Planar heating element

Abstract

The purpose of the present invention is to provide a planar heating element capable of reducing any irregulaties in surface temperature in a variety of electrode forms and electrode positions, and capable of being manufactured using a simple method. A planar heating element 1 of the present invention has a sheet-form heating layer 10, a sheet-form base 2 for supporting the heating layer 10, and at least a pair of electrodes E1, E2. Holes 11 are provided at random or in the form of a grid in the heating layer 10, and the weight per unit area of the heating layer 10 is gradually changed in the direction the electrodes E1 and E2 face each other and the direction of positioning of the electrodes E1 and E2 by adjusting the open area ratio or adopting other approaches. The heating layer 10 can be formed, for example, by printing a conductive paint containing a conductive filler.

Description

Planar heating element

[0001] The present invention relates to a sheet heating element having a sheet-like heating layer.

[0002] Unlike conventional heaters that use electric heating wires, planar heating elements with a sheet-like heating layer can generate heat over the entire surface, are less susceptible to problems such as loss of heat due to wire breakage, and are flexible enough to be installed in small spaces. Therefore, they are used in a variety of applications, such as defrosters for buildings, home appliances, and cars, seat heaters, snow-melting heaters, and wearable heaters (Patent Document 1).

[0003] Sheet heating elements are processed into various shapes depending on the actual usage, and in this case, differences in the shape, arrangement, and distance between the electrodes used to pass electricity through the heating layer of the sheet heating element cause unevenness in the heating temperature. In response to this, for example, in a transparent sheet heating element for a defroster in a full-face helmet, where one of a pair of opposing electrodes is straight and the other is arc-shaped, it has been proposed to adjust the thickness of the heating layer according to the change in the distance between the electrodes (Patent Document 2).

[0004] Patent No. 6940711 Publication JP-A-3-62492

[0005] However, simply adjusting the thickness of the sheet-like heating layer according to the distance between the opposing electrodes makes it difficult to sufficiently uniformize the surface temperature of the heating layer in sheet heating elements with various electrode shapes and electrode arrangements. For example, as shown in Figures 8A and 8B, in a sheet heating element 1x, an eccentric, annular heating layer 10x is laminated on a substrate 2, and electrodes E1 and E2 are located on the inner and outer edges of the heating layer 10x, and the distance between these electrodes varies. If the heating layer 10x is a solid layer of conductive paint and its thickness is uniform, when current is applied to electrodes E1 and E2 to generate heat in the sheet heating element 1x, localized high-temperature and low-temperature regions will be generated, as shown in Figure 9. It is difficult to uniformize such uneven surface temperature using conventional methods.

[0006] In response to this, an object of the present invention is to provide a sheet heating element that reduces unevenness in surface temperature even with various electrode shapes and electrode arrangements and that can be manufactured using a simple method.

[0007] The inventors have conceived the following: in a planar heating element having a sheet-like heating layer, if an area is provided in which the weight per unit area of ​​the heating layer gradually changes not only in the direction in which the electrodes face but also in the direction in which the electrodes are arranged, the surface temperature of the heating layer can be adjusted and unevenness in the surface temperature can be reduced regardless of the electrode shape or electrode arrangement, or the shape of the heating layer; if the heating layer is formed by applying a conductive paint in which conductive filler is dispersed in a binder, the heating layer can be manufactured at low cost using a printing method; and in this case, if the printing pattern is a pattern with a large number of dispersed holes, the weight per unit area of ​​the heating layer can be precisely adjusted by the opening area ratio of the holes, etc., and have completed the present invention.

[0008] That is, the present invention provides a sheet-shaped heating element having a sheet-shaped heating layer, a sheet-shaped substrate supporting the heating layer, and at least one pair of electrodes, wherein the weight per unit area of ​​the heating layer has an area in which it gradually changes in the direction in which the electrodes are arranged and in the direction in which the electrodes face each other.

[0009] The present invention also provides a method for manufacturing the above-mentioned sheet-shaped heating element, which is a method for manufacturing a sheet-shaped heating element in which a sheet-shaped heating layer is formed on a sheet-shaped substrate and at least a pair of electrodes is provided, and in which the heating layer is formed so that the weight per unit area of ​​the heating layer gradually changes in the direction of the electrode arrangement and the direction in which the electrodes face each other in a predetermined region of the heating layer.

[0010] According to the sheet-shaped heating element of the present invention, the sheet-shaped heating element has a sheet-shaped heating layer on a sheet-shaped substrate and has at least one pair of electrodes, and has an area in which the weight per unit area of ​​the heating layer gradually changes in the direction in which the electrodes are arranged and in the direction in which the electrodes face each other.Therefore, regardless of the shape and arrangement of the electrodes on the sheet-shaped heating element or the shape of the heating layer, it is possible to adjust the surface temperature of the heating layer and reduce unevenness in the surface temperature.

[0011] Furthermore, with regard to the method for manufacturing the heat generating layer of the planar heating element of the present invention, although the conductive film can be formed by vacuum deposition as described in Patent Document 2, the heat generating layer can also be formed by applying a conductive paint in which conductive filler is dispersed in a binder using a printing method such as silk screen printing or gravure printing, thereby making it possible to manufacture the heat generating layer at low cost.

[0012] FIG. 1 is a plan view of the heat generating layer of a sheet heating element of an example. FIG. 2 is a partially enlarged view of the heat generating layer shown in FIG. 1. FIG. 3 is a cross-sectional view of a sheet heating element having recesses as holes in the heat generating layer. FIG. 4 is a cross-sectional view of a sheet heating element having through-holes as holes in the heat generating layer. FIG. 5 is a cross-sectional view of a sheet heating element in which a solid base-side heat generating layer and a surface-side heat generating layer having through-holes are laminated. FIG. 6 is a cross-sectional view of a sheet heating element in which multiple heat generating layers having through-holes are laminated. FIG. 7 is a thermograph of a sheet heating element of an example when current is applied. FIG. 8A is a plan view of a sheet heating element 1x of a comparative example. FIG. 8B is a cross-sectional view of a sheet heating element 1x of a comparative example. FIG. 9 is a thermograph of a sheet heating element 1x of a comparative example.

[0013] The present invention will now be described in detail with reference to the drawings, in which the same reference numerals represent the same or equivalent components.

[0014] (Basic Structure of Sheet Heating Element) The sheet heating element of the present invention has a sheet-like heating layer, a sheet-like substrate supporting the heating layer, and at least one pair of electrodes. There are no particular limitations on the shape, arrangement direction, or arrangement position of the electrodes, or the shape of the heating layer. The sheet-like heating layer may be a conductive film formed by vacuum deposition, as described in Patent Document 2, or may be formed from a coating of a conductive paint in which a conductive filler such as carbon nanotubes is dispersed in a binder, as described in Patent Document 1.

[0015] For example, Fig. 1 is a plan view of the heat generating layer 10 of a sheet heating element 1 according to one embodiment of the present invention. This heat generating layer 10 has a circular shape in plan view, similar to the heat generating layer 10x of the sheet heating element 1x shown in Figs. 8A and 8B. Similarly to the sheet heating element 1x shown in Figs. 8A and 8B, a pair of electrodes E1, E2 are provided on the inner and outer edges of the heat generating layer 10, and the distance between the electrodes varies. Here, the inter-electrode distance refers to the shortest distance from any point P on the electrode to the opposing electrode.

[0016] If necessary, an insulating protective layer may be provided on the heat generating layer 10. The constituent materials of the substrate 2, heat generating layer 10, electrodes E1, E2, and protective layer may be the same as those of known planar heating elements described in Patent Documents 1 and 2, etc.

[0017] (Heat-generating layer) The heat-generating layer 10 of the sheet heating element of the present invention is characterized by having an area in which the weight per unit area of ​​the heat-generating layer gradually changes in the direction of arrangement of the electrode E1 or electrode E2 and in the direction in which the electrodes E1 and E2 face each other. The gradual change includes a gradual increase, a gradual decrease, or a mixture of both. The gradual change is preferably in the form of a gradation.

[0018] The gradual change in the weight per unit area in a certain region of the heat generating layer in the direction of arrangement of electrode E1 or E2 and in the direction in which the electrodes face each other can be confirmed by sampling multiple locations on the heat generating layer and measuring the weight per unit area, or by measuring the light transmittance if the heat generating layer is light-transmitting.

[0019] One method for gradually changing the weight per unit area of ​​the heat generating layer 10 in the direction of electrode placement and the direction of facing the electrodes is to provide a large number of holes in the heat generating layer 10 arranged randomly or in a grid pattern in a planar view, and gradually change the opening area ratio of the holes in the direction of electrode placement and the direction of facing the electrodes.

[0020] Furthermore, when the heat generating layer is a coating of a conductive paint containing a conductive filler, methods for gradually changing the weight per unit area of ​​the heat generating layer in the direction of arrangement of the electrodes and the direction of facing the electrodes include a method of gradually changing the content of the conductive filler in the heat generating layer in the direction of arrangement of the electrodes and the direction of facing the electrodes. When the content of the conductive filler in the heat generating layer 10 is changed, inkjet printing can be performed using multiple types of conductive paint that have different specific gravities due to different contents of conductive filler.

[0021] In order to eliminate unevenness in the surface temperature when current is applied, it is not necessary to change the thickness of the heat generating layer formed by inkjet printing in some areas, and the designed layer thickness can be made constant over the entire area of ​​the heat generating layer. Here, "constant designed layer thickness" includes cases where, when the heat generating layer is formed by laminating printed layers of a predetermined thickness, the number of layers is constant over the entire area of ​​the heat generating layer, but the layer thickness of the heat generating layer changes slightly due to distortion of the heat generating layer after formation.

[0022] On the other hand, if the opening area ratio of the holes is gradually changed to gradually change the weight per unit area of ​​the heat generating layer in the electrode arrangement direction and the opposing direction, the content of the conductive filler in the conductive paint can be constant. Here, the holes may be recesses or through-holes. A heat generating layer with a varying opening area ratio of the holes can be easily formed using a printing method such as screen printing or gravure printing, as described below. In this case, too, it is not necessary to change the number of stacked printed layers in some areas of the heat generating layer to eliminate uneven surface temperature, and the number of stacked printed layers can be constant across the entire area of ​​the heat generating layer.

[0023] The heat-generating layer 10 of the sheet heating element 1 of the embodiment shown in Figure 1 has a large number of holes 11 arranged in a square lattice pattern, and the size of the holes 11 varies, resulting in a region in which the opening area ratio gradually changes in the direction of the electrodes E1 and E2 (i.e., in the direction along the electrodes if the electrodes are strip-shaped) and in the direction in which the electrodes face each other. In Figure 1, numbers (1) to (13) represent division lines when the heat-generating layer 10 is divided into 24 equal-angle parts (12 parts in the simplified symmetrical view shown in Figure 1) by a line passing through the center O of the heat-generating layer 10's outer shape. Figure 2 is an enlarged view of the region between division lines (8) and (11). As can be seen from Figure 2, this heat-generating layer 10 has circular through-holes as holes 11 at the vertices of a square lattice with a lattice pitch p, as indicated by the dashed lines in the figure. In the heat generating layer 10, at least in the region surrounded by the dividing lines (8) to (11), the opening area ratio of the holes 11 gradually changes in the direction in which the electrodes E1, E2 are arranged and in the opposing direction, and as a result, the weight per unit area of ​​the heat generating layer 10 gradually changes, and therefore the amount of conductive filler attached per unit area of ​​the heat generating layer 10 gradually changes, thereby adjusting the amount of heat generated.

[0024] Although FIG. 2 shows a circular through-hole as the hole 11, in the present invention, the hole 11 may be a recess. Whether the hole 11 is a recess or a through-hole, there are no particular restrictions on the shape of the opening, and it may be a circle, an ellipse, a rectangle, a polygon, or the like.

[0025] There are no particular limitations on the opening area of ​​each hole 11 or the lattice pitch, but in order to ensure a uniform surface temperature when electricity is applied to the heat generating layer 10, the lattice pitch p is preferably 0.1 mm to 6.0 mm, and more preferably 0.5 mm to 3.0 mm.

[0026] The opening area of ​​each hole 11 is preferably 85% or less of the area of ​​the unit lattice, and therefore the opening area ratio of the region where the opening area ratio gradually changes within the heat-generating layer 10 is preferably 0% or more and 85% or less. If the opening area ratio is too large, the conductive paths formed in the parts other than the holes 11 will become narrower, making heat generation in those parts unstable or causing localized heat generation, making heat generation control difficult and making it difficult to uniform the surface temperature of the entire heat-generating layer.

[0027] In this embodiment, holes 11 are provided at the vertices of a square lattice, but the lattice is not limited to a square lattice, and may be a rectangular lattice, a rhombic lattice, a hexagonal lattice, a parallelepiped lattice, or the like.

[0028] The holes 11 may be arranged randomly instead of in a lattice arrangement. Even in the random arrangement, the amount of conductive filler attached per unit area can be adjusted by controlling the opening area ratio of the holes 11.

[0029] Furthermore, the surface temperature of the heating layer may be made uniform by adjusting the number density of the holes while keeping the size of the holes constant, or by adjusting both the size and number density of the holes to adjust the amount of conductive filler attached per unit area.

[0030] (Cross-sectional structure of heat generating layer) As described above, the holes 11 provided in the heat generating layer 10 may be recesses, through-holes, or a mixture of holes and through-holes. For example, as in the planar heating element 1A shown in Fig. 3, a single-layer heat generating layer 10a having recesses 12 as the holes 11 in a sheet-like substrate 2 may be provided, or as in the planar heating element 1B shown in Fig. 4, a single-layer heat generating layer (hereinafter also referred to as a perforated layer) 10b having through-holes 13 as the holes 11 in a sheet-like substrate 2 may be provided.

[0031] In cases where the surface of the heat generating layer is a perforated layer 10b with a high opening area ratio of the through holes 13 and it is difficult to ensure a stable conductive path with the perforated layer 10b alone, a solid layer 10c, which is a heat generating layer with no holes formed, may be provided between the surface perforated layer 10b and the substrate 2, as in the sheet heating element 1C shown in Figure 5. This results in the holes 11 in the heat generating layer becoming depressions 12 (11) with thin bottoms. Note that when the heat generating layer 10 has a layered structure of at least one perforated layer and at least one solid layer, there are no restrictions on the layering positions of the perforated layer and the solid layer, and the solid layer may be on the outermost surface of the heat generating layer.

[0032] The total resistance of the heat generating layer 10 may be adjusted by stacking multiple perforated layers each having a through hole 13 formed therein. For example, three perforated layers 10b1, 10b2, and 10b3 may be stacked as in the planar heating element 1D shown in FIG. 6. In this case, it is preferable that the positions of the through holes 13 are offset between the surface perforated layer 10b1 and the inner perforated layers 10b2 and 10b3, and it is also preferable that holes that look like recesses 12 (11) in plan view are formed. On the other hand, holes that look like through holes 13 (11) in plan view may also be present.

[0033] When the heat generating layer 10 has a laminated structure of perforated layers 10b1, 10b2, and 10b3, the opening area ratio, opening pattern, and layer thickness of the holes of the individual perforated layers may be the same or different. Furthermore, when the laminated perforated layers 10b1, 10b2, 10b3, and 10b or the solid layer 10c are printed layers of conductive paint, the content of the conductive filler in each conductive paint may be the same or different.

[0034] (Thickness and deposition amount of heat generating layer) The heat generating layer 10 may have a single layer structure as described above, or a laminated structure, but from the viewpoint of manufacturing costs, the lower limit of the total thickness is preferably 1.0 μm or more, more preferably 3.0 μm or more, and even more preferably 5.0 μm or more, and the upper limit is preferably 100.0 μm or less, more preferably 50.0 μm or less, and even more preferably 30.0 μm or less.

[0035] The weight per unit area of ​​the heat generating layer 10 (i.e., the amount of coating when the heat generating layer is formed from a coating of conductive paint) has a lower limit of 0.1 g / m ２ Above, 1.0g / m 2 or more, or 3.0 g / m 2 The upper limit is 100.0 g / m 2 or less than 30.0 g / m 2 It can be as follows:

[0036] (Method of Designing the Heat-Generating Layer) Up to a thickness of about 30 μm, the surface resistivity of the heat-generating layer 10 is highly dependent on the layer thickness. Furthermore, the path of current flowing through a heat-generating layer with numerous holes 11 on its surface is complex and difficult to simulate. Therefore, a method of designing the heat-generating layer involves fabricating a heat-generating layer 10 for final use, with a predetermined electrode shape, a predetermined electrode arrangement, and a predetermined heat-generating layer shape, having a hole 11 pattern and stacking configuration that are empirically predicted to generate heat to a certain degree of uniformity, measuring the heat generation distribution, and then performing a feedback design to adjust the weight per unit area of ​​the heat-generating layer, the amount of conductive filler attached per unit area, the hole 11 pattern, and the stacking configuration of the heat-generating layer, which are necessary to generate heat uniformly.

[0037] For example, taking the sheet heating element shown in Figure 8A as an example, the voltage applied between the electrodes is the same in both the left region in the figure where the inter-electrode distance is small and the right region in the figure where the inter-electrode distance is large, but the left region where the inter-electrode distance is small has a smaller inter-electrode resistance than the right region where the inter-electrode distance is large, so the current when current is applied is larger and the surface temperature when current is applied is higher (Figure 9). Therefore, in order to reduce the surface temperature when current is applied in the left region in the figure where the inter-electrode distance is small, it is necessary to increase the resistance in that region. To do this, it is preferable to reduce the amount of conductive filler attached in this region, and therefore it is preferable to increase the opening area ratio of the holes.

[0038] Furthermore, because the sheet heating element shown in Fig. 8A has a pair of electrodes formed in a roughly concentric pattern, the current density is higher at the inner periphery of the sheet heating element compared to the outer periphery, resulting in a higher surface temperature when current is applied (Fig. 9). Therefore, in order to make the temperature uniform when current is applied, it is necessary to reduce the resistance of the inner periphery compared to the outer periphery, and it is preferable to increase the amount of conductive filler attached at the inner periphery compared to the outer periphery, and for this purpose it is preferable to reduce the opening area ratio of the holes at the inner periphery.

[0039] (Composition of Heat-Generating Layer) The heat-generating layer 10 may be a transparent conductive film formed by vacuum deposition as described in Patent Document 2, but from the viewpoint of easily forming a heat-generating layer by a printing method, it is preferable to form it from a conductive paint in which a conductive filler is dispersed in a binder.

[0040] The composition of the heat generating layer can be a known composition described in Patent Document 1, etc., and for example, the conductive filler can be carbon nanotubes (multi-walled carbon nanotubes and single-walled carbon nanotubes), graphite, other carbon-based materials, metal-based fillers such as metal fibers, conductive polymers, etc.

[0041] It is preferable to use carbon nanotubes as the main component of the conductive filler, and it is particularly preferable to use multi-walled carbon nanotubes. By using carbon nanotubes as the main component, deterioration of the heating layer due to oxidation, corrosion, heat, etc. can be suppressed. Carbon nanotubes have a small fiber diameter and a large aspect ratio, which stabilizes the conductive path and reduces the risk of breakage due to external forces such as bending compared to other metal-based fillers. Furthermore, carbon nanotubes emit a larger amount of far-infrared heat than metal-based fillers, which can also suppress the power consumption of the heating layer. Furthermore, by using single-walled carbon nanotubes as the main component of the conductive filler, it is possible to form a transparent heating layer.

[0042] Carbon nanotubes with a smaller average fiber diameter have poorer dispersibility in conductive paints, whereas larger average fiber diameters result in higher surface resistivity. Therefore, a preferred average fiber diameter is selected depending on the type of binder, the amount of carbon nanotubes, etc. Typically, the average fiber diameter of multi-walled carbon nanotubes can be 1 nm or more and 80 nm or less, and preferably 12 nm or more and 70 nm or less. This average fiber diameter can be measured using an electron microscope.

[0043] The average fiber length of the multi-walled carbon nanotubes is preferably 1 μm or more and 1000 μm or less, and particularly 10 μm or more and 50 μm or less.

[0044] The proportion of carbon nanotubes in the conductive filler is preferably 50% by mass or more and 100% by mass or less.

[0045] The content of the conductive filler in the conductive paint is preferably 1% by mass or more and 40% by mass or less, more preferably 3% by mass or more and 30% by mass or less. If the content of the conductive filler in the conductive paint is too low, the surface resistivity may increase, and there is a risk of forming an area where the heat generation amount is significantly reduced, and there is also a risk of the heat generation amount varying. Conversely, if the content of the conductive filler is too high, the dispersibility of the conductive filler deteriorates, the viscosity of the conductive paint increases, and it becomes difficult to form a uniform coating film. In addition, adhesion to the substrate also decreases.

[0046] (Binder) The type of binder is not particularly limited, and can be appropriately selected from various thermoplastic resins and thermosetting resins in view of heat resistance, coating film stretchability, moldability, and low surface resistivity.

[0047] Examples of resins that can be used to form the binder include polyester resins, acrylic resins ((meth)acrylic homopolymers, (meth)acrylic copolymers, acrylic-styrene copolymers), polyamide resins, polyurethane resins, fluorine-containing resins, olefin resins, vinyl chloride resins, polyvinyl alcohol resins, polyamide-imide resins, polyimide resins, various ionomer resins, various elastomers, and various rubbers (NBR, SBR, silicone rubber). Thermoplastic polyurethane resins are particularly preferred.

[0048] Here, XX-based resin means a resin containing XX as the main component, and a portion of the resin may be substituted or modified with other monomers or functional groups.

[0049] The preferred binder content in the conductive paint is 40% by mass or more and 95% by mass or less. If the binder content is too low, the heat generating layer will not have sufficient mechanical coating strength, will have poor chemical resistance, and will be difficult to ensure adhesion to the substrate.

[0050] (Dispersants and other additives) The conductive paint may contain a dispersant. Examples of dispersants for water-based paints include polymer dispersants having a carboxylate functional group, such as ammonium salt of carboxymethyl cellulose, and polymer dispersants having a sulfonate functional group.

[0051] As a dispersant for a solvent-based paint, a polymer dispersant having a carboxyl group or a carboxylate functional group can be used.

[0052] The content of the dispersant in the conductive coating material can be 20.0 parts by mass or more and 300.0 parts by mass or less per 100 parts by mass of the conductive filler.

[0053] Other additives that can be added to the conductive coating include surfactants, antifoaming agents, viscosity modifiers, ultraviolet absorbers, light stabilizers, antioxidants, crosslinking agents, and the like.

[0054] (Surface Resistivity of Heat-Generating Layer) By forming the heat-generating layer as a coating film of the above-mentioned conductive paint, the surface resistivity of the heat-generating layer can be formed in the range of 1.0 Ω / □ to 5000.0 Ω / □, and it is preferable to set the range appropriately depending on the application, etc. For example, for battery-powered applications such as automobile seat heaters and wearable heaters, it is preferable to set the surface resistivity to 1.0 to 500.0 Ω / □.

[0055] Furthermore, the surface resistivity of the heat generating layer decreases as the amount of conductive filler attached increases, so when generating heat at a high temperature with a low voltage, it is preferable to attach a large amount.

[0056] (Substrate) As the sheet-like substrate 2, known substrates described in, for example, Patent Document 1 can be used. Generally, resin sheets, rubber, and nonwoven fabrics can be used, and it is particularly preferable to use a resin sheet. Among them, as the resin constituting the resin sheet, thermoplastic resins or thermosetting resins such as polyester resins (PET, A-PET, PET-G, PBT, PEN), acrylic resins, olefin resins, polyamide resins, polyurethane resins, polycarbonate resins, polyimide resins, polyetherimide resins, polyphenylene sulfide resins, polyethersulfone resins, vinyl chloride resins, and various elastomers are preferred.

[0057] The thickness of the sheet-like substrate 2 is preferably 1 μm or more and 1000 μm or less, and more preferably 6 μm or more and 300 μm or less.

[0058] (Method for manufacturing a sheet-shaped heating element) A method for manufacturing the sheet-shaped heating element of the present invention includes, for example, applying a conductive paint having a conductive filler dispersed in a binder to a sheet-shaped substrate and drying it to form a heating layer, and providing at least one pair of electrodes on the heating layer.

[0059] In this case, the conductive paint can be applied by printing as described above. Also, in order to make the surface temperature uniform when current is applied, the amount of conductive filler attached per unit area of ​​the heating layer is gradually changed in the direction of the electrodes and the direction in which the electrodes face each other in a predetermined region of the heating layer, thereby providing an area in which the weight per unit area of ​​the heating layer is adjusted.

[0060] The sheet heating element thus obtained has a uniform surface temperature without forming localized areas of increased or decreased temperature when energized. For example, when a predetermined voltage is applied between the pair of electrodes E1 and E2, the sheet heating element 1 of the embodiment shown in the plan view of FIG. 1 has no localized areas of increased or decreased temperature, as observed in the thermograph shown in FIG. 7. Instead, the temperature range within 0 to -20% of the maximum temperature of 46°C accounts for preferably 80% or more, and more preferably 90% or more, of the entire surface. In the thermograph of the comparative sheet heating element shown in FIG. 9, when heating is performed, the temperature range of 36.8 to 46.0°C, which is within 0 to -20% of the maximum temperature of 46.0°C, accounts for approximately 50% of the entire surface, demonstrating that the present embodiment achieves a uniform surface temperature.

[0061] (Uses of Sheet Heating Element) The sheet heating element of the present invention can be used, for example, to melt snow on roofs and stairs of buildings, melt snow on LED signals, windows and headlights, keep pipes warm, defrosters and seat heaters in cars, wearable heaters, etc.

[0062] REFERENCE SIGNS LIST 1, 1A, 1B, 1C, 1D Sheet heating element of example 1x Sheet heating element of comparative example 2 Base material 10, 10a Heating layer 10b, 10b1, 10b2, 10b3 Heating layer (perforated layer) 10c Solid layer 10x Heating layer of comparative example 11 Hole 12 Depression 13 Through-hole E1, E2 Electrode

Claims

1. A sheet-shaped heating element having a sheet-shaped heating layer, a sheet-shaped substrate supporting the heating layer, and at least one pair of electrodes, wherein the weight per unit area of ​​the heating layer has an area in which it gradually changes in the direction in which the electrodes are arranged and in the direction in which the electrodes face each other.

2. A sheet heating element according to claim 1, wherein the heat generating layer has holes arranged randomly or in a grid pattern in a plan view, and the opening area ratio of the holes has a region where it gradually changes in the direction of the electrodes and in the direction of the electrodes facing each other.

3. The sheet heating element according to claim 2, wherein at least one of the number density and size of the holes is gradually changed, thereby gradually changing the open area ratio of the holes.

4. The sheet heating element according to claim 1 or 2, wherein the heat generating layer is a coating of conductive paint.

5. A sheet heating element according to any one of claims 1 to 3, wherein the amount of conductive filler contained in the conductive paint attached per unit area in the heat generating layer gradually changes in the direction of the electrodes and in the direction in which the electrodes face each other.

6. The sheet heating element according to claim 4, wherein the conductive paint contains carbon nanotubes.

7. The sheet heating element according to claim 4, wherein the heat generating layer has a laminated structure of at least one porous layer having through holes as holes and at least one solid layer having no through holes.

8. The sheet heating element according to claim 7, wherein the number of porous layers or solid layers laminated in the heating layer is constant.

9. The sheet heating element according to any one of claims 1 to 3, wherein the sheet-like substrate is thermoplastic.

10. A method for manufacturing a sheet-like heating element as described in claim 1, which comprises forming a sheet-like heating layer on a sheet-like substrate and providing at least one pair of electrodes, and which forms the heating layer so that the weight per unit area of ​​the heating layer gradually changes in the direction of the electrodes and the direction in which the electrodes face each other in a predetermined region of the heating layer.

11. The method for producing a sheet heating element according to claim 10, wherein the heat generating layer is formed by printing a conductive paint.

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