Propylene composition with high stiffness and impact strength

1. A process for the production of a polypropylene composition comprising a propylene homo- or copolymer (A) with a polydispersity index (PI), determined according ISO 6721-1, of at least 5.8 Pa −1 , in an amount of at least 80 wt %; and an inorganic filler (B);

wherein the process comprises preparing the propylene homo- or copolymer (A) in one or more reactors wherein in at least one reactor the process comprises the following steps:

a) feeding one or more (co)monomers and hydrogen to the reactor, whereby the hydrogen concentration in said reactor is periodically varied;

b) preparing the propylene homo- or copolymer (A) in the presence of an olefin polymerization catalyst; and

c) withdrawing the propylene homo- or copolymer (A) from said at least one reactor, whereby the following relation is fulfilled:

P/Tau>2.0, wherein

P is the time of one variation period of the hydrogen concentration in said at least on reactor; and

Tau is the average residence time of the polymer in said at least one reactor.

2. The process according to claim 1 , wherein the following relation is fulfilled by the polypropylene composition:

(80 F+ 1700)MPa≦ T

wherein

F are the parts per weight of component (B) based on 100 parts per weight of the total amount of (A)+(B);

T is the tensile modulus in MPa, determined according to ISO 527-2, of the polypropylene composition measured on a test specimen prepared by injection molding according to ISO 1873-2.

3. The process according to claim 1 , wherein component (B) of the polypropylene composition is selected from natural silicates or glass platelets or fibers.

4. The process according to claim 1 , wherein the average aspect ratio of the filler of the polypropylene composition is at least 5.0.

5. The process according to claim 1 , wherein the MFR 2 of the polypropylene composition, determined according to ISO 1133 at 230° C. and under a load of 2.16 kg, is from 0.01 to 100 g/10 min.

6. The process according to claim 1 , wherein the polypropylene composition has a tensile modulus determined according to ISO 527-2 of at least 2000 MPa measured on a test specimen prepared by injection molding according to ISO 1873-2.

7. The process according to claim 1 , wherein the polypropylene composition has an impact strength in a charpy notch test according to ISO 179/1eA:2000 at +23° C. of at least 3.8 kJ/m 2 measured on a V-notched test specimen prepared by injection molding according to ISO 1873-2.

8. The process according to claim 1 , wherein the polypropylene composition has a longitudinal shrinkage of not more than 2.2%.

9. The process according to claim 1 , wherein the polypropylene composition has a transversal shrinkage of not more than 2.0%.

10. The process according to claim 1 , wherein the inorganic filler (B) of the polypropylene composition is present in an amount from 4.0 to 18.0 parts per weight, based on 100 parts per weight of (A)+(B).