Pallets containing smaller packages are regularly stabilised using polymer films. While shrink wrapping has been used, the more economic and safer process of stretch wrapping is presently being adopted for the stabilisation of pallet loads of packages and other packaging applications. Stretch wrap films incorporate a cling additive that binds each layer of film so that effectively a single unit is obtained and the inherent elasticity of the film provides a rebound force which prevents a load shifting during storage and transport.

Stretch wrap films desirably have a number of attributes including high resistance to puncture and tear when stretched and under tension, high elastic rebound, high adhesion between layers of stretched film, high degree of prestretching possible prior to wrapping of the articles, and good optical properties. In order to obtain improvements in some or all of these film properties, multilayer film structures have been employed.

Linear low density polyethylene (LLDPE) has been adopted for stretch wrap applications due to its combination of a number of the desirable properties described above. Linear low density polyethylenes used for blown film stretch wrap applications are typically produced by solution polymerisation using octene as the comonomer. Linear low density polyethylene may also be produced by gas phase polymerisation, for example, using hexene as a comonomer. LLDPEs produced by gas phase polymerisation have a molecular structure which mitigates against the use of these LLDPEs in blown film stretch wrap applications.

Gas phase polymerisation processes using hexene as a comonomer provide a linear low density polyethylene which is particularly suited for cast stretch wrap applications. However its use in blown stretch wrap applications is limited.

We have now found that by modifying linear low density polyethylene it is possible to obtain a blown stretch wrap film monolayer having improved properties which render it particularly suited for stretch wrap applications. In the context of LLDPEs produced by gas phase polymerisation the modification according to the present invention allows the gas phase polymerized LLDPEs to be used advantageously in stretch wrap applications. In the context of LLDPEs generally, the modification according to the present invention provides the LLDPEs with improved properties, such as improved migration of cling additive to the surface of the film and an increase in the amount of pre-stretching possible.

According to the present invention there is provided a polymer blend comprising a linear low density polyethylene, further including from 0.5 to 5% (w/w) of a low density polyethylene wherein said low density polyethylene has a long-chain branch frequency of greater than 2 /1000C and a molecular weight fraction (Mw) over Sx105 g/mol of greater than 2.5% by weight of the low density polyethylene, and further including an effective amount of a cling additive.

According to a further aspect there is provided a monolayer film manufactured from a polymer blend comprising a linear low density polyethylene, further including from 0.5 to 5 % (w/w) of a low density polyethylene wherein said low density polyethylene has a long- chain branch frequency of greater than 2/1000C and a molecular weight fraction (Mw) over Sx105 g/mol of greater than 2.5% by weight of the low density polyethylene, and further including an effective amount of a cling additive.

It will be understood that the term,"linear low density polyethylene"is meant to include copolymers of ethylene and at least one alpha-olefin comonomer. The term includes copolymers, terpolymers, etc. Linear low density polyethylenes are generally copolymers of ethylene and alpha-olefins such as propene, butene, 4-methyl-pentene, hexene, octene and decene.

Linear low density polyethylenes suitable for use in the present invention include those

produced by commercial polymerisation processes such as gas phase polymerisation, slurry polymerisation and solution polymerisation, including multireactor systems of any combination of these processes. Catalysts suitable for use in the production of linear low density polyethylenes include homogeneous and heterogeneous catalysts.

Linear low density polyethylenes generally have a density in the range of from 910 to 960 kg/m3. Typically, linear low density polyethylenes suitable for blown film extrusion have a melt flow index (MFI) in the range of from 0.1 to 7.0 g/600s.

Linear low density polyethylenes typically have peak melting points in the range of from 110 °C to 130 °C. The heat of fusion of linear low density polyethylenes is typically in the range of from 18 to 42 cal/g. Linear low density polyethylenes typically have a crystallinity in the range of from 25 to 60%.

Preferred linear low density polyethylenes for use in the present invention are polymerized in gas-phase reactors using heterogeneous catalysts. The preferred linear low density polyethylene is polymerized in a gas-phase reactor using a Ziegler/Natta catalyst system.

Such preferred polymers preferably have a density in the range of from 913 to 930 kg/m3 and a melt index in the range of from 0.4 to 2.5 g/600s. Solution-phase and slurry-phase polymerized linear low density polyethylenes may also be used advantageously in the polymer blends of the present invention.

Low density polyethylenes suitable for use in the present invention include ethylene homopolymers and copolymers having up to 30% (w/w) of a comonomer such as vinyl acetate, butyl acrylate and the like. Low density polyethylenes most suitable for use in the present invention are produced by free radical polymerisation via a high pressure autoclave process. Preferably the low density polyethylenes for use in the present invention are polymerised under conditions which are known to those skilled in the art to favour the production of polymers with broad molecular weight distributions and a high degree of long- chain branching such as in an autoclave reactor. It is particularly preferred that the low

density polyethylenes have a bi-modal molecular weight distribution and a high degree of long-chain branching in the high molecular weight fraction of the low-density polyethylene.

The low density polyethylenes generally have peak melting temperatures in the range of from 80 °C to 115 °C. Typically their heat of fusion is in the range of from 18 to 30 cal/g and their crystallinity is in the range of from 20 to 45 %.

Preferably the low density polyethylenes selected for use in the compositions of the present invention have melt indices in the range of from 0.1 to 7 g/600s, more preferably 0.3 to 4 g/600s. Preferably the low density polyethylenes have a density in the range of from 913 to 930 kg/m3, more preferably in the range of from 917 to 922 kg/m3. Preferably the low density polyethylenes have a polydispersity index (MW/M, of greater than 15 and more preferably greater than 22. The M, value of low density polyethylenes is preferably greater than 2 x 106 g/mol, more preferably greater than 3 x 106 g/mol. The long-chain branch frequency as determined by 13C nuclear magnetic resonance spectroscopy is greater than 2 /1000C and preferably greater than 3/1000C. The high molecular weight fraction over M, 5 x 105 is greater than 2.5%, preferably greater than 5%, and more preferably greater than 10% of the total LDPE polymer by weight.

The cling additive for use in the present invention may be any suitable cling additive. Examples of suitable cling additives include migratory tackifying agents such as low molecular weight polyisobutylenes or polybutenes, atactic polypropylenes, glycerol monooleate and sorbitol monooleate. Preferably, the cling additive is a low molecular weight polyisobutylene, such as a poly (isobutylene-co-butylene) having a number average molecular weight in the range of from 300 to 5000 g/mol. More preferably the low molecular weight polyisobutylene has a molecular weight in the range of from 500 to 3000 g/mol.

The cling additive is present in the modified linear low density polyethylene in an effective amount, preferably in the range of from 0.1 % to 7 % (w/w).

The polymer blends according to the present invention may include a variety of other additives such as those known to persons skilled in the art. Such additives include process, thermal, light and weathering stabilisers, pigments and dyes, surface friction modifiers and the like.

The polymer blends of the present invention may be compounded on a variety of extrusion equipment such as is well known to those skilled in the art. Extrusion equipment is generally capable of providing dispersive and distributive mixing capabilities. Suitable extrusion equipment includes single and multi-screw extruders, multi-rotor type extruders and the like. Preferably the polymer blend is blended on a twin-screw extruder. While such continuous processing equipment is generally preferred it is possible to form the polymer blends of the present invention in batch processes.

The polymer blends of the present invention may be mixed and blended in an extruder which is ultimately used in the film blowing process if it is equipped with screw mixing elements, static mixer units, gear pumps or combinations of such mixing units capable of providing sufficient dispersive and distributive mixing. Preferably the polymers and additives used to form the polymer blend of the present invention are blended in a separate process using a single or multi-screw extruder specifically designed for compounding polymer blends. Most preferably, the polymers and additives used to form the polymer blends of the present invention are compounded ex-LLDPE reactor on a co-rotating twin-screw extruder in an inert atmosphere and the subsequently formed polymer blend is pelletised for use in a separate film manufacturing process.

The manufacture of film suitable for use in stretch wrap applications may be performed in blown film and cast film processes which are well known to those skilled in the art. Blown film processes are preferred in the manufacture of stretch wrap applications using polymer blends of the present invention. The addition of cling additives such as polyisobutylene is usually conducted by injection into the extruder between the feed throat and the compression zone of the screw using a metering pump. Careful control over the addition rate of the

polyisobutylene, the film cooling rate and the film winding tension are required to provide optimum properties in the stretch wrap film. Such film manufacturing techniques are well known to those skilled in the art.

The polymer blend of the present invention permits the manufacture of stretch wrap film having substantially improved properties. The cling additive in the stretch wrap film manufactured from a polymer blend according to the present invention advantageously exhibits faster and more consistent migration to the surface of the film, particularly in gas- phase polymerised linear low density polyethylene, immediately after manufacturing thereby allowing a reduction in the storage time required before such films are suitable for use. Prior art films generally require storage in excess of 7 days prior to use.

Films manufactured from the polymer blend of the present invention may advantageously demonstrate improved retention of cling properties subsequent to the film being stretched.

The improved retention of cling is particularly evident once the film has been stretched to over 250% of its original length with the consequent reduction in thickness. The integrity of films manufactured from polymer blends of the present invention is substantially improved and can be seen in the ability of the films to maintain the loading even at higher stretch ratios.

The ability to stretch the films produced from the polymer blends of the present invention to a greater extent provides for more economical use of film through the use of thinner stretch gauges. The films are able to be stretched to a greater level before the effects of strain hardening are observed when further stretching is not generally practical.

The increased ability of the films made from the polymer blends of the present invention to be stretched provides a concomitant increase in the films ability to maintain the stability of loads on wrapped pallets. The films produced from polymer blends of the present invention also advantageously demonstrate increased optical clarity which permits easier use of optical character recognition equipment, such as barcode readers, in the identification of the product wrapped under the stretch wrap film on the pallet.

Without wishing to be bound by theory it is believed that the addition of the particular low density polyethylenes described to the linear low density polyethylene improves the properties of the linear low density polyethylene film by the following mechanism. During the film blowing process, the polymer melt is extruded as a thin-walled cylinder that is subjected to extensional stress. The further thinning of the film and induction of a degree of orientation among the polymer chains permit the film to rapidly solidify and results in extremely rapid crystallisation whilst the partially oriented polymer melt is under extensional stress. As the molten film is cooled, crystallisation is believed to initially occur in regions of high molecular weight, lightly branched chains of linear low density polyethylene. Crystallisation fronts subsequently propagate through the molten film. It is believed that the crystallisation process may be usefully considered in two stages, primary and secondary. The primary crystallisation process is believed to form a network of oriented lamellae, which is followed by a secondary crystallisation within the highly supercooled film and which continues until an equilibrium level of crystallinity is achieved. It is believed that the minor component of low density polyethylene dispersed through the polymer blend acts to partially disrupt this primary crystallisation process, therefore altering the morphology created in the film. The changes in the crystalline morphology may be inferred from a decrease observed in crystallisation- induced haze which suggests a decrease in the average size of the crystallites. Changes in the intensity of x-ray scattering at low angles (SAXS) are consistent with this theory.

The efficacy of the low-density polyethylene modifier has been found to be greater with low density polyethylenes having higher molecular weight and higher degrees of long chain branching. It is believed that it is the small amount of extremely high molecular weight, highly long-chain branched molecules that provide the modification to the morphology of the polyethylene in a manner that is particularly advantageous to the stretch wrap properties as have been described above.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word"comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps

but not the exclusion of any other integer or step or group of integers or steps.

The present invention is further described by reference to the following non-limiting examples.

Example 1 An LLDPE base resin was produced in a gas-phase (Unipol) reactor with the comonomer being 1-hexene. The resin had a melt flow index (MFI) at 190°C and 2.16 kg mass of 1.0 g/600s and a density at 25°C of 920 kg/m3. The LDPE used was produced in a high pressure autoclave reactor. The LDPE had the following properties: Melt Index 1.7 g/600s Density 918 kg/m3 mw/m. 23 M, 3.5 x 106 g/mol Long Chain Branch Frequency 3.5/1000C HMW Fraction over M, 5 x 10'10. 8% of LDPE A blend of the LLDPE and LDPE at the level of 2% (w/w) of LDPE was prepared on a compounding extruder. Films were produced at 21-22, us gauge on a blown film line typical of those used in industry, a 90 mm Battenfeld extruder fitted with a 400 mm oscillating die with a 2.4 mm die gap, and the film was blown at a blow up ratio of 2.8. The films contained, in addition, 4% (w/w) of a polyisobutene cling modifier (BP Hyvis 30, with a molecular weight of approximately 3000 g/mol) added during processing.

The films were tested using an instrumented stretch wrapper based on a LANTECH GP2000 in addition to all standard film tests. Standard film properties are shown in Table 1, while Table 2 lists the results of the automated stretch wrap tests.

Example 2 An LLDPE base resin of Example 1 was modified with an LDPE having the properties listed below: Melt Index 1.6g/600s Density 921 kg/m3 M,/Mn 23 Mz 2.4 x 106 g/mol Long Chain Branch Frequency 3.6/1000C HMW Fraction over Mw 5 x 105 5.2% of LDPE A blend of the above two polyolefins with a level of 4 % (w/w) of LDPE was prepared on a compounding extruder. Films were produced and tested in accordance with Example 1.

Standard film properties are shown in Table 1, while Table 2 lists the results of the automated stretch wrap tests.

Example 3 The LLDPE base resin of Example 1 was blended with a LDPE having the properties listed below: Melt Index 1. Og/600s Density 922 kg/m3 MW/Mn 7.4 Mz 1.0 x 106 g/mol Long Chain Branch Frequency 2.4/1000C HMW Fraction over Mw 5 x 105 1. 8% of LDPE A blend of the above two polyolefins containing a level of 5 % (w/w) of LDPE was prepared on a compounding extruder. Films were produced and tested in accordance with Example 1. Standard film properties are shown in Table 1, while Table 2 lists the results of the automated stretch wrap tests.

Table 1

Haze Gloss NAS Puncture Energy Example (%) (units) (units) 100 % LLDPE 5.3 81 3. 8 3.0 Example 13. 0871. 92. 9 3.6803.13.1Example2 6.1774.63.1Example3 Example 4 A LLDPE resin as of Examples 1-3 was modified with the same LDPE used in Example 1. A blend of the two polymers with a level of 1.5 % (w/w) of LDPE was prepared by tumble blending.

Films from this blend were produced at an average gauge of 21 urn on a Reifenhauser blown film extrusion line. The film line extruder was of 70 mm diameter with a L/D ratio of 30, with a grooved barrel feed section and a barrier screw incorporating both dispersive and distributive mixing elements at the discharge end. The die diameter was 350 mm with a 2.6 mm die gap. Film was produced at a blow up ratio of 2.3. The films contained, in addition, 5 % (w/w) of a polyisobutylene cling additive (BP Hyvis 30) added by direct injection through the screw into the feed section of the extruder.

The films were aged for 7 days, then tested on a LANTECH GP2000 instrumented stretch wrapper. The results are shown in Table 2 below.

Example 5 A blend of the same polymers as in Example 4 was tumble blended to give 4.5 % (w/w) of the LDPE. Films were produced and tested in accordance with Example 4 and the results listed in Table 2.

Example 6 A LLDPE resin as of Example 4 was tumble blended with a LDPE having the properties listed below: Melt Index 8 g/600 s Density 918 kg/m3 MW/MN 25 Mz 4.0 X106 g/mol Long Chain Branch Frequency 3.6/1000C HMW Fraction over Mw 5xlO'7. 8% A blend of the two polymers was produced with a level of 4.5% (w/w) LDPE by tumble blending. Films were produced and tested in accordance with Example 4 and the results listed in Table 2.

Example 7 A LLDPE resin as of Example 4 was tumble blended with a LDPE having the properties listed below: Melt Index 0.3 g/600 s Density 921 kg/m3 MW/MN 9. 0

Mz 2.0 x106 g/mol Long Chain Branch Frequency 3.2/1000C HMW Fraction over Mw 5x105 3. 0% A blend of the two polymers was produced with a level of 4.5% (w/w) LDPE by tumble blending. Films were produced and tested in accordance with Example 4 and the results listed in Table 2.

Example 8 A LLDPE resin as of Example 4 was tumble blended with a LDPE having the properties listed below: Melt Index 1.0 g/600 s Density 921 kg/m3 MW/MN 7.4 Mz 1.0 X106 g/mol Long Chain Branch Frequency 2.7/1000C HMW Fraction over Mw 5xlO'1. 8% A blend of the two polymers was produced with a level of 4.5% (w/w) LDPE by tumble blending. Films were produced and tested in accordance with Example 4 and the results listed in Table 2.

Table 2 Example Effective Stretch Maximum Stretch Adhesion @ 400% stretch1)(%)(%) 100 % LLDPE 335 510 3 Example 1 380 560 9 Example 2 355 510 5 3304950Example3

Example 4 420 600 9 Example 5 400 570 9 Example 6 330 535 7 Example 7 320 575 4 Example 8 350 515 4 Adhesion was assessed on a scale of 0-9 where: 0 = Non-existent 3 = Poor 6 = Good 9 = Excellent Example 9 The average unwinding force of a stretch wrap film roll is a measure of the cling, and therefore when measured as a function of aging, it is a measure of the migration of cling modifier to the film surface. Table 3 compares the unwinding force for two rolls of film, measured throughout the period in which cling developed. The first film was prepared from an unmodified LLDPE and the second film prepared from a blend of the same LLDPE and 2 % (w/w) of the LDPE described in Example 1. Full development of cling was apparent for the modified composition substansially prior to that of the unmodified LLDPE.

Table 3 Film Aging 1 day 3 days 5 days 7 days 10 days 100% LLDPE Unwinding Force (N) 4.9 13.7 19.6 22.6 26.5 1 Example 1 Unwinding Force (N) 3.9 17.7 23.5 25.5 24.5