JP2638862B2 - Positive low temperature coefficient heating element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive resistance temperature coefficient heating element used for a heating appliance or a general heating appliance.

2. Related Art A composition in which conductive fine powder such as carbon black is dispersed in a crystalline polymer such as polyethylene, ethylene-vinyl acetate copolymer, anoimer, polypropylene, and polyvinylidene fluoride has a crystalline portion near its melting point. It is known that the resistance value sharply increases due to a sudden change in physical properties during amorphousization. Applying its characteristics, when the temperature reaches a predetermined temperature, the power drops rapidly, and the resistor itself prevents the temperature runaway, and the power is kept in a direction to keep the temperature constant according to the fluctuation of the thermal load. Has been applied as a so-called self-controlled heating element that is automatically controlled.

FIG. 2 shows an example of a heating element having a positive resistance temperature coefficient based on the prior art, as represented by Japanese Patent Publication No. 55-40161. 2a and 2b are electrodes. Reference numeral 3 denotes a positive resistance temperature coefficient resistor mainly composed of a crystalline polymer and carbon black. In general, a ceramic-based sintered body exhibits extremely good heat conduction characteristics while being an electrical insulator, and therefore has a high ability to maintain a uniform temperature distribution over almost the entire surface of the positive resistance temperature coefficient resistor. It is possible to maintain a stable heat generation state due to normal resistance value distribution and potential distribution, which is very advantageous when a high-output positive resistance temperature coefficient heating element is formed. However, when a heating element having a large area or a long heating element is formed, a ceramic-based material such as an alumina sintered body cannot be practically used in terms of manufacturing technology and strength.

Therefore, as a substitute for a ceramic substrate material, Japanese Patent Publication No. 57-43995 or a composite material substrate composed of an electrically insulating film 4 and a metal heat equalizing plate 5 as shown in FIG. 3 has been used. The thermal conductivity of the resin was about two orders of magnitude lower than ceramic, and the thermal conductivity of the composite did not exceed that of ceramic. Therefore, the power density of these heating elements was at an extremely low level. As a result, in many applications, the output of the heating element is insufficient or the mounting area of the heating element becomes unnecessarily large, leakage current due to induction reaches dangerous levels, and material cost becomes an alternative. The use was extremely limited, and the use was extremely limited.

Therefore, paying attention to the structure of the positive temperature coefficient heating element, as shown in JP-A-60-28195 and FIG. 4, the distance between the pair of electrodes 2a and 2b is made closer to each other, so that Rather than relying on the soaking effect, a method to greatly increase the heat diffusion capability of the resistor 3 itself has been studied, and a way to realize a high output positive resistance temperature coefficient heating element with a wide range of applications has been opened. I've been killed.

However, the positive resistance temperature coefficient heating element as shown in FIG. 4 is very excellent as a structure for generating high output, but has a relatively low resistance such as carbon black. The problem that must be solved in view of the withstand voltage breakdown characteristics of the positive temperature coefficient resistor formed by dispersing the conductive fine powder and the volume resistivity range where extremely high resistance is required Was piled up.

First, in order to construct a positive resistance temperature coefficient heating element in which the electrode spacing is very close, not only is it necessary to select a conductive fine powder having excellent withstand voltage breakdown characteristics, but also by obtaining sufficient resistance temperature characteristics. It was very important to take care not to runaway beyond the peak resistance area of the resistance temperature characteristic. The volume specific resistance value of the conventional 10 ^{0 -} 10 ²
A semiconductor region of 103 to 105 Ωcm is required for Ωcm, and the composition ratio of the conductive fine powder must be greatly reduced. As a result, the number of contact points between the conductive fine powders is drastically reduced, and the resistance temperature characteristic is not only controlled by the melting point of the crystalline polymer, but also unstable due to the linear expansion coefficient at a lower temperature range. It now contains more components. Furthermore, in the course of aging, the resistance value and the temperature coefficient of resistance change significantly due to the crystal growth of the crystalline polymer and the aggregation of the conductive fine powder.

When used for a heating element, for example, when a material composition exhibiting excellent positive resistance temperature characteristics such that the ratio of the resistance value at room temperature to the peak resistance value is 4 to 6 digits is selected, the resistance value at room temperature and The ratio to the resistance value in the stable temperature range is unnecessarily increased, and in a device requiring a large amount of power, the rush power greatly exceeds the allowable value, and the possibility that the current breaker malfunctions cannot be avoided. Also, in the change over time,
It lacked stability in temperature and power, and was not practical. Furthermore, in addition to this, there is a very difficult subject of how to secure stable electrical connection between the electrode and the resistor. At the interface where the conductive material and the metallic material have a very small number of contact points between the conductive fine powder and a uniform conductive composition having a volume resistivity exceeding 10 ³ Ωcm, a surface skin layer with poor conductivity is easily formed, It was not easy to secure electrical connection even though it could be physically connected. Of course, if there is no physical connection, stable electrical connection cannot be realized, and technical means satisfying both are desired. These problems have been fatal for a positive resistance temperature coefficient heating element having an adhesive structure as shown in FIG. As described above, by simply adjusting the composition ratio of the conductive fine powder, the specific resistance value is 10 ³ Ωcm
Above, it was not possible to produce useful positive resistance temperature coefficient heating element using peak resistance / cold resistance of 10 ⁴ or more positive resistance temperature coefficient resistor.

An object of the present invention is to solve the above problems and to provide a positive resistance temperature coefficient heating element capable of increasing the output by shortening the distance between the electrodes.

Means for Solving the Problems In order to solve the above-mentioned problems, the positive resistance temperature coefficient heating element of the present invention is a non-crosslinked low-volume specific resistance value obtained by dispersing a conductive fine powder in a crystalline polymer composition. After cross-linking the conductive composition with an electron beam or an organic peroxide, etc., it is finely divided into fine powders and dispersed in a metal-adhesive polymer composition containing a carboxyl group to have a high volume resistivity. A positive resistance temperature coefficient resistor composition formed in a thin shape, and a pair of electrodes provided to apply a voltage in the thickness direction of the composition, the composition and the electrodes are integrally formed. It is.

In the above configuration, the conductive material fine powder has a structure in which the conductive material fine powder is agglomerated, and the portion where the conductive material fine powder is agglomerated can contact the electrode with the help of the metal adhesive polymer composition. It is possible to form an adhesive positive resistance temperature coefficient resistor composition. Such compositions can be considered 10 ⁰ volume resistivity of Ωcm level a set of parts having both adhesion, as a whole composition, excellent very high resistance while low resistance compositions It has physical properties. When forming a heating element that applies a voltage in the thickness direction of a thin-walled resistance element, a resistor with a significantly higher volume resistivity than a heating element in both width directions is required to obtain a predetermined resistance value. However, the above resistor is indispensable for achieving such an object. Further, since the heating element having the thin structure is thin and the heat diffusion ability is greatly improved, it is possible to increase the output, and a high-output and highly reliable positive-resistance temperature coefficient heating element can be obtained. Further, since the crosslinked conductive composition is once dispersed into the metal adhesive polymer composition after being pulverized to a fine powder, molding by heat is easy,
It can be formed integrally with the electrode.

Embodiments Hereinafter, embodiments will be described with reference to the accompanying drawings. First
In the drawing, 6 is a positive resistance temperature coefficient resistor having a thickness of 1 mm,
Reference numerals 7 and 8 denote a pair of metal foil electrodes joined to the positive temperature coefficient resistor. The positive resistance temperature coefficient resistor 6 has a positive resistance temperature characteristic in which the ratio of the peak resistance value to the normal temperature resistance value exceeds three digits by using high density polyethylene and furnace black as basic materials. . In order to apply 100 V to the heating element having the structure shown in FIG. 1 and maintain the heat generation, it is necessary to connect the extremely high resistance value having a specific resistance of 10 ⁴ Ωcm to the electrical and physical connection with the metal foil electrode. Is essential. Conductive fine powder was added to the crystalline polymer composition at 10 ⁴ Ωc.
These objects cannot be achieved only by adding while adjusting the specific resistance value to m. Therefore, the positive temperature coefficient resistor 6 was manufactured by the following procedure.

First, add high density polyethylene and furnace black
While kneading at a ratio of 1.2, dicumyl peroxide was added as a cross-linking agent to the polyethylene at 3.5%, and the heat treatment was performed to complete the cross-linking reaction. Obtained. Thereafter, the pulverized product was kneaded in maleic acid-grafted high-density polyethylene at a composition ratio of carbon black of 30%, and a copper foil / resistor / copper foil laminated structure was prepared by hot pressing at 200 ° C. Furthermore, a volume resistivity value of 10 ⁴ Ωcm is obtained by performing annealing for 3 hours in a nitrogen atmosphere at 190 ° C.
Was obtained.

On the other hand, as a conventional example, instead of maleic acid-grafted high-density polyethylene, a non-modified high-density polyethylene was used to prepare a 10 ⁴ Ωcm positive resistance temperature coefficient composition in the same procedure as in the example, and a comparative sample was prepared. I got

Next, an evaluation test was performed on the resistor composition sample based on the present invention while comparing the sample of the conventional example.
The results are shown in Table 1.

As is evident from Table 1, in the examples according to the present invention, in the heat resistance, the change in the resistance value is small, and it is considered that it is almost stable. On the other hand, in the conventional example, the peel strength is insufficient, the resistance value fluctuates drastically, and once decreases slightly, then starts to increase. This is presumably because the resistance inside the resistor decreases due to the aggregation phenomenon of carbon black, while the electrical coupling state at the interface with the electrode becomes unstable, and the resistance increases.

Next, the results obtained by examining the relationship data between the peel strength and the heat resistance of the resistor composition based on the method of the present invention by replacing part of the composition of the maleic acid-grafted high-density polyethylene with high-density polyethylene. It is shown in FIG.

As is clear from Table 2, as the composition ratio of the maleic acid-grafted high-density polyethylene increases, the peel strength at the interface with the electrode increases, and the stability of the resistance value in the heat resistance test tends to be improved. From this result, it can be concluded that the peel strength capable of securing the electrical connection is at least 500 g / 25 mm width.

Next, the relationship between the volume resistivity and the heat resistance was examined. Table 3 shows the results.

As is clear from Table 3, the present invention has a specific resistance of 10 ⁴
Even in a high resistance region exceeding Ωcm, it is possible to maintain the stability of not only the resistor but also the interface resistance value with the electrode.

As described above, in the present invention, in the high resistance region,
It is intended to provide a means capable of maintaining good contact between conductive fine powders or conductive fine powders and electrodes. It is particularly effective. The material is not limited to the examples, and as the crystalline polymer, low-density polyethylene, linear low-density polyethylene, polypropylene, or the like, and any copolymer or derivative thereof can be used. In addition, as the conductive fine powder, carbon black, thermal black, acetylene black, carbon black, and other carbon blacks exhibiting a remarkable positive resistance temperature characteristic can be used. Examples of the metal-adhesive polymer containing a carboxy group include anoima, ethylene-vinyl acetate copolymer, ethylene / ethyl acrylate, polyethylene and polypropylene grafted with acrylic, etc., all polymers containing a carboxyl group, and furthermore, Many materials are available, such as a composite of a polymer with another polymer. Adhesion is not particularly required for the polymer of the partner in the case of compounding, and many materials can be selected in consideration of heat resistance, workability, compatibility, mechanical properties, and the like. Particularly preferred are polypropylene, ethylene, propylene thermoplastic elastomer, styrene / isobutylene thermoplastic elastomer, polyester-based thermoplastic elastomer, and the like. The thermoplastic elastomer does not require crosslinking and is a polymer that can provide the most preferable physical properties as a resistor composition.

Effect of the Invention As is clear from the description of the above examples, an uncrosslinked conductive composition obtained by dispersing a conductive fine powder in a crystalline polymer composition and a positive resistance temperature coefficient resistor composition is formed by an electron beam or After cross-linking with an organic peroxide or the like, the mixture is finely divided and dispersed in a metal-adhesive polymer composition containing a carboxy group to form a thin film, thereby shortening the distance between the electrodes and increasing the positive resistance. It was possible to produce a high-resistance and high-stability adhesive positive resistance temperature coefficient resistor composition suitable for applications such as a temperature coefficient heating element. This composition has not only the stability of the resistance value inside the resistor but also the adhesive strength enough to maintain the stability of the resistance value of the electrical coupling part with the electrode body. The temperature coefficient heating element can solve the problem of heat generation temperature and long-term stability of electric power, and can be developed for many uses.

[Brief description of the drawings]

FIG. 1 is a perspective view of a part of a positive resistance temperature coefficient heating element showing one embodiment of the present invention. FIGS. 2, 3 and 4 are perspective views of a positive resistance temperature coefficient heating element based on the prior art. 6 Positive temperature coefficient resistor, 7, 8 Metal plate electrode.

Claims (4)

(57) [Claims] An uncrosslinked conductive composition having a low volume resistivity, which is obtained by dispersing a conductive fine powder in a crystalline polymer composition, is crosslinked with an electron beam or an organic peroxide, and then the fine powder is formed. Positive resistance temperature coefficient resistor composition formed by dispersing in a metal-adhesive polymer composition containing a carboxyl group to have a high volume specific resistance value and forming into a thin shape, and A positive resistance temperature coefficient heating element comprising a pair of electrodes provided to apply a voltage in a thickness direction, wherein the composition and the electrodes are integrally formed. 2. The positive temperature coefficient heating element according to claim 1, wherein an adhesive peel strength of the positive temperature coefficient coefficient resistor composition to the electrode is 500 (g / 25 mm width) or more. 3. The positive resistance temperature coefficient resistor has a volume specific resistance value of
^{3. The} heating element according to claim 1, wherein said heating element has a resistance value higher than 10 ³ Ωcm. 4. The heating element according to claim 1, wherein said metal-adhesive polymer composition is a composite composition with a thermoplastic elastomer composition. .