POLYETHYLENE FOR PIPES

Abstract

The present invention provides a process for the preparation of a multimodal polyethylene comprising: (i) polymerising ethylene and optionally an α-olefin comonomer in a first polymerisation stage to produce a first ethylene polymer; and (ii) polymerising ethylene and optionally an α-olefin comonomer, in the presence of said first ethylene polymer, in a second polymerisation stage, wherein the first and second polymerisation stages are carried out in the presence of an unsupported metallocene catalyst and each polymerisation stage produces at least 5% wt of the multimodal polyethylene, and the multimodal polyethylene has a multimodal molecular weight distribution, a molecular weight of at least 50,000 g/mol and a bulk density of at least 250 g/dm.sup.3, and wherein a solution of the unsupported metallocene catalyst in a solvent is employed. The present invention also provides a multimodal polyethylene, a process for preparing a pipe comprising preparing a multimodal polyethylene and extruding the multimodal recycle polyethylene to produce a pipe, and a pipe obtained by such a process.

Claims

1. A process for the preparation of a multimodal polyethylene comprising: (i) polymerising ethylene and optionally an α-olefin comonomer in a first polymerisation stage to produce a first ethylene polymer; and (ii) polymerising ethylene and optionally an α-olefin comonomer, in the presence of said first ethylene polymer, in a second polymerisation stage, wherein said first and second polymerisation stages are carried out in the presence of an unsupported metallocene catalyst and each polymerisation stage produces at least 5% wt of said multimodal polyethylene, and said multimodal polyethylene has a multimodal molecular weight distribution, a molecular weight of at least 50,000 g/mol and a bulk density of at least 250 g/dm.sup.3, and wherein a solution of said unsupported metallocene catalyst in a solvent is employed.

2. A process as claimed in claim 1, wherein said solvent is a C.sub.3-10 saturated alkane or an aromatic hydrocarbon.

3. A process as claimed in claim 2, wherein said solvent is a C.sub.4-10 saturated alkane which is preferably selected from hexane and cyclohexane.

4. A process as claimed in claim 2, wherein said solvent is toluene.

5. A process as claimed in any one of claims 1 to 4, wherein the metallocene catalyst is a complex of a group 3 to 10 metal having at least two ligands, wherein each ligand is a substituted or unsubstituted cyclopentadienyl, substituted or unsubstituted indenyl, substituted or unsubstituted fluorenyl or substituted or unsubstituted tetrahydroindenyl.

6. A process as claimed in any one of claims 1 to 5, wherein the metallocene catalyst is a complex of a metal ion formed by a metal selected from Zr, Hf or Ti.

7. A process as claimed in any one of claims 1 to 6, wherein said metallocene is a compound of formula (I):
(Cp).sub.2L.sub.nMX.sub.2  (I) wherein each Cp is independently a cyclic group having a delocalised system of pi electrons; L is a bridge of 1-7 atoms; n is 0 or 1; M is a transition metal of Group 3 to 10; and each X is independently a sigma-ligand.

8. A process as claimed in claim 7, wherein Cp is a substituted or unsubstituted cyclopentadienyl, substituted or unsubstituted indenyl, substituted or unsubstituted fluorenyl or substituted or unsubstituted tetrahydroindenyl.

9. A process as claimed in claim 7 or 8, wherein each Cp is unsubstituted.

10. A process as claimed in any one of claims 7 to 9, wherein n is 0.

11. A process as claimed in any one of claims 7 to 9, wherein n is 1.

12. A process as claimed in claim 11, wherein L is a methylene, ethylene or silyl bridge.

13. A process as claimed in any one of claims 7 to 12, wherein M is Ti, Zr or Hf.

14. A process as claimed in any one of claims 7 to 13, wherein each X is independently selected from H, halogen, C.sub.1-20 alkyl, C.sub.1-20 alkoxy, C.sub.2-20 alkenyl, C.sub.2-20 alkynyl, C.sub.3-12 cycloalkyl, C.sub.6-20 aryl, C.sub.6-20 aryloxy, C.sub.7-20 arylalkyl, C.sub.7-20 arylalkenyl, —SR″, —PR″, SiR″.sub.3, —OSiR″.sub.3, —NR″.sub.2 or —CH.sub.2Y wherein Y is C.sub.6-20 aryl, C.sub.6-20 heteroaryl, C.sub.1-20 alkoxy, C.sub.6-20 aryloxy, —NR″.sub.2, —SR″, —PR″.sub.3, —SiR″.sub.3, or —OSi″.sub.3 and wherein each R″ is independently a hydrogen or hydrocarbyl; or in the case of —NR″.sub.2, the two substituents R″ can form a ring, together with the nitrogen atom to which they are attached.

15. A process as claimed in any one of claims 1 to 8, wherein said metallocene is of formula (II): ##STR00005## wherein M is a transition metal of Group 4 to 6; each X is independently a sigma-ligand; R.sup.1 and R.sup.1′ are each independently C.sub.1-20 hydrocarbyl; and R.sup.2, R.sup.2′, R.sup.3, R.sup.3′, R.sup.4, R.sup.4′, R.sup.5 and R.sup.5′ are each independently H or a C.sub.1-20 hydrocarbyl.

16. A process as claimed in any one of claims 1 to 8, wherein said metallocene is of formula (III): ##STR00006## wherein M is a transition metal of Group 4 to 6; each X is independently a sigma-ligand; R.sup.1, R.sup.1′ R.sup.2, R.sup.2′, are each independently H or a C.sub.1-20 hydrocarbyl; and L is a bridge of 1-4 C-atoms and 0-4 heteroatoms, wherein each of the bridge atoms may bear independently substituents; or a bridge of 1-3 hetero atoms.

17. A process as claimed in claim 16, wherein M is Ti, Zr or Hf.

18. A process as claimed in claim 16 or 17, wherein each X is independently H, halogen, C.sub.1-20 alkyl, C.sub.1-20 alkoxy, C.sub.2-20 alkenyl, C.sub.2-20 alkynyl, C.sub.3-12-cycloalkyl, C.sub.6-20 aryl, C.sub.6-20 aryloxy, C.sub.7-20 arylalkyl, C.sub.7-20 arylalkenyl, —SR″, —PR″.sub.3, —SiR″.sub.3, —OSiR″.sub.3, —NR″.sub.2 or —CH.sub.2—Y, wherein Y is C.sub.6-20 aryl, C.sub.6-20 heteroaryl, C.sub.1-20 alkoxy, C.sub.6-20 aryloxy, NR″.sub.2, —SR″, —PR″.sub.3, —SiR″.sub.3, or —OSiR″.sub.3 and wherein each R″ is independently a hydrogen or hydrocarbyl; or in the case of —NR″.sub.2, the two substituents R″ can form a ring, together with the nitrogen atom to which they are attached.

19. A process as claimed in claim 18 wherein each X is independently halogen, C.sub.1-20 alkyl, C.sub.1-20 alkoxy, C.sub.6-20 aryl, C.sub.7-20 arylalkenyl or —NR″.sub.2 as defined above.

20. A process as claimed in any one of claims 16 to 19, wherein R.sup.1, R.sup.1′, R.sup.2, R.sup.2′ are each independently H.

21. A process as claimed in any one of claims 16 to 20, wherein L is a methylene, ethylene or silyl bridge.

22. A process as claimed in any one of claims 1 to 21, wherein said metallocene is (nBuCp).sub.2HfCl.sub.2, (Ind).sub.2ZrCl.sub.2, Et-(tetrahydroindenyl).sub.2ZrCl.sub.2 or (nBuCp).sub.2ZrCl.sub.2.

23. A process as claimed in any one of claims 1 to 22, wherein an aluminoxane cocatalyst is employed.

24. A process as claimed in claim 23, wherein a solution of said aluminoxane cocatalyst in a solvent, preferably a C.sub.3-10 saturated alkane or an aromatic hydrocarbon, is employed.

25. A process as claimed in claim 24, wherein said solvent is a C.sub.4-10 saturated alkane which is preferably selected from hexane and cyclohexane.

26. A process as claimed in claim 24, wherein said solvent is toluene.

27. A process as claimed in claim 23, wherein a mixture of an aluminoxane cocatalyst and metallocene, diluted in a C.sub.4-10 saturated alkane or toluene, is employed.

28. A process as claimed in any one of claims 1 to 27, wherein said first polymerisation stage is in slurry conditions.

29. A process as claimed in any one of claims 1 to 28, wherein said second polymerisation stage is in slurry conditions.

30. A process as claimed in claim 28 or 29, wherein said slurry polymerisation is carried out in an aliphatic hydrocarbon diluent.

31. A process as claimed in any one of claims 1 to 30, wherein said first polymerisation stage is carried out in the presence of hydrogen.

32. A process as claimed in any one of claims 1 to 31, wherein there is no reactor fouling in said first polymerisation stage.

33. A process as claimed in any one of claims 1 to 32, wherein said second polymerisation is carried out in the presence of hydrogen.

34. A process as claimed in any one of claims 1 to 32, wherein said second polymerisation is carried out in the absence of hydrogen.

35. A process as claimed in any one of claims 1 to 34, wherein there is no reactor fouling in said second polymerisation stage.

36. A process as claimed in any one of claims 1 to 35, wherein said process consists of a first polymerisation stage and a second polymerisation stage.

37. A process as claimed in any one of claims 1 to 36, wherein said first polymerisation stage produces 1 to 65% wt of said multimodal polyethylene.

38. A process as claimed in any one of claims 1 to 37, wherein said second polymerisation stage produces 35 to 99% wt of said multimodal polyethylene.

39. A process as claimed in any one of claims 1 to 38, wherein said process consists of a first polymerisation stage, a second polymerisation stage and a third polymerisation stage.

40. A process as claimed in claim 39, wherein said third polymerisation is carried out in slurry conditions.

41. A process as claimed in claim 39 or 40, wherein said third polymerisation stage produces 0.5-30 wt % of said multimodal polyethylene.

42. A process as claimed in any one of claims 39 to 41, comprising the sequential steps (a)-(c): (a) polymerising ethylene and optionally an α-olefin comonomer in a first polymerisation stage to produce a lower molecular weight ethylene (LMW) polymer; (b) polymerising ethylene and optionally an α-olefin comonomer in a second polymerisation stage to produce a first higher molecular weight ethylene polymer (HMW1); and (c) polymerising ethylene and optionally an α-olefin comonomer in a third polymerisation stage to produce a second higher molecular weight ethylene polymer (HMW2).

43. A process as claimed in any one of claims 39 to 41, comprising the sequential steps (a)-(c): (a) polymerising ethylene and optionally an α-olefin comonomer in a first polymerisation stage to produce a lower molecular weight ethylene polymer (LMW); (b) polymerising ethylene and optionally an α-olefin comonomer in a second polymerisation stage to produce a second higher molecular weight ethylene polymer (HMW2); and (c) polymerising ethylene and optionally an α-olefin comonomer in a third polymerisation stage to produce a first higher molecular weight ethylene polymer (HMW1).

44. A process as claimed in any one of claims 1 to 43, wherein there is no reactor fouling in said second and/or third polymerisation stage.

45. A process as claimed in any one of claims 1 to 44, wherein said process is semi-continuous or continuous, preferably continuous.

46. A process as claimed in any one of claims 1 to 45, wherein said multimodal polyethylene has a bimodal or trimodal molecular weight distribution.

47. A process as claimed in any one of claims 1 to 46, wherein said multimodal polyethylene has a Mw of 100,000 to 250,000 g/mol.

48. A process as claimed in any one of claims 1 to 47, wherein said multimodal polyethylene has a Mn of 18,000 to 40,000 g/mol.

49. A process as claimed in any one of claims 1 to 48, wherein said multimodal polyethylene has a MWD of 1 to 25.

50. A process as claimed in any one of claims 1 to 49, wherein said multimodal polyethylene has a MFR.sub.2 of 0.005 to 0.2 g/10 min.

51. A process as claimed in any one of claims 1 to 50, wherein said multimodal polyethylene has a MFR.sub.5 of 0.05 to 1 g/10 min.

52. A process as claimed in any one of claims 1 to 51, wherein said multimodal polyethylene comprises 0.5 to 10% wt comonomer.

53. A process as claimed in any one of claims 1 to 52, wherein said multimodal polyethylene has a density of 920 to 980 kg/dm.sup.3.

54. A process as claimed in any one of claims 1 to 53, wherein said multimodal polyethylene has a bulk density of 250 to 400 g/dm.sup.3.

55. A process as claimed in any one of claims 1 to 54, wherein said multimodal polyethylene has an ash content of 0 to 800 wt ppm.

56. A process as claimed in any one of claims 1 to 55, wherein said multimodal polyethylene contains less than 100 wtppm of material of hardness more than 3 on Moh's scale.

57. A process as claimed in any one of claims 1 to 56, wherein said multimodal polyethylene is in the form of particles.

58. A process as claimed in any one of claims 1 to 57, wherein said first ethylene polymer has a MFR.sub.2 of at least 10 g/10 min.

59. A process as claimed in any one of claims 1 to 58, wherein said first ethylene polymer has a MFR.sub.2 of 10 to 1000 g/10 min.

60. A multimodal polyethylene obtainable by a process as claimed in any one of claims 1 to 59.

61. A multimodal polyethylene obtained by a process as claimed in any one of claims 1 to 59.

62. A metallocene multimodal polyethylene comprising: i) a multimodal molecular weight distribution; ii) a molecular weight of at least 50,000 g/mol; iii) a MFR.sub.2 of less than 0.2 g/10 min; iv) a MFR.sub.5 of less than 1 g/10 min; v) a bulk density of at least 250 g/dm.sup.3; and vi) an ash content of less than 800 ppm wt.

63. A metallocene multimodal polyethylene as claimed in claim 62, wherein said metallocene multimodal polyethylene contains less than 100 wtppm of material of hardness more than 3 on Moh's scale.

64. A process for preparing a pipe comprising: i) preparing a multimodal polyethylene by the process claimed in any one of claims 1 to 59; and ii) extruding said multimodal polyethylene to produce pipe.

65. A pipe obtainable by a process as claimed in claim 64.

66. A pipe obtained by a process as claimed in claim 64.

67. A pipe comprising metallocene multimodal polyethylene as claimed in any one of claims 60 to 63.

Description

[0296] The invention will now be described with reference to the following non-limiting examples and Figures wherein:

[0297] FIG. 1 is a schematic of a process of the present invention;

[0298] FIG. 2 is a graph showing polymerisation activity for a process of the invention utilising unsupported rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride catalyst and a comparative process utilising a supported rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride catalyst; and

[0299] FIG. 3 is a graph showing polymerisation activity for a process of the invention utilising unsupported bis-n-butylcyclopentadienylzirconium dichloride catalyst and a comparative process utilising a supported bis-n-butylcyclopentadienylzirconium dichloride catalyst

[0300] FIG. 4A is a light microscopy picture of a pressed thin film sample of bimodal polyethylene produced by a bimodal polymerisation process of the invention utilising unsupported bis-n-butylcyclopentadienylzirconium dichloride catalyst

[0301] FIG. 4B is a light microscopy picture of a pressed thin film sample of bimodal polyethylene produced by a comparative bimodal polymerisation process utilising a supported bis-n-butylcyclopentadienylzirconium dichloride catalyst

EXAMPLES

Determination Methods for Polymers

[0302] Unless otherwise stated, the following parameters were measured on polymer samples as indicated in the Tables below.

[0303] Melt indexes (MFR.sub.2 and MFR.sub.5) were measured according to ISO 1133 at loads of 2.16 and 5.0 kg respectively. The measurements were at 190° C.

[0304] Molecular weights and molecular weight distribution, Mn, Mw and MWD were measured by Gel Permeation Chromatography (GPC) according to the following method: 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 Alliance GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 1 PLgel GUARD+3 PLgel MIXED-B and 1,2,4-trichlorobenzene (TCB, stabilised with 250 mg/l 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160° C. and at a constant flow rate of 1 ml/min. 206 μl of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 15 narrow molecular weight distribution polystyrene (PS) standards in the range of 0.58 kg/mol to 7500 kg/mol. These standards were from Polymer Labs and had Mw/Mn from 1.02 to 1.10. Mark Houwink constants were used for polystyrene and polyethylene (K: 0,19×10.sup.−5 dl/g and a: 0.655 for PS and K: 3.9×10.sup.−4 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 stabilised TCB (same as mobile phase) and keeping for 3 hours at 140° C. and for another 1 hour at 160° C. with occasional shaking prior to sampling into the GPC instrument.

[0305] Comonomer content (% wt) was determined based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with C13-NMR.

[0306] Density of materials was measured according to ISO 1183:1987 (E), method D, with isopropanol-water as gradient liquid. The cooling rate of the plaques when crystallising the samples was 15° C./min. Conditioning time was 16 hours.

[0307] Rheology of the polymers was determined by frequency sweep at 190° C. under nitrogen atmosphere according to ISO 6721-10, using Rheometrics RDA II Dynamic Rheometer with parallel plate geometry, 25 mm diameter plate and 1.2 mm gap. The measurements gave storage modulus (G′), loss modulus (G″) and complex modulus (G*) together with the complex viscosity (η*), all as a function of frequency (ω). These parameters are related as follows: For any frequency ω: The complex modulus: G*=(G′2+G″2).sup.1/2. The complex viscosity: η*=G*/ω. The denomination used for modulus is Pa (or kPa) and for viscosity is Pa s and frequency (1/s). η*.sub.0.05 is the complex viscosity at a frequency of 0.05 s.sup.−1 and η*.sub.300 is the complex viscosity at 300 s.sup.−1. According to the empirical Cox-Merz rule, for a given polymer and temperature, the complex viscosity as a function of frequency measured by this dynamic method is the same as the viscosity as a function of shear rate for steady state flow (e.g. a capillary).

[0308] The shear thinning index SHI (0.05/300) is defined as the ratio of the two viscosities eta0.05 (η*.sub.0.05) and eta300 (η*.sub.300).

[0309] Polymerisation activity (kg PE/mol metal*h) was calculated in each polymerisation stage based on polymer yield, molar level of the metallocene complex and residence time in the reactor.

[0310] Polymerisation productivity (kg PE/mol metal) was calculated in each polymerisation stage based on polymer yield and molar level of the metallocene complex.

[0311] Total activity and total productivity are based on the polymer yields and residence times in each reactor, taking also into account the polymer samples taken out of the reactor between the different stages.

[0312] As used herein, bulk density is measured on polymer powder. The bulk density of a powder (loose bulk density) is the ratio of the mass of an untapped powder sample and its volume (g/dm.sup.3). The bulk density of a polymer powder was determined by measuring ca. 100 g of powder sample and let it flow freely through a funnel into a 100 ml cylinder with certified volume and measuring the powder weight.

[0313] Particle size of the polymer was analysed from the dry powder by using Malvern Mastersizer 2000.

[0314] For particle size distributions the median is called the d50. The d50 has been defined as the diameter where half of the population lies below this value. Similarly, 90 percent of the distribution lies below the d90, and 10 percent of the population lies below the d10.

[0315] Ash content of the polymer samples was measured by heating the polymer in a microwave oven at 650° C. during 20 minutes according to ISO 3451-1.

[0316] The foreign particle content of the polymer samples was analysed using light microscopy (Leica MZ16a; Contrast mode: Transmitted light/dark field) on the pressed thin film sample. The samples were prepared by melting one gram of the polymer powder and hot-pressing it to a film between two Mylar sheets, with thickness approx. 200 μm. The quantification of the foreign particles was done by image analysis on the pressed thin film samples (3,3×2.5 mm).

[0317] Al/Me is the ratio in the polymerisation (mol/mol) of aluminium in the aluminoxane to the metal ion (e.g. Zr) of the metallocene. The aluminium level is calculated from MAO and the metal level from the metallocene complex.

Experiments and Results

Experimental

[0318] The following unsupported single site catalyst was used in the polymerisations: [0319] SSC 1: rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride (Mw=426.2 g/mol; commercially available from MCAT, Germany). [0320] SSC 2: bis-n-butylcyclopentadienyl zirconium dichloride (Mw=404.2 g/mol; commercially available from STREM, Germany).

[0321] As a reference, two supported single site catalysts were used. The catalysts were: [0322] comparative catalyst 1: supported rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride) metallocene complex (Zr level 0.2% wt). This catalyst was synthesised according to U.S. Pat. No. 6,291,611; [0323] comparative catalyst 2: supported bis-n-butylcyclopentadienyl zirconium dichloride metallocene complex (Zr level 0.2% wt). This catalyst was synthesised according to WO93/023439. This is the same catalyst used in the examples of WO98/58001; and

[0324] Polymerisations were carried out in a 3,5 and 8 litre reactors fitted with a stirrer and a temperature control system. The same comonomer feeding system was used for all runs. The procedure comprised the following steps:

Polymerization of Lower Molecular Weight Ethylene Polymer:

[0325] The reactor was purged with nitrogen and heated to 110° C. 1200/3500 ml of liquid diluent was then added to the reactor and stirring started at 270 rpm. The reactor temperature was 80° C. Unsupported metallocene catalyst and methylaluminoxane (MAO) were then pre-contacted for 5 min and loaded into the reactor with 300 ml of diluent. Ethylene and hydrogen were then fed to achieve a certain total pressure. Ethylene and hydrogen were then fed continuously. When sufficient amount of powder was made, the polymerization was stopped and the hexane was evaporated.

Polymerization of Higher Molecular Weight Ethylene Polymer:

[0326] 1500/3500 ml of liquid diluent was then added to the reactor and stirring started at 270 rpm. The reactor temperature was 80° C. Ethylene, hydrogen and 1-butene were then fed to achieve a certain total pressure. Ethylene, hydrogen and 1-butene were then fed continuously. When sufficient amount of powder was made, the polymerization was stopped and the hexane was evaporated.

Polymerization of Second Higher Molecular Weight Ethylene Polymer:

[0327] 1500/3500 ml of liquid diluent was then added to the reactor and stirring started at 270 rpm. The reactor temperature was 80° C. Ethylene, hydrogen and 1-butene were then fed to achieve a certain total pressure. Ethylene, hydrogen and 1-butene were then fed continuously. When sufficient amount of powder was made, the polymerization was stopped and the hexane was evaporated.

[0328] Two comparative bimodal polymerisations were also carried out. The first comparative polymerisation (C1) was carried out in the same manner as above except that instead of using unsupported metallocene catalyst and MAO a supported catalyst with rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride) metallocene complex was used. The second comparative polymerisation (C2) was carried out in the same manner as above except that instead of using unsupported metallocene catalyst and MAO a supported catalyst with bis-n-butylcyclopentadienyl zirconium dichloride metallocene complex was used.

[0329] No reactor fouling on the walls occurred in any of the polymerisations E1-E5, C1 or C2. Further details of the polymerisation procedure and details of the resulting polyethylene polymers are summarised in Table 1 below wherein RI refers to the polymerisation in and the product of the first reactor, RII refers to the polymerisation in the second reactor and the product of the first and second reactor together, which is the final polyethylene product in a two stage polymerisation and RIII refers to the polymerisation in the third reactor and the product of the first, second and third reactors together, which is the final product in a three stage polymerisation.

Results

[0330] The results consistently showed the following: [0331] The polymerisation activity (per mol metal) is much higher for the unsupported catalyst than the supported catalysts. This is particularly clear from FIGS. 2 and 3 wherein blue shows the activity of the first polymerisation, red shows the activity of the second polymerisation and green shows the total activity. [0332] Catalyst productivity is high compared to supported metallocene catalyst (per mol metal) [0333] The first polymerisation yields free flowing polyethylene particles with a bulk density of 100-200 g/dm.sup.3 and the second polymerisation yields free flowing polyethylene particles with a bulk density of 200-300 g/dm.sup.3

[0334] The polymerisations carried out in example 1 and comparative example 1 are under identical conditions and with the same catalyst except that in example 1 the catalyst is unsupported, rather than supported as in comparative example 1. The polymerisations were run without the use of hydrogen in the second stage in order to produce bimodal polymers of the highest MW possible in the conditions employed.

[0335] A comparison of the results for example 1 and comparative example 1 in tables 1 and 2 show the following: [0336] The use of an unsupported single site catalyst in a bimodal polymerisation gives rise to polyethylene of significantly higher MW (142,000 c.f. 120,000 g/mol) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. [0337] The use of an unsupported single site catalyst in a bimodal polymerisation gives rise to polyethylene of significantly lower MFR.sub.5 (0.38 c.f. 1.14 g/10 min respectively) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. The MFR values obtained using the unsupported single site catalyst render the bimodal polyethylene suitable for the production of pipe. [0338] The use of the unsupported single site catalyst in the bimodal polymerisation surprisingly did not lead to any reactor fouling. [0339] The use of the unsupported single site catalyst in the bimodal polymerisation produced polymer particles having good morphology and reasonably high bulk density (310 vs. 350 g/dm.sup.3) [0340] The use of the unsupported single site catalyst in the bimodal polymerisation produced polyethylene having a significantly lower ash content (500 c.f. 1310 wt ppm) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. [0341] The use of the unsupported single site catalyst in the bimodal polymerisation produced polyethylene having significantly lower gels than polymerisation with a supported version of the same catalyst under otherwise identical conditions.

[0342] A comparison of the results for example 2 and comparative example 2 in tables 1 and 2 show the following: [0343] The use of an unsupported single site catalyst in a bimodal polymerisation gives rise to polyethylene of significantly higher MW (100,000 c.f. 60,000 g/mol) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. [0344] The use of an unsupported single site catalyst in a bimodal polymerisation gives rise to polyethylene of significantly lower MFR.sub.2.16 and MFR.sub.5 (2.3 c.f. 13 g/10 min and 4.4 c.f. 31 g/10 min respectively) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. [0345] The use of the unsupported single site catalyst in the bimodal polymerisation surprisingly did not lead to any reactor fouling. [0346] The use of the unsupported single site catalyst in the bimodal polymerisation produced polymer particles having good morphology and reasonably high bulk density. [0347] The use of the unsupported single site catalyst in the bimodal polymerisation produced polyethylene having a significantly lower ash content (470 c.f. 910 wt ppm) than polymerisation with a supported version of the same catalyst under otherwise identical conditions. [0348] The use of the unsupported single site catalyst in the bimodal polymerisation produced polyethylene having significantly lower gels than polymerisation with a supported version of the same catalyst under otherwise identical conditions.

[0349] The foreign particle content of the bimodal polymer product of example 2 (E2-RII) and comparative example 2 (C2-RII) was analysed using light microscopy as described above. The results are shown in FIGS. 4A (E2-RII) and 4B (C2-RII). A comparison of the results shows that when an unsupported single site catalyst was used no foreign particles were found on the sample plate. When a supported version of the same catalyst under otherwise identical conditions was used, a large amount of foreign particles (identified as silica) were found with light microscopy on the sample plate.

[0350] Table 3 shows the quantification of the foreign particles in the bimodal polymer product of example 2 (E2-RII) and comparative example 2 (C2-RII). This was done using image analysis. The particles were divided into different size categories (est. diameter).

[0351] A comparison of the results shows that no foreign particles were found on the sample plate when unsupported catalyst was used. When supported catalyst was used mostly particles with diameter sizes 20-40 μm and 40-60 μm were observed. This shows that the use of the unsupported single site catalyst in the bimodal polymerisation produced polyethylene having a significantly lower foreign particle, e.g. silica, content than polymerisation with a supported version of the same catalyst under otherwise identical conditions.

[0352] The polymerisations carried out in example 3 and comparative example 2 are with the same catalyst except that in example 3 the catalyst is unsupported, rather than supported as in comparative example 2, but under different conditions designed to yield the same bimodal polyethylene. In example 3, in the second stage polymerisation, hydrogen is present as a MW regulator whereas in comparative example 2 no hydrogen is used in either the first or second stage of polymerisation.

[0353] A comparison of the results for example 3 and comparative example 2 in tables 1 and 2 shows that the polymers produced have comparable density, MFR.sub.2.16, MFR.sub.5 and molecular weights. The polymer produced in example 3 using an unsupported single site catalyst, however, has a much lower ash content (320 wt c.f. 910 wt ppm).

[0354] The results for examples 4 and 5, which are both three stage polymerisations, show the following: [0355] The use of an unsupported single site catalyst in a trimodal polymerisation gives rise to polyethylene having a high MW (125,000 and 138,000 g/mnol), a density of circa 950 kg/dm.sup.3 and a MFR.sub.5 value of 0.47 or 0.49 g/10 min. The MFR values obtained using the unsupported single site catalyst render the trimodal polyethylenes suitable for the production of pipe. [0356] The use of the unsupported single site catalyst in the trimodal polymerisation surprisingly did not lead to any reactor fouling. [0357] The use of the unsupported single site catalyst in the trimodal polymerisation produced polymer particles having good morphology [0358] The use of the unsupported single site catalyst in the trimodal polymerisation produced polyethylene having a low ash content and low gels.

[0359] The polymerisations carried out in examples 6-8 show the reactor fouling behaviour in first step homopolymerisation where MFR2 is less than 10. Tests E6, E7 and E8 were made to confirm the effect of polymer melt index to reactor fouling behavior. In E6 and E7 unsupported rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride) metallocene complex and in E8 unsupported bis-n-butylcyclopentadienyl zirconium dichloride metallocene complex was used. For E6 and E7, El-RI is used as reference and for E8, E2-RI is used as reference.

[0360] In the reference tests El-RI and E2-RI with MFR2>10, no reactor fouling or clump formation was observed. When the first stage polymerisation tests, E6, E7 and E8, were made by producing polymer with MFR2<10 (0.39, 2.1, 8.8, respectively), significant reactor fouling was observed in all tests. The level of reactor fouling was increased with decreasing melt index; in the tests E6, E7 and E8 about 40, 20 and 5 wt %, respectively, of the total polymer amount was attached on the reactor equipment surfaces. Besides the fouling on the reactor walls, also agglomeration and polymer clumps were obtained with E6 and E7. In all the examples E6, E7 and E8, reactor fouling and clumps caused major problems with reactor operation, e.g. with reactor cooling and stirring.

TABLE-US-00002 TABLE 1 Example Nos. E1-RI E1-RII C1-RI C1-RII C2-RI C2-RII E2-RI E2-RII Catalyst type Unsupported Supported Catalyst Supported Catalyst Unsupported Catalyst Catalyst Complex type Et(tetrahydroind) Et(tetrahydroind) (nBuCp)2ZrCl2 (nBuCp)2ZrCl2 2ZrCl2 2ZrCl2 Mw of complex g/mol 426.2 426.2 426.2 426.2 404.2 404.2 404.2 404.2 Al/Me mol/mol 1000 1000 1000 1000 Complex amount mg 9.9 9.84 1.8 1.68 Metal amount mmol 0.023 0.023 0.02 0.02 0.011 0.011 0.004 0.004 MAO amount g 4.598 0.882 MAO amount ml 5.11 0.98 Catalyst mg 924 917 490 490 POLYMERISATION Homo Copo Homo Copo Homo Copo Homo Copo Temperature ° C. 80 80 80 80 80 80 80 Total pressure bar 18.6 19.6 18.6 19.6 7.8 8.8 7.8 8.8 Solvent i-butane i-butane i-butane i-butane hexane hexane hexane hexane Partial pressure of solvent bar 13.6 13.6 13.6 13.6 2.8 2.8 2.8 2.8 Amount of solvent ml 3800 3800 3800 3800 1500 1500 1500 1500 Stirring speed rpm 270 270 270 270 270 270 270 270 Ethylene partial pressure bar 5 6 5 6 5 6 5 6 Hydrogen (in C2= feed) vol ppm 3950 0 3950 0 3000 0 3000 0 Comonomer type — 1- — 1-butene — 1-butene — 1- butene butene Comonomer total ml 0 56 0 56 0 56 0 10 Running time min 40 38 40 39 40 15 20 10 Reactor split w % 50 50 50 50 50 50 52 48 Yield g 890 885 670 660 260 255 150 140 Activity kg PE/mol Me * h 58059 61115 49489 50376 36214 94714 101050 202100 Total Activity kg PE/mol Me * h 59548 49927 51662 134733 Activity kg pol/g cat h 1.09 1.11 0.80 2.08 Productivity kg PE/mol Me 38706 38706 32992 32744 24143 23679 33683 33683 Total Productivity kg PE/mol Me 77412 65737 47357 68570 Example Nos. E3-RI E3-RII E4-RI E4-RII E4-RIII E5-RI E5-RII E5-RIII Catalyst type Unsupported Unsupported Catalyst Unsupported Catalyst Catalyst Complex type (nBuCp)2ZrCl2 Et(tetrahydroind)2ZrCl2 Et(tetrahydroind)2ZrCl2 Mw of complex g/mol 404.2 404.2 426.2 426.2 426.2 426.2 426.2 426.2 Al/Me mol/mol 1000 1000 1000 1000 1000 1000 1000 1000 Complex amount mg 1.7 1.58 16.2 15.9 15.6 16.2 15.9 15.6 Metal amount mmol 0.004 0.004 0.038 0.037 0.036 0.038 0.037 0.037 MAO amount g 0.833 7.524 7.524 MAO amount ml 0.93 8.36 8.36 Catalyst mg POLYMERISATION Homo Copo Homo Copo Copo Homo Copo Copo Temperature ° C. 80 80 80 80 80 80 80 Total pressure bar 7.8 8.8 16.2 16.6 16.6 16.2 16.6 16.6 Solvent hexane hexane i- i- i- i- i- i-butane butane butane butane butane butane Partial pressure of solvent bar 2.8 2.8 13.6 13.6 13.6 13.6 13.6 13.6 Amount of solvent ml 1500 1500 3800 3800 3800 3800 3800 3800 Stirring speed rpm 270 270 270 270 270 270 270 270 Ethylene partial pressure bar 5 6 5 6 6 5 6 6 Hydrogen (in C2= feed) vol ppm 3000 280 2400 150 0 2400 170 0 Comonomer type — 1- — 1- 1- — 1- 1- butene butene butene butene butene Comonomer total ml 0 10 5 90 5 80 Running time min 20 8 40 27 5 40 36 5 Reactor split w % 52 48 51 40 9 51 40 9 Yield g 140 130 570 440 100 500 610 110 Activity kg PE/mol Me * h 99861 249653 22494 26183 32881 19731 27293 36027 Total Activity kg PE/mol Me * h 142659 24889 24260 Activity kg pol/g cat h Productivity kg PE/mol Me 33287 33287 14996 11783 2740 13154 16376 3002 Total Productivity kg PE/mol Me 66574 29867 32752 Example Nos. E6-RI E7-RI E8-RI Catalyst type Unsupported catalyst Unsupported catalyst Unsupported catalyst Complex type Et(tetrahydroind)2ZrCl2 Et(tetrahydroind)2ZrCl2 (nBuCp)2ZrCl2 Mw of complex g/mol 426.2 426.2 404.2 Al/Me mol/mol 1000 1000 1000 Complex amount mg 3.6 1.4 2 Metal amount mmol 0.008 0.003 0.005 MAO amount g 1.672 0.650 0.979 MAO amount ml 1.86 0.72 1.09 Catalyst mg POLYMERISATION Homo Homo Homo Temperature ° C. 80 80 80 Total pressure bar 6.4 6.4 6.4 Solvent hexane hexane hexane Partial pressure of solvent bar 2.8 2.8 2.8 Amount of solvent MI 1500 1500 1500 Stirring speed Rpm 270 270 270 Ethylene partial pressure bar 5 5 5 Hydrogen (in C2= feed) vol ppm 0 900 900 Comonomer type Comonomer total ml 0 0 0 Running time min 40 60 20 Reactor split w % 100 100 100 Yield g 100 160 200 Activity kg PE/mol 17758 48709 121260 Me * h Total Activity kg PE/mol 17758 48709 121260 Me * h Activity kg pol/g cat h Productivity kg PE/mol Me 11839 48709 40420 Total Productivity kg PE/mol Me 11839 48709 40420

TABLE-US-00003 TABLE 2 POLYMER ANALYSES Example/Run Nos E1-RI E1-RII C1-RI C1-RII C2-RI C2-RII E2-RI E2-RII E3-RI E3-RII E4-RI Density kg/dm3 948 947 942 938.8 941 MFR2.16 270 290 13 285 2.3 12 106 MFR 5 0.38 1.1 31 4.4 32 eta0.05 (η*.sub.0.05) 740 7740 660 eta300 (η.sub.300) 165 680 120 SHI 4 11 5 Mw 142000 120000 60000 100000 60000 Mn 24700 15300 18000 26000 19000 MWD 5.8 7.8 3.3 3.9 3.2 d10 μm 180 155 45 d50 μm 435 240 110 d90 μm 800 440 400 C4 content (FTIR) w % 0.5 0.9 1.9 1.6 1.4 Melting temperature ° C. 132.4 131.8 129.3 127.4 132.9 127.9 132.5 Crystallisation temperature ° C. 116.9 116.2 114.7 113 116.9 112.6 115.7 Heat of Fusion J/g 273 232 230 181 233 193 251 Crystallinity % 94 80 79 62.5 80.5 66.5 86.5 BD g/dm3 310 350 260 130 190 Ash content wtppm 500 1310 910 470 320 Example/Run Nos E4-RII E4-RIII E5-RI E5-RII E5-RIII E6-RI E7-RI E8-RI Density kg/dm3 949.1 952.8 949 940.1 954.5 957.2 MFR2.16 0.39 2.1 8.8 MFR 5 0.47 0.49 eta0.05 (η*.sub.0.05) 56156 4686 1019 eta300 (η.sub.300) 857 809 346 SHI Mw 125000 138000 165000 105000 80000 Mn 29000 29700 41000 42000 34000 MWD 4.3 4.7 4 2.5 2.3 d10 μm d50 μm d90 μm C4 content (FTIR) w % 0.5 0.5 Melting temperature ° C. 133.5 133.1 132.8 Crystallisation temperature ° C. 117.6 117.1 116.8 Heat of Fusion J/g 234 240 235 Crystallinity % 81 82.5 81 BD g/dm3 240 200 Ash content wtppm

TABLE-US-00004 TABLE 3 Foreign particle analysis E2-RII C2-RII Particle No. of No. of diameter (μm) particles Area fraction (%) particles Area fraction (%)  0-20 0 0 56 11 20-40 0 0 32 32 40-60 0 0 17 47 60-80 0 0 2 11