Planar heating element

Abstract

The technical problem of the present disclosure is to provide a planar heating element in which the resistance value is easy to increase in a short time after reaching a certain temperature, and the resistance value is high after the temperature is increased. To solve the technical problem, the planar heating element comprises: a substrate; a pair of electrodes disposed on the substrate; and a PTC resistor body disposed between the pair of electrodes on the substrate, the PTC resistor body containing conductive particles, a binder resin, and a wax, the wax containing a wax of natural origin and / or a polyolefin wax, and in a DSC curve obtained by raising the polyolefin wax from 25 DEG C to 150 DEG C at a temperature raising rate of 10 DEG C / min using a differential scanning calorimeter, the coefficient of thermal conductivity of the PTC resistor body is greater than that of the conductive particles, and the coefficient of thermal conductivity of the PTC resistor body is greater than that of the PTC resistor body in a DSC curve obtained by raising the polyolefin wax from 25 DEG C to 150 DEG C at a temperature raising rate of 10 DEG C / min. The number of observed melting peaks is 1 or 2.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a planar heating element. BACKGROUND

[0002] In order to remove frost and dew attached to a door mirror of an automobile or the like, a heater is sometimes installed on the back of the mirror. In such a heater, a planar heating element having a PTC (Positive Temperature Cofficient) characteristic without requiring an expensive temperature control device is widely used.

[0003] A planar heating element having a PTC characteristic generally includes a pair of electrodes and a resistance body having a PTC characteristic (also referred to as a "PTC resistance body" in the present specification) provided between the electrodes. As the PTC resistance body, a resistance body containing a crystalline resin and electrically conductive particles is known (for example, Patent Literature 1). In a planar heating element having such a PTC resistance body, when a voltage is applied between the electrodes, current is conducted in the PTC resistance body, and the temperature thereof rises. On the other hand, as the temperature of the planar heating element (PTC resistance body) rises, the crystalline resin contained therein thermally expands, the distance between the electrically conductive particles expands, and the resistance value rises. Then, the resistance value sharply rises near the softening temperature or melting point of the crystalline resin, and current becomes difficult to conduct. That is, the planar heating element is controlled so that the temperature does not reach a certain temperature or more.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-227081 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] However, in the conventionally known planar heating element containing a PTC resistance body using a crystalline resin as a binder, thermal expansion at the time of temperature rise is still insufficient, and there is a technical problem that even if the temperature reaches a certain temperature, an increase in the resistance value takes time. In addition, it is further required to increase the resistance value of the planar heating element after temperature rise.

[0009] Therefore, an object of the present disclosure is to provide a planar heating element in which the resistance value easily rises in a short time after reaching a certain temperature, and the resistance value after temperature rise is high.

[0010] MEANS OF SOLVING THE PROBLEM

[0011] One embodiment of the present disclosure provides a planar heating element including: a substrate; a pair of electrodes disposed on the substrate; and a PTC resistor disposed between the pair of electrodes on the substrate, the PTC resistor containing electrically conductive particles, a binder resin, and a wax, the wax containing a naturally derived wax and / or a polyolefin-based wax, the number of melting peaks observed in a DSC curve obtained by using a differential scanning calorimeter to raise the polyolefin-based wax from 25°C to 150°C at a temperature raising rate of 10°C / minute is 1 or 2.

[0012] Effects of Invention

[0013] According to the present disclosure, a planar heating element that easily increases in resistance value in a short time after reaching a certain temperature and has a high resistance value after temperature rise can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a plan view showing an example of the structure of the planar heating element of the present disclosure.

[0015] Figure 2 A is a DSC (Differential Scanning Calorimaetry) curve of the polyolefin-based wax used in Example A-1 and Example A-2, Figure 2 B is a DSC curve of the ultra-high molecular weight polyethylene used in Comparative Example A-2 and Comparative Example A-3. DETAILED DESCRIPTION

[0016] In the present specification, a numerical range indicated by "to" means a numerical range including the numerical values described before and after "to".

[0017] Next, the structure of the planar heating element 100 of the present disclosure will be described. As shown in the plan view of Figure 1 The planar heating element 100 has, for example, a substrate 1, a pair of electrodes 21, 22 disposed on the substrate 1, and a PTC resistor 3 disposed between the pair of electrodes 21, 22. However, the structure of the planar heating element 100 is not limited to this structure, and can have, for example, an electrically conductive coating film (not shown) disposed on the electrode 2, and the like.

[0018] The substrate 1 is not particularly limited as long as it has insulating properties and can laminate the electrode 21, 22 and the PTC resistor 3, and can be appropriately selected according to the use of the planar heating element 100. Specific examples of the substrate 1 include a resin-made film such as a polyester film, and the like.

[0019] The pair of electrodes 21, 22 can be configured to include main electrodes 21a, 22a and comb-tooth electrodes 21b, 22b. The electrodes 21, 22 can be composed of a material capable of conducting electricity. The electrodes 21, 22 are usually composed of metal, for example, can be composed of aluminum or the like.

[0020] The method of producing the electrodes 21, 22 is not particularly limited, for example, an aluminum foil or the like formed in a pattern can be attached to the substrate 1, or a metal layer disposed on the substrate 1 can be punched by a die or the like to be patterned.

[0021] Further, in this planar heat generating body 100, the thickness of the PTC resistor 3 is preferably 10 μm or more, and more preferably 20 to 80 μm. If the thickness of the PTC resistor 3 is 10 μm or more, a sufficient amount of heat can be obtained. On the other hand, if the thickness is 80 μm or less, the thickness of the planar heat generating body 100 can be made thin, and it is more easily applicable to various uses. As for the PTC resistor 3, it will be described in detail later.

[0022] In use of the planar heat generating body 100, a terminal 4 connected to each of the electrodes 21, 22 is disposed on the substrate 1, and the terminal 4 is connected to an external electrode. Then, by applying a voltage between the electrodes 21, 22, the temperature of the PTC resistor 3 can be raised to a desired temperature.

[0023] The planar heat generating body 100 described above can be used for a variety of uses. For example, it can be used for a heater for an automobile door mirror defogging heater, an automobile anti-collision device in-vehicle camera defogging heater, an automobile anti-collision device in-vehicle millimeter wave radar defogging heater, and the like, an antenna cover, a thermistor, a pipe freeze prevention heater, a floor heating, a chair heating sheet, an armrest heating sheet, and the like.

[0024] Hereinafter, the PTC resistor 3 possessed by the planar heat generating body 100 will be described in detail.

[0025] (Regarding the PTC resistor)

[0026] As described above, the PTC resistor of the conventional planar heat generating body mainly imparts the PTC characteristic by utilizing thermal expansion and recrystallization of a crystalline resin. However, this method has a low rising amplitude of the resistance value after reaching a certain temperature, and the rising amplitude of the resistance value is also small. In view of this, the PTC resistor 3 of the planar heat generating body 100 of the present disclosure contains electrically conductive particles and a binder resin, and a specific polyolefin-based wax or a wax of natural origin. In this PTC resistor 3, these waxes melt near their melting points when the temperature rises. The volume increase due to the melting of these waxes is significantly larger than the volume increase of general resins. Furthermore, since these waxes easily melt, the resistance value increases significantly in a short time. Therefore, the planar heat generating body of the present disclosure has a larger change in the resistance value at the required temperature than the conventional planar heat generating body, and has a higher resistance value after the temperature rises.

[0027] Hereinafter, the case where the above-described PTC resistor contains a polyolefin-based wax (first mode) and the case where it contains a wax of natural origin (second mode) are described.

[0028] (1) PTC resistor of the first mode

[0029] The PTC resistor of the first mode contains a polyolefin-based wax, a binder resin, and electrically conductive particles, the number of melting peaks observed on a DSC curve when the DSC curve is plotted using a differential scanning calorimeter (hereinafter referred to as "DSC") at a temperature rising rate of 10°C / min from 25°C to 150°C is one or two. Hereinafter, each component is described.

[0030] • Polyolefin-based wax

[0031] In the present specification, a wax refers to an organic substance that is in a solid or semi-solid state at ordinary temperature, and melts without decomposing when heated. Furthermore, a polyolefin-based wax refers to a wax containing 50% by mass or more of an olefin-derived component. The polyolefin-based wax used in the present mode has one or two melting peaks observed on a DSC curve when the DSC curve is plotted using a DSC at a temperature rising rate of 10°C / min from 25°C to 150°C. The number of observed melting peaks is preferably one. In the present specification, a melting peak in the DSC curve refers to an endothermic peak having an absolute value of peak height of 0.5 mW or more, excluding noise having a height of less than 0.5 mW. Furthermore, the DSC curve in the present specification is a value obtained using a 5 mg sample.

[0032] In the present specification, the peak height is set to a value obtained as follows. First, in the DSC curve, the position where the line descends and the position where the line rises in the region protruding downward (endothermic reaction region) are confirmed. If both can be confirmed, they are connected with a line and set as a baseline. It should be noted that, in the present specification, the baseline is plotted only for a large fluctuation region where the rise and descent exceed 1 mW. For example, as shown in FIG. 1, the baseline is not plotted for the region where the rise and descent are less than 1 mW. Figure 2B, when a case where a small peak (B, C, and D) of 1 mW or less is included in a large fluctuation region (a region bulging downward), a baseline is drawn only for the large fluctuation region. Further, as shown in Figure 2 B, when a case where two or more large fluctuation regions (regions bulging downward) exist, a baseline is drawn for each region (two in B). On the other hand, as shown in Figure 2 A, when a case where either of the line rising position and the line falling position of a region bulging downward cannot be confirmed, a baseline is drawn linearly along the edge of the rising position or the falling position that can be confirmed. Figure 2

[0033] Then, a line perpendicular to the X axis is drawn from each peak top of the melting peaks of the DSC curve (A in A, B, C, D, and E in B), and the distance from the intersection of the perpendicular line and the baseline to the peak top is taken as the height of the respective peak. Figure 2 Figure 2

[0034] The absolute value of the height of the melting peak in the DSC curve obtained according to the above-described method is preferably 5 mW or more. If the absolute value of the height of the melting peak is 5 mW or more, many polyolefin-based waxes are likely to melt at the melting peak temperature. Note that, when two melting peaks exist in the DSC curve, the absolute value of the height of either of them can be 5 mW or more.

[0035] Further, the melting peak in the DSC curve of the above-described polyolefin-based wax is preferably in a range of 80°C or higher and 150°C or lower, more preferably in a range of 90°C or higher and 140°C or lower, and further preferably in a range of 100°C or higher and 130°C or lower. By this, a PTC resistor that shows an increase in resistance value at 80°C or higher and 150°C or lower can be obtained.

[0036] The weight average molecular weight of the above-described polyolefin-based wax is preferably 1,000 or more and 1,500,000 or less, and more preferably 3,000 or more and 20,000 or less. When the weight average molecular weight of the polyolefin-based wax is in this range, it is likely to have a melting peak in the above-described temperature range. The above-described weight average molecular weight is a polystyrene conversion value measured by gel permeation chromatography (GPC).

[0037] ​​​The PTC resistor can contain only one kind of the above polyolefin wax, or two or more kinds. Examples of the polyolefin wax include homopolymers of olefins, copolymers of two or more kinds of olefins, copolymers of olefins and non-olefin monomers, and the like. More specific examples of the polyolefin wax include low-density polyethylene wax, medium-density polyethylene wax, high-density polyethylene wax, and the like; polypropylene wax; polybutene wax; ethylene-propylene copolymer wax; ethylene-propylene-butene copolymer wax; and the like. In addition, it can also be an oxidized polyolefin wax obtained by oxidizing these by a known method. Among these, from the viewpoint of easy availability and PTC characteristics and the like, a polyethylene wax is preferable.

[0038] In the PTC resistor, the amount of the polyolefin wax is preferably 6% by mass or more and 24% by mass or less, and more preferably 12% by mass or more and 20% by mass or less. When the amount of the polyolefin wax in the PTC resistor is 6% by mass or more, the PTC ratio of the PTC resistor is further increased, and the rate of increase in the resistance value after the PTC resistor is warmed up and reaches a certain temperature is also more easily increased. On the other hand, when it is 24% by mass or less, the amount of the conductive particles is sufficient, and the PTC characteristics are more easily made good.

[0039] • Binder resin

[0040] The binder resin can be any resin that binds the above polyolefin wax or the conductive particles described later, and binds these to a substrate or the like, but it is preferable that the resin does not interfere with the volume change caused by melting of the above polyolefin wax. In addition, the binder resin itself can be a resin that expands by being warmed, thereby contributing to the increase in the resistance value of the PTC resistor. The PTC resistor can contain only one kind of the binder resin, or two or more kinds.

[0041] Examples of the binder resin include known thermoplastic resins. Specific examples include thermoplastic polyurethane resin, polyester resin, polyacrylate resin, polysiloxane resin, halogenated vinyl resin, vinyl resin, polyimide resin, phenoxy resin, polyether resin, polyketone resin, polyvinyl butyral resin, polyvinyl pyrrolidone resin, polyacrylate resin, SEBS resin (styrene-ethylene-butylene-styrene copolymer) and hydrogenated products thereof, SEPS resin (styrene-ethylene-propylene-styrene copolymer) and hydrogenated products thereof, and the like. Among these, from the viewpoint of easy deformation and non-interference with the volume change of the above polyolefin wax, SEBS resin and hydrogenated SEPS resin are more preferable.

[0042] In the PTC resistor, the amount of the binder resin is preferably 6 mass% or more and 30 mass% or less, and more preferably 15 mass% or more and 28 mass% or less. When the amount of the binder resin in the PTC resistor is 6 mass% or more, the composition including the conductive resin, the polyolefin-based wax, and the binder resin is more easily printed, and the fixability after printing is also better. In addition, the strength of the PTC resistor is also more easily improved. On the other hand, when the amount of the binder resin is 30 mass% or less, the amount of the conductive particles is sufficient, and the PTC characteristics are more easily made good.

[0043] • Conductive particles

[0044] The conductive particles are not particularly limited as long as they are particles having conductivity, but are preferably particles having a coefficient of thermal expansion of 20 x 10 -6 / °C or less, and more preferably 6.0 x 10 -6 / °C or less. The coefficient of thermal expansion of the conductive particles can be determined depending on the material of the conductive particles. The PTC resistor can include only one kind of conductive particles, or can include two or more kinds.

[0045] Examples of the conductive particles include carbon-based particles such as graphite, carbon black, carbon nanotubes, and graphene; metal-based particles such as nickel powder, copper powder, silver powder, and tungsten powder; and the like. Among these, from the viewpoint that the affinity with the above-described polyolefin-based wax and the binder resin and the like is high and the particles are not easily precipitated in the composition for manufacturing the PTC resistor, nickel powder, silver powder, tungsten powder, and graphite are preferred.

[0046] In addition, the shape of the conductive particles is not particularly limited, and can be, for example, spherical, amorphous, tubular, rod-like, flat, broken, or the like. The specific surface area of the conductive particles is preferably 5 m 2 / g or less. The specific surface area is a value measured by a gas adsorption method.

[0047] The size of the conductive particles is appropriately selected depending on the kind of the conductive particles and the like. For example, when the conductive particles are spherical or close to spherical, the average particle diameter is preferably 30 μm or less, and more preferably 1 μm or more and 10 μm or less. When the average particle diameter is 30 μm or less, the PTC ratio of the PTC resistor is more easily made good. The average particle diameter is a value measured by a laser diffraction and scattering method, and is a median value (D50) in a cumulative particle size distribution.

[0048] The content of the conductive particles in the PTC resistor is preferably 45% by mass or more and 80% by mass or less, more preferably 50% by mass or more and 70% by mass or less, and further preferably 55% by mass or more and 67.5% by mass or less. When the amount of the conductive particles in the PTC resistor is 45% by mass or more, the PTC characteristics are more likely to be stable. On the other hand, when the amount of the conductive particles in the PTC resistor is 80% by mass or less, the amounts of the above-mentioned polyolefin-based wax and the binder resin increase, and the resistance value at the time of temperature rise is more likely to increase.

[0049] • Other

[0050] The PTC resistor according to the present embodiment can further include other components as necessary, without impairing the objects and effects of the present embodiment, in addition to the above-mentioned polyolefin-based wax, the binder resin, and the conductive particles. Examples of the other components include various additives such as an antioxidant and a flame retardant.

[0051] • Method for manufacturing a PTC resistor

[0052] The PTC resistor according to the present embodiment can be manufactured by coating a composition including the above-mentioned polyolefin-based wax, the above-mentioned binder resin, the above-mentioned conductive particles, and a solvent as necessary, and heating and curing the composition.

[0053] The solvent can be any solvent that can uniformly dissolve or disperse the above-mentioned polyolefin-based wax, the binder resin, the conductive particles, and the like, and the kind of the solvent is not particularly limited. However, the boiling point of the solvent is preferably 100°C or higher, more preferably 100 to 330°C, and further preferably 150 to 250°C. When the boiling point of the solvent is within this range, the storage stability and the like of the composition are improved, and the composition is more easily coated.

[0054] The solvent is appropriately selected depending on the kind of the polyolefin-based wax, the kind of the binder resin, and the like. Examples of the solvent include alcohol, ketone, ester, glycol ester, glycol ether, ether, aromatic hydrocarbon, and a mixture of these, and are preferably menthol, butyl carbitol acetate, tetralin, toluene, and a mixture of these.

[0055] The amount of the solvent is appropriately selected depending on the viscosity and the like of the desired composition, and generally, the amount of the solvent is preferably 20 to 70 parts by mass, and more preferably 25 to 65 parts by mass, with respect to 100 parts by mass of the total amount of the composition (ink). When the amount of the solvent is within this range, the viscosity of the composition (ink) is more easily within the desired range.

[0056] The viscosity of the composition is preferably selected as appropriate depending on the method of forming the PTC resistor. For example, in the case where the composition is printed by screen printing and cured to obtain a PTC resistor, the viscosity of the composition is preferably 100 to 400 dPa-s. The viscosity is a value measured at 25°C by a cylinder-type rotational viscometer (manufactured by RION Corporation). When the viscosity of the composition is in this range, the composition can be applied to a desired thickness, and a uniform film can be formed.

[0057] The method of producing the above composition is not particularly limited, and the polyolefin-based wax, the resin binder, the electrically conductive particles, and the solvent can be mixed at the same time. On the other hand, the resin binder and the solvent can be mixed first, and then the polyolefin-based wax and the electrically conductive particles can be mixed.

[0058] Further, the method of applying the above composition is not particularly limited, and examples thereof include screen printing, roll coating, and application using a dotter.

[0059] Further, the method of curing the composition can be a method of heating to about 100 to 200°C. At this time, the heating time is preferably about 1 to 30 minutes. When the composition is heated to the above degree, the solvent in the composition is removed.

[0060] (2) PTC resistor of the second mode

[0061] The PTC resistor of the second mode contains a naturally derived wax, a binder resin, and electrically conductive particles. Hereinafter, each component will be described.

[0062] • Naturally derived wax

[0063] In the present specification, a wax refers to an organic substance that is in a solid or semi-solid state at normal temperature (25°C) and melts without decomposing upon heating. A naturally derived wax refers to a plant-based wax derived from a plant or an animal-based wax derived from an animal. Further, a plant-based wax refers to a wax that uses a plant as a raw material. An animal-based wax refers to a wax derived from an animal. The PTC resistor can contain only one kind of naturally derived wax, or can contain two or more kinds. The kind of naturally derived wax contained in the PTC resistor is not particularly limited, but generally requires that the PTC resistor have an increase in resistance value at 40°C or higher and 100°C or lower. Therefore, the melting point of the naturally derived wax is preferably 40°C or higher and 100°C or lower, and more preferably 50°C or higher and 90°C or higher.

[0064] The wax ester component contained in the naturally derived wax is preferably 10% by mass or more. When the wax ester component is 10% by mass or more, the melting point of the naturally derived wax easily falls within the above range. Further, melting in the vicinity of the melting point is more easily performed in a short time.

[0065] Here, the wax ester component refers to an ester of a higher fatty acid and a higher monohydric alcohol combined in a 1 to 1 ratio. The wax ester component is, for example, an ester of a higher fatty acid having a carbon number of 10 or more and 50 or less and a higher monohydric alcohol having a carbon number of 10 or more and 50 or less. The total number of carbons constituting the wax ester component is preferably 20 or more and 100 or less, more preferably 30 or more and 50 or less.

[0066] The natural source wax can also contain free saturated fatty acids (the above higher fatty acid) and free alcohols (the above higher monohydric alcohol). The amount of free saturated fatty acids is preferably 20% by mass or less of the total mass of the natural source wax. On the other hand, the amount of free alcohols is preferably 15% by mass or less of the total mass of the natural source wax.

[0067] Further, the iodine value of the above natural source wax is preferably 80 or less, more preferably 30 or less, and further preferably 25 or less. The iodine value indicates the amount of unsaturated bonds in the hydrocarbon chains of the components contained in the natural source wax. It is preferable that the amount of unsaturated double bonds in the natural source wax be small. When the amount of unsaturated double bonds is large, the natural source wax is difficult to melt even if the temperature of the PTC resistor body rises. That is, when the iodine value exceeds 80, the resistance value is difficult to increase upon temperature rise. In contrast, when the iodine value is 80 or less, the natural source wax is easily melted upon temperature rise of the PTC resistor body, and the resistance value is easily increased.

[0068] Examples of the plant-based wax, which is one of the above natural source waxes, include candelilla wax, carnauba wax, rice wax, wood wax, hydrogenated jojoba wax, and the like. Among these, from the viewpoint of easy availability and easy increase in PTC ratio, carnauba wax, candelilla wax, and rice wax are preferable.

[0069] On the other hand, examples of the animal-based wax, which is one of the natural source waxes, include beeswax, lanolin wax, and the like.

[0070] In the PTC resistor body, the amount of the natural source wax is preferably 5% by mass or more and 50% by mass or less, more preferably 7% by mass or more and 40% by mass or less. When the amount of the natural source wax in the PTC resistor body is 5% by mass or more, the resistance value upon temperature rise of the PTC resistor body is more easily increased. On the other hand, when it is 50% by mass or less, the amount of the conductive particles described later is relatively sufficient, and the PTC characteristics of the PTC resistor body are more easily made good.

[0071] • Binder resin

[0072] The binder resin is only required to be a resin capable of binding the above-mentioned natural origin wax or the later-described conductive particles, and binding these to a substrate or the like, but is preferably a resin that does not hinder the volume change caused by melting of the above-mentioned natural origin wax. Furthermore, the binder resin itself can also be a resin that expands by heating, and contributes to the increase in the resistance value of the PTC resistor. The PTC resistor can contain only one kind of binder resin, or two or more kinds. The binder resin is the same as the binder resin of the first mode PTC resistor.

[0073] The amount of the binder resin in the PTC resistor of the present mode is preferably 5% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% by mass or less. When the amount of the binder resin in the PTC resistor is 5% by mass or more, the composition containing the conductive resin and the natural origin wax, the binder resin is easily printed, and the fixability after printing is also more favorable. Furthermore, the strength of the PTC resistor is also more easily increased. On the other hand, when the amount of the binder resin is 50% by mass or less, the amount of the conductive particles is relatively sufficient, and the PTC characteristics are more easily made favorable.

[0074] • Conductive particles

[0075] The conductive particles are not particularly limited as long as they are particles having electrical conductivity, and are the same as the conductive particles of the above-mentioned first mode PTC resistor.

[0076] The content of the conductive particles in the PTC resistor of the present mode is preferably 45% by mass or more and 80% by mass or less, and more preferably 50% by mass or more and 70% by mass or less. When the amount of the conductive particles in the PTC resistor is 45% by mass or more, the PTC characteristics are more easily stabilized. On the other hand, when the amount of the conductive particles in the PTC resistor is 80% by mass or less, the amounts of the above-mentioned natural origin wax and the binder resin increase, and the resistance value at the time of heating is more easily increased.

[0077] • Other

[0078] The PTC resistor can contain other components as necessary within a range not impairing the objects and effects of the present mode, in addition to the above-mentioned natural origin wax, binder resin, and conductive particles. Examples of the other components include various additives such as antioxidants, flame retardants, and the like.

[0079] • Method for manufacturing PTC resistor

[0080] The PTC resistor of the present mode can be manufactured by coating a composition containing the above-mentioned natural origin wax, the above-mentioned binder resin, the above-mentioned conductive particles, and a solvent as necessary, and heating and curing.

[0081] The solvent is not particularly limited as long as it uniformly dissolves or disperses the natural source wax, the binder resin, the electrically conductive particles, and the like. However, the boiling point of the solvent is preferably 100°C or higher, more preferably 100 to 330°C, and further preferably 150 to 250°C. When the boiling point of the solvent is within this range, the storage stability and the like of the composition are improved, and the composition is further easily applied.

[0082] The solvent is appropriately selected depending on the type of the natural source wax and the type of the binder resin, and the like. Examples thereof include alcohol, ketone, ester, glycol ester, glycol ether, ether, aromatic hydrocarbon, and a mixture thereof, and are preferably menthol, butyl carbitol acetate, tetrahydronaphthalene, toluene, and a mixture thereof.

[0083] The amount of the solvent is appropriately selected depending on the viscosity and the like of the desired composition (ink), and generally, the amount of the solvent is preferably about 40 to 90 parts by mass, and more preferably 40 to 85 parts by mass, with respect to 100 parts by mass of the total amount of the composition (ink). When the amount of the solvent is within this range, the viscosity of the composition easily falls within the desired range.

[0084] The preferred viscosity of the composition is appropriately selected depending on the method of forming the PTC resistor. For example, in the case where the composition is printed by screen printing and cured to obtain the PTC resistor, the viscosity of the composition is preferably 100 to 400 dPa-s. This viscosity is a value measured at 25°C by a cylinder-type rotational viscometer (manufactured by RION Corporation). When the viscosity of the composition is within this range, the composition can be applied to the desired thickness, and a uniform film can be formed.

[0085] The method of producing the above-described composition is not particularly limited, and the natural source wax, the resin binder, the electrically conductive particles, and the solvent can be mixed at the same time. Alternatively, the resin binder and the solvent can be mixed first, and then the natural source wax and the electrically conductive particles can be mixed.

[0086] Furthermore, the method of applying the above-described composition is not particularly limited, and examples thereof include screen printing, roll coating, and a doctor blading method.

[0087] In addition, the method of curing the composition can be exemplified by a method of heating to about 100 to 200°C. At this time, the heating time is preferably about 1 to 30 minutes. When the composition is heated to the above-described degree, the solvent in the composition is removed.

[0088] Example

[0089] Hereinafter, specific examples of the present application will be described. Furthermore, these examples do not limit the explanation of the scope of the present application.

[0090] [First Method]

[0091] 1. Preparation of Materials

[0092] As a material for manufacturing a PTC resistor, the following compounds were prepared.

[0093] (Polyolefin component)

[0094] • Polyethylene wax (CERAFLOUR) 961 (manufactured by BYK)

[0095] • Ultra-high molecular weight polyethylene (MIPELON PM-200 (manufactured by Mitsui Chemicals, Inc.)

[0096] (Binder resin)

[0097] • SEBS resin (styrene-ethylene-butylene-styrene block copolymer, TUFTEC 1913, manufactured by Asahi Kasei Corporation)

[0098] • Hydrogenated SEPS resin (hydrogenated styrene-ethylene-propylene-styrene block copolymer, SEPTON 2002, manufactured by Kuraray Co., Ltd.)

[0099] (Conductive particles)

[0100] • PWSH: crushed carbon (manufactured by Nippon Graphite Industries, Ltd., graphite)

[0101] • PG11: spherical carbon (manufactured by Nippon Graphite Industries, Ltd., graphite)

[0102] • ICB: spherical carbon (manufactured by Nippon Carbon Co., Ltd., graphite)

[0103] For the above polyolefin component, a DSC curve was obtained from 25°C to 150°C at a temperature increase rate of 10°C / min using DSC. The number of melting peaks and the absolute value of the height of the melting peaks at that time were determined. These results are shown in Table 1. In addition, the DSC curve of the polyethylene wax used in Example A-1 and Example A-2 is shown in FIG. 1. Figure 2 A, the DSC curve of the ultra-high molecular weight polyethylene used in Comparative Example A-2 and A-3 is shown in FIG. 2. Figure 2 B.

[0104] The average particle diameter D50 of the above conductive particles was measured by a laser diffraction and scattering method. In addition, the thermal expansion coefficient was set to the literature value. Furthermore, the specific surface area was determined by a gas adsorption method. These results are shown in Table 1.

[0105] 2. Preparation of PTC resistor

[0106] Each PTC resistor was prepared according to the following procedure.

[0107] [Example A-1]

[0108] A varnish was prepared by mixing 8.0 parts by mass of hydrogenated SEPS resin with 51.0 parts by mass of an organic solvent (terpineol) at 100°C for 1 hour with a stirrer. This varnish was mixed with 9.0 parts by mass of a polyolefin-based wax (polyethylene wax) and 32.0 parts by mass of electrically conductive particles (spherical carbon), and ground with a grinder to obtain the desired ink for screen printing.

[0109] A substrate with a pair of interdigital electrodes was prepared, and the above ink was printed between the interdigital electrodes by screen printing. Then, the substrate was heated to 150°C to prepare a planar heating element having a desired PTC resistor.

[0110] [Comparative Example A-1]

[0111] A planar heating element having a PTC resistor was prepared in the same manner as in Example A-1, except that no polyolefin-based wax was used and a paste was prepared in the composition shown in Table 1.

[0112] [Examples A-2, and Comparative Examples A-2 and A-3]

[0113] A planar heating element having a PTC resistor was prepared in the same manner as in Example A-1, except that the kind and amount of each component and the organic solvent were changed as shown in Table 1.

[0114] [Evaluation]

[0115] The following tests were performed on the planar heating elements prepared in Examples A-1 and A-2 and Comparative Examples A-1 to A-3. The results are shown in Table 1.

[0116] (Determination of temperature rise rate)

[0117] The planar heating element was allowed to warm up in a room temperature environment (25°C), and the temperature at which the temperature stabilized and the time required to reach that temperature were determined. Then, determination was made by the following equation.

[0118] (Stable temperature - initial temperature) / (time required for stabilization)

[0119] (Determination of surface resistance)

[0120] The surface resistance of each planar heating element was measured by the 2-terminal method or the 4-terminal method.

[0121] (Determination of maximum PTC ratio)

[0122] The resistance values of each planar heating element when warmed from -40°C to 120°C were measured. Then, the maximum resistance value and the minimum resistance value were determined, and the ratio thereof (maximum resistance value / minimum resistance value) was calculated as the maximum PTC ratio. In addition, the measurement of each resistance value was performed by the 2-terminal method or the 4-terminal method.

[0123] (Determination of maximum PTC ratio in room temperature environment)

[0124] The electric resistance value of the sheet-like heat generating body immediately after manufacture was measured at 25°C. Next, the sheet-like heat generating body was subjected to temperature change between -40°C and 120°C, and then returned to 25°C, and the electric resistance value was measured again. Then, the ratio thereof (electric resistance value at 25°C after temperature change / electric resistance value at 25°C before temperature change) was calculated as the maximum PTC ratio in room temperature environment.

[0125] (Determination)

[0126] "0" when the maximum PTC ratio is 100 or more, and "X" when it is less than 100.

[0127] [Table 1]

[0128]

[0129] As shown in the above table, the sheet-like heat generating bodies using PTC resistors containing polyolefin-based wax (Examples A-1 and A-2) had very high maximum PTC ratio and also had very fast temperature increase rate, as compared with the sheet-like heat generating bodies using PTC resistors not containing the wax (Comparative Examples A-1 to A-3).

[0130] [Second method]

[0131] 1. Preparation of materials

[0132] As the materials for preparing PTC resistors, the following compounds were prepared.

[0133] (Naturally derived wax (plant-based wax))

[0134] • Carnauba wax: melting point 80 to 86°C, iodine value 5 to 14, wax ester content 80 to 85 mass%

[0135] • Candelilla wax: melting point 68 to 73°C, iodine value 5 to 14, wax ester content 80 to 85 mass%

[0136] • Rice wax: melting point 80°C, iodine value 7.4, wax ester content 93 to 97 mass%

[0137] (Plant-based oil)

[0138] • Castor oil: no melting point, iodine value 83 to 89, wax ester content 1.7%

[0139] (Others)

[0140] • Conductive particles (artificial graphite, average particle diameter D50 4 μm, specific surface area 5.0 m2 / g, thermal expansion coefficient 6.0 x 10 -6 / °C)

[0141] • Binder resin: SEBS resin (styrene-ethylene-butylene-styrene block copolymer, TUFTEC 1913, manufactured by Asahi Chemical Industry Co., Ltd.)

[0142] • Solvent: terpineol

[0143] 2. Preparation of a planar heating element having PTC resistors

[0144] A planar heating element having PTC resistors was prepared according to the following procedure.

[0145] [Example B-1]

[0146] A varnish was prepared by mixing 4.9 parts by mass of SEBS resin and 63.5 parts by mass of an organic solvent (terpineol) at 100°C for 1 hour using a stirrer. This varnish was mixed with 11.5 parts by mass of a natural wax (carnauba wax) and 20.1 parts by mass of conductive particles (artificial graphite), and ground using a grinder, to obtain the desired ink for screen printing.

[0147] A substrate with a pair of comb electrodes was prepared, and the above ink was printed between the comb electrodes by screen printing. Then, the substrate was heated to 150°C, to prepare a planar heating element having the desired PTC resistors.

[0148] [Example B-2]

[0149] A planar heating element having PTC resistors was prepared in the same manner as in Example B-1, except that the type of natural wax was changed to candelilla wax.

[0150] [Example B-3]

[0151] A planar heating element having PTC resistors was prepared in the same manner as in Example B-1, except that the type of natural wax was changed to rice wax.

[0152] [Comparative Example B-1]

[0153] A planar heating element having PTC resistors was prepared in the same manner as in Example B-1, except that castor oil was used instead of natural wax.

[0154] [Evaluation]

[0155] The sheet-like heat generating bodies prepared in Examples B-1 to B-3 and Comparative Example B-1 were each measured for resistance value at 25°C to 120°C. Then, the maximum PTC ratio was calculated as (maximum resistance value) / (resistance value at 25°C). Note that the resistance value was measured by the 2-terminal method or the 4-terminal method. The results are shown in Table 2. Further, the criteria were as follows: "O" when the maximum PTC ratio was 100 or more, "Δ" when the maximum PTC ratio was 2 or more but less than 100, and "X" when the maximum PTC ratio was less than 2.

[0156] [Table 2]

[0157]

[0158] As shown in the above table, the PTC ratio of the sheet-like heat generating body using a plant-based oil (Comparative Example B-1) did not sufficiently increase, whereas the maximum PTC ratio was very high in the sheet-like heat generating bodies using PTC resistors containing waxes of natural origin (Examples B-1 to B-3). Further, in Examples B-1 to B-3, it was confirmed that the temperature stabilized in about 1 minute from the start of heat generation at room temperature (25°C).

[0159] This application claims priority based on Japanese Patent Application No. 2023-046590 filed on March 23, 2023. The contents described in the specification and drawings of the application are incorporated into the specification of the present application.

[0160] Industrial Applicability

[0161] The PTC resistor of the present disclosure is likely to increase the resistance value in a short time after reaching a certain temperature, and the resistance value after the temperature increase is high. Therefore, it is very suitable for the manufacture of various sheet-like heat generating bodies.

Claims

1. A surface heating element comprising: a substrate; a pair of electrodes disposed on the substrate; and a positive temperature coefficient resistor disposed between the pair of electrodes on the substrate, the positive temperature coefficient resistor containing electrically conductive particles, a binder resin, and a wax, the number of melting peaks observed in a differential scanning calorimetry curve obtained by using a differential scanning calorimeter to raise the temperature of the polyolefin-based wax from 25°C to 150°C at a temperature raising rate of 10°C / minute is 1 or 2.

2. The surface heating element according to claim 1, wherein 3. The surface heating element according to claim 1, wherein The thermal expansion coefficient of the conductive particles is 20 x 10 -6 / °C or less. the average particle diameter of the electrically conductive particles is 30 μm or less.

4. The surface heating element according to claim 1, wherein 5. The surface heating element according to any one of claims 1 to 4, wherein The specific surface area of the conductive particles is 5 m 2 / g or less. the wax contains the polyolefin-based wax, the absolute value of the height of at least one melting peak in the differential scanning calorimetry curve is 5 W or more.

6. The surface heating element according to any one of claims 1 to 4, wherein the wax contains a naturally derived wax, the naturally derived wax contains 10 mass% or more of a wax ester component.

7. The surface heating element according to claim 6, wherein the iodine value of the naturally derived wax is 80 or less. ​

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