, the z. B. from zirconium tetra-π-propylate can be prepared by reaction with hydrochloric acid in the presence of acetylacetone.

The sol from the at least one metal or semimetal oxide or its precursors are then mixed with the solution or the sol of magnesium fluoride or its precursors. The ratio can vary widely. In general, however, the amounts are chosen such that the molar ratio of the amount of magnesium (Mg) in magnesium fluoride or its precursors to metal or semimetal (M) in the at least one metal or semimetal oxide or its precursors Mg / M in the coating composition in the range of 1: 0.01 to 1: 1.8, more preferably in the range from 1: 0.05 to 1: 0.5 or 1: 0.1 to 1: 0.5 and particularly preferably from 1: 0.1 to 1 : 0.2 lies.

In addition to magnesium fluoride or its precursors and the at least one metal or semimetal oxide or its precursors, the solvent or solvents and optionally complexing agents or adhesion promoters, the coating composition preferably comprises essentially no further components. However, it is conceivable to add other additives.

Therefore, magnesium fluoride or its precursors and the at least one metal or semimetal oxide or its precursors preferably make up at least 80% by weight, more preferably at least 90% by weight and particularly preferably at least 95% by weight of the solids content of the coating composition. The proportion of magnesium fluoride or its precursors is preferably at least 10% by weight, more preferably at least 20% by weight and particularly preferably at least 30% by weight, based on the solids content of the coating composition.

The coating composition is applied to a substrate. In principle, all substrates come into consideration. Examples of a suitable substrate are substrates made of metal, semiconductor, glass, ceramic, glass ceramic, plastic, crystalline substrates or inorganic-organic composite materials. Preferably substrates are used which are stable to a thermal treatment of the coating. The substrates can be pretreated, e.g. for cleaning, through a corona treatment or with a pre-coating (e.g. a paint or a metallized surface).

The layers obtained are used in particular for optical coatings or optical or optoelectronic applications. Preferred substrates are in particular those which are translucent at least in a certain region or in certain regions of the light spectrum from UV light to visible light to infrared light. Transparent substrates with translucency in the range of visible light are particularly useful. Examples of plastic substrates are polycarbonate, polymethyl methacrylate, polyacrylate, polyethylene terephthalate. Transparent plastics, glasses (e.g. silicate glasses, such as window glass or optical glasses, silica glass, quartz glass, borosilicate glass or soda lime silicate glass, chalcogenide and halide glasses etc.) and crystalline substrates (e.g. sapphire, silicon or lithium niobate) are preferred.

Suitable substrates for optical applications are e.g. Flat glasses, watch glasses, instrument covers, lenses and other optical elements, plastic films or transparent containers.

All common wet chemical methods for producing optical layers, such as, for. B. dipping, spinning, spraying, roll coating techniques or combinations of these, as well as common printing processes, eg. B. screen printing, flexographic printing or pad printing can be used. Other coating processes are knife coating, casting, brushing, flood coating, slot coating, meniscus coating or curtain coating.

After the applied coating composition has dried, the coating is subjected to a thermal aftertreatment, for example above 50 ° C. The temperature used can vary within wide ranges, preferably a heat treatment in the temperature range from 100 ° C. and 600 ° C., more preferably from 300 to 500 ° C., particularly preferably from 400 to 450 ° C. The optical properties (eg reflection, refractive index) and the mechanical properties can be controlled by the choice of temperature. They depend on the optical properties of the substrate (refractive index), on the intended optical purpose (anti-reflective coating, interference layer package), on the thermal resistance of the substrate and on the desired application (outdoor application, indoor application). The heat treatment can, for example, harden or densify and / or convert the precursors into MgF ₂ or the oxide.

The ratio of Mg to metal or semimetal in the finished view corresponds at least approximately to the ratio in the coating composition. As there, the ratio can vary widely. In general, the material Quantity ratio of magnesium (Mg) in magnesium fluoride to metal or semimetal (M) in the at least one metal or semimetal oxide in the coating in the range from 1: 0.01 to 1: 1.8, more preferably in the range from 1: 0.05 to 1: 0.5 or 1: 0.1 to 1: 0.5 and particularly preferably from 1: 0.1 to 1: 0.2.

The layers preferably consist essentially of MgF ₂ and the at least one semimetal or metal oxide. If necessary, the above-mentioned complexing agents or adhesion promoters or other additives can be contained in the finished coating in relatively small amounts. Organic components used, such as complexing agents or from the adhesion promoter, can optionally be volatile during the heat treatment or burned out. It is usually mostly or essentially inorganic layers.

Magnesium fluoride and the at least one metal or semimetal oxide preferably make up at least 80% by weight, more preferably at least 90% by weight and particularly preferably at least 95% by weight of the coating. The proportion of magnesium fluoride in the coating is preferably at least 10% by weight, more preferably at least 20% by weight and particularly preferably at least 30% by weight.

The layer thickness can vary within wide ranges, but is usually in the range from 20 nm to 1 μm, preferably 30 to 500 nm and particularly preferably 50 to 250 nm.

The coating according to the invention can be used as a single layer or as a layer from a multi-layer package. The other layers can be the same, possibly with different ratios, or different, usually also optical layers. Accordingly, further layers can be applied to the substrate in a conventional manner before and / or after the coating.

As a rule, the coating is used as an optical coating. The coating is particularly suitable for anti-reflective coatings, in particular as a single layer, and for interference layer packages. These antireflective and interference boundary layers are preferably used on transparent substrates or substrates which are translucent in at least one region of the wavelength range from UV light to IR light.

The following examples further illustrate the invention without restricting it. The solution or sol with MgF ₂ or its precursors is simply called MgF ₂ sol, even if it is a solution of MgF ₂ precursors.

Examples

A) Production of the brine

MgF ₂ sol: At room temperature, 25.396 g (0.22 mol) of magnesium ethylate are added to 522.810 g of 2-propanol. 51.016 g (0.35 mol) of trifluoroacetic acid (TFA) are added to the stirred dispersion and the mixture is stirred at room temperature. A slight warming of the reaction mixture is observed at the start of the reaction. As the reaction progresses, the reaction mixture becomes increasingly clear. After 2 hours, any insoluble constituents present are separated off using a syringe filter (1.2 μm) and the reaction mixture is then left to stand at room temperature. A colorless precipitate forms overnight, which is separated off using a pleated filter. The filtrate is filtered again through a 1.2 μm syringe filter and a yellow solution results. The coating composition is stable in storage at room temperature for at least 4 weeks.

SiO ₂ sol: At room temperature, 13.29 g (87.3 mmol) of tetramethoxysilane (TMOS) are dissolved in 11.80 g of ethanol. A mixture of 13.40 g (744.4 mmol) of water, 0.30 g of hydrochloric acid (37%) and 11.80 g of ethanol is added with stirring. The mixture is stirred at room temperature for at least 2 h (brief heating of the reaction mixture after addition) and diluted with 130 g of 2-propanol. Immediately after the dilution, 0.30 g (1.52 mmol) of 3-glycidyloxypropyltrimethoxysilane (GPTS) and 18 mg (0.08 mmol) of 3-aminopropyltriethoxysilane in 26 g are stirred 2-propanol added. The mixture is stirred at room temperature for 1 h and results in a colorless, clear sol which is stable in storage at 4 ° C. for at least 4 weeks.

Al ₂ O ₃ sol: 40 g (0.16 mol) of aluminum tri-sec-butoxide are dissolved in 240 g of 2-propanol at room temperature with stirring. 8 g (0.08 mol) of acetyl acetone and 3.2 g (0.18 mol) of water are added with stirring. The mixture is stirred at room temperature for 1 h and filtered through a 0.45 μm syringe filter. The result is a yellow, clear sol.

Zr0 ₂ sol: 24 g (51.3 mmol) of zirconium tetra-n-propylate (70% by weight in 1-propanol) are dissolved in 240 g of 2-propanol at room temperature. 2.553 g (25.5 mmol) of acetylacetone are added with stirring and the mixture is stirred for 10 min. Then 1.8 g of concentrated salsic acid are added and the mixture is stirred at room temperature for 1 h. Filtration through a 5 μm syringe filter results in a yellow, clear sol.

B) Preparation of the MgF ₂ composites

By simply mixing the MgF ₂ sol with the appropriate amounts of SiO ₂ , Al ₂ O ₃ or ZrO ₂ sol, MgF ₂ composite sols are produced.

C) Coating process

Lime sodium silicate glass panes are cleaned by rubbing them with ethanol and coated with the respective sol using the immersion process (3.5 mm / s). The coating is cured at 450 ° C. for 30 minutes.

D) characterization

The scratch resistance is tested with a steel wool test (steel wool 0000, 250 g / 1 cm ² , 10 cycles). The damage (number of scratches generated) is assessed using light microscopy. The reflectivity is determined spectroscopically. Example 1 MgF ₂ / Si0 ₂ composites

The behavior of the layers obtained with respect to the transmission was examined and is shown in FIG. 1.

A significant improvement in scratch resistance is achieved with a mixing ratio of MgF ₂ sol / SiO ₂ sol of 50/60. The transmission is higher than in the case of the uncoated glass and also higher than in the case of a pure SiO ₂ layer.

Example 2 MgF ₂ / Al ₂ θ ₃ composites

The behavior of the layers obtained with respect to the transmission was examined and is shown in FIG. 2.

The scratch resistance of MgF ₂ layers is improved by adding Al ₂ O ₃ sols. MgF ₂ / AI ₂ O ₃ mixtures of 80/20 show the best performance with improved scratch resistance with a transmission of up to approx. 99%. Example 3: MgF ₂ / Zr0 ₂ composites

The behavior of the layers obtained with respect to the transmission was examined and is shown in FIG. 3.

The significant improvement in scratch resistance is achieved with a mixing ratio of MgF ₂ sol / ZrO ₂ sol of 80/20. The transmission is still very good with a maximum of approx. 98%.

Determination of refractive indices and layer thicknesses by ellipsometry for Examples 2 and 3

Claims

1. Substrate with an abrasion and scratch resistant coating with a low refractive index, comprising magnesium fluoride and at least one metal or semimetal oxide.

2. Substrate according to claim 1, characterized in that magnesium fluoride and the at least one metal or semimetal oxide make up at least 80% by weight of the coating.

3. Substrate according to claim 1 or 2, characterized in that the molar ratio of the amount of magnesium (Mg) in magnesium fluoride to metal or semimetal (M) in the at least one metal or semimetal oxide Mg / M in the coating in the range of 1: 0.01 to 1: 1.8.

4. Substrate according to one of claims 1 to 3, characterized in that the coating comprising magnesium fluoride and at least one metal or semimetal oxide is a single layer or a component of a multilayer system.

5. Substrate according to one of claims 1 to 4, characterized in that the at least one metal or semimetal oxide is ZrO ₂ , TiO _2r Al ₂ O _3ι Ta ₂ O and / or SiO ₂ .

6. Substrate according to one of claims 1 to 5, characterized in that the substrate is translucent in at least one region of the wavelength range from UV light to IR light.

7. Substrate according to one of claims 1 to 6, characterized in that the substrate is a transparent plastic, a glass or a crystalline substrate.

8. Substrate according to one of claims 1 to 7, characterized in that magnesium fluoride makes up at least 10% by weight of the coating.

9. Substrate according to one of claims 1 to 8, characterized in that the substrate is a flat glass, a watch glass, an instrument cover, a lens or another optical element, a plastic film or a transparent container.

10. A method for producing a substrate with an abrasion and scratch-resistant coating with a low refractive index according to one of claims 1 to 9, characterized in that one comprises a coating composition, the magnesium fluoride or a precursor thereof and at least one metal or semimetal oxide or a precursor thereof contains, applied to the substrate and heat treated.

11. The method according to claim 10, characterized in that the coating composition is applied to the substrate by a coating or printing process.

12. The method according to claim 10 or 11, characterized in that the heat treatment is carried out at a temperature of 100 to 600 ° C.

13. The method according to any one of claims 10 to 12, characterized in that the substrate is provided with one or more identical or different layers before and / or after the coating.

14. Use of a substrate with an abrasion and scratch-resistant coating with a low refractive index according to one of claims 1 to 9 as an interference layer package on a transparent substrate or as a single anti-reflective layer on a transparent substrate.

15. A coating composition for a low refractive index abrasion and scratch resistant coating comprising magnesium fluoride or a precursor thereof and at least one metal or semimetal oxide or a precursor thereof.

16. Coating composition according to claim 15, characterized in that it comprises magnesium fluoride or a precursor thereof as a sol or in solution and the at least one metal or semimetal oxide or a precursor thereof as a sol.

17. A coating composition according to claim 15 or claim 16, characterized in that magnesium fluoride or a precursor thereof and the at least one metal or semimetal oxide or a precursor thereof make up at least 80% by weight of the solids content of the coating composition.

18. A method for producing a coating composition according to any one of claims 15 to 17, in which a sol or a solution of magnesium fluoride or a precursor thereof and a sol of at least one metal or semimetal oxide or a precursor thereof are mixed together.

19. The method according to claim 18, characterized in that the sol or the solution of magnesium fluoride or a precursor thereof is obtained by reacting a magnesium compound with a fluorinated organic compound in an organic solvent.

20. The method according to claim 19, characterized in that the fluorinated organic compound is a compound having a CF ₃ group, preferably a ketone or a carboxylic acid, particularly preferably trifluoroacetic acid.

21. The method according to claim 19 or 20, characterized in that the magnesium compound is a magnesium alcoholate, preferably magnesium ethylate.

22. The method according to any one of claims 19 to 21, characterized in that the organic solvent is an alcohol.

23. The method according to any one of claims 19 to 22, characterized in that the sol of at least one metal or semimetal oxide or a precursor thereof is obtained by hydrolysis of a hydrolyzable metal or semimetal compound, preferably a metal or semimetal alcoholate.

24. The method according to claim 23, characterized in that a complexing agent and / or an adhesion promoter is added to the coating composition.