CA1284707C

CA1284707C - Oriented films from propylene copolymers and unplastized saran

Info

Publication number: CA1284707C
Application number: CA000495738A
Authority: CA
Inventors: Henry George Schirmer
Original assignee: WR Grace and Co Conn
Current assignee: Cryovac LLC
Priority date: 1985-01-30
Filing date: 1985-11-20
Publication date: 1991-06-11
Anticipated expiration: 2008-06-11
Also published as: JPH0725153B2; AU5115485A; NZ214460A; ZA859537B; BR8600321A; US4608302A; AU586999B2; JPS61175038A

Abstract

ABSTRACT
A heat-shrinkable thermoplastic film includes a food contact layer; a second layer of propylene homopolymer or copolymer, ethylene copolymer, butylene homopolymer or blends thereof; an adhesive layer; a layer of unplasticized vinylidene chloride copolymer; another adhesive layer; and an optical clarity layer.

Description

6~536-564 ORIENTED FILMS FROM PROPYLENE COPOLYMERS AND ~NPLASTICIZED SARAN

BACKGROUND OF THE INVENTION
This invention relates generally to heat shrinkable, relatively gas and moisture impermeable, thermoplastic packaging films which can be heat sealed.to ~lemselves ~o~forn aflexible package. The invention relates more partlcularly to packaging films for food products such as bakery goods, snack foods, cheese products, and candy where a high moisture vapor barrier and oxygen barrier packa~ing material is required.
There is a need i~ the food packaging industry for a packaging film which has improved moisture vapor and oxygen barrler characteristics.
A pa~kaging film for use ln connection with food products should exhibit several desirable characteristics. The film should be heat shrinkable so as to provide an attractively packaged food product. Additionally, ehe film should be heae sealable and have seal integrity under processing conditions, and in particular the heat sealed seams should resist being pulled apart during heat shrin~ing operations. The film should also be heat sealable to itself. Furthermore, the food contact layer of such a film must qualify under the appropriate food laws and regulations for safe food contact. Very importantly, the film must provide a good oxygen and vapor barrier, i.e. must possess a low permeability to both oxygen and ?~
. . ~

84~07 ~53~-564 water vapor to maintain the freshness of the food contained therein.
Also, the film must possess sufficient shrink energy such that upon the heat shrinking of a food product in a package made from the fllm in accordance with the inventlon, the film will shrink snuggly around the product contained therein, representatively about 30 to 50% blaxlal shrinkage at about 90C. In additlon to the foregoing, the film should desirably possess optical clarity i.e. the film should not become cloudy upon exposure ~o conditions of heat shrink, so that the consumer appeal of the packaged product is maintained.
ID general, such a multi-layer film seructure will have the minimal structure (sealing and food contact layer/shrink layer/barrier layer/sealing and food contact layer), a composite structure being re-guired to achieve the desired composite properties of the packaging film.
A preferred barrier material is unplasticized saran which refers to a family of thermoplastic resins, such as vinylidene chloride polymers, and copolymers of vinylidene chloride with other monomers such as vinyl chloride> acry'onitrile, methyl methacrvlate, acrylates, acrylate esthers, and vinyl acetate. Copolymers of vinylidene chloride with other monomers are generally described in terms of the weight ratio of the monomer units in the copolymer. Saran by definition, has at least 50% vinylidene chloride. The unplasticized saran of the present invention is a low viscoslty saran.
. ~ .
A heat shrinkable, thermoplastic, mulei-layer packaglng film which has enjoyed considerable com~ercial success for making bags having low gas transmission is described in U.S. Patent No. 3,741,253 issued on June 26, 1973 to Brax et al, which relates to a multi-layer film compris-in~ a first outside layer of an irradiated ethylene vinyl acetate ,, copolymer, a core layer of polyvinylidene chloride copol~mer, and a second outside layer of an ethylene vinyl aceta~e copolymer. Typlcally the process of manufacturing this type of oriented heat shrinkable film is a tubular orientation process wherein a primary tube of the fllm i6 biaxially oriented by stretching with internal pressure in ~he transverse dlrection and the use of pinch rolls at different speeds in the machine direction. This "blown bubble" technique is well known in the art. After the bubble is collapsed, the film is wound up as flattened, seamless, tubular film to be used later to make bags, for example either end seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-sealed bags in which the trans-verse seals form the bag sides and one edge of the tubing Eormsthe bag bottom.
This type of bag is used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gath-ered mouth of the bag to for~ a hermetic seal, and then im~erslng the bag in a hot water bath at approximately the same eemperature at which the film was stretch-oriented, typically about 160 to 205F> hot water immersion being one of the quickest and most economical means of transfer-ring sufficient heat to the film to shrink it uniformly. One problem which has been encountered is the failure of the bag seals at the bottom of the bags as the bag is shrunk around a product, the shrink forces tending to pull the seal apart.
Of interest concerning the present invent~on, is the disclosure of U.S. Patent No. 3,808,304 for "Oriented Blends of Polypropylene and . - . , ~ . . .
Poly-Butene-l" issued Aprll 30, 1974 to Schirmer, being directed to ., 70~
6~536~56 pachaging film made of such blends and having improved heat sealing and heat shr1nking properties in near boiling water.
Of interest is the disclosure of V.S. Patent No. 3,634,553 for "Heat Shrinkable Films of Polypropylene and an Ethylene/Butene CopolyMer"
issued January 11, 1972 to Foglia et al, being directed to or~ented thermoplastic films formed from blends of a high isotactic content polypropylPne with an ethylenelbutene-1 copolymer containing a minor a~ount of ethylene.
Of interest is the disclosure of U.S. Patent No. 4,207,363 for "Flexible Heat Shrinkable Multilayer Film For Packaging Primal Meat"
issued June 10, 1980 to Lustig et al, being directed to a heat shrinkable multi-layer film including a first outer layer comprising a blend of a propylene-ethylene copolymer, a butene-1/ethylene copolymer, and a thermoplastic elastomer; a first core layer capable of being stretched durlng orientation of the multi-layer film; a second core layer serving as an oxygen barrler and being suitable for the biaxial orienting and heat shrinking of the film; and a second outer layer comprising an ethylene/
vinyl acetate copoly~er; the multi-layer film being biaxially oriented.
Of interest is the disclosure of U.S. Patent No. 3,891,008 for "Multilayer Thermoplastic Film and Method for Making Same" issued June 24, 1975 to D'Entremont, being directed to a polymeric composition comprising a blend of polybutene-1 with an ethylene-propylene copolymer which, when combined with a shrinkable, cross-linked polymer of ethylene produces a laminate having improved abuse and tear resistance and reduced self-adherence.
Of interest is the disclosure of U.S. Patent No. 3,754,063 for "Method for Casting an Inflated Tubular Polyethylene ~ilm with Mixed ,, ~

1~8~7~7 6~536-564 Polymer of Polypropylene-Polybutene-1" issued August 21, 1973 to Schirmer, - being directed to a thermoplastic film having improved abuse characteris-tics including a layer of ethylene vinyl acetate copolymer on a tubular substrate of a predominantly ethylene polymer material with an outer layer of admixed lsotactic polypropylene, polybutene-l, and atactic polypropylene, the resulting film being oriented.
Of interest is the disclosure of U.S. Patent 4,448,792 for "Pasteurizable and Cook-in Shrink Bag Constructed of a Multilayer Film"
issued May 15, 1984 to Schirmer and assigned to a common ass;gnee with the present application. This disclosure is directed to a multilayer film lncluding a layer of propylene homopolymer or copolymer, a layer of a blend of the propylene polymer with butylene homopolymer or copolvmer, an adhesive layer comprising a copolymer of ethylene, a fourth layer of vinylidene chloride copolymer, a fifth adhesive layer of ethylene copolymer, and another layer of propylene homopolymer or copolymer, the - film being oriented and irradiated to at least a dosage sufficient to render the film delamination resistant during pasteuri~ng or cooking.

SUMMARY OF THE I~VENTION

The instant invention is directed to a multi-layer film sui~able for the packaging of food products.

In accordance with the present invention, there is provided a heat shrinkable thermoplastic fllm suitable for the packaging of food products which comprises a first, heat-sealablP food contact layer com-prising propylene homopolymer or copolymer; a second layer comprising a ~8~707 64536~564 propylene homopolymer or copolymer, ethylene copo~ymer of fractional melt index, butylene homopolymer, or blends thereof; a third or adheslve layer comprising a copolymer of ethylene; a fourth or barrier layer comprising an unplasticized vinylidene chloride copoly~er, a fifth or adheslve layer as in said third layer; and a sixth or optical c~arity layer as in said first or second layer.
The method of the invention provides for a process for making a heat shrinkable film comprising melt forming a tubular film of the above described structure; carrying said film through a hot air oven; and orlenting the film.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details are giYen below w~th reference to the drawings wherein:
- FIG. 1 is a schematlc cross-section of a preferred embodiment of a multi-layer fil~ of the invention; and FIG. 2 is a schematic illustration of a preferred mode of making the film of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to the drawings, in FIG. 1, a schematic cross-seetion of a preferred embodiment of the multiply film of the invention is s~own the film structure is directed to a multi-layer `.. ^ ; ..

6~536--56~

composlte having ~ general structure, beginnin~ with the inside layer, A/B/C/D/C/A where A is a sealing and food contact layer, B is a shrink layer, C is an adhesive layer, and D is a barrier layer.
Specifically, inside layer 11 serves as a heat sealing layer and as a food contact layer and is propylene homopolymer or copolymer, and more speclfically ls a propylene-ethylene copolymer, the ethylene content being minor in extent and sufficient to promote orientability. A repre-sentative commercially available resin suitable for inside layer 11 is W
827 polypropylene copolymer available from Cosden (formerly Arco) Chemical Company. This resin has a density of .890 grams per cubic centimeter and a melt flow index of 4Ø Other suitable resins include ARCO J258*avail able from Cosden (formerly ARCO) Chemical Company, a propylene copoly~er containing about 4.25% ethylene; and Eltex-P* KS400~ from Solvay, a polypropylene resin with about 3.8% ethylene by weight.
Shrink layer 12 can be composed of a propylene homopolymer or copolymer, and preferably the same propylene homopolymer or copolymer as in inside layer 11, or it may contain an ethylene copolymer to assist in orientation su~h as an EVA of 10-12% vinyl acetate content and of frac-tional melt index. "Fractional melt index" is used herein to define a material with a melt index of less than 1Ø "Melt Index" is used herein to indicate the amount, in grams, of a thermoplastic resin which can be forced through a given orifice under a specified temperature and pressure with ten minutes. The value should be determined in accordance with ASTM
D 1238.
Adhesive layer 13 is compatible with adjacent layers to provide delamination resistance and representatively is ethylene methacrylaee (EMA) or vinyl ac2tate (EVA) copolymer, such as commercially available *Trademark ; ~ 7 : ,., . , ~

6~536-564 Gulf resin 2205 being ethylene methacrylate copoly~er with 20%
methacrylate and balance ethylene and having a density of 0.942 and a melt index of about 2.4, or DuPont Alathon*3170.
An important feature of the present invention i6 barrier layer 14 which serves to inhibit transmission of oxygen and water vapor and ls compatible with said adhesive layer to provide delamination resistance, and more specifically is vinylidene chloride copolymer. Even more specif-ically, the barrier layer 14 is a low viscosity low molecular weight unplasticized saran. Conventionally, the addition of plasticizers to vinylidene chloride copolymers is necessary to allow for coextrusion of saran with other resins in a typical coextrusion process. It has been found th~t the use of a low viscosity unplasticized saran in accordance with the present invention permits coextrusion of the saran in combination with the other materials of the present film without the need for plasti-cizers, and also results in improved barrier characteristics in themultiply film. The improved barrier characteristics include those apply~
ing to water vapor transmission as well as to oxygen transm~ssion through the film. Suitable unplasticized sarans include either PV 858 or PV 864 manufactured by Solvay. These are similar resins, wieh PV 864 having a larger particle size than PV 858. These resins are representative of low viscosity vinylidene chloride copolymers. The relative viscosity of these representative examples is about 1.32. This value derived by calculating the ratio between the dynamic viscosity of 1% of the unplasticized saran in a tetrahydrofuran solution, and the dynamic viscosity of pure tetrahydrofuran. Melt viscosity of these examples is about 750 pascals (PA.S.) at 100 reciprocal seconds and 170C (measured by cone and plate rheometer). More generally, unplasticized sarans with viscosities of less *Trademark , ~

~l~89~ 7 6~536-564 than 10,000 polse and preferably between 1,000 and 5,000 polse at 100 reciprocal seconds at 170C are preferred. A small percentage of suitable solid saran stabilizers and suitable extrusion aid may be added to the saran.
Suitable unplasticized sarans to be used in conjunction wlth the present invention may also be characterized by molecular weight, and specifically number average molecular weights ranging fro~ 25,000 to 34,000, weight average molecular weights ranging from 53,000 to 68,000, and Z-avera~e molecular weights ranging from 78,000 eo 134,000 (tested by gel permeation chromotography compared ~ith known sample such as poly-styrene~.
Adhesive layer 15 is as ln said layer 13. Optical clarity layer 16 is as in said layer 11 and additionally provides a pleasingly clear appearance of the film.
As further discussed below the film is oriented. Representative-ly, the multiply film will have a thickness of about 1 mil, layer 11 will have a thickness of about .2 mils, layer 12 about .2 mils, layer 13 about .1 mils, layer 14 about .2 mils, layer 15 about .1 mils, and layèr 16 about .2 mils.
The term "oriented" and/or "orientation" are used herein to described the procèss and resulting product characteristics obtained by stretching a resinous orientable polymer thermoplastic material which is heated to its oriene~tion temperature range and then cooled in order loc~-in or freeze the molecular alignment of the material in the direction o~ stretching. This action improves the mechanical properties of the film, such as, for example, shrink tension and orientation release stress.
These properties may be measured in accordance with ASTM D 2838.

70~7 64536-s6a~
Alternatively, the extent of orientation may be characterized by the extent of unrestrained shrinkage with accordance and ASTM D
2732. The orientation temperature range for a glven film will vary with the different resinous thermoplastic materials or blends thereof which comprise the Eilm. However, the orientation temperature range may generally be stated to be above room temperature and below the melting point of the thermoplastic material or blend of materials. Orientation temperature ranges for the materials encompassed by the present application are preferably between about 190F and 210F, and a particularly suitable temperature range for the multiply film of the present invention is about 210F. When the stretching force is applied in one direction uniaxial orientation results; when the stretching force is applied in two directions, biaxial orientation results.
In a film made in accordance with the present invention, the degree of stretching to achieve desirable biaxial orientation is preferably about three times the original dimension in the machine direction and àbout four times the original dimension in the transverse direction.
In FIG. 2, a preferred method of making the film of the invention is schematically illustrated. A preferred film is melt-formed conventionally as by coextrusion (not shown) and the advancing tubular film at 21 is directed through a hot air oven 30 and onto nip rollers 22a,b and is biaxially oriented between nip roll sets 24a,b and 22a,b while subjected to internal pressurization at 23. The oriented film is advanced from nip rollers 24 and directed to take up roll 26 and is then ready for ..

. . .

8~707 64536-5~4 use in the packaging of food products.
It is emphasized that the film oE the present invention is preferably not irradiated and does not require an irradiation step, such as that shown for example in U.S. Patent No. 4,448,792 issued to the common assignee of the present invention.
Referring to U.S. Patent No. 4,448,7g2, that reEerence discloses an irradiated multi-layer film including a second or heat shrinkable layer of a blend of propylene polymer with buty-lene homopolymer or copolymer. An unirradiated specimen of about 0.051 mm film thickness showed a water vapor transmission at 100F
of .43 (grams/24 hours, 645 square cms) (Table l of the refer-- ence). An unirradiated specimen of 0.051 mm film thickness also showed an oxygen transmission at 73F, 0% ~H of 27 (CC/24 hours, square meter, atmosphere) ~Table l of the reference). The saran layer of the reference has a thickness of about 0.0051 mm.
In contrast, and by way of example, improved oxygen and moisture vapor barrier characteristics are obtained through the - practice of the present invention. ~ one mil blown film contain-ing an unplasticized saran, PV 864 vinylidene chloride copolymer manufactured by Solvay, the saran layer having a thickness of about 0.0063 mm, displayed a moisture vapor transmission of about .35 grams/645 square cms 24 hours 100F at lO0~ relative humidity.
This blown film included a heat-sealable layer of ethylene vinyl acetate copolymer, a second layer of linear low density poly-ethylene, a third or adhesive layer of ethylene vinyl acetate, a barrier layer of unplasticized vinylidene chloride copolymer, a fifth adhesive layer as in the ~8~7~7 , third layer, and a sixth layer of ethylene vinyl acetate. This film specimen also displayed a rate of oxygen transmission of : about 10 CC/square meter 24 hours at 73F and 0~ relativ0 humidity, and about 5 CC/square me-ter 24 hours 73F at 100 relative humidity.

lla :;.. ~' -: 6~536~56 Testing of moisture vapor transmission was done accordin~ to ASTM F 372. Testing of oxygen transmisslon was done according to ASTM D 3985.

Table 1 Samples Properties Sample 1 Sample 2Sample 3Reference ~ater Vapor .34 .33 .39 .43 Transm~ssion (1j @ 100F

10 Oxygen 11.5 8.5 10.5 27 Transmission (2) @ 73F, 0Z RH

Ilotes-~1) Units of grams/~24 hours, 645 square cms); appsoximate at ~ mil~
15 film thickness; ASTM F 372.

(2) Units of cubic centimeters STP (24 hours, square me~er, atmosphere); approximate at 2 mils film thickness; ASTM D 3985.
In another example, four films were prepared in accordance with the present invention, each film including a first heat sealable layer of propylene ethylene copolymer; a second layer of a blend of 50% ethylene propylene copolymer and 502 polybutylene; a third or adhesive layer of ethylene methacrylate copolymer; a barrier layer of unplasticized vinylidene chlor~de copolymer; a fifth adhesive layer of ethylene vinyl acetate with 17% vinyl acetate content; and a sixth layer as in said first layer. The first of these four films contained a barrier layer of PV 867 unplàsticized saran manufactured by Solvay; the second film, labeled Sample 5, contained a barrier layer of PV 864 unplasticized saran;

, .

~84707 64536-56~

Sa~ple 6 cnntained a barrier layer wlth a ~ixture of 50Z PV 864 and 50~ PV
867 and Sample 7 contained a barrier layer of PV 867 of greater ~hickness than that in the samples discussed above.
Generally, each of these film ~amples had a total thickness of about 2 ~ils.
Physical properties for Samples 1 through 4 appear below in Table 2.

Sample Sample Sa~ple Sa~ple Tensile at Break and 73F (PSI) Av. Long. 81.7 x 100 76.3 x 10072.0 x 100 74.3 x 100 Std. Dev. 4.2 x 100 3.7 x 1002.1 x 100 2.0 x 100 95Z C.L.2 6.8 x 100 5.9 x 1003.3 x 100 3.3 x 100 Av. Trans. 72.9 x 100 65.1 x 10062.3 x 100 60.2 x 100 Std. Dev~ 4.4 x 100 4.0 x 1003.5 x 100 4.4 x 100 95~ C.L. 6.9 x 100 6.3 x 1005.6 x 100 7.1 x 100 Elongation at Break and 73F ~PSI) 3 Av. Long. 162 124 123 127 Std. Dev. 9 5 3 95% C.L. 15 ~8 5 9 Av. Trans. 179 175 160 178 Std. Dev. 12 16 18 18 95Z C.L. 18 25 29 28 , ' ' 6~536-564 MOdU1US at 73F (PSI)4 AV. LOng 94.1 X 100081.5 X 100078.2 X 1000 93.7 X 1000 Std. DeV. 4.3 X 10001.3 X 10004.7 X 100~4.7 X 1000 95% C.L. 6.8 X 10002.0 X 10007.5 X 10007.6 X 1000 AV. Tran6. 96.8 X 100079.2 X 100069.3 X 1000 96.2 X 1000 Std. DeV. 5.7 X 10002.9 X 10003.3 X 10003.3 X 1000 95% C.L. 9.1 X 10004.6 X 10005.2 X 10005.3 X 1000 Ba11 BUrSt ImPaCt 10 at 73~ 1.00 ID.
Diam. SPhere Hd.
( cm . x kg . ) 5 AVerage 16.8 8.8 10.4 5.3 Std. DeV. 3.2 2.1 2.1 0.5 95Z C.L. 5.1 3.3 3.3 0.8 Water VaPOr Tr8nS~iSSiOn at 100F
SamP1e 1 0.18 0.20 0.25 0.19 Sample 2 0.23 0.21 0.24 0.18 Sample 3 0. 20 0. 24 0.29 0.16 OXYgen TranSmiSSiOn at 73F, 0Z RH 7 -Sample 1 4.6 5.7 4.9 2.8 SamP1e 2 3.2 S.4 4.5 3.3 Sa~P1e 3 4.6 5.9 ;6.6 3.1 Oxygen TranSmiSSiOn at 73F, 100% RH 8 ;
.~ ~
; 14 ''' ~

~84~)7 ' ~6~536-56~

Sample 1 4-4 5 9 3-7 2.1 Sample 2 3.0 5.5 3.6 2.8 Sample 3 4-4 6.0 5.0 2,6 5 Sample 1 4.5 6.0 3.6 2.0 Sample 2 3,1 5.6 4.0 2.9 Sample 3 4.5 6,0 5,2 2.2 Day 4 Sample 1 4.2 5,6 3.5 2.0 10Sample 2 2.7 5.1 4.0 2,9 Sample 3 4.1 5.6 5.2 2.2 Day 15 Sample 1 1.3 2.5 3.5 2.0 Sample 2 1.6 1.8 3.9 2.9 15Sample 3 2.1 1.3 5.2 2.2 Saran Gauge9 (Water Vapor Transmission Samples) Sa~ple 1 Avg. 0.64 0.42 0.30 0.46 20Std. Dev. 0.04 0.04 0.01 0.02 95% C.L. 0.05 0.05 0.01 0.03 Sample 2 Avg. 0.38 0.48 0.35 0.44 Std. Dev. 0.03 0.03 0.04 0.02 95% C.L. 0.04 0.04 0.05 0.03 25Sample 3 Avg. 0.44 0.33 0.27 0.60 Std. Dev. 0.02 0.05 0.01 0.01 95% C.L. 0.03 0.06 0.02 0.01 . ~ ~5 ! 64536-564 (Oxygen Transmission Samples) Sample 1 Avg. 0.40 0.40 0.41 0.69 Std. Dev. 0.01 0.01 0.02 0.02 95% C.L. 0.02 0.01 0.03 0.03 Sample 2 Avg. 0.60 0.39 0.46 0.55 Std. Dev. 0.02 0.03 0.02 0.03 95% C.L. 0.03 0.03 0.02 0.04 Sample 3 Avg. 0.39 0.41 0.27 0.54 Std. Dev. 0.02 0.02 0.01 0.02 95~ C.L. 0.02 0.03 0.01 0.03 Notes:

1. ASTM D882-81.
2. C.L. is confidence limit - For example, if the reported 15average value was 10 and the 95Z C.L. was 2, then of 100 replicate read-ings, 95 would have a value between 8 and 12 inclusive.

3. ASTM D-882-81.

4. ASTM D-882-81.

5. ASTM D-3420-80.
206. Units of grams/~24 hours, 100 square inches); approximate at 2 mils film thickness; ASTM F372.
7. Units of cubic centimeters STP/(24 hours, square meter, a~mosphere); approximate at 2 mils film thickness; ASTM D 3985.
8. Testing methodology is outlined in ASTM Journal of Testing 25and E~aluation, V~lume 12, Number 3, May 1984, pp. 149-1~1. Equipment is 1~8~707 64536-56~

Ox-Tran Oxygen Permeability Test Equipment. "Sandwich Method" of ASTM
article used in conjunction with manufacturer's (Modern Controls, Inc.) suggested procedures.
9. 100 gauge is equal to 1 mil. Saran ~au~e figules were optically measured, and correspond to data samples re: oxygen and mois-ture transmission rates found in Table 2.

A significant aspect of the present invention is the use of a low viscosity unplasticized saran as a barrier layer in the preseDt film.
The saran which is used is without the plasticizers such as those commonly used in the art in connectios with the coextrusion of fiIms including a saran layer. The present invention allows for the use of an unplasticized saran with a melt index and viscosity sufficiently low to still permit coextrusion of the saran along with the other layers of the present film.
It has been found that the use of a low viscosity unplasticized saran in combiDation with the present multiply film provides a heat shrinkable film suitable for food packaging wherein improved water vapor and oxygen transmission barrier characteristics are obtained; As an example, refer-ence is made to Table 1 and specifically to the water vapor and oxygen transmission data for the present invention as compared with the analogous data for the multi-layer film disclosed in U.S. Patent No. 4,448,792.
In use, the film of the invention is very suitable in con-ventional heat-shrinking operations to package food products such as snack foods, bakery goods, cheese products and candy.
Although the present invention has been described in conjunction with preferred embodiments it is to be understood that modifications and variations may be utilized without departing from the principles and scope ~, 7~
. .

of the invention, as those skilled in the art will readily understand. Accordingly such modifications and variations may be practiced within the scope of the following claims.

:'

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat-shrinkable thermoplastic film suitable for the packaging of food products comprising:
(a) a first, heat-sealable food contact layer comprising propylene homopolymer or copolymer;
(b) a second layer taken from the group consisting of propylene homopolymer, propylene copolymer, ethylene copolymer, butylene homopolymer, or blends thereof;
(c) a third or adhesive layer comprising a copolymer of ethylene;
(d) a fourth or barrier layer comprising a low viscosity unplasticized vinylidene chloride copolymer;
(e) a fifth or adhesive layer; and (f) a sixth or optical clarity layer comprising a propylene polymer.

2. The film of claim 1 wherein said first layer comprises a propylene ethylene copolymer, the ethylene content being minor in extent and sufficient to promote orientability.

3. The film of claim 1 wherein said second layer comprises a propylene homopolymer or copolymer as in said first layer.

4. The film of Claim 1 wherein said second layer comprises ethylene copolymer.

5. The film of Claim 4 wherein the ethylene copolymer of the second layer has a vinyl acetate content of between about 10 and 12% and a fractional melt index.

6. The film of claim 1 wherein said third layer comprises ethylene methacrylate copolymer.

7. The film of claim 1 wherein said third layer comprises ethylene vinyl acetate copolymer.

8. A method of producing a heat-shrinkable thermoplastic film suitable for packaging food products comprising:
(a) melt forming a tubular multi-layer film including a first, heat-sealable food contact layer comprising propylene homo-polymer or copolymer; a second layer comprising a propylene homo-polymer, propylene copolymer, or ethylene copolymer; a third or adhesive layer comprising a copolymer of ethylene; a fourth or barrier layer comprising a low viscosity unplasticized vinylidene chloride copolymer; a fifth or adhesive layer as in said third layer; and a sixth or optical clarity layer as in said first or second layer;
(b) passing the coextruded film through a hot air oven;
and (c) orienting said tubular film.

9. A method according to claim 8 wherein the coextruded film is passed through a hot air oven and thereby heated to a temperature of between about 190 F and 210 F.

10. A method according to claim 8 wherein the tubular film is oriented to a stretching ratio of about 3:1 in the machine direction, and about 4:1 in the transverse direction.