JPS6259415B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a heater or safety element having a positive temperature coefficient of resistance (hereinafter abbreviated as PTC).

Structures of Conventional Examples and Their Problems Conventionally, as resistor compositions having PTC characteristics, there have been known resistor compositions in which conductive fine powder such as carbon black is dispersed in a crystalline polymer such as polyethylene. The mechanism that produces the PTC property is that when the crystalline polymer transitions to an amorphous state near its crystalline melting point, the coefficient of expansion rapidly increases, and the chain of dispersed carbon black is broken, resulting in a decrease in resistance. This is expected to increase. Therefore, it can be said that the temperature range in which remarkable PTC characteristics are obtained is determined by the crystal melting point of the crystalline polymer, and the magnitude of the temperature coefficient is determined by the magnitude of the crystallinity. The PTC characteristics of this composition are relatively stable and reproducible, although there is some hysteresis due to thermal cycling, etc.
Its value as a material for PTC heaters has been recognized. A conventional example of a PTC heater using such a PTC composition is shown in FIG. In FIG. 1, 1 is a PTC composition made of a mixture of polyethylene and carbon black, and 2 and 2' are a pair of silver paste baked electrodes provided on the surface of the PTC composition.
This structure has a large cross-sectional area and a small thickness, so it is possible to obtain a sufficiently low resistance value even if the weight composition ratio of carbon black is small. The feature is that it is easy to take out. Therefore, it has an ideal shape and structure for the purpose of constructing a PTC heater with low resistance and high output. but,
Many polymers with a high degree of crystallinity, such as polyethylene, have almost no polar groups and have poor adhesive strength, causing problems in electrical or mechanical bonding with electrodes. For example, even when using a baked silver paste electrode using an epoxy resin with particularly good adhesion as a binder, the lack of strength of the coating film itself resulted in an impractical electrode structure. . The preferred electrode material is a metal foil with high resistance and strength, such as copper foil or nickel foil, but it is difficult to obtain an adhesive strength that exceeds that of silver paste using epoxy resin, and in any case, the application The current situation was that it had no choice but to be used in a limited manner.

Purpose of the Invention The present invention provides a means for obtaining a PTC composition that exhibits good adhesion and resistance temperature characteristics to metal foils such as copper foil and nickel foil that have excellent physical properties as electrode materials. High output with wide value adjustment range
The purpose is to provide PTC heaters.

Structure of the Invention The present invention comprises a pair of parallel plate electrodes provided close to each other, a crystalline polymer containing a functional group having good adhesion to metal and a conductive material provided between the pair of parallel plate electrodes. The basic structure of the heater is a heater having PTC characteristics, which is composed of a resistor composition mainly composed of a magnetic fine powder and a resistor composition mainly composed of the resistor composition.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described based on FIGS. 2 and 3. 3 and 3' are electrolytic copper foil electrodes with a plate thickness of 35 μm, and 4 is a PTC sheet made of a mixture of a crystalline polymer in which a carboxyl group is introduced at the end of a low-density polyethylene molecule and oil furnace carbon black. It is a resistor composition. This carboxyl group-introduced low-density polyethylene has carboxyl groups introduced mainly at the ends of each molecule, and unlike copolymers, it has little effect on the physical properties such as crystallinity and crystal melting point of low-density polyethylene. First, strong adhesion to metals, which is unique to carboxyl groups, can be obtained.

As a first example, the amount of carbon black added to the PTC resistor composition 4 is 60% by weight, and the carbon black has an average particle diameter of 500 Å that can be observed with an electron microscope, and a shape that can also be observed with an electron microscope. uses an almost spherical shape.
The sheet thickness of PTC resistor composition 4 is approximately 300 μm
As shown in curve A in Figure 3, the resistance value at 20°C is extremely low, 20 mΩ per square centimeter, and in the temperature range exceeding 80°C, the resistance value begins to increase rapidly, reaching the crystal melting point of 110°C. At ℃, it reaches approximately 2 kΩ, achieving an extremely excellent PTC resistance temperature characteristic of approximately 5 orders of magnitude. In this way, resistors with low resistance at room temperature and excellent resistance-temperature characteristics of PTC can be used as compact heaters with excellent quick heating properties used in low-voltage power supplies such as batteries, or when excessive current is generated. It can be used as a safety element connected in series with a load in case of current or temperature. When creating resistors with the same constituent materials at the same composition ratio using low-density polyethylene without introducing carboxyl groups,
Although it was possible to create a resistor that looked identical in appearance, the resistance value at 20°C was as high as 30 to 60 mΩ, and the variation in resistance was large, and there was contact resistance between the electrode and the resistor composition. It shows that you are doing it. In addition, the peel strength was low, and the contact state was unstable even when subjected to external forces and thermal cycles, so that it was not possible to obtain a product that could withstand practical use. In this case, the only options are to find a special adhesive treatment, use the electrodes after they are crimped, or reduce the carbon black blending ratio to 50% or less to improve the adhesion to some extent. This was also a major constraint, making it impossible to obtain a PTC heater with low resistance and high reliability.

A carboxyl group is the best functional group to introduce into a crystalline polymer, but the most common method of introduction is through copolymerization with acrylic acid, etc., which has a carboxyl group. Acrylic acid copolymers and the like are easily available. However, in the method of introducing a functional group that can produce a copolymer, although adhesion to metals is certainly improved, physical properties such as crystallinity and crystal melting point also tend to decrease. Therefore, in the PTC characteristics, the low resistance value starts to increase at a lower temperature, and the rate of change in the low resistance value also tends to decrease. Therefore, it is not convenient when it is desired to make use of the characteristic values of low-density polyethylene, high-density polyethylene, etc. to impart only adhesive properties. For such purposes, instead of adding a functional group as a comonomer and polymerizing it, a polymer such as polyethylene that has already been polymerized contains a carboxyl group such as an acid anhydride and reacts with the polyolefin. By selectively reacting mainly at the ends of each polyethylene molecule using the obtained material, and then removing unreacted substances,
Physically, it is mostly polyethylene, but due to the effective presence of carboxyl groups, a crystalline polymer with excellent adhesion to metals can be obtained. Polyolefins with functional groups based on a similar concept include the Admar series by Mitsui Petrochemical Industries, Ltd., and while these materials exhibit PTC properties that are almost indistinguishable from those of the base polyolefin, they do not contain copper. Extremely excellent adhesion to metals such as foil can be obtained. Examples of functional groups other than carboxyl groups include hydroxyl groups, amino groups, epoxy groups, and composite groups thereof, which can be applied in the same manner as in the case of carboxyl groups.

Next, the influence of the particle size, shape, and composition ratio of carbon black and the more preferable range will be described. As shown in Fig. 4, the carbon black to be dispersed in the PTC resistor composition is particularly preferably a nearly spherical material with an average particle diameter and shape of 400 Å or more that can be observed with an electron microscope;
Materials with a thickness of ~300 Å or non-spherical materials with a large surface area have a sufficient resistance change factor.
Not only is it difficult to obtain PTC characteristics, but the resistance-temperature characteristics tend to become negative in the temperature range above the crystal melting point, and when high voltage is applied or used in a high temperature atmosphere, as the temperature increases, There is a possibility that a so-called runaway phenomenon, in which the power increases as a result, occurs. Furthermore, even if spherical carbon black has an average particle size of 400 Å or more, the temperature coefficient of resistance of PTC will increase as the weight ratio in the PTC resistor composition is reduced, but the temperature coefficient of resistance above the crystal melting point will increase. becomes a negative characteristic, and if left at that temperature for a long time, the resistance value will decrease over time. These phenomena occur because carbon black with a small average particle size or non-spherical carbon black has a large surface area and has a large filling effect on the crystalline resin, which inhibits the expansion of the resin as the temperature increases, resulting in a large PTC resistance temperature. The rate of change in properties cannot be obtained, and since the cohesive force is large, redispersion occurs in the direction of forming chains of carbon blacks in the amorphous dispersion state above the crystal melting point, resulting in negative resistance-temperature properties and It is thought that a phenomenon in which the resistance value decreases occurs. Carbon black, which is spherical and has a large average particle size, has only one point of contact with each other, and even the slightest movement causes non-contact, compared to carbon black that is non-spherical and has multiple complex contact points.
Naturally, it is easier to obtain a change in resistance value, and the cohesive force is also small, so even in a temperature range above the crystal melting point, it is thought that the resistance value continues to increase, clearly reflecting the expansion tendency of the resin. In addition, regarding the influence of the weight composition ratio of carbon black, the smaller the ratio, the smaller the filling effect on the resin, so it does not inhibit expansion, and the rate of change in the resistance temperature characteristics of PTC increases, but conversely, the crystal melting point In the amorphous state in the above temperature range, distortion occurs due to internal stress due to insufficient filling effect, and due to the movement of carbon black, orientation and redispersion phenomena occur with respect to the initial uniformly dispersed state, resulting in resistance. It is thought that the value will decrease. As shown in Figure 5, this tendency is observed in the composition ratio of spherical carbon black with an average particle size of 500 Å and low-density polyethylene.
In the PTC resistor composition, it is thought that this becomes noticeable when the weight composition ratio of carbon black is less than 40%, and reaches its maximum at the minimum composition ratio of 10% where conductivity can be confirmed.

The above-mentioned negative resistance-temperature characteristics and decrease in resistance above the crystal melting point may not be much of a problem depending on the application. However, it cannot be said that it is sufficiently safe in cases where abnormal voltage application is expected, in applications where it is used at high temperatures for long periods of time, or in cases where it is used as a final safety mechanism. Since the PTC heater of this example has electrodes of different polarities close to each other and has a large area, the voltage gradient applied to the PTC resistor composition is quite steep.
The structure is such that even if there is a defect in a very small part, that part can generate abnormal heat. From this perspective,
The PTC characteristics must have sufficient margin, and the use conditions must be carefully examined before applying a device that exhibits a negative characteristic region or a resistance value drop phenomenon.

Next, as a second example, a relatively high resistance
We will describe the method for manufacturing a PTC heater. In order to create a high-resistance PTC heater, the blending ratio of carbon black must be 50% by weight or less, and in this case, negative resistance temperature characteristics and a decrease in resistance occur in the temperature range above the crystal melting point. do. In order to prevent this, it is recommended to add a high resistance or electrically insulating filler that can provide a filling effect corresponding to a weight composition ratio of 40 to 70% of carbon black. The weight composition ratio of carbon black is
70% is a tentative limit, and if it exceeds this, not only will it become brittle against bending, but the rate of change in PTC will decrease, making kneading impossible. The filling effect should be judged based on the mechanical properties, PTC properties, and kneading properties, and the optimal composition ratio will vary depending on the filler material and particle size. By adding a filler, the negative resistance-temperature characteristics above the crystal melting point are hardly observed, but this effect is due to the fact that carbon black is also considered to be a filler with a type of filling effect, and when the composition ratio is 40% or more, crystallinity It is thought that the filling effect can be obtained even in the microscopic parts of the resin, and the same effect can be obtained even with fillers other than carbon black, although there are some differences.

In the PTC heater of the first embodiment shown in FIG. 2, the amount of carbon black added to the total amount of resin and carbon black in PTC resistor composition 4 was reduced to 35% by weight, and 40% by weight of alumina powder with a particle size of 1 μm
As a result, as shown in curve B in Figure 3, the resistance value per square centimeter at 20°C is 120Ω, and the resistance value at 110°C is 120Ω.
Excellent resistance-temperature characteristics of 800 kΩ, about four orders of magnitude less, were obtained, and compared to curve C in which no alumina powder was added, there was almost no tendency for negative characteristics or a decrease in resistance at temperatures above 110°C. this
PTC resistance temperature characteristics are 60 for carbon black only.
The temperature coefficient is slightly smaller than that of the first example in which % by weight was added, but the characteristics are as if they were shifted in parallel to the high resistance side by about 4 orders of magnitude, so there is no other difference other than the difference in resistance value. I can't even see it. Furthermore, by adjusting the composition ratio of carbon black and filler, it is possible to create a PTC heater with arbitrary resistance-temperature characteristics. The filler can be used in any shape such as granular, fibrous, or leaf-like, and regardless of whether it is organic or inorganic, and is effective as long as it is a high-resistance or electrically insulating material. Particularly effective materials include materials with a wide particle size distribution or mixtures of materials with two or more types of particle sizes. In order to prevent the temperature coefficient of negative characteristics in the temperature range above the crystal melting point, it is desirable to use particles with a small particle size that has excellent filling effects in the microscopic range, but since they also have a strong tendency to inhibit PTC characteristics, it is necessary to The most desirable method is to use a material with a large particle size that has an excellent filling effect in a macroscopic range and does not inhibit PTC characteristics, and to obtain a mutual effect of both.

Regarding the method of adjusting the resistance value, it is possible to change the thickness of the resistor composition of the PTC heater, but
If the resistor is made too thick, for example exceeding 1 mm, the thermal resistance inside the resistor increases and a considerable temperature gradient is required to match the heat flux passing through the resistor.
Also, the temperature distribution in the voltage application direction inside the resistor is 1
Even at ℃, the temperature coefficient of resistance exceeds 0.21/℃
In a PTC resistor, a resistance value distribution of 20% occurs, and there is a heat generation density distribution with a difference of 20% proportional to the resistance value. If the temperature distribution exceeds 5℃, a heat generation density distribution of 1.2 to the fifth power, or 2.5 times, will occur, and the resistor at the highest temperature will reach a heat value that exceeds its limit, causing redispersion of carbon black. However, a change in resistance value that cannot be recovered will occur. The thermal conductivity of general crystalline polymers is
Since it is in the range of 0.1 to 0.4 Kcal/mh℃, room temperature 20
In a PTC resistor that is stable at 80℃ and generates and dissipates heat at 600Kcal/m ² h,
The temperature difference per thickness should be 1.5-6°C. Furthermore, considering a high temperature PTC resistor or a case where the load is large, it is necessary to assume a temperature of 1000 to 10000 Kcal/m ² , and in that case, the temperature difference per 1 mm thickness will be as high as 2.5 to 100°C. 10000Kcal／
If m ² is generated, it is considered to be near room temperature, where the PTC temperature coefficient is small, and the temperature is rising or the load is sufficiently cold, and the heat generation density distribution depending on the temperature coefficient may not be that great. Even so, a temperature difference exceeding 100°C is considered to be close to the limit for an organic heater. For example, when constructing a submerged heater that heats a liquid at 20°C using a PTC heater whose resistance value increases rapidly in a temperature range of 100°C or higher, even if the surface of the heater is 20°C, Inside is
It would have to reach 120°C, and the distribution of resistance values would be very large. In this case, the portion that essentially functions as a heat generating portion is considered to be a very thin portion with high resistance, high voltage gradient, and high power density. The other parts are simply heat transfer paths, and rather serve only as heat insulators. From the above point of view, PTC with a thickness of 1 mm or more
It seems that the resistor has no meaning at all. In addition, as a means to alleviate the above-mentioned problems, there is a method of adding a filler with high thermal conductivity.For example, high-purity alumina powder with a thermal conductivity of 30 Kcal/mh°C or carbonized filler with a thermal conductivity of 90 Kcal/mh°C is available. By adding silicon powder, the thermal conductivity of the resistor can be increased from 0.5 to
Since it is technically easy to set the temperature to 1 Kcal/mh°C, it is possible to achieve a temperature distribution of 1 to 20°C in a heater with a thickness of 1 mm and an output of 10000 Kcal/m ² h. This filler can be used in combination with the filler for improving the negative resistance-temperature characteristics in the high temperature range, or can be added independently. As a filler for improving thermal conductivity, it is relatively advantageous to use a filler with a large particle size, and in extreme cases, it may span between a pair of electrodes. Summarizing the results described above, the thermal resistance value of the PTC resistor in the thickness direction can be defined as 0.01 h°C/Kcal or less per square meter.

Next, we will discuss the heat resistance stability of this PTC resistor. The above-mentioned PTC resistor itself is highly stable and can withstand long-term use, but when polyolefin-based materials are used for the resin, heavy metal ions used in the electrodes can damage the polyolefin. may undergo significant oxidative deterioration. Since the PTC heater of the present invention is constructed very close to copper foil electrodes, etc., special attention must be paid to this point. In addition, in order to prevent the deterioration process caused by the scission of the main chain of polyolefin and the crosslinking that occurs as a secondary reaction, the best effect can be obtained by using a combination of phenol and thioether stabilizers, but the presence of thioether As a result, electrodes such as copper foil may deteriorate. In order to prevent such a phenomenon, it is effective to add a heavy metal ion deactivator that can combine with heavy metal ions to form metal complex compounds, and is necessary to obtain long-term reliability. The blending ratio of these stabilizers must be sufficient to overcome the inhibitory effect of carbon black, with 1 to 10% of phenol and thioether stabilizers based on the resin, and 1 to 10% of heavy metal ion deactivators. It is effective to add 0.5 to 2%, which is a larger amount than usual.

Effects of the Invention As described above, the heater of the present invention provides the following effects.

(1) Low resistance PTC containing a large amount of conductive fine powder
Since good adhesion to metal can be obtained even with the composition, by providing a resistor between the parallel plate electrodes, a highly reliable heater with extremely low resistance can be produced.

(2) It has a large PTC resistance change rate and small internal thermal resistance, and has a wide resistance value adjustment range, and has a wide range of applications as a heater and safety element.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a conventional heater, FIG. 2 is a perspective view of a heater that is an embodiment of the present invention, FIG. 3 is a diagram showing the relationship between the resistance value and temperature of the heater of the embodiment, and FIG. Figure 4 shows the PTC characteristics of compositions of low density polyethylene and various carbon blacks.
The figure shows the relationship between the composition ratio of low density polyethylene and carbon black having an average particle size of 500 Å and the PTC characteristics. 3, 3'...Copper foil electrode, 4...PTC resistor composition.

Claims (1)

[Scope of Claims] 1. A pair of parallel plate-like electrodes provided close to each other, and a crystalline polymer containing a functional group with good adhesion to metal provided between the pair of parallel plate-like electrodes. 1. A heater having a positive temperature coefficient of resistance, comprising a resistor composition mainly composed of a conductive powder and a conductive fine powder. 2 The functional group-introduced crystalline polymer is composed of a functional group-introduced crystalline polymer in which functional groups with good adhesion to metals are placed mainly at the ends of the molecules of the base crystalline polymer, and the crystallinity and crystallinity before and after the introduction of functional groups are The heater according to claim 1, wherein the change in physical property value of melting point is very small. 3. The heater according to claim 1 or 2, wherein the functional group introduced is a carboxyl group. 4. The heater according to claim 1 or 2, wherein the conductive fine powder is made of carbon black having an average particle diameter of 400 Å or more and a substantially spherical shape. 5. The heater according to claim 1 or 2, wherein the proportion of carbon black in the resistor composition is 10 to 70% by weight. 6 Carbon black composition ratio when the resistor composition contains an electrically insulating or high-resistance filler as a subcomponent, and the mechanical strength of the resistor composition in tensile, bending, and shearing does not include the filler.
3. The heater according to claim 1 or 2, wherein the mechanical strength is within the range of 40 to 70% of the mechanical strength of the resistor composition. 7. The heater according to claim 1 or 2, wherein the resistor composition contains an electrically insulating or high-resistance filler having a thermal conductivity of 20 Kcal/mh°C or more as a subcomponent. 8 Thermal resistance between a pair of electrodes per square meter
The heater according to claim 1 or 2, which has a temperature of 0.01 h°C/Kcal or less. 9. The heater according to claim 1 or 2, which contains a deactivating agent capable of forming a metal complex with heavy metal ions as part of the stabilizer of the resistor composition. 10 The crystalline polymer of the resistor composition is a polyolefin-based resin, contains a phenol-based or thioether-based stabilizer, and a deactivating agent capable of forming a metal complex compound with respect to heavy metal ions, and the electrode contains copper. 3. A heater according to claim 1 or 2, which is made of a metal containing: