Preparing multimodal polyethylene having controlled long chain branching distribution
A process to prepare a multimodal polyethylene with controlled LCB distribution is disclosed. In the first stage, ethylene is polymerized in the presence of a Ziegler catalyst that results in a homopolyethylene component having a higher LCB concentration. In the second stage, ethylene is copolymerized with a 1-olefin in the presence of the Ziegler catalyst and a lower concentration of hydrogen resulting in a copolymer component with a lower LCB concentration. The homopolyethylene component and the copolymer component are combined to form a novel multimodal polyethylene.
1 . A process of preparing a multimodal polyethylene, which comprises:
(a) a first stage of homopolymerizing ethylene with a Ziegler catalyst and a co-catalyst to form a homopolyethylene component having a rheological dispersity (R D ) within the range of about 1 to about 12;
(b) a second stage of copolymerizing ethylene and at least one C 3 to C 10 1-olefin with the catalyst and the co-catalyst to form a copolymer component having a R D within the range of about 0.1 to about 8; and
(c) mixing the homopolyethylene component and the copolymer component to form the multimodal polyethylene.
2 . The process of claim 1 wherein the catalyst comprises: (i) the transition metal compound selected from the group consisting of M(OR′) a X 4-a and MOX 3 , in which M is a transition metal selected from the group consisting of titanium, vanadium, and zirconium, R′ is a C 1 to C 19 alkyl group, X is a halogen, and a is zero or an integer less than 4; (ii) a magnesium-aluminum complex, (MgR 2 ) m (AlR 3 ) n , in which R is a C 1 to C 12 alkyl group, and m/n is 0.5 to 10; and (iii) a silica or alumina; and wherein the co-catalyst is a trialkyl aluminum compound.
3 . The process of claim 1 wherein the transition metal compound is selected from the group consisting TiCl 4 , Ti(OR′)Cl 3 , Ti(OR′) 2 Cl 2 , Ti(OR′) 3 Cl, VOCl 3 , VCl 4 , and mixture thereof.
4 . The process of claim 1 wherein the transition metal compound is TiCl 4 .
5 . The process of claim 1 wherein the magnesium-aluminum complex is {(C 4 H 9 ) 2 Mg} 6.5 {(C 2 H 5 ) 3 Al}.
6 . The process of claim 1 wherein the first stage is performed at a higher temperature than the second stage.
7 . The process of claim 1 wherein the first stage is performed at a higher hydrogen concentration than the second stage.
8 . The process of claim 1 wherein the homopolyethylene component prepared in the first stage has a higher melt index MI 2 than the copolymer component prepared in the second stage.
9 . The process of claim 1 wherein the first stage and the second stage are performed in two parallel reactors.
10 . The process of claim 1 wherein the first stage and the second stage are performed in two sequential reactors.
11 . A multimodal polyethylene which comprises
(a) a homopolyethylene component having
(i) a rheological dispersity (R D ) within the range of about 2 to about 12;
(ii) a density of greater than 0.96 g/cm 3 ;
(iii) a melt elasticity (ER) within the range of about 0.3 to about 2; and
(iv) a melt index (MI 2 ) within the range of about 0.1 g/10 min to 500 g/10 min; and
(b) an ethylene-1-olefin copolymer component having
(i) a R D within the range of about 0.1 to about 8;
(ii) a density of less than or equal to 0.955 g/cm 3 ;
(iii) an ER within the range of about 0.1 to about 1.2; and
(iv) an MI 2 within the range of about 0.001 g/10 min to 5 g/10 min.
12 . The multimodal polyethylene of claim 11 wherein the homopolyethylene component has a R D within the range of about 3 to about 10, and the copolymer component has a R D within the range about 0.5 to about 6.
13 . The multimodal polyethylene of claim 11 wherein the homopolyethylene component has a R D within the range of about 4 to about 8, and the copolymer component has a R D within the range about 2 to about 4.
14 . The multimodal polyethylene of claim 11 wherein the homopolyethylene component has a density greater than or equal to 0.96 g/cm 3 , and the copolymer component has a density within the range about 0.9 g/cm 3 to about 0.955 g/cm 3 .
15 . The multimodal polyethylene of claim 11 wherein the homopolyethylene component has an MI 2 within the range of 0.5 g/10 min to about 200 g/10 min, and the copolymer component has an MI 2 within the range about 0.1 g/10 min to about 5 g/10 min.
16 . The multimodal polyethylene of claim 11 wherein the 1-olefin is a C 3 -C 10 olefin.
17 . The multimodal polyethylene of claim 11 wherein the 1-olefin is selected from the group consisting of propylene, 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene, and mixtures thereof.
18 . The multimodal polyethylene of claim 11 wherein the 1-olefin is 1-hexene.
19 . The multimodal polyethylene of claim 11 having a weight ratio of homopolyethylene component/copolymer component within the range of about 10/90 to about 90/10.
20 . The multimodal polyethylene of claim 11 having a weight ratio of homopolyethylene component/copolymer component within the range of about 20/80 to about 80/20.