NZ248250A - Multilayer thermoformable film which is permeable to oxygen comprising a sealing layer and an outer heat-resistant layer which has a melting point higher than the sealing layer - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £48250 24 82 5 0 Priority Date(s): Comp'et" Specification Filed: Ciass: [t-) e»ts.^.t3s y.4-o :: 'blicalion Date: 2.7.FEB, 1996 0. Jc.'.rncJ No.1 ddbfrri! COMPLETE SPECIFICATION MULTILAYERED THERMOPLASTIC PACKAGING FILM WITH IMPROVED OXYGEN PERMEABILITY We, W. R. GRACE & CO.-CONN., a corporation of the State of Connecticut, United States of America, 1114 Avenue of the Americas, New York, NY 10036, United States of America . hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- NEW ZEALAND PATENTS ACT, 1953 No.: Date: (followed by page -la-) 24 8250 Background of the Invention The present invention relates to a multi-layer film which is particularly useful as packaging material. More specifically, this invention relates to an oxygen permeable, thermoformable, multilayer film especially useful for packaging products that require oxygen such as fresh poultry and frozen red meat.

Description of the Prior Art Numerous film products are employed for packaging and for delivery of food products. These films were developed to have particular properties and often employ multiple layers to obtain the desired properties. For example, it is well known to use polyolefin based films which are characterized by high strength, excellent moisture and water vapor resistance, fair chemical resistance and variable processability. These polymers are often used in combination with other polymers. It has been found that no single polymer or copolymer however can possess all the desired properties and thus the proper combination of different polymers and multilayered structures have been found to provide a good balance depending on the end use of the film.

Many films designed for packaging applications in the food industry incorporate "barrier" polymers to prevent the la 2 4 8 2 5 0 passage of oxygen. The present invention however is directed to films to be used for pachug.i.ng certain food products such as fresh poultry which must necessarily possess high oxygen permeability.

For many such packaging applications it is also desirable that the oxygen permeable film is capable of thermoforming. Most typically, a non-thermoforming film or web will be used in combination with a thermoforming film or web to produce a final package. In a typical operation a forming web is formed into a mold to provide a film cavity in which a food product is placed. A non-forming web can then be placed over the cavity and vacuum sealed by means well known in the art to the periphery of the forming web. Many meat products are packaged in this manner.

A prior art non-thermoformable web is available which employs biaxially oriented polypropylene as an outermost heat resistant layer. However, heretofore there has been only one commercially available thermoformable film having good oxygen permeability properties. That is, it is generally known that thermoformable mono-layer ionomers provide high oxygen permeability. Generally speaking, ionomers are metal neutralized salts of ethylene acrylic acid or methacrylic acid copolymers and are most typically sold under the trade name Surlyn(TM) by DuPont. Although monolayer Surlyn produced by a blown or cast process provides a heat sealable thermoformable oxygen permeable film, product failure rate is high because the periphery of the web must be heated to its softening point in order for sealing to occur. Thus, burn through is common.

Thus there is a need in the art for a heat-sealable thermoformable oxygen permeable film or web which does not degrade upon sealing. /920821.5/OLDFLDR 2 248 250 Summary of the Invention It is thus an object of the present invention to provide heat-sealable thermoplastic multilayer film having good oxygen permeability. tion to provide a suitable thermoplastic material for packaging fresh poultry or frozen meat.

It is yet another object of the present invention to provide a multilayer thermoplastic film having an oxygen transmission rate greater than 2000 cc. mil/m2-24 hr. atm. at 73 Such objects are generally achieved by providing a multilayer, thermoformable film which includes a predominantly ethylene based sealing layer and an outer heat resistant layer, preferably of a propylene based polymer, wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer.

Such objects are more particularly achieved by providing such a film wherein the outer heat resistant layer has a melting point at least about 10°F greater than that of the sealing layer.

The present invention also provides a package comprising: It is a more particular object of the present inven- F.

M.Z. PATENT OFFICE " 7 DEC 1995 received 248250 a first web, thermofonned into a cavity for receiving meat, said first web comprising a sealing layer and an outer heat resistant layer wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer; and a nonforming web comprising a sealing layer an outer heat resistant layer wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer; and wherein the nonforming web is vpositioned immediately adjacent the thermoformed web such that the respective sealing layers of each web are in direct contact and a heat seal is formed about the periphery of the thermoformed cavity.

Description of Preferred Embodiments The present invention provides a multilayer packaging film characterized by having excellent oxygen permeability and thermoformability properties which includes at least a sealing layer and a heat resistant layer. The sealing layer most preferably includes an ethylene based polymer such as low density polyethylene or a copolymer of ethylene 24 825 and one or more comonomers. Preferred ethylene copolymers include ethylene/alpha-olefins, ethylene vinyl acetates, ethylene alkyl acrylates, ethylene acrylic acid copolymers as well as the metal neutralized salts of ethylene acrylic or methacrylic acid copolymers commonly referred to as ionomers.

Ethylene alpha-olefins are, generally speaking, copolymers of ethylene with one or more comonomers selected from C3 to about C10 alpha olefins but especially comprises ethylene copolymers with C4 to about Cxo alpha olefins such as butene-1, pentane-1, hexene-1, octene-1, and the like, in which the polymer molecules comprise long chains with few side chains or branches and sometimes are referred to as linear polymers. These polymers are obtained by low pressure polymerization processes and the side branching which is present will be short compared to non-linear ethylenes. Ethylene/alpha-olefin copolymers have a density in the range of from about 0.860 g/cc to about 0.940 g/cc. The term linear low density polyethylene is generally understood to include that group of ethylene/alpha-olefin copolymers which fall into the density range of about 0.915 to about 0.940 g/cc. Sometimes linear polyethylene in the density range from about 0.926 to about 0.940 is referred to a linear medium density polyethylene (LMDEE). Lower density ethylene alpha olefins may be referred to as very low density polyethylene (VLDPE, typically used to refer to the ethylene butene copolymers supplied by Union Carbide) and ultra-low density polyethylene (ULDPE, typically used to refer to the ethylene octene copolymers supplied by Dow). It should be noted although specific density ranges is for VLDPE, ULDPE, LLDPE, and LMDPE have been set forth herein, that no bright line can be drawn for density classification and such will vary by supplier. /920821.5/OLDFLDR 4 248250 Recently a new type of ethylene based linear polymers have been introduced. These new resins are produced by metallocene catalyst polymerization and are characterized by narrow or more homogenous compositional properties, such as molecular weight distribution, than resins produced by conventional Ziegler-Natta polymerization processes. Conventional Ziegler-Natta polymerization systems have discreet catalyst composition differences which are manifested as different catalyst reaction sites with each site having different reaction rates and selectivities. Metallocene catalyst systems are characterized as a single identifiable chemical type which has a singular rate in selectivity. Thus the conventional systems produce resins that reflect the differential character of the different catalyst sites while resins produced by metallocene systems reflect the single catalytic site. However, it should be noted that at least some previously available ethylene based linear polymers approximated the physical and compositional properties achieved by the present metallocene catalyzed polyolefins. That is traditional metallic catalyzed polymerization processes operating at low reaction rates can produce relatively homogenous resins that compare favorably with the homogeneity of metallocene catalyzed resins. An example of such are the resins sold under the trade name Tafmer(TM) by Mitsui. Both metallocene catalyzed ethylene alpha olefins and the Tafmer-type of resins are appropriate for use in the heat seal layer of the present invention.

Another resin which may be used in the present heat seal layer is a butadiene styrene copolymer (BDS) such as DK10, one of the K resin series available from Phillips Chemical Company. Also, within the scope of the present heat resistant layer are modified polyvinyl chlorides (PVC) such as supplied by B.F. Goodrich. Inclusion of such resins provides stiffness to the overall film structure as well as imparting excellent oxygen permeability qualities. /920821.5/OLDFLDR 248250 As noted above, ionomers are also within the scope of the present seal layer. As has been seen in the prior art monolayer Surlyn films, such resin provides excellent heat sealability as well as high oxygen permeability. However, unlike the mono-layer films of the prior art, the present film structure will also include a heat resistant layer such that upon sealing the entire thickness of the film structure is not heated to its softening point.

Accordingly, the present film structure includes a heat resistant layer having a melting point greater than that of the sealing layer. Thus, when a non-forming web in accordance with the present invention is positioned above a thermoformed web containing product and sealed about the periphery thereof, the outermost surfaces of each of the two webs need not be heated to the point of softening by the sealing mechanism in order for adequate sealing to occur between the two respective seal layers.

Furthermore, it should be noted that although the present invention is generally directed to thermoformable webs, non-forming webs are also within the scope of the invention. That is, thermoformable webs which are not thermoformed in the end-use application are considered non-forming and are covered by the present invention.

Most preferred for use in the heat resistant layer of the present film structure are propylene based resins such as propylene homopolymers and propylene copolymers. Ethylene propylene copolymers which have a major portion of propylene and a minor portion of ethylene are desirable for use in the present heat resistant layer as such do not become brittle at freezing temperatures. Conventional polypropylenes are desirable because of their high melting point, approximately 160 C, but may become brittle at freezing temperatures. Thus, the end-use application as well as the melting point of the polymeric composition of the heat /920821.5/OLDFLDR 6 24 825 0 seal layer must be considered in choosing the polymeric composition of the heat resistant layer. Specifically, the heat resistant layer must have a melting point greater than that of the heat seal layer and most preferably at least 10°F greater than that of the heat seal layer.

Other resins appropriate for use in the heat resistant layer include polystyrene and styrene butadiene copolymer. Although such resins have melting points less than that of polypropylene, they can be employed in accordance with the present invention so long as the polymeric components of the heat seal layer have an even lower melting point.

It has further been found that advantages of different heat resistant polymers may be incorporated into a single structure by incorporating two or more of such resins into a single film, either in separate layers or by blending. For example, in a preferred embodiment, the outermost layer is a polypropylene but an internal layer is included of an ethylene propylene copolymer (EPC). The internal EPC layer is, of course, more heat resistant than the seal layer but adds a pliancy to the structure not afforded by the more heat resistant but less pliant polypropylene. Thus, cracking at freezing temperatures is reduced. Other polymers appropriate for use in the outer heat resistant layer may also be used internally in the structure although it would generally be preferred to provide the most heat resistant polymeric composition in the outermost layer.

Also within the scope of the present invention are internal tie layers which add bulk and prevent delamination without decreasing the oxygen transmitability of the entire structure. Preferred tie layers include ethylene vinyl acetates, ethylene methyl acrylates, ethylene butyl acry-lates, very low density polyethylenes, ultra low density polyethylenes, Tafmers, as well as metallocene catalyzed ethylene alpha-olefins of lower densities. Generally speak- /920821.5/OLDFLDR 7 ing, most resins suitable for use in the seal layer will also serve appropriately as tie layer resins. A preferred tie resin is a high vinyl acetate EVA which promotes adhesion between the various layers of the film structure.

The following examples are intended to illustrate the preferred embodiments of the invention.

EXAMPLE 1 Sample films were prepared by coextrusion, i.e., all of the layers are extruded at once. The polymer melt from the extrusion dies was cooled and cast into solid sheets having a thickness of 7.46 mils.

P.P. EVA P.P. EVA P.P. EVA LLDPE 22% 12% 7% 6% 7% 8% 38% P.P. = Quantum Petrothene Polypropylene Homopolymer PP 2004-MR EVA = Exxon Ethylene Vinyl Acetate LD 720.92 19% V.A. LLDPE= Dow Dowlex Linear Low Density Polyethylene 2044A One of the sheets was tested for oxygen permeability on an OX-TRANS Ten-Fifty Oxygen Permeability Tester (Test Method E-160) and the results for three cuts were 1061.8; 1077.4; and 1037.0 cc/m2/24 hr.-atm, respectively. /920821.5/OLDFLDR 8 248250 TABLE I LONGITUDINAL Specimen Stress at Strain at Modulus Number Max. Load Max. Load (psi) (%) (PSIX1000) 1 4017.5 942.0 34.128 2 4055.9 941.5 29.417 3 4020.3 942.0 42.016 Mean 4031.2 941.8 .187 TRANSVERSE Specimen Stress at Strain at Modulus Number Max. Load Max. Load (psi) (%) (PSIX1000) 1 3236.9 942.0 41.791 2 3407.0 942.0 43.214 3 3029.8 942.0 42.251 Mean 3224.6 942.0 42.418 EXAMPLE 2 Another sample film was prepared by blending 93% of LLDPE (Dowlex 2044A) and 15% ULDPE (Attane 4201) with about 2% of a master batch concentrate containing slip and antiblock additives. The antiblock master batch included Ampacet (10853). This heat sealing layer was coextruded into a film structure containing alternating layers of ethylene vinyl acetate and ethylene-polypropylene copolymer.

Films having the following formulation were cast into sheets having a thickness of 3.0, 3.5, 5.0 and 11 mils. 98% P.P.

EVA EPC EVA EPC EVA % ULDPE 2% Amp.l 83% LLDPE 2% Amp.2 12% % 13% 7% 13% 7% 33% /920821.5/OLDFLDR 9 24 & p.p.

EVA LLDPE Ainp.l Amp. 2 EPC ULDPE Exxon Escorene Polypropylene Homopolymer PD 3345-88 Exxon Ethylene Vinyl Acetate LD 720.92 19% V.A.

Dow Dowlex Linear Low Density Polyethylene 2044A Ampacet PP based slip masterbatch / Ampacet 40604 Ampacet LLDPE based masterbarch / Ampacet 10853 Exxon Escorene PD9302 3.3% Ethylene 3.8 M.F.

Dow Attane 4201 - 1.0 M.F. 0.912 Density A sheet of 3.0 and 3.5 mil film was tested for oxygen permeability on an OX-TRANS Ten-Fifty Oxygen Permeability Tester (Test Method E-160). The resultant transmission rates shown below in Table II are not as high as would be expected, especially in view of the data given on Example 1 for a 7.46 mil film. It is believed that the Permeability Tester employed is somewhat inaccurate at higher permeability. However, the present numbers represent minimum transmission rates which are well within the preferred transmissibility per mil range of 2000-3000 cc. mil/m2-24hr. atm at 73°F.

TABLE II 02 Transmission Rate (cc/m2/24hr-atm) SAMPLE 3.0 mil 3.5 mil 1st Cut 1776 1779 2nd Cut 1876 1799 3rd Cut 1965 1889 Samples of the 3.0 mil, 3.5 mil and 5.0 mil films were tested for tensile strength, elongation and modulus.. The results are shown in the tables below. /920821.5/OLDFLDR 24 82 5 TABLE III LONGITUDINAL1" Specimen Number Stress at Max. Load (psi) Strain at Max. Load (%) Modulus (PSIX1000) 1 2 3 5613.0 6052.8 6271.0 825.0 868.0 840. 31.732 28.928 30.373 Mean 5978.9 844.3 .345 L Mean thickness was 2 .793 mils.

TRANSVERSE2 Specimen Number 1 2 3 Stress at Max. Load (psi) 3643.9 3548.1 3917.8 Strain at Max. Load (%) 941.5 941.5 941.5 Modulus (PSIX1000) 30.595 25.263 29.452 Mean 3703.3 941.5 28.437 2Mean thickness was 2. 940 mils.

TABLE IV LONGITUDINAL1 Specimen Number Stress at Max. Load (psi) Strain at Max. Load (%) Modulus (PSIX1000) 1 2 3 6315.6 6101.6 6071.0 941.5 941.5 891.0 33.297 32.113 39.997 Mean 6162.0 924.7 .136 1 Mean thickness was 3 .68 mils.

TRANSVERSE2 Specimen Number 1 2 3 Stress at Max. Load (psi) 3548.6 3310.8 3559.3 Strain at Max. Load (%) 941.5 941.5 941.5 Modulus (PSIX1000) 35.332 28.852 25.209 Mean 3506.3 941.5 29.798 2 Mean thickness was 5.120 mils. /920821.5/OLDFLDR 11

Claims (21)

248 250 LONGITUDINAL3 Specimen Number 1 2 3 Stress at Max. Load (psi) 3204.4 3303.6 4573.1 TABLE V Strain at Max. Load (%) 751.5 751.5 942.0 Modulus (PSIXIOOO) 26.051 29.232 30.113 Mean 3703.7 815.0 28.465 1 Mean thickness was 5 .120 mils. TRANSVERSE2 Specimen Number 1 2 3 Stress at Max. Load (psi) 3491.8 3395.6 3484.1 Strain at Max. Load (%) 941.5 941.5 942.5 Modulus (PSIXIOOO) 35.107 33.745 29.576 Mean 3457.1 941.8 32.809 1 Mean thickness was 4 .980 mils. Although illustrated embodiments of the invention have been described in detail hereinabove, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be readily effected by persons of ordinary skill without departing from the scope of the invention as defined in the following claims. For example, although it is generally preferred that film structures in accordance with the present invention are coextruded, films which are laminated such as by adhesive lamination, heat and pressure or corona lamination are also within the scope of the present invention. 5/920821.5/OLDFLDR N.Z. PATENT On"ICE 12 [■ 7 DEC V.'uAT WE CLAIM IS: 248 250

1. A multilayer, thermoforaiable film comprising: a predominantly ethylene based sealing layer; and an outer heat resistant layer, wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer.

2. A multilayer film as set forth in claim 1, wherein said heat resistant layer comprises a propylene based polymer .

3. A multilayer film as set forth in'claim 1, wherein said heat resistant layer comprises a styrene based polymer.

4. A multilayer film as set forth in claim 1, wherein the melting point of the polymeric composition of the outer heat resistant layer is at least 10°F greater than that of the sealing layer.

5. A multilayer film as set forth in claim 1, wherein said sealing layer comprises an ethylene based polymer selected from the group consisting of low density polyethylene, ethylene alpha-olefin copolymers, ethylene vinyl acetate copolymers, ethylene alkyl acrylate copolymers, ethylene acrylic acid copolymers, and metal neutralized salts of ethylene acrylic acid or methacrylic acid copolymers.

6. A multilayer film as set forth in claim 5, wherein said sealing layer comprises a blend of two or more ethylene alpha olefin copolymers of differing densities and viscosities. !!"•11 ■■ — j N.Z. PATENT QrTiCn^;| 13 - 7 DEC m I 248250

7. A multilayer film as set forth in claim 1,' further including at least one internal layer.

8. A multilayer film as set forth in claim 7, wherein said internal layer is a heat resistant layer.

9. A multilayer film as set forth in claim 8, wherein said internal layer comprises a propylene based polymer.

10. A multilayer film as set forth in claim 9, wherein said propylene based polymer is polypropylene.

11. A multilayer film as set forth in claim 9, wherein said propylene based polymer is an ethylene propylene; copolymer having a major portion of propylene and a minor portion of ethylene. %

12. A multilayer film as set forth in claim 7, wherein said internal layer is a tie layer.

13. A multilayer film as set forth in claim 1, wherein said film is coextruded.

14. A multilayer film as set forth in claim 1, wherein at least said sealing layer and at least said outer heat resistant layer are bonded together by lamination.

15. A multilayer thermoformable film characterized by having an oxygen permeability of greater than 2000 cc. mil/m2 24hr. atm. at 73°F., comprising: . • . r a predominantly ethylene based sealing layer; and an outer heat resistant layer comprising a propylene based polymer. 248250

16. A multilayer film as set forth in claim '15, further including at least one internal layer.

17. A multilayer thermoformable web for use in a non-forming application, comprising: a predominantly ethylene based sealing layer; and an outer heat resistant layer wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer.

18. A package comprising: a first web, thermoformed into a cavity for receiving meat, said first web comprising a sealing layer and an outer heat resistant layer wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer; and a nonforming web comprising a sealing layer an outer heat resistant layer wherein the polymeric composition of the outer heat resistant layer has a melting point greater than that of the sealing layer; and wherein the nonforming web is positioned immediately adjacent the thermoformed web such that the respective sealing layers of each web are in direct contact and a heat seal is formed about the periphery of the thermoformed cavity. N.Z. PATE NT OFFICE 5/920821.5/OLDFLDR J 15 "7 DEC 1995 received 248 250

19. A multilayer, thermoformable film substantially as herein described with reference to the included examples.

20. A multilayer, thermoformable web substantially as herein described with reference to the included examples.

21 . A package substantially as herein described with reference to the included examples. f C C-O By We f their authorised Agents a.j. park & SON. t _n.z. patent office 16 " 1 DEC 1995 RECEIVED