The present invention relates to a biaxially oriented heat-shrinkable laminated film which has good heat shrinkability, excellent melt hole resistance and heat sealing resistance (the heat resistance of a sealed portion), which remains transparent after being shrunk and serves as a packaging material suitable for packaging foods, which exhibits a shrinkage percentage of 15% or more at 90.degree. C. and excellent gas barrier properties, and which comprises a laminate having a layer composed of a vinylidene chloride copolymer composition containing 0.5 to 9.1 % by weight of polyfunctional (metha)crylate, in which the vinylidene chloride copolymer is crosslinked by applying electron beams at a dosage of 1 to 10 megarads. The present invention also relates to a method of producing such a heat-shrinkable laminated film.
Shrink packaging is generally the most convenient form of packaging for food products such as fatty foods like raw meat, processed meat, cheese and the like, all of which are irregular in form. Since such foods are required to be stored for a long period, the packaging of such foods must have such characteristics as excellent gas barrier properties (below 200 cc/m.sup.2.day.atm at 30.degree. C. and 100% RH), excellent heat sealing properties, cold resistance, melt hole resistance, heat sealing resistance (the heat resistance of a sealed portion) and the like, as well as having transparency after being shrunk which is one important factor that affects the appearance of products. The packaging and sterilization of fatty foods often involves problems in that a film softened by oil and heat may be stretched to become thin and thus break (melt hole) and in that a sealed part or parts in the vicinity thereof may be broken owing to heat shrinkage stress during sterilization. The industry has therefore required for heat-shrinkable films having gas barrier properties, melt hole resistance, heat sealing resistance and excellent transparency after being shrunk.
Simple films of vinylidene chloride copolymer (referred to as PVDC hereinafter) not only have shrinkability but also display excellent characteristics with respect to gas barrier properties, oil resistance, ligating properties and the like, and these films are thus widely used.
However, since ordinary simple PVDC films contain 6 to 10% by weight of such additives as plasticizers and stabilizers and so on which are added for the purpose of providing these films with cold resistance, flexibility for improving their ligating properties and good film-forming properties, certain problems occur because the additives migrate to foods and are thus sanitarily undesirable according to the kind of foods packaged, and also because the strength, particularly the cold resistance, of the films may in some cases be insufficient under severe conditions such as the packaging of heavy foods. There is therefore a demand for excellent packaging materials having none of these problems.
In order to remove such problems, a proposal has been made (see Canadian Patent Application No. 982923) regarding a three-layer film formed by co-extrusion and comprising an intermediate PVDC layer containing very small amounts of additives such as plasticizers and stabilizers, or substantially no additives, and two outer layers of ethylene-vinyl acetate copolymer (EVA) which have excellent low-temperature resistance and good adhesion to the PVDC layer, these outer layers being provided on both sides of the PVDC layer. This three-layer film involves no hygienic problems as with the simple PVDC film and also display an improved cold resistance.
The films described below have also been proposed.
A flexible laminate was disclosed in Japanese Patent Publication No. 43024/1983 (U.S. Pat. No. 3,741,253) which has (1) a first layer containing an organic polymer; (2) a PVDC-based gas barrier layer with oxygen permeability that is sufficiently low to ensure that the oxygen permeation coefficient thereof is below 70 cc/m.sup.2 /24 hours/atm (measured at 22.8.degree. C. and relative humidity 0% in accordance with ASTM Standard D 1434); and (3) a layer containing an organic polymer having resistance to cruel use, and which is suitable for use in heat-shrinkage packaging, the laminate being characterized in that the first layer (1) is composed of an oriented copolymer of ethylene and vinyl acetate containing 5 to 20% by weight of the unit derived from vinyl acetate, the copolymer being crosslinked by irradiation, in that the gas barrier layer (2) contains 70 to 85% by weight of the unit derived from vinylidene chloride and 30 to 15% by weight of the unit derived from vinyl chloride, and in that the layer (3) contains (i) a copolymer of ethylene and vinyl acetate having 5 to 20% by weight of the unit derived from vinyl acetate, or (ii) a blend of isotactic polypropylene, atactic polypropylene and polybutene-1.
A heat resistant laminate was disclosed in Japanese Patent Laid-Open No. 11342/1985 (U.S. Pat. No. 4,699,846) which has three or more layers comprising a resin layer having gas barrier properties, a layer of thermoplastic resin that differs from the gas barrier resin and an adhesive layer interposed therebetween, the laminate being characterized in that the adhesive layer is composed of 100 parts by weight of adhesive resin which can be melt-extruded and 0.1 to 50 parts by weight of radiation sensitive compound and is crosslinked by radiation.
An oriented film laminate was disclosed in Japanese Patent Publication No. 47859/1986 (U.S. Pat. No. 4,044,187 which is formed by irradiating the whole of a film laminate composing a film layer (1) which serves as a base layer and contains an .alpha.-monoolefin polymer crosslinked by irradiation, and a film layer (2) containing a polymer crosslinkable by irradiation so that the polymer in the film layer (2) is crosslinked and the polymer in the film layer (1) is further crosslinked.
A heat-shrinkable biaxially oriented multi-layer film is disclosed in Japanese Patent Laid-Open No. 3948/1987 which contains a gas barrier layer composed of a vinylidene chloride-methyl acrylate copolymer, which is irradiated at a dosage of about 1 to 5 megarads and which is suitable for packaging primal and subprimal slices of meat and processed meat.
A molecule oriented multi-layer polymer film is disclosed in Japanese Patent Laid-Open No. 23752/1987 which comprises first and second layers each composed of ethylene-vinyl acetate copolymer as a principal component and a third layer disposed between the first and second layers and composed of PVDC, each of the first, second and third layers being crosslinked by irradiation at a dosage equal to that of electron beams of 1.5 megarads or more.
In addition, laminated films having as an outer layer a polyolefin layer or EVA layer crosslinked by irradiation have been proposed in Japanese Patent Laid-Open No. 3456/1972, Japanese Patent Publication No. 20549/1979, U.S. Pat. Nos. 4,044,187, 4,064,296, 4,352,844 and 4,501,780, Japanese Patent Publication Nos. 5553/1968, 20599/1971, 44019/1976 and 44020/1976, British Patent No. 2,040,804, U.S. Pat. Nos. 4,391,862, 4,448,792, 4,514,465 and 4,551,380, etc.
However, although conventional laminated films such as EVA/PVDC/EVA films are laminated films which can be heat-sealed and exhibit good cold resistance and excellent gas barrier properties and which compensate for the deficiency of simple PVDC films, they lack melt hole resistance and heat sealing resistance. Furthermore, since a laminate composed of a PVDC layer and two polyolefin layers provided on both sides thereof is generally oriented at a temperature up to 40.degree. C. below the melting point of the polyolefin crystal in order to provide the laminate with heat shrinkability, the effect of stretch orientation cannot be sufficiently imparted to the PVDC layer. The PVDC layer thus exhibits a poor heat shrinkage percentage and, when the laminate is heat-shrunk, the PVDC layer has the tendency to bend limply because the behavior of the shrinkage is slower than those of the other layers and this consequently obstructs the laminate's transparency after shrinkage.
For this reason, there is a strong demand in the field of food packaging for a heat-shrinkable film that not only display good gas barrier properties and cold resistance, excellent melt hole resistance and heat sealing resistance, but also exhibits excellent transparency after being shrunk.
As a result of the energetic research performed by the inventors with a view to solving the above-described problems of the prior art, the inventors found that heat shrinkability can be given to a PVDC layer by adding a crosslinking agent to the PVDC intermediate layer and then crosslinking it by irradiation with electron beams at a low dosage and by the following stretching process. This invention could be attained based on this finding.