JPH0429162B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a novel electromagnetic shielding material. Recently, with the spread of various electronic devices, electromagnetic noise caused by these devices is becoming a serious social problem. For this reason, there has been an increasing need for electromagnetic shielding materials to prevent disturbances caused by such noise. [Prior Art] As electromagnetic shielding materials, materials sprayed with metal or coated with conductive paint have been known so far, but these are expensive and have poor durability. Therefore, it cannot necessarily be said that it is practically satisfactory. To overcome these problems, plastics have been proposed in which conductive fillers such as metal fibers and metal powders are blended into plastics. This results in a decrease in mechanical strength and limits the use of the material. [Problems to be Solved by the Invention] The present inventors have overcome the drawbacks of such conventional electromagnetic shielding materials, and have developed a material that exhibits excellent electromagnetic shielding effects, high strength, and good formability. In order to develop a material that can be produced at a low cost, we have conducted extensive research and developed a material that uses natural mica as a material, and after pretreating the surface of this material with a hydrochloric acid solution of palladium chloride, we perform a chemical plating process. Chemically plated with a metal coating, which has a weight ratio of 40% or more on the surface When plastic is incorporated with a volume fraction of mica at a volume fraction of 14 to 28%, a material with excellent electromagnetic shielding properties can be obtained at a low cost, and it is also strong. The present inventors have discovered the unexpected fact that the amount of water increases significantly, and based on this finding, the present invention has been completed. [Means for Solving the Problems] According to the present invention, mica having a metal coating firmly adhered by chemical plating on the surface with a chemical plating weight ratio of 40% or more is 14 to 28% by volume. Mica with a metal coating is made of a plastic compounded with An electromagnetic shielding material is provided. Unlike metal fibers or metal powder, the mica with a metal coating used as a filler in the present invention is a stable inorganic substance inside and only has metal on the surface, so it is not only chemically stable but also When blended into plastics, it has the advantage that it can provide high conductivity with a small metal content without impairing moldability, and can also provide a high reinforcing effect. Mica with a metal coating can be obtained by plating mica using a chemical plating method, but with the conventional chemical plating method,
Prior to chemical plating treatment, it is necessary to perform a base treatment. In the past, this surface treatment consisted of a complicated process, for example, immersion treatment using an aqueous solution containing stannous chloride and hydrochloric acid, followed by treatment with an aqueous solution containing palladium chloride and hydrochloric acid. methods etc. are being carried out. However, since this surface treatment has a large impact on product cost, it is clear that it is desirable to simplify it as much as possible. The present inventors used natural mica as a material,
When chemical plating is applied to this, it has been found that a sufficient activation effect can be obtained by simply treating the surface with a hydrochloric acid aqueous solution containing palladium chloride. The material used in the present invention is natural mica, and synthetic mica is unsuitable. Examples of natural mica include conventionally known mica, such as muscovite mica and phlogopite mica. It is associated with the use of natural mica that the mica surface treatment is accomplished in such a simplified process. That is, unlike synthetic mica, natural mica contains a small amount of metal impurities such as iron, so when natural mica is brought into contact with an aqueous hydrochloric acid solution containing palladium chloride, due to the action of the metal impurities such as iron, Palladium chloride is reduced and deposited on the mica surface as metallic palladium, forming a suitable base for chemical plating. To produce mica having a metal coating used in the present invention, finely pulverized natural mica is
Palladium chloride 0.1-1g/and hydrochloric acid 1-30ml/
After performing a base treatment using an aqueous solution containing the following, a chemical plating treatment may be performed by a conventional method. In this case, the metal coating formed on the mica surface by chemical plating includes conventionally known metals, such as Ag,
Examples include Au, Cu, Pd, Pt, Rh, Ru, Fe, etc. In addition, the metal film formed on the surface of mica can be made of not only individual metals but also alloys such as Ni-Co and Ni.
-W, Ni-Fe, Co-W, Co-Fe, Ni-Cu, etc., but when forming an alloy film, add multiple metal salts to the plating solution as desired. Just add it. After the above-mentioned surface treatment is completed, the mica powder is subjected to chemical plating treatment. In this case, chemical plating treatment can be performed according to a conventionally known method, and generally metal salts, reduction A plating solution containing agents, complexing agents, buffers, stabilizers, etc. is used. in this case,
As reducing agents, sodium hypophosphite, sodium borohydride, aminoborane, formalin, etc. are used, and as complexing agents and buffers, formic acid, acetic acid, succinic acid, citric acid, tartaric acid, malic acid, glycine, Ethylenediamine, EDTA, triethanolamine, etc. are used. Typical compositions of chemical plating solutions include, for example, metal salt 10-200g/hypophosphite 0.3-50g/
, 5 to 300 g of a PH buffer, and preferably 5 to 200 g of glycine as a reinforcing additive can be added to such a plating solution. Also,
Other plating liquids include metal salts 10-200g/carboxylate 10-100g/alkali hydroxide 10-200g/
Copper and silver are representative metals that can be plated. The chemical plating treatment is usually preferably carried out at a temperature of 20 to 95° C. while stirring, for example, air bubbling, so that a uniform film is formed on the powder surface. In this way, plating is continued until a chemical plating weight ratio of 40% by weight or more is finally obtained. In the present invention, the mica powder having the metal film on the surface obtained as described above is added to plastic as a filler at a volume fraction of 14 to 28% to obtain the desired electromagnetic shielding material. . The plastic used in the present invention may be either a thermoplastic plastic or a thermosetting plastic, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, polycarbonate, polyacetal resin, polyurethane resin. , nylon 6, ethylene-vinyl acetate copolymer, ethylene-
Acrylic acid copolymer, ABS resin, epoxy resin,
Unsaturated polyester resin, phenol resin, etc. are used. The electromagnetic shielding material of the present invention can be applied in a conventionally used shape, and the manufacturing method of the product is appropriately carried out depending on the type of resin to be applied, the purpose, the shape, etc. In order to form a molded product into a cylinder shape, pellet shape, etc., a mixture of plastic and filler is processed by vacuum forming, extrusion molding, injection molding,
Methods such as calendar molding and compression molding are used,
In addition, in order to obtain a coating composition for film formation, a filler may be added to and mixed with a plastic emulsion or a plastic solvent solution.Furthermore, when used as a rubber composition, a conventional rubber composition may be added to the plastic emulsion or a plastic solvent solution. The above-mentioned filler may be added to the material. The electromagnetic shielding material of the present invention has excellent electromagnetic shielding properties and high strength, and is used for various electronic devices,
It is particularly useful as equipment for communication equipment, medical equipment, measuring equipment, etc. The characteristics of the electromagnetic shielding material using the test piece as an example will be explained in more detail below. The chemical plating weight percentage of mica having a metal coating in the following is defined as follows. Chemical plating weight ratio (%) = weight of formed and deposited metal/mica weight + weight of formed and deposited metal x 100 [Example] Test piece No. 1 shown in Table 1 which is an example of the present invention.
2, 3, 4 and test pieces No. 1 and No. 5 for comparison
The manufacturing method of ~No. 8 and the characteristic tests for those test pieces are shown below. Characteristic Test 1 Probaptite mica (average particle size: 60 mesh) powder was immersed in an acidic aqueous solution of palladium chloride in hydrochloric acid for surface treatment. Next, this ground-treated mica powder was put into a plating solution having the following composition, and heated at a temperature of 60 to 90°C while stirring the air.
Treated for 10-30 minutes. Plating liquid composition: Nickel sulfate...30g/Sodium hypophosphite...10g/Sodium citrate...10g/PH...4 to 6 The obtained plated product was dried to give a product. This product was kneaded with polypropyline using a Brabender plastomill, and then 2
The surface resistance and volume resistivity of the material molded to mm thickness were measured. The results are shown in Table 1. For comparison, this table also shows a reference example using a filler without a metal coating.

【table】

[Table] *1 No plating film *2 Example of the present invention (metal-coated mica filling)
For test pieces No. 1 to 3 in Table 1, the volume fraction of mica in polypropylene is approximately 14 to 20% when the volume of the nickel film on the mica surface is ignored.
Similarly, No. 4 is a molding material obtained by filling mica to a volume fraction of about 30%. As the thickness of the plating film (specific gravity: 7.95) increases, the electrical resistance decreases exponentially. For compression molded test pieces, when filled with plate-like and fibrous fillers,
Anisotropy is seen in the volume resistivity. The specific gravity of phlogopite mica is 2.80 to 2.90, and conductivity can be obtained just by coating the surface with nickel, so a molding material with a lower specific gravity than filling with nickel metal foil was effective. Nos. 5 to 8 showed the results using general conductive fillers used to produce conductive molding materials. They showed higher electrical resistance values than No. 3, which had approximately the same volume fraction. However, although No. 7 is fibrous and has a lower electrical resistance value than the others, it does not have an appearance suitable for use as a product. Characteristic Test 2 Regarding the test piece shown in Characteristic Test 1, the transmittance and reflectance of 4 GHz electromagnetic waves (microwaves) were measured. Measurements were made using the cross-sectional size (58.1 x 29.1 mm) of a 4 GHz band rectangular waveguide (model name: WRJ-4).
The sample was cut and inserted into the waveguide. The transmittance was measured by keeping the output of the oscillator constant and calculating the ratio of the readings on the wattmeter after inserting the sample and before inserting the sample. Transmission loss (dB) is the common logarithm of the reciprocal of transmittance multiplied by 10. In the device used this time, the maximum power that reaches the wattmeter before inserting the sample is 1.5 mW, and the minimum power that can be read by the wattmeter is 0.1 μW, so the minimum measurable transmittance is 0.007% and the maximum transmission loss. is approximately 40dB. Regarding the reflectance, the ratio S (voltage standing wave ratio) between the maximum value and the minimum value of a standing wave caused by interference between an incident wave and a reflected wave was measured. Relationship between S and power reflectance γ Reflectance γ was determined from the formula γ=(S−1/S+1) ² . However, in the case of a sample with high conductivity, the value of S becomes large, and the measurement accuracy deteriorates because it becomes more susceptible to the characteristics of the detector of the standing wave measuring device. Therefore, in this measurement, in order to avoid this, the distance △l between two points where the standing wave power is doubled (√2 times in voltage) on both sides of the minimum point lmin
was measured, and S was calculated using the following formula. λg: Channel wavelength (9.81cm at 4.00GHz) The above measurement results are shown in Table 2. The test piece No. shown here corresponds to the test piece No. shown in Table 1 of Characteristic Test 1.

【table】

[Table] The shielding property indicated by the transmission loss of the polypropylene matrix is 0.10 dB, and mica alone without a plating film in No. 1 has no effect, but the test pieces in No. 3 and No. 4 In this case, more than 30dB is achieved. At this time, the weight proportion of nickel in the nickel-plated mica is about 50%. No. 5 to 8 and No.
When compared with No. 3, even with almost the same volume fraction, No.
3 has better shielding properties than other molding materials,
It has better shielding properties than No. 7 and 8, which are aluminum-filled molding materials. Characteristics test 3 In an electromagnetic shield room, use a high-pressure discharge (25KV, 200mA) spark plug as a noise source.
The shielding effect was measured in the frequency range of 0 to 1 Hz. Determine the distance between the half-wave dipole antenna and the object to be measured.
The distance was kept constant at 500 mm, and the received signal was analyzed using a spectrum analyzer. Frequency analysis range 0~200MHz
The antennas were tuned to 50MHz for 1GHz and 220MHz for 0 to 1GHz. The test piece is a full opening (113 x 113 mm) of a copper box (500 x 500 x 500 mm).
installed on. Table 3 shows the transmission loss (dB). The test piece numbers correspond to those in Table 1. However, the average thickness is 1.16
mm. In addition, an aluminum plate was used as a comparison sample.

[Table] Molding material filled with nickel-plated mica flakes exhibits higher transmission loss in the high frequency band of 750MHz compared to aluminum metal plates. Table 3 shows transmission losses at representative peaks. Although the weight percentage of nickel in the nickel-plated mica of Nos. 3 and 4 is approximately 50%, a considerable shielding effect is observed. No. 8 is a molding material filled with aluminum flake and has almost no shielding properties. Characteristics test 4 A DC motor housed in an iron plate case is rotated by a 3V dry cell battery in an electromagnetic shield room, noise radio waves are generated from the brush, and the opening (155 x 60 mm) is
The radio waves leaking through the test piece attached to the
The signal was received by a half-wavelength dipole antenna located at a distance of mm, and measured using a spectrum analyzer in the same manner as in characteristic test 1. Table 4 shows the transmission loss (dB). The test piece is the first
This is equivalent to No. 3 in the table.

[Table] The aluminum plate shows a stable transmission loss of around 35 dB over the entire measurement wavelength range (0 to 1 GHz), but the molded product containing nickel-plated mica shows almost the same transmission loss as in Characteristic Test 3. Table 4 shows the transmission loss at the peak. Depending on the frequency, there are places where only about 5 dB of transmission loss can be obtained, but at high frequencies (500 MHz), an average transmission loss of about 15 dB can be obtained. Characteristic Test 5 The tensile strength of the electromagnetic shielding material of the present invention was determined. Dumbbell-shaped tensile test specimens were cut from compression-molded plates approximately 1.2 mm thick. Seven tensile test pieces were prepared for each test piece number, and the average values of those seven test pieces are shown in Table 5. The tensile speed was 5 mm/min. The test piece numbers correspond to Table 1.

[Table] Even if the mica surface is coated with a nickel film, the material strength does not decrease. Although the volume filling ratio is almost the same as No. 3 and No. 7 using aluminum fiber, the material filled with nickel-plated mica shows superior values in both strength and elastic modulus. Characteristic test 6 Mica coated with nickel (chemical plating weight percentage
53%) was added to polypropylene at a ratio of 55% by weight, and kneaded for 6 minutes at 230°C in a Prabender Plastomill to prepare an electromagnetic shielding material with a filler volume fraction of 25%. Next, add this material
Compression molding was performed at 220° C. for 5 minutes to obtain a 2 mm thick sheet. The volume resistivity of this product is 5.1×10 ^-1
It was Ω・cm. Table 6 shows the electric field shielding characteristics and magnetic field shielding characteristics of the samples thus obtained.

【table】

[Table] Characteristics Test 7 Chemically plated nickel-coated mica with a weight ratio of 45% was mixed with various thermoplastic plastics in proportions such that the volume fraction was 15%, 20%, and 25%, and the tests were carried out in the same manner as in Characteristics Test 5. The electromagnetic shielding material was adjusted using
Table 7 shows the results of measuring the volume resistivity and electromagnetic wave transmission loss (4 GHz) of these materials.

【table】

[Table] Characteristics Test 8 Electromagnetic shielding materials were prepared in the same manner as in Characteristics Test 6 using thermosetting plastics of epoxy resin, unsaturated polyester resin, and phenol resin as the plastics. Table 8 shows the results of measuring the volume resistivity and electromagnetic wave transmission loss (4HGz) of these materials.

【table】

〔Effect of the invention〕

The electromagnetic shielding material obtained by the present invention has a lower resistance value than acetylene black, graphite, and aluminum flakes except for aluminum fibers with almost the same filling amount, and has excellent shielding properties as indicated by transmission loss. In addition, when comparing the strength of a material made of plastic filled with aluminum fiber, which has excellent shielding properties, and the strength of a composite material filled with almost the same amount of metal-coated mica in the example of the present invention, it is found that the composite material of the present invention shows a better value. In the case of producing an electromagnetic shielding material by filling plastic with the filler according to the embodiment, the most suitable thermoplastic plastics are polyethylene and polypropylene resin, and the most suitable thermosetting plastics are epoxy, unsaturated polyester, and phenolic resin. From the above, the electromagnetic shielding material of the present invention has excellent properties.

Claims (1)

[Scope of Claims] 1. After subjecting natural mica to a base treatment with an aqueous solution of palladium chloride in hydrochloric acid, a metal coating formed firmly by chemical plating treatment is applied by weight percentage (chemical plating weight percentage). An electromagnetic shielding material characterized by being made of plastic containing 14 to 28% by volume of metal-coated mica having a volume fraction of 40% or more. 2. The electromagnetic shielding material according to claim 1, wherein the metal film is nickel.