Note: Descriptions are shown in the official language in which they were submitted.
S61
miS invention relates to heating elements conprising
conductive polymers.
It i5 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 Patent 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. ~owever, our researches have
shown that such articles suffer from certain disadvantages. In
particular we have found that 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. We have also
; found that if the current is passed from end to end of the conductive
polymer member, as suggested by British Patent No. 1,265,194, the member
is often not heated as uniformly as is desirable to achieve satisfactory
shrinkage and to avoid local overheating.
We have now discovered that if a laminar conductive polymer
member is sandwiched between a pair of laminar flexible 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.
~hus in its first aspect, the invention provides a heating
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1100561
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 which has a
resistivity greater than lO ohm.cm at room temperature; 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; (iii) are co~posed of a material having
a resistivity of less than 10 ohm.cm, preferably less than 1
ohm.cm,at room temperature; and (iv) 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 (or, in
the case of an element which has been deformed in the plane thereof,
corresponding to) apertures of a size, shape and distribution which
permit at least one of the dimensions of the element in the plane
thereof to be changed at the melting point of the polymer 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.
.~:
The term "without complete rupture of ~he electrode at any
point" means that each electrode maintains an electrical pa~hway completely
surrounding each aperture; thus the electrode may tear partially but
lloas6l
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 ther~oplastic polymers, is the softening
point of the polymer.
mese 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%. We
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.
lS As indicated above, the apertures must be 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
;~` same stretching or contraction of the element material itself is not
~ 20 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 ofthe 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
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llQ(~S61
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. m e 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. We 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 pluralit~ 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
; V2 inch (1 25 cm) long, especially 5 to 20 tim~s 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 known 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 Ts) is used to denote the
llO~S6~
temperature at which the rapid increase takes place. When the
increase takes place over a temperature range (as is often the
case~ then Ts 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 Ts
above 50C, often above 100C, and a resistivity at temperatures
below Ts from about 25 to about 105 ohm.cm. It is also desirable
that the increase in resistance above Ts should be sufficiently
high that member A is effectivel~ converted from an electrical
conductor to an electrical insulator by a relatively limited
increase in temperature. A convenient expression of this re-
quirement is that the material should have an R14 value of at
least 2.5 or an R100 value of at least 10, and preferably an R30
value of at least 6, where R14 is the ratio of the resistivities
at the end and beginning of the 14C range showing the sharpest
increase in resistivity, R100 is the ratio of the resistivities
at the end and beginning of the 100C range showing the sharpest
increase in resistivity, and R30 is the ratio of the resistivities
at the end and beginning of the 30C range showing the sharpest
ncrease 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 composi-
tions are disclosed in Polymer Engineering and Science, November
1973, 13 No. 6, pages 462-468, and United States Patents Nos.
2,978,665, 3,243,753, 3,412,358, 3,591,526, 3,793,716, 3,823,217,
and 3,914,363. For details of recent developments in this
field, reference may be made to Application Nos. 236,506, 236,456
236,482 (all filed September 26, 1975~ and Nos. 258,294, 258,295,
and 258,297 (all filed August 3, 1976).
~i '
~ , .
ilO~561
The use of a PTC material for member A prevents the member
from being electrically heated to a temperature above its Ts~ The TS f
PTC materials is usually very much dependent upon the tensile stress
thereof, and in the absence of the perforations, the Ts Of member A
would alter considerably when its planar dimensions were changed.
However, we have found that, although parts of member A are under
considerable stress, the overall Ts of member A is not substantially
changed by expansion or contraction.
m e 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
:j `
as high as aluminium are preferred. m e 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 we have found can arise, especially
when using metal electrodes and/or when the apertures are diamond-
shaped, is that short circuits can occur between electrodes of opposite
polarity 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-iinked polymer, which has a softening temperature
above the highest t~mperatuIe likely to be reached by the electrode. ït
is desirable that the edges as well as the planar surfaces of the
ll(;~C~S61
electrodes should be coated, and slit apertures should therefore 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
!
;~ 15 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 Ts of member A; (b) is
composed of a material which ccmprises an organic polymer and
electrically conductive particles dispersed in the polymer in amount
sufficient to render the member electrically conductive; (c) has a
; 25 resistivity greater than 10 ohm.cm; and (d) is interposed between the
member A and an electrode. Preferably there is one suchlintermediate
layer either side of member A. The term "constant wattage behavior"
means that the layer undergoes an increase in resistance of less than
561
six-fold in any 30C range below the Ts of member A and preferably
between room temperature and Ts of the member A.
It is preferred that the constant wattage layers have
resistances at room temperature which are higher than the resistance of
member A, in order that they can 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 Ts of member A, i.e. with a higher
Ts. mis iS useful in preventing the overheating of ~he 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. m e 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 Ts
at or near the crystalline melting point of the polymer, which may be
cross-linked to give the composition improved stability above Ts. A
preferred composition for me~ber 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 Ts of about 120C. A
preferred composition for a constant wattage intermediate layer
comprises an ethylene/vinyl acetate copolymer (61% by weight) and carbon
black (39% by ~ight).
ll(P~S61
In formulating the ccrpositions 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.
; 5 Having regard to the disclosure herein, the selection of suitable
comeositions will present no difficulties to those skilled in the art.
Preferably the element is of symmetrical construction.
; m e 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 ~e
placed between the layers, especially between metal electrodes and
;i adjacent polymeric layers, to ensure adequate adhesion between the
layers. A suitable adhesive camprises about 65% by weight of an
lS 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
--10--
561
formation so that in effect the slits are made one row at a time; a
stripFer 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.
m e 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 i5 capable of changing its dimensions in the required way
when the article is heated.
5~1
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. Patent No. 3,086,242) and include polymers of one or more olefins
and/or one or more ethylenically unsaturated monomers containing polar
groups.
Articles camprising 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. Patent 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
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S61
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 heater 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
2d 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
conposition 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
-13-
~lQ~S61
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 materi~l 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 heater element is on
the outside, such sleeves can conveniently be made by assembling a
potentially heat-recoverable sleeve inside a heating element with an
intenmediate 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.
A tubular heating element can conveniently be made by joining
opFosite edges of a sheet material through a strip of insulating
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11(;1~561
material, e.g. of an organic polymer, to which the edges can be
bonded by a suitable adhesive.
A particularly valuable use o~ the heating elements
of the invention is in the novel splice cases described in our
copending Application ~o. 267,266 filed 7 December, 1976.
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 processes
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.
- 15 -
,~
S~;~
me invention is illustrated in the accompanying drawing, in
which the Figure is an isometric view of a part of a heating elenent of
the invention. The element comprises a layer 12 composéd of a
conductive polymer which exhibits PTC behavior. Adherent to layer 12
; 5 are constant wattage layers 13a and 13b which are ccmposed of aconductive polymer and preferably exhibit PTC behavior with a Ts 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. ~hen these edge portions are separated, the apertures
become diamond-shaped.
The invention is further illustrated by the following Example
in which the percentages are by weight.
_ample
Laminar members of the compositions and thicknesses shown were
assembled in the order shown:
13 3~ai / C/ec~r~
(1) Lead~ 4 mils (0.01 cm) thick.
(2) A mixture of an ethylene/acrylic acid ccpolymer
(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.
-16-
110(~56
-
(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 heat-
recoverable polymeric sheet was obtained as follows. A sheet .08 inch
(0.2 cm) thick was extruded frcm 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 megarad~
radiation; a sample 10 x 4 inch (25 x 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 x 4 inch (25 x
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 elenent while hot
was bonded to the sampie of t~e 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 lOO~C. The heat produced by the heater softened the
c~ r k s
1561
:'
adhesive and by application of pressure the heater was ~used 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 115C in about
1 min. Shrinkage of the heat-recoverable member and heater
; 10 occurred smoothly.
It will be appreciated that as the heating element
lS not necessarily flat, for example, in those embodiments in
which it is a sleeve, the terms "plane" and "planar" refer not
only to the plane actually occupied by the element or portion
thereof, but also that plane in which the element or portion
thereof would lie if it were flat.
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