Process for preparing a polypropylene composition

Abstract

The invention relates to a process for producing a polypropylene composition by sequential polymerization the polypropylene composition having an improved balanced combination of high flowability, high stiffness and impact, and high level of optical properties.

Claims

1. A process for producing a polypropylene composition by sequential polymerization comprising the steps: a) polymerizing in a first reactor monomers comprising propylene and optionally one or more comonomers selected from ethylene and C.sub.4-C.sub.10 alpha-olefins, to obtain a first propylene polymer fraction having a comonomer content in the range of 0.0 to 1.0 wt %; b) polymerizing in a second reactor monomers comprising propylene and one or more comonomers selected from ethylene and C.sub.4-C.sub.10 alpha-olefins, in the presence of the first propylene polymer fraction, to obtain a second propylene polymer fraction having a comonomer content in the range of 0.3 to 2.0 wt %; c) polymerizing in a third reactor monomers comprising propylene and one or more comonomers selected from ethylene and C.sub.4-C.sub.10 alpha-olefins, wherein the ratio of the one or more comonomers to propylene is in the range of 45.0 to 170.0 mol/kmol, in the presence of the second propylene polymer fraction to obtain a third propylene polymer fraction having a comonomer content in the range of from 1.5 to 5.0 wt %; and d) extruding the third propylene polymer fraction in the presence of at least one alpha-nucleating agent; wherein the polypropylene composition has an MFR.sub.2 in the range of from 12.0 to 60.0 g/10 min, as measured according to ISO 1133 at 230° C. under a load of 2.16 kg.

2. The process according to claim 1, wherein the polymerization in steps a), b), and c) is carried out in the presence of a Ziegler-Natta catalyst.

3. The process according to claim 2, wherein the Ziegler-Natta catalyst is free of a phthalic compound.

4. The process according to claim 1, wherein the process is operated in the presence of a Ziegler-Natta catalyst with a transition metal of group 4 to 6 of the periodic table, the catalyst comprising an internal donor, wherein the internal donor is a non-phthalic internal donor.

5. The process according to claim 4, wherein the non-phthalic internal donor is selected from (di)esters of non-phthalic carboxylic (di)acids, wherein the (di)ester belongs to the group consisting of malonates, maleates, succinates, citraconates, glutarates, cyclohexene-1,2-dicarboxylates, benzoates, derivatives thereof, and mixtures thereof.

6. The process according to claim 4, wherein the catalyst further comprises a co-catalyst, an external donor, or a combination thereof.

7. The process according to claim 1, wherein the comonomers in step a), step b), step c), or a combination thereof are selected from one or more comonomers comprising ethylene.

8. The process according to claim 1, wherein: a) the hydrogen/propylene (H.sub.2/C.sub.3) ratio in the first reactor is in the range of 1.5 to 6.0 mol/kmol; b) the hydrogen/propylene (H.sub.2/C.sub.3) ratio in the second reactor is in the range of 12.0 to 70.0 mol/kmol; and c) the hydrogen/propylene (H.sub.2/C.sub.3) ratio in the third reactor is in the range of 15.0 to 80.0 mol/kmol.

9. The process according to claim 1, wherein the third propylene polymer fraction is extruded in the presence of an amount of the at least one alpha-nucleating agent in the range of from 0.01 to 1.0 wt %, relative to the total amount of polypropylene composition.

10. The process according to claim 1, wherein the polypropylene composition has a haze value <20%, as measured according to ASTM D1003 on injection molded plaques having 1 mm thickness produced as described in EN ISO 1873-2.

11. The process according to claim 1, wherein the first reactor comprises a slurry reactor.

12. The process according to claim 1, wherein the second reactor comprises a gas phase reactor.

13. The process according to claim 1, wherein the third reactor comprises a gas phase reactor.

Description

EXAMPLES

I. Measuring Methods

(1) a) Melt Flow Rate

(2) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability and hence the processability of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR.sub.2 of polypropylene is determined at a temperature of 230° C. and under a load of 2.16 kg.

(3) b) DSC Analysis

(4) The melting temperature and the crystallisation temperature are measured with a TA Instrument Q2000 differential scanning calorimetry device (DSC) according to ISO 11357/3 on 5 to 10 mg samples, under 50 mL/min of nitrogen atmosphere. Crystallisation and melting temperatures were obtained in a heat/cool/heat cycle with a scan rate of 10° C./min between 30° C. and 225° C. Crystallisation and melting temperatures were taken as the peaks of the endotherms and exotherms in the cooling step and the second heating step respectively.

(5) c) Xylene Soluble Content (XCS, Wt %)

(6) The content of the polymer soluble in xylene is determined according to ISO 16152; 5.sup.th edition; 2005-07-01 at 25° C.

(7) d) Tensile Modulus

(8) Tensile Modulus is measured according to ISO 527-1:2012/ISO527-2:2012 at 23° C. and at a cross head speed=50 mm/min; using injection moulded test specimens as described in EN ISO 1873-2 (dog bone shape, 4 mm thickness).

(9) e) Charpy Notched Impact

(10) Charpy notched impact strength is determined according to ISO 179/1eA at 23° C. on injection moulded test specimens as described in EN ISO 1873-2 (80×10×4 mm).

(11) f) Haze

(12) Haze is determined according to ASTM D1003 on injection moulded plaques having 1 mm thickness and 60×60 mm.sup.2 area produced as described in EN ISO 1873-2.

(13) g) Comonomer Content

(14) Poly(Propylene-Co-Ethylene)-Ethylene Content by IR Spectroscopy

(15) Quantitative infrared (IR) spectroscopy was used to quantify the ethylene content of the poly(ethylene-co-propene) copolymers through calibration to a primary method.

(16) Calibration was facilitated through the use of a set of in-house non-commercial calibration standards of known ethylene contents determined by quantitative .sup.13C solution-state nuclear magnetic resonance (NMR) spectroscopy. The calibration procedure was undertaken in the conventional manner well documented in the literature. The calibration set consisted of 38 calibration standards with ethylene contents ranging 0.2-75.0 wt % produced at either pilot or full scale under a variety of conditions. The calibration set was selected to reflect the typical variety of copolymers encountered by the final quantitative IR spectroscopy method.

(17) Quantitative IR spectra were recorded in the solid-state using a Bruker Vertex 70 FTIR spectrometer. Spectra were recorded on 25×25 mm square films of 300 um thickness prepared by compression moulding at 180-210° C. and 4-6 mPa. For samples with very high ethylene contents (>50 mol %) 100 um thick films were used. Standard transmission FTIR spectroscopy was employed using a spectral range of 5000-500 cm.sup.−1, an aperture of 6 mm, a spectral resolution of 2 cm.sup.−1, 16 background scans, 16 spectrum scans, an interferogram zero filling factor of 64 and Blackmann-Harris 3-term apodisation. Quantitative analysis was undertaken using the total area of the CH.sub.2 rocking deformations at 730 and 720 cm.sup.−1 (A.sub.Q) corresponding to (CH.sub.2).sub.>2 structural units (integration method G, limits 762 and 694 cm.sup.−1). The quantitative band was normalised to the area of the CH band at 4323 cm.sup.−1 (A.sub.R) corresponding to CH structural units (integration method G, limits 4650, 4007 cm.sup.−1). The ethylene content in units of weight percent was then predicted from the normalised absorption (A.sub.Q/A.sub.R) using a quadratic calibration curve. The calibration curve having previously been constructed by ordinary least squares (OLS) regression of the normalised absorptions and primary comonomer contents measured on the calibration set.

(18) Poly(Propylene-Co-Ethylene)-Ethylene Content for Calibration Using .sup.13C NMR Spectroscopy

(19) Quantitative .sup.13C{.sup.1H} NMR spectra were recorded in the solution-state using a Bruker Avance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for .sup.1H and .sup.13C respectively. All spectra were recorded using a .sup.13C optimised 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 (6 k) transients were acquired per spectra. Quantitative .sup.13C{.sup.1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. 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) and the comonomer fraction calculated as the fraction of ethylene in the polymer with respect to all monomer in the polymer: fE=(E/(P+E) 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. For systems with very low ethylene content where only isolated ethylene in PPEPP sequences were observed the method of Wang et. al. was modified reducing the influence of integration of sites that are no longer 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)) 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)) 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. The mole percent comonomer incorporation was calculated from the mole fraction: E [mol %]=100*fE. The weight percent comonomer incorporation was calculated from the mole fraction: E [wt %]=100*(fE*28.06)/((fE*28.06)+((1−fE)*42.08)).

II. Inventive and Comparative Examples

a) Catalyst Preparation

(20) For the preparation of the catalyst 3.4 litre of 2-ethylhexanol and 810 ml of propylene glycol butyl monoether (in a molar ratio 4/1) were added to a 20.0 l reactor. Then 7.8 litre of a 20.0% solution in toluene of BEM (butyl ethyl magnesium) provided by Crompton GmbH, were slowly added to the well stirred alcohol mixture. During the addition, the temperature was kept at 10.0° C. After addition, the temperature of the reaction mixture was raised to 60.0° C. and mixing was continued at this temperature for 30 minutes. Finally after cooling to room temperature the obtained Mg-alkoxide was transferred to a storage vessel.

(21) 21.2 g of Mg alkoxide prepared above was mixed with 4.0 ml bis(2-ethylhexyl) citraconate for 5 min. After mixing the obtained Mg complex was used immediately in the preparation of the catalyst component.

(22) 19.5 ml of titanium tetrachloride was placed in a 300 ml reactor equipped with a mechanical stirrer at 25.0° C. Mixing speed was adjusted to 170 rpm. 26.0 g of Mg-complex prepared above was added within 30 minutes keeping the temperature at 25.0° C. 3.0 ml of Viscoplex® 1-254 and 1.0 ml of a toluene solution with 2 mg Necadd 447™ was added. Then 24.0 ml of heptane was added to form an emulsion. Mixing was continued for 30 minutes at 25.0° C., after which the reactor temperature was raised to 90.0° C. within 30 minutes. The reaction mixture was stirred for a further 30 minutes at 90.0° C. Afterwards stirring was stopped and the reaction mixture was allowed to settle for 15 minutes at 90.0° C. The solid material was washed 5 times: washings were made at 80.0° C. under stirring for 30 min with 170 rpm. After stirring was stopped the reaction mixture was allowed to settle for 20-30 minutes and followed by siphoning. Wash 1: washing was made with a mixture of 100 ml of toluene and 1 ml donor Wash 2: washing was made with a mixture of 30 ml of TiCl4 and 1 ml of donor. Wash 3: washing was made with 100 ml of toluene. Wash 4: washing was made with 60 ml of heptane. Wash 5: washing was made with 60 ml of heptane under 10 minutes stirring.

(23) Afterwards stirring was stopped and the reaction mixture was allowed to settle for 10 minutes while decreasing the temperature to 70° C. with subsequent siphoning, followed by N.sub.2 sparging for 20 minutes to yield an air sensitive powder.

b) Inventive Examples (IE1 and IE2)

(24) The third propylene polymer fractions related to the inventive examples (IE) were produced in a pilot plant with a prepolymerization reactor, one slurry loop reactor and two gas phase reactors. The solid catalyst component described above along with triethyl-aluminium (TEAL) as co-catalyst and dicyclo pentyl dimethoxy silane (D-donor) as external donor, were used in the inventive process.

(25) The polymerization process conditions and properties of the propylene polymer fraction are described in Table 1.

(26) The inventive polypropylene compositions were prepared by extruding the respective third propylene polymer fraction with a nucleating agent in a co-rotating twin screw extruder type Coperion ZSK 40 (screw diameter 40 mm, LID ratio 38). The temperatures in the extruder were in the range of 190-230° C. In each of the inventive examples 0.05 wt % of Irganox 1010 (Pentaerythrityl-tetrakis(3-(3′,5′-di-tert. butyl-4-hydroxyphenyl)-propionate, CAS No. 6683-19-8, commercially available from BASF AG, Germany), 0.05 wt % of Irgafos 168 (Tris (2,4-di-t-butylphenyl) phosphite, CAS No. 31570-04-4, commercially available from BASF AG, Germany), 0.10 wt % of Calcium stearate (CAS. No. 1592-23-0, commercially available under the trade name Ceasit Fl from Baerlocher GmbH, Germany) and 0.06 wt % of Glycerol monostearate (CAS No. 97593-29-8, commercially available with 90% purity under the trade name Grindsted PS 426 from Danisco NS, Denmark) were added to the extruder as additives.

(27) Following the extrusion step and after solidification of the strands in a water bath, the resulting polypropylene composition was pelletized in a strand pelletizer.

(28) The polypropylene composition properties are described in Table 2.

c) Comparative Examples (CE1 and CE2)

(29) CE-1 is a C.sub.2 propylene random copolymer having an MFR.sub.2 of 13.0 g/10 min, produced in one reactor process and distributed by Borealis under the Trade name RE420MO.

(30) CE-2 is a C.sub.2 propylene random copolymer having an MFR.sub.2 of 20.0 g/10 min, produced in one reactor process and distributed by Borealis under the Trade name RF365MO.

(31) TABLE-US-00001 TABLE 1 Polymerization process conditions and propylene polymer fractions properties IE1 IE2 Pre-polymerization reactor Temperature [° C.] 30 30 Catalyst feed [g/h] 2.5 2.6 D-Donor [g/t 41 41 propylene] TEAL/propylene [g/t 170 170 propylene] Al/D-Donor [Co/ED] [mol/mol] 8.3 8.2 Al/Ti [Co/TM] [mol/mol] 217 211 Residence Time [h] 0.3 0.3 Loop reactor (first propylene polymer fraction) Temperature [° C.] 70 70 Pressure [kPa] 5270 5270 Residence time [h] 0.4 0.4 Split [%] 47 36 H.sub.2/C.sub.3 ratio [mol/kmol] 2.4 2.3 C.sub.2/C.sub.3 ratio [mol/kmol] 3.6 3.6 MFR.sub.2 [g/10 min] 24.8 21.9 C.sub.2 content [wt %] 0.5 0.4 First gas-phase reactor (second propylene polymer fraction) Temperature [° C.] 80 80 Pressure [kPa] 2400 2400 Residence time [h] 1.4 1.4 Split [%] 38 48 H.sub.2/C.sub.3 ratio [mol/kmol] 28.7 27.2 C.sub.2/C.sub.3 ratio [mol/kmol] 9.2 9.1 MFR.sub.2 [g/10 min] 22.4 19.0 C.sub.2 content [wt %] 0.98 0.93 Second gas-phase reactor (third propylene polymer fraction) Temperature [° C.] 80 80 Pressure [kPa] 2350 2350 Residence time [h] 0.9 0.9 Split [%] 15 16 H.sub.2/C.sub.3 ratio [mol/kmol] 36.9 45.4 C.sub.2/C.sub.3 ratio [mol/kmol] 69.3 71.2 MFR.sub.2 [g/10 min] 19.0 18.4 C.sub.2 content [wt %] 2.6 3.0 *Split relates to the amount of propylene polymer produced in each specific reactor.

(32) TABLE-US-00002 TABLE 2 Extrusion process conditions and polypropylene composition properties. IE1 IE2 CE1 CE2 Nucleating agent wt % 0.17 0.17 0.0 0.0 (Millad 3988 ®) Composition properties* MFR.sub.2 [g/10 min] 19.0 18.0 13 20 C.sub.2 content [wt %] 2.6 2.9 3.4 3.4 XCS [wt %] 6.9 8.6 5.8 6.8 Tensile Modulus [MPa] 1305 1240 921 1138 Charpy notched impact [kJ/m.sup.2] 5.5 7.22 5.3 4.5 strength Haze (1 mm) [%] 17 15 23 20 Tm [° C.] 160 159 150 151 Tc [° C.] 128 128 120 120 *measured on pellets obtained after the extrusion process.

(33) From Table 2 it can be derived that the polypropylene compositions (inventive examples) show an improved balanced combination of high flowability, high stiffness and impact, and high level of optical properties (low haze value), compared to the comparative examples.