MULTI-MODAL LINEAR LOW DENSITY POLYETHYLENE POLYMER

This invention relates to a multimodal linear low density polymer suitable for the manufacture of films which possess excellent impact properties as well as to films and other articles made from the polymer. The polymer can be used to form monolayer films or a single layer in a multilayer film, e.g. a cast film or blown film. Further the invention relates to a process for producing the polymers as well to polymers having some special improved features.

Over the past ten years there has been a rapid growth in the market for linear low density polyethylene (LLDPE). A broad range of LLDPE's are now used in injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, usually about 3 to 10 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer and the catalyst type used in the polymerization.

Many LLDPE resins typically incorporate 1-butene or 1-hexene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethyl ene/1- butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-l-pentene, and 1-octene. The bulk of the LLDPEs manufactured today are used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes them well suited for a broad spectrum of applications. LLDPE films are often characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.

These properties together with toughness are enhanced by increasing the molecular weight. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly the extrudability (from the lower molecular weight component) can be improved.

The blending of polymers is successfully achieved in a staged reactor process such as those described in United States patents 5,047,468 and 5,126,398 but while the in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, industry continues to seek films with characteristics tailored to particular applications.

WO03/066699 describes films formed from an in situ blend of two polymer components in which a metallocene catalyst is used to manufacture the polymer. The films are said to have excellent sealing properties.

WO2005/014680 describes further in situ multimodal LLDPE polymers which have applications in injection moulding. The polymers are again manufactured using metallocene catalysis.

In EP-A-778289 a two stage tandem reactor process is described for the formation of a multimodal polymer. Ziegler-Natta catalysis is employed but the LMW component is typically a homopolymer or substantially a homopolymer. In WO03/020821 , multimodal LLDPE polymers are also described. The

LLDPE's are generally made using particular constrained geometry metallocene catalysts and are for use in pipe manufacture.

The present inventors sought new multimodal polymers and films made therefrom that possess particularly good impact properties without compromising processability. This allows, for example, the formation of strong films with lower material cost.

The present inventors have now prepared a new polymer with remarkably high impact strength as well as good tear resistance which also possess desirable processability. Good processability means, in general, higher output with less energy needed.

Thus, viewed from one aspect the invention provides a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m ³ , and containing at least two α-olefin comonomers in addition to ethylene comprising:

(A) 25 to 45 wt% of a lower molecular weight component being an ethylene homopolymer or a copolymer of ethylene and at least one α-olefin;

and

(B) 75 to 55 wt% of a higher molecular weight component being a copolymer of ethylene and at least one α-olefϊn; wherein the multimodal LLDPE has a dart drop of at least 700 g; and wherein components (A) and (B) are preferably obtainable using a Ziegler-Natta catalyst.

Alternatively viewed, the invention provides a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m ³ , and containing at least one α-olefin comonomer in addition to ethylene comprising:

(A) less than 41 wt% of a lower molecular weight component being an ethylene homopolymer or a copolymer of ethylene and at least one α-olefϊn;

and

(B) more than 59 wt% of a higher molecular weight component being a copolymer of ethylene and at least one α-olefin, having a density in the range 902 to 912 kg/m ³ ; and wherein components (A) and (B) are preferably obtainable using a

Ziegler-Natta catalyst.

Viewed from another aspect the invention provides a process for the manufacture of a multimodal LLDPE as hereinbefore described comprising: in a first stage polymerising ethylene and optionally at least one α-olefin so as to form 25 to 45 wt% of a lower molecular weight component and; transferring the product of the first stage to a second stage and in a second stage polymerising ethylene and at least one α-olefin to form 75 to 55 wt% of a higher molecular weight component; ; wherein components (A) and (B) are preferably obtainable using a Ziegler- Natta catalyst.

Viewed from another aspect the invention provides a composition comprising a multimodal linear low density polymer as hereinbefore described.

Viewed from another aspect the invention provides an article, preferably a film comprising a multimodal linear low density polymer as hereinbefore described.

Viewed from another aspect the invention provides use of a film as hereinbefore described in packaging as well as an article packaged using said film. For the avoidance of doubt, by a multimodal polymer having a dart drop of at least 700 g is meant that when said multimodal polymer is formulated as a film of thickness 40 μm following the protocol set out in the film blowing example below, the dart drop of the formed film when measured using ISO 7765-1, method "A" (A dart with a 38 mm diameter hemispherical head is dropped from a height of 0.66 m onto a film clamped over a hole. If the specimen fails, the weight of the dart is reduced and if it does not fail the weight is increased. At least 20 specimens are tested. The weight resulting in failure of 50% of the specimens is calculated), a value of at least 700 g is obtained.

Properties of the Multimodal LLDPE

This invention relates to a multimodal linear low density polyethylene having at least two components, a lower molecular weight component (A) and a higher molecular weight component (B). The multimodal LLDPE polymer of the invention should have a density of

900 to 940 kg/m ³ , preferably less than 935 kg/m ³ , e.g. 905-939 kg/m ³ , preferably in the range of from 910 to 930 kg/m ³ , such as 910 to 926 kg/m ³ , e.g. 918 to 923 kg/m ³ (ISO 1183-1 :2004 "Immersion method"). The MFR ₂ of the multimodal LLDPE is preferably be in the range 0.001 to 10 g/lOmin, preferably 0.01 to 5 g/lOmin, e.g. 0.05-1 g/10min. Generally, MFR ₂ is less than 5, especially less than 3 g/10min (ISO 1133, 190 °C/min, 2,16 kg load).

The MFR ₅ of the multimodal LLDPE is preferably be in the range 0.05 to 10 g/10min, preferably 0.1 to 5 g/10min, e.g. 0.5-3 g/10min, especially 0.8 to 3 g/10min (ISO 1133, 190 °C/min, 5,0 kg load). The MFR _2I for multimodal LLDPE should be in the range 5 to 150, preferably 10 to 100 g/10min, e.g. 15 to 70 g/10 min (ISO 1133, 190 °C/min, 21,6 kg load).

The FRR (MFR ₂₁ /MFR ₂ ) of the polymer of the invention may be 10 to 100.

The Mw of multimodal LLDPE should be in the range 100,000 to 400,000, preferably 130,000 to 300,000. The Mn should be in the range 5000 to 35,000, preferably 8,000 to 25,000. The Mw/Mn for multimodal LLDPE should be in the range 5 to 25, e.g. 7 to 22.

The multimodal LLDPE of the invention possess a low xylene soluble fraction. The XS may be less than 20 wt%.

The multimodal LLDPE may formed from ethylene along with at least one other α-olefϊn comonomer, preferably at least one C3-12 α-olefin comonomer, more preferably at least one C4-12 α-olefin comonomer, e.g. 1-butene, 1-hexene or 1- octene. The HMW component can contain at least one comonomer which is the same as one employed in the LMW component but ideally both components are not polymers of ethylene and butene alone. It is possible for both components to be polymers of ethylene and hexene (or ethylene and octene and so on) although it will be appreciated that both components are different even if the same comonomer is used in both components. By definition for example, the molecular weight of the two components must be different.

Preferred comonomer combinations include (LMW/HMW) butene/hexene, hexene/butene and hexene/hexene. The multimodal LLDPE is preferably formed from ethylene along with at least two other α-olefin comonomers, preferably C3-12 α-olefin comonomers. Preferably, the multimodal LLDPE is a terpolymer, i.e. the polymer contains ethylene and two comonomers. It is also preferred if the HMW component contains at least one comonomer which is different from that employed in the LMW component. It is also preferred if the HMW component contains at least one comonomer which is of higher molecular weight to that employed in the LMW component. The HMW component can comprise the same monomer as used in the LMW component as long as the HMW component additionally contains a comonomer different from and preferably heavier than that used in the LMW component.

The amount of comonomer present in the multimodal LLDPE as a whole is preferably 1 to 20 wt%, e.g. 2 to 15% wt% relative to ethylene, especially 5 to 13

wt%. Preferably the 1-hexene content of the LLDPE of the invention is less than 8 mol%. Ideally, the butene content of the multimodal polymer of the invention is less than 5 mol%, more preferably less than 4 mol%, determined by C ¹³ NMR. Where both butene and hexene are present, the molar ratio of these components may be in the range 5:1 to 1 :5, e.g. 3:1 to 1 :3, preferably 1 :1 to 1 :3. As butene is a cheaper comonomer than hexene any increase in the butene content without loss of properties is advantageous.

As noted above, the multimodal LLDPE of the invention comprises at least a lower molecular weight component (LMW) and a higher molecular weight (HMW) component.

Usually, a polyethylene, e.g. LLDPE composition, comprising at least two polyethylene fractions, which have been produced under different polymerization conditions resulting in different (weight average) molecular weights and molecular weight distributions for the fractions, is referred to as "multimodal". The prefix "multi" relates to the number of different polymer fractions present in the polymer. Thus, for example, multimodal polymer includes so called "bimodal" polymer consisting of two fractions. The form of the molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as a function of its molecular weight, of a multimodal polymer, e.g. LLDPE, will show two or more maxima or is typically distinctly broadened in comparison with the curves for the individual fractions. For example, if a polymer is produced in a sequential multistage process, utilizing reactors coupled in series and using different conditions in each reactor, the polymer fractions produced in the different reactors will each have their own molecular weight distribution and weight average molecular weight. When the molecular weight distribution curve of such a polymer is recorded, the individual curves from these fractions form typically together a broadened molecular weight distribution curve for the total resulting polymer product.

The LMW component has a lower molecular weight than the higher molecular weight component. Preferably there may be a difference in molecular weight of at least 1000, preferably at least 5000, especially at least 20,000 between components.

The multimodal LLDPE of the invention is preferably bimodal or trimodal, especially bimodal.

LMW Component

The lower molecular weight component of the multimodal LLDPE preferably has a MFR ₂ of at least 50, preferably at least 100 g/10min, preferably 110 to 3000 g/10min, e.g. 110 to 500 g/lOmin, especially 150 to 400 g/lOmin. The molecular weight of the low molecular weight component should preferably range from 15,000 to 50,000, e.g. 20,000 to 40,000.

It is preferred if the Mw/Mn of the LMW component is in the range 3 to 10, e.g. 5 to 8.

The density of the lower molecular weight component may range from 930 to 980 kg/m ³ , e.g. 940 to 970 kg/m ³ preferably 945 to 965 kg/m ³ , especially 947 to 955 kg/m ³ . It is a feature of the invention that the density of the LMW component is comparatively low for a multimodal polymer. It is preferred however if the density is 950 kg/m ³ or more.

The amount of LMW component is critical. In order to ensure the very high dart drop values required of the polymers of the invention, the HMW component needs to be in excess. The lower molecular weight component should preferably form 25 to 45 wt%, e.g. 30 to 43% by weight, especially 32 to 42 wt%, most especially 33 to 40 wt% of the multimodal LLDPE. Ideal splits are around 35 or 36% LMW component. In a further preferred embodiment, the LMW component forms less than 41 wt% of the multimodal LLDPE, e.g. 30 to 40 wt%. Especially preferably the lower molecular weight component forms 39 wt% or less of the multimodal LLDPE.

The lower molecular weight component can be an ethylene homopolymer (i.e. where ethylene is the only monomer present) but is preferably an ethylene copolymer, especially where only one comonomer is present. Especially preferably it is a copolymer of ethylene and 1-butene .

The comonomer content in the LMW component is preferably kept as low as possible. Comonomer contents of the order of less than 3 wt% are appropriate, preferably less than 2wt% .

HMW Component

The higher molecular weight component should have a lower MFR ₂ and a lower density than the lower molecular weight component.

The higher molecular weight component should have an MFR ₂ of less than 1 g/10 min, preferably less than 0.5 g/10 min, especially less than 0.2 g/lOmin. The MFR ₂₁ of the HMW component should be in the range 0.1 to 20, preferably 1 to 10 g/10min, e.g. 2 to 8 g/10 min.

The higher molecular weight component should have a density of less than 915 kg/m , e.g. less than 913 kg/m , preferably less than 912 kg/m , especially less than 910 kg/m ³ . It is a feature of the invention that the HMW component possesses a very low density. It is also preferred however if the density of the HMW component is greater than 902 kg/m ³ . Ideally, the density should be in the range 902 to 912 kg/m ³ .

This combination of features is believed to improve the processability of the polymers of the invention making them, for example, easy to extrude on a film line and hence capable of increased film line output.

The Mw of the higher molecular weight component may range from 100,000 to 1,000,000, preferably 150,000 to 500,000.

It is preferred if the Mw/Mn of the HMW component is in the range 3 to 10, e.g. 5 to 8.

The higher molecular weight component forms 70 to 40 wt%, e.g. 65 to 45% by weight, more preferably 60 to 50, especially 60 to 52 wt% of the multimodal LLDPE. In a further highly preferred embodiment, the HMW component forms more than 59 wt% of the multimodal LLPDE, preferably more than 60 wt%, e.g. 61 wt% or more, e.g. more than 61 wt%. In some embodiments, the HMW component can form 62 wt% or more, e.g. 63 wt% or more, such as 64 wt% or more of the multimodal LLDPE.

The higher molecular weight component is preferably an ethylene copolymer, in particular a binary copolymer (i.e. where only one comonomer is present) or a terpolymer (with two comonomers). It is preferred if the HMW component contains at least one α-olefin which is not present in the LMW component. It is also preferred if the HMW component contains at least one comonomer of greater molecular weight than those used in the LMW component.

Where the LMW component is a homopolymer, the HMW component preferably contains at least 2 α-olefin comonomers.

Especially preferably the HMW component is a binary copolymer of ethylene and hexene or a terpolymer of ethylene, butene and hexene.

The amount of comonomer present in the HMW component may range from 1 to 6 wt%, e.g. 2 to 5 wt%, especially 3 to 5 wt%. It should be noted that comonomer amounts in HMW component can not be measured directly (in a process where the HMW component is formed second in a multistage process), but may be calculated based on the amount of the LMW component present and of the final polymer as well as knowledge of the production split.

Especially preferably there is a low butene content in this component and a higher content of hexene. The butene content may be of the order of less than 1 mol%, e.g. 0.1 to 1 mol%, such as 0.2 to 0.8 mol% and the hexene content in the range 1 to 5 mol%.

Where both butene and hexene are present in the HMW component, the molar ratio between these two may be at least 1 :2, e.g. at least 1 :4, preferably at least 1 :6, especially at least 1 :10.

In the most highly preferred embodiments of the invention the LMW component is an ethylene butene copolymer and the HMW component is an ethylene hexene copolymer or ethylene butene hexene terpolymer. Alternatively, the LMW component is an ethylene homopolymer and the HMW component is an ethylene butene octene terpolymer.

Viewed from another aspect the invention therefore provides a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m ³ , and containing butene and hexene in addition to ethylene comprising:

(A) 39 wt% or less of a lower molecular weight component being an ethylene butene copolymer;

and

(B) 61 wt% or more of a higher molecular weight component being a ethylene butene hexene terpolymer or ethylene hexene copolymer and having a density in the range 902 to 912 kg/m ³ ; and wherein components (A) and (B) are obtainable using a Ziegler-Natta catalyst.

Viewed from another aspect the invention therefore provides a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m ³ , and containing butene and hexene in addition to ethylene comprising:

(A) 39 wt% or less of a lower molecular weight component being an ethylene butene copolymer and having a density of 950 kg/m ³ or more;

and

(B) 61 wt% or more of a higher molecular weight component being a ethylene butene hexene terpolymer or ethylene hexene copolymer and having a density in the range 902 to 912 kg/m ³ ; and wherein components (A) and (B) are obtainable using a Ziegler-Natta catalyst. Viewed from another aspect the invention therefore provides a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m ³ , and containing butene and hexene in addition to ethylene comprising:

(A) 39 wt% or less of a lower molecular weight component being an ethylene butene copolymer and having a density of 950 kg/m ³ or more;

and

(B) 61 wt% or more of a higher molecular weight component being a ethylene butene hexene terpolymer or ethylene hexene copolymer and having a density in the range 902 to 912 kg/m ³ ; wherein components (A) and (B) are preferably obtainable using a Ziegler-

Natta catalyst and wherein the molar ratio of the butene content to hexene content in the polymer is 1 :1 to 1 :3; and preferably wherein the tear resistance of the polymer in the transverse direction is at least ION.

Further Components of the Multimodal Polymer

The multimodal LLDPE may comprise other polymer components over and above the LMW and HMW components. For example, the polymer may contain up to 10 % by weight of a polyethylene prepolymer (obtainable from a prepolymerisation step as well known in the art). In case of such prepolymer, the prepolymer component may be comprised in one of LMW and HMW components, preferably LMW component, as defined above.

Further Properties

The polymer of the invention can exhibit very high dart drop. Thus for a 40 μm film manufactured as described below, Dart drop F50 (ISO 7765/1) may be at least 240, e.g. at least 350 g, preferably at least 400 g, e.g. at least 50Og, more preferably at least 700 g, especially at least 800 g, most especially at least 900 g. Some polymers of the invention exhibit dart drop values of over 1000 g.

The multimodal LLDPE polymer of the invention also exhibits excellent tear resistance. When measured on a film of 40 μm of the polymer of the invention and manufactured as described in the examples tear resistance in the machine direction may be at least IN, preferably at least 1. IN, especially ay least 1.2N. The tear resistance in the transverse direction may be at least ION, preferably at least 11 N, especially at least 12 N.

The polymer of the invention can exhibit excellent shear thinning properties. The SHI 1/100 values may be 11 or more, preferably 12 or more.

The SHI 2.7/210 values are typically 35 or more, e.g. 40 or more.

Where a property is measured on a film of 40 μm, that film is made as a monolayer film using the polymer of the invention only following the protocol in the examples.

Manufacture

Multimodal LLDPE polymers may be prepared for example by two or more stage polymerization or by the use of two or more different Ziegler Natta polymerization catalysts in a one stage polymerization. It is important, however, to ensure that the higher and lower molecular weight components are intimately mixed prior to extrusion. This is most advantageously achieved by using a multistage process.

Preferably the multimodal LLDPE is produced in a two-stage polymerization using the same Ziegler-Natta catalyst in both steps. Two-stage polymerisation can be carried out in one reactor or e.g. in two different reactors. In the latter case, for example, two slurry reactors or two gas phase reactors could be employed. Preferably however, the multimodal LLDPE is made using a slurry polymerization in a loop reactor followed by a gas phase polymerization in a gas phase reactor.

A loop reactor - gas phase reactor system is marketed by Borealis as a BORSTAR reactor system. Any multimodal LLDPE of the invention is preferably formed in a two stage process comprising a first slurry loop polymerisation followed by gas phase polymerisation.

The conditions used in such a process are well known. For slurry reactors, the reaction temperature will generally be in the range 60 to 110°C (e.g. 85-110°C), the reactor pressure will generally be in the range 5 to 80 bar (e.g. 50-65 bar), and the residence time will generally be in the range 0.3 to 5 hours (e.g. 0.5 to 2 hours). The diluent used will generally be an aliphatic hydrocarbon having a boiling point in the range -70 to +100°C. In such reactors, polymerization may if desired be effected

under supercritical conditions. Slurry polymerisation may also be carried out in bulk where the reaction medium is formed from the monomer being polymerised.

For gas phase reactors, the reaction temperature used will generally be in the range 60 to 115°C (e.g. 70 to HO ⁰ C), the reactor pressure will generally be in the range 10 to 25 bar, and the residence time will generally be 1 to 8 hours. The gas used will commonly be a non-reactive gas such as nitrogen or low boiling point hydrocarbons such as propane together with monomer (e.g. ethylene).

Preferably, the lower molecular weight polymer fraction is produced in a continuously operating loop reactor where ethylene is polymerised in the presence of comonomer(s), a Ziegler Natta polymerization catalyst with conventional cocatalysts, i.e. compounds of Group 13 metal, like Al alkyl compounds, and a chain transfer agent such as hydrogen. The diluent is typically an inert aliphatic hydrocarbon, preferably isobutane or propane. Preferably the C4/C2 ratio in the first stage is 200 to 600 mol/kmol. The hydrogen feed may be of the order of 50 to 150 g/h.

The higher molecular weight component can then be formed in a gas phase reactor using the same catalyst.

The split between the two components is critical. Whilst higher concentrations of HMW component increase the dart drop and tear resistance, it becomes more difficult to run a process for the manufacture of the polymer when the proportion of HMW component becomes too high. It is a feature of the invention that very high dart drop values can be achieved at conventional LMW/HMW split ratios.

Preferably therefore the LMW and HMW components are made in situ. Where the HMW component is made as a second step in a multistage polymerisation it is not possible to measure its properties directly. However, e.g. for the above described polymerisation process of the present invention, the density, MFR2 etc of the HMW component can be calculated using Kim McAuley's equations. Thus, both density and MFR2 can be found using K. K. McAuley and J. F. McGregor: On-line Inference of Polymer Properties in an Industrial Polyethylene Reactor, AIChE Journal, June 1991, Vol. 37, No, 6, pages 825-835. The density is calculated from McAuley's equation 37, where final density and density after the

first reactor is known. MFR2 is calculated from McAuley's equation 25, where final MFR2 and MFR2 after the first reactor is calculated.

All components of the multimodal LLDPE of the invention are preferably made using a Ziegler Natta catalyst. Preferred Ziegler-Natta catalysts comprise a transition metal component and an activator. The transition metal component comprises a metal of Group 4 or 5 of the Periodic System (IUPAC) as an active metal. In addition, it may contain other metals or elements, like elements of Groups 2, 13 and 17. Preferably, the transition metal component is a solid. More preferably, it has been supported on a support material, such as inorganic oxide carrier or magnesium halide. Examples of such catalysts are given, among others in WO

95/35323, WO 01/55230, EP 810235 and WO 99/51646. The catalysts disclosed in WO 95/35323 are especially useful as they are well suited in production of both a polyethylene having a high molecular weight and a polyethylene having a low molecular weight. Thus, especially preferably the transition metal component comprises a titanium halide, a magnesium alkoxy alkyl compound and an aluminium alkyl dihalide supported on an inorganic oxide carrier.

In one embodiment a catalyst of Ziegler Natta type, wherein the active components are dispersed and solidified within Mg-based support by the emulsion/solidification method adapted to PE catalyst, e.g. as disclosed in WO03106510 of Borealis, e.g. according to the principles given in the claims thereof.

In another preferable embodiment, the catalyst is a non-silica supported catalyst, i.e. the active components are not supported to an external silica support. Preferably, the support material of the catalyst is a Mg-based support material. Examples of such preferred Ziegler-Natta catalysts are described in EP 0 810 235. Multimodal (e.g. bimodal) polymers can also be made by mechanical blending of the polymer in a known manner.

In a very preferable embodiment of the invention the polyethylene composition is produced using a ZN catalysts disclosed in EP 688794. Conventional cocatalysts, supports/carriers, electron donors etc can be used.

Ideally, the Ziegler Natta catalyst is a Mg complex formed with BOMAG (butyl octyl magnesium) and 2-ethylhexanol. The molar ratio of these components can be in the range EHA/Mg molar ratio 1.5 to 2.5.

The catalyst is ideally supported on a carrier such as silica. Ethyl aluminium dichloride is the preferred Al compound and TiCl ₄ the preferred titanium species.

As noted above, a crucial aspect of the invention is the very high amount of HMW polymer present in the multimodal LLDPE. To achieve such a marked split between lower and higher molecular weight components requires manipulation of the polymerisation parameters. For example, higher levels of catalyst than normal might be used, the ethylene partial pressure may be lowered in the LMW phase compared to conventional levels but elevated in the HMW phase. It may also be necessary to increase flushing from the LMW stage.

Compositions

The multimodal polymer of the invention can be combined with other polymer components, e.g. LDPE, LLDPE components or HDPE polymers to form a composition comprising the polymer of the invention. It is also possible to combine two polymers of the invention to make a highly preferred composition. Preferably however, no other polymer components are present and the multimodal polymer of the invention is the only polymer component used in the manufacture of a film (or layer of a film). The polymer can however form a composition with conventional additives such as antioxidants, UV stabilisers, acid scavengers, nucleating agents, anti-blocking agents as well as polymer processing agent (PPA).

Film Formation and Properties

The polymer of the invention can be in the form of powder or pellets, preferably pellets. Pellets are obtained by conventional extrusion, granulation or grinding techniques and are an ideal form of the polymer of the invention because they can be added directly to converting machinery. Pellets are distinguished from polymer powders where particle sizes are less than 1 mm.

The use of pellets ensures that the composition of the invention is capable of being converted in a film, e.g. monolayer film, by the simple in line addition of the pellets to the converting machinery.

For film formation using a polymer mixture it is important that the different polymer components be intimately mixed prior to extrusion and casting/blowing of the film as otherwise there is a risk of inhomogeneities, e.g. gels, appearing in the film.

The polymers of the invention have been found to allow the formation of films having an ideal balance of properties. They have excellent mechanical properties and are readily processed. In particular, films exhibit high dart impact strengths, high tear strengths, sealability and good processability.

The films of the invention are preferably monolayer films or the polymer of the invention is used to form a layer within a multilayer film. Any film of the invention may have a thickness of 10 to 250 μm, preferably 20 to 200 μm, e.g. 30 to 150 μm, such as e.g. 30 to 135 μm, preferably 30 to 60 μm.

The films of the invention can be manufactured using simple in line addition of the polymer pellets to an extruder. For film formation using a polymer mixture it is important that the different polymer components be intimately mixed prior to extrusion and blowing of the film as otherwise there is a risk of inhomogeneities, e.g. gels, appearing in the film. Thus, it is especially preferred to thoroughly blend the components, for example using a twin screw extruder, preferably a counter- rotating extruder prior to extrusion and film blowing. Sufficient homogeneity can also be obtained by selecting the screw design for the film extruder such that it is designed for good mixing and homogenising. The film of the invention can be blown or cast, preferably blown. Blown films will typically be produced by extrusion through an annular die, blowing into a tubular film by forming a bubble which is collapsed between nip rollers after solidification. This film can then be slit, cut or converted (e.g. gusseted) as desired. Conventional film production techniques may be used in this regard. Typically the composition will be extruded at a temperature in the range 160°C to 240°C, and cooled by blowing gas (generally air) at a temperature of 10 to 50°C to provide a

frost line height of 1 or 2 to 8 times the diameter of the die. The blow up ratio should generally be in the range 1.5 to 4, e.g. 2 to 4, preferably 2.5 to 3.

The film of the invention can also be a cast film. The cast film process involves the extrusion of polymers melted through a slot or flat die to form a thin, molten sheet or film. This film is "pinned" to the surface of a chill roll (typically water-cooled and chrome-plated) by a blast of air from an air knife or vacuum box. The film quenches immediately and then has its edges slit prior to winding. Because of the fast quenching step, a cast film generally has better optical properties than a blown film and can be produced at higher line speeds. The films of the invention exhibit high dart impact strengths and tear strengths, especially in the machine direction. In the passages which follow, certain parameters are given based on a specific film thickness. This is because variations in thickness of the film cause a change to the size of the parameter in question so to obtain a quantitative value, a specific film thickness is quoted. This does not mean that the invention does not cover other film thicknesses rather it means that when formulated at a given thickness, the film should have the given parameter value.

Thus for a 40 μm film manufactured as described below, Dart drop F50 (ISO 7765/1) may be at least 240, e.g. at least 350 g, preferably at least 400 g, e.g. at least 500g, more preferably at least 700 g, especially at least 800 g, most especially at least 900 g. Some polymers of the invention exhibit dart drop values of over 1000 g.

Elmendorf Tear resistances in the machine direction for a 40 μm film, especially a monolayer film, more especially a monolayer film consisting essentially of the multimodal polymer of the invention may be at least 1.5 N.

Elmendorf Tear resistances in the transverse direction for a 40 μm film prepared as described in the examples section may be at least IO N, 12N, especially at least 15N. In general as TD tear resistance improves, MD resistance decreases. The tear resistance in the transverse direction may be at least 1 ON, preferably at least 11 N, especially at least 12 N.

The films of the invention, e.g. monolayer films may be laminated on to barrier layers as is known in the art. For food and medical applications for example, it may be necessary to incorporate a barrier layer, i.e. a layer which is impermeable to water and oxygen, into the film structure. This can be achieved using

conventional lamination techniques. Suitable barrier layers are known and include polyamide, ethylene vinyl alcohol, PET and metallised Al layers.

Viewed from another aspect therefore the invention provides a laminate comprising a film as hereinbefore defined laminated onto a barrier layer. In such an embodiment it may be convenient to laminate the barrier layer onto two monolayer films as hereinbefore described thereby forming a 3 layer structure in which the barrier layer forms the middle layer.

The films of the invention have a wide variety of applications but are of particular interest in packaging of food and drink, consumer and industrial goods, medical devices and in heavy duty packaging. Specific applications include industrial liners, heavy duty shipping sacks, carrier bags, bread bags and freezer bags.

Other Applications

The polymers of the invention may also be used in rotomoulding, injection moulding, blow moulding, extrusion coating, wire, cable and pipe formation.

The invention will now be described further with reference to the following non-limiting examples:

Analytical Tests

The following methods were used to measure the properties that are defined generally above and in examples below. The material and film samples used for the measurements and definitions were prepared as described under the particular method or in tables.

Density

Density of the materials is measured according to ISO 1183-1 :2004 "Immersion method".

MFR

MFR2/5/21 are measured according to ISO 1133 at 190°C at loads of 2.16, 5 and

21.6 kg respectively.

Dart Drop

Impact resistance is determined on Dart-drop (g/50%). Dart-drop is measured using ISO 7765-1, method "A". A dart with a 38 mm diameter hemispherical head is dropped from a height of 0.66 m onto a film clamped over a hole. If the specimen fails, the weight of the dart is reduced and if it does not fail the weight is increased. At least 20 specimens are tested. The weight resulting in failure of 50% of the specimens is calculated.

The films were produced as described below in the film preparation example.

Tear resistance (determined as Elmendorf tear (N))

The tear strength is measured using the ISO 6383/2 method. The force required to propagate tearing across a film specimen is measured using a pendulum device. The pendulum swings under gravity through an arc, tearing the specimen from a pre-cut slit. The specimen is fixed on one side by the pendulum and on the other side by a stationary clamp. The tear strength is the force required to tear the specimen. The relative tear resistance (N/mm) is then calculated by dividing the tear resistance by the thickness of the film.

The films were produced as described below in the film preparation example

Molecular weights, molecular weight distribution ( Mn, Mw, MWD) The weight average molecular weight Mw and the molecular weight distribution (MWD = Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) is measured by a method based on ISO 16014-4:2003. A Waters 150CV plus instrument, equipped with refractive index detector and online viscosimeter was used with 3 x HT6E styragel columns from Waters (styrene-divinylbenzene) and 1 ,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 140 °C and at a constant flow rate of 1 mL/min. 500 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014- 2:2003) with 10 narrow MWD polystyrene (PS) standards in the range of 1.05 kg/mol to 11 600 kg/mol. Mark Houwink constants were used for polystyrene and polyethylene (K: 19 xlO ^"3 dL/g and a: 0.655 for PS, and K: 39 xlO ^"3 dL/g and a: 0.725 for PE). All samples were prepared by dissolving 0.5 - 3.5 mg of polymer in 4 mL (at 140 °C) of stabilized TCB (same as mobile phase) and keeping for 2 hours at 140 ⁰ C and for another 2 hours at 160 °C with occasional shaking prior sampling in into the GPC instrument.

Comonomer Content

Comonomer content of the obtained products was measured with Fourier transform infrared spectroscopy (FTIR) calibrated with ¹³ C-NMR.

Dynamic viscosity and Shear thinning index

Dynamic rheological measurements were carried out with a rheometer, namely

Rheometrics RDA-II, on compression moulded samples under nitrogen atmosphere at 190 °C using 25 mm diameter plates and plate and plate geometry with a 1.2 mm gap. The oscillatory shear experiments were done within the linear viscosity range of strain at frequencies from 0.05 to 300 rad/s (ISO 6721-1). Five measurement points per decade are made.

The values of storage modulus (G'), loss modulus (G") complex modulus (G*) and complex viscosity (η*) were obtained as a function of frequency (η). η 100 is used as abbreviation for the complex viscosity at the frequency of 100 rad/s. Shear thinning index (SHI), which correlates with MWD and is independent of Mw, was calculated according to Heino ("Rheological characterization of polyethylene fractions" Heino, E. L., Lehtinen, A., Tanner J., Seppala, J., Neste Oy, Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th (1992), 1, 360-362, and "The influence of molecular structure on some rheological properties of polyethylene", Heino, EX., Borealis Polymers Oy, Porvoo, Finland, Annual Transactions of the Nordic Rheology Society, 1995.)

SHI value is obtained by calculating the complex viscosities η*(1.0 kPa) and η*(100 kPa) at a constant value of complex modulus of 1.0 kPa and 100 kPa, respectively. The shear thinning index SHI(I /100) is defined as the ratio of the two viscosities η*(l kPa) and η*(100 kPa), i.e. η(l)/η(100).

Values at 2.7 and 210 kPa are also measured.

Differential Scanning Calorimeter (DSC)

The Melting Temperature (Tm) and the Crystallization Temperature (Tcr) were measured with Mettler TA820 differential scanning calorimeter (DSC) on 3±0.5 mg samples. Both crystallization and melting curves were obtained during 10°C/min cooling and heating scans between -10 - 200°C. Melting and crystallization temperatures were taken as the peaks of endotherms and exotherms, respectively. The degree of crystallinity was calculated by comparison with heat of fusion of a perfectly crystalline polyethylene, i.e. 290 J/g.

Preparation of the catalyst:

Mg complex preparation: BOMAG (butyl octyl magnesium, 20wt% sol in toluene 1,4 mol Mg/kg of carrier) was fed to a reactor. 2-ethylhexanol (2-EHA -

2,56 mol/kg of carrier, dried on molecular sieves) was slowly added (at least Ih addition). The EHA/Mg molar ratio was 1,83. Reaction temperature was kept below 38°C. The solution was mixed for 5h minimum.

480 kg of silica carrier, calcinated at 600°C for 4h, was added to a reactor. EADC (1 ,6mol/kg of carrier) and pentane (0, 19kg/kg of carrier) were mixed together and then slowly transferred to the reactor containing silica. During this operation the reactor temperature is kept between 30 and 40°C. The solution EADC/pentane was mixed with the silica for 2h. Then, Mg-complex (1,4 mol Mg/kg of carrier) was added during minimum 90 min to the reactor and the temperature was kept under 40°C during the addition. The mixture was mixed during Ih at a temperature between 30 and 40°C.

Then pentane (0,55kg/kg of carrier) was added to the reactor and mixed for 5h. The temperature was kept under 40°C. TiCl ₄ (0,7 mol/kg of carrier) was added during at least 45 min and the reactor temperature was kept. The mixture was mixed during 5h at a temperature between 40 and 50°C.

After the catalyst was dried under nitrogen flow during 2h at 60°C and 2h at 80°C.

Polymerisation

The polymerisation were carried out in a two stage process comprising a slurry loop polymerisation followed by a gas phase polymerisation.

The first stage of the polymerisations below was carried out in a 500 dm ³ loop reactor in the presence of ethylene, comonomer, propane and hydrogen in the amounts specified in table 1. The temperature was 85°C The catalyst was added directly to the loop reactor as well as the cocatalyst. The cocatalyst (TEA) as 10 wt% solution in pentane) are further diluted with propane to have a final concentration between 1 and 2 wt%. The amount of cocatalyst fed is calculated in order to maintain an Al/Ti ratio of 20 mol/mol. The polymer containing active catalyst was separated from the reaction medium and transferred to a gas phase reactor operated at 20 bar pressure and 85°C

where additional ethylene, hydrogen and comonomer were added and the amount are also specified in Table 1.

After the gas phase, the polymer was degassed and conveyed to the extruder. Other polymers were produced under similar conditions described further in table 1.

Film Blowing

Preparation Method of Film Samples:

40 μm thick films were blown on a Reifenhauser monofilm line with extruder 25d, die diameter 250mm, and die gauge 1.5 mm. The blow up ratio was 2.5:1, and frost line height, FLH, 600 mm. The temperature profile range was 180 -210°C.

As noted above, various properties of the polymer/film of the invention are dependent on the nature of the film. Such properties are measured on a film made by the above protocol.

Table 1 Loop Conditions

N)