A polyethylene composition for producing blow-molded hollow articles, having the following features: 1) density from greater than 0.957 to 0.965 g/cm.sup.3, determined according to ISO 1183-1 at 23 C.; 2) ratio MIF/MIP from 12 to 25; 3) MIF from 18 to 40 g/10 min.; 4) Mw equal to or greater than 230,000 g/mol; 5) .sub.0.02 from 35,000 to 55,000 Pa.Math.s; 6) long-chain branching index, LCBI, equal to or greater than 0.55; and 7) ratio (.sub.0.02/1000)/LCBI from 55 to 75.

A polyethylene composition for producing blow-molded hollow articles, having the following features: 1) density from greater than 0.957 to 0.965 g/cm.sup.3, determined according to ISO 1183-1 at 23 C.; 2) ratio MIF/MIP from 12 to 25; 3) MIF from 18 to 40 g/10 min.; 4) Mw equal to or greater than 230,000 g/mol; 5) .sub.0.02 from 35,000 to 55,000 Pa.Math.s; 6) long-chain branching index, LCBI, equal to or greater than 0.55; and 7) ratio (.sub.0.02/1000)/LCBI from 55 to 75.

The present invention relates to pellets comprising a polyethylene composition, a pipe or pipe system comprising the pellets as well as a process for the preparation of such a pipe or pipe system.

Ethylene-based polymers having a density of 0.952 to 0.968 g/cm.sup.3, a ratio of HLMI/MI from 185 to 550, an IB parameter from 1.46 to 1.80, a tan at 0.1 sec.sup.1 from 1.05 to 1.75 degrees, and a slope of a plot of viscosity versus shear rate at 100 sec.sup.1 from 0.18 to 0.28 are described, with low melt flow versions having a HLMI from 10 to 30 g/10 min and a Mw from 250,000 to 450,000 g/mol, and high melt flow versions having a HLMI from 30 to 55 g/10 min and a Mw from 200,000 to 300,000 g/mol. These polymers have the processability of chromium-based resins, but with improved stress crack resistance and topload strength for bottles and other blow molded products.

Ethylene-based polymers having a density of 0.952 to 0.968 g/cm.sup.3, a ratio of HLMI/MI from 185 to 550, an IB parameter from 1.46 to 1.80, a tan at 0.1 sec.sup.1 from 1.05 to 1.75 degrees, and a slope of a plot of viscosity versus shear rate at 100 sec.sup.1 from 0.18 to 0.28 are described, with low melt flow versions having a HLMI from 10 to 30 g/10 min and a Mw from 250,000 to 450,000 g/mol, and high melt flow versions having a HLMI from 30 to 55 g/10 min and a Mw from 200,000 to 300,000 g/mol. These polymers have the processability of chromium-based resins, but with improved stress crack resistance and topload strength for bottles and other blow molded products.

Process for preparing an olefin polymer including the step of polymerizing an olefin in the presence of a polymerization catalyst and hydrogen as molecular weight regulator in a gas-phase polymerization reactor to yield growing polymer particles, the reactor including three or more polymerization zones and at least two thereof are sub-zones of a polymerization unit wherein the growing polymer particles flow downward in a densified form and at least one polymerization zone has a ratio of hydrogen to the sum of olefins which is a factor of at least 1.5 lower than the ratio of hydrogen to the sum of olefins in the polymerization zone having the highest ratio of hydrogen to the sum of olefins and a factor of at least 1.5 higher than the ratio of hydrogen to the sum of olefins in the polymerization zone having the lowest ratio of hydrogen to the sum of olefins.

Process for preparing an olefin polymer including the step of polymerizing an olefin in the presence of a polymerization catalyst and hydrogen as molecular weight regulator in a gas-phase polymerization reactor to yield growing polymer particles, the reactor including three or more polymerization zones and at least two thereof are sub-zones of a polymerization unit wherein the growing polymer particles flow downward in a densified form and at least one polymerization zone has a ratio of hydrogen to the sum of olefins which is a factor of at least 1.5 lower than the ratio of hydrogen to the sum of olefins in the polymerization zone having the highest ratio of hydrogen to the sum of olefins and a factor of at least 1.5 higher than the ratio of hydrogen to the sum of olefins in the polymerization zone having the lowest ratio of hydrogen to the sum of olefins.

A polyethylene composition for producing blow-molded hollow articles, having the following features: 1) density from greater than 0.957 to 0.965 g/cm.sup.3, determined according to ISO 1183-1 at 23 C.; 2) ratio MIF/MIP from 12 to 25; 3) MIF from 18 to 40 g/10 min.; 4) Mw equal to or greater than 230,000 g/mol; 5) .sub.0.02 from 35,000 to 55,000 Pa.Math.s; 6) long-chain branching index, LCBI, equal to or greater than 0.55; and 7) ratio (.sub.0.02/1000)/LCBI from 55 to 75.

A polyethylene composition for producing blow-molded hollow articles, having the following features: 1) density from greater than 0.957 to 0.965 g/cm.sup.3, determined according to ISO 1183-1 at 23 C.; 2) ratio MIF/MIP from 12 to 25; 3) MIF from 18 to 40 g/10 min.; 4) Mw equal to or greater than 230,000 g/mol; 5) .sub.0.02 from 35,000 to 55,000 Pa.Math.s; 6) long-chain branching index, LCBI, equal to or greater than 0.55; and 7) ratio (.sub.0.02/1000)/LCBI from 55 to 75.

The present invention relates to pellets comprising a polyethylene composition, a pipe or pipe system comprising the pellets as well as a process for the preparation of such a pipe or pipe system.