Food packaging film

Vinylidene chloride polymer compositions comprising a vinylidene chloride polymer and from about 2 to about 12 weight percent of a glycerin ester provide films which exhibit oxygen transmission rates sufficiently low to protect gassy cheeses from spoilage but sufficiently high to let any excess CO.sub.2 produced during the cheese curing process escape from the packaged gassy cheeses.

BACKGROUND OF THE INVENTION
 This invention relates to food packaging films and more particularly to
 packaging films for gassy cheeses.
 Vinylidene chloride polymers have good barrier to oxygen and are well-known
 to be useful in the fabrication of packaging films for oxygen-sensitive
 materials, such as fresh and processed meats. Gassy cheeses are specialty
 products which give off substantial amounts of CO.sub.2 after packaging.
 Although gassy cheeses require some oxygen protection, the packaging bag
 must be able to expel the excess CO.sub.2 produced during the cheese
 curing process. Thus, packaging films which are suitable for fresh and
 processed meats are not necessarily suitable for gassy cheeses.
 It would be desirable to provide a packaging film having an oxygen
 transmission rate which is sufficiently low to protect the gassy cheese
 from spoilage but sufficiently high to let any excess CO.sub.2 produced
 during the cheese curing process to escape from the packaged gassy cheese.
 SUMMARY OF THE INVENTION
 The present invention is, in one aspect, a vinylidene chloride polymer
 composition comprising (1) a vinylidene chloride polymer formed from a
 monomer mixture wherein the major component is vinylidene chloride and the
 remainder is at least one monoethylenically unsaturated monomer
 copolymerizable therewith, excluding vinyl chloride and (2) a glycerin
 ester represented by the formula:
 ##STR1##
 wherein each of R.sub.1, R.sub.2 and R.sub.3 is independently a hydrogen
 atom or a mixture of a first acyl group and a second acyl group, the first
 acyl group has 2 carbon atoms and the second acyl group has 10 to 14
 carbon atoms, the first acyl group is present in an amount, on the
 average, of up to 2 mol per mol of glycerin, the second acyl group is
 present in an amount, on the average, of at least 0.9 mol per mol of
 glycerin and the total amount of the acyl groups is, on the average, 2.7
 to 3.0 mol per mol of glycerin;
 the glycerin ester being present in an amount sufficient to provide the
 composition with an oxygen transmission rate of less than about 9
 cc-mil/100 in.sup.2 -atm-day.
 In a second aspect, this invention is a packaging film comprising the
 vinylidene chloride polymer composition described previously.
 In a third aspect, this invention is a multilayer film comprising a core
 layer of the vinylidene chloride polymer composition described previously
 and one or more outer layers of a polyolefin film.
 The vinylidene chloride polymer composition of the present invention can be
 used in fabricating packaging films for gassy cheeses.
 DETAILED DESCRIPTION OF THE INVENTION
 Vinylidene chloride polymers are known and are commercially available.
 Processes for preparing them, such as by emulsion or suspension
 polymerization process, are also familiar to persons of ordinary skill in
 the art. See, for example, U.S. Pat. Nos. 2,558,728; 3,007,903 and
 3,879,359, all of which are incorporated herein by reference. Typically, a
 mixture of vinylidene chloride monomers and one or more monoethylenically
 unsaturated monomers is emulsified or suspended in an aqueous phase. The
 polymerization of the monomer mix is usually carried out with heating and
 agitation. After polymerization is complete, the resulting suspension or
 emulsion slurry of vinylidene chloride copolymer has a majority of an
 aqueous phase. The resultant slurry is vacuum stripped. Thereafter, the
 slurry is cooled down, unloaded and dewatered, and the resin is collected
 and further dried.
 As used herein, the term "vinylidene chloride polymer" encompasses
 homopolymers of vinylidene chloride, and also copolymers, terpolymers,
 etc. thereof, wherein the major component is vinylidene chloride and the
 remainder is one or more monoethylenically unsaturated comonomer
 copolymerizable with the vinylidene chloride monomer, excluding vinyl
 chloride.
 Monoethylenically unsaturated monomers which can be employed in the
 practice of the present invention are those which can be polymerized with
 the vinylidene chloride monomers, excluding vinyl chloride, to form the
 vinylidene chloride polymers. Preferred monoethylenically unsaturated
 monomers include alkyl acrylates, alkyl methacrylates, acrylic acid,
 methacrylic acid, itaconic acid, acrylonitrile, methacrylonitrile, and the
 like. Preferred monoethylenically unsaturated monomers include
 acrylonitrile, methacrylonitrile, alkyl acrylates, and alkyl
 methacrylates. More preferred monoethylenically unsaturated monomers
 include acrylonitrile, methacrylonitrile, and the alkyl acrylates and
 alkyl methacrylates having from about 1 to about 8 carbon atoms per alkyl
 group. Most preferably, the alkyl acrylates and alkyl methacrylates are
 methyl acrylates, ethyl acrylates, and methyl methacrylates. The most
 preferred monoethylenically unsaturated monomer is methyl acrylate.
 Most preferred vinylidene chloride polymers include polymers formed from
 about 91 to about 94 weight percent vinylidene chloride and from about 6
 to about 9 weight percent of methyl acrylate.
 Glycerin esters which can be employed in the practice of the present
 invention include those which provide vinylidene chloride polymer films an
 oxygen transmission rate which is sufficiently low to protect the gassy
 cheese from spoilage but sufficiently high to let any excess CO.sub.2
 produced during the cheese curing process escape from the packaged gassy
 cheese. The preferred glycerin esters are those described previously in
 the summary of the invention. These glycerin esters are known in the art.
 See, for example, U.S. Pat. No. 4,426,477, incorporated herein by
 reference.
 Most preferred glycerin esters are the distilled acetylated monoglycerides
 commercially available from Eastman Chemical Company under the trademark
 KODAFLEX.TM. EPZ or MYVACET.TM. 9-08.
 While the amount of glycerin esters employed depends on a variety of
 factors, including barrier requirements and processability of the gassy
 cheese, in general, the glycerin ester is used in an amount of from about
 2 to about 12 weight percent, based on the weight of the composition. For
 certain gassy cheese formulations, the preferred amount of glycerin ester
 is from about 6 to about 9 weight percent, based on the weight of the
 composition and, for other formulations, the preferred amount of glycerin
 ester is from about 10 to about 11 weight percent, based on the weight of
 the composition.
 The vinylidene chloride composition of the present invention may also
 contain various additives to impart desirable properties such as, for
 example, heat stabilizers, light stabilizers, pigments, processing aids,
 lubricants and the like. Each of these additives is known and several
 types of each are commercially available.
 The vinylidene chloride polymer composition of the present invention can be
 prepared by adding the glycerin esters to the monomer mix before or during
 polymerization or to the dry vinylidene chloride resin by dry blending.
 Preferably, the composition is prepared by adding the glycerin ester to
 the monomer mix to ensure a more uniform distribution of the glycerin
 ester in the resulting polymer.
 If the composition is prepared by dry blending, the components should be
 mixed to form a visually uniform admixture. Suitable dry blending
 equipment includes Hobart mixers, Welex mixers, Henschel high intensity
 mixers, and the like.
 The vinylidene chloride polymer composition of the present invention can be
 coextruded into a multilayer film with conventional coextrusion machines.
 Multilayer films employed in packaging gassy cheeses, for example, can
 have one or more layers comprising the vinylidene chloride polymer
 composition of the present invention and one or more layers comprising a
 polyolefin, such as polyethylene. The multilayer film can also have a glue
 layer disposed between a vinylidene chloride polymer layer and an adjacent
 polyolefin layer. The glue layer typically comprises ethylene vinyl
 acetate or any polymer or copolymer having good adhesive properties.
 Films fabricated from the vinylidene chloride polymer compositions of the
 present invention containing from about 2 to about 12 weight percent
 glycerin ester, based on the weight of the composition, exhibit oxygen
 transmission rates of from about 0.6 to about 9 cc-mil/100 in.sup.2
 -atm-day. If the vinylidene chloride polymer composition contains from
 about 6 to about 9 weight percent glycerin ester, based on the weight of
 the composition, films fabricated therefrom exhibit oxygen transmission
 rates of from about 1.5 to about 2.5 cc-mil/100 in.sup.2 -atm-day. If the
 vinylidene chloride polymer composition contains from about 10 to about 11
 weight percent glycerin ester, based on the weight of the composition,
 films fabricated therefrom exhibit oxygen transmission rates of from about
 5 to about 6 cc-mil/100 in.sup.2 -atm-day.
 The following working examples are given to illustrate the invention and
 should not be construed as limiting its scope. Unless otherwise indicated,
 all parts and percentages are by weight.

EXAMPLE 1
 Fifty-pound blends of SARAN.TM. MA 119 (trademark of The Dow Chemical
 Company) (92.36 weight percent vinylidene chloride, 7.64 weight percent
 methyl acrylate) resin with various levels of distilled acetylated
 monoglyceride and other additives are prepared using a conventional high
 intensity blender provided with a jacket containing cooling water. The
 cooling water maintains the temperature of the blend at a temperature not
 exceeding 50.degree. C. The resin and additives are blended for 5 minutes
 and the blends aged for 2 weeks to allow the plasticizer to migrate into
 the resin particles. The blends are then extruded into blown films using a
 2.5-inch blown film extruder. Mineral oil is added to the internal side of
 the molten films to aid in bubble formation. Immediately after exiting the
 extruder, the films are quenched in a cool water bath to crystallize the
 polymer and then passed through a reheating bath to soften the films. The
 composition of the blends and the film properties are shown in Table I.
 TABLE I
 Epoxidized Acetylated
 SARAN soybean mono-
 MA EVA Stearamide oil glyceride
 (wt %) (wt %) (wt %) (wt %) (wt %) OTR.sup.1
 1 91.8 5.0 0.2 1 2.0 0.664
 2 89.8 5.0 0.2 1 14.0 0.693
 3 87.8 5.0 0.2 1 6.0 1.4715
 4 85.8 5.0 0.2 1 8.0 2.157
 5 83.8 5.0 0.2 1 10.0 4.6505
 6 81.8 5.0 0.2 1 12.0 9.02
 .sup.1 Reported as cc-mil/100-in.sup.2 -atm-day and determined for blown
 films (5-10 mil) at 23.degree. C. and 60% relative humidity according to
 ASTM Method D-3985-81.
 EXAMPLE 2
 Into a ten-gallon stirred polymerization reactor is loaded 14000 grams of a
 monomer mixture which comprises 91.42 weight percent of vinylidene
 chloride monomer and 8.58 weight percent of methyl acrylate. To the
 mixture in the reactor is added 896 grams of a distilled acetylated
 monoglyceride which is commercially available from Eastman Chemical
 Company under the trademark MYVACET.TM. 9-08 and thoroughly mixed with the
 monomers. Deionized water (18000 grams) containing 16.2 grams METHOCEL.TM.
 (trademark of The Dow Chemical Company) suspending agent is then added to
 the mixture.
 The reactor is sealed, purged with nitrogen and elevated to a temperature
 of about 25.degree. C. After polymerization has begun, the temperature is
 raised to 80.degree. C. and polymerization is allowed to continue for
 about 8 hours. The polymer slurry is then vacuum stripped of residual
 monomers, dried, and recovered.
 The resultant polymeric composition is extruded into blown films as
 described in Example 1. The oxygen transmission rate of the films is about
 2.5 cc-mil/100 in.sup.2 -atm-day as determined for 5-10 mil blown films at
 23.degree. C. and 60 percent relative humidity according to ASTM Method
 D-3985-81.