METHOD OF MANUFACTURING AN ELECTRICAL DEVICE

Abstract: An electrical device, especially an overcurrent protective device, with a polymer composition arranged between two electrodes comprising a polymer material and an electrically conductive powdered material, distributed in the polymer material, is manufactured by mixing the polymer material in thermoplastic state and in powdered state with a grain size of less than 100 mu m, and of less than 40 mu m in at least 50 % of the material, in solid, dry state with the electrically conductive powdered material in a grain size of less than 100 mu m into a mixture in which the polymer material constitutes at least 30 per cent and the electrically conductive powdered material at least 20 per cent of the total volume of these materials, and by subjecting the mixture together with the electrodes to a pressing and a heating to a temperature at which the polymer material melts at least on the surface of the grains while forming a permanently coherent body of the mixture and while fixing the electrodes to the coherent body.

Description: Method of manufacturing an electrical device The present invention relates to a method of manufacturing an electrical device, particularly an overcurrent protective device, comprising a body, provided with two parallel sur¬ faces, of an electrically conductive polymer composition with a resistivity of at most 100 mΩ cm and two electrodes arranged in contact with the parallel surfaces, the polymer composition comprising a polymer material and an electri¬ cally conductive powdered material distributed in the poly¬ mer material. One type of such an electrical device in the form of an overcurrent protective device is a PTC element (Positive Temperature Coefficient) , that is, an element whose resistivity has a positive temperature coefficient. The resistance of a PTC element of the above kind is low, for example a few hundredths Ω, in the normal working range of the element which may extend to, for example, 80°C and increases slightly with the temperature. If the temperature of the element exceeds this value, for example because of overcurrent, the resistance increases more rapidly, and upon exceeding a certain critical temperature, the element suddenly changes from a low-resistance to a high-resistance state in which the resistance may amount to 10 kΩ and more . Another type of such an electrical device in the form of an overcurrent protective device is a thermal overload relay. When manufacturing the body of electrically conductive polymer composition for an electrical device of the described kind with hitherto used methods, the used polymer material, if it is a thermoplastic resin, for example polyethylene, is melted and mixed (compounded) with the conductive powdered material which normally consists of carbon in some form or of a metallic material or of a mixture of carbon and a metallic material. This technique involves a number of limitations. Thus, to introduce the conductive material, the polymer material must have a relatively low viscosity and even if this is the case, it may be impossible to intermix the desired, sufficiently high contents of carbon. The powerful processing during the compounding operation also entails a risk that the conductive material is crushed or otherwise influenced so as to undergo undesirable changes. After the compounding, the produced mixture is subjected to a processing in connection with it being formed by extrusion, compression moulding, or in some other way, which entails a risk of an undesired anisotropy arising in the material in the formed product. The processes described may lead to problems in achieving reproducible properties in the manufactured, finished body. After the forming of the body of polymer composition, the body is provided with electrodes according to applied technique. The electrodes usually consist of metal foils and are applied by being pressed onto the body while being heated. According to the present invention, the mentioned limita¬ tions connected with prior art technique are eliminated and a considerable simplification of the manufacture of the electrical device is obtained. Thus, the invention makes possible the use of polymer material with a considerably higher viscosity than what has previously been possible. It provides a considerably greater freedom of choice as regards types and contents of conductive materials. The conductive material is not sub¬ jected to any processing with undesired consequences. No forming, entailing a risk of anisotropy, occurs. The electrodes may be applied without having to carry out a separate operating step. A particularly important advantage is that the method makes it possible to obtain devices with very low transition resistance between electrode and polymer composition. According to the invention, the favourable results are obtained by mixing the polymer material in thermoplastic state and in powdered form with a grain size of below 100 μm, and of below 40 μm in at least 50% of the material, in solid, dry state with the electrically conductive powdered material in a grain size of below 100 μm into a mixture in which the polymer material constitutes at least 30 per cent and the electrically conductive powdered material at least 20 per cent of the total volume of these materials and by subjecting the mixture together with the electrodes to a pressing and a heating to a temperature at which the polymer material melts at least on the surface of the grains while forming a permanently coherent body of the mixture and while fixing the electrodes to the coherent body. The grains of the polymer material then entirely lose their identity. According to a favourable embodiment of the invention, which provides a particularly good reproducibility in the manu¬ factured device, the mixture of polymer material and elec¬ trically conductive powdered material is subjected to a pressing operation at room temperature or other temperature which is considerably lower than the temperature at which the polymer material melts while forming a preformed body, before the mixture in the form of the preformed body together with .the electrodes is subjected to the pressing and the heating for forming the permanently coherent body and for fixing the electrodes. As electrodes there are preferably used prefabricated plates of a powdered metallic material, which have a porous struc¬ ture on the side facing the mixture and are impermeable to the polymer material so that they prevent penetration of the polymer material to the side facing away from the mixture. When using electrodes of this kind, the electrodes are effectively secured to the conductive polymer composition without the polymer material providing any insulating or any possibly slightly conductive coating on the outside of the electrodes. The polymer material penetrates, in its ther- moplastic state, into the pores of the electrodes without penetrating the electrodes. Plates which are completely porous from the beginning may be made impermeable to the penetration of polymer material by being provided on the outside with a metallic coating, for example electro- lytically. It is also possible to make the outside tight by causing a superficial layer on the porous electrodes to melt and solidify, for example by using laser technique, the electrodes otherwise being maintained unchanged in their porous state. Another way of maintaining a porous surface structure towards the side facing the mixture and making the plates tight against the penetration of polymer material is to sinter the plates, normally in reducing atmosphere, at a temperature necessary therefor, which is considerably lower than the melting temperature for the used metallic electrode material, and, after the sintering, to grind or otherwise mechanically treat the outside of the plate. However, it is possible to use electrodes of a type different from that described above. Thus, for example, the electrode material in powdered state may be applied in the form of layers on the surfaces of the conductive polymer composition, before it is subjected to pressing and heating for forming the permanently coherent body and for fixing the electrodes. Also in such a case, measures are preferably taken for ensuring that the electrodes are tight against the pene¬ tration of polymer material, for example by providing them with a tight coating of metallic material on that side facing away from the mixture. The metallic material in the electrodes may advantageously consist of nickel or copper but also other metallic materials in the form of pure metal or metal alloy with sufficient electric conductivity may be used. A suitable grain size of the metallic material is 0.5 μm - 20 μm and a suitable thickness of an electrode 100-1000 μm. If the electrodes are provided with a metallic coating, for example electrolytically, the coating may advantageously consist of copper which provides a well scattered electrical conduction in the lateral direction within the electrode in question, irrespective of the material on which the electrode is otherwise based. The tight surface layer on an otherwise porous electrode may advantageously have a thickness which amounts to 3-30% of the thickness of the entire electrode. According to an advantageous embodiment of the invention, a crosslinkable linear polymer is used as polymer material in the manufacture of the device. If the crosslinking ability of the polymer material is utilized, the crosslinking is carried out after the mixture of the polymer material and the conductive material together with the electrodes has been subjected to the pressing and the heating for forming the permanently coherent body and for fixing the electrodes. By crosslinking polymer material, a greater mechanical and thermal stability of the manufactured device may be achieved. The polymer material preferably consists of a polyolefin, such as polyethylene, polypropylene, polybutene, or a copolymer of ethylene and propylene. Specially preferred is HD polyethylene. However, it is possible to use other liner polymers which may be made sufficiently fine-grained and be mixed in dry state with the conductive material and be transferred into thermoplastic state when the mixture is subjected to pressing and heating for forming a permanently coherent body of the polymer composition and for fixing the electrodes. Examples of such other linear polymers are polyamide, polethylene terephthalate, polybutene terephthalate and polyoxymethylene. The polymer material preferably has a crystallinity of at least 5%. The grain size of the polymer material is preferably 5-100 μm, of which at least 50% of the material has a grain size smaller than 40 μm. As examples of suitable electrically conductive materials in the polymer composition may be mentioned carbon in the form of conductive carbon powders such as carbon black, metallic materials such as, for example, nickel, tungsten, molybdenum, cobalt, copper, silver, aluminium and brass, borides such as, for example, ZrB 2 and iB 2 , nitrides such as, for example, ZrN and TiN, oxides such as, for example, V 2 O 3 and TiO, carbides such as, for example, TaC, WC and ZrC, as well as mixtures of two or more of the exemplified materials such as, for example, a mixture of soot and nickel. The grain size of the conductive carbon powders such as carbon black is usually 0.01-0.10 μm, the grain size of metallic material preferably 0.5-100 μm and the grain size of borides, nitrides, oxides, and carbides preferably 0.01-100 μm. Preferably, at least part of the electrically conductive powdered material has a grain size which is smaller than the cross section of the pores of the electrode, so that at least that part of the powdered material can accompany the polymer material when it penetrates into the pores of the electrode. The polymer material suitably constitutes 30-80 per cent and the electrically conductive powdered material 20-70 per cent of the total volume of these materials in the polymer composition built up of the mixture. If the electrically conductive material consists of a mixture of carbon and metallic material, there are preferred a content of polymer material of 65-80 per cent and a content of electrically conductive powdered material of 20-35 per cent of the total volume of these materials in devices with a pronounced PTC effect. Of the electrically conductive powdered material, the carbon preferably constitutes 5-75 per cent by volume and the metallic material 25-95 per cent by volume. The invention will be explained in greater detail by describing a number of examples. Example 1 75 parts by volume HD polyethylene (NB 6081 from PLAST-LABOR S.A., Bulle, Switzerland) with a melting index (MI 190/2) of 40 g/10 min, a density of 0.960 g/cm 3 and a grain size of 5- 90 μm, thereof a grain size of 24-36 μm in more than 50% of the material, is mixed with 13 parts by volume nickel powder with a grain size of less than 7 μm and with 12 parts by volume carbon black of type N550 (ASTM) with a grain size of 0.040-0.048 μm into a mixture in the form of a polymer composition. The mixture is pressed at room temperature and a pressure of 70 MPa in a forming tool with a cylindrical cavity and one or two movable cylindrical dies into a preformed round plate with a diameter of 25 mm and a height of 1.5 mm. In the same forming tool there are prepared two electrodes in the form of plates with a thickness of 0.6 mm by pressing of nickel powder with a grain size of 4-7 μm. The pressing is carried out at room temperature and a pressure of 70 MPa. The plates are porous with through-going pores. Each plate is electrolytically provided on one side with a 20 μm thick layer of copper which, for one thing, eliminates the presence of through-going pores by the layer being tight, and, for another, provides a radially scattered surface layer of high conductivity. The plate of the polymer composition with one of the nickel electrode plates on each flat side is again placed in the forming tool with the copper layers facing outwards, whereupon the stack of the three plates is first pressed at room temperature and a pressure of 70 MPa and then at 150°C without changing the pressure. The polymer composition then forms a permanently coherent body, to which the electrodes are mechanically secured in an effective manner with a low transition resistance between electrode and polymer com¬ position by the polymer composition having penetrated into the pores of the electrodes or at least the polymer material and the part of the electrically conductive powdered mat¬ erial having a grain size which is smaller than the pores of the electrode. The polymer composition has a resistivity of less than 50 mΩ cm. The manufactured device is excellently suited for use as PTC element. Example 2 A device is manufactured in the manner stated in Example 1 with the difference that the copper layers on the outside of the electrodes are not applied until the stack of the plates of the polymer composition and of the nickel electrodes has been pressed at room temperature into a coherent body. After application of the copper layers on the outside of the electrodes, the coherent body is subjected to a pressing at a pressure of 70 MPa and a temperature of 150°C. Example 3 A device is manufactured in the manner described in Example with the difference that the electrode plates pressed at room temperature, instead of being provided with tight copper layers on the outside, are subjected to a sintering in hydrogen gas atmosphere at about 400°C for four hours followed by a grinding of the side facing away from the polymer composition using 320 mesh wet grinding paper. The grinding entails a deformation of the surface layer so that this becomes tight. This resuls in electrode plates with a porous surface structure on the side facing the polymer composition but without through-going pores. They will therefore become impermeable to polymer material during the hot pressing. Example 4 A device is manufactured in the manner described in Example 1 with the difference that the porous electrodes, instead of being provided on the outside with a copper layer, are made tight there by melting down the surface layer to a depth of about 50 μm and causing it to solidify by the use of laser. The device is excellently suited for use as PTC element. Example 5 •a i A device is manufactured in any of the ways described in Examples 1-4. After the last pressing with heat treatment, the polymer composition is subjected to a crosslinking by electron-irradiating the device in its entirety until the crosslinking degree of the polymer material amounts to 80% . The polymer composition has a resistivity of less than 50 mΩ cm. The device is excellently suited for use as PTC element. Example 6 Electrodes are manufacturd in a manner described in Example 1, Example 3 or Example 4. The electrodes are placed together with the polymer composition described in Example 1 in the form of a powder, that is, without preforming, in the cavity of a forming tool of the kind described in Example 1 with the electrodes on either side of the polymer composition and with the tight layers (Examples 1 and 4) facing outwards. The polymer composition and the electrodes are subjected to a pressing with a pressure of 70 MPa at a temperature of 150°C. The polymer composition has a resistivity of. less than 50 mΩ cm. The device is excellently suited for use as PTC element . Example 7 A device is manufactured in any of the ways described in Examples 1-6 but with the difference that, instead of polyethylene mentioned there, there is used an LD poly¬ ethylene (HX 1681 from PLAST-LABOR S.A.) with a melting index of 70 g/10 min, a density of 0.916 g/cm 3 and a grain size of 5-35 μm, thereof a grain size of 10-14 μm in more than 50% of the material. The polymer composition has a resistivity of less than 50 mΩ cm. The produced device is excellently suited for use as a PTC element. Example 8 A device is manufactured in any of the ways described in Example 1, 2, 3, 4 or 6, with the difference that instead of polyethylene mentioned there, there is used polypropylene (PB 0580 from PLAST-LABOR S.A.) with a melting index (MI 230/5) of 100 g/10 min, a density of 0.905 g/cm 3 , and a grain size of 5-90 μm, thereof a grain size of 24-36 μm in more than 50% of the material and the difference that the hot pressing is carried out at 170°C. Example 9 A device is manufactured in any of the ways described in Examples 1-8, with the difference that instead of the electrically conductive powdered materials in the form of 13 parts by volume nickel powder and 12 parts by volume carbon black mentioned there, there are used 50 parts by volume ZrN with a grain size of less than 45 μm. The polymer composition has a resistivity of less than 50 mΩ cm. Example 10 A device is manufactured in the manner described in Example 9, with the difference that instead of ZrN there is used TiN with a grain size of less than 6-10 μm. The polymer composition has a resistivity of less than 35 mΩ cm. Exa ple 11 A device is manufactured in any of the ways described in Examples 1-8, with the difference that instead of the electrically conductive powdered materials in the form of 13 parts by volume nickel powder and 12 parts by volume carbon black mentioned there, there are used 13 parts by volume of the same carbon black and 52 parts by volume TiN with a grain size of 6-10 μm. The polymer composition has a resistivity of less than 35 Ωm cm. Example 12 A device is manufactured in the manner described in Example 10, with the difference that instead of TiN there is used ZrB 2 with a grain size of less than 45 μm. The polymer composition has a resistivity of less than 30 mΩ cm. Example 13 A device is manufactured in the manner described in Example 9, with the difference that instead of ZrN there is used TiB 2 with a grain size of less than 45 μm. Example 14 A device is manufactured in any of the ways described in Examples 1-8, but with the difference that instead of the electrically conductive powdered materials mentioned there in the form of 13 parts by volume nickel powder and 12 parts by volume carbon black, there is used 120 parts by volume soot of the same kind as in Example 1. Example 15 A device is manufactured in any of the ways described in Examples 1-8, with the difference that instead of the electrically conductive powdered materials mentioned there in the form of 13 parts by volume nickel powder and 12 parts by volume carbon black, there is used 60 parts by volume nickel powder of the same kind as in Example 1. In all of the cases mentioned in Examples 1-15, the grains of the polymer material entirely lose their identity.

Claims: CLAIMS 1. A method of manufacturing an electrical device, especially an overcurrent protective device, comprising a body, provided with two parallel surfaces, of an electrically conductive polymer composition with a resistivity of at most 100 mΩ cm and two electrodes arranged in contact with the parallel surfaces, the polymer composition comprising a polymer material and an electrically conductive powdered material distributed in the polymer material, characterized in that the polymer material in thermoplastic state and in powdered stated with a grain size of less than 100 μm, and of less than 40 μm in at least 50% of the material, is mixed in solid, dry state with the electrically conductive powdered material in a grain size of less than 100 μm into a mixture in which the polymer material constitutes at least 30 per cent and the electrically conductive powdered material at least 20 per cent of the total volume of these material, and that the mixture together with the electrodes is subjected to a pressing and a heating to a temperature at which the polymer material melts at least on the surface of the grains while forming a permanently coherent body of the mixture and while fixing the electrodes to the coherent body. 2. A method according to claim 1, characterized in that the mixture is subjected to a pressing at room temperature or another temperature which is considerably lower than the temperature at which the polymer material melts, while for¬ ming a preformed body, before the mixture in the form of the preformed body together with the electrodes is subjected to the pressing and the heating for forming the permanently co¬ herent body and for fixing the electrodes. 3. A method according to claim 1 or 2, characterized in that as electrodes there are used prefabricated plates of a powdered metallic material which have a porous structure on the side facing the mixture and are tight against the pene- tration of polymer material to the side facing away from the mixture. 4. A method according to claim 3, characterized in that the plates are tight on that side facing away from the mixture by being provided on said side with a tight coating of a metallic material. 5. A method according to claim 3, characterized in that the plates are tight on that side facing away from the mixture by being arranged on said side with a melted and solidified surface layer of the powdered metallic material in the plates. 6. A method according to claim 3, characterized in that the plates are impermeable to the penetration of polymer material by through-going pores being removed by sintering of the plates and a subsequent mechanical treatment of that side facing away from the mixture. 7. A method according to any of claims 1-6, characterized in that as polymer material there is used a crosslinkable linear polymer and that the polymer material is crosslinked after the mixture together with the elec¬ trodes has been subjected to the pressing and the heating for forming the permanently coherent body and for fixing the electrodes. 8. A method according to any of claims 1-7, characterized in that as polymer material there is used a polyolefin. 9. A method according to claim 8, characterized in that as polymer material there is used polyethylene. 10. A method according to any of claims 1-9, characterized in that the polymer material constitutes 30-80 per cent and the electrically conductive powdered material 20-70 per cent of the total volume of these materials in the mixture. 11. A method according to any of claims 1-10, characterized in that the polymer material constitutes 65-80 per cent and the electrically conductive powdered material 20-35 per cent of the total volume of these materials in the mixture. 12. A method according to any of claims 1-11, characterized in that as electrically conductive powdered material there is used carbon in the form of carbon black. 13. A method according to any of claims 1-11, characterized in that as electrically conductive powdered material in the mixture there is used a metallic material. 14. A method according to any of claims 1-11, characterized in that as electrically conductive powdered material in the mixture there is used a mixture of carbon in the form of carbon black and a metallic material. 15. A method according to any of claim 13 and 14, characterized in that the metallic material consists of nickel. 16. A method according to claim 14, characterized in that the carbon constitutes 5-75 per cent and the metallic material 25-95 per cent of the total volume of these materials in the mixture thereof.

Inventors: HANSSON TOMAS, HENRIKSSON ULF, JOHANSSON HAAKAN, HANSSON, TOMAS, HENRIKSSON, ULF, JOHANSSON, HAAKAN
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: H01B1
IPC codes: H01B1, H01C7