Heating elements comprising conductive polymers capable of dimensional change

The invention relates to heating elements comprising conductive polymers which are capable of undergoing a change in dimension. The heating elements comprise a laminar member composed of a conductive polymer and two laminar electrodes connected directly or indirectly to opposite faces of the laminar member, and there are apertures, for example slits, passing through the thickness of the element so that at least one planar dimension of the element can be changed by changing the shape of the apertures. The apertures through the element facilitate its dimensional change. Preferably the laminar member exhibits PTC characteristics. The heating elements are particularly useful in the form of articles in which the element is attached to a heat-responsive sheet, for example a heat-recoverable polymeric film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to heating elements comprising conductive polymers. 
2. Description of the Prior Art 
It is well known that polymers can be made electrically conductive by 
dispersing therein suitable amounts of finely divided conductive fillers. 
It is also well known that certain polymeric articles can be rendered 
heat-recoverable. It has been proposed (see British Pat. No. 1,265,194) to 
make a heat-recoverable article comprising a first heat-recoverable member 
composed of a conductive polymer and a second heat-recoverable member 
which is not electrically conductive, and to cause such an article to 
recover by passing an electric current through the first member. However, 
such articles suffer from certain disadvantages. In particular the 
electrical characteristics of the conductive polymer member are liable to 
change excessively if any of the dimensions of the member are changed by 
more than 30%, which is less than is generally desirable for 
heat-recoverable articles. Furthermore, the presence of the conductive 
polymer layer increases the force needed to deform the article and can 
adversely affect recovery. It has also been found that if the current is 
passed from end to end of the conductive polymer member, as suggested by 
British Pat. No. 1,265,194, the member is often not heated as uniformly as 
is desirable to achieve satisfactory shrinkage and to avoid local 
overheating. 
SUMMARY OF THE INVENTION 
I have not discovered that if a laminar conductive polymer member is 
sandwiched between a pair of laminar flexibel electrodes, and suitable 
apertures are formed in the resulting laminate, it is possible to obtain 
an easily expandable and/or contractable product having greatly improved 
electrical characteristics. 
DETAILED DESCRIPTION OF THE INVENTION 
Thus in its first aspect, the invention provides a heating element which 
comprises: 
(A) a laminar member composed of a material which comprises an organic 
polymer and electrically conductive particles dispersed in the polymer in 
amount sufficient to render the member electrically conductive; and 
(B) a pair of laminar flexible electrodes which (i) are connected (directly 
or indirectly) to opposite faces of said laminar member; (ii) are 
substantially coplanar with said laminar member; and (iii) are adapted to 
be connected to an external source of power to cause current to pass 
through said laminar member; said laminar member and laminar electrodes 
having a plurality of apertures through the thickness thereof, the 
apertures being of a size, shape and distribution which permit at least 
one of the dimensions of the element in the plane thereof to be changed by 
a change in the shape of the apertures. Preferably at least one of the 
planar dimensions of the element can be changed at the melting point of 
the polymer (and preferably at some lower temperature, e.g. 25.degree. or 
50.degree. C.) by a percentage which is at least 30%, without complete 
rupture of the electrode at any point, the dimensional change being 
accomodated by a change in the shape of the apertures. It is to be 
understood that the said dimensional change may be an increase as a result 
of stretching, or a decrease as a result of compression, or both (i.e. an 
element which can be stretched by X% and compressed by Y% in the same 
dimension, where X+Y is at least 30). 
The term "without complete rupture of the electrode at any point" means 
that each electrode maintains an electrical pathway completely surrounding 
each aperture; thus the electrode may tear partially but not completely at 
any point. The "melting point of the polymer" referred to above is for 
crystalline thermoplastic polymers the temperature at which melting of 
crystalline material begins, and for other polymers, e.g. elastomers and 
non-crystalline thermoplastic polymers, is the softening point of the 
polymer. 
These novel heating elements are useful in situations in which it is 
desirable to have a heater which can readily change at least one of its 
planar dimensions without excessive change in its electrical 
characteristics. Preferably the element is deformable as set out above by 
a said percentage which is at least 50%, e.g. at least 70%, especially at 
least 100%, and for some purposes at least 250%. I have surprisingly found 
that even when the element is deformed by a high percentage, its 
resistance does not generally increase by more than 20%, which is very 
valuable. 
As indicated above, the apertures are preferably capable of changing in 
shape so as to accomodate a change in a planar dimension of at least 30% 
(or preferably a greater percentage as set out above), but it is to be 
understood that at greater percentage changes, the possibility of some 
stretching or contraction of the element material itself is not excluded. 
The term "apertures" is used herein to include slits which open up into, 
for example, diamond-shaped openings when the element is extended. In 
general it will be convenient for the apertures in the undeformed element 
to be such that substantial planar deformation of the element is possible 
only by stretching, i.e. the element is expandable; and the invention will 
be chiefly so described. However, the invention includes, for example, 
elements which in the undeformed state are expandable or contractable in 
one direction, and elements having apertures such that the element can be 
stretched simultaneously in two directions. 
The choice of apertures will be dependent on the degree of expandability 
required and the ductility of the electrodes and member A. The apertures 
must be elongated and must overlap each other, but a wide variety of 
apertures fulfilling these requirements can be used. Thus the apertures 
may be regular or irregular and may for example be straight or wave-form 
slits or slots, oval holes or diamond-shaped holes. It will generally be 
convenient that the apertures should be regularly spaced and of the same 
size and shape. It is generally desirable that the apertures should be 
such that the element responds symmetrically through the thickness thereof 
to extensions in the plane of the element, in order to avoid buckling of 
the element. I have obtained good results with elements in which the 
distance between the edges of adjacent apertures is 1/10 to 1/2 inch (0.25 
to 1.25 cm). Preferably the apertures are a plurality of identical 
straight slits in parallel equally-spaced rows with the slits in adjacent 
rows overlapping each other; the length of the slits are preferably at 
least 1/2 inch (1.25 cm) long, especially 5 to 20 times the distance 
between adjacent rows of slits. Elements which can be stretched to at 
least three times their original length can be obtained in this way. 
In a preferred embodiment, Member A is composed of a material which is one 
of the small proportion of conductive polymers which exhibits what is know 
as PTC (positive temperature coefficient) behavior, i.e. a rapid increase 
in resistivity at a particular temperature or over a particular 
temperature range. The term "switching temperature" (usually abbreviated 
to T.sub.s) is used to denote the temperature at which the rapid increase 
takes place. When the increase takes places over a temperature range (as 
is often the case) then T.sub.s can conveniently be designated as the 
temperature at which extensions of the substantially straight portions of 
the plot of the log of the resistance against the temperature (above and 
below the range) cross. 
PTC materials used in member A will generally have a T.sub.s above 
50.degree. C., often above 100.degree. C. 
It is also desirable that the increase in resistance above T.sub.s should 
be sufficiently high that member A is effectively converted from an 
electrical conductor to an electrical insulator by a relatively limited 
increase in temperature. A convenient expression of this requirement is 
that the material should have R.sub.14 value of at least 2.5 or an 
R.sub.100 value of at least 10, and preferably an R.sub.30 value of at 
least 6, where R.sub.14 is the ratio of the resistivities at the end and 
beginning of the 14.degree. C. range showing the sharpest increase in 
resistivity; R.sub.100 is the ratio of the resistivities at the end and 
beginning of the l00.degree. C. range showing the sharpest increase in 
resistivity; and R.sub.30 is the ratio of the resistivities at the end and 
beginning of the 30.degree. C. range showing the sharpest increase in 
resistivity. 
For a general survey of conductive polymers, reference may be made to 
"Conductive Rubbers and Plastics" by R. H. Norman, published in 1970 by 
Elsevier Publishing Company. PTC compositions are disclosed in Polymer 
Engineering and Science, Nov. 1973, 13 No. 6, pages 462-468, and U.S. Pat. 
Nos. 2,978,665; 3,243,753; 3,412,358; 3,591,526; 3,793,716, 3,823,217; and 
3,914,363, the disclosures of which are hereby incorporated by reference. 
For details of recent developments in this field, reference may be made to 
the following patent applications all filed Aug. 4, 1975: Horsma and 
Lyons, Ser. No. 601,638; Whitney and Horsma, Ser. No. 601,427 (now U.S. 
Pat. No. 4,017,715); Horsma and Hammack, Ser. No. 601,639; Moyer, Ser. No. 
601,424 (now abandoned); Horsma and Diaz, Ser. No. 601,549, (now 
abandoned); and Horsma and Diaz, Ser. No. 601,344, the disclosures of 
which are hereby incorporated by reference. 
The use of a PTC material for member A prevents the member from being 
electrically heated to a temperature above its T.sub.s. The T.sub.s of PTC 
materials is usually very much dependent upon the tensile stress thereof, 
and in the absence of the performations, the T.sub.s of member A would 
alter considerably when its planar dimensions were changed. However, I 
have found that, although parts of member A are under considerable stress, 
the overall T.sub.s of member A is not substantially changed by expansion 
or contraction. 
The resistivity of the laminar member A should be sufficient to provide 
adequate heating in the operating range of the element. In the case of 
materials which do not exhibit PTC characteristics, a resistivity of at 
least 10 ohm.multidot.cm at room temperature is generally necessary. With 
PTC materials, however, the increase in resistivity as the temperature 
rises can be sufficient to permit the use of materials whose room 
temperature resistivity is relatively low, e.g. 1 to 10 ohm.multidot.cm. 
Particularly is this so when the resistivity of the electrodes or of other 
layers in the element is such as to provide additional heating. 
The electrodes may be of any suitable material, for example of metal or a 
highly conductive polymer, and may comprise bussing sections which do not 
contain apertures and which run at right angles to the direction of major 
dimensional change. Metals are generally preferred because they have high 
conductivity coupled with elongations which are generally high enough for 
most uses; metals having a ductility at least as high as aluminium are 
preferred. The tendency of metal foil electrodes to tear can be decreased 
(and at the same time flexibility increased) by corrugating the foil, for 
example by an amount which shortens it by about 15%. Suitable metals 
include copper, lead and aluminium. 
One of the problems which I have found can arise, especially when using 
metal electrodes and/or when the apertures are diamond-shaped, is that 
short circuits can occur between the electrodes of opposite polarity if 
the element is locally distorted from a generally planar configuration, 
especially at the edges of the element, or if the element is partially 
broken. This problem can be alleviated by coating the exposed surfaces of 
the electrodes with an insulating material, for example a polymer, 
especially a cross-linked polymer, which has a softening temperature above 
the highest temperature likely to be reached by the electrode. It is 
desirable that the edges as well as the planar surfaces of the electrodes 
should be coated, and slit apertures should therefore preferably be opened 
out by expanding the element prior to coating. Suitable coating techniques 
include electroplating, electrostatic spraying, and dipping into a 
suitable powdered insulator, followed by curing of the coating by heat. An 
alternative way of reducing the likelihood of shorting is to use apertures 
such as slots or ovals which have substantial width even when the element 
is completely contracted. Another solution is to use two coextensive 
electrodes which are composed of conductive polymer and which are 
therefore less likely to short than a metal electrode, and if necessary or 
desirable to provide satisfactory electrical characteristics by the use of 
additional metal electrodes which do not overlap; such an arrangement also 
gives rise to changed electrical characteristics because of non-uniform 
current flow. 
Generally speaking the electrodes will be coextensive with member A. 
However, this is not essential provided that in use current is passed 
through substantially the whole of the member A so as to provide 
satisfactory heating thereof. 
Particularly useful heating elements are those in which member A exhibits 
PTC characteristics and which also comprise at least one intermediate 
layer which (a) exhibits constant wattage behavior (as hereinafter 
defined) at temperatures below the T.sub.s of member A; (b) is composed of 
a material which comprises an organic polymer and electrically conductive 
particles dispersed in the polymer in amount sufficient to render the 
member electrically conductive; (c) has a resistivity greater than 10 ohm 
cm; and (d) is interposed between the member A and an electrode. 
Preferably there is one such intermediate layer either side of member A. 
The term "constant wattage behavior" means that the layer undergoes an 
increase in resistance of less than six-fold in any 30.degree. C. range 
below the T.sub.s of member A and preferably between room temperature and 
T.sub.s of the member A. 
It is preferred that the constant wattage layers have resistivities at room 
temperature which are higher than the resistivity of member A, so that 
they help to control the level of current inrush when the heating element 
is initially connected to a power supply. It is also preferred that the 
intermediate layers should exhibit PTC behavior at temperatures above the 
T.sub.s of member A, i.e. with a higher T.sub.s. This is useful in 
preventing the overheating of the intermediate layer which would otherwise 
take place if the electrode was completely ruptured at any point, thus 
causing current to pass through the intermediate layer to bridge the 
rupture; this can cause severe overheating if the intermediate layer does 
not shut itself off at some suitable temperature. 
The conductive particles in member A and any intermediate layers are 
preferably of carbon black, particularly when PTC characteristics are 
needed. In electrode layers comprising conductive polymers, the conductive 
particles are preferably of carbon black or a metal. The particles may be 
of any shape, including fibres. Examples of suitable compositions are to 
be found in the prior publications and patent applications referred to 
above. The PTC compositions are preferably based on crystalline polymers, 
which compositions have a T.sub.s at or near the crystalline melting point 
of the polymer, which may be cross-linked to give the composition improved 
stability above T.sub.s. A preferred composition for member A is a mixture 
comprising high density polyethylene (45% by weight) an ethylene-propylene 
rubber (5% by weight) and carbon black (50% by weight), which has a 
T.sub.s of about 120.degree. C. A preferred composition for a constant 
wattage intermediate layer comprises an ethylene/vinyl acetate copolymer 
(61% by weight) and carbon black (39% by weight). In formulating the 
compositions for the different layers, it is, of course, necessary to 
consider the physical, as well as the electrical, properties thereof, for 
example flexibility, adhesion to adjacent layers and resistance to flow at 
operating temperatures. Having regard to the disclosure herein, the 
selection of suitable compositions will present no difficulties to those 
skilled in the art. Preferably the element is of symmetrical construction. 
The novel heating elements can be prepared by assembling the various 
layers; bonding them together with the aid of heat and pressure; and then 
creating the apertures in the bonded assembly. Suitable conductive 
adhesives, e.g. carbon-loaded hot melt adhesives, can be placed between 
the layers, especially between metal electrodes and adjacent polymeric 
layers, to ensure adequate adhesion between the layers. A suitable 
adhesive comprises about 65% by weight of an ethylene/acrylic acid 
copolymer and about 35% carbon. 
When it is desired that the polymer in member A and in any other polymeric 
layers should be cross-linked, as may be preferred, the polymers initially 
employed must, of course be cross-linkable. Cross-linking is preferably 
carried out after the bonding step but before the apertures are created, 
for example by use of ionising radiation of sufficient dosage, e.g. 5 to 
20 megarads. Alternatively a chemical cross-linking agent can be 
incorporated in the polymers, and cross-linking effected during the 
bonding step, or in a separate heating step after the bonding step but 
before the apertures are created. 
Slits are in general easier to create in the element than openings such as 
slots or ovals. Slits can be simply cut by means of a sharp blade, for 
example a plurality of blades operating in staggered formation so that in 
effect the slits are made one row at a time; a stripper pad may be used to 
prevent the blade from tearing the element as it is withdrawn. Openings, 
on the other hand, must be punched out. 
The novel heating elements are particularly useful as components of 
articles which comprise a heat-responsive (as hereinafter defined) sheet 
material and adjacent to one face of the sheet material a heating element 
as defined above. The heating element may be in direct contact with the 
sheet material, for example secured thereto by an adhesive, or may be 
separated therefrom by an intermediate layer provided that there is 
adequate heat transfer between the heating element and the sheet material. 
The article is preferably flexible, at least at the temperature at which 
the sheet material becomes responsive. 
The term "heat-responsive" is used herein to mean that when the sheet 
material is heated to a suitable temperature it either (a) undergoes a 
spontaneous change in at least one dimension in the plane thereof; and/or 
(b) undergoes some other change, e.g. it softens (including flows), which 
substantially reduces the external forces (e.g. manual forces) required to 
change at least one dimension of the sheet material in the plane thereof. 
The sheet material preferably comprises an organic polymer, for example a 
polymeric film which is heat-recoverable or can be rendered 
heat-recoverable, an adhesive (for example a hot-melt or heat-activatable 
adhesive) or a mastic. 
It will, of course, be apparent that in such articles the heating element 
should be placed adjacent the sheet material in such a way that it is 
capable of changing its dimensions in the required way when the article is 
heated. 
The articles of the invention will generally have one heater element and 
one sheet material, but may contain more than one element and/or more than 
one sheet material; for example they may comprise an element sandwiched 
between two sheet materials or one sheet material sandwiched between two 
elements. 
When the sheet material is a polymeric film which is heat-recoverable or 
can be rendered heat-recoverable, it preferably comprises a crystalline 
cross-linked polymer. Suitable polymers for heat-recoverable sheet 
materials are well known in the art, see for example U.S. Pat. No. 
3,086,242, the disclosure of which is hereby incorporated by reference, 
and include polymers of one or more olefins and/or one or more 
ethylenically unsaturated monomers containing polar groups. 
Articles comprising a heat-recoverable polymeric film can be made by 
deforming an article which comprises (a) a film which can be rendered 
heat-recoverable and (b), attached to one face of the film, a heating 
element (preferably one which has not been substantially deformed in the 
plane thereof), the deformation being carried out at a temperature above 
the crystalline melting point of the polymer in the sheet material, 
followed by cooling the article while it is in the deformed state. 
Suitable techniques are well known in the art (see for example U.S. Pat. 
No. 3,086,242). Such articles can also be made by assembling a 
heat-recoverable sheet material and a heating element, preferably one that 
has been deformed in the plane thereof in the direction opposite to the 
direction of heat recovery of the sheet material. 
As noted above, the sheet material will generally be secured to the heating 
element by means of an adhesive. The adhesive need not be a very powerful 
one since the dimensions of the heating element are easily changed. This 
is an important advantage over similar articles comprising a heating 
element without apertures, which generally require a powerful adhesive to 
ensure that the element satisfactorily follows dimensional change of the 
sheet material. The adhesive is preferably one which at the operating 
temperature, e.g. the recovery temperature of the article, permits 
slippage between the heating element and the sheet material, but does not 
flow into the apertures of the heating element and thus interfere with 
dimensional change thereof. Suitable adhesives are, for example, included 
in the disclosure of British Specification No. 1,440,810, and other 
adhesives containing labile ionic bonds. Preferably the adhesive is one 
whose Vicat melting point is below the operating temperature and whose 
ring and ball softening point is below the operating temperature. 
Particularly preferred adhesives are thixotropic at the operating 
temperature, e.g. the recovery temperature. 
The sheet material and heating element can, for example, be secured to each 
other by assembling them with a layer of a suitable hot melt or 
heat-activatable adhesive between them, and heating the assembly under 
pressure, e.g. from a pair of rollers. If the sheet material has been 
rendered heat-recoverable prior to assembly and preparation of the 
assembly involves use of a temperature above the recovery temperature, 
then steps must be taken to prevent complete recovery of the sheet 
material. 
When the heating element provides one face of the article, it may be 
desirable that at least some of the apertures therein contain a 
composition which flows at the operating temperature of the article, for 
example a solder or a mastic or a hot melt adhesive. This is particularly 
useful when the article is heat-recoverable, for example a heat-shrinkable 
sleeve having the heating element on the inside, and recovery of the 
article brings the heating element into contact with a substrate to be 
covered and thus causes the composition to be squeezed from the apertures. 
Presence of the composition can also improve heat transfer from the heater 
element to the sheet material. 
The articles of the invention may be of any suitable shape, and can be part 
of a larger object. Thus the invention includes objects having one or more 
sections which are articles according to the invention. Particular useful 
articles are sleeves, i.e. hollow articles of closed cross-section having 
at least one open end, e.g. tubular articles of circular or other 
cross-section, especially such sleeves which contract to a smaller 
diameter on heating. When the heater element is on the inside, such 
sleeves can conveniently be made by expanding a heating element sleeve by 
placing it over a mandrel; surrounding the exterior of the element with a 
suitable adhesive (e.g. a preformed tube thereof); surrounding the 
exterior of the adhesive with a sleeve of sheet material which is 
heat-shrinkable to a diameter less than the external diameter of the 
element; heating the assembly to cause the sleeve to shrink down and bond 
to the element via the adhesive; cooling the assembly; and removing the 
mandrel. When the heating element is on the outside, such sleeves can 
conveniently be made by assembling a potentially heat-recoverable sleeve 
inside a heating element with an intermediate layer of adhesive; heating 
the assembly and pneumatically expanding it against an external die; and 
cooling the assembly while maintaining the assembly in expanded condition. 
An tubular heating element can conveniently be made by joining opposite 
edges of a sheet material through a strip of insulating material, e.g. of 
an organic polymer, to which the edges can be bonded by a suitable 
adhesive. 
A particularly valuable use of the heating elements of the invention is in 
the novel splice cases described in copending Application of Diaz, Ser. 
No. 683,687 filed Dec. 8, 1975 (now abandoned), the disclosure of which is 
hereby incorporated by reference. 
The invention includes processes in which an article is prepared by 
connecting the electrodes of an article as defined above to an external 
source of power which causes current to pass through the laminar member A 
and to provide at least part of the heat needed to heat the sheet material 
to a temperature at which it becomes responsive; and allowing or forcing 
the sheet material at said temperature to undergo dimensional change in 
the plane thereof. The process is particularly useful when a substrate is 
covered by positioning the article adjacent to the substrate and 
connecting the electrodes so that the heated sheet material undergoes 
dimensional change such that the article conforms to the surface of the 
substrate. If desired the article can surround the substrate or can 
cooperate with another covering member to surround the substrate. The 
external source of power used in these proceses is conveniently DC of 
about 12 volts from a battery or AC of about 115 or about 220 volts from a 
mains source.It may be desirable to continue heating the article (by 
continuing to pass current through the heating element or otherwise) after 
the planar dimensions thereof have changed, for example to heat a 
substrate brought into contact therewith to ensure adequate adhesion 
between the article and the substrate, by a heat-activatable adhesive or 
otherwise.

Referring now to the drawings, FIG. 1 is an isometric view of a part of a 
heating element of the invention. The element comprises a layer 12 
composed of a conductive polymer which exhibits PTC behavior. Adherent to 
layer 12 are constant wattage layers 13a and 13b which are composed of a 
conductive polymer and preferably exhibit PTC behavior with a T.sub.s 
higher than layer 12. Layer 13a and 13b are secured to corrugated metal 
foil layers 15a and 15b via layers of adhesive 14a and 14b. There are a 
plurality of slits formed in parallel staggered rows 17. The slits will 
generally be somewhat longer than is shown in the drawings. The edge 
portions of the sheet parallel to the perforations do not contain 
perforations. When these edge portions are separated, the apertures become 
diamond-shaped, as illustrated in FIG. 2. 
FIG. 3 illustrates how the problem of shortcircuiting referred to above can 
arise. When edge portion 19 is displaced, short circuits can occur across 
the spaces 20 and 21 between the oppositely-charged electrodes as the 
element contracts. When this problem arises it can be cured by one of the 
solutions referred to previously. 
FIG. 4 illustrates the installation of a heating element on the inside of 
the heat shrinkable sleeve. Tubular member 24 represents an expanded 
cylindrical heating element positioned over a mandrel 25. The mandrel can 
be used to expand the element to its desired diameter. Member 26 is a 
tubular layer of adhesive and member 27 is a sleeve of heat recoverable 
material having a larger diameter than the final heat recoverable diameter 
desired. Members 26 and 27 are concentrically arranged over heater 24 and 
mandrel 25 and the assembly heated, for example, in an oven, to shrink the 
sleeve 27 and activate the adhesive. In shrinking, the sleeve 27 provides 
the pressure for a secure bond between it and the heating element. The 
assembly is then cooled and the mandrel 25 removed. When subsequently 
heated by means of heating element 24, the sleeve can continue to shrink 
to its heat stable diameter or into contact with a substrate. 
FIG. 5 illustrates the installation of a heating element on the outside of 
a heat shrinkable tube. Sleeve 28 is a tube of polymer capable of being 
rendered heat recoverable. Members 29 and 24 are, respectively, a tubular 
adhesive layer and the heating element to be joined to sleeve 28. Member 
30 is a pneumatic expansion die. The various elements are concentrically 
arranged and tube 28 heated and pneumatically expanded to render it heat 
recoverable. In the process of expansion, the heat can activate adhesive 
layer 29 to form a bond with heating element 24. Expansion is limited by 
die 30. When the assembly is cool, pneumatic pressurization of sleeve 28 
is terminated and die 31 removed from the assembly. 
FIG. 6 illustrates a cylindrical heating element 24 that can be employed in 
the process of FIGS. 4 and 5. As shown, the ends are joined by means of an 
"I" beam member 31, preferably a polymeric member, that insulates the ends 
from each other to prevent short circuits. The "I" beam member is 
preferably bonded to the heater by a suitable adhesive. 
The invention is further illustrated by the following Example in which the 
percentages are by weight. 
EXAMPLE 
Laminar members of the compositions and thicknesses shown were assembled in 
the order shown: 
(1) Lead; 4 mils (0.01 cm) thick. 
(2) A mixture of an ethylene/acrylic acid copolymer (65%) and carbon black 
(35%); 5 mils (0.0125 cm) thick. 
(3) A mixture of an ethylene/vinyl acetate copolymer (61%) and carbon black 
(39%); 10 mils (0.025 cm) thick. 
(4) A mixture of high density polyethylene (45%), an ethylene/propylene 
rubber (5%) and carbon black (50%); 20 mils (0.051 cm) thick. 
(5) as laminar member (3). 
(6) as laminar member (2). 
(7) as laminar member (1). 
These layers were bonded together with heat and pressure and then exposed 
to 6 megarads of ionising radiation. An expandable heating element was 
made by creating in the bonded assembly slits 0.5 inch (1.25 cm) long in 
parallel but offset rows 0.025 inch (0.06 cm) apart; the slits in a row 
were spaced apart 0.10 inch (0.25 cm). A heatrecoverable polymeric sheet 
was obtained as follows. A sheet 0.08 inch (0.2 cm) thick was extruded 
from a mixture of an ethylene/ethyl acrylate copolymer (88.4%) (DPD 6181 
from Union Carbide), a dispersion of 1 part of carbon black in 3 parts of 
an ethylene/vinyl acetate copolymer (9%) (Colorant CC 004), finely divided 
silica (3%) (Cabosil) and an antioxidant (0.6%); the sheet was crosslinked 
with 12 megarads radiation; a sample 10.times.4 inch (25.times.10 cm) was 
cut from the sheet, stretched to 20 inch (50 cm), and held there until 
cool. A section of a heating element prepared as described above and about 
10.times.4 inch (25.times.10 cm) was connected to a 24 volt power source 
and allowed to heat, and was then stretched to 20 inch (50 cm). The 
heating element while hot was bonded to the sample of the heat-recoverable 
sheet by means of a 5 mil (0.0125 cm) thick layer of an adhesive that 
would soften but not flow at about 100.degree. C. The heat produced by the 
heater softened the adhesive and by application of pressure the heater was 
fused to the recoverable sheet. The recoverable sheet was restrained to 
prevent its recovery. The resulting article was allowed to cool to room 
temperature and the restraint on the heat-recoverable member removed. The 
heater was then connected to a 24 volt power source. The heat-recoverable 
member and heating element recovered to their original dimensions within 2 
min. The heater reached its control temperature of about 115.degree. C. in 
about 1 min. Shrinkage of the heat-recoverable member and heater occurred 
smoothly.