Ethylene copolymer having multiple pitch in molecular weight distribution and the method of preparing the same
View Patent ↗Disclosed are an ethylene polymerization process, a catalyst for use in the process, a production method employing the catalyst, and a product produced thereby. More specifically, disclosed is a process of producing an ethylene copolymer from ethylene and an alpha-olefin comonomer, in which the produced ethylene copolymer has a multimodal molecular weight distribution and excellent processability and physical properties, and thus can increase the value and productivity of products, including pipes and films. Particularly, the produced ethylene copolymer has a trimodal or higher molecular weight distribution or density distribution, and thus, when it is a linear low-density copolymer, it has an excellent effect of improving the impact strength of films, and when it is a medium-density ethylene copolymer, it can be produced into pipes, which have slow crack growth rate and can be used even at high temperature.
1. An ethylene copolymer resin having trimodal or higher molecular weight distribution, which has an ethylene content of more than 60 wt % and a density of 0.900 to 0.950 g/cm 3 , the ethylene copolymer resin is prepared by a multistage solution polymerization using at least three polymerization reactors, the multistage solution polymerization comprising the following steps:
(a) polymerizing ethylene with at least one C4-C10 α-olefin in a first reactor using a metallocene catalyst represented by the following formula 1, which comprises a cyclopentadienyl group containing ligand around a transition metal and aryloxide ligand or ligands substituted with aryl derivatives at the ortho-positions, the ligands not being bridged, thus producing a first copolymer solution in which a first polymer fraction has a density of 0.860-0.920 g/cm 3 and a melt index (MI) of 0.001-1.0 g/10 min, wherein the melt index (MI) is measured in accordance with ASTM D 1238;
(b) passing the first copolymer solution through a second reactor, which contains the same metallocene catalyst as used in step (a), ethylene and at least one C4-C10 α-olefin, at a temperature higher than the reaction temperature of step (a), thus producing a second copolymer solution in which a second polymer fraction has a density of 0.900-0.940 g/cm 3 and a melt index (MI) of 0.1-10.0 g/10 min, wherein the melt index (MIT) is measured in accordance with ASTM D 1238; and
(c) passing the second copolymer solution through a third reactor, which contains a Ziegler-Natta catalyst different from the catalyst used in step (b), ethylene and at least one C4-C10 α-olefin, at a temperature higher than the reaction temperature of step (b), thus producing a third copolymer solution containing a third polymer fraction:
wherein M is a group-IV transition metal in the periodic table; Cp is cyclopentadienyl or a derivative thereof, which forms an η5-bond with the central metal; R1, R2, R3, R4, R5, R6, R7, R8 and R9 on the aryloxide ligands are each independently a hydrogen atom, a halogen atom, a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a silyl group containing a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a C6-C30 aryl group optionally substituted with at least one halogen atom, an C7-C30 arylalkyl group optionally substituted with at least one halogen atom, an alkoxy group having a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a siloxy group substituted with C3-C20 alkyl or C6-C20 aryl, an amido or phosphido group having a C1-C20 hydrocarbon group, or a mercapto or nitro group substituted with C1-C20 alkyl, wherein these substituents are optionally bound to each other to form a ring; and X is selected from the group consisting of a halogen atom, a C1-C20 alkyl group other than a Cp derivative, a C7-C30 arylalkyl group, an alkoxy group having a C1-C20 alkyl group, a siloxy group substituted with C3-C20 alkyl, and an amido group having a C1-C20 hydrocarbon group; and n is an integer between 1 and 3,
wherein the contents of the first polymer fraction, the second polymer fraction and the third polymer fraction in the ethylene copolymer resin are in the range of 2 to 20 wt %, 5 to 78 wt % and 20 to 75 wt %, respectively, based on the ethylene copolymer resin.
2. The ethylene copolymer resin of claim 1 , wherein the first reactor is operated at a temperature of 70 to 190° C. and a pressure of 30 to 500 atm.
3. The ethylene copolymer resin of claim 1 , wherein the second reactor is operated at a temperature of 110 to 200° C. and a pressure of 30 to 500 atm.
4. The ethylene copolymer resin of claim 1 , wherein the third reactor is operated at a temperature of 170 to 300° C. and a pressure of 30 to 500 atm.
5. The ethylene copolymer resin of claim 1 , wherein the ethylene copolymer passed through the first and second reactors has a molecular weight distribution index of more than 3.
6. The ethylene copolymer resin of claim 1 , wherein the α-olefin which is a comonomer for ethylene in each of the steps is 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene or 1-decene.
7. The ethylene copolymer resin of claim 1 , wherein the ratio between the amounts of ethylene supplied into the first, second and third reactors is in the range of 1-1.6:3-5.2:3.2-6.
8. The ethylene copolymer resin of claim 1 , wherein the ethylene content is more than 75 wt %.
9. The ethylene copolymer resin of claim 1 , wherein the ethylene copolymer resin is a linear low-density polyethylene having a density of 0.910 to 0.940 g/cm3.
10. The ethylene copolymer resin of claim 1 , wherein the ethylene copolymer resin is a very low-density or ultra low-density polyethylene having a density of less than 0.910 g/cm3.
11. The ethylene copolymer resin of claim 1 , wherein the ethylene copolymer resin has a molecular weight distribution index of 3.80 to 4.31, a melt index (MI) of 0.70-2.49 g/10 min and a density of 0.916 to 0.939 g/cm3.
12. A blown film comprising the ethylene copolymer resin of claim 1 .
13. A cast film comprising the ethylene copolymer resin of claim 1 .
14. A cold/hot water pipe comprising the ethylene copolymer resin of claim 1 .