Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
CONDUCTIVE POLYMERIC CONDUIT HEATER
This invention relates to a conduit comprising conduc-
-tive polymeric material, which can be used to heat a fluid
passing through the conduit.
Conductive polymeric materials and devices incorporating
them are also well known. ~eference may be made for example
to U.S. Patents Nos. 2,952,761, 2,978,665, 3,243,753,
3,351,882, 3,571~777, 3,658,976, 3,757,086, 3,793,7].6,
3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715,
4,072,848, 4,085,286, 4,117,312, 4,151,126, 4,177,376,
4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468,
4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471,
4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231,
4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704,
4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084,
4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877,
4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,470,898,
4,481,498, 4,476,450, 4,502~92g; 4,514,620, 4,517,449,
4,534,889, and 4,560,498; J. Applied Polymer Science 19,
813-815 (1975), Klason and Kubat; Polymer Engineering and
Science 18, 649-653 (1978), Narkis et al; European
Application Nos. 38,713, 38,714, 38,718, 74,281, 92,406,
119,807, 133,748, 134,145, 144,187, 157,640, 158,410,
175,550 and 176,284; and Japanese Published Patent
Application No. 59-122,524.
The use of articles comprising conductive polymeric
~ materials as heaters is known. One of the most common types
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of conductive polymeric heaters is a strip heater. This is
typically in the form of an elongate strip of conductive
polymeric material with two elongate electrodes o-f opposite
polarity embedded in the strip on either side thereof,
extending along the lenyth of the heater. When the electro-
des are connected to a power source, curren-t flows across the
width of the strip to heat the strip. Such a heater is
typically employed by wrapping it around, or taping it to,
the substrate -to be protected, for example around or to a
pipeline. It may be used for example as a frost protection
device, or to maintain a fluid carried by the pipeline at a
certain temperature.
Typically strip heaters exhibit PTC behaviour ~positive
temperature coefficient of resistivity), that is a sharp
rise in resistivity at a particular temperature, or over a
short range of temperatures. This temperature or tem-
perature range is known as the anomaly or switching tem-
perature. Typically the material, which is cross-linked, is
selected to exhibit the sharp rise in resistivity at, or
slightly above, the melting temperature of the composition.
The PTC behaviour is advantageous since it means that if one
region of the article preferentially draws more current and
consequently becomes hotter than adjacent regions, the
resistivity of that region rises significantly, mitigating
the effect of the preferred current draw to that region.
Hence heating is stabilised and overhea-ting of any par-
ticular region avoided. Materials which exhibit a PTC
effect are typically referred to simply as PTC materials.
The terms "composition exhibiting PTC behaviour" and
"PTC composition" are used in this specification to denote a
composition which has an R14 value of at least 2.5 or an Rloo
value of at least 10, and preferably both, and particularly
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one which has an R30 value of at least 6, where R14 ;s the
ratio of the resistivities at the end and the beginning of a
14C range, R1oo is the ratio of the resistivities at the
end and the beginning of a 100C range, and R30 is the
ratio of the resistivities at the end and the beginning of
a 30C range.
Another use of known conductive polymeric heaters is as
a heating element for an electric blanket. One such heater,
which in fact uses a PTC material, is described in US
4271350 (Sunbeam). In one embodiment (Figure 2 of the
reference) the heater comprises a flexible insulating centre
core formed of stranded glass or rayon. Wrapped on the core
are helically wound spaced conductors separated by insu-
lating spacers, and surrounding these an extruded sleeve of
uncross-linked PTC material and surrounding jacket of a
thermoplastic coating. The PTC material must be annealed to
give it its required temperature resistance characteristics.
The thermoplastic coating material has a higher melting
point than the uncross-linked PTC material, and hence can main
tain the integrity of the PTC material during the annealing
process. Also the spacers between the electrodes prevent
physical contact of the electrodes during softening of the
PTC material during the annealing process.
Another use of a conductive polymeric heater is
described in GB 2065430A (Junkosha). This heater uses a
conductive material comprising sintered carbon-~illed PTFE
made by blending 10 wt % of electroconductive carbon with
PTFE fine powder and a liquid lubricant, ram extruding the
material and subsequently removing the lubricant. The con-
ductive material may be made in tape form wrapped around an
inner tube. Alternatively the conductive material may be
made in tube form itself. Electrodes are positioned on the
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conductive materia1 and powered to pass electrical current
through the conductive material, thereby heating it.
We have discovered a new method and article employing
conductive polymeric material which exhibits PTC behaviour
for use as a heater. The article and method involve using a
conductive polymeric material in the form of a condui-t,
which can be used directly to heat fluid flowing
therethrough.
A first aspect of the invention provides a hea-table
conduit suitable for heating a fluid passing through the
conduit, comprising
(a) an electrically conductive polymeric material which
exhibits PTC behaviour and comprises an organic
polymer, and dispersed in the polymer a particulate
conductive filler, and
(b) at least one pair of electrodes of opposite polarity
which are helically arranged and in contact with
the conductive material, which when connected to a
source of electrical power cause a substantial por-
tion of the electrical current to flow between the
electrodes substantially parallel to the faces of
the conduit, thereby heating the electrically con-
ductive polymeric material and hence the fluid.
~ second aspect of the invention provides a heatable
conduit suitable for heating a fluid passing through the
conduit, comprising
(a) an electrically conductive polymeric material, which
exhibits PTC behaviour and comprises an organic
polymer, and dispersed in the polymer a particulate
conductive filler,
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(b) an electrically insulating hollow tubular liner, and (c)
at least one pair of electrodes of opposite polarity which
are in electrical contact with the conductive polymeric
material and which when connected to a source of electrical
power cause a substantial portion of the electrical current
to flow between the electrodes substantially parallel to the
faces of the conduit, thereby heating the electrically
conductive polymeric material and hence the fluid.
More particularly the invention provides a heatable conduit
suitable for heating a fluid passing through the conduit,
comprising (a) a tubular member prepared by melt extruding,
in the form of a tube, an electrically conductive polymeric
material which exhibits PTC behaviour and comprises an
organic polymer, and dispersed in the polymer a particulate
conductive filler, and (b) at least two spaced apart
electrodes of opposite polarity each of which is helically
arranged in relation to the tubular member and is in contact
with the conductive material, so that when the electrodes are
connected to a source of electrical power, electrical current
flows between them, with a substantial portion of the
electrical current being substantially parallel to the faces
of the tubular member, thereby heating the electrically
conductive polymeric material and hence the fluid.
A urther aspact o~ the inven~isn provides method of
heating a fluid comprising: (a) providing a conduit as
defined immediately above, and (b) connecting the electrodes
to a source of electrical power to cause a substantial
portion of the electrical current to flow between the
electrodes substantially parallel to the faces of the
conduit, thereby heating the conduit and hence the fluid.
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The fluid passing through the tube may be a liquid or a gas.
The conductive polymeric material exhibits PTC behaviour.
This has two advantages. First the article is self
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regulating. This means tha-t for a constant voltage supply,
the power output adjusts automatically, compensating automa-
tically for fluctuations in ambient conditions, to maintain
a constant temperature in the conduc-tive polymeric material.
Secondly, the article is self limiting. This means there is
a maximum temperature which the article never exceeds since
at the anomaly temperature or temperature range the conduc-
tive polymeric material becomes non-conductive, and effec-
tively "switches itself off", so that no further heating
above the anomaly temperature takes place.
The article according to the invention may be operated
at temperatures where it is self regulating, or at tem-
peratures where it is self limiting or both.
In a preferred method according to the invention the
electrodes are connected to a source of electrical power
which is operated so that a sufficient power is applied so
that the article operates in a self regulating manner. For
some applications, the conduit is maintained at a tem-
perature which is preferably in excess of 50C, more pre-
ferably in excess of 80C, and for some applications in
excess of 100C. For other applications the condui-t is pre-
ferably maintained at lower temperatures, e.g. at less than
25C, for examp'le at about 10C for frost protection or at
about -10C for prevention of waxing in diesel fuel.
In preferred embodiments according to the invention the
conduit is both the heater for the fluid and also the con-
tainer for the fluid. In other embodiments the fluid is
contained in another tubular vessel, and the conduit
according to the invention is placed to surround that vessel
in thermal contact therewith.
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The heating of fluids in pipes by wrapping conductive
polymeric strip heaters around the pipes is known in the
prior art. However, an advantage of the conduit o~ the pre-
sent invention compared to strip heaters is that the conduit
can be in contact with the entire outer surface of the
vessel containing the fluid; or where the conduit is the
container as well as the heater for the fluid, it is in
direct contact with the volume of fluid. Strip heaters in
contrast are typically wrapped around pipes to be heated
with spaces between adjacent turns, i.e. they are not in
contact with the entire outer surface of pipes. ~
Another advantage of the conduit according to the inven-
tion is that, being tubular, it can be used around, or to form
small diameter conduits for the passage of fluids.
Typically it is difficult to wrap traditional heater strips
around small diameter pipes where the angle of bend is
extreme.
When the conduit is not the container for the fluid
(i.e. when the fluid is in another vessel in thermal contact
with the conduit) the conduit may be a push fit over the
container, or it may be a loose fit over the container and a
thermally conductive medium introduced between the conduit
and the container. The thermally conductive medium may
comprise, for example a grease, or a thermally conductive
adhesive. Preferably the thermally conductive medium has a
sufficiently low viscosity that it can flow to fill the
space between the conduit and the container.
The conductive polymer material is preferably in the
form of a tube. In this case the conductive polymeric
material preferably forms a continuum between the elec-tro-
des, i.e. there are no air gaps or electrically insulating
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gaps between the electrodes. This is to be contrasted with
the embodiment in GB 2065430A (Junkosha) in which a conduc-
tive tape with electrodes on the long edges thereof is
wrapped around a core tube. In that case there is an air
gap between the edges of adjacent turns of the tape, and
hence between adjacent electrodes on those turns.
In another embodiment the conductive polymeric material
is in the form of discrete members extending between the
electrodes. For example, the discrete members may be fibres
or tapes. These are preferably helically arranged and used
with helical bus bars or electrodes. The helical busbars
are preferably helically wound in the opposite sense to the
helix of conductive polymeric fibres or tapes.
The conduit according to the invention may be lined with
a layer of electrically insulating material. This is par-
ticularly advantageous in conditions where the medium in
contact with the inner surface of the conduit is electri-
cally conductive, since in these conditions the electrically
insulating lining prevents short circuiting of the electri-
cal current through the electrically conduc-tive medium in
contact with the inner surface of the conduit. For example,
where the conduit is also the fluid container, and the fluid
is electrically conductive, the electrically insulating
layer prevents short circuiting through the fluid.
Similarly, where the conduit surrounds a vessel for the
fluid, and the vessel is electrically conductive, the
electrically insulating layer prevents short circuiting
through the vessel. The conduit may also have an outer
coating of an electrically insulating layer. This may be
advantageous where the conduit is in environments which
might cause short circuiting, e.g. in damp or wet environ-
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Where an electrically insulating inner or jacket, orboth, are used, these members may also be selected for their
thermal properties. In one embodiment the electrically
insulating inner is preferably relatively thermally conduc-
tive. For example it may include additives toincreaseits
thermal conduc-tivity compared to the material thereof without
the additives. A suitable liner is an electrically insu-
lating polymer with particulate fillers of high thermal con-
ductivity, such as zinc oxide. Iiners without additives,
i.e. unfilled polymers, may be acceptable in many applica-
tions, particu~arly if they are thin and thus do not consti-
tute a very large thermal barrier. Preferably the lining
has an electrical resistivity of at least 101 ohm.cm.
In another embodiment, the electrically insula-ting
jacket is also thermally insulating, to minimise heat losses
externally from the conduit. For example, it may comprise a
foam or other material having a very low thermal
conductivity.
Where a liner or outer jacket or both are used, -the
composite structure may be constructed in the factory. This
has a number o~ advantages compared to installation of one
of the layers, e.g. the outer layer, in the field. First
there is no risk of there being any gaps between the conduit
and i-ts jacket which may result in non uniform or incorrect
thermal properties. Secondly the type and nature of the
lining and jacket can be determined in the factory having
regard to the required electrical and thermal proper-ties and
the installation design. This avoids the common problem of
the wrong type or thickness of jacket being installed in the
field. Thirdly a factory-installed outer jacket provides
mechanical and chemical protection for the conduit while it
is in transit. The composite design can advantageously be
cut to length in the field.
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Where an outer jacket is used this may be applied in
the factory at the same time as the conductive polymeric
material of the conduit, e.g. by co-extrusion, or sub-
sequently, e.g. by tandem extrusion~
Where the conduit is in direct contact with the fluid
the inner electrically insulating layer is preferably
selected to be chemically resistant to the fluid. For
example, where the fluid is water, the inner electrically
insulating layer is preferably polyethylene and where the
fluid is diesel fuel, it may be for example nylon, polych-
loroprene or polyvinylidene fluoride.
Where the fluid carried by the conduit is electricaly
insulating, there is no requirement for an electrically
insulating layer to be used. In -this case a liner may be
omitted, but the conductive polymeric material of the con-
duit itself is preferably chemically resistant to the fluid.
The outer electrically insulating layer, where used, may
be any suitable material depending on the environment to
which the heater is exposed. If it is to be used in damp
environments where there is the possibility of water short
circuiting the e?ectrodes a preferred outer electrically
insulating layér is polyethylene. If the heater is prone to
splashing by e.g. diesel fuel, a preferred outer electri-
cally insulating layer is nylon, polychloroprene or polyvi-
nylldene fluoride.
Where either inner or outer electrically insulating
~layers, or both, are used the electrodes may be posi-tioned
~between the inner layer and the electrically conductive
polymeric material, or between the outer layer and the
electrically conductive polymeric material. Alternatively
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the electrodes may be embedded in the electrically conduc-
tive polymeric material.
Where a multi layer structure is used, each layer may be
added sequentially for example by sequential extrusion, or
the layers may be integrally formed, e.g. by coextrusion.
The electrodes may be any suitable shape. Examples are
wire, foil strips, metallic braid, conductive mesh, corru-
gated metal strip, silver paint or spray coated metal. The
electrodes may be ultrasonically bonded into thermal contact
with the conductive polymeric material.
The electrodes extend generally longitudinally. They
may extend substantially parallel to the major axis of con-
duit or be helically wrapped.
The use of helically wrapped electrodes has a number of
advantages. An important advantage is improved flexibility,
as compared, for example, with straight electrodes extending
parallel to the axis of the conduit. Another advantage of
helically winding the electrodes is that it enables two (or
a small number) of electrodes to be used around even large
diameter pipes while maintaining a small electrode separa-
tion. This is important -to avoid hot-lining which is a
known problem encountered with the use of conductive poly-
meric materials which exhibit PTC behaviour. With these
materials, when the electrical current flows parallel to the
surface, there is a tendency for a zone of high resistance
and consequent high voltage gradient to develop between, and
generally parallel to, the electrodes during electrical
powering. This phenomenom is known as "hot-lining" and may
eventually lead to damage to the heater. Hot-lining is
minimised by minimising the electrode separation. The heli-
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cal configuration enables not only small, but also large,
diameter substrates to be electroded using two electrodes
while maintaining the same optimum small electrode separa-
tion. Other features which affect hot-lining are the
applied voltage and the material properties. Taking these
into consideration, the pitch of the helical winding of the
electrodes (which defines the electrode separation) can be
selected to avoid hot-lining while providing the required
heat output. Thus the simple procedure of varying the pitch
of the electrode windings increases the ability to select
different materials and diameters for the conduit. Changing
the pitch of the helix can be used to alter the range of
power per unit length for conduit of given size, and also to
vary the power output along its length. Other factors which
are also relevant to affecting the power output, which can
be changed, include the wall thickness of the conductive
polymeric layer, and the resistivity of the conductive
material.
A.C. or D.C. power sources may be used to supply power
to the articles.
There may be a single pair of electrodes of opposite
polarity, or multiples of pairs. For example multiples of
~pairs of helically wrapped electrodes may be used.
A'so three electrodes, (or groups of three) may be used
with a three phase power system. For example three elec-tro-
des may be wrapped helically.
It rnay be useful to include a layer of material adja-
cent to the electrodes having a resistivity lower than that
of the bulk of the material, for example to minimise heating
~ around the electrodes. This additional layer may exhibit
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ZTC (zero temperature coefficient) NTC (negative temperature
coefficient) or PTC behaviours. The layer may be, for
example, a conductive adhesive.
~ n additional conductive polymeric layer (more, less or
as conduc-tive as the bulk of the material adjacent the
electrodes) may also be used to minimise the contact
resistance between the electrodes and the conductive poly-
meric material in the bulk of the article.
It is known from GB 1600256 and GB 1600257 that for
strip heaters comprising a conductive polymeric composition
in contact with electrodes, the longer they are in service
the higher the resistance and the lower the power output of
the heaters. It is also known from those references that
the lower the initial contact resistance between the
electrodes and the conductive polymer composition the
smaller the increase in total resistance with time.
According to the references, the contact resistance can be
decreased by a process which comprises:
(1) heating a conductive polymer composition to a tem-
perature (Tp) above its melting point (Tm);
(2) heating an electrode, out of contact with the conductive
polymer composition, to a temperature (Te) above the
melting point of the conductive polymer composition;
(3) contacting the electrode and conductive polymer com-
position in contact therewith.
- In one case according to the present invention the
; electrodes are helically positioned. In another case
according to the invention an inner lining layer is used,
and this may have a melting point lower than the processing
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temperature of the conductive polymeric composition and/or
lower than the preferred preheat temperature (Te) of the
electrodes. Thus in these cases it is not possible, or at
least extremely difficult to provide a hollow conduit using
the heating process o-f GB 1600256 to minimise the contact
resistance between the electrodes and the conductive polymer
composition. Thus according to the present invention an
additional conductive polymeric layer is preferably provided
around the electrodes before positioning in contact with the
inner lining layer (where present) and the bulk conductive
polymeric material. The additional conductive polymeric
layer is applied to electrodes using wire heating according
to GB 1600256 to minimise contact resistance therebetween.
The additional conductive polymer layer and the bulk conduc-
tive polymeric material are compatible. Thus the contact
resistance between the electrodes and the bulk conductive
polymeric material is minimised.
A preferred process for minimising contact resistance
according to the present invention comprises the following
stages. The electrode and the additional conductive poly-
meric layer are heated to a temperature above the melting
temperature of the material of the additional conductive
polymeric layer and then brought into contact with each
other. This produces a low contact resistance between the
electrode and the additional conductive polymeric layer (as
described in GB 1600256). At some subsequent stage (which
may be immediate or some time later) the electrodes are
wrapped around an inner electrically insulating liner or
core (e.g. nylon). This wrapped core tube is then fed into
a cross-head extruder die. A preheat oven may be used just
before the cross-head to heat the coated wires on the core
tabe to a temperature high enough to soften the wlre
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coating, but not high enough to mel-t the core tube The bulk
conductive polymeric ma-terial is extruded over the liner at
a temperature below -the melting tempera-ture of the inner
electrically insu1ating liner. This temperature is suf-
ficient to bond the polymeric material of the conductive
layer around the electrodes to the bulk conductive polymeric
material, but is not sufficiently high to melt the inner
electrically insulating liner.
Where mechanical Flexibility is important the material
and thickness of the conduit, i.e. the conductive polymeric
and electrically insulating layer(s) are preferably selected
to allow flexibility. For some applications the conduit is
sufficiently flexible that it can be bent around a ~ inch
diameter mandrel by hand, or more preferably around a 1 inch
diameter mandrel by hand. For other applications less
flexibility is required. The preferred helical arrangement
of electrodes, where used, also enhances flexibility.
The conduit of the present invention may be used for
example, to heat water flowing through a tube. The conduit
may be the container for the water or heat a tube already
containing the water. Applications are, for example, the
prevention of freezing of water in conduits leading to
windscreen washers for automobiles, and the heating of water
for domestic use. Similar examples are for example
heating diesel fuel. For these applications, long lengths
(e.g. several metres) of heated conduit may be used.
A'ternatively a short length of heated condui-t may be used
at one end of a conduit containing fluid to effec-t the
heating, e.g. at the end of a shower hosing. The conduit is
particularly advantageous for heating fluids where small
bore conduits are required, especially those having an
~internal diameter less than 25mm, more preferably less than
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lOmm, especially less than 5mm. Such objects are difficult
to heat or are heated inefficiently using strip heaters,
since such heaters either have to be wrapped around -the
small diameter, or applied longitudinally along one side
resulting in non-uniform heating.
Some embodiments according to the invention involve
delivering heated fluid from the conduit. Examples of this
which may be mentioned included heated conduits for
windscreen washer water on automobiles, and heated showers.
~n these cases a jet is usually included to direct the
water. The jet may be incorporated in a conduit according
to the invention. The jet may also be heated, separately or
in conjunction with the conduit. The jet may be made
integrally with the conduit.
The heated conduit according to the invention may be
electrically or mechanically connected, or both, to members
other than a jet assembly. For example it may be connected
to lengths of pipe extending from or through the conduit
(including junctions, connections, T's and Y's) or to taps
or to valves. Thus the present invention also includes a
heatable conduit according to the invention in combination
with electrical and/or mechanical connection means which
connect the conduit to other objects, for example to the
power supply, or to parts extending through or from the
heatable conduit.
Any suitable material can be used for the conductive
polymeric material of the article, including any of the
materials disclosed in the references to conductive polymeric
materials and devices incorporating them mentioned above.
The polymer preferably comprises at least one thermoplastic
crystalline polymer. Particularly useful polymers are ole-
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fin polymers, including homopolymers, particularly polyethy-
lene and the polyalkenamers obtained by polymerizing
cycloolefins; copolymers of two or more olefins; and copoly-
mers of one or more olefins, e.g. ethylene or propylene,
with one or more olefinically unsaturated comonomers, pre-
ferably polar comonomers, e.g. vinyl acetate, acrylic acid,
methyl acrylate and ethyl acrylate. A'so particularly use-
ful are fluoropolymers (which may be olefin polymers), in
particular polyvinylidene fluoride and copolymers of ethy-
lene with te-trafluoroethylene and/or a perfluorocomonomer.
Mixtures of polymers can be used, including mixtures of
crystalline and amorphous, e.g. elastomeric, polymers. One
example that can be used is a semiconductive copolymer com-
pound DHDM 7704 black supplied by BP Chemicals ~td (which
comprises a dispersion of carbon black in an ethylene
ethylacrylate copolymer).
Within the conductive polymeric material the conductive
particles are preferably substantially uniformly distributed
in the polymer. Preferably the conductive PTC polymeric
material is made by melt processing. This method advan-
tageously results in uniform dispersion.
The conductive material can be shaped into a conduit by
any suitable method, especially by a melt processing tech-
nique, e.g. by melt extrusion.
Preferably the amount of conductive filler used is at
least 10 weight %. Preferably the conductive filler is carbon
black.
The conductive polymeric material useb in the conduit is
preferably cross-linked. It may be cross-linked by irra-
diating it with high energy electrons to a beam dose in
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the range 2-35 MRads especially 2-25 MRads for example 10
or 15 MRads. The cross-linking may also be effected
chemically.
The degree of cross-linking of the conductive polymer
may be expressed in terms of gel content (ANSl ASTM
D2765-68) of the polymeric matrix of the condutive polymer
(i.e. excluding the conductive filler or any other non-
polymeric additives present). Preferably the gel content of
the polymeric matrix is at least 10%, more preferably at
least 20%, e.g. at least 30~, more preferably at least 40%.
The cross-linked conductive material used in the invention
will have a mel-ting point at least as high, or higher than
its typically immediately surrounding jacket layer or any
surrounding jacket layer.
Embodiments of the invention will now be described, by
way of example, with reference to the accompanying drawings
wherein:
Figure 1 is a first embodiment of conduit according to
the invention,
Figures 2, 3 and 4 show alternative embodiments of conduit
according to the invention heating a fluid passing
therethrough;
Figure 5 shows the conduit of Figure 4 heating a vessel
containing fluid;
Figure 6 shows the conduit of Figure 1 in conjunction
with a jet for delivering fluid passing through the conduit.
Referring now to the drawings, Figure 1 shows a heatable
conduit 2. Two wire electrodes 4 of opposite polarity are
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helically wound around an electrically insulating lining
nylon core 5. ~ conductive polymer tube 3 surrounds the
wrapped electrodes 4 and can be extruded over the electrode
covered nylon core 5. Finally an electrically insulating
polyethylene sheath 7 surrounds -the conductive polymer tube
3. The helical wrapping enables close spacing of the
electrodes to be easily achieved. It also facilitates
flexibility of the conduit. The electrodes can be connected
to a power source. Passage of electrical current can be
used to heat the conductive polymer layer 3, the nylon core
5 and hence heat a fluid (not shown) passing through the
core 5 or any substrate positioned in thermal contact with
core 5.
Figure 2 shows a similar embodiment in which the
electrodes 4 are embedded directly in the walls of the
conductive polymer layer 3, rather than between the conduc-
tive polymer layer 3 and the core 5. Water 14 is shown
flowing through the core 5.
Figure 3 shows another conduit 2 similar to that of
Figure 1 heating water 14 flowing therethrough. In this
case the electrodes are in the form of flat metal braids 4.
The braids 4, like the wires of Figure 1 are sandwiched bet-
ween the conductive polymer layer 3 and the inner electri-
cally insulating layer 5.
Figure 4 is identical to Figure 3 except that the
electrodes 4 are sandwiched between the conductive polymeric
layer 3 and the outer electrically insulating layer 7.
Figure 5 shows the conduit of Figure 4 positioned around
a vessel 12~ containing water 14. In this case the water 14
is hea-ted via the vessel 12 rather than being in direct con-
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tact with the conduit 2. The conduit 2 may be a push fit on
vessel 12.
Figure 6 shows the conduit of Figure 1 in conjunction
with a jet for delivering water. A metal jet 22 mounted in
a plastic casing 24 is positioned at the end of the core 5
of the conduit 2 so that water passing through the core 5 is
delivered to the jet 22. The plastic casing 24 surrounds
an end length of the conduit 2 to hold the jet and conduit
in co-operation. The jet 22 itself may also comprise a
heater.
Electrical connection to the conduits of Figures 2 - 5
may be at the end of the conduit where the electrodes pro-
ject therefrom, Alternatively, connection may be made by
use of a crimp which penetrates the outer layers to contact
the electrodes 4.
Conduits according to the invention were made, as
described in the following examples.
Example 1
An electrically powerable conduit having the structure of
Figure 4 was made by the following method.
A conductive polymer composition containing carbon black
and polyethylene, and exhibiting PTC behaviour, was extruded
onto a tube of nylon.
Strips of tin plated copper .005 inches thick and .120
inches wide were attached to opposite sides of a 7 inch
length of the tubing by means of conductive silver loaded
epoxy adhesive. An insulating outer jacket of heat
shrinkable polyethylene based tubing was then recovered on
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to the outside of the assembly to secure the two conductors
while the epoxy cured, and to provide external insulation.
(In this construction the purpose of the epoxy adhesive was
to lower the contact resistance between the electrodes and
the conductive material.)
The volume resistivity of the tubing material at 25C
was then found to be approximately 5 ohm centimetres.
The article had the following dimensions.
Inner tube of nylon I.D. 0.13" O.D. 0.1875"
Conductive tube I.D. 0.187" O.D. 0.265"
Outer jacket I.D. 0.265" O.D. 0.295"
A potential of 12 volts was applied across the conductors
and the current measured at intervals of 10 seconds while
the temperature of the tubing increased. After several
minutes the temperature was approximately 23 degrees cen-
tigrade above ambient and the current had dropped to about
half of the initial value.
It was clear that the increase of temperature of the
tubing resulted in an increase in volumetric resistivity of
the tubing material and a subsequent decrease in current.
Example 2
.
Electrically powered conduit heaters, having the struc-
ture shown in Figure 1 and designed -For operation at 12 V,
were made by the following method.
28 AWG silver plated copper wire, consisting of 7
strands of 36 AWG, were coated by melt extrusion with a thin
layer of conductive polymeric material, using a wire preheat
as described earlier, to form the electrodes 4. The purpose
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22 -
of this coating was to obtain good bonding and electrical
cont~ct between the wire electrodes 4 and the bulk jacket of
the conductive polymeric material 3 of the heater over the
whole length of the electrodes 4.
A core tuble of nylon 5 having an outside diameter of
3/16" was used to carry the fluid and around this were heli-
cally wound -two of the coated wires 4 separated by a
distance of 3/8" and with a helical pitch of 3/4".
This tubing was then preheated using a tube -Furnace to
a temperature just below the melting temperature of the core
tube 5 in order to soften the conductive polymeric material
on the wires 4, and then immediately passed through a cross
heat extruder die. This coated the tube with a range of
thicknesses of conductive polymer compound 3 exhibiiting PTC
behaviour and with a room temperature volume resistivity in
the range 9 to 13 ohm cm. The thicknesses of these conduc-
tive layers were in the range of .005" to .030".
Heater samples were prepared by cutting test lengths of
this tubing, trimming back the conductive compound to expose
the ends of the core tube, and exposing the bus wires for
electrical connection. Electrical insulating was supplied
in the form of heat shrink tubing recovered over the length
of conductive polymeric material. Alternatively, insulation
could have been applied by melt processing during the
earlier extrusion process.
A lft length of such a heater was powered at 12V at
room temperature with no fluid flow, and found to have a
steady state surface temperature of approximately 45C, i.e.
a delta T of 25C, at a steady state power of approximately
12 W. When the ambient temperature was reduced to -30C
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the power delivered rose to 24 W, with a delta T of 50C.
When powered in a 40C ambient, the maximum temperature
reached was 55C, i.e. a delta T of only 15C, with a steady
state power of only 8 W. This demonstrates the self regu-
lating nature o-f the heater, the power output changing with
the ambient temperature.
EX~MPLE 3
An electrically powered conduit heater, in conjunction
with a jet assembly, (as shown in Figure 6), was made by the
following method.
Where the conduit heater or hose was desired for
windscreen wash water heating, a length of conduit produced
as described in Example 4 above was coupled with a jet
assembly. The nylon core tube 5 was a push fit into an inner
diameter of the jet assembly 24 and the conduit and any
exposed bus wires were potted into the assembly with a
curable resin or adhesive, to secure the hose within the
assembly and to insulate the wire ends from contact with the
water.
EXAMPLE 4
-
An electrically powered conduit heater, having the
structure shown in Figure 1 and dèsigned for operation at
mains voltage, was made by a similar method to that
described in Example 2 above. There were, however, a number
of slight diFferences:
The wire used for the electrodes was 22 ~WG silver
plated copper, consisting of 19 strands of 34 AWG. This was
coated with conductive polymeric material as described in
Example 2.
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The nylon core tubing used was 3/8" O.D. and the wires
were wrapped with a 3/8" spacing on a helical pitch of 3/4ll.
This wrapped core tube was fed through a preheat oven
and into an extruder cross head die as before, where it was
coated with approximately lmm of conductive polymeric
material, of a type normally used for making PTC strip
heaters, exhibiting PTC behaviour and having a room tem-
perature volume resistivity of around 2000 ohm cm.
A lft long sample of this tubing was prepared and tested
in the same manner as described in Example 2, but powered at
110V a.c., with the following results.
Ambient Power Output Delta T
Temperature (Steady State) (Heater surface temp.
~ ambient)
-30C I34 W 160C
20C I21 W 136C
40C I15 W 124C
These values again illustrate the self regulating
nature of the heater. Similar behaviour was obta~ned in
testing at 240V a.c.
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