PTC resistor

Abstract

The present invention addresses the problem of providing a PTC resistor wherein the resistance value is easily increased within a short time after reaching a certain temperature and the resistance value after temperature increase is high. A PTC resistor according to the present invention, which solves the above-described problem, comprises conductive particles, a polyolefin-based wax and a binder resin. With respect to the DSC curve of the polyolefin-based wax as obtained by raising the temperature from 25°C to 150°C at a heating rate of 10°C / min using a differential scanning calorimeter, the number of melting peaks found in the DSC curve is 1 or 2.

Description

PTC resistor

[0001] The present disclosure relates to PTC resistors.

[0002] To remove frost and dew from automobile door mirrors, heaters are often installed on the rear surface 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 sheet heating element with PTC characteristics typically has a pair of electrodes and a resistor with PTC characteristics (also referred to herein as a "PTC resistor") 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 sheet 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 sheet 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 sheet heating element is controlled so as not to exceed a certain level.

[0004] JP 2012-227081 A

[0005] However, conventional PTC resistors using crystalline resins as binders have the problem that their thermal expansion during temperature rise is still insufficient, and it takes time for the resistance value to increase even after the temperature reaches a certain level. There is also a need to further increase the resistance value of PTC resistors after temperature rise.

[0006] Therefore, an object of the present disclosure is to provide a PTC resistor whose resistance value is likely to increase in a short time after reaching a certain temperature and whose resistance value after temperature rise is high.

[0007] One embodiment of the present disclosure provides a PTC resistor comprising conductive particles, a polyolefin wax, and a binder resin, wherein the number of melting peaks observed in a DSC curve of the polyolefin wax obtained by heating the polyolefin wax from 25°C to 150°C at a heating rate of 10°C / min using a differential scanning calorimeter is one or two.

[0008] According to the present disclosure, a PTC resistor can be obtained 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 rises.

[0009] FIG. 1A is a DSC curve of the polyolefin wax used in Examples 1 and 2, and FIG. 1B is a DSC curve of the ultra-high molecular weight polyethylene used in Comparative Examples 2 and 3.

[0010] In this specification, a numerical range indicated by "to" means a numerical range including the numerical values ​​written before and after "to".

[0011] As described above, conventional PTC resistors have been imparted with PTC characteristics primarily by utilizing the thermal expansion and recrystallization of crystalline resins. However, with this method, the rate at which the resistance value increases after reaching a certain temperature is slow, and the degree of increase in the resistance value is also small. In contrast, the PTC resistor disclosed herein contains a specific polyolefin wax in addition to conductive particles and a binder resin. In this PTC resistor, the polyolefin wax melts near its melting point when the temperature increases. The volume increase due to melting of the polyolefin wax is significantly greater than the volume increase of typical resins.

[0012] Furthermore, when a DSC curve is created for this polyolefin wax using a differential scanning calorimeter (hereinafter also referred to as "DSC") by heating from 25°C to 150°C at a heating rate of 10°C / min, the number of melting peaks observed in the DSC curve is one or two. A small number of melting peaks in the DSC curve indicates very few impurities and means that the wax melts at the melting peak temperature in a short time. Therefore, in a PTC resistor containing this polyolefin wax, when the temperature rises, the polyolefin wax melts in a short time at the melting peak temperature of the polyolefin wax, and the resistance value increases significantly in a short time. A PTC resistor with such excellent PTC properties can be suitably used in various planar heating elements, etc. Below, each component of the PTC resistor of the present disclosure will be described.

[0013] (Polyolefin Wax) In this specification, wax refers to an organic substance that is solid or semi-solid at room temperature and melts without decomposition when heated. Furthermore, polyolefin wax refers to a wax containing 50% by mass or more of an olefin-derived component. The polyolefin wax used in this disclosure has one or two melting peaks observed in the DSC curve when heated from 25°C to 150°C at a heating rate of 10°C / min by DSC. It is more preferable that one melting peak is observed. Note that, in this specification, the melting peak in the DSC curve refers to an endothermic peak whose absolute value of the peak height is 0.5 mW or more, and does not include noise whose height is less than 0.5 mW. Note that all DSC curves in this specification are values ​​obtained using 5 mg of sample.

[0014] In this specification, the peak height is defined as a value calculated as follows. First, in the DSC curve, the falling and rising positions of the line in the downward convex region (region where endothermic reaction occurs) are confirmed. If both can be confirmed, they are connected with a line, which is used as the baseline. Note that in this specification, a baseline is drawn only for large peak regions where the rising and falling powers exceed 1 mW. For example, as shown in FIG. 1B, if a single large peak region (downward convex region) contains small peaks (B, C, and D) with rising or falling powers of 1 mW or less, only one baseline is drawn for the large peak region. Furthermore, as shown in FIG. 1B, if there are two or more large peak regions (downward convex regions), a baseline is drawn for each of them (two in FIG. 1B). On the other hand, as shown in FIG. 1A, if either the rising or falling position of the line in the downward convex region cannot be confirmed, a linear baseline is drawn along the base of the side where the rising or falling position can be confirmed.

[0015] Then, a line perpendicular to the X-axis is drawn from each peak top of the melting peak of the DSC curve (A in FIG. 1A, and B, C, D, and E in FIG. 1B), and the distance from the intersection of the perpendicular line with the baseline to the peak top is defined as the height of each peak.

[0016] The absolute value of the melting peak height in the DSC curve obtained by the above method is preferably 5 mW or more. When the absolute value of the melting peak height is 5 mW or more, many polyolefin waxes tend to melt at the melting peak temperature. When there are two melting peaks in the DSC curve, it is sufficient that the absolute value of the height of either one of them is 5 mW or more.

[0017] The melting peak in the DSC curve of the polyolefin wax 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 provides a PTC resistor that shows an increase in resistance value at 80° C. to 150° C.

[0018] The weight-average molecular weight of the polyolefin wax example is preferably 1,000 to 1,500,000, more preferably 3,000 to 20,000. When the weight-average molecular weight of the polyolefin wax is within this range, it is likely to have a melting peak within the above temperature range. The weight-average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0019] The PTC resistor may contain only one type of the polyolefin wax, or may contain two or more types. Examples of polyolefin waxes include olefin homopolymers, copolymers of two or more types of olefins, and copolymers of olefins and monomers other than olefins. More specific examples of polyolefin waxes include 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. Alternatively, oxidized polyolefin waxes obtained by oxidizing these waxes using known methods may be used. Among these, polyethylene waxes are preferred from the viewpoints of availability and PTC characteristics.

[0020] The amount of polyolefin wax in the PTC resistor 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 polyolefin wax in the PTC resistor is 6% by mass or more, the PTC magnification of the PTC resistor is further increased, and the rate of increase in the resistance value after the PTC resistor has reached a constant temperature after heating is likely to be further increased. On the other hand, when the amount is 24% by mass or less, the amount of conductive particles is sufficiently large relative to the PTC resistor, and the PTC characteristics are likely to be further improved.

[0021] (Binder Resin) The binder resin may be any resin capable of binding the polyolefin wax and the conductive particles described below, or binding them to a substrate or the like, but it is preferable that the binder resin does not prevent the volume change caused by melting the polyolefin wax. Furthermore, the binder resin itself may expand with increasing temperature, contributing to an increase in the resistance value of the PTC resistor. The PTC resistor may contain only one type of binder resin, or may contain two or more types.

[0022] Examples of the binder resin include known thermoplastic resins. Specific examples 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, SEBS resins (styrene-ethylene-butylene-styrene copolymers) and their hydrogenated products, and SEPS resins (styrene-ethylene-propylene-styrene copolymers) and their hydrogenated products. Among these, SEBS resins and hydrogenated SEPS resins are preferred because they are easily deformable and do not interfere with the volume change of the polyolefin wax.

[0023] The amount of binder resin in the PTC resistor is preferably 6% by mass or more and 30% by mass or less, and more preferably 15% by mass or more and 28% by mass or less. When the amount of binder resin in the PTC resistor is 6% by mass or more, it becomes easier to print conductive resins, polyolefin waxes, and compositions containing the binder resin, and the fixation after printing becomes good. Furthermore, the strength of the PTC resistor is likely to be further increased. On the other hand, when the amount of binder resin is 30% by mass or less, the amount of conductive particles becomes sufficiently large relative to the amount of conductive particles, and the PTC characteristics are likely to be further improved.

[0024] (Conductive Particles) The conductive particles are not particularly limited as long as they are conductive particles. －６ / °C or less, and －６ / °C or less is more preferred. The thermal expansion coefficient of the conductive particles can be determined from the material of the conductive particles. The PTC resistor may contain only one type of conductive particles or may contain two or more types of conductive particles.

[0025] Examples of 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; etc. Among these, nickel powder, silver powder, tungsten powder, and graphite are preferred because they have high affinity with the above-mentioned polyolefin wax and binder resin, and are less likely to precipitate in the composition for producing a PTC resistor.

[0026] The shape of the conductive particles is not particularly limited, and may be, for example, spherical, irregular, tubular, rod-like, flat, crushed, etc. The specific surface area of ​​the conductive particles is 5 m ２ The specific surface area is a value measured by a gas adsorption method.

[0027] The size of the conductive particles is selected appropriately depending on the type of conductive particles, etc. For example, when the conductive particles are spherical or similar in shape, 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 magnification of the PTC resistor tends to be better. The average particle diameter is a value measured by a laser diffraction / scattering method, and is the median (D50) in the cumulative particle size distribution.

[0028] The content of 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 even more preferably 55% by mass or more and 67.5% by mass or less. When the amount of conductive particles in the PTC resistor is 45% by mass or more, the PTC characteristics tend to be more stable. On the other hand, when the amount of conductive particles in the PTC resistor is 80% by mass or less, the amount of the polyolefin wax and binder resin increases, and the resistance value when heated tends to increase.

[0029] (Others) In addition to the polyolefin wax, the binder resin, and the conductive particles, the PTC resistor may contain other components as needed, provided that the purpose and effect of the present disclosure are not impaired. Examples of other components include various additives such as antioxidants and flame retardants.

[0030] (Method for manufacturing PTC resistor) The PTC resistor of the present disclosure can be manufactured by applying a composition containing the polyolefin wax, the binder resin, the conductive particles, and optionally a solvent, and then heating and curing the composition.

[0031] The type of solvent is not particularly limited as long as it can uniformly dissolve or disperse the polyolefin wax, binder resin, conductive particles, etc. 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 composition is improved and the composition is easier to apply.

[0032] The solvent is appropriately selected depending on the type of polyolefin wax, the type of binder resin, etc. Examples of the solvent 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.

[0033] The amount of the solvent is appropriately selected depending on the desired viscosity of the composition, and 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 the composition (ink). When the amount of the solvent is within this range, the viscosity of the composition (ink) tends to fall within the desired range.

[0034] The preferred viscosity of the composition is appropriately selected depending on the method for forming the PTC resistor. For example, when the composition is printed by screen printing and then 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 using a cylindrical rotational viscometer (manufactured by Rion Co., Ltd.). When the viscosity of the composition is within this range, the composition can be applied to a desired thickness and can form a film without unevenness.

[0035] The method for preparing the composition is not particularly limited, and the polyolefin wax, resin binder, conductive particles, and solvent may be mixed at once, or the resin binder and solvent may be mixed first, and then the polyolefin wax and conductive particles may be mixed.

[0036] The method for applying the composition is not particularly limited, and examples thereof include screen printing, roll coating, and application with a dispenser.

[0037] The composition can be cured by heating it to about 100 to 200° C. The heating time is preferably about 1 to 30 minutes. Heating the composition to this extent removes the solvent in the composition.

[0038] (Uses of PTC Resistors) The PTC resistors can be combined with, for example, a pair of electrodes to form a planar heating element. When heated above a certain temperature, the PTC resistors exhibit a high resistance in a short time, and this resistance is significantly higher than the resistance at room temperature (25°C). Therefore, they can be used in a wide variety of applications. For example, they can be used in heaters such as defogging heaters for automobile door mirrors, heaters for defogging interior cameras in automobile rear-end collision prevention systems, heaters for defogging interior millimeter-wave radars in automobile rear-end collision prevention systems, antenna covers, thermistors, heaters for preventing the freezing of pipelines, floor heating, heated seats for chairs, heated seats for handrails, etc.

[0039] Specific examples of the present invention will be described below, but the scope of the present invention should not be construed as being limited by these examples.

[0040] 1. Preparation of Materials The following compounds were prepared as materials for producing a PTC resistor.

[0041] (Polyolefin-based components) Polyethylene wax (CERAFLOUR 961 manufactured by BYK) Ultra-high molecular weight polyethylene (MIPERON PM-200 manufactured by Mitsui Chemicals)

[0042] (Binder resin) SEBS resin (styrene-ethylene-butylene-styrene block copolymer, Tuftec 1913, manufactured by Asahi Kasei Corporation) Hydrogenated SEPS resin (hydrogenated styrene-ethylene-propylene-styrene block copolymer, Septon 2002, manufactured by Kuraray Co., Ltd.)

[0043] (Conductive particles) PWSH: crushed carbon (graphite, manufactured by Eitan Co., Ltd.) PG11: spherical carbon (graphite, manufactured by Eitan Co., Ltd.) ICB: spherical carbon (graphite, manufactured by Nippon Carbon Co., Ltd.)

[0044] A DSC curve was obtained for the polyolefin component in the range of 25°C to 150°C at a heating rate of 10°C / min using a DSC. The number of melting peaks and the absolute values ​​of the melting peak heights were determined. These results are shown in Table 1. The DSC curves for the polyethylene wax used in Examples 1 and 2 are shown in Figure 1A, and the DSC curves for the ultra-high molecular weight polyethylene used in Comparative Examples 2 and 3 are shown in Figure 1B.

[0045] The average particle diameter D50 of the conductive particles was measured by a laser diffraction / scattering method. The thermal expansion coefficient was measured using a literature value. The specific surface area was measured by a gas adsorption method. The results are shown in Table 1.

[0046] 2. Preparation of Sheet Heating Elements Containing PTC Resistors Sheet heating elements containing each PTC resistor were prepared according to the following procedure. [Example 1] 8.0 parts by mass of hydrogenated SEPS resin and 51.0 parts by mass of an organic solvent (terpineol) were mixed in a stirrer at 100°C for 1 hour to prepare a varnish. This varnish was mixed with 9.0 parts by mass of polyolefin wax (polyethylene wax) and 32.0 parts by mass of conductive particles (spherical carbon) and crushed in a crusher to obtain the desired screen printing ink. A substrate equipped 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 prepare a sheet heating element with the desired PTC resistor.

[0047] Comparative Example 1 A sheet heating element having a PTC resistor was produced in the same manner as in Example 1, except that a paste was produced with the composition shown in Table 1 without using any polyolefin wax.

[0048] Example 2 and Comparative Examples 2 and 3 A sheet heating element having a PTC resistor was produced in the same manner as in Example 1, except that the organic solvent and the types and amounts of each component were changed as shown in Table 1.

[0049] [Evaluation] The following tests were carried out on the sheet heating elements produced in Examples 1 and 2 and Comparative Examples 1 to 3. The results are shown in Table 1.

[0050] (Measurement of Temperature Rise Rate) The sheet heating element was heated in a room temperature environment (25°C), and the temperature when it stabilized and the time it took to reach that temperature were determined. This was determined using the following formula: (stable temperature - initial temperature) / (time until stabilization).

[0051] (Measurement of Sheet Resistance) The sheet resistance of each sheet heating element was measured by the two-terminal method or the four-terminal method.

[0052] (Determination of 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 maximum resistance value and the minimum resistance value were then determined, and the ratio of these (maximum resistance value / minimum resistance value) was calculated as the maximum PTC magnification. The measurement of each resistance value was performed using the two-terminal method or the four-terminal method.

[0053] (Determining the maximum PTC magnification in a room temperature environment) The resistance value of the sheet heating element at 25°C was measured immediately after production. Next, the temperature of the sheet heating element was changed from -40°C to 120°C, and then returned to 25°C, and the resistance value was measured again. The ratio of these values ​​(resistance value at 25°C after the temperature change / resistance value at 25°C before the temperature change) was calculated as the maximum PTC magnification in a room temperature environment.

[0054] (Evaluation) A maximum PTC magnification of 100 times or more was evaluated as ◯, and a maximum PTC magnification of less than 100 times was evaluated as x.

[0055]

[0056] As shown in the table above, the sheet heating elements using PTC resistors containing polyolefin wax (Examples 1 and 2) had a much higher maximum PTC magnification and a faster temperature rise rate than the sheet heating elements using PTC resistors that did not contain this wax (Comparative Examples 1 to 3).

[0057] This application claims priority based on Japanese Patent Application No. 2023-046587, filed March 23, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0058] The PTC resistor of the present disclosure has a resistance that easily rises in a short time after reaching a certain temperature, and has a high resistance after the temperature rise, making it extremely useful for producing various sheet heating elements.

Claims

1. Conductive particles; Polyolefin wax, A binder resin, Including, the binder resin contains at least one of a styrene-ethylene-butylene-styrene copolymer resin and a hydrogenated styrene-ethylene-propylene-styrene copolymer, the number of melting peaks observed in a DSC curve of the polyolefin wax obtained by heating the wax from 25°C to 150°C at a heating rate of 10°C / min using a differential scanning calorimeter is 1 or 2; PTC resistor.

2. The absolute value of at least one melting peak height in the DSC curve is 5 mW or more; 2. The PTC resistor of claim 1.

3. The conductive particles have a thermal expansion coefficient of 20×10 －６ / °C or less, 2. The PTC resistor of claim 1.

4. The conductive particles have an average particle size of 30 μm or less.

2. The PTC resistor of claim 1.

5. The conductive particles have a specific surface area of ​​5 m ２ / g or less, 2. The PTC resistor of claim 1.