JP7508602B2 - Sheet heating element and manufacturing method thereof - Google Patents

Description

The present invention relates to a planar heating element having a planar heating layer containing a conductive material, and a method for producing the same. The planar heating element of the present invention can be used as a heat source for, for example, electric carpets, floor heating equipment, wall heating equipment, heaters for melting snow on roads and roofs, seat heaters used in automobiles, etc., heaters for preventing fogging of mirrors, heaters used for heating and keeping warm pipelines and agricultural greenhouses, heaters used for heating batteries in electric automobiles, etc.

Conventionally, sheet heating elements have been developed for purposes such as floor heating and melting snow on roads. Patent Document 1 is an invention related to a sheet heating element obtained using an aqueous dispersion of fine carbon fibers. Patent Document 1 discloses a method for manufacturing a sheet heating element in which an aqueous dispersion of fine carbon fibers is applied to the surface of an insulating substrate, the aqueous dispersion is dried to form a sheet heating layer, and electrodes are further formed on the sheet heating layer (Patent Document 1 [Claim 21]). Patent Document 1 [Example 1] employs a method for manufacturing electrodes in which (1-1) silver paste is applied to the surface heating layer, a copper plate cut into a T shape is placed on top of the silver paste, and silver paste is applied again on top of the copper plate.

Patent Document 2 is an invention related to a heater having a heat generating layer which is a conductive metal oxide layer, and includes a substrate, a conductive metal oxide layer (heat generating layer), and a power supply electrode electrically connected to the metal oxide layer (Patent Document 2 [Claim 1]).
Regarding the formation of the power supply electrodes, Patent Document 1 [0044] discloses the following three methods.
(2-1) A method in which a metal film is formed on a main surface of a conductive metal oxide layer by a dry process such as chemical vapor deposition or physical vapor deposition, and then the thickness of the metal film is increased by a wet process such as plating, after which a masking film is placed, unnecessary metal film is removed by etching, and then the masking film is removed.
(2-2) A method in which a metal film is formed on the main surface of a conductive metal oxide layer by a dry process such as chemical vapor deposition or physical vapor deposition, a masking film is then placed and the thickness of the metal film is increased by a wet process such as plating, the masking film is then removed, and the portion that was covered by the masking film is removed by etching.
(2-3) A method in which a conductive ink is applied in a predetermined pattern onto the main surface of a conductive metal oxide layer, and the applied conductive ink is cured.

Patent Document 3 is an invention related to a flexible sheet-like heating element, and discloses a heating element in which an insulating substrate, a nanocarbon material layer (heating layer), and an electrode layer are laminated (Patent Document 3 [Claim 1]). Patent Document 3 discloses a manufacturing method for a heating element, including a step of i) applying a dispersion liquid containing a nanocarbon material and a surfactant to the surface of an insulating substrate and drying the dispersion liquid to form the nanocarbon material layer (heating layer), and a step of ii) forming a plurality of electrode layers on a part of the surface of the nanocarbon material layer (heating layer) (Patent Document 3 [Claim 14]). Furthermore, Patent Document 3 discloses a method of ii) forming an electrode layer on a carbon material layer, including a method of (3-1) pressing a rolled metal foil onto a part of the surface of the nanocarbon material layer where an electrode is to be formed, (3-2) plating the part with a metal, and (3-3) applying a metal paste to the part by screen printing and drying (Patent Document 3 [0055]).

WO2009/081986 JP 2020-47370 A JP 2020-47519 A

When electrodes are formed by the methods disclosed in the above-mentioned inventions, the following problems arise.
The methods (1-1), (2-3), and (3-3) using metal paste or conductive ink cannot efficiently supply power to the heating layer because the resistance of the metal paste or conductive ink is high. In addition, the silver paste is expensive, so the manufacturing cost of the electrode is high. Furthermore, the methods disclosed in (2-1), (2-2), and (3-2) require chemical vapor deposition, physical vapor deposition, plating, etching, etc., all of which require expensive manufacturing equipment and are also complicated manufacturing processes. Furthermore, the method (3-1) of bonding rolled metal foil with pressure has a problem that the adhesive strength between the heating layer and the metal foil is very weak. In order to increase the adhesive strength, the method (3-1-1) of using a conductive adhesive and the method (3-1-2) of heating during compression can be considered, but the use of the conductive adhesive (3-1-1) has a problem that the electrical resistance value increases and the heating efficiency decreases. Furthermore, the method (3-1-2) of heating during compression has a problem that the heating layer and the insulating base material are thermally deformed and the thickness becomes uneven.

The objective of the present invention is to extremely simply form electrodes that can efficiently supply power to a heat generating layer. It is also an object of the present invention to provide a sheet heating element that has good heat generation efficiency and low manufacturing costs.

In order to increase the heat generation efficiency, the inventors decided to use metal foil with low electrical resistance as the material for the electrodes. They then discovered a method for fixing an electrode made of metal foil onto a heat generation layer, in which the electrode is fixed onto an insulating substrate and then laminated with the heat generation layer, rather than directly onto the heat generation layer. Specifically, they discovered a method in which an adhesive layer is provided on one side of the electrode, the electrode is attached to the insulating substrate in advance by the adhesive layer, and then a heat generation layer is formed on the insulating substrate, leading to the present invention.

That is, according to the present invention, the following sheet heating element is provided.
[1] A sheet-shaped heating element comprising an insulating base material, a heating layer containing a conductive material, and an electrode for supplying power to the heating layer, wherein the electrode is made of metal foil, an adhesive layer is laminated on one side of the electrode, and the electrode and the adhesive layer are disposed between the insulating base material and the heating layer such that the adhesive layer is in contact with the insulating base material and the heating layer is in contact with the metal foil.
[2] The sheet heating element according to [1], characterized in that the conductive material is carbon nanotubes and/or graphene.
[3] The sheet heating element according to [1] or [2], wherein the thickness of the adhesive layer is 1 to 30 μm.
[4] A sheet heating element as described in any one of [1] to [3], characterized in that a cover film made of an insulating material is provided on the surface of the heat generating layer opposite the insulating substrate.

The following method for producing a sheet heating element is also provided.
[5] A method for manufacturing a sheet heating element as described in any one of [1] to [4], characterized in that it comprises, in order, a first step of laminating an electrode and an adhesive layer to produce an electrode/adhesive layer laminate, a second step of attaching the electrode to an insulating substrate using the adhesive layer, and a third step of forming a heating layer on the surface of the insulating substrate on which the electrode is laminated.
[6] The method for manufacturing a sheet-shaped heating element described in [5], characterized in that in the first step, the method for laminating the electrode and the adhesive layer is a method of peeling off one of the release films of a laminated film in which the adhesive layer is sandwiched between two release films, and laminating it with the electrode.
[7] The method for manufacturing a sheet heating element according to [5] or [6], characterized in that in the third step, the method for forming the heating layer is a method for printing a dispersion of a conductive material.

In the sheet heating element of the present invention, the electrodes made of metal foil are in direct contact with the heating layer, so that the heating layer can be efficiently supplied with power. In addition, the electrodes are in contact with the insulating substrate via the adhesive layer, so that the electrodes can be arranged at desired positions very easily.
Furthermore, when the conductive material added to the heating layer is carbon nanotubes or graphene, which have excellent electrical heat conductivity, high surface heating performance can be obtained. Furthermore, carbon nanotubes and graphene also have excellent electromagnetic wave absorption performance and far-infrared radiation performance, so these functions can be added to the sheet heating element. By using a sheet heating element with excellent far-infrared radiation performance, the growing period of agricultural crops can be shortened.
Furthermore, when the thickness of the adhesive layer is 2 to 20 μm, the occurrence of cracks in the heat generating layer due to the thickness of the adhesive layer can be suppressed.
Furthermore, if the sheet heating element is provided with a cover film made of an insulating material on the surface of the heating layer opposite the insulating substrate, deterioration of the heating layer can be suppressed.

According to the manufacturing method of the present invention, a sheet heating element with excellent heat generation efficiency can be manufactured extremely simply.
Furthermore, when the adhesive layer of the sheet heating element of the present invention is formed using a laminate film in which the adhesive layer is sandwiched between two release films, a thin adhesive layer with excellent thickness uniformity can be provided on the back surface of the electrode.
Furthermore, if the heat generating layer is formed on the electrodes by printing a dispersion liquid containing a conductive material, the sheet heating element can be easily manufactured.

1A is a plan view of a sheet heating element of the present invention, (B) is a schematic cross-sectional view of the sheet heating element along the a-a' line, and (C) is an enlarged partial cross-sectional view. 1 is a schematic cross-sectional view of a sheet heating element of the present invention. FIG. 2 is a plan view showing an example of a state in which an electrode is attached onto an insulating base material. 4 is a graph showing the relationship between the application time and the surface temperature of the sheet heating elements in Examples 1 and 2. 2 is a SEM photograph of a cross section of the sheet heating element of Example 1.

The following describes an embodiment of the present invention. The scope of the present invention is not limited to these descriptions, and other than the following examples, the present invention can be modified and implemented as appropriate without departing from the spirit of the present invention.

1A is a plan view showing one embodiment of the sheet heating element of the present invention, (B) is a schematic cross-sectional view of the sheet heating element along the line aa', and (C) is an enlarged cross-sectional view of the sheet heating element. Each drawing is made for ease of understanding, and thicknesses and the like differ from the actual ones.
The sheet heating element 1 shown in FIG. 1 has an insulating substrate 11, an electrode 12, a heating layer 13, and an adhesive layer 14 as main components.

[Insulating substrate]
The insulating substrate used in the sheet heating element of the present invention is a sheet-like insulating substrate having a strength sufficient for use. For example, an inorganic film such as a glass film or an organic film such as a thermoplastic resin film can be used as the insulating substrate. When the insulating substrate is a thermoplastic resin film, it is excellent in transparency, flexibility, processability, etc.

The thermoplastic resin film in the present invention is a general term for films that melt or soften with heat, and is not particularly limited. Representative examples include polyester films such as polyethylene terephthalate and polybutylene terephthalate, polyolefin films such as polypropylene and polyethylene films, polylactic acid films, polycarbonate films, acrylic films such as polymethyl methacrylate and polystyrene films, polyamide films such as nylon, polyvinyl chloride films, polyurethane films, fluorine-based films, polyphenylene sulfide films, polyimide films, and polyamideimide films. In consideration of heat resistance, it is preferable to use polyester films such as polyethylene terephthalate and polybutylene terephthalate, fluorine-based films, and polyphenylene sulfide films, while in consideration of ease of availability, variety, price, and the like, polyester films such as polyethylene terephthalate and polybutylene terephthalate, and especially polyethylene terephthalate films, are preferred.

The thickness of the insulating base material is not particularly limited as long as it is within a range that provides flexibility according to the application, but it is preferably in the range of, for example, 2.5 μm to 10 mm. In consideration of flexibility and strength, it is preferably 10 to 500 μm, and more preferably 100 to 300 μm.
In addition, various additives such as antioxidants, heat stabilizers, weather stabilizers, ultraviolet absorbers, organic lubricants, pigments, dyes, organic or inorganic fine particles, fillers, antistatic agents, nucleating agents, etc. may be added to this thermoplastic resin film to the extent that they do not deteriorate its properties.

[electrode]
The electrode of the present invention is made of a metal foil. The type of metal foil is not particularly limited, and for example, gold foil, silver foil, copper foil, aluminum foil, tin foil, platinum foil, etc. can be used. In view of heat generation efficiency, it is preferable to use silver foil or copper foil. Furthermore, aluminum foil is particularly suitable for the electrode of the present invention because it is inexpensive and has a relatively low electrical resistance.
The thickness of the metal foil used as the electrode in the present invention is preferably 5 to 1000 μm, more preferably 10 to 200 μm, and even more preferably 20 to 100 μm. If the metal foil is thinner than the above-mentioned thickness, it becomes difficult to supply sufficient power to the heat generating layer. Furthermore, if the thickness is thicker than the above-mentioned thickness, there is a risk that the heat generating layer will crack at the step portion (X in FIG. 1) when the heat generating layer is provided on the electrode.

[Heat generating layer]
The heat generating layer is a layer that generates heat by electric power and contains a conductive material such as metal or carbon. In applications where transparency is not required, it is desirable to use a carbon-based material as the conductive material from the viewpoints of economy and heat generation efficiency. In particular, carbon black, carbon nanotubes (CNT), carbon nanofibers, fullerenes, graphene, or derivatives thereof, or mixtures thereof can be used. From the viewpoint of realizing high electrical conductivity, it is preferable to use carbon nanotubes and/or graphene.

Carbon nanotubes suitable for use in the heat generating layer include single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, and mixtures thereof. The average diameter of the carbon nanotubes may be about 1 nm to 1 μm, and preferably 1 nm to 50 nm. The average length may be about 100 nm to 10 mm, and preferably 500 nm to 100 μm, and more preferably 500 nm to 10 μm.
The graphene (including graphene oxide) preferably used in the heat generating layer may have a ratio of the average length to the average width of the sheet structure of 1:0.1 to 1:10, preferably 1:0.5 to 1:5. The average length of the sheet structure may be 100 nm to 1000 μm, preferably 200 nm to 50 μm.

The surface temperature of the sheet heating element can be controlled by adjusting the type and amount of conductive material, the amount of power supplied to the heating layer, etc. Changing the width and thickness of the electrodes and the wiring pitch of the electrodes are also effective ways of adjusting the amount of power supplied to the heating layer.

If necessary, an adhesive aid such as diethylene glycol can be added to the heat generating layer of the present invention. The adhesive aid contributes to improving the adhesion between the base film and the heat generating layer. In addition, when the heat generating layer is formed by printing as described below, it is desirable to adjust the viscosity of the layer using a thickener or the like.
Furthermore, the heat generating layer of the present invention may appropriately contain known additives such as plasticizers, dispersants, coating surface conditioners, flowability conditioners, ultraviolet absorbers, storage stabilizers, thermoplastic polymers, slip agents, leveling agents, ultraviolet absorbers, polymerization inhibitors, antistatic agents, inorganic fillers, organic fillers, and inorganic fillers that have been subjected to a surface organification treatment.

[Adhesive layer]
The adhesive constituting the adhesive layer is not particularly limited as long as it can fix an electrode made of a metal foil on an insulating substrate. For example, an acrylic adhesive containing an acrylic resin having a glass transition temperature (Tg) of 0° C. or less and a crosslinking agent, the acrylic resin being obtained by radically polymerizing an acrylic monomer composition containing a (meth)acrylic acid ester as a main component and a small amount of a (meth)acrylic monomer having a functional group in the presence of a polymerization initiator.
The thickness of the adhesive layer is not particularly limited, but is preferably 1 to 30 μm, more preferably 2 to 20 μm, and even more preferably 3 to 8 μm. If the adhesive layer is too thick, the heat generating layer may crack due to the thickness of the adhesive layer, specifically at the step X in FIG. 1. If the adhesive layer is too thin, it is difficult to sufficiently fix the electrode to the insulating substrate.

[Surface heating element]
As shown in Fig. 1, the sheet heating element of the present invention sequentially comprises an insulating substrate 11, an adhesive layer 14, an electrode 12, and a heating layer 13. It is preferable that the size of the electrode 12 and the size of the adhesive layer 14 are roughly the same in a plan view. If the adhesive layer 14 is small compared to the electrode 12, wrinkles or the like may occur in the electrode 12, and if the adhesive layer 14 is large compared to the electrode 12, it may cause foreign matter to adhere.
2, the sheet heating element of the present invention may be provided with a cover film 25 on the surface of the heating layer 23 opposite the insulating substrate 21. The cover film 25 may be one of the insulating substrates mentioned above. The cover film 25 is attached to cover the heating layer 23, for example, with an adhesive 26. The cover film 25 prevents the heating layer 23 from cracking or peeling off.

[Method of manufacturing sheet heating element]
The present invention also proposes a method for producing a sheet heating element, which includes a first step of laminating an electrode and an adhesive layer to produce an electrode/adhesive layer laminate, a second step of bonding the electrode to an insulating substrate with the adhesive layer, and a third step of forming a heating layer on the surface of the insulating substrate on which the electrode is laminated.

[First step]
In the first step, an electrode made of a metal foil and an adhesive layer are laminated. This can be done by applying an adhesive to the metal foil. However, in the present invention, a method is proposed in which an adhesive layer is first formed on a release film, and then the adhesive layer is transferred to the electrode. With this method, even a very thin adhesive layer can be easily laminated on the back surface of the electrode.

When the adhesive is the acrylic adhesive described above, the adhesive is diluted with a solvent to form an acrylic adhesive solution, which is then coated onto a release film and heated at 60-120°C for 0.5-10 minutes to evaporate the organic solvent to form an adhesive layer. After laminating the electrode onto this adhesive layer, it is desirable to age the layer for 5-20 days in an atmosphere at a temperature of 23°C and a humidity of 65%, for example, to allow the crosslinking agent (C) to react sufficiently. The release film should be removed before proceeding to the second step described below.

It is also possible to use a laminated film in which the adhesive layer is sandwiched between two release films by coating an acrylic adhesive solution on a release film and then laminating the release film. It is also desirable to mature the laminate for about 5 to 20 days in an atmosphere of, for example, 23°C and 65% humidity to allow the crosslinking agent to react sufficiently. The release film on one side of the laminated film is peeled off to laminate the adhesive layer on the electrode, and the other release film is peeled off before moving to the second step. In addition, commercially available "non-carrier adhesive film/sheet" (manufactured by Lintec Corporation, Nitto Denko Corporation, etc.) can also be used as the laminated film. A laminated film in which the adhesive layer is sandwiched between two release films is particularly suitable for use because it has excellent processability.

[Second step]
In the second step, the electrode is attached to the insulating substrate by the adhesive layer laminated in the first step. Fig. 3 is a plan view showing an example of a state in which an electrode 32 is attached to an insulating substrate 31. Although it is a plan view, the electrode portion is hatched so that it is easy to see.

Prior to lamination, the electrode/adhesive layer laminate is trimmed into a desired shape. For example, the electrode/adhesive layer may be cut into a tape shape. The tape-shaped electrode/adhesive layer laminate may be arranged in the shape of the electrode 32 shown in FIG. 3. However, this operation is very cumbersome. In addition, since it is necessary to connect the electrodes at the intersection Y, there is a risk that the electrical resistance will be high at the Y portion. Therefore, in the present invention, a method is proposed in which a sheet-shaped electrode/adhesive layer laminate is manufactured and punched into the shape of the electrode. According to this method, no seam is formed at the intersection Y of the electrodes, and an increase in the electrical resistance value can be suppressed.
In the first step, if a method is adopted in which the adhesive layer is once formed on a release film and then the adhesive layer is transferred to the electrode, a laminate of electrode/adhesive layer/release film can be obtained. Thus, the electrode/adhesive layer laminate can be punched into a desired shape with the release film attached, improving the handleability of the laminate during the punching operation.

After the electrode/adhesive layer laminate is adjusted to the desired shape, the release film is removed as necessary and the laminate is bonded to an insulating substrate. For example, a sheet-fed bonding machine HAL-TEC manufactured by Sankyo Co., Ltd. can be used for bonding.

[Third step]
In the third step, a heat generating layer is formed on the surface of the insulating substrate on which the electrodes are laminated. As long as the conductive material such as metal or carbon can be laminated without unevenness in thickness, the method is not particularly limited. For example, the conductive material, surfactant, polymer compound, dispersant, etc. may be diluted with a solvent such as water or a solvent, and applied to the surface of the insulating substrate on which the electrodes are laminated by roll-to-roll printing or screen printing. After application, the solvent is dried and removed to complete the heat generating layer.
Finally, a cover film is laminated as necessary. For example, an adhesive may be used for laminating the cover film. Alternatively, an insulating film having an adhesive layer on the back side may be used.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these.

[Example 1]
<First step>
First, an aluminum foil having a width of 500 mm, a length of 500 mm, and a thickness of 30 μm, and a non-carrier adhesive film having a width of 500 mm, a length of 500 mm, and a thickness of 45 μm (a laminate of release film/acrylic adhesive layer/release film, adhesive layer thickness 5 μm) were prepared. Next, one release film of the non-carrier adhesive film was peeled off, and the film was laminated with the aluminum foil to obtain a laminate of electrode (aluminum foil)/adhesive layer/release film.

<Second step>
Prior to the second step, the electrode/adhesive layer/release film laminate was punched out into the shape shown in FIG. 3. A Thomson cutter was used for punching. A polyethylene terephthalate film with a width of 550 mm, a length of 550 mm, and a thickness of 188 μm was prepared as an insulating substrate. Next, the release film was peeled off from the electrode/adhesive layer/release film laminate, and this was attached to the insulating substrate.

<Third process>
An aqueous dispersion of graphene (graphene concentration: 20 wt%) was applied by screen printing to the surface of the insulating substrate on which the electrodes were laminated. The amount of application was 46 g/ ^m2 . The water in the dispersion was dried and removed to obtain a sheet heating element of the present invention.

[Example 2]
In Example 2, a planar heating element was obtained in the same manner as in Example 1, except that an aqueous dispersion of carbon nanotubes (concentration of carbon nanotubes: 7% by weight) was used instead of the aqueous dispersion of graphene (concentration of graphene: 20% by weight).

A voltage of 15 V was applied to the electrodes of the sheet heating elements obtained in Examples 1 and 2, and the surface temperature of the heating layer was measured. The relationship between the application time and the surface temperature is shown in FIG.
5 shows an SEM photograph of a cross section of the sheet heating element of Example 1. The heating layer 43 covering the electrodes 42 was in close contact with the electrodes 42 even at the edge portions of the electrodes 42.

Reference Signs List 1, 2 Planar heating element 11, 21, 31, 41 Insulating substrate 12, 22, 32, 42 Electrode 13, 23, 43 Heating layer 14, 24, 44 Adhesive layer 25 Cover film

Claims (5)

A method for manufacturing a sheet heating element including an insulating substrate, a heat generating layer including a conductive material, and an electrode for supplying power to the heat generating layer, comprising the steps of:
The electrode is made of a metal foil, and an adhesive layer is laminated on one surface of the electrode;
the electrode and the adhesive layer are disposed between the insulating base material and the heat generating layer such that the adhesive layer is in contact with the insulating base material and the heat generating layer is in contact with the metal foil ;
a first step of peeling off one of the release films of a laminate film in which an adhesive layer is sandwiched between two release films, thereby laminating the electrode and the adhesive layer, thereby producing an electrode/adhesive layer laminate;
A second step of attaching the electrode to the insulating substrate with the adhesive layer;
a third step of forming the heat generating layer on the surface of the insulating base material on which the electrodes are laminated . 2. The method for producing a sheet heating element according to claim 1, wherein the conductive material is carbon nanotubes and/or graphene. 3. The method for producing a sheet heating element according to claim 1, wherein the adhesive layer has a thickness of 1 to 30 μm. 4. The method for manufacturing a sheet heating element according to claim 1, further comprising providing a cover film made of an insulating substrate on a surface of the heat generating layer opposite to the insulating substrate. 5. The method for producing a sheet heating element according to claim 1, wherein in the third step, the heating layer is formed by printing a dispersion of a conductive material.

Images