EA044845B1 - METHOD FOR PRODUCING MULTIMODAL HIGH DENSITY POLYETHYLENE - Google Patents

Description

The present invention relates to a process for producing a polyethylene polymer which is suitable for injection molded or compression molded articles, in particular for the manufacture of caps and closures. The present invention also relates to a polyethylene polymer as such, to an injection molded article containing said polymer, and to the use of said polymer to produce an injection molded article such as a cap or closure. The process of the invention comprises three separate polymerization steps that produce a multimodal high density polyethylene with a specific set of properties enabling the production of molded articles having the preferred attributes in terms of good processing properties and resistance to stress cracking.

State of the art

Injection molding can be used to make a wide variety of products, including those that have relatively complex shapes and a wide range of sizes. Injection molding, for example, is suitable for the manufacture of products used as caps and closures for food and beverage products, such as bottles containing carbonated or still drinks, or for non-food applications, such as containers for cosmetics and pharmaceuticals.

Injection molding is a molding process in which a polymer is melted and then injected into a mold. During the initial injection, high pressure is applied and the polymer melt is compressed. Thus, when injected into a mold, the polymer melt first expands, or relaxes, and fills the mold. The mold, however, has a lower temperature compared to the polymer melt, and thus, as the polymer melt cools, it tends to shrink. To compensate for this effect, back pressure is applied. After this, the polymer melt is further cooled to allow the molded product to be removed from the mold without deformation.

An important property of an injection molded product is its resistance to stress cracking. It should be understood that the injection molded articles of the invention should not be susceptible to brittle fracture and thus should have high resistance to stress cracking. An increase in stress cracking resistance, measured as FNCT (6 MPa, 50°C), however, is generally associated with a decrease in tensile strength, such as tensile modulus. Thus, lower density provides increased ESCR (FNCT) but decreased tensile strength. It should also be understood that injection molded products are preferably rigid. For example, thinner lids require higher rigidity.

The decrease in tensile modulus associated with an increase in FNCT is especially pronounced in the case of HDPE. The present inventors set out to find new HDPEs, developed particularly for the cap and closure market, that have improved resistance to stress cracking while still maintaining good rigidity. However, this problem is complicated by the fact that these improvements must not come at the expense of a deterioration in the processing characteristics of the polymer or the appearance of any manufactured product. Technological characteristics must be maintained or even improved to meet consumer needs. Injection molded products are produced quickly, and any degradation in process performance can increase cycle times and therefore reduce process efficiency.

Multimodal HDPE for caps and closures is typically produced in a two-step process, such as described, for example, in WO 2014/180989 and WO 2017/093390. Before these methods, a prepolymerization step can be carried out.

The present inventors have surprisingly discovered that the use of at least a three-step polymerization process, optionally preceded by a prepolymerization step, results in polymers, particularly HDPE homopolymers, that have an improved balance of processability and mechanical properties such as ESCR and stiffness.

Multi-stage polymerization processes comprising three stages have been described, for example, in WO 2010/054732, WO 2016/198271, WO 2015/086812 and WO 2015/086812. However, the suitability of products manufactured by these processes, in particular for applications requiring injection molding, for example in the field of caps and closures, is not discussed in any of these documents.

Brief description of the invention

According to one aspect, the invention provides a method for producing multimodal high density polyethylene (HDPE) having a melt flow rate (MFR ₂ ) from 0.5 to 1.3 g/10 min, MFR ₅ from 2.5 to 12 g/10 min and resistance to stress cracking (FNCT) (6 MPa, 50°C) for at least 50 hours, including:

- 1 044845 (i) polymerizing ethylene in a first polymerization step in the presence of a Ziegler-Natta catalyst to obtain a first ethylene homopolymer having an MFR2 of 10 to 500 g/10 min;

(ii) polymerizing ethylene in a second polymerization step in the presence of said catalyst and said first ethylene homopolymer to produce a mixture of ethylene homopolymers containing said first ethylene homopolymer and a second ethylene homopolymer, wherein said mixture has an MFR2 of 50 to 1000 g/10 min; and (iii) polymerizing ethylene and at least one alpha-olefin comonomer in a third polymerization step in the presence of said catalyst and said mixture of ethylene homopolymers to produce said multimodal HDPE.

According to another aspect, the invention provides multimodal high density polyethylene (HDPE) having a melt flow rate (MFR2) from 0.5 to 1.3 g/10 min, MFR ₅ from 2.5 to 12 g/10 min and FNCT (6 MPa, 50°C) for at least 50 hours, obtained by a method such as defined above herein.

According to an additional aspect, the invention provides multimodal high density polyethylene (HDPE) having a density of 940 to 970 kg/m ³ and a melt flow rate (MFR2) of 0.5 to 1.3 g/10 min, MFR ₅ of 2.5 up to 12 g/10 min and FNCT (6 MPa, 50°C) for at least 50 h, wherein said HDPE contains a low molecular weight ethylene homopolymer component and a high molecular weight ethylene copolymer component, wherein said HDPE has a Mz/Mn of at least 95 and/or Mz is at least 680,000, and wherein said low molecular weight ethylene homopolymer component is a mixture of ethylene homopolymers containing a first ethylene homopolymer and a second ethylene homopolymer.

According to another aspect, the invention provides an injection molded or compression molded article, preferably a cap or closure, comprising, for example, a multimodal HDPE, as defined herein above, for example.

According to another aspect, the invention provides a cap or closure comprising, for example, consisting of, multimodal HDPE, as defined herein above.

According to a further aspect, the invention provides a method for producing an injection molded or compression molded article, such as a cap or closure, comprising: producing a multimodal HDPE as defined hereinabove;

it is not necessary to granulate HDPE to produce pellets; and injection molding or compression molding of the resulting HDPE pellets to produce said product.

According to another aspect, the invention provides the use of multimodal HDPE, such as defined above herein, to produce an injection molded or compression molded article, such as a cap or closure.

Detailed Description of the Invention

Multimodal HDPE.

It has been found that the high density polyethylene polymer according to the invention provides an improved material for compression molding or, in particular, injection molding, in particular for lid and closure applications, which combines very good mechanical properties, e.g. FNCT and tensile modulus, and excellent processing properties (eg in terms of flowability).

The polymer of the invention is a high density multimodal ethylene polymer and is an ethylene copolymer. By ethylene copolymer is meant a polymer the majority of which by weight is derived from ethylene monomer units (i.e. at least 50 wt.% ethylene relative to the total weight of the copolymer). The comonomer proportion is preferably up to 10 mol%, more preferably up to 5 mol%. However, ideally, in the polymers of the present invention, the comonomers are present in very low amounts, such as 0.1 to 2.0 mol%, for example 0.1 to 1.0 mol%.

The other copolymerizable monomer(s) is preferably C3-12, in particular C3-10, alpha-olefin comonomers, in particular mono- or polyethylenically unsaturated comonomers, in particular C3- 10-alpha-olefins such as propene, but-1-ene, hex1-ene, oct-1-ene and 4-methylpent-1-ene. Particular preference is given to the use of 1-hexene, 1-octene and 1-butene, in particular 1-butene. Ideally, only one comonomer is present.

The polymer according to the invention is multimodal and thus contains at least two components. The polymer according to the invention preferably contains:

(A) a lower molecular weight component of a first ethylene homopolymer, and (B) a higher molecular weight component of a second ethylene copolymer.

In general, the higher molecular weight component will preferably have a Mw that is at least 5000 Da greater than the lower molecular weight component, for example at least 10,000 Da greater.

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HDPE according to the invention is multimodal. Typically, a polyethylene composition containing at least two polyethylene fractions, which are obtained under different polymerization conditions that provide fractions with different (weight average) molecular weights and molecular weight distributions, is called multimodal. Accordingly, in this respect the compositions according to the invention are multimodal polyethylenes. The prefix multi- refers to the number of different polymer fractions that make up the composition.

On the molecular weight distribution curve, i.e. on the graph of the dependence of the mass fraction of the polymer on its molecular weight, for the specified multimodal polymer, two or more maxima are observed or at least there is a noticeable broadening compared to the curves for individual fractions.

For example, if a polymer is produced in a sequential multi-step process using reactors connected in series and using different conditions in each reactor, then each of the polymer fractions produced in the different reactors will have its own molecular weight distribution and weight average molecular weight. When constructing a molecular weight distribution curve for a specified polymer, the individual curves for the specified fractions, when superimposed, form a molecular weight distribution curve for the resulting polymer product as a whole, usually in the form of a curve with two or more pronounced maxima.

HDPEs according to the invention have a melt flow rate (MFR2) from 0.1 to 4.0 g/10 min. Typically, HDPE has an MFR2 of 3.0 g/10 min or less, preferably 2.0 g/10 min or less, preferably 1.3 g/10 min or less, for example 0.9 g/10 min or less. The polymer preferably has an MFR2 of at least 0.01 g/10 min, such as greater than 0.05 g/10 min, preferably at least 0.10 g/10 min, ideally at least 0.3 g/10 min, in particular 0.5 g/10 min or more. Thus, particularly suitable MFR2 values are from 0.5 to 1.5 g/10 min, for example from 0.5 to 1.3 g/10 min.

The polymer of the invention preferably has an MFR21 of 55 to 200 g/10 min, for example 60 to 150 g/10 min, most preferably 65 to 125 g/10 min.

The polymer according to the invention preferably has an MFR ₅ of 0.5 to 20 g/10 min, for example 1.5 to 15 g/10 min, preferably 2.5 to 12 g/10 min.

The polymer of the invention preferably has a melt flow rate ratio (FRR) for MFR21/MFR2 of at least 70.0 or more, for example at least 80.0 or more, more preferably at least 90.0.

The polymer of the invention may also have a melt flow rate ratio MFR ₂₁ /MFR ₅ of at least 18.5 or more, for example at least 20.0 or more, more preferably at least 21.0 or more.

The density of the polymer is preferably 940 kg/m ³ or more. The polymers according to the invention are therefore high density polyethylenes, HDPE. More preferably, the polymer has a density of 950 kg/m ³ or more, for example 954 kg/m ³ or more.

Moreover, the density of the polymer is preferably 970 kg/m ³ or less, and more preferably 965 kg/m ³ or less. The typical density range is from 940 to 970 kg/m ³ , ideally from 950 to 960 kg/m ³ .

Preferably, the HDPE polymer has a tensile modulus of at least 850 MPa, more preferably at least 900 MPa. Typically, HDPE has a tensile modulus of less than 970 MPa, for example less than 950 MPa.

The polymer preferably has an environmental stress cracking resistance, measured as FNCT (6 MPa, 50°C), 30 hours or more, for example 50 hours or more, more preferably 60 hours or more, more preferably 80 hours or more . Although the FNCT may ideally be unlimited, a typical upper limit may be 200 hours, such as 150 hours. The polymer will particularly preferably have specified high FNCT and MFR ₂ values of 0.5 to 1.3 g/10 min, preferably MFR ₂ of 0.5 to 1.0 g/10 min. The polymers according to the invention have improved FNCT compared to existing multimodal mixtures and at the same time have improved processing characteristics.

The polyethylene polymer preferably has a molecular weight distribution Mw/Mn, which is the ratio of the weight average molecular weight Mw to the number average molecular weight Mn, of 10 or more, for example 15 or more, more preferably 18 or more.

The polymer preferably has a Mw/Mn of 30 or less, more preferably 25 or less, for example 22 or less.

The weight average molecular weight Mw of the polymer is preferably at least 80 kDa, more preferably at least 100 kDa, and most preferably at least 115 kDa. Moreover, the Mw of the composition is preferably not more than 300 kDa, more preferably not more than 230 kDa, for example not more than 180 kDa.

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The Mz/Mn ratio is preferably at least 80, more preferably at least 90, in particular at least 100.

The Mz/Mn ratio is preferably up to 200, more preferably 180, in particular up to 150.

Accordingly, the Mz/Mn ratio may be in the range of 80 to 200, for example in the range of 90 to 180, more preferably in the range of 100 to 150.

The actual value of Mz is preferably in the range from 600 to 900 kDa, for example from 670 to 850 kDa, for example from 680 to 800 kDa.

In one preferred embodiment, the MFR ₂ of the polymer composition is less than 1.30 g/10 min, and the Mz is greater than 670 kDa, preferably greater than 690 kDa, in particular greater than 700 kDa.

In addition, the compositions of the present invention should preferably have a shear viscosity reduction ratio (SHI 2.7/210, which is defined as the ratio of the shear viscosity of 2.7 kPa and 210 kPa) of at least 25, preferably at least 30, more preferably at least 35. The shear thinning factor is a measure of the processing properties of the material. In general, the higher the SHI value, the better the fluidity, thereby improving the processing characteristics.

As noted above, the polymers of the invention preferably contain at least a lower molecular weight component (A) and a higher molecular weight component (B). In one particularly preferred embodiment, the HDPE is composed of components (A) and (B). The weight ratio of fraction (A) to fraction (B) in the composition is generally in the range of 30:70 to 70:30, more preferably 35:65 to 65:35, most preferably 40:60 to 60:40 . In some embodiments, the ratio may be 45 to 55 wt% fraction (A) and 55 to 45 wt% fraction (B), such as 50 wt% fraction (A) and 50 wt% fraction (B) .

Fraction (A) is an ethylene homopolymer component, and fraction (B) is an ethylene copolymer component. By ethylene homopolymer is meant a polymer containing at least 97% by weight (eg at least 98% by weight, in particular at least 99.5% by weight) of ethylene monomer units. Typically, fraction (B) consists of a single ethylene copolymer. Fraction (A) can be considered as a mixture of ethylene homopolymers containing (eg, consisting of) a first ethylene homopolymer and a second ethylene homopolymer. Fraction (A) can be unimodal or multimodal.

HDPE according to the invention is produced in a multi-stage process, in which fractions (A) and (B) are obtained in successive stages. In this case, the properties of the fractions obtained in the second stage (or additional stages) of a multi-stage process can be determined by the polymers that are obtained separately in one stage using polymerization conditions (for example, identical temperature, partial pressure of reagents/diluents, suspension medium, reaction time) , identical to the stage of the multi-stage method in which this fraction is obtained, and using a catalyst on which the previously obtained polymer is not deposited. Alternatively, the properties of fractions obtained at a later stage of a multi-stage process can also be calculated, for example, in accordance with the work of B. Hagstrom, Conference on Polymer Processing (The Polymer Processing Society), Extended Abstracts and Final Program, Gothenburg, August 19 to 21, 1997, 4:13.

Thus, although the properties of the products of a multi-step process cannot be measured directly, the properties of the fractions obtained in later stages of the multi-step process can be determined by any one or both of the above methods. A specialist will be able to choose the appropriate method.

HDPE polymers produced in a multi-step process are also called in situ blends. The resulting final product consists of a homogeneous mixture of polymers from two or more reactors, the different molecular weight distribution curves for said polymers being superimposed to form a molecular weight distribution curve having a broad maximum or two or more maximums, i.e. the final product is a multimodal polymer blend.

The lower molecular weight fraction (A) preferably has an MFR ₂ of 10 g/10 min or more, more preferably 50 g/10 min or more, and most preferably 100 g/10 min or more.

Moreover, fraction (A) preferably has an MFR2 of 500 g/10 min or less, preferably 450 g/10 min or less, and most preferably 400 g/10 min or less.

Preferably, fraction (A) is an ethylene homopolymer with a density of at least 965 kg/m ³ .

Preferably, fraction (B) is an ethylene copolymer with a density of 920 to 960 kg/m ³ , for example 930 to 940 kg/m ³ .

Fraction (B) is a copolymer. Preferred ethylene copolymers use alpha olefins (eg, C3-12 alpha olefins) as comonomers. Examples of suitable al-pha-olefins include but-1-ene, hex-1-ene and oct-1-ene. But-1-ene is a particularly preferred comonomer.

Polymerization method

The multimodal HDPE according to the invention is produced by a multi-stage polymerization process comprising at least three polymerization stages. Briefly, the method includes:

(i) polymerizing ethylene in a first polymerization step in the presence of a Ziegler-Natta catalyst to produce a first ethylene homopolymer having an MFR2 of 10 to 500 g/10 min;

(ii) polymerizing ethylene in a second polymerization step in the presence of said catalyst and said first ethylene homopolymer to produce a mixture of ethylene homopolymers containing said first ethylene homopolymer and a second ethylene homopolymer, wherein said mixture has an MFR2 of 50 to 1000 g/10 min; and (iii) polymerizing ethylene and at least one alpha-olefin comonomer in a third polymerization step in the presence of said catalyst and said mixture of ethylene homopolymers to produce said multimodal HDPE.

First and second stages of polymerization.

The first polymerization stage produces a first ethylene homopolymer, which is then fed to the second polymerization stage. In the second polymerization step, a second ethylene homopolymer is produced, thereby forming a mixture of ethylene homopolymers, which is then fed to the third polymerization step.

The first and/or second polymerization stages are preferably suspension polymerization stages. More preferably, both the first and second polymerization stages are suspension polymerization stages.

Suspension polymerization is usually carried out in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes, etc., or mixtures thereof. Preferably, the diluent is a low boiling hydrocarbon containing from 1 to 4 carbon atoms, or a mixture of these hydrocarbons. A particularly preferred diluent is propane, possibly containing small amounts of methane, ethane and/or butane.

The ethylene content in the liquid phase of the suspension can be from 1 to 50 mol%, preferably from 2 to 20 mol% and in particular from 2 to 10 mol%. The advantage of a high ethylene concentration is that it increases catalyst performance, but the disadvantage is that more ethylene must be reused than at a lower concentration.

The temperature in each of the first and second polymerization stages is generally from 60 to 100°C, preferably from 70 to 90°C. The temperature should not be allowed to rise excessively to prevent partial dissolution of the polymer in the diluent and clogging of the reactor. The pressure is from 1 to 150 bar (0.1 to 15 MPa), preferably from 40 to 80 bar (4 to 8 MPa).

Suspension polymerization can be carried out in any known reactor used for suspension polymerization. Said reactors include a continuous stirred reactor and a loop reactor. The polymerization is particularly preferably carried out in a loop reactor. In these reactors, the suspension is moved at high speed through a closed pipe using a circulation pump. Loop reactors are generally known in the art and examples are given, for example, in US-A-4582816, US-A-3405109, US-A-3324093, EP-A-479186 and US-A5391654. Thus, the first and second polymerization stages are preferably carried out as slurry polymerization in two loop reactors connected in series.

The slurry can be removed from each reactor either continuously or periodically. The preferred method of batch removal is the use of settling tanks in which the slurry is allowed to concentrate before removing a batch of the concentrated slurry from the reactor. The use of settling tanks is described, among other things, in US-A-3374211, US-A-3242150 and EP-A1310295. Continuous removal is described, inter alia, in EP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. Continuous removal is preferably used in combination with a suitable concentration method, as described in EP-A-1310295 and EP-A-1591460. Preferably, the suspension in each of the first and second polymerization stages is removed continuously.

In the first and second polymerization stages, hydrogen is typically introduced to control the MFR ₂ of the first and second ethylene homopolymers. The amount of hydrogen required to achieve the desired MFR depends on the catalyst used and the polymerization conditions. A polymer with the desired properties can be obtained by slurry polymerization in a loop reactor at a molar ratio of hydrogen to ethylene from 100 to 1000 mol/kmol (or mol/1000 mol) and preferably from 200 to 800 mol/kmol.

The average residence time in the reactor for each of the first and second polymerization stages is typically from 20 to 120 minutes, preferably from 30 to 80 minutes. It is well known in the art that the average residence time t in the reactor can be calculated using Equation 1 below:

Equation 1. Reactor residence time ^T Q _a where V _R is the volume of the reaction space (in the case of a loop reactor, the volume of the reactor; in the case of a fluidized bed reactor, the volume of the fluidized bed), and Qo is the volumetric flow rate of the product (including polymer product and liquid reaction mixture).

The productivity is adjusted accordingly using the catalyst feed rate. Performance can also be influenced by selecting the appropriate monomer concentration. The target monomer concentration, in turn, can be achieved by properly controlling the ethylene feed rate.

According to the present invention, the polymer, monomer and catalyst particles in the first and second polymerization stages should preferably have a narrow residence time distribution in the reactor. This has been confirmed to provide advantages in terms of particle uniformity, namely more uniform catalyst activity in producing the ethylene copolymer in the subsequent third polymerization step, resulting in a more uniform distribution of the ethylene copolymer within/around said particles, more predictable comonomer incorporation and reducing the amount of easily extracted low molecular weight fractions.

Without being bound by any theory, the inventors believe that a certain minimum residence time in the reactor during the first and second stages of polymerization affects the activity of the catalyst, as a result of which the density and flow rate of the final polymer product melt can be finely tuned.

Thus, the inventors of the present invention have discovered a method for producing a more homogeneous polymer fraction by dividing the production process into two successive polymerization steps. The specified separate production mode ensures a more uniform residence time of particles in the reactor before entering the third stage of polymerization, and therefore more uniform properties of the particles obtained at it in terms of viscosity and density. These two properties, namely the combination of viscosity and density, have a decisive influence on the final properties of the final multimodal HDPE and any products made from it.

The first and second ethylene homopolymers may be unimodal in terms of their molecular weight and/or density, or they may be bimodal in terms of their molecular weight and/or density. However, they are preferably unimodal in terms of their molecular weight and density. Thus, the mixture of ethylene homopolymers is preferably a unimodal mixture. It will be apparent to one skilled in the art that when producing the first and second ethylene homopolymers in two reactors connected in series, the MFR2 and density values of each fraction may differ slightly, but are still considered unimodal.

The ratio of the amounts used in the first and second polymerization stages may be in the range from 30:70 to 70:30, preferably from 40:60 to 60:40, for example from 45:55 to 55:45, for example 50: 50.

Third stage of polymerization.

In the third polymerization step, ethylene is co-polymerized with at least one alpha-olefin comonomer in the presence of a catalyst and a mixture of ethylene homopolymers. Thus, it will be understood that the third polymerization step produces an ethylene copolymer which, when combined with a mixture of ethylene homopolymers, forms the HDPE of the invention. Preferred comonomers are discussed above, however, it should be noted that it is especially preferred that at least one alpha-olefin is butene.

The third polymerization step is preferably a gas phase polymerization step, i.e. carried out in a gas-phase reactor. Any suitable gas phase reactor known in the art, such as a gas phase fluidized bed reactor, can be used.

In gas-phase reactors, the temperature of the reaction mixture is generally in the range from 60 to 115°C (for example, from 70 to 110°C), the pressure in the reactor is generally in the range from 10 to 25 bar (from 1 .0 to 2.5 MPa), and the residence time in the reactor, in general, ranges from 1 to 8 hours. The gas is traditionally a non-reactive gas such as nitrogen or low boiling hydrocarbons such as propane together with a monomer (eg ethylene).

A chain transfer agent (eg hydrogen) is typically added in the third polymerization step, preferably in amounts of 50 to 500 mol H ₂ /kmol ethylene.

The ratio of the amounts used in the third polymerization stage and in the first and second polymerization stages in total (i.e. between gas phase polymerization and suspension polymerization) may be in the range from 30:70 to 70:30, preferably from 40:60 to 60:40, for example - 6 044845 for example, from 45:55 to 55:45, for example 50:50.

Within the scope of the invention, the density of the ethylene copolymer should preferably be lower than the density of the mixture of ethylene homopolymers.

Prepolymerization.

Before the polymerization steps discussed above, a prepolymerization step can be performed. The purpose of prepolymerization is to polymerize a small amount of polymer on a catalyst at low temperature and/or low monomer concentration. Prepolymerization makes it possible to improve the performance of the catalyst in suspension and/or modify the properties of the final polymer. The prepolymerization step is carried out in suspension.

Thus, the prepolymerization step can be carried out in a loop reactor. Thus, the prepolymerization is preferably carried out in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes, etc., or mixtures thereof. Preferably, the diluent is a low boiling hydrocarbon containing from 1 to 4 carbon atoms, or a mixture of these hydrocarbons.

The temperature in the prepolymerization step is generally from 0 to 90°C, preferably from 20 to 80°C, and more preferably from 55 to 75°C.

Pressure is not critical and is typically between 1 and 150 bar (0.1 and 15 MPa), preferably between 40 and 80 bar (4 and 8 MPa).

The amount of monomer is typically such that from 0.1 to 1000 grams of monomer per gram of solid catalyst component is polymerized in the prepolymerization step. As is known to those skilled in the art, not all catalyst particles released from a continuous prepolymerization reactor contain the same amount of prepolymer. On the contrary, each particle contains an amount characteristic only of it, which depends on the residence time of the specified particle in the prepolymerization reactor. Given that some particles remain in the reactor for a relatively long period of time, and some for a relatively short period, the amounts of prepolymer in different particles differ, and some individual particles may contain prepolymer in amounts that are outside the specified amounts. above the limits. However, the average amount of prepolymer on the catalyst is generally within the above limits.

The molecular weight of the prepolymer can be adjusted using hydrogen, as is known in the art. In addition, antistatic additives can be used to prevent particles from sticking to each other or to the reactor walls, as described in WO-A-96/19503 and WO-A96/32420.

All catalyst components are preferably introduced into the prepolymerization step if a prepolymerization step is carried out. However, if the solid catalyst component and the cocatalyst can be fed separately, it is conceivable that only a portion of the cocatalyst may be introduced in the prepolymerization step, with the remainder added in subsequent polymerization steps. In addition, in these cases, at the prepolymerization stage, it is necessary to introduce such an amount of cocatalyst that allows the polymerization reaction to be properly carried out.

It should be understood that, within the scope of the invention, the amount of polymer produced during prepolymerization is typically 1-5 wt.% relative to the final multimodal HDPE.

Catalyst.

Polymerization is carried out in the presence of an olefin polymerization catalyst. The catalyst may be any catalyst that can produce the desired ethylene polymer. Suitable catalysts include, but are not limited to, transition metal Ziegler-Natta catalysts such as titanium, zirconium and/or vanadium catalysts. Ziegler-Natta catalysts are effective because they produce polymers with a wide range of molecular weights at high productivity.

Ziegler-Natta catalysts are preferred within the scope of the invention.

Suitable Ziegler-Natta catalysts preferably contain a magnesium compound, an aluminum compound and a titanium compound, optionally on a particulate support.

The particulate support may be an inorganic oxide support such as silica, alumina, titanium dioxide, silica-alumina, and silica-titania. Preferably the support is silica.

The average particle size of the silica substrate can typically range from 10 to 100 microns. However, it has been shown that specific advantages can be obtained if the substrate has a median particle size of 6 to 40 μm, preferably 6 to 30 μm.

A magnesium compound is a product of the reaction of dialkyl magnesium with alcohol. The alcohol is a linear or branched aliphatic monohydric alcohol. Preferably, the alcohol contains from 6 to 16 carbon atoms. Branched alcohols are particularly preferred, and 2-ethyl-1-hexanol is one example of preferred alcohols. Dialkylmagnesium may be any compound of magnesium bonded to two alkyl groups, which may be the same or different. Butyl octyl magnesium is one of the preferred examples of dialkyl magnesium.

The aluminum compound is a chlorinated aluminum alkyl. Particularly preferred compounds are alkyl aluminum dichlorides and alkyl aluminum sesquichlorides.

The titanium compound is a halogen-containing titanium compound, preferably a chlorine-containing titanium compound. A particularly preferred titanium compound is titanium tetrachloride.

The catalyst can be prepared by sequentially bringing the support into contact with the above compounds, as described in EP-A-688794 or WO-A-99/51646. Alternatively, it can be prepared by first preparing a solution from the components and then contacting the solution with a carrier as described in WO-A-01/55230.

Another group of suitable Ziegler-Natta catalysts contains a titanium compound together with a magnesium halide compound acting as a support. Thus, the catalyst contains a titanium compound supported by a magnesium dihalide, such as magnesium dichloride. Said catalysts are described, for example, in WO-A-2005/118655 and EP-A-810235.

An additional type of Ziegler-Natta catalysts are catalysts obtained by a process in which an emulsion is obtained in which the active components form a dispersed, i.e. discontinuous phase, in an emulsion of at least two liquid phases. The dispersed phase in the form of droplets solidifies in the emulsion, thereby forming a catalyst in the form of solid particles. The basis for the preparation of these types of catalysts is given in the publication WO-A-2003/106510 by Borealis.

The Ziegler-Natta catalyst is usually used in conjunction with an activator. Suitable activators are alkyl metal compounds and in particular alkyl aluminum compounds. These compounds include alkyl aluminum halides such as ethylaluminum dichloride, diethylaluminum chloride, ethylaluminum sesquichloride, dimethylaluminum chloride, etc. They also include trialkylaluminum compounds such as trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum and tri-n-octylaluminum. Additionally, they include alkylaluminum oxy compounds such as methylaluminum oxane (MAO), hexyisobutylaluminum oxane (HIBAO), and tetraisobutylaluminum oxane (TIBAO). Other alkylaluminum compounds such as isoprenylaluminum may also be used. Particularly preferred activators are trialkylaluminum compounds, of which triethylaluminum, trimethylaluminum and triisobutylaluminum are particularly suitable.

The amount of activator used depends on the specific catalyst and activator. Typically, triethylaluminum is used in such an amount that the mole ratio of aluminum to transition metal, eg Al/Ti, is from 1 to 1000, preferably from 3 to 100 and in particular from about 5 to about 30 mol/mol.

When preparing the multimodal HDPE according to the present invention, a blending step is preferably used, wherein the base resin composition, i.e. the mixture, which is typically prepared as a powder base resin in a reactor, is extruded using an extruder and then granulated to form polymer pellets in a manner known in the art.

The polyethylene composition may also contain minor amounts of additives such as pigments, nucleating agents, antistatic agents, fillers, antioxidants, etc., generally in amounts up to 10% by weight, preferably up to 5% by weight.

Additives or other polymer components may optionally be added to the composition at the blending stage in amounts such as those described above. Preferably, the composition according to the invention obtained in the reactor is mixed in an extruder together with additives in a manner known in the art.

The multimodal HDPE of the invention can also be combined with other polymer components, such as other polymers of the invention, with other HDPE, or with other polymers, such as LLDPE or LDPE. However, articles of the invention, such as caps and closures, preferably contain at least 75% by weight, eg at least 90% by weight, of the polymer of the invention, eg at least 95% by weight. In one embodiment, the articles consist essentially of a polymer according to the invention. The term consists essentially of meaning that the polymer of the invention is the only non-additive polyolefin present. However, it should be understood that the polymer may contain standard polymer additives, some of which may be contained on a polyolefin support (a so-called masterbatch, well known in the art). The term consists essentially of does not exclude the presence of the specified additive on the substrate.

Applications.

Furthermore, the present invention relates to an injection molded or compression molded article, preferably a cap or closure containing HDPE such as described above, and the use of said polyethylene composition to produce the article.

- 8 044845 manufactured by injection molding or compression molding, preferably a lid or closure. Preferably, the products are manufactured by injection molding.

Injection molding of the composition described above herein can be performed using any conventional injection molding equipment. A typical injection molding process can be carried out at temperatures ranging from 190 to 275°C.

Preferably, the composition according to the invention is used for the manufacture of caps or closures.

The caps and closures according to the invention can be of a conventional size and are thus intended for bottles, etc. They have an outer diameter of approximately 2 to 8 cm (measured by the hard top surface of the cap) depending on the bottle and are provided with threads. The height of the cover can be from 0.8 to 3 cm.

Caps and closures may be provided in conjunction with tear strips from which the cap is separated upon first opening, as is well known in the art. Covers may also be provided in conjunction with gaskets.

Thus, according to another aspect, the invention provides a cap or closure, such as defined above, that is screwed onto a beverage container containing the beverage, which exerts internal pressure on the cap. Thus, preferably the invention relates to a cap according to the invention that is screwable onto a beverage container containing a carbonated beverage. The beverage container itself can be manufactured using traditional methods, for example using PET.

It should be understood that any parameter stated above is measured according to the tests detailed below. For any parameter, if narrower and more general implementation options are described, then these implementation options are described together with narrower and more general implementation options for other parameters.

The present invention will now be described using the following non-limiting examples.

Test methods.

Melt flow rate.

Melt flow rate (MFR) was determined according to ISO 1133 and reported in g/10 min. MFR is a measure of the melt viscosity of a polymer. For PE, MFR was determined at 190°C. The load at which the melt flow rate is determined is usually indicated as a subscript, for example MFR ₂ is measured at a load of 2.16 kg (condition D), MFR ₅ is measured at a load of 5 kg (condition T), or MFR21 is measured at a load of 21. 6 kg (condition G).

FRR was defined as the ratio of melt flow rates at different loads.

FRR 21/5 is the ratio of MFR21 to MFR ₅ .

FRR 21/2 is the ratio of MFR21 to MFR2.

Density.

The polymer density was measured according to ISO 1183/1872-2B.

For purposes of the present invention, the density of the mixture can be calculated from the densities of the components in accordance with the equation:

where p _b represents the density of the mixture, wi represents the mass fraction of component i in the mixture, and pi represents the density of component i.

Quantification of microstructure by NMR spectroscopy.

Quantitative nuclear magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of polymers.

Quantitative ¹³ C{1H} NMR spectra were obtained in the melt on a Bruker Advance III 500 NMR spectrometer at 500.13 and 125.76 MHz for 1H and ¹³ C, respectively. All spectra were acquired using a 7 mm ¹³ C optimized probe head with magic angle spin (MAS) at 150°C using nitrogen gas in all pneumatic assemblies. Approximately 200 mg of the substance was placed in a zirconia MAS rotor with an outer diameter of 7 mm and rotated at 4 kHz. We used standard single-pulse excitation in the non-stationary nuclear power mode with a short recovery time of 3 s {pollard04, klimke06} using an RS-HEPT decoupling circuit {fillip05, griffin07}. In total, 1024 (1k) transitions were recorded for one spectrum. This set of parameters was chosen due to its high sensitivity to low comonomer content.

Quantitative ¹³ C{1H} NMR spectra were processed, integrated and quantitative characteristics were determined using special programs for automating spectral analysis. All chemical shifts were internally referenced to the main methylene signal (δ+) at 30.00 ppm {randall89}.

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The characteristic signals corresponding to the incorporation of 1-butene (randall89) were observed and the content of all other monomers present in the polymer was calculated.

Characteristic signals corresponding to the isolated incorporation of 1-butene were observed, i.e. EEEE comonomer sequences. The level of isolated 1-butene incorporation was quantified by integrating the signal at 39.84 ppm, which corresponded to the *B2 sites, taking into account the number of indicated sites per comonomer:

B = 1*B2

Since other signals corresponding to other comonomer sequences, i.e. sequential insertion of the comonomer were absent, then the total content of 1-butene comonomer was calculated solely from the number of sequences with isolated 1-butene:

In general V

The relative ethylene content was quantified by integrating the main methylene signal (δ+) at 30.00 ppm:

Ε =(1/2)*Ιδ+

The total ethylene comonomer content was calculated from the main methylene signal, taking into account ethylene units present in other observed comonomer sequences or end groups:

E _O total= E + (5/2)*B

The total mole fraction of 1-butene in the polymer was calculated as follows:

fB ( Total / ( Total ”6 Total )

The total level of incorporation of 1-butene comonomer in mole percent was calculated from the mole fraction using the standard method:

B [mol%] = 100 * fB

The total level of incorporation of 1-butene comonomer in mass percent was calculated from the mole fraction in the standard way:

B [mass%] = 100 * (fB * 56.11)/((fB * 56.11) + (fH* 84.16) + ((l-(fB + fH)) * 28.05))

A detailed description of these methods can be found in Klimke et al. Macromol. Chem. Phys. 2006;207:382, Pollard et al. Macromolecules 2004;37:813, Filip et al. J. Mag. Resn. 2005, 176, 239, Griffin et al. Mag. Res. in Chem. 2007 45, S1, S198, and J. Randall Rev. Macromol. Chem. Phys. 1989, C29, 201.

Molecular mass.

Average molecular weights, molecular weight distribution (Mn, Mw, Mz, MMP)

The average molecular weights (Mz, Mw and Mn), the molecular weight distribution (MMP) and its width, described by the polydispersity coefficient, PDI= Mw/Mn (where Mn is the number average molecular weight and Mw is the mass average molecular weight), were determined by gel permeation chromatography (GPC) according to ISO 16014-1:2003, ISO 16014-2:2003, ISO 16014-4:2003 and ASTM D 6474-12 using the following formulas:

Μ = ^Σ ^= ^1Λί - (1) ^p Σ^^/ΜΟ

Μ = χMj)

For a constant elution volume, the interval ΔV _i , where Ai and Mi represent the chromatographic peak fragment area and polyolefin molecular weight (MM), respectively, is related to the elution volume, Vi, where N corresponds to the number of experimental points obtained in the chromatogram between the integration limits.

A high temperature GPC device was used, equipped with either an infrared (IR) detector (IR4 or IR5 from PolymerChar (Valencia, Spain) or a differential refractometer (RI) from Agilent Technologies, and 3 x Agilent-PLgel Olexis and 1x Agilent-PLgel Olexis Guard columns 1,2,4-trichlorobenzene (TCB), stabilized with 250 mg/l 2,6-di-tert-butyl-4-methylphenol, was used as a solvent and mobile phase. The chromatographic system was operated at 160°C at a constant flow rate of 1 mL/min. For analysis, 200 μl of sample solution was injected. Data acquisition was performed using either Agilent Cirrus software version 3.3 or PolymerChar GPC-IR control software.

A set of columns were calibrated by universal calibration (according to ISO 16014-2:2003) using 19 narrow MMD polystyrene (PS) standards ranging from 0.5 kg/mol to 11500 kg/mol. PS standards were dissolved at room temperature for several hours. The molecular weight of the polystyrene peak was converted to the molecular weight of the polyolefin using the Mark-Kuhn-Houwink equation and the following Mark-Kuhn-Houwink constants:

Kps = 19 x 10' ³ ml/g, aps = 0.655

Kpe = 39 x 10' ³ ml/g, “pe = 0.725

Cpp = 19 x 10' ³ ml/g, app = 0.725

To substitute the calibration data, a third-order polynomial transformation was used.

All samples were obtained at concentrations in the range of 0.5-1 mg/ml and dissolved at 160°C for 2.5 hours in the case of PP or 3 hours in the case of PE with continuous gentle shaking.

Resistance to environmental stress cracking

Environmental Stress Cracking Resistance (ESCR) can be measured by Full Notch Creep Test (FNCT) according to ISO/DIS 16770 at 50°C with a notch depth of 1 mm and sample dimensions of 6 mm x 6 mm x 90 mm . The solvent used was 2 wt.% Arcopal N110 in deionized water. We used samples made by pressing (ISO 1872-2), cooling rate during pressing: 15 K/min. Time to failure ( _tf ) was measured at 4 different stress levels (σ) in the range of 5-7 MPa. We carried out a linear approximation of the graph of log(t _f ) versus log(σ) and obtained an equation of the form log(t _f ) = A log(σ)+B. The FNCT value at 6 MPa stress was then calculated by linear interpolation using the equation and reported as FNCT (6 MPa, 50°C).

Tensile modulus

The tensile properties were measured on injection molded specimens according to ISO 527-2, specimen type multipurpose bar 1A, thickness 4 mm. The elastic modulus was measured at a speed of 1 mm/min. Sample preparation was carried out according to ISO 1872-2.

Rheological measurements.

Definition of SHI (2.7/210).

The rheological properties of the polymers were determined using an Anton Paar MCR 501 rheometer. Measurements were carried out using a disk from a polymer sample made by pressing. The disk was approximately 2 m thick and 26 mm in diameter. The disk was placed in the device and heated at a set temperature of 190°C in a nitrogen atmosphere. The molten polymer sample was then held at 190°C for 5 minutes to ensure uniform melting. A frequency sweep was carried out from 628 rad/s to 0.01 rad/s. The measurements were carried out in logarithmic mode, using 5 measuring points per frequency decade. The rheometer had a slab-to-plate geometry of 25 mm diameter, 1.3 mm gap (1.4 mm sample removal position), and strain was within the linear viscoelastic range, typically <10%. As a result of measurements, the values of the dynamic modulus of elasticity (G') and loss modulus (G), as well as the dependence of the absolute value of complex viscosity (η*) on frequency (ω) or the absolute value of the complex modulus of elasticity (G*) are obtained.

η*=1(θ' ² +θ ² )/ω gM(G' ² +G' ² )

According to the Cox-Mertz rule, the complex viscosity function, η*(ω), is the same as the traditional viscosity function (viscosity versus shear rate) if the frequency is expressed in rad/s. If the specified empirical equation is correct, then the absolute value of the complex modulus of elasticity corresponds to the shear stress in the traditional measurement of viscosity (that is, at rest). This means that the function η* (G*) coincides with the dependence of viscosity on shear stress.

In the proposed method, low shear viscosity, or η* at low G* (which is used as an approximation of so-called zero viscosity), is used as a measure of average molecular weight. On the other hand, a decrease in shear viscosity, i.e. the decrease in viscosity with decreasing G* becomes more pronounced with a wider molecular weight distribution. This characteristic can be approximated by determining the so-called shear thinning factor, SHI, which is the ratio of the viscosities at two different shear stresses. In the examples below, shear moduli (or G*) of 2.7 kPa and 210 kPa were used. Thus:

SHl2.7/210 - η*2.7/η*210 where η* ₂ , ₇ represents the complex viscosity at G* = 2.7 kPa, and P* ₂₁₀ represents the complex viscosity at G* = 210 kPa. Determination of SHI (5/300).

- 11 044845

SHI ₅ /3oo = η*5/η*3θο η* ₅ is the complex viscosity at G* = 5.0 kPa, and η* ₃₀₀ is the complex viscosity at G* = 300 kPa.

Since G* at 300 kPa is outside the experimental range, extrapolation of G* to 300 kPa is required. It is carried out using Rheoplus software developed by Anton Paar. Logarithmic interpolation is carried out based on the following equation:

y is the complex viscosity η* in Pa s, and x is the complex shear modulus G* in Pa.

To calculate η* ₃₀₀ , G* is extrapolated to 300 kPa at two angular frequency points at 0.0100 and 0.0158 rad/s.

yi and yii represent two values of η*, and xi and xii represent values of G* for two angular frequency points of 0.0100 and 0.0158 rad/s. The values of yi, yii, xi and xii are available experimentally. To limit the extrapolation error, the calculated value is limited to 10% of the G* value for the last experimental point.

Spiral flow test.

The spiral flow test was carried out on an Engel ES330/65 cc90 injection molding machine and a spiral mold at 1000 bar (100 MPa);

screw diameter: 35mm;

max. piston working volume: 150 cm ^3; tool shape: oval; provided by Axxicon; thickness 2 mm, width: 5 mm;

temperature in the pre-treatment chamber and matrix: 220°C;

temperature in zone 2/zone 3/zone 4/zone 5: 220°C/230°C/225°C/200°C;

injection cycle: injection time including dwell time: 15 s;

cooling time: 15 s;

injection pressure: depends on the specified length of the material being tested;

holding pressure = injection pressure;

screw rotation speed: 30 rpm;

system pressure: 160 bar (16 MPa);

method of administration: the stroke of the metering piston must be set so that the screw stops 20 mm before the end position upon completion of pressurization during holding;

instrument temperature: 40°C.

The flow length in the spiral form can be determined immediately after the injection operation.

Experimental part.

Preparation of the catalyst.

Obtaining the complex.

87 kg of toluene was added to the reactor. 45.5 kg Bomag A in heptane was then added to the reactor. Then 161 kg of 99.8% 2-ethyl-1-hexanol was introduced into the reactor at a flow rate of 24-40 kg/h. The molar ratio of BOMAG-A to 2-ethyl-1-hexanol was 1:1.83.

Preparation of the solid catalyst component.

275 kg of silica (ES747JR manufactured by Crossfield, having an average particle size of 20 mm) activated at 600°C in a nitrogen atmosphere was placed in the reactor to produce the catalyst. 411 kg of 20% EADC (2.0 mmol/g silica) diluted in 555 L of pentane were then added to the reactor at ambient temperature over one hour. Then the temperature was raised to 35°C and the treated silica was stirred for one hour. The silica was dried at 50°C for 8.5 hours. Then 655 kg of the complex prepared as described above (2 mmol Mg/g silica) was added at 23°C for ten minutes. 86 kg of pentane was added to the reactor at 22°C for ten minutes. The suspension was stirred for 8 hours at 50°C. Finally, 52 kg TiCl ₄ was added over 0.5 h at 45°C. The suspension was stirred at 40°C for five hours. The catalyst was then dried by purging with nitrogen.

Preparation of multimodal HDPE according to the invention (IE1 - IE3).

Three examples according to the invention (IE1 to IE3) were prepared by a three-stage polymerization method under the conditions indicated in table. 1, using a catalyst such as defined above. The properties of different fractions and the final HDPE are also given in Table. 1. Melt flow data are given in table. 3. Data on molecular weight and comonomer content are shown in the table. 4 and 5. Other properties of the resulting HDPE are given in table. 6-8.

- 12 044845

Table 1

Data on obtaining the proposed HDPE

IE1 IE2 IE3 Loop reactor A21 Pace. (°C) 95 95 95 H2/C2 ratio (mol/kmol) 549.1 566.2 601.6 Rel. number of reagents (%) 25.5 25.7 25.8 MFR2 (g/10 min) 112 184 202 Loop reactor A2 Pace. (°C) 95 95 95 H2/C2 ratio (mol/kmol) 486.2 481.8 489.1 C4/C2 ratio (mol/kmol) 0 0 0 Rel. number of reagents (%) 34.5 34.5 34.5 MFR2 (g/10 min) 352 360 280 AZ GPR (gas phase reactor) Pace. (°C) 80 80 80 H2/C2 ratio (mol/kmol) 135.2 151.2 208.5 C4/C2 ratio (mol/kmol) 83.7 81.3 76.2 C6/C2 ratio (mol/kmol) 0 0 0 Rel. number of reagents (%) 40 40 40 Pellet MFR2 (g/10 min) 0.65 0.77 1.27 MFR5 (g/10 min) 2.9 3.32 4.9 Density (kg/m3) 955.6 955.6 956.7

Preparation of reference HDPE (CE1).

A two-stage polymerization was carried out using a catalyst such as described in example 1 in WO 2014/180989. Properties are shown in table. 2. Melt flow data are given in table. 3. Data on molecular weight and comonomer content are shown in table. 4 and 5. Other properties of the resulting HDPE are given in table. 6-8.

table 2

Properties of HDPE comparison

CE1 Loop reactor Rel. number of reagents, % 47.4 GPR Rel. number of reagents, % 50.4 Pellet Density, kg/m ³ 954.2 MFR2, g/10 min 1.35 MFRs, g/10 min 4.94

Table 3

Melt Flow Data

MFR2 (g/10 min) MFR5 (g/10 min) MFR21 (g/10 min) FRR(21/5) FRR(21/2) I.E.I. 0.65 2.9 71.9 24.8 110.6 IE2 0.77 3.3 79.6 24.0 103.4 IE3 1.27 4.9 112.2 22.9 88.3 CE1 1.35 4.94 88.9 18.0 65

Table 4

Molecular weight data

MP (kDa) Mw (kDa) Mz (kDa) Mv (kDa) Mw/Mn Mz/Mn I.E.I. 6670 137500 762000 89300 20.6 114.2 IE2 6635 133000 744500 86650 20.1 112.2 IE3 6470 122000 695500 79450 18.8 107.5 CE1 7000 114000 624000 84000 16.3 89.4

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Table 6

Table 5

Comonomer content data_____

comonomer mol.% wt.% I.E.I. butene 0.48 0.95 IE2 butene 0.47 0.94 IE3 butene 0.46 0.91 CE1 butene 0.55 1.09

Rheological data

IE1 Ш2 IE3 CE1 SHI (0 / 50) [~] 15.4 14.49 14.38 7.15 SHI (0/100) n 36.05 33.45 32.64 13.9 SHI (2.7/210) n 45.13 41.17 40.8* 20.46 SHI (5 / 300)* n 75.4 66.0 66.5 32.8 this 0 [Pass] 48890 40670 26500 26530 eta (0.05 rad/s) [Pa. With] 21465 18773 13213 17337 eta (300 rad/s) [Pass] 573 556 482 702

(*) values calculated by extrapolation of dynamic rheological curves.

Table 7

Flowability of PE-based materials in a spiral flow test

Flow properties in spiral form P, bar (MPa) I.E.I. IE2 IE3 CE1 Melt temperature, °C 190 230 190 230 190 230 190 220 Flow length, mm 600 (60) 263 312 272 320 293 347 1000 (100) 407 471 420 479 443 517 389 439 1400 (140) 555 631 574 634 598 682

Table 8

Mechanical characteristics

FNCT (6 MPa, 50 °C), h Tensile modulus (MPa) (± rms) IE1 98 905 (±9) IE2 102 903 (±5) IE3 32 937 (±4) CE1 56.8 916

CLAIM

Claims (15)

1. A method for producing multimodal high-density polyethylene (HDPE) having a melt flow rate (MFR2) from 0.5 to 1.3 g/10 min, MFR5 from 2.5 to 12 g/10 min and resistance to stress cracking ( FNCT) (6 MPa, 50°C) at least 50 hours, including: (i) polymerizing ethylene in a first polymerization step in the presence of a Ziegler-Natta catalyst to produce a first ethylene homopolymer having an MFR2 of 10 to 500 g/10 min; (ii) polymerizing ethylene in a second polymerization step in the presence of said catalyst and said first ethylene homopolymer to produce a mixture of ethylene homopolymers containing said first ethylene homopolymer and a second ethylene homopolymer, wherein said mixture has an MFR2 of 50 to 1000 g/10 min; and (iii) polymerizing ethylene and at least one alpha-olefin comonomer in a third polymerization step in the presence of said catalyst and said mixture of ethylene homopolymers to produce said multimodal HDPE. 2. The method according to claim 1, wherein said first polymerization step and/or said second polymerization step is carried out in a slurry loop reactor. 3. The method according to claim 1 or 2, wherein said third polymerization step is carried out in a gas-phase reactor. 4. The method according to any one of claims 1 to 3, wherein at least one said alpha-olefin contains from 3 to 12 carbon atoms and is preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene, more preferably 1-butene. - 14 044845 5. The method according to any one of claims 1-4, in which the specified multimodal HDPE has a density of 950 to 960 kg/ ^m3 . 6. The method according to any one of claims 1 to 5, wherein said multimodal HDPE has a tensile modulus of at least 850 MPa, more preferably at least 900 MPa. 7. Multimodal high density polyethylene (HDPE) having a melt flow rate (MFR ₂ ) from 0.5 to 1.3 g/10 min, MFR ₅ from 2.5 to 12 g/10 min and FNCT (6 MPa, 50 °C) for at least 50 hours, obtained by a method as defined in any of claims 1 to 6. 8. Multimodal high-density polyethylene (HDPE), having a density from 940 to 970 kg/m ³ , melt flow rate (MFR ₂ ) from 0.5 to 1.3 g/10 min, MFR ₅ from 2.5 to 12 g /10 min and FNCT (6 MPa, 50°C) for at least 50 h, wherein said HDPE contains a low molecular weight ethylene homopolymer component and a high molecular weight ethylene copolymer component, wherein said HDPE has an Mz/Mn of at least 95 and/or an Mz of at least at least 680,000, and wherein said low molecular weight ethylene homopolymer component is a mixture of ethylene homopolymers containing a first ethylene homopolymer and a second ethylene homopolymer. 9. An injection molded or compression molded article containing multimodal HDPE, such as defined in any one of claims 1 to 8. 10. An article comprising a cap or closure containing multimodal HDPE, such as defined in any one of claims 1 to 8. 11. A method for producing a product made by injection molding or compression molding, including: obtaining multimodal HDPE, such as defined in any of claims 1 to 8; and injection molding or compression molding of the multimodal HDPE to produce said article. 12. A method for producing an injection molded or compression molded article as defined in claim 11, wherein said multimodal HDPE is pelletized before injection or compression molding to form said article. 13. A method for producing an injection molded or compression molded product such as those defined in π. 11, wherein said article is a cap or closure. 14. The use of multimodal HDPE, such as defined in any of claims 1 to 8, to produce an injection molded or compression molded product. 15. The use of multimodal HDPE, such as defined in claim 14, wherein said product is a cap or closure.