SE460534B - Multilayer film with core layers of polyene and polyethylene outer layers - Google Patents

Description

4so 534 *, with "heat shrinkability", these terms denoting a material which has been subjected to stretching and curing or solidification by cooling in its stretched dimensions. An oriented (ie, heat-shrinkable) material tends to return to its original unstretched dimensions when heated to the appropriate temperature below its melting temperature range.

Returning to the basic process for producing the film as discussed above, it can be seen that the film, once extruded and initially cooled to cool, is then reheated to its orientation temperature range and oriented . The stretching for orientation can be achieved in many ways, such as e.g. by the technique of "blown bubble" or "tenter framing". These terms are well known to those skilled in the art and refer to orientation steps, whereby the material is stretched in the cross-sectional or transverse direction (TD) and in the longitudinal or machine direction (MD). Once stretched, the film is rapidly cooled to cure or lock in the oriented molecular configuration.

After locking in the oriented molecular configuration, the film can then be stored in rolls and used to tightly package a variety of objects. In this respect, the product to be packaged is first enclosed in the heat-shrinkable material by heat sealing the shrink film to itself, when necessary. Thereafter, the sealed product is exposed to elevated temperature, e.g. by causing the product to pass through a hot, air or hot water tunnel. This causes the film to shrink around the product to provide a tight wrap that accurately matches the contours of the product.

The above general description of the production of films is not intended to be exhaustive, as this process is well known to those skilled in the art. See e.g. U.S. Patents 4,274,900, 4,229,241, 4,194,039, 4,188,443, 4,048,428, 3,821,182 and 3,022,543. The contents of these patents are hereby incorporated into the present text.

Many variations of the general process idea described above are available to those skilled in the art depending on the end use for which the film is to be used and the characteristics which it is desired to impart to the film. For example, the molecules in the film may be crosslinked during processing to improve the film's resistance to degradation and other characteristics. Crosslinking and methods of crosslinking are well known in the art. Crosslinking can be accomplished by irradiating the film, or alternatively it can be chemically accomplished using peroxides. Another possible process variation involves applying a fine mist of silicone spray to the inside of the newly extruded material to improve the further machinability of the material. One way of accomplishing such internal application is described in Canadian Patent No. 1,198,561, wherein its contents are hereby incorporated into the present text.

The polyolefin family and especially the polyethylene family of shrink films provide a wide range of physical and performance characteristics, such as shrinkage force (the force exerted by a film per unit area of its cross section during shrinkage), the degree of free shrinkage (the reduction in linear dimension in a particular direction that a material undergoes when exposed to elevated temperature in the free state), tensile strength (the maximum force that can be applied to a unit's surface of the film before it begins to tear, sealability, shrinkage temperature curve (the relationship between shrinkage and temperature), rivinitization and durability (the force at which a film begins to tear and continues to tear), optical properties (gloss, light diffusion and transparency of the material) and dimensional stability (the ability of the film to return to its original dimensions during different types of storage - conditions) Film characteristics play a significant role in choice t of a particular film, and they differ for each type of packaging application and for each package. The size, weight, shape, rigidity of the product, the number of product components, other packaging materials that can be used with the film and the available type of packaging equipment must be taken into account.

In view of the physical properties of polyethylene films discussed above and also in view of the myriad applications for which these films have already been used and for which they may be used in the future, it is readily apparent that the need to continually improve any of or all of the above-described physical properties of these films are large and, of course, present all the time. OBJECTS OF THE INVENTION Accordingly, a general object of the invention is to provide a heat-shrinkable polyolefin film which represents an improvement over the films which have been used previously according to the prior art.

Another object of the present invention is to provide a polyolefin film with improved shrinkage stress.

A further object of the present invention is to provide an improved polyolefin shrink film with improved optical properties. Yet another object of the invention is to provide a polyolefin shrink film having a wide shrinkage temperature range.

Another object of the present invention is to provide an improved polyolefin shrink film with improved sealability. Yet another object of the present invention is to provide a polyolefin shrink film with improved tear resistance.

Another object of the invention is to provide a polyolefin shrink film with improved machinability.

A further object of the present invention is to provide an improved polyethylene shrink film, in which either a low density linear polyethylene or a medium density linear polyethylene is used as a component in a core and / or intermediate layer.

These and other objects are achieved by the polyolefin shrink film described herein.

DEFINITIONS Unless otherwise indicated and defined or limited, the terms "polymer" or "polymer resin" as used herein generally include homopolymers, copolymers, terpolymers, block polymers, graft polymers, random and alternating polymers.

As used herein, the term "melt flow" or "melt flow index" refers to the amount, expressed in grams, of a thermoplastic resin that can be forced through a given orifice under a particular pressure and at a particular temperature for ten minutes, such as is described in ASTM D 1238. ~ ß. 460 534 The term "core" or "core layer" in the present case means a layer in a multilayer film which is surrounded on both sides by additional layers.

As used herein, the term "skin" or "skin layer" means an outer (ie, surface) layer of a multilayer film.

The term "intermediate" or "intermediate layer" in the present case refers to a layer of a multilayer film which is neither a core layer nor a skin layer.

As used herein, the term "low density polyethylene" (LDPE) refers to homopolymers of ethylene having a density of from 0.910 to 0.925. As used herein, the term "low density linear polyethylene" (LLDPE) refers to a copolymer of ethylene and 8% or less of butene, octene or hexene having a density of from 0.910 to 0.925 and wherein the molecules comprise long chains having few or no branches or cross-linked structures.

As used herein, the term "medium density linear polyethylene" (LMDPE) refers to a copolymer of ethylene and less than 8% butene, octene or hexene having a density of from 0.926 to 0.940 and wherein the molecules comprise long chains with few or no branches or cross-linked structures.

The term "ethylene-vinyl acetate copolymer" (EVA) in the present case refers to a copolymer formed of ethylene and vinyl acetate monomers in which the units derived from ethylene are present in major amounts and the units derived from vinyl acetate are present in minority amounts.

As used herein, the term "ethylene-propylene copolymer" (EPC) refers to a copolymer formed of ethylene and propylene monomers in which the propylene-derived moieties are present as the major component and the ethylene-derived moieties are present as a minority component.

The term "propylene homopolymer" (BP) in the present case refers to a thermoplastic resin having a density of approximately 0.90 and obtained by polymerizing propylene with suitable catalysts, as is well known in the art. SUMMARY OF THE INVENTION It has been found that a flexible, heat-shrinkable thermoplastic packaging film with the desired combination of physical properties, such as shrinkage stress, optical characteristics, cutability, sealability, shrinkage temperature range and tear resistance, has been achieved by the flexible, multilayer thermoplastic packaging film of the present invention. This multilayer film contains a core layer comprising low density linear polyethylene or medium density linear polyethylene and two skin or outer layers comprising an ethylene-propylene copolymer, and it has an average value of the free machine shrinkage at 93 ° C of at least about 12% and an average value of the free shrinkage in the transverse direction at 93 ° C of at least about 17%. A preferred three-layer embodiment also comprises, in addition to the above-mentioned "core" layers, two skin layers, each comprising a mixture of a propylene homopolymer and an ethylene-propylene copolymer. Preferably, the multilayer film is oriented so that it is heat-shrinkable in at least one direction.

The multilayer film can be combined with other polymeric materials for specific applications. For example, relatively thin layers can be added to either or both sides of the basic preferred three-layer structure to improve the sealing strength or to lower the gas and moisture permeability.

Another embodiment of the present invention is a five layer film structure. A preferred five-layer structure includes the same core and skin layers as the three-layer structure discussed above and further includes two intermediate layers, each comprising a blend of an ethylene-vinyl copolymer and either a low density ionomeric resin or a linear polyethylene. It is currently assumed that a linear polyethylene with medium density can also replace the linear polyethylene with low density in the intermediate layer.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a preferred three-layer embodiment of the present invention.

Figure 2 is a cross-sectional view of a preferred five-layer embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS From Figure 1, which is a cross-sectional view of a preferred three-layer embodiment of the present invention, it is seen that this embodiment comprises a core layer 2 and skin layers 1 and 3. The preferred thickness ratio for the three layers of 1/3/1 are demonstrated in Figure 1. A preferred component of the core layer 2 comprises a linear low density polyethylene polymer.

However, it can be assumed that medium density linear polyethylene polymer can instead be used as a component in the core layer without appreciable change in the properties of the final film product.

The core layer 2 comprises linear low density polyethylene (alternatively linear with medium density), or the core layer 2 may comprise a copolymer mixture of low density linear polyethylene (alternatively linear polyethylene with medium density) with either one (a) ethylene-propylene copolymer or (b) ethylene-vinyl 8C6tais fi mPolymef or (c) ethylene-vinyl acetate copolymer blended with an ionomeric resin or (d) low density polyethylene. Thus, the various mixtures of the core layer 2 in accordance with the present invention can be selected from the following groups. (1) 10-100% LLDPE mixed with o-90% EPC or 11 (2) 10-100% LMDPE mixed with 0-90% EPC or (3) 10-80% LLDPE mixed with 20-90% EVA or (4 ) 10-80% LMDPE mixed with 20-90% EVA or ä (S) 10-80% LLDPE mixed with 10-80% EVA and 10-80% ionomeric resin or (6) 10-80% LMDPE mixed with 10- 80% EVA and 10-80% nominal resin or n (7) 10-80% LLDPE mixed with 20-90% LDPE or (8) 10-80% LMDPE mixed with 20-90% LDPE.

LLDPE is used herein as an abbreviation for low density linear polyethylene as defined above. LMDPE is used herein as an abbreviation for medium density linear polyethylene as defined above. EPC is used herein as the abbreviation for an ethylene-propylene copolymer as defined above. EVA is used herein as an abbreviation for an ethylene-vinyl acetate copolymer as defined above. The term ionomeric resin is used herein to describe the group of ionomeric resins in general terms. One of the more notable of the ionomeric resins is marketed under the trademark Surlyn by du Pont.

My experiments have revealed that a particularly preferred composition for the core layer consists mainly of linear low density polyethylene. This material is available from Dow Chemical Company ZS 460 534 under the tradename Dowlex 2045.

To return to Figure 1 and especially to skin layers 1 and 3, suitable compositions for the skin layer can be selected from the following groups: (1) EPC or (2) 70-90% EPC mixed with 10 = soz PP or (3) 70-90% EPC mixed with 10-30% LLDPE or (4) 70-90% EPC mixed with 10-30% LMDPE.

For all abbreviations, what has been stated above applies in connection with the compositions for the core layers. In addition, PP is used herein as an abbreviation for a propylene homopolymer as defined above. Experiments have also shown that a particularly preferred composition for the skin layer essentially consists of a mixture of 20% PP and 80% EPC.

The propylene homopolymer can be obtained from Hercules Chemical Company under the tradename PD064. The ethylene-propylene copolymer can be obtained from Soltex Chemical Company under the tradename 42X01 or alternatively from Solvay Chemical Company under the tradename KS400.

For the present description and requirements, it applies that all percentages refer to weight percent. For the present description and requirements, it further applies that all references to density refer to g / cm 3.

In summary, my experiments have shown that a particularly preferred embodiment of the present invention comprises a core layer consisting essentially of linear low density polyethylene and skin layers consisting essentially of a mixture of ZOE propylene homopolymer and 80% ethylene-propylene copolymer. . Although the above-mentioned three-layer compositions are generally preferred over structures with more than three layers as a result of the economics associated with manufacturing, I have also produced various five-layer compositions, which are also satisfactory in terms of physical properties. However, the cost of making a five layer film is usually greater than that of a three layer film.

Figure 2, which is a cross-sectional view of a preferred five layer film of the present invention, shows the preferred layer thickness ratio of 2/2/1/2/2. The core layer 6 may comprise any of the core layer compositions discussed above in connection with the core layer 2 in the three layer embodiment. Dess- - Alathon 3137. The ionomeric resin can be obtained from du Pont Chemical, Acrawax C). A preferred silicone spray is a liquid polyorgano 9,460,554 except the core layer may consist essentially of either (1) an ethylene-propylene copolymer (EPC) or (2) an ethylene-vinyl acetate copolymer (EVA).

The skin layers 4 and 8 in the five-layer embodiment may comprise any of the skin layer compositions discussed above in connection with the skin layers 1 and 3 in the embodiment of Figure 1 with three layers.

The five-layer embodiment of Fig. 2 also includes intermediate layers 5 and 7. These intermediate layers may comprise any of the compositions described above in connection Z with core layer 2 in the three-layer embodiment. In addition, the composition of the intermediate layers 5 and 7 can be selected from the following additional groups. (1) EVA or (2) 20-80% LLDPE mixed with 20-80% ionomeric resin or (3) 20-80% LMDPE mixed with 20-80% ionomeric resin. pg) My experiments have shown that a particularly preferred five-layer structure comprises skin layers 4 and 8, which consist essentially of an ethylene-propylene copolymer (EPC), intermediate layers 5 and 7, which consist essentially of a mixture of 90% ethylene vinyl acetate copolymer and% ionomeric resin, and a core layer 6, which consists essentially of a linear low density polyethylene. The EPC in question can be obtained from Soltex Chemical Company under the tradename 42X01. EVA is available from du Pont Chemical Company under the trade name Company under the trademark Surlyn and has the name Surlyn 1601.

LLDPE can be obtained from Dow Chemical Company under the tradename Dowlex 2045. Those skilled in the art will readily appreciate that all of the above percentages by weight may be subject to certain variations. In addition, these percentages may vary somewhat as a result of incorporating or utilizing additives, such as the silicone mist mentioned above, or agents such as lubricants and anti-blocking agents. A preferred anti-blocking agent is silica, which is available from Johns Manvílle under the trade name White Mist.

Preferred lubricants are erucamide (marketed by Humko Chemical under the tradename Kemamide E) and stearamide (marketed by Humko Chemical Company under the tradename Kemm ß ß S) and N, N'-dioleoylethylenediamine (marketed by Glyco Chemical under the trade name Silox 4). General Electric under the tradename General Electric SF18-polydimethylsiloxane.

The general ranges for use of these additives are as follows: (1) Silica - 250 - 3000 ppm (2) Acrawax C: 200 - 4000 ppm (3) Erucamide: 200 - 5000 ppm (4) Stearamide: 200 - 5000 ppm (5 ) Silicone spray: 0,5ngft2 - and more When the term "consists essentially of" is used in the present description and claims, it is not intended to exclude certain percentage variations or additives and agents of this kind.

Additional layers and / or minor amounts of additives of the types described above may be added to either the three-layer or five-layer structure of the present invention, as described, but care must be taken not to adversely alter desirable compressive stresses, shrinkage properties, optical properties and other characteristics of the multilayer film of the present invention.

In the preferred process for making the low density or medium density linear polyethylene shrink film of the present invention, the basic steps are mixing the polymers for the various layers, coextrusion of the layers, forming a multilayer film, and then sanding. of the film to biaxially orient it. These steps and further desirable steps will be explained in detail below.

The process begins by mixing the raw materials (ie, polymeric resins) in the desired proportions and ranges as indicated above. The resins are usually purchased from a company in pellet form and can be mixed in any of a number of commercially available mixers, as is well known in the art. During the mixing process, any additives and / or agents that you wish to use are also incorporated.

The mixed resins and the used additives and / or agents are then fed into the hopper to extruders = * machines, which feed extruder nozzles. For the three-layer film, at least three extruders must be used, if each layer is to have a different composition. Two extruders feed the materials desired for the inner and outer skin layers, and the other extruder feeds the low or medium density linear polyethylene material desired for use in the core layer. . Additional extruders can be used, if desired. Preferably, the materials are co-extruded in the form of a tube with a diameter which depends on the stretching ratio and the desired final diameter. This coextruded tube is relatively thick and is called the "tape". Circular co-extruder nozzles are well known in the art and can be purchased from a number of manufacturers. In addition to tubular co-extrusion, slit nozzles can be used to co-extrude the material in a flat form. Well-known single or multilayer extrusion coating processes can also be used, if desired.

An additional process step that can be used is to irradiate the tape or the unexpanded tube or sheet by bombarding it with high energy electrons from an accelerator to crosslink the materials in the tape. The crosslinking significantly increases the structural strength of the film or the force with which the material can be stretched before tearing, when the film materials are predominantly ethylene, such as polyethylene or ethylene-vinyl acetate. The irradiation also improves the optical properties of the film and changes the properties of the film at higher temperatures.

If an irradiation step is used, a preferred irradiation dose level is in the range of 0, SMR to 12.0MR. MR is an abbreviation for megarad. A megarad is 1 x 106 rows, one row being the quantity of ionizing radiation that results in the absorption of 100 erg energy per gram of irradiated material regardless of the radiation source. In some cases, it may be desirable to first stretch the multilayer film and then irradiate it, or, if sequential coating is used, a layer or group of layers may be irradiated and then another layer or layers may be added before the final stretching. and the orientation step.

As stated above, a further possible process step involves applying a fine silicon spray to the inside of the freshly extruded tape. The details of this process step are described in Canadian Patent Specification No. 1198 SH, VBTS contents are hereby incorporated into the present text.

After co-extrusion, cooling or quenching and, if desired, irradiation, the extruded tape is reheated and subjected to continuous inflation by means of an i 'n __ _. _... ..., ..._ .. i ....-......-. ., ..... v ..- «_« .-- w- ~ .-, fi -. 1S Zö 460 534 ll internal air pressure to a bubble, whereby the narrow tape with thick walls is transformed into a wide film with thin walls of the desired film thickness. This process is sometimes referred to as the "bubble entrapment technique" for orientation or "stretching". After stretching, the bubble is emptied of air and the film is wound up on semi-finish rolls called "factory rolls". The stretching process orients the film by stretching it transversely and to some extent longitudinally to regroup the molecules and thus impart shrinkage properties to the film and modify the physical characteristics of the film. Further longitudinal or machine direction stretching can be achieved by rotating the compression rollers which contribute to the "blown bubble" collapsing at a higher speed than for the rollers which help to transport the reheated "tape" to the stretching surface or surface of the blown bubble. All of these orientation methods are well known in the art. To further describe and explain the scope of my invention to those skilled in the art, the following examples are presented.

EXAMPLE I A fine layer structure with an approximate layer thickness ratio of 2/2/1/2/2 was extruded using four extruders. Extruder No. 1, which fed the nozzle openings for the two intermediate layers, was fed a mixture of 90% ethylene-vinyl acetate copolymer (121 vinyl acetate) [Älathon 3137 (melt index 0.517 and 10% ionomeric resin [Éurlyn 1601 (density 0.940, melt mixture 1.41] The mixture contained 1100 ppm erucamide [Chemamide 57 and 1100 ppm stearamide [Chemamide Qj. Extruder No. 2, which fed the nozzle orifice for the core layer, was fed with 100% linear low density polyethylene [Dowlex 2045 (density 0.920, melt index 1, 01] The extruders Nos. 3 and 4, each feeding a nozzle orifice for a skin layer, were charged with both 100% ethylene-propylene copolymer (3.5% ethylene) Lšoltex 42X01 (melt flow 4.51] mixed with 100 ppm erucamide [Chemamide §], 1100 ppm stearamide [Xenamide S] and 1100 ppm silica L fi white Mist fl j. 'Extruder No. 1 was maintained at a temperature in the range 199-218 ° C. Extruder No. 2 was maintained at a temperature in the range 210-243 ° C. Extruder No. 3 was maintained at a temperature in the range of 177-188 ° C and extruder No. 4 was maintained at a temperature in the range of 174-185 ° C. The circular nozzle was maintained at a temperature in the range of 199-224 ° C.

After extruding the layers through the 254 mm circular die orifice, the tubular extrudate, which had a tape thickness of about 0.38 mm and a tube width of about 24.3 cm, was cooled by passing it through a cold water bath at a speed of about 11.3 m per minute. The tube was then reheated for orientation by passing it through a heating zone or oven at 11.6 m per minute. The furnace was heated by means of horizontal, vertical and steam heating elements. In this example, the horizontal heating element was maintained at 93 ° C. The vertical heating elements were maintained at 149 ° C and the steam element, which supplied steam by passage through pipes or jackets located inside the furnace, was fed with steam at 13.8 kPa.

After heating in the manner described above, the tubular extrudate was subjected to inflation and stretching in the transverse direction approximately to 4.8 to 1 and in the longitudinal direction approximately 4.3 to 1. Thereafter, the film was cooled by rapid cooling with water to lock in the oriented the structure. The final film thickness was approximately 75 gauge.

The experimental data obtained for this film composition are presented in Table I, which follows the description of the other examples.

EXAMPLE II A five layer structure with an approximate layer thickness ratio of 2/2/1/2/2 was extruded using four extruders. Extruder No. 1, which fed the nozzle openings for the two intermediate layers, was fed a mixture comprising 50% ethylene-vinyl acetate copolymer (12% vinyl acetate) [Älathon 3137 (melt index 0.51] and 50% low density linear polyethylene [Dowlex 2045 ( density 0.920, melt index 1.01], which also contained 1100 ppm erucamide [Chemamide No. Extruder No. 2, which fed the nozzle orifice to the core layer, was supplied with 100% low density linear polyethylene [Dowlex 2045 (density 0.920, melt index 1.01].

Extruders 3 and 4, each of which fed a nozzle orifice to a skin layer and both of which were supplied with 100% ethylene-propylene copolymers ($ 2.7 ethylene) [Anco x-193 (small surface 2,317 with 1100 ppm erucamide [Chemamide E]) and 1100 ppm silicon [White Mistj.

In Extruder No. 1 was maintained at a temperature in the range of 207-213 ° C. Extruder No. 2 was maintained at a temperature in the range of 204-246 ° C. Extruder No. 3 was maintained at a temperature within 460,554 N in the range of 179-224 ° C and extruder No. 4 was maintained at a temperature in the range of 179-210 ° C. The circular nozzle was maintained at a temperature in the range of 199-218 ° C.

After extrusion of the layers into the die orifice, the tubular extrudate having an approximate tape width of 22.2 cm and a thickness of 0.43 mm was cooled by passing it through a cooling water bath at a rate of about 11.9 m minute. The tubular extrudate was then reheated for orientation by passing it through a heating zone or oven at approximately The oven was heated by horizontal, vertical and angular elements. In this Example 7-1, the horizontal heating element was maintained at 100 ° C. The vertical heating element was maintained at 173 ° C and the steam element, which supplied steam by passage through pipes or jackets located inside the furnace, was supplied at 48.3 kPa.

After heating, the tubular extrudate was subjected to inflation and stretching in the transverse direction to about 4.8 to 1 and in the longitudinal direction to about 4.5 to 1. Thereafter, the film was cooled by water rapid cooling to lock the oriented molecular structure.

The final film thickness was approximately 75 gauge.

The data obtained during testing of this material are reported by the 254 mm circular 11.6 m per minute. in Table I below.

EXAMPLE III A three-layer structure with an approximate layer-thickness ratio of 1/3/1 was extruded using four extruders. Extruders No. 1 and 2, which fed the nozzle orifice to the core layer, were fed with 100% low density linear polyethylene [Dowlex 2045 (density 0.920, melt index 1.01] with 3300 ppm erucamide [Chemamide No.). Extruders No. 3 and 4, which each fed a nozzle orifice to a skin layer and which were both bathed with a mixture of 80% ethylene-propylene copolymer ($ 3.5 ethylene) ¿Soltex 42X01 (melt flow 4.51] and 20% propylene homopolymer [Hercules PD064 (density 0.906, melt flow 3) , 51] and with 3300 ppm erucamide [Chememide E] and 1100 ppm silicon dioxide [White Mist] and 1650 ppm Acrawax C.

The temperature for extruder No. 1 was kept within the temperature range 202-252 ° C. Extruder No. 2 was kept in the temperature range 218-243 ° C. Extruder No. 3 was kept in the temperature range 188-191 ° C. Extruder No. 4 was kept within the temperature range 460 554 188-191 ° C. The circular nozzle was maintained at a temperature of 196 ° C. The actual temperature ranges at which the extruders and nozzle were held were not recorded in this example.

After extruding the layers through the 254 mm circular die orifice, the tubular extrudate was cooled by passing it through a cold bath at a rate of about 12.0 m per minute. When extruding the tape, a fine silica mist was applied to the inside of the extruded tube at a rate of S-7 mg ftz. The cooled tubular extrudate was then reheated for orientation by passing it through a heating zone or oven at a speed of about 11.5 m per minute. The furnace was heated by means of a horizontal, vertical and steam heating element. In this example, the horizontal element was kept at 93 ° C. The vertical element was maintained at 152 ° C and the steam element, which supplied steam by passage through pipes or jackets located in the furnace, was supplied at 24.1 kPa.

After heating, the tubular extrudate was subjected to transverse stretching to about 4.8 to 1 and longitudinally to about 4.5 to 1. Thereafter, the film was cooled by water cooling to load the oriented structure. The final film thickness was approximately 75 gauge.

The data obtained during testing of this film are reported in Table I below. 460 534 Exemgel Layer Ratio Tensile Strength (MP8) 1 MD TD Elongation ($) 2 MD TD Modu1 (MPa) 3 MD TD Tear Expansion (g) 4 MD TD Tear Resistance (kg) 5 MD TD Shrinkage properties at 9s ° c Free shrinkage (%) 6 MD TD Shrinkage stress (MPa) 7 MD Tu at 1z1 ° c Free Shrinkage (%) 6 MD TD Shrinkage stress (MPa) 7 MD rn at 13s ° c Free shrinkage (%) 6 MD TD Shrinkage stress (MPa) 7 MD.

TD Optical Properties Light Diffusion (%) c1ans (4s °) Total transmission TABLE II 2/2/1/2/2 67.0 76.2 86 65 623 700 z, p 94 92.5 Footnote to Table I, see next page II 2/2/1/2/2 86.3 72.5 98 107 678 685 2.1 ss 92.2 III 1/3/1 62 64 2.98 0 2.0 ss 92.2 17 460 534 Footnote to table I 'Asrm D ssz ZASTM D ssz 3AsrM D 882 “ASTM D 1938 5AsrM D 1004 6AsTM D 2732 7AsTM n zsss 8AsTM D 1003 All data reported in the above Table I are averages obtained by procedures in accordance with the specified ASTM- standard.

It should be noted that the detailed description and the concrete examples relating to the present preferred embodiments of the invention are given for illustrative purposes only, as various changes and modifications within the scope of the invention will be apparent to one skilled in the art upon a study of the above detailed description and examples.

Claims (10)

18 PATENT REQUIREMENTS A multilayer film having a core layer of linear polyethylene and a skin or outer layer of ethylene polymer, characterized in that it contains a core layer comprising linear low density polyethylene or linear polyethylene having a medium density and two skin or outer layers comprising an ethylene propylene copolymer, and that it has an average value of the free shrinkage in the machine direction at 93 ° C of at least about 12% and an average value of the free shrinkage in the transverse direction at 93 ° C of at least about 17%. A film according to claim 1, characterized in that the two skin layers comprise a mixture of from 70 to 90% by weight of an ethylene-propylene copolymer and from 10 to about 30% by weight of a propylene homopolymer. A film according to claim 1, characterized in that the two skin layers comprise a mixture of from 70 to 90% by weight of an ethylene-propylene copolymer and from 10 to 30% by weight of a linear low density polyethylene. A film according to claim 1, characterized in that the two skin layers comprise a mixture of from 70 to 90% by weight of an ethylene-propylene copolymer and from 10 to 30% by weight of a linear polyethylene of medium density. 5. A film according to any one of the preceding claims, characterized in that it further comprises two intermediate layers, which contain a mixture of about 90% by weight of ethylene-vinyl acetate copolymer and about 10% by weight of an ionomeric resin. Film according to any one of claims 1 to 4, characterized in that it further comprises two intermediate layers, which contain a mixture of from 20 to 90% by weight of an ethylene-vinyl acetate copolymer and from 10 to 80% by weight of linear polyethylene. with low density. Film according to one of Claims 1 to 4, characterized in that it consists of three layers. The film of claim 2, characterized in that the skin layers comprise a mixture of about 80% by weight of an ethylene-propylene copolymer with about 20% by weight of a propylene homopolymer. Film according to any one of claims 1-8, characterized in any of the following groups: 8 .. b. 10. 460 534 in that the core layer comprises a composition selected from 10 to 100% by weight of a low density linear polyethylene and from 0 to 90% by weight of an ethylene-propylene copolymer; 10-100 density and 0-90% by weight of an ethylene-propylene weight percent of a linear polymeric polyethylene; 10-80% by weight of a low density linear polyethylene and 20-90% by weight of an ethylene-vinyl acetate copolymer; 10-80% by weight of a medium density linear polyethylene and 20-90% by weight of an ethylene-vinyl acetate copolymer; 10-80% by weight of a linear low density polyethylene and 10-80% by weight of an ethylene-vinyl acetate copolymer 3 and 10-80% by weight of an ionomeric resin; 10-80% by weight of a linear density of medium density polyethylene and 10-80% by weight of an ethylene-vinyl acetate copolymer and 10-80% by weight of an ionomeric resin; 10-80% by weight of a linear low density polyethylene and 20-90% by weight of a low density polyethylene; 10-80% by weight of a medium density linear polyethylene and 20-90% by weight of a low density polyethylene. Film according to any one of claims 1-4, 8 or 9, characterized in that it additionally contains intermediate layers comprising a composition selected from any of the following groups: 31 b. 10-100% by weight of a linear polyethylene having the the site and 0-90% by weight of an ethylene-propylene copolymer; 10-100% by weight of a medium density linear polyethylene and 0-90% by weight of an ethylene-propylene copolymer; 10-80% by weight of a low density linear polyethylene and 20-90% by weight of an ethylene-vinyl acetate copolymer; V 10-80% by weight of a linear-polyethylene with medium density and -20-90% by weight of an ethylene-vinyl acetate copolymer; 460 554 I.) -_ _. 10-80% by weight of a low density linear polyethylene and 10-80% by weight of an ethylene-vinyl acetate copolymer and 10-80% by weight of an ionomeric resin; 10-80% by weight of a linear polyethylene of medium density and 10-80% by weight of an ethylene-vinyl acetate copolymer and 10-80% by weight of an ionomeric resin; 10-80% by weight of a linear low density polyethylene and 20-90% by weight of a low density polyethylene; 10-80% by weight of a medium density linear polyethylene and 20-90% by weight of a low density polyethylene; an ethylene-vinyl acetate copolymer; 20-80% by weight of low density linear polyethylene and 20-80% by weight of an ionomeric resin; 20-80% by weight of linear polyethylene with medium density and 20-80% by weight of an ionomeric resin.