Rotomolding compositions with low relative elasticity
High density polyethylene compositions having a high flow index and a bimodal composition provides an outstanding combination of processability, stiffness and ductility in rotomolded articles. The compositions have a low relative elasticity (G′/G″, measured at 0.05 rad/sec) of less than 0.03.
1. A bimodal polyethylene composition having
1) A molecular weight distribution, Mw/Mn, of from 2.3 to 5.5;
2) A density of from 0.940 to 0.957 g/cc;
3) A melt index, I 2 , as measured by ASTM D1238 at 190° C. using a 2.16 kg load of from 4 to 10 grams per 10 minutes; and
4) A relative elasticity, G′/G″ when measured at 190° C. and 0.05 rad/second of less than 0.03 rad/sec,
wherein the bimodal polyethylene composition comprises:
A. from 10 to 70 weight % of a first ethylene copolymer having:
A.i. a melt index, I 2 , as measured by ASTM D1238 at 190° C. using a 2.16 kg load of from 0.4 to 5 grams per 10 minutes;
A.ii. a molecular weight distribution, Mw/Mn, of from 1.8 to 3.0; and
A.iii. a density of from 0.920 to 0.950 g/cc;
B. from 90 to 30 weight % of a second ethylene copolymer having:
B.i. a melt index, I 2 , as measured by ASTM D1238 at 190° C. using a 2.16 kg load of from 4 to 1500 grams per 10 minutes;
B.ii. a molecular weight distribution, Mw/Mn, of from 2.3 to 6.0; and
B.iii. a density that is greater than the density of the first ethylene copolymer but less than 0.967 g/cc;
with the proviso that the density of the first ethylene copolymer is lower than the density of the second ethylene copolymer by an amount of from 0.010 to 0.035 g/cc.
2. The bimodal polyethylene composition of claim 1 , wherein the first ethylene copolymer further exhibits:
A.iv. a number average molecular weight, Mn, of from 35,000 to 80,000;
A.v. a weight average molecular weight, Mw, of from 70,000 to 150,000;
A.vi. an Mz of from 120,000 to 250,000;
A.vii. Mw/Mn of from 2 to 3;
A.viii. a number of short chain branches (SCB1) per thousand carbon atoms of from 1 to 5; and
A.ix. a melt index, I 2 , as measured by ASTM D1238 at 190° C. using a 2.16 kg load of from 0.5 to 4.0 grams per 10 minutes.
3. The bimodal polyethylene composition of claim 1 , wherein the second ethylene copolymer further exhibi:
A.iv. a number average molecular weight, Mn, of from 12,000 to 30,000;
A.v. a weight average molecular weight, Mw, of from 28,000 to 72,000;
A.vi. an Mz of from 70,000 to 150,000;
A.vii. an Mw/Mn of from 2.3 to 5.0;
A.viii. a number of short chain branches (SCB2) per thousand carbon atoms of from 0.1 to 2;
B.ix. a density greater than the density of the first ethylene copolymer but less than 0.965 g/cc;
B.x. a melt index, I 2 , as measured by ASTM D1238 at 190° C. using a 2.16 kg load of from 4 to 100 grams per 10 minutes;
with the proviso that the density of said first ethylene copolymer is lower than the density of the second ethylene copolymer by an amount of from 0.010 to 0.030 g/cc.
4. A rotomolded part comprising the bimodal polyethylene composition of claim 1 .
5. The rotomolded part of claim 4 having a ductility index of from 80 to 100%.
6. The rotomolded part of claim 5 having a mean failure energy greater than 120 ft·lb on 0.250 inches thick specimens tested according to ASTM D5628 at a test temperature of −40° C.
7. The rotomolded part of claim 4 , wherein the first ethylene copolymer is prepared with a single site catalyst and the second ethylene copolymer is prepared with a Ziegler Natta catalyst.
8. The bimodal polyethylene composition of claim 1 , wherein the bimodal polyethylene composition has a comonomer content of less than 1.2 mol % as determined by FTIR method.
9. The bimodal polyethylene composition of claim 1 , wherein the first and second ethylene copolymers are copolymers of ethylene and 1-octene.
10. The bimodal polyethylene composition of claim 1 , wherein the 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.
11. A process for the production of polyolefin hollow articles, the process comprising charging the bimodal polyethylene composition of claim 1 into a mold, heating the mold in an oven to above 280° C., such that the stabilized polyolefin fuses, rotating the mold around at least 2 axes, the plastic material spreading to the walls, cooling the mold while still rotating, opening it, and taking the resultant hollow article out.