Process for preparing polypropylene composition

Process for producing a polypropylene composition by a sequential polymerization process wherein the polypropylene composition is a binary blend comprising two propylene polymer fractions PPF1 and PPF2 and wherein the process comprises at least two reactors connected in series, said process comprising the steps:

• a) polymerizing in a first reactor (R-1), propylene and one comonomer selected from C4-C10 alpha-olefin to obtain a propylene polymer fraction (PPF1), the first reactor (R-1) being a slurry reactor and the propylene polymer fraction (PPF1) being a propylene copolymer,
• b) transferring the propylene polymer fraction (PPF1) and unreacted comonomers of the reactor (R-1) into a second reactor (R-2), the second reactor (R-2) being a first gas-phase reactor-1 (GPR-1),
• c) polymerizing in the gas-phase reactor-1 (GPR-1) propylene, ethylene and one comonomer selected from C4-C10 alpha-olefin in the presence of the propylene polymer fraction (PPF1), to obtain a propylene polymer fraction (PPF2), the propylene polymer fraction (PPF2) being a polypropylene terpolymer,
• d) forming the polypropylene composition from the propylene polymer fraction (PPF2) and the propylene polymer fraction (PPF1), and
• e) recovering the polypropylene composition wherein the mol ratio of C4-C10 alpha-olefin content in the propylene copolymer fraction PPF1 (C4-C10-PPF1)/C4-C10 alpha-olefin content in the polypropylene composition (C4-C10-T) [(C4-C10-PPF1) /(C4-C10-T)] is in the range of 0.10 to 0.95,
• and wherein the polypropylene composition has a melting temperature (Tm) in the range of 135 to 160° C. as determined by DSC according to ISO 11357,
• wherein the polymerization process is carried out in the presence of a Ziegler-Natta catalyst that is free of phthalic compound, wherein the Ziegler-Natta catalyst comprises a transition metal of Group 4 to 6 (TM) and a non-phthalic internal donor (ID).