Bimodal polyethylene compositions for blow molding applications
Bimodal polyethylene compositions and methods for making the same are provided. In at least one specific embodiment, the bimodal polyethylene composition can include a high molecular weight component having a weight average molecular weight (Mw) of from about 400,000 to about 950,000. The bimodal polyethylene composition can also include a low molecular weight component having a weight average molecular weight (Mw) of from about 3,000 to about 100,000. The high molecular weight component can be present in an amount ranging from about 25 wt % to about 40 wt % of the bimodal polyethylene composition. The bimodal polyethylene composition can also have a percent die swell of less than about 80%.
1. A method for making a bimodal polyethylene composition, comprising:
polymerizing ethylene and a comonomer in the presence of a mixed catalyst comprising at least one Group-15 containing catalyst in a gas phase polymerization reactor to produce a bimodal polyethylene;
heating the bimodal polyethylene to produce a substantially melted bimodal polyethylene;
contacting the substantially melted bimodal polyethylene with a gas mixture comprising from about 8 vol % to about 20 vol % oxygen to produce an oxygen tailored bimodal polyethylene, wherein during the oxygen tailoring the amount of phosphorous containing compounds present is less than 0.05 wt % based on the combined weight of the polyethylene resin and phosphorous containing compounds; and
cooling the oxygen tailored bimodal polyethylene to produce a bimodal polyethylene composition comprising:
a high molecular weight component having a weight average molecular weight (Mw) of from about 500,000 to about 950,000;
a low molecular weight component having a weight average molecular weight (Mw) of from about 3,000 to about 100,000,
wherein the high molecular weight component is present in an amount ranging from about 25 wt % to about 40 wt % of the bimodal polyethylene composition;
wherein the ratio of the weight average molecular weight of the high molecular weight component to the weight average molecular weight of the low molecular weight component is from about 15 to about 35;
a melt flow ratio (I 21 /I 2 ) of equal to 200; and
a percent die swell of less than about 80%.
2. The method of claim 1 , wherein the substantially melted bimodal polyethylene is heated to a temperature of from about 240° C. to about 260° C.
3. The method of claim 1 , wherein the bimodal polyethylene composition further comprises an elasticity (G′/G″) of about 0.8 or more.
4. The method of claim 1 , further comprising forming the bimodal polyethylene composition into a blow molded article.
5. A method for making a bimodal polyethylene composition, comprising:
polymerizing ethylene and a comonomer in the presence of a mixed catalyst comprising at least one Group-15 containing catalyst in a gas phase polymerization reactor to produce a bimodal polyethylene;
conveying the bimodal polyethylene through a mixer/extruder, wherein the mixer/extruder comprises a melt-conveying zone, and wherein the bimodal polyethylene is substantially melted in the melt-conveying zone; and
contacting the substantially melted polyethylene with a gas mixture comprising from about 8 vol % oxygen to about 20 vol % oxygen within the melt-conveying zone, wherein during the oxygen tailoring the amount of phosphorous containing compounds present is less than 0.05 wt % based on the combined weight of the polyethylene resin and phosphorous containing compounds, to produce a bimodal polyethylene composition comprising:
a high molecular weight component having a weight average molecular weight (Mw) of from about 500,000 to about 950,000;
a low molecular weight component having a weight average molecular weight (Mw) of from about 3,000 to about 100,000, wherein the high molecular weight component is present in an amount ranging from about 25 wt % to about 40 wt % of the bimodal polyethylene composition, and wherein a ratio of the weight average molecular weight of the high molecular weight component to the weight average molecular weight of the low molecular weight component ranges from about 15 to about 35;
a melt flow ratio (I 21 /I 2 ) of greater than or equal to 200; and
a percent die swell of less than about 80%.
6. The method of claim 5 , further comprising pelletizing the bimodal polyethylene composition to produce a pelletized polyethylene product.
7. The method of claim 6 , further comprising forming the pelletized polyethylene product into a blow molded article.
8. The method of claim 5 , wherein the bimodal polyethylene composition further comprises an elasticity (G′/G″) of about 0.8 or more.
9. The method of claim 5 , wherein the bimodal polyethylene composition further comprises a density of from about 0.95 g/cm 3 to about 0.97 g/cm 3 .
10. The method of claim 5 , wherein the bimodal polyethylene composition further comprises a flow index (I 21 ) of from about 20 g/10 min to about 40 g/10 min.