JPH04306582A - Material to ptc heat emitting element - Google Patents

Abstract

PURPOSE:To provide a PTC heat emitting substance, whose electroconductivity secular change and mechanical brittleness are improved. CONSTITUTION:A PTC heat emitting substance as per invention consists of an admixture of crystalline polymer, a polymer not perfectly compatible with the crystalline polymer, and electroconductive particles, or otherwise consists of a dispersion in which the crystalline polymer region and the polymer region segregated from the crystalline polymer are existing mixedly in fine distribution and in which electroconductive particles exist in the mentioned crystalline polymer region eccentrically. In the later case, it may happen that the crystalline polymer phase in which the electroconductive particles and the polymer phase segregated from this polymer phase form a mutually continued phase.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to heating element materials, particularly PTC (Positive
Temperature Coefficient)
Regarding heating element materials.

0002

2. Description of the Related Art One of the materials used as a heating element that generates heat when energized is a PTC heating element material whose resistance value increases as the temperature rises (that is, it has a positive temperature coefficient of resistance). The electric heater using this PTC heating element material is
It is known as a useful and safe heater because it naturally has a self-temperature control function that increases the resistance value when the temperature rises, reduces the current, suppresses the amount of heat generated, and prevents a unilateral temperature rise. .

[0003] Conventionally, as such a PTC heating element material, there is a polymer material made of a crystalline polymer in which conductive particles are dispersed. Here, examples of the crystalline polymer include polyethylene, ethylene-vinyl acetate copolymer, ionomer, polyvinylidene fluoride, etc., and examples of the conductive particles include fine carbon black powder, fine graphite powder, and the like.

This conductive particle-dispersed crystalline polymer is PT
The reason for exhibiting the C characteristic is as follows. As the temperature of the conductive particle-dispersed crystalline polymer increases to the melting point of the crystals, a large increase in volume occurs as the crystals melt. This increase in volume causes the breakage of conductive chains generated by contact between dispersed conductive particles and the expansion of the spacing between conductive particles. The breaking of these conductive chains and the expansion of the spacing between conductive particles lead to a decrease in the conductive effect of the conductive powder, and as a result,
This means that the resistance value increases rapidly.

[0005]

[Problem to be solved by the invention] However, the above P
TC heating element materials have insufficient electrical properties and mechanical properties. The reason why the electrical characteristics cannot be said to be sufficient is that the electrical conductivity (resistance value) changes significantly over time. That is, the state of dispersion of the conductive particles changes over time, and the conductivity deviates from its initial value. Crystalline polymers and conductive particles have different surface energies, and conductive particles in polymers have a very strong tendency to aggregate and stabilize. On the other hand, in the case of a conductive particle-dispersed crystalline polymer obtained by kneading a molten crystalline polymer and a conductive powder, it is difficult to produce the conductive powder in a stable state of dispersion in the crystalline polymer. Therefore,
When used in a heater, the conductive particles move in an attempt to reach a stable dispersion state while the crystals are repeatedly melting and recrystallizing, and the dispersion state of the conductive particles gradually changes. Changes in the dispersion state of the conductive particles cause fluctuations in the conductive action of the conductive particles, and as a result, PTC
There will be a large change in the electrical conductivity of the heating element material over time.

[0006] The reason why the mechanical properties are not sufficient is that the impact resistance etc. are low and there is a strong tendency to be brittle. The resistance value of the PTC heating element material varies depending on the application and heater structure, and is usually 100 to 105 Ω·cm, but a lower resistance value is preferable in order to derive a sufficient amount of heat generation. To lower the resistance (higher conductivity), add a large amount of (low structure) carbon black. However, adding a large amount of carbon black reduces the flexibility of the polymer and makes it brittle. If carbon black powder with a high structure is used, the amount added can be reduced, but the electrical properties are unstable, making it impractical and not an appropriate solution.

[0007] In view of the above-mentioned circumstances, an object of the present invention is to provide a PTC heating element material in which changes in electrical conductivity over time and mechanical fragility are improved.

[0008]

[Means for Solving the Problems] In order to solve the above problems, the PTC heating element material according to claim 1 comprises a crystalline polymer (first polymer) and a polymer that is not completely miscible with the crystalline polymer. The PTC heating element material according to claim 2 has a structure in which a (second polymer) and conductive particles are kneaded, and the crystalline polymer (first polymer) region and the same crystalline polymer are phase separated. Polymer (second polymer)
The conductive particles are dispersed so as to be unevenly distributed in the crystalline polymer regions.

[0009] This invention will be specifically explained below. Examples of the first polymer in the PTC heating element material of the present invention include polyethylene, polypropylene, polymethylpentene, EVA (ethylene-vinyl acetate resin), EEA (ethylene-ethyl acrylate resin), and EMA (ethylene-vinyl acetate resin).
ethylene copolymer polymers such as methyl acrylate resin), ionomers, polyvinylidene fluoride, polyamides,
Examples include polyester. On the other hand, examples of second polymers that are not completely compatible with the first polymer include polyethylene, polypropylene, polymethylpentene, ethylene copolymer (EVA, EEA, EMA, etc.), ionomer, polyvinylidene fluoride, polyamide, polyester, etc. Examples include crystalline polymers and amorphous polymers such as PMMA, polystyrene, polycarbonate, polysulfone, polyvinyl chloride, and copolymers thereof. Note that the second polymer that is not completely compatible with the first polymer is a polymer that is almost incompatible with the first polymer, or a polymer that is slightly or partially compatible with the first polymer and that is compatible with both polymers. It is a polymer that creates a so-called polymer alloy with 1 and 2 polymers. Regarding such polymer alloys, please refer to "Polymer Alloys - Basics and Applications" (edited by The Society of Polymer Science, Inc., 1981), "Polymer Alloys" (edited by The Society of Polymer Engineers, Inc., written by Takashi Inoue and Toshio Nishi, 1988), Blend" published by CMC, 1979). It goes without saying that the entire region of the crystalline polymer does not need to be in a crystalline state, and that an amorphous portion may coexist. It is known that the crystallinity of commercially available crystalline polymers is not very high under normal molding conditions. For example, nylon 6-6 is 30-35%, polypropylene is 50-7%
0%, 65-90% for polyethylene, 25-35% for PBT (polybutylene terephthalate), and 15-20% for PET (polyethylene terephthalate).

[0010] Examples of the conductive particles include powder or fiber particles made of carbon, graphite, or metal. Finely divided carbon black powder is commonly used. It is preferable that the first and second polymers form a continuous phase in the alloy and constitute the polymer alloy in a mutually continuous state in which they are intertwined with each other in the form of a network. For this purpose, the combination of the first and second polymers and
The blending ratio may be appropriately selected. For example, do the following:

Specific examples of combinations of the first polymer and the second polymer include those shown in Table 1 below.

0012

[Table 1]

The ratio of the first polymer to the second polymer is usually in the range of 20 parts by weight or more and 80 parts by weight or less, when the total polymer is 100 parts by weight. The amount of conductive particles added is usually 100% of the polymer
The range is 1 to 40 parts by weight.

[0014] This PTC heating element material is manufactured, for example, as follows. First, a second polymer is added to a first polymer and kneaded. A compatibilizer may be added to control or stabilize the phase separation structure. Next, conductive particles are added and further kneaded, molded under heat and pressure, and cooled to obtain a PTC heating element material. The order of adding these raw materials is not limited to the above, and the second polymer may be added and kneaded after the first polymer and conductive particles are kneaded,
These three components may be added and kneaded at the same time. During manufacturing, stabilizers, antioxidants, lubricants, surfactants, flame retardants, inorganic fillers (aluminum hydroxide, silica, etc.), nucleating agents, etc. are added at appropriate stages as necessary. Good too. Further, the polymer may be crosslinked using a crosslinking agent or using radiation.

[0015]

[Function] In the PTC heating element material according to claim 1, changes in electrical conductivity over time and mechanical fragility are improved by using a second polymer that is not completely compatible with the first polymer. Ru. This is presumed to be because the conductive particles are unevenly distributed and present on average throughout the material. It is considered that the dispersion state in which the conductive particles are unevenly distributed throughout the material is stable, and the dispersion state of the conductive particles is difficult to change, so that the conductivity does not fluctuate. Also,
On the other hand, there are areas throughout the material with a small amount of conductive particles where conductive particles are not unevenly distributed, and areas where the decrease in flexibility is suppressed with a small amount of conductive particles exist throughout the material, reducing brittleness. It is thought that it has an improving effect.

[0016] In the PTC heating element material according to the second aspect, a first polymer region in which conductive particles are unevenly distributed and a second polymer region in which the same crystalline polymer is phase-separated are finely mixed. In this case, the movement of the conductive particles is restricted within a microscopic region due to the presence of the second polymer, so that the dispersion state of the conductive particles becomes more difficult to change and the conductivity is stabilized. A second polymer region with a lower amount of conductive particles and reduced flexibility is present throughout the material and serves to improve brittleness.

The reason why the conductive particles become unevenly distributed in the first polymer region is due to the difference in compatibility between the first and second polymers, and
It is presumed that the main cause is the difference in surface energy between the first and second polymers and the conductive particles. These differences act as a driving force that causes the conductive particles to migrate into the first polymer region during kneading. The higher the affinity between the conductive particles and the first polymer, the smoother the transition of the conductive particles to the first polymer region. Therefore, the first, second
It is preferable that the polymer has a large surface energy difference, the first polymer and the conductive particles have a small surface energy difference and have high affinity, and the second polymer and the conductive particles have a large surface energy difference and lack affinity. .

[0018] When a polymer whose thermal softening temperature is higher than that of the first polymer (higher crystal melting point or glass transition point) is used as the second polymer, the fine phase separation structure of the first polymer and the second polymer is stabilized. It is preferable because it has excellent properties and, as a result, the stability of the dispersion state of the conductive particles increases. Especially the first
, when the second polymer forms a continuous phase and forms a mutually continuous structure, the fine phase separation structure is more stable, and furthermore,
This is preferable because the stability of the dispersion state of the conductive particles is high.

Furthermore, with such a combination of polymers,
When the ratio of the second polymer to the first polymer is increased,
This is advantageous because it can be used as a PTC heating element material with excellent heat deformation resistance. In other words, the characteristics of the alloy material of the present invention are that the properties necessary as a heating element such as PTC properties can be designed using the first polymer and conductive particles, and the heat resistance and mechanical properties can be designed using the second polymer. be. In order to exhibit such characteristics, it is necessary for the two phases to form a mutually continuous phase, and it goes without saying that the first polymer phase, in which conductive particles are dispersed, is in the sea of the second polymer phase. If they are individually dispersed into islands, the conductivity will be low and the material will be unsuitable as a heating element. Such a sea-island phase structure tends to occur when the melt viscosity of the first polymer is significantly lower than that of the second polymer. In addition to compatibility, matching of melt viscosity is required.

In addition, when the first polymer is crosslinked, even if the first polymer undergoes repeated crystal melting and recrystallization, the crystal structure of the first polymer is highly stable, and the conductive particles are not dispersed. Conditions tend to change less. As a result of our studies on heating element materials with excellent PTC properties and thermal stability in the temperature range from room temperature to 90°C, we found that polyethylene was used as the first polymer, polypropylene was used as the second polymer, and polypropylene was used as the conductive particle. It has been discovered that by using carbon black and controlling its type and blending amount within a specific range, it is possible to produce a heating element material with even better PTC characteristics and thermal stability.

[0021]

[Embodiments] Examples of the present invention will be described below. It goes without saying that this invention is not limited to the following embodiments. Table 2 shows the physical properties of the raw material resin used.

[0022]

[Table 2]

[0023] The raw materials shown in Table 2 were used in the formulations shown in Table 3. The conditions for kneading and molding are shown below. Kneading machine: 1.8 (1) T-type intensive mixer (hand mixer) Compound amount: Approx.
kg) Kneading conditions: 120 rpm Mixer jacket heating temperature 120-130°C, kneading time 6 minutes Molding conditions: Polyethylene alone 170°C
2 minutes Pressure 10 seconds Polypropylene alloy 240℃ x 2 minutes
Pressurize for 10 seconds

[0024]

[Table 3]

[0025] The electrical properties of the obtained press sample were measured by gluing an aluminum foil with a conductive adhesive as an electrode and measuring the specific impedance (100 Hz) in the thickness direction while changing the temperature. The results obtained are shown in Table 4.

[0026]

[Table 4]

In Example 1, the CB amount was 17% and Z was 104
In Example 2, the characteristic impedance of the order of CB
It showed a specific impedance of the order of 105 when the amount was 14.6%. However, a single non-alloy polymer (Comparative Example 2) has a high impedance of the order of 108 even if the CB content is 23%. The reason for this is thought to be that the polyethylene phase and the polypropylene phase are phase separated, CB is selectively incorporated into the polyethylene phase (concentration effect), and the polyethylene phase is mostly a continuous phase.

On the other hand, when comparing Example 2 and Comparative Example 1, it is found that although both are alloy-based and have the same CB content, Comparative Example 1 has considerably higher Z20. This is considered to be because in Comparative Example 1, the polyethylene phase did not form a continuous phase but was dispersed in the form of islands, and a large amount of CB was incorporated therein. This shows that the fluidity of the polyethylene phase is the same as in Example 2.
Compared to Comparative Example 1, it is considerably higher, and therefore,
It seems that the polyethylene phase is dispersed in island shapes. In this way, if the fluidity of the first polymer (CB unevenly distributed phase) is too large than the fluidity of the second polymer, the first polymer phase tends to become island-like. It is considered necessary to match the liquidity of PE and PP.

Next, attention will be paid to the aspect of thermal stability. H. T
．． is a thermal stability evaluation method conducted by the present inventors, and 1
It shows how much the impedance at 20°C changed due to heat treatment at 50°C for 5 minutes. Among Comparative Examples 2, 3, and 4 prepared using only the first polymer, in 3 and 4, 20
The impedance at ℃ increases and decreases by more than an order of magnitude. Comparative Example 2 uses a single polymer system, which has a very high impedance of the order of the 7th power, so H. T. It is thought that the change was difficult to show. On the other hand, in alloy-based Examples 1 and 2 and Comparative Example 1, H. T. It can be seen that thermal stability is improved by forming a very small alloy.

When looking at the degree of deformation of the shape of the sample after heat treatment, it was found that the thermal deformation was large in the plain system, whereas
There was no change at all in the alloy type. In this way, heat deformation resistance can be improved.

[0031]

EFFECTS OF THE INVENTION The PTC heating element material according to claim 1 uses a second polymer that is not completely compatible with the first polymer, so that changes in electrical conductivity over time and mechanical brittleness are prevented. has been improved. In the PTC heating element material according to claim 2, the first polymer region in which the conductive particles are unevenly distributed and the second polymer region in which the same crystalline polymer is phase-separated are finely mixed, and the conductive particles are separated from each other. The conductivity is stable because the movement is confined within the microscopic region due to the presence of the second polymer. Furthermore, since the second polymer region with a small amount of conductive particles and reduced flexibility is present throughout the material, brittleness is improved.

[0032] When a polymer whose thermal softening temperature is higher than that of the first polymer is used as the second polymer, the stability of the fine phase separation structure of the first polymer and the second polymer is excellent, and as a result, the conductive particles are The stability of the dispersed state increases. In particular, when the first and second polymers form a continuous phase and form a mutually continuous structure, the fine phase separation structure is more stable.
This further increases the stability of the dispersion state of the conductive particles.

According to the present invention, in such a combination of polymers, by increasing the ratio of the second polymer to the first polymer, a PTC heating element material having excellent heat deformation resistance can be obtained. That is, the properties necessary for a heating element such as PTC properties can be designed using the first polymer and conductive particles, and the heat resistance and mechanical properties can be designed using the second polymer.

In the present invention, when the first polymer is crosslinked, even if the crystals of the first polymer are repeatedly melted and recrystallized, the crystal structure of the first polymer is highly stable, and the conductive particles The change in the dispersion state of will be smaller. Furthermore, when polyethylene is used as the first polymer, polypropylene is used as the second polymer, and carbon black is used as the conductive particles, a heating element material with even better PTC characteristics and thermal stability can be produced.

Claims (4)

[Claims] 1. A PTC heating element material, which is obtained by kneading a crystalline polymer, a polymer that is not completely compatible with the crystalline polymer, and conductive particles. 2. A PTC in which a crystalline polymer region and a phase-separated polymer region are finely mixed, and conductive particles are dispersed so as to be unevenly distributed in the crystalline polymer region. Heating element material. 3. The PTC heating element material according to claim 2, wherein a crystalline polymer phase in which conductive particles are dispersed and a polymer phase separated from the same polymer phase form a mutually continuous phase. 4. The PTC heating element material according to claim 1, wherein the crystalline polymer is polyethylene, the polymer which is not completely compatible with the crystalline polymer is polypropylene, and the conductive particles are carbon black.