EP3479651B1

EP3479651B1 - Heating element

Info

Publication number: EP3479651B1
Application number: EP17736731.5A
Authority: EP
Inventors: Emiliano BILOTTI; Harshit PORWAL; Yi Liu; Mark Newton; Jamie EVANS
Original assignee: LMK Thermosafe Ltd
Current assignee: LMK Thermosafe Ltd
Priority date: 2016-06-30
Filing date: 2017-06-29
Publication date: 2020-04-22
Anticipated expiration: 2037-06-29
Also published as: EP3479651A1; GB2551789B; WO2018002633A1; ES2787033T3; CA3027473C; CA3027473A1; GB2551789A; GB201611397D0

Description

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 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. CPCs 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 PTC 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 CPCs represent a compromise between low percolation threshold and large PTC intensity.
In order to try to overcome this compromise, CPCs have been prepared containing combinations of two fillers (so-called "mixed-filler" composites): one filler exhibiting a large PTC intensity and the other exhibiting a low percolation threshold. However, in such mixed-filler composites the PTC intensity is dominated by the filler with the lowest PTC intensity, even at very low loadings. WO2016/012762A1 relates to a conductive polymer composite.
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:

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 µm, 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 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.
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 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.
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.
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.
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 µm. The second conductive particles preferably have a longest dimension of from 20 to 150 µm, more preferably from 40 to 60 µm. When the second conductive particles are in the form of a sphere, the longest dimension is the diameter of the 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 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.
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 Estane® 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 ® 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 ºC, whereas the use of TPU may result in a maximum temperature of around 120 ºC. 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, 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 VERSIFY™ 2200 plastomers and elastomers) and TPU (e.g. Estane® 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 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.
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 µm,
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.

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 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 ºC.
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 WO2016/012762 . 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.
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.
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:

The preferable and optional features of the first aspect apply also to this aspect.
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
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 µm, 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 Estane® 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.
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 rpm, processing temperature of 200 °C, and a residhg 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 composite was directly collected 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.
The produced compounded strands were chopped into pellets and compression moulded into sample bar with the dimension of 28mm×10mm×2mm 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 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 polyurethane (TPU)	Lubrizol Estane® 58437	Density 1.19 gcm^-1
Polymer	High density polyethylene (HDPE)	Rigidex® HD5218EA	Density 0.952 gcm^-1
Conductive filler	Multi-wall carbon nanotubes (MWCNTs)	Nanocyl S.A. Product No. C7000	Average diameter of 9.5 nm, carbon purity 90%
	Silver coated glass spheres (AgS)	Potters industries Ltd.	Average diameter of 50 µm
	Graphene nanoplatelets (GNPs)	xGnP® Grade M	Average particle diameter of 15 µm, 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 corotating 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 L/D). The throughput was of 2 kg/h, using a screw speed of 50 rpm, and a temperature ranging between 190 °C and 220 °C over 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 × 10 mm × 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 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.

Percolation curve:

Percolation curves are shown in Figure 6 of: a) TPU/CNT composites showing a relatively low percolation threshold (ϕ_c) 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.

PTC 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 PTC 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.

Tri-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 (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).

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 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.

Example 3 - Polymer Blends

Materials:

PPE - VERSIFY™ 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 - Estane® 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:

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.

PTC behaviour:

PTC 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 PTC 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 PTC 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 PTC 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 apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents

Claims

A self-regulating heating element 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); 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 µm, characterised in that the first conductive particles have an aspect ratio greater than 100 and the first conductive polymer composite and the second conductive polymer composite are arranged in series between the pair of electrodes.
The heating element of claim 1, wherein:
the heating core has a positive temperature coefficient intensity of greater than 1, preferably greater than 3, more preferably greater than 5, even more preferably greater than 6; and/or
the first conductive particles comprise carbon nanotubes; and/or

the first conductive particles have an aspect ratio of greater than 150, preferably greater than 500, more preferably greater than 1000; and/or

the first conductive polymer composite comprises from 0.1 to 10 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite, preferably from 0.5 to 10 wt.% based on the total weight of the first conductive polymer composite, more preferably from 0.5 to 5 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite, even more preferably from 2 to 3 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite; and/or

the second conductive particles comprise spheres and/or flakes; and/or

the second conductive particles comprise one or more of silver particles and silver-coated glass particles; and/or

the second conductive particles comprise graphene nanoplatelets; and/or

the second conductive particles have an aspect ratio of from 1 to 10; and/or

the second conductive particles have a longest dimension of from 20 to 150 µm, preferably from 40 to 60 µm.
The heating element of claim 1 or claim 2, wherein the second conductive polymer composite comprises from 10 to 60 wt.% of the second conductive particles based on the total weight of the conductive polymer composite, preferably from 30 to 40 wt.% of the second conductive particles based on the total weight of the second conductive polymer composite.
The heating element of any preceding claim, wherein the polymer of the first polymer matrix and/or the polymer of the second polymer matrix comprise a plastomer and/or an elastomer.
The heating element of claim 4, wherein the plastomer is an olefin-based plastomer or a polyurethane-based plastomer.
The heating element of claim 4 or claim 5, wherein the elastomer is a cross-linked elastomer.
The heating element of any preceding claim, wherein 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.
The heating element of any preceding claim wherein:
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 µm,

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.
The heating element of any of claims 1 to 7, wherein:
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.
The heating element of any preceding claim, wherein the heating element is a thermal switch.
A circuit comprising the thermal switch of claim 10 and an electronic component, wherein the thermal switch is connected in series to the electronic component.
The circuit of claim 11 comprising 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.
A container heater comprising the heating element of any of claims 1 to 10.
A heating jacket comprising the heating element of any of claims 1 to 10, preferably wherein the heating jacket is a flexible heating jacket.
A trace heater comprising the heating element of any of claims 1 to 10.