Self-regulating heater

Abstract

A flat sheet electrical heater comprising a single semiconductive layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; wherein said semiconductive layer has a top surface and a bottom surface; a plurality of conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer.

Description

[0001] Self-regulating heater

[0002] This invention relates to a self-regulating heater and to processes for the preparation of such a structure. In particular, the invention relates to a flat sheet electrical heater comprising a semiconductive layer comprising an electrically semiconductive composition with a positive temperature coefficient where a plurality of conductors are located on the electrically semiconductive layer. The invention also relates to a moulded electric heater having a body with top and bottom surfaces comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler. The moulded heater is provided with a plurality of conductors in contact with the top and / or bottom surface of the moulded heater. In one embodiment these conductors are conductor foils which cover the majority of the top and bottom surface of said heater wherein the distance between the conductor films is substantially uniform Such a heater may be in the form of a cylinder or cuboid.

[0003] When the power is switched on, the flow of electrical energy causes the semiconductive composition to heat to a steady state temperature.

[0004] Background of Invention

[0005] Parallel resistance self-regulating heating cables are known. Such cables normally comprise two conductors extending longitudinally along the cable. Typically, the conductors are embedded within a resistive polymeric heating element, the element being extruded continuously along the length of the conductors. The cable thus has a parallel resistance form, with power being applied via the two conductors to the heating element connected in parallel across the two conductors. The heating element usually has a positive temperature coefficient of resistance. Thus as the temperature of the heating element increases, the resistance of the material electrically connected between the conductors increases, thereby reducing power output. Such heating cables, in which the power output varies according to temperature, are said to be self-regulating or self-limiting. Thus, to avoid overheating and potential destruction of the object, they are self-limiting and require no regulating electronics.

[0006] Self-regulation utilises a conversion from electrical to thermal energy by allowing a current to pass through a semiconductive medium with Positive temperature coefficient (PTC) characteristics, which elevates the object temperature above that of its surroundings, until a steady state is reached (self-regulation). A material with a PTC has an electrical resistance that increases with temperature, and is the mechanism behind the self-regulating function. These PTC cables are often used in underfloor heating or wrapped around pipes for e.g. anti-freeze purposes. Cables however, do not offer a significant surface area of heat so it takes a large number of cables to provide underfloor heating for example.

[0007] The use of flat sheet self-regulating heaters is also known therefore. In W02014 / 188190, an electrical heater is described that comprises conductors and a heating element disposed between the conductors wherein the heating element comprises an electrically conductive material distributed within a first electrically insulating material. The insulating material separates the conductor from the electrically conductive material.

[0008] US7250586 describes a surface heating system for a car seat or the like comprising a support and a heating layer that contains an electrically conductive plastic, which is characterized by the fact that the heating layer is formed by a flexible film and that the support is flexible.

[0009] US4247756 describes a heated floor mat in which two electrically conductive inner layers sandwich conductors. These conductors are adhered to the inner layers.

[0010] US7053344 describes a flexible heater for a fabric.

[0011] US2021112631 describes a heating element that has at least one film of an electrically conductive polymer material.

[0012] US5451747 discloses a heat mat with PTC material.

[0013] US2023 / 0092379 describes a PTC material with graphene as the filler. Para 0006 describes a flat sheet heater with polymer and filler with upper and lower foil conductors. US 2016 / 021705 a heater comprising a thermoplastic elastomer with metal filler. US9955531 describes a method for making a PTC material via a polymer emulsion. The invention requires crosslinking of the polymer.

[0014] US2016 / 0264809 concerns a resistor composition and the addition of BN as a conductive fdler to that composition.

[0015] EP1009196 describes a sheet shaped heater in which a covering member covers electrodes that are located on the edges of the sheet surface. A plurality of electrodes are positioned on the ends of the flat sheet and are covered by a sheath which sits on the flat sheet.

[0016] US5111025 describes heaters for car seats. The invention describes a flat sheet support and a heating element in the form of a tape which wraps around the support and is based on UHMWPE and a filler.

[0017] WO2008 / 133562 describes a heating device comprising two elongated electrodes arranged at a distance and being inter-connected by a semiconducting heat generating member of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections of a low resistivity PTC material compared with the PTC material of intermediate section.

[0018] EP1275274 describes a device for floor heating comprising a bendable, electrically conductive, thermoplastic mat. The device is provided with at least two electrodes. Current is conducted through the device, which heats up and emits heat.

[0019] WO2021 / 188595 describes a blanket comprising a first outer panel, a selfregulating heating element proximate to said first outer panel and a second outer panel proximate to said self-regulating heating element joined to said first outer panel wherein said first outer panel and said second outer panel contain said selfregulating heating elements.

[0020] WO2022 / 129251 describes a self-regulating flat sheet heater prepared by coextrusion where conductors are embedded within the semiconductive composition.

[0021] In the present invention conductors are arranged in contact with / adhered to the semiconductive layer. Only one semiconductive layer is required. The fact that conductors can be applied directly to the semiconductive layer also simplifies the installation process as no semiconductive material needs to be removed to make electrical connections. The conductors can also be readily located above and below the semiconductive layer to avoid any risk of short circuit. Applying conductors after formation of the semiconductive layer or 3D object also opens up the possibility to place conductors in a tailored pattern after production, either manually or with machine / robot.

[0022] The application of the conductors to the surface of the semiconductor layer also opens up the possibility of applying film like conductors covering a majority of the semiconductor layer surface, e.g. one above and one below. If the semiconductor layer is relatively thin, conductor distance can become very short > 0,5 mm.

[0023] Applying the conductors post formation also opens up the option to manufacture 3D moulded objects, that can be produced with various techniques, for example injection molding. Such 3D moulded objects might be in the form of a cylinder or cuboid and should have top and bottom surfaces of semiconductor composition that are preferably evenly spaced apart. A conductor foil can then be applied to the top and bottom surface to allow heating of the semiconductor composition that forms the body of the object. In this way a standalone electrical heater can be produced using simple manufacturing techniques.

[0024] Summary of Invention

[0025] Viewed from one aspect the invention provides a flat sheet electrical heater comprising a single semiconductive layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; wherein said semiconductive layer has a top surface and a bottom surface; a plurality of conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer.

[0026] The term single is used herein to imply the presence of one semiconductive layer only.

[0027] Viewed from another aspect, the invention provides a moulded electric heater having a body with a top surface and a bottom surface said body comprising an electrically semi conductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; a plurality of conductors arranged on and in contact with the top and / or bottom surface of the body.

[0028] Viewed from another aspect the invention provides a moulded electric heater having a body with a top surface and a bottom surface, said body comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; wherein a first conductor foil is arranged on and in contact with the top surface of the body and covers a majority of the top surface of said body and a second conductor foil is arranged on and in contact the bottom surface of the body and covers a majority of the bottom surface of said body and wherein the distance between the first conductor foil and the second conductor foil is substantially uniform.

[0029] In particular, the invention provides a moulded, cylindrical electric heater having a top with a top surface and a bottom with a bottom surface and a cylindrical body, wherein said top and bottom and said cylindrical body comprise an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler. A foil conductor is placed in contact with or adhered to the top surface of the top and a foil conductor is placed in contact with or adhered to bottom surface of the bottom of the heater wherein the distance between the conductor films is substantially uniform.

[0030] Viewed from another aspect the invention provides a process for the preparation of a flat sheet electrical heater as hereinbefore defined comprising the steps of (a)

[0031] - providing and melt mixing an electrically semiconductive composition comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler, and forming a layer thereof; (b) applying a plurality of conductors onto the layer such that they are arranged on and in contact with the layer.

[0032] Viewed from another aspect, the invention provides a process for the preparation of a moulded electrical heater comprising the steps of (a)

[0033] - providing an electrically semiconductive composition comprising a polyethylene, a polypropylene or a mixture thereof, and a conductive filler,

[0034] - melt mixing said electrically semiconductive composition;

[0035] - moulding, e.g. injection moulding, an object having a body with a top surface and a bottom surface, said body comprising said electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler;

[0036] (b) applying a plurality of conductors onto the top and / or bottom surface of the heater such that they are arranged on and in contact with the heater.

[0037] Viewed from another aspect, the invention provides a process for the preparation of a moulded electrical heater comprising the steps of (a)

[0038] - providing an electrically semiconductive composition comprising a polyethylene, a polypropylene or a mixture thereof, and a conductive filler,

[0039] - melt mixing said electrically semiconductive composition;

[0040] - moulding, e.g. injection moulding, an object having a body with a top surface and a bottom surface, said body comprising said electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler;

[0041] (b) applying a first conductor foil onto the top surface of said body and a second conductor foil onto the bottom surface of said body wherein the first conductor foil covers a majority of the top surface and the second conductor foil covers a majority of the bottom surface, and wherein the first conductor foil and the second conductor foil are applied such that the distance therebetween is substantially uniform.

[0042] The foil conductor is therefore a flat sheet conductor that covers preferably the majority of the top and bottom surface, such as substantially all the top and bottom surface of the heater. Detailed Description of Invention

[0043] The present invention relates to an electrical heater comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler. Conductors are placed on the outside of the semiconductor composition, e.g. in a regular arrangement, in contact with the semiconductive composition. In one embodiment, the electrical heater is in the form of a flat sheet and comprises therefore a semiconductive layer comprising the semiconductive composition. In an alternative embodiment, the electric heater is a moulded 3D heater with a body and top and bottom surfaces onto which conductors can be applied. The body of the heater comprises the semiconductive composition.

[0044] The heater provides heat in a safe, cheap and simple manner. The heater of the invention uses the principle of positive temperature coefficient (PTC). The term positive temperature coefficient (PTC) term, defines the non-linear increase in electrical resistivity as a function of temperature. To avoid overheating and potential destruction of the heater, the heat generated is self-limiting and requires no regulating electronics. As temperature increases within the semiconductive composition caused by the power applied, resistance within the semiconductive composition increases until a steady state is reached and no further heating takes place. In one embodiment therefore, the heater of the invention contains no regulating electronics, e.g. a heat cut off to prevent overheating.

[0045] The electrically semiconductive composition cannot overheat and requires no overheat protection. The technical solution in this particular invention utilises conversion from electrical to thermal energy by allowing a current to pass through a semiconductive composition with PTC characteristics, which elevates the object temperature above that of its surroundings, until a steady state is reached (selfregulation).

[0046] The electrically semiconductive composition comprises a polypropylene or polyethylene and a conductive filler (e.g. carbon black). The self-regulating thermal phenomenon occurs due to two parallel antagonistic processes: a. Poor conduction of electrons through the semiconductive medium generates electrical losses, manifested in heat emission. b. Thermal expansion of the non-conductive part of the material leads to further decrease of the conductivity by separation of the conductive filler particles.

[0047] Once the two processes have equalised, a steady elevated temperature plateau is reached.

[0048] The temperature increase in the electrically semiconductive composition is governed by a number of factors. These include the thickness of the electrically semiconductive composition (e.g. the thickness of the semiconductor layer in which it is present), the amount of conductive filler present in the electrically semiconductive composition, and the applied voltage. The distance between the conductors that supply current to the semiconductive composition is also a factor.

[0049] A thicker electrically semiconductive layer increases the temperature at which a steady elevated temperature plateau is reached.

[0050] Increases in conductive filler content in the semiconductive layer increases the temperature at which a steady elevated temperature plateau is reached.

[0051] It is preferred that the steady state elevated temperature is no more than 50°C, such as no more than 45°C, especially where the object may be in contact with humans. The heater should ideally achieve a temperature of at least 30 °C.

[0052] This gives the product designer freedom to alter the level of heat generated through manipulation of the size and shape of the object, the make-up of the semiconductive composition and applied voltage in order to reach a pre-determined target temperature.

[0053] Electrically semiconductive composition

[0054] The heater of the invention comprises an electrically semiconductive composition. In one embodiment, the semiconductive composition is in the form of a layer. In another embodiment, the semiconductive composition is shaped into a 3D object with a body, top and bottom surfaces onto which a conductor foil can be applied. In one embodiment, the semiconductive layer comprises or consists of the electrically semiconductive composition. The electrically semiconductive composition comprises a polyethylene, polypropylene or mixture thereof. In a preferred embodiment, the heater of the invention is essentially a flexible flat sheet which can be manipulated into desired shapes (such as a cylinder, cuboid and so on) as required.

[0055] The heater therefore comprises an electrically semiconductive composition, e.g. in the form of a single semiconductive layer. This electrically semiconductive composition comprises a polyethylene, a polypropylene or a mixture thereof.

[0056] It is preferred if the semiconductive composition comprises a polyethylene although preferably it is not an ultra-high molecular weight polyethylene such as one having a Mw of at least 2,000,000. It preferred if the polyethylene is one prepared in a high temperature autoclave or tubular process such as a LDPE homopolymer or copolymer.

[0057] Although the term LDPE is an abbreviation for low density polyethylene, the term is understood not to limit the density range, but covers the LDPE-like high pressure (HP) polyethylenes. The term LDPE describes and distinguishes only the nature of HP polyethylene with typical features, such as different branching architecture, compared to the polyethylene produced in the presence of an olefin polymerisation catalyst.

[0058] The term LDPE means a low density homopolymer of ethylene (referred herein as LDPE homopolymer) or a low density copolymer of ethylene with one or more comonomer(s) (referred herein as LDPE copolymer).

[0059] It is preferred if the electrically semiconductive composition comprises an LDPE copolymer. The one or more comonomers of LDPE copolymer are preferably selected from the polar comonomer(s), non-polar comonomer(s) or from a mixture of the polar comonomer(s) and non-polar comonomer(s). Moreover, said LDPE homopolymer or LDPE copolymer may optionally be unsaturated.

[0060] As a polar comonomer for the LDPE copolymer, comonomer(s) containing carboxyl and / or ester group(s) are used as said polar comonomer. Still more preferably, the polar comono mer(s) of LDPE copolymer is selected from the groups of acrylate(s), methacryl ate(s) or acetate(s), or any mixtures thereof. If present in said LDPE copolymer, the polar comonomer(s) is preferably selected from the group of alkyl acrylates, alkyl methacrylates or vinyl acetate, or a mixture thereof. The use of ethylene alkyl acylates or ethylene vinyl acetate is preferred.

[0061] Further preferably, said polar comonomers are selected from Ci- to Ce-alkyl acrylates, Ci- to Ce-alkyl methacrylates or vinyl acetate. Still more preferably, said LDPE copolymer is a copolymer of ethylene with Ci- to C4-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, or vinyl acetate, or any mixture thereof. The use of ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), ethylene butyl acrylate (EBA) or ethylene vinyl acetate (EVA) is preferred.

[0062] As the non-polar comonomer(s) for the LDPE copolymer preferred options are polyunsaturated comonomers comprising C and H atoms only. In a preferred embodiment, the polyunsaturated comonomer consists of a straight carbon chain with at least 8 carbon atoms and at least 4 carbon atoms between the non-conjugated double bonds, of which at least one is terminal.

[0063] A preferred diene compound is 1,7-octadiene, 1,9-decadiene, 1,11- dodecadiene, 1,13 -tetradecadiene, or mixtures thereof. Furthermore, dienes like 7- methyl-l,6-octadiene, 9-methyl-l,8-decadiene, or mixtures thereof can be mentioned.

[0064] If the LDPE polymer is a copolymer, it preferably comprises 1.0 to 40 wt.- %, more preferably 5.0 to 35 wt.-%, still more preferably 10 to 30 wt%%, of one or more comonomer(s).

[0065] Where there is apolar comonomer, the comonomer content is preferably 5.0 to 30 wt%, such as 7.5 to 20 wt% in the polymer. The use of ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), ethylene butyl acrylate (EBA) or ethylene vinyl acetate (EVA) is preferred especially where there is 5 to 30 wt%, such as 7.5 to 20 wt% comonomer present.

[0066] The MFR of the polymer when measured under a load of 21.6 kg / 125°C may be 4.0 g / lOmin or more are required, such as at least 6.0 g / 10 min, even more preferably 8.0 to 15 g / 10 min, and most preferably at least 10.0 g / 10 min. An upper limit of 25 g / 1 Omin is preferred, such as 18 g / 1 Omin. Using higher MFR values appears to give more uniform layer thicknesses and allows higher extrusion output without changing the ability of the polyethylene to act as a heater through PTC principles.

[0067] Any LDPE homopolymer or copolymer may have a density of 905 to 935 kg / m3, such as 910 to 925 kg / m3.

[0068] The polyethylene can be produced by any conventional polymerisation process. Preferably, it is an LDPE and is produced by radical polymerisation, such as high pressure radical polymerisation. High pressure polymerisation can be effected in a tubular reactor or an autoclave reactor. Preferably, it is a tubular reactor. In general, the pressure can be within the range of 1200-3500 bars and the temperature can be within the range of 150°C-350°C. Further details about high pressure radical polymerisation are given in WO93 / 08222, which is herewith incorporated by reference.

[0069] The semiconductive composition may also comprise a polypropylene polymer, such as a polypropylene copolymer. Such a copolymer might be a random polypropylene copolymer with ethylene or C4-10 alpha olefin.

[0070] The electrically semiconductive composition may comprise at least 50 wt% of the polyethylene, polypropylene or mixture thereof, such as at least 60 wt%. Any layer in which the electrically semiconductive composition is present may consist of the electrically semiconductive composition. Thus, any layer in which the electrically semiconductive composition is present may comprise at least 50 wt% of the polyethylene, polypropylene or mixture thereof, such as at least 60 wt%. The polyethylene, polypropylene or mixture thereof will form the balance of the electrically semiconductive composition once all other components are determined.

[0071] The semiconductive composition may also form the 3D objects in the second embodiment described further below.

[0072] Conductive Filler

[0073] According to the present invention, the electrically semiconductive composition further comprises a conductive filler such as carbon black. The conductive filler should not be metallic.

[0074] Suitable conductive fillers include graphite, graphene, carbon fibres, carbon nanotubes, metal powders, metal strands or carbon black. The use of carbon black is preferred. The use of boron nitride should be avoided. It is preferred therefore if the semi conductive composition of the invention is free of boron nitride and / or metallic conductive fdlers. Most preferably the conductive filler is carbon black and no other conductive fillers should be present.

[0075] The semiconductive properties result from the conductive filler added. Thus, the amount of conductive filler is at least such that a semiconducting composition is obtained. Depending on the desired use and conductivity of the composition, the amount of conductive filler can vary. Preferably, the electrically semiconductive composition comprises 5-50 wt% conductive filler, such as 15 to 50 wt%. In other preferred embodiments, the amount of conductive filler is 5-48 wt.-%, 10-45 wt%, 20-45 wt%, 25-45 wt% or 30-41 wt%, based on the weight of the electrically semiconductive composition.

[0076] Any carbon black can be used which is electrically conductive. Examples of suitable carbon blacks include furnace blacks, channel blacks, gas blacks, lamp blacks, thermal blacks and acetylene blacks. Additionally, graphitised furnace blacks (as produced by Imerys) and high structure blacks (known as Ketjenblacks produced by Nouryon) may also be used. Mixtures may also be used. Where a blend of carbon blacks is used then this percentage refers to the sum of the carbon blacks present.

[0077] The carbon black may have a nitrogen surface area (BET) of 5 to 1500 m2 / g, for example of 10 to 300 m2 / g, e.g. of 30 to 200 m2 / g, when determined according to ASTM D3037-93. Further, the carbon black may have one or more of the following properties: i) a primary particle size of at least 5 nm which is defined as the number average particle diameter according to ASTM D3849-95a, ii) iodine adsorption number (IAN) of at least lOmg / g, for example 10 to 300 mg / g, e.g. 30 to 200 mg / g, when determined according to ASTM D-1510; and / or iii) DBP (dibutyl phthalate) absorption number (= oil absorption number) of at least 30 cm3 / 100g, for example 60 to 300 cm3 / 100g, e.g. 70 to 250 cm3 / 100g, for example 80 to 200 cm3 / 100g, e.g. 90 to 180 cm3 / 100g, when measured according to ASTM D 2414. Furthermore, the carbon black may have one or more of the following properties: a) a primary particle size of at least 15 nm which is defined as the number average particle diameter according ASTM D3849-95a; b) iodine number of at least 30 mg / g according to ASTM DI 510; c) oil absorption number of at least 30 ml / lOOg which is measured according to ASTM D2414.

[0078] Furnace carbon blacks are preferred. This is a generally acknowledged term for the well-known carbon black type that is produced continuously in a furnacetype reactor. As examples of carbon blacks, the preparation process thereof and the reactors, reference can be made to i.a. EP-A-0629222 of Cabot, US 4,391,789, US 3,922,335 and US 3,401,020. As an example of commercial furnace carbon black grades described in ASTM D 1765-98b i.a. N351, N293 and N550, can be mentioned.

[0079] Other components

[0080] The semiconductive composition may be crosslinked using peroxide or silane moisture curing systems. Crosslinking may also be effected using irradiation to avoid the need for a crosslinking agent.

[0081] Preferably, crosslinking is avoided and the resulting heater is a more recyclable product. The semiconductive composition of the invention is preferably not crosslinked.

[0082] Viewed from another aspect therefore, the invention provides a flat sheet electrical heater comprising a single non-crosslinked semiconductive layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene and a carbon black conductive filler; wherein said semiconductive layer has a top surface and a bottom surface; a plurality of wire conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer.

[0083] Viewed from another aspect therefore the invention provides a flat sheet electrical heater comprising a single non-crosslinked semiconductive layer consisting of an electrically semiconductive composition with a positive temperature coefficient comprising at least a polyethylene and conductive filler consisting of carbon black (i.e. no other conductive filler is present); wherein said semiconductive layer has a top surface and a bottom surface; a plurality of wire conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer. The wires should not contact any other surface of the flat sheet heater.

[0084] Antioxidant

[0085] The semiconductive composition may contain an antioxidant. As antioxidant, sterically hindered or semi-hindered phenols, aromatic amines, aliphatic sterically hindered amines, organic phosphates, thio compounds, polymerized 2,2,4- trimethyl-l,2-dihydroquinoline and mixtures thereof, can be mentioned.

[0086] More preferred, the antioxidant is selected from the group of 4,4'- bis(l,l'dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4’-thiobis (2 -tert, butyl-5-methylphenol), polymerized 2,2,4-trimethyl-l,2- dihy droquinoline, 4-( 1 -methyl- 1 -phenylethyl)N- [4-( 1 -methyl- 1 -phenyl ethyl )phenyl] aniline or derivatives thereof.

[0087] More preferred, the antioxidant is selected from the group (but not limited to) of 4,4'- bis(l,l'dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4’-thiobis (2 -tert, butyl-5-methylphenol), 2,2’- thiobis(6-t-butyl-4-methylphenol), distearylthiodipropionate, 2,2’-thio-diethyl- bis-(3-(3,5-di-tertbutyl-4-hydroxyphenyl)propionate, polymerized 2,2,4- trimethyl-l,2-dihy droquinoline, or derivatives thereof. Of course, not only one of the above- described antioxidants may be used but also any mixture thereof.

[0088] The amount of antioxidant, optionally a mixture of two or more antioxidants, can range from 0.005 to 2.5 wt-%, such as 0.01 to 2.5 wt-%, preferably 0.01 to 2.0 wt-%, more preferably 0.03 to 2.0 wt-%, especially 0.03 to 1.5 wt-%, more especially 0.05 to 1.5 wt%, or 0.1 to 1.5 wt% based on the weight of the semiconductive composition. The semiconductive composition may comprise further additives. As possible additives stabilisers, processing aids, flame retardant additives, acid scavengers, inorganic fillers, voltage stabilizers, or mixtures thereof can be mentioned.

[0089] Preferably, the first electrically semiconductive composition has a volume resistivity, measured at 40°C, of less than 20 Ohm • cm Preferably, the first electrically semiconductive composition has a volume resistivity, measured at 25°C, of less than 12 Ohm • cm

[0090] Form of the Heater

[0091] The semiconductive composition can be formed into a variety of shapes in order to form the electrical heater of the invention. In one embodiment, the semiconductive composition can be formed into a single layer in a flat sheet heater. The semiconductive layer may have a thickness of 50 to 3000 pm, such as 75 to 2000 pm, especially 100 to 1000 pm. It is especially preferred if the thickness is 100 to 900 pm, such as 125 to 800 pm. In such a flat sheet heater there is only one semiconductive layer present.

[0092] The semiconductive composition can also be moulded into a 3D object. Where the semiconductive composition is shaped into a 3D object such as a cuboid or cylinder then the thickness of the semiconductive composition is likely to be much higher. A heater can therefore be moulded into a shape such as a cylinder or cuboid.

[0093] In one embodiment, the semiconductor composition is moulded into a shape with a body and top and bottom surfaces that are preferably evenly separated, i.e. the distance between the top and bottom surfaces is the same as these are parallel. For example, the shape, such as a cylinder, may be injection moulded or rotomoulded. Any moulded electrical heater of the invention can have at least one axis of symmetry, such as at least two axes of symmetry.

[0094] It is preferred if the moulded electrical heater has a top surface which is solid and contains no holes. It is preferred if the moulded electrical heater has a bottom surface which is solid and contains no holes. The moulded electric heater of the invention can have a top surface and a bottom surface onto which a conductor, such as a conductor foil can be applied.

[0095] For example, a semiconductive composition can take the form of a moulded cylindrical body of semiconductive composition with top and bottom surfaces. More generally, the semiconductive composition can take the form of a body (such as a hollow cuboid) with top and bottom surfaces. The shape of the body can be readily designed by the person skilled in the art and there are an enormous variety of shapes that can be envisaged (e.g. an hourglass shape, , a cuboid, etc). Ideally, the structure will have a top surface parallel to the bottom surface, the shape of the body in between being of a variety of design.

[0096] Conductors can be applied to the top and / or bottom surface of the moulded electrical heater such that these are in contact with the top and / or bottom surface, optionally using an adhesive. These conductors are preferably evenly spaced apart, i.e. the distance between conductors is the same (see e.g. figure 5). The conductors are ideally not therefore embedded within the single semiconductive layer.

[0097] It is preferred if the conductors are in the form of wires. It is preferred if the conductors are not themselves provided with a covering member to separate the conductors from the flat sheet. The conductors should be in direct contact with the flat sheet (although an adhesive might be used).

[0098] Ideally wires are in contact with the top or bottom surface of the flat sheet heater only.

[0099] In one embodiment, foil type conductors can be used. Where foil conductors contact a moulded electrical heater, these are preferably in contact with the majority such as the whole of the top and bottom surface of the heater. The conductor foil may, in this embodiment, be in the form a circular disc that contacts the top and bottom surface of the cylinder, as illustrated in figure 4.

[0100] Conductors

[0101] The heater of the invention comprises a plurality of conductors. The term plurality is used herein to imply at least 2, preferably at least 4 conductors. In use, the conductors have alternate polarity. The conductors can be made from any suitable conductive metal, typically copper or aluminium. The conductor may be in the form of a tape, foil or wire.

[0102] In the flat sheet embodiment, conductors may have a diameter or thickness of 1.0 pm to 2.0 mm, such as 5.0 pm to 1.0 mm. Conductors can be spherical in cross-section in which case the diameter is given above. Some conductors may have a width of 0.5 to 15 mm, such as 1.0 to 10 mm. The length of the conductor is governed by the size of the heater into which the film will be incorporated.

[0103] In one embodiment, the conductors are elongate which implies that the conductors are long and thin compared to their width and thickness. The conductors are often wires or tapes, which can be made via well-known processes such as extrusion.

[0104] Each conductor may be provided with an electrode to allow the plurality of conductors to be interconnected and to allow the application of an external power source to create a circuit and hence heat. The conductors may be designed to be directly solderable for ease of installation. It is also possible that at least one of the conductors acts as a connecting wire to a power source.

[0105] The heater may comprise a minimum of 2 separate conductors but it may contain many more conductors. The conductors are spaced apart from each and hence do not touch. The conductors are preferably substantially parallel to each other. All conductors should preferably be evenly spaced from each other to ensure an even temperature on application of power. By evenly spaced means that the distance between adjacent conductors is always the same. The conductors are preferably linear. In theory however the conductors might be curved (SS shaped for example) such that they remain equidistant from each other at all times. We regard this as being “parallel”.

[0106] In one embodiment, the gap between the conductors is 20 to 150 mm, preferably 30 to 90 mm, such as 40 to 80 mm.

[0107] It is preferred therefore if the heater comprises a plurality of conductors that are evenly spaced apart from and substantially parallel to each other, e.g. wherein the distance between conductors is 20 to 150 mm.

[0108] In the moulded heater embodiment, the conductors can also be elongate which implies that the conductors are long and thin compared to their width and thickness. The conductors are often wires or tapes, which can be made via well- known processes such as extrusion. These conductors can be evenly spaced apart and the discussion above on conductors in flat sheet heaters applies.

[0109] In the moulded electric heater embodiment, the conductors may be a foil. Such foils can have any dimension to suit the heater to which they are applied. Where the conductor is placed on a 3D object, it is preferred if the conductor is in the form of a foil and hence it can cover the majority of the top and bottom surface of the heater.

[0110] The conductor should be in contact with or adhered to the top and bottom surface of any heater. The top and bottom surfaces should preferably be uniformly spaced apart. In one embodiment, conductor foils cover the majority of the top and bottom surface of the heater, ideally where the conductor foils are also uniformly spaced apart, i.e. the distance between the conductor foils is substantially uniform. This ensures even heating in the heater when the power is switched on.

[0111] No semiconductive material needs to be removed for electrical connections to be made, simplifying installation.

[0112] In the flat sheet embodiment, it is preferred if the plurality of conductors are located alternately above and below the semiconductive layer or wherein all conductors are located on one side of the layer. If located alternatively above and below the semiconductive layer, the risk of short circuits is reduced.

[0113] The conductors are in direct contact with the electrically semiconductive composition. In this embodiment therefore there should not be a layer separating the electrically semiconductive composition from the conductor.

[0114] An adhesive may however be employed. This adhesive partially or completely covers the conductor and not only adheres to the semiconductive layer but provides an electrical contact with the semiconductive layer.

[0115] Suitable adhesives are conductive adhesives such as those comprising silicone or epoxy resins filled with metallics or conductive carbon fragments.

[0116] Exposed parts of the conductor are preferably protected in some fashion, e.g. via a coating or protective top layer. An electric heater can also be provided with a frame to support the semiconductive body, top and bottom. Other layers

[0117] In any embodiment of the invention, exposed conductors are preferably protected from damage by an additional insulation layer.

[0118] Whilst there is a single semi conductive layer present it is possible to provide the flat sheet heater with non semiconductive layers such as insulation layers or protective layers.

[0119] The flat sheet heater of the invention may be provided with one or more additional layers to protect the semiconductive composition and conductors. For example, an aesthetic top layer can be textile fabric, non-woven or solid sheet (rubber, plastic, paper, wood, metal, etc.).

[0120] The top layer may be extrudable, e.g. a polyolefin layer. Such a layer may be transparent.

[0121] In a preferred embodiment the heater is provided with an insulation layer or heat reflective layer at the base of the flat sheet heater.

[0122] The moulded electric heater of the invention may also be provided with an insulation layer, heat reflective layer or aesthetic layer as described above.

[0123] The body of a moulded 3D heater may be provided with a heat reflective layer inside the body. The 3D heater may also comprise a protective layer or any frame required to support the structure.

[0124] A protective layer may be electrically insulating, thermally insulating or both. Such a layer increases the heating effectiveness of the heater. Such a layer may comprise a polyolefin such as a polyethylene, especially an LDPE, e.g. an LDPE homopolymer. Preferred insulation layers use polypropylene as the only polymer component. Polypropylene offers a higher melting point facilitating lamination or coextrusion to the semiconductive composition.

[0125] The heater may be provided with a support to provide mechanical strength.

[0126] In use current is applied to the heater via the conductors to generate heat. Typically voltages are 10 to 70 v, such as 12 to 40 v. The heater can be heated therefore using batteries or via the mains with suitable transformer. The application of the power to the heater leads to almost instant heat. There is no risk of electrocution as the voltage used does not need to be high. The heater can be prepared in any desired dimensions. The width of the heater can be adjusted readily to any possible use. The width may be a function of the coextrusion apparatus and sheets from 5 cm to 5 metres can be produced readily.

[0127] As previously discussed, the heating power of the heater can be controlled through the thickness of the semiconductive layer, separation of the conductors, conductive filler content and applied voltage. Moving the conductors closer together increases the wattage and hence the heat generated. The relationship can be expressed as power = [voltage]2 / resistance.

[0128] Thicker semiconductive layers tend to increase the power output.

[0129] Production

[0130] The heater can be produced by several techniques including:

[0131] Extrusion

[0132] Injection moulding

[0133] Compression moulding

[0134] Lamination

[0135] Vacuum forming.

[0136] The use of extrusion is most convenient for flat sheet heaters. The films may be effected using cast or blown film extrusion. In cast film extrusion, a slit die is employed positioned vertically so as to extrude a fine melt film onto a highly polished, high speed chill roll. The melt is pinned to the surface of the chill roll by either the pressure from an air knife or a vacuum box located close to the roll. This causes the fine film to be rapidly quenched, which improves its mechanical properties and clarity. The film then travels through a further series of chill, polishing and nip rolls, which help to draw the film down to the correct thickness, before its edges are trimmed and it is wound onto a drum for storage.

[0137] In blown film extrusion, the molten plastic from the extruder passes through an annular die and emerges as a thin tube. A supply of air to the inside of the tube prevents it from collapsing and may be used to inflate it to a larger diameter. Initially the bubble consists of molten plastic but a jet of air around the outside of the tube (cooling ring) promotes cooling and at a certain distance from the die exit, a freeze line can be identified. Eventually the cooled film passes through collapsing guides and nip rolls before being taken off to storage drums or, for example, gussetted and cut to length.

[0138] Some flat sheets might be prepared by moulding techniques however, such as that shown in figure 5. If the flat sheet is moulded then the semi conductive layer might be a lot thicker than a semi conductive layer produced by cast or blown film extrusion. Thicknesses of 0.5 to 5.0 cm are possible.

[0139] The heater of the invention can therefore be prepared continuously.

[0140] The heater of the invention is cheap. It is also thin and flexible.

[0141] This process can be readily adapted to include further layers above or below the semiconductive layer.

[0142] In one embodiment, crosslinking conditions can then be applied to cause a crosslinking reaction. It is preferred however if no crosslinking reaction is used.

[0143] Melt mixing means mixing above the melting point of at least the major polymer component(s) of the mixture and is typically carried out in a temperature of at least 10-15°C above the melting or softening point of polymer component(s).

[0144] The term coextrusion means herein that two or more layers are extruded in the same extrusion step. The term coextrusion means that all or part of the layer(s) are formed simultaneously using one or more extrusion heads.

[0145] In one embodiment, the semiconductive composition can be moulded, such as injection molded, e.g. to form a cylinder or any other shape governed by the mold. In particular the use of back injection molding is possible. Injection moulding can therefore be used to prepare an article having a body and top and bottom surfaces that are preferably evenly spaced apart.

[0146] The semiconductive composition can therefore be injection molded to form the sides and the ends of a cylinder to which conductors can then be applied.

[0147] Applications

[0148] The heater of the invention can be utilised in many fields. Applications of the technology described herein are therefore widespread.

[0149] We often provide thermal comfort in winter by heating the entire volume of air in a room or building. In earlier times, our ancestor’s concept of heating was more localized: heating people, not places. They used radiant heat sources that warmed only certain parts of a room, creating micro-climates of comfort. These people countered the large temperature differences with insulating furniture, such as hooded chairs and folding screens, and they made use of additional, personal heating sources that warmed specific body parts. It would make a lot of sense to restore this old way of warming, especially since modem technology has made it so much more practical, safe and efficient.

[0150] The heater of the invention may therefore be employed within an item of furniture such as a screen, chair or sofa.

[0151] In one embodiment the heater of the invention might be used in a heated garment. Heated garments available today have small wires (often made of brittle carbon fibres) built into them. They heat up when a low voltage electric current is passed through. There are two main types of heated clothing, battery powered or powered by a vehicle (e.g. heated gloves on a motorcycle). The heaters of the invention are ideally suited for use in both these applications.

[0152] The heater may also be used in a blanket. A major concern with electric heating blankets on the market today is fire risk. These blankets tend to overheat. Using the heater of the present invention that risk is eliminated. In general therefore the invention provides a textile comprising the heater of the invention.

[0153] Radiators are large, immobile and often unattractive. In many parts of the world, radiators are hidden behind more aesthetically pleasing covers of various designs. These covers may also reduce noise or protect against the touching of radiators that get excessively hot. But hiding the radiator is not efficient because adding a radiator cover slows the movement of heat out of the radiator and into the room. The rate of heat loss out through the building’s exterior wall is likely to be increased.

[0154] The heaters of the invention can replace radiators or be used in walls, under floors, in ceilings as heaters. The heaters could even be included within a carpet or rug or other floor covering.

[0155] Electric cars generate next to no heat as opposed to conventional passenger vehicles, which produce more than enough engine heat to heat the interior. An additional electric heater is therefore required in an electric vehicle to heat the interior. This heater is supplied with power by the same battery that provides the engine with energy. This can reduce the maximum possible drive distance by a considerable amount.

[0156] Thus, there is a need for heating e-vehicles as efficiently as possible. The present invention might be used to heat inner contact surfaces such as steering wheel, armrest, door panels, seats within the vehicle. More efficient heating can be envisaged compared to heating the entire inner volume of the car, especially for short journeys.

[0157] The heater of the invention could be used to prevent ice or snow build up on a critical surface such as a solar panel. Heaters might therefore have utility in deicing operations. Other surfaces might be wing mirrors.

[0158] 3D heaters of the invention might simply be an attractive room heater.

[0159] The heaters are flexible and might be wrapped around pipes to prevent liquid freezing therein. Heaters can furthermore be used to keep fluids heated e.g. in swimming pools or liquid containers. The skilled person can device many applications of these versatile heaters.

[0160] The invention will now be described with reference to the following nonlimiting examples and figures.

[0161] Brief Description of the Figures

[0162] Figure 1 is a schematic of a flat sheet heater with parallel conductors arranged on and in contact with the top bottom surface of the single semi conductive layer of semiconductive composition.

[0163] Figure 2 is a schematic of a flat sheet heater with parallel conductors arranged on and in contact with the top and bottom surface of the single semiconductive layer in an alternating pattern.

[0164] Figure 3 is a schematic of a flat sheet heater in which the conductors are extended to connect directly to the power source.

[0165] Figure 4 shows a moulded heater in the form of a cylinder having a body with top and bottom surfaces, where the body and top and bottom surfaces are formed from a semiconductive composition. The top and bottom surfaces are provided with a foil conductor. Figure 5 shows a moulded heater in the form of a cuboid having a body with top and bottom surfaces, where the body and top and bottom surfaces are formed from a semiconductive composition. The top and bottom surfaces are provided with evenly spaced conductors.

[0166] Melt Flow Rate

[0167] The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g / 10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 230°C for polypropylene and may be determined at different loadings such as 2.16 kg (MFR2) or 21.6 kg (MFR21). The MFR is determined at 190°C for the HDPE and may be determined at different loadings such as 2.16 kg (MFR2) or 21.6 kg (MFR21).

[0168] The MFR is determined at 125°C for polyethylene present within the semiconductive composition. It may be determined at loading of 21.6 kg (MFR21).

[0169] Examples

[0170] The following materials are used herein:

[0171] Semiconductive composition EVAI - Ethylene vinyl acetate copolymer with 17 wt% comonomer and a density of 920 kg / m3and combined with carbon black (38 wt%) and having an MFR21 of 7.0 g / lOmin measured at 125°C.

[0172] Example 1 - monolayer film

[0173] A monolayer layer film was prepared using a SML cast film co-extrusion line. The 3-layer line is equipped with an 1200 mm Cloeren and a flexible die gap of 0,5 - 1,0 mm. The film thickness is adjusted automatically by using a radioactive thickness measurement device in combination with automatic controlled heating bolts at the die. The chill roll temperature was set to 15 °C. The film was wounded on 3” cores. EVAI was extruded in layers 1 to 3 to form a monolayer. The overall film thickness was 125 pm.

[0174] Elongate conductor wires are applied to the monolayer film alternately to the top and bottom surfaces of the film as shown in figure 2. The conductors are parallel and separated from each other longitudinally.

Claims

Claims1. A flat sheet electrical heater comprising a single semi conductive layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; wherein said semiconductive layer has a top surface and a bottom surface; a plurality of conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer.

2. A flat sheet electrical heater as claimed in claim 1 wherein the plurality of conductors are arranged (i) in contact with the top surface of the single semiconductive layer, (ii) in contact with the bottom surface of the single semiconductive layer, or (iii) alternately in contact with the top surface and the bottom surface of the single semiconductive layer.

3. A flat sheet electrical heater as claimed in any preceding claim wherein the semiconductive layer and any conductors in contact therewith are coated with a protective layer.

4. A flat sheet electrical heater as claimed in claim 3 wherein the protective layer is transparent.

5. A flat sheet electrical heater as claimed in any preceding claim wherein the conductors are substantially parallel.

6. A flat sheet electrical heater as claimed in any preceding claim wherein the conductors are evenly spaced apart.

7. A flat sheet electrical heater as claimed in any preceding claim wherein the conductors are wires.

8. A flat sheet electrical heater as claimed in any preceding claim wherein at least one of the conductors is configured to act as a connecting wire to a power source.

9. A moulded electric heater, such as an injection mou Ided electric heater, having a body with a top surface and a bottom surface, preferably evenly spaced apart, said body comprising an electrically semi conductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; a plurality of conductors, preferably evenly spaced apart, arranged on and in contact with the top and / or bottom surface of the body.

10. A moulded electric heater as claimed in claim 9 having a body with a top surface and a bottom surface, said body comprising an electrically semi conductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; wherein a first conductor foil is arranged on and in contact with the top surface of the body and covers a majority of the top surface of said body and a second conductor foil is arranged on and in contact the bottom surface of the body and covers a majority of the bottom surface of said body and wherein the distance between the first conductor foil and the second conductor foil is substantially uniform11. A flat sheet electrical heater according to any one of claims 1-8 or a moulded electrical heater according to claim 9, wherein the semiconductive composition comprises a LDPE homopolymer or LDPE copolymer or a polypropylene copolymer or a mixture thereof.

12. A flat sheet electrical heater or a moulded electrical heater according to claim11, wherein the semiconductive composition comprises an ethylene alkyl acrylate or ethylene vinyl acetate polymer.

13. A flat sheet electrical heater according to any one of claims 1-8, 11 to 12 or a moulded electrical heater according to any of claims 9-10, wherein the conductive filler comprises or consists of carbon black.

14. A flat sheet electrical heater according to claim 12 or a moulded electrical heater according to claim 12, wherein the conductive filler is free of boron nitride and / or metallic conductive fillers preferably wherein carbon black is the only conductive filler present.

15. A flat sheet electrical heater according to any one of claims 1-8 or 11-14 or a moulded electrical heater according to any of claims 9-14, wherein the electrically semiconductive composition comprises 15-50 wt% conductive filler based on the total weight of the semiconductive composition.

16. A flat sheet electrical heater as claimed in claim 1-8 or 10-15 wherein the semiconductive layer has a thickness in the range of 50 to 900 pm, such as from 125 to 800 pm.

17. A flat sheet electrical heater comprising a single non-crosslinked semiconductive layer consisting of an electrically semiconductive composition with a positive temperature coefficient comprising at least a polyethylene and conductive filler consisting of carbon black (i.e. no other conductive filler is present); wherein said semiconductive layer has a top surface and a bottom surface; a plurality of wire conductors, preferably evenly spaced apart from and preferably substantially parallel to each other, arranged on and in contact with the top and / or bottom surface of the single semiconductive layer.

18. A process for the preparation of a moulded electrical heater comprising the steps of (a)- providing an electrically semiconductive composition comprising a polyethylene, a polypropylene or a mixture thereof, and a conductive filler,- melt mixing said electrically semiconductive composition;- moulding, e.g. injection moulding, an object having a body with a top surface and a bottom surface, said body comprising said electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene, polypropylene or a mixture thereof and a conductive filler; (b) applying a first conductor foil onto the top surface of said body and a second conductor foil onto the bottom surface of said body wherein the first conductor foil covers a majority of the top surface and the second conductor foil covers a majority of the bottom surface, and wherein the first conductor foil and the second conductor foil are applied such that the distance therebetween is substantially uniform.

19. An article comprising a flat sheet electrical heater as claimed in claim 1 to 8 or 10-15, e.g. heated clothing or a vehicle, such as interior heating.

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