This invention relates generally to heat shrinkable, relatively gas impermeable, thermoplastic packaging film which can be heat sealed to itself to form a flexible package. The invention relates more particularly to bags of such film for food products in which the packaged product is submerged in heated water, or exposed to steam or dry air heat or autoclaved for a substantial period of time for pasteurizing or cooking, the bag structure being shrinkable and nondegradable under such conditions.
There is a need in the food packaging industry for a packaging film from which bags can be made which are of improved structural soundness such that they may be fully characterized as pasteruizable and/or cook-in. Further, it is desirable to have a precooked food product which is attractively packaged inside the film within which it was precooked.
The term "pasteurizable" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to pasteurizing conditions while containing a food product. Many food products require pasteurization after they have been hermetically packaged to destroy harmful microbes which grow in the absence of air. Specific pasteurization requirements tend to vary by country; however, limiting conditions probably are submersion of the hermetically sealed product in water at 95.degree. C. for 1 hour. Thus, for a bag to characterized as pasteurizable, structural integrity of the bag must be maintained during pasteurization, i.e. the bag must have superior high temperature seal strength and must be delamination resistant under such time-temperature conditions. Additionally, the packaging material should be heat shrinkable under pasteurizing conditions so as to provide an attractively packaged pasteurized food product.
The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods that go directly to the consumer in that configuration which may be consumed with or without warming. Cook-in time-temperature conditions typically refer to a long slow cook, for example submersion in water at 70.degree.-80.degree. C. for 4-6 hours. Such cook-in time-temperature requirements are representative of institutional cooking requirements. Submersion at 80.degree. C. for 12 hours probably represents the limiting case. Under such conditions, a packaging material properly characterized as cook-in will maintain seal integrity and will be delamination resistant. Additionally, the packaging film should be heat shrinkable under these conditions so as to form a tightly fitting package and preferably should have some tendency for product adhesion to prevent "cook-out" or collection of juices between the surface of the food product and the interior surface of the packaging material.
The term "ethylene alkyl acrylate copolymer" (EAA) is used herein to define a copolymer formed from ethylene and alkyl acrylate comonomers wherein the ethylene derived units in the copolymer are present in major amounts, and the alkyl groups may include e.g. ethyl, methyl and butyl groups.
Generalizing, there are a number of requirements for a pasteurizable, cook-in packaging material. It is the purpose of the present invention to provide a pasteurizable, cook-in packaging film meeting all of these requirements. First, bags made from such film must have seal integrity under such conditions, i.e. the heat sealed seams should resist being pulled apart during heat shrinking. As a corollary, the film should be heat sealable to itself. Second, such bags must be delamination resistant, i.e. the multilayers making up the film must not separate or blister. Third, the food contact layer of such film must qualify under the appropriate food laws and regulations for safe food contact. Fourth, the film must provide an oxygen and vapor barrier, i.e. must possess a low permeability to maintain the freshness of the food contained therein. Fifth, the film must be heat shrinkable in hot water under these time-temperature conditions, i.e. the film must possess sufficient shrink energy such that upon the packaged food product being submerged in hot water the packaging film will shrink snugly around the product contained therein, representatively about 30-50% biaxial shrinkage at about 90.degree. C. Sixth, the film should possess optical clarity, i.e. the film should not become cloudy upon exposure to these time-temperature conditions so as to maintain eye appeal of the packaged product.
In general, such a multilayer film structure will have the minimal structure (sealing and food contact layer)/(shrink layer)/(barrier layer)/(abuse layer), a composite structure being required to achieve the desired composite properties of the packaging film.
A heat shrinkable, thermoplastic, barrier packaging film for making bags which has enjoyed considerable commercial success is described in U.S. Pat. No. 3,741,253 issued on June 26, 1973 to Brax et al, which relates to a multilayer film comprising a first outside layer of an irradiated ethylene-vinyl acetate copolymer, a core layer of vinylidene chloride copolymer, and a second outside layer of an ethylene-vinyl acetate copolymer. In manufacturing this type of heat shrinkable film, a tubular orientation process is utilized wherein a primary tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and with the use of pinch rolls at different speeds in the machine direction. Then the bubble is collapsed, and the film is wound up as flattened, seamless, tubular film to be used later to make bags, e.g. either end-seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-seal bags in which the transverse seals form the bag sides and one edge of the tubing forms the bag bottom.
This type of bag is typically used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to form a hermetic seal, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented, typically about 160.degree. to 205.degree. F., hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. Alternatively, the bag may serve as a liner of a cooking mold. One problem which has been encountered is the failure of the bag seals at the bottom of the bags as the bag is shrunk around a product, the shrink forces tending to pull the seal apart.
Of interest is U.S. Pat. No. 4,469,742 directed to pasteurizable, cook-in shrink film having six layers, the second or shrink layer comprising a cross-linked ethylene homopolymer or copolymer, and having a sealant layer of a nonlipophillic material such as polypropylene or ionomer.
Of general interest concerning the present invention, is the disclosure of U.S. Pat. No. 4,352,702 for "Method of Making a Thermoplastic Receptacle Having Improved High Temperature Seal" issued Oct. 5, 1982 to Bornstein, being directed to a pasteurizable shrink bag from tubular film having a layer of hydrolyzed ethylene-vinyl acetate copolymer and an interior surface layer of a polyolefin which is cross-linkable by ionizing radiation, with the layers being directly melt-joined without an adhesive disposed therebetween and the film being irradiatively cross-linked and oriented. A second irradiation treatment is carried out on receptacles made from the film to condition their heat seals for pasteurizing conditions.
Also of interest is U.S. Pat. No. 4,424,243 issued to Nishimoto et al and disclosing a multilayer film having outer surface layers of linear low density polyethylene or a blend of this linear polyethylene with alphaolefin polymers such as ethylene alkyl acrylate copolymers. Thermoplastics resins used as adhesive layers include ethylene alkyl acrylates having 1 to 6 carbon atoms in the alkyl group.
Of general interest is the disclosure of U.S. Pat. No. 4,064,296 for "Heat Shrinkable Multilayer Film of Hydrolyzed Ethylene-Vinyl Acetate and a Cross-linked Olefin Polymer" issued Dec. 20, 1977 to Bornstein et al, being directed to a coextruded tubular film having a layer of hydrolyzed ethylene-vinyl acetate copolymer layer between two other polymeric layers at least one of which being irradiatively cross-linkable, the film being irradiated and oriented.
Of general interest is the disclosure of U.S. Pat. No. 3,595,740 for "Hydrolyzed Ethylene-Vinyl Acetate Copolymer as Oxygen Barrier Layer" issued July 27, 1971 to Gerow, being directed to oxygen barrier films having an interior barrier layer of a melt extrudable hydrolyzed ethylene-vinyl acetate copolymer and a heat sealing layer of an ethylene polymer or copolymer.
Of general interest is the disclosure of U.S. Pat. No. 4,233,367 for "Coextruded Multilayer Film" issued Nov. 11, 1980 to Ticknor et al, being directed to a coextruded multilayer film having adhesive interlayers of a chemically modified polyolefin, such as Plexar.TM. adhesive, with functional groups selected for their strong affinity for nylon under heat and pressure conditions of coextrusion.
Of general interest is the disclosure of U.S. Pat. No. 4,276,330 for "Trilaminate Film For Forming Sideweld Bags" issued June 30, 1981 to Stanley et al, being directed to sideweld bags made from trilaminate film having a core layer of propylene-ethylene block copolymer between surface layers of propylene-ethylene random copolymer.