BACKGROUND OF THE INVENTION

This invention relates to an electrically conductive laminate having improved resistance stability, and in particular its use in a heating element for conductive, convective and radiant heating.

A number of electric radiant heaters utilizing resistor wires, metal foils and electrically conductive films have been described in the prior art. These heaters are used in a variety of applications, including personal comfort heating, agribusiness and industrial processes. For example, radiant heating panels have been fabricated by embedding resistive wires in an insulating substrate, such as gypsum board, or by applying the wires directly to the insulating substrate. A variety of types of etched or pierced aluminum and other metal foil heating elements have also been proposed. For instance, a commercially available unit comprises a metal foil- laid between two layers of clear polyester film.

Resistive wire heaters and metal foil heaters suffer a major deficiency in that all of the current is usually carried by a single continuous conductor. A break anywhere in the electrically conductive path renders the entire heater inoperable. The substrate interposes an additional insulative layer between the heating element and the intended receptor and thereby reduces the effectiveness of heat transmission.

Metallic powders, transparent vapor-deposited metals or metal oxides, and binders which contain conductive carbon black or graphite have been proposed for use as electrically conductive films in radiant heaters. These materials may be deposited between layers of silicone rubber, polyester film or asbestos-like paper.

Devices using electroconductive films have not proved entirely satisfactory. When a metal is used in the electrically conductive material, the metal is frequently of high cost. This is particularly true of thick-film heaters

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based on noble metals. Heaters based on aluminum, tin oxide, indium oxide and similar materials are also costly. Con¬ ductive metal powder coatings eventually develop oxides on their particulate surfaces, which raise their resistance to such a high extent that they become unstable and eventually are rendered inoperative. The electrical resistivity of electrically conductive films comprised mainly of carbon may vary with age and the conditions of use. Moreover, installation of radiant heaters based on electroconductive films is usually difficult because it is necessary to cut into the conductive film and through the dielectric insulating cover. This raises a variety of potential problems ranging from corrosion of exposed connections, with a resulting increase in resistance, to the possibility of mechanical damage or corrosion to the interior of the heating panel.

Furthermore, the electrical stability of conductive plastic matrices containing conductive carbon black or graphite tends to be difficult to control. Generally, this is manifested by changes in the resistance of the conductive matrix through various manufacturing processes (to form the matrix or specific end products) , and through the lif -cycle of these specific end-products. In many types of applications and end-uses, high levels of stability in such conductive matrices are not critical - e.g., anti-static applications. However, in other applications, and particularly when these conductive matrices form the basis for heating element systems, the constancy of the output (constant wattage) of the heater during constant or cyclical use, or flexing or oh thermal shock is critically dependent on the resistance stability of the system. A recent recognition of difficulties in this area appeared in a symposium held in London, England, on October 30, 31, 1980, in a paper entitled "Conductive Carbon Black Filled Thermo¬ plastics" by A. Wathers of the Cabot Corporation, where he stated: "A much patented but little applied use of conductive plastics is in heating elements . . . conductive plastics have the right level of resistance to make use of voltage and current at their normally available levels to provide heat when a current is passed. Applications came to mind like space

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heating and deicing. These applications have not developed, probably because of contact resistance and resistance stability problems." Similar references to resistance stability problems can be found in the patent literature - e.g., U.S. Patent 3,179,544.

Although a limited number of relatively stable constant voltage conductive carbon black or graphite filled systems have been introduced to the market, most, if not all, have been based on coating technology, where a carrier substrate is coated or impregnated with a conductive carbon black or graphite filled vehicle - such as a curable adhesive or coating resin. Practical problems with the resistance stability of these types of systems have already been alluded to, particularly when such systems are flexed or subjected to thermal shock.

Other techniques have been employed in plastic based systems to form conductive matrices which could serve as the basis for heaters. A particular variety of this type of system is believed to be based on the compounding of the conductive carbon black or graphite into the resin system by conventional thermoplastic melt processing techniques. Rather than operating on constant voltage, these types of systems are commonly designed to be reversibly self-limiting, with resistance increasing with the temperature of the system.

To summarize, commercially available constant wattage heaters are based on conductive carbon black (or graphite) coating or impregnation technology, and these systems are difficult to maintain in a constant output mode (stable resistance, constant wattage, constant temperature) under a range of practical operating conditions.

Accordingly, there exists a need in the art for an electrically conductive layer having improved resistance stability, which can be employed in a laminated article useful, for example, as a radiant, conductive or convective heater. The electrically conductive layer should enable the manufacture of a laminated article useful at relatively low operating temperatures, such as about 250°F or less, and also at higher temperatures, if desired. When used as a heating

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element, the article should be of a unitary structure making it unnecessary to cut or otherwise open or separate the lamina in order to install the article in its intended location. It should also be possible to make electrical connections to the article without damaging or destroying the external laminae. The article should be water-proof and substantially impermeable to moisture and harmful gases and vapors. The device should exhibit a stable resistivity. Moreover, the resistivity should be controllable, reproducible and predictable over a range of operating conditions. The article should be fire-resistant and safe if accidentally punctured. It should also be capable of being operated at 110 volts AC or other voltages. The article should have good aesthetic qualities and be capable of being produced at relatively high speed using conventional equipment.

SUMMARY OF THE INVENTION Accordingly, this invention aids in fulfilling these needs in the art. The invention provides an electrically conductive layer having improved resistance stability which can be employed in a laminated article useful, for example, as a radiant conductive or connective heater, or as an infra¬ red radiating device. More specifically, the electrically conductive layer comprises an amorphous, solvent-cast thermoplastic film and conductive carbon black, the carbon black being dispersed in the film, such that the electrically conductive laminate has improved resistance stability.

The laminated article in which the electrically conductive layer can be employed is a unitary, composite laminated article of manufacture comprising an electrically insulating top layer having an inner surface and an outer surface. An electrically insulating bottom layer having an inner surface and an outer surface is also provided. The aforesaid electrically conductive layer is substantially continuous and of substantially uniform thickness, and is interposed between the inner surfaces of the top and bottom layers. Electrical conductor means are provided in contact with the electrically conductive layer. The electrically conductive layer is capable of emitting electromagnetic suεε> t i i ⁱ - -*— * - _B U _R E _A

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radiation when an electric current is passed therethrough. The top layer and the bottom layer have edges that are sealed together to thereby form an enclosed laminate. More particularly, the electrical conductor means are enclosed between the top and bottom layers which have been secured together. The laminated article is characterized by an improved resistance stability, and the article is particularly useful as a radiant heat source.

This invention also provides a process for preparing the unitary, composite laminated article of this invention. The process comprises substantially covering an inner surface ^" of an electrically insulating top layer having an inner surface and an outer surface with the previously described ^' substantially continuous electrically conductive layer of substantially uniform thickness. The process includes providing electrical conductor means in contact with the electrically conductive layer. An electrically insulating bottom layer is provided in contact with the electrically conductive layer. The top layer and bottom layer have edges, which are bonded together to thereby form an enclosed laminate containing the electrically conductive layer.

Further, this invention provides a method of using the unitary, composite, laminated article of the present invention. More particularly, the electrically conductive layer in the article of this invention is connected to an electric power supply. An electric current is passed through the electrically conductive layer, which results in the article radiating infra-red energy.

BRIEF DISCUSSIGN OF THE DRAWINGS This invention will be more fully understood by reference to the drawings in which like reference numerals identify like parts.

Figure 1 is a plan view of a laminate of the invention in a three-busbar configuration comprised of a central hot line and two lateral neutral lines the various layers are progressively peeled away to illustrate their relationship; and

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Figure II is a view through section A-A of Figure I.

DETAILED DESCRIPTION

Referring to Figure I, a unitary, composite laminated article useful as a radiant heating panel is comprised of an electrically insulating bottom layer 1, such as a polyester sheet. The electrically conductive layer 3 is deposited on the inner surface of layer 1. Adhesive layer 4 contacts the electrically conductive layer 3 and is.applied to the inner surface of electrically insulating top layer 6. I will be understood that an adhesive layer can be applied to either or both of the inner surfaces of the top and bottom layers.

Busbars 7 and 8 are shown as having been laid on top of the conductive layer 3. A transverse interconnecting line 9 between the outer busbar 8 is positioned outside the electrically conductive layer 3 so as not to interfere with the conductive paths between central busbar 7 and lateral busbars 8. Alternatively, the transverse interconnecting line 9 could be laid over the electrically conductive layer 3 if line 9 is electrically isolated from layer 3, such as by a strip of high-dielectric film. Lead wires 12 and 13 are connected to the busbars at 10 and 11. In still another embodiment, transverse interconnecting line 9 can be omitted and wire pigtails can be directly connected to busbars 7 and 8, such as by the use of crimped connectors secured to the busbars.

Decorative layer 18 is an optional layer applied to either side of the article. Since the function of the decorative layer 18 is to mask the usual black color of the electrically conductive layer 3 and the busbars 7 and 8 from view, or to match the decor of the surrounding area in which the article is used, it is between the viewer and these parts.

The water-tight, sealed construction of the laminate can be more fully appreciated from Figure II showing a cross-section of a heating panel near one of the lateral edges of the panel. A compression seal 14 is formed by laminating under pressure or pressure and heat various

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layers comprising the article. By sealing all of the exposed edges of the article, the electrically conductive layer and internal electrical circuitry are protected from the elements. It will also be understood that the laminate can be water¬ proofed and protected with a second insulating or protective layer heat sealed to encase the laminate. Also shown in Figure IIis a solder bead 15, which bonds the transverse connector 9 to one of the longitudinal busbars 8. Adhesive layers 16 bond the busbars 8 and 9 to the layers with which they are in contact.

The various parts that make up the laminated article of this invention can now be considered in greater detail. First of all, the top layer and the bottom layer are each electrically insulating layers. They can be comprised of the same material or can be of different materials. Preferably, the top and bottom layers are each comprised of a polymeric film, especially a flexible polymeric film. Typical of such material are films comprised of polyesters, acrylics, ABS, cellulosics, fluorocarbons, polyethylene, polypropylene, polystyrenes, silicones, polyvinylidene chloride, polyvinylidene fluoride, other fluorine-containing thermoplastics, polyether-imide, styrene-acrylonitrile polymer, polycarbonate, chlorosulfonated polyethylene, polyetherether ketone, polysulfone, polyether sulfone, polyamid -imide, and related alloys and blends of the above. Preferred polymeric films are polyesters. A particularly preferred polyester is polyethylene terephthalate, such as Mylar (manufactured by E. I. DuPont de Nemours and Co.).

The top and bottom layers can have thicknesses up to about 30 mils, but generally have thicknesses of about 0.35 to about 14 mils, preferably about 0.5 to about 5 mils. The thickness of the layers will depend upon where the laminated article is to be used. For instance, the top and bottom layers can each be about 2 mils if a panel is ceiling-mounted as a radiant heating device or about 5 mils if wall mounted. Layers less than 5 mils thickness can even be employed in wall mounted radiant heaters, but an additional layer or facing may be required for safety purposes

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8 to protect the laminate of the invention from mechanical damage. The preferred range of thickness gives the desired resiliency, bending properties and resistance to tearing required for most uses.

The top and bottom layers must each be electrically insulating. These layers are characterized by dielectric strengths of at least about 400 volts AC per mil, preferably at least about 1000 volts AC per mil. Dielectric strengths of at least about 7500 volts AC per mil are particularly preferred. Dielectric strength as referred to herein is determined according to ASTM D 149.

Typically, the top and bottom layers will exhibit volume resistivities of at least about 4.5 x 10 13, preferably about 1 x 10 15 to about 1 x 1018. Volume resistivity is expressed as ohm-cm and is determined by ASTM D 257.

Typically, the dielectric constants for the top and bottom layers as determined by ASTM D 150 will be at least about 2.0 at 10 cps, at least about 2.0 at 10 cps, q and at least about 2.0 at 10 cps. Preferred values are at

3 6 9 least about 3.2 at 10 cps, 3.0 at 10 cps, and 2.8 at 10 cps.

The particularly preferred polyethylene terephthalate polyester film employed in this invention exhibits a volume

^■ j o ~) resistivity of 10 ohm/cπr at 50% RH and 23°C, is void free and has a relatively low dielectric constant of about 2.8 to 3.2 at from 10 3 to 109 cps. It also has a very high arc resistance (121-200 seconds) when tested by ASTM D 495. It exhibits a dielectric strength of about 7500 VAC/(l-mil).

These properties make it excellent for use as an ^' electrical insulator in preventing any shock hazard thereby insuring the safety of the device when used as a radiant heater.

The mechanical properties of the top and bottom layers will depend upon the conditions to which the laminate is exposed. For a particularly preferred laminate of the invention, each of these layers will have a tensile strength

(ASTM D 882) of about 20,000 psi. Tear strength (ASTM D

1922) will be about 20 g/mil. The percent elongation (ASTM

D 882) of each layer is about 50%.

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In a preferred embodiment of this invention, the laminated article is flexible. This invention fulfills a need in the art for a light-weight heating element that can be rolled into the form of a tube for compactness in shipping and ease of handling and installation. In this embodiment, the top and bottom layers should both be flexible. Therefore, the folding endurance for each of these layers should be at least about 400 folds, preferably at least 10,000 folds, as determined by ASTM D 2176.

The thermal resistance of each of the top and bottom layers will be dependent upon the conditions under which the laminate of the invention is used. If the laminate is used as a radiant heater in extremely cold climates, it is desirable for the components to resist embrittlement at the low temperatures to which it will be exposed. The preferred polyester film of this invention is capable of withstanding - 100°F when tested by ASTM D 759. A preferred 1200 watt heating panel generally will not exceed about 160°F in normal operation. Other panels, however, when insulated, may attain temperatures of about 200° to about 250°F. Thus, the heat resistance of the top and bottom layers when tested by ASTM d 759 will preferably be at least about 275°F without degradation. In certain circumstances, - it may be necessary to go as high as 300°F in order to assure long term aging resistance at maximum operating temperatures. The preferred polyethylene terephthalate film is capable of withstanding 300°F.

Since the top and bottom layers of the laminate of this invention will generally be exposed to the surrounding atmosphere in which the laminate is used, these layers should possess the required chemical resistance. Preferably, their resistance to acids, alkalies, greases, oils and organic solvents should be rated as good when tested according to ASTM D 543. This is an indication that there is no adverse chemical reaction when the films are in contact with these materials.

The top and bottom layers should also be resistant to water, and thus will exhibit water adsorption values

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(ASTM D 570) of about 0.8 to about 2.9% in 24 hours, preferabl less than about 0.8% in 24 hours. Similarly, the rate of water vapor transmission (ASTM E 96-E) expressed as gm/100 sq in/24 hr/mil at 37.8°C will be about 1 to about 5.4 preferably not more than about 1. Permeability to gases (ASTM D 1434) expressed as cc/100 sq in/mil/24hrs/atm at 25°C should not exceed about:

Preferably Especially co ₂ 50 25 15

H ₂ 300 150 50 ₂ 10 1 1

°2 50 10 3

The top and bottom layers are bonded to each other in order to enclose the electrically conductive layer and electrical conductor means. Preferably, the layers are bonded by adhesive means. A large number of adhesives can be suitable for bonding the top and bottom layers together to form an enclosed laminate. The adhesive must be effective at performance temperatures, and can be selected from a group consisting of most pressure sensitive adhesives (acrylics, silicones, etc.), most thermosets, i.e., curable systems such as polyesters, polyurethanes, etc., and thermoplastics having melting points above operating temperatures. Silicone adhesives are preferred.

When the laminated article of the present invention is employed as a heating panel, it is important to consider the flammability characteristics of the top and bottom layers. Preferably, the layers will be non¬ flammable or at least self-extinguishing as determined by ASTM D 1433-58. Alternatively, secondary flame retardant layers can be employed.

The electrically conductive layer employed in the laminate of this invention comprises carbon or graphite and is capable of emitting electromagnetic radiation in the middle infra-red range (i.e., about 3 to about 30 microns

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in wavelength) when electric current is passed therethrough, It is employed in a layer of substantially uniform thickness, which is important in order to ensure uniform electrical and thermal characteristics in the layer. One of the problems encountered with laminated articles having electrically conductive layers comprising carbon is that the electrical resistivity of the electrically conductive layer varies over a period of time in different types of environ¬ ments. This is unacceptable when the articles are used as radiant heaters. Specifically, such heaters must usually be approved or certified by recognized testing laboratories or by Government agencies. One of the requirements is that resistivity be maintained within relatively narrow limits over extended periods of time.

The present invention overcomes the problem of varying electrical resistivity of electrically conductive layers that comprise carbon. Important characteristics of the electrically conductive film of the present invention re its amorphous polymer structure and the fact that the film is produced by a solvent casting technique. A preferred film layer having these characteristics is marketed by the Schweitzer Division of the Kimberly-Clark Corporation under the trademark KIMFLO. Surprisingly, it has been found that, when employed in a laminated heating element, the electrically conductive layer possesses greatly improved resistance stability under a variety of operating conditions, relative to layers which do not possess the important characteristics of the conductive layer.

A number of different thermoplastic materials comprising polymer of amorphous structure are suitable for use in the electrically conductive layer. These materials include polycarbonates, polysulfones, polyvinylchloride, polyimides (including polyether imides such as Ulte , manufactured by General Electric Company) , silicones and other elastomers and various thermoset ing resins. To achieve the improved electrical stability of the electrically conductive layer, the solvent cast material must be capable of producing a film comprising polymer of substantially non-crystalline, amorphous structure. Preferably, the solvent cast, amorphous plastic

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can also be formed into a flexible film, . so that the resulting heating element can be flexed and/or folded.

The electrically conductive layer is produced by a solvent casting method to form an electrically stable, ultra-thin film comprising polymer of amorphous structure. Generally speaking, solvent casting involves forming a solution of an appropriate thermoplastic material, which includes carbon black as a disperse phase, and casting the element from solution. Examples of solvent casting techniques are given at pages 238 and 239 of Volume 10 of Kirk-Othmer, Encyclopedia of Chemical ^" Technology, Third Edition, and in U. S. Patent No. 3,793,716, issued to Smith-Johannsen. Both of these references are hereby incorporated by reference.

The electrically conductive layer has substantially uniformly dispersed therethrough finely divided particles of carbon black. The particles can have various configurations and aggregate size, and structure, depending on the process in which they are made. Typical suitable, commercially available conductive carbon blacks are: Conducte SC (Columbia Chemical), acetylene black (Shawinigan) , Kjetenblack (Noury) , Black Pearls 2000 and Vulcan XC-72 (Cabot) . The preferred carbon black is Vulcan XC-72, although others are also acceptable. The carbon black can comprise about 5% to about 50%, by weight of the electrically conductive layer, and it is preferable that the carbon black comprise about 15% to about 30% by weight of the layer.

The solvent cast electrically conductive layer generally has a thickness of about 0.1 to about 2 mils. Preferably, the thickness is about 0.4 to about 0.8 mils. Additives can be included in the electrically conductive layer, and the selection of these additives is possible by one having ordinary skill in the art. These additives include antioxidants, flame retardants, and other stabilizers or processing aids.

The application of a barrier layer to the inner surfaces of the top and bottom layers surrounding the electri- . cally conductive layer is optional. The barrier layers are substantially impermeable to moisture and water vapor and

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substantially cover the inner surfaces of the top and bottom layers as described in U. S. Patent 4,250,398.

In another preferred embodiment of this invention, the electrically conductive layer is adhesively bonded to the inner surfaces of the top and bottom layers. The adhesive layer should be substantially non-flammable or at least self-extinguishing, very flexible, elastomeric, thermoplastic or thermosetting, resistant to high temperatures, and free of out-gasing. The adhesive layer can be applied in single or multiple coats. Suitable adhesives include most pressure- sensitive adhesives (acrylics, silicones, and others), most thermosets, i.e., curable systems (polyesters, polyurethanes, and others) , and thermoplastics that have melting points above operating temperatures. Particularly preferred adhesives for use in the present invention are silicone adhesives. Adhesive layers will generally have a thickness of about 0.1 to about 0.5 mils, preferably about 0.1 to about 0.3 mils, but other thickness can be employed.

In general, the adhesive should be able to bond the dissimilar layers in which it is in contact; to withstand prolonged exposure to operational temperatures without degrading, outgasing, discoloring or relaxing its tensile bond strength; to stretch under the stresses imposed by the coefficients of expansion of the various layers while still maintaining bond integrity; to form a bond under contact pressure (and heat) after being air dried; and not to migrate into the conductive coating after lamination.

Busbars in parallel are utilized to conduct electricity along the length of the electrically conductive layer to present an equidistant path of resistance to the passage of electric current. Non-corroding metal foil busbars are generally employed, but wires or bars, etched copper claddings, and even vapor-deposited or painted metallic coatings can be employed. Any of numerous types of metal foils can be employed, including aluminum, lead, stainless steel, silver, brass, bare copper or tin-plated copper and the like. Electroless copper and nickel busbars are also suitable.

In the preferred laminate described herein, copper foil tape is utilized as the busbar material because of its

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14 high conductivity and malleability. Preferably, the tape is backed with a high temperature, electrically conductive heat activated or pressure sensitive adhesive to adhere it to the substrate. Adhesive can be made electrically conductive by dispersing conductive carbon or other conductive particles therethrough. In one form of tape utilized in the practice of this invention, a copper foil tape carries a 0.1-mil thick layer of high temperature, electrically conductive acrylic adhesive. The foil can be applied either underneath the conductive layer or on the top of the conductive layer. When laminated under pressure, the acrylic adhesive offers little resistance to the flow of current. It is important to note that the thickness and behavior of the adhesive under pressure should be investigated for any tendency to bleed into the busbar conductive layer junction where it may cause interfacial problems. The heater of the preferred design described herein utilizes copper foil 0.001 inches thick, by 0.750 inches wide, to carry the complete 1200-watt load. The copper busbars of this design do not heat signifi¬ cantly above ambient temperature, and thus can safely be employed.

Mounting the laminate in the form of a conductive, convective or radiant heating panel to a variety of substrates can be accomplished in a number of ways. For example, the panel can be surrounded by a frame and the frame attached by fasteners to a substrate. The sealed edges surrounding the conductive area of the panel can be stapled directly to the substrate. The ^" panel can be backed with a high- emperature- resistant contact adhesive that will ensure smooth attachment.

What has been described above is a novel laminated article that is especially useful as a radiant, convective and conductive flexible heater. The article of this invention is also useful for signaling, information transmission, status reporting, transmitting an electrical current and providing electrical continuity. One feature is the high resistance stability of the laminate. The electrically conductive layer in the laminate exhibits a resistivity that is very stable, predictable, controllable and reproducible.

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To further assist one of ordinary skill in the art to make and use the invention, the entire disclosure of U. S. Patent 4,250,398, issued to Ellis et al. , is hereby incorporated by reference.

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