Process for preparing polypropylene composition

A process for producing a polypropylene composition by sequential polymerization comprising the steps:

• a) polymerizing in a first reactor monomers consisting of propylene and a comonomer selected from C4-C10 alpha-olefins, to obtain a first propylene polymer fraction having a comonomer content in a range of from 2.0 to 8.0 mol %,
• b) polymerizing in a second reactor monomers consisting of propylene, ethylene, and a comonomer selected from C4-C10 alpha-olefins, in the presence of the first propylene polymer fraction, to obtain a second propylene polymer fraction, and
• c) polymerizing in a third reactor monomers consisting of propylene, ethylene, and optionally one or more C4-C10 alpha-olefins, in the presence of the second propylene polymer fraction, to obtain a polypropylene composition having an ethylene comonomer content in a range of from 0.5 to 5.0 mol % and a C4-C10 alpha-olefin comonomer content in a range of from 3.0 to 10.0 mol %, relative to a total amount of monomers present in the polypropylene composition, and
• wherein the process is operated in the presence of a Ziegler-Natta catalyst comprising a transition metal of group 4 to 6 of the periodic table, and an internal donor, optionally a co-catalyst and optionally an external donor, wherein the internal donor is a non-phthalic internal donor.