Process for the preparation of multimodal high density polyethylene

1. A process for the preparation of a multimodal high density polyethylene (HDPE) having a melt flow rate (MFR 2 ) of 0.1 to 4.0 g/10 min and a MFR 5 of 2.5 to 12 g/10 min, wherein the multimodal HDPE has a FNCT (6 MPa, 50° C.) of at least 30 hours, said process comprising:

(i) polymerising ethylene in a first polymerisation stage in the presence of a Ziegler-Natta catalyst to prepare a first ethylene homopolymer having a MFR 2 from 10 to 500 g/10 min;

(ii) polymerising ethylene in a second polymerisation stage in the presence of said catalyst and said first ethylene homopolymer to prepare an ethylene homopolymer mixture comprising said first ethylene homopolymer and a second ethylene homopolymer, said mixture having a MFR 2 from 50 to 1000 g/10 min; and

(iii) polymerising ethylene and at least one alpha-olefin comonomer in a third polymerisation stage in the presence of said catalyst and said ethylene homopolymer mixture to prepare said multimodal HDPE.

2. The process as claimed in claim 1 , wherein said first polymerisation stage and/or said second polymerisation stage are carried out in a slurry loop reactor.

3. The process as claimed in claim 1 , wherein said third polymerisation stage is carried out in a gas phase reactor.

4. The process as claimed in claim 1 , wherein said at least one alpha-olefin has 3 to 12 carbon atoms.

5. The process as claimed in claim 4 , wherein said at least one alpha-olefin is selected from the group consisting of 1-butene, 1-hexene and 1-octene.

6. The process as claimed in claim 4 , wherein said at least one alpha-olefin is 1-butene.

7. The process as claimed in claim 1 , wherein the multimodal HDPE has a density of 950 to 960 kg/m 3 .

8. The process as claimed in claim 1 , wherein the multimodal HDPE has a MFR 2 of 0.5 to 1.5 g/10 min.

9. The process as claimed in claim 1 , wherein the multimodal HDPE has a tensile modulus of at least 850 MPa, more preferably at least 900 MPa.

10. A multimodal high density polyethylene (HDPE) having a melt flow rate (MFR 2 ) of 0.1 to 4.0 g/10 min obtained by a process as defined in claim 1 .

11. A multimodal high density polyethylene (HDPE) having a density of 940 to 970 kg/m 3 and a melt flow rate (MFR 2 ) of 0.1 to 4.0 g/10 min, a MFR 5 of 2.5 to 12 g/10 min, wherein said multimodal HDPE comprises a low molecular weight ethylene homopolymer component and a high molecular weight ethylene copolymer component and wherein said multimodal HDPE has a Mz/Mn of at least 95 and/or Mz of at least 680000;

wherein said low molecular weight ethylene homopolymer component is an ethylene homopolymer mixture comprising a first ethylene homopolymer and a second ethylene homopolymer; and

wherein the multimodal HDPE has a FNCT (6 MPa, 50° C.) of at least 30 hours.

12. An injection or compression molded article, comprising the multimodal HDPE as defined in claim 11 .

13. The injection or compression molded article of claim 12 , wherein the injection or compression molded article is a cap or a closure.

14. A cap or closure comprising the multimodal HDPE as defined in claim 11 .

15. A process for the manufacture of an injection or compression molded moulded article, said process comprising preparing the multimodal HDPE as defined in claim 11 ;

optionally pelletizing the multimodal HDPE to form pellets; and

injection or compression moulding the formed pellets or HDPE to form said article.

16. A process for a manufacture an injection or compression molded cap or closure comprising:

injection or compression molding of the multimodal HDPE of claim 11 .