CA1062755A - Layered self-regulating heating article - Google Patents

Abstract

ABSTRACT

A self-regulating heating article comprising a layer of material exhibiting a positive temperature coefficient of resistance (PTC) and said PTC layer having at least partially contiguous therewith at least one layer of constant wattage (CW) output material. The article operates such that when connected to an electric power source, the current flows through at least a portion of the thickness of the PTC layer and of the constant wattage layer.

In a preferred embodiment, upon heating the article, change in dimensions as well as activation of an adhesive occurs.

Description

7cjs This invention relates to shaped structures of electrically conductive polymeric compositions having a positive temperature coefficient of resistance (PTC), especially to heating elements comprising PTC materials.
An improvement in electrical heating devices in recent years has been the provision of self-regulating heating systems which utilize materials exhibiting certain types of PTC
characteristics, namely that upon attaining a certain temperature a substantial rise in resistance occurs. EIeaters utilizing PTC
materials reportedly exhibit more or less sharp rises in resistance within a narrow temperature range but below that temperature range exhibit only relatively small changes in resistance with temperature. The temperature at which the resistance commences to increase sharply is often designated the switching or anomaly temperature (Ts) since on reaching that temperature the heater exhibits an anomalous change in resistance and, for practical purposes, switches off. Self regulating . , .
heaters utilizing PTC materials have advantages over conventional heating apparatus in that they generally eliminate the need for separate thermostats, fuses or in-line electrical resistors.
~; The most wi~ely used PTC material has been doped barium `~ ~ titanate which has been utilized for self-regulating ceramic ~ heaters employed in such applications as food warming trays and -'~ other small porta~le heating appliances. Although such ceramic '~ 25 ~ PTC materials are in common use for heating application,~their rigidity severely limits the type of application for which the~
can be used. PTC materials comprising electrically conductive . ~ :

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polymeric compositions are also known, some of which are stated to possess the special characteristics described hereinabove.
However, the use of such polymeric PTC materials has been relatively limited, primarily due to their low heating capacity.
Such materials generally comprise one or more conductive fillers, for example carbon black or powdered metal, dispersed in a crystalline thermoplastic polymer. PTC compositions prepared from highly crystalline polymers generally exhibit a steep rise in resistance commencing a few degrees below their crystalline melting point similar to the behaviour of their ceramic counterparts at the Curie temperature (the Ts for ceramics). PTC compositions derived from homopolymers and copolymers of lower crystallinity, for example, less than about 50%, exhibit somewhat less steep increases in resistance which increase commences at a less well defined temperature in a range often considerably ~elow the polymer's crystalline melting point. In the extreme case some polymers of low crystallinity yield resistance vs temperature curves which are m~re or less concave (from above). Other types of thermoplastic polymers yield resistances which increase fairly smoothly and more or less steeply but continuously with temperature. The behaviour of .i such compositions is illustrated by Figures l and 2 of the ., accompanying drawings, which show graphically the variation of resistance with temperature. Figure l of the accompanying drawings illustrates characteristics curves for the aforementioned ~ ~ different types of PTC compositions. In Figure l curve I exhibi~s ', the sharp virtually instantaneous increase in resistance (herein-I after known as type I behaviour~ generally characteristic of ~ . . .
~ inter alia polymers having high crystallinity, curve II shows l 30 the more gradual increase at lower temperatures (relative to ~ 3 -. , . ~ , ; , .. , . ., - . ,:,, . . . - , , , . :
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the polymer melting point) hereinafter known as type II behaviour ~enerally characteristic of lower crystallinity polymers. Curve III illustrates the concave (from above) curve characteristic (Type III behaviour) of many very low crystallinity polymers while curve IV illustrates the large increase in resistance without a region of more or less constant resistance (Type IV behaviour) at least in the temperature range of commercial interest seen with some materials. Curve V illustrates the gently increasing resistance temperature characteristic (Type V behaviour) shown by many "normal" electrical resistors. Although the above types of behaviour have been illustrated mostly by reference to ; specific types of polymeric material it will be realized by those ~ -skilled in the art that the particular type of behaviour manifested is also very dependent on the type and amount of conductive filler and, particularly in the case of carbon black, on its particle size and shape, surface characteristics, tendency to agglomerate and the shape of the particle agglomerates ti.e., its tendency to structure).
It should be noted that the preferred PTC compositions disclosed in the prior art are all stated to manifest essentially Type I behaviour. In fact, the prior art does not spec1fically recognize Types II to IV behaviour notwithstanding the fact that many of the PTC compositions disclosed in the prior art in fact exhibit not ~ype I, but rather Type II, III or IV behaviour.
With Type I resistance temperature characteristics, the increase in resistance above Ts is rapid so that Ts may ~e regarded as the temperature at which the device switches off.

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6Z755i However, with type II or type III PTC materials the transition from a resistance relatively stable as temperature is increased ; to a resistance rising steeply with temperature is much less well defined, and the anomaly temperature or Ts is frequently not an exact temperature. In the present specification, although a device may ~e described as shutting off at a given Ts, it will be understood by those skilled in the art that in many practical instances it may be more appropriate to understand Ts as being the lowest temperature of a range in temperature over which the device switches off or, to consider Ts to be a relatively narrow temperature range rather than a specific temperature.
Previously disclosed self-regulating thermal devices utilizing a PTC material are described as having extremely steep (Type I) R = f (T) curves so that above a certain tempera-ture the device will in effect shut off, while below thattemperature a relatively constant wattage output at constant voltage is achieved. At temperatures below Ts the resistance is at a relatively low and constant level and thus the current flow is relatively high for any given applied voltage. The power generated by this current flow is dissipated as heat, i.e. heat i~ generated by electrical resistance and warms . .
up the PTC material. As the tempexature rises, the resistance stays at this relatively low level until about the Ts temperature, at which point a rapid increase in ~ -resistance occurs. With the increase in resistance there is a concomitant decrease in power, thereby limiting the amount of ;~ heat generated so that when TS is reached heating is essentially stopped. Then, upon a lowering of the temperature of the device 5 _ . ~ ~

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below Ts by dissipation of heat to the surroundings, the resistance drops, thereby increasing the power output.
At a steady state, the heat generated will essentially balance the heat dissipated. Thus, when an applied voltage is directed across a PTC heating element, the Joule heat causes heating of the PTC element up to about its Ts the rapidity of such heating depending on the applied voltage and type of PTC
element, after which little additional temperature rise will occur because of the increase in resistance. Because of the resistance rise, a PTC heating element will ordinarily reach a steady state at approximately Ts thereby self-regulating the heat output of the element without resort-to fuses or thermo-stats. The davantages of such a self contained heat regulating element in many applications will be apparent.
Kohler, U.S. Patent 3,243,753 discloses carbon filled ;~ polyethylene wherein the conductive carbon particles are in sub stantial contact with one another. Kohler describes a product containing 40% polyethylene and 60'yO carbon particles so as to give a resistance at room temperature of about 0.4 ohm/cm. As is typical of the alleged performance of the prior art materials, ~; Kohler's PTC product is described as having a relatively flat -; curve o~ electrical resistance versus temperature below the switching temperature, followed by a sharp rise in resistivity of at least 250% over a 14C range. The mechanism suggested by - 25 Kohler for khe sharp rise in resisti~ity is that such change is a function of the di~ference in thermal expansion of the materials, i.e. polyethylene and particulate carbon. It is suggested that ~' ~

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the composition's high level of conductive filler forms a conductive network through the polyethylene polymer matrix, thereby giving an initial constant resistivity at lower temperatures. However, at about its crystalline melt point, the polyethylene matrix rapidly expands, such expansion causing a breakup of many of the conductive networks, which in turn results in a sharp increase in the resistance of the composition.
Other theories proposed to account for the PTC phenomenon in conductive particle filled polymer compositions include complex mechanisms based upon electron tunnelling through inter-grain gaps between particles of conductive filler or some mechanism based upon a phase change from crystalline to amorphous regions in the polymer matrix. A background discussion of a n~ber of proposed alternative mechanisms for the PTC phenomenon is found in "Glass Transition Temperature as a Guide to the Selection of Polymers Suitable for PTC Materials", J. Meyer, Polymer Engineering and Science, November, 1973, 13, No~ 6. In U.S.
;~ Patent 3,673,121 Meyer suggests that, based upon a phase change theory, to attain a steeply sloped PTC o~ resistance with a sharp cutoff (Type I) the polymex matrix should comprise a crystalline pol~mer having a narrow molecular weight distribution. Kawashima ~: et al, in U.S. Patent 3,591,526, disclose a PT~ moldin~ composition -` in which the conductive particles, such as carbon black, are first disparsed in a thermoplastic material, and thereafter this dispersed mixture is blended into a molding resin. Kawashima et al likewise suggest the desirability of an extremely steep ; temperature-resistance curve (that is, R = f (T) curve at a Ts of about 100- 130C.
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62'755 Because of their flexibility, comparatively low cost, and ease of installation, PTC strip heaters comprising conductive particles dispersed in a crystalline polymer have recently found wide use as pipe tracing heaters on industrial piping and in related applications. For example, such polymeric PTC
heaters, because of their self-regulating features, have been used for wrapping pipes in chemical plants to protect against freezing, or for maintaining a constant temperature which in turn permits aqueous or other solutions to flow through the pipes without ~'salting out".
In such applications, heaters ideally attain and are -` maintained at a temperature at which the energy lost through heat transfer to the surroundings equals that gained from the current. Such heaters ordinarily consist of relatively narrow and thin ribbon or strip of carbon filled polymeric material having electrodes (such as embedded copper wixes) at opposite :
edges along the long axis of the strip. Thus an electrical potential gradient along the plane of, and transverse to the long axis of, the strip has generally been contemplated, an applied voltage between the opposite electrodes r~sulting in h~ating of the entixe strip, usually to approximately its Ts.
` Obviously, from the preceding discussion it is apparent that Type I materials have significant advantages over the other types of PTC material enumerated hereinbefore in most applications. Types II and III have a disadvantage in that ~ because of the much less sharp transition the steady state ;~ tempexature of the heater is more dependent on the thermal load , ~- ... .

1~)6Z7S5 placed on it. Such compositions also suffer from a current inxush problem as described in greater detail hereinafter.
Types IV and V materials, because they lack a useful temperature range in which the power output changes from being independent of temperature to being dependent on temperature, - have not so far been considered as suitable materials for practical heaters under ordinary circumstances.
In such uses as have been described above and in others there exists a need for flexible strip heaters with much higher power ou~.put densities and/or higher operation temperatures than are contemplated by the prior art. It does not appear possible to operate heaters, particularly strip heaters fabricated from prior art compositions and according to prior art designs, at higher power outputs, i~e., higher wattage levels (above 1.5 watts/sq. in) and/or higher temperatures (above about 100C).
me actual wattage delivered by prior art heaters is far less than that which would be expected based on the heater area and heat transfer considerations apparently because the heat is produced in a very thin band down the longitudinal axis of the strip between the two electrodes. Such a phenomenon, is herein formed a hotline. Thi9 hotline results in an inadeguate and nonuniform heating performance and renders the entire heating device useless for most of the heating cycle in applications where high wattage outputs, especially at temperatures above 100~, are desired. More specifically, because the heat output is confined to a narrow band or line transverse to-a current pathr the high reslstance of thls line prevents the flow of ~ _ 9 '` :. . ,. ' :
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current across the path, in effect causing the entire heater to shut off until the temperature of the hotline drops below TS again, It has now been discovered that this hotline condition occurs in most if not all prior art design polymeric PTC
strip heaters where a voltage is applied, and the current flows, transversely across the strip, the extent of such condition being generally dependent upon the amount of applied voltage as well as the thermal conductivity of the polymer and the extent of non-uniform heat dissipation. The hot-line along the longitudinal axis of the strip, between the electrodes, effectively shuts down the heating device even though only a ; small portion of the surface area of the film, i.e. the hot line, has achieved Ts. This, in many cases, will destroy the heater or at the very least render it so inefficient that it appears to exhibit the very low heating capability found to be generally associated with the PTC polymeric strip heaters of the prior art.
` From the Coregoing discussion, it is apparent that the elimination of hotline is important for the efficient operation of a PTC self-regulating heater, especially one with a high power output and/or high operating temperature.
It would also be most advantageous if a PTC self-regulating heater could be fabricated wherein the heating surface was of .~ ` 25 d shape other than a relatively long, narrow strip e.g., a square or round~heating pad. Also desirable would be a PTC self-~.~
-~i regulating heater which could be fabricated into relatively , .. ' ' .

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complex three-dimensional configurations, e.g. one capable of making effective contact with essentially the entire outside surface of a chemical process vessel. Unfortunately, the tendency to hotline is particularly prevalent when the current path distance, i.e. the distance between electrodes, is large relative to the cross sectional area per unit length of PTC
material through which the current must flow~ For example, in the case o a heating strip with electrodes at the strip edges, a relatively wide short strip has a greater tendency to hotline than a narrow strip of the same length, composition and thickness. Likewise, for the same length and width, the thinner the strip the greater the tendency to hotline. Increas-ing the strip length while holding the width and thickness constant, ha~ no significant effect on hotlining tendency. The problem of hotlining has apparently not previously been pr~perly recognized, and certainly no suggestion for a composition or construction to reduce it has been proposed.
Polymeric PTC compositions have also been suggested for heat shrinkable articles. For e~2mple, Day in U.S. Patent 20 Office Defensive Publication T905,001 teaches the use of a PTC
heat shrinkable plastic film. However, the Day shrinkable Eilm suffers from the rather serious shortcoming that since Ts is no greater than the crystalline melting point of the film, very little recovery force can be generated. Buiting et al, 25 in U.S. Patent No. 3,413,442, suggest a heater constructions involving sandwiching a polymeric layer between silver electrodes.
A significant shortcoming of the Buiting et al con~truction is .

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its lac~ of flexibility. Additionally, neither Buiting et al nor any of the other previously discussed prior art teachings even addresses, much less solves, certain additional problems inherent in all prior art PTC heaters.
First, is the problem of current inrush. This problem is particularly severe when it is desired to provide a heater having a Ts in excess of about 100C. Many applications could advantageously utilize self-regulating heaters having a ~s of 200C or even more. Unfortunately, as heretofore indicated, previously proposed PTC heater constructions are essentially unsuitable -for such high Ts applications.
With materials having a TS substantially above 100C, the ; resistance of such material at or just below Ts may be as much as 10 times its resistance at ambient temperature. Since the PTC heater ordinarily functions at or slightly below its Ts, its effective heat output is determined by its resistance at slightly below Ts. ~herefore, a PTC heater drawing, for example, 15 amps at 200C could easily draw 150 amps at ambient temperature. Such a heater system would require a current carrying ca~acity vastly in excess o~ that required for steady .~, i' state operation or, alternatively, require the installation of `~ complex and generally fragile or expensive control circuitry to prevent the 150 amp initial current inrush from burning out the heater or lead wires thereto when the heater is first connected to an electrical source.
` ~ Referring to Figure 2 o~ the accompanying drawings, which is a graph of resistance v temperature, the preferred type of .

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' . , . ' ' ! ' , ' . . . ' , . ~ ~ ' . , . , . , , . . ' ~IL06~7S5 heater characteristic (line ABC) in its ideal form has a constant resistance ~denoted by the line AB) up to Ts and a resistance which increases extremely rapidly (denoted by the line BC) above the Ts. Thus, the operating range, say from its maximum rate to ~ O current drawn, is as shown by the dotted lines intersecting the resistance temperature curve at B and D. The power output of the ideal heater is unaffected by changes in temperature below Ts but changes over its whole range in a very small range of temperatures above Ts. Unfortunately, as hereinbefore described, very few, if any, PTC materials actually display this ideal characteristic. The nearest one can usually get with practical heaters is shown by the lines AB'C. If the maximum permissible power drawn from the electrical circuit is given by the resistance at A, then the operating range for s~lf limiting or "controlling" is given by the portion of the ;~ line B'C' lying between the dottecl lines. Obviously, the heater temperature, when operating under "controlling" conditions, varies much more in this latter instance and the available power range in the "controlled " region is less than that in the ; 20 ideal case. I~ a power range equal to that of the ideal case is desired, then a resistance characteristic such as A'B"C"
s necessary.
~ Referring again to Figure 2, curve AEF represents a portion - ~ of~the resistance characteristic of a PT material of type II.
If, as in the previous instance, the operating power range is - set by the dotted resistance lines, it can be readily appreciated ` that the temperature of the heater will vary o~er quite wide , ... . . : . . . - .: :
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6'~7~5 limits in operation depending on the thexmal load.
Although, as hereinabove mentioned, the prior art recog-nizes the considerable advantage of having a heater composition which possesses a resistance temperature characteristic of Type I, many of the compositions alluded to in the prior art show behaviour more closely resembling Type II, or ~ven Type III
- behaviour. The optimum (Type I) characteristic is shown only by a limited selection of compositions and there has been a long felt need for a means of modifying compositions showing Type II or III behaviour so that the behaviour becomeq or at least more closely approaches that of Type I.
An additional problem inherent in all prior art PTC strip heaters is that when it is desired to heat an irregularly shaped substrate the heater must be wrapped around the substrate, `
generally resulting in certain portions of the strip fully or partially overlapping other portions. This overlap can cause irregular heating.
It is thus apparent that while a variety of PTC compositions and constructions are well-known to the prior art, all such compositions and constructions and indeed, any apparent combination thereof,poss~ss~ serious shortcomings which severely limit the use of self-regulating PTC heating articles.
The present invention provides an article comprising at least one first electrically resistive layer and at least one ~ 25 second electrically resistive layer at least a part of a `~ surface of a first layer being contiguous with at least part of i a surface of a second layer and providing electrical and '' . ':' ' - 14 _ ~: ~

~;2~755 thermal contact between them, the first layer exhibiting a positive temperature coefficient (hereinafter PTC~ of resistance and having an anomaly temperature, above which it is substantially non-conducting, and the second layer ha~ ng a substantially constant resistance (hereinafter CW) at least below the anomaly temperature of the first layer.
The present invention also provides a self regulating heating article comprising a laminate and at least a pair of electrodes so positioned that, when there is a potential difference ~etween the electrodes, at ambient temperature current will pass between the electrodes through at least a portion of at least one first layer and of at least one second layer.
When, however, the temperature of the heater, or heating article, reaches the higher of (A) the temperature at which the resistance of the first layer exceeds that of the second layer (i.e., the resistances of their respective portions of current path between the electrodes) (B) the anomaly temperature of the first layer the predominant current flow between the electrodes will ~e along . , a line which minimizes the path length through the first ' layer.
The present invention also provides a self-regulating ` 25 heating article comprising a first layer of material which -e~hibits a positive temperature coefficient of resistance and having at least partially contiguous therewith a second layer ; ' .~

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~2 1~55 of constant wattage material, and said first layer being connectable to an electric power input source whereby current flow is through at least a portion of said first layer and through at least a portion of said second layer, whereby there is both direct electrical and thermal coupling batween said first and second layers and whereby, at the higher of the temperature at which the resistance o~ said first layer exceeds the resistance of said second layer or the anomaly temperature of said first layer current flow predominantly follows a path the length of which through the first layer is as short as possible.
- Preferably, the length of path through the PTC layer doesnot exceed its thickness (measured perpendicularly to the line between the electrodes) by more than 5~/0, preferably not by lS more than 20%.
Advantageously, the PTC layer has two substantially planar surfaces, which may be parallel, each of which is in at least paxtial contact with a surface of a CW layer.
In one series of embodiments the conductivity of the CW
layer or layers is so chosen that the material, while being sufficiently resistive to generate heat when connected to the - appropriate electrical source, is sufficiently conductive to act as electrode material also~ -Alternatively, the electrode may be a metal, which may be embedded in or in contact with a surface of either the PTC layer the CW layer, or in contact with a surface of either (i.e., at ~" a surface remote from the contiguous surface) or both, ' ~

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~(~62'755 at an interface between them. The electrode may be a fabric, braid, a grid (e.g., a series of parallel electrodes, or a mesh or network), and in the form of wire, strip or sheet. It may also be a fiber. Where the article is to be positioned over a conductive substrate, e.g. a metallic pipe, the substrate may itself form one electrode.
The article may comprise a plurality of electrodes intended for connexion to each of the terminals of the electrical power source, the plurality being referred to herein as a set.
The electrodes in a given set are preferably parallel and equi-spaced. The two sets may be positioned parallel to each other, or transverse, especially perpendicular, preferably lying in - parallel planes. Where the sets are parallel an electrode in one set may be positioned opposite an electrode in the other set, or it may be positioned opposite a space between two electrodes in the other set. The distance between adjacent electrodes in a given set, and that between the electrodes in one set and those in the other, together with the posikioning of the sets relative to the CW or the PTC layers, and the interface between them, may 20 all influence the performance of the heater, as described in more detail below.
The article may comprise a single electrode of one polarity, and a set of electrodes of the ot~er. Similarly, the CW
material may serve as at least one set, or may act as a single 25 ~ electrode for one polarity.
~ The article of the present invention may have any of a -` large number~of configurations, some of which are described and : :
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illustrated below. For example, it may comprise a laminate of two layers or sheets, one CW and the other PTC material, or a ; sandwich of a single layer of one of the materials between two layers of the other. The layer of one material may be completely S surrounded by the other, the PTC material may be in the form of a layer only immediately surrounding one or both of a pair of elonyate electrodes; or the PTC material may be in the form of a single layer surrounding the elongate electrodes and forming a web between them.
1~ In a further embodiment, the article has a generally rectangular cross-section, having a diagonal~ layer of one material, preferably the PTC material, one electrode being in - each of the remaining sub~tantially triangular regions. In a `~ material of similar overall cross-section, one triangular region may be of èach material. It will be appreciated that many configurations of one or more CW layers and one or more PTC
layers may be used, with the electrode positioning taking into account the requirements for appropriate current flow.
The article may be covered on one, or more, or all sides with an insulating layer. There may alternatively or also be ~` provided on at least one surface preferablyj a heat-activated, ~` ~ adhesive, or sealant. In some embodiments, the CW layer may serve this purpose.
..j1 AdvantageQusly, the first and second layers are polymeric materials having conductive particles, for example, carbon blac~, metal powders, or conductive fibers or fibrils dispensed therein. The CW layer may in a prefexred embodiment have the .'~ , .

2~55 fibers or fibrils as well as carbon particles. The layer may, on the other hand, comprise barium titanate.
~ dvantageously the article is heat recoverable. Preferably, the whole of the article is heat recoverable, i.e., all the layers are individually capable of returning to or towards a heat-stable configuration, but in some embodiments some layers may simply be passive, and allow the recovery of the article as a unit. Preferably the recovery temperature of the article is within the operating range of the article as a heater. The article may be laminated to heat recoverable articles~when preferably the article is itself heat recoverable.
The article may be of any of a number of configurations, advantageously being an elongate flexible strip with current passing, in operation, in a direction substantially transverse to the longitudinal axis, rather than along it.
Advantageously, the article has an effective Ts above 90C, which is greater than the inherent Ts of the first layer, this layer is advantageously a polymeric layer, preferably a cross-linked polymeric layer, and its crystalline melting point is less than the ef~ective T9.
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; ~t ambient temperatures, the resistivity of the first and second layers may be in the ratio 0.1:1.0 to 20.0:1Ø
The invention also provides a method of heatlng a substrate ~, which comprises positioning the article of the invention in thermal and, where necessary, electrical contact with it, and energizing the heating,element by connexion to an electrical `, power source.
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9L~)6~S~i The invention further provides a method of recovering an article according to the invention that is heat recoverable by connecting it to a power source for a time sufficient to course recovery.
The invention further provides a method of covering a substrate which comprises applying a heat recoverable article of the invention to the substrate, and causing recovery thereof, preferably by energizing the heating element thereof, and a substrate covered thereby.
The configuration and positional relationship of the PTC
and CW layers and the electrodes are subject to certain limitations, and the following requirements should be met:
1. At any temperature at least some of the current flow between electrodes of opposite polarity ~ through at least a portion of at least one PTC layer and also through at least a portion of at least one CW layer.
2. There is both electrical and thermal contact (and hence coupling) between PTC and CW layers. The electrical and thermal ~ -gradients may be parallel ~r non parallel to each other.
As hereinafter described in greater detail, certain articles constructed in accordance with the invention manifest an anomaly temperature higher than the intrinsic Ts of the PTC
layer itself. The Ts of the article is termed the effective Ts~
Advantageously, the thermal and ele~trical gradients in the PTC layer are predominantly along the same line or axis at or above the Ts of the PTC layer or the effective Ts if the latter ~ ~
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3. At or above Ts, or the effective Ts, if the latter is greater, the line of maximum current ~low is the line with the minimum path length through the PTC layer or layers, even though a longer path length through the CW layer or layers is occasioned thereby.
The configuration of the article will in certain instances preferably be such that the directionally shortest current path through the PTC layer does not dimensionally exceed the maximum thickness of the PTC layer in a planè perpendicular to the plane joining the electrodes and perpendicular to the current flow by more than about 5~/O, preferably by more than about 20%.
The term thickness as used herein is intended to connote the dimension between any two surfaces (interior and exterior) of the PTC layer which is the dimension of least measure. In most heater designs in accordance with the present inven-tion current flow through the PTC material at or above Ts will be predominantly perpendicular to the! interface between the PTC and CW layers.
; Among other advantages of the present invention, hotlining may be substantially reduced, or even eliminated, even at extremely high wattage outputs and/or operating temperatures, by providing current flow through the thickness of the PTC

., layer as opposed to along its length or width.

Other unexpected advantages of forming a laminate of the , PT~ material with at least one CW material, are tha the heaters may be used at outputs and for applications not only not ; contemplated but indeed virtually unattainable by previously proposed designs.

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6;2~55 q~e CW layer or layers, if sufficiently conductive, may be connected directly to a power source so as to function as and be considered to be an electrode. Alternatively, the CW
layer may ha~-e impregnated therein or thereover electrodes to conduct current therethrough. Such CW layer-electrode combina-tions differ critically from previously proposed electrode-PTC
sandwiches since with such prior art designs the electrode ; layers served only as conductors and not as additional resistive heating elements. In contradistinction, in the structures of the present invention, the CW layer, which is in direct contact - with the PTC layer, acts both as an electrode and also as an efficient heat output source.
In accordance with the present invention, thermoplastic polymer compositions having PTC characteristics can suitably be employed as a heating element that approaches more nearly Type I
; characteristics than does the PTC material per se, which would ordinarily manifest Type II, III, or IV characteristics. In ;, particular, virtually all of the previously proposed polymeric - PTC materials may be used a3 the P'rC layer in a heating element constructed according to the present invention. Additionally, the novel PTC materials described in Canadian application serial ~o. 236,456 entitled "Positive Temperature Coefficient of Resistance Compositions" filed September 26, 1975 by D. A. Horsma et al. are suitable for use in the present invention.
Suitable conductive fillers for the polymeric PTC composition useful in the present invention in add1tion to particular carbon ' , . . : . , ~ . . , , , ,., . . . ~ , ~ .~ - '- " . ' , . . , .. ' ' .~ ' ' ' - , . . ~.
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black include graphite, metal powders, conductive metal salts and oxides, and boron or phosphorus doped silicon or germanium.
Preferably the PTC material exhibits an increase in resistance of at least a factor of six for a temperature increase of 30 deg. C starting at Ts, or exhibits an increase of a factor OI SiX for a temperature increase of less than 30 deg. C
starting at Ts.
As mentioned herein, although prior art disclosures stress the practical advantages and importance of providing resistive compositions manifesting a type I resistance temperature ; characteristic, the number of such compositions available is relatively small notwithstanding the claims of the prior art.
Most of the hitherto disclosed compositions in fact possess type II and type III resistance characteristics. Thus a method of enabling a PTC material compositions having inherent type II
or III resistance characteristics to manifest more closely type I behaviour very greatly increases the number of compositions available for use in heating or otl~er resistive devices. Thus, lone can select a PTC material on tl~e basis of its Ts and /or ;`20 other desirable physical and/or chemical properties and by using :'.
,the present invention provide a heating article more clearl~
manifesting type I behaviour.
`~The electrical resistivity of most electrically conductive ;~materials, both PTC and non PTC, is found to increase or decrease -,25 more or less markedl~ with temperature. The magnitude of this ~ariation ranges from the less than +0.5% per deg. C characteristic of most metals to the t I to 5% or higher per deg. C. changes ','` ' ' :

: ,, : , , :: - , , ........ - . . , . . , - -:, -; . ,,: ., . ~ . : . :, :

exhibited by most conductive thermoplastic polymer compositions.
With most materials, however, the direction and magnitude of the change is such that when operated as an electric resistance heater the temperature attained by the heater is predominantly determined by the rate of:thermal conduction or radiation to its surrounding environment and not predominantly by the switching mechanism heretofore described for commercially useful PTC heater materials. Thus, the term CW material or CW
output material as used herein denotes a material whose resistance does not increase by more than a factor of six in any 30 deg. C
segment below the Ts of the PTC material it is in contact with.
; Preferably, the CW material has a~resistivity of at least 1 ohm/cm - at 25C. It should, of course, also be noted that when combined :
with a PTC material the CW layer or layers can yield a heater ~hich, below its Ts, will show changes in resistivity within the above indicated limits although such layer or layers comprise materials, which if their intrinsic resistivity is measured independently, will show resistivity changes outside these limits.
Additionally, since many PTC materials are constant wattag~
materials up to about their Ts, the term constant wattage as used herein encompasses materials which manifest PTC characteris-tlCS, provided, however, that they are used in conjunction with A PTC material having a lower Ts. Under these circumstances the PTC material of higher Ts will not reach its TS and hence in use will manifest only essentially constant wattage character-istics.
Constant wattage materials suitable for use in the instant invention are well known to the prior art. Suitable in this . ~
.

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. .. : ' ' . . ; .: ' ' : . '. , ' ' .; . . . .. . . .

~ 06Z755 respect are polymers, especially thermoplastic polymers, containing high loadings of conductive particulate materials, for example, carbon black or metals~ Where the thermoplastic material undergoes a large change in volume at its melting or softening point so as - to tend to decrease the number of conductive paths between the particles at or about that temperature and thereby cause its resistflnco to incres~se, ~uch incroascs ma~ bo avoided by mul~iplyine the number of alternative conductive paths, for ex~mple, by increasing the loading of conductive material and/or using a more s~ructured form of the conductive material. Structured as used herein connotes both the shape of the individual particles (for ., example spherical, lenticular or fibrillar) and the tendency of such particles agglomerate when incorporated into the polymeric matrix. Also suitable are essentially inorganicJ flexible constant wattage materials including carbon coat~ad asbestos paper as taught for example in Smith-Johannsen, U.S. Patent 2,952,761.
Of course~ in some applications it i9 not necessary for a high degre~ of flexibility to be present and resistive metal wire :.:
heaters supported by inorganic insulating materials may be utili~ed as the constan~ wattage layer. In such a case one end `~ wi~e o~ the resistive metal heater may be electrically connected . .
to the PTC layer via an electrode coplanar wlth the PTC layer ... .
~ surface but not necessarily coextensive with the PTC la~er In . . .
yet other applications a high degree of flexibility may only be advantageous or desired in the process of forming the ar~icle, for example, by vacuum or thermoforming. In such instances the PTC layer may be formed over a lay~r or sandwiched between ' ' :. '~
, : ~ : ' - ` ~ ' ,:

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. . .

. .: , , . ~ . . ~ . : , ., : . , ` ~06~:'7SS
layers of relatively rigid constant wattage material in the configuration of the desired article so as to maintain good thermal coupling between the layers, the current flow being either directly across the interfacing contiguous plane or by means of an intervening electrode on the surface of the PTC
; layer interleaved between the PTC layer and the constant wattage layer or layers. In these types of embodiments almost any type of constant wattage material contemplated by the prior art relating to electrical heaters may suitably be used.
In certain embodiments of this invention the constant wattage layer can serve as an electrode by being conductively connected directly to the electric powex source. If the constant wattage heating layer is not sufficiently conductive to act as an electrode, a metal or other highly conductive material electrode, for example, a metal grid may be embedded therein, such electrode being conductively connected to an outside power - source. In certain embodiments it may be advantageous to disperse in the constant wattage layer twhich may already contain a conductive filler) an additional quantity of highly conductive (preferably metal) ~iller in the form of fi~ers of fibrils. This embodiment is particularly advantageous when the electrodes are not co-extensive with the whole planar sur~ace of the constant wattage layer but are contiguous either with ; ~ said surface or with the interface between the constant wattage and PTC layer or are embedded in said constant wattage layer.
It should be noted that the structure constructed in . ~ .
~ ~ accordance with the invention may have any of ai`wide ~ariety `~ of electrode conf~gurations, types, positioning, and materials.
. ~ .

' :
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For example, metal fabric mesh or grid, flexible metal strip, convoluted wires, conductive paint, solid carbon e.g., carbon fibers, graphite impregnated fiber, metal coated fiber e.g., copper or stainless steel, solid metal conductor of various geometries and other electrodes as known in the art are all suitable. An electrode, whether connected to the constant wattage layer or to the PTC layer or both, can be fully or partially coplanax with the outer surface thereof. By outer surface of the PTC layer is meant a surface thereof not conti-guous with a constant wattage layer and, conversely, for the constant wattage layer, the outer surface thereof is a surface not contiguous with the PTC layer. Alternatively, the electrode may be embedded in the PTC or in a constant wattage layer. Yet ; another construction involves one electrode being embedded in or on the outer surface of the PTC layer and the other electrode being located at the interface between the PTC and constant wattage layers. Of course, if des:ired a plurality of electrodes which are shunt connected for each polarity can be utilized with the same variety of placement~3 being suitable.
As herein above indicated, w~th prior art PTC compositions `~ and~-~also with the novel PTC compositions of the above-mentioned ` Application No. 236456 certain embodiments of the present invention significantly affect the operating characteristics of ., a heater utilizing the PTC composition. More particularly, when ` 25 embedded or abutting electrodes whose surface is not co-extensive with that of the CW or PTC layer are used, the placement of the .~ .

' . .

-~z~ss electrodes having an opposite polarity with respect to each other can signiEicantly modify the operating characteristics of the apparatus. Thus, if strip electrodes of opposite polarity, coplanar but not coextensive with, the outer sur~aces of the CW and PTC layers, are placed directly opposite and parallel with each other different operating characteristics are obtained from those which result when the electrodes are parellel but laterally displaced with respect to one another or when the perpendicular projections of the electrodes on one another intersect. Although the invention is not to be limited by any particular theoretical interpretation it is believed that electrode placement has an effect on the favored current paths at different temperatures. Thus for the case of electrodes directly opposite to one another current flow is predominantly ; 15 normal to the plane of the PTC layer. However, if electrodes are displaced in some manner from this arrangement and the resistance for the CW layer is initially (i.e., at lower temperatures) greater than that of the PTC layer, the predomin-; a~t conduction path at lower temperatures may be normal to the plane of and through the thickness of the C~ layer and diagon-ally through the thickness of the PTC layer. At some higher temperature, ~here the resistances of the CW and PTC layers ` b~come equal, conduction occurs predominantly diagonally through the thickness of both layers while at yet highar temperatures the prefarred conductive path may be normal to the plane of and through the thickness of the PTC layer but diagonally `
` through the thickness of the CW layer.
'-'. ' '`

.. . . ." . . , ~. . ~ , - . -~ ~0~ 7~5 In general, opposing the electrodes will yield an apparatus having a resistance-temperature curve similar but not identical to that obtained by having electrodes contiguous with the ~hole surface of each outer layex. As the lateral and/or angular displacement of the electrodes from an opposing parallel configuration is increased the electrical characteris-tics tend to deviate more from that expected for a simple series connection, as described in greater detail in the examples.
More specifically, where pairs of electrodes are opposed (i.e., their centres are on a line perpendicular to the inter-face between PTC and CW layers, and the current path is perpen-dicular through the PTC and CW layers, the effective Ts will be that characteristic of the particular combination of layered materials. However, if one electrode (or the electrodes of one polarity) is shifted in the plane i.e., parallel to the inter-; face of the layers such that the current path is diagonal, the effective Ts is increased. Generally, the more diagonal (the ` more displaced from transverse to the interface) the current path between electrodes, the higher the effective Ts. Indeed, where the resistance of the CW exceeds that of that PTC layerat the latter~'s intrinsic Ts and where such electrode placement i9 utilized ~he effective Ts may b2 substantially above the crystalline melting point on the PTC material. Thus, regardless of the relative positions of opposed electrodes as the resistivity of the constant wattage layer relative to that of the PTC layer is raised, the èffective TS also tends to increase~
~ he electrodes may have a vaxiety of shapes for e~ample, their cross~ections may be square, rectangular, or circular, ~' ~ - 29 -.'~ ' -:
.~ .

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'. . ' '.
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3L~627S5 they may be rectilinear, planar or curved strips, spiral (with the pitch of the spiral for each electrode being the same or different) or rectilinear spiral and, as hereinbefore mentioned, the electrodes may be directly opposite or laterally or otherwise displaced with respect to one another and ei~her or both electrodes may be monolithic or multiple in nature. It is thus apparent that the heat output and TS characteristics of the article of the instant invention can be varied by an appropriate choice of electrode shape and/or position, that selected being dependent upon the use to which the structure is to be put and a suitable arrangement being ascertainable by routine experimen-tation.
Although in most embodiments the PTC layer and the CW
layer or layers will be fully contiguous (i.e., the whole of one face of one layer will be in contact with the whole of the corresponding face of the other) in some circumstances it is advantageous for the PTC and CW 12lyers not to be fully contiguous over the entire respective opposing surfaces. Particularly where high Joule outputs at high t:emperature are desired, it ` 20 is advantageous to generate the major portion of the heat ; output in the constant wattage layer. In many such instances the PTC layer will preferably be contiguous with onl~y a portion . ~
of the opposing surface of the CW layer. Such arrangements `~ tend to reduce the effective Ts. When the PTC layer is contiguous with only a part of the surface of the constant ` wattage,layer, said PTC lay can experience wide variations in ., .
power generation~ Therefore, good thermal coupling and balancing of the relative power-levels is desirable.
' ~; ~ 30 -:' :`

: :: ,: : : : :

:~6~75~;
The articles provided by the present invention have utility in a wide variety of applications. For example, they may be used as heaters for causing heat recoverable articles to recover on to a substrate whether ~`y being an integral part of the heat recoverable article or by being placed in substan-tially abutting heat transferring relation thereto. In applications where heat activation of an adhesive is required, the high temperatures and high outputs attainable by heaters constructed acaording to the present invention render them particularly desirable. The articles are also useful where uniform heating of a substantial area is required as, for example, in heated ducts for fluid flow or as enclosure walls or panels as in ovens, residences or transportation vehicles. Other uses include heaters for industrial process pipes and vessels requiring uniform heating and/or temperature control, and ; de-icing heaters on roads and airc!raft wings. The laminar form and uniform heating characteristics of many of these articles render them particularly useful ae~ heaters for waterbeds, warming tray~ and bowls and medicall heating pads while their capacity for high wattage output at high temper~tures in addition renders them particularly attracti~e as heaters for cooking appliances such as griddles and frying pans.
~ Most PTC materials comprise a crystalline thermoplastic ; ; ~ matrix having a conductive, usually particulate, filler dispersed ; therein. For example, the previously mentioned Kohler, U.S.
Patent 3,243,753 discloses a polyethylene or polypropylene ` carbon black composition, in which the polyolefin has been :''' ~,.

. .
.' ~

.

p~lymerized 1n situ, such materials exhibiting the PTC anomaly temperature close to the melt temperature of the polymers, i.e., about 110C - 120 C. Likewise, Kohler et al, in U.S.
Patent 3,351,882, disclose carbon particles dispersed in polyethylene in which the composition may be crosslinked, or may contain ~hermosetting resins to add strength or regidity to the system. However, the Ts temperature still remains just below the crystalline melting point of the thermoplastic polyolefin. Hummel et al, in U.S. Patent 3,412,358, disclose a PTC polymeric material comprising carbon black or other conductive part~cles previously dispersed in an insulating material, the homogeneous mixture in turn being dispersed in a thermoplastic resin binder. The PTC characteristics are apparently achieved by the interaction of ~he carbon black and - 15 the insulating material and it i9 suggested by Hummel et al that the insulating material must have a specific electrical re~istance and a coefficient of thermal expansion higher than ~- that of the conductive particle.
U.S. Patent 3,823,217 to Kanlpe discloses a wide range of conductive particle ~illed crystalline polymers which exhibit ~3 ~; PTC characteristics. These polymers include polyolefins, for ` example, low, medium and high density polyethylenes and poly-propylenes, poly(butene-l), poly(dodecamethylene pyromellitimide~, ethylenepropylene copolymers and terpolymers w~th-non-conjugated dienec, poly~vinylidene fluoride) and vinylidene fluoride-tetra-fluoxethylene copolymers. It is also suggested that blends of .:
~ polymers containing carbon black can suitably be employed, for ~.
example polyethylene with an ethylene-ethyl acrylate copolymer.
' ` 32 -'' '' .. . .: : . . ... ... . . . .
,, . , .. , ,. , . .,. . , , ~ , . . . .

~L~627~
Kampe achieves lower resistance levels by cycling his products above and below the melt temperature of the polymers. Changes in resistance caused by the thermal history of ~he sample have also been found to be minimized by thermal cycling. U.S.
Patent 3,793,716 to Smith-Johannsen discloses conductive pol~mer compositions exhibiting PTC characteristics in which a crystalline polymer having carbon black dispersed therein is dissolved in a - suitable solvent and the solution impregnated into a substrate followed by evaporation of the solvent yielding articles having decreased room temperature resistivities for a given level of conductive filler. However, Ts ~till occurs just below the crystalline melting point of the polymer. Similarly, Kawashima et al, in U.S. Patent 3,591,526, disclose carbon black cantaining polymer blends exhibiting PTC characteristics with the Ts temperature occurring at about the crystalline melting point of a thermoplastic material added to a second material for the purpose of molding the mixture.
A particularly unexpected feature of the present invention is that when compositions of the type described in the prior art as being useful for PTC or for CW heaters are used in multi-layer heaters designed in accordance with certain embodiments of the present inv0ntion they manifest resistance/temperature characteristics which would in no way be expected from a ^ consideration of the resistance/temperature characteristics of the individual layers or indeed that expected to result when such layers are connected together in series to form an electri-cal circuit. Fabrication of a multilayer heater in accordance :` :

: .
:

~ ~6~7~
with the teaching of the present invention utilizing layers having appropriately chosen specific resistivities may substantially alter the Ts of the article containing the PTC
layer to a temperature at or in excess of the melting or softening point of the polymeric constituent of the PTC l~yer.
Thus, though the prior art suggests that Ts is independent of the geometrical configuration of the heater, it has most unexpectedly been discovered that certain of the geometrical arrangements contemplated herein can result in substantial increases in Ts even to above the polymer melting point, thus greatly increasing the utility and versatility of both previously proposed and other compositions.
In a preferred embodiment, a layered article of this invention comprises a middle layer of condùctive polymeric PTC
material interleaved or sandwiched between two CW layers. The CW layers may have embedded therein or deposited thereover electrodes (ordina~ily metal) such that upon application of a voltage across the electrodes the current will flow through the ~-`` PTC layer and thereby cause heating of both the PTC layer and the CW output layers.
` In another preferred embodiment, the heating element may .'' , .
be bonded to a heat recoverable material or be itself rendered heat recoverable, to provide a heat recoverable article which ~` can be made to recover by means of internally generated as . ~
~`; 25 ~opposed to externaLly applied heat. Such an article thus . . .
~ advantageously avoids the requirement of an outside heating .:
source to effect recovery, requiring only attachment to an -~ electrical power source.
.

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~L~6275S

In a particularly preferred embodiment of the present invention of great utility to the manufacturer of heat recoverable devices, PTC compositions disclosed in the above-mentioned Application No. 236456 are used. Such compositions comprise blends of thermoplastic and elastomeric materials having conductive materials dispersed therein. As pointed out in ~he above mentioned application, such blends exhibit a steep rise in resistance at about the melting point of the thermo-plastic component, the resistance continuing to rise with temperature thereafter. Because of the increased safety margin given by the further increases of resistance above the melting point such heaters can be designed to control at temperature above Ts and at resistances well in excess of that at Ts but yet avoid the risk of thermal runaway and/or burn out which occurs when prior art PTC compositions are uqed in such designs.
Such preferred heaters, especially when the increase in resistance with temperature above Ts is very steep, are very demand ; insensitive, that i9, the operating temperature of the PTC
material varies very little with thermal lo~d. They can also be designed to generate very high powers up to Ts when electr~cally connected to a power source. Because of their excellent temperature control, they can be employed to activate adhesives ; and cause heat recoverable devices to recover around substrates e.g., thermoplastic telephone cable jackets without fear of melting or deforming the substrate even if left connected for - considerable periods of time.

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~ii27~;5 In this preferred embodiment, a heater PTC core is combined with a constant wattage outer layer whose thermo-plastic polymer ingredients, if any, have a melting point not greater than that of the thermoplastic polymer component of the PTC composition. The constant wattage layer, if comprising thermoplastic polymers, can be made heat recoverable and/or optionally but preferably an additional member comprising a layer of a heat recoverable polymer composition having a recovery temperature less than the melting point of the thermoplastic component of the PTC composition is also provided.
An additional layer of a hot melt adhesive or mastic may also be provided, the hot melt, if used, having a melting point similar to that of the heat recoverable member and an activation -~ temperature less than the melting point of the thermoplastic component of the PTC composition. The electrodes are advantage-ously formed from flattened braided wires which are produced by extruding a braid over a thermoplastic cora, and flattening the product while the thermoplast:ic is soft. Such an embodiment has been found to be particularly advantageous as hereinabove mentioned, where the substrate is heat sensitive i.e., if warmed above its melting point wi:ll deform or flow. Such applications include telephone splice cases and many other applications in the communication industry.
~ The invention will now be described in greater~detail,- -~
by way of e~ample onIy, with reference to the accompanying drawings, in which:
.
:' '', ;' ~

' .

. .

. .

Figures 1 and 2 w~ich have already been discussed, illustrate the resistance t.emperature characteristics of various PTC materials, Figures 3 to 5 are perspectives of prior art structures utilizing PTC compositions' Figures 6 to 12, 13b and 15 to 34 are perspectives of or serve to ullustrate and explain various articles constructed . in accordance with the invention' Figure 13a is a cross section of the embodiment shown in Figure 13b, while Figure 14 is a cross section of the embodiment . -shown in Figure 15, Figure 35 illustrates an embodiment in which in effect .~ point electrodes are provided at intervals along the length of the article, Figures 36 and 37 illustrates the power-temperature relationship for products described in certain of the examples.
Referring now more especial:Ly to Figures 3 to 5, there ,:~
are shown various prior art structures utilizing PTC composi-tions. Figure 3 shows a strip heater similar to that disclosed ; 20 by Buiting et al in U.S. Patent '3,413,442, wherein thin sheets - , of sil~er, 1 and 3, are positioned on each side of a PTC
`material 2. This is not in accordance with the present invention, ~i even though a laminated configuration is disclosed, since -`~ material contiguous with a PTC layer is so condùctive that : ~ .
:~ 25 lt will not itself act as a heater. :~
~ ~ Fiyure 4 depicts a strip heater according to Kohler, U.S.
`~ Patent 3,243,753, wherein a PTC material 6 has on each edge :? ~ 37 -:~ :
. - .
.

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` ~6Z~i;S
thereof conductive grid electrodes 5 and 7.
Figure 5 represents a previously proposed strip heater, in which a PTC material 10 having a cross section in the shape o~ a dumbbell has conductive wire electrodes 8, 9, positioned along its length.
Turning now to configurations constructed in accordance with the invention, Figure 6 depicts a PTC layer 11 having contiguous, or partially contiguous, therewith a CW heating layer 12. Overlying the surface o~ the constant wattage layer -is a grid electrode 13 while the second grid electrode 14 is contiguous with the surface of the PTC layer remote from constant wattage layer 12.
In Fig~re 7, a plurality of strip electrodes 16, connected in parallel, embedded in a CW layer 15. The opposite electrode 18 is a continuous sheet, applied to the remote surface of the PTC material 17.
Figure 8 depicts a further variation in which electrodes 20 and 22 are strip electrodes (electrodes 20 being paralle~, or shunt, connected, electrodes 22 likewise), the electrodes 20 20 being sandwiched between a PTC layer 21 and a CW layer 19.
In this ~onfiguration a low resi~tance CW layer is de~irable bècause the gradient potential along the interface between layers 21 and 19 is reduced.
Figure 9 depicts a configuration similar to Figure 6, with ~ 25 a grid electrode 23 overlying the CW layer 2~ which in turn `~ is contiguous with a PTC layer 25. Howe~er, the other electrode is grid electrode 26, sandwiched within the PTC layer.

"; :, : . : : :: , , : : , :: . :: : . . - , :,,: .

Turning to Figure 10, a CW layer 27 has embedded therein a first set of electrodes 28, while a PTC layer 29 has embedded therein a second set of electrodes 30.
It will be understood that the various embodiments depicted in Figures 6 to 10 may be utilized in accordance with this invention, in any combination. More specifically, grid electrodes, as shown in Figures 6 and 9, film electrodes as shown in Figure 7 or strip electrodes as shown in Figure 8 may be utilized in any of the embodiments, and combination of two or more different type electrodes may be utilized in a given configuration. A first electrode may be positioned over the CW layer, embedded in the CW layer or be positioned between the CW layer and the PTC layer. A second electrode may be positioned on the opposite side of the PTC layers over, within or between a second CW layer or beneath or embedded in the PTC layer.
Figure 11 shows strip electrodes 32 and 34 embedded in two CW layers 31 and 35, the electrode CW layers sandwiching ` a PTC layer 33 therebetween. Of course, as previously discussed, ; 20 the electrode may have a grid, film or other construction.
.: ~ Figure 12 represents a particular embodiment of the present invention which has been found useful for increa~ing Ts. ~s previously discussed, by staggering the elect~odes, so that the . . ~ ..
current path has a component across the layers as opposed to being perpendicular through, then the effective T~ may be `~ increased. Thus, in Figure 12 strip electrodes 37 are staggered ~ between the geometrical perpendicular projections of strip - electrodes 39, the sets of electrodei 37 and 39 being embedded ~ _ 39 _ ,~; . -i i ~0~i2~55 in CW layers 36 and 40, a PTC layer 38 being sandwiched therebetween.
Figures 13a and 13b are a cross section and perspective view of a preferred embodiment. A plurality of wire electrodes, 42 shunt connected, is embedded within a CW layer 41 and similarly a plurality 45 in the layer 44, a PTC layer 43 being sandwiched between the layers 41 and 44. Wires 42 are preferably substantially all in one direction, with wires 45 being in a second direction substantially perpendicular to that of the first. Further, the overall layer configuration may take the form of a disk, such form being particularly well suited for a number of heating applications.
Turning to Figures 14 and 15, a layered configuration particularly suited for the making of heat recoverable :i encapsulating articles, as described fully in Canadian application serial No. 236,482 entitled "Heat Recoverable Self-Heating Sealing Article and Method of Sealing a Splice Therefrom", filed 26 September, 1975 by D. A. Horsma et al is shown. For this purpose, the layers are generally of a flexible, polymeric material, with any or all of the layer9 being rendered heat recoverable. For a more detailed description of heat recoverable articles, and their applications, see the herein-above referred to application No. 236,482. If the article is to be used for sealing an electrical splice, utilizing the ~- 25 layer composite of this invention, an outer layer 46 is . ~ , . .
- provided, which may be insulating material, which may or may not be heat recoverable. Next in the laminate is a CW
material having embedded therein electrodes 48 which may be of a braided, serrated, or convoluted configuration, and which , .

, .

~62755 axe shunt connected to a power source. A PTC material layer 49 follows, with a second set of electrodes 51 embedded within a second CW layer 50. A second insulating material layer 53, which may be heat recoverable, is placed adjacent the heating layers, and on the face surface of this layer 53 is an adhesive layer 54, which is heat activated by the heating element of this invention.
Referring now to Figures 16 to 34, the electrodes, of whatever form, are denoted by reference numerals 55 and 56, CW layers are denoted by 57 and 58, PTC layers by 59 and 60 and a conductive substrate, e.g., a pipe by 61.
Figure 16 represents an embodiment in which the dimensions (for example thickness) of a particular layer and, as a result, the relative thicknesses, of the CW and PTC layers, are locally varied to alter the power output density and/or the effective Ts. Figure 17 represents an embodiment in which the PTC and/or CW layer have different compositions in different areas to alter the watt density and/or effective Ts.
; Figure 18 is a cross section of an embodiment in which ;~ 20 the substrate, for example, a metal pipe, is part of the electrical circuit, that is, it forms one of the electrodes.
Figure 19 represents an embodiment where the individual layers ~ - :
are wrapped consecutively around an object that is required to be heated ~o as to form a layered heater in situ. The layers can be caused to adhere together by heating either externally or by passage of an electric current or the layers can be formed . .: .
` from materials which adhere together at the temperature at which ,.`' :' ~ - 41 -,. .

~ :`
, . . .

~(~6~55 the article is applied. This is an example of an embodiment in which it may be especially useful to have the substrate form part of the electrical circuit. Figures 20 to 26 show another group of embodiments. In a modification (not shown) of the construction shown in Figure 20, the electrode 56 may also have a coaxial layer of PTC material as shown for the electrode 55. The constructions shown in Figures 23 to 25 are examples of heaters in which conduction below the effective s (depending on the relative resistivities of the PTC and CW layers) may be predominantly across the PTC material between the electrodes. However, when the PTC layer heats up to a temperature above its Ts conduction then occurs predominantly or almost entirely from one electrode through the thickness of the PTC layer by way of the shortest possible path from that electrode to the constant wattage layer then,through the constant wattage layer to the other electrode (again through the minimum thickness of any PTC material which may be inter-' vening).
It will be appreciated that the "predominant" current flow referred to herein relates to the path along which thegreatest current Iflux' exists. Although theoretically this ~, path will not always be exactly the shortest path in the PTC
~',' layer, because e~en at or abo~e Ts there will be some portion of the current that is carried by the rest of the PTC material, this portion may be ignored for practical purposes e.g., in a ~' configuration such that as in Figure 24, as shown in the drawing, ,~ current will for practical purposes predominantly flow perpendi-cularly upward and downward through the PTC layer 59, and along

- 4~ -~

.. , .. , ... , . .. . . , . . - .. - .. - ~ . - - - - . .

` ~iL1~6;27S~
the layers 57 and 58, although there must be a very slight component toward the other electrode in the path of the pre-dominant current flow in the PTC layer. This is small enough to be ignored for practical purposes.
In a variation of the construction illustrated in Figure 25, the layer 59 may be omitted, and the electrode 56 positioned in contact with the layers 57 and 58, spaced apart from the electrode 55.
- Figures 26 and ~7 illustrate embodiments in which the PTClayer is contiguous with only a part o~ the CW layer. We have ~ound that as the fraction of the total CW surface area in contact with the PTC surface area is reduced the ambient ;` temperature at which for a given applied voltage the haater limits its power output is also reduced.
Figure 28 shows another variant of the embodiment shown in Figuire 21, in a variant of Figure 28, there may be a single CW layer 57 which is positioned where the layer 59 isi illustrated ,~ and a pair of PTC layers 59, 60 wl~ich replace the illustrated CW layers 57, 58.
~` 20 Figures 29 and 30 show further variants of the basic layered heater having the same general form and manner of function as Figures 23 to 25.
, .
j~i - Figures 31 and 32 illustrate other forms of the embodiment .:. : : . .
shown in Figure 12 wherein the effective Ts of the heater may be advantageously different from that of the PTC material alone ~- ~ as hereinbefore described.

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Figures 33 and 34 show how useful layered heaters can be formed by combining extrusion coated wireswherein the coatings have PTC or CW characteristlcs.
Referring now to Figure 35, there is shown an additional article constructed in accordance with the invention wherein conductors 55 and 56, which in operation are of different polarity, have a concentric layer of insulation 62 around them.
Referen~e numeral 59 represents the PTC material and 57 the CW
material. The layer 62 is discontinuous over the surface of the conductor in that as shown in an article of substantially linear elongate configuration, segments of the insulation are ; removed intermittently along the length of the conductors.
As can be seen, where the insulation has been removed the conductor is in direct conductive contact with the CW material.
`15 Such areas of contact for each electrode are not opposite each other but in fact diagonally opposed along the long axis of the article. The advantage of the present embodiment is that of necessity current flow between the electrodes of opposite polarity is not merely across the width of the article, i.e., ~20 dis~ance X but in ~act curri~nt must flow distance Y so that the ;current path is down a portion of the length of the article. A
long current path is desirable in that it enables one to utilize a CW material of low resistivity (enabling higher voltages to - be used) without manifesting a tendency to burn out. Of course, ~"25 alternative configurations which ensure that the current flow ,. .
is at least partially down the length of the article are readily : .
`constructed. For example, in a construction in which a PTC
.
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. ' ~ , ' ' , ' .' ,, '. '' .. . , .' ' . ~ .1 . , , ,' ;, , .:: ' ...... 1 . ' .. . ' ~;, ' , ' : ,: . ., ' . ' ' ' : , ` ' ' ' , . '` ' " ' ' : ' ' ~CI 6;~75~i layer is sandwiched between two CW layers with strip electrodes disposed on the outer surface of the CW layers, an intermittent insulating layer may be positioned between each constant wattage layer and the electrode disposed on the surface thereof. Or, where a continuous insulating layer is disposed on the outer surface, the electrodes may alternatively pass through the insulating layer and contact the CW layer.
The following examples illustrate the invention: Articles constructed in accordance with the invention may be made in a variety of ways known per se. For polymer heaters the individual layers may be extruded separately and thereafter laminated, bonded or otherwise affixed together, the electrodes being inserted during extrusion or lamination as desired. The layers may otherwise be made by calendering or coextrusion, the electrodes, as previously indicated, being inserted at any suitable stage in the operation. A preferred method of fabricating a particular embodimeTlt of a heater in accordance with the present invention is de~3cribed in the hereinabove mentioned application ~o. 236,482.
Methods of construction of nonpolymeric conductive compositions suitable for utilization in t~e present invention, for e~ample ceramics or carbon loaded asbestos paper, are well known in the art. The layers may be affixed to other layer~3 by bonding, welding, gluing and other well known processes which preserve or maintain conductive contact between the layers.

: ' 1 ' :

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- . , .,, , , : : ,: . . .

~6~7S5i Example 1 ~ laminate was constructed as generally shown in Fig. 14 having a PTC layer as described in Example 5, Blend 2 and a constant wattage layer as descri~ed in Example 3, with the insulating layer comprising a blend of polyethylene and a low structure, low conductivity black. The adhesive layer was a hot melt adhesive with a ring and ball softening temperature of 110C. The laminate was irradiated to effect cross-linking prior to coating with the adhesive, hot stretched perpendicular to the convoluted wire electrodes and cooled. The expanded sheet was wrapped around a polyethylene jacketed telephone cable and the opposing ends held together. On connecting the electrode wires to a 12 volt lead-acid battery, the laminate shrank smoothly and uniformly onto the telephone cable.

Example 2 A 2.5 x 15.2 x 0.05 cm strip to which was attached on , ~
opposite edges along its length copper electrodes and having the composition 7~O of a medium density polyethylene, 18%
ethylene/eth~l acrylate copolymer and l~/o XC72 carbon black from Cabot Corp. was anne~led at 150C in vacuum for 16 hours and then irradiated to a dose of 20 Mrads and coated with a temperature indicating paint (Templace* 76C indicating paint).

;

~ The electrodes were connected to a 100 volt A.C. supply.
, `j Within less than a minute the white paint had melted in a thin region approximately one tenth of an inch wide and roughly equi-distant between the electrodes, a "hotline". T~e qurface temper~-ture in t~e middle of the hotline~was estimated to be close to ` 85C which is just above ~s for this particular composition.
. .
*"Templace" is a trade mark.

,; - 46 -,~ 6;~P7~5iS

Regions only 0.5 cm away from the hotline were below 50C.
In this condition the element was generating substantially all its power from the hotline area. In a similar experiment in which the element was insulated, placed in water and connected to a power source a similar "hotline" was noted. Then the composition of this example was fabricated into a laminated . core sandwiched between CW layers o-f carbon black filled silicone rubber, each CW layer carrying a 20 AWG (about 0.081 cm : diameter) multi strand copper bus in its center. The element heated smoothly to a uniform surface temperature of about 65C
` in air, the core temperature being about 80C. Thus, layering of the PTC layer between constant wattage layers e7iminated the hotline for this PTC composition.
, .

Example 3 ':~

A series of laminated heater~ was constructed usin~ a constant wattage layer consisting of ethylene-propylene rubber, 35 parts, ethylene-~inyl acetate copolymer, 30 parts and carbon ~:

.~. black, 35 parts and a PTC core composition as described in Table I below in which the carbon black was dispersed in the polypropylene befo~e the TPR 1900 rub~er was blended in.
. ' . , .
ABLE

. Sample No. 1 2 3 ~ 5 6 ~: ~PR 1900 (thermoplastic 72.5 70.0 68.7567.5 66.25 65.0 . ethylene-propylene . rubber *rorn Uniroyal : :Corporation) `~' Profax* 6524 (polypropy- 16.5 18.0 18.7519.5 20.25 21.0 -:
. : lene from Hercules i Corporation) 25 XC72 (carbon black 11.0 12.0 12.5 13.0 13.5 14.~ :
from Cabot Corp.) .. . .
. *"Profax" is a trade mark~ :-.,'.' .
47 ~

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. .
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The CW and PTC materials were hydraulically pressed at 200C
into 15.2 x 15.2 x 0.05 cm slabs for one minute and the heater constructions comprising a PTC layer sandwiched between two CW layers laminated at 200C for two minutes and then annealed at 200C for 10 minutes and irradiated. Heater segments 2.5 x 3.75 cm were cut from each specimen and 2.5 x 0.635 cm electrodes of conductive silver paint were painted adjacent to diagonally opposite 2.5 edges of the CW layers, one electrode to each CW layer, resulting in-a heater construction similar to that of Figure 12. The effect of varying composition on the inrush/operating current ratio and self regulating tempera-' ture can be seen from the inrush ratio and Ts in Table II below:

TABLE II

Carbon black Room Temp.
. . 1 1 resistance Inrush T
15 Composltlon core % of laminate Ratio* ( C)**

:
PTC Core Alonë 12.5 8 85 1 11 21,000 8 90 3 12.5 245 4.4 125 ~ 13 230 3.9 165 .~ .
13.5 220 3.7 185 *~efined as the ratio of resistance at Ts to resistance at room ~` temperature.

-~ ~ 25 **Melting point of P~C approximately 165 C.

- As is appa~ent, minor alteration of compositioni of the PTC

.
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.. . : , . , , , . , .. .. .. . . i ... -.~ :- .

i62755 material with the CW material being held constant can signi-ficantly alter the TS and the inrush ratio when used in heater constructed according to the invention. Specifically, Ts can be varied to above the melting point of the PTC. Furthermore when a PTC material having Ts of 85C and containing 12.5%
of carbon black was sandwiched between CW layers, the effective Ts was raised to 125C, the resistance temperature characteristic of the latter as shown by the inrush ratio being much closer to Type I behaviour (which by definition has an inrush ratio of 1).
Example 4 A 0.063 cm thick slab of PTC material having the composition ` described in Example 2 was laminated between two 0.063 cm thick CW layers having the composition of the CW layers of Example 3.
15 The laminate was annealed at 150C for 16 hours and then irradiated to a dose of about 10 megrads. A 2.5 cm square piece cut from the laminate and paintecl with conductive silver paint over the entire outer surfaces of the CW layers, i.e. of similar basic construction to Figure 11, was found to have a Ts of 70C.
20 A similar sample in which two 2.5 x 0.63 cm strip electrodes were affixed to diagonally opposite planar surfaces of the constant w~ttage layer (one to each layer) ~i.e~ similar to Eigure 12) was found to have a Ts in excess of 90C. It is thus apparent that electrode placement can significantly alter the Ts of constructions in accordance with the present invention.
Exam~le 5 PTC compositions having the formulation and characteristics ` shown in Table III were prepared ~y mill blending, then .~.
. ~ .
~ 4 .

. .:: . . . , : ~ , ij;, -. . .. : . , ,: ~. ~ . . .

- ~62~5S
hydraulically pressed into slabs of 0.025 cm thickness and irradiated to effect cross-linking. Layered heaters were constructed by sandwiching the PTC slab between two CW layers of resistivity 7 ohm-cm prepared from a conductive silicone rubber (R1515) either 0.025 or 0.10 cm thick.
TABLE III

Sample Marlex* 6003 Sterling* SRF~S dose Resistance of 0.025 cm film Nos. % % Mrads.ohm-cm .

5-1 58 42 12 1.5 Electrodes of 2.5 x 0.63 cm size wera applied to the ~` outer surface of heater segments as in Example 4. The heater was then placed on and in good thermal contact with a stainless ; steel block equipped with a thermometer mounted on a temperature ; 15 controlled hot plate whereby the temperature of the block could ~ be varied. The heater was connected to a voltage source of -,i such a magnitude that it generated about 0.31 watts/sq. cm at about ambient room temperature. The power output of the heater was monitored as the temperature of the metal block was raised.
`~ 2Q For results see Figure 36.
Figure 37 shows how the power/temperature curve of a . i heater constructed from a 0.025 cm layer of composition 5-2 with an unirradiated 0.025 cm layer of constant wattage silicone varies with the electrode configuration. Unirradiated silicone constant wattage layers were chosen because their resistance :. .
*UMarlex" and "sterling" are trade marks.

~- - 50 -,' ;, .. ..... .. .. .. .

~ ~6Z7~5 changes very little with temperature and thus the observed changes can be ascribed to geometrical effects and changes in the PTC layer resistance. Three configurations were compared: A) in which the electrodes covered the whole of the upper and lower surfaces of the specimen (i.e. similar to Figure 6 except that two CW layers were employed and the electrodes were silver paint, not mesh~, B) in which opposed silver paint electrodes 0.63 x 2.5 cm were disposed across the upper and lower surfaces (two on each side, the electrodes 10 on each side being spaced 2.5 cm apart), and C) in which one upper and one lower electrode 0.63 x 2.5 cm were alternated 2.5 cm apart in a staggered configuration. The power density/
temperature relations for these three configurations as shown ~ in Figure 37 demonstrate that the power/temperature curve can .~` 15 be changed dramatically and unexpectedly by changes in electrode configuration. For many applications the power curve denoted C is preferred and Figure 37 showql that with the compositions and resistances chosen this can be obtained with an alternating or laterally displaced electrode con~iguration. However, even when the electrode9 cover the whole upper and lower surfaces ~:of the CW layer, a curve of the C type can be obtained by , i .
appropriate selection of the resistivity of the PTC and CW :~
layer as shown in Figure 36 which indicates tEatto obtain a type C power curve the ~oom temperature resistivity of the PTC
layer should be le99 than that of the CW layer. However, with .. . . .
.:~.alternating, laterally displaced electrodes, type C power .
~curves are obtained by choosing a PTC layer with a reslstivity.

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6~75S
higher than that of the CW layers.
Example 6 Heaters were constructed according to configuration A
of Example 5 and of the same compositions as in Example 5.
However, in certain specimens as shown below, the CW layer was 0.10 cm thick. The heaters were tested while mounted on a stainless steel block as described in Example 5. The block temperature at which the power generated by the heater commenced to drop is shown in Table IV. The results show t~at by varying the relative resistances of the PTC and CW layers the drop off temperature and hence Ts can be varied quite significantly.
;~ Likewise, the degree of change of power with temperature is significantly affected. As is apparent, resistance for the CW
layer is altered by increasing its thickness. In the last two experiments shown in Table IV the size of the PTC core layer was reduced while keeping the CW layers constant.
Depending upon the ratio of interface area of the PTC layer to , the CW layers, the drop of temperature can be varied quite significantly.
TABLE IV

Heater CW layer Power drop-off Power at 23.9 C
PTC thickness temperature C Power at 85C
"core" cm 5-1 0.025 124 1.31 0.1 127 1.15 5-2 0.025 110 1.06 - 25 0.1 113 1.06 5-3 0.025 77 1.27 ` 0.1 80 1 30 ;~ ~ 5-2* 0.025 93 - --** 0.1 80 ----. - , .
* PTC layer covers 1/3 of CW layer ` ** PTC layer covers 1/6 of CW layer .

. .

f6Z7S~

A particular advantage of the thicker, i.e. higher resistance CW layers is that resistance variations in the PTC, layer do not have such a great impact on the power output, i.e. there is less temperature variation in power output. In 5 this way, one can use a highly crystalline, high molecular weight polymer with a highly structured carbon black for the PTC layer (such combinations yield the desired behaviour, approximately Type I, but show extreme sensitivity of the resistance obtained to processing and thermal history)O By 10 combining such compositions with CW layers of much higher resistivity as may be prepared from blends of low crystallinity -or amorphous polymers with medium or high structure blacks (which give resistivities of lower sensitivity to processing or thermal history), one can provide a heater of much greater 15 uniformity, reproducibility and functional usefulness than has hitherto been available.
As mentioned hereinabove, an important feature of a ; functional heater is the ratio of resistance at room temperature to that at the desired operating temperature. This ratio is 20 related to but not identical with the inrush ratio. Furt~er-~, more, lower values of this resistance ratio also indicate a closer approach to a Type I resistance characteristic. For tha heaters described in this example, an operating range in the neighborhood of 85C is considered optimum. To obtain low 25 ratios, PTC to CW volume resistivity ratios (at 2~C) between about 0.1:1 and 20:1 (the exact ratio depending on the relative thickness of the layers) are preferred, those between 1 and g 10 being particularly pre~erred.
- ~ :
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, . . . ..
:. ., ,, : . , .. - ., ~. ., . . , . :

6Z~7S~i Example 7 PTC materials were made up as in the previous example~
having the compositions given in Table V. 0.05 cm thick slabs of these compositlons were laminated between two 0.05 cm slabs of a mixture of 20% Black Pearls carbon black in Silastic* 437 (resistivity 400 ohm-cm) and the laminate then irradiated with 12 Mrads of ionizing radiation to effect cross-linking throughout.

TAsLE V

Sample Marlex 6003 SRF-~S PTC layer Power curve ~os. (%) (%) resistivity Type (Fig.35 _ ohm-cm

7-1 58 42 100 B

7-2 60 40 240 C but some drop off near room temp.
7-3 62 38 400 Very good C type This example demonstrates how the shape of the power curve can be modified by the selection of appropriate resistivity ratios for the PTC and cW layers The power te~perature relation is, of course, equatable with the temperature resistance relationship accordinS~ to the formula P = I R or P = E . The curve labelled C is close to the R
ideal expected from a heater having a resistance temperature characteristic of Type I.
Example 8 Two 30 cm long sections of flat strip heater constructed in accordance with ~.S. Patent ~umber 3,861,029 and having a . , .
~ *"Silastic" is a trade mark ...

... . .
.~. .

,. ~ . . , . . . . ,. , ~ : .. : . ~ : .. : . . :

i2~55 PTC core of composition similar to that used in Example 1, and shaped like Fi~ure 5 (0.8 cm wide) were affi~ed to an aluminium block maintained at 18 C by circulating water. The other side of each of the heater sections was painted with temperature indicating paint. The voltage applied to the sections was varied so as slowly to increase their power out-put. One section had a resistance of 488 ohms per meter. This section could be operated at up to about 5.~8 watts per metre without formation of a hotline, but with the core operating at temperature less than its Ts, At a power output of about 6.1 watts per metre at which power level the core warmed to its Ts a hotline was formed. The other heater section, which had a resistance of about 8080 ohms per metre could likewise be operated at about 4.88 watts per metre without hotlining, but ; 15 hotlined when operated above about 6.1 watts per metre.
Attempts to operate both these heaters at higher voltage levels resulted in concomitant drops in current so that under the ` experimental conditions these heaters did not consume more than about 9.3 watts per metre and their maximum output under these conditions was about 0.15 watt per square cm. Thus, attempts .., ; to operate the strip heater at power levels greater than about 0.08 per square cm resulted in hotlining.

Example 9 layered heater, was constructed in which a PTC layer 2~ (0.n75 cm thick) had the composition 47% Marlex*6003 5% Epsyn*

~~ 5508 tethylene-propylenediene modified rubber) and 48% Sterlin~*

`~SRF-~S (carbon ~lack~. Two CW layers 0.15 cm thick having the *"~psyn" is a trade mark -~ *Trade ~ark 55 ,, ,: . .. . .~ : - ::: :
' ' " ,... , .. ~ :
, ~316Z755 composition 60% Elvax* 250(ethylene-vinyl acetate copolymer) and 40% Cabot~ XC72 (carbon blac~) and having embedded therein flattened wire braid electrodes 0.95 cm wide and 0.95 cm apart (thre~ in all to each CW layer) were applied to each side of the PTC layer so that the electrodes were opposed to one another i.e., similarly to Fi~jure 11 except that the electrodes were braided rather than strips. The dimensions of the heater were 7.5 cm by 15 cm with the electrodes running along the long dimension with electrodes of opposite polarity extending beyond the polymeric layers at opposite ends of the heater.
The layers were carefully laminated together and the article then heated at 200 ~ for 10 minutes to anneal out any stress, then cooled and irradiated to 12 ~rads dose using Cobalt-60 gamma rays whilst enclosed in a container containing nitrogen.
The heater was sandwiched between 0.025 cm thick insulating layers comprising crosslinked low density polyethylene and pressed firmly to a cooled aluminium block as in the previous example and tamperature indicating paint applied to the upper surface of ~he heater. Blectrodes of opposite polarity were 2Q connected to a 12 volt battery. The heater consumed more than 70 amps while warming up, i.e., more than 5.4 watts/cm2.
Fo~ a period of several minutes the heater stabilized at a current of over 2Q amps, i.e.~ greater than 15.5 watts/cm .
j Pinally, the aluminium block started to warm up despite the applied cooling and the heater PTC layer warmed up to its Ts ~`~ Ca~out 120C). The t~mperature indicating paint melted during *Trade Mark ~. . ' ', ':
~"''''' ::

1~627SS

this last stage starting in the center and proceeding rapidly and smoothly to the edges. In this final condition the heater maintained itself at a temperature very close to its Ts and was consuming about lO amps, i.e., a heat output of about 7.1 watts/cm2 when the aluminium block was replaced by a slab of thermally insulating msterial. The current fell to much less than one amp, i.e., less than 0.67 watts/cm2 at a heater temperature still very close to T , the whole surface of the heater being at about this temperature. It is thus apparent that a heater in accordance with the present invention can . . .
operate at high power outputs at T temperatures well in excess of lO0 C without hotlining.
It will be apprecîated that references herein to a PTC layer being or becoming substantially non-conducting are relative to the electrical properties of the CW layer. It is not appropriate to give absolute values for such properties since these d0pend among other factors, on the relative configurations of the various layers, but in a simple laminate as illu~trated for example in Figure 23, as soon as the PTC
lager exceeds its ~D~ temperature the electrical fluxdensi~y through the CW layer i3 many time that through the PTC layer in any portion of the laminate where the two layers are electrically in parallel. Advantageous1y, where the two types of layers are electrically in parallel, the proportion of current passing through a C~ layer is at least lO, preferably at least 25 times that passing through a PTC layer at above it~ anomoly temperature, although in certain cases, for example if the article is in proximity to a relatively large heat 3ink, lower ratio~, of 5 or less may be adequate.

.' . .

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Claims (56)

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

1. An article comprising at least one first electrically resistive layer and at least one second electrically resistive layer at least a part of a surface of a first layer being contiguous with at least part of a surface of a second layer and providing electrical and thermal contact between them, the first layer exhibiting a positive temperature coefficient (hereinafter PTC) of resistance and having an anomaly temperature, above which it is substantially non-conducting, (hereinafter Ts) and the second layer having a substantially constant resistance (hereinafter CW) at least below the anomaly temperature of the first layer.

2. A self-regulating heating article comprising a laminate as claimed in claim 1 and at least a pair of electrodes so positioned that, when there is a potential difference between the electrodes, at ambient temperature current will pass between the electrodes through at least a portion of at least one first layer and of at least one second layer.

3. A self-regulating heating article comprising a first layer of material which exhibits a positive temperature coefficient of resistance and has an anomaly temperature and which layer has at least partially con-tiguous therewith a second layer of constant wattage material, and said first layer being connectable to an electric power input source whereby current flow is through at least a portion of said first layer and through at least a portion of said second layer, whereby there is both direct electrical and thermal coupling between said first and second layers, and whereby at the higher of the temperature at which the resistance of said first layer exceeds the resistance of said second layer or the anomally temperature of said first layer current flow predominantly follows a path the length of which through the first layer is as short as possible.

4. An article as claimed in claim 2, wherein the length of path through the first layer does not exceed the thickness thereof by more than 50%.

5. An article as claimed in any one of claim 1 to 3, wherein the PTC
layer has two substantially planar surfaces and has a CW layer at least partially contiguous with each of the planar surfaces.

6. An article as claimed in any one of claims 1 to 3, wherein the CW
layer serves, or the CW layers serve, as electrodes.

7. An article as claimed in any one of claims 1 to 3, which comprises or also comprises a conductive fiber electrode.

8. An article as claimed in claim 1, which comprises or also comprises a metal electrode.

9. An article as claimed in claim 8, wherein the metal electrode is a fabric, braid or grid in the form of a wire, strip or sheet.

10. An article as claimed in claim 8 or 9, wherein the material of the electrodes is in the form of a wire, strip or sheet.

11. An article as claimed in any one of claims 1 to 3, wherein at least one electrode is embedded in a CW layer.

12. An article as claimed in any one of claims 1 to 3, wherein at least one electrode is positioned on a face of a CW layer remote from the face in contact with a PTC layer.

13. An article as claimed in any one of claims 1 to 3, wherein at least one electrode is embedded in a PTC layer.

14. An article as claimed in any one of claims 1 to 3, wherein the first layer and the second layer are each disposed around an electrode or electrodes.

15. An article as claimed in any one of claims 1 to 3, wherein at least one electrode is positioned at an interface between a CW layer and a PTC layer.

16. An article as claimed in claim 1, which comprises at least one set of electrodes connected together in parallel.

17. An article as claimed in claim 16, which comprises two sets of electrodes, one set being positioned in a plane parallel to the plane of the other set.

18. An article as claimed in claim 17, wherein the electrodes in one set are transverse to the electrodes in the other set.

19. An article as claimed in claim 17, wherein the electrodes of one set are positioned in lines parallel to those of the other set.

20. An article as claimed in claim 19, wherein the electrodes of one set are opposite spaces between electrodes in the other set.

21. An article as claimed in any one of claims 1 to 3, which com-prises a layer of a CW material sandwiched between two layers of PTC mate-rial.

22. An article as claimed in claim 1, which comprises a layer of a PTC material sandwiched between two layers of CW material.

23. An article as claimed in claim 1, wherein the first layer is surrounded by the second, or wherein the second layer is surrounded by the first.

24. An article as claimed in claim 23, wherein the layers are co-axial.

25. An article as claimed in claim 22, wherein the article is of substantially rectangular cross-section and the first or second material forms a diagonal layer, the second or first material forming the remainder of the cross-section.

26. An article as claimed in any one of claims 1 to 3, wherein the article is of substantially rectangular cross-section, the boundary between the first and second materials being a diagonal of the rectangle.

27. An article as claimed in any one of claims 1 to 3, which is covered at least partially by an insulating layer.

28. An article as claimed in claim 1, which also comprises a sealant or adhesive on at least one surface, the sealant or adhesive being heat-activatable at a temperature at which the article acts as a heater.

29. An article as claimed in claim 20, wherein the CW layer is a sealant or adhesive.

30. An article as claimed in claim 1, wherein the first layer com-prises a polymeric composition.

31. An article as claimed in claim 1, wherein the second layer com-prises a polymeric composition.

32. An article as claimed in claim 30 or claim 31, wherein the polymeric material has dispersed therein carbon black.

33. An article as claimed in any one of claims 1 to 3, wherein the second layer is polymeric and has dispersed therein carbon black and con-ductive fibers or fibrils.

34. An article as claimed in claim 1, which is heat-recoverable.

35. An article as claimed in claim 34, wherein the article is heat-recoverable at a temperature at which the article acts as a heater.

36. An article as claimed in claim 34, or claim 35 which comprises an electrical insulating layer, which layer is heat-recoverable.

37. An article as claimed in any one of claims 1 to 3, wherein the first layer comprises barium titanate.

38. An article as claimed in any one of claims 1 to 3, which is an elongate flexible strip.

39. An article as claimed in claim 1, which has an effective Ts above 90°C, which is greater than the Ts of the first layer.

40. An article as claimed in claim 39, wherein the first layer com-prises a polymer and the effective Ts is greater than its melting point.

41. An article as claimed in claim 40, wherein the said polymer is cross-linked.

42. An article as claimed in any one of claims 1 to 3, wherein the resistivity ratio of the first and second layers is in the ratio of from 0.1 to 20 at 24°C.

43. A method of covering a substrate which comprises applying to the substrate an article, as claimed in any one of claims 1 to 3, that is heat-recoverable and heating it to cause recovery thereof to cover the sub-strate.

44. A method of covering a substrate which comprises applying to the substrate an article as claimed in any one of claims 1 to 3, that is heat-recoverable, and heating it by connecting it to an electrical power source to cause recovery to cover the substrate.

45. A method of heating a substrate which comprises positioning an article as claimed in any one of claims 1 to 3 in thermal contact with the substrate and energizing it by connecting the electrodes to an electrical power source.

46. A method of heating or covering a substrate which comprises positioning an article as claimed in any one of claims 1 to 3 over the sub-strate, and connecting the article to an electrical power source, the sub-strate forming one electrical contact to the source, the article being heat-recoverable at least if the substrate is to be covered.

47. A method of recovering an article as claimed in any one of claims 1 to 3, that is heat-recoverable, which comprises connecting the electrodes to an electrical power source for a time sufficient to effect recovery.

48. A substrate whenever covered by an article as claimed in any one of claims 1 to 3.

49. A substrate covered by applying an article as claimed in any one of claims 1 to 3 that is heat recoverable and heating it to cause recovery.

50. A substrate covered by applying an article as claimed in any one of claims 1 to 3, that is heat recoverable and heating it by connecting it to an electrical power source.

51. A substrate covered by positioning an article as claimed in any one of claims 1 to 3, that is heat recoverable, over the substrate, and con-necting the article to an electrical power source, the substrate forming one electrical contact to the source.

52. A process for supplying heat and automatically controlling the maximum temperature reached which comprises passing electrical current thourgh an article which comprises a first electrically resistive layer and a second electrically resistive layer, at least a part of a surface of the first layer being contiguous with at least a part of a surface of the second layer along an interface which provides direct electrical and thermal contact between the layers, the first layer being composed of a material which exhibits a positive temperature coefficient of resistance and having an anomaly temperature above which it is substantially non-conducting and the second layer being composed of a constant resistance material, the current passing sequentially through the two layers, the resistance of the first layer being greater than the resistance of the second layer, and the predominant current path through the first layer being as short as possible at the higher of the temperature at which the resistance of the first layer exceeds the resistance of the second layer and the anomaly temperature (TS).

53. A process as claimed in claim 52 wherein the current path through the first layer is predominantly perpendicular to the interface between the first and second layers.

54. A process as claimed in either of claims 52 or 53, wherein the article is generating at least 1.5 watts per square inch.

55. A process as claimed in either of claims 52 or 53, wherein the PTC material is at a temperature in excess of 100°C.

56. A process as claimed in either of claims 52 or 53, wherein equilibrium is maintained between the heat supplied and the heat lost.