JPH069913B2 - Synthetic resin laminate - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a transparent synthetic resin laminate capable of retort sterilization and having a high gas barrier property, during retort sterilization treatment,
Further, the present invention relates to a synthetic resin laminate for use in bags and containers that retain a high gas barrier property even after treatment and can store foods or pharmaceuticals for a long period of time.

[Prior Art and its Problems] Saponified ethylene-vinyl acetate copolymers are packaged by using as a gas barrier layer of packaging materials such as multi-layer containers due to their excellent gas barrier properties, aroma retention and transparency. It is widely used because it prevents the deterioration of foods or pharmaceuticals due to oxygen, etc. and enables long-term storage.

However, the saponified ethylene-vinyl acetate copolymer shows excellent oxygen gas barrier property in a low humidity state,
The oxygen gas barrier property is greatly reduced when the humidity is high or the moisture content is high. Therefore, generally, a low moisture-permeable hydrophobic polymer such as polyethylene or polypropylene is laminated on both sides and used.

As a reason that the oxygen gas barrier property of the saponified ethylene-vinyl acetate copolymer is excellent, intermolecular or intramolecular hydrogen bonding is stronger than other polymers.
The symmetry and polarity of the molecular chains are synergistically contributing. On the other hand, when the water content of the saponified ethylene-vinyl acetate copolymer becomes higher, the adsorbed water molecules first bond to the hydrophilic OH group, and the adsorbed water becomes intermolecular hydrogen as the water content increases. It is believed to break the bonds and allow the molecular motions necessary for oxygen molecule diffusion, resulting in an increase in the oxygen permeability coefficient. It is considered that if the water content further increases from this state, free water will exist in addition to the adsorbed water, the intermolecular force will further weaken, and the oxygen permeability coefficient will increase further due to the plasticizing effect on the molecular motion. There is.

When such an ethylene-saponified vinyl acetate copolymer is used as a multi-layer container for retort sterilization, from the viewpoint of heat resistance to heat absorption or steam of about 120 ° C. in retort sterilization, saponified ethylene-vinyl acetate copolymer is used. It is common to laminate polypropylene layers on both sides of the.

However, the moisture permeability of the polypropylene layer in the heated and pressurized state during retort sterilization increases by 15 to 20 times as compared with that at room temperature, so that the water content of the saponified layer of ethylene-vinyl acetate copolymer increases rapidly and oxygen The gas barrier property is greatly reduced. The multilayer container whose oxygen gas barrier property is greatly reduced by such retort sterilization is stored after retort sterilization, and the moisture in the saponified ethylene-vinyl acetate copolymer layer is gradually released to the outside. Although the barrier property is restored, it requires a long period of time, and its use was limited to contents having a relatively large permissible oxygen content against deterioration and those having a short storage period.

In order to improve these problems, three methods have been mainly investigated, one of which is a method of imparting hot water resistance to the saponified product of the ethylene-vinyl acetate copolymer itself, and the other of which is the ethylene-vinyl acetate. This is a method of accelerating the release rate of water absorbed by the copolymer and accelerating the recovery of the oxygen gas barrier property.
This method is provided on both sides of the vinyl acetate copolymer layer to suppress deterioration of the oxygen gas barrier property.

Regarding the first method, the water resistance and hot water resistance are improved by increasing the ethylene content in the saponified ethylene-vinyl acetate copolymer, but the original oxygen gas barrier property is drastically reduced accordingly. Because it is not practical. In the second method, the saponified ethylene-vinyl acetate copolymer is stored during storage after retort sterilization by making the outer polypropylene layer thinner than the inner polypropylene layer. It accelerates the release rate of water to the outside air and accelerates the recovery of the oxygen gas barrier property. However, in this method, since the outer layer polypropylene is thin, the water absorption amount of the saponified ethylene-vinyl acetate copolymer layer 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 cumulative amount of permeated oxygen can be slightly reduced in the long term. Furthermore, in the initial stage after retort sterilization, conversely, Since the oxygen concentration becomes high, deterioration may be promoted depending on the contents.

As a method for improving these first and second methods, a method using a desiccant has been proposed. That is,
57-170748 gazette, according to this description,
By protecting both sides of the saponified ethylene-vinyl acetate copolymer layer with a layer containing a desiccant, the water that invades during retort sterilization is trapped and the increase in water content of the saponified ethylene-vinyl acetate copolymer layer is reduced. It is a method of suppressing the deterioration of the oxygen gas barrier property. In this method, even though the deterioration of oxygen gas barrier property due to retort sterilization can be suppressed, calcium chloride, sodium chloride, dibasic sodium phosphate, ammonium chloride, potassium carbonate, potassium alum, magnesium sulfate, magnesium chloride, sodium nitrate, ammonium sulfate, It is intended to obtain an effect by mixing a desiccant such as monobasic ammonium phosphate and potassium bromide with 10 to 20% by weight of the adhesive resin layer.

As is well known, since these desiccants have the property of actively absorbing water under normal conditions, they have the property of always absorbing water in the air from the time of mixing with the adhesive resin. Therefore, the resin may absorb moisture before molding. Therefore, if an attempt is made to obtain a synthetic resin laminate using a resin containing a mixture of these adhesives, unless the resin is extruded in a sufficiently dry state with the utmost care and molded, foaming due to moisture, etc. Trouble-prone and requires extremely advanced technology and management.

Further, even if a synthetic resin laminate is obtained by using a resin mixed with such a desiccant by a high technology and management, the desiccant contained therein always removes moisture from the air or surrounding inclusions. Moisture that has been taken up and once taken up will not be released again unless considerable energy is applied.Therefore, the laminate such as the container packaging material made of the synthetic resin laminate obtained should be as soon as possible after manufacturing. If the contents are not filled in, the effect of the intended retort may be diminished.

Furthermore, if a desiccant necessary for absorbing water is added to the adhesive layer, the adhesive strength between the gas barrier layer and the adhesive layer may be reduced.

As described above, the conventional ethylene-saponified vinyl acetate copolymer as the oxygen gas barrier layer, in a synthetic resin laminate for a multi-layer container, it is possible to prevent deterioration of the oxygen gas barrier property due to retort sterilization, and A synthetic resin laminate that is easy to manufacture, has good stability over time before filling the contents, and is inexpensive has not been obtained, and development has been earnestly desired.

[Means for solving problems]

The present invention has been obtained as a result of various studies in view of the above circumstances. That is, the present invention is a synthetic resin laminate in which at least 7 or more synthetic resin layers are laminated, the inner and outer layers are made of a polyolefin layer, the intermediate layer is made of a saponified ethylene-vinyl acetate copolymer, It is a synthetic resin laminate in which a mixed resin layer containing a water absorbent resin is laminated with an intermediate layer via an adhesive layer. As a result, a synthetic resin laminate for producing a flexible multilayer bag or a rigid multilayer container that retains a high gas permeation barrier property even after retort sterilization is obtained. The present invention will be described in detail.

FIG. 1 is a sectional view of a laminate of the present invention. (1) shows a polyolefin layer as an outer layer of the synthetic resin laminate, (5) shows a polyolefin layer as an inner layer, (2) shows an ethylene-vinyl acetate copolymer saponified layer as an intermediate layer,
(3) shows an adhesive layer for joining them. A mixed resin layer (6) made of a resin mixed with a water-absorbent resin is interposed between the adhesive layer (3) and the intermediate layer (2).

Here, 7 layers may be simultaneously extruded to form a multilayer synthetic resin laminate, or a film obtained by simultaneously extruding the inner and outer layers and the mixed resin layer may be laminated on the intermediate layer by a dry lamination method. The resin used for this mixed resin layer is preferably the same resin as the inner and outer layers.

In the adhesive layer (3), the water absorbent resin (4) is mixed and coexistent in the adhesive.

The water-absorbent resin described in the present invention is insoluble in water, and is a starch-sodium acrylate graft polymer, a hydrolyzate of a starch-acrylonitrile graft polymer, a partially crosslinked starch-poly (meth) acrylic acid copolymer. , Partially crosslinked starch-polymethylmethacrylate hydrolyzate and starch-graft polymerization system such as salts of such substances, or partially crosslinked polyacrylate, polyisobutylene-maleic anhydride copolymer , Cross-linked synthetic resin type represented by a hydrolyzate of methyl methacrylate-vinyl acetate copolymer, etc., and these are basically those obtained by partially cross-linking a water-soluble polymer to make it insoluble. When it comes into contact with water or an aqueous solution such as a salt, it quickly absorbs water, swells, and retains water. The ability to absorb and retain water is generally 100 to 1,000 times the weight ratio, and is so large that it cannot be compared with the value of the desiccant.

Specifically, Sumika gel manufactured by Sumitomo Chemical Co., Ltd., Aquaprene manufactured by Meisei Chemical Co., Ltd., Aquakeep manufactured by Steel Manufacturing Co., Ltd., KI gel manufactured by Kuraray Isoprene Chemical Co., Sunwet manufactured by Sanyo Chemical Co., Ltd., Showa There are pre-apple made by Denko and SGP absorber polymer made by Henkel.

Here, it is noteworthy that these water-absorbent resins absorb water in the atmosphere, but the amount of moisture absorption changes depending on the temperature and humidity, and the water-absorbent resin that absorbs moisture under high humidity is stored in a low-humidity atmosphere. When it is placed inside, it releases moisture. That is, it absorbs moisture contained in the environmental atmosphere, but does not actively absorb moisture, and the water-absorbent resin selectively absorbs moisture from the environmental atmosphere to lower the relative humidity of the environmental atmosphere, that is, as a desiccant. It does not work. Therefore, by storing the water absorbent resin at a constant relative humidity, the water content of the water absorbent resin is kept at an equilibrium water content corresponding to the atmosphere. On the other hand, a water-absorbent resin that exhibits such properties with respect to moisture in an environmental atmosphere,
Once in direct contact with water, as described above, it quickly absorbs water and swells, and can hold 100 to 1,000 times the weight of water. Therefore, even if a water-absorbent resin having such properties is mixed in the resin and the synthetic films are laminated, there is no property to actively absorb moisture in the environment, and therefore the resin is preserved. As long as the environment is kept constant, the water content in the resin does not exceed the ambient atmosphere, and it is possible to prevent problems such as foaming of the resin during molding.

When the synthetic resin laminate is used for retort sterilization in the form of a bag or a molded product, high temperature and high pressure steam during retort sterilization against moisture entering through the synthetic resin film layer, It plays a major role as a hygroscopic agent.

With the polyolefin layer used for the inner and outer layers in the present invention,
Polyethylene, polypropylene and the like can be used, but polypropylene is generally suitable, and among polypropylene, homopolypropylene and ethylene-propylene copolymer are suitable. Furthermore, in addition to these polypropylene, by using a synthetic resin in which a high density polyethylene having a density of 0.95 g / cm ³ or more is mixed in a weight ratio of 5 to 40% by weight, the moisture permeability becomes smaller than that of polypropylene alone. The infiltration of water during retort sterilization can be further reduced, and the effect of the invention can be further enhanced.

The saponified ethylene-vinyl acetate copolymer described in the present invention has a vinyl acetate content of 10 to 60% by weight and a saponification degree of 95% or more, preferably a vinyl acetate content of 20 to 50% by weight and a saponification degree. 99% or more is suitable.

In the case of a dry laminate used as the adhesive layer in the present invention, polyester-based, polyurethane-based or epoxy-based adhesives are generally used. Specifically, Adcoat (hereinafter referred to as AD) 527 (polyester urethane prepolymer), AD503-35E (epoxy modified polyester)
AD960 (〃) or higher Toyo Morton Company and EPS-75
(Polyester type), EPS781 (Polyurethane type) or higher
Good results can be obtained by using a two-component curing type adhesive containing an isocyanate component such as I (xylene diisocyanate), TDI (xylene diisocyanate) and HMDI (hexamethylene diisocyanate).

Further, as the adhesive resin used as the adhesive layer for coextrusion described in the present invention, a copolymer resin of ethylene and an unsaturated carboxylic acid or a derivative thereof, or ethylene vinyl acetate and an unsaturated carboxylic acid or a derivative thereof The copolymer resin is used, and more specifically, a resin obtained by graft-polymerizing a carboxylic acid group, an acid anhydride group and a derivative thereof on a polyethylene or ethylene vinyl acetate copolymer resin is used. Specific examples of the carboxylic acid, acid anhydride and derivatives thereof include methacrylic acid, maleic acid, fumaric acid, methacrylic acid anhydride, maleic anhydride, ethyl methacrylate, glycidyl acrylate, diglycidyl methacrylate and the like. . Among them, particularly good results were obtained with a copolymer resin of glycidyl acrylate and diglycidyl methacrylate.

[Example 1] Using a coextrusion multilayer sheet manufacturing apparatus, the resins shown in Table 1 were simultaneously extruded from four extruders, joined at the molten resin joining portion, and then extruded into a sheet from a T-die and cooled. Thus, a 4 type 7 layer sheet having a total thickness of 5200 μm and containing a saponified ethylene-vinyl acetate copolymer as an intermediate layer was obtained.

The thickness of each layer is the same for both No.1 and No.2.
μ, mixed resin layer 100 μ, adhesive layer 40 μ, intermediate layer 40
It was μ.

Next, each sheet is plug-assisted vacuum forming method,
Molded into a tray container with a drawing ratio (depth / opening diameter) of 0.2,
After filling water as the content, biaxially stretched polyester (12
(μ) / aluminum foil (9μ) polypropylene (50μ). Further, the filled and sealed multi-layer container is treated with a retort sterilizer for 120
Hot water retort sterilization was performed at -30 ° C for 30 minutes.

Table 2 shows the results of measuring the oxygen permeability of the multilayer container obtained by the retort sterilization as described above after 1 day of retort sterilization and after storage at 20 ° C.-65% RH for 1 month.

From the above results, as a material for a multi-layer container for retort sterilization having an saponified ethylene-vinyl acetate copolymer as an intermediate layer, an outer layer and an inner layer are polyolefin resins, and a mixed resin layer is made to coexist a water-absorbent resin in the polyolefin resin. Thus, it was confirmed that a synthetic resin laminate having a small degree of decrease in oxygen gas barrier property due to retort sterilization can be obtained.

[Example 2] With the coextrusion multilayer sheet manufacturing apparatus, the resins shown in Table 3 were simultaneously extruded from four extruders as in Test Example 1, joined at the molten resin joining section, and then formed into a sheet from the T-die. By extruding and cooling, a saponified ethylene-vinyl acetate copolymer as shown in FIG. 1 having a desiccant and a water-absorbent resin in the adhesive resin layer is used as an oxygen gas barrier layer to a total thickness of 52.
A 0 type 4 type 7 layer sheet was obtained.

The thickness of each layer is the same for both No.3 and No.4.
0μ, mixed resin layer 100μ, adhesive layer 40μ intermediate layer 40
It was μ.

Next, each sheet was formed into a tray container having a drawing ratio (depth / opening diameter) of 0.2 by plug-assisted vacuum forming method immediately after production, and after filling water as the content, biaxially stretched polyester (12μ) / Aluminum foil (9μ) / polypropylene (5
The lid material having a composition of 0 μ) was heat-sealed and hermetically sealed. further,
The filled and sealed multilayer container was subjected to hot water retort sterilization at 120 ° C. for 30 minutes by a retort sterilizer.

On the other hand, after each sheet was stored at room temperature for 1 year, it was similarly formed into a tray container with a drawing ratio (depth / opening diameter) of 0.2 by the plug-assisted vacuum forming method, filled with water as the contents, and then Axial stretched polyester (12μ) / aluminum foil (9
A lid member composed of (μ) / polypropylene (50μ) was heat-sealed and hermetically sealed. Further, the filled and sealed multi-layer container was similarly subjected to hot water retort sterilization at 120 ° C. for 30 minutes by a retort sterilizer.

Table 4 shows the results of measuring the oxygen permeation rates of various multilayer containers obtained by retort sterilization as described above after 1 day of retort sterilization and after storage at 20 ° C-65% RH for 1 month.

As a result, whether the drying agent coexisted in the adhesive resin layer or the water-absorbent resin coexisted, the molded product in the tray container immediately after the sheet production was almost the same, the oxygen gas barrier property by the retort sterilization. A synthetic resin laminate with a small degree of decrease was obtained, but in the case where it was molded in a tray container after it was stored in a sheet state for 1 year at room temperature, the one with the drying agent added may have absorbed the drying agent during storage. , But the oxygen gas barrier property by retort sterilization decreased. On the other hand, in the case of using the water-absorbent resin, the one stored for one year showed almost the same numerical value as that immediately after the production.

<Effects of the Invention> The multi-layer container using the synthetic resin laminate of the present invention has much less degree of reduction in oxygen gas barrier property of the saponified ethylene-vinyl acetate copolymer due to retort sterilization than the conventional multi-layer container. As a result, the cumulative amount of oxygen permeated into the contents through the container is reduced, and the storability of the contents can be greatly improved. In addition, stability over time is also good.

Further, the present invention is economical because the conventional synthetic resin laminate manufacturing apparatus and process can be used as they are, and the manufacturing cost can be increased without significantly increasing the material cost.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of the synthetic resin laminate of the present invention.

Claims (1)

[Claims] 1. A synthetic resin laminate comprising a laminate of at least 7 synthetic resin layers, wherein the inner and outer layers are polyolefin layers and the intermediate layer is a saponified ethylene-vinyl acetate copolymer, and water is absorbed between these layers. A synthetic resin laminate obtained by laminating a mixed resin layer, which is a mixture of functional resins, with an intermediate layer via an adhesive layer.