JPH0636859A - Manufacture of ptc heating element - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a PTC heating element in which the reduction in resistance value is minimized even if it is allowed to stand at a high temperature for a long time, and the danger of short-circuit is thus minimized. CONSTITUTION:A PTC heating element is manufactured by use of a molding material obtained by mutually kneading a first crystalline polymer, a second polymer which is never perfectly compatible with the first polymer and has a melting point or thermal deforming temperature higher than the melting point of the first polymer, and a conductive particle. After forming a molded product, it is subjected to electron beam cross-linking treatment and successively to thermal treatment at a temperature which is the melting point of the first polymer or more and lower than the melting point or thermal deforming temperature of the second polymer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a heating element, and in particular, a PTC (Positive T) having a positive temperature coefficient of resistance.
The present invention relates to a method for manufacturing an heating element.

[0002]

2. Description of the Related Art One of the heating elements that generate heat when energized is a PTC heating element whose resistance value increases with increasing temperature (that is, has a positive temperature coefficient of resistance). The electric heater using the PTC heating element naturally has a self-temperature control function that the resistance value increases as the temperature rises, the current decreases, the heat generation amount is suppressed, and the one-way temperature rise is prevented. Therefore, it is known as a useful and safe heater. As a material for manufacturing such a PTC heating element, a conductive particle-dispersed crystalline polymer obtained by dispersing conductive particles in a crystalline polymer is known. Examples of the crystalline polymer include polyethylene, ethylene-vinyl acetate copolymer, ionomer, polyvinylidene fluoride and the like, and examples of the conductive particles include carbon black fine powder and graphite fine powder.

This conductive particle-dispersed crystalline polymer is PT
The reason why the C characteristic is exhibited is as follows. When the temperature of the conductive particle-dispersed crystalline polymer rises to the melting point of the crystalline polymer, a large volume increase occurs as the crystals melt. This increase in volume causes breakage of the conductive chains generated by the contact between the dispersed conductive particles and expansion of the spacing between the conductive particles. The cutting of the conductive chains and the expansion of the spacing between the conductive particles cause a decrease in the conductive action of the conductive particles, resulting in a sharp increase in the resistance value.

However, the above-mentioned conductive particle-dispersed crystalline polymer has a large change in conductivity (resistance value) with time,
There was a problem that the electric conductivity deviated from the initial value. Therefore, as a means for solving this problem, the inventors of the present invention disclosed in Japanese Patent Application No. 3-310804 the first crystallinity
And a second polymer that is not completely compatible with the first polymer and has a melting point or a heat distortion temperature higher than the melting point of the first polymer and a conductive material. In the method for producing a PTC heating element according to claim 1, after being formed into a molded product, heat treatment is performed at a temperature not lower than the melting point of the first polymer and not higher than the melting point or thermal deformation temperature of the second polymer. Proposing a body manufacturing method.

[0005]

However, in the PTC heating element produced by the above-mentioned manufacturing method, if the PTC heating element is left at a temperature higher than the melting point of the first polymer for a long time, the resistance value is lowered, and there is a danger of short circuit (short circuit). However, there is a problem that the safety of the PTC heating element is not sufficient.

In view of the above-mentioned circumstances, the present invention has a low resistance value reduction even if it is left at a temperature above the melting point of the first polymer for a long time, and therefore there is little risk of short circuit.
It is intended to provide a method for producing a C heating element.

[0007]

SUMMARY OF THE INVENTION The present invention is a crystalline first
And a second polymer that is not completely compatible with the first polymer and has a melting point or a heat distortion temperature higher than the melting point of the first polymer and a conductive material. In the method for producing a PTC heating element by using a molded article, a molded article is subjected to electron beam cross-linking treatment, and then heat-treated at a temperature not lower than the melting point of the first polymer and not higher than the melting point or heat distortion temperature of the second polymer. P which is characterized by
This is a method of manufacturing a TC heating element.

Now, the process of the present invention will be described.
In the conductive particle-dispersed crystalline polymer, there is a problem that the electrical conductivity (resistance value) changes greatly with time, and the electrical conductivity deviates from the initial value.The present inventors, as a means for solving this problem, A PTC containing a crystalline first polymer, a second polymer which is not completely compatible with the first polymer, and a second polymer having a melting point or a heat distortion temperature higher than that of the first polymer and conductive particles. It was found that it is effective to use a heating element molding material, and the application is filed as Japanese Patent Application No. 3-88881. The PTC heating element obtained using this means realizes a fine mutual continuous structure by matching the melt viscosities of the first polymer and the second polymer, and the first polymer and the second polymer. An alloy material in which conductive particles are unevenly distributed in the region of the first polymer, and the conductive particles are unevenly distributed in the region of the first polymer, and Since the polymer region is isolated by the second polymer having high heat resistance, the movement of the conductive particles is limited, and the PTC heating element has a stable electric conductivity. Further, as a means for reducing the change in conductivity over time, as described above, Japanese Patent Application No.
-310804, a crystalline first polymer;
Using a molding material obtained by kneading a conductive polymer and a second polymer that is not completely compatible with the first polymer and has a melting point or a heat distortion temperature higher than the melting point of the first polymer, P
In the method for producing a TC heating element, after forming a molded article,
It proposes a method for producing a PTC heating element, which is characterized in that heat treatment is performed at a temperature not lower than the melting point of the first polymer and not higher than the melting point or heat distortion temperature of the second polymer. The present invention has been reached as a result of further progress of the above series of studies.

In the present invention, the conductive particles are unevenly distributed in the region of the first polymer mainly because of the difference in surface energy between the first polymer, the second polymer and the conductive particles. The difference in the surface energy between the conductive particles and the first polymer is smaller than the difference in the surface energy between the conductive particles and the second polymer, which is a driving force, and the conductive particles are being kneaded into the region of the first polymer. To move to. Table 1 shows the surface tension of the raw material. That is, for example, when polyethylene is used as the first polymer, polypropylene is used as the second polymer, and carbon black is used as the conductive particles, from the surface tension values shown in Table 1, the carbon black is mixed in the polyethylene phase having a small difference in surface tension during kneading. It will shift.

[0010]

[Table 1]

The present invention will be specifically described below. Examples of the crystalline first polymer used in the present invention include polyethylene, polypropylene, polymethylpentene, EVA (ethylene-vinyl acetate resin), EEA (ethylene-ethyl acrylate resin), and EMA (ethylene-methyl acrylate resin). Examples thereof include ethylene copolymer polymers, ionomers, polyvinylidene fluoride, polyamides and polyesters. On the other hand, as the second polymer which is not completely compatible with the first polymer, polyethylene, polypropylene, polymethylpentene, ethylene copolymers (EVA, EEA, EMA, etc.), ionomer, polyvinylidene fluoride, polyamide, Examples thereof include crystalline polymers such as polyester, and amorphous polymers such as PMMA (polymethylmethacrylate), polystyrene, polycarbonate, polysulfone, polyvinyl chloride and copolymers thereof.

The second polymer used in the present invention, which is not completely compatible with the first polymer, means the polymer which is almost incompatible with the first polymer or the first polymer. It is a polymer that is compatible with a small amount or partially and forms a polymer alloy that is not a compatible system with the first and second polymers. Regarding such a polymer alloy, (“Polymer alloy-basics and applications-”; edited by Japan Society of Polymer Science, 1981),
("Polymer Alloy"; edited by The Society of Polymer Science, Japan, Takashi Inoue, Toshio Nishinishi, 1988), ("Polymer Blend"; published by CMC, 1979). Needless to say, the crystalline polymer does not have to be in a crystalline state over the entire region, and an amorphous portion may coexist. Further, it is known that the crystalline polymer that is actually commercially available does not have such high crystallinity under ordinary molding conditions. For example, nylon 6-6
Is 30 to 35%, polypropylene is 50 to 70%, polyethylene is 65 to 90%, PBT (polybutylene terephthalate) is 25 to 35%, and PET (polyethylene terephthalate) is about 15 to 20%.

It is important that the second polymer used in the present invention has a melting point or heat distortion temperature higher than the melting point of the first polymer, and a specific combination of the first polymer and the second polymer is used. Examples include the combinations shown in Table 2.

[0014]

[Table 2]

The blending ratio of the first polymer and the second polymer is not particularly limited, but 10 parts by weight of the entire polymer is used.
When the amount is 0 part by weight, the amount of the first polymer is preferably in the range of 20 parts by weight or more and 80 parts by weight or less in order to realize the mutual continuous structure of the first polymer and the second polymer. Further, in the present invention, a third polymer component such as a high-molecular compatibilizing agent may be added as long as it does not impair the object of the present invention.

The conductive particles used in the present invention include powdery or fiber-like particles made of carbon, graphite and metal, and fine carbon black powder is usually used. The amount of conductive particles added is not particularly limited, but is preferably 1 to 40 parts by weight with respect to 100 parts by weight of the polymer. This is because if the amount is less than 1 part by weight, the intended current does not flow, and if it exceeds 40 parts by weight, the intended self-temperature control function cannot be obtained.

In the present invention, P prepared by kneading the above raw materials
A TC heating element molding material is used, and as a method for producing this PTC heating element molding material, for example, the following method is exemplified. First, the second polymer is added to the first polymer and kneaded. At this time, a compatibilizer may be added to control or stabilize the phase separation structure. Next, conductive particles are added and further kneaded to obtain a PTC heating element molding material. The order of adding these raw materials is not limited to the above, and the second polymer may be added and then kneaded after the first polymer and the conductive particles are kneaded. You may knead. In the production of the PTC heating element molding material, if necessary, a stabilizer, an antioxidant, a lubricant, a surfactant, a flame retardant, an inorganic filler (aluminum hydroxide, silica, etc.), a nucleating agent, etc. may be appropriately added. It may be added in stages.

In the present invention, P produced as described above
A PTC heating element is produced by molding using a TC heating element molding material, but the method of molding is not particularly limited, and for example, direct pressure molding or extrusion molding into a desired shape such as a sheet shape or a wire shape. The method may be used to obtain a molded product.

The feature of the present invention is that after the molded product is subjected to electron beam cross-linking treatment, it is then heat-treated at a temperature not lower than the melting point of the first polymer and not higher than the melting point or heat distortion temperature of the second polymer. In point. The electron beam irradiation amount in this electron beam crosslinking treatment is appropriately in the range of several Mrad to several tens of Mrad. Specifically, it is preferable that the irradiation amount of the electron beam is 5 Mrad or more, and the electron beam crosslinking treatment is such that the gelation rate of the first polymer is 50% or more. The reason for using the electron beam cross-linking treatment instead of the chemical cross-linking treatment is that the degree of the cross-linking reaction is difficult to be constant in the chemical cross-linking treatment and the treatment varies widely, whereas the degree of the cross-linking reaction in the electron beam cross-linking treatment is large. This is because it is possible to obtain a product that is uniform, has a small variation in processing, and has a good yield and uniform characteristics.

After the electron beam cross-linking treatment as described above, the melting point of the first polymer is not lower than the second melting point.
By heat-treating the polymer at a temperature not higher than the melting point or the heat distortion temperature of the polymer, the temporal change of the electric conductivity of the obtained PTC heating element is reduced, and the stability of the electric conductivity is improved. The heat treatment time is not particularly limited, but it is preferable to perform the heat treatment for 30 seconds or more in order to ensure the effect of improving the stability of the electric conductivity.

[0021]

In the present invention, the electron beam cross-linking treatment after forming a molded article has the function of cross-linking the first polymer in which the conductive particles are dispersed. Then, by crosslinking the first polymer, the viscosity of the first polymer increases and the conductive particles become difficult to migrate. Therefore, the obtained PTC heating element is left at a temperature equal to or higher than the melting point of the first polymer. In this case, it is considered that the effect of reducing the resistance value is reduced and the risk of short circuit is reduced.

[0022]

EXAMPLES The present invention will be described below with reference to examples. It goes without saying that the present invention is not limited to the following examples. (Examples 1 and 2 and Comparative Examples 1 and 2) As the crystalline first polymer, MFR = 0.23, low-density polyethylene having a melting point of 122 ° C., and as the second polymer MFR = 1.2,
Polypropylene having a melting point of 166 ° C. and carbon black having a particle diameter of 62 nm were used as conductive particles. In addition, MFR has shown the melt flow rate. A 1.8 (l) T-type intensive mixer (Banbury mixer) was used as a kneading machine, and about 1 kg of the raw material was mixed at the mixing ratio (% by weight) shown in Table 3 and kneaded under the following kneading conditions. Got the material. Kneading conditions: rotation speed 120 rpm, heating temperature of mixer jacket 120 to 130 ° C., kneading time 6 minutes, and then molding using the molding material obtained under molding conditions of 170 ° C. × 2 minutes and pressure of 10 seconds to form a sheet-like molded article. Got

[0023]

[Table 3]

Next, in Examples 1 and 2, the sheet-shaped molded product obtained above was subjected to electron beam crosslinking treatment at an irradiation dose of 30 Mrad. The gelation rate of the first polymer in the obtained molded product was about 95%. Next, for Examples 1 and 2, the obtained electron beam cross-linked molded articles were used, and for Comparative Examples 1 and 2, molded articles not subjected to electron beam cross-linking treatment were used at 150 ° C. for 5 minutes. Heat treatment of
A PTC heating element was obtained. Using the PTC heating element thus obtained, the electrical characteristics were tested.

In this electrical test, a test piece prepared by sticking aluminum foil with a conductive adhesive material as electrodes on both sides of the above-mentioned sheet-shaped molded product was used to measure the impedance of the PTC heating element in the thickness direction. (100Hz)
Was measured. First, the impedance of each test piece was measured by changing the ambient temperature. The results are shown as graphs in FIGS. 1 and 2. In addition, the impedance in the atmosphere of 140 ℃ obtained in this test is 140 ℃
Impedance immediately after being left in the atmosphere (denoted as Z140-)
The obtained numerical values are shown in Table 4. And then 1
Impedance (Z140-
Is expressed) and the obtained numerical values are shown in Table 4. Further, the rate of change in impedance after leaving for 1 hour in a 140 ° C. atmosphere was calculated by the following formula, based on the impedance immediately after leaving in a 140 ° C. atmosphere, and is also shown in Table 4.

Impedance change rate (%) after standing in 140 ° C. atmosphere for 1 hour = 100 × (Z140 --- Z140-)
/ Z140-

[0027]

[Table 4]

From the above results, in Comparative Examples 1 and 2, the first
It was confirmed that when left at 140 ° C., which is higher than the melting point of the polymer of 1., for 1 hour, the impedance was significantly reduced, but in Examples 1 and 2, the reduction rate was small. That is, it was confirmed that the electron beam crosslinking reduced the decrease in the resistance value when left at a temperature equal to or higher than the melting point of the first polymer, and therefore reduced the risk of short circuit.

[0029]

The PTC heating element produced by the method of the present invention has a melting point or a melting point of the first polymer which is not completely compatible with the crystalline first polymer and the first polymer. In a method for producing a PTC heating element using a molding material obtained by kneading a higher second polymer and conductive particles, a molded article is subjected to electron beam crosslinking treatment, and then the melting point of the first polymer. As described above, since the heat treatment is performed at a temperature equal to or lower than the melting point or the heat distortion temperature of the second polymer, the PTC heating element has a restricted migration of the conductive particles. Therefore, the present invention is a method useful for producing a PTC heating element in which there is little decrease in resistance value when left at a temperature above the melting point of the first polymer for a long time, that is, there is little risk of short circuit. is there.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between an impedance (100 Hz) in a thickness direction and an ambient temperature in Example 1 of the present invention and Comparative Example 1.

FIG. 2 is a graph showing the relationship between the impedance (100 Hz) in the thickness direction and the ambient temperature in Example 2 and Comparative Example 2 of the present invention.

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[Procedure amendment]

[Submission date] January 20, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0027

[Correction method] Change

[Correction content]

[0027]

[Table 4]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masao Sumita 2-4 Terao 2-3, Terao, Ayase City, Kanagawa Prefecture

Claims (3)

[Claims] 1. A crystalline first polymer and a second polymer and conductive particles which are not completely compatible with the first polymer and have a melting point or heat distortion temperature higher than that of the first polymer. In a method for producing a PTC heating element using a molding material obtained by kneading and, a molded product is subjected to electron beam cross-linking treatment, and then the melting point of the first polymer is higher than that of the second polymer. A method for producing a PTC heating element, which comprises performing heat treatment at a temperature equal to or lower than a melting point or a heat distortion temperature. 2. The method for producing a PTC heating element according to claim 1, wherein the electron beam crosslinking treatment is carried out at an irradiation dose of 5 Mrad or more. 3. The method for producing a PTC heating element according to claim 1, wherein the first polymer is polyethylene, the second polymer is polypropylene, and the conductive particles are carbon black.