Process for the preparation of multimodal polyethylene resins
A two-stage cascade polymerization process for the production of multimodal polyethylene film resins with improved bubble stability is provided. The process comprises polymerizing ethylene or a mixture of ethylene and a C 4-8 α-olefin in two reactors arranged in series using a mixed single-site catalyst comprised of a bridged and a non-bridged indenoindolyl transition metal complex to form a multimodal polyethylene resin comprised of a lower molecular weight, higher density component and a higher molecular weight, lower density component.
1. A process for making multimodal polyethylene resin comprising:
(a) polymerizing ethylene or mixture of ethylene and C 3-8 α-olefin in a first reactor in the presence of hydrogen and a catalyst system comprised of a supported mixed indenoindolyl single site catalyst comprised of (5,10-dihydro-5,8-dimethylindeno[1,2-b]indole-10-yl)(cyclopentadienyl) zirconium dichloride and rac-ansa (methylene)bis(2,5-dimethyl-indeno[2,1-b]indolyl)zirconium dichloride, and an activator, to produce a polymerizate containing a first polyethylene resin having a density of 0.942 g/cm 3 or greater and MI 2 from 0.1 to 10 g/10 min;
(b) removing substantially all hydrogen from the polymerizate and transferring to a second reactor; and
(c) polymerizing ethylene and a C 3-8 α-olefin in the second reactor to produce a second polyethylene resin of relatively higher molecular weight and lower density than said first polyethylene resin and obtain a multimodal polyethylene resin product comprised of 30 to 55 weight percent first polyethylene resin and 45 to 70 weight percent second polyethylene resin and having a density from 0.940 to 0.960 g/cm 3 and HLMI from 2 to 50 g/10 min.
2. The process of claim 1 wherein the mixed indenoindolyl single site catalyst is immobilized in an inorganic support selected from the group consisting of silica, alumina, silica-alumina, magnesia, titania, zirconia and magnesium chloride and the mole ratio of nonbridged to bridged indenoindolyl catalyst components is from 1.75:1 to 1:2.
3. The process of claim 2 wherein the multimodal resin product has a density from 0.942 to 0.958 g/cm 3 , HLMI from 7 to 30 g/10 min and is comprised of 35 to 48 weight percent first polyethylene resin and 52 to 65 weight percent second polyethylene resin.
4. The process of claim 3 wherein the multimodal polyethylene resin has a density from 0.945 to 0.956 g/cm 3 and HLMI from 7 to 20 g/cm 3 .
5. The process of claim 1 wherein the polymerizations are carried out in an inert hydrocarbon.
6. The process of claim 1 wherein ethylene is polymerized in the first reactor and a mixture of ethylene and C 3-8 α-olefin are polymerized in the second reactor.
7. The process of claim 1 wherein the α-olefin is butene-1.
8. The process of claim 1 wherein the catalyst system includes an organoaluminum cocatalyst of the formula
AlR 1 n Y 3−n
wherein R 1 is a C 1-8 hydrocarbon group, Y is a halogen or alkoxy group and n is 1, 2 or 3.
9. The process of claim 8 wherein the organoaluminum cocatalyst is triethylaluminum or triisobutylaluminum.