Stretched polyolefin film, production method for the same, and protective film for build-up film

Abstract

To provide a stretched polyolefin film that makes it possible to prevent blocking without containing an anti-blocking agent.SOLUTION: The present invention is a stretched polyolefin film comprising a polyolefin resin, the stretched polyolefin film being stretched in at least one axial direction among the MD direction and the TD direction, wherein skewness (Ssk) as measured in a surface roughness measurement of at least one surface is 1.0 or less, and the height (Sk) of a core part is 0.04 μm or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a stretched polyolefin film, a method for producing the same, and a protective film for a build-up film. [Background technology]

[0002] Polyolefin films, such as polyethylene films and polypropylene films, are widely used in various fields, such as protective films for electronic devices, packaging films, and optical films, and are highly valuable materials.

[0003] For example, it is known that polyolefin films are suitable as protective films for build-up films used in build-up methods (for example, Patent Document 1, etc.). The build-up method is an essential technology for high integration of printed wiring boards. In such build-up methods, a build-up film is attached to a circuit board, the resin in the build-up film is cured by heat, and then vias are formed by laser processing and the resulting resin residue is removed, followed by copper plating and patterning. High integration of printed wiring boards is achieved by repeating this operation.

[0004] The build-up film used in the build-up method is formed of an adhesive layer containing a thermosetting resin such as an epoxy resin, and is an insulating material. A base film made of polyethylene terephthalate (PET) is provided on one side of the adhesive layer, and a protective film made of the above-mentioned polyolefin film is provided on the other side. In the build-up method, the protective film provided on the build-up film is peeled off in advance to expose the adhesive layer (build-up film), and then the build-up film is attached to the circuit board as described above. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-122472 A [Patent Document 2] JP 2019-163358 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, a general polyolefin film contains an antiblocking agent to prevent blocking of the film (for example, Patent Document 2 above). For this reason, the antiblocking agent may fall off from the polyolefin film. The fallen antiblocking agent may adhere to or transfer to other components, causing problems such as a decrease in performance of the other components. For example, in the case of the build-up film described above, when the protective film formed of the polyolefin film is peeled off from the build-up film (adhesive layer), the antiblocking agent present in the polyolefin film may fall off and transfer to the adhesive layer. The transfer of the antiblocking agent to the adhesive layer causes the thickness of the adhesive layer to become non-uniform, which may cause a decrease in insulation performance.

[0007] The present invention has been made in consideration of the above, and aims to provide a stretched polyolefin film that can prevent blocking without containing an antiblocking agent, a manufacturing method thereof, and a protective film for a build-up film. [Means for solving the problem]

[0008] As a result of intensive research into achieving the above-mentioned object, the inventors have found that the above-mentioned object can be achieved by stretching in at least one axial direction, i.e., MD and TD, and by setting the skewness (Ssk) measured by surface roughness measurement on at least one surface to 1.0 or less and the height (Sk) of the core portion to 0.04 μm or more, thereby completing the present invention.

[0009] That is, the present invention includes, for example, the subject matter described in the following sections. Item 1 A stretched polyolefin film containing a polyolefin resin, It is stretched at least uniaxially in the MD direction and the TD direction, A stretched polyolefin film having a skewness (Ssk) of 1.0 or less as measured by surface roughness measurement on at least one surface, and a core height (Sk) of 0.04 μm or more. Section 2 Item 2. The stretched polyolefin film according to item 1, wherein the polyolefin resin contains a polyethylene resin. Section 3 Item 3. The stretched polyolefin film according to item 1 or 2, which does not contain an inorganic antiblocking agent. Section 4 Item 4. The stretched polyolefin film according to any one of items 1 to 3, which is a biaxially stretched film stretched in both the MD direction and the TD direction. Section 5 Item 5. The stretched polyolefin film according to any one of items 1 to 4, which is used as a protective film for a build-up film. Section 6 Item 6. A protective film for a build-up film, comprising the stretched polyolefin film according to item 5. Section 7 A method for producing a stretched polyolefin film according to any one of items 1 to 6, The raw sheet containing the polyolefin resin is stretched in at least one axial direction of the MD direction and the TD direction, A method for producing a stretched polyolefin film, wherein the raw sheet is preheated to a temperature of 155°C or higher in the stretching treatment. Effect of the Invention

[0010] The stretched polyolefin film of the present invention can prevent blocking without containing an antiblocking agent. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present invention will be described in detail. In this specification, the expressions "containing" and "comprise" include the concepts of "containing", "comprises", "consists essentially of" and "consists only of".

[0012] 1. Stretched polyolefin film The stretched polyolefin film of the present invention is stretched at least uniaxially in the MD and TD directions, and has a skewness (Ssk) of 1.0 or less as measured by surface roughness measurement on at least one surface, and a core height (Sk) of 0.04 μm or more.

[0013] The stretched polyolefin film of the present invention can prevent blocking even without containing an antiblocking agent. Generally, since a stretched film is easily formed flat, blocking between the surfaces of the stretched film is easily caused. In order to prevent this, it is necessary to contain an antiblocking agent in the stretched film. In this respect, in the stretched polyolefin film stretched at least uniaxially in the MD direction and the TD direction, the skewness (Ssk) measured by surface roughness measurement is controlled to 1.0 or less, and the height of the core part measured by surface roughness measurement is controlled to 0.04 μm or more. By carrying out such control, the present inventors have found for the first time that it is possible to provide a stretched polyolefin film that can prevent blocking even without containing an antiblocking agent, and have completed the present invention.

[0014] The stretched polyolefin film of the present invention can be used in various applications, particularly in fields where the dropping off of antiblocking agents has been a problem, and is particularly suitable as a protective film for build-up films.

[0015] The stretched polyolefin film of the present invention contains a polyolefin resin. Specifically, the stretched polyolefin film of the present invention is a stretched film formed of a resin containing a polyolefin resin as a main component.

[0016] The polyolefin resin may be, for example, a wide variety of polyolefin resins used for producing stretched films. For example, the polyolefin resin may be a polymer obtained by polymerizing an olefin compound. Such an olefin compound may preferably be an olefin compound having 2 to 20 carbon atoms, more preferably having 2 to 10 carbon atoms, and even more preferably having 3 to 6 carbon atoms. Specific examples of the polyolefin resin include polyethylene resin, polypropylene resin, poly(1-butene) resin, polyisobutene resin, poly(1-pentene) resin, and poly(4-methylpentene-1) resin. The polyolefin resin may also be a copolymer containing two or more structural units derived from different olefin compounds, such as an ethylene-propylene copolymer.

[0017] Among them, the polyolefin resin preferably contains at least one selected from the group consisting of polyethylene resins and polypropylene resins, and more preferably contains a polyethylene resin, because the skewness (Ssk) value and the core height (Sk) value of the stretched polyolefin film can be easily adjusted to the desired range and blocking can be more easily prevented. Among the polyethylene resins, low-density polyethylene or high-density polyethylene is preferable, and linear low-density polyethylene, so-called LLDPE, is more preferable.

[0018] The polyolefin resin contained in the stretched polyolefin film of the present invention may contain other resins as long as the effects of the present invention are not impaired.

[0019] The polyolefin resin preferably contains 80% by mass or more of the polyethylene resin, more preferably 90% by mass or more, further preferably 95% by mass or more, and particularly preferably 99% by mass or more. The polyolefin resin may consist of only polyethylene resin.

[0020] The stretched polyolefin film of the present invention may not contain an inorganic antiblocking agent. As described above, the stretched polyolefin film of the present invention has a skewness (Ssk) value and a core height (Sk) value adjusted to an appropriate range, so that the occurrence of blocking is suppressed. When the stretched polyolefin film of the present invention contains an inorganic antiblocking agent, the content ratio is 5 mass% or less, preferably 1 mass% or less, more preferably 0.1 mass% or less, and particularly preferably 0 mass% based on the total mass of the stretched polyolefin film, that is, the stretched polyolefin film of the present invention does not contain an inorganic antiblocking agent.

[0021] The stretched polyolefin film of the present invention may contain an organic antiblocking agent as long as the effect of the present invention is not impaired. When the stretched polyolefin film of the present invention contains an organic antiblocking agent, the content is 5% by mass or less, preferably 1% by mass or less, more preferably 0.1% by mass or less, and particularly preferably 0% by mass, based on the total mass of the stretched polyolefin film. In other words, the stretched polyolefin film of the present invention does not contain any organic antiblocking agent.

[0022] The stretched polyolefin film of the present invention may contain other components (excluding antiblocking agents) other than the polyolefin resin, as long as the effects of the present invention are not impaired. Examples of other components include additives contained in known stretched films, such as heat stabilizers, antioxidants, organic and inorganic lubricants, chlorine traps, antistatic agents, antifogging agents, hydrolysis inhibitors, etc. When the stretched polyolefin film of the present invention contains the other components (excluding antiblocking agents), the content is 10% by mass or less, preferably 5% by mass or less, more preferably 1% by mass or less, and particularly preferably 0.5% by mass, based on the total mass of the stretched polyolefin film.

[0023] As described above, the stretched polyolefin film of the present invention is stretched at least uniaxially in the MD direction and the TD direction, and preferably, the stretched polyolefin film of the present invention is stretched at least in the TD direction.

[0024] The stretched polyolefin film of the present invention may be a uniaxially stretched film obtained by stretching in one direction, or a biaxially stretched film obtained by stretching in two directions. When the stretched polyolefin film of the present invention is a biaxially stretched film, it is preferably a biaxially stretched film stretched in two directions, MD and TD.

[0025] The stretched polyolefin film of the present invention may have a single layer structure or a multilayer structure. When the stretched polyolefin film of the present invention has a multilayer structure, each layer contains the above-mentioned polyolefin resin. In this case, the polyolefin resins contained in each layer may be the same as each other, or at least one or all of them may be different.

[0026] A specific example of the stretched polyolefin film having the multilayer structure is a stretched polyolefin film having a base layer and a surface layer formed on each side of the base layer. The base layer and the surface layers formed on both sides are layers containing a polyolefin resin. The polyolefin resins contained in each layer may be the same as each other, or at least one or all of them may be different. Specific examples of stretched polyolefin films having a multilayer structure include a stretched polyolefin film in which all layers are LLDPE, a stretched polyolefin film in which the base layer is HDPE and the surface layers on both sides are LLDPE, and a stretched polyolefin film in which the base layer is polypropylene and the surface layers on both sides are LLDPE. It is preferable that no layer is formed between the base layer and the surface layer.

[0027] The thickness ratio between the base layer and the surface layer is not particularly limited, and for example, when the thickness of the surface layer is 3, the thickness of the base layer is preferably 4 to 50, more preferably 7 to 40, and further preferably 10 to 30. It is preferable that the thicknesses of the surface layers formed on both sides of the base layer are the same.

[0028] When the stretched polyolefin film of the present invention has a multilayer structure, a preferred embodiment of such a multilayer structure is a stretched polyolefin film in which the base layer is made of a polyethylene resin and a pair of surface layers provided on both sides of the base layer are also made of a polyethylene resin. In this case, it is particularly preferred that all layers are made of the same type of LLDPE.

[0029] Here, one of the surfaces of the stretched polyolefin film of the present invention is referred to as surface A, and the other surface is referred to as surface B. When the stretched polyolefin film of the present invention has a base layer and a surface layer on each of both sides of the base layer as described above, the exposed surface of one surface layer is surface A, and the exposed surface of the other surface layer is surface B.

[0030] In the stretched polyolefin film of the present invention, at least one of surface A and surface B has a skewness (Ssk) measured by surface roughness measurement of 1.0 or less and a core height (Sk) measured by surface roughness measurement of 0.04 μm or more. This prevents blocking between films in the stretched polyolefin film of the present invention even without containing an antiblocking agent.

[0031] The skewness (Ssk) and the core height (Sk) are indexes of surface roughness defined in ISO 25178, and both can be measured using a commercially available surface roughness measuring instrument. In particular, in the present invention, the skewness (Ssk) and the core height (Sk) of the stretched polyolefin film can be measured using an optical interference type non-contact surface profile and surface roughness measuring instrument "VertScan2.0 (model: R5500GML)" manufactured by Ryoka Systems Co., Ltd.

[0032] The value of the skewness (Ssk) is preferably 0.5 or less, more preferably 0.3 or less, and even more preferably 0.1 or less. The value of the skewness (Ssk) may be less than 0. There is no particular lower limit to the value of the skewness (Ssk), and for example, it is preferably -1.5 or more, and more preferably -1.1 or more.

[0033] In the stretched polyolefin film of the present invention, both surface A and surface B preferably have a skewness (Ssk) value of 1.0 or less.

[0034] The value of the core height (Sk) is preferably 0.05 μm or more, and more preferably 0.06 μm or more, in that blocking between films is more easily suppressed. The upper limit of the value of the core height (Sk) is not particularly limited, and is, for example, preferably 0.5 μm or less, more preferably 0.3 μm or less, even more preferably 0.2 μm or less, and particularly preferably 0.15 μm or less. In the stretched polyolefin film of the present invention, only one of the surfaces A and B may have a core height (Sk) of 0.04 μm or more, or both surfaces may have the core height (Sk).

[0035] In view of the above, the stretched polyolefin film of the present invention has at least one of surface A and surface B having a skewness (Ssk) measured by surface roughness measurement of 1.0 or less and a core height (Sk) of 0.04 μm or more, and in a particularly preferred embodiment, both surface A and surface B have a skewness (Ssk) value of 1.0 or less and at least one surface has a core height (Sk) of 0.04 μm or more.

[0036] The method of adjusting the skewness (Ssk) and the height (Sk) of the core part is not particularly limited, and various methods can be adopted. Preferably, as described later, in the manufacturing process of the stretched polyolefin film, the skewness (Ssk) and the height (Sk) of the core part can be adjusted by adjusting the temperature of the preheating treatment performed when the raw sheet is stretched and the temperature of the relaxation treatment performed thereafter. This will be described in detail in the section on the manufacturing method of the stretched polyolefin film.

[0037] The value of the protruding peak height (Spk) of the stretched polyolefin film of the present invention is not particularly limited, and for example, the protruding peak height (Spk) of at least one of surface A and surface B can be 0.015 μm or more. In particular, it is preferable that the surface that satisfies the skewness (Ssk) of 1.0 or less and the core height (Sk) of 0.04 μm or more has a protruding peak height (Spk) of 0.015 μm or more. The upper limit of the protruding peak height (Spk) is not particularly limited, and is, for example, 0.1 μm or less, preferably 0.05 μm or less.

[0038] The value of the protruding valley height (Svk) of the stretched polyolefin film of the present invention is not particularly limited, and for example, the protruding peak height (Spk) of at least one of the surfaces A and B can be 0.015 μm or more. In particular, it is preferable that the surface having a skewness (Ssk) of 1.0 or less and a core height (Sk) of 0.04 μm or more has a protruding valley height (Svk) of 0.015 μm or more. The upper limit of the protruding valley height (Svk) is not particularly limited, and is, for example, 0.1 μm or less, preferably 0.08 μm or less.

[0039] The peak height (Spk) and valley height (Svk) are also one of the indexes of surface roughness defined in ISO 25178, and can be measured, for example, by the commercially available surface roughness measuring device mentioned above.

[0040] The thickness of the stretched polyolefin film of the present invention is not particularly limited, and can be a desired thickness depending on the intended use.For example, when the stretched polyolefin film is used as a protective film for a build-up film, the thickness of the stretched polyolefin film can be 5 to 100 μm, preferably 10 to 50 μm, and more preferably 15 to 30 μm.In addition, when the stretched polyolefin film has the above-mentioned multi-layer structure, the thickness of the stretched polyolefin film means the total value of the thicknesses of each layer.

[0041] The stretched polyolefin film of the present invention is unlikely to cause blocking even without the inclusion of an antiblocking agent, and therefore can be applied to various applications, particularly to fields where the dropping off of the antiblocking agent has been a problem. In particular, the stretched polyolefin film of the present invention is particularly preferably used as a protective film for a build-up film. That is, the stretched polyolefin film of the present invention can be particularly suitably used as a protective film for a build-up film.

[0042] The type of build-up film to which the stretched polyolefin film of the present invention can be applied as a protective film is not particularly limited, and for example, it can be widely applied to known build-up films. Such a protective film is provided to protect the adhesive layer of the build-up film.

[0043] In the build-up film having the stretched polyolefin film of the present invention as a protective film, the antiblocking agent does not fall off and is not transferred to the adhesive layer even when the protective film is peeled off. Therefore, the build-up film is less likely to cause a decrease in insulation performance and is suitable for the build-up method.

[0044] The peeling force of the stretched polyolefin film of the present invention is not limited. For example, from the viewpoint of easily increasing the adhesion to the adherend, the peeling force of the surface having a skewness (Ssk) of 1.0 or less and a core height (Sk) of 0.04 μm or more is preferably 0.005 N / 25 mm or more, more preferably 0.01 N / 25 mm or more, and even more preferably 0.02 N / 25 mm or more. When the surface A and surface B of the stretched polyolefin film of the present invention contain LLDPE, the peeling force is preferably 1.2 N / 25 mm or less, more preferably 0.9 N / 25 mm or less, and even more preferably 0.6 N / 25 mm or less. When the surface A and surface B of the stretched polyolefin film of the present invention contain HDPE, the peeling force is preferably 4 N / 25 mm or less, more preferably 3 N / 25 mm or less. When the surface A and surface B of the stretched polyolefin film of the present invention contain polypropylene, the peeling force is preferably 6 N / 25 mm or less, more preferably 5 N / 25 mm or less.

[0045] 2. Manufacturing method of stretched polyolefin film The method for producing the stretched polyolefin film of the present invention is not particularly limited, and for example, the same method as that for known stretched films can be widely adopted. Specifically, the stretched polyolefin film of the present invention can be produced by a production method including a step of stretching a raw sheet containing the polyolefin resin at least in one axial direction of the MD direction and the TD direction.

[0046] The method for producing the raw sheet is not particularly limited, and the raw sheet can be obtained by a method similar to a known method. Specifically, the raw sheet can be formed by extruding the raw material containing the polyolefin resin using an extruder.

[0047] Even when producing a stretched polyolefin film having a multilayer structure, the raw sheet for producing such a film can be produced by a known method. For example, when producing a stretched polyolefin film having a surface layer on both sides of a base layer, a raw sheet having a laminated structure can be formed using a polyolefin resin for forming the base layer and a polyolefin resin for forming the surface layer. For the raw sheet having a laminated structure, for example, a co-extrusion method, a lamination method, a heat sealing method, etc. can be used. Examples of the co-extrusion method include a pre-die lamination method in which molten resin is contacted in a feed block in front of a mold, an in-die lamination method in which molten resin is contacted in a path inside a mold, for example, a multi-manifold die, and an outside-die lamination method in which the resin is discharged from multiple concentric lips and contacted. For example, in the case of the in-die lamination method, a multi-layer die such as a three-layer multi-manifold die can be used. The raw sheet formed from the polyolefin resin is cooled by a cooling drum and then wound up.

[0048] The raw sheet is stretched under appropriate conditions to obtain the desired stretched polyolefin film. The stretching method is not particularly limited, and any known method can be widely used. For example, known methods such as stretching between rolls with a difference in peripheral speed, tenter method, tubular method, etc. can be used as the stretching method.

[0049] The stretching direction can be uniaxial or biaxial. The stretching direction is preferably at least uniaxial in the MD and TD directions. When the stretched polyolefin film is a uniaxially stretched film, it is more preferable to stretch the raw sheet in the TD direction, and when it is a biaxially stretched film, it is more preferable to stretch the raw sheet in each of the MD and TD directions.

[0050] In the case of uniaxial stretching, the raw sheet is stretched in the MD direction or the TD direction. In this case, the raw sheet is preheated in advance. The preheating temperature is preferably 155°C or higher. When the preheating temperature of the raw sheet in the stretching process is 155°C or higher, the obtained stretched polyolefin film can easily satisfy the desired skewness (Ssk) and core height (Sk) described above. In particular, in the case of a uniaxially stretched film, it is preferable to stretch the raw sheet in the TD direction, and it is preferable to preheat the raw sheet at a temperature of 155°C or higher. A more preferable preheating temperature in the uniaxial stretching is 160°C or higher, and an even more preferable preheating temperature is 165°C or higher. The preheating temperature is not particularly limited, but from the viewpoint of increasing the thickness accuracy in the TD direction by suppressing the preheating temperature, the preheating temperature in the biaxial stretching is preferably 190°C or lower, more preferably 185°C or lower, and even more preferably 180°C or lower.

[0051] The method of preheating the raw sheet can be widely adopted from known methods, specifically, a method of preheating the raw sheet at a predetermined temperature by hot air from an oven heated to a predetermined temperature by a tenter, a method of preheating at a predetermined temperature by an IR heater provided between metal rolls, etc. After this preheating, the raw sheet is stretched in a stretching zone. The temperature of the stretching zone is not limited, and is, for example, 110 to 130°C.

[0052] The stretching ratio of the raw sheet by uniaxial stretching is not particularly limited, and the stretching ratio can be, for example, 1.1 to 15 times.

[0053] After the raw sheet is uniaxially stretched, the obtained stretched film is subjected to relaxation treatment. The relaxation treatment method is not particularly limited, and known relaxation treatment methods can be widely adopted. In the uniaxial stretching, the temperature of the relaxation treatment can be, for example, 100°C or higher, preferably 110°C or higher, more preferably 115°C or higher, and even more preferably 120°C or higher. This makes it easier for the obtained uniaxially stretched polyolefin film to satisfy the desired skewness (Ssk) and core height (Sk) described above. However, when the temperature of the preheating treatment described above is 160°C or lower, the temperature of the relaxation treatment is more preferably 140°C or higher, even more preferably 150°C or higher, even more preferably 160°C or higher, and particularly preferably 165°C or higher.

[0054] As the relaxation treatment method, a wide variety of known methods can be adopted. Specifically, the stretched film can be relaxed by hot air from an oven heated to a predetermined temperature by a tenter.

[0055] On the other hand, when the stretching process is the biaxial stretching described above, for example, both sequential stretching and simultaneous stretching can be applied as the biaxial stretching method. Among these, simultaneous biaxial stretching by a tenter method, sequential biaxial stretching by a tenter method, and sequential biaxial stretching in which longitudinal (flow, MD) stretching is performed between rolls with a peripheral speed difference and then transverse (width, TD) stretching is performed by a tenter method are preferred because they are easy to obtain the desired stretched film.

[0056] In the case of biaxial stretching, the raw sheet is stretched in the MD direction and the TD direction, for example, first in the MD direction and then in the TD direction.

[0057] Even in the case of biaxial stretching, the raw sheet is preheated in advance. The temperature of the preheating treatment in biaxial stretching is preferably 155°C or higher. By preheating the raw sheet in the biaxial stretching treatment at a temperature of 155°C or higher, the obtained stretched polyolefin film can easily satisfy the desired skewness (Ssk) and core height (Sk) described above. The temperature of the preheating treatment when stretching in the MD direction in biaxial stretching is preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher. The temperature of the preheating treatment is not particularly limited, but from the viewpoint that adhesion to the metal roll can be suppressed by suppressing the preheating temperature, the temperature of the preheating treatment in the MD stretching in biaxial stretching is preferably 120°C or lower, more preferably 115°C or lower, and even more preferably 110°C or lower. The temperature of the preheating treatment when stretching in the TD direction is preferably 155°C or higher, more preferably 160°C or higher, and even more preferably 165°C or higher. The temperature of the preheating treatment is not particularly limited, but from the viewpoint of increasing the thickness accuracy in the TD direction by suppressing the preheating temperature, the temperature of the preheating treatment in biaxial stretching is preferably 190°C or less, more preferably 185°C or less, and even more preferably 180°C or less.

[0058] In the biaxial stretching, the method of preheating the raw sheet can be widely adopted from known methods, specifically, a method of preheating the raw sheet at a predetermined temperature by hot air from an oven heated to a predetermined temperature by a tenter, a method of preheating at a predetermined temperature by an IR heater provided between metal rolls, etc. After this preheating, the raw sheet is stretched in a stretching zone. The temperature of the stretching zone is not limited, and is, for example, 110 to 130°C.

[0059] The stretching ratio of the raw sheet by biaxial stretching is not particularly limited. For example, the stretching ratio in the MD direction can be 2 to 10 times, and the stretching ratio in the TD direction can be 2 to 15 times.

[0060] After the raw sheet is biaxially stretched, the resulting stretched film is subjected to a relaxation treatment. The relaxation treatment method is not particularly limited, and any known relaxation treatment method can be widely adopted. The relaxation treatment is performed after stretching in the MD direction and after stretching in the TD direction.

[0061] The temperature of the relaxation treatment after stretching in each direction in the biaxial stretching can be, for example, 100°C or higher, preferably 110°C or higher, more preferably 115°C or higher, and even more preferably 120°C or higher. This makes it easier for the obtained stretched polyolefin film to satisfy the desired skewness (Ssk) and core height (Sk) described above. However, when the temperature of the preheating treatment described above is 160°C or lower, the temperature of the relaxation treatment is more preferably 140°C or higher, even more preferably 150°C or higher, even more preferably 160°C or higher, and particularly preferably 165°C or higher.

[0062] As the relaxation treatment method, a wide variety of known methods can be adopted. Specifically, the stretched film can be relaxed by hot air from an oven heated to a predetermined temperature by a tenter.

[0063] The stretched film obtained by uniaxial or biaxial stretching is wound up, and then, after being subjected to an aging treatment, it can be cut to a desired product width.

[0064] As described above, a preferred embodiment of the method for producing a stretched polyolefin film of the present invention includes a step of stretching an original sheet containing the polyolefin resin at least uniaxially in the MD direction and the TD direction, and the preheating temperature of the original sheet in the stretching process is 155° C. or higher. This allows the obtained stretched polyolefin film to easily satisfy the desired skewness (Ssk) and core height (Sk) described above. In particular, when all the polyolefin resins contained in the original sheet are LLDPE, it is particularly easy to adjust the skewness (Ssk) and core height (Sk) to the desired range.

[0065] In the manufacturing process of the stretched polyolefin film obtained by the above manufacturing method, the raw sheet comes into contact with the cooling drum for cooling as described above. At this time, the surface opposite to the surface in contact with the cooling drum satisfies "skewness (Ssk) of 1.0 or less and core height (Sk) of 0.04 μm or more". For example, when surface B of the stretched polyolefin film comes into contact with the cooling drum, surface A, which is the opposite surface, satisfies "skewness (Ssk) of 1.0 or less and core height (Sk) of 0.04 μm or more". Of course, surface B (i.e., the surface in contact with the cooling drum) as well as surface A may also satisfy skewness (Ssk) of 1.0 or less, and core height (Sk) of 0.04 μm or more.

[0066] In specifying the invention included in the present disclosure, the configurations (properties, structures, functions, etc.) described in each embodiment of the present disclosure may be combined in any manner. In other words, the present disclosure includes all subject matter consisting of all combinations of the configurations that can be combined as described in this specification. EXAMPLES

[0067] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the embodiments of these examples.

[0068] The polyolefin resins used in the examples and comparative examples are as follows. PE-1: Dow Chemical Company's "TF80" (LLDPE) PE-2: SABIC "BX202" (LLDPE) PE-3: LG Chem "LO4904P" (HDPE) PP-1: Prime Polymer Co., Ltd. "F-300SP"

[0069] Example 1 As shown in Table 1 below, a stretched polyolefin film was produced in which a surface layer A and a surface layer B were arranged on both sides of a base layer. First, 100 parts by mass of PE-1 (hereinafter referred to as pellet 1) was prepared as a polyolefin resin for forming a base layer, and 100 parts by mass of PE-1 (hereinafter referred to as pellet 2) was prepared as a polyolefin resin for forming the surface layer A and the surface layer B. Pellet 1 was fed from a hopper into a single-screw type extruder a, and pellet 2 was fed from a hopper into a single-screw type extruder b, which was different from the extruder a. The pellets 1 and 2 were melted, and stacked in a three-layer configuration inside a three-layer multi-manifold die, and then cooled and solidified while being pressed against a cooling drum with air pressure using an air knife. As a result, a raw sheet was obtained in which a layer of PE-2 derived from pellet 2 was directly formed on both sides of a layer of PE-1 derived from pellet 1.

[0070] Next, the raw sheet was stretched using a tenter-type sequential biaxial stretching machine. In this stretching process, the preheating temperature when stretching in the MD direction was 100°C, the preheating temperature when stretching in the TD direction was 165°C, and the relaxation temperature after stretching in the TD direction was 120°C. Specifically, the raw sheet was preheated at 100°C with a longitudinal stretching roll and stretched in the MD direction at a stretching ratio of 6 times. This stretched raw sheet was preheated in a tenter oven at 165°C, and then stretched in the TD direction at a stretching ratio of 7.8 times in the stretching zone at 125°C. The stretched film obtained was relaxed in the TD direction by 10% at 120°C to obtain a stretched film with a thickness of 20 μm. The obtained stretched film was a stretched polyolefin film in which the surface layer A, the base layer, and the surface layer B were laminated in this order, and the thickness ratio was 3:14:3 (surface layer A: base layer: surface layer B). In such a stretched polyolefin film, the surface layer B is a layer that is in direct contact with the cooling drum, and the surface layer A is a layer that is not in contact with the cooling drum.

[0071] Example 2 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the preheating temperature during stretching in the TD direction was changed to 175°C.

[0072] Example 3 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the polyolefin resin for forming the base layer was changed to PE-2, and the polyolefin resin for forming the surface layer A and the surface layer B was changed to PE-2.

[0073] Example 4 A stretched polyolefin film was obtained in the same manner as in Example 2, except that the polyolefin resin for forming the base layer was changed to PE-2, and the polyolefin resin for forming the surface layer A and the surface layer B was changed to PE-2.

[0074] Example 5 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the polyolefin resin for forming the surface layer A and the surface layer B was changed to PE-2.

[0075] Example 6 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the stretching ratio in the MD direction was changed to 5 times.

[0076] Example 7 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the stretching ratio was changed to 5 times in the MD direction and 10 times in the TD direction.

[0077] Example 8 As shown in Table 1 below, a stretched polyolefin film was produced in which a surface layer A and a surface layer B were arranged on both sides of a base layer. First, 100 parts by mass of PE-1 (hereinafter referred to as pellet 1) was prepared as a polyolefin resin for forming a base layer, and 100 parts by mass of PE-1 (hereinafter referred to as pellet 2) was prepared as a polyolefin resin for forming the surface layer A and the surface layer B. Pellet 1 was fed from a hopper into a single-screw type extruder a, and pellet 2 was fed from a hopper into a single-screw type extruder b, which was different from the extruder a. The pellets 1 and 2 were melted, and stacked in a three-layer configuration inside a three-layer multi-manifold die, and then cooled and solidified while being pressed against a cooling drum with air pressure using an air knife. As a result, a raw sheet was obtained in which a layer of PE-2 derived from pellet 2 was directly formed on both sides of a layer of PE-1 derived from pellet 1.

[0078] Next, the raw sheet was stretched using a uniaxial stretching machine. In this stretching process, the raw sheet was stretched only in the TD direction, and the preheating temperature during stretching was 165°C, the stretching temperature was 125°C, and the relaxation temperature after stretching was 120°C. Specifically, the raw sheet was preheated in a tenter oven at 165°C, and then stretched in the TD direction at a stretching ratio of 10 times in a stretching zone at 125°C. The stretched film obtained in this way was relaxed 10% in the TD direction at 120°C to obtain a stretched film with a thickness of 20 μm. The obtained stretched film was a stretched polyolefin film in which the surface layer A, the base layer, and the surface layer B were laminated in this order, and the thickness ratio was 3:14:3 (surface layer A: base layer: surface layer B).

[0079] Example 9 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the preheating temperature during stretching in the TD direction was changed to 155°C and the relaxation temperature after stretching in the TD direction was changed to 165°C.

[0080] Example 10 A stretched polyolefin film was obtained in the same manner as in Example 2, except that the polyolefin resin for forming the surface layer A and the surface layer B was changed to PE-3.

[0081] Example 11 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the polyolefin resin for forming surface layer A and surface layer B was changed to PP-1 and the preheating temperature during stretching in the TD direction was changed to 185°C.

[0082] Comparative Example 1 A stretched polyolefin film was obtained in the same manner as in Example 1, except that the preheating temperature during stretching in the TD direction was changed to 155°C.

[0083] Comparative Example 2 A stretched polyolefin film was obtained in the same manner as in Example 3, except that the preheating temperature during stretching in the TD direction was changed to 155°C.

[0084] Comparative Example 3 99.7 parts by mass of PE-1 as a polyolefin resin and 0.3 parts by mass of "Silisia 430" (manufactured by Fuji Silysia Chemical Co., Ltd.) as an antiblocking agent (AB-1) were mixed to prepare pellet 3. A raw sheet was obtained using this pellet. Next, the raw sheet was stretched using a tenter-type sequential biaxial stretching machine. In such a stretching process, the preheating temperature when stretching in the MD direction was 100°C, the preheating temperature when stretching in the TD direction was 155°C, the temperature of the TD stretching zone was 125°C, and the relaxation temperature after stretching in the TD direction was 120°C. Specifically, the raw sheet was preheated at 100°C with a longitudinal stretching roll and stretched in the MD direction at a stretch ratio of 6 times. This stretched raw sheet was preheated in a tenter in an oven at 155°C, and then stretched in the TD direction at a stretch ratio of 7.8 times in an oven at 125°C. The thus obtained stretched film was subjected to a 10% relaxation treatment in the TD direction at 120° C. to obtain a stretched film having a thickness of 20 μm.

[0085] (surface roughness measurement) For the surface roughness measurement, a non-contact optical interference surface profiler, "VertScan2.0 (model: R5500GML)" manufactured by Ryoka Systems Co., Ltd., was used. As a measurement sample, each film of the examples and comparative examples was cut into an arbitrary size of about 20 cm square, and in a state where wrinkles were sufficiently stretched out so that they were no longer wrinkled, they were set on the measurement stage using an electrostatic contact plate or the like. First, the measurement was performed using WAVE mode, a 530 white filter and a 1× BODY lens barrel, and a 10× objective lens to measure (470 μm × 353 μm) per field of view. This operation was performed at 10 points at 1 cm intervals in the flow direction from the center of the surface of the skin layer of the target sample film in both the flow direction and width direction. Next, an analysis was performed using the "ISO parameters" in the "Bearing" plug-in function of the "VS-Viewer" analysis software of "VertScan2.0" to determine Ssk, Sk (μm), Spk (μm), and Svk (μm), and the average value of each value obtained at the above 10 locations was calculated.

[0086] (Anti-blocking agent (AB agent) falling off) In the surface roughness measurement, the presence or absence of AB agent falling off was evaluated by observing the convex shape according to the size of the AB agent. Specifically, the part where the AB agent fell off becomes a depression of the same size, so the convex part was observed using this depression as a mark. A maximum of 10 convex parts were confirmed, and if one or more convex parts were observed on at least one side, it was marked with ×, and if they were not observed, it was marked with ◯.

[0087] (Evaluation of blocking resistance) The stretched film was cut into a size of 5 x 10 cm, and the front and back sides of the film were overlapped and pressed against each other with a pressure of 1 kg / cm 2 A load of 10 ... ◎: Peels off easily. ◯: There is some resistance but it peels off. ×: Does not peel off.

[0088] (Evaluation of peeling force) A 50 mm wide x 150 mm long adhesive tape (Nitto Denko Corporation No. 31B tape, acrylic adhesive) was attached to the surface of the film on the surface layer side of the peelable film by rolling a 2 kg roller back and forth twice to obtain a laminate. When the stretched film had a surface with a skewness (Ssk) of 1.0 or less and a core height (Sk) of 0.04 μm or more, the adhesive tape was attached to that surface. The obtained laminate was left to stand for 20 hours in an environment of a temperature of 70°C and a humidity of 50%, and then cut into a 25 mm wide piece to be used as a measurement sample. The measurement sample was peeled at 180° at a speed of 50 mm / min using a tensile tester (Minebea Corporation's universal tensile tester "Technograph TGI-1kN"), and the peel force was measured.

[0089] (Evaluation Results) Table 1 shows the production conditions and evaluation results of the films obtained in each Example and Comparative Example. In Table 1, surface layer A is a layer that was not in contact with either the longitudinal stretching roll or the transverse stretching roll during the stretching process in the production of the stretched polyolefin film, and surface layer B is a layer that was in direct contact with the longitudinal stretching roll and the transverse stretching roll. Surface layer A is a layer having the above-mentioned surface A, and surface layer B is a layer having the above-mentioned surface B.

[0090] [Table 1]

[0091] It can be seen from Table 1 that the stretched polyolefin films obtained in each Example had a skewness (Ssk) of 1.0 or less, measured by measuring the surface roughness of at least one side (particularly the side opposite to the side that contacted the roll in the preheating treatment), and a core height (Sk) of 0.04 μm or more. In addition, the stretched polyolefin films obtained in each Example did not suffer from blocking. In contrast, the stretched polyolefin films obtained in Comparative Examples 1 and 2 did not satisfy the requirements of at least one side skewness (Ssk) of 1.0 or less and a core height (Sk) of 0.04 μm or more, and therefore suffered from significant blocking. Although no blocking occurred in Comparative Example 3, the antiblocking agent (AB agent) was included, and thus the antiblocking agent was seen to fall off from the film.

[0092] From the above, it has been demonstrated that the stretched polyolefin films obtained in the examples have a specified skewness (Ssk) value and a core height (Sk) value, and therefore can prevent blocking even without containing an antiblocking agent.

Claims

1. A stretched polyolefin film containing a polyolefin resin, It is stretched at least uniaxially in the MD direction and the TD direction, A stretched polyolefin film having a skewness (Ssk) of 1.0 or less as measured by surface roughness measurement on at least one surface, and a core height (Sk) of 0.04 μm or more.

2. The stretched polyolefin film according to claim 1 , wherein the polyolefin resin comprises a polyethylene resin.

3. 2. The stretched polyolefin film according to claim 1, which does not contain an inorganic antiblocking agent.

4. 2. The stretched polyolefin film according to claim 1, which is a biaxially stretched film stretched in both the MD and TD directions.

5. The stretched polyolefin film according to any one of claims 1 to 4, which is used as a protective film for a build-up film.

6. A protective film for a build-up film, comprising the stretched polyolefin film according to claim 5 .

7. A method for producing the stretched polyolefin film according to any one of claims 1 to 4, The process includes a step of stretching the raw sheet containing the polyolefin resin in at least one axial direction of the MD direction and the TD direction, The method for producing a stretched polyolefin film, wherein the raw sheet is preheated to a temperature of 155° C. or higher in the stretching treatment.

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