Differentially cross-linked multilayer film

- Z018807 DIFFERE~TIAL~Y CROSS-~INKED MU~TI~ER FILM Field of the Invention This invention relates to an irradiated multilayer film which can be formed into packaging bags for esample for foods such as primal and sub-primal meat cuts, and poultry. In particular, this invention relates to a tubular article formed of biasially oriented irradiated multilayer film wherein at least one of the layers is cross-linked to a different extent than the other layers.

~k5~ound o ~he Invention Primal meat cuts are generally smaller than a side of beef, but larger than the ultimate cut sold to the retail consumer. Primal cuts are prepared at the slaughter house and shipped to retail meat stores or restaurants where they are butchered into smaller cuts of meat commonly referred to as sub-primal meat cuts. Sub-primal cuts, however, may also be prepared at the slaughter house.

D-20029 2~18807 Primal and sub-primal cuts are packaged to minimize air (i.e. oxygen) esposure and prevent meat spoilage and discoloration during shipping and handling. One common way to package primal and sub-primal cuts and protect them from the adverse effects of air is to shrink package the cuts in a film having good osygen barrier properties.

For esample, a multilayer film having a barrier layer containing polyvinylidene chloride ~PVDC) is a well known packaging material. PVDC is known by those skilled in the art to have escellent osygen barrier properties. The other layers of the multilayer film function to protect the PVDC layer and provide the reguisite low temperature and abrasion resistance properties which the PVDC layer lacks.

There are other types of biasially oriented heat shrinkable multilayer films which do not reguire osygen barrier properties, for esample films to be used for shrink wrapping poultry.

Shrinkage properties may be produced in a film by biasially stretching the film in the machine and transverse direction. The resulting ilm will heat shrink within a specified range of percentages such as from about 20 to about 50 percent at about 90C ~ Multilayer heat shrinkable film may also be formed into packaging bags. ~ags are generally made by transversely sealing a tubular stock of multilayer film and cutting or splitting the tube, or by superimposing flat sheets of multilayer film and sealing on three sides, or by end folding flat D-20029 .,f ~" 3807 -- 3 sheets and sealing the two sides. A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal in which the adjacent thermoplastic layers are sealed together by application of heat and pressure across the area to be sealed, using contact with opposing bars of which at least one is heated to cause the adjacent layers to fusion bond. For esample, bags manufactured from a tube stock generally require one hot bar seal transverse to the tube. This seal i5 also referred to as a bottom seal. Once the bottom seal is applied, the tube stock is transversely cut to form the mouth of the bag. The strength of seals may be measured by determining the time for a seal to fail when under certain conditions the seal is immersed in hot water at 95C i.e., the hot water seal strength (~HWSS~) may be measured by a test such as that described as the ~restrained shrinkage-seal strength test~ in Funderburk et al U. S.

Patent No.

3,900,6~5.

Once meat or poultry is inserted into the bag, the package i8 evacuated and the bag mouth must be sealed. At one time, the 8tandard method for sealing a bag was to asten a clip around the mouth of the bag. More recently, impulse heat sealing techniques have been employed to seal the bag mouth. In general, a bag mouth is impulse sealed by application of heat and pressure using opposing bars at least one of which has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name ~impulse~) to cause the adjacent film layers to fusion bond.

D-20029 2018807 -- 4 - Following the impulse of heat the bars are cooled while continuing to hold the bag inner surfaces together to achie~e adequate sealing strength.

One problem with impulse heat sealing is that the film in the seal area often becomes e~truded during sealing. This elongation of the product results in thinning of the film and in an estreme situation severing of the thinned film. The latter is known to those skilled in the art as burn-through. One solution to this "burn-through~ problem is to irradiate the film prior to manufacture of the bag.

Irradiation of a multilayer film causes the various irradiated layers in the film to cross-link. Under controlled conditions, cross-linking by irradiation not only provides a higher temperature impulse heat sealing range, but also enhances the pu'ncture resistance of the film.

Unfortunately, cross-linked thermoplastic films are not easy to melt, making it difficult for food packagers to achieve strong seals, parSicularly by impul~e sealing the bag mouths after filling with meat or poultry. All of the bag seals ~including both the sealed bottom as for esample made by the bag manufacturers with a hot bar and the impulse-sealed bag mouth by the food processor) must m,aintain their integrity when the food-containing package is immersed in hot water to shrink the film against the packaged food. Thus, there is a need for a multilayer film which can be made into a bag having both strong bottom or side seals effected by hot bar sealing or even impulse sealing, and strong D-20029 20lssa7 mouth s~als effected by impulse heat sealing. The film outer layer, which typically contacts the sealing apparatus, should provide high burn-through resistance during sealing and the inner layer should be capable of forming seals having high hot water seal strength.

It is known that heat shrinkable bags for food packaging may be fabricated from multilayer film having a difference in degree of cross-linking o the respective layer9 of the film. This diference by cross-linking can be obtained by irradiation of individual layers followed by lamination or estrusion coating. These technigues, however, employ multiple irradiation and/or multiple estrusion or lamination steps.

Canadian Patent 1,125,229 discloses a film structure having a heat sealable inner layer and an outer layer wherein the outer layer is cross-linked to a larger estent than the heat sealable layer.

The differential cross-linking is achieved by adding a cross-linking enhancer compound to the outer layer, forming the structure, and then irradiating.

The irradiation enhancer allows the irradiation dosage to be lowered to a point where the heat sealable inner layor i9 not ad~er~ely afectèd ~n its heat sealing characteristics by the radiation.

However, a major problem with this film structure is that by lowering the irradiation dosage, the other beneficial effects which irradiation provides, such a~ puncture resiætance, are surrendered.

Sun U. S.

Patent No. 4,724,176 describes a heat shrinkable bag with an unirradiated inner D-20029 .. .. . ..

807 6 - layer, an osygen barrier core layer, and an irradiated outer layer. The film used to fabricate this bag is prepared by coestrusion and then controlled irradiation to only cross-link the outer layer. This approach requires precise control of the cross-linking and does not permit enhancement of inner layer puncture strength by cross-linking.

It is also known that manufacturers of thermoplastic re~ins freguently include limited guantities of antiosidants to improve thermal stability of the ~esin during storage and processing. These antiosidants tend to retard cross-linking in film estruded from the resin.

The discovery of a differentially cross-linked multilayer film which can be produced by esposure to only one dosage of irradiation and yet still retains the other beneficial effects of irradiation would be widely accepted by those skilled in the art and meet with substantial commercial success.

It is therefore an ob~ect of the present invention to provide a differentially cross-linked multilayer film having high hot water seal strength, high burn-through resistance, a broad impul8e sealing range and enhanced puncture resistance. It is a further ob~ect of the present invention to provide a differentially cross-linked multilayer film having these improvements after esposing the film to only one irradiation dosage. It is still a further object of the present invention to provide a multilayer film structure which can be more easily and efficiently formulated into a packaging bag. It D-20029 .,. . i ,~ j .

2018807 7 - is yet another object of the present invention to provide a meat and poultry packaging bag having both improved bottom and edge seals formed by e.g.hot bar sealing and after food is packaged therein, and improved mouth seals formed by impulse heat sealing. It is still yet another object of the present invention to provide a method for manufacturing a multilayer film having the differential crose-linking characteristics described above.

It is to be understood that these objectives as well as others which are apparent in view of the specification are not to be considered a limitation of the present in~ention, the scope of which is defined by the appended claims.

In accordance with one aspect of the invention, a tubular article is provided which is formed of biasially oriented, differentially cross-linked, heat shrinkable multilayer film irradiated to at least about 2 megarad (MR). This ilm compri8es a thermoplastic inner layer and a thermoplastic outer layer, the irradiated inner layer containing an antiosidant in sufficient guantity to ad~ust its melt flow inde~ to at least about 0.5. The irradiated outer layer has a melt flow indes of less than about 0.7, and the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5. The inner and outer irradiated layers may be adhered together but in a preferred embodiment the inner and outer layers are separated by an osygen barrier layer.

D-20029 _ 2018807 -- 8 - At least the irradiated inner layer must be heat sealable to itself, i.e., be 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 therebetween without loss of the film integrity. Also, the bond interface must be sufficiently thermally stable to pre~ent gas or liquid leakage therethrough when e~po8ed to abo~e or below ambient temperatures during processing of food within the tube when sealed at both ends, i.e., in bag form. Finally, the bond interface between contiguous inner layers must have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube.

As used herein, ~antiosidant~ means an additive to the first layer resin which retards osidation, i.e., cross-linking, of that layer on irradiation.

Another aspect of the invention relates to a method of manufacturing a tubular article formed of biasially orientèd differentially cross-linked, heat shrinkable multilayer film. In this method a tube of multilayer blown film is formed comprising a thermoplastic inner layer and a thermoplastic outer layer with the inner layer containing an antio~idant. The multilayer film is biasially oriented to make same heat shrinkable. The bia2ially oriented heat shrinkable multilayer film is then irradiated at a dosage of at least about 2 D-20029 - - 9 2alss~7 - .

MR to simultaneously adjust the inner layer melt flow indes to at least about 0.5 and adjust the outer layer melt flow indes to less than about 0.7 such that the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5.

A still further aspect of the invention relates to a food storage bag formed of biasially oriented, differentially cross-linked and heat shrinkable multilayer film lrradiated to at least about 2 MR. Thi8 film comprisa8 a thermoplastic inner layer and a thermoplastic outer layer, the irradiated inner layer containing an antiosidant in sufficient quantity to adjust its melt flow indes to greater than about 0.5. The irradiated outer layer has a melt flow indes of less than about 0.7 such that the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5. One end of the bag is heat sealed by adhesion between contiguous inner layer surfaces in a direction transverse to the oppo8itely located side walls of the bag. The mouth end of the bag is impulse heat sealable by fu~ion bonding between contiguous inner layer surfaces after filling the bag with food.

Another aspect of this invention is as follows:

- 9a 2alssa7 A tubular article formed of biasially oriented, differentially cross-linked heat shrinkable multilayer film irradiated to at least about 2 MR comprising a thermoplastic inner layer, a thermoplastic outer layer and an osygen barrier layer between said inner and outer layers and adherëd thereto on oppo8ite sides, the irradiated inn0r layer containing an antiosldant in suficient quantity to adjust it~ melt 1OW indes to at lea~t about 0.7 and the irradiated outer layer ha~ing a melt f low indes of less than about 0.7, such that the ratio of said melt flow indices of the inner layer to the outer layer is at least about 1.5.

Detailed Description of Preferred Embodiments Upon esposure to irradiation, polymers undergo a seguence o chemical reactions resulting in radical ormation and peroside osidation.

Irradiation initiJtos radical ormation and in the presence of osygen the radical and the osygen units produce a perosy radical. The perosy radical causes auto-osidation by abstracting a neighboring labile D-20029 Z018807 -- 10 - hydrogen to form a hydropero~ide and another free radical. These reactions propagate until inert products are formed through free radical combinations, terminating the auto-osidation. This termination is commonly referred to as cross-linking and results in an increase in viscosity and toughness of the polymer. The estent of cross-linking can be measured by a change in melt low inde~ ~MFI).

Melt 1OW inda~ is an indication of the rate of estrusion of a molten thermoplastic resin through a die of a specified length and diameter under prescribed conditions of temperature, load and piston position in the barrel as the time measurement is being made.

Under controlled conditions, polymeric multilayer films can be irradiated to produce a cross-linked product having greater puncture resistance characteristics and other enhancements known to those skilled in the art. However, esposure to irradiation also causes a reduction of heat sealability in other layers at conventional temperature8, pre9sures and timas using typical heat sealing equipment thareby resulting in poor bottom and edge seals in bags made from the film. However, in the practice of this invention it has been found that the addition of an antiosidant in sufficient quantity to the heat sealing first (inner) layer inhibits cross-linking to the e~tent that a relatively high melt flow inde~ of at least about 0.5 is obtained. At the same time the second (outer) layer of the multilayer film is selected so D-20029 2~188~7 as to have a relatively low level of antiosidant and optionally may contain a cross-linking enhancer, thereby providing a relatively low melt flow index of less than about 0.7 in this layer. Further, these constituent additives or reductions are such that the multilayer film, which only requires a single irradiation treatment, provides a combination of improved hot water puncture resistance and high burn-through ro~istance. ~a8ed on information avàilable to the inventors, this particular beneficial combination has not boen achieved in prior art biasially oriented heat shrinkable multilayer films and represents a substantial improvement in this art.

As u~ed in accordance with the present invention, the term ~antiosidant~ is a chemical composition which, when blended with a polymer, produces a material which cross-links to a lesser estent at a given radiation dosage than does the same polymer without the cross-linking inhibitor.

Thus, any chemical composition which, when blended with a polymer, produces upon irradiation a cros8-linked product having a higher melt 10w inde~ than the same polymer at the same irradiation dosage level without the chemical composition is a cross-linking inhibitor.

According to the present invention, preferred cross-linking inhibitors include those chemical compositions known as free radical scavengers. Free radical scavengers inhibit or retard osidation and the degradative effects on a polymer system which occur as a result of osidation D-20029 .~1&

Z018807 - 12 upon irradiation. These compositions inhibit osidation by their reaction with chain propagating radicals resulting in the transfer of hydrogen to the radical. Free radical scavengers are also known as primary antioxidants.

These primary or chain terminating antiosidants generally have a reactive NH or OH functional group as secondary aryl amines or hindered phenols. Specific antiosidant~ contemplated by the pre5ent invention include but are not limited to 2,6 ditert butyl-4-ethyl phenol ~BHE~ and butylated hydrosytoluene (BHT). Another suitable antiosidant agent is tetrakis ~methylene 3-~3', 5'-di-t-butyl-4'- hydrosyphenyl) propionate] methane. This material is believed to be a symmetrical molecule which includes four sterically hindered phenolic hydrosyl group~ and has a molecular weight of about 1178. This material is available from by Ciba-Geigy under the trade designation Irganos~ 1010.

The antiosidants contemplated by the present invention are not believed to completely inhibit osidation of the polymer, but when employed in the amount contemplated, they suficiently retard the rate of auto-osidation throughout the usèful life of the polymer material.

The guantity of antiosidant required to achieve melt flow indes values of at least 0.5 in the heat sealing layer of this multilayer film is believed to be substantially higher than the quantity added by resin manufacturers for stability. For esample, a currently available EVA D-20029 Z018807 - 13 resin with 12 wt% VA contains BHT antio~idant and has a MFI of about 0.43 when irradiated to about 3 MR.

In accordance with one embodiment of the present invention, an antiosidant cross-linking inhibitor is added to a heat sealable inner thermoplastic layer of a multilayer film in the form of a tubular article. The antio~idant is added in sufficient guantity to ad~u8t the melt flow index of the layer in the irradiated state to at least about 0.5. Also the melt flow inde~ of the outer thermoplastic layer in the irradiated state is adjusted to below about 0.7. These adjustments are such that the ratio of the melt flow indices of the first outer layer to the second outer layer is at least about 1.5. Suitably, the inhibitor is added in an amount between about 500 to about 2000 parts per million (PPM), preferably about 800 to about 1600 PPM by weight of the layer based on antiosidant equivalency to 8HT ~BHT Eguivalency~). The antio~idant may be added in any manner known in the art, including blending with the polymer in the solid form or as a hot melt. As u~ed herein BHT Equivalency i9 tha amount of an alternative antio~idant required to manufacture a multilayer film of the invention with an equivalent melt flow ratio to an otherwise identical mu-ltilayer film with a specific amount of BHT in the heat sealable inner layer of the film, and produced by the identical method. By way of e~ample, assume 1000 ppm 8HT is included in an EVA inner layer prepared from a resin having 10% vinyl acetate content and the melt flow D-20029 - 14 2018807 lndex of this layer in a particular film irradiated to 4 MR is 1.00. If only 7SO ppm of a particular alternative antiosidant is required in the inner layer of an otherwise identical film to provide an inner layer melt flow index of 1. 00, the BHT Equivalency of this film i5 750 ppm.

Decreasing the amount of antio~idant in the heat sealing inner layer results in greater cros8-linking while increasing antio~idant concentration tends to reduce the impulse voltage sealing range and impair the improved puncture resistance from cross-linking. Optimum amounts of added antiosidant will of course vary, depending upon such parameters as the particular polymers used to form the inner layer, the amount of antio~idant already present in the as-manufactured polymeric resin, and anticipated sealing conditions to which the film will be subjected. Suitable amounts of antiozidant to be added to the inner layer can be easily determined by one skilled in the art.

The heat sealable inner layer of the multilayer film of the present invention may be of any material conventionally used for its heat sealing capabllity. Conventional heat seal~ng materials comprise thermoplastic polymèrs or polymer blends. Thus, according to one embodiment, the heat sealable layer may comprise any thermoplastic polymer or polymer blend which is cross-linkable by irradiation and may be inhibited in its cross-linking ability by the presence of antiozidants contemplated.

D-20029 c,q~ 2018807 Irradiated heat sealable thermoplastic polymers are recognized by those skilled in the art as being capable of heat sealing to themselves at a variety of time, pressure and temperature conditions. For esample, 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 recognize that depending upon such factors as the type of heat sealing equipment used, sealing parameters such as temperature, pressure and time o application may be selected without undue esperimentation.

The preferred material contemplated by the present invention for use as the heat sealable inner layer is ethylene vinyl acetate as at least the major constituent. Further, the inner layer more preferably comprises at least 50 weight percent EVA and most preferably at least 70 weight percent EVA.

The EVA may contain between about 3% and about 18% vinyl acetate. Other suitable polymers and copolymers include alpha olefins such as members of the polyethylene family such as linear low density polyethylene (~DPE), very low den8ity polyethylene sometimes referred to as ultra low density polyethylene ~VLDPE and ULDPE) respectively. Still other suitable thérmoplastic polymers as the heat sealable inner layer include polypropylene, ethylene-propylene copolymer or an ionomer. It is to be understood however that the thermoplastic polymers mentioned herein are not intended to be an e~haustive list, but merely e~emplary.

D-20029 Z018807 - 16 Upon e~posure to irradiation sufficient to cause cross-linking, heat sealable layers generally tend to diminish their heat sealing ability.

However, in accordance with the present invention, the addition of antio~idant to the heat sea}able inner layer of the tubular article inhibits the cross-linking within the polymer, thereby reducing the adverse effects of radiation upon the heat sealing properties. Atdition of an ant~osidant further allow8 the irradiation dosage to be sufficiently high to allow other layers of the multilayer film to retain the beneficial effects of irradiation.

In accordance with a preferred embodiment, the present in~ention in the form of a tubular article comprises a multilayer film having a thermoplastic inner layer, an o~ygen barrier layer, and a thermoplastic outer layer. The inner layer contains an antiosidant in sufficient quantity such that upon irradiation of the multilayer film the inner layer's melt flow indes is adjusted to at least about 0.5 preferably 0.7 and the outer irradiated outer layer's melt flow inde~ i9 le~s than 0.7. The ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5 preferably at least about 2 and most preferably at least about 4. The tubular multilayer film may be formed into bags useful for the packaging of meats.

The multilayer film is especially useful for those bags manufactured by hot bar or impulse sealing or a combination thereof, and after meat is inserted therein, the mouth is sealed by impulse heat sealing.

D-20029 ~o~wa7 - 17 _ Accordingly, the bag from the tubular multilayer film is manufactured such that the inner layer forms the inside portion of the bag while the outer layer forms the outside ~ortion of the bag.

Accordingly, the inner layer of the tubular multilayer film is the heat sealable layer which is easily sealed by hot bar sealing due to its lower degree of cros8-linking. Moreover, because of the latter, the mouth of the bag may be more easily sealed by impulse heat sealing when desired.

Preferably, the heat sealable inner layer of the multilayer film is an ethylene-vinyl acetate copolymer (EVA) containing from about 3 to about 18 weight percent of vinyl acetate (VA~, based on the weight of the copolymer. The vinyl acetate content of the EVA should be at least about 3 wt% to provide the desired high shrink, but no higher than about 18 wt~ to allow the needed bia~ial orientation. Higher vinyl acetate content makes the film escessively ~oft and not capable of bia~ial orientation. A preferred balance between these characteristics is a vinyl acetate content of between about 6 and about 15 wt% of the ethylene v~nyl acetate.

The thermoplastic inner layer has a melt flow inde~ of at least about 0.5 g/10 min and preferably at least about 1.0 g/10 min. An inner layer having a melt flow indes below about 0.7 g/10 min is difficult to melt and form, therefore not acceptable for the purpose of hot bar sealing.

Preferably, the heat sealable inner layer will have a melt flow inde~ between about 0.7 and about 2.0 g/10 min.

D-20029 2018807 - 18 The thickness of the heat sealable inner thermoplastic first layer is preferably between about 0.5 and about 2.0 mils. Thinner layers may not perform the aforedescribed functions while thicker layers do not appreciably improve processability of the film and may reduce total film performance. Accordingly, they would be uneconomical.

In a preferred embodiment, the inner and outer thermoplastic layers are adhered to opposite sides of a barrier core layer. The functional requirement of the barrier layer is that together with the other layers it must provide an osygen transmission rate through the entire multilayer film below about 5 cc/100 in.2/ 24 hrs/Atm. This is necessary to avoid spoilage of certain food products, for esample meat enclosed in the multilayer film package due to osygen passage from the environment through the film wall. This reguirement may be satisfied by numerous well-known barrier layer materials as for esample certain of the polyamide3 ~Nylon), hydrolyzed ethylene vinyl acetate copolymer ~EV0~) and preerably a vinylidene copolymer. Vinylidene chloride-vinyl chloride (PVDC) is the most commonly used copolymer but the vinyl chloride discolors on e~posure to substantial irradiation dosage (more than about 2 MA) so the vinylidene chloride-methyl acrylate copolymer ~A-VDC) is preferred to avoid discoloration on irradiation. The use of this preferred copolymer as a barrier layer in a multilayer film is described in Lustig et al U. S.

Patent No . 4, 714, 638. Another D-20029 -- 19 - - 2~18807 satisfactory o~ygen barrier layer is a blend of vinylidene chloride-methyl acrylate copolymer and vinylidene chloride-vinyl chloride copolymer having 2.9 - 13.5 wt% methyl acrylate and 2.9 - 11.6 wt% vinyl chloride. This blend is described in Schuetz U. S.

Patent No. 4,798,751. As e~plained therein, this blend provides the advantages o no discoloration on lrradlatlon with MA-VDC and proces#ing ease o PVDC.

The barrier layer thickness is preferably between about 0.1 and about 0.5 mils. Thinner barrier layers may not perform the intended functions and thicker layers do not appreciably improve performance.

In the barrier layer embodiment of this invention the outer thermoplastic layer of the enclosing multilayer ilm is on the opposite side of the core layer from the inner layer, and in the preferred three layer embodiment this outer layer is both directly adhered to the core layer and in direct contact with the environment. Since it is seen by the use~con#umer, it must enhance optical properties of the ilm. Also, it must withstand contact with sharp objects s~ it i9 termed the abuse layer and provides abrasion resistance.

The outer layer is preferably formed of ethylene vinyl acetate as at least the major constituent, more preferably at 50% weight percent EVA and most preferably at least 70 weight percent EVA. Also, the outer layer preferably has the same range of between about 3% and about 18~ vinyl acetate content and for the same reasons as in the inner layer.

D-20029 201~3807 - 20 Alternatively, the outer layer may be ; formed of other thermoplastic materials as for esample polypropylene, ethylene - propylene copolymer, ionomer, or an alpha olefin and in particular a member of the polyethylene family such as linear low density polyethylene (L~DPE), very low density polyethylene (VLDPE and ULDPE) respectively, or blends of the8e material8.

The thermopla8tic outer layer thickness is r preferably between about 0.5 and 1.0 mils.

Thinner layers may be less effective in performing the abuse resistance function, while thicker layers may reduce s~ film stretchability.

; The outer layer must be formed of a material which is readily cross-linkable so that on irradiation its melt flow indes is reduced below about 0.7.

From this standpoint, EVA with 10% vinyl acetate content is preferred as at least the major constituent.

In accordance with another embodiment the j second outer layer may contain a cross-linking I enhancer.

As used horein, the term ~cross-linking enhancer~ i5 a chemical composition which when blended with a polymer produces a material which cross-links to a greater estent at a given radiation dosage than does the same polymer without the cross-linking enhancer.

Thus, any chemical composition which when blended with a polymer produces upon irradiation a cross-linked product having a lower melt flow indes than the same polymer at the same irradiation dosage level without the chemical composition is a cross-linking enhancer.

D-20029 2018807 Many cross-linking enhancers are known in the art. Accordingly, any such enhancer may be employed in the second outer layer of the multilayer film. Organic perosides are suitable cross-linking enhancers for use in this invention. These include but are not limited to, dichlorobenzoyl peroside, benzoyl peroside, dicumyl peroside, di-tert-butyl peroside, 4,5-dimethyl-2,5-di(perosy benzoate)hesyne-3, 1,3-bis~tert-butyl perosy isopropyl)benzene, lauroyl pero~ide, tert-butyl peracetate, 2,5-dimethyl-2,5-di(tert-butyl perosy)hesyne-3, 2,5-dimethyl-2,5-di(tert-butyl perosy)hesane and tertbutyl perbenzoate. However, the cross-linking enhancers most preferred include trimethylopropane and trimethylacrylate. It is to be understood that the cross-linking enhancers mentioned herein are not to be a limitation of the present invention but are merely esemplary.

The cross-linking enhancer may be added to the second outer layer by any known conventional technigue. Accordingly, the cross-linking enhancer may be mised with the polymers making up a layer in the powder ~orm by blending the enhancer into the polymer while in the molten state, by difusing the enhancer in a gaseous or liguid form into the polymer, and the like. Preferably, the enhancer is mised with the polymer in the powder form prior to melting. The enhancer may be employed in amounts between about 0.001 to about 0.025 wt% of the layer preferably about 0.007 to about 0.015 wt%. It is to be understood, however, that the esact amount of enhancer is not critical to the invention and is D--2002g Z018807 - 22 dependent upon the estent of differential cross-linking in the multilayer film intended.

The thermoplastic outer layer preferably has a thickness of from about 0.4 mil to about 1 mil to perform the aforedescribed functions, and the preferred three layer film will generally have a total thickness of from about 1.6 mils to about 3.0 mils preferably of from about 1.8 mils to about 3.0 mils. ~ower thicknesses reduce the efectiveness of at least one o the three layers while higher thicknesses reduce the film flesibility and do not appreciably improve its performance.

In general, various conventional additives such as slip agents and pigments can be incorporated in the film in accordance with conventional practice.

While this preferred embodiment of the invention is specifically descri~ed in terms of three layers, it should be understood that one or more additional layers may be directly adhered to the outside of the outer layer or between the barrier layer and the inner layer, or in place of the barrier layer, but not inside the inner layer.

This additional layer may for esample be EVA, ~DPE, V~DPE, polypropylene, EVOH, polyurethane, acrylonitrile nylon, ionomer, or blends thereof.

For esample, if the outer layer of a barrier-type film is EVA a fourth layer may be interposed between the EVA layer and the barrier layer.

The multilayer film of this invention can be produced by known techniques such as by co-estruding the multiple layers into a primary tube, followed by biasially stretching the tube by D-20029 - 23 20188~7 known techniques to form a heat shrinkable film.

The ~double bubble~ technique disclosed in Pahl~e U. S.

Patent No. 3,456,044, can be used to produce the film of this invention. Alternatively, the film may be a slot cast co-estruded multilayer film which is subsequently biasially stretched. After biasial stretching, the multilayer film is then irradiated preferably to a dosage level of between about 2 megartd~ and about 5 megarad~, such as by pa8sing it through an electron beam irradiation unit. The multilayer film may then be employed to manufacture heat shrinkable bags useful in packaging primal and sub-primal meat cuts and processed meats.

Bags may be produced from the multilayer film of this invention by any suitable method, but preferably by hot bar heat sealing. For instance, 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. I the film of this invention is made in the form o flat sheet~, bag9 can be ormed by hot bar sealing three edges of two superimposed sheets of film. When carrying out the hot bar sealing operations, the surfaces which are heat sealed to each other to form seams are the first outer layers containing the antiosidant of the multilayer films of the invention. Thus, for esample, when forming a bag by heat sealing one edge of the length of tubular film, the inner surface of the tube, i.e., the surface D-20029 .,, Z01880~ - 24 which will be heat sealed to itself, will be the inner layer of the tubular multilayer film.

Esample 1 In this series of tests, bags of the same size ~22 inches long s 12 inches wide) were abricated from three layer films compri~ing a heat sealable thermoplastic inner layer with ethylene vinyl acetate ~EYA) as at least the ma~or constituent, a vinylidene chloride copolymer type osygen barrier layer adhered on one side to the sealable inner layer, and a second thermoplastic outer layer adhered to the opposite side of the osygen barrier layer and also formed of EVA. The three layer film was the biasially oriented heat shrinkable type, prepared by the double or trapped bubble method as broadly described in the aforementioned Palkhe U. S.

Patent No. 3,456,044.

In particular, all three layers were simultaneously coestruded. Certain of the film samples were cross-linked by irradiation after biasial orientation, i.e. po~t-irradiated, in the manner generally de~cribed in ~ustig et al U. S.

Patent No.

4,737,391. The total thickness of the multilayer films was the same for all samples, i.e., 2.17 mils, but the thicknesses of the inner and outer layers were varied.

Certain physical properties of bags were measured by either of the test procedures discussed below.

Melt Flow Indes: ASTM D-1238, condition F (at 190C with 21.60 kg load on film). This ASTM D-20029 ` Z018807 - 25 test method is incorporated by reference into this disclosure.

Shrinkage values were obtained by measuring unrestrained shrink of the stretched film -at 90C for five seconds.

Four test specimens were cut from a qiven sample of the oriented film to be tested. The specimens were cut to 10 cm. in the machine direction by lO cm. in the transverse direction. Each specimen was completely immersed for 5 seconds in a 90~C water bath. The dista~ce between the snds of the shrunken specimen was measured. The difference in the measured distance for the shrunken specimen and the original 10 cm.

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 voltage ranges for impulse sealing plastic films. A Sentinel Model 12-12AS }aboratory sealer manuactured by Packaging Industries Group, Inc., Hyannis Mass. was used.

This impulse sealer was equipped with a replacement sealing ribbon for a Multivac AG100. The ribbon is available from Xoch Supplies of Kansas City, Missouri. In this test, two four inch wide (T.D.

direction) samples are cut from a tubular film. The impulse sealer is equipped with controls for coolant flow, impulse voltage and time, and seal bar pressure. These controls e~cept for impulse voltaqe were set at the following conditions:

D-20029 201880~ .

0.5 seconds impulse time (upper ribbon only) 2.2 seconds cooling time 50 psi (345 kPa) jaw pressure 0.3 gallon per minute (1 liter per minute) cooling water flow One of the samples was folded in half for use in determining a minimum sealing voltage~ This folding simulates fold~ng which may inadvertently occur durlng con~entlonal bag sealing operatlons. The folded sample which now had four layers was placed into the sealer and by trial and error the minimum ~oltage to seal the bottom two layers to each other was determined.

The ma~imu~ voltage was then determined for the two layer sample by placing it in the sealer and then activating the seal bar. The film sample was manually pulled with about 0.5 lbs. of force and the voltage which does not cause burn-through or significant distortion of the seal was determined.

As previously esplained, in commercial use bags were filled with food product, as or esample poultry, evacuated through the mouth end and sealed, as for example, by an impulse sealing machine. The seal strength of the sealed test bags was determined by a Hot Water Seal Strength (HWSS) test designed to simulate a poultry packaging application. A metal frame fabricated from wire was employed to simulate the poultry to be packaged, and the frame was placed inside the test bag. The so-opened bag and test frame was then immersed in water at 95C _ 0.5C with the seals at the bottom end, and the time to D-20029 2018807 - 27 failure of the seal was measured. This procedure for measuring HWSS is described more completely in the previously mentioned Funderburk et al U. S.

Patent No. 3,900,635, incorporated herein by reference.

The results of these tests are summarized in Table A.

D-20029 ;

2018~07 ~,~ a~ _ _ _ _ C C C C C o V o U~ ~ ~ o ~ I~ ~ ~s o ,~ o , o ~ C~ ~ ~ o X ~ c~ ~ o o~ u~ o. ~ o , o o o o o o o o o3 O -~ ~ ~ ~ ~ ~ ~ o. ~ ~- ~ o o o ~ O o O o O O ~ 1_1 o~ ~ O O O 4 ~ ~ ~ 3 ~ o o ,.~ o ~. ~ o oo ~ ~ ~ ~ N COoo o .~ ~ e ~.~ ~ a ~ 3 a ~ O 00 ~ .0 0 0 ~ O O ro ~ I r~ :~ d ~ o o ~o ~N ~O ~ O O -- ~ O ?le C 1.~ ~ V~ ~ O O r~i r~ ~ O N ~ O~ C N e~ ') N 1-1~ 1~ O O O r~ l ~ r-~ O O ~ r~l r; r; oo ,0 r~ .~ ~ ~ ~ r "` ~ ~ C~ h ~ o o O o o o o o ~ ~11 0 0 o O ~ O ~ o ~ ~o 1` ' D.'~ r~ ~ ~ r~ Q ~ c c o E~ ~ 0~ c O ~ ~ 0 aJ ,~ D ~ ~ ~ ~ 0 ~ X C C El ~ r1 C~ ~ ~ u') ~ r~ CO 0~ O U~ ~-20029 ,~ ~ - 29 2018807 Samples 1 and 10 are a type of bag in commercial use without estra antiosidant in the inner layer, and is the control for the esperimental bags. The EVA outer layer (type H) was a blend of 74% of a 12% vinyl acetate (VA) type and 23.5% of a 4.5% VA type, the balance being slip agent. The low VA type EVA contained substantial antiosidant which was added by the resin manufacturer for thermal stability. The multilayer film of Samples 1 and 10 wa~ post-lrradlated at 3 MR in accordance with the teachings of U. S.

Patent No. 4,737,391. In Sample 2, 1600 ppm BHT antiosidant was added to the inner layer and the post-irradiation level was increased to 4 MR. The inner layer MFI increased substantially ~from 0.14 to 0.71) and the outer layer MFI decreased (from 1.72 to 0.47). This substantially increased the Hwæs (from 10.8 to 17.2 after 6 days). However, the burn-through voltage undesirably slightly decreased rom 46.0 to 43.8 but this level was still considered commercially acceptable. Accordingly, Sample 2 is an embodiment of the inventlon.

Samples 3-9 employed a dlfferent tyye of EVA outer layer having lOS vinyl acetate content and substantially lower antiosidant content (indicated by the resin manufacturer) than the type used in Samples 1, 2, and 10. In Sample 3, there was no antiosidant added to the inner layer. The burn~through voltage was about the same as Sample 1 and the HWSS was significantly higher (16.9 vs.

10.8).

Sample 3 is a commercially acceptable article and is not an embodiment of this invention.

D-20029 . . .

- 2018807 , - 30 In Sample 4, the outer layer percent thickness was increased from 26% to 38% of the total multilayer film (so the inner layer thickness was simultaneously decreased), and 1600 ppm. ~HT antiosidant was added to the inner layer. However, there was no benefit from these changes as both the burn-through voltage and the HWSS declined. Sample 4 shows the disadvantage5 of under cross-linking the inner layer, i.e., the high level o antioxidant limited the cross-linklng and the melt flow index was relatively high, i.e., 3.64. The total structure did not have sufficient resistance to seal without burn-through under severe sealing conditions and the burn-through voltage was significantly reduced ~47.2 to 39.8).

When the irradiation level was increased from 3 MR to 4 MR in Sample 5, there was improvement in both burn-through voltage ~39.8 to 45.6) and HWSS remained about the same ~12.3 to 15.9 after 6 days). That is, it was possible to increase the irradiation voltage and obtain more cross-linking in the outer layer ~while retarding cross-linking in the inner layer beCause of the antiosidant) because o the lower ant~osidant content included by the EVA resin ffl the manufacturer. The melt flow indes of the inner and outer layers were 0.94 and 0.10 respectively, and the ratio was 9.4. Accordingly, Sample 5 is another embodiment of the invention.

Samples 6 and 7 differ from Samples 1-5 in two respects. The heat sealable inner layer contains 10 wt~ very low density polyethylene (~VLDPE~), i.e.

Attane type 4001 manufactured and D-20029 2018807 - 31 sold by Dow Chemical Company. Also, the bag was end sealed by a hot bar instead of the impulse side sealing used to fabricate bag Samples 1-5. Sample 7 differs from Sample 6 only by decreasing the outer layer percent thickness from 38% to 26% of the total multilayer film, so the inner layer thickness was simultaneously increased. It will be noted that the HWSS was substantially higher than the other comparable Samples 1-5 employing the type E outer layer. This was due to a difference in data between end seal and side seal test equipment and the desire to test commercially average sealing conditions with these samples instead of the severe sealing conditions used in Samples 1-5.Samples 6 and 7 show substantially improved HWSS compared to Sample 10 (22.2 and 28.5 respectively vs. 10) and similar burn-through voltages ~47.4 and 44.2 compared to 46). The melt flow indeses of the Sample 6 inner and outer layers are 0.74 and 0.19 respectively, and the ratio is 3.9. The melt flow inde~es of the Sample 7 inner and outer layers are 0.74 and 0.25, and the ratio is 3Ø Accordingly, Samples 6 and 7 are also embodiments of the invention.

Sample 8 differs from Samples 6 and 7 by the inclu8ion of 30 wt~ VLDPE in the heat sealable inner layer. Sample 9 differs from Sample 8 by increasing the inner layer thickness and decreasing the outer layer thickness from 38% to 26% of the total film. The principal effect was beneficial, i.e., there was less inner layer cross-linking than Sample 10, so higher MFI and the HWSS increased substantially. It should be understood that in D-20029 2018807 general high concentrations of VLDPE adversely affect the optic properties of EVA type layer films, and also diminish the shrink properties to a limited e~tent. Nevertheless, Samples 8 and 9 are embodiments of the invention.

Although not clearly demonstrated in Esample 1 and Table A, an improved, i.e., broader impulse scaling range is espected from the practice of this invention because the ~nner layer will flow more essily under typical pres~ure/temperature~dwell settings of the sealer to create a seal, and the outer layer will not estrude away from the seal ribbon, thus preven ting burn-through.

Esample 2 In another series of tests, esperiments were conducted on a packing plant production line using various types of bags to package ham sections, i.e., butts. The purpose of these tests was to compare the freguency of leakage in impulse-sealed bags of various types under typical commercial packaging conditions. The film~ used to fabricate the~e bag samples were certain of the types described ln E~ample 1, and addltional as an additional control Sample 11. This Sample 11 is a 2.5 mil thick, four layer film of the type EVA~VLDPE~PVDC~EVA that was manufactured by the Cryovac Division of W. R.

Grace Company, and marketed as type BSS0. The two inner layers are irradiated. This film is sold for heavy duty packaging of processed meats. The antiosidant level i~ the inner layer is believed to be only that D-20029 i ' f .~.

Z018807 - 33 included by the EVA resin manufacturer for thermal stability.

On the first day of operation, Sample 10 (not of the invention) and Samples 5, 6 and 7 type bags (of the invention) were used to package ham butts and sealéd with a Cryovac Series 8600 impulse sealing machine operating at a voltage of about 25-28 volts. During the sealing operation, air was evacuated from the ham butt containing bag and the evacua~ed bag was temporarily sealed by mechanically clamping it at a location across the mouth end and then spaced inwardly from the clamp the bag was impulse sealed. The escess film was severed from the sealed bag by a knife cutting across the mouth end between the impulse seal and the clamp. Thirty meat product-containing bags of each type were prepared and the number of leaking bags attributed to bad seals were determined and are reported in Table B. ~eakage was determined by visual ~nspection of the packaged meat. After the packaged product emerged from the shrink tunnel those packages that had a 1008e film appearance and/or bulges were removed. Packages w~th a good vacuum closely follow the contour of the meat. These bags are inflated by puncturing the film with a stainless steel needle through which sufficient air is added to e-pand the bag so that it has enough air to allow determination of leaks by submersion of the bag in a tank of water followed by visual inspection for air bubbles. Care is taken not to admit so much air as to stress the bags and its seals.

D-20029 Z0~8807 - 34 On the second day of operation, Sample 11 (control not of the invention) was used to package ham butts and Sample 1 (control not of the invention) was used to package a misture of ham butts and shanks. The number of leaking bags attributable to bad seals was determined. For Sample 1 the bags determined to be leakers were 8 of 204 and for Sample 11 the leake~ rate wa~ 9 of 100.

D-20029 - 201880~ 35- v a o o o o J~ ~ ~ ~ ~ a ~ ~ ~ ~ ~,q a o~ ~1 .a ~: ~ ~ a~ . _, ~a al .,~ o a ~ o o O v a~E~ e ~ ~:C ~ .~ Vl al~, a D.

ael :~: e al' co ~ 0 0 ~:

_I ~., . . . . a~ .,1 al E m ~ ,~ o u a~ v ~ al e ~ 3 v o ,~ ~ c~ I o E , ~v bq a~ o C al z ~q 1 D-20029 , 201880~ - 36 Table B shows that with respect to the number of leaking seals, Sample No. 7 (an embodiment of this invention) was far superior to the other samples, consistent with its high HWSS. This was probably due to its relatively thick inner layer and the inner layer's 10% VLDPE content. Sample No. 6 (of the invention) also contained 10% VLDPE and had improved leaker rate, whereas Sample No. 5 (also an embodimènt o thi9 invention) was comparable with the control Sample 1.

Summarizing Esample 2 and Table B, they demonstrate that certain embodiments of this invention provide esceptionally strong impulse seals (relatively few leaks~ in the bag packaging of meat products. Moreover, these qualities may be achieved with relatively low VLDPE content in the EVA inner layer, so that optical properties may be masimized (for esample, high gloss and low haze) and high shrink percentages are realized despite the use of V$DPE.

~m~ple 3 Still another series o tests were conducted on a packing plant production line using two types of bags to shrink package boneless turkey breasts. The purpose of these tests was to compare the freguency and types of leakage in impulse-sealed bags of two types under typical commercial package conditions and rates. The films used to fabricate these bag samples were two types described in Esample 1, i.e., control Sample 3 without antio~idant added to the inner layer and Sample 7 D-20029 Z018807 with added antiosidant. However, the control Sample 3 was relatively thick, i.e., 3 mils instead of the 2.17 mil thickness of Sample 7. A relatively thick Sample 3 was used because the thinner version used in Esample 1 does not provide sufficient puncture resistance for packaging turkey breasts. A Cryovac Series 8600 impulse sealing machine was used and operated at a seal bar pressure of 1.4 kg/cm2.

The general procedure for packaging, evacuation and sealing in the Sample 3 and 7 multilayer film bags was the same ag outlined in Esample 2. However, the Sample 3 and 7 bags were used to surround an inner ~Curlon~ bag manufactured by Curwood Packaging Company. The turkey breast was first vacuum packaged in the Curlon bag, then the formed package was heat shrunk in hot water. Nest the heat shrunk package was inæerted in the test bag and further packaged in the Esample 1 manner. Then the double packaged turkey breasts were cooked and chilled.

For gample 3, different packaging rates and sealing voltages were used, i.e., piece rates per minute between 20 and 31 and sealing voltages between 27 and 32 volts. No combination of machine conditions resùlted in consistently qatisfactory seals. The predominant cause of leakage was failure of the bag to seal through folds at the mouth.

For Sample 7, an embodiment of the invention there were no incidences of leaks at folds and no evidence of burn-through of the bag tails or charring of the seals. With one esception, the leaks using the Sample 7 bags were due to deficiencies unrelated to these bags, primarily D-20029 2~1880~ - 38 puncturing of the Sample 7 outer bag by sharp edges of the inner Curlon bag. Reproductive results of these E~ample 3 tests are summarized in Table C.

Summarizing Esample 3 and Table C, they show that at least one embodiment of this invention is suitable for packaging boneless turkey breasts under typical commercial conditions and at acceptable rebag rate. That is, the packer advised that for commercial acceptance the rebag ~leakage) rate should not e~ceed S~, and the esperimental data indicates that only one o the Sample 7 leaks was attributable to the bag itself. This was the leak due to a cold tweak) seal. Accordingly, the effective leak rate was 1~38 bags or 2.6%.

While this invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention.

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