Bimodal polyethylene film

1. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

2. A film according to claim 1 , wherein the polyethylene resin composition is a multimodal composition comprising from 30 to 70 wt % of an ethylene polymer (A) having a melt index MI 2 of 5 to 1000 g/10 min and a density of at least 965 kg/m 3 , and from 30 to 70 wt % of an ethylene polymer (B) having a melt index MI 5 of 0.001 to 2 g/10 min and a density of 910 to 945 kg/m 3 .

3. A film according to claim 2 , wherein the ethylene polymer (A) is a homopolymer, and the ethylene polymer (B) is a copolymer of ethylene and a C 4 -C 8 alpha-olefin.

4. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3850, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

5. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3900, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

6. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=550, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

7. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=450, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

8. A film formed from a polyethylene resin composition which obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=350, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

9. A film according to claim 1 , wherein G′(G″=3000) is less than 2650.

10. A film according to claim 9 , wherein G′(G″=3000) is less than 2550.

11. A film according to claim 1 , wherein the composition has been subjected to reticulation during a compounding step which has increased the value of z and decreased the value of y.

12. A film comprising a blend of 20 to 80 wt % of a first polyethylene resin composition and 80 to 20 wt % of a second polyethylene resin composition, wherein said first and second polyethylene resin compositions each obey a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

13. A film according to claim 1 , which is formed from a composition capable of being obtained by a process in which:

in a first reactor, ethylene is polymerized in suspension in a first mixture comprising a diluent, hydrogen, a catalyst based on a transition metal and a cocatalyst, so as to form from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A);

said first mixture is withdrawn from said reactor and is subjected to a reduction in pressure, so as to degas at least a portion of the hydrogen to form an at least partially degassed mixture, and said at least partially degassed mixture, together with ethylene and a C 4 -C 8 alpha-olefin and, optionally, at least one other alpha-olefin, are introduced into a subsequent reactor and a suspension polymerization is carried out therein in order to form from 70 to 30% by weight with respect to the total weight of the composition of a copolymer of ethylene and a C 4 -C 8 alpha-olefin (B).

14. A process for making a film, comprising the steps of polymerizing ethylene and optionally comonomer, and then compounding the resultant polyethylene composition to form it into pellets, and then forming the composition into a film, wherein the film obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709.

15. A process for making a film according to claim 14 , wherein the polyethylene composition is a multimodal polyethylene resin composition comprising from 30 to 70 wt % of an ethylene homopolymer (A) having a melt index MI 2 of 5 to 1000 g/10 min and a density of at least 965 kg/m 3 , and from 70 to 30 wt % of an ethylene copolymer (B) having a melt index MI 5 of 0.001 to 2 g/10 min and a density of 910 to 945 kg/m 3 , wherein said multimodal polyethylene resin composition is obtained by polymerizing, in a first reactor, ethylene in suspension in a first mixture comprising a diluent, hydrogen, a polymerization catalyst and a cocatalyst, so as to form from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A), withdrawing said mixture from said reactor and subjecting said mixture to a reduction in pressure so as to degas at least a portion of the hydrogen to form an at least partially degassed mixture, then introducing said at least partially degassed mixture, together with ethylene and a C 4 -C 8 alpha-olefin and, optionally, at least one other alpha-olefin, into a subsequent reactor and carrying out a suspension polymerization therein in order to form from 70 to 30% by weight, with respect to the total weight of the composition, of a copolymer of ethylene and of C 4 -C 8 alpha-olefin (B).

16. A process for making a film according to claim 15 , wherein the polymerization catalyst is a mixed alcoholate catalyst containing 5 to 30% by weight of transition metal, 0.5 to 20% by weight of magnesium, 20 to 60% by weight of chlorine and 0.1 to 10% by weight of aluminium, and having a residual organic radical content in the precipitated catalyst of less than 35 wt %.

17. A process for making a film according to claim 16 , wherein the catalyst contains 8 to 16% by weight of transition metal, 5 to 15% by weight of magnesium, 40 to 60% by weight of chlorine and 0.1 to 5% by weight of aluminium, and has a residual organic radical content in the precipitated catalyst of less than 35 wt %, with less than 20 wt % total alkyl benzoate.

18. A process for making a film according to claim 16 , wherein the compounding conditions are selected so as to reticulate the composition such that after compounding the value of z is greater than before compounding, and the value of y is less than before compounding.

19. A process for making a film according to claim 18 , wherein the compounding step increases the value of z by at least 100, and decreases the value of y by at least 100.

20. A process for making a film according to claim 19 , wherein the compounding step increases the value of z by 400 to 1200.

21. A process for making a film according to claim 19 , wherein the compounding step decreases the value of y by 400 to 1200.

22. A process for making a film according to claim 14 , wherein the film is a blown film.

23. A process for making a film, comprising the steps of polymerizing ethylene and optionally comonomer, and then compounding the resultant polyethylene composition to form it into pellets, and then forming the composition into a film, wherein the film obeys a dynamic rheological relationship at 190° C. between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η* 100 , measured in Pa·s at 100 rad/s, such that (a) G′(G″=3000)>−0.86η* 100 +z where z=3800, and at the same time (b) G′(G″=3000)>0.875η* 100 −y where y=650, and having an impact strength (DDT) of at least 250 g, measured on 15 μm thick film (blown under conditions with BUR=5:1 and Neck Height=8×D) conditioned for 48 hours at 20-25° C., according to ASTM D1709, wherein the polyethylene composition comprises a blend of 20 to 80 wt % of a first composition and 80 to 20 wt % of a second composition, wherein said first and second compositions are each made by a process in which:

in a first reactor, ethylene is polymerized in suspension in a first mixture comprising a diluent, hydrogen, a catalyst based on a transition metal and a cocatalyst, so as to form from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A);

said first mixture is withdrawn from said reactor and is subjected to a reduction in pressure, so as to degas at least a portion of the hydrogen to form an at least partially degassed mixture, and said at least partially degassed mixture, together with ethylene and a C 4 -C 8 alpha-olefin and, optionally, at least one other alpha-olefin, are introduced into a subsequent reactor and a suspension polymerization is carried out therein in order to form from 70 to 30% by weight with respect to the total weight of the composition of a copolymer of ethylene and a C 4 -C 8 alpha-olefin (B).