WO2016193000A1 - Coating composition containing aluminum oxide and method for coating separators therewith - Google Patents

Abstract

The invention relates to a coating composition, which can be obtained by mixing an aluminum-oxide-containing dispersion and a binder, wherein A) the aluminum-oxide-containing dispersion contains a) at least 40 wt% of aluminum oxide in the form of aggregated primary particles having a BET surface area of 20 to 200 m²/g and an average particle diameter d₅₀ in the dispersion of less than 100 nm, b) at most 60 wt% of a liquid phase, which consists of at least 90 wt% of one or more organic solvents having a boiling point of 50°C to 250°C, wherein the organic solvent consists of more than 90 wt% of one or more C₁-C₄ alcohols, c) 1-5 wt% of at least one mineral acid and/or one C₁-C₅ carboxylic acid, d) 0.1-1 wt% of at least one organophosphonic acid, e) 0.1-1 wt% of at least one sulfonic acid and/or salt thereof, and B) the binder f) is selected from the group consisting of polyethylene oxide, polyvinylidene fluoride, polyvinylidene chloride, polytetrafluoroethylene, polyacrylonitrile, polyimides, polyetheretherketone, polymethyl methacrylate, polytetraethylene glycol diacrylate, polyvinylidene fluoride/hexafluoropropylene copolymer, polyvinylidene fluoride/chlorotrifluoroethylene copolymer, and mixtures thereof, and g) is in the form of a solution or dispersion in an organic solvent having a boiling point of 50°C to 250°C, and C) the weight ratio of aluminum oxide to binder is 50:50 to 95:5.

Description

 Aluminum oxide-containing coating composition and

 Process for coating separators with it

The invention relates to a coating composition obtainable by mixing an alumina-containing dispersion and a binder. The invention further relates to a method for coating a membrane by means of this coating composition.

Furthermore, the invention relates to the use of the alumina-containing dispersion for producing a coating composition for a separator of an electrochemical cell and the coating of powdered cathode materials.

In the manufacture of energy storage devices, such as a lithium ion battery, polymer membranes are used. Their essential task is to separate the cathode and anode from each other and to maintain the ion conduction. The heat resistance of the polymer membrane is limited. You try to increase them by coating them.

 WO2009 / 079889 discloses a process for producing a non-woven fabric-reinforced polymer membrane in which a colloidal polymer emulsion is treated for several hours in a ball mill. The colloidal polymer emulsion is prepared by copolymerizing 100 parts of a water-soluble polymer, 30-500 parts of a hydrophobic monomer, 0-200 parts of a hydrophilic monomer, and 1-5 parts of an initiator. The emulsion thus prepared is added with 0-100% of an inorganic filler and 20-100% of a plasticizer to give a slurry. Suitable plasticizers are alkyl phosphates, as inorganic fillers are metal oxides in question. The slurry thus obtained is coated on both sides of a nonwoven fabric and dried. A disadvantage of this procedure is that the entire, containing filler and plasticizer, colloidal polymer emulsion must be ground. This is time consuming and large volumes must be moved.

In WO2013 / 10791 1, a separator for an electrochemical cell is disclosed, which is a porous

Layer and at least one alumina or aluminum hydroxide. The porous layer is formed by a block copolymer having three or more polymer blocks. As the alumina of the formula Al2O3 x H2O ^■ is used with x from 0 to 1; 5. Preferably, a water-containing material is used. This includes aluminas and hydroxides called alpha or gamma alumina. The BET surface area is preferably in the range of 10 to 250 m ² / g. These primary particles can be grouped together to form a secondary structure. The resulting secondary particles preferably have a particle size of 50 nm to 2000 nm. Fumed aluminum oxides should also be usable. The alumina is provided in the form of a dispersion, which may also contain inorganic or organic acids.

In DE-A-102007021 199 nanocomposites are disclosed in which 0, 1-80 wt .-% of particles of alumina, zirconia, yttriumdotiertes zirconia, titania, silica, antimony oxide, zinc oxide, cerium oxide, iron oxide, palladium dioxide, indium tin oxide or antimony tin oxide in a matrix are embedded from an organic polymer. Suitable polymers include polyphenylene ethers, polyolefins, polyketones, polyethersulfones, polyether ketones, polysulfone, polyphenylene sulfones, polyvinyl chlorides, polyimides, PVDF, ETFE, EFEP and mixtures thereof. A first step is to produce by means of a high energy mill a dispersion comprising the metal oxide, solvent, phosphoric acid, phosphorous acid, hypophosphorous acid, phosphonic acid, sulfonic acid,

Sulfuric acid, sulfuric acid, boric acid and oxygen acids based on tin and bismuth compounds. Explicitly disclosed is a dispersion containing about 35% by weight.

Contains alumina in ethanol. Disadvantages of this dispersion are the rather high mean particle size of 140 nm and the very high proportion of phosphoric acid. Despite this high proportion, the dispersion has only limited stability.

 The object of the present invention was to provide a coating composition with a high proportion of aluminum oxide, which has advantages over the prior art in terms of viscosity, stability and processability.

 The object of the invention was also to provide an alumina-containing dispersion for the preparation of a coating composition.

 The invention relates to a coating composition obtainable by mixing an alumina-containing dispersion and a binder, wherein

 A) the alumina-containing dispersion

a) at least 40% by weight, preferably 40-60% by weight, of aluminum oxide in the form of aggregated primary particles having a BET surface area of 20 to 200 m ² / g and a middle,

 Particle diameter dso in the dispersion of less than 100 nm, and

 b) at most 60% by weight, preferably 40-60% by weight, of a liquid phase which consists of at least 90% by weight of one or more organic solvents having a boiling point of 50 ° C to 250 ° C and wherein the organic solvent consists of more than 90% by weight of one or more C 1 -C 4 -alcohols,

 c) 1 - 5 wt .-% of at least one mineral acid and / or at least one d-Cs carboxylic acid, d) 0, 1 - 1 wt .-% of at least one organophosphonic acid and

 e) 0.1 to 1% by weight of at least one sulfonic acid and / or salt thereof

 contains and

 B) the binder

 f) from the group consisting of polyethylene oxide, polyvinylidene fluoride, polyvinylidene chloride, polytetrafluoroethylene, polyacrylonitrile, polyimides, polyetheretherketone,

 Polymethyl methacrylate, polytetraethylene glycol diacrylate, polyvinylidene fluoride /

 Hexafluoropropylen copolymer, polyvinylidene fluoride / chlorotrifluoroethylene copolymer, and mixtures thereof is selected, and

 g) is present as a solution or dispersion in an organic solvent having a boiling point of 50 ° C to 250 ° C, and

C) the weight ratio of alumina / binder is 50:50 to 95: 5. The alumina is in the form of aggregated primary particles. The primary particles are non-porous and have hydroxyl groups on their surface. The alumina is preferably obtained by a pyrogenic process. These are organic or inorganic

Aluminum compounds usually reacted in a hydrogen / oxygen flame.

The alumina comprises as crystalline phases chi, kappa, gamma, delta and theta

Alumina. In addition to these crystalline constituents, small amounts of amorphous aluminum oxide may also be present. The main constituent is preferably gamma-alumina or mixtures of gamma-alumina with delta-alumina and / or theta-alumina. The alumina-containing dispersion does not contain alpha alumina. It has a BET surface area of 20-200 m ² / g, preferably 50-150 m ² / g, more preferably 90-1 10 m ² / g or 50-80 m ² / g. The BET surface area is determined according to DIN 66131.

 The aluminum oxide particles in the dispersion have an average particle diameter d 50 of at most 100 nm, preferably 50-100 nm. It can be determined by the usual methods of light scattering known to those skilled in the art for determining particle size distributions in dispersions, such as dynamic light scattering, for example by means of a Zetasizer series instrument from Malvern Instruments.

 The liquid phase of the dispersion is intended to include the organic solvent, as well as phosphoric acid, organophosphonic acid and sulfonic acid. The proportion of the liquid phase is at most 60% by weight, based on the dispersion, preferably 40-60% by weight. In addition, the liquid phase may also contain small amounts of water. This is true when, for example, aqueous

Solutions of organophosphonic acid and / or sulfonic acid are used. The proportion of water is not more than 3 wt .-%. The solvent should have a certain volatility, the boiling point is therefore 50 ° C to 250 ° C, preferably 60 to 100 ° C. The proportion of the solvent in the liquid phase is at least 90% by weight, preferably more than 95% by weight. Suitable organic solvents are alcohols, esters, ethers, ketones, amides and hydrocarbons.

Explicit are methanol, ethanol, isopropanol, n-propanol, butanol, diethyl ether,

Propylene monomethyl ether, acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, toluene, xylene, benzene, hexane and heptane. Particularly preferred is ethanol. Alkyl phosphates, such as

Trimethyl phosphate, triethyl phosphate, tri-n-propyl phosphate, triisopropyl phosphate,

 Methyldiethylphosphat are in the liquid phase of the dispersion to a maximum of 10 wt .-% before. The dispersion is preferably free of alkyl phosphates.

Another essential ingredient of the dispersion is a mineral acid and / or a C1-C5 carboxylic acid. The carboxylic acid may also be a hydroxycarboxylic acid or a dicarboxylic acid. Hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, formic acid, acetic acid and 2-hydroxysuccinic acid may be mentioned explicitly. Particular preference is given to using phosphoric acid. The mineral acid or d-Cs-carboxylic acid has a proportion of 1 to 5 wt .-%, preferably 2 to 4 wt .-%. Another essential component of the dispersion is an organophosphonic acid. This may be an organophosphonic according to the general formula RR ² N- (X-NY) _a -X-NR ³ R ⁴ , with

X = C ¹ -C ¹⁰ -alkyl radical, Y = (XN) _b R ⁵ R ⁶ ; R1, R ^2, R ^3, R ^4, R ^5, R ⁶ = in each case independently H or -CH 2 PO (OH) 2; a and b are each independently 0 - 2500. Examples of preferred

Organophosphonic acids having this structure are ethylenediamine tetramethylenephosphonic acid,

Diethylenetriaminepentamethylenephosphonic acid, hydroxyethylethylenediaminetrimethylenephosphonic acid, pentaethylenehexaminoctamethylenephosphonic acid, hexamethylenediamine tetramethylenephosphonic acid, diethylenetriamine monocarboxymethyltetramethylenephosphonic acid and / or salts thereof.

The organophosphonic acid may furthermore be one of the general formula H ² N-R ³ CR R ² PO (OH) 2, where R is C 1 -C ₄ -alkyl, C 1 -C 10 -aryl, -PO (OH) ₂ ; R ² = C 1 -C ₄ -alkyl radical, Ce-Cio-aryl radical;

R ³ = C 1 -C 6 -alkylene radical or R and R ³ together with the C-atom to which they are attached form an aromatic ring, where R ² is absent in this case. Examples more preferred

Organophosphonic acids having this structure are 2-amino-1-phenethylphosphonic acid,

1-aminoethanedisphosphonic acid, 1-aminopropanediphosphonic acid and

Aminophenylmethylendiphosphonsäure.

Furthermore, the organophosphonic acid HOCH ₂ -CH (OH) PO (OH) ₂ , (HO) ₂ OP-CH (OH) -PO (OH) ₂ , (HO) ₂ OP-CH ₂ -CH ₂ -CH (OH) PO ( OH) 2 and H0 ₂ C-CR (OH) PO (OH) ₂ with R = Me, Et, Pr, Bu.

Particularly preferred is hydroxyethylaminobis (methylenephosphonic acid).

 Another essential ingredient of the dispersion is a sulfonic acid. This is preferably selected from the group consisting of methanesulfonic acid, dodecylsulfonic acid,

p-toluenesulfonic acid, p-dodecylbenzenesulfonic acid, vinylsulfonic acid and polyvinylsulfonic acid.

 The binder of the present invention is polyethylene oxide,

Polyvinylidene fluoride, polyvinylidene chloride, polytetrafluoroethylene, polyacrylonitrile, polyimides,

Polyetheretherketone, polymethyl methacrylate, polytetraethylene glycol diacrylate, polyvinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride / chlorotrifluoroethylene copolymer, and

Mixtures thereof. The binder is as a solution or dispersion in an organic

Solvent with a boiling point of 50 ° C to 250 ° C before. The solvents are the same, which are also suitable as solvents for the dispersion. In this case, the solvent of the dispersion and the binder may be the same or different. While the solvent in the dispersion has only small amounts of alkyl phosphate, the

 Solvent of the binder partially or entirely consist of alkyl phosphates. The solvents of the dispersion and the binder should be adjusted so that only forms a liquid phase.

Another object of the invention is a method for coating a membrane, wherein the dispersion of the invention is applied to a membrane and the organic solvent and optionally water is removed. Another object of the invention is the use of an alumina-containing dispersion for producing a coating composition for a separator of an electrochemical cell or the coating of powdered cathode materials, wherein the alumina-containing dispersion

a) 40 to 60% by weight of aluminum oxide in the form of aggregated primary particles having a BET surface area of 20 to 200 m ² / g and an average particle diameter d 50 in the dispersion of less than 200 nm,

b) 40-60 wt .-% of a liquid phase which consists of at least 90 wt .-% of one or more organic solvents having a boiling point of 50 ° C to 250 ° C, wherein the liquid phase has a proportion of alkyl phosphates of a maximum 10 wt .-%, wherein the organic

Solvent consists of more than 90 wt .-% of one or more Ci-C4 alcohols, c) 1 to 5 wt .-% phosphoric acid, and

d) 0, 1 - 1 wt .-% of at least one organophosphonic acid

e) 0.1 to 1% by weight of at least one sulfonic acid and / or salt thereof

contains.

 The alumina is in the form of aggregated primary particles. The primary particles are non-porous and have hydroxyl groups on their surface. The alumina is preferably obtained by a pyrogenic process. These are to be understood as flame hydrolysis and / or flame oxidation. In this case, organic or inorganic aluminum compounds are usually reacted in a hydrogen / oxygen flame.

 The alumina comprises as crystalline phases chi-, kappa, gamma, delta and theta alumina. In addition to these crystalline constituents, small amounts of amorphous aluminum oxide may also be present. The main constituent is preferably gamma-alumina or mixtures of gamma-alumina with delta-alumina and / or theta-alumina. In the alumina-containing dispersion of the present invention, no alpha-alumina is detectable.

The aluminum oxide has a BET surface area of 20 to 200 m ² / g, preferably 50 to 150 m ² / g, particularly preferably 90-1 10 m ² / g or 50-80 m ² / g. The BET surface area is determined according to DIN 66131.

The aluminum oxide particles in the dispersion have an average particle diameter d 50 of at most 100 nm, preferably 50-100 nm. It can be determined by the usual methods of light scattering known to those skilled in the art for determining particle size distributions in dispersions, such as dynamic light scattering, for example by means of a Zetasizer series instrument from Malvern Instruments.

The liquid phase of the dispersion is said to be the organic solvent, as well as phosphoric acid,

Organophosphonic acid and sulfonic acid include. The proportion of the liquid phase is at most 60% by weight, based on the dispersion, preferably 40-60% by weight. In addition, the liquid phase may also contain small amounts of water. This applies if, for example, aqueous solutions of the organophosphonic acid and / or sulfonic acid are used. The proportion of water is not more than 3% by weight. The solvent should have a certain volatility, the boiling point is therefore 50 ° C to 250 ° C, preferably 60 to 100 ° C. The proportion of the solvent in the liquid phase is at least 90% by weight, preferably more than 95% by weight. Suitable organic solvents are alcohols, esters, ethers, ketones, amides and hydrocarbons.

Explicit are methanol, ethanol, isopropanol, n-propanol, butanol, diethyl ether,

 Propylene monomethyl ether, acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, toluene, xylene, benzene, hexane and heptane. Particularly preferred is ethanol. Alkyl phosphates, such as

Trimethyl phosphate, triethyl phosphate, tri-n-propyl phosphate, triisopropyl phosphate,

Methyldiethylphosphat are in the liquid phase of the dispersion to a maximum of 10 wt .-% before.

The dispersion is preferably free of alkyl phosphates.

 Another essential ingredient of the dispersion is a mineral acid and / or a

Ci-Cs-carboxylic acid. The carboxylic acid may also be a hydroxycarboxylic acid or a

Be dicarboxylic acid. Explicitly mentioned are hydrochloric acid, sulfuric acid, nitric acid,

Called phosphoric acid, formic acid, acetic acid and 2-hydroxysuccinic acid. Particular preference is given to using phosphoric acid. The mineral acid or C 1 -C 8 -carboxylic acid has a proportion of 1 to 5% by weight, preferably 2 to 4% by weight.

Another essential component of the dispersion is an organophosphonic acid. This may be an organophosphonic according to the general formula RR ² N- (X-NY) _a -X-NR ³ R ⁴ , with

X = C ¹ -C ¹⁰ -alkyl radical, Y = (XN) _b R ⁵ R ⁶ ; R1, R ^2, R ^3, R ^4, R ^5, R ⁶ = in each case independently H or -CH 2 PO (OH) 2; a and b are each independently 0 - 2500. Examples of preferred

Organophosphonic acids having this structure are ethylenediamine tetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hydroxyethylethylenediaminetrimethylenephosphonic acid, pentaethylenehexaminoctamethylenephosphonic acid, hexamethylenediamine tetramethylenephosphonic acid, diethylenetriamine monocarboxymethyltetramethylenephosphonic acid and / or salts thereof.

The organophosphonic acid may furthermore be one of the general formula H ² N-R ³ CR R ² PO (OH) 2, where R is C 1 -C ₄ -alkyl, C 1 -C 10 -aryl, -PO (OH) ₂ ; R ² = C 1 -C ₄ -alkyl radical, Ce-Cio-aryl radical;

R ³ = C 1 -C 6 -alkylene radical or R and R ³ together with the C-atom to which they are attached form an aromatic ring, where R ² is absent in this case. Examples more preferred

Organophosphonic acids having this structure are 2-amino-1-phenethylphosphonic acid,

1-aminoethanedisphosphonic acid, 1-aminopropanediphosphonic acid and

Aminophenylmethylendiphosphonsäure.

Furthermore, the organophosphonic acid HOCH ₂ -CH (OH) PO (OH) ₂ , (HO) ₂ OP-CH (OH) -PO (OH) ₂ , (HO) ₂ OP-CH ₂ -CH ₂ -CH (OH) PO ( OH) 2 and H0 ₂ C-CR (OH) PO (OH) ₂ with R = Me, Et, Pr, Bu.

Particularly preferred is hydroxyethylaminobis (methylenephosphonic acid). Examples

 Example 1: Aluminum oxide-containing dispersion

 In a 100 l stainless steel batch tank 77 kg of ethanol were submitted. Subsequently, under shear conditions (Ystral Conti-TDS 3 (stator slots: 4 mm ring and 1 mm ring, rotor / stator distance about 1 mm), initially 4.74 kg phosphoric acid 85%, 0.56 kg polyvinyl sulfonic acid (Na salt, 25% in water, Sigma-Aldrich) and 1, 39 kg Cublen P 50 (, 50% solution of

2-phosphonobutane-1,2,4-tricarboxylic acid in water; Zschimmer & Schwarz GmbH) and subsequently 55.8 kg of AEROXIDE ^® Alu C (BET 100 m ² / g), Evonik Industries, added to the batch tank. After completion of the addition, the mixture was post sheared at 3000 rpm for 30 minutes.

This predispersion was passed in two passes through the high-energy mill Sugino Ultimaizer HJP-25050 at a pressure of 2500 bar and diamond nozzles of 0.25 mm diameter, thus further intensively ground. The proportion of aluminum oxide is 40 wt .-%

 After dispersing, a particle size dso of 80 nm was determined by light scattering (Zetasizer 3000 Hsa, Malvern Instruments, UK).

Example 2: Coating Compositions

 The three coating compositions shown in Table 1 are prepared by combining appropriate amounts of the dispersion of Example 1, a 5% by weight mixture, PVDF (Arkema, "Kynar Superflex 2500-20") in ethanol and appropriate amounts of ethanol.

Table 1: Coating compositions

The coating composition is μιτι by die method on a polypropylene LIB Separator "2500", Celgard ^®, thickness 25 is applied. The separator is at a distance of 10 μιη adjusted to the slot. The slot is then at a speed of 0.4 m / min along the separator and the coating composition at a flow rate of 0.5 to 1 ml / min, depending on the desired layer thickness, followed by drying at a temperature of 50 ° C. for 2 hours.

 The so-coated separators show little shrinkage under thermal stress and no adverse effect on the pore structure.

 With the coating composition according to the invention which contains high proportions of aluminum oxide, suitable coatings of separators can be produced cost-effectively.

It has proven to be essential that only a combination of partially unusual additives leads to a processable, low-viscosity coating composition.

Claims

claims  A coating composition obtainable by mixing an alumina-containing dispersion and a binder, wherein  A) the alumina-containing dispersion  a) at least 40% by weight of aluminum oxide in the form of aggregated primary particles having a BET Having a surface area of from 20 to 200 m ² / g and a mean particle diameter d 50 in the dispersion of less than 100 nm,  b) at most 60% by weight of a liquid phase consisting of at least 90% by weight of one or more organic solvents having a boiling point of from 50 ° C to 250 ° C, the organic solvent being more than 90% by weight. % consists of one or more C 1 -C 4 -alcohols,  c) 1 to 5% by weight of at least one mineral acid and / or one d-Cs carboxylic acid, d) 0.1 to 1% by weight of at least one organophosphonic acid,  e) 0, 1 - 1 wt .-% of at least one sulfonic acid and / or its salt and  B) the binder  f) from the group consisting of polyethylene oxide, polyvinylidene fluoride, polyvinylidene chloride, polytetrafluoroethylene, polyacrylonitrile, polyimides, polyetheretherketone,  Polymethyl methacrylate, polytetraethylene glycol diacrylate, polyvinylidene fluoride /  Hexafluoropropylen copolymer, polyvinylidene fluoride / chlorotrifluoroethylene copolymer, and mixtures thereof is selected, and  g) is present as a solution or dispersion in an organic solvent having a boiling point of 50 ° C to 250 ° C, and  C) the weight ratio of alumina / binder is 50:50 to 95: 5. 2. Coating composition according to claim 1, characterized in that  at least one organophosphonic acid from the group consisting of the general formula RR ² N- (X-NY) _a -X-NR ³ R ^{4 is} selected, with X = Ci-Cio-alkyl, Y = (XN) _b R ⁵ R ⁶ ; R1, R ^2, R ^3, R ^4, R ^5, R ⁶ = in each case independently H or -CH 2 PO (OH) 2; a and b are each independently 0 - 2500 selected.  3. Coating composition according to claims 1 or 2, characterized in that the at least one organophosphonic acid from the group consisting of the general formula H ₂ NR ³ CR R ² PO (OH) _{2 is} selected, with R = C 1 -C ₄ -alkyl radical, Ce-Cio-aryl radical, -PO (OH) ₂ ; R ² = C ¹ -C ⁴ -alkyl radical, C 6 -C 10 -aryl radical; R ³ = C 1 -C 6 -alkylene radical or R and R ³ together with the C-atom to which they are attached form an aromatic ring, where R ² is absent in this case. 4. Coating composition according to claims 1 to 3, characterized in that at least one organophosphonic acid from the group consisting of HOCH ₂ -CH (OH) PO (OH) _2, (HO) ₂ OP-CH (OH) -PO (OH) _2, HOH ₂ CH ₂ N [(CH ₂ PO (OH) _{2)] 2,} (HO) ₂ OP-CH ₂ -CH ₂ -CH (OH) PO (OH) ₂ and H0 ₂ C-CR (OH) PO Me, Et, Pr, Bu is selected (OH) ₂ with R =. Coating composition according to claims 1 to 4, characterized in that at least one sulfonic acid is selected from the group consisting of methanesulfonic acid, Dodecylsulfonic acid, p-toluenesulfonic acid, p-dodecylbenzenesulfonic acid, vinylsulfonic acid and polyvinylsulfonic acid. A process for coating a membrane, wherein the coating composition according to claims 1 to 5 is applied to a membrane and the organic solvent and optionally water is removed. Use of an alumina-containing dispersion for the production of a A coating composition for a separator of an electrochemical cell or the coating of powdered cathode materials, wherein the alumina-containing dispersion a) 40 to 60% by weight of aluminum oxide in the form of aggregated primary particles having a BET surface area of 20 to 200 m ² / g and an average particle diameter d 50 in the dispersion of less than 200 nm, b) 40-60 wt .-% of a liquid phase which consists of at least 90 wt .-% of one or more organic solvents having a boiling point of 50 ° C to 250 ° C, wherein the liquid phase has a proportion of alkyl phosphates of a maximum 10 wt .-%, wherein the organic solvent consists of more than 90 wt .-% of one or more Ci-C4 alcohols, c) 1 to 5 wt .-% phosphoric acid, and d) 0, 1 - 1 wt .-% of at least one organophosphonic acid e) 0.1 to 1% by weight of at least one sulfonic acid and / or salt thereof contains.