Polyethylene composition for high pressure resistant pipes with improved homogeneity

1. A polyethylene composition comprising:

a base resin, which is a polymeric part of the polyethylene composition without fillers, having a density of from 953.0 kg/m 3 to 960.0 kg/m 3 , determined according to ISO 1183, wherein

the polyethylene composition has a melt flow rate MFR 21 (190° C., 21.16 kg), of from 1.0 to 7.5 g/10 min, determined according to ISO 1133, a complex viscosity at a frequency of 0.05 rad/s eta 0.05 of from 750 kPa·s to 1900 kPa's, determined according to ISO 6721-1 and ISO 6721-10, a white spot rating of 3.0 to 12.0, determined according to ISO 18553, after a single compounding step in a twin screw extruder at an extruder temperature before die plate of 200° C. to 300° C. and a specific energy input SEI of 200 kwh/ton to 350 kWh/ton, a weight average molecular weight Mw of 300 kg/mol to 600 kg/mol, determined by GPC, a molecular weight distribution Mw/Mn of from 30 to 65, and a tensile modulus of 1200 MPa to 2000 MPa, determined according to ISO 527-2:1993.

2. The polyethylene composition according to claim 1 , wherein the base resin comprises an ethylene copolymer having ethylene monomer units and comonomer units selected from at least one alpha-olefin having from 4 to 12 carbon atoms.

3. The polyethylene composition according to claim 1 , wherein the polyethylene composition has a content of comonomer units selected from at least one alpha-olefin having from 4 to 12 carbon atoms, of from 0.01 to 0.50 mol %, based on the total molar amount of monomer units in the polyethylene composition.

4. The polyethylene composition according to claim 1 having a weight average molecular weight Mw of from 380 kg/mol to 500 kg/mol, determined by GPC.

5. The polyethylene composition according to claim 1 having a z average molecular weight Mz of from 1200 kg/mol to 3000 kg/mol, determined by GPC.

6. The polyethylene composition according to claim 1 having a polydispersity index PI of from 2.5 Pa −1 to 7.5 Pa −1 , determined according to ISO 6721-1 and ISO 6721-10.

7. The polyethylene composition according to claim 1 , wherein the base resin comprises at least four ethylene homo- or copolymer fractions (A), (B), (C) and (D),

wherein fractions (A) and (B), (C) are all an ethylene homopolymer, and fraction (D) is a copolymer of ethylene and at least one alpha-olefin comonomer with 4 to 12 carbon atoms,

whereby the combined fractions (A) and (B) have a melt flow rate MFR 2 (190° C., 2.16 kg) of 5.0 to 250 g/10 min,

the combined fractions (A), (B) and (C) have a melt flow rate MFR 2 (190° C., 2.16 kg) of 80 to 250 g/10 min and

wherein fraction (A) is present in an amount of 0.5 to 10 wt.-% with respect to the base resin,

and combined fractions (A), (B) and (C) are present in an amount of 35 to 70 wt.-% with respect to the base resin.

8. A polyethylene composition of claim 1 obtainable by a multistage process, the multistage process comprising the following steps:

a) polymerizing ethylene in the presence of a solid Ziegler-Natta catalyst component in a first reactor for obtaining a first intermediate material,

b) transferring the first intermediate material to a second reactor

(i) feeding ethylene to the second reactor

(ii) further polymerizing the first intermediate material

for obtaining a second intermediate material, the second intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 5.0 to 250 g/10 min; and

c) transferring the second intermediate material to a third reactor

(i) feeding ethylene to the third reactor

(ii) further polymerizing the second intermediate material

for obtaining a third intermediate material, the third intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 80 to 250 g/10 min;

d) transferring the third intermediate material to a fourth reactor

(i) feeding ethylene and comonomer selected from alpha-olefins having from 4 to 12 carbon atoms to the fourth reactor

(ii) further polymerizing the third intermediate material in the presence of a silane type external donor

for obtaining a base resin having density of from 953.0 kg/m 3 to 960.0 kg/m 3 , determined according to ISO 1183, and

e) extruding the base resin into a polyethylene composition having a melt flow rate MFR 21 (190° C., 21.16 kg), of from 1.0 to 7.5 g/10 min, determined according to ISO 1133, a complex viscosity at a frequency of 0.05 rad/s eta 0.05 of from 750 kPa's to 1900 kPa·s, determined according to ISO 6721-1 and ISO 6721-10, a white spot rating of 3.0 to 12.0, determined according to ISO 18553, after a single compounding step in a twin screw extruder temperature before die plate of 200° C. to 300° C. and a specific energy input SEI of 200 kWh/ton to 350 kWh/ton, a weight average molecular weight Mw of from 300 kg/mol to 600 kg/mol, determined by GPC, a molecular weight distribution Mw/Mn of from 30 to 65, and a tensile modulus of 1200 MPa to 2000 MPa, determined according to ISO 527-2:1993.

9. A process for producing the polyethylene composition according to claim 1 , wherein the base resin is polymerized in a multistage process in at least four sequential reactor stages in any order in the presence of a solid Ziegler-Natta catalyst component.

10. The process according to claim 9 , wherein the multistage process comprises the following steps:

(a) polymerizing ethylene in the presence of the solid Ziegler-Natta catalyst component in a first reactor for obtaining a first intermediate material,

(b) transferring the first intermediate material to a second reactor

(i) feeding ethylene to the second reactor

(ii) further polymerizing the first intermediate material

for obtaining a second intermediate material, the second intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 5 to 250 g/10 min; and

(c) transferring the second intermediate material to a third reactor

(i) feeding ethylene to the third reactor

(ii) further polymerizing the second intermediate material

for obtaining a third intermediate material, the third intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 80 to 250 g/10 min;

(d) transferring the third intermediate material to a fourth reactor

(i) feeding ethylene and comonomer selected from alpha-olefins having from 4 to 12 carbon atoms to the fourth reactor

(ii) further polymerizing the third intermediate material in the presence of a silane type external donor

for obtaining a base resin having density of from 953.0 kg/m 3 to 960.0 kg/m 3 , determined according to ISO 1183, and

(e) extruding the base resin into a polyethylene composition.

11. The process according to claim 9 wherein the solid Ziegler-Natta catalyst component is a solid MgCl 2 supported catalyst component comprising an internal organic compound of formula (I) or isomers or mixtures therefrom

and

wherein in the formula (I)

R 1 to R 5 are the same or different and can be hydrogen, a linear or branched C 1 to C 8 -alkyl group, or a C 3 -C 8 -alkylene group, or two or more of R 1 to R 5 can form a ring,

the two oxygen-containing rings are individually saturated or partially unsaturated or unsaturated.

12. The process according to claim 9 wherein the solid Ziegler-Natta catalyst is pre-polymerized with one or more olefin monomer(s) selected from C2, C3 or C4 olefin monomers and mixtures thereof to obtain a pre-polymerized solid Ziegler-Natta catalyst before the first polymerization step (a).

13. An article comprising the polyethylene composition according to claim 1 .

14. The article according to claim 13 being a pipe or pipe fitting.

15. The article according to claim 13 being a pipe which has a pressure resistance of at least 100 h, determined according to ISO 1167-1:2006 at a hoop stress of 6.8 MPa at a temperature of 80° C., and/or a pressure resistance of at least 2000 h, determined according to ISO 1167-1:2006 at a hoop stress of 6.2 MPa at a temperature of 80° C.