Ultra-high molecular weight polyethylene submicron thin film and method for manufacturing the same

Abstract

Provided are: a sub-micron thin film made of an ultra-high molecular weight polyethylene, the sub-micron thin film containing, as a main component, an ultra-high molecular weight polyethylene that has a viscosity average molecular weight of 1,000,000 to 15,000,000, and having a film thickness of less than 1 μm and a tensile strength at break of 100 MPa or higher; and a method for manufacturing the same.

Description

[Technical Field]

[0001] This disclosure relates to a submicron thin film made of ultra-high molecular weight polyethylene and a method for producing the same, and more particularly to a submicron thin film made of ultra-high molecular weight polyethylene having a thickness of less than 1 μm, high visible light transmittance, and high tensile breaking strength and tear strength, and a method for producing the same. [Background technology]

[0002] Polymer thin films are generally recognized as films thinner than 1 μm formed on a substrate by methods such as solution casting or vapor deposition polymerization, and it has been revealed that they are in a state where molecular mobility is excited compared to the bulk state (Non-Patent Literature 1). On the other hand, attempts have also been made to create self-supporting thin films with a thickness of nm, and methods have been proposed in which polymer chains are arranged by self-assembly on a water surface, or in which a polymer solution is cast onto a substrate and then peeled off (Non-Patent Literature 2). However, these methods make it difficult to prepare large-area films.

[0003] Here, ultra-high molecular weight polyethylene (hereinafter also referred to as "UHMW-PE") refers to polyethylene with a molecular weight of 1 million or more, and due to its high molecular weight, it possesses excellent properties such as high strength, abrasion resistance, and chemical stability. However, because it contains many molecular chain entanglements, it has a high melt viscosity and is difficult to mold. In particular, its thin film production is done by the skiving method, which involves cutting from a pre-sintered block, and the film thickness is limited to about 100 μm. Therefore, the transparency of UHMW-PE films molded by this method is low.

[0004] Methods for molding UHMW-PE include gel stretching (Non-Patent Document 3) and thermally induced phase separation (Non-Patent Document 4), both of which are used in the production of high-strength fibers or battery separators. However, these molding methods use large amounts of organic solvents, raising concerns about increased recovery costs and environmental impact due to volatilization.

[0005] In contrast, the "melt stretching method" utilizes the molecular chain entanglement of UHMW-PE as a point of deformation stress transfer (Non-Patent Literature 5). In this melt stretching process, "extended chain crystals" are formed when the polyethylene molecular chains are fully stretched and crystallized. These extended chain crystals are components of high-strength polyethylene fibers and exhibit high strength. The higher the stretching ratio, the more of these extended chain crystals there are, resulting in a stronger film. On the other hand, during the cooling process, a structure is formed in which easily deformable folded chain crystals epitaxially grow relative to the extended chain crystals. A key feature of this molding method is that UHMW-PE thin films can be molded without using any organic solvents.

[0006] Using this method, a UHMW-PE thin film with a thickness of 5 μm is formed by stretching it 8 × 8 times in the x and y axes (Patent Document 1). Furthermore, by increasing the stretching ratio to 16 × 16 times, a UHMW-PE thin film with a thickness of 2 μm is also formed (Patent Document 2). For this purpose, a biaxial stretching apparatus capable of stretching to high magnifications (Patent Document 3) has been developed.

[0007] It has also been reported that a UHMW-PE thin film can be obtained by dropping a UHMW-PE solution onto a glass substrate, melting it above the melting point of UHMW-PE, and then picking it up with tweezers (Non-Patent Literature 6).

[0008] Patent Document 1: Japanese Unexamined Patent Publication No. 2010-201649 Patent Document 2: Japanese Unexamined Patent Publication No. 2019-193997 Patent Document 3: International Publication WO2018 / 039436

[0009] Non-patent document 1: K. Tanaka et al., ACS Macro Lett., Vol. 7, p. 1198 (2018) Non-patent document 2: S.Takeoka et al, Adv. Mater.,Vol.25, p.545 (2013) Non-patent document 3: P. Smith et al, J. Mater. Sci., Vol. 15, p. 505 (1980) Non-Patent Document 4: A. Pankaj et al, Chem. Rev., Vol. 104, p. 4419 (2004) Non-Patent Document 5: H. Uehara et al., Adv. Funct. Mater., Vol. 22, p. 2048 (2012) Non-Patent Document 6: Thomas Keller, Matthias Grosch, and Klaus D. Jand. Macromolecules 2007, 40, 5812 - 5819

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, when the draw ratio is further increased, the film breaks, making it difficult to further reduce the film thickness. Also, with the manufacturing method of Non-Patent Document 6, although partial thinning is possible, it is difficult to obtain a UHMW-PE film of a practical size including uniform thinning. Therefore, physical property values such as tensile breaking strength, tear strength, nitrogen permeability coefficient, and light transmittance have not been obtained.

[0011] The problem to be solved by one embodiment of the present disclosure is to provide a submicron thin film made of ultra-high molecular weight polyethylene with a film thickness of less than 1 μm, high visible light transmittance, high tensile breaking strength, and high tear strength, and a method for manufacturing the same.

Means for Solving the Problems

[0014] Furthermore, to confirm that the ultra-high molecular weight polyethylene submicron thin films disclosed herein contain UHMW-PE, it is effective to examine the molecular weight distribution by gel permeation chromatography (GPC) measurement using trichlorobenzene or tetrachlorobenzene as a solvent. GPC measurement can be performed using the method described in International Publication WO2014 / 0344484. Furthermore, to determine the viscosity-average molecular weight of the UHMW-PE constituting the ultra-high molecular weight polyethylene submicron thin film of this disclosure, it is effective to measure the intrinsic viscosity in decalin solvent (135°C). [Effects of the Invention]

[0015] According to one embodiment of the present disclosure, it is possible to provide a submicron thin film made of ultra-high molecular weight polyethylene with a film thickness of less than 1 μm, high visible light transmittance, and high tensile breaking strength and tear strength, as well as a method for manufacturing the same. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a schematic diagram illustrating the raw film preparation process in Example 1. [Figure 2] Figure 2 shows scanning electron microscope (SEM) images of cross-sections of the UHMW-PE submicron thin films obtained in Examples 1 and 2. [Figure 3] Figure 3 is a graph showing the melting profiles of the films or thin films obtained in Example 1, Comparative Example 1, and Control Example 1 when measured using differential scanning calorimetry (DSC). [Figure 4] Figure 4 is a graph showing the melting profiles of the films or thin films obtained in Examples 2-3 and Comparative Examples 2-3 when measured using differential scanning calorimetry (DSC). [Figure 5] Figure 5 shows scanning electron microscope (SEM) images of the film or thin film surface obtained in Examples 1-2, Comparative Examples 1-3, and Control Example 1. [Figure 6]Figure 6 is a schematic diagram illustrating the adhesion test method for the films or thin films obtained in Examples 1-2, Comparative Examples 1-3, and Control Example 1. [Modes for carrying out the invention]

[0017] In this specification, numerical ranges indicated using "~" represent a range that includes the numbers before and after "~" as the lower and upper limits, respectively. In this specification, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, provided that the intended purpose of the process is achieved. Furthermore, in this specification, the amount of each component in a composition means the total amount of multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition. Furthermore, unless otherwise specified, the term "substituent" is used to encompass both unsubstituted and further substituted elements. For example, when the term "alkyl group" is used, it encompasses both unsubstituted alkyl groups and alkyl groups with further substituents. The same applies to other substituents. In the numerical ranges described stepwise within this specification, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise. Furthermore, in the numerical ranges described within this specification, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples. Furthermore, in this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In this specification, room temperature means 20°C.

[0018] The following describes in detail the ultra-high molecular weight polyethylene submicron thin film and its manufacturing method (hereinafter, the ultra-high molecular weight polyethylene submicron thin film of this disclosure will also be referred to as "UHMW-PE thin film" or "thin film"). In this disclosure, "submicron thin film" refers to a thin film with a thickness of 1 μm or less, and is generally sometimes simply called "thin film" or "ultrathin film".

[0019] <Submicron thin film made of ultra-high molecular weight polyethylene> The thin film of this disclosure contains ultra-high molecular weight polyethylene (UHMW-PE) with a viscosity-average molecular weight of 1 million to 15 million as its main component, has a film thickness of less than 1 μm, and a tensile breaking strength of 100 MPa or more.

[0020] (Ultra-high molecular weight polyethylene) The components comprising the thin film of this disclosure, including UHMW-PE, will be described in detail in the method for manufacturing the thin film of this disclosure. Furthermore, "containing UHMW-PE as the main component" indicates that UHMW-PE is the most abundant component among the components contained in the thin film. Specifically, "containing UHMW-PE as the main component" means that the thin film contains 50% by mass, 60% or more by mass, 70% or more by mass, 80% or more by mass, or 90% or more by mass of UHMW-PE.

[0021] (film thickness) The film thickness of the thin film of this disclosure is 1 μm or less, but is preferably 900 nm or less, more preferably 850 nm or less, and even more preferably 500 nm or less. The film thickness of the thin film described herein is measured by the method shown in the examples described later.

[0022] (Diffuse light transmittance, parallel light transmittance, and haze value in the visible light region) The diffuse light transmittance of the thin film of this disclosure in the visible light region is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less. The parallel light transmittance of the thin film of this disclosure in the visible light region is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. The broadened haze value of the thin film of this disclosure in the visible light region is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less. The diffuse light transmittance, parallel light transmittance, and haze value of the thin film in the visible light region of this disclosure are values ​​(%) in the wavelength range of 360 to 750 nm and are measured by the method shown in the examples described later.

[0023] (Tensile breaking strength) The tensile breaking strength of the thin film of this disclosure is 100 MPa or more, preferably 150 MPa or more, and more preferably 300 MPa or more. The tensile fracture strength of the thin film described herein is measured by the method shown in the examples described later.

[0024] (Tear strength) The tear strength of the thin film of this disclosure is preferably 1 N / mm or more, more preferably 5 N / mm or more, and even more preferably 10 N / mm or more. The tear strength of the thin film described herein is measured by the method shown in the examples described later.

[0025] (Nitrogen permeability coefficient) The nitrogen permeability coefficient of the thin film disclosed herein is 1 × 10⁻⁶ -14 mol·m / (m 2 Preferably less than 5 × 10 -15 mol·m / (m 2 ·s·Pa) or less is more preferable, 1 × 10 -15 mol·m / (m 2 The following are even more preferable: s·Pa) The nitrogen permeability coefficient of the thin film of this disclosure is measured by the method shown in the examples described later.

[0026] (Melting profile) The thin film of this disclosure preferably has one or more endothermic peaks in the melting profile recorded by a differential scanning calorimeter at (1) 130°C or higher and less than 140°C, (2) 140°C or higher and less than 150°C, and (3) 150°C or higher. When the thin film of this disclosure is obtained by the two-stage melt biaxial stretching described later, the above melting profile is obtained. The melting profile recorded by the differential scanning calorimeter is the melting profile obtained by the "temperature-increasing measurement of DSC in this specification" described later.

[0027] (Adhesion) The adhesion coefficient obtained by the peritoneal adhesion test in this disclosure is preferably 1000 N / m or more, more preferably 2000 N / m or more, and even more preferably 3000 N / m or more. When the thin film of this disclosure is in close contact with a mating material, it exhibits excellent adhesion by conforming to the shape of the mating material, reflecting its thinness. The adhesion coefficient of the thin film described herein, obtained by adhesion testing, is measured by the following method. -Adhesion Test- Cut the target film and the corresponding material, a Niraco aluminum plate (product number AL-013421, thickness 300 μm), into strips 1 cm wide x 5 cm long. Apply 20 μmL of liquid paraffin (viscosity 0.87 g / mL: 20℃) evenly to the entire 1 cm x 3 cm area at the tip of the target film. After bringing the target film and the 1 cm x 3 cm area at the tip of the aluminum plate into close contact so that the top and bottom of the target film and the strips of the aluminum plate are staggered, pull the ends of the strips opposite the contact point upwards and downwards. The maximum recorded load is divided by the thickness of the target film to obtain a value (N / m) which is used as an indicator of the adhesion of the target film (i.e., the adhesion coefficient). Details of the adhesion coefficient obtained from adhesion tests of the thin films of this disclosure will be shown in the examples described later.

[0028] <Method for manufacturing submicron thin films made of ultra-high molecular weight polyethylene> The present disclosure is a method for manufacturing a thin film comprising a first molten biaxial stretching step, a cooling step, and a second molten biaxial stretching step, and is a method for obtaining a submicron thin film made of UHMW-PE with a thickness of less than 1 μm. The method for manufacturing a thin film according to this disclosure may use a commercially available raw material film, but may also include a raw material film preparation step. The method for manufacturing a thin film according to this disclosure may include a molten biaxial shrinkage step between the first molten biaxial stretching step and the cooling step. Note that "melt biaxial stretching" is a method of stretching a raw film or a stretched film along the x-axis direction and the y-axis direction at a temperature above its melting point. Hereinafter, each step will be described.

[0029] (Raw film preparation step) In the raw film preparation step, a ultra-high molecular weight polyethylene (UHMW-PE) raw material powder having a viscosity average molecular weight of 1 million to 15 million is formed into a film at a temperature above the melting point of the ultra-high molecular weight polyethylene raw material powder. Thereby, a raw film made of ultra-high molecular weight polyethylene is obtained.

[0030] The UHMW-PE raw material powder is a powdery polyethylene raw material powder having a viscosity average molecular weight (Mv) of 1 million to 15 million, and a polyethylene raw material powder having an Mv of 1 million to 10 million is more preferable, and a polyethylene raw material powder having an Mv of 1.2 million to 6 million is even more preferable. That is, the Mv of the UHMW-PE contained in the thin film of the present disclosure is preferably within the above range. Note that the viscosity average molecular weight is a value measured in a decalin solvent (135 ° C), and the intrinsic viscosity ([η]) is preferably 7 dl / g to 45 dl / g, more preferably 7 dl / g to 35 dl / g, and even more preferably 8 dl / g to 24 dl / g. In UHMW-PE, it is known that the above viscosity average molecular weight and intrinsic viscosity are in the relationship represented by the following formula, as described in, for example, JP-A-2005-314544 and JP-A-2005-313391. Mv = 5.37×10 4 [η] 1.49 Using the above formula, the viscosity average molecular weight can be obtained from the measured intrinsic viscosity. The viscosity average molecular weight in this specification adopts the value obtained as described above.

[0031] As previously described, the molecular weight of UHMW-PE can be measured. However, if dissolution in decalin solvent is difficult, the molecular weight of UHMW-PE can be measured by the following method. This method is an application of the ASTM D 1430-65T method. First, a film made of ultra-high molecular weight polyethylene is prepared, and the molecular weight is calculated by measuring the deformation yield stress of this film in a molten state. The UHMW-PE raw material powder whose molecular weight is to be measured is formed into a film by melt-press molding to create a dumbbell-shaped test specimen as specified in the ASTM D 1430-65T method. Multiple dumbbell-shaped test specimens are prepared, and different loads are applied to each, and they are immersed in a glycol bath heated to 150°C. As the test specimen stretches due to the applied load, the time required to elongate by 600% is measured. The time required for the elongation obtained above is plotted on a logarithmic coordinate system against the tensile stress applied to the test specimen (the value obtained by dividing the load by the cross-sectional area of ​​the test specimen). Linearity is observed in the plotted values, and from this graph, the stress called the yield value (N / mm²) required for 10 minutes of elongation is determined. 2 ) is required. The yield value for UHMW-PE is 0.05 N / mm². 2 ~1.5 N / mm 2 It is preferable that the range is within this range. For example, according to literature such as the Hostalene GUR catalog for ultra-high molecular weight polyethylene (PE-UHMW) (Hoechst Aktiengesellschaft, August 1993), the yield value and the viscosity-average molecular weight measured by the viscosity method are correlated, so the molecular weight can be detected by the yield value measurement method.

[0032] The particle size of the UHMW-PE raw material powder is the volume average particle size (D 50 ) preferably 2000 μm or less, more preferably 1 μm to 2000 μm, and even more preferably 10 μm to 1000 μm.

[0033] UHMW-PE can be a commercially available product, for example, Mitsui Chemicals' Hyzex Million 340M (product name: Mv=3.5×10 6), Ticona's hostalene GUR4113 (product name: Mv=3.2×10 6 ), Hostalen GUR4150 (product name: Mv=7.3×10 6 ) are some examples. UHMW-PE can be any polymer polymerized using a known catalyst, but powdered polyethylene polymerized using a Ziegler catalyst or a metallocene catalyst is preferably used. Generally, the molecular weight distribution of polyethylene synthesized with Ziegler catalysts is broader than that of polyethylene synthesized with metallocene catalysts, and the Mw / Mn ratio is larger in the former. Here, Mw and Mn are the weight-average molecular weight and number-average molecular weight, respectively, and can be determined by GPC measurement.

[0034] UHMW-PE is preferably composed solely of ethylene as its constituent unit due to its high crystallinity and excellent physical properties such as strength. However, it may also be a polymer or copolymer containing constituent units derived from ethylene. When UHMW-PE is a copolymer, constituent units that make up the copolymer together with the ethylene constituent unit include propylene, 1-butene, 1-hexene, 1-octene, and α-olefins such as 4-methyl-1-pentene and their derivatives. In other words, the name UHMW-PE in this specification also includes copolymers of ethylene and α-olefins. Therefore, UHMW-PE also includes polyethylenes containing long-chain branching, such as linear low-density polyethylene and low-density polyethylene.

[0035] In the raw material film preparation process, other components besides UHMW-PE may be used in combination with the UHMW-PE raw material powder, to the extent that the effect is not impaired. In other words, the thin film of this disclosure may contain other components besides UHMW-PE.

[0036] Other components besides UHMW-PE include, for example, polymers such as polyethylene with a lower molecular weight than UHMW-PE; and known additives, such as plasticizers, antioxidants, weathering agents, light stabilizers, UV absorbers, heat stabilizers, lubricants, mold release agents, antistatic agents, flame retardants, foaming agents, fillers such as silica, antibacterial agents, antifungal agents, nucleating agents, and colorants such as pigments, which are commonly added to polyolefins. One or more of the other components mentioned above may be included in the UHMW-PE raw material powder, depending on the purpose, as long as it does not impair the effectiveness. Among the other components, suitable antioxidants include phenolic antioxidants such as BASF's Irganox 1076 (trade name); phosphorus-based antioxidants such as Asahi Denka Kogyo's Adeka Stab HP-10 (trade name); and sulfur-based antioxidants. Methods for incorporating other components into UHMW-PE raw material powder include, in addition to directly mixing the other components with the raw material powder, dispersing or dissolving the other components in another solvent, then mixing or spraying this mixture with the raw material powder and volatilizing and removing only the solvent, and known addition methods such as kneading the compounding agent while the ultra-high molecular weight polyethylene raw material is molten.

[0037] In the raw material film preparation process, the UHMW-PE raw material powder described above is formed into a film at a temperature above the melting point of the UHMW-PE raw material powder to obtain the raw material film. By forming the UHMW-PE raw material powder into a film at a temperature above the melting point of the UHMW-PE raw material powder, a raw material film with improved fusion properties between the UHMW-PE raw material powder particles can be obtained.

[0038] Specifically, from the viewpoint of improving the fusion properties between UHMW-PE raw material powders, the film-forming temperature is preferably 130°C to 250°C, and more preferably 150°C to 200°C.

[0039] In this specification, "melting point" refers to the endothermic peak temperature (°C) of the melting profile obtained by temperature rise measurement using a differential scanning calorimeter (DSC). Although it varies depending on the manufacturing method and molecular weight of the UHMW-PE raw material, the melting point of UHMW-PE raw material powder is in the range of approximately 135°C to 145°C. If multiple melting peaks exist in the melting profile, the temperature of the peak with the greatest intensity (endothermic amount) is taken as the melting point. In this specification, the DSC temperature measurement is based on results obtained by heating the sample to a temperature range of 30°C to 180°C at a heating rate of 10°C / min under a nitrogen atmosphere (measuring device: PerkinElmer Diamond DSC). Approximately 4 mg of the sample (raw material powder, film) is sealed in an aluminum pan and subjected to DSC measurement. Temperature and heat values ​​are calibrated using standard substances (indium and tin).

[0040] For forming the film, press molding is preferred. Furthermore, press molding is more preferably carried out under reduced pressure. The press pressure in press forming is preferably 0.01 MPa to 100 MPa, and more preferably 0.1 MPa to 50 MPa. The reduced pressure in press forming is preferably 10 Torr or less, and more preferably 1 Torr or less. Press molding under reduced pressure improves the fusion properties between UHMW-PE raw material powders, resulting in a unified structure. This allows for the production of a UHMW-PE thin film that fully exhibits the properties of UHMW-PE.

[0041] The film-forming process may also be roll forming. Examples of roll forming include the "first roll processing step" and the "second roll processing step" in the method for manufacturing polyethylene film described in Japanese Patent Application Publication No. 2019-193997. Furthermore, the film-like molding may be a combination of press molding and roll molding.

[0042] From the viewpoint of obtaining a UHMW-PE thin film with a thickness of less than 1 μm, the film thickness of the raw material film obtained in the raw material film preparation process is preferably 1000 μm or less, more preferably 300 μm or less, and even more preferably 100 μm or less.

[0043] (First melt biaxial stretching process) In the first melt biaxial stretching step, a raw film (or a raw film formed in the raw film preparation step) containing ultra-high molecular weight polyethylene with a viscosity-average molecular weight of 1 million to 15 million as its main component is melt biaxially stretched along the x and y axes at a temperature above the melting point of the raw film.

[0044] The first molten biaxial stretching may be sequential biaxial stretching, where the material is stretched in the x-axis direction first, and then in the y-axis direction perpendicular to the x-axis direction, or it may be simultaneous biaxial stretching, where the material is stretched simultaneously in the x-axis direction and the y-axis direction perpendicular to the x-axis direction. Here, in the first molten biaxial stretching, if stretching is performed in the x-axis direction followed by stretching in the y-axis direction perpendicular to the x-axis direction, the time difference is, for example, 0.1 to 100 mins. Furthermore, the stretching speed in the x-axis direction and the stretching speed in the y-axis direction may be the same or different. If the stretching speed in the x-axis direction and the stretching speed in the y-axis direction are different, the speed difference is, for example, 0.1 to 1000 mm / min.

[0045] The first melt biaxial stretching temperature shall be above the melting point of the raw film. Stretching the raw film at a temperature above its melting point allows for stretching to high magnifications, which is advantageous for thinning and increasing strength. The first molten biaxial stretching temperature is preferably 120°C to 180°C, more preferably 130°C to 180°C, even more preferably 136°C to 180°C, and most preferably 136°C to 170°C. The temperature may be varied during the first molten biaxial stretching within this temperature range.

[0046] The temperature conditions for the first melt biaxial stretching can be appropriately selected depending on the viscosity-average molecular weight (Mv) and copolymerization composition of the UHMW-PE raw material powder, which is the raw material for the base film. For example, if the viscosity-average molecular weight of UHMW-PE is around 1 million, a temperature of around 136°C to 145°C near the melting point is preferable. However, as the molecular weight increases, the thermal properties of the molded film change, making it possible to perform biaxial stretching at higher temperatures.

[0047] In the first melt biaxial stretching, the stretching ratio is preferably at least twice the length of the original film in both the x-axis and y-axis directions, and more preferably at least five times. The stretching ratios in the x-axis and y-axis directions may be the same or different.

[0048] The stretching speed in the first molten biaxial stretching is preferably in the range of 1 mm / min to 1000 mm / min, and more preferably in the range of 10 mm / min to 500 mm / min. When first molten biaxial stretching is performed at a stretching speed of less than 1 mm / min, the molten film bends and sags downwards, resulting in a thin, high-strength film that cannot be obtained.

[0049] The process may include a holding step in which the material is held at the temperature for the first molten biaxial stretching for a certain period of time before the first molten biaxial stretching. In this case, the time for holding the temperature is preferably 1 minute to 180 minutes, more preferably 1 minute to 10 minutes.

[0050] In the first molten biaxial stretching process, in order to efficiently perform biaxial stretching in a molten state, it is preferable to use a hot air blowing type biaxial stretcher or the like to melt only the center of the raw film roll while keeping the chuck portion (end) from melting during stretching. In this case, it is preferable that the biaxial stretcher is equipped with a stress detection mechanism so that it can be confirmed that the biaxial stretching is being performed in a molten state. Furthermore, since the film thickness at the chuck portion (end) gradually thins and becomes slippery as stretching progresses, it is preferable that the biaxial stretcher is equipped with a chuck mechanism, such as an air chuck mechanism, that applies a constant gripping force at all times.

[0051] From the viewpoint of obtaining a UHMW-PE thin film with a thickness of less than 1 μm, the thickness of the stretched film obtained in the first melt biaxial stretching step is preferably 500 μm or less, more preferably 150 μm or less, and even more preferably 50 μm or less.

[0052] (Melting biaxial shrinkage process) In the molten biaxial shrinkage process, before the cooling process, the stretched film obtained in the first molten biaxial stretching process (hereinafter also referred to as the "first stretched film") is subjected to molten biaxial shrinkage along the x-axis and y-axis directions at a temperature above the melting point of the stretched film. The molten biaxial shrinkage process is an optional process.

[0053] After the first molten biaxial stretching, when the first stretched film is melt-shrunk, the folded chain crystals of UHMW-PE become uniform. Therefore, using this UHMW-PE structure as the starting point, if the first stretched film is subjected to the second molten biaxial stretching described later, it becomes possible to perform the second molten biaxial stretching to even higher magnifications due to the uniformity of the film structure. As a result, it becomes easier to obtain UHMW-PE thin films with a thickness of less than 1 μm.

[0054] The molten biaxial shrinkage may be sequential biaxial shrinkage, where the material shrinks in the x-axis direction first, and then in the y-axis direction perpendicular to the x-axis direction, or it may be simultaneous biaxial shrinkage, where the material shrinks simultaneously in the x-axis direction and the y-axis direction perpendicular to the x-axis direction.

[0055] Molten biaxial shrinkage may be performed immediately after the first molten biaxial stretching, or it may be performed after the first molten biaxial stretching, while the material is held at the temperature for molten biaxial shrinkage for a certain period of time (for example, about 1 to 10 minutes).

[0056] The temperature for melt biaxial shrinkage is preferably 80°C to 180°C, more preferably 120°C to 165°C, even more preferably 136°C to 165°C, and particularly preferably 140°C to 155°C. The temperature may be varied during melt biaxial shrinkage within this temperature range.

[0057] The shrinkage rate in molten biaxial shrinkage is preferably 5% to 95% of the length after shrinkage (length of the first stretched film immediately after the first molten biaxial stretching) in both the x-axis and y-axis directions, and more preferably 20% to 75%. The shrinkage rates in the x-axis and y-axis directions may be the same or different.

[0058] (cooling process) In the cooling process, the first stretched film (or the first stretched film shrunk in the first melt biaxial stretching process) is cooled to below the melting point of the first stretched film. Specifically, in the cooling process, the first stretched film is cooled to a temperature above room temperature but below the melting point of the first stretched film, for example, at a cooling rate of 1°C / min to 1000°C / min.

[0059] (Second melt biaxial stretching process) In the second melt biaxial stretching step, the first stretched film, which has been cooled in the cooling step, is again melt biaxially stretched along the x-axis and y-axis directions at a temperature above the melting point of the first stretched film.

[0060] The second molten biaxial stretching may be sequential biaxial stretching, where the material is stretched in the x-axis direction first, and then in the y-axis direction perpendicular to the x-axis direction, or it may be simultaneous biaxial stretching, where the material is stretched simultaneously in the x-axis direction and the y-axis direction perpendicular to the x-axis direction. Here, in the second molten biaxial stretching, if stretching is performed in the x-axis direction first, and then in the y-axis direction perpendicular to the x-axis direction, the time difference is, for example, 0.1 to 100 mins. Furthermore, the stretching speed in the x-axis direction and the stretching speed in the y-axis direction may be the same or different. If the stretching speed in the x-axis direction and the stretching speed in the y-axis direction are different, the speed difference is, for example, 0.1 to 1000 mm / min.

[0061] The second melt biaxial stretching temperature is set to be above the melting point of the first stretched film. Stretching the first stretched film at a temperature above its melting point allows for stretching to high magnifications, which is advantageous for thinning and increasing strength. Furthermore, the resulting UHMW-PE thin film is a non-porous film, resulting in a thin film with high visible light transmittance and barrier properties.

[0062] The second melt biaxial stretching temperature is preferably 120°C to 180°C, more preferably 130°C to 180°C, even more preferably 136°C to 180°C, and most preferably 136°C to 170°C. The temperature may be varied during the second melt biaxial stretching process within this temperature range.

[0063] The temperature conditions for the first melt biaxial stretching can be appropriately selected depending on the viscosity-average molecular weight (Mv) and copolymerization composition of the UHMW-PE raw material powder, which is the raw material for the base film. For example, if the viscosity-average molecular weight of UHMW-PE is around 1 million, a temperature of around 136°C to 145°C near the melting point is preferable. However, as the molecular weight increases, the thermal properties of the molded film change, making it possible to perform biaxial stretching at higher temperatures.

[0064] In the second melt biaxial stretching, the stretching ratio is preferably 5 times or more the length of the original film in both the x-axis and y-axis directions, more preferably 10 times or more, and even more preferably 20 times or more. The stretching ratios in the x-axis and y-axis directions may be the same or different.

[0065] Here, the ratio of the stretching ratio in the first molten biaxial stretching to the stretching ratio in the second molten biaxial stretching (stretching ratio in the second molten biaxial stretching / stretching ratio in the first molten biaxial stretching) is preferably 1.1 to 20 times, and more preferably 2 to 10 times, in both the x-axis and y-axis directions.

[0066] The stretching speed in the second molten biaxial stretching is preferably in the range of 1 mm / min to 1000 mm / min, and more preferably in the range of 10 mm / min to 500 mm / min. When second-stage molten biaxial stretching is performed at a stretching speed of less than 1 mm / min, the molten film bends and sags downwards, resulting in a thin, high-strength film that cannot be obtained.

[0067] The process may include a holding step in which the material is held at the temperature for the first molten biaxial stretching for a certain period of time before the second molten biaxial stretching. In this case, the time for holding the temperature is preferably 1 minute to 180 minutes, more preferably 1 minute to 10 minutes.

[0068] In the second molten biaxial stretching process, in order to efficiently perform biaxial stretching in a molten state, it is preferable to use a hot air blowing type biaxial stretcher or the like to melt only the center of the first stretched film while keeping the chuck portion (end) from melting. In this case, it is preferable that the biaxial stretcher is equipped with a stress detection mechanism so that it can be confirmed that the biaxial stretching is being performed in a molten state. Furthermore, since the film thickness at the chuck portion (end) gradually thins and becomes slippery as stretching progresses, it is preferable that the biaxial stretcher is equipped with a chuck mechanism, such as an air chuck mechanism, that applies a constant gripping force at all times.

[0069] The second fused biaxial stretching is carried out until the film thickness of the resulting second stretched film is less than 1 μm. After the second fused biaxial stretching, the resulting second stretched film is cooled to room temperature, for example.

[0070] Heat treatment may be performed after the second molten biaxial stretching or after cooling. The heat treatment temperature is preferably 100°C to 180°C, and more preferably 120°C to 160°C. The temperature may be varied during the heat treatment within this temperature range. In this case, the time for heat treatment is preferably 1 minute to 180 minutes, more preferably 1 minute to 10 minutes.

[0071] In the thin film manufacturing method of this disclosure, a UHMW-PE thin film with a thickness of less than 1 μm can be obtained through the steps described above. The method for manufacturing a thin film according to the present disclosure includes a third step, which includes at least one of the following: a second melt biaxial stretching step, a molten biaxial stretching step, a molten biaxial stretching step, a molten biaxial shrinking step, a molten shrinking step, a molten biaxial shrinking step, a molten shrinking step, a molten shrinking step, a molten shrinking step, a molten shrinking step, a molten shrinking step, a molten shrinking step, a molten shrinking step, and a molten shrinking step, which includes at least one of the following: a second melt biaxial stretching step, a molten biaxial shrinking step, a molten third step, which may be repeated once or multiple times.

[0072] In the thin film manufacturing method of this disclosure, post-processing may be performed on the obtained UHMW-PE thin film. Post-processing methods include crosslinking the UHMW-PE thin film by irradiating it with an electron beam or radiation. Post-processing can further improve the chemical resistance, dimensional stability, and heat resistance of UHMW-PE thin films. [Examples]

[0073] The manufacturing method of the present disclosure will be described in detail below with reference to examples. The following examples are merely representative examples, and various modifications can be implemented as long as they do not exceed the spirit of the present disclosure.

[0074] [Example 1] First, the UHMW-PE base film was formed as follows (base film preparation process). As shown in Figure 1, a 125 μm thick polyimide release film (2) was placed on a 150 mm diameter × 2 mm thick disc-shaped stainless steel plate (1). Next, a 100 mm × 100 mm rectangular window (indicated as area A in Figure 1) was cut out of a 150 mm diameter × 0.30 mm thick disc-shaped stainless steel plate (3), and approximately 3.0 g of powdered UHMW-PE raw material (Mitsui Chemicals Hizex Million 340M, viscosity average molecular weight 3.3 million, average particle size 150 μm) was placed inside rectangular window A. A 125 μm thick polyimide release film (4) was placed on top of that, and then a 150 mm diameter × 2 mm thick (5) was placed on top of that.

[0075] The entire assembly is placed between the upper and lower press plates in a press machine (manufactured by Baldwin Corporation) installed in a vacuum chamber at room temperature (25°C), and 1 × 10 -1 After depressurizing to Torr using a rotary pump, the upper and lower press plates were brought as close together as possible to avoid stress, heated to 200°C, and maintained at 200°C for 5 minutes. Then, while pressed at a pressure of 4.5 MPa (cylinder pressure 60 MPa), the heater power was turned off and the material was slowly cooled to room temperature (25°C) under reduced pressure. After that, the vacuum chamber was opened and the UHMW-PE raw film was removed. The thickness of the obtained UHMW-PE raw film was approximately 0.30 mm.

[0076] The obtained UHMW-PE raw film was cut to an initial length of 65 mm x 65 mm and placed in a large biaxial stretcher described in specification WO2018 / 088280, which is equipped with an air chuck function and a stress sensor, and heated by blowing hot air onto it. After holding at 155°C for 5 minutes, simultaneous biaxial stretching was performed in the x-axis and y-axis directions at a stretching temperature of 155°C and a stretching speed of 180 mm / min (first melt biaxial stretching step). The stretching ratio was 7x7x. This was cooled to room temperature and removed from the large twin-screw stretcher (cooling process). The obtained first stretched film was cut to an initial length of 35 mm x 35 mm and set in a planar expansion stretcher (manufactured by Island Industries) equipped with an air chuck function and a stress sensor, and heated by blowing hot air onto it. After holding at 155°C for 5 minutes, simultaneous biaxial stretching was performed along the x-axis and y-axis directions at a stretching temperature of 155°C and a stretching speed of 20 mm / min (second melt biaxial stretching process). The stretching ratio was 4x4x. Therefore, the total stretching ratio based on the original film (press-molded UHMW-PE original film) was 28x28x. SEM observation of the cross-section of the obtained UHMW-PE thin film revealed a film thickness of 853 nm (Figure 2(A)). The film thickness of the UHMW-PE thin film is the average of the thicknesses measured at five points on the cross-section using SEM observation. Furthermore, the obtained UHMW-PE thin film had a tensile breaking strength of 201 MPa, a tear strength of 14.5 N / mm, and a nitrogen permeability coefficient of 1.48 × 10⁻⁶. -16 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 88.6%, the diffuse light transmittance was 32.2%, the parallel light transmittance was 56.3%, and the haze value was 36.4%. Furthermore, when the resulting UHMW-PE thin film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 3660 N / m was obtained.

[0077] [Example 2] A UHMW-PE raw material film was formed in the same manner as in Example 1 (raw material film preparation process). In the first melt biaxial stretching process, the stretching ratio was set to 10x10x, and while maintaining the temperature at 155°C, simultaneous biaxial shrinkage was performed in the x-axis and y-axis directions at a shrinkage rate of 180 mm / min until the stretching ratio reached 7x7x (melt shrinkage process). In the same manner as in Example 1, the film was cooled to room temperature and removed from the large biaxial stretcher (cooling process). In the second melt biaxial stretching process, the stretching ratio was set to 5x5x. Therefore, the total stretching ratio was 35x35x. SEM observation of the cross-section of the obtained UHMW-PE thin film revealed that its thickness was 354 nm (Figure 2(B)). Furthermore, the obtained UHMW-PE thin film had a tensile breaking strength of 304 MPa, a tear strength of 36.7 N / mm, and a nitrogen permeability coefficient of 2.63 × 10⁻⁶. -15 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 88.6%, the diffuse light transmittance was 25.4%, the parallel light transmittance was 63.1%, and the haze value was 28.7%. Furthermore, when the obtained UHMW-PE thin film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 4810 N / m was obtained.

[0078] [Example 3] The UHMW-PE thin film was prepared in the same manner as in Example 2, except that the stretching ratio in the second melt biaxial stretching step was set to 4x4. Therefore, the total stretching ratio was 28x28. SEM observation of the cross-section of the obtained UHMW-PE thin film revealed that its thickness was 841 nm. Furthermore, the obtained UHMW-PE thin film had a tensile breaking strength of 156 MPa, a tear strength of 15.2 N / mm, and a nitrogen permeability coefficient of 6.10 × 10⁻⁶. -18 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 88.6%, the diffuse light transmittance was 31.0%, the parallel light transmittance was 57.6%, and the haze value was 35.0%. Furthermore, when the obtained UHMW-PE thin film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 3340 N / m was obtained.

[0079] [Comparative Example 1] A UHMW-PE film was formed in the same manner as in Example 1 (raw film preparation process). The stretching ratio in the first melt biaxial stretching process was 7x7x, and the film was cooled to room temperature in the same manner as in Example 1 and removed from the large biaxial stretcher (cooling process). The thickness of the obtained stretched UHMW-PE film was measured with a micrometer and found to be 23 μm. Furthermore, the resulting stretched UHMW-PE film had a tensile breaking strength of 65.1 MPa, a tear strength of 12.3 N / mm, and a nitrogen permeability coefficient of 1.56 × 10⁻⁶. -16 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 93.6%, the diffuse light transmittance was 78.5%, the parallel light transmittance was 15.1%, and the haze value was 83.9%. Furthermore, when the resulting stretched UHMW-PE film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 165 N / m was obtained.

[0080] [Comparative Example 2] A UHMW-PE raw material film was formed in the same manner as in Example 1 (raw material film preparation process). The stretching ratio in the first melt biaxial stretching process was set to 10x10x, and in the same manner as in Example 2, simultaneous biaxial shrinking was performed to a stretching ratio of 7x7x while maintaining the temperature at 155°C (shrinking process), and the film was cooled to room temperature and removed from the biaxial stretcher (cooling process). The thickness of the obtained stretched UHMW-PE film was measured with a micrometer and found to be 26 μm. Furthermore, the resulting stretched UHMW-PE film had a tensile breaking strength of 36.8 MPa, a tear strength of 9.9 N / mm, and a nitrogen permeability coefficient of 1.39 × 10⁻⁶. -16 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 96.5%, the diffuse light transmittance was 87.8%, the parallel light transmittance was 8.7%, and the haze value was 90.4%. Furthermore, when the resulting stretched UHMW-PE film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 667 N / m was obtained.

[0081] [Comparative Example 3] A stretched film made of UHMW-PE was prepared in the same manner as in Comparative Example 1, except that the stretching ratio in the first melt biaxial stretching process was set to 10x10x. The thickness of the obtained stretched UHMW-PE film was measured with a micrometer and found to be 15 μm. Furthermore, the tensile breaking strength of the obtained UHMW-PE stretched film was 93 MPa, and the tear strength was 14.1 N / cm.

[0082] [Comparative Example 4] To verify the effectiveness of inserting a cooling step between the first and second fused biaxial stretching steps, we attempted to perform the first and second fused biaxial stretching steps consecutively without removing the UHMW-PE stretched film from the stretcher (without cooling it). Here, in order to reduce the film thickness of a UHMW-PE raw material film with a thickness of approximately 0.30 mm to less than 1 μm by melt biaxial stretching, an at least stretching ratio of 24x24 is required, by analogy from Example 1. However, the maximum stretching ratio of the large biaxial stretching machine described in WO2018 / 088280 is 20x20. Therefore, in order to achieve a film thickness of less than 1 μm by performing the first melt biaxial stretching process and the second melt biaxial stretching process consecutively, it is necessary to thin the UHMW-PE raw material film. Thus, by making the thickness of the disc-shaped stainless steel plate with the rectangular window cut out in Figure 1 150 μm, a UHMW-PE raw material film with a thickness of approximately 0.15 mm was obtained (raw material film preparation process). This was set in the large biaxial stretcher described above, similar to Example 1, and held at 155°C for 5 minutes. Then, simultaneous biaxial stretching was performed along the x-axis and y-axis directions at a stretching temperature of 155°C and a stretching speed of 180 mm / min (first melt biaxial stretching step). The stretching ratio was 6x6x. Subsequently, without removing the material from the large biaxial stretcher, it was held at 155°C for 5 minutes, and then simultaneously biaxial stretched along the x and y axes at a stretching temperature of 155°C and a stretching speed of 20 mm / min (second melt biaxial stretching process). However, the film fractured before reaching a total stretching ratio of 11x11, and a thin film made of UHMW-PE could not be obtained.

[0083] [Comparative Example 5] Similar to Comparative Example 4, in order to verify the effectiveness of inserting a cooling step between the molten biaxial shrinkage step and the second molten biaxial stretching step after the first molten biaxial stretching step, we attempted to perform the first molten biaxial stretching step, the molten biaxial shrinkage step, and the second molten biaxial stretching step in succession without removing the UHMW-PE stretched film from the stretcher (without cooling). Similar to Comparative Example 4, a UHMW-PE base film with a thickness of approximately 0.15 mm was formed (base film preparation step). In the first melt biaxial stretching step, the stretching ratio was set to 6x6x, and while maintaining the temperature at 155°C, simultaneous biaxial shrinkage was performed in the x-axis and y-axis directions at a shrinkage rate of 180 mm / min until the stretching ratio reached 4x4x (melt shrinkage step). Similar to Example Y, the film was not removed from the large biaxial stretching machine, and after being held at 155°C for 5 minutes, simultaneous biaxial stretching was performed in the x-axis and y-axis directions at a stretching temperature of 155°C and a stretching speed of 20 mm / min (second melt biaxial stretching step). However, the film ruptured before reaching a total stretching ratio of 11x11x, and a UHMW-PE thin film could not be obtained.

[0084] [Contrast Example 1] A UHMW-PE raw material film was formed in the same manner as in Example 1 (raw material film preparation process). The thickness of the obtained UHMW-PE raw material film was measured with a micrometer and found to be 351 μm. Furthermore, the obtained UHMW-PE raw material film had a tensile breaking strength of 33.5 MPa, a tear strength of 32.2 N / cm, and a nitrogen permeability coefficient of 2.83 × 10⁻⁶. -15 mol·m / (m 2 The values ​​were (·s·Pa). Furthermore, in the visible light region, the total light transmittance was 80.9%, the diffuse light transmittance was 65.9%, the parallel light transmittance was 15.0%, and the haze value was 81.5%. Furthermore, when the obtained UHMW-PE raw film was subjected to an adhesion test using an aluminum plate as the mating material, an adhesion coefficient of 6.68 N / m was obtained.

[0085] <Performance Evaluation> 1. Tensile test The tensile breaking strength of the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1 was evaluated. From the film or thin film, dumbbell-shaped sections with an initial length of 12 mm and a width of 3 mm were cut along the x-axis for tensile testing. These test pieces were subjected to tensile testing using an ORIENTEC RTC-1325A Tensilon universal testing machine at a test speed of 20 mm / min and at room temperature. The tensile breaking strength was defined as the maximum stress in the recorded stress chart divided by the cross-sectional area of ​​the film.

[0086] As a result, as described above, the tensile breaking strength of the UHMW-PE thin films in Examples 1 to 3 was 100 MPa or higher, despite being thin films of less than 1 μm, compared to the UHMW-PE films in Comparative Examples 1 to 3 and Control Example 1.

[0087] 2. Tear test The tear strength of the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1 was evaluated. A test specimen measuring 40 mm in length and 12.5 mm in width was cut from a film or thin film along the x-axis. A 20 mm vertical cut was made in the center of the specimen, and the remaining 20 mm was stretched vertically to tear the specimen. The maximum stress recorded when the specimen was torn was recorded, and the tear strength was defined as this value divided by the film thickness. For this measurement, an ORIENTEC Tensilon universal tester RTC-1325A was used, and the tear test was performed at a test speed of 100 mm / min. and at room temperature to determine the tear strength.

[0088] As a result, as described above, the tear strength of the UHMW-PE thin films in Examples 1 to 3 was 10 N / mm or higher, despite being thin films of less than 1 μm, compared to the UHMW-PE films in Comparative Examples 1 to 3 and Control Example 1.

[0089] 3. Differential Scanning Calorimeter (DSC) Measurement For the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1, the melting behavior was compared by DSC measurement to investigate the structural changes in each process. DSC measurements were performed using a Perkin Elmer Diamond DSC under a nitrogen atmosphere, in a temperature range of 30°C to 180°C, with a heating rate of 10°C / min and a sample weight of approximately 4 mg. Indium and tin were used as standard materials for temperature and calorific value correction.

[0090] Figure 3 shows the DSC melting curves for the original film (Control Example 1), a UHMW-PE stretched film obtained by first melt biaxial stretching the original film to 7x7x (Comparative Example 1), and a UHMW-PE thin film obtained by second melt biaxial stretching the first melt biaxially stretched UHMW-PE stretched film to 4x4x (total stretching ratio of 28x28x) (Example 1). In the original film (Control Example 1), only an endothermic peak originating from folded chain crystals (FCC) is observed. In contrast, in the stretched film obtained by first melt biaxial stretching (Comparative Example 1), a peak originating from fully extended chain crystals (ECC) is observed around 153°C. In the UHMW-PE thin film obtained by second melt biaxial stretching of this stretched film (Example 1), the peak originating from ECC increases, and a new endothermic peak appears around 143°C, which is midway between these peaks. Thus, the UHMW-PE thin film of Example 1, which was second-molten biaxially stretched, has one or more endothermic peaks in the melting profile recorded by a differential scanning calorimeter at (1) 130°C to less than 140°C, (2) 140°C to less than 150°C, and (3) 150°C or higher.

[0091] Figure 4 also shows the DSC melting curves for a stretched UHMW-PE film (Comparative Example 3) obtained by first melt biaxial stretching of the original film (Reference Example 1) to 10x10x, a stretched UHMW-PE film (Comparative Example 2) obtained by first melt biaxial stretching to 10x10x and then melt shrinking to 7x7x, a thin film made of UHMW-PE (Example 3) obtained by further second melt biaxial stretching of the melt-shrunk UHMW-PE stretched film (Comparative Example 2) to 4x4x (total stretching ratio of 28x28x), and similarly, a thin film made of UHMW-PE (Example 2) obtained by further second melt biaxial stretching of the melt-shrunk UHMW-PE stretched film (Comparative Example 2) to 5x5x (total stretching ratio of 35x35x). In the stretched UHMW-PE film (Comparative Example 2) that underwent melt shrinkage after the first melt biaxial stretching, the peak width derived from FCC was narrower compared to the stretched UHMW-PE film (Comparative Example 3) that underwent the first melt biaxial stretching, indicating that structural homogenization occurred. The UHMW-PE thin films (Examples 3 and 2) obtained by second melt biaxial stretching of a melt-shrunk UHMW-PE stretched film (Comparative Example 2) show a peak around 143°C, similar to the UHMW-PE thin film (Example 1) obtained by two-stage melt biaxial stretching (first melt biaxial stretching and second melt biaxial stretching) without a melt shrinkage step, as shown in Figure 3. In other words, the UHMW-PE thin films of Examples 2 and 3 also have one or more endothermic peaks in their melting profiles recorded with a differential scanning calorimeter at (1) 130°C to less than 140°C, (2) 140°C to less than 150°C, and (3) 150°C or higher.

[0092] 4. Scanning electron microscope (SEM) observation For the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1, SEM observation of the film surface was performed to confirm the crystal structure (folded chain crystal: FCC structure, unfolded chain crystal: ECC structure). SEM measurements were performed using a Hitachi High-Technologies Corporation S-4800 electrolytic emission scanning electron microscope (FE-SEM). The measurements were performed under conditions of an acceleration voltage of 2.0 kV and an emission voltage of 10 μA.

[0093] As shown in Figure 5, in the original film (Control Example 1), a spherulite-derived FCC structure was observed. However, in the stretched film (Comparative Example 1) obtained by first melt biaxial stretching of the original film (Control Example 1) up to 7x7 times, fibril structures belonging to the ECC structure and FCC structures grown from them were observed. Similarly, in the stretched films of Comparative Examples 2 and 3, fibril structures belonging to the ECC structure and FCC structures grown from them were also observed. In the thin film (Example 1) obtained by second-stage molten biaxial stretching (total stretching ratio of 28x28x) of a first-stage molten biaxially stretched film (Comparative Example 1) up to 4x4x, it can be seen that a more narrowly curved ECC structure is exhibited. This was also observed in the thin film (Example 2) obtained by two-stage molten biaxial stretching including a melt shrinkage step (total stretching ratio: 35x35x). It is presumed that the coexistence of these two types of ECC structures is what allowed for the creation of a self-supporting thin film even when thinned to a thickness of less than 1 μm.

[0094] 5. Measurement of total light transmittance, diffuse light transmittance, parallel light transmittance, and haze value. For the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1, haze value measurements were performed to evaluate the visible light transmittance. The haze value measurements were performed using a Suga Test Instruments HZ-2 haze meter in accordance with JIS K 7361: (1997), measuring the total light transmittance (%), diffuse light transmittance (%), parallel light transmittance (%), and haze value (%) in the visible light region (range of 360 to 750 nm) as indicators of cloudiness.

[0095] As a result, as described above, the UHMW-PE thin films of Examples 1 to 3 exhibited high transparency in the visible light region, with a total light transmittance of 80% or more, a diffuse light transmittance of 40% or less, a parallel light transmittance of 40% or more, and a haze value of 40% or less.

[0096] 6. Nitrogen permeability coefficient To evaluate the barrier properties of the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1, the nitrogen permeability coefficient was measured at room temperature using a Tsukuba Rika Seiki K-315N-01 film diffusion analyzer. Nitrogen was used as the permeate gas, and a Graphtec GL20 data logger was connected to the permeater. The voltages on the low-pressure and high-pressure sides (1V = 1.6MPa) and permeation time were recorded, and the nitrogen permeability coefficient ρ was calculated using the following equations (1) and (2). The sample pieces were cut into circles with a diameter of 30 mm.

[0097] Q = (V / (R×T×P×A))×(dp / dt) Equation (1) ρ = Q×L Equation (2) Q: Nitrogen permeability [mol / (m³) 2 (·s·Pa) ρ: Nitrogen permeability coefficient [mol m / (m 2 (·s·Pa) V: Low-pressure side volume of the cell [L] A: Transmission area [m 2 ] T: Test temperature [K] P: Differential pressure of supplied gas [Pa] dp / dt: Change in pressure (p) on the low-pressure side per unit time (t) [Pa / s] L: Sample thickness [m]

[0098] 7. Adhesion Test Adhesion tests were performed on the UHMW-PE films or thin films of Examples 1 to 3, Comparative Examples 1 to 3, and Control Example 1 using the following method. As shown in Figure 6, the target film or thin film was cut into strips measuring 1 cm wide x 5 cm long, and 20 μL of liquid paraffin (viscosity 0.87 g / mL: 20℃) was uniformly applied to the entire 1 cm x 3 cm area at the tip. A 300 μm thick Niraco aluminum plate (product number AL-013421), cut into strips measuring 1 cm wide x 5 cm long, was then placed on top of the target film and the aluminum plate strips, with the top and bottom alternating. This allowed the 1 cm x 3 cm area at the tip of the target film or thin film strip and the 1 cm x 3 cm area at the tip of the aluminum plate strip to be brought into close contact. Similar to the tensile test described above, an ORIENTEC RTC-1325A Tensilon universal testing machine was used. The ends of both strips, opposite the contact points, were fixed to the upper and lower chucks, and the strips were pulled vertically at a test speed of 5 mm / min at room temperature. The maximum load recorded in the stress chart was divided by the film thickness (N / m) of the film or thin film in question and used as an index of adhesion (i.e., adhesion coefficient). In Figure 6, 10 represents the target film or thin film, 11 represents an aluminum plate, and 12 represents liquid paraffin.

[0099] As a result, as described above, the nitrogen permeability coefficient of the UHMW-PE thin films of Examples 1 to 3 was similar to or lower than that of the UHMW-PE films of Comparative Examples 1 to 3 and Control Example 1, despite being thin films of less than 1 μm, demonstrating excellent barrier properties.

[0100] From the above, it can be seen that the UHMW-PE thin films of Examples 1 to 3 have a thickness of less than 1 μm and high tensile fracture strength. Furthermore, the UHMW-PE thin films of Examples 1-3 exhibit high tensile and tear strengths. They also demonstrate low nitrogen permeability and high barrier properties. Additionally, they exhibit high total light transmittance, low diffuse light transmittance, high parallel light transmittance, and low haze. Therefore, they possess excellent transparency. [Explanation of Symbols]

[0101] 1. Disc-shaped stainless steel plate 2. Polyimide film for mold release 3. A disc-shaped stainless steel plate with a rectangular window cut out. 4. Polyimide film for mold release 5. Disc-shaped stainless steel plate

[0102] Furthermore, the disclosure of Japanese Patent Application No. 2021-091702 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. A submicron thin film made of ultra-high molecular weight polyethylene, containing ultra-high molecular weight polyethylene with a viscosity-average molecular weight of 1 million to 15 million as its main component, having a film thickness of less than 1 μm and a tensile breaking strength of 100 MPa or more, and exhibiting one or more endothermic peaks in the melting profile recorded by a differential scanning calorimeter at (1) 130°C to less than 140°C, (2) 140°C to less than 150°C, and (3) 150°C or above.

2. A submicron thin film made of ultra-high molecular weight polyethylene according to claim 1, wherein the tear strength is 5 N / mm or more.

3. Nitrogen permeability coefficient is 1 × 10 －１４ mol・m / (m) ２ A submicron thin film made of ultra-high molecular weight polyethylene according to claim 1 or claim 2, wherein the s·Pa is less than or equal to s·Pa.

4. A submicron thin film made of ultra-high molecular weight polyethylene according to claim 1 or claim 2, wherein the haze value in the visible light region is 50% or less.

5. A submicron thin film made of ultra-high molecular weight polyethylene according to claim 1 or claim 2, wherein the adhesion coefficient determined by an adhesion test is 1000 N / m or more.

6. A first melt biaxial stretching step involves melt biaxial stretching a raw film containing ultra-high molecular weight polyethylene with a viscosity-average molecular weight of 1 million to 15 million as its main component, along the x-axis and y-axis directions at a temperature above the melting point of the raw film. A cooling step is performed to cool the stretched film, which has been stretched in the first melt biaxial stretching step, to room temperature. A second melt biaxial stretching step is performed in which the stretched film cooled in the above cooling step is again melt biaxially stretched along the x-axis and y-axis directions at a temperature above the melting point of the stretched film. It has, A method for producing an ultra-high molecular weight polyethylene submicron thin film, wherein the thickness of the resulting ultra-high molecular weight polyethylene submicron thin film is less than 1 μm.

7. A method for producing a submicron thin film made of ultra-high molecular weight polyethylene according to claim 6, further comprising a molten biaxial shrinkage step, in which the stretched film obtained in the first molten biaxial stretching step is molten biaxially shrunk along the x-axis and y-axis directions at a temperature above the melting point of the stretched film, before the cooling step.

8. The third step includes at least one of the following: a molten biaxial stretching and cooling step, in which, after the second molten biaxial stretching step, the stretched film is cooled to below its melting point, then molten biaxially stretched again along the x-axis and y-axis directions at a temperature above its melting point, and then cooled to below its melting point; and a molten shrinkage stretching and cooling step, in which, after the second molten biaxial stretching step, the stretched film is molten biaxially shrinked along the x-axis and y-axis directions at a temperature above its melting point, then cooled to below its melting point, then molten biaxially stretched again along the x-axis and y-axis directions at a temperature above its melting point, and then cooled to below its melting point. The method for producing a submicron thin film made of ultra-high molecular weight polyethylene according to claim 6 or claim 7, wherein the third step is repeated once or more times.

9. A method for producing a submicron thin film made of ultra-high molecular weight polyethylene according to claim 6 or claim 7, comprising a raw material film preparation step of forming an ultra-high molecular weight polyethylene raw material powder having a viscosity-average molecular weight of 1 million to 15 million into a film at a temperature above the melting point of the ultra-high molecular weight polyethylene raw material powder.

10. The method for producing a submicron thin film made of ultra-high molecular weight polyethylene according to claim 9, wherein the raw material film preparation step is a step of forming the ultra-high molecular weight polyethylene raw material powder into a film by press molding.

11. The method for producing a submicron thin film made of ultra-high molecular weight polyethylene according to claim 10, wherein the press molding is performed under reduced pressure.

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