Thermoplastic films for use in stretch/cling applications

Abstract: A thermoplastic film is provided which is especially suited for use in stretch/cling applications such as, for example, the bundling, packaging and unitizing of foods and other goods. The film comprises a cling layer comprising a polymer of at least ethylene and acrylate, preferably without a tackifying additive. The film further comprises a slip layer comprising a polyolefin preferably including an anticling (slip and/or antiblock) additive. Preferred polyolefins include linear low density polyethylene and polypropylene.

Description: The present invention relates generally to thermoplastic films and, more particularly, to thermoplastic films having properties making them especially well suited for use as stretch/cling wraps in various bundling, packaging and palletizing operations. Stretch/cling films have found utility in a wide variety of fields including the bundling and packaging of food and other goods. One application of particular, but not limiting, interest to the present invention is in the bundling of goods for shipping and storage such as, for example, the bundling of large rolls of carpet, fabric or the like for shipping from the manufacturer to a retail outlet. An important subset of these bundling applications is in the containment and unitizing of pallet loads. The load of a pallet may be unitized or "bundled " by stretch-wrapping a film several times around the articles to be palletized. There exist a variety of stretch-wrapping techniques, two of which are commonly employed. In one technique, the loaded pallet is placed on a rotating turntable and the end of a continuous roll of film attached to the load. As the turntable rotates, the film is continuously wrapped around the pallet and load. Tension is applied to the film roll to cause the film to stretch as it is applied. In another technique, the converse occurs, i.e., the pallet and load are kept stationary and the continuous roll of film rotated around the pallet and load. Again tension is applied to the film roll to cause the film to stretch as it is applied. Because the film is in a stretched condition, it is placed under considerable tension and will have a tendency to return to its original, unstretched state. This tension can cause the film to unravel from the wrapped pallet, thereby jeopardizing the integrity of the unitized load. It is desirable, therefore, that the film have cling properties to prevent unraveling of the film from the pallet. To impart cling properties to, or improve the cling properties of, a particular film, a number of well-know tackifying additives have been utilized. Common tackifying additives include polybutenes, terpene resins, alkali metal stearates and hydrogenated rosins and rosin esters. The cling properties of a film can also be modified by the well-known physical process referred to as corona discharge. The use of tackifying additives, however, is not desirable. These additives have a tendency to accumulate on the stretch wrapping apparatus often requiring additional periodic cleaning and maintenance. They also can migrate into the bundled or unitized articles resulting in damage to such articles, as well as migrating throughout a film, even a multilayer film, causing tackiness on both sides. In palletizing operations, this may cause the film on adjacent pallets to cling together resulting in tear, puncture or other damage to the wrap and jeopardizing the integrity of the unitized load. For this reason, it is desirable for the film to have slip properties on its "outer" side to prevent this interpallet cling. Slip is defined in terms of coefficient of friction. In other words, it is desirable that the "outer" side of the film have a low coefficient of friction in contact with another object, particularly another like film. As with cling, slip can be imparted to the film or improved through the use of various well-known slip and/or antiblock additives including silicas, silicates, diatomaceous earths, talcs and various lubricants. Under highly stretched conditions, however, the coefficient of friction in the films tends to increase and even the slip additives may not provide the desired slip properties. The tension in the stretched film may also cause the film to be more susceptible to punctures and tears. It is, therefore, also desirable for the film, as a whole, to have good stretch, tensile, puncture resistance and tear resistance properties. Additionally, thermal stability of the various film components is important for the recycling of edge trim and film scrap generated in the various film production processes. A wide variety of thermoplastic polymers such as, for example, polyvinyl chloride, polyethylene, polypropylene and various polymers of ethylene and other comonomers, most notably vinyl acetate, have been used as stretch/cling films. These materials standing alone, however, suffer from a number of shortcomings. Most do not possess desirable cling properties without the use of tackifying additives. Further, most cannot be stretched to a great extent without adversely affecting their slip, tensile, tear resistance and puncture resistance properties. For the particular case of ethylene-vinyl acetate polymers, thermal stability becomes a problem on the reprocessing of trim and scrap. More recently, the use of multilayer films has gained popularity. With a multilayer film, one can obtain a stretch/cling wrap having cling properties on one side and slip properties on the other side. For example, U.S. Patent No. 4518654 discloses a multilayer film having an A/B construction wherein the A side has cling characteristics and the B side has slip characteristics. In the aforementioned patent, the A side is said to comprise a polyethylene or an ethylene-monoolefin polymer, preferably linear low density polyethylene (LLDPE). To provide the LLDPE with the desired cling properties, a tackifying agent is added to the polymer. The B side is said to comprise a low density polyethylene (LDPE) with an anticling additive added to impart the desired slip properties to the LDPE. This patent is hereby incorporated by reference herein for all purposes as if fully set forth. Other multilayer films comprising layers of the various aforementioned stretch/cling materials are disclosed in U.S. Patent Nos. 3508944, 3748962, 3817821, 4022646. 4082877, 4147827, 4189420, 4194039, 4303710, 4399180, 4364981, 4418114, 4425268, 4436788, 4504434, 4588650 and 4671987; U.K. Patent Appln. No. 2123747; French Patent No. 2031801; and European Patent Appln. No. 0198091. These multilayer films are generally produced by one of a number of well-known coextrusion processes also disclosed in the aforementioned references. Many of the multilayer films, however, still suffer from shortcomings possessed by their individual layers. For instance, most still require the use of tackifying additives to impart cling properties to the film. As previously mentioned, these tackifying additives have a tendency to accumulate on dispensing apparatus and may migrate through the films to the slip side. Also, films containing a tackifying additive may be prepared and used in such a manner that the tackifying additive is "picked off" and onto the slip side of the film because the slip and cling layers of the film are in intimate contact on the film roll. Others do not possess desired slip properties, particularly when in a highly stretched state. Still others do not possess a desirable combination of stretch, tensile, tear resistance, puncture resistance and thermal stability properties. Summary of Invention The present invention, therefore, provides a thermoplastic film having properties especially well suited for use as a stretch/cling wrap. The present invention also provides a thermoplastic film having excellent cling properties, even without the use of a tackifying additive. The present invention further provides a multilayer stretch/cling film having excellent cling properties on one side and excellent slip properties on an opposite side, even in a highly stretched state. Still further, the present invention provides a multilayer stretch/cling film which, as a whole, possesses desirable stretch, tensile strength, puncture resistance, tear resistance and thermal stability properties. US-A-4,367,112 describes a biaxially oriented (i.e. non-stretchable) heat-sealable film which is a laminate of a polypropylene layer and a layer comprising a copolymer of ethylene and from 6 to 30 wt.% of methylacrylate, and non-oriented intermediate products. No slip or cling properties are described, and no slip (or other anticling) agents are mentioned. In accordance with the invention there is provided a thermoplastic stretch/cling film formed by coextrusion of a cling layer and a slip layer opposite to the cling layer, wherein that said cling layer consists essentially of a copolymer of ethylene and from 20 to 35 wt.% of an acrylate, based on the weight of copolymer and the film exhibits cling at 100 % stretch greater than 50 grams. Preferably the acrylate content is from 24 to 28 wt.%. The slip layer may comprise any suitable polyolefin. Particularly preferred are linear low density polyethylene (LLDPE) and polypropylene. Additionally the slip layer may include one or more well-known anticling (slip and/or antiblock) additives. The film may include an additional layer e.g. of a polyolefin formed by coextrusion intermediate the slip and cling layers. The additional layer may constitute at least 50% of the thickness of the film. The invention includes a method for bundling, packaging or unitizing an article or plurality of articles by stretch-wrapping said article or articles with a stretch/cling film as described above. The invention also includes a package comprising a stretch-wrapped article or plurality of articles, characterised in that said film is a film as described above. The invention also includes the use of a copolymer of ethylene and from 20 to 35 wt.% of an acrylate, based on the weight of copolymer, to form a cling layer of sufficient tackiness without the use of additives in a thermoplastic stretch/cling film having coextruded slip and cling layers. The film may be produced utilizing any one of a number of well known coextrusion processes. The cling layer will preferably comprise from 5% to 95%, and the slip layer will preferably comprise from 95% to 5%, of the combined thickness of the cling and slip layers. Thermoplastic films produced in accordance with the present invention have excellent cling properties on the cling layer and slip properties on the slip layer. Particularly, the use of tackifying additives can be avoided because of the excellent cling properties of the cling layer EA polymer. The present thermoplastic films, as a whole, additionally have desirable stretch, tensile, puncture resistance and tear resistance properties. Further, the thermoplastic compounds used for the cling and slip layers have excellent thermal stability, and edge trim scrap can be processed without significant loss of film performance. This combination of properties makes the thermoplastic films of the present invention especially well suited for use as stretch/cling wraps. These and other features and advantages of the present invention will be more readily understood by those skilled in the art from a reading of the following detailed description Detailed Description of the Preferred Embodiments As previously indicated, the present invention provides a thermoplastic film having excellent cling, slip, stretch, tensile, tear resistance, puncture resistance and thermal stability properties, making such film especially well suited for use as a stretch/cling wrap. Acrylate, in the singular, refers to both a single acrylate and combinations of different acrylates. These copolymers will generally be referred to as ethylene-acrylate (or EA) copolymers. In the preferred embodiment, the EA copolymer comprises an acrylate content from about 24% to 28%, by weight based upon the weight of the EA copolymer. The EA copolymer may have a wide range of melt indexes (MI), generally between about 0.1 to about 30, more preferably between about 1 to about 10, g/10 min. (ASTM D-1238, Condition E). Specific examples of preferred acrylates include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, octadecyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate and the like. Particularly preferred are methyl acrylate, ethyl acrylate and n-butyl acrylate. The EA copolymer may be produced by any one of a number of well-known processes such as, for example, those described in U.S. Patent No. 3350372. Generally ethylene and acrylate are metered into, for example, a high pressure autoclave reactor along with any one of a number of well-known free radical polymerization initiators (catalysts) suitable for producing EA copolymers. Particularly preferred catalysts include organic peroxides such as, for example, lauroyl peroxide, di-tert butyl peroxide, tert butyl peroxide and various azo compounds. Typically, the catalyst will be dissolved in a suitable organic liquid such as benzene, mineral oil or the like. Ordinarily the catalyst is used at a level of between about 50 to about 20000, more preferably between about 100 to about 250, ppm based upon the weight of the monomers. The EA copolymer may, if desired, also include one or more other well-known additives such as, for example, antioxidants, ultraviolet absorbers, antistatic agents, release agents, pigments, colorants or the like; however, this should not be considered a limitation of the present invention. The film of the present invention comprises a slip layer opposite the cling layer, the slip layer comprising any suitable polyolefin or combination of polyolefins such as polyethylene, polypropylene, copolymers of ethylene and propylene, and polymers obtained from ethylene and/or propylene copolymerized with minor amounts of other olefins, particularly C 4 to C 12 olefins. Particularly preferred are polypropylene and linear low density polyethylene (LLDPE), i.e., a copolymer of ethylene with up to about 20% by weight C 4 to C 10 olefin(s). Especially preferred olefins include 1-butene, 1-hexene, 1-octene and 4-methyl pentene-1. Suitable LLDPEs include those having a density greater than about 0.900 g/cm 3 , more preferably in the range of from about 0.900 to about 0.940 g/cm 3 . The LLDPEs may also have wide ranging Mls, generally up to about 30 g/10 min., preferably between about 0.5 to about 10 g/10 min. Such LLDPEs and methods for making the same are well-known in the art and are readily available commercially. Suitable polypropylene is normally solid and isotactic, i.e., greater than 90% hot heptane insolubles, having wide ranging melt flow rates of from about 0.1 to about 300 g/10 min. As is known, such polypropylene is normally crystalline with a density range of from about 0.89 to about 0.91 g/cc for isotactic polypropylene. Such polypropylene and methods for making the same are well-known in the art and are readily available commercially. Additionally, the slip layer may include one or more anticling (slip and/or antiblock) additives which may be added during the production of the polyolefin or subsequently blended in to improve the slip properties of this layer. Such additives are well-known in the art and include, for example, silicas, silicates, diatomaceous earths, talcs and various lubricants. These additives are preferably utilized in amounts ranging from about 100 ppm to about 20000 ppm, more preferably between about 500 ppm to about 10000 ppm, by weight based upon the weight of the B layer. The slip layer may, if desired, also include one or more other well-known additives such as, for example, antioxidants, ultraviolet absorbers, antistatic agents, release agents, pigments, colorants or the like; however, this again should not be considered a limitation of the present invention. Additionally, normal trim and scrap from the film production process can be recycled into either the cling or slip layers, but preferentially to the slip layer of a two-layer film or the core structural layer of a three-layer film. The present invention may also include one or more intermediate layers between the cling and slip layers for any one of a number of well-known purposes such as, for example, to modify the overall physical properties balance of the film, to utilize the recycle trim and scrap or to provide a barrier layer to oxygen or other gases. As just indicated, this intermediate layer may comprise the recycle trim and scrap, or may comprise any other suitable polymer. The intermediate layer(s), however, is optional and should not be considered a limitation on the present invention. In preparing the thermoplastic stretch/cling films of the present invention, any one of a number of well-known coextrusion techniques as disclosed in the previously incorporated references may be utilized. As preferred examples, any of the blown or chill roll cast processes as disclosed and described in those references is suitable for use in producing thermoplastic stretch/cling films in accordance with the present invention. The cling layer preferably comprises between about 5% to about 95%, more preferably between about 5% to about 35%, most preferably between about 10% to about 20%, of the combined thickness of the cling and slip layers. Conversely, the slip layer preferably comprises between about 5% to about 95%, more preferably between about 65% to about 95%, most preferably between about 80% to about 90%, of the combined thickness of the cling slip layers. As previously mentioned, the thermoplastic films of the present invention have properties making them especially well suited for use as stretch/cling films; however, this use should not be considered a limitation on the present invention. For example, these films can be made into other forms, such as a tape, by any one of a number of well-known cutting, slitting and/or rewinding operations. Physical properties including, but not limited to, tensile strength, tear strength and elongation can be adjusted over wide ranges by altering the resin types and specifications as appropriate to meet the requirements of a given wrapping, bundling or taping application. For bundling, packaging and unitizing applications, the thermoplastic film of the present invention is stretch-wrapped by any one of a number of well-known procedures (such as those disclosed in the aforementioned incorporated references) around an article or a plurality of articles preferably so that the cling layer faces inside (towards the article) and the slip layer faces outside (away from the article). Typical of articles suitable for bundling, packaging and unitizing with the present thermoplastic film include, but are not limited to, various foodstuffs (canned or fresh), rolls of carpet, liquid containers and various like goods normally containerized and/or palletized for shipping, storage and/or display. The foregoing more general discussion of this invention will be further exemplified by the following specific examples offered by way of illustration and not limitation of the above-described invention. EXAMPLES In the following examples, property evaluations were made in accordance with the following tests: (1) Coefficient of Friction (COF) -- ASTM D-1894. (2) Cling -- cling is reported as the force in grams required to partially peel apart two strips of film. A first film strip is attached to a 30° inclined plane with the outside surface (slip) facing upward. A second 2.54 X 20.3 cm (1" X 8") strip is placed on top of the first strip with the inside surface (cling) facing downward. Pressure is applied to the second strip to cause the two strips to stick together. If an evaluation of cling under stretched conditions is desired, both film strips are prestretched and allowed to relax before testing. The end of the second strip at the base of the inclined plane is attached, by clip and string, to an apparatus which can exert a strain at a constant rate (Instron 1130). The two strips are then pulled apart at a crosshead speed of 10 cm/min until the aforementioned string is parallel with the base of the inclined plane. The force at this point is reported as cling. Examples 1-9, Comparative Example 1 An ethylene-methyl acrylate (EMA) copolymer, with an Ml of 5 g/10 min., was coextruded with a LLDPE (ethylene-hexene) having a density of 0.918 g/cm 3 and an Ml of 3.2 g/10 min. (commercially available from Exxon Chemical Company, Houston, Texas, under the trade designation LL-3003). Slip (Kememide E, a commercial erucimide available from Humko Chemical Company, Memphis, Tennessee) and antiblock (Super Floss™, a commercial silica available from Johns Manville) were added to the LLDPE. Other properties of the EMA and LLDPE are listed below in Table 1. The film was produced by coextruding the EMA copolymer and LLDPE on a Black Clawson cast film line with two 8.89 cm (3.5") extruders, respectively, for the EMA and LLDPE layers. The EMA was extruded at a melt temperature of 205°C (400°F), while the LLDPE was extruded at a melt temperature of 232°C (450°F). The chill roll temperature was set to 27°C (80°F) and the line speed to 137 m (450 feet) per minute. resulting film had a gauge of 20.3 µm (0.8 mil), with the EMA layer comprising 20% of the total film thickness. The O/O slip (LLDPE/LLDPE) was then measured as the coefficient of friction (COF) for 0%, 100% and 200% stretch, and I/O cling (EMA/LLDPE) was measured for 100% and 200% stretch. The results are presented below in Table 1. Ex. %MA AB (ppm) SLIP (ppm) COF (kinetic) CLING (grams) 0% 100% 200% 100% 200% Comp.1 24 10000 500 0.19 0.21 0.35 20 30 1 24 0 500 >1 >1 >1 107 90 2 24 5000 1000 0.20 0.35 0.44 50 50 3 24 5000 0 0.45 0.83 >1 90 85 4 24 5000 500 0.26 0.61 >1 77 72 5 28 10000 500 0.23 0.28 0.42 77 65 6 28 0 500 >1 >1 >1 175 126 7 28 5000 1000 0.21 0.32 0.51 83 75 8 28 5000 0 0.70 >1 >1 149 113 9 28 5000 500 0.45 0.75 >1 114 86 Examples 10 and 11 A 28% MA, 3 g/10 min. Ml, EMA copolymer was coextruded with LL-3003 as in Examples 1-9. EMA extrusion melt temperature was 227°C (440°F) and LLDPE extrusion melt temperature was 243°C (470°C). Slip and antiblock additives were again added to the LLDPE. The resulting film had a gauge of 21.6 µm (0.85 mil), with the EMA layer comprising 15% of the total film thickness. The O/O slip (LLDPE/LLDPE) was then measured as the coefficient of friction (COF) for 0%, 100% and 200% stretch, and I/O cling (EMA/LLDPE) was measured for 0%, 100% and 200% stretch. The results are presented below in Table II. These films were also evaluated on a commercial Lantech V-Series Power Prestretch machine at 160% and 330% stretch with the results presented in Table III. Ex. AB (ppm) SLIP (ppm) COF (kinetic) CLING (grams) 0% 100% 200% 0% 100% 200% 10 5000 500 0.39 0.48 0.80 165 170 130 11 10000 1000 0.25 0.33 0.54 70 110 75 Ex. SLIP CLING 160% 330% 160% 330% 10 good fair good good 11 excell. v. good v. good good Example 12 A 24% MA, 5 g/10 min. Ml, EMA copolymer was coextruded with a 12 melt flow rate (ASTM D-1238, condition L) polypropylene (commercially available from Exxon Chemical Company, Houston, Texas, under the trade designation PP-3014) as in Examples 1-9. No slip and/or antiblock additives were added to the polypropylene. The EMA copolymer layer comprised about 30% of the total film thickness. The O/O slip (polypropylene/polypropylene) was then measured as the COF, and the I/O (EMA/polypropylene) cling measured as in Examples 10 and 11. The results are presented in Table IV below. Example 13 A 24% MA, 5 g/10 min. Ml, EMA copolymer was coextruded with a blend of 10 wt% PP-3014, 90 wt% LL-3003, 5000 ppm antiblock and 500 ppm slip (as in Examples 1-9). The EMA copolymer layer comprised about 30% of the total film thickness. The 0/0 slip was then measured as the COF, and the I/O cling measured as in Examples 10 and 11. The results are presented in Table IV below. Ex. AB (ppm) SLIP (ppm) COF (kinetic) CLING (grams) 0% 100% 200% 0% 100% 200% 12 0 0 0.66 0.86 >1 339 267 234 13 5000 500 0.73 0.90 >1 179 164 128 The results of Tables I-IV show that films in accordance with the present invention exhibited good physical properties, extrusion processability and utility appropriate for stretch/cling wrap applications. The results further demonstrate the ability to tailor film cling and slip properties and balances by altering variables including the EA polymer comonomer contents, resin type(s) of the slip layer, slip additive content of the slip layer and antiblock additive content of the slip layer. The results of Table III further indicate that the overall performance of the films met or exceeded the desired balance of cling, slip, toughness and stretch performance required of stretch/cling wraps.

Claims: A thermoplastic stretch/cling film formed by coextrusion of a cling layer and a slip layer opposite to the cling layer, wherein said cling layer consists essentially of a copolymer of ethylene and from 20 to 35 wt.% of an acrylate, based on the weight of copolymer and the film exhibits cling at 100% stretch of at least 50 grams. A film according to claim 1 wherein said copolymer has an acrylate content of from 24 to 28 wt.%. A film according to claim 1 or claim 2, wherein said acrylate is selected from methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, octadecyl acrylate, methyl methacrylate, ethyl methacrylate and n-butyl methacrylate. A film according to any preceding claim wherein said acrylate is selected from methyl acrylate, ethyl acrylate, and n-butyl acrylate. A film according to any preceding claim wherein said slip layer comprises a polyolefin. A film according to any preceding claim wherein said slip layer comprises polypropylene or linear low density polyethylene. A film according to any preceding claim wherein said slip layer comprises a slip and/or antiblock additive. A film according to any preceding claim wherein said cling layer constitutes from 10 to 20% of the combined thickness of the slip and cling layers. A film according to any preceding claim including an additional layer formed by coextrusion intermediate the slip and cling layers. A film according to claim 9 wherein the additional layer constitutes at least 50 % of the thickness of the film. A film according to any of the preceding claims in which the cling layer is substantially free of tackifying additives. A method for bundling, packaging or unitizing an article or plurality of articles by stretch-wrapping said article or articles with a stretch/cling film, wherein said film is a film according to any of the preceding claims. A package comprising a stretch-wrapped article or plurality of articles, wherein said film is a film according to any of the preceding claims 1 to 11.

Inventors: GERMAN PAUL MASTEN, GERMAN, PAUL MASTEN
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1988
Main classification: B65D65
IPC codes: C09J7, B32B27, C08F8, B29C47, C08F210