AU2006233253A1 - Novel polyethylene films - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: BP CHEMICALS LIMITED Invention Title: NOVEL POLYETHYLENE FILMS The following statement is a full description of this invention, including the best method of performing it known to me/us:

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0 IND NOVEL POLYETHYLENE FILMS O The present invention relates to copolymers of ethylene and alpha-olefins in particular to low density copolymers and also to novel films produced from said copolymer having improved properties in particular improved stretch and creep characteristics.

In recent years there have been many advances in the production of polyolefin copolymers due to the introduction ofmetallocene catalysts. Metallocene catalysts offer the advantage of generally higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single-site in nature. Because of their singlesite nature the polyolefin copolymers produced by metallocene catalysts often are quite uniform in their molecular structure. For example, in comparison to traditional Ziegler produced materials, they have relatively narrow molecular weight distributions (MWD) and narrow Short Chain Branching Distribution (SCBD).

Although certain properties of metallocene products are enhanced by narrow MWD, difficulties are often encountered in the processing of these materials into useful articles and films relative to Ziegler produced materials. In addition, the uniform nature of the SCBD of metallocene produced materials does not readily permit certain structures to be obtained.

Recently a number of patents have published directed to the preparation of films based on low density polyethylenes prepared using metallocene catalyst compositions.

WO 94/14855 discloses linear low density polyethylene (LLDPE) films prepared using a metallocene, alumoxane and a carrier. The metallocene component is typically a bis (cyclopentadienyl) zirconium complex exemplified by bis (n-butylcyclopentadienyl) I zirconium dichloride and is used together with methyl alumoxane supported on silica.

O The LLDPE's are described in the patent as having a narrow Mw/Mn of 2.5-3.0, a melt c flow ratio (MPR) of 15-25'and low zirconium residues.

o WO 94/26816 also discloses films prepared from ethylene copolymers having a 0 5 narrow composition distribution. The copolymers are also prepared from traditional metallocenes (eg bis (1-methyl, 3-n-butylcyclopentadienyl) zirconium dichloride and methylalumoxane deposited on silica) and are also characterised in the patent as having Sa narrow Mw/Mn values typically in the range 3-4 and in addition by a value of Mz/Mw n of less than 0, 10 However, it is recognised that the polymers produced from these types of O catalyst system have deficiencies in processability due to their narrow Mw/Mn. Various approaches have been proposed in order to overcome this deficiency. An effective method to regain processability in polymers of narrow Mw/Mn is by the use of certain catalysts which have the ability to incorporate long chain branching (LCB) into the polymer molecular structure. Such catalysts have been well described in the literature, illustrative examples being given in WO 93/08221 and EP-A-676421.

Furthermore, WO 97/44371 discloses polymers and films where long chain branching is present, and the products have a particularly advantageous placement of the comonomer within the polymer structure. Polymers are exemplified having both narrow and broad Mw/Mn, for example from 2.19 up to 6.0, and activation energy of flow, which is an indicator of LCB, from 7.39 to 19.2 kcal/mol (31.1 to 80.8kJ/mol).

However, there are no examples of polymers of narrow Mw/Mn, for example less than 3.4, which also have a low or moderate amount of LCB, as indicated by an activation energy of flow less than 11.lkcal/mol (46.7kJ/mol).

We have now found that it is possible to prepare copolymers of ethylene and alpha-olefins having narrow Mw/Mn and low or moderate amounts of LCB. These polymers are suitable for many applications which will be known to those skilled in the art, but in particular are advantageous for preparing films with an excellent balance of processing, optical and mechanical properties.

In particular the present invention is particularly directed to stretch films with excellent cling properties and to blown films suitable for use for heavy duty sacks.

Our copending application WO 00/68285 describes copolymers of ethylene and ND an alpha olefin having 3 to 10 carbon atoms, said copolymers having 0 a density in the range 0.900 to 0.940 C an apparent Mw/Mn of 2 3.4 O 2 1 /2 from 16 to 24 activation energy of flow from 28 to 45 kJ/mol a ratio EaHMW)/Ea(LMW) and a ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.

C These copolymers may be used to prepare the full range of products normally cr manufactured from polyethylene copolymer products in the density range 0.900 to 0.940 ,D 10 kg/m Examples of applications for the copolymers include injection moulding, Srotomoulding, extrusion into pipes, sheets, films, fibres, non-woven fabrics, cable coverings and other uses which will be known to those skilled in the art are particularly suitable for the production of films and sheets prepared using traditional methods well known in the art Examples of such methods are film blowing, film casting and orientation of the partially crystallised product. The films exhibit good processability, improved optical and mechanical properties and good heat sealing properties.

WO 00/68285 described blown films from such copolymers having haze ranging from 3 to 20, dart impact 100g and hexane extractables in the range 0.1 Such films also exhibited a MD tear strength in the range 106 210g/25pm.

The application of polyethylene films in stretch wrapping has been considerably enhanced by the use of linear low density polyethylene (LLDPE) type products. When formed into a film for stretch wrap application, LLDPE products typically combine a high extensibility with good mechanical properties to provide a wrapping or collation function to be achieved in an economic and effective manner. In this respect, LLDPE has significant advantages over LDPE which, due to both its behaviour in extension and its mechanical performance, is not normally regarded as a product of choice for stretch wrapping applications.

Application of stretch wrap films may be either by hand or by machine. The film may be either wrapped directly onto the article or articles to be packaged, or it may undergo a pre-stretching operation prior to wrapping. Pre-stretching typically enhances the mechanical property of the film and provides a more effective packaging and more efficient coverage for a given unit mass of film. Hence the response of the film to either IN a pre-stretch or the stretch applied during wrapping is an important parameter affecting Sfilm performance. In particular for a given film width and thickness the efficiency with which an object is wrapped is affected by the degree to which the film can be thinned O during the stretching and the loss of film width which may occur at the same time. The 0 5 resistance to sudden impact events, puncture by sharp objects and the ability to maintain a tension sufficient to maintain the package in the desired shape and configuration are Salso important parameters.

cA further requirement in many stretch wrapping applications is that the film c displays a certain degree of adhesive or cling behaviour enabling a firm closure of the 0O 10 package to be achieved without resort to use of additional securing measures such as 0 straps, glues or heat sealing operations. For monolayer films, such adhesion may be provided by the intrinsic film properties or by using a "cling" additive in the film formulation. An example of a cling additive which is widely used is poly(isobutene) (PIB) which term is taken to include polybutenes produced from mixed isomers of butene. For multi-layer films, it is relatively easy to provide one or more surface layers which are specifically formulated to provide cling. In general this method allows a more flexible approach to film manufacture as choice of product for the main body of the film may be made on the basis of mechanical performance and the surface layers can be specially formulated for adhesion. Those skilled in the art will appreciate the multiplicity and flexibility of the choices ofpossible film structures.

A further requirement for the film producer is that the fabrication of the film is made as easy as possible by the use of polyethylenes having processing characteristics which allow film extrusion to be carried out as easily as possible. The use of a product of lower molecular weight or broader molecular weight distribution provides easier processability, but normally at the expense of a reduction in mechanical performance of the film. Similarly the use of products such an LDPE containing long chain branches (LCB) may assist processability but at the expense of stretchability in the subsequent wrapping process.

We now found that a particularly advantageous combination of film properties may be obtained by producing a stretch film from the novel copolymers described in the aforementioned WO 00/68285. The films have a particularly advantageous combination of properties, combining high impact resistance with easy processability and good performance in stretch wrapping and when combined with polyisobutene as a cling 0 enhancer, the films show a particularly advantageous control of cling force.

C Thus according to the present invention there is provided a stretch film Scomprising a cling additive in amount 0.5% and having dart impact of> 450 g MD tear strength of> 190 S(c) MD elongation at break of> 450 said film comprising a copolymer of ethylene and an alpha-olefin having from 3 Sto 10 carbons atoms, said copolymer having ,O 10 a density in the range 0.900 to 0.940

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S(b) an apparent Mw/Mn of 2 3.4 21 /12 from 16 to 24 activation energy of flow from 28 to 45 kJ/mol a ratio Ea(HMW)Ea(LMW) and a ratio g'(HMW)g'(LMW) from 0.85 to 0.95.

The preferred stretch films according to the present are those having a dart impact of> 600 g and most preferably 1100 g.

The preferred films show an elongation of> 500 The cling additive may be present in amount 2% and most preferably in amount of greater than or equal to 4 The preferred cling additiveis polyisobutene (PIB).

The novel stretch fils of the present invention may also be utilised in multi layer films, for example in 3-layer films wherein the other layers comprise polymers of lower density or copolymers.as described above.

When extruded into a-stretch film by film blowing, the products of the invention give produce films with a particularly advantageous balance of properties. The processability of the ethylene copolymers during the film production process is typically comparable if not better than an LLDPE type polymer produced from a ziegler catalyst.

The processability is assessed from measures such as the melt pressure in extrusion, the output rate for a given set ofextruder conditions and the motor load. Such processing performance allows these products to be a "drop-in" for existing LLDPE grades of similar specification withouthaving to make expensive changes to extrusion machinery IND or suffering a handicap in terms of extrusion performance.

SAs regards mechanical performance the dart impact of the films is vey high Scompared to a ziegler product of similar specification, being typically more than 600g O and preferably more than 1 l00g for a film of thickness 25pm for a product of melt index 0 5 about 1 and density 917. Film elongation is maintained at more than 500% despite the presence of LCB. It is impoiant that the film can be stretched to 300% or more without en fracturing.

CN Due to their unique structure, the films of the invention show an advantageous Sbehaviour whilst undergoing stretching that the film width is not unduly reduced. For a pre-stretch of 70% the films retain over 75% of their initial width., this property being 0 retained during storage of the film roll for up to one month or more.

The films of the invention show a hi-cling force as assessed by a Thimon stretch wrapping machine. A particularly advantageous behaviour is that the cling force varies only weakly with the amount of PIB cling agent added to the film. Hence there is a wide latitude for addition levels ofPIB to vary without causing either too much or too little cling to develop in the film.

Good elongation coiibined with outstanding impact resistance provides significant advantages in wrapping applications.

In the application of polyethylene copolymer products in blown films, a key performance compromise is the balance between the modulus of the film and its impact performance. In general, alterations to the polymer structure such as increasing the crystallinity lead to increased modulus but at the expense of reduced impact performance. The advent ofmetallocene catalysed products has lead to a redefinition of this performance compromise. It is generally acknowledged that blown films from copolymers produced from metallocene catalysts have a different balance of properties when compared to LLDPE type products produced by the more well established ziegler catalysts. When comparing products of the same basic specification in melt index and density, the metallocene products tend to have very high impact properties due to narrow molecular weight distribution and reduced modulus due to homogeneity of comonomer distribution.

We have found that the copolymers of the present invention can offer increased modulus and impact when compared to more conventional ziegler products while at the IND same time having no penalty in extrusion performance. For a given balance ofI 0 ~performance in impact and modulus, the creep performance of the inventive resins is also better than conventional Ziegler products, as are the film optical properties. Sealing o is also improved. Hence the resins of the invention show many advantages without S 5 displaying any disadvantage in processing.

A particular application of blown films is for use in heavy duty sacks for example for use for fertilisers, plastic pellets, etc. The mechanical properties of stffes, impact and creep resistance are of prime importance for the suitability of the M copolymer product Because of the intrinsic high impact resistance, the stiffness of the ICO 10 copolymers can be increased while maintaining a better impact resistance compared 0 with conventional products. Also due to the superior SCBD of the copolymners the cereep resistance (creep elongation) is significantly improved leading to advantages in handling of the filled bags and provides a potential for significant downgauging while maintaining similar performance to reference proprietary products.

For this application the films of the present invention suitably comprise copolymers of density 0.920.

Thus according to another aspect of the present invention there is provided a blown film having dart impact of 450 g MD tear stregth 190 g/25 pm MD elongation 450 said film comprising a copolymer of ethylene and an alpha-olefin having from 3 to 10 carb~ons atoms, said copolymer having a density 0.920; an apparent MwlAn-of 2 -3.4 1 21 1 2 from 16to 24 activation energy of flow from 28 to 45 kJ/mol a ratio Ea(HNM/Ea(LMW) 1, and a ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.

The preferred blown films according to this aspect of the present invention are those having a dart impact of >600 g and most preferably 1 100 g.

The preferred blown films show an elongation of 500 ID The novel blown films of the present invention may suitably be utilised in blends, for example with medium density polyethylenes.

The most preferred copolymers for use in the novel stretch films of the present O invention are those having 0 5 a density in the range 0.900 to 0.940 an apparent Mw/Mn in the range 2.5 to 3 S(c) 1 2 1 /12 from 18 24 NC activation energy of flow from 30 to 35 kJ/mol C a ratio Ea(HMW)/Ea(LMW) and ,D 10 a ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.

By apparent Mw/Mn is meant a value of Mw/Mn uncorrected for long chain branching.

The significance of the parameters Ea(HMW)/Ea(LMW) and g'(HMW)/g'(LMW) is described below. The experimental procedures for their measurements are described later in the text The polymers contain an amount of LCB which is clearly visible by techniques such as GPC/viscometry and flow activation energy. The content of LCB is lower than reported in many earlier publications, but is still sufficient, when coupled with broadened Mw/Mn, to give improved processability compared to linear polymers of narrow MWD (Mw/Mn less than about which do not contain LCB.

For the measurement of LCB, we have found that the most useful techniques are those which have a particular sensitivity to the presence of LCB in the high molecular weight chains. For these high molecular weight molecules, the physical effects of LCB on the solution and melt properties of the polymer are maximised. Hence detection of LCB using methods based upon solution and melt properties is facilitated.

Activation energy of flow is commonly used as an indicator of the presence of LCB in polyethylenes as summarised in the aforementioned WO 97/44371. For lower amounts of LCB, for which the global activation energy is of the order of 28 to kJ/mol, it is found that the LCB has a strong effect upon the activation energy as measured at low test rates ie the region in which the heology is dominated by the high molecular weight (HMW) species. Therefore, the ratio of activation energy derived from the low rate data Ea(HMW) tends to exceed that derived from the high rate data, O Ea(LMW). Hence polymers containing LCB predominantly in the high molecularweight chains tend to show the ratio Ea(HMW)/ Ea(LMW) greater than umty.

c A further well established method indicating the presence of LCB is gel O permeation chromatography with on-line detection of viscosity (GPC/OLV). By combining the data from 2 detectors, the ratio g' can be derived as a function of molecular weight; g' is the ratio of the measured intrinsic viscosity[rj] divided by the mc intrinsic viscosity [Tjlinar of a linear polymer having the same molecular weight In Cpolymers containing LCB, the g' measured at high molecular weights tends to be less c than that measured at low molecular weights. To quantify this effect, we have used a S 10 simple ratio g'(HMW)/g'(LMW). g'(HMW) is the weighted mean value of g' calculated for the 30% of the polymer having the highest molecular weight, while g'(LMW) is the weighted mean value of g' calculated for the 30% of the polymer having lowest molecular weight For linear polymers, g' is equal to 1 at all molecular weights, and so g'(HMW)/g'(LMW) is also equal to 1 when there is no LCB present.

For polymers containing LCB, g'(HMW)/g'(LMW) is less than 1. It should be noted that the g' data can be corrected for the effect of short chain branching (SCB). This would normally be done using a mean value of SCB content, the correction being applied uniformly at all molecular weights. Such a correction has not been applied here because in measuring the ratio g'(HMW)/g'(LMW) the same correction would apply to both g' values and there would be no net effect on the results reported here.

Another method to quantify LCB content in polyethylenes is by carbon-13 Nuclear Magnetic Resonance (13C-NMR). For the low amounts of LCB observed for polymers of the invention it is generally accepted that this technique can give a reliable quantification of the number of LCB points present in the polymer when the polymer is a homopolymer or a copolymer of ethylene and propylene or butene-l. For the purposes of this specification, a measurement of LCB by 13C- NMR is achieved in such polymers by quantification of the isolated peak at about 38.3ppm corresponding to the CH carbon of a tri-functional long chain branch. A tri-functional long chain branch is taken to mean a structure for which at least the first four carbon atoms of each of the 3 chains radiating from the CH branch carbon are all present as CH2 groups. Care must be exercised in making such measures to ensure that sufficient signalmoise is obtained to quantify the resonance and that spurious LCB structures are not generated during the sample heating IO by oxidation induced free-radical reactions.

SThe above described analysis of LCB by 13C-NMR is much more dificult when the copolymer contains hexene-1. This is because the resonance corresponding to an O LCB is very close to or overlapping that for the CH carbon at the branch site of the n- O 5 butyl branch obtained from this comonomer. Unless the two CH resonances can be resolved, which is unlikely using NMR equipment currently available, LCB could only c be determined for an ethylene/hexene-1 copolymer using the above described technique In C if the amount of n-butyl branches was so low, in comparison to the amount of LCB c present, that it could either be ignored or a reliable subtraction carried out on the CH Q 10 resonance at about 38.3ppm.

SUsing the preferred catalyst system of the present invention an ethylene/butene-l copolymer containing 6.5wt% butene-1 has been prepared using a continuous gas phase reactor. This polymer contained 0.12 LCB/10,000 total carbons using the 13C-NMR technique described above. The spectrum was obtained from a 600MHz NMR spectrometer after 912,000 scans. The polymer also contained 025 n-butyl branches/ 0,000 total carbons. No detectable oxidation was observed during this analysis with a limit of detection of approximately 0.05/10,000 total carbons.

Despite a relatively low average LCB content, it would be expected that such polymers would show distinctly modified theological behaviour in comparison with truly linear polymers. If the LCB is concentrated in the molecules of higher molecular weight, as is known to be the case, then an average value of 0.12 LCB/10,000 total carbons in the whole polymer could correspond to about 0.3 or more LCB/10,000 for molecules of molecular weight about one million. Hence these molecules would be expected to contain at least 2 LCB points per molecule, equivalent to a branched structure with 5 arms. Such molecules are known to display very different rheological properties to linear molecules.

The preferred polymers of the invention also show quite low amounts of vinyl unsaturation as determined by either infra-red spectroscopy or preferably proton NMR.

For a polymer of melt index (2.16kg) about 1, values are less than 0.05 vinyl groups per 1000 carbon atoms or even as low as less than 0.02 vinyl groups per 1000 carbon-atoms.

Again, for melt index (2.16kg) about 1, total unsaturations are also low compared to some other metallocene polymers containing LCB, the total unsaturations as measured by proton NMR to be the sum of vinyl, vinylidene, tri-substituted and cis+trans di- 0 substituted internal unsaturation being in the range of less than 0.2 to 0.5 per 1000 C carbon atoms. Products with higher or lower melt index, and hence lower or higher Snumber average molecular weights, may show respectively higher or lower terminal unsaturations, in proportion to the total number of chain ends present Hence the total unsaturations per 1000 carbon atoms are less than 17500/Mn where Mn is the number average molecular weight uncorrected for LCB and the vinyl unsaturations are less than S 1750/Mn.

SThe comonomer present in the preferred polymers of the invention is not 10 randomly placed within the polymer structure. If the comonomer was randomly placed, Sit would be expected that the elution trace derived from temperature rising elution fractionation (TREP) would show a single narrow peak, the melting endotherm as measured by differential scanning calorimetry would also show a substantially singular and narrow peak. It would also be expected that little variation would be expected in either the amount of comonomer measured as a function of molecular weight by techniques such as GPC/FTIR, or the molecular weight of fractions measured as a function of comonomer content by techniques such as TREF/DV. These techniques for structure determination are also described in the aforementioned WO 97/44371, the relevant parts of which are incorporated herein by reference.

However, the comonomer may be placed in a way as to give a distinct broadening of the TREF ehition data, often with the appearance of one or two or even three peaks. At a polymer density of about 918kg/m 3 the TREF data typically show two main peaks, one at about 87°C and another distinct but smaller peak at about 72C, the latter being about 2/3 of the height of the former. These peaks represent a heterogeneity in the amount of comonomer incorporated in the polymer chains. A third peak is often visible at about 100 C. Without being bound by any theory this peak is considered to be nothing other than a consequence of the fact that the polymer molecules of low comonomer content tend to crystallise into large chain folded crystals which melt and dissolve in the TREF experiment in a narrow range of temperatures at about 100*C. The same peak is very clearly visible in certain types of LLDPE polymers produced by ziegler catalysts and it is present in TREF analysis of MDPE and HDPE type polyethylenes. Thus, without being bound by any theory, the third peak at about 100°C I is more a result of the crystallisation of linear or near-linear molecules, than a feature O which can be simply interpreted as representing a particular and separate polymer Sspecies.

O The CDBI (Composition Distribution Branch Index) of the polymers is between 0 5 55 and 75%, preferably 60 to 75%, reflecting the fact that the polymers are neither highly homogeneous (CDBI about 90%) nor highly heterogeneous (CDBI about c The CDBI of a polymer is readily calculated from techniques known in the art, C such as, for example, temperature rising elution fractionation (TREF) as described, for Sexample, in Wild et al., Journal of Polymer Science, Polymer Phys.Ed., Vol 20, p 4 4 1 o 10 (1982), or in US patent 4,798,081.

SThe behaviour seen in melting endotherms by DSC reflects the behaviour in TREF in that one, two or three peaks are typically seen. For example three peaks are often seen for the preferred polymers of density about 918 kg/mn, when heated at after crystallisation at the same rate. As is usual, it would be expected that the peaks seen in TREF and DSC would move to lower temperatures for polymers of lower density and to higher temperatures for polymer of higher density. The peak melting temperature Tp (the temperature in °C at which the maximum heat flow is observed during the second heating of the polymer) can be approximated by the following expression within normal experimental errors: Tp 462 x density 306 The amount of comonomer measured as a function of molecular weight by GPC/FTIR for the preferred polymers shows an increase as molecular weight increases.

The associated parameter Cpfis greater than 1.1. The measurement of Cpf is described in WO 97/44371.

The preferred copolymers exhibit extensional rheological behaviour, in particular strain-hardening properties, consistent with the presence of long chain branching.

The copolymers may suitably be prepared by use of a metallocene catalyst system comprising, for example a traditional bisCp metallocene complex or a complex having a 'constrained geometry' configuration together with a suitable activator.

Suitable complexes, for example, are those disclosed in WO 95/00526 the disclosure of which is incorporated herein by reference.

Suitable activators may comprise traditional aluminoxane or boron compounds for example borates again disclosed in the aforementioned. WO 95100526.

c-i Preferred metallocene complexes for use in the preparation of the copolymers may be represented by the general formula: wherein:- R! each occurrence is independently selected. from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof; said R! having up to nonhydrogen atoms, and optionally, two R! groups (where R! is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is a neutral 44 bonded diene group having iup to 30 non-hydrogen atoms, which forms a complex with M; Y is M is titanium or zirconiumn in the 2 formal oxidation state; Z* is SiR 1 2

CR

41 2 SiR* 2 S1R* 2

CR

1 2

CR

1 2 CR*=CR*, CR 1 2

SLR"'

2 Or GeR* 2 wherein: R* each occurrence is independently hydrogen, or a member selected from hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof; said R* having up to 10 non-hydrogen atoms, and optionally, two R 1 groups from Z* (when R* is not hydrogen), or an R* group from Z*and an R 1 group from Y form a ring system.

Examples of suitable X groups include s-trans-4 4 ,4-diphenyl-1,3butadiene, s-trans-4 4 -3-methyl-l ,3-pentadiene; s-trans-4 4 -2,4-hexadiene; s-trans-44- IND 1,3-pentadiene; s-ftans-4 4 -1,4-ditolyl-1,3-butadiene; s-trans-4 4 1,4- 1 bis(trimethylsilyl)-1,3-butadiene; SCiS__4 -3-methyl-1 ,3-pentadiene; s-ciS_4 4 _1 Adibenzyl-1,3-butadiene; s-cis-41-l,3-pentadiene; s-cis-4 4 -l ,4-bis(trimethylsilyl)-l ,3-

C.)

C) 5 Most preferably R! is hydrogen, methyl, ethyl, propyl, butyl, pentyl, hexyl, benzyl, or phenyl or 2 R' groups (except hydrogen) are linked together, the entire

CSR'

4 group thereby being, for example, an indenyl, tetrahydroindenyl, fluorenyl, terahydrofluorenyl, or octahydrofluorenyl group.

Hfighly preferred Y groups are nitrogen or phosphorus containing groups ,O 10 containing a group corresponding to the formula -NWR)- or wherein e is Cw.

1 hydrocarbyl.

Most preferred complexes are amidosilane or amidoalkanediyl complexes.

Most preferred complexes are those wherein M is titanium.

Specific complexes suitable for use in the preparation of the novel copolymers of the present invention are those disclosed in the aforementioned WO 95/00526 and are incorporated herein by reference.

A particularly prefenred complex for use in the preparation of the novel copolymers of the present invention is (t-butylamido) (tetramethyl-4 5 cyclopentadienyl) dimethyl silanetitanium. _44 -1,3-pentadiene.

The activator may preferably be a boron compound for example a borate such as ammonium salts, in particular.

triethylammonium terahenylborate triethylammonim tetraphenylborate, tripropylammonium, ttraphenylborate, tri(n-butyl)ammoniumn tetraphenylborate, trift-butyl)ammoniumi tetraphenylborate, N,N-dimethylaniliniinn tetraphenylborate, N,N-diethylanilinium tetraphenylborate, trimethylammonium tetralcis(pentafluorophenyl) borate, triethylammonium tefrakis(pentafluorophenyl) borate, trpoyammonium tetrakis(pentafluorophenyl) borate, tri(n-butyl)ammoniumn tetrakis(pentafluorophenyl) borate, N,N-dimethylanilinium tetrakis(pentafluorophenyl) borate, 0 N,N-diethylanilinium tetrakis(pentafluorphenyl) borate.

CI Another type of activator suitable for use with the metallocene complexes are the Sreaction products of ionic compounds comprising a cation and an anion wherein the anion has at least one substituent comprising a moiety having an active hydrogen and an organometal or metalloid compound wherein the metal or metalloid is from Groups 1-14 of the Periodic Table.

S' Suitable activators of this type are described in WO 98/27119 the relevant portions of which are incorporated herein by reference.

S 10 A particular preferred activator of this type is the reaction product obtained from alkylammonium tris(pentafluorophenyl) 4-(hydroxyphenyl) borates and trialkylaluminium. For example a preferred activator is the reaction product of bis(hydrogenated tallow alkyl) methyl ammonium tris (pentafluorophenyl) (4hydroxyphenyl) borate and triethylaluminium.

The molar ratio of metallocene complex to activator employed in the process of the present invention may be-in the range 1:10000 to 100:1. A preferred range is from 1:5000 to 10:1 and most preferred from 1:10 to 10:1.

The metallocene catalyst system is most suitably supported. Typically the support can be an organic or inorganic inert solid. However particularly porous supports such as talc, inorganic oxides and resinous support materials such as polyolefins which have well-known advantages in catalysis are preferred. Suitable inorganic oxide materials which may be used include Group 2, 13 14 or 15 metal oxides such as silica, alumina, silica-alumina and mixtures thereof Other inorganic oxides that may be employed either alone or in combination with the silica, alumina or silica-alumina are magnesia, titania or zirconia. Other suitable support materials may be employed such as finely divided polyolefins such as polyethylene.

The most preferred support material for use with the supported catalysts is silica.

Suitable silicas include Crosfield ES70 and Grace Davison 948 silicas.

The support material may be subjected to a heat treatment and/or chemical treatment to reduce the water content or the hydroxyl content of the support material.

Typically chemical dehydration agents are reactive metal hydrides, aluminium alkyls and halides. Prior to its use the support material may be subjected to treatment at 1000C 0 to 1000 0 C and preferably at 200 to 850 0 C in an inert atmosphere under reduced c pressure, for example, for 5 hrs.

o The support material may be pretreated with an aluminium alkyl at a temperature o 5 of-20 0 C to 150°C and preferably at 20°C to 1000C.

The pretreated support is preferably recovered before use in the preparation of Sthe supported catalysts.

SThe copolymers comprise copolymers of ethylene and alpha-olefns having 3 to ecn 10 carbon atoms. Preferred alpha olefins comprise 1-butene, 1-hexene and 4-methyl-1- ,0 10 pentene. A particularly preferred alpha olefin is 1-hexene.

O The copolymers are most suitably prepared in the gas phase in particular in a continuous process operating at a temperature >60°C and most preferably at a temperature of 75 0 C or above. The preferred process is one comprising a fluidised bed reactor. A particularly suitable gas phase process is that disclosed in EP 699213 incorporated herein by reference.

When prepared by use of the preferred catalyst systems described above the copolymers have a titanium content in the range 0.1 to 2.0 ppm.

Examples Catalyst preparation Treatment of silica A suspension of Grace 948 silica (13kg, previously calcined at 250°C for hours) in 110 litres of hexane was made up in a 240L vessel under nitrogen. IL of a hexane solution containing 2g/L of Stadis 425 was added and stirred at room temperature for 5 minutes. 29.1L of a 892mmolAl/L solution of triethylaluminium (TEA) in hexane was added slowly to the stirred suspension over 30 minutes, while maintaining the temperature of the suspension at 30°C. The suspension was stirred for a further 2 hours. The hexane was filtered, and the silica washed with hexane, so that the aluminium content in the final washing was less than 0.5 mmol Al/litre. Finally the suspension was dried in vacuo at 60°C to give a free flowing treated silica powder with residual solvent less than (ii) Catalyst fabrication All steps, unless otherwise stated, of the catalyst fabrication were carried out at 3L of toluene was added to a 24L vessel equipped with a turbine stirrer, and Sstirred at 300rpm. 5.01L of a 9.5wt% solution in toluene of bis(hydrogenated tallow C alkyl) methyl ammonium tris(pentafluorophenylX4-hydroxyphenyl)borate was added O during 15 minutes. Then 1.57L of a 250mmolAl/L solution in toluene of triethylaluminium was added during 15 minutes and mixture stirred for 30 minutes. The solution obtained was then transferred under nitrogen, with stirring during 2 hours, to an c 80L vessel containing 10kg of the TEA treated silica described above. 60L ofhexane 'was then rapidly introduced and mixed for 30 minutes. 1.83 kg of a 7.15wt% solution in Cr heptane of (t-butylamido)(tetramethyl-T5-cyclopentadienyl) dimethylsilanetitanium -i 4 0 10 1,3-pentadiene was added during 15 minutes. Mixing was continued for 1 hour and 1L Sof a 2g/L hexane solution of stadis 425 was added. The catalyst slurry was then transferred to a vessel of volume 240L and 70L ofhexane added. Excess solvent was removed by decantation, and a further 130L ofhexane added. This process was repeated until less than 0.2L of tolueneremained in the solvent 1L of a 2g/L hexane solution of stadis 425 was then added aid-the catalyst dried under vacuum at 400C to a residual solvent level of lwt/%.

(iii) Polymerisation using continuous fluidised bed reactor Example 1 Ethylene, I-hexene, hydrogen and nitrogen were fed into a continuous fluidised bed reactor of diameter 45cm. Polymerisation was performed in the presence of a catalyst similar to that prepared above. Polymer product was continuously removed from the reactor. Operating conditions.are given in Table 1.

Example 2 The procedure for example -',was scaled up to produce a catalyst of batch size approximately 75 kg. This catalyst was used to produce a copolymer in a commercial gas phase scale reactor of diameter 5 metres again using the conditions shown in Table 1.

TABLE 1 Example 1 2 total pressure (bar) 20.0 19.8 temperature C) 80 ethylene pressure (bar) 7.5 8.1

H

2

/C

2 ratio 0.0025 0.0023 C6/C 2 ratio 0.0055 0.0050 production (kg/hr) 74 8700 Comparative Example 1 A film from Dowlex 2045 was used for comparison.

3 layer films were produced on a coextrusion operating line at about 100kg/hr.

This line was equipped with 4 25L/D LLDPE extruders and a 300mm diameter die with 1.2mm die gap. The film was of thickness 25mm and the blow up ratio 2.5:1. The inner cling layer was formed from an EVA copolymer containing TAC 100 (50% PIB). The other layers were formed from the test polymer containing TAC 100.

Details of the copolymers prepared and films produced are given in Table 2.

TABLE 2 Film properties Example Comp Comp Comp Ic la lb ic 2a la lb MI/2.16 g/10mn 0.91 0.91 0.91 1.18 1.18 1.18 13 HLMI g/10nm 25.8 25.8 25.8 23.70 23.70 23.70 25.80 MFR 28.4 28.4 28.4 20.1 20.1 20.1 19.8 Density ks/m 919.4 919.4 919.4 916.6 916.6 916.6 916.9 EXTRUSION CONDIT ONS Melt pressure bar 533 494 460 508 496 467 454 Melt temperature °C 232 232 231 229 233 230 228 Output kg/h 95 95 95 110 110 110 110 Motor Load A 55 50 50 54 51 49 49 Blend 4%PIB 5%PIB 6%PIB 4%PIB 5%PIB 6%PIB nECHAAICAL

PROPERTIES

Dart Impact g 265 350 310 >1100 >1100 >1100 >1100 Elmendorftear str. MD /25 _m 255 1207 196 TD Ig/25m 656 577 572 Elongation at break MD 670 640 600 TD 780 660 680 Example 3 A resin was produced in the gas phase using a similar catalyst system to that described above with melt index 1 and density 923.6 kg/m 3 This was extruded into film 150pm thick on a Reifenhauser blown film line equipped with a die of diameter 150mm and die gap 2.3mm. The product was extruded both pure and blended with 20 of a medium density polyethylene of density about 938 kg/m 3 melt index about 0.2 produced using a chromium catalyst system.

Comparative example 2 Dowlex 2045 was used as a comparative example.

The blown film properties are given in Table 3 below. The films were also tested in creep at 60°C under 5Mpa load. After 200 minutes, the deformation of the film of example lb was 57% compared to 63% for comparative example 2 19 SUBSTITUTE SHEET (RULE 26) TABLE 3 Example Ia lb 22M -I M112.16 g/10rnn 1.00 1.00 0.94 HLMI g/l0mn 23.46 23.46 26.8 MFR 23.5 23.5 28.5 Density kg/rn 923.6 923.6 919.7 EXTRUSION Die mm 150 150 150 Die gap mm 2.3 2.3 2.3 Screw speed rm83.4 85 89.2 Melt pressure bar 267 283 268 Melt temperature 216.7 217 217.1 output 50/ s 50 BUR 2:1 2:1 2:1 Motor Load A 62 65 61 Specific energy KWtVKg 0.22 0.23 0.23 Thickness Jim_ 150 150 150 Blend pure 20% MDPE 20% MDPE MECHANICAL PROPERTIES Dart Impact! 1295 1084 890 Edge fold Impact (Staircase Metod) 805 735 650 Eimendorf tear str. MD g/25pm 260 210 341 TD g/25ipm 418 471 573 Tensile str. at yield MD MPa 12.9 14.4 12.5 TD MPa 14 14.6 13.4 Tensile str. at break MD MPa 48 45.6 43.9 TD MPa 47.5 41.6 42.5 Elongation at break MD 1250 862 930 TD 1000 917 1000 Secant modulusl1% MD MPa 235 263 208 TD MPa 285 298 239 Haze 23.8 22.5 19.8 Gloss 450 57.7 49.4 47.9 Methods of test Melt index (190/2.16) was measured according to ISO 1133.

Meltnfow ratio (MFR) was calculated from the ratio of flow rates determined according to ISO 1133 under condition (190/21.6) and condition (190/2.16).

was measured using a density column according to ISO 1872/1-1986, except that the melt index extrUdates Were not annealed but were left to cool on a sheet of 'Polymeric material for 30 minutes.

Dart imi~act was measured by AS'IM DI709, tear strengib by ASThID1922, and hazeby ASTM D1003.

Claims (8)

1. A stretch film comprising a cling additive in amount 0.5% and having dart impact of> 450 g MD tear strength of> 190 MD elongation at break of> 450 O/o, said film comprising a copolymer of ethylene and an alpha-olefin having from 3 to 10 carbons atoms, said copolymer having a density in the range 0.900 to 0.940 an apparent Mw/Mn of 2 3.4 (c)12 1 /1 2 from 16 to 24 activation energy of flow from 28 to 45 kJ/mol a ratio Ea(HMW)/Ea(LMW) and a ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.

2. A stretch film according to claim 1 having a dart impact 600 g.

3. A stretch film according to either of the preceding claims having a dart impact 1100 g.

4. A stretch film according to any of the preceding claims having a MD elongation of> 500 A stretch film according to any of the preceding claims wherein the cling additive is present in amount 2

6. A stretch film according to any of the preceding claims wherein the cling additive is polyisobutene (PIB).

7. A blown film having 0r 0r o' m, lam 7. A blown film having ()dart impact of 450 g ND tear strength 190 g/25 jum MD elongation 450 C.) C) 5 to 10 carbons atoms, said copolymer having a densityr> 0.920, an apparent Mw/Mn of 2 -3.4 12 1 1 2 from 16 to24 activation energy of flow from 28 to 45 kJ/mol a ratio Ea(HMW)/Ea(LMW) 1, and a ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.

8. A blown film according to claim 7having adart impact >600 g.

9. A blown film according to claims 7 or 8 having a dart impact 1100 g. A blown film according to claims 7 to 9 having a TOD elongation of 500 g.