JPS6255496B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing electrically conductive plastic molded articles. In order to impart electrical conductivity to plastics or elastomers, it is known to mix and compose powders of electrically conductive materials such as carbon and metals. When forming the composite, the conductive material is uniformly dispersed using a rolling roll, a Brabender, or the like. When producing plastic conductive materials using such conventional methods, it is necessary to mix a large amount of conductive material to increase the conductivity, and the mechanical strength may decrease due to the mixing of a large amount of conductive material. I had a big problem. The present invention provides a method of manufacturing a plastic conductive material that is free from such problems. That is,
The method for producing a plastic conductive material of the present invention includes mechanically mixing thermoplastic plastic powder and conductive powder having an average particle size of 1/10 or less of the average particle size of the plastic powder. A step of coating the powder surface with the conductive powder, a step of press-molding the plastic powder coated with the conductive powder to form a molded object, and a step of molding the obtained molded object into the molded object. This process is characterized by a step of performing heat treatment while maintaining the shape at a temperature as high as possible. The thermoplastic plastic powder herein refers to powdered materials such as polyethylene, polystyrene, and nylon. The original form obtained by polymerization of many thermoplastic plastics is powder, and in the present invention, powder in such original form can be used. Regarding the particle size of the powder, the finer it is, the more preferable it is in order to improve the mechanical strength of the conductive material to be manufactured. However, at the same time, it is necessary to increase the amount of conductive powder mixed. In order to satisfy the conflicting goals of not requiring too much conductive powder and having high mechanical strength, it is preferable to use plastic powder with a wide particle size distribution.
For example, polyethylene powder in its original manufacturing form has a particle size of
30 to 40% of fine powder is less than 250 mμ, 60 to 70% is coarse powder of 250 mμ or more, and the average particle size is 400 to 500 mμ, but it is preferable to use powder with a wide particle size range. Examples of conductive powder include carbon powder, nickel powder,
Metal powder such as copper powder can be used. The finer the particle size of the conductive powder, the better, but the conductive properties of the resulting plastic conductive material are determined primarily by the relationship with the particle size of the plastic powder. That is, the average particle size of the conductive powder is preferably 1/10 or less of the average particle size of the plastic powder. When the average particle size of the conductive powder exceeds 1/10 of the average particle size of the plastic powder, the contribution of increasing the amount of the conductive powder to the improvement of the conductive properties becomes low. The mixing ratio of plastic powder and conductive powder can be arbitrarily selected depending on the desired conductive properties. For the target conductive properties,
The minimum blending ratio of the conductive powder is the blending ratio necessary to form a minimum double layer on the surface of the plastic powder. The following theoretical formula holds true between the blending ratio of the conductive powder required to form this double layer and the ratio of the average particle diameters of the plastic powder and the conductive powder. β=100 [1+(φ/4)α] ^-1 Here, β is the blending ratio of conductive powder (%) α is the average particle size ratio of both powders φ is a factor related to the shape of the conductive powder 1.11～
It is 1.37. From this theoretical formula, when α is 1/10, β is 22 to 26
%, when α is 1/50, β is 5.5 to 6.5%, α is 1/10
When β is 0, β is 2.9 to 3.5%, and when α is 1/1000, β is 0.29 to 0.36%. However, the experimental values vary greatly from the theoretical values, and the blending ratio of plastic powder and conductive powder must be arbitrarily selected based on the desired conductive properties as well as the conductive properties of the conductive powder. In addition to the plastic powder and the conductive powder, reinforcing agents such as glass fibers can be added depending on the purpose of use. Note that various additives for plastics, such as antioxidants and fillers such as calcium carbonate, can be used by uniformly dispersing them in individual plastic powders. As a means for mechanically mixing the plastic powder and the conductive powder, a grinding mixer such as a mixer, a ball mill, a crusher, or other powder mixer that does not involve grinding can be used. When using a grinding mixer such as a crusher, the conductive powder may stick to the surface of the plastic powder during mixing, resulting in a stable coating layer of conductive powder on the surface of the plastic powder. can get. The process of pressure-molding the mixed powder of raw materials is a process of obtaining a molded body in which the mixed powder of a certain shape is pressed and compacted in a mold or between forming rollers. When pressurizing in a mold, a molded product corresponding to the shape of the mold can be obtained, and when using a molding roller, a plate-shaped or rod-shaped molded product can be obtained. No special heating is required during pressure molding. It can usually be carried out at room temperature. It is also possible to perform pressure molding under heating for the purpose of increasing the bonding strength of the compacted body. However, fluidization of the raw material mixed powder due to heating must be avoided. If the raw material mixed powder is fluidized and undergoes large plastic deformation, large deformation occurs in the next heat treatment step, making it impossible to obtain a material in the desired shape. Moreover, the conductive properties are also deteriorated. The higher the pressure during molding, the better it is for improving the mechanical strength of the molded product, but it is sufficient if the pressure is sufficient to prevent the shape from deforming during transfer to the heat treatment step. It is usually molded at around 2 tons/ ^cm2 . The heat treatment process is a process in which the compacted molded body is held at a high temperature to maintain the molded shape and join the plastic powders together. This process bonds the plastic powders that are in contact with each other, creating a strong bond. Further, the conductive powder covering the surface of the plastic powder is forced into the gap formed by the plastic powder, and the conductive powder itself is bonded to each other to form a network structure. Therefore, by applying heat treatment, both the mechanical strength and conductive properties of the molded body are improved. Among the heat treatment conditions, first, the temperature should be set around the melting point or glass transition point of the plastic powder. However, in cases where the plastic has very high molecular weight and poor fluidity, it is also possible to treat it at a temperature 30° C. or more higher than the melting point or glass transition point. The longer the heat treatment time, the better. Generally, heat treatment time is closely related to heat treatment temperature, and if the heat treatment temperature increases by 10°C, the same heat treatment effect can be expected with approximately 1/2 the heat treatment time. Note that in order to prevent deterioration during heat treatment, it is also preferable to perform heat treatment under an inert gas. It is also possible to shorten the heat treatment time by using induction heating using high frequency irradiation. The plastic conductive material is completed through the above heat treatment process. After this heat treatment step, applying a high voltage to the resulting plastic conductive material will result in
Conductive properties are further improved. The voltage and time to be applied are several hundred volts/cm or more, preferably 2000 V/cm or more. The current application time may be 1 to 2 seconds. Although the voltage is much lower than the dielectric breakdown, the possibility of carbonization of plastic due to local discharge is
Since no change in mechanical strength was observed even after repeated application of high voltage, it is presumed that this was simply a rearrangement of the conductive powder. Hereinafter, the present invention will be explained in further detail by showing test results. Test Example 1 High-density polyethylene powder (hereinafter referred to as HDPE powder) in its original form was used as the thermoplastic plastic powder. The particle size distribution of this HDPE powder is the particle size
30% for 500mμ or more, 35% for 500-250mμ, 250
~150mμ 17%, 150mμ or less 18%. Acetylene black (hereinafter referred to as AB powder) with a particle size of about 500 Å was used as the conductive powder. Raw materials with 14 different compositions were made by blending this HDPE powder and AB powder at a volume percentage of AB powder of 0.25% to 15%, and each raw material was crushed for 1 hour using a crusher with a capacity of 1 to mix the raw materials. A powder was prepared. Next, the raw mixed powder is put into a cylindrical mold and compacted under a pressure of 5t/ ^cm2 at room temperature.
A plate-shaped molded product having a size of 100 mm and a thickness of 5 mm was obtained. Next, regarding plate-shaped compacts with a blending ratio of AB powder of 3% or less,
1 hour at 134℃, 140 for 4% plate-shaped compacts
℃ for 1 hour, for plate-shaped compacts of 6% or more.
Heat treatment was performed at 160°C for 1 hour. In this way, 14 types of conductive materials were obtained. Next, the specific volume resistivity (Pv) of each conductive material was determined. The results are shown by the solid line (white circles) in Figure 1 in relation to the amount of AB powder blended.
For reference, a raw material was prepared by mixing HDPE powder and AB powder in an extruder, and the relationship between the specific volume resistivity (Pv) of the injection-molded material and the amount of AB blended is shown by the broken line in Figure 1. Indicated by From FIG. 1, it can be seen that the conductive material obtained by the manufacturing method of the present invention has a small volume resistivity (Ωcm) and excellent conductive properties even if the blending ratio of conductive powder is low. The tensile strength of the conductive material obtained by the method of the present invention was 205 to 215 Kg/cm ² , which was almost the same as the tensile strength of the material obtained by melt-pressing. Test Example 2 Using the same HDPE powder as used in Test Example 1 and nickel metal powder (hereinafter referred to as Ni powder) with a particle size of 5 to 6 mμ as a conductive powder, various blending ratios were prepared. A mixed powder was prepared, and each powder was crushed in a crusher for 1 hour to prepare a raw material mixed powder. Next, pressure molding was performed in the same manner as in Test Example 1 to obtain a compacted molded body. For reference, the volume resistivity (Ωcm) of some compacted bodies was measured. Next, conductive materials were manufactured by heat treatment at 134° C. for 60 minutes, and the specific volume resistivity of each conductive material was measured. Next, for some of the conductive materials, a high voltage of 2000 V/cm was applied for 1 minute, and the specific volume resistivity was measured. The relationship between these specific volume resistivities and the Ni powder content is shown in FIG. 2. In Figure 2, the white diamond mark indicates the volume resistivity of the compacted compact, the open circle mark indicates the volume resistivity of the conductive material after heat treatment, and the black circle mark indicates the volume resistivity of the conductive material subjected to high voltage application treatment after heat treatment. Indicates the specific volume resistivity of the material. From FIG. 2, it can be seen that the conductive properties of the material are significantly improved by heat treatment, or by heat treatment and high voltage application treatment. For example, the amount of Ni powder required to obtain a material with Pv10 ² - 10 ³ Ω-cm is 20% in a pressure molded product, but it can be increased to 12% by heat treatment to 15°C and further by applying a high voltage. can be reduced to moreover,
Quality variations that tend to occur when handling powder can be homogenized. Incidentally, according to the melt-pressing method with uniform dispersion, the amount of Ni powder required to obtain the same level of conductivity is 25
~30%, so according to the present invention, it is possible to reduce the amount of Ni powder blended to 1/2 or less. Next, the relationship between the starting raw material HDPE powder, particle size, and tensile strength of the molded body is shown in the table. However, the amount of Ni powder mixed is 20% by volume.

[Table] The raw materials whose particle sizes are specified in the table are the starting materials that have been sieved in advance. In addition, the values obtained using the melt pressure molding method are also listed. As is clear from the table, the tensile strength is maximum when using the starting material with a particle size of 150 mμ or less in the method of the present invention, but even when using the unsieved powder in its original form, it is A conductive material having tensile strength comparable to that obtained by pressure molding can be obtained.

[Brief explanation of the drawing]

FIG. 1 shows the test results of the amount of acetylene carbon black blended according to the present invention and the volume resistivity Ω-cm of a molded article, using high-density polyethylene powder in its original form as a starting material. FIG. 2 shows experimental results showing the effect of increasing the volume resistivity of high-density polyethylene powder and nickel powder according to the present invention by subjecting compression molded bodies to heat treatment and high voltage application treatment. However, the particle size of high-density polyethylene is 250 to 500 mμ.

Claims (1)

[Claims] 1. Thermoplastic plastic powder and conductive powder having an average particle size of 1/10 or less of the average particle size of the plastic powder are mechanically mixed, and the surface of the plastic powder is coated on the surface of the plastic powder. a step of coating the conductive powder; a step of press-molding the plastic powder coated with the conductive powder to form a molded body; 1. A method for producing a plastic conductive material, comprising the steps of heat treatment while holding the material at a temperature as high as possible. 2. The method for producing a plastic conductive material according to claim 1, wherein a high voltage is applied to the molded body after the heat treatment step.