JP2688061B2 - Positive resistance temperature coefficient heating element - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive resistance temperature coefficient heating element which is useful in a heating appliance and a general heating device.

2. Description of the Related Art A conventional heating element having a positive temperature coefficient of resistance has been self-controlled to an appropriate temperature by the positive resistance temperature characteristic of a resistor between a pair of electrodes.

However, especially when a large power density or high temperature is required, it is essential to make the temperature distribution always good in the direction between the pair of electrodes in order to make the temperature distribution of the heating element itself uniform. As a solution, a method was adopted in which the distance between a pair of electrodes is made close to each other as shown in FIG. In FIG. 6, 1 and 2 are a pair of parallel plate electrodes provided close to each other, and by disposing a resistor 3 formed by mixing and dispersing conductive fine powder in a crystalline polymer between them, It has been found that a high-power positive resistance temperature coefficient heating element may emerge.

However, the conventional positive resistance temperature coefficient heating element as described above was very excellent as a structure for producing a high output, but a relatively low resistance conductive material such as carbon black. Many problems have to be solved in consideration of the breakdown voltage breakdown characteristics of a positive temperature coefficient resistor made by mixing and dispersing conductive fine powder, and the volume resistivity region where extremely high resistance is required. Had the problem of. For example, in order to construct a positive resistance temperature coefficient heating element in which the electrode interval is very close, not only conductive fine powder having excellent withstand voltage breakdown characteristics is selected, but also a sufficiently large positive resistance temperature characteristic is obtained. Therefore, it was indispensable to take precautions so as not to run over the peak resistance value. The volume resistivity of the conventional 10 ⁰ ~10 ² Ωcm, 10
A semiconductor region of ³ to 10 ⁵ Ωcm is required, and the composition ratio of the conductive fine powder has to be significantly reduced. As a result, the number of contact points between the conductive fine powders is drastically reduced, and the resistance-temperature characteristic is controlled not only by the melting point of the crystalline polymer in which the conductive fine powders are mixed and dispersed, but also by the heat in the lower range. The unstable component, which is considered to be caused by the thermal stress of various constituent materials such as expansion and thermal contraction, dramatically increased, resulting in extremely unstable characteristics. In addition, due to changes over time, crystal resistance of the crystalline polymer, thermal stress in various parts of the heating element, or agglomeration of conductive fine powder will cause a large change in resistance and temperature coefficient of resistance. Was not stable, had a very short heat generation life, and had the risk of abnormal overheating, smoke emission, ignition, etc., and was far outside the practically acceptable range.
As described above, it was not possible to produce a useful positive resistance temperature coefficient heating element having a volume resistivity value of 10 ³ Ωcm or more only by adjusting the composition ratio of the conductive fine powder.

The present invention solves such a problem, and an object of the present invention is to provide a material structure of a positive resistance temperature coefficient heating element capable of realizing excellent resistance stability that can withstand practical use.

Means for Solving the Problems In order to solve the above problems, the positive resistance temperature coefficient heating element of the present invention is obtained by crosslinking and subdividing a conductive composition in which conductive fine powder is mixed and dispersed in a crystalline polymer. And the type of the crystalline polymer so that the temperature coefficient of resistance is different, the type of the conductive fine powder, the composition ratio of the crystalline polymer and the conductive fine powder, crosslinking conditions, at least one of the particle size Thin-type positive resistance temperature coefficient resistor formed by further mixing and dispersing a plurality of types of particulate conductive composition (hereinafter referred to as conductive filler) prepared by mixing one in a crystalline polymer, and its thickness It is provided with a pair of electrode bodies provided to apply a voltage in the vertical direction.

With the above configuration, the material configuration of the positive resistance temperature coefficient resistor is divided into a portion in which the conductive fine powder is dispersed in the crystalline polymer at a high ratio and a portion in which the conductive fine powder is hardly dispersed, and both of them are separated. Since it is arranged in a sea-island shape, the conductive filler obtained by crosslinking and subdividing the conductive composition obtained by mixing and dispersing conductive fine powder in a crystalline polymer corresponds to the former part and has a volume specific resistance were very stable even 10 ⁰ to 10 ¹ [Omega] cm level and if I is crosslinked by electron beam or an organic peroxide, the conductive fine powder is securely fixed with a conductive filler in, It is possible to exhibit stable resistance characteristics over time. However, especially in the latter case where the crystalline polymer portion is large, the movable zone of the conductive filler becomes large and easy to move, so that the aggregate structure of the conductive filler during annealing due to stress relaxation due to processing, heat, crystal growth, etc. Changes such as being alleviated,
Although the conductive path becomes unstable, the ratio of the conductive filler to the resistor by introducing another type of high-resistance conductive filler that does not make a large contribution in a certain temperature range, which corresponds to the actual use state. Not only becomes larger and becomes hard to move, but also the density distribution in the resistor of the conductive fine powder itself becomes smaller, so that extremely stable resistance stability can be maintained. The type of crystalline polymer, the type of the conductive fine powder, the composition ratio of the crystalline polymer and the conductive fine powder, the cross-linking conditions, the resistance temperature coefficient and the volume specific resistance value are different by changing the particle diameter. A conductive filler can be formed. Since these conductive fillers are crosslinked, independent fillers can be formed. Since these conductive fillers are crosslinked, they can form independent fillers. Since these conductive fillers are crosslinked, they are dispersed in the crystalline polymer as independent filler particles. Thus, 10
It is possible to achieve excellent stability of a high resistance resistor of ³ Ωcm or more, and to realize a high output positive resistance temperature coefficient heating element.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

As shown in FIG. 1, the positive resistance temperature coefficient heating element of one embodiment of the present invention has electrodes 5 and 6 bonded to the upper and lower surfaces of the positive resistance temperature coefficient resistor 4 having a thickness of 0.4 mm, and Exterior materials 7 and 8 are provided on both sides. The positive resistance temperature coefficient resistor 4 is formed as follows. That is, while kneading 55 wt% of thermal black as a conductive fine powder and 45 wt% of low density polyethylene as a crystalline polymer, 2.5 wt% of dicumyl peroxide, which is an organic peroxide, with respect to low density polyethylene After the addition and heat treatment to complete the crosslinking reaction, a pulverized product having an average particle diameter of 60 μm, that is, a first conductive filler was obtained by freeze pulverization. Next, in the same manner, a fine powder in which furnace black and thermal black were uniformly dispersed at 1: 1 was prepared, and 42 wt% of this conductive fine powder was kneaded with 58 wt% of high density polyethylene which is a crystalline polymer. Meanwhile, the organic peroxide, dicumyl peroxide, was added to the high-density polyethylene in an amount of 3 wt% and the heat treatment was applied to complete the cross-linking reaction.
To obtain a pulverized product, that is, a second conductive filler. After that, the first and second conductive fillers are added in a ratio of both.
It is 3: 1 and the ratio of both conductive fillers to the total amount of resistors is 54.3 wt%, and it is uniformly dispersed and kneaded in maleic acid-modified high-density polyethylene as another crystalline polymer. A positive resistance temperature coefficient resistor 4 was obtained. Further, after processing the resistor 4 into a heating element as described above,
It was annealed to obtain a predetermined resistance characteristic. Here, in order to examine its effectiveness, in addition to the No1 specification according to the above specifications, the following No2, 3, and 4 specification samples in which the first and second conductive fillers were appropriately replaced were processed in the same manner. And first
A sample of the table was obtained.

Here, the value of the specific resistance is a volume specific resistance value when the temperature of the heating element is 20 ° C., and is a value measured from the materials of the first and second conductive fillers before fine division. Although this value causes a slight difference in the absolute value of the measured value depending on the measuring method, it is the same measuring method.
At some temperature in the temperature range from room temperature to usable temperature,
It is applied to a positive resistance temperature coefficient resistor having two or more different kinds of conductive fillers, Nos. 1 to 3 show this example, and No. 4 shows a conventional comparative example. No.1 and
The resistance temperature characteristic diagram of No. 4 is shown in FIG. 2. The first conductive fillers of both are the same, and No. 1 further contains the second conductive filler. Further, the resistance temperature characteristic charts of Nos. 2 and 3 are shown in FIGS. 3 and 4, respectively, and refer to examples in which the difference in resistance temperature characteristic is small and examples in which the difference is large.

Actually, comparative experiments were conducted by conducting tests of No. 1 to 4 samples. The energization mode was evaluated by intermittent energization every 10 minutes in order to evaluate acceleration by the heat cycle. The result is shown in FIG. As is clear from FIG. 5, in the sample of the conventional comparative example of No. 4, the maximum temperature of the intermittent energization cycle starts to drop from 1500h to 2400h.
After that, the resistance became so high that almost no heat was generated, and the life was extremely short. On the other hand, the No. 1 sample in which the second conductive filler was kneaded maintained the initial temperature even at 7500h, has very stable heat generation characteristics, and has an extremely high resistance temperature characteristic. Was stable to. The other No. 2 and 3 samples also had extremely stable heat generation characteristics as of No. 1 sample at 7500h. Although it has a different life depending on the current-carrying mode actually used, the introduction of another type of conductive filler has brought about an epoch-making resistance stabilization method that maintains long-life and extremely stable resistance stability. I knew I could do it.

It is considered that this is due to the following mechanism. That is, the conductive filler obtained by crosslinking and subdividing the conductive composition obtained by mixing and dispersing the conductive fine powder in the crystalline polymer is extremely stable even if the volume resistivity is 10 ⁰ to 10 ¹ Ωcm level. In addition, since the conductive fine powder is fixed in the conductive filler because it is crosslinked by the electron beam or the organic peroxide, it becomes possible to exhibit stable resistance characteristics over time. However, especially when the crystalline polymer portion that binds the conductive filler is large, since the movable zone of the conductive filler becomes large and easy to move,
Stress relaxation due to processing, heat, crystal growth, etc. causes changes such as relaxation of the agglomerate structure of the conductive filler during annealing, making the conductive path unstable, but at a certain temperature range corresponding to the actual use state. Introducing other types of high-resistance conductive fillers that do not contribute significantly in, the ratio of the conductive filler in the resistor increases, not only becomes difficult to move, in the resistor of the conductive fine powder itself Since the density distribution of is also reduced, extremely stable resistance stability can be maintained. In this way, it is possible to achieve excellent stability of a high resistance resistor of 10 ³ Ωcm or more, and to realize a high output positive resistance temperature coefficient heating element.

The material constituting the conductive filler is not limited to the above combination, but includes low density polyethylene, medium density polyethylene, high density polyethylene, linear polyethylene, ethylene vinyl acetate copolymer, ethylene acrylic acid copolymer. Coalesced, ionomer, polypropylene,
Among crystalline polymers such as polyamide, polyvinylidene fluoride, polyester, and organic acid-modified polyethylene such as acrylic acid and maleic acid, and carbon black such as thermal black, furnace black, channel black, and acetylene black It may be an appropriate combination with a conductive material exhibiting resistance-temperature characteristics, as long as a plurality of types of this conductive filler are used, and preferably the volume resistivity is in the temperature range from room temperature to usable temperature. If the positive resistance temperature coefficient resistor has two or more kinds of conductive fillers that differ by 1.5 times or more at that temperature, as is clear from the examples of Nos. 2 and 3, the volume resistivity of different conductive fillers Even if the ratio is small or large, the same effect can be obtained.

Similarly, the crystalline polymer in which the conductive filler is kneaded is not limited to high density polyethylene, and may be the same as the crystalline polymer in the conductive filler. Further, if at least one of the plural kinds of conductive fillers has a volume resistivity of 10 ² Ωcm or more, this conductive filler hardly contributes to the resistance temperature characteristic, so that it is added as a resistance stability imparting agent. It is also possible to do so. further,
The ratio of the conductive filler in the positive resistance temperature coefficient resistor is preferably 50 wt% or more,
Resistance stability can be further improved. In addition, by increasing the average particle size of the conductive filler to 80 μm or less and making it compatible with the crystalline polymer prepared by mixing and dispersing the conductive filler, the safety against sparks caused by microcracks and the like is improved. be able to. Also,
By giving the crystalline polymer an organic acid-modified functional group, not only can the adhesive properties with metal electrodes such as copper and copper alloys be enhanced, but the crystalline polymer in the conductive filler can also have an organic acid-modified functional group. The effect of increasing the cross-linking level and further improving the resistance stability can be obtained by providing the above. Furthermore, it is also possible to obtain a high degree of resistance stability by adopting a structure in which another conductive filler is contained in the conductive filler. The thickness of the positive temperature coefficient resistor is preferably 1 mm or less.

EFFECTS OF THE INVENTION As described above, in order to realize a high output / high temperature positive resistance temperature coefficient heating element, a resistor having a volume specific resistance value around the semiconductor region is required. Since the number of contact points of the conductive fine powders is drastically reduced only by adjusting the composition ratio of the conductive fine powders, the resistance temperature characteristic is not only controlled by the melting point of the crystalline polymer, but also in the lower temperature range. Unstable components, which are considered to be caused by thermal stress of various constituent materials such as thermal expansion and thermal contraction, will dramatically increase, resulting in extremely unstable characteristics and extremely short heat generation life. Although there was a risk of abnormal overheating, smoking, ignition, etc., the positive resistance temperature coefficient heating element of the present invention can solve these problems,
By introducing other high resistance and stable conductive filler to the stable conductive filler in which the conductive fine powder is fixed by cross-linking, not only does it increase the ratio of the conductive filler in the resistor to make it difficult to move. In addition, the density distribution of the conductive fine powder itself in the resistor can be made small, excellent resistance stability can be realized for a long period of time, and a high output, long life positive resistance temperature coefficient heating element can be obtained. Further, it has an effect that it is possible to finely adjust the variation of the resistance value depending on the material lot, the processing lot, etc., only by adjusting the mixing ratio of different conductive fillers, which is extremely advantageous in practical use. .

[Brief description of the drawings]

FIG. 1 is a perspective view of a positive resistance temperature coefficient heating element of a first embodiment of the present invention, FIG. 2 is a resistance temperature characteristic diagram of a conductive filler of the heating element, and FIG. 3 is a second embodiment. Resistance temperature characteristic diagram of the conductive filler of the positive resistance temperature coefficient heating element, FIG. 4 is a resistance temperature characteristic diagram of the conductive filler of the positive resistance temperature coefficient heating element of the third embodiment, and FIG. 3 is a characteristic diagram showing changes with time in the surface temperature of the example 3 and the conventional positive resistance temperature coefficient heating element, and FIG. 6 is a perspective view of the conventional positive resistance temperature coefficient heating element. 4 ... Positive resistance temperature coefficient resistor, 5,6 ... Electrode, 7,8 ... Exterior material.

Claims (3)

(57) [Claims] 1. A conductive polymer composition comprising a conductive fine powder mixed and dispersed in a crystalline polymer, which is obtained by crosslinking and finely dividing the conductive composition, and the type of the crystalline polymer and the conductive material have different resistance temperature coefficients. A plurality of types of particulate conductive composition prepared by adjusting at least one of the kind of fine powder, the composition ratio of the crystalline polymer and the conductive fine powder, the crosslinking condition, and the particle diameter (hereinafter referred to as conductive filler (Hereinafter referred to as)) is further mixed and dispersed in a crystalline polymer to form a thin positive temperature coefficient resistor having a positive resistance temperature, and a positive resistance temperature having a pair of electrode bodies provided to apply a voltage in the thickness direction thereof. Coefficient heating element. 2. A positive resistance temperature coefficient heating element according to claim 1, wherein the positive resistance temperature coefficient heating element has a temperature range in which a ratio of volume specific resistance values is at least 1.5 times or more among a plurality of kinds of conductive fillers. 3. The positive resistance temperature coefficient heating element according to claim 1, wherein at least one of the plurality of kinds of conductive fillers has a volume resistivity value of 10 ² Ωcm or more.