Laminated packaging film having low vapor and gas permeability

摘要

A laminated film suitable as a food packaging film comprised of a base layer of an oriented polypropylene film, a layer of a vinylidene chloride polymer adhered to one surface of the base layer, a layer of first ethylene/vinyl acetate copolymer having a vinyl acetate content greater than 10 percent by weight adhered to the vinylidene chloride polymer layer and a top layer of a second ethylene/vinyl acetate copolymer having a vinyl acetate content less than 10 percent adhered to the first ethylene/vinyl acetate copolymer layer.PP BACKGROUND OF THE INVENTIONPP1. Field of the Invention PP This application is a continuation-in-part of copending application Ser. No. 81,941 filed Oct. 19, 1970, and now abandoned. PPThis invention relates to an improved packaging film and more particularly relates to a laminated packaging film having low permeability to gases and a method for the preparation thereof.P P2. The Prior ArtPPFilms and other shaped articles of polypropylene have found widespread use as a wrapping and packaging material for many articles of commerce because of their flexibility, transparency, low water vapor permeability and an excellent capability for being heat sealed to provide strong permanent joints. PPAlthough polypropylene films have low water vapor permeability, the films exhibit poor resistance to the permeation or transmission of air and oxygen and for this reason have not found wide application in food packaging where extended shelf-life is a requirement. PPSaran coatings which are basically vinylidene chloride polymers containing minor proportions of a copolymerized ethylenically unsaturated monomer such as vinyl chloride or acrylonitrile have the outstanding property in comparison to other synthetic coatings of their excellent resistance to the transmission of oxygen or air. For this reason, it has previously been the practice to coat polypropylene films with either saran emulsions or lacquers. These coatings once applied to the polypropylene films render the films sufficiently impervious to air and oxygen to permit the use of the films for food packaging applications. PPThe application of saran coatings to polypropylene films has not been without problems. When saran emulsions are employed, at least two coating applications are required in order to eliminate pinholes in the saran coating that is deposited. The resultant high coating weight required when using the emulsions may result in impairment of the impact strength of the coated film. When saran lacquers are employed, the organic solvents used in the lacquers are absorbed by the polypropylene film producing objectionable odors which are difficult to eliminate.P POne approach to the solvent absorption problem has been to use lacquers containing extremely volatile solvents and to wind the saran coated polypropylene film into rolls. The rolls are then stored for time periods of at least 48 hours before any further use is made of the coated film. During this storage period, the solvents absorbed from the saran coating operation rapidly permeate the polypropylene film layer and concentrate between the roll windings. After the storage period, the film roll is unwound and subjected to a jet of high velocity air directed on the uncoated side of the polypropylene film layer which effectuates the removal of the absorbed solvent which has permeated out of the film. PPIt has been found that excellent gas transmission properties can be imparted to polypropylene films with very low coating weights of saran, e.g. in the range of 0.3 to 0.5 pounds per 1000 square feet of film surface, using surface oxidized films fabricated from polypropylene and employing a saran lacquer as the coating medium. PPThese saran coated polypropylene films cannot, however, without further modification, be employed as packaging films, as the saran layer is easily scratched or damaged during the packaging operation which causes a marked diminution in the gas barrier properties of the coated film. In order to protect the saran coating from damage by packaging operations, attempts have been made to laminate the saran coated polypropylene films to other film substrates such as those fabricated from nylon, polyethylene terephthalate or another polyolefin with the saran layer forming the intermediate layer of the laminate. PPIn forming the laminated film, an adhesive agent such as synthetic rubber or polyurethane is generally applied in an organic solvent to the second film layer and then combined with the coated side of the saran coated polypropylene immediately after the solvent has been evaporated from the applied adhesive layer. Another method to effect lamination is to extrude a molten layer of nylon or a polyolefin onto the coated side of a saran coated polypropylene, after first priming the saran layer with an organic titanate or a polyurethane.PP Although the solvent absorption problem encountered in the saran coating of polypropylene films is substantially eliminated by the roll storage procedure discussed above, the solvent absorption problem is frequently encountered again when the saran coated polypropylene surface is laminated to a second film layer using adhesive solutions. Removal of the absorbed adhesive solvent from the laminated structure is extremely difficult and may not be apparent for several weeks after the structure is fabricated so that when the absorbed solvent slowly permeates through the structure, it can contaminate food stuffs packaged therein.P PThe problem is particularly severe when the second film layer laminated to the saran coated polypropylene film is a film which has good gas barrier properties such as nylon or polyethylene terephthalate. However, the problem also exists when the second film layer is a polyolefin such as an ethylene/vinyl acetate copolymer, for although the ethylene/vinyl acetate copolymer films are readily permeable to many solvents, it has been found that the solvents such as the aliphatic and aromatic hydrocarbons used in preparing adhesive solutions commonly employed in film laminating operations are generally less volatile than the solvents used in the saran coating operations, and do not readily permeate out of the film during extended storage periods.P P SUMMARY OF THE INVENTIONPPIn accordance with the present invention there is provided a laminated film structure having improved gas barrier properties, the structure being comprised of a base layer of an oriented polypropylene film, a continuous layer of a saran polymer coated on one surface of the base layer, a layer of a first ethylene/vinyl acetate copolymer having a vinyl acetate content of greater than 10 percent by weight adhered to the saran layer and a top layer of a second ethylene/vinyl acetate copolymer having a vinyl acetate content less than 10 percent adhered to the first ethylene/vinyl acetate copolymer layer.PPThe laminated films of the present invention do not require the use of adhesive solutions in their preparation. As no adhesive solutions are employed, there is no requirement for solvent removal during the laminating operation and the problem of solvent absorption is thereby substantially eliminated. PP The laminated film structures prepared in accordance with the present invention are substantially free of absorbed quantities of organic solvents and have particular utility as wrapping and packaging materials for foodstuffs and other articles where it is desirable to provide packages having maximum resistance to the permeation of gases.