CA1325090C - Multilayer film including layer of very low density polyethylene - Google Patents

Abstract

ABSTRACT
Improved shrink, toughness and barrier properties are achieved by a multi-layer, thermoplastic, shrink film having a substrate layer of very low density polyethylene and a gas barrier layer of vinylidene chloride copolymer or ethylene-vinyl-alcohol.
A thermoplastic polymeric layer is located on the side of the barrier layer opposite to the polyethylene layer. The film is particularly useful for making bags for packaging large cuts of fresh red meat.

Description

This applicatisn is a divisional application of - -:
Canadian Patent Application Serial No. 502,615 filed on ~:
February 25, 1986. ~:
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FIELD OF THE INVENTION ~ .

Thi~ i~vention relates to thermopla~tlc packagin~ fllms and r-cept-cle~ such as pouches, ~J8, ant caslngs made tberefro~ In partic~
ul-r, ~hi~ lDveDtlon relates to plastlc fil3~ and ~ags which are heat ~brlDk~bl- aDd hav~ i~provet shrink, tear, barrler a~t puncture reslstance prop-rtle~
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~ACKGROUND O~ TH~ INVENTION ~ `
Shr~nkabl~ thermopla~tlc fllms having a gas barrier have found ~ -Y-ny u~eful applic-tloD~ in packaglng of meats, cheeses, poultry, ~od ~u~erou~ other food aDt ~o~-food -products. There is alway~ the search -~
lor i~prove~eDt ln thesc fil~s to make the~ have better barrle~ proper-tl~-, better ~bu~e resistance, bettcr tear resl~tance, lmproved clarity, snd ea der ~ ndl~ag.- Ona fll~ of thl- type 1~ a multi-layer fll~ having ~y~ of polyethylcn~/s~n/polyethyleD- ~hlch 1~ di~elosed in U S
Patent No 3,821,182 whlch issuet on June 28, i974 to Wllliam G Baird, .

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Jr. ee. al. The shrlnk and abuse resistance of such a film 18 1mproved by lrratiatiog the film to cros8-link the polyethylene layers prior to heating and orienting the fllm by the trapped bubble technique.
Another film laminate that has been quite successful 18 that dlsclosed in U.S. Patent No. 3,741,253 which is6ued on June 26, 1973 to Harri J. Brax et. al. Di~clo8ed in this patent is a multi-ply laminate which has a first layer of cross-llnked ethylene-vinyl acetate copolymer directly ~oined to a middle layer of a copolymer of vinylldene chloride which is ~oined to another ethylene-vinyl acetate copolymer layer. The ethylene-yinyl acetate copolymer (hereinafter EVA) layer has lmproved properties over the previously used polyethylene and, i4 the extruslon coatlng method used to produce the multi-layer fllm according to the Brax et. al. patent, the substrate EVA layer is preferably cros6-linked by irratiatiOD before the saran layer i8 extru~ion coated thereon thus avolding lrradlation of the saran layer. Saran (vinylidene chloride copolymer) tends to discolor under high energy irradiation.
An alternate and successful ~ulti-layer film where a hydrGlyzed ethylene-vin~l acetate copolymer is used as a barrier layer instead of saran i~ discloset in U.S. Patent No. 4,064,296 which issued on December 29, 1977, to Norman D. Bornstein et. al. A heat shrinkable multi-layer film i~ formet by coextruding the hydrolyzed ethylene-vlnyl acetate copoly er (someti~es abbreviated "~EVA" or called ethylene-vinyl alcohol and -abbre~iated "EVAL" or "EVO~".) Since EVOH does not suffer fro~ the effects of radiatio~ a coextruded product such as EVA/EVOH/EVA
can readily be cross-linked by irradiation before orientation.
~ nother way of improving the perfor~ance of packaging films has been to blend various polymers. In U.S. Patent No. 3,090,770 which 1s6ued : - ...... , ., ,:.:, -~.,: - -. . , . . ., .- ,.:. . - . ,. -. ,, .. :: -:. :- .. .... .-. .. ., : -on May 21, 1973 to Razmlc S- Gre~orian, the blending of cross-llnked polyethylene with non-cross-~inket polyethylene i8 di~closet to improve the clarlty of a film. Such blend6 were accompli6hed by using dlfferlng proportions of hlgh, low and medium denslty polyethylene. This Patent also ti6closet a cro~8-llnked polyethylene; and, U.S. Patent 3,118,866~
which issued on January 28, 1964 to the same inventor, is dlrected to an ethylene composltlon and the process of cross-llnklng by chemlcal means.
The olefln polymers and copolymers have been partlcularly attractive because of low cost, availabillty, and wlte range of satisfactory charac-terlstlcs for packaglng fllms.
Recently, the medium and low denslty l~Dear polyethylenes have become commercially avallable and have begun to be used ln a number of pac~aging appllcatlons. One of the early patents in thls fleld is U.S.

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Patent No. 4,076,698 which lssued on February 28, 1978 to Arthur Wlllia~
Anterson and dlscloses an interpolymer composed of ethylene and mono-alpha-olefinlc hydrocarbons conta~ning flve to ten carbon atoms per ~oleculc ant the proportion o~ the mono-oleflnic hytrocarbon belng 3 to 7 percent of the welght of the lnterpolymer with a melt index from 0.3 to 20 and a tenslty of 0.93 to 0.94. Llnear polymers of thls type are charac-terlzet by actually belng an lDterpolymer-or copolymer with another olefin ant havlng a relatlvely straight ~olecular chaln, that 16, having a chaln wlth no slde branches or llmlted slte branching. Low density versions of this type of fllm where denslty 18 in the range of O.g20 to 0.926 are produced by a low pressure process as opposed to ths high pressure process which produces a branched, low denslty polyethylene. Linear low denslty polyethylene~ abbrevlated herei~after as "LLDPE", has found many applica-tlons and uses as exempllflet by U.S. Patent No. 4,364,981 whlch lssued on " ~, 401/850409/2/3 ~
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,ecember 21, 1982 to Jerome T. Horner and tiscloses an EVA/LLDPE/EVA
structure a8 does al80 U-S- Patent No. 4,399,180 which lssued on August 16, 1983 to Wllliam F- Brlggs et. al. In U.S. Patent No. 4,457,960 a multi-layer structure is disclosed of EVA/Saran/EVA-LLDPE-blent.
Still another polymeric material hag mo~e recently entered the market having different propertles from the copolymers which comprlse the LLDPE cla6s of materials. The~e copolymers are known as very low density polyethylene ~hereinafter abbrevlated "VLDPE") and whereas conventional polyethylenes and LLDPEs have densltie~ as low as 0.912, the VLDPE' 8 currently on the market have densltles below 0.910, speciflcally, 0.900 to 0.906, and lt is thought that densltles as low as 0.860 will be forthcoming. European Patent Application No. 120,503 ~Union Carbide) has been published dlscloslng a method of maklng VLDPE. In "Plastics Technology" msgazine for Septembes 1984 at page 113, a news item eDtitled "Introduclng Very Low Density PE" briefly describet some of VLDPE's propertles and ~tated that lt's what the manufacturer "Calls an entirely new class of polyethylene, consistlng of llnear copolymers that can be produced at tensitles down to 0.89 or lower. What makes them special i8 an unique comblnst~on of properties in between those of standard PE's and polyoleflDlc rubbers". In the October 1984 ls6ue of "Plastics Technology"
at page 13 another artlcle appeared entitled "New Kind of Polyethjlene Combines nexlblllty, Toughness, Heat Resi~tance". This article lists a number of the properties of VLDPE and compares them with EVA and states that uses for thi~ material is for squeeze tubes, bottles, hoses, tubing, drum liners and film. VLDPE is also listed as having potential a8 an adtitive. It is expected to be used as a blending resin in high density polyethylene, polypropylene, EVA, and some EPR's, wlth all of which VLDPE
is compatlble.
401/85~409/2/4 . - . - . ... . . . . .. . .

Accortlng to the article, the first two commercially available grade6 are from Unlon Carbide and are de6ignated "DFDA-1l37 NT7", which has a narrow moleculsr weight dl8trlbution, higher toughness, clarity, ant glos6 and FDA clearance for food contact. The other secin 18 DFDA-1138 whlch 18 almed partlcularly at film, has a broad molecular weight dlstrlbutlon, and 18 cuperlor in processablllty. On page 15 in the ~ame article, lt is stated that "the new reslns have been in~ectlon molded, extruded, blow molded, and thermoformed on standard equipment". It is noted that blown fllm can be extruded on systems deslgned either for conventionsl LDPE or for LLDPE. ~owever, the company generally reco ends LLDPE-type scre~
de6igns in higher torque capabllity, especially with narrow-MWD grades.
The artlcle observes that the enlarged dle gaps requlred by LLDPE are not requlred for VLDPE and that conventlonal blown fllm dle gaps of 30-40 mil have proven satlsfactory at blow up ratios of 2-3:1. For blown fllm, DFDA-1137 and 1138 are sald to extrude much like Z-~I LLDPE or 0.5~
LDPF. An artlcle slmilar to the one ln "Plastics Technology" appeared ln the October 1984 lssue of "Plastlcs World" at page 86.
In the abo~e mentloned European Patent Applicatlon publlcatlon No. 120,503, publlshed October 3, 1984, a process for preparing very low denslty ethylene polymers ln a fluldlzed bet is described. These ethylene polymers are classlfled as ha~ing a density of less than 0.9l rams per cublc centlmeter and havlng a melt flow lntex whlch is preferably from 0.2 to 4.0-Now, returnlng to the speclfic application of polymers to~ackaglng fllm and-receptacles made therefrom, a very successful and useful fllm ls made accordlng to the process shown ln U.S. Patent No.
3,~41,253 mentioned above. A heat shrinkable bag can be made from ~uch 40ll850409~2/~

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1 3i~ '9`:~
fllm which has wite appllcatlon, partlcularly for meat, poultry, ant dairy products. In fact, heat shrinkable polymeric fllms have gained rather ~lde spread acceptance for packaglng meat, particularly fre~h meat and processed meat. Bags made from the hea~t ghrlnkable film are supplled to a meat packer being sealet at one end with the other end open and ready to receive a meat product. After the cut of meat is placed in the bag, the bag will normally be evacuated and the open end of the bag closed by heat sealing or by applying a metal clip. This process is advantageously c~rried out within'a vscuum chamber where the evacuatlon and application of the clip or heat seal ls done automatically. After the ba8 i8 removed from the chamber lt iB heat shrunk by applying heat which process can be ~-'-performet by i ersing the filled bag into a hot water bath or conveying it through a hot air tunnel.
In the usual distribution chain, a whole primal or subprimal i~
packaged ~ithln shrink bags of this type. The meat within the bag will --travel from a central ~laughter house ~here it hss been packaged to a retall supermsrket where the bag wlll ~e opened and the meat will be cut for re~all portions. Thu~, the bags of thi~'type must fiatlsfy a number of requirements whlch are lmposet by both the slaughter house or pa~king house and by the bag user. Furthermore, often the bag i8 placed in the show ca~e at the retail supermsrket for special promotions when a whole loin, for example, l~ to be sold to the consumer for his U8~. Ae thi~
poi~t, it i8 tesirabl~ to have an attractive package whereby there has been relatlvely complete 6hrinkage of the bag around the product 50 that the bag is not wrinkled and the blood and ~uices are not trapped in the folds of the ~rinkle~. Accordingly, it is one ob~ect of the present invention to provide a film sod receptacle made therefrom which has lmproved shrink characteristics over ba~s used in the past. ~' -4~1/850409/2~6 -. . :......... . .. . -, . .. : , : . . .

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Another lmportant characteristlc of a ba8 i8 the capablllty of the bag to physically survive the proce~s of be$ng fllled, evacuated, sealet, closed, he-t ~hrunk, boxed, shlpped about the country, unloaded, ant stored at the retail supermarket. Thi8 type of abuse rules out many polymeric films. Accordingly, it is another ob~ect of the present lnvention to provlde a new comblnat~on of polymerlc fllms whlch wlll withstand the abuRe of pack~ng, shlpplng, ant storlng.
Anotber feature requiret by bags uset for the foregoing tescribed applicatlon is that the bag must also be strong enough to survive the handllng involvet in moving packaged meat which may welgh 100 pounds or more or large chunks of cheese welghing 60 lbs. or more. In particular, when the chunk of ueat or cube of cheese i8 pushet into the bag its bottom seal must withstand the force of the meat or cheese as it hits the seal.
Al~o, in bags that are made by folding a sheet wlth the fold as the bottom of the ~ag and by ~ealing the side~, seal strength i8 quite an important factor. Accordingly, lt is stlll another ob~ect of the present invention to provide a bag which has improved seal streDgth over prevlously avallabie bag~.
It l~ also vcry desirable for the bag to serve as a barrler agalnst oxygen from the surroundlng atmosphere whlch will detrimentally affect the fresh meat protuct. Accortlngly, $t i8 yet another ob~ect of the present invention to provlde a flexible fll~ product which will ~alntaln during its packagi~g life tlme an effective barrier to gases and oxygen, in particular.
One of the more common hazards $n packaging ant distrlbuting products in flexlble packaglng materials is the hazart of the materlal recelvlng a puncture whlch will release the vacuu~ inside the bag ant allcw oxygen to enter. Anyehing from the appllcation of the clip to the : .

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pre~ence of a bone ln the meat can c8~8e a puncture. Accordiogly, it 16 an important ob~ect of the pre6ent lnvention to provide ~ fllm whlch has - 6uperior puncture resistance.

SUMMARY OF THE INVENTION
It ha~ been found that the above mentioned ob~ects are achieved by the present invention which, in one aspect, is a multi-layer, thermoplastlc barrier film includin& a layer compris$ng very low density polyeth~lene (VLDPE) having a density of 0. io6 grsms per cubic centl-~eter or lower; a barrier layer comprising a material selected from the group consisting of: (1) copolymers of vinylldene chloride and (2) hydrolized ethylene-~inyl acetate copolymers; and a thermoplastic polymeric layer, said layer being on the side of the barrier layer oppo-s$te that of the layer comprising very low density polyethylene. The thermoplastic poly~eric layer may advantageously compri6e a material ~elected fro~ the group consi~ting of: (a) ethylene. polymer~ and copoly~ers and (b) blend~ of polymers and copolymers.selected from group - (a). A pasticularly advantageous thermoplastic polymeric layer i8 one whlch comprises ethylene-vlnyl acetate copolymers or a very low density polyethylene. An unexpected result which .the.multilayer fllm of the invention glves 18 that while VLDP~ has a high melting point, e.g., 244-F, it can.be oriented out of.hot.water, l.e.r below the.boiling point sf ~aeer and hence it will be shrinkable below the boillng polnt. Such propertle~ greatly enhance its usefulness in psckaglng appllcatlons.
I~ another- aspect. the fil~ of the present in~ention i8 a multl-layer thermoplastlc barrler fllm includlng a layer comprlslng a very lo~ denslty polyethylene; and a layer comprising a polymeric barrier ,-:`: . ::~ ; :- :- . , , .: ,,: :

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material. Advantageously, the very low density polyethylene material may be cross-linked. A preferable method of cross-linking is to cross-link by irradiation although the material may be cross-linked by chemical means. Also, in certain instances where the barrier material is hydrolyzed ethylene-vinyl acetate copolymer, it may be advantageous to cross-link the barrier material.
In still another aspect, the present invention is a seamless tubular film made from any one of the multi-layer film combinations set forth above by a tubular or annular extrusion or coextrusion process.
In yet another aspect, the invention is a bag or pouch made from the film of the invention.
In a further aspect, the invention is a process for - making a multi-layer, thermoplastic barrier film having a layer of very low density polyethylene by orienting it below 100C (212F) -~
according to the process step~ described hereinafter. A feature of this aspect of the invention is that the very low density polyethylene layer of the multi-layer film may be cross-linked.
Accordingly, the invention herein comprises a multi~
layer, thermoplastic barrier film comprising: (a) a layer ~-comprising very low density polyethylene havinq a density of less than 0.910 gms/cc; ~b) a barrier layer comprising a material selected from the group consisting of: (1) copolymers or vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate ;
- copolymers; (c) a thermoplastic polymeric layer, said layer being ~
on the side of the barrier layer opposite to that of layer (a); ;

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said multi-layer film being oriented and heat shrinkable at a temperature below 100C (212F).
The invention also comprises a multi-layer thermoplastic barrier film comprising: (a) a layer comprising cross-linked very low density polyethylene having a density of less than 0.910 -gms/cc; (b) a layer comprising a polymeric barrier material; (c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite that of layer (a).

DEFINITIONS :

The term "polyethylene" (PE) as used herein refers to a family of resins obtained by polymerizing the gas ethylene, C2H4.
By varying the catalysts and methods of polymerization, properties such as density, melt index, cry~tallinity, degree of branching and cross-linking, molecular weight and molecular weight distribution can be regulated over wide ranges. Polyethylenes having densities ranging from about 0.915 to 0.925 are called "low density polyethylenesn. Those having densities from about 0.926 to about 0.940 are called "medium den~ity polyethylenes", and those 9a ` ~ 3250~0 having densitles above about 0.940 are callet "high den~lty polyethyl-enes". (See "Wh~ttington'g Dlctlonary of Pla6tlcg, 1978). Molecule6 ln conventional low denglty polyethylene are branched and linket in random fa6hlon, those ln the hlgher den~ity polyethylene6 are llnket in longer chalns with fewer 61de branches. Conventlonal low density PE i~ ~ometlmes called branched low denslty PE.
The term "llnear low density polyethylene ~LLDPE)" a~ used herein refers to copolymer6 of ethylene wlth one or more co-monomers selected from C4 to Cio alpha oleflns such as butene-l, octene-l, hexene-l, pentene-l, etc. in whlch the molecules thereof comprlse long chalns wlth fe~ branche~ or cross-linked structures. Thi6 copolymer i~ sometlmes called "lo~ pressure", low denslty polyethylene thereby referring to the polymerizatlon process which produces ie. Preferably, the den~ity ~hould be malntalned between 0.916 and 0.925.
The term "very low density polyethylene" as used hereln means llDear PE copol~mers havlng a denslty of less than O.9lO gm/cc and as low aa 0.ô60 or even lover. .
The ten~ "ethylene-vinyl acetate copoly~er" (EVA) as used hérein .
refer~ to a copolymer formed from ethylene and vlnyl acetate monomers ~herein the ethylene units are present in a ma~or amount and the vinyl-acetate units are present in a minor amount.
An "orlented" or "heat ~hrinkable" material ls defined herein ~8 a material which, when h~ated to an appropriate temperature absve room temperature-(to, for example 96C) will have a frse shrink of 5Z or greater in at lea~ one linear direction.

4Dl/850409/2/lO

.: : : . .: . : -1 3250qo DETAILED DESCRIPTION
A preferred method of making the film of the present invention is that according to the process outlined and described in U.S. Patent No. 3,741,253 (Brax et. al.). In this process the first or substrate layer of the film is extruded through a conven-tional tubular extruder whose die is modified to handle very low density polyethylene resin to form a tubular tape or film. The preferred resin is "DFDA-1137 NT7" from Union Carbide Corporation which has a density of approximately 0.906 grams per cubic centi-meter, a melt index of 0.721 grams/l0 minutes, a melting point of 244~F, anc is a butene based copolymer. The extrudate has a dia-meter of about 3 1/2 inches with a wall thickness of 19 to 20 mils as it leaves the die. After leaving the die the substrate i9 cooled and flattened. At this point it may be sent through an irradiation vault where it will be irradiated by high energy elec-trons to a dosage of preferably about 4.5 MR. Depending on the characteristics desired, this dosage can vary from 2 to 20 MR.
After leaving the irradiation vault the substrate i8 again inflat-ed and sent through a first tubular extrusion coating die where it receives a coating of about 3.5 mils of vinylidene chloride copolymer. After receiving the coating of vinylidene chloride copolymer the still inflated and now double-walled film passes through a second tubular extrusion coating die where it receives a layer of about 5.0 mils of ethylene vinyl acetate copolymer which has a vinyl acetate content of about 5%. After receiving the final coating, the film i~ cooled, collap~ed and rolled up. It is ` 1 325090 now a three layer tubular tape having a wall thickness of approxi-mately 27.5 mils. This tape is subsequently unrolled, fed through a bath of hot water held at 205 to 210F, preferably close to 210F, and as it leaves the hot .

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water lt i~ lnflated and blown into thln tublng where its wall thlckne6s will be 2.4 mlls. Thls 16 the trapped bubble technlque whlch 18 well known in the art. The fllm 18 rapldly coolet to set the orlentatlon and then rollet up for further processlng.
It i8 qulte fiurprlslng that the VLDPE havlng a melt temperature of 244F can be oriented at 205F, particularly, when the VLDPE 18 the substrate of i- barrler fllm. Normally, an olefin based poly~er would be expected to orient at not more than 10F to 15F below its melting polnt. For exa~ple, in the above mentioned U.S.P. 3,741,253 the EV~ h~s a melt polnt of sbout 205F and iA oriented at about 190F.
Slnce films shrink at or near orientation temperature, this means that packages mate from fllm according to the invention can be shrunk in hot water baths.
One further processing step can be taken to make end ~eal bag~ by transversely sealiDg and ~evering acros~ the seamless tubular film as it is laid flat to make indivldual bags. Side sealed bags may be made by ~lltting the se~less tubular f$1m along one of its edges after which it 1~ transversely sealed ant severed into bags. The ~ide seals are the sealiDg and ~everiDg seams aDd the bottom of the bag is the unsllt edge of the fil~. -Other bag and pouch making methods known ln the art may be readily adapted to making receptacle~ from the multi-layer film of the present i~vention.
The substrate film of ~ery low density p,olyethylene may be extruded as a monolayer substrate or coextruded as a multi-layer substrate and then irradiated depenting upon the desired char-cteristic~ of the final fllm. Furthermore, adtltlonal layers may be extrusion coated upon the lnflated substrate 80 that fllms havlng 4, 5, and 6 or more layers ~ay result.

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1 3250qO
~ n alternate method of manufacturing a fllm accordiog to the present invention i~ to employ the method dlsclosed ln U.S. Patent No.
4,379,117 whereln the extruder6 feed to a common coextruslon die whereln the inner layer 18 VLDPE polymer, the center layer 18 a ~inylltene chlo-rlde copolymer and the outer layer wlll be an ethylene vlnyl acetate copolymer. Thl8 fllm can be blown lnto a tube lmmedlately upon leavlng the coextruslon dle. If lt is desired to irradlate all three layers of film, lt is preferable to use EVOH in place of the vinylidene chlorlde copolymer layer as vinylldene chloride copolymers do not react well to radlatlon tendlng to turn an undesirable brow~lsh color. Agaln, the fllm 18 not limlted to three layers and will preferably be extruded ln the form of an uDstretchet tape after whlch it 18 lrradlated and then oriented by the ~ame type trapped bubblè technique as descrlbed above and ~hown and descrlbed U.S. Patent ~o. 4,379,117.
Returnlng no~ the process of U.S. Patent No. 3,741,253, three runs were made whlch de Dstrate the lmproved a~d unexpected results of u~ing a YLDPE layer iD conJuDctlon wlth a vlnylidene chlorlde copolymer (PYDC) layer to ma~e an improved barrier fllm and receptacle.
In the control ssmple a film accorting to U.S. Patent No.
3,471,253-wa~ produced~ l~e single_substrate 1ayer 8s disclosed in that patent is, ln the control sample actually two layers, l.e., the two layers to the left of t~e PVDC_film were coextruded_~nt irradiatet pFlor to receivlng ~ coating of PVDC aDd followed by a coating of EVA. The layer constructlon with the thickness i~ mils of each layer ls as follow~:
Conerol: EVA/EV~/PVDCtEYA
14.5/3.0/3.5/6.5 1~

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.: ~ . . : . . .. ~. . ... . . .. .. . .
:.: . -. . .- : - ~ -- ~ - , -The two EVA layers to the left were lrradistet to a do~age of 4.5 MR and represent the sub6trate. The tape was oriented from a hot water bath at 210-F and stretched to a wall thickDess of 2.4 mll6 by the trapped bubble technique.

EXAMPL~ 1 The thicker 6ubstrate layer (14.5 mils) was DFDA-1137 NT7, a VLDPE res~n fro~ Union Carbide. The process Dset was the 6ame as for the coDtrol sample and the layer construction was as follow6 having the same thicknesses as the control 6a~ple:
VIDPE/EVA/PYDC/EVA
t~is extruslon coating process, the EVA layer interposed between the VLDPE surface or 6kin layer and the PVDC layer is ~o placed to pro~ote adhesion between the layers snd to lessen any tendency of the film to delaminate as VLDPE does not adhere to PVDC as well as does EYA.
- . . ,'' ' '' The secoDd example ~as run in the same ~aDner as the first ~xample and-the control sample excepe that both outer layers of the film ~ere VLDPE DXFP-1137 resln. This;structure was as follows: -VLDPE/EVA/PVDC/VLDPF ' :-The tapes produced ln Examples i and 2 were oriented $n the same ~ -manner a~ the eontrol sa~ple. The physical properties of the film6 are set forth ln Table I below.

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TABLE I
Para~eter Control Example I Exsmple 2 Te~t Procedure Ten~ile Strength (PSI) Longitutlnal 8,490 8,430 11,300 ASTM D882-81 Transver~e 10,920 10,750 11,090 Elongation at Break (Percentage) 206 277 314 ASTM D882-81 Tear Propagation at ?3F (Grams) Longitutinal 22.75 480 258.75 ASTM D1938-79 Transverse 21.0 240 265.5 Free Shrlnk at l90-F
(Percent) Longltudinal 37 33 38 ASTM D2732-70 Transverse 51 48 47 Shrlnk Tenslon at 195~ (PSI) Longltudinal 248 183 234 ASTM D2838-81 Transverse - 303 342 385 Ball Burst I3pact at 73-F (CM~KG) 12.5 20.6 28.1 ASTM D3420-80 Optical Properties at 73-~
Haze (Percent) 2.8 4.1 8.7 ASTM D1003-61 -Water Vapor Transmisslo~
at 100-~ :
Grams/24 hours, lOb sq. ASTM F 372 in. 100~ R~ 0.76 0.49 0.48 (Gauge-mils) (2.21) (2.48) (2.38) Oxygen Trans2i~slon at 73-~
CC (STP/24 ~ours, sq. ~-~eter, ATM~ 39.2 31.3 26.4 ASTM D3985-81 (Gauge-mil~) - (2.42) (2.38)(2-37) The e~amples show a slgnificant and unexpected improvement in ball burst ~trengt~ which i~ direcsly related to the puncture resistance and 18 a highly-desirsble property~ln a film which wlll be used to package 401/85~40912J15 , ' ' a wlde variety of irregular article~, particularly those whicb may contaln bones and wlll be subJected to a variety of abuslve condltlons.
Another u~expected and beneflclal result is the fact that there wa8 8 reduction in the oxygen ~ransmi6~ion ra~e 80 that the film of Examples l and 2 demonstrated improved barrier properties. Normally, one would not expect the substitution of a lower density material (VLDPE) for a blgher density one (EVA) to result in a better barrier, partlcularly, siDce the polyethylenes, in general, have poor oxygen barrler properties as compared to saran, EVOH, nylon, etc.
The same result occurred with water vapor transmis6ion ln that lt was lowered by the 6ub6titutlon of the lower density VLDPE layerfi for tbe hlgher density EVA layers.
Furthermore, both films containing VIDPE layer~ ~howed i~proved elongatlon before breakage too~ place thus lndicating the film wlll stand more abuse before rupturing or tearing.
And, as stated above, a very surprising and unexpectet re~ult was that the VLDPE, having a melt point of 244F, could be oriented out of a hot water bath somc 40-F below lts melt point.
Bss-d on the ~ork tone above, three layer fllms c~mprising the follo~iDg ~tructures can-be made. The particular four layer structures mate a~o~e in the control and in Examples-l and 2 were-made fro~ a four e~trùder configuration using an--EV~-layer- between-the VLDPE-and-PVDC to ~:
promote layer-to-ls~er adhesion. Thus, all samples were run on the same equlpment. $he followlng-examples can be ~ade based on this work.
EYample j:--VLDPE/PVDC/EVA

(InsideOutsite tube wall) tube wall) -~,.

~ b Example 4: EVA/PVDC/VLDPE
(Inside (Outside tube wall) tube wall) Example 5: VLDPE/EVA/PVDC/VLDPE
(In3ide) (Outside) In this structure it i8 preferred that the inside VLDPE
and EVA layers be irradiated before the PVDC and outer VLDPE
layer~ are coated thereon.

Example 6: VLDPE/Adhesive/PVDC/Adhesive/VLDPE
(Inside) (Out~ide) This structure is preferred for maximum delamination protection. The adhesive can be an EVA of 10% or greater vinyl -~
acetate content, a Plexar* brand adhesive from Chemplex Company of -~
Rolling Meadow~, Illinois, or a CXA brand adh0sive from duPont Corporation of Wilmington, Delaware. The inside VLDPE and adhe-sive layers are preferably coextruded a~ a tube, cooled, flattened and irradiated while the remaining three layers are sequentially extru~ion coated onto the substrate after it has been irradiated and inflated.
Example 7: By coextruding according to the process of the above mentioned USP 3,821,182, a preferred structure as follows may be obtained: ~-*Trade-marX

VLDPE/Adhesive/HEVA/Adhesive/YLDPE, where the adhesives are selected as in Example 6 and the entire structure may be irradiated and oriented.
. .

~hile the present lnvention has been te~cribed with reference to tubular coextruslon and stretching by ehe trapped bubble technique other method~ of manufacture are available 6uch as coextruding the multilayer fll~ through a slot die and then stretchlng the fllm by u~e of tenter frames In adtltlon, ln certain applicatio~ blend~ of VLDPE, LLDPE and/or EVA may be u~et to achleve desired prope~tles.

Numerous layer combinatlon~ will become evident to tho~e skilled ln th- art upon readlDg thla dl~closure. The scope of ~he invention i- ll lt-d only by the followiDg claims~

;~ ' ."'' ''~

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401J850409/2/18 l~ :

Claims (34)

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

1. A multi-layer, thermoplastic barrier film comprising:
(a) a layer comprising very low density polyethylene having a density of less than 0.910 gms/cc;
(b) a barrier layer comprising a material selected from the group consisting of: (1) copolymers of vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate copolymers;
(c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite to that of layer (a); said multi-layer film being oriented and heat shrinkable at a tempera-ture below 100°C (212°F).

2. The film of claim 1 wherein the thermoplastic polymer of layer (c) comprises a material selected from the group consisting of: (1) ethylene polymers and copolymers, and (2) blends of poly-mers or copolymers selected from group (1).

3. The film of claim 2 wherein layer (c) comprises an ethylene-vinyl acetate copolymer.

4. The film of claim 2 wherein layer (c) comprises a very low density polyethylene.

5. The film of claim 2 wherein layer (c) comprises a linear low density polyethylene.

6. The film of claim 2 wherein layer (c) comprises a blend of a linear low density polyethylene and a very low density poly-ethylene.

7. The film of claim 1 wherein said film is in the form of a seamless tube with layer (a) being the face of the inner tube wall.

8. The film of claim 1 wherein the material of the layer comprising a very low density polyethylene has been cross-linked.

9. The film of claim 8 wherein the material of the layer comprising a very low density polyethylene has been cross-linked by radiation.

10. A bag formed from tubular film according to claim 7 wherein the bottom of the bag has been formed by a transverse seal across the flattened tube and the mouth has been formed by sever-ing the tube at a pre-selected distance from said transverse seal.

11. A multi-layer thermoplastic barrier film comprising:
(a) a layer comprising a cross-linked very low density poly-ethylene having a density of less than 0.910 gms/cc;
(b) a layer comprising a polymeric barrier material;
(c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite that of layer (a).

12. The film of claim 11 wherein the polymeric barrier material is cross-linked.

13. The multi-layer film of claim 11 wherein an adhesive layer is interposed between the barrier layer and the layer of very low density polyethylene.

14. The multi-layer film of claim 11 wherein said film com-prises at least five layers in which the center layer comprises a barrier material, the layers on each side of the barrier layer comprise an adhesive, and the surface layers comprise very low density polyethylene.

15. The film of claim 14 wherein the film has been formed by coextrusion.

16. The film of claim 14 wherein at least one layer is a cross-linked substrate layer upon which subsequent layers have been extrusion coated.

17. A multi-layer, thermoplastic barrier film having at least three layers comprising:
(a) a layer consisting essentially of very low density poly-ethylene having a density of less than 0.910 gms/cc;
(b) a barrier layer comprising a material selected from the group consisting of: (1) copolymers of vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate copolymers;
(c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite to that of layer (a); and, (d) the shrinkage of layer (a) controlling the shrinkage of the entire multi-layer barrier film said multi-layer film having been oriented and rendered heat shrinkable at a temperature below 100° C. (212° F.), said orientation temperature being about 40°F.
or more below the melt temperature of said very low density poly-ethylene.

18. The film of claim 17 wherein the thermoplastic polymer of layer (c) comprises a material selected from the group consist-ing of: (1) ethylene polymers and copolymers, and (2) blends of polymers or copolymers selected from group (1).

19. The film of claim 18 wherein layer (c) comprises an ethylene-vinyl acetate copolymer.

20. The film of claim 18 wherein layer (c) comprises a very low density polyethylene.

21. The film of claim 18 wherein layer (c) comprises a linear low density polyethylene.

22. The film of claim 18 wherein layer (c) comprises a blend of a linear low density polyethylene and a very low density poly-ethylene.

23. The film of claim 17 wherein said film is in the form of a seamless tube with layer (a) being the face of the inner tube wall.

24. The film of claim 17 wherein the material of layer 1 has been cross-linked.

25. The film of claim 24 wherein the material of layer 1 has been cross-linked by radiation.

26. A bag formed from tubular film according to claim 23 wherein the bottom of the bag is formed by a transverse seal across the flattened tube and the mouth is formed by severing the tube at a pre-selected distance from said transverse seal.

27. A multi-layer thermoplastic barrier film comprising:
(a) at least one layer consisting essentially of a very low density polyethylene having a density of less than 0.910 gms/cc and a melt temperature of about 244° F., said layer or layers of very low density polyethylene comprising at least 65% of the thickness of said multi-layer film; and, said very low density polyethylene layer being orientable below the boiling point of water;
(b) a layer comprising a polymeric barrier material; and, (c) a thermoplastic polymeric layer comprising a material selected from the group consisting of: (1) ethylene polymers and copolymers, and (2) blends of polymers or copolymers selected from group (1).

28. The film of claim 27 wherein the very low density poly-ethylene is cross-linked.

29. The film of claim 27 wherein the thermoplastic barrier material is cross-linked.

30. The multi-layer film of claim 27 wherein an adhesive layer is interposed between the barrier layer and the layer of very low density polyethylene.

31. The multi-layer film of claim 27 wherein said film com-prises at least five layers in which the center layer comprises a barrier material, the layers on each side of the barrier layer comprise an adhesive, and the surface layers comprise very low density polyethylene.

32. The film of claim 31 wherein the film has been formed by coextrusion.

33. The film of claim 31 wherein at least one layer is a cross-linked substrate layer upon which subsequent layers have been extrusion coated.

34. A side-sealed bag formed from the film of claim 27, said film being folded so that the fold forms the bottom of the bag and side seals close the side of the bag.