POLYETHYLENE COMPOSITION FOR HIGH PRESSURE RESISTANT PIPES

1 : A polyethylene composition comprising

a base resin having a density of from 950.0 kg/m 3 to 962.0 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 9.0 g/10 min, determined according to ISO 1133 and a viscosity at a constant shear stress of 747 Pa eta 747 of from 3500 kPa·s to 20000 kPa·s.

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 comonomer 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 comonomer 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 complex viscosity at a frequency of 0.05 rad/s eta 0.05 of from 600 kPa·s to 2000 kPa·s, determined according to ISO 6721-1 and ISO 6721-10.

5 : The polyethylene composition according to claim 1 , having a weight average molecular weight Mw of from 300 kg/mol to 750 kg/mol, determined by GPC and/or a molecular weight distribution Mw/Mn of from 25 to 100.

6 : The polyethylene composition according to claim 1 , having a z average molecular weight Mz of from 1800 kg/mol to 3000 kg/mol, determined by GPC.

7 : The polyethylene composition according to claim 1 , having a polydispersity index PI of from 2.0 Pa −1 to 7.5 Pa −1 , determined according to ISO 6721-1 and ISO 6721-10.

8 : A polyethylene composition 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, the first intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 5.0 to 300 g/10 min,

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 80 to 300 g/10 min; and

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

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

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

for obtaining a base resin having density of from 950.0 kg/m 3 to 962.0 kg/m 3 , determined according to ISO 1183, and

d) 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 9.0 g/10 min, determined according to ISO 1133, and a viscosity at a constant shear stress of 747 Pa eta 747 of from 3500 kPa·s to 20000 kPa·s.

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 three 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 a solid Ziegler-Natta catalyst component in a first reactor for obtaining a first intermediate material, the first intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of 5.0 to 300 g/10 min,

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 80 to 300 g/10 min; and

c) transferring the second intermediate material to a third 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 second intermediate material in the presence of a silane type external donor,

for obtaining a base resin having density of from 950.0 kg/m 3 to 962.0 kg/m 3 , determined according to ISO 1183, and

d) extruding the base resin into the polyethylene composition.

11 : The process according to claim 10 , wherein the polymerization step a) is preceded by an additional polymerization step b),

b) polymerizing ethylene and optionally comonomer selected from alpha olefins having from 4 to 12 carbon atoms in the presence of the solid Ziegler-Natta catalyst component in a pre-first reactor for obtaining a pre-first intermediate material and transferring said pre-first intermediate material to the first reactor.

12 : An article comprising the polyethylene composition according to claim 1 .

13 : The article according to claim 12 , being a pipe or pipe fitting.

14 : The article according to 12 , being a pipe which has a pressure resistance of at least 75 h, determined according to ISO 1167-1:2006 at a hoop stress of 13.9 MPa at a temperature of 20° C., and/or a pressure resistance of at least 1000 h, determined according to ISO 1167-1:2006 at a hoop stress of 12.9 MPa at a temperature of 20° C.

15 . (canceled)