JP-2026075060-A - Biaxially oriented polypropylene film and packaging

Abstract

[Problem] To provide 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 the following: propylene-ethylene copolymer, propylene-ethylene-butene copolymer, or propylene-1/butene copolymer. 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. [Selection Diagram] Figure 1

Inventors

• 豊田 梨帆
• 長江 秀敏
• 中島 禎彦
• 濱田 俊一郎
• 寺本 靖丈

Assignees

• フタムラ化学株式会社

Dates

Publication Date

20260507

Application Date

20250930

Priority Date

20241021

Claims (5)

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. A biaxially oriented polypropylene film characterized in that, when the sealing layers are heat-sealed together under the conditions of 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. 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. The biaxially oriented polypropylene film according to claim 1, wherein when the overlapping 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.
3. The biaxially oriented polypropylene film according to claim 1 or 2 , wherein the low-density polyethylene has a density of 0.930 g/cm³ or less, and 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.

Description

This invention relates to 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 from this biaxially oriented polypropylene film. For example, in food packaging, stretched polypropylene film with excellent properties such as transparency and rigidity is preferably used. This packaging is manufactured by methods such as pillow packaging, gusset packaging, three-side sealed bags, four-side sealed bags, and heat-sealed bags, where the film surfaces are heat-sealed or otherwise processed. In recent years, packaging for rice balls, bread, and other similar products sold in convenience stores has seen increasing diversification of packaging forms. For example, in these types of packages, features such as easy opening are sometimes required, allowing consumers to consume the food without soiling their hands. These packages are often made using heat sealing or other methods to create special bag shapes tailored to the contents, and the heat-sealed portion can be easily peeled off for opening. In packaging with easy opening at the seal, an example of a polypropylene film for packaging is a stretched polypropylene film comprising at least two layers, including a seal layer, where the seal layer is composed of a mixed material of polypropylene resin and low-density polyethylene (see Patent Document 1). In this polypropylene film, the seal 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 incompatibility of the polypropylene resin and polyethylene resin in the seal layer causes cohesive breakdown due to a sea-island dispersion structure, resulting in easy opening. Incidentally, when opening a package by peeling off the seal portion of a manufactured film, it is necessary to apply a predetermined tearing force to initiate the peeling of the seal. Even if the 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, 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. Therefore, as a film with good peelability and ease of opening, for example, 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-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). 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 sealed portion is peeled off at a speed of 200 mm/min in the TD direction, the maximum heat seal strength is within 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, while the heat seal strength of the above-mentioned polypropylene film is maintained at a certain percentage above the maximum heat seal strength when the sealed portion is peeled off and reaches a specific travel distance, the property of a large drop in heat seal strength after peeling has not been improved. Therefore, the stability during peeling is insufficient, and the need for special production equipment with a four-layer structure makes it economically disadvantageous. Japanese Patent Publication No. 2021-095162Patent No. 7018793 This is a chart showing the heat seal strength of a biaxially oriented polypropylene film according to one embodiment of the present invention.This is a chart showing the heat seal strength of conventional biaxially oriented polypropylene films.These are schematic plan and cross-sectional views illustrating the seal structure formed at the opening of the packaging. A biaxially oriented polypropylene film according to one embodiment of the present invention is a biaxially oriented film for packaging, composed of at least two layers including a base layer and a sealing layer, both mainly made of polypropylene resin. During the packaging process, the sealing layers are heat-fused together to form an easily openable heat-sealed section. Packaging made from this biaxially oriented polypropylene film is primarily used as packaging for food products, and is particularly suitable for packaging rice balls, bread, and similar items. In the biaxially oriented polypropylene film of the present invention, the resin material used in each layer described later is app