Polyethylene for pipes
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 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 polyethylene to produce a pipe, and a pipe obtained by such a process.
1. A metallocene multimodal polyethylene comprising:
i) a multimodal molecular weight distribution;
ii) a weight average molecular weight of at least 50,000 g/mol;
iii) a MFR 2 of less than 0.2 g/10 min;
iv) a MFR 5 of less than 1 g/10 min;
v) a bulk density of at least 250 g/dm 3 ; and
vi) an ash content of less than 800 ppm wt;
wherein said multimodal polyethylene is prepared with a single catalyst, wherein said single catalyst is an unsupported metallocene catalyst.
2. A metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene contains less than 100 wt ppm of material of hardness more than 3 on Moh's scale.
3. A pipe comprising metallocene multimodal polyethylene as claimed in claim 1 .
4. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a Mw of 100,000-250,000 g/mol.
5. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a Mn of 18,000 to 40,000 g/mol.
6. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a MWD of 1 to 25.
7. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a MFR 2 of 0.005-0.2 g/10 min.
8. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a MFR 5 of 0.05-1 g/10 min.
9. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a density of 920-970 kg/m 3 .
10. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene is in the form of powder.
11. The metallocene multimodal polyethylene as claimed in claim 10 , wherein said metallocene multimodal powder has a bulk density of 250-400 g/dm 3 .
12. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene is in the form of particles.
13. The metallocene multimodal polyethylene as claimed in claim 12 , wherein said metallocene multimodal particles have a bulk density of 250-400 g/dm 3 .
14. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has an ash content of 0 to 600 wt ppm.
15. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has an ash content of 0 to 400 wt ppm.
16. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a fluorocarbon and fluorocarbyl content of less than 20 wt ppm.
17. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene has a bimodal or trimodal molecular weight distribution.
18. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene does not contain silica.
19. The metallocene multimodal polyethylene as claimed in claim 1 , wherein said metallocene multimodal polyethylene comprises 98 to 99.9% wt ethylene monomer.