JPH01154738A - Multi-layer structure - Google Patents

Abstract

PURPOSE:To improve the dispersibility of desiccant, check the lowering of bond strength and at the same time contrive to improve the retorting resistance and long-time keeping quality after the filling of contents by a method wherein the desiccant, which is filled into porous material, is compounded with a multi- layer structure having a gas barrier so as to protect the gas barrier layer. CONSTITUTION:When the kneading of desiccant with polyolefin resin is intended, once the desiccant is kneaded with porous material and, after that, kneaded with aimed polyolefin resin. Concretely, the desiccant and the porous material are kneaded together and pelletized by means of a double-screw co-extruding device. The resultant desiccant-filled porous material is further kneaded with ethylene-propylene random copolymer and formed into pellets. In a multi-layer sheet co-extruding device, ethylene-propylene random copolymer, saponified ethylene-vinyl acetate copolymer and ethylene-propylene random copolymer, to which adhesive resin and the desiccant-filled porous material are added, are made into a sheet. Thus, in a desiccant-filled system multi-layer structure, the dispersibility of the desiccant is improved, the lowering of bond strength due to deliquescence is checked, the generation of blister is suppressed and no drying effect is impaired.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a synthetic resin laminate that can be retort sterilized and has high gas barrier properties, and maintains high gas barrier properties during and after retort sterilization treatment. The present invention relates to a synthetic resin laminate used in containers capable of storing foods, foods, or medicines for a long period of time.

<Prior art and its problems> Saponified ethylene-vinyl acetate copolymer has excellent gas barrier properties, fragrance retention properties, and transparency, and can be used as a gas barrier layer in packaging materials such as multilayer containers. It is widely used because it prevents food or medicine from deteriorating due to oxygen, etc., and enables long-term storage.

However, although saponified ethylene-vinyl acetate copolymer exhibits excellent oxygen gas barrier properties under low humidity conditions,
When the humidity is high or the moisture content is high, the oxygen gas barrier properties are significantly reduced. Therefore, it is generally used by laminating low moisture permeability hydrophobic polymers such as polyethylene and polypropylene on both sides.

The reason why the saponified ethylene-vinyl acetate copolymer has excellent oxygen gas barrier properties is that the intermolecular and intramolecular hydrogen bonds are stronger than those of other polymers.
One example is the synergistic contribution of molecular chain symmetry, polarity, etc. On the other hand, when the water content of the saponified ethylene-vinyl acetate copolymer increases, the adsorbed water molecules first bond to hydrophilic OH groups, and as the water content increases, the adsorbed water becomes intermolecular hydrogen. It is thought that it breaks bonds and enables the molecular movement necessary for oxygen molecule diffusion, leading to an increase in the oxygen permeability coefficient. It is thought that if the water content increases further from this state, free water will exist in addition to adsorbed water, and the intermolecular force will further weaken, and the oxygen permeability coefficient will further increase due to the plasticizing effect on molecular motion. There is.

When using such saponified ethylene-vinyl acetate copolymer as a multilayer container for retort sterilization, saponified ethylene-vinyl acetate copolymer is recommended because of its heat resistance to hot water or steam of about 120°C during retort sterilization. It is common to laminate polypropylene layers on both sides of the compound.

However, in the heated and pressurized state during retort sterilization, the moisture permeability of the polypropylene layer increases by 15 to 20 times compared to that at room temperature, so the water content of the saponified ethylene-vinyl acetate copolymer layer increases rapidly, and at the same time oxygen Gas barrier properties are greatly reduced. When stored after retort sterilization, the moisture in the saponified ethylene-vinyl acetate copolymer layer is gradually released to the outside of multi-layered containers, whose oxygen gas barrier properties have been greatly reduced due to retort sterilization. Although the barrier properties can be restored, it takes a long time, and the application is limited to contents that have a relatively high oxygen tolerance against deterioration,
It was limited to items with a short shelf life.

Three methods are currently being considered to improve these problems; the first is to impart hot water resistance to the saponified ethylene-vinyl acetate copolymer itself;
Method 3 is a method that accelerates the release rate of water absorbed by the ethylene-vinyl acetate copolymer and accelerates the recovery of oxygen gas barrier properties.
Providing the vinyl acetate copolymer layer on both sides is a method of suppressing a decrease in oxygen gas barrier properties.

Regarding the first method, by increasing the ethylene content in the saponified ethylene-vinyl acetate copolymer, water resistance and hot water resistance are improved, but the original oxygen gas barrier properties are significantly reduced. Therefore, it is not practical. In addition, as a second method, by making the thickness of the outer polypropylene layer thinner than the thickness of the inner polypropylene layer, the saponified ethylene-vinyl acetate copolymer is absorbed during storage after retort sterilization. This speeds up the release of water into the outside air and speeds up the recovery of oxygen gas barrier properties. However, in this method, since the outer layer polypropylene is thin, the amount of water absorbed by the saponified ethylene-vinyl acetate copolymer layer during retort sterilization is large, so the oxygen gas barrier properties are greatly reduced, and even if the recovery speed is fast, it will take a long time. In general, the cumulative amount of permeated oxygen can be slightly reduced, and furthermore, in the early stage after retort sterilization, the oxygen concentration inside the container increases, so there is a risk that the contents may accelerate deterioration.

A method using a desiccant has been proposed as a method for further improving the first and second methods.

That is, it is described in Japanese Patent Application Laid-Open No. 57-170748,
According to this description, by protecting both sides of the saponified ethylene-vinyl acetate copolymer layer with a layer containing a desiccant, water that enters during retort sterilization is captured, and the ethylene-vinyl acetate
This method aims to reduce the increase in water content of the saponified vinyl acetate copolymer layer and suppress the decrease in oxygen gas barrier properties. Although this method can suppress the deterioration of oxygen gas barrier properties due to retort sterilization, calcium chloride,
Add 10 to 20% by weight of a drying agent such as sodium chloride, dibasic sodium phosphate, ammonium chloride, potassium carbonate, potassium alum, magnesium sulfate, magnesium chloride, sodium nitrate, ammonium sulfate, primary ammonium phosphate, or potassium bromide to the adhesive resin layer. The idea is to obtain the effect by mixing them.

As is well known, these desiccants have the property of actively absorbing moisture under normal conditions, so from the time they are mixed with the adhesive resin, they constantly try to absorb moisture from the air. Therefore, there is a risk that the resin will absorb moisture before being molded. Therefore, if you try to obtain a synthetic resin laminate using a resin mixed with these drying agents, problems such as foaming due to moisture will occur unless you take extreme care to extrude and mold the resin in a sufficiently dry state. easily occur and require extremely sophisticated technology and management.

Furthermore, even if a synthetic resin laminate is obtained using a resin mixed with such a desiccant through advanced technology and management, the desiccant contained therein will always remove moisture from the air or surrounding inclusions. Since the water that is tried to be absorbed and taken in will not be released again unless a considerable amount of energy is applied, laminates such as containers and packaging materials made of the resulting synthetic resin laminate should be kept as long as possible after manufacturing. If the contents are not filled promptly, the desired effect during retorting may be diminished.

As described above, in a multilayer structure having a saponified ethylene-vinyl acetate copolymer as a gas barrier layer, deterioration of the gas barrier layer due to retort sterilization treatment can be prevented,
Moreover, a multilayer structure that is easy to manufacture, has good stability over time before being filled with contents, and is inexpensive has not yet been obtained, and its development has been eagerly desired.

Problems to be Solved> A method has been proposed in which a desiccant is used in a conventional multilayer structure having a saponified ethylene-vinyl acetate copolymer as a gas barrier layer.
According to Japanese Patent No. 748, this is a multilayer structure in which a polyolefin resin or an adhesive resin is kneaded to serve as a desiccant, a saponified ethylene-vinyl acetate copolymer is used as a gas barrier layer, and a film is formed by coextrusion.

However, this method has a problem in that the desiccant in powder form has poor dispersibility and tends to aggregate when kneaded with the polyolefin resin, affecting the adjacent adhesive layer and causing a decrease in adhesive strength. In addition, as the desiccant absorbs moisture, its adhesive strength decreases due to the influence of moisture due to deliquescent action.
There is a tendency to decrease more and more.

Furthermore, if the tray is filled with contents after molding and stored for a long period of time, the desiccant absorbs moisture and the adhesive strength decreases, and in the worst case, blisters occur in some thin-walled tray corners. This blistering is caused by the excessive hygroscopic action of the desiccant, resulting in a problem that significantly reduces the value as a product. In order to solve these problems, we have improved the dispersibility of the desiccant, suppressed the decline in adhesive strength, and achieved retort resistance after filling the contents, and the contents remained good even when stored for a long time. The present invention provides a laminate having the following properties.

Means for Solving the Problems> That is, the present invention has solved the problem by kneading the drying agent into a polyolefin resin, first by kneading it into a porous body, and then kneading it into the target polyolefin resin.

That is, the multilayer structure is characterized in that a desiccant is filled into a porous body and blended into a multilayer structure having a gas barrier layer to protect the gas barrier layer.

[Effect]

Commercially available desiccants are usually in powder form, and when mixed directly into polyolefin resin, they tend to aggregate.
The desiccant has poor dispersibility, thus affecting the adjoining adhesive layer, and since the desiccant has deliquescent properties, when excessive moisture is absorbed, the deliquesced water reduces the adhesive strength. In particular, when a tray is formed, filled with contents, and stored for a long period of time, the adhesive strength at the tray corners decreases, and in the worst case, blisters occur due to deliquescence.

This blistering significantly lowers the commercial value and makes commercialization difficult even if the barrier properties are good. Therefore, in order to solve this problem, the porous body is filled with a desiccant agent in advance, and the desiccant is sealed in, thereby preventing the deliquescence phenomenon. In addition, the desiccant is surrounded by a porous material with fine pores, so the original drying effect is not lost. Furthermore, since it is filled into a porous body, agglomeration is prevented, dispersibility is improved, and a decrease in adhesive strength can be suppressed.

The problem was solved by using the above method.

Examples of porous materials include silicon dioxide, silica gel, activated carbon, calcium carbonate, and zeolite.

<Example-1> Using a twin-screw coextrusion device, a desiccant (dibasic sodium hydrogen phosphate) and a porous body (silica gel) were kneaded and pelletized, and the resulting desiccant-added porous body was further mixed with ethylene-propylene random. Copolymer (Tissopolypro: F-302
0) to form pellets. The mixing ratio of each was 1:2:17.

From the above, using a multilayer sheet coextrusion device, ethylene-propylene random copolymer (same as above) and saponified ethylene-vinyl acetate copolymer (Kuraray ■: F1 old) (hereinafter EVOH)
A sheet was formed using the above-mentioned ethylene-propylene random copolymer containing a desiccant-added porous material, an adhesive resin (P300F manufactured by Mitsubishi Yuka Co., Ltd.), and a total thickness of 600 μm. The formed sheet was molded into a tray using a pressure forming machine, and the adhesive strength of the resulting tray was measured using a tensile tester. As a comparison, a film was formed in the same manner using only a drying agent without adding a porous material, and a tray was formed, and the adhesive strength was measured.

Furthermore, using the two types of trays produced, they were filled with contents (water) and subjected to retort treatment, and changes in adhesive strength during long-term storage and the condition of the trays were checked over time. The results are shown in Table 1.

Table 1 The retort conditions were a rotational retort at 120°C for 30 minutes.

From the above, as the porous material-free product was stored, the adhesive strength at the tray corner decreased significantly, and blisters appeared at the tray corner after 6 months, and the product lost its value as a product. In addition, the porous material-added product prevented a decrease in adhesive strength during storage and did not cause blisters, and was in good condition.

<Example-2> Using a twin-screw co-extrusion device, a desiccant (dibasic sodium hydrogen phosphate) and a porous material (zeolite) were kneaded and pelletized, and a film was formed using a multilayer sheet pressing device in the same manner as in Example-1. death,
Tray molding was performed using a pressure molding machine.

The resulting tray was placed in a boiling tank at 100°C, and an excessive amount of water was added to perform a boiling acceleration test. For comparison, a boiling acceleration test was similarly conducted after forming a film-forming tray using only a desiccant agent without adding a porous material (zeolite).

Table 2 shows the test results.

Table 2 From the above, the porous material-free product developed blisters at the corner of the tray after 24 hours and 36 hours of boiling, and lost its value as a product.

Note that the porous material-added product did not cause blisters and was in good condition, confirming the effect of the porous material addition.

<Effects of the Invention> As described above, the present invention improves the dispersibility of the desiccant in a desiccant-added multilayer structure, suppresses the decrease in adhesive strength due to deliquescent property, suppresses the occurrence of blisters, and Does not impair the drying effect of the desiccant.

Therefore, by sharing resins such as saponified ethylene-vinyl acetate copolymer whose gas permeation blocking ability tends to decrease due to the influence of moisture, this ability can be maintained and its commercial value will be increased when filled with contents. It is in good condition without any damage.

The obtained multilayer structure can be used for retort containers, etc.
It can be used for multiple purposes such as moisture-proof containers in hyperhumid environments.

Patent applicant Toppan Printing Co., Ltd. Representative: Kazuo Suzuki

Claims (2)

[Claims] (1) A multilayer structure, characterized in that a protective layer in which a porous material is filled with a desiccant agent is disposed at a position protecting the gas barrier layer of a multilayer structure having a hygroscopic gas barrier layer. (2) The multilayer structure according to claim 1, wherein the protective layer is an inner layer, and a single layer or multiple layers are arranged outside the gas barrier layer.