Polyethylene composition for pressure pipes with enhanced flexibility

1. A polyethylene composition comprising a base resin which comprises

(a) an ethylene homo- or copolymer fraction (A); and

(b) an ethylene homo- or copolymer fraction (B),

wherein

(i) fraction (A) has a lower average molecular weight than fraction (B);

(ii) the base resin has a density of 940 to 947 kg/m 3 , determined according to ISO 1183-2;

(iii) the polyethylene composition has an MFR 5 of 0.1 to 0.5 g/10 min, determined according to ISO 1133;

(iv) the polyethylene composition has an SHI (2.7/210) of 10 to 45;

(v) fraction (B) comprises 0.3 to 6.0 mol % of at least one alpha-olefin comonomer; and

(vi) fraction (A) has a density of 955 to 980 kg/m 3 , determined according to ISO 1183-2.

2. The polyethylene composition according to claim 1 further comprising a nucleating agent.

3. The polyethylene composition according to claim 1 , wherein fraction (A) has a MFR 2 of 10 to 300 g/10 min, determined according to ISO 1133.

4. The polyethylene composition according to claim 3 , wherein fraction (A) has a MFR 2 of 30 to 100 g/10 min, determined according to ISO 1133.

5. The polyethylene composition according to claim 1 , wherein the shear stress of the polyethylene composition η 2.7 kPa is 80 to 230 kPas.

6. The polyethylene composition according to claim 1 , wherein the flexural modulus of the polyethylene composition is 500 to 900 MPa, determined according to ISO 178.

7. The polyethylene composition according to claim 1 , wherein the weight split between fraction (A) and fraction (B) is (30-47):(70-53).

8. The polyethylene composition according to claim 1 wherein the rapid crack propagation resistance of a pipe made of the polyethylene composition as measured in the S4 test results in a T crit of <−12° C., determined according to ISO 13477:1997 (E).

9. The polyethylene composition according to claim 1 wherein the slow crack propagation resistance of a pipe made of the polyethylene composition as measured according to ISO 6252 (CTL) with a notch according to ASTM 1473 is at least 500 h.

10. The polyethylene composition according to claim 1 wherein the base resin comprises 0.2 to 3.0 mol % of at least one alpha-olefin comonomer.

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

12. The article according to claim 11 wherein the article is a pipe.

13. The pipe according to claim 12 which fulfils the MRS 10.0 requirement according to ISO 9080.

14. The polyethylene composition of claim 1 , wherein the base resin has a density of 940 to 946 kg/m 3 , determined according to ISO 1183-2.

15. The polyethylene composition of claim 1 , wherein the base resin has a density of 941 to 945 kg/m 3 , determined according to ISO 1183-2.

16. The polyethylene composition of claim 1 , wherein the polyethylene composition has an SHI (2.7/210) of 15 to 35.

17. The polyethylene composition of claim 1 , wherein the polyethylene composition has an SHI (2.7/210) of 15 to 23.1.

18. A polyethylene composition comprising a bimodal base resin which comprises

(a) an ethylene homo- or copolymer fraction (A) that has a MFR 2 of 10 to 300 g/10 min, determined according to ISO 1133; and

(b) an ethylene homo- or copolymer fraction (B),

wherein

(ii) fraction (A) has a lower average molecular weight than fraction (B);

(ii) the base resin has a density of 941 to 945 kg/m 3 , determined according to ISO 1183-2;

(iii) the polyethylene composition has an MFR 5 of 0.2 to 0.4 g/10 min, determined according to ISO 1133;

(iv) the polyethylene composition has an SHI (2.7/210) of 15 to 35;

(v) the polyethylene composition further comprises a nucleating agent; and

(vi) fraction (B) comprises 0.3 to 6.0 mol % of at least one alpha-olefin comonomer.