Polyethylene composition for pipe applications with improved sagging and extrusion properties
The present invention relates to a polyethylene composition comprising, a base resin (A) comprising a copolymer of ethylene and at least one comonomer selected from alpha-olefins having from three to twelve carbon atoms, wherein the ethylene copolymer comprises a low molecular weight component (A-1) and a high molecular weight component (A-2) with the low molecular weight component (A-1) having a lower weight average molecular weight than the high molecular weight component (A-2), (B) carbon black in an amount of 1.0 to 10 wt % based on the total amount of the polyethylene composition, and (C) optional further additives other than carbon black; wherein the low molecular weight component (A-1) has a melt flow rate MFR 2 (190° C., 2.16 kg) of equal to or more than 150 g/10 min to equal to or less than 400 g/10 min, determined according to ISO 1133, the base resin (A) has a density of equal to or more than 943 kg/m 3 to equal to or less than 957 kg/m 3 , determined according to ISO 1183, and the composition has a melt flow rate MFR 5 (190° C., 5 kg) of equal to or more than 0.14 g/10 min to equal to or less than 0.30 g/10 min, determined according to ISO 1133, a viscosity at a constant shear stress of 747 Pa, eta 747 , of equal to or more than 800 kPa*s to equal to or less than 1300 kPa*s, and complies with the in-equation (I) eta 747 >−1800×MFR 5 +1200 (I), a process for producing said polyethylene composition and an article comprising said polyethylene composition.
1. Polyethylene composition consisting of:
a base resin (A) comprising
a copolymer of ethylene and at least one comonomer selected from alpha-olefins having from three to twelve carbon atoms,
wherein the ethylene copolymer comprises a low molecular weight component (A-1) and a high molecular weight component (A-2) with the low molecular weight component (A-1) having a lower weight average molecular weight than the high molecular weight component (A-2),
(B) carbon black in an amount of 1.0 to 10 wt % based on the total amount of the polyethylene composition, and
(C) optional further additives other than carbon black which are antioxidants, metal scavengers, UV stabilizers, or antistatic agents;
wherein the low molecular weight component (A-1) has a melt flow rate MFR 2 (190° C., 2.16 kg) of equal to or more than 150 g/10 min to equal to or less than 400 g/10 min, determined according to ISO 1133,
the base resin (A) has a density of equal to or more than 943 kg/m 3 to equal to or less than 957 kg/m 3 , determined according to ISO 1183, and
the composition has a melt flow rate MFR 5 (190° C., 5 kg) of equal to or more than 0.14 g/10 min to equal to or less than 0.30 g/10 min, determined according to ISO 1133, the composition has a complex viscosity at the frequency of 0.05 rad/s, eta 0.05 , of 170 kPa*s to 270 kPa*s, a viscosity at a constant shear stress of 747 Pa, eta 747 , of equal to or more than 800 kPa*s to equal to or less than 1300 kPa*s, and complies with the in-equation (I)
eta 747 [kPa*s]>−1800×MFR 5 [g/10 min]+1200 (I)
with eta 747 referring to said viscosity at a constant shear stress of 747 Pa of the polyethylene composition and MFR 5 referring to said melt flow rate MFR 5 (190° C., 5 kg) of the polyethylene composition; and
the composition has a flow rate ratio FRR 21/5 , being the ratio of melt flow rate MFR 21 (190° C., 21.6 kg) to melt flow rate MFR 5 (190° C., 5 kg), determined according to ISO 1133, of 30 to 40.
2. The polyethylene composition according to claim 1 , wherein the polyethylene composition complies with in-equation (II)
Eta
747
Eta
0.05
≥
3.8
(
II
)
eta 747 referring to said viscosity at a constant shear stress of 747 Pa of the polyethylene composition and eta 0.05 referring to the complex viscosity at the frequency of 0.05 rad/s [kPa*s] of the polyethylene composition.
3. The polyethylene composition according to claim 1 , wherein the composition has a shear thinning index SHI 2.7/210 of equal to or more than 70 to equal to or less than 130.
4. The polyethylene composition according to claim 1 , wherein the low molecular weight component (A-1) is an ethylene homopolymer and the high molecular weight component (A-2) is a copolymer of ethylene and a comonomer selected from alpha-olefins having from three to twelve carbon atoms.
5. The polyethylene composition according to claim 1 , wherein the weight ratio of the low molecular weight component (A-1) to the high molecular component (A-2) is from 40:60 to 55:45.
6. The polyethylene composition according to claim 1 , wherein the composition has a melt flow rate MFR 21 (190° C., 21.6 kg) of 5.0 g/10 min to 8.0 g/10 min, determined according to ISO 1133.
7. The polyethylene composition according to claim claim 1 , wherein the composition has a flow rate ratio FRR 21/5 , being the ratio of melt flow rate MFR 21 (190° C., 21.6 kg) to melt flow rate MFRS (190° C., 5 kg), determined according to ISO 1133 , of 32 to 38.
8. The polyethylene composition according to claim 1 , wherein the composition has a density of equal to or more than 953 kg/m 3 to equal to or less than 967 kg/m 3 , determined according to ISO 1183.
9. An article comprising the polyethylene composition according to claim 1 .
10. The article according to claim 9 being a pipe or pipe fitting.
11. The article according to claim 10 being a pipe having a wall thickness of at least 100 mm.
12. The polyethylene composition of claim 1 , wherein the complex viscosity at the frequency of 0.05 rad/s, eta 0.05 , is 180 kPa*s to 260 kPa*s.
13. The polyethylene composition of claim 1 , wherein the complex viscosity at the frequency of 0.05 rad/s, eta 0.05 , is 190 kPa*s to 250 kPa*s.
14. The polyethylene composition of claim 1 , wherein the polyethylene composition has a complex viscosity at the frequency of 300 rad/s of 1250 Pa*s to 3000 Pa*s.
15. A polyethylene composition obtainable by a multistage process, the process comprising the steps of
a) polymerizing ethylene in the presence of a Ziegler-Natta catalyst for obtaining an intermediate material having a melt flow rate MFR 2 (190° C., 2.16 kg) of equal to or more than 150 g/10 min to equal to or less than 400 g/10 min, determined according to ISO 1133,
b) transferring the intermediate material to a gas phase reactor
(i) feeding ethylene and an alpha-olefin comonomer having from 3 to 12 carbon atoms to the gas phase reactor
(ii) further polymerizing the intermediate material
to obtain a base resin (A) which comprises the intermediate material polymerized in step a) and a material polymerized in step b) which has a higher weight average molecular weight as the intermediate material of step a), wherein the base resin (A) has a density of equal to or more than 943 kg/m 3 to equal to or less than 957 kg/m 3 , determined according to ISO 1183,
c) extruding the base resin (A) in the presence of 1 to 10 wt % carbon black (B), based on the amount of the polyethylene composition, and optional further additive(s) (C) which are antioxidants, metal scavengers, UV stabilizers, or antistatic agents, into a polyethylene composition having a melt flow rate MFR 5 (190° C., 5 kg) of equal to or more than 0.14 g/10 min to equal to or less than 0.30 g/10 min, determined according to ISO 1133, the composition has a complex viscosity at the frequency of 0.05 rad/s, eta 0.05 , of 170 kPa*s to 270 kPa*s, a viscosity at a constant shear stress of 747 Pa, eta 747 , of equal to or more than 800 kPa*s to equal to or less than 1300 kPa*s, and complying with the in-equation (I)
eta 747 >−1800×MFR 5 +1200 (I)
with eta 747 referring to said viscosity at a constant shear stress of 747 Pa of the polyethylene composition and MFR 5 referring to said melt flow rate MFR 5 (190° C., 5 kg) of the polyethylene composition;
the composition has a flow rate ratio FRR 21/5 , being the ratio of melt flow rate MFR 21 (190° C., 21.6 kg) to melt flow rate MFR 5 (190° C., 5 kg), determined according to ISO 1133, of 30 to 40; and
the polyethylene composition consists of the base resin (A), the carbon black (B), and the optional further additives (C).
16. The polyethylene composition according to claim 15 , wherein the composition has a flow rate ratio FRR 21/5 , being the ratio of melt flow rate MFR 21 (190° C., 21.6 kg) to melt flow rate MFR 5 (190° C., 5 kg), determined according to ISO 1133, of 30 to 40.
17. An article comprising the polyethylene composition according to claim 15 .
18. The article according to claim 17 being a pipe or pipe fitting.
19. The article according to claim 18 being a pipe having a wall thickness of at least 100 mm.
20. A process for producing the polyethylene composition according to claim 1 , wherein the base resin (A) is produced in a multi-stage process comprising two reactors connected in series.