Radio wave absorber and method for manufacturing the same

Abstract

To provide a radio wave absorber which can exhibit high radio wave absorption performance while maintaining a stable absorption layer even in a case where a finished article is processed later, and a manufacturing method thereof.SOLUTION: A radio wave absorber has a paper absorption layer 20 containing insulating fibers 10, a binder resin 12, and pitch-based carbon fibers 11 having an average fiber length of 0.3 mm or more and 2.0 mm or less, wherein a percentage content of the pitch-based carbon fibers 11 to the insulating fibers 10 is 5 mass% or more and 30 mass% or less, volume resistivity thereof is 1×109 Ω cm or more and 1×1012 Ω cm or less, and the radio wave absorber absorbs a radio wave of a frequency of a predetermined frequency region within a range of 26.5 GHz or more and 90 GHz or less.SELECTED DRAWING: Figure 1

Description

[Technical Field] 【0001】 The present invention relates to a radio wave absorber having excellent absorption performance over a wide frequency band, and to a method for manufacturing the same. [Background technology] 【0002】 Radio wave absorbers are primarily used to absorb radio waves in the millimeter wave and sub-millimeter wave regions, which have wavelengths of approximately 1 to 10 mm and frequencies of 30 to 300 GHz. These radio wave absorbers are used, for example, in wireless communication systems that utilize radio waves to prevent system malfunctions caused by radio wave intrusion from the external environment. 【0003】 As a conventional radio wave absorber, Patent Document 1 discloses a radio wave absorber having a paper-based absorption layer made by wet-processing conductive carbon filler and insulating filler containing fine fibrous cellulose or cellulose nanofibers and fibers, which has a radio wave absorption effect even when thin. [Prior art documents] [Patent Documents] 【0004】 [Patent Document 1] International Publication No. 2019 / 130627 [Overview of the project] [Problems that the invention aims to solve] 【0005】 In the technology disclosed in Patent Document 1, carbon black (primary particle size: 36 nm to 40 nm) as a carbon filler is fixed to fine fibers as an insulating filler using a cationic fixing agent such as aluminum sulfate. However, because the carbon black is minute, it has the problem of easily detaching from the insulating filler. In particular, when processing the radio wave absorber by cutting or other means, the carbon black is prone to detachment. In radio wave absorbers where the carbon black is easily detached in this way, it is not possible to stabilize the radio wave absorption performance, and there is room for improvement. 【0006】 The present invention has been made in view of the above-mentioned problems, and its objective is to provide a radio wave absorber that can exhibit high radio wave absorption performance while maintaining a stable absorption layer even when the finished product is processed afterward, and a method for manufacturing the same. [Means for solving the problem] 【0007】 The characteristic configuration of the radio wave absorber for achieving the above objective is as follows: The absorbent layer is made of paper and contains insulating fibers, a binder resin, and pitch-based carbon fibers having an average fiber length of 0.3 mm to 2.0 mm. The pitch-based carbon fibers have a content of 5% to 30% by mass relative to the insulating fibers, and the volume resistivity is 1 × 10⁻⁶. 9 Ω cm or more 1×10 12 The key features are that it has a capacitance of Ω·cm or less and absorbs radio waves in a predetermined frequency range between 26.5 GHz and 90 GHz. 【0008】 The aforementioned radio wave absorber has a paper-based absorption layer composed of insulating fibers and binder resin, as well as pitch-based carbon fibers with a relatively large fiber diameter and a bent shape. As a result, these pitch-based carbon fibers easily intertwine with the insulating fibers and are less likely to fall off. This allows for the maintenance of a stable absorption layer even when the radio wave absorber is cut or otherwise processed. Furthermore, the inventors have set the average fiber length of the pitch-based carbon fibers to be 0.3 mm or more and 2.0 mm or less, the pitch-based carbon fiber content to the pitch-based carbon fibers and insulating fibers to be 5% by mass or more and 30% by mass or less, and the volume resistivity to be 1 × 10⁻⁶ 9 Ω cm or more 1×10 12By setting it to less than Ω·cm, as shown in the examples described later, in a predetermined frequency range within the range of 26.5 GHz or more and 90 GHz or less, particularly in a frequency band of about 27 GHz or more and 28 GHz or less used in the fifth-generation mobile communication system (hereinafter sometimes abbreviated as 5G), and in a frequency band of about 76 GHz or more and 80 GHz or less used in automotive radar, it has been experimentally confirmed that high absorption performance (maximum) of about -20 dB or less can be exhibited. From the above, even when the finished product is processed later, a radio wave absorber that can maintain a stable absorption layer and exhibit high radio wave absorption performance can be realized. 【0009】 A further characteristic configuration of the radio wave absorber is that the carbon content of the pitch-based carbon fiber is 90% by mass or more. 【0010】 As in the above characteristic configuration, by setting the carbon content of the pitch-based carbon fiber to 90% by mass or more, appropriate conductivity can be imparted to the absorption layer. In the absorption layer, after converting radio wave energy into a minute current, it is converted into heat energy, and the radio wave can be effectively absorbed. 【0011】 A further characteristic configuration of the radio wave absorber is that the pitch-based carbon fiber is isotropic. 【0012】 As in the above characteristic configuration, by using an isotropic pitch-based carbon fiber, the volume resistivity can be increased compared to the case of using anisotropic carbon fiber or PAN-based carbon fiber, so the performance difference caused by a slight difference in the blending amount can be reduced. In addition, since the skin irritation is small, the influence on the human body during the processing of the radio wave absorber can be suppressed. 【0013】 A further characteristic configuration of the radio wave absorber is that the insulating fiber is selected from at least one or more of nylon fiber, polyester fiber, acrylic fiber, and pulp. 【0014】 In the manufacture of a radio wave absorber having an absorption layer according to the present invention, since the insulating fibers are dissolved and mixed with materials such as insulating fibers in water and then papermaking is performed, the insulating fibers need to be water-soluble. As described in the above characteristic configuration, by making the insulating fiber be selected from at least one or more of nylon fiber, polyester fiber, acrylic fiber, and pulp, the insulating fiber can be made more compatible with water in the manufacturing process. 【0015】 A further characteristic configuration of the radio wave absorber is that the binder resin is selected from at least one or more of polyvinyl alcohol, acrylic resin, and polyacrylamide. 【0016】 As described in the above characteristic configuration, by making the binder resin be selected from at least one or more of polyvinyl alcohol, acrylic resin, and polyacrylamide, a radio wave absorber having a certain degree of flexibility can be realized after drying in papermaking. 【0017】 A further characteristic configuration of the radio wave absorber is that the thickness is 0.3 mm or more and 1.0 mm or less. 【0018】 Conventionally, in a conductive sheet-like radio wave absorber using a conductive material such as metal fiber, many of them have a thickness set to 1 / 4 of the wavelength λ of the radio wave. The radio wave absorber is called a "λ / 4 type" and has a reflection layer arranged on the back side of the absorption layer. Thereby, the radio wave reflected on the surface of the absorption layer and the radio wave that passed through the absorption layer and was reflected by the reflection layer and then passed through the absorption layer are shifted in phase by 180° and interfered with each other to cancel each other out, reducing the radio wave. However, in the conventional radio wave absorber that reduces radio waves by such a mechanism, since the thickness of the absorption layer cannot be made less than λ / 4, it has been difficult to make it thinner. On the other hand, in the radio wave absorber according to the present invention described so far, since the radio wave is absorbed and reduced by the absorption layer without forming a reflection layer, the thickness can be made 0.3 mm or more and 1.0 mm or less, and it can be made thinner. Particularly, in an example of the frequency within the frequency band of the absorption target of the radio wave absorber according to the present invention, the length of λ / 4 is as shown in [Table 1] below. 【0019】 【Table 1】 【0020】 A further characteristic configuration of the radio wave absorber is that the specific resistance of the pitch-based carbon fiber is 1.0×10 2 Ω·cm or less. 【0021】 As described in the above characteristic configuration, by setting the specific resistance of the pitch-based carbon fiber to 1.0×10 2 Ω·cm or less, radio wave energy can be converted into a minute current and further into thermal energy, thereby enabling good attenuation of radio waves, that is, absorption of radio waves. 【0022】 A further characteristic configuration of the radio wave absorber is that the specific resistance of the insulating fiber is 1.0×10 8 Ω·cm or more. 【0023】 When the specific resistance of the insulating fiber is less than 1×10 8 Ω·cm, when making a paper-based radio wave absorber, the electrical conductivity of the radio wave absorber cannot be made appropriate, and radio waves are reflected on the surface of the absorption layer. As a result, there is a risk that the radio wave absorption performance cannot be exhibited. As described in the above characteristic configuration, by setting the specific resistance of the insulating fiber to 1.0×10 8 Ω·cm or more, the electrical conductivity of the radio wave absorber is made appropriate, reflection of radio waves on the surface of the absorption layer is suppressed, and the radio wave absorption performance can be exhibited well. 【0024】 A method for manufacturing a radio wave absorber for achieving the above object is The method includes a step of dissolving insulating fibers, a binder resin, and pitch-based carbon fibers having an average fiber length of 0.3 mm to 2.0 mm in water, with the pitch-based carbon fibers being present in a ratio of 5% to 30% by mass relative to the pitch-based carbon fibers and insulating fibers, stirring the resulting mixture, and then wet-processing and drying the mixture. This manufacturing method makes it possible to produce radio wave absorbers that exhibit the unique effects described above in good quality. [Brief explanation of the drawing] 【0025】 [Figure 1] This is a schematic diagram of a radio wave absorber according to an embodiment of the present invention. [Figure 2] This graph shows the radio wave absorption performance of the radio wave absorbers in the 26.0 to 40.0 GHz frequency band in the examples and comparative examples. [Figure 3] This graph shows the radio wave absorption performance of the radio wave absorbers in the 40.0 to 60.0 GHz frequency band in the examples and comparative examples. [Figure 4] This graph shows the radio wave absorption performance of the radio wave absorbers in the 60.0 to 90.0 GHz frequency band in the examples and comparative examples. [Modes for carrying out the invention] 【0026】 The present invention relates to a radio wave absorber and a method for manufacturing the same that can exhibit high radio wave absorption performance while maintaining a stable absorption layer even when the finished product is processed afterward. The following describes the radio wave absorber and its manufacturing method based on the drawings. 【0027】 As shown in Figure 1, the radio wave absorber 100 according to the embodiment has a paper absorption layer 20 formed in a sheet shape, and is configured to absorb radio waves of a specific wavelength, which will be described later. The absorption layer 20 is composed of insulating fibers 10, a binder resin 12, and pitch-based carbon fibers 11, and has an appropriate electrical resistance, thereby converting the radio wave energy of incident radio waves into a small current and then into thermal energy for absorption. 【0028】 [Radio wave absorber 100 (absorption layer 20)] The radio wave absorber 100 having the absorption layer 20 has a volume resistivity of 1 × 10 9 Ω cm or more 1×10 12 Ω·cm or less, more preferably 1 × 10⁻¹⁰ 10 Ω cm or more 1×10 12 It has a density of Ω·cm or less and absorbs radio waves in a predetermined frequency range between 26.5 GHz and 90 GHz. Furthermore, its thickness is 0.3 mm to 1.0 mm, more preferably 0.5 mm to 0.8 mm, and is adjusted according to the required absorption performance. 【0029】 [Pitch-based carbon fiber 11] The pitch-based carbon fibers 11 constituting the absorption layer 20 can preferably be isotropic, and as shown in Figure 1, they have a bent shape along the longitudinal direction and are relatively large in diameter, and have the characteristic of easily intertwining with the insulating fibers 10 and being difficult to detach. The pitch-based carbon fiber 11 has a single fiber thickness of 5 μm to 30 μm, preferably 8 μm to 20 μm, more preferably 10 μm to 15 μm, an average fiber length of 0.3 mm to 2.0 mm, more preferably 0.3 mm to 1.5 mm, a carbon content of 90% by mass or more, more preferably 93% by mass or more, and a resistivity of 1.0 × 10⁻¹⁶ 2 It is less than or equal to Ω·cm. 【0030】 [Insulating fiber 10] As the insulating fiber 10, a water-soluble material is preferably used, and it is preferable to select at least one of nylon fibers, polyester fibers, acrylic fibers, and pulp. The insulating fiber 10 has a single fiber thickness of 5 μm to 50 μm, preferably 10 μm to 40 μm, more preferably 20 μm to 30 μm, an average fiber length of 1 mm to 20 mm, more preferably 5 mm to 15 mm, and a resistivity of 1.0 × 10⁻¹⁰ 8 Ω·cm or more, more preferably 1.0 × 10 10 It is greater than or equal to Ω·cm. 【0031】 [Binder resin 12] The binder resin 12 is preferably selected from at least one of polyvinyl alcohol, acrylic resin, and polyacrylamide. This makes it possible to create an electromagnetic wave absorber with a certain degree of flexibility after papermaking and drying. 【0032】 [Method for manufacturing the radio wave absorber 100] The absorption layer 20, which serves as the radio wave absorber 100, is obtained by dissolving insulating fibers 10 having high electrical resistance, conductive pitch-based carbon fibers 11, and binder resin 12 in a predetermined amount of water, stirring the mixture, pouring it onto carrier paper laid on a frame, wet-processing the paper, removing excess water, and then drying it at a predetermined temperature (for example, 70°C). After drying, the binder resin 12 remains in the absorption layer 20, suppressing the detachment of the pitch-based carbon fibers 11 from the insulating fibers 10. 【0033】 <Examples and Comparative Examples> Examples and comparative examples of the radio wave absorber 100 will be described below. In Example 1, the pitch-based carbon fiber 11 was used in a mixture of pitch-based carbon fiber 11 and insulating fiber 10 at a ratio of 10% by mass (0.2g), the insulating fiber 10 was 2.1g, and the binder resin 12 was 40g. This mixture was stirred and mixed with 200g of water in a container. The mixture was poured onto carrier paper laid in a 10cm × 15cm frame, and after papermaking to remove excess water, it was dried at 70°C to obtain the radio wave absorber 100. Other conditions are as shown in Table 2. Furthermore, the average fiber diameter of the pitch-based carbon fiber 11 is 13 μm, and its resistivity is 1.0 × × 10 ―2 The material has a density of Ω·cm, and the nylon fiber used as the insulating fiber 10 is Tough Binder (registered trademark) manufactured by Toray Amtex Co., Ltd., with an average fiber length of 5 mm. The binder resin 12 is polyvinyl alcohol (Kaneyonol manufactured by Kaneyo Soap Co., Ltd.: PVA effective ratio 8%). 【0034】 [Table 2] 【0035】 [Table 3] 【0036】 The radio wave absorbers 100 for Examples 2-5 and Comparative Examples 1-6 were also manufactured using the same manufacturing method as in Example 1, with the conditions set to those in Table 2 or Table 3. 【0037】 <Method for measuring volume resistivity> Incidentally, for Examples 1-5 and Comparative Examples 1 and 6, which have relatively high resistivity, the volume resistivity was measured at five points near the center and four corners of the surface of a 150mm x 100mm sheet-shaped radio wave absorber 100 using a high resistivity meter (Highresstar UX MCP-HT800: manufactured by Mitsubishi Analytec Corporation) with a voltage of 100V applied and a USR probe (probe diameter: approximately 30mm) (in accordance with JIS K 6911). On the other hand, for comparative examples 2 to 5, which had relatively low resistivity, five points near the center and four corners of the surface of a 150 mm x 100 mm sheet-shaped radio wave absorber 100 were measured using a low resistivity meter (Rolestar EP MCP-T360: manufactured by Mitsubishi Chemical Corporation) (in accordance with JIS K 7194). 【0038】 <Method for measuring carbon content> The carbon content was measured using a measuring device (vario EL cube: manufactured by Elementor Japan Co., Ltd.) in CHNS mode, which is capable of measuring and analyzing carbon (C), hydrogen (H), nitrogen (N), and sulfur (S), the basic components of organic matter. The combustion tube temperature was set to 1150°C and the reduction tube temperature to 850°C. 2 mg of pitch-based carbon fiber, used as the sample for measurement, was wrapped in a tin board and analyzed. 【0039】 <Method for measuring resistivity> The resistivity of the carbon fiber was measured using a measuring device (PC700 digital multimeter: manufactured by Sanwa Electric Instrument Co., Ltd.) in accordance with JIS R 7609. The resistivity of the insulating fiber was measured using a measuring device (High Resistivity Meter Hi-Resta-UX MCP-HT800: manufactured by Mitsubishi Analytec Co., Ltd.) with an applied voltage of 500V, referring to "JIS K 6911". 【0040】 <Method for measuring average fiber length> The average fiber length was measured using the following method. Carbon fibers were placed in a container of pure water and dispersed using a surfactant. Approximately 20 ml of the resulting carbon fiber aqueous dispersion was transferred to a 100 mm square petri dish and set up under a microscope. An image analysis device equipped with image analysis and measurement software (automatic image processing and analysis device "LUZEX(registered trademark) AP": manufactured by Nireco) was used to calculate the average fiber length from the lengths of 6000 randomly selected carbon fibers. The average fiber length (Lv) is defined as shown in [Equation 1] below. 【0041】 [Formula 1] Lv=(l1×l1+l2×l2+…+l n ×ln ) / (l1+l2+…+l n ) Furthermore, in [Equation 1], l1~l n These represent the lengths of the 1st to nth carbon fibers, respectively. 【0042】 <Method for measuring radio wave absorption performance> The radio wave absorption performance, expressed as the ratio of reflected electric field strength to incident electric field strength, was measured using the free-space method (compliant with JIS R 1679) with a pair of horn antennas (free-space measuring device FS-110: EM Lab Co., Ltd.) and a network analyzer (N5290A 900Hz to 110GHz PNA mm-wave system: Keysight Co., Ltd.). To further explain, a pair of horn antennas connected to the network analyzer were installed facing each other, and a radio wave absorber 100 (length: 150 mm, width: 100 mm, thickness: as shown in Tables 2 and 3) was placed between the pair of horn antennas. The radio wave absorber 100 was positioned so that its thickness direction was aligned with the direction in which the horn antennas faced each other. In addition, a metal plate was placed on one side of the radio wave absorber 100 in the thickness direction. In this state, radio waves (26.2-40 GHz, 40-60 GHz, 60-90 GHz) were radiated from a horn antenna provided on the other side in the thickness direction of the radio wave absorber 100. The radio waves that passed through the radio wave absorber 100 and reflected off the metal plate were received by the horn antenna on the other side in the thickness direction to determine the radio wave absorption performance. 【0043】 In Examples 1 to 5, the main parameters were adjusted to create a radio wave absorber 100 by adjusting the average fiber length of the pitch-based carbon fibers 11 to 0.37 mm or more and 1.5 mm or less, and the content of the pitch-based carbon fibers 11 in relation to the insulating fibers 10 to 5% by mass or more and 10% by mass or less. In all of Examples 1 to 5, the volume resistivity was set to 4.0 × 10⁻⁶ 10 Ω cm or more 3.6×10 11 The results showed that the values ​​were less than Ω·cm, and each exhibited a high maximum absorption performance of approximately -19dB or less at a given frequency. Furthermore, the inventors have confirmed that the frequency at which maximum absorption performance is achieved can be changed by changing the average fiber length of the pitch-based carbon fiber 11. 【0044】 In contrast, when the average fiber length of the pitch-based carbon fiber 11 was increased compared to the examples, to 3.3 mm as shown in Comparative Example 4 and 10 mm as shown in Comparative Example 5, the volume resistivity was 2.6 × 10⁻⁶ in Comparative Example 4. 3 Ω·cm, 1.7 × 10 in Comparative Example 5 2 The values ​​were relatively large at Ω·cm, and the maximum absorption performance was also lower than in the examples, at approximately -10dB for both. On the other hand, when the average fiber length of the pitch-based carbon fiber 11 is shortened to 0.04 mm, as shown in Comparative Example 6, compared to the example, the volume resistivity is 6.9 × 10⁻⁶. 11 The result was slightly higher than the example, at Ω·cm, and the maximum absorption performance was approximately -0.7dB, indicating that it exhibited almost no absorption performance. 【0045】 Furthermore, when the pitch-based carbon fiber content 11 relative to the insulating fiber 10 was increased to 50% by mass as shown in Comparative Example 2 and 90% by mass as shown in Comparative Example 3, compared to the examples, the volume resistivity in Comparative Example 2 was 4.1 × 10⁻⁶. -1 Ω·cm, 7.8 × 10 in Comparative Example 3 -1 The Ω·cm value was relatively small, and the maximum absorption performance was lower compared to the examples, at -5.6 dB in Comparative Example 2 and -5.7 dB in Comparative Example 3. On the other hand, when the pitch-based carbon fiber 11 content relative to the insulating fiber 10 is lower than in the example, as shown in Comparative Example 1 (0% by mass), the volume resistivity is 1.0 × 10⁻⁶. 13 The value was relatively large at Ω·cm, and the maximum absorption performance was -0.4dB, indicating that it exhibited almost no absorption performance. 【0046】 Figure 2 shows the radio wave absorption performance of the radio wave absorber 100 in the 26.0 to 40.0 GHz frequency band for Examples 1 to 5 and Comparative Examples 1 to 6, Figure 3 shows the radio wave absorption performance of the radio wave absorber 100 in the 40.0 to 60.0 GHz frequency band, and Figure 4 shows the radio wave absorption performance of the radio wave absorber 100 in the 60.0 to 90.0 GHz frequency band. However, since the data for each graph was measured separately, the absorption performance may differ at the boundary frequencies of each graph (40.0 GHz and 60.0 GHz). This is a phenomenon of the measurement method and falls within the range of measurement error. 【0047】 Incidentally, the pitch-based carbon fiber content 11 and the content of the binder resin 12 relative to the pitch-based carbon fiber 11 and insulating fiber 10 as shown in this specification are the same values ​​in the manufacturing stage and in the finished product. 【0048】 Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Moreover, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the purpose of the present invention. In particular, the dimensions, materials, shapes, relative arrangements, etc. of materials described in the embodiments are not limited thereto unless specifically stated. Also, the size and positional relationships of each component shown in the drawings may be exaggerated to clarify those relationships. [Industrial applicability] 【0049】 The radio wave absorber and its manufacturing method of the present invention can be effectively used as a radio wave absorber and its manufacturing method that can exhibit high radio wave absorption performance while maintaining a stable absorption layer even when the finished product is processed afterward. [Explanation of Symbols] 【0050】 10: Insulating fibers 11: Pitch-based carbon fiber 12: Binder resin 20: Absorption layer 100: Radio wave absorber

Claims

[Claim 1] The absorbent layer is made of paper and contains insulating fibers, a binder resin, and pitch-based carbon fibers having an average fiber length of 0.3 mm to 2.0 mm. The pitch-based carbon fibers have a content of 5% to 30% by mass relative to the insulating fibers, and the volume resistivity is 1 × 10⁻⁶. ９ Ω・cm or more 1×10 １２ A radio wave absorber having a density of Ω·cm or less and absorbing radio waves in a predetermined frequency range within the range of 26.5 GHz to 90 GHz. [Claim 2] The radio wave absorber according to claim 1, wherein the carbon content of the pitch-based carbon fiber is 90% by mass or more. [Claim 3] The radio wave absorber according to claim 2, wherein the pitch-based carbon fibers are isotropic. [Claim 4] The radio wave absorber according to any one of claims 1 to 3, wherein the insulating fiber is selected from at least one of nylon fiber, polyester fiber, acrylic fiber, and pulp. [Claim 5] The radio wave absorber according to any one of claims 1 to 3, wherein the binder resin is selected from at least one of polyvinyl alcohol, acrylic resin, and polyacrylamide. [Claim 6] The radio wave absorber according to claim 5, wherein the thickness is 0.3 mm or more and 1.0 mm or less. [Claim 7] The resistivity of the pitch-based carbon fiber is 1.0 × 10 ２ A radio wave absorber according to any one of claims 1 to 3, wherein the density is Ω·cm or less. [Claim 8] The resistivity of the aforementioned insulating fiber is 1.0 × 10 ８ A radio wave absorber according to any one of claims 1 to 3, wherein the density is Ω·cm or greater. [Claim 9] A method for producing an electromagnetic wave absorber, comprising the steps of dissolving insulating fibers, a binder resin, and pitch-based carbon fibers having an average fiber length of 0.3 mm or more and 2.0 mm or less in water, such that the pitch-based carbon fibers are present in a ratio of 5% by mass or more and 30% by mass or less relative to the pitch-based carbon fibers and insulating fibers, stirring the resulting mixture, wet-processing the mixture, and then drying it.

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