Polypropylene-based films and laminates for retort food packaging

Abstract

This invention provides a polypropylene film that combines low-temperature heat sealability, heat resistance, low-temperature impact resistance, and steam permeability during microwave heating, as well as a laminate for retort food packaging using the same. [Solution] A polypropylene-based unoriented film laminated with a substrate, comprising at least two layers: a base layer (layer A) and a seal layer (layer B), wherein layer A, which is laminated with the substrate layer, has a melt flow rate of 0.5 to 10 g / 10 min at 230°C under a load of 21.18 N, the amount of xylene-soluble part CXS at 20°C is 8% by mass or more and 25% by mass or less, the intrinsic viscosity [η]CXS of the xylene-soluble part CXS at 20°C is 1.5 dl / g or more and 3.5 dl / g or less, and the intrinsic viscosity [η]CXIS of the xylene-insoluble part CXIS at 20°C is 1.5 dl / g or more and 2 The resin composition consists of a polypropylene-impact copolymer with a concentration of 0.2 dl / g or less as the main component, and layer B, which is laminated with layer A, mainly composed of copolymer polypropylene obtained by copolymerizing propylene with α-olefins such as ethylene and butene. Layer A, layer B, or both layers consist of a polypropylene copolymer (e) that is compatible with polypropylene, and the film surface average roughness Ra of the surface of layer B is 0.05 μm or more, the aspect ratio Str of the surface properties is 0.1 to 1.0, and the peak count RPc with a peak height of 0.3 μm or more is 100 or more per 40 mm. 2 The above-mentioned polypropylene film and laminate for retort food packaging using the same.

Description

[Technical Field]

[0001] The present invention relates to a polypropylene film and a laminate for retort food packaging using the same. [Background technology]

[0002] The packaging for retort foods uses laminates that combine heat-resistant substrates such as biaxially oriented polyethylene terephthalate film (sometimes referred to as PET), which has excellent heat resistance; biaxially oriented polyamide 6 film (sometimes referred to as ON), which has excellent strength; and aluminum foil (sometimes referred to as AL foil), which has excellent barrier properties; with unoriented polypropylene film (sometimes referred to as CPP), which can be heat-sealed.

[0003] In recent years, Japan has seen an increase in microwave-safe packaging materials using PET, which has superior barrier properties, instead of aluminum foil. Furthermore, in terms of environmental considerations, there is progress in exploring plastic-reducing packaging materials that do not use iontophoresis (ON), and monomaterial packaging materials that consider recycling.

[0004] Retort foods range in weight from individual servings of around 100g to commercial-use products weighing over 1kg, and the CPP used in their packaging requires excellent impact strength.

[0005] Monomaterial packaging for retort foods involves a PP monomaterial combining biaxially oriented polypropylene film (hereinafter sometimes referred to as OPP) and CPP. However, OPP has a lower melting point than PET and lower strength than ON. Therefore, the CPP used in combination with OPP needs to have both heat resistance suitable for retort processing and low-temperature heat sealability, as well as excellent low-temperature impact strength.

[0006] For retort food packaging, propylene-impact copolymer resin (hereinafter sometimes referred to as impact PP), which has excellent low-temperature impact strength, is often used.

[0007] Impact PP is primarily composed of polypropylene (hereinafter sometimes referred to as PP), a homopolymer of propylene, and is blended with elastomer (hereinafter sometimes referred to as PER), a copolymer of propylene and ethylene, in a polymerization facility.

[0008] Although attempts have been made to further improve the low-temperature impact resistance of this impact PP by adding an elastomer, which is a copolymer of ethylene and α-olefin, these ethylene-α-olefins are immiscible with PP, causing the phases to separate, resulting in a sea-island morphology where PP is the matrix (sea) and the elastomer is the domain (island).

[0009] When phase-separated PP and elastomer are subjected to less entanglement of their respective molecular chains, a large amount of elastomer added leads to a significant decrease in cohesive force within the film. When bag making and sealing is performed using laminates made with such CPP, a challenge has been that in high-speed bag making with short sealing times or single-sided heat sealing, there is little entanglement of molecular chains due to heat melting, resulting in insufficient seal strength.

[0010] Furthermore, since both the PP component, which is the main component of Impact PP, and the PP used in OPP are homopolymers of polypropylene, there is almost no difference in their melting points. For this reason, a challenge is that when the sealing temperature during bag making is increased in PP monomaterial packaging materials that combine OPP and Impact PP, the OPP melts and the bag shrinks.

[0011] One possibility is to composite laminate impact PP with copolymer polypropylene resin (hereinafter sometimes referred to as copolymer PP), which has a lower melting point due to copolymerization of propylene with α-olefins such as ethylene and butene, so that it can be heat-sealed at low temperatures. However, if copolymer PP is subjected to retort processing at temperatures exceeding 120°C, the CPP (inner surface of the bag) may fuse together.

[0012] Furthermore, the heat from the retort process promotes crystallization of polypropylene, which reduces the entanglement of molecular chains at the interface between impact PP and copolymer PP, leading to a decrease in heat seal strength.

[0013] Patent Document 1 proposes a polypropylene multilayer film in which both surface layers consist of a propylene homopolymer and a non-crosslinked olefin-based thermoplastic elastomer, and the intermediate layer consists of a propylene-ethylene block copolymer and an ethylene-propylene copolymer elastomer. However, because the melting peak temperature of the polypropylene homopolymer in the heat-sealed surface layer is 155°C or higher, it was difficult to heat-seal at low temperatures.

[0014] Patent Document 2 proposes a multilayer film consisting of two surface layers mainly composed of propylene-based random copolymer and an intermediate layer mainly composed of propylene-ethylene block copolymer. Since the melting point of the propylene-based random copolymer in the surface layer is in the range of 135°C to 145°C, it has good low-temperature heat sealability. However, fusion of the CPP film (inner surface of the bag) is a problem in high-retort processing at temperatures above 120°C and below 130°C.

[0015] Patent Document 3 proposes a polypropylene multilayer film with a sea-island structure, in which polyethylene, propylene-ethylene elastomer, or ethylene-α-olefin elastomer are added as dispersion components to a seal layer in which the matrix is ​​a propylene-ethylene random copolymer and a base layer in which block polypropylene. However, when resins with low compatibility are melt-blended, each phase separates to form a sea-island structure, resulting in less entanglement of molecular chains between the matrix phase and the domain phase, and a problem in that the cohesive force inside the film is low, leading to low heat seal strength. [Prior art documents] [Patent Documents]

[0016] [Patent Document 1] Patent No. 7537235 [Patent Document 2] Patent No. 6734656 [Patent Document 3] Patent No. 7632716 [Overview of the project] [Problems to be Solved by the Invention]

[0017] The problem of the present invention is to provide a CPP used in a laminate for retort food packaging with a reduced number of layers in consideration of the environment or a laminate for retort food packaging made of a polypropylene single material in consideration of recycling, which has low-temperature heat-sealability, heat resistance, low-temperature impact resistance, and steam permeability during microwave heating. The present invention aims to provide a polypropylene-based film having these properties and a retort food packaging laminate using the same. [Means for Solving the Problems]

[0018] In order to solve the above problems, the present invention has the following configuration. (1) A polypropylene-based non-stretched film laminated with a base material, wherein the polypropylene-based non-stretched film is composed of at least two layers, namely a base layer (A layer) and a seal layer (B layer). The layer (A layer) on the surface to be bonded to the base material layer has a melt flow rate of 0.5 to 10 g / 10 minutes at 230 °C under a load of 21.18 N, the amount of 20 °C xylene-soluble part CXS is 8% by mass or more and 25% by mass or less, the intrinsic viscosity [η] CXS of the 20 °C xylene-soluble part CXS is 1.5 dl / g or more and 3.5 dl / g or less, and the intrinsic viscosity [η] CXIS of the 20 °C xylene-insoluble part CXIS is 1.5 dl / g or more and 2.2 dl / g or less. The polypropylene-impact copolymer is the main component. The B layer laminated with the A layer is mainly composed of a copolymer polypropylene obtained by copolymerizing propylene with an α-olefin such as ethylene or butene. The A layer or the B layer, or both layers, are made of a resin composition containing a polypropylene-based copolymer (e) compatible with polypropylene. The average roughness Ra of the film surface on the B layer surface is 0.05 μm or more, the aspect ratio Str of the surface property is 0.1 to 1.0, and the peak count number RPc with a peak height of 0.3 μm or more is 100 or more per 40 mm. 2 A polypropylene-based film characterized by the above. (2) The polypropylene-based copolymer (e) compatible with the polypropylene is a copolymer of ethylene and an α-olefin with a propylene ratio in the range of 51 to 97 mol%, and has a density in the range of 861 to 891 kg / m 3 The polypropylene-based film according to (1), which is in the range. (3) The polypropylene-based copolymer (e) compatible with the polypropylene has a polypropylene polymerized with propylene as a hard segment and ethylene-α-olefin copolymerized as a soft segment. The polypropylene-based film according to (1) or (2). (4) The polypropylene-based film according to any one of (1) to (3), wherein the addition amount of the polypropylene-based copolymer (e) compatible with the polypropylene is in the range of 3% to 50% by weight of the entire film. (5) The polypropylene-based film according to any one of (1) to (4), wherein the ethylene-α-olefin copolymer containing an α-olefin having 3 to 10 carbon atoms is contained in the entire film at 40% by weight or less. (6) The polypropylene-based film according to any one of (1) to (5), wherein there is no fusion of the film in a 130°C blocking test in which the seal surfaces are overlapped. (7) The polypropylene-based film according to any one of (1) to (6), wherein when the seal surfaces are overlapped and heat-sealed, the heat-sealing start temperature when the heat-sealing strength becomes 3 N / 15 mm or more is 150°C or less. (8) The polypropylene-based film according to any one of (1) to (7), wherein when the B surfaces are heat-sealed in the laminate of the base material and the polypropylene-based non-stretched film, the seal strength after retort is 35 N / 15 mm or more at 23°C in an atmosphere. (9) The polypropylene-based film according to any one of (1) to (8), wherein when the B surfaces are heat-sealed in the laminate of the base material and the polypropylene-based non-stretched film, the seal strength in an atmosphere of 100°C after retort is in the range of 4 N / 15 mm or more and 30 N / 15 mm or less. (10) A polypropylene film according to any of (1) to (9), wherein the film thickness is in the range of 20 μm or more and 150 μm or less. (11) A polypropylene film according to any one of (1) to (10), wherein the thickness ratio of the base layer to the seal layer is in the range of 10:1 or more and 2:1 or less. (12) The polypropylene film according to any one of (1) to (11), wherein the copolymer polypropylene obtained by copolymerizing propylene and α-olefin in the B layer is mainly composed of a crosslinked compound resin obtained by melt-extruding a mixture of propylene, polyethylene and an organic peroxide crosslinking agent. (13) A polypropylene film according to any one of (1) to (12), wherein the extrapolation starting point (according to JIS K7121-1987) of the temperature change of the storage modulus (E') of the polypropylene film is 110°C or higher and 160°C or lower. (14) A laminate for retort food packaging comprising two or fewer base material layers having at least one selected from the group consisting of biaxially oriented polyamide film, biaxially oriented polyethylene terephthalate film, biaxially oriented polypropylene film, biaxially oriented polybutylene terephthalate film, biaxially oriented polyester / polyamide hybrid film, uniaxially oriented polyamide film, uniaxially oriented polyethylene terephthalate film, uniaxially oriented polypropylene film, and uniaxially oriented polybutylene terephthalate film, and at least one selected from the group consisting of a vapor-deposited film, synthetic paper, and aluminum foil, wherein at least one of these films is subjected to metal vapor deposition, inorganic vapor deposition, or metal oxide vapor deposition, and a polypropylene film according to any one of (1) to (13). (15) The laminate for retort food packaging according to (14), wherein the base material is a stretched polypropylene film, and the proportion of polypropylene resin in the laminate with the polypropylene film is 80% by mass or more.

[0019] A particularly preferred embodiment of the present invention is a laminate for retort food packaging, comprising a heat-resistant substrate and a polypropylene-based unoriented film, wherein the polypropylene-based unoriented film consists of at least two layers: a base layer and a seal layer, the base layer mainly composed of impact PP, and the seal layer mainly composed of copolymer PP. A polypropylene-based copolymer, characterized by its compatibility with polypropylene, is added to both the base layer and the seal layer, or to either the base layer or the seal layer, in a total amount ranging from 3% to 50% by weight. The copolymer PP in the seal layer has a composition of crosslinked polyethylene compounded, the average surface roughness Ra of the film is 0.05 μm or more, the aspect ratio Str of the surface properties is 0.1 to 1.0, and the peak count RPc of the peak height of 0.3 μm or more is 100 or more per 40 mm. 2 The laminated material for retort food packaging is characterized by the above features. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a laminate for retort food packaging that has excellent impact resistance at low temperatures, low-temperature heat sealability, and resistance to fusion during retort processing, and furthermore, an environmentally friendly PP monomaterial packaging material can be provided. [Modes for carrying out the invention]

[0021] The following describes in detail the polypropylene film of the present invention and the laminate for retort food packaging using the same.

[0022] The laminate according to the present invention is a laminate of a substrate (preferably a heat-resistant substrate) and a polypropylene-based unoriented film, wherein the polypropylene-based unoriented film consists of at least two layers: a base layer and a sealing layer.

[0023] The above base layer preferably contains 50% by mass or more of impact PP. If the impact PP content is less than 50% by mass, the laminate formed by laminating with a heat-resistant substrate may have poor low-temperature impact resistance when used as a packaging bag.

[0024] The above impact PP has a 20°C xylene-soluble portion CXS of 8% by mass or more and 25.0% by mass or less, an intrinsic viscosity [η]CXS of the 20°C xylene-soluble portion CXS of 1.5 dl / g or more and 3.5 dl / g or less, an intrinsic viscosity [η]CXIS of the 20°C xylene-insoluble portion CXIS of 1.5 dl / g or more and 2.2 dl / g or less, preferably a lower limit of 11% by mass or more and an upper limit of 22% by mass or less for the amount of xylene-soluble portion CXS, and a lower limit of 1.7 dl / g or more and an upper limit of 3.2 dl / g for the xylene-soluble portion [η]CXS. The following ranges are preferable for obtaining the characteristics of the laminate for retort food packaging targeted by the present invention: the amount of xylene is dl / g or less, the lower limit of the xylene-insoluble portion [η]CXIS is 1.6 dl / g or more and the upper limit is 2.1 dl / g or more, more preferably the lower limit of the amount of the xylene-soluble portion CXS is 13% by mass or more and the upper limit is 20% by mass or less, the lower limit of the xylene-soluble portion [η]CXS is 2.0 dl / g or more and the upper limit is 3.0 dl / g or less, and the lower limit of the xylene-insoluble portion [η]CXIS is 1.7 dl / g or more and the upper limit is 2.0 dl / g or more.

[0025] Here, the above-mentioned 20°C xylene-insoluble portion CXIS and soluble portion CXS refer to the following: when the above-mentioned polypropylene film is completely dissolved in boiling xylene, the temperature is lowered to 20°C, left for 4 hours or more, and then filtered to separate the precipitate from the solution, the precipitate is referred to as the 20°C xylene-insoluble portion CXIS (hereinafter sometimes referred to as the xylene-insoluble portion CXIS), and the solution portion (filtrate) is dried to dryness under reduced pressure at 70°C to obtain the portion referred to as the 20°C xylene-soluble portion CXS (hereinafter sometimes referred to as the xylene-soluble portion CXS).

[0026] The xylene-insoluble portion CXIS at 20°C corresponds to pure polypropylene, while the xylene-soluble portion CXS corresponds to the elastomer component.

[0027] If the amount of xylene-soluble part CXS in the base layer is less than 8% by mass, the low-temperature impact resistance may be poor, the decrease in heat seal strength of the laminate in a 100°C atmosphere may be small, and steam permeability may be poor when heated in a microwave oven. If the amount of CXS exceeds 25% by mass, the film becomes more prone to blocking, the decrease in heat seal strength at 100°C is significant, and steam may pass through before the contents are sufficiently heated in a microwave oven.

[0028] If the intrinsic viscosity ([η]CXS) of the xylene-soluble portion CXS in the above base layer is less than 1.5 dl / g, the low-temperature impact resistance decreases, and if it is greater than 3.5 dl / g, the dispersibility into the xylene-insoluble portion CXIS deteriorates, which may worsen the melt extrudeability.

[0029] Furthermore, if the intrinsic viscosity ([η]CXIS) of the xylene-insoluble portion CXIS in the above-mentioned polypropylene film is less than 1.5 dl / g, the low-temperature impact resistance decreases, and if it is greater than 2.2 dl / g, the dispersibility of the xylene-soluble portion CXS may deteriorate.

[0030] The melt flow rate (hereinafter sometimes referred to as MFR) of the above-mentioned impact PP is preferably in the range of 0.5 to 10 g / 10 min, more preferably with a lower limit of 1.0 g / 10 min or more and an upper limit of 6 g / 10 min or less, and even more preferably with a lower limit of 1.5 g / 10 min or more and an upper limit of 4 g / 10 min or less, from the viewpoint of extrusion stability and low-temperature impact resistance, at 230°C and a load of 21.18 N.

[0031] By adding an ethylene-α-olefin copolymer to the above base layer in an amount of 10% to 50% by mass, the low-temperature impact resistance of the laminate for retort packaging can be improved.

[0032] Ethylene-α-olefin copolymers include linear low-density polyethylene (LLDPE), which is copolymerized with ethylene and 1-butene, 1-hexene, 1-octene, etc., having 4 to 10 carbon atoms, and elastomers (EBR), which are randomly copolymerized with ethylene and butene, etc.

[0033] From the viewpoint of extrusion stability and melt miscibility with impact PP, the ethylene-α-olefin copolymer is preferably given an MFR in the range of 0.5 to 10 g / 10 min at 230°C and a load of 21.18 N.

[0034] If the ethylene-α-olefin copolymer content is less than 10% by mass, low-temperature impact resistance may be insufficient, and if it exceeds 50% by mass, the seal strength decreases significantly, making it difficult to achieve a PP ratio of 80% or more as a PP monomaterial in a laminate.

[0035] The sealing layer is preferably a copolymer PP with a melting temperature peak in the range of 135°C to 145°C. If the melting temperature peak is below 135°C, it tends to fuse easily with a retort treatment at 130°C, and if the melting temperature peak exceeds 145°C, the heat sealing start temperature at which the seal strength is 3N / 15mm or more exceeds 150°C, and in laminates bonded with OPP, the problem of thermal shrinkage of the OPP occurs.

[0036] The above-mentioned sealing layer has a film surface average roughness Ra of 0.05 μm or more, a surface texture aspect ratio Str of 0.1 to 1.0, and a peak height of 0.3 μm or more, with a peak count RPc of 100 or more per 40 mm. 2 Preferably, the surface texture has an Ra of 0.1 μm or more, an aspect ratio Str of 0.2 to 1.0, and a peak height of 0.3 μm or more, with a peak count RPc of 150 or more per 40 mm. 2 More preferably, the Ra is 0.15 μm or greater, the surface texture aspect ratio Str is 0.3 to 1.0, and the peak height is 0.3 μm or greater, with a peak count RPc of 200 or more per 40 mm. 2 It is preferable that the above conditions are met.

[0037] To prevent CPP from fusing together when retorted at 130°C, the shape and density of protrusions are important, along with the average surface roughness of the sealing surface.

[0038] One method for creating protrusions on the film surface is to add inorganic or organic particles. However, to achieve a roughness that prevents fusion at 130°C, it is necessary to add a large amount of large-diameter particles, which often leads to problems such as particle adhesion to the die during film formation and particle detachment from the film.

[0039] In this invention, the main component is a crosslinked compound resin, which is obtained by mixing linear low-density polyethylene with an organic peroxide crosslinking agent as the copolymer PP and melt-extruding it using a twin-screw extruder, thereby crosslinking the linear polyethylene into granular form. This makes it possible to obtain a film with a protrusion shape and protrusion density that does not fuse even after retort treatment at 130°C, and also provides good film-forming properties.

[0040] The amount of the above-mentioned organic peroxide added is preferably in the range of 0.1 to 10% by mass relative to the resin component, and more preferably in the range of 0.5 to 5% by mass, as this allows for good dispersibility in the resin and enables the crosslinking agent effect on the polyethylene to be exerted.

[0041] The above organic peroxides are not particularly limited, but examples include alkyl peroxides, diacyl peroxides, peroxide esters, and peroxide carbonates. Examples of alkyl peroxides include dicumyl peroxide, di-tert-butyl peroxide, di-tert-butylcumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butyl-oxy)hexyne-3, tert-butylcumyl, 1,3-bis(tert-butylperoxyisopropyl)benzene, and 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane.

[0042] Examples of diacyl peroxides include benzoyl peroxide, lauroyl peroxide, and decanoyl peroxide. Examples of peroxide esters include 1,1,3,3-tetramethylbutyl peroxyneodecanoate, α-cumyl peroxyneodecanoate, tert-butyl peroxyneodecanoate, tert-butyl peroxyneoheptanoate, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-amyl peroxyl-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyisobutyrate, di-tert-butyl peroxyhexahydroterephthalate, tert-amyl peroxy 3,5,5-trimethylhexanoate, tert-butyl peroxy 3,5,5-trimethylhexanoate, tert-butyl peroxyacetate, Examples include tert-butyl peroxybenzoate and dibutyl peroxytrimethyl adipate.

[0043] Examples of peroxide carbonates include di-3-methoxybutyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, diisopropyl peroxybonate, tert-butyl peroxyisopropyl carbonate, di(4-t-butylcyclohexyl) peroxydicarbonate, dicetyl peroxydicarbonate, and dimyristyl peroxydicarbonate.

[0044] Furthermore, it is preferable from a food hygiene perspective for retort food packaging that the above-mentioned organic peroxides do not remain in the sealing layer.

[0045] In addition to the method of adding inorganic or organic particles, another method is to use a film-forming nip roller to impart an embossed shape to one side of the film (the sealing layer). There are no particular restrictions on the film-forming nip roller as long as it is capable of forming the sealing layer in the present invention, but for example, the film-forming nip rollers disclosed in International Publication No. 2013 / 80925 and Japanese Patent Application Publication No. 2020-55189 can be preferably used. Specifically, an embossing nip roller having an arithmetic mean roughness Ra of 0.2 μm or less, a ten-point mean roughness Rz of 2 to 8 μm, and an average spacing Sm of the bumps and grooves of 90 μm or less is preferred.

[0046] The base layer and seal layer of the above-mentioned polypropylene-based unoriented film may contain antioxidants, heat stabilizers, neutralizing agents, antistatic agents, hydrochloric acid absorbers, antiblocking agents, lubricants, etc., to the extent that they do not impair the objectives of the present invention. These additives may be used individually or in combination of two or more.

[0047] Specific examples of antioxidants include hindered phenols such as 2,6-di-t-butylphenol (BHT), n-octadecyl-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate ("Irganox" 1076, "Sumilizer" BP-76), tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane ("Irganox" 1010, "Sumilizer" BP-101), and tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate ("Irganox" 3114, Mark AO-20).

[0048] Examples of phosphite-based (phosphorus-based) antioxidants include tris(2,4-di-t-butylphenyl) phosphite ("Irgafos" 168, Mark 2112), tetrakis(2,4-di-t-butylphenyl)-4-4'-biphenylene-diphosphonite ("Sandstab" P-EPQ), bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite ("Ultranox" 626, Mark PEP-24G), and distearyl pentaerythritol diphosphite (Mark PEP-8).

[0049] Among these, 6-[3-(3-t-butyl-4-hydroxy-5-methyl)propoxy]-2,4,8,10-tetra-t-butyldibenz[d,f][1,3,2]-dioxaphosfepine ("Sumilizer" GP) and 2[1-2-hydroxy-3,5-di-t-pentylphenyl]ethyl]-4,6-di-t-pentylphenyl ("Sumilizer" GS) which possess the functions of both hindered phenols and phosphites are preferred. In particular, the combined use of these two is preferred because it is effective in suppressing resin decomposition during film formation and contributes to achieving both heat seal strength and blocking resistance.

[0050] The amount of antioxidant added can be set appropriately within the range of 100 to 10,000 ppm, depending on the type of antioxidant used.

[0051] As neutralizing agents, hydrotalcite compounds and calcium hydroxide are preferred for reducing smoke generation during film formation.

[0052] Methods for obtaining the above-mentioned polypropylene-based unstretched film include, for example, a method in which the above-mentioned resin and additive composition is melted and kneaded in a single-screw extruder, a twin-screw coaxial rotary extruder, or a twin-screw anomalous rotary extruder, then extruded from a multi-layer T-type die onto a cooling drum and cooled and solidified on a casting drum, or a method of melt-extruding onto a heat-resistant substrate and laminating. However, in the present invention, the method of extruding from a multi-layer T-type die onto a cooling drum and cooling and solidifying is preferred because it provides high heat seal strength.

[0053] The polypropylene copolymer added as a compatibilizer for ethylene-α-olefin copolymers, which are immiscible with polypropylene and cause a decrease in seal strength, is a copolymer with ethylene-α-olefins in which the propylene ratio is in the range of 51 to 97 mol%, and has a density of 861 to 891 kg / m³. 3 It is within the range of being completely compatible with impact PP, the main component of the base layer, and copolymer PP (matrix resin), the main component of the seal layer, and is incompatible with PP but is compatible with the copolymer of ethylene and α-olefin, which is a domain resin.

[0054] The polypropylene copolymers compatible with polypropylene mentioned above are polypropylene copolymers in which polypropylene is a hard segment and polypropylene is copolymerized with a soft segment which is a random copolymer of propylene and α-olefins such as ethylene. Similar effects were observed with random copolymers of butene, in which the proportion of propylene is 50% or more.

[0055] To obtain the characteristics of the laminate for retort packaging of the present invention, the amount of polypropylene copolymer compatible with polypropylene added is in the range of 3% to 50% by weight of the total film, preferably with a lower limit of 4% or more by weight and an upper limit of 40% or less by weight, and more preferably with a lower limit of 5% or more by weight and an upper limit of 30% or less by weight.

[0056] While it is preferable that the base layer and the seal layer have the same concentration of additive, if neither layer contains an additive amount of polypropylene-compatible polypropylene copolymer, or if the concentration is low, it is preferable to increase the concentration of the other layer so that the overall concentration of the film does not exceed 50% by weight.

[0057] To improve low-temperature impact resistance, ethylene-α-olefin copolymer can be added up to a maximum of 40% by weight of the total film. However, in order to obtain the characteristics of the laminate for retort food packaging of the present invention, it is necessary to add a polypropylene copolymer compatible with polypropylene in an amount equivalent to the amount of ethylene-α-olefin copolymer added. Therefore, the amount of ethylene-α-olefin copolymer added is preferably 3% by weight or more at the lower limit and 30% by weight or less at the upper limit, and more preferably 25% by weight or less at the upper limit when using a PP monomaterial packaging material.

[0058] In a PP monomaterial packaging material constructed with OPP laminate, for the OPP to be sealed without heat shrinkage, the heat-sealing start temperature at which the seal strength is 3N / 15mm or more when the sealing layer surfaces are overlapped and heat-sealed according to JIS Z1713:1999 must be 150°C or lower. Preferably, the heat-sealing start temperature is 147°C or lower, and more preferably 145°C or lower.

[0059] In order to ensure that the laminate for retort food packaging of the present invention, which consists of a heat-resistant substrate and a polypropylene-based unoriented film, meets the legally mandated heat seal strength of 23 N / 15 mm or more, a heat seal strength of 35 N / 15 mm or more in a 23°C atmosphere after retort processing is deemed acceptable.

[0060] Furthermore, products with a seal strength of 4N / 15mm or more and 30N / 15mm or less in a 100°C atmosphere after retorting were deemed acceptable because they allowed for both heating of the contents and steam release during microwave heating. If the heat seal strength in a 100°C atmosphere after retorting was less than 4N / 15mm, steam would escape from the seal before the food contents were sufficiently heated. If the heat seal strength in a 100°C atmosphere exceeded 30N / 15mm, the internal pressure of the bag became too high, resulting in an explosive steam release.

[0061] Therefore, in order for steam to escape with proper heating, the lower limit of the heat seal strength in a 100°C atmosphere after retorting is preferably 5 N / 15 mm or more, and the upper limit is preferably 25 N / 15 mm or less, and more preferably the lower limit of the heat seal strength in a 100°C atmosphere after retorting is 6 N / 15 mm or more, and the upper limit is 20 N / 15 mm or less.

[0062] The film thickness of the above-mentioned polypropylene-based unoriented film is preferably 20 μm or more and 150 μm or less, more preferably with a lower limit of 40 μm and an upper limit of 120 μm. The thickness ratio of the base layer to the seal layer is preferably 10:1 or more and 2:1 or less, more preferably 6:1 or more and 3:1 or less. In this case, stable film formation, low-temperature impact resistance, and heat seal strength of the film can be obtained, and bag-making properties can also be maintained.

[0063] In food packaging, there are standing bags, which are made by folding the bottom material into the bag and sealing it to give it self-supporting structure. In standing bags, gaps can occur at the overlapping part where the bottom material is folded into the body material and sealed. If molten resin does not flow into these gaps, the bag cannot be sealed.

[0064] While heat sealing at high temperatures is effective in filling the gaps between the overlapping sections of the body and base materials, in the case of a PP monomaterial packaging structure laminated with OPP, it is advantageous for the polypropylene-based unoriented film to soften quickly until it completely melts, as this is beneficial for filling the gaps between the overlapping sections of the body and base materials.

[0065] The relationship between the storage modulus (E') and temperature change was investigated as an indicator of the softening temperature of polypropylene unoriented film. Using the extrapolation start point according to JIS K7121-1987 as an indicator of when softening begins, it was found that if the extrapolation start point is 160°C or lower, the OPP does not heat shrink and the gap in the overlapping part of the body and bottom material can be filled. Therefore, in order to fill the gap in the overlapping part of the body and bottom material with a sealing time of 0.5 seconds during bag making, the extrapolation start point is preferably 160°C or lower, more preferably 155°C or lower, and even more preferably 150°C or lower. There is no lower limit for the extrapolation start point from the point of filling the gap in the overlapping part of the body and bottom material, but in order for the polypropylene unoriented film to not block at 130°C, the outer layer start point is preferably 110°C or higher, more preferably 115°C or higher, and even more preferably 120°C or higher.

[0066] As the above-mentioned base material, particularly as a heat-resistant base material, a base material layer of two or fewer layers is preferred, comprising at least one selected from the group consisting of biaxially oriented polyamide film, biaxially oriented polyethylene terephthalate film, biaxially oriented polypropylene film, biaxially oriented polybutylene terephthalate film, biaxially oriented polyester / polyamide hybrid film, uniaxially oriented polyamide film, uniaxially oriented polyethylene terephthalate film, uniaxially oriented polypropylene film, and uniaxially oriented polybutylene terephthalate, as well as a vapor-deposited film obtained by applying at least one selected from the group consisting of metal vapor deposition, inorganic vapor deposition, and metal oxide vapor deposition to one of the above films, and at least one selected from the group consisting of synthetic paper and aluminum foil. In the case of PP monomaterial packaging, it is preferable to use a base material of biaxially oriented polypropylene film or uniaxially oriented polypropylene film.

[0067] When the base material for the PP monomaterial packaging is a stretched polypropylene film, it is preferable that the proportion of polypropylene resin in the laminate with the polypropylene film is 80% by mass or more.

[0068] The method for laminating the above heat-resistant substrate and the above polypropylene-based unoriented film is not particularly limited, but a dry lamination method is preferred from a productivity standpoint.

[0069] While there are no particular limitations on adhesives for dry lamination, examples include two-component reactive aromatic adhesives, two-component reactive aliphatic adhesives, polyurethane adhesives, acrylic adhesives, epoxy adhesives, polyolefin adhesives, elastomer adhesives, fluorine adhesives, and the like, which consist of a first liquid comprising one or more polyols selected from the group consisting of polyurethane polyols, polyester polyols, and polyether polyols, and a second liquid (curing agent) comprising an isocyanate.

[0070] The thickness of the adhesive layer is preferably 0.5 to 5 μm, and more preferably 0.5 to 3 μm. If the thickness of the adhesive layer is 0.5 μm or more, it becomes easier to control the film thickness, and if it is 5 μm or less, it becomes easier to shorten the drying time and reduce production costs while providing sufficient adhesive strength.

[0071] The above-mentioned laminate for retort food packaging can be used after being processed into flat bags (flat pouches), standing pouches, etc., using a polypropylene-based unstretched film as the inner surface of the bag as a sealing layer. [Examples]

[0072] The present invention will be specifically described below with reference to examples, but the scope of the present invention is not limited thereto. Furthermore, the measured values ​​of each evaluation item in the detailed description of the present invention and in the examples were measured by the following method.

[0073] (1) Melting temperature peak Using a differential scanning calorimeter (Shimadzu DSC-60), the highest peak temperature of the melting peak when the temperature was increased from 20°C at a rate of 10°C / min to 250°C was defined as the melting temperature peak.

[0074] (2) Density of polyethylene (unit: g / cm³) 3 ) Measurements were taken according to Method A (water displacement method) of JIS-K7112:1999.

[0075] (3) Melt Flow Rate (MFR) In accordance with JIS K7210:1999, propylene-based random copolymers, propylene-ethylene block copolymers, and propylene-α-olefin copolymer elastomers were measured at a temperature of 230°C, while polyethylene-based resins and ethylene-α-olefin copolymer elastomers were measured at a temperature of 190°C, each under a load of 21.18 N.

[0076] (4) Content of xylene-soluble portion CXS and insoluble portion CXS at 20°C 5 g of film or polymer was completely dissolved in 500 ml of boiling xylene (Grade 1, manufactured by Kanto Chemical Co., Ltd.), then cooled to 20°C and left for at least 4 hours. After that, the mixture was filtered into precipitate and solution to separate the xylene-soluble and xylene-insoluble parts. The mass of the xylene-insoluble part was determined by measuring the mass at 23°C after drying the precipitate under reduced pressure at 70°C. The xylene-soluble part CXS was also determined by measuring the mass after drying the filtrate under reduced pressure at 70°C.

[0077] (5) Intrinsic viscosity of xylene-insoluble portion CXIS and soluble portion CXS of film and polymer The samples separated by the method described above were measured using an Ubbelohde viscometer in tetralin at 135°C.

[0078] (6) Film surface average roughness Ra, peak count RPc based on roughness curve elements, peak count of 0.3 μm or more Using a fully automatic micro-shape measuring machine (SURFCORDER ET4000A) manufactured by Kosaka Laboratory Co., Ltd., the film surface was measured in the direction perpendicular to the film flow direction (TD) according to the measurement method specified in JIS B-0601:1982, under the following conditions, to determine the average surface roughness Ra, the aspect ratio Str of the surface properties, and the peak count RPc based on the roughness curve elements, with peak counts of 0.3 μm or more being determined. Measurement length: 4mm Measurement length in the Y direction: 10 mm Measurement pitch in the Y direction: 0.1 mm Number of scans: 100 times.

[0079] (7) Film thickness and thickness composition The film thickness was measured at 10 arbitrary points on the film in accordance with the JIS K7130 (1992) A-2 method using a dial gauge. The average value was taken as the film thickness.

[0080] (8) Temperature at which the heat seal strength is 3 N / 15 mm or more In accordance with JIS Z1713:2009, samples with the heat seal temperature changed by overlapping the heat seal surfaces of polypropylene-based non-stretched films were measured for heat seal strength at a peeling speed of 300 mm / min using a tensilon manufactured by Orientec Co., Ltd. In this measurement method, a heat seal start temperature at which the heat seal strength at 23°C is 3 N / 15 mm or more and 150°C or less was considered to have good low-temperature heat sealability.

[0081] (9) Heat seal strength before and after retort Using an aliphatic ester-based adhesive (Mitsui Chemicals, Inc.'s "Takelac" (registered trademark) A385 / "Takenate" (registered trademark) A50, adhesive layer thickness 2 μm), a heat-resistant base material layer and a polypropylene-based non-stretched film were laminated by the normal dry lamination method and aged at 40°C for 3 days to prepare a laminate. The polypropylene-based non-stretched films of the laminate were overlapped, and as conditions corresponding to single-sided heat seal for bag making, the upper plate of the heat sealer was set at 190°C, the lower plate at 80°C, pressure 2 kg / cm 2 , and samples heat-sealed for 1 second were subjected to a retort treatment at 130°C for 30 minutes, and the seal strength before and after the retort treatment was measured at a speed of 300 mm / min using a tensilon manufactured by Orientec Co., Ltd. In this measurement method, a seal strength of 35 N / 15 mm or more after the retort treatment was considered suitable for retort packaging.

[0082] (10) Blocking shear force at 130°C A 30mm wide and 100mm long film sample was prepared from a polypropylene unoriented film. The sealing layers were overlapped over a 30mm x 40mm area, a 10g load was applied, and the samples were heated in a 130°C oven for 30 minutes. After being left in an atmosphere of 23°C and 65% humidity for at least 30 minutes, shear peeling force was measured using an Orientec Tensilon at a tensile speed of 300mm / min. Films that peeled without plastic deformation exceeding the yield point stress using this measurement method were considered to have good high-temperature blocking resistance.

[0083] (11) Bag breakage retention rate (low temperature impact resistance) A 60 μm thick unoriented polypropylene film was bonded to one side of a heat-resistant substrate layer of a 15 μm thick biaxially oriented polyamide film using an aliphatic ester adhesive (Mitsui Chemicals, Inc. "Takelac" (registered trademark) A385 / "Takenate" (registered trademark) A50, adhesive layer thickness 2.5 μm) via a standard dry lamination method, and then aged at 40°C for 3 days to create a laminate.

[0084] Using the aforementioned laminate, standing bags were manufactured using a bag-making machine (model: SBM-350-SST) manufactured by Seibu Machinery Co., Ltd. For bag sealing, sufficient heat was applied to the polypropylene film. The bottom seal was done with one pass at 210°C / 210°C for both top and bottom, the point seal was done with one pass at 210°C / 210°C for both top and bottom, and the side seal was done with two passes at 210°C / 80°C and 80°C / 210°C. The sealing time was 0.5 seconds, and the number of shots was 50 shots / minute. The finished bags were 130 mm wide and 210 mm high, filled with 350 g of saline solution, stored at 0°C for 3 days, and then dropped vertically from a height of 0.7 m in a 0°C environment. Under the same conditions, 20 bags were dropped 20 times, and the percentage that did not break was defined as the bag break retention rate. A bag break retention rate of 50% or more was considered to indicate good low-temperature impact strength.

[0085] (12) Heat seal strength in a 100°C atmosphere (steam permeability during microwave heating) Using the packaging bags obtained in (11) above, a retort treatment was performed at 130°C for 30 minutes. The heat seal strength was measured using Orientec's Tensilon at a 100°C atmosphere with a peeling rate of 300 mm / min. A value of 30 N / 15 mm or less was evaluated as good steam permeability during microwave heating, and a value exceeding 30 N / 15 mm was evaluated as poor steam permeability.

[0086] (13) Bag making test A bag-making test of standing bags was conducted using a bag-making machine (model: SBM-350-SST) manufactured by Seibu Machinery Co., Ltd., by laminating two 20μm thick biaxially oriented polypropylene films with a 60μm thick unoriented polypropylene film. The bag-making seal was set to the upper limit temperature at which the biaxially oriented polypropylene film does not shrink due to heat. The bottom seal was a single pass at 185°C / 185°C for both top and bottom, the point seal was a single pass at 185°C / 185°C for both top and bottom, and the side seal was a double pass at 195°C / 80°C and 80°C / 195°C. The sealing time was 0.5 seconds, and the number of shots was 50 shots / minute. An Ageless Seal Check Spray manufactured by Mitsubishi Chemical Corporation was sprayed onto the overlapping area of ​​the bottom and body materials. Bags where the permeating liquid could not penetrate to the other side were marked as "successful gap filling" (○), and those where it could penetrate were marked as "failure to fill the gap" (×).

[0087] (14) Storage modulus (E') Using a TA Instruments RSA-G2 dynamic viscoelasticity analyzer, a 60 μm thick unoriented polypropylene film was cut into 4 mm wide strips and stacked in three layers. The measurement temperature was increased from 40 °C to 170 °C at a rate of 3 °C / min, and the viscoelasticity was measured at a frequency of 1 Hz and a strain of 0.05%. The extrapolation starting point for the temperature dependence of the storage modulus (E') of the polypropylene film (according to JIS K7121-1987) was determined as the inflection point (°C) of the storage modulus.

[0088] (15) Percentage of polypropylene resin in a laminate with polypropylene film Three sheets of biaxially oriented polypropylene film (20 μm thick), biaxially oriented polypropylene film (20 μm thick), and unoriented polypropylene film (60 μm thick) were laminated together with an aliphatic ester adhesive (adhesive layer thickness 2.5 μm). The proportion of polypropylene resin in the laminated polypropylene film was divided by the total thickness of the laminated film to determine the proportion of polypropylene resin.

[0089] (Examples 1-11, Comparative Examples 1-9) The various raw material compositions used in this invention are described below. Furthermore, the properties of the polypropylene-based unoriented film and the laminate based on these raw material formulations are summarized in Table 1 (Examples) and Table 2 (Comparative Examples).

[0090] (1) Polypropylene-impact copolymer (a) MFR: 2.1g / 10min (230℃) CXS amount: 20% by mass [η]CXS: 3.2 dl / g [η]CXIS: 1.8 dl / g Peak melting temperature: 163°C

[0091] (2) Copolymer polypropylene (b) Ethylene-propylene random copolymer. Ethylene content: 4% by mass MFR: 3.0g / 10min (230℃) Melting temperature peak: 142°C

[0092] (3) Ethylene-α-olefin copolymer (c) Linear low-density polyethylene copolymerized with 1-octene MFR: 0.8g / 10min (190℃) Density: 925kg / m 3 Peak melting temperature: 125°C

[0093] (4) Random copolymer of ethylene and butene (d) Ratio of ethylene to butene: 84% and 16% MFR: 2.2g / 10min (230℃) Density: 885kg / m 3 Peak melting temperature: 66°C

[0094] (5) Polypropylene copolymer (e1) in which polypropylene is the hard segment and a random copolymer of propylene and ethylene is the soft segment Ratio of propylene to ethylene: 85% and 15% MFR: 20g / 10min (230℃) Density: 861kg / m 3 Vicat softening point: 48℃

[0095] (6) Polypropylene copolymer (e2) in which polypropylene is the hard segment and a random copolymer of propylene and ethylene is the soft segment Propylene to ethylene ratio: 96.4% and 3.6% MFR: 8g / 10min (230℃) Density: 891kg / m 3 Vicat softening point: 105℃

[0096] (7) Polypropylene copolymer in which polypropylene is the hard segment and propylene and ethylene α-olefin copolymer is the soft segment (e3) Ratio of propylene to ethylene α-olefin: 84% and 16% MFR: 6g / 10min (230℃) Density: 868kg / m 3

[0097] (8) Random copolymer of polypropylene and butene (e4) Propylene to butene ratio: 70% and 30% MFR: 7g / 10min (230℃) Density: 885kg / m 3

[0098] (9) Compound resin of polypropylene and cross-linked polyethylene (f) The compound resin (f) of polypropylene and crosslinked polyethylene is prepared by thoroughly cooling 88.6% by mass of a propylene-based random copolymer (b) and 9% by mass of an ethylene-α-olefin copolymer (c) with liquid nitrogen, then grinding it into a powder using an impeller mill. To this, 2.4% by mass of “Perhexa” (registered trademark) 25B (manufactured by Nippon Oil & Fats Co., Ltd., chemical name: 2,5-dimethyl-2,5-di(t-butylperoxy)hexane) is added as a peroxide, and further, tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (“Sumilizer”) is added to 100 parts by mass of the total amount of this mixed composition. 0.05 parts by mass of (registered trademark) BP-101 and 0.01 parts by mass of "Irganox" (registered trademark) 1076 (manufactured by Ciba Specialty Chemicals) were added and mixed in a Henschel mixer for 3 minutes. The mixture was then supplied to a twin-screw extruder heated to 260°C and melt-kneaded to obtain the product.

[0099] The polypropylene-based unoriented film was obtained by using a T-die film-making machine temperature-controlled to 250°C, cooling and solidifying it at a take-up speed of 60 m / min and a cooling roll temperature of 45°C to form a 60 μm thick film, and then corona-discharging one side of it. The thickness ratio of the base layer to the seal layer was set to 5:1.

[0100] The heat-resistant substrate was prepared by laminating a 12 μm thick transparent vapor-deposited biaxially oriented polyethylene terephthalate film (1011 "Barrierox" (registered trademark) SBR2, manufactured by Toray Film Processing Co., Ltd.) and a 15 μm thick biaxially oriented polyamide 6 film (ONMB, manufactured by Unitika Ltd.) using an aliphatic ester adhesive ("Takelac" (registered trademark) A385 / "Takenate" (registered trademark) A50, manufactured by Mitsui Chemicals, Inc., with an adhesive layer thickness of 2.5 μm). After laminating the aforementioned unoriented polypropylene film to the biaxially oriented polyamide 6 side of the substrate film, the laminate was aged at 40°C for 3 days to obtain a laminate.

[0101] The properties of the obtained polypropylene-based unoriented film and the properties of the laminate with a heat-resistant substrate are shown in Tables 1 and 2.

[0102] In Example 1, the average surface roughness Ra of the B layer surface of the polypropylene film was 0.16 μm, the aspect ratio Str of the surface properties was 0.8, and the peak count RPc of peaks with a peak height of 0.3 μm or more was 350 or more per 40 mm. 2 Therefore, the blocking shear force at 130°C is 5 N / 12 cm. 2 There was no fusion of the bags, and the heat seal temperature for 3N / 15mm was 143℃, indicating good low-temperature heat sealability.

[0103] The heat seal strength of the laminated material with a heat-resistant substrate was 50 N / 15 mm after retort treatment at 130°C, and the bag breakage retention rate was 80%, satisfying all the required characteristics of the present invention.

[0104] The results for Examples 2-11 and Comparative Examples 1-9 are shown in Tables 1 and 2.

[0105] [Table 1]

[0106] [Table 2]

Claims

1. A polypropylene-impact copolymer is the main component of a polypropylene-impact copolymer containing 50% or more by mass of the following: a polypropylene-impact copolymer is a polypropylene-impact copolymer that is laminated with a substrate, the polypropylene-impact copolymer is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is a polypropylene-impact copolymer that is the main component of the polypropylene-impact copolymer, the polypropylene-impact copolymer is a polypropylene-impact copolymer that The B layer, which is laminated with the A layer, contains 66% by mass or more of copolymer polypropylene as the main component, which is obtained by copolymerizing propylene with ethylene and an α-olefin selected from 1-butene, 1-hexene, or 1-octene. The A layer, the B layer, or both layers are made of a resin composition containing a polypropylene copolymer (e) that is compatible with polypropylene, and the amount of the polypropylene copolymer (e) added is in the range of 3% to 50% by weight of the entire film. The average surface roughness Ra of the B layer surface is 0.05 μm or more, the aspect ratio Str of the surface properties is 0.1 to 1.0, and the peak count RPc with a peak height of 0.3 μm or more is 100 or more per 40 mm. ２ A polypropylene-based film characterized by the above.

2. The polypropylene copolymer (e) compatible with the polypropylene is a copolymer of ethylene-α-olefin with a propylene ratio in the range of 51 to 97 mol%, and has a density of 861 to 891 kg / m³. ３ A polypropylene film according to claim 1, which is within the range of the specified range.

3. The polypropylene film according to claim 1, wherein the polypropylene copolymer (e) compatible with the polypropylene is a polypropylene film in which polypropylene polymerized into a hard segment is copolymerized into a soft segment with ethylene-α-olefin.

4. The polypropylene film according to claim 1, wherein an ethylene-α-olefin copolymer containing an α-olefin having 3 to 10 carbon atoms is contained in an amount of 40% by weight or less of the total film.

5. The polypropylene film according to claim 1, wherein no film fusion occurs in a 130°C blocking test when the sealing layer surfaces are stacked together.

6. The polypropylene film according to claim 1, wherein when the sealing layer surfaces are overlapped and heat-sealed, the heat sealing start temperature at which the heat sealing strength becomes 3 N / 15 mm or more is 150°C or less.

7. The polypropylene film according to claim 1, wherein when the B-layer surfaces of the laminated material and the polypropylene-based unoriented film are heat-sealed together, the seal strength after retorting is 35 N / 15 mm or more in a 23°C atmosphere.

8. The polypropylene film according to claim 1, wherein when the B-layer surfaces of the laminated material and the polypropylene-based unoriented film are heat-sealed together, the seal strength in a 100°C atmosphere after retorting is in the range of 4 N / 15 mm or more and 30 N / 15 mm or less.

9. The polypropylene film according to claim 1, wherein the film thickness is in the range of 20 μm or more and 150 μm or less.

10. The polypropylene film according to claim 1, wherein the thickness ratio of the base layer to the seal layer is in the range of 10:1 or more and 2:1 or less.

11. The polypropylene film according to claim 1, wherein the copolymer polypropylene obtained by copolymerizing propylene and α-olefin in the B layer contains 75% by mass or more of a crosslinked compound resin, which is mainly composed of propylene and polyethylene and an organic peroxide crosslinking agent, and is melt-extruded.

12. The polypropylene film according to claim 1, wherein the extrapolation starting point (according to JIS K7121-1987) for the temperature dependence of the storage modulus (E') of the polypropylene film is 110°C or higher and 160°C or lower.

13. A laminate for retort food packaging comprising two or fewer substrate layers having at least one selected from the group consisting of biaxially oriented polyamide film, biaxially oriented polyethylene terephthalate film, biaxially oriented polypropylene film, biaxially oriented polybutylene terephthalate film, biaxially oriented polyester / polyamide hybrid film, uniaxially oriented polyamide film, uniaxially oriented polyethylene terephthalate film, uniaxially oriented polypropylene film, and uniaxially oriented polybutylene terephthalate film, and at least one selected from the group consisting of a vapor-deposited film, synthetic paper, and aluminum foil, wherein at least one of these films is subjected to metal vapor deposition, inorganic vapor deposition, or metal oxide vapor deposition, and a polypropylene-based film according to any one of claims 1 to 12 is laminated with the aforementioned polypropylene-based film.

14. The laminate for retort food packaging according to claim 13, wherein the base material is a stretched polypropylene film, and the proportion of polypropylene resin in the laminate with the polypropylene film is 80% by mass or more.

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