Packaging paper for sealed containers and method for manufacturing the same

Abstract

To provide packaging base paper for sealed containers that is resistant to tearing even when the paper is folded, and is also resistant to tearing against external shocks.SOLUTION: The present invention relates to packaging base paper for sealed containers, which is mainly composed of pulp. At least 70 mass% or more of the pulp is softwood pulp, and a dry paper strength enhancer and a wet paper strength enhancer are contained. Carboxymethyl cellulose is applied to at least one side of the base paper.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a base paper for packaging sealed containers, and more particularly to a base paper for packaging sealed containers that is resistant to tearing even when folded and resistant to tearing from external impacts, and a method for manufacturing the same. [Background technology]

[0002] In recent years, the plastic waste problem has become increasingly serious. Global plastic production is said to exceed 400 million tons per year, with the packaging sector accounting for a particularly large portion and being the main cause of plastic waste. The most commonly used plastics for packaging are polyethylene terephthalate (PET), used in beverage bottles, and polyethylene (PE) and polypropylene (PP), used in shopping bags and container laminations. Plastics do not decompose semi-permanently, and their waste breaks down into microplastics in the natural environment, causing serious damage to ecosystems. In particular, marine pollution by microplastics is severe, and it is said that this plastic waste is impossible to recover. Reducing plastic use in the future is necessary for the global environment.

[0003] In response to this problem, there is a growing movement to replace plastic packaging with paper packaging. For example, for airtight packaging, various products are available that use a paper base with a laminate or heat-seal layer. However, paper is more prone to bending than plastic, so if its bending strength is insufficient, it can easily tear at the folds. Furthermore, it is susceptible to punctures and tears from impacts such as contact with sharp objects or protrusions during distribution. When such problems occur, the contents can leak, making it unsuitable as an airtight container.

[0004] To avoid these problems, it is necessary to increase the strength of the paper, especially its folding strength and resistance to impacts such as contact with sharp objects and protrusions. However, no technology has been reported to date that can increase the aforementioned resistance to impacts.

[0005] As a technique for imparting strength to a paper substrate, Patent Document 1 proposes a packaging paper characterized by the addition of 0.01 to 3.0% by mass of carboxymethylcellulose with a substitution degree of 0.3 to 0.6 in order to obtain excellent sizing properties and paper strength in both wet and dry conditions. Furthermore, Patent Document 2 proposes an impregnated paper in which an impregnation resin is made up of an acrylic acid ester copolymer, characterized by containing 75 to 98% by mass of constituent units derived from 10 to 100% by mass of methyl acrylate and 0 to 90% by mass of methyl methacrylate and / or ethyl acrylate, and 2 to 15% by mass of constituent units derived from one or more monomers selected from (meth)acrylamide, N-alkyl substituted derivatives of (meth)acrylamide and (meth)acrylic acid, and having a glass transition temperature (Tg) of -30 to +30°C, in order to obtain excellent internal bonding strength and folding strength. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2003-027399 [Patent Document 2] Japanese Patent Publication No. 2005-281431 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the packaging paper described in Patent Document 1 had problems such as low resistance to impact from contact with sharp objects or protrusions (impact resistance), making it prone to holes and tears during distribution. Furthermore, the invention described in Patent Document 2 required a large amount of resin impregnation to obtain excellent strength, resulting in the problem of excessive plastic usage.

[0008] This invention was made in view of these problems, and aims to provide a base paper for sealing containers that has high strength against bending, excellent impact resistance, and maintains strength that makes it difficult to tear even when subjected to impacts during distribution after the contents have been packaged. [Means for solving the problem]

[0009] The packaging paper for sealed containers of the present invention is a packaging paper for sealed containers mainly composed of pulp, wherein 70% or more by mass of the pulp is softwood pulp, and it contains a dry paper strength enhancer and a wet paper strength enhancer, and is characterized in that carboxymethylcellulose is coated on at least one surface. With this configuration, high folding strength can be obtained, and the strength against bending is increased. In addition, it is possible to obtain a sealed container that has high resistance to impact from contact with sharp objects or protrusions (impact resistance) and is less likely to tear even after the contents have been packaged. In this invention, the term "sealed container" includes not only relatively airtight containers such as retort pouches for food, but also containers that, although permeable to gas, prevent solid or liquid foreign matter from entering under normal handling, transportation, or storage conditions, thereby preventing loss, efflorescence, deliquescence, or evaporation of the contents.

[0010] Furthermore, the base paper for sealed containers of the present invention may also contain a sizing agent. With such a configuration, carboxymethylcellulose is more easily distributed on the surface of the base paper, making it possible to obtain a base paper for sealed containers with superior impact resistance.

[0011] Furthermore, the basis weight of the base paper for sealing containers according to the present invention is 50 to 120 g / m². 2 This is also acceptable. This configuration makes it easier to ensure various strengths, such as impact resistance, while maintaining the flexibility required for packaging paper for sealed containers.

[0012] Furthermore, the sealed container made from the packaging paper of the present invention is resistant to tearing even when subjected to impacts during distribution after the contents have been packaged, making it a suitable container for storage and transportation, and is particularly suitable as a container for liquids and powders.

[0013] Furthermore, the present invention relates to a method for producing base paper for sealed containers, comprising the steps of: preparing a pulp slurry containing 70% by mass or more of softwood pulp; preparing a raw material slurry by adding a dry paper strength enhancer, a wet paper strength enhancer and a sizing agent to the pulp slurry; producing base paper using the raw material slurry in a paper machine; and impregnating or coating the base paper with carboxymethylcellulose during the papermaking process. [Effects of the Invention]

[0014] The packaging paper for sealed containers of the present invention has high strength against bending and excellent resistance to impacts during distribution after the contents have been packaged, making it suitable for use as a sealed container such as a packaging bag, and is suitable for use as a base paper to replace some or all of conventional plastic packaging with paper packaging. [Modes for carrying out the invention]

[0015] Next, the present invention will be described in detail with reference to embodiments, but the present invention is not to be construed as being limited to these descriptions. Various modifications of the embodiments are possible as long as the effects of the present invention are achieved.

[0016] The base paper for packaging airtight containers used in the present invention mainly consists of pulp. Examples of the pulp used here include unbleached chemical wood pulps represented by hardwood unbleached kraft pulp (LUKP), softwood unbleached kraft pulp (NUKP), hardwood unbleached sulfite pulp (LUSP), softwood unbleached sulfite pulp (NUSP); bleached chemical wood pulps represented by hardwood bleached kraft pulp (LBKP), softwood bleached kraft pulp (NBKP), hardwood bleached sulfite pulp (LBSP), softwood bleached sulfite pulp (NBSP); and mechanical pulps represented by thermomechanical pulp (TMP), refiner ground pulp (RGP), and groundwood pulp (GP). The proportion of pulp in the base paper for packaging airtight containers of the present invention is not particularly limited, but it is preferably 90% by mass or more. For example, in the base paper for packaging airtight containers, the pulp content is preferably 90 - 99% by mass. Although it is possible to lower the proportion of pulp by adding fillers such as fillers, this is not preferable because it also causes a decrease in various strengths of the paper at the same time.

[0017] In the present invention, 70% by mass or more of the above pulp is softwood pulp. Preferably it is 75% by mass or more, 80% by mass or more, and more preferably 85% by mass or more. For example, in all the pulp, the softwood pulp is preferably 70 - 100% by mass, further preferably 75 - 100% by mass, 80 - 100% by mass, 85 - 100% by mass, 90 - 100% by mass, for example, 95 - 100% by mass. Examples of the softwood pulp include, but are not particularly limited to, NUKP, NUSP, NBKP, or NBSP. Among these, NUKP, for which strength physical properties such as folding endurance are more likely to be improved, is more preferable. When the proportion of softwood pulp in all the pulp is less than 70% by mass, it becomes difficult to satisfy the strength against bending, and at the same time, sufficient impact resistance cannot be obtained because the ratio of long fiber pulp in the paper decreases.

[0018] The pulp used in the present invention is preferably adjusted to a freeness of 400 ml CSF to 700 ml CSF, particularly 500 ml CSF to 650 ml CSF, for example, 550 ml CSF to 600 ml CSF, measured in accordance with JIS P 8121-2012 "Pulp - Drainage Degree Test Method - Part 2: Canadian Standard Drainage Degree Method" by beating. By setting it within such a range, it is possible to obtain paper with good formation while maintaining strength.

[0019] The beating method of the raw material pulp is not particularly limited, and any beating machine such as a beater, Jordan, deluxe finisher, double disc refiner, etc. may be used alone or in combination.

[0020] The packaging base paper for the sealed container of the present invention contains a dry paper strength enhancer and a wet paper strength enhancer as paper strength enhancers. Although its action is not clear, by containing both a dry paper strength enhancer and a wet paper strength enhancer, it is possible to obtain a packaging base paper for a sealed container that is more excellent in impact resistance than when each is used alone. In addition, since it is possible to impart wet paper strength more than when using only a dry paper strength enhancer, it is possible to obtain a packaging base paper for a sealed container that is not easily torn even when wetted with water. [[ID=^{}9]]

[0021] Examples of the dry paper strength enhancer used in the present invention include starches such as oxidized starch, cationized starch or modified starch, polyacrylamide-based resins, urea formalin resins, melamine formalin resins, plant gums, polyvinyl alcohol, modified polyvinyl alcohol, rubber latex, polyethylene oxide, polyamide resins, etc., and one or more of these can be used. Among these, polyacrylamide-based resins, particularly amphoteric polyacrylamide-based resins with excellent fixing properties, are preferred. The method of applying these to the base paper is not particularly limited, and an internal addition method or an external addition method can be used, but by adding (internally adding) to the pulp slurry and using it, it becomes easier to obtain a base paper with excellent impact resistance.

[0022] The dry strength enhancer content in the base paper for sealed container packaging is preferably 0.12 to 2.2 parts by mass, and more preferably 0.15 to 2.0 parts by mass, per 100 parts by mass of pulp. More preferably, it is 0.2 to 2.0 parts by mass. Below 0.12 parts by mass, the impact resistance may be poor. Conversely, above 2.2 parts by mass, aggregates derived from the strength enhancer may be generated in the paper machine, potentially causing contamination of the paper.

[0023] Examples of wet strength enhancers used in the present invention include urea-formaldehyde resin, melamine-formaldehyde resin, and polyamide-epichlorohydrin-based epoxy resins, and one or more of these can be used. The method of application to the base paper is not particularly limited, and internal or external addition methods can be used, but adding them to the pulp slurry (internal addition) makes it easier to obtain base paper with excellent impact resistance. In particular, adding both the dry strength enhancer and the wet strength enhancer to the pulp slurry (internal addition) makes it even easier to obtain base paper with excellent impact resistance.

[0024] The content of the wet strength enhancer in the base paper for sealed container packaging is preferably 0.12 to 2.2 parts by mass, and more preferably 0.15 to 2.0 parts by mass, per 100 parts by mass of pulp. More preferably 0.2 to 2.0 parts by mass. Below 0.12 parts by mass, the impact resistance may be poor. Conversely, above 2.2 parts by mass, aggregates derived from the strength enhancer may be generated in the paper machine, potentially causing contamination of the paper.

[0025] In the present invention, a sizing agent may be further included in the base paper for sealed containers. One or more sizing agents can be used from among paraffin wax-based sizing agents, microcrystalline wax-based sizing agents, carnauba (carnauba wax)-based sizing agents, alkyl ketene dimer-based sizing agents, rosin-based sizing agents, alkenyl succinic anhydride-based sizing agents, styrene-acrylic sizing agents, etc. As described later, the base paper for sealed containers of the present invention has carboxymethylcellulose coated on the surface of the base paper. By including a sizing agent in the base paper, the penetration of carboxymethylcellulose into the base paper is suppressed, and it is more likely to remain on the surface of the base paper. As a result, a base paper for sealed containers with superior impact resistance can be obtained. The method of applying the sizing agent to the base paper is not particularly limited; internal or external application methods can be used, but it is preferable to add it to the pulp slurry (internal application). When external application is used, it is preferable to apply the sizing agent to the base paper first, and then apply the carboxymethylcellulose. The sizing agent content is preferably 0.1 to 0.5 parts by mass, and more preferably 0.15 to 0.35 parts by mass, per 100 parts by mass of pulp. In the present invention, for example, it is preferable to add 0.1 to 0.5 parts by mass of a rosin-based sizing agent, preferably a modified rosin emulsion, per 100 parts by mass of pulp.

[0026] The packaging paper for sealed containers of the present invention may contain various papermaking materials such as aluminum sulfate, fillers, yield improvers, coloring dyes, coloring pigments, and bulking agents, to the extent that they do not impair the effects intended for the present invention. As fillers, one or more known fillers such as hydrated silicic acid, white carbon, talc, kaolin, clay, calcium carbonate, titanium dioxide, aluminosilicate, calcined clay, barium sulfate, and synthetic resin fillers can be used. For example, 0.1 to 2.0 parts by mass of aluminum sulfate may be included per 100 parts by mass of pulp.

[0027] In this invention, the papermaking method for the base paper is not particularly limited, and it can be made using conventionally known papermaking machines such as cylinder papermaking machines, short-screen papermaking machines, long-screen papermaking machines, and combination papermaking machines of these machines. The drying method in the papermaking machine is not particularly limited, and a multi-cylinder dryer system, a Yankee dryer system, an air drying system represented by hot air drying, or a radiant drying system represented by an infrared device can be used. Among these, the multi-cylinder dryer system or the Yankee dryer system is preferred, and the Yankee dryer system is particularly preferred. By drying with the Yankee dryer system, the mirror surface of the Yankee dryer is transferred to one side of the base paper, resulting in a paper with one-sided gloss, and a base paper for sealed container packaging with superior impact resistance can be obtained.

[0028] In the present invention, it is preferable that the base paper for sealing containers be made of two or more layers. This configuration suppresses unevenness in the formation and makes it possible to obtain a base paper for sealing containers with superior impact resistance. If unevenness in the formation is present, the paper may tear easily in areas where the formation is thin, and there is a risk of localized areas with poor impact resistance.

[0029] The packaging paper for sealed containers of the present invention has carboxymethylcellulose coated on at least one surface of the base paper. By coating the surface of the paper base with carboxymethylcellulose, effects such as reinforcing interfiber bonds, filling voids between fibers, and providing strength through a coating are obtained, improving folding strength and elongation at break, as well as impact resistance. Furthermore, the higher the moisture content in the paper, the better the impact resistance. Since carboxymethylcellulose has high water retention, it is expected to have the effect of maintaining a high moisture content in the paper, and is therefore expected to improve impact resistance. The packaging paper for sealed containers of the present invention preferably has a moisture content (hereinafter sometimes referred to as "equilibrium moisture content") of 6.5 to 8.5% by mass after 24 hours of humidity control in a 23°C × 50% RH environment. More preferably, it is 7.0 to 8.0% by mass. If the equilibrium moisture content is less than 6.5% by mass, the effect of improving impact resistance may be poor. Conversely, even if the equilibrium moisture content exceeds 8.5% by mass, the effect of improving impact resistance plateaus. However, achieving an equilibrium moisture content of 8.5% by mass or higher would impart excessive water retention to the paper, potentially reducing its suitability for processing into packaging bags and the like. Furthermore, it was found that when the packaging base paper for sealed containers of the present invention is conditioned for 24 hours in an environment of, for example, 40°C × 90% RH, the moisture content may become around 12% by mass, but even in this case, the impact resistance remains good. Therefore, containers using the packaging base paper for sealed containers of the present invention can be made into containers with sufficient impact resistance even in high-humidity environments.

[0030] The carboxymethylcellulose used in this invention can, for example, have a degree of substitution of 0.6 to 0.7, a degree of polymerization of 220 to 250, and a molecular weight of 47,000 to 135,000. A molecular weight of 47,000 to 54,000 is particularly preferred. Within this range, it dissolves easily in water, has low viscosity, resulting in good operability, easily penetrates and impregnates the base paper uniformly, and easily improves impact resistance.

[0031] The method of applying carboxymethyl cellulose is not particularly limited, and various known coating devices or impregnation devices can be used. For example, coating methods such as size press method, dipping method, coater method, spray method, etc. can be used. By attaching carboxymethyl cellulose to the surface of the base paper, the surface strength of the paper is improved, and abrasion resistance and impact resistance are improved, so it can be preferably used as a packaging base paper for sealed containers. The adhesion amount of carboxymethyl cellulose to the base paper is 0.4~2.2 g / m 2 , for example, 0.4~2.0 g / m 2 is preferred. More preferably, it is 0.5~1.5 g / m 2 . If it is less than 0.4 g / m 2 , it is difficult to obtain the effect of maintaining a high equilibrium moisture content, and there is a risk of inferior improvement in impact resistance. If it exceeds 2.2 g / m 2 , the equilibrium moisture content of the paper can be made higher, but the stiffness of the packaging base paper for sealed containers becomes too high, and paper cracking is likely to occur, and there is a risk of being easily torn, and the processing suitability for packaging bags, etc. may also decrease.

[0032] The drying method after applying carboxymethyl cellulose to the base paper is not particularly limited, and air drying methods represented by hot air drying such as multi-cylinder dryer method, Yankee dryer method, air dryer method, etc., radiation drying methods represented by infrared devices, etc. can be used.

[0033] In this embodiment, the basis weight of the packaging base paper for sealed containers is preferably 50~120 g / m 2 , more preferably 80 g / m 2 ~100 g / m 2 . Within such a range, the folding endurance and impact resistance are excellent. If the basis weight is less than 50 g / m 2 , there is a risk of inferior folding and impact resistance. Also, if it exceeds 120 g / m 2 , the impact resistance is high, but the stiffness is also too high, so there is a risk of decreasing the processing suitability for packaging bags, etc.

[0034] In this invention, the base paper for sealed container packaging can be calendered. The calendering method is not particularly limited, and machine calenders, supercalenders, etc., can be used. By crushing the voids between fibers through calendering, it is possible to reduce the number of areas that are locally weak against impact due to a large number of voids and low fiber density, thereby producing a base paper for sealed container packaging with superior impact resistance.

[0035] The packaging paper for sealed containers of the present invention can be used as a bag-shaped sealed container by applying an adhesive or a heat-sealable resin, for example, and then directly processing it into a bag. Alternatively, it can be laminated with polyethylene or the like on one side before processing it into a sealed container. While such lamination involves the use of plastic materials, the fact that part of the sealed container is made of paper contributes to a reduction in the amount of plastic used. The form of bag processing is not particularly limited, and depending on the application, for example, pillow packaging, caramel packaging, or a combination of these can be used.

[0036] Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. In the examples, "parts" refers to "solid parts" unless otherwise specified. Note that the number of added parts is a value based on solid content.

[0037] (Example 1) <Manufacturing of base paper> A pulp slurry was prepared by dispersing 100 parts of Freeness 580ml CSF unbleached softwood kraft pulp in water. To the pulp slurry, 0.2 parts each of a dry paper strength enhancer (product name: DS4433 / Seiko PMC, polyacrylamide resin) and a wet paper strength enhancer (product name: WS4030 / Seiko PMC, polyacrylic epichlorohydrin resin) were added, and 0.25 parts of a sizing agent (product name: AL1300 / Seiko PMC, modified rosin emulsion) was added. 0.5 parts of acid band were added relative to the pulp weight, and after papermaking using a standard square sheet machine, the paper was dried in a hot air dryer at 120°C, resulting in a basis weight of 80 g / m². 2 We obtained the base paper.

[0038] <apply> The obtained base paper was impregnated with an aqueous solution of carboxymethylcellulose (product name: Cellogen PR, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., degree of substitution 0.6-0.7, degree of polymerization 220-250, molecular weight 47000-54000), and dried in a rotary dryer at 130°C to obtain the target base paper for sealed container packaging. The amount of carboxymethylcellulose adhering to the base paper was 1 g / m² per side of the base paper. 2 That was the case.

[0039] (Example 2) In Example 1, a base paper for packaging for sealed containers was obtained in the same manner as in Example 1, except that 75 parts of bleached softwood kraft pulp and 25 parts of bleached hardwood kraft pulp were used.

[0040] (Example 3) A base paper for sealed container packaging was obtained in the same manner as in Example 1, except that the amount of dry paper strength enhancer and wet paper strength enhancer added was 0.15 parts.

[0041] (Example 4) A base paper for sealed container packaging was obtained in the same manner as in Example 1, except that the amount of dry paper strength enhancer and wet paper strength enhancer added was 2 parts.

[0042] (Example 5) In Example 1, the amount of carboxymethylcellulose attached was 0.5 g / m². 2 A base paper for sealing container packaging was obtained in the same manner as in Example 1, except that the above was done.

[0043] (Example 6) In Example 1, the amount of carboxymethylcellulose applied was 2 g / m². 2 A base paper for sealing container packaging was obtained in the same manner as in Example 1, except that the above was done.

[0044] (Comparative Example 1) In Example 1, a base paper for sealed container packaging was obtained in the same manner as in Example 1, except that a dry paper strength enhancer and a wet paper strength enhancer were not added.

[0045] (Comparative Example 2) In Example 1, a base paper for sealed container packaging was obtained in the same manner as in Example 1, except that a wet-strength enhancer was not added.

[0046] (Comparative Example 3) In Example 1, a base paper for sealed container packaging was obtained in the same manner as in Example 1, except that a drying strength enhancer was not added.

[0047] (Comparative Example 4) In Example 1, a base paper for sealed container packaging was obtained in the same manner as in Example 1, except that 0.4 parts of a dry paper strength enhancer was added as the paper strength enhancer.

[0048] (Comparative Example 5) In Example 1, a base paper for sealed container packaging was obtained in the same manner as in Example 1, except that 0.4 parts of a wet paper strength enhancer was added as the paper strength enhancer.

[0049] (Comparative Example 6) In Example 1, a base paper for sealing container packaging was obtained in the same manner as in Example 1, except that carboxymethylcellulose impregnation was not performed.

[0050] (Comparative Example 7) In Example 1, a base paper for sealing containers was obtained in the same manner as in Example 1, except that impregnation with carboxymethylcellulose was omitted and carboxymethylcellulose was added in part to the pulp slurry.

[0051] (Comparative Example 8) In Example 1, a base paper for packaging for sealed containers was obtained in the same manner as in Example 1, except that 25 parts of bleached softwood kraft pulp and 75 parts of bleached hardwood kraft pulp were used.

[0052] (Comparative Example 9) In Example 1, carboxymethylcellulose impregnation was omitted, and similarly water-retentive polyethylene glycol was added at a rate of 1 g / m². 2 A base paper for sealing container packaging was obtained in the same manner as in Example 1, except that it was impregnated.

[0053] The obtained packaging paper for sealed containers was evaluated using the method described below. The results are shown in Table 1.

[0054] (Number of bending cycles) The number of folds that the base paper for sealed containers could withstand was measured according to the method specified in JIS P 8115 "Paper and cardboard - Test method for folding strength - MIT testing machine method".

[0055] (Tensile strength) The tensile strength (kN / m) of the base paper for sealing containers was measured according to the method specified in JIS P 8113 "Paper and paperboard - Test methods for tensile properties - Part 2: Constant-rate elongation method".

[0056] (Impact resistance) The test specimens were fixed in place, and a conical jig measuring 25 mm ± 0.7 mm was struck with a wooden mallet to create a hole in the specimen. The resistance felt was evaluated on a 5-point scale. A score of 5 indicated the strongest resistance, while a score of 1 indicated the weakest resistance and the easiest hole to create. A score of 3 or higher was considered a passing grade. The test was conducted without the testers knowing which specimens were being tested.

[0057] (Equilibrium moisture content) After conditioning the test specimens at 23°C and 50% RH for 24 hours, the moisture content of the test specimens was measured according to the method specified in JIS P 8203 "Paper, cardboard and pulp - Method for determining oven dryness - Method using a drying oven".

[0058] Table 1 shows an overview of the composition and evaluation results of the base paper for sealing containers obtained in each example and comparative example. The base paper for sealing containers obtained in each example exhibited excellent impact resistance.

[0059] [Table 1]

Claims

1. A base paper for sealing containers, comprising pulp as the main component, wherein 70% by mass or more of the pulp is softwood pulp, and it contains a dry paper strength enhancer and a wet paper strength enhancer, and carboxymethylcellulose is coated on at least one side of the base paper, and the amount of carboxymethylcellulose adhering to the base paper is 0.4 to 2.2 g / m².

2. The packaging base paper for sealed containers according to claim 1, further characterized by containing a sizing agent.

3. Grain weight: 50-120 g / m² ２ Packaging base paper for sealed containers according to claim 1 or 2, characterized in that it is the same as described above.

4. A method for producing base paper for sealed containers, comprising the steps of: preparing a pulp slurry containing 70% by mass or more of softwood pulp; preparing a raw material slurry by adding a dry paper strength enhancer, a wet paper strength enhancer and a sizing agent to the pulp slurry; producing base paper using the raw material slurry in a paper machine; and impregnating or coating the base paper with carboxymethylcellulose during the papermaking process so that the amount of carboxymethylcellulose adhering to it is 0.4 to 2.2 g / m².

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