Process for producing polyolefin film composition and films prepared thereof

1. A process for producing multimodal ethylene polymer film compositions comprising the steps of

(i) copolymerising ethylene and an alpha-olefin comonomer in a first polymerisation step in the presence of a silica supported Ziegler-Natta polymerisation catalyst to produce a first ethylene copolymer (PE1) having a density of from 940 to 980 kg/m 3 and a melt flow rate MFR 2 of from 400 to 1000 g/10 min;

(ii) copolymerising ethylene and an alpha-olefin comonomer in a second polymerisation step in the presence of the first ethylene copolymer to produce a first ethylene polymer mixture (PEM1) comprising the first ethylene copolymer and a second ethylene copolymer, said first ethylene polymer mixture having a density of from 940 to 980 kg/m 3 and a melt flow rate MFR 2 of from 150 to 800 g/10 min, and wherein the MFR 2 of the first ethylene copolymer (PE1) is higher than the MFR 2 of the first ethylene polymer mixture (PEM1) and

(iii) copolymerising ethylene and an alpha-olefin comonomer in a third polymerisation step in the presence of the first ethylene polymer mixture to produce a second ethylene polymer mixture (PEM2) comprising the first ethylene polymer mixture and a third ethylene copolymer, said second ethylene polymer mixture having a density of from 918 to 925 kg/m 3 and a melt flow rate MFR5 of from 0.3 to 3 g/10 min, wherein the second ethylene polymer mixture (PEM2) comprises from 10 to 35% by weight of the first ethylene copolymer, from 10 to 35% by weight of the second ethylene copolymer and from 45 to 70% by weight of the third ethylene copolymer.

2. The process according to claim 1 wherein the α-olefin comonomer in at least one of steps (i), (ii), or (iii) is selected from the group consisting of α-olefins having from 4 to 10 carbon atoms and their mixtures.

3. The process according to claim 1 wherein the third polymerisation step is conducted in gas phase.

4. The process according to claim 1 wherein at least one of the first and the second polymerisation step is conducted as a slurry polymerisation in a loop reactor.

5. The process according to claim 4 wherein a diluent in the slurry polymerisation comprises at least 90% of hydrocarbons having from 3 to 5 carbon atoms.

6. The process according to claim 1 , wherein a ratio MFR 2 (PEM1)/MFR 2 (PE1) is at most 0.8.

7. A multimodal film composition having density of from 918 to 925 kg/m 3 and a melt flow rate MFR 5 of from 0.3 to 3 g/10 min and produced according to claim 1 .

8. A multimodal ethylene-alpha-olefin copolymer film composition produced according to the process of claim 1 .

9. The multimodal film composition according to claim 8 , wherein a ratio MFR 2 (PEM1)/MFR 2 (PE1) is at most 0.8.

10. A process for producing a film, comprising the steps of

(i) copolymerising ethylene and an alpha-olefin comonomer in a first polymerisation step in the presence of a silica supported Ziegler-Natta polymerisation catalyst to produce a first ethylene copolymer (PE1) having a density of from 940 to 980 kg/m 3 and a melt flow rate MFR 2 of from 400 to 1000 g/10 min;

(ii) copolymerising ethylene and an alpha-olefin comonomer in a second polymerisation step in the presence of the first ethylene copolymer to produce a first ethylene polymer mixture (PEM1) comprising the first ethylene copolymer and a second ethylene copolymer, said first ethylene polymer mixture having a density of from 940 to 980 kg/m 3 and a melt flow rate MFR 2 of from 150 to 800 g/10 min, and wherein the MFR 2 of the first ethylene copolymer (PE1) is higher than first ethylene polymer mixture (PEM1);

(iii) copolymerising ethylene and an alpha-olefin comonomer in a third polymerisation step in the presence of the first ethylene polymer mixture to produce a second ethylene polymer mixture (PEM2) comprising the first ethylene polymer mixture and a third ethylene copolymer, said second ethylene polymer mixture having a density of from 918 to 925 kg/m 3 and a melt flow rate MFR 5 of from 0.3 to 3 g/10 min, wherein the second ethylene polymer mixture (PEM2) comprises from 10 to 35% by weight of the first ethylene copolymer, from 10 to 35% by weight of the second ethylene copolymer and from 45 to 70% by weight of the third ethylene copolymer;

iv) pelletizing the second polymer mixture; and

v) providing a film by blow molding.

11. The process according to claim 10 wherein the α-olefin comonomer in at least one of steps (i), (ii), or (iii) is selected from the group consisting of α-olefins having from 4 to 10 carbon atoms and their mixtures.

12. A film comprising the multimodal film composition of claim 7 .

13. The process according to claim 2 wherein the α-olefin comonomer is selected from the group consisting of α-olefins having from 4 to 8 carbon atoms and their mixtures.

14. The process according to claim 4 wherein both the first and the second polymerisation steps are conducted as a slurry polymerisation in a loop reactor.

15. The process according to claim 1 wherein the second ethylene polymer mixture (PEM2) comprises from 13 to 26% by weight of the first ethylene copolymer, from 16 to 26% by weight of the second ethylene copolymer and from 50 to 65% by weight of the third ethylene copolymer.

16. The process according to claim 1 wherein the first ethylene copolymer has a melt flow rate MFR 2 of from 500 to 800 g/10 min.

17. The process according to claim 6 , wherein a ratio MFR 2 (PEM1)/MFR 2 (PE1) is at most 0.7.