Polyethylene composition and film having retained dart impact
A polyethylene composition including a first polyethylene which is an ethylene copolymer having a weight average molecular weight of from 70,000 to 250,000 and a molecular weight distribution M w /M n of <2.3, a second polyethylene which is an ethylene copolymer or homopolymer having a weight average molecular weight of less than 75,000 and a molecular weight distribution M w /M n of <2.3, and a third polyethylene which is an ethylene copolymer or homopolymer having a weight average molecular weight of from 100,000 to 300,000 and a molecular weight distribution M w /M n of >2.3, where the first polyethylene has more short chain branching than the second polyethylene or the third polyethylene. The polyethylene composition has a soluble fraction in a CEF analysis of at least 15 weight percent. A film made from the polyethylene composition may have a machine direction 1% secant modulus of ≥200 MPa (at a film thickness of about 1 mil) and an oxygen transmission rate (OTR) of ≥700 cm 3 per 100 inch 2 per day (at a film thickness of about 1 mil). A film made from the polyethylene composition retains much of its dart impact performance on downgauging from a thickness of 1 mil to a thickness of 0.75 mil.
1. A polyethylene composition comprising:
from 15 to 75 wt % of a first polyethylene which is an ethylene copolymer, the first polyethylene having a weight average molecular weight Mw of from 70,000 to 250,000, a molecular weight distribution M w /M n of <2.3 and from 5 to 100 short chain branches per thousand carbon atoms;
from 5 to 80 wt % of a second polyethylene which is an ethylene copolymer or an ethylene homopolymer, the second polyethylene having a weight average molecular weight Mw of less than 75,000, a molecular weight distribution M w /M n of <2.3 and from 0 to 20 short chain branches per thousand carbon atoms; and
from 5 to 60 wt % of a third polyethylene which is an ethylene copolymer or an ethylene homopolymer, the third polyethylene having a weight average molecular weight Mw of from 100,000 to 300,000, a molecular weight distribution M w /M n of >2.3 and from 0 to 75 short chain branches per thousand carbon atoms; wherein
the number of short chain branches per thousand carbon atoms in the first polyethylene (SCB PE-1 ) is greater than the number of short chain branches per thousand carbon atoms in the second polyethylene (SCB PE-2 ) and the third polyethylene (SCB PE-3 );
the number of short chain branches per thousand carbon atoms in the third polyethylene (SCB PE-3 ) is greater than the number of short chain branches per thousand carbon atoms in the second polyethylene (SCB PE-2 ); and
the weight average molecular weight of the second polyethylene is less than the weight average molecular weight of the first polyethylene and the third polyethylene; wherein,
the polyethylene composition has a density of ≤0.939 g/cm 3 , a melt index I 2 , determined at 190° C., of from 0.1 to 10 dg/min, a melt flow ratio, I 21 /I 2 of greater than 40 and has a soluble fraction in a crystallization elution fractionation (CEF) analysis of at least 15 weight percent.
2. The polyethylene composition of claim 1 wherein the polyethylene composition has a bimodal profile in a gel permeation chromatograph (GPC).
3. The polyethylene composition of claim 1 wherein the polyethylene composition has a soluble fraction in a crystallization elution fractionation (CEF) analysis of at least 20 weight percent.
4. The polyethylene composition of claim 1 wherein the polyethylene composition has a melting peak temperature in a differential scanning calorimetry (DSC) analysis at above 125° C.
5. The polyethylene composition of claim 1 wherein the polyethylene composition has two melting peaks in a differential scanning calorimetry (DSC) analysis.
6. The polyethylene composition of claim 1 wherein the first polyethylene has from 25 to 75 short chain branches per thousand carbon atoms.
7. The polyethylene composition of claim 1 wherein the second polyethylene is an ethylene homopolymer.
8. The polyethylene composition of claim 1 wherein the third polyethylene is an ethylene copolymer and has from 5 to 50 short chain branches per thousand carbon atoms.
9. The polyethylene composition of claim 1 wherein the first polyethylene has a weight average molecular weight, Mw of from 75,000 to 200,000.
10. The polyethylene composition of claim 1 wherein the second polyethylene has a weight average molecular weight, Mw of from 15,000 to 40,000.
11. The polyethylene composition of claim 1 wherein the third polyethylene has a weight average molecular weight, Mw of from 100,000 to 240,000.
12. The polyethylene composition of claim 1 wherein the first polyethylene has a density of from 0.865 to 0.916 g/cm 3 .
13. The polyethylene composition of claim 1 wherein the second polyethylene is an ethylene homopolymer having a density of from 0.940 to 0.980 g/cm 3 .
14. The polyethylene composition of claim 1 wherein the third polyethylene is an ethylene copolymer having a density of from 0.880 to 0.936 g/cm 3 .
15. The polyethylene composition of claim 1 wherein the first polyethylene is present in from 20 to 70 wt %.
16. The polyethylene composition of claim 1 wherein the second polyethylene is present in from 15 to 60 wt %.
17. The polyethylene composition of claim 1 wherein the third polyethylene is present in from 5 to 50 wt %.
18. The polyethylene composition of claim 1 wherein the first polyethylene has a CDBI 50 of at least 75 wt %.
19. The polyethylene composition of claim 1 wherein the third polyethylene is a copolymer with a CDBI 50 of less than 75 wt %.
20. The polyethylene composition of claim 1 wherein the first polyethylene is a homogeneously branched ethylene copolymer.
21. The polyethylene composition of claim 1 wherein the third polyethylene is a heterogeneously branched ethylene copolymer.
22. The polyethylene composition of claim 1 wherein the first polyethylene is a made with a single site catalyst.
23. The polyethylene composition of claim 1 wherein the third polyethylene is made with a Ziegler-Natta catalyst.
24. The polyethylene composition of claim 1 wherein the polyethylene composition has a molecular weight distribution M w /M n of from 2.3 to 8.0.
25. The polyethylene composition of claim 1 wherein the polyethylene composition has a molecular weight distribution M w /M n of from 2.5 to 6.5.
26. The polyethylene composition of claim 1 wherein the polyethylene composition has a density of <0.935 g/cm 3 .
27. The polyethylene composition of claim 1 wherein the polyethylene composition has a density of from 0.880 to 0.932 g/cm 3 .
28. The polyethylene composition of claim 1 wherein the polyethylene composition has a melt index, I 2 of from 0.1 to 3.0 dg/min.
29. The polyethylene composition of claim 1 wherein the polyethylene composition has a Mz/M w of less than 4.0.
30. The polyethylene composition of claim 1 wherein the polyethylene composition has a melt index ratio, I 21 /I 2 of from greater than 40 to 100.
31. The polyethylene composition of claim 1 wherein the polyethylene composition has a melt index ratio, I 21 /I 2 of greater than 45.
32. A film layer having a thickness of from 0.5 to 10 mil, comprising the polyethylene composition of claim 1 .
33. The polyethylene composition of claim 1 , which when made into a film layer having a thickness of about 1 mil exhibits a machine direction (MD) 1% secant modulus of ≥200 MPa.
34. The polyethylene composition of claim 1 , which when made into a film layer having a thickness of about 1 mil exhibits an oxygen transmission rate (OTR) of ≥700 cm 3 per 100 inches 2 per day.
35. The polyethylene composition of claim 1 , which when made into a film layer having a thickness of about 1 mil exhibits a machine direction (MD) 1% secant modulus of ≥200 MPa and an oxygen transmission rate (OTR) of ≥700 cm 3 per 100 inches 2 per day.
36. The polyethylene composition of claim 1 , which when made into a film layer having a thickness of about 1 mil exhibits a first dart impact value in g/mil and when made into a film layer having a thickness of about 0.75 mil exhibits a second dart impact value in g/mil; wherein the second dart impact value is within 15 percent of the first dart impact value.
37. A film comprising the polyethylene composition of claim 1 , the film satisfying the following relationship:
oxygen transmission rate (OTR in cm 3 per 100 inches 2 per day)>−5.4297(machine direction (MD) 1% secant modulus in MPa)+1767.8;
wherein the OTR is measured at a film thickness of about 1 mil, and the machine direction (MD) 1% secant modulus is measured at a film thickness of about 1 mil.
38. The polyethylene composition of claim 1 wherein the second polyethylene is a made with a single site catalyst.