Polyethylene compositions and closures for bottles
A dual reactor solution process gives high density polyethylene compositions containing a first ethylene copolymer and a second ethylene copolymer and which have good processability, toughness, and environmental stress crack resistance combined with good organoleptic properties. The polyethylene compositions are suitable for compression molding or injection molding applications and are particularly useful in the manufacture of caps and closures for bottles.
1. A bimodal polyethylene composition comprising:
(1) 10 to 70 wt % of a first ethylene copolymer having a melt index I 2 , of less than 0.4 g/10 min; a molecular weight distribution M w /M n , of less than 3.0; and a density of from 0.920 to 0.955 g/cm 3 ; and
(2) 90 to 30 wt % of a second ethylene copolymer having a melt index I 2 , of from 100 to 10,000 g/10 min; a molecular weight distribution M w /M n , of less than 3.0; and a density higher than the density of said first ethylene copolymer, but less than 0.967 g/cm 3 ;
wherein the density of said second ethylene copolymer is less than
0.037 g/cm 3 higher than the density of said first ethylene copolymer; the ratio (SCB1/SCB2) of the number of short chain branches per thousand carbon atoms in said first ethylene copolymer (SCB1) to the number of short chain branches per thousand carbon atoms in said second ethylene copolymer (SCB2) is greater than 0.5; and wherein said bimodal polyethylene composition has a molecular weight distribution M w /M n , of from 3 to 11; a density of at least 0.949 g/cm 3 ; a melt index I 2 , of from 0.4 to 5.0 g/10 min; an M z of less than 400,000; a stress exponent of less than 1.50; and an ESCR Condition B (10% IGEPAL) of at least 20 hrs.
2. The bimodal polyethylene composition of claim 1 wherein the ratio (SCB1/SCB2) of the number of short chain branches per thousand carbon atoms in said first ethylene copolymer (SCB1) to the number of short chain branches per thousand carbon atoms in said second ethylene copolymer (SCB2) is at least 1.0.
3. The bimodal polyethylene composition of claim 1 wherein the ratio (SCB1/SCB2) of the number of short chain branches per thousand carbon atoms in said first ethylene copolymer (SCB1) to the number of short chain branches per thousand carbon atoms in said second ethylene copolymer (SCB2) is at least 1.5.
4. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has an ESCR Condition B (10% IGEPAL) of at least 60 hrs.
5. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has an ESCR Condition B (10% IGEPAL) of at least 120 hrs.
6. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has a molecular weight distribution, M w /M n , of from 4.5 to 10.0.
7. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has melt index I 2 , of from 0.4 to 3.0 g/10 min.
8. The bimodal polyethylene composition of claim 1 wherein said first ethylene copolymer has a density of from 0.925 to 0.950 g/cm 3 .
9. The bimodal polyethylene composition of claim 1 wherein said second ethylene copolymer has a density of less than 0.965 g/cm 3 .
10. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has a density of from 0.951 to 0.957 g/cm 3 .
11. The bimodal polyethylene composition of claim 1 wherein the density of said second ethylene copolymer is less than 0.035 g/cm 3 higher than the density of said first ethylene copolymer.
12. The bimodal polyethylene composition of claim 1 wherein said second ethylene copolymer has a melt index I 2 , of greater than 1500 g/10 min.
13. The bimodal polyethylene composition of claim 1 wherein said first and second ethylene copolymers have a M w /M n of less than 2.5.
14. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has a composition distribution breadth index (CDBI) of greater than 65%.
15. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition comprises:
from 30 to 60 wt % of said first ethylene copolymer; and
from 70 to 40 wt % of said second ethylene copolymer.
16. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition has a comonomer content of less than 0.75 mol % as determined by 13 C NMR.
17. The bimodal polyethylene composition of claim 1 wherein the bimodal polyethylene composition further comprises one or more nucleating agents.
18. The bimodal polyethylene composition of claim 1 wherein said first and second ethylene copolymers are copolymers of ethylene and 1-octene.
19. The bimodal polyethylene composition of claim 1 wherein said bimodal polyethylene composition is prepared by contacting ethylene and an alpha-olefin with a polymerization catalyst under solution polymerization conditions in a least two polymerization reactors.
20. The bimodal polyethylene composition of claim 1 wherein the density of said second ethylene copolymer is less than 0.031 g/cm 3 higher than the density of said first ethylene copolymer.