Polyethylene pipe resins

1. A polyethylene resin comprising from 47 to 55 wt % of a high molecular weight polyethylene fraction, and from 45 to 53 wt % of a low molecular weight polyethylene fraction;

the high molecular weight polyethylene fraction comprising a linear low density polyethylene having a density of from 0.913 to 0.923 g/cm 3 , and an HLMI of from 0.02 to 0.2 g/10 min;

and the low molecular weight polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm 3 and an MI 2 (8/2) of greater than 100 g/10 min;

wherein the relationship between the density D of the resin in g/cm 3 and the weight fraction of the low molecular weight fraction P 1 is defined by 0.055P 1 +0.916<D<0.034P 1 +0.937.

2. Polyethylene resin according to claim 1 , having a dynamical viscosity η 0.01 , measured at 0.01 radian/second, of greater than 200,000 Pa·s and a ratio of dynamical viscosities measured at respectively 0.01 and 1 radian/second, η 0.01 /η 1 , of greater than 8.

3. Polyethylene resin according to claim 1 , having a time to failure under a stress of 11.2 MPa and at a temperature of 23° C. in a creep test performed on a 2 mm thick dogbone-shaped plaque of said resin of greater than 500 hours.

4. A polyethylene resin according to claim 1 having a dynamical viscosity η 0.01 , measured at 0.01 radian/second, of greater than 200,000 Pa·s and a ratio of dynamical viscosities measured at respectively 0.01 and 1 radian/second, η 0.01 /η 1 , of greater than 8, and a time to failure, at a stress of 11.2 MPa and a temperature of 23° C., in a creep test performed on a 2 mm thick dogbone-shaped plaque of said resin of greater than 500 hours.

5. Polyethylene resin according to any one of claims 2 to 4 , wherein the dynamical viscosity η 0.01 , measured at 0.01 radian/second, is greater than 500,000 Pa·s, and the ratio of dynamical viscosities measured at respectively 0.01 and 1 radian/second, η 0.01 /η 1 , is greater than 10.

6. A polyethylene resin according to claim 1 , wherein the density of the resin is at least 0.945 g/cm 3 .

7. A polyethylene resin according to claim 1 , wherein the HLMI of the high molecular weight fraction is from 0.02 to 0.15 g/10 min.

8. A polyethylene resin according to claim 1 , wherein the MI 2 of the low molecular weight fraction is from 200 to 1000 g/10 min.

9. A polyethylene resin according to claim 1 , wherein the density of the high molecular weight fraction is from 0.915 to 0.922 g/cm 3 .

10. A polyethylene resin according to claim 1 , wherein the density of the low molecular weight fraction is from 0.970 to 0.990 g/cm 3 .

11. A polyethylene resin according to claim 1 , wherein the polydispersity index D of the low molecular weight fraction is from 2 to 6.

12. A polyethylene resin according to claim 1 , wherein the polydispersity index D of the high molecular weight fraction is from 2 to 6.

13. A polyethylene resin according to claim 1 , wherein the density is from 0.948 to 0.954 g/cm 3 .

14. A polyethylene resin according to claim 1 , wherein the HLMI is from 3 to 50 g/10 min.

15. A polyethylene resin according to claim 1 , wherein the relationship between the density D of the resin in g/cm 3 and the weight fraction of the low molecular weight fraction P 1 is defined by 0.055P 1 +0.919<D<0.0341 P 1 +0.937.

16. A polyethylene resin according to claim 1 , wherein the ratio HLMI/MI 5 is at least 30.

17. A polyethylene resin according to claim 1 , which has a carbon black dispersion parameter of 2 or better, and a carbon black distribution parameter of B2 or better, as measured by microscopy in accordance with ISO18533, following extrusion and pelletisation in a single pass.

18. A polyethylene resin according to claim 17 , wherein the carbon black dispersion parameter is 1 or better, and the carbon black distribution parameter is B1 or better.

19. A pipe or a fitting comprising a polyethylene resin as defined in claim 1 .

20. A polyethylene resin according to claim 16 , wherein the ratio of HLMI/MI 5 is at least 35.