JPH0440804B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a conductive sheet having excellent conductivity, particularly high electromagnetic shielding properties.

<Conventional technology> With the rapid development of computer equipment in recent years, it has become a problem that the many ICs and LSIs used in them generate high-frequency pulses that have a large impact on surrounding video, radio, and peripheral equipment. ing.

Conversely, it is also conceivable that computers used in various fields such as factories, offices, Shinkansen trains, online banks, etc. may malfunction due to unnecessary radio waves from outside, causing serious social confusion.

In particular, in the past, sheet metal housings were used as storage cases for computer equipment due to its large size and low volume production, but recently, office computers,
As the use of plastic housing increases with the spread of personal computers, the problem of electromagnetic interference is once again being brought into focus. In other words, plastic itself has high electrical insulating properties and allows electromagnetic waves to pass through it, so it is not effective in shielding the electromagnetic waves that are generated by digital devices and become noise that interferes with the reception functions of radios, televisions, etc. A common method for shielding plastic is to form a conductive layer on the surface, but this method involves molding the case, applying a surface treatment, and then applying a conductive agent, which is costly. There was a problem with the durability of the surface coating layer. In addition, the method of kneading conductive filler into plastic to form a shield has poor dispersion of the conductive filler, and when the kneaded material is injection molded, the conductive filler tends to accumulate in the nozzle or bent part. Moreover, it is difficult to obtain a uniform seal, and furthermore, the contact between the conductive fillers is easily inhibited by the plastic, so that the shielding effect is often insufficient.

<Problems to be Solved by the Invention> The present invention provides a novel conductive sheet that can replace the above-mentioned plastic materials, and has excellent conductivity, especially high electromagnetic shielding properties, which could not be obtained with the prior art. The purpose is to obtain a conductive sheet that is inexpensive and durable.

<Means for solving the problems> The feature of the present invention is that by adopting a wet papermaking method as a sheet forming method, a high blending ratio of ultrafine stainless steel fibers with a fiber diameter of 12 μm or less is achieved. There is a particular thing. That is, the fiber diameter
A sheet consisting of 60 to 95% by weight of ultrafine stainless steel fibers of 12 μm or less and the rest of easily soluble PVA fibers is wet-formed, heat-pressed, and then pressure-treated.
The present invention provides a conductive sheet having a volume resistivity of 10 ^-1 to ^{10 -6} Ω·cm.

The stainless steel fiber used in the present invention has a fiber diameter of
Ultrafine fibers of 12 μm or less, preferably 4 to 8 μm are applied. If this is larger than 12μm,
In the wet papermaking process, a dense entangled structure is not formed, which not only makes it impossible to obtain the desired density and electromagnetic shielding properties, but also makes it difficult to obtain the physical strength of the sheet due to the reduced contact area between the stainless steel fibers. Become. The stainless steel fibers used are fibers having a predetermined fiber diameter that are bundled with a binding agent and cut into lengths of 1 cm or less, preferably 4 to 8 mm.

On the other hand, it is easy to dissolve when mixed with stainless steel fibers.
For example, PVA fiber (Fibribond VPB105) manufactured by Kuraray Co., Ltd. is used as the PVA fiber, which has the adhesive ability to strongly bond stainless steel fibers with a small amount. Easily soluble PVA fibers retain their fiber form on a wet paper machine until the wet paper forming process, and when heated and pressed onto a drying drum during the drying process, they easily dissolve between the stainless steel fibers and act as a binder. It is the origin of function.

Since the blending ratio of stainless steel fibers and easily soluble PVA fibers has a great effect on the electromagnetic shielding properties, the inventors of the present invention conducted extensive studies on the blending ratio, and found that stainless steel fibers account for 60 to 95% of the total sheet weight. It has been discovered that good properties can be obtained by blending in such a manner as to achieve the desired weight percentage. In this case, the stainless steel fiber is 95% by weight or more,
The wet strength necessary for forming continuous sheets cannot be obtained during the wet papermaking process, and the resulting conductive sheets have weak physical properties such as tensile strength, which poses a practical problem.
On the other hand, if the stainless steel fiber is less than 60% by weight, the volume resistivity becomes high due to the presence of easily soluble PVA fibers made of insulating organic matter, and sufficient electromagnetic shielding properties cannot be obtained. The method for blending stainless steel fibers at a high blending ratio of 60 to 95% by weight of the total sheet weight is as follows.

First, stainless steel fibers treated with a sizing agent and easily soluble PVA fibers are dispersed in cold or hot water,
Gently stir with a low-speed stirrer such as an agitator to disintegrate the fibers and create a slurry. The obtained slurry is compressed and dried on a drying drum using a wet paper machine. In this case, the wet paper machine does not matter whether it is fourdrinier or circular,
Particular attention must be paid to the setting conditions of press pressure and dryer temperature, as these are important points in obtaining a uniform sheet. The obtained sheet is subjected to pressure treatment using a press device such as a calendar at room temperature or under heating. This improves the state of contact between the stainless steel fibers, resulting in a low and stable volume resistivity and improved electromagnetic shielding effect.

As mentioned above, by using a high blending ratio of ultrafine stainless steel fibers with a fiber diameter of 12 μm or less, combined use of easily soluble PVA fibers, and pressure treatment after wet papermaking, the volume resistivity can be reduced to 10 ^-1 to 10 ^-4 Ω.・cm conductive sheets can be obtained.

Note that the measurement of volume resistivity in the present invention is based on the length
Both lengthwise ends of a sample piece of 70 mm and width 50 mm were held between clip-shaped electrodes so that the span was 50 mm, and current was applied to measure the voltage and current. The measured resistance was calculated using the following formula.

Volume resistivity (ρv) = R x S/l However, R: Actual resistance (Ω) S: Cross-sectional area of sample (cm ² ) l: Span distance (cm) The conductive sheet of the present invention is used for purposes other than electromagnetic shielding. It can be used in a wide range of applications, including as a conductive material to avoid static electricity problems.

<Examples> The present invention will be described in detail below with reference to Examples and Comparative Examples.

Example 1 90 parts by weight of stainless steel fiber (Sasmic, manufactured by Tokyo Seizuna Co., Ltd.) with a fiber diameter of 8 μm and a fiber length of 6 mm and a fiber length of 3 mm.
PVA fiber (Kuraray Co., Ltd. Fibribond VPB105
-2) 10 parts by weight was dispersed in water at room temperature and stirred in an agitator for 10 minutes. Separately, 3 parts by weight of acrylamide (Acriperth manufactured by Diafloc Co., Ltd.) was added as an anti-settling agent to prepare a slurry with a solid content concentration of 0.05%. This slurry was processed using a Fourdrinier paper machine with a weight of 55 m.
A wet fiber sheet obtained by paper making with a target of g/m ² and dehydration pressing was pressed and heated using a dryer heated to 100° C. to obtain a dry sheet. This sheet was further pressure-treated in a calender at a pressure of 100 kg/cm ² to obtain a conductive sheet having a basis weight of 56 g/m ² and a thickness of 31 μm.
This sheet had a volume resistivity of 3.7×10 ⁻³ Ω·cm and had sufficient electrical conductivity.

In addition, Takeda Riken's spectrum analyzer
When we measured the amount of attenuation against the electric field at 500MHz using the TR4172 model, we obtained 40dB, confirming the excellent shielding effect. Furthermore, the physical strength such as tensile strength was sufficient for practical use.

Example 2 Using the manufacturing conditions of Example 1, the basis weight was 105 g/m ² ,
A conductive sheet with a thickness of 58 μm was obtained. This sheet had a volume resistivity of 6.8×10 ^-3 Ω·cm and had sufficient electrical conductivity.

Furthermore, when we measured the amount of attenuation against the electric field at 500MHz, we obtained 45dB, confirming the excellent shielding effect. Furthermore, the physical strength such as tensile strength was sufficient for practical use.

Example 3 75 parts by weight of stainless steel fiber (Sasmic, manufactured by Tokyo Seami Co., Ltd.) with a fiber diameter of 8 μm and a fiber length of 6 mm and a fiber length of 3 mm.
PVA fiber (Kuraray Co., Ltd. Fibribond VPB105
-2) 25 parts by weight was dispersed in water at room temperature and stirred in an agitator for 10 minutes. Separately, 3 parts by weight of acrylamide (Acryperse manufactured by Diafloc Co., Ltd.) was added as an anti-settling agent to prepare a slurry with a solid content concentration of 0.05%.

This slurry was processed using a Fourdrinier paper machine to produce a paper weight of 70g/m2 ^.
A wet fiber sheet obtained by paper making and dehydration pressing was pressed and heated using a dryer heated to 95°C to obtain a dry sheet. Add this sheet by 100Kg/
Heat treated with a cm ² calendar and has a basis weight of 71.6g/m ² ,
A conductive sheet with a thickness of 40 μm was obtained. The volume resistivity value of this sheet was 1.0×10 ^-3 Ω·cm, and it had sufficient electrical conductivity.

Furthermore, when we measured the amount of attenuation against the electric field at 500MHz, we obtained 37dB, confirming the excellent shielding effect. Furthermore, the physical strength such as tensile strength was sufficient for practical use.

Comparative example: 50 parts by weight of stainless steel fiber (Sasmic, manufactured by Tokyo Seizuna Co., Ltd.) with a fiber diameter of 8 μm and a fiber length of 6 mm and a fiber length of 3 mm.
PVA fiber (Kuraray Co., Ltd. Fibribond VPB105
-2) 20 parts by weight and 30 parts by weight of NBKP wood pulp were dispersed in water and stirred in an agitator for 10 minutes. Separately, 2 parts by weight of acrylamide (Acriperth manufactured by Diaflick Co., Ltd.) was added as an anti-settling agent, and the solid content concentration was
A 0.05% slurry was prepared. This slurry was made into paper using a Fourdrinier paper machine with a paper weight of 55 g/m ² , and the wet fiber sheet obtained by dehydration pressing was heated and dried to produce stainless steel fibers with a weight of 53 g/m ² and a thickness of 68 μm. A blending sheet was obtained. Although the physical strength such as tensile strength of this sheet was sufficient for practical use, the volume resistivity value was 3×10 ⁰ Ω·cm, which was unsatisfactory in terms of conductivity.

Furthermore, when we measured the amount of attenuation against the electric field at 500MHz, we obtained a shielding characteristic of 20dB, which poses a problem in mounting.

<Effects of the Invention> Since the present invention has the above-described configuration, the conductivity of the stainless steel fiber itself can be fully utilized in the sheet-like material, and excellent electromagnetic shielding properties can be obtained due to a stable and low electrical resistance value. was completed. In addition, it is cheaper and more durable than conventional shielding materials, so it can be used in a wider range of applications.

Claims (1)

[Claims] 1 60 ultra-fine stainless steel fibers with a fiber diameter of 12 μm or less
A sheet consisting of ~95% by weight and the rest being easily soluble PVA fibers is wet-formed, heat-pressed, and then pressure-treated to have a volume resistivity of 10 ^-1 to ^{10 -4} Ω・cm. A method for manufacturing a conductive sheet.