Polyethylene composition for injection moulding with improved stress crack/stiffness relation and impact resistance

1. A multimodal polyethylene composition wherein

(i) the composition has an MFR 2 of 0.05 to 100 g/10 min, and

(ii) the environmental stress crack resistance ESCR measured in hours according to ASTM 1693, condition B and E-modulus EM measured according to ISO 527-2:1993 in MPa satisfy the following relation:

ESCR≧−EM h/MPa+1150h

and wherein the composition comprises:

(A) a fraction of a copolymer of ethylene with one or more alpha-olefins, with a weight average molecular weight M w of 300,000 g/mol or more;

(B) a first ethylene homo- or copolymer fraction; and

(C) a second ethylene homo-or copolymer fraction;

wherein fraction (A) is present in the composition in an amount of from 1 to 20 wt %;

wherein fraction (B) has a lower average molecular weight than fraction (C).

2. A polyethylene composition according to claim 1 , wherein the Charpy impact strength CIS (23° C.) measured in kJ/m 2 according to ISO

179:2000 and the MFR 2 in g/10 min satisfy the following relation:

CIS≧− 12 log( MFR 2 /g/10min)kJ/m 2 +21.6kJ/m 2 .

3. A polyethylene composition according to claim 1 , wherein the Charpy impact strength CIS (23° C.) measured in kJ/m 2 according to ISO 179:2000 and the flowability (FA) measured in terms of the spiral length in cm at a pressure of 1000 bar satisfy the following relation:

CIS≧− 0.5 FA kJ/m 2 /cm+39kJ/m 2 .

4. A polyethylene composition according to claim 1 wherein fraction (A) has a M w of 320,000 to 1,000,000 g/mol.

5. A polyethylene composition according to claim 1 wherein the comonomer content of fraction (A) is from 1 to 15 mol %.

6. A polyethylene composition according to wherein fraction (A) has a density of 940 kg/m 3 or less.

7. A polyethylene composition according to claim 1 wherein the M w of fraction (A) is higher than the M w of fractions (B) and (C) by a factor of from 2 to 6.

8. A polyethylene composition according to claim 1 wherein fractions (A), (B) or (C) are produced in a multistage reaction.

9. A polyethylene composition according to claim 1 wherein the composition has a density of 940 kg/m 3 or higher.

10. An injection or compression moulded article comprising the polyethylene composition according to claim 1 .

11. A process for preparing an injection or compression moulded article according to claim 10 wherein the process comprises injection or compression moulding the polyethylene composition into an article.

12. A polyethylene composition according to claim 1 adapted for injection or compression moulding.

13. A polyethylene composition according to claim 2 , wherein the Charpy impact strength CIS (23° C.) measured in kJ/m 2 according to ISO 179:2000 and the flowability (FA) measured in terms of the spiral length in cm at a pressure of 1000 bar satisfy the following relation:

CIS≧− 0.5 FA kJ/m 2 /cm+39kJ/m 2 .

14. A polyethylene composition according to claim 4 wherein the comonomer content of fraction (A) is from 1 to 15 mol %.

15. A polyethylene composition according to claim 1 , wherein the composition has an MFR 2 of 1.0 to 100 g/10 min and the ESCR is at least 150 h.