Polyethylene for pipes
The present invention provides a process for the preparation of a multimodal polyethylene comprising: (i) polymerizing ethylene and optionally an α-olefin comonomer in a first polymerization stage to produce a first ethylene polymer; and (ii) polymerizing ethylene and optionally an α-olefin comonomer, in the presence of said first ethylene polymer, in a second polymerization stage, wherein the first and second polymerization stages are carried out in the presence of an unsupported metallocene catalyst and each polymerization 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 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.
1. A process for the preparation of a multimodal polyethylene, comprising:
(i) polymerizing ethylene and optionally an α-olefin comonomer in a first slurry polymerization stage to produce a first ethylene polymer; and
(ii) polymerizing ethylene and optionally an α-olefin comonomer, in the presence of said first ethylene polymer, in a second slurry polymerization stage,
wherein said first and second polymerization stages are carried out in the presence of an unsupported metallocene catalyst, a solution of said unsupported metallocene catalyst in a solvent is employed, and each polymerization stage produces at least 5% wt of said multimodal polyethylene, and
said multimodal polyethylene has a multimodal molecular weight distribution, a weight average molecular weight of at least 50,000 g/mol and a bulk density of at least 250 kg/m 3 .
2. A process as claimed in claim 1 , wherein said solvent is a C 3-10 saturated alkane or an aromatic hydrocarbon.
3. A process as claimed in claim 2 , wherein said solvent is a C 4-10 saturated alkane.
4. A process as claimed in claim 1 , wherein said metallocene is a compound of formula (I):
(Cp) 2 L n MX 2 (I)
wherein
each Cp is independently a cyclic group having a delocalized 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.
5. A process as claimed in claim 4 , wherein Cp 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 claim 4 , wherein L is a methylene, ethylene or silyl bridge.
7. A process as claimed in claim 4 , wherein M is Ti, Zr or Hf.
8. A process as claimed in claim 4 , wherein each X is independently selected from H, halogen, C 1-20 alkyl, C 1-20 alkoxy, C 2-20 alkenyl, C 2-20 alkynyl, C 3-12 cycloalkyl, C 6-20 aryl, C 6-20 aryloxy, C 7-20 arylalkyl, C 7-20 arylalkenyl, —SR″, —PR″, —SiR″ 3 , —OSiR″ 3 , —NR″ 2 or —CH 2 Y wherein Y is C 6-20 aryl, C 6-20 heteroaryl, C 1-20 alkoxy, C 6-20 aryloxy, —NR″ 2 , —SR″, —PR″ 3 , —SiR″ 3 , or —OSi″ 3 and wherein each R″ is independently a hydrogen or hydrocarbyl; or in the case of —NR″ 2 , the two substituents R″ can form a ring, together with the nitrogen atom to which they are attached.
9. A process as claimed in claim 1 , wherein said metallocene is of formula (II):
wherein
M is a transition metal of Group 4 to 6;
each X is independently a sigma-ligand;
—R 1 and —R 1′ are each independently C 1-20 hydrocarbyl; and
—R 2 , —R 2′ , —R 3 , —R 3′ , —R 4 , —R 4′ , —R 5 , and —R 5′ are each independently H or a C 1-20 hydrocarbyl.
10. A process as claimed in claim 1 , wherein said metallocene is of formula (III):
wherein
M is a transition metal of Group 4 to 6;
each X is independently a sigma-ligand;
—R 1 , —R 1′ , —R 2 , and —R 2′ are each independently H or a C 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.
11. A process as claimed in claim 10 , wherein each X is independently H, halogen, C 1-20 alkyl, C 1-20 alkoxy, C 2-20 alkenyl, C 2-20 alkynyl, C 3-12 -cycloalkyl, C 6-20 aryl, C 6-20 aryloxy, C 7-20 arylalkyl, C 7-20 arylalkenyl, —SR″, —PR″ 3 , —SiR″ 3 , —OSiR″ 3 , —NR″ 2 or —CH 2 —Y, wherein Y is C 6-20 aryl, C 6-20 heteroaryl, C 1-20 alkoxy, C 6-20 aryloxy, NR″ 2 , —SR″, —PR″ 3 , —SiR″ 3 , or —OSiR″ 3 and wherein each R″ is independently a hydrogen or hydrocarbyl; or in the case of —NR″ 2 , the two substituents R″ can form a ring, together with the nitrogen atom to which they are attached.
12. A process as claimed in claim 10 , wherein —R 1 , —R 1′ , —R 2 , and —R 2′ are each independently H.
13. A process as claimed in claim 10 , wherein L is a methylene, ethylene or silyl bridge.
14. A process as claimed in claim 1 , wherein said metallocene is (nBuCp) 2 HfCl 2 , (Ind) 2 ZrCl 2 , Et-(tetrahydroindenyl) 2 ZrCl 2 or (nBuCp) 2 ZrCl 2 .
15. A process as claimed in claim 1 , wherein an aluminoxane cocatalyst is employed.
16. A process as claimed in claim 15 , wherein said solvent is a C 4-10 saturated alkane.
17. A process as claimed in claim 1 , wherein said slurry polymerization is carried out in an aliphatic hydrocarbon diluent.
18. A process as claimed in claim 1 , wherein said first and/or said second polymerization stage is carried out in the presence of hydrogen.
19. A process as claimed in claim 1 , wherein there is no reactor fouling in said first and/or said second polymerization stage.
20. A process as claimed in claim 1 , wherein said process consists of a first polymerization stage and a second polymerization stage.
21. A process as claimed in claim 1 , wherein said process consists of a first polymerization stage, a second polymerization stage and a third polymerization stage.
22. A process as claimed in claim 21 , comprising the sequential steps (a)-(c):
(a) polymerizing ethylene and optionally an α-olefin comonomer in a first polymerization stage to produce a lower molecular weight ethylene (LMW) polymer;
(b) polymerizing ethylene and optionally an α-olefin comonomer in a second polymerization stage to produce a first higher molecular weight ethylene polymer (LMW); and
(c) polymerizing ethylene and optionally an α-olefin comonomer in a third polymerization stage to produce a second higher molecular weight ethylene polymer (HMW2);
or
comprising the sequential steps (a)-(c):
(a) polymerizing ethylene and optionally an α-olefin comonomer in a first polymerization stage to produce a lower molecular weight ethylene polymer (LMW);
(b) polymerizing ethylene and optionally an α-olefin comonomer in a second polymerization stage to produce a second higher molecular weight ethylene polymer (HMW2); and
(c) polymerizing ethylene and optionally an α-olefin comonomer in a third polymerization stage to produce a first higher molecular weight ethylene polymer (HMW1).
23. A process as claimed in claim 21 , wherein there is no reactor fouling in said first and/or second and/or third polymerization stage.
24. A process as claimed in claim 1 , wherein said multimodal polyethylene comprises:
a Mw of 100,000 to 250,000 g/mol, and/or;
a MWD of 1 to 25 and/or;
a MFR 2 of 0.005 to 0.2 g/10 min and/or;
a MFR 5 of 0.05 to 1 g/10 min and/or;
0.5 to 10% wt comonomer and/or;
a density of 920 to 980 kg/m 3 and/or;
a bulk density of 250 to 400 kg/m 3 and/or;
an ash content of 0 to 800 wt ppm and/or;
less than 100 wtppm of material of hardness more than 3 on Moh's scale.
25. A process as claimed in claim 1 , wherein said multimodal polyethylene is in the form of particles.
26. A process for preparing a pipe comprising:
i) preparing a multimodal polyethylene by the process claimed in claim 1 ; and
ii) extruding said multimodal polyethylene to produce pipe.