ELECTRICAL DEVICE WITH PTC-BEHAVIOR

Abstract: An electrical device (1) which exhibits PTC behavior and which comprises a resistive element (7) electrically connected to first and second electrodes (3, 5). The resistive element is prepared from a composition comprising a curable polymeric component (9), a first particulate filler (13) which is both electrically conductive and magnetic, and an optional second particulate filler (15). The first particulate filler is dispersed in the polymeric component in discrete regions (11) which extend from the first electrode to the second electrode. Devices of the invention can be used as circuit protection devices or heaters. Also disclosed is a method of making an electrical component in which a composition comprising an electrically conductive and magnetic particulate filler is positioned on a layer which comprises a magnetic pattern to form an element, and is then cured so that discrete regions of the particulate filler form corresponding to the pattern.

Description: Electrical device with PTC-behaviour BACKGROUND OF THE INVENTION Field of the Invention This invention relates to electrical devices comprising conductive polymer compositions and methods for making such devices. Introduction to the Invention Conductive polymer compositions and electrical devices comprising them are well-known. Such compositions generally comprise a polymeric component, and dispersed therein, a particulate electrically conductive filler. Electrical devices such as circuit protection devices and heaters are prepared from such compositions by connecting electrodes to an element prepared from a conductive polymer composition. When connected to a source of electrical power, current passes through the conductive polymer element from a first electrode to a second electrode. Particularly useful are devices prepared from conductive polymer compositions which exhibit a positive temperature coefficient of resistance, i.e. exhibit PTC behavior. Such compositions exhibit a sharp increase in resistivity over a relatively narrow temperature range. The temperature at which this increase occurs is the switching temperature T s and may be defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of a PTC element against temperature which lie on either side of the portion of the curve showing a sharp change in slope. When the polymer matrix is crystalline, the PTC anomaly generally occurs slightly below the melting point Tm of the polymer, the melting point being defined as the peak of the endotherm on a differential scanning calorimetry (DSC) curve. The increase from the resistivity at a specified low temperature, e.g.20°C (p20) > to peak resistivity (ppeak > i-e. the maximum resistivity which the composition exhibits above Ts) is the PTC anomaly height. For many applications it is desirable that the composition have as low a resistivity and as high a PTC anomaly as possible. A low resistivity allows preparation of small devices which have a low resistance and may occupy little space when installed into an electrical circuit or positioned on a printed circuit board. A high PTC anomaly allows the device to withstand necessary applied voltage. For many conventional compositions, the particulate conductive filler is carbon black. Compositions with lower resistivities can often be made by replacing some or all of the carbon black by a conductive filler which is metal. Examples of such metal-filled compositions are disclosed in U.S. Patents Nos. 4,545,925 (Fouts Jr. et al) and 5,378,407 (Chandler et al). Even when very conductive metal fillers are used, it is often necessary to use a substantial amount of filler to achieve the desired low resistivity, and when crystalline polymers are used, the resulting composition may be highly viscous. Such highly viscous compositions are difficult to process and form into devices, and once formed, the resulting device may exhibit electrical instability on repeated exposure to an applied voltage. Furthermore, the switching temperature of such devices is dependent on the melting temperature of available crystalline polymers. Generally the composition must be processed at a temperature above the melting temperature of the polymer, limiting the available mixing techniques. It may be difficult to form complex shapes if it is necessary to do so above the melting temperature of the polymer. Compositions in which relatively low loadings of magnetic metal particles are aligned in a magnetic field are known. See, for example, V.E. Gul et al, "Formation of Electrically Conductive Structures in a Polymeric Material Under the Action of a Magnetic Field", Plast. Massy. No. 4, p. 46 (1968), Jin et al, J. Appl. Phys., vol. 64, no. 10, p. 6008 (November 15, 1988), and U.S. Patent No. 3,359,145 (Salyer). Such compositions have not been used, however, to prepare electrical devices which exhibit PTC behavior. SUMMARY OF THE INVENTION We have now discovered that electrical devices exhibiting high PTC anomalies can be prepared from low resistivity compositions which comprise very little filler if the filler is ferromagnetic, ferrimagnetic, or paramagnetic and is aligned in discrete domains which extend from one electrode to another. In addition, such compositions are not limited to crystalline polymers but can be prepared from curable or "settable" polymers. The alignment occurs when the polymeric component is cured in the presence of a magnetic field. The switching temperature can be "selected" based on the curing temperature. In a first aspect, this invention provides an electrical device which exhibits PTC behavior and which comprises ( 1 ) a resistive element prepared from a composition which comprises (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component in discrete regions, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first filler and the second filler being at most 50% by volume of the total composition; and (2) first and second electrodes which are electrically connected to the resistive element and can be connected to a source of electrical power, the discrete regions extending through the resistive element from the first electrode to the second electrode. Particular advantages in terms of electrical stability can be achieved if the conductive filler is aligned between two electrodes, each of which is magnetic. Thus, in a second aspect, this invention provides a method of preparing an electrical device, said method comprising ( 1 ) preparing a mixture of (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first filler and the second filler being at most 50% by volume of the total composition; (2) positioning the mixture adjacent a first electrode which is electrically conductive and magnetic to form an element precursor which (i) comprises a second surface opposite the first electrode and (ii) has a thickness; and (3) curing the mixture in the presence of a magnetic field so that the first filler aligns in discrete regions through the thickness of the element precursor from the first electrode to the second surface. In a preferred embodiment, the curing is conducted when the mixture is in direct contact with one or both electrodes. We have also found that the formation and position of discrete domains of the conductive filler can be controlled by the use of at least one electrode which is in the form of a particular pattern, e.g. a mesh. In addition, the position of the domains can be controlled by the use of a layer which is not one of the electrodes. Thus in a third aspect, this invention provides a method of preparing an electrical component, said method comprising (1) preparing a mixture of (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first and second fillers being at most 50% by volume of the total composition; (2) positioning the mixture in proximity with a layer to form an element having a thickness, said layer comprising a pattern prepared from a magnetic material; and (3) curing the mixture in the presence of a magnetic field so that the first filler aligns in discrete regions through the thickness of the element, the discrete regions corresponding to the pattern. This method can be used to prepare electrical devices by the addition of one or more electrodes to the component. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross-sectional view of an electrical device according to the first aspect of the invention; Figures 2 and 3 are schematic cross-sectional views of electrical devices made according to the second or third aspects of the invention; Figure 4 is a schematic cross-sectional view of an electrical component made according to the third aspect of the invention; and Figures 5 to 10 show resistivity as a function of temperature curves for devices of the invention. DETAILEP DESCRIPTION OF THE INVENTION The devices of the invention generally exhibit positive temperature coefficient (PTC) behavior. In this specification, the term "PTC" is used to mean a composition or device which has an R]4 value of at least 2.5 and/or an RlOO value of at least 10, and it is preferred that the composition or device should have an R30 value of at least 6, where Rl4 is the ratio of the resistivities at the end and the beginning of a 14°C range, RlOO is the ratio of the resistivities at the end and the beginning of a 100°C range, and R30 is the ratio of the resistivities at the end and the beginning of a 30°C range. Generally the devices of the invention which exhibit PTC behavior show increases in resistivity which are much greater than those minimum values. Devices of the invention comprise a resistive element positioned between and in electrical contact with first and second electrodes. The resistive element is prepared from a composition which, when cured, has a resistivity of less than 10^ ohm-cm, preferably less than 10-* ohm-cm, particularly less than 100 ohm-cm, more particularly less than 10 ohm-cm, especially less than 1 ohm-cm, most especially less than 0.5 ohm-cm, e.g. 0.01 to 0.5 ohm-cm. The composition comprises a curable polymeric component, a first particulate filler, and one or more optional second particulate fillers. The curable polymeric component is one that undergoes a physical and/or chemical change on exposure to an appropriate curing condition, e.g. heat, light, radiation, microwaves, a chemical component, or a temperature change. The polymeric component may be any appropriate polymer, e.g. a thermoplastic material such as a polyolefin, a fluoropolymer, a polyamide, a polycarbonate, a polyester, or a styrene-butadiene-styrene material; but it is preferred that the polymeric component be a thermosetting resin. Suitable thermosetting resins include silicone elastomers, acrylates, epoxies, polyurethanes, polyesters, and liquid ethylene/propylene/diene monomers. Particularly preferred are curable liquid silicone elastomers made from functionalized precursors, including those with functional groups such as vinyl, hydride, silanol, amino, aminoalkyl, alkoxy, acetoxy, methacryloxyalkyl, mercapto, carboxy alkyl, carbinol, chloromethyl, e.g. vinyldimethylsiloxy terminated polydimethylsiloxane. In addition, ultraviolet light- curable materials such as acrylates, silicones and epoxies are preferred. The curable component is present in an amount 50 to 99.99% by volume, preferably 60 to 99.9% by volume, particularly 70 to 99.9% by volume, especially 80 to 99.9% by volume, more especially 90 to 99% by volume of the total composition. It is preferred that, prior to curing, the unfilled resin have a viscosity at room temperature of at most 200,000 cps, preferably at most 100,000 cps, particularly at most 10,000 cps, in order to allow the particulate filler to move under the influence of a magnet. The curable component may be cured by any suitable means, including heat, light, microwaves, electron beam, or gamma irradiation (e.g. a Co^O source). A catalyst, e.g. a platinum catalyst, may be added to initiate the cure and control the rate and/or uniformity of the cure. The first particulate filler is present at 0.01 to 50% by volume, preferably 0.1 to 40% by volume, particularly 0.1 to 30% by volume, especially 0.1 to 20% by volume, more especially 1 to 10% by volume of the total composition. The first filler is both electrically conductive and magnetic. In this specification the term "electrically conductive" is used to mean a filler which is conductive or semiconductive and which has a resistivity of less than 1 x 10^ ohm-cm and is preferably much lower, i.e. less than 1 ohm-cm, particularly less than 1 x 10 " ohm-cm, especially less than 1 x 10 ohm-cm. The term "magnetic" is used to mean ferromagnetic, ferrimagnetic, and paramagnetic materials. The filler may be completely magnetic, e.g. a nickel sphere; it may comprise a non-magnetic core with a magnetic coating, e.g. a nickel-coated ceramic particle; or it may comprise a magnetic core with a non-magnetic coating, e.g. a silver-coated nickel particle. Suitable first fillers include nickel, iron, cobalt, ferric oxide, silver-coated nickel, or silver-coated ferric oxide, or alloys of these materials. Any shape particle may be used, although approximately spherical particles are preferred. In general, the primary particle size of the first filler is less than 300 microns, preferably less than 200 microns, particularly less than 150 microns, especially less than 100 microns, and is preferably in the range of 0.05 to 40 microns, particularly 1 to 10 microns. Because processing techniques, e.g. coating the primary particle, may result in agglomeration, it is possible that the first filler, as mixed into the polymeric component, may have an agglomerate size of as much as 300 microns. For some applications, a mixture of different particle sizes and/or shapes and/or materials may be desirable. The optional second particulate filler may be electrically conductive or non- conductive, but is generally not magnetic. Such fillers are present to modify the physical and/or thermal and/or electrical properties of the composition, e.g. viscosity, thermal conductivity, and/or resistivity. The second filler is present in amount 0 to 20% by volume, preferably 0.1 to 20% by volume, particularly 0.2 to 15% by volume of the total composition. When the second filler is present, the total volume of the first filler and the second filler is at most 50% by volume of the total composition. Suitable second fillers include silica, alumina, alumina trihydrate, magnesium hydroxide, zinc borate, and carbon black, or mixtures thereof. The composition may comprise additional components including antioxidants, radiation crosslinking agents (often referred to as prorads or crosslinking enhancers), stabilizers, dispersing agents, coupling agents, acid scavengers, flame retardants, arc suppressants, or other components. These components generally comprise at most 10% by volume of the total composition. If the composition is to be used as an electrical component rather than as a device which exhibits PTC behavior, the composition generally comprises 40 to 95% by volume, preferably 45 to 90% by volume, particularly 50 to 85% by volume of the total composition of the curable component, and 5 to 60% by volume, preferably 10 to 55% by volume, particularly 15 to 50% by volume of the total composition of the first particulate filler. The second particulate filler may be present at 0 to 10% by volume of the total composition. The total volume of the first and second fillers is at most 50% by volume of the total composition. The dispersion of the first and second fillers, as well as other components, into the polymeric component may be achieved by any suitable means of mixing. Because it is preferred that the polymeric component have a relatively low viscosity prior to curing, the fillers can be mixed into the polymeric component by hand or by the use of a mechanical stirrer. Mixing is done until a uniform dispersion of the filler particles is achieved. In addition, conventional mixing equipment may be suitable or may be adapted for use. Solvent-mixing may also be used. The resistive element is in physical and electrical contact with at least one electrode that is suitable for connecting the element to a source of electrical power. The type of electrode is dependent on the shape of the element, but is preferably laminar and in the form of a metal foil, metal mesh, or metallic ink layer. Particularly suitable metal foil electrodes comprise microrough surfaces, e.g. electrodeposited layers of nickel or copper, and are disclosed in U.S. Patents Nos.4,689,475 (Matthiesen) and 4,800,253 (Kleiner et al), and in International Application No. PCT US95/07888 (Raycem Corporation, filed June 7, 1995). Particularly good electrical stability for devices of the invention can be achieved if at least one and preferably both of the electrodes is both electrically conductive and has at least some portion which is magnetic. Electrodes of this type include nickel, nickel-coated copper, and stainless steel. It is preferred that the entire surface of the electrode comprise the magnetic material. The electrode may be electrically connected to the resistive element by means of a conductive tie layer. Electrical devices of the invention can be used as circuit protection devices, heaters, sensors (e.g. to measure changes in pressure and/or temperature, or the presence of a chemical species), or resistors. The shape of the device depends on the application, but particularly useful devices comprise two laminar electrodes, preferably metal foil electrodes, and a resistive element comprising a conductive polymer element sandwiched between them. Additional metal leads, e.g. in the form of wires or straps, can be attached to the foil electrodes to allow electrical connection to a circuit. In addition, elements to control the thermal output of the device, e.g. one or more conductive terminals, can be used. Circuit protection devices generally have a resistance at 20°C, R20, of less than 100 ohms, preferably less than 20 ohms, particularly less than 10 ohms, especially less than 5 ohms, most especially less than 1 ohm. For many applications, the resistance of the circuit protection device is much less than 1 ohm, e.g. 0.010 to 0.500 ohm. Heaters generally have a resistance at 20°C of at least 100 ohms, preferably at least 250 ohms, particularly at least 500 ohms. When the electrical device is a heater, the resistivity of the conductive polymer composition is preferably higher than that of circuit protection devices, e.g. 10^ to 10^ ohm-cm, preferably 10^ to 10^ ohm-cm. The resistance of the device will be affected by the loading and type of the first filler and optional second filler, the alignment of the particles, the particle size, and the geometry of the device, among other factors. In order to achieve good electrical properties, it is necessary to align the first and second components in discrete regions in the polymeric component, e.g. as a column that extends through the polymeric component from one side to the other, or, when electrodes are present, as a column that extends through a resistive element from the first electrode to the second electrode. Such domains can be formed in the presence of a magnetic field that causes the magnetic first and second filler particles to align. When such alignment occurs during curing of the resin, the alignment is maintained in the cured resin. Any type of magnetic field that is capable of supplying a field strength sufficient to align the particles may be used. We have found that for uncured resins having a viscosity of less than 200,000 cps, magnetic field strengths of between 80 and 1200 gauss are strong enough. A conventional magnet of any type, e.g. ceramic or rare earth, may be used, although for ease in manufacture, it may be preferred to use an electromagnet with suitably formed coils to generate the desired magnetic field. It is often preferred that the uncured resin be positioned between two magnets during the curing process, although for some applications, e.g. a particular device geometry, or the need to cure by means of ultraviolet light, it can be sufficient that there be only one magnet that is positioned on one side of the resin. The resin is often separated from direct contact with the magnets by means of an electrically insulating spacing layer, e.g. a polycarbonate, polytetrafluoroethylene, wax, or silicone sheet, or by means of first and second electrodes. However, depending on the type of the resin and the electrode, it may be desirable to cure the resin directly in contact with one of the electrodes in order to form an element precursor, or in contact with both electrodes in order to form a resistive element. The element precursor can be converted into an element by attachment of a second electrode. Alternatively, it is possible to cure the resin partially or completely before attaching the electrodes to the cured resin to form the resistive element. The latter technique is especially appropriate for use with mesh or other foraminous electrode materials. A conductive tie layer, e.g. a conductive adhesive, may be used to enhance the bond between the electrodes and the resin. In order to control the thickness of the resistive element, the uncured resin may be poured or otherwise positioned within a mold of specified thickness, and then cured. The uncured resin may be cured by any suitable means, including heat, light, microwave, electron beam, or gamma irradiation, and is often cured by using a combination of time and temperature suitable to substantially cure the resin. The curing temperature T c may be at any temperature that allows substantial curing of the resin, i.e. that cures the resin to at least 70%, preferably at least 80%, particularly at least 90% of complete cure. When the curable polymeric component is a thermosetting resin which has a glass transition temperature T g , it is preferred that the curing be conducted at a curing temperature T c which is greater than T g . The switching temperature, T s , i.e. the temperature at which the PTC anomaly occurs, is dependent on the curing temperature T c and curing rate. We have found that, in general, T s is approximately (T c ± 40°C). By controlling variables such as viscosity, field strength, particle size, particle shape, particle composition, and cure rate, it is possible to reproducibly control T s of the device to within ± 5°C of a desired temperature. Particularly good electrical stability for devices of the invention can be achieved if one and preferably both of the electrodes are magnetic and during the curing and alignment process the uncured resin is in direct contact with at least one of the electrodes. The magnets may be in direct physical contact with the electrodes, or may be separated from them, e.g. by means of air, an electrically insulating polymeric sheet, or an electrically conductive sheet. For some applications it is desirable to ensure that the particles are aligned in one or more discrete sections of the composition. Such alignment may be achieved by the use of a layer which comprises a pattern prepared from a magnetic material. When the uncured resin is positioned in proximity with the layer and then is cured in the presence of a magnetic field, the alignment occurs in discrete regions corresponding to the pattern. Thus it is not necessary to have a magnet in the shape of the desired alignment. The term "in proximity with" means that the uncured resin is either in direct physical contact with the layer or is separated from direct contact by means of a spacing layer. The layer may comprise one of the electrodes, or electrodes may be attached to the cured element. If a metal mesh which comprises a magnetic material is used as the layer, the alignment will occur along the wires and at the intersection of the wires of the mesh. Useful electrical components, suitable for use, for example, as EMI shielding apparatus or gaskets, may be prepared by using a layer with a pattern. If electrodes are subsequently added, the electrical component can be converted into an electrical device. Under some circumstances it is possible to prepare electrical devices or electrical components using methods of the invention, but instead of using a first filler which is electrically conductive and magnetic, a filler which is electrically conductive and non¬ magnetic is dispersed in a curable magnetic fluid (i.e. a ferrofluid). A magnetic fluid is a colloidal suspension of small magnetic particles, e.g. magnetite, in a nonmagnetic carrier fluid. Generally the magnetic particles have a particle size of less than 1 micron, preferably less than 0.1 micron, particularly less than 0.01 micron. The electrically conductive particles have a particle size of at least 1 micron, preferably at least 10 microns, and are at least 10 times larger, preferably at least 100 times larger, than the magnetic particles. Suitable electrically conductive particles include metal particles and metal-coated glass or polymer particles. The fluid is a curable or settable polymeric component such as those described above. Particularly preferred are polymeric components with low viscosity, i.e. less than 10,000 cps, such as liquid EPDM or liquid silicone elastomers. In the presence of a magnetic field, the magnetic particles align and induce alignment in the larger, nonmagnetic particles. The invention is illustrated by the drawing in which Figure 1 shows device 1 in which first electrode 3 and second electrode 5 sandwich resistive element 7. Aligned throughout polymeric component 9 are discrete conductive chains 11, each of which is made of individual particles 13 of electrically conductive and magnetic filler. Also present in polymeric component 9 are particles 15 of second particulate filler. Figure 2 shows a device 1 which is similar to that of Figure 1 except that first and second electrodes 3,5 are in the form of a magnetic mesh. Conductive chains 11 are aligned under the mesh. Figure 3 shows a device 1 in which resistive element 7 is sandwiched between first electrode 3 which is magnetic and has a pattern and second electrode 5 which is solid and magnetic. Conductive chains 11 are formed in the region of the pattern. Figure 4 shows an electrical device 17 prepared from a component in which resistive element 7 is positioned between first and second electrodes 3,5 which are nonmagnetic. Conductive chains 11 have been formed due to the presence of magnetic layers 19,21 during the curing process. The invention is illustrated by the following examples in which Example 1 is a comparative example. Each of the compositions was prepared according to the Mixing Process unless otherwise indicated. The following components were used: Polymeric Component Silicone A Liquid curable silicone elastomer containing 98.4 g vinyldimethylsiloxy- terminated polydimethylsiloxane (Petrarch Inc. catalog number PS443 or Huls PS443; component 1), 0.2 g trimethylsiloxy-terminated polydimethylsiloxane (Union Carbide L45/50; component 2), 0.2 g 1,3,5,7- tetravinyltetramethyl-cyclotetrasiloxane (Petrarch Inc.; component 3), 0.014 g platinum-based catalyst (Petrarch Inc. PC075 or Gelest SIP6830.0; component 4), and 1.24 g tetrakis(dimethyl siloxy) silane (component 5). Silicone B Liquid curable silicone elastomer containing 98.6 g component 1, 0.4 g component 2, 0.002 g component 3, 0.014 g component 4, and 1.03 g component 5. Silicone C Liquid curable silicone elastomer containing 98.4 g component 1, 0.2 g component 2, 0.2 g component 3, 0.029 g component 4, and 1.24 g component 5. Silicone D Liquid curable silicone elastomer containing 98.6 g component 1, 0.4 g component 2, 0.02 g component 3, 0.014 g component 4, and 1.04 g component 5. Silicone E Liquid curable silicone elastomer containing 48.3 g component 1, 49.79 g component 2, 0.2 g component 3, 0.014 g component 4, 0.18 g component 5, and 1.52 g hydride-terminated polydimethylsiloxane (Petrach Inc. catalog number PS537; component 6). Silicone F Liquid curable silicone elastomer containing 49.61 g component 1, 49.24 g component 2, 0.0023 g component 3, 0.064 g component 4, 0.62 g component 5, and 4.52 g component 6. Epoxy Mastermend™ epoxy available from Loctite Corporation. Acrylate 1 Ebecryl™ 270 thermally curable acrylate available from UCB Radcure Inc. Acrylate 2 NOA65 uv-curable acrylate available from Loctite Corporation. Ella _*__ Type Shape Size Supplier Name (microns) 1 Ag-coated sphere 6-80 Aesar 13793 Ni (agglomerated) 2 Ag-coated sphere 6-80 INCO Ni (agglomerated) 3 Ag-coated sphere 1-20 Potters Conduct-o- Fe3θ4 fil™ 4 Pd-coated Ni platelet 1-50 5 Ni-coated Al chunk ~60 INCO 6 Ni sphere 1-80 Aesar 10581 7 Ni sphere 150 Aesar 10579 8 Ni fiber <750 9 Ni-coated sphere 10-50 Spectro Ni-ceno- glass Dynamics sphere 10 Co-coated sphere 10-50 Spectro Co-ceno- glass Dynamics sphere 11 Fe-coated sphere 10-50 Carbosphere glass 12 Carbon black aggregated sphere 0.030 Cabot Vulcan™ XC-72 13 Fumed Silica 0.014 Cabot Cab-o-Sil™ M-5 14 Zinc borate <10 U.S. Borax Firebreak™ ZB Electrode No. Metal Typ__ Name Supplier I Cu electrodeposited foil Gould II Ni electrodeposited foil Type 31 Fukuda III Ni mesh 3NΪ7-2/0 Exmet Magnets NI Size [mm finch " )] Supplier i 9.53 x 102 x 152 (0.375 x 4 x 6) Magnet Sales and Manufacturing Co. ii 25.4 x 102 x 152 (1 x 4 x 6) Magnet Sales and Manufacturing Co. Mixing Process The first filler, any second filler, and the liquid curable polymer component were mixed by hand or with a mechanical stirrer in a beaker until a uniform mixture was achieved. The mixture was poured onto a first electrode into a reservoir having a thickness of 0.76 mm (0.030 inch) unless otherwise specified and made from polytetrafluoroethylene, silicone elastomer, or metal. A second electrode sheet was positioned over the resin. Both electrodes were larger than the reservoir and thus extended over the edge of the mixture. The filled assembly was placed between two polycarbonate sheets, each with a thickness of 9.5 mm (0.375 inch), and two ceramic magnets were placed over the polycarbonate sheets so that the orientation of the magnetic field was across the thickness of the uncured resin. The assembly was placed in an oven at the specified temperature Tc for the specified time t c to cure the resin. If required to achieve a particular size, the cured resin was then cut into devices. Electrical contact could be made to the devices by connecting leads to the electrode overhanging the cured resin. R(T) Test The resistance versus temperature for a device was measured by placing the device in an oven at a specified initial temperature Tj (either 0°C or 20°C), and then cycling the device from Tj to an elevated temperature T f at a specified rate (e.g. in 1 to 20°C increments) and back to Tj three times. The resistance was measured periodically and the resistivity was calculated, giving p; as the initial resistivity, p 1 as the resistivity at the start of the second cycle, and p 2 as the resistivity at the start of the third cycle. Example 1 Following the Mixing Process, the ingredients shown in Table 1 were mixed and poured between electrodes into a reservoir having a diameter of 22.2 mm (0.875 inch). No magnets were in place during the curing process at 80°C. At 0°C, the resistivity of the device was 1 x 10 to 1 x 10 ohm-cm, well above the resistivity values achieved for comparable loadings which were aligned during the curing stage. E amples 2 to 4 These devices were made following the procedure of Example 1 except that two ceramic magnets (Magnet i) were in place during curing. The results are shown in Table I. R(T) curves for Examples 1, 2 and 3 are shown in Figure 5; an R(T) curve for Example 4 is shown in Figure 6. The switching temperature, T s , was determined from the R(T) curves, as was the temperature Tj 000 , at which the device underwent a lOOOx increase in resistivity from the initial resistivity p,. Also recorded was the ratio of pi/pj and p 2 pι- It is apparent that the devices with magnetic foils, i.e. types II and III, had improved stability and limited increase in resistivity over devices made with non-magnetic foils. TABLE I Examples 5 to 7 These devices were prepared following the Mixing Procedure and were cured at the specified temperature using two magnets. Devices of Example 5 were cycled three times from 0°C to 120°C; devices of Example 6 were cycled three times from 20°C to 110°C; devices of Example 7 were cycled three times from 0° to 130°C. The results are shown in Table II which includes the resistivity at 0°C and 20°C. The first cycle curve for Examples 5 to 7 is shown in Figure 7. It is apparent that the maximum increase in resistivity, as measured over a 14°C range, is a function of the cure temperature of the polymer; i.e. a higher cure temperature gives a higher switching temperature. TABLE II Example 5 6. 1 Polymer B B C Filler 1 1 1 Filler Loading (Vol%) 1.0 5.0 1.0 Electrode Type 2 1 2 Magnet ii i ii T C (°C) 25 60 80 t c (hours) 1.5 1.5 1.5 Device Shape round round rectangular Device Size [mm(inch)] 14.3 (0.56) 22.2 (0.875) 20 x 15 (0.8 x 0.2) pi at 0°C (Ω-cm) 6.7 - 1.67 p at 20°C (Ω-cm) 439 175 3.36 Tiooo (°C) 25-30 55-60 90-95 Examples 8 to 17 These devices of the compositions shown in Table III were prepared following the Mixing Procedure and were cured at the specified temperature using two magnets. For Example 13, instead of the Mixing Procedure, the silicone resin was poured into the reservoir and the nickel particles were sprinkled over the top of the resin before the second electrode was applied. Devices were prepared in either round, rectangular ("rec ") or square configuration. R(T) curves were measured over the specified temperature range and the temperature range indicating the largest change in resistivity ΔP max was recorded. R(T) curves for the second cycle for Examples 8 to 10 are shown in Figure 8, and for Examples 13 to 16 in Figure 9. The discontinuities shown in the R(T) curves are an artifact of the measurement equipment. Examples 18 to 20 The compositions of Table IV were prepared according to the Mixing Procedure and were cured using two magnets (Magnet i) and copper electrodes (Electrode I). Round devices with a diameter of 22 mm were prepared. The R(T) curves for the first thermal cycle are shown in Figure 10. TABLE IV Example 18. 12 2Q Polymer E Epoxy Acrylate 1 Filler 1 1 1 Filler Loading (Vol%) 3.0 1.0 1.0 T c (°C) 80 25 100 t c (hours) 1.5 1.5 1.5 Pi at 0°C (Ω-cm) 30-50 - 4-11 Δ Pmax (°C) 70-90 25-35 65-90 Example 21 The composition was prepared by mixing 1.0 volume % of Filler 1 into Acrylate 2 and pouring the mixture onto Electrode I to give a thickness of 0.38 mm. The assembly was placed on top of one Magnet i in a UV chamber and irradiated approximately 60 seconds until the resin hardened. Silver paint was painted on the top surface to form the second electrode and to connect two copper wires to that electrode. Two additional copper wires were soldered to the bottom copper electrode in order to make electrical connection and allow four-point resistance measurements. The resistivity at 0°C was about 100 ohm-cm, and the largest change in resistivity occurred in the range of 20 to 35°C when an R(T) curve was measured. Example 22 A composition was prepared by mixing 1.0 volume % Filler 1 into Silicone B. The mixture was poured onto Parafilm™ "m" laboratory film (American National Can) into a reservoir with a thickness of 0.76 mm and a second piece of Parafilm film was placed over the resin. A piece of stainless steel chicken wire with strands of about 0.75 mm diameter and a square opening of about 6.35 mm was placed on top of the Parafilm film. The assembly was placed between polycarbonate sheets and then placed between two magnets (Magnet i) and cured at 25°C for two hours. It was apparent that the filler particles were concentrated directly under the strands of the wire. Examples 23 to 28 Devices were prepared from compositions made following the Mixing Procedure using the components shown in Table V. The compositions were cured at 80°C for 1.5 hours between two magnets (Magnet ii) using nickel electrodes (Electrode II) to give a cured layer of 0.76 mm thickness. Rectangular devices of 20 x 15 mm (Examples 23 and 24) or 10 x 15 mm (Examples 25 to 27) were cut from the cured material. Devices were tested by inserting the device into a circuit in which the device was in series with an AC or a DC power supply, a load resistor, and a switch. After measuring the initial resistance at about 25°C, the resistor was selected so that when the voltage was at a specified level, the current in the circuit was limited to a specified level. The switch was closed to supply power to the device for a period of 0.03 to 60 seconds, depending on the time to trip the device, and then was opened, allowing the device to cool for 3 to 5 minutes if tested with AC, and for at least 5 minutes if tested with DC. Probes inserted in the circuit detected the current as the device tripped into a high resistance state. The time to trip into the high resistance state, t^, and the resistance after cooling down to about 25°C, R x , were recorded, where "x" indicates the cycle number. For the devices tested using AC, the current and voltage traces were recorded on an oscilloscope. Example 23, which contained no carbon black, showed substantial current and voltage spikes in the tripped state, while Example 24, with carbon black, exhibited few, if any spikes. Examples 25 to 28 tripped repeatedly under the test. After 100 cycles, for Example 25, ^ was 2.24 seconds and R x was 0.010. After 25 cycles, for Example 26, ^ was 0.24 seconds and R x was 0.017. After 110 cycles, for Example 27, t j was 0.16 seconds and R x was 0.014. After 900 cycles, for Example 28, ^ was 1.74 seconds and R x was 0.013. TABLE V Example 22 24 25. 26 22 2S Polymer C C C C C C Filler 1 1 1 1 1 1 Loading 1 (Vol%) 2.0 2.0 5.09 4.83 5.0 9.75 Second filler - 12 12 12/13 12/14 12 Loading 2 (Vol%) - 0.5 0.5 0.5/0.46 0.48/0.49 0.5 t t , (sec) 0.25 1.4 2.87 0.03 0.03 16.18 R l (ohms) - - 0.014 0.022 0.011 0.012 Volts 12 12 4 12 12 4 Current (A) 15 15 11 15 15 7 AC/DC AC AC DC DC DC DC Exam le 29 A chemical sensor was prepared from a composition made by mixing 5% Filler 1 into Silicone F. The composition was poured onto one electrode (Electrode II) in a mold with a thickness of 1 mm and the mold was positioned between a top magnet (Magnet ii) and a bottom magnet (Magnet i). The material was cured at 25°C for 10 minutes and then at 80°C for 2 hours. Following cure, a device was prepared by cutting a 1 cm x 1 cm square sample and attaching a top electrode (Electrode II). Wire leads were soldered to both electrodes. The room temperature resistance was 500 to 1000 ohms. Following immersion in hexane, the device resistance increased in less than 1 second to >10 6 ohms, the limit of the measurement equipment. When the device was removed from the hexane, the resistance decreased within 10 seconds to 500 to 1000 ohms. The procedure was repeated several times with similar results.

Claims: What is claimed is: 1. An electrical device which exhibits PTC behavior and which comprises (1) a resistive element prepared from a composition which comprises (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component in discrete regions, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first filler and the second filler being at most 50% by volume of the total composition; and (2) first and second electrodes which are electrically connected to the resistive element and can be connected to a source of electrical power, the discrete regions extending through the resistive element from the first electrode to the second electrode. 2. A device according to claim 1 wherein the curable polymeric component comprises a thermosetting resin, preferably a silicone elastomer, an acrylate, an epoxy, or a polyurethane. 3. A device according to claim 1 or claim 2 wherein the first particulate filler comprises nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials. 4. A device according to claim 1 or claim 2 wherein the second particulate filler is electrically conductive and nonmagnetic, and preferably comprises carbon black. 5. A device according to claim 1 or claim 2 wherein the second filler comprises silica, alumina, alumina trihydrate, magnesium hydroxide, or zinc borate. 6. A device according to claim 1 or claim 2 wherein at least one of the first and second electrodes comprises a region composed of a material which is electrically conductive and magnetic, and preferably the region comprises nickel or stainless steel. 7. A device according to any one of the preceding claims wherein the resistive element has a resistivity at 20°C of less than 1 ohm-cm. 8. A method of preparing an electrical device, said method comprising ( 1 ) preparing a mixture of (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first filler and the second filler being at most 50% by volume of the total composition; (2) positioning the mixture adjacent a first electrode which is electrically conductive and magnetic to form an element precursor which (i) comprises a second surface opposite the first electrode and (ii) has a thickness; and (3) curing the mixture in the presence of a magnetic field so that the first filler aligns in discrete regions through the thickness of the element precursor from the first electrode to the second surface. 9. A method according to claim 82 wherein the mixture is positioned between the first electrode and a second electrode which is adjacent the second surface and which is electrically conductive and magnetic to form an element, preferably wherein the first and second electrodes comprise nickel or stainless steel. 10. A method of preparing an electrical component, said method comprising ( 1 ) preparing a mixture of (a) 50 to 99.99% by volume of the total composition of a curable polymeric component, (b) 0.01 to 50% by volume of the total composition of a first particulate filler which is (i) electrically conductive and magnetic, and (ii) is dispersed in the polymeric component, and (c) 0 to 20% by volume of the total composition of a second particulate filler, the total volume of the first filler and the second filler being at most 50% by volume of the total composition; (2) positioning the mixture in proximity with a layer to form an element having a thickness, said layer comprising a pattern prepared from a magnetic material; and (3) curing the mixture in the presence of a magnetic field so that the first filler aligns in discrete regions through the thickness of the element, the discrete regions corresponding to the pattern.

Inventors: MUNCH MARK R, REAMEY ROBERT H, ROBISON JENNIFER L, TOTH JAMES, YOM CHI SUK, MUNCH, MARK, R., REAMEY, ROBERT, H., ROBISON, JENNIFER, L., TOTH, JAMES, YOM, CHI, SUK
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1996
Main classification: H01C7
IPC codes: H01C7