Porous sheet

Abstract

To provide a sheet with even greater infrared reflectivity than conventional sheets. [Solution] A porous sheet containing thermoplastic resin and inorganic particles, having numerous pores. The numerous pores communicate with each other, thereby giving the porous sheet breathability. Each surface of the porous sheet has a region not covered by a solid resin layer. The solid resin layer is either a resin layer without pores, or a resin layer with pores that open on the surface and do not have pores that communicate one surface with the other. The porous sheet has a pore volume ratio of 10% or more. Preferably, the porous sheet has a plurality of stem fibers extending in one direction and fibril fibers extending between the stem fibers and having a smaller diameter than the stem fibers.

Description

[Technical Field]

[0001] This invention relates to a porous sheet. [Background technology]

[0002] In order to reduce greenhouse gas emissions and achieve carbon neutrality, the Building Energy Conservation Act was amended in 2022, requiring further reductions in energy consumption in the building sector. For example, as part of efforts to improve energy efficiency in homes, attempts are being made to improve the cooling efficiency of homes during the summer. For this purpose, heat-shielding sheets made of high-density polyethylene nonwoven fabric coated with aluminum are being used as building materials (see Non-Patent Document 1).

[0003] Furthermore, Patent Document 1 describes a material comprising metal particles and a non-metallic filler, wherein voids are formed in the areas of the non-metallic filler by stretching in at least one axial direction, and the water vapor permeability resistance is 0.04 to 0.19 m. 2 It has been proposed to use a porous film with a density of s·Pa / μg, an infrared reflectivity of 60% or more, and an infrared transmittance of 30% or less as a heat shielding sheet. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2014-139305 [Non-patent literature]

[0005] [Non-Patent Document 1] DuPont Tyvek Silver, [online], Asahi DuPont Flashspan Co., Ltd., [searched November 8, 2023], Internet <URL: https: / / www.tyvek.co.jp / construction / product / silver / > [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] To improve the heat-shielding performance of heat-shielding sheets, it is advantageous to increase the infrared reflectivity. Although the sheets described in Patent Document 1 and Non-Patent Document 1 mentioned above have high infrared reflectivity, there is a need for sheets with even higher infrared reflectivity. Therefore, the object of the present invention is to provide a sheet with even greater infrared reflectivity than the conventional technology described above. [Means for solving the problem]

[0007] In order to solve the aforementioned problems, the inventors conducted diligent research and discovered that by incorporating inorganic particles into a porous resin sheet, the infrared reflectivity is dramatically improved. The present invention is based on this discovery and solves the aforementioned problems by providing a porous sheet containing a thermoplastic resin and inorganic particles and having a large number of pores. In one embodiment of the present invention, the numerous holes communicate with one another, thereby giving the porous sheet breathability. In one embodiment of the present invention, The porous sheet has a region on each surface of the porous sheet that is not covered by a solid resin layer. The solid resin layer is either a resin layer without pores, or a resin layer having pores, but the pores are open on the surface and do not connect one surface to the other. In one embodiment of the present invention, the void volume fraction is 10% or more.

[0008] Furthermore, the present invention provides a method for manufacturing the porous sheet described above. In one embodiment of the present invention, the manufacturing method includes a step of stretching a resin sheet containing the thermoplastic resin and the inorganic particles to form the numerous holes in the resin sheet. [Effects of the Invention]

[0009] According to the present invention, a sheet is provided in which the infrared reflectivity is further improved compared to conventional sheets. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a scanning electron microscope image representing one embodiment of the porous sheet of the present invention. [Modes for carrying out the invention]

[0011] The present invention will be described below based on its preferred embodiments. The present invention relates to a porous sheet. The porous sheet of the present invention contains a thermoplastic resin and has numerous pores. Preferably, these pores are formed by stretching a resin sheet that is substantially free of pores, as described later. The numerous pores formed in the porous sheet of the present invention are of the open-cell type, communicating with one another. Therefore, the porous sheet of the present invention has multiple pores that connect one surface to the other. In other words, the porous sheet of the present invention has numerous through-holes that connect one surface to the other. The fact that the numerous pores communicate with one another can be confirmed, for example, by the air permeability resistance of the porous sheet measured in accordance with JIS P8117 being 40,000 s / 100 mL or less.

[0012] The sheet of the present invention is given breathability by having numerous interconnected pores. The breathability of the sheet of the present invention is advantageous in that condensation is less likely to occur when the sheet is used, for example, as building material or agricultural material. Furthermore, due to the presence of numerous interconnected pores and the synergistic effect with the inorganic particles described later, the sheet of the present invention exhibits high infrared reflectivity. Specifically, the inventors believe that the sheet of the present invention exhibits high infrared reflectivity due to the refraction of light by the inorganic particles and the difference between the refractive index of the thermoplastic resin constituting the sheet and the refractive index of air. However, the scope of the present invention is not limited to this theory.

[0013] The fact that the sheet of the present invention has a high infrared reflectance is advantageous in that it can suppress the temperature rise in a room or the like when the sheet is used, for example, as a building material or an agricultural material. From this viewpoint, the porous sheet of the present invention preferably has an infrared reflectance of 80% or more, more preferably 85% or more, still more preferably 90% or more, and even more preferably 95% or more. The method for measuring the infrared reflectance will be described in the examples described later.

[0014] Examples of the thermoplastic resin constituting the porous sheet of the present invention include polyolefin resins such as polyethylene (PE), linear low-density polyethylene (LLDPE), branched low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), ethylene-propylene copolymer, and polybutene; polyester resins such as polyethylene terephthalate (PET); polyamide resins; vinyl resins such as polyvinyl chloride and polystyrene; acrylic resins such as polyacrylic acid and polymethyl methacrylate; and fluorine resins such as polyperfluoroethylene. These resins can be used alone or in combination of two or more. Among these, from the viewpoint of enhancing the moldability of the sheet, it is preferable to contain a polyolefin resin, and it is more preferable to contain polypropylene.

[0015] The porous sheet of the present invention contains inorganic particles in addition to the thermoplastic resin. The inorganic particles are preferably present in the porous sheet in a state kneaded into the thermoplastic resin. As the inorganic particles, those made of a material having no compatibility with the thermoplastic resin are preferably used because a large number of pores can be successfully formed in the porous sheet of the present invention. Also, the inorganic particles preferably have a high refractive index of light. Further, it is also preferable that the inorganic particles are likely to reflect and / or scatter light in the infrared wavelength region. Specific examples of inorganic particles include minerals such as gypsum, talc, clay, kaolin, silica, mica, zeolite, and diatomaceous earth; metal carbonates such as calcium carbonate, magnesium carbonate, and barium carbonate; metal oxides such as aluminum oxide (alumina), aluminum oxide, zinc oxide, and titanium oxide; metal sulfates such as sodium sulfate, calcium sulfate, magnesium sulfate, and barium sulfate; metal phosphates such as calcium phosphate; metal hydroxides such as aluminum hydroxide; carbon particles such as activated carbon and carbon black; and metal powders such as aluminum powder, iron powder, and copper powder. These can be used individually or in combination of two or more types. The shape of the inorganic particles may be spherical, lumpy, fibrous, or amorphous.

[0016] In particular, in order to impart high infrared reflectivity to the porous sheet of the present invention, it is preferable to use titanium dioxide, zinc oxide, aluminum oxide, silicon dioxide, and zirconium oxide as inorganic particles, and it is especially preferable to use titanium dioxide. When titanium dioxide is used as inorganic particles, any of the rutile, anatase, or brookite crystal structures may be used. However, rutile-type titanium dioxide is preferred due to its high refractive index.

[0017] In the porous sheet of the present invention, from the viewpoint of exhibiting high infrared reflectivity, the pore volume ratio is preferably 10% or more, and more preferably 30% or more. Similarly, in the porous sheet of the present invention, the pore volume ratio is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less.

[0018] In the porous sheet of the present invention, from the viewpoint of exhibiting high infrared reflectivity, the pore diameter is preferably 35 nm or larger, and more preferably 250 nm or larger. Similarly, in the porous sheet of the present invention, the pore diameter is preferably 50 μm or smaller, and more preferably 30 μm or smaller.

[0019] From a similar viewpoint, the cumulative pore volume of the pores formed in the porous sheet of the present invention is preferably 0.4 mL / g or more, and more preferably 0.8 mL / g or more. Furthermore, the cumulative pore volume is preferably 5 mL / g or less, more preferably 3 mL / g or less, and even more preferably 2 mL / g or less.

[0020] The pore volume ratio, pore diameter, and cumulative pore volume of the porous sheet of the present invention can be measured by the following method in accordance with the mercury injection method specified in JIS R 1655. In detail, a 2-3 g sample is cut from the sheet to be measured, and the measurement cell containing the sample is set in a mercury porosimeter (Autopore IV9520, manufactured by Micromeristics), and the pore volume of the sample is measured. Then, the relationship between the converted pore diameter D0, which is calculated according to the following formula (1), and the pore volume is plotted. D0 = -4γcosθ / P ···(1) (γ: surface tension of mercury, θ: contact angle, P: pressure)

[0021] The above measurements are performed at 22°C and under 65% RH conditions. The surface tension γ of mercury is 480 dyn / cm, the contact angle θ is 140°, and the mercury injection pressure P is in the range of 0 psia to 60,000 psia. Based on the distribution curve of the equivalent pore size D0 obtained under these measurement conditions, the cumulative sum of pore volumes corresponding to the equivalent pore size in the range of 0.0018 μm to 100 μm is defined as the cumulative pore volume (mL / g), and the median value of the pore size in the distribution curve is defined as the pore size (μm) of the present invention. The pore volume fraction (%) is the value calculated from the following formula (2). Pore ​​volume fraction (%) = 100 × ((Density of inorganic particle-containing thermoplastic resin [g / cm³]) 3 ])-(Density of porous sheet [g / cm³] 3 ])) / (Density of inorganic particle-containing thermoplastic resin [g / cm³ 3 ]) ···(2)

[0022] The size of the inorganic particles contained in the porous sheet of the present invention is related to its infrared reflectivity. The size of the inorganic particles is also related to the strength and moldability of the porous sheet. From these viewpoints, the average particle size W3 of the inorganic particles is preferably 30 nm or more, preferably 150 nm or more, and more preferably 200 nm or more. Furthermore, the average particle size W3 of the inorganic particles may be 5 μm or less, 4 μm or less, or 3 μm or less.

[0023] The particle size of inorganic particles is defined, for example, as the average value obtained by observing 50 or more inorganic particles observed on the surface of a sheet using a scanning electron microscope and measuring the maximum diameter (Ferret diameter) of each inorganic particle. When a porous sheet contains multiple types of inorganic particles, the average particle size W3 of the inorganic particles is the value measured by the method described above for all types of inorganic particles.

[0024] From the viewpoint of achieving high infrared reflectivity, the inorganic particle content in the porous sheet of the present invention is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more. Furthermore, the inorganic particle content is preferably 70% by mass or less, more preferably 65% ​​by mass or less, and even more preferably 60% by mass or less. It is preferable that the porous sheet of the present invention contains only inorganic particles as particles and does not contain metal particles (including both elemental metal particles and alloy particles).

[0025] From the viewpoint of achieving a high level of both sheet strength and infrared reflectivity, the content of thermoplastic resin in the porous sheet is preferably 33% by mass or more, more preferably 38% by mass or more, even more preferably 40% by mass or more, and even more preferably 50% by mass or more. From a similar viewpoint, the content of thermoplastic resin in the porous sheet is preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and even more preferably 55% by mass or less.

[0026] As previously stated, the porous sheet of the present invention has breathability because one surface and the other surface are connected by pores. The breathability of the porous sheet of the present invention is preferably 4000 s / 100 mL or less, more preferably 3000 s / 100 mL or less, even more preferably 2000 s / 100 mL or less, and even more preferably 1000 s / 100 mL or less, as measured in accordance with JIS P8117. Furthermore, the air permeability resistance of the porous sheet of the present invention is preferably 10 s / 100 mL or more, more preferably 30 s / 100 mL or more, even more preferably 50 s / 100 mL or more, and even more preferably 600 s / 100 mL or more.

[0027] To ensure sufficient breathability, it is preferable that each surface of the porous sheet of the present invention has an area that is not covered by a solid resin layer (hereinafter, this area is also referred to as the "exposed area"). As a result, at least a portion of the surface of the porous sheet of the present invention is exposed to the outside world, so that sufficient breathability can be ensured. From this viewpoint, it is preferable that the exposed area of ​​each surface of the porous sheet of the present invention is 20% or more, more preferably 30% or more, and even more preferably 40% or more. Most preferably, in the porous sheet of the present invention, there is no solid resin layer on any of the surfaces. In other words, the exposed area is 100%. If the porous sheet of the present invention does not have a solid resin layer on any of its surfaces, the porous sheet may be a single-layer sheet made entirely of porous material, or it may be a multilayer sheet having two or more single-layer sheets made of porous material. A solid resin layer includes both a resin layer that has no pores at all, and a resin layer that has pores but does not have pores that open on the surface and do not connect one surface to the other. There are no restrictions on the type of resin. That the porous sheet has an exposed area can be confirmed by elemental analysis such as SEM-EDS. Specifically, when elements derived from inorganic particles are detected by elemental analysis of the sheet surface, it can be determined that the porous sheet has an exposed area. When only elements derived from the resin are detected, it can be determined that the porous sheet does not have an exposed area.

[0028] In addition to having air permeability as described above, it is also preferable that the porous sheet of the present invention has high water resistance. That the porous sheet has water resistance is advantageous when the porous sheet is used, for example, as a building material or an agricultural material. From this perspective, the porous sheet of the present invention preferably has a water pressure resistance of 3000 mmAq or more, more preferably 10000 mmAq or more, still more preferably 20000 mmAq or more, and even more preferably 27000 mmAq or more. The water pressure resistance is measured in accordance with Method A (low water pressure method) of the water resistance test (hydrostatic pressure method) of JIS L1092-1998.

[0029] From the perspective of improving the infrared reflection ability, air permeability, and water resistance in a well-balanced manner, the basis weight of the porous sheet is preferably 10 g / m 2 or more, more preferably 20 g / m 2 or more, still more preferably 30 g / m 2 or more, and even more preferably 45 g / m 2 or more. Also, the basis weight of the porous sheet is preferably 200 g / m 2 or less, more preferably 150 g / m 2 or less, still more preferably 100 g / m 2 or less, and even more preferably 80 g / m 2 or less.

[0030] In relation to the above-mentioned basis weight, the thickness of the porous sheet is preferably 15 μm or more, more preferably 25 μm or more, still more preferably 30 μm or more, and even more preferably 50 μm or more. Furthermore, the thickness of the porous sheet is preferably 150 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less. The thickness of the porous sheet within this range provides a balance between infrared reflectivity, breathability, water resistance, and strength. The thickness of the porous sheet can be measured using a micrometer and the method specified in JIS K 7130:1999.

[0031] Figure 1 shows a scanning electron microscope image illustrating one embodiment of the porous sheet of the present invention. As shown in the figure, the porous sheet 1 has a plurality of stem fibers 2 extending in one direction. When focusing on any one of the plurality of stem fibers 2, the stem fibers 2 include the stem fiber 2a and other stem fibers 2b that branch off from the stem fiber 2a. The stem fiber 2a has a structure that branches off from other stem fibers and has a joint point 2p where the stem fibers 2 are partially joined together. The thickness of each stem fiber 2a, 2b is thinner than the thickness of the part where the joint point 2p is formed. The thickness of the stem fibers 2 is the same as or different from that of the other stem fibers 2. The cross-sectional shape of the stem fibers 2 can be a perfect circle, an ellipse or other circular shape, a polygon such as a triangle, a square, a pentagon, a plate shape, or a combination thereof. A single stem fiber 2 may have a combination of these shapes, and each stem fiber 2 may have one of the shapes described above.

[0032] Between each stem fiber 2a, 2b, there are three-dimensional voids that do not contain thermoplastic resin, penetrating in the sheet surface direction and thickness direction. These voids are interconnected, forming fine pores in the porous sheet 1. As mentioned earlier, these pores have an open-cell structure.

[0033] The porous sheet 1 has multiple fibril fibers 3 that are smaller in diameter than the stem fibers 2. The fibril fibers 3 extend between the stem fibers 2, 2 and are formed to branch off from at least one stem fiber 2. Possible forms of the fibril fibers 3 include, for example, (i) branching off from a single stem fiber 2, with one end of the fibril fiber 3 being a fixed end bonded to the stem fiber 2 and the other end being a free end not bonded to any other fiber, or (ii) branching off from a single stem fiber 2, with one end of the fibril fiber 3 bonded to the stem fiber 2 and the other end being a fixed end bonded to the same or a different stem fiber 2, or to another fibril fiber 3. Of these, from the viewpoint of increasing infrared reflectivity, it is preferable that the fibril fibers 3 exist in the form of (ii) described above.

[0034] As described above, the porous sheet 1 of this embodiment has a plurality of stem fibers 2 and fibril fibers 3 that are smaller in diameter than the stem fibers 2. Both the stem fibers 2 and the fibril fibers 3 contain a thermoplastic resin. In this specification, fibril fibers refer to fibers with a diameter of 250 nm or less. Fibers with a diameter greater than 250 nm are considered stem fibers.

[0035] The fibril fibers 3 are formed to branch off from the stem fibers 2, and the fibril fibers 3 contain the same type of thermoplastic resin as the stem fibers 2, preferably both the stem fibers 2 and the fibril fibers 3 are formed from the same thermoplastic resin. The thickness of the fibril fibers 3 is the same as or different from that of the other fibril fibers 3. The cross-sectional shape of the fibril fibers 3 can be a circular shape such as a perfect circle or ellipse, a polygonal shape such as a triangle, square, or pentagon, a plate-like shape, or a combination thereof. A single stem fiber 2 may have a combination of these shapes, and each fibril fiber 3 may have one of the shapes described above.

[0036] In addition to the spaces between the stem fibers 2, there are also spaces between the stem fibers 2 and the fibril fibers 3, and spaces between the fibril fibers 3 themselves, where thermoplastic resin is absent, and these spaces penetrate the sheet surface direction and thickness direction in three dimensions. This forms a three-dimensional matrix in which the fibers are arranged in a mesh-like manner, and the spaces formed between the fibers communicate with each other to form fine pores in the porous sheet 1. As mentioned earlier, these pores have an open-cell structure.

[0037] Examples of thermoplastic resins constituting the stem fibers 2 and fibril fibers 3 include polyethylene (PE), linear low-density polyethylene (LLDPE), branched low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polyolefin resins such as ethylene-propylene copolymer and polybutene, polyester resins such as polyethylene terephthalate (PET), polyamide resins, vinyl resins such as polyvinyl chloride and polystyrene, acrylic resins such as polyacrylic acid and polymethyl methacrylate, and fluororesins such as polyperfluoroethylene. These resins can be used individually or in combination of two or more. Of these, it is preferable to include a polyolefin resin, and even more preferable to include polypropylene, from the viewpoint of improving the moldability of the sheet.

[0038] The porous sheet 1 contains the inorganic particles described above, in addition to the stem fibers 2 and fibril fibers 3. As shown in Figure 1, the porous sheet 1 has multiple inorganic particles 5 held by the stem fibers 2. The inorganic particles 5 are dispersed in the planar direction of the porous sheet 1, and are arranged so that some or all of the inorganic particles 5 are exposed from the stem fibers 2. When some of the inorganic particles 5 are exposed, the remainder of the particles 5 are embedded in the fibers. The inorganic particles have the effect of making the sheet porous during the manufacturing of the fiber sheet, and are used to adsorb and retain oily stains such as oils and sebum when the sheet is used. Furthermore, inorganic particles 5 are present in greater quantities in the stem fibers 2 than in the fibril fibers 3. As a result, the inorganic particles 5 are stably retained within the porous sheet 1, and a high infrared reflectivity is stably maintained.

[0039] The porous sheet of this embodiment, having the above configuration, has stem fibers and fibril fibers that are smaller in diameter than the stem fibers, and fine pores consisting of air gaps are formed three-dimensionally between the fibers. As a result, the specific surface area is very high, and consequently, the difference in refractive index between air and the porous sheet is significantly apparent. Due to this, and the presence of inorganic particles, the porous sheet has a very high infrared reflectivity.

[0040] In particular, in the preferred mode of existence for fibril fibers, as described in (ii) above, where they branch off from a single stem fiber, with one end of the fibril fiber bonded to the stem fiber and the other end of the fibril fiber bonded to the same or a different stem fiber, or to another fibril fiber as a fixed end, sufficient fine pores are formed between the stem fiber and the fibril fiber. Therefore, compared to the case where the fibril fiber mainly exists with a free end, as described in (i) above, the infrared reflectivity is further increased.

[0041] In this embodiment, it is preferable that the porous sheet has a three-dimensional matrix in which stem fibers and fibril fibers form a network, with voids formed between each fiber. Having such a structure in the porous sheet makes it easy to set the void volume ratio of the porous sheet within the range described above.

[0042] From the viewpoint of facilitating the formation of pores within the preferred range described above and improving the manufacturing efficiency of porous sheets having stem fibers and fibril fibers, it is preferable that the porous sheet of the present invention is obtained by uniaxially stretching a resin sheet containing a thermoplastic resin and inorganic particles. In Figure 1, the direction indicated by the symbol X is the stretching direction of the resin sheet.

[0043] From a similar viewpoint, the thermoplastic resin contained in the porous sheet preferably has a melt flow rate of 0.5 g / min or more, more preferably 1 g / 10 min or more, and even more preferably 5 g / 10 min or more, as measured in accordance with JIS K 7210. Furthermore, the melt flow rate of the thermoplastic resin is preferably 60 g / 10 min or less, more preferably 40 g / 10 min or less, and even more preferably 20 g / 10 min or less. The melt flow rate can be measured according to JIS K 7210, by heating and applying load depending on the type of polyolefin resin. For example, when polypropylene, a polyolefin resin, is used as the thermoplastic resin, the measurement is performed under conditions of a temperature of 230°C and a load of 21.18 N.

[0044] From the viewpoint of further enhancing infrared reflectivity, the porous sheet of the present invention preferably has a number of fibers with a diameter of 500 nm or less among all the fibers constituting the porous sheet that is 1% or more, and more preferably 5% or more, of the total number of constituent fibers. Furthermore, it is preferable that the number of fibers with a diameter of 500 nm or less among all the fibers constituting the porous sheet be 99% or less, more preferably 95% or less, and even more preferably 90% or less. The diameter and number of fibers are determined by observing the sheet to be measured using a scanning electron microscope. For 100 or more observed fibers, the maximum length in the direction perpendicular to the direction of extension of each fiber is defined as the diameter, and the number of fibers with a diameter of 500 nm or less is measured. Then, based on the total number of observed fibers, the percentage of fibers with a diameter of 500 nm or less is calculated.

[0045] From the viewpoint of improving infrared reflectivity, breathability, and water resistance in a balanced manner, the fiber diameter W1 of the main fiber is preferably 500 nm or more, more preferably 750 nm or more, and even more preferably 1000 nm or more. Furthermore, the fiber diameter W1 of the stem fibers is preferably 20 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. On the other hand, the fiber diameter W2 of the fibril fiber is preferably 100 nm or more, more preferably 150 nm or more, and even more preferably 200 nm or more, provided that it is thinner than the fiber diameter W1 of the stem fiber. Furthermore, the fiber diameter W2 of the fibril fiber is preferably 1000 nm or less, more preferably 800 nm or less, and even more preferably 500 nm or less. The fiber diameters W1 and W2 of the stem fibers and fibril fibers are synonymous with the fiber diameters described above and can be measured in the same manner as described above.

[0046] From the viewpoint of further enhancing the infrared reflectivity of the porous sheet, the ratio of the average particle size W3 (μm) of inorganic particles to the fiber diameter W1 (μm) of the main fibers (W3 / W1) is preferably 0.1 or higher, more preferably 0.2 or higher, and even more preferably 0.3 or higher. Furthermore, the value of W3 / W1 is preferably 5 or less, more preferably 3 or less, and even more preferably 2 or less.

[0047] From the viewpoint of further enhancing the infrared reflectivity of the porous sheet, the ratio of the average particle size W3 (μm) of inorganic particles to the fiber diameter W2 (μm) of fibril fibers (W3 / W2) is preferably 0.2 or more, more preferably 0.5 or more, and even more preferably 1 or more. Furthermore, the value of W3 / W2 is preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less.

[0048] From the viewpoint of improving the dispersibility of inorganic particles and forming many fine pores throughout the sheet, thereby further enhancing infrared reflectivity, the porous sheet of the present invention preferably contains an inorganic particle dispersant. Examples of inorganic particle dispersants include silicone oil, fatty acids, maleic acid-modified polypropylene, maleic acid-modified polyethylene, etc. These can be used individually or in combination. Of these dispersants, it is even more preferable for the porous sheet to contain silicone oil from the viewpoint of providing excellent dispersibility of inorganic particles. The dispersant for inorganic particles is contained in the stem fibers and fibril fibers.

[0049] When using silicone oil as a dispersant, examples of silicone oils that can be used include dimethylpolysiloxane, dimethylcyclopolysiloxane, methylphenylpolysiloxane, methylhydrogenpolysiloxane, and higher alcohol-modified organopolysiloxane. These can be used individually or in combination of two or more. Of these, dimethyl silicone oil is even more preferable from the viewpoint of improving compatibility with thermoplastic resins that are raw materials for porous sheets and further improving the dispersibility of inorganic particles.

[0050] When the porous sheet contains an inorganic particle dispersant, the content of the inorganic particle dispersant in the porous sheet is preferably 0.5% by mass or more, more preferably 1.5% by mass or more, and even more preferably 3% by mass or more. Furthermore, the content of the dispersant for inorganic particles in the porous sheet is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably 8% by mass or less. The aforementioned content of the dispersant allows for uniform porosity of the raw resin sheet during the manufacturing of the porous sheet, and further enhances the infrared reflectivity of the porous sheet.

[0051] When a porous sheet contains an inorganic particle dispersant, the inorganic particles may be treated products that have been surface-treated with the dispersant beforehand. When treating the surface of the inorganic particles with a dispersant, it is preferable to use a dispersant that hydrophobicizes the surface. As such a dispersant, for example, the dispersants described above can be used as appropriate.

[0052] When polypropylene is used as the thermoplastic resin in the porous sheet of the present invention, it is preferable that the porous sheet contains a resin crystal nucleating agent from the viewpoint of forming many fine pores uniformly throughout the sheet and further improving the infrared reflectivity. In particular, it is even more preferable that the porous sheet contains a polypropylene β-crystal nucleating agent, which is used as a resin crystal nucleating agent to facilitate the formation of a β-crystal structure in the polypropylene resin. By incorporating resin nucleating agents such as β-nucleating agents into porous sheets, the physical properties of thermoplastic resins such as polypropylene are altered, increasing the efficiency of fibril fiber formation during the manufacturing of porous sheets and enabling the production of uniform porous sheets. Simultaneously, fine pores between the stem fibers and fibril fibers can be efficiently formed throughout the sheet. As a result, the infrared reflectivity, breathability, and water resistance of the porous sheet can be improved in a balanced manner. When a porous sheet is manufactured using a resin nucleating agent, the resin nucleating agent is contained within both the stem fibers and fibril fibers.

[0053] When polypropylene is used as the thermoplastic resin, suitable β-nucleating agents for the present invention include, for example, 2-N,6-N-dicyclohexylnaphthalene-2,6-dicarboxamide, quinacridone, N,N'-diphenylhexanediamide, 1-N,4-N-dicyclohexylbenzene-1,4-dicarboxamide, N-[4-(cyclohexanecarbonylamino)phenyl]cyclohexanecarboxamide, N-(4-benzamidecyclohexyl)benzamide, N-(5-benzamidenaphthalene-1-yl)benzamide, N-[4-(cyclohexanecarbonylamino)cyclohexyl]cyclohexanecarboxamide, 4-(cyclohexanecarbonylamino)-N-cyclohexylbenzamide, and N-(5-anilinopentyl)benzamide. These can be used individually or in combination of two or more.

[0054] The content of the resin nucleating agent in the porous sheet is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.08% by mass or more, relative to the thermoplastic resin used. Furthermore, the content of the resin nucleating agent in the porous sheet is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less, relative to the thermoplastic resin used. Even when using the above-mentioned β-nucleating agent as a resin crystal nucleating agent, the content of the β-nucleating agent can be within the range described above.

[0055] In addition to the components described above, the porous sheet of the present invention may contain additives depending on the specific application of the porous sheet. Examples of additives include resin plasticizers, crosslinking agents, antistatic agents, pigments, antioxidants, etc. The additive content in the porous sheet can be 0.001% by mass or more and 10% by mass or less, expressed as the total amount of all additives.

[0056] Next, a preferred method for manufacturing the porous sheet of the present invention will be described. The method for manufacturing the porous sheet includes at least a step of uniaxially stretching a resin sheet containing a thermoplastic resin and inorganic particles.

[0057] First, a resin composition containing a thermoplastic resin and inorganic particles is molded into a resin sheet. The resin composition can be obtained, for example, by heating and kneading the thermoplastic resin and inorganic particles in a single-screw or twin-screw extruder to obtain a molten mixture. The heating temperature during heating and kneading can be set according to the type of thermoplastic resin; for example, for polyolefin resin, it can be 120°C to 210°C. The content of the thermoplastic resin and inorganic particles in the resin composition can be appropriately adjusted so that they fall within the above-mentioned content ranges.

[0058] From the viewpoint of improving the dispersibility of inorganic particles in the resin composition and forming finer fibril fibers, it is preferable to use a resin composition containing an inorganic particle dispersant. It is even more preferable to use silicone oil as the dispersant. If necessary, a resin crystal nucleating agent such as the β-crystal nucleating agent mentioned above may be added to the resin composition for purposes such as forming fibril fibers throughout the sheet and making the resulting porous sheet uniformly porous. The content of the inorganic particle dispersant and the content of additives such as resin crystal nucleating agents in the resin composition should be appropriately adjusted so as to fall within the above-mentioned content range.

[0059] Next, the molten resin composition containing the above-mentioned components is molded to obtain a resin sheet made of the resin composition. Since the resin sheet is formed from a resin composition containing thermoplastic resin and inorganic particles, the resin composition and the resin sheet have substantially the same composition. Therefore, the resin sheet contains thermoplastic resin and inorganic particles. The resin sheet is a non-porous sheet as it is in its pre-stretched state.

[0060] From the viewpoint of improving the efficiency of sheet manufacturing, the melt flow rate of the resin composition is preferably 0.5 g / 10 min or more, and more preferably 1 g / 10 min or more. Furthermore, the upper limit is preferably 60 g / 10 min or less, and more preferably 15 g / 10 min or less.

[0061] As a molding apparatus for forming a molten resin composition into a sheet, known molding apparatuses such as T-die molding apparatuses and inflation molding apparatuses can be used. The heating temperature during molding is not particularly limited as long as it is at least a temperature at which the resin composition can melt, but from the viewpoint of ease of molding and prevention of thermal decomposition of components in the resin composition, it is preferable that the temperature is (M+20)°C or higher and (M+60)°C or lower, where M (°C) is the melting point of the thermoplastic resin used as the raw material. If the resin composition is composed of multiple resins, the melting point M of the resin composition is set to the highest melting point among the constituent resins. When forming a molten resin composition into a sheet, the molded body may be allowed to cool naturally, or it may be forcibly cooled using cooling means as needed.

[0062] Next, the resin sheet obtained through the above-described process, i.e., the unstretched sheet, is stretched at least uniaxially. By stretching, interfacial delamination occurs between the thermoplastic resin and inorganic particles contained in the resin composition, making the sheet porous. As a result, the fibers containing the thermoplastic resin form a three-dimensional matrix, and a mesh-like porous sheet with fine pores can be obtained. The porous sheet produced in this way is obtained by stretching a resin sheet that contains a thermoplastic resin and inorganic particles, and preferably further contains at least one additive such as an inorganic particle dispersant and a resin crystal nucleating agent. Therefore, the porous sheet and the resin sheet have substantially the same composition.

[0063] Methods for obtaining porous sheets by uniaxial stretching include, for example, the roll method and the tenter method. Furthermore, when obtaining porous sheets by uniaxial stretching, it is preferable to heat the resin sheet. When heat treatment is performed, the resin sheet is preferably heated to (M-80)°C or higher, more preferably (M-70)°C or higher, and preferably (M-15)°C or lower, and even more preferably (M-20)°C or lower, when the melting point of the thermoplastic resin used as the raw material is M (°C). If the resin sheet contains multiple thermoplastic resins, the melting point M is defined as the melting point of the thermoplastic resin with the highest melting point among the resins.

[0064] Specifically, when polypropylene (melting point M: 165°C) is used as the thermoplastic resin, the resin sheet is preferably heated to 85°C or higher, more preferably 90°C or higher, and even more preferably 100°C or higher, and then uniaxially stretched. The heating temperature when uniaxially stretching polypropylene is preferably 140°C or lower, more preferably 135°C or lower, and even more preferably 130°C or lower. When polyethylene (melting point M: 122°C) is used as the thermoplastic resin, the resin sheet is preferably heated to 42°C or higher, more preferably to 52°C or higher, and even more preferably to 60°C or higher, and then uniaxially stretched. The heating temperature when uniaxially stretching polyethylene is preferably 107°C or lower, more preferably 102°C or lower, and even more preferably 100°C or lower. By performing uniaxial stretching at the above heating temperatures, the resin sheet can be uniformly stretched while interfacial delamination occurs, successfully forming a porous sheet having stem fibers, fibril fibers, and voids. There are no particular restrictions on the method of heating the resin sheet; for example, the resin sheet can be brought into contact with a heated roll, or the resin sheet can be heated with hot air or a heater.

[0065] A resin sheet using polypropylene as the thermoplastic resin and containing a resin crystal nucleating agent, preferably a β-crystal nucleating agent for polypropylene, is designed to readily exhibit a β-crystal crystalline structure. Compared to the α-crystal crystalline structure that polypropylene can stably adopt, the β-crystal crystalline structure has a lower melting point and softer physical properties. When such a resin sheet is uniaxially stretched, it is easily stretched uniformly and without unevenness, so that a three-dimensional matrix having stem fibers and fibril fibers is uniformly formed throughout the sheet, making it easy to produce a porous sheet with a mesh-like structure containing fine pores. Furthermore, the resulting porous sheet exhibits even greater infrared reflectivity.

[0066] From the viewpoint of successfully forming a porous sheet having stem fibers, fibril fibers, and voids while preventing the sheet from breaking during stretching, the stretching speed of the resin sheet in uniaxial stretching is preferably 7000 mm / second or less, and more preferably 5000 mm / second or less. Furthermore, from the viewpoint of productivity, it is preferably 500 mm / second or more, and more preferably 1000 mm / second or more.

[0067] When obtaining a porous sheet by uniaxial stretching of a resin sheet, it is preferable to stretch the resin sheet so that its area doubles or more, more preferably so that its area increases by 3.5 times or more, and even more preferably so that its area increases by 4 times or more. The upper limit of the stretching ratio is preferably 8 times or less, more preferably 6.5 times or less, and even more preferably 6 times or less. By stretching the resin sheet to achieve this stretch ratio, interfacial delamination can be induced, successfully producing a porous sheet containing stem fibers and fibril fibers. The stretch ratio of the resin sheet can be achieved by appropriately adjusting the heating temperature and stretching speed as described above.

[0068] The porous sheet manufactured through the above process contains a thermoplastic resin, inorganic particles, preferably a dispersant, and preferably a resin crystal nucleating agent such as a β-crystal nucleating agent, and has multiple fine pores. The porous sheet of the present invention has a three-dimensional matrix formed by its constituent fibers, stem fibers and fibril fibers, and inorganic particles are dispersed within it, thus possessing high infrared reflectivity. Taking advantage of this benefit, the porous sheet of the present invention is useful as an agricultural material and a building material. Examples of agricultural materials include films for covering the exterior of greenhouses, films for covering the interior of greenhouses, tunnel films, and mulch films. Examples of building materials include sheets used for the walls, roofs, and floors of buildings, such as building exterior wall underlayment materials.

[0069] Conventional building exterior wall substrates, such as the one described in Non-Patent Document 1 above, consist of a porous resin sheet with aluminum vapor-deposited onto it. Other building exterior wall substrates also exhibit infrared reflectivity through a combination of a porous sheet and metal foil. In contrast to this, the porous sheet of the present invention exhibits infrared reflectivity superior to that of the building exterior wall substrate described in Non-Patent Document 1, on its own. Thus, when the porous sheet of the present invention is used as a building exterior wall substrate, it is advantageous in that it eliminates the need for metal vapor deposition or metal foil.

[0070] Although the present invention has been described above based on its embodiments, the present invention is not limited to the above embodiments. For example, in the above embodiments, uniaxial stretching of a resin sheet was exemplified as a preferred method for manufacturing the porous sheet of the present invention, but the method for manufacturing the porous sheet is not limited thereto.

[0071] Furthermore, although the above embodiments mainly described the use of the porous sheet of the present invention alone, it may also be used in combination with other sheets, etc., as long as the effects of the present invention are not impaired. [Examples]

[0072] 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 these examples. Unless otherwise specified, "%" means "mass%".

[0073] [Example 1] Polypropylene (melting point: 165°C, melt flow rate: 11g / 10min) was used as the thermoplastic resin, titanium dioxide (rutile type) with an average particle size of 1μm was used as the inorganic particles, silicone oil (dimethyl silicone oil) was used as the dispersant for the inorganic particles, and a β-crystal nucleating agent (NJester NU-100, Shin-Nippon Rika Co., Ltd.) was used as the resin crystal nucleating agent. These were heated and kneaded using a twin-screw extruder to obtain a resin composition. The obtained resin composition was molded using a T-die molding apparatus to obtain a resin sheet. The resin sheet contained 53% thermoplastic resin, 42% by mass of inorganic particles, 4.95% by mass of dispersant, and 0.05% by mass of β-nucleating agent. The resulting resin sheet (melt flow rate: 12 g / 10 min) is uniaxially stretched so that the area of ​​the resin sheet is 5 times its original size, resulting in a yield of 70 g / m². 2 A porous sheet with a basis weight was manufactured. The conditions for uniaxial stretching were a heating temperature of 100°C and a stretching speed of 3000 mm / second. Microscopic observation of the obtained porous sheet revealed a porous structure with stem fibers and fibril fibers similar to that shown in Figure 1. The fiber diameter W1 of the stem fibers was 5000 nm, and the fiber diameter W2 of the fibril fibers was 250 nm. The proportion of fibers with a diameter of 500 nm or less was 50%.

[0074] [Examples 2 to 6] A porous sheet was obtained in the same manner as in Example 1, except that the composition of the resin composition was as shown in Table 1 below. Microscopic observation of the obtained porous sheet confirmed a porous structure having stem fibers and fibril fibers similar to those shown in Figure 1.

[0075] [Examples 7 to 10] Titanium dioxide (rutile type) with an average particle size of 250 nm was used as the inorganic particles. The composition of the resin composition was as shown in Table 1 below. A porous sheet was obtained in the same manner as in Example 1, except for these factors. Microscopic observation of the obtained porous sheet confirmed a porous structure with stem fibers and fibril fibers similar to those shown in Figure 1.

[0076] [Examples 11 and 12] Titanium dioxide (rutile type) with an average particle size of 35 nm was used as the inorganic particles. The composition of the resin composition was as shown in Table 1 below. A porous sheet was obtained in the same manner as in Example 1, except for these factors. Microscopic observation of the obtained porous sheet confirmed a porous structure with stem fibers and fibril fibers similar to those shown in Figure 1.

[0077] [Comparative Examples 1 to 3] The composition of the resin composition was as shown in Table 1 below. A porous sheet was obtained in the same manner as in Example 1, except for the composition of the resin composition.

[0078] [Examples 13 to 15] As inorganic particles, zinc oxide with the average particle size shown in Table 2 was used. The composition of the resin composition was as shown in Table 2 below. A porous sheet was obtained in the same manner as in Example 1, except for these factors. Microscopic observation of the obtained porous sheet confirmed a porous structure with stem fibers and fibril fibers similar to those shown in Figure 1.

[0079] [Example 16] As inorganic particles, silica with the average particle size shown in Table 2 was used. The composition of the resin composition was as shown in Table 1 below. A porous sheet was obtained in the same manner as in Example 1, except for these factors. Microscopic observation of the obtained porous sheet confirmed a porous structure with stem fibers and fibril fibers similar to those shown in Figure 1.

[0080] 〔evaluation〕 The infrared reflectance of the sheets obtained in the examples and comparative examples was measured using the following method. Furthermore, the air permeability resistance and water pressure resistance were measured using the method described above. In addition, the basis weight and thickness were measured. These results are shown in Tables 1 and 2 below.

[0081] [Measurement of infrared reflectance] Measurements were performed using a spectrophotometer (Shimadzu UV-3600) with an integrating sphere, with barium sulfate as the reference sample. The average value of the reflectance in the 350-2100 nm range was defined as the infrared reflectance.

[0082] [Table 1]

[0083] [Table 2]

[0084] As is clear from the results shown in Tables 1 and 2, stretching an unstretched resin sheet increases its infrared reflectivity. In particular, using titanium dioxide with an average particle size of 250 nm or more as inorganic particles further increases the infrared reflectivity.

Claims

1. A porous sheet containing thermoplastic resin and inorganic particles, having numerous pores, The numerous holes communicate with each other, thereby giving the porous sheet breathability. Each surface of the porous sheet has a region that is not covered by a solid resin layer. The solid resin layer is either a resin layer without pores, or a resin layer having pores but with openings on its surface and without pores connecting one surface to the other. A porous sheet having a void volume ratio of 10% or more.

2. The porous sheet according to claim 1, wherein the infrared reflectance is 80% or more.

3. The porous sheet according to claim 1, wherein the average particle size of the inorganic particles is 5 μm or less.

4. The porous sheet according to claim 1, wherein the average particle size of the inorganic particles is 30 nm or more.

5. The porous sheet according to claim 1, wherein the inorganic particle content is 30% by mass or more.

6. The porous sheet according to claim 1, wherein the inorganic particles contain titanium oxide.

7. The porous sheet according to claim 1, wherein the air permeability resistance is 4000 s / 100 mL or less.

8. The porous sheet according to claim 1, wherein the water pressure resistance is 3000 mmAq or more.

9. The system comprises a plurality of stem fibers containing the aforementioned thermoplastic resin and extending in one direction, and fibril fibers containing the same type of thermoplastic resin as the stem fibers, extending between the stem fibers and having a smaller diameter than the stem fibers, The plurality of stem fibers include one stem fiber and other stem fibers branched from said stem fiber, and have voids between the stem fibers. The porous sheet according to claim 1, wherein a plurality of the inorganic particles are held in the stem fibers.

10. An agricultural material comprising a porous sheet according to any one of claims 1 to 9.

11. A building exterior wall substrate material comprising a porous sheet as described in any one of claims 1 to 9.

12. A method for manufacturing a porous sheet according to any one of claims 1 to 9, A method for manufacturing a porous sheet, comprising the step of stretching a resin sheet containing the thermoplastic resin and the inorganic particles to form the numerous holes in the resin sheet.

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