Electrical Heating Device

This invention relates to electrical heaters.

U.S. patent application Ser. No. 181,974, filed Aug. 28, 1981 and now abandoned and Ser. No. 295,400, filed Aug. 21, 1981, both of which are now owned by Flexwatt Corporation, the assignee of the present application, disclose a flexible sheet heater in which a semi-conductor pattern including a pair of longitudinally-extending stripes is printed on a substrate. A thin copper electrode is placed on each strip and is there held in place by a cover sheet that is attached, at opposite sides of the electrode, to closely-adjacent portions of the underlying substrate.

The present invention is directed to improvements in such sheet heaters.

I have discovered that the ability of such sheet heaters to flex without adversely effecting the electrical contact between the copper electrodes and the semi-conductor pattern is greatly improved if a pair of thin, rectangular-in-cross-section electrodes are placed, one on top of the other, on each semi-conductor strip.

I have discovered also that such a double electrode structure makes it possible simply and permanently to connect a wire to each electrode, even when the electrodes are tightly sealed between a pair of plastic insulating sheets.

FIG. 1 is a plan view of a sheet heater embodying the present invention.

FIG. 2 is a section, portions of which have been enlarged, taken at 2--2 of FIG. 1.

FIG. 3 is a section, portions of which have been enlarged, taken at 3--3 of FIG. 4.

FIG. 4 is an enlarged view of portions of the heater of FIG. 1.

FIG. 1 illustrates a sheet heater, generally designated 10, comprising a substrate (typically paper or plastic) 12 on which is printed (typically by screening) an about 0.0005 inch thick semi-conductive pattern of colloidal graphite. Such semi-conductor patterns are described in detail in the above-referenced U.S. Patent Applications, both of which are here incorporated by reference.

As shown, the semi-conductor pattern includes a pair of parallel longitudinal stripes 14, each 5/32 inch wide, and a plurality of identical, regularly-spaced semi-conductor bars 18 extending between stripes 14. Each bar 18 is 1/4 inch wide measured perpendicularly to the edges of the bars, and thus somewhat longer than 1/4 measured along the intersection between bars 18 and stripes 14, The substrate 12 on which the pattern is printed is of sufficient width to provide an uncoated boundary 16 (about 1/32 inch wide) along the outside edge of each stripe 14. The spaces 20 between adjacent bars 18, each about half as wide as a bar 18 (i.e., about 1/8 inch wide), are also uncoated, i.e., are free from the semi-conductor pattern.

In the illustrated embodiment, the distance between stripes 14 is about 6 inches. Each of bars 18 is in the form of a chevron, each straight line segment 18a, 18b of which extends angularly half the distance between the stripes and forms an angle, .alpha., of about 10.degree. with a line extending perpendicularly between stripes 14. The included angle between segments 18a, 18b is, thus, about 160.degree.. In embodiments in which the distance between stripes 14 is less than about 41/2 inches, each bar typically will comprise a single straight line segment extending the full distance between stripes 14.

A pair of tin-plated copper electrodes 22a, 22b, each typically 1/8 inch wide and 0.003 inch thick, is placed on top of each longitudinal stripe 14 with electrode 22a engaging the underlying stripe 14 and an electrode 22b on top of electrode 22a with the upper surface of electrode 22a and the lower surface of electrode 22b in face-to-face engagement with each other. The two electrodes 22a, 22b of each pair are not attached to each other and, except to the extent they are restrained by other portions of the laminate, are free to slide relative to each other.

Substrate 12, the graphite pattern printed thereon, and electrodes 22 are hermetically sealed between a pair of thin plastic sheets 23, 24. Each of sheets 23, 24 is a co-lamination of a 0.002 inch thick polyester ("Mylar") dielectric insulator 23a, 24a and a 0.003 inch thick polyethylene adhesive binder 23b, 24b. Plastic adheres poorly to graphite, but the polyethylene layers 23b, 24b heat seal well to substrate 12 and to each other. The polyethylene layer 23b on top of substrate 12 bonds to both the uncoated paper boundary outside stripes 14 and, between stripes 14, to the uncoated substrate spaces 20 between adjacent bars 18. Stripes 14 are slightly wider than electrodes 22 to insure proper electrical connection between the electrodes and underlying stripes, but both this extra width and the distance from the stripes 14 to the side edges of the substrate are kept to a minimum so that the electrodes will be held tightly down on the stripes.

It will be noted that sheets 23, 24 are both wider and longer than substrate 12 and are tightly sealed to each other in the areas outside the longitudinal and transverse edges of substrate 12. Electrodes 22, however, extend the full length of heater 10. The fact that the transverse edges of the substrate 12 are spaced inwardly from the ends of the heater, coupled with the fact that the polyethylene layers 23b, 24b seal tightly around the periphery of the electrode portions in the areas between the transverse edges of the substrate 12 and the ends of the heater where there is no substrate, ensure that all the edges of the heater are effectively hermetically sealed.

Preferred apparatus for making the heater of the present invention is disclosed in my copending application Ser. No. 478,080, filed Mar. 23, 1983, which is here incorporated by reference. As there shown, the various layers of the heated are laminated, i.e., heat sealed, together by passing them between a pair of heated nip rolls. As shown in FIG. 2, the heat from the nip rolls partially melts the polyethylene layer 23b of sealing layer 23, causing the melted plastic to flow partially around the side edges of the top electrode 22b. The polyethylene layer 23 seals to the top electrode 22b, to the uncoated boundary space 16 along one edge of electrode 22, and to the uncoated space 20 between adjacent bars 18 at the other edge of electrode 22. It also bonds, but only lightly, to the semi-conductor portion, e.g., to the exposed portions of the upper surfaces of stripes 14 and to the top of bars 18. The pressure of the nip rolls also forces the bottom electrode 22a slightly down into paper substrate 12, and the slightly compressed paper acts somewhat as a spring, maintaining positive pressure between the engaged electrodes 22a, 22b.

Reference is now made to FIGS. 3 and 4 which illustrate the manner in which connecting wires 28 are attached to heater 10. At the end 34 of the heater 10 to which connection is to be made, a wedge-shaped portion 32 of the heater laminate, about 1/2 inch long and 3/16 inch wide at the end 34 of the heater, is cut out, and removed, along each side of each of electrodes 22. As indicated, one edge 33 of each cut-out extends longitudinally along a side edge of an electrode 22 (very close to or slightly within the electrode), thus breaking the seal between the tightly sealed together plastic sheets 23, 24 at the edge of the electrode and permitting the upper electrode 22b (and the portion of sealing layer 23 on top of it) to be folded back upon itself. The upper electrode 22b and overlying sealing layer 23 are folded back so that their free end 35 is located about 3/8 inch from the edge 34 of heater 10, thus exposing an about 3/8 inch long strip of electrode 22 (formed, as shown in FIG. 3, by a 3/16 inch length of the exposed top of electrode 22a and an equal length of the exposed bottom surface of folded-back electrode 22b) to which the stripped end 30 of connecting wire 28 is soldered. It is important to note that the total length of exposed electrode (about 3/8 inch) is shorter than the length (1/2 inch) of the removed wedge 32. This permits an electrically insulating sleeve 36 to be slipped over, and cover, the entire connection area. As shown in FIG. 1, sleeve 36 is positioned with its end abutting the inner ends 38 of the cut-out portion 32 on each side of electrode 22 and heat shrunk to hold it in place. For clarity, sleeve 36 is not shown in FIGS. 1A and 3.

At the other end 48 of heater 10, a elongated hole 50 about 1/4 inch wide is punched through each electrode 22 and the superposed, sealed-together sheets 13, 14 at a point a short distance from end 48, i.e., between heater end 48 and the adjacent end of substrate 12. The plastic around the edge of the holes 50 is melted, using a conventional spot heater, to cause plastic to flow over and seal the ends of the conductors 22. As will be apparent, this both insures that the ends of conductors 22 at heater end 48 are not "live", and also maintains the desired water-tight seal around the entire heater periphery.

Other embodiments will be within the scope of the following claims.