Planar heating element, laminate, and vehicle interior material

Abstract

A planar heating element 10 comprises a substrate 15 that includes a foam body 11, and a heating layer 20 that is disposed on one surface 15A of the substrate 15.

Description

Sheet heating element, laminate, and vehicle interior material

[0001] The present invention relates to a sheet heating element used as a heat generating device, and a laminate and vehicle interior material including the sheet heating element.

[0002] Conventionally, heaters have been installed in vehicle interior materials such as automobiles to improve passenger comfort in cold environments. Heaters installed in interior materials are generally constructed with electric heating wires made of metal materials such as nichrome or copper alloys. The nichrome wires are attached to components constituting the interior materials, such as resin sheets, woven fabrics, and nonwoven fabrics. Generally, nichrome wires have a thickness of several hundred micrometers to several millimeters. Therefore, when attached to interior materials, the nichrome wires can create an uneven texture, potentially reducing passenger comfort. Furthermore, nichrome wires are less flexible than other components used in interior materials, such as skins and nonwoven fabrics, and therefore cannot conform to stresses caused by passengers sitting on the seats, potentially reducing passenger comfort.

[0003] For this reason, conventionally, as disclosed in Patent Document 1, for example, a groove or the like is provided in the component constituting the interior material, and the interior material is placed inside the groove, thereby preventing the unevenness of the nichrome wire from appearing on the surface.

[0004] Japanese Patent Application Laid-Open No. 2019-111656

[0005] However, as in Patent Document 1, providing grooves or the like in the components that make up the interior material increases the number of steps required to process the interior material, resulting in a decrease in production efficiency. Furthermore, the heat generated by the nichrome wire dissipates in all directions, making it difficult to efficiently heat the outer surface of the interior material. Furthermore, simply enclosing the nichrome wire in a groove does not solve the problem of the nichrome wire losing its flexibility and impairing its soft feel.

[0006] Therefore, the present invention aims to provide a sheet heating element that can efficiently heat the surface of a sheet without reducing production efficiency, with a simple configuration, and while preventing the sheet from having an uneven feel.

[0007] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by disposing a heat-generating layer on one surface of a substrate containing a foam, and have thus completed the present invention as described below. Specifically, the present invention provides the following items [1] to

[17] . [1] A sheet heating element comprising a substrate containing a foam and a heat-generating layer disposed on one surface of the substrate. [2] The sheet heating element described in [1] above, in which the heat-generating layer is disposed so as to be in direct contact with the foam. [3] The sheet heating element described in [1] or [2] above, in which a primer layer is formed on the surface of the substrate that is in contact with the heat-generating layer. [4] The sheet heating element described in any of [1] to [3] above, in which the heat-generating layer is formed by printing. [5] The sheet heating element described in any of [1] to [4] above, in which at least a portion of the heat-generating layer is embedded inward from the surface of the foam. [6] The sheet heating element described in any of [1] to [5] above, in which a skin layer is formed on at least one surface of the foam, and the heat-generating layer is formed on the skin layer. [7] The sheet heating element according to any one of [1] to [6] above, wherein the foam is an olefin resin-based foam. [8] The sheet heating element according to [7] above, wherein the resin constituting the foam includes a polypropylene resin. [9] The sheet heating element according to any one of [1] to [8] above, wherein the foam is an electron beam crosslinked product.

[10] The sheet heating element according to any one of [1] to [9] above, wherein the foam is a closed-cell foam.

[11] The sheet heating element according to any one of [1] to

[10] above, wherein the surface roughness Ra of the surface of the substrate on which the heating layer is provided is 5 μm or less.

[12] The sheet heating element according to any one of [1] to

[11] above, wherein the heating layer has a thickness of 100 μm or less and is formed of a conductive material.

[13] A laminate comprising the sheet heating element according to any one of [1] to

[12] above, and a skin bonded to the sheet heating element directly or via another layer.

[14] The laminate according to

[13] above, wherein the skin is adhered to the surface of the sheet heating element on which the heat-generating layer is provided.

[15] The laminate according to

[13] or

[14] above, wherein at least a portion of the heat-generating layer is embedded inward from the surface of the foam.

[16] A vehicle interior material comprising the sheet heating element according to any one of [1] to

[12] above, or the laminate according to any one of

[13] to

[15] above.

[17] The vehicle interior material according to the above

[16] , wherein the heat generating layer is disposed on the interior side and the substrate is disposed on the opposite side to the interior side.

[0008] According to the planar heating element of the present invention, the surface can be efficiently heated without reducing production efficiency, with a simple configuration, while preventing the surface from feeling uneven to the touch.

[0009] Fig. 1 is a schematic cross-sectional view of a sheet heating element according to one embodiment of the present invention; Fig. 2 is a schematic cross-sectional view of a sheet heating element according to another embodiment of the present invention; Fig. 3 is a plan view of a sheet heating element according to one embodiment of the present invention; Fig. 4 is a schematic cross-sectional view of a laminate according to one embodiment of the present invention.

[0010] <Sheet Heating Element> The sheet heating element of the present invention will be described below with reference to embodiments. Fig. 1 shows a sheet heating element according to one embodiment of the present invention. As shown in Fig. 1, the sheet heating element 10 according to one embodiment includes a base material 15 made of a foam 11 and a heating layer 20 disposed on one surface 15A of the base material 15.

[0011] With the above-described configuration, when the sheet heating element 10 is used or assembled into a vehicle interior material, the sheet heating element 10 is pressed from the side where the heating layer 20 is provided, thereby allowing at least a portion of the heating layer 20 to be embedded within the foam 11. This prevents the surface where the heating layer 20 is provided from feeling uneven. Furthermore, the foam 11 in the base material 15 acts as an insulating layer, preventing heat generated in the heating layer 20 from dissipating from the surface of the base material 15 of the sheet heating element 10. This allows the surface of the sheet heating element 10 where the heating layer 20 is provided to be efficiently heated. Furthermore, since special processing such as providing grooves in the base material 15 is not required, the sheet heating element 10 can be manufactured with high productivity. In addition, using the foam 11 as the base material 15 also allows the sheet heating element 10 to be lightweight.

[0012] Each component constituting the sheet heating element will be described in more detail below. [Substrate and foam] The foam 11 is a resin foam. Examples of resins constituting the foam include polyolefin resins, urethane resins, acrylic resins, and elastomer resins. The resin used for the foam 11 may be a single type, or two or more types may be used in combination. Of the above, the foam 11 is preferably a polyolefin resin foam, which uses a polyolefin resin as the resin. Using a polyolefin resin for the foam improves flexibility and mechanical strength.

[0013] (Polyolefin Resin) Examples of polyolefin resins include polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, etc., and among these, polypropylene resin is preferred. By using polypropylene resin, it becomes easier to impart heat resistance to the base material 15, making it suitable for use as a planar heating element.

[0014] The polyethylene resin is low-density polyethylene (density: 0.930 g / cm ３ less than 0.930 g / cm ３ 0.942g / cm or more ３ less than 0.942 g / cm ３ The density of the linear low-density polyethylene is 0.870 to 0.910 g / cm ３ is preferably 0.875 to 0.907 g / cm ３ More preferably, 0.880 to 0.905 g / cm ３ is more preferable. As the polyethylene resin, a plurality of polyethylene resins may be used, and a polyethylene resin having a density outside the above-mentioned range may also be used. Examples of the ethylene-vinyl acetate copolymer used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.

[0015] The polypropylene resin is not particularly limited, and examples thereof include propylene homopolymers (homopolypropylenes) and copolymers of propylene and other olefins. The copolymers of propylene and other olefins may be block copolymers, random copolymers, or random block copolymers, with random copolymers (random polypropylenes) being preferred. Examples of copolymers of propylene and other olefins include propylene-α-olefins containing, for example, preferably 75% by mass or more, more preferably 90% by mass or more, of propylene. Examples of other olefins copolymerized with propylene include α-olefins such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, and 1-decene, with ethylene being preferred among these. Therefore, ethylene-propylene random copolymers are more preferred as random polypropylenes.

[0016] When polypropylene resin is used as the polyolefin resin, polypropylene resin may be used alone, or may be used in combination with other polyolefin resins other than polypropylene resin or resins other than polyolefin resins. For example, polyolefin resins other than polypropylene resins may be used in combination, or resins other than polyolefin resins may be used in combination. Examples of resins other than polyolefin resins include elastomer resins. Examples of elastomer resins include various rubber components such as ethylene-propylene-diene rubber (EPDM), ethylene-propylene rubber (EPM), polybutadiene rubber, polyisoprene rubber, styrene-butadiene copolymer (SBR), or styrene rubbers such as hydrogenated styrene-butadiene copolymer (HSBR). Other examples include thermoplastic elastomers such as olefin thermoplastic elastomers and styrene thermoplastic elastomers. From the viewpoint of heat resistance, the higher the content of polypropylene resin in the foam, the better. For example, the content is preferably 50 to 100 mass%, more preferably 70 to 100 mass%, and even more preferably 80 to 100 mass%, based on the total amount of resin contained in the foam.

[0017] Furthermore, from the viewpoint of imparting both heat resistance and flexibility, the resin constituting the foam is preferably a polypropylene-based elastomer containing a polypropylene resin and a rubber component. The polypropylene resin and the rubber component may be separate or copolymerized in the polypropylene-based elastomer. Polypropylene-based elastomers are classified into reactor-type simple blend types and dynamic crosslinking types depending on the production method, with the reactor type being preferred because the domain diameter of the dispersed rubber component is small and transparency is high. Commercially available reactor-type polypropylene-based elastomers, such as "Catalloy" (manufactured by SunAllomer Co., Ltd.), can also be used.

[0018] (Foaming Agent) The foam 11 is preferably a foam obtained by foaming a foamable composition containing the resin and a foaming agent. Examples of the foaming agent include thermally decomposing foaming agents, and organic and inorganic foaming agents can be used as thermally decomposing foaming agents. Thermally decomposing foaming agents typically have a decomposition temperature higher than the melting temperature of the resin, e.g., 140 to 270°C. Specific examples of organic foaming agents include azo compounds such as azodicarbonamide, azodicarboxylic acid metal salts (e.g., barium azodicarboxylate), and azobisisobutyronitrile; nitroso compounds such as N,N'-dinitrosopentamethylenetetramine; hydrazine derivatives such as hydrazodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), and toluenesulfonylhydrazide; and semicarbazide compounds such as toluenesulfonylsemicarbazide. Examples of inorganic foaming agents include ammonium carbonate, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate. Among these, azo compounds are preferred, and azodicarbonamide is particularly preferred, from the viewpoints of obtaining fine bubbles, economy, and safety. These thermally decomposable foaming agents can be used alone or in combination of two or more. The amount of the thermally decomposable foaming agent in the foamable composition can be adjusted depending on the expansion ratio of the foam, but is preferably 0.5 to 20 parts by mass, more preferably 1 to 15 parts by mass, and even more preferably 2 to 10 parts by mass per 100 parts by mass of the resin.

[0019] However, as the foaming agent, a foaming agent other than a pyrolytic foaming agent may be used, for example, a physical foaming agent may be used. As the physical foaming agent, a high-pressure inert gas is preferably used. The inert gas is not particularly limited as long as it is inert to the resin composition and can be impregnated, and examples thereof include carbon dioxide, butane gas, nitrogen gas, and air. These gases may be used in combination. Of these, carbon dioxide and butane gas are preferred from the viewpoint of easily increasing the expansion ratio of the foam. The inert gas used for impregnation is preferably in a supercritical or subcritical state.

[0020] (Other Additives) The foam or foamable composition may contain, as needed, additives commonly used in foams, such as crosslinking agents, crosslinking aids, antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, fillers, decomposition temperature adjusters, etc. Among these, it is preferable to use antioxidants and decomposition temperature adjusters.

[0021] (Expansion Ratio) The foam 11 has an expansion ratio of 4 to 50 cm ３ / g, and 8 to 30 cm ３ / g, and more preferably 10 to 20 cm ３ / g. When the expansion ratio of the foam 11 is equal to or greater than the above lower limit, it undergoes large compressive deformation under a relatively low load, making it easier for the heat generating layer 20 to be embedded within the substrate 15 during use or assembly, and more effectively preventing the irregularities of the heat generating layer 20 from appearing on the surface. When the expansion ratio is equal to or less than the above upper limit, a certain level of mechanical strength is imparted to the foam 11, making it easier to improve durability, etc. The expansion ratio is the reciprocal of the apparent density, which can be measured, for example, in accordance with JIS K7222.

[0022] (Closed Cell Ratio) The foam 11 may be an open-cell foam, a semi-closed-cell foam, or a closed-cell foam, but is preferably a closed-cell foam. The foam 11 being a closed-cell foam improves the heat insulating properties and further improves the heat generating efficiency of the surface of the sheet heating element 10 on which the heat generating layer 20 is provided. Furthermore, even if the heat generating layer 20 is formed directly on the surface of the foam 11, or if a base layer 16 (see FIG. 2 ) is formed as described below, the ink used to form the heat generating layer 20 or base layer 16 is less likely to penetrate into the interior, improving the film formability of the base layer 16 and heat generating layer 20.

[0023] A closed-cell foam is one in which most of the bubbles contained in the foam are closed cells. Specifically, the closed-cell ratio is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. The upper limit of the closed-cell ratio is not particularly limited, and is 100%. Having a closed-cell ratio equal to or greater than the lower limit improves heat insulation and more easily prevents ink from seeping into the foam. Furthermore, a high closed-cell ratio increases the mechanical strength of the foam 11, which also tends to improve the durability of the substrate 15.

[0024] The closed cell ratio can be measured according to the method of ASTM D2856 (1998). Specifically, it is recommended to measure it as follows. First, a flat square test piece with a side length of 5 cm is cut out from the foam. Then, the thickness of the test piece is measured to determine the apparent volume V of the test piece. １ Calculate the weight of the test piece W １ Next, measure the volume V occupied by the bubbles. ２ is calculated based on the following formula: The density of the matrix resin constituting the test piece is ρ (g / cm ３ ) The volume occupied by the bubbles is V ２ =V １ -W １ Next, the test piece is submerged in distilled water at 23°C to a depth of 100 mm from the water surface, and a pressure of 15 kPa is applied to the test piece for 3 minutes. After that, the pressure is released in the water, and the test piece is left to stand for 1 minute. Then, the test piece is taken out of the water, and the water adhering to the surface of the test piece is removed, and the weight W of the test piece is measured. ２ The open cell rate F and closed cell rate F were measured based on the following formula: ２ Calculate the open cell ratio F １ (%) = 100 × (W ２ -W １ ) / V ２ Closed bubble rate F ２ (%) = 100 - F １

[0025] (Degree of Crosslinking) The foam 11 is preferably a crosslinked foam, and more preferably an electron beam crosslinked foam crosslinked by an electron beam. When the foam 11 is a crosslinked foam, and especially an electron beam crosslinked foam, durability, moldability, etc. are improved. The degree of crosslinking of the foam 11 is not particularly limited, but is, for example, 10 to 70 mass %, more preferably 20 to 60 mass %. By setting the degree of crosslinking of the foam 11 within the above range, it is easy to improve the mechanical strength, flexibility, etc. of the substrate 15. Furthermore, it becomes possible to appropriately foam the foam 11. The method for measuring the degree of crosslinking is as follows. A test piece of approximately 100 mg is taken from the foam, and the weight A (mg) of the test piece is precisely weighed. Next, this test piece is immersed in 30 cm of xylene at 120°C. ３ After immersion for 24 hours, the insoluble matter on the mesh is filtered through a 200-mesh wire netting, collected, and vacuum-dried. The weight B (mg) of the insoluble matter is then precisely weighed. The degree of crosslinking (mass%) is calculated from the obtained value using the following formula: Degree of crosslinking (mass%) = 100 × (B / A).

[0026] (Thickness) The thickness of the foam 11 is not particularly limited, but is preferably 0.5 to 100 mm, and more preferably 1 to 50 mm. When the thickness is within this range, the sheet heating element 10 can be suitably used as a vehicle interior material. Furthermore, by making the thickness equal to or greater than the above lower limit, the heating layer 20 becomes more easily embedded inside the substrate 15, and the heat insulating properties of the substrate 15 are also improved.

[0027] (Surface Roughness) The surface 15A of the substrate 15 on which the heat generating layer 20 is provided preferably has a surface roughness Ra of 5 μm or less. Because the substrate 15 is made of foam 11, air bubbles can cause surface irregularities. However, by setting the surface roughness Ra to 5 μm or less, the air bubbles are less likely to appear on the surface of the substrate 15. This makes it easier to prevent the air bubbles from causing a decrease in adhesion of the heat generating layer 20 to the substrate 15. The surface roughness Ra of the surface 15A is more preferably 3 μm or less. The smaller the surface roughness Ra of the surface 15A, the better, but practically, it is preferably 1 μm or more. The surface 15A of the substrate 15 may be made of the surface of the foam 11, but it may also be made of a material other than the foam 11. Specifically, the surface 15A may be made of a non-foamed surface layer (not shown) that serves as a skin layer (described later), a base layer 16 (see FIG. 2), or the like.

[0028] The surface roughness Ra can be adjusted to a certain value or less, as described above, by adjusting the surface condition of the foam 11, for example. For example, the foam 11 may be sliced. In this case, the surface of the foam 11 (non-sliced ​​surface) other than the surface formed by slicing (sliced ​​surface) can be used as the surface 15A on which the heat generating layer 20 is provided. The surface roughness Ra can also be reduced by appropriately controlling the degree of crosslinking of the foam 11, the expansion ratio of the foam sheet, etc. Furthermore, as described below, the surface roughness Ra can also be reduced by forming a skin layer or a base layer 16. The surface roughness Ra is the arithmetic mean roughness value determined in accordance with JIS B0601 (2001).

[0029] The substrate 15 may be made of a foam alone, or a layer other than a foam may be provided on the surface of the substrate 15. For example, a layer other than the foam 11 may be provided on the surface 15A of the substrate 15 on which the heat generating layer 20 is provided. Specifically, a non-foamed layer (not shown) that will become a skin layer, which will be described later, may be provided, or as shown in Fig. 2, a base layer 16 such as a deterioration-resistant film or an adhesive coating film may be provided.

[0030] (Skin Layer) The foam 11 may have a skin layer (not shown) formed on at least one surface thereof, and the heat-generating layer 20 may be formed on the skin layer. The skin layer is a layer in which the influence of unevenness due to air bubbles is reduced compared to the interior of the foam 11. Specifically, the skin layer may be formed by a non-sliced ​​surface, as described below, and may be a thin film layer on the surface of the foam 11. The skin layer may also be a non-foamed layer (also referred to as a "surface non-foamed layer") or a foamed layer (also referred to as a "surface foamed layer") having a lower expansion ratio than the foam (core foam layer) provided inside the skin layer. The surface non-foamed layer or surface foamed layer may be composed of a resin layer, and the resin used for the resin layer may be appropriately selected from the resins exemplified for the resins constituting the foam described above, as described in detail for the foam. The presence of the skin layer may also prevent ink used for printing from seeping into or bleeding into the interior when forming the heat-generating layer or base layer, as described below.

[0031] Furthermore, when the skin layer is a surface foam layer, the foam 11 may include a core foam layer made of a foam and a surface foam layer disposed on the core foam layer. The surface foam layer may be disposed on at least the surface 15A of the substrate 15 on which the heat-generating layer 20 is disposed, but a surface foam layer may also be disposed on both sides of the core foam layer. That is, when the skin layer is a surface foam layer, the foam 11 disposed on the substrate may have a laminated structure of surface foam layer / core foam layer / surface foam layer, or may have a laminated structure of surface foam layer / core foam layer. Furthermore, it may have a laminated structure of surface foam layer / core foam layer / non-foamed surface layer.

[0032] The core foam layer has the same configuration as that of the foam described above except for the thickness. The surface foam layer has the same configuration as that of the foam described above except for the thickness and expansion ratio. The expansion ratio of the surface foam layer is preferably lower than that of the core foam layer, and specifically, is 1 to 5 cm. ３ / g, and 1.1 to 3 cm ３ / g, and 1.2 to 2 cm ３It is preferable that the total thickness of the core foam layer and the surface foam layer is the same as the thickness of the foam, but from the viewpoint of ensuring the flexibility of the foam, it is preferable that the thickness of the core foam layer is greater than the thickness of each surface foam layer.

[0033] When the skin layer is a non-foamed surface layer, as described above, the skin layer may be a non-foamed resin layer substantially free of bubbles. Furthermore, when the skin layer is a non-foamed surface layer, the substrate 15 may include a core foam layer made of foam 11 and a non-foamed surface layer disposed on the core foam layer. The non-foamed surface layer may be disposed on at least the surface 15A of the substrate 15 on which the heat-generating layer 20 is disposed, but non-foamed surface layers may also be disposed on both sides of the core foam layer. That is, when the skin layer is a non-foamed surface layer, the foam 11 disposed on the substrate may have a laminated structure of non-foamed surface layer / core foam layer / non-foamed surface layer, or may have a laminated structure of non-foamed surface layer / core foam layer. Furthermore, it may have a laminated structure of non-foamed surface layer / core foam layer / foamed surface layer.

[0034] As described above, the thickness of the skin layer (non-foamed surface layer or foamed surface layer) on the surface 15A on which the heat generating layer 20 is disposed is not particularly limited, but is preferably, for example, approximately 5 to 500 μm, more preferably 8 to 300 μm, and even more preferably 10 to 100 μm. By making the thickness of the non-foamed surface layer or foamed surface layer at least a certain level, the effect of providing a skin layer made of a non-foamed layer or foamed surface layer can be more easily achieved. Furthermore, by making the thickness of the non-foamed surface layer or foamed surface layer at or below a certain level, the flexibility of the substrate 15 is not impaired, and the heat generating layer 20 can be more easily embedded inside the foam 11.

[0035] (Underlayer) In the substrate 15, a substrate surface 15A that contacts the heat generating layer 20 may be provided with a substrate underlayer 16 as shown in FIG. 2 . Examples of the underlayer 16 include a deterioration-resistant film and an adhesive coating film. The underlayer 16 may also function as both of these films. The deterioration-resistant film is a film for preventing deterioration of the heat generating layer 20. In the present invention, since the substrate 15 is made of foam 11, the adhesive surface of the heat generating layer 20 attached to the substrate 15 is prone to deterioration due to air flow from the substrate 15 side. However, providing a deterioration-resistant film can help to suppress such deterioration. Specifically, providing the substrate 15 with a deterioration-resistant film can improve at least one of, and preferably all of, the oxidation resistance, sulfidation resistance, and moisture resistance of the heat generating layer 20.

[0036] When the base layer 16 is an adhesive coating film, the base layer 16 can improve the adhesive strength between the heat generating layer 20 and the foam 11. The heat generating layer 20 may not have sufficient adhesive strength when directly adhered to the foam 11, but the provision of an adhesive coating film can prevent poor adhesion of the heat generating layer 20 to the substrate 15.

[0037] The base layer 16 is preferably formed directly on the surface of the foam 11, but may also be formed on the surface of a layer other than the foam formed on the surface of the foam 11, such as a non-foamed surface layer that constitutes a skin layer. When the base layer 16 is provided, the heat-generating layer 20, which will be described later, is preferably formed directly on the base layer 16. The base layer 16 may be provided on the entire surface 15A of the substrate 15 as shown in Fig. 2, but it does not have to be provided on the entire surface; it is preferable that the base layer 16 be provided at least in the region where the heat-generating layer 20, which will be described later, is to be formed.

[0038] The base layer 16 may be formed from a resin composition containing a resin such as a thermosetting resin or a thermoplastic resin, without any particular limitation. Specific examples of resins used in the base layer 16 include urethane resin, acrylic resin, cycloolefin polymer, and cycloolefin copolymer. These resins may be used alone, or may be a mixture or copolymer of two or more selected resins. The use of a urethane resin or an acrylic resin in the base layer 16 makes it suitable for use as a deterioration-resistant film that improves the aforementioned deterioration resistance or as an adhesive coating film that improves the adhesion between the heat-generating layer 20 and the foam 11. The resin composition for forming the base layer may contain additives and colorants other than the resin components, or may contain a solvent and be diluted with the solvent.

[0039] The base layer 16 is preferably formed using a printing ink, and the printing ink may be a colorless medium ink. When the base layer 16 is formed using a printing ink, it may be applied to the surface of the foam 11 or the surface of a layer other than the foam, such as a skin layer, using a known printing method such as screen printing or inkjet printing. By forming the base layer 16 using a printing ink, both the base layer 16 and the heating layer 20 can be formed using a known printing method, so the sheet heating element 10 can be efficiently manufactured even when the base layer 16 is provided. The printing ink for forming the base layer 16 is not particularly limited, and examples include thermosetting inks, photocurable inks, and two-component curable inks made of acrylic resins, urethane resins, cycloolefin polymers, cycloolefin copolymers, etc.

[0040] The thickness of the underlayer 16 is not particularly limited, but is preferably about 0.1 to 50 μm, and more preferably 0.5 to 20 μm.

[0041] The deterioration-resistant film described above is not limited to being provided as the base layer 16 on the surface 15A of the substrate 15 described above, but may also be formed on the surface of the heating layer 20 described below opposite the surface facing the substrate 15. When the deterioration-resistant film is formed on the surface of the heating layer 20 opposite the surface facing the substrate 15, the base layer 16 as a deterioration-resistant film provided on the surface of the foam may be omitted, but it is preferable to form the base layer 16 as a deterioration-resistant film. With this configuration, deterioration-resistant films are formed on both the surface facing the substrate 15 and the surface opposite thereto, so that deterioration of the heating layer 20 can be more effectively suppressed.

[0042] (Method for Producing Foam) The foam 11 can be produced by, without particular limitation, foaming a foamable composition containing a resin component and a foaming agent with the foaming agent. In this case, foaming with the foaming agent may be performed by heating or the like. The foam 11 is preferably obtained by crosslinking the foamable composition and foaming the crosslinked foamable composition. In addition to the resin component and the foaming agent, the foamable composition may contain additives as necessary.

[0043] Specifically, it is industrially advantageous to produce the foam 11 by a method comprising the following steps (1) to (3): Step (1): A step of supplying the raw materials of the foamable composition to a kneading device, kneading them, and then obtaining a sheet-like foamable composition (foamable sheet); Step (2): A step of irradiating the foamable composition obtained in step (1) with ionizing radiation to crosslink it; Step (3): A step of heating the foamable composition crosslinked in step (2) to a temperature equal to or higher than the decomposition temperature of the foaming agent to foam it, thereby obtaining a sheet-like foam.

[0044] In the step (1), the raw materials constituting the foamable composition are kneaded using a kneader such as a Banbury mixer or a pressure kneader, and then the kneaded mixture is continuously extruded using an extruder, a calender, conveyor belt casting, or the like, to produce a sheet-like foamable composition.

[0045] Examples of the ionizing radiation used in step (2) include α-rays, β-rays, γ-rays, and electron beams, with electron beams being preferred. The dose of ionizing radiation may be any dose sufficient to achieve the desired degree of crosslinking; it is preferably 0.1 to 10 Mrad, more preferably 0.2 to 5 Mrad, and even more preferably 0.5 to 3 Mrad. In step (3), the temperature at which the foamable composition is heated and foamed depends, for example, on the decomposition temperature of the thermally decomposable foaming agent used as the foaming agent, but is typically 140 to 300°C, preferably 150 to 280°C. In step (3), the foam may be stretched in either or both MD and TD after or while foaming. Alternatively, step (3) may be performed after step (1) without passing through step (2). That is, the sheet-like foamable composition may be foamed without crosslinking.

[0046] Furthermore, if the foam obtained by the above-described manufacturing method is thick, it may be sliced ​​appropriately. Slicing is preferably performed in a direction perpendicular to the thickness direction of the sheet-like foam. Therefore, both surfaces of the foam 11 may be sliced ​​surfaces formed by slicing, or one surface may be a sliced ​​surface and the other surface may be an unsliced ​​surface. Of course, the foam 11 may not be sliced, and both surfaces may be unsliced. However, it is preferable that at least one surface is an unsliced ​​surface. The unsliced ​​surface is the foam surface itself formed by foaming the foamable composition described above, and a thin film formed by the cell partitions is formed. This prevents the bubbles from being exposed and forming holes, and also facilitates a low surface roughness Ra. The thin film referred to here is sometimes referred to as a skin layer, as described above.

[0047] When a foam has a non-foamed surface layer or a foamed surface layer as a skin layer, as described above, the resin layer or foamable composition for forming the non-foamed surface layer or the foamed surface layer may be laminated onto a sheet-shaped foamable composition for forming the core layer by coextrusion or the like. Specifically, in the above step (1), the resin layer for forming the non-foamed surface layer or the foamed surface layer may be laminated onto a sheet-shaped foamable composition for forming the core layer by coextrusion. This configuration facilitates the production of a substrate having a skin layer and makes it easier to obtain a substrate with high adhesion between the skin layer and the core layer. Furthermore, since foaming in the foamed surface layer can be performed in step (3), the production process can be simplified. However, the non-foamed surface layer or the foamed surface layer may also be formed by bonding it to a pre-prepared foam for forming the core layer.

[0048] [Heat-generating Layer] The heat-generating layer 20 disposed on one surface 15A of the substrate 15 is not particularly limited as long as it is a conductive layer capable of generating heat by passing an electric current through it, but is preferably a metal layer formed from a metal. Examples of metals include metals or alloys such as silver, copper, gold, nickel, aluminum, and iron. Among these, it is preferable to use a metal with high ductility. The heat-generating layer 20 is highly ductile, and by maintaining its thickness below a certain level as described below, it can easily follow the deformation of the sheet heating element 10. Therefore, even if the sheet heating element 10 is formed into a predetermined shape by vacuum molding or the like, the heat-generating layer 20 is less likely to break, and can easily generate heat with low resistance and low voltage. Specific examples of highly ductile metals include silver, copper, and gold. Among these, silver is preferred from the standpoints of cost, ductility, durability, and the like.

[0049] The thickness T of the heat generating layer 20 is preferably 500 μm or less, more preferably 200 μm or less, even more preferably 100 μm or less, and even more preferably 50 μm or less. If the thickness T of the heat generating layer 20 is equal to or less than the above-mentioned upper limit, a portion of the heat generating layer 20 is embedded in the foam 11 during use, making it difficult to feel the unevenness of the heat generating electrode. Therefore, unevenness caused by the heat generating layer 20 is less likely to appear on the surface of the sheet heating element 10 on which the heat generating layer 20 is provided during use or assembly, resulting in a good feel. Furthermore, if the thickness of the heat generating layer is equal to or less than the above-mentioned upper limit, it will be significantly thinner than the substrate 11 and the skin 31, and the presence or absence of the heat generating layer will have a smaller effect on the overall thickness, making it difficult to feel the unevenness of the heat generating electrode. The thickness T of the heat generating layer 20 is not particularly limited, but from the viewpoint of maintaining the strength as a heat generating layer and ensuring conductivity even after molding, for example, it is preferably at least a certain value; specifically, it is preferably at least 0.05 μm, more preferably at least 0.1 μm, even more preferably at least 5 μm, and even more preferably at least 20 μm.

[0050] The heating layer 20 preferably comprises wiring. The planar shape of the heating layer 20 is not particularly limited. However, as shown in FIG. 3, for example, linear wiring extends in a serpentine pattern from one terminal 20E to the other terminal 20E so as to heat a certain area uniformly. In this case, the heating layer 20 has, for example, a plurality of parallel linear portions 20A, with one end or the other end of adjacent linear portions 20A, 20A alternately connected via connecting portions 20B along the parallel direction, and the other ends of the linear portions 20A, 20A on both sides become the respective terminals 20E, 20E. The terminals 20E, 20E may be wider than the portions other than the terminals 20E. The heating layer 20 generates heat when a voltage is applied between the terminals 20E, 20E.

[0051] In another aspect, the heating layer 20 may preferably have a planar shape other than the linear shape described above, such as a rectangle or square. In this case, more uniform heating is possible throughout the entire surface. In this case, it is desirable to place electrodes for conducting electricity on both ends of the planar heating layer. To ensure uniform heating of the heating layer, the heating layer and electrodes must have uniform resistance between the electrodes and within the surface. This allows the heating layer to heat uniformly when voltage is applied. In the case of a rectangular or square shape, the resistance between the electrodes can be made uniform by suppressing the in-plane variation in the resistance of the heating layer. On the other hand, in the case of an irregular shape such as a trapezoid, ellipse, or circle, it is necessary to make the power flowing through the heating layer uniform by inserting slits in the heating layer or inserting auxiliary electrodes between the electrodes. Furthermore, it is desirable for the resistance of the electrodes to be lower than that of the heating layer. This prevents the electrodes from generating more heat than the heating layer. Furthermore, when the surface shape is rectangular, square, or any of the irregular shapes described above, there are no steps between the heating layers on the surface 15A, eliminating unevenness in the heating electrodes.

[0052] As shown in Figures 1 and 2, the heat generating layer 20 preferably has a width W greater than the thickness T, for example, a rectangular cross section. The rectangular cross section of the heat generating layer 20 makes it easier to keep a portion of the heat generating layer 20 embedded inside the foam 11 during use or assembly. The width W should be sufficiently greater than the thickness T, and is, for example, 1 to 1000 mm, preferably 2 to 700 mm, and more preferably 3 to 200 mm. It is preferable that the portion of the heat generating layer 20 other than the terminals 20E has the above width W.

[0053] The heat-generating layer 20 is preferably formed from a metal nanoink, particularly a metal nanoink paste. Metal nanoink is an ink in which metal nanomaterials such as metal nanoparticles, metal nanowires, and metal nanorods are dispersed in a dispersion medium such as water or an organic solvent. If necessary, the ink may further contain a binder component, which is a resin component, or a dispersant. The resin component is not particularly limited, but a thermosetting resin is preferably used. The type of metal used in the metal nanoink is as described above, and a silver nanomaterial containing silver as the main component, such as silver nanoparticles or silver nanowires, is preferred. Therefore, the metal nanoink is preferably a silver nanoink, and more preferably a silver nanoink paste. Forming the heat-generating layer 20 from a metal nanoink facilitates low resistance and the ability to generate heat at low voltage. Generally, the foam 11 constituting the substrate 15 does not have high insulation resistance. However, the heat-generating layer 20's ability to generate heat at low voltage prevents electrical leakage from the substrate 15.

[0054] Metal nanoparticles are particles with a nanoscale particle size. The average particle size of the metal nanoparticles is preferably 1 nm or more and less than 1000 nm, more preferably 1 to 300 nm, and even more preferably 5 to 100 nm. The "average particle size" refers to the volume-average particle size (median diameter, D50), which can be measured, for example, with a laser particle size distribution analyzer. Commercially available metal nanoinks can also be used, such as those sold under the trade name "DOTITE" (manufactured by Fujikura Kasei Co., Ltd.), "Moldable Conductive Paste" (manufactured by Toyobo MC Co., Ltd.), and "Low-Temperature Curing Conductive Paste" (manufactured by Taiyo Ink Mfg. Co., Ltd.).

[0055] The heat-generating layer 20 may be formed by applying a metal nanoink to the substrate 15 using a known printing method such as screen printing or inkjet printing, followed by drying and, if necessary, heating. The metal nanoparticles may be subjected to pseudo-sintering or thermal aggregation on the substrate 15 to form a metal layer. The heating temperature after application of the metal nanoink is, for example, 80 to 150°C, preferably 100 to 140°C. Setting the heating temperature at or above the lower limit facilitates pseudo-sintering and thermal aggregation, making it easier to achieve low resistance and the ability to generate heat at low voltage. Setting the heating temperature at or below the upper limit prevents thermal degradation of the foam-containing substrate.

[0056] However, the heating layer 20 may be formed from materials other than metal nanoink. For example, it may be formed from a conductive material other than metal, such as a conductive polymer such as polythiophene-based conductive polymer (PEDOT / PSS). Using a conductive polymer can result in a more flexible heating layer and a pleasant tactile sensation. Furthermore, using a conductive polymer can result in a transparent heating layer, and using a light-transmitting foam as the substrate can result in a light-transmitting heater. The heating layer 20 may also be formed by forming a metal film by physical vapor deposition such as vacuum deposition or sputtering, or by plating. The heating layer 20 may also be formed by laminating a metal foil such as gold foil. The heating layer 20 may also be formed from carbon materials such as graphite, carbon nanotubes, and graphene, or carbon fiber fabric. In particular, using carbon nanomaterials such as carbon nanotubes and carbon nanoparticles can result in a more flexible and light-transmitting heating layer. Furthermore, the heating layer 20 may be formed from a conductive ink in which conductive carbon or metal particles are dispersed in a resin. The heat generating layer may be made of one material, or may be made of a mixture of multiple materials, or may be made of multiple materials laminated together.

[0057] As shown in Figure 1, the heat generating layer 20 is preferably disposed so as to be in direct contact with the surface of the foam 11. By laminating the heat generating layer 20 directly onto the surface of the foam 11, the heat generating layer 20 can be easily embedded inside the foam 11. Furthermore, since the sheet heating element 10 can be made with a simple configuration, the sheet heating element 10 can be manufactured with higher production efficiency.

[0058] However, the heat generating layer 20 does not need to be laminated directly on the surface of the foam 11, and may be formed so as to contact the surface of a member other than the foam 11. For example, if the above-mentioned underlayer 16 is formed on the surface 15A of the substrate 15, the heat generating layer 20 may be formed so as to contact the underlayer 16. Furthermore, if a non-foamed surface layer is provided as a skin layer, the heat generating layer 20 may be formed so as to contact the non-foamed surface layer. The line width size of the heat generating layer is not particularly limited, and the foam layer may be sized to cover, for example, the entire surface or almost the entire surface of the foam 11. The resistance value of the heat generating layer is not particularly limited, but is preferably less than 1 KΩ / sq, more preferably less than 500 Ω / sq, and particularly preferably less than 100 Ω / sq. When it is within this range, a heater of the same size can be heated at a lower voltage.

[0059] <Laminate> The sheet heating element 10 may be further laminated with other members to form a laminate. One embodiment of the laminate is shown in Figure 4. As shown in Figure 4, the laminate 30 preferably comprises the above-mentioned sheet heating element 10 and a skin 31 adhered to the sheet heating element 10. The skin 31 is adhered to the surface of the sheet heating element 10 on which the heat-generating layer 20 is provided. Therefore, the skin 31 can decorate the surface of the sheet heating element 10 on which the heat-generating layer 20 is provided, and the laminate 30 including the skin 31 is suitable for use as a vehicle interior material.

[0060] Examples of the surface 30 include resin sheets such as polyvinyl chloride sheets and sheets made of a mixed resin of polyvinyl chloride and ABS resin, thermoplastic elastomer sheets, woven fabrics, knitted fabrics, and nonwoven fabrics made from natural or artificial fibers, fake leathers such as artificial leather and synthetic leather, and metals. Also, genuine leather or leathers with a design such as leather grain or wood grain pattern on the surface, which is made using a silicone stamper or the like with indentations transferred from stone or wood, may be used.

[0061] The skin 31 may be directly adhered to the surface of the sheet heating element 10 on which the heat generating layer 20 is provided. As shown in FIG. 4 , the skin 31 may be laminated on the sheet heating element 10 with a portion adhered to the substrate 15 and a portion adhered to the heat generating layer 20. However, the skin 31 does not necessarily have to be directly adhered to the sheet heating element 10, and may be adhered to the surface of the sheet heating element 10 on which the heat generating layer 20 is provided via another layer (not shown), such as an adhesive layer. In this case, the skin 31 may be laminated on the sheet heating element 10 with a portion adhered to the substrate 15 via another layer, such as an adhesive layer, and a portion adhered to the heat generating layer 20 via another layer, such as an adhesive layer. Furthermore, the skin 31 does not necessarily have to be adhered to the heat generating layer 20.

[0062] 1 and 4, when the foam 11 constitutes the surface 15A of the substrate 15, the skin 31 may be adhered directly to the foam 11 or may be adhered to the foam 11 via another layer such as an adhesive layer. However, when the surface 15A of the substrate 15 is provided with a skin layer constituted by the base layer 16 (see FIG. 2) or a non-foamed surface layer (not shown), the skin 31 may be adhered directly to the base layer 16 or the non-foamed surface layer or may be adhered to the base layer 16 or the non-foamed surface layer via another layer such as an adhesive layer.

[0063] Methods for adhering the skin 30 to the planar heating element 10 include, for example, extrusion lamination, adhesive lamination in which an adhesive is applied and then the two are bonded together, thermal lamination (thermal fusion), hot melt, high-frequency welder, and in the case of metals, electroless plating, electrolytic plating, and vapor deposition, but any method of adhesion may be used.

[0064] In the laminate 30, it is preferable that at least a portion of the heat generating layer 20 is embedded inside the surface of the foam 11. In this case, it is preferable that the skin 30 is adhered to the sheet heating element 10 so as to press against the heat generating layer 20, as shown in FIG. 4 . With this configuration, it is possible to more appropriately prevent the surface of the laminate 30 on which the skin 30 is provided from having an uneven texture. However, the heat generating layer 20 does not need to be embedded in the foam 11 by being pressed by the skin 30; it may be pressed by another member, or may be embedded in the foam 11 without being pressed. For example, the heat generating layer 20 may be embedded in the foam 11 in the state of the sheet heating element 10 without the skin 30.

[0065] <Applications> The sheet heating element of the present invention is preferably used in vehicle interior materials, and more preferably as a laminate 30 provided with a surface skin 31. Sheet heating elements are particularly suitable for use in vehicle interior materials in the automotive field, and are preferably used to construct ceiling materials, doors, instrument panels, and the like. The sheet heating element and laminate of the present invention are used as heating devices in which the surface of the sheet heating element on which the heating layer is provided generates heat by passing an electric current through the heating layer as described above. Therefore, by applying the sheet heating element and laminate to vehicle interior materials, it is possible to heat the vehicle interior materials. In the sheet heating element 10, the heating layer 20 (i.e., the surface skin 31 in the laminate 30) is preferably disposed on the interior side, and the substrate 15 is preferably disposed on the opposite side from the interior side. The interior side refers to the side decorated by the vehicle interior material, i.e., the outer peripheral surface side of the body to which the interior material is attached.

[0066] The sheet heating element and laminate of the present invention are preferably formed and used in a desired shape. The sheet heating element and laminate are suitable for use as interior materials after being formed and shaped. When forming the sheet heating element and laminate, the heating layer 20 may also be formed together with the foam 11 and the skin 31. Methods for forming the sheet heating element and laminate include stamping molding, vacuum forming, compression molding, injection molding, and the like. Of these, stamping molding and vacuum forming are preferred. As the vacuum forming method, either male-draw vacuum forming or female-draw vacuum forming can be used, but male-draw vacuum forming is more preferred.

[0067] REFERENCE SIGNS LIST 10 Planar heating element 11 Foam 15 Base material 15A Surface 16 Underlayer 20 Heat generating layer 30 Laminate 31 Skin

Claims

1. A sheet heating element comprising a substrate containing a foam and a heating layer disposed on one surface of the substrate.

2. The sheet heating element according to claim 1, wherein the heat generating layer is disposed so as to be in direct contact with the foam.

3. The sheet heating element according to claim 1, wherein a base layer is formed on the surface of the substrate that contacts the heat generating layer.

4. The sheet heating element according to any one of claims 1 to 3, wherein the heat generating layer is formed by printing.

5. A sheet heating element according to any one of claims 1 to 3, wherein a skin layer is formed on at least one surface of the foam, and a heating layer is formed on the skin layer.

6. The sheet heating element according to any one of claims 1 to 3, wherein the foam is an olefin resin foam.

7. The sheet heating element according to claim 6, wherein the resin constituting the foam includes a polypropylene resin.

8. The sheet heating element according to any one of claims 1 to 3, wherein the foam is an electron beam crosslinked product.

9. The sheet heating element according to any one of claims 1 to 3, wherein the foam is a closed-cell foam.

10. A sheet heating element according to any one of claims 1 to 3, wherein the surface roughness Ra of the surface of the base material on which the heat generating layer is provided is 5 µm or less.

11. The sheet heating element according to any one of claims 1 to 3, wherein the heat generating layer has a thickness of 100 μm or less and is made of a conductive material.

12. A laminate comprising the sheet heating element according to claim 1 and a skin adhered to the sheet heating element directly or via another layer.

13. The laminate according to claim 12, wherein the skin is adhered to the surface of the sheet heating element on which the heat generating layer is provided.

14. A vehicle interior material comprising the sheet heating element according to claim 1 or the laminate according to claim 12.

15. The vehicle interior material according to claim 14, wherein the heat generating layer is disposed on the interior side and the substrate is disposed on the opposite side to the interior side.