JP6972899B2 - Zinc oxide particles and a resin composition containing them - Google Patents

Description

The present invention relates to zinc oxide particles and a resin composition containing the same. More specifically, the present invention relates to zinc oxide particles that can be suitably used as a material for a resin varistor, and a resin composition containing the same.

A varistor is an electronic component that has the property that the electrical resistance is high when the voltage between the two terminals is low, and the electrical resistance drops sharply when the voltage exceeds a certain level, and protects other electronic components from high voltage. It is used in many electronic circuits as a bypass to do so.
As a varistor _{, metal oxides such as Bi 2} O ₃ , Co ₂ O ₃ , MnO, NiO, Cr ₂ O ₃ , and Sb ₂ O ₃ are mixed with zinc oxide having a particle diameter of 1 μm or less and calcined, and then the particles are calcined to each other. The most common is a ceramic varistor, which is a sintered body in which zinc oxide has a structure surrounded by an insulating grain boundary layer of the above-mentioned added metal oxide, which is obtained by sintering the above-mentioned, but the ceramic varistor has a desired shape. Since it is difficult to mold the zinc oxide into a varistor and it is difficult to control the size of zinc oxide by sintering, the varistor characteristics are unstable.

From the viewpoint of increasing the degree of freedom in molding, zinc oxide contains a sintered body containing zinc oxide as a main component, a semi-conductive whisker, a varistor material containing a catalyst and an epoxy resin, and a predetermined metal component in zinc oxide to provide non-linear resistance. A technique has been proposed in which a matrix composite of a varistor powder having improved properties and, for example, an epoxy resin is used as a varistor (see Patent Documents 1 and 2). These are referred to as resin varistor or plastic varistor.
Further, the resin is replaced with a highly insulating resin such as a polyacetal resin, and is regarded as an insulating layer in place of the added metal oxide. That is, the zinc oxide particles of the semiconductor are dispersed in the highly insulating resin to exhibit the varistor characteristics. A technique has been proposed (see Patent Document 3).

Japanese Unexamined Patent Publication No. 2015-101714 Japanese Unexamined Patent Publication No. 2008-218949 International Publication No. 2016/133460

As described above, resin varistor or plastic varistor for the purpose of increasing the degree of freedom of molding of the varistor has been proposed, but the problem that the composition and the particle size are difficult to control and the varistor characteristics are not stable has not been solved. ..
Further, it is not easy to highly fill zinc oxide with a highly heat-resistant resin having a low affinity for fillers in general, and it has not been possible to obtain varistor characteristics comparable to those of conventional ceramic varistor. As described above, the conventional varistor is not sufficient in terms of performance, and there is room for further improvement.

In view of the above situation, it is an object of the present invention to provide a material having excellent varistor characteristics, which enables the realization of a varistor having excellent moldability as well as stability of the characteristics.

The present inventors have investigated a material that enables the realization of a plastic varistor having excellent properties, stability and moldability, and as a result, zinc oxide particles are contained in a high proportion in the resin in order to obtain desired performance. In the process of kneading and molding the resin composition (sometimes referred to as high compounding), it was found that the zinc oxide particles were partially cracked or crumbled. Normally, if the amount of zinc oxide particles is such that they are uniformly dispersed in the resin, the frequency with which the zinc oxide particles come into contact with the machine wall of the processing machine and the frequency with which the particles come into contact with each other are very low. Since the zinc oxide particles in the varistor are contained in a very high proportion, the frequency of contact between the zinc oxide particles and the machine wall or the frequency of contact between the zinc oxide particles is significantly increased, resulting in cracking or collapse. It is estimated to be. Furthermore, it has been found that the particle size in the molded product becomes non-uniform due to the occurrence of cracking and collapse of the particles, which may lead to destabilization of the varistor characteristics.
Therefore, the present inventors have investigated zinc oxide particles capable of stabilizing the varistor characteristics, and have investigated an aggregate composed of zinc oxide particles, a resin composition in which zinc oxide particles and a resin are kneaded, or the resin composition. One of the molded bodies obtained by molding the above is cut with a cross-section polisher, the cross section is photographed with a scanning electron microscope, and pores of 0.1 μm or more in the cross section of 100 zinc oxide particles arbitrarily extracted are formed. ^{When counted, zinc oxide particles having an average number of pores per 100 μm 2} cross-sectional area of less than 8 are less likely to be destroyed during kneading with the resin or molding, and are at a high ratio with respect to the resin. It was found that it is possible to mix. Then, they have found that by preparing a composition in which such zinc oxide particles are blended in a high proportion in a resin, a varistor material having excellent moldability as well as varistor characteristics and stability of characteristics can be obtained. It came to be completed.

That is, in the present invention, either an aggregate composed of zinc oxide particles, a resin composition obtained by kneading zinc oxide particles and a resin, or a molded body obtained by molding the resin composition is cut by a cross section polisher. When the cross section was photographed with a scanning electron microscope and the pores of 0.1 μm or more in the cross section of 100 arbitrarily extracted zinc oxide particles were counted, the average number of pores per ^{100 μm 2 cross section was eight.} Zinc oxide particles characterized by less than.

The present invention is also a resin composition containing 70% by weight or more of the zinc oxide particles of the present invention with respect to the resin.

The resin is preferably a highly heat-resistant resin having a melting point of 200 ° C. or higher or a thermosetting resin having a pyrolysis temperature of 350 ° C. or higher.

The present invention is also a plastic varistor using the resin composition of the present invention.

The zinc oxide particles of the present invention are strong particles that are less likely to crack or collapse even when they are highly blended in a resin and kneaded. , A varistor material having excellent varistor characteristics, stability of characteristics, and moldability can be obtained.

It is a figure which showed the scanning electron micrograph of the zinc oxide particle A obtained in Example 1. FIG. It is a figure which showed the particle size distribution of the zinc oxide particle A obtained in Example 1. FIG. It is a figure which showed the cross-sectional photograph of the disk-shaped molded body 1 which hardened by heat after press-molding the resin composition containing zinc oxide particles A and maleimide resin obtained in Example 1. FIG. It is a figure which showed the current-voltage non-linear resistance characteristic graph of the disk-shaped compact 1 which was heat-cured after press-molding the resin composition containing zinc oxide particles A and maleimide resin obtained in Example 1. FIG. .. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the disk-shaped molded body 1 obtained in Example 1. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the disk-shaped molded body 1 obtained in Example 1. FIG. It is a figure which showed the current-voltage non-linear resistance characteristic graph of the disk-shaped compact 2 obtained in Example 2. FIG. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the disk-shaped molded body 2 obtained in Example 2. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the disk-shaped molded body 2 obtained in Example 2. FIG. It is a figure which showed the scanning electron micrograph of the zinc oxide particle B obtained in Example 3. FIG. It is a figure which showed the particle size distribution of the zinc oxide particle B obtained in Example 3. FIG. It is a figure which showed the cross-sectional photograph of the disk-shaped molded body 3 which hardened by heat after press-molding the resin composition containing zinc oxide particles B and maleimide resin obtained in Example 3. FIG. It is a figure which showed the current-voltage non-linear resistance characteristic graph of the disk-shaped compact 3 obtained in Example 3. FIG. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the disk-shaped molded body 3 obtained in Example 3. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the disk-shaped molded body 3 obtained in Example 3. FIG. It is a figure which showed the scanning electron micrograph of the zinc oxide particle C obtained in Example 4. FIG. It is a figure which showed the particle size distribution of the zinc oxide particle C obtained in Example 4. It is a figure which showed the cross-sectional photograph of the disk-shaped molded body 4 which hot-press molded the resin composition containing the zinc oxide particle C and the polyamide 66 resin obtained in Example 4. FIG. It is a figure which showed the current-voltage non-linear resistance characteristic graph of the disk-shaped compact 4 which hot-press molded the resin composition containing the zinc oxide particle C and the polyamide 66 resin obtained in Example 4. FIG. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the disk-shaped molded body 4 obtained in Example 4. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the disk-shaped molded body 4 obtained in Example 4. FIG. It is a figure which showed the scanning electron micrograph of the zinc oxide particle E obtained in the comparative example 1. FIG. It is a figure which showed the particle size distribution of the zinc oxide particle E obtained in the comparative example 1. FIG. It is a figure which showed the cross-sectional photograph of the comparative disk-shaped molded body 1 which hot-press molded the resin composition containing the zinc oxide particle E and the polyamide 66 resin obtained in the comparative example 1. It is a figure which showed the current-voltage linear resistance characteristic graph of the comparative disk-shaped compact 1 which heat-press molded the resin composition containing the zinc oxide particle E and the polyamide 66 resin obtained in the comparative example 1. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the comparative disk-shaped molded body 1 obtained in the comparative example 1. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the comparative disk-shaped molded body 1 obtained in the comparative example 1. FIG. It is a figure which showed the scanning electron micrograph of the zinc oxide particle F obtained in the comparative example 2. FIG. It is a figure which showed the particle size distribution of the zinc oxide particle F obtained in the comparative example 2. FIG. It is a figure which showed the cross-sectional photograph of the comparative disk-shaped molded body 2 which hot-press molded the resin composition containing the zinc oxide particle F and the polyamide 66 resin obtained in the comparative example 2. FIG. It is a figure which showed the current-voltage linear resistance characteristic graph of the comparative disk-shaped compact 2 which heat-press molded the resin composition containing the zinc oxide particle F and the polyamide 66 resin obtained in the comparative example 2. It is a figure which showed the scanning electron micrograph of the residue which removed the resin component from the comparative disk-shaped molded body 2 obtained in the comparative example 2. FIG. It is a figure which showed the particle size distribution of the residue which removed the resin component from the comparative disk-shaped molded body 2 obtained in the comparative example 2. FIG.

Hereinafter, preferred embodiments of the present invention will be specifically described, but the present invention is not limited to the following description, and can be appropriately modified and applied without changing the gist of the present invention.

1. 1. Zinc Oxide Particles The zinc oxide particles of the present invention are either aggregates made of zinc oxide particles, a resin composition obtained by kneading zinc oxide particles and a resin, or a molded product obtained by molding the resin composition. When the cross section was cut with a cross-section polisher and the cross section was photographed with a scanning electron microscope and the pores of 0.1 μm or more in the cross section of 100 arbitrarily extracted zinc oxide particles were counted, the pores per ^{100 μm 2 cross section were counted.} The average number of particles is less than eight.
Cracking or collapsing of zinc oxide particles during kneading or molding of zinc oxide particles and resin causes deterioration of varistor characteristics and stability of characteristics. Oxidation satisfying such conditions. Zinc particles are less likely to crack or crumble when kneaded with resin or molded. Therefore, in the present invention, zinc oxide particles can be highly blended in the resin, and even in that case, the particles are less likely to crack or collapse, so that a material having excellent varistor characteristics, stability of characteristics, and moldability can be obtained. be able to.
The number of pores in the cross section of the zinc oxide particles taken with a scanning electron microscope may be measured, for example, on the zinc oxide particles obtained by compacting the zinc oxide particles with a press machine or the like into aggregates, and the zinc oxide particles and the zinc oxide particles may be measured. It may be performed on a resin composition kneaded with a resin, or may be performed on a molded product obtained by molding a resin composition.
Further, in the present invention, the “pore of 0.1 μm or more” means a pore in which the maximum value of the distance between two points on the outer periphery of the pore is 0.1 μm or more.
In the present invention, the zinc oxide particles may refer to one zinc oxide particle or a powder composed of zinc oxide particles, and in the case of zinc oxide particles as a material to be kneaded with a resin, zinc oxide particles. Refers to powder consisting of.

The zinc oxide particles of the present invention are cross-section polishers obtained from either an aggregate composed of zinc oxide particles, a resin composition obtained by kneading zinc oxide particles and a resin, or a molded product obtained by molding the resin composition. When the cross section was photographed with a scanning electron microscope and the pores of 0.1 μm or more in the cross section of arbitrarily extracted 100 zinc oxide particles were counted, the average number of pores per ^{100 μm 2 cross section.} The number of pores may be less than 8, but the average number of pores is preferably less than 7. More preferably, it is less than 6, still more preferably less than 5, and particularly preferably less than 4.

The zinc oxide particles of the present invention are obtained by cutting either a resin composition obtained by kneading zinc oxide particles and a resin or a molded product obtained by molding the resin composition with a cross-section polisher and scanning a cross section with electrons. When the pores of 0.1 μm or more in the cross section of 100 zinc oxide particles arbitrarily extracted by taking a picture with a microscope are counted, the ratio of the particles having less than 8 pores per ^{100 μm 2 cross section is} It is preferably 60% or more. Zinc oxide particles satisfying such conditions can be said to be stronger particles, and are more suitable as materials for plastic varistor.
The above ratio is more preferably 70% or more, still more preferably 80% or more.

The zinc oxide particles of the present invention preferably have an average particle size of 1 to 100 μm. With such an average particle size, it is possible to blend the resin in a high concentration, and the material obtained by kneading with the resin has good and stable varistor characteristics.
The average particle size of the zinc oxide particles is more preferably 1 to 80 μm, still more preferably 5 to 60 μm.
The average particle size of the zinc oxide particles can be measured by the method described in Examples described later.

The zinc oxide particles of the present invention preferably have a D95 / D10 of less than 3.5 in the particle size distribution. When the zinc oxide particles have such a particle size distribution, the uniform dispersibility when kneaded with the resin becomes excellent, and the plastic varistor produced using such zinc oxide particles has more varistor characteristics and characteristics. It has excellent stability.
The D95 / D10 in the particle size distribution is more preferably less than 3.3, still more preferably less than 3.1, particularly preferably less than 2.8, and most preferably less than 2.5. be.
The particle size distribution of the zinc oxide particles can be measured by the method described in Examples described later.

For the zinc oxide particles, D50 of the particle size distribution of the zinc oxide particles before kneading with the resin is defined as (a), and i) a resin composed of 30% by weight or less of the polyamide 66 resin and 70% by weight or more of the zinc oxide particles. The composition was kneaded with a twin-screw extruder (kneading temperature 200 to 600 ° C., residence time 1 minute) and hot press molded with a press machine (molding temperature 200 to 600 ° C., press pressure 25 MPa), or ii) maleimide. A resin composition consisting of 30% by weight or less of the resin composition and 70% by weight or more of the zinc oxide particles was kneaded with two rolls (kneading temperature 20 to 600 ° C.) for 25 minutes, and the kneaded product was kneaded with a press machine (without heating, 25 MPa). ), And then heat-cured at 20-600 ° C, the resin component is incinerated and removed by heating at 500-900 ° C in the air, and the particle size distribution of the incinerated residue is measured. When D50 is (b), the value obtained by (ab) / a × 100 (hereinafter referred to as the rate of change before and after the resin kneading test) is preferably less than 10.
Since the incinerated residue is considered to be zinc oxide particles that cracked, collapsed, or did not occur during processing, the above values indicate the proportion of zinc oxide particles that maintain their shape after kneading with the resin and molding. ..
The above value may be less than 10, but is preferably less than 8. It is more preferably less than 6, and even more preferably less than 4.

2. 2. Method for Producing Zinc Oxide Particles The method for producing zinc oxide particles of the present invention is not particularly limited, but a step of mixing the raw material zinc oxide and a melting agent, a step of drying and granulating the mixture, and a step of granulating the mixture are used. It is preferable to manufacture by a manufacturing method including a step of firing.

The melting agent is not particularly limited as long as it is an organic acid containing no halogen component, a metal salt of an organic acid, an inorganic acid, an organic base, or an inorganic base, but for example, acetic acid, polycarboxylate, formic acid, and propion. Acids, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, oleic acid, linoleic acid, linolenic acid, arachidonic acid, d. Icosapentaenoic acid, docosahexaenoic acid, sorbic acid, lactic acid, malic acid, citric acid, benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, salicylic acid, gallic acid, melitonic acid, silicic acid, oxalic acid, malonic acid, succinic acid, Glutal acid, adipic acid, pimelli acid, fumaric acid, suberic acid, maleic acid, magellanic acid, sebacic acid, aconitic acid, trimeric acid, nitrate, sulfuric acid, hydrogen peroxide, ammonia, pyridine, piperazine, imidazole, zinc nitrate, acetic acid. One kind or two or more kinds of zinc salts such as zinc and zinc sulfate can be used.

The amount of the flux used is preferably 0.1 to 20% by mass with respect to 100% by mass of the raw material zinc oxide. More preferably, it is 0.1 to 10% by mass.

When mixing the raw zinc oxide and the flux, it is preferable to mix the flux with the raw zinc oxide in the state of a slurry. Water (ion-exchanged water, purified water, distilled water) is preferable as the solvent for forming the slurry. At this time, a dispersant such as a polycarboxylic acid ammonium salt may be used.

The method of mixing the raw material zinc oxide and the flux and then drying is not particularly limited, but if it is a mixture of the raw material zinc oxide slurry and the flux, spray drying with a spray dryer is preferable, and the raw material zinc oxide containing water and the flux are used. , A spartan crown, a spartan mixer, a henschel mixer, a mulmerizer, or the like may be used for granulation, and then heating or static drying with a vacuum dryer may be performed.

The temperature at which the granulated product obtained by drying the mixture of the raw material zinc oxide and the flux is fired is preferably 121 to 1600 ° C. By firing at such a temperature, the pores inside the particles are reduced, and densely sintered strong zinc oxide particles can be obtained. The firing temperature is more preferably 1200 to 1500 ° C, still more preferably 1230 to 1400 ° C.

The time for calcining the granulated product obtained by drying the mixture of the raw material zinc oxide and the flux is preferably 0.5 to 10 hours after reaching the maximum temperature. It is more preferably 1 to 6 hours, and even more preferably 2 to 4 hours.

The method for producing zinc oxide particles may include other steps other than those described above. Examples of other steps include a classification step using a sieve, an elutriation, an air flow classifier, and the like.

The zinc oxide particles may be surface-treated for the purpose of improving the affinity with the resin and arbitrarily controlling the varistor characteristics by changing the volume resistivity of the surface of the zinc oxide particles. The surface treatment is not particularly limited, and a surface having a film formed by at least one compound selected from the group consisting of silicon oxide, silicon oxide hydrate, aluminum oxide and aluminum hydroxide. Examples thereof include treatment, surface treatment with an organic compound, surface treatment with a coupling agent such as a silane coupling agent and a titanium coupling agent, and the like. Further, these two or more types of surface treatments may be combined and performed.

Examples of the coupling agent include vinyl trichlorosilane, vinyl trimethoxy silane, vinyl triethoxysilane, 2- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, 3-glycidoxypropyltrimethosixylan, and 3-glycid. Xipropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxy Silane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2- (aminoethyl) -3-aminopropylmethyldimethoxysilane, N-2- (aminoethyl) -3-aminopropyl Trimethoxysilane, N-2- (aminoethyl) -3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-trietoxysilyl-N- (1,3-) Dilane coupling agents such as dimethyl-butylidene) propylamine, N-phenyl-3-aminopropyltrimethoxysilane, 3-ureidopropyltriethoxysilane, tetraisopropyl titanate, tetranormal butyl titanate, butyl titanate dimer, tetra (2- Ethylhexyl) titanate, tetramethyl titanate, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetacetate, titanium octane diolate, titanium lactate, titanium triethanolaminate, polyhydroxytitanium stearate and other titanium coupling agents. Can be mentioned. Further, tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, hexamethyldisilazane, hexyltrimethoxysilane, decyltrimethoxylane, methylhydrogensiloxane, etc. A silane-based surface treatment agent can also be mentioned. Organic compounds that can be used as coupling agents other than the above include caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, araquinic acid, behenic acid, lignoceric acid, cellotic acid, and montanic acid. Saturated fatty acids such as mericic acid; unsaturated fatty acids such as oleic acid, lignoric acid and lignoceric acid; and the like. In the surface treatment with a coupling agent, one kind of coupling agent may be used, or two or more kinds of coupling agents may be used in combination.

3. 3. Resin Composition The resin composition of the present invention is characterized by containing the zinc oxide particles of the present invention in a proportion of 70% by weight or more with respect to the resin. Since such a resin composition is excellent in varistor characteristics, stability of characteristics, and moldability, it can be suitably used as a material for plastic varistor. The proportion of zinc oxide particles in the resin composition is preferably 72% by weight or more. More preferably, it is 74% by weight or more. Further, from the viewpoint of molding processability, it is preferably 96% by weight or less. More preferably, it is 94% by weight or less.

The resin composition of the present invention preferably contains a highly heat-resistant resin. As such a resin, a highly heat-resistant resin having a melting point of 200 ° C. or higher or a thermosetting resin having a pyrolysis temperature of 350 ° C. or higher is suitable.
If the heat resistance of the resin used for the plastic varistor is low, the resin may deteriorate or deteriorate depending on the heat generated by the lightning surge or the environment exposed to high temperatures, and the varistor characteristics may be lost. The melting point is 200. By using a highly heat-resistant resin, which is either a thermoplastic resin having a temperature of ° C. or higher or a thermosetting resin having a thermal decomposition temperature of 350 ° C. or higher, such a risk can be reduced.
Highly heat-resistant resins, either thermoplastic resins with a melting point of 200 ° C or higher or thermosetting resins with a thermal decomposition temperature of 350 ° C or higher, generally have low affinity for fillers in general, so a high amount of filler may be added. High heat-resistant resins, which are difficult and have a low zinc oxide compounding ratio, tend to have linear resistance characteristics in the current-voltage characteristics and may not exhibit varistor characteristics. However, since the zinc oxide particles of the present invention can be highly blended even with a highly heat-resistant resin, it is possible to produce a plastic varistor having excellent varistor characteristics and not losing the characteristics in a high temperature environment. Is.

The highly heat-resistant resin of either a thermoplastic resin having a melting point of 200 ° C. or higher or a thermosetting resin having a thermal decomposition temperature of 350 ° C. or higher includes polyamide, polyimide, maleimide, thiol-modified maleimide, polyamideimide, or polybenzoimidazole. , Polyethylene terephthalate, polyvinyl alcohol, polyvinylidene chloride, polybutylene terephthalate, polyetherimide, polysulfone, ethylene-tetrafluoroethylene, polychlorotrifluoroethylene, polytetrafluoroethylene, cellulose acetate, polyetherketone, polyetheretherketone, Polyetherketone Etherketoneketone, polyphenyllensulfide, polyethersulfone, liquid crystal polymer, polydialylphthalate, silicone and the like can be mentioned, and one or more of these can be used.

The thermoplastic resin preferably has a melting point of 210 ° C. or higher, and more preferably 220 ° C. or higher. The thermosetting resin preferably has a thermal decomposition temperature of 360 ° C. or higher, and more preferably a thermal decomposition temperature of 370 ° C. or higher.

The high heat resistant resin is also preferably a resin having ^{a volume resistivity of 10 11 Ωcm or more.} The inclusion of such a high insulating resin makes the resin composition more suitable as a material for the varistor. A resin having a volume resistivity of 10 ¹² Ωcm or more is more preferable, and a resin having a volume resistivity of 10 ¹³ Ωcm or more is more preferable.

The resin composition of the present invention may contain other resins in addition to the above-mentioned high heat-resistant resin as long as the performance as a plastic varistor is not impaired. Other resins include polyethylene, polypropylene, polystyrene, high density polyethylene, low density polyethylene, acrylonitrile styrene, acrylonitrile-butadiene-styrene copolymer, polyvinyl chloride, acrylic resin, polyvinylidene fluoride, polyacetal, polycarbonate, phenol resin, etc. Uria resin, melamine resin, polyester resin, unsaturated polyester, polyurethane, epoxy resin, furan resin, nitrocellulose, propionate cellulose, ethyl cellulose, polymethylpentene, polyphenylene ether, ethylene vinyl acetate copolymer, ethylene-ethyl acrylate Examples thereof include copolymers, polyphenylene oxides, thermoplastic elastomers, and the like, and one or more of these can be used.

When the resin composition of the present invention contains a highly heat-resistant resin, the varistor obtained from the composition has a ratio of a voltage having a current value of 1 mA to a voltage having a current value of 9.9 _{mA (V 9.9} mA / V _{1 mA).} ) Will be lower, and the non-linear resistance characteristics will be better.
V _9.9 mA / V _{1 mA} is preferably 2.8 or less. It is more preferably 2.5 or less, still more preferably 2.2 or less.

The resin composition of the present invention may contain other components other than the resin and the zinc oxide particles of the present invention. Examples of other components include mold release agents and pigments used in injection molding and hot press molding. Further, when a thermosetting resin is used in the present invention, it may unintentionally contain a deactivated polymerization initiator or curing accelerator.
The content of other components is preferably 1.0% by mass or less with respect to 100% by mass of the entire resin composition. More preferably, it is 0.5% by mass or less.

The resin composition of the present invention can be produced by kneading the resin and the zinc oxide particles of the present invention. The temperature and time for kneading and the kneading speed may be appropriately adjusted according to the type of resin used, etc., but when the thermosetting resin and the zinc oxide particles of the present invention are kneaded, the kneading temperature is 20. The temperature is preferably ~ 600 ° C, more preferably 100 to 400 ° C. When the thermoplastic resin and the zinc oxide particles of the present invention are kneaded, the kneading temperature is preferably 200 to 600 ° C.
The kneading time is preferably 5 to 60 minutes, more preferably 10 to 40 minutes.
When the kneading speed is controlled by the number of revolutions, it is preferably 5 to 600 rpm, more preferably 10 to 300 rpm.
Examples of the kneading device include a Bambari mixer equipped with a heating mechanism, a Henschel mixer, a ribbon blender, a kneader, 2 to 4 rolls, an extruder and the like.

4. Plastic Varistor The present invention is also a plastic varistor using the resin composition of the present invention. The plastic varistor uses the resin composition of the present invention for hot press molding, injection molding, extrusion molding, roll molding, blow molding, compression molding, sheet molding compound molding, bulk molding compound molding, transfer molding, vacuum molding, pressure molding, and calendar. It can be manufactured by molding by molding, three-dimensional printer molding or the like.
As described above, the resin composition of the present invention contains a high amount of the zinc oxide particles of the present invention in the resin, and the zinc oxide particles of the present invention are less likely to crack or collapse even during press molding. The plastic varistor using the resin composition of the present invention has excellent varistor characteristics, stability, and moldability.

The molding temperature, molding pressure, and molding time when the resin composition of the present invention is molded by hot press molding may be appropriately adjusted according to the type of resin contained in the resin composition, etc., but the molding temperature is 150 to 150. 600 ° C. is preferable, and 200 to 400 ° C. is more preferable.
The molding pressure is preferably 5 to 400 MPa, more preferably 10 to 300 MPa.
The molding time is preferably 0.1 to 30 minutes, more preferably 0.5 to 15 minutes.

Further, when a plastic varistor is manufactured using a thermosetting resin, a step of further molding and curing the molded product obtained by the molding process may be performed after the step of molding the resin composition of the present invention.
The curing temperature in the curing step is preferably 20 to 600 ° C, more preferably 100 to 400 ° C, although it depends on the heat resistance of the resin.
The curing time is preferably 10 to 1200 minutes, more preferably 60 to 600 minutes.

Since the plastic varistor of the present invention has excellent varistor characteristics, stability, and moldability, it can be suitably used as a component of various electronic devices such as a lightning surge protection device.

For the zinc oxide particles, the above-mentioned D50 of the particle size distribution of the zinc oxide particles before the kneading process with the resin is subjected to (a), and the composition of the zinc oxide particles and the predetermined resin is subjected to a predetermined treatment to obtain a resin. When D50 when measuring the particle size distribution of the incineration residue after the components are incinerated and removed is (b), the value obtained from (ab) / a × 100 is preferably less than 10.
More specifically, for example, a resin composition consisting of 10% by weight of a polyamide 66 resin (Leona 1300S manufactured by Asahi Kasei Chemicals Co., Ltd., melting point 260 ° C.) and 90% by weight of the zinc oxide particles is used in a twin-screw extruder (kneading temperature 280 ° C.). , Kneaded with a residence time of 1 minute) and hot press molded with a press machine (molding temperature 280 ° C., press pressure 25 MPa), or a maleimide resin composition (BMI-1100H manufactured by Daiwa Kasei Kogyo Co., Ltd. and Daiwa Kasei Kogyo Co., Ltd.) A mixture of DABPA in a molar ratio of 3: 1. Thermal decomposition temperature of the cured product (440 ° C.) 2 rolls of a resin composition consisting of 12.5% by weight and 87.5% by weight of the zinc oxide particles (kneading temperature). After kneading at 140 ° C. for 25 minutes and molding the kneaded product with a press machine (without heating, 25 MPa), 180 ° C. for 2 hours, followed by 200 ° C. for 2 hours, then 230 ° C. for 2 hours, and then 250. One of the products that had been heat-cured at ° C for 2 hours was heated at 500 to 900 ° C in the air to remove the resin component by incineration, and the particle size distribution of the incinerated residue was measured as D50 (b). ), The value obtained from (ab) / a × 100 is less than 10.

Examples are given below to illustrate the invention in detail, but the invention is not limited to these examples alone. Unless otherwise specified, "%" means "% by mass (% by weight)" and "parts" means "parts by mass (parts by weight)".

Various measurements were performed as follows.
<Electron microscope photography>
The appearance of the zinc oxide particles was observed with a scanning electron microscope JSM-6510A (manufactured by JEOL Ltd.). The cross section of the zinc oxide particles was observed by cutting the kneaded product or the molded product with a cross section polisher (manufactured by JEOL Ltd.) and using a scanning electron microscope JSM-7000F (manufactured by JEOL Ltd.).
<Specific surface area measurement>
The specific surface area (%) was measured by a fully automatic BET specific surface area measuring device Macsorb Model HM-1200 (manufactured by Muntech).
<ZnO purity measurement>
The ZnO purity (%) was measured by a fluorescent X-ray analyzer ZSX PrimusII (manufactured by Rigaku Denki Kogyo Co., Ltd.).
<Measurement of particle size distribution and processing change rate>
In the present specification, D50, D10, and D95 are values of volume conversion diameter measured by a laser diffraction / scattering type particle size distribution measuring device Microtrack MT-3300 EXII (manufactured by Nikkiso Co., Ltd.). The measurement was carried out using water as a solvent at the time of measurement.
The processing change rate was measured as follows.
A resin composition composed of 10% by weight of polyamide 66 resin (Leona 1300S manufactured by Asahi Kasei Chemicals Co., Ltd., melting point 260 ° C.) and 90% by weight of zinc oxide particles was kneaded with a twin-screw extruder (kneading temperature 280 ° C., residence time 1 minute). A disc-shaped molded body obtained by hot press molding with a press machine (molding temperature 280 ° C., press pressure 25 MPa), or a maleimide resin composition (BMI-1100H manufactured by Daiwa Kasei Kogyo Co., Ltd. and DABPA manufactured by Daiwa Kasei Kogyo Co., Ltd.). A mixture of 3: 1 molar ratio, a resin composition consisting of 12.5% by weight of the cured product (thermosetting temperature of 440 ° C.) and 87.5% by weight of zinc oxide particles is rolled into two rolls (kneading temperature of 140 ° C.). After kneading at 180 ° C. for 25 minutes and molding the kneaded product with a press machine (without heating, 25 MPa), 180 ° C. for 2 hours, then 200 ° C. for 2 hours, then 230 ° C. for 2 hours, and then 250 ° C. for 2 hours. One of the disc-shaped molded bodies that had been heat-cured for hours was used for the test.
For the processing change rate (%), D50 of the particle size distribution of the zinc oxide particles before the kneading process with the resin is set as (a), and the resin component of the molded product obtained by the molding process is incinerated and removed at 500 to 900 ° C. Assuming that D50 of the particle size distribution of the zinc oxide particles thus obtained is (b), it is obtained by (ab) / a × 100. Here, if the incinerator removal temperature is less than 500 ° C., the resin may remain, and if it exceeds 900 ° C., the particles may be fused and the rate of change may not be accurately measured.
In the calculation of the processing change rate (%), when measuring the particle size distribution when the zinc oxide particles hardly change before and after processing, the value of b becomes slightly larger than a due to the influence of measurement variation, and the processing change rate. The value of may be negative. If the value at this time is 0 to −0.1, the processing change rate is set to 0 because it is within the range of measurement variation. If it is less than -0.1, remeasure.
<Measurement of current-voltage non-linear resistance characteristics>
It was measured with a withstand voltage tester TOS5051A (manufactured by Kikusui Electronics Co., Ltd.). Of the examples and comparative examples, those showing the current-voltage non-linear resistance characteristic are shown in Table 1 as ◯, and those showing the non-linear resistance characteristic and having a simple linear resistance characteristic are shown in Table 1.

Example 1
10 kg of fine zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., specific surface area: 10 m ² / g) is repulped into pure water to prepare a slurry having a zinc oxide concentration of 200 g / L, and then 61.5 g of acetic acid (to zinc oxide). (0.615% by weight), and 250 g (2.5% by weight based on zinc oxide) of Poise 532A (polycarboxylic acid ammonium salt manufactured by Kao Co., Ltd.) were added. Subsequently, the granulated product obtained by spray drying was fired at 1250 ° C. for 3 hours to obtain zinc oxide particles A having a particle size of 25.79 μm and a ZnO purity of 99.5%. The scanning electron micrograph of the obtained zinc oxide particles A is shown in FIG. 1, and the particle size distribution is shown in FIG.
The obtained zinc oxide particles A are mixed with a maleimide resin composition (BMI-1100H manufactured by Daiwa Kasei Kogyo Co., Ltd. and DABPA manufactured by Daiwa Kasei Kogyo Co., Ltd. in a molar ratio of 3: 1, and the thermal decomposition temperature of the cured product is 440 ° C.). Weighed so that the filler filling ratio was 87.5% by volume (60% by volume), and kneaded with two rolls (desktop roll mill manufactured by Toyo Seiki Seisakusho Co., Ltd.) at a kneading temperature of 140 ° C., a kneading speed of 20 rpm, and a kneading time of 25 minutes. , The kneaded product was molded by a press machine (no heating, molding pressure 25 MPa, molding time 10 minutes). Then, the disc-shaped molded product 1 (by heat curing treatment at 180 ° C. for 2 hours, subsequently at 200 ° C. for 2 hours, then at 230 ° C. for 2 hours, and then at 250 ° C. for 2 hours in a box dryer). 20 mm in diameter × 2.5 mm in thickness) was obtained. FIG. 3 shows a photograph of a cross section of the disc-shaped molded body 1 cut by a cross section polisher. When 100 particles were arbitrarily extracted from the cross-sectional photograph of the disk-shaped molded body 1 and ^{the number of pores of 0.1 μm or more per 100 μm 2} was confirmed, all of them were less than 8, and the average was 3. There were two.
The current-voltage non-linear resistance characteristic of this disk-shaped molded body 1 was measured by the following method. The graph of the obtained current-voltage non-linear resistance characteristic is shown in FIG. In addition, V _9.9 mA / V _{1 mA} was 1.18. The change width (%) with respect to the average value _{of V 1 mA} per 10 molded products was -13.8 to 13.3.
Further, the resin component was removed by firing the disc-shaped molded product 1 at 800 ° C. for 3 hours, and the residue was observed with a scanning electron microscope. As a result, there was no change in the morphology of the zinc oxide particles A immediately after production, and the processing change rate Was 0.50%. From this, it was confirmed that the obtained zinc oxide particles were robust and strong zinc oxide particles. The scanning electron micrograph of the extracted residue is shown in FIG. 5, and the particle size distribution is shown in FIG. Table 1 shows the preparation conditions of the zinc oxide particles and the resin composition containing the zinc oxide particles, the molding conditions of the molded body, the measurement results of various physical properties, and the like.

Example 2
The zinc oxide particles A obtained in Example 1 were weighed in a polyamide 66 resin (Leona 1300S manufactured by Asahi Kasei Chemicals Co., Ltd., melting point 260 ° C.) so as to have a filler filling rate of 90.0% by weight (65% by volume), and biaxial. Knead with an extruder (Laboplast Mill manufactured by Toyo Seiki Seisakusho) at a kneading temperature of 280 ° C, a kneading speed of 40 rpm, and a kneading time of 1 minute. Molding was performed to obtain a disc-shaped molded body 2 (diameter 22 mm × thickness 2.5 mm). When 100 particles were arbitrarily extracted from a photograph of a cross section of the disc-shaped molded product 2 cut with a cross-section polisher and ^{the number of pores of 0.1 μm or more per 100 μm area 2} was confirmed, all of them were less than 8. Yes, the average was 3.4.
The current-voltage non-linear resistance characteristics of the obtained disc-shaped molded body 2 were measured. The graph of the obtained current-voltage non-linear resistance characteristic is shown in FIG. Further, V _9.9 mA / V _{1 mA} was 1.75. The change width (%) with respect to the average value _{of V 1 mA} per 10 molded products was -16.7 to 16.7.
Further, the resin component was removed by firing the disc-shaped molded body 2 at 800 ° C. for 3 hours, and the residue was observed with a scanning electron microscope. As a result, there was no change in the morphology of the zinc oxide particles A immediately after production, and the processing change rate Was 1.20%. From this, it was confirmed that the zinc oxide particles maintained a robust and strong state even in the disc-shaped molded body 2. The scanning electron micrograph of the extracted residue is shown in FIG. 8, and the particle size distribution is shown in FIG. Table 1 shows the preparation conditions of the zinc oxide particles and the resin composition containing the zinc oxide particles, the molding conditions of the molded body, the measurement results of various physical properties, and the like.

Example 3
The zinc oxide particles A obtained in Example 1 were airflow-classified using an airflow classifier Turboclassifier TC-25M (manufactured by Nisshin Engineering Co., Ltd.) to achieve a particle size of 27.09 μm and a ZnO purity of 99.5%. Zinc oxide particles B were obtained. The scanning electron micrograph of the obtained zinc oxide particles B is shown in FIG. 10, and the particle size distribution is shown in FIG. Using the obtained zinc oxide particles B, molding was performed in the same manner as in Example 1 except that the molding conditions shown in Table 1 were met, to obtain a disk-shaped molded body 3 having the sizes shown in Table 1. A photograph of a cross section of the obtained disc-shaped molded body 3 cut by a cross section polisher is shown in FIG. 12, and a graph of the results of measuring the current-voltage non-linear resistance characteristics is shown in FIG. Further, FIG. 14 shows a scanning electron micrograph of the residue after the disk-shaped molded body 3 was fired by the same method as in Example 1 to remove the resin component, and FIG. 15 shows the particle size distribution.
Table 1 shows the measurement results and the like performed on the obtained disc-shaped molded body 3 by the same method as in Example 1.

Example 4
10 kg of fine zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., specific surface area: 10 m ² / g) is repulped into pure water to prepare a slurry having a zinc oxide concentration of 600 g / L, and then 61.5 g of acetic acid (to zinc oxide). (0.615% by weight), and 350 g (3.5% by weight based on zinc oxide) of Poise 532A (polycarboxylate ammonium salt manufactured by Kao Co., Ltd.) was added. Subsequently, the granulated product by spray drying was fired at 1250 ° C. for 3 hours to obtain zinc oxide particles C having a particle size of 41.09 μm and a ZnO purity of 99.7%. The scanning electron micrograph of the obtained zinc oxide particles C is shown in FIG. 16, and the particle size distribution is shown in FIG. Further, the obtained zinc oxide particles C were molded by the same method as in Example 2 except that the molding conditions shown in Table 1 were met, to obtain a disk-shaped molded body 4 having the sizes shown in Table 1. A photograph of a cross section of the obtained disc-shaped molded body 4 cut by a cross section polisher is shown in FIG. 18, and a graph of the results of measuring the current-voltage non-linear resistance characteristics is shown in FIG. Further, FIG. 20 shows a scanning electron micrograph of the residue after the disk-shaped molded body 4 was fired by the same method as in Example 1 to remove the resin component, and FIG. 21 shows the particle size distribution.
Table 1 shows the measurement results and the like performed on the obtained disc-shaped molded body 4 by the same method as in Example 2.

Example 5
10 kg of fine zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., specific surface area: 10 m ² / g) is repulped into pure water to prepare a slurry having a zinc oxide concentration of 200 g / L, and then 61.5 g of acetic acid (to zinc oxide). (0.615% by weight), and 250 g (2.5% by weight based on zinc oxide) of Poise 532A (polycarboxylic acid ammonium salt manufactured by Kao Co., Ltd.) were added. Subsequently, granulation was performed by spray drying and calcined at 1220 ° C. for 3 hours to obtain zinc oxide particles D having a particle diameter of 23.62 μm and a ZnO purity of 99.5%.
Using the obtained zinc oxide particles D, molding was performed in the same manner as in Example 2 except that the molding conditions shown in Table 1 were met, to obtain a disk-shaped molded body 5 having the sizes shown in Table 1.
Table 1 shows the measurement results and the like performed on the obtained disc-shaped molded body 5 by the same method as in Example 2.

Comparative Example 1
10 kg of fine zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., specific surface area: 10 m ² / g) is repulped into pure water to prepare a slurry having a zinc oxide concentration of 200 g / L, and then 61.5 g of acetic acid (to zinc oxide). 0.615% by weight), 250 g of Poise 532A (polycarboxylate ammonium salt manufactured by Kao Co., Ltd.) (2.5% by weight with respect to zinc oxide) was added, and the granulated product was spray-dried at 1100 ° C. for 3 hours. By firing, zinc oxide particles E having a particle diameter of 22.24 μm and a ZnO purity of 99.4% were obtained. The scanning electron micrograph of the obtained zinc oxide particles E is shown in FIG. 22, and the particle size distribution is shown in FIG. 23.
The obtained zinc oxide particles E are weighed in a polyamide 66 resin having a heat resistance of 280 ° C. (Leona 1300S manufactured by Asahi Kasei Chemicals Co., Ltd., melting point 260 ° C.) so that the filler filling rate is 90.0% by weight (65% by volume). , Kneading with a twin-screw extruder (lab plast mill manufactured by Toyo Seiki Seisakusho) at a kneading temperature of 280 ° C, a kneading speed of 40 rpm, and a kneading time of 1 minute. A comparative disc-shaped molded product 1 (diameter 22 mm × thickness 2.5 mm) was obtained by hot press molding. FIG. 24 shows a photograph of a cross section of the comparative disc-shaped molded product 1 cut by a cross section polisher. When 100 particles were arbitrarily extracted from the cross-sectional photograph of the comparative disc-shaped molded body 1 and ^{the number of pores of 0.1 μm or more per 100 μm area 2} was confirmed, the number was 20, and the average number of pores was 20. , 24.3 pieces.
When the current-voltage non-linear resistance characteristic of this comparative disk-shaped molded body 1 was measured by the same method as above, the current-voltage characteristic showed a simple linear resistance characteristic, and the varistor characteristic could not be obtained. A graph showing the current-voltage linear resistance characteristics is shown in FIG. V _9.9mA / V _1mA was 3.40. The change width (%) with respect to the average value _{of V 1 mA} per 10 molded products was -63.0 to 66.7.
Further, when the resin component was removed by firing the comparative disk-shaped molded body 1 at 800 ° C. for 3 hours and the residue was observed with a scanning electron microscope, the zinc oxide particles were cracked as compared with the zinc oxide particles E immediately after production. The state of collapse was observed, and the processing change rate was 17.22%. From this, it was confirmed that the zinc oxide particles E were not robust and strong zinc oxide particles. The scanning electron micrograph of the extracted residue is shown in FIG. 26, and the particle size distribution is shown in FIG. 27. Table 1 shows the preparation conditions of the zinc oxide particles and the resin composition containing the zinc oxide particles, the molding conditions of the molded body, the measurement results of various physical properties, and the like.

Comparative Example 2
10 kg of fine zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., specific surface area: 10 m ² / g) is repulped into pure water to prepare a slurry having a zinc oxide concentration of 200 g / L, and then 61.5 g of acetic acid (to zinc oxide). 0.615% by weight), Poise 532A (manufactured by Kao, polycarboxylic acid ammonium salt) 250 g (2.5% by weight with respect to zinc oxide) was added to make a spray-dried granulated product, and the granulated product was spray-dried and dried at 1200 ° C. for 3 hours. By firing, zinc oxide particles F having a particle diameter of 22.26 μm and a ZnO purity of 99.4% were obtained. The scanning electron micrograph of the obtained zinc oxide particles F is shown in FIG. 28, and the particle size distribution is shown in FIG. 29.
FIG. 30 shows a photograph of a cross section of the kneaded product obtained by kneading with the polyamide 66 resin in the same manner as in Comparative Example 1 except that the obtained zinc oxide particles F were used, and cut by a cross section polisher. Further, molding was performed in the same manner as in Comparative Example 1 except that the molding conditions shown in Table 1 were met, to obtain a comparative disk-shaped molded product 2 having the sizes shown in Table 1. When the current-voltage non-linear resistance characteristic of the obtained comparative disk-shaped molded body 2 was measured, the current-voltage characteristic showed a simple linear resistance characteristic, and the varistor characteristic could not be obtained. A graph of the results showing the current-voltage linear resistance characteristics is shown in FIG. Further, FIG. 32 shows a scanning electron micrograph of the residue after the comparative disk-shaped molded body 2 was fired to remove the resin component by the same method as in Comparative Example 1, and FIG. 33 shows the particle size distribution.
Table 1 shows the measurement results and the like performed on the obtained comparative disc-shaped molded product 2 molded product by the same method as in Comparative Example 1.

From the cross-sectional photographs of 100 zinc oxide particles arbitrarily extracted from Examples 1 to 5 taken with a scanning electron microscope, when the pores of 0.1 μm or more in the cross section are counted, the fine ^{particles per 100 μm 2 cross-sectional area are counted.} It was confirmed that the zinc oxide particles having an average number of pores of less than 8 are robust and strong zinc oxide particles in which the particles are not destroyed in the kneading process with the resin and the molding process, and the processing change rate is low. Further, such zinc oxide particles are filled and kneaded at 70% by weight or more with a highly heat-resistant resin of either a thermoplastic resin having a melting point of 200 ° C. or higher or a thermosetting resin having a pyrolysis temperature of 350 ° C. or higher. It was confirmed that a molded product exhibiting good varistor characteristics can be obtained by molding. Further, from Example 3, the smaller the D95 / D10 in the particle size distribution of such robust and strong zinc oxide particles, that is, the more uniform the particle size, the more _{stable the value of V1 mA} , that is, the molded product. It was confirmed that the change width (%) with respect to the average value of V _{1 mA per 10 particles can be reduced.} In addition, it was easy to form a clean disc-shaped molded product during press molding or hot press molding. Since it is considered that the resin fluidity at the time of molding is high, it is presumed that the zinc oxide particles are finely dispersed in the resin.
On the other hand, from Comparative Examples 1 and 2, the zinc oxide particles having an average number of pores of 8 or more do not become robust and strong zinc oxide particles, and the zinc oxide particles are kneaded with the resin and molded. Was destroyed, and the processing change rate was high. It was confirmed that the molded product obtained by kneading and molding the zinc oxide particles and the resin did not have good varistor characteristics. Further, in the resin composition containing zinc oxide particles of Comparative Example 1, it was difficult to form a clean disc-shaped molded product during hot press molding, and some sink marks (chips) and voids were often generated. It was speculated that the dispersibility of the zinc oxide particles in the polyamide resin was poor and the fluidity of the resin was low.

Claims (4)

An aggregate composed of zinc oxide particles, a resin composition obtained by kneading zinc oxide particles and a resin, or a molded body obtained by molding the resin composition is cut by a cross-section polisher to scan a cross section. When the pores of 0.1 μm or more in the cross section of 100 zinc oxide particles arbitrarily extracted by taking a picture with a microscope are counted, the average number of pores per ^{100 μm 2 cross section is less than 8.} Characterized zinc oxide particles. A resin composition comprising the zinc oxide particles according to claim 1 in a proportion of 70% by weight or more with respect to the resin. The resin composition according to claim 2, wherein the resin is a highly heat-resistant resin, which is either a thermoplastic resin having a melting point of 200 ° C. or higher or a thermosetting resin having a pyrolysis temperature of 350 ° C. or higher. thing. A plastic varistor using the resin composition according to claim 2 or 3.

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