Heterophasic polyolefin composition having improved optical properties

1. A heterophasic propylene copolymer (HECO), said heterophasic propylene copolymer (HECO) comprises a matrix (M) being a propylene homopolymer (H-PP) and an elastomeric propylene copolymer (EPC) dispersed in said matrix (M),

wherein the heterophasic propylene copolymer (HECO) has

(a) a melt flow rate MFR 2 (2.16 kg, 230° C.) measured according to ISO 1133 in the range of 0.2 to 8.0 g/10 min,

(b) a xylene cold soluble content (XCS) determined according ISO 16152 (25° C.) in the range of 10.0 to 20.0 wt %, and

(c) a total ethylene comonomer content in the range of more than 2.5 to 8.0 wt %,

wherein further;

(d) the melt flow rate MFR 2 (2.16 kg, 230° C.) measured according to ISO 1133 of the matrix (M) is in the range of 0.2 to 4.0 g/10 min

(e) the ethylene comonomer content of xylene cold soluble (XCS) fraction of the heterophasic propylene copolymer is in the range of 25.0 to 40.0 wt %,

(f) the intrinsic viscosity (IV) determined according to DIN ISO 1628/1 (in decalin at 135° C.) of the xylene cold soluble (XCS) fraction of the heterophasic propylene copolymer is in the range of more than 1.0 to below 2.9 dl/g, and

(g) the relative content of isolated to block ethylene sequences (I(E)) of the XCS fraction fulfilling the in equation (I):

I ( E )(XCS)<54−0.81× C 2(XCS)  (I)

wherein C2 is the comonomer content of the XCS fraction, XCS is the amount of xylene cold soluble fraction in, and

wherein the I(E) content is defined by equation (II):

I ( E )= f PEP/(( f EEE+ f PEE+ f PEP))×100  (II)

wherein

I(E) is the relative content of isolated to block ethylene sequences of the XCS fraction;

fPEP is the mol fraction of propylene/ethylene/propylene sequences (PEP) of the XCS fraction;

fPEE is the mol fraction of propylene/ethylene/ethylene sequences (PEE) and of ethylene/ethylene/propylene sequences (EEP) of the XCS fraction;

fEEE is the mol fraction of ethylene/ethylene/ethylene sequences (EEE) of the XCS fraction,

wherein all sequence concentrations are based on a statistical triad analysis of 13C-NMR data from the XCS fraction of the heterophasic copolymer.

2. The heterophasic propylene copolymer (HECO) according to claim 1 , which further comprises component (c) being a crystalline ethylene copolymer (CEC), and/or

(d) a nucleating agent.

3. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the heterophasic propylene copolymer (HECO) comprises 82.0 to 93.0 wt %, based on the total weight of the heterophasic propylene copolymer HECO, of the propylene homopolymer (H-PP), 7.0 to 18.0 wt %, based on the total weight of the heterophasic propylene copolymer (HECO), of the elastomeric propylene copolymer (EPC), 0.0 to 5.0 wt %, based on the total weight of the heterophasic propylene copolymer (HECO), of a crystalline ethylene copolymer (CEC) and from 0.0 to 0.1 wt %, based on the total weight of the heterophasic propylene copolymer (HECO), of a nucleating agent.

4. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein xylene solubles fraction of the propylene homopolymer matrix is in the range of 0.5 wt % to 3.0 wt %.

5. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the ethylene content of the cold insoluble fraction (XCI) of the heterophasic propylene copolymer (HECO) is in the range of 1.0 to 3.5 wt %.

6. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the matrix (M) is unimodal or bimodal.

7. The heterophasic propylene copolymer (HECO) according to claim 6 , wherein the matrix is bimodal and consists of two propylene homopolymer fractions (H-PP-1) and (H-PP-2).

8. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the heterophasic propylene copolymer (HECO) is a non-visbroken heterophasic propylene copolymer (HECO).

9. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the heterophasic propylene copolymer (HECO) has been visbroken with a visbreaking ratio (VR) as defined by the inequality VR>1.0,

wherein VR=MFR final /MFR start ,

wherein MFR final is the MFR 2 (230° C./2.16 kg) of the heterophasic propylene copolymer (HECO) after visbreaking, and

wherein MFR start is the MFR 2 (230° C./2.16 kg) of the heterophasic propylene copolymer (HECO) before visbreaking.

10. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the heterophasic propylene copolymer (HECO) has a haze determined according to ASTM D 1003-00 measured on a 50 μm cast film of below 30%.

11. The heterophasic propylene copolymer (HECO) according to claim 1 , wherein the heterophasic propylene copolymer (HECO) has a haze determined according to ASTM D 1003-00 measured on a 50 μm cast film of below 20%.

12. A polymerization process for producing the heterophasic propylene copolymer (HECO) according to claim 1 comprising polymerizing propylene in at least two subsequent polymerization steps in the presence of a Ziegler-Natta catalyst (ZN-C), whereby:

a) in the first polymerization reactor (R1) propylene is polymerized obtaining either a first propylene homopolymer fraction (H-PP-1) or the propylene homopolymer matrix (M), transferring said first propylene homopolymer fraction (H-PP-1) or propylene homopolymer matrix (M) to a second polymerization reactor (R2),

b) in the second polymerization reactor (R2) either a second propylene homopolymer fraction (H-PP-2), forming together with the first propylene homopolymer fraction (H-PP-1) the propylene homopolymer matrix (M) or,

the propylene copolymer fraction (EPC) in the presence of the propylene homopolymer matrix (M) produced in earlier steps is produced, whereupon

c) if in the first and the second polymerization reactor the propylene homopolymer matrix (M) is produced, in an optional third polymerization reactor the propylene copolymer fraction (EPC) in the presence of the propylene homopolymer matrix (M) produced in earlier steps is produced and

d) optionally subsequent visbreaking of the heterophasic propylene copolymer (HECO) obtained after step b) respectively c).

13. The process according to claim 12 , wherein the process further comprises a prepolymerization step prior to step a), whereby propylene is polymerized in a pre-polymerization reactor (PR) in the presence of a Ziegler-Natta catalyst (ZN-C) to obtain a mixture (MI) of the Ziegler-Natta catalyst and a polypropylene (Pre-PP), and subsequently the mixture (MI) of the Ziegler-Natta catalyst and the polypropylene (Pre-PP) is transferred to the first polymerization reactor (R1).