WO1997028960A2

WO1997028960A2 - Heat sealable, peelable film and method of making same

Info

Publication number: WO1997028960A2
Application number: PCT/US1996/020509
Authority: WO
Inventors: Stephen G. Junker; Mark A. Schroeder
Original assignee: Tredegar Industries, Inc.
Priority date: 1996-02-09
Filing date: 1996-12-16
Publication date: 1997-08-14
Also published as: WO1997028960A3; AU1344997A

Abstract

The present invention provides a heat sealable, yet peelable film. The film comprises at least one metallocene catalyst based polyethylene sealant material, and at least one peelable additive material such as polybutylene and/or polypropylene, copolymers and terpolymers thereof.

Description

DESCRIPTION HEAT SEALABLE, PEELABLE FILM AND METHOD OF MAKING SAME

Technical Field

This invention relates to a heat sealable, yet peelable film.

Background of the Invention Many items are packaged today, primarily in the area of foods, where it is vitally important to achieve a quick seal of the film comprising the packaging. In many instances, it is desired that the package have the capability of being opened without destroying the film. In many instances, it is also desired that the film possess adequate moisture, oxygen, and aroma barrier properties. One of the first packaging materials attempting to solve this need was wax coated paper. Cellulosic films also were used early on. Later synthetic polymers were developed and films such as polyethylene were commonly used. As the demands grew, it was discovered that the properties of the polymer films could be improved by orienting the films. The most common orienting methods are to stretch the film in one direction (uniaxial orientation) or two directions (biaxial orientation). One "stretching" technology is called "tentering" and involves machinery which literally grabs the film and stretches it. With oriented polypropylene films, it was found that biaxial orientation provides an increase in moisture barrier .properties and yields a film with greater tensile strength in both machine and transverse directions, as well as excellent optical clarity; hence, it is the most commonly practiced technology today.

Whatever the means of orientation used, once the film is created with an adequate moisture, gas and aroma barrier properties, the film must then be made into the form and embodiment of the desired package, most usually a bag. To form the bag, one must be able to seal the film to itself. The most common means for sealing is "heat sealing". Many kinds of machinery have been constructed for the purpose of forming the bags while simultaneously filling the bags with the desired content. These machines are typically known as vertical form fill-and- seal and horizontal form fill-and-seal machines.

These machines typically have forming collars or bars that shape a flat piece of film into the more tubular shape of the bag. Hot metal sealing jaws are moved from an open position to a closed position, contacting the film in order to seal it into its bag-shape. It is important that the outside of the film, which comes in direct contact with hot metal surface of the sealing jaws, have a higher melting temperature than the inside of the film. The heat can transfer through the outside of the film to melt and fuse the inner, sealant side to form the seal. When the jaws reopen, the outside of the film, which has not melted, is not stuck to the sealing jaws. Since one polymeric material cannot have two diverse melting points, a multilayer film is used.

The multilayer materials are commonly made using lamination technology, wherein a film of barrier material is laminated to a film of sealant material via any of several means. Solvent based adhesive laminations are common, as are water based adhesive laminations. Thermal, sonic or radio frequency bonding can also be utilized. In blown or cast films, coextrusion technology is common. However, these films, if unoriented, can not provide the same barrier properties as oriented films unless they are much thicker.

Another technology used is extrusion coating, where a sealant polymer is generally extruded onto a barrier film and adheres because the sealant polymer is molten. Emulsion coating of barrier films with sealant materials is also known. It is desired to produce a material especially useful for making a

"sealable" and "peelable" film for use in providing packages. The

"sealable" characteristic of the film is needed to provided a strong and quick seal on a package, while the "peelable" characteristic is needed to provide an easily openable seal on the package.

In the past, the sealable and peelable films have been made using a lamination technology. The barrier film materials have had a cling-and- peel sealant layer coated or laminated onto the surface of a film. However, it is difficult to achieve a sealable, yet clean peelable film which readily seals but which, upon being opened or torn, produces no aesthetically displeasing effect, such as string or hair-like strands of film. The strands are present due to the delamination of the sealant layer from a further layer. These stringy, hairlike fibers are not aesthetically acceptable and pose of risk of contamination into the packaged product. In addition to the aforementioned methods of providing a heat sealable barrier film, which are done as secondary operations, there is a coextrusion of a heat sealable resin layer with a barrier layer disclosed in the U.S. Patent No. 4, 189,519, assigned to the American Can Company. The heat sealable resin comprises a blend of polybutylene and a copolymer of ethylene and an ethylenically unsaturated ester. However, this sealant layer does not possess adequate "hot tack" and "low seal initiation temperature" characteristics often needed in the very rapid bag forming and filling applications required by today's packaging industries. The "hot tack" property of a film refers to the film's ability to adequately seal to itself while still warm. The film must rapidly cool and yet still have high bond strength so that the product can be loaded into the bag immediately after formation of the bag. A competing consideration, however is the "seal initiation temperature" of the film as it seals to itself. It is desired that a film have a sufficiently low seal initiation temperature so that the film is cool enough to seal quickly, thereby allowing the packages to more quickly fill packages.

It is therefore the object of the present invention to provide a heat sealable, peelable film and a method for the production of such film which possesses good hot tack and seal initiation temperature properties.

The present invention also provides one or two-side heat sealable, peelable multilayer films comprising a sealant layer of the heat sealable, peelable film and a barrier layer of a film having moisture and/or gas barrier properties.

Description of the Invention

According to one aspect of the present invention, a heat sealable, peelable film is created by blending a metallocene catalyzed polyethylene sealant material with a contaminant-type peelable additive material. In preferred embodiments, a heat sealable, yet peelable film is produced using a metallocene catalyzed sealant material comprising, for example, metallocene catalyzed-linear low density polyethylene blended with a peelability additive material comprising, for example, either a homopolymer, copolymer or terpolymer polybutylene or a homopolymer or copolymer polypropylene, and combinations thereof, to obtain a clean, peelable aesthetically pleasing opening seal.

The present invention provides a sealable film which is considered to be "clean peelable." That is, separation of the seal at the sealant-to- sealant interface results in a low or no stringy appearance. This clean peel seal also provides an easy-to-open package.

It has been further found that a variety of additional sealant materials such as ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene methacrylic acid (EMMA), ethylene acrylic acid (EEA) sealant materials are useful in combination with the blend of metal catalyzed sealant materials and polybutylene and/or polypropylene peelable additive materials.

In another aspect of the present invention, the sealant layer can comprise a blend of the polypropylene peelable additive material and a primary EVA-type sealant material. In another aspect, the EVA-type primary sealant material can be combined with a secondary sealant material, such as a suitable ionomer (Surlyn^®, IMAC^®), EMA, EMAA, EAA, a metallocene catalyzed polyethylene, and blends thereof.

It has been found that films having only the EVA-type base sealant materials lack adequate hot tack characteristics such that the film immediately adheres to itself during operation. Further, the EVA-type films lack the desired seal initiation temperatures. According to the present invention, it was found that a blend of a metallocene catalyzed polyethylene material (either with or without an additional sealant material such as an EVA-type material) and a peelable additive material such as polypropylene and polybutylene provides a film having improved characteristics. The film has increased hot tack, higher seal strength, lower seal initiation temperatures, greater impact strength or puncture resistivity, and higher orientation rates over previously made films. Another aspect of the present invention comprises a multilayer film comprising a sealant layer, comprising the blend of the sealant material and contaminant-type peelable material, and a barrier layer. The sealant blend material can be coextruded with a suitable barrier layer at a constant roll ratio with annealing temperatures that preferably range between about 160 to about 195° F. Although higher annealing temperatures create better clean peelability, annealing above the sealant material's crystalline point increases the sealant material's crystallinity. When the sealant material has an increased crystallinity higher seal initiation temperatures and lower seal strength at the initiation temperature are produced. Therefore, in certain embodiments it is preferred to anneal at the relatively lower temperature ranges. By manipulating two variables (the roll ratio and the post annealing temperatures) in the film making process, heat sealable, clean peelable films are produced. Still another aspect of the present invention comprises a multilayer coextruded film comprising a sealant layer and a barrier layer. In a preferred method, the co-extruded layers are compression rolled at a lower temperature that is required to orient the low melting point sealant layer. Compression rolling is a solid-state contact orientation process. As such, the mill roll surface temperature is lowered to orient an input film(s) having a low melting point sealant polymer contacting at least one mill roll. Normally, these lower temperatures result in less orientation (thickness reduction) and consequently less desirable moisture barrier performance. Even at the lower mill roll surface temperature required to avoid melting the sealant polymer, orientation (thickness reduction) ratios of 4: 1 or more are possible.

In certain embodiments of the present invention the sealant layers are produced with adequate levels of dimensional stability (resistance to shrinkage in use) by annealing after orientation at the required lower temperatures for longer than normal time periods.

A new film is created by coextruding at least one layer comprising barrier material such as a high melting point polymer (for example, a high density polyethylene homopolymer) with at least one sealant layer comprising a sealant material comprising a metallocene sealant material (for example, metal catalyzed linear low density polyethylene) and a peelable additive material (such as polybutylene and/or polypropylene).

The coextrusion of the multi-layer film may utilize cast film, cast sheet, or blown film systems. Until the present invention, there has not been a coextruded multi-layer film comprising at least one heat sealable polymer layer and at least one barrier polymer layer which can be produced without orientation, or, alternatively can be produced at a lower orientation ratio than previously made films.

According to one embodiment where compression rolled orientation is used, a multi-layer coextrusion system is used to create a blown tube with a lower melting point polymer on the outside of the tube and a higher melting point polymer on the inside of the tube. The tube is collapsed to form a two-ply material. The collapsed tube thus comprises in this order: first sealant layer, first barrier material, second barrier material and second sealant layer. The collapsed tube is then transported to a compression rolled orientation mill.

Another aspect of the present invention relates to a blown one- side heat sealable film produced in a similar manner, wherein after formation of the tube as described above having two-plies or webs, the tube is slit and one side or web of the tube is inverted before the two webs are transported into a compression mill. This film places a heat sealable layer inside the film structure, such that the finished film structure has the following layers: heat sealable polymer, high melting point barrier polymer, heat sealable polymer, high melting point barrier polymer.

Brief Description of the Drawings Fig. 1 is a simplified schematic plan view of a coextruded, compression rolled orientation method using a blown film process.

Fig. 2 is a cross-sectional view of the blown film tubing taken along the line 2-2 in Fig. 1 .

Fig. 3 is an enlarged, cross-sectional view of the area shown in Fig. 1 and is a depiction of the tubing near the air flotation flattener.

Fig. 4 is a cross-sectional view of the flattened tubing taken along the line 4-4 in Fig. 1 . Fig. 5 is a cross-sectional view of the two webs after one-ply is inverted taken along the line 5-5 in Fig. 1.

Fig. 6 is a cross-sectional view of a two-layer, two-ply, heat sealable compression rolled film having sealant layers as both outer surfaces of the film.

Fig. 7 is a simplified schematic plan view of a coextruded, compression rolled orientation method using a film casting process.

Fig. 8 is a cross-sectional view of a three-layer, single ply, heat sealable compression rolled film having sealant layers as both outer surfaces.

Fig. 9 is a cross-sectional view of a two-layer, single ply, one-side heat sealable compression rolled oriented film.

Fig. 10 is a simplified schematic drawing of a coextruded, compression rolled orientation method using a sheet extrusion casting process.

Fig. 1 1 is a cross-sectional view of a three-layer, two-ply, single- side heat sealable compression rolled film having a sealant layer as one outer surface of the film, and a barrier layer as the other outer surface.

Fig. 12 is a cross-sectional view of another embodiment of a five- layer, two-ply, heat sealable compression rolled film, having moisture and gas barrier properties.

Fig. 13 is a cross-sectional view of yet another embodiment of a six-layer, two-ply, heat sealable compression rolled film having moisture and gas barrier properties.

Best Mode of Carrying Out Invention According to the present invention, a film is provided which has improved hot tack properties, improved seal strength properties, improved heat sealable properties, improved clean peelability properties, improved impact strength (puncture resistance) properties, and improved orientation rates.

The heat sealant, peelable film comprises a blend of at least one sealant material having various heat seal properties (such as low seal initiation temperature, hot tack strength and coefficient of friction) with at least one peelability additive material. Various useful sealant materials comprise metallocene catalyst based polyethylenes (such as linear low density polyethylene (MET-LLDPE) and the like) which are copolymerized with 10-20% octane, hexane, butane or mixtures thereof. The metallocene catalyzed polyethylenes provide additional puncture and tear resistance. The presence of the metallocene sealant polyethylene materials takes advantage of the constrained geometry of the metallocenes. That is, the enucleating agents and molecules polymerize around the agent such that the metallocene polyethylenes do not have a broad range of molecular weights.

In various embodiments, the sealant material can further comprise ethylene vinyl acetate copolymers (EVA), ethylene methyl acrylate copolymers (EMA), ethylene acrylic acid copolymer (EAA), ethylene methacrylic acid (EMAA) copolymers (such as Nucrel^®), hexane-butene copolymers, ionomers such as Surlyn^®, acid and anhydride modified ethylene vinyl acetates such as Bynel^®, medium density polyethylene (MDPE), low density polyethylene (LDPE), ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE), linear polyethylenes (PE), butane, hexane, octane linear copolymers of polyethylene (PE), and blends thereof.

Various useful peelable additive materials comprise polybutylene (PB), polypropylene (PP, isotactic, atactic, syndiotactic) homopolymers, copolymers and terpolymers and blends of these materials. The presence of the polybutylene and/or polypropylene act as an immiscible contaminant or peelability component. The sealant material tends to stick to itself everywhere. Where there is polypropylene and/or polybutylene present in the film there is no sticking of the sealant material to the polypropylene and/or polybutylene peelable component. For example, the polybutylene (PB) does not stick to metallocene catalyzed polyethylene or the other PB components. The sealing of the film occurs where the same of sealant component is adjacent the another portion of the sealant component present in the film. This altering of seal and peel components in the film affects both the hot tack characteristics and the peelability characteristics of the sealant layer. The addition of polybutylene/polypropylene peelable material adds a "clean peel" characteristic to the sealant layer such that no or low stringiness occurs. The peelable material provides a nondestructive seal and is immiscible with the sealant material. The peelable material lowers the overall seal strength (by acting as a contaminant or providing portions of the film where no sealing occurs) and provides a nondestructive seal. Use of polypropylene is also contemplated, especially in embodiments where higher melting temperatures than can be used for polybutylene would be desired. In additional, copolymers and terpolymers of polybutylene and/or polypropylene are useful, especially in embodiments where lowering the sealing temperatures would be desired. The copolymer/terpolymers of polybutylene and polypropylene are also immiscible with the sealant component.

During production of the package, the film must be able to rapidly cool and yet have sufficient bond strength to quickly adhere to an adjacent film to form the package. The product is immediately loaded into the package after the seal is made. Often, the product is loaded into the packages from heights that can range from about 4 to 10 feet is therefore, it is desired to produce a film for bag-in-box packaging having quick hot tack characteristics. This hot tack characteristic must be balanced however, with attempting to achieve a suitably low sealant initiation temperature since, the lower the temperature at which the film begins to seal to itself and cool, the faster the product can be loaded into the sealed film.

Especially suitable sealant materials and peelable additive materials are listed in Table I below, along with the preferred percents of the co¬ monomer and/or ionomer content, the preferred range of the co-monomer content, and an example of one especially suitable sealant material. The especially suitable peelable additive components are listed along with the preferred percents of any copolymer content or other terpolymers and an example of especially suitable peelable additives.

It is to be understood that the range of the sealant component can range from about 5 to about 90% and preferably about 80%. The peelable additive component can range from about 10 to about 50% and preferably about 20%. In certain preferred embodiments the sealant layer of the film comprises the metallocene catalyzed based polyethylene present in percent by weight of the blend, in an amount of about 10 to about 80%, and, more preferably, about 40%; about 20% to about 90%, ethylene vinyl acetate and, more preferably about 40%; and about 5 to about 50%, peelability additive component, and most preferably about 20%.

In another embodiment the heat sealable peelable film comprises a blend of at least one sealant material comprising a ethylene vinyl acetate and at least one peelable additive material comprising polypropylene and homopolymers, comonomers and/or terpolymers thereof and blends thereof.

It is to be understood that the appropriate amount of slip, anti- block and other processing aides which are desired for manufacturing the film, as well as for providing benefits to the customers (e.g., low coefficient of friction) can be added to the blend and are considered to be within the scope of the invention. Additives such as TiO₂ or other colorants can also be added for aesthetic performance.

The resulting heat sealable, peelable films produce seal strengths of about 200 to about 2000 grams per inch. The overall effective seal range of the film is from about 200°F to about 265 °F using resistant sealing equipment.

Table I

SEALANT COMPONENTS* Example

Co-Monomer Content Range Preferred Range and/or Ionomer TvDe Co-Monomer Content Co-Monomer Content

MET-LLDPE 14.5% Octene butene, hexene, octene at 10% to 20% butene, any FDA approved % hexene or octene comonomer content

EVA 1 2% A 4% VA to 30% VA 12% VA to 1 8% VA

EMA 20% MA 12% MA to 24% MA

EMAA 9% MAA 3% MAA to 1 5% MAA 9% MAA to 12% MAA

EAA 6.5%

EMA or EMAA IONOMERS Zinc and/or Sodium

Co- or Ter-lonomers All materials may be acid or anhydride modified, or other derivatives of the base copolymer including terpolymers.

PEELABLE ADDITIVE COMPONENTS Range Other

Example Co-Polvmer Tvoe TerDolvmer TVDΘ

POLYBUTYLENE (PB)* Homopolymer Ethylene, Propylene Ethylene-Propylene-PB*

POLYPROPYLENE (PP) Homopolymer Ethylene-PP Ethylene-Butylene-PP

(isotactic, atactic, syndiotactic)

PB* & PP BLENDS

*PB: is based on a 1 -butene monomer

It is also within the contemplated scope of the present invention that various other materials can be included as an additional layer to form a multilayer film. The various additional layers can be provided to increase the desired properties of the film such as puncture resistance, tear resistance, opacity level and moisture, aroma and/or gas barrier properties. These various additional materials can include barrier materials, other sealant layers, metallic particles layers, and layers which include trim or excess from the film material. It is also contemplated that a color component (such as titanium dioxide) can be added to render any desired level of opacity or color to the film.

The present invention also provides, in part, a multi-layer film and a process for simultaneously milling both soft and stiff polymers to form a sealable, clean peelable, high barrier film. In embodiments where the film of the present invention is coextruded with a barrier layer to form a multilayer film, the sealant layer is typically a thinner layer than other layers comprising the multi-layer film.

The present invention provides, in part, a multi-layer film comprising two or more layers of film. A first layer is a barrier layer which comprises a gas, aroma and/or moisture barrier material, such as high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE), linear polyethylenes (such as butane, hexane, octane copolymers), polypropylene, nylon, ethylene vinyl alcohol (EVOH), polyester, polyacrylonitrile, polyvinylidene chloride (PvDC) and blends thereof. The barrier layer is comprised of a material which has a higher melting point than the sealant layer. In various embodiments, it is preferred to have high density polyethylene as a primary component of the barrier layer. Typically, it is noted that heat sealant layers have lower melting points than barrier layers. The heat sealant layer is tacky and tends to adhere to itself and other materials. During the processing of producing the film, the manipulation of "roll ratio", (which is defined as the mill roll speed divided by the tensioner speed) is an important factor in determining clean peelability. Once the roll ratio is at a satisfactory level, the annealing temperatures improve upon the performance by relaxing the orientation of the sealant/peelable additive. The orientation of the HDPE, from which the moisture barrier is obtained, is not affected since the temperatures are well below its softening point.

Another way to impart stress relieving into the sealant layer of the mono-axially oriented co-extruded film is to allow the finished film, or roll of film, to heat up utilizing a secondary operation. This secondary step can involve such techniques as 1 ) storage of the finished roll(s) in a heated warehouse or 2) heating the film in-line at the converter. This was tested by subjecting a 6.0 roll ratio peelable blend film, which did not have an adequate clean peel performance, placing it in a heated oven for 15 seconds at 120°F and allowing it to cool for 1 minute. After sealing the film to itself, the sealant side to sealant side peeled cleanly.

It has also been found that the combination of compression rolling to monoaxially orient the film and the stress relieving heating step is an important factor in providing a film which provides satisfactory moisture barrier characteristics combined with both sealability and peelability where the peeled seal has a good visual appearance.

By manipulating variables such as the roll ratio and post-annealing temperatures, the monoaxial orientation process utilized produces clean peelable seals using a variety of sealants and peelable additives.

The present invention provides film thicknesses ranging from about 0.30 mil to about 4.00-5.00 mil. The preferred film ranges from about 1.0 mil to about 2.5 mil and most preferably from about 1 .3 to about 2.0 mil. The sealant layers preferably comprise about 2.5 to about 80% of the total thickness of the film. In embodiments where there are two or more sealant layers, it is preferred that each sealant layer ranges from about 2.5% to about 40% based on the total thickness of the film. The most preferred thickness layer of each sealant layer ranges from about 8 to about 18% and, in certain embodiments, it is preferred to be about 12.5%. It is to be understood that in embodiments where the film is formed by a blown film method, the identical amount is contained in each web of the film, as will be understood readily by the following explanation.

It should be understood that according to the present invention the coextruded film comprising the sealant layer having the components as described above and the barrier layer can be made without using any orientation of the film. The coextruded film of the present invention can be, in certain embodiments, made using the straight blown film which is unoriented. In other embodiments, it may be desirable to orient using, for example, compression rolled orientation. It is to be understood that when the film has no orientation the best peelable seal is achieved. However this is balanced against the fact if there is no orientation, there is a loss of barrier properties to the film. In various embodiments, the process conditions include orienting the film about a 4:1 ratio. It is to be understood however that the orientation ratio can range from about 7.5:1 to no orientation. If, for example, the orientation is at about a 5:1 ratio, then the film can be annealed to relieve stress. Further, it is to be understood that the barrier layer and the sealant layer have different tolerances to the orientation. It is further to be understood that thinner films cool more quickly than thicker films. However, a thicker film has a thicker sealant layer which would have a proportionately larger bond strength.

One preferred method for making a multilayer film is shown in Fig. 1 . It is to be understood that other multi-layer films can be produced according to the method of the present invention and that the following description is merely illustrative.

Referring now to Fig. 1 , each material which is to ultimately form a layer of the film is extruded from an extruder. In the embodiment shown in Fig. 1 , three" extruders, 12, 14 and 16 are shown. It is understood that either two or more extruders, including others not shown, can be utilized in order to make a multi-layer film of the present invention. With reference to Figs. 2-5, it is understood that three extruders are used in order to make a multi-layer film described according to those figures. The extruded materials pass through a coextrusion adapter 18 and pass through a coextruder die 20. An air ring 22 forces cold air in the direction of the arrows 24 such that a blown film bubble 30 is formed. The temperature of the film is greater than the melting point of the coextruded materials at point 32, such that the film is molten. As the film cools, a frost line 34 occurs generally as shown with vertical lines. The film is quenched or cooled such that the temperature of the film drops and at point 36 the film is generally below the temperature of the recrystallization (Tc).

The frost line area 34 is determined by the amount of coolant air on the bubble 30, as well as melt temperatures exiting the die and polymer flow rate (pounds per hr.). The distance of the frost line 34 from the air ring 22 impacts the barrier properties of the film. The higher the frost line 34 or farther away from the air ring 22, the more crystalinity is imparted to the film. Also, when the frost line 34 is higher, the temperature drop of the molten materials is slower and the film spends more time at the maximum crystallization rate temperature (Tc) which also provides a higher density to the film. Higher density films yield better barrier properties in the final oriented film product. The lower the frost line 34, or closer to the air ring 22, the better thickness control for the film is possible. Referring now to Fig. 2, one-side of a multi-layer blown film bubble 30 is shown prior to collapse in the flattener.

The multi-layer film being extruded in Fig. 1 comprises an adhesive or sealant layer A, a barrier layer B and a middle or intermediate layer M. In various embodiments, the intermediate layer M can comprise barrier material and trim or scrap material from the film process. The adhesive layer A is on the outside of the bubble 30. Continuous portions of the bubble 30 pass by air flatteners 40. The air flatteners 40 contain a plurality of apertures 42 which allow air to flow from the air flattener 40 in the direction of the bubble 30. The air flatteners 40 provide internal pressure which is forced out through the apertures 42 to provide an air cushion 44 so that the bubble 30 slides on air. The adhesive or sealant layer A generally comprises materials having a high coefficient of friction. As shown in the enlargement in Fig. 3, a cushion of air 44 is provided by the air flowing through the apertures 42 such that the tacky adhesive layer A does not adhere to the air flatteners 40 or become wrinkled as the bubble 30 is being collapsed. The bubble 30 collapses at a point adjacent to the trailing edges 46 of the air flatteners 40 and collapses to form a two-ply or web material 56. The two-ply material 56 passes through squeeze rolls 52 and 54. In preferred embodiments, the squeeze roll 52 comprises a steel material, while the squeeze roll 54 comprises a rubber material on its surface.

Referring now to Fig . 4, a cross-section is taken along the lines 4-4 in Fig. 1 . As seen in Fig. 4, the two-ply material 50 comprises in the following order the adhesive layer A, the intermediate layer M, the barrier layer B, the barrier layer B', the intermediate layer M' and the adhesive layer A'. The two plies are in contact with each other, but are not bonded or welded together.

The collapsed film shown in Fig. 4 proceeds to a web separating mechanism 60 which splits or separates the collapsed bubble into two webs 62 and 64. The first web 62 proceeds to a web inverter mechanism 70 such as those made by the Collier Coding Machinery Corporation of Greentown, Ohio which flips or turns the web 180°. This can be seen in Fig. 1 once the web separating mechanism 60 has separated the webs 62 and 64. The web 62 has an outside surface designated O and an inside surface designated I and the web 64 has an inside surface I' and an outside surface O'. In the embodiment shown, the web inverter mechanism 70 generally allows the web 62 to pass over a first bar 72 which turns the film 45° . The web 62 passes over a second bar 74 which turns the film 90° and passes over a third bar 76 which turns the web 62 another 45°. After the web 62 is inverted, the outside surface O of the web 62 is adjacent the inside surface I' of the web 64. The webs 62 and 64 are brought together by being pulled over at least one roll 78 to form a recombined web material 80. Referring now to Fig. 5, the recombined multi-layer film is shown after passing through the web inverter mechanism 70. The recombined web comprises the following layers: first sealant layer A, first intermediate layer M, first barrier layer B, second sealant layer A', intermediate layer M' and second barrier layer B'. The interface between the first barrier layer B and second sealant layer A' is not welded together at this point, as is shown by a small area 81 present between the first barrier layer B and second sealant layer A'.

The recombined web 80 passes through a pair of compression rolled orientation milling rolls 82 and 84 which cause the webs 62 and 64 of the recombined web 80 to be bonded or welded together and form a multi-layer film 86.

A multi-layer film comprising a sealant layer and a barrier film are coextruded and compression rolled together. During the compression rolled orientation of the multi-layer film, the two dissimilar polymers pass through the same heat, pressure, shear or extensional forces. The thinner lower melting point sealant layer of the thinner film is not compression rolled at a rate different from the thicker higher melting point barrier layer film. Both the sealant layer and barrier layer are readily oriented together. This is especially surprising since each polymer has unique and different rheological, thermal and morphological properties. The most notable difference is the difference in extensional viscosity or ductility which would cause one to expect that the lower melting sealant layer would be thinned at a different rate from the barrier layer material. The sealant layer, when compression rolled in the multi-layer film of the present invention, readily compression roll orients at the same rate as the intermediate and barrier layers. The ability to compression orient soft polymers and hard/stiff polymers simultaneously without the softer polymer being displaced disproportionately provides an improved multi¬ layer film. Further, there is an improved rolling ratio. The multi-layer film 86 passes through a post-annealing mechanism 90 comprising a plurality of post-annealing rolls 91 , 92, 93 and 94. It is to be understood that the number of post-annealing rolls can be varied. Thereafter, in certain embodiments, the film 86 can pass through a corona treatment mechanism 100, chill roll 102, and to a winding section 104. The chill roll 102 removes latent heat in the film 86. When the latent heat is not removed, the film 86 can have stress relief or shrinkage after winding which can crush the core on which the film is wrapped.

The post-annealing of the film provides stress relief and dimensional stability to the film at elevated temperatures. The dimensional stability reflects the film's ability to shrink at elevated temperatures. In preferred embodiments, it is desired to have the temperature on the surfaces of the rolls 91-94 be slightly lower temperature than conventional post-annealing methods such that the sealant layer A does not stick to the annealing rolls. Further, in various embodiments, it has been found to be useful to have larger than standard annealing rolls such that time and temperature can be manipulated to also increase dimensional stability of the multi-layer film. One advantage of time and temperature manipulation is that the high density polyethylene is kept at a higher temperature which provides the multi¬ layer film with better stress relief.

However, it is to be understood that various other multi-layer films can be produced according to the present invention. It is possible to provide a two-side sealable film in which case the film is not split or separated and inverted. The process can be then shown as in Fig. 1 by not having the two web material 56 pass through the web separating mechanism 60 and the film inverter 70. The material 56 instead directly proceeds from being collapsed at the bubble to compression rolled orienting nip rolls. The tube or bubble is flattened and then fed directly into a compression rolling mill. The combination of heat and pressure produces a two-sided sealable film having a high bond strength between the barrier layers. As shown in Fig 6, the two-sided sealable film comprises sealant layer A, barrier layer B, a second barrier layer B' and a second sealant layer A'. Another method within the scope of this invention includes passing the collapsed bubble web 56 of Fig. 1 through the web separator 60 and dividing it into individual webs 62 and 64. The web 62 is not inverted in web inverter 70, rather each web 62 and 64 are further separated and fed into independent compression roll orienting mills, post annealers and the independent winders.

In still another method, one of the webs can be wound onto an intermediate winder (not shown) and retained on the wound roll to be compression roll oriented at a later date or on an independent compression rolled orientation milling machine. It is to be understood that other coextrusion methods can be utilized to provide a multi-layer film. A multi-layer heat sealable film may be produced by casting a three layer thick film using slot-die film casting technology and compression rolled orienting the resulting film. A single side heat sealable film is produced from a two layer coextrusion system. Fig. 7 shows a film casting which involves extruding molten polymers through a flat die 200 which preferred embodiments have a die gap of about 0.01-0.06 inches and drawing the multi-layer extruded materials down to a thin film 202 from the extrusion die 200 using a large metal casting roll 204. The metal roll 204 may enter a water bath (not shown) to quench the film. The film 202 may be pinned to the casting roll 204 by an air knife 206, vacuum box, electrostatic charge or rubber nip roll. It has been found in preferred embodiments that the best barrier properties are obtained by utilizing relatively low melt temperatures, high casting roll temperatures and no water bath. In preferred embodiments, a stripper roll 208 is utilized to maximize the film contact time on the casting roll 204. The film 202 passes through a compression rolled orientation mechanism 210 and through a post- annealing mechanism 220 as in a manner described above. In embodiments where a rubber nip roll is utilized (not shown), it is desired to have a highly polished smooth surface on the rubber nip roll. Fig. 8 shows a two-side heat sealable compression rolled film having a first sealant layer A, a barrier layer B and second sealant layer A'. An alternative embodiment comprises a one-side heat sealable cast compression roll oriented film as shown in Fig. 9 which comprises an sealant layer A and a barrier layer B.

Yet another casting process can be utilized to form a multi-layer sheet which comprises sheet extrusion technology. This method is useful for producing thick films with higher crystalinity and excellent smooth or high polished surfaces. One example of a sheet extrusion method is generally shown in Fig. 10, wherein a coextrusion die 300 extrudes a sheet 302 material over a plurality of temperature controlled polish rolls 304. In preferred embodiments, there is a controlled gap of about between 75 to about 90% of the nominal sheet thickness, such that the sheet 302 passes over the temperature controlled polish rolls 304. A cooling roll 306 is preferably used adjacent the plurality of polished rolls 304. The multi-layer sheet 302 produced according to this method can be one or two-side heat sealable thick films having thicknesses ranging from 0.006 to about 0.50". The multi-layer sheet 302 passes around a stripper roll 308 which is utilized to maximize the sheet contact time on the cooling roll 306. The sheet 302 passes through a compression rolled orientation mill apparatus 310 and a post- annealing apparatus 320 in a manner as described above. It is contemplated that other coextrusion cast die systems are useful, including ones wherein the various viscosity and flow rate of the polymers can be adjusted. The ability to adopt the process to the differences of various polymers viscosity allows the choice of a great variety of polymers. For example, the Cloeren coextrusion cast die system by the Cloeren Company, which utilizes flow dividers that can be adjusted or moved in response to the various viscosity and flow rates of the polymers flowing beside the flow dividers are useful in the present invention in cast die systems.

Fig. 1 1 shows a cross-sectional view of a multi-layer sealable barrier film comprising adhesive layers A and A' which have lower melting points than the barrier layers B and B'. The heat and pressure from the compression orienting mills will not melt the barrier layers B and B' of the film, but will cause the sealant layer A' to soften and fuse to the barrier layer B in the middle portion of the film. The intermediate layers M and M' can be any desired material. In one example, the intermediate layers M and M' comprise barrier resin plus trim or the excess width trimmed off the edges of the film and incorporated back into an extruder to save costs. The trim material contains both the barrier material and sealant material which in certain embodiments helps to bond the layers of the film together. The trim is not a requirement for bonding the adhesive layer to the barrier layer, however.

Fig. 12 is a cross-sectional view showing a multi-layer material having an adhesive layer A, barrier layer B and intermediate layers M1 , M2 and M3. The multi-layer film shown in Fig. 12 has ten layers. The intermediate layer M1 can be a tie or adhesive layer comprising specialized polymers designed to have an affinity to each of two diverse polymers that have no affinity to each other. An example is the DuPont's Bynel^® adhesive material. The tie layer can be used to tie a layer M2 comprising, for example, a gas barrier polymer, including for example nylon or ethylene vinyl alcohol (EVOH) to the barrier or sealant layer. Gas barrier polymers such as nylon and EVOH are polar materials and are hygroscopic. Such polymers absorb water vapor which reduces the gas barrier performance of the film proportionately. The multi-layer film shown in Fig. 12 has two layers of barrier material B and B' on both sides of one of the gas barrier layer M2'. The gas barrier layer M2' remains protected from moisture adsorption and provides more stable gas barrier properties with less loss of gas barrier properties over time and moisture exposure. The M3 layer can be comprised of trim reclaim material from the process which includes both the tie layer material M1 and trim comprising all layers A, B, M1 , M2 and M3. Fig. 13 is a further embodiment showing a multi-layer film having two webs, each comprising an sealant layer barrier layer B and four intermediate layers, M1 , M2, M3 and M4. It is contemplated that the M1 layer can be a tie material while the layers M2 and M3 can be barrier property materials such as a nylon layer M2 and EVOH layer M3, while the M4 layer can comprise the tie material, trim or scrap material. In preferred embodiments, the sealant layer comprises about 2.5 to about 80% adhesive layer, based on the layer thickness of the multi¬ layer film and is preferably about 8 to about 18%, and in certain embodiments about 12.5%. The barrier layer (and intermediate layers together, if present) comprises about 20 to about 97.5% and preferably about 82 to about 92% and in certain embodiments about 87.5%. The total thickness of the compression rolled film ranges from about .30 mil to about 4.0 mil and is preferably 0.80 to 1 .80 miles. For a single ply film having about 1 .4 mil thickness, the sealant layer preferably ranges from about 0.035 to about 1 .12 mil and preferably about 0.175 mil while the barrier layer ranges from about 0.28 to about 1.365 mil and preferably about 1.225 mil.

Blown coextruded, compression rolled films may be single or two- ply multi-layer structures. The following layer ratio and layer thickness, based on a 1.40 mil. preferred thickness are as shown in Table II below.

Table II Blown Input Film Oriented Single Ply, Two-Layer Thickness Ratio (%) Film Layer Range Preferred Example

A 5-80 8-18 12.5 B 20-95 82-92 87.5

Oriented, 1.40 Mil, Single Ply, Two-Layer

Film Layer Thicknesses (Mil)

Film Layer Range Preferred Example

A 0.07-1.12 0.1 12-0.252 0.175 B 0.28-1.33 1.148-1 .288 1.225

Oriented Two-Ply, Two-Layer Thickness Ratio (%) Film Layer Range Preferred Example

A 2.5-40 8-18 12.5

B 10.-47.5 32-42 37.5

A' 2.5-40 8-18 12.5 B' 10.-47.5 32-42 37.5

1 .40 Mil, Two-Ply, Four-Layer

Film Layer Thicknesses (Mil)

Film Layer Range Preferred Example

A 0.035-0.560 0.1 12-0.252 0.175 B 0.140-0.665 0.448-0.588 0.525

A' 0.035-0.560 0.112-0.252 0.175

B' 0.140-0.665 0.448-0.588 0.525

Table III below shows for oriented single ply, multi-layer cast films, the layer ratio, the percentage and film thickness for the preferred ranges and one example. The thicknesses are based on a 1.4 mil thick film.

Table III Cast Input Film Oriented, Single Ply, Two-Layer Thickness Ratio (%) Film Layer Range Preferred Example A 2.5-80 6-20 12.5

B 20-97.5 80-94 87.5

Oriented, 1.40 Mil, Single Ply, Two-Layer Film Layer Thicknesses (Mil) Film Laver Ranαe Preferred Example

A 0.035-1.120 0.084-0.280 0.175

B 0.280-1 .365 1.120-1.316 1.225 Table IV below shows a blown film with trim reclaim as a middle layer. It is to be understood that the middle layer can contain trim alone, trim and barrier layer material, or trim and a polymer of choice and an inorganic filler of choice for achieving such additional desirable properties as opacity, puncture strength, and/or tear strength. This description is for a two-ply, three-layer coextrusion, compression rolled together as shown in Fig. 1 1 .

Table IV 3-Layer Coextrusion, 2-Ply Multi-Layer Film

Film Layer Ranαe Preferred Example

B' 5-20 7.5-15 12.5

M' 10-40 20-30 25.0

A' 5-20 7.5-15 12.5

B 5-20 7.5-15 25.0

M 10-40 20-30 25.0

A 5-20 7.5-15 12.5 100.0%

The data in Table V below demonstrates superior barrier properties in a coextruded compression roll oriented heat sealable barrier film of the present invention as compared to other films. The first three examples showthe prior technology of monolayer high density polyethylene (HDPE) films having moisture vapor transmission rates ranging from about .10 to about .12 using a compression rolled orientation process.

The next three examples show a coextruded high density polyethylene/sealant film produced by Mobil Corporation's OHD process having moisture vapor transmission rates of about .34 The Mobil OHD process stretches the film in a machine direction using two sets of nip rolls. The second set of nip rolls is set at a higher speed than the first set, such that the film is stretched in between the first and second rolls.

The film is then stretched in a transverse direction using a tentering methodology.

The next two examples show monolayer and coextruded layers of polypropylene which were biaxially tentered, as shown by the Hercules

B500 film and the Borden OPPtiwrap^tmCD442 films. The moisture vapor transmission rates were .30 and .35, respectively.

The next example shows a coextruded high density polyethylene/sealant film, which was blown with no orientation process, having a moisture vapor transmission rate was about .34.

In comparison, the coextruded high density polyethylene barrier film/sealant layer which was compression roll oriented according to the present invention had a moisture vapor transmission rate of about .16.

It is important to note that the difference in moisture vapor transmission rates between .16 and .23 is of great significance to those in the industry. A package designer who wants a barrier property of about .10

MVTR must utilize a Mobil OHD film having a thickness of about 2.3 mils. However, the package designer can utilize the coextruded barrier/sealant film compression roll oriented according to the present invention having a thickness of about 1.6 mil. This is a significant difference when the package designer must produce millions of packages. The savings of about 0.7 mil difference becomes significant while the MVTR performance criteria are still met.

Table VI below shows the annealing temperature, tear strength, hot tack and seal strength for various examples of heat sealable, clean peelable films. The barrier layer comprises HDPE for each example. The heat sealant, clean peelable layer is as stated below for each example. Example 1 comprises a 1.5 mil film with a sealant layer comprising ethylene vinyl acetate (EVA) and polypropylene (PP). Example 2 comprises a 1.5 mil film with a sealant layer having metallocene catalyzed linear low density polyethylene (MET) with polybutylene (PB). Example 3 comprises a 1.5 mil film with a sealant layer comprising about 40% EVA copolymer, about 40% MET-LLDPE, and about 20% homopolymer polybutylene. Examples 4 and 5 use the same barrier layer and sealant layer as for Example 3 but are processed at different annealing temperatures of about 160°, 180° and 190° F. As can be seen, the combination of sealant material and the processing parameters provide a film with superior hot tack, seal initiation and tear strength properties.

Table VI

Example 1 Example 2 Example 3 Example 4 Example EVA/PP MET-LLPPE/PB EVA/MET-LLDPE/PB

ANNEAL TEMP. 190 190 160 180 195

TEAR STRENGTH

Machine Direction grams 75 55 90 56 52

HOT TACK cooling time(s)

40 psi 250 ms 17 - 35 42 - 70 45 - 75 30 - 64 25 - 47

0.5 seconds 750 ms 45 - 100 130 + 1 10 + 50 - 125 50 - 125 flat jaws

200°F - 225°F

SEAL STRENGTH 200 F 145 80 250 250 170

40 psi 210 F 315 215 395 355 305

0.5 seconds 220 F 335 350 440 400 350 horizontal jaws 230 F 380 425 505 455 425

240 F 450 455 615 630 545

250 F 600 600 975 920 780

The present invention has been described in detail by reference to a preferred embodiment. However, it is apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. Several changes or modifications have been briefly mentioned for purposes of illustration.

Claims

WE CLAIM:

1. A heat sealable peelable film comprising a blend of at least one sealant material comprising a metallocene catalyst based polyethylene, with at least one peelable additive material comprising polybutylene, polypropylene and homopolymers, comonomers and/or terpolymers thereof and blends thereof.

2. The film of claim 1 wherein the sealant material comprises at least one metallocene catalyst based polyethylene containing comonomers of butene, hexene or octene.

3. The film of claim 1 , wherein the sealant material further comprises a material selected from the group comprising: ethylene vinyl acetate polymer; ethylene methyl acrylate copolymer; ethylene acrylic acid copolymer; ethylene methacrylic acid copolymers; ionomers, acid anhydride modified ethylene vinyl acetate copolymers; butene, hexene and octene copolymers of polyethylene; hexane-butene copolymers; medium density polyethylene; low density polyethylene; ultra low density polyethylene; very low density polyethylene; and blends thereof.

4. The film of claim 1 wherein the film is oriented by tensile force.

5. The film of claim 1 wherein the film is oriented by compression rolled orientation.

6. A multilayer film comprising the film of claim 1 as a sealant layer and further comprising at least one barrier layer of a thermoplastic material.

7. The film of claim 6, wherein the barrier layer comprises a material selected from the group comprising high density polyethylene, medium density polyethylene, low density polyethylene, linear polyethylene, polypropylene, nylon, ethylene vinyl alcohol, polyester, polyacrylonitrile, polyvinylidene chloride and blends thereof.

8. The film of claim 6 wherein the film is oriented by tensile force.

9. The film of claim 6 wherein the film is oriented by compression rolled orientation.

10. The film of claim 6 wherein the sealant layer is laminated to the barrier layer.

11. The film of claim 9 having an average moisture vapor transmission rate less than about 0.2 grams/100 square inches per day at 100°F and 90° relative humidity per mil of thickness.

12. The film of claim 6, wherein at least one intermediate layer is positioned between the sealant layer and the barrier layer.

13. The film of claim 6, comprising about 2.5 to about 80% of at least one sealant layer and about 20 to about 97.5% of at least one barrier layer, based on the thickness of the film.

14. The film of claim 2 wherein the sealant material comprises a blend of a metallocene catalyzed based polyethylene and an ethylene vinyl acetate polymer.

15. The film of claim 14 wherein the metallocene catalyzed based polyethylene is present in percent by weight of the blend, in an amount of about 10 to about 80%, and, more preferably, about 40%; about 20% to about 90%, ethylene vinyl acetate and, more preferably about 40%; and about 5 to about 50%, peelability additive component, and most preferably about 20%.

16. A heat sealable peelable film comprising a blend of at least one sealant material comprising a ethylene vinyl acetate and at least one peelable additive material comprising polypropylene and homopolymers, comonomers and/or terpolymers thereof and blends thereof.

17. The film of claim 16 wherein the film is oriented by tensile force.

18. The film of claim 16 wherein the film is oriented by compression rolled orientation.

19. The multilayer film comprising the film of claim 16 as a sealant layer and further comprising at least one barrier layer of a thermoplastic material.

20. The film of claim 19 wherein the barrier layer comprises a material selected from the group comprising high density polyethylene, medium density polyethylene, low density polyethylene, linear polyethylene, polypropylene, nylon, ethylene vinyl alcohol, polyester, polyacrylonitrile, polyvinylidene chloride and blends thereof.

21 . The film of claim 19 wherein the film is oriented by tensile force.

22. The film of claim 19 wherein the film is oriented by compression rolled orientation.

23. The film of claim 22 having an average moisture vapor transmission rate less than 0.2 grams/100 square inches per day at 100°F and 90° relative humidity per mil of thickness.

24. The film of claim 19 wherein at least one intermediate layer is positioned between the sealant layer and the barrier layer.

25. The film of claim 16 comprising about 2.5 to about 80% of at least one sealant layer and about 20 to about 97.5% of at least one barrier layer, based on the thickness of the film.