JP6933891B2 - Rubber molded body - Google Patents

Description

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

Electronic devices may cause malfunctions and noise due to electromagnetic waves emitted by other devices, and electromagnetic waves emitted by themselves may also cause malfunctions and noise of other devices. In order to prevent such electromagnetic interference, an electromagnetic wave shielding material capable of attenuating the electromagnetic wave causing the interference and shielding the circuit from the electromagnetic wave is effective. Various types of such electromagnetic shielding materials have been proposed. For example, a metal layer such as a metal foil and an anisotropic conductive adhesive layer are laminated (see Patent Document 1), or conductive fibers. A laminate (see Patent Document 2) and the like including a rigidity-retaining member made of fiber-reinforced plastic mixed with the above are known.

The electromagnetic wave shielding materials described in Patent Documents 1 and 2 have a laminated structure and are said to have not only an electromagnetic wave shielding function but also a signal transmission function and an electromagnetic wave transmission function, but are excellent in flexibility. There is also a demand for an electromagnetic wave shielding material having a single-layer structure. Examples of the electromagnetic wave shielding material having flexibility include rubber-conductive fiber composites (see Patent Document 3) in which conductive fibers such as stainless metal fibers are mixed in rubber, gas phase growth nanofibers, and the like. A rubber molded body (see Patent Document 4) made of a rubber composition containing short fibers and a base rubber material is known. Further, Patent Document 3 discloses, as a comparative example, an electromagnetic wave shielding material in which ferrite powder is added to rubber.

Japanese Unexamined Patent Publication No. 2016-36044 International Publication No. 2016/002456 Japanese Unexamined Patent Publication No. 6-816 Japanese Unexamined Patent Publication No. 2005-29018

However, it is difficult to achieve excellent electromagnetic wave shielding properties in a wide range of high frequency bands (500MHz to 18GHz) with a flexible rubber molded body, and in particular, a thin sheet with a thickness of 1 mm or less ensures sufficient electromagnetic wave shielding properties. It was difficult to do. For example, in the rubber-conductive fiber composite described in Patent Document 3 and the rubber molded body described in Patent Document 4, when the blending amount of conductive fibers and short fibers is increased in order to improve the electromagnetic wave shielding property, If the flexibility is reduced and the thickness of the sheet is reduced, the electromagnetic wave shielding property is reduced. Therefore, both the flexibility and thinness and the excellent electromagnetic wave shielding property in a wide range of high frequency bands should be compatible at a high level. I couldn't. In addition, if the amount of conductive fibers or short fibers blended is large, 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 situation, and an object of the present invention is to provide a rubber molded product which is thin and flexible and exhibits excellent electromagnetic wave shielding property 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 wave shielding properties in a wide range of high frequency bands with a rubber molded body having a thickness of 1 mm or less, the present inventors have conducted a pitch system oriented in the thickness direction in the rubber molded body. The present invention has been completed by finding that the transmission attenuation in the frequency band of 500 MHz to 18 GHz can be increased to 30 dB or more by blending carbon fibers.

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

The 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 rubber component is preferably silicone rubber.

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

It is an image of a cross section parallel to the thickness direction of the rubber molded body of the present invention taken with a scanning electron microscope (SEM). It is sectional drawing which showed the extruder used for molding the rubber molded article of this invention. It is a 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 product of the present invention is a rubber molded product containing a rubber component and pitch-based carbon fibers, and the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded product. The thickness is 1 mm or less, and the transmission attenuation amount in the frequency band of 500 MHz to 18 GHz is 30 dB or more.

The rubber molded product 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 product. Pitch-based carbon fibers function as an anisotropic conductive filler. That is, the pitch-based carbon fibers are conductive, and by orienting the pitch-based carbon fibers in the thickness direction, it is possible to impart conductivity anisotropy to the rubber molded product of the present invention. By orienting such pitch-based carbon fibers in the thickness direction, the electromagnetic wave shielding property can be remarkably improved as compared with the case where the pitch-based carbon fibers are oriented in the plane direction, and even if the thickness is 1 mm or less. 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 be a simple structure having a single layer structure. Further, the rubber molded product of the present invention has an advantage that the sheet weight is light because the specific gravity thereof can be set to about 1.5.

FIG. 1 is an image of a cross section of the rubber molded body of the present invention parallel to the thickness direction taken with a scanning electron microscope (SEM). The image on the left side of FIG. 1 shows a rubber molded product of the present invention produced by extrusion molding described below. That is, in the rubber molded product of the present invention, a thin sheet in which pitch-based carbon fibers are oriented and dispersed in the rubber component is sent out, and the sheet is folded a plurality of times in the direction perpendicular to the traveling direction of the sheet and folded in a bellows shape. It can be formed by welding. In this case, the rubber molded body formed has a sheet shape in which a large number of weld lines extending in the thickness direction thereof are formed. Here, the weld line is not limited to a perfect straight line, and may be curved in an arc shape. The plurality of weld lines are arranged close to each other in the plane direction of the sheet-shaped rubber molded body. As a result, 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 significantly improved, and the amount of loss absorption of electromagnetic waves transmitted through the rubber molded body can be increased. Due to these synergistic effects, the rubber molded product of the present invention can obtain excellent electromagnetic wave shielding properties.

Further, the image on the right side of FIG. 1 shows a rubber molded product of the present invention produced by laminating and laminating flaky rubber molded products. 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 laminating and laminating the obtained flaky rubber molded bodies.

Examples of the rubber component include styrene-based thermoplastic elastomers such as styrene-butadiene copolymers or hydrogenated polymers thereof, styrene-isoprene block copolymers or hydrogenated polymers thereof, 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 rubbers. These may be used alone or in combination of two or more. Among them, silicone rubber has excellent electromagnetic wave shielding properties and has a smooth surface because the fibers can be sufficiently dispersed even when a large amount of pitch-based carbon fibers are mixed and kneaded. It is suitable for producing a sheet to have. Further, the silicone rubber can also disperse carbon fibers other than the pitch type and conductive fillers such as graphite (graphite). Furthermore, silicone rubber is easy to obtain flexibility (flexibility) even if a large amount of filler is blended, and is also excellent in heat resistance. Further, as the rubber component, it is preferable to use a peroxide crosslinked vulcanized rubber.

The pitch-based carbon fiber used in the present invention is a carbon fiber made from petroleum pitch or coal pitch. By using pitch-based carbon fibers, it is possible to make the rubber molded body exhibit good electromagnetic wave shielding properties. Of these, pitch-based carbon fibers having developed graphitization properties, which are calcined at a high temperature of 2000 to 3200 ° C. using mesophase pitch as a raw material, are preferable from the viewpoint of improving electromagnetic wave shielding properties. The pitch-based carbon fibers may be surface-treated 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 the pitch-based carbon fibers having a length substantially the same as the thickness of the rubber molded body is included. Is preferable. When the length of the pitch-based carbon fiber is shorter than the thickness of the molded rubber molded product, the molded rubber molded product can be sliced to a thickness thinner than the length of the pitch-based carbon fiber. , The rubber molded product of the present invention may be obtained. Further, the pitch-based carbon fibers shorter than the thickness of the rubber molded body may come into contact with each other to form a conductive path in the substantially thickness direction of the rubber molded body.

When the average length of the pitch-based carbon fibers is 50 μm to 1 mm, it is possible to align the fibers in the thickness direction and disperse and orient them well at the time of producing the rubber molded product, thereby exhibiting excellent electromagnetic wave shielding properties. Is possible. If the average length is less than 50 μm, it becomes difficult to uniaxially orient the pitch-based carbon fibers in the rubber component. Further, when slicing to prepare a rubber molded product, it is difficult to slice it to a thickness of less than 50 μm. If the average length exceeds 1 mm, it becomes difficult to uniaxially orient the pitch-based carbon fibers 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 product may not be sufficiently obtained.

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

The pitch-based carbon fibers preferably have an average aspect ratio of 10 to 500. Thereby, a rubber molded product having low hardness and good electromagnetic wave shielding property can be obtained. If the average aspect ratio is less than 10, it may be difficult to uniaxially orient the pitch-based carbon fibers in the rubber component, and if it exceeds 500, the dispersion of the pitch-based carbon fibers in the rubber component becomes poor. There is a risk. The average aspect ratio is more preferably 15 to 300.

The average length and average aspect ratio of the pitch-based carbon fibers can be measured by 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, 5), and the arithmetic mean value thereof is used. 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 fibers is preferably 30 parts by weight or less, more preferably 25 parts by weight or less, based on 100 parts by weight of the rubber component. If the weight standard content is less than 10 parts by weight, the electromagnetic wave shielding effect of the rubber molded product may not be sufficiently obtained. If the weight standard content exceeds 30 parts by weight, dispersion by kneading may not be possible. The volume fraction (volume filling rate) of the pitch-based carbon fibers is, for example, 20 to 80% by volume with respect to the rubber component (100% by volume). If the volume fraction is less than 20% by volume, the electromagnetic wave shielding effect of the rubber molded product may not be sufficiently obtained. Further, when the volume fraction exceeds 80% by volume, when the rubber precursor described later passes through the first gap, the rubber precursor is folded in a direction substantially perpendicular to the flow direction in the first gap. There is a possibility that a problem may occur in which it becomes difficult for them to fuse with each other. On the other hand, if the volume fraction exceeds 80% by volume, the processability (formability) when producing the rubber molded product and the surface smoothness and flexibility of the obtained rubber molded product may decrease. Therefore, in order to enhance the electromagnetic wave shielding effect of the rubber molded body and facilitate extrusion molding, the volume fraction of the pitch-based carbon fibers is preferably 30 to 70% by volume, preferably 50 to 65% by volume. Is more preferable.

In the rubber molded product of the present invention, the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded product. Here, in the present invention, "the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded product" means that the longitudinal direction of each fiber of the pitch-based carbon fibers in the rubber molded product is the main component of the rubber molded product. It means that the fibers are aligned in the direction perpendicular to the surface (thickness direction). Specifically, it is preferable that the orientation ratio of the pitch-based carbon fibers in the rubber molded body in the thickness direction is 80% or more. In the present invention, "alignment rate of 80% or more" means that the ratio of pitch-based carbon fibers aligned in the thickness direction to 80% or more of all pitch-based carbon fibers arranged in the rubber molding body (based on the number of fibers). ). The orientation ratio is more preferably 90% or more, and further preferably 95% or more. Further, in the present invention, "arranged uniformly in the thickness direction" means that the angle (orientation inclination angle) formed by the thickness direction of the rubber molded body and the orientation direction of each pitch-based carbon fiber is 30 ° or less. The angle (orientation distribution angle) between the direction in which any pitch-based carbon fibers are oriented in the rubber molded body and the direction in which other pitch-based carbon fibers are oriented is also 30 ° or less. Means that The orientation inclination angle is more preferably 10 ° or less. Further, the orientation distribution angle is more preferably 10 ° or less. The orientation direction of the pitch-based carbon fibers can be determined, for example, by cutting the rubber molded body and observing the cut surface with an electron microscope, and by image processing the data of the obtained cut surface. The orientation rate can be obtained.

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 type, graphene, graphite, carbon nanotubes and the like can be used. Carbon materials may be used.

The shape of the conductive filler is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include scale-like, plate-like, film-like, columnar, prismatic, elliptical and flat shapes. The aspect ratio is preferably 2 or more from the viewpoint of easily imparting conductivity and easily orienting in the rubber component.

The volume fraction of the conductive filler with respect to the rubber component can be appropriately determined, 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 cross-linking agent, a cross-linking accelerator, a cross-linking accelerator, an antiaging agent, and a tackifier. It may contain additives such as an antistatic agent, a kneaded adhesive, a flame retardant, and a coupling agent. As the cross-linking agent, a peroxide-based cross-linking agent is preferably used.

The thickness of the rubber molded product of the present invention is 1 mm or less. The rubber molded product of the present invention is excellent in that a large electromagnetic wave shielding effect can be obtained while ensuring sufficient flexibility (flexibility) with a thickness of 1 mm or less. The thickness of the rubber molded product may be adjusted so that the properties such as electromagnetic wave shielding property and flexibility are desired. The thickness of the rubber molded product is preferably 0.1 mm or more. Further, the rubber molded product of the present invention is preferably a sheet-like material (rubber sheet) having a substantially constant thickness.

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

The rubber molded body of the present invention has high conductivity performance by orienting pitch-based carbon fibers in the thickness direction in a dense (high density) and high orientation ratio. For example, the surface resistance value is set to 10 to 10. 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 its hardness is, for example, the hardness when measured with a type E durometer tester specified in JIS K 6253 (Shore E). ) Can be adjusted within the range of 40 to 90, and Asker C specified in the Japanese Industrial Standards (SRIS) can be adjusted within the range of 40 to 90.

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

FIG. 2 is a schematic cross-sectional view showing an extruder used for molding the rubber molded product of the present invention, and shows an outline of a cross section of a tip portion of the extruder and a T-die. The rubber composition containing the rubber component, pitch-based carbon fibers 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 band. 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-shaped rubber precursor.

When the thin sheet-shaped rubber precursor in which the 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 expanded and the vertical direction is increased. Due to the increased length, the flow of the rubber precursor changes in the vertical direction. After that, when the rubber precursor passes through the second gap 14, the rubber precursor is fused while being folded in a direction substantially perpendicular to the flow direction in the first gap 12, and a sheet-shaped rubber molded body is formed. ..

Although the typical embodiments of the present invention have been described above, the present invention is not limited to these, and various design changes are possible, and such design changes are also included in the present invention.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Example 1)
For 100 parts by weight of silicone rubber (manufactured by MOMENTIVE, "TSE201") as a rubber component, 58 parts by weight of pitch-based carbon fiber (manufactured by Nippon Graphite Fiber, "XN100 (CF)") as a conductive filler and graphite 27 By weight, 3 parts by weight of peroxide (Nippon Oil Co., Ltd., "Park Mill D") as a cross-linking agent, and 20 parts by weight of silicone oil (Shin-Etsu Chemical Co., Ltd., "KF-96") as a plasticizing agent. Was kneaded with two rolls to prepare a sheet-shaped rubber composition. The volume fraction of the pitch-based carbon fibers (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 is formed into a sheet having a thickness of 10 mm by a short-screw extruder for rubber having a tip portion of a structure shown in FIG. 2 and a T-die, and further at 180 ° C. using a press vulcanizer. It was subjected to a peroxide cross-linking treatment for 10 minutes. A die (base) for vertical orientation was used for the extruder. Subsequently, the obtained rubber sheet having a thickness of 10 mm after cross-linking was sliced in the direction of the sheet surface to prepare a rubber sheet having a thickness of 1.0 mm. The hardness (shore E) when three obtained rubber sheets were laminated and measured with a type E durometer tester specified in JIS K 6253 was 40 (corresponding to 40 in ASKERC).

(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 (“S-4800” manufactured by Hitachi High-Technologies Corporation). By SEM observation, it was confirmed that the pitch-based carbon fibers were oriented in the sheet thickness direction in the rubber sheet of Example 1 (the orientation rate 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. rice field.

(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 by a punching machine to prepare a sample for measuring the electromagnetic wave shielding effect. Using the network analyzer "N5222A" of Keysight Technologies, Inc. and "GCP-7" of Keycom, the transmission attenuation in the high frequency region of 500MHz to 18GHz was measured by the coaxial tube method, and the thickness was 1. The electromagnetic wave shielding effect of the 0.0 mm sample was confirmed. Tables 1 and 3 below show the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Example 1.

(Example 2)
Similar to Example 1 except that the crosslinked rubber sheet having a thickness of 10 mm was sliced to a thickness of 0.5 mm, the pitch-based carbon fibers were oriented in the sheet thickness direction to a thickness of 0.5 mm. The rubber sheet was prepared and the electromagnetic wave shielding effect was measured. Tables 1 and 3 below show the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Example 2.

(Comparative Example 1)
When slicing a rubber sheet having a thickness of 10 mm after cross-linking, the pitch-based carbon fibers were oriented in the direction of the sheet surface in the same manner as in Example 1 except that the rubber sheet was sliced in the sheet thickness direction. The rubber sheet was prepared and the electromagnetic wave shielding effect was measured. Tables 1 and 3 below show the measurement results of the electromagnetic wave shielding effect of the rubber sheet of Comparative Example 1.

(Comparative Example 2)
Similar to Comparative Example 1 except that the crosslinked rubber sheet having a thickness of 10 mm was sliced to a thickness of 0.5 mm, the pitch-based carbon fibers had a thickness of 0.5 mm oriented in the sheet surface direction. A rubber sheet was prepared and the electromagnetic wave shielding effect was measured. Tables 1 and 3 below 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 the 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. It was confirmed that it showed the above transmission attenuation and had excellent electromagnetic wave shielding properties. On the other hand, the rubber sheets of Comparative Examples 1 and 2 in which the pitch-based carbon fibers were oriented in the direction of the sheet surface did not obtain a transmission attenuation of 30 dB or more in the entire high frequency band of 500 MHz to 18 GHz, and were found in Examples 1 and 2. It was confirmed that the electromagnetic wave shielding property was inferior to that of the rubber sheet.

(Comparative Example 3)
In the formulation of the rubber composition of Example 1, instead of 58 parts by weight of the pitch-based carbon fiber, the same amount of vapor-phase carbon fiber (manufactured by Showa Denko Co., Ltd., “VGCF-H”, fiber length 6 μm small diameter) was used. 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 appropriately dispersed by kneading, and it cannot be formed into a sheet shape.

8 Screw 10 Flow path 12 First gap 14 Second gap

Claims (1)

A rubber molded product containing silicone rubber and pitch-based carbon fibers.
The pitch-based carbon fibers are oriented in the thickness direction of the rubber molded product, and the pitch-based carbon fibers are oriented in the thickness direction of the rubber molded product.
The average length of the pitch-based carbon fibers is 50 μm or more and 1 mm or less.
The content of the pitch-based carbon fiber is 10 parts by weight or more with respect to 100 parts by weight of the silicone rubber.
The rubber molded body is 0.5mm or more thickness is at 1mm or less, and Der transmission attenuation is 30dB or more in the frequency band of 500MHz~18GHz is, that having a plurality of weld lines extending in the thickness direction A rubber molded body characterized by this.

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