CA1298681C

CA1298681C - Thermoplastic packaging film of low

Info

Publication number: CA1298681C
Application number: CA 564228
Authority: CA
Inventors: Thomas C. Warren
Original assignee: WR Grace and Co Conn
Current assignee: WR Grace and Co Conn
Priority date: 1987-09-17
Filing date: 1988-04-15
Publication date: 1992-04-07
Anticipated expiration: 2009-04-07
Also published as: BR8804799A; AU2231988A; US4820557A; MX165400B; JPH0199840A; JP2701238B2; AR245403A1; NZ226196A; AU617119B2

Abstract

ABSTRACT

Disclosed is a high abuse-resistance film suitable for making bags and pouches. The film has at least one layer comprising a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less.
The copolymer has a comparatively lower I10/I2 ratio than the corresponding ethylene/alpha-olefin with the same comonomer and essentially the same density and essentially the same melt index at condition 190/2.16.

Description

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THERMOPLASTIC PACKAGING FILM OF LOW Ilo/I~

The invention relates to thermoplastic, paclcaging films and bags or pouches made therefram. In particular this invention relates to films and bags having excel-lent abuse-resistance properties.

Thermoplastic films are being used in packaging of non-food and food products like meat, cheese, poultry and the like. Many attempts have been made to improve abuse-resistance (toughness or strength) without losing other properties such as processability characteristics.
Or if the films are oriented, without losing shirnk characteristics.

BACKGROUND OF THE INVENrION

A film known from U. S. Patent No. 3,741,Z53 to Brax et al comprises a core layer of a vinylidene chloride copolymer (saran) between a layer of ethylene-vinyl acetate copolymer and layer of cross-linked ethylene-vinyl acetate copolymer. Ethylene-vinyl acetate copolymer (EVA) has same improved properties over the previously used polyethylene. Vinylidene chloride copolymers are knawn barrier materials to fluids such as oxygen.

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404/870~07/7/1 ~

361~

As disclosed in U. S. Patent No. 4,064,296 to Bornstein et al the core layer may also be a hydrolyzed ethylene-vinyl acetate copolymer (EVOH). It has similar oxygen barrier properties as vinylidene chloride copolymers and offers the advantage that it may be irradiated without discoloration, which is further dis-cussed below.

Blends of linear low density polyethylene and ethylene-vinyl acetate copolymer in oriented barrier films are disclosed in U. S. Patent 4,457,960 to Newscme, which claims an oriented multiple layer poly-meric fi~m, comprising (a) a first barrier layer, said first layer having two opposing surfaces; (b) a second layer adhered to one said surEace, said second layer being 10% to 90% linear low density polyethylene and 90%
to 10% ethylene vinyl acetate; and (c) a third layer adhered to the other said surface, the composition of said third layer being selected fro.m the group consist-ing of (i) ethylene-vinyl acetate, and (ii) blends of 10% to 90% linear low density polyethylene with 90% to 10% ethylene-vinyl acetate.

U. S. Patent 4,640,856 to Ferguson et al, commonly assigned to W. R. Grace & Co., discloses a multi-layer, thermoplastic barrier filn having at least three layers comprising: (a) a layer consisting essentially of 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); and (d) the shrinkage of layer (a) controlling the shrinkage of the entire multi-layer barrier film, said multi-layer filn having been oriented and rendered heat shrinkable at a te~pera-ture below 100C. (212F.), said orientation temperature ,~

, being about 40F. or more below the melt temperature of said very low density polyethylene.
"Ll,DPE Properties Tied to ~ranch Distribution", Plastics Enqine~ , January 1987, by Larry D. Cady of Dow Chemical Co~pany, ~eep~ e~as ~isc~sses ~o~ linear l~w density ~o yet~y~enes ~ ~o ~a~e ~e sa~e ~ensi~y o~ ~.922, t~e same melt index of 1.0 to 1.1, the same I1o/I2 ratio of 7.5 to 7.6, and the same octene co~onomer, yet the LLDPE with an elution ~0 te~erature around B5C, ~Yigure 3~ exhibite~ a ~i~her dart impaet of 330g ~Table) t~an the LLDPE with an elution temperature around 100C (Figure 3) which exhibited a lower dart impact of 193g (Table).
~his lnvention seeks to provide a packaging film and hags made therefrom having excellent abuse resistance and thereby provide a minimal risk of breakages when bags made of the film material are utilized in automated loading processes. The main use of the bags is in packaging large cuts of meat, which often have bony projections and large cavities.
This invention also seeks to provide a heat-shrinkable material for films and bags having the above advantage yet retaining good shrinkability characteristics~ and good orientation processing characteristics.
SUMMARY OF THE INV~NTION
Therefore, the present invention provides a heat-shrinkable packaging film of improved abuse resistance comprising at least one layer of a linear copolymer of ethylene and an alpha-~: f,~

3L~ 6~L
64536-6~2 olefin with a density of about 0.935 ytcc or less selected from ethylene/alpha-olefin copolymer having (a) a low Ilo/I2 melt flow ratio of about 7.2 when said alpha-olefin is octene or (b) a low Ilo/I2 melt flow ratio of about 9.9 and a comonomer of hexene.
The corresponding film with a linear ethylene/alpha-olefin copolymer with the same comonomer and essentially the same density and essentially the same melt index at condition 190/2.16, but with a comparatively higher Ilo/I2 ratio, will not exhibit as good abuse resistance as the film of the invention. Ilo/I2 is a melt flow ratio of condition 190/10 to condition 190/2.16 as per ASTM D
1238. The "190" refers to the temperature in degrees centigrade;
the "10" and "2.16" refer to the kilogram loading. In a preferred embodiment the film is heat-shrinkable and/or contains a barrier layer.
In a preferred embodiment the invention also provides a thermoplastic, multi-layer, heat-shrinkabla packaging film having çxcellent abuse resistance, comprising an outside polymer layer, a heat sealing layer and an interior layer be~ween said sealing and said outside layers, wherein said interior layer comprises a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less, said ethylene/alpha-olefin copolymer having a comparatively low Ilo/I2 ratio. In an even more preferred embodiment, this shrinkable film contains a barrier layer.
The invention also provides a process for manufacturing such a thermoplastic, optionally multi-layer, heat-shrinkable packaging film comprising (I) extruding at least one layer of a 2~6~
6~536-642 linear copolymer of ethylene and an alpha-olefin with a den~ity of ahout 0.935 g/cc or less, said ethylene/alpha-olefin copolymer having a low Ilo~I2 ratio as deflned above. This process may fuxther include (II) orlen~ing the extruded polymer in at least one direction, and (III) recovering a heat-shrinkable polymeric film.
The invention also provides a thermoplastic, heat-shrinkable packaging film of improved abuse resistance, comprising (I~ a layer of a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having ~a) a low Ilo/I2 ratio of about 7.~ when said alpha-olefin is octene or (b) a low Ilo/I2 ratio of about 9.9 and a comonomer of hexene and, (II) a layer of barrier material.
The invention also provides a heat-shrinkable packaging film of imp~oved abuse resistance comprising a layer of a linear copolymer o~ ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a~ a low Ilo/I2 melt flow ratio of about 7.2 and a comonomer of octene or (b) a low Ilo/I2 melt flow ratio of about 9.9 when said alpha-olefin is hexene, said ethylene/alpha-olefin copolymer having a comparatively lower Ilo~I2 ratio than the corresponding ethylene/alpha-olefin copolymer with the ~ame comonomer and essentially the same density but a comparatively higher Ilo/I2 ratio, said packaging film exhibiting better abuse resistance than the corresponding film made of the ethylene/alpha-olefin copolymer having the hiyher Ilo/I2 ratio, wherein said 4a B

~86~l lower I1o/I2 ratio and said higher I1oJI2 ratio differ by an amount above about 0.~.
The invention further provides a thermoplastic, multi-layer, heat-shrinkable packaging film having excellent abuse resistance, comprising in direct surfaca-to-surface contact at least the 4-layer structure; an outside polymer layer, a barrier layer, and interior layer, a heat sealing layer, wherein said - interior layer comprlses a linear copolymer of ethylene and an alpha-olefin with a density of ahout 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I1o~I2 mel~
flow ratio of about 7.2 when said alpha-olefin is octene or (b) a low I1o/I2 melt flow ratio of about 9.9 and a comonomer of hexene.
In another aspect the invention also provides side sealed and/or end sealed bags made from the above-mentioned inventive film.

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DETAIT~n DESCRIPTION OF THE INVENTION

Suitable linear ethylene/alpha-olefin copolymers with a density of about 0.935 g/cc or less, for use in the films of the invention, belong to the class of polymers known as linear low density polyethylene (LLPPE) or very low density linear polyethylene (VLDPE).
They ~IUSt have a low Ilo/I2 ratio. By "low Ilo/I2 ratio" is meant that if other factors, such as (a) the melt index at condition 190/2.16 of ASTM D-1238 is essentially the same and (b) the density is essentially the same and (c) the comonomer is the same for two linear ethylene/alpha-olefin copolymers, then the film of the invention made with the one with the lower Ilo/I2 ratio will exhibit better abuse-resistance than the same film but made with the other. By "essentially the same melt index" is meant that the two melt indices, when measured at condition 190/2.16 of ASTM D1238, are preferably within 0.3 dg/minute, re preferably within 0.2 dg/minute, of each other. Most preferably the two are the same dg/minute. By "essentially the same density" is meant the two densities are preferably within about +0.05 g/cc, re preferably within about +0.03 g/cc, of each other. The difference between the higher Ilo/I2 ratio and the lower Ilo/I2 ratio should be above about 0.3, more preferably above about 1.0, most preferably above about 2Ø The greater the difference in Ilo/I2 ratios, the greater the abuse-resistance im-provement. For instance, Test Sample 2 in Table II be-low showed an Ilo/I2 lower than its ccmparison by about 8 and showed about a 100% improvement in abuse-resistance over its co~,parison. VLDPE and LLDPE are further discussed below.

~, 6~u The so called linear low density polyethylenes are copolymers of ethylene and varying amounts of higher alpha-olefins with e.g. 5 to 10 carbon atoms per mole-cule (U. S. Patent No. 4,076,698) or 3 to 8 carbon atoms per lecule (Published European Patent Application 120503 published October 3, 1984, assigned to Union Carbide), for example copolymers of ethylene and butene-l, copolymers of ethylene and octene-l, and the like. Depending on their density these materials are referred to as linear low density polyethylene ~LLDPE) or very low density linear polyethylene (VLDPE), the separation line being at a density of abcut 0.910 g/cc.
Some properties of VLDPE have been described in Plastics Technology, September 1984, page 113. In October 1984, on page 13 of Plastics Technology, was another article describing VLDPE entitled "New Kind of Polyethylene Combines Flexibility, Toughness, Heat Resistance", which lists properties of VLDPE and compares them with EVA. According to the article, two commercially avail-able grades from ~nion Carbide of VLDPE are designated "DFDA-1137 NT7", which has a narrow molecular weight distribution, higher toughness, clarity, and gloss and FDA clearance for food contact, and "DFDA-1138" which is aimed particularly at film, has a broad ~olecular weight distribution, and is superior in processability. VLDPE
is also described in a company brochure published in Febr~ary 1984 by DSM in the Netherlands and entitled "Stamylex PE". Their properties are said to be a unique combination between those of standard polyethylene and polyolefinic rubbers. Their sealability and their compatibility with other polymers has been mentioned.

Accordingly, the term "linear low density poly-ethylene" (LLDPE), as used herein, refers to the newer copolymers of a major amount of ethylene with a minor amount of one or more comonomers selected from C3 to .~
~ 40~/870807/7/6 ~z~

about C10 or higher alpha-olefins such as butene-l, pentene-l, hexene-l, octene-l, etc. in which the mole-cules thereof cGhprise long chains with few side chains or branched structures achieved by low pressure poly-merization. The side branching which is present will beshort as conpared to non-linear polyethylenes. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus may be weakened by energy applied in the form of heat. Linear low density polyethylene has a density preferably in the range from about 0.911 g/cc to about 0.935 g/cc, more preferably in the range of from about 0.912 g/cc to about 0.928 g/cc for film making purposes. The melt flow index of linear low density polyethylene generally ranges from between about 0.1 to about 10 grams per ten minutes and preferably between from about 0.5 to about 3.0 grams per ten minutes. LLDPE resins of t~is type are co~!mercially available and are manufactured in low pressure vapor phase and liquid phase processes us-ing transition metal catalysts. The very low density linear low density polyethylenes (VLDPE) have a density from about 0.910 g/cc to about 0.860 g/cc, or even lower.

Optionally, the films of the invention have a barrier layer such as a layer of EVOH or saran or nylon.
Also, the films may be heat-shrinkable, i.e. oriented, i-f desired. In the oriented, heat-shrinkable, preferred embodiment, the low Ilo/I2 typically does not cause a loss in other properties, i.e. orientation speed and percentage shrink remain essentially the same and sometimes even improve.

Typically, in the manufacture of films, a suitable polymer usually in the form of pellets or the like, is brought into a heated area where the polymer feed is melted and heated to its extrusion temperature and ex-truded as a tubular "blc~n bubble" through an annular 404/~70807/7/7 - ~L2~

die. Other methods, such as "slot die" extrusion wherein the resultant extrudate is in planar, as opposed to tubular, form are also well known. If heat shrink-able film is desired, then after extrusion, the filln is typically cooled and stretched, i.e. oriented by "-tenter framing" or by inflating with a "trapped bubble", to impart the heat-shrinkable property to the fi~n, as is further described below. If desired, irradiation, typically via an electron beam, may take place after but preferably takes place prior to the stretching for orienting the fi~n. However, for the present invention, such irradia~ion is not necessary since a very suitable packaging fi~n is obtained without irradiation.

In the preferred embodiment as illustrated in the Egample below, the fi~n is an oriented, barrier fi~n with some of its layers irradiated; therefore, below is first described in detail the general process for making and orienting fi~n. Then irradiation is described in detail.

More particularly, the manufacture of shrink, i.e.
oriented, films may be generally accomplished by ex-trusion (single layer films) or coextrusion (~,ulti-layer films) of thermoplastic resinous materials which have been heated to or above their flow or melting point f-r~n an extrusion or coextrusion die in, for example, either tubular or planar (sheet) form, followed by a post ex-trusicn cooling. The stretching for orienting the film may be conducted at some point during the cool dcwn ; while the film is still hot and at a temperature within its orientation temperature range, followed by complet-ing the cooling. Alternatively, after the post extrusion cooling, the relatively thick "tape" extrudate is then reheated to a temperature within its orientation temper-ature range and stretched to orient or align the crystallites and/or molecules of the material, and then -` 129~

cooled again. The orientation temperature range for a given material or materials will vary with the different resinous polymers and/or blends thereof which colprise the material. However, the orientation temperature range for a given thermoplastic material may generally be stated to be below the crystalline melting point of the material but above the second order tr~nsition temperature ~sometimes referred to as the glass transi-tion point) thereof. Within this temperature range, the material may be effectively stretched to provide a heat-shrinkable filn.

The terms "orienting" or "oriented" are used herein to describe generally the process steps and resultant product characteristics obtained by stretching, trans-versely, longitudinally, or both (whether during the post extrusion cool dcwn or during reheating after the post extrusion cool dcwn as described in the paragraph above~ and substantially immediately cooling a resinous thermoplastic polymeric material which has been heated to a temperature within its orientation temperature range so as to revise the inte D lecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to ~ prove certain mechanical properties of the filn such as, for example, shrink tension and release stress. Both of these properties may be measured in accordance with ASTM
D 2833-81. When the stretching force is applied in one direction, monoaxial orientating results. ~hen the stretching force is applied in two directions, biaxial orientating results. The term oriented is also herein used interchangeably with the term "heat-shrinkable"
with these terms designating a material ~hich has been stretched and set by cooling while substantially retain-ing its stretched dimensions. An oriented (i.e.
heat-shrinkable) material will tend to return to its original unstretched (unextended) dimensions when heated ~04/870807/7/9 L2~
to an appropriate elevated temperature. However, by "orientation processing characteristics or properties"
as that te~m is used herein, it is specifically intended to mean the orientation speed during processing in making the oriented film. When it is intended to refer to the percent shrink of the film and bags made there-from, then the term "heat-shrinkability characteristics or properties" or the term "shrink characteristics or properties" is employed herein.

Returning to the basic process for manufacturing film as discussed above, it can be seen that the film, once extruded (or coextruded if it is a multi-layer film), is then oriented by stretching within its orien-tation temperature range. The stretching to orient may be accomplished in many ways such as, for example, by "trapped bubble" techniques or "tenter frami~g". These processes are well kno~n to those in the art and refer to orienting procedures whereby the material is stretched in the cross or transverse direction (TD) ; 20 and/or in the longitudinal or machine direction (MD).
After being stretched, the film is quickly cooled while substantially retaining its stretched dimensions to cool the film rapidly and thus set or lock-in the oriented molecular configuration.

The film which has been made may then be stored in rolls and utilized to package a wide variety of items.
If the material was manufactured by "trapped bubble"
tech~iques the material may still be in tubular form or it may have been slit and opened up to form a sheet of film material. In this regard, a product to be packaged may first be enclosed in the material by heat sealing the film to itself where necessary and appropriate to form a pouch or bag and then inserting the product therein. Alternatively, a sheet of the material may be ~` 35 utilized to overwrap the product. These packaging methods are all well kno~n to those of skill in the art.
404/870~07/7/10 ~3 6 ~3L

Wh~n a n~terial is of the heflt-sllrinkable ~i.e.
oriented) type, then af~er wrapplng, the enclosed pro-duct mHy be st1bjected to e]evated temperatures, for ex~mple, hy pfl~q~{n~ ~he ~nc]o.cted product thr~lgh a hot air tunnel or by placing the enclosed prodhtct in hot w~ter. This causes the enclosing heat shrir~cable film to sbrink around the product to produce a ti~ht wrappin~
that c]osely co~forms to the contour of the product. As stated above, the film s~teet or tube may be formed into bags or pouches and thereaf~er utilized to package a product. In this case, if the filn has been formed as a tube it may be preEerable first to slit the tubular film to fo~m a film sheet and thereafter form the sheet into ba~s or pouches. Such bags or pouches forming methods, llk~wlcse, are well known to those of sk;ll in the art.
:
The above general outline for manufacturing of films is not meant to be all inclusive since such pro-cescte~ are well known to thoce in the art. For exa~,ple, see U.S. Pat. Nos. 4,274,900; 4,299,241; 4,194,039;

4,18~,443; 4,048,428: 3,8~1,182; 3,555,604 and 3,022,563. The discloc7ures of these patents are g~ner~
ally r~presentative of such processes.

,, Alternative methods of producing filns of this type are kn~wn to those in the art. One w~ knt~n alterna-tive is t~e method of for~ing a multi-layer film by an extrusion coati~g in combination with an extrusion or coextru~ion process as was discu~sed above. In extrusion coating a first tubular layer or layers is extruded and t~ereater an addi~ional layer or layers is simultane-au91y or sequentially coated ~nto the outer surface of the irst tubular layer or a successive layer.

404/~70~07/7/ll '\

M~ly other process variations for forming films are well known to those in the art. For example, con-ventional thermoforming or laminating techniques may be employed. For instance~ multiple substrate layers may be first coextruded via a blown bubble tube with addi-tional layers thereafter being extrusion coated or laminated thereon, or two multi-layer tubes may be co-extruded with one of the tubes thereafter being ex-trusion coated or laminated onto the other.

In the preferred embodiments as illustrated in the ex~mple below, the multi-layer film of the invention contains a barrier layer. The layer is a ba-rrier to fluids such as gas. The barrier layer may be composed of a layer comprising vinylidene chloride copolymer (cammonly kncwn as saran), or composed of a layer cam-prising hydrolyzed ethylene-vinyl acetate copolymer (EVOH), preferably hydrolyzed to at least about 50%, most preferably to greater than about 99%, or composed of both a layer comprising vinylidene chloride copolymer and a layer ccmprising EVO~. The term "saran" or "PVDC", as used herein, refers to a vinylidene chloride copolymer wherein a major amount of the copolymer comprises vinylidene chloride and a minor amount of the copolymer camprises one or more unsaturated monomers copolymerizable therewith. Examples of unsaturated monomers copolym~rizable with the vinylidene chloride ' are vinyl chloride, acrylonitrile, and alkyl acrylates having 1 to 18 carbon atoms in the alkyl group. When the barrier layer is camposed of a layer comprising EVOH, the mole percent of vinyl acetate prior to hydrol-ysis shculd be at least about 29%, since for lesser amounts the effectiveness of the hydrolyzed copolymer as a barrier to fluids such as gas is substantially dimin-ished. It is further preferred that the barrier copolymer have a melt flow being generally campatible with that of the other components of the multi-layer ., . . ~ . ~ . . .

-~Z~3136i~
film, preferably in the range of about 3-10 (melt flow being determined generally in accordance with ASTM
D1238). The gas of main concern is oxygen and transmis-sion is considered to be sufficiently 1OWJ i.e. the barrier material is relatively gas i~,permeable, when the transmission rate is below 70 cc/m2/mil thickness/24 hours/atms, as measured according to the procedures of ASTM Method D-1434. The barrier layer of t~e m,ulti-layer barrier shrink film according to the pre-ferred film embodiment of the present invention has atransmission rate below this value. EVOH can be advan-tageously utilized in the film of the invention since irradiative high energy electron treatment of the fully coextruded film does not degrade an EVOH barrier layer, as could be the case for a vinylidene chloride copolymer barrier layer. It is also well known that many polyamides, i.e. commonly kn~wn a nylons, have an oxygen transmission rate below the 70 cc and thus also will serve well as the barrier material.
.:
When, as further discussed below, a vinylidene chloride copolymer (PVDC) is employed instead of or together with EVOH as the barrier layer, then the irradiation preferably should take place prior to application of the saran layer to avoid degradation thereof. This application may be achieved by well known extrusion coating methods, as discussed above. More particularly, the extrusion coating method of filn formation is preferable to coextruding the entire filn when it is desired to subject one or re layers of the film to a treatment which may be harmful to one or more of the other layers. Exemplary of such a sîtuation is a case where it is desired to irradiate with high energy electrons one or more layers of a filn containing a barrier layer comprised of one or re copolymers of vinylidene chloride (i.e. saran), such as of vinylidene chloride and acrylonitrile or such as of vinylidene ; 40~/870807/7/13 ' .,, , .. , - , 6~
chloride and vinyl chloride or such as of vinylidene chloride and methyl acrylate. (XU32027.01, supplied by Dow Chemical, is a copolymer of vinylidene chloride with methyl acrylate.) In other words, the barrier layer includes a saran layer in addition to or ins.ead of an EVOH layer. Those of skill in the art generally recognize that irradiation with high energy elec-t~ons is generally harmful to such saran barrier layer compositions, as irradiation may degrade and discolor saran, making it turn brownish. Thus, if full coext~usion and orientation followed by high energy electron irradiation of the ~lulti-layer structure is carried out on a filn having a saran layer, the irradia-tion should be done at low levels with care. Alterna-tively, this situ tion may be avoided by using extrusioncoating. Accordingly, by means of extrusion coating, one m~y first extrude or coextrude a first layer or layers, subject that layer or layers to high energy electron irradiation and thereafter extrusion coat the saran barrier layer and, for that matter, si~ultaneously or sequentially extrusion coat other later layers (which may or may not have been irradiated) onto the outer surface of the extruded previously irradiated tube and then orient the resultant. This sequence allows for the irradiative treatment of the first and later layer or layers without subjecting the saran barrier layer to the harmful discoloration effects thereof.

Irradiation may be accomplished by the use of high energy electrons, ultra violet radiation, X-rays, gamma rays, beta particles, etc. Preferably, electrons are employed up to about 20 megarads (MR) dosage level. The irradiation source can be any electron beam generator operating in a range of about 150 kilovolts to about 6 ~; megavolts with a pGwer output capable of supplying the desired dosage. ~he voltage can be adjusted to appro-priate levels which may be for example 1,000,000 or ~%~

2,000,000 or 3,000,000 or 6,000,000 or higher or lower.
Many apparatus for irradiating films are kno~n to those of skill in the art. The irradiation is usually carried out at a dosage up to about 20 MR, typically between about 1 MR and about 20 MR, with a preferred dosage range of about 2 MR to about 12 ~ . Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, 0C to 60C
may be employed.

In the Example below the multi-layer fi~ns were made by a conventional method of manufacturing, ccmbin-ing tubular coextrusion (colloquially called the hot blown bubble technique) with extrusion coating to achieve an oriented (heat-shrinkable) film. ~ tubular process was utilized wherein a coextruded tube of a multi-layer substrate core was extrusion coated with saran and another layer simultaneously, then the resultant structure was cooled and collapsed, and then reheated and biaxially stretched in the transverse direction and in the longitudinal machine direction via inflating the tube with a bubble. Then the stretched bubble was cooled and collapsed, and the deflated oriented filn wound up as flattened, seamless, tubular film to be used later to make bags, overwrap, et cetera.
Prior to the coating of the saran layer and the addi-tional layer, the substrate core was guided through an ionizing radiation field; for example, through the beam of an electron accelerator to receive a radiation dosage in the range of about 4 to 5 megarads (MR).

The LLDPE of low Ilo/I2 or VLDPE of low Ilo/I2 may be blended with one or more various other compatible polymers, said one or more other polymers preferably being present in a weight amount up to abcut 50%, more preferably less than about 35%, most preferably less than about 25%. These various other polymers also may 6~

be employed for the ir~ler heat sealing layer of the preferred ~lulti-layer barrier films of the present invention. Many of these other polymers are also suitable for use in any other layers of the films of the present invention, whether or not the films are barrier films and whether or not the films are oriented.
Suitable other polymers include, but are not limited to, ethylene vinyl acetate (EVA) copolymers, LDPE, HDPE, MDPE, polypropylene, ethylene/ propylene copolymers, ethylene/alkyl-acrylate copolymers (EAA~ [such as ethylene/methyl-acrylate (EMA), ethylene/ethyl-acrylate (EEA), and ethylene/butyl-acrylate (EBA~], acid modified EVA, copolymers of (i) and (ii) where (i) is an alpha-olefin of the formula RHC=CH2 wherein R is H or Cl to C8 alkyl and (ii) is an alpha, beta-ethylenically unsaturated carboxylic acid, and the like and mixtures thereof. Preferably, in the RHC=CH2 copolymer of an olefin and a carboxylic acid, the olefin is ethylene and the carboxylic acid is acrylic acid or methyl acrylic acid. Materials, which are the copolymer of an alpha-olefin having the formula RHC=CH2 wherein R is H
or Cl to C8 alkyl and an alpha, beta-ethylenically unsaturated carboxylic acid, representatively may be one of the Primacor (TM) polymers, supplied by Dow Chemical Company, Midland, Michigan. Primacor is produced by the free radical copolymerization of ethylene and a car-boxylic acid ccmonamer therefor such as acrylic acid or methacrylic acid. Also, the copolymer of an alpha-olefin having the formula RHC=CH2 wherein R is H or Cl to C8 alkyl and an alpha, beta-ethylenically unsaturated carboxylic acid may be metal salt neutralized such as with sodium, Na. Thus, the copolymer may be an ionamer.
Representatively, such an ionomeric material is con-mercially available as Surlyn (TM) from the E. I. du Pont de Nemours Company of Wilmington, Delaware, and is described in detail in U.S. Patent 3,355,319 and U.S.
Patent 3,845,163.

-" ~2~ 361~

DEFINITIONS

By "low Ilo/I2 ratio'l is meant the following. ~or two linear ethylene/alpha-olefin copolymers, if other factors, such as melt index at condition 190/2.16 is essentially the same and density is essentially the same and the comonomer is the same for the two linear ethylene/alpha-olefin copolymers, then the film of the invention made with the copolymer having the lower Ilo/I2 will exhibit better abuse-resistance than the corresponding film made with the copolymer having the higher Ilo/I2 ratio. By "essentially the same melt index" is meant that the two melt indices, when measured at condition 190/2.16 of ASTM D1238, are preferably within 0.3 dg/minute, more preferably within 0.2 dg/~inute, of each other. Most preferably the two are the same dg/minute. By "essentialy the same density" is meant the two densities are preferably within about i 0.05 g/cc, re preferably within about i 0.03 g/cc, of each other.

As used herein the term "extrusion" or the term "extruding" is intended to include coextrusion, ex-trusion coating, or combinations thereof, whether by ;~ tubular methods, planar methods, or combinations thereof.
~, A~ "oriented" or "heat shrinkable" material is de-fined herein as a material which, when heated to an appropriate temperature above room temperature (for exam~le 96C), will have a free shrink of about 5% or greater in at least one linear direction.

Unless specifically set forth and defined or otherwise limited, the terms "polymer" or "polymer resin" as used hereln generally include, but are not limited -to, hom3polymers, copolymers, such as, for example block, graft, randam and alternating copolymers, terpolymers, etc. and blends and modifications thereof.
Furthermore, unless otherwise specifically limited the term "polymer" or "polymer resin" shall include all possible molecular configurations of the material.
These structures include, but are not limited to~
isotactic, syndiotactic and randcm molecular configurations.
' 10The term "polyethylene", as used herein, refers to families of resins obtained by substantially polymeriz-ing the gas ethylene, C2H4. By varying the comonomers, catalysts and methods of polymerization, properties such ; as density, melt index, crystallinity, degree of branch-ing, molecular weight and molecular weight distribution can be regulated over wide ranges. Further modifica-tions are obtained by other processes, such as halogenation, and compounding additives. Low ~lecular weight polymers of ethylene are fluids used as lubri-;20 cants; medium weight polymers are waxes miscible with paraffin; and the high molecular weight polymers are ;resins generally used in the plastics industry. Poly-ethylenes having densities ranging from about 0.900 g/cc to about 0.935 g/cc are called low density polyethylenes (LDPE) while those having densities from a~out 0.935 g/cc to about 0.940 g/cc are called medium density polyethylenes (MDPE), and those having densities from about 0.941 g/cc to about 0.965 g/cc and over are called high density polyethylenes tHDPE). The older, classic low density types of polyethylenes are usually polymer-ized at high pressures and temperatures whereas the older, classic high density types are usually polymer-ized at relatively low temperatures and pressures.

., ~ 404/870807/7/18 %~
The term "ethylene vinyl acetate copolymer" (EVA) as used herein refers to a copolymer formed from ethy-lene and vinyl acetate monomers wherein the ethylene derived units in the copolymer are present ,n major amounts and the vinyl acetate (V~) derived units in the copolymer are present in minor amounts. For film form-ing purposes, it is desirable that the VA~content of the EVA be fron about 3% to about 25%.

The term "ethylene/alkyl-acrylate copolymer" (EAA) as used herein refers to a copolymer formed from ethy-lene and alkyl-acrylate wherein the alkyl moiety has 1 to 8 carbon atoms and the ethylene derived units in the copolymer are present in major amounts and the alkyl-acrylate derived units in the copolymer are present in minor amounts. Thus, the term "ethylene/methyl acrylate copolymer" (EMA) as used herein refers to a copolymer formed fron ethylene and methyl acrylate monomers. The term "ethylene/ethyl acrylate copolymer" ~EEA~ as used herein refers to a copolymer formed fram ethylene and ethyl acrylate monomers. The term "ethylene/butyl acrylate copolymer" (ERA) as used herein refers to a copolymer formed from ethylene and butyl acrylate ; monomers. Many suitable EBA's are commercially avail-able and these have a butyl acrylate content fram about 3% up to about 18% by weight. USI is a commercial supplier of Resin No. 4895, which is an EBA having about 3% by weight butyl acrylate and a melt index of 3 and a melting point of about 106 to 107C.

The follawing Example is intended to illustrate the preferred embodiments of the invention and comparisons ~; thereto. It is not intended to limit the invention thereby.

36~

MATERIALS EMPLOYED IN THE EXAMPLE

A suitable adhesive type of polymer employed in the preferred fi ~s of the invention is cammercially avail-able as Bynel C~A 3101. It is an ethylene-based adhe-sive with a cambination of ester and acid camonomerfunctionally, (i.e. an acid-modified EVA~ and is sup-plied by du Pont.

S~me of the LLDPE employed in the examples was Dowle~2045.03 having a melt index of 1.1 at candition 190/2.16 and a density of 0.920. It was supplied by Dow Chemical. The comonomer is octene-l.

Some of the l~,~PE employed in the Examples was XPRD545-36568-llA having a melt index of 1.0 at condition 190/2.16 and a density of 0.920. The co-monamer is octene-l. It was supplied by Dow Chemical.

Same of the VLDPE employed in the Examples was XPR05~5-37904-4H, having a melt index of 0.8 at condition 190/2.16 and a density of 0.905. The co-mon~mer is octene~l. It was supplied by Dow Ch~mical.

Dow XU 61512.08L is a VLDPE supplied by Dow Chemi-cal. It has octene-l as the comonomer. The density =
0.905; M¢ = 0.80 at condition 190/2.16.
;

Same of the LLDPE emplo~ed was DEFD 1630 supplied by Unian Carbide. The comonomer is hexene-l; Ml = 0.5 25 at condition 190/2.16; and density = 0.913.

Some of the VLDPE employed was DEFD 1629 supplied by Union Carbide. The camonamer of DEFD 1629 is hexene-l; Ml = 0.5 at candition 190/2.16; and density =
: O . 910 .

. . . -36~l The saran employed in some of the laboratory ex-a~lples was Ixan (TM) WV320 supplied by Solvay Corpora-tion. It is a copolymer of vinylidene chloride with vinyl chloride.

The EVA e~ployed in the laboratory examples was LD318.92, which is an EVA containing 9% vinyl acetate and having a melt index of 2.0 at condition 190/2.16. It was supplied by Exxon EXAMPLE
-Percentages indicated in the Example were calculat-ed as % by weight.

The films were made by first hot blowing -through an annular die a two-layer extruded tube of the structure:
LAYER l/LAYER 2 as the substrate. Then with a two-ply die, a layer of saran (barrier layer 3) and another layer (outside layer 4) were extrusion coated on. The resultant 4-layer structure was then cooled and col-lapsed. The tube was then reheated and oriented by stretching via a trapped bubble 4:1 in the transverse direction and 3:1 in the longitudinal direction for an overall biaxial orientation of 12:1. When such films were made into bags, the heat sealing layer 1 was the "inner" or "inside" layer as it was the bag "inside", and "outside" layer 4 was the bag "outside". The test layer 2 and the barrier layer 3 were "interior" layers of the multi-layer film.

The two-layer substrate was irradiated at 4.5 MR
prior to ~he coating on of saran and the outer layer.
Various properties, i.e. orientation speed, abuse resistance (Carson Dart Drop, or ball burst), Ilo/I2 ,. , ~8~
melt flow ratio and % shrink, were measured for the films as noted in the Table below. The ball burst and shrink were measured in accordance with procedures set out in ASTM D 3420 and ASTM D 2732, respectively. Ilo~I2 was measured in accordance with ASTM D 1238, and the ratio is that of condition 190/10 in dg/minute to condition 190/2.16 in dg/mi~lute.

The Carson Dart Drop is an impact test that mea-sures the impact resistance of film by the free-falling dart method. The apparatus employed for the free-falling dart impact was as described in ASTM D
1709. The dart weighed about 41.5 grams. Film samples of about 7 x 7 inches (17.8 x 17.8 cm) were cut.
Samples and equipment were allowed to equilibrate at room temperature for 36-40 hours prior to testing.
Three specimens of each kind of film were sequentially placed in the clamp of the apparatus with the inside (layer 1 of the bel~w films~ of the sample up. A
failure height was selected and the dart released. The height was lowered by 1 inch (2.54 cm) and testing of another three specimens repeated. The 1 inch lowering was successively repeated un-til all three samples did not break from the impact of the dart.

Films having 4 layers were made and as indicated in Table I below, the polymers for Layer 2 were varied for the various films that were made? whereas the polymers for Iayers 1, 3, and 4 were kept the same.

IABLE I

E~rRUSION
~; SUBSTRATE LAYERS COATED LAYERS
SEALING * BARRIER OUTSIDE

Polymer: EVA: LLDPESA~AN EVA
or VLDPE
:: Before 10 Orientation Thickness: 3 14.53.5 6.5 (mils)**
*Layer 2 was a blend of 7% Bynel CXA3101 and 93% LLDPE
by weight or was a blend of 7% Bynel CXA3101 and 93%
VLDPE by weight.

**After orientation, the total thickness of the 4-layer fi~m was about 2.2 mils to about 2.5 mils.

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As can be seen, for instance, fron Comparative Sa~ple 1 and Test Sample 1, the test film where the TTnPE of layer 2 had the lower Ilo/I2 ratio (7.18~
exhibited better abuse resistance (dart drop = 48.3 cm and ball burst = 46 cm-kg) than the comparative filn wherein the TI~npE of layer 2 'nad the higher Ilo/I2 ratio (8.07) which had a worse abuse resistance (dart drop -40.6 cm and ball burst = 45 cm-kg).

Likewise, as can be seen, from Comparative Sa~lple 2 and Test Sample 2, the test film where LLDPE of layer 2 had the lower Ilo/I2 ratio (9.90) exhibited better abuse resistance (dart drop = 27.9 cm and ball burst = 31 cm-kg) than the co~lparative film wherein the LLDPE of layer 2 had the higher Ilo/I2 ratio (17.9) which had a worse abuse resistance (dart drop = 15.2 cm and ball burst = 14 cm-kg).

Likewise also, for Co~parative Sample 3 and Test Sample 3, the film where the LLDPE of layer 2 had the lower Ilo/I2 ratio (7.29) exhibited better abuse 20 resistance (dart drop = 40.6 cm and ball burst = 46 cm-kg) than the film wherein the LIDPE of layer 2 had ~-~ the higher Ilo/I2 ratio (9.05) which had a worse abuse resistance (dart drop = 33.0 cm and ball burst = 37 cm-kg~.

W~ile certain representative embodiments and details have been shown for the purpose of illustration, numerous modifications to the for~,ulations described above can be made withou~ departing from the invention disclosed.

Claims

1. A heat-shrinkable packaging film of improved abuse resistance comprising at least one layer of a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I10/I2 melt flow ratio of about 7.2 when said alpha-olefin is octene or (b) a low I10/I2 melt flow ratio of about 9.9 and a comonomer of hexene.

2. The film of claim 1, wherein said copolymer of ethylene and an alpha-olefin of low I10/I2 ratio is in blend with another polymer compatible therewith.

3. The film of claim 2, wherein said layer of ethylene/alpha-olefin copolymer having a low I10/I2 ratio further includes up to 50% by weight, based on the layer composition, of a polymer which is compatible with said ethylene/alpha-olefin copolymer and is selected from ethylene/alkyl-acrylate copolymer, high density polyethylene (HDPE), linear medium density polyethylene (LMDPE), linear high density polyethylene (LHDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), ethylene/vinyl acetate (EVA), acid-modified EVA, polypropylene, ethylene/propylene copolymers, copolymers of an alpha-olefin having the formula RHC=CH2 wherein R is H or C1 to C8 alkyl and an alpha, beta-ethylenically unsaturated carboxylic acid, and mixtures thereof.

4. The film of claim 1, wherein said film is a multi-layer film including a barrier layer.

5. The film of claim 1, wherein said film is a multi-layer film and at least one layer of copolymer of ethylene/alpha-olefin of low I10/I2 ratio is an interior layer of the multi-layer film.

6. The film of claim 1, wherein said at least one layer of ethylene/alpha-olefin copolymer of low I10/I2 ratio has been irradiated at a dosage up to about 20 MR.

7. A bag formed from the film of claim 1, said bag having end seal(s), side seal(s), or a combination thereof.

8. The bag of claim 7, wherein the film of the bag is a multi-layer film including a barrier layer.

9. The bag of claim 8, wherein at least one layer of copolymer of ethylene/alpha-olefin of low I10/I2 ratio is an interior layer of the multi-layer film.

10. The bag of claim 7, wherein the film of the bag is a heat-shrinkable film.

11. A thermoplastic, heat-shrinkable packaging film of improved abuse resistance, comprising (I) a layer of a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I10/I2 ratio of about 7.2 when said alpha-olefin is octene or (b) a low I10/I2 ratio of about 9.9 and a comonomer of hexene and, (II) a layer of barrier material.

12. A heat-shrinkable packaging film of improved abuse resistance comprising a layer of a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I10/I2 melt flow ratio of about 7.2 and a comonomer of octene or (b) a low I10/I2 melt flow ratio of about 9.9 when said alpha-olefin is hexene, said ethylene/alpha-olefin copolymer having a comparatively lower I10/I2 ratio than the corresponding ethylene/alpha-olefin copolymer with the same comonomer and essentially the same density but a comparatively higher I10/I2 ratio, said packaging film exhibiting better abuse resistance than the corresponding film made of the ethylene/alpha-olefin copolymer having the higher I10/I2 ratio, wherein said lower I10/I2 ratio and said higher I10/I2 ratio differ by an amount above about 0.5.

13. A thermoplastic, multi-layer, heat-shrinkable packaging film having excellent abuse resistance, comprising in direct surface-to-surface contact at least the 4-layer structure; an outside polymer layer, a barrier layer, and interior layer, a heat sealing layer, wherein said interior layer comprises a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I10/I2 melt flow ratio of about 7.2 when said alpha-olefin is octene or (b) a low I10/I2 melt flow ratio of about 9.9 and a comonomer of hexene.

14. The film of claim 13, wherein said interior layer further includes up to 50% by weight, based on the layer composition, of a polymer which is compatible with said ethylene/alpha-olefin copolymer and is selected from ethylene/alkyl-acrylate copolymer, high density polyethylene (HDPE), linear medium density polyethylene (LMDPE), linear high density polyethylene (LHDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), ethylene/vinyl acetate (EVA), acid-modified EVA, polypropylene, ethylene/propylene copolymers, copolymers of an alpha-olefin having the formula RHC=CH2 wherein R
is H or C1 to C8 alkyl and an alpha, beta-ethylenically unsaturated carboxylic acid, and mixtures thereof.

15. The film of claim 8, wherein said interior layer of ethylene/alpha-olefin having a low I10/I2 ratio has been irradiated at a dosage up to about 20 MR.

16. A process for manufacturing heat shrinkable packaging film of improved abuse resistance comprising (I) extruding at least one layer of a linear copolymer of ethylene and an alpha-olefin with a density of about 0.935 g/cc or less selected from ethylene/alpha-olefin copolymer having (a) a low I10/I2 melt flow ratio of about 7.2 when said alpha-olefin is octene or (b) a low I10/I2 melt flow ratio of about 9.9 and a comonomer of hexene.

17. The process of claim 16 further including the steps of (II) orienting the extruded polymer in at least one direction, and (III) recovering a heat-shrinkable polymeric film.

18. The process of claim 17 further including extruding a barrier layer prior to the orienting step.

19. The process of claim 16, wherein said copolymer of ethylene and an alpha-olefin of low I10/I2 ratio is in blend with another polymer.

20. The process of claim 16 further including irradiating said layer of ethylene/alpha-olefin copolymer of low I10/I2 ratio at a dosage up to about 20 MR prior to the orienting step.