Polyolefin pipe resin with very good sagging and slow crack growth resistance

1 . A polyethylene composition comprising a base resin which comprises

(A) a first ethylene homo- or copolymer fraction, wherein fraction (A) has melt flow rate, MFR 2 , as measured in accordance with ISO 1133 from 100 to 600 g/10 min; and

(B) a second ethylene-1-hexene copolymer fraction,

wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 50.0 to 58.0 wt. % based on the total weight of the base resin;

wherein the base resin has a content of units derived from 1-hexene from 0.44 to 0.79 mol % based on the total amount of base resin;

wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32 to 40 and the base resin has a Z average molecular weight, Mz, of more than 1,500 kg/mol;

wherein the polyethylene composition has a melt flow rate MFR 5 from 0.10 to 0.25 g/10 min; and a melt flow rate ratio, FRR 21/5 , from 30 to 42; and

wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of −10° C. or lower and not less than −25° C.

2 . Polyethylene composition according to claim 1 ,

wherein the polyethylene composition has a strain hardening modulus of 75 MPa or higher; or

wherein the polyethylene composition has a failure time in accelerated creep test (ACT) of more than 1500 h; or

wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 5.4 MPa at 80° C. of at least 200 h; or

wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 12.0 MPa at 20° C. of at least 130 h; or

wherein the polyethylene composition has a stress at yield at 80° C. from 6.0 to 7.0 MPa; or

wherein the base resin has a viscosity at a shear stress of 747 Pa (eta 747 ) of more than 700 kPa*s.

3 . Polyethylene composition according to claim 1 ,

wherein fraction (A) has a melt flow rate, MFR 2 , as measured in accordance with ISO 1133 from 100 to 230 g/10 min; or

wherein fraction (A) has a melt flow rate, MFR 2 , as measured in accordance with ISO 1133 of 231 to 550 g/10 min.

4 . Polyethylene composition according to claim 1 ,

wherein fraction (B) of the base resin has a content of units derived from 1-hexene from 0.81 to 1.60 mol %, based on the total amount of fraction (B); or

wherein fraction (B) is present in an amount of from 51.0 to 57.0 wt. %, based on the total weight of the base resin.

5 . Polyethylene composition according to claim 1 ,

wherein the base resin has a number average molecular weight, Mn, of 7,300 g/mol or higher; or

wherein the base resin has a number average molecular weight, Mn, of 9,150 g/mol or lower; or

wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32.5 to 39.5; or

wherein the base resin has a Z average molecular weight, Mz, of more than 1,600 kg/mol.

6 . Polyethylene composition according to claim 1 ,

wherein the base resin has a density of at least 945 kg/m 3 ; or

wherein the polyethylene composition has a density of at least 953 kg/m 3 ; or

wherein the base resin has a content of units derived from 1-hexene from 0.45 to 0.78 mol %, based on the total amount of the base resin.

7 . Polyethylene composition according to claim 1 ,

wherein the polyethylene composition has a melt flow rate, MFR 5 , from 0.12 to 0.22 g/10 min; oe

wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of −12° C. or lower and/or not less than −23° C.

8 . A process for producing a polyethylene composition according to claim 1 , comprising the steps of:

a) polymerizing ethylene in the presence of

a Ziegler-Natta catalyst,

in one or more loop reactor(s), in the presence of an alkyl aluminium compound and a chain transfer agent for obtaining fraction (A), the fraction (A) having a melt flow rate, MFR 2 , from 100 to 600 g/10 min; and

b) transferring fraction (A) to a gas phase reactor

feeding ethylene and comonomer to the gas phase reactor,

further polymerizing to obtain a base resin comprising fraction (A) obtained in step a) and fraction (B) obtained in step b),

wherein fraction (B) of the base resin has a content of units derived from 1-hexene from 0.81 to 1.60 mol %, based on the total amount of fraction (B); and

wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 50.0 to 58.0 wt. %, based on the total weight of the base resin; and

c) extruding the base resin into a polyethylene composition having a melt flow rate, MFR 5 , from 0.10 to 0.25 g/10 min, and having a melt flow rate ratio, FRR 21/5 , from 30 to 42.

9 . The polyethylene composition according to claim 8 , wherein the polymerization catalyst is a Ziegler-Natta catalyst.

10 . An article comprising the polyethylene composition according to claim 1 .

11 . The article according to claim 10 being a pipe or pipe fitting.

12 . A pipe according to claim 11 ,

wherein the pipe has a critical temperature, Tc, in the rapid crack propagation test of −10° C. or lower and not less than −25° C.; or

wherein the pipe has a failure time in the accelerated creep test (ACT) of more than 1500 h; or

wherein the pipe has a failure time in the short term pressure resistance (STPR) test at a stress level of 5.4 MPa at 80° C. of at least 200 h; or

wherein the pipe has a failure time in the short term pressure resistance (STPR) test at a stress level of 12.0 MPa at 20° C. of at least 130 h.

13 . A method for producing an article, comprising extruding the composition of claim 1 .