Polyethylene moulding composition with improved stress crack/stiffness relationship and impact resistance

1. A high density polyethylene moulding composition having a multimodal molecular weight distribution, having a density according to ISO 1183 at 23° C. in the range of 945 to 965 kg/m 3 and an MFR 190/2 according to ISO 1133 in the range of 0.05 to 25 g/10 min, said polyethylene moulding composition comprising at least three ethylene polymer fractions having distinct molecular weights:

A) 15 to 50 wt % of a low molecular weight ethylene homopolymer fraction with a weight average molecular weight Mw in the range of 15 to 40 kg/mol;

B) 15 to 50 wt % of a medium molecular weight ethylene homopolymer fraction with a weight average molecular weight Mw in the range of 70 to 180 kg/mol; and

C) 15 to 50 wt % of a high molecular weight ethylene copolymer fraction with a weight average molecular weight Mw in the range of 200 to 400 kg/mol;

and wherein the composition has:

a tensile modulus according to ISO 527-2:1993 measured on compression moulded samples of at least 800 MPa;

a Charpy impact strength CIS (23° C.) according to ISO 179:2000 measured on V-notched samples produced by compression moulding of at least 30 kJ/m 2 ; and

an environmental stress crack resistance ESCR measured as FNCT full notch creep test according to ISO/DIS 16770.3 at 50° C. and 6 MPa of at least 20 h.

2. A composition as claimed in claim 1 having a Charpy impact strength (−20° C.) according to ISO 179:2000 measured on V-notched samples produced by compression moulding of at least 10 kJ/m 2 .

3. A composition as claimed in claim 1 in which there is at least 20 wt % of each fraction A), B) and C).

4. A composition as claimed in claim 1 wherein component B has a density of 940 to 965 kg/m 3 .

5. A composition as claimed in claim 1 wherein component C has a density of less than 915 kg/m 3 .

6. A composition as claimed in claim 1 wherein fraction C is an ethylene hexene copolymer.

7. A composition as claimed in claim 1 wherein fraction C forms at least 25 wt % of the composition.

8. A composition as claimed in claim 1 having an environmental stress crack resistance ESCR measured as FNCT full notch creep test according to ISO/DIS 16770.3 at 50° C. and 6 MPa of at least 20 h.

9. A composition as claimed in claim 1 wherein the fractions A, B and C are manufactured using a single site catalyst.

10. A moulded article comprising a composition as claimed in claim 1 .

11. A high density polyethylene moulding composition having a multimodal molecular weight distribution, having a density according to ISO 1183 at 23° C. in the range of 945 to 965 kg/m 3 and an MFR 190/2 according to ISO 1133 in the range of 0.05 to 25 g/10 min, said polyethylene moulding composition comprising at least three ethylene polymer fractions having distinct molecular weights:

A) 15 to 50 wt % of a low molecular weight ethylene homopolymer fraction with an MFR 190/2 of 50 to 1000 g/10 min;

B) 15 to 50 wt % of a medium molecular weight ethylene homopolymer fraction with an MFR 190/21 of 6 to 50 g/10 min; and

C) 15 to 50 wt % of a high molecular weight ethylene copolymer fraction with an MFR 190/21 of 0.05 to 5 g/10 min;

and wherein the composition has:

a tensile modulus according to ISO 527-2:1993 measured on compression moulded samples of at least 800 MPa;

a Charpy impact strength CIS (23° C.) according to ISO 179:2000 measured on V-notched samples produced by compression moulding of at least 30 kJ/m 2 ; and

an environmental stress crack resistance ESCR measured as FNCT full notch creep test according to ISO/DIS 16770.3 at 50° C. and 6 MPa of at least 20 h.

12. A process for the manufacture of a polyethylene moulding composition, the process comprising blending

A) 15 to 50 wt % of a low molecular weight ethylene homopolymer fraction with a weight average molecular weight Mw in the range of 15 to 40 kg/mol;

B) 15 to 50 wt % of a medium molecular weight ethylene homopolymer fraction with a weight average molecular weight Mw in the range of 70 to 180 kg/mol; and

C) 15 to 50 wt % of a high molecular weight ethylene copolymer fraction with a weight average molecular weight Mw in the range of 200 to 400 kg/mol so as to form a high density polyethylene moulding composition having a multimodal molecular weight distribution,

a density according to ISO 1183 at 23° C. in the range of 945 to 965 kg/m 3 and an MFR 190/2 according to ISO 1133 in the range of 0.05 to 25 g/10 min,

a tensile modulus according to ISO 527-2:1993 measured on compression moulded samples of at least 800 MPa;

a Charpy impact strength CIS (23° C.) according to ISO 179:2000 measured on V-notched samples produced by compression moulding of at least 30 kJ/m 2 ; and

an environmental stress crack resistance ESCR measured as FNCT full notch creep test according to ISO/DIS 16770.3 at 50° C. and 6 MPa of at least 20 h.