SHEET HEATERS

Abstract: Electrical sheet heaters which comprise electrodes secured to a surface of a resistive element, a dielectric layer intimately bonded to the resistive element and to the electrodes, and an outer insulating layer. The heater is illustrated in Figure 1.

Description: This invention relates to sheet heaters. Sheet heaters typically comprise a laminar resistive element and two or more electrodes. The resistive element may be composed of a conductive polymer, i.e. a mixture of a conductive filler and an organic polymer (this term being used to include polysiloxanes), the filler being dispersed in, or otherwise held together by, the organic polymer. The resistive element may exhibit PTC behavior, thus rendering the heater self-regulating. In some sheet heaters, the electrodes are positioned on one face of the resistive element, e.g. by printing a conductive ink onto the heating element. Particularly relevant in this regard are EP-A-158 410, 175,550 and 176 284. Other documents describing conductive polymer compositions and devices comprising them include U.S. Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4;427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,473,450, 4,481,498, 4,476,450, 4,502,929, 4,514,620, 4,519,449, 4,529,866, 4,534,889 and 4,560,498; J. Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978), Narkis et al; and EP-A-38,713, 38,714, 38,718; 74,281, 92,406, 119,807, 134,145, 133,748 and 0,144,187. As disclosed in our earlier application EPA-175,550, we have found, during our work with laminar heaters comprising electrodes positioned on a surface of a laminar resistive element, that separation of the electrodes from the resistive element is liable to take place. As disclosed in that earlier application, one solution to this difficulty is to use an insulating layer which is not secured to the electrode-bearing surface; however, the use of such dissociated insulation has the disadvantage that, if there is even a very small hole in the insulation, moisture entering through the hole can accumulate under the insulation and cause a short between the electrodes. DE-A-2 006 165 relates to a flexible heating plate having an applied solid insulation layer of PVC material. US-A-4 250 398 relates to a composite laminated heating article having two insulating layers with a conductive layer therebetween. We have now discovered that the problems outlined above can be solved through the use of insulation which comprises (a) a first layer which is bonded to the electrodes and the resistive element and is composed of a cured polymeric composition having a relatively low tensile strength so that it can accommodate to the stresses imposed by flexing and/or thermal cycling, and (b) a second, outer, layer which may be a conventional insulating material except that it is not bonded, or is at most lightly bonded, to the first layer. The first layer (which is often referred to herein as the dielectric layer) is formed by applying to the electrode-bearing surface a composition which is liquid when it is applied and which is cured in situ so that it is intimately bonded to at least part of the electrodes and preferably also to at least part of the resistive element, and especially to the surface as a whole. We have also found that the applied dielectric layer provides improved electrical properties, in particular improved electrical safety, which eliminate, or at least reduce, the possibility of sparking and burning if one of the electrodes is broken. Thus, the present invention provides an electrical sheet heater which comprises: (1) a laminar resistive element (2) which (a) is composed of a conductive polymer composition which exhibits PTC behaviour and (b) comprises a first laminar surface and a second laminar surface; (2) two or more interdigitated electrodes (4, 6) which are secured to the first surface of the element (2) and which are positioned so as to leave a part of that first surface exposed; (3) a first insulating layer (8) which (a) is positioned over and directly contacts the electrodes (4, 6) and the exposed first surface of the resistive element (2), and (b) comprises an organic polymeric composition which is applied in the liquid form and which, when cured, has a tensile strength of less than 275.79 bar (4,000 psi) at 23°C; and (4) a second insulating layer (16) which is positioned over the electrodes (4, 6), the resistive element (2) and the first insulating layer (8); wherein any bond between the first (8) and second (16) insulating layers is such that if the second insulating layer (16) is peeled away from the resistive element (2), the peel strength of the bond between the first (8) and second (16) insulating layers is less than the peel strength of the bond between the first insulating layer (8) and the electrodes (4, 6). The dielectric layer covers at least a part of the electrodes, and preferably the whole of the electrodes except for those parts which are connected to electrical leads (for connecting the heater to a power supply) or to connection members which connect spaced-apart electrodes or which provide a bus along an edge or central portion of an electrode, eg. an electrode having arms which extend from the connection member. If such connection members are present, the dielectric layer should be formed after the connection members have been applied, or should not cover the parts of the electrodes to which the connection members will later be applied. The dielectric layer preferably also covers at least part, and preferably all, of the exposed part of the electrode-bearing surface(s) of the resistive element, particularly those parts which are adjacent the electrodes. This is particularly useful when the polymeric component of the conductive polymer comprises, eg. contains at least 75% by weight of, a polymer of low surface energy, for example less than 40, particularly less than 35, especially less than 30, dynes/cm, since it is difficult to bond to such conductive polymers by the melt-bonding and adhesive techniques which are conventionally used for applying insulation. The dielectric layer is formed by applying a suitable liquid composition, ie. one containing a curable organic polymer or suitable precursor(s) for such a polymer, and then solidifying and curing the composition in situ , if necessary after removing any solvent present in the liquid composition. The composition is preferably applied, solidified and cured at a temperature which avoids possible damage to the resistive element and/or electrodes. Curing can result merely from the selection of appropriate ingredients for the liquid composition or can depend on, or be accelerated by, an external stimulus such as moisture, heat, and/or irradiation. Particularly suitable materials are two-part silicone systems, one part containing the monomer and the other containing the catalyst, for example Sylgard 577 as supplied by Dow-Corning. Flexible two-part epoxy systems can also be used. Preferably, the peel strength of the bond between the cured material and the electrodes is at least 1, particularly at least 2, especially at least 3, pounds per linear inch at 20°C, such that when the dielectric layer is peeled away from the electrodes, it fails cohesively (ie. the layer is torn apart, leaving the interface substantially unchanged) rather than adhesively (ie. the layer separates from the substrate along the interface). The cured material should have a tensile strength at 23°C of at most 4,000 psi, preferably at most 3,000 psi, particularly at most 2,000 psi, especially at most 1,000 psi, so that it can conform to changes in the dimensions of the electrodes and the resistive element as a result of flexing and/or differences in thermal expansion and contraction. The thickness of the dielectric layer is preferably at least 0.002 inch, particularly at least 0.004 inch, especially at least 0.006 inch, in order to ensure that it is free from pinholes. The dielectric layer preferably has a dielectric strength of at least 1,000 volts per inch at all temperatures likely to be encountered during use of the heater, eg. at all temperatures from room tempeature to the switching temperature of a PTC conductive polymer composition in a self-regulating heater. The second insulating layer is preferably not bonded to the dielectric layer or to any parts of the electrodes and the electrode-carrying surface of the resistive element which are not covered by the dielectric layer. If there is any such bonding, it is preferably such that if the second insulating layer is peeled away from the resistive element, the dielectric layer remains in place, the peeling taking place through cohesive failure of the dielectric layer or through adhesive failure at the interface between the dielectric layer and the second insulating layer (ie. the peel strength of the bond between the dielectric and second layers is less than the peel strength of the bond between the dielectric layer and the electrodes). The second insulating layer is preferably a polymeric material, eg. a preformed flexible sheet thereof, which has satisfactory resistance to physical stresses and to chemicals. An important reason for the presence of the second insulating layer is that the compositions suitable for use in the dielectric layer have poor resistance to physical stresses and/or to chemicals. When, as is preferred, both or all of the electrodes are secured to the same surface of the resistive element, the heater will normally include a third insulating layer which covers the non-electrode-carrying surface of the resistive element (and may be bonded thereto) and which is bonded to the second insulating layer along the edges thereof. The heaters according to the invention are preferably flexible, by which is meant that at 23°C, and preferably at -20°C, they can be wrapped around a 4 inch diameter mandrel without damage. The conductive polymer preferably has a resistivity at 23°C of at least 0.5 ohm.cm, particularly 0.5 to 100,000 ohm.cm. It is preferably cross-linked, particularly by radiation, eg. electron beam or gamma radiation, eg. to a uniform dose of at least 5 Mrads, preferably at least 12 Mrads. The invention is particularly useful when the electrodes have been formed by printing, particularly silk screen printing, a conductive ink onto the resistive element, or by a like technique which results in somewhat fragile electrodes, eg. electrodes formed by the use of polymer thick film technology, or by sputtering, or by a process comprising an etching step, because application of the dielectric has little or no effect on the electrodes or the resistive element/electrode interface. The electrodes preferably comprise a conductive polymer, for example in the form of an ink, in which the conductive filler consists of or contains a metal, preferably silver, or a mixture of silver and graphite. The electrodes preferably have a resistivity in the range 2.5 x 10⁻⁴ to 1 x 10⁻³ ohm.cm. Preferably there is, between the electrodes and the resistive element, a contact layer having a resistivity between the resistivities of the electrodes and the resistive element, as described in European Application No. 176,284. The electrodes are preferably positioned on the same surface of the resistive element, so that current passing between them flows mainly parallel to the surface, but they can be on both surfaces. It is particularly preferred that the electrodes be interdigitated as disclosed in European Application No. 185,410. When the heater requires a ground plane, eg. if it is to be used in hazardous location, it preferably includes a laminar metallic element which functions as a ground plane and which is not bonded directly or indirectly to the resistive element or the insulating elements of the heater. The novel heaters are substantially safer than identical heaters without the dielectric layer. Thus the dielectric layer both increases the force rquired to damage an electrode, and reduces the dangers resulting from damage to the electrodes. Without the dielectric layer, if there is a break in one of the electrodes, arcing, sparking and subsequent burning of the resistive element can occur. With the dielectric layer, even though a break in the electrodes can result in arcing across the break, it does not lead to sparking and subsequent burning. Without limiting the invention in any way, it is thought that the absence of sparking and burning may be due to the fact that the dielectric layer prevents, or at least minimizes, access of oxygen to the break in the electrode, so that sparking and burning cannot be sustained. The material of the dielectric layer preferably has a high resistance to tracking, so that it helps to extinguish any continued sparking. Also the dielectric layer prevents water or any other electrolyte contacting and bridging the electrodes, and therefore avoids the possibility of short circuits between the electrodes and the problems of consequent sparking and burning of the resistive element. In this respect the invention is particularly useful when adjacent electrodes are less than 1 inch apart, and easily short-circuited. Referring now to the drawing, the Figures illustrate a heater which comprises a heating element comprising a laminar conductive polymer resistive element 2 having printed on the top surface thereof inter-digitated electrodes 4 and 6. A dielectric layer 8 overlies the interdigitating portions of the electrodes, but does not extend to the longitudinal margins of the electrodes. The dielectric layer 8 comprises a polysiloxane obtained by curing a liquid two-part system applied over the element 2 and electrodes 4 and 6, and then heated to 275°F for 10 minutes. The dielectric layer 8 is intimately bonded to the underlying element 2 and the electrodes 4 and 6. Bus bars 10 and 12, composed of metal mesh, are folded around uncovered marginal portions of the element 2 and the electrodes 4 and 6 respectively. An insulating jacket (shown in Figure 1 only) is formed around the heating element, and bus bars by a polymeric bottom sheet 14 and a polymeric top sheet 16. Sheet 14 is secured to the bottom of the heating element 2, and to the edge portions of the top sheet by a substantially continuous layer of adhesive 17 (as shown), or by melt bonding (not shown). The top sheet 16 is adjacent to but not secured to the bus bars 10 and 12, the dielectric 8, the electrodes 4 and 6, or the resistive element 2. On top of the top sheet there is a metallic, e.g., copper, foil 18 which is maintained in position by an outer polymeric insulating sheet 20 whose marginal portions are secured to the marginal portions of the sheet 16 by adhesive layers 22 and 24 (as shown) or by melt bonding. As shown in Figure 2, the electrodes have width t and length l and are separated by a distance d, the bus bars have width x, and the dielectric layer a length y parallel to the length of the electrodes. Typical values for these variables are t 0.03 - 0.02 inch l 0.5 - 6.0 inch d 0.1 - 0.3 inch x 0.2 - 0.8 inch y 1 + 1 inch The invention is further illustrated by the following Example. Example A heater as illustrated in Figures 1 and 2 was made in the following way. The ingredients listed below were compounded together and melt extruded at 450°F as a sheet 0.0175 inch thick. The sheet was irradiated to a dose of 14 Mrads (7 Mrads each side) thus cross-linking the polymer. An electrode pattern as illustrated in Figure 2 was deposited on the strips by screen printing a layer comprising a graphite-and silver-containing composition, having a resistivity of 1.3 × 10⁻² ohm.cm, followed by drying. The distance (d) between adjacent electrodes was 0.25 inch; the width (t) of each electrodes was 0.0625 inch, and the length (l) of each electrode was 5.4 inch. Then the sheet was heated to 175°F for 1 hour and slit into strips 7.25 inches wide. A layer 8 to 10 mils thick of a curable two part silicone liquid (Sylgard 577, sold by Dow Corning ) was then applied to the strips and the strips were placed in an oven at 275°F for 5 to 10 minutes to cure the silicone. Bus bars of nickel-coated copper expanded metal, 1.5 inch wide, were folded around the edges of the electrode-bearing strip. The resulting assembly was laminated between (A) a bottom sheet of ethylene-chlorotrifluoroethylene copolymer ("Halar") 8.5 inch wide and 0.020 inch thick, coated on the whole of its top surface with a layer 0.002 inch thick of a silicone adhesive sold by Adhesives Research Corporation under the trade name "Arclad", and (B) a top sheet of ethylene-chlorotrifluoroethylene copolymer ("Halar") 8.5 inch wide and 0.010 inch thick, which was coated on 0.5 inch wide edge portions of its bottom surface with a layer 0.002 inch thick of the same adhesive. Thus the dielectric layer and the top surface of the bus bars were not contacted by adhesive. Lamination was carried out at 125°F and 100 psi. A sheet of copper, 0.002 inch thick and 7.25 inch wide, was placed on the exposed surface of the top sheet, and the copper was covered by an outer sheet of ethylene-chlorotrifluoroethylene copolymer ("Halar"), 8.5 inch wide and 0.005 inch thick, which was coated on 0.5 inch wide edge portions of its bottom surface with a layer 0.002 inch thick of the same adhesive. The outer sheet was laminated to the top sheet (but not to the copper foil) at 125°F and 100 psi.

Claims: An electrical sheet heater which comprises: (1) a laminar resistive element (2) which (a) is composed of a conductive polymer composition which exhibits PTC behaviour and (b) comprises a first laminar surface and a second laminar surface; (2) two or more interdigitated electrodes (4, 6) which are secured to the first surface of the element (2) and which are positioned so as to leave a part of that first surface exposed; (3) a first insulating layer (8) which (a) is positioned over and directly contacts the electrodes (4, 6) and the exposed first surface of the resistive element (2), and (b) comprises an organic polymeric composition which is applied in the liquid form and which, when cured, has a tensile strength of less than 275.79 bar (4,000 psi) at 23°C; and (4) a second insulating layer (16) which is positioned over the electrodes (4, 6), the resistive element (2) and the first insulating layer (8); wherein any bond between the first (8) and second (16) insulating layer is such that if the second insulating layer (16) is peeled away from the resistive element (2), the peel strength of the bond between the first (8) and second (16) insulating layers is less than the peel strength of the bond between the first insulating layer (8) and the electrodes (4, 6). A heater according to claim 1 wherein the second insulating layer is not bonded to the first insulating layer. A heater according to claim 1 or 2 wherein the first insulating layer is (0.0508 to 0.0762 mm) (0.002 to 0.003 inch) thick, preferably 0.1016 to 0.508 mm (0.004 to 0.020 inch) thick, and is composed of a material having a tensile strength of less than 206.843 bar (3,000 psi) at 23°C, preferably less than 137.895 bar (2000 psi) at 23°C. A heater according to claim 3 wherein the first insulating layer is 0.1524 to 0.3048 mm (0.006 to 0.012 inch) thick and is composed of a material having a tensile strength of less than 68.948 bar (1,000 psi) at 23°C. A heater according to any one of the preceding claims wherein the first insulating layer is composed of a polysiloxane. A heater according to any one of the preceding claims wherein the electrodes are positioned on the same face of the resistive element so that when current passes between them, a substantial proportion of the current is parallel to the faces of the resistive element. A heater according to claim 6 which comprises interdigitated electrodes which are positioned on the same surface of a resistive element, and the resistive element is composed of a conductive polymer composition exhibiting PTC behavior. A heater according to claim 7 wherein the conductive polymer composition has been melt-extruded and has a resistivity of 0.5 to 100,000 ohm.cm at 23°C. A heater according to any one of the preceding claims wherein the electrodes were formed by a process which comprises printing a conductive ink onto the resistive element.

Inventors: BATLIWALLA, NEVILLE SAM, OSWAL, RAVINDER K., SHAFE, JEFF
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: H05B3
IPC codes: H05B3