High-density polyethylene for caps and closures

A cap or closure comprising:

• a polyethylene composition having a density of at least 940 kg/m3 and of at most 970 kg/m3 as determined according to ISO 1183 at 23° C., and a high load melt index HLMI of at least 35 dg/min and of at most 180 dg/min as determined according to ISO 1133, condition G, at 190° C. and 21.6 kg;
• wherein the polyethylene composition comprises a first polyethylene fraction and a second polyethylene fraction;
• wherein the first polyethylene fraction has a density of at least 920 kg/m3 and of at most 945 kg/m3 as determined according to ISO 1183 at 23° C., and a melt index HL275 of at least 3 dg/min and of at most 12 dg/min, wherein the melt index HL275 is determined in the same way as the HLMI except with a die opening having a diameter of 2.75 mm;
• and
• wherein the second polyethylene fraction has a density of at least 960 kg/m3 and of at most 980 kg/m3 determined according to ISO 1183 at 23° C.

A process for the production of a cap or closure, the process comprising:

• (a) polymerizing ethylene in the presence of a Ziegler-Natta polymerization catalyst in a first polymerization reactor to produce a first polyethylene fraction having a density of at least 920 kg/m3 and of at most 945 kg/m3 as determined according to ISO 1183 at 23° C., and a melt index HL275 of at least 3 dg/min and of at most 12 dg/min;
• (b) polymerizing ethylene and at least one comonomer in the presence of a Ziegler-Natta polymerization catalyst in a second polymerization reactor to produce a second polyethylene fraction having a density of at least 960 kg/m3 and of at most 980 kg/m3 as determined according to ISO 1183 at 23° C.;
• (c) blending the first polyethylene fraction and the second polyethylene fraction to obtain a polyethylene composition having a density of at least 940 kg/m3 and of at most 970 kg/m3 as determined according to ISO 1183 at 23° C., and a high load melt index HLMI of at least 35 dg/min and of at most 180 dg/min as determined according to ISO 1133, condition G, at 190° C. and 21.6 kg, and wherein the melt index HL275 is determined in the same way as the HLMI except with a die opening having a diameter of 2.75 mm;
• and
• (d) subsequently injection molding the polyethylene composition to obtain a cap or closure;
• with the provision that the density and melt index of the second polyethylene fraction are chosen such that the density and melt index of the polyethylene composition are as defined.