Polyethylene composition for pipe applications with improved sagging properties

Abstract

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.

1 - 15 . (canceled) 
     
     
         16 . 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 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 0.12 g/10 min to equal to or less than 0.20 g/10 min, determined according to ISO 1133, a shear thinning index SHI 2.7/210  of equal to or more than 80 to equal to or less than 130 and a viscosity at a constant shear stress of 747 Pa, eta 747 , of equal to or more than 700 kPas to equal to or less than 1200 kPas.   
     
     
         17 . The polyethylene composition according to  claim 16 , wherein the low molecular weight component (A-1) is an ethylene homopolymer. 
     
     
         18 . The polyethylene composition according to  claim 16 , wherein the low molecular weight component (A-1) has a melt flow rate MFR 2  (190° C., 2.16 kg) of 200 g/10 min to 400 g/10 min, determined according to ISO 1133. 
     
     
         19 . The polyethylene composition according to  claim 16 , wherein 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. 
     
     
         20 . The polyethylene composition according to  claim 16 , wherein the weight ratio of the low molecular weight component (A-1) to the high molecular component (A-2) is from 45:55 to 55:45. 
     
     
         21 . The polyethylene composition according to  claim 16 , wherein the composition has a melt flow rate MFR 21  (190° C., 21.6 kg) of 5.6 g/10 min to 7.0 g/10 min, determined according to ISO 1133. 
     
     
         22 . The polyethylene composition according to  claim 16 , 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. 
     
     
         23 . The polyethylene composition according to  claim 16 , wherein the composition has a shear thinning index SHI 5/200  of equal to or more than 50 to equal to or less than 100. 
     
     
         24 . The polyethylene composition according to  claim 16 , wherein the composition has at least one, preferably both, of the following properties a) a weight average molecular weight, Mw, of 250 to 360 kg/mol, more preferably 260 to 340 kg/mol, and/or b) Mz value of 1600 to 2500 kg/mol, preferably 1700 to 2200 kg/mol, even more preferably 1750 to 2150 kg/mol. 
     
     
         25 . The polyethylene composition according to  claim 16 , wherein the composition has a molecular weight distribution Mw/Mn, of at least 25, preferably 30 to 50, more preferably 30 to 45. 
     
     
         26 . The polyethylene composition according to  claim 16 , wherein the composition has a tensile modulus of equal to or less than 1250 MPa, determined according to ISO 527-2:1993. 
     
     
         27 . A polyethylene composition obtainable by a multistage process, the process comprising
 a) polymerizing ethylene in the presence of a Ziegler-Natta catalyst for obtaining an intermediate material,   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), into a polyethylene composition having a melt flow rate MFR 5  (190° C., 5 kg) of equal to or more than 0.12 g/10 min to equal to or less than 0.20 g/10 min, determined according to ISO 1133 (type II), a shear thinning index SHI 2.7/210  of equal to or more than 80 to equal to or less than 130 and a viscosity at a constant shear stress of 747 Pa, eta 747 , of equal to or more than 700 kPas to equal to or less than 1200 kPas.   
     
     
         28 . An article comprising the polyethylene composition according to  claim 16 . 
     
     
         29 . The article according to  claim 28  being a pipe or pipe fitting, preferably pipe having a wall thickness of at least 60 mm. 
     
     
         30 . An article comprising the polyethylene composition of  claim 27 . 
     
     
         31 . The article according to  claim 30  being a pipe or pipe fitting leaving a wall thickness of at least 60 mm.