WO2002000430A2

WO2002000430A2 - Multi-layer film with high gauge evoh heat seal layer

Info

Publication number: WO2002000430A2
Application number: PCT/US2001/020681
Authority: WO
Inventors: Lyle J. Harley
Original assignee: Exxonmobil Oil Corporation
Priority date: 2000-06-28
Filing date: 2001-06-27
Publication date: 2002-01-03
Also published as: AU2001271601A1; WO2002000430A3

Abstract

A multi-layer film has a core layer, such as a biaxially oriented polypropylene layer, and a relatively thick ethylene vinyl alcohol (EVOH) skin layer. The multi-layer film also has a heat seal, such as a crimp seal, formed by contacting a surface of the EVOH heat seal layer with another film surface under sufficient conditions of heat and pressure to form a seal.

Description

MULTI-LAYER FILM WITH HIGH GAUGE EVOH HEAT SEAL LAYER

BACKGROUND

The present invention relates to multi-layer films. For example, there is provided a new composite multi-layer package film having an oxygen barrier.

Packaging technology has over the years required the development of many disciplines. Currently, packaging technologists integrate elements of engineering, chemistry, food science, metallurgy, and other technologies in order to provide the consumer fresh, healthy food product. In those cases where packages are prepared from multi-layer film, it is desirable to be able to provide a barrier which does not permit passage of air or water vapor.

In recent years, containers produced out of multiple-layer flexible film, such as bags and pouches, predominate the marketplace. In order to utilize continuous multiple-layer flexible film, the industry generally employs form/fill/seal packaging techniques. The type of product packaged dictates whether or not the technique will include horizontal form/fill/seal packaging (HFFS) or vertical form/fill/seal packaging (VFFS). It is important for the packaging artisan to be able to select a multi- layer film having optimum barrier properties for storage of the food items and be confident of providing a high quality seal using high speed packaging apparatus. For example, it is known that stereoregular polypropylene, e.g., oriented polypropylene, is quite useful in the manufacture of packages from flexible films. Using oriented polypropylene as a core layer, additional layers in the way of coatings, co-extrusions, laminations, and combinations thereof are added to improve barrier properties of the film. In certain cases, films can be prepared which exclude moisture and oxygen, but permit the passage of light. In other cases, it is also important to prevent light from passing through the film barrier. Barrier properties can also be modified and/or enhanced by treatments such as heat and flame treatment, electrostatic discharge, chemical treatments, halogen treatment, ultra-violet light, and combinations thereof.

A primary concern for designing multiple-layer films for packaging is to ensure they can be processed on high speed form/fill/seal machinery. Form/fill/seal package apparatus operates by unwinding continuous film from bulk film rolls, followed by forming pouches therefrom, filling the pouches, and, finally, sealing the pouch closed. Thus, the film must have sufficient flexibility to undergo machine folding from a flat orientation to a folded condition, and be subjected to a sealing function which is part of high-speed packaging apparatus. In selecting the optimum multi-layer film for its barrier properties, high-speed unrolling and folding are a primary concern. An additional, and very important aspect of the packaging process, however, is the ability to effectively seal the pouch after it is filled with the product. High-speed horizontal and vertical form/fill/seal apparatus include sealing functions at various stages of the packaging process. In a horizontal form/fill/seal apparatus, individual pouches are formed by folding the multi-layer film in half followed by providing vertical seals along the length of the folded web and separating the pouches along the seals formed by vertical sealing. (Optionally, the bottoms of the pouches can also be sealed). After the pouch thusly formed is filled, the top of the pouch is sealed.

Similarly, in vertical form/fill/seal apparatus, the continuous web is formed around a tube and the web is immediately joined together by a longitudinal sealing jaw as either a lap seal or a fin seal. A lap seal is depicted schematically in Figures 1 and 1a of U.S. Patent No. 5,888,648 to Donovan, et al. A fin seal is depicted schematically in Figures 2 and 2a of U.S. Patent No. 5,888,648.

A second sealing function is present in a VFFS configuration which consists of a combination top- and bottom-sealing section (with a bag cutoff device in between). The top-sealing portion seals the bottom of an empty bag suspended from the bag forming tube while the bottom portion seals the top of a filled bag.

As a consequence of processing high-barrier property multi-layer films in high speed form/fill/seal apparatus, damage can occur to metallized layers for providing a barrier to oxygen and water vapor. In order, therefore, to provide high-barrier multi-layer film with hermetic seals, several factors must be considered. It is important to provide a sealing capability at as low a temperature as possible in order to retain, among other things, stereoregularity imposed during orientation, little or no film shrinkage, retention of film and/or chemical additive properties, and highly consistent quality sealing capabilities. Furthermore, the film must have surface characteristics which permit it to be readily used on high-speed machinery. For example, the coefficient of friction must be such that it can be readily unrolled from a high volume roll of film and passed through the packaging machinery. Undesirable sticking or friction characteristics can cause bag imperfections and interruption of high-speed processing. Moreover, seals formed during process must have good seal strength.

More recently, the packaging artisan has been concerned with the ability to provide quality seals which preserve the freshness of the contents while providing the consumer with an easily openable and reclosable container. Innovations to date have been primarily concerned with the components of the seal material. For example, U.S. Patent No. 3,202,528 to James describes a oriented polypropylene film having an adherent heat-sealable coating which includes a material from the group consisting of copolymers of vinylidene chloride and acrylonitrile, copolymers of vinyl chloride and vinyl acetate, chlorinated rubbers, nitrocellulose and polyamide which melts below 160°C and an acidic material provided in an amount of about 20 to about 60% by weight of the film forming material. This adhesive is coated and dried on the film. U.S. Patent No. 4,020,228 to Eastes describes a gel composition which provides a heat sealable surface to polyolefinic materials or cellulosic sheet materials. U.S. Patent No. 4,121 ,956 discloses an ionomer adhesive adhered to an outer ionomeric surface of package wrapping for attachment of labels.

U.S. Patent No. 4,292,882 to Andrews et al. discloses an oriented heat-sealable anti-static polypropylene film manufactured by applying to a surface of a base polypropylene film a heat-sealable olefinic polymer containing between 0.2 and 10% by weight of an anionic hydrocarbyl sulfonate. Andrews, et al. also provide that a slip agent can be incorporated for ease of handling. U.S. Patent No. 4,389,450 to Schaefer, et al. describes a multi-layer packaging film in which the outer polymeric layers cooperate to provide a relatively constant coefficient of friction differential. This enhances the ability to use the film in high speed processing to form fin seal and lap seals. Schaefer, et al. have addressed the problem of providing the proper coefficient of friction for use of the film in high-speed processing apparatus.

U.S. Patent No. 5,049,436 to Morgan, et al. discloses a multi-layer film which is hermetically heat sealable over a broad temperature range. The Morgan, et al. patent describes a heat-sealable layer which includes an ethylene-propylene copolymer and/or an ethylene-propylene-butene terpolymer with an inorganic anti-block agent and a fatty acid amide.

U.S. Patent No. 5,153,074 to Migliorini discloses a high barrier film which has been metallized. The Migliorini '074 patent describes a metallized multi-layer film having a polymer substrate at least one surface of which includes a maleic anhydride modified polypropylene homopolymer or copolymer, and at least one surface having a skin layer thereon of ethylene-vinyl alcohol copolymer, such skin layer having an aluminum layer directly thereon. The ethylene-vinyl alcohol copolymer layer provides excellent oxygen barrier properties. U.S. Patent No. 5,888,648 to Donovan, et al. discloses a multi-layer film which is hermetically heat sealable. The thickness and composition of layers are selected to avoid tunneling in seals, as well as to avoid z- direction tears when sealed bags are opened.

SUMMARY

There is provided a multilayer polymeric film comprising: (a) a core layer comprising a thermoplastic polymer;

(b) a heat seal layer having a thickness of 0.10 mil to 0.50 mil, said heat seal layer comprising an ethylene vinyl alcohol copolymer; and

(c) at least one heat seal formed by contacting a surface of said heat seal layer with another film surface under sufficient conditions of heat and pressure to form a seal. DETAILED DESCRIPTION

The core layer comprises a thermoplastic polymer which has properties suitable for extrusion or coextrusion. The extruded or coextruded film may be biaxially oriented in the machine and transverse directions under elevated temperature so as to form a multi-layer film. Although the preferred thermoplastic polymer of the core layer is a polypropylene homopolymer, other polymers may be used. These polymers include polymers made from one or more 2 to 4 carbon atom olefins, such as ethylene or butene-1 , or a polymer made predominantly of propylene with minor amounts of another olefin, usually a 2 or 4 carbon atom olefin.

Optionally, the core layer comprises an antistatic agent or other additives in addition to the thermoplastic polymer. The antistatic agent may be selected from, e.g., glycerol monostearate (GMS) and a blend of GMS and tertiary amine. Suitable amounts for the antistatic agent may range from about 0.05% to about 3 weight%, based upon on the weight of the core layer.

Another optional component of the core layer is an agent to promote the adhesion of an EVOH layer to the surface of the core layer. Such adhesion promoting agents include maleic anhydride-modified polyolefin, e.g., polyethylene or polypropylene. Such adhesion promoting agents may be blended with the main polymer, e.g., polypropylene, of the core layer prior to extrusion. The use of maleic anhydride-modified polyolefins to promote adhesion of an EVOH layer to a polypropylene layer is described in U.S. Patent No. 5,153,074 and in U.S. Patent No. 5,827,615. If it is desired to produce a film which is opaque after being subjected to uniaxial or biaxial orientation as described herein, microspheres may optionally be dispersed in the core layer polymer before extrusion and orientation of the film. Such microspheres are composed of a material higher melting than and immiscible with the core layer base polymer and the core layer may be any of those disclosed, for example, in U.S. Patent Nos. 4,377,616 and 4,632,869. Thus, the microspheres may be composed of a polymer, e.g., a polyester such as polybutylene terephthalate (PBT) or polyethylene terephthalate (PET), a nylon, an acrylic resin, or polystyrene, or an inorganic material such as glass, metal or ceramic. The preferred material for the microspheres is PBT. The particle size of the microspheres may be, for example, about 0.1 to 10 microns, preferably about 0.75 to 2 microns. The microspheres may be present in the core layer in an amount of up to about 20 wt. %, preferably about 4 to 12 wt. %, based on the total weight of the polymer matrix in the portion of the core layer containing the microspheres. To preserve the structural integrity of the microsphere-containing core layer, a thin layer of core layer polymer in the absence of microspheres may be coextruded on one or both sides of the microsphere-containing core layer polymer. In this case, the total of the microsphere-containing polymer layer and the non-microsphere-containing polymer layers may be considered the overall core layer of the film on one side of which is either a maleic anhydride- modified polyolefin tie layer if such modified polyolefin is not present in the core polymer, or an EVOH copolymer skin layer if such modified polyolefin is present in the core layer, and the other side of which optionally is a skin layer having a lower melting temperature than the core layer. When such a polymer substrate is subjected to uniaxial or biaxial orientation, a cavity forms around each microsphere giving the oriented film an opaque appearance.

The polypropylene of the core layer may be the homopolymer Fina 3371 sold by the Fina Oil and Chemical Company. The polypropylene of the core layer may be a homopolymer or a copolymer. Propylene homopolymers for the core layer include isotactic polypropylene, preferably 80-100% isotactic polypropylene, most preferably about 95% isotactic polypropylene. The propylene homopolymers preferably have a melt flow (measured in accordance with the standard ASTM D1238 method) ranging from about 1.2 to about 10 g/10 minutes, most preferably from about 2.5 to about 6 g/10 minutes. Particular propylene copolymers include (98-93)/(2-7) propylene/ethylene copolymers.

When coextruded, the ethylene vinyl alcohol (EVOH) layer may adhere well to a polypropylene layer, provided that a suitable adhesion promoting agent is blended with the polypropylene. Accordingly, a tie layer or an adhesive layer may not be needed to laminate the EVOH heat seal layer to the core layer. However, one or more such tie or adhesive layers may be optionally used. Such a tie layer may be composed of a maleic anhydride-modified polyolefin. The maleic anhydride-modified polyolefin, e.g., polypropylene, which is present in the core layer or in a tie layer on one side of the core layer may be prepared by any process, for example, that disclosed in U.S. Patent Nos. 3,433,777 and 4,198,327. A commercially available maleic anhydride-modified polypropylene or propylene copolymer has the following physical characteristics: density 0.89-0.91 (ASTM D1505), Vicat softening point 100 °C-150 °C. (ASTM D1525); Shore hardness 50-70 (ASTM D2240); melting point 140 °C-160 °C. (ASTM D2117). If the maleic anhydride-modified polyolefin is blended with the base polymer of the core layer, it is generally present in an amount, for example, under about 10 wt. %, preferably about 0.5 to 1.5 wt. %, based on the combined weight of base polymer and modified polyolefin. Examples of particular maleic anhydride-modified polyolefins are Admer AT 1179E and Admer AT 1152A, avalilable from Mitsui Petrochemical Industries, Ltd., Tokoyo, Japan.

The core layer may have a thickness of 20-100 gauges (0.2-1.0 mil). The ethylene vinyl alcohol (EVOH) copolymer referred to herein can be obtained from any commercial source. For example, extrusion grade ethylene vinyl alcohol copolymer is available under the name EVAL from Kuraray C. Ltd. of Japan. EVOH copolymer is conventionally prepared by saponifying ethylene vinyl acetate copolymer having a polymerized ethylene content of from about 20-70 mol % to a saponification degree of at least about 90 %. Thus, the ethylene vinyl alcohol copolymer employed herein can have an ethylene content ranging from about 20-70 mol %. The final thickness of the EVOH, e.g., after optional biaxial orientation, may be 0.10-0.50 mil, for example, from 0.10 to 0.40 mil. One or more layers may optionally be applied to the surface of the core layer opposite from the surface of the EVOH layer. These layers may be formed from thermoplastics, such as polyolefins or blends thereof, which may be the same or different from the polyolefins of blends used for forming the core layer. The exposed surface of the film on the opposite side from the EVOH layer may also be treated, primed, coated, metallized, printed upon or laminated to another surface or film. The surface of the EVOH skin layer may be untreated, e.g., uncoated and metal-free.

The oxygen transmission rate of the final film product may be measured by a reliable method, such as ASTM D3985. In particular, OTR may be measured with a Mocon OXYTRAN 1000 instrument (available from Modern Controls, Inc., Elk River, MN) at 23°C and 0% relative humidity.

The water vapor transmission rate of the final film product may also be measured by a reliable method, such as ASTM F1249. In particular, WVTR may be measured with a Mocon PERMATRAN W600 instrument (available from Modern Controls, Inc., Elk River, MN) at 38°C and 90% relative humidity. In order to modify or enhance certain properties of multi-layer films, it is possible for one or more of the layers to contain appropriate additives in effective amounts. Such additives include, but are not limited to anti- , blocks, anti-static agents, coefficient of friction (COF) modifiers, processing aids, colorants, clarifiers, and other additives known to those skilled in the art.

Also, the exposed layers of multi-layer films opposite the EVOH surface can be surface-treated to render the films receptive to printing inks, adhesives and/or coatings. These surface-treated layers may be subsequently laminated onto other films or surfaces. The surface treatment can be carried out by any method known in the art such as corona discharge treatment, plasma treatment or flame treatment.

Optionally, a coating may be applied to an exposed surface of an outermost layer of a film opposite to the EVOH surface to facilitate lamination. Prior to application of the coating material, the film may be surface treated or may be primed with a primer layer. Appropriate coatings contemplated include acrylic coatings such as those described in U.S. Patent Nos. 3,753,769 and 4,865,908, and PVDC coatings such as those described in U.S. Patent No. 4,214,039; 4,447,494; 4,961,992; 5,019,447 and 5,057,177. A vinyl alcohol polymer may also be used as a coating composition, such as VINOL 325, available from Air Products Inc..

Appropriate primer materials are poly(ethyleneimine), epoxy primers, and the like.

The outer surface of a film may be treated as noted above to increase its surface energy and therefore insure that the coating layer will be strongly adherent thereto thereby reducing the possibility of the coating peeling or being stripped from the film. This treatment can be accomplished employing known techniques, such as, for example, film chlorination, i.e., exposure of the film surface to aqueous chlorine, treatment with oxidizing agents such as chromic acid, hot air or steam treatment, and the like. Although any of these techniques are effectively employed to pretreat the film surface, a particularly desirable method of treatment is the so-called corona treatment method which comprises exposing the film surface to a high voltage corona discharge while passing the film between a pair of spaced electrodes. After corona treatment of the film surface, the coating composition is then applied thereto. Treated or untreated surfaces may be laminated together with a suitable adhesive, e.g., a hot melt adhesive such as low density polyethylene, ethylene-methacrylate copolymer, water-based adhesives, such as polyvinylidene chloride latex, and the like.

The extruded film can be stretched in the machine direction, coated with a coating composition and then stretched perpendicularly in the transverse direction. In yet another embodiment, coating can be carried out after biaxial orientation is completed.

The film of the invention may have a total thickness ranging from about 0.3 mil to about 5 mils, specifically from about 0.4 mil to about 2.5 mils.

Multi-layer films may be prepared employing commercially available systems for coextrusion.

It is preferred that layers containing polypropylene are coextruded. Thereafter, the film is biaxially oriented. Specifically, the polymers are brought to the molten state and coextruded from a conventional extruder through a flat sheet die, the melt streams are combined in an adapter prior to being extruded from the die or within the die. After leaving the die, the multi-layer web is chilled and the quenched web is reheated and oriented. The film is oriented by biaxially stretching the film. The film can be oriented by stretching from about 3 to about 11 times in the machine direction (MD) at temperatures ranging from about 105 °C to about 150 °C and from about 3 to about 12 times in the transverse direction (TD) at temperatures ranging from about 150 °C to about 165 °C. EXAMPLES Five-layered film samples were prepared. The order of layers in these film samples is referred to herein as A B/C/D/E for the five layers. Skin layer A was prepared from a polypropylene homopolymer. Intermediate layer B was also prepared from a polypropylene homopolymer. Cavitated core layer C was prepared from a polypropylene homopolymer blended with 2 wt % polybutylene terephthalate (PBT). Tie layer D was prepared from maleic anhydride-modified polypropylenes, i.e. either Admer AT 1179E or Admer AT 1152A, avalilable from Mitsui

Petrochemical Industries, Ltd., Tokoyo, Japan. Skin layer E was prepared from an EVOH copolymer, i.e. EVAL EPG 156B, available from Kuraray C. Ltd. of Japan.

The five-layered film was oriented about 5 times in the machine direction and 8 times in the transverse direction by the tenter frame process.

Layers A, B and D were each 0.10 mil in thickness. Core layer C had a thickness of 0.75 mil. The thickness of EVOH skin layer E was either 0.10 mil or 0.30 mil. Samples of the five-layered film were tested for oxygen transmission rate (OTR). In a first OTR test, a first film sample with an EVOH skin layer having a thickness of 0.10 mil had an OTR at 0% relative humidity of 1.84 cc/100 in²/day and an OTR at 50% relative humidity of 0.91 cc/100 in²/day. In a second OTR test, this first film sample with an EVOH skin layer having a thickness of 0.10 mil had an OTR at 0% relative humidity of 1.34 cc/100 in²/day and an OTR at 50% relative humidity of 0.81 cc/100 in²/day.

In a first OTR test, a second film sample with an EVOH skin layer having a thickness of 0.30 mil had an OTR at 0% relative humidity of 0.93 cc/100 in²/day and an OTR at 50% relative humidity of 0.37 cc/100 in²/day. In a second OTR test, this second film sample with an EVOH skin layer having a thickness of 0.30 mil had an OTR at 0% relative humidity of 0.70 cc/100 in²/day and an OTR at 50% relative humidity of 0.38 cc/100 in²/day. Crimp seals were formed by contacting the EVOH sides of two five- layered film specimens and forming a crimp seal. In particular, minimum seal temperature was determined using a Wrap-Aide Crimp Sealer Model J or K. The crimped sealer is set to a dial pressure of about 20, dwell time of 0.75 seconds and starting temperature of about 93 °C. A film specimen was prepared so that when two surfaces were placed together the resulting film was approximately 6.35 cm in the transverse direction by 7.62 cm in the machine direction. The specimen was then inserted squarely, smoothly and flatly into the crimp sealer jaws so that a small amount protruded beyond the back end of the jaws. The transverse direction of the film was parallel to the sealer jaws.

The jaws were closed and immediately after the sealing bar was risen the specimen was removed from the jaws of the sealer. A JDC-type cutter was used to cut the film into a one inch strip. The amount of force needed to separate the seal was determined on an Alfred-Suter crimp seal strength testing unit. The amount of force needed to pull the seal apart was recorded in N/m. In order to determine the minimum temperature required to form a seal requiring about 77.03 N/m peel force, the crimp seals were formed at temperatures raised by 2.8 degree increments until one temperature yielded a seal value of less than about 77.03 N/m and the next temperature yielded a seal value of greater than or equal to about 77.03 N/m.

A chart method (using an established chart) for a 77.03 N/m minimum seal temperature (MST) was used in the present Examples. However, a calculation method, as described in U.S. Patent No. 5,858,552, may also be used.

The minimum seal temperature (MST) of a sample of a crimp seal with two 0.10 mil EVOH layers was 251 °F (122 °C), and the minimum seal temperature (MST) of a sample of a crimp seal with two 0.30 mil EVOH layers was 253 °F (123 °C).

Claims

WHAT IS CLAIMED IS:

1. A multilayer polymeric film comprising:

(a) a core layer comprising a thermoplastic polymer;

(c) at least one heat seal formed by contacting a surface of said heat seal layer with another film surface under sufficient conditions of heat and pressure to form a seal.

2. A film according to claim 1 , wherein a tie layer is between said core layer and said heat seal layer, and said tie layer is a maleic anhydride-modified polyolefin layer.

3. A film according to claim 1 , wherein said heat seal layer has a thickness of 0.10 to 0.40 mil.

4. A film according to claim 3, wherein said heat seal is a lap seal, a fin seal or a crimp seal.

5. A film according to claim 4, wherein said core layer is a polypropylene layer.

6. A film according to claim 5 which is biaxially oriented.

7. A film according to claim 6, wherein said heat seal is a crimp seal formed by contacting a surface of said heat seal layer with another surface of said heat seal layer under sufficient conditions of heat and pressure to form a crimp seal.

8. A film according to claim 6 further comprising a skin layer on the surface opposite from said heat seal layer.

9. A film according to claim 6 further comprising a polypropylene intermediate layer and a polypropylene skin layer on the surface opposite from said heat seal layer.

10. A film according to claim 1 having a cavitated core layer.