JP2581063B2 - Multi-layer structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is a multilayer structure using an ethylene-vinyl alcohol polymer as a gas barrier layer, characterized in that a decrease in gas barrier properties due to heat sterilization during retort sterilization is reduced. And a multilayer structure.

(Prior art)

BACKGROUND ART Ethylene-vinyl alcohol resins have excellent gas barrier properties and transparency and are widely used as gas barrier layers constituting container films and the like. With this gas barrier layer, the deterioration of the contents such as foods and pharmaceuticals due to oxidation can be prevented, and the contents can be stored for a long time. The ethylene-vinyl alcohol-based resin herein has the same meaning as the ethylene-vinyl acetate copolymer and the saponified product.

However, while the ethylene-vinyl acetate copolymer exhibits excellent oxygen gas barrier properties in low humidity conditions, the oxygen gas barrier properties are significantly reduced in high humidity conditions and high moisture content. Therefore, a hydrophobic polymer having low moisture permeability such as polyethylene or polypropylene is generally used by laminating on both sides.

The reason why the saponified ethylene-vinyl acetate copolymer has an excellent oxygen gas barrier property is that the intermolecular or intramolecular hydrogen bond is stronger than other polymers, as well as the symmetry of the molecular chain and the polarity. Are synergistically contributing. On the other hand, when the water content of the saponified ethylene-vinyl acetate copolymer increases, the adsorbed water molecules first bind to hydrophilic OH groups, and as the water content increases, the adsorbed water becomes intermolecular hydrogen. It is believed that it breaks the bonds and enables the molecular movement required for oxygen molecular diffusion, resulting in an increase in the oxygen permeability coefficient.

If the water content further increases from this state, free water will be present in addition to the adsorbed water, the intermolecular force will further weaken, and the oxygen permeation coefficient will increase further due to the plasticizing effect on molecular motion. ing.

When such a saponified ethylene-vinyl acetate copolymer is used as a transparent multi-layer container for sterilizing retorts, from the viewpoint of heat resistance to hot water or steam at about 120 ° C. in retort sterilization, ethylene-vinyl acetate copolymers are used. It is common to laminate polypropylene on both sides of the saponified material.

However, in the heating and pressurizing state during retort sterilization,
Since the water vapor permeability of polypropylene increases by 15 to 20 times as compared with that at normal temperature, the water content of the saponified ethylene-vinyl acetate copolymer layer rapidly increases, and the oxygen gas barrier property greatly decreases.

By such retort sterilization, the multi-layer container in which the oxygen gas barrier property is greatly reduced, by preservation after the retort sterilization, the water content of the saponified ethylene-vinyl acetate copolymer layer is gradually released to the outside, Although the oxygen gas barrier property is restored, it requires a long period of time, and its use is limited to contents having a relatively large allowable oxygen amount for deterioration and those having a short storage period.

In order to improve these problems, three methods are mainly studied, one of which is a method of imparting body water to the saponified ethylene-vinyl acetate copolymer itself, and the other of which is two. This is a method of accelerating the release rate of water absorbed by the ethylene-vinyl acetate copolymer and quickening the recovery of oxygen gas barrier properties. The third method is to provide a protective layer for preventing water intrusion on both sides of the ethylene-vinyl acetate copolymer. This is a method for suppressing a decrease in oxygen gas barrier property.

In the first method, the water resistance and the hot water resistance are improved by increasing the ethylene water content in the saponified ethylene-vinyl acetate copolymer, but the original oxygen gas barrier property is significantly reduced. It is not practical. As a second method, the thickness of the outer polypropylene layer is made thinner than the thickness of the inner polypropylene layer so that the water absorbed by the saponified ethylene-vinyl acetate copolymer during storage during retort sterilization is used. To accelerate the release rate of oxygen into the outside air, and to speed up the recovery of oxygen gas barrier properties. However, in this method, since the outer layer polypropylene is thin, the water absorption of the saponified ethylene-vinyl acetate copolymer during retort sterilization is large.

Therefore, the degree of decrease in the oxygen gas barrier property is large, and even if the recovery speed is fast, the accumulated permeated oxygen amount can be slightly reduced in the long term, and further, in the initial stage after retort sterilization, Because the oxygen concentration increases,
Depending on the contents, deterioration may be promoted.

As a third method, according to Japanese Patent Publication No. 61-34392, by protecting the saponified ethylene-vinyl acetate copolymer layer on both sides with a layer containing a desiccant, water entering during retort sterilization is captured. A method has been proposed in which the increase in the water content of the saponified ethylene-vinyl acetate copolymer layer is reduced, and the decrease in the oxygen gas barrier property is suppressed. In this method, the decrease in oxygen gas barrier property due to retort sterilization can be suppressed, but by including a desiccant (10 to 20 wt%), transparency is sacrificed as a matter of course, resulting in an opaque container, and the product appeal effect is reduced by half. There were drawbacks.

Although the above method focuses on preventing oxygen intrusion in order to suppress oxidative degradation of foods, it is actually important to prevent oxygen intrusion and suppress transmission of ultraviolet rays that promote the oxidation reaction. It is. Ultraviolet rays dramatically increase the production of oxygen radicals, functional groups such as oils and fats that react with them, and radicals. In order to prevent the transmission of the ultraviolet rays, usually a pigment or the like is buried in the container, or an ultraviolet absorbing layer is provided by printing or the like. However, this method naturally impairs the visibility of the contents and significantly lowers the merchantability.

[Problem to be solved]

A multilayer container or film having an ethylene-vinyl alcohol-based resin as a gas barrier layer has excellent gas barrier properties, but when subjected to heat sterilization such as retort sterilization, the gas barrier properties are reduced due to the influence of heat and moisture.

In order to improve this drawback, as shown in JP-B-61-34392, a protective layer containing a desiccant is provided at a position for protecting the gas barrier layer, thereby adsorbing moisture invading during retort sterilization and extending to the gas barrier layer. A method of maintaining gas barrier properties by preventing moisture from reaching has been proposed.

However, in this method, as a matter of course, the addition of the desiccant may make the container opaque or the appearance may be deteriorated due to the desiccant present in a dispersed state.

An object of the present invention is to provide a multilayer structure such as a container and a film which can solve the above-mentioned drawbacks and can suppress a decrease in a barrier layer due to retort sterilization.

[To solve the problem and means]

A gas barrier layer made of an ethylene-vinyl alcohol copolymer, and a hydrophilic resin layer having an adhesiveness to the gas barrier layer on both sides of the gas barrier layer and having a saturated water content in water of 1% or more, The problem was solved by providing a polyolefin resin layer on the surface of the resin layer. Specifically, as shown in FIG. 1, both sides of the gas barrier layer 1 made of an ethylene-vinyl alcohol copolymer have adhesiveness to the gas barrier layer, and the saturated water content in water is 1% or more. It is a multilayer structure comprising hydrophilic resin layers 2, 2 and polyolefin-based resin layers 3, 3 outside the hydrophilic resin layers 2, 2.

In the present invention, the polyolefin-based resin layer is a low-density polyethylene, a high-density polyethylene, a polypropylene, or a mixture thereof, and can be used properly depending on the sterilization temperature. That is, retort sterilization (115 ° C-60 minutes, 120 ° C
-30 minutes), polypropylene, high-density polyethylene, or a mixture thereof is preferred from the viewpoint of heat resistance. Next, the saponified ethylene-vinyl acetate copolymer used in the present invention has an ethylene content of 20 to 60 mol%, in order to improve the storage stability of the contents due to high gas barrier properties.
The saponification degree is 96% or more, preferably 99% or more.

Next, the hydrophilic resin layer of the present invention will be described. The hydrophilic resin of the hydrophilic resin layer in the present invention, the water absorbing ability to capture water that penetrates through the outer or inner polyolefin resin layer during sterilization of boiling or retort, the outer or inner polyolefin resin layer, and ethylene- It also has an adhesive ability to thermally bond the saponified vinyl acetate copolymer layer. Therefore, the saturated water content of the hydrophilic resin in water must be at least 1% or more, preferably 2% or more.
Here, the saturated water content in water refers to the conversion of plastics to water at 23 ° C.
It shows the rate of weight increase when immersed for 24 hours.

The relationship between the saturated water content in water and the water content in a high temperature state during sterilization of a boil or retort cannot be determined unconditionally depending on the composition of the hydrophilic resin and the sterilization temperature, but the high-temperature state has a water content of 1 to 1.5 times. As shown in the figure, the water content rate is greatly increased.

Since the water content of the hydrophilic resin is the product of the water content and the weight of the resin, the thicker the hydrophilic resin is, of course, the more advantageous, the higher the effect of trapping infiltration water during retort sterilization and the like.

As described above, the water content and the water content rate increase in a high-temperature and high-humidity state, and the water content increases as the hydrophilic resin becomes thicker, but the sterilization temperature condition range (80 ° C.) at which the multilayer structure of the present invention is provided is provided. -60 minutes boil sterilization to 121 ° C -30 minutes retort sterilization), and in the range of practically acceptable hydrophilic adhesive resin thickness range (5μ to 100μ), if the saturated water content of the hydrophilic resin in water is 1% or less, practical The deterioration of the oxygen gas barrier property of the saponified ethylene-vinyl acetate copolymer (2) cannot be prevented.

The resin of the hydrophilic resin layer in the present invention will be described more specifically. The single polymer has a water absorption of a saturated water content of 1% or more in water, a polyolefin resin layer and a saponified ethylene-vinyl acetate copolymer layer. A polymer that satisfies both the adhesive strength required for thermal bonding and a polymer that has thermal adhesive strength but has a saturated water content of 1% or less in water or a polymer that needs to have higher performance even if it is 1% or more, and a polymer with a high water content Are classified, and both can be used.

Examples of the former include an ethylene-vinyl acetate copolymer saponified product having an ethylene water content of 50 to 90 mol% and a saponification degree of 70 to 95%, or an unsaturated carboxylic acid modified product thereof. Ethylene content 50-90 mol%, saponification degree 50-
70% of a saponified ethylene-vinyl acetate copolymer or an unsaturated carboxylic acid-modified polyolefin, and a polymer having a high water content, for example, polyvinyl alcohol, polyvinyl acetate,
It is a mixture of polyvinyl butyral, nylon and the like in a range where the thermal adhesive force can be maintained to a practically acceptable level. Here, the unsaturated carboxylic acid modification means a carboxylic acid group,
It means that an acid anhydride group and a derivative thereof are copolymerized or graft-polymerized, and specifically, as a carboxylic acid group, an acid anhydride group and a derivative thereof, methacrylic acid, maleic acid, fumaric acid, and methacrylic anhydride. Products, maleic anhydride, ethyl methacrylate, glycidyl acrylate, diglycidyl methacrylate, and the like.

Next, a method for manufacturing the multilayer structure of the present invention will be described. The multilayer structure of the present invention is formed by an extrusion molding method or an injection molding method in which a thermoplastic resin constituting a multilayer is heated and melted by an extruder and laminated, and more specifically, a co-extrusion T-die casting method, Extrusion inflation method,
It can be formed by a multilayer blow molding method or a multilayer injection blow molding method.

Of these, the co-extrusion T-die casting method will be further described as an example. The polyolefin-based resin and the saponified ethylene-vinyl acetate copolymer hydrophilic adhesive resin constituting the multilayer structure of the present invention are separately extruded. , And heated and melted in each extruder, and then laminated in a molten state at the junction of the molten resin so as to have a layer structure shown in FIG.
By extruding into a sheet from a die and cooling, a five-layer film or sheet structure is obtained. These multilayer films or sheets can be used as multilayer containers by heat sealing in a bag shape or deep drawing thermoforming by vacuum or pressure forming.

The above manufacturing method and the manufacturing method are not special and are generally adopted in manufacturing a multilayer structure, and the multilayer structure of the present invention can be easily manufactured by existing equipment as before. Can be manufactured. Hereinafter, the present invention will be described with reference to examples.

[Example -1] The resin shown in Table 1 was simultaneously extruded from three extruders by a co-extrusion multilayer sheet manufacturing apparatus, merged at a molten resin merging section, extruded into a sheet shape from a T-die, and cooled. By this, a total thickness of 520μ of a saponified ethylene-vinyl acetate copolymer copolymer having an intermediate layer of a hydrophobic adhesive resin conventionally used and a hydrophilic resin of the present invention as an adhesive layer, respectively. Three types of five-layer sheets were obtained.

Note that the thickness of each layer is the same for both No. 1 and No.
0 μ, the adhesive layer was 40 μ, and the intermediate layer was 40 μ.

Next, each sheet is formed into a tray container having a draw ratio (depth / opening diameter) of 0.2 by plug-assist vacuum forming method, filled with water as contents, and then biaxially stretched polyester (12μ) / aluminum foil (9μ). ) / Polypropylene (50μ)
The lid material of the composition was heat-sealed and sealed. Furthermore, the filled and sealed multi-layer container is retorted by a retort sterilizer.
Hot water retort sterilization was performed at 120 ° C. for 30 minutes.

Table 2 shows the results of measuring the oxygen permeability of the multilayer container obtained by retort sterilization as described above one day after retort sterilization and one month storage at 20 ° C-65% RH.

From the above results, as a multilayer container for disinfecting retort having a saponified ethylene-vinyl acetate copolymer as an intermediate layer, the outer layer and the inner layer as polyolefin resin layers, by using a hydrophilic resin for the adhesive layer, by retort disinfection. A multilayer container with a small decrease in oxygen gas barrier properties was obtained.

[Example 2] The resin shown in Table 3 was simultaneously extruded from three extruders as in Example 1 by a co-extrusion multilayer sheet manufacturing apparatus, and after joining at the junction of the molten resin, a sheet was formed from a T-die. By extruding and cooling, a saponified ethylene-vinyl acetate copolymer as an intermediate layer as shown in FIG. 1 having an adhesive resin containing an inorganic desiccant and a hydrophilic resin of the present invention as an adhesive layer was used as an intermediate layer. A 520 micron thick three-layer, five-layer sheet was obtained.

The thickness of each layer was the same for No. 3, No. 4, and No. 5, and the inner and outer layers were 200 μm, the adhesive resin layer was 40 μm, and the intermediate layer was 40 μm.

Next, each sheet is formed into a tray container with a draw ratio (depth / opening diameter) of 0.2 by plug-assist vacuum forming immediately after production, filled with water as the contents, and then biaxially stretched polyester (12μ) / aluminum foil (9μ) / polypropylene (50
The lid material of μ) was heat-sealed and sealed. Further, the filled and sealed multilayer containers were subjected to a hydrothermal retort sterilization at 120 ° C. for 30 minutes using a retort sterilizer.

On the other hand, after storing each sheet at room temperature for one year, it was similarly formed into a tray container having a drawing ratio (depth / opening diameter) of 0.2 by plug-assist vacuum forming method, filled with water as contents, and then biaxially stretched. Polyester (12μ) / Aluminum foil (9μ)
/ Lid (50μ) was heat-sealed and sealed. In addition, those filled sealed multi-layer containers,
Similarly, hot water retort sterilization was performed at 120 ° C. for 30 minutes using a retort sterilizer.

The oxygen permeability of various multilayer containers obtained by retort sterilization as described above was measured one day after retort sterilization and after storage at 20 ° C-65% RH for one month.

 The results are shown in Table-4.

As a result, even when a desiccant was coexisted in the adhesive resin layer, even when a hydrophilic resin was used, the one molded into a tray container immediately after the seal was manufactured was almost equivalent to the oxygen gas barrier property by retort sterilization. Although a multi-layer container with a small degree of decrease was obtained, it was stored in a sheet state for one year at room temperature and then molded into a tray container. The one with a desiccant added was probably due to moisture absorption of the desiccant during storage, or retort sterilization. The oxygen gas barrier property was reduced. On the other hand, those using the hydrophilic adhesive resin showed almost the same values as those immediately after the production even when stored for one year.

[Example-3] In the same manner as in Example-1 and Example-2, Table-
5, the resin shown in Fig. 5 is co-extruded, and three types of saponified ethylene-vinyl acetate copolymers of samples No. 6 to No. 8 are made of hydrophilic resins having different saturated water contents in water as an adhesive layer. A three-layer, five-layer sheet having a total thickness of 520 µ and an inter-adhesion force of 1.2 kg / 25 mm or more was obtained.

The thickness of each layer is the same for No. 6 to No.
μ, hydrophilic adhesive resin layer 40 μ, and intermediate layer 40 μ.

Next, each sheet was formed into a multilayer container having a squeezing ratio of 0.2 by a plug assist vacuum forming method, filled with water and sealed under the same conditions as in Example 1, and then sterilized by retort.

Table 6 shows the results of measuring the oxygen permeability of the multilayer container thus obtained one day after retort sterilization and one month storage at 20 ° C.-65% RH.

From the above results, No. 6 using the hydrophilic adhesive resin having a saturated water content of 0.6% in water was compared with No. 1 using the hydrophobic adhesive resin of Example 1 in oxygen gas barrier property by retort sterilization. Although the degree of decrease is slightly small, it is insufficient for practical use. On the other hand, No. 7 and No. 8 using a hydrophilic resin having a saturated water content of 1% or more in water show a practical effect,
In particular, a remarkable effect was observed when a hydrophilic adhesive resin having a saturated water content of 2% or more in water was used. In addition, each sample was a highly practical multi-layer container without any peeling or poor appearance due to retort sterilization.

〔The invention's effect〕

The multilayer container using the hydrophilic adhesive resin of the present invention, the degree of decrease in oxygen gas barrier properties of the saponified ethylene-vinyl acetate copolymer by retort sterilization is very small as compared to conventional multilayer containers, and as a result, the container The amount of accumulated permeation that penetrates the contents through the through hole is reduced, and the preservability of the contents can be greatly improved. In addition, since it has low hygroscopicity during storage at room temperature, it has good stability over time, and its effect lasts for a long period of time.

Further, the present invention is economical because the conventional manufacturing apparatus and process can be used as it is and the manufacturing can be performed without increasing the material cost.

[Brief description of the drawings]

 FIG. 1 is a cross-sectional view showing a multilayer structure of the present invention. (1) ... gas barrier layer, (2) ... hydrophilic resin layer (3) ... polyolefin resin layer

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location B29L 22:00

Claims (1)

(57) [Claims] 1. A gas barrier layer comprising an ethylene-vinyl alcohol-based polymer, and a hydrophilic resin layer having adhesiveness to the gas barrier layer on both sides of the gas barrier layer and having a saturated water content in water of 1% or more, Further, a multilayer structure suitable for heat sterilization of a structure in which a polyolefin resin layer is provided on the surface of the hydrophilic resin layer.