Method of making self-temperature regulating electrical heating cable

Disclosed are improved melt processable, self-temperature regulating, irradiation cross-linkable, electrically semi-conductive polymeric compositions which in conjunction with annealing at a temperature at or above their melt point temperatures subsequent to their having been radiation cross-linked provide for improved self-temperature regulating electrical heating devices including flexible electrical heating cables. Heating cables made in accordance with the invention comprise two or more elongate substantially parallel spaced-apart electrical conductors that are electrically inter-connected by means of extruded forms of the compositions which have been annealed at a temperature at or above their melt point temperatures prior and subsequent to their having been cross-linked by irradiation. The compositions of the invention have an amount of electrically conductive particles, such as carbon black, dispersed therein, that is controlled within the range of 17% to 25% by weight to the total weight of the compositions. The semi-conductive compositions are characterized by exhibiting a positive temperature coefficient of electrical resistance and by having sufficient crystallinity in their polymeric portion to provide attractive self-temperature heat regulating characteristics in conjunction with a lessening of criticality in their annealing requirements from that heretofore associated with the process of making electrical heating cables utilizing electrically semi-conductive polymeric materials.

This invention relates generally to improved melt processable, 
self-temperature regulating, irradiation cross-linked electrically 
semi-conductive polymeric compositions having a positive temperature 
coefficient of electrical resistance and their use in flexible electrical 
heating devices and in particular to their use in flexible electrical 
heating cables having extruded, irradiation cross-linked, forms of the 
polymeric compositions and more particularly to improved melt processable 
self-temperature regulating irradiation cross-linked semi-conductive 
polymeric compositions which contain an amount of electrically conductive 
particles, such as carbon black, dispersed therein that is controlled 
within the range of 17% to 25% by weight to the total weight of the 
semi-conductive composition and which have been annealed, at a temperature 
at or above their melt point temperatures subsequent to their having been 
radiation cross-linked in conjunction with their use in making electrical 
heating devices and the method of making flexible electrical heating 
cables using extruded forms of the compositions whereby the compositions 
are annealed at a temperature at or above their melt point temperatures 
prior and subsequent to their having been cross-linked by radiation. 
BACKGROUND OF THE INVENTION 
Self-regulating heaters utilizing electrically semi-conductive compositions 
having a positive temperature coefficient of electrical resistance and 
containing restrictively prescribed amounts of electrically conductive 
particles, such as carbon black, are well known in the prior art. 
Generally, a material which exhibits a positive temperature coefficient of 
electrical resistance is a material whose electrical resistance increases 
as a result of an increase in its temperature. It is believed by many that 
polymeric compositions containing dispersed electrically conductive 
particles, such as carbon black, exhibit a positive temperature 
coefficient of electrical resistance as a result of the polymeric matrix 
expanding at a rate greater than that of the electrically conductive 
particles when subjected to an increase in temperature. It has been 
theorized that such polymeric matrix expansion tends to increase, or 
otherwise alter, the spacial relationship between the electrically 
conductive particles in such a manner as to result in an increase in the 
electrical resistance of the polymeric composition. An increase in the 
electrical resistance of the polymeric composition would correspondingly 
reduce the amount of electrical current derived from a fixed electrical 
potential placed across the composition and reduce the amount of heat 
generated by the electrical current according to the established 
relationship of heat equals I.sup.2 R. 
It is the theory of others that the amount of crystallinity present in a 
polymeric composition containing electrically conductive particles is an 
important factor in providing a useful positive temperature coefficient of 
electrical resistance. According to this train of thought, an increase in 
electrical resistance may arise as a result of the reorientation of the 
crystalline-amorphic boundaries when the polymeric composition's 
temperature is caused to increase and which, aside from whether or not the 
composition expands during its increase in temperature, tends to 
electrically insulate the conductive particles (or groups of the 
electrically conductive particles) more effectively from each other and 
thereby contributes to an increase in the all-over electrical resistance 
of the composition. 
Previous studies of polymeric compositions containing varying amounts of 
dispersed electrically conductive carbon blacks have shown certain 
characteristics as to the magnitude of increase of electrical resistance 
per thermal unit of temperature increase. Such studies have also resulted 
in derived terminology that is useful in describing certain relationships. 
Generally, the type and make-up of the polymeric composition; the nature, 
physical size and amount of electrically conductive particles; and the 
method by which they are dispersed in the polymeric matrix determines the 
value of derived terms such as, for example, R.sub.25 (electrical 
resistance at 25.degree. C.); T.sub.c (controlling temperature about which 
the electrical resistance increases or decreases in response to an 
electrical current having a fixed potential; R.sub.p (peak electrical 
resistance above which the electrical resistance of the semi-conductive 
composition begins to reverse itself and decrease rapidly in response to 
an increase in temperature in association with the melt phase of the 
polymeric composition; and R.sub.p /R.sub.25 (the ratio of the above 
described electrical resistances generally depicting the range of 
resistance between the given two temperature points. 
Until the time of the present invention, it was thought that in order to 
provide a useful electrically semi-conductive heating device the amount of 
electrically conductive carbon black particles dispersed in the polymeric 
composition must be either 15% or less or 25% or more, by weight, of the 
total weight of the composition. An example of such compositions can be 
found in Kohler's U.S. Pat. No. 3,243,573 wherein the electrically 
semi-conductive compositions are described as containing 25 to 75 percent 
by weight carbon black as a result of in-situ polymerization. Although 
such compositions may be useful for some heating purposes, it has been 
found that polymeric compositions containing more than 25% by weight of 
carbon black generally possess poor cold temperature properties; exhibit 
inferior elongation characteristics; and generally do not possess good 
electrical current regulating characteristics in response to changes in 
temperature. As noted above, it has also been proposed that electrically 
semi-conductive compositions must not have more than 15% by weight of 
carbon black in order to provide a useful self-regulating heating device. 
Such teaching can be found, for example, in U.S. Pat. No. 3,793,716 in 
which a process is described for making a self-regulating heating element 
utilizing a composition having less than 15% by weight of carbon black 
incorporated therein. This contention is also maintained in U.S. Pat. No. 
3,861,029 wherein a polymeric material containing not more than about 15% 
by weight of carbon black is subjected to a prolonged annealing procedure 
to reduce its electrical volume resistivity at room temperature to from 
about 5 to about 100,000 ohm-cm. 
A further extension of this belief can be found in U.S. Pat. No. 3,914,363 
wherein a shape retaining thermoplastic jacket is disposed about 
self-regulating conductive articles utilizing crystalline polymeric 
compositions containing not more than about 15% by weight of conductive 
carbon black and the combination thereof is subjected to an annealing 
procedure whereby the room temperature electrical volume resistivity of 
the polymeric composition is reduced to within the range of from about 5 
to about 100,000 ohm-cm. This contention is also reiterated in U.S. Pat. 
No. 3,823,216 wherein a cyclic annealing process is disclosed and claimed 
for reducing the electrical volume resistivity to a value within the range 
of from about 5 to about 100,000 ohm-cm at 70.degree. F. for compositions 
disclosed therein which are used in self-temperature regulating articles 
and which contain carbon lack dispersed therein in an amount not greater 
than about 15% by weight to the total weight of the composition. 
Electrically conductive compositions can additionally be found, for 
example, in U.S. Pat. No. 2,750,482 in which is disclosed an amorphous 
polyisobutylene material containing conducting particles for use in high 
temperature alarms and in U.S. Pat No. 2,905,919 in which an electrical 
heating cable is described as containing a semi-conductive body of 
pulverulent inorganic material. A further example of an electrically 
semi-conductive composition can be found in U.S. Pat. No. 3,179,544 in 
which an electrically conductive article is produced by depositing an 
electrically conductive composition comprising an aqueous dispersion of 
graphite particles upon an insulating base. Still further examples of 
electrically semi-conductive compositions can be found in U.S. Pat. No. 
2,803,566 in which an article is disclosed having a coating thereupon of a 
mixture of colloidal silica, substantially free of alkalai and in U.S. 
Pat. No. 3,413,442 in which a semi-conductive material is disclosed having 
a steep slooped positive temperature coefficient for use in electrical 
heating devices in the form of an open ended container. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide improved melt processable, 
self-temperature regulating, irradiation cross-linkable, electrically 
semi-conductive polymeric compositions adapted for use in electrical 
heating devices wherein the compositions contain an amount of electrically 
conductive particles, such as carbon black, dispersed therein that is 
controlled within the range of 17% to 25% by weight to the total weight of 
the composition and exhibit a positive coefficient of electrical 
resistance and which in conjunction with annealing at a temperature at or 
above their melt point temperatures subsequent to their having been 
radiation cross-linked provide for improved uniformity and stability in 
their self-temperature regulating electrical heating characteristics. It 
is yet another object of this invention to provide improved electrical 
heating devices utilizing two or more spaced apart electrical conductors 
that are electrically interconnected by means of electrically 
semi-conductive polymeric compositions made and processed in accordance 
with the present invention. It is a further object of this invention to 
provide improved, flexible, self-temperature regulating electrical heating 
cables comprising two or more elongate substantially parallel spaced-apart 
electrical conductors electrically interconnected by means of extruded 
forms of electrically semi-conductive compositions made and processed in 
accordance with the present invention. It is yet a further object of this 
invention to provide a method of manufacturing improved, flexible, 
self-temperature regulating electrical heating cables utilizing extruded 
forms of electrically semi-conductive compositions made and processed in 
accordance with the present invention.

DESCRIPTION OF SOME OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of the invention wherein generally tubular 
shaped flexible heating cable 1 has a generally circular transverse 
cross-section having longitudinally extended electrical conductor 2 
disposed along the central longitudinal axis thereof. Electrical conductor 
4, in the form of a metallic layer, surrounds conductor 2 and is 
substantially coaxial therewith and radially spaced apart therefrom. 
Barrier layer 3 surrounds and encloses conductors 2 and 4. Extruded and 
irradiation cross-linked electrically semi-conductive composition 5 made 
and processed in accordance with the invention is disposed intermediate 
conductor 2 and conductor 4 so as to provide an electrical interconnection 
therebetween. Outer protective jacket 6 is disposed in encompassing 
relationship about layer 3 in order to provide an electrically insulative 
protective outer covering. In the embodiment shown in FIG. 1, conductor 2 
is in the form of a metallic wire. Although conductor 2 may be made from 
nickel-chromium alloys commonly known as, Nichrome, it is preferred that 
conductor 2 be made from suitable alloys of copper or aluminum having low 
electrical resistance. Conductor 2 may be made from uncoated or 
conductively coated solid or stranded wire and is preferably sized from 
about 10 AWG to about 22 AWG and more preferrably from about 14 AWG to 
about 18 AWG. Although it is preferred that conductor 2 be in the form of 
a wire, it may have any cross-sectional shape suitable for the purpose 
intended for a particular heating cable made in accordance with the 
invention. Although it is preferred that conductor 2 be made from a 
metallic material, it may be made from a non-metallic material or from 
combinations of metallic and non-metallic material provided its electrical 
resistance is sufficiently lower than that of composition 5 to provide 
effective electrical current carrying capacity along the axial length of 
cable 1 necessary for the operation of heating cables made in accordance 
with the invention. Electrically conductive layer 4 shown in FIG. 1 
surrounds and is spaced radially apart from conductor 2 to provide a 
second electrical current carrying conductor required for operation of 
cable 1. Although conductor 4 (as in the case of conductor 2) may be made 
from an electrically conductive non-metallic material or combinations of 
non-metallic and metallic materials, it is preferred that conductor 4 be 
made from a metallic material such as suitable alloys of copper or 
aluminum. Although conductor 4 is shown in FIG. 1 as having a continuous 
transverse cross-section, it can readily be seen that conductor 4 may be 
in the form of a plurality of separate electrical conductors such as, for 
example, braided or spirally wound wire or in the form of a longitudinally 
folded or spirally wound tape. In the example shown in FIG. 1, conductor 4 
is surrounded by layer 3. Although layer 3 is not essential to the 
construction, its incorporation into cable 1 is preferred so as to provide 
improved resistance to penetration of moisture and other fluids and vapors 
from outside of cable 1. Conductor 4 and layer 3 may be bonded together. 
Conductor 4 and layer 3 may comprise a combination wherein layer 3 is a 
polymeric film such as, for example, poly(alkylene)terrephthalate and 
conductor 4 is an electrically low resistance coating thereupon such as 
copper or aluminum metal. A preferred combination of conductor 4 and layer 
3 is where conductor 4 is in the form of an aluminum or copper coating 
disposed upon a film form of layer 3 that is made from 
poly(ethylene)terrephthalate such as "Mylar" sold by E. I. du Pont de 
Nemours Company. Typically a "Mylar" film layer 3 having a 1/2 mil copper 
coating as conductor 4 may be used to advantage. As described above, it is 
preferred, but not essential, that conductor 4 be in the form of a coating 
on layer 3. Conductor 4 may be in the form of a tape with or without the 
presence in the construction of a layer 3 and may be longitudinally 
folded, spirally wound or otherwise disposed in a spaced-apart surrounding 
relationship to conductor 2. 
Outer protective jacket 6, shown in FIG. 1, is disposed in encompassing 
relationship about layer 3 to provide protection and electrical 
insulation. Although jacket 6 may be made from any suitable flexible 
material possessing the electrically insulative and protective properties 
required, it is preferred that jacket 6 be made from an extrudable 
polymeric material such as, for example, nylon, polyurethane, polyvinyl 
chloride, rubber, rubber-like elastomers, and the like possessing such 
properties. The selection of a material for use in jacket 6 is typically 
based upon combining toughness, weatherability, chemical and heat 
resistance and electrical insulating characteristics combined with 
suitable flexibility characteristics. Jacket 6 is typically in the order 
of 15 to 60 mils in thickness and may be made from crystalline, 
semi-crystalline, amorphous or elastomeric materials which may, if 
desired, be cross-linkable by means of chemical vulcanization or 
irradiation. Since part of the process of making electrical heating 
devices under this invention requires that the compositions of the 
invention be annealed at a temperature at or above their melt point 
temperatures subsequent to their having been melt-processed and 
cross-linked by irradiation, it is required, in order to retain the shape 
thereof, that covering materials present during the annealing process such 
as jacket 6 or that covering which may be temporarily used to retain the 
processed shape, have a melt point temperature higher than the temperature 
used to anneal the particular composition made in accordance with this 
invention. Although it is preferred that jacket 6 be extruded about layer 
3, it can be readily seen that jacket 6 may also be in the form of a 
winding, such as a tape, which is either spirally wound or longitudinally 
folded about layer 3 and may be suitably bonded thereto or, in the absence 
of layer 3, then either extruded, wound about, or longitudinally folded 
directly about conductor 4 and bonded thereto by suitable means, is such 
is desired, to provide the electrically insulative, protective and 
handling characteristics required. Although not shown in the preferred 
embodiments of the figures, flexible armour or other protective means may 
be disposed about the outer surface of jacket 6 to provide increased 
protection, if such is desired. 
Semi-conductive composition 5 is disposed between conductor 2 and conductor 
4 and provides an electrical interconnection therebetween. Composition 5 
is an extruded, flexible, self-regulating irradiation cross-linked 
electrically semi-conductive material containing one or more polymeric 
components and has a positive temperature coefficient of electrical 
resistance provided by an amount of electrically conductive particles, 
such as carbon black, dispersed therein that is controlled within the 
range of from 17% to 25% by weight to the total weight of composition 5. 
Composition 5 has been annealed for a period of time suitable to promote 
the electrical characteristics desired thereof at a temperature that is at 
or above its melt point temperature prior to and subsequent to its having 
been radiation cross-linked and possesses sufficient crystallinity to 
provide the self-temperature regulating characteristics desired. 
FIG. 2 illustrates an embodiment of heating cable 1 made in accordance with 
the invention wherein cable 1 has a generally bar-bell transverse 
cross-section. Shown in FIG. 2 are a pair of elongate substantially 
parallel electrical conductors 2 in the form of solid wires that are 
spaced apart along the longitudinal length of cable 1 and electrically 
interconnected by means of an extruded and irradiation cross-linked 
composition 5 made and processed in accordance with the invention. As in 
all embodiments of extruded forms of composition 5, made and processed in 
accordance with the invention, composition 5 has been annealed at a 
temperature at or above its melt point temperature prior and subsequent to 
its having been cross-linked by means of radiation. Protective jacket 6 is 
disposed in encompassing relationship about conductors 2 and composition 5 
and may comprise materials and be formed by methods hereinbefore 
described. 
As in all embodiments of the invention where jacket 6 is in direct contact 
with composition 5, it may be bonded to composition 5, if such is desired, 
and there may be additional bonded or unbonded layers about the outer 
surface of jacket 6 such as, for example, a protective flexible armour. 
There may also be a barrier layer such as, for example, "Mylar" film and 
the like, as hereinbefore described, disposed intermediate jacket 6 and 
composition 5 and which may or may not be bonded to composition 5 and/or 
jacket 6. 
FIG. 3 illustrates an embodiment similar to that shown in FIG. 1. Shown in 
FIG. 3 is generally tubular shaped heating cable 1 having a generally 
circular transverse cross-section having longitudinally extending 
electrical conductor 2, in the form of a stranded wire, located generally 
along the central longitudinal axis thereof. Electrical conductor 8 is 
substantially parallel to and spaced radially apart from conductor 2 along 
the longitudinal length of cable 1 and is in electrical contact with 
electrical conductor 7. Electrical conductor 7 in FIG. 1 is a tubular 
spaced metallic film which may be disposed coaxially about conductors 6 
and 8 by means of longitudinally folding or spirally wrapping a flexible 
tape form of conductor 7. Conductor 8 is in the form of a wire in the 
embodiment shown in FIG. 3 and is in electrical contact with the inner 
surface of conductor 7 to act as a drain wire for assisting conductor 7 in 
the transfer of electrical current along the longitudinal length of cable 
1. Conductor 2 and the combination of conductors 7 and 8 are electrically 
interconnected by means of extruded, radiation cross-linked, electrically 
semi-conductive composition 5, made and processed in accordance with the 
invention, disposed between conductor 2 and the combination of conductors 
7 and 8. Protective jacket 6 is disposed in encompassing relationship 
about conductor 7 and may or may not be bonded thereto dependent upon the 
performance or handling characteristics desired. Jacket 6, as for all 
embodiments of the invention, may have additional bonded or unbonded 
layers disposed about its outer surface such as, for example, flexible 
armour where such is desired. Cable 1 of FIG. 3 may also have a barrier 
layer disposed between conductor 7 and jacket 6 such as, for example, a 
"Mylar" film for improved resistance against fluid or water vapor 
penetration into cable 1 as herein before described. Conductor 7 may 
comprise a conductive coating upon a flexible polymeric film, as earlier 
described, such as "Mylar" wherein the conductive coating is in direct 
electrical contact with conductor 8 and the polymeric film portion is in 
contact with the inner surface of jacket 6. As in all embodiments of the 
invention, the various layers chosen may or may not be bonded together as 
desired so long as such bonding does not interfere with the ability of 
composition 5 of the invention to electrically inter-connect the two or 
more spaced-apart electrical conductors forming a part of cable 1. 
FIG. 4 illustrates yet another embodiment of the invention wherein a tape 
form of cable 1 has more than two elongate substantially parallel 
electrical conductors spaced apart along the longitudinal length of cable 
1. Such an example is for illustrative purposes only and is included 
merely to show that electrical cables made in accordance with the present 
invention are not limited to having only two spaced-apart electrical 
conductors. Cable 1 of FIG. 4 has a longitudinally extending conductor 2 
in the form of a stranded wire generally centrally located along the 
longitudinal axis of cable 1 and is electrically inter-connected by means 
of extruded, radiation cross-linked, composition 5 made and processed in 
accordance with the invention, disposed between itself and two 
diametrically apposed substantially parallel electrical conductors 9 
spaced-apart therefrom along the longitudinal axis of cable 1. Although 
conductors 2 and 9 are shown in the form of a stranded wire, it is to be 
understood, as earlier described, that electrical conductors used in 
heating devices utilizing compositions made and processed in accordance 
with the invention may be of any form suitable for the characteristics 
desired. 
Where in previous examples, a suitably selected electrical potential 
(voltage) is placed across the spaced-apart conductors to derive the 
electrical current which passes through composition 5 from one conductor 
to the other conductor to create the heating characteristics desired, so 
it is in the case where more than two conductors are utilized in heating 
cables made in accordance with the present invention. Although it is 
preferred to impose a suitably derived and controlled alternating 
electrical potential across the spaced apart electrical conductors 
utilized in heating devices of the invention, a controlled direct 
electrical potential can be used where desired. Generally, in embodiments 
of heating cables of the invention having a centrally located conductor 
such as, for example, as shown in FIGS. 1 and 3, the central conductor is 
generally preferred as the "hot" line (high potential side) and the 
conductors spaced apart therefrom towards the protective jacket are 
preferred as the "ground" (low potential side). In an embodiment such as 
shown in FIG. 3, either conductor may be used as the ground or low 
potential line. An embodiment, such as shown in FIG. 4, can be used to 
advantage that centrally located conductor 2 can be used either as the 
high or low potential line whilst the conductors 9 spaced apart therefrom 
can both be used as a carrier of electrical potential of higher or lower 
magnitude than that of central conductor 2. For example, when central 
conductor 2 is used as the "ground" or low potential line, both the 
electrical conductors 9 spaced therefrom can be used as the "hot" or high 
potential line or vice versa. A construction, such as shown in FIG. 4, 
permits wider configurations of heating cables to be made in accordance 
with the invention since the distance between conductors is an important 
factor in conjunction with the semi-conductive nature of the composition 
electrically inter-connecting the conductors whereby such distances can be 
reduced by the use of more than two conductors and thereby reduce the 
amount of electrical potential required to drive the desired electrical 
current through the semi-conductive composition to create the heating 
characteristics required. Cable 1 of FIG. 4 has flexible protective jacket 
6 disposed about electrically semi-conductive composition 5 and conductors 
2 to provide the protective and electrical insulating characteristics 
desired. As in all embodiments of the invention, jacket 6 may have 
additional bonded or unbonded barriers disposed between it and composition 
5, as hereinbefore described, and may be surrounded by bonded or unbonded 
layers such as, for example, a flexible armour. 
Although the electrically conducting particles used in compositions of the 
invention may be metallic in nature such as, for example, silver, 
aluminum, iron, or the like, it is preferred that carbon particles such as 
carbon black or graphite be used and more preferred that a highly 
electrically conductive furnace black be used such as, for example, Vulcan 
XC-72 sold by Cabot Corporation. Although the amount of electrically 
conductive particles present in the compositions of the invention is 
controlled within the range of 17% to 25% by weight to the total weight of 
the particular composition, it is preferred that the amount of conductive 
particles be from about 20% to about 22% by weight to the total weight of 
the particular composition. 
Compositions of the invention may be made from polymeric, homopolymers or 
copolymers of crystalline materials such as, for example, polyethylene, 
polypropylene and blends thereof. Generally, the compositions of the 
invention contain one or more melt-processable crystalline and/or 
semi-crystalline polymeric materials which may be combined with suitably 
selected amorphous and/or elastomeric polymeric materials provided that 
the completed compositions of the invention made therefrom remains 
melt-processable. A composition made in accordance with the invention may, 
for example, contain a copolymer or blend of low density polyethylene and 
ethylene vinyl acetate as the crystalline melt-processable component 
thereof. Generally the type and crystalline aspects of a particular 
polymer or combination of polymers selected for use in making compositions 
of the invention determines the hereinbefore described controlling 
temperature "T.sub.c " about which the composition will self-temperature 
regulate. Thus, for example, a composition of the invention based upon a 
particular low density polyethylene might be made to self-temperature 
regulate about 70.degree. C. whereas a composition of the invention based 
upon a polypropylene might be made to self-temperature regulate about 
90.degree. C. Higher controlling temperature "T.sub.c " may be provided by 
formulating compositions of the invention to include melt-processable 
fluorinated and/or fluorochlorinated materials such as, for example, 
polyvinylidene fluoride and copolymers thereof with tetrafluoroethylene, 
and the like. Generally, the one or more polymers chosen for use in making 
a particular composition of the invention are selected on the basis of 
their nature and crystalline contents in conjunction with the hereinbefore 
described electrically conductive particles and other additives (if such 
are desired) to provide a melt-processable composition that provides a 
controlling temperature "T.sub.c " after being processed in accordance 
with the invention that is satisfactorily beneath the long-term heat 
exposure degradation level determined or known for the particular 
composition. 
Compositions of the invention may contain other additives such as, for 
example, processing aids, fillers, anti-oxidants, heat stablizers, and the 
like, provided that the resultant composition remains melt-processable and 
radiation-cross-linkable while providing the physical, chemical, heat 
resistance and self-temperature regulating characteristics desired. 
The flexibility of compositions made in accordance with the invention is 
accordingly dependent upon the crystallinity and nature of the polymers 
selected for their making in addition to the effects created by the 
incorporation of the controlled amount of electrically conductive 
particles of the invention and other additives which may be included as 
described above. Thus compositions made in accordance with the invention 
may range from relative rigid versions having melt processability 
characteristics more suitable for injection molding to more flexible 
versions having melt-processing characteristics more suitable to the 
process of extrusion such as, for example, for use in making the flexible 
heating cables of the invention. Generally, the method of melt-processing 
a particular composition made in accordance with the invention can be 
determined by means of experimentation and examination of the rheological 
aspects of the particular composition. Although electrical heating cables 
made from extruded forms of the compositions of the invention require 
annealing prior and subsequent to their cross-linking by radiation, 
compositions melt processed by other methods to make electrical heating 
devices of the invention may not require annealing prior to their 
radiation cross-linking. 
It is required that compositions of the invention be cross-linked by 
radiation subsequent to their having been melt-processed into the form 
required for the particular self-temperature regulating device desired. In 
making electrical heating cables of the invention, it is preferred that 
the compositions of the invention be extruded since it provides economic 
savings and other advantages associated with the capability of producing 
long continuous lengths. Although any suitable means of radiation may be 
used to cross-link compositions of the invention, it is preferred that 
they are cross-linked by means of suitable exposure to high speed 
electrons such as, for example, as produced by a high energy electron Beam 
Generator. Other components used in electrical heating devices in 
combination with compositions of the invention (such as, for example, the 
outer protective jacket of flexible heating cables of the invention) may 
also be cross-linked by irradiation during the process of making the 
device if such is desired. The irradiation cross-linkability of 
compositions of the invention may be improved by the incorporation therein 
of radiation sensitizing materials such as, for example, m-phenylene 
dimaleimide sold under the name of "HVA-2" E. I. du Pont de Nemours and 
Company in the event it is determined that such is required. 
It has been found that the incorporation of a controlled amount of 
electrically conductive particles, such as carbon black, into compositions 
of the invention and subsequently cross-linking them by radiation, after 
their having been melt-processed, in combination with the annealing 
thereof at a temperature at or above their melt point temperature 
subsequent to radiation cross-linking provides improved self-temperature 
regulating electrical heating devices that have been heretofore 
unavailable. It has been found that the incorporation of between 17% to 
25%, by weight, of carbon black, such as Vulcan XC-72, into compositions 
of the invention results in an electrical resistance at 25.degree. C. 
(R.sub.25) which is low enough to permit effective heating whilst using an 
effective level of electrical current yet provides a controlling 
temperature (T.sub.c) for keeping the heat generated sufficiently below 
the long-term maximum continuous use temperature associated with the 
composition in combination with an effective peak electrical resistance 
(R.sub.p) to protect the composition from self-destructing. 
An example of a flexible heating cable made in accordance with the 
invention and its comparison to heating cables containing less than 15% 
carbon black in conjunction with variations in annealing techniques is 
illustrated in the following table. 
__________________________________________________________________________ 
SAMPLE* 
A B C D E 
__________________________________________________________________________ 
Polymeric 
All are low density Polyethylene 
Component 
% Carbon Black 
11 22 22 11 22 
(Vulcan XC-72) 
Annealing 
1 2 3 4 5 
Schedule*** 
R.sub.25 (ohm/ft.) 
3.2 .times. 10.sup.4 
5.4 .times. 10.sup.2 
3.9 .times. 10.sup.3 
1.1 .times. 10.sup.8 
5 .times. 10.sup.2 
R.sub.p (ohm/ft.) 
4 .times. 10.sup.8 
1.1 .times. 10.sup.5 
3.9 .times. 10.sup.7 
1.8 .times. 10.sup.9 
Not Tested 
R.sub.p /R.sub.25 
12,500 
204 10,000 
16 Not Tested 
Current Draw on 
4 230 23 Not Tested 
Not Tested 
Energizing 
(mA)** 
T.sub.c (.degree. C.)** 
22 66 31 Not Tested 
Not Tested 
Controlling 
3.5 42 13 Not Tested 
Not Tested 
Current (mA)** 
__________________________________________________________________________ 
*The compositions are blends of low density polyethylene and the indicate 
amount of carbon black without additional additives. The heating cables 
containing the compositions were made by extruding the compositions about 
a pair of spaced apart 18 AWG (19 Strand) tinned copper conductors such 
that the cables assumed a barbell transverse crosssectional shape such as 
shown in FIG. 2. A shaperetaining jacket of polyurethane was extruded 
about the extruded composition and conductors to prevent deformation 
during the annealing process. 
**Ambient Temperature 17.degree. C. 
***Annealing Schedule: 
(1) 24 hr. at 150.degree. C. without any crosslinking or annealing 
thereafter. 
(2) Same as (1) above. 
(3) 24 hr. at 150.degree. C. prior and 1 hr. at 150.degree. C. subsequent 
to crosslinking by electron irradiation. 
(4) Same as (3) above. 
(5) 24 hr. at 150.degree. C. prior to crosslinking by electron radiation. 
The above comparison illustrates that Sample "C" (made and processed in 
accordance with the invention) possesses an effectively low (R.sub.25); an 
attractively high (R.sub.p); and effective (R.sub.p /R.sub.25); and an 
attractive (T.sub.c). 
It has been found that compositions made and processed in accordance with 
the present invention exhibit improved long-term operating stability over 
that of Sample "A" at a (T.sub.c) attractively below the long-term maximum 
use temperature established for the composition as a result of the 
controlled amount of carbon black of the invention. It has also been found 
that heating cables such as Sample "B" above which contain more than 15% 
carbon black and which have not been cross linked by radiation and 
subsequently annealed at a temperature at or above the melt point 
temperature of the respective compositions tend to either fail or exhibit 
erratic heating performance in actual use which is believed to be the 
result of their having an extremely low R.sub.25 ; low R.sub.p /R.sub.25 ; 
and high T.sub.c. It has been found that heating cables processed in 
accordance with Sample "B" may fail catastrophically after energization. 
It has also been found that compositions such as Sample "D" having less 
than 15% carbon black and processed in accordance with the invention tend 
to have a high R.sub.25 causing them to perform relatively ineffectively 
as heaters. 
Sample "E" above is the same as Sample "C" except it has not been annealed 
at a temperature at or above its melt point temperature after having been 
cross-linked by radiation. Sample "E" illustrates that by not annealing 
the composition after cross-linking the R.sub.25 of the composition 
remains low in comparison to that shown for Sample "C" above. It has been 
determined that a low R.sub.25 such as found in Sample "E" provides poor 
heat regulating characteristics. 
FIG. 5 illustrates, by means of block diagrams, the basic steps of the 
preferred process by which flexible heating cables utilizing extruded 
compositions of the present invention can be made. Generally, the 
hereinbefore described polymeric components, conductive particles and 
additional additives, if any, of the present invention are uniformly mixed 
and blended by suitable means such as, for example, by use of a Brabender 
Batch type or Henschel continuous type mixer, extruder, and the like. 
Although it is preferred that the components be mixed and blended in 
conjunction with sufficient heat to promote uniform distribution of the 
conductive particles prior to the extrusion of the compositions, as shown 
in Step "A", into a flexible heating cable, the components, dependent on 
the particular composition, may be dry blended and extruded directly to 
electrically inter-connect the one or more electrical conductors making up 
the particular heating cable provided that such blending disperses the 
conductive particles uniformily. Although the annealing step shown in Step 
"C" may not be required in certain melt-processing techniques other than 
extrusion, it has been found that, because of the disruptive effect of 
extrusion upon the electrical characteristics of the compositions of the 
invention, annealing is required prior to irradiation cross-linking in 
making electrical heating cables under the present invention in order to 
achieve the characteristics desired. Since the annealing Step "C" is at a 
temperature that is at or above the melt point temperature of the 
composition, it is required that a shape retaining covering be disposed 
thereabout as illustrated by Step "B" of FIG. 5. The shape retaining cover 
is required to have a melt point temperature that is higher than that of 
the annealing temperature in order to prevent or minimize deformation of 
the extruded composition. The covering, dependent upon the particular 
heating cable being made, may be temporary or permanent in nature. If it 
is permanent in nature such as, for example, an extruded jacket, barrier, 
or conductor, it must be penetrable by the radiation of Step "D" in order 
that the composition beneath the covering can be cross-linked and, 
dependent upon materials used; may themselves be cross-linked by radiation 
during the process of cross-linking the composition of the invention. If 
the covering is temporary and provides no other function other than shape 
retainment and is intended to be removed after annealing then it is 
required to have a melt point temperature higher than the annealing 
temperature and may or may not be penetrable by radiation depending upon 
whether it was removed after annealing Step "C" and before Step "D" or 
after annealing Step "E". The extruded form of the electrical cable having 
a shape retaining cover is annealed in Step "C" at a temperature that is 
at or above the melt point temperature of the composition for a period of 
time sufficient to effect the characteristics desired. Generally, 
annealing Step "C" is required in order to reduce the electrical 
resistance elevations resulting from the disruptive effects of extrusion. 
Although not shown in FIG. 5, it is to be understood that cooling the 
composition of the invention from a higher temperature to a lower 
temperature is included in the process of making heating devices such as 
heating cables under the invention. Although, it is within the scope of 
the invention that certain types of heating devices may be made under the 
invention in a continuous manner without substantial cooling excepting 
after its annealing after cross-linking by radiation, it is preferred that 
the composition be cooled at least to a temperature sufficient to provide 
suitable handling characteristics subsequent to its melt processing and 
annealing steps and after the shape retaining covering step, if such is 
applied by melt processing such as, for example, by extruding a shape 
retaining jacket about the composition of the invention. Obviously all 
compositions of the invention are cooled to ambient temperature after 
their annealing subsequent to having been cross-linked by radiation. The 
process of the invention also includes the simultaneous melt processing of 
compositions of the invention in conjunction with the application of a 
shape retaining covering thereabout such as, for example, extruding a 
composition of the invention into a form suitable for use as a heating 
cable whilst simultaneously extruding a shape retaining protective jacket 
thereabout. Compositions of the present invention can be satisfactorily 
annealed both in Steps "C" and "E" by exposure for a period of time 
sufficient to promote the electrical characteristics desired thereof at a 
temperature of the composition. After the annealing of Step "C", the 
composition (in the form of a completed or semi-finished heating cable as 
the case may be) is cross-linked by means of radiation (preferably 
electron radiation) in Step "D". The finished or semi-finished electrical 
cable, as the case may be, having the extruded and radiation cross-linked 
composition, as a part thereof, is annealed at a temperature at or above 
the melt point temperature of the composition in Step "E". Whether 
electrical cables of the invention enter into Steps "C", "D" and "E" as a 
finished product would, as described above, depend upon the particular 
cable and the melt point and radiation penetrability of any barrier, 
conductor, covering or jacket which might be placed about the outer 
surface of the extruded composition prior to the annealing and/or 
radiation steps. 
Although the invention is described in detail for the purpose of 
illustration, it is to be understood that such detail is solely for that 
purpose and that variations can be made therein by those skilled in the 
art without departing from the spirit and scope of the invention.