Process for preparing polypropylene composition

A film comprising a polypropylene composition produced by a sequential polymerization process wherein the polypropylene composition is a binary blend comprising two propylene polymer fractions PPF1 and PPF2, wherein the propylene polymer fraction PPF1 is present in an amount of 30 to 50 wt % relative to the sum of the propylene polymer fractions PPF1 and PPF2 and is a propylene copolymer comprising 1.50 to 7.00 mol % of 1-butene (C4) relative to the total amount of monomers in the propylene copolymer (PPF1) and the propylene polymer fraction PPF2 is present in an amount of 50 to 70 wt % relative to the sum of the propylene polymer fractions PPF1 and PPF2 and is a propylene terpolymer comprising 0.30 to 12.00 mol % of ethylene comonomer and 1.50 to 17.00 mol % of 1-butene (C4), the amount of ethylene and 1-butene (C4) comonomers relative to the total amount of monomers in the propylene terpolymer (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 1-butene (C4) to obtain the 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 1-butene (C4) in the presence of the propylene polymer fraction (PPF1) to obtain 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 1-butene (C4) content in the propylene copolymer fraction PPF1 (C4-PPF1)/1-butene (C4) content in the polypropylene composition, (C4-T), [(C4-PPF1)/(C4-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 143 to 160° C. as determined by DSC according to ISO 11357, a xylene soluble fraction (XS) determined at 25° C. according to ISO 16152, 5th edition, 2005 Jul. 1 in the range of 4 to 30 wt %, and a melt flow rate (MFR2) determined according to ISO 1133 at a temperature of 230° C. under a load of 2.16 kg in the range of 3 to 20.0 g/10 min,
• wherein the polymerization process is carried out in the presence of a Ziegler-Natta catalyst that is free of phthalic compound, a co-catalyst (Co) being triethylaluminum (TEAL), and an external donor being dicyclo pentyl dimethoxy silane, wherein the Ziegler-Natta catalyst comprises a transition metal of Group 4 to 6 (TM) and a non-phthalic internal donor (ID) being bis(2-ethylhexyl) citraconate, and
• wherein the polypropylene composition satisfies the equation
• Delta=Tm−SIT
• wherein Delta is in the range of 41 to 50° C., and wherein
• Tm is the melting temperature of the polypropylene composition, and
• SIT is the sealing initiation temperature, in ° C., of the polypropylene composition.