Stretched film

Abstract

The present invention addresses the problem of providing a film that exhibits excellent handling performance after thermal processing and in which zipping and / or poor adhesion are suppressed. This problem is solved by a stretched film comprising a polyethylene resin, the stretched film having an average linear thermal expansion coefficient of 8.00 × 10-4 / °C or less in at least one of the longitudinal and width directions when heated from 30°C to 85°C at a rate of 10°C / min, and the stretched film having a peel strength of 4.00 N / 25 mm or less as measured in a 180° peel test conducted on at least one surface at a peel rate of 50 mm / min.

Description

STRETCHED FILM

[0001] The present invention relates to a stretched film and the like.

[0002] Polyolefin films, such as polyethylene film and polypropylene film, are excellent in lightness, thermal stability, and mechanical properties, and are thus widely used as industrial material films, such as those for packaging. Particularly in recent years, due to their excellent release properties, polyolefin films have been widely used, for example, as protective materials and release materials that are used in the manufacturing processes of electronic components and electronic substrates or used for a thermosetting resin member, such as fiber-reinforced plastics, thus increasing their utility value.

[0003] When used as a protective film, polyolefin film is sometimes thermally laminated with other layers to form a laminate. During thermal lamination, curl may occur, which deteriorates handling performance in subsequent steps and reduces production efficiency, thus being problematic. Patent Literature (PTL) 1 discloses a polyethylene film, produced by the inflation method, intended for use as a cover film for dry film resist, and characterized by a reduced occurrence of fish-eye defects. However, this film is prone to curl.

[0004] Polyolefin film, used as a protective film for an adhesive resin layer, may have a zipping (stick-slip) problem when the protective film is peeled off at an operation speed, resulting in the formation of a pattern of horizontal bars on the adhesive surface, and may also have a protective film-lifting problem due to poor adhesion.

[0005] WO2020 / 255194

[0006] An object of the present invention is to provide a film that exhibits excellent handling performance after thermal processing and in which zipping and / or poor adhesion are suppressed.

[0007] In view of the above problems, the present inventors conducted extensive research and found that the problems can be solved by a stretched film comprising a polyethylene resin that has an average linear thermal expansion coefficient of 8.00×10-4 / °C or less in at least one of the longitudinal and width directions when heated from 30°C to 85°C at a temperature rise rate of 10°C / min and that has a peel strength of 4.00 N / 25 mm or less as measured in a 180° peel test conducted on at least one surface at a peel rate of 50 mm / min. The present inventors have accomplished the present invention as a result of further research based on this finding. Specifically, the present invention includes the following embodiments.

[0008] Item 1. A stretched film comprising a polyethylene resin, the stretched film having an average linear thermal expansion coefficient of 8.00 × 10-4 / °C or less in at least one of the longitudinal and width directions when heated from 30°C to 85°C at a rate of 10°C / min, and the stretched film having a peel strength of 4.00 N / 25 mm or less as measured in a 180° peel test conducted on at least one surface at a peel rate of 50 mm / min.

[0009] Item 2. The stretched film according to Item 1, wherein the average linear thermal expansion coefficient is 1.20 × 10-4 / °C or more and 8.00 × 10-4 / °C or less, and the peel strength is 0.05 N / 25 mm or more and 4.00 N / 25 mm or less.

[0010] Item 3. The stretched film according to Item 1 or 2, having a curl curvature of less than 0.21 / mm when laminated at a temperature of 85°C.

[0011] Item 4. The stretched film according to any one of Items 1 to 3, having a tensile strength of 30 MPa or more in the longitudinal direction, and a tensile strength of 50 MPa or more in the width direction.

[0012] Item 5. The stretched film according to any one of Items 1 to 4, which is a biaxially stretched film.

[0013] Item 6. The stretched film according to any one of Items 1 to 5, having a thickness of 10 μm or more and 50 μm or less.

[0014] Item 7. A protective film, release film, or packaging film comprising the stretched film of any one of Items 1 to 6.

[0015] Item 8. A laminate comprising the stretched film of any one of Items 1 to 6 and one or more other layers.

[0016] The present invention can provide a film that exhibits excellent handling performance after thermal processing and in which zipping and / or poor adhesion are suppressed.

[0017] In the present specification, the terms “comprise,” “contain,” and “include” encompass the concepts of comprising, consisting essentially of, and consisting of.

[0018] In the present specification, “to” in a numerical range means “more than or equal to” and “less than or equal to.” That is, the expression “from α to β” means “more than or equal to α and less than or equal to β,” or “more than or equal to β and less than or equal to α,” and the range includes α and β.

[0019] When multiple lower limit values and multiple upper limit values are listed separately in the present specification, the range formed by any combination of a lower limit value and an upper limit value is also disclosed in the present specification.

[0020] In specifying the subject matter included in the present disclosure, various characteristics (properties, structures, functions, etc.) described in the embodiments of the present disclosure may be combined in any way. In other words, the present invention encompasses all subject matter including any combination of the combinable features described in the present specification.

[0021] 1. Stretched Film In one embodiment, the present invention provides a stretched film comprising a polyethylene resin, the stretched film having an average linear thermal expansion coefficient of 8.00 × 10-4 / °C or less in at least one of the longitudinal and width directions when heated from 30°C to 85°C at a rate of 10°C / min, and the stretched film having a peel strength of 4.00 N / 25 mm or less as measured in a 180° peel test conducted on at least one surface at a peel rate of 50 mm / min (which may also be referred to herein as “the stretched film of the present invention”). This film is described below.

[0022] The stretched film of the present invention exhibits excellent handling performance after thermal processing, and the occurrence of zipping and / or poor adhesion is suppressed in the stretched film.

[0023] The present inventors found that in terms of handling performance after thermal processing, zipping (in particular, zipping under high-speed operation), poor adhesion, etc., parameters of the average linear thermal expansion coefficient in at least one of the longitudinal and width directions when heated from 30°C to 85°C at a temperature rise rate of 10°C / min and peel strength as measured in a 180° peel test conducted at a peel rate of 50 mm / min (which may be simply referred to below as “peel strength”) are important. Adjusting these parameters to fall within the ranges described above can improve handling performance after thermal processing, and suppress zipping and / or poor adhesion.

[0024] In terms of handling performance after thermal processing (in particular, in terms of handling performance after thermal processing), the average linear thermal expansion coefficient is preferably 0.80×10-4 / °C or more and 8.00×10-4 / °C or less, more preferably 0.90×10-4 / °C or more and 8.00×10-4 / °C or less, even more preferably 1.00×10-4 / °C or more and 8.00×10-4 / °C or less, particularly preferably 1.10×10-4 / °C or more and 7.50×10-4 / °C or less, particularly more preferably 1.20×10-4 / °C or more and 7.00×10-4 / °C or less, particularly even more preferably 1.30×10-4 / °C or more and 6.50×10-4 / °C or less, and particularly still even more preferably 1.30×10-4 / °C or more and 6.00×10-4 / °C or less. In one embodiment of the present invention, the average linear thermal expansion coefficient is, for example, 1.50×10-4 / °C or more, 2.00×10-4 / °C or more, 2.50×10-4 / °C or more, 3.00×10-4 / °C or more, or 3.50×10-4 / °C or more, and is also, for example, 5.50×10-4 / °C or less, or 5.00×10-4 / °C or less.

[0025] The average linear thermal expansion coefficient is a value measured according to Method (4-1) described in the Examples below.

[0026] In terms of zipping (in particular, zipping under high-speed operation), poor adhesion, and the like (in particular, zipping and poor adhesion), the peel strength is preferably 0.05 N / 25 mm or more and 4.00 N / 25 mm or less, more preferably 0.05 N / 25 mm or more and 3.50 N / 25 mm or less, even more preferably 0.05 N / 25 mm or more and 3.00 N / 25 mm or less, still even more preferably 0.05 N / 25 mm or more and 2.50 N / 25 mm or less, and particularly preferably 0.06 N / 25 mm or more and 2.30 N / 25 mm or less. In one embodiment of the present invention, the peel strength is, for example, 0.10 N / 25 mm or more, 0.20 N / 25 mm or more, 0.30 N / 25 mm or more, 0.40 N / 25 mm or more, 0.50 N / 25 mm or more, or 0.60 N / 25 mm or more, and is also, for example, 2.00 N / 25 mm or less, 1.50 N / 25 mm or less, 1.20 N / 25 mm or less, or 1.00 N / 25 mm or less.

[0027] The peel strength is a value measured according to Method (4-2) described in the Examples below.

[0028] In terms of handling performance after thermal processing and the like (in particular, in terms of handling performance after thermal processing), the stretched film of the present invention preferably has a curl curvature of less than 0.21 / mm at a lamination temperature of 85°C. The curl curvature is more preferably 0.01 / mm or more and less than 0.21 / mm, more preferably 0.01 / mm or more and 0.20 / mm or less, even more preferably 0.02 / mm or more and 0.20 / mm or less, particularly preferably 0.03 / mm or more and 0.20 / mm or less, particularly more preferably 0.05 / mm or more and 0.20 / mm or less, particularly even more preferably 0.07 / mm or more and 0.20 / mm or less, particularly still even more preferably 0.07 / mm or more and 0.18 / mm or less, and especially preferably 0.07 / mm or more and 0.16 / mm or less. In one embodiment of the present invention, the curl curvature is, for example, 0.09 / mm or more, 0.11 / mm or more, or 0.12 / mm or more, and is also, for example, 0.15 / mm or less.

[0029] The curl curvature is a value measured according to Method (4-3) described in the Examples below.

[0030] In terms of film breakage, handling performance after thermal processing, zipping (in particular, zipping under high-speed operation), and poor adhesion, the stretched film of the present invention preferably has a tensile strength of 30 MPa or more in the longitudinal direction and a tensile strength of 50 MPa or more in the width direction.

[0031] In terms of suppressing film breakage, the tensile strength in the longitudinal direction is preferably 30 MPa or more and 200 MPa or less, more preferably 30 MPa or more and 150 MPa or less, even more preferably 30 MPa or more and 120 MPa or less, and particularly preferably 30 MPa or more and 100 MPa or less. In one embodiment of the present invention, the tensile strength in the longitudinal direction is, for example, 40 MPa or more, 50 MPa or more, or 60 MPa or more, and is also, for example, 90 MPa or less.

[0032] The tensile strength in the width direction is preferably 50 MPa or more and 350 MPa or less, more preferably 70 MPa or more and 300 MPa or less, and even more preferably 90 MPa or more and 280 MPa or less. In one embodiment of the present invention, the tensile strength in the width direction is, for example, 100 MPa or more, 120 MPa or more, 140 MPa or more, or 160 MPa or more, and is also, for example, 250 MPa or less, 230 MPa or less, or 220 MPa or less.

[0033] The tensile strength in the longitudinal direction / width direction is a value measured according to Method (4-4) described in the Examples below. Increasing the tensile strength can improve impact resistance.

[0034] The stretched film of the present invention contains a polyethylene resin. The stretched film of the present invention contains a polyethylene resin as a main component. In the present specification, the expression “contains a polyethylene resin as a main component” means that the polyethylene resin is contained in an amount of 30 mass% or more relative to the entire stretched film (when the entire stretched film is taken as 100 mass%). The polyethylene resin content relative to the entire stretched film of the present invention is preferably 50 mass% or more, more preferably 60 mass% or more, even more preferably 70 mass% or more, still even more preferably 80 mass% or more, particularly preferably 90 mass% or more, particularly more preferably 95 mass% or more, and especially preferably 99 mass% or more. The upper limit content of the polyethylene resin is, for example, 100 mass% or 99.9 mass% relative to the entire polyethylene film of the present invention.

[0035] The polyethylene resin is not particularly limited in terms of its origin and may be derived from petroleum-based raw materials or from plant-based sources (commonly known as biomass plastics).

[0036] The polyethylene resin is preferably low-density polyethylene or high-density polyethylene, and more preferably linear low-density polyethylene, known as “LLDPE.”

[0037] In terms of thickness uniformity, mechanical properties, thermo-mechanical properties, and the like, the weight average molecular weight (Mw) of the polyethylene resin is preferably 200000 or more and 400000 or less, and more preferably 210000 or more and 300000 or less.

[0038] In terms of suppressing the elastic modulus after stretching and obtaining a flexible film, the number average molecular weight (Mn) of the polyethylene resin is preferably 80000 or less, and more preferably 10000 or more and 70000 or less.

[0039] The molecular weight distribution (Mw / Mn) of the polyethylene resin, which is calculated as a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), is preferably 3 or more and 13 or less, and more preferably 3.3 or more and 12 or less, in terms of achieving appropriate resin fluidity during biaxial stretching and efficiently obtaining a film without breakage.

[0040] The melt flow rate (MFR) of the polyethylene resin at 190°C under a load of 2.16 kg is not particularly limited, and is preferably 5 g / 10 min or less in terms of reducing the mechanical load in the film-forming process. The MFR is more preferably 0.2 g / 10 min or more and 4 g / 10 min or less in terms of making the thickness of the stretched film of the present invention uniform. The MFR is preferably 0.8 g / 10 min or more, and more preferably 1.2 g / 10 min or more in terms of handling performance after thermal processing and the like.

[0041] The Z average molecular weight (Mz) of the polyethylene resin is, for example, 500000 or more and 1800000 or less.

[0042] The density of the polyethylene resin is, for example, 0.910 g / cm3or more and 0.970 g / cm3or less, preferably 0.915 g / cm3or more and 0.960 g / cm3or less, more preferably 0.918 g / cm3or more and 0.950 g / cm3or less, even more preferably 0.920 g / cm3or more and 0.940 g / cm3or less, and particularly preferably 0.922 g / cm3or more and 0.930 g / cm3or less.

[0043] The average molecular weight and the molecular weight distribution of the polyethylene resin are values measured according to Method (2-1) described in the Examples below. The MFR of the polyethylene resin is a value measured according to Method (2-2) described in the Examples below. The density of the polyethylene resin is a value measured according to Method (2-3) described in the Examples below.

[0044] The stretched film of the present invention may contain resins other than the polyethylene resin as long as the effects of the present invention are not impaired. Examples of such other resins include polyolefin resins other than polyethylene resins. Examples of polyolefin resins include a wide range of polyolefin resins that are used for producing films. Examples of polyolefin resins include polymers obtained by polymerizing one or more olefin compounds. Examples of such olefin compounds include preferably C2-C20, more preferably C2-C10, even more preferably C3-C6olefin compounds. Specific examples of polyolefin resins include polypropylene resins, poly(1-butene) resins, polyisobutene resins, poly(1-pentene) resins, and poly(4-methylpentene-1) resins. The polyolefin resin may be a copolymer containing two or more structural units derived from different olefin compounds, such as an ethylene-propylene copolymer.　The content of other resins relative to the entire stretched film of the present invention is, for example, less than 70 mass%, preferably 50 mass% or less, more preferably 40 mass% or less, even more preferably 30 mass% or less, still even more preferably 20 mass% or less, particularly preferably 10 mass% or less, particularly more preferably 5 mass% or less, and especially preferably 1 mass% or less. In one embodiment, the stretched film of the present invention does not contain other resins.

[0045] The stretched film of the present invention can contain components other than resins as long as the effects of the present invention are not impaired. For example, such other components include a wide range of additives contained in known resin films. Examples include antioxidants, heat stabilizers, light stabilizers, UV absorbers, plasticizers, lubricants, crosslinking agents, flame retardants, antistatic agents, heat-resistance improvers, anti-blocking agents, inorganic particles, resin particles, chlorine scavengers, anti-fogging agents, and hydrolysis inhibitors. These components can be used alone or in a combination of two or more, as needed. When the stretched film of the present invention contains such one or more other components, the content of other components relative to the total mass of the stretched film of the present invention is 10 mass% or less, preferably 5 mass% or less, more preferably 1 mass% or less, and particularly preferably 0.5 mass% or less.

[0046] The stretched film of the present invention is a film obtained by being stretched in at least one axial direction, either in the longitudinal direction (MD direction) or the width direction (TD direction). The stretched film may also be a biaxially stretched film obtained by being stretched in two axial directions. When the stretched film of the present invention is a biaxially stretched film, it is preferably a biaxially stretched film obtained by being stretched in both the longitudinal direction and width direction. It is particularly preferable that the polyethylene film of the present invention is a sequentially biaxially stretched film in terms of stably achieving thickness uniformity and easily adjusting the mechanical strength of the film.

[0047] The stretched film of the present invention can have a single-layer structure, or a multilayer structure (for example, consisting of 2 to 7 layers, 2 to 5 layers, 2 to 3 layers, or 3 layers).　The stretched film of the present invention preferably has a multilayer structure.　When the stretched film of the present invention has a multilayer structure, at least one layer (for example, 2 to 6 layers, 2 to 4 layers, or 2 layers, preferably all layers) contains the polyethylene resin described above (preferably as a main component).　When multiple layers contain polyurethane resin, the polyurethane resin present in each layer may be the same, or at least one or all of the polyurethane resins may be different.

[0048] The thickness of the stretched film of the present invention is not particularly limited. The stretched film can have a desired thickness depending on the intended application. In terms of avoiding film breakage and stably obtaining uniform thickness, the lower limit of thickness is preferably 2 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and still even more preferably 15 μm or more. On the other hand, in terms of reducing production costs and mechanical load in the film-forming process, the upper limit of thickness is preferably 100 μm or less, more preferably 70 μm or less, even more preferably 50 μm, and still even more preferably 30 μm or less. When the stretched film of the present invention has a multilayer structure, the thickness of the stretched film of the present invention means the total thickness of the layers.

[0049] 2. Production Method The method for producing the stretched film of the present invention is not particularly limited. For example, a wide range of methods of producing known films can be used. Specifically, for example, the stretched film of the present invention can be produced by a production method comprising the steps of obtaining a cast sheet comprising a polyethylene resin, and stretching the cast sheet in the MD direction and the TD direction. This method, which is one example, is described in detail below.

[0050] The cast sheet, which is an unstretched precursor,　can be obtained by using a known method. For example, polyethylene resin pellets, dry-mixed polyethylene resin pellets, or mixed polyethylene resin pellets prepared by melt kneading in advance are supplied to an extruder, melted by heating, filtered to remove foreign matter and degraded polymers, extruded into a sheet from a T-die, and cooled and solidified on at least one cooling drum, whereby a cast sheet can be obtained.

[0051] In the extruder, the polyethylene resin is denatured to some degree due to thermal degradation and oxidative degradation. To suppress such polymer denaturation, the resin temperature during melt extrusion is 170°C or higher and 320°C or lower, and preferably 200°C or higher and 300°C or lower. Deterioration can be suppressed, for example, by replacement of the atmosphere inside the extruder with nitrogen, the screw shape, the internal shape of the T-die during casting, or the amount of antioxidant added.

[0052] The cooling drum temperature is preferably maintained at 20°C or higher and 90°C or lower, more preferably 40°C or higher and 80°C or lower. Any method, such as an air knife method, touch roll method, electrostatic application method, or water-cooled casting method, can be used to bring the sheet resin into close contact with the cooling drum. However, the air knife method is preferred because adjustment when the sheet resin is brought into close contact with the cooling drum is easy and it is easy to handle. When the air knife is used, the blowout air temperature (AK air temperature) is preferably 10°C or higher and 90°C or lower, and more preferably 20°C or higher and 80°C or lower.

[0053] In the cast sheet obtained by adjusting the cooling drum temperature and the AK air temperature within the above ranges, crystallization is suppressed, and the mechanical load during stretching is small. Presumably, this facilitates obtaining the desired physical properties of the present invention.

[0054] In one embodiment, the cast sheet is biaxially stretched in the longitudinal direction and the width direction (MD direction and TD direction), whereby the stretched film of the present invention can be obtained.

[0055] First, the cast sheet is heated to 70°C or higher and 130°C or lower, preferably to 80°C or higher and 120°C or lower. Any heating method may be used. However, it is preferable to use a set of four or more rolls arranged in the flow direction to alternately heat both sides of the cast sheet and then simultaneously heat both sides of the sheet immediately before stretching in the longitudinal direction. With the temperature ranges described above, the cast sheet does not undergo excessive thermal expansion and can be stretched in the longitudinal direction as described below while maintaining planarity.

[0056] The cast sheet is stretched in the longitudinal direction (MD stretch) and is then immediately relaxed to obtain an MD stretched sheet. The stretch ratio (MD stretch ratio) in the longitudinal direction is 3 or more and 11 or less, and preferably 3.5 or more and 9 or less. The relaxation ratio (MD relaxation ratio) is preferably 0% or more and 12% or less, more preferably 5% or more and 11% or less, and still more preferably 6% or more and 10% or less.

[0057] The MD stretched sheet obtained by adjusting the MD stretch ratio and the MD relaxation ratio within the above ranges maintains planarity and suppresses oriented crystallization. This makes it possible to reduce the mechanical load during stretching in the width direction described below. Any method can be used to stretch and relax the sheet in the longitudinal direction. However, a preferred method is one that uses the difference in peripheral speed between two or more rolls arranged in the flow direction.

[0058] Next, the stretched sheet is introduced into a tenter and stretched in the width direction (TD stretch). The stretching temperature in the width direction (TD temperature) is preferably 150°C or higher and 190°C or lower, preferably 155°C or higher and 185°C or lower, more preferably 160°C or higher and 180°C or lower, and even more preferably 165°C or higher and 180°C or lower. The stretch ratio in the width direction (TD ratio) is 4 or more and 13 or less, preferably 5 or more and 12 or less, and more preferably 6 or more and 11 or less.

[0059] By adjusting the stretching conditions within the above ranges, stretching breakage caused by a residual portion that has not been stretched (unstretched portion) can be suppressed, and a stretched film with uniform thickness can be efficiently obtained.

[0060] Lastly, the biaxially stretched film is relaxed in the width direction, whereby the stretched film of the present invention is obtained.

[0061] The relaxation ratio in the width direction (TD relaxation ratio) is 0% or more and less than 23%, preferably 5% or more and 20% or less, and more preferably 8% or more and 15% or less.

[0062] Presumably, adjusting the relaxation ratio within the above ranges facilitates obtaining the desired properties of the present invention. In particular, the desired physical properties of the present invention can be readily achieved by adjusting the stretching temperature (in particular, the preheating temperature for TD stretching) within the above specified range, and further relaxing the film after MD (machine direction) stretching and TD (transverse direction) stretching to achieve a certain level of or higher relaxation ratio in at least one (preferably both) of the MD and TD directions. More specifically, stretching induces orientation and crystallization and can adjust the average linear thermal expansion coefficient. By setting a higher preheating temperature for TD stretching, rapid heating suppresses the formation of fine crystals, allowing crystals to grow larger, thereby enabling control over the linear thermal expansion coefficient. By setting a higher MD relaxation ratio, the MD relaxation loosens the unoriented polymer chains, thereby facilitating their orientation and crystallization during the subsequent TD stretching. By setting a higher TD relaxation ratio, the stress in the TD direction is reduced, making it easier for stress to be applied in the MD direction, thereby promoting stronger molecular orientation and crystallization along the MD axis.

[0063] The film delivered from the tenter is wound into a roll by a winding machine, whereby the stretched film of the present invention can be obtained. Further, the stretched film of the present invention can be surface-treated depending on the intended application as long as its properties are not impaired. Examples of such surface treatments include corona discharge treatment, plasma treatment, and flame treatment.

[0064] 3. Application The stretched film of the present invention can be used for various applications. Among the various applications, the stretched film of the present invention is particularly suitable as a protective film for electronic components. The stretched film of the present invention can also be used as a protective film different from the above, a release film, and a packaging film.

[0065] The stretched film of the present invention can be used as a protective film for dry film resist. The type of dry film resist is not particularly limited. The stretched film is widely applicable, for example, to known dry film resist. Such a protective film is provided to protect an adhesive layer of the dry film resist. In one embodiment, the dry film resist can be a film formed by stacking the stretched film of the present invention, a resist layer, and a base material film (e.g., a film comprising polyethylene terephthalate (PET)) in this order.

[0066] The stretched film of the present invention can have one or more other layers disposed on one or both sides as needed, and can be used for various applications. In one example, the stretched film of the present invention can have a release layer containing a release agent (e.g., silicone coating) on one or both sides as needed, and can be used, for example, for the above protective film or release film. In another example, the stretched film of the present invention can have a coating layer (e.g., a gas barrier layer) on one or both sides as needed, and can be used, for example, for a packaging film.

[0067] The present invention is described in detail below with reference to Examples. However, the present invention is not limited to these Examples.

[0068] (1) Preparation of Resins The following resins were used in the Examples and the Comparative Examples. PE represents polyethylene resin. PP represents polypropylene resin. - PE1: TF80 (LLDPE) available from Dow Chemical Company - PE2: BX202 (LLDPE) available from SABIC - PE3: LO4904P (HDPE) available from LG Chem - PE4: LC520 (LDPE) available from Japan Polyethylene Corporation - PE5: LF128 (LDPE) available from Japan Polyethylene Corporation - PP: F-300SP available from Prime Polymer Co., Ltd. Physical properties of these resins are shown below. The measurement methods are as follows. In the table, “-” indicates that the measurement was not conducted.

[0069]

[0070] (2) Measurement of Physical Properties of Resins(2-1) Measurement of Various Average Molecular Weights and Various Molecular Weight Distributions of Resins Using size exclusion chromatography (SEC), various average molecular weights and various molecular weight distributions were measured under the following conditions. Device: HLC-8321 GPC / HT (detector: differential refractometer (RI)) (available from Tosoh Corporation) Columns: TSKgel guardcolumn HHR(30) HT (7.5 mm I.D. × 7.5 cm) × 1 + TSKgel GMHHR-H (20) HT (7.8 mm I.D. × 30 cm) × 3 (available from Tosoh Corporation) Eluent: 1,2,4-trichlorobenzene (for GPC available from FUJIFILM Wako Pure Chemical Corporation) + dibutylhydroxytoluene (0.05%) Flow rate: 1.0 mL / min Detection conditions: polarity = (-) Injection volume: 300 μL Column temperature: 140°C System temperature: 40°C Sample concentration: 1 mg / mL Pre-treatment: Each sample was weighed, and a solvent (1,2,4-trichlorobenzene with 0.1% dibutylhydroxytoluene added) was added. The mixture was dissolved under stirring at 140°C for 1 hour. Subsequently, the mixture was hot-filtered through a 0.5 μm sintered filter. No insoluble substances were observed in the sample solution upon visual inspection. Calibration curve: A fifth-order approximation calibration curve was created using standard polystyrene available from Tosoh Corporation. Therefore, the obtained values are polystyrene-equivalent molecular weights.

[0071] From the obtained calibration curve and SEC chromatogram, the number average molecular weight (Mn), the weight average molecular weight (Mw), and the Z average molecular weight (Mz) were obtained using analytical software for measurement devices. The molecular weight distribution (Mw / Mn) was obtained using the values of Mw and Mn.

[0072] (2-2) Measurement of Melt Flow Rate (MFR) The melt flow rate (MFR) of each resin in the form of raw material resin pellets was measured using a melt indexer available from Toyo Seiki Co., Ltd., according to JIS K 7210:2014. Specifically, first, 4 g of the sample was weighed and inserted into a cylinder heated to a test temperature (190°C for PE and 230°C for PP) and was preheated for 3.5 minutes under a load of 2.16 kg. Subsequently, the weight of the sample extruded from a bottom hole over a period of 30 seconds was measured to calculate the MFR (g / 10 min). This measurement was repeated three times, and the average value was taken as the measured MFR value.

[0073] (2-3) Measurement of Density For each resin, the density of the raw resin pellets was measured using a Type A density gradient column apparatus available from Shibayama Scientific Co., Ltd., in accordance with JIS K 7112:1999. Specifically, an ethanol / water mixture was used as the gradient liquid, and the measurement was conducted at 23°C approximately 24 hours after introducing the gradient liquid. The above measurement was repeated three times, and the average value was taken as the measured density.

[0074] (3) Production of Stretched Film Example 1 As shown in Table 2 below, a stretched film was formed in which surface layer A was disposed on one surface of the base material layer and surface layer B was disposed on the other surface of the base material layer so that both surfaces of the base material layer had a surface layer. First, 100 parts by mass of PE1 (referred to below as Pellet 1) was prepared as a resin for forming the base material layer, and 100 parts by mass of PE1 (referred to below as Pellet 2) were prepared as a resin for forming surface layer A and surface layer B. Pellet 1 was fed through a hopper into a single-screw extruder, Extruder a, and Pellet 2 was fed through a hopper into another single-screw extruder, Extruder b, separate from Extruder a. Pellet 1 and Pellet 2 were individually melted at 260°C and laminated into a three-layer structure within a three-layer multi-manifold die. The laminated structure was then cooled and solidified on a cooling drum while being pressed with air pressure using an air knife. The temperature of the blowing air was set to 25°C. As a result, a raw sheet was obtained in which a layer of PE2 derived from Pellet 2 was directly formed on both surfaces of the PE1 layer derived from Pellet 1.

[0075] The obtained cast sheet was pre-heated at a temperature of 100°C, stretched 6-fold in the longitudinal direction (MD direction), and subsequently relaxed by 0% in the same direction. The temperature was then immediately returned to room temperature. Subsequently, the stretched film was introduced into a tenter. The stretched film was gripped at both ends with clips, preheated at 155°C, stretched 7.8-fold in the width direction (TD direction) in a 125°C stretching zone, and then relaxed by 10% in the same direction, whereby a 20 μm thick biaxially stretched polyethylene film. The resulting stretched film was a film formed by stacking surface layer A, a base material layer, and surface layer B in this order at a thickness ratio (surface layer A : base material layer : surface layer B) of 1:2:1. In the stretched film, surface layer B is a layer formed by direct contact with the cooling drum, whereas surface layer A is a layer that is not brought into contact with the cooling drum.

[0076] The film thickness was measured using a micrometer (JIS B 7502 (2016)) according to JIS C 2330 (2014).

[0077] A stretched film was obtained in the same manner as in Example 1, except that the film was relaxed by 8% in the longitudinal direction and relaxed by 0% in the width direction.

[0078] Example 3 A stretched film was obtained in the same manner as in Example 1, except that the film was relaxed by 8% relaxation in the longitudinal direction.

[0079] Example 4 A stretched film was obtained in the same manner as in Example 3, except that the preheating temperature before stretching in the width direction was set to 165°C.

[0080] Example 5 A stretched film was obtained in the same manner as in Example 3, except that the preheating temperature before stretching in the width direction was set to 175°C.

[0081] Example 6 A stretched film was obtained in the same manner as in Example 5, except that the resin for forming the base material layer was PE2 and the resin for forming surface layer A and surface layer B was PE2.

[0082] Example 7 A stretched film was obtained in the same manner as in Example 5, except that the resin for forming the base material layer was PE3 and the resin for forming surface layer A and surface layer B was PE3.

[0083] Example 8 A stretched film was obtained in the same manner as in Example 1, except that the resin for forming the base material layer was changed to PP1 and the preheating temperature before stretching in the TD direction was 195°C.

[0084] Comparative Example 1 PE4 pellets were fed into a single-screw extruder through a hopper and melted at a resin temperature of 200°C. After foreign matter and degraded polymers were removed by using a filter installed midway in the polymer tube, the melted pellets were extruded using a ring die, passed through pinch rolls, and wound up. Subsequently, air was blown into the ring to inflate the resin into a balloon-like shape between the ring die and the pinch rolls. After edge portions of the balloon folded by being passed through the pinch rolls were cut off, the resulting balloon films were individually wound as two sheets of film to obtain a polyethylene film with a thickness of 20 μm.

[0085] Comparative Example 2 A polyethylene film was obtained in the same manner as in Comparative Example 1, except that the resin fed into the extruder was PE5.

[0086] Comparative Example 3 A stretched film was obtained in the same manner as in Example 1, except that the film was relaxed by 0% in the width direction.

[0087] Comparative Example 4 A stretched film was obtained in the same manner as in Example 3, except that the preheating temperature before stretching in the width direction was set to 145°C.

[0088] Comparative Example 5 A polyethylene film was obtained in the same manner as in Example 5, except that the resin fed into the extruder was changed to PP, the preheating temperature before stretching in the MD direction was set to 130°C, and the stretching zone temperature in stretching in the TD direction was set to 160°C.

[0089] (4) Measurement of Physical Properties of Films(4-1) Measurement of Average Linear Thermal Expansion Coefficient The average linear thermal expansion coefficient (1 / °C) of the films of the Examples and Comparative Examples in the longitudinal direction was determined using a thermomechanical analyzer (TMA / SS6000, produced by Seiko Instruments Inc.) in accordance with JIS K 7197:1991 in the following manner. The film was cut into samples of 30 mm in longitudinal length and 4 mm in width, and the sample was set in a thermomechanical analyzer with a chuck-to-chuck distance of 15 mm. While applying a tensile load of 0.2 N / mm in the tensile direction, the temperature was raised from 25°C to 105°C at a temperature rise rate of 10°C / min (as ambient temperature) and maintained for 7 minutes. The average linear thermal expansion coefficient (1 / °C) in the temperature range of the sample from 30°C to 85°C (as sample temperature) was determined.

[0090] (4-2) Measurement of Peel Strength A 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, total thickness: 53 μm, acrylic adhesive) was applied to one of the surfaces of each film obtained in the Examples and the Comparative Examples, which is the side to be bonded to an adherend, by rolling a 2 kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. After the obtained laminate was allowed to stand for 20 hours under an environment of 70°C and 50% humidity, the laminate was allowed to cool at room temperature. The resulting laminate was used as a measurement sample. Each measurement sample was subjected to 180° peel testing at a peel rate of 50 mm / min using a tensile testing machine (TechnoGraph TGI-1kN universal tensile testing machine available from Minebea Co., Ltd.), and the peel strength was measured. The measurements were performed with n = 10 for each peel rate, and the average value was taken as the measurement result.

[0091] (4-3) Measurement of Curl Curvature A 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, total thickness: 53 μm, acrylic adhesive) was applied to one of the surfaces of each film obtained in the Examples and the Comparative Examples, which is the side to be bonded to an adherend, by rolling a 2 kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. The obtained laminate was placed on a 0.1 mm thick SUS plate with the 31B tape side facing up. A 38 μm thick PET film was overlaid to cover the laminate, and passed through a laminator (LM-A3 available from Aurora Japan Corporation). The heating temperature confirmed using a thermo label was 85°C. After being passed through the laminator, the laminate was removed from the paper and the SUS plate, and cut into a size of 25 mm (width) × 70 mm (length). Subsequently, the laminate was allowed to stand on graph paper with its long side surface facing down to determine the curl curvature of the laminate. The curl curvature was calculated as the reciprocal of the curl radius (2 divided by the curl diameter). Measurements were performed with n = 3, and the average value was taken as the measurement result.

[0092] (4-4) Measurement of Tensile Strength The tensile strength of the films of the Examples and Comparative Examples in the longitudinal direction and the width direction was measured using a “Tensilon RTG-1210” (trade name) universal testing machine available from A&D Company, Limited. Specifically, each film was cut into 15 × 150 mm samples (with the long side being in the width direction) and 150 × 15 mm samples (with the long side being in the longitudinal direction). Tensile strength was measured with a chuck-to-chuck distance of 100 mm at a tensile speed of 200 mm / min to determine the strength at break (unit: MPa).

[0093] (4-5) Measurement of Impact Resistance The films obtained in the Examples and Comparative Examples were tested for impact strength using a film impact tester available from Yasuda Seiki in accordance with ASTM-D3420. For this measurement, a 15 kgf・cm weight was used, and a metal ball with a radius of 6.35 mm was used as the impact ball. Impact strength was measured three times per sample, and the average value was taken as the impact strength."

[0094] (5) Evaluation of Film Performance(5-1) Evaluation of Handling Performance A 150 mm (width) × 250 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, total thickness: 53 μm, acrylic adhesive) was applied to one of the surfaces of each film obtained in the Examples and the Comparative Examples, which is the side to be bonded to an adherend, by rolling a 2 kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. The obtained laminate was placed on a 0.1 mm thick SUS plate with the 31B tape side facing up. A 38 μm thick PET film was overlaid to cover the laminate and the covered laminate was passed through a laminator (LM-A3 available from Aurora Japan Corporation). The heating temperature confirmed using a thermo label was 85°C. After the obtained sample was allowed to stand at room temperature for 24 hours, the stretched film was peeled off again. The resulting 31B tape was applied to a 50 μm thick PET film using a desktop laminator, and inspected for the presence of any defects. The evaluation was conducted with n=10. The number of measurement samples showing appearance defects, such as folds, creases, and uneven surfaces, was counted. The handling performance was evaluated according to the following criteria.

[0095] ◎: Appearance defects occurred in 0 measurement samples.

[0096] ο: Appearance defects occurred in 1 to 5 measurement samples.

[0097] ×: Appearance defect occurred in 6 to 10 samples.

[0098] (5-2) Zipping Assessment A 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, total thickness: 53 μm, acrylic adhesive) was applied to one of the surfaces of each film obtained in the Examples and the Comparative Examples, which is the side to be bonded to an adherend, by rolling a 2 kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. The obtained laminate was allowed to stand for 20 hours under an environment of 70°C and 50% humidity. The laminate was allowed to cool at room temperature. The resulting laminate was used as a measurement sample. Each measurement sample was subjected to 180° peel testing at a peel rate of 1250 mm / min using an adhesive and film peeling analysis device (“VPA” available from Kyowa Interface Science Co., Ltd.), whereby the occurrence of zipping was inspected. The measurements were performed with n = 10. The number of measurement samples in which zipping occurred was counted. Zipping was evaluated according to the following evaluation criteria.

[0099] ◎: Zipping occurred in 0 measurement samples.

[0100] ο: Zipping occurred in 1 to 5 measurement sample.

[0101] x: Zipping occurred in 6 to 10 measurement samples.

[0102] (5-3) Evaluation of Poor Adhesion A 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, acrylic adhesive) was applied to one of the surfaces of each film obtained in the Examples and the Comparative Examples, which is the side to be bonded to an adherend, by rolling a 2 kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. The obtained laminate was allowed to stand for 20 hours under an environment of 70°C and 50% humidity. The laminate was then allowed to stand at room temperature for 24 hours and the occurrence of lifting from the adhesive tape was inspected. The measurements were performed with n = 10. The number of measurement samples in which lifting occurred (spontaneous delamination) was counted. Poor adhesion was evaluated according to the following evaluation criteria.

[0103] ◎: No lifting occurred in any of the measurement samples.

[0104] ο: Lifting occurred in 1 to 5 measurement sample.

[0105] x: Lifting occurred in 6 to 10 measurement samples.

[0106] Table 2 shows the resin compositions, production conditions, physical property measurement results, and performance evaluation results.

[0107]

Claims

1. A stretched film comprising a polyethylene resin, the stretched film having an average linear thermal expansion coefficient of 8.00 × 10-4 / °C or less in at least one of the longitudinal and width directions, when heated from 30°C to 85°C at a rate of 10°C / min, and the stretched film having a peel strength of 4.00 N / 25 mm or less as measured in a 180° peel test conducted on at least one surface at a peel rate of 50 mm / min.

2. The stretched film according to claim 1, wherein the average linear thermal expansion coefficient is 1.20 × 10-4 / °C or more and 8.00 × 10-4 / °C or less, and the peel strength is 0.05 N / 25 mm or more and 4.00 N / 25 mm or less.

3. The stretched film according to claim 1, having a curl curvature of less than 0.21 / mm when laminated at a temperature of 85°C.

4. The stretched film according to claim 1, having a tensile strength of 30 MPa or more in the longitudinal direction, and a tensile strength of 50 MPa or more in the width direction.

5. The stretched film according to claim 1, which is a biaxially stretched film.

6. The stretched film according to claim 1, having a thickness of 10 μm or more and 50 μm or less.

7. A protective film, release film, or packaging film comprising the stretched film of any one of claims 1 to 6.

8. A laminate comprising the stretched film of any one of claims 1 to 6 and one or more other layers.

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