Porous film and producing method thereof

Abstract

To provide a porous film in which generation of pin holes is suppressed at the time of production.SOLUTION: It is a porous film having at least a two-layer structure. An olefin resin composition, an inorganic filler of 50 pts.mass or more and 400 pts.mass or less with respect to 100 pts.mass of the olefin-based resin composition, and a fatty acid of 0.5 pts.mass or more and 5.0 pts.mass or less with respect to 100 pts.mass of the inorganic filler. The olefin resin composition includes a low melting point olefin resin having a melting point of less than 80°C, and a high melting point olefin resin having a melting point of 80°C or higher. The two opposing layers in the two-layer structure are made of materials having the same composition, and the peel strength between the two opposing layers is 5 N / m width or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a porous film and a method for producing the same. [Background technology]

[0002] Conventionally, a porous film obtained by stretching a composition containing a thermoplastic resin and an inorganic filler has been known. The porous film has high water pressure resistance and moisture permeability, and is therefore used in various absorbent articles and the like. For example, Patent Document 1 describes a material with a density of 0.910 g / cm 3 A layer (I) made of a resin composition containing 25% by mass to 54% by mass of the above olefin resin and 46% by mass to 75% by mass of an inorganic filler, and a layer (I) having a density of 0.850 g / cm 3 More than 0.910g / cm 3 The document describes a stretched porous laminated non-woven film having at least two layers of layer (II) made of a resin composition containing 25% by mass to 54% by mass of an olefin resin and 46% by mass to 75% by mass of an inorganic filler. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-131694 A Summary of the Invention [Problem to be solved by the invention]

[0004] By the way, a porous film is generally manufactured by melt molding a composition containing a thermoplastic resin and an inorganic filler, and stretching the obtained resin sheet. By carrying out this stretching process, fine holes can be formed inside the resin sheet, and moisture permeability can be imparted to the porous film. However, in this stretching process, holes (hereinafter also referred to as "pinholes") may occur in the porous film, although the frequency is relatively low. Since pinholes are one of the causes of a decrease in the water pressure resistance of the porous film, for example, when a porous film having pinholes is used in a diaper, it causes excrement to seep out. Therefore, when manufacturing a porous film, it is necessary to monitor the occurrence of pinholes and remove the areas where pinholes have occurred. However, this work is one of the causes of increasing the manufacturing cost of the porous film.

[0005] Therefore, an object of the present invention is to provide a porous film that has both high moisture permeability and suppresses the occurrence of pinholes during production, and a method for producing the same. [Means for solving the problem]

[0006] The present invention relates to a porous film having at least a two-layer structure. In one embodiment, the porous film preferably contains an olefin-based resin composition, an inorganic filler in an amount of 50 parts by mass or more and 400 parts by mass or less per 100 parts by mass of the olefin-based resin composition, and a fatty acid in an amount of 0.5 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the inorganic filler. In one embodiment, the olefin resin composition preferably contains a low-melting-point olefin resin having a melting point of less than 80° C. and a high-melting-point olefin resin having a melting point of 80° C. or higher. In one embodiment, it is preferred that the two opposing layers in the two-layer structure are made of materials of the same composition, and that the peel strength between the two opposing layers is 5 N / m width or more.

[0007] The present invention further relates to a method for producing a porous film having at least a two-layer structure. In one embodiment, the production method includes an inflation molding step of inflation molding an olefin resin composition containing a low-melting point olefin resin having a melting point of less than 80°C and a high-melting point olefin resin having a melting point of 80°C or higher, an inorganic filler, and a composition containing a fatty acid to obtain a tubular film. In one embodiment, the manufacturing method includes a flattening step of flattening the tubular film to form a flat body having two opposing surfaces. In one embodiment, the manufacturing method includes a stretching step of stretching the flat body. In one embodiment, the manufacturing method includes a heat treatment step of heat treating the flat body after stretching at a temperature equal to or higher than the melting point of the low-melting point olefin-based resin to fuse two opposing surfaces of the flat body. In one embodiment, the composition contains 50 parts by mass or more and 400 parts by mass or less of the inorganic filler per 100 parts by mass of the olefin-based resin composition, and contains 0.5 parts by mass or more and 5 parts by mass or less of the fatty acid per 100 parts by mass of the inorganic filler. Effect of the Invention

[0008] According to the present invention, there is provided a porous film having high moisture permeability and suppressing the occurrence of pinholes. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing one embodiment of the porous film of the present invention, and is a cross-sectional view that typically shows a cross section along the thickness direction of the porous film. [Diagram 2] FIG. 2 is a schematic diagram showing one embodiment of a porous film production apparatus used in the method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention will be described below based on preferred embodiments with reference to the drawings. The porous film of the present invention has at least a two-layer structure. Figure 1 shows a porous film 3 having a two-layer structure consisting of a first layer 1 and a second layer 2 as one embodiment of the present invention.

[0011] The two opposing layers in the two-layer structure of the porous film 3, the first layer 1 and the second layer 2, are fused together, and the two layers are in close contact with each other with high peel strength. There are substantially no voids between the two layers due to non-fusion. Therefore, the porous film 3 is a single film material having a two-layer structure.

[0012] The first layer 1 and the second layer 2, which are two opposing layers in the two-layer structure of the porous film 3, are made of materials of the same composition. Therefore, the first layer 1 and the second layer 2 cannot be distinguished by their compositions. However, by observing a cross section of the porous film 3 along the thickness direction with a scanning electron microscope, an interface is observed between the first layer 1 and the second layer 2, which are fused to each other, and it can be confirmed that the porous film 3 has the first layer 1 and the second layer 2. In this specification, "materials of the same composition" means that the types of components constituting the first layer 1 and the second layer 2 are the same, and the content ratios of each component are the same. As long as the first layer 1 and the second layer 2 are made of materials with the same composition, their thicknesses and basis weights may be the same or different.

[0013] The first layer 1 and the second layer 2 contain an olefin resin composition as a constituent resin. In this specification, the term "olefin resin composition" includes both a case where only one type of various olefin resins is contained and a case where two or more types are contained. In addition, the term "olefin resin composition" is a concept that is composed only of various olefin resins and does not contain other resins or components other than resins. It is not prohibited that the first layer 1 and the second layer 2 contain a resin other than the olefin resin. The olefin resin composition used in the present invention is composed mainly of a polymer or copolymer of a monoolefin such as ethylene, propylene, butene, etc. Examples of such a polymer or copolymer include high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, an olefin elastomer, and a mixture of any two or more of these.

[0014] In the present invention, from the viewpoint of imparting flexibility to the porous film 3 and from the viewpoint of facilitating fusion of the first layer 1 and the second layer 2 in the manufacturing method described below, it is preferable that the olefin-based resin composition contains a low-melting point olefin-based resin.

[0015] The low-melting-point olefin resin preferably has a melting point less than 80° C., more preferably less than 75° C., and even more preferably less than 70° C. By setting the melting point of the low-melting-point olefin resin to this temperature or lower, the low-melting-point olefin resin becomes easier to melt when producing the porous film 3, and the occurrence of pinholes becomes easier to suppress. In order to obtain dimensional stability of the porous film 3, the low-melting-point olefin resin preferably has a melting point of 40° C. or higher. The melting point of the low melting point olefin resin is measured by the following method. Using approximately 2.0 mg of the porous film 3 as a sample, differential scanning calorimetry (DSC) is performed using a differential scanning calorimeter (DSC7000X, manufactured by Hitachi High-Tech Science Corporation) under conditions of a measurement temperature range of 10°C to 260°C, a heating rate of 10°C / min, and an air environment. In the obtained DSC curve, an endothermic peak is observed in a temperature range lower than 80°C, which occurs when the low melting point olefin resin melts. The melting point of the low melting point olefin resin is the temperature at the apex of the observed endothermic peak.

[0016] From the viewpoint of further suppressing the occurrence of pinholes during the production of the porous film 3, the low melting point olefin resin has a density of 0.895 g / cm 3 It is preferable that the density is 0.890 g / cm or less. 3 More preferably, it is 0.886 g / cm or less. 3It is even more preferable that: From the viewpoint of maintaining the strength of the porous film 3, the low melting point olefin resin has a density of 0.840 g / cm 3 It is preferable that the content is 0.850 g / cm or more. 3 More preferably, it is 0.860 g / cm or more. 3 More preferably, the above is the case. Considering the above, the density of low-melting-point olefin resin is 0.840 g / cm 3 More than 0.895g / cm 3 It is preferable that the density is 0.850 g / cm or less. 3 More than 0.890g / cm 3 More preferably, it is 0.860 g / cm or less. 3 More than 0.886g / cm 3 It is even more preferable that:

[0017] In order to easily enable the low-melting-point olefin-based resin to satisfy the above-mentioned preferred density range and the above-mentioned preferred melting point range, the low-melting-point olefin-based resin is preferably a random copolymer of olefin-based monomers.

[0018] In particular, the low melting point olefin resin is preferably a copolymer of ethylene and an α-olefin (hereinafter also referred to as "ethylene-α-olefin copolymer"). Examples of α-olefins include propylene, 1-butene, 1-pentene, and 1-hexene. In particular, a copolymer of ethylene and an α-olefin polymerized by a metallocene catalyst is more preferred because it further improves the strength of the film against tearing, punch-through, and the like.

[0019] The metallocene catalyst is a combination of a metallocene, which is a compound having a structure in which a transition metal such as titanium, zirconium, or hafnium is sandwiched between unsaturated cyclic compounds containing a π-electron system cyclopentadienyl group or a substituted cyclopentadienyl group, and a cocatalyst such as an aluminum compound. Examples of the metallocene include titanocene and zirconocene. Examples of the aluminum compound include alkylaluminoxane, alkylaluminum, aluminum halide, and alkylaluminum halide.

[0020] In the olefin resin composition used in the present invention, the content of the low melting point olefin resin is preferably 40% by mass or more from the viewpoint of imparting a satisfactory flexibility to the porous film 3 and from the viewpoint of appropriately controlling the peel strength between two opposing layers described below to suppress the occurrence of pinholes during production. In order to make this advantage more prominent, the content of the low melting point olefin resin in the olefin resin composition is more preferably 50% by mass or more. In addition, the content of the above-mentioned low melting point olefin resin in the olefin resin composition used in the present invention is preferably 95% by mass or less, since blocking is less likely to occur in the porous film 3. From the viewpoint of making this advantage more prominent, the content of the low melting point olefin resin in the olefin resin composition is more preferably 92% by mass, and even more preferably 90% by mass. Taking the above into consideration, the content of the above-mentioned low melting point olefin resin in the olefin resin composition is preferably 40 mass% or more and 95 mass% or less, more preferably 50 mass% or more and 92 mass% or less, and even more preferably 50 mass% or more and 90 mass% or less.

[0021] The olefin resin composition used in the present invention preferably contains a high-melting point olefin resin in addition to the above-mentioned low-melting point olefin resin, from the viewpoint of imparting further heat resistance, morphological stability, processability and moisture permeability to the porous film 3. From the viewpoint of achieving both the heat resistance, morphological stability and processability achieved by using a high-melting point olefin resin and the flexibility resulting from a low-melting point olefin resin, it is preferable that the high-melting point olefin resin has a relatively low density, specifically, 0.950 g / cm. 3 It is preferable that the density is 0.940 g / cm or less. 3 More preferably, it is 0.930 g / cm or less. 3 It is even more preferable that: In order to prevent blocking, the density of the high melting point olefin resin is set to 0.900 g / cm. 3 More preferably, it is equal to or greater than this. Considering the above, the density of high melting point olefin resin is 0.900g / cm 3 More than 0.950g / cm 3 It is preferable that the density is 0.900 g / cm or less. 3 More than 0.940g / cm 3 More preferably, it is 0.900 g / cm or less. 3 More than 0.930g / cm 3 It is even more preferable that:

[0022] As the high melting point olefin resin having the above density, it is preferable to use polyethylene such as low density polyethylene or linear low density polyethylene, and in particular, it is preferable to use linear low density polyethylene because it improves heat resistance during stretching and enables uniform stretching. In particular, linear low density polyethylene polymerized by a metallocene catalyst is more preferable because it further improves the strength of the film against tearing, punch-through, etc.

[0023] The olefin resin composition used in the present invention preferably contains 5 parts by mass or more of a high melting point olefin resin having the above-mentioned density per 100 parts by mass of the olefin resin composition, from the viewpoint of imparting further heat resistance, morphological stability, and processability to the porous film 3. From the viewpoint of making this advantage more prominent, the high melting point olefin resin is more preferably contained in an amount of 8 parts by mass or more, and even more preferably contained in an amount of 10 parts by mass or more, per 100 parts by mass of the olefin resin composition. Moreover, the olefin resin composition used in the present invention preferably contains the above-mentioned high melting point olefin resin in an amount of 60 parts by mass or less per 100 parts by mass of the olefin resin composition, from the viewpoint of achieving both the heat resistance, morphological stability, and processability achieved by using the high melting point olefin resin, and the flexibility of the porous film 3. To make this advantage even more remarkable, the high melting point olefin resin is more preferably contained in an amount of 50 parts by mass or less per 100 parts by mass of the olefin resin composition. Taking the above into consideration, the olefin-based resin composition contains a high melting point olefin-based resin in an amount of preferably 5 parts by mass or more and 60 parts by mass or less, more preferably 8 parts by mass or more and 50 parts by mass or less, and even more preferably 10 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the olefin-based resin composition.

[0024] The high melting point olefin resin used in the present invention preferably has a melting point of 80°C or higher, more preferably 90°C or higher, and even more preferably 95°C or higher, so as to achieve rapid solidification in a short period of time, in order to achieve high speed molding of the melt-molded porous film 3. The melting points of the high melting point olefin resins contained in the first layer 1 and the second layer 2 are measured by the following method. Using approximately 2.0 mg of the porous film 3 as a sample, differential scanning calorimetry (DSC) is performed using a differential scanning calorimeter (DSC7000X, manufactured by Hitachi High-Tech Science Corporation) under conditions of a measurement temperature range of 10°C to 260°C, a heating rate of 10°C / min, and an air environment. An endothermic peak that occurs when the high melting point olefin resin melts is observed in a temperature range of 80°C or higher on the obtained DSC curve. The apex of the endothermic peak is the melting point of the high melting point olefin resin.

[0025] The first layer 1 and the second layer 2 contain an inorganic filler. The inorganic filler is a substance that causes peeling at the interface with the olefin resin composition to form micropores. From this viewpoint, the inorganic filler has an average particle diameter D 50 is preferably 30 μm or less, and more preferably 10 μm or less. The inorganic filler has an average particle size D 50 The average particle diameter D of the inorganic filler is preferably 0.5 μm or more, and more preferably 1.0 μm or more. 50 refers to the weight cumulative particle size at a cumulative weight of 50% by mass as measured by a laser diffraction / scattering particle size distribution measurement method.

[0026] Examples of inorganic fillers include calcium carbonate, gypsum, talc, clay, kaolin, silica, diatomaceous earth, magnesium carbonate, barium carbonate, magnesium sulfate, barium sulfate, calcium phosphate, aluminum hydroxide, zinc oxide, titanium oxide, alumina, mica, zeolite, carbon black, and mixtures thereof. In particular, it is preferable to use calcium carbonate because it is easy to adjust the particle size to the above-mentioned size.

[0027] The inorganic filler is preferably contained in an amount of 50 parts by mass or more per 100 parts by mass of the olefin resin composition in order to form a sufficient amount of fine pores and sufficiently increase the moisture permeability of the porous film 3, more preferably 60 parts by mass or more, and even more preferably 80 parts by mass or more. In addition, it is preferable for the inorganic filler to be contained in an amount of 400 parts by mass or less per 100 parts by mass of the olefin resin composition, from the viewpoint of sufficiently enhancing the leakproofness of the porous film 3, more preferably 350 parts by mass or less, and even more preferably 200 parts by mass or less. Taking the above into consideration, the inorganic filler is preferably contained in an amount of 50 parts by mass or more and 400 parts by mass or less, more preferably 60 parts by mass or more and 350 parts by mass or less, and even more preferably 80 parts by mass or more and 200 parts by mass or less, per 100 parts by mass of the olefin-based resin composition.

[0028] The first layer 1 and the second layer 2 preferably contain a fatty acid. The fatty acid is used as a dispersant for the inorganic filler. The fatty acid is preferably one that can hydrophobize the surface of the inorganic filler, and specific examples of the fatty acid include caprylic acid, palmitic acid, stearic acid, capric acid, oleic acid, myristic acid, and lauric acid. In particular, it is preferable that the chain length of the hydrocarbon chain in the fatty acid is the same as the chain length of the hydrocarbon chain in the fatty acid constituting the metal soap described below, since this allows the metal soap to be transferred more smoothly to the inorganic filler surface-modified with the fatty acid.

[0029] In order to enhance the dispersibility of the inorganic filler, the first layer 1 and the second layer 2 preferably contain 0.5 parts by mass or more of the fatty acid relative to 100 parts by mass of the inorganic filler. To make this advantage more pronounced, the fatty acid is more preferably contained in an amount of 0.8 parts by mass or more, and even more preferably 1.0 part by mass or more, relative to 100 parts by mass of the inorganic filler. In order not to impair the formability of the film, it is preferable that the first layer 1 and the second layer 2 contain 5.0 parts by mass or less of the fatty acid relative to 100 parts by mass of the inorganic filler. To make this advantage more pronounced, the fatty acid is more preferably contained in an amount of 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less, relative to 100 parts by mass of the inorganic filler.

[0030] The first layer 1 and the second layer 2 may contain additives. Examples of such additives include metal soaps. By adding metal soaps, the generation of micropores during stretching of a resin film containing an olefin resin composition and an inorganic filler can be made smoother, and the moisture permeability of the porous film 3 can be further increased.

[0031] As the metal soap, metal salts of saturated or unsaturated fatty acids are preferably used. Examples of fatty acids include caprylic acid, palmitic acid, stearic acid, capric acid, oleic acid, myristic acid, lauric acid, etc. Examples of metal salts include calcium, aluminum, magnesium, zinc, etc. salts of these fatty acids.

[0032] In the first layer 1 and the second layer 2, the metal soap is preferably contained in an amount of 0.5 part by mass or more per 100 parts by mass of the olefin resin composition from the viewpoint of successfully generating micropores. From the viewpoint of making this advantage more prominent, the metal soap is preferably contained in an amount of 1.0 part by mass or more, and more preferably 2.0 parts by mass or more, per 100 parts by mass of the olefin resin composition. In order to maintain good moldability, the metal soap is preferably contained in an amount of 20 parts by mass or less per 100 parts by mass of the olefin-based resin composition in the first layer 1 and the second layer 2. To make this advantage more pronounced, the metal soap is preferably contained in an amount of 15 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the olefin-based resin composition.

[0033] The amount of metal soap contained in the first layer 1 and the second layer 2 is also related to the amount of inorganic filler contained in the first layer 1 and the second layer 2. In particular, in the first layer 1 and the second layer 2, the metal soap is preferably contained in an amount of 0.5 parts by mass or more per 100 parts by mass of the inorganic filler, from the viewpoint of successfully generating micropores. From the viewpoint of making this advantage more prominent, the metal soap is preferably contained in an amount of 1.5 parts by mass or more, and even more preferably 2.0 parts by mass or more, per 100 parts by mass of the inorganic filler. In order to maintain good moldability, the metal soap is preferably contained in an amount of 15 parts by mass or less per 100 parts by mass of the inorganic filler in the first layer 1 and the second layer 2. To make this advantage more pronounced, the metal soap is preferably contained in an amount of 10 parts by mass or less per 100 parts by mass of the inorganic filler, more preferably 9.0 parts by mass or less, and even more preferably 8.0 parts by mass or less.

[0034] The first layer 1 and the second layer 2 may contain triglyceride as an additive. By containing triglyceride, the water repellency of the porous film 3 is increased, and the leakproofness of the porous film 3 is improved.

[0035] In order to make the above-mentioned advantages more pronounced, the amount of triglyceride blended is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, and even more preferably 1.0 part by mass or more, per 100 parts by mass of the olefin-based resin composition. From the viewpoint of film formability, the blending amount of triglyceride is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of the olefin resin composition. Taking the above into consideration, the amount of triglyceride to be blended is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 0.5 parts by mass or more and 25 parts by mass or less, and even more preferably 1.0 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of the olefin-based resin composition.

[0036] The triglyceride used in the present invention is preferably a hydrocarbon group that contains a group derived from a fatty acid having 16 to 22 carbon atoms and that does not have an unsaturated bond or a substituent. By using such a triglyceride, the appearance of the porous film 3 containing the triglyceride can be easily improved.

[0037] The triglyceride used in the present invention is preferably the following (A) or (B). (A) A mixture of a triglyceride containing at least one group derived from a saturated fatty acid having 18 carbon atoms in one molecule and a triglyceride containing at least one group derived from a saturated fatty acid having 16 to 22 carbon atoms (excluding fatty acids having 18 carbon atoms) in one molecule. (B) A triglyceride containing, in one molecule, at least one group derived from a saturated fatty acid having 18 carbon atoms and at least one group derived from a saturated fatty acid having 16 to 22 carbon atoms (excluding fatty acids having 18 carbon atoms).

[0038] The first layer 1 and the second layer 2 may contain additives other than the additives described above. Such additives can impart various additional properties to the porous film 3. Examples of such additives include plasticizers, water repellents, antioxidants, ultraviolet absorbing agents, and colorants.

[0039] From the viewpoint of enhancing the dimensional stability of the porous film 3, the peel strength between the first layer 1 and the second layer 2 is preferably 5 N / m width or more, more preferably 10 N / m width or more, even more preferably 13 N / m width or more, and particularly preferably 15 N / m width or more. The peel strength between the first layer 1 and the second layer 2 is measured by the following method.

[0040] <Method for measuring peel strength> The porous film 3 is cut in the machine direction to a width of 30 mm and a length of 150 mm to prepare a test piece. The first layer 1 and the second layer 2 are peeled off from a 50 mm area at one end of the length of the test piece beforehand before the test. Next, the first layer 1 and the second layer 2 that have been peeled off beforehand are clamped in the chucks of a tensile tester. Then, the zero point of the load read by the tensile tester is adjusted, and the first layer 1 and the second layer 2 of the test piece are T-peeled off at a tensile speed of 200 mm / min. The maximum strength is read from the obtained data, and the value obtained by dividing the maximum strength by the width of the test piece is the peel strength (N / m width). The distance between the chucks before the test is 20 mm.

[0041] In order to set the peel strength between the first layer 1 and the second layer 2 within the above-mentioned range, for example, the ratio of the low-melting point olefin-based resin in the olefin-based resin composition contained in the first layer 1 and the second layer 2 may be appropriately adjusted. Specifically, for example, in order to make the peel strength between the first layer 1 and the second layer 2 10 N / m width or more, it is preferable that the proportion of the low-melting point olefin resin in the olefin resin composition is 40 mass % or more. In order to ensure that the peel strength between the first layer 1 and the second layer 2 is 13 N / m width or more, the proportion of the low-melting point olefin resin in the olefin resin composition is preferably 50 mass % or more.

[0042] From the viewpoint of suppressing the occurrence of pinholes during the production of the porous film 3, the peel strength between the first layer 1 and the second layer 2 is preferably 60 N / m width or less, more preferably 55 N / m width or less, even more preferably 50 N / m width or less, and particularly preferably 45 N / m width or less. When the peel strength is equal to or less than the upper limit, for example, when a pinhole occurs in the first layer 1 in the case of producing the porous film 3 by stretching the flat body 5 described later, the second layer 2 is pulled by the first layer 1, and the occurrence of a pinhole in the same place in the second layer 2 is suppressed. As a result, even if a pinhole occurs in the first layer 1, a pinhole does not occur in the second layer 2 at the same position, and therefore a pinhole penetrating the entire porous film 3 does not occur. As described above, pinholes do not occur frequently, and therefore the probability that pinholes will occur by chance at the same positions in both the first layer 1 and the second layer 2 is extremely low. Therefore, the porous film 3 of the present invention is a porous film that is substantially free of pinholes.

[0043] Taking the above into consideration, the peel strength between the first layer 1 and the second layer 2 of the porous film 3 is preferably 5 N / m width or more and 60 N / m width or less, more preferably 10 N / m width or more and 55 N / m width or less, even more preferably 13 N / m width or more and 50 N / m width or less, and particularly preferably 15 N / m width or more and 45 N / m width or less.

[0044] The porous film 3 shown in FIG. 1 has a two-layer structure, but instead, the porous film 3 may have a 2n-layer structure (n is an integer of 1 or more). In this case, the nth layer and the n+1th layer counted from one outer surface of the porous film 3 correspond to the first layer 1 and the second layer 2 in the embodiment shown in FIG. 1, and the above description of the first layer 1 and the second layer 2 applies to the nth layer and the n+1th layer. For example, the nth layer and the n+1th layer are made of a material of the same composition, and the composition is the same as that described as the composition of the first layer 1 and the second layer 2 in FIG. 1. In addition, the peel strength between the nth layer and the n+1th layer is preferably 5 N / m width or more and 60 N / m width or less, more preferably 10 N / m width or more and 55 N / m width or less, even more preferably 13 N / m width or more and 50 N / m width or less, and particularly preferably 15 N / m width or more and 45 N / m width or less.

[0045] In summary, the porous film of one embodiment of the present invention has a 2n-layer structure (n is an integer of 1 or more), in which the nth layer and the n+1th layer in the 2n-layer structure are made of materials of the same composition, and the peel strength between the nth layer and the n+1th layer is 5 N / m width or more.

[0046] In particular, each of the n-th layer and the (n+1)-th layer in the 2n-layer structure contains an olefin-based resin composition, an inorganic filler in an amount of 50 parts by mass or more and 400 parts by mass or less per 100 parts by mass of the olefin-based resin composition, and a fatty acid in an amount of 0.5 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the inorganic filler, The olefin resin composition preferably contains a low-melting-point olefin resin having a melting point of less than 80°C and a high-melting-point olefin resin having a melting point of 80°C or higher.

[0047] From the viewpoint of ease of production, it is preferable that the 1st layer to the nth layer and the (n+1)th layer to the 2nth layer in the 2n-layer structure are made of materials of the same composition, i.e., the types and total contents of all components constituting the 1st layer to the nth layer are the same as the types and total contents of all components constituting the (n+1)th layer to the 2nth layer.

[0048] Next, a preferred method for producing the porous film 3 will be described. The present manufacturing method includes the following steps. (a) An inflation molding step of extruding an olefin resin composition, an inorganic filler, and a fatty acid (hereinafter, also referred to as a "compound") into a tubular film. (b) A flattening step in which the tubular film is flattened to form a flat body having two opposing surfaces. (c) A stretching step for stretching the flattened body. (d) A heat treatment process in which the flattened body after stretching is heat treated at a temperature equal to or higher than the melting point of the low-melting point olefin-based resin to fuse the two opposing surfaces of the flattened body. Details of the olefin-based resin composition, inorganic filler, fatty acid, and other components contained in the compound are the same as those of the olefin-based resin composition, inorganic filler, fatty acid, and other components in the first layer 1 and the second layer 2 described above.

[0049] Fig. 2 shows a schematic diagram of an embodiment of a manufacturing apparatus for a porous film 3 used in the manufacturing method of the present invention. The apparatus 10 shown in the figure has a melt-kneading section 20, a molding section 30, a stretching section 40, and a heating section 50, arranged in this order along the conveying direction. Each section of the apparatus 10 will be described below.

[0050] The melt-kneading section 20 has an extruder 22. The extruder 22 has a normal structure and is provided with a screw (not shown) inside a cylinder. The extruder 22 has a hopper 21 for supplying a compound that is a raw material for the porous film. Furthermore, the extruder 22 has a metering pump 23 at its tip for discharging the molten compound (hereinafter also referred to as "molten resin"). Depending on the required quantitation of the discharge, the device 10 may not have to have the metering pump 23. The molten resin discharged from the metering pump 23 is sent to the molding section 30 after passing through a filter 24 to remove foreign matter.

[0051] The forming section 30 includes a ring die 31, take-up rolls 32, 32 each consisting of a pair of nip rolls, and a guide plate 33. As the forming section 30, those generally used in inflation molding can be used. The molten resin fed to the die 31 is inflation-molded in the forming section 30 to form a tubular film, and then passes through the guide plate 33 and the take-up rolls 32, 32 and is sent out to the stretching section 40.

[0052] The stretching section 40 is equipped with a roll stretching machine. The roll stretching machine includes a first roll pair 41 consisting of a first nip roll 41a and a first stretching roll 41b, and a second roll pair 42 consisting of a second nip roll 42a and a second stretching roll 42b located downstream of the first roll pair 41. In the first roll pair 41, the first nip roll 41a and the first stretching roll 41b are rotated in opposite directions and at the same peripheral speed. In the second roll pair 42, the second nip roll 42a and the second stretching roll 42b are rotated in opposite directions and at the same peripheral speed. When the peripheral speed of the first stretching roll 41b is V1 and the peripheral speed of the second stretching roll 42b is V2, V1 and V2 can be set independently. In this embodiment, the peripheral speeds of the respective nip rolls are set so that V1 < V2. Thereby, the formed film is uniaxially stretched along the conveyance direction between the first stretching roll 41b and the second stretching roll 42b. Although not shown, one or more roll pairs each consisting of a nip roll and a stretching roll may be further installed between the first roll pair 41 and the second roll pair 42 to perform stretching in multiple stages. In that case, when the peripheral speeds of the stretching rolls in the one or more roll pairs are V1', V1'', etc., the peripheral speeds of the respective stretching rolls are set so that V1 < V1' (<V1''...) < V2, that is, the peripheral speeds of the respective stretching rolls gradually increase as they go downstream in the conveyance direction.

[0053] The heating section 50 includes a first heat roll 51 and a second heat roll 52. The first and second heat rolls 51, 52 are arranged so that their rotation axes are parallel and their peripheral surfaces are spaced apart. Each of the heat rolls 51, 52 is provided with a heating means (not shown) and can be heated to an independent temperature. A method for producing a film using the apparatus 10 having the above configuration will be described below.

[0054] (a) Inflation molding process First, a compound containing an olefin resin composition, an inorganic filler, and a fatty acid is supplied to a hopper 21. The blending amounts of the olefin resin composition, the inorganic filler, the fatty acid, and other components contained in the compound are the same as the blending amounts of these components contained in the first layer 1 and the second layer 2. Furthermore, the types and amounts of optional components contained in the compound are also the same as the types and amounts of optional components contained in the first layer 1 and the second layer 2.

[0055] The compound supplied to the hopper 21 is heated and kneaded in the extruder 22 to become a molten resin. The molten resin is discharged quantitatively by a metering pump 23 through a filter 24. The extruder 22 may be a single-screw extruder or a twin-screw extruder. The discharged molten resin is extruded through a ring-shaped die 31 and is inflation-molded to obtain a tubular film 4.

[0056] (b) Flattening process Next, the tubular film 4 passes through a guide plate 33 and a take-up roll 32 to be flattened into a flat body 5 having two opposing surfaces. The flat body 5 is then transported to a stretching section 40. At this point, the two opposing surfaces of the flat body 5 are not completely fused.

[0057] (c) Stretching process In the extension part 40, the flat body 5 undergoes an extension process while being conveyed. The extension process is achieved by making the peripheral speed V2 of the second roll pair 42 higher than the peripheral speed V1 of the first roll pair 41 while gripping the flat body 5 with the first roll pair 41 and then gripping the flat body 5 with the second roll pair 42, thereby uniaxially extending the flat body 5 along its conveyance direction (i.e., the longitudinal direction). Since the flat body 5 contains an inorganic filler, a large number of micropores starting from the inorganic filler are formed in the extended flat body 5. As described above, when one or more roll pairs (not shown) rotating at peripheral speeds V1’, V1’’... are further installed between the first roll pair 41 and the second roll pair 42, the peripheral speeds of the respective extension rolls can be set to V1 < V1’ (< V1’’...) < V2, and the extension can be performed in multiple stages. When performing the extension process using the peripheral speed difference of the extension rolls, the extension ratio refers to the value of V2 / V1.

[0058] Regardless of whether the extension is performed in one stage or multiple stages, from the perspective of successfully generating the micropores of the porous film 3, it is preferable that the extension ratio of the flat body 5 in the extension process is extended at a ratio of 1.1 times or more and 5.5 times or less in the uniaxial direction. The extension temperature of the flat body 5 depends on the type of the olefin resin composition. For example, setting it to 30°C or higher and 100°C or lower, preferably 35°C or higher and 95°C or lower, and more preferably 40°C or higher and 90°C or lower is advantageous from the point of view of successfully performing the extension. Also, from the perspective of increasing the moisture permeability of the porous film 3, it is advantageous to perform the extension process at a temperature preferably 20°C or higher and 50°C or lower, more preferably 23°C or higher and 45°C or lower, and even more preferably 25°C or higher and 40°C or lower, lower than the temperature of the heat treatment step described later.

[0059] (d) Heat treatment step The flat body 5 after stretching is subjected to a heat treatment in the heating section 50 while being transported. This ensures that the two opposing surfaces of the flat body 5 after stretching are fused together to obtain a porous film 3. The heat treatment also has the following purpose in addition to fusing the two opposing surfaces. That is, in the flat body 5 after stretching, the polymer chains of the resin constituting the flat body 5 are oriented in the stretching direction (i.e., the transport direction). Since the molecularly oriented state is a thermodynamically unstable state, it is preferable from the viewpoint of the dimensional stability of the flat body 5 to relax the molecular orientation to make it a thermodynamically stable state. The heat treatment relaxes the molecular orientation of the polymer chains of the resin constituting the flat body 5 and causes crystallization of the molecular chains.

[0060] From the viewpoint of more reliably fusing the two opposing surfaces of the flat body 5 after stretching, it is preferable that the heating temperature of the flat body 5 in the heating section 50 is a temperature equal to or higher than the melting point of the low-melting-point olefin resin contained in the flat body 5. From this viewpoint, the temperature in the heat treatment step is preferably 80° C. or higher, more preferably 85° C. or higher, and even more preferably 90° C. or higher. It is also preferable that the temperature in the heat treatment step is equal to or lower than the melting point of the high-melting-point olefin resin contained in the flat body 5. Setting the temperature in the heat treatment step in this manner can prevent the flat body 5 after stretching from being melted and broken by the heat treatment step. From this viewpoint, the temperature in the heat treatment step is preferably equal to or lower than 100°C, more preferably equal to or lower than 98°C, and even more preferably equal to or lower than 95°C, provided that the temperature is equal to or lower than the melting point of the high-melting-point olefin resin. Considering the above, the heating temperature of the flat body 5 after stretching is preferably 80° C. or higher and 100° C. or lower, more preferably 80° C. or higher and 98° C. or lower, and even more preferably 85° C. or higher and 95° C. or lower. The heating temperature of the flat body 5 after stretching in the heating section 50 refers to the temperature of the circumferential surfaces of the first and second heat rolls 51, 52. When the temperatures of the circumferential surfaces of the first and second heat rolls 51, 52 are different, the heating temperature refers to the higher temperature.

[0061] Relaxation of the orientation of the polymer chains by heating the flat body 5 after stretching is accompanied by shrinkage of the flat body 5 in the stretching direction (i.e., the conveying direction). Therefore, from the viewpoint of ensuring relaxation of the orientation of the polymer chains, it is preferable to make the conveying speed of the flat body 5 in the heating section 50 lower than the conveying speed of the flat body 5 in the stretching section 40. This weakens the force pulling the film in the conveying direction in the heating section 50, thereby promoting shrinkage of the flat body 5 after stretching by the heating section 50. In other words, the flat body 5 after stretching can be shrunk in the conveying direction while being subjected to heat treatment. However, if the conveying speed of the flat body 5 in the heating section is made too low, the film will slacken and wrap around the roll. From this viewpoint, when the conveying speed of the flat body 5 in the stretching section 40 is V2 and the conveying speed of the flat body 5 in the heating section 50 is V3, it is advantageous to set V3 / V2 to preferably 0.50 or more and 0.85 or less, more preferably 0.52 or more and 0.82 or less, and even more preferably 0.55 or more and 0.80 or less. It is preferable to set the shrinkage ratio of the flat body 5 when heat-treated with this speed relationship (hereinafter also referred to as "return ratio") to preferably 15% or more, more preferably 18% or more, and even more preferably 20% or more. In addition, it is preferable to set the return ratio to preferably 50% or less, more preferably 48% or less, and even more preferably 45% or less. Taking the above into consideration, the return ratio is preferably 20% or more and 50% or less, more preferably 23% or more and 48% or less, and even more preferably 25% or more and 45% or less.

[0062] Setting the return ratio within the above range makes it possible to sufficiently relax the molecular orientation of the polymer chains of the resin constituting the flat body 5. Since resins with relaxed molecular orientation can easily melt into each other, the two opposing surfaces of the flat body 5 can be more easily fused.

[0063] The porous film obtained through the heat treatment step can be subjected to post-treatment steps, such as a cooling step and a cutting step, which are usually carried out in the production of films, as necessary.

[0064] The porous film 3 produced by the above-mentioned production method has a two-layer structure, but the porous film 3 produced by the production method of the present invention is not limited to a two-layer structure. For example, when a molten resin is extruded from the ring-shaped die 31, if multiple types of compounds are used and multiple types of molten resins are extruded by co-extrusion, a porous film 3 having a 2n (n is an integer of 1 or more) layer structure can be obtained.

[0065] Although the present invention has been described above based on its preferred embodiment, the present invention is not limited to the above embodiment. For example, the heating section 50 in the device 10 shown in Fig. 2 is provided with two heat rolls, the first and second heat rolls 51 and 52, but depending on the type and thickness of the resin constituting the flat body 5, only one heat roll may be used, or three or more heat rolls may be used. EXAMPLES

[0066] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0067] [Examples 1 to 8] (1) Compound manufacturing The low melting point olefin resin A, the high melting point olefin resin B, the inorganic filler, the fatty acid, the metal soap and the triglyceride were weighed. The low-melting-point olefin resin A was an ethylene-α-olefin copolymer produced with a metallocene catalyst (melting point 44°C, density 0.864 g / cm 3 ) was used. As the high melting point olefin resin B, a linear low density polyethylene (melting point 116°C, density 0.924g / cm3) produced by a metallocene catalyst was used. 3 The amount of high melting point olefin resin B was set so that the ratio of low melting point olefin resin A in the olefin resin composition (low melting point olefin resin and high melting point olefin resin) was as shown in Table 1 below. As inorganic filler, ground calcium carbonate (average particle size D 50The olefin resin composition was used in an amount of 167 parts by mass, with a diameter of 1.8 μm, per 100 parts by mass of the composition. As the fatty acid, stearic acid was used in an amount of 1.5 parts by mass per 100 parts by mass of the inorganic filler. As the metal soap, zinc stearate was used in an amount of 5.7 parts by mass per 100 parts by mass of the olefin resin composition, except for Example 4, where no metal soap was used. As the triglyceride, 2.8 parts by mass of extremely hardened palm oil was used per 100 parts by mass of the olefin resin composition. The weighed components were mixed in a Henschel mixer (manufactured by Kawata Corp.) The mixture obtained was kneaded by the extruder 22 using a twin-screw extruder manufactured by Toyo Seiki Co., Ltd., and a pelletized compound was obtained. (2) Inflation molding process The ring-shaped die 31 used was an inflation molding machine with a discharge part diameter of 100 mm and a slit gap of 1.0 mm. The molten compound was molded into a tubular film 4 using this die 31. The die temperature was set to 200°C, and the blow ratio was 2.5 times. The discharge speed of the molten resin was 100 rpm when the basis weight of the porous film 3 was 37 g / m 2 (The basis weight of the first layer 1 and the second layer 2 is 18.5 g / m 2 ) was adjusted. (3) Flattening process The cylindrical film obtained in (2) above was passed through a guide plate 33 and a take-up roll 32 to be flattened, thereby obtaining a flat body 5. The take-up speed was 10 m / min. (4) Stretching process In this step, the flat body was uniaxially stretched in the conveying direction D by using a roll stretching method. The stretching temperature was as shown in Table 1, and the stretching ratio was 3.5 times. (5) Heat treatment process The heat treatment temperature was as shown in Table 1, and the return ratio was 41%.

[0068] Comparative Example 1 A compound was produced without using low melting point olefin resin A, zinc stearate, or palm hardened oil. The basis weights of the first layer 1 and the second layer 2 were 37 g / m 2 The inflation molding process was carried out at a discharge speed such that the first layer 1 and the second layer 2 were peeled off after the heat treatment process to obtain a porous film having a single layer structure. The stretching ratio was 2.5 times, and the return ratio was 13%. Otherwise, the porous film having a single layer structure was produced in the same manner as in Example 1.

[0069] Comparative Example 2 The first layer 1 and the second layer 2 of the porous film 3 each have a basis weight of 37 g / m 2 After the heat treatment step, the first layer 1 and the second layer 2 were peeled off to obtain a porous film having a single layer structure.

[0070] Comparative Example 3 A molten resin X was prepared by melting a compound consisting of a high melting point olefin resin C having a melting point of 120°C and an inorganic filler of 100% by mass relative to the high melting point olefin resin C. A molten resin Y was prepared by melting a polyurethane resin. The molten resins X and Y were co-extruded and inflation-molded to obtain a tubular film. The tubular film was flattened and then subjected to a stretching process. In the stretching process, tenter stretching was performed at a stretching temperature of 40°C and a stretching ratio of 2.5 times. No heat treatment was performed after the stretching process. A two-layer film in which a porous film layer (first layer 1) derived from the molten resin X and a moisture-permeable resin layer (second layer 2) derived from the molten resin Y were laminated was produced in the same manner as in Comparative Example 1 except for this. The basis weight of the obtained film was 12 g / m for the porous film layer. 2 , moisture permeable resin layer is 4g / m 2 It was.

[0071] (evaluation) The results of various measurements and evaluations of the obtained film are shown in Table 1. The peel strength between the first layer 1 and the second layer 2 was measured by the method described above. Other measurement items were measured as follows. When the porous film 3 is used in an absorbent article, the moisture permeability is set to 1.0 g / 100 cm from a practical standpoint. 2 From the same viewpoint, the elastic modulus is preferably 0.005 N / mm width / (g / m 2 ) or more 0.060N / mm width / (g / m 2 ) or less.

[0072] <Method of measuring moisture permeability> The moisture permeability of the porous film was measured by the following method in accordance with JIS L 1099 A-2. Diameter 2.03cm (area 3.23cm 2 Approximately 25 mL of ion-exchanged water was placed in a 100% glass bottle (Labolan screw bottle No. 8, AS ONE Co., Ltd.), the mouth of the glass bottle was covered with one test piece so that there were no gaps, and the test piece of porous film 3 was fixed to the glass bottle with a rubber band to prepare an evaluation sample. After measuring the mass (W1) of the evaluation sample, the sample was stored in a thermostatic chamber controlled at 40°C and 20% RH for 10 to 15 hours. After storage, the mass (W2) of the evaluation sample was measured and the storage time (T1, unit: h) was recorded, and the moisture permeability was calculated using the following formula (s): Moisture permeability (g / 100cm 2 / h)=(W1-W2) / (T1×3.23)×100 (s)

[0073] <Method of measuring elastic modulus> The porous film was cut into three pieces measuring 150 mm in the machine direction and 30 mm in the width direction. The cut test pieces were fixed to a tensile tester (product name: AG-1S, manufactured by Shimadzu Corporation) so that the initial length L0 of the test piece was 100 mm. After fixing, the load read by the tensile tester was set to zero, and the test piece was elongated to 1.3 times L0 at a deformation rate of 200 mm / min, and then immediately contracted to L0 at a deformation rate of 200 mm / min to perform a cyclic test. From the obtained data, the load (F) at the time of 1.03 times deformation during the elongation process was calculated. 3% ) and calculate the elastic modulus (N / (mm width·(g / m 2 ))) was calculated. Elastic modulus (N / (mm width·(g / m 2 )))=F3% (N) / (0.03×30(mm width)×film basis weight(g / m 2 ))

[0074] <40m 2 How to measure the number of pinholes per unit> The number of pinholes was visually confirmed and counted for each measurement sample produced in the Examples and Comparative Examples. 2 ) / (area of ​​the measured sample) to get 40m 2 The results were converted into the number of pinholes per hole.

[0075] [Table 1]

[0076] As can be seen from Table 1, the porous film 3 of each Example had excellent moisture permeability, and no pinholes were observed. On the other hand, pinholes were observed in both the porous films of Comparative Examples 1 and 2 having a single-layer structure. The porous film of the two-layer structure produced in Comparative Example 3 had a peel strength between layers exceeding 60 N / m width due to the two layers being formed by coextrusion. No pinholes were observed in this porous film. This is because the moisture-permeable resin layer does not contain an inorganic filler. However, the porous film of Comparative Example 3 did not have sufficient moisture permeability for practical use as a whole porous film 3 due to the low moisture permeability of the moisture-permeable resin layer. It was also confirmed that the porous film of each Example contained the low melting point olefin resin A, and therefore had the characteristics of being a soft film with a small elastic modulus. [Explanation of symbols]

[0077] 1 First layer 2 Second layer 3. Porous Film 10 Manufacturing equipment 20 Melt kneading section 30 Molding section 40 Stretching section 50 Heating section

Claims

1. A porous film having at least a two-layer structure, an olefin-based resin composition; an inorganic filler in an amount of 50 parts by mass or more and 400 parts by mass or less per 100 parts by mass of the olefin-based resin composition; and a fatty acid in an amount of 0.5 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the inorganic filler; the olefin resin composition contains a low-melting-point olefin resin having a melting point of less than 80°C and a high-melting-point olefin resin having a melting point of 80°C or higher, A porous film, wherein the two opposing layers in the two-layer structure are made of materials of the same composition, and the peel strength between the two opposing layers is 5 N / m width or more.

2. The porous film according to claim 1, wherein the olefin resin composition contains the low-melting-point olefin resin in an amount of 50% by mass or more.

3. The porous film according to claim 1 or 2, wherein the peel strength between the two opposing layers in the two-layer structure is 13 N / m width or more.

4. The porous film according to claim 1 or 2, wherein the peel strength between the two opposing layers in the two-layer structure is 60 N / m width or less.

5. an inflation molding step of inflation molding an olefin resin composition containing a low-melting-point olefin resin having a melting point of less than 80°C and a high-melting-point olefin resin having a melting point of 80°C or higher, an inorganic filler, and a composition containing a fatty acid to obtain a tubular film; a flattening step of flattening the tubular film into a flat body having two opposing surfaces; a stretching step of stretching the flat body; a heat treatment step of heat-treating the flat body after stretching at a temperature equal to or higher than the melting point of the low-melting point olefin-based resin to fuse two opposing surfaces of the flat body, The composition contains 50 parts by mass or more and 400 parts by mass or less of the inorganic filler per 100 parts by mass of the olefin-based resin composition, and 0.5 parts by mass or more and 5 parts by mass or less of the fatty acid per 100 parts by mass of the inorganic filler.

6. The method according to claim 5 , wherein the temperature in the heat treatment step is equal to or lower than the melting point of the high-melting-point olefin-based resin.

7. The manufacturing method according to claim 5 or 6, wherein a return ratio of the flattened body in the heat treatment step is 20% or more.

8. The method according to claim 5 or 6, wherein the stretching step is carried out at a temperature that is 20°C or more lower than the temperature of the heat treatment step.

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