WO2020109556A1

WO2020109556A1 - Polymer composition and process for making the same

Info

Publication number: WO2020109556A1
Application number: PCT/EP2019/083098
Authority: WO
Inventors: Franz Ruemer; Ravindra Tupe; Pauli Leskinen; Qizheng Dou; John Jamieson
Original assignee: Borealis Ag
Priority date: 2018-11-29
Filing date: 2019-11-29
Publication date: 2020-06-04
Also published as: EP3887411A1

Abstract

The present invention relates to a polymer composition, especially for pipe, and a process for making the composition. This process for making a polymer composition for pipe comprises polymerizing ethylene in at least two slurry reactors and at least one gas phase reactor connected in series, whereby the polyethylene composition comprises a) an ethylene copolymer as fraction (A), and b) an ethylene homo- or copolymer as fraction (B), with fraction (A) having a lower molecular weight than fraction (B), wherein a single-site catalyst (SSC) is used in the polymerisation of at least one of fractions (A) and/or (B), the composition having (i) a density of between 932 and 955 kg/m3, and (ii) a MFR5 (at 190°C / 5.00 kg) of between 0.5 and 3.5 g/10 min, wherein fraction (A) comprises two fractions (A1) and (A"), whereby fraction (A1) is produced in a first slurry loop reactor and fraction (A") is produced in a second loop reactor.

Description

Polymer composition and process for making the same

The present invention relates to a polymer composition, especially for pipe, and a process for making the same.

Pipes, in particular pressure pipes, are used in various applications like the transport of drinking water, sewage, different industrial applications, gas and more.

Based on the polymer strength, polyethylene pipes for pressurised systems can be classified in different categories, such as PE63, PE80 or PE100. The higher the number, the longer the service life under high pressure. However, polyethylene has a limited pressure resistance at elevated temperature. Especially, it is difficult to combine good pressure resistance at higher temperatures with suitable flexibility and/or processability of the piping materials.

The classic tool to improve the pressure resistance of a pipe at elevated temperature is to cross-link the material. However, the inferior purity of cross-linked resins can be an obstacle for their use in pipes which are in contact with drinking water and/or food. Furthermore, the recycling of cross-linked material is difficult. Thus, thermoplastic solutions would be preferred if the technical performance, such as pressure resistance at elevated temperatures, can be sufficiently improved. Many attempts to design such materials have been made.

Presently, the best polyethylene pressure pipes are prepared in a multistage process with Ziegler-Natta catalysts. The densities of such polyethylene resins are high in order to reach a high pressure resistance. However, high density gives a high stiffness, which is a drawback e.g. when installing the pipes.

There has also been an intensive research on polyolefin resins produced with metallocene or“single-site” catalysts, but still the introduction of such resin into the market is low. The main areas where single site resins have been introduced are film or extrusion coating, as disclosed e.g. in WO 03/066699. The films disclosed in this document have excellent mechanical properties and outstanding sealability.

However, it is known that the catalytic activity of single-site catalysts is moderate and the highest activity is reached in the medium to low density regions.

Furthermore, pressure pipe resins prepared by single-site catalysts of the state of the art, as described e.g. in WO 02/34829, have traditionally a density significantly higher than 940 or even 945 kg/m³. The consequence is that the flexibility of the pipes is rather low.

Still further, for the production of pressure pipes it is necessary that the polyethylene compositions used have a suitable melt flow rate and molecular weight distribution, in order to ensure a good processability of the composition during the extrusion process.

Hence, the object of the present invention is to provide a polymer composition for pipe that allows for having improved/good pressure resistance, improved/good processability, improved/good flexibility and/or improved/good stress at yield and/or to achieve a good balance of at least two or three or all of these properties.

It has now surprisingly been found that can be produced by a process according to the invention. Therefore, the present invention provides a process for making a polymer composition for pipe, comprising polymerizing ethylene in at least two slurry reactors and at least one gas phase reactor connected in series, whereby the polyethylene composition

comprises a. an ethylene copolymer as fraction (A), and

b. an ethylene homo- or copolymer as fraction (B), with fraction (A) having a lower molecular weight than fraction (B), wherein a single-site catalyst (SSC) is used in the polymerisation of at least one of fractions (A) and/or (B), the composition having

(i) a density of between 932 and 955 kg/m³, and

(ii) a MFR.5 at 190 °C / 5.00 kg of between 0.5 and 3.5 g/10 min, wherein fraction (A) comprises two factions (A') and (A"), whereby fraction (A') is produced in a first slurry loop reactor and fraction (A") is produced in a second loop reactor.

In a process for making a polymer composition for pipe according both slurry loops reactors may thus be operated conditions that are different from each other.

In a process for making a polymer composition for pipe according to the invention the fraction(s) (A') and/or (A") may have a density between > 940 kg/m³ and < 957, preferably between > 945 kg/m³ and < 955 kg/m³, preferably between > 945 or 946 kg/m³ and < 954 kg/m³, and/or preferably the density of fraction (A') is > 950 kg/m³ and/or preferably the density of fraction (A") is < 950 kg/m³.

In a process for making a polymer composition for pipe according to the invention, fraction(s) (A') and/or (A") may be a copolymer(s) of ethylene and C4 to C20 alpha-olefin comonomers, preferably of ethylene and 1 -butene.

In a process for making a polymer composition for pipe according to the invention, fraction (A') may have a melt flow rate MFR2 15 to 250 g/lOmin, 20 to 200 g/l Omin, preferably > 20 to 100 g/10 min, further preferred of 15 to 50 g/10 min, preferably 20 and 35 g/10 min, and/or fraction (A") may have a melt flow rate MFR2 of 55 to 150 g/10 min, preferably 75 and 125 g/10 min and/or fraction (A") may have a melt flow rate MFR2 higher than fraction (A'). This may contribute for example to improve pressure resistance and/or especially slow crack propagation.

In a process for making a polymer composition for pipe according to the invention, the weight ratio between fraction (A) and fraction (B) may be between 60 : 40 to 40 : 60, preferably the weight ratio between fraction (A') and fraction (A") may be between < 1 to 1.5, preferably < 1.

In a process for making a polymer composition for pipe according to the invention, the ethylene copolymer (A) and ethylene homo- or copolymer (B) are polymerised in the presence of the same single-site catalyst.

In a process for making a polymer composition for pipe according to the invention, the polymer composition may have a density between > 932 and < 947 kg/m3, preferably between 935 and 945 kg/m³, preferably between 937 and 943 kg/m³, and/or the polymer composition may have a MFR₅ at 190 °C / 5.00 kg of between 1 and < 3.5 g/10 min, preferably between 1.5 and 3 g/10 min, preferably 1.7 and 2.7 g/10 min, and/or the polymer composition may have a MFR21 at 190 °C / 21.60 kg of between 15 and 30 g/10 min, preferably 17 and 27 g/10 min and/or the polymer composition may have a MFR2 at 190 °C / 2.16 kg of between 0.1 and 1.5 g/10 min, preferably between 0.2 and < 1.5 g/10 min, preferably 0.5 and 1 g/10 min.

The present invention thereby also concerns a polymer composition for pipe, produced by polymerizing ethylene in at least two slurry reactors and at least one gas phase reactor connected in series, whereby the polyethylene composition comprises a. an ethylene copolymer as fraction (A), and

(i) a density of between 932 and 955 kg/m³, and

(ii) a MFR5 at 190 °C / 5.00 kg of between 0.5 and 3.5 g/10 min, wherein fraction (A) comprises two factions (A') and (A"), whereby fraction (A') is produced in a first slurry loop reactor and fraction (A") is produced in a second loop reactor.

The present invention may thereby especially also concern a pipe

comprising or consisting of a polymer composition as described herein. In a polymer composition for pipe according to the invention, the fraction(s) (A') and/or (A") may have a density between has/have a density between > 940 kg/m³ and < 957, preferably between > 945 kg/m³ and < 955 kg/m³, further preferred between > 945 or 946 kg/m³ and < 954 kg/m³, and/or preferably the density of fraction (A') may be > 950 kg/m³ and/or preferably the density of fraction (A") may be < 950 kg/m³. Density as reported herein may be measured according to ISO 1 183.

In a polymer composition for pipe according to the invention the fraction(s) (A') and/or (A") may be (a) copolymer(s) of ethylene and C4 to C20 alpha- olefin comonomers, preferably of ethylene and 1 -butene.

In a polymer composition for pipe according to the invention fraction (A') may have a melt flow rate MFR2 of 15 to 50 g/ 10 min, preferably 20 and 35 g/10 min, and/or fraction (A") may have a melt flow rate MFR2 of 55 to 150 g/10 min, preferably 75 and 125 g/10 min and/or fraction (A") may have a melt flow rate MFR2 higher than fraction (A').

In a polymer composition for pipe according to the invention the weight ratio between fraction (A) and fraction (B) may be between 60 : 40 to 40 : 60, preferably the weight ratio between fraction (A') and fraction (A") may be between < 1 to 1.5, preferably < 1.

In a polymer composition for pipe according to the invention the ethylene copolymer (A) and ethylene homo- or copolymer (B) may be polymerised in the presence of the same single-site catalyst.

In a polymer composition for pipe according to the invention the polymer composition may have a density between > 932 and < 947 kg/m³, preferably 935 and 945 kg/m³, preferably between 937 and 943 kg/m³, and/or the polymer composition may have a MFR5 at 190 °C / 5.00 kg of between 1 and < 3.5 g/10 min, preferably 1.5 and 3 g/10 min, preferably 1.7 and 2.7 g/10 min, and/or the polymer composition may have a MFR21 at 190 °C / 21.60 kg of between 15 and 30 g/10 min, preferably 17 and 27 g/10 min and/or the polymer composition may have a MFR2 at 190 °C / 2.16 kg of between 0.1 and 1.5 g/10 min, preferably between 0.2 and < 1.5 g/10 min, preferably 0.5 and 1 g/10 min.

The present invention also concerns the use of a polymer composition according to the invention for pipe.

A polymer composition for pipe according to the invention allows for having improved pressure resistance, improved process ability and/or improved stress at yield. It may also contribute to reaching the PE 125 pipe standard.

Moreover, for pipe applications a good processability of the polyethylene composition is important. High molecular weight is needed for meeting the requirement of good pressure resistance at elevated temperatures and low creep, however, processing of such high molecular weight resins is more difficult. Improved processability is reached by the multimodal design of the base resin. This means at least one lower molecular weight fraction (A) giving easier processability and one fraction with a higher molecular weight (B) contributing to mechanical strength, are present in the polymer composition used for pipes of the invention.

Usually, a polyethylene composition comprising at least two polyethylene fractions, which have been produced under different polymerisation conditions resulting in different (weight average) molecular weights for the fractions, is referred to a“multimodal”. The prefix“multi” relates to the number of different polymer fractions the composition is consisting of. Thus, for example, a composition consisting of two fractions only is called “bimodal”.

The form of the molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight, of such a multimodal polyethylene will show two or more maxima or will at least be distinctly broadened in comparison with the curves for the individual fractions.

For example, if a polymer is produced in a sequential multistage process, utilising reactors coupled in series and using different conditions in each reactor, the polymer fractions produced in different reactors will each have their own molecular weight distribution and weight average molecular weight. When the molecular weight distribution curve of such a polymer is recorded, the individual curves from these fractions are superimposed into the molecular weight distribution curve for total resulting polymer product, usually yielding a curve with two or more distinct maxima.

The polymer composition for pipe according to the present invention may be a multimodal-, or more preferred trimodal comprising fractions (A'), (A") and (B) as defined above, wherein fraction (A) has a lower molecular weight than fraction (B). This may mean that fraction (A) thereby comprises both factions (A') and (A"), which each have a lower molecular weight than fraction (B).

In the preferred embodiment the polymer composition for pipe according to the present invention may further comprise a prepolymer fraction, preferably in an amount of 0.1 to 1 w%, preferably > 0.1 to < 1 w%, further preferred 0.1 to < 0.7 w %. Furthermore, in the present invention fraction (A) is an ethylene copolymer and fraction (B) can be an ethylene homo- or copolymer. However, it is preferred that fraction (B) is also an ethylene copolymer.

The used comonomers of both fractions may be the same or different, preferably different, preferably 1 -butene for fraction (A) and 1 -hexene for fraction (B). This may mean that 1 -butene may be use for both fractions (A') and (A"). Alternatively, different comonomers can also be used for factions (A') and (A").

As comonomers various alpha-olefins with C4 to C20 carbon atoms may be used, but the comonomers are preferably a C4-C20 alkene selected from the group of 1 -butene, 1 -pentene, 4-methyl- 1 -pentene, 1 -hexene, 1-heptene, 1- octene, 1 -decene and 1 -eicosene. In particular preferred embodiment, the comonomer is 1 -butene and/or 1 -hexene.

The polyethylene base resin of the present invention may also comprise a terpolymer, which means that at least on of the fractions (A) and (B) consists of ethylene and two different comonomer units.

Preferably, fraction (B) is an ethylene copolymer, and the comonomer used is an alpha-olefin with 4, more preferably 6, or more carbon atoms, more preferably is 1 -hexene or 1 -octene.

In a further preferred embodiment, fraction (B) is an ethylene copolymer comprising ethylene, 1 -butene and 1 -hexene.

To produce multimodal, in particular bimodal, olefin polymers, such as in the present invention, two or more reactors or zones connected in series as described in EP 517 868, which is hereby incorporated by way of reference in its entirety, can be used. According to the present invention, the main polymerisation stages are preferably carried out as a combination of slurry polymerisation/gas-phase polymerisation. The slurry polymerisation is preferably performed in a so- called slurry loop reactor.

Optionally and advantageously, the main polymerisation stages may be preceded by a pre-polymerisation, in which case a prepolymer in the amount as described above, most preferably in an amount of for example 0.1 to 5% or 1 to 3 % by weight of the total amount of polymers is produced. The pre-polymer may be an ethylene homo- or copolymer.

If a pre-polymerisation takes place, in this case all of the catalyst is preferably charged into the first prepolymerisation reactor and the pre polymerisation is performed as slurry polymerisation. Such a polymeri sation leads to less fine particles being produced in the following reactors and to a more homogeneous product being obtained in the end. Generally, this technique results in a multimodal polymer mixture through polymerisation with the aid of a catalyst, in the present invention with the aid of a single site catalyst.

The single-site catalyst used in the examples of the present invention has been disclosed in EP 1 462 464, example 5, catalyst 3.

In the process of the invention for producing the base resin of the polymer composition of the invention, at least fraction (A) or fraction (B) are produced in a polymerisation reaction in the presence of a single-site catalyst.

It is, however, preferred that both fractions (A) and (B) are prepared in the presence of a single-site catalyst. Furthermore, it is preferred that fraction (A) and fraction (B) are polymerised in the presence of the same single-site catalyst. This also means that fractions (A') and (A") are preferably prepared in the presence of the same single-site catalyst.

That a single-site catalyst is used in the polymerization of at least one of fractions (A) and/or (B) may thereby mean for example that at least one or preferably at least two or preferably all of the fractions (A) and/or (B) and/or (A') and/or (A") may have for example a MWD=Mw/Mn determined by GPC of 1.5 to 6.5, preferably 2 to 5.5, further preferred > 2 to < 5 or < 4.5.

According to the invention it is preferred that fraction(s) (A), (A') and/or (A") is/are produced in a slurry loop reactor under certain conditions with respect to hydrogen, monomer and comonomer concentration, temperature, pressure, and so forth.

Furthermore, it is preferred that fraction (B) is produced in a gas-phase reactor.

Still further, preferably, after the polymerisation fraction (A) including the catalyst is transferred to the reactor, preferably a gas-phase reactor, where fraction (B) is produced under different conditions.

The resulting end product consists of an intimate mixture of the polymers from the three main reactors, the different molecular-weight-distribution curves of these polymers together forming a molecular-weight-distribution curve having a broad maximum or three maxima, i.e. the end product is a trimodal polymer mixture. It is most preferred that the polymerization is carried out in a pre polymerization reactor/two slurry loop reactors/ a gas-phase reactor. Preferably, the polymerization conditions in the preferred four-step method are chosen so that fraction (A') is produced in one step in a first slurry loop reactor, fraction (A") is produced in a second step in a second slurry loop reactor and fraction (B) is produced in further step, preferably the third reactor. The order of these steps may, however, be reversed.

In the present invention it is preferred that the pre -polymerisation operates at a temperature between 40 to 70 °C, more preferred between 50 to 65 °C and preferably at a pressure of 50 to 70 bar, more preferably of 55 to 65 bar.

In the first slurry loop reactor the polymerisation temperature is preferably between 60 to 100 °C, more preferably between 70 to 90 °C, and preferably at a pressure of 40 to 70 bar, more preferably of 50 to 60 bar.

In the second slurry loop reactor the polymerisation temperature is preferably between 60 to 100 °C, more preferably between 70 to 90 °C, and preferably at a pressure of 40 to 70 bar, more preferably of 50 to 60 bar.

In the third reactor the temperature is preferably between 60 to 105 °C, more preferably between 70 and 90 °C and preferably at a pressure of 10 to 40 bar, more preferably of 15 to 20 bar.

The weight ratio between both fractions (A) and (B) may preferably be for example from 60 : 40 to 40 : 60, more preferably from 55 : 45 to 45 : 55.

The polymer composition for pipe of the invention may also comprise additives like process aids, antioxidants, pigments, UV-stabilizers and the like. Usually, the amount at those additives is 10 wt% or lower, based on the total composition.

A pipe may be prepared from the polymer composition for pipe according to the invention in any conventional manner, preferably by extrusion of the polyolefin composition in an extruder. This is a technique well known to the person skilled in the art.

Such a pipe may thereby show good stress resistance.

Methods and Examples Melt flow rate (MFR) The MFR is determined according to ISO 1 133 and is indicated in g/l Omin. For polyethylene resins, a temperature of 190 °C is applied. The MFR is determined at different loadings such as 2.16 kg (MFR₂; ISO 1 133), 5 kg (MFRs; ISO 1 133) or 21.6 kg MFR₂₁ (ISO 1 133). The flow rate ratio, FRR is the ratio between MFR_Weighti and MFR_weight2, i.e. FRR21/5 means the ratio between MFR₂₁ and MFR₅.

Molecular weight

The weight average molecular weight M_w and the molecular weight distribution (MWD = M_w/M_n, wherein M_n is the number average molecular weight and M_w is the weight average molecular weight) is measured by based on ISO 1014-4:2003. A Waters 150CV plus instrument was used with column 3 x HT&E styragel from Waters (divinylbenzene) and trichlorobenzene (TCB) as solvent at 140 °C. The column set was calibrated using universal calibration with narrow MWD PS standards (the Mark Howings constant K: 9.54 * 10 ⁵ and a: 0.725 for PS, and K: 3.92 * 10 ⁴ and a: 0.725 for PE). The ratio of M_w and M_n is a measure of the broadness of the distribution, since each is influenced by opposite end of the“population”.

Pressure test on un-notched pipes

The pressure test on un-notched 32 mm pipes is carried out according to ISO 1 167 3 at 3.8, 4 and 4.2 MPa and 95 °C. The time to failure is determined in hours.

Examples

Comparative example

A commercial bimodal high density polyethylene (not produced with a single site catalyst) with an MFRs of 0.24 g/10 min, a density of 939 g/cm³ available from Borealis AG under the designation BorSafe HE3493-LS-H.

Catalyst preparation

130 grams of a metallocene complex bis(l -methyl-3 -n-butylcyclopentadienyl) zirconium (IV) dichloride (CAS no. 151840-68-5), and 9.67 kg of a 30% solution of commercial methylalumoxane (MAO) in toluene were combined and 3.18 kg dry, purified toluene was added. The thus obtained complex solution was added onto 17 kg silica carrier Sylopol 55 SJ (supplied by Grace) by very slow uniform spraying over 2 hours. The temperature was kept below 30°C. The mixture was allowed to react for 3 hours after complex addition at 30°C.

Example 1

Into a 50 dm³ loop reactor 32 kg/h propane and 8.3 g/h hydrogen were added. The operating temperature was 50°C and operating pressure 57 bar. The single site catalyst prepared as disclosed above was continuously fed at a rate of 42 g/hour into the loop reactor.

The slurry was taken out of the reactor and transferred into a 150 dm³ loop reactor. The reactor was operated at 85 °C and 55 bar pressure. Additional ethylene, 1 -butene, propane diluent and hydrogen were continuously introduced into the reactor so that the MFR2 of the polymer was 27.5 g/l Omin and the density of polymer was 948 kg/m³.

The slurry was transferred into a second 300 dm³ loop reactor. The reactor was operated at 85 °C and 54 bar pressure. Additional ethylene, 1 -butene, propane diluent and hydrogen were continuously introduced into the reactor so that the MFR2 of the polymer was 65 g/l Omin and the density of polymer was 950 kg/m³.

The slurry was continuously withdrawn from the reactor to a flash stage where hydrocarbons were removed from the polymer. The polymer was then transferred into a gas phase reactor where the polymerisation was continued. The reactor was operated at 85°C temperature and 20 bar pressure. Ethylene, hydrogen, 1 -butene and 1 -hexene were fed into the reactor to obtain such conditions that the MFR₅ of the polymer was 2.4 g/l Omin, MFR₂ of the polymer was

0.9 g/lOmin and the density 942 kg/m³. The productivity of the catalyst was 2.4 kg/g catalyst.

The ratio between polymer amounts produced in the slurry loop reactor 1 , the slurry loop reactor 2 and gas phase reactor 3 was 19.4:20:58.1 (the remainder being attributed to the pre-polymerization).

The polymer was then compounded in with 1500 ppm Calcium stearate and 3000 ppm B225. The properties of the compounded resin are given in Table 1 , where also the reaction conditions for the production of the base resin are shown.

The compounded material was extruded into pipes having an external diameter of about 1 10 mm and thickness of about 10 mm and 32 mm and a thickness of 3 mm respectively. The production conditions for example 1 are given in Table 1 (the density and MFR values indicated in Table 1 are the ones for the overall product obtained after polymerization in one or more reactors) and the pressure test result of the pipe is given in Table 2.

Table 1 polymerization conditions:

Despite having a significantly higher MFR5 (consequently improved

processability) and a similar density, inventive Example 1 performs much better than the comparative example especially for example on pressure resistance as shown above by the results at 4 and 4.2 MPa.

Claims

1. Process for making a polymer composition for pipe,

comprising polymerizing ethylene in at least two slurry reactors and at least one gas phase reactor connected in series, whereby the polyethylene composition comprises a. an ethylene copolymer as fraction (A), and

(i) a density of between 932 and 955 kg/m³, and

2. Process for making a polymer composition for pipe according to claim 1 , wherein the fractions (A') and/or (A") has/have a density between > 940 kg/m³ and < 957, preferably between > 945 kg/m³ and < 955 kg/m³, further preferred between > 945 or 946 kg/m³ and <

954 kg/m³, and/or preferably the density of fraction (A') is > 950 kg/m³ and/or preferably the density of fraction (A") is < 950 kg/m³.

3. Process for making a polymer composition for pipe according to any of the proceeding claims, wherein fraction (A') and/or (A") is/are a copolymer(s) of ethylene and C4 to C20 alpha-olefin comonomers, preferably of ethylene and 1 -butene.

4. Process for making a polymer composition for pipe according to any of the proceeding claims, wherein fraction (A') has a melt flow rate MFR2 of 15 to 50 g/10 min, preferably 20 and 35 g/10 min, and/or fraction (A") has a melt flow rate MFR2 of 55 to 150 g/10 min, preferably 75 and 125 g/10 min and/or fraction (A") has a melt flow rate MFR2 higher than fraction (A').

5. Process for making a polymer composition for pipe according to any of the proceeding claims, wherein the weight ratio between fraction (A) and fraction (B) is between 60 : 40 to 40 : 60, preferably the weight ratio between fraction (A') and fraction (A") is between < 1 to 1.5, preferably < 1.

6. Process for making a polymer composition for pipe according to any of the proceeding claims, wherein the ethylene copolymer (A) and ethylene homo- or copolymer (B) are polymerised in the presence of the same single-site catalyst.

7. Process for making a polymer composition for pipe according to any of the proceeding claims, wherein the polymer composition has a density between 935 and 945 kg/m³, preferably between 937 and 943 kg/m³, and/or wherein the polymer composition has a MFR5 at 190 °C / 5.00 kg of between 1.5 and 3 g/10 min, preferably 1.7 and 2.7 g/10 min, and/or wherein the polymer composition has a MFR21 at 190 °C / 21.60 kg of between 15 and 30 g/10 min, preferably 17 and 27 g/10 min and/or wherein the polymer composition has a MFR.2 at 190 °C / 2.16 kg of between 0.2 and 1.5 g/10 min, preferably 0.5 and 1 g/10 min.

8. A polymer composition for pipe, produced by polymerizing ethylene in at least two slurry reactors and at least one gas phase reactor connected in series, whereby the polyethylene composition comprises a. an ethylene copolymer as fraction (A), and

(i) a density of between 932 and 955 kg/m³,

and

9. A polymer composition for pipe according to claim 1, wherein the fractions (A') and/or (A") has/have a density between > 940 kg/m3 and < 957, preferably between > 945 kg/m3 and < 955 kg/m3, further preferred between > 945 or 946 kg/m3 and < 954 kg/m3, and/or preferably the density of fraction (A') is > 950 kg/m3 and/or preferably the density of fraction (A") is < 950 kg/m3.

10. A polymer composition for pipe according to any of the proceeding claims, wherein fraction (A') and/or (A") is/are a copolymer(s) of ethylene and C4 to C20 alpha-olefin comonomers, preferably of ethylene and 1 -butene.

1 1. A polymer composition for pipe according to any of the proceeding claims, wherein fraction (A') has a melt flow rate MFR2 of 15 to 250 g/l Omin, 20 to 200 g/lOmin, preferably > 20 to 100 g/10 min, further preferred 15 to 50 g/10 min, preferably 20 and 35 g/10 min, and/or fraction (A") has a melt flow rate MFR2 of 55 to 150 g/10 min, preferably 75 and 125 g/10 min and/or fraction (A") has a melt flow rate MFR2 higher than fraction (A').

12. A polymer composition for pipe according to any of the proceeding claims, wherein the weight ratio between fraction (A) and fraction (B) is between 60 : 40 to 40 : 60, preferably the weight ratio between fraction (A') and fraction (A") is between < 1 to 1.5, preferably < 1.

13. A polymer composition for pipe according to any of the proceeding claims, wherein the ethylene copolymer (A) and ethylene homo- or copolymer (B) are polymerised in the presence of the same single-site catalyst.

14. A polymer composition for pipe according to any of the proceeding claims, wherein the polymer composition has a density between 935 and 945 kg/m³, preferably between 937 and 943 kg/m³, and/or wherein the polymer composition has a MFR5 at 190 °C / 5.00 kg of between 1.5 and 3 g/10 min, preferably 1.7 and 2.7 g/10 min, and/or wherein the polymer composition has a MFR21 at 190 °C / 21.60 kg of between 15 and 30 g/10 min, preferably 17 and 27 g/10 min and/or wherein the polymer composition has a MFR2 at 190 °C / 2.16 kg of between 0.2 and 1.5 g/10 min, preferably 0.5 and 1 g/10 min.

15. Use of a polymer composition according to any of the preceding claims for pipe.