Resin film and method for manufacturing resin film

Abstract

The present invention provides a resin film which has a thin thickness and is still capable of having improved mechanical strength such as stiffness and rigidity.　The present invention relates to a resin film which has at least a plate-like filler layer 1, a base material layer, and a plate-like filler layer 2 in this order, wherein: the plate-like filler layer 1 and the plate-like filler layer 2 are each independently a stretched layer that contains a polyolefin-based resin and a plate-like inorganic filler and has a porosity of less than 20%, with the content of the plate-like inorganic filler being 20 mass% or more; the base material layer contains a polyolefin-based resin; and the content of the plate-like inorganic filler is 10 mass% or less.

Description

Resin film and method for producing the same

[0001] The present invention relates to a resin film and a method for producing a resin film.

[0002] A resin film (synthetic paper) in sheet form has been proposed and put to practical use, which is made by stretching a resin composition containing a thermoplastic resin and an inorganic filler to form a porous layer. These resin films are useful as materials for printing paper, labels, etc.

[0003] Resin films are sometimes required to have appropriate mechanical strength depending on the application, such as printing, etc. Patent Document 1 discloses an extruded sheet made of a polypropylene resin composition containing a specific propylene polymer or the like having a high degree of crystallinity, as an extruded sheet from which a lightweight molded product with excellent rigidity can be obtained.

[0004] Japanese Patent Application Publication No. 07-33920

[0005] In recent years, environmental considerations and other demands have led to a demand for reduced weight in resin products. Therefore, there is a demand for thinner resin films to reduce their weight. However, thin resin films, such as those with thicknesses of 300 μm or less, are particularly susceptible to deterioration in mechanical strength, such as stiffness and rigidity, and the thinner they are, the more difficult it is to maintain practical handling. Resin films made from materials such as polycarbonate and polyethylene terephthalate have high stiffness and rigidity even when thin, but these resins have a relatively high density, which tends to require a large amount of resin to be used in the film. On the other hand, because polyolefins have a low resin density, resin films made from them are easily lightweight, meaning that the amount of resin used can be reduced, but thinner films tend to lack stiffness and rigidity. Furthermore, making resin films porous can further reduce the amount of resin used in the film, but stiffness and rigidity tend to be further reduced. Resin films are also required to be more easily recycled.

[0006] Therefore, the present invention provides a resin film that can improve mechanical strength such as stiffness and rigidity even when the film is thin, and also provides a resin film that can reduce impurities and is easy to recycle when recovered and recycled after use.

[0007] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have arrived at the present invention having the following gist.

[0008] That is, the present invention is as follows: [1] A resin film having at least a plate-like filler layer 1, a base layer, and a plate-like filler layer 2, in this order, wherein the plate-like filler layer 1 and the plate-like filler layer 2 each independently contain a polyolefin resin and a plate-like inorganic filler, are stretched layers with a plate-like inorganic filler content of 20% by mass or more, and have a porosity of less than 20%, and the base layer contains a polyolefin resin and a plate-like inorganic filler content of 10% by mass or less. [2] The resin film according to [1], wherein the plate-like inorganic filler is at least one selected from the group consisting of talc, mica, clay, diatomaceous earth, and glass flakes. [3] The resin film according to [1] or [2], wherein the orientation angle of the plate-like inorganic filler in the plate-like filler layer 1 and the plate-like filler layer 2 is each independently 10° or less. [4] The resin film according to any one of [1] to [3], wherein the thicknesses of the plate-like filler layer 1 and the plate-like filler layer 2 are each independently 10 μm or less. [5] The resin film according to any one of [1] to [4], wherein the thicknesses of the plate-like filler layer 1 and the plate-like filler layer 2 are each independently 20% or less of the total thickness. [6] The resin film according to any one of [1] to [5], wherein the total thickness is 200 μm or less. [7] The resin film according to any one of [1] to [6], wherein at least one of the plate-like filler layer 1 and the plate-like filler layer 2 is located on the surface side 20 μm or less from the outermost surface in the thickness direction. [8] The resin film according to any one of [1] to [7], wherein the porosity of the base layer is 10% or more.[9] A method for producing a resin film, comprising: a lamination step of laminating sheet-like resin compositions for forming plate-like filler layers on both sides of a sheet-like resin composition for forming a base layer to form a laminate composed of at least plate-like filler layer 1, the base layer, and plate-like filler layer 2 in that order; and a stretching step of stretching the laminate, wherein the plate-like filler layer 1 and the plate-like filler layer 2 each independently contain a polyolefin resin and a plate-like inorganic filler in the resin composition, and the content of the plate-like inorganic filler is 20 mass% or more, the base layer contains a polyolefin resin in the resin composition, and the content of the plate-like inorganic filler is 10 mass% or less, and the plate-like filler layer 1 and the plate-like filler layer 2 after the stretching step each independently have a porosity of less than 20%.

[10] A method for producing a resin film according to [9], wherein the base layer contains 15 mass% or more of calcium carbonate.

[0009] According to the present invention, it is possible to provide a resin film that can improve mechanical strength such as stiffness and rigidity even when the film is thin.

[0010] 1 is a schematic cross-sectional view of a resin film according to an embodiment; FIG. 2 is a schematic cross-sectional view of a resin film showing the orientation angle of a plate-like inorganic filler according to an embodiment;

[0011] The present invention will be described in detail below, but the following description is an example (typical example) of the present invention and is not intended to limit the present invention.

[0012] (Resin film)

[0013] The resin film of this embodiment has, in this order, a plate-like filler layer (hereinafter also referred to as "plate-like filler layer 1") which is an oriented layer containing a plate-like inorganic filler at a predetermined content or more, a base material layer, and a plate-like filler layer (hereinafter also referred to as "plate-like filler layer 2"). Even in the case of a thin film having a thickness of 300 μm or less, the resin film of this embodiment has adequate mechanical strength (particularly rigidity) and good handleability. The configurations (composition, thickness, etc.) of plate-like filler layer 1 and plate-like filler layer 2 may be the same or different.

[0014] <Configuration of Resin Film> An example of the configuration of a resin film will be described with reference to FIG. 1 . The resin film 10 has a plate-like filler layer 100, a base layer 200, and a plate-like filler layer 300, in this order. That is, the plate-like filler layers 100 and 300 are laminated on both surfaces (both outer main surfaces) of the base layer 200. The plate-like filler layers 100 and 300 contain plate-like inorganic fillers 110 and 310, respectively. The resin film 10 may be a laminate of the plate-like filler layer 100, the base layer 200, and the plate-like filler layer 300, and the order of lamination is not important. For example, the resin film 10 may have the plate-like filler layer 100 laminated on one main surface of the base layer 200 and the plate-like filler layer 300 laminated on the other main surface.

[0015] The plate-like filler layers 100, 300 are stretched layers containing 20% ​​by mass or more of plate-like inorganic filler 110, 310, respectively, and the resin film 10 is easily improved in rigidity by having the above-described configuration in which two plate-like filler layers 100, 300 are arranged on both sides of the base layer 200. Specifically, the resin film 10 has appropriate mechanical strength, particularly by (1) having stretched layers (plate-like filler layers) 100, 300 containing a predetermined amount or more of plate-like inorganic filler 110, 310, and (2) having two plate-like filler layers 100, 300 arranged so as to sandwich the base layer 200.

[0016] The resin film 10 may have one or more layers other than the base material layer 200 and the plate-like filler layers 100, 300. For example, the resin film 10 may have an intermediate layer between the base material layer 200 and the plate-like filler layer 100, or may have a surface layer on the main surface of the plate-like filler layer 100 opposite the base material layer 200. The resin film 10 may also have a back surface layer or a heat seal layer on the main surface of the plate-like filler layer 300 opposite the base material layer 200. These layers other than the base material layer 200 and the plate-like filler layers 100, 300 may be arranged asymmetrically in the resin film 10 with respect to the base material layer 200.

[0017] The plate-like filler layer 100 is a stretched layer containing a plate-like inorganic filler 110, and the plate-like filler layer 300 is a stretched layer containing a plate-like inorganic filler 310. The plate-like inorganic filler 110 and the plate-like inorganic filler 310 may be the same or different. The resin film 10 of the present invention having such plate-like filler layers 100, 300 is likely to have improved rigidity. The inventors believe that one of the reasons for this improved rigidity is that the plate-like filler layers 100, 300 are stretched layers (stretched layers), and as shown in FIG. 1, the plate-like inorganic fillers 110, 310 in the layers are easily formed so that their plate-like surfaces are aligned along the stretching direction. In this way, it can be inferred that when the plate-like inorganic fillers 110, 310 are aligned in the layer, the degree of freedom of deformation of the resin in the plate-like filler layers 100, 300 is limited, making it easier to maintain the mechanical strength of the resin film 10.

[0018] The resin film 10 of the present invention also has a plate-like filler layer 100, a base layer 200, and a plate-like filler layer 300, in this order. That is, the plate-like filler layer 100 and the plate-like filler layer 300 are located on both sides of the resin film 10 nearer the surface side (or the outermost layer) than the base layer 200. The portions of the resin film 10 near the surface side are susceptible to tensile or compressive forces when an external force is applied to the film by bending, etc. Therefore, the mechanical strength of these layers near the surface side of the resin film 10 is likely to contribute to improving the rigidity of the entire film. As described above, the plate-like filler layer 100 and the plate-like filler layer 300, which are located particularly near the surface, effectively improve the rigidity of the resin film 10.

[0019] Furthermore, the resin film 10 has a configuration in which the plate-like filler layers 100, 300, which mainly contribute to improving stiffness, are functionally separated as layers separate from the base material layer 200, thereby reducing the amount of impurities contained in the resin of the entire resin film 10 when recycled after use, making it easier to recycle. Specifically, the stiffness of the resin film 10 can be improved while reducing the amount of plate-like inorganic fillers 110, 310 contained in the entire resin film 10.

[0020] <Thickness of Resin Film> From the viewpoint of ease of production and reduction of the amount of resin, the resin film preferably has a total thickness of 200 μm or less. From the viewpoint of ease of handling and reduction of the amount of resin, the total thickness of the resin film is more preferably 120 μm or less, even more preferably 100 μm or less, and particularly preferably 80 μm or less. From the viewpoint of easy maintenance of the strength of the resin film, the total thickness of the resin film is more preferably 40 μm or more, even more preferably 50 μm or more, and particularly preferably 60 μm or more.

[0021] Furthermore, the resin film of the present invention is characterized in that it is easy to improve mechanical strength such as stiffness and rigidity even when it is thin. From this point of view, the thickness of the resin film of the present invention may be 80 μm or less, or may be 70 μm or less. Even a resin film of such a thickness is easy to maintain mechanical strength that allows it to be appropriately handled in various applications such as printing.

[0022] <Stiffness of Resin Film> The stiffness of a resin film can be measured, for example, by Clark stiffness. The Clark stiffness in the present invention can be measured by a method in accordance with JIS P8143:1996. The suitable Clark stiffness range for the resin film of the present invention varies depending on the thickness of the resin film, its intended use, etc. For example, when the resin film is thin (300 μm or less), the Clark stiffness of the resin film is preferably 26 or more, more preferably 29 or more, and particularly preferably 31 or more. Furthermore, when the resin film is used as printing paper, the Clark stiffness is preferably 26 to 100. This is because a resin film within this range is likely to exhibit good printability as printing paper.

[0023] (Flat-Like Filler Layer) Next, the plate-like filler layers (plate-like filler layer 1 and plate-like filler layer 2) containing a plate-like inorganic filler laminated on the base layer will be described. Plate-like filler layer 1 and plate-like filler layer 2 are each stretched layers containing a polyolefin resin and a plate-like inorganic filler, with a plate-like inorganic filler content of 20% by mass or more and a porosity of less than 20%. Note that the plate-like inorganic filler layer 1 and plate-like filler layer 2 arranged on both sides of the base layer may have the same composition or different compositions. Hereinafter, when simply referred to as a "plate-like filler layer," this refers to matters that apply to both the plate-like filler layer 1 and the plate-like filler layer 2. The plate-like filler layer will be described in detail.

[0024] <Plate-Like Inorganic Filler> The plate-like inorganic filler is a plate-like filler made of an inorganic material. The plate-like filler has a flake or scale-like shape, and the aspect ratio, i.e., the ratio of the major axis to the thickness (major axis / thickness), is preferably 1.2 to 100, more preferably 2 to 50, and even more preferably 5 to 35. By setting the aspect ratio at or above the lower limit, the mechanical strength effect of the resin film becomes significant, while by setting it at or below the upper limit, the formability of the resin film and flexibility suitable for various applications can be easily ensured. Furthermore, the major axis of the plate-like inorganic filler is preferably approximately 1 to 45 μm, more preferably approximately 3 to 15 μm. The thickness of the plate-like inorganic filler is preferably approximately 0.05 to 1.5 μm, more preferably approximately 0.1 to 0.8 μm. The aspect ratio, major axis, and thickness are average values ​​obtained by measuring 18 particles of filler using a scanning electron microscope (SEM).

[0025] The plate-like inorganic filler is preferably at least one selected from the group consisting of talc, mica, clay, diatomaceous earth, and glass flakes. Among these, talc is preferred as the plate-like inorganic filler because it easily maintains its shape when kneaded and dispersed in a resin and has good heat resistance.

[0026] The content of the plate-like inorganic filler in the plate-like filler layer is 20% by mass or more. From the viewpoint of easily improving the rigidity of the resin film, the content of the plate-like inorganic filler is preferably 25% by mass or more, more preferably 40% by mass or more. From the viewpoint of easily maintaining appropriate flexibility, the content of the plate-like inorganic filler is preferably 80% by mass or less, more preferably 70% by mass or less, particularly preferably 60% by mass or less.

[0027] Average particle diameter D of plate-like inorganic filler ５０ The average particle diameter D of the plate-like inorganic filler is preferably less than 7 μm, from the viewpoints of preventing voids from being formed during stretching of the layer and facilitating improvement in the rigidity of the resin film. ５０ The average particle diameter D of the plate-like inorganic filler is more preferably 6 μm or less, and even more preferably 5 μm or less. ５０ The lower limit of the average particle diameter (D ５０ ) is the particle diameter corresponding to 50% cumulatively (cumulative 50% particle diameter) in a particle size distribution measured with a particle measuring device, for example, a laser diffraction particle measuring device "Microtrac" (trade name, manufactured by Nikkiso Co., Ltd.).

[0028] The apparent density of the plate-like inorganic filler is 0.05 to 0.35 g / cm ３ is preferably 0.10 to 0.30 g / cm ３ More preferably, it is 0.13 to 0.28 g / cm ３ It is more preferable that the density is 0.05 to 0.35 g / cm ３ This makes it easier to improve the stiffness of the resin film.

[0029] <<Orientation angle of plate-like inorganic filler>> The orientation angle of the plate-like inorganic filler in the plate-like filler layer is preferably 10° or less. Here, the orientation angle of the plate-like inorganic filler refers to the angle, determined by the following method, formed between an extension line in the surface direction of the plate-like filler layer and an extension line in the longitudinal direction of the cross section of the plate-like inorganic filler contained in the plate-like filler layer.

[0030] The orientation angle of the plate-like inorganic filler will be specifically described with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view of a resin film 10 observed in a cross section along the extension direction of the plate-like filler layer, specifically, a cross section parallel to the plane formed by the thickness direction of the resin film and the extension direction of the plate-like filler layer. When measuring the orientation angle of the plate-like inorganic filler, an SEM photograph of an arbitrary cross section of the resin film 10 is used. In FIG. 2, the orientation angle α of the plate-like inorganic filler 110 can be measured as the angle α between a straight line along the surface 100A of the plate-like filler layer 100 and the longitudinal extension line of the cross section of the plate-like inorganic filler 110B. The orientation angle of the plate-like inorganic filler 110 in the resin film 10 can be calculated as the average value of the orientation angles α measured as described above. From the viewpoint of more easily improving the rigidity of the resin film, the orientation angle of the plate-like inorganic filler is more preferably less than 8°, even more preferably less than 6°, and particularly preferably less than 5°. The lower limit of the orientation angle of the plate-like inorganic filler is not particularly limited, and may be greater than 0°.

[0031] <Porosity of plate-like filler layer> The porosity of the plate-like filler layer is less than 20%. From the viewpoint of easily improving the rigidity of the resin film, the porosity is preferably 10% or less, more preferably 8% or less, and particularly preferably 5% or less. The lower limit of the porosity of the plate-like filler layer is not particularly limited, and may be 0% (below the measurement limit). The above porosity can be determined from the ratio of the area occupied by pores to a certain region of the cross section of a sample observed with an electron microscope.

[0032] <Resin Forming the Plate-Like Filler Layer> The plate-like filler layer contains a polyolefin-based resin. As the polyolefin-based resin, a polypropylene-based resin with a higher modulus of elasticity is preferred from the viewpoint of ensuring the rigidity of the plate-like filler layer. Examples of polypropylene-based resins include isotactic homopolypropylene and syndiotactic homopolypropylene, which are obtained by homopolymerizing propylene, as well as polypropylene-based copolymers with various stereoregularities, which are mainly composed of propylene and copolymerized with α-olefins such as ethylene, 1-butene, and 1-pentene. The propylene-based copolymer may be a binary system or a multi-component system of ternary or higher components, and may also be a random copolymer or a block copolymer. Among these, homopolypropylene (h-PP) is preferred because it increases the rigidity of the resin film. In particular, the content of homopolypropylene in the polyolefin-based resin is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. Note that a high content of homopolypropylene tends to make voids more likely to form in the layer upon stretching. Since the rigidity of the resin film decreases as the porosity of the layer increases, the porosity of the plate-like filler layer is adjusted to less than 20%, for example, by increasing the stretching temperature, adjusting the stretching ratio, or by using a small amount of a resin with a lower melting point than homopolypropylene. Only one type of polyolefin-based resin may be used, or two or more types may be used in combination. From the viewpoint of formability, the polyolefin-based resin is preferably contained in an amount of 20 to 80% by mass, more preferably 30 to 75% by mass, and even more preferably 40 to 60% by mass, of the resin composition forming the plate-like filler layer.

[0033] <Thickness of plate-like filler layer> The thickness of each plate-like filler layer is preferably 35 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. This is because within this range, the rigidity of the resin film is improved while also making it easy to improve recyclability. From the viewpoint of improving recyclability, the thickness of the plate-like filler layer is more preferably 8 μm or less, and even more preferably 6 μm or less. From the viewpoint of preventing the plate-like inorganic filler from protruding from the layer, the thickness of the plate-like filler layer is preferably 1 μm or more, and more preferably 3 μm or more.

[0034] The thickness of each of the plate-like filler layers relative to the overall thickness of the resin film is preferably 20% or less. As described above, the resin film of the present invention has two plate-like filler layers on both surfaces of the substrate layer, which makes it easy to effectively improve the rigidity of the resin film. Therefore, even if the thickness of each plate-like filler layer relative to the overall thickness of the resin film is within the above range, it is easy to maintain mechanical strength that allows the resin film to maintain ease of handling in various applications. From the viewpoint of improving recyclability, the total thickness of the plate-like filler layers (the total thickness of the two plate-like filler layers) relative to the overall thickness of the resin film is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less. From the viewpoint of improving the rigidity of the resin film, the total thickness of the plate-like filler layers relative to the overall thickness of the resin film is preferably 3% or more, more preferably 5% or more, and even more preferably 10% or more.

[0035] The plate-like filler layer described above is a stretched layer, and is a layer formed by stretching. The fact that it is a stretched layer can be confirmed, for example, from the storage modulus of the plate-like filler layer. Specifically, the storage modulus of the plate-like filler layer is measured in the MD (longitudinal) direction and the TD (transverse) direction. For example, in the case of a plate-like filler layer containing a polypropylene-based resin, if the storage modulus in the MD direction (longitudinal direction, machine direction, the direction in which the resin flows) or the TD direction (transverse direction, transverse direction, the direction perpendicular to the longitudinal direction) exceeds 1500 MPa, it can be confirmed that it is a stretched layer. (A stretch ratio of 2500 MPa or more can be confirmed as 4 times or more, and a stretch ratio of 5000 MPa or more can be confirmed as 9 times or more.)

[0036] The storage modulus can be measured by cutting a film into a test piece measuring 30 mm in length and 15 mm in width and using a solid viscoelasticity measuring device (RSA-III manufactured by TA Instruments Japan, Inc.) under the following measurement conditions: chuck distance 20 mm, measurement frequency 10 Hz, strain amount 0.1%, temperature rise rate 10 degrees / min, tensile mode, and temperature 23°C.

[0037] The resin film preferably has at least one plate-like filler layer on the surface side, no more than 20 μm from the outermost surface in the thickness direction. As described above, the vicinity of the outermost surface of the resin film (the outermost layer in the laminated structure of the resin film) is susceptible to tensile and compressive forces due to external forces such as bending. Therefore, by arranging each plate-like filler layer near this outermost surface (on the surface side), the rigidity of the resin film can be effectively improved. From the viewpoint of easily preventing curling, it is particularly preferable that each plate-like filler layer be located near the outermost surface on both main surfaces of the resin film (both the front and back sides). Here, "having each plate-like filler layer on the surface side, no more than 20 μm from the outermost surface in the thickness direction" means that at least the center position of the thickness of each plate-like filler layer is located no more than 20 μm from the outermost surface on the surface side in the thickness direction. Note that even when each plate-like filler layer is located near the outermost surface (on the surface side) of the resin film, other layers may be located closer to the outermost surface than each plate-like filler layer, as long as the rigidity of the resin film can be maintained. For example, it is possible to provide a thin printability-imparting layer having a thickness of 10 μm or less. As described above, each flake-like filler layer is preferably present on the surface side 20 μm or less from the outermost surface in the thickness direction, more preferably 10 μm or less from the outermost surface, and particularly preferably 8 μm or less from the outermost surface.

[0038] The two plate-like filler layers disposed on both main surfaces of the base layer may be layers composed of resin compositions with different constituent materials and blending ratios as long as the effects of the present invention are achieved. From the viewpoint of easily preventing curling of the resin film, it is preferable that the two plate-like filler layers be layers composed of resin compositions with the same constituent materials and blending ratios.

[0039] (Substrate Layer) The substrate layer contains a polyolefin resin, and the content of the plate-like inorganic filler is 10% by mass or less. That is, in the resin film of this embodiment, the content of the plate-like inorganic filler in the substrate layer is a low content of 10% by mass or less, unlike the plate-like filler layer. As such, the resin film is configured such that the content of the plate-like inorganic filler is concentrated in the plate-like filler layer, rather than blending the plate-like inorganic filler so that it is uniform throughout the entire layer. With this configuration, the resin film of this embodiment can effectively increase the rigidity of the entire resin film. Furthermore, the rigidity can be improved while reducing the content of the plate-like inorganic filler in the substrate layer. Therefore, the content of the plate-like inorganic filler in the entire resin film can be reduced, making it easier to use as a recycled resin. Note that, from the viewpoint of easily improving recyclability, the content of the plate-like inorganic filler in the substrate layer is preferably 5% by mass or less, and more preferably 0% by mass, i.e., no filler is contained.

[0040] <Resin Forming the Base Layer> The resin forming the base layer includes a polyolefin-based resin. As the polyolefin-based resin, a polypropylene-based resin is preferred from the viewpoint of ensuring the rigidity of the base layer. Examples of polypropylene-based resins include isotactic homopolypropylene and syndiotactic homopolypropylene obtained by homopolymerizing propylene, as well as polypropylene-based copolymers having various stereoregularities, mainly composed of propylene and copolymerized with α-olefins such as ethylene, 1-butene, and 1-pentene. The propylene-based copolymer may be a binary system, a ternary or higher multi-component system, or a random or block copolymer. Among these, homopolypropylene is preferred because of its high elastic modulus and ease of improving the rigidity of the layer. In particular, the content of homopolypropylene in the polyolefin-based resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. Only one type of polyolefin-based resin may be used, or two or more types may be used in combination. From the viewpoint of moldability and stiffness (having the hardness required for handling and printing), the polyolefin resin is preferably contained in the resin composition forming the base layer at 40% by mass or more, more preferably 50 to 95% by mass, even more preferably 60 to 90% by mass, and particularly preferably 60 to 80% by mass. Furthermore, when the resin composition forming the base layer does not contain a plate-like inorganic filler or "other fillers" described below, the polyolefin resin may be 100% by mass in the resin composition forming the base layer.

[0041] <Porosity of base layer> From the viewpoint of increasing the opacity of the resin film, the base layer preferably has a porosity of 10% or more. From the viewpoint of easily improving the whiteness of the resin film, the porosity of the base layer is more preferably 20% or more, and even more preferably 25% or more. From the viewpoint of preventing sheet breakage, the porosity of the base layer is preferably 50% or less, more preferably 40% or less, and even more preferably 35% or less.

[0042] <Other Fillers> The substrate layer may contain "other fillers" other than the plate-like inorganic filler. Examples of other fillers contained in the substrate layer include inorganic fillers other than the plate-like inorganic fillers, organic fillers, and the like. The organic filler is not particularly limited, but is preferably an organic particle that is incompatible with the thermoplastic resin, has a melting point or glass transition temperature higher than that of the thermoplastic resin, and is finely dispersed under the melt-kneading conditions of the thermoplastic resin. The melting point (°C) and glass transition temperature (°C) of the resin can be measured by differential scanning calorimetry (DSC).

[0043] Inorganic fillers other than the above-mentioned plate-like inorganic fillers (hereinafter sometimes simply referred to as "inorganic fillers") are not particularly limited. Examples of inorganic fillers include heavy calcium carbonate, light calcium carbonate, calcined clay, talc, diatomaceous earth, titanium oxide, zinc oxide, barium sulfate, silicon oxide, magnesium oxide, and inorganic particles obtained by surface-treating these with a fatty acid, a polymer surfactant, an antistatic agent, or the like. Among these, calcium carbonate such as heavy calcium carbonate and light calcium carbonate is preferred because it has good pore-forming properties and is inexpensive. One of the above inorganic fillers may be selected and used alone, or two or more may be used in combination.

[0044] The content of the other filler in the base layer is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of easily adjusting the porosity within the preferred range. From the viewpoint of maintaining strength and formability, the content of the other filler in the base layer is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. When calcium carbonate is used as the other filler, it is preferable that the calcium carbonate content be 15% by mass or more, from the viewpoint of easily adjusting the porosity within the preferred range.

[0045] The average particle diameter of other fillers (D ５０) is not particularly limited. The average particle diameter of the other fillers is preferably large from the viewpoint of ease of mixing with the thermoplastic resin, and is preferably small from the viewpoint of preventing troubles such as film breakage during stretching. Specifically, the average particle diameter of the other fillers is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more. The average particle diameter of the other fillers is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. The average particle diameter (D ５０ ) has the same definition as that of the above-mentioned plate-like inorganic filler and can be measured by the same measuring method as above.

[0046] <Thickness of Base Material Layer> The thickness of the base material layer is preferably 30 to 130 μm. Within this range, it is easy to achieve the thickness of the resin film described above. From the viewpoint of handleability, the thickness of the base material layer is more preferably 100 μm or less, and even more preferably 80 μm or less. From the viewpoint of formability, the thickness of the base material layer is more preferably 40 μm or more, and even more preferably 50 μm or more.

[0047] (Uses of Resin Film) The resin film of the present embodiment is suitable for use in various printing papers, adhesive labels, etc. As described above, the resin film of the present invention is easy to improve the stiffness of the resin film, and therefore, even a thin resin film having a thickness of 300 μm or less can maintain an appropriate mechanical strength, making it particularly suitable for uses where environmental consideration is required.

[0048] (Method for producing resin film) The resin film of this embodiment can be produced by a method comprising: a lamination step of laminating sheet-like resin compositions that form plate-like filler layers on both sides of a sheet-like resin composition that forms a base layer to form a laminate composed of at least plate-like filler layer 1, the base layer, and plate-like filler layer 2 in that order; and a stretching step of stretching the laminate, wherein the plate-like filler layer 1 and the plate-like filler layer 2 each independently contain a polyolefin-based resin and a plate-like inorganic filler in the resin composition, and the content of the plate-like inorganic filler is 20% by mass or more; the resin composition of the base layer contains a polyolefin-based resin, and the content of the plate-like inorganic filler is 10% by mass or less; and the plate-like filler layer 1 and the plate-like filler layer 2 after the stretching step each independently have a porosity of less than 20%. The method for producing the resin film of this embodiment will now be described in detail.

[0049] <Lamination Step> The manufacturing method of this embodiment includes a lamination step of forming a laminate composed of at least the plate-like filler layer (plate-like filler layer 1), the base layer, and the plate-like filler layer (plate-like filler layer 2) in this order. In the lamination step, sheet-like resin compositions that form each plate-like filler layer are laminated on two opposing main surfaces of a sheet-like resin composition that forms the base layer. The base layer may be uniaxially stretched before each plate-like filler layer is laminated thereon, or each plate-like filler layer may be laminated on an unstretched sheet-like base layer.

[0050] In the lamination process, the resin composition forming each plate-like filler layer contains a polyolefin resin and a plate-like inorganic filler, and the content of the plate-like inorganic filler is 20% by mass or more. In the lamination process, the resin composition forming the base layer contains a polyolefin resin, and the content of the plate-like inorganic filler is 10% by mass or less. The resin composition of the base layer may contain fillers other than the above-mentioned plate-like inorganic filler. The resin composition forming each layer may further contain additives such as pigments, heat stabilizers (antioxidants), light stabilizers, dispersants, lubricants, or nucleating agents, as necessary.

[0051] <Stretching Step> The stretching step is a step of stretching the laminate. In the stretching step, it is preferable to stretch each plate-like filler layer so that the stretching ratio is 3 times or more. The stretching ratio may be different for each layer of the stretched resin sheet. When the stretching ratio of each plate-like filler layer is 3 times or more, the orientation angle of the plate-like inorganic filler is likely to be small, and the rigidity of the resin film is likely to be improved. The stretching ratio of each plate-like filler layer is more preferably 4 times or more, and even more preferably 8 times or more, from the viewpoint of easily setting the orientation angle of the plate-like inorganic filler to 10° or less. From the viewpoint of maintaining strength, the stretching ratio of each plate-like filler layer is preferably 40 times or less, and even more preferably 20 times or less.

[0052] In the stretching step, the stretching ratio of the laminate may be appropriately determined so that the stretching ratio of each flake filler layer falls within the above-mentioned preferred range. For example, when the laminate is stretched in one direction, the stretching ratio is usually about 1.2 times or more, preferably 2 times or more, and usually 12 times or less, preferably 10 times or less. Furthermore, when the laminate is stretched biaxially, the stretching ratio is usually 1.5 times or more, preferably 8 times or more, more preferably 15 times or more, in terms of areal stretching ratio. On the other hand, from the viewpoints of strength and manufacturing difficulty, the areal stretching ratio is usually 40 times or less, preferably 20 times or less.

[0053] The stretching method (stretching method) in the stretching step is not particularly limited. Examples include a longitudinal stretching method using the difference in peripheral speed between rolls, a transverse stretching method using a tenter oven, a sequential biaxial stretching method combining these, a rolling method, a simultaneous biaxial stretching method using a tenter oven and a pantograph, and a simultaneous biaxial stretching method using a tenter oven and a linear motor. Also usable is a simultaneous biaxial stretching (inflation molding) method in which a molten resin is extruded into a tubular shape using a circular die connected to a screw extruder, and then air is blown into the extruded tubular shape.

[0054] The stretching temperature when performing stretching may be set appropriately. The stretching temperature is preferably within a range below the melting point of the resin forming the base layer. Specifically, the stretching temperature is preferably 2 to 60°C lower than the melting point of the resin forming the base layer. When the base layer contains the aforementioned "other fillers" and forms a base layer with high opacity, the stretching temperature is more preferably 5 to 50°C lower, and even more preferably 10 to 30°C lower, than the melting point of the resin forming the base layer, from the viewpoint of easily increasing the porosity of the base layer while easily preventing breakage, etc. Furthermore, when the plate-like filler layer contains a large amount of homopolypropylene as the polyolefin resin, stretching at a low temperature tends to generate many voids in the layer originating from the plate-like inorganic filler. If many voids are present in each plate-like filler layer, the rigidity of the layer decreases, making it difficult to obtain rigidity for the resin film. Therefore, stretching is preferably performed at a temperature lower than the melting point of the homopolypropylene in the layer. For example, the difference between the stretching temperature and the melting point of the homopolypropylene is greater than 0° C. and not more than 20° C., preferably about 5° C. to 10° C. The stretching speed is not particularly limited, but is preferably within the range of 20 to 350 m / min from the viewpoint of stable stretch molding.

[0055] After the above-described stretching process, each plate-like filler layer has a porosity of less than 20%. The porosity can be appropriately adjusted, for example, by blending about 20% by mass or less of a low-melting-point polyolefin resin in each plate-like filler layer, or by appropriately setting the stretching temperature.

[0056] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts", "%", and the like are based on mass unless otherwise specified.

[0057] (Preparation of Resin Composition) Resin films of Examples 1 to 4 and Comparative Examples 1 to 5 were produced according to the following procedure. Details of the materials used in each Example and Comparative Example are summarized in Table 1. Table 2 also summarizes the types and blending ratios (mass%) of materials used in producing the resin films of each Example and Comparative Example, as well as stretching conditions and evaluations. The material symbols shown in Table 2 correspond to the material symbols shown in Table 1.

[0058]

[0059] [Example 1] The resin composition for forming the substrate layer was a composite of 75 mass % h-PP, which is a polypropylene resin, and heavy calcium carbonate particles (CaCO ３ A resin composition was prepared by melt-kneading 20% ​​by mass of PEG-1, a sintered inorganic filler, and 5% by mass of talc-1, a plate-like inorganic filler, in an extruder set at 270°C. This resin composition was extruded into a sheet through a die and cooled to 70°C with a cooling roll to obtain a single-layer unstretched film. This unstretched film was reheated to 150°C and then stretched four times in the sheet flow direction (longitudinal direction) using the speed difference between multiple rolls to obtain a base layer (longitudinal uniaxially stretched layer).

[0060] Next, as the resin composition for forming the plate-like filler layers (plate-like filler layer 1 and plate-like filler layer 2), 55% by mass of h-PP, a polypropylene resin, and 45% by mass of talc-1, a plate-like inorganic filler, were melt-kneaded in an extruder set at 270°C to prepare a resin composition. This resin composition was extruded into a sheet through a die and cooled to 70°C with a cooling roll to obtain a single-layer unstretched film. Two layers of this unstretched film were formed in the same manner, and each was laminated on both sides (both main surfaces) of the above-mentioned base layer.

[0061] The obtained laminate was heated to 150°C using an oven, stretched 9 times in the transverse direction using a tenter stretching machine, and then heat-treated at 170°C to obtain a resin film consisting of a plate-like filler layer (uniaxially stretched layer) / base layer (biaxially stretched layer) / plate-like filler layer (uniaxially stretched layer).

[0062] The thickness of the obtained resin film was 60 μm, and the thickness of the plate-like filler layer was 4 μm and the porosity was 3%. The thickness of the base material layer was 52 μm and the porosity was 25%. The method for measuring the thickness and porosity of the resin film etc. will be described later.

[0063] Example 2 A resin film was obtained in the same manner as in Example 1, except that 45% by mass of talc-2 was used in place of talc-1 for the flake filler layer.

[0064] Example 3 A resin film was obtained in the same manner as in Example 1, except that the amount of Talc-1 added to the flake filler layer was 30% by mass.

[0065] Example 4 A resin film was obtained in the same manner as in Example 1, except that the amount of Talc-1 added to the flake filler layer was 20% by mass.

[0066] Comparative Example 1 A resin film was obtained in the same manner as in Example 1, except that the amount of Talc-1 added to the flake filler layer was 10% by mass.

[0067] Comparative Example 2 A resin film was obtained in the same manner as in Example 1, except that 45% by mass of Talc-3 was used in place of Talc-1 for the flake filler layer.

[0068] [Comparative Example 3] For the flake filler layer, CaCO was used instead of Talc-1. ３ A resin film was obtained in the same manner as in Example 1, except that 45% by mass of -1 was used.

[0069] [Comparative Example 4] After obtaining a laminate of a plate-like filler layer / base material layer (uniaxially stretched layer) / plate-like filler layer, a (unstretched) resin film was obtained in the same manner as in Example 1, except that the 9-fold stretching using a tenter stretching machine was not performed.

[0070] [Comparative Example 5] The amount of talc-1 added to the flake filler layer was set to 10% by mass, and 10% by mass of talc-1 was also added to the base layer. ３ A resin film was obtained in the same manner as in Example 1, except that the amount of -1 added was 15% by mass.

[0071] [Various Measurements and Evaluations] The stretched resin sheets of Examples 1 to 4 and Comparative Examples 1 to 5 obtained above were subjected to various evaluations by the following methods.

[0072] <Layer Thickness (μm)> The thickness (μm) of the resin film was measured in accordance with JIS K7130:1999 using a constant pressure thickness measuring instrument (manufactured by Teclock Corporation, product name: PG-01J). The thickness (μm) of each layer was determined by cooling a piece of resin film (sample) to a temperature of −60° C. or lower with liquid nitrogen, cutting the resin film piece placed on a glass plate with a razor blade (manufactured by Schick Japan Co., Ltd., product name: Proline Blade) at a right angle to prepare a sample for cross-sectional observation, observing the cross-section of the obtained sample using a scanning electron microscope (manufactured by JEOL Ltd., product name: JSM-6000), determining the boundary lines of the layers from the composition appearance, and multiplying the total thickness of the resin film by the thickness ratio of each observed layer.

[0073] <Orientation angle of plate-like inorganic filler> In the same manner as in the measurement of the <layer thickness> above, the resin film was cooled and cut, and the cross section was measured in a field of view of 5 μm (thickness direction) × 10 μm (direction perpendicular to the thickness direction) = 50 μm ２ The range was photographed using a scanning electron microscope (manufactured by JEOL Ltd., product name: JSM-6000), and the orientation angle (°) was measured based on the observation photograph. The cut surface was parallel to the plane formed by the thickness direction of the resin film and the stretching direction (TD direction) of the plate-like filler layer. The orientation angle of the plate-like inorganic filler was taken as the angle formed by a straight line along the surface of the plate-like filler layer and the stretching line in the longitudinal direction of the cross section of the plate-like inorganic filler. The orientation angle was confirmed for the plate-like inorganic filler in the observation photograph, and the average value was taken as the orientation angle.

[0074] <Porosity (%)> After embedding a piece of resin film in epoxy resin and solidifying it, a cut surface parallel to the plane formed by the thickness direction and the TD direction of the film was prepared using a microtome. This cut surface was then metallized by vapor deposition, and a scanning electron microscope (trade name: JCM-6000, manufactured by JEOL Ltd.) was used to observe an area of ​​30 μm (thickness direction) × 40 μm (perpendicular to the thickness direction) at 3000 times magnification. The pore portion of each layer was binarized using image processing software (Inkscape), and the area ratio (%) of pores occupying the measurement area was calculated to obtain the porosity (%) of each layer.

[0075] <Clark stiffness (TD) of flake-like filler layer> The Clark stiffness was measured in accordance with JIS P8143: 1996. Table 2 shows the evaluation results of the Clark stiffness according to the following criteria: ◎: Clark stiffness is 31 or more; ○: Clark stiffness is 29 or more; △: Clark stiffness is 26 or more; ×: Clark stiffness is less than 26

[0076] <Opacity (%) of Plate-Like Filler Layer> In accordance with the method described in JIS-P-8149:2000, the luminous reflectance measured with a standard black board as the backing was divided by the intrinsic luminous reflectance measured with a standard white board as the backing, and the opacity was calculated as a percentage.

[0077] The evaluation results for the resin sheets of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 2.

[0078] The resin films of Examples 1 to 4 all had a Clark stiffness of more than 25. In contrast, the resin films of Comparative Examples 1 and 5, in which the content of plate-like inorganic filler in each plate-like filler layer was less than 20% by mass, the resin film of Comparative Example 2, in which the porosity of each plate-like filler layer was 20% or more, and the resin film of Comparative Example 3, in which each plate-like filler layer did not contain a plate-like inorganic filler, all had a Clark stiffness of 25 or less. The unstretched resin film of Comparative Example 4 also had a Clark stiffness of 25 or less.

[0079] 10 Resin film 100, 300 Plate-like filler layer 100A Surface of plate-like filler layer 110, 110B, 310 Plate-like inorganic filler 200 Base layer α Orientation angle

Claims

1. A resin film having at least a plate-like filler layer 1, a base layer, and a plate-like filler layer 2, in this order, wherein the plate-like filler layer 1 and the plate-like filler layer 2 each independently contain a polyolefin resin and a plate-like inorganic filler, the plate-like inorganic filler content is 20% by mass or more, and the base layer is an extension layer with a porosity of less than 20%, and the base layer contains a polyolefin resin, and the plate-like inorganic filler content is 10% by mass or less.

2. The resin film according to claim 1, wherein the plate-like inorganic filler is at least one selected from the group consisting of talc, mica, clay, diatomaceous earth, and glass flakes.

3. The resin film according to claim 1 or 2, wherein the orientation angles of the plate-like inorganic filler in the plate-like filler layer 1 and the plate-like filler layer 2 are each independently 10° or less.

4. The resin film according to claim 1 or 2, wherein the thickness of said flake filler layer 1 and said flake filler layer 2 is independently 10 μm or less.

5. A resin film according to claim 1 or 2, wherein the thickness of said flake filler layer 1 and said flake filler layer 2 is independently 20% or less of the total thickness.

6. The resin film according to claim 1 or 2, having an overall thickness of 200 μm or less.

7. A resin film according to claim 1 or 2, wherein at least one of the plate-like filler layer 1 and the plate-like filler layer 2 is present on the surface side within 20 μm from the outermost surface in the thickness direction.

8. The resin film according to claim 1 or 2, wherein the porosity of the substrate layer is 10% or more.

9. A method for manufacturing a resin film, comprising: a lamination step of laminating sheet-like resin compositions that form plate-like filler layers on both sides of a sheet-like resin composition that forms a base layer to form a laminate composed of at least plate-like filler layer 1, the base layer, and plate-like filler layer 2 in that order; and a stretching step of stretching the laminate, wherein the plate-like filler layer 1 and the plate-like filler layer 2 each independently contain a polyolefin resin and a plate-like inorganic filler in the resin composition, and the content of the plate-like inorganic filler is 20 mass% or more; the base layer contains a polyolefin resin in the resin composition, and the content of the plate-like inorganic filler is 10 mass% or less; and the plate-like filler layer 1 and the plate-like filler layer 2 after the stretching step each independently have a porosity of less than 20%.

10. The method for producing a resin film according to claim 9, wherein the base layer contains 15% by mass or more of calcium carbonate.

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