Polyolefin-based multilayer film

Abstract

To provide a polyolefin-based multilayer film which is excellent in straight traveling properties and sealing packaging properties, even in packaging using a high-speed packaging machine, while having performances such as transparency and gas barrier properties similar to conventional polyolefin-based multilayer films.SOLUTION: A polyolefin-based multilayer film includes two or more polyolefin-based resin layers, and includes a first surface layer (S1) containing a polyethylene-based resin and a second surface layer (S2) containing a polyolefin-based resin as the polyolefin-based resin layers, wherein the first surface layer (S1) has a projected structure having a height of 30 nm or more in an amount of 0.2 to 5.8 pieces / 100 μm2, when the surface is observed.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a polyolefin-based multilayer film having two or more polyolefin-based resin layers. [Background technology]

[0002] Resin films have traditionally been used as packaging materials. For example, resin films made of polyolefins are widely used as sealant films because they are flexible, transparent, and have excellent heat-sealing properties. Resin films made of polyolefins usually cannot be used as substrates due to their lack of strength and heat resistance, and are therefore used in combination with polyester or nylon films. For this reason, typical packaging containers are made of laminated films in which the substrate and sealant film are made of different resin materials (see, for example, Patent Document 1).

[0003] Conventionally, packaging using the above-mentioned resin film has been performed using a packaging method called stretch packaging, in which the edges of the film are stretched to wrap around a tray, and the edges of the film folded back on the back of the tray are self-adhered or are further pressed against a heating plate to thermally adhere the films together. Stretch shrink packaging is also performed, in which a heat-shrinkable film is used for stretch packaging, and then the film is heat-shrunk, and films suitable for this have also been developed (see, for example, Patent Documents 2 and 3).

[0004] Furthermore, in recent years, packaging has also been performed using high-speed packaging machines, such as a pillow packaging machine as shown in Figure 1. In such pillow packaging machines, a sheet-like polyolefin multilayer film is fed, the film wraps the packaged item (tray with contents on it) from above into a cylindrical shape (bag formation), both ends of the film are folded together, and the surface layers of the film are heat-sealed together (bottom center seal) while a rotary roll-type heat sealer located below the conveyor is rotated, and then, immediately after sealing the front side of the package with the front and rear heat sealers, the front package unit is separated by serrated blades contained in the front and rear heat sealers, and then the rear side of the rear package unit is similarly sealed with the front and rear heat sealers. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-202519 [Patent Document 2] Japanese Patent Application Publication No. 8-156975 [Patent Document 3] Japanese Patent Application Publication No. 6-179466 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when packaging using conventional polyolefin multilayer films is carried out using a high-speed packaging machine such as the pillow packaging machine described above, once the unwound polyolefin multilayer film becomes misaligned, it is difficult to correct the misalignment in a short time, which deteriorates the straight running properties of the film and makes it difficult to perform sufficient sealed packaging.The films used for stretch packaging and stretch shrink packaging disclosed in Patent Documents 2 and 3 are also not sufficiently compatible with high-speed packaging, and further improvements in the films have been desired.

[0007] Therefore, an object of the present invention is to provide a polyolefin-based multilayer film that has the same performance as conventional films, such as transparency and gas barrier properties, and also has excellent straight running properties and airtight packaging properties, even when packaged using a high-speed packaging machine. [Means for solving the problem]

[0008] The inventors conducted research to solve the above-mentioned problems with polyolefin-based multilayer films having two or more polyolefin-based resin layers, and noticed that, once a shift occurs in the unwound polyolefin-based multilayer film, conventional films have such high adhesion that the film runs without being centered, and the shift cannot be corrected before the center seal is performed. After further intensive research, the inventors discovered that by providing a polyolefin resin layer with a first surface layer (S1) containing a polyethylene resin and a second surface layer (S2) containing a polyolefin resin, and by forming specific protrusion structures on the first surface layer (S1), it is possible to impart a certain degree of slipperiness to the surface of the polyolefin multilayer film that comes into contact with the roller, enabling centering of the film when it is fed out, and thus correcting any misalignment before center sealing, thereby achieving excellent straight running properties and center sealing even when packaging using a high-speed packaging machine.In addition, the inventors discovered that because the second surface layer (S2) constituting the polyolefin multilayer film is imparted with the same slipperiness and strength as conventional films, good sealed packaging performance can be maintained without deterioration of slipperiness with the packaging machine, leading to the completion of the present invention.

[0009] The present invention has been made based on the above findings, and the gist of the present invention is as follows. (1) A polyolefin-based multilayer film having two or more polyolefin-based resin layers, The polyolefin-based resin layer includes a first surface layer (S1) containing a polyethylene-based resin and a second surface layer (S2) containing a polyolefin-based resin, When the surface of the first surface layer (S1) was observed, protrusion structures having a height of 30 nm or more were found at a rate of 0.2 to 5.8 / 100 μm. 2 A polyolefin-based multilayer film characterized by having a film thickness in the range of (2) The polyolefin multilayer film according to (1), characterized in that the tensile modulus of the polyolefin multilayer film is the product of MD (length direction) and TD (width direction) of 170,000 or more. (3) The package according to (1) or (2), wherein the protrusion-like structures are spherulite-derived protrusion-like structures made of a thermoplastic resin having a melting point of 125°C or higher. (4) The polyolefin-based multilayer film according to any one of (1) to (3), characterized in that the difference in methyl flow rate (MFR) between the thermoplastic resin constituting the protrusion structures and the polyethylene-based resin serving as the base resin of the first surface layer (S1) measured at a measurement temperature of 190°C and a load of 2.16 kg (MFR of S1 - MFR of protrusion structures) is 7 or less. (5) The polyolefin-based multilayer film according to any one of (1) to (4), further comprising a substrate layer between the first surface layer (S1) and the second surface layer (S2). (6) The polyolefin-based multilayer film according to (5), further comprising a first internal layer (I1) provided between the first surface layer (S1) and the base layer, and a second internal layer (I2) provided between the second surface layer (S2) and the base layer. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a polyolefin-based multilayer film that has performance such as transparency and gas barrier properties similar to those of conventional films, and that has excellent straight running properties and airtight packaging properties even when packaged using a high-speed packaging machine. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram schematically illustrating an example of a pillow packaging machine. [Figure 2]FIG. 1 is a cross-sectional view schematically illustrating a state in which misalignment occurs in a polyolefin-based multilayer film during center sealing, preventing the edges of the film from being fully folded. [Figure 3] FIG. 2 is a diagram schematically showing a cross section of a first surface layer (S1) of the polyolefin-based multilayer film of the present embodiment. [Figure 4] 1 is a diagram schematically illustrating a cross section of a laminated state of a polyolefin-based multilayer film according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. The following embodiments are merely examples for explaining the present invention, and are not intended to limit the present invention to the following contents. The present invention can be implemented by appropriately modifying it within the scope of its gist.

[0013] Here, Fig. 1 shows an example of a pillow packaging machine, Fig. 2 shows a schematic diagram of the state of the film and packaged items when the polyolefin multilayer film is center-sealed, Fig. 3 shows a schematic diagram of a cross section of the first surface layer (S1) of the polyolefin multilayer film of this embodiment, and Fig. 4 shows a schematic diagram of an example of the laminated state of the polyolefin multilayer film of this embodiment. It should be noted that the dimensions and proportions of the components shown in FIGS. 1 to 3 are different from the actual ones for the sake of convenience of explanation.

[0014] <Polyolefin multilayer film> First, the polyolefin-based multilayer film (hereinafter sometimes simply referred to as "film") of this embodiment will be described. The polyolefin-based multilayer film of this embodiment is a polyolefin-based multilayer film having two or more polyolefin-based resin layers. Specifically, as shown in FIG. 4, the polyolefin resin layer includes at least a first surface layer (S1) containing a polyethylene resin and a second surface layer (S2) containing a polyolefin resin.

[0015] As shown in FIG. 3, in the polyolefin-based multilayer film of this embodiment, when the surface of the first surface layer (S1) is observed, protrusion-like structures having a height of 30 nm or more are present at a density of 0.2 to 5.8 / 100 μm. 2 It is characterized in that it has a range of When packaging with polyolefin multilayer film is performed using a high-speed packaging machine such as the one shown in Figure 1, if the unwound polyolefin multilayer film becomes misaligned (misaligned in the width direction of the film), it is difficult to correct the misalignment in a short time due to the strong adhesion between the film and the roller that feeds the film, and the film is fed out in a misaligned state, resulting in a problem of poor straight running properties of the film.If the straight running properties of the film deteriorate, the edges of the film will not be folded properly during center sealing, as shown in Figure 2, causing a misalignment of the center and poor sealing packaging properties of the film.

[0016] Therefore, in the present invention, by providing specific protrusion structures on the first surface layer (S1) containing a polyethylene-based resin as described above, when packaging with a polyolefin-based multilayer film using a high-speed packaging machine, it is possible to impart appropriate slipperiness to the surface (S1) that comes into contact with the roller that feeds the polyolefin-based multilayer film.As a result, as the film is fed out, centering of the film can be performed and any misalignment of the film can be corrected before center sealing, resulting in excellent straight running properties and center sealing.

[0017] Each layer constituting the polyolefin-based multilayer film of this embodiment will be described below.

[0018] (First surface layer (S1)) As shown in FIG. 4, the first surface layer (S1) (hereinafter, sometimes referred to as "S1 layer") is the innermost layer of the film of this embodiment (the layer that comes into contact with the packaged item when packaging is performed using a high-speed packaging machine), and is a layer that is folded over at the bottom of the packaged item when center-sealing is performed during high-speed packaging, and contains a polyethylene-based resin.

[0019] Examples of the polyethylene resin include polyethylene, ethylene-α-olefin copolymer, etc. Examples of polyethylene include medium-density polyethylene, low-density polyethylene (LDPE), and very-low-density polyethylene. Examples of very-low-density polyethylene include linear very-low-density polyethylene (also known as "VLDPE" or "ULDPE").

[0020] Here, polyethylene can be classified by density based on JISK 6922. Specifically, the density is 0.942 g / cm 3 The above is called high density polyethylene. , density is 0.930g / cm 3 More than 0.942g / cm 3 Less than medium density polyethylene and its density is 0.910 g / cm 3 More than 0.930g / cm 3 Less than this is called low density polyester. It is called polyethylene and has a density of 0.910 g / cm 3 Anything less than this is called ultra-low density polyethylene.

[0021] The polyethylene resin has a density of 0.920 g / cm 3 The polyethylene having a viscosity of 0.930 g / cm or more is preferable in terms of imparting slipperiness to the film. 3 More preferably, the polyethylene has a viscosity of 0.940 g / cm or more. 3It is more preferable that the polyethylene resin is an ultra-low density polyethylene or a linear low density polyethylene in order to provide low temperature sealing properties. In this case, the density of the polyethylene is 0.880 to 0.925 g / cm. 3 is preferably 0.895 to 0.915 g / cm 3 and more preferably 0.900 to 0.910 g / cm 3 Two or more of these polyethylenes may be used in view of sealing properties and preventing stickiness.

[0022] The ethylene-α-olefin copolymer refers to a copolymer of ethylene and at least one selected from the above-mentioned α-olefins. The ethylene-α-olefin copolymer is preferably a soft copolymer in which the proportion of α-olefin in all the monomers constituting the copolymer (based on the charged monomers) is 5 to 30% by mass. Furthermore, as the ethylene-α-olefin copolymer, a copolymer of ethylene and at least one comonomer selected from a propylene comonomer, a butene comonomer, a hexene comonomer, and an octene comonomer is generally easily available and can be suitably used.

[0023] The polyethylene resin may be polymerized using a known catalyst such as a single-site catalyst or a multi-site catalyst, and from the viewpoint of achieving even better transparency, it is preferable to polymerize using a single-site catalyst. The polyethylene resin has a density of 0.860 to 0.925 g / cm from the viewpoint of further improving physical properties such as heat sealing properties at low temperatures. 2 It is preferable that the concentration is 0.870 to 0.920 g / cm 2 More preferably, it is 0.880 to 0.915 g / cm 2 It is more preferable that:

[0024] The first surface layer (S1) has a protrusion structure as shown in FIG. The protrusion structure in the S1 layer 10 is composed of protrusions 20 having a height of 30 nm or more when observed from the surface, and the number of protrusions is 0.2 to 5.8 per 100 μm. 2 This structure existed within the range of

[0025] The height of the protrusions constituting the protrusion structure is set to 30 nm or more because, when the height of the protrusions is 30 nm or more, the contact area between the S1 layer and the mechanism for feeding the film (rollers, etc.) becomes smaller, thereby achieving good slip properties.

[0026] The density of the protrusions constituting the protrusion structure is 0.2 to 5.8 pieces / 100 μm 2 The reason is that the range of protrusions is 0.2 to 5.8 pieces / 100 μm. 2 In this case, the contact area between the S1 layers becomes appropriately small, and good sliding properties can be obtained. From the same viewpoint, the density of the protrusions constituting the protrusion structure is 0.2 to 5.8 pieces / 100 μm 2 It is preferable that the number of particles is 0.9 to 5.8 particles / 100 μm. 2 It is more preferable that:

[0027] The range of the protrusions is an average value, for example, a range of 100 μm selected randomly when observing the surface. 2 The number of protrusions present in the range can be calculated as the average value of measurements taken at nine locations. Specifically, measurements can be made using an atomic force microscope (AFM). Measurement conditions include a DimensionIcon (manufactured by Bruker) device, an NCH-type Si single crystal probe as the probe, and tapping mode as the measurement mode. Images of the surface shape were acquired with a resolution of 512 x 512 pixels or more over a field of view of 30 μm on a side. The 30 μm field image was divided into nine parts to create images with a 10 μm field of view, and Ra for each image was calculated. Images with an Ra of 10 nm or less over a 10 μm field of view were then selected to measure the height and number of surface protrusions. In this case, protrusions of 30 nm or more were counted as protrusion-like structures. The threshold value was set at 30 nm in order to exclude protrusions due to undulations on the substrate surface and reduce the variability in the measurement values. The average number of these protrusion-like structures was set to 100 μm. 2 The number of protrusion structures per unit area (protrusion structure density) can be used.

[0028] The method for forming the protrusions that make up the protrusion-like structure is not particularly limited as long as it satisfies the above-mentioned conditions (protrusion height, range of existence). For example, a method can be used in which a flat S1 layer is first formed and then textured, or a method can be used in which an S1 layer with pre-formed texture is created by imprinting or the like and then attached to the multilayer film. Furthermore, as shown in Figure 3, a protruding structure can be formed on the surface of the S1 layer by incorporating particles 21 made of a material different from the polyethylene resin 10 that constitutes the S1 layer.

[0029] Among the methods for forming the protrusion structure described above, it is preferable to incorporate particles 21 made of a material different from the polyethylene resin that is the base resin of the S1 layer 10, as shown in Figure 3, in order to form a fine protrusion structure more easily and reliably. As shown in Figure 3, when the particles 21 present in the S1 layer 10 form a protrusion structure, even if the particles 21 present inside the S1 layer 10 are the same as the particles 21 that form the protrusions 20, they do not form the protrusion structure 20 if they do not protrude from the surface of the S1 layer 10.

[0030] When the protrusion structure is composed of particles, the particles may be inorganic particles, various thermoplastic resins, etc. Among these particles, spherulites made of a thermoplastic resin having a melting point of 125°C or higher are preferred from the viewpoints of preventing the particles from falling out of the S1 layer and maintaining good transparency.

[0031] The thermoplastic resin having a melting point of 125°C or higher is not particularly limited as long as it is different from the polyethylene resin 10 constituting the S1 layer, and can be appropriately selected depending on the required performance. Examples of the thermoplastic resin include high-density polyethylene (HDPE), polypropylene, and polyethylene terephthalate. Among these, high-density polyethylene (HDPE) is preferred from the viewpoints of preventing particles from falling out of the S1 layer, maintaining good transparency, and being able to be realized by polymer blending.

[0032] Furthermore, it is preferable that the difference in methyl flow rate (MFR) between the thermoplastic resin constituting the particles and the polyethylene resin that is the base resin of the first surface layer (S1) measured at a measurement temperature of 190°C and a load of 2.16 kg (MFR of S1 - MFR of protrusion structures) is 7 or less. By making the difference in methyl flow rate (MFR) 6 or less, it is possible to more reliably provide the protrusion structures that satisfy the above-mentioned conditions of height and existence range.

[0033] Furthermore, when the protrusion structures are made of high density polyethylene (HDPE) particles, the content of HDPE in the S1 layer is preferably 2 to 5 mass %, more preferably 3 to 4 mass %. When the HDPE content is 3% by mass or more, the protrusion structure can be formed more reliably, and when the HDPE content is 4% by mass or less, the physical properties such as transparency and durability of the S1 layer are not deteriorated.

[0034] In addition to the polyethylene resin and particles constituting the protrusion structure, the first surface layer may further contain additives such as fatty acid esters and ethylene oxide adducts of polyhydric alcohols such as glycerin, polyglycerin, sorbitan, and pentaerythritol, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene fatty acid esters.

[0035] (Second surface layer (S2)) The second surface layer (S2) (hereinafter sometimes referred to as "S2 layer") is the outermost layer of the film of this embodiment (the layer that comes into contact with the roller or belt that feeds the film), as shown in Figure 4, and is a layer that contains a polyolefin resin.

[0036] Examples of the polyolefin resin include at least one product selected from polypropylene, low-density polyethylene (LDPE), high-density polyethylene (HDPE), low-density linear polyethylene (LLDPE), ultra-high molecular weight polyethylene (UHMWPE), very low-density polyethylene (VLDPE) obtained by metallocene catalysis, polyethylene, alpha-olefins (ethylene, propylene, butene-1, octene-1, butadiene, etc.), diolefins, styrene / butadiene / styrene block copolymer (SBS), its hydrogenated product styrene / ethylene-butylene / styrene block copolymer (SEBS), styrene / butadiene copolymer (SBR), its hydrogenated product styrene / ethylene / butylene copolymer (HSBR), styrene / isoprene / styrene block copolymer (SIS), and its hydrogenated product styrene / ethylene-propylene / styrene block copolymer (SEPS).

[0037] Among these, it is preferable that the S2 layer contains at least a polypropylene resin from the viewpoints of slipperiness with rollers or belts, strength, and the like. The polypropylene-based resin is not particularly limited and can be appropriately selected depending on the required performance. For example, the polypropylene-based resin may be a propylene homopolymer, or a copolymer of propylene and another α-olefin (for example, an α-olefin having 4 to 20 carbon atoms, such as 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, or 1-eicosane, preferably an α-olefin having 4 to 8 carbon atoms).

[0038] The polypropylene may be one polymerized using a known catalyst such as a single-site catalyst or a multi-site catalyst, and from the viewpoint of achieving even better transparency, one polymerized using a single-site catalyst is preferred. The polypropylene may be a resin polymerized using a catalyst such as a Ziegler-Natta catalyst, or a resin polymerized using a metallocene catalyst. That is, for example, syndiotactic polypropylene, isotactic polypropylene, etc. can also be used as the polypropylene.

[0039] Furthermore, copolymers with propylene or ethylene can also be used. Examples of such copolymers include elastomers and plastomers containing structural units derived from propylene, elastomers and plastomers containing structural units derived from ethylene, elastomers and plastomers containing structural units derived from ethylene and structural units derived from propylene, and elastomers and plastomers containing structural units derived from ethylene, structural units derived from propylene, and structural units derived from other α-olefins (for example, α-olefins having 4 to 20 carbon atoms such as butene).

[0040] The polyolefin resin described above is sufficient as long as it contains at least one type of polypropylene resin, and it may be composed of only one type or a plurality of types.

[0041] The melt flow rate (MFR) of the polypropylene resin, measured at a temperature of 230°C and a load of 2.16 kg, is preferably 1 to 20 g / 10 min, more preferably 1.5 to 10 g / 10 min, and even more preferably 2 to 8 g / 10 min, from the viewpoints of adhesive strength between adjacent layers and stretch moldability. When a plurality of polypropylene resins are contained, it is preferable that at least one of them satisfies the above MFR, and it is more preferable that all of them satisfy the above MFR.

[0042] The melting point (Tm) of the polypropylene resin is not particularly limited, but from the viewpoint of adhesive strength with adjacent layers and extrusion moldability, it is preferably 70 to 190°C, more preferably 75 to 180°C, and even more preferably 80 to 170°C. When a plurality of polypropylene-based resins are contained, it is preferable that at least one of them has a melting point above, and it is more preferable that all of them have a melting point above.

[0043] The S2 layer may further contain, as components other than the polypropylene-based resin, additives such as various surfactants, tackifying resins, antiblocking agents, antistatic agents, lubricants, plasticizers, antioxidants, UV absorbers, colorants, and inorganic fillers.

[0044] (Base material layer (C1)) Moreover, the polyolefin-based multilayer film of the present embodiment preferably further comprises a base layer (C1) between the first surface layer (S1) and the second surface layer (S2), as shown in Fig. 4. The base layer is the central layer of the multilayer film, and can impart gas barrier properties and water vapor barrier properties.

[0045] Therefore, it is more preferable that the base layer contains a barrier resin. The type of the barrier resin is not particularly limited as long as it can impart barrier properties, and can be appropriately selected depending on the required performance. Examples include saponified ethylene-vinyl alcohol copolymer (EVOH), polyamide resins, polyester resins, polyvinylidene chloride resins, etc. Among these, from the viewpoint of particularly excellent gas barrier properties and stretch moldability, saponified ethylene-vinyl alcohol copolymer (EVOH) or polyamide resins are preferred, and saponified ethylene-vinyl alcohol copolymer (EVOH) is more preferred. The barrier resin may be one type or a combination of two or more types.

[0046] The saponified ethylene-vinyl alcohol copolymer tends to have better stretchability because the crystallinity and melting point decrease as the content of structural units derived from ethylene (sometimes referred to as ethylene content in this specification) increases; generally, the melting point is 183°C when the ethylene content is 32 mol%, 173°C when it is 38 mol%, and 163°C when it is 44 mol%. The ethylene content is preferably 30 mol% or more and 60 mol% or less, more preferably 31 mol% or more and 50 mol% or less, and even more preferably 32 mol% or more and 45 mol% or less. When the ethylene content is within the above range, a film with excellent stretchability and barrier properties can be obtained.

[0047] The barrier performance tends to be better as the interaction between molecular chains is stronger, and saponified ethylene-vinyl alcohol copolymers in particular have high barrier performance due to their strong interaction between molecules. However, because the interaction between molecular chains is strong, the heat shrinkage stress is large, and when used as a shrink (heat-shrinkable) film, the container of the package is likely to be deformed.

[0048] By controlling the crystal structure, the saponified ethylene-vinyl alcohol copolymer can achieve both a melting point of 180°C or less (preferably 170°C or less) and an ethylene content of 45 mol% or less. Furthermore, by using a saponified ethylene-vinyl alcohol copolymer with a larger supercooling temperature difference (the difference between the melting point and the crystallization temperature by cooling) than that of a typical saponified ethylene-vinyl alcohol copolymer, the lamella thickness is reduced and the crystals are more likely to be uniform. In the case of a core layer containing such a saponified ethylene-vinyl alcohol copolymer, stress concentration is reduced, and container deformation during gas pack packaging can be suppressed. The supercooling temperature difference is preferably 25°C or more, more preferably 26°C or more, and even more preferably 27°C or more. The reason for this effect is not entirely clear, but is thought to be as follows. That is, the heat shrinkage properties of a multilayer film are exhibited when the amorphous portions, which have been stretched due to the molecular orientation of the constituent resins, relax and return to a non-oriented state, and the crystalline portions serve to prevent the shrinkage of the amorphous portions up to near the melting temperature. Generally, saponified ethylene-vinyl alcohol copolymers have strong intermolecular interactions and large heat shrinkage stress, which tends to dominate the heat shrinkage stress of the entire multilayer film. However, it is thought that by controlling the crystalline structure, stress concentration due to relaxation of the orientation of the amorphous and crystalline portions can be suppressed, and a multilayer film with excellent barrier properties can be provided.

[0049] Examples of the polyamide resin include polyamides with a head group of polyamide 6, polyamide 66, polyamide 610, polyamide 11, polyamide 12, polyamide 6 / 66, polyamide 6 / 12, polyamide 6 / 610, polyamide 6 / 66 / 12, polyamide 6 / 66 / 610, and polyamide 6 / 66 / 612.

[0050] The gas barrier properties of the entire multilayer film including the core layer are as follows: oxygen permeability at 23°C and 65% RH is 300 cc / (m 2It is preferable that the oxygen permeability is less than 1·day·MPa. The oxygen permeability can be measured, for example, in accordance with JIS K7126-1 (differential pressure method) under conditions of 23°C and 65% RH.

[0051] The gas barrier property, water vapor barrier property, and other performances can be appropriately adjusted by, for example, the composition of the resin constituting the base layer, the thickness of the base layer, and the like.

[0052] (Internal layer (I1), internal layer (I2)) As shown in FIG. 4, the polyolefin-based multilayer film of this embodiment may further comprise a first internal layer (I1) (hereinafter sometimes simply referred to as "I1 layer") provided between the first surface layer (S1) and the base layer, and a second internal layer (I2) (hereinafter sometimes simply referred to as "I2 layer") provided between the second surface layer (S2) and the base layer. The first inner layer (I1) and the second inner layer (I2) are layers for imparting adhesion between the base layer and the first surface layer (S1) and the second surface layer (S2).

[0053] Here, the I1 layer and the I2 layer contain at least a resin, and may further contain an additive. The resin constituting the inner intermediate layer and the outer intermediate layer may be a thermoplastic resin, such as an acid-modified olefin resin, a modified olefin resin such as modified LLDPE, or an olefin resin.

[0054] Examples of the acid-modified olefin resin include polyolefins modified with carboxylic acid (including carboxylic acid anhydride). The acid-modified olefin resin may be either a homopolymer or a copolymer, and preferably has repeating units derived from ethylene and / or an α-olefin, such as propylene, 1-butene, or 1-octene.

[0055] Examples of the olefin structure constituting the acid-modified olefin resin include homopolymers such as polyethylene, polypropylene, polybutene, and polyoctene, ethylene-propylene copolymers, ethylene-1-butene copolymers, propylene-1-butene copolymers, propylene-1-hexene copolymers, propylene-4-methyl-1-pentene copolymers, propylene-1-octene copolymers, propylene-1-decene copolymers, propylene-1,4-hexadiene copolymers, propylene-dicyclopentadiene copolymers, propylene-5-ethylidene-2-norbornene copolymers, propylene-2,5-norbornadiene copolymers, propylene-5-ethylidene-2-norbornene copolymers, 1-octene-ethylene copolymers, 1-butene-propylene copolymers, 1-octene-ethylene copolymers, 1-butene-propylene copolymers, 1-octene-ethylene copolymers, 1-butene-propylene copolymers, 1-butene-propylene copolymers, 1-hex ... Binary copolymers such as ethylene-propylene-1-butene copolymer, 1-butene-1-hexene copolymer, 1-butene-4-methyl-1-pentene copolymer, 1-butene-1-octene copolymer, 1-butene-1-decene copolymer, 1-butene-1,4-hexadiene copolymer, 1-butene-dicyclopentadiene copolymer, 1-butene-5-ethylidene-2-norbornene copolymer, 1-butene-2,5-norbornadiene copolymer, 1-butene-5-ethylidene-2-norbornene copolymer, ethylene-propylene-1-butene copolymer, ethylene-propylene-1-hexene copolymer, ethylene-propylene-1-octene copolymer, ethylene-propylene-1-octene copolymer, ethylene-propylene-1,4-hexadiene copolymer, ethylene-propylene-1,4-Hexadiene copolymer, Ethylene-propylene-dicyclopentadiene copolymer, Ethylene-propylene-dicyclopentadiene copolymer, Ethylene-propylene-5-ethylidene-2-norbornene copolymer, Ethylene-propylene-5-ethylidene-2-norbornene copolymer, Ethylene-propylene-2,5-norbornadiene copolymer, Ethylene-propylene-2,5-norbornadiene copolymer, Ethylene-propylene-5-ethylidene-2-norbornene copolymer, Ethylene-propylene-5-ethylidene-2-norbornene copolymer, 1-butene-ethylene-propylene copolymer, 1-butene-ethylene-1-hexene copolymer, 1-butene-ethylene-1-octene copolymer, 1-butene-propylene Examples include multi-component copolymers such as 1-octene copolymer, 1-butene-ethylene-1,4-hexadiene copolymer, 1-butene-propylene-1,4-hexadiene copolymer, 1-butene-ethylene-dicyclopentadiene copolymer, 1-butene-propylene-dicyclopentadiene copolymer, 1-butene-ethylene-5-ethylidene-2-norbornene copolymer, 1-butene-propylene-5-ethylidene-2-norbornene copolymer, 1-butene-ethylene-2,5-norbornadiene copolymer, 1-butene-propylene-2,5-norbornadiene copolymer, 1-butene-ethylene-5-ethylidene-2-norbornene copolymer, and 1-butene-propylene-5-ethylidene-2-norbornene copolymer.

[0056] The acid-modified olefin resin may be produced by a conventional method, for example, by graft polymerizing an unsaturated carboxylic acid onto the polyolefin under conventional conditions, for example, by stirring under heating, or may be a commercially available product, such as a polymer obtained by grafting maleic anhydride onto LDPE, HDPE, LLDPE, PS, or PP.

[0057] Examples of the olefin (olefin monomer) constituting the olefin-based resin include ethylene, propylene, 1-butene, 3-methyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0058] The melt flow rate (MFR) of the resin contained in the I1 layer and the I2 layer is preferably 1 to 20 g / 10 min, more preferably 1.5 to 10 g / 10 min, and even more preferably 2 to 8 g / 10 min, from the viewpoints of adhesive strength with adjacent layers and stretch moldability. The melt flow rate can be measured, for example, in accordance with JIS K7210 or ASTM D1238 under conditions of a temperature of 230° C. and a load of 2.16 kg. At least one of the resins contained in the inner intermediate layer and the outer intermediate layer preferably satisfies the above MFR, and more preferably all of them satisfy the above MFR.

[0059] (Method for producing multilayer film) The method for producing the multilayer film is not particularly limited, but the following method can be mentioned in this example. The method for producing the multilayer film according to this embodiment preferably includes a step of laminating a laminate having at least two resin layers (hereinafter sometimes referred to as "unstretched raw sheet") by a co-extrusion method and heat-stretching the laminate. The co-extrusion method will be described below.

[0060] In the coextrusion method, each material is melt-extruded from a separate extruder, laminated in a multilayer die, melt-coextruded, and quenched to obtain an unstretched raw sheet. The melt-coextrusion method is not particularly limited, and examples include methods using a multilayer T-die or a multilayer circular die. Among these, methods using a multilayer circular die are preferred. The use of a multilayer circular die is advantageous in terms of the required space and investment amount for equipment, is suitable for high-mix, low-volume production, and makes it easier to obtain the desired heat shrinkage rate.

[0061] As the coolant used for rapid cooling, water at 60°C or below is usually suitably used. The coolant can be brought into direct contact with the molten resin or can be used indirectly as an internal coolant for the metal roll. When used as an internal coolant, other known coolants such as oil can be used in addition to water, and in some cases, it can also be used in combination with blowing cold air.

[0062] In the stretching step, the obtained unstretched raw sheet is heated to a temperature equal to or higher than the softening temperature of the resin constituting the unstretched raw sheet, and stretched, for example, in the MD (Machine Direction: longitudinal direction (length direction)) by 1.5 times or more and in the TD (Transverse Direction: transverse direction (width direction)) by 1.5 times or more. By such a stretching step, the above-mentioned multilayer film can be easily obtained.

[0063] The stretching ratio is appropriately selected depending on the purpose, and if necessary, heat treatment (thermal relaxation treatment) can be performed after stretching. The thermal relaxation treatment relaxes the molecular orientation of the multilayer film, thereby further suppressing dimensional changes during transportation and / or storage.

[0064] The stretching step can also be carried out by a direct inflation method, in which air or nitrogen is blown into a tube immediately after melt extrusion to perform stretching. This method also makes it possible to easily obtain a multilayer film having a predetermined heat shrinkage rate. However, to more reliably achieve an appropriate heat shrinkage rate, a biaxial stretching method is preferred, and a tubular method (also called a double bubble method) in which an unstretched raw sheet obtained using the above-mentioned circular die is heated and biaxially stretched is more preferred. In other words, the multilayer film of this embodiment is preferably a biaxially stretched multilayer film produced by a tubular method in which biaxial stretching is performed.

[0065] The above-described production method may include a crosslinking step of crosslinking the resin before or after stretching. When crosslinking is performed, it is preferable to perform the crosslinking treatment by irradiating with energy rays before heating and stretching the resin. This increases the melt tension of the laminate during heat stretching, making it possible to further stabilize the stretching. Note that the laminate after stretching may also be irradiated with energy rays to crosslink the resin. Examples of energy rays that can be used include ionizing radiation such as ultraviolet rays, electron beams, X-rays, and gamma rays. Of these, electron beams are preferred.

[0066] <Physical properties of polyolefin multilayer film> The polyolefin-based multilayer film of this embodiment preferably has a tensile modulus, expressed as the product of MD (length direction) and TD (width direction), of 170,000 or more. This is because a high tensile modulus can provide excellent strength and make the film less likely to wrinkle during packaging. From the same viewpoint, the tensile modulus, expressed as the product of MD (length direction) and TD (width direction), is more preferably 210,000 or more, and even more preferably 250,000 or more. The tensile modulus can be increased by appropriately adjusting the stretching temperature or the stretching ratio.

[0067] Furthermore, the polyolefin-based multilayer film of the present embodiment has high transparency, and the haze is preferably 3.0 or less, more preferably 2.7 or less, and even more preferably 2.4 or less.

[0068] Furthermore, when gas barrier properties are imparted to the polyolefin-based multilayer film of this embodiment, the oxygen permeability at 23°C and 65% RH is 300 cc / (m 2 It is preferable that the oxygen permeability is less than 1·day·MPa. The oxygen permeability can be measured, for example, in accordance with JIS K7126-1 (differential pressure method) under conditions of 23°C and 65% RH. [Example]

[0069] The present invention will be described in detail below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.

[0070] [Examples 1 to 10 and Comparative Examples 1 to 10] (1) Preparation of multilayer film Each sample of the multilayer film of the examples and comparative examples was produced under the conditions shown in Tables 1-4. Although the method for producing the multilayer film is not particularly limited, in this example, each sample was produced by coextrusion. The melt coextrusion conditions were a multilayer circular die (annular die) and water at 60°C or less was used as a refrigerant. In addition, in the stretching process, a tubular method was used in which the unstretched raw material obtained with the circular die was biaxially stretched. In each of the multilayer films, the first surface layer (S1) is used as a sealing layer, and from the outermost surface, the first internal layer (I1), the base layer (C1), the second internal layer (I2), and the second surface layer (S2) are sequentially formed.

[0071] The materials used for each layer are shown below. PP1: Propylene copolymer (MFR 3.5g / 10min (230℃), melting point 161℃) PP2: Propylene copolymer (MFR 5.3 g / 10 min (230°C), melting point 135°C) EVAC1: Ethylene-vinyl acetate copolymer (MFR 2.3 g / 10 min (190°C), melting point 90°C, vinyl acetate content 15%) PO elastomer 1: ethylene-propylene copolymer (MFR 2.0 g / 10 min (230°C), melting point 82°C) AF1: Diglycerol oleate / glycerol monooleate Adhesive PE: Acid-modified polyolefin composition (MI 2.3 g / 10 min (190°C), melting point 120°C) HDPE1: High-density polyethylene (MFR 0.66 g / 10 min (190 °C), melting point 132 °C) HDPE2: High-density polyethylene (MFR 1.35g / 10min (190℃), melting point 136℃) HDPE3: High-density polyethylene (MFR 0.25g / 10min (190℃), melting point 130℃) HDPE4: High-density polyethylene (MFR 2.5g / 10min (190℃), melting point 127℃) LLDPE1: Linear low-density polyethylene (MFR 2.0 g / 10 min (190°C), melting point 116°C) EVOH: Ethylene-vinyl alcohol copolymer saponification product (MI 4.0 g / 10 min (210°C), melting point 160°C, ethylene content 38 mol%) Adhesive PP: Acid-modified polyolefin composition (MI 7.7 g / 10 min (230°C), melting point 140°C) VLDPE1: Ultra-low density polyethylene (MFR 3.8g / 10min (190℃), melting point 98℃) VLDPE2: Ultra-low density polyethylene (MFR 2.0 g / 10 min (190°C), melting point 99°C) VLDPE3: Ultra-low density polyethylene (MFR 8.0 g / 10 min (190°C), melting point 102°C) VLDPE4: Ultra-low density polyethylene (MI 7.5g / 10min (190℃), melting point 97℃)

[0072] (tensile modulus) The tensile modulus of the obtained multilayer film was measured by conducting a tensile test using an Autograph AG-IS MO series manufactured by Shimadzu Corporation under conditions of 23°C, 50% RH, using a sample slit to a width of 10 mm, a sample length of 120 mm, a chuck distance of 50 mm, and a tensile speed of 200 mm / min.

[0073] (Hayes) The haze of the obtained multilayer film was measured using a HAZE METER NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd., by attaching a slit sample having a width of 100 mm and a length of 100 mm to a jig, according to JIS K 7136.

[0074] <Packaging machine evaluation> The multilayer film samples obtained as described above were packaged using a high-speed packaging machine and evaluated as follows: The high-speed packaging machine used was a pillow packaging machine "S5000X" manufactured by Omori Machinery Industry Co., Ltd., and high-speed packaging was performed under the conditions of a packaging speed of 60 packs / min, a center seal temperature of 140°C during packaging, and a top seal temperature of 125°C. The evaluation results are shown in Tables 1 to 4.

[0075] (1) Center sealing For multilayer film samples, the number of samples that could be continuously packaged was counted because the film shifted during packaging at the bag-making section, preventing the center seal from joining together, resulting in a center seal. Evaluation was conducted according to the following criteria. ◎: The number of samples that can be continuously packaged is 200 or more. 〇: The number of samples that can be continuously packaged is between 100 and 200 △: The number of samples that can be continuously packaged is between 50 and 100. ×: The number of samples that can be continuously packaged is less than 50

[0076] (2) Foldability of both film ends For multilayer film samples, since soft films tend to sag in the bag-making section during packaging and both ends of the film tend to fold and seal tightly, when 30 samples were packaged, the film edges were checked for folds and evaluated according to the following criteria. 〇: The edge of the film is not folded ×: The edge of the film was folded.

[0077] (3) Hot tack The packaging speed was 60 packs / minute, and gas was added while adjusting the amount of gas inside the package using a pressure device attached to the packaging machine. The package was wrapped so that the center of the top surface of the film expanded 1 cm above the edge of the tray. Immediately after wrapping, a load was applied to the top seal using a shrink tunnel "G1500" manufactured by KYU Systems Co., Ltd., using the thermal shrinkage force generated when the film was shrunk at a tunnel temperature of 150°C. The top seal of the package was then inspected and evaluated according to the following criteria. 〇: The top seal remains sealed ×: Opened and gas cannot be sealed

[0078] [Table 1] [Table 2] [Table 3] [Table 4]

[0079] The results in Tables 1 and 2 show that the samples of the Examples are well-balanced and superior in all aspects compared to the samples of the Comparative Examples. On the other hand, it was found that the samples of the comparative examples showed results that were significantly inferior to the samples of the examples in at least one evaluation item. In particular, when the transparency of the entire film is poor, the waviness of the film surface is greater than that of the protrusion-like structures, and it is not possible to ensure a waviness-free field of view, making it impossible to measure the density of the protrusion-like structures. This waviness on the film surface is thought to be caused by additive domains that are formed due to poor kneading of the resin during extrusion, and then spread during stretching. Furthermore, when PET was added instead of HDPE, it was found to be incompatible with the base resin, tending to fall off the film surface, and the transparency of the entire film was significantly reduced, making it unsuitable for this application. [Industrial Applicability]

[0080] According to the present invention, it is possible to provide a polyolefin-based multilayer film that has performance such as transparency and gas barrier properties similar to those of conventional films, and that has excellent straight running properties and airtight packaging properties even when packaged using a high-speed packaging machine. [Explanation of symbols]

[0081] 10 First surface layer (S1) 20 protrusions 21 particles

Claims

1. A polyolefin-based multilayer film having two or more polyolefin-based resin layers, The polyolefin-based resin layer includes a first surface layer (S1) containing a polyethylene-based resin and a second surface layer (S2) containing a polyolefin-based resin, When the surface of the first surface layer (S1) is observed, protrusion structures having a height of 30 nm or more are present at a density of 0.2 to 5.8 / 100 μm. ２ A polyolefin-based multilayer film characterized by having a film thickness in the range of

2. 2. The polyolefin multilayer film according to claim 1, wherein the polyolefin multilayer film has a tensile modulus, expressed as the product of MD (length direction) and TD (width direction), of 170,000 or more.

3. 3. The polyolefin multilayer film according to claim 1, wherein the protrusion-like structures are derived from spherulites made of a thermoplastic resin having a melting point of 125°C or higher.

4. 3. The polyolefin-based multilayer film according to claim 1, wherein the difference in methyl flow rate (MFR) between the thermoplastic resin constituting the protrusion-like structures and the polyethylene-based resin serving as the base resin of the first surface layer (S1) (MFR of S1 - MFR of protrusion-like structures) measured at a measurement temperature of 190°C and a load of 2.16 kg is 7 or less.

5. The polyolefin-based multilayer film according to claim 1 or 2, further comprising a substrate layer between the first surface layer (S1) and the second surface layer (S2).

6. 6. The polyolefin-based multilayer film according to claim 5, further comprising a first internal layer (I1) provided between the first surface layer (S1) and the base material layer, and a second internal layer (I2) provided between the second surface layer (S2) and the base material layer.

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