Electromagnetic wave shielding powder, composition including the same, and method for manufacturing the same

Abstract

To provide a powder for electromagnetic wave shielding that has a high attenuation effect over a wide frequency range, particularly in a frequency range of GHz or higher.SOLUTION: An electromagnetic wave shielding powder contains shredded fibers made from burned bamboo rayon fibers. The shredded fibers preferably have an average fiber length of 50 μm or more and 500 μm or less. It is also preferable that the average aspect ratio, defined as the average fiber length / average fiber diameter, is 20 or more and 60 or less. It is also preferable that the shredded fibers are such that no diffraction peaks derived from graphite are observed in X-ray diffraction measurements. It is also preferable that the shredded fibers are amorphous.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic shielding powder and a method for producing the same, and also to an electromagnetic shielding composition containing the electromagnetic shielding powder. [Background technology]

[0002] Recent advances in microelectronics technology have led to the widespread use of a wide variety of electronic devices, including personal computers and mobile phones. In these environments, the impact of electromagnetic noise caused by unwanted electromagnetic waves on electronic devices has been noted. Furthermore, measures are required for in-vehicle radars and radars for speed measurement, etc., to receive only the electromagnetic waves necessary for measurement and to block unwanted electromagnetic waves. Therefore, there is a demand for devices to prevent malfunctions and breakdowns, suppress the radiation of unwanted electromagnetic waves, and provide sufficient resistance to external electromagnetic waves.

[0003] Countermeasures against electromagnetic waves can be broadly divided into methods that reflect electromagnetic waves and methods that absorb them. For example, metals are commonly used as electromagnetic shields that reflect electromagnetic waves. On the other hand, to prevent malfunctions of equipment due to electromagnetic waves generated by the equipment itself, electromagnetic wave absorbers are required rather than reflectors. Rubber or plastics with ferrite kneaded into them are the mainstream for such products.

[0004] For example, Patent Document 1 proposes an electromagnetic wave absorber for the GHz band that is made of a three-dimensional network-like carbon fiber structure made up of carbon fibers with an outer diameter of 15 to 100 nm. Patent Document 2 proposes an electromagnetic wave absorber formed by laminating two or more conductive fiber sheets, each of which is a fiber sheet substrate coated with a conductive polymer. Patent Document 3 proposes an electromagnetic wave absorber that is made of a laminate having a plurality of first layers containing pulp and carbon fiber and a second layer containing pulp but not carbon fiber, with the second layer being provided between two first layers in the laminate. Non-Patent Document 1 examines the electromagnetic wave shielding effect of bamboo charcoal, and reports that bamboo charcoal carbonized at 750°C or higher is found to have the effect of reflecting 4GHz electromagnetic waves. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-115854 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-39017 [Patent Document 3] Patent Publication No. 2021-158175 [Non-patent literature]

[0006] [Non-Patent Document 1] "Study on the Electrical Properties and Electromagnetic Wave Shielding Effect of Bamboo Charcoal," Kagoshima Prefectural Industrial Technology Center Research Report No. 12, 1998 Summary of the Invention [Problem to be solved by the invention]

[0007] With technological advances, the wavelength range of electromagnetic waves used in various fields has expanded, and there is a demand for materials that can shield with sufficient attenuation from relatively long wavelengths to short wavelengths such as millimeter waves. In particular, there is a demand for shielding materials for short wavelengths of 1 GHz or more. Therefore, an object of the present invention is to provide a powder for electromagnetic wave shielding that has a high attenuation effect over a wide frequency range, and a method for producing the same. [Means for solving the problem]

[0008] The present invention provides a powder for electromagnetic shielding containing shredded fibers made from burned bamboo rayon fibers.

[0009] The present invention also provides a method for producing the electromagnetic wave shielding powder, comprising the steps of: Bamboo rayon fiber is baked in multiple stages under vacuum at a maximum temperature of 650°C to 1400°C to obtain a baked body. The present invention also provides a method for producing an electromagnetic wave shielding powder, which comprises subjecting the sintered body to a shredding step. [Effects of the Invention]

[0010] According to the present invention, there are provided a powder for electromagnetic shielding that has a high attenuation effect over a wide frequency range, particularly in the frequency range of GHz or higher, and a method for producing the powder. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described below based on preferred embodiments thereof. In this specification, "electromagnetic wave shielding" refers to the phenomenon of absorbing or reflecting electromagnetic waves. In the following description, "electromagnetic wave shielding" refers to absorbing electromagnetic waves, reflecting electromagnetic waves, or both absorbing and reflecting electromagnetic waves, depending on the context. Furthermore, in the present invention, "powder for electromagnetic wave shielding" refers to a powder capable of absorbing or reflecting electromagnetic waves.

[0012] The electromagnetic shielding powder of the present invention is made from shredded bamboo rayon fiber (hereinafter also referred to as "burned bamboo fiber"). The use of burned bamboo fiber as a raw material in the electromagnetic shielding powder of the present invention is advantageous in that it enhances the electromagnetic wave shielding effect over a wide frequency range, while particularly enhancing the electromagnetic wave absorption effect in the GHz and higher frequency ranges. Furthermore, because bamboo rayon fiber is a naturally derived material, it also has the advantage of being environmentally friendly.

[0013] Bamboo rayon fiber is made by chemically modifying natural bamboo cellulose fibers to dissociate the hydrogen bonds between the molecules, creating a colloidal solution, which is then reconstituted into cellulose molecules, allowing the polymers to reassemble and regenerate the fibers. Examples of bamboo rayon fibers include rayon (viscose rayon), cuprammonium rayon (cupro), polynosic, lyocell (Tencel), modal, acetate, triacetate, and refined cellulose. Bamboo rayon fibers may also be regenerated cellulose fibers that have been crosslinked with a crosslinking agent.

[0014] In particular, among bamboo rayon fibers, it is preferable to use viscose rayon fibers made from bamboo by the viscose method, because they have an extremely high shielding effect against electromagnetic waves over a wide frequency range, especially an extremely high electromagnetic wave absorption effect in the frequency range of 1 GHz or higher, and because bamboo rayon fibers are easily available.

[0015] Bamboo rayon fiber is preferably produced using bamboo pulp or bamboo pulp and general wood pulp other than bamboo in the production of rayon. In order to obtain the excellent properties derived from bamboo, it is preferable that 50% by mass or more of the total cellulose raw material is derived from bamboo, more preferably 70% by mass or more, even more preferably 90% by mass or more, and most preferably 100% by mass is derived from bamboo.

[0016] The shape of the single yarn of burned bamboo fiber can be confirmed using a standard scanning electron microscope. The cross-sectional shape of the single yarn can be selected from various shapes depending on the shape of the spinneret, and may be, for example, circular, flat, or sawtooth.

[0017] There is no particular limitation on the type of bamboo used as the raw material for bamboo rayon fiber, and examples that can be used include Madake, Moso Bamboo, Awatake, and Metake Bamboo. These various types of bamboo may be used alone or in combination of two or more types.

[0018] Bamboo burned fiber is preferably produced by burning bamboo rayon fiber in multiple stages at a temperature of 650°C or higher and 1400°C or lower under vacuum. One of the features of the present invention is that bamboo rayon fiber is sintered under vacuum conditions. Sintering under vacuum conditions makes it easy to obtain an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly in the GHz frequency range or higher. From this perspective, the degree of vacuum during sintering is preferably within the range of -0.001 kPaG to -85 kPaG in gauge pressure.

[0019] The bamboo rayon fiber may be burned as it is in the form of fiber, or may be burned after being processed into a fabric. The fabric may be either a woven fabric or a knitted fabric. After placing the bamboo rayon fiber in a baking furnace, the pressure inside the baking furnace is reduced. After the baking furnace reaches a predetermined vacuum level, the baking furnace is heated. The maximum baking temperature is preferably 650°C or higher and 1400°C or lower, and particularly 700°C or higher and 1300°C or lower. The time from the start of heating (usually room temperature) to the target maximum temperature is preferably 200 minutes or higher, particularly 300 minutes or higher. Furthermore, the time from the start of heating to the target baking temperature is preferably 1000 minutes or less, particularly 800 minutes or less.

[0020] It is preferable to carry out the heating in two or more stages from the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly a high electromagnetic wave absorption effect in a frequency range of GHz or higher. The multi-stage firing includes at least two stages: a first firing and a second firing. It is preferable not to perform a temperature lowering operation during the multi-stage firing in order to successfully obtain an electromagnetic wave shielding powder having the desired electromagnetic wave shielding effect.

[0021] In the first baking step of the multi-stage baking process, it is preferable to raise the temperature of the bamboo rayon fiber at a slow rate. Specifically, the first heating rate from room temperature to the first baking temperature is preferably 0.4°C / min or more, and more preferably 0.5°C / min or more. Furthermore, it is preferable that the first heating rate is 3.0°C / min or less, and more preferably 2.0°C / min or less. The temperature increase in the first baking may be linear with the passage of time, may be stepwise, or may be along a predetermined curve.

[0022] From the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly an electromagnetic wave shielding powder that has a high electromagnetic wave absorption effect in a frequency range of GHz or higher, the first firing temperature is preferably set to 300°C or higher and 600°C or lower, and more preferably set to 350°C or higher and 500°C or lower.

[0023] Once the first firing temperature is reached, it is preferable to maintain that temperature for 60 minutes or more and 180 minutes or less, from the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly a high electromagnetic wave absorption effect in the frequency range of GHz or more. To further enhance this advantage, once the first firing temperature is reached, it is even more preferable to maintain that temperature for 80 minutes or more and 150 minutes or less.

[0024] After the first firing is completed, the firing furnace is further heated to a second firing temperature, that is, a target temperature (hereinafter also referred to as "target temperature"), in which the temperature is increased from the first firing temperature to the second firing temperature. In the second firing, the second heating rate is preferably 0.15°C / min or more, and more preferably 0.2°C / min or more. Furthermore, the second heating rate is preferably 7.0°C / min or less, and more preferably 6.0°C / min or less. The temperature increase in the second baking may be linear with the passage of time, may be stepwise, or may be along a predetermined curve.

[0025] From the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly an electromagnetic wave shielding powder that has a high electromagnetic wave absorption effect in a frequency range of GHz or higher, the target temperature is preferably set to 650°C or higher and 1400°C or lower, more preferably 1100°C or higher and 1400°C or lower, and even more preferably 1100°C or higher and 1300°C or lower.

[0026] Once the target temperature is reached, it is preferable to maintain that temperature for 60 minutes or more and 200 minutes or less, from the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly a high electromagnetic wave absorption effect in a frequency range of GHz or more. To further enhance this advantage, once the target temperature is reached, it is even more preferable to maintain that temperature for 70 minutes or more and 190 minutes or less.

[0027] The degree of vacuum in the firing furnace may be the same or different between the first firing and the second firing. From the perspective of successfully obtaining an electromagnetic shielding material that has a high electromagnetic wave shielding effect over a wide frequency range, particularly a high electromagnetic wave absorption effect in the frequency range of GHz or higher, it is preferable that the degree of vacuum in the second firing be higher than that in the first firing.

[0028] After the second firing is completed, the firing furnace is cooled. Cooling may be natural or forced. The cooling rate is preferably 2.0 to 15°C / min, particularly 2.8 to 10°C / min, from the viewpoint of successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly a high electromagnetic wave absorption effect over a frequency range of GHz or higher.

[0029] When baking bamboo rayon fiber, an intermediate baking may be performed between the first and second baking steps. By performing the intermediate baking, it is possible to more successfully obtain an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, especially in the GHz frequency range or higher.

[0030] In the intermediate firing, the intermediate temperature rise rate from the first firing temperature to the intermediate firing temperature is preferably 0.4°C / min or more, particularly 0.5°C / min or more, and is preferably 20°C / min or less, particularly 10°C / min or less. The temperature increase in the intermediate baking may be linear with the passage of time, may be stepwise, or may be along a predetermined curve.

[0031] From the viewpoint of more successfully obtaining an electromagnetic shielding powder that has a high electromagnetic wave shielding effect over a wide frequency range, particularly an electromagnetic wave shielding powder that has a high electromagnetic wave absorption effect in a frequency range of GHz or higher, the intermediate firing temperature is preferably set to 650°C or higher and 1050°C or lower, and more preferably set to 700°C or higher and 1000°C or lower.

[0032] Once the burned bamboo fiber is obtained in this way, it is shredded into powder. As a result of the inventor's research, it was unexpectedly discovered that using shredded burned bamboo fiber as an electromagnetic wave shielding material has a higher electromagnetic wave absorption effect than using the burned bamboo fiber itself. The reason for this is unclear, but the inventor believes as follows. The higher the temperature at which burned bamboo fiber is fired, the more the carbon crystallizes, resulting in a higher electromagnetic wave reflection effect. Meanwhile, we believe that the loss of the amorphous state observed in medium-temperature firing significantly reduces the absorption of electromagnetic waves in the 2.4 GHz band. In contrast, in the present invention, we believe that the shredding process exposes the interior of the burned bamboo fiber, which is in an amorphous state where the carbon is not fully crystallized, and that shredding also creates a fine structure on the surface, thereby enhancing the absorption of electromagnetic waves. Various grinders can be used for shredding. The shredded burned bamboo fiber, i.e., the shredded material, is preferably in the form of fine fibers with an average fiber length of preferably 50 μm or more, from the viewpoint of enhancing the electromagnetic wave shielding effect. From this viewpoint, the average fiber length of the shredded burned bamboo fiber is more preferably 100 μm or more, and even more preferably 200 μm or more. From the same viewpoint, the average fiber length is preferably 500 μm or less, more preferably 450 μm or less, and even more preferably 400 μm or less. The average fiber length of shredded bamboo fiber is measured by the following method. The average fiber length of shredded burned bamboo fiber can be measured by observing the fibers using Keyence Corporation's VHX digital microscope at a magnification of 20x to 2000x. Ten samples are measured, and the maximum and minimum values ​​are excluded. The average value is used as the average fiber length of shredded burned bamboo fiber. Furthermore, there is no substantial change in the average fiber length of shredded burned bamboo fiber before and after shredding.

[0033] Furthermore, from the viewpoint of enhancing the electromagnetic wave shielding effect, the average diameter of the cross-section of the shredded bamboo burned fiber, i.e., the average fiber diameter, is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more. From the same viewpoint, the average fiber diameter is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. The fiber diameter can be measured by observing the fibers using Keyence Corporation's VHX digital microscope at magnifications of 20x to 2000x. Ten samples are measured, and the maximum and minimum values ​​are excluded. The average value is used as the diameter of the shredded bamboo fiber. Furthermore, there is no substantial change in the average fiber diameter before and after shredding the bamboo fiber.

[0034] Furthermore, from the viewpoint of enhancing the electromagnetic wave shielding effect, the average aspect ratio of the shredded bamboo burned fiber, defined as the average fiber length / the average fiber diameter, is preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more, provided that the average fiber length and average fiber diameter are within the above-mentioned ranges. From the same viewpoint, the average aspect ratio is preferably 60 or less, more preferably 50 or less, and even more preferably 40 or less.

[0035] The electromagnetic shielding powder of the present invention, made from shredded bamboo fiber, is preferably an aggregate of shredded pieces of various lengths, from the viewpoint of enhancing the electromagnetic shielding effect. Specifically, when a graph of fiber length distribution is created with the length of the shredded pieces on the horizontal axis and the frequency of shredded pieces at that length on the vertical axis, it is preferable that the fiber length peaks in the range of 50 μm to 500 μm, particularly in the range of 100 μm to 450 μm, and especially in the range of 200 μm to 400 μm. Shredded bamboo fiber with such a fiber length distribution can be obtained by appropriately setting the shredding conditions for the bamboo fiber.

[0036] When shredded bamboo fiber is subjected to X-ray diffraction analysis, it is preferable that no diffraction peaks derived from graphite are observed, from the viewpoint of enhancing the electromagnetic wave shielding effect. In particular, when the shredded bamboo fiber is subjected to X-ray diffraction analysis, it is preferable that no diffraction peaks are observed, from the viewpoint of further enhancing the electromagnetic wave shielding effect. In other words, it is preferable that the shredded bamboo fiber is amorphous.

[0037] The electromagnetic shielding powder of the present invention may be composed solely of the shredded burned bamboo fiber described above, or may be composed of other materials in addition to the shredded burned bamboo fiber. Examples of other materials include pulverized burned bamboo. The inclusion of pulverized burned bamboo in addition to the shredded burned bamboo fiber in the electromagnetic shielding powder of the present invention has the advantage of further enhancing the electromagnetic shielding effect.

[0038] Bamboo burned bodies are obtained by cutting felled bamboo into chips as needed, then burning and carbonizing them. Prior to burning, the bamboo may be subjected to wet de-oiling in hot water. There are no particular restrictions on the type of bamboo.

[0039] Bamboo is preferably fired in a vacuum at a temperature of 650°C or higher and 1400°C or lower, from the viewpoint of obtaining a pulverized fired bamboo product that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder. From the same viewpoint as above, the degree of vacuum during firing is preferably set within the range of -0.001 kPaG to -85 kPaG in gauge pressure.

[0040] After placing the bamboo in the kiln, the pressure inside the kiln is reduced. After the specified vacuum level is reached, the kiln is heated. The maximum temperature reached during firing is preferably between 650°C and 1400°C, and particularly between 700°C and 1300°C. The time from the start of heating (usually room temperature) to the target maximum temperature is preferably 200 minutes or more, and particularly 300 minutes or more. Furthermore, the time from the start of heating to the target maximum temperature is preferably 1000 minutes or less, and particularly 800 minutes or less.

[0041] Heating in two or more stages is preferable from the viewpoint of obtaining a pulverized bamboo product that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder. Multi-stage firing includes at least two stages: a first firing and a second firing. It is preferable not to perform a temperature lowering operation during the multi-stage firing in order to successfully obtain an electromagnetic wave shielding powder having the desired electromagnetic wave shielding effect.

[0042] In the first firing step of the multi-stage firing, it is preferable to raise the temperature of the bamboo at a slow rate. Specifically, the first heating rate from room temperature to the first firing temperature is preferably 0.4°C / min or more, and more preferably 0.5°C / min or more. Furthermore, it is preferable that the first heating rate is 3.0°C / min or less, and more preferably 2.0°C / min or less. The temperature increase in the first baking may be linear with the passage of time, may be stepwise, or may be along a predetermined curve.

[0043] From the viewpoint of obtaining pulverized bamboo burned body that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder, the first firing temperature is preferably set to 300°C or higher and 600°C or lower, and more preferably set to 350°C or higher and 500°C or lower.

[0044] Once the first firing temperature is reached, it is preferable to maintain that temperature for 60 to 180 minutes in order to obtain a pulverized bamboo product that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder.To further enhance this advantage, it is even more preferable to maintain the first firing temperature for 80 to 150 minutes in order to further enhance the electromagnetic wave shielding effect of the EMI shielding powder.

[0045] After the first firing is completed, the firing furnace is further heated to a second firing temperature, that is, a target temperature (hereinafter also referred to as "target temperature"), in which the temperature is increased from the first firing temperature to the second firing temperature. In the second firing, the second heating rate is preferably 0.15°C / min or more, and more preferably 0.2°C / min or more. Furthermore, the second heating rate is preferably 7.0°C / min or less, and more preferably 6.0°C / min or less. The temperature increase in the second baking may be linear with the passage of time, may be stepwise, or may be along a predetermined curve.

[0046] From the viewpoint of obtaining pulverized bamboo burned body which can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder, the target temperature is preferably set to 650°C or higher and 1400°C or lower, more preferably 1100°C or higher and 1400°C or lower, and even more preferably 1100°C or higher and 1300°C or lower.

[0047] Once the target temperature is reached, it is preferable to maintain that temperature for 60 to 200 minutes in order to obtain a pulverized bamboo product that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder.To further enhance this advantage, once the target temperature is reached, it is even more preferable to maintain that temperature for 70 to 190 minutes in order to further enhance this advantage.

[0048] The degree of vacuum in the firing furnace for the first firing and the second firing may be the same or different. From the viewpoint of obtaining a pulverized bamboo product that can further enhance the electromagnetic wave shielding effect of the electromagnetic wave shielding powder, it is preferable that the degree of vacuum in the second firing is higher than that in the first firing.

[0049] Once the second firing is complete, the firing furnace is cooled. Cooling can be natural or forced. The cooling rate during cooling is preferably 2.0 to 15°C / min, and more preferably 2.8 to 10°C / min, from the perspective of obtaining a pulverized fired bamboo product that can further enhance the shielding effect of the electromagnetic wave shielding powder.

[0050] Once the burned bamboo body is obtained in this way, it is crushed to produce a crushed product. There are no particular restrictions on the crushing equipment, and the same equipment as that used for crushing the burned bamboo fiber described above can be used.

[0051] The particle size of the burned bamboo pulverized in this manner is preferably as small as possible to further enhance the electromagnetic wave shielding effect. Specifically, the particle size of the burned bamboo pulverized material is preferably 1000 μm or less, more preferably 500 μm or less, and even more preferably 300 μm or less. The lower limit of the particle size can be, for example, 5 μm or more, or 20 μm or more, or even 50 μm or more. The particle size of the crushed burned bamboo can be measured by observing the fibers using Keyence Corporation's VHX digital microscope at a magnification of 20x to 2000x. Measurements are taken for 10 samples, and the maximum and minimum values ​​are excluded, and the average value is used as the particle size of the crushed burned bamboo.

[0052] When the pulverized burned bamboo material is subjected to X-ray diffraction analysis, it is preferable that no diffraction peaks derived from graphite are observed, from the viewpoint of enhancing the electromagnetic wave shielding effect. In particular, when the pulverized material is subjected to X-ray diffraction analysis, it is preferable that no diffraction peaks are observed, from the viewpoint of further enhancing the electromagnetic wave shielding effect. In other words, it is preferable that the pulverized material is amorphous.

[0053] When the electromagnetic wave shielding powder of the present invention contains shredded bamboo burned fiber and crushed burned bamboo, the ratio of shredded bamboo burned fiber to the electromagnetic wave shielding powder is preferably 5% by mass or more and 45% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 15% by mass or more and 35% by mass or less. On the other hand, the ratio of crushed burned bamboo material to the electromagnetic wave shielding powder is preferably 55% by mass or more and 95% by mass or less, more preferably 60% by mass or more and 90% by mass or less, and even more preferably 65% ​​by mass or more and 85% by mass or less.

[0054] The electromagnetic shielding powder of the present invention may contain other powder components in addition to shredded burned bamboo fiber and pulverized burned bamboo, as needed, although from the perspective of enhancing the electromagnetic shielding effect, it is preferable that the electromagnetic shielding powder is composed only of shredded burned bamboo fiber and pulverized burned bamboo, as needed.

[0055] The electromagnetic shielding powder of the present invention can be used as it is, or it can be used as an electromagnetic shielding composition obtained by mixing the electromagnetic shielding powder of the present invention with other components. When the electromagnetic shielding powder of the present invention is used as a composition, the composition can be used, for example, as a paint, a three-dimensional resin molded product, or a resin film. For example, by applying a paint containing the electromagnetic shielding powder of the present invention to the exterior surfaces of vehicles and aircraft, the electromagnetic shielding effect can be imparted to these. Alternatively, by attaching a resin film containing the electromagnetic shielding powder of the present invention to the exterior surfaces of vehicles and aircraft, the electromagnetic shielding effect can also be imparted to these. Furthermore, by using a three-dimensional resin molded product containing the electromagnetic shielding powder of the present invention as various electronic devices and housings for electronic devices, the electromagnetic shielding effect can be imparted to these devices while reducing their weight.

[0056] Although the present invention has been described above based on the preferred embodiments thereof, it goes without saying that the present invention is not limited to the above embodiments. [Example]

[0057] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "parts" means "parts by mass."

[0058] Example 1 The bamboo rayon fiber (obtained by the viscose rayon method and containing at least 90% by mass of bamboo as the cellulose raw material) was made by twisting two 80 count single yarns (indicated by British cotton count). Several of these yarns were bundled together to make a yarn bundle with a diameter of 5.8 μm, which was then twisted to make a twisted yarn with a diameter of approximately 150 μm, and used as a plain weave fabric. Using a baking oven, the fabric was subjected to a three-stage baking process consisting of a first baking, an intermediate baking, and a second baking under vacuum. In the first firing, the temperature was increased from room temperature (25°C) to the first firing temperature (300°C) at a rate of 2°C / min and maintained at the first firing temperature for 120 minutes. The degree of vacuum in the firing furnace was set to -0.01 kPaG. During the intermediate firing, the temperature was increased from the first firing temperature (300°C) to the intermediate firing temperature (800°C) at a rate of 10°C / min, and the intermediate firing temperature was maintained for 120 minutes. The degree of vacuum in the firing furnace was set to -0.01 kPaG. In the second firing, the temperature was increased from the intermediate firing temperature (800°C) to the second firing temperature (1200°C) at a rate of 4°C / min and maintained at the second firing temperature for 150 minutes. The degree of vacuum in the firing furnace was set to -0.01 kPaG. After the second baking was completed, the inside of the baking furnace was cooled at a temperature decreasing rate of 10°C / min to room temperature, and a baked fabric was obtained. The resulting burned cloth was shredded using a grinder (Super Rotary Cutter, Yoshiko Co., Ltd.), and the shredded material was sieved through a 1 mm mesh, and the sieved material was collected. The shredded material thus obtained had an average fiber length of 246 μm, an average fiber diameter of 7 μm, and an average aspect ratio of 35. The fiber length peak was 334 μm. X-ray diffraction analysis of this shredded material revealed no diffraction peaks attributable to graphite, and it was confirmed to be amorphous. This shredded bamboo burned fiber was used as the electromagnetic wave shielding powder of this example.

[0059] Example 2 The green bamboo was crushed into chips and then fired in the following manner. The chips were subjected to a three-stage baking process under vacuum using a baking oven. First, the temperature was increased from room temperature (25°C) to 300°C at a rate of 2°C / min, and the first firing temperature was maintained for 120 minutes. The degree of vacuum in the firing furnace was set to -0.01 kPaG. Next, the temperature was increased from 300° C. to 800° C. at a rate of 10° C. / min, and held at 800° C. for 120 minutes. The degree of vacuum in the firing furnace was set to −0.01 kPaG. Finally, the temperature was increased from 800°C to 1200°C at a rate of 4°C / min and held at 1200°C for 150 minutes. The degree of vacuum in the firing furnace was set to -0.01 kPaG. After the firing was completed, the inside of the firing furnace was cooled at a temperature drop rate of 10°C / min to room temperature, and a fired bamboo body was obtained. The obtained sintered body was pulverized using a pulverizer (ANJIZERUN, 2500G powder mill), and the pulverized material was sieved in two stages. In the first stage, a 1 mm mesh sieve was used, and the pulverized material that passed through the sieve was collected. In the second stage, a 0.5 mm mesh sieve was used, and the pulverized material that passed through the sieve was collected. The average particle size of the pulverized material thus obtained was 246 μm. As a result of X-ray diffraction measurement of this pulverized material, no diffraction peaks derived from graphite were observed, and it was confirmed that it was amorphous. Three parts of the crushed burned bamboo material obtained in this manner were mixed with one part of shredded burned bamboo fiber obtained in Example 1 to produce the electromagnetic wave shielding powder of this example.

[0060] Example 3 Because green bamboo contains a lot of oil, potassium, and sodium, it is easy to imagine that it would place a load on the kiln during the firing process. Therefore, chips made from bamboo that had been wet-deoiled using hot water treatment were fired under the same conditions as in Example 2 to obtain a pulverized product derived from deoiled bamboo. The electromagnetic shielding powder of this example was obtained in the same manner as in Example 2, except for this.

[0061] [Rating 1] The electromagnetic wave shielding effect of the electromagnetic wave shielding powder obtained in each example was evaluated by the following method. 5 mL of conductive paint was applied to an 11 cm square polypropylene plate, and the electromagnetic shielding powder obtained in each example was sprinkled on top of it. The amount sprinkled was 1 g in Example 1, and 4 g in Examples 2 and 3. The solvent contained in the conductive paint was dried and removed at 60°C, and the electromagnetic shielding powder was fixed. A polypropylene plate identical to the polypropylene plate was placed on the surface of the polypropylene plate on which the electromagnetic wave shielding powder had been sprinkled, to prepare a measurement sample. The measurement sample was fixed in a cassette in an electromagnetic wave anechoic box, and the electromagnetic wave shielding effect was evaluated. For the measurements, an anechoic box consisting of an inner box and an outer box, both made of aluminum, was used. A horn antenna installed in the inner box irradiated the sample with 2.4 GHz electromagnetic waves, and the amount of electromagnetic waves that passed through the sample was measured using a log-periodic antenna. The walls of both the inner and outer boxes were covered with electromagnetic absorbing material to suppress diffuse reflection of the electromagnetic waves from the walls. The results are shown in Table 1 below. For comparison, a measurement sample was also prepared without the electromagnetic shielding powder being sprinkled thereon, and this was designated Comparative Example 1.

[0062] [Table 1]

[0063] As is clear from the results shown in Table 1, the electromagnetic shielding powders of the examples have a high electromagnetic shielding effect in the frequency range of GHz or higher.

[0064] [Rating 2] The electromagnetic wave absorbing properties of the electromagnetic wave shielding powder obtained in each example were evaluated by the following method. 10 mL of conductive paint was applied to a 22 cm square steel plate, and the electromagnetic shielding powder obtained in each example was sprinkled on top of it. The amount sprinkled was 4 g in Example 1, and 16 g in Examples 2 and 3. The solvent contained in the conductive paint was dried and removed at 60°C, and the electromagnetic shielding powder was fixed to prepare a measurement sample. The measurement sample was fixed in a cassette in an electromagnetic wave anechoic box, and the electromagnetic wave absorption effect was evaluated. The measurement was performed using the system described in Evaluation 1 above. 2.4 GHz electromagnetic waves were irradiated toward the sample at a 45-degree angle from a horn antenna installed on the outer box, and the amount of electromagnetic waves reflected by the sample was measured using a log-periodic antenna installed at a 90-degree angle from the horn antenna. In this way, the electromagnetic wave absorption by the electromagnetic wave shielding powder during the process of total reflection of the electromagnetic waves was evaluated. The results are shown in Table 2 below. For comparison, a measurement sample was also prepared without the electromagnetic shielding powder being sprinkled thereon, and this was designated Comparative Example 2. Furthermore, for comparison, the sintered cloth (basis weight: 54.8 g / m) obtained in the process of producing the electromagnetic wave shielding powder in Example 1 was 2 ) was also evaluated for electromagnetic wave absorption. This was designated Comparative Example 3.

[0065] [Table 2]

[0066] As is clear from the results shown in Tables 1 and 2, the electromagnetic wave shielding powders of the examples have electromagnetic wave absorption properties in the frequency range of GHz or higher. Furthermore, as is clear from the comparison between Example 1 and Comparative Example 3, burned bamboo fiber itself does not exhibit electromagnetic wave absorption properties, but when the burned bamboo fiber is processed into shredded material, electromagnetic wave absorption properties are exhibited.

Claims

1. An electromagnetic wave shielding powder containing shredded fibers made from burned bamboo rayon fibers.

2. 2. The electromagnetic shielding powder according to claim 1, wherein the shredded material has an average fiber length of 50 μm or more and 500 μm or less.

3. 3. The electromagnetic shielding powder according to claim 2, wherein an average aspect ratio defined by the average fiber length / average fiber diameter is 20 or more and 60 or less.

4. 3. The electromagnetic shielding powder according to claim 1, wherein the shredded material does not exhibit any diffraction peaks derived from graphite in X-ray diffraction measurement.

5. 3. The electromagnetic shielding powder according to claim 1, wherein the shredded material is amorphous.

6. 3. The electromagnetic shielding powder according to claim 1, further comprising a pulverized burned bamboo material.

7. 7. The electromagnetic shielding powder according to claim 6, wherein the average particle size of the pulverized product is 50 μm or more and 2000 μm or less.

8. An electromagnetic wave shielding composition comprising the electromagnetic wave shielding powder according to claim 1.

9. The electromagnetic wave shielding composition according to claim 6, which is a paint.

10. The electromagnetic wave shielding composition according to claim 8, which is a three-dimensional resin molded product.

11. The electromagnetic wave shielding composition according to claim 8 , which is a resin film.

12. A method for producing the electromagnetic shielding powder according to claim 1, comprising: Bamboo rayon fiber is calcined in multiple stages under vacuum at a maximum temperature of 650°C or higher and 1400°C or lower to obtain a calcined body; The method for producing an electromagnetic wave shielding powder comprises subjecting the sintered body to a shredding step.

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