HEATING ELEMENT AND ELECTRICALLY CONDUCTIVE RADIATOR

Description

[0001] The invention relates to a heating medium in the form of an electrically conductive coating and an electrically conductive heating element produced therewith. The invention also relates to a use of the heating medium.

[0002] In radiators with a functional coating, this coating contains electrically conductive components that convert electrical energy into heat due to ohmic resistance when current flows.

[0003] WO 2012 / 143221 discloses a composition which contains inorganic binder, carbon black and carbon fibers of 50 - 8000 µm length and optionally graphite.

[0004] US 2009 / 0328411 describes a manufacturing process for a heating element consisting of carbon microfibers 5-20 mm in length in a paper pulp, carbon powder and a conductive polymer, as well as a binder.

[0005] US patent 4,534,886 discloses a non-woven fabric for a heating element containing conductive carbon fibers of 3.18 - 50.8 mm length and electrically conductive carbon black particles.

[0006] EP 2023688 B1 claims a surface heating system made of a thermoplastic matrix and carbon fibers of 0.1 - 30 mm length and 1 - 15 µm thickness.

[0007] With almost all known heating elements or heating compositions based on a heating coating, satisfactory heating performance cannot be achieved with relatively small coating thicknesses (surface area) and the same applied voltage. The few examples of particularly high-performance heating coatings currently available are almost exclusively based on electrically conductive nanoparticles such as carbon nanotubes (CNTs). The health risks of such nano-based materials, like carbon nanotubes, for biological organisms have not yet been definitively clarified due to a lack of necessary long-term data.

[0008] WO 95 / 20005 A1 concerns a polyimide composition with improved properties. The idea is to use 8–69 wt.% of at least one polyimide, 30–80 wt.% of natural graphite powder, and 1–12 wt.% of carbon or graphite fibers as fillers. The natural graphite powder is intended to ensure reduced wear and friction, while the carbon fibers are meant to significantly reduce thermal expansion. For example, a carbon fiber with an average length of 130 µm and a diameter of 7 µm can be used.

[0009] WO 03 / 103 955 A1 concerns a sliding bearing composite material in which graphite particles can be used in weight percentages of 5 - 15.

[0010] DE 10 2011 007 834 A1 discloses a mineral composition for the production of electrical heating layers. An inorganic binder is to be used. Fine-grained graphite particles are mentioned in an amount of 0.5–5 wt.%.

[0011] WO 2008 / 105121 A1 concerns a semiconducting resin composition with two conductive fillers that can be used in various cast products.

[0012] Database WPI, Week 200914, Thomson Scientific, London, GB, AN 2009-B30953 and KR 2008 0030410 A, 4 April 2008, disclose a conductive ink composition for a flat-shaped radiator, which contains 2-12 wt.% carbon black, 2-12 wt.% graphite, 1-10 wt.% carbon fibers with a length of 0.1-0.5 mm, 20-27% of a polymer resin, 1-3 wt.% of a dispersant, 1-60 wt.% of an organic solvent, 1-5 wt.% polyethylene wax, 10-20% slow-drying solvent and 3-10 wt.% of a rubber-like adhesive.

[0013] DE 2 326 936 A relates to a conductive coating agent in the form of a film-forming, rheologically advantageous liquid mixture and its use for electrical resistance elements, for example, for converting electrical energy into heat energy. The aim is to find a conductive, film-forming, rheologically advantageous, liquid coating agent with which durable electrical resistance elements with constant electrical resistance values ​​under continuous load can be produced. Therefore, conductive fiber materials should also be present, so that the coating agent, as a hardened layer, should have a specific resistance value between 10⁻³ and 10⁺¹⁰ Ω·cm.

[0014] KR 2000 0053890 A discloses a heating device based on electrical resistance. A mixture of a far-infrared light pigment and graphite is applied to fiberglass.

[0015] The present invention is therefore based on the objective of providing a heating medium in the form of an electrically conductive coating, in which a significantly higher heating power is achieved through a targeted selection of the conductive components and a targeted combination of their dimensions, and thereby the surface area of ​​such a functional coating can be reduced.

[0016] Another objective of this invention is to avoid the potential risk of health hazards from nanomaterials in the manufacture of such a heating system by excluding nanoparticles from the outset.

[0017] Another object of this invention is to ensure a correspondingly more efficient resistance heating system with the same surface area and the same other preconditions, and thus to produce a textile-improved radiator in the form of a heating fabric, knitted fabric or non-woven fabric.

[0018] These tasks are solved by a heating medium according to claim 1, an electrically conductive heating element according to claim 7, and a use according to claim 9. Advantageous further developments are set out in the dependent claims.

[0019] According to the invention, the heating medium, in the form of an electrically conductive coating for such an electrically conductive heating element, comprises an elastomer as a base material and embedded therein a homogeneous mixture of a carbon fiber with a mean fiber length of 70–180 µm, measured by light microscopy, wherein mean fiber length means that approximately 50% of the fiber has this length, and a mean fiber diameter of 4–15 µm, and dispersed platelet-shaped graphite particles with a mean particle diameter d50 of 1–10 µm, measured by laser diffraction according to ISO 13320 using a laser granulometer, wherein the heating medium comprises: 5 - 30 wt.% elastomer, preferably 5 - 20 wt.%, 8 - 25 wt.% carbon fibers, preferably 10 - 20 wt.%, 50 - 85 wt.% graphite particles, preferably 60 - 85 wt.%, each based on the total weight of the dried heating medium.

[0020] The base material is an elastomer.

[0021] Such elastomers can be selected from polymers such as polyacrylates, polyurethanes, various synthetic and natural rubbers, and various mixtures of these polymers, provided they exhibit heat resistance of at least 100°C for the intended application. The required heat resistance of such a base material is, of course, determined by those skilled in the art based on the desired heating output, the temperature to be achieved, and also the environment in which the heating medium according to the invention is to be introduced.

[0022] The term "embedded in it" means that the homogeneous mixture is mixed with the base material and the carbon fibers and graphite particles are distributed within the base material.

[0023] The heating medium according to the invention can be applied to any body of the base material to form a radiator. This can, for example, be a textile surface material made of, for example, polypropylene, polyethylene, glass fibers or mixtures of these materials.

[0024] Preferably, for the present invention, fiberglass fabrics, woven fabrics, or knitted fabrics are used as the base material for a radiator, in particular fiberglass mesh fabrics with mesh openings in the range of 1 × 1 to 20 × 20 mm, and especially 3 × 3 to 10 × 10 mm. Such fiber mesh fabrics made of fiberglass yarns or fiberglass rovings can advantageously also be arranged on a fiberglass fleece and thereby optionally form a composite.

[0025] The glass from which the textile fiber of such woven, knitted and non-woven fabrics is produced is preferably E-glass, ECR-glass, C-glass or similar glass.

[0026] A particularly preferred base material is a fiberglass fabric, especially a fiberglass mesh fabric with mesh openings of 4 × 4 mm, 6 × 6 mm or 10 × 10 mm (e.g. from Vitrulan Technical Textiles GmbH, Oberland am Rennsteig, Germany).

[0027] The proportion of the elastomeric base material in the heating medium is in the range of 5 to 30 wt.%, preferably 5 to 20 wt.%.

[0028] Preferred elastomers are styrene-butadiene rubbers (SBR), isoprene rubbers, polyacrylates, or rubbers containing S, O, or Cl groups, or styrene. A styrene-butadiene copolymer or styrene-butadiene rubber, which is commercially available, for example, as an aqueous dispersion (or latex), is particularly preferred.

[0029] The proportion of carbon fibers in the heating medium according to the invention is in the range of 8 - 25 wt.%, preferably 10 - 20 wt.%, in particular 10 to 15 wt.%.

[0030] The average filament length of the carbon fibers used is advantageously in the range of 70–160 µm, particularly 90–130 µm, and especially 90–110 µm. If the product is not commercially available, the desired average fiber length can also be achieved by milling a coarser carbon fiber material with longer fiber lengths, for example, using a vibratory mill and suitable grinding media. This is standard practice and readily achievable by a person skilled in the art.

[0031] The average fiber diameter is advantageously 7 - 13 µm, and such fiber thicknesses are available on the market.

[0032] A particularly preferred carbon fiber of the invention presented herein has a mean fiber length of 90 - 130 µm and a mean fiber thickness of 13 µm, more specifically of 6 - 8 µm, in particular 7 µm.

[0033] The mean fiber lengths are measured using light microscopy. Similar to the diameter of the graphite particles, "mean" fiber length means that at least approximately 50% of the fiber has this length.

[0034] The proportion of graphite particles used in the heating medium according to the invention is in the range of 50 - 85 wt.%, based on the total weight of the hardened heating medium.

[0035] The graphite particles in the heating medium according to the invention have a crystalline, scaly, or platelet-shaped structure and are not fine-grained and dense like graphitoids. The diameter d90 of the particles is < 10 µm, and the mean diameter d50 of these platelets is 1 to 10 µm, in particular 3 to 10 µm. "d50" means that at least about 50% of the particles lie within this diameter range, and "d90" means that at least about 90% of the particles lie within this range. The measurement is carried out by laser diffraction according to ISO 13320 using a laser granulometer, e.g., CILAS 1090LD.

[0036] The term "dispersed" in connection with the graphite particles means that they are present in an aqueous dispersion, possibly with common dispersing agents such as carboxymethylcellulose, isopropanol, and / or thickeners. The proportion of such agents in the dispersion is less than 3% by weight.

[0037] Also in contrast to many prior art heating media, no carbon black is used in the heating layer according to the invention. In principle, carbon black would be well suited for a conductive network in such a functional coating due to its high internal surface area. However, this material—due to its amorphous components—does not provide good conductivity in the heating coating and is therefore unsuitable for solving the problem described above. Furthermore, carbon black also contains nanoparticles; however, unlike the health risks of CNTs, the harmful effects of carbon black are well known.

[0038] As already mentioned, the concentration values ​​of the elastomer, carbon fiber, and graphite particles in all cases refer to the total weight of the dried heating medium. In a preferred embodiment of the invention, the heating medium comprises 5 to 10 wt.% elastomer, in particular SBR, 10 to 15 wt.% carbon fibers, and 80 to 85 wt.% graphite particles.

[0039] In a further preferred embodiment of the invention, the heating medium comprises 10 to 30 wt.% elastomer, in particular SBR, 10 to 20 wt.% carbon fibers and 60 to 70 wt.% graphite particles.

[0040] The efficiency of the heating medium according to the invention results from the fact that, under the same physical, electrical and coating-related boundary conditions (substrate, electrode spacing, applied voltage, coating quantity per m²), surprisingly high heating powers of 1500 to over 5000 W / m² can be achieved, whereby even values ​​of 2000 - 5000 W / m² or 3000 - 5000 W / m² can be achieved with voltages of 24, 48 or 60 V.

[0041] These performance levels are up to 100% or more higher than those of comparable state-of-the-art products.

[0042] Without being bound to a specific theory, it seems that for the most efficient contacting of the conductive particles and thus for the electrical conduction in the heating coating, there is an optimal interplay of certain carbon fiber lengths, carbon fiber thicknesses and graphite particle diameters, with which a surprisingly high conductivity and performance of the heating medium can be achieved.

[0043] The heating medium according to the invention can be subjected to known voltages, i.e., those in the low-voltage range of, for example, 12 V, 24 V, 48 V or 60 V, or also in the higher voltage range of 110 to 240 V. Very good results at 240 V can already be achieved with graphite particle concentrations of 50–70 wt.%, 15–25 wt.% carbon fiber and 15–30 wt.% elastomers.

[0044] The invention also relates to an electrically conductive heating element comprising a base material and a heating medium in the form of an electrically conductive coating arranged thereon or therein, wherein the heating medium comprises an elastomer as a base material and a homogeneous mixture of a carbon fiber with a mean fiber length of 70–180 µm, measured by light microscopy, where mean fiber length means that approximately 50% of the fiber has this length, and a mean fiber diameter of 4 to 15 µm, and dispersed platelet-shaped graphite particles with a mean particle diameter d50 of 1 to 10 µm, measured by laser diffraction according to ISO 13320 using a laser granulometer, wherein the heating medium comprises: 5–30 wt.% elastomer, preferably 5–20 wt.%, 8–25 wt.% carbon fibers, preferably 10–20 wt.%, 50–85 wt.% graphite particles, preferably 60–85 wt.%, each based on the total weight of the dried heating medium. The heating element also has power supply lines.

[0045] The heating medium according to the invention can be applied to the base material of the radiator using known methods, such as rolling, fouling, dipping, doctoring, spraying, as well as screen or roller printing. Such methods commonly used in coating technology are also preferred for the base material favored in this invention – woven, non-woven, and knitted fabrics made of glass fibers, e.g., glass fiber mesh fabrics. The coating process can be repeated so that a double or triple layer can be produced, thereby achieving an even higher heating output.

[0046] The radiator can have any shape, e.g., flat or with curved surfaces. The base material of the radiator can be rigid or flexible. A flexible radiator is particularly preferred, especially a flexible panel made of a structured fabric, such as woven, non-woven, or knitted fabrics made of glass, plastic, or a glass / plastic fiber blend. Glass fiber mesh fabrics are especially preferred.

[0047] In a preferred embodiment, a glass fiber fabric carries a flexible heating element, in which the base material is an elastomer, e.g. a styrene-butadiene rubber, in which the carbon fibers and the graphite particles are present in a homogeneous distribution.

[0048] The base material itself may already have another coating before the heating medium according to the invention is applied, e.g. to ensure certain structural properties of the base material.

[0049] The base material can be not only glass fabric but also a polymeric material, such as polyethylene, polypropylene, polyacrylate, rubbers, and their respective copolymers, or glass composites. The base material, i.e., the elastomer, comprises 5–30% by weight of the heating medium. The proportion of carbon fibers ranges from 8–25% by weight, particularly 10–20% by weight, and the proportion of graphite particles ranges from 50–85% by weight, particularly 60–85% by weight. These concentration values ​​are based on the weight of the dried heating medium.

[0050] The heating element according to the invention can be contacted in a conventional and known manner by means of current supply electrodes. Due to the very high heating power of the heating medium according to the invention, either the amount applied per unit area (surface coverage) can be reduced, or, in the case of higher application amounts, the electrode spacing for the current supply can be significantly increased. Such parameters are defined by the respective desired application or according to the desired power of such a heating element.

[0051] A method for producing a planar heating element from a fiberglass fabric is also conceivable, characterized in that a dispersion of platelet-shaped graphite particles with a mean diameter d 50 of the platelets of 1 to 10 µm is mixed with carbon fibers of a fiber length of 70 - 180 µm and a fiber diameter of 4 - 15 µm and with a dispersion of an elastomer and homogenized at 1000 to 1500 rpm, the homogeneous mixture thus obtained is applied as a heating medium to a fiberglass fabric, is then cured, and with an application quantity of 60 - 80 g / m 2< the distances of the current supply electrodes are in the range of 10 - 50 cm in order to achieve heating powers of 1500 to 5000 W / m 2< at applied voltages of 24V, 48V or 60V.

[0052] The invention will be explained in more detail below by means of examples. All percentages are percentages by weight. Example 1

[0053] A dispersion of platelet-shaped graphite particles is mixed with a quantity of carbon fibers and an SBR dispersion using a paddle stirrer and homogenized at 1200 rpm. The quantities used are selected so that, after subsequent drying of the mixture, a concentration of 60% graphite (1-10 µm) 10% carbon fiber (7 µm thickness, 100 µm length) 30% SBR (styrene-butadiene rubber) results.

[0054] The resulting heating agent mixture is applied to a textile base material (open-mesh grid fabric) made of glass fibers (warp length 20.5 cm; weft length 14.5 cm) by rollers and dried at 150 °C. Before coating with the heating agent, the textile substrate is fitted with two current supply electrodes spaced 10.5 cm apart on both long sides.

[0055] The surface area coverage is 76.13 g / m².

[0056] With an applied voltage of 24 V, the resulting heating element achieves a heating output of 1671 W / m². This corresponds to a surface- and contact-related heating output Psurface-contact of 738 W / (gm²). Example 2

[0057] The process is the same as in Example 1. The concentrations in the cured coating are... 60% graphite (1 - 10 µm) 20% carbon fiber (7 µm thickness, 100 µm length) 20% SBR Surface coverage: 85.11 g / m² Heating power at 24 V: 2140 W / m² (surface coverage = 845 W / (gm²)). Example 3

[0058] The procedure is the same as in Example 1. The concentrations are... 70% graphite (1-10 µm) 10% carbon fiber (7 µm thickness, 160 µm length) 20% SBR Surface covering 66.44 g / m² Heating power at 24 V: 1792 W / m² (907 W / (gm²)) Example 4

[0059] The procedure is the same as in Example 1. The concentrations are... 80% graphite (1-10 µm) 10% carbon fiber (7 µm thickness, 180 µm length) 10% SBR Surface covering 69.27 g / m² Heating power at 24 V: 2196 W / m² (1066 W / (gm²)) Example 5

[0060] The procedure is the same as in Example 1. The concentrations are... 80% graphite (1-10 µm) 10% carbon fiber (7 µm thickness, 70 µm length) 10% SBR Surface covering 59.88 g / m² Heating power at 24 V: 1760 W / m² (989 W / (gm²)) Example 6

[0061] The procedure is the same as in Example 1. The concentrations are... 75% graphite (1-10 µm), 5% carbon fiber (7 µm thickness, 100 µm length), 20% SBR. Surface covering: 66.17 g / m². Heating power at 24 V: 1534 W / m² (780 W / gm²). Example 7 Comparative example 1

[0062] 10% carbon fiber (13 µm thickness, 130 µm length) 90% SBR Surface covering 43.40 g / m²< Heating power at 24 V: 0 W / cm²< Example 8 Comparative example 2

[0063] State-of-the-art polyurethane heating dispersion with carbon nanomaterials and graphite as conductive components (Carbo e-Therm PUR-160 1 W)

[0064] Surface area 30.05 g / m², distance between power supply electrodes 10.5 cm, heating power at 24 V: 1558 W / m² (758 W / (gm²) Example 9 Comparative example 3

[0065] State-of-the-art heating dispersion 61% graphite (1-10 µm), 9% carbon nanotubes, 10% SBR, surface area 35.90 g / m², distance between power supply electrodes 10.5 cm, heating power at 24 V: 525 W / m² (492 W / (gm²))

[0066] The comparison of the inventive examples 1 - 6 with heating outputs P surface area of ​​845 - 1066 W / (gm 2< ) compared to those with 492 and 758 W / (gm 2< ) shows the significant superiority of the heating medium and thus also of the radiators of the invention.

Claims

1. Heating composition in the form of an electrically conductive coating for an electrically conductive heating body, comprising an elastomer as a base substance and embedded therein a homogeneous mixture of a carbon fibre having an average fibre length of 70-180 µm measured by optical microscopy, wherein average fibre length is to be understood as meaning that about 50% of the fibres have this length, and an average fibre diameter or 4 to 15 µm and dispersed floccular graphite particles having an average diameter d50 of 1 to 10 µm measured by laser diffraction according to ISO 13320 with a laser granulometer, wherein the heating composition comprises: 5-30% by weight of elastomer, preferably 5-20% by weight, 8-25% by weight of carbon fibres, preferably 10-20% by weight, 50-85% by weight of graphite particles, preferably 60-85% by weight, in each case based on the total weight of the dried heating composition.

2. Heating composition according to Claim 1, wherein the average length of the carbon fibres is in the range from 70 to 160 µm, in particular 90-130 µm, especially 90-110 µm.

3. Heating composition according to Claim 1, wherein the average thickness of the diameters of the carbon fibres is in the range of 7-13 µm.

4. Heating composition according to any of Claims 1 to 3, wherein said composition contains carbon fibres having average lengths of 90-130 µm and average fibre diameters of not more than 13 µm, in particular 6 to 8 µm.

5. Heating composition according to any of Claims 1-4, comprising 5-10% by weight of elastomer, 10-15% by weight of carbon fibres, 80-85% by weight of graphite particles, in each case based on the total weight of the dried heating composition.

6. Heating composition according to any of Claims 1 to 4, comprising 10-30% by weight of elastomer, 10-20% by weight of carbon fibres, 60-70% by weight of graphite particles, in each case based on the total weight of the dried heating composition.

7. Electrically conducting heating body, characterized by a main material and arranged thereupon or therein a heating composition in the form of an electrically conductive coating, wherein the heating composition comprises an elastomer as a base substance and a homogeneous mixture of a carbon fibre having an average fibre length of 70-180 µm measured by optical microscopy, wherein average fibre length is to be understood as meaning that about 50% of the fibres have this length, and an average fibre diameter of 4-15 µm and dispersed floccular graphite particles having an average particle diameter d50 of 1-10 µm measured by laser diffraction according to ISO 13320 with a laser granulometer, wherein the heating composition comprises: 5-30% by weight of elastomer, preferably 5-20% by weight, 8-25% by weight of carbon fibres, preferably 10-20% by weight, 50-85% by weight of graphite particles, preferably 60-85% by weight, in each case based on the total weight of the dried heating composition.

8. Heating body according to Claim 7, wherein the main material is a glass fibre fabric.

9. Use of a heating composition according to any of Claims 1-6 for producing a flat heating body.