High melt flow polymer of improved durability for pipe applications

1. A process for preparing a polypropylene containing polymer composition having a tensile modulus, determined according to ISO 527-2/1 B at 1 mm/min. and 23° C., of at least 1400 MPa, comprising the following steps in any sequence:

(i) preparing a polypropylene homopolymer which has a melt flow rate MFR 2.16 kg/230° C. of less than 20 g/10 min,

(ii) copolymerization of propylene with an olefin comonomer to result in a polypropylene random copolymer comprising from 0.2 to 5.0 wt % of comonomer units, and

(iii) copolymerization of propylene with an olefin comonomer to result in an elastomeric copolymer having an intrinsic viscosity of from 3 to 6 dl/g measured in tetraline;

using a Ziegler-Natta catalyst including an electron-donor;

wherein the Ziegler-Natta catalyst comprises a cocatalyst component which has been prepared by bringing together magnesium dichloride, a lower alcohol selected from methanol, ethanol or mixtures thereof, a titanium compound and an ester of phthalic acid having an alkoxy group of at least five carbon atoms.

2. The process according to claim 1 , wherein the process steps (i) and (ii) are carried out in at least one loop reactor and/or at least one gas phase reactor.

3. The process according to claim 1 , wherein all process steps (i) to (iii) are carried out in at least one loop reactor and/or at least one gas phase reactor.

4. The process according to claim 1 , wherein the process steps are carried out in the following sequence: (i)→(ii)→(iii).

5. The process according to claim 1 , wherein the process steps are carried out in the following sequence: (ii)→(i)→(iii).

6. The process according to claim 2 , wherein the first process step is carried out in a loop reactor, optionally followed by polymerization in a gas phase reactor, and the second and third process step are carried out in separate gas phase reactors.

7. The process according to claim 2 , wherein the one or more loop reactors are operated at a temperature of at least 70° C. and a pressure of 4600 to 10000 kPa.

8. The process according to claim 7 , wherein at least one loop reactor is operated at supercritical conditions.

9. The process according to claim 2 , wherein the one or more gas phase reactors for preparing the polypropylene homopolymer and/or polypropylene random copolymer are operated at a temperature of 60° C. to 100° C. and a pressure of 1000 kPa to 4000 kPa.

10. The process according to claim 9 , wherein a split between the process step (i) and the process step (ii), irrespective of their sequence, is from 80:20 to 20:80.

11. The process according to claim 1 , wherein the Ziegler-Natta catalyst is modified by polymerizing, in the presence of the catalyst, a vinyl compound of the formula

wherein R 1 and R 2 together form a 5 or 6-membered saturated, unsaturated or aromatic ring or independently represent an alkyl group comprising 1 to 4 carbon atoms, and the modified catalyst is used for the preparation of the polymer composition.

12. The process according to claim 1 , wherein a nucleating agent is added selected from talc, polymerized vinyl compounds, dibenzylidene sorbitol, sodium benzoate, di(alkylbenzylidene)sorbitol, or mixtures thereof.

13. The process of claim 1 , wherein the polymer composition has a melting point from 155.6° C. to 163.7° C.