| 主权项 |
1. A polymerization process for preparing a polyethylene composition having the following features:
A) density from 0.945 to less than 0.952 g/cm3, preferably from 0.948 to 0.951 g/cm3, determined according to ISO 1183 at 23° C.; B) ratio MIF/MIP from 15 to 30, where MIF is the melt flow index at 190° C. with a load of 21.60 kg, and MIP is the melt flow index at 190° C. with a load of 5 kg, both determined according to ISO 1133; C) SIC Index from 2.5 to 5.5; wherein the SIC Index is the Shear-Induced Crystallization Index, determined according to the following relation:
SIC Index=(tonset,SIC@1000×tonset,quiescent)/(HLMI*100) where tonset,SIC@1000 is measured in seconds and is the time required for crystallization onset under shear rate of 1000 s−1, the tonset,quiescent is measured in seconds and is the crystallization onset time at temperature of 125° C. under no shear, determined in isothermal mode by differential scanning calorimetry; HLMI is the melt flow index determined at 190° C. with a load of 21.6 kg, according to ISO 1133; said polymerization process comprising the following steps, in any mutual order: a) polymerizing ethylene with one or more comonomers, in a gas-phase reactor in the presence of hydrogen; b) copolymerizing ethylene with one or more comonomers in another gas-phase reactor in the presence of an amount of hydrogen less than step a); where in at least one of said gas-phase reactors the growing polymer particles flow upward through a first polymerization zone under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone through which they flow downward under the action of gravity, leave said second polymerization zone and are reintroduced into the first polymerization zone, thus establishing a circulation of polymer between said two polymerization zones, all the polymerization steps being carried out in the presence of a Ziegler-Natta polymerization catalyst supported on MgCl2 |
