Positive temperature coefficient (PTC) resistor body

Abstract

The technical problem of the present disclosure is to provide a PTC resistor, the resistance value of which is easy to increase in a short time after reaching a certain temperature, and the resistance value after temperature increase is high. The PTC resistor body for solving the technical problem comprises conductive particles, natural-source wax and binder resin.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a PTC resistor. 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 resistor having a PTC characteristic (also referred to as a "PTC resistor" in the present specification) provided between the electrodes. As the PTC resistor, a resistor including a crystalline resin and electrically conductive particles is known (for example, Patent Literature 1). In a planar heating element having such a PTC resistor, when a voltage is applied between the electrodes, a current is conducted in the PTC resistor, and the temperature thereof rises. On the other hand, as the temperature of the planar heating element (PTC resistor) rises, the crystalline resin contained therein thermally expands, the distance between the electrically conductive particles expands, and the electric resistance value rises. Then, the electric resistance value sharply rises near the softening temperature or the melting point of the crystalline resin, and the 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 PTC resistor using a crystalline resin as a binder, the 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, the increase in the electric resistance value takes time. In addition, it is further required to increase the electric resistance value of the PTC resistor after temperature rise.

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

[0010] SOLUTION TO PROBLEM

[0011] One embodiment of the present disclosure provides a PTC resistor including: electrically conductive particles; a naturally derived wax; and a binder resin.

[0012] Inventive Effects

[0013] According to the present disclosure, a PTC resistor body that has a resistance value that easily increases in a short time after reaching a certain temperature and has a high resistance value after the temperature increase can be obtained. DETAILED DESCRIPTION

[0014] In the present specification, a numerical range indicated by "~" means a numerical range including the numerical values described before and after the "~".

[0015] As described above, conventional PTC resistor bodies mainly impart PTC characteristics by using thermal expansion and recrystallization of crystalline resin. However, this method has a low increase amplitude of the resistance value after reaching a certain temperature, and the increase amplitude of the resistance value is also small. In view of this, the PTC resistor body of the present disclosure contains electrically conductive particles, a binder resin, and a naturally derived wax. In this PTC resistor body, when the temperature increases, the naturally derived wax melts near its melting point. The volume increase of this naturally derived wax due to melting is significantly larger than the volume increase of general resins and occurs in a short time. Therefore, when the PTC resistor body reaches a predetermined temperature, the resistance value increases in a short time, and the increase amplitude of this resistance value is also very large. A PTC resistor body having such excellent PTC characteristics can be applied to various planar heat generating bodies and the like.

[0016] Hereinafter, each component in the PTC resistor body of the present disclosure will be described.

[0017] (Naturally Derived Wax)

[0018] 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 when heated. A naturally derived wax refers to a plant-based wax derived from plants or an animal-based wax derived from animals. In addition, a plant-based wax refers to a wax that uses plants as a raw material. An animal-based wax refers to a wax derived from animals. The PTC resistor body can contain only one kind of naturally derived wax, or two or more kinds. The kind of naturally derived wax contained in the PTC resistor body is not particularly limited, but generally requires the PTC resistor body to have an increase in the 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.

[0019] 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. In addition, melting near the melting point is more easily performed in a short time.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] (Binder resin)

[0027] 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 of the above-mentioned natural origin wax due to melting. 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 can contain two or more kinds.

[0028] Examples of the binder resin include well-known thermoplastic resins. Specific examples include thermoplastic polyurethane resins, polyester resins, polyacrylate resins, polysiloxane resins, halogenated ethylene resins, vinyl resins, polyimide resins, phenoxy resins, polyether resins, polyketone resins, polyvinyl butyral resins, polyvinyl pyrrolidone resins, polyacrylate resins, SEBS resins (styrene-ethylene-butylene-styrene copolymers) and hydrogenated products thereof, SEPS resins (styrene-ethylene-propylene-styrene copolymers) and hydrogenated products thereof, and the like. Of these, from the viewpoint of being easily deformable and not hindering the volume change of the above-mentioned natural origin wax, SEBS and hydrogenated SEPS resins are preferred.

[0029] The amount of the binder resin in the PTC resistor is preferably 5 mass% or more and 50 mass% or less, and more preferably 10 mass% or more and 40 mass% or less. When the amount of the binder resin in the PTC resistor is 5 mass% or more, the composition containing the conductive resin and the natural origin wax, the binder resin is easily printable, and the fixability after printing is also more favorable. Furthermore, the strength of the PTC resistor is also more easily improved. On the other hand, when the amount of the binder resin is 50 mass% or less, the amount of the conductive particles is relatively sufficient, and the PTC characteristics are more easily made favorable.

[0030] (Conductive particles)

[0031] The conductive particles are not particularly limited as long as they are particles having electrical conductivity, but are preferably particles having a thermal expansion coefficient of 20 x 10 -6 / °C or less, and more preferably 6.0 x 10 -6 / °C or less. The thermal expansion coefficient of the conductive particles can be determined depending on the kind or the like of the conductive particles. The PTC resistor can contain only one kind of conductive particles, or can contain two or more kinds.

[0032] 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. Of these, from the viewpoint of having high affinity with the above-mentioned natural origin wax and the binder resin or the like, and not easily precipitating in the composition for manufacturing the PTC resistor, nickel powder, silver powder, tungsten powder, and graphite are preferred.

[0033] Further, 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.

[0034] The size of the conductive particles is appropriately selected depending on the kind of the conductive particles or the like. For example, in the case where the conductive particles are spherical or nearly spherical, the average particle diameter is preferably 30 μm or less, more preferably 1 μm or more and 10 μm or less. When the average particle diameter is 30 μm or less, the PTC characteristics of the PTC resistor are more easily obtained. 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.

[0035] The content of the conductive particles in the PTC resistor is preferably 45 mass% or more and 80 mass% or less, more preferably 50 mass% or more and 70 mass% or less. When the amount of the conductive particles in the PTC resistor is 45 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 mass% or less, the amounts of the above-mentioned naturally derived wax and the binder resin increase, and the resistance value at the time of temperature rise is more easily increased.

[0036] (Other)

[0037] The PTC resistor can contain other components as necessary within a range not impairing the objects and effects of the present disclosure, in addition to the above-mentioned naturally derived wax, the binder resin, and the conductive particles. Examples of the other components include various additives such as an antioxidant and a flame retardant.

[0038] (Method for manufacturing PTC resistor)

[0039] The PTC resistor of the present disclosure can be manufactured by coating a composition (ink) containing the above-mentioned naturally derived wax, the above-mentioned binder resin, the above-mentioned conductive particles, and a solvent as necessary, and heating and curing.

[0040] The solvent can be any solvent as long as it can uniformly dissolve or disperse the above-mentioned naturally derived wax, the binder resin, the conductive particles, or the like, and the kind thereof 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 or the like of the composition is improved, and the composition is further easily coated.

[0041] The solvent is appropriately selected depending on the kind of the naturally derived wax and the kind of the binder resin or the like. Examples thereof include alcohol, ketone, ester, glycol ester, glycol ether, ether, aromatic hydrocarbon, and a mixture thereof, and are preferably pinol, butyl carbitol acetate, tetrahydronaphthalene, toluene, and a mixture thereof.

[0042] The amount of the solvent is appropriately selected in accordance with the viscosity of the composition (ink) desired, 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, relative to 100 parts by mass of the total amount of the composition (ink). When the amount of the solvent is in this range, the viscosity of the composition easily falls within the desired range.

[0043] The preferred viscosity of the composition is appropriately selected in accordance with 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. 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 in this range, the composition can be applied to the desired thickness, and a uniform film can be formed.

[0044] The method of producing the above composition is not particularly limited, and the naturally derived 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 naturally derived wax and the electrically conductive particles can be mixed.

[0045] Furthermore, the method of applying the above composition is not particularly limited, and examples thereof include screen printing, roll coating, and a doctor blading method.

[0046] 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 degree, the solvent in the composition is removed.

[0047] (Use of the PTC resistor)

[0048] The above PTC resistor can be combined with a pair of electrodes to form a planar heating element, for example. The above PTC resistor increases in resistance value in a short time after exceeding a certain temperature at the time of temperature increase, and the resistance value is very high relative to the resistance value at normal temperature (25°C). Therefore, it 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.

[0049] Example

[0050] Hereinafter, specific examples of the present application will be described. In addition, these examples do not limit the explanation of the scope of the present application.

[0051] 1. Preparation of materials

[0052] As a material for manufacturing the PTC resistor, the following compounds were prepared.

[0053] (Naturally derived wax (plant-based wax))

[0054] • Carnauba wax: melting point 80-86°C, iodine value 5-14, wax ester content 80-85 mass%

[0055] • Candelilla wax: melting point 68-73°C, iodine value 5-14, wax ester content 80-85 mass%

[0056] • Rice wax: melting point 80°C, iodine value 7.4, wax ester content 93-97 mass% (plant-based oil)

[0057] • Castor oil: no melting point, iodine value 83-89, wax ester content 1.7%

[0058] (Other)

[0059] • 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)

[0060] • Binder resin: SEBS resin (styrene-ethylene-butylene-styrene block copolymer, TUFTEC 1913, manufactured by Asahi Kasei Corporation)

[0061] • Solvent: terpineol

[0062] 2. Preparation of a planar heat generating body containing a PTC resistor

[0063] A planar heat generating body containing each PTC resistor was prepared according to the following procedure.

[0064] [Example 1]

[0065] 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 naturally derived 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.

[0066] 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 heat generating body having the desired PTC resistor.

[0067] [Example 2]

[0068] A planar heat generating body having a PTC resistor was produced in the same manner as in Example 1 except that the kind of the natural source wax was changed to candelilla wax.

[0069] [Example 3]

[0070] A planar heat generating body having a PTC resistor was produced in the same manner as in Example 1 except that the kind of the natural source wax was changed to rice wax.

[0071] [Comparative Example 1]

[0072] A planar heat generating body having a PTC resistor was produced in the same manner as in Example 1 except that castor oil was used instead of the natural source wax.

[0073] [Evaluation]

[0074] The planar heat generating bodies produced in Examples 1 to 3 and Comparative Example 1 were each measured for the 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 measurement of the resistance value was performed by the 2-terminal method or the 4-terminal method. The results are shown in Table 1. Further, the criteria were as follows: “O” when the maximum PTC ratio was 100 times or more, “Δ” when the maximum PTC ratio was 2 times or more and less than 100 times, and “X” when the maximum PTC ratio was less than 2 times.

[0075] [Table 1]

[0076]

[0077] As shown in the above table, the PTC ratio of the planar heat generating body using the plant-based oil (Comparative Example 1) did not sufficiently increase, whereas in the planar heat generating bodies using the PTC resistor containing the natural source wax (Examples 1 to 3), the maximum PTC ratio was very high. Further, in Examples 1 to 3, it was confirmed that when heating started from room temperature (25°C), the temperature stabilized in about 1 minute.

[0078] This application claims priority based on Japanese Patent Application No. 2023-046588 filed on March 23, 2023. The contents described in the specification of this application are incorporated in the specification of this application.

[0079] Industrial Applicability

[0080] 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 rise is high. Therefore, it is very suitable for the production of various planar heat generating bodies.

Claims

1. A positive temperature coefficient resistor comprising: a conductive particle; a naturally derived wax; and a binder resin.

2. The positive temperature coefficient resistor of claim 1, wherein, The naturally derived wax contains 10% by mass or more of a wax ester component.

3. The positive temperature coefficient resistor of claim 1, wherein, The naturally derived wax has an iodine value of 80 or less.

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