JP7585029B2 - Easy-to-open co-extruded film - Google Patents

Description

The present invention relates to an easy-to-open co-extruded film.

Traditionally, various packages have been developed and provided that use films or container-shaped molded bodies made of plastic materials to house contents and seal the opening by heating. The contents range from a wide range, including food, medical products, industrial parts, and daily necessities, and the shapes of the packages used are adapted to the contents and the manner of use, including bags, cups, deep-draw base materials, blister packs, and skin packs. In recent years, efforts have been made to improve the ease of opening of these packages, so that they can be opened with light force, and various easy-to-open films have been developed to suit the shape and use of the packages.

A widely used easy-open film is one that utilizes a cohesive failure function in which, when opened, the inside of the seal layer undergoes cohesive failure while the surface of the seal layer of the easy-open film remains in close contact with the surface of the adherend, and this cohesive failure propagates within the layer, allowing the film to be opened and peeled off with light force.
For example, Patent Document 1 discloses an easy peel sealant that is extremely useful, having heat sealability, excellent airtightness, and easy opening, and that is made of at least two co-extruded laminated films, a base layer and a seal layer, and further, the base layer is constituted of a resin layer made of a resin composition mainly composed of medium-density polyethylene, high-density polyethylene, or polypropylene, and the seal layer is constituted of a resin layer made of a resin composition mainly composed of a mixture of polypropylene having a melt flow rate (MFR) in the range of 10 to 40 g/10 min and low-density polyethylene, the blending ratio being 50 to 90 parts by weight of the former and 10 to 50 parts by weight of the latter, and the easy peel sealant is characterized in that the easy peel sealant is made of a co-extruded laminated film of both resin layers.

On the other hand, there is a demand for easy-open films for packages in which the internal pressure gradually increases after the package is sealed, such as when the contents are fermented foods. These packages require stronger sealing and closure properties due to the internal pressure, and therefore use high temperature and/or high pressure heat sealing conditions. Specifically, productivity is reduced when high temperature and/or high pressure is applied all at once with general heat sealing, so two-stage sealing (two-step sealing) is performed, in which the heat sealing process is carried out twice, and in some cases, a strong heat seal is achieved by combining solid sealing and line sealing.

However, when strong heat sealing such as double sealing is performed, the resin in the film portion that is exposed to the heat and pressure of the heat seal melts significantly, and the molten resin moves to the outside of the seal width, thinning the film in the seal width portion. Therefore, when double sealing is performed on a conventional easy-to-open film of the cohesive failure type such as that in Patent Document 1, the film becomes thinner and the seal layer becomes too thin, so that cohesive failure of the seal layer does not occur and easy-to-open properties cannot be achieved.

JP 2007-168257 A

The present invention aims to provide a sealant film that can be easily opened even when heat sealed under conditions that involve high heat and/or pressure loads, such as double sealing.

As a result of intensive research aimed at solving the above problems, the present inventors have completed the following invention.
The first invention relates to an easy-open coextruded film having at least a seal layer and an adjacent layer, characterized in that when the coextruded film is heat-sealed to an adherend and then opened, the opening of the heat-sealed portion proceeds in the following order of (1) to (3).
(1) From the opening start point to the heat-sealed part, the seal layer undergoes cohesive failure. (2) At the heat-sealed part, delamination occurs between the seal layer and the adjacent layer. (3) After passing through the heat-sealed part, the seal layer undergoes cohesive failure.

In the first aspect of the present invention, it is preferable that the sealing layer contains 50 to 95% by mass of polypropylene resin and 5 to 50% by mass of low-density polyethylene resin, and the adjacent layer contains 10 to 90% by mass of low-density polyethylene resin.

In the first aspect of the present invention, it is preferable that the thickness of the sealing layer is 5 to 15 μm.

The second invention is a package in which the easy-open co-extruded film of the first invention is used as a lid material, and the lid material is heat-sealed to a base container made of a polypropylene-based resin.

In the second invention, it is preferable that the easy-open coextruded film undergoes interlaminar delamination at the heat-sealed portion of the package and cohesive failure occurs at a portion other than the heat-sealed portion.

The easy-to-open co-extruded film of the present invention (hereinafter sometimes referred to as the film of the present invention) exhibits easy-to-open properties even when heat-sealed under conditions of high heat and/or pressure load, such as double sealing, for example, by combining a solid seal with a line seal, because the sealing layer has a cohesive failure function and has a delamination function between the sealing layer and the adjacent layer. Therefore, by using the film of the present invention, it is possible to achieve both sufficient sealing strength and easy opening, even in packages of fermented foods, etc., in which the internal pressure increases after the package is sealed.

FIG. 1(a) is a schematic diagram of a package (before heat sealing) in which the film of the present invention is combined with a base material 20 as a lid material 10 to house contents 30, and FIG. 1(b) is an enlarged view of the flange portion of the package. 2(a) and (b) are enlarged views of the flange portion of the package after the package has been heat-sealed twice.

In the present application, the term "easy opening property" is synonymous with "easy peel property" and "easy peelability." Furthermore, the sealant film may be called a "heat seal film" or a "heat sealable film."
The peel strength used as an index of ease of opening is synonymous with seal strength, and indicates the peelability between heat-sealed packaging materials and the adhesion of the heat seal, and is sometimes called opening strength.
When a numerical range is expressed as "○ to ○○", it means "0 or more and ○○ or less", and preferably means "greater than ○ and smaller than ○○".
The term "main component" means 50% by mass or more, preferably 60% by mass or more, and more preferably 70% by mass or more, when the total of the constituent raw materials is 100% by mass.

<Easy-to-open co-extruded film>
The film of the present invention comprises a coextruded film having at least a seal layer and an adjacent layer.

(Sealing layer)
The film of the present invention has a seal layer on the surface layer on the side that comes into contact with an adherend, and the seal layer and the adherend are brought into contact with each other and heat sealed, thereby bringing them into close contact with each other.
The seal layer of the film of the present invention has a cohesive failure function, which means that the seal layer has a sea/island dispersion structure in which multiple incompatible resins are dispersed, and the force applied to pull the film when opening the package causes microscopic peeling at the sea/island phase interface in the layer, which then propagates within the layer, causing the layer to fail cohesively.

The resin composition of the sealing layer is not particularly limited as long as it has a cohesive failure function, but examples include a combination of polypropylene resin and polyethylene resin, a combination of polypropylene resin and polybutene resin, and a combination of polyethylene resin and polybutene resin. In addition, each of these polypropylene resins, polyethylene resins, and polybutene resins may be one type, or two or more types may be included.

When the film of the present invention is used as a lid material for a package that generates internal pressure, polypropylene-based resins are often used for the base container, so in terms of heat seal adhesion between the base container and the lid material, the resin composition of the seal layer of the film of the present invention preferably contains 50 to 95 mass% polypropylene-based resin, more preferably 60 to 90 mass%, and even more preferably 70 to 90 mass%.
In addition, since the seal layer needs to have interlayer adhesion with the adjacent layer during coextrusion, and since the use of a polyethylene-based resin is suitable for the adjacent layer as described below, the resin to be combined with the polypropylene-based resin of the seal layer is preferably a polyethylene-based resin, and among them, low-density polyethylene is preferred from the viewpoint of extrusion characteristics. The resin composition of the seal layer made of low-density polyethylene is preferably 5 to 50 mass%, more preferably 10 to 40 mass%, and even more preferably 10 to 30 mass%.

As the polypropylene-based resin, a polypropylene-based resin produced using a single-site catalyst is used, which has narrow molecular weight distribution and composition distribution, high tensile strength, high impact strength, and excellent low-temperature sealability. For example, the molecular weight distribution is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less, as the ratio Mw/Mn of the weight average molecular weight to the number average molecular weight. For example, the composition distribution is preferably 40 or less in short chain branching (SCB), more preferably 30 or less, and even more preferably 20 or less. The type of single-site catalyst is not particularly limited, but a representative example is a metallocene catalyst. The density of the polypropylene-based resin is generally preferably 880 kg/m ³ or more and 920 kg/m ³ or less, and more preferably 890 kg/m ³ or more and 910 kg/m ³ or less. The melting point of the polypropylene-based resin is preferably 135° C. or less, more preferably 130° C. or less, from the viewpoint of heat sealability.
The melt flow rate (MFR) of the polypropylene resin is preferably from 2.0 to 15.0 g/10 min, more preferably from 5.0 to 10.0 g/10 min, from the viewpoint of film formability.

The low-density polyethylene resin preferably has a density of 900 to 930 kg/m ³ , more preferably a lower limit of 910 kg/m ³ or more, and even more preferably 920 kg/m ³ or more. The melt flow rate (MFR) is preferably 1.0 g/10 min or less, more preferably 0.8 g/10 min or less, and the lower limit is preferably 0.4 g/10 min or more, and more preferably 0.5 g/10 min or more. When the density and MFR are within such ranges, dispersibility with the polypropylene resin used in the seal layer is good, and sufficient easy peel property due to cohesive failure of the seal layer is easily exhibited.

The thickness of the sealing layer is preferably 5 to 15 μm, and more preferably 5 to 10 μm. If the thickness is 5 μm or more, sufficient cohesive failure performance is exhibited even if the sealing layer is deformed when sealing is performed under conditions of high heat and/or pressure load, such as double sealing.

(adjacent layer)
The film of the present invention has an adjacent layer adjacent to the seal layer by coextrusion.
The film of the present invention has a delamination function between the seal layer and the adjacent layer, which means that when a force is applied to peel the film from the adherend, the seal layer peels off from the adjacent layer while remaining in close contact with the adherend.

The resin composition of the adjacent layer is not particularly limited as long as it has interlayer adhesion during coextrusion and interlayer peeling function with the seal layer when the package is opened, but polyethylene-based resins are preferred from the standpoint of economy and productivity of the coextruded film.
The polyethylene resin may be, for example, one or a mixture of two or more selected from low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ethylene-(meth)acrylic acid resins, and ionomers. Among them, low density polyethylene and linear low density polyethylene are preferred from the viewpoints of production stability and interlayer adhesion with the seal layer, and since low density polyethylene has the effect of increasing production stability and linear low density polyethylene has the effect of increasing adhesion with the seal layer, good interlayer peeling with the seal layer can be achieved by mixing low density polyethylene and linear low density polyethylene in a specific ratio.

The low density polyethylene used may be the same as the low density polyethylene used in the sealing layer.
The linear low-density polyethylene is preferably a linear low-density polyethylene produced using a single-site catalyst, which has narrow molecular weight distribution and composition distribution, high tensile strength, high impact strength, and excellent low-temperature sealability. The ratio of weight-average molecular weight to number-average molecular weight (Mw/Mn) is, for example, preferably 4.0 or less, more preferably 3.5 or less. The composition distribution is, for example, preferably a short chain branching degree (SCB) of 40 or less, more preferably 30 or less, and even more preferably 20 or less.
The density of the linear low density polyethylene is preferably 910 kg/m ³ or more and 940 kg/m ³ or less.
The melt flow rate (MFR) of the linear low density polyethylene is preferably 0.5 g/10 min or more and 5.0 g/10 min or less, and more preferably 0.6 g/10 min or more and 3.0 g/10 min or less.
From the viewpoint of preventing interlayer delamination between the seal layer and the adjacent layer, when the total of the low-density polyethylene and the linear low-density polyethylene is taken as 100% by mass, the composition of the low-density polyethylene is preferably 10 to 90% by mass, more preferably 20 to 80% by mass, and even more preferably 20 to 70% by mass.

There are no particular limitations on the thickness of the adjacent layer, but from the viewpoint of production stability, it is preferable that the thickness be thicker than the sealing layer. For example, 10 to 40 μm is preferable, and 12 to 35 μm is more preferable.

(Other layers)
The film of the present invention may be co-extruded with other layers in the following order: seal layer/adjacent layer/other layer. The other layers may be one layer or two or more layers.
The resin composition of the other layers is not particularly limited, but examples thereof include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, and polypropylene.
The thickness of the other layer is preferably from 5 to 30 μm, and more preferably from 5 to 20 μm.

(Laminated film)
Various films or resin layers can be laminated on the surface of the film of the present invention opposite the seal layer, i.e., on the surface of an adjacent layer or on the surface of another layer, using a known method such as dry lamination, extrusion lamination, or thermal lamination.
For example, other films include unstretched and stretched films of polypropylene, polyester, polyamide, etc., and films provided with a gas barrier layer such as a vinylidene chloride coating layer, an ethylene-vinyl alcohol layer, or an inorganic oxide vapor deposition layer.

(Film characteristics)
The film of the present invention can provide both heat seal adhesion and ease of opening even when heat sealed under conditions of high heat and/or pressure load such as double sealing.
Fig. 1(a) shows a schematic diagram of a package (before heat sealing) in which the film of the present invention is combined as a lid material 10 with a base material 20 to house contents 30. Fig. 1(b) is an enlarged view of a flange portion of the package. The film constituting the lid material 10 includes, from the base material 20 side, a sealing layer 12, an adjacent layer 14, another layer 16, and another film 18, as an example.
2(a) and (b) show enlarged views of the flange portion of the package after double heat sealing.

In the case of conventional easy-open films of the cohesive failure type, for example, cohesive failure propagates within the seal layer, and the film peels off from the adherend macroscopically. However, when heat sealing is performed under conditions of high heat and/or pressure load, such as double sealing, as shown in Figure 2(a), the strong heat and/or pressure load of the heat seal melts the film resin located in the seal width W1 and moves to the outside of the seal width W1, and when the film or seal layer in the seal width W1 part becomes thin, the peeling between the sea/island in the thinned part does not propagate within the seal layer 12 and stops.

On the other hand, in the case of the easy-open coextruded film of the present invention, when the film at the end of the package (the tab portion T1) is pulled to open it, cohesive failure of the seal layer 12 occurs and propagates from the opening start point S1, as in the conventional technology. Then, when the cohesive failure of the seal layer 12 reaches the part where the film is thinned in the seal width (the heat seal portion W1), it transitions to interlayer peeling between the seal layer 12 and the adjacent layer 14. Then, when it passes the thin seal width (the heat seal portion W1), the interlayer peeling transitions again to cohesive failure of the seal layer 12, and the film is peeled off from the adherend, allowing it to be opened.
In FIG. 2(b), for reference, the flange portion is further enlarged, with the cohesive failure of the seal layer 12 indicated by dotted line L1 and the delamination of the heat seal portion W1 indicated by solid line L2.

Thus, when the film of the present invention is opened after being sealed to the adherend (base material) 20, first, cohesive failure of the seal layer 12 occurs from the opening start point S1 to the heat seal section W1, then delamination occurs between the seal layer 12 and the adjacent layer 14 at the heat seal section W1, and then, after passing through the heat seal section W1, cohesive failure of the seal layer 12 occurs, resulting in easy opening. Due to these characteristics, the film (covering material) 10 of the present invention firmly adheres to the adherend (base material) 20 when subjected to strong heat and/or pressure such as double sealing, and is capable of combining the seemingly contradictory characteristics of maintaining adhesion even when internal pressure of the package is generated, and easy opening.

When sealing is performed twice, the thickness of the sealing layer 12 is reduced by half or more. For example, when the sealing layer 12 is thinned to 0.5 μm to 5 μm, or even 0.5 to 3 μm, the sealing layer 12 is likely to undergo cohesive failure, leading to delamination between the sealing layer 12 and the adjacent layer 14.

The film of the present invention is easy to open even when heat-sealed under conditions of high heat and/or pressure load such as double sealing. The peel strength at which peeling starts when opening is not particularly limited, but for example, the upper limit of the peel strength after double sealing is preferably 18.0 N/15 mm, more preferably 15.0 N/15 mm or less, even more preferably 12.0 N/15 mm or less, and particularly preferably 10.0 N/15 mm or less. If it is 18.0 N/15 mm or less, it can be peeled cleanly from the sheet to be adhered with light force. The lower limit is sufficient as long as sufficient heat seal adhesion is obtained, and is, for example, 5.0 N/15 mm or more.
Furthermore, the increase in peel strength of the film of the present invention after second sealing is preferably within 2.0 N/15 mm, more preferably within 1.5 N/15 mm, and even more preferably within 1.0 N/15 mm, compared to the peel strength after first sealing. If the increase in peel strength after second sealing is small, it means that even if the film at the heat-sealed portion is thinned by second sealing, easy-opening properties are sufficiently exhibited by the chain of cohesive failure, delamination, and cohesive failure described above.
The peel strength of the film of the present invention can be measured by the evaluation method of the Examples described later. The peel strength of the film of the present invention is the maximum stress during the peel test as described in the Examples, but in the film of the present invention, the peel strength of cohesive failure is higher than that of interlayer peeling, and the maximum stress is at the start of peeling.

The film of the present invention has a sealing strength that allows it to maintain its adhesion even when the internal pressure of the package is applied. The sealing strength is not particularly limited, but is generally preferably 50 to 300 mmHg, more preferably 100 to 280 mmHg, and even more preferably 150 to 280 mmHg.
The seal strength is measured using a seal strength measuring device (for example, 304-AW device manufactured by Sun Scientific Co., Ltd.) by gradually introducing air into the package and measuring the internal pressure when the package bursts.

<Method of producing the film of the present invention>
The film of the present invention can be produced by a known coextrusion method. For example, it can be produced by an inflation method, a T-die method, etc. The method for producing the coextruded film is not particularly limited, but a known feed block method, a multi-manifold method, or a combination thereof can be used.
The film may be unstretched, uniaxially stretched, simultaneously biaxially stretched, or sequentially biaxially stretched. If necessary, the film may be subjected to a surface treatment or finish such as corona treatment, printing, coating, or vapor deposition.

Additives can be mixed into each layer of the film of the present invention as long as the purpose is not impaired. Examples of additives include colorants, inorganic fillers, organic fillers, UV absorbers, light stabilizers, heat stabilizers, antioxidants, lubricants, anti-blocking agents, hydrolysis inhibitors, plasticizers, and flame retardants.

<Packaging>
The configuration, shape, and manufacturing method of the packaging material using the film of the present invention are not limited, but examples include bags and pouches in which the sealing layers of the film are placed opposite each other, and packaging materials in which the film of the present invention is used as a lid material and a cup, tray, or deep-drawn molded article made of another material is used as a base material, and the like, and these can be manufactured by known methods.
When the film of the present invention is used as a lid material, it is preferable to use a polypropylene-based resin as the material constituting the base material, because this has good heat seal adhesion and sealing strength when the internal pressure inside the package increases, and further because the above-mentioned easy-open phenomenon and functions can be fully exhibited.
Even if the package of the present invention is subjected to strong heat sealing such as double sealing, it will undergo cohesive failure in areas other than the heat-sealed areas and will undergo delamination at the heat-sealed areas, making it easy to open.

The present invention will be described below using examples, but the scope of the present invention is not limited to the following examples.
(Preparation of co-extruded film)
Using each of the raw materials indicated by the abbreviations below, a coextruded film having a three-layer structure as shown in Table 1 was produced by inflation molding. The melt flow rate (MFR) of the raw materials was measured in accordance with JIS K6922-2:2018 for the polyethylene resin and in accordance with JIS K7210-1:2014 for the polypropylene resin. The density of the raw materials was measured in accordance with JIS K7112:1999.

(raw materials)
PP1: Propylene-ethylene random copolymer polypropylene produced using a metallocene catalyst, density 900 kg/m ³ , MFR 7.0 g/10 min, melting point 125° C.
LD1: low density polyethylene resin, density 924 kg/m ³ , MFR 0.7 g/10 min LL1: linear low density polyethylene resin produced using a metallocene catalyst, density 937 kg/m ³ , MFR 1.8 g/10 min LL2: linear low density polyethylene resin produced using a metallocene catalyst, density 923 kg/m ³ , MFR 1.5 g/10 min

(Preparation of Laminated Film)
A 15 μm thick biaxially oriented polyamide film was dry laminated to the other layer side of the obtained coextruded film using a two-liquid curing polyester-based polyurethane adhesive. During dry lamination, the machine direction (MD) of the biaxially oriented polyamide film was aligned with the machine direction (MD) of the coextruded film. After that, aging was performed at 45° C. for 48 hours to produce a laminated film.

(Peel strength measurement)
A single-sealed test piece was prepared by performing a solid seal once between the seal layer side of the laminated film and a 300 μm-thick homopolypropylene resin sheet used as an adherend under conditions of a sealing pressure of 0.2 MPa, a sealing time of 1 second, and a predetermined sealing temperature shown in Table 1. In addition, a double-sealed test piece was prepared by performing sealing under the same conditions using the single-sealed test piece.
Next, a strip of the laminate film having a length of 50 mm in the machine direction (MD) and a width of 15 mm in the horizontal direction (TD) was cut out, and in accordance with JIS Z0238:1998, each end of the laminate film and the adherend were attached to the gripping tools of a tensile tester with the heat-sealed portion of the strip at the center. The strip was pulled in the length direction of the strip, i.e., in the machine direction (MD) of the laminate film and the co-extruded film, until it peeled off or broke, at a peel angle of 180 degrees and a peel speed of 300 mm/min, and the maximum stress measured was measured as the peel strength (unit: N/15 mm width).

In Examples 1 to 5, even though the film at the heat-sealed portion became thinner after two seals, upon opening, the seal layer experienced cohesive failure, followed by delamination between the seal layer and the adjacent layer, and then cohesive failure of the seal layer, resulting in a peel strength of 18 N/15 mm or less, making the product easy to open. Furthermore, the peel strength after two seals was about the same as after one seal, making the product suitable for a wide range of packaging applications, both for one-sealing and two-sealing. In particular, Example 3 showed excellent ease of opening, with a peel strength of 10 N/15 mm or less after both one-sealing and two-sealing.

In Comparative Example 1, the peel strength after double sealing at a heat sealing temperature of 180°C exceeded 18 N/15 mm, and ease of opening was insufficient. This was thought to be because the heat-sealed portion of the film was crushed by the heat and pressure load caused by double sealing, and because the adjacent layers were composed of 100% linear low-density polyethylene by mass, which has strong interlayer adhesion, no interlayer peeling occurred.

The easy-to-open coextruded film of the present invention exhibits easy-to-open properties even when heat-sealed under conditions of high heat and/or pressure load, such as double sealing, because the sealing layer has a cohesive failure function and has a delamination function between the sealing layer and the adjacent layer. Therefore, even in packages of fermented foods and the like, in which the internal pressure increases after the package is sealed, the film of the present invention can be used to achieve both sufficient sealing strength and easy opening, which were not possible with conventional technology, making it extremely useful as a food packaging film.

10: Lid material 20: Base material 30: Contents 12: Sealing layer 14: Adjacent layer 16: Other layer 18: Other film T1: Handle portion W1: Heat seal portion, seal width S1: Opening start point L1: Cohesive failure L2: Interlayer peeling

Claims (5)

The present invention provides an easily openable coextruded film having at least a seal layer and an adjacent layer, the seal layer containing 50 to 95% by mass of a polypropylene-based resin and 5 to 50% by mass of a low-density polyethylene resin, and the adjacent layer consisting of only a polyethylene-based resin and containing 10 to 90% by mass of a low-density polyethylene resin, and when the coextruded film is heat-sealed to an adherend and then opened, the opening of the heat-sealed portion proceeds in the following order of (1) to (3).
(1) From the opening start point to the heat-sealed part, the seal layer undergoes cohesive failure. (2) At the heat-sealed part, delamination occurs between the seal layer and the adjacent layer. (3) After passing through the heat-sealed part, the seal layer undergoes cohesive failure. The easy-open coextruded film according to claim 1, wherein the sealing layer comprises 50 to 75 % by mass of a polypropylene-based resin and 25 to 50% by mass of a low-density polyethylene resin. The easy-to-open co-extruded film according to claim 1 or 2, wherein the thickness of the sealing layer is 5 to 15 μm. A package comprising a lid material made of the easy-open co-extruded film of any one of claims 1 to 3, and the lid material is heat-sealed to a base container made of a polypropylene-based resin. The package of claim 4, in which the easy-to-open coextruded film delaminates at the heat-sealed portion of the package and fails cohesively at the portion other than the heat-sealed portion.