Note: Descriptions are shown in the official language in which they were submitted.
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Heating Element
The invention relates to a heating element. In particular, the invention
relates to a
heating element for use in, for example, a flexible heating jacket or a trace
.. heater.
The current heating elements used in container heaters typically require the
use
of a thermostat to control the temperature. This is not ideal when the heater
is
used to heat a flammable and/or explosive material, since an electric device
such
.. as a thermostat may provide an igniting spark.
The first self-regulated heater was made by Raychem and revolutionized the
trace heating market. What made this invention revolutionary at the time was
the
ability of the material to limit power outputs based on the temperature
changes on
the surface of the item being heated. Not only did the material allow power
control, it also made it easier to design with, install and maintain by making
it
feasible to cut to length on the field.
A schematic of a conventional self-regulated heater or cable is shown in
Figure 1.
Self-regulated heaters or cables are made up of a semi conductive polymer
composite 1 (usually cross-linked high density polyethylene filled with carbon
black) extruded between two parallel bus conductors 2. The semi conductive
polymer composite 1 acts as the heating core. This core is then covered by an
insulating polymer jacket 3 and a tinned copper braid 4. An optional
additional
jacket 5 can be used to provide mechanical or corrosion protection for the
device.
Self-regulated heaters or cables work by changing their electrical
resistivity, and
hence the power output, with change in temperature. At high temperatures, the
resistivity increases and the heat output generated by the self-regulated
heaters
is reduced accordingly. This is caused by a disruption in the electrical
pathways
within the conductive filler (e.g. carbon black) network of the heating core.
One
possible explanation is that the conductive paths formed by the conductive
filler
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get broken due to expansion of the polymer matrix. This reduces the number of
effective conductive paths and this leads to a reduction in heat output.
Reversely,
as the temperature reduces, the polymer matrix contracts and this reduces the
distance between the conductive fillers therefore helping in the re-formation
of
conductive pathways. This results in an increase in heat output. This
mechanism
is depicted in Figure 2.
Conductive polymer composites (CPC) are formed of insulated polymers filled
with conductive fillers. CFCs provide a way of controlling the temperature of
a
heater by changing its resistivity suddenly within a narrow temperature range.
This is known as the positive temperature coefficient (PTC) effect.
The intensity of the FTC effect increases with increasing size of the
conductive
filler. However, the electrical percolation threshold also increases with
increasing
filler size. Higher filler contents are then required in order to make the CPC
conductive, with detrimental consequences for the flexibility, processability,
cost
and recyclability of the CPC. Accordingly, conventional CFCs represent a
compromise between low percolation threshold and large FTC intensity.
In order to try to overcome this compromise, CFCs have been prepared
containing combinations of two fillers (so-called "mixed-filler" composites):
one
filler exhibiting a large FTC intensity and the other exhibiting a low
percolation
threshold. However, in such mixed-filler composites the FTC intensity is
dominated by the filler with the lowest FTC intensity, even at very low
loadings.
The present invention seeks to tackle at least some of the problems associated
with the prior art or at least to provide a commercially acceptable
alternative
solution thereto.
In a first aspect, the present invention provides a self-regulating heating
element
comprising a heating core disposed between a pair of electrodes, the heating
core comprising:
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a first conductive polymer composite comprising first conductive particles
dispersed in a first polymer matrix, the first conductive particles having an
aspect
ratio greater than 100; and
a second conductive polymer composite comprising second conductive
particles dispersed in a second polymer matrix, the second conductive
particles
having an aspect ratio of from 1 to 100 and a longest dimension of greater
than
urn, wherein the first conductive polymer composite and the second
conductive polymer composite are arranged in series between the pair of
electrodes.
The heating element may exhibit an advantageous combination of an overall low
percolation threshold and a large positive temperature coefficient (PTC)
intensity.
As a result, the heating element may be particularly effective at self-
regulating its
temperature, while also being flexible and easy and low cost to manufacture.
Each aspect or embodiment as defined herein may be combined with any other
aspect(s) or embodiment(s) unless clearly indicated to the contrary. In
particular,
any features indicated as being preferred or advantageous may be combined
with any other feature indicated as being preferred or advantageous.
The term "self-regulating" as used herein may encompass the ability of a
heating
element to reduce its power output on reaching a certain pre-determined
temperature and/or to control the current that flows though it as a function
of
temperature.
The term "heating element" used herein may encompass an element capable of
converting electricity into heat through the process of resistive or Joule
heating.
Without being bound by theory, it is considered that electric current passing
through the element encounters resistance, resulting in heating of the
element.
The term "heating element" may encompass, for example, a "heater element", in
which the generation of heat may be the main purpose of the heating element.
It
may also encompass, for example, and a "thermal switch", in which the
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temperature of the heating element may control the current capable of passing
through the heating element.
The term "positive temperature coefficient" (PTC) as used herein may
encompass the ability of a material to exhibit an increase in electrical
resistance
when its temperature is raised.
The term "positive temperature coefficient intensity" (PTC intensity) as used
herein is defined as log10 (maximum resistivity / minimum resistivity). When
the
PTC intensity is large, typically greater than 1, the resistivity of the
material
changes suddenly within a narrow temperature range.
The term "aspect ratio" as used herein may encompass the ratio of the longest
dimension of the particle to the shortest dimension of the particle. Such
aspect
ratios may be determined by, for example, a combination of optical microscopy
and SEM. When the particle is a sphere, the aspect ratio will be 1.
The heating element is self-regulating. In other words, once the heating
element
reaches a certain pre-determined temperature, the power output is reduced,
typically to zero.
The heating core of the heating element may exhibit a low percolation
threshold
compared with conventional conductive polymer composite-containing heating
elements. In other words, conductive pathways may form in the heating core of
the heating element with only low levels of conductive particles. This may
result
in the heating element exhibiting higher flexibility and reduced manufacturing
costs in comparison to conventional heating elements.
The heating core of the heating element may exhibit a large positive
temperature
coefficient (PTC) intensity. Accordingly, the heating element may be
particularly
good at self-regulating its temperature. This may render the heating element
particularly suitable to heat, for example, a flammable and/or explosive
material.
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The first conductive polymer composite and the second conductive polymer
composite are arranged in series between the pair of electrodes. This means
that, in use, current flowing between the pair of electrodes flows through the
first
5 -- conductive polymer composite followed by the second conductive polymer
composite, or through the second conductive polymer composite followed by the
first conductive polymer composite.
The first conductive polymer composite and the second conductive polymer
-- composite typically have similar volumes within the heating core. The ratio
of the
volume of the first conductive polymer composite to the volume second
conductive polymer composite is typically in the range of from 5:1 to 1:5.
The first conductive polymer composite and the second conductive polymer
composite are conductive to the extent that they can be used in a heating
element.
The first conductive polymer composite and the second conductive polymer
composite may be flexible. This may enable the heating element to be
-- advantageously employed in a flexible heating jacket. As discussed in more
detail
below, such a flexible heating jacket may be folded over on itself a large
number
of times without causing significant damage to the conductive polymer
composite.
The conductive polymer composites may exhibit a storage modulus measured by
dynamic mechanical analysis (DMA) at room temperature of less than 1000 MPa,
-- typically less than 900 MPa, even more typically less than 800 MPa, even
more
typically less than 500 MPa, still even more typically less than 100 MPa,
still even
more typically less than 800 kPa, still even more typically from 10 to 500
kPa.
The heating core may contain more than one of each of the first conductive
-- polymer composite and the second conductive polymer composite. In this
case,
the first conductive polymer composite and the second conductive polymer
composite will typically alternate in series between the pair of electrodes.
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The electrodes may be conventional electrodes known in the art. The electrodes
may be, for example, bus conductors. The electrodes may comprise, for
example, copper.
The polymer of the first polymer matrix and the polymer of the second polymer
matrix typically exhibit a high resistivity. The polymer of the first polymer
matrix
and the polymer of the second polymer matrix are preferably flexible. This may
enable the heating element to be used, for example, in a flexible heating
jacket.
The polymer of the first polymer matrix and the polymer of the second polymer
matrix may be the same or different.
The heating core preferably has a positive temperature coefficient intensity
of
greater than 1, more preferably greater than 3, even more preferably greater
than
5, still even more preferably greater than 6. In a preferred embodiment, the
heating core has a positive temperature coefficient intensity of about 7 to 8.
A
greater positive temperature coefficient intensity results in the resistivity
of the
heating core changing more suddenly within a narrow temperature range. This
may enable the heating element to more accurately regulate its temperature.
Accordingly, the heating element may be used advantageously to heat materials
requiring very precise temperature control, such as flammable and/or explosive
materials.
The first conductive particles preferably comprise carbon nanotubes (CNTs).
The
carbon nanotubes may comprise, for example, single wall carbon nanotubes
(SWCNTs) and/or multi-wall carbon nanotubes (MWCNTs). Carbon nanotubes
are particularly effective as the first conductive particles since they
exhibit
particularly favourable conductivities and aspect ratios. When the first
conductive
particles comprise carbon nanotubes, the heating core may exhibit a
particularly
low percolation threshold. The heating core may also exhibit particularly
favourable Joule heating.
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The first conductive polymer composite preferably comprises from 0.1 to 10
wt.%
of the first conductive particles based on the total weight of the first
conductive
polymer composite, more preferably from 0.5 to 10 wt.%, even more preferably
from 0.5 to 5 wt.%, still even more preferably from 2 to 3 wt.% of the first
conductive particles based on the total weight of the first conductive polymer
composite. In a preferred embodiment, the first conductive polymer composite
comprises about 2.5 wt.% of the first conductive particles based on the total
weight of the first conductive polymer composite. Higher levels of the first
conductive particles may result in increased materials and manufacturing
costs.
The first conductive particles have an aspect ratio greater than 100.
Preferably,
the first conductive particles having an aspect ratio greater than 150, more
preferably greater than 500, even more preferably greater than 1000. Larger
aspect ratios may reduce the percolation threshold. The aspect ratio is
typically
less than 10000.
The second conductive particles may be in the form of, for example, spheres,
rods, fibres and/or flakes. The second conductive particles preferably
comprise
spheres and/or flakes. Spheres and flakes may exhibit particularly favourable
aspect ratios. Furthermore, spheres and flakes may be easier to handle,
thereby
reducing manufacturing costs.
The second conductive particles may comprise, for example, one or more of
carbon particles, carbon-coated particles, metal particles, metal oxide
particles,
alloy particles, metal-coated glass particles, metal-coated polymer particles,
conductive polymer-coated particles and graphene nanoplatelets (GNPs). The
metal may be selected from, for example, copper, silver, nickel, aluminium,
titanium, zinc and/or gold.
The second conductive particles preferably comprise one or more of silver
particles (e.g. silver flakes) and silver-coated glass particles. Use of such
particles may result in a particularly pronounced PTC effect.
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The second conductive particles preferably comprise GNPs. While GNPs have a
slightly less pronounced PTC effect than, for example, silver-coated glass
spheres (AgS), they are lower in weight, provide lower percolation and are
more
cost effective than AgS. GNPs are also less sensitive to damage than AgS,
thereby providing a more stable heating element.
Specific particles that are advantageously used as the second conductive
particles include, for example:
1. GNP, preferably size (i.e. longest dimension) 5-100 micron
2. Nickel or Nickel coated spheres or flakes, preferably size 5-100 micron
3. Aluminium or Aluminium coated spheres or flakes, preferably size 5-100
micron
4. Gold or Gold coated spheres or flakes, preferably size 5-100 micron
5. TiO2 or TiO2 coated spheres or flakes, preferably size 5-100 micron
6. ZnO2 or ZnO2 coated spheres or flakes, preferably size 5-100 micron
7. Carbon or Carbon coated spheres or flakes, preferably size 5-100
micron
The second conductive particles may substantially all be the same shape and
size. Alternatively, the second conductive particles may have different shapes
and sizes.
The second conductive particles have an aspect ratio of from 1 to 100. The
second conductive particles preferably have an aspect ratio of from 1 to 10.
Higher aspect ratios may result in a reduced PTC intensity and/or reduced
flexibility.
The second conductive particles have a longest dimension of greater than 10
pm.
The second conductive particles preferably have a longest dimension of from 20
to 150 pm, more preferably from 40 to 60 pm. When the second conductive
particles are in the form of a sphere, the longest dimension is the diameter
of the
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sphere. The longest dimension may be measured by, for example, a combination
of optical microscopy and SEM. Smaller particles may exhibit an unfavorably
low
PTC intensity. Larger particles may result in an unfavorably low percolation
threshold.
The second conductive polymer composite preferably comprises from 10 to 60
wt.% of the second conductive particles based on the total weight of the
second
conductive polymer composite, more preferably from 30 to 40 wt.% of the second
conductive particles based on the total weight of the second conductive
polymer
composite. When the second conductive particles comprise AgS, the second
conductive polymer composite preferably comprises from 30 to 40 wt.% of the
second conductive particles based on the total weight of the second conductive
polymer composite. When the second conductive particles comprise GNPs, the
second conductive polymer composite preferably comprises from 10 to 30 wt.%
of the second conductive particles based on the total weight of the second
conductive polymer composite, more preferably from 15 to 25 wt.%, even more
preferably from 17 to 19 wt.%, still even more preferably about 18 wt.%.
Higher
levels of the second conductive particles may result in an unfavourable low
PTC
intensity. Higher levels of the second conductive particles may result in
increased
manufacturing costs. Furthermore, the flexibility of the heating core may be
reduced.
The polymer of the first polymer matrix and/or the polymer of the second
polymer
matrix comprise a plastomer and/or an elastomer. Such species may increase
the flexibility of the first and second conductive polymer composites, thereby
making the heating element more suitable for incorporation into a heater
requiring
flexibility such as, for example, a drum heater or trace heater. The term
"elastomer" as used herein encompasses a family of polymers exhibiting rubbery
behaviour at room temperature and having a glass transition temperature of
less
than 20 C, more typically of from -150 C to -50 C. Elastomers typically
comprise long polymer chains, and typically contain at least some chemical
cross-linking. The term "plastomer" as used herein encompasses a thermoplastic
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elastomer, i.e. an elastomer that can be processed via the melt. Plastomers
typically contain physical cross-linking rather than chemical cross-linking,
meaning that the cross-linking may disappear on heating but reform on cooling,
thereby allowing melt processing of the polymer.
5
The plastomer preferably comprises an olefin-based plastomer or a polyurethane
based plastomer (TPU). Such plastomers exhibit advantageous levels of
flexibility
and processability. An example of a commercially available plastomer suitable
for
use in the present invention is Lubrizol Estanee 58437.
The elastomer preferably comprises a cross-linked elastomer. Such elastomers
exhibit advantageous levels of flexibility.
The polymer of the first polymer matrix and/or the polymer of the second
polymer
matrix may comprise high density polyethylene (HDPE). A commercially available
HDPE suitable for use in the present invention is Rigidex 0 HD5218EA. HDPE
may provide a sharp transition at melting point (and therefore a stable
switching
temperature), and may contribute to the large PTC.
The polymer of the first polymer matrix and/or the polymer of the second
polymer
matrix may be chosen so as to fine tune the maximum temperature that the
heating element can reach. For example, when a higher switching temperature is
required, a polymer with a higher melting temperature / glass transition
temperature / softening temperature may be selected. Alternatively, when a
lower
switching temperature is required, a polymer with a lower melting temperature
/
glass transition temperature / softening temperature may be selected. For
example, the use of HDPE may result in a maximum temperature of around 130
C, whereas the use of TPU may result in a maximum temperature of around 120
C. The polymer of the second polymer matrix typically exhibits more control
over
the temperature than the polymer of the first polymer matrix. Accordingly, in
order
to fine tune the maximum temperature that the heating element can reach,
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selection of the polymer of the second polymer matrix is more important than
selection of the polymer of the first conductive matrix.
The polymer of the first polymer matrix and/or the polymer of the second
polymer
matrix may comprise a polymer blend. The particular components of the polymer
blend may be chosen so as to fine tune the maximum temperature that the
heating element can reach. The polymer blend may comprise, for example, one
or more thermoplastic polymers (e.g. HDPE) and/or one or more thermoplastic
elastomers (e.g. TPU). The polymer blend may comprise, for example, one or
more of HDPE, styrene ethylene butylene styrene (SEBS, e.g. Kraton FG1901 G
¨ a clear, linear triblock copolymer based on styrene and ethylene/butylene
with a
polystyrene content of 30%), propylene-ethylene copolymers (PPE, e.g. the
VERSIFYTM 2200 plastomers and elastomers) and TPU (e.g. Estanee 58437 ¨
an aromatic polyester-based thermoplastic polyurethane). Again, it is the
identity
of the second polymer matrix that exhibits a greater effect over the maximum
temperature compared to the identity of the first polymer matrix.
In a preferred embodiment, the polymer blend of the first polymer matrix
and/or
the polymer blend of the second polymer matrix may comprise a thermoplastic
polymer and a thermoplastic elastomer. In a preferred embodiment, the polymer
blend of the first polymer matrix and/or the second polymer matrix (preferably
at
least the second polymer matrix) comprises HDPE and one or more of SEBS,
TPU and PPE. In a particularly preferred embodiment, the polymer blend of the
first polymer matrix and/or the second polymer matrix (preferably at least the
second polymer matrix) comprises HDPE and PPE. The addition of SEBS, TPU
and/or PPE to HDPE may improve the flexibility of the heating element. It may
also help to fine tune the maximum temperature of the heating element. In
these
embodiments, the polymer blend preferably comprises up to 65 wt.% of the
SEBS, TPU and/or PPE, more preferably from 10 to 60 wt.%, even more
preferably from 20 to 55 wt.%, still even more preferably from 45 to 55 wt.%.
In a
particularly preferred embodiment, the polymer blend comprises about 50 wt.%
SEBS, TPU and/or PPE. Lower levels may exhibit only a limited increase in
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flexibility of the heating element. Higher levels may exhibit an unfavourable
drop-
off in Joule heating property.
The polymer blend may be a binary polymer blend or a tertiary polymer blend.
Polymer blends comprising a greater number of polymers are also possible. The
polymers of the polymer blend may be miscible or immiscible. Immiscible
polymers may result in a co-continuous blend. Alternatively, immiscible
polymers
may exhibit a "drop-shaped" blend, i.e. with one polymer present as the
continuous phase and another polymer dispersed within the continuous phase as
"droplets".
When the first and/or second polymer matrix comprises a polymer blend, the
corresponding conductive particles may be dispersed, for example, in only one
of
the polymers of the polymer blend, and/or in more than one polymer of the
polymer blend, and/or in all polymers of the polymer blend, and/or in
interfaces
between polymers of the polymer blend. When the polymer blend is a binary
polymer blend, typically the conductive particles are dispersed in both of the
polymers of the polymer blend.
In one embodiment, the heating core comprises either:
an additional first conductive polymer composite, and the second
conductive polymer composite is sandwiched between the two first conductive
polymer composites; or
an additional second conductive polymer composite, and the first
conductive polymer composite is sandwiched between the two second
conductive polymer composites.
Preferably the heating core comprises an additional first conductive polymer
composite, and the second conductive polymer composite is sandwiched
between the two first conductive polymer composites.
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The "sandwich" arrangement of the first and second conductive polymer
composites may take the form of, for example, a sheet or a cable. In "sheet"
form,
the first conductive polymer composite(s) and second conductive polymer
composite(s) may lie along the elongated axis (i.e. plane) of the sheet. In
"cable"
form the first conductive polymer composite(s) and second conductive polymer
composite(s) may lie perpendicular to the elongated axis of the cable.
In "sheet" form, the sheet may be flexible and/or flat and may be
advantageously
incorporated into a heating jacket. In "cable" form, the cable may be in the
form of
.. a trace heating cable.
The first conductive polymer composite and the second conductive polymer
composite may vary in their relative thicknesses (in the direction of the
plane in
which both electrodes sit). Reducing the thickness of the second conductive
polymer composite may reduce the weight of the heating element and may also
result in the heating element heating up more quickly. For example, when the
second conductive polymer composite is sandwiched between the two first
conductive polymer composites, the thickness ratios are preferably, for
example,
1-1-1 of 2-1-2 rather than, for example, 1-2-1.
In a particularly preferred embodiment:
the first conductive particles comprise carbon nanotubes,
the polymer of the first polymer matrix comprises thermoplastic
polyurethane,
the first polymer matrix comprises from 3 to 8 wt.% of the first conductive
particles,
the second conductive particles comprise silver coated glass spheres
and/or silver flakes having a longest dimension of from 40 to 60 pm,
the polymer of the second polymer matrix comprises thermoplastic
polyurethane, and
the first polymer matrix comprises from 30 to 40 wt.% of the second
conductive particles.
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In a particularly preferred embodiment:
the first conductive particles comprise carbon nanotubes,
the polymer of the first polymer matrix comprises HDPE,
the first polymer matrix comprises from 3 to 8 wt.% of the first conductive
particles,
the second conductive particles comprise GNPs,
the polymer of the second polymer matrix comprises HDPE, and
the first polymer matrix comprises from 10 to 30 wt.% of the second
conductive particles.
In this preferred embodiment, the second polymer matrix preferably further
comprises one or more of SEBS, TPU and PPE, more preferably PPE, even
more preferably from 45 to 55 wt.% PPE.
In a preferred embodiment, the heating element is a thermal switch. When
incorporated into a circuit, the thermal switch may allow current to pass
through
when the heating element is at a certain temperature, but then prevent current
from passing though at higher temperatures. This may serve to prevent
overheating of electronic components connected in series to the switch in a
circuit. It may also serve to control the heat output of a conventional
electric
heater, i.e. by switching off the heater when a particular temperature is
achieved.
When the electronic component is a thermal switch, the heating element is
likely
to be smaller than when used as the main body of a container heater or a
heating
jacket (i.e. when the heating element is a "heater element"). This is because
the
switch would not need to be substantially co-extensive with a surface, axis or
plane of the heater or heating jacket.
The thermal temperature switch may be capable of drawing current ratings well
above the levels available in state of the art semi-conductor switches. This
may
be useful either for control or for over temperature protection. The switch
may be
15
used in diverse harsh industrial applications whether or not there are risks
of
explosion due to presence of gases or dusts.
Existing electromechanical "bi-metallic" switches are currently used in most
commercial applications (up to typically 25 amps rating) selected primarily on
cost grounds, but these are unreliable for general industrial use due to
mechanical wear and tear and lightweight case construction. Very small heat-
detecting semi-conductor (thermistor) sensors are often incorporated into
domestic equipment such as washing machine and vacuum cleaner motors, but
these rely on external electronic circuit boards to determine the switch point
and
to disconnect power through an additional switching device.
The limitations of current rating and mechanical reliability of prior art
switches can
both be solved by the use of the switch described herein. Carbon loading and
filler can be adjusted to accommodate higher current switching whilst the
temperature switch point can be formulated over a wide range up to typically
130
C.
The switch may be manufactured as a two wire canister, or in a flat sheet
orientation, or even as a flexible cord. The first and/or second conductive
polymer
composite may be as described in W02016/012762. The use of such a switch in
a heating system may increase the safety of the heating system. Since the core
is flexible, there is no compromise of a surface temperature measurement in
any
way. This is particularly important when the switch is used in a flexible
heater.
This is because, when used with flexible heaters, even a small rigid sensor
can
cause temperature measurement inaccuracies.
In a further aspect, the present invention provides a circuit comprising the
thermal
switch described herein and an electronic component, wherein the thermal
switch
and electronic component are connected in series.
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The electronic component may be selected from, for example, a motor, a pump, a
heater and electronic circuit board with components that potentially generate
heat
in use such as, for example, a diode, and LED, a light bulb, a transistor and
a
solid state device.
Preferably, the circuit comprises two or more of the thermal switches and two
of
more electronic components, wherein each thermal switch is connected in series
to one or more electronic components and wherein the two or more heating
elements are connected in parallel.
In such a circuit, only the component experiencing a fault (such as
overheating)
would be switched off.
In a further aspect, the present invention provides a container heater
comprising
the heating element described herein.
The container heater may have a capacity of from 20 to 2000 litres. The
container
heater may have a generally cylindrical shape. Alternatively, the container
heater
may have a generally prismatic shape with a rectangular base. The prismatic
shape may have curved corners.
In a further aspect, the present invention provides a heating jacket
comprising the
heating element as described herein. The heating jacket is preferably a
flexible
heating jacket. Due to the flexibility of the conductive polymer composite,
the
flexible heating jacket may advantageously be capable of rolling up on itself
like a
camping mattress, or at the very least folding over on itself so that it can
be
stored in between uses. Typically, this may cause no damage to the conductive
polymer composite for the normal life of the jacket, which is typically
expected to
be a number of years. Typically, the flexibility of the conductive polymer
composite allows the flexible heating jacket to be folded over on itself, e.g.
to
form a tube at the least. This may allow the flexible heating jacket to
effectively
heat an element to be heated, such as, for example, a pipe.
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The flexible heating jacket may comprise a layer of thermal insulation and/or
one
or more outer protective layers covering the conductive polymer composite.
With
the additional layers, the flexible heating jacket typically has a thickness
of from 5
to 25 mm. Even with such additional layers, due to the flexibility of the
conductive
polymer composite, the flexible heating jacket may typically still be able to
at least
fold over on itself. In one typical embodiment, when the conductive polymer
composite is assembled into a finished heating jacket of thickness typically 5
to
25 mm including insulation/additional layers, the finished product can be
folded
over upon itself for storage without significant damage to the heater, however
many times this action is performed. The flexible heating jacket is typically
capable of being folded over on itself at least 100 times, more typically at
least
500 times, even more typically at least 1000 times, still even more typically
at
least 10000 times without causing significant damage to the conductive polymer
composite.
In a further aspect, the present invention provides a trace heater comprising
the
heating element described herein.
In a further aspect, the present invention provides a thermal switch
comprising a
core disposed between a pair of electrodes, the core comprising:
a first conductive polymer composite comprising first conductive particles
dispersed in a first polymer matrix, the first conductive particles having an
aspect
ratio greater than 100; and
a second conductive polymer composite comprising second conductive
particles dispersed in a second polymer matrix, the second conductive
particles
having an aspect ratio of from 1 to 100 and a longest dimension of greater
than
10 rim, wherein the first conductive polymer composite and the second
conductive polymer composite are arranged in series between the pair of
electrodes.
The preferable and optional features of the first aspect apply also to this
aspect.
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A description of the non-limiting Figures appended hereto is as follows:
Figure 1 is a schematic of a trace heater of the prior art.
Figure 2 is a schematic of the PTC effect for a CPC.
Figure 3 shows a schematic of a heating element according to an embodiment of
the present invention.
Figure 4 shows results of PTC intensity testing of a sample of Example 1.
Figure 5 shows results of percolation threshold testing and PTC intensity
testing
of samples of Comparative Example 1.
Figure 6 shows percolation curves of samples according to Example 2.
Figure 7 shows pyro-resistive behaviours of samples according to Example 2.
Figure 8 shows pyro-resistive behaviours of tri-component series assemblies
according to Example 2.
Figure 9 shows Joule heating performance of tri-component series assemblies of
Example 2.
Figure 10 shows schematics, Joule heating behaviours and IR images of various
composites of Example 2.
Figure 11 shows results of flexibility measurements of various composites of
Example 2.
Figure 12 shows SEM images of various samples of Example 3
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Figure 13 shows the electrical conductivity properties of various samples of
Example 3.
Figure 14 shows results of PTC intensity testing of samples of Example 3.
Referring to Figure 3, there is shown a self-regulating heating element A
comprising a heating core B disposed between a pair of electrodes C, the
heating
core B comprising: a first conductive polymer composite D comprising first
conductive particles E dispersed in a first polymer matrix F, the first
conductive
particles having an aspect ratio greater than 100; and a second conductive
polymer composite G comprising second conductive particles H dispersed in a
second polymer matrix I, the second conductive particles H having an aspect
ratio of from 1 to 100 and a longest dimension of greater than 10 pm, wherein
the
first conductive polymer composite D and the second conductive polymer
composite G are arranged in series between the pair of electrodes. The two
fillers
may form continuous (conductive) networks.
The invention will now be described in relation to the following non-limiting
examples.
Example 1
Heating elements were prepared as follows.
The triple section series composite samples were fabricated using TPU
(Lubrizol
Estanee 58437, density 1.19 g/cm3) as the polymer matrix, MWCNTs (Nanocyl
S.A. Product No. C7000) and silver coated glass spheres (AgS) with average
diameter of 50 micron (Potters Industries Ltd.) as the conductive filler. All
the
TPU pellets are dried overnight at 80 C before compounding.
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Melt compounding process was used to disperse the fillers (AgS and CNTs) into
polymer matrix. To have a good dispersion of AgS and in the meantime avoid
silver surface damage on the AgS, DSM X'plore 15 mini twin-screws extruder
(the Netherlands) was used to produce the compound with screw speed of 50
5 rpm, processing temperature of 200 C, and a residiig time of 5 minutes
in
nitrogen gas flow atmosphere. The desired amount of CNTs (5 wt.%) was mixed
with TPU by Dr Collin twin-screw compounder (ZK35, 35mm). The throughput
was of 2 kg/hr, with screw speed of 50rpm, and temperature ranging between
190 C and 220 C over 8 heating zones. The compoab was directly collected
10 into a water bath for consolidation and pelletised inline after removing
excess of
water with an air-blade. 5 wt.% CNTs/TPU composites are used as master batch
to dilute into lower concentration using DSM X'plore 15 mini twin-screws
extruder
with the same processing condition as AgS/TPU composite.
15 The produced compounded strands were chopped into pellets and
compression
moulded into sample bar with the dimension of 28mmx10mmx2mm using Collin
hot press P300E (Germany), at 220 C for 5 minutes.Two pieces of copper mesh
(0.263 mm aperture and 0.16 mm wire diameter) were pre-embedded on both
side of the sample as electrode for electrical test during hot pressed.
The serial samples were manufactured by cutting desired length of each
section,
melting and combining the sections together.
Scanning electron microscope (SEM) images were taken by a FEI Inspector-F,
both the cross-section area and interfacial area between the CNTs/TPU and
AgS/TPU were examined (immersed in liquid nitrogen for 5 minutes and then
fractured). Gold sputtered were applied on the surface before imaging.
The conductivity of all samples were measured by a simple two-point
measurement with a combination of a picometer (Keithley 6485) and a DC
voltage source (Agilet 6614C). A minimum 5 samples were measured for the
conductivity data point. PTC testing was conducted also on the rectangular
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samples subjected to the certain heating profile in the oven, while the
conductivity, time and sample temperature were monitored simultaneously.
Example results of the PTC testing are shown in Figure 4 (cycle 1: top, cycle
2:
middle, cycle 3: bottom ¨ CNT-AgSm-CNT, length ratio 1:1:1, middle part about
10 mm). It can be seen that the heating element exhibited a high PTC intensity
(around 7-8 orders of magnitude, similar to pure AgS/TPU ¨ see comparative
example below) with a low percolation threshold.
Changing the length ratio of the different composites did not change the
result,
and nor did inverting the position of the two composites.
Comparative Example 1
Figure 5 shows the results of conductivity vs. filler loading and resistivity
vs.
temperature for two reference example conductive polymer composites: (i)
containing just CNTs dispersed in TPU, and (ii) containing just silver spheres
dispersed in TPU. The results indicate that CPC (i) exhibited a low
percolation
threshold (0.5 ¨ 1 wt.%) but small PTC intensity (< 1), whereas CPC (ii)
exhibited
a high percolation threshold (35-40 wt.%) but large PTC intensity (7-8 orders
of
magnitude). In the key of the bottom left hand side plot, the lines are (from
top to
bottom): first cycle, second cycle, third cycle, fourth cycle.
Example 2
Materials:
Material Trade Name Information
Polymer Thermoplastic Lubrizol Estane6 Density 1.19 gcm-1
polyurethane (TPU) 58437
High density Rigidex Density 0.952 gcm-
polyethylene (HDPE) HD5218EA 1
Conductive Multi-wall carbon Nanocyl S.A. Average diameter
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filler nanotubes (MWCNTs) Product No. of 9.5 nm, carbon
C7000 purity 90%
Silver coated glass Potters industries Average diameter
spheres (AgS) Ltd. of 50 pm
Graphene nanoplatelets xGnP Grade M Average particle
(GNPs) diameter of 15
density 2.2 gcm-1,
carbon content
>99.5%
All the polymers are in the form of pellets and dried overnight at 80 C
before
compounding.
Sample preparation:
Twin-screw melt compounding was employed to achieve a good level of
dispersion for both fillers (AgS and CNTs) within the polymer matrix. In order
to
avoid damage to the silver coating on AgS particles during compounding, a co-
-- rotating DSM X'plore (Netherlands) 15 mini-extruder was used to produce the
compound, with a modest rotating speed at 50 rpm for 5 min, at a temperature
of
200 C, and under nitrogen atmosphere. CNTs (5 wt.%) were compounded with
TPU using a Dr. Collin (Germany) twin-screw compounder (ZK35 with a screw
length of 32 LID). The throughput was of 2 kg/h, using a screw speed of 50
rpm,
and a temperature ranging between 190 C and 220 Cover 8 heating zones.
The produced TPU/CNT (5 wt.%) composite was used as master batch that was
diluted into desired concentrations using the DSM X'plore 15 mini-extruder
with
the same mild processing conditions used for TPU/AgS composites. The
compounded strands that were produced were then chopped into pellets and
compression moulded into bar shaped samples with dimensions of 30 mm x 10
mm x 2 mm, using a Dr. Collin hot press P300E, at 220 C for 5 min and 60 bar
pressure. Two pieces of copper mesh (0.263 mm aperture and 0.16 mm wire
diameter) were embedded on both sides of the sample during compression
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moulding for use as the electrodes. The series and parallel samples were
manufactured by cutting the desired lengths of each composite, and hot welded
the cut composite sections together using the same compression moulding
equipment.
Characterisation:
A scanning electron microscope (SEM) (FEI Inspector-F, Netherlands) was used
to examine the morphology of sample cross-sections as well as the interfacial
area between the TPU/CNT and TPU/AgS, with the aim to characterize the filler-
filler, filler-polymer and composite-composite interaction. Brittle fracture
was
induced by immersing the specimens into the liquid nitrogen for 5 min. All the
surfaces analysed were gold sputtered before imaging.
The pyro-resistive behaviour of all samples were tested with an apparatus
consisting of a temperature controlled oven (heating rate of 2 C/min) and a
two-
point resistance measurement unit, obtained by combining a picometer (Keithley
6485) with a DC voltage source (Agilent 6614C).
The thermocouple was placed close to, but not touching, the specimen to ensure
accurate reading. A constant voltage (1V) was applied during heating and
cooling
cycles on the rectangular specimens while the current and temperature were
monitored and recorded simultaneously.
To evaluate the Joule heating behaviour of the series composites, direct
voltage
was applied to the sample whilst two thermal infrared cameras (FLIR A35 and
E40) recorded thermal images during heating.
To examine the increased flexibility of the produced specimens, the electrical
resistance was measured in-situ during bending tests on the tri-component
series
assembly. The specimen was bent around an insulating cylindrical object with
known radius while the electrical resistance was measured and recorded.
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Percolation curve:
Percolation curves are shown in Figure 6 of: a) TPU/CNT composites showing a
relatively low percolation threshold ((Pc) of 0.32 wt.%, calculated by fitting
experimental data with Equation 2 (inset); and b) TPU/AgS composites showing a
sharp "on-off" behaviour in electrical conductivity in correspondence with the
percolation threshold.
FTC of Mono-filler systems:
The pyro-resistive behaviours are shown in Figure 7 of: a) TPU/CNT composites
(5 wt.% (filled squares) and 0.4 wt.% (open squares)), showing a slightly NTC
effect at both loadings; b) TPU/AgS composites (45 wt.% (filled squares) and
50
wt.% (open squares)), with a clear FTC effect at similar temperature.
Parallel and series connected systems:
c) and d) of Figure 7 show predicted (symbols) and experimentally measured
.. (lines) electrical resistivity of TPU/AgS ¨TPU/CNT composites in parallel
and
series connection, respectively, as a function of temperature. TPU/AgS
composite (45 wt.%), TPU/CNT composite (5 wt.%) and TPU/CNT composite
(0.4 wt.%) are referred to as R1, R2 and R3, respectively. (Dashed lines
replicate
the resistivity of TPU/CNT composites, 5 wt.% and 0.4 wt.% in c and d,
respectively). Excellent agreement between the experimental data and predicted
pyro-resistive behaviour has been obtained.
Tr-component series assembly with different switching unit length:
The pyro-resistive behaviours of tri-component series assembly are shown in
Figure 8: a) with three representative switching unit length ratio (TPU/AgS
composites) in the mid-section (2:1:2 (squares), 1:1:1 (triangles), and 1:2:1
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(circles)); and b) three repeated heating cycles on the tri-component series
sample with the smallest switching unit portion (2:1:2), showing good
repeatability
of presented systems (cycle 1: diamonds, cycle 2: stars, cycle 3: triangles).
5 Joule heating behaviour:
Joule heating performance of tri-component series assembly with different
ratio of
HDPE/GNP composite as the switching part are shown in Figure 9: a) Electrical
power changes with increasing temperature and stabilised at the PTC switching
10 temperature, indicating no further heating up will occur (2-1-2:
squares, 1-2-1:
circles); b) Resistivity increases with electrical heating for both of
assembly, and
also shows the stable final resistivity at self-regulating temperature (1-1-1:
squares, 1-2-1: circles). Initial resistivity differences are observed with
the two
switching unit ratio, indicating different heating up rate.
Illustrations, Joule heating behaviours, and IR images are shown in Figure 10
of:
TPU/AgS composite reached 60 C after 60 min (a and b); TPU/CNT composite
reached over 150 C after 15 min (c and d); Sandwich-structured tri-component
series assembly with TPU/AgS as the switching unit which stabilised at 110 C
(e,
.. f and g); Linear tri-component series assembly with HDPE/GNP as the
switching
part, showing universality of the current design with two switching unit
length ratio
(1:1:1 and 1:2:1) (h, i and j). Uniform heating of the samples via Joule
heating
was confirmed by the IR images.
Flexibility measurement:
Figure 11 shows: a) Illustration of the tri-component series assembly based on
TPU matrix, b) relative resistance change of tri-component series assembly
upon
bending at different radius of curvatures, confirming the good flexibility and
reliability of the presented composites. Both c) and d) demonstrate the
flexibility
of the specimen and the IR image under Joule heating.
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Example 3 - Polymer Blends
Materials:
PPE - VERSIFYTM 2200 Plastomers and Elastomers are a versatile family of
specialty propylene-ethylene copolymers.
Kraton - FG1901 G is a clear, linear triblock copolymer based on styrene and
ethylene/butylene with a polystyrene content of 30%.
TPU - Estanee 58437 is an 85A aromatic Polyester-Based Thermoplastic
Polyurethane (TPU).
Morphology:
The SEM images in Figure 12 below indicate that the location of GNPs in the
polymer matrix.
a). 12 wt.% of GNPs in HDPE matrix. Directly diluted from masterbatch in
Mini
Extruder (ME).
b). 12 wt.% of GNPs in HDPE/PPE blends. Mixed by adding PPE into the
masterbatch in ME. From the image, it looks like the GNP prefers to stay in
the
HDPE phase.
c). 12 wt.% of GNPs in Kraton/PPE blends. Mixed by adding Kraton into the
masterbatch in ME. Kraton stays more compatible with HDPE and there is no
phase separation.
d). 12 wt.% of GNPs in TPU/HDPE blends. Mixed by adding TPU into the
masterbatch in ME. It can be seen that most of the GNPs still stay in the HDPE
phase. However, some of them migrate to the TPU phase.
Electrical property:
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Figure 13 shows the electrical conductivity of different concentration of
HDPE/GNP composite and polymer blends/GNP composite (PPE: squares,
Kraton: circles, TPU: triangles).
The percolation threshold of HDPE/GNP composite is 8.8 wt.% (4.0 vol.%).
By adding the same amount of second polymer (PPE, Kraton and TPU) into the
masterbatch, the blends show different conductivity level. PPE blends show the
highest conductivity, while kraton blends show the most conductivity drop.
This
may correlate with the morphology of the blends, in accordance with GNP
conductive pathways.
FTC behaviour:
FTC behaviour of a number of composites is shown in Figure 14. The plots
relate
to composites comprising GNP (top left; squares: 12%, circles: 15%, stars:
18%,
triangles: 22%, diamonds: 24%), PPE (top right; squares: 10%, circles: 20%,
starts 35%, triangles: 50%), Kraton (bottom left; squares: 10%, circles: 20%,
stars: 35%, triangles: 50%) and TPU (bottom right; squares: 10%, circles: 20%,
stars: 35%, triangles: 50%). The FTC intensity of HDPE/GNP composite is larger
with lower filler content, more than 3 orders of resistivity change has been
observed from 18 wt.% GNP filled HDPE composite. PPE blends show the most
attractive feature of different filler loading. The FTC behaviour of each
filler
contents shows quite similar trend, while the intensity is also about 3 orders
of
magnitude. Kraton blends have been influenced most when filler loading
decreases. The initial conductivity level of Kraton blends changes more than
HDPE/GNP composites. The FTC behaviour of TPU blends sits in the middle of
these two blends. These interesting observations may relate to the filler
location
and polymer blending morphology.
The foregoing detailed description has been provided by way of explanation and
illustration, and is not intended to limit the scope of the appended claims.
Many
variations in the presently preferred embodiments illustrated herein will be
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apparent to one of ordinary skill in the art and remain within the scope of
the
appended claims and their equivalents.
