Inclined porous body

Abstract

In a gradient porous material where the porosity changes in a gradual manner along the thickness direction between opposing main surfaces, the relative permittivity on the main surface with high porosity can be controlled effectively. [Solution] The porosity changes in a gradual manner along the thickness direction between opposing main surfaces, forming regular or irregular irregularities on the main surface with high porosity.

Description

[Technical Field]

[0001] The present invention relates to a gradient porous body in which the porosity changes in a gradient along the thickness direction between opposing main surfaces. [Background technology]

[0002] For example, Patent Document 1 discloses a gradient foamed plastic sheet manufactured by dissolving a gaseous gas at room temperature and atmospheric pressure into plastic under high temperature and high pressure, then exposing the plastic to an atmosphere with a pressure lower than the pressure at which the gas dissolved, and finally exposing both sides of the sheet to atmospheres with different temperatures. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2002-363324 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, even when using gradient foaming as disclosed in Patent Document 1, there was room for improvement depending on the application, such as difficulty in controlling foaming on the high porosity side or the formation of a non-foamed skin layer on the surface. For example, in automobile collision avoidance systems, electromagnetic waves such as millimeter-wave radar are generally used, and in applications where a material is interposed between the electromagnetic wave path and the material to improve the permeability to the material, attempts have been made to suppress the reflection of electromagnetic waves by reducing the dielectric constant on the incident side by using high foaming and changing the porosity in a gradient along the thickness direction, but there was room for improvement in controlling the dielectric constant on the high porosity side.

[0005] Therefore, in view of the above background art, the present inventors have conducted intensive studies to enable good control of the relative permittivity on the main surface on the high porosity side in an inclined porous body in which the porosity changes gradually along the thickness direction between the opposing main surfaces, and as a result, have completed the present invention.

Means for Solving the Problems

[0006] The inclined porous body according to the present invention is an inclined porous body in which the porosity changes gradually along the thickness direction between opposing main surfaces, and is configured such that regular or irregular concavo-convexities are formed on the main surface on the high porosity side.

Advantages of the Invention

[0007] According to the present invention, in an inclined porous body in which the porosity changes gradually along the thickness direction between opposing main surfaces, it is possible to enable good control of the relative permittivity on the main surface on the high porosity side.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is an explanatory view schematically showing an example of an inclined porous body according to an embodiment of the present invention. In the inclined porous body 1, the ratio (porosity) occupied by pores c changes gradually along the thickness direction between opposing main surfaces. [Figure 2] A graph in which reflectance [dB] is taken on the vertical axis and the relative permittivity change rate is taken on the horizontal axis, and these are plotted. [Figure 3] A graph in which deflection δ [mm] is taken on the vertical axis and L / D is taken on the horizontal axis, and these are plotted.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[0010] FIG. 1 is an explanatory view schematically showing an example of an inclined porous body according to the present embodiment. In the inclined porous body 1, the ratio (porosity) occupied by pores c changes gradually along the thickness direction between opposing main surfaces.

[0011] In this embodiment, the inclined porous body 1 can preferably be in the form of a sheet with a thickness of 1 to 10 mm, but is not limited thereto. The shape and dimensions of the inclined porous body 1 can be appropriately changed depending on its application.

[0012] Furthermore, the gradient porous body 1 is made of synthetic resin, and the base resin can be, for example, polyethylene, polypropylene, polystyrene, polyester, polyamide, polyvinyl chloride, polyvinylidene chloride, polybutene, polyacetal, polyphenylene oxide, polymethyl methacrylate, polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, polyamideimide, polycarbonate, polyarylate, polyimide, fluororesin, ethylene-propylene resin, ethylene-ethyl acrylate, epoxy resin, urethane resin, imide resin, acrylic resin, or norbornene-based resin. These may be used individually or in combination, and may be polymer alloys or composite materials of a resin matrix and fillers. The base resin is not limited to these and can be appropriately selected according to the physical properties required for the application of the gradient porous body 1, but it is preferable to use polyolefin resins such as low-density polyethylene, linear low-density polyethylene, high-density polyethylene, or ethylene-propylene copolymer as the base resin.

[0013] The porosity of the inclined porous body 1 decreases from one main surface to the other, and regular or irregular irregularities are formed on the main surface on the side with high porosity. By forming irregularities on the main surface on the side with high porosity, the relative permittivity (average value of relative permittivity) is reduced by the amount of voids caused by the depressions. Therefore, when forming irregularities on the main surface on the side with high porosity, the relative permittivity on the main surface on the side with high porosity can be well controlled by appropriately adjusting the proportion of voids caused by the depressions (porosity) according to the shape of the irregularities.

[0014] Here, P is the void ratio, porosity, or sum thereof of the portion partitioned in layers by any plane perpendicular to the thickness direction of the inclined porous body 1, and ε is the relative permittivity of air. air, let the relative permittivity of the base resin be ε base , then the average value ε ave of the relative permittivity at that part can be obtained by the following formula (1). ε ave = ε air ·P + ε base ·(1 - P) ···(1)

[0015] Also, let the relative permittivity on the incident side be ε i , and the relative permittivity on the transmission side be ε i+1 , then the reflectivity R of the incident electromagnetic wave can be obtained by the following formula (2). R = ((1 - (ε i / ε i+1 )) 1 / 2 ) / (1 + (ε i / ε i+1 )) 1 / 2 )) 2 ···(2) Based on this formula, with the reflectivity R [dB] on the vertical axis and the relative permittivity change rate ε i / ε i+1 on the horizontal axis, a graph plotting these is shown in Figure 2.

[0016] In the graph shown in Figure 2, it can be observed that the slope of the graph changes significantly when the relative permittivity change rate ε i / ε i+1 is near 0.9. Therefore, in the part partitioned into layers as described above, when the change rate of the average value of the relative permittivity between adjacent layers is 0.9 or less, it is considered that the reflection of electromagnetic waves between these can be efficiently suppressed. From this perspective, when forming irregularities on the main surface on the high porosity side, it is preferable to adjust the porosity so that the change rate between the average value of the relative permittivity at the deepest part (the boundary of the irregularity formation region) of the region where the irregularities are formed and the average value of the relative permittivity of the region where the adjacent pores are formed (the boundary of the pore formation region) is 0.9 or less. Furthermore, from the perspective of approaching the relative permittivity of air and making the change rate with respect to the relative permittivity of air as small as possible, it is preferable to set the porosity of the pore formation region boundary to 50% or more and make the porosity of the irregularity formation region boundary larger than that.

[0017] Furthermore, the average value of the relative permittivity on the incident side is ε i(ave) , porosity etc. P i Substituting these into equation (1) above, we obtain equation (3) below. ε i(ave) =ε air ·P i +ε base (1-P i ) ···(3) Similarly, the relative permittivity of the transmitting side is ε i+1(ave) , porosity etc. P i+1 Substituting these into equation (1) above, we obtain equation (4) below. ε i+1(ave) =ε air ·P i+1 +ε base (1-P i+1 ) ···(4) From these equations, equation (5) below can be derived. ε i(ave) -ε i+1(ave) =ε air ·(P i -P i+1 )-ε base ·(P i -P i+1 ) =( ε air -ε base )·(P i -P i+1 ) ···(5) When this is rearranged, we get equation (6) below. P i -P i+1 =( ε i(ave) -ε i+1(ave) ) / (ε air -ε base ) ···(6)

[0018] The relative permittivity of air is ε air 1. The relative permittivity ε of the base resin. base Taking the case where is 2.5 as an example, when the porosity of the boundary of the void formation region (transmission side) is 50% or more, the average value of its relative permittivity is 1.75 or less, and if the rate of change between this and the average value of the relative permittivity of the boundary of the unevenness formation region (incident side) is 0.9 or less, then the difference in the average values ​​of the relative permittivity between these adjacent layers is |εi(ave) -ε i+1(ave) | will be 0.175 or less. Therefore, according to equation (6) above, the difference in porosity, etc. between these adjacent layers |P i -P i+1 | will be 11.6% or less.

[0019] From this perspective, when forming irregularities on the main surface on the high-porosity side, it is preferable to form them such that the following conditions are met. Specifically, near the surface of the main surface on the high-porosity side, a plane parallel to the center line of the irregularities, including the lowest valley bottom of the irregularities, is defined as the reference plane, the area outside the reference plane in the thickness direction is defined as the irregularity formation region, the area inside the reference plane in the thickness direction is defined as the void formation region, the area within the irregularity formation region that is partitioned parallel to the reference plane by a thickness of 100 μm from the reference plane is defined as the irregularity formation region boundary, and the area within the void formation region that is partitioned parallel to the reference plane by a thickness of 100 μm from the reference plane is defined as the void formation region boundary, it is preferable that the difference between the porosity of the irregularity formation region boundary and the porosity of the void formation region boundary is 11.6% or less.

[0020] The void ratio and porosity can be determined, for example, from cross-sectional CT images. The center line of the surface irregularities is the center line of the roughness curve obtained by measuring the surface roughness of the main surface on which the irregularities are formed.

[0021] Furthermore, in order to reduce the reflection of electromagnetic waves such as millimeter-wave radar, taking into account the wavelength of the incident electromagnetic wave, it is preferable that the difference between the porosity of the boundary

[0022] When forming irregularities on the main surface on the high-porosity side, it is preferable to form the irregularities so that the protrusions taper towards the top, as shown in the example in Figure 1, so that the porosity in the irregularity-forming region gradually decreases toward the reference surface. By doing so, the relative permittivity of the portion at the tip of the protrusions among the layered portions as described above, i.e., the outermost layer of the inclined porous body 1, is minimized and brought closer to the relative permittivity of air, thereby further reducing the reflection of electromagnetic waves such as millimeter-wave radar incident on the inclined porous body 1. In this case, taking into account the wavelength of the incident electromagnetic wave, it is preferable that the maximum width D of the protrusions along the direction perpendicular to the thickness direction is 1 / 10 or less of the wavelength of the incident electromagnetic wave.

[0023] Figure 1 schematically shows an example of a gradient porous body 1, and a graph shows the change in relative permittivity along the thickness direction. The horizontal axis of the graph is relative permittivity, and the slope of the graph differs between the unevenness-forming region and the void-forming region. It goes without saying that it is ideal to control the relative permittivity so that the slope of the graph is constant over the entire thickness direction including the unevenness-forming region, but even if the relative permittivity does not change smoothly between the unevenness-forming region and the void-forming region, it is preferable to form irregularities on the main surface on the high-porosity side so that there is a tendency for the relative permittivity to change in a gradient over the entire thickness direction including the unevenness-forming region, and more preferably so that the difference in the average value of the relative permittivity between adjacent layers is 0.9 or less. In this way, when changing the porosity in a gradient along the thickness direction between opposing main surfaces, it is preferable that the porosity near the surface of the low-porosity main surface facing the high-porosity main surface is 40% or less.

[0024] Furthermore, voids c may exist in the protrusions of the unevenness formed on the main surface on the high-porosity side. In this case, the voids c in the protrusions are also considered as voids, and the porosity in the unevenness-forming region is calculated accordingly. This allows the porosity in the unevenness-forming region to be adjusted by the voids caused by the recesses and the voids c in the protrusions, thereby expanding the range of adjustment. A skin layer may be formed on the surface of the protrusions, as long as it does not hinder the adjustment of the porosity in the unevenness-forming region.

[0025] It should be noted that in the example shown in Figure 1, a single void c is depicted in the convex portion, but this is merely a schematic representation of the presence of a void c in the convex portion.

[0026] The shape of the irregularities formed on the main surface on the high-porosity side may be arranged in a striped pattern with protrusions and / or recesses extending in any direction perpendicular to the thickness direction, or with protrusions and / or recesses regularly arranged in a grid pattern, or with protrusions and / or recesses irregularly arranged in a scattered pattern. The shape of the protrusions is not limited to the tapered shape shown in Figure 1; for example, their longitudinal cross-section may be rectangular. The shape of the protrusions does not have to be the same for all protrusions. The shape and spacing of the irregularities can be arbitrarily set so that the proportion of voids due to the recesses is suitable for the application.

[0027] Here, the amount of deflection δ when a load P is applied to the free end of a cantilever beam of length L can be calculated by the following equation (7). δ=PL 3 / 3EI ···(7) In equation (7), E is the Young's modulus and I is the second moment of area. The second moment of area can be calculated by equation (8) below when the cross-section is a square with length and width D. I=D 4 / 12 ···(8) Based on these equations, taking the case where the load P is 1N and the Young's modulus E is 2000MPa as an example, Figure 3 shows a graph plotting the deflection δ [mm] on the vertical axis and L / D on the horizontal axis.

[0028] The graph in Figure 3 shows that the amount of deflection δ changes significantly when L / D is around 2. In light of this, in order to suppress the deflection deformation of the protrusions of the unevenness formed on the main surface on the high porosity side, it is preferable that the ratio L / D of the height L of the protrusions in the thickness direction to the maximum width D of the protrusions in the direction perpendicular to the thickness direction is 2 or less.

[0029] The gradient porous body 1 described above can be manufactured, for example, as follows.

[0030] [Manufacturing Example 1] First, a foaming agent is dissolved in a molten and kneaded base resin to prepare a foaming resin. Examples of foaming agents include physical foaming agents such as carbon dioxide and nitrogen, organic thermal decomposition type chemical foaming agents such as azodicarbonamide, and inorganic thermal decomposition type chemical foaming agents such as sodium bicarbonate, as well as heat-expandable capsules containing foaming components. However, these are not limited to these, and can be appropriately selected depending on the solubility in the base resin.

[0031] Next, a molding die is used in which a temperature difference is created between the molding surface that molds one side of the main surface of the inclined porous body 1 and the molding surface that molds the other side of the main surface of the inclined porous body 1. A predetermined uneven shape is engraved on the molding surface that is heated to a higher temperature, and molten foaming resin is injected into the cavity of the molding die. As a result, when the foaming resin filled in the cavity foams and bubbles grow, larger bubbles grow on the side that is heated to a higher temperature, and smaller bubbles grow on the side that is heated to a lower temperature. In this way, bubbles grow to different sizes in accordance with the temperature distribution, and unevenness is formed on the main surface on the side with high porosity where the bubbles have grown larger.

[0032] In this manufacturing example, a gradient porous body 1 with irregularities formed on the main surface on the high porosity side is manufactured as described above. However, it is preferable to expand the cavity volume by sliding the mold in the mold opening direction after filling the cavity with foaming resin, or during the process in which bubbles grow before or after this. By doing so, it is possible to control the growth of bubbles so that they extend in the thickness direction while suppressing the growth of bubbles in the direction perpendicular to the thickness direction. This makes it less likely for undulations and wrinkles to occur during the bubble growth process, and a gradient porous body 1 with good flatness can be manufactured. In the gradient porous body 1 manufactured in this way, the cross-sectional shape of the pores is a long, flattened shape along the direction from one side of the main surface to the other side.

[0033] [Manufacturing Example 2] The foamed resin prepared in the same manner as in Manufacturing Example 1 is injection molded into a predetermined shape while remaining unfoamed or slightly foamed. Then, one of the opposing main surfaces of the resulting molded intermediate is constrained so as to restrict movement in the direction perpendicular to the thickness direction by vacuum suction, and the other main surface is heated in that state.

[0034] This initiates foaming, and bubbles grow to different sizes in response to the temperature distribution between the two main surfaces, with larger bubbles growing on the heated other main surface. During this process, the bubbles grow while one main surface is constrained, thus controlling bubble growth so that bubbles extend in the thickness direction while suppressing bubble growth in the direction perpendicular to the thickness direction. This reduces the likelihood of undulations and wrinkles during bubble growth, allowing bubbles to grow while maintaining good flatness. The pores formed by these grown bubbles have a cross-sectional shape that is elongated and flattened along the direction from one side of the main surface to the other.

[0035] In this manufacturing example, after growing the bubbles as described above, a stamper with a predetermined uneven shape is pressed against the main surface of the molded intermediate, where the bubbles have grown larger, to form the uneven surface. This produces an inclined porous body 1 in which an uneven surface is formed on the main surface on the high porosity side.

[0036] [Manufacturing Example 3] A foamed resin prepared in the same manner as in Manufacturing Example 1 is injection molded into a predetermined shape while remaining unfoamed or slightly foamed. At this time, a predetermined uneven shape is engraved on the molding surface of the mold, and injection molding is performed so that the unevenness is formed on one of the main surfaces. Next, the resulting molded body is restrained so that its movement in the direction perpendicular to the thickness direction is restricted, and the main surface side on which the unevenness is formed is heated in that state.

[0037] This initiates foaming, and bubbles grow to different sizes in response to the temperature distribution between opposing main surfaces, with larger bubbles growing on the main surface side where the irregularities are formed. In this process, bubbles can be grown while maintaining good flatness, and the pores formed by the grown bubbles have a long, flattened cross-sectional shape along the direction from one side of the main surface to the other, similar to manufacturing example 2.

[0038] In this manufacturing example, a gradient porous body 1 is produced by heating the main surface side on which the irregularities of an unfoamed or slightly foamed molding intermediate are formed, thereby creating irregularities on the main surface on the high porosity side.

[0039] [Manufacturing Example 4] The foamed resin, prepared in the same manner as in Manufacturing Example 1, is extruded into a cylindrical shape and formed into a predetermined hollow shape by direct blow molding using a mold with a predetermined uneven surface engraved on its molding surface. At this time, the temperature of the mold is controlled so that the inner side of the extruded cylindrical parison is at a lower temperature, and the outer side, which comes into contact with the molding surface and forms the uneven surface, is at a higher temperature. As a result, bubbles grow to different sizes in response to the temperature distribution between the inner and outer sides, so that bubbles grow larger on the outer side where the uneven surface is formed.

[0040] In this manufacturing example, a hollow molded intermediate, in which irregularities are formed on the outer surface where the bubbles have grown larger, is cut into a sheet, thereby producing an inclined porous body 1 in which irregularities are formed on the main surface on the high porosity side.

[0041] [Manufacturing Example 5] The foamed resin, prepared in the same manner as in Manufacturing Example 1, is extruded into a sheet and sandwiched between a shaping roll, which has a predetermined uneven shape engraved on it, and a pressing roll that is paired with the shaping roll, thereby forming the unevenness on one side. At this time, the temperature of the pressing roll is controlled so that it is at a lower temperature and the shaping roll is at a higher temperature. As a result, bubbles grow to different sizes in response to the temperature distribution between the two sides, with larger bubbles growing on the side where the unevenness is formed.

[0042] In this manufacturing example, an inclined porous body 1 is produced in which irregularities are formed on the main surface on the high-porosity side by creating irregularities on one side of the molded intermediate extruded into a sheet shape, while growing larger bubbles on the side where the irregularities are formed.

[0043] In addition, in manufacturing examples 4 and 5, foaming resins with different foaming agent contents may be extruded in layers. In this case, it is preferable to have a higher concentration of foaming agent on the side where the irregularities are formed, and for the foaming agent content to decrease gradually on the opposite side.

[0044] Although the present invention has been described above with reference to preferred embodiments, it goes without saying that the present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the present invention.

[0045] The inclined porous body 1 according to the present invention can be applied to a wide range of uses in various fields. In addition to being applicable to civil engineering and construction materials, automotive materials, and packaging materials that utilize properties such as heat insulation, shock absorption, and sound absorption, it is particularly suitable for applications such as interposing it between components in the path of electromagnetic waves to improve the permeability of such components in automobile collision avoidance systems and radomes that serve as covers to protect antennas. [Explanation of symbols]

[0046] 1 Gradient porous material c vacancy D Maximum width of the protrusion Height of the L-shaped protrusion

Claims

1. A gradient porous body in which the porosity changes in a gradient along the thickness direction between opposing main surfaces, An inclined porous body characterized by having regular or irregular irregularities formed on the main surface on the high-porosity side.

2. Near the surface of the main surface on the high porosity side, The plane parallel to the center line of the unevenness, including the lowest valley bottom of the recess of the unevenness, is used as the reference plane. The area outside the thickness direction relative to the aforementioned reference surface is defined as the region where irregularities are formed. The area inside the thickness direction relative to the aforementioned reference surface is defined as the void formation region. Within the aforementioned unevenness formation region, the area that is demarcated parallel to the reference surface by a thickness of 100 μm from the reference surface is defined as the boundary portion of the unevenness formation region. When the void-forming region is defined as the void-forming region boundary, which is a section of the void-forming region that is separated from the reference surface by a thickness of 100 μm and parallel to the reference surface, The gradient porous body according to claim 1, wherein the difference between the porosity of the boundary portion of the unevenness-forming region and the porosity of the boundary portion of the void-forming region is 11.6% or less.

3. The inclined porous body according to claim 2, wherein the protrusions of the unevenness are formed in a tapered shape, and the porosity in the unevenness formation region gradually decreases toward the reference surface.

4. The inclined porous body according to claim 1, wherein voids are present in the convex portions of the aforementioned irregularities.

5. The inclined porous body according to claim 4, wherein a skin layer is formed on the surface of the protruding portion of the aforementioned irregularities.

6. The inclined porous body according to claim 1, wherein the ratio of the height of the protrusions along the thickness direction to the maximum width along the direction perpendicular to the thickness direction of the protrusions is 2 or less.

7. The inclined porous body according to claim 1, wherein the cross-sectional shape of the void is a long, flattened shape along the direction from one side to the other side of the main surface.

8. A gradient porous body that can be used as an electromagnetic wave low reflectance high transmittance member, Near the surface of the main surface on the high porosity side, The plane parallel to the center line of the unevenness, including the lowest valley bottom of the recess of the unevenness, is used as the reference plane. The area outside the thickness direction relative to the aforementioned reference surface is defined as the region where irregularities are formed. The area inside the thickness direction relative to the aforementioned reference surface is defined as the void formation region. Within the aforementioned unevenness formation region, the area that is demarcated parallel to the reference surface with a thickness of 1 / 40 of the wavelength of the incident electromagnetic wave from the reference surface is defined as the boundary portion of the unevenness formation region. When the boundary of the void formation region is defined as the area within the void formation region that is partitioned parallel to the reference surface with a thickness of 1 / 40 of the wavelength of the incident electromagnetic wave from the reference surface, The gradient porous body according to claim 1, wherein the difference between the porosity of the boundary portion of the unevenness-forming region and the porosity of the boundary portion of the void-forming region is 11.6% or less.

9. The gradient porous body according to claim 8, wherein the maximum width along the direction perpendicular to the thickness direction of the protrusions of the aforementioned irregularities is 1 / 10 or less of the wavelength of the incident electromagnetic wave.

10. The gradient porous body according to claim 2, 3, 8, or 9, wherein the porosity of the boundary portion of the void-forming region near the surface of the main surface on the high-porosity side is 50% or more, and the porosity of the main surface on the low-porosity side near the surface opposite the main surface on the high-porosity side is 40% or less.

11. The gradient porous body according to claim 10, which is made of an olefin resin.

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