PTC resistor, ink for PTC resistor, and planar heating element

Abstract

To provide a PTC resistor for obtaining a planar heating element having good PTC characteristics.SOLUTION: The foregoing problem is solved by a PTC resistor that includes a binder resin and multi-wall carbon nanotubes.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a PTC resistor, ink for a PTC resistor, and a sheet heating element. [Background technology]

[0002] To remove frost and dew from car door mirrors, heaters are often installed on the rear of the mirror. These heaters often use planar heating elements with PTC (positive temperature coefficient) characteristics, which do not require expensive temperature control devices.

[0003] A planar heating element with PTC characteristics typically has a pair of electrodes and a resistor with PTC characteristics (also referred to as a "PTC resistor" in this specification) disposed between the electrodes. A known PTC resistor is one containing a crystalline resin and conductive particles (see, for example, Patent Document 1). In a planar heating element with such a PTC resistor, when a voltage is applied between the electrodes, electricity flows through the PTC resistor, causing its temperature to rise. Meanwhile, as the temperature of the planar heating element (PTC resistor) rises, the crystalline resin contained therein thermally expands, increasing the distance between the conductive particles and increasing the resistance value. The resistance value then rises sharply near the softening temperature or melting point of the crystalline resin, making it difficult for electricity to pass through. In other words, the temperature of the planar heating element is controlled so that it does not rise above a certain level. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-227081 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0005] Here, the conductive particles used in the PTC resistors are often carbon black, metal particles, etc. However, depending on the type of conductive particles, sufficient PTC characteristics may not be obtained, and further improvements have been required.

[0006] A primary object of the present disclosure is to provide a sheet heating element having good PTC characteristics, and a PTC resistor for obtaining such a sheet heating element. [Means for solving the problem]

[0007] In order to solve the above problems, the present disclosure provides the following PTC resistor. [1] A PTC resistor comprising a binder resin and multi-walled carbon nanotubes. [2] The PTC resistor according to [1], wherein the binder resin contains a thermoplastic styrene-based elastomer. [3] The PTC resistor according to [1] or [2], wherein the binder resin contains a semi-crystalline polymer. [4] The PTC resistor according to any one of [1] to [3], wherein the aspect ratio of the multi-walled carbon nanotubes is 50 or less.

[0008] In order to solve the above problems, the present disclosure provides the following sheet heating element and ink for PTC resistors. [5] A planar heating element comprising a substrate, a pair of electrodes arranged on the substrate, and a PTC resistor according to any one of [1] to [4] arranged on the substrate and between the pair of electrodes. [6] An ink for a PTC resistor, comprising a binder resin, multi-walled carbon nanotubes, and a solvent. [Effects of the Invention]

[0009] According to the present disclosure, a sheet heating element having good PTC characteristics is provided, and a PTC resistor for obtaining the sheet heating element is provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a plan view showing an example of the structure of a sheet heating element according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this specification, a numerical range indicated by "to" means a numerical range including the numerical values ​​written before and after "to".

[0012] 1. PTC resistors and ink for PTC resistors The PTC resistor of the present disclosure includes a binder resin and multi-walled carbon nanotubes. The PTC resistor may also include other components as long as the objects and effects of the present disclosure are not impaired.

[0013] As mentioned above, conventional PTC resistors typically use metal particles or carbon black as conductive particles. While carbon nanotubes are also known as conductive particles, they have been considered unsuitable as a material for PTC resistors for the following reasons: Carbon nanotubes are particles composed of tubular graphene (tubular graphene). Depending on the orientation of the carbon atoms constituting the tubular graphene, carbon nanotubes can be classified as either metallic or semiconducting, depending on the orientation of the carbon atoms constituting the tubular graphene. Single carbon nanotubes composed of single-layer tubular graphene typically contain a 1:2 mixture of metallic and semiconducting nanotubes. In semiconducting carbon nanotubes, electrons tend to move from the valence band to the conductor as the temperature increases. Therefore, semiconducting carbon nanotubes have a tendency to decrease in resistance with increasing temperature. On the other hand, as mentioned above, PTC resistors are required to decrease in resistance with increasing temperature. In other words, one reason for this is that semiconducting carbon nanotubes have properties that are contrary to the characteristics required for PTC resistors.

[0014] In response to this, the inventors conducted extensive research and found that while it is difficult to obtain PTC properties with the above-mentioned single-walled carbon nanotubes, very good PTC properties can be obtained with multi-walled carbon nanotubes, in which tubular graphene are arranged concentrically. Although the reason for this is unclear, it is thought that because multi-walled carbon nanotubes contain multiple tubular graphene, even if some of the tubular graphene are semiconducting, their properties are suppressed by the metallic tubular graphene. It is also thought that the unique shape of multi-walled carbon nanotubes allows them to exhibit better PTC properties than general conductive particles. Each component constituting the PTC resistor of the present disclosure will be described in detail below.

[0015] (binder resin) The binder resin of the PTC resistor of the present disclosure is an insulating resin that bonds the multi-walled carbon nanotubes (described later) to the substrate. The binder resin increases in volume as the temperature of the PTC resistor increases, widening the distance between adjacent multi-walled carbon nanotubes and preventing electrical conduction between them. The binder resin may contain only one type of resin, or may contain two or more types. Examples of resins that can be suitably used as the binder resin in the PTC resistor of the present disclosure include crystalline or amorphous thermoplastic resins and semi-crystalline polymers. The binder resin may contain only one of these, or may contain both. In particular, containing both makes it easier to obtain better PTC characteristics.

[0016] Examples of thermoplastic resins include thermoplastic polyurethane resins, polyester resins, polyacrylate resins, polysiloxane resins, vinyl halide resins, vinylidene resins, polyimide resins, phenoxy resins, polyether resins, polyketone resins, polyvinyl butyral resins, polyvinylpyrrolidone resins, polyacrylate resins, and thermoplastic elastomers. The binder resin may contain only one type of thermoplastic resin, or two or more types. Among the above thermoplastic resins, thermoplastic elastomers are preferred because they are easily deformable.

[0017] Specific examples of thermoplastic elastomers include olefin-based elastomers such as ethylene-propylene-diene copolymers; amide-based elastomers; ester-based thermoplastic elastomers; urethane-based elastomers; and styrene-based elastomers (thermoplastic styrene-based elastomers). Among these, styrene-based elastomers are preferred in terms of availability and ease of handling. Specific examples of styrene-based elastomers include styrene-ethylene-butylene-styrene copolymers (SEBS) and their hydrogenated products, and styrene-ethylene-propylene-styrene copolymers (SEPS) and their hydrogenated products. SEBS and hydrogenated SEPS are particularly preferred in terms of their ease of deformation.

[0018] On the other hand, in this specification, a semi-crystalline polymer refers to a polymer that has both amorphous and crystalline regions and has both a clear melting point and glass transition temperature. The melting point of the semi-crystalline polymer is preferably 40°C or higher and 150°C or lower, and more preferably 60°C or higher and 130°C or lower. When the melting point of the semi-crystalline polymer is within this range, the semi-crystalline polymer melts near the melting point when the temperature of the PTC resistor increases. The increase in volume of the semi-crystalline polymer then facilitates a significant increase in resistance. The binder resin may contain only one type of semi-crystalline polymer, or two or more types.

[0019] Examples of semi-crystalline polymers include hydrocarbon waxes such as polyethylene waxes, such as low-density polyethylene wax, medium-density polyethylene wax, and high-density polyethylene wax, polypropylene wax, polybutene wax, ethylene-propylene copolymer wax, and ethylene-propylene-butene copolymer wax; vegetable waxes such as candelilla wax, carnauba wax, rice wax, Japan wax, and hydrogenated jojoba wax; animal waxes such as beeswax and lanolin wax; montan wax; ozokerite; ceresin; paraffin wax; microcrystalline wax; mineral wax; petroleum wax; higher fatty acids such as stearyl stearate and behenyl behenate; These include ester waxes derived from higher alcohols; ester waxes derived from higher fatty acids and monohydric or polyhydric lower alcohols, such as butyl stearate, propyl oleate, glyceride monostearate, glyceride distearate, and pentaerythritol tetrabehenate; ester waxes derived from higher fatty acids and polyhydric alcohol multimers, such as diethylene glycol monostearate, dipropylene glycol distearate, diglyceryl distearate, and triglyceryl tetrastearate; sorbitan higher fatty acid ester waxes, such as sorbitan monostearate; and cholesterol higher fatty acid ester waxes, such as cholesteryl stearate. As used herein, wax refers to an organic substance that is solid or semi-solid at room temperature and melts without decomposition upon heating. Among these, the semi-crystalline polymer is preferably a hydrocarbon wax or a naturally derived wax (plant-based or animal-based wax).

[0020] When the semi-crystalline polymer is a hydrocarbon wax, a differential scanning calorimeter (hereinafter also referred to as "DSC") is used to generate a DSC curve by heating the hydrocarbon wax from 25°C to 150°C at a heating rate of 10°C / min. The melting peak observed in the DSC curve is preferably in the range of 80°C to 150°C, more preferably in the range of 90°C to 140°C, and even more preferably in the range of 100°C to 130°C. This results in a PTC resistor that shows an increase in resistance value at 80°C to 150°C.

[0021] Furthermore, when the semi-crystalline polymer is a hydrocarbon wax, its weight-average molecular weight is preferably 1,000 to 1,500,000, more preferably 3,000 to 20,000. When the weight-average molecular weight of the hydrocarbon wax is within this range, it is likely to have a melting peak in the desired temperature range. The weight-average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0022] On the other hand, when the semi-crystalline polymer is a naturally occurring wax, the wax ester component preferably accounts for 10% by mass or more of the composition. The wax ester component refers to an ester in which a higher fatty acid and a higher monohydric alcohol are bonded in a 1:1 ratio. When the wax ester component accounts for 10% by mass or more, the melting point tends to fall within the desired range, and melting near the melting point tends to proceed in a shorter time. The total number of carbon atoms constituting the wax ester component is preferably 20 to 100, more preferably 30 to 50.

[0023] Furthermore, when the semi-crystalline polymer is a naturally occurring wax, it may further contain free saturated fatty acids (the above-mentioned higher fatty acids) and free alcohols (the above-mentioned higher monohydric alcohols). The amount of free saturated fatty acids is preferably 20% by mass or less relative to the total mass of the naturally occurring wax. On the other hand, the amount of free alcohols is preferably 15% by mass or less relative to the total mass of the naturally occurring wax.

[0024] When the semi-crystalline polymer is a naturally occurring wax, its iodine value is preferably 80 or less, more preferably 30 or less, and even more preferably 25 or less. The iodine value represents the amount of unsaturated bonds in the hydrocarbon chains of the components contained in the naturally occurring wax. The iodine value, i.e., the amount of unsaturated double bonds, in the naturally occurring wax is preferably low. If the iodine value is 80 or less, the naturally occurring wax is likely to melt when the temperature of the PTC resistor rises, and its resistance value is likely to increase.

[0025] The total amount of binder resin in the PTC resistor of the present disclosure is preferably 60% by mass or more and 90% by mass or less, and more preferably 70% by mass or more and 85% by mass or less, relative to the total amount of the PTC resistor. When the total amount of binder resin is 60% by mass or more, the binding strength of the multi-walled carbon nanotubes is further improved. Furthermore, the resistance value of the PTC resistor after temperature rise is likely to be high. On the other hand, when the total amount of binder resin is 90% by mass or less, the amount of multi-walled carbon nanotubes becomes relatively large, and the resistance value of the PTC resistor at low temperatures is likely to be low.

[0026] On the other hand, the amount of the thermoplastic resin (e.g., thermoplastic elastomer) relative to the total amount of the PTC resistor is preferably 15% by mass or more and 85% by mass or less, and more preferably 20% by mass or more and 80% by mass or less. When the amount of the thermoplastic resin is within this range, the multi-walled carbon nanotubes are less likely to fall off from the PTC resistor even when the PTC resistor is subjected to repeated temperature changes. On the other hand, the amount of the semi-crystalline polymer relative to the total amount of the PTC resistor is preferably 70% by mass or less, and more preferably 40% by mass or more and 60% by mass or less. When the amount of the semi-crystalline polymer is within this range, the PTC characteristics of the PTC resistor are likely to be further improved.

[0027] (multi-walled carbon nanotubes) As described above, the multi-walled carbon nanotubes contained in the PTC resistor of the present disclosure have a structure in which multiple cylindrical graphite particles are arranged concentrically. The multi-walled carbon nanotubes may have a structure in which two or more cylindrical graphite particles are stacked, and the number of layers is usually two or more.

[0028] The average diameter of the multi-walled carbon nanotubes is preferably 2 nm to 400 nm, more preferably 5 nm to 200 nm. The average diameter refers to the outer diameter of the outermost carbon nanotubes. The average length of the multi-walled carbon nanotubes is preferably 100 μm or less, more preferably 5 μm or less. The aspect ratio of the multi-walled carbon nanotubes may be greater than 1, preferably 10 or more. The aspect ratio is preferably 3000 or less, more preferably 2000 or less. When the average diameter, average length, and aspect ratio of the multi-walled carbon nanotubes are within the above ranges, better PTC properties are likely to be exhibited. The average diameter and average length of the multi-walled carbon nanotubes can be measured, for example, from an image obtained by observation with a TEM (transmission electron microscope). The average diameter and average length are values ​​calculated by arithmetic averaging the diameters and lengths of any 50 or more multi-walled carbon nanotubes. The aspect ratio is a value determined from the average diameter and the average length, and specifically, is a value determined by the ratio of average length to average diameter.

[0029] The content of multi-walled carbon nanotubes is preferably 4% by mass or more and 35% by mass or less, and more preferably 10% by mass or more and 30% by mass or less, relative to the total amount of the PTC resistor. When the content of multi-walled carbon nanotubes is 4% by mass or more, the resistance value of the PTC resistor at low temperatures tends to be low, and when it is 10% by mass or more, the resistance value tends to be particularly low. On the other hand, when the content of multi-walled carbon nanotubes is 35% by mass or less, the amount of binder resin becomes relatively large enough, and when it is 30% by mass or less, the multi-walled carbon nanotubes tend to be particularly firmly bound. Furthermore, in the planar resistor of the present disclosure, since the shape of the multi-walled carbon nanotubes is cylindrical rather than spherical, even with a relatively small amount of multi-walled carbon nanotubes, the multi-walled carbon nanotubes tend to easily contact each other and achieve electrical continuity. Therefore, the content of multi-walled carbon nanotubes can be 30% by mass or less. A low content of multi-walled carbon nanotubes can also result in a PTC resistor with high optical transparency.

[0030] (others) In addition to the binder resin and the multi-walled carbon nanotubes, the PTC resistor may contain other components as needed, as long as the purpose and effect of the present disclosure are not impaired. Examples of other components include various additives such as antioxidants and flame retardants.

[0031] (Manufacturing method of PTC resistor) The PTC resistor of the present disclosure can be produced by applying an ink for a PTC resistor containing the binder resin, the multi-walled carbon nanotubes, and optionally a solvent, and then heating and curing the ink. That is, the ink for a PTC resistor of the present disclosure (hereinafter also referred to as "ink") can be a composition containing the binder resin, the multi-walled carbon nanotubes, and a solvent.

[0032] The type of solvent that the ink may contain is not particularly limited as long as it can uniformly dissolve or disperse the binder resin and multi-walled carbon nanotubes. However, the boiling point of the solvent is preferably 100°C or higher, more preferably 100 to 330°C, and even more preferably 150 to 250°C. When the boiling point of the solvent is within this range, the storage stability of the ink is improved and the ink is easier to apply.

[0033] The solvent is appropriately selected depending on the type of binder resin, etc. Examples include alcohols, ketones, esters, glycol esters, glycol ethers, ethers, aromatic hydrocarbons, and mixtures thereof, with terpineol, butyl carbitol acetate, tetralin, toluene, and mixtures thereof being preferred.

[0034] The amount of the solvent is selected appropriately depending on the desired viscosity of the ink, but is usually preferably about 20 to 70 parts by mass, and more preferably 25 to 65 parts by mass, per 100 parts by mass of the total amount of ink. If the amount of solvent is within this range, the viscosity of the ink tends to fall within the desired range.

[0035] The preferred viscosity of the ink is selected appropriately depending on the method for forming the PTC resistor. For example, when the ink is printed by screen printing and then cured to obtain a PTC resistor, the viscosity of the ink is preferably 100 to 400 dPa·s. This viscosity is measured at 25°C using a cylindrical rotational viscometer (manufactured by Rion Co., Ltd.). When the ink viscosity is within this range, the ink can be applied to the desired thickness and can form a film without unevenness.

[0036] The method for preparing the ink is not particularly limited, and the binder resin, multi-walled carbon nanotubes, and solvent may be mixed at once. On the other hand, when the binder resin contains a thermoplastic resin and a semi-crystalline polymer, the thermoplastic resin and solvent may be mixed first, and then the multi-walled carbon nanotubes and semi-crystalline polymer may be mixed.

[0037] The method for applying the ink is not particularly limited, and examples thereof include screen printing, roll coating, and application with a dispenser.

[0038] The ink can be cured by heating to about 100 to 200° C. The heating time is preferably about 1 to 30 minutes. Heating the ink to this extent removes the solvent from the ink.

[0039] 2. Planar heating element The above-mentioned PTC resistor can be used in a sheet heating element. The configuration of such a sheet heating element 100 will be described. As shown in the plan view of Fig. 1, the sheet heating element 100 has a substrate 1, a pair of electrodes 21, 22 arranged on the substrate 1, and a PTC resistor 3 arranged between the pair of electrodes 21, 22. However, the configuration of the sheet heating element 100 is not limited to this configuration and may further include any optional components.

[0040] The substrate 1 is not particularly limited as long as it is an insulating substrate on which the electrodes 21, 22 and the PTC resistor 3 can be laminated, and may be appropriately selected depending on the application of the sheet heating element 100. Specific examples of the substrate 1 include resin films, such as polyester films.

[0041] The pair of electrodes 21, 22 may have a structure including main electrodes 21 a, 22 a and comb-shaped electrodes 21 b, 22 b. The electrodes 21, 22 may be made of any material that can conduct electricity. The electrodes 21, 22 are typically made of metal, and may be made of aluminum, for example.

[0042] The method for producing the electrodes 21 and 22 is not particularly limited; for example, a patterned aluminum foil or the like may be attached to the substrate 1, or a metal layer placed on the substrate 1 may be patterned by cutting it out with a blade or the like.

[0043] The PTC resistor 3 is the same as the PTC resistor described above. The PTC resistor may be disposed between the pair of electrodes 21, 22, and its shape in plan view may be selected appropriately depending on the shape of the pair of electrodes 21, 22. The thickness of the PTC resistor 3 in the sheet heating element 100 is preferably 0.1 μm or more, and more preferably 0.5 to 10 μm. If the thickness of the PTC resistor 3 is 0.1 μm (more preferably 0.5 μm) or more, a sufficient amount of heat can be generated. On the other hand, if the thickness is 10 μm or less, the sheet heating element 100 becomes thin, and if it is 1 μm or less in particular, transparency is improved, making it easier to apply to various uses.

[0044] When using the sheet heating element 100, terminals 4 connected to the electrodes 21 and 22 are placed on the substrate 1, and the terminals 4 are connected to external electrodes. Then, by applying a voltage between the electrodes 21 and 22, the temperature of the PTC resistor 3 can be raised to a desired temperature.

[0045] The sheet heating element 100 can be used for a wide variety of purposes, such as heaters for defogging door mirrors in automobiles, heaters for defogging cameras inside automobile rear-end collision prevention devices, heaters for defogging millimeter-wave radar inside automobile rear-end collision prevention devices, antenna covers, thermistors, heaters for preventing pipeline freezing, floor heating, heated seats for chairs, heated seats for handrails, etc. [Example]

[0046] 1. Material Preparation The following components were prepared as materials for the PTC resistor.

[0047] (multi-walled carbon nanotubes) MW1: Average particle size: 100-200 nm, average length: 4.0 μm, aspect ratio: 20-40 MW2: Average particle size: 9.5 nm, average length: 1.5 μm, aspect ratio: 157.9 MW3: Average particle size: 16 nm, average length: 3.0 μm, aspect ratio: 187.5 MW4: Average particle size: 150 nm, average length: 4.0 μm, aspect ratio: 26.7

[0048] (single-walled carbon nanotubes) SW: Average particle diameter: 1.6 nm, average length: 5.0 μm, aspect ratio: 3125

[0049] (binder resin) Styrene-based thermoplastic elastomer (SEBS) Semi-crystalline polymer (hydrocarbon wax, melting point 78°C)

[0050] (solvent) Terpineol

[0051] 2. Preparation of PTC resistor (sheet heating element) [Example 1] 79.7 parts by mass of styrene elastomer and an appropriate amount of solvent (terpineol) (approximately 7 to 15 times the amount of styrene elastomer) were mixed in a stirrer at 100°C for 1 hour to prepare a varnish. 20.3 parts by mass of multi-walled carbon nanotubes (MW1) were added to the varnish. The mixture was then crushed in a crusher to obtain the desired screen printing ink. A substrate (PET: polyethylene terephthalate) with a pair of comb-shaped electrodes was prepared, and the ink described above was printed between the comb-shaped electrodes by screen printing. The substrate was then heated to 150°C to produce a sheet heating element with the desired PTC resistor (thickness: approximately 2.5 μm).

[0052] [Examples 2 to 5 and Comparative Example 1] The amount of styrene elastomer shown in Table 1 and an appropriate amount of solvent (terpineol) (approximately 7 to 15 times the amount of the styrene elastomer) were mixed in a stirrer at 100°C for 1 hour to prepare a varnish. The amounts of hydrocarbon wax and multi-walled carbon nanotubes or single-walled carbon nanotubes shown in Table 1 were added to the varnish. The mixture was then crushed in a crusher to obtain the desired screen printing ink. Then, in the same manner as in Example 1, a planar heating element having the desired PTC resistor (thickness: approximately 2.5 μm in each case) was produced.

[0053] 3. Evaluation The sheet resistance and maximum PTC resistance ratio of the PTC resistor were evaluated by the following method.

[0054] (Surface resistance measurement) The surface resistance of each sheet heating element at 25°C was measured by the two-terminal method or the four-terminal method.

[0055] (Maximum PTC magnification) The resistance value of each sheet heating element was measured when the temperature was raised from -40°C to 120°C. The resistance value at 25°C and the maximum resistance value were then determined, and the ratio of these (maximum resistance value / resistance value at 25°C) was taken as the maximum PTC multiplier. The results are shown in Table 1.

[0056] (judgement) The maximum PTC multiplication factor was evaluated according to the following criteria, and the results are shown in Table 1. 〇: Maximum PTC multiplier is 1.5 or more △: Maximum PTC magnification is over 1.0 and less than 1.5 ×: Maximum PTC magnification is 1.0 or less

[0057] [Table 1]

[0058] As shown in Table 1 above, the PTC resistor (sheet heating element) using single-walled carbon nanotubes had a high sheet resistance and a maximum PTC magnification of 1, clearly indicating that it did not function as intended (Comparative Example 1). In contrast, the PTC resistor (sheet heating element) using multi-walled carbon nanotubes had a very low sheet resistance and a PTC magnification of 1.8 or more. In other words, the sheet resistance was very low and the PTC magnification was sufficiently high. [Industrial Applicability]

[0059] The PTC resistor of the present disclosure has excellent PTC characteristics and is therefore very useful for producing various types of planar heating elements. [Explanation of symbols]

[0060] 1 board 3 PTC resistor 4 terminals 21, 22 electrodes 21a, 22a Main electrode 21b, 22b comb-shaped electrode 100 Planar heating element

Claims

1. A binder resin, Multi-walled carbon nanotubes, A PTC resistor comprising:

2. The binder resin contains a thermoplastic styrene-based elastomer.

2. The PTC resistor of claim 1.

3. The binder resin comprises a semi-crystalline polymer.

2. The PTC resistor of claim 1.

4. The aspect ratio of the multi-walled carbon nanotubes is 50 or less.

2. The PTC resistor of claim 1.

5. A substrate; a pair of electrodes disposed on the substrate; The PTC resistor according to any one of claims 1 to 4, which is disposed on the substrate and between the pair of electrodes; Including, Surface heating element.

6. A binder resin, Multi-walled carbon nanotubes, A solvent, Including, Ink for PTC resistors.

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