JP2018076408A - Rubber molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rubber molding that is thin and flexible, and shows excellent electromagnetic wave shielding properties in a wide range of high frequency bands (500 MHz-18 GHz).SOLUTION: A rubber molding contains a rubber component and pitch-based carbon fibers. The pitch-based carbon fibers are oriented in the thickness direction of the rubber molding, and the rubber molding has a thickness of 1 mm or less, and has a transmission attenuation in the frequency bands of 500 MHz-18 GHz of 30 dB or more. Preferably, the rubber component is a silicone rubber.SELECTED DRAWING: Figure 1

Description

The present invention relates to a rubber molded body. More specifically, the present invention relates to a rubber molded body used for electromagnetic wave shielding, electromagnetic noise suppression and the like.

An electronic device may cause malfunction or noise due to electromagnetic waves emitted by other devices, and may cause malfunction or noise of other devices even by electromagnetic waves emitted by itself. In order to prevent such an electromagnetic wave interference, an electromagnetic wave shielding material capable of attenuating the electromagnetic wave causing the failure and shielding the circuit from the electromagnetic wave is effective. As such an electromagnetic wave shielding material, various things are proposed, for example, what laminated metal layers, such as metal foil, and an anisotropic conductive adhesive layer (refer to patent documents 1), and conductive fiber A laminate (see Patent Document 2) and the like including a rigid holding member made of fiber-reinforced plastic mixed with bismuth is known.

The electromagnetic wave shielding material described in Patent Documents 1 and 2 has a laminated structure and has not only an electromagnetic wave shielding function but also a signal transmission function and an electromagnetic wave transmission function, but has excellent flexibility. There is also a demand for an electromagnetic shielding material having a single layer structure. As for the electromagnetic wave shielding material having flexibility, for example, a rubber-conductive fiber composite (refer to Patent Document 3) in which conductive fibers such as stainless steel metal fibers are mixed in rubber, vapor grown nanofibers, etc. A rubber molded body made of a rubber composition containing short fibers and a base rubber material (see Patent Document 4) is known. Patent Document 3 discloses an electromagnetic wave shielding material in which ferrite powder is added to rubber as a comparative example.

Japanese Unexamined Patent Publication No. 2016-36044 International Publication No. 2016/002456 JP-A-6-816 JP 2005-290018 A

However, it is difficult to realize excellent electromagnetic shielding properties in a wide range of high frequency bands (500 MHz to 18 GHz) by a flexible rubber molded body, and particularly sufficient electromagnetic shielding properties are secured with a thin sheet having a thickness of 1 mm or less. It was difficult to do. For example, when the rubber-conductive fiber composite described in Patent Document 3 and the rubber molded body described in Patent Document 4 increase the blending amount of conductive fibers and short fibers in order to improve electromagnetic shielding properties, When flexibility is reduced and sheet thickness is reduced, electromagnetic shielding properties are reduced. Therefore, both flexibility and thinness and excellent electromagnetic shielding properties in a wide range of high frequency bands must be achieved at a high level. I could not. Moreover, when the compounding quantity of the conductive fiber or the short fiber is increased, it may not be possible to form a thin sheet having a smooth surface.

The present invention has been made in view of the above-described present situation, and an object thereof is to provide a rubber molded body that is thin and flexible, and exhibits excellent electromagnetic shielding properties in a wide range of high frequency bands (500 MHz to 18 GHz). And

As a result of various studies on means for realizing excellent electromagnetic shielding properties in a wide range of high frequency bands by a rubber molded body having a thickness of 1 mm or less, the present inventors have found that a pitch system oriented in the thickness direction in the rubber molded body. It has been found that by blending carbon fiber, the transmission attenuation in the frequency band of 500 MHz to 18 GHz can be increased to 30 dB or more, and the present invention has been completed.

The rubber molded body of the present invention is a rubber molded body containing a rubber component and pitch-based carbon fibers, wherein the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded body, and the rubber molded body is The thickness is 1 mm or less, and the transmission attenuation in a frequency band of 500 MHz to 18 GHz is 30 dB or more.

The pitch-based carbon fiber content is preferably 10 parts by weight or more with respect to 100 parts by weight of the rubber component.

The rubber component is preferably silicone rubber.

According to the present invention, it is possible to provide a rubber molded body which is thin and flexible and exhibits excellent electromagnetic shielding properties in a wide range of high frequency bands (500 MHz to 18 GHz).

It is the image which image | photographed the cross section parallel to the thickness direction of the rubber molding of this invention with the scanning electron microscope (SEM). It is the cross-sectional schematic diagram which showed the extruder used for shaping | molding of the rubber molding of this invention. It is the graph which showed the measurement result of the electromagnetic wave shielding effect of the rubber sheet of an Example and a comparative example.

The rubber molded body of the present invention is a rubber molded body containing a rubber component and pitch-based carbon fibers, wherein the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded body, and the rubber molded body is The thickness is 1 mm or less, and the transmission attenuation in a frequency band of 500 MHz to 18 GHz is 30 dB or more.

The rubber molded body of the present invention contains a rubber component and pitch-based carbon fibers, and the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded body. The pitch-based carbon fiber functions as an anisotropic conductive filler. That is, the pitch-based carbon fiber is electrically conductive, and the anisotropic conductivity can be imparted to the rubber molded body of the present invention by orienting the pitch-based carbon fiber in the thickness direction. Such pitch-based carbon fibers are oriented in the thickness direction, so that the electromagnetic wave shielding property can be remarkably improved as compared with the case where the pitch-type carbon fibers are oriented in the plane direction. A very high shielding effect can be obtained. Therefore, the rubber molded body can exhibit excellent electromagnetic wave shielding properties without adding another layer such as a metal foil layer, and can have a simple structure of a single layer structure. Moreover, since the specific gravity of the rubber molded body of the present invention can be about 1.5, there is an advantage that the sheet weight is light.

FIG. 1 is an image obtained by photographing a cross section parallel to the thickness direction of a rubber molded body of the present invention with a scanning electron microscope (SEM). The image on the left side of FIG. 1 shows a rubber molded body of the present invention produced by extrusion molding described below. That is, the rubber molded body of the present invention is obtained by feeding a thin sheet in which pitch-based carbon fibers are oriented and dispersed in the rubber component, bending it in a direction perpendicular to the traveling direction of this sheet, and folding it into a bellows shape. It can be formed by welding. In this case, the formed rubber molded body has a sheet shape in which a large number of weld lines extending in the thickness direction are formed. Here, the weld line is not necessarily a complete straight line, and may be curved in an arc shape. The plurality of weld lines are arranged close to each other in the surface direction of the sheet-shaped rubber molded body. Thus, the pitch-based carbon fibers can be arranged in the thickness direction of the rubber molded body in a dense (high density) and high orientation ratio. As a result, a conductive path is formed in the thickness direction, the conductivity in the thickness direction can be greatly improved, and the loss absorption amount of electromagnetic waves that pass through the rubber molded body can be increased. Due to these synergistic effects, the rubber molded body of the present invention can have excellent electromagnetic shielding properties.

Further, the image on the right side of FIG. 1 shows the rubber molded body of the present invention produced by laminating and bonding flaky rubber molded bodies. That is, the rubber molded body of the present invention can also be formed by slicing (cutting) a rubber molded body having a thickness of 1 mm or more to a thickness of less than 1 mm and stacking and bonding the obtained flaky rubber molded bodies.

Examples of rubber components include styrene-butadiene copolymers or their hydrogenated polymers, styrene-based thermoplastic elastomers such as styrene-isoprene block copolymers or their hydrogenated polymers, olefin-based thermoplastic elastomers, and vinyl chloride-based thermoplastics. Examples thereof include elastomers, polyester-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, and silicone rubber. These may be used individually by 1 type and may use 2 or more types together. Among them, silicone rubber can sufficiently disperse fibers even when a large amount of pitch-based carbon fibers is blended and kneaded, so that it has excellent electromagnetic shielding properties and a smooth surface. It is suitable when producing the sheet | seat which has. Silicone rubber can also be dispersed in conductive fibers such as carbon fibers and graphite (graphite) other than pitch. Furthermore, silicone rubber is easy to obtain flexibility (flexibility) even when a large amount of filler is blended, and is excellent in heat resistance. Further, as the rubber component, it is preferable to use peroxide-crosslinked vulcanized rubber.

The pitch-based carbon fibers used in the present invention are carbon fibers made from petroleum pitch or coal pitch. By using pitch-based carbon fiber, the rubber molded body can exhibit good electromagnetic wave shielding properties. Of these, pitch-based carbon fibers having developed graphitization properties that are fired at a high temperature of 2000 to 3200 ° C. using mesophase pitch as a raw material are preferable from the viewpoint of improving electromagnetic shielding properties. The pitch-based carbon fiber may be subjected to a surface treatment with a silane coupling agent or the like.

The average length (fiber length) of the pitch-based carbon fibers dispersed in the rubber molded body is preferably 50 μm or more, and includes pitch-based carbon fibers having substantially the same length as the thickness of the rubber molded body. Is preferred. In addition, when the length of the pitch-based carbon fiber is shorter than the thickness of the molded rubber molded body, the molded rubber molded body is sliced to a thickness thinner than the length of the pitch-based carbon fiber. The rubber molded body of the present invention may be obtained. Moreover, the pitch system carbon fiber shorter than the thickness of a rubber molding may contact, and the form by which the conductive path was formed in the substantially thickness direction of the rubber molding may be sufficient.

When the pitch carbon fiber has an average length of 50 μm to 1 mm, the fibers can be well dispersed and oriented in the thickness direction during the production of a rubber molded body, and excellent electromagnetic shielding properties can be exhibited. Is possible. When the average length is less than 50 μm, it becomes difficult to uniaxially orient the pitch-based carbon fiber in the rubber component. Moreover, when producing a rubber molded body by slicing, it is difficult to slice to a thickness of less than 50 μm. When the average length exceeds 1 mm, it becomes difficult to uniaxially orient the pitch-based carbon fiber in the rubber component, and it becomes difficult to stably exhibit the electromagnetic wave shielding property. Further, if the average length exceeds 1 mm, the flexibility of the rubber molded body may not be sufficiently obtained.

The average filament diameter (fiber diameter) of the pitch-based carbon fiber is, for example, 4 to 20 μm.

The pitch-based carbon fiber preferably has an average aspect ratio of 10 to 500. Thereby, it is possible to obtain a rubber molded body having low hardness and good electromagnetic shielding properties. When the average aspect ratio is less than 10, it may be difficult to uniaxially orient the pitch-based carbon fiber in the rubber component. When the average aspect ratio exceeds 500, dispersion of the pitch-based carbon fiber in the rubber component is deteriorated. There is a fear. The average aspect ratio is more preferably 15 to 300.

In addition, the average length and average aspect-ratio of pitch type carbon fiber can be measured from observation with a scanning electron microscope (SEM). Specifically, SEM photography is performed, and the length and aspect ratio (fiber length / fiber diameter) of carbon fibers are measured for an arbitrary number (for example, five), and an arithmetic average value thereof is calculated. can do.

The weight-based content of the pitch-based carbon fiber is preferably 10 parts by weight or more with respect to 100 parts by weight of the rubber component. The weight-based content of the pitch-based carbon fiber is preferably 30 parts by weight or less, and more preferably 25 parts by weight or less with respect to 100 parts by weight of the rubber component. When the content by weight is less than 10 parts by weight, the electromagnetic wave shielding effect of the rubber molded body may not be sufficiently obtained. If the content by weight exceeds 30 parts by weight, dispersion by kneading may not be possible. Moreover, the volume fraction (volume filling factor) of the pitch-based carbon fiber is set to, for example, 20 to 80% by volume with respect to the rubber component (100% by volume). When the said volume fraction is less than 20 volume%, there exists a possibility that the electromagnetic wave shielding effect of a rubber molding may not fully be acquired. Further, when the volume fraction exceeds 80% by volume, the rubber precursor described later is folded in a direction substantially perpendicular to the flow direction in the first gap when passing through the first gap. There is a possibility that a problem that it becomes difficult to fuse each other may occur. Moreover, when the said volume fraction exceeds 80 volume%, there exists a possibility that the workability (moldability) at the time of producing a rubber molding, and the surface smoothness and flexibility of a rubber molding obtained may fall. Therefore, in order to enhance the electromagnetic wave shielding effect of the rubber molded body and facilitate extrusion, the volume fraction of the pitch-based carbon fiber is preferably 30 to 70% by volume, and 50 to 65% by volume. It is more preferable.

In the rubber molded body of the present invention, the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded body. Here, in the present invention, “the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded body” means that the longitudinal direction of each fiber of the pitch-based carbon fibers in the rubber molded body is the main of the rubber molded body. It means a state where they are aligned in a direction (thickness direction) perpendicular to the surface. Specifically, the orientation ratio in the thickness direction of the pitch-based carbon fibers in the rubber molded body is preferably 80% or more. In the present invention, “the orientation ratio is 80% or more” means that the ratio of pitch-based carbon fibers arranged in the thickness direction is 80% or more (number basis) among all pitch-based carbon fibers arranged in the rubber molded body. ). The orientation ratio is more preferably 90% or more, and still more preferably 95% or more. Further, in the present invention, “arranged in the thickness direction” means that the angle formed by the thickness direction of the rubber molded body and the orientation direction of each pitch-based carbon fiber (orientation inclination angle) is 30 ° or less. And the angle (orientation distribution angle) formed by the direction in which any pitch-based carbon fiber in the rubber molded body is oriented and the direction in which other pitch-based carbon fibers are oriented is also 30 ° or less. It means that. The orientation tilt angle is more preferably 10 ° or less. The orientation distribution angle is more preferably 10 ° or less. The orientation direction of the pitch-based carbon fiber can be determined, for example, by cutting a rubber molded body and observing the cut surface with an electron microscope, and by image processing the obtained data of the cut surface. The orientation rate can be determined.

The rubber molded body of the present invention may contain a conductive filler other than the pitch-based carbon fiber (hereinafter simply referred to as “conductive filler”) in addition to the rubber component and the pitch-based carbon fiber. As such a conductive filler, various conventionally known materials can be used as long as the effects of the present invention are not impaired. For example, carbon fibers other than pitch-based materials, graphene, graphite (graphite), carbon nanotubes, etc. A carbon material may be used.

The shape of the conductive filler is not particularly limited and can be appropriately selected according to the purpose. Examples thereof include a scale shape, a plate shape, a film shape, a columnar shape, a prismatic shape, an elliptical shape, and a flat shape. The aspect ratio is preferably 2 or more from the viewpoint of easily imparting conductivity and easy orientation in the rubber component.

The volume fraction of the conductive filler relative to the rubber component can be determined as appropriate, and is, for example, 20 to 80% by volume.

In addition to the rubber component and pitch-based carbon fiber, the rubber molded body of the present invention includes a reinforcing agent, a filler, a softening agent, a plasticizer, a crosslinking agent, a crosslinking accelerator, a crosslinking accelerator, an antiaging agent, a tackifier, You may contain additives, such as an antistatic agent, a kneading adhesive agent, a flame retardant, and a coupling agent. As the crosslinking agent, a peroxide-based crosslinking agent is preferably used.

The rubber molded body of the present invention has a thickness of 1 mm or less. The rubber molded body of the present invention is excellent in that a large electromagnetic shielding effect is obtained while ensuring sufficient flexibility (flexibility) as a thickness of 1 mm or less. What is necessary is just to adjust the thickness of a rubber molding so that characteristics, such as electromagnetic wave shielding property and flexibility, may become desired. The thickness of the rubber molded body is preferably 0.1 mm or more. Moreover, it is preferable that the rubber molded body of the present invention is a sheet (rubber sheet) having a substantially constant thickness.

The rubber molded body of the present invention has a transmission attenuation amount of 30 dB or more in the frequency band of 500 MHz to 18 GHz. The greater the transmission attenuation, the greater the electromagnetic wave shielding effect. When the transmission attenuation amount in the frequency band of 500 MHz to 18 GHz is 30 dB or more, practical performance can be secured when the rubber molded body of the present invention is used for electromagnetic wave shielding and electromagnetic wave noise suppression in electronic devices. For example, if there is a shielding performance of 30 dB (shielding ratio 96 to 97%) or more, communication of a mobile phone can be blocked.

The rubber molded body of the present invention has high conductivity performance by orienting pitch-based carbon fibers densely (high density) and with a high orientation ratio in the thickness direction. It can be in the range of 1000Ω / □.

The rubber molded body of the present invention is suitable for applications requiring flexibility and bendability, and the hardness thereof is, for example, the hardness (Shore E) measured by a type E durometer tester defined in JIS K 6253. ) In the range of 40 to 90, or Asker C (ASKER C) stipulated in the Japan Rubber Association Standard (SRIS).

The method for producing the rubber molded body of the present invention is not particularly limited as long as the pitch-based carbon fibers can be oriented in the thickness direction in the obtained rubber molded body. For example, the rubber molded body is produced by the following method. be able to.

FIG. 2 is a schematic cross-sectional view showing an extruder used for molding a rubber molded body of the present invention, and shows an outline of a cross-section of a tip portion of the extruder and a T die. A rubber composition containing a rubber component, pitch-based carbon fiber, and the like is stirred and kneaded by the screw 8 and introduced into the first gap 12 along the flow path 10.

The flow of the rubber composition is squeezed in the vertical direction (thickness direction) by the first gap 12 to form a thin strip shape. When passing through the first gap 12, a shearing force acts on the rubber composition, and the pitch-based carbon fibers mixed in the rubber component are oriented in the flow direction of the rubber composition. In this case, the pitch-based carbon fibers are oriented in the plane direction of the sheet-like rubber precursor.

When a thin sheet-like rubber precursor in which pitch-based carbon fibers are oriented in the flow direction of the rubber composition completely passes through the first gap 12, the cross-sectional area of the flow path 10 is enlarged, Since the length becomes longer, the flow of the rubber precursor changes in the vertical direction. Thereafter, when the rubber precursor passes through the second gap 14, it is fused while being folded in a direction substantially perpendicular to the direction of flow in the first gap 12, thereby forming a sheet-like rubber molded body. .

As mentioned above, although typical embodiment of this invention was described, this invention is not limited only to these, Various design changes are possible, and such a design change is also contained in this invention.

EXAMPLES Hereinafter, although an Example is hung up and demonstrated in more detail about this invention, this invention is not limited only to these Examples.

Example 1
For 100 parts by weight of silicone rubber (made by MOMENTIVE, “TSE201”) as a rubber component, 58 parts by weight of pitch-based carbon fiber (made by Nippon Graphite Fiber, “XN100 (CF)”) and 27 parts by weight of graphite are used. Parts by weight, 3 parts by weight of a peroxide as a cross-linking agent (manufactured by NOF Corporation, “Park Mill D”), and 20 parts by weight of silicone oil as a plasticizer (“KF-96”, manufactured by Shin-Etsu Chemical Co., Ltd.) Was kneaded with two rolls to prepare a sheet-like rubber composition. The volume fraction of the pitch-based carbon fiber (specific gravity 1.8) with respect to the entire rubber composition (specific gravity 1.5) was 45% by volume. The average length of the pitch-based carbon fibers (fiber length) was 150 μm.

The obtained rubber composition was formed into a sheet having a thickness of 10 mm using a short shaft extruder for rubber having a tip portion having a structure shown in FIG. 2 and a T die, and further at 180 ° C. using a press vulcanizer. A peroxide crosslinking treatment for 10 minutes was performed. In the extruder, a vertical alignment die (die) was used. Subsequently, the obtained rubber sheet having a thickness of 10 mm after crosslinking was sliced in the sheet surface direction to produce a rubber sheet having a thickness of 1.0 mm. The hardness (Shore E) when three of the obtained rubber sheets were laminated and measured with a type E durometer tester defined in JIS K 6253 was 40 (corresponding to 40 in ASKER C).

(Orientation rate)
The rubber sheet of Example 1 was sliced to a thickness of about 100 μm, and a cross section parallel to the sheet thickness direction was observed with a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, “S-4800”). SEM observation confirmed that the pitch-based carbon fibers were oriented in the sheet thickness direction in the rubber sheet of Example 1 (orientation ratio of the pitch-based carbon fibers in the sheet thickness direction: 99%). The orientation rate in the sheet thickness direction is obtained by confirming the orientation direction of 30 fibers arbitrarily extracted from the cross-sectional image and calculating the ratio of the number of fibers oriented in the sheet thickness direction. It was.

(Electromagnetic wave shielding effect)
The rubber sheet of Example 1 was processed into a ring shape having an inner circle diameter of 2.5 mm and an outer circle diameter of 13.0 mm using a punching machine, and a sample for measuring the electromagnetic shielding effect was produced. Using the network analyzer “N5222A” of Keysight Technologies, Inc. and “GCP-7” of Keycom, the transmission attenuation in the high frequency range of 500 MHz to 18 GHz band is measured by the coaxial tube method. The electromagnetic wave shielding effect by a 0.0 mm sample was confirmed. In Table 1 and FIG. 3 below, the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Example 1 are shown.

(Example 2)
A thickness of 0.5 mm in which pitch-based carbon fibers are oriented in the sheet thickness direction in the same manner as in Example 1 except that a rubber sheet having a thickness of 10 mm after crosslinking is sliced to a thickness of 0.5 mm. A rubber sheet was prepared and the electromagnetic shielding effect was measured. The measurement results of the electromagnetic wave shielding effect of the rubber sheet of Example 2 are shown in Table 1 and FIG.

(Comparative Example 1)
When slicing a rubber sheet having a thickness of 10 mm after crosslinking, the thickness of 1.0 mm in which pitch-based carbon fibers are oriented in the sheet surface direction is the same as in Example 1 except that the sheet is sliced in the sheet thickness direction. A rubber sheet was prepared and the electromagnetic shielding effect was measured. Table 1 and FIG. 3 show the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Comparative Example 1.

(Comparative Example 2)
A pitch-based carbon fiber having a thickness of 0.5 mm in which the pitch-based carbon fibers are oriented in the sheet surface direction is the same as Comparative Example 1 except that a rubber sheet having a thickness of 10 mm after crosslinking is sliced to have a thickness of 0.5 mm A rubber sheet was prepared and the electromagnetic shielding effect was measured. Table 1 and FIG. 3 show the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Comparative Example 2.

As can be seen from Table 1 above, the rubber sheets of Examples 1 and 2 in which pitch-based carbon fibers are oriented in the sheet thickness direction are as thin as 1 mm or less, but 30 dB in the entire high frequency band of 500 MHz to 18 GHz. The above transmission attenuation was shown, and it was confirmed that it had excellent electromagnetic shielding properties. On the other hand, in the rubber sheets of Comparative Examples 1 and 2 in which pitch-based carbon fibers are oriented in the sheet surface direction, a transmission attenuation amount of 30 dB or more cannot be obtained in the entire high frequency band of 500 MHz to 18 GHz. It was confirmed that the electromagnetic wave shielding property was inferior to that of the rubber sheet.

(Comparative Example 3)
In the blending of the rubber composition of Example 1, the same amount of vapor grown carbon fiber (Showa Denko Co., Ltd., “VGCF-H”, fiber length 6 μm fine diameter) was used instead of 58 parts by weight of pitch-based carbon fiber. Kneaded with two rolls. However, since the fiber length of the fiber is too short and the reinforcing property is too strong, the fiber cannot be properly dispersed by kneading and cannot be formed into a sheet.

8 Screw 10 Channel 12 First gap 14 Second gap

Claims (3)

A rubber molded body containing a rubber component and pitch-based carbon fiber,
The pitch-based carbon fiber is oriented in the thickness direction of the rubber molded body,
The rubber molded body has a thickness of 1 mm or less and a transmission attenuation in a frequency band of 500 MHz to 18 GHz of 30 dB or more. The rubber molded body according to claim 1, wherein a content of the pitch-based carbon fiber is 10 parts by weight or more with respect to 100 parts by weight of the rubber component. The rubber molding according to claim 1 or 2, wherein the rubber component is silicone rubber.

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