Production of polypropylene with low volatiles

A process for producing polypropylene pellets comprising the steps in the following order:

• (a) polymerizing propylene and optionally at least one α-olefin selected from ethylene, 1-butene and 1-hexene in the presence of a metallocene catalyst, obtaining thereby a polypropylene in the form of a powder,
• wherein the obtained polypropylene powder has a melt flow rate MFR2 (230° C., 2.16 kg) measured according to ISO 1133 in the range of 1.0 to 100 g/10 min,
• (b) optionally mixing the polypropylene powder with additives,
• (c) visbreaking the polypropylene powder of step (a) or the mixture of step (b) with a visbreaking agent in a continuous melt-mixing device obtaining thereby a polypropylene in form of pellets,
• wherein the polypropylene pellets have a melt flow rate MFR2 (230° C., 2.16 kg) measured according to ISO 1133 being higher than the melt flow rate MFR2 (230° C., 2.16 kg) of the polypropylene powder of step (a),
• wherein further the visbreaking ratio (VR) is in the range of 2.0 to 25.0, wherein the visbreaking ratio (VR) is determined according to equation
• VR=MFR2(FINAL)/MFR2(START)
• wherein
• MFR2(FINAL) is melt flow rate MFR2 (230° C.;
• 2.16 kg) measured according to ISO 1133 of the polypropylene pellets after visbreaking,
• MFR2(START) is melt flow rate MFR2 (230° C.;
• 2.16 kg) measured according to ISO 1133 of the polypropylene powder before visbreaking,
• (d) aerating the polypropylene pellets of step (c) in an aeration vessel for 1 to 15 hours with an aeration gas having a temperature of at least 40° C. but lower than the heat deflection temperature (HDT) measured according to ISO 75 B method at a stress of 0.45 MPa of the polypropylene pellets of step (c), and
• (e) discharging the polypropylene pellets of step (d) from the aeration vessel.