Polyethylene composition for pressure pipes with enhanced flexibility
View Patent ↗The present invention relates to 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 weight average molecular weight than fraction (B); (ii) the base resin has a density of 932 to 938 kg/m 3 ; (iii) the polyethylene composition has an MFR 5 of 0.1 to 0.6 g/10 min; and (iv) the polyethylene composition has a shear stress η2.7 kPa of 85 to 230 kPas. Furthermore, the present invention relates to an article, preferably a pipe comprising said composition and to the use of said composition for the production of an article, preferably a pipe.
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 weight average molecular weight than fraction (B);
(ii) the base resin has a density of 932 to 938 kg/m 3 ;
(iii) the polyethylene composition has an MFR 5 of 0.1 to 0.6 g/10 min; and
(iv) the polyethylene composition has a shear stress η 2.7 kPa (viscosity at a shear stress of 2.7 kPa at 190° C.) of 85 to 230 kPas.
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 400 g/10 min.
4. The polyethylene composition according to claim 3 , wherein fraction (A) has an MFR 2 of 25 to 100 g/10 min.
5. The polyethylene composition according to claim 1 , wherein fraction (A) has a density of 960 to 980 kg/m 3 .
6. The polyethylene composition according to claim 1 , wherein the SHI (2.7/210) (ratio of the viscosity, measured at a shear stress of 2.7 kPa and 210 kPa at 190° C.) of the polyethylene composition is 10 to 49.
7. The polyethylene composition according to claim 1 , wherein the flexural modulus of the polyethylene composition is 300 to 700 MPa.
8. The polyethylene composition according to claim 1 , wherein the weight split between fraction (A) and fraction (B) is (30-47): (70-53).
9. 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 <−6° C.
10. 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 13479:1997 is >500 h.
11. 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.
12. The polyethylene composition according to claim 1 , wherein fraction (B) comprises 0.4 to 6.0 mol % of at least one alpha-olefin comonomer.
13. An article comprising a polyethylene composition according claim 1 .
14. The article according to claim 13 wherein the article is a pipe.
15. The pipe according to claim 14 which fulfils the MRS 8.0 requirement according to ISO 9080.
16. 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 weight average molecular weight than fraction (B);
(ii) the base resin has a density of 932 to 938 kg/m 3 ;
(iii) the polyethylene composition has an MFR 5 of 0.1 to 0.6 g/10 min;
(iv) the polyethylene composition has a shear stress η 2.7 kPa (viscosity at a shear stress of 2.7 kPa at 190° C.) of 85 to 230 kPas;
(v) the SHI(2.7/210) (ratio of the viscosity, measured at a shear stress of 2.7 kPa and 210 kPa at 190° C.) of the polyethylene composition is 15 to 35; and
(vi) the flexural modulus of the polyethylene composition is 400 to 700 MPa.