Conventionally, for the multi-layered structure to be used for the above-mentioned uses, resins with high oxygen gas barriers such as saponified ethylene vinyl acetate copolymer, polyvinyl alcohol, acrylonitrile-methyl acrylate-butadiene copolymer, nylon (polyamide resin), or vinylidiene chloride copolymer are used for the core. On both sides of this core layer, hydrophobic polyolefins such as polyethylene, polypropylene, or ethylene-olefin copolymer are used.
In low humidity, due to the gas barrier layer, the multi-layered structure does not allow the passage of oxygen and prevents oxidation of the contents. As a result, over longer periods the quality of the contents would stay consistently good. However, when using this multi-layered structure for packaging food in the retort process which requires steam treatment at 90.degree.-135.degree. C., moisture would go through the outer layer and be absorbed by the barrier layer. In cases where the barrier layer is made of hygroscopic materials, such as saponified ethylene vinyl acetate copolymer or polyvinyl alcohol, or polyamide, due to the moisture absorption by the barrier resin, the oxygen barrier properties of the barrier layer would drop dramatically and the gas barrier layer would lose a significant portion of its function. Therefore, because of this water absorption and retention of the barrier layer, the multi-layered structure sometimes could not be used for retort food products.
FIG. 1 shows the relationship of the amount of moisture absorbed by the gas barrier layers and the amount of oxygen that passes through when a multi-layered coextruded structure made up of polypropylene/adhesive/saponified ethylene vinyl acetate copolymer/adhesive/polypropylene with a total thickness of 100 .mu.m is tested in several retort processes.
From this figure, it is apparent that in temperatures above 120.degree. C., which is the common range for retort processes, the water absorption amount is above 7% and the oxygen passage increases to over 1000 times its normal rate.
To correct this problem, in EP59274, U.S. Pat. Nos. 407,897, 464,443, 425,410 and Tokkaisho Japan No. 57-170748, a procedure where a drying agent derived from an inorganic metal is added to the adhesive layer between the outer and gas barrier layer was presented. However, in this procedure, problems arose such as the addition of desiccant decreasing the strength of the adhesion between the adhesive layer and barrier layer. Also, when the adhesive layer was being extruded, the generation of fisheye gels was sometimes observed.
Also, in U.S. Pat. Nos. 347,337 and 289,830, there is described a procedure to increase water resistance by adding phenolic compound to the saponified ethylene vinyl acetate copolymer and cross-linking an ether linkage through the dehydration reaction of two hydroxyl groups of the saponified ethylene vinyl acetate copolymer.
Additionally, there is the technique of making the outside layer a very water-permeable polycarbonate and the gas barrier layer as saponified ethylene vinyl acetate copolymer. In this case, the water absorbed from the retort process by the saponified ethylene vinyl acetate copolymer is dispersed in an early stage after the retort process. This would increase the speed of recovery of the gas barrier properties of the barrier material.
However, in this method, there is the problem of the polycarbonate being high in price. Also, its required molding temperature is high, making the multi-layer coextrusion difficult and requiring special extrusion equipment.
Also, there is the technique of placing the saponified ethylene vinyl acetate copolymer on the outside of the multi-layer structure. This increases the speed of recovery of the gas barrier properties. However, with this technique, during the retort process, due to the water absorption the outside layer expands. This causes the structure to lose shape and renders this technique impractical.
The inventors have developed a multi-layered structure that does not need a special extruder machine, and which exhibits superior gas barrier properties and solves the foregoing problems, at a low cost.