WO1998008891A1

WO1998008891A1 - Cross-linked film for the packaging of flowable materials

Info

Publication number: WO1998008891A1
Application number: PCT/US1997/014901
Authority: WO
Inventors: Jacques Laurent; Victor Holbert
Original assignee: Tetra Laval Holdings & Finance, S.A.
Priority date: 1996-08-26
Filing date: 1997-08-25
Publication date: 1998-03-05
Also published as: AU4235397A

Abstract

A cross-linked film for the packaging of flowable materials such as fruit juices, milk and the like. The packaging may be a pouch having transversely heat sealed ends fabricated using a standard vertical form, fill and seal machine. The cross-linked film has at least one layer composed of a linear low density polyethylene material having a density range of approximately 0.900 to 0.950 and optionally a second polymeric material having a density range of approximately less than 0.950. Further, the layer may be composed of a multitude of components in addition to the previously mentioned polymeric materials. The cross-linked film may undergo cross-linking through a number of possible processes including high energy electrons, X-rays, ultraviolet rays and chemical agents. A preferable method of cross-linking is through irradiation by an electron beam at a dosage of approximately between 1 and 20 megarads. The electron beam may have an accelerating voltage between 80 kilovolts and 6 megavolts. The cross-linked film may be composed of a multitude of layers having at least one core layer. The multilayer film may be subject to differential cross-linking where one or more of the layers will not undergo cross-linking. Such differential cross-linking is accomplished through varying the accelerating voltage of the electron beam, introduction of cross-linking inhibitor agents or varying the density of the layers. The cross-linked film demonstrates a variety of enhanced properties of the pouch including compressionability, drop height, puncture resistance, tensile strength, heat resistance, chemical resistance and heat sealing range. The cross-linked film may have a thickness of between 10 and 200 microns.

Description

TITLE CROSS-LINKED FILM FOR THE PACKAGING OF FLOWABLE

MATERIALS

TECHNICAL FIELD The present invention relates to cross-linked films for the packaging of flowable materials. Specifically, the present invention relates to a cross-linked polyethylene film for the packaging of flowable materials and a method for producing the cross-linked polyethylene film.

BACKGROUND ART It is well known that heat-sealable polyethylene film may be fabricated into disposable pouches for the packaging of liquids and other flowable materials. These pouches are commonly used as consumer packaging for containing fruit juices and milk. The pouches, once filled with a desired contents and sealed at a production center, are transported to a store for the ultimate distribution to the consumer. The consumer, with his or her subjective preferences, will have the choice of purchasing a flowable material contained in a disposable pouch or in a more traditional package such as a plastic bottle, paper carton or metal can. In order to overcome the consumer's time- fortified preferences for the more traditional packaging, a disposable pouch must be able to provide the consumer with an assurance that the pouch is safe (the contents) and is able to withstand the same handling treatment as the traditional package.

A polyethylene pouch as consumer packaging for flowable materials has numerous advantages over traditional packages such as glass bottles, paper cartons and high density polyethylene jugs. Compared to these traditional packages, a polyethylene pouch consumes less raw material, requires less space in a landfill, is recyclable, can be processed easily, requires less storage space, requires less energy for chilling because of increased heat transference, and can be readily incinerated. A polyethylene pouch is also reusable as a general purpose storage bag. Flowable materials are usually packaged in a polyethylene pouch through a form, fill and seal machine. The operation of the form, fill and seal machine commences with the unwinding of a web of thermoplastic film which is then formed into a continuous tube in a tube forming section, by sealing the longitudinal edges of the film together to form a fin seal (the forming aspect of the machine). The tube thus formed is pulled vertically toward a filling station. The tube is then collapsed across a transverse cross-section of the tube, the position of the cross s-section being at a sealing device. The sealing device makes a first transverse heat seal at the collapsed portion of the tube thereby creating an airtight seal across the tube. The sealing device generally comprises a pair of jaws which contain a heating element, the jaws closing upon the tube and the heating element heating to form the airtight seal. Subsequent to sealing the tube and prior to the opening of the jaws, a predetermined quantity of a desired contents is deposited into the tube to a predetermined level upward from the first transverse seal (the filling aspect of the machine). The tube is then moved downward to a predetermined position whereby the level of the desired contents is below the jaws of the sealing device. The jaws of the sealing device are again closed, thereby collapsing the tube at a second transverse section and creating a second transverse heat seal (the sealing aspect of the machine). During the second closing of the jaws, the sealing device clamps, seals and severs the tube transversely. A pouch filled with a flowable material is now ready for further transportation. Thus, during the second closing, the sealing device has sealed the top of the filled pouch, severed this pouch from the rest of the tube, and sealed the bottom of the next-to-be filled pouch. An example of a form, fill and seal machine is the TETRA POUCH™, available from Tetra Pak, Inc., Chicago,

Illinois.

In the production of flowable materials in polyethylene pouches, efficiency is measured by the speed in which such pouches may be formed, filled and sealed. The limiting factor is the sealing of the pouch which is usually accomplished through heat sealing as described above. Some factors which affect the heat sealing include the composition of the film, the thickness of the film and the structure of the film. For some films, if the temperature of the heating element rises above a certain level then the film will stick to the sealing jaws thereby decreasing production of the pouches. The force required to open the seal is very important to the integrity of the pouch. The strength of the seal should be sufficient to maintain the integrity of the pouch during transport and storage. The sealing time may be decreased by expanding the heat seal range and by lowering the temperature for which a sufficient seal may be made by the sealing device. Also, expanding the heat seal range allows for application of a film to various form, fill and seal machines.

An example of such a film is Lustig et al, U.S. Patent No. 4,963,419, for a Multilayer Film Having Improved Heat Sealing Characteristics which discloses a multilayer film having one of the layers which contains siloxane cross-linking bonds. In Lustig et al, the heat sealing layer is substantially free from cross- linking bonds and it is disclosed that "ethylene copolymers which contain siloxane cross-linking bonds cannot be heat sealed under conventional impulse sealing conditions." Another example of such a film is Chum et al, U.S. Patent 5,089,321, for a Multilayer Polyolefinic Film Structures Having Improved Heat Seal Characteristics. Chum et al discloses a film structure composed of a first linear polyethylene layer and a second linear polyethylene layer which have improved heat seal performance after irradiation. Chum et al does disclose a film structure having an improved heat seal performance, however the costs of the materials used in the film are much higher than other available polyethylenes. Still another example of such a film is Lustig et al, U.S. Patent 4,997,690, for an Irradiated Multilayer Film For Primal Meat Packaging. Lustig et al '690, discloses a heat-shrinkable multilayer film having an outer layer of ethylene- vinyl acetate copolymers and a barrier layer of a polyvinylidene chloride- vinyl chloride copolymer wherein the film has been subjected to post-biaxially stretched irradiation of at least 1 megarad. The irradiated film of Lustig et al '690 is ideal for packaging primal meat cuts, however such a film is undesirable for the packaging of flowable materials. The foregoing patents, although efficacious in the protection of their contents, are not the denouement of the problems of the packaging industry. There are still unresolved problems which compel the enlargement of inventions in the packaging industry.

DISCLOSURE OF THE INVENTION The present invention resolves some of the problems of the packaging industry by providing a cross-linked film for the packaging of flowable materials which has an expanded heat sealing temperature range and which is applicable to many form, fill and seal machines. The cross-linked film of the present invention may also undergo differential cross-linking whereby only certain layers of the film undergo cross-linking.

It is a primary object of the present invention to provide a method for differently cross-linking a pouch for containing flowable material.

It is a further object of the present invention to provide a cross-linked pouch having a plurality of enhanced inherent properties. It is a further object of the present invention to provide a method for corss-

1 inking a pouch.

BRIEF DESCRIPTION OF THE DRAWINGS Several features of the present invention are further described in connection with the accompanying drawings in which: There is illustrated in FIG. 1 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in mm) of a material with a density of 1.0 g/cm for different accelerating voltages. There is illustrated in FIG. 2 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in gsm) of a material with a density of 1.0 g/cm for different accelerating voltages. There is illustrated in FIG. 3 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in gsm) of a material with a density of 1.16 g/cm for an accelerating voltage of 100 kilovolts. There is illustrated in FIG. 4 a side perspective of an embodiment of the cross-linked film of the present invention. There is illustrated in FIG. 5 a perspective view of a pouch package of the present invention.

There is illustrated in FIG. 6 a bar graph of compression break load (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 7 a bar graph of drop height (measured in cm) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 8 a bar graph of puncture break load (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 9 a bar graph of puncture energy to break (measured in Joules) versus electron beam dosage (measured in megarads) of two different films of the present invention. There is illustrated in FIG. 10 a bar graph of tensile break (measured in megaPascals ) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 1 1 a bar graph of tensile energy (measured in Joules) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 12 a bar graph of the hot tack maximum force (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention.

There is illustrated in FIG. 13 a plot of the hot tack force (measured in Newtons) versus hot tack temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels.

There is illustrated in FIG. 14 a plot of the hot tack force (measured in Newtons) versus the hot tack temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels. There is illustrated in FIG. 15 a plot of the heat seal peak load (measured in Newtons) versus heat seal temperature (measured in degrees Celsius) of one film irradiated at three different levels. There is illustrated in FIG. 16 a plot of the heat seal energy (measured in Joules) versus heat seal temperature (measured in degrees Celsius) of one film irradiated at three different levels.

There is illustrated in FIG. 17 a bar graph of a drop height (measured in millimeters) versus electron beam dosage (measured in megarads) of three films of the present invention having three different thickness.

There is illustrated in FIG. 18 a plot of the drop height (measured in millimeters) versus the heat setting (measured in generic units particular to a certain machine) for one film of the present invention irradiated at three different dosages.

BEST MODE FOR CARRYING OUT THE INVENTION

The cross-linked film of the present invention is usually employed in the packaging of flowable materials, and particularly liquids. "Flowable materials" are defined as solid (particularly granular or more finely divided) or liquid materials which are flowable through pumping or due to gravitational forces. Examples of some flowable materials includes: aqueous liquids such as milk, water and fruit juices; oleaginous liquids such as cooking oil or motor oil; emulsions such as margarine or mayonnaise; pastes such as tomato pastes and peanut butter; preserves such as jams, jellies and marmalades; dough such as bread dough, cookie dough or biscuit dough; ground meat such as ground beef or ground turkey; greases for cooking, lubrication, and other purposes; powders such as gelatin powders and baking powder; granular solids such as nuts, grains or sugars; and many other materials.

The term "polyethylene" as used herein refers to the families of resins obtained by substantially polymerizing the gas ethylene, C₂H . By varying the comonomers, catalysts and methods of polymerization, properties such as density, melt index, crystallinity, degree of branching, molecular weight and molecular weight distribution may be regulated over wide ranges. High molecular weight polymers of ethylene are resins generally used in the plastics industry while low molecular weight polymers are fluids and used as lubricants. The cross-linked film of the present invention may be a monolayer film or a multilayer film. As a multilayer film, the film should consist of an interior layer, an exterior layer and a plurality of core layers. However, the film may only consists of an interior layer and an exterior layer. If the film is monolayer, then the monolayer has preferably the same composition as the interior layer of the multilayer film.

The interior layer (or the monolayer) is composed of a first polymeric material having a density approximately less than 0.950 g/cm³ and optionally a second polymeric material having a density approximately less than 0.950 g/cm³. The first polymeric material should be 10-100 wt. % of the layer and the optionally second polymeric material should be between 0-90 wt. % of the layer. The interior layer may further consists of an additive. The interior layer may still further consists of a multitude of additional components selected from polymeric materials, non-polymeric materials and further additives. The first polymeric material may be a linear low density polyethylene

("LLDPE") or a metallocene linear low density polyethylene. For the present invention, the LLDPE is defined as having a density range between approximately 0.915 to 0.939 g/cm³ and as containing from approximately 5 to 10% by weight of a C₄ or higher alkyl comonomer. Several examples of such material are ESCORENE resin sold by Exxon Chemical Polymers Group; DOWLEX™ resin sold by DOW Plastics; and FLEXIRENE resin sold by Enichem. The density of polyethylene is lowered by copolymerizing ethylene with minor amounts of an alpha, beta-ethylenically unsaturated alkene(s) having from 3 to 20 carbons per alkene molecule (e.g. 1-propene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1,9- decadiene and 1 ,7-octadiene), and preferably from 4 to 8 carbon atoms (e.g. 1 - butene, 1-pentene, 1-hexene and 1-octene).

The second polymeric material may be a low density polyethylene ("LDPE"), a medium density polyethylene ("MDPE"), a high density polyethylene ("HDPE"), a very low density polyethylene ("VLDPE"), a LLDPE, a metallocene LLDPE, a ethylene vinyl acetate copolymer ("EVA"), ethylene methyl acrylate ("EMA"), ethylene ethylacrylate ("EEA"), ethylene butyl acrylate ("EBA"), or an ionomer. The copolymer content of the EVA, EMA, EEA and EBA is between 2 to 40 %. For example, the EVA would have an ethylene range of between 60 to 98% and a vinyl acetate range of between 2 to 40%. For the present invention, the LDPE is defined as having a density ranging from approximately 0.900 to 0.930 g/cm3. The MDPE is defined as having a density of 0.935 to 0.950 g/cm3. An exemplary MDPE is MARLEX polyethylene, available from Phillips Petroleum Company , Bartlesville, Oklahoma. Blending an MDPE with a LLDPE raises the average density of the layer.

Cross-linking of the films greatly increases several inherent properties of the film, and pouches fabricated from the film. Many of these increased properties are illustrated in the examples, however several are not illustrated. One of the properties which are not illustrated is the chemical resistance of the film to the contents of the container. The cross-linking of the film reduces reactivity between the contents and the film thereby lowering contamination of the contents and absorption by the film.

Additives/Fillers Additives and fillers may be blended with the polymeric materials in one or more of the layers of the cross-linked film of the present invention. The additives and fillers are often added one or more layers to provide or enhance certain desirable properties in the cross-linked film. The additives and fillers will also increase or decrease the density of the layers depending on the density of the additives and fillers. This adjustment in the density of the layers will affect the cross-linking process as discussed below.

Those skilled in the pertinent art are well-aware of many suitable additives and fillers which may be employed in practicing the present invention. A list of such suitable additives may include, but not be limited by, pigments, antioxidants and processing aids. These or other additives and fillers may be added to one or more layers of the cross-linked film of the present invention. However, the amount of additive or filler should not detract from the utility of the cross-linked film.

For certain applications of the cross-linked film, a pigment additive may be added for whitening or blackening purposes. An exemplary pigment is titanium dioxide, optionally provided as a masterbatch by dispersing it in high pressure LDPE. The LDPE generally has a density of approximately 0.916 to approximately 0.930 g/cm , and has a melt index of from approximately 0.1 to approximately 10 g/10 minutes. A suitable masterbatch is sold by A. Schulman Inc., Akron, Ohio. The titanium dioxide may be used to impart a white, opaque film color suitable as a background for printing. Carbon black may be added to one or more layers to increase the opacity of the cross-linked film thereby preventing the exposure of the contents to light, particularly ultraviolet light. A representative carbon black formulation is sold by A. Schulman Inc., Akron, Ohio. It may be convenient to use a masterbatch of the pigment in LDPE or another polyethylene. An exemplary masterbatch is sold by A. Schulman Inc., Akron, Ohio. The pigment is included in a "batch" with a low density polyethylene material. The weight percentage of the batch may be between 5 and 8 % of a layer of the cross-linked film. Some additives may serve as cross-linking inhibitor agents in order to prevent substantial cross-linking of a particular layer of the film. One such cross- linking inhibitor agent is anti-oxidants which compete for the free-radicals which occur during cross-linking as explained below. Cross-linking inhibitor agents may be added to one or more layers to prevent cross-linking in order to create a specific film architecture.

Cross-linking Cross-linking is a well-known practice which may trace its roots back to the vulcanization of rubber, circa the mid-nineteenth century. In cross-linked polymers the molecular chains are joined together by covalent bonds and so the chains cannot slide past one another upon the application of heat and pressure.

Polymers of this kind are infusible and are said to be thermoset-like as opposed to thermoplastic polymers which are reversible fusible. Cross-linking may be accomplished by irradiation of the film using high energy electrons, ultra violet radiation, x-rays, beta particles and the like. Another possibility is chemical cross-linking where chemicals are added to the polymer materials to promote cross-linking. Lustig et al, U.S. Patent No. 4,963,419, for a Multilayer Film Having Improved Heat Sealing Characteristics discloses such cross-linking using siloxane to promote the chemical cross-linking.

Cross-linking by irradiation can be accomplished by any electron beam generator operating in a range of approximately 80 kilovolts to approximately 6 megavolts with a power output capable of supplying the desired dosage. The irradiation is usually carried out at a dosage up to 20 megarads (200 kGy), and typically between 1 megarad (lOkGy) and 20 megarads (200 kGy), with a preference for a dosage range of 2 megarads (20kGy) to 12 megarads (120 kGy). Irradiation may be carried out at room temperature, although higher or lower temperatures may be employed.

When irradiation is performed by an electron beam generator, there are five parameters which specify the transfer of energy from the electron beam into a particular material. These five parameters are: (1) absorbed dose; (2) depth of penetration; (3) uniformity; (4) throughput and (5) the thickness of the material. For the present purposes, the "absorbed dose" is defined as the amount of energy deposited into a specified mass of material. The unit of the adsorbed dose is either the kilogray or the megarad. A kilogray (kGy) is defined as the number of joules of energy deposited into 1 kilogram of matter. One megarad is equivalent to 10 kilograys. The penetrating power of the electron beam is related to the accelerating voltage, the density of the material, and the thickness of the material. Higher voltages will cause deeper penetration by the electron beams, while denser material will reduce the depth of penetration. There is illustrated in FIG. 1 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in mm) of a material with a density of 1.0 g/cm³ for different accelerating voltages. FIG. 1 is from an Industrial Electron Beam Systems brochure from RPC Industries of Hay ward, California. These curves relate the dose, normalized to the dose received by the surface of the product, to the depth of penetration in a material with a mass equivalent to 1.0 g/cm³. The electron beam penetration into materials with greater or lesser densities may be estimated by multiplying the penetration depth by the ratio of a density of 1.0 g/cm to the density of the material. For example, assuming a density for a polyethylene material to be 0.915 , the electron beam to be 300 kV, and the dose point on the plot of FIG. 1 to be 0.762 mm, then the electron beam will have a depth of 0.832 mm in the polyethylene material. Dose uniformity is a direct function of the electron beam uniformity. The throughput is a measure of the energy deposition rate and relates directly to the amount of material that can be processed within a given time interval.

There is illustrated in FIG. 2 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in gsm) of a material with a density of 1.0 g/cm for different accelerating voltages. The "gsm" (grams per square meter) of a film is the weight of the material which has the components of density and thickness. The gsm is very important in measuring the depth of penetration of an electron beam. FIG. 2 shows typical depth dose profile curves plotted as film weight (x-axis) versus dose received as a percentage of that received at the front surface of the film (y-axis) for four different accelerating voltages. The plot may be interpreted in the following manner. The layers of a film may be represented by the x-axis, with the surface of the film at zero. The depth of the film increases by moving to the right along the x-axis. For example, a 100 gsm film (a film having a density of 1.00 g/cm and a thickness of 100 microns) and an accelerating voltage of 150 kV will give a dose at the bottom of the film which is only 50% of that at the top surface of the film ( where the top surface of the film is the surface facing the electron source). However, for the same film, if the accelerating voltage is increased to 200 kV, the dose at the bottom of the film will increase to 95% of the top surface of the film. There is illustrated in FIG. 3 a plot of normalized dose (measured in percentages of the surface dosage) versus depth of penetration (measured in gsm) of a material with a density of 1.16 g/cm for an accelerating voltage of 100 kilovolts. As is apparent from FIG. 3, an accelerating voltage of 100 kilovolts or lower may be employed in cross-linking the film structure. At an accelerating voltage of 100 kilovolts and 2.93 megarads, the normalized dosage at 10 microns was 94% of that at the surface. Thus, it is evident from FIGS. 1 , 2 and 3 that by adjusting either the accelerating voltage, the density of the film, or the thickness of the film, one may control the depth of penetration of an effective dosage of the electron beam.

This control of the depth of penetration of an effective dosage of the electron beam allows for "differential cross-linking" of the film. For purposes of the present invention, "differential cross-linking" is defined as the ability to crosslink specific layers of a multilayer film while rendering other layers untreated. For example, if a particular film was to have cross-linked outer layers for enhanced mechanical properties while the inner layers were to be untreated for heat sealing reasons, then the acceleration voltage of the electron beam would be adjusted accordingly. Information on numerous factors concerning a particular film should be accumulated in order to adjust the acceleration voltage properly. This information may include the following: (1) the thickness of each layer as well as the entire film; (2) the overall density of the film as well as the density of each particular layer which is to be cross-linked by irradiation; (3) the effectiveness of a particular dosage of irradiation; and (4) the positioning of the layers to be cross-linked. For example, using the plots in FIGS. 1, 2 and 3, if a particular film with a known density of approximately 1.0 is effectively cross- linked at a dosage which is 95% of that at the surface, then to effectively cross- link a layer which is 25 microns below the top surface would require an acceleration voltage of at least 150 kV. Thus, the acceleration voltage of the electron beam should be adjusted to a particular film structure's density and thickness, or alternatively a film's structure should be adjusted (the placement of particular layers and the thickness of each layer) to a particular acceleration voltage.

Another factor which may affect the cross-linking of a film is the presence of additives or fillers in one or more layers of the film. A discussion of possible additives and fillers was discussed in the section on additives/fillers. The addition of any additive or filler will almost always affect the density of the film or layer. However, certain additives influence the cross-linking of the film or particular layers. For example, the addition of cross-linking inhibitor agents to the film or a particular layer will result in a decrease in the cross-linking effectiveness since the agents scavenge the radicals (discussed below) which are responsible for a large part of the cross-linking mechanism. Other agents might result in an increase in the cross-linking effectiveness. Thus, the control of additives/agents in various layers of the film may lead to differential cross-linking by virtue of their inhibiting or promoting actions.

Through the use of differential cross-linking it may be possible to create films with almost any combination of cross-linked and non cross-linked layers. This ability to control the film architecture allows for the creation of films having many different desirable properties such as an expanded heat sealing range, greater compressionability and the like. One important aspect of the present invention is that this flexibility in the film architecture is possible without the use of exotic and very expensive materials such as very low density polyethylene ("VLDPE"). In this manner, the present invention is able to provide films with the capabilities of many recent films but without the higher costs of materials. Film Fabrication

The film of the present invention may be a monolayer or multilayer film structure. The multilayer film may be a coextruded film, a coated film, a laminated film or a combination of these. Adhesives may optionally be used to join any two contiguous layers. A multitude of fabrication techniques for these multilayer films are within the scope of one skilled in the pertinent art.

The film can be fabricated by the blown extrusion method or the cast extrusion method, two exemplary methods well known in the art. The blown extrusion method is described for example in Modern Plastics Mid-October 1989 Encyclopedia issue, Volume 66, Number 11, pages 264 to 266. The cast extrusion method is described, for example, in Modern Plastics Mid-October 1989

Encyclopedia issue, Volume 66, Number 1 1, pages 256 to 257. The blown extrusion film may either be used as extruded (in tubular form) or slit before use to form a flat ribbon. Alternatively, the film may be formed as a flat film.

There is illustrated in FIG. 4 a side perspective of an embodiment of the cross-linked film of the present invention. As shown in FIG. 4, a cross-linked multilayer film is generally designated 20, and is composed of five layers 22, 24, 26, 28 and 30. The cross-linked film 20 may have been fabricated by any of the previously mentioned techniques. The cross-linked film 20 may have also been cross-linked through any of the previously mentioned methods. A preferred method of cross-linking is irradiation by electron beams at a dosage of between 2 megarads and 20 megarads. However, those skilled in the pertinent art will recognize that other dosages having greater ranges and that other cross-linking methods may be employed in practicing the present invention.

The layer 22 is the interior layer of the film 20 and would be in contact with any flowable material if a pouch or other package was fabricated from the film 20. The layer 30 is the exterior layer of the film 20 and would be in contact with the environment if a pouch or other package was fabricated from the film 20. The layer 30 may also be a print layer. The layers 24, 26 and 28 are the core layers of the film 20 and are located between the interior layer 22 and the exterior layer 30. One of the core layers 24, 26 and 28 may be composed of a barrier material. The barrier material may be a polymeric material such as PVDC, EVOH or a polyamide, or the it may be a non-polymeric material, for example, a material coated with silicon oxide. This barrier layer may also be cross-linked along with the entire film structure.

The thickness of the cross-linked film 20 of the present invention is from about 0.39 mils (10.0 microns) to approximately 10 mils (254 microns), preferably from about 1 mil (25.4 microns) to approximately 5 mils (127 microns); and more preferably from approximately 2 mils (50.8 microns) to about 4 mils ( 100 microns). Each of the five layers 22, 24. 26, 28 and 30 are approximately 10 microns in thickness, with the entire film 20 50 microns in thickness. As can be seen from the different embodiments of the present invention, the film 20 of the present invention has design flexibility. Different materials may be used in the interior layer 22, the exterior layer 30 and the core layers 24, 26 and 28 to optimize specific film properties such as stiffness, impermeability and elongation. Thus, the film 20 can be optimized for specific applications. Pouch Fabrication

The present invention encompasses various packaging for flowable materials. A preferred packaging is a flexible pouch for flowable materials. An example of such a pouch is illustrated in FIG. 5. The pouch of FIG. 5 may be fabricated on a vertical form fill and seal ("VFFS") machine as previously described. A pouch embodying one aspect of the present invention is generally designated 32. The pouch 32 has a first transverse seal 34, a second transverse seal 36, a longitudinal seal 38, and an unsealed longitudinal overlap 40.

Fabrication of the pouch 32 commences with a film, which may have a film structure such as previously described in FIG. 4, being fed into a VFFS machine whereby it is converted into a tubular form. The film is converted to the tubular form by sealing the longitudinal edges of the film together. The sealing may be accomplished either by overlapping the film and sealing the film using an inside/outside seal, or by fin sealing the film using an inside/inside seal. An example of a inside/outside seal is illustrated in FIG. 5 where longitudinal seal 38 overlaps the inside surface of a film with the outside surface of the film thereby creating longitudinal seal 38 and the unsealed overlap 40. The first transverse seal 34 is formed by a sealing bar which transversely seals the bottom of pouch 32. Then, a flowable material such as fruit juice is longitudinally added to the pouch 32. The sealing bar then seals the top end of the pouch 32 thereby creating the second transverse seal 36. The sealing bar also detaches pouch 32 from the tubular film by burning through the film or by cutting through the film. The capacity of pouch 32 may vary from 10 milliliters to 10 liters. The present invention enables a VFFS machine to operate at a higher capacity while producing pouches with fewer defects.

Packaging for flowable materials should have certain properties which are measured to demonstrate the efficacy of the packaging. The following is a general discussion of many of these properties which a specific discussion of the properties of the present invention disclosed in the examples.

Hot Tack Strength The hot tack strength of a film is the force required to separate a heat seal before the seal has had a chance to completely cool (crystallize). The hot tack strength is in contrast to the seal strength which is the strength of the seal after it has completely cooled (crystallized). The hot tack strength of the films can be measured using the Hot Tack Test Method. This method simulates the filling of flowable material into a pouch before the bottom transverse seal has had a chance to cool. The procedure uses a tensile type instrument to pull the sealed specimen apart at a predetermined interval after the seal is made on the package.

Hot tack testing is very important in the food packaging industry to establish seal performance. The sealed area of packages frequently are subjected to disruptive forces before the seal has cooled (form and fill applications). In these applications, cold seal testing cannot be used to establish seal performance. The ability to directly measure a heat seal's hot tack tensile strength under simulated production conditions provides a method to predict film structure performance during actual packaging of flowable materials.

The Hot Tack Test Method may be performed using a Dynamic Testing Consultants Hottack Tester according to the following conditions: Specimen Width (transverse direction): 25.4 mm

Specimen Length (machine direction): 304.8 mm Number of Specimen/Temperatures: 5 (min.)

Temperature Increments: 10°C

Temperature Range: 90 °C - 170 °C

Seal Dwell Time: 0.5 seconds

Seal Pressure: 40 psi Delay Time: 0.1 seconds

Peel Speed: 200mm/sec

The "Hot tack Seal Initiation Temperature" ("T,") is the lowest temperature at which a seal is formed. If a seal force of 0.5 N is selected as the force required to form an adequate seal, then T, will be found at a force of 0.5 N. A low T, and a broad heat seal range are very important for pouch packaging. A low T, and a broad heat seal range allow a packaging machine to operate at faster line speeds by allowing the sealing jaws of the machine to close for short periods of time while still obtaining an adequate heat seal. The temperature between T, and the temperature of the maximum hot tack strength indicates the breadth of the hot tack sealing range.

Seal Strength The seal strength of the film sample may be determined according to the method of ASTM D882. The force required to pull open a seal is relevant to the integrity of a package. For certain applications it is desirable to control the seal strength of a package to facilitate opening. The seals must be of sufficient strength to protect and contain the intended product during its intended life cycle.

The Test Machine Incorporated Heatsealer is designed to mimic a production scenario in producing a seal between overlapping layers of thin thermoplastic films. A MTS SINTECH # 1/D Universal Tester may then be used to measure the seal strength. The following test conditions may be employed: Direction of Pull: 90o to seal

Crosshead Speed: 508 mm/minute

Full Scale Load: 5 kg

Number of Samples: 5

Break Criterion: 80% Gauge Length: 50.8 mm

Sample Width: 25.4 mm

Seal Force: 25 psi

Temperature: 270F

Dwell Time: 0.5 seconds The "Heat Seal Initiation Temperature" ("Heat Seal T,") is the lowest temperature at which a seal is formed. If a seal load of 2065 Gm is selected as the load required to form an adequate seal, then Heat Seal T, will be found at a force of 2065 Gm.

A low Heat Seal T, and a broad heat seal range are very important for poach packaging. A low Heat Seal T, and a broad heat seal range allow a packaging machine to operate at faster line speeds by allowing the sealing jaws of the machine to close for short periods of time while still obtaining an adequate heat seal. The temperature between T, and the temperature of the maximum heat seal strength indicates the breadth of the heat sealing range. Tensile Testing Tensile testing of polyethylene films will provide measurements of properties which are useful for the identification and characterization of materials for control and specification purposes. For the present purposes, "tensile" is defined as the stiffness of the plastic sheeting. These properties include the yield, the yield elongation, the break and the break elongation. The "Tensile Energy to Break" is defined as the total energy absorbed per unit volume of the specimen up to the point of rupture.

The seal strength of the film sample may be determined according to the method of ASTM D882. A MTS SINTECH # 1/D Universal Tester may then be used to measure the various properties. The following test conditions may be employed:

Film Thickness: <0.025 mm

Specimen Width: 25 mm

Specimen Length: 150 mm Number of Specimen (machine direction): 5

Number of Specimen (transverse direction): 5 Speed: 20in/minute

Initial Jaw Separation: 2 in

Test specimens with parallel sides are cut from the film or sheeting with the large dimensions in the desired direction to be tested. The specimen is mounted between two clamps, one stationary and one movable. The movable clamp, or crosshead, is moved away from the stationary clamp at a constant speed of 20 inches per minute. The force is recorded in table form, graphically or both. The resulting stress-strain curve is then used to calculate several tensile properties.

Drop Height The drop impact resistance/ mean failure height covers drop tests on loaded containers or packages in order to measure the ability of the container or package to withstand handling or for comparative evaluation of various container or package constructions. This test is determined according to the method set forth in ASTM D 2463-74. This test provides a measure of the drop impact resistance of a group of containers or package from which the test specimens were selected. This procedure will evaluate the effect of construction, materials and processing conditions on the impact resistance of the container or package. This test method is an aid in the design and improvements of the containers or packages. Test samples should be collected from the packaging machine during steady state production. Samples should be filled with liquid to the required level with a deviation of less than 1%. Only "tight" containers should be used for this test. All samples should be checked for leakage before the test. During collection, samples must be marked to be able to differentiate the seals on the samples. The number of samples should be t least 40. From this 40, 5 random samples should weighed and the average value recorded. The samples should be dropped on the same side for every drop, and this side should be recorded. The surface on which the samples are dropped should be flat and hard (e.g. concrete or metal). The temperature of the liquid inside of the sample should be measured with a thermocouple and the value should be recorded. If a mean failure height is expected, begin at that height. Increase or decrease the height by 15 cm depending on the results. If the mean failure height is unknown, begin at an arbitrary height based on previous tests. After each drop, if failure does not occur, increase the drop height by 30 cm. If there is a failure, decrease the drop height by 30 cm. Once two failures occur consecutively, calculate the average of these heights.

The floor must be wiped after every failure and each sample must drop on the proper side. Any visible rupture on the container or package other than the seal is considered as a failure. Samples should be squeezed gently after impact to determine any pinhole type failures. The total number of drops should be at least 30.

Compression Testing Compression testing is used to determine a package's integrity. The force a package will withstand before failure, and the energy it absorbs will assist in determining how a filled package will survive during transportation. Also, failure type (e.g. the seal or material) is examined to determine the "weak link" in the package. This testing method utilizes a load frame with a "compression chamber" attached to the frame. The chamber contains two plattens. The bottom platten is secured to the load frame base and remains stationary while the upper platten is secured to a load cell on a crosshead and is capable of moving vertically at a constant speed. A MTS Sintech load frame may be used for the compression testing. The load frame should have a crosshead capable of maintaining a consistent speed. The load frame must also be equipped with a load cell which is rated for loads up to 1000 lb. Each material to be tested should be represented by at least 5 samples. The 5 samples should be free from defects and should be able to be classified as "nonleakers." Each sample should be placed on the center of the lower platten. The chamber is closed and the upper platten is adjusted to the testing speed. The upper platten continues at a consistent speed until the package bursts, or the operator terminates the testing. Data gathered from this testing will provide measurements on the peak and break load for each sample. Puncture Testing

Puncture testing through a slow rate penetration resistance method characterizes flexible barrier films and laminates by slow rate penetration resistance to a driven probe. For the present purposes, "elongation" is defined as the elastic deformation (as opposed to plastic deformation) of flexible sheet material under penetration by a driven probe. "Perforation" is defined as the development of a measurable flaw through a barrier film undergoing penetration. "Puncture" is defined as the brittle elastic fracture of a flexible sheet material under penetration by a driven probe. Penetration resistance is an important end- use performance of thin flexible materials where a sharp-edged product can breach the integrity of a barrier wrap. A breach of the integrity may result in the egress or ingress of gases, odors and unwanted contaminates thereby causing potential harm to the end product and reducing its shelf-life. The results will differ based on the film thickness, elastic modulus, rate of penetration, temperature, shape and type of probe used for the testing. This test is determined according to the method set forth in ASTM F

1306-90. An Universal Testing Machine manufactured by MTS Sintech may be used to perform the test in conjunction with a Dell computer system to gather data and calculate the results of the test. The test is performed by applying a biaxial stress at a single test velocity on the material until perforation occurs. The force, energy, and elongation to perforation are determined by this test. This method is applicable to two types of puncture tests: (1) the slow rate puncture as the probe is driven up through the material; and (2) the slow rate puncture as the probe is driven down through the material. Both of these types of tests are operated at 25mm/min. The number of specimens should be 5 for each sample packaging. Each specimen should be removed from the filling process, and each should contain a product. The specimens should be able to be classified as "nonleakers." Each sample should be placed on the tray of the testing machine directly below the probe. The probe should be positioned less than 2 mm from the specimen. The probe is directed downward until the specimen is penetrated by the probe or the test is terminated by the operator.

INDUSTRIAL APPLICABILITY The present invention will be described in the following examples which will further demonstrated the efficacy of the novel cross-linked film, however, the scope of the present invention is not to be limited by these examples.

EXAMPLE ONE Tables One and Two provide the material specifications for the cross- linked films which are embodied in the present invention. Each of the cross- linked films specified in the Tables have five layers. However, those skilled in the pertinent art will recognize that the present invention is not limited to five layers and may have less than or more than five layers. The five layers are blends of polymeric materials with some of the layers also containing additives as described in Tables. The additives described in the Tables are Masterbatch White ("MBW") which is a pigment additive, or Slip and antiblock ("AB") which are processing agents which are added for extrusion purposes only. The cross- linked films vary in thickness from 40 to 50 microns, and have an interior layer , an exterior layer and three core layers. The dominant component of each of the five layers is a LLDPE which consists of between 10-100 weight percent of each of the layers of the present invention. A possible LLDPE is ESCORENE from Exxon Chemical Company. The LLDPE may also be a metallocene LLDPE, and a possible metallocene is EXCEED from Exxon Chemical Company.

A second component of the layers of each cross-linked film is either a LDPE, a MDPE, a HDPE, a VLDPE, an EVA, an EMA, an EEA, an EBA, an ionomer, a LLDPE or a metallocene LLDPE. The second component consists of between 0-90 weight percent of each of the layers. A possible LDPE is

PETROTHENE from Quantum Chemical. A possible MDPE is MARLEX from Phillips Petroleum. A possible VLDPE is FLEXOMER from Union Carbide. The following Examples will further describe the analysis of the present invention by focusing on Materials A and B from Tables One and Two. Materials A and B were both irradiated with a high energy electron beam at an acceleration voltage of 190 kilovolts for the dosages at 2.0 and 3.5 megarads. Referring to the previously mentioned FIGS. 1 and 2, an acceleration voltage of 190 kilovolts will penetrate a film with a 50 micron thickness at a dosage near 100% of the surface dosage. Thus, all five layers of Materials A and B were irradiated with a similar dosage of electron beams thereby providing uniform cross-linking throughout the films. However, those skilled in the pertinent art will recognize that differential cross-linking may occur by lowering the acceleration voltage, increasing the thickness of the film, or increasing the density of the film. The three films of Material A and the three films of Material B were fabricated into pouches and filled with water to conduct the following tests.

TABLE ONE

(Material A)

LAYER LAYER 2 LAYER 3 LAYER 4

LAYER 5

Type LDPE MDPE MDPE MDPE LDPE

Density 0.923 0.939 0.939 0.939 0.923

% of layer 20 20 20 20 20

Type C6-LLDPE C6-LLDPE C6-LLDPE C6-LLDPE C6-LLDPE

Density 0.921 0.921 0.921 0.921 0.921

% of layer 80 72 72 72 75

Type MBW MBW MBW B Batch(slip+

A AB)

Density 1.000 1.000 1.000 1 1..000

% of layer 8 8 8 5

25

TABLE TWO

(Material B)

LAYER 1 LAYER 2 LAYER 3 LAYER 4

LAYER 5

Type LDPE MDPE MDPE MDPE LDPE

Density 0.923 0.939 0.939 0.939 0.923

% of layer 20 20 20 20 20

Type C6-LLDPE C6-LLDPE C6-LLDPE C6-LLDPE C6-LLDPE

Density 0.917 0.917 0.917 0.917 0.917

% of layer 80 72 72 72 75

Type MBW MBW MBW Batch(slip+ AB)

Density 1.000 1.000 1.000 1.000

% of layer 8 8 8 5

EXAMPLE TWO The pouches composed of Materials A and B from Example One were subjected to compression tests which were conducted in accordance with the description previously mentioned in the specification. There is illustrated in FIG. 6 a bar graph of compression break load (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention. In FIG. 6, Material A and Material B subjected to zero, 2 and 3.5 megarads were measured for each of their respective compression break loads. As is shown in FIG. 6, irradiation of the cross-linked films, Materials A and B, increases the compression break load which is dramatically demonstrated in the irradiation of Material B with a dosage of 2 megarads.

EXAMPLE THREE The pouches composed of Materials A and B from Example One were subjected to drop height tests which were conducted in accordance with the method set forth in ASTM D 2463-74 and the description previously mentioned in the specification. There is illustrated in FIG. 7 a bar graph of a drop height (measured in centimeters) versus electron beam dosage (measured in megarads) of two different films of the present invention. In FIG. 7, Material A and Material B subjected to zero, 2 and 3.5 megarads were measured for each of their respective mean failure heights. As is shown in FIG. 7, irradiation of the cross- linked films, Materials A and B, increases the mean failure drop height which is dramatically demonstrated in the irradiation of Material B with a dosage of 3.5 megarads. Material B with an irradiation dosage of 3.5 megarads has a mean failure drop height of almost three times that of Material B with zero irradiation.

EXAMPLE FOUR The pouches composed of Materials A and B from Example One were subjected to puncture tests through a slow rate penetration resistance method which were conducted in accordance with the method set forth in ASTM F 1306- 90 and the description previously mentioned in the specification. There is illustrated in FIG. 8 a bar graph of a puncture break load (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention. In FIG. 8, Material A and Material B subjected to zero, 2 and 3.5 megarads were measured for each of their respective puncture break loads. As is shown in FIG. 8, irradiation of the cross-linked films, Materials A and B, slightly increased the puncture break load of Material A while slightly decreasing the puncture break load of Material B.

There is illustrated in FIG. 9 a bar graph of puncture energy to break (measured in Joules) versus electron beam dosage (measured in megarads) of two different films of the present invention. As is shown in FIG. 9, irradiation of the cross-linked films, Materials A and B, slightly increased the puncture energy to break of Material A while slightly decreasing the puncture energy to break of Material B.

EXAMPLE FIVE The pouches composed of Materials A and B from Example One were subjected to tensile tests which were conducted in accordance with the method set forth in ASTM D882 and the description previously mentioned in the specification. There is illustrated in FIG. 10 a bar graph of tensile break (measured in megaPascals) versus electron beam dosage (measured in megarads) of two different films of the present invention. In FIG. 10, Material A and

Material B subjected to zero, 2 and 3.5 megarads were measured for each of their respective tensile break strengths. As is shown in FIG. 10, irradiation of the cross-linked films, Materials A and B, had minimal effect on the tensile break strength of Material A while slightly decreasing the tensile break strength of Material B at a dosage of 2 megarads and slightly increasing the tensile break strength of Material B at a dosage of 3.5 megarads.

There is illustrated in FIG. 1 1 a bar graph of tensile energy (measured in Joules) versus electron beam dosage (measured in megarads) of two different films of the present invention. As is shown in FIG. 1 1, irradiation of the cross- linked films, Materials A and B, had minimal effect on the tensile energy of

Material A while slightly decreasing the tensile energy of Material B at a dosage of 2 megarads and slightly increasing the tensile energy of Material B at a dosage of 3.5 megarads.

EXAMPLE SIX The pouches composed of Materials A and B from Example One were subjected to hot tack strength tests which were conducted in accordance with the description previously mentioned in the specification. There is illustrated in FIG. 12 a bar graph of hot tack maximum force (measured in Newtons) versus electron beam dosage (measured in megarads) of two different films of the present invention. In FIG. 12, Material A and Material B subjected to zero, 2 and 3.5 megarads were measured for each of their respective hot tack maximum force. As is shown in FIG. 12, irradiation of the cross-linked films. Materials A and B, minimally decreases the hot tack maximum force.

There is illustrated in FIG. 13 a plot of the hot tack force (measured in Newtons) versus the hot tack temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels. As is shown in FIG. 13, a plot of the hot tack range for Material A subject to zero, 2 and 3.5 megarads demonstrates that irradiation of the film does not substantially decrease the hot tack range. There is illustrated in FIG. 14 a plot of the hot tack force (measured in

Newtons) versus the hot tack temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels. As is shown in FIG. 14, a plot of the hot tack range for Material B subject to zero, 2 and 3.5 megarads demonstrates that irradiation of the film does not substantially decrease the hot tack range.

EXAMPLE SEVEN The pouches composed of Materials A and B from Example One were subjected to seal strength tests which were conducted in accordance with the method set forth in ASTM D882 and the description previously mentioned in the specification. There is illustrated in FIG. 15 a plot of the heat seal peak load

(measured in Gm) versus the heat seal temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels. As is shown in FIG. 15, a plot of the heat seal range for Material B subject to zero, 2 and 3.5 megarads demonstrates the expanded heat seal range of the cross-linked film. There is illustrated in FIG. 16 a plot of the heat seal energy (measured in Joules) versus the heat seal temperature (measured in degrees Celsius) of one film of the present invention irradiated at three different levels. As is shown in FIG. 16, a plot of the heat seal range for Material B subject to zero, 2 and 3.5 megarads demonstrates the expanded heat seal range of the cross-linked film.

Referring again to FIGS. 15 and 16, the sample film receiving zero irradiation, the "blank", has a discontinuity at approximately 120 °C. Above this sealing temperature, the blank will partially melt and may adhere

("tackiness") to the sealing jaws and/or heating element of the sealing device. This tackiness is unacceptable in the production of pouches for flowable materials. Hence, the discontinuity in the plot of the blank. In comparison, the film which has been irradiated at a dosage of 2 megarads does not have a discontinuity until approximately 150 °C, and the film which has been irradiated at a dosage of 3.5 megarads does not have a discontinuity until approximately 170 °C. The cross-linking of the film improves the film's heat resistance without deterring from its sealability thereby allowing for higher heat sealing temperatures. This thirty to fifty degree expansion of the heat sealing temperature range allows the production line of the form, fill and seal machine to operate faster because the sealing may occur at a higher temperature allowing for a shorter dwell time for the pouch at the sealing device. The expanded heat sealing temperature range also allows for application of the cross-linked films to a wider variety of form, fill and seal machines. Thus, the cross-linked films of the present invention may be utilized in a greater number of form, fill and seal machines than non cross-linked films and previously disclosed cross-linked films.

EXAMPLE EIGHT There is illustrated in FIG. 17 a bar graph of a drop height (measured in millimeters) versus electron beam dosage (measured in megarads) of three films of the present invention having three different thickness. The three films were fabricated into pouches and filled with water to conduct the drop height tests. The pouches were subjected to drop height tests which were conducted in accordance with the method set forth in ASTM D 2463-74 and the description previously mentioned in the specification. As shown in FIG. 17, the films had a thickness of 40 microns, 55 microns and 70 microns. In FIG. 17, each of the three films subjected to zero, 2 and 4 megarads were measured for each of their respective mean failure heights. As is shown in FIG. 17, irradiation of the cross-linked films, except for the 55 microns films at a dosage of 4 megarads, increases the mean failure drop height which is dramatically demonstrated in the irradiation of each of the three films at a dosage of 2 megarads. The 40 microns thickness film with an irradiation dosage of 2 megarads has a mean failure drop height of almost 160% that of the 40 microns thickness film with zero irradiation.

EXAMPLE NINE There is illustrated in FIG. 18 a plot of the drop height (measured in millimeters) versus the heat setting (measured in generic units particular to a certain machine) for one film of the present invention irradiated at three different dosages. As shown in FIG. 18, Material B from Example One has been irradiated at dosages of zero, 2 and 3.5 megarads. Pouches were then fabricated from the materials and subjected to different heat settings on the VFFS machine. These pouches were then subjected to drop height tests which were conducted in accordance with the method set forth in ASTM D 2463-74 and the description previously mentioned in the specification. As is shown in FIG. 18, cross-linking of the films greatly improved the drop height of the pouches.

Claims

CLAIMSWe claim as our invention:

1. A crosslinked multi-layer film for packaging of pumpable foods, the crosslinked multi-layer film comprising: an exterior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the exterior layer having a thickness range of 10 to 20 microns, the exterior layer having an average density of approximately 0.922 to 0.925 g/cc, the exterior layer having an exterior surface; an interior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the interior layer having a thickness range of 10 to 20 microns, the interior layer having an average density of approximately 0.918 to 0.922 g/cc; and a core layer disposed between the exterior layer and the interior layer, the core layer comprising blend of linear low density polyethylene and a medium density polyethylene, the core layer having a thickness range of 30 to 60 microns, the core layer having an average density of 0.928 to 0.930 g/cc; whereby the crosslinked multi-layer film has been irradiated with a high energy electron beam for a dosage of 2.5 or 3.5 megarads to provide approximately 100% crosslinking at the exterior surface.

2. The crosslinked multi-layer film according to claim 1 wherein the crosslinked multi-layer film is fabricated into a pouch for pumpable foods, the pouch having a longitudinal seal and a first and second transverse seals.

3. The crosslinked multi-layer film according to claim 2 wherein the crosslinked multi-layer film has been subjected to a 2.5 megarad dosage, the pouch having a heat sealing range extending to 150 °C.

4. The crosslinked multi-layer film according to claim 2 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a heat sealing range extending to 170 °C.

5. The crosslinked multi-layer film according to claim 2 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a mean failure drop height at least twice as high as a similar pouch fabricated from a similar non-irradiated multi-layer film.

6. A method for producing a crosslinked multi-layer film for packaging of a pumpable food, the method comprising the steps of: providing a multi-layer film, the multi-layer film comprising an exterior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the exterior layer having a thickness range of 10 to 20 microns, the exterior layer having an average density of approximately 0.922 to 0.925 g/cc, the exterior layer having an exterior surface, an interior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the interior layer having a thickness range of 10 to 20 microns, the interior layer having an average density of approximately 0.918 to 0.922 g/cc, and a core layer disposed between the exterior layer and the interior layer, the core layer comprising blend of linear low density polyethylene and a medium density polyethylene, the core layer having a thickness range of 30 to 60 microns, the core layer having an average density of 0.928 to 0.930 g/cc; and irradiating the exterior surface with a high energy electron beam for a dosage of 2.5 or 3.5 megarads to provide approximately 100% crosslinking at the exterior surface.

7. The method according to claim 6 wherein the core layer is subject to 100% crosslinking.

8. The method according to claim 6 further comprising the step of biaxially orienting the crosslinked film.

9. The method according to claim 6 further comprising the step of fabricating a pouch for pumpable foods, the pouch having a longitudinal seal and a first and second transverse seals.

10. The method according to claim 9 wherein the crosslinked multi-layer film has been subjected to a 2.5 megarad dosage, the pouch having a heat sealing range extending to 150 °C.

1 1. The method according to claim 9 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a heat sealing range extending to 170 °C.

12. The method according to claim 9 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a mean failure drop height at least twice as high as a similar pouch fabricated from a similar non- irradiated multi-layer film.

13. A pouch for pumpable foods, the pouch composed of a crosslinked multilayer film, the pouch comprising: an exterior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the exterior layer having a thickness range of 10 to 20 microns, the exterior layer having an average density of approximately 0.922 to 0.925 g/cc, the exterior layer having an exterior surface; an interior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the interior layer having a thickness range of 10 to 20 microns, the interior layer having an average density of approximately 0.918 to 0.922 g/cc; and a core layer disposed between the exterior layer and the interior layer, the core layer comprising blend of linear low density polyethylene and a medium density polyethylene, the core layer having a thickness range of 30 to 60 microns, the core layer having an average density of 0.928 to 0.930 g/cc; whereby the crosslinked multi-layer film has been irradiated with a high energy electron beam for a dosage of 2.5 or 3.5 megarads to provide approximately 100% crosslinking at the exterior surface.

14. The pouch according to claim 13 wherein the crosslinked multi-layer film has been subjected to a 2.5 megarad dosage, the pouch having a heat sealing range extending to 150 °C.

15. The pouch according to claim 13 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a heat sealing range extending to 170 °C.

16. The pouch according to claim 13 wherein the crosslinked multi-layer film has been subjected to a 3.5 megarad dosage, the pouch having a mean failure drop height at least twice as high as a similar pouch fabricated from a similar non- irradiated multi-layer film.

17. A method for producing a differently crosslinked multi-layer film for packaging of a pumpable food, the method comprising the steps of: providing a multi-layer film, the multi-layer film comprising an exterior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the exterior layer having a thickness range of 10 to 20 microns, the exterior layer having an average density of approximately 0.922 to 0.925 g/cc, the exterior layer having an exterior surface, an interior layer comprising a blend of linear low density polyethylene and a low density polyethylene, the interior layer having a thickness range of 10 to 20 microns, the interior layer having an average density of approximately 0.918 to 0.922 g/cc, the interior layer having an interior surface, and a core layer disposed between the exterior layer and the interior layer, the core layer comprising blend of linear low density polyethylene and a medium density polyethylene, the core layer having a thickness range of 30 to 60 microns, the core layer having an average density of 0.928 to 0.930 g/cc; and irradiating the exterior surface with a high energy electron beam for a predetermined dosage to provide approximately 100% crosslinking at the exterior surface and to provide less than 100% crosslinking at the interior surface.

18. The method according to claim 17 wherein the differential crosslinking is accomplished through varying the density of the multi-layer film.

19. The method according to claim 17 wherein the differential crosslinking is

accomplished through varying an acceleration voltage of the high electron beam.