Copolymers of ethylene and vinyl acetate (EVA) are extensively used by the package industry, because they provide a number of very desirable properties for typical applications. Depending largely upon proportion of ethylene and vinyl acetate in such copolymers, they may exhibit very desirable heat sealing properties, good adhesion to other polymers, good strength and toughness, abrasion resistance, clarity, gloss, and low temperature flex; moreover, EVA copolymers may be readily processed and molecularly oriented, and they are relatively inexpensive.
Due to the foregoing, such resins are widely utilized in laminates with other polymers, which may be employed to enhance the strength and barrier properties of the composite. An especially desirable film of this sort is one which is produced by coating EVA copolymer upon a substrate of a vinylidene chloride-vinyl chloride copolymer (commonly known as "saran"). Such laminates are widely used in the meat packaging industry.
More particularly, an increasingly prevalent method of handling beef is to prepare primal cuts at the slaughterhouse, which are then shipped to a retail meat store or other outlet, where they are butchered into smaller cuts. This necessitates packaging the meat at the slaughterhouse in such a way so to protect it against oxygen exposure during shipping and handling, to minimize spoilage and discoloration. An especially effective method of so packaging the meat is to enclose it within a bag of a thermoplastic film having good oxygen barrier properties and the capacity to shrink under heat. Following evacuation and sealing of the bag about the meat, the package is subjected to an elevated temperature, whereupon the film shrinks and tightly encases the meat therewithin, thus producing an attractive and conveniently handled package, in which the meat is well protected and secure.
Notwithstanding the advantages, shrink-bag packaging of meat is not without its difficulties, many of which are attributable to limitations inherent in the films presently available for such applications. As will be appreciated, such a technique exposes the film to rather severe conditions, due to the nature of the contained product, as well as to the conditions under which the package must be formed.
Firstly, commercially available shrink films tend to have a fairly narrow range of operability, from the standpoint of the temperatures at which they must be shrunk. The film must respond to heat rapidly enough for commercial practicality, and yet must not exhibit such a level of shrink energy as would cause the film to pull apart during shrinkage, under its own internal forces. It is especially important to appreciate that the film is particularly vulnerable at the conditions of operation, due to the relatively high temperatures to which it must be exposed in order to produce a satisfactory level of response in the film, for commercial production.
Moreover, the shrink-related problems are seriously exascerbated when the contained cut of meat is one which includes protruding bones and/or significant depressions in its surface. Needless to say, the hard, irregular and often sharp-edge nature of bony material promote penetration through the film, and thus represent a primary source of failure in bags so utilized. The problem can be alleviated either by utilizing heavier gauge films or stronger materials, or by the inclusion in the package of an added member which covers the exposed bone, and thereby protects the bag against penetration.
Insofar as the presence of significant depressions in the meat surface is concerned, because packaging is carried out under vacuum, a pressure differential will develop in the overlying area of the bag, exposing that area to additional stress and making it a likely site for failure. Specifically, there is a tendency for shrink-holes to develop in such areas, under the combined effects of elevated temperatures and negative pressure; such holes, of course, admit oxygen into the bag, leading intolerably to spoilage of the contained meat.
Exemplary of the known prior art which has utilized EVA resins in combination with other plastics, to produce film laminates suitable for use in packaging applications, are the following U.S. Pats. No. 3,713,965, issued on Jan. 30, 1973 to Widiger, et al; No. 3,741,253, issued on June 26, 1973 to Brax, et al; No. 3,817,821, issued on June 18, 1974 to Gellini; and No. 3,924,051, issued on Dec. 2, 1975 to Wiggins, et al. Also of interest is United Kingdom Specification No. 1,385,196, published Feb. 26, 1975.
Specifically, U.S. Pat. No. 3,713,965 discloses a non-blocking film made from vinylidene chloride-vinyl chloride copolymer, and an EVA copolymer, which has blended with it a small amount of calcium carbonate and a fatty amide. U.S. Pat. No. 3,741,253 discloses laminates, especially useful for meat packaging, which include two layers of EVA copolymer and an intermediate layer of saran; one of the EVA layers is cross-linked, as by electron irradiation. In the U.S. Pat. No. 3,817,821, films are disclosed in which a blend EVA resins may be applied to a core of polyethylene; the copolymer blend layer produced a peelable heat-seal with itself. U.S. Pat. No. 3,924,051, discloses laminates of EVA and saran, products produced therefrom, and methods for the production thereof. Finally, the United Kingdom Specification discloses heat-shrinkable films consisting of layers of EVA copolymer on saran, wherein the EVA copolymer extracts plasticizer from the saran to maximize its oxygen barrier capability.
Accordingly, it is an object of the present invention to provide novel EVA compositions, which exhibit such levels of molecular orientability, and such other properties, as will render the compositions particularly well suited for use in a heat-shrinkable film.
It is also an object of the invention to provide novel heat-shrinkable films utilizing such EVA compositions, which films exhibit outstanding properties.
A specific object of the invention is to provide novel film laminates which are especially adapted to the packaging of meat products, and which exhibit high resistance to failure, especially due to shrink-hole formation, while preserving or improving other desirable properties which are typical of films of similar composition.
Another object of the invention is to provide novel bags produced from laminates of the foregoing type, which may be especially suitable for the packaging of meat.
Still another object of the invention is to provide a novel method by which heat-shrinkable films can be produced.