WO2002032658A1

WO2002032658A1 - Cook-in film

Info

Publication number: WO2002032658A1
Application number: PCT/AU2001/001301
Authority: WO
Inventors: Rodney L. Wallace; Stamatis Ginosatis
Original assignee: Sporos Sa; Sporos Australia Pty Ltd
Priority date: 2000-10-17
Filing date: 2001-10-17
Publication date: 2002-04-25
Also published as: AUPR082700A0

Abstract

A heat shrinkable multilayer cook-in film for packaging food products including a heat sealable layer and a heat shrink layer each modified by blending with an adhesive resin selected to adhere to said heat sealable layer; a thermoplastic oxygen barrier layer adjacent said heat shrink layer, said heat shrink layer being further modified by blending with an adhesive resin selected to be adherent to said oxygen barrier layer, and an abuse layer adjacent said oxygen barrier layer and comprising a polymer blend including an adhesive resin selected to be adherent to said oxygen barrier layer. Advantageous the films of the invention do not require discrete adhesion layers to achieve acceptable layer adhesion when exposed to pasteurization and cooking temperatures.

Description

COOK-IN FILM

Field of the Invention

This invention relates to a multilayer shrink cook-in film.

This invention has particular, but not exclusive application, to a heat shrinkable multilayer film suitable for packaging of food products and which are subject to cooking and/or pasteurization processes. For illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as for packaging products other than food that may require heating in a packaged form.

Background of the Invention

The term "cook-in" is used to refer to a thermoplastic packaging material in which food products are heated to a cooked and/or pasteurized state. Many food products require pasteurization after packaging to destroy harmful microbes. Cook-in packaging material, such as films, sheets, tubes, bags and the like, must be structurally capable of withstanding exposure to pasteurization and cooking temperature conditions over a suitable period of time while containing a food product. Cook-in films are typically shrinkable to enable the formation a tightly fitting package. Suitable cook-in films should be delamination resistant and have superior high temperature seal strength.

European Patent Application No. 99122609.3 (EP1000973A1 ) describes a film comprising a three component polymer blend of an ethylene octene-1 copolymer and films an ethylene alpha olefin copolymer, and an ethylene vinyl ester copolymer. Though they describe the suitability of the films for cook-in and pasteurization processes, there are no tests to indicate if the integrity of such films remains intact during submersion in hot water over suitable time periods for cook-in and pasteurization processes without delamination.

To enable pasteurization submersion is generally required for at least 3 minutes at about an about 95°C. Cooking times may require submersion for up to one hour at temperature of 95°C. Under such conditions adhesive layers have typically been employed to maintain the integrity of the film. The film described in European Patent Application No, 99122609.3 is unusual in that it does not specifically include an adhesive resin.

Conventional prior art cook-in films, such as described in United States Patent Nos. 4,448,792 and 4,469,742 require discrete adhesive layers and irradiation to avoid delamination.

Summary of the Invention

In one aspect this invention aspect in a thermoplastic heat shrinkable cook-in film including: a heat sealable layer and a heat shrink layer each modified by blending with adhesive resin selected to adhere to said heat sealable layer; a thermoplastic oxygen barrier layer adjacent said heat shrink layer, said heat shrink layer being further modified by blending with an adhesive resin selected to be adherent to said oxygen barrier layer, and an abuse layer adjacent said oxygen barrier layer and including a polymer blend including an adhesive resin selected to be adherent to said oxygen barrier layer.

Polymers or polymer blends that may be used in the heat sealable, heat shrink, barrier and abuse layers of cook-in films are known in the art. Therefore, the adhesive resin or resins may be selected on the basis of compatibility for bonding with the polymers in the various layers of a cook-in film.

There is an unexpected result in taking one or more adhesive resins known to be used in discrete adhesive layers of prior art cook-in films and blending with the polymers used in the various layers of a cook-in film. Not being bound by theory, it is thought that by blending an adhesive resin with the respective polymers of adjacent layers of a cook-in film the adhesive resin may intermingle to form a better bond between adjacent layers then just relying on the affinity of the adhesive resin to a different polymer. The oxygen barrier layer, being the exception, does not include an adhesive resin, as it must maintain a minimal oxygen transmission rate to avoid spoilage of the packaged food product. Therefore, the adhesive resin used in the layers adjacent to the oxygen barrier layer is selected to adhere to the barrier layer. If the adhesive resin similarly promotes adherence to the heat sealable layer and the barrier layer, each of the heat sealable, the heat shrink and abuse layers may be modified with the same adhesive resin. Alternatively, the heat sealable, heat shrink and abuse layers may each be modified with one or more adhesive resins selected to promote adherence to its respective adjacent layer or layers.

By "heat sealable" it is meant that the layer is capable of fusion bonding by conventional indirect heating means which generate sufficient heat on at least one film contact surface for conduction to the contiguous film contact surface and formation of a bond interface between without loss of the film integrity. The bond interface should be sufficiently thermally stable to prevent gas or liquid leakage when during the cooking process of food within the tube when sealed at both ends, i.e., in bag form. Further, the bond interface between contiguous layers must have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube. Heat sealable thermoplastic polymers are recognized by those skilled in the art as being capable of heating sealing to themselves at a variety of time, pressure and temperature conditions. For example, at a given pressure either a relatively high temperature may be applied briefly or a lower temperature may be applied for a longer period of time to obtain similarly suitable seals. One of ordinary skill will recognise that, depending upon such factors as the type of heat sealing equipment used, sealing parameters such as temperature, pressure and time of application may be selected without undue experimentation. The heat sealable layer may include any suitable thermoplastic polymer or polymer blend capable of maintaining a heat seal during cook-in conditions. As the heat sealable layer is also the food contact layer, the choice of the polymers must be satisfy food use requirements.

In a preferred embodiment, the heat sealable layer includes a polypropylene homopolymer or copolymer. The polypropylene copolymer may be propylene ethylene copolymer. Suitably, the ethylene content of propylene ethylene copolymer may be relatively minor in extent and sufficient to promote orientabilϊty. The adhesive resin selected to adhere to the heat sealable layer may be chosen to adhere to a wide range of thermoplastic materials and more suitably to a polypropylene homopolymer or copolymer. The heat sealable and shrink layers include different adhesive resins that adhere to similar polymers. In a preferred embodiment, the heat sealable and shrink layers may be mutually modified with the same adhesive resin. For example, the adhesive resin may be anhydride modified polypropylene homopolymer or copolymer, as it promotes bonding to polypropylene homopolymers or copolymers, as well as to a variety of other materials. The heat sealable blend may be from between about 30% to about 70% of the adhesive resin, and between about 30% to about 100% of polypropylene homopolymer or copolymer. The thickness of the thermoplastic heat seal layer may be between 30-60μ.

As used herein, the term "shrink layer" is intended to refer to the shrink- controlling layer that enables compatible shrinkage of the overall multilayer structure. Suitably, the heat shrink layer includes a blend of at least one adhesive resin and at least one ethylene α-olefin copolymer. The ethylene α- olefin copolymer may be selected from copolymers of ethylene with one or more C₃ to C₁₀ α-olefins, such as propene, butene-1 , pentene-1 , hexene-1 , octene-1 etc. The heat shrink polymer blend may also include ethylene vinyl acetate copolymer.

In one embodiment, the shrink layer includes a blend of very low density polyethylene (VLDPE), ethylene α-olefin plastomer copolymer and one or more adhesive resins. VLDPE is sometime referred to an ultra low density polyethylene (ULDPE). VLDPE generally has a density ranging from between about 0.915 g/cm³ down to about 0.860 g/cm³. The VLDPE may be an ethylene 1 -octene copolymer. Ethylene α-olefin plastomer copolymers generally have a density less than 0.90 g/cm³ and a melting point less than 100°C. The ethylene α-olefin plastomer copolymer may be an ethylene butene copolymer or an ethylene octene copolymer. In another embodiment, the shrink layer includes a blend of polypropylene homopolymer or copolymer, polybutylene such as a butylene homopolymer or copolymer and at least one adhesive resin. The butylene copolymer may be a copolymer of butene-1 and at least one α-olefin. For example, the butylene copolymer may be butene-1 ethylene copolymer. The polypropylene copolymer may be propylene ethylene copolymer.

The heat shrink layer may include an adhesive resin selected to promoted bonding to both the heat sealable and barrier layers. For example, the adhesive resin may be an anhydride-modified polypropylene homopolymer or copolymer that may adhere to a variety of materials, such a polypropylene homopolymers and copolymers and barrier materials such as hydrolyzed ethylene-vinyl acetate copolymer (EVOH). Alternatively, the heat shrink layer may include two or more adhesives resins, wherein at least one of the adhesive resins promotes bonding to the heat sealable layer and another adhesive resin promotes bonding to the barrier layer. For example, the heat shrink layer may include two adhesive resins such as anhydride-modified polypropylene homopolymer or copolymer, which adheres to polypropylene homopolymers and copolymers and acid/acrylate-modified ethylene vinyl acetate polymers, which adheres to barrier material such as vinylidene chloride copolymer.

The heat shrink layer polymer blend may include between about 20% to about 60% of adhesive resin, between about 15% to about 25% of VLDPE and between about 20% to about 40% of plastomer of the total blend. The shrink layer blend may also include other suitable polymers and copolymers such as linear low-density polyethylene (LLDPE), ethylene vinyl acetate, polypropylene, ethylene-propylene copolymer, an ionomer or a combination of one or more thereof. It is to be understood however that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary. The thickness of the shrink layer is selected such that the shrink temperature of the shrink layer controls the shrinkage of the entire multilayer film, when oriented. The thickness of the shrink layer may be between 10-40μ.

As used herein the term "barrier layer" means oxygen barrier layer unless otherwise specified. The barrier layer may serve to inhibit transmission of oxygen and water vapor. The functional requirement of the barrier layer, together with the other layers, may provide an oxygen transmission rate through the entire multilayer cook-in film below about 20cc/1 m²/24hrs/atm. This may avoid spoilage of certain food products, for example meat enclosed in the multilayer film package due to oxygen passage from the environment through the film wall.

The barrier layer may be selected from copolymers of vinylidene chloride, EVOH, certain polyamides such as nylon. In preferred embodiments, the barrier layer may include vinylidene chloride vinyl chloride copolymer (PVDC-VC), vinylidene chloride-methyl acrylate (PVDC-MA) or a blend thereof. The thickness of the barrier layer may be between about 3-9μ. Thinner barrier layers may not perform the intended functions and thicker layers may not appreciably improve performance. The abuse layer is typically termed as such as it should withstand contact with relatively sharp objects and provide abrasion resistance. The abuse layer may be directly adhered to the barrier layer and may prevent degradation of the barrier layer. Thermoplastic polymers that are suitable for use in the abuse layer are recognized by those skilled in the art. The thermoplastic polymers may be selected to enable a broad heat sealing range while preventing burn- through during impulse heat sealing.

In a preferred embodiment, the abuse layer includes a blend of polypropylene homopolymer or copolymer, polybutylene such as a butylene homopolymer or copolymer and at least one adhesive resin. The butylene copolymer may be a copolymer of butene-1 and at least one α-olefin. For example, the butylene copolymer may be butene-1 ethylene copolymer. The polypropylene copolymer may be propylene ethylene copolymer. Alternatively, the abuse layer may include a polymer blend substantially similar to the shrink layer, such as a blend of VLDPE, an ethylene α-olefin plastomer copolymer and adhesive resin. Other thermoplastic materials may be included in the abuse layer blend. For example, the abuse layer blend may further include LLDPE, VLDPE or ULDPE, EVA, ethylene-propylene copolymer, an ionomer or a combination thereof.

The adhesive resin suitably bonds to a wide range of thermoplastic materials and more specifically bonds to the barrier layer thermoplastic polymer or copolymer. For example, the second adhesive resin may be an acid/acrylate- modified ethylene vinyl acetate polymer. The thickness of the abuse layer may be between about 9-25μ. Thinner layers may be less effective in performing the abuse resistance function, while thicker layers may reduce film stretchability. In one embodiment, the heat shrink layer and the abuse layer may have a substantially similar composition.

The heat shrinkable cook-in film may also include an outer optically clear layer adjacent the abuse layer to improve the optical properties of the film. Not being bound by theory, it is believed that in some cases the abuse layer does not shrink as much as the inner heat sealable and heat shrink layers, causing the abuse layer to become wrinkled. This wrinkling may cause the light to refract making the film appear hazy or white. By adding an optically clear layer over the abuse layer, it is believed that the voids created by the wrinkles are filled allowing light to pass cleanly through the film. Since the light is not refracted the film appears clear.

Suitably, the optically clear layer is a thermoplastic polymer blend including an adhesive resin that promotes bonding to the abuse layer. For example, the adhesive resin may adhere to a polypropylene homopolymer or copolymer, such an anhydride-modified polypropylene homopolymer or copolymer. If the cook-in film includes an optically clear layer, the abuse layer may further modified by blending an adhesive resin selected to adhere to the optically clear layer. For example, the abuse layer may further include anhydride-modified polypropylene homopolymer or copolymer. The optically clear layer may include a blend of adhesive resin and a thermoplastic polymer selected from a polypropylene homopolymer or copolymer, VLDPE, LLDPE, EVA or a combination thereof. For example, the optically clear layer may include a blend of a polypropylene homopolymer or copolymer and adhesive resin. Alternatively, the optically clear layer may include a blend of ethylene α-olefin plastomer copolymer with VLDPE or LLDPE and adhesive resin. In another alternative, the outer layer may include a blend of EVA, an ethylene α-olefin plastomer copolymer, a VLDPE or LLDPE and adhesive resin. The optically clear layer may have the same composition as the heat sealable layer. The thickness of the fifth optional optically clear layer may be between about 6-12μ. Thinner layers may result in poorer optical properties while thicker layers may not contribute to an improved film.

It should be understood that the optically clear layer is optional. Suitably the film may not become cloudy upon exposure to time-temperature conditions in order to maintain eye appeal of the packaged product. However if some cases, such as when the film is coloured or printed, then clear optics of the film of the invention may not be essential.

The four-layer cook-in film of the invention may have a total thickness of from about 40-90μ, more suitably from about 50-70μ. Lower thickness may reduce the effectiveness of at least one of the four layers while higher thickness may reduce the film flexibility and may not appreciably improve its performance.

In one embodiment, the heat sealable and heat shrink layers may have the same composition as the abuse and optically clear layers, respectively. In this embodiment, the film has a layer structure of A/B/C/B/A where the heat sealable and heat shrink layers can functionally act as abuse and optically clear layer respectively and vise versa. This type of layer structural arrangement may be suitable for form and fill application as described in more detail below.

While embodiments of the invention is specifically described in terms of the above described four or five layers, it should be understood that one or more additional layers of thermoplastic polymers may be directly adhered to the:

1 ) outside of the outer abuse layer or optically clear layer; or

2) between the barrier layer and shrink layer and/or abuse layer. This additional layer may, for example, be EVA, LLDPE, VLDPE, polypropylene, EVOH, polyurethane, acrylonitrile nylon, ionomer, or blends thereof. In general, various conventional additives such as slip agents, processing aids and pigments can be incorporated into any layer of the film in accordance with conventional practice.

Advantageous the films of the invention do not require separate or discrete adhesion layers and irradiation to achieve acceptable layer adhesion. Though a film in accordance with the invention does not require irradiation, it is to be understood that the film could be produced with one or more layers cross- linked by irradiation to further broaden and/or increase the heat sealing range.

The multilayer film of this invention can be produced by known techniques such as by co-extruding the multiple layers into a primary tube, followed by biaxially stretching the tube by known techniques to form a heat shrinkable film. Alternatively, the film may be a slot cast co-extruded multilayer film, which is subsequently biaxially stretched. In a still further alternative, the film may made by coating lamination, followed by biaxially stretching. The resulting film may shrink from about 20% to about 50% in at least one of machine and transverse directions by measuring unrestrained shrink of the film at 90°C for about five seconds, or equivalent shrinkage thereof.

The film of the present invention may be in the form of a sheet or a tubular form. The sheet may be formed into a tube around a mandrel and the inner heat sealable layer may be sealed to the outer abuse or optically clear layer. The sheet may be useful for form and film applications where the sealed film is filled or stuffed with food products such as meat or cheese and the open ends of the tube are either clipped or heat sealed. In the tubular form, bags may be formed by sealing the inner heat sealable layers of the tube together at one end. Once food is inserted into the bag, the bag mouth may be sealed. Heat sealing procedures, such hot bar sealing or impulse sealing, are known in the art.

In order that this invention may be more readily understood and put into practical effect, reference will now be made to the following examples which illustrate preferred embodiments of the invention.

Definitions

As used herein, the term "film" is used in a generic sense to include a plastic web, regardless of whether it is a film or a sheet. Preferably, films of use in the present invention have a thickness of 150μ or less, more preferably of from about 20 to about 120μ, and even more preferably of from about 40 to about 100μ. As used herein, the term "polymer" refers to the product of a polymerization reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc.

As used herein, the term "homopolymer" is used with reference to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of repeating unit.

As used herein, the term "copolymer" refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the term "copolymer" includes the copolymehsation reaction product of ethylene and an α-olefin, such as 1 -hexene. However, the term "copolymer" is also inclusive of, for example, the copolymerisation of a mixture of ethylene, propylene, 1 -hexene, and 1 -octene.

As used herein, the term "polyethylene" will be used (unless indicated otherwise) to refer to ethylene homopolymers as well as copolymers of ethylene with α-olefins and the term will be used without regard to the presence or absence of substituent branch groups.

As used herein, the phrases "ethylene alpha-olefin copolymer" or "ethylene α-olefin copolymer" refer to such heterogeneous materials as linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene catalyzed or other single site catalyzed polymers. These materials generally include copolymers of ethylene with one or more comonomers selected from C₃ to C₁₀ α-olefins such as propene, butene-1 , hexene-1 , octene- 1 , etc in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. Suitable ethylene α-olefin copolymers are EXCEED™ and EXACT™ materials supplied by Exxon, TAFMER™ materials supplied by Mitsui Petrochemical corporation and AFFINITY™, ATTANE™, ELITE™, ENGAGE™ and DOWLEX™ materials supplied by Dow Chemical Company. As used herein, the term "plastomer " as used herein refers generally to a class of ethylene based copolymers with density of less than about 0.900 g/cm³ down to about 0.865 g/cm³ at a molecular weight MW greater than about 20,000 (about 200 Ml and lower). Plastomers have an ethylene crystallinity between plastics (i.e. linear low density and very low density polyethylenes) and ethylene/alpha-olefin elastomers. Examples of plastomers are sold under the trade names EXACT™, TAFMER™ and AFFINITY™ as mentioned above.

As used herein, the phrase "heterogeneous polymer" refers to polymerization reaction products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, i.e., typical polymers prepared, for example, using conventional Ziegler-Natta catalysts. Heterogeneous polymers are useful in various layers of the film used in the present invention. Although there are a few exceptions (such as TAFMER.TM. linear homogeneous ethylene/alpha-olefin copolymers produced by Mitsui Petrochemical Corporation, using Ziegler-Natta catalysts), heterogeneous polymers typically contain a relatively wide variety of chain lengths and comonomer percentages.

As used herein, the phrase "homogeneous polymer" refers to polymerization reaction products of relatively narrow molecular weight distribution and relatively narrow composition distribution. Homogeneous polymers are structurally different from heterogeneous polymers, in that homogeneous polymers exhibit a relatively even sequencing of comonomers within a chain, a mirroring of sequence distribution in all chains, and a similarity of length of all chains, i.e., a narrower molecular weight distribution. Furthermore, homogeneous polymers are typically prepared using metallocene, or other single-site type catalysts, rather than using Ziegler-Natta catalysts.

As used herein, the term "polypropylene" refers to any polymer including propylene polymerization units, regardless of whether the polymer is a homopolymer or a copolymer, and further includes blends of such homopolymers and copolymers.

As used herein, the term "polybutylene" refers to any polymer comprising butene-1 polymerization units, regardless of whether the polymer is a homopolymer or a copolymer, and further includes blends of such homopolymers and copolymers. These materials generally include copolymers of butene-1 with or without α-olefins monomers, such as butene-1 -ethylene, butene-1 -propylene, and butene-1 -α-olefins having from 5 to 8 carbon atoms.

As used herein, the terms "polyvinylidene chloride" or "PVDC" refers to a vinylidene chloride copolymer wherein a major amount of the copolymer comprises vinylidene chloride and a minor amount of the copolymer comprises one or more unsaturated monomers copolymerizable therewith, such as vinyl chloride, alkyl acrylates (e.g. methyl acrylate), acrylonitrile, methyl metacrylate, acrylate esters, vinyl esters or to a blend thereof in different proportions. Preferred barrier materials are vinylidene chloride vinyl chloride copolymer (PVDC-VC), vinylidene chloride-methyl acrylate (PVDC-MA) or blends thereof. As used herein, the terms "hydrolyzed ethylene-vinyl acetate copolymer",

"ethylene vinyl alcohol copolymer" or "EVOH" refers to saponified products of ethylene-vinyl ester copolymers, generally of ethylene-vinyl acetate copolymers, wherein the ethylene content is typically comprised between 20 and 60% by mole and the degree of saponification is generally higher than 85%, preferably higher than 95%.

Detailed Description of the Invention

In accordance with one preferred embodiment of this invention, there is provided a heat shrinkable cook-in film including: an inner heat sealable layer including a blend of polypropylene homopolymer or copolymer and a first adhesive resin; a heat shrink layer adjacent said heat sealable layer and including a blend of a VLDPE, an ethylene α-olefin plastomer copolymer, said first adhesive resin and a second adhesive resin, an abuse layer including a blend of a polypropylene homopolymer or copolymer, a polybutylene homopolymer or copolymer and said second adhesive, and a thermoplastic polymer oxygen barrier layer between said heat shrink layer and said abuse layer, wherein said first adhesive resin promotes bonding between the heat sealable layer and the heat shrink layer and wherein said second adhesive resin promotes bonding of the heat shrink and abuse layers to the barrier layer. Preferably, the oxygen barrier layer is PVDC-VA, PVDC-MA or a blend thereof.

Suitably, this embodiment further includes an optically clear layer including a polymer blend of polypropylene homopolymer or copolymer blended with the first adhesive. The first adhesive may be an anhydride-modified polypropylene homopolymer or copolymer and the second adhesive may be an acid/acrylate-modified ethylene vinyl acetate polymer.

In a second preferred embodiment, there is provided a heat shrinkable cook-in film including an inner heat sealable layer and outer optically clear layer each including a blend of polypropylene homopolymer or copolymer and a first adhesive resin; a heat shrink layer and an abuse layer adjacent said sealable and optically clear layer respectively, said shrink and abuse layers each including a blend of a polypropylene homopolymer or copolymer, a polybutylene homopolymer or copolymer, said first adhesive resin and a second adhesive resin, and a thermoplastic polymer oxygen barrier layer between said shrink and abuse layers, wherein said first adhesive resin promotes bonding between the heat sealable layer and the heat shrink layer and between the optically clear layer and the abuse layer, and wherein the second adhesive resin promotes bonding of the heat shrink and abuse layers to the barrier layer

The multilayer film of this invention can be produced by known techniques such as by coextruding the multiple layers into a primary tube, followed by biaxially stretching the tube by known techniques to form a heat shrinkable film The "double bubble" technique disclosed in United States Patent No 3,456,044 can be used to produce the film of this invention Alternatively, the film may be a slot cast coextruded multilayer film, which is subsequently biaxially stretched In a further alternative, the multilayer film may be formed by coating lamination Coating lamination procedures are described in US Patent 3,741 ,253 In other embodiments, the film may be formed as a sheet For example, a biaxially stretched coextruded multilayer tube may be split at the sides to form a two sheet

If the film of this invention is produced in a tubular stock, bags can be produced by hot bar sealing one end of a length of the tubular film or at any number of longitudinally spaced positions across the tube width, and then cutting the tube or splitting one edge to form the bag mouth If the film of this invention is made in the form of flat sheets, bags can be formed by hot bar sealing three edges of two superimposed sheets of film When carrying out the hot bar sealing operations, the surfaces that are heat sealed to each other to form seams are the inner heat sealable layers of the multilayer films of the invention The inner heat sealable layer forms the inside portion of the bag while the outer abuse or optional optically clear layer forms the outside portion of the bag The mouth of the bag may be sealed by impulse heat sealing after filling Once food is inserted into the bag, the package is evacuated and the bag mouth sealed, generally by impulse heat sealing techniques The filled heat- shπnkable bags can then be subjected to cooking and/or pasteurisation processes When tubular film is heat sealed the two inner surfaces are melted together to form the heat seal. For this application it may be desirable to have the inner layer thicker than the outer layers. When the film is in the form of a sheet, a tube is formed around a mandrel such as circular shoe or horn by sealing the outer abuse or optional optically clear layer is sealed to the inner heat sealable layer. In this application it is desirable to make the inner and outer layers nearly the same thickness and formulation. In fact, the film structure of the sheet film may be balanced to the extent that the inside layer may function as the outer layer or vice versa. Sheet film according to the invention is suitable for applications known as form and fill. Once the sheet film is sealed meat or cheese is stuffed into the formed tube. The open ends of the formed tube are either clipped or heat sealed.

Certain physical properties of bags were measured by either of the test procedures discussed below. Shrinkage values were obtained by measuring unrestrained shrink of the stretched film at 90°C for five seconds. Four test specimens were cut from a given sample of the oriented film to be tested. The specimens were cut to 10cm. in the machine direction by 10cm in the transverse direction. Each specimen was completely immersed for 5 seconds in a 90°C water bath. The distance between the ends of the shrunken specimen was measured. The difference in the measured distance for the shrunken specimen and the original 10cm was multiplied by ten to obtain the percent of shrinkage for the specimen. The shrinkage for the four specimens was averaged for the MD shrinkage values of the given film sample, and the shrinkage for the four specimens was averaged for the TD shrinkage value.

The impulse sealing range test is run to determine and compare the acceptable impulse sealing range for the test films. An evacuator impulse sealer used by the meat industry to evacuate and seal products placed into the bags was used. Boss Vacuum Packaging Machines manufactures this evacuator sealer. This evacuator sealer is equipped with impulse sealing ribbons covered by a Teflon cloth. A constant voltage is applied to the sealing ribbons and is not adjustable. (This is typical for evacuator sealers used commercially by the industry.) The time that the voltage is applied to the sealing ribbons is adjustable to obtain a leak proof strong seal The time is adjustable from 0 to 4 0 seconds or from 0 to 10 on the indicator on the machine

Film samples are placed in the evacuator sealer and the sealing time is adjusted to determine the minimum time that is required to obtain a strong seal and the maximum time that a good seal can be obtained without burn through The resistance of the film to delaminate is first determined by placing a 10cm square of the film in 90°C water for 5sec and a second piece in 90°C water for 5 minutes The results of these tests are summarized in Table A

The test films that exhibited acceptable resistance to delamination were selected to be tested in 95°C water for one hour to simulate actual cook-in conditions For this test water was placed were placed into bags, and the bags heat sealed on commercial equipment The water filled bags were submerged then in 95°C water for one hour The water filled bags were inspected for delamination, seal failures, or other flaws that would make the bags not usable Hams were placed in some selected bags, evacuated, and heat sealed on commercial equipment After the bags containing the hams were submerged in 95°C water for 1 hour, they were inspected for delamination and other flaws Comparing these results with bags filled with water indicated that submersing water filled bags in 95°C water is a more severe test than when actual hams are used (Bags that showed signs of delamination when filled with water had no signs of delamination when actual hams were used )

The results are summarized in table A Table A describes the various test film properties Table B defines the film structure and formulations of the test films Table C provides the specifications of the various polymers used in the test films

Table A

* "Yes" indicates the film showed signs of delamination. The numbers indicate the degree of delamination. 1 indicates very little delamination. 5 indicates a high degree of delamination.

** "Clear" indicates the film was clear after shrinking at 90°C. The numbers indicate the degree of clarity. 1 indicates the film was very clear. 5 indicates the film was not so clear.

*** Indicates degree of delamination after filling a bag with water and placing it in a water bath at 95°C for 1 hour. 1 indicates very little delamination. 5 indicates a high degree of delamination. OK means there was no visible sign of delamination. Table B

C'ont Table B

Table C

Test film 8288 is an example of the films according to the invention. It has a four-layer structure that has not been irradiated, but exhibits excellent performance in the simulated cook-in tests. The optical properties of this film are not as good as that of test 8287, which includes a fifth outer optically clear layer adjacent to the abuse layer and also exhibited excellent performance in the simulated cooking test.

Test film 1565 is a 4-layer structure that has nearly as good a performance as test films 8288 and 8287. Close examination of the mode of failure of test film 1565 indicated the failure occurred at the interface of the PVDC barrier layer with the sealing and abuse layers. This problem was resolved by the addition of an adhesive resin that promotes adhesion to PVDC, such as an acid/acrylate-modified ethylene vinyl acetate polymer.

Test film 8291 is an example of a suitable film structure according to the invention that may be used in sheet form for form and fill applications. The heat sealable layer has the same polymer composition as optically clear layer and the heat shrink layer has the same polymer composition as the abuse layer to provide a balanced film where the inner layers are functionally the same as the outer layers or vice versa.

It will of course be realised that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A thermoplastic heat shrinkable cook-in film including: a heat sealable layer and a heat shrink layer each modified by blending with an adhesive resin selected to adhere to said heat sealable layer; a thermoplastic oxygen barrier layer adjacent said heat shrink layer, said heat shrink layer being further modified by blending with an adhesive resin selected to be adherent to said oxygen barrier layer, and an abuse layer adjacent said oxygen barrier layer and comprising a polymer blend including an adhesive resin selected to be adherent to said oxygen barrier layer.

2. A cook-in film according to claim 1 , wherein said heat sealable layer includes a polypropylene homopolymer or copolymer.

3. A cook-in film according to claim 2, wherein said polypropylene copolymer is a propylene ethylene copolymer.

4. A cook-in film according to any one of claims 1 to 3, wherein said heat shrink layer is a thermoplastic polymer blend including at least one ethylene α- olefin copolymer.

5. A cook-in film according to claim 4, wherein said thermoplastic polymer blend includes very low density polyethylene and ethylene α-olefin plastomer copolymer.

6. A cook-in film according to any one of claims 1 to 5, where said heat sealable layer and said heat shrink layer are mutually modified with the same adhesive resin.

7. A cook-in film according to claim 6, wherein said adhesive resin is anhydride modified polypropylene homopolymer or copolymer.

8. A cook-in film according to claim 1 , wherein the thermoplastic oxygen barrier layer is selected from the group of ethylene vinyl alcohol copolymer, vinylidene chloride vinyl chloride copolymer and vinylidene chloride-methyl acrylate.

9. A cook-in film according to claim 1 , wherein said abuse layer includes a blend of polypropylene homopolymer or copolymer and polybutylene.

10. A cook-in film according to claim 1 or claim 9, wherein said abuse layer includes acid/acrylate-modified ethylene vinyl acetate polymer.

11 . A cook-in according to any one of claims 1 , 4 to 7, 9 and 10, wherein said heat shrink layer includes acid/acrylate-modified ethylene vinyl acetate polymer.

12. A cook-in film according to any one of claims 1 to 1 1 , wherein said film includes an outer optically clear layer adjacent said abuse layer.

13. A cook-in film according to claim 12, wherein said optically clear layer includes a polypropylene homopolymer or copolymer.

14. A cook-in film according to claim 12 or claim 13, wherein said optically clear layer includes anhydride modified polypropylene homopolymer or copolymer.

15. A cook-in film according to claim 1 , wherein said heat sealable layer is a blend of a polypropylene homopolymer or copolymer and anhydride modified polypropylene homopolymer or copolymer.

16. A cook-in film according to claim 1 , wherein said heat shrink layer is a blend of very low density polyethylene, ethylene α-olefin plastomer copolymer, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate- modified ethylene vinyl acetate polymer.

17. A cook-in film according to claim 1 , wherein said abuse layer is a blend of polypropylene homopolymer or copolymer, polybutylene, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate-modified ethylene vinyl acetate polymer.

18. A cook-in film according to claim 1 , wherein said heat shrink layer and said abuse layer have substantially the same composition.

19. A cook-in film according to claim 18, wherein said composition is a blend of polypropylene homopolymer or copolymer, polybutylene, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate-modified ethylene vinyl acetate polymer.

20. A cook-in film according to any one of claims 1 to 19, wherein said film is formed by coextruding said layers into a tube followed by biaxially stretching the tube.

21. A cook-in film according to any one of claims 1 to 19, wherein said film is a sheet.

22. A cook-in film according to any one of claims 1 to 21 , wherein said film is formed into a bag.

23. A thermoplastic heat shrinkable cook-in film according to claim 12, wherein: the heat sealable layer is a blend of polypropylene homopolymer or copolymer and anhydride modified polypropylene homopolymer or copolymer; the heat shrink layer is a blend of very low density polyethylene, ethylene α-olefin plastomer copolymer, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate-modified ethylene vinyl acetate polymer; the thermoplastic oxygen barrier layer is vinylidene chloride copolymer; the abuse layer is a blend of polypropylene homopolymer or copolymer, polybutylene, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate-modified ethylene vinyl acetate polymer, and the optically clear layer is a blend of anhydride modified polypropylene homopolymer or copolymer and anhydride modified polypropylene homopolymer or copolymer.

24. A thermoplastic heat shrinkable cook-in film according to claim 12, wherein: the heat sealable layer and the optically clear layer are each a blend of a polypropylene homopolymer or copolymer and anhydride modified polypropylene homopolymer or copolymer; the thermoplastic oxygen barrier layer is vinylidene chloride copolymer, and the heat shrinkable layer and the abuse layer are each a blend of polypropylene homopolymer or copolymer, polybutylene, anhydride modified polypropylene homopolymer or copolymer and acid/acrylate-modified ethylene vinyl acetate polymer.