CA1226027A

CA1226027A - Laminar electrical heaters

Info

Publication number: CA1226027A
Application number: CA000454040A
Authority: CA
Inventors: Michael J. Katila; David Chazan
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1983-05-11
Filing date: 1984-05-10
Publication date: 1987-08-25
Also published as: KR850000165A; EP0128664B1; GB8411927D0; ES532367A0; JPS59214187A; EP0128664A1; BR8402218A; ATE36796T1; AU580552B2; US4517449A; AU2789684A; DE3473721D1; GB2141610A; ES8600599A1

Abstract

ABSTRACT OF THE DISCLOSURE

Laminar heaters comprise a luminary heating element sandwiched between two laminar electrodes having substantial electrical resistance along the thickness thereof. The heater has a heating area of irregular shape, and at least one of the electrodes extends over a connection area in which heat is not generated but which provides a shorter and/or more uniform current path to the heat-generating parts of the heater.

Description

~Z2~;~);27 This invention relates to luminary electrical heaters.

Luminary electrical heaters, including those which comprise a luminary resistive heating element composed of a conductive polymer, are known. Some of these heaters comprise a pair of luminary electrodes, e.g. of metal foil, an aperture (e.g. pierced, slit or punched) metal foil or sheet, with the heating element sandwiched between them.
The heating element preferably exhibits PTC behavior, thus rendering the heater self-regulating. It is also known to make such heaters which are heat-recoverable or which are deformable so that they can be attached to heat-recoverable articles and will not prevent heat-recovery thereof.
Reference may be made for example to US. Patents Nos.
4,085,286, 4,177,376, 4,177,446~ 4,223,209, and 4,3l8,220, and published European Application No. 63,440. For further details concerning conductive polymers and electrical devices concerning them, reference may be made to US.
Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757r086, 3,793,716, 3~823,217, 3,858,144, 3,861,029, 4,017,715, 4,072,848, 4,117,312, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,315~237, 4,317,027, 4,318,881 and 4,330,704; J. Applied Polymer Science 19, 813-815 (1975), Klason and Cubit; Polymer Engineering and Science 18, 649-653 (1978), Narkis et at; German OWLS 2,634,999; 2,755,077;

2,745,602; 2,755,076; 2,821,799; European Application Nos. 38713, 38714, 38715, 38716 and ~87l8.

For a number of uses of luminary heaters, it is desirable to make use of a heater having an irregular shape.

Jo ~6~27 For example, when such a heater is used as part of a heat-recoverable article, e.g. to cover a splice between telephone cables, recovery of only certain parts of the article may be desired or necessary. Alternatively, if recovery of parts of the article is all that is needed, then even if recovery of the whole article is not harmful, limitations on the available power supply (e.g. a battery or limited I power) may make it necessary or desirable to restrict heating to those parts of the article which need to recover (and/or provide other thermal response such as heat-activation of an adhesive).

We have discovered that when it is desired to make use of a luminary heater which comprises a heating element sand-wicked between two luminary electrodes and which has an irreg-ular shape, especially one having a cut-out portion (or "window") or a deeply reentrant portion, serious problems can arise, for example a failure to generate sufficient heat in some areas and/or generation ox excessive heat in some areas.

These problems appear to arise from the fact that the luminary electrodes have significant resistance along the thickness thereof, leading to a loss of potential difference between the electrodes at points which can only be reached by a circuitous path from one or both of the bus connectors, and/or to excessive current densities at restricted portions of the electrodes.

We have discovered that these problems can be mitigated or overcome by making use of a heater in which at least one of the electrodes extends over a connection area in which heat is not generated but which provides a shorter and/or more uniform current path to the heat-generating parts of the heater.

:~L226~;27 In one aspect, the invention provides a luminary heater which comprises (1) a first luminary electrode which has low electrical resistance through the -thickness thereof but has substantial electrical resistance along the thick-news -thereof; (2) a firs-t electrical connection means which (a) makes distributed electrical contact with the first electrode in a firs-t contact zone, and (b) can be connected to a source of electrical power; (3) a second luminary elect-rode which has low electrical resistance -through -the thick-news thereof but has substantial electrical resistance along -the -thickness thereof; (4) a second electrical connection means which (a) makes distributed electrical contact with the second electrode in a second contact zone which is off-set from the first contact zone, and (b) can be connected to a source of electrical power; and (5) a luminary resistive element which has substantial electrical resistance through the thickness -thereof and which is sandwiched between the first and second electrodes; said heater comprising (a) a heating area which lies between -the first contact zone and the second contact zone, in which, when the first and second connection means are connected to a source of electrical power, current flows through the thickness of the resistive element between the electrodes; and (b) a connection area which lies between -the first contact zone and the second contact zone in which -the resistive element and the second electrode are present, and in which, if both electrodes are present, a-t least one of -the electrodes is electrically in-sulfated from the resistive element so that, when the first and second connection means are connected to a source of electrical power, heat is not genera-ted in the resistive element in the connection area; and the second electrical connection means being so connected to the second electrode that there are points on the second electrode which are in the heating area and which are connected to the second con-section means by a path of least electrical resistance which passes through the connection area.

122~);27 though the heaters of the invention can have substantially zero resistance, i.e. have an equipotential surface, the invention is of particular value for heaters in which (i) each of -the electrodes is a luminary electrode which has low electrical resistance through the thickness thereof but substantial electrical resistance along the thickness thereof, and (ii) the heating element is sand-wicked between the electrodes. It is to be understood that the resistance of the electrodes is not usually high in an absolute sense, merely high enough -to give rise to the probe lets noted above. Generally such heaters comprise first and second electrical connection means which (a) can be connect ted to a source of electrical power and (b) make distributed electrical contact with the first and second electrodes, respectively; there being points on at least one of the electrodes in the - pa -., ~L2~6~2~7 heating area which are connected to the respective con-section means by a path of least electrical resistance which passes through the connection area.

In another aspect, the invention provides a preferred method of making a luminary electrical heater as defined above which generates heat only in preselected areas, which method comprises (1) providing a luminary heater comprising (a) first and second luminary electrodes, (b) a luminary resistive heating element sandwiched between the electrodes; and (2) cutting through the first electrode, but not the second electrode, alienage line which electrically isolates an area of the first electrode.

This method results in a heater in which the con-section area comprises in addition to the second electrode, a first luminary member which is substantially the same as the luminary resistive heating element except that current does not pass through it when the connection means are connected to a source of electrical power; and a second luminary member which is substantially the same as the first electrode and is separated from the first electrode by a discontinuity through which current cannot pass.

~22~i~27 Another method of making a heater as defined above is to prepare a laminate of the heating element and -the so-cord electrode, and to apply a suitably shaped first elect trove to the opposite face of the heating element. Another method is to apply suitably shaped electrodes to opposite faces of a luminary heating element.

In another aspect, the invention provides a method of covering a substrate which comprises: (A) placing adja-cent the substrate a heat-shrinkable article comprising a luminary electrical heater which comprises (1) a first luminary electrode which has low electrical resistance through the thickness thereof but has substantial electrical resistance along -the thickness thereof; (2) a first electric eel connection means which (a) makes distributed electrical con-tact with -the first electrode in a first contact zone, and (b) can be connected to a source of electrical power;

(3) a second luminary electrode which has low electrical resistance through the thickness thereof but has sub Stan-trial electrical resistance along the thickness thereof; (4) a second electrical connection means which (a) makes disk tribute electrical contact with the second electrode in a second contact zone which is offset from the first contact zone, and (b) can be connected to a source of electrical power; and (5) a luminary resistive element which has sub-staunchly electrical resistance -through -the thickness thereof and which is sandwiched between the first and second elect-nodes; said heater comprising a heating area which lies be-t-wren the first con-tact zone and the second contact zone, and in which, when the first and second connection means are connected to a source of electrical power, current flows through the thickness of the resistive element between the electrodes, and (b) a connection area which lies between the first contact zone and the second contact zone, in which the resistive element and the second electrode are present, and in which, if both electrodes are present, at least one of .

~226C~;Z7 the electrodes is electrically insulated from the resistive element so that, when -the first and second connection means are connected to a source of electrical power, heat is not generated in the resistive element in the connection area;
and the second electrical connection means being so connect ted to the second electrode that there are points on the second electrode which are in the heating area and which are connected to the second connection means by a path of least electrical resistance which passes through the connection area, and (B) connecting the first and second electrical connection means to a source of electrical power to cause heating of selected areas of the article, at least some of the selected areas shrinking towards the substrate.

For the most uniform heating, preferably all points in the heating area of each electrode are connect ted to -the respective connection means by a path of least electrical resistance which is a straight line on the sun-face of the electrode. The term "a straight line on -the surface of the electrode" is used to mean that if the heater is flat (i.e. planar), or if i-t is no-t flat and is (notion-ally) flattened out, the current path is a geometrically straight line. Louvre, in many cases, sufficiently uniform heating can be obtained even when parts of the heating area are heated via current paths which, from one of -the connect lion means, are not straight. Thus we have found that, when a generally - pa -.

~L~26~ 7 rectangular connection area (window) is to be produced in a generally rectangular heater, between the first and second connection means and closer to one of the connection means, very much better results are obtained if the electrode which spans the connection area is the one whose connection means is closer to the window. Putting this another way, the distance from the first connection means to the window is preferably greater than the distance from the second con-section means to the window.

The heating element can be of any type but preferably it comprises a conductive polymer. It may be of uniform composition, e.g. a single PTC layer, or comprise two or more layers of different conductive polymers e.g. at least one layer of a ZTC conductive polymer and a layer of a PTC
conductive polymer.

The heater can be used mere to maintain selected areas of a substrate or the surrounding atmosphere at an elevated temperature. However, the heater can itself undergo a thermal response when it is connected to a suitable power source or it can be secured to an element, usually a non-conductive element, which undergoes a thermal response when the heater is connected to a suitable power source.
The term "undergo a thermal response" is used herein to denote a change in physical and/or chemical state, in addition to a change in temperature, for example a physical change such as a change in shape, or a chemical reaction.
Thus the heater may comprise a conductive polymer element which is heat-recoverable. Alternatively the heater can be secured to an article, e.g. of an organic polymer or a metal, which is heat-recoverable, e.g. heat-shrinkable, or to an element which melts and flows when it is heated, e.g.

I '7 a heat-activated adhesive or a mastic, or to an element which comprises two components which react together when heated, e.g. a two-part latent adhesive. We have found heaters of the invention to be particularly useful for securing to heat-shrinkable wraparound articles for covering elongate substrates such as cable splices, the heating areas of the heater being arranged to heat only the areas of the article which require to be heated, in particular the end sections of the article, which shrink into contact with the substrate and which are normally coated with a heat-activated adhesive, and at least one of the longitudinal edge sections of the article, which are held together by a closure device and which are likewise normally coated with a heat-activated adhesive so as to seal the closure area.

If the heater is itself heat-shrinkable or is secured to an article which is heat-shrinkable, care may be needed to ensure that the desired electroding pattern retains in effect.

Referring now to the drawings, Figures 1, 3 and 5 are diagrammatic plan views of heaters of the invention and Figures 2, 4 and 6 are diagrammatic cross-sections taken on lines 2, 4 and 6 of Figures 1, 3 and 5 respectively In each of Figures 1-6, a first electrode 1 at the top of the heater is contacted by a first distributed connection means 2, a second electrode 3 at the bottom of the heater is contacted by a second distributed connection means 4, and a conductive polymer heating element 5 is sandwiched between the electrodes. The heaters comprise a heating area (shown cross-hatched in Figures 1, 3 and 5) and at least one connection area (shown horizontally hatched in Figures 1, 3 and 5) The heaters have been prepared from rectangular 3L2;;~

luminary heaters of the same cross-section throughout, by cutting a channel 11 into the first electrode (and in Figures 3 and 4, by also cutting a channel 31 into the second electrode 3). Also shown. are the current paths from the first connection means 2 (solid lines) and from the second connection means 4 (dashed lines) to various points X, Y and Z in the heating areas. It will be seen that in each case at least one of the current paths is a straight line. Also shown in Figure 6 is a heat-responsive member 6, e.g. a heat-shrinkable sheet and/or a heat-activated adhesive.

Figure 7 is a diagrammatic perspective view of a heat-shrinkable splice case being applied around a telephone cable splice, the interior of the cover having secured thereto a heater of the invention whose heating area is indicated by the dotted lines.

The invention is further illustrated in the following Example.

EXAMPLE

A "slit heater" of the kind described in Patent No. 4,177,446 was made by slitting a rectangular laminate consisting of a sheet 40 miss thick of a PTC conductive polymer sandwiched between two aluminum foils each 0065 mix thick, and expanding it 4.5 times to give a heater 38.5 inch long and 18.5 inch wide, with 36 diamond-shaped apertures across its width A copper bus bar way attached to one electrode along one side of the sheet and a second copper bus bar was attached to the other electrode along the other side of the sheet. Each bus bar was 0.3125 inch wide and 16 ~L2~;02~

mix thick. Using a router equipped with a carbide tip, a channel was cut into the upper surface of the heater in the shape shown in Figure 1, the "window" connection area thus created being about 13.5 inches high and about 18.5 inches wide, the top line of the window being about 4 inches from the top of the heater, and the bottom line of the window being about 1 inch from the bottom of the heater.

The heater was secured with polyurethane adhesive to the interior of a heat-shrinkable polymeric sleeve, and a 30 mix thick layer of heat-activated adhesive was then applied over the heater.

The resulting product was wrapped around a substrate, the edges secured together, and the heater powered. The resultant heating caused the heat-shrinkable sleeve to shrink into contact with the substrate and to seal along the bond line, while the window area remained unheated and did not shrink.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A laminar heater which comprises (1) a first laminar electrode which has low electrical resistance through the thickness thereof but has substantial electrical resistance along the thickness thereof; (2) a first electri-cal connection means which (a) makes distributed electrical contact with the first electrode in a first contact zone, and (b) can be connected to a source of electrical power;
(3) a second laminar electrode which has low electrical resistance through the thickness thereof but has substan-tial electrical resistance along the thickness thereof; (4) a second electrical connection means which (a) makes dis-tributed electrical contact with the second electrode in a second contact zone which is offset from the first contact zone, and (b) can be connected to a source of electrical power; and (5) a laminar resistive element which has sub-stantial electrical resistance through the thickness thereof and which is sandwiched between the first and second elec-trodes; said heater comprising (a) a heating area which lies between the first contact zone and the second contact zone, in which, when the first and second connection means are connected to a source of electrical power, current flows through the thickness of the resistive element between the electrodes, and (b) a connection area which lies between the first contact zone and the second contact zone in which the resistive element and the second electrode are present, and in which, if both electrodes are present, at least one of the electrodes is electrically insulated from the resistive element so that, when the first and second connection means are connected to a source of electrical power, heat is not generated in the resistive element in the connection area;
and the second electrical connection means being so con-nected to the second electrode that there are points on the second electrode which are in the heating area and which are connected to the second connection means by a path of least electrical resistance which passes through the connection area.

2. A heater according to claim 1 wherein all the points on the second electrode which are in the heating area are connected to the second connection means by a path of least electrical resistance which is a straight line on the surface of the second electrode.

3. A heater according to claim 1 wherein the dis-tance from the first connection means to the connection area is greater than the distance from the second connection means to the connection area.

4. A heater according to claim 1 which comprises a second connection area in which the resistive element and the first electrode are present, and in which, if both elec-trodes are present, at least one of the electrodes is elect-rically insulated from the resistive element so that, when the first and second connection means are connected to a source of electrical power, heat is not generated in the resistive element in the second connection area.

5. A heater according to claim 1 wherein the resistive element comprises a conductive polymer.

6. A heater according to claim 1 wherein the resistive element comprises a layer of a PTC conductive polymer.

7. A heater according to claim 1 wherein the resistive element comprises a first layer of a ZTC material adjacent the first electrode, a second layer of ZTC material adjacent the second electrode, and a layer of a PTC conduc-tive polymer between the ZTC layers.

8. A heater according to claim 1 which, when it is connected to a suitable source of electrical power, un-dergoes a change in shape.

9. A heater according to claim 8 wherein the resistive element comprises a heat-shrinkable PTC conductive polymer element.

10. A heater according to claim 1 wherein the con-nection area comprises a second laminar member which is sub-stantially the same as the first electrode and is separated from the first electrode by a discontinuity through which current cannot pass.

11. A heater according to claim 1 which further comprises (6) a non-conductive element which undergoes a thermal response when the connection means are connected to a suitable source of electrical power.

12. A heater according to claim 11 wherein (a) the element (6) comprises a heat-shrinkable polymeric sheet and (b) the electrodes and resistive element are capable of deformation to permit shrinkage of the sheet.

13. A heater according to claim 11 wherein the element (6) comprises a material which melts and flows when it is heated.

14. A heater according to claim 13 wherein the element (6) comprises a hot melt adhesive.

15. A heater according to claim 1 wherein all the points on the first electrode which are in the heating area are connected to the first connection means by a path of least electrical resistance which is a straight line on the surface of the electrode.

16. A heater according to claim 1 wherein the heating area surrounds the connection area.

17. heater according to claim 1 wherein the con-nection area is re-entrant in the heating area.

18. A method of providing a protective covering over a substrate, which method comprises: (A) placing adja-cent the substrate a heat-shrinkable article comprising a laminar electrical heater which comprises (1) a first lami-nar electrode which has low electrical resistance through the thickness thereof but has substantial electrical resis-tance along the thickness thereof; (2) a first electrical connection means which (a) makes distributed electrical contact with the first electrode in a first contact zone, and (b) can be connected to a source of electrical power;
(3) a second laminar electrode which has low electrical resistance through the thickness thereof but has substan-tial electrical resistance along the thickness thereof; (4) a second electrical connection means which (a) makes dis-tributed electrical contact with the second electrode in a second contact zone which is offset from the first contact zone, and (b) can be connected to a source of electrical power; and (5) a laminar resistive element which has sub-stantial electrical resistance through the thickness thereof and which is sandwiched between the first and second elec-trodes; said heater comprising a heating area which lies between the first contact zone and the second contact zone, and in which, when the first and second connection means are connected to a source of electrical power, current flow through the thickness of the resistive element between the electrodes, and (b) a connection area which lies between the first contact zone and the second contact zone, in which the resistive element and the second electrode are present, and in which, if both electrodes are present, at least one of the electrodes is electrically insulated from the resistive element so that, when the first and second connection means are connected to a source of electrical power, heat is not generated in the resistive element in the connection area;
and the second electrical connection means being so con-nected to the second electrode that there are points on the second electrode which are in the heating area and which are connected to the second connection means by a path of least electrical resistance which passes through the connection area, and (B) connecting the first and second electrical connection means to a source of electrical power to cause heating of selected areas of the article, at least some of the selected areas shrinking towards the substrate.

19. A method according to claim 18 wherein the article comprises a hot-melt adhesive coated on the interior surface of the article in at least one of the selected areas.

20. A method according to claim 18 wherein the article is a wrap-around article which has been wrapped around the substrate and secured in place by means of a closure device which is secured to longitudinal edge sec-tions of the article; and the selected areas of the article comprise (a) end sections of the article which shrink down towards the substrate and (b) at least one of the longi-tudinal edge sections of the article.

21. A method according to claim 18 wherein all the points on the second electrode which are in the heating area are connected to the second connection means by a path of least electrical resistance which is a straight line on the surface of the second electrode.

22. A method according to claim 21 wherein the distance from the first connection means to the connection area is greater than the distance from the second connection means to the connection area.

23. A method according to claim 18 wherein the heater comprises a second connection area in which the resi-tive element and the first electrode are present, and in which, if both electrodes are present, at least one of the electrodes is electrically insulated from the resistive ele-ment so that, when the first and second connection means are connected to a source of electrical power, heat is not gene-rated in the resistive element in the second connection area.

24. A method according to claim 18 wherein the resistive element comprises a conductive polymer.

25. A method according to claim 18 wherein the resistive element comprises a layer of a PTC conductive polymer.

26. A method according to claim 18 wherein the resistive element comprises a first layer of a ZTC material adjacent the first electrode, a second layer of a ZTC
material adjacent the second electrode, and a layer of a PTC
conductive polymer between the ZTC layers.

27. A method according to claim 18 wherein the resistive element comprises a heat-shrinkable PTC conductive polymer element.

28. A method according to claim 18 wherein the connection area comprises a second laminar member which is substantially the same as the first electrode and is separa-ted from the first electrode by a discontinuity through which current cannot pass.

29. A method according to claim 18, wherein the heat-shrinkable article comprises a heat-shrinkable poly-meric sheet and the laminar heater is capable of deformation to permit shrinkage of the sheet.

30. A method according to claim 18 wherein all the points on the first electrode which are in the heating area are connected to the first connection means by a path of least electrical resistance which is a straight line on the surface of the electrode.

31. A method according to claim 18 wherein the heating area surrounds the connection area.

32. A method according to claim 18 wherein the connection area is re-entrant in the heating area.