Polyethylene composition for pipe and pipe coating applications

1. A polyethylene composition comprising

a base resin being a copolymer of ethylene with at least one alpha olefin comonomer having from 3 to 12 carbon atoms, the base resin having a density of more than 940.0 kg/m 3 and equal to or less than 952.5 kg/m 3 , determined according to ISO 1183-1:2004,

wherein the composition has a melt flow rate MFR 5 (190° C., 5 kg) of 0.10 to 3.0 g/10 min, determined according to ISO 1133, a storage modulus of G′ (2 kPa) of 600 Pa to 900 Pa, a shear thinning index SHI 2.7/210 of 20 to 50 and

the composition has an elongation at break (EB) at −45° C. in dependence of the molar comonomer content (mol. CC) of the base resin following the following equation

EB[%]≧175[%/mol %]·mol. CC[mol %],

whereby the elongation at break (EB) is determined according to ISO 527-1 at a temperature of −45° C. and the molar comonomer content (mol. CC) reflects the molar content of alpha-olefin comonomer units in the total content of monomer units of the base resin.

2. The polyethylene composition according to claim 1 having a polydispersity index PI within the range of equal to or higher than 1.5 Pa −1 and equal to or less than 3.0 Pa −1 .

3. The polyethylene composition according to claim 1 , wherein the base resin has a content of units derived from at least one alpha olefin comonomer having from 3 to 12 carbon atoms of 0.1 to 3.0 mol.-%.

4. The polyethylene composition according to claim 1 , wherein the composition has a tensile modulus (23° C.) of at least 900 MPa.

5. The polyethylene composition according to claim 1 , wherein the composition has a Charpy Notched Impact Strength at 0° C. of at least 10 kJ/m 3 .

6. The polyethylene composition according to claim 1 , wherein the composition has a flow rate ratio FRR 21/5 , being the ratio of MFR 21 to MFR 5 , of 15 to 40.

7. The polyethylene composition according to claim 1 , wherein the composition has a complex viscosity at 0.05 rad/s eta* 0.05 rad/s of 100 000 Pa·s to 220 000 Pa·s.

8. The polyethylene composition according to claim 1 , wherein the composition has a molecular weight distribution, being the ratio of Mw/Mn, of 10 to 30.

9. The polyethylene composition according to claim 1 , wherein the base resin comprises at least two ethylene homo- or copolymer fractions (A) and (B) being different in their weight average molecular weight M w ,

wherein fraction (A) is an ethylene homopolymer and fraction (B) is a copolymers of ethylene and at least one alpha-olefin comonomer units with 3 to 12 carbon atoms,

whereby fraction (A) has a melt flow rate MFR 2 (190°, 2.16 kg) of 100 to 400 g/10 min, and

wherein fraction (A) is present in an amount of 44 to 54 wt.-% with respect to the base resin.

10. A polyethylene composition obtainable by a multistage process, the multistage process comprising

a) polymerizing ethylene in the presence of

a silica supported Ziegler Natta catalyst having a molar composition of the catalyst including

Al 1.30 to 1.65 mol/kg silica,

Mg 1.25 to 1.61 mol/kg silica,

Ti 0.70 to 0.90 mol/kg silica

and having a mean particle size (D50) of 7 to 15 μm, preferably of 8 to 12 μm

in a first reactor

for obtaining an intermediate material, the intermediate material having a has a melt flow rate MFR 2 (190°, 2.16 kg) of 100 to 400 g/10 min; and

b) transferring the reaction product to gas phase reactor

(i) feeding ethylene and comonomer to the gas phase reactor

(ii) further polymerizing the intermediate

to obtain a base resin having a density of more than 940.0 kg/m 3 and equal to or less than 952.5 kg/m 3 , determined according to ISO 1183-1:2004,

c) extruding the base resin into a polyethylene composition having a melt flow rate MFR 5 (190° C., 5 kg) of 0.10 to 3.0 g/10 min, determined according to ISO 1133, a storage modulus of G′ (2 kPa) of 600 Pa to 900 Pa, a shear thinning index SHI 2.7/210 of 20 to 50, and the composition has an elongation at break (EB) at −45° C. in dependence of the molar comonomer content (mol. CC) of the base resin following the following equation

EB[%]≧175[%/mol %]·mol. CC[mol %],

whereby the elongation at break (EB) is determined according to ISO 527-1 at a temperature of −45° C. and the molar comonomer content (mol. CC) reflects the molar content of alpha-olefin comonomer units in the total content of monomer units of the base resin.

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

12. The article according to claim 11 being a pipe or pipe fitting.

13. The article according to claim 11 being a pipe, the outer surface of which is coated with a coating layer comprising a polyethylene composition which comprises

a base resin being a copolymer of ethylene with at least one alpha olefin comonomer having from 3 to 12 carbon atoms, the base resin having a density of more than 940.0 kg/m 3 and equal to or less than 952.5 kg/m 3 , determined according to ISO 1183-1:2004,

wherein the composition has a melt flow rate MFR 5 (190° C., 5 kg) of 0.10 to 3.0 g/10 min, determined according to ISO 1133, a storage modulus of G′ (2 kPa) of 600 Pa to 900 Pa, a shear thinning index SHI 2.7/210 of 20 to 50 and

the composition has an elongation at break (EB) at −45° C. in dependence of the molar comonomer content (mol. CC) of the base resin following the following equation

EB[%]≧175[%/mol %]·mol. CC[mol %],

whereby the elongation at break (EB) is determined according to ISO 527-1 at a temperature of −45° C. and the molar comonomer content (mol. CC) reflects the molar content of alpha-olefin comonomer units in the total content of monomer units of the base resin.

14. An article comprising the polyethylene composition according to claim 10 .

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

16. The article according to claim 11 being a pipe, the outer surface of which is coated with a coating layer comprising a polyethylene composition obtainable by

a multistage process, the multistage process comprising

d) polymerizing ethylene in the presence of

a silica supported Ziegler Natta catalyst having a molar composition of the catalyst including

Al 1.30 to 1.65 mol/kg silica,

Mg 1.25 to 1.61 mol/kg silica,

Ti 0.70 to 0.90 mol/kg silica

and having a mean particle size (D50) of 7 to 15 μm, preferably of 8 to 12 μm

in a first reactor

for obtaining an intermediate material, the intermediate material having a has a melt flow rate MFR 2 (190°, 2.16 kg) of 100 to 400 g/10 min; and

e) transferring the reaction product to gas phase reactor

(iii) feeding ethylene and comonomer to the gas phase reactor

(iv) further polymerizing the intermediate

to obtain a base resin having a density of more than 940.0 kg/m 3 and equal to or less than 952.5 kg/m 3 , determined according to ISO 1183-1:2004,

f) extruding the base resin into a polyethylene composition having a melt flow rate MFR 5 (190° C., 5 kg) of 0.10 to 3.0 g/10 min, determined according to ISO 1133, a storage modulus of G′ (2 kPa) of 600 Pa to 900 Pa, a shear thinning index SHI 2.7/210 of 20 to 50, and the composition has an elongation at break (EB) at −45° C. in dependence of the molar comonomer content (mol. CC) of the base resin following the following equation

EB[%]≧175[%/mol %]·mol. CC[mol %],

whereby the elongation at break (EB) is determined according to ISO 527-1 at a temperature of −45° C. and the molar comonomer content (mol. CC) reflects the molar content of alpha-olefin comonomer units in the total content of monomer units of the base resin.