Heating elements and a process for their manufacture

Abstract: Disclosed is a heating element having improved performance, particularly at high power densities and high temperatures. The heating element comprises a substrate having a first layer comprising a silicon based electrically insulating material on it surface. On a surface of the first layer is a second layer comprising a silicon based electrically resistive material. Attached to the second layer is at least two separate areas of silicon based electrically conductive material, each of said areas suitable for connection to a power supply.

Description: The present invention relates to heating elements and to a process for their manufacture. Heating elements are known in the art. For example, EP0248781 describes a heating element which comprises an insulating support sheet with an electrically conductive layer applied on one of its faces. The electrically conductive layer is derived from a composition consisting of hollow particles of carbon black dispersed in a silicone resin which is soluble in organic solvents. This composition is thermo-hardened to form the electrically conductive layer. A problem with heating elements known in the art is their poor mechanical and heating performance after repeated exposure to the high temperatures (e.g., 200°C) and with high power densities (e.g., > 10 W/cm 2 ). This poor performance can include thermally generated stress and undesired hot spots which often lead to device failure. For example, assemblies comprising such heating elements often fail after a relatively short period of time (e.g. 50 hours or less) when submitted to 220 Volts. One object of the present invention is to provide a heating element having improved performance, particularly at high power densities and high temperatures. The invention provides in one of its aspects a heating element comprising a substrate; on a surface of the substrate, a first layer of material, said first layer being electrically insulating and obtained by curing a composition comprising a silicone resin; on a surface of the first layer, a second layer of material, said second layer being electrically resistive and obtained by curing a composition comprising a silicone resin and electrically conductive material; attached to the second layer are at least two separate areas of a third material, each of said areas of third material being electrically conductive and suitable for connection to a power supply, said areas of third material obtained by curing a composition comprising a silicone resin and electrically conductive material, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. In another of its aspects, the invention provides a process of manufacturing a heating element comprising supplying a substrate; applying a first composition comprising a silicone resin on a surface of the substrate; curing the first composition to form an electrically insulating layer; applying a second composition comprising a silicone resin and electrically conductive filler for forming an electrically resistive element on the electrically insulating layer; heating the second composition for a time and at a temperature sufficient to partially cure the second composition; applying a third composition comprising a silicone resin and electrically conductive filler for forming electrically conductive elements on at least two separate areas of the second composition, each of said areas suitable for connection to a power supply; and curing the second and third compositions, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. Surprisingly, when such heating elements are connected to 220 Volts, power densities higher than 10 W/cm 2 and temperatures of 250°C and more can be achieved and maintained for periods in excess of 1000 hours without failing. Such properties allow the heating elements of the invention to satisfy European Standard EN60335-1 relating to high voltage insulation and leakage current at room temperature. The silicone resin used to make the electrically insulating layer, the electrically resistive layer and the electrically conducting areas of the heating element of this invention can be the same or different and are restricted only by their compatibility with each other and the substrate, their ability to be applied to the substrate and cured to a solid material, and their resistance to the temperature to be achieved by the element. Preferably, the silicones used in each of these layers have the same or a similar modulus versus temperature curve to prevent the generation of stress as the devices are repeatedly heated. As long as the above objects are achieved, nearly any silicone resin can be used. Such resins are known in the art and can be produced by known techniques. Generally, these resins have the structure: (R 1 R 2 R 3 SiO 0.5 ) w (R 4 R 5 SiO) x (R 6 SiO 1.5 ) y (SiO 4/2 ) z In this structure, R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are independently selected from the group consisting of hydrogen and hydrocarbons of 1-20 carbon atoms. The hydrocarbons can include alkyls such as methyl, ethyl, propyl, butyl and the like, alkenyls such as vinyl, allyl and the like, and aryls such as phenyl. w , x , y and z in this structure comprise the molar ratio of the units with the total of w + x + y + z = 1. Generally, any value for w , x , y and z which result in the formation of a branched polymer (resin, DS < 1.8)) are functional herein (i.e., either y or z >0). Mixtures of resins are also useful herein. In a preferred embodiment of the invention, some of the above R groups are phenyl. Such materials often form better coatings and have improved properties at high temperatures. Especially preferred silicone resins include units of the structure [MeSiO 3/2 ], [MePhSiO 2/2 ], [PhSiO 3/2 ] and [Ph 2 SiO 2/2 ], where Me denotes a methyl group and Ph a phenyl group. Such resins are known in the art and commercially available. Generally, silicone resins are diluted/dissolved in solvents for the processing herein. Suitable solvents are known in the art and can include, for example, organic solvents such as aromatic hydrocarbons (e.g., xylene, benzene or toluene), alkanes (e.g., n- heptane, decane or dodecane), ketones, esters, ethers, or inorganic solvents such as low molecular weight dimethylpolysiloxanes. The amount of solvent used varies depending on the resin, any additives and the processing but can be, for example, in the range of between about 10 and about 90 wt. % based on the weight of the resin. The first layer of material in the present invention is characterised in that it is electrically insulating (insulating element). In a preferred embodiment, the first layer is also thermally conductive to transfer a high amount of heat from the electrically resistive layer. To achieve the electrical insulation and thermal conductivity, the first layer often includes a filler in addition to the silicone resin. Suitable thermally conductive, electrically insulating fillers are known in the art and can include, for example, alumina, silicon carbide, silicon nitride, zirconium diboride, boron nitride, silica, aluminium nitride, magnesium oxide, mixtures of the above and the like. Generally, these filler are included in an amount of greater than 30 wt. %, for example 50 to 90 wt. %. The second layer in the present invention is characterised in that it is electrically resistive (resistive element). To achieve this, the silicone resin is loaded with sufficient electrically conductive fillers to form an electrically resistive layer (e.g., resistivity p>0.1 ohm.cm). Such electrically conductive fillers can include, for example, graphite, carbon black, silver, nickel, nickel coated graphite, silver coated nickel, and mixtures of the above. The amount of filler used in this layer varies depending on the filler but, generally it is in the range of greater than 5 wt. %, for example 10 to 80 wt. %. The third, electrically conductive material in the present invention is characterised in it comprises at least two separate areas, each of said areas being suitable for connection to a power supply (conductive elements). To achieve this, the silicone resin is loaded with sufficient electrically conductive filler to form electrically conductive material (e.g., resistivity ρ<10 -3 Ohm.cm.). Suitable electrically conductive fillers include, for example, silver, gold, platinum, nickel and the like. The amount of filler used is generally greater than 40 wt. %, for example 60 to 80 wt.%. In a preferred embodiment of the invention, the heating element can have a fourth layer covering the top surface of the electrically resistive element (second layer) and the electrically conductive elements (third layer). This layer protects the elements from the environment (moisture, chemicals, etc.) and forms an electrically insulating layer. The fourth layer can comprise any of the well known electrical protection compounds known in the electronics industry such as epoxy, polyimide, PCB, silicones and the like. In a preferred embodiment of the invention, the fourth layer is a silicone with the same or similar modulus versus temperature curve as the first three layers. Each of the above four layers may also contain other ingredients which are conventional in the formulation of silicone resins. These can include, for example, fillers such as fumed or precipitated silica, crushed quartz, diatomaceous earth, calcium carbide, barium sulfate, iron oxide, titanium dioxide, and the like, pigments, plasticisers, agents for treating fillers, rheological additives, adhesion promoters, and heat stabilising additives such as zirconium or titanium containing methyl polysiloxane. The proportions of such optional ingredients are tailored to deliver the desired properties to the layer. The substrates used in the present invention include those which are conventionally used for heating elements and which are compatible with the final utility. These include, for example, metals such as anodised aluminium, aluminium, stainless steel, enamelled steel or copper or a non-metallic substrate, e.g. polyimide or mica. Obviously, if the substrate is electrically insulating and can disperse the heat effectively, the first layer of electrically insulating material may not be necessary. The substrate may be a flat plate, a tube or may have any other configuration. The heating elements of the present invention can be made by any desirable process. In a preferred embodiment of the invention, the heating elements are made by first supplying a substrate. The above composition comprising a silicone resin used to make the first layer is then applied on a surface of the substrate. This can be achieved by any of the well known techniques. These include, for example, dipping, spraying, painting, screen printing, etc. The composition used to form the first layer is then cured. The time and temperature used to cure the composition will depend on the silicone used as well as any fillers or additives used. As an example, however, the composition can be cured by heating in a range of 150 to 400°C for 1 to 4 hours. If desired, additional layers of the insulating material may be applied to assure electrical insulation. Next, the composition comprising a silicone resin and sufficient electrically conductive filler to form an electrically resistive element is applied on a surface of the electrically insulating layer. This composition can be applied via any of the methods described above for the first layer. The composition used to form the second layer is then cured as with the first layer. In a preferred embodiment of the invention, however, the second layer is only partially cured at this stage. By 'partially cured' it is meant that the composition used to form the second layer has been cured to a state sufficient to prevent diffusion of the composition used to form the electrically conductive areas through it and yet not cured to its final state. By not completely curing the second layer, the inventors have discovered that the physical properties of the heating element are improved. The time and temperature used for the partial curing will depend on the silicone used as well as the fillers. Generally, however, the composition can be cured by heating in a range of 100 to 300°C for 30 seconds up to several hours. The third material comprising a silicone resin and sufficient electrically conductive filler to form electrically conductive areas is applied on at least two separate and distinct areas of the electrically resistive layer. These areas can be, for example, on the top surface of the electrically resistive layer, on the ends of the electrically resistive layer or in any other configuration. These electrically conductive areas each allow for connection to a power supply. In a preferred embodiment, the third material is applied at 2 distinct distant ends of the electrically resistive layer. This material can be applied via any of the methods described above for the first layer. The materials used to form the electrically conducting areas (and the second layer, if it was not previously cured) are then cured. As with the previous cure steps, the time and temperature used for the curing will depend on the silicone used as well as the fillers and additives. Generally, however, the compositions can be cured by heating in a range of range of 150 to 350°C for 1 to 4 hours. If desired, the electrically resistive layer and the electrically conducting areas can be coated with the composition used to form the top protective layer. This composition can be applied via any of the methods described above for the first layer. The composition used to form the fourth layer is then cured. As with the previous cure steps, the time and temperature used for the curing will depend on the material used as well as the fillers and additives. The resultant heating elements of the invention are especially suitable for use in areas where high temperature elements are required. The applications include, for example, domestic appliances such as dry and steam irons, coffee machines, deep fryers, grills, space heaters, waffle irons, toasters, cookers, ovens, cooking hobs, water flow heaters, and the like, industrial equipment such as heaters, steam generators, process and pipe heating and the like and in the transportation industry such as for fuel and coolant preheating. In order that the invention may become more clear there now follows a description to be read with the accompanying drawings of one example heating element according to the invention. In this description all parts are by weight unless the context indicates otherwise. Example 1 In the drawings, Figure 1 is a sectional view of the example heating element. Figure 2 is a top view of the example heating element. The example heating element comprises the first, electrically insulating layer (2) formed on an anodised aluminium base plate (1), an electrically resistive layer (3) on top of the insulating layer, and at least two electrically conductive areas (4) thereon which are suitable for connection to a power supply. The heating element was formed by applying the composition used to form the first electrically insulating layer (2) onto an anodised aluminium base plate by means of a screen printer. This composition comprised 100 parts of a methyl phenyl silicone resin of the structure [MeSiO 3/2 ] 0.25 [MePhSiO 2/2 ] 0.5 [PhSiO 3/2 ] 0.15 [Ph 2 SiO 2/2 ] 0.10 in 100 parts xylene, 190 parts of alumina supplied by Alcoa under the trade name CL3000FG and 10 parts of silica supplied by Cabot under the trade name Cabosil® LM150. The finished layer had a uniform thickness of about 100 microns. The layer was cured by heating to 250°C for 1 hour. The composition used to form the second electrically resistive layer (3) was applied on top of the insulating layer (2) by means of a screen printer. This composition comprised 100 parts of the same methyl phenyl silicone resin used in layer 1, in 100 parts xylene, 140 parts of graphite supplied by Lonza under the trade name SFG6 and 10 parts particles of carbon black supplied by Cabot under the trade name Vulcan XC72 R. The finished layer had a uniform thickness of about 75 microns. The composition used to form the third electrically conductive elements was applied as two areas (4) on top of the electrically resistive layer (3) by dispensing the composition in the form of parallel tracks at either side of the electrically resistive layer (3). This composition comprised 100 parts of the same methyl phenyl silicone resin used in layers 1 and 2, in 100 parts xylene and 200 parts of silver flakes (type SF10E supplied by DEGUSSA). The second and third layers were finally cured by heating to 325°C for 3 hours. The fourth insulating protective top layer (5) was applied covering the layer (3) and the areas (4). The material used to apply this layer was a an addition cured highly filled silicone elastomer and was applied by screen printing and cured by heating to 150° C for 30 minutes. The resultant heating element was connected to a power supply of 220 volts at a specific power density of 10 watt/cm2 and submitted to a test cycle of 1000 hours. This test simulated normal use of a heating element as an appliance unit and comprised: 1- heating the element for a period of 1 hour during which the temperature is regulated with a thermal switch keeping the temperature about 250°C. 2- switching off the power and allowing the element to cool to a temperature of 50°C or below over a period of 30 minutes. No failure was observed. The example heating element was also submitted to a continuous heating test. In one such test, the power remained stable at a temperature of 250°C for 1000 hours. In a second test the power remained stable at a temperature of 170°C for 1600 hours. Neither test resulted in a failure. Example 2 The heating element was formed in a manner similar to Example 1. The composition used to form the first electrically insulating layer was applied to the anodised aluminium substrate as in Example 1 and comprised 75 parts of methyl phenyl silicone flakes having the structure: [MeSiO 3/2 ] 0.45 [MePhSiO 2/2 ] 0.05 [PhSiO 3/2 ] 0.40 [Ph 2 SiO 2/2 ] 0.10 dissolved in 75 parts xylene, 25 parts of the methyl phenyl silicone resin used in Example 1 in 25 parts xylene, 180 parts of alumina supplied by Alcoa under the trade name CL3000FG and 10 parts as of silica supplied by Cabot under the trade name Cabosil ® TS720. The layer was cured by heating to 250°C for 30 minutes. A second layer of the same electrically insulating material used to form the first layer was applied on the first layer and cured by heating to 250°C for 1 hour. The composition used to form the electrically resistive layer was applied as in Example 1 and comprised 95 parts methyl phenyl silicone flakes described above in this Example dissolved in 95 parts xylene, 5 parts of the methyl phenyl silicone resin used in Example 1 in 5 parts xylene, 130 parts of graphite supplied by Lonza under the trade name SFG6 and 20 parts particles of carbon black supplied by Cabot under the trade name Vulcan XC72 R. The layer was partially cured by heating to 200°C for 2 minutes under infra-red lamps. The composition used to form the electrically conductive layer was applied as in Example 1 and comprised 100 parts of the methyl phenyl silicone resin used in Example 1 in 100 parts xylene and 200 parts of silver flakes (type SF10E supplied by DEGUSSA). The second and third layers were cured by heating to 300°C for 1 hour. The resultant heating element met European Standard EN 60335-1 relating to high voltage insulation and leakage at room temperature. The heating element was connected to a power supply of 220 volts at a specific power density of 20 watt/cm2 and submitted to the test cycle of Example 1. No failure was observed. The power loss was less than or equal to 10%.

Claims: A heating element comprising: a substrate; on a surface of the substrate, a first layer of material, said first layer being electrically insulating and obtained by curing a composition comprising a silicone resin; on a surface of the first layer, a second layer of material, said second layer being electrically resistive and obtained by curing a composition comprising a silicone resin and electrically conductive filler; and attached to the second layer at least two separate areas of a third material, each of said areas of third material being electrically conductive and suitable for connection to a power supply, said areas of third material obtained by curing a composition comprising a silicone resin and electrically conductive filler, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. A heating element according to Claim 1 wherein the substrate is selected from the group consisting of anodised aluminium, aluminium, stainless steel, enamelled steel, and copper. A heating element according to any of the preceding Claims wherein the electrically insulating layer contains a thermally conductive filler. A heating element according to any of the preceding Claims wherein the electrically resistive layer comprises particles selected from the group consisting of graphite and carbon black. A heating element according to any of the preceding Claims wherein the third material comprises particles of silver. A heating element according to any of the preceding Claims wherein the silicone resin used in the electrically insulating layer, the electrically resistive layer and the electrically conductive areas comprises silicon-bonded phenyl groups. A heating element according to any of the preceding Claims wherein an insulating protective top layer covers the electrically resistive layer and the electrically conductive areas. A heating element comprising: a substrate comprising a first electrically insulating, thermally conductive material; on a surface of the substrate, a second layer of material, said second layer being electrically resistive and obtained by curing a composition comprising a silicone resin and electrically conductive filler; and attached to the second layer at least two separate areas of a third material, each of said areas of third material being electrically conductive and suitable for connection to a power supply, said areas of third material obtained by curing a composition comprising a silicone resin and electrically conductive filler, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. A process of manufacturing a heating element comprising: supplying a substrate; applying a first composition comprising a silicone resin on a surface of the substrate; curing the first composition to form an electrically insulating layer; applying a second composition comprising a silicone resin and electrically conductive filler for forming an electrically resistive element on a surface of the electrically insulating layer; heating the second composition for a time and at a temperature whereby the second composition is partially cured; applying a third composition comprising a silicone resin and electrically conductive filler for forming electrically conductive elements on at least two separate areas of the second composition, each of said areas suitable for connection to a power supply; and curing the second and third compositions, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. A process of manufacturing a heating element comprising: supplying a first electrically insulating, thermally conductive substrate; applying a second composition comprising a silicone resin and electrically conductive filler for forming an electrically resistive element on a surface of the substrate; heating the second composition for a time and at a temperature sufficient to partially cure the second composition; applying a third composition comprising a silicone resin and electrically conductive filler for forming electrically conductive elements on at least two separate areas of the second composition, each of said area suitable for connection to a power supply; and curing the second and third compositions, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element. A process of manufacturing a heating element comprising: supplying a substrate; applying a first composition comprising a silicone resin on a surface of the substrate; curing the first composition to form an electrically insulating layer; applying a second composition comprising a silicone resin and electrically conductive filler for forming an electrically resistive element on a surface of the electrically insulating layer; curing the second composition to form an electrically resistive element; applying a third composition comprising a silicone resin and electrically conductive filler for forming electrically conductive elements on at least two separate areas of the electrically resistive element, each of said areas suitable for connection to a power supply; and curing the third composition to form an electrically conductive element, whereby the silicon resins of the layers are compatible with each other and the substrate and resistant to a temperature to be achieved by the heating element.

Inventors: PAQUET RENE L, VANLATHEM ERIC, PAQUET, RENE L., VANLATHEM, ERIC
Applicants: DOW CORNING SA, DOW CORNING S.A.
Application year: 1997
Main classification: H05B3
IPC codes: H05B3, H01C7