Biaxially oriented polyethylene film

Abstract

The present invention aims to provide a polyethylene film in which heat-induced curl is more suppressed. The present invention provides a biaxially oriented polyethylene film comprising polyethylene resin, wherein (a) a heat shrinkage ratio (MDHS / TDHS) of heat shrinkage in a longitudinal direction (MDHS) measured at 100°C according to JIS Z 1712 (2009) to heat shrinkage in a width direction (TDHS) measured at 100°C according to JIS Z 1712 (2009) is 0.35 or more and 2.50 or less, and / or (b) a planar orientation coefficient ΔP is 0.006 or more and 0.035 or less as calculated from refractive indices in a width direction (Ny), a longitudinal direction (Nx), and a thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz.

Description

BIAXIALLY ORIENTED POLYETHYLENE FILM

[0001] The present invention relates to a biaxially oriented polyethylene film and the like.

[0002] Polyethylene films are widely used as packaging and industrial material films because they are excellent in light weight, thermal stability, and mechanical characteristics. Particularly in recent years, due to their excellent release properties, polyethylene films have been widely used as protective materials, release materials, and the like for thermosetting resin members, such as fiber-reinforced plastics in manufacturing processes of electronic components and electronic substrates.

[0003] When used as a protective film, it is sometimes thermally laminated with other layers to obtain a laminate. During thermal lamination, curl caused by thermal shrinkage occurs, which reduces the handling performance in subsequent steps and lowers production efficiency. Conventional technology discloses a technique for controlling the mechanical strength and heat shrinkage of biaxially oriented polypropylene films within specific ranges to suppress film curl while maintaining flexibility and tensile strength (PTL 1).

[0004] JP2015-178651A

[0005] However, in conventional technology, curl occurs in the longitudinal direction, depending on temperature conditions, making it impossible to completely peel off a protective film from a substrate in a subsequent step after thermal lamination, and the protective film is sometimes carried over to the subsequent steps.

[0006] The present invention aims to provide a polyethylene film in which heat-induced curl is more effectively suppressed.

[0007] As a result of extensive research in light of the above problem, the present inventors found that the problem can be solved by a biaxially oriented polyethylene film containing polyethylene resin in which (a) the heat shrinkage ratio (MDHS / TDHS) of heat shrinkage in a longitudinal direction (MDHS) measured at 100°C according to JIS Z 1712 (2009) to heat shrinkage in a width direction (TDHS) measured at 100°C according to JIS Z 1712 (2009) is 0.35 or more and 2.50 or less, and / or (b) the planar orientation coefficient ΔP is 0.006 or more and 0.035 or less as calculated from refractive indices in a width direction (Ny), a longitudinal direction (Nx), and a thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz. As a result of further research based on the above finding, the present inventors completed the present invention. Specifically, the present invention includes the following embodiments.

[0008] Item 1. A biaxially oriented polyethylene film comprising polyethylene resin, wherein (a) a heat shrinkage ratio (MDHS / TDHS) of heat shrinkage in a longitudinal direction (MDHS) measured at 100°C according to JIS Z 1712 (2009) to heat shrinkage in a width direction (TDHS) measured at 100°C according to JIS Z 1712 (2009) is 0.35 or more and 2.50 or less, and / or (b) a planar orientation coefficient ΔP is 0.006 or more and 0.035 or less as calculated from refractive indices in a width direction (Ny), a longitudinal direction (Nx), and a thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz.

[0009] Item 2. The biaxially oriented polyethylene film according to Item 1, wherein a peel force measured in a 180° peel test on at least one surface is 1.00 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 300 mm / min, 1.60 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 1000 mm / min, and 2.30 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 2500 mm / min.

[0010] Item 3. The biaxially oriented polyethylene film according to Item 1 or 2, wherein an elastic modulus in a thickness direction at 23°C measured by the nanoindentation method on at least one surface is 2.00 GPa or less.

[0011] Item 4. The biaxially oriented polyethylene film according to any one of Items 1 to 3, wherein a melt flow rate (MFR) at 190°C measured according to JIS K 7210 (1999) is 0.8 g / 10 min or more.

[0012] Item 5. The biaxially oriented polyethylene film according to any one of Items 1 to 4, which has a thickness of 10 μm or more and 50 μm or less.

[0013] Item 6. A protective film, a release film, or a packaging film comprising the biaxially oriented polyethylene film according to any one of Items 1 to 5.

[0014] Item 7. A laminate comprising the biaxially oriented polyethylene film according to any one of Items 1 to 5, and another layer.

[0015] The present invention can provide a polyethylene film in which heat-induced curl is more suppressed.

[0016] In the present specification, the terms “comprise,” “contain,” and “include” encompass the concepts of comprising, consisting essentially of, and consisting of.

[0017] In the present specification, “to” in a numerical range means “greater than or equal to” and “less than or equal to.” That is, the expression “from α to β” means “greater than or equal to α and less than or equal to β,” or “greater than or equal to β and less than or equal to α,” and the range includes α and β.

[0018] In the present specification, when upper-limit and lower-limit values are described separately, ranges obtained by arbitrarily combining the described upper-limit and lower-limit values are also disclosed herein.

[0019] Further, the various characteristics (properties, structures, functions, and the like) described for each embodiment of the present invention mentioned above may be combined in any way when identifying the subject matter included in the present invention. In other words, the present invention encompasses all subject matter including all combinations of each of the combinable features described in the present specification.

[0020] 1. Biaxially Oriented Polyethylene Film According to an embodiment, the present invention relates to a biaxially oriented polyethylene film comprising polyethylene resin, wherein (a) a heat shrinkage ratio (MDHS / TDHS) of heat shrinkage in a longitudinal direction (MDHS) measured at 100°C according to JIS Z 1712 (2009) to heat shrinkage in a width direction (TDHS) measured at 100°C according to JIS Z 1712 (2009) is 0.35 or more and 2.50 or less, and / or (b) a planar orientation coefficient ΔP is 0.006 or more and 0.035 or less as calculated from refractive indices in a width direction (Ny), a longitudinal direction (Nx), and a thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz (in the present specification, the biaxially oriented polyethylene film is sometimes referred to as “the polyethylene film of the present invention”). A description is given below.

[0021] The polyethylene film of the present invention is one in which heat-induced curl is more suppressed.

[0022] As a result of further research on curl, the present inventors found that with respect to curl, what is important is not the numerical value of the heat shrinkage in the longitudinal direction or width direction (MDHS or TDHS), but the heat shrinkage ratio (MDHS / TDHS). Adjusting the heat shrinkage ratio within the above range makes it possible to prevent non-uniform contraction within the film plane and allows for suitable thermal lamination while suppressing the occurrence of curl. In addition, when peeling off the film, it is possible to cleanly peel the film without damaging the appearance of the substrate. In cases outside the above range, wrinkles or lifting occurs in the longitudinal direction or width direction, and so curl easily occurs.

[0023] Further, the present inventors proceeded with examination from a different perspective and found that the planar orientation coefficient ΔP is important. Adjusting this value within the above range also allows for suitable thermal lamination while suppressing the occurrence of curl.

[0024] When a laminate with a protective film is transported at the operating speed of the processing step and the protective film is peeled off at the operating speed, zipping (stick-slip) may occur, resulting in horizontal stripe patterns on the adhesive surface and / or lifting of the protective film due to poor adhesion. Since the operating speed can vary depending on the nature of the processing and other factors, it is important to suppress the above problems even under such variable conditions. According to the present invention, adjusting the heat shrinkage ratio (MDHS / TDHS) and / or the planar orientation coefficient ΔP within the above ranges also makes it possible to suppress zipping and adhesion failure under various operating speeds.

[0025] In terms of suppression of heat-induced curl, suppression of zipping, suppression of adhesion failure, and the like, the heat shrinkage ratio (MDHS / TDHS) is preferably 0.35 or more and 2.20 or less, more preferably 0.35 or more and 2.00 or less, still more preferably 0.37 or more and 2.00 or less, yet still more preferably 0.37 or more and 1.80 or less, particularly preferably 0.37 or more and 1.50 or less, more particularly preferably 0.37 or more and 1.20 or less, still more particularly preferably 0.40 or more and 1.20 or less, and most particularly preferably 0.42 or more and 1.10 or less.

[0026] The heat shrinkage ratio (MDHS / TDHS) is a value measured according to Method (4-1) in the Examples described below.

[0027] In terms of suppression of heat-induced curl, suppression of zipping, suppression of adhesion failure, and the like, the planar orientation coefficient ΔP is preferably 0.007 or more and 0.035 or less, more preferably 0.007 or more and 0.033 or less, still more preferably 0.007 or more and 0.031 or less, yet still more preferably 0.007 or more and 0.028, particularly preferably 0.007 or more and 0.025 or less, more particularly preferably 0.007 or more and 0.022 or less, and most particularly preferably 0.008 or more and 0.022 or less.

[0028] The planar orientation coefficient ΔP is a value measured according to Method (4-2) in the Examples described below.

[0029] In the polyethylene film of the present invention, in terms of suppression of zipping, suppression of adhesion failure, and the like, the peel force measured in 180° peel test on at least one surface is 1.00 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 300 mm / min, 1.60 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 1000 mm / min, and 2.30 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 2500 mm / min.

[0030] In terms of suppression of zipping, suppression of adhesion failure, and the like, the peel force at a peel rate of 300 mm / min is preferably 1.05 N / 25 mm or more and 3.95 N / 25 mm or less, more preferably 1.10 N / 25 mm or more and 3.90 N / 25 mm or less, still more preferably 1.20 N / 25 mm or more and 3.50 N / 25 mm or less, and yet still more preferably 1.20 N / 25 mm or more and 3.00 N / 25 mm or less.

[0031] In terms of suppression of zipping, suppression of adhesion failure, and the like, the peel force at a peel rate of 1000 mm / min is preferably 1.65 N / 25 mm or more and 3.95 N / 25 mm or less, more preferably 1.70 N / 25 mm or more and 3.90 N / 25 mm or less, still more preferably 1.80 N / 25 mm or more and 3.70 N / 25 mm or less, yet still more preferably 1.90 N / 25 mm or more and 3.50 N / 25 mm or less, and particularly preferably 1.95 N / 25 mm or more and 3.35 N / 25 mm or less.

[0032] In terms of suppression of zipping, suppression of adhesion failure, and the like, the peel force at a peel rate of 2500 mm / min is preferably 2.35 N / 25 mm or more and 3.90 N / 25 mm or less, more preferably 2.40 N / 25 mm or more and 3.80 N / 25 mm or less, still more preferably 2.40 N / 25 mm or more and 3.40 N / 25 mm or less, and yet still more preferably 2.40 N / 25 mm or more and 3.15 N / 25 mm or less.

[0033] The peel force is a value measured according to Method (4-3) in the Examples described below.

[0034] In the polyethylene film of the present invention, the elastic modulus in the thickness direction at 23°C measured by the nanoindentation method on at least one surface is preferably 2.00 GPa or less in terms of suppression of zipping, suppression of adhesion failure, and the like. The elastic modulus is more preferably 0.40 GPa or more and 1.95 GPa or less, still more preferably 0.50 GPa or more and 1.90 GPa or less, yet still more preferably 0.55 GPa or more and 1.80 GPa or less, and particularly preferably 0.60 GPa or more and 1.60 GPa or less.

[0035] The elastic modulus is a value measured according to Method (4-4) in the Examples described below.

[0036] In the polyethylene film of the present invention, the melt flow rate (MFR) at 190°C measured according to JIS K 7210 (1999) is preferably 0.8 g / 10 min or more in terms of suppression of heat-induced curl and the like. The MFR is more preferably 1.2 g / 10 min or more, and still more preferably 1.5 g / 10 min or more.

[0037] The MFR is a value measured according to Method (2-2) in the Examples described below.

[0038] The polyethylene film of the present invention contains polyethylene resin. The polyethylene film of the present invention contains polyethylene resin as a main component. In the present specification, the expression “contains polyethylene resin as a main component” means that the polyethylene resin is contained at an amount of 50 mass% or more relative to the entire polyethylene film (when the entire polyethylene film is taken as 100 mass%). The content of the polyethylene resin relative to the entire polyethylene film of the present invention is preferably 60 mass% or more, more preferably 70 mass% or more, still more preferably 80 mass% or more, yet still more preferably 90 mass% or more, particularly preferably 95 mass% or more, and most particularly 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.

[0039] Polyethylene resin from any source may be used. For example, it can be a resin derived from petroleum-derived raw materials, or it can be a resin derived from plant-based raw materials (known as “biomass plastic”).

[0040] A preferred polyethylene resin is low-density polyethylene or high-density polyethylene. A more preferred polyethylene resin is linear low-density polyethylene, known as “LLDPE.”

[0041] 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, in terms of thickness uniformity, mechanical properties, thermo-mechanical properties, and the like.

[0042] 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, in terms of obtaining a flexible film by suppressing the elastic modulus after stretching.

[0043] The molecular weight distribution (Mw / Mn) of the polyethylene resin calculated as the 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.

[0044] The melt flow rate (MFR) of the polyethylene resin at 190°C under a load of 2.16 kg is not particularly limited, preferably 5 g / 10 min or less in terms of reduction in mechanical load in the film-forming process. It is more preferably 0.2 g / 10 min or more and 4 g / 10 min or less in terms of uniform thickness of the polyethylene film of the present invention. In terms of suppression of heat-induced curl and the like, the MFR is preferably 0.8 g / 10 min or more, more preferably 1.2 g / 10 min or more, and still more preferably 1.5 g / 10 min or more.

[0045] The Z average molecular weight (Mz) of the polyethylene resin is 500000 or more and 1800000 or less, for example.

[0046] The average molecular weight and the molecular weight distribution of the polyethylene resin are values measured according to Method (2-1) in the Examples described below. The MFR of the polyethylene resin is a value measured according to Method (2-2) in the Examples described below.

[0047] The polyethylene film of the present invention can contain other components in addition to the polyethylene resin as long as the effect of the present invention is not impaired. For example, the 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 as needed. When the polyethylene film of the present invention contains the other one or more components, the content relative to the total mass of the polyethylene film is 10 mass% or less, preferably 5 mass% or less, more preferably 1 mass% or less, particularly preferably 0.5 mass% or less.

[0048] The polyethylene film of the present invention is a biaxially oriented film obtained by being biaxially stretched in the longitudinal direction (MD direction) and the width direction (MD direction). It is particularly preferable that the polyethylene film of the present invention is a sequentially biaxially oriented film in terms of stably achieving thickness uniformity and easily adjusting the mechanical strength of the film.

[0049] The polyethylene film of the present invention can have a single layer structure or a multilayer structure. Preferably, the polyethylene film of the present invention has a single layer structure. When the polyethylene film of the present invention has a multilayer structure, each layer contains the polyethylene resin described above. In this case, the polyethylene resin contained in each layer may be the same, or at least one polyethylene resin may be different or all the polyethylene resins may be different.

[0050] The thickness of the polyethylene film of the present invention is not particularly limited. The polyethylene film can have a desired thickness depending on the intended application. In terms of avoiding film breakage and stably obtaining uniform thickness, the lower thickness limit is preferably 2 μm or more, more preferably 5 μm or more, still more preferably 10 μm or more, and yet still more preferably 15 μm or more. Meanwhile, in terms of suppression of production costs and reduction in mechanical load in the film-forming process, the upper thickness limit is preferably 100 μm or less, more preferably 70 μm or less, still more preferably 50 μm, and yet still more preferably 30 μm or less. When the polyethylene film of the present invention has a multilayer structure, the thickness of the polyethylene film of the present invention means the total thickness of the layers.

[0051] 2. Production Method Any production method may be used to produce the polyethylene film of the present invention. For example, a wide range of methods of producing known films can be used. Specifically, for example, the polyethylene film of the present invention can be produced by a production method including a step of obtaining a cast sheet including polyethylene resin, and stretching the cast sheet in the MD direction and the TD direction. Such a method, which is one example, is described in detail below.

[0052] A cast sheet as an unstretched precursor can be obtained 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 under heat, filtered to remove foreign matter and degraded polymer, extruded into sheets from a T-die, and cooled and solidified on at least one cooling drum, whereby a cast sheet can be obtained.

[0053] 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 by replacing the inside of the extruder with nitrogen, adjusting the screw shape, adjusting the internal shape of the T-die during casting, adjusting the amount of antioxidant added, and the like.

[0054] 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, may be used to bring the sheet resin into close contact with the casting drum. However, the air knife method is preferred because it facilitates easy adjustment when bringing the sheet resin into close contact with the cooling drum 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.

[0055] 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.

[0056] The cast sheet is biaxially stretched in the longitudinal direction and the width direction (MD direction and TD direction), whereby the polyethylene film of the present invention can be obtained.

[0057] 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.

[0058] 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 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 12% or less, more preferably 11% or less, and still more preferably 10% or less.

[0059] The MD stretched sheet obtained by adjusting 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 may be used to stretch and relax 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.

[0060] 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 130°C or higher and 190°C or lower, preferably 140°C or higher and 185°C or lower, and more preferably 150°C or higher and 175°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.

[0061] By adjusting within the above ranges, stretching breakage caused by a residual portion that has not been stretched (unstretched portion) can be suppressed, and a polyethylene film with uniform thickness can be efficiently obtained.

[0062] Lastly, the biaxially oriented film is relaxed in the width direction, and subsequently, clips are released at a film temperature described below, whereby the polyethylene film of the present invention is obtained.

[0063] The relaxation ratio in the width direction (TD relaxation ratio) is 5% or more and less than 23%, preferably 8% or more and less than 22%, and more preferably 9% or more and less than 21%. The film temperature when the clips are released afterward is 50°C or higher and lower than 98°C, preferably 55°C or higher and lower than 97°C, and more preferably 60°C or higher and lower than 96°C.

[0064] By adjusting within the above ranges, the residual stretching stress in the biaxially stretched film is uniformly relaxed, resulting in a film having appropriate flexibility while maintaining planarity. Presumably, this facilitates obtaining the desired properties of the present invention.

[0065] The film delivered from the tenter is rolled into a roll by a winding machine, whereby the polyethylene film of the present invention can be obtained. Also, the polyethylene film of the present invention can be surface-treated depending on the application within a range that does not impair its properties. Examples of surface treatment include corona discharge treatment, plasma treatment, and flame treatment.

[0066] 3. Application The polyethylene film of the present invention can be used for various applications. Among the various applications, the polyethylene film of the present invention is particularly suitable as a protective film for electronic components. The polyethylene film of the present invention can also be used as a protective film different from the above, a release film, and a packaging film.

[0067] The polyethylene 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 polyethylene film is widely applicable to known dry film resist, for example. Such a protective film is provided to protect an adhesive layer of the dry film resist. According to an embodiment, the dry film resist can be a film obtained by sequentially laminating the polyethylene film of the present invention, a resist layer, and a base film (e.g., a film including polyethylene terephthalate (PET)).

[0068] The polyethylene 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 polyethylene film of the present invention can have one or more release layers containing a release agent (e.g., silicone coating) on one or both sides as needed and can be used for the protective film, the release film, and the like described above. In another example, the polyethylene film of the present invention can have coating layers (e.g., gas barrier layers) on one or both sides as needed and can be used for a packaging film and the like.

[0069] The present invention will be described in detail below with reference to the Examples. However, the present invention is not limited to these Examples.

[0070] (1) Preparation of Polyethylene Resin The following polyethylene resins were used in the Examples and the Comparative Examples. - PE1: TF80 (LLDPE) available from Dow Chemical Company - PE2: SP3010 (LLDPE) available from Prime Polymer Co., Ltd. - PE3: BX202 (LLDPE) available from SABIC - PE4: LO4904P (HDPE) available from LG Chem Physical properties of these polyethylene resins are shown below. The measurement methods are as follows.

[0071]

[0072] (2) Measurement of Physical Properties of Polyethylene Resins(2-1) Measurement of Various Average Molecular Weights and Various Molecular Weight Distributions of Polyethylene 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 / mLPre-treatment: The 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.

[0073] 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 the measurement device. Then, the molecular weight distribution (Mw / Mn) was obtained using the values of Mw and Mn.

[0074] (2-2) Measurement of Melt Flow Rate (MFR) For each resin in the form of raw material resin pellets, the melt flow rate (MFR) was measured using a melt indexer available from Toyo Seiki Co., Ltd., according to condition M of JIS K 7210 (1999). Specifically, first, 4 g of the sample was weighed and inserted into a cylinder heated to a test temperature (190°C) 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 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.

[0075] (3) Production of Biaxially Oriented Polyethylene Films Example 1 PE1 was supplied to an extruder and melted at a resin temperature of 260°C. Then, after removing foreign matter and degraded polymer with a filter installed midway along a polymer pipe, the molten resin was extruded using a T-die and wound around a casting drum maintained at a surface temperature of 50°C for solidification, whereby a cast sheet was produced.

[0076] As a method of bringing the cast sheet into close contact with the casting drum, an air knife was used at a blowout air temperature of 25°C.

[0077] The obtained cast sheet was pre-heated at a temperature of 100°C, stretched 6-fold in the longitudinal direction, and subsequently relaxed by 3.5% in the same direction. Then, the temperature was immediately returned to room temperature.

[0078] Subsequently, the stretched film was introduced into a tenter. The stretched film was gripped at both ends with clips at 110°C, preheated at 165°C, stretched 7.8-fold in the width direction, and then relaxed by 10% in the same direction. Subsequently, the biaxially stretched film temperature was cooled to 60°C, and the clips of the tenter were then released, whereby a 20 μm-thick biaxially oriented polyethylene film.

[0079] The film thickness was measured using a micrometer (JIS B 7502 (2016)) according to JIS C 2330 (2014).

[0080] Example 2 A biaxially oriented polyethylene film was obtained as in Example 1, except that PE2 was used as the resin to be supplied to the extruder.

[0081] Example 3 A biaxially oriented polyethylene film was obtained as in Example 1, except that PE3 was used as the resin to be supplied to the extruder.

[0082] Example 4 A biaxially oriented polyethylene film was obtained as in Example 1, except that the polyethylene film was stretched 8-fold in the longitudinal direction and 11-fold in the width direction.

[0083] Example 5 A biaxially oriented polyethylene film was obtained as in Example 1, except that the resin temperature for melting was set to 240°C and the surface temperature of the casting drum and the blowout air temperature of the air knife were set to 70°C.

[0084] Example 6 A biaxially oriented polyethylene film was obtained as in Example 1, except that the film temperature when the clips of the tenter were released was set to 95°C.

[0085] Example 7 A biaxially oriented polyethylene film was obtained as in Example 1, except that the polyethylene film was relaxed by 10% in the longitudinal direction and 20% in the width direction.

[0086] Example 8 A biaxially oriented polyethylene film was obtained as in Example 4, except that the polyethylene film was relaxed by 20% in the width direction and that the film temperature when the clips of the tenter were released was set to 95°C.

[0087] Example 9 A biaxially oriented polyethylene film was obtained as in Example 1, except that PE4 was used as the resin to be supplied to the extruder.

[0088] Comparative Example 1 A biaxially oriented polyethylene film was obtained as in Example 1, except that the polyethylene film was relaxed by 0% in the longitudinal direction and that the film temperature when the clips of the tenter were released was set to 45°C.

[0089] Comparative Example 2 A biaxially oriented polyethylene film was obtained as in Example 1, except that the polyethylene film was relaxed by 13% in the longitudinal direction and by 23% in the width direction and that the film temperature when the clips of the tenter were released was set to 98°C.

[0090] Comparative Example 3 A biaxially oriented polyethylene film was obtained as in Example 4, except that the resin temperature for melting was set to 240°C and the surface temperature of the casting drum and the blowout air temperature of the air knife were set to 70°C.

[0091] Comparative Example 4 A biaxially oriented polyethylene film was obtained as in Example 2, except that the polyethylene film was relaxed by 13% in the longitudinal direction and by 23% in the width direction and that the film temperature when the clips of the tenter were released was set to 98°C.

[0092] Comparative Example 5 A biaxially oriented polyethylene film was obtained as in Example 9, except that the resin temperature for melting was set to 245°C, the surface temperature of the casting drum was set to 65°C, and the blowout air temperature of the air knife were set to 70°C.

[0093] (4) Measurement of Physical Properties of Biaxially Oriented Polyethylene Films(4-1) Measurement of Heat Shrinkage and Heat Shrinkage Ratio For each of the biaxially oriented polyethylene films of the Examples and Comparative Examples, the heat shrinkage in the width direction (TDHS) and the heat shrinkage in the longitudinal direction (MDHS) were measured at 100°C according to JIS Z 1712 (2009), and the ratio of MDHS to TDHS (heat shrinkage ratio, MDHS / TDHS) was calculated. Specifically, except that the measurement temperature was set to 100°C ± 3°C, MDHS and TDHS were measured in accordance with JIS Z 1712 (2009) in a constant-temperature oven with air circulation maintained at the above temperature, and the heat shrinkage ratio was calculated by dividing the value of MDHS by the value of TDHS.

[0094] (4-2) Measurement of Refractive Index and Planar Orientation Coefficient For each of the biaxially oriented polyethylene films of Examples and Comparative Examples, the planar orientation coefficient ΔP was calculated from refractive indices in the width direction (Ny), the longitudinal direction (Nx), and the thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz. The refractive indices were measured in accordance with JIS K 7142 (2014).

[0095] (4-3) Measurement of Peel Force For each of the biaxially oriented polyethylene films of the Examples and the Comparative Examples, the peel force was measured as follows. (1) A 50 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, acrylic adhesive) was applied to the surface of one of the sides of the biaxially oriented polyethylene film, which is to be bonded to a substrate, by rolling a 2-kg roller over the adhesive tape twice back and forth, whereby a laminate was obtained. (2) The obtained laminate was allowed to stand for 20 hours under an environment of 70°C and 50% humidity. Then, the laminate was cut into 25 mm widths to obtain measurement samples. (3) The measurement samples were subjected to 180° peel testing at a constant peel rate (50 mm / min, 300 mm / min, 1000 mm / min, and 2500 mm / min) using a tensile testing machine (TechnoGraph TGI-1kN universal tensile testing machine available from Minebea Co., Ltd.), and the peel force was measured. (4) The measurements were performed with n = 10 for each peel rate, and the average value was taken as the measurement result.

[0096] (4-4) Measurement of Elastic Modulus For each of the biaxially oriented polyethylene films of the Examples and the Comparative Examples, the elastic modulus in the thickness direction at 23°C was measured by the nanoindentation method on the side used for the measurement of the peel force in (4-3).

[0097] The measurements were performed according to the method prescribed in ISO 14577 (2002) using a DUH-211S dynamic ultra-micro hardness tester available from Shimadzu Corporation. One drop of Aron Alpha (registered trademark) Pro Impact Resistant available from Toagosei Co., Ltd., was applied to the biaxially oriented polyethylene film, which was then fixed to a dedicated sample holder using an instant adhesive. Of the sides of the film, the one to be bonded to a substrate was used as the measurement side and subjected to measurement. A triangular-pyramidal diamond indenter with an inter-edge angle of 115° (Berkovich indenter) was used for the measurement. The measurement data were processed using dedicated analysis software, and the indentation elastic modulus EIT (GPa) was calculated with a Poisson’s ratio of 0.44. The measurements were performed with n = 10, and the average value was obtained. The measurement conditions were as follows.

[0098] - Measurement mode: load-unload test - Maximum load: 0.5 mN - Holding time at the maximum load: 5 sec. - Loading rate, unloading rate: 0.02 mN / sec.

[0099] (5) Performance Evaluation of Biaxially Oriented Polyethylene Films(5-1) Evaluation of Curl For each of the biaxially oriented polyethylene films of the Examples and the Comparative Examples, a 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, acrylic adhesive) was applied to the surface of the side opposite to the cast roll-contacted side (i.e., the side adjacent to the air knife) 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, which was then sandwiched between pieces of 0.1-mm-thick paper and passed through a laminator (LM-A3 available from Aurora Japan Corporation). The heating temperature confirmed using a thermo label was 75°C.

[0100] 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) × 25 mm (length). Subsequently, the laminate was placed on a flat table and left to stand with the 31B tape side facing down. The maximum height of lifting (curl) at the four corners of the laminated body was measured. The measurements were performed with n = 10, and the number of measurement samples in which the height of the lifting (curl) exceeded 10 mm was counted. The curl was evaluated based on the following evaluation criteria.

[0101] Evaluation Criteria A: Curl occurred in 0 measurement samples. B: Curl occurred in 1 measurement sample. C: Curl occurred in 2 to 4 measurement samples. D: Curl occurred in 5 or more measurement samples.

[0102] (5-2) Zipping Evaluation For each of the biaxially oriented polyethylene films of the Examples and the Comparative Examples, a 50 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, acrylic adhesive) was applied to the surface of one of the sides, which is to be bonded to a substrate, 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. Then, the laminate was cut into 25 mm widths to obtain measurement samples. The measurement samples were subjected to 180° peel testing at each of the following constant peel rates: 50 mm / min, 300 mm / min, 1000 mm / min, and 2500 mm / min, using a tensile testing machine (TechnoGraph TGI-1kN universal tensile testing machine available from Minebea Co., Ltd.), whereby the occurrence of zipping was checked. The measurements were performed with n = 10 for each peel rate. The number of measurement samples in which zipping occurred was counted. Among the results at each peel rate, the result with the highest number of occurrences was used as the measurement outcome, and zipping was evaluated based on the following evaluation criteria.

[0103] Evaluation Criteria A++: Zipping occurred in 0 out of 10 measurement samples. A+: Zipping occurred in 1 out of 10 measurement samples. A: Zipping occurred in 2 to 4 out of 10 measurement samples. B: Zipping occurred in 5 out of 10 measurement samples. C: Zipping occurred in 6 or more out of 10 measurement samples.

[0104] (5-3) Evaluation of Adhesion Failure For each of the biaxially oriented polyethylene films of the Examples and the Comparative Examples, a 25 mm (width) × 150 mm (length) adhesive tape (No. 31B tape available from Nitto Denko Corporation, acrylic adhesive) was applied to the surface of one of the sides, which is to be bonded to a substrate, 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. Then, the laminate was used as measurement samples. The measurement samples were subjected to 180° peel testing at each of the following constant peel rates: 50 mm / min, 300 mm / min, 1000 mm / min, and 2500 mm / min, using a tensile testing machine (TechnoGraph TGI-1kNuniversal tensile testing machine available from Minebea Co., Ltd.), whereby the occurrence of film lifting was checked. The measurements were performed with n = 10 for each peel rate. The number of measurement samples in which lifting occurred was counted. Among the results at each peel rate, the result with the highest number of occurrences was adopted as the measurement outcome, and adhesion failure was evaluated based on the following evaluation criteria.

[0105] Evaluation Criteria A++: Lifting occurred in 0 out of 10 measurement samples. A+: Lifting occurred in 1 out of 10 measurement samples. A: Lifting occurred in 2 to 4 out of 10 measurement samples. B: Lifting occurred in 5 out of 10 measurement samples. C: Lifting occurred in 6 or more out of 10 measurement samples.

[0106] (6) Results Table 2 shows the resin compositions and the production conditions. Table 3 shows the measurement results of physical properties and the performance evaluation results.

[0107]

[0108]

Claims

1. A biaxially oriented polyethylene film comprising polyethylene resin, wherein (a) a heat shrinkage ratio (MDHS / TDHS) of heat shrinkage in a longitudinal direction (MDHS) measured at 100°C according to JIS Z 1712 (2009) to heat shrinkage in a width direction (TDHS) measured at 100°C according to JIS Z 1712 (2009) is 0.35 or more and 2.50 or less, and / or (b) a planar orientation coefficient ΔP is 0.006 or more and 0.035 or less as calculated from refractive indices in a width direction (Ny), a longitudinal direction (Nx), and a thickness direction (Nz) measured according to JIS K 7142 (2014), using the following formula: planar orientation coefficient ΔP = (Nx + Ny) / 2 - Nz.

2. The biaxially oriented polyethylene film according to claim 1, wherein a peel force measured in a 180° peel test on at least one surface is 1.00 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 300 mm / min, 1.60 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 1000 mm / min, and 2.30 N / 25 mm or more and 4.00 N / 25 mm or less at a peel rate of 2500 mm / min.

3. The biaxially oriented polyethylene film according to claim 1, wherein an elastic modulus in a thickness direction at 23°C measured by the nanoindentation method on at least one surface is 2.00 GPa or less.

4. The biaxially oriented polyethylene film according to claim 1, wherein a melt flow rate (MFR) at 190°C measured according to JIS K 7210 (1999) is 0.8 g / 10 min or more.

5. The biaxially oriented polyethylene film according to claim 1, which has a thickness of 10 μm or more and 50 μm or less.

6. A protective film, a release film, or a packaging film comprising the biaxially oriented polyethylene film according to any one of claims 1 to 5.

7. A laminate comprising: the biaxially oriented polyethylene film according to any one of claims 1 to 5; and another layer.

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