EP0175550A1

EP0175550A1 - Sheet heaters having dissociated insulation

Info

Publication number: EP0175550A1
Application number: EP85306476A
Authority: EP
Inventors: Neville S. Batliwalla; Ravinder K. Oswal; Michael Charles Jones; Jeff Shafe
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1984-09-14
Filing date: 1985-09-12
Publication date: 1986-03-26
Also published as: JPS6174285A

Abstract

57 Electrical sheet heaters which comprise electrodes (12, 13) secured to a surface of a resistive element (11) and whose insulation comprises a laminar insulating element (18) adjacent to, but not secured to, the electrodes and resistive element. Such an insulating element permits relative movement of the insulation and the electrodes, e.g. as a result of flexing or of different expansions on thermal cycling, and thus reduces the danger that the electrodes will become detached from the resistive element.

Description

SHEET HEATERS HAVING DISSOCIATED INSULATION

This invention relates to sheet heaters.
Sheet heaters of many kinds are known. They often comprise a laminar heating element which comprises a laminar resistive element and two or more electrodes which can be connected to a suitable power source, and an insulating jacket which is firmly secured to the heating element. 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, and 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. Documents describing conductive polymer compositions and devices comprising them include U.S. Patents Nos.

and 4,442,139; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978), Narkis et al; and commonly assigned U.S. Serial Nos. 601,424 now abandoned, published as German OLS No. 1,634,999; 732,792 (Van Konynenburg et al), now abandoned, published as German OLS No. 2,746,602; 798,154 (Horsma et al), now abandoned, published as German OLS No. 2,821,799; 134,354 (Lutz); 141,984 (Gotcher et al), published as European Application No. 38,718; 141,988 (Fouts et al), published as European Application No. 38,718, 141,989 (Evans), published as European Application No. 38,713, 141,991 (Fouts et al), published as European Application No. 38,714, 150,909 (Sopory), published as UK Application No. 2,076,106A, 184,647 (Lutz), 250,491 (Jacobs et al) and 254,352 (Taylor), published as European Application No. 63,440, 272,854 and 403,203 (Stewart et al), published as European Patent Application No. 67,679, 274,010 (Walty et al), 300,709 and 423,589 (Van Konynenburg et al), published as European Application No. 74,281, 349,505 (McTavish et al), published as European Applicaton No. 87,884, 369,309 (Midgley et al), 380,400 (Kamath), published as European Application No. 96,492, 474,390 (Leary), 483,633 (Wasley), 485,572 (Nayak et al), 493,445 (Chazan et al), 493,390 (Leary et al), 509,897 (Masia et al), 524,482 (Tomlinson et al), 534,913 (McKinley), 535,449 (Cheng et al) 552,649 (Jensen et al), and 904,736, published as UK Patent Nos. 1,470,502 and 1,470,503. The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
We have discovered that when the electrodes of a sheet heater are positioned on a face of the resistive element, serious difficulties can arise if the insulating element on that side of the heater is secured firmly thereto in the known ways, e.g. through the use of an adhesive or a melt bond. Thus we have found that if the electrodes are secured to the insulating layer and to the resistive element, the bond to the insulating element can cause the electrode to become detached from the resistive element, resulting in loss of power and/or dangerous short circuits. Such detachment can occur, for example, as a result of flexing the heater (if it is flexible) and/or as a result of thermal cycling which causes different parts of the heater to expand and contract at different rates. The present invention provides improved sheet heaters which mitigate or overcome these difficulties by using an insulating layer which is adjacent to the electrodes and the surface of the resistive element bearing the electrodes, but which is not secured to the electrodes and preferably is not secured to the electrodes or to the resistive element. An added benefit of such heaters is that the separation between the resistive element and the insulation provides a thermal barrier such that heat can be directed towards the substrate to be heated, which is preferably placed on the opposite side from the electrodes. Insulation of the heating element is normally completed by a second insulating layer which is adjacent the surface of the resistive element which does not bear the electrodes.
Accordingly, in its first aspect the invention provides an electrical sheet heater which comprises

(1) a laminar heating element which comprises
- (a) a laminar resistive element having a first face and a second face, and
- (b) at least two electrodes which are positioned on the first face of the resistive element and which can be connected to a source of electrical power to cause current to pass through the resistive element and cause resistive heating thereof, and
(2) a first laminar insulating element which is adjacent to the electrodes and the first face of the resistive element but is not secured to the electrodes.

In a second aspect, the invention provides a method of heating a substrate by placing a heater of the invention adjacent the substrate and passing current through the heater.
Insulation of the heater is preferably completed by means of a second laminar insulating element which is secured to the second face of the resistive element (preferably by means of a substantially continuous layer of adhesive) and to the edge portions of the first insulating element, e.g. by means of an adhesive or a melt bond. The insulating elements are preferably flexible polymeric sheets having a melting point substantially above the operating temperature of the heater. When using a heater comprising such insulating elements, the second element is preferably placed adjacent the substrate to be heated, since the adhesive layer assists heat transfer, whereas the separation of the first element from the heating element results in a relative thermal barrier. The first insulating layer preferably covers the electrodes and the first face of the resistive element, but is not secured directly thereto.
The heaters of the invention are preferably flexible, by which is meant that at 23°C, and preferably at -20°C, they can be wrapped around a 18 cm (4 inch) diameter mandrel, preferably around a 2.5 cm (1 inch) diameter mandrel, without damage.
The laminar resistive element of the present invention can be a layer of any resistive material, either PTC or ZTC, but is preferably composed of a conductive polymer. The conductive polymer is preferably melt-shaped, particularly melt-extruded, in which case the resistive element will usually be at least 0.0051 cm (0.002 inch) thick, preferably 0.025 to 0.635 cm (0.01 to 0.25 inch) thick, particularly 0.025 to 0.25 cm (0.01 to 0.1 inch) thick. However, the conductive polymer can also be shaped as a composition containing a solvent or liquid dispersing medium which is subsequently evaporated.
The invention is particularly useful when the electrodes are placed on the resistive element by a process which results in a bond which is vulnerable to damage by flexing or thermal cycling. The electrodes can for example be formed by printing, particularly silk screen printing, a conductive ink onto the resistive element, or by the use of polymer thick film technology, or by sputtering, or by a process comprising an etching step.
The electrodes are preferably arranged in a way similar to that disclosed in European Patent Application No. 85300415.8 corresponding to U.S. Serial No. 573,099 (MP0897 Batliwalla et al), i.e. a plurality of ribbon-shaped electrodes which are dimensioned and positioned on a surface of the resistive heating element so that

(a) when current passes between the electrodes, a substantial proportion of the current is parallel to the faces of the resistive element, and
(b) the ratio of the average width of the electrodes, measured parallel to the faces of the resistive element, to the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element, is at least 0.01:1, particularly at least 0.1:1.

Preferably the electrodes are so positioned and dimensioned that, at all points, the distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element, is not more than three times the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element. It is particularly preferred that the ratio of the average width of the electrodes to the average distance between the electrodes between which current passes is from 0.4:1 to 5:1, especially an arrangement in which the electrodes comprise a plurality of parallel bars which are preferably spaced apart from each other by substantially the same distance. When the conductive polymer has been melt-extruded, the electrodes are preferably arranged so that the current flows along the direction of extrusion.
When the heater requires a ground plane, e.g. 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 preferably positioned adjacent the face of the first laminar insulating element remote from the resistive element, and/or adjacent the face of the second insulating element remote from the resistive element. The ground plane can be of a known kind, but is preferably arranged so as to permit relative movement between the ground plane and the adjacent insulating jacket.
When the heater comprises a plurality of electrodes which are positioned on a surface of the resistive element and connected by bus bars, the bus bars are preferably in the form of laminar members as disclosed in the the Application filed contemporaneously herewith (our Ref. MP0961), corresponding to U.S. Serial Nos. 650,920, 663,014 and 735,408, the entire disclosure of which is incorporated herein by reference. The bus bars can be, but preferably are not, secured to the first insulating element, and when the bus bars are folded around the edge of the heating element, as disclosed in said application, can be, but preferably are not, secured to the second insulating element. The electrodes are preferably formed by printing a conductive PTF ink onto the resistive element in the desired electrode pattern. Preferably a resistive PTF ink (having a resistivity intermediate that of the electrodes and the substrate) is positioned between the electrodes and the resistive element to improve the physical and electrical contact therebetween. Such an arrangement is described in the the Application filed contemporaneously herewith (our Ref. MP0961), corresponding to U.S. Serial Nos. 650,920, 663,014 and 735,408, the entire disclosure of which is incorporated herein by reference.
A preferred embodiment of heater according to the invention comprises a flexible electrical sheet heater which comprises

(1) a laminar heating element which comprises
- (a) a laminar resistive element which has a first face and a second face, which is at least 0.002 inch thick, and which is composed of a conductive polymer composition comprising an organic polymer and, dispersed in the polymer, a particulate conductive filler; and
- (b) a plurality of electrodes, at least two of which can be connected to a source of electrical power to cause current to pass through the resistive element, and which are dimensioned and positioned on the first face of the resistive element so that
  - (i) when current passes between the electrodes, a substantial proportion of the current is parallel to the faces of the resistive element, and
  - (ii) the ratio of the average width of the electrodes, measured parallel to the faces of the resistive element, to the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element, is at least 0.01:1;
(2) a first laminar polymeric insulating element which covers the electrodes and the first face of the resistive element but is not secured to the electrodes or to the first face of the resistive element; and
(3) a second laminar polymeric insulating element which is secured to the second face of the resistive element by a substantially continuous layer of adhesive and to the first insulating element.

Another preferred embodiment of heater according to the invention comprises a flexible, self-regulating, electrical sheet heater which comprises

(1) a laminar heating element which comprises
- (a) a laminar resistive element which has a first face and a second face, which is 0.01 to 0.1 inch thick and is composed of a conductive polymer composition which
  - (i) exhibits PTC behavior, and
  - (ii) comprises a crystalline organic polymer and, dispersed in the polymer, carbon black; and
- (b) a plurality of ribbon-shaped electrodes which are dimensioned and positioned on a surface of the laminar resistive element so that
  - (i) when current passes between the electrodes, a substantial proportion of the current is parallel to the faces of the resistive element, and
  - (ii) the ratio of the average width of the electrodes, measured parallel to the faces of the resistive element, to the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element, is at least 0.1:1;

(2) a first laminar polymeric insulating element which covers the electrodes and the first face of the resistive element but is not secured to the electrodes or the first face of the resistive element; and
(3) a second laminar polymeric insulating element which is secured to the second face of the resistive element by a substantially continuous layer of adhesive and which is secured to the first insulating element.

In heaters according to the invention, the resistive element has a thickness of 0.025 to 0.0254 cm (0.01 to 0.1 inch). Preferably the resistive element comprises a conductive polymer composition that has been melt extruded, and the electrodes are so positioned on the element that current passing between the electrodes follows a path which is substantially parallel to the direction of extrusion.
Preferably the ratio of the average width of the electrodes to the average distance between adjacent electrodes between which current passes is at least 0.1:1, preferably from 0.4:1 to 5:1.
In preferred embodiments the electrodes comprises a plurality of spaced bars. The bars are preferably spaced apart from each other by substantially the same distance.
An embodiment of the invention is now described, by way of example, with reference to the accompanying drawings wherein:

Figure 1 is a cross-section through a heater of the invention, and
Figure 2 is a plan view of the resistive element, electrodes and bus bars of Figure 1.

Referring now to the drawing, the Figures illustrate a heater which comprises a heating element comprising a laminar conductive polymer resistive element 11 having printed on the top surface thereof interdigitated electrodes 12 and 13. Bus bars 15 and 16, composed of metal mesh, are folded around marginal portions of the element 11 and the electrodes 12 and 13 respectively. An insulating jacket is formed around the heating element, and bus bars by a polymeric bottom sheet 17 and a polymeric top sheet 18. Sheet 17 is secured to the bottom of the heating element, to the bottom of the bus bars and to edge portions of the top sheet by a substantially continuous layer of adhesive 21. The top sheet is adjacent to, but not secured to, the bus bars, electrodes and resistive element. On top of the top sheet there is a metallic, e.g. copper, foil 19 which is maintained in position by an outer polymeric insulating sheet 20, whose marginal portions are secured to the marginal portions of the sheet 18 by adhesive layers 22 and 23. As shown in Figure 2, the electrodes have a width t and a length 1, and are separated by a distance d, and the bus bar have a width x. Typical values for these variables are

t 0.08 - 0.51 cm (0.03 - 0.2 inch)
1 1.27 - 15.24 cm (0.5 - 6.0 inch)
d 0.25 - 0.76 cm (0.1 - 0.3 inch)
x 0.06 - 2.04 cm (0.2 - 0.8 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 232°C (450°F) as a sheet 0.04 cm (0.0175 inch) thick.
The sheet was irradiated to a dose of 14 megarads, thus cross-linking the polymer. The sheet was then heated and split into strips 18.42 cm (7.25 inch) wide. An electrode pattern as illustrated in Figure 1 was deposited on the strips, by screen-printing a graphite-and-silver-containing composition onto the strip, followed by drying. The distance (d) between adjacent electrodes was 0.64 cm (0.25 inch); the width (t) of each electrode was 0.16 cm (0.0625 inch); and the length (1) of each electrode was 13.72 cm (5.4 inch).
Bus bars of nickel-coated copper expanded metal, 3.81 cm (1.5 inch) wide, are folded around the edges of the electrode-bearing strip, and the assembly laminated between (A) a bottom sheet of polyvinylidene fluoride ("Kynar") 21.6 cm (8.5 inch) wide and 0.0127 cm (0.005 inch) thick, coated on the whole of its top surface with a layer 0.005 cm (0.002 inch) thick of a silicone adhesive sold by Flexcon Corporation under the trade name "Densil", and (B) a top sheet of polyvinylidene fluoride ("Kynar") 21.6 cm (8.5 inch) wide and 0.025 cm (0.010 inch) thick, placed in contact with the printed electrodes, which was coated on 1.27 cm (0.5 inch) wide edge portions of its bottom surface with a layer 0.005 cm (0.002 inch) thick of the same adhesive. Lamination was carried out at 52°C (125°F) and 690 KPa (100 psi). There was no adhesive between the top sheet and the bus bars, or between the top sheet and the conductive polymer sheet, or between the top sheet and the electrodes. A sheet of copper, 0.005 cm (0.002 inch) thick and 18.42 cm (7.25 inch) wide, was placed on the exposed surface of the top sheet, and an outer sheet of polyvinylidene fluoride ("Kynar"), 21.6 cm (8.5 inch) wide and 0.0127 cm (0.005 inch) thick, was placed over the copper sheet and laminated [at 52°C (125°F) and 690 KPa (100 psi)] to the edge portions of the bottom sheet (but not the copper foil), through 1.27 cm (0.5 inch) wide layers of 0.005 cm (0.002 inch) thick "Densil" adhesive on edge portions of the outer sheet. There was no adhesive between the outer sheet and the copper foil.

Claims

1. An electrical sheet heater which comprises

(1) a laminar heating element which comprises

(a) a laminar resistive element having a first face and a second face, and

(b) at least two electrodes which are positioned on the first face of the resistive element and which can be connected to a source of electrical power to cause current to pass through the resistive element and cause resistive heating thereof, and

(2) a first laminar insulating element which is adjacent to the electrodes and to the first face of the resistive element but is not secured to the electrodes.

2. A heater according to claim 1, wherein the first insulating element is not secured to the electrodes or to the first face of the resistive element.

3. A heater according to claim 1 or 2, which comprises a second laminar insulating element which is secured to the second face of the resistive element and to the first insulating element.

4. A heater according to claim 3, wherein the second insulating element is secured to the resistive element by means of a substantially continuous layer of adhesive.

5. A heater according to claim 3 or 4, wherein the first and second insulating elements are secured to each other by adhesive or by melt-bonding.

6. A heater according to any preceding claim, wherein the resistive element is composed of a conductive polymer composition which comprises an organic polymer and, dispersed in the polymer, a particulate conductive filler.

7. A heater according to any preceding claim, wherein the resistive element is at least 0.002 inch thick.

8. A heater according to any preceding claim, wherein the resistive element is melt-extruded and the electrodes are so positioned that current passing between the electrodes follows a path which is substantially parallel to the direction of extrusion.

9. A heater according to any preceding claim, wherein the electrodes were formed by a process comprising printing a conductive ink onto the first face of the resistive element or through the use of polymer thick film technology.

10. A heater according to any preceding claim comprising a plurality of electrodes, at least two of which can be connected to a source of electrical power to cause current to pass through the resistive element, and which are dimensioned and positioned on the first face of the resistive element so that

(i) when current passes between the electrodes, a substantial proportion of the current is parallel to the faces of the resistive element, and

(ii) the ratio of the average width of the electrodes, measured parallel to the faces of the resistive element, to the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the resistive element, is at least 0.01:1.