Title of Invention	CONNECTING PINS FOR CARBON MATERIAL ELECTRODES
Abstract	Connecting pins for carbon material electrodes characterized in that the connecting pins contain oxidatively activated carbon fibres which additionally have a carbonized coating , the carbonized coating being the carbonizetion product of a coating material selected from wax,pitch,natural resinam thermoplastic and thermasetting polymers ,and processes for the production thereof.

Full Text

Connecting pieces for carbon material electrodes The invention relates to connecting pieces for carbon material electrodes. In particular it relates to connecting pieces which are produced by mixing cokes, pitches and carbon fibres to give a material and shaping this material, and are used for connecting carbon material electrodes, in particular graphite electrodes. Carbon material electrodes, in particular graphite electrodes, are used in electric arc furnaces in the steel industry. These electrodes consist of individual cylindrical elements connected to one another, further elements being added in each case depending on the firing. The electrodes are usually connected to one another mechanically and in an electrically conductive manner by connecting pieces (connecting pins). Here, the connecting pieces have the shape of a double cone (two truncated pyramids joined to one another at the base) having threads on the lateral surfaces, which fit into corresponding recesses of the cylindrical electrodes, which recesses are arranged centrally in the end faces. Owing to the thermal load, it is necessary to adapt the thermal expansion of the connecting pieces and of the electrodes to one another in such a way as to prevent the formation of stresses which may lead to fracture or other damage at the connecting point. In the past, connecting pieces reinforced with carbon fibres have been proposed. Document US-A 4,998,709 describes graphite connecting pieces which are reinforced by carbon fibres which are obtained from mesophase pitch and are present in a proportion by mass of from 8 to 2 0% in the moulding material used for the production. WO 01/62667 describes a similar process, but the fibres likewise obtained from mesophase pitch have a lower modulus of elasticity and are used in a smaller proportion by mass (from 0.5 to 5% in the moulding material) . This process leads to a reduction in the coefficient of thermal expansion in the direction of extrusion and of the main axis of the connecting piece. The stresses usually occurring in fibre-reinforced materials are due to the different coefficients of thermal expansion of fibres and matrix. Carbon fibres are practically dimensionally stable in the fibre direction (i.e. the coefficient of thermal expansion is small and negative) , whereas, for example, in the case of glassy carbon this coefficient is of the order of magnitude of 3-10-6 K-1. In the investigations on which the present invention is based, it has now been found that further improvement compared with the known processes cited (US 4998709, WO 01/626667) can be achieved if carbon fibres whose surface has been treated before mixing into the moulding material and has been provided with a polymer coating are used in the moulding material for the production of the connecting pieces. Connecting pieces according to the invention have a reduced longitudinal (in the direction of the axis of the cylindrical electrode and of the connecting piece) coefficient of thermal expansion and increased strength. Owing to these properties, the connecting pieces according to the invention withstand not only the carbonization temperatures below 1000°C but also a subsequent graphitizing treatment at above 3 000°C. It is known that carbon fibres can be anodically oxidized in an electrolyte solution (J.B. Donnet, R.C. Bansal, Carbon Fibers, Marcel Dekker Inc. New York (1990)), In this surface treatment, oxygen-containing groups form on the fibre surface, as a rule strongly or weakly acidic carboxyl groups and hydroxyl groups, C-H groups activated by carbonyl groups (C-H acidic groups), and basic pyrone-1ike surface groups (H.P. Boehm, E. Diehl, W. Heck, R. bappoK,Angew. chem. 76 (1964) , 742; B .R. Puri, in Walker: Chemistry and Physics of Carbon, Vol. 6, Marcel Dekker, New York (1971), 191). Oxygen groups can be produced on the fibre surface also by thermal oxidation without the necessity of subsequent washing out for removing electrolytes. Oxygen in various concentrations, oxygen-halogen mixtures, ozone, carbon dioxide or oxides of nitrogen are described as an oxidation medium. A detailed discussion of these topics is to be found in J. Cziollek ("Studien zur Beeinflussung des Verstarkungsverhaltens von Kohlenstoffasern durch Oberflachenbehandlung der Fasern und durch Verwendung eines Kohlenstoff/Kohlenstoff-Skelettes als Verstarkungskomponente" [Studies on influencing the reinforcing behaviour of carbon fibres by surface treatment of the fibres and by the use of a carbon/carbon skeleton as a reinforcing component], Thesis, University of Karlsruhe (1983), page 40 et seq.). In the case of carbon fibre-reinforced carbon ("CFC"), reduced reactivity of the fibre surface is regarded as a basic requirement for good utilization of the fibre properties. The reduced reactivity is intended to enable the matrix to shrink away from the fibre surface. The shrinkage cracks are then filled by repeated reimpregnation/recarbonization steps (impregnation, for example, with pitches and combustion in the absence of oxidizing agents). Consequently, carbon bonding bridges between fibre surface and matrix are produced (J. Cziollek, op. cit.). The restoration of the weakened fibre/matrix adhesion brought about by newly created surface groups as a result of the reimpregnation step has also been discussed (K.H. Giegl: "Studien zur Oberflachenchemie von Kohlenstoffasern und zur Entwicklung von Kohlenstoff-Hohlfasern" [Studies of the surface chemistry of carbon fibres and of the development of hollow carbon fibres], Thesis, University of Karlsruhe (1979)). In contrast to these experimental findings in the case of CFC composites, an oxidative surface treatment of the fibres in the present invention has surprisingly-proved to be advantageous for the properties of a composite produced therewith. The present invention therefore relates to connecting pieces for carbon material electrodes, the connecting pieces containing carbon fibres whose surface has been oxidatively activated and which additionally have a carbonized coating. This surface coating is the carbonization product of a coating material (sizing) selected from wax, pitch, natural resins, thermoplastic and thermosetting polymers. The invention furthermore relates to a process for the production of connecting pieces which contain the fibres treated according to the invention. In the process according to the invention, the surface of the carbon fibres is activated by oxidation in a first step, the fibres are then provided, in a second step, with a surface coating comprising a coating material selected from wax, pitch, natural resins or thermoplastic or thermosetting polymers, the coated fibres are optionally treated in a third step at a temperature of from 750 to 1 300°C for carbonization of the coating, are mixed in a fourth step with coke having a mean particle size in the range from 0.05 to 4 mm, pitch having a softening temperature in the range from 70°C to 150°C and optionally further additives and shaped to give cylindrical bodies, the cylindrical mouldings are carbonized and then graphitized in a fifth step, and the graphitized mouldings are turned, in a sixth step, to give the connecting pieces having threads. It is preferable to use carbon fibres in the form of fibre tow comprising from 1 000 to 60 00 0 individual filaments, which, after the third process step, are cut to give short fibres having an average length of from 0.5 to 40 mm. It is furthermore preferable to use carbon fibres in the form of heavy tow comprising from 4 0 00 0 to 2 000 0 00 individual filaments, which, after the third process step, are cut to give short fibres having an average length of from 0.5 to 40 mm. It is furthermore preferable if the activated carbon fibres are coated, in the second step, in an aqueous or solvent-containing bath containing a dispersion or solution of a coating material selected from wax, pitch, natural resins or thermoplastic or thermosetting polymers. It is furthermore preferable if the coated carbon fibres are treated, in the third step, at a temperature of from 900 to 1 200°C for carbonization of the coating. It is furthermore preferable if, in the fourth step, a mixture containing for each 100 kg of coke, from 10 to 40 kg of a pitch and from 0.2 to 2 0 kg of carbon fibres is prepared. It is furthermore preferable to add, as further additive, from 0.1 to 1 kg of an iron oxide pigment having a mean particle size of from 0.1 to 2 μm.. The surface coating is effected in particular using polymers which have a sufficient carbon yield at carbonization temperatures of, preferably, from about 750 to about 1 3 0 0 ° C. Polyurethane resins, phenol resins and pitches having a C residue of at least 40% of the mass of the coating material used are particularly suitable. The carbonization of the coating material can be effected in a thermal treatment step prior to mixing into the moulding material, or preferably simultaneously with the combustion after green production. Carbon fibres which are obtainable by carbonization of oxidatively stabilized polyacrylonitrile fibres in a known manner are preferably used. A thermal treatment of the fibres in the range from 1500°C to the graphitizing temperature (1800°C to about 3000°C, in some cases also above 3000°C) prior to mixing in can be dispensed with. The modulus of elasticity of the fibres is preferably from 200 to 250 GPa. The surface activation of the carbon fibres is effected by oxidation in an aqueous bath or by an oxygen-enriched gas stream at a temperature of from 400 to 600°C, the gas stream also serving for fanning out the fibre tow. In a preferred manner, it is also possible to oxidize the carbon fibres electrochemically, i.e. anodically in aqueous baths. Aqueous solutions of salts of oxidizing acids, such as nitrates, sulphates, chlorates, bromates and iodates, and the stated acids themselves are suitable as the oxidation bath; solutions which contain volatile oxidizing agents are preferred, the reduction products of these oxidizing agents preferably likewise being volatile. Here, substances which are defined as being volatile are those which are removed completely or, substantially completely (with an evaporation residue which is not more than 0.5% of the mass of the treated fibres) on drying the treated fibres, for example in an air stream or on godets. Particularly preferred oxidizing agents are oxidizing acids, such as nitric acid, chloric acid or mixtures of these with neutral or salt-like inorganic oxidizing agents, such as hydrogen peroxide, chromates, permanganates and hypochlorites (KMnO4/H2SO4, K2Cr2O7/H2SO4, HOCl/H2O/NaOCl) , Mixtures of nonoxidizing acids with salt-like oxidizing agents are also suitable, for example the mixtures of hydrochloric acid and chlorates, known as euchlorine. In an electrochemical (anodic) oxidation, it is sufficient to establish adequate conductivity by dissolving acids, bases or salts in water. After an electrochemical treatment (anodic oxidation) and with the use of oxidizing solutions (in particular salt solutions) , it is necessary to wash the fibre tow with demineralized water, preferably at least two baths being arranged in series. The acid-treated fibres may also be washed, it being possible to omit the drying step. The carbon fibres activated in this manner with oxygen-containing groups are then provided with the abovementioned surface coating, the dried or only washed fibres being passed through an aqueous impregnating bath, the excess solution containing coating materials being squeezed out in a known manner, and the fibre tow being dried, for example on heated godets. The impregnating bath is preferably an aqueous formulation of said coating materials, for example an aqueous dispersion of waxes, in particular polyolefin waxes based on polyethylene or polypropylene, and montan waxes, or waxes synthesized by esterification of fatty alcohols with long-chain fatty acids having 12 to 40 carbon atoms. Furthermore, it is possible to use dispersions of pblyurethane resins, of activated polyolefins (activated, for example, by grafting with maleic anhydride) or the copolymers thereof (for example with vinyl alcohol or vinyl acetate) or of phenol resins. It is also possible to treat the activated carbon fibres with organic compounds, in particular those based on pitches, dissolved in organic solvents. Pure pitches in a suitable low-viscosity form may also be used for the coating. The concentrations of the coating formulations are usually such that a solids mass fraction of from 0.5 to 30%, originating from the coating, results on the fibre surface. A range of from 3 to 15% is preferred. This results in a mass fraction of, preferably, from 0.2 to 15% of the carbonized coating on the fibres. Carbon fibres based on (carbonized) polyacrylonitrile are preferably used, since it has been found that these undergo the least damage by the mixing and shaping process on mixing to give the moulding materials according to the invention. Their modulus of elasticity is as a rule not as high as that of carbon fibres based on mesophase pitch. This means lower rigidity and therefore also less sensitivity to shearing. Both the additionally applied coating and the conversion of this coating into a carbon layer lead to additional mechanical protection. It was observed that the comminution of the fibres on mixing in is substantially reduced compared with the HM fibres (high-modulus fibres) sensitive to shearing. The degree of utilization of the amount of fibres used is thus higher, leading to a further cost benefit in addition to the lower costs of the HT fibres (high-tenacity fibres). The meterability of the fibre bundle cut into shprt fibres (preferably having an average length of from 0.5 to 40 mm) is also improved by the sizing applied. Uncoated individual filaments having lengths of more than 2 mm tend to agglomerate and therefore cannot be metered in a controllable manner. The fibre tows used preferably have from 1 000 to 60 00 0 individual filaments, and multifilament tows based on heavy tow having more than 40 000 filaments and up to 2 000 000 filaments are also preferred. It is also possible for the carbon fibres to be present in the form of parallel filaments (so-called "UD tapes"), woven fabrics, warp-knitted fabrics, knitted fabrics and/or nonwoven fabrics. The connecting pieces according to the invention preferably have a linear coefficient of thermal expansion in the extrusion direction of from -0.5 to +0.1 μm/(K'm). The extrusion direction is the direction parallel to the lateral surface of the generally cylindrically shaped blanks which, after the carbonization and combustion, are processed by turning or milling and into which the required threads are cut. Perpendicular to the extrusion direction, the linear coefficient of thermal expansion is preferably from 1.7 to 2.1 μm/ (K-m) . The mass fraction of carbon fibres in the connecting pieces is preferably from 0.2 to 10%. It was surprisingly found that connecting pieces which were produced using such carbon fibres not only are distinguished by the desired low values for the coefficients of thermal expansion but also have increased strength. Both are indicative of good fibre-matrix adhesion. This can be shown, for example, by preparing an electron micrograph of the fracture surface of connecting pieces -destroyed in a tensile test or in a bending test and comparing this with a fracture surface of connecting pieces which contain fibres without such a treatment. This finding is illustrated by the microphotographs. Fig. 1 shows an electron micrograph of a fracture surface of a connecting piece (connecting pin) in which carbon fibres according to the invention were used, and Fig. 2 shows an electron micrograph of a fracture surface of a connecting piece (connecting pin) in which carbon fibres from mesophase pitch were used as reinforcement. From the comparison of the two micrographs, it is clear that, in the case of fibres from mesophase pitch without the treatment according to the invention (fig. 2), said fibres are simply pulled out of the matrix in the event of a fracture and leave behind a hole, whereas, in the case of a connecting piece comprising fibres treated according to the invention by activation and coating, said fibres adhere firmly in the matrix and are not pulled out of the matrix in the event of a fracture (fig. 1). In a connecting . piece which is produced according to the invention, the fracture surface displays matrix cracks and cracks of the fibres in the fracture surface. However, the matrix reveals no holes from which the reinforcing fibres were pulled out on failure. The adhesion of the fibres to the matrix is evidently so great that the force required for pulling the fibres out of the matrix ("pull-out") is greater than the tensile strength of the fibres. In a comparison with a connecting piece which was produced according to the prior art using carbon fibres obtained from mesophase pitch and without the treatment according to the invention, the pull-out holes of the fibres from the fracture surface are clearly detectable. It is furthermore surprising that, as explained above, no destruction of the fibres as a result of internal stresses occurs in spite of the presumably better binding of the fibres in the matrix due to the surface treatment. The graphitized bodies which are produced from the materials according to the invention have the following properties: * Parallel to the extrusion direction Connecting pieces comprising these graphitized bodies lead, in a practical test, to substantially reduced susceptibility to cracking due to thermal stresses. The invention is explained by the following examples. Example 1 A fibre tow (7 x 60 000 filaments having a fibre diameter of 7μm) comprising . carbonized polyacrylo--nitrile fibres was subjected to an anodic oxidation. For this purpose, the fibre tow was passed through a bath having an effective length of about 1 m and containing an aqueous solution of sodium hydroxide (5 g in 100 g of the solution) at a speed of 1 m/min according to the method stated in US 4,704,196, example 3, the bath having been continuously circulated. A sinusoidal voltage of 5 V was applied, and the current was about 70 A. Thereafter, the fibre tow was washed out in a two-stage wash bath containing demineralized water and was squeezed out. The tow was then passed through a sizing bath containing 10 g of aqueous dispersed polyurethane resin in 100 g of the dispersion and having an effective length of 0.5m, squeezed out, and dried over godets at 120°C. The fibre tow was cut to give staple fibres about 6 mm long. Example 2 A moulding material was prepared from 100 kg of needle coke having a mean particle size of 0.5 mm, 26 kg of cold tar pitch having a softening temperature (SPM) of 110°C and 3 kg of PAN-based carbonized carbon fibres having a diameter of 7μm and an average length of 6 mm, which were anodically oxidized and provided with a polyurethane coating according to example 1, and 0.5 kg of iron oxide of pigment quality (particle size range from 0.1 to 2μm) , The material was mixed for 0.5 hour in a kneader-mixer at 160oC, extruded at 120oC to give a cylindrical extrudate and, after cutting to a length of about 3 000 mm, combusted at 800°C for 500 hours. The combusted cylindrical carbon bodies were then impregnated three times with an impregnating pitch (SPM 80°C) and subsequently combusted at 800°C.. The impregnated and subsequently combusted carbon bodies were graphitized in a conventional manner at about 3000oC. The following values were measured on the cylindrical graphite bodies having a diameter of 3 05 mm and a length of 2 300 mm (a corresponding mixture without addition of fibres was prepared as a comparison, and the measured values for the graphite bodies produced therefrom are shown in brackets): Longitudinal coefficient of in μm/(K-m) 0.06 (0.14) thermal expansion Transverse coefficient of in μm/(K.m) 1.88 (1.88) thermal expansion Flexural strength parallel to in MPa 28.5 (26.0) the extrusion direction WE CLAIM: 1. A connecting pieces for carbon material electrodes, characterized in that the connecting pieces contain carbon fibres whose surface has been oxidatively activated and which additionally have a carbonized coating, the carbonized coating being the carbonization product of a coating material selected from wax, pitch, natural resins, thermoplastic and thermosetting polymers. 2. The connecting pieces as claimed in claim 1, wherein the carbon fibres have a modulus of elasticity of from 200 to 250 GPa. 3. The connecting pieces as claimed in claim 1, wherein they have a linear coefficient of thermal expansion of from -0.5 to +0.1 |im/(K"m) in the direction parallel to the lateral surface and from 1.7 to 2.1 |jm/(K'm) in the plane perpendicular thereto, 4. The connecting pieces as claimed in claim 1, wherein the carbon fibres have an average length of from 0.5 to 40 mm. 5. The connecting pieces as claimed in claim 1, wherein the mass fraction of carbon fibres in the connecting pieces is from 0.2 to 10%. 6. The connecting pieces as claimed in claim 1, wherein the mass fraction of the carbonized coating on the carbon fibres, based on the mass of the carbon fibres, is from 0.2 to 15%. 7. The connecting pieces as claimed in claim 1, wherein the carbon fibres are those based on polyacrylonitrile. 8. The connecting pieces as claimed in claim 1, wherein the carbon fibres are present in the form of parallel filaments, woven fabrics, layered fabrics, warp-knitted fabrics, knitted fabrics and/or nonwoven fabrics. 9. A process for the production of connecting pieces containing carbon fibres and intended for carbon material electrodes, characterized in that the surface of the carbon fibres is activated by oxidation in a first step, that the fibres are then provided, in a second step, with a surface coating comprising a coating material selected from wax, pitch, natural resins or thermoplastics or thermosetting polymers, the coated fibres are optionally treated in a third step at a temperature of from 750 to 1 300°C for carbonization of the coating, are mixed in a fourth step with coke having a mean particle size in the range from 0.05 to 4 mm, pitch having a softening temperature in the range from 70°C to 150°C and optionally further additives and shaped to give cylindrical bodies, the cylindrical mouldings are carbonized and then graphitized in a fifth step, and the graphitized mouldings are turned, in a sixth step, to give the connecting pieces having threads. 10. The process as claimed in claim 9, wherein carbon fibres in the form of fibre tow comprising from 1 000 to 60 000 individual filaments are used, which, after the third process step, are cut to give short fibres having an average length of from 0.5 to 40 mm. 11. The process as claimed in claim 9, wherein carbon fibres in the form of heavy tow comprising from 40 000 to 2 000 000 individual filaments are used, which, after the third process step, are cut to give short fibres having an average length of from 0.5 to 40 mm. 12. The process as claimed in claim 9, wherein the carbon fibres are activated in an aqueous bath containing an oxidizing agent. 13. The process as claimed in claim 9, wherein the carbon fibres are activated in an aqueous bath by anodic oxidation. 14. The process as claimed in claim 9, wherein the carbon fibres are activated in a gas stream containing an oxidizing agent. 15. The process as claimed in claim 9, wherein the activated carbon fibres are coated in the second step, in an aqueous or solvent-containing bath containing a dispersion or solution of a coating material selected from wax, pitch, natural resins or thermoplastic or thermosetting polymers. 16. The process as claimed in claim 9, wherein the coated carbon fibres are treated in the third step, at a temperature, of from 900 to 1 200°C for carbonization of the coating. 17. The process as claimed in claim 9, wherein in the fourth step, a mixture containing for each 100 kg of coke, from 10 to 40 kg of a pitch and from 0.2 to 20 kg of carbon fibres is prepared. 18. The process as claimed in claim 17, wherein from 0.1 to 1 kg of an iron oxide pigment having a mean particle size of from 0.1 to 2μm is added as a further additive. Dated this 19 day of March 2004

Documents:

250-che-2004-abstract.pdf

250-che-2004-claims duplicate.pdf

250-che-2004-claims original.pdf

250-che-2004-correspondnece-others.pdf

250-che-2004-correspondnece-po.pdf

250-che-2004-description(complete) duplicate.pdf

250-che-2004-description(complete) original.pdf

250-che-2004-drawings.pdf

250-che-2004-form 1.pdf

250-che-2004-form 19.pdf

250-che-2004-form 26.pdf

250-che-2004-form 3.pdf

250-che-2004-form 5.pdf