GB2551789A

GB2551789A - Heating element

Info

Publication number: GB2551789A
Application number: GB1611397.9A
Authority: GB
Inventors: Bilotti Emiliano; Porwal Harshit; Liu Yi; Newton Mark; Evans Jamie
Original assignee: LMK Thermosafe Ltd
Current assignee: LMK Thermosafe Ltd
Priority date: 2016-06-30
Filing date: 2016-06-30
Publication date: 2018-01-03
Anticipated expiration: 2036-06-30
Also published as: EP3479651B1; EP3479651A1; WO2018002633A1; CA3027473C; GB2551789B; CA3027473A1; GB201611397D0; ES2787033T3

Abstract

A self-regulating heating element A comprises a positive temperature coefficient heating core B disposed between a pair of electrodes C, the heating core 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 having an aspect ratio of from 1 to 100 and a longest dimension of greater than 10µm, wherein the first conductive polymer composite and the second conductive polymer composite are arranged in series between the pair of electrodes. The first conductive particles may comprise carbon nanotubes and the second conductive particles may comprise spheres and/or flakes and may be silver particles or silver-coated glass particles.

Description

(54) Title of the Invention: Heating element Abstract Title: Heating element (57) A self-regulating heating element A comprises a positive temperature coefficient heating core B disposed between a pair of electrodes C, the heating core 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 having an aspect ratio of from 1to 100 and a longest dimension of greater than 10pm, wherein the first conductive polymer composite and the second conductive polymer composite are arranged in series between the pair of electrodes. The first conductive particles may comprise carbon nanotubes and the second conductive particles may comprise spheres and/or flakes and may be silver particles or silver-coated glass particles.

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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 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, precessability, 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.

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 pm, 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.

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 “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 log-ιο (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 0.5 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 5 wt.%, 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, metal particles, alloy particles, metal-coated glass particles, metal-coated polymer particles and conductive polymer-coated particles. The metal may be selected from, for example, copper, silver and/or gold. The second conductive particles preferably comprise one or more of silver particles (e.g. silver flakes) and silver-coated glass particles.

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 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. 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. 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 and processability.

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.

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.

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.

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.

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 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 residing 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 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 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). 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).

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

Claims:

1. A self-regulating heating element comprising a heating core disposed between a pair of electrodes, the heating core comprising:

2. 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.

3. The heating element of claim 1 or claim 2, wherein the first conductive particles comprise carbon nanotubes.

4. The heating element of any preceding claim, wherein the first conductive particles have an aspect ratio of greater than 150, preferably greater than 500, more preferably greater than 1000.

5. The heating element of any preceding claim, wherein the first conductive polymer composite comprises from 0.5 to 10 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite, preferably from 0.5 to 5 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite, more preferably from 2 to 3 wt.% of the first conductive particles based on the total weight of the first conductive polymer composite.

6. The heating element of any preceding claim, wherein the second conductive particles comprise spheres and/or flakes.

7. The heating element of any preceding claim, wherein the second conductive particles comprise one or more of silver particles and silver-coated glass particles.

8. The heating element of any preceding claim, wherein the second conductive particles have an aspect ratio of from 1 to 10.

9. The heating element of any preceding claim, wherein the second conductive particles have a longest dimension of from 20 to 150 pm, preferably from 40 to 60 pm.

10. The heating element of any preceding claim, 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.

11. 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.

12. The heating element of claim 11, wherein the plastomer is an olefin-based plastomer or a polyurethane-based plastomer.

13. The heating element of claim 11 or claim 12, wherein the elastomer is a cross-linked elastomer.

14. The heating element of any preceding claim, wherein the heating core comprises either:

15. 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 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.

16. A container heater comprising the heating element of any preceding claim.

17. A heating jacket comprising the heating element of any of claims 1 to 15.

18. The heating jacket of claim 17, wherein the heating jacket is a flexible heating jacket.

19. A trace heater comprising the heating element of any of claims 1 to 15.

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