BOPP film with improved stiffness/toughness balance

1. A biaxially oriented polypropylene (BOPP) film comprising a polypropylene (PP), wherein the biaxially oriented polypropylene (BOPP) film has

a) a modulus of elasticity in transverse direction (TD) of at least 5 000 N/mm 2 , and

b) an elongation at break in machine direction (MD) of at least 170%, and

wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having

i) a melting temperature (T m ) measured by differential scanning calorimetry (DSC) in the range of from 164 to 169° C.,

ii) a xylene cold soluble fraction (XCS) determined at 25° C. according to ISO 16152 in the range of from 1.0 to 3.5 wt %, and

iii) a content of ethylene units in the range of from 0.01 to 0.5 wt %, based on the total weight of the random polypropylene copolymer (C-PP).

2. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the biaxially oriented polypropylene (BOPP) film has

a) a modulus of elasticity in machine direction (MD) of at least 2 000 N/mm 2 , and/or

b) an elongation at break in transverse direction (TD) of at least 46%.

3. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the biaxially oriented polypropylene (BOPP) film has a tensile strength in machine direction (MD) of at least 110 N/mm 2 and/or a tensile strength in transverse direction (TD) of at least 300 N/mm 2 .

4. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having

a) a crystallization temperature (T c ) measured by differential scanning calorimetry (DSC) of at least 115° C., and/or

b) a melt flow rate MFR 2 (230° C.) measured according to ISO 1133 of ≦7.0 g/10min, and/or

c) an mmmm pentad content of ≧95.0% determined by 13 C NMR spectroscopy, and/or

d) 2,1 erythro regio-defect content of below 1.0% determined by 13 C NMR spectroscopy.

5. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having

a) a crystallization temperature (T c ) measured by differential scanning calorimetry (DSC) of at least 118° C., and/or

b) a melt flow rate MFR 2 (230° C.) measured according to ISO 1133 of ≦7.0 g/10 min,

c) an mmmm pentad content of 95.0 to 98.0% determined by 13 C NMR spectroscopy, and/or

d) 2,1 erythro regio-defect content of below 0.5% determined by 13 C NMR spectroscopy.

6. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having a comonomer corrected meso sequence length (MSL4′) in the range of from 130 to 250 as determined by the following formula (IV)

MSL 4′=(((1−( fE* 5))*[ mmmm ])/(((1−( fE* 5))*0.5*[ mmmr ])+(0.5*2* fE )))+4   (IV).

7. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having a ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) [M w /M n ] of at least 4.0.

8. The biaxially oriented polypropylene (BOPP) film according to claim 7 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having a ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) [M w /M n ] of at least 5.0.

9. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the polypropylene (PP) is a random polypropylene copolymer (C-PP) having a polydispersity index of ≧2.5.

10. The biaxially oriented polypropylene (BOPP) film according to claim 1 , wherein the biaxially oriented polypropylene (BOPP) film has

a) a draw ratio in machine direction (MD) of ≧3.0 and/or

b) a draw ratio in transverse direction (TD) of ≧6.0.

11. The biaxially oriented polypropylene (BOPP) film according to claim 10 , wherein the biaxially oriented polypropylene (BOPP) film has

a) a draw ratio in machine direction (MD) of ≧4.0, and/or

b) a draw ratio in transverse direction (TD) of ≧7.0.

12. A process for the preparation of a biaxially oriented polypropylene (BOPP) film according to claim 1 , the process comprising at least the steps of

i) providing a polypropylene (PP) which is a random polypropylene copolymer (C-PP) having

a) a crystallization temperature (T c ) measured by differential scanning calorimetry (DSC) of at least 115° C., and/or

b) a melt flow rate MFR 2 (230° C.) measured according to ISO 1133 of ≦7.0 g/10 min, and/or

c) an mmmm pentad content of ≧95.0% determined by 13 C NMR spectroscopy, and/or

d) 2,1 erythro regio-defect content of below 1.0% determined by 13 C NMR spectroscopy, and

ii) stretching the polypropylene (PP) of step i) in machine direction (MD) and transverse direction (TD), wherein the stretching in machine direction (MD) and transverse direction (TD) is carried out in consecutive steps.

13. The process according to claim 12 , wherein the polypropylene (PP) has

a) a drawing temperature (T draw′ ) when drawn in transverse direction in the range of the inequation (II),

Tm− 25 ≦T draw′≦ Tm+ 10  (II),

wherein

Tdraw′ is the drawing temperature (T draw′ ) in ° C. of the first segment of the drawing zone (DZT) in the oven where the polypropylene (PP) is drawn in transverse direction, wherein further this first segment of the drawing zone (DZT) is defined as the zone where the draw ratio of the drawn polypropylene (PP) is at least 1.1; and

Tm is the melting temperature (T m ) of the polypropylene (PP) in ° C.;

and optionally

b) a drawing temperature (T draw″ ) when drawn in transverse direction in the range of the inequation (III),

Tm≦T draw″≦ Tm+ 18  (III),

wherein

Tdraw″ is the drawing temperature (T draw″ ) in ° C. of the heating zone (HZT) in the oven where the polypropylene (PP) is drawn in transverse direction, wherein further the heating zone (HZT) is the zone upstream to the drawing zone (DZT); and

Tm is the melting temperature (T m ) of the polypropylene (PP) in ° C.

14. The process according to claim 12 , wherein the polypropylene (PP) has a drawing temperature (T draw ) when stretched in machine direction in the range of the inequation (I)

Tm− 50 ≦T draw≦ Tm− 15  (I),

wherein

Tdraw is the drawing temperature (T draw ) in ° C., wherein the drawing temperature (T draw ) is defined as the temperature (in ° C.) of the first roll (R 1 ) of two successive rolls (R1, R2) of all rolls in the oven consecutively arranged in machine direction where the polypropylene (PP) is drawn in machine direction, wherein further said two successive rolls (R1, R2) when locking in machine direction have as the first pair of successive rolls for the first time a roll speed difference of at least 20 m/min; and

Tm is the melting temperature (T m ) of the polypropylene (PP) in ° C.

15. The process according to claim 12 , wherein step b) is carried out in that the polypropylene (PP) of step a) is stretched

a) in machine direction (MD) with a draw ratio of ≧3.0 and/or

b) in transverse direction (TD) with a draw ratio of ≧6.0.

16. An article comprising a biaxially oriented polypropylene (BOPP) film as defined in claim 1 .