US3366912A - Electrical heating element - Google Patents

Description

United States Patent 3,366,912 ELECTRICAL HEATING ELEMENT Leo P. riubbuch, Lima, Pa., assignor to E. I. du Pont de Nernours and Company, Wilmington, Del., 2 corporation of Delaware No Drawing. Fitted Aug. 25, 1965, Ser. No. 482,596 3 Ciaims. (Cl. 338-408) ABSTRACT OF THE DISCLOSURE A flexible electrical heating element comprising (A) flexible heat stable electrically insulating threads interlaced with (B) a continuous strand having an electric resistivity of less than about 120 ohm-circular mils per foot, a deflection weight of less than about 4 ounces, and a heat stable electrically insulating coating selected from the group of an aromatic polyimide, an aromatic polyimide, an aromatic polyarnide-imide, a polybenzamidizole, and a polyoxadiazole. These heating elements have long useful lives and produce heat with a high degree of uniformity, even when they are operated at temperatures of up to about 250 C.

This invention cencerns flexible electrical heating elements.

Electrical heating elements which consist of heat generating high resistance Wire, such as Nichrome, are widely used in heating pads, electric blankets and cooking utensils. Heat production by many of these high resistance wire heating elements is noticeably irregular and non-uniform over the elements surface. Elements having more uniform heat producing ability have been prepared using high resistance Wire, but only by sacrificing a large amount of flexibility and at a substantial increase in cost.

The electrical heatin elements of this invention are flexible, have a long useful life, even when operated at temperatures up to about 250 C., and produce heat with a high degree of uniformity. These elements consist essentially of (A) flexible heat stable electrically insulating threads interlaced with (B) a continuous strand having an electric resistivity less than about 120 ohm-circular mils per foot, a deflection weight less than about 4 ounces, and a heat stable electrically insulating coating. Heat production is essentially uniform over the surfaces of these elements which makes the elements particularly useful in applications such as electric blankets, heating pads, frying pans, etc.

The term deflection Weight is used in this application to refer to the weight necessary to produce a deflection of one centimeter in a ten centimeter span of a bar supported at the span ends. Deflection weight is related to bar size and to material by the equation for expressing Youngs modulus by bending. For circular bars this equation is 'rngZ where M is Youngs modulus for a particular metal, in is the mass which produces a deflection s when applied midway between supports separated by distance a to a bar having radius r, and g is the gravitational acceleration constant. The product mg is the deflection weight 1. After substituting values for l and s in the equation, it can be rearranged to F=3.89 Mr where deflection Weight f is in ounces, Youngs modulus M, a force, is in pounds per square inch and radius r is in inches. Deflection weight can be readily calculated from this equation for a circular bar With radius r of any material having Youngs modulus M.

If desired, the above equation can be rewritten to express r in terms of Youngs modulus and deflection weight; in this form, bar radius for a material having a specified deflection weight is more readily calculated. For materials having a rectangular cross section r is replaced by (1% where a is the vertical dimension and b is the horizontal dimension. Values of Youngs modulus, which is also called the tensile modulus of elasticity, and electrical resistivity for numerous materials can be found in Perrys Chemical Engineers Handbook, Section 23 (4th ed. 1963). Tables on pages 31-47 and 58 of section 23 are particularly pertinent. Deflection weight can be determined by direct measurement on bars for materials lacking values of Youngs modulus.

It should be emphasized that deflection weight concerns both the size and the material content of wires useful in this invention. For example, the deflection weight of hard drawn copper wire having a tensile modulus of about 15.95 1O pounds per square inch in gage size (B and S gage, 0.032 inch in diameter) is about 4.06 ounces; in 10 gage size (0.102 inch in diameter), about 419 ounces; and in 32 gage size (0.0080 inch in diameter), about 0.0155 ounce while aluminum wire having a tensile modulus of about 10.1 10 pounds per square inch in 20 gage size has a deflection weight of about 2.57 ounces.

Values of electrical resistivity of wires used in elements of this invention are measured at 68 F. In addition to copper and aluminum, useful wire materials include tin, tungsten, zinc, brass, silver, nickel and chromium separately, most carbon steels, etc. Preferred heating elements of this invention having an excellent combination of flexibility and heat generating ability contain conductive wires having electrical resistivities less than ohm-circular mils per foot and having a deflection weight up to about 0.04 ounce, such as the smaller copper and aluminum wires.

Coatings, for electrically insulating and continuously protecting wires, useful in this invention preferably are those that can be maintained at temperatures exceeding about 240 C. for at least a year without cracking, loss of flexibility, or other detrimental effects. Materials especially useful as these wire coatings because of excellent protection and endurance at high temperatures are the aromatic polyimides, aromatic polyamides, aromatic polyamide-imides, polybenzimidazoles and polyoxydiazoles. The polyimides can be prepared by the methods disclosed in United States Patent No. 3,179,634 to Edwards, and this disclosure is hereby incorporated into this specification. Briefly these polyimide coatings are prepared by reacting a diamine with a tetracarboxylic dianhydride in a solvent to produce a polyamic acid solution, coating wire with this solution, and curing the polyamide acid to polyimide by baking. Useful polyamides can be prepared by the method disclosed in Hill et al., United States Patent No. 3,066,899, which is hereby incorporated into this specification. The polybenzimidazoles are prepared by the process described by H. Vogel and C. S. Marvel in Journal of Polymer Science, 501511 (New York, 1961). The polyoxadiazoles are prepared by the process described in Journal of Polymer, part A, volume 3, No. 1, (January 1965). Instead of using precoated Wires in practicing this invention, the elements of this invention can be coated with the insulation disclosed above by dipping, brushing, etc. It is also practical to coat elements con taining either previously coated or uncoated wire. The use of precoated wire is preferred.

Heat stability of the insulating strands, useful in this invention must be sufficient to prevent substantial breaking, cracking, loss of flexibility or tensile strength when operated at temperatures of about 250 C. for periods exceeding a year. Useful insulating strand materials include glass fiber, asbestos, aromatic polyimides, aromatic polyamides, and any other material stable under the temperature conditions of use, in the form of mono-filaments and multi-filament threads, yarns, rope, roving, etc.

The term strand refers generally to mono-filament and multi-filament yarns of staple and continuous fibers and wires, and includes threads, strings, roving, rope and the like.

The term interlaced, as used in this specification with respect to linear heating elements, refers to strands in which one or more insulating strands is intermixed with one or more wire strands, and, with respect to planar heating elements, refers to woven, non-woven and knitted fabrics in which conductive and non-conductive strands are intermixed in any fashion (as by needlin or otherwise united to form a flexible heating element. A woven structure in which insulating threads are the warp strands and a continuous strand of wire is the woof strand has a preferred combination of surface smoothness and flexibility and is easily manufactured.

Connection of the electrical heating elements of this invention to an electrical power source is usually made by stripping the coatings from the wire ends and mechanically fastening the ends to a binding post or soldering the ends to electrical leads. When weaving the elements of this invention, a short length of wire is usually provided at the wire ends to allow connection. If desired, connections can be made at a point along the length of the wire by means of clamps, soldering, etc. Multiple connections can be made to large elements or" this invention in this manner. Elements of this invention can be made to produce heat at rates exceeding 1000-2000 watts per square foot by providing multiple connections to the elements at suitable intervals.

Example 1 A planar heating element about 1% inches wide and 8 inches long is woven on a small loom using about 60 warp threads per inch of electrical grade glass yarn (Warp threads in this element run the 8 inch length) and woof threads per inch of polyimide coated Brown and Sharpe gage No. 32 copper wire which had been coated with the polyimide precursor solution described below and then heated to convert the precursor to polyimide. Thickness of the cured coating on the wire is about 0.7-0.8 mil. This element can be wound around a inch mandrel without adverse effects. The glass yarn is made from continuous filament strands having a diameter of about 0.29 mil and a strand count of 225 by first twisting two single strands into a ply and then twisting two plies with about 4.4 turns per inch in an S twist to form the yarn. (This yarn is designated ECE 225 2/24.4S and can be obtained from the Owens Corning Fiberglas Corp, Huntingdon, Pa.)

Two planar elements about 1 /8 x 4 inches are made by cutting this element in half. Each of these smaller elements has a continuous wire as the woof thread about 90 inches long.

A polyamic acid solution is prepared by stirring 74.53 parts by weight of 4,4-diaminodiphenylether into 678.90 parts by weight of ILN-dimethylacetamide and slowly adding 78.75 parts by weight pyromellitic dianhydride over a 30 minute period at a reaction temperature of about C. Up to 0.82 part of pyromellitic dianhydride are then added over an additional minute period. Stirring is continued throughout the reaction which is carried out under a nitrogen atmosphere. A viscous solution of 18.5% solids by weight is produced. This solution is diluted to about 13% solids with N,N-dimethylacetamide.

One of the glass fabric elements is dipped into the solution and drained in a vertical position. Excess soiution is removed by pulling opposed glass rods over the surface of the element. The coated element is then baked 4 10 minutes at C., 20 minutes at C. and 1 minute at 400 C.

Applying about 2.1 amperes at 4.3 volts AC to each Of the dipped and undipped elements produces a uniform surface temperature of about 180 C., and about 2.6 amperes at 6 volts produces surface temperatures of about 250 C. In heat aging tests conducted at higher temperatures both of these elements had a projected 242 C. resistance increase over a period of one year, of about 10%.

Example 2 Using the procedure of Example 1, planar heating elements are prepared using uncoated 32 gage copper wire. One of these elements is coated with a polyimide precursor solution and then heated to convert the coating to polyimide, using the materials and procedure of Example 1; the other element is left uncoated. In heat aging tests at elevated temperatures, both the coated and uncoated elements had a projected resistance increase of 10% in about 5 months at 242 C. Application of the heat stable coating to the wire has a decidedly greater protective effect than application to the woven structure. The protection resulting from precoating the wire is very pronounced. Difiiculty in coating the individual wires after formation of the woven structure may be a possible explanation for the very pronounced superior endurance of heating elements of this invention containing precoated wire compared to similar elements comprising uncoated wire, even when both elements are ultimately coated overall with one of the above mentioned heat-stable insulation coatings, even though the latter elements are far superior to prior art heating elements in flexibility and heat uniformity.

Example 3 A linear heating element is prepared by mixing 25 parts by weight staple fibers of heat stable poly(m-phenylene isophthalamide) with 75 parts by weight of short lengths (6-8 inches) 32 gage uncoated copper wire. The mixed fibers are carded and formed into a yarn and then coated with the polyimide precursor solution of Example 1, which is then cured by heating. This linear heating element is very flexible being capable of winding around a /8 inch mandrel without adverse effect upon it and provides good heat uniformity over its surface.

Another linear heating element is prepared by twisting a continuous yarn of 32 gage copper wire with a continuous yarn of heat stable poly(m-phenylene isophthalamide) fibers. The yarn of copper wire is precoated with the polyimide precursor solution of Example 1 and cured by heating prior to combining with the insulating polyarnide strand. The composite yarn can be used as thus prepared or can be further coated with the polyimide precursor solution of Example 1 and again cured to provide greater protection for the element without significant effect on its flexibility, which is similar to that referred to above. This composite also provides good heat uniformity over its surface.

I claim:

1. A planar flexible electrical heating element in the form of sheet fabric comprising:

(A) flexible heat-stable electrically insulating strands interlaced with (B) a strand of wire selected from the group of copper,

aluminum, and silver, said wire having an electrical resistivity of less than about 25 ohm-circular mils per foot and a deflection weight of less than about 0.04 ounce, and said wire being electrically insulated by a heat-stable flexible coating selected from the group consisting of an aromatic polyimide, an aromatic polyamide, an aromatic polyamide-imide, a polybenzimidazole, and a polyoxadiazole.

2. A heating element of claim .1 wherein the electrically insulating strands are selected from the group of glass,

asbestos, aromatic polyimides, and aromatic polyarnides.

3. A planar heating element in the form of a woven fabric in which the warp strands are heat-stable synthetic organic polymer yarns and the woof strands are wires selected from the group of copper, aluminum and silver, said wires having an electrical resistivity of less than 25 ohm-circular mils per foot and a deflection weight of less than 0.04 ounce, and said Wires being electrically insulated by a heat-stable flexible coating selected from the group consisting of an aromatic polyimide, an aromatic polyamide, an aromatic polyamide-imide, a. polybenzimidazole and a polyoxadiazole.

References Cited UNITED STATES PATENTS Jahr 338-208 X Harris 338-208 X Jacob 338-208 Levers et al. 338-208 X Jones et al. 219-545 X Crump 338-208 X 10 RICHARD M. WOOD, Primary Examiner.

V. MAYEWSKY, Assistant Examiner.