Method for preparing a polypropylene composition

Abstract

The present invention relates to a process for preparing a polypropylene composition by sequential polymerization, the polypropylene composition being improved in terms of a balanced combination of high flowability, high stiffness and impact resistance, and high optical properties.

Claims

1. A method for preparing a polypropylene composition by sequential polymerization, comprising the following steps: a) In a first reactor, a monomer containing propylene and optionally one or more comonomers selected from ethylene and C 4 -C 10 α-olefins are polymerized to obtain a first propylene polymer fraction having a comonomer content in the range of 0.0 to 1.8 wt% and an MFR determined according to ISO 1133 at 230 °C and a load of 2.16 kg 2 in the range of 15.0 to 40.0 g / 10 min, b) In a second reactor, monomers containing propylene and one or more comonomers selected from ethylene and C 4 -C 10 α-olefins are polymerized in the presence of the first propylene polymer fraction to obtain a second propylene polymer fraction having a comonomer content in the range of 2.0 to 12.0 wt%, c) extruding the second propylene polymer fraction in the presence of at least one α-nucleating agent, wherein the second propylene polymer fraction comprises the first propylene polymer fraction and a second propylene polymer, and the ratio of the comonomer content of the second propylene polymer in wt% to the comonomer content of the first propylene polymer fraction in wt% is in the range of 7 to 50; wherein the polypropylene composition has an MFR measured according to ISO 1133 at 230 °C and a load of 2.16 kg in the range of 15.0 to 60.0 g / 10 min 2 , and wherein the method is operated in the presence of a Ziegler-Natta catalyst having a transition metal from Groups 4 to 6 of the Periodic Table, and the catalyst comprises an internal donor, an optional cocatalyst, and an optional external donor, wherein the internal donor is a non-phthalic acid internal donor, and wherein the non-phthalic acid internal donor is selected from esters or diesters of carboxylic acids or dicarboxylic acids of non-phthalic acids, and the ester or diester belongs to the group comprising malonic esters, maleic esters, citraconic esters, glutaric esters, cyclohexene-1,2-dicarboxylic esters, and their derivatives or mixtures thereof.

2. The method according to claim 1, wherein the first reactor is a slurry reactor.

3. The method according to claim 1, wherein the second reactor is a gas-phase reactor.

4. The method according to claim 1, wherein the comonomer is selected from one or more comonomers comprising ethylene.

5. The method according to claim 1, wherein the comonomer is only selected from ethylene.

6. The method according to claim 1, wherein: a) the hydrogen / propylene ratio in the first reactor is in the range of 1.5 to 6.0 mol / kmol, and b) the hydrogen / propylene ratio in the second reactor is in the range of 12.0 to 70.0 mol / kmol.

7. The method according to claim 1, wherein the second propylene polymer fraction is extruded in the presence of at least one α-nucleating agent in an amount in the range of 0.01 to 1.0 wt% relative to the total amount of the polypropylene composition.

8. The method according to claim 1, wherein The ratio of one or more comonomers in the first reactor to C 3 is from 0.0 to 14.0 mol / kmol, and / or The ratio of one or more comonomers in the second reactor to C 3 is from 15.0 to 85.0 mol / kmol.

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