PRODUCTION OF POLYPROPYLENE WITH LOW VOLATILES

Abstract

Process for producing a polypropylene comprising the steps of polymerizing the polypropylene in the presence of a metallocene catalyst, visbreaking said polypropylene and subsequently aerating the polypropylene at elevated temperature.

Claims

1. 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 MFR.sub.2 (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 MFR.sub.2 (230° C., 2.16 kg) measured according to ISO 1133 being higher than the melt flow rate MFR.sub.2 (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=MFR.sub.2(FINAL)/MFR.sub.2(START) wherein MFR.sub.2(FINAL) is melt flow rate MFR.sub.2 (230° C.; 2.16 kg) measured according to ISO 1133 of the polypropylene pellets after visbreaking, MFR.sub.2(START) is melt flow rate MFR.sub.2 (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.

2. The process according to claim 1, wherein the visbreaking agent is a peroxide or a hydroxylamine ester.

3. The process according to claim 1, wherein the metallocene catalyst has the formula (I): ##STR00006## wherein M is Ti, Zr or Hf. Z is an oxygen atom or a sulfur atom, R.sup.30, R.sup.31, R.sup.32 and R.sup.33 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having a carbon number of 1 to 6, an alkoxy group having a carbon number of 1 to 6, a halogen-containing alkyl group having a carbon number of 1 to 6, or an aryl group having a carbon number of 6 to 18, Q is a carbon atom, a silicon atom or a germanium atom, each of X.sub.1 and X.sub.2 is independently a halogen atom, an alkyl group having a carbon number of 1 to 6, an aryl group having a carbon number of 6 to 18, an amino group substituted with an alkyl group having a carbon number of 1 to 6, an alkoxy group having a carbon number of 1 to 6, a halogen-containing alkyl group having a carbon number of 1 to 6, or a halogen-containing aryl group having a carbon number of 6 to 18, R.sup.7 and R.sup.17 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having a carbon number of 1 to 6, an alkoxy group having a carbon number of 1 to 6, a halogen-containing alkyl group having a carbon number of 1 to 6, an aryl group having a carbon number of 6 to 18, or a halogen-containing aryl group having a carbon number of 6 to 18, and when either one of R.sup.7 and R.sup.17 is a hydrogen atom, the other is a substituent except for a hydrogen atom, R.sup.8 and R.sup.18 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having a carbon number of 1 to 6, an alkoxy group having a carbon number of 1 to 6, a halogen-containing alkyl group having a carbon number of 1 to 6, or a halogen-containing aryl group having a carbon number of 6 to 18, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.9, R.sup.12, R.sup.13, R.sup.14, R.sup.15, R.sup.16 and R.sup.19 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having a carbon number of 1 to 6, an alkoxy group having a carbon number of 1 to 6, a halogen-containing alkyl group having a carbon number of 1 to 6, an aryl group having a carbon number of 6 to 18, a halogen-containing aryl group having a carbon number of 6 to 18, A is a divalent hydrocarbon group having a carbon number of 3 to 12 and forming a ring together with Q to which it is bonded, and may contain an unsaturated bond, R.sup.10 is a substituent on A and is an alkyl group having a carbon number of 1 to 6, a halogen containing alkyl group having a carbon number of 1 to 6, a trialkylsilyl group-containing alkyl group having a carbon number of 1 to 6, a silyl group containing a hydrocarbon group having a carbon number of 1 to 6, an aryl group having a carbon number of 6 to 18, or a halogen-containing aryl group having a carbon number of 6 to 18, m represents an integer of 0 to 24, and when m is 2 or more, R.sup.10s may combine with each other to form a new ring structure.

4. The process according to claim 1, wherein the polymerization of step (a) takes place in two reactors connected in series.

5. The process according to claim 1, wherein in step (c) additionally additives are added into the continuous melt-mixing device.

6. The process according to claim 1, wherein step (b) is omitted.

7. The process according to claim 1, wherein polypropylene pellets after step (e) consist of: (a) 95.0 to 99.95 wt. % of a polypropylene, and (b) 0.05 to 5.0 wt. % of additives, based on the total amount of the polypropylene pellets.

8. The process according to claim 1, wherein the polypropylene pellets after step (e) have a melt flow rate MFR.sub.2 (230° C., 2.16 kg) measured according to ISO 1133 in the range of 40 to 1500 g/10 min.

9. The process according to claim 1, wherein: (a) the polypropylene pellets have a molecular weight distribution (MWD) determined by gel permeation chromatography (GPC) in the range of 1.0 to below 3.2, and (b) the polypropylene of the polypropylene pellets has 2,1 regio-defects determined by .sup.13C-NMR spectroscopy in the range of 0.10 to 0.90%.

10. The process according to claim 1, wherein the polypropylene of the polypropylene pellets has a comonomer content of not more than 5.0 wt. %.

11. The process according to claim 1, wherein the polypropylene of the polypropylene pellets is a monophasic polypropylene.

12. The process according to claim 11, wherein the monophasic polypropylene is a propylene homopolymer.

13. The process according to claim 1, wherein the polypropylene powder has a molecular weight distribution (MWD) determined by gel permeation chromatography (GPC) in the range of 2.0 to below 4.0, wherein further the ratio MWD.sub.(start)/MWD.sub.(Final) is in the range of 1.01 to 2.50, wherein MWD.sub.(start) is the molecular weight distribution (MWD) determined by gel permeation chromatography (GPC) of the polypropylene powder before visbreaking; MWD.sub.(Final) is the molecular weight distribution (MWD) determined by gel permeation chromatography (GPC) of the polypropylene pellets after visbreaking.

14. The process according to claim 1, wherein the polypropylene pellets after step (e) have a VOC (volatile organic compounds) value determined according to VDA 278 October 2011 in the range of 3.0 to 50 μg/g and a FOG (low volatility or condensable organic compounds) value determined according to VDA 278 October 2011 in the range of 10 to 80 μg/g.

15. The process according to claim 1, wherein the aeration gas is air.

16. The process according to claim 4, wherein the first reactor is a bulk reactor and the second reactor is a gas phase reactor.

17. The process according to claim 7, wherein the polypropylene pellets have a median particle size d50 in the range of 2.5 to 5.0 mm.

Description

EXAMPLES

[0389] 1. Determination Methods

[0390] The following definitions of terms and determination methods apply for the above general description of the invention as well as to the below examples unless otherwise defined.

[0391] a) Melt Flow Rate

[0392] The melt flow rate (MFR.sub.2) is determined according to ISO 1133 and is indicated in g/10 min. The MFR.sub.2 of polypropylene is determined at a temperature of 230° C. and under a load of 2.16 kg.

[0393] b) Xylene Cold Soluble Fraction (XCS, Wt %)

[0394] The amount of the polymer soluble in xylene is determined at 25.0° C. according to ISO 16152; 1th edition; 2005 Jul. 1.

[0395] c) Heat Deflection Temperature

[0396] Heat deflection temperature (HDT) is measured according to ISO 75 B method at a stress of 0.45 MPa using test bars of 80×10×4 mm.sup.3 injection moulded in line with EN ISO 1873-2.

[0397] d) Melting Temperature T.sub.m

[0398] The melting temperature T.sub.m is determined by differential scanning calorimetry (DSC) according to ISO 11357-3 with a TA-Instruments 2920 Dual-Cell with RSC refrigeration apparatus and data station. A heating and cooling rate of 10° C./min is applied in a heat/cool/heat cycle between +23 and +210° C. The melting temperature (T.sub.m) and melting enthalpy (H.sub.m) are being determined in the second heating step.

[0399] e) Quantification of Copolymer Microstructure by .sup.13C-NMR Spectroscopy

[0400] Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers. Quantitative .sup.13C{.sup.1H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for 1H and 130 respectively. All spectra were recorded using a .sup.13C optimized 10 mm extended temperature probehead at 125° C. using nitrogen gas for all pneumatics. Approximately 200 mg of material was dissolved in 3 ml of 1,2-tetrachloroethane-d.sub.2 (TCE-d.sub.2) along with chromium-(III)-acetylacetonate (Cr(acac).sub.3) resulting in a 65 mM solution of relaxation agent in solvent (Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatory oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimised tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128). A total of 6144 (6k) transients were acquired per spectra. Quantitative .sup.13C{.sub.1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs. All chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. Characteristic signals corresponding to the incorporation of ethylene were observed Cheng, H. N., Macromolecules 17 (1984), 1950).

[0401] With characteristic signals corresponding to 2,1 erythro regio defects observed (as described in L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100 (4), 1253, in Cheng, H. N., Macromolecules 1984, 17, 1950, and in W-J. Wang and S. Zhu, Macromolecules 2000, 33 1157) the correction for the influence of the regio defects on determined properties was required. Characteristic signals corresponding to other types of regio defects were not observed.

[0402] The comonomer fraction was quantified using the method of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157) through integration of multiple signals across the whole spectral region in the .sup.13C{.sup.1H} spectra. This method was chosen for its robust nature and ability to account for the presence of regio-defects when needed. Integral regions were slightly adjusted to increase applicability across the whole range of encountered comonomer contents.

[0403] For systems where only isolated ethylene in PPEPP sequences was observed the method of Wang et. al. was modified to reduce the influence of non-zero integrals of sites that are known to not be present. This approach reduced the overestimation of ethylene content for such systems and was achieved by reduction of the number of sites used to determine the absolute ethylene content to:

E=0.5(Sββ+Sβγ+Sβδ+0.5(Sαβ+Sαγ))

[0404] Through the use of this set of sites the corresponding integral equation becomes:

E=0.5(I.sub.H+I.sub.G+0.5(I.sub.C+I.sub.D)) [0405] using the same notation used in the article of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157). Equations used for absolute propylene content were not modified.

[0406] The mole percent comonomer incorporation was calculated from the mole fraction:

E [mol %]=100*fE

[0407] The weight percent comonomer incorporation was calculated from the mole fraction:

E [wt %]=100*(fE*28.06)/((fE*28.06)+((1−fE)*42.08))

[0408] f) Number Average Molecular Weight (Mn), Weight Average Molecular Weight (Mw) and the Molecular Weight Distribution (Mw/Mn)

[0409] Number average molecular weight (M.sub.n), weight average molecular weight (M.sub.w) and the molecular weight distribution (M.sub.w/M.sub.n) were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-4:2003 and ASTM D 6474-99. A PolymerChar GPC instrument, equipped with infrared (IR) detector was used with 3× Olexis and 1× Olexis Guard columns from Polymer Laboratories and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160° C. and at a constant flow rate of 1 mL/min. 200 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11 500 kg/mol. Mark Houwink constants for PS, PE and PP used are as described per ASTM D 6474-99. All samples were prepared by dissolving 5.0-9.0 mg of polymer in 8 mL (at 160° C.) of stabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hours for PE at max. 160° C. under continuous gentle shaking in the autosampler of the GPC instrument.

[0410] g) VOC and FOG

[0411] VOC values and FOG values were measured according to VDA 278 (October 2011; Thermal Desorption Analysis of Organic Emissions for the Characterization of Non-Metallic Materials for Automobiles, VDA Verband der Automobilindustrie) after sample preparation of injection moulding plaques according to EN ISO 19069-2:2016. These plaques were packed in aluminium-composite foils immediately after production and the foils were sealed. According to the VDA 278 October 2011 the VOC value is defined as “the total of the readily volatile to medium volatile substances. It is calculated as toluene equivalent. The method described in this recommendation allows substances in the boiling/elution range up to n-pentacosane (025) to be determined and analyzed.”

[0412] The FOG value is defined as “the total of substances with low volatility, which elute from the retention time of n-tetradecane (inclusive)”. It is calculated as hexadecane equivalent. Substances in the boiling range of n-alkanes “C.sub.14” to “C.sub.32” are determined and analysed.

[0413] h) Diameter of Polypropylene Pellets

[0414] A sieve analysis according to ISO 3310 was performed. The sieve analysis involved a nested column of sieves with wire mesh screen with the following sizes: >20 μm, >32 μm, >63 μm, >100 μm, >125 μm, >160 μm, >200 μm, >250 μm, >315 μm, >400 μm, >500 μm, >710 μm, >1 mm, >1.4 mm, >2 mm, >2.8 mm, >4 mm. The samples were poured into the top sieve, which has the largest screen openings. Each lower sieve in the column has smaller openings than the one above (see sizes indicated above). At the base is the receiver. The column was placed in a mechanical shaker. The shaker shook the column. After the shaking was completed the material on each sieve was weighed. The weight of the sample of each sieve was then divided by the total weight to give a percentage retained on each sieve. The particle size distribution and the characteristic median particle size d50 was determined from the results of the sieve analysis according to ISO 9276-2.

[0415] 2. Preparation of the Polypropylene Pellets

[0416] Preparation of the Metallocene Catalyst System

[0417] (a) Acid and Base Treatment of Ion-Exchangeable Layered Silicate Particles

[0418] Benclay SL, whose major component is 2:1-layered montmorillonite (smectite), was purchased from Mizusawa Industrial Chemicals, Ltd, and used for catalyst preparation. Benclay SL has the following properties: [0419] Median size D50=46.9 μm [0420] Chemical composition [wt.-%]: Al 9.09, Si 32.8, Fe 2.63, Mg 2.12, Na 2.39, Al/Si 0.289 mol/mol

[0421] Acid Treatment

[0422] To a 2 L-flask equipped with a reflux condenser and a mechanical agitation unit, 1300 g of distilled water and 168 g of sulfuric acid (96%) were introduced. The mixture was heated to 95° C. by an oil bath, and 200 g of Benclay SL was added. Then the mixture was stirred at 95° C. for 840 min. The reaction was quenched by pouring the mixture into 2 L of pure water. The crude product was filtrated with a Buechner funnel connected with an aspirator and washed with 1 L of distilled water. Then the washed cake was re-dispersed in 902.1 g of distilled water. The pH of the dispersion was 1.7.

[0423] Base Treatment

[0424] The aqueous solution of LiOH was prepared by solving 3.54 g of lithium hydroxide monohydrate into 42.11 g of distilled water. Then the aqueous LiOH solution was introduced to a dropping funnel and dripped in the dispersion obtained above at 40° C. The mixture was stirred at 40° C. for 90 min. The pH of the dispersion was monitored through the reaction and stayed less than 8. The pH of the reaction mixture was 5.68. The crude product was filtrated with a Buechner funnel connected with an aspirator and washed 3 times with 2 L of distilled water each.

[0425] The chemically treated ion-exchangeable layered silicate particles were obtained by drying the above cake at 110° C. overnight. The yield was 140.8 g. Then the silicate particles were introduced into a 1 L-flask and heated to 200° C. under vacuum. After confirming that gas generation was stopped, the silicate particles were dried under vacuum at 200° C. for 2 h. The catalyst component for olefin polymerization of the present innovation was obtained.

[0426] Preparation of Catalyst

[0427] (b) Reaction with Organic Aluminum

[0428] To a 1000 ml flask, 10 g of the chemically treated ion exchangeable layered silicate particles obtained above (the catalyst component for olefin polymerization of the present invention) and 36 ml of heptane were introduced. To the flask, 64 ml of heptane solution of tri-n-octyl-aluminium (TnOA), which includes 25 mmol of TnOA, was introduced. The mixture was stirred at ambient temperature for 1 h. The supernatant liquid was removed by decantation, and the solid material was washed twice with 900 ml of heptane. Then the total volume of reaction mixture was adjusted to 50 ml by adding heptane.

[0429] (c) Prepolymerization

[0430] To the heptane slurry of the ion-exchangeable layered silicate particles treated with TnOA as described above, 31 ml of heptane solution of TnOA (12.2 mmol of TnOA) was added.

[0431] To a 200 ml flask, 283 mg of (r)-dichlorosilacyclobutylene-bis [2-(5-methyl-2-furyl)-4-(4-t-butylphenyl)-5,6-dimethyl-1-indenyl] zirconium (300 μmol) and 30 ml of toluene were introduced. Then the obtained complex solution was introduced to the heptane slurry of the silicate particles. The mixture was stirred at 40° C. for 60 min.

[0432] Then the mixture was introduced into a 1 L-autoclave with a mechanical stirrer, whose internal atmosphere was fully replaced with nitrogen in advance of use. The autoclave was heated to 40° C. After confirming the internal temperature was stable at 40° C., propylene was introduced at the rate of 10 g/h at 40° C. Propylene feeding was stopped after 2 h and the mixture was stirred at 40° C. for 1 h.

[0433] Then the residual propylene gas was purged out and reaction mixture was discharged into a glass flask. The supernatant solvent was discharged after settling enough. Then 8.3 ml of heptane solution of TiBAL (6 mmol) was added to the solid part. The mixture was dried under vacuum. The yield of solid catalyst for olefin polymerization (prepolymerized catalyst) was 35.83 g. Prepolymerization degree (the weight of prepolymer divided by the weight of solid catalyst) was 2.42.

[0434] Step (a): Process of the Propylene Homopolymer Powder

TABLE-US-00001 TABLE 1 Polymerization conditions for producing the polypropylene powder, i.e. the propylene homopolymer powder HPP HPP Prepolymerization Temperature [° C.] 25 Pressure [kPa] 5145 Catalyst feed [g/h] 2.5 C3 feed [kg/h] 47 H2/C3 ratio [mol/kmol] 0.07 Residence time [h] 0.4 Loop (Reactor 1) Temperature [° C.] 70 Pressure [kPa] 5382 H2/C3 ratio [mol/kmol] 0.24 Residence time [h] 0.39 Loop reactor split [wt.-%] 60 MFR.sub.2 [g/10 min] 21 GPR (Reactor 2) Temperature [° C.] 80 Pressure [kPa] 2500 H2/C3 ratio [mol/kmol] 1.5 Polymer residence time [h] 2.0 GPR reactor split [wt.-%] 40 Powder properties Tm [° C.] 160 MWD [—] 3.3 XCS [wt.-%] 0.60 MFR.sub.2 [g/10 min] 25.0 <2.1> defects [%] 0.25 HDT ISO 75 B [° C.] 94

[0435] No step (b) had been applied. The propylene homopolymer powder was directly used in step (c).

[0436] Step (c): Visbreaking of the Propylene Homopolymer Powder HPP to the Propylene Homopolymer Pellets HPP1

[0437] In a second step the propylene homopolymer powder HPP was visbroken to propylene homopolymer pellets HPP1 by using a co-rotating twin-screw extruder at 200-230° C. and using (tert.-butylperoxy)-2,5-dimethylhexane (Trigonox 101, distributed by Akzo Nobel, Netherlands) in an appropriate amount to achieve the target MFR.sub.2 as indicated in table 2. Additionally to the peroxide the following combination of additives was used in compounding: 0.0525 wt % of Tris (2,4-di-t-butylphenyl) phosphite (CAS-No. 31570-04-4, commercially available as Irgafos 168 from BASF AF, Germany), 0.0525 wt % of Pentaerythrityl-tetrakis(3-(3′,5′-di-tert. butyl-4-hydroxyphenyl)-propionate (CAS-No. 6683-19-8, commercially available as Irganox 1010 from BASF AG, Germany) and 0.03 wt.-% of calcium stearate of FACI SpA, Italy. The amounts of additives are based on the weight of the propylene homopolymer pellets. The remaining part is the propylene homopolymer (see table 2 below).

[0438] The properties of the propylene homopolymer pellets HPP1 are summarized in table 2.

TABLE-US-00002 TABLE 2 Properties of propylene homopolymer pellets HPP1 HPP1 Properties Tm [° C.] 160 HDT ISO 75 B [° C.] 94 MWD [—] 2.8 XCS [wt.-%] 0.70 MFR.sub.2 [g/10 min] 119 <2.1> defects [%] 0.25 VB ratio [—] 4.8 MWD.sub.(Start)/MWD.sub.(Final) [—] 1.18 Total amount of additives [wt.-%] 0.135 d50 [mm] 3.5

[0439] Steps (d) and (e): Aeration and Discharging

[0440] Aeration was carried out in an insulated cylindrically shaped silo with dimensions of 1.5 m.sup.3. The pellets had a median particle size d50 of 3.5 mm (ISO 3310, evaluation according to ISO 9276-2).

[0441] The aeration process was carried at different time intervals as indicated in table 3. A gas flow rate of 260 m.sup.3/h was used. This corresponds to a normalised gas flow of 2.6 Nm.sup.3/kg. The gas was fed into the silo at the bottom, passed through the propylene homopolymer pellets HPP1 and was released at the top of the silo. The gas was hot air with temperatures as indicated in table 3. The propylene homopolymer pellets HPP1 were fed to the silo before starting the aeration and discharged after ending.

[0442] The amount of volatiles of the propylene homopolymer pellets HPP1 under different aeration conditions is summarized in Table 3.

TABLE-US-00003 TABLE 3 Volatiles of the propylene homopolymer pellets under different conditions IE1 IE2 CE1* CE2 CE3** Conditions Time [h] 6 3 0 6 0 Gas temperature [° C.] 60 90 23 23 23 Properties MWD [—] 2.8 2.8 2.8 2.8 3.7 MFR.sub.2 [g/10 min] 119 119 119 119 125 VOC [μg/g] 37 9 79 63 299 FOG [μg/g] 70 40 86 84 607 *the propylene homopolymer pellets of comparative example CE1 have been not subjected aeration step (d); the volatiles were measured on the propylene homopolymer pellets after the visbreaking step (c). **Comparative example CE3 is the commercial PP homopolymer HK060AE of Borealis AG, Austria being produced by visbreaking from a base polymer having an MFR of 87 and based on a conventional 4.sup.th generation Ziegler-Natta type catalyst. The VB ratio is 1.4, the final polymer has a melting point of 165° C., an HDT ISO 75 B off 101° C. and an XCS content of 2.8 wt.-%.