Biaxially oriented polypropylene film

Abstract

The present invention provides a biaxially oriented polypropylene film and packaging that suppress the drop in heat seal strength after peeling of the sealed portion, thereby enabling more stable peeling of the sealed portion. [Solution] The seal consists of at least two layers, including a base layer and a seal layer. The base layer is made of a resin material containing polypropylene resin and polyethylene resin. The polypropylene resin in the base layer is a mixture of a propylene homopolymer and a propylene-based elastomer with a melting point of 150°C or higher. When the seal layers are heat-sealed at 130°C, 0.35 MPa, and for 1 second, the heat seal strength is 2.0 N / 15 mm to 6.0 N / 15 mm, and the standard deviation of the heat seal strength is 0.45 N / 15 mm or less.

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film, and more particularly to a biaxially oriented polypropylene film for packaging that forms an easily openable heat-seal portion. [Background technology]

[0002] For example, in food packaging, stretched polypropylene film with excellent properties such as transparency and rigidity is preferably used. This packaging is manufactured by a bag-making method such as pillow packaging, in which the surfaces of the film are heat-sealed. In recent years, packaging for bread and snack foods sold in convenience stores and the like requires properties such as easy opening, which allows the sealed portion, such as the heat seal, to be easily peeled off when opening.

[0003] In packaging with easy opening at the sealed portion, examples of packaging films include, for example, a stretched polypropylene film having at least two layers including a sealing layer, wherein the sealing layer is composed of a mixed material of polypropylene resin and low-density polyethylene (see Patent Document 1).

[0004] In this polypropylene film, the sealing layer is composed of a polypropylene resin manufactured using a single-site catalyst and two types of low-density polyethylene resins with different melt viscosities. The immaturity of the polypropylene resin and polyethylene resin in the sealing layer causes cohesive failure due to a sea-island dispersion structure, resulting in easy opening. However, the above polypropylene film lacks transparency and was therefore unsuitable for food packaging films where high transparency is required.

[0005] Furthermore, as a film with good peelability and ease of opening, for example, a stretched polypropylene film containing an anti-fogging agent has been proposed, which has a base layer and a seal layer laminated to the base layer via an adhesive layer and an intermediate layer, wherein the seal layer is composed of a mixed material of a propylene-based random copolymer and a butene-based elastomer, and the intermediate layer is composed of a mixed material of an amorphous or low-crystalline ethylene-α-olefin copolymer and low-density polyethylene (see Patent Document 2).

[0006] In this polypropylene film, the intermediate layer of the four-layer structure is composed of 70-95% by weight of ethylene-α-olefin copolymer and 5-30% by weight of low-density polyethylene. When the seal portion is peeled off at a speed of 200 mm / min in the TD direction, the maximum heat seal strength is in the range of 2.5-4.5 N / 10 mm, and the heat seal strength at the point when the chuck has moved 10 mm is maintained at 60% or more of the maximum heat seal strength. However, although the heat seal strength at the point when the seal portion is peeled off is maintained at a certain percentage or more of the maximum heat seal strength when it reaches a certain distance, the property of large drops in heat seal strength after peeling has not been improved. Therefore, the stability during peeling is insufficient, and it is economically disadvantageous because it requires special production equipment for the four-layer structure.

[0007] Furthermore, to prevent peeling of the seal during transport, a film is provided that achieves both sealing strength and ease of peeling. For example, it comprises an outer layer containing a propylene polymer, an intermediate layer containing 5 to 35% by weight of an ethylene homopolymer and two or more propylene polymers, and a seal layer containing a propylene random copolymer with a melting point of 150°C or lower, wherein at least one of the propylene polymers in the intermediate layer is a propylene homopolymer or a propylene random copolymer, at least one of the propylene polymers is a propylene block copolymer, and the density of the ethylene homopolymer contained in the intermediate layer is 930 kg / m³. 3The following multilayer film has been proposed (see Patent Document 3). However, this multilayer film is based on the premise of lamination with a surface substrate, and when used as a film for food packaging, the processing costs are high, making it economically disadvantageous. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2021-095162 [Patent Document 2] Patent No. 7018794 [Patent Document 3] Patent No. 7243462 [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention has been proposed in view of the above points, and provides a low-cost biaxially oriented polypropylene film with excellent transparency that suppresses the drop in heat seal strength after peeling of the sealed portion, enabling more stable peeling of the sealed portion. [Means for solving the problem]

[0010] In other words, the first invention relates to a biaxially oriented polypropylene film for packaging, comprising at least two layers including a base layer and a seal layer mainly composed of polypropylene resin, wherein the seal layers are heat-fused together to form an easily openable heat-seal portion, wherein the base layer is composed of a resin material comprising polypropylene resin and polyethylene resin, the polypropylene resin of the base layer is a mixture of a propylene homopolymer and a propylene-based elastomer having a melting point of 150°C or higher, and the heat seal strength when the seal layers are heat-sealed at 130°C, 0.35 MPa, and for 1 second is 2.0 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the heat seal strength is 0.45 N / 15 mm or less.

[0011] The standard deviation of the heat seal strength was calculated from 12 points: the maximum heat seal strength and the heat seal strength at 1 mm intervals within a range where the chuck moved 5 mm to 15 mm, after heat sealing the seal layers together at 130°C, 0.35 MPa, and for 1 second, with a seal width of 10 mm in the MD direction and 15 mm in the TD direction. The test specimen was fixed in the chuck of a tensile testing machine and peeled off at a speed of 200 mm / min in the MD direction.

[0012] The second invention relates to a biaxially oriented polypropylene film in which, in the first invention, the base layer comprises 65 to 90% by weight of the propylene homopolymer, 5 to 15% by weight of the propylene-based elastomer, and 1 to 13% by weight of the polyethylene resin, wherein the proportion of the polyethylene resin is less than or equal to the proportion of the propylene-based elastomer.

[0013] The third invention is that, in the first or second invention, the polyethylene resin has an MFR of 0.01 to 10 g / 10 min and a density of 0.900 to 0.970 g / cm³, as measured under conditions of 190°C and a load of 2.16 kg in accordance with JIS K 7210. 3 This relates to a biaxially oriented polypropylene film.

[0014] The fourth invention relates to a biaxially oriented polypropylene film in which, in the first or second invention, the sealing layer is mainly composed of a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer.

[0015] The fifth invention relates to a biaxially oriented polypropylene film in which the sealing layer is mainly composed of a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer.

[0016] The sixth invention relates to a biaxially stretched polypropylene film in the first or second invention, wherein the haze of the biaxially stretched film is 15% or less.

[0017] The seventh invention relates to a biaxially stretched polypropylene film in the third invention, wherein the haze of the biaxially stretched film is 15% or less.

[0018] The eighth invention relates to a biaxially stretched polypropylene film in the fourth invention, wherein the haze of the biaxially stretched film is 15% or less.

[0019] The ninth invention relates to a biaxially stretched polypropylene film in the fifth invention, wherein the haze of the biaxially stretched film is 15% or less.

Advantages of the Invention

[0020] According to the biaxially stretched polypropylene film of the first invention, it is composed of at least two or more layers including a base material layer mainly composed of a polypropylene resin and a seal layer, and a heat-sealable heat-sealing part is formed by heat-sealing the seal layers together. The base material layer is composed of a resin material containing a polypropylene resin and a polyethylene resin. The polypropylene resin in the base material layer is a mixture of a propylene homopolymer and a propylene-based elastomer having a melting point of 150 °C or higher. When the seal layers are heat-sealed under the conditions of 130 °C, 0.35 MPa, and 1 second, the heat-sealing strength is 2.0 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the heat-sealing strength described below is 0.45 N / 15 mm or less. Therefore, stable easy-openability and excellent transparency can be obtained, and when used as a film for a package, lamination of a surface base material or the like is not required, and the processing cost can be reduced.

[0021] According to the biaxially stretched polypropylene film of the second invention, in the first invention, the propylene homopolymer is 65 to 90% by weight, the propylene-based elastomer is 5 to 15% by weight, and the polyethylene resin is 1 to 13% by weight. Since the blending ratio of the polyethylene resin is not more than the blending ratio of the propylene-based elastomer, a film excellent in stiffness and transparency can be obtained.

[0022] According to the package of the third invention, in the first or second invention, the polyethylene resin has an MFR of 0.01 to 10 g / 10 min and a density of 0.900 to 0.970 g / cm 3 measured under the conditions of 190 °C and a load of 2.16 kg in accordance with JIS K 7210, so that both easy opening property and moldability can be achieved.

[0023] According to the package of the fourth invention, in the first or second invention, the seal layer is mainly composed of a polypropylene-based copolymer composed of any one or a mixture of two or more of a propylene-ethylene copolymer, a propylene-ethylene-butene copolymer, and a propylene-1-butene copolymer. Therefore, due to the presence of low-melting ethylene and butene in the copolymerization, an appropriate heat seal strength is obtained, and a film excellent in packaging suitability can be obtained.

[0024] According to the package of the fifth invention, in the third invention, the seal layer is mainly composed of a polypropylene-based copolymer composed of any one or a mixture of two or more of a propylene-ethylene copolymer, a propylene-ethylene-butene copolymer, and a propylene-1-butene copolymer. Therefore, due to the presence of low-melting ethylene and butene in the copolymerization, an appropriate heat seal strength is obtained, and a film excellent in packaging suitability can be obtained.

[0025] According to the package of the sixth invention, in the first or second invention, since the haze of the biaxially stretched film is 15% or less, a film with a transparent feeling can be obtained.

[0026] According to the packaging of the seventh invention, in the third invention, since the haze of the biaxially oriented film is 15% or less, a transparent film can be made.

[0027] According to the packaging of the eighth invention, in the fourth invention, since the haze of the biaxially oriented film is 15% or less, a transparent film can be made.

[0028] According to the packaging of the ninth invention, in the fifth invention, since the haze of the biaxially oriented film is 15% or less, a transparent film can be made. [Brief explanation of the drawing]

[0029] [Figure 1] This is a chart showing the heat seal strength of a biaxially oriented polypropylene film according to one embodiment of the present invention. [Figure 2] This is a chart showing the heat seal strength of conventional biaxially oriented polypropylene films. [Modes for carrying out the invention]

[0030] A biaxially oriented polypropylene film according to one embodiment of the present invention is a biaxially oriented film for packaging, comprising at least two layers including a base layer and a sealing layer, the sealing layers being heat-fused together during the packaging process to form an easily openable heat-sealed section. The packaging made from this biaxially oriented polypropylene film is mainly used as packaging for food products, and is particularly suitable for packaging bread and snack foods.

[0031] In the biaxially oriented polypropylene film of the present invention, the resin material used in each of the layers described later is appropriately selected from resins produced from appropriate starting materials such as petroleum-derived, biomass-derived, material-recycled, and chemical-recycled materials.

[0032] The biaxially oriented polypropylene film of the present invention can be obtained by known film forming methods such as the T-die method or the inflation method. In particular, it is preferable that a sheet formed by the T-die method is stretched and then formed. Film forming by the T-die method has the advantage of being able to obtain high thickness-to-thinness accuracy.

[0033] Furthermore, the biaxially oriented polypropylene film is a biaxially oriented film that has been stretched in two axial directions: the longitudinal (MD) direction and the transverse (TD) direction of the film. Both sequential biaxial stretching and simultaneous biaxial stretching are well used. Because the formation of a biaxially oriented film results in resin orientation in both the longitudinal (MD) and transverse (TD) directions, it is possible to improve the accuracy of thickness, such as thinness, and mechanical properties such as strength, and it is also excellent for mass production. The stretching ratio is approximately 2 to 8 times in the longitudinal (MD) direction and 4 to 12 times in the transverse (TD) direction.

[0034] The base layer is formed with a thicker layer thickness compared to other layers and is the main layer of the biaxially oriented polypropylene film, defining the basic properties of the film such as its rigidity. It is composed of a resin material containing polypropylene resin and polyethylene resin.

[0035] The sealing layer is the inner layer of the bag after it has been manufactured, and it has properties such as low-temperature sealing ability. This sealing layer is formed by heat fusion of the sealing layers when, for example, biaxially oriented polypropylene film is manufactured into a bag, thereby forming a heat-sealed portion. The sealing layer is composed of a resin material mainly composed of polypropylene resin.

[0036] The biaxially oriented polypropylene film of the present invention is formulated by combining a base layer composed of a resin material containing polypropylene resin and polyethylene resin, and a seal layer composed of a resin material mainly composed of polypropylene resin. In this case, since the resin material constituting the base layer contains polypropylene resin and polyethylene resin, which are incompatible, a sea-island structure is formed in the base layer. When the seal layers are heat-sealed to form a heat-sealed portion, the sea-island structure in the base layer causes delamination between the base layer and the seal layer, resulting in easy opening. Therefore, in the biaxially oriented polypropylene film of the present invention, the heat seal strength when the seal layers are heat-sealed under the conditions of 130°C, 0.35 MPa, and 1 second is set to be 2.0 N / 15 mm or more and 6.0 N / 15 mm or less.

[0037] Heat seal strength is one of the indicators of processability, such as ease of opening and resistance to tearing, and is measured in accordance with JIS Z 0238 (1998). If the heat seal strength is too low, the heat fusion between the seal layers will be insufficient, which may cause the package to tear easily when used. If the heat seal strength is too high, the seal layers may adhere more tightly than necessary, which may prevent easy opening. By having an appropriate heat seal strength, the heat-sealed area will have resistance to tearing, and peeling will begin easily, resulting in good easy opening.

[0038] Furthermore, the biaxially oriented polypropylene film of the present invention has a standard deviation of heat seal strength of 0.45 N / 15 mm or less. This standard deviation of heat seal strength is calculated from 12 points, which are obtained by heat-sealing two seal layers together at 130°C, 0.35 MPa, and for 1 second, fixing a test piece with a seal width of 10 mm in the MD direction and 15 mm in the TD direction in the chuck of a tensile testing machine, and peeling it off at a speed of 200 mm / min in the MD direction, and summing the maximum heat seal strength and the heat seal strength at 1 mm intervals within the range where the chuck moves from 5 mm to 15 mm.

[0039] Figure 1 is an example of a chart showing the heat seal strength when the heat-sealed portion of the biaxially oriented polypropylene film of the present invention is peeled off under the above conditions. As shown in the figure, in the biaxially oriented polypropylene film of the present invention, peeling of the heat-sealed portion begins near the maximum heat seal strength indicated by the symbol P1, and then the rapid decrease in heat seal strength is suppressed, and the heat seal strength remains almost constant, indicating that the standard deviation of the heat seal strength is small.

[0040] On the other hand, Figure 2 is an example of a heat seal strength chart when the heat seal portion of a conventional biaxially oriented polypropylene film is peeled off under the above conditions. As shown in the figure, in the conventional biaxially oriented polypropylene film, immediately after peeling of the heat seal portion begins near the maximum heat seal strength indicated by symbol P2, the heat seal strength decreases sharply and the overall waveform changes in a concave shape, indicating that the standard deviation of the heat seal strength is large.

[0041] If the standard deviation of the heat seal strength is too large, the stability of the heat seal strength during peeling will be insufficient, resulting in large variations in heat seal strength after peeling or large drops in heat seal strength after peeling. Therefore, it may be difficult to peel the heat-sealed area stably. On the other hand, if the standard deviation of the heat seal strength is small, specifically 0.45 N / 15 mm or less, the drop in heat seal strength after peeling of the heat-sealed area will be suppressed, and the heat-sealed area can be peeled continuously and stably. Thus, since the standard deviation of the heat seal strength is calculated based on the maximum heat seal strength and the heat seal strength at multiple points over a predetermined range of movement distance after the start of peeling of the heat-sealed area, it can be used as an indicator of the stability of the heat seal strength during peeling of the heat-sealed area.

[0042] In biaxially oriented polypropylene films, a suitable resin material is selected that satisfies the heat seal strength requirements for achieving the above-mentioned easy-open properties, by combining a resin material containing polypropylene resin and polyethylene resin for the base layer and a resin material mainly composed of polypropylene resin for the seal layer.

[0043] The polypropylene resin of the base layer is a mixture of a propylene homopolymer and a propylene-based elastomer with a melting point of 150°C or higher. The propylene homopolymer (homopolypropylene) has high crystallinity and excellent heat resistance, chemical resistance, and strength, which enhances the rigidity and processability of the film. The melt flow rate (MFR) of the propylene homopolymer is not particularly limited, but for example, the MFR measured under conditions of 230°C and a load of 2.16 kg in accordance with JIS K 7210 is preferably 1.5 to 10 g / 10 min, and more preferably 2 to 8 g / 10 min. If the MFR is too low, the resin pressure in the extruder will increase, putting a load on the extruder and potentially impairing productivity due to poor thickness accuracy. If the MFR is too high, the high fluidity of the resin may make it difficult to mold into a sheet and may cause problems with stretchability. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0044] Propylene elastomers have high compatibility with propylene homopolymers, and mixing them can improve the strength and transparency of the film. Furthermore, because propylene elastomers have a high amorphous content, they provide flexibility to the film, mitigating the unavoidable stretching of the film after stretching. For example, in packaging with a heat-sealed portion, they can relieve the stress acting on the heat-sealed portion. Additionally, their flexibility improves the impact resistance of the film, mitigating the impact when the heat-sealed portion is peeled off. Therefore, tearing of the base layer can be prevented, and easy opening can be appropriately provided. The propylene elastomer mixed with the propylene homopolymer is preferably one with a melting point of 150°C or higher. If a propylene elastomer with a low melting point is used, mixing it with the propylene homopolymer may not adequately improve strength or transparency.

[0045] While the melt flow rate (MFR) of propylene-based elastomers is not particularly limited, it is preferable, and more preferably, 2 to 10 g / 10 min, when measured under conditions of 230°C and a load of 2.16 kg, as compliant with JIS K 7210. If the MFR is too low, the resin pressure inside the extruder will increase, putting a load on the extruder and potentially impairing productivity due to poor thickness accuracy. If the MFR is too high, the high fluidity of the resin may make it difficult to mold into a sheet and may cause problems with stretchability. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0046] Furthermore, while the density of propylene-based elastomers is not particularly limited, for example, 0.860~0.880 g / cm³ 3 Preferably, it is 0.865 to 0.870 g / cm³. 3 That is the case.

[0047] The polyethylene resin used in the base layer is not particularly limited in type, but from the viewpoint of achieving both easy opening and moldability, it is preferable to adjust the melt flow rate (MFR) and density to a predetermined range.

[0048] The MFR of polyethylene resin is preferably 0.01 to 10 g / 10 min, and more preferably 0.04 to 5.0 g / 10 min, as measured under conditions of 190°C and a load of 2.16 kg in accordance with JIS K 7210. If the MFR is too high, the high fluidity may make it difficult to form a sea-island structure, potentially resulting in poor openability. If the MFR is too low, the extremely poor fluidity may lead to poor mixing with other resins, potentially causing molding defects such as frequent fisheye formation.

[0049] Furthermore, polyethylene resin has a density of 0.900 to 0.970 g / cm³. 3 Preferably, it is 0.910 to 0.960 g / cm³. 3 If the density is too high, easy opening may not be achieved. If the density is too low, transparency may be reduced.

[0050] The type of polyethylene resin used in the base layer is not particularly limited, but it is preferable to select from one or more types of polyethylene resins, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE, LLDPE), or a mixture of two or more types. In particular, when using a mixture of two or more types of polyethylene resin, the MFR and density of the polyethylene resin used in the base layer can be easily adjusted by mixing multiple types of resins with different MFRs and densities. The MFR and density when multiple types of polyethylene resins are mixed are values ​​calculated from the MFR and density of each resin and their respective mixing ratios (calculated MFR, calculated density).

[0051] The polypropylene resin used in the sealing layer is preferably composed of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer. By using these resins, the presence of copolymerized low-melting-point ethylene and butene provides appropriate heat seal strength, resulting in a film with excellent packaging suitability.

[0052] For polypropylene resins used in sealing layers, the melt flow rate (MFR) is not particularly limited, but preferably, the MFR measured under conditions of 230°C and a load of 2.16 kg in accordance with JIS K 7210 is 0.1 to 20 g / 10 min, and more preferably 1.0 to 10 g / 10 min. If the MFR is too low, the resin pressure in the extruder will increase, putting a load on the extruder and potentially impairing productivity due to poor thickness accuracy. If the MFR is too high, the high fluidity of the resin may make it difficult to mold into a sheet. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0053] In the biaxially oriented polypropylene film of the present invention, in addition to the above-mentioned stable and easy-to-open properties, in order to provide a film suitable for food packaging that is flexible, has excellent rigidity, and is highly transparent, the polypropylene resin of the base layer is a mixture of a propylene homopolymer and a propylene-based elastomer with a melting point of 150°C or higher. Particularly, it is preferable that the proportion of propylene homopolymer (A) is greater than the proportion of propylene-based elastomer (B), and that the proportion of propylene homopolymer (A) is greater than the proportion of polyethylene resin (C) (A>B, A>C). It is more preferable that the film contains 65-90% by weight of propylene homopolymer (A), 5-15% by weight of propylene-based elastomer (B), and 1-13% by weight of polyethylene resin (C), with the proportion of polyethylene resin (C) being less than or equal to the proportion of propylene-based elastomer (B) (B≧C).

[0054] If the proportion of propylene homopolymer in the base layer is too low, the film strength may be insufficient. If the proportion of propylene elastomer is too low, easy opening properties may not be achieved and transparency may be insufficient. If the amount of polyethylene resin is too low, easy opening properties may not be achieved. It is believed that stable easy opening properties and excellent transparency can be obtained by combining a specific formulation of the base layer with the constituent materials of the seal layer.

[0055] Furthermore, when compounding the base layer, if the polyethylene resin is a mixture of high-density polyethylene and low-density polyethylene, it is preferable to use 3 to 10% by weight of high-density polyethylene and 1 to 3% by weight of low-density polyethylene. High-density polyethylene tends to have a relatively large effect in increasing transparency and a relatively small effect in increasing ease of opening. On the other hand, low-density polyethylene tends to have a relatively large effect in increasing ease of opening and a relatively small effect in increasing transparency. Based on these considerations, by mixing high-density polyethylene and low-density polyethylene within the appropriate range described above, it is possible to easily achieve both ease of opening and transparency.

[0056] The transparency of biaxially oriented polypropylene film is evaluated, for example, by its haze value. Haze (%) is an indicator of the film's cloudiness and is measured in accordance with JIS K 7136 (2000). A lower value indicates higher transparency. The preferred transparency of the biaxially oriented polypropylene film of the present invention is a haze value of 15% or less. If the haze is too high, the transparency will be insufficient, and the visibility required for packaging food and other products may be impaired. By having an appropriate haze value, a transparent film can be made, which is particularly suitable as a film for packaging food and other products.

[0057] Furthermore, in the biaxially oriented polypropylene film of the present invention, it is preferable that the sum of the elastic modulus in the longitudinal (MD) direction and the elastic modulus in the transverse (TD) direction (MD+TD) is 3.0 GPa or higher, and more preferably that the tensile modulus in the longitudinal (MD) direction is 1.0 GPa or higher and the tensile modulus in the transverse (TD) direction is 2.0 GPa or higher. The tensile modulus (GPa) is used as an indicator of the stiffness of the film and is measured based on the test method for plastic tensile properties in accordance with JIS K 7127 (1999). If the tensile modulus is insufficient, the stiffness will be insufficient, and the film may have poor processing suitability, such as being prone to deformation. By having an appropriate tensile modulus, sufficient stiffness is ensured, the stiffness is increased, and good processing suitability is obtained.

[0058] In the biaxially oriented polypropylene film of the present invention, a surface layer may be laminated on the side of the base layer opposite to the seal layer, if necessary. The surface layer is the outer layer of the package after bag making and protects the biaxially oriented polypropylene film. The surface layer is preferably composed of a resin composition mainly consisting of a propylene polymer from the viewpoint of heat resistance, chemical resistance, strength, etc. The polypropylene resin used for the surface layer is selected from, for example, at least one of propylene homopolymer, propylene-α-olefin random copolymers such as propylene-ethylene random copolymer or propylene-ethylene-butene random copolymer, or propylene-ethylene block copolymer. In particular, it is preferable to use a propylene homopolymer as the main component from the viewpoint of rigidity, etc.

[0059] The polypropylene resin used for the surface layer does not have any particular limitations on its melt flow rate (MFR). However, from the viewpoint of extrusion moldability, for example, the MFR measured under conditions of 230°C and a load of 2.16 kg in accordance with JIS K 7210 is preferably 1 to 15 g / 10 min, more preferably 1.5 to 10 g / 10 min, and even more preferably 2.0 to 8 g / 10 min. If the MFR is too low, the resin pressure inside the extruder will increase, putting a load on the extruder and potentially impairing productivity due to poor thickness accuracy. If the MFR is too high, the high fluidity of the resin may make it difficult to mold into a sheet. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0060] This surface layer can be subjected to surface treatments such as printing as needed. In this case, surface treatments such as corona treatment may be applied to the surface to improve the printability of the film surface.

[0061] In the biaxially oriented polypropylene film of the present invention, various additives such as antiblocking agents, lubricants, antistatic agents, antifogging agents, heat stabilizers, antioxidants, light stabilizers, crystal nucleating agents, neutralizing agents, colorants, and other scraps can be added to each layer as needed, within the limits that do not impair the objectives of the present invention. These additives may be added directly to the powder after polymerization of each resin, or they may be mixed in any step from preparing a high-concentration masterbatch to obtaining the film. When using a masterbatch, a small amount of resin may be unintentionally incorporated, but this can be used within the limits that do not impair the properties of each layer.

[0062] As described above, the biaxially oriented polypropylene film of the present invention is formulated by combining a base layer made of a resin material containing a mixture of a propylene homopolymer and a propylene-based elastomer with a melting point of 150°C or higher, and polyethylene resin, with a seal layer made of a resin material mainly composed of polypropylene resin. The heat seal strength is 2.0 to 6.0 N / 15 mm, and the standard deviation of the heat seal strength is 0.45 N / 15 mm or less, resulting in stable easy opening, as well as flexibility, strong rigidity, and excellent transparency. Furthermore, when this biaxially oriented polypropylene film is used as a packaging film, lamination of the surface material, etc., is unnecessary, reducing processing costs compared to conventional products and making it economically advantageous.

[0063] Furthermore, the biaxially oriented polypropylene film of the present invention can be used to produce food packaging with good ease of opening by manufacturing it using known bag-making methods such as pillow packaging. In particular, since the difference in heat seal strength is suppressed when peeling (opening) the heat-sealed portion, the heat-sealed portion can be peeled off continuously and stably, and the film does not tear or the contents spill out when opened, making it easy to open. [Examples]

[0064] [Fabrication of biaxially oriented polypropylene film] For prototypes 1 to 10, each material described below was fed into three extruders, kneaded and melted, and the layers were set to be laminated in the order of surface layer, base layer, and seal layer. The films were co-extruded from a three-layer T-die set to 200-250°C, cooled and solidified on a cooling roll at 50°C to obtain a raw material sheet. Next, the raw material sheet was stretched five times in the MD direction on a heating roll set to a temperature of 110-130°C, then preheated in a tenter to a temperature of 180-190°C, stretched eight times in the TD direction at a stretching temperature of 160-170°C, and annealed at a temperature of 165-175°C. After that, the surface layer was subjected to corona discharge treatment, and the film was wound on a winding machine to obtain a biaxially oriented polypropylene film.

[0065] [Materials used] As the resin material for each layer, the following resins were used. As the characteristics of each resin, the melt flow rate (MFR) complied with JIS K 7210 (2014), and the values measured at 230°C and 2.16 kg for the polypropylene resin and at 190°C and 2.16 kg for the ethylene resin were adopted. Also, the melting point complied with the measurement of differential scanning calorimetry (DSC) of JIS K 7121 (2012). Using a differential scanning calorimeter (manufactured by Netzsch Japan Co., Ltd.; "DSC 214 Polyma"), the melting peak temperature was determined from the DSC curve obtained when the temperature was raised at a heating rate of 10°C / min and taken as the melting point. When multiple peaks could be confirmed, the highest melting peak temperature was taken as the melting point. In Prototype Examples 1 to 10, the blending ratio of the resin for each layer was blended so that it would be 100% by weight, and additives such as an anti-blocking agent were omitted. The resin compositions of each layer in Prototype Examples 1 to 10 are shown in Tables 1 and 2 described later.

[0066] [[ID=]3] <Polypropylene resin> ·PP1: Polypropylene homopolymer, MFR (230°C / 2.16 kg) 3.5 g / 10 min, melting point 164°C ·PP2: Polypropylene homopolymer, MFR (230°C / 2.16 kg) 7.3 g / 10 min, containing 8% of an antistatic agent component ·PP3: Polypropylene-based elastomer, MFR (230°C / 2.16 kg) 6.0 g / 10 min, melting point 160°C, density 0.868 g / cm 3 ·PP4: Propylene-ethylene-butene random copolymer, MFR (230°C / 2.16 kg) 5.0 g / 10 min, melting point 125°C ·PP5: Propylene-1-butene random copolymer, MFR (230°C / 2.16 kg) 7.0 g / 10 min, melting point 75°C

[0067] <Ethylene resin><0000​​​PE2: High-density polyethylene, MFR (190℃ / 2.16kg) 0.05g / 10min, melting point 132℃, density 0.950g / cm³ 3 PE3: Low-density polyethylene, MFR (190℃ / 2.16kg) 0.35g / 10min, melting point 112℃, density 0.920g / cm³ 3

[0068] [Prototype Example 1] Prototype Example 1 is a biaxially oriented polypropylene film in which the surface layer is composed of PP1:100% by weight, the base layer is composed of propylene resin (PP1:69% by weight, PP2:5% by weight, and PP3:14% by weight) and ethylene resin (PE1:6% by weight, PE2:4% by weight, and PE3:2% by weight), and the sealing layer is composed of PP4:84% by weight and PP5:16% by weight. The layer ratio of each layer in this film (surface layer / base layer / sealing layer) was 1 / 20 / 1, and the thickness was 22 μm.

[0069] [Prototype Example 2] Prototype Example 2 is a biaxially oriented polypropylene film that differs from Prototype Example 1 in that the ethylene-based resin is not incorporated into the base layer, and the propylene-based resin is changed to PP1:81% by weight, PP2:5% by weight, and PP3:14% by weight, while the rest of the composition remains the same. The layer ratio of each layer in this film (surface layer / base layer / seal layer) was 1 / 22 / 1, and the thickness was 24 μm.

[0070] [Prototype Example 3] Prototype Example 3 is a biaxially oriented polypropylene film in which the propylene resin of the base layer has been changed from that of Prototype Example 1 to PP1:76% by weight, PP2:5% by weight, and PP3:7% by weight, while the rest of the composition remains the same. The layer ratio of each layer of this film (surface layer / base layer / seal layer) was 1 / 21 / 1, and the thickness was 23 μm.

[0071] [Prototype Example 4] Prototype Example 4 is a biaxially oriented polypropylene film in which the propylene resin in the base layer is changed from Prototype Example 1 to PP1:75% by weight, PP2:5% by weight, and PP3:14% by weight, and the ethylene resin in the base layer is changed to PE1:5% by weight and PE3:1% by weight, while all other aspects of the composition remain the same. The layer ratio of each layer in this film (surface layer / base layer / seal layer) is 1 / 18 / 1, and the thickness is 20 μm.

[0072] [Prototype Example 5] Prototype Example 5 is a biaxially oriented polypropylene film in which the ethylene-based resin in the base layer of Prototype Example 4 was changed to PE1:3 wt%, PE2:2 wt%, and PE3:1 wt%, while the rest of the composition remained the same. The layer ratio of each layer in this film (surface layer / base layer / seal layer) was 1 / 17 / 1, and the thickness was 19 μm.

[0073] [Prototype Example 6] Prototype Example 6 is a biaxially oriented polypropylene film in which the propylene resin of the base layer was changed from that of Prototype Example 5 to PP1:82% by weight, PP2:5% by weight, and PP3:7% by weight, while the rest of the composition remained the same. The layer ratio of each layer of this film (surface layer / base layer / seal layer) was 1 / 21 / 1, and the thickness was 23 μm.

[0074] [Prototype Example 7] Prototype Example 7 is a biaxially oriented polypropylene film in which the propylene resin in the base layer was changed from Prototype Example 1 to PP1:76% by weight, PP2:5% by weight, and PP3:14% by weight, and the ethylene resin in the base layer was changed to PE1:5% by weight, while all other aspects of the formulation remained the same. The layer ratio (surface layer / base layer / seal layer) of this film was 1 / 18 / 1, and the thickness was 20 μm.

[0075] [Prototype Example 8] Prototype Example 8 is a biaxially oriented polypropylene film in which the propylene resin in the base layer is changed from Prototype Example 1 to PP1:80% by weight, PP2:5% by weight, and PP3:14% by weight, and the ethylene resin in the base layer is changed to PE3:1% by weight, while all other aspects of the composition remain the same. The layer ratio of each layer in this film (surface layer / base layer / seal layer) is 1 / 18 / 1, and the thickness is 20 μm.

[0076] [Prototype Example 9] Prototype Example 9 is a biaxially oriented polypropylene film that differs from Prototype Example 1 in that the substrate layer does not contain ethylene-based resin, and the propylene-based resin is changed to PP1:90% by weight and PP2:10% by weight, and the seal layer is changed to PP4:100% by weight, while all other aspects of the composition remain the same. The layer ratio of each layer in this film (surface layer / substrate layer / seal layer) is 1 / 18 / 1, and the thickness is 20 μm.

[0077] [Prototype Example 10] Prototype 10 is a biaxially oriented polypropylene film in which the propylene resin of the base layer was changed from that of Prototype 4 to PP1:89% by weight and PP2:5% by weight, while the rest of the composition remained the same. The layer ratio of each layer of this film (surface layer / base layer / seal layer) was 1 / 18 / 1, and the thickness was 20 μm.

[0078] [Table 1]

[0079] [Table 2]

[0080] For the biaxially oriented polypropylene films of prototypes 1 to 10, the density (calculated density) and MFR (calculated MFR) of the ethylene resin were calculated, and the haze, tensile modulus, and heat seal strength were measured. The standard deviation of the heat seal strength was also determined. Furthermore, heat-sealed sections were formed using the films of prototypes 1 to 10, and the ease of opening the heat-sealed sections was evaluated. The results of each test are shown in Tables 3 and 4 below.

[0081] [MFR of ethylene-based resins (calculated MFR)] For the biaxially oriented polypropylene films of prototype examples 1, 3 to 8 and 10, the calculated MFR (g / 10min) of the ethylene resin in the base layer was determined. The calculated MFR of the ethylene resin was obtained from the MFR and blending ratio of each ethylene resin (PE1, PE2, PE3) contained in the base layer of each prototype example. Prototype examples 2 and 9 were omitted as they do not contain ethylene resin.

[0082] [Density of ethylene-based resins (calculated density)] For the biaxially oriented polypropylene films of prototype examples 1, 3 to 8, and 10, the calculated density of the ethylene resin in the substrate layer (g / cm³) 3 The calculated density of the ethylene resin was determined from the density and blending ratio of each ethylene resin (PE1, PE2, PE3) contained in the substrate layer of each prototype example. Prototype examples 2 and 9 were omitted because they do not contain ethylene resin.

[0083] [Measurement of haze] Haze (%) measurement is an indicator of transparency and was performed in accordance with JIS K 7136 (2000) using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd.; "NDH-5000"). For prototype examples 1 to 10 films, a measurement result of 15.0% or less was classified as "Good (〇)" and an amount exceeding 15.0% was classified as "Poor (×)".

[0084] [Tensile modulus of elasticity] For prototype examples 1-10, the tensile modulus (GPa) was measured in accordance with JIS K 7127 (1999). A tensile testing machine (manufactured by A&D Co., Ltd.; "Tensilon Universal Material Testing Machine RTF-1310") was used to measure the tensile modulus in two directions: the longitudinal (MD) direction corresponding to the winding direction of each film, and the transverse (TD) direction perpendicular to it. The sum of the tensile moduli in the longitudinal (MD) and transverse (TD) directions (GPa) was then calculated. In the evaluation of the tensile modulus, a film was judged as "Good (○)" if it met all of the following conditions: a tensile modulus in the longitudinal (MD) direction of 1.0 GPa or higher, a tensile modulus in the transverse (TD) direction of 2.0 GPa or higher, and a sum of the tensile moduli in the longitudinal (MD) and transverse (TD) directions (MD+TD) of 3.0 GPa or higher. A film was judged as "Unacceptable (×)" if any of these conditions were not met.

[0085] [Measurement of heat seal strength] The heat seal strength (N / 15mm) was measured as follows. For prototype examples 1 to 10, two films were cut to 200mm in the MD direction and 50mm in the TD direction. The sealing layers were placed together, and a thermal gradient tester (Toyo Seiki Seisakusho Co., Ltd.; "HG-100-2") was used to heat seal each film under multiple heat seal temperature conditions, with a seal width of 10mm in the MD direction and 25mm in the TD direction, a seal pressure of 0.35MPa, and a seal time of 1 second. The heat seal temperature conditions ranged from 110°C to 130°C in 5°C increments. After heat sealing, a test piece was cut out so that the seal width was 10mm in the MD direction and 15mm in the TD direction. The heat-sealed portion of the test piece was in the center, and both ends were opened 180° to face the opposite direction. Both ends were fixed with chucks on a tensile tester (Shimadzu Corporation; "AGS-X"), and the peel strength was measured by peeling at a speed of 200mm / min in the MD direction. The maximum strength during peeling at each heat sealing temperature was defined as the heat seal strength. In the evaluation of heat seal strength, a measurement result of 6.0 N / 15 mm or less at a heat sealing temperature of 130°C was classified as "Good (〇)", and a result exceeding 6.0 N / 15 mm was classified as "Poor (×)".

[0086] [Standard deviation of heat seal strength] For prototypes 1-10, test specimens were prepared using the same procedure as for the heat-seal strength measurements described above. The heat-seal conditions were set to a seal width of 10 mm in the MD direction and 25 mm in the TD direction, a seal temperature of 130°C, a seal pressure of 0.35 MPa, and a seal time of 1 second. The heat-sealed portion of the test specimen was placed in the center, and both ends were opened 180° to face the opposite direction. Both ends were fixed with chucks on a tensile testing machine (Shimadzu Corporation; "AGS-X"), and the specimens were peeled off at a speed of 200 mm / min in the MD direction to create a chart of the heat-seal strength of prototypes 1-10.

[0087] From the heat seal strength charts of the 10 prototype films, the maximum strength within the range from the start to the end of the test was read and defined as the maximum heat seal strength (N / 15mm) for prototypes 1-10. Furthermore, from the heat seal strength charts of the 10 prototype films, the heat seal strength at 1mm intervals (5mm, 6mm, 7mm, ..., 14mm, 15mm) within the range where the chuck moved 5mm to 15mm was read, and the standard deviation was calculated from these 12 points, including the maximum heat seal strength. The test was performed 10 times, and the average of the calculated standard deviations was used as the standard deviation (N / 15mm) of the heat seal strength for prototypes 1-10.

[0088] [Easy opening of the heat-sealed portion] For prototype examples 1 to 10, two films were cut to 200 mm in the MD direction and 50 mm in the TD direction. The sealing layers were aligned, and a thermal gradient tester (Toyo Seiki Seisakusho Co., Ltd.; "HG-100-2") was used to heat-seal the films under the following conditions: sealing width 10 mm in the MD direction and 25 mm in the TD direction, sealing temperature 130°C, sealing pressure 0.35 MPa, and sealing time 1 second. Test specimens with a heat-sealed portion were obtained. Next, the testers grasped the area near the heat-sealed portion of the obtained test specimen and pulled in the opposite direction to peel off the heat-sealed portion and evaluate the ease of opening. For the evaluation of the ease of opening of the heat-sealed portion, it was judged as "Good (○)" if the heat-sealed portion could be easily peeled off with a nearly constant force and the base layer was delaminated without breakage, and as "Poor (×)" if the base layer broke and the heat-sealed portion could not be properly peeled off.

[0089] [Table 3]

[0090] [Table 4]

[0091] [Results and Discussion] Prototypes 1-10 are biaxially oriented polypropylene films in which the surface layer, base layer, and seal layer are mainly composed of polypropylene resin. As can be seen from Tables 3 and 4, prototypes 1-10 all exhibited good rigidity (tensile modulus) and heat seal strength.

[0092] First, we compare prototypes 1, 2, and 9. Prototype 1 is an example where the base layer is a mixture of polypropylene resin (propylene homopolymer and polypropylene elastomer) and polyethylene resin; prototype 2 is an example where the base layer is polypropylene resin (propylene homopolymer and polypropylene elastomer) but does not contain polyethylene resin; and prototype 9 is an example where the base layer is propylene homopolymer and does not contain polypropylene elastomer or polyethylene resin. Prototype 1 showed good transparency and ease of opening. On the other hand, prototypes 2 and 9 showed good transparency but insufficient ease of opening. Regarding ease of opening, the standard deviation of the heat seal strength of the film in prototype 1 was extremely small compared to prototypes 2 and 9.

[0093] The standard deviation of heat seal strength represents the degree of variation in heat seal strength during peeling, that is, the stability of heat seal strength during peeling. Therefore, it can be said that the film of prototype example 1 has the property that the heat seal strength during peeling does not vary significantly from the maximum strength. On the other hand, it can be said that the films of prototype examples 2 and 9 have the property that the heat seal strength during peeling varies relatively significantly from the maximum strength. Specifically, in the films of prototype examples 2 and 9, the peeling interface was unstable, cracks formed in the substrate layer, and the film eventually tore. In contrast, with the film of prototype example 1, peeling of the heat seal portion was easy to initiate, and it was possible to continue peeling smoothly with almost the same force as when opening it.

[0094] These findings indicate that in biaxially oriented polypropylene films, stable and good ease of opening can be achieved by including an appropriate amount of polyethylene resin in the base layer. This is thought to be because the inclusion of polyethylene resin, which is incompatible with polypropylene resin, in the base layer, which is mainly composed of polypropylene resin, forms a sea-island structure within the base layer, resulting in the ease of opening.

[0095] Furthermore, comparing prototype example 4 and prototype example 10, prototype example 4 is an example where the material is a mixture of polypropylene resin (propylene homopolymer and polypropylene-based elastomer) and polyethylene resin, while prototype example 10 is an example where the base layer is a mixture of polypropylene resin (propylene homopolymer) and polyethylene resin, and the polypropylene resin does not contain polypropylene-based elastomer.

[0096] In prototype example 4, both transparency and ease of opening were good. On the other hand, in prototype example 10, transparency was good, but ease of opening was insufficient. In particular, in the film of prototype example 10, the heat seal strength was lower than that of prototype example 4, making it easy to initiate peeling of the heat-sealed portion. However, the standard deviation of the heat seal strength was extremely large compared to prototype example 4, and it was not possible to continue peeling smoothly with almost the same force after the start of peeling. From these results, it was shown that even if the base layer contains both polypropylene resin and polyethylene resin, it is necessary to include a polypropylene-based elastomer in the polypropylene resin in order to obtain stable ease of opening.

[0097] We will compare prototype example 1 with prototype example 3, and further compare prototype example 5 with prototype example 6. Prototype example 3 is an example in which the proportion of polypropylene elastomer was reduced from prototype example 1. The heat seal strength and standard deviation of the heat seal strength were almost the same as prototype example 1, and stable easy opening properties were obtained, but the haze was higher than that of prototype example 1, resulting in insufficient transparency. On the other hand, prototype example 6 is an example in which the proportion of polypropylene elastomer was reduced from prototype example 5. The heat seal strength and standard deviation of the heat seal strength were almost the same as prototype example 5, and stable easy opening properties were obtained, and transparency was also good.

[0098] Comparing prototypes 1 and 3 with prototypes 5 and 6, prototypes 5 and 6 differ in that the proportion of polyethylene resin is lower than that of prototypes 1 and 3. Furthermore, in prototypes 3 and 6, which are examples where the proportion of polypropylene elastomer is reduced, prototype 3 differs in that the proportion of polypropylene elastomer is lower than that of polyethylene resin, while prototype 6 differs in that the proportion of polypropylene elastomer is higher than that of polyethylene resin. Thus, from the viewpoint of transparency, it is preferable for the polyethylene resin in the base layer to be relatively low, and for the polypropylene elastomer in the base layer to be higher than that of polyethylene resin.

[0099] Comparing prototype example 7 and prototype example 8, prototype example 7 is an example where the ethylene-based resin of the base layer is high-density polyethylene (5% by weight), while prototype example 8 is an example where the ethylene-based resin of the base layer is low-density polyethylene (1% by weight). Since both prototype examples 7 and 8 exhibit stable easy-opening properties and good transparency, it was demonstrated that it is possible to use either high-density polyethylene or low-density polyethylene as the ethylene-based resin of the base layer. [Industrial applicability]

[0100] The biaxially oriented polypropylene film of the present invention is a biaxially oriented film for packaging in which a heat-seal portion is formed. When peeling (opening) the heat-seal portion, the drop in heat-seal strength is suppressed, and the heat-seal portion can be peeled continuously and stably, resulting in good easy opening of the heat-seal portion. In addition, it has excellent transparency, and when used as a film for packaging, lamination of surface materials, etc., is unnecessary, reducing processing costs. For this reason, it is a promising alternative to conventional biaxially oriented polypropylene films for packaging. [Explanation of symbols]

[0101] P1 Maximum heat seal strength of the film of the present invention P2 Maximum heat seal strength of conventional films

Claims

1. A biaxially oriented film for packaging, comprising at least two layers including a base layer and a sealing layer mainly composed of polypropylene resin, wherein the sealing layers are heat-fused together to form an easily openable heat-sealed portion, The aforementioned substrate layer is composed of a resin material containing polypropylene resin and polyethylene resin. The polypropylene resin of the substrate layer is a mixture of a propylene homopolymer and a propylene-based elastomer with a melting point of 150°C or higher. The heat seal strength obtained when the aforementioned sealing layers are heat-sealed at 130°C, 0.35 MPa, and for 1 second is 2.0 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the aforementioned heat seal strength is 0.45 N / 15 mm or less. A biaxially oriented polypropylene film characterized by the following features. Standard deviation of heat seal strength: The standard deviation was calculated from 12 points, which consist of the maximum heat seal strength and the heat seal strength at 1 mm intervals within a range of 5 mm to 15 mm of movement of the chuck of a tensile testing machine, when a test piece with a seal width of 10 mm in the MD direction and 15 mm in the TD direction was fixed to the chuck of a tensile testing machine and peeled off at a speed of 200 mm / min in the MD direction.

2. The biaxially oriented polypropylene film according to claim 1, wherein the base layer comprises 65 to 90% by weight of the propylene homopolymer, 5 to 15% by weight of the propylene-based elastomer, and 1 to 13% by weight of the polyethylene resin, wherein the proportion of the polyethylene resin is less than or equal to the proportion of the propylene-based elastomer.

3. The polyethylene resin, as measured under conditions of 190°C and a load of 2.16 kg in accordance with JIS K 7210, has an MFR of 0.01 to 10 g / 10 min and a density of 0.900 to 0.970 g / cm³. ３ The biaxially oriented polypropylene film according to claim 1 or 2.

4. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the sealing layer is mainly composed of a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1 / butene copolymer.

5. The biaxially oriented polypropylene film according to claim 3, wherein the sealing layer is mainly composed of a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1 / butene copolymer.

6. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the haze of the biaxially oriented film is 15% or less.

7. The biaxially oriented polypropylene film according to claim 3, wherein the haze of the biaxially oriented film is 15% or less.

8. The biaxially oriented polypropylene film according to claim 4, wherein the haze of the biaxially oriented film is 15% or less.

9. The biaxially oriented polypropylene film according to claim 5, wherein the haze of the biaxially oriented film is 15% or less.

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