Caps and closures

1. A process for preparing an article comprising:

injection molding a bimodal high density polyethylene (HDPE) resin having a molecular weight distribution (MWD), defined by the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), of at least 4.0 and of up to 6.0;

wherein the bimodal HDPE resin comprises two polyethylene fractions A and B, fraction A being substantially free of comonomer and having a lower weight average molecular weight and a higher density than fraction B, each fraction prepared in different reactors of two reactors connected in series in the presence of a metallocene-containing catalyst system;

wherein when fraction A is prepared in the first reactor of the two reactors connected in series, the ratio of the MI 2 of fraction A to the MI 2 of the bimodal HDPE resin is in the range of 1.05 to 2000;

wherein when fraction B is prepared in the first reactor of the two reactors connected in series, the ratio of the HLMI of fraction B to the MI 2 of the bimodal HDPE resin is in the range of 0.06 to 0.95; and

wherein the metallocene is selected from at least one of the following formula (I) or (II):

wherein each R is the same or different and is selected from hydrogen or XR′ v in which X is chosen from carbon, silicon, germanium, oxygen or nitrogen, and each R′ is the same or different and is chosen from hydrogen or a hydrocarbyl of from 1 to 20 carbon atoms and v+1 is the valence of X;

wherein R″ is a structural bridge between the two indenyl or tetrahydrogenated indenyls to impart stereorigidity that comprises a C 1 -C 4 alkylene radical, a dialkyl germanium, silicon or siloxane, or an alkyl phosphine or amine radical;

wherein Q is a hydrocarbyl radical having from 1 to 20 carbon atoms or a halogen; and

wherein M is a transition metal Group 4 of the Periodic Table or vanadium.

2. The process of claim 1 , wherein the metallocene comprises a bridged unsubstituted bis(tetrahydroindenyl).

3. The process of claim 2 , wherein the metallocene is selected from ethylene-bis(tetrahydroindenyl)zirconium dichloride and ethylene-bis(tetrahydroindenyl)zirconium difluoride.

4. The process of claim 1 , wherein the two reactors connected in series are a double loop reactor.

5. The process of claim 4 , wherein the double loop reactor is a liquid full double loop reactor.

6. The process of claim 1 , wherein a melt index MI 2 of the bimodal HDPE resin, measured following the method of standard test ASTM D 1238 at a temperature of 190° C. and under a load of 2.16 kg, is from 0.5 to 50 dg/min, and a density of the bimodal HDPE resin, measured following the method of standard test ASTM 1505 at a temperature of 23° C., is from 0.940 to 0.965 g/cm 3 .

7. The process of claim 1 , wherein the polyethylene fraction A is obtained from the first reactor and the polyethylene fraction B is obtained from the second reactor.

8. The process of to claim 7 , wherein the polyethylene fraction A has a melt index MI 2 , measured following the method of standard test ASTM D 1238 at a temperature of 190° C. and under a load of 2.16 kg, of from 10 to 1000 dg/min, and a density, measured following the method of standard test ASTM 1505 at a temperature of 23° C., of from 0.960 to 0.980 g/cm 3 .

9. The process of claim 1 , wherein the polyethylene fraction B is obtained from the first reactor and the polyethylene fraction A is obtained from the second reactor, and wherein fraction B has an HLMI ranging from 3 to 20 dg/min.

10. The process of claim 1 , wherein a rheological horizontal activation energy of the resin at a frequency of 1 s −1 is more than 45 kJ/mol and at a frequency of 100 s −1 is less than 45 kJ/mol.

11. The process of claim 10 , wherein the rheological horizontal activation energy of the resin at a frequency of 1 s −1 is more than 50 kJ/mol and at a frequency of 100 s −1 is less than 40 kJ/mol.

12. The process of claim 1 , wherein the bimodal HDPE resins has a Bell Environmental Stress Crack Resistance (ESCR) F0 of at least 1500 hours and a volatile organic compound concentration of at most 250 ppm, measured by:

heating the sample of the resin to 150° C.,

adsorbing the thereby released volatile organic compounds from the sample on a trap at −40° C.

followed by flash desorption of the trap at 230° C.

and detecting and determining the amount of desorbed volatile organic compounds using a gas chromatography—flame ionization detector.

13. The process of claim 1 , wherein the article is a cap or a closure.

14. The process of claim 1 , wherein R is selected from hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl or t-butyl.

15. The process of claim 1 , wherein Q is chlorine or fluorine.

16. The process of claim 1 , wherein the bimodal HDPE resin comprises from 40 to 65 wt. % of fraction A and from 35 to 60 wt. % of fraction B.