JPH04273104A - Conductive composite and manufacture - Google Patents

Abstract

PURPOSE:To provide a conductive composite which is thermally stable, whose electric resistance value is well reproducible and which displays a positive temperature coefficient and to provide a method to easily manufacture the composite. CONSTITUTION:A conductive composite is composed mainly of an ultrahigh- molecular-weight polyethylene fine powder and a carbon fiber. The conductive composite is obtained as follows: the ultrahigh-molecular-weight polyethylene fine powder, the carbon fiber and a latex are dispersed to water; an aqueous slurry is prepared; a polymer coagulant is added to the obtained aqueous slurry; a solid content is coagulated; and the coagulated solid content is heated, compressed and molded.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to a conductive composite material which is thermally stable and exhibits a positive characteristic temperature coefficient (hereinafter abbreviated as PTC) with good electrical resistance value reproducibility. The present invention relates to a manufacturing method thereof. [0002] PTC is a coefficient that indicates the property that the electrical resistance value increases rapidly as the temperature rises in a specific temperature range. Conventionally, as a conductive composition exhibiting PTC,
Compositions obtained by mixing carbon black and metal powder with crystalline polymers such as polyethylene, polypropylene, or nylon are known. However, conventional products have a problem of poor thermal stability and poor PTC reproducibility. Therefore, as a method for obtaining a thermally stable conductive composite with good PTC reproducibility, for example,
JP-A No. 61-218117 discloses a method for obtaining a conductive crosslinked polymer exhibiting PTC by mixing 0 to 400,000 polyethylene and carbon black into pellets, and then irradiating the resulting molded product with an electron beam. It is disclosed in the publication No. In addition, JP-A-62-167358 discloses a method of melt-mixing an organic polymer such as polyethylene and carbon black, pulverizing the mixture, irradiating it with electron beams, and then blending it with a matrix polymer to obtain a molded conductive polymer composition. It is disclosed in the publication No. In these methods, the polymer is crosslinked by irradiating it with radiation such as an electron beam before or after molding, thereby thermally stabilizing it and improving the reproducibility of PTC. However, it is difficult to uniformly crosslink polymers, and as a result, it has been difficult to obtain a product that is thermally stable and has good PTC reproducibility. Furthermore, in order to crosslink the polymer uniformly, it is extremely difficult to select the radiation source, intensity, energy, and irradiation method, and the process becomes complicated. [Problems to be Solved by the Invention] Therefore, an object of the present invention is to provide a conductive composite that is thermally stable and has good PTC reproducibility, and a conductive composite that can be easily obtained. The purpose is to provide a manufacturing method. [Means for Solving the Problems] As a result of intensive research in order to solve the above problems, the present inventors have found that ultra-high molecular weight polyolefin with excellent heat resistance is mixed with a specific carbon fiber as described below. The composite composed of the composition is thermostable and PTC
The present invention was achieved based on the discovery that the reproducibility of the conductive composite is good and that such a conductive composite can be easily produced by the method described below. That is, the gist of the present invention is that the main components are ultra-high molecular weight polyolefin and the fiber length is 0.2 to 50 mm.
It is a conductive composite consisting of carbon fibers having a volume resistivity of 100 Ωcm or less at 25°C, and an electrical resistance value exhibiting a positive characteristic temperature coefficient. The conductive composite is prepared by dispersing ultra-high molecular weight polyolefin, carbon fiber with a fiber length of 0.2 to 50 mm, and latex in water to prepare an aqueous slurry.
It can be produced by adding a polymer flocculant to the resulting aqueous slurry to flocculate the solids, and then molding the flocculated solids. The present invention will be explained in detail below. The conductive composite of the present invention has a volume resistance of 100Ω at 25°C.
It is preferable to have a resistance value of 10 cm or less and an R15 value of 10 or more or an R100 value of 100 or more in the operating temperature range of 0 to 200°C, especially one with an R15 value of 10 or more and an R100 value of 100 or more. . Here, the R15 value represents the ratio of the initial and final resistivity in the temperature range of 15°C, where the increase in resistivity is the largest in the operating temperature range of 0 to 200°C, and the R100 value represents the ratio of the resistivity between 0 and 200°C.
It represents the ratio of the initial and final resistivity in the temperature range of 100°C, where the increase in resistivity is the largest in the operating temperature range of 200°C. [0008] The conductive composite according to the present invention is mainly composed of an ultra-high molecular weight polyolefin and carbon fibers, and examples of the ultra-high molecular weight polyolefin include ethylene, propylene, 1-butene, 1-hexene, and 1-hexene. Homopolymers or copolymers of α-olefins such as octene, 1-decene, 1-dodecene, 4-methyl-1-pentene, or mixtures thereof are used, and among these, homopolymers of ethylene are particularly used. preferable. The molecular weight of ultra-high molecular weight polyolefin is 1 million or more, especially 300~
8 million is preferred. When the molecular weight is less than 1 million, it is difficult to obtain a PTC with good thermal stability and reproducibility. Also, if the polymer is too high, moldability will be poor. The ultra-high molecular weight polyolefin used in the present invention preferably has a particle size of 10 to 800 μm. If the particle size is less than 10 μm, secondary aggregation is likely to occur, and if the particle size is more than 800 μm, it is difficult to obtain a composite with sufficient strength. The content of ultra-high molecular weight polyolefin is 10 to 99.5% by weight, especially 20 to 99.5% by weight.
It is preferably 96% by weight; if it is less than 10% by weight, it will be difficult to obtain a composite with sufficient strength, and if it exceeds 99.5% by weight, it will be difficult to obtain a composite with sufficient conductivity. The carbon fibers used in the present invention include carbon fibers obtained by carbonizing acrylonitrile fibers, pitch fibers, phenol fibers, rayon fibers, etc. Carbon fibers with high strength and high modulus of elasticity are particularly preferred. Carbon fibers having an average length of 0.2 to 50 mm are used. If the average length is shorter than 0.2 mm, it is difficult to obtain sufficient strength, and if the average length exceeds 50 mm, it is difficult to disperse in an aqueous medium. Further, it is preferable to use fibers having an aspect ratio (length to diameter ratio) of greater than 40. When the aspect ratio is 40 or less, it is difficult to obtain sufficient strength. That is, the fiber length is 0.5 to 16 mm.
It is particularly preferable to use one having a diameter of 13 to 25 μm. Further, a composite using carbon fiber exhibits PTC and also has high mechanical strength. [0011] The content of carbon fiber in the conductive composite is
It is usually 0.5 to 90% by weight, particularly preferably 3 to 70% by weight. If it is less than 0.5% by weight, it is difficult to obtain sufficient conductivity, and if it exceeds 90% by weight, the strength of the conductive composite becomes weak and it becomes difficult to mold it. Next, a preferred example of manufacturing the conductive composite of the present invention will be explained. After thoroughly stirring the ultra-high molecular weight polyolefin, carbon fiber, and latex in an aqueous medium and confirming that they are neatly dispersed, a polymer flocculant is added to destabilize the solid content in the aqueous slurry and cause it to flocculate. After the solid content is collected, dehydrated, and dried, the solid content is molded by the method described below to obtain a conductive composite. When collecting the solid content by agglomerating it, collecting the solid content in the form of a sheet using a sheet machine is suitable for obtaining a sheet-like composite, and the subsequent steps are easy. [0013] The latex used here is one in which a synthetic or natural polymer is dispersed in water, and examples of such latex include acrylic polymer latex produced by emulsion polymerization, and acrylic polymer latex containing ethylenically unsaturated groups. Examples include homopolymers or copolymers of monomers having the same, and latexes of these polymer derivatives. The polymer in the latex serves to bind or adhere the solids in the aqueous slurry as they are recovered. The amount of latex added to the aqueous slurry is preferably 0.1 to 10% by weight of the composite in terms of solid content. If it is less than 0.1% by weight, it will be difficult to obtain a conductive composite with sufficient strength, and if it exceeds 10% by weight, the amount of coagulant used will be large, making it uneconomical. [0014] Furthermore, the polymer flocculant plays the role of destabilizing and flocculating the solid content in the aqueous slurry. The polymer flocculant is preferably a natural or synthetic polymer that can form a film at room temperature or lower temperatures (40° C. or lower). Particularly when going through a wet lamination mat forming process, it is preferred to use a polymer that is film-forming at that temperature. Examples of the polymer flocculant include partially hydrolyzed polyacrylamide, sodium polyacrylate, partially hydrolyzed polyacrylonitrile, modified cationic polyacrylamide, dialkylaminoethyl methacrylate polymer, vinylimidazoline polymer, Examples include diarylamine polymers. The amount of polymer flocculant added is preferably 0.1 to 10% by weight of the composite in terms of solid content. If it is less than 0.1% by weight or more than 10% by weight, it is difficult to obtain a conductive composite with sufficient strength. [0016] The molding conditions are: temperature: melting point of ultra-high molecular weight polyolefin to melting point +50°C, especially melting point to melting point +30°C
, Press pressure: 100 to 1500 kg/cm2, especially 120 to 1000 kg/cm2, Press time: 0.2
-900 seconds, particularly preferably 0.5-400 seconds. If the temperature during molding is lower than the melting point, it will be difficult to obtain sufficient strength, and if the temperature is higher than the melting point +50°C, the ultra-high molecular weight polyolefin will cover the surface of the carbon fiber, making it difficult to obtain sufficient electrical conductivity. If the press pressure is lower than 100kg/cm2 and the press time is shorter than 0.2 seconds, it is difficult to obtain sufficient strength, and the press pressure is 1500kg/cm2.
Exceeding this value increases fuel consumption and is not economical. If the pressing time exceeds 900 seconds, the molding cycle becomes long, which is not economical. EXAMPLES Next, the present invention will be specifically explained using examples. Example 1 Ultra-high molecular weight polyethylene fine powder (PE-COMP-14
07, manufactured by Toyo Ink Co., Ltd.) 9g, carbon fiber chopped strand (fiber length: 3mm, HTA-C3-P
L, manufactured by Toho Rayon Co., Ltd.) 1g, latex (SFC-5
5. Manufactured by Dainippon Ink Co., Ltd., solid content 3% by weight) 10g to 1 part water
1 and stirred for 5 minutes to prepare an aqueous slurry. A polymer flocculant (UF-105,
Add 100 g of Unitika (manufactured by Unitika, solid content 0.05% by weight) to coagulate the solid content to obtain a mat, dry this mat,
The mixture was filled into a mold, pressed at 150° C. and 150 kg/cm 2 for 30 seconds, and cooled under pressure to obtain a conductive composite. Table 1 shows the characteristic values of the obtained conductive composite. Example 2 Ultra-high molecular weight polyethylene fine powder (PE-COMP-14
07, manufactured by Toyo Ink Co., Ltd.) 8g, carbon fiber chopped strand (fiber length: 3mm, HTA-C3-P
Example 1 except that it was changed to 2g (L, manufactured by Toho Rayon Co., Ltd.)
A conductive composite was obtained in the same manner as above. Table 1 shows the characteristic values of the obtained conductive composite. Example 3 Carbon fibers were made into carbon fiber chopped strands (
A conductive composite was obtained in the same manner as in Example 1 except that the fiber length was changed to 6 mm, HTA-C6-PL (manufactured by Toho Rayon Co., Ltd.). Table 1 shows the characteristic values of the obtained conductive composite. [Table 1] [Effects of the Invention] As described above, the conductive composite of the present invention has the following properties:
PTC exhibits thermal stability and electrical resistance values with good reproducibility. Therefore, they are ideally used as fuses in electronic devices, or in planar heating elements. Also,
According to the method of the present invention, a conductive composite can be easily produced.

Claims (2)

[Claims] Claim 1: A composite whose main components are ultra-high molecular weight polyolefin and carbon fibers with a fiber length of 0.2 to 50 mm, which has a volume resistivity of 100 Ωcm or less at 25°C and an electrical resistance value of A conductive composite characterized by exhibiting a positive characteristic temperature coefficient. [Claim 2] An aqueous slurry is prepared by dispersing ultra-high molecular weight polyolefin, carbon fibers having a fiber length of 0.2 to 50 mm, and latex in water, and a polymer flocculant is added to the obtained aqueous slurry. 2. The method for producing a conductive composite according to claim 1, further comprising aggregating the solid content and molding the agglomerated solid content.