JPS6225103B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a laminated bag that can be sterilized by retort, and more particularly to a laminated bag that can be sterilized by retort and has excellent stacking pressure resistance. Conventionally, laminated sheets having a heat-sealable crystalline olefin resin layer provided on one surface of a flexible gas barrier substrate such as aluminum foil have been widely used as sealed packaging bags for retort sterilization. This laminated sheet is stacked in a bag shape so that the olefin resin layers face each other, and the periphery is heat sealed to form a bag, which is then filled with contents such as food, degassed and sealed, and then placed in a sterilizer called a retort. Heat sterilize inside the
It becomes a package that can be stored at room temperature. The crystalline olefin resin layer of this laminated sheet is
It is required to act as a heat sealant (thermal sealing material) and an inner surface protection material, and it is also subjected to harsh heat sterilization treatment.
Various restrictions exist. Low-density polyethylene, which has traditionally been most widely used in such applications, has the advantage of excellent heat sealability, but has the disadvantage of poor heat resistance and extraction resistance, and cannot be used at high temperatures during heat sterilization. There are disadvantages in that the bag may break and its performance as an inner coating material deteriorates, or the resin component may migrate into the contents, impairing flavor characteristics and sanitary characteristics. On the other hand, although medium to high density polyethylene has excellent heat resistance and extraction resistance, it has the disadvantage of being susceptible to environmental stress cracking. When packaging bags are piled up and stored in a piled-up state, the heat-sealed portion becomes brittle, leading to a fatal defect in which the bags easily break with the slightest impact. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a retort-sterilizable laminate that effectively eliminates the above-mentioned drawbacks that occur when medium to high-density polyethylene is used as the inner surface material and heat sealing material of a laminate bag. Another object of the present invention is to provide a retort sterilizable laminated bag that has a crystalline olefin resin layer on its inner surface that has an excellent combination of heat resistance, extraction resistance, heat sealability, and pressure resistance. The present inventor has discovered that the load strength of heat sterilized sealed packaging bags is closely related to the molecular weight distribution of polyethylene used as the inner material and heat sealing material, and that the flow ratio (Kp) described in detail below is within a specific range. By selectively using a certain medium to high density polyethylene,
It has been found that the stacking strength can be significantly improved compared to conventional laminated bags. According to the present invention, a laminated sheet consisting of a flexible gas barrier substrate and a heat-sealable crystalline olefin resin layer provided on one surface of the substrate is placed in a bag such that the olefin resin layer faces each other. In a laminated bag formed by stacking the layers in a shape and joining the peripheries by heat sealing, the crystalline olefin resin layer has the following formula Kp=100log MI ₂₀ /MI ₂ where MI ₂ is the olefin resin measured according to JIS K6760. Represents the melting index (g/10 minutes) of the resin,
MI ₂₀ represents the melting index (g/10 min) of olefin resin measured in the same manner as JIS K6760 except that the load was 20.0 kg and the sample collection time was 30 seconds.The flow ratio (Kp) defined by is 165. That's all,
Provided is a retort sterilizable laminated bag with excellent loading pressure resistance, characterized by being made of polyethylene having a melting index (MI ₂ ) of 0.05 to 15 g/10 minutes and a density of 0.935 g/cc or more. . The present invention will be explained in detail below. In FIG. 1 showing a cross-sectional structure of an example of a laminate sheet used in the present invention, a laminate sheet 1 includes an aluminum foil 2, a crystalline olefin resin layer 4 bonded to one surface of the aluminum foil 2 via an adhesive layer 3, and other parts. It consists of a heat-resistant resin layer 6 bonded to one surface via an adhesive layer 5. Further, if necessary, a shock absorbing layer can be provided between the aluminum foil 2 and the heat-resistant resin layer 6 via an adhesive layer. Furthermore, the impact-reducing layer can also be provided between the modified olefin resin layer 3 and the crystalline olefin resin layer 4. In this case, an adhesive layer is required between the impact mitigation layer and the crystalline olefin resin layer. In manufacturing the sealed package, as shown in FIG. 2, two laminated sheets 1, 1 are stacked so that the crystalline olefin resin layer 4 is on the inside, and the periphery 7 is heat-sealed. The bag has a food storage section 8 inside. For simplicity, the adhesive layer 3 and the adhesive layer 5 are omitted in FIG. 2. An important feature of the present invention is that, as the crystalline olefin resin layer 4, a polyethylene film having a flow ratio (Kp) of 165 or more and a density of 0.935 g/cc or more, which has not been used at all in conventional film production, is used. It consists in using it. Conventionally, the inner material for heat sealing of laminated bags has been either laminated with commercially available medium- to high-density polyethylene films, or extrusion-coated with film-grade medium- to high-density polyethylene. All high-density polyethylenes have a flow ratio (Kp) of 160 or less as expressed by the above formula (1), and such conventional polyethylene films themselves have excellent heat resistance as shown in Comparative Example 1 described later. Even if the bag exhibits seal strength, if the bag is filled with liquid contents, sealed and sterilized, and then stored under load, it will exhibit significantly low compressive strength, and such laminated bags will be difficult to store or handle. Sometimes even the slightest impact can cause the bag to break. In contrast, according to the present invention, the flow ratio (Kp)
If polyethylene film with a density of 165 or higher and a density of 0.935 g/cc or higher is used as the inner surface material and heat sealing material for laminated bags, it is possible to improve the stacking strength under similar conditions to an order of magnitude higher. Therefore, the effects of the present invention are completely unexpected from conventional laminated bags using medium to low density polyethylene films. In the above formula (1), MI ₂ in the denominator is a melting index in the usual sense and is an index that correlates with molecular weight, while MI ₂₀ in the numerator represents the melting index at high shear rates, and MI ₂₀ / MI ₂ represents the degree of non-Newtonianity. Thus, the value of Kp is closely related to the molecular weight distribution of polyethylene;
The higher this value, the wider the molecular weight distribution. In the present invention, by setting the flow ratio (Kp) to 165 or more, particularly in the range of 170 to 250, it is possible to significantly improve the stacking pressure strength of the laminated bag.
In addition, the density of polyethylene must be increased to 0.935 g/cc or more.
A range of 0.940 to 0.960 is also important in terms of heat resistance, extraction resistance, and mechanical strength of the inner surface material. Also, the melting index (MI ₂ ) of the polyethylene used
is preferably in the range of 0.05 to 15 from the viewpoint of film formability and mechanical properties. Even if the polyethylene used in the present invention is an ethylene homopolymer, it generally contains propylene, butene-1, pentene-1, and 4-methylpentene-1 in an amount of 3% by weight or less as long as the crystallinity is not impaired. It may also contain comonomers such as other olefins. In order to improve the impact resistance and blocking properties of polyethylene, polyisobutylene, butyl rubber,
Elastomers such as styrene-butadiene rubber and ethylene-propylene rubber, or α-olefin copolymers such as ethylene-propylene copolymers and ethylene-butene copolymers can also be blended in an amount of 1 to 50% by weight. The polyethylene used in the present invention can be modified in many ways within the range that satisfies the above-mentioned limitations. For example, instead of using a single polyethylene, it is also possible to use two or more types of polyethylene in the form of a blend. It is. The polyethylene used in the present invention is easily available as medium- to high-density polyethylene for blow molding, and can be easily formed into a film by means known per se, such as an inflation film forming method or a T-die method. be able to. The thickness of the film is desirably in the range of 10 to 300 μm, particularly 30 to 100 μm in terms of heat sealability. Furthermore, in terms of heat sealability,
The film is preferably unstretched, but an orientation similar to that of a T-die film is acceptable. As a flexible substrate with gas barrier properties, aluminum foil with a thickness of 6 to 80 μm is suitable, but other metal foils such as steel foil, cellulose film, and untreated or acetoalkali-treated polyvinyl are also suitable. It is also possible to use an alcohol film or an ethylene vinyl alcohol copolymer film. As the adhesive layer for bonding the two, in addition to thermosetting adhesives such as urethane adhesives and epoxy adhesives, acid-modified polyolefin adhesives such as maleic acid-modified polyethylene can also be used. As the heat-resistant resin layer applied to the other surface of the gas barrier substrate, a thermoplastic resin or a thermosetting resin having a higher melting temperature or decomposition temperature than the above-mentioned crystalline olefin resin is used. Examples of heat-resistant thermoplastic resins include polyesters such as polyethylene terephthalate, and nylon.
Examples of thermosetting resins include polyamides such as 6, nylon-6 and nylon-6, polycarbonates, cellulose esters, fluororesins, etc.; , a heat-resistant polymer containing a heterocyclic ring such as an oxadiazole ring or a thiazole ring, such as polyimide, polyamideimide,
Polyesterimide, polyamideimide ester, polyesteramideimide, polyimideimidazopyrrolone, etc. can be used. Alternatively, epoxy/phenolic resin paints, phenolic resin paints, unsaturated polyester resin paints, oleogenous paints, etc. can also be used. Heat-resistant thermoplastic resins are easily available as unstretched or biaxially stretched films, and these films can be bonded to aluminum foil or sheets using known adhesives such as epoxy adhesives and polyurethane adhesives. Laminated, heat-infusible heat-resistant resins are formed by applying a solution of a precursor polymer of these resins to the foil or sheet and then baking. As the impact-relaxing layer, when provided on the outside of the gas barrier substrate, stretched or unstretched films such as polyamide or copolyamide, polycarbonate, polyester-polyether, polyester-polylactone, biaxially stretched polyester film, etc. can be used. It can be used. In addition, when provided inside the aluminum foil, polyamide, copolyamide, or the like that has adhesive properties with modified olefin resin can be used. The various films described above can be laminated by dry lamination, a method known per se. The thus produced laminated sheet can be made into a flexible bag-like container by stacking the two pieces so that the crystalline olefin resin layer is on the inside and heat sealing the three peripheral parts. For heat sealing, use a heating bar,
heating knife, heating wire, impulse seal,
This can be easily done using ultrasonic sealing, induction heating sealing, etc. These containers are filled with perishable foods, especially liquid foods, and if necessary, gases such as air that are harmful to storage are removed using vacuum degassing, hot filling, etc.
After removing the gas by steam degassing, steam injection, degassing by deforming the container, or the like, the filling port is sealed by the heat sealing method described above. This package is then filled into a retort device and heat sterilized at a temperature of 100°C or higher. In the retort sterilization sealed package according to the present invention, there is no change in the flavor of the contents even during heat sterilization, and there is no damage to the sealed portion not only after sterilization but also when subjected to drop impact, etc. It has this remarkable advantage. The invention is illustrated by the following example. The load strength was measured by applying a predetermined load (17.0 kg/bag) to a bag filled with contents (water: 140 c.c., detergent: 40 c.c.) under accelerated conditions, that is, at an ambient temperature of 55°C. The time taken for the contents to leak was measured and expressed as this time. Example 1 12μ thick polyethylene terephthalate film, 9μ thick aluminum foil, flow ratio (Kp)
180.0, a 3-layer sheet consisting of a 70μ thick inner film of high-density polyethylene (MI: 0.5) with a density of 0.958g/cc was used to make a bag 170mm long and 130mm wide, and 180g of water/oil suspension was added to the bag. filled and sealed. After processing this under the conditions shown in Table 1, the adhesive strength (Kg/15mm) between the aluminum foil and the inner film was measured, and the temperature in the bag was kept at 0 to 2°C, and a 1.2m high The bag was dropped 10 times vertically onto the concrete surface, and the number of broken bags was determined.
The load of the bag was placed on a sealed bag filled with water/detergent: 140/40 c.c., and the time until the contents leaked (load strength) was examined, and the results shown in Table 1 were obtained. .

[Table] Comparative Example 1 A bag was made from the same three-layer sheet as in Example 1 using a high-density polyethylene film with a flow ratio (Kp) of 160.0 and a density of 0.953 g/cc (MI1.2) instead of the inner film of Example 1. made, filled and sealed under the same conditions,
Adhesive strength, load strength and drop tests were conducted, and Table 2
The results shown are obtained.

【table】 [Brief explanation of the drawing]

FIG. 1 is a sectional view of the laminated sheet, and FIG. 2 is a sectional view of the sealed package. 1... Laminated sheet, 2... Aluminum foil or sheet, 3... Adhesive layer, 4... Crystalline olefin resin layer, 5... Adhesive layer, 6... Heat resistant resin layer, 7... Surroundings, 8... Food storage section.

Claims (1)

[Scope of Claims] 1. A laminated sheet consisting of a flexible gas barrier substrate and a heat-sealable crystalline olefin resin layer provided on one surface of the substrate so that the olefin resin layer faces each other. In a laminated bag formed by overlapping the layers into a bag shape and joining the periphery by heat sealing, the crystalline olefin resin layer is formed by the following formula Kp=100log MI ₂₀ /MI ₂ where MI ₂ is measured according to JIS K6760. represents the melting index (g/10 min) of the olefin resin,
MI ₂₀ represents the melting index (g/10 min) of olefin resin measured in the same manner as JIS K6760 except that the load was 20.0 kg and the sample collection time was 30 seconds.The flow ratio (Kp) defined by is 165. That's all,
A retort sterilizable laminated bag with excellent loading pressure resistance, characterized by being made of polyethylene having a melting index (MI ₂ ) of 0.05 to 15 g/10 minutes and a density of 0.935 g/cc or more. 2. On the other surface of the gas barrier substrate,
The laminated bag according to claim 1, further comprising a layer of thermoplastic resin or thermosetting resin having a higher melting temperature or decomposition temperature than that of the crystalline olefin resin.