Process for preparing a fluorine-doped tin oxide powder

Abstract: A fluorine-doped tin oxide powder is prepared by combining a solution of a stannic salt with a solution of a fluoride, separating the tin hydroxide from the liquid and drying it at a temperature not exceeding about room temperature and heating the dried product to at least 500°C, until the fluorine content is les than 10% and preferably is between 1 and 5% by weight.

Description: The invention relates to a process for preparing a fluorine-doped tin oxide in the form of a fine powder. A frequent technical requirement is to provide electrically insulating materials with an anti-static or electrically conductive coating in order to prevent electrostatic charges from accumulating on the material or in order to make the material suitable for a specific use. The coating must usually satisfy the requirement of having sufficiently high electrical conductivity under extreme conditions, particularly under very dry conditions. An additional requirement is frequently that the coating should be transparent. This latter requirement applies, for example, to conductive coatings of exposure windows or exposure cylinders of copying machines or original scanners, to transparent film materials and to electrically conductive coloured toner powders which have been given the required conductivity by the application of an electrically conductive coating to the surface of the toner powder particles. Transparent electrically conductive or anti-static coatings can be formed by applying a metal, metal oxide or metal salt, e.g. gold, chromium, nickel, palladium, indium oxide, stannic oxide or cupro-iodide in the form of a thin film to the insulating surface, e.g. by sputtering or vapour-coating in a vacuum. These methods, however, are expensive and in many cases do not give the required good coating adhesion. Moreover, these coating methods are not always applicable, for example they are not usable for giving fine toner powder particles an electrically conductive layer. US Patent 4 431 764 describes compositions for forming transparent electrically conductive coatings comprising a film-forming binder and finely divided tin oxide having a particle size less than 0.4 micrometer, the tin oxide being doped with 0.1 to 20% by weight of antimony, in the form of Sb₂O₃ or Sb₂O₅. The tin oxide described in that patent can also be used to form conductive layers without the aid of a film-forming binder, by softening the (thermoplastic) surface of the area to be coated, softening being carried out either by heat or by means of a solvent or solvent vapour, and by covering the softened and thus tacky surface with the tin oxide. For example, electrically conductive toner powder can be made by softening the toner powder particles in a hot gas stream and bringing the fine tin oxide powder into the gas stream, or by intensively mixing a mixture of tone powder and fine tin oxide in a ball mill for some time so that the toner powder particles soften as a result of the frictional heat produced and the tin oxide particles are deposited on the softened surface. Disadvantages of antimony-doped tin oxide in accordance with the said US patent are that its electrical resistance is still relatively high, and that the dry powder itself is not loose but has fairly poor flow properties, so that it is difficult to divide it finely. Conductive toner powders made by means of this tin oxide contain a fairly considerable amount of loose tin oxide or tin oxide insufficiently bounded to the toner powder, so that considerable soiling of the apparatus in which the toner powder is used occurs. Japanese application PN 62-2203617 describes the production of a tin oxide sol useful for the formation of electrical conductive coatings on articles. The preparation of a stable, highly conductive, free flowing doped tin oxide powder is not disclosed in this reference. The invention provides a tin oxide powder having a higher conductivity and better flow properties than the prior art antimony-doped tin oxide, so that it can be used for many applications in which the known antimony-doped tin oxide has proved to be unsuitable or less suitable, e.g. the manufacture of conductive toner powder. More specifically the invention relates to a process for the preparation of fluorine-doped tin oxide powder, wherein an alcoholic solution of a stannic salt is combined with an aqueous solution of a fluoride, while the pH of the mixture is maintained at a value of about 3.5, the tin hydroxide is precipitated from the liquid and separated, the separated product is dried at a temperature not exceeding about room temperature and the dried powder is heated to a temperature of at least 500°C. According to the invention a doped tin oxide powder is provided that consists of mainly crystalline tetragonal tin oxide which is doped with fluorine and which has a primary particle size less than 0.2 micrometer and a specific electrical resistance of at most 50 ohms.metre. The fluorine-doped tin oxide powder according to the process of the invention differs from the known commercially available antimony-doped tin oxide powder according to US Patent 4 431 764 in that it has a higher electrical conductivity and better flow properties. The powder according to the invention consists mainly of crystalline, tetragonal tin oxide containing bonded fluorine (probably in the form of Sn-F bonds), has a primary particle size of less than 0.2 micrometer and a specific electrical resistance of not more than 50 ohms.metre. The percentage of fluorine in the tin oxide should be at least 1% by weight and is preferably between 1 and 10, more particularly between 1 and 5% by weight. These latter products in particular are distinguished by a low specific electrical resistance, which is often well below 25 ohms.metre, and is usually between 1 and 15 ohms.metre depending on the preparation conditions. The resistance of the powder is measured as follows: A cylindrical container having an inside diameter of 17.2 mm, the base of which consists of brass having a thickness of 1.5 mm, and the wall, which has an internal height of 22.9 mm, consists of Teflon having a thickness of 9 mm, is filled with an excess of powder. The filling is then compressed by crushing it ten times in a crusher made by Engelsmann A.G., of Ludwigshafen, Germany. This filling procedure is repeated twice. Excess powder is then wiped off with a ruler and a brass lid having a diameter of 17.2 mm and a mass of 55 g is placed on the column of powder. The filled container is placed in a Faraday cage and a 10 volt D.C. is applied between the base and lid. The current intensity is measured after about 20 seconds. The measuring procedure (container filling and current measurement) is repeated three times, whereafter the average current intensity of the three measurements is calculated. The resistance of the powder follows from the formula: ρ = U Ig x A h where: U = the applied voltage (= 10 volts) A = contact area of lid and powder column (= 2.32x10⁻⁴m²) h = height of powder column (=2.29x10⁻²m) Ig = average current strength (in amps). The fluorine content of the tin oxide powder is determined potentiometrically by means of an ion-selective fluorine electrode such as electrode number 60502150 manufactured by Messrs Metrohm A.G., Herisau, Switzerland. The procedure used for this is the standard solution addition method as described in the brochure supplied by Metrohm A.G. and entitled "Messen in der Chemie". The solution in which the fluorine analysis is carried out is prepared as follows: The following are successively weighed out into a platinum crucible with a lid, the weight of which has been accurately determined: 1g of sodium potassium carbonate, about 0.05g of flourine-doped tin oxide powder and 1g of sodium potassium carbonate. The closed crucible is calcined for 2 hours and, after cooling, transferred to a 250 ml beaker, the weight of which has been accurately determined and which is filled with 30g of demineralised water, 30g of concentrated hydrochloric acid (specific gravity 1.19) and 10g of a 2% solution of tartaric acid in demineralised water. The liquid is heated and stirred until the crucible contents are completely dissolved. After cooling, the pH of the solution is brought to between 4 and 8 by the addition of a concentrated potassium hydroxide solution (47% by weight). After the beaker and contents have been accurately weighed, the solution is filtered over a folded filter into a plastic bottle. The potentiometric analysis of the fluorine content is carried out with a liquid containing: 40g of the tin oxide solution described above, 40g of buffer solution and 0.4g of standard sodium fluoride solution in demineralised water. The buffer solution is prepared as follows: 58.4g of sodium chloride and 5g of titriplex IV (Article number 8424 of Merck AG, Darmstadt, Germany) are dissolved in about 600g of demineralised water, whereafter 60.7g of acetic acid are added to the solution and it is made up to about 900g with demineralised water. By the addition of sodium hydroxide pellets the pH of the solution is then brought to between 5 and 5.5. The mass of the solution is then made up to 1000g by the addition of demineralised water. The concentration of the standard sodium fluoride solution is determined by measuring, by means of the ion-selective fluorine electrode, the potential of a liquid containing 10g of the above-described tin oxide solution and 10g of buffer solution and calculating the fluorine concentration by reference to the potential as measured. For the standard solution the fluorine concentration used is 100 x the calculated concentration. The fluorine-doped tin oxide powder is prepared according to the invention by combining a solution of a stannic salt in a water-miscible alcohol with an aqueous solution of a fluoride while controlling the pH of the mixture at a value of about 3.5, and separating the resulting precipitate of tin hydroxide from the liquid. This precipitate is then dried at a temperature not exceeding about room temperature and then calcined by heating it for some time to a temperature of at least 500°C. During calcination, it is heated at least until the fluorine content of the powder is at most 10% by weight and is preferably between 1 and 5% by weight. The known stannic salts soluble in water-miscible alcohols can be used, such as stannic sulphate and stannic chloride. Stannic chloride is preferably used, dissolved in ethanol or methanol. The water-soluble fluoride may, for example, be: sodium fluoride, ammonium fluoride or stannous fluoride. The latter substance is preferred because it generally gives tin oxide powders having a lower resistance than the powder prepared under otherwise identical condition from sodium or ammonium fluoride. Calcination of the dried tin hydroxide is carried out by heating the tin hydroxide for some time in air or an inert gas such as nitrogen. In the case of calcination in air, a situation in which the fluorine content of the powder is between 1 and 5% by weight and the resistance has reached a minimum value is fairly quickly obtained, within 15 minutes already. If calcination is carried out in nitrogen the process is slower, so that it can be more satisfactorily controlled to give a product of constant quality. The calcination time can then run up to some hours. Calcination of the tin hydroxide can be carried out directly after drying of the tin hydroxide or some time thereafter. Before the tin hydroxide is calcined it is dried, since a preceding drying step has a favourable influence on the particle size of the final tin oxide powder. Drying is carried out under moderate conditions, preferably at room temperature or lower. One attractive drying method is freeze-drying. Moreover, in the preparation of the tin oxide it is possible to promote the formation of fine particles by combining the salt solutions rapidly and with intensive agitation and adding the stannic salt preferably to the fluorine solution. The tin oxide powder according to the invention can be used for the same purposes as described in the above US Patent 4 431 764 in connection with antimony-doped tin oxide. The fluorine-doped tin oxide is applied, together with one or more binders and any other additives, to the surface to be coated. The fluorine-doped tin oxide powder according to the invention is particularly attractive for the manufacture of electrically conductive toner powder, in which toner particles containing thermoplastic resin are coated with a thin layer of tin oxide on their surface. Preferably, the tin oxide powders according to the invention used for this application are those which, measured by the known BET method, have a specific area of at least 50 m2/g and preferably more than 70 m2/g. The coating is made in manner known per se by softening the surface of the toner particles and anchoring the fine tin oxide particles on the softened surface. The good flow properties of the powder according to the invention give a good coating in a short time. The fluorine-doped tin oxide powder according to the invention has the advantage over antimony-doped tin oxide of a higher electrical conductivity so that less tin oxide has to be deposited on the toner particles to achieve the same resistance level. The invention will now be explained with reference to examples. Example 1 Preparation of fluorine-doped tin oxide powder A solution of 25 ml of stannic chloride in 600 ml of ethanol was rapidly added to a solution of 25g of stannorns fluoride in 1500 ml of water at 85°C with intensive stirring. During the addition of the stannic chloride solution the pH of the reaction mixture was kept at about 3.5 by dropwise addition of ammonia. After the stannic chloride solution had been completely added, the mixture was brought to pH 7 by the addition of ammonia, whereupon the ethanol was evaporated from the mixture. The precipitate was separated from the liquid and dried by freeze-drying. Two tin hydroxides prepared in the above way were heated under the following conditions directly after drying: Powder 1: 15 minutes at 500°C in air Powder 2: 60 minutes at 500°C in nitrogen. The fluorine content and the electrical resistance of the resulting fluorine-doped crystalline tin oxide powders were determined in the manner described hereinbefore. Result: Powder 1: 3.6% by weight of fluorine, ρ = 1.9 ohms.metre Powder 2: 1.7% by weight of fluorine, ρ = 1.6 ohms.metre The primary particle size of the two tin oxide powders was less than 0.1 micrometer. Five other tin hydroxides (powders 3 to 7) prepared in the above-described manner were processed into tin oxide under the following conditions two months after drying: Powder 3: 15 minutes at 500°C in air Powder 4: 30 minutes at 500°C in nitrogen Powder 5: 60 minutes at 500°C in nitrogen Powder 6: 120 minutes at 500°C in nitrogen Powder 7: 180 minutes at 500°C in nitrogen. The resulting tin oxide powders had a primary particle size of less than 0.1 micrometer. The fluorine content and the electrical resistance of these powders were as follows: Powder 3: 4.4% by weight of fluorine, ρ = 9.2 ohms.metre Powder 4: 2.2% by weight of fluorine, ρ < 1.3 ohms.metre Powder 5: 3.0% by weight of fluorine, ρ < 1.3 ohms.metre Powder 6: 2.0% by weight of fluorine, ρ = 1.3 ohms.metre Powder 7: 1.2% by weight of fluorine, ρ = 1.9 ohms.metre For comparison: the commercially available antimony-doped tin oxide, Mitsubishi T1, prepared to US Patent 4 431 764, has a resistance of about 80 - 100 ohms.metre, a primary particle size less than 0.2 micro-meter and a specific area of 65 - 75 m²/g. Example 2 Tin hydroxide was prepared in the manner described in Example 1 except that 25g of ammonium fluoride were used instead of 25g of stannous fluoride. Four tin hydroxides prepared in this way were processed into tin oxide in the manner described below. Powder 1: Heating in air at 500°C for 15 minutes directly after drying. Powder 2: Heating in nitrogen at 500°C for 60 minutes directly after drying. Powder 3: Heating in air at 500°C for 15 minutes two months after drying. Powder 4: Heating in nitrogen at 500°C for 60 minutes two months after drying. The fluorine content and the electrical resistance of the resulting tin oxides were as follows: Powder 1: 4.4% by weight of fluorine, ρ = 21.5 ohms.metre Powder 2: 3.0% by weight of fluorine, ρ = 5.5 ohms.metre Powder 3: 6.2% by weight of fluorine, ρ = 33.7 ohms.metre Powder 4: 4.1% by weight of fluorine, ρ = 14.4 ohms.metre Example 3 Two tin oxide powders 1 and 2 were prepared by the method of Example 1, the hydrates of the two powders being heated in nitrogen at 500°C for 60 minutes two months after drying. The hydrate of powder 1 was brought to 500°C from room temperature in 15 minutes, whereas the heating-up time for the hydrate of powder 2 was about 60 minutes. The following properties were measured for the resulting fluorine doped tin oxide powders: Powder 1: 1.5% by weight of fluorine, ρ = 1.2 ohms.metre, specific area = 63 m²/g. Powder 2: 3.9% by weight of fluorine, ρ = 1.8 ohms.metre, specific area = 100 m²/g. Example 4 The procedure of Example 1 was repeated, however the hydrate after it had been separated from the precipitation liquid was washed with demineralised water of 70°C and not dried until after this. A portion of the hydrate was heated in nitrogen at 500°C for 60 minutes directly after drying, the heating time to 500°C being 60 minutes. The resulting tin oxide powder had the following properties: 1.1% by weight fluorine, ρ = 1.6 ohms.metre, specific area = 43 m²/g. Another portion of the hydrate was processed in the same way as the first portion two months after drying. The resulting tin oxide powder had: 1% by weight of fluorine, ρ = 2.5 ohms.metre, specific area = 78 m²/g.

Claims: A process for the preparation of a fluorine-doped tin oxide powder, characterised in that a solution of a stannic salt is combined with a solution of a fluoride while the pH of the mixture is maintained at a value of about 3.5, the tin hydroxide is precipitated from the liquid and separated, the separated product is dried at a temperature not exceeding about room temperature and the dried powder is heated to a temperature of at least 500°C. A process according to claim 1 characterised in that the pH of the mixture is controlled by the addition of ammonia. A process according to claim 1, characterised in that the stannic salt is stannic chloride. A process according to claim 1, characterised in that the fluoride is stannous fluoride. A process according to claim 1, characterised in that stannic chloride and stannous fluoride are used. A process according to claim 1, characterised in that the separated product is dried by freeze-drying. A process according to claim 1, characterised in that the tin hydroxide is heated in an inert gas atmosphere to a temperature of at least 500°C. A process according to claim 6 in which the inert gas is nitrogen. A process for the preparation of a fluorine-doped tin oxide powder, characterised in that a solution of stannic chloride in an alcohol is combined with an aqueous stannous fluoride solution while maintaining the pH of the mixture at a value of about 3.5, the precipitate is separated from the liquid and is dried by freeze-drying and the dried product is heated in an inert gas to a temperature of at least 500°C.

Inventors: GROOTHUIJSE RONALD, VERCOULEN GERARDUS CORNELIS PE, GROOTHUIJSE, RONALD, VERCOULEN, GERARDUS CORNELIS PETER
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1991
Main classification: C01G19
IPC codes: G03G9, H01B1, C01G19