Polyolefin film with excellent ease of cutting

Abstract

To provide a polyolefin film that has high easy-cuttability, appropriate heat-sealability, and slipperiness on the film surface.SOLUTION: Provided is a polyolefin film, which is a polyolefin film that includes at least a substrate layer (A) and a surface layer (B), and in which the substrate layer (A) contains homopolyethylene and homopolypropylene by 90 mass% or more and 100 mass% or less in total, the surface layer (B) is an ethylene / propylene random copolymer containing ethylene units by 0.9 mass% or more and 1.5 mass% or less, and the Charpy impact value in TD direction is 0.50 MJ / m2 or more and 1.50 MJ / m2 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin film that is easy to cut. [Background technology]

[0002] Polyolefin films have been widely used in a variety of food packaging and industrial applications. They are sometimes made into multilayer laminates to provide gas barrier properties and decorative features, which require sufficient heat-sealing properties. For food packaging, in particular, the ability to easily tear a film in a specific direction allows for easy tearing without notching, allowing for easy access to the contents.

[0003] Conventional easy-to-tear films for food packaging use a low-melting-point polyolefin surface layer, which is a co-extrusion of polypropylene resin and ethylene / butene-propylene copolymer. However, to improve heat-sealability, the film must contain several tens of percent of low-melting components. However, when using raw materials with a high content of low-melting components, they tend to contaminate metal rolls during the film-making process, significantly affecting process contamination. Therefore, it is preferable to keep the content of low-melting components in the raw materials low.

[0004] Furthermore, in the process of laminating a gas barrier layer or a design layer onto an easy-to-cut food packaging film, the easy-to-cut food packaging film must have appropriate slip properties.In addition, smoothness is also required, as a rough film surface may cause defects such as scratches and creases during transportation.

[0005] Various studies have been conducted to solve these problems. For example, a method for improving heat-sealability has been disclosed in which a laminate film is formed having an intermediate layer in which a cyclic olefin resin is added to a mixture of two types of ethylene-α-olefin copolymers with different flow activation energies, and a surface layer in which the mixture is made of two types of ethylene-α-olefin copolymers with different flow activation energies (Patent Document 1). Another example is a pressure-sensitive adhesive tape that combines processability and ease of cutting by forming an adhesive layer on one or both surface layers of a base film containing at least one base layer primarily composed of a mixed resin of a cyclic polyolefin resin and a polyethylene resin (Patent Document 2). Furthermore, a heat-sealable stretched polypropylene laminate film that combines low thermal shrinkage and high rigidity has been disclosed by providing a heat-seal layer made of a polypropylene random copolymer and / or a polypropylene block copolymer on at least one side of a polypropylene resin layer and stretching the layer (Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-72985 [Patent Document 2] Japanese Patent Application Publication No. 2019-131660 [Patent Document 3] WO2015 / 012165 publication Summary of the Invention [Problem to be solved by the invention]

[0007] However, it is difficult to obtain a polyolefin film that has all of the properties of easy cutting, heat sealing, and slipperiness using the methods of Patent Documents 1 to 3, and when the films obtained by these methods are used as packaging materials, there are problems with ease of content removal and sealing properties. The present invention has been made against the background of such problems of the conventional technology, and an object of the present invention is to provide a polyolefin film that is excellent in easy cutting, heat sealing properties, quality, and slipperiness, and can improve ease of content removal and sealing properties when used as a packaging material. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above object, and have completed the present invention. Specifically, the present invention relates to a polyolefin film comprising at least a base layer (A) and a surface layer (B), wherein the base layer (A) contains a total of 90% by mass to 100% by mass of homopolyethylene and homopolypropylene, and the surface layer (B) is an ethylene-propylene random copolymer containing 0.9% by mass to 1.5% by mass of ethylene units, and the polyolefin film has a Charpy impact value in the TD direction of 0.50 MJ / m 2 More than 1.50MJ / m 2 The polyolefin film is as follows: [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a polyolefin film that is easy to cut, has appropriate heat sealability, quality, and smoothness of the film surface, and is suitable for packaging applications. DETAILED DESCRIPTION OF THE INVENTION

[0010] The polyolefin film of the present invention is described below. The polyolefin film of the present invention is a polyolefin film comprising at least a base layer (A) and a surface layer (B), wherein the base layer (A) contains homopolyethylene and homopolypropylene in total at 90% by mass or more and 100% by mass or less, and the surface layer (B) is an ethylene-propylene random copolymer containing 0.9% by mass or more and 1.5% by mass or less of ethylene units, and has a Charpy impact value in the TD direction of 0.50 MJ / m 2 More than 1.50MJ / m 2 The following is the result.

[0011] Here, polyolefin film refers to a film whose main component is polyolefin resin. Polyolefin resin refers to a resin containing more than 50 mol% but not more than 100 mol% of olefin units in total, when all structural units constituting the molecular chain are taken as 100 mol%. Furthermore, the main component refers to a component that is contained in an amount of more than 50 mol% but not more than 100 mol% when all components constituting the film are taken as 100 mass%, and unless otherwise specified, the same interpretation can be used for the main component hereinafter. Note that when a film contains multiple types of polyolefin resins, if the total content of the polyolefin resins satisfies the above requirements, it is considered to contain polyolefin resin as the main component.

[0012] The polyolefin resin may contain copolymerization components based on other unsaturated hydrocarbons, or may be a blend of multiple polymers, as long as the objectives of the present invention are not impaired. Examples of monomer components constituting such copolymerization components or blends include ethylene, propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. From the viewpoint of dimensional stability, the copolymerization amount or blend amount is preferably 5 mol% or less, preferably less than 1 mol%, and the blend amount is preferably less than 10 mass%. Here, the copolymerization amount refers to the amount (mol %) obtained by excluding the most abundant structural unit from all structural units constituting the polyolefin resin, and the blend amount refers to the amount (mass %) obtained by excluding the most abundant resin in the polyolefin resin.

[0013] The polyolefin film of the present invention is preferably composed mainly of polypropylene resin, i.e., is a polypropylene film. Polypropylene resin refers to a resin containing more than 50 mol % but not more than 100 mol % of propylene units, when all structural units constituting the molecular chain are taken as 100 mol %.

[0014] The polyolefin film of the present invention may contain various additives other than the polyolefin resin, such as a crystal nucleating agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, an antiblocking agent, a filler, a viscosity modifier, and a color inhibitor, as long as the effects of the present invention are not impaired. However, it is preferable that the film does not contain an antiblocking agent. This is because the antiblocking agent may fall off during the process, accumulate, and re-adhere to the film, causing surface defects or forming voids during stretching, with the antiblocking agent acting as a nucleus.

[0015] The polyolefin film of the present invention is a polyolefin film comprising at least a base layer (A) and a surface layer (B), wherein the base layer (A) contains a total of 90% by mass or more and 100% by mass or less of homopolyethylene and homopolypropylene, and the surface layer (B) is an ethylene-propylene random copolymer containing 0.9% by mass or more and 1.5% by mass or less of ethylene units. Here, "homopolyethylene" refers to a resin that contains 95 mol% to 100 mol%, preferably 99 mol% to 100 mol%, and more preferably 99.7 mol% to 100 mol% of ethylene units, when all structural units constituting the molecular chain are taken as 100 mol%, and "homopolypropylene" refers to a resin that contains 95 mol% to 100 mol%, preferably more than 99 mol% to 100 mol%, and more preferably 99.7 mol% to 100 mol% of propylene units, when all structural units constituting the molecular chain are taken as 100 mol% (however, both of these exclude "ethylene-propylene random copolymers containing 0.9 mass% to 1.5 mass% ethylene units," described below).

[0016] Furthermore, "containing 90% by mass or more and 100% by mass or less of homopolyethylene and homopolypropylene in total" means that when all components constituting the target layer are taken as 100% by mass, the total of homopolyethylene and homopolypropylene in the layer is 90% by mass or more. In this case, it is not essential that both components are contained in the layer; as long as the layer contains only one of homopolyethylene and homopolypropylene and the content of that component is 90% by mass or more, the layer also satisfies the requirements of the base layer (A).

[0017] The base layer (A) contains a total of 90% by mass or more and 100% by mass or less of homopolyethylene and homopolypropylene. By having such a base layer (A), it is possible to maintain high levels of breaking strength, breaking elongation, and Charpy impact value. From the above viewpoints, the total amount of homopolyethylene and homopolypropylene in the base layer (A) is preferably 94% by mass or more and 100% by mass or less, more preferably 96% by mass or more and 100% by mass or less.

[0018] The surface layer (B) of the polyolefin film of the present invention is an ethylene-propylene random copolymer containing 0.9 to 1.5% by mass of ethylene units in order to enhance heat-sealability. Here, "surface layer" refers to a layer located on at least one surface. In other words, "surface layer (B)" must not only satisfy the above compositional requirements but also be located on the surface. Furthermore, "containing 0.9 to 1.5% by mass of ethylene units" means that 0.9 to 1.5% by mass of ethylene units are contained in 100% by mass of all structural units constituting the molecular chain. "The surface layer (B) is an ethylene-propylene random copolymer containing 0.9 to 1.5% by mass of ethylene units" means that 95 to 100% by mass of the surface layer (B) is an ethylene-propylene random copolymer containing 0.9 to 1.5% by mass of ethylene units. From the above viewpoints, the surface layer (B) is more preferably an ethylene-propylene random copolymer containing 0.9% to 1.2% by mass of ethylene units. If the ethylene-propylene random copolymer constituting the surface layer (B) contains less than 0.9% by mass of ethylene units, the heat seal strength decreases, and peeling may occur during multilayer lamination or after sealing. On the other hand, if the ethylene unit content exceeds 1.5% by mass, heat degradation of low-molecular-weight components causes contamination of metal rolls during the film-forming process, resulting in adhesion of contaminants to the film and degrading its quality.

[0019] The melt flow rate (hereinafter referred to as MFR) of the ethylene-propylene random copolymer constituting the surface layer (B) of the polyolefin film of the present invention, when measured in accordance with JIS K 7210 (1995) condition M (230°C, 2.16 kg), is preferably 4.3 to 10 g / 10 min, more preferably 4.3 to 8.0 g / 10 min, and even more preferably 4.3 to 6.0 g / 10 min, from the viewpoints of film formation stability and thickness unevenness. When the MFR of the polypropylene resin of the surface layer (B) is 4.3 g / 10 min or more, a polyolefin film can be obtained with excellent film formation properties and stability, and this also leads to reduced thickness unevenness and an appropriate range of heat seal strength. On the other hand, when the MFR of the polypropylene resin of the surface layer (B) is 10 g / 10 min or less, the decrease in heat shrinkage rate can be reduced. To maintain the MFR of the polypropylene resin within the above range, methods such as controlling the average molecular weight and molecular weight distribution are preferably employed.

[0020] From the viewpoint of achieving both strength and ease of cutting, the polyolefin film of the present invention has a Charpy impact value in the TD direction of 0.50 MJ / m 2 More than 1.50MJ / m 2 The following is the definition. Here, TD direction refers to the direction in the film plane that is perpendicular to the direction in which the film runs during the film manufacturing process (MD direction, longitudinal direction), and in the state of a film roll, this corresponds to the direction parallel to the central axis. Note that TD direction is also known as the "width direction." The Charpy impact value in the TD direction refers to the impact value when the film is torn in the TD direction. A Charpy impact value in the TD direction of 0.50 MJ / m 2 If the Charpy impact strength is less than 1.50 MJ / m, the strength of the film itself will decrease, adversely affecting the ease of cutting when used as a laminated film or packaging material. 2 If the Charpy impact value in the TD direction exceeds 1.00 MJ / m, the cuttability will decrease and a large force will be required to tear the film in the TD direction. 2 More than 1.50MJ / m 2 It is preferable that:

[0021] The Charpy impact value in the TD direction can be measured by the following procedure. First, a sample is cut out to 10 mm (TD direction) x 150 mm (MD direction), and its thickness is measured using a dial gauge. Next, the sample is set in a Charpy impact tester so that the MD direction is horizontal, and the pendulum of the impact tester is brought into contact with the center of the sample to measure the impact strength required to break it. The Charpy impact value is then calculated using the following formula. Formula: Charpy impact value (MJ / m 2 ) = Impact strength (kg·cm / 10mm) / Thickness (mm).

[0022] Charpy impact value in the TD direction is 0.50MJ / m 2 More than 1.50MJ / m 2 Methods for achieving the above or following preferred ranges include setting the surface temperature of the casting drum during the manufacturing process to 15 to 100°C, preferably 80 to 95°C, or using an embodiment having a layer strongly oriented in the TD direction. By setting the surface temperature of the casting drum within the above range, the degree of crystallization of the polyolefin resin can be maintained at an appropriate level, thereby allowing the Charpy impact value to fall within a preferred range. More specifically, by setting the surface temperature of the casting drum to 80°C or higher, sufficient crystallinity is ensured, thereby preventing an excessive increase in the Charpy impact value in the TD direction. As a result, the polyolefin film is easily cut. Furthermore, by using an embodiment having a layer strongly oriented in the TD direction, the polyolefin film becomes difficult to tear in the MD direction but easy to tear in the TD direction due to molecular orientation.

[0023] The layer structure of the polyolefin film of the present invention is not particularly limited as long as it has a base layer (A) and at least one surface is a surface layer (B). It may have another layer between the base layer (A) and the surface layer (B), or another layer on the opposite side of the surface layer (B) so that the base layer (A) serves as an intermediate layer. In this case, the "another layer" may have a different composition from the base layer (A) or the surface layer (B), or it may have the same composition as either one. If there are multiple layers that satisfy the composition requirements of the base layer (A), but one of them is uniaxially oriented, the uniaxially oriented layer is treated as the base layer (A). The thickness composition ratio of the base layer (A) to the surface layer (B) is preferably base layer (A layer) / surface layer (B layer) = 15-20 / 0.5-10, and more preferably 15-20 / 0.5-3.0.

[0024] From the viewpoint of achieving both improved heat sealability and reduced blocking, the polyolefin film of the present invention preferably has a heat seal strength at 155°C of 0.3 N / cm or more and 1.0 N / cm or less, more preferably 0.4 N / cm or more and 0.9 N / cm or less, and even more preferably 0.5 N / cm or more and 0.8 N / cm or less. A heat seal strength of 0.3 N / cm or more at 155°C provides sufficient heat sealability and prevents delamination during film lamination. On the other hand, a heat seal strength of 1.0 N / cm or less at 155°C makes it difficult for the polyolefin films to adhere to each other due to internal pressure when stored as a roll, thereby reducing blocking. Therefore, when the polyolefin film is unwound from the film roll, wall breakage of the surface layer on the core side is reduced.

[0025] The heat seal strength at 155°C can be measured by the following procedure. First, the surface layers (B) of the polyolefin films are overlapped and heat-sealed using a heat sealer heated to 155°C. A sample measuring 80 mm in the width direction and 20 mm in the longitudinal direction is cut out from the resulting laminate. The sample is attached to a shopper with backing on both sides so that the width direction is the peel direction, and the shopper is lowered so that the sample is at a 180° angle. The peel strength is measured, and the heat seal strength is calculated based on the peel strength. Formula: Heat seal strength (N / cm) = Peel strength (N) / Sample width (cm).

[0026] A method for adjusting the heat seal strength at 155°C to 0.3 N / cm or less and 1.0 N / cm or less, or to the above-mentioned preferred range, can be, for example, a method using an ethylene-propylene random copolymer, preferably an ethylene-propylene random copolymer containing 0.9 mass % or more and 1.5 mass % or less of ethylene units, as a raw material for the surface layer (B).

[0027] The polyolefin film of the present invention preferably has a surface roughness Sa of 50 nm to 100 nm, more preferably 60 nm to 90 nm, and even more preferably 70 nm to 80 nm. Having a surface roughness Sa of 50 nm or more on at least one surface prevents the surface from becoming excessively smooth, reducing meandering during processing or transport or slippage during winding. On the other hand, having a surface roughness Sa of 100 nm or less on at least one surface prevents roughening and the resulting air entrapment during winding, reducing the occurrence of wrinkles. To achieve the above effects, the surface roughness Sa of one surface needs only to be 50 nm to 100 nm or less, or within the above-mentioned preferred range, and the Sa of the other surface is not particularly limited.

[0028] The surface roughness (Sa) is measured using a non-contact surface / layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. Specifically, measurements are taken at three locations randomly selected in the longitudinal direction at the center position in the width direction of the film, and the average value is taken as the surface roughness (Sa) of the sample. Methods for making the surface roughness Sa of at least one surface 50 nm to 100 nm or within the above-mentioned preferred range include, for example, a method in which the cast drum temperature of the surface layer (B) is set within the above-mentioned range, or a method in which the temperature in the longitudinal stretching step is set within the range described below, and these methods may be combined as appropriate.

[0029] The polyolefin film of the present invention preferably has a protruding peak height Spk of 100 nm or more and 150 nm or less, more preferably 110 nm or more and 140 nm or less, and even more preferably 120 nm or more and 130 nm or less on its surface. When the protruding peak height Spk is 100 nm or more, the surface of the polyolefin film becomes moderately rough, stabilizing running during processing. On the other hand, when the protruding peak height Spk is 150 nm or less, the polyolefin film does not become excessively rough, reducing the occurrence of transfer marks when rolled.

[0030] In addition, the polyolefin film of the present invention has a peak density Spd of 100 / mm 2 More than 400 / mm 2 It is preferable that the ratio is 120 / mm or less, and more preferably 120 / mm 2 More than 380 / mm 2 or less, and more preferably 140 / mm 2 More than 370 / mm 2 The peak density Spd is 100 / mm 2 By setting the peak density Spd at 400 / mm or more, the number of peaks per unit area is ensured, and running performance during processing is stabilized. 2 By satisfying the above condition, it becomes easy to achieve sufficient heat sealability.

[0031] The protruding peak height Spk and the peak apex density Spd are measured using a non-contact surface / layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. More specifically, measurements are taken at three locations randomly selected in the longitudinal direction at the center position in the width direction of the film, and the average values ​​are taken as the protruding peak height Spk and the peak apex density Spd of that sample. Methods for setting the protruding peak height Spk and the peak apex density Spd within the above ranges include, for example, setting the cast drum temperature for the surface layer (B) within the above ranges, or setting the temperature in the longitudinal stretching step within the ranges described below, and these may be combined as appropriate.

[0032] From the viewpoint of achieving all of processability, dent resistance, and heat sealability, the surface layer (B) preferably has Sa of 50 nm or more and 100 nm or less, or the preferred range described above, a protruding peak height Spk of 100 nm or more and 150 nm or less, or the preferred range described above, and a peak density Spd of 100 / mm 2 More than 400 / mm 2 It is preferred that it is below or within the above preferred range.

[0033] From the viewpoints of appearance during printing and visibility of contents when used for packaging purposes, the polyolefin film of the present invention preferably has a haze of 6.0% or less, more preferably 5.0% or less. A haze of 6.0% or less means that the polyolefin film has high transparency, and such an embodiment improves the appearance during printing and visibility of contents when used for packaging purposes.

[0034] The haze can be measured using a known haze meter in accordance with JIS 7105 (1981). Methods for adjusting the haze to 6.0% or less or within the above-mentioned preferred range include, for example, adjusting the longitudinal stretching temperature of the surface layer (B) or the transverse stretching temperature after laminating the surface layer (B) and the base layer (A) within the ranges described below, and these may be used in combination as appropriate.

[0035] From the viewpoint of strength, the polyolefin film of the present invention preferably has a MD breaking strength of 40 MPa or more, more preferably 50 MPa or more, and even more preferably 60 MPa or more. By having an MD breaking strength of 40 MPa or more, the strength of the film is maintained and the occurrence of breakage during the film-forming process or transportation is suppressed. Furthermore, the TD breaking strength is preferably 300 MPa or more, more preferably 340 MPa or more, and even more preferably 380 MPa or more. The upper limit of the TD breaking strength is 500 MPa.

[0036] Furthermore, from the viewpoint of improving runnability, the polyolefin film of the present invention preferably has a breaking elongation in the MD direction of 100% or more, more preferably 130% or more. Similarly, the breaking elongation in the TD direction is preferably 10% or more, more preferably 20% or more. A method for achieving the breaking strength and breaking elongation in the above ranges in both the MD and TD directions can be achieved by using the above-mentioned polypropylene resin and setting specific conditions for the longitudinal stretching step, transverse stretching step, and heat treatment step during film production, as described below.

[0037] The breaking strength and breaking elongation can be measured by the following procedure. First, a sample is cut out of the polyolefin film in the longitudinal or transverse direction, measuring 200 mm in the measurement direction and 10 mm in the direction perpendicular to the measurement direction, and marks are placed 50 mm from both ends. A known film strength and elongation measuring device (AMF / RTA-100) is then used to perform a tensile test at a pulling rate of 300 mm / min in an atmosphere of 23°C and 65% RH, to measure the breaking strength and breaking elongation in each direction. The breaking strength and breaking elongation can be adjusted to the above ranges, for example, by setting the cast drum temperature and longitudinal stretching temperature of the surface layer (B), the longitudinal stretching ratio, and the transverse stretching temperature and transverse stretching ratio after lamination of the surface layer (B) and the base layer (A) within the ranges described below.

[0038] The polyolefin film of the present invention can be a uniaxially oriented film, a biaxially oriented film, or a laminate film of a uniaxially oriented layer and a biaxially oriented layer. However, from the viewpoint of improving ease of cutting, a laminate film of a uniaxially oriented layer and a biaxially oriented layer is preferred. Here, a uniaxially oriented layer refers to a layer that has been uniaxially stretched, and a biaxially oriented layer refers to a layer that has been stretched in two perpendicular directions. The orientation of each layer can be determined, for example, by measuring the planar orientation coefficient in each direction using a known method. A laminate film of a uniaxially oriented layer and a biaxially oriented layer can be obtained by stretching an unstretched sheet in the MD direction to obtain a biaxially oriented layer, extruding a resin composition for the uniaxially oriented layer onto the obtained uniaxially oriented film and laminating it, and then stretching the resulting laminate in the TD direction. In this case, it is more preferable that stretching in the MD direction is performed using a pair of stretching rolls with different peripheral speeds, and stretching in the TD direction is performed using a tenter.

[0039] The polyolefin film of the present invention is not particularly limited with respect to the orientation of the base layer (A) and the surface layer (B). However, from the viewpoint of improving ease of cutting, it is preferable that the base layer (A) and the surface layer (B) have different orientations. Here, "different orientations" means that one is a uniaxially oriented layer and the other is a biaxially oriented layer. From the viewpoint of ease of cutting, it is more preferable that the base layer (A), which is relatively thick and likely to affect cuttability, is a uniaxially oriented layer. Specific examples of such an embodiment include a two-kind, two-layer structure consisting of a biaxially oriented surface layer (B) and a uniaxially oriented base layer (A), and a three-kind, four-layer structure (surface layer (B) / layer (C) / layer (B') / base layer (A)) in which, for example, a biaxially oriented layer (layer (C)) composed mainly of a homopolypropylene resin and a biaxially oriented layer (layer (B')) having the same composition and orientation as the surface layer (B) are disposed between the biaxially oriented surface layer (B) and the uniaxially oriented base layer (A). In the latter case, it becomes easier to achieve both ease of cutting and strength.

[0040] The polyolefin film of the present invention may have a functional layer laminated on at least one side thereof for the purpose of imparting various effects. The laminate structure may include a base layer (A) and a surface layer (B), and may be a three-layer laminate or a greater number of laminates, as long as the surface layer (B) is located on at least one surface. The lamination method may be, for example, a feedblock method or a multi-manifold method using coextrusion, or a method in which films are bonded together by lamination.

[0041] Next, the method for producing the polyolefin film of the present invention will be explained using a polypropylene film consisting of a uniaxially oriented base layer (A) and a biaxially oriented surface layer (B) as an example, although the method for producing the polyolefin film and its layer structure are not necessarily limited to this.

[0042] First, the above-described preferred polypropylene resin as the raw material for the surface layer (B) is fed into a melt extruder and melt-extruded at 230 to 260°C. Next, foreign matter and modified polymers are removed using a filter installed midway through the polymer tube, and the resulting film is discharged onto a casting drum through a T-die to obtain an unstretched sheet. Furthermore, the surface temperature of the casting drum is preferably 15 to 100°C, more preferably 80 to 95°C, in order to control the Charpy impact value in the TD direction and the film surface roughness within appropriate ranges. At this time, methods such as electrostatic application, air knife, nip roll, and underwater casting can be used to adhere the molten sheet discharged from the T-die to the casting drum. However, the air knife method is preferred from the viewpoint of reducing foreign matter and cooling the film.

[0043] The unstretched sheet obtained by the above method is then stretched in the longitudinal direction using a pair of stretching rolls with different peripheral speeds. The film temperature during this process is preferably 120 to 160°C, more preferably 130 to 155°C, from the viewpoints of controlling the haze within a preferred range and ensuring film formation stability. The film temperature can be adjusted using a temperature-controlled rotating roll or a hot air oven. The stretching ratio during this process is preferably 3.5 to 6.0 times, more preferably 4.0 to 5.0 times, in order to control the heat shrinkage and elongation at break within preferred ranges. A stretching ratio of 3.5 times or more enables more uniform stretching, thereby suppressing the occurrence of uneven thickness in the film. On the other hand, a stretching ratio of 6.0 times or less reduces film breakage during the longitudinal stretching step and the subsequent transverse stretching step.

[0044] Subsequently, the above-mentioned preferred polypropylene resin is fed to a melt extruder as a raw material for the base layer (A), and melt extrusion is carried out at 230 to 250° C. Next, foreign matter, modified polymers, etc. are removed using a filter installed midway through the polymer pipe, and then the resulting mixture is extruded from a T-die onto a uniaxially stretched film corresponding to the surface layer (B) that has been running on a casting drum in advance, and laminated to form a laminate of the surface layer (B) and the base layer (A).

[0045] Thereafter, this laminate is stretched in the width direction. As a method for stretching in the width direction, it is preferable to adopt a stretching method using a tenter-type stretching machine from the viewpoints of film formation stability and thickness uniformity. When using a tenter-type stretching machine, the laminate is introduced into the tenter-type stretching machine while holding both widthwise end portions of the laminate with clips, and stretched in the width direction. The stretching temperature is preferably 145 to 170°C, more preferably 150 to 165°C, from the viewpoints of thickness unevenness and film formation stability. The stretching ratio is 7 to 12 times, preferably 8 to 11 times.

[0046] The stretched laminate thus obtained is then heat-treated directly in a tenter-type stretching machine. The heat-treatment temperature is preferably 110 to 160° C., and more preferably 120 to 150° C. Furthermore, the heat-treatment may be carried out while relaxing the film in the TD direction. In particular, a relaxation rate in the TD direction of 5 to 15%, more preferably 6 to 10%, is preferred from the viewpoint of reducing the thermal shrinkage rate in the width direction and ensuring appropriate dimensional stability.

[0047] The polyolefin film obtained as described above can be used for various purposes such as packaging films, surface protection films, process films, sanitary products, agricultural products, construction products, medical products, etc. In particular, by laminating a biaxially oriented layer and a uniaxially oriented layer and further using the above-mentioned preferred materials as the raw materials for each layer, the cuttability of the obtained polyolefin film is improved, making the polyolefin film more preferably usable as a packaging film. [Example]

[0048] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods. However, Example 4 is a reference example.

[0049] (1) Film thickness Measurements were taken at five points using a micro thickness meter (manufactured by Anritsu Corporation), and the average value was calculated.

[0050] (2) Hayes Measurement was performed according to JIS 7105 (1981) using a haze meter manufactured by Suga Test Instruments Co., Ltd. This measurement was performed at three points in the width direction, and the average value was taken as the haze of the easy-to-cut polyolefin film of the present invention.

[0051] (3) Breaking strength and elongation Five samples were cut from the polyolefin film in either the longitudinal or transverse direction, measuring 200 mm in the measurement direction and 10 mm perpendicular to the measurement direction, and marks were placed 50 mm from each end to give a test length of 100 mm. Measurements were performed at a tension rate of 300 mm / min in an atmosphere of 23°C and 65% RH using an Orientec Co., Ltd. film strength and elongation measuring device (AMF / RTA-100). The average values ​​of five samples in both the longitudinal and transverse directions were used to determine the breaking strength and breaking elongation.

[0052] (4) Charpy impact value in the TD direction Thirteen samples were cut out from the polyolefin film to form rectangles measuring 10 mm in the TD direction and 150 mm in the MD direction, and the thickness of each sample was measured using a dial gauge. The samples were then placed in a Charpy impact tester with the MD direction aligned horizontally, and the impact strength required to break the sample was measured by contacting the pendulum of the impact tester with the center of the sample. The Charpy impact value was calculated using the following formula. The calculated Charpy impact values ​​of the 13 samples were averaged to determine the Charpy impact value of the polyolefin film in the TD direction. Formula: Charpy impact value (MJ / m 2 ) = Impact strength (kg·cm / 10mm) / Thickness (mm).

[0053] (5) Heat seal strength The surface layers (B) of the easy-to-cut polyolefin films were overlapped and heat-sealed under the following conditions using a heat sealer heated to 155°C. Three samples measuring 80 mm in the width direction and 20 mm in the length direction were cut out of the sealed film, and each sample was attached to a shopper with backing on both sides in the width direction. The shopper was lowered so that the sample was at a 180° angle, and the peel strength was measured. The heat seal strength was calculated using the following formula based on the above peel strength, and the average value was used as the heat seal strength of the easy-to-cut polyolefin film of the present invention. Formula: Heat seal strength (N / cm) = Peel strength (N) / Sample width (cm) <Sealing conditions> Heat sealer crimping part shape: Dice Heat sealing pressure: 1.0 kg / cm 2 Heat seal time: 5 seconds.

[0054] (6) Arithmetic mean height (Sa), peak height (Spk), peak density (Spd) Measurements were made using a non-contact surface / layer cross-sectional shape measurement system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. Samples were taken from the slit film roll, and measurements were taken at three locations randomly selected in the longitudinal direction at the center position in the width direction of the film, and the average values ​​were taken as the arithmetic mean height (Sa), protruding peak height (Spk), and peak density (Spd) of the sample. The detailed conditions for one measurement were as follows. One field of view (field area 1,252 μm × 939 μm = 1,175,628 μm) was used for one measurement. 2 ) was observed.

[0055] A. Measurement conditions CCD camera: SONY HR-57 1 / 2” Objective lens: 10X Lens tube: 0.5X BODY Wavelength filter: 530 white Measurement mode: Wave Field of view size: 640 x 480 Scan range: (Start) 5μm, (Stop) -5μm.

[0056] B.Measurement method A dedicated sample holder was used for film measurements. The sample holder consists of two detachable metal plates with a circular hole in the center, and the film was sandwiched between them without any wrinkles, and measurements were taken of the film in the central circular part.

[0057] C. Analysis method The data obtained from the above measurements was analyzed using VertScan 2.0's image analysis software, VS-Viewer. First, noise was removed using a median filter (5 × 5), and waviness components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, the arithmetic mean height (Sa), peak height (Spk), and peak density (Spd) defined in ISO 25178 were measured using the "ISOPara" function. Note that the S-Filter in the "ISOPara" function was set to 6.0 μm.

[0058] (8) Ease of cutting evaluation Regarding ease of cutting, when the sample was held by the edges of the film with both hands and pulled in the TD direction, if the crack went straight through, it was marked as ◯; if the crack went straight through but cracked, it was marked as △; if the crack did not go straight through but cracked, it was marked as ×.

[0059] (9) Evaluation of workability The processability of the polyolefin film was evaluated as follows. Good: No shifting or wrinkles during running or winding, and has sufficient heat sealing properties. △: There is misalignment or wrinkles during running or winding, or the heat sealability is insufficient. ×: Misalignment or wrinkles were observed during running and winding, and the heat sealability was insufficient.

[0060] (10) Evaluation of quality The quality of the polyolefin film was evaluated according to the following criteria: The presence or absence of scratches and dirt was judged visually. 〇: When polyolefin film is continuously produced for 72 hours, there are no scratches or stains on the film surface. △: When polyolefin film is continuously produced for 72 hours, either scratches or dirt are found on the film surface. ×: When the polyolefin film was continuously produced for 72 hours, both scratches and dirt were found on the film surface.

[0061] (11) Heat shrinkage rate Two rectangular samples, each measuring 100 mm in the measurement direction and 10 mm perpendicular to the measurement direction, were cut out in either the MD or TD direction of the polyolefin film, and the reference length was measured. The samples were then heat-treated for 15 minutes in a 120°C atmosphere using a hot air circulation oven (GPS22) manufactured by Tabai Espec Corporation. The length of the samples after cooling was measured, and the heat shrinkage was calculated using the following formula. The average value of three samples in both the MD and TD directions was used as the heat shrinkage of the polyolefin film. Formula: Heat shrinkage rate (%) = (reference length (mm) - length after heat treatment (mm)) / reference length (mm) x 100.

[0062] Example 1 Two melt extruders were used. The first extruder melt-extruded a homopolypropylene resin (Sumitomo Chemical Co., Ltd., MFR = 2.2 g / 10 min) as the raw material for layer (C), and the second extruded an ethylene-propylene random copolymer (Sumitomo Chemical Co., Ltd., MFR = 4.6 g / 10 min) with an ethylene unit content of 1.2% by mass as the raw material for layer (B). The layers were then laminated in a die in the order of layer (B) raw material / layer (C) raw material / layer (B) raw material using a T-die method and co-extruded at 240 °C into a sheet. The resulting sheet was then cooled and solidified on a 90 °C cooling roll to obtain an unstretched sheet with a two-component, three-layer structure. This unstretched sheet was then stretched longitudinally at 140 °C at a stretch ratio of 4.7 to obtain a uniaxially oriented film. Onto the obtained uniaxially stretched film, a homopolypropylene resin (manufactured by SunAllomer, MFR = 18 g / 10 min) melted at 240 °C was extruded from a third extruder as the raw material for the base layer (A), forming a layer that would become the base layer (A). Furthermore, both ends of the resulting three-type, four-layer laminate in the width direction were clamped with clips and introduced into a tenter-type stretching machine. After preheating at 161 °C, it was stretched 10 times in the width direction at 156 °C, and then heat-treated at 155 °C while applying 8.3% relaxation in the transverse direction. After that, a discharge of 38 W·min / m was applied to both sides of the film. 2The film was then subjected to a corona treatment and wound up on a winder to obtain a polyolefin film with a three-type, four-layer structure: surface layer (B) / layer (C) / layer (B') / base layer (A). (Only the base layer (A) is a uniaxially oriented layer, and the others are biaxially oriented layers. Note that the surface layer (B) and layer (B') have the same composition, and the layer located on the surface is referred to as the surface layer (B).) The evaluation results of the obtained polyolefin film are shown in Table 1.

[0063] Example 2 Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer (manufactured by Sumitomo Chemical Co., Ltd., MFR = 4.4 g / 10 min) having an ethylene unit content of 1.2 mass% was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0064] Example 3 Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer (manufactured by Sumitomo Chemical Co., Ltd., MFR = 4.3 g / 10 min) having an ethylene unit content of 0.9% by mass was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0065] Example 4 Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer (manufactured by Sumitomo Chemical Co., Ltd., MFR = 4.7 g / 10 min) having an ethylene unit content of 1.5 mass% was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0066] (Comparative Example 1) Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer having an ethylene unit content of 0.8% by mass (manufactured by Prime Polymer Co., Ltd., MFR = 4.0 g / 10 min) was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0067] (Comparative Example 2) Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer (manufactured by Sumitomo Chemical Co., Ltd., MFR = 4.2 g / 10 min) having an ethylene unit content of 0.4 mass% was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0068] (Comparative Example 3) A film was produced in the same manner as in Example 2, except that the cooling roll temperature when obtaining the unstretched sheet was 50° C. The evaluation results of the obtained polyolefin film are shown in Table 1.

[0069] Comparative Example 4 Film formation was carried out in the same manner as in Example 1, except that an ethylene-propylene random copolymer (manufactured by Sumitomo Chemical Co., Ltd., MFR = 4.8 g / 10 min) having an ethylene unit content of 1.6 mass% was used as the raw material for the surface layer (B) and layer (B'). The evaluation results of the obtained polyolefin film are shown in Table 1.

[0070] [Table 1]

[0071] The arithmetic mean height (Sa), protruding peak height (Spk), and peak density (Spd) of each example and comparative examples 1 and 2 were measured first for the surface layer (B). 2 More than 400 / mm 2 In the following cases (each example), measurements were not taken on the opposite side. When any of the above requirements was not met (Comparative Examples 1 and 2), measurements were also taken on the opposite side, but the measured values ​​were the same. In Comparative Example 3, the outermost layer was the same, so there was a high probability that the measured values ​​would be the same on both sides, and therefore measurements were taken on only one layer. [Industrial Applicability]

[0072] The present invention provides a polyolefin film having high cuttability, suitable heat sealability, quality, and surface smoothness. The polyolefin film of the present invention can be used for various purposes such as packaging films, surface protection films, processing films, sanitary products, agricultural products, construction products, and medical products, and is particularly suitable for use as a packaging film because of its excellent cuttability.

Claims

1. A polyolefin film comprising at least a base layer (A) and a surface layer (B), wherein the base layer (A) contains a total of 90% by mass or more and 100% by mass or less of homopolyethylene and homopolypropylene, and the surface layer (B) is an ethylene-propylene random copolymer containing 0.9% by mass or more and 1.2% by mass or less of ethylene units, and has a Charpy impact value in the TD direction of 0.50 MJ / m ２ 1.50MJ / m or more ２ The polyolefin film has the following structure, wherein the substrate layer (A) and the surface layer (B) have different orientations.

2. The polyolefin film according to claim 1, having a heat seal strength at 155°C of 0.3 N / cm or more and 1.0 N / cm or less.

3. The polyolefin film according to claim 1 or 2, wherein at least one surface has a surface roughness Sa of 50 nm or more and 100 nm or less.

4. On at least one surface, the protruding peak height Spk is 100 nm or more and 150 nm or less, and the peak density Spd is 100 / mm ２ More than 400 / mm ２ The polyolefin film according to any one of claims 1 to 3, wherein:

5. The polyolefin film according to any one of claims 1 to 4, having a haze of 6.0% or less.

6. The polyolefin film according to any one of claims 1 to 5, wherein the substrate layer (A) is a uniaxially oriented layer.

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