Biaxially oriented polypropylene film and packaging

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 seal layer is made of a resin material containing polypropylene resin and polyethylene resin. The polypropylene resin in the seal layer is a polypropylene copolymer consisting of one or more of propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer, or a mixture of two or more of these. The polyethylene resin in the seal layer is low-density polyethylene. When the seal layers are heat-sealed at 160°C, 0.35 MPa, and for 1 second, the heat seal strength is 1.5 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the heat seal strength is 0.3 N / 15 mm or less.

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene film and packaging, and more particularly to a biaxially oriented polypropylene film for packaging that forms an easily openable heat-seal portion, and to packaging made of this biaxially oriented polypropylene film. [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 processing the film surfaces together, such as by heat sealing, using bag-making methods such as pillow packaging, gusset packaging, three-side sealed bags, four-side sealed bags, and heat-sealed bags.

[0003] In recent years, packaging for rice balls, bread, and other products sold at convenience stores and other retailers has seen increasing diversification of packaging forms. For example, in these types of packages, polypropylene film is sometimes heat-sealed to create special bag shapes that suit the contents, allowing for easy opening and consumption without soiling one's hands. Features such as easy opening, where the heat-sealed portion can be peeled off to allow for easy opening, are sometimes required.

[0004] In packaging with easy opening at the sealed portion, an example of a film for packaging is 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). 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, and easy opening is achieved by cohesive breakdown due to a sea-island dispersion structure caused by the incompatibility of the polypropylene resin and polyethylene resin in the sealing layer.

[0005] Incidentally, when opening a package by peeling off the sealed portion of a film bag, it is necessary to apply a predetermined tearing force to initiate the peeling of the seal. In such cases, even if opening (peeling) is easy, if the heat seal strength after opening is extremely low compared to the heat seal strength at the time of opening, resulting in a large difference between the two, the seal may peel off too forcefully, causing the contents to spill out or the film to tear unnecessarily at the opening, making stable opening impossible.

[0006] Therefore, as a film that is easy to open and provides good peelability and openness, for example, there is a stretched polypropylene film having 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 random copolymer and a butene 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).

[0007] 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. As a result, when the sealed 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.

[0008] However, with the above-mentioned polypropylene film, although the heat seal strength at the point of reaching a certain travel distance when the sealed portion is peeled is maintained at a certain percentage or higher relative to the maximum heat seal strength, the property of a large drop in heat seal strength after peeling has not been improved. As a result, the stability during peeling is insufficient, and it is economically disadvantageous because it requires special production equipment for a four-layer structure. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2021-095162 [Patent Document 2] Patent No. 7018793 [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention has been proposed in view of the above points, and provides a biaxially oriented polypropylene film and packaging that suppress the difference in heat seal strength after peeling of the sealed portion and enable more stable peeling of the sealed portion. [Means for solving the problem]

[0011] 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 seal layer is composed of a resin material comprising polypropylene resin and polyethylene resin, the polypropylene resin of the seal layer is a polypropylene copolymer consisting of any one or more of propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer, or a mixture of two or more such copolymers, the polyethylene resin of the seal layer is low-density polyethylene, and the heat seal strength when the seal layers are heat-sealed together under the conditions of 160°C, 0.35 MPa, and 1 second is 1.5 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the heat seal strength is 0.3 N / 15 mm or less.

[0012] 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 160°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.

[0013] The second invention relates to a biaxially oriented polypropylene film in which, in the first invention, the base layer is mainly composed of a polypropylene homopolymer, the seal layer contains 60 to 85% by weight of the polypropylene copolymer and 15 to 40% by weight of the low-density polyethylene, and when the stacked seal layers are heat-sealed at 280°C to form an easily openable heat-sealed portion, the heat-seal strength of the heat-sealed portion is 9 N / 15 mm or less.

[0014] The third invention is that, in the first or second invention, the low-density polyethylene has a density of 0.930 g / cm³. 3 The following pertains to biaxially oriented polypropylene films, where the melt flow rate (MFR) (190°C, 2.16 kg) measured in accordance with JIS K 7210 (2014) is 0.1 g / 10 min to 15 g / 10 min.

[0015] The fourth invention relates to a packaging body characterized by being made of a biaxially oriented polypropylene film as described in the first or second invention.

[0016] The fifth invention relates to a packaging body characterized by being made of the biaxially oriented polypropylene film described in the third invention. [Effects of the Invention]

[0017] The biaxially oriented polypropylene film of the first invention is a biaxially oriented 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 seal layer is composed of a resin material comprising polypropylene resin and polyethylene resin, the polypropylene resin of the seal layer is a polypropylene copolymer consisting of any one or more of propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer, or a mixture of two or more of these, the polyethylene resin of the seal layer is low-density polyethylene, and the heat seal strength when the seal layers are heat-sealed together under the conditions of 160°C, 0.35 MPa, and 1 second is 1.5 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the heat seal strength is 0.3 N / 15 mm or less, so that the drop in heat seal strength after peeling of the heat-seal portion is suppressed and the heat-seal portion can be peeled off continuously and stably.

[0018] According to the biaxially oriented polypropylene film of the second invention, in the first invention, the base layer is mainly composed of a polypropylene homopolymer, the seal layer contains 60 to 85% by weight of the polypropylene copolymer and 15 to 40% by weight of the low-density polyethylene, and when the overlapping seal layers are heat-sealed at 280°C to form an easily openable heat-sealed portion, the heat-sealing strength of the heat-sealed portion is 9 N / 15 mm or less, so the heat-sealed portion can be easily and stably broken, and good easy-to-open properties of the heat-sealed portion can be obtained.

[0019] According to the biaxially oriented polypropylene film of the third invention, in the first or second invention, the low-density polyethylene has a density of 0.930 g / cm³. 3 As described below, the melt flow rate (MFR) (190℃, 2.16kg) measured in accordance with JIS K 7210 (2014) is 0.1g / 10min to 15g / 10min, which facilitates the formation of a sea-island structure with polypropylene resin, thereby improving ease of opening.

[0020] According to the package of the fourth invention, since it is made of the biaxially stretched polypropylene film described in the first or second invention, a food package having good easy-opening property can be obtained.

[0021] According to the package of the fifth invention, since it is made of the biaxially stretched polypropylene film described in the third invention, a food package having good easy-opening property can be obtained.

Brief Description of the Drawings

[0022] [Figure 1] It is a chart showing the heat seal strength of the biaxially stretched polypropylene film according to an embodiment of the present invention. [Figure 2] It is a chart showing the heat seal strength of a conventional biaxially stretched polypropylene film. [Figure 3] It is a schematic plan view and a schematic cross-sectional view showing the seal structure formed at the opening part of the package.

Modes for Carrying Out the Invention

[0023] The biaxially stretched polypropylene film according to an embodiment of the present invention is a biaxially stretched film for a package composed of at least two or more layers including a base material layer mainly made of polypropylene resin and a seal layer. When the package is formed into a bag, the seal layers are heat-sealed to form an easy-opening heat-seal part. The package formed by using this biaxially stretched polypropylene film is mainly used as a food package, and is particularly suitable for packaging items such as onigiri and bread.

[0024] In the biaxially stretched polypropylene film of the present invention, the resin materials used in each layer described later are appropriately selected from resins produced from appropriate starting materials such as petroleum-derived, biomass-derived, material-recycled, and chemical-recycled materials.

[0025] 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.

[0026] 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.

[0027] The base layer is the main layer of the biaxially oriented polypropylene film, and is formed with a thicker layer thickness compared to other layers. It defines the basic properties of the film, such as its rigidity. As the resin material constituting the base layer, a general-purpose polypropylene resin with excellent heat resistance, chemical resistance, and strength is mainly used. The polypropylene resin used in the base layer can be a material generally recognized as homopolypropylene, and in addition to propylene homopolymers, propylene-ethylene random copolymers with an ethylene content of 2% by weight or less can be used. In particular, from the viewpoint of rigidity, it is preferable to mainly use propylene homopolymers.

[0028] While the melt flow rate (MFR) of polypropylene resin is not particularly limited, 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 in the extruder will increase, putting a load on the extruder and potentially impairing productivity due to deterioration in 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.

[0029] The sealing layer is the inner layer of the bag after it has been manufactured, and it has properties such as low-temperature sealing and easy opening. This sealing layer is formed by heat fusion between sealing layers when a biaxially oriented polypropylene film is manufactured, for example, to form an easy-open heat-sealed section. The sealing layer is composed of a resin material containing polypropylene resin and polyethylene resin. In the sealing layer, since the constituent materials, polypropylene resin and polyethylene resin, are immiscible, a sea-island structure is formed in the resin layer. This sea-island structure makes it possible to cause cohesive failure at the heat-seal peeling interface in the sealing layer, thus providing easy opening.

[0030] The polypropylene resin used in the sealing layer is preferably selected as a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1·butene copolymer.

[0031] While there are no particular restrictions on the melt flow rate (MFR) of polypropylene resin, it is preferable, and more preferably, 1.0 to 10 g / 10 min, to have an MFR of 0.1 to 20 g / 10 min, 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 thickness accuracy and reducing productivity. If the MFR is too high, the resin's fluidity will be too high, potentially worsening thickness accuracy. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0032] Low-density polyethylene (LDPE) is preferably selected as the polyethylene resin used in the sealing layer. Low-density polyethylene has a long-chain branched structure, high melt tension, and readily forms a sea-island structure with polypropylene. Therefore, biaxially oriented polypropylene films can be opened with appropriate ease of opening. From the viewpoint of effectively forming a sea-island structure with polypropylene resin, a density of 0.930 g / cm³ is preferred for low-density polyethylene. 3 The following conditions are met, and it is preferable that the MFR (190℃, 2.16kg) is between 0.1g / 10min and 15g / 10min.

[0033] The density of low-density polyethylene is measured, for example, by the density gradient pipe method in accordance with JIS K 7112 (1999). As the density of low-density polyethylene decreases, the difference in melt tension with polypropylene resin increases, making it easier to form a sea-island structure and improving ease of opening. A preferred density for low-density polyethylene is 0.930 g / cm³. 3 The following is the case: If the density of the low-density polyethylene is too high, the difference in melt tension between it and the polypropylene resin becomes small, which may prevent the proper formation of the sea-island structure and reduce the ease of opening.

[0034] The melt flow rate (MFR) of low-density polyethylene is measured, for example, in accordance with JIS K 7210, under conditions of 190°C and a load of 2.16 kg. A higher MFR tends to indicate a smaller molecular weight, less entanglement between molecules, and higher fluidity. The preferred MFR for low-density polyethylene is 0.1 g / 10 min to 15 g / 10 min, and more preferably 0.2 g / 10 min to 10 g / 10 min. If the MFR is too low, the pressure of the extruder used for molding may become excessively high, potentially reducing productivity. If the MFR is too high, there may be insufficient entanglement between molecules, making it difficult to form a sea-island structure with the polypropylene resin, potentially impairing the ease of opening. By appropriately adjusting the MFR of low-density polyethylene, the sea-island structure with the polypropylene resin is more easily formed, improving the ease of opening.

[0035] In the biaxially oriented polypropylene film of the present invention, the heat seal strength when the seal layers are heat-sealed at 160°C, 0.35 MPa, and for 1 second is set to 1.5 N / 15 mm or more and 6.0 N / 15 mm or less. 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 portion will have resistance to tearing, and peeling will begin easily, resulting in good easy opening.

[0036] Furthermore, the biaxially oriented polypropylene film of the present invention has a standard deviation of heat seal strength of 0.3 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 160°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 measuring 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.

[0037] 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 heat seal strength remains almost constant, indicating that the standard deviation of the heat seal strength is small.

[0038] 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.

[0039] 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. As a result, it may become difficult to stably peel off the heat seal, such as the heat seal peeling off too forcefully after the start of peeling, or the peeling interface becoming unstable due to inadequate cohesive failure of the seal layer, leading to cracks in the substrate layer and tearing of the film. In contrast, if the standard deviation of the heat seal strength is small, specifically 0.3 N / 15 mm or less, the drop in heat seal strength after peeling of the heat seal can be suppressed, allowing for continuous and stable peeling of the heat seal. 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 within a predetermined range of movement distance after the start of peeling of the heat seal, it can be used as an indicator of the stability of the heat seal strength during peeling of the heat seal.

[0040] Furthermore, in addition to the continuous and stable ease of opening of the heat-sealed portion, it is preferable that the biaxially oriented polypropylene film of the present invention also has the ease of opening of the heat-sealed portion formed by the heat-sealing of the overlapping seal layers.

[0041] In recent years, the diversification of packaging forms for food products has led to the proposal of special bag shapes. For example, some packaging materials are processed using heat sealing in addition to thermal sealing during the bag manufacturing process. In packaging materials using thermal sealing, a heated cutting blade is pressed against a film that has been folded and overlapped with the sealing layer facing inward. This cuts the film, and the overlapping sealing layers are heat-sealed together (thermal sealing), forming a thermal sealed section. Normally, when bags are manufactured using thermal sealing, the strength of the thermal sealed section (thermal seal strength) is increased to suppress bag breakage. However, from the perspective of improving functionality in response to the diversification of packaging forms, it is also recommended that the thermal sealed section be easily opened.

[0042] Therefore, in the biaxially oriented polypropylene film of the present invention, the ease of opening of the heat-sealed portion is ensured by setting the heat-seal strength of the heat-sealed portion to 9 N / 15 mm or less when the overlapping seal layers are heat-sealed at 280°C to form the heat-sealed portion. The heat-seal strength (N / 15 mm) indicates the sealing force of the heat-sealed portion and can be used as an indicator of the ease of opening and tear resistance of the heat-sealed portion. The heat-seal strength is obtained by pulling a test piece, cut so that the heat-sealed portion is positioned in the center, using a tensile testing machine, and measuring the strength at the point when the heat-sealed portion breaks.

[0043] If the heat-seal strength is too high, the sealing force of the heat-sealed portion may be too strong, making it difficult to break and potentially preventing easy opening. If the heat-seal strength is too low, the sealing force of the heat-sealed portion may be insufficient, potentially leading to bag rupture. If the heat-seal strength of the heat-sealed portion is 9N / 15mm or less, the heat-sealed portion can be easily and stably broken, resulting in good easy opening of the heat-sealed portion.

[0044] In the biaxially oriented polypropylene film of the present invention, in order to provide easy opening of the heat-sealed portion as well as the slash-sealed portion, it is preferable that the base layer is mainly composed of a polypropylene homopolymer, and the seal layer is composed of a blend of 60 to 85% by weight of a polypropylene copolymer and 15 to 40% by weight of low-density polyethylene. It is believed that the ease of opening of the slash-sealed portion is improved by the combination of the constituent materials of the base layer and the specific blend of the seal layer. In particular, when a crystalline structure is formed in the heat-sealed portion by slash sealing, the presence of low-density polyethylene, which is immiscible with polypropylene resin, causes the crystalline structure to become irregular, thereby appropriately suppressing the strength of the slash-sealed portion. If the combination of the base layer and the seal layer does not satisfy the above conditions, it is possible that the ease of opening of the slash-sealed portion cannot be obtained.

[0045] 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.

[0046] While the melt flow rate (MFR) of polypropylene resin is not particularly limited, 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 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 resin's fluidity will be too high, potentially worsening thickness accuracy. By having an appropriate MFR, good thickness accuracy and moldability can be achieved.

[0047] 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.

[0048] 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.

[0049] 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, gusset packaging, three-side sealed bags, four-side sealed bags, and heat-sealed bags. 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, preventing tearing of the film or spillage of contents during opening, making it easy to open. Furthermore, in a more preferred embodiment, the heat-sealed portion can be easily and stably broken, providing packaging with an easily openable heat-sealed portion and facilitating adaptation to various packaging forms.

[0050] Figure 3 is a schematic plan view (a) and a schematic cross-sectional view (b) along line AA showing an example of a seal structure formed in the opening portion 10 of a package made of biaxially oriented polypropylene film according to one embodiment of the present invention. In this package opening portion 10, the first film portion 21 and the second film portion 22 of the film body 20 are folded back and overlapped via a folded portion 23 so that the seal layer is on the inside. Heat-sealed portions 30, 30 are formed on a part of both sides of the overlapping portion 24 of the first film portion 21 and the second film portion 22, and heat-sealed portions 31, 31 are formed on the entire side of both sides of the overlapping portion 24. An opening start portion 25 is provided on the end side of the second film portion 22 (opposite the folded portion 23).

[0051] In the seal structure of the opening portion 10 described above, a first seal structure 35 consisting only of a heat-sealed portion 31 is continuously formed on the opening start portion 25 side of the overlapping portion 24, and a second seal structure 36 consisting of a composite seal portion of a heat-sealed portion 30 and a heat-sealed portion 31 is continuously formed on the folded portion 23 side of the overlapping portion 24. When opening, the opening start portion 25 is pulled in the opening direction D, causing the tip portion 32 of the heat-sealed portion 31, which is the first seal structure 35, to break and opening to begin. As the opening start portion 25 continues to be pulled and the breakage of the heat-sealed portion 31 progresses to the second seal structure 36, peeling of the heat-sealed portion 30 begins as the breakage of the heat-sealed portion 31 progresses. In the second seal structure 36, the breakage of the heat-sealed portion 31 and the peeling of the heat-sealed portion 30 proceed almost simultaneously, resulting in opening.

[0052] The biaxially oriented polypropylene film according to one embodiment of the present invention provides stable and continuous easy opening at the heat-sealed portion, as well as stable and easy opening at the heat-sealed portion. Therefore, at the opening portion 10 of the package, the first seal structure 35 can be easily opened by tearing from the tip 32 of the heat-sealed portion 31, and at the second seal portion, the heat-sealed portion 30 can be peeled off stably and continuously as the heat-sealed portion 31 is torn, allowing for opening. Thus, it is possible to provide a package that can be opened at the heat-sealed portion or the heat-sealed portion, and it is possible to appropriately respond to the diversification of packaging forms. [Examples]

[0053] [Fabrication of biaxially oriented polypropylene film] For prototypes 1-13, 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 100-130°C, then preheated in a tenter to a temperature of 175-190°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. The materials used in each layer of prototypes 1-13, along with the film thickness, are shown in Tables 1 and 2 below.

[0054] [Materials used] The following resins were used as the resin material for each layer. For the properties of each resin, the melt flow rate (MFR) was measured in accordance with JIS K 7210 (2014), with propylene-based resins measured at 230°C and 2.16 kg, and ethylene-based resins measured at 190°C and 2.16 kg. The melting point was determined in accordance with differential scanning calorimetry (DSC) measurement in JIS K 7121 (2012), using a differential scanning calorimeter (NETC Japan Co., Ltd.; "DSC 214 Polyma"), and the melting peak temperature was obtained from the DSC curve obtained when the temperature was increased at a heating rate of 10°C / min. In prototype examples 1 to 13, the resin blending ratio for each layer was 100% by weight, and additives such as antiblocking agents were omitted.

[0055] <Propylene resin> PP1: Polypropylene homopolymer, MFR (230℃ / 2.16kg) 3.5g / 10min, melting point 164℃ • PP2: Polypropylene homopolymer, MFR (230℃ / 2.16kg) 7.3g / 10min, contains 8% antistatic agent. PP3: Propylene-ethylene-butene random copolymer, MFR (230℃ / 2.16kg) 5.0g / 10min, melting point 125℃ ·PP4: Propylene-1-butene random copolymer, MFR (230 °C / 2.16 kg) 7.0 g / 10 min, melting point 75 °C

[0056] <Ethylene-based resin> ·PE1: Low-density polyethylene, MFR (190 °C / 2.16 kg) 2.5 g / 10 min, melting point 110 °C, density 0.922 g / cm 3 ·PE2: Low-density polyethylene, MFR (190 °C / 2.16 kg) 2.8 g / 10 min, melting point 113 °C, density 0.925 g / cm 3 ·PE3: Low-density polyethylene, MFR (190 °C / 2.16 kg) 7.0 g / 10 min, melting point 106 °C, density 0.918 g / cm 3 ·PE4: Low-density polyethylene, MFR (190 °C / 2.16 kg) 4.0 g / 10 min, melting point 116 °C, density 0.933 g / cm<A000008> ·PE5: High-density polyethylene, MFR (190 °C / 2.16 kg) 7.5 g / 10 min, melting point 134 °C, density 0.963 g / cm 3 ·PE6: Low-density polyethylene, MFR (190 °C / 2.16 kg) 0.4 g / 10 min, melting point 112 °C, density 0.920 g / cm 3 ·PE7: Low-density polyethylene, MFR (190 °C / 2.16 kg) 22 g / 10 min, melting point 106 °C, density 0.921 g / cm 3

[0057] [Prototype Example 1] Prototype Example 1 is a biaxially oriented polypropylene film in which the surface layer is 100% by weight of PP1, the base layer is 90% by weight of PP1 and 10% by weight of PP2, and the seal layer is formulated with 78% by weight of PP3 and 2% by weight of PP4 of a propylene-based resin and 20% by weight of PE1 of an ethylene-based resin, respectively, and the film thickness is 30 μm.

[0058] [Prototype Example 2] Prototype Example 2 is a biaxially oriented polypropylene film that is identical to Prototype Example 1, except that the sealing layer does not contain ethylene-based resin and is instead composed of propylene-based resins of 97% by weight of PP3 and 3% by weight of PP4.

[0059] [Prototype Example 3] Prototype Example 3 is a biaxially oriented polypropylene film that is identical to Prototype Example 1 except that the sealing layer is made of propylene resin (83% by weight of PP3 and 2% by weight of PP4) and ethylene resin (15% by weight of PE2), and the film thickness is 20 μm.

[0060] [Prototype Example 4] Prototype Example 4 is a biaxially oriented polypropylene film that is identical to Prototype Example 3, except that the sealing layer is changed to a propylene-based resin (78% by weight of PP3 and 2% by weight of PP4) and an ethylene-based resin (20% by weight of PE2).

[0061] [Prototype Example 5] Prototype Example 5 is a biaxially oriented polypropylene film that is identical to Prototype Example 3, except that the sealing layer is changed to a propylene-based resin (68% by weight of PP3 and 2% by weight of PP4) and an ethylene-based resin (30% by weight of PE2).

[0062] [Prototype Example 6] Prototype Example 6 is a biaxially oriented polypropylene film that is identical to Prototype Example 3, except that the sealing layer is changed to a propylene-based resin (58% by weight of PP3 and 2% by weight of PP4) and an ethylene-based resin (40% by weight of PE2).

[0063] [Prototype Example 7] Prototype Example 7 is a biaxially oriented polypropylene film that was manufactured to have a film thickness of 20 μm, while remaining identical to Prototype Example 2 in all other respects.

[0064] [Prototype Example 8] Prototype Example 8 is a biaxially oriented polypropylene film that is identical to Prototype Example 3, except that the propylene resin (87% by weight of PP3 and 3% by weight of PP4) and the ethylene resin (10% by weight of PE2) have been changed.

[0065] [Prototype Example 9] Prototype Example 9 is a biaxially oriented polypropylene film that is identical to Prototype Example 4 except that the ethylene-based resin in the sealing layer has been changed to PE3.

[0066] [Prototype Example 10] Prototype 10 is a biaxially oriented polypropylene film that is identical to prototype 4 except that the ethylene-based resin in the sealing layer has been changed to PE4.

[0067] [Prototype Example 11] Prototype 11 is a biaxially oriented polypropylene film that is identical to Prototype 4 except that the ethylene-based resin in the sealing layer has been changed to PE5.

[0068] [Prototype Example 12] Prototype 12 is a biaxially oriented polypropylene film that is identical to Prototype 4 except that the ethylene-based resin in the sealing layer has been changed to PE6.

[0069] [Prototype Example 13] Prototype 13 is a biaxially oriented polypropylene film that is identical to prototype 4 except that the ethylene-based resin in the sealing layer has been changed to PE7.

[0070] [Table 1]

[0071] [Table 2]

[0072] For the biaxially oriented polypropylene films of prototypes 1 to 13, the heat seal strength was measured to determine the maximum heat seal strength and the standard deviation of the heat seal strength, and the heat-seal strength was also measured. In addition, heat-sealed and heat-sealed sections were formed using the films of prototypes 1 to 13, and the ease of opening of the heat-sealed and heat-sealed sections was evaluated. The results of each test are shown in Tables 3 and 4 below.

[0073] [Measurement of heat seal strength] The heat seal strength (N / 15mm) was measured as follows. For prototypes 1 to 13, two films were cut to 200mm in the MD direction and 50mm in the TD direction. The sealing layers were placed together and heat-sealed using a thermal gradient tester (Toyo Seiki Seisakusho Co., Ltd.; "HG-100-2"). The heat sealing conditions were: seal width 10mm in the MD direction and 25mm in the TD direction, seal temperature 160℃, seal pressure 0.35MPa, and seal time 1 second. 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 piece was peeled off at a speed of 200mm / min in the MD direction to create a heat seal strength chart for prototypes 1 to 13.

[0074] [Maximum heat seal strength] From the heat seal strength charts of prototype examples 1 to 13 films, created by measuring heat seal strength, 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 prototype examples 1 to 13.

[0075] [Standard deviation of heat seal strength] From the heat seal strength charts of prototype examples 1-13 films created by measuring heat seal strength, the heat seal strength was read at 1mm intervals (5mm, 6mm, 7mm, ..., 14mm, 15mm) within the range where the chuck moved 5mm to 15mm. The standard deviation was calculated from a total of 12 points, including the maximum heat seal strength. The test was performed 10 times, and the average of the standard deviations calculated for each test was used as the standard deviation of heat seal strength for prototype examples 1-13 (N / 15mm).

[0076] [Measurement of thermal seal strength] The heat-seal strength (N / 15mm) was measured as follows. For prototype examples 1 to 13, two films were cut to 10cm in the TD direction and 15cm in the MD direction, the sealing layers were aligned, and a heat-sealing device (Tester Industries Co., Ltd.; "TP-701-B") was used. Two sheets of cellophane film (20μm) were placed between the cut films to prevent resin adhesion to the heat-sealing blade, and the sealing was performed with the sealing bar perpendicular to the film flow direction. The heat-sealing conditions were a cutting blade angle of 60 degrees, a temperature of 280℃ on the upper side of the sealing bar and room temperature on the lower side, a sealing time of 0.2 seconds, and a sealing pressure of 0.15 MPa. Subsequently, the cellophane film was removed, and a test specimen was cut so that the width of the heat-sealed portion was 15 mm and perpendicular to the heat-sealed portion. The test specimen was then fixed in the chuck of a tensile testing machine (Shimadzu Corporation; "AGS-X") so that the heat-sealed portion was horizontal, and the strength at which the heat-sealed portion broke (maximum strength) was measured at room temperature at a tensile speed of 200 mm / min. The test was performed with n=3, and the average value was used as the heat-sealed strength for prototypes 1 to 13.

[0077] [Easy opening of the heat-sealed portion] For prototype examples 1 to 13, 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 160°C, sealing pressure 0.35 MPa, and sealing time 1 second. Test specimens with a heat-sealed portion were obtained. Next, the testers pinched 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, "Good (○)" was rated if the heat-sealed portion was easy to peel off and could be peeled off with a nearly constant force after the start of peeling, and "Poor (×)" was rated if the heat-sealed portion was difficult to peel off or could not be peeled off stably.

[0078] [Easy opening of the heat-sealed portion] For prototypes 1 to 13, two films were cut to 10 cm in the TD direction and 15 cm in the MD direction. The sealing layers were aligned, and a heat-sealing device (Tester Industries Co., Ltd.; "TP-701-B") was used. To prevent resin from adhering to the heat-sealing blade, two sheets of cellophane film (20 μm) were placed between the cut film. The heat-sealing was performed with the blade angle set to 60 degrees, the sealing bar temperature set to 280°C on the upper side and room temperature on the lower side, the sealing time to 0.2 seconds, and the sealing pressure to 0.15 MPa, so that the sealing bar was perpendicular to the film flow direction. Test specimens with a heat-sealed portion were obtained. Next, the tester grasped one end of the film that was not heat-sealed and pulled it in the opposite direction to break the heat-sealed portion and evaluate the ease of opening. To evaluate the ease of opening the heat-sealed portion, a "Good (○)" rating was given if the heat-sealed portion could be easily broken and the breakage continued smoothly after the start of the break, while a "Poor (×)" rating was given if the heat-sealed portion was difficult to break or if the film tore after the start of the break.

[0079] [Table 3]

[0080] [Table 4]

[0081] [Results and Discussion] Prototype 1 is a biaxially oriented polypropylene film whose sealing layer is composed of propylene resin and ethylene resin, while Prototype 2 is a biaxially oriented polypropylene film whose sealing layer is composed of propylene resin and does not contain ethylene resin. Comparing the maximum heat seal strength and the standard deviation of heat seal strength of Prototype 1 and Prototype 2, there was no significant difference in the maximum heat seal strength, but the standard deviation of heat seal strength for Prototype 1 was significantly smaller than that of Prototype 2.

[0082] 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 film of prototype example 2 has the property that the heat seal strength during peeling varies relatively significantly from the maximum strength.

[0083] Therefore, when examining the ease of opening the heat-sealed portion, in the film of prototype example 2, although the peeling of the heat-sealed portion was easy to initiate, the peeling interface was unstable, causing cracks in the base layer and ultimately tearing the film. On the other hand, in the film of prototype example 1, the peeling of the heat-sealed portion was easy to initiate, and it was possible to continue peeling smoothly with almost the same force as at the start of opening. From this, it is thought that in biaxially oriented polypropylene film, by composing the seal layer with a propylene-based resin and an ethylene-based resin, the peeling of the heat-sealed portion becomes easier to initiate, and stable and continuous peeling becomes possible.

[0084] As shown in Tables 3 and 4, among the prototypes 1, 3 to 6, and 8 to 13, which contain ethylene-based resin in the sealing layer, the ease of opening the heat-sealed portion was good for prototypes 1, 3 to 6, 9, and 12, while it was insufficient for prototypes 8, 10, 11, and 13. Of prototypes 8, 10, 11, and 13, prototype 8 and 11, which had relatively high maximum heat-seal strength, did not exhibit easy opening due to excessive film adhesion. Furthermore, although prototypes 10 and 13 had the same maximum heat-seal strength as prototypes 1, 3 to 6, 9, and 12, the standard deviation of the heat-seal strength was relatively high, making it impossible to stably continue peeling the heat-sealed portion after the start of peeling.

[0085] On the other hand, in prototype examples 1, 3 to 6, 9, and 12, peeling of the heat-sealed portion was easy to initiate, and stable and continuous peeling was possible in all cases. From these findings, it is considered that the heat seal strength conditions for easy peeling initiation and stable and continuous peeling of the heat-sealed portion are a heat seal strength of approximately 1.5 N / 15 mm to 6.0 N / 15 mm, and a standard deviation of the heat seal strength of approximately 0.3 N / 15 mm or less.

[0086] Next, the ease of opening the heat-sealed portion was examined. As shown in Tables 3 and 4, among prototypes 1 to 13, prototypes 1, 3 to 6, 9, 10, 12, and 13 showed good ease of opening the heat-sealed portion, while prototypes 2, 7, 8, and 11 showed insufficient ease of opening. Comparing prototypes 1, 3 to 6, 9, 10, 12, and 13 with prototypes 2, 7, 8, and 11, prototypes 1, 3 to 6, 9, 10, 12, and 13, which showed good ease of opening the heat-sealed portion, had lower heat-seal strength values ​​than prototypes 2, 7, 8, and 11. In the films of prototypes 2, 7, 8, and 11, tearing at the heat-sealed portion was difficult when the film edge was pulled, making it difficult to open at the heat-sealed portion.

[0087] On the other hand, in prototype examples 1, 3-6, 9, 10, 12, and 13, the heat-sealed portion could be easily broken, and the heat-sealed portion could be broken stably even after the breakage began. From these results, it is considered that the heat-sealed strength required for the heat-sealed portion to break easily and to break stably and continuously is a heat-sealed strength of approximately 9 N / 15 mm or less.

[0088] Next, regarding the propylene-based resin and ethylene-based resin constituting the seal layer, the blending ratios that easily yield the preferred heat seal strength (maximum strength, standard deviation) and cut seal strength were investigated from prototype examples 3 to 8. In prototype examples 3 to 8, the blending ratio of the propylene-based resin and ethylene-based resin constituting the seal layer was adjusted within the range of 0% to 40% by weight, and the ease of opening was evaluated.

[0089] Prototype Example 7 is an example where the seal layer does not contain ethylene-based resin (0% by weight), resulting in relatively high standard deviations for heat seal strength and heat-seal strength, and insufficient ease of opening of the heat-sealed and heat-sealed sections. Prototype Examples 3-6 and 8 are examples where the ethylene-based resin in the seal layer was adjusted within the range of 10% to 40% by weight. In Prototype Example 8, where the ethylene-based resin was 10% by weight, the maximum strength, standard deviation of heat seal strength, and heat-seal strength were all high, resulting in insufficient ease of opening of the heat-sealed and heat-sealed sections.

[0090] On the other hand, in prototypes 3-6, where the ethylene-based resin content was 15% to 40% by weight, the maximum heat seal strength, standard deviation, and heat-seal strength were all lower compared to prototype 8, resulting in good ease of opening at the heat-sealed and heat-sealed sections. In particular, as the proportion of ethylene-based resin in the seal layer increased, the values ​​of the maximum heat seal strength, standard deviation, and heat-seal strength tended to decrease, improving the ease of opening at the heat-sealed and heat-sealed sections. Therefore, it is considered that desirable heat seal strength (maximum strength, standard deviation) and heat-seal strength can be easily obtained by adjusting the proportion of ethylene-based resin in the seal layer to approximately 15% to 40% by weight.

[0091] Next, regarding the ethylene-based resin contained in the sealing layer, properties that easily yield the desirable heat seal strength (maximum strength, standard deviation) and cut seal strength were investigated from prototype examples 1, 4, 9-13. In prototype examples 1, 4, 9-13, the type of ethylene-based resin in the sealing layer was changed to 20% by weight, and the ease of opening was evaluated. Prototype example 11 used high-density polyethylene as the ethylene-based resin, and compared to the other prototype examples 1, 4, 9, 10, 12, and 13, the maximum strength and standard deviation of the heat seal strength and the cut seal strength were all high, but the ease of opening of the heat-sealed and cut-sealed parts was insufficient.

[0092] In prototype examples 1, 4, 9, 10, 12, and 13, low-density polyethylene was used as the ethylene-based resin. Comparative example 10 used low-density polyethylene with a relatively high density as the ethylene-based resin, resulting in a slightly higher standard deviation of heat seal strength and heat-seal strength, and insufficient ease of opening of the heat-sealed and heat-sealed sections. Prototype example 13 used low-density polyethylene with a relatively high MFR, resulting in good heat-seal strength and easy opening of the heat-sealed section. Although the maximum heat seal strength was good, the standard deviation was relatively high, resulting in insufficient ease of opening of the heat-sealed section.

[0093] These findings indicate that ethylene-based resins with relatively high density and MFR are unsuitable for use in the sealing layer. Therefore, it is believed that desirable heat seal strength (maximum strength, standard deviation) and cut seal strength can be easily obtained by selecting an ethylene-based resin with relatively low density and low MFR. For this reason, low-density polyethylene is preferred as the ethylene-based resin, and among them, a density of 0.930 g / cm³ is preferred. 3 Low-density polyethylene with an MFR of approximately 0.1 g / 10 min to 15 g / 10 min is considered preferable. [Industrial applicability]

[0094] 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 difference in heat-seal strength is suppressed, and the heat-seal portion can be peeled continuously and stably, resulting in good ease of opening of the heat-seal portion. Furthermore, when a heat-seal portion is formed, the heat-seal portion can be easily and stably broken, resulting in good ease of opening of the heat-seal portion. For this reason, it is promising as a replacement for conventional biaxially oriented polypropylene films for packaging. [Explanation of symbols]

[0095] 10. Opening part of the packaging 20 film bodies 21. Film Section 1 22. Second Film Section 23 Turning section 24 Overlapping parts 25 Opening start part 30 Heat seal section 31. Fusible sealing section 32 Tip of the heat-sealed section 35. First seal structure 36. Second seal design D Opening direction 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 sealing layer is made of a resin material containing polypropylene resin and polyethylene resin. The polypropylene resin in the sealing layer is a polypropylene copolymer consisting of one or more of the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1-butene copolymer. The polyethylene resin in the sealing layer is low-density polyethylene. The heat seal strength obtained when the aforementioned sealing layers are heat-sealed at 160°C, 0.35 MPa, and for 1 second is 1.5 N / 15 mm or more and 6.0 N / 15 mm or less, and the standard deviation of the aforementioned heat seal strength is 0.3 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 aforementioned substrate layer is mainly composed of a polypropylene homopolymer, The sealing layer comprises 60 to 85% by weight of the polypropylene copolymer and 15 to 40% by weight of the low-density polyethylene. When the overlapping seal layers are heat-sealed at 280°C to form an easily openable heat-sealed portion, the heat-sealing strength of the heat-sealed portion is 9 N / 15 mm or less. The biaxially oriented polypropylene film according to claim 1.

3. The low-density polyethylene has a density of 0.930 g / cm³. ３ The biaxially oriented polypropylene film according to claim 1 or 2, wherein the melt flow rate (MFR) (190°C, 2.16 kg) measured in accordance with JIS K 7210 (2014) is 0.1 g / 10 min to 15 g / 10 min.

4. A packaging body characterized by being made of a biaxially oriented polypropylene film as described in claim 1 or 2.

5. A packaging body characterized by being made of the biaxially oriented polypropylene film described in claim 3.

Images