Polyolefin pipe resin with very good anti-sagging property and slow crack growth property

Abstract

The present invention relates to a polyethylene (PE) composition comprising a matrix resin, the matrix resin comprising (A) a first ethylene homopolymer or copolymer component, wherein component (A) has a melt flow rate MFR 2 of 100 g / 10 min to 600 g / 10 min; and component (B) a second ethylene-1-hexene copolymer component, wherein component (A) has a lower molecular weight than component (B), wherein, based on the total weight of the matrix resin, component (B) is present in an amount of 50.0 wt.% to 58.0 wt.%; wherein, based on the total amount of the matrix resin, the matrix resin has a content of units derived from 1-hexene of 0.44 mol% to 0.79 mol%; wherein, the matrix resin has a molecular weight distribution (the ratio of Mw / Mn) of 32 to 40, the matrix resin has a Z-average molecular weight Mz of greater than 1500 kg / mol; wherein, the polyethylene composition has a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min; a melt flow rate ratio FRR 21 / 5 of 30 to 42; and wherein, the polyethylene composition has a critical temperature Tc of -10 °C or lower and not lower than -25 °C in a rapid crack propagation test. The present invention also relates to a PE composition obtainable by a multi-stage process, to an article comprising the PE composition, to a pipe, and to the use of a PE composition comprising a matrix resin for the production of an article.

Description

TECHNICAL FIELD

[0001] The present invention relates to a polyethylene composition comprising a matrix resin, to a polyethylene composition obtainable by a multi-stage process, to an article comprising the polyethylene composition, to a pipe, and to the use of a polyethylene composition comprising a matrix resin for the production of an article. BACKGROUND ART

[0002] Polyolefin pipes, especially polyethylene pipes, are generally used for the transportation of water, gas, and industrial liquids and slurries. Due to their wide range of uses, ease of production and installation, and non-corrosiveness, their use is increasing continuously.

[0003] The fluid to be transported may have different temperatures, generally in the temperature range of about 0 °C to about 50 °C. According to ISO9080, polyethylene pipes are classified according to their minimum required strength, i.e., their ability to withstand different circumferential stresses for 50 years at 20 °C without rupture. Therefore, when the circumferential stress is 8.0 MPa (MRS 8.0 ), it is classified as a PE80 pipe, and when the circumferential stress is 10.0 MPa (MRS 10.0 ), it is classified as a PE100 pipe. The use temperature of PE 100 is generally in the range of about 0 °C to 50 °C.

[0004] In order to meet the PE80 requirements of multi-modal resins produced by conventional Ziegler-Natta catalysts, their density needs to be at least 940 kg / m 3 , and in order to meet the PE100 requirements, the density needs to be greater than 945 kg / m 3 . However, the density of polyethylene resin is directly related to its crystallinity. The higher the crystallinity of polyethylene resin, the lower its resistance to slow crack growth. In other words, all polyethylene materials used for pressure pipes are affected by the correlation between crystallinity and density and by slow crack growth. When the density increases, the performance of resistance to slow crack growth (SCG) decreases.

[0005] For example, Scheirs et al. discussed the manufacture of polyethylene materials for pressure pipes in an article (Scheirs, Bohm, Boot and Leevers: PE 100 Resins for Pipe Applications, TRIP Vol.4, No 12 (1996) pp.408-415).

[0006] The aim of pipes for transporting pressurized fluids (so-called pressure pipes) to withstand increasingly high (internal) design stresses involves higher creep resistance and higher hardness. On the other hand, pressure pipes must also meet the requirements of their rapid and slow crack growth resistance and must have low brittleness and high impact strength. However, these properties are mutually opposed, so it is difficult to provide a composition for the pipes that simultaneously has all these properties. In addition, since polymer pipes are usually manufactured by extrusion or, to a lesser extent, by injection molding, the polyethylene composition must also have good processability. Finally, the polymer composition for pipes must also show good weldability, since pipe systems are usually constructed by welding or fusion, either as a general connection method between pipe system components or as a connection between layers, for example in a multi-layer pipe structure, such as butt fusion welding, electrofusion, spin welding (friction welding) and manual or automatic welding using additional welding materials. Therefore, it is important that the composition used must show a certain minimum welding strength. It is known that the welding strength of filled polymer compositions is generally poor.

[0007] It is known that in order to meet the mutually opposed requirements of pipe materials, bimodal polyethylene compositions can be used. Such compositions are described in EP 0 739 937 and WO 02 / 102891. The bimodal polyethylene compositions described in these documents generally comprise a low molecular weight polyethylene component and a high molecular weight component of an ethylene copolymer containing one or more α-olefin comonomers.

[0008] EP 1 987 097 discloses, in the name of Chevron Phillips Chemical Company, a polyethylene suitable for pipes with a pellet density of 947 kg / m 3 to 954 kg / m 3 and an MFR 21 (ASTM D1238, 21.6 kg load) of 1 g / 10 min to 30 g / 10 min. The example resins show a weight average molecular weight of 278 kg / mol to 346 kg / mol and an Mw / Mn of 30.5 to 35.1.

[0009] EP 1 781 712 discloses various compositions in the name of Univation Tech LLC [US], including but not limited to high-strength bimodal polyethylene compositions with a density of 0.940 g / cc or higher, which comprise a high molecular weight polyethylene component (MwHMW) with a higher weight average molecular weight and a low molecular weight polyethylene component (MwLMW) with a lower weight average molecular weight, wherein the ratio of the higher weight average molecular weight to the lower weight average molecular weight (MwHMW:MwLMW) is 30 or more; and the composition meets the PE100 material such that, according to ISO 1167, when the internal pipe resistance curve is extrapolated to 50 years or 100 years according to ISO 9080:2003, the pipe subjected to internal pipe resistance formed from this composition has an extrapolated stress of 10 MPa or higher.

[0010] EP 1 922 342 of the Ineos group discloses that at a melt flow rate (5 kg load) of 0.15 g / 10 min to 0.5 g / 10 min, the natural density is 935 kg / m 3 to 956 kg / m 3 of the composition, the comonomer is 1-hexene, and the dynamic viscosity at 100 rad / s and 190 °C is not more than 2500 Pa·s.

[0011] EP 1 985 660 A1 discloses a pipe or piping product with improved resistance to slow crack growth, which comprises a polyethylene composition, and the polyethylene composition comprises a matrix resin, and the matrix resin comprises a first ethylene homopolymer or copolymer component (A) and a second ethylene homopolymer or copolymer component (B), wherein component (A) has a lower average molecular weight than component (B), and wherein the density of the matrix resin is in the range of 945 kg / m 3 to 949 kg / m 3 range, MFR 5 is in the range of 0.2 g / 10 min to 0.4 g / 10 min, the content of the comonomer is higher than 2.0 wt.% and SHI (2.7 / 210) is in the range of 55 to 100.

[0012] New installation techniques require PE resins to have increasingly high resistance to slow crack growth. The requirements for slow crack growth are becoming increasingly strict, and many existing products cannot always meet these requirements. At the same time, it is also necessary to improve the impact resistance of HDPE pipe resins to avoid pipe failure due to rapid crack propagation.

[0013] Accordingly, there is still a need for polyethylene compositions having favorable resistance to slow crack growth, while having very good sag resistance and low critical temperature as well as good impact resistance. In addition, these polyethylene compositions should also meet the requirements of PE100 resins. Summary of the Invention

[0014] The object of the present invention is to provide a polyethylene composition that meets the requirements of PE100 resins, which has excellent resistance to slow crack growth, while having very good sag resistance and low critical temperature as well as good impact resistance.

[0015] The present invention is based on the surprising discovery that such a pipe material can be provided by selecting a specific combination of properties of a multimodal polyethylene matrix resin, which combination relates to the melt flow rate of a selected low molecular weight (LMW) component, a selected high Mw component having a certain molecular weight distribution (MWD), and the 1-hexene content in the matrix resin.

[0016] Accordingly, the present invention provides a polyethylene composition comprising a matrix resin, which matrix resin comprises the following, or consists of the following,

[0017] (A) A first ethylene homopolymer or copolymer component, wherein component (A) has a melt flow rate MFR 2 , measured according to ISO1133 as 100 g / 10 min to 600 g / 10 min, preferably 100 g / 10 min to 550 g / 10 min, and more preferably 115 g / 10 min to 450 g / 10 min; and

[0018] (B) A second ethylene-1-hexene copolymer component,

[0019] wherein component (A) has a lower molecular weight than component (B), and based on the total weight of the matrix resin, component (B) is present in an amount of 50.0 wt.% to 58.0 wt.%, preferably in an amount of 51.0 wt.% to 57.0 wt.%, more preferably in an amount of 52.0 wt.% to 56.0 wt.%, and even more preferably in an amount of 53.0 wt.% to 55.0 wt.%;

[0020] wherein, based on the total amount of the matrix resin, the matrix resin has a content of units derived from 1-hexene of 0.44 mol% to 0.79 mol%, preferably 0.45 mol% to 0.78 mol%, more preferably 0.46 mol% to 0.77 mol%, and even more preferably 0.46 mol% to 0.76 mol%;

[0021] Among them, the molecular weight distribution (Mw / Mn ratio) of the matrix resin is from 32 to 40, preferably from 32.5 to 39.5, more preferably from 33 to 39, and the Z-average molecular weight Mz of the matrix resin is greater than 1500 kg / mol, preferably greater than 1600 kg / mol, and more preferably greater than 1700 kg / mol;

[0022] Among them, the polyethylene composition has a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min, preferably 0.12 g / 10 min to 0.22 g / 10 min, and more preferably 0.14 g / 10 min to 0.20 g / 10 min; and the melt flow rate ratio FRR 21 / 5 is from 30 to 42; and

[0023] Among them, the critical temperature Tc of the polyethylene composition in the rapid crack propagation test is -10 °C or lower, preferably -12 °C or lower, and not lower than -25 °C, preferably not lower than -23 °C.

[0024] Surprisingly, when the MFR 5 and FRR 21 / 5 of the polyethylene composition and the Mz value and molecular weight distribution of the matrix resin are within specific ranges, excellent resistance to slow crack growth, low critical temperature, and excellent anti-sag performance can be obtained. In addition, the melt flow rate of the low molecular weight (LMW) component (component (A)) of the polyethylene composition needs to be within a certain range, and the matrix resin needs to contain a specific amount of 1-hexene. In addition, the ratio of the low molecular weight (LMW) component (component (A)) to the high molecular weight (HMW) component (component (B)) of the polyethylene composition has a positive effect on processability and property balance. Surprisingly, the polyethylene composition of the present invention has improved properties, especially the balance of the improved properties as described above.

[0025] The present invention further provides a polyethylene composition obtainable by a multi-stage process, the multi-stage process comprising or consisting of the following steps

[0026] a) Polymerize ethylene in the presence of

[0027] - a catalyst

[0028] - in one or more loop reactors, preferably in one or two loop reactors, obtain component (A) in the presence of an alkylaluminum compound and a chain transfer agent, and component (A) has a melt flow rate MFR of 100 g / 10 min to 600 g / 10 min 2, preferably from 100 g / 10 min to 550 g / 10 min, and more preferably from 115 g / 10 min to 450 g / 10 min; and

[0029] b) Transfer component (A) to a gas-phase reactor

[0030] - Feed ethylene and a comonomer into the gas-phase reactor,

[0031] - Further polymerize to obtain a matrix resin comprising component (A) obtained in step a) and component (B) obtained in step b),

[0032] wherein, based on the total amount of component (B), the matrix resin has a content of units derived from 1-hexene in component (B) of 0.81 mol% to 1.60 mol%, preferably 0.83 mol% to 1.59 mol%, and more preferably 0.85 mol% to 1.58 mol%; and

[0033] wherein component (A) has a lower molecular weight than component (B), and wherein, based on the total weight of the matrix resin, the content of component (B) present is 50.0 wt.% to 58.0 wt.%, preferably 51.0 wt.% to 57.0 wt.%, more preferably 52.0 wt.% to 56.0 wt.%, and even more preferably 53.0 wt.% to 55.0 wt.%;

[0034] c) Extrude the matrix resin into a polyethylene composition having a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min, preferably 0.12 g / 10 min to 0.22 g / 10 min, and more preferably 0.14 g / 10 min to 0.20 g / 10 min, and having a melt flow rate ratio FRR 21 / 5 of 30 to 42.

[0035] The above object can be further achieved by an article, preferably a pipe or fitting, more preferably a pipe.

[0036] Furthermore, the above object can be achieved by using the above polyethylene composition to produce an article, preferably a pipe.

[0037] The polyethylene composition according to the present invention represents a polymer derived from at least 50 mol% of ethylene monomer units and additional 1-hexene units.

[0038] According to the present invention, the expression "ethylene homopolymer" relates to an ethylene polymer which consists essentially of ethylene, i.e. at least 99% by weight, more preferably at least 99.5% by weight, even more preferably at least 99.8% by weight, and most preferably an ethylene polymer which only comprises ethylene monomer units.

[0039] The term "matrix resin" is the entirety of the polymer components in the polyethylene composition according to the present invention, i.e. it represents the polymer part of the composition without fillers such as carbon black. The matrix resin generally accounts for at least 90 wt% of the total composition. Preferably, the matrix resin consists of polymer component (A) and component (B), optionally further comprising a prepolymer component, the content of which is at most 10 wt%, more preferably at most 7 wt%, and most preferably at most 5 wt% of the total matrix resin. Those skilled in the art will understand that the presence of a stabilizer is required to measure the properties of the matrix resin.

[0040] All rheological tests can be carried out on the matrix resin and the composition. By definition, all rheological properties preferably also apply to the composition.

[0041] The term "catalytic system" shall mean the composition formed by the catalyst and the cocatalyst.

[0042] The melt flow rate (MFR) is an important property of the multimodal polyethylene according to the present invention for pipes. The MFR is determined according to ISO 1133 and expressed in g / 10 min. The MFR is an indicator of the polymer fluidity and thus the processability. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined under different loads, such as 2.16 kg (MFR 2 ; ISO 1133, condition D), 5 kg (MFR 5 ; ISO 1133, condition T) or 21.6 kg (MFR 21 ; ISO 1133, condition G).

[0043] The polyethylene composition according to the present invention preferably has a melt flow rate MFR 21 of 3 g / 10 min to 8 g / 10 min, more preferably 4 g / 10 min to 7 g / 10 min, and even more preferably 4.5 g / 10 min to 6.5 g / 10 min.

[0044] Preferably. The polyethylene composition has a melt flow rate MFR 5 of 0.12 g / 10 min to 0.22 g / 10 min, and more preferably 0.14 g / 10 min to 0.20 g / 10 min.

[0045] In a preferred embodiment, component (A) has a melt flow rate MFR 2 of from 100 g / 10 min to 550 g / 10 min, and more preferably from 115 g / 10 min to 450 g / 10 min.

[0046] According to a preferred embodiment, component (A) has a melt flow rate MFR 2 of from 100 g / 10 min to 230 g / 10 min, more preferably from 115 g / 10 min to 210 g / 10 min, and even more preferably from 130 g / 10 min to 190 g / 10 min.

[0047] According to another preferred embodiment, component (A) has a melt flow rate MFR 2 of from 231 g / 10 min to 550 g / 10 min, more preferably from 250 g / 10 min to 450 g / 10 min, and even more preferably from 270 g / 10 min to 380 g / 10 min.

[0048] In a preferred embodiment, the matrix resin has a molecular weight distribution (the ratio of Mw / Mn) of from 32.5 to 39.5, and preferably from 33 to 39.

[0049] For example, it is also more important to ensure that the polymer composition does not flow from the upper part to the lower part of the pipe under the action of gravity after being extruded into the pipe and before being cooled, thereby causing uneven distribution of the polymer around the cross-section of the pipe. This phenomenon is called the tendency of the polymer to exhibit gravitational flow or "sag". The sag resistance is the property of the polymer pipe to withstand this tendency. The sag resistance can be measured by the rheological method or the melt index method.

[0050] The present invention is measured by a method closely related to the above melt index method, and this method is related to the rheology of the polymer. This method is based on the measurement of the polymer viscosity at a very low constant shear stress of 747 Pa (eta747). The viscosity of the polymer at this shear stress is measured at a temperature of 190 °C and is found to be inversely proportional to the gravitational flow of the polymer, that is, the greater the viscosity, the lower the gravitational flow. According to the present invention, the polyethylene composition has a viscosity shown at eta 747 preferably greater than 700 kPa*s, preferably greater than 730 kPa*s, and even more preferably greater than 750 kPa*s. The viscosity shown at eta 747The viscosity at [specific condition] is not greater than 1400 kPa·s. Generally, the matrix resin of the polyethylene composition of the present invention has a viscosity of 700 kPa·s to 1400 kPa·s, preferably 730 kPa·s to 1300 kPa·s, more preferably 750 kPa·s to 1200 kPa·s, and even more preferably 750 kPa·s to 1100 kPa·s at a shear stress (eta747) of 747 Pa.

[0051] Equally importantly, the polyethylene composition has excellent resistance to slow crack growth.

[0052] According to a preferred embodiment, the polyethylene composition has a strain hardening modulus of 75 MPa or higher, more preferably 80 MPa or higher, still more preferably 85 MPa to 100 MPa, and most preferably 87 MPa to 100 MPa. A further preferred strain hardening modulus can be 87 or higher. Preferably, the strain hardening modulus is 75 MPa to 110 MPa, preferably 80 MPa to 105 MPa, and more preferably 85 MPa to 100 MPa; and / or

[0053] The polyethylene composition preferably has a failure time of greater than 1500 h, more preferably greater than 2000 h, and even more preferably greater than 3000 h in an accelerated creep test (ACT); and / or

[0054] The polyethylene composition preferably has a failure time of at least 200 h, more preferably at least 400 h, and even more preferably at least 500 h in a short-term compressive strength (STPR) test at a stress level of 5.4 MPa and a temperature of 80 °C; and / or

[0055] The polyethylene composition preferably has a failure time of at least 130 h, more preferably at least 150 h, and even more preferably at least 175 h, and still more preferably at least 200 h in a short-term compressive strength (STPR) test at a stress level of 12.0 MPa and a temperature of 20 °C; and / or

[0056] The polyethylene composition preferably has a yield stress of 6.0 MPa to 7.0 MPa, preferably 6.2 MPa to 6.9 MPa, and more preferably 6.3 MPa to 6.8 MPa at 80 °C; and / or

[0057] The matrix resin preferably has a viscosity of greater than 700 kPa·s, more preferably greater than 730 kPa·s, even more preferably greater than 750 kPa·s and not greater than 1400 kPa·s at a shear stress (eta 747 ) of 747 Pa; and / or

[0058] The polyethylene composition preferably has a critical temperature Tc of -12 °C or lower and / or not less than -23 °C in a rapid crack propagation test.

[0059] Further preferably, the polyethylene composition has a failure time of greater than 1000 h, more preferably greater than 2000 h, and even more preferably greater than 3000 h in an accelerated point load test (PLT+).

[0060] As indicated by the balance of the above-described properties, the polyethylene composition has good impact resistance and very good sag resistance. In addition, the composition has good rapid crack propagation resistance.

[0061] Component (B) constitutes the high molecular weight (HMW) component of the polyethylene composition, and the polyethylene composition is a copolymer of ethylene and 1-hexene.

[0062] In a preferred embodiment, based on the total amount of component (B), component (B) has a content of units derived from 1-hexene of 0.81 mol% to 1.60 mol%, preferably 0.83 mol% to 1.59 mol%, and more preferably 0.85 mol% to 1.58 mol%.

[0063] According to a preferred embodiment, the present invention relates to a polyethylene composition, wherein, based on the total amount of the matrix resin, the matrix resin has a content of units derived from 1-hexene of 0.45 mol% to 0.78 mol%, preferably 0.46 mol% to 0.77 mol%, and more preferably 0.46 mol% to 0.76 mol%.

[0064] Component (A) of the matrix resin may be a homopolymer or copolymer of ethylene.

[0065] If component (A) is an ethylene copolymer, it is preferably a copolymer of ethylene and an α-olefin having 3 to 8 carbon atoms, more preferably a copolymer with an α-olefin having 4 to 6 carbon atoms, and most preferably a copolymer with 1-butene or 1-hexene. Preferably, the amount of comonomer (if present) in component (A) is 1 mol% or less, for example 0.1 mol% to 0.5 mol%.

[0066] However, preferably, component (A) of the matrix resin is a homopolymer of ethylene.

[0067] Component (A) preferably has a density of 960 kg / m 3 to 980 kg / m 3 3.

[0068] Component (A) and / or component (B) can consist of a single polymer component prepared in one reactor, or can consist of two or more partial components prepared in different reactors.

[0069] Preferably, component (A) and / or component (B) consist of two partial components or a single component.

[0070] Most preferably, component (A) consists of a single component or two partial components, preferably produced in one or two loop reactors respectively, and component (B) consists of a single component, preferably produced in a gas phase reactor.

[0071] The matrix resin of the polyethylene composition of the present invention comprises (at least) component (A) and component (B) with different molecular weights. This resin is designated as multimodal polyethylene. The prefix "multi" is related to the number of different polymer components that make up the matrix resin. Thus, for example, a matrix resin consisting of only two different components is called "bimodal".

[0072] The form of the molecular weight distribution curve of this multimodal polyethylene (i.e., the appearance of the graph of the mass fraction of the polymer as a function of the molecular weight) will show two or more maxima, or at least be significantly broader compared to the curve of a single component.

[0073] For example, if the polymer is produced in a continuous multi-stage process, using reactors connected in series and different conditions in each reactor, the polymer components produced in different reactors will have their respective molecular weight distributions and weight average molecular weights. When the molecular weight distribution curve of such a polymer is recorded, the individual curves from these components are superimposed on the molecular weight distribution curve of the resulting total polymer product, usually producing a curve with two or more different maxima.

[0074] In the present invention, the matrix resin is preferably a bimodal polyethylene matrix resin, that is, the matrix resin preferably consists of component (A) and component (B) and optionally a small amount of prepolymer, which is considered to be part of the polymer component produced in the first reactor.

[0075] If one or more components of the matrix resin consist of partial components from separate reactors, the reaction conditions are preferably selected so as to produce substantially the same polymer therein. This means that, for example, preferably, if the matrix resin consists of component (A) and component (B), and component (A) is produced in two separate loop reactors in the form of two partial components under conditions such that the polymers produced therein are the same or substantially the same, then the matrix resin will still be a bimodal resin because it consists of two components of different polymers.

[0076] Another feature of the present invention is the density of the polyethylene matrix resin. Due to strength reasons, the density is in the medium to high density range, particularly in the range of 930 kg / m 3 to 965 kg / m 3 range. Preferably, a density of 935 kg / m 3 to 960 kg / m 3 is used. Preferably, the density is not higher than 965 kg / m 3 . Using high density multimodal polyethylene may result in pressure pipes with higher design stress ratings compared to using medium density multimodal polyethylene.

[0077] Preferably, the matrix resin has a density of at least 945 kg / m 3 , preferably 945 kg / m 3 to 951 kg / m 3 , more preferably 945 kg / m 3 to 949 kg / m 3 , and even more preferably 945.5 kg / m 3 to 948.5 kg / m 3 .

[0078] According to a preferred embodiment, the present invention relates to a polyethylene composition, wherein the polyethylene composition has a density of at least 953 kg / m 3 , preferably 953 kg / m 3 to 962 kg / m 3 , and even more preferably 955 kg / m 3 to 961 kg / m 3 .

[0079] Based on the total amount of the matrix resin determined by quantitative melt state nuclear magnetic resonance (NMR) spectroscopy, the matrix resin preferably has a content of units derived from 1-hexene of 0.45 mol% to 0.78 mol%, preferably 0.46 mol% to 0.77 mol%, and more preferably 0.46 mol% to 0.76 mol%.

[0080] Furthermore, the polyethylene composition preferably has a failure time of at least 3000 h, preferably at least 4000 h, and even more preferably at least 4682 h in the notched pipe test (NPT).

[0081] Preferably, the polyethylene composition has a Charpy notched impact strength (CIS 23 °C) of at least 30 kJ / m 2 , and more preferably at least 32 kJ / m 2 , determined at 23 °C according to ISO 179 / 1eA:2000, as described in the experimental section below.

[0082] Preferably, the polyethylene composition has a Charpy notched impact strength (CIS 0 °C) of at least 20 kJ / m 2 , and more preferably at least 22 kJ / m 2 , determined at 0 °C according to ISO 179 / 1eA:2000, as described in the experimental section below.

[0083] Even more preferably, the polyethylene composition has a Charpy notched impact strength (CIS -20 °C) of 13 kJ / m 2 , and more preferably at least 14 kJ / m 2 , determined at -20 °C according to ISO 179 / 1eA:2000, as described in the experimental section below.

[0084] In addition to the matrix resin, additives commonly used for polyolefins, such as pigments, stabilizers (antioxidants), antacids and / or UV agents, antistatic agents and utilization agents (such as processing aids) may be present in the polyethylene composition. Preferably, the addition amount of these additives is 10 wt.% or less of the total polyethylene composition, more preferably 8 wt.% or less of the total polyethylene composition, and even more preferably 4 wt.% or less of the total polyethylene composition.

[0085] The composition may contain carbon black in an amount of 8 wt% or less of the total composition, more preferably 1 wt% to 4 wt% of the total composition, and even more preferably 2.0 wt% to 2.5 wt% of the total composition.

[0086] Preferably, the matrix resin constitutes at least 90 wt% of the polyethylene composition, more preferably at least 92 wt%, and even more preferably at least 95 wt%.

[0087] In addition, preferably, the polyethylene composition consists of the matrix resin, carbon black and any amount of further (common) additives described herein.

[0088] According to a preferred embodiment, the LMW component (Component A) is a homopolymer of ethylene, and the HMW component (Component (B)) is a copolymer of ethylene. The HMW component (Component (B)) is a copolymer of ethylene and 1-hexene.

[0089] According to a preferred embodiment, based on the total weight of the matrix resin, Component (B) is present in an amount of 51.0 wt.% to 57.0 wt.%, more preferably 52.0 wt.% to 56.0 wt.%, and even more preferably 53.0 wt.% to 55.0 wt.%.

[0090] In the present invention, the ratio of the LMW component (component (A)) to the HMW component (component (B)) (also referred to as the "share" between the components) is appropriately selected. Component (A) may also contain components obtained in prepolymerization. Specifically, the weight ratio of the LMW component to the HMW component should be in the range of (40 - 55):(60 - 45), preferably (42 - 51):(58 - 49), more preferably (43 - 49):(57 - 51), and even more preferably (45 - 47):(55 - 53). If the share is within these ranges, the proportion of the HMW component results in high strength values (such as slow crack growth and pressure resistance) as well as low amounts of gel and good processability.

[0091] The molecular weight distribution of the multimodal polyethylene is defined by the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (i.e., Mw / Mn), and this ratio can range from quite narrow to quite wide and is in the range of 32 to 40, preferably 32.5 to 39.5, and even more preferably 33 to 39. Further, the number average molecular weight Mn of the matrix resin is 7300 g / mol or higher, preferably 8000 g / mol or higher. The matrix resin has a number average molecular weight Mn of 9200 g / mol or lower, preferably 9150 g / mol or lower. Preferably, the number average molecular weight is in the range of 7300 g / mol to 9200 g / mol, more preferably 8000 g / mol to 9150 g / mol. The weight average molecular weight Mw of the matrix resin is in the range of 270000 g / mol to 350000 g / mol, preferably 280000 g / mol to 340000 g / mol.

[0092] Preferably, the matrix resin has a Z average molecular weight Mz greater than 1500 kg / mol, preferably greater than 1600 kg / mol, and more preferably greater than 1700 kg / mol, and preferably not greater than 2200 kg / mol. Preferably, the Z average molecular weight Mz of the matrix resin is in the range of 1500 kg / mol to 2200 kg / mol, more preferably 1600 kg / mol to 2100 kg / mol, and even more preferably 1700 kg / mol to 2000 kg / mol.

[0093] According to a further preferred embodiment, the polyethylene composition has a melt pressure of 20 MPa to 25 MPa (200 bar - 250 bar) (both for 32 mm and 110 mm pipes).

[0094] In another preferred embodiment, the present invention relates to a polyethylene composition, wherein the matrix resin is produced by a multi-stage process in the presence of a Ziegler - Natta catalyst.

[0095] A multi-stage process is a process that uses at least two reactors, one for producing low molecular weight components and a second for producing high molecular weight components. These reactors can be used in parallel, in which case the components must be mixed after production. More commonly, the reactors are used in series, such that the product of one reactor is used as the starting material for the next reactor. For example, one component is formed in the first reactor and a second component is formed in the second reactor in the presence of the first component. In this way, the two components are more closely mixed together because one is formed in the presence of the other.

[0096] The polymerization reaction at each stage may involve the use of conventional reactors, such as conventional ethylene homopolymerization or copolymerization reactions in a loop reactor, a gas phase reactor, a batch reactor, etc., such as gas phase polymerization, slurry phase polymerization, liquid phase polymerization.

[0097] The polymerization can be carried out continuously or batchwise, preferably, the polymerization is carried out continuously.

[0098] Known two-stage processes are, for example, liquid-liquid processes, gas-gas processes and liquid-gas processes. It is also known that these two-stage processes can be further combined with one or more additional polymerization steps selected from a gas phase polymerization process, a slurry phase polymerization process or a liquid phase polymerization process.

[0099] In a preferred multi-stage process, the lower molecular weight and higher molecular weight polymers, component (A) and component (B), are produced in any order in different polymerization steps.

[0100] In the first polymerization step, an LMW polymer (component (A)) can be prepared, and in the second polymerization step, an HMW copolymer (component (B)) can be prepared. This can be referred to as the normal mode and is preferred.

[0101] It is also possible to prepare a part of the HMW copolymer (B) in the first polymerization step and a part of the LMW polymer (A) in the second polymerization step. This can be referred to as the reverse mode.

[0102] If the LMW component is produced in the first polymerization step, the melt flow rate of the first ethylene component (A) can be measured directly as described herein. If the LMW component is produced in the second polymerization step, the melt flow rate of the LMW ethylene component (A) can be calculated based on the weight ratio of the LMW component and the HMW component and the molecular weight of the total polyethylene composition.

[0103] In addition, when the components of each polymer are known, subtracting the GPC curve can also determine the melt flow rate of the polymer produced in the second stage of the multi-stage polymerization process.

[0104] For example, the two-stage process can be a slurry-slurry process or a gas-phase-gas-phase process, particularly preferably a slurry-gas-phase process. Optionally, the process according to the invention can comprise one or two additional polymerization steps.

[0105] These one or two optional additional polymerization steps preferably comprise a slurry polymerization step.

[0106] The slurry and gas-phase stages can be carried out using any conventional reactor known in the art. For example, slurry-phase polymerization can be carried out in a continuously stirred tank reactor, a batch-operated stirred tank reactor or a loop reactor. Preferably, the slurry-phase polymerization is carried out in a loop reactor. In such a reactor, the slurry is circulated at high speed along a closed pipeline by using a circulation pump. Loop reactors are well known in the art and some examples are given, such as those given in US 4582816A, US 3405109A, US 3324093A, EP 479186A and US 5391654A.

[0107] The art-recognized gas-phase reactors include any mechanically mixed, fluidized bed reactor, fast fluidized bed reactor or sedimentation bed reactor or gas-phase reactor having two separate zones, such as a fluidized bed combined with a sedimentation bed zone. Preferably, the gas-phase reactor for the second polymerization step is a fluidized bed reactor.

[0108] In a preferred embodiment of the invention, the LMW component is first produced and the HMW component is produced in the presence of the LMW component. In this case, the LMW component is the first polyethylene component (A) and the HMW component is the second polyethylene component (B).

[0109] The resulting final product consists of a homogeneous mixture of polymer components from the reactor, and the different molecular weight distribution curves of these polymers together form a molecular weight distribution curve having a broad maximum or multiple maxima, that is, the final product is a multimodal polymer mixture.

[0110] Preferably, the multimodal matrix resin of the polyethylene composition according to the invention is a bimodal polyethylene mixture composed of polymer components (A) and (B), and this bimodal polyethylene mixture optionally further comprises a small pre-polymerized component. It is also preferred that such a bimodal polymer mixture has been produced by the above polymerization reaction under different polymerization conditions in two or more polymerization reactors connected in series. Due to the flexibility of the reaction conditions thus obtained, the polymerization reaction is most preferably carried out in a loop reactor / gas-phase reactor combination.

[0111] According to a preferred embodiment of the present invention, the process comprises a slurry phase polymerization stage and a gas phase polymerization stage. A suitable reactor configuration comprises one to two slurry reactors, preferably loop reactors, and a gas phase reactor. Such polymerization configurations are described, for example, in patent documents such as WO92 / 12182A1, WO96 / 18662A1 and WO2010054732 of Borealis and are known as Borstar technology.

[0112] The catalyst can be transferred to the polymerization zone by any method known in the art. Thus, the catalyst can be suspended in a diluent and maintained as a homogeneous slurry. As disclosed in WO2006 / 063771, it is particularly preferred to use an oil with a viscosity of 20 mPa*s to 1500 mPa*s as the diluent. The catalyst can also be mixed with a viscous mixture of grease and oil, and the resulting paste can be fed into the polymerization zone. In addition, for example, in the manner disclosed in EP 428 054A1, the catalyst can also be allowed to settle, and a portion of the resulting catalyst slurry can be introduced into the polymerization zone.

[0113] The polymerization reaction in the slurry usually occurs in an inert diluent, usually a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentane, hexane, heptane, octane, etc., or a mixture thereof. Preferably, the diluent is a low-boiling hydrocarbon having 1 to 4 carbon atoms or a mixture of such hydrocarbons. A particularly preferred diluent is propane, which may contain small amounts of methane, ethane and / or butane.

[0114] The temperature of the slurry polymerization is generally 40°C to 115°C, preferably 60°C to 110°C, especially 70°C to 100°C. The pressure is 1 bar to 150 bar, preferably 10 bar to 100 bar.

[0115] The slurry polymerization can be carried out in any known reactor for slurry polymerization. Such reactors include continuous stirred tank reactors and loop reactors. It is particularly preferred to carry out the polymerization in a loop reactor. As is known in the art, optionally, hydrogen is fed into the reactor to control the molecular weight of the polymer.

[0116] In addition, one or more α-olefin copolymers can be added to the reactor to control the density and morphology of the polymer product. The actual amounts of such hydrogen and comonomer depend on the required melt index (or molecular weight) and density (or comonomer content) of the resulting polymer.

[0117] The gas phase polymerization can be carried out in a fluidized bed reactor, or in a fast fluidized bed reactor, or in a sedimentation bed reactor, or in any combination of these reactors.

[0118] Typically, the operating temperature range of a fluidized bed or precipitation bed polymerization reactor is from 50°C to 100°C, more preferably from 65°C to 90°C. The pressure is suitably from 10 bar to 40 bar, more preferably from 15 bar to 30 bar.

[0119] In addition, if desired, an antistatic agent can be introduced into the slurry and / or gas phase reactor.

[0120] The process may further include pre-reactors and post-reactors.

[0121] Before the polymerization step, there may be a pre-polymerization step. The pre-polymerization step can be carried out in slurry or gas phase. Preferably, the pre-polymerization is carried out in slurry, especially in a loop reactor. The temperature of the pre-polymerization step is generally from 0°C to 90°C, preferably from 20°C to 80°C, and more preferably from 30°C to 70°C.

[0122] The pressure is not crucial and is generally from 1 bar to 150 bar, preferably from 10 bar to 100 bar.

[0123] The polymerization can be carried out continuously or batchwise, preferably continuously.

[0124] In the first example of the process, the polymerization of the olefin is carried out in a multi-stage polymerization process that includes at least one gas phase reactor for producing ethylene (co)polymers.

[0125] In the second example of the process, the polymerization of ethylene and 1-hexene described herein is carried out in a multi-stage polymerization process that includes at least one slurry reactor, such as one or two slurry reactors, preferably two slurry reactors, and a gas phase reactor.

[0126] A chain transfer agent, preferably hydrogen, is added to the reactor as needed. Preferably, when producing LMW components in the reactor, 100 mol to 1400 mol of H 2 / kmol of ethylene is added to the reactor; when producing HMW components in the reactor, 0 mol to 70 mol of H 2 / kmol of ethylene is added to the gas phase reactor.

[0127] The polymerization reaction is carried out in the presence of an olefin polymerization catalyst. The catalyst is preferably a Ziegler-Natta (ZN) catalyst, which generally comprises at least one catalyst component formed from the following components: transition metal compounds of Groups 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), metal compounds of Groups 1 to 3 of the Periodic Table (IUPAC), optionally a Group 13 compound of the Periodic Table (IUPAC), and optionally an internal organic compound (such as an internal electron donor). The ZN catalyst may also contain further catalyst components, such as a cocatalyst and optionally an external additive.

[0128] A suitable ZN catalyst preferably comprises a magnesium compound, an aluminum compound, and a titanium compound supported on a particulate carrier.

[0129] The particulate carrier may be an inorganic oxide carrier, such as silica, alumina, titanium dioxide, silica-alumina, silica-titanium dioxide, or a MgCl 2 -based carrier. Preferably, the carrier is silica or a MgCl 2 -based carrier.

[0130] A particularly preferred Ziegler-Natta catalyst is the catalyst as described in EP 1378528A1.

[0131] If used, the magnesium compound is preferably the reaction product of a dialkylmagnesium and an alcohol. The alcohol is a straight-chain or branched-chain aliphatic monohydric alcohol. Preferably, the alcohol has 6 to 16 carbon atoms. Branched-chain alcohols are particularly preferred, and 2-ethyl-1-hexanol is an example of a preferred alcohol. The dialkylmagnesium can be any compound in which magnesium is combined with two alkyl groups, which may be the same or different. Butyloctylmagnesium is an example of a preferred dialkylmagnesium.

[0132] The aluminum compound is an alkylaluminum containing chlorine. Particularly preferred compounds are aluminiumalkyl dichloride and aluminium alkyl sesquichlorides.

[0133] The transition metal compounds of Groups 4 to 6 are preferably titanium compounds or vanadium compounds, more preferably halogen-containing titanium compounds, and most preferably chlorine-containing titanium compounds. A particularly preferred titanium compound is titanium tetrachloride.

[0134] As described in EP 688794 or WO 99 / 51646, the catalyst can be prepared by contacting the support with the above compounds in sequence. Alternatively, as described in WO 01 / 55230, the catalyst can be prepared by first preparing a solution from the components and then contacting the solution with the support.

[0135] Another suitable ZN catalyst comprises a titanium compound and a magnesium halide compound as the support. Thus, the catalyst comprises a titanium compound and optionally a Group 13 compound, such as an aluminum compound supported on magnesium dihalide (such as magnesium dichloride). For example, such catalysts are disclosed in WO 2005 / 118655, EP 810235, WO2014 / 096296 and WO2016 / 097193.

[0136] Suitable activators are Group 13 metal compounds, typically alkyl compounds of Group 13, especially alkylaluminum compounds, wherein the alkyl contains 1 to 16 C atoms. These compounds include trialkylaluminum compounds such as trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum and tri-n-octylaluminum; alkylaluminum halides such as dichloroethylaluminum, diethylaluminum chloride, sesquiethylaluminum chloride, dimethylaluminum chloride, etc. Particularly preferred activators are trialkylaluminums, among which triethylaluminum, trimethylaluminum and triisobutylaluminum are especially used.

[0137] The amount of the activator used depends on the specific catalyst and activator. Generally, triethylaluminum is used in an amount such that the molar ratio of aluminum to the transition metal (such as Al / Ti) is from 1 mol / mol to 1000 mol / mol, preferably from 3 mol / mol to 100 mol / mol, especially from 5 mol / mol to about 30 mol / mol.

[0138] The optional internal organic compound can be selected from the following categories: ethers, esters, amines, ketones, alcohols, acid anhydrides or nitriles or mixtures thereof. Preferably, the optional internal organic compound is selected from ethers and esters, and most preferably from ethers. Preferred ethers are ethers having 2 to 20 carbon atoms, especially monocyclic, bicyclic or polycyclic saturated or unsaturated ethers containing 3 to 6 ring atoms. Typical cyclic ethers applicable in the present invention (if used) are tetrahydrofuran (THF); substituted THF such as 2-methyltetrahydrofuran; bicyclic ethers such as 2,2-bis(2-tetrahydrofuran)propane, 2,2-bis(2-furan)propane or its isomers or mixtures thereof. The internal organic compound is also often referred to as an internal electron donor.

[0139] The composition according to the present invention is preferably produced in a process comprising a compounding step, wherein the composition, i.e., the mixture, is generally obtained as a polyolefin matrix resin powder from a reactor, extruded in an extruder, and then made into polymer pellets in a manner known in the art. The extruder can be, for example, any conventionally used extruder. For example, the extruder used for the current compounding step can be provided by Japan Steel works, Kobe Steel or Farrel-Pomini, such as JSW460P or JSW CIM90P.

[0140] In certain embodiments, the feed rate employed in the extrusion step is from 100 kg / h to 500 kg / h, and more preferably from 150 kg / h to 300 kg / h.

[0141] The screw speed of the extruder can be from 200 rpm to 500 rpm, and more preferably from 300 rpm to 450 rpm.

[0142] In certain embodiments, in the extrusion step, the SEI (specific energy input) of the extruder can be from 100 kWh / ton to 400 kWh / ton, and more preferably from 125 kWh / ton to 300 kWh / ton.

[0143] The melt temperature in the extrusion step is preferably from 200 °C to 300 °C, and more preferably from 230 °C to 270 °C.

[0144] According to a further aspect, the present invention relates to a polyethylene composition obtainable by a multi-stage process as described above, in particular the multi-stage process comprises or consists of the following steps

[0145] a) polymerizing ethylene in the presence of

[0146] - a catalyst

[0147] - in one or more loop reactors, preferably in one or two loop reactors, in the presence of an alkylaluminum compound and a chain transfer agent to obtain component (A), which has a melt flow rate MFR of from 100 g / 10 min to 600 g / 10 min 2 , preferably from 100 g / 10 min to 550 g / 10 min, and more preferably from 115 g / 10 min to 450 g / 10 min; and

[0148] b) transferring component (A) to a gas phase reactor

[0149] - feeding ethylene and a comonomer into the gas phase reactor,

[0150] - Further polymerization to obtain a matrix resin, which comprises component (A) obtained in step a) and component (B) obtained in step b),

[0151] wherein, based on the total amount of component (B), the content of units derived from 1-hexene in component (B) of the matrix resin is 0.81 mol% to 1.60 mol%, preferably 0.83 mol% to 1.59 mol%, and more preferably 0.85 mol% to 1.58 mol%; and

[0152] wherein component (A) has a lower molecular weight than component (B), and wherein, based on the total weight of the matrix resin, the content of component (B) present is 50.0 wt.% to 58.0 wt.%, preferably 51.0 wt.% to 57.0 wt.%, more preferably 52.0 wt.% to 56.0 wt.%, and even more preferably 53.0 wt.% to 55.0 wt.%,

[0153] c) Extruding the matrix resin into a polyethylene composition having a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min, preferably 0.12 g / 10 min to 0.22 g / 10 min, and more preferably 0.14 g / 10 min to 0.20 g / 10 min, and having a melt flow rate ratio FRR 21 / 5 of 30 to 42.

[0154] According to a preferred embodiment, the process comprises a prepolymerization step before step a).

[0155] A further preferred polymerization catalyst is a ZN catalyst.

[0156] According to a preferred embodiment, the matrix resin has a molecular weight distribution (the ratio of Mw / Mn) of 32 to 40, preferably 32.5 to 39.5, and more preferably 33 to 39; and / or the matrix resin has a Z-average molecular weight Mz greater than 1500 kg / mol, preferably greater than 1600 kg / mol, and more preferably greater than 1700 kg / mol; and / or the matrix resin has a viscosity greater than 700 kPa*s at a shear stress of 747 Pa (eta 747 )), preferably greater than 730 kPa*s, more preferably greater than 750 kPa*s and not greater than 1400 kPa*s; and / or the polyethylene composition has a critical temperature Tc of -10 °C or lower in a rapid crack propagation test, preferably -12 °C or lower and not lower than -25 °C, preferably not lower than -23 °C.

[0157] In a further aspect, the present invention relates to an article comprising or consisting of a polyethylene composition according to any one of the embodiments described herein.

[0158] Preferably, the article is a pipe or a pipe fitting.

[0159] A further preferred article is a pipe, wherein the pipe has a stress crack resistance measured by the notched pipe test (NPT) of at least 3000 h, more preferably at least 4000 h, and even more preferably at least 4682 h.

[0160] A further preferred article is a pipe, wherein the pipe has a critical temperature Tc of -10 °C or lower, preferably -12 °C or lower, and more preferably -13 °C or lower and not lower than -25 °C, preferably not lower than -23 °C; and / or wherein the pipe has a failure time greater than 1500 h in an accelerated creep test (ACT), preferably greater than 2000 h, more preferably greater than 3000 h; and / or wherein the pipe has a failure time of at least 200 h, preferably at least 400 h, and preferably at least 500 h in a short-term pressure resistance (STPR) test at a stress level of 5.4 MPa and a temperature of 80 °C; and / or wherein the pipe has a failure time of at least 130 h, preferably at least 150 h, more preferably at least 175 h, and even more preferably at least 200 h in a short-term pressure resistance (STPR) test at a stress level of 12.0 MPa and a temperature of 20 °C.

[0161] This demonstrates a good balance between the pressure resistance, rapid crack propagation resistance, and slow crack growth resistance of the pipes of the present invention.

[0162] According to another aspect, the present invention relates to the use of a polyethylene composition according to any one of the embodiments described herein for the production of an article (preferably a pipe or a pipe fitting).

[0163] Unless otherwise explicitly stated, it is to be understood that the description of the present invention is such that any one or more of the preferred embodiments of the invention described above can be combined with the invention described in the most general terms.

[0164] For the sake of completeness, it should be noted that although certain properties (such as short-term pressure resistance) are tested on specific test pipe samples (such as pipes of a specific thickness and diameter), they are still properties of the polymer composition used to make the test pipe samples.

[0165] The methods for measuring and determining the parameters used herein are given below, and the present invention is further illustrated by the invention examples and comparative examples. Detailed Description

[0166] Methods for Measurement and Determination

[0167] Unless otherwise specified, the following methods are used to determine the properties of the polymer compositions or their components described in the specification or in the experimental part below and in the claims. Unless otherwise stated, the samples used for testing consist of the polymer composition to be tested or, respectively, of the polymer components to be tested as specified.

[0168] Melt flow rate

[0169] The melt flow rate (MFR) is determined in accordance with ISO 1133 and is expressed in g / 10 min. The higher the melt flow rate, the lower the viscosity of the polymer. For polyethylene, the MFR is determined at 190 °C and at a load of 2.16 kg (MFR 2 ), 5.00 kg (MFR 5 ), or 21.6 kg (MFR 21 ). The quantity of FRR (flow rate ratio) is an indicator of the molecular weight distribution and represents the ratio of the flow rates at different loads. Thus, FRR 21 / 5 represents the value of MFR 21 / MFR 5 .

[0170] Molecular weight properties

[0171] In accordance with ISO 16014-4:2003 and ASTM D 6474-99, the number-average molecular weight (Mz, Mw, and Mn), molecular weight distribution (MWD), and its breadth, described by the polydispersity index, PDI = Mw / Mn (where Mn is the number-average molecular weight and Mw is the weight-average molecular weight), are determined by gel permeation chromatography (GPC) using the following formula:

[0172]

[0173]

[0174]

[0175] For a constant elution interval ΔV i , where A i and M i are the chromatographic peak slice area and the polyolefin molecular weight (MW).

[0176] PolymerChar GPC instrument, equipped with an infrared (IR) detector, used with 3x Olexis and 1x Olexis Guard columns from Polymer Laboratories, and using 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg / l of 2,6-di-tert-butyl-4-methyl-phenol) as the solvent, at a temperature of 160 °C and a constant flow rate of 1 ml / min. 200 μL of the sample solution was injected for each analysis. The column set was calibrated by universal calibration using at least 15 narrow MWD polystyrene (PS) standards ranging from 0.5 kg / mol to 11,500 kg / mol (according to ISO 16014-2:2003). The Mark Houwink constants for PS, PE, and PP are as described in ASTM D 6474-99. All samples were prepared as follows: In the autosampler of the GPC instrument, 5.0 mg to 9.0 mg of the polymer was dissolved in 8 ml (at 160 °C) of stabilized TCB (the same as the mobile phase), and under continuous gentle shaking at 160 °C, it was dissolved for 2.5 hours for PP or 3 hours for PE.

[0177] Density

[0178] The density of the polymer was measured on compression-molded specimens prepared according to EN ISO 1872-2 (February 2007) in accordance with ISO 1183-1:2004 (Method A), in kg / m 3 Provided.

[0179] Notched Pipe Pressure Test (NPT)

[0180] The resistance to slow crack growth was determined according to ISO 13479-2009 by the number of hours the pipe withstood a certain pressure at a certain temperature before failure. The pressure test was carried out on an SDR 11 notched pipe with an outer diameter of 110 mm. The pressure used was 9.2 bar and the temperature was 80 °C. A notch was formed using a climb milling cutter conforming to ISO 6108 and including a V-shaped milling cutter with a 60° angle, with a cutting rate of 0.010 ± 0.002 (mm / rev) / tooth. The cutter used had 24 teeth and the cutter speed was 680 rpm. The remaining ligament was 0.82 - 0.78 times the minimum wall thickness. The depth of the notch was calculated by the following formula. h is the depth of the notch, expressed in mm. These four notches were evenly distributed around the circumference of the pipe.

[0181] The length of the notch was 110 ± 1 mm.

[0182]

[0183] Where

[0184] b s : Width of the notched machining surface, expressed in millimeters;

[0185] d em : Measured average outside diameter of the pipe, expressed in millimeters.

[0186] Fast Crack Propagation (Critical Temperature, Tc)

[0187] The resistance of a pipe to fast crack propagation (RCP) can be determined by a method known as the S4 test (Small-Scale Steady State), which was developed by Imperial College (London) and described in ISO 13477:2008. The outside diameter of the pipe is approximately 110 mm or greater, and the wall thickness is approximately 10 mm or greater. When determining the RCP performance of the pipe of the present invention, an outside diameter and a wall thickness of 110 mm and 10 mm, respectively, are selected. The length of the pipe is 785 mm. When the outside of the pipe is at ambient pressure (atmospheric pressure), the inside of the pipe is pressurized, and the pressure inside the pipe is maintained constant at a positive pressure of 4.0 bar. The length of the gauge is 590 mm. The pipe and the equipment around it are adjusted to a predetermined temperature. A number of discs are installed on the axis inside the pipe to prevent decompression during the test. In the so-called initiation zone, a knife projectile with a well-defined shape and a mass of 1500 g is launched along the pipe towards one of its ends to initiate a fast-running axial crack. The speed of the knife is 16 + / - 1 m / s. The initiation zone is provided with abutments to avoid unnecessary deformation of the pipe. The test equipment is adjusted to cause cracks in the material involved, and several tests are carried out at different temperatures. In each test, the axial crack length is measured in a measurement area with a total length of 4.7 times the diameter, and a curve is plotted according to the set test temperature. If the crack length exceeds 4.7 times the diameter, the crack is considered to have propagated. If the pipe passes the test at a given temperature, the temperature is successively lowered until a certain temperature is reached at which the crack propagation exceeds 4.7 times the pipe diameter and the pipe no longer passes the test.

[0188] Critical temperature (T c ) measured according to ISO 13477:2008, which is the ductile-brittle transition temperature and is the lowest temperature at which the pipe passes the test. The lower this critical temperature is, the better, as it extends the applicability of the pipe.

[0189] Short-Term Pressure Resistance (STPR)

[0190] The pressure test of a 32 mm SDR 11 pipe without notch and with a length of 450 mm was carried out in internal and external water environments in accordance with ISO 1167-1:2006. End caps of type A were used. The failure time was expressed in hours. The applied circumferential stresses were 5.4 MPa, 5.7 MPa and 12.0 MPa, and the temperature was 80 °C.

[0191] Charpy impact strength (CIS)

[0192] According to ISO179 / 1eA:2000, for 80*10*4 mm 3 V-notch samples, the Charpy impact strength was determined at 23 °C (Charpy impact strength (23 °C)), 0 °C (Charpy impact strength (0 °C)) and -20 °C (Charpy impact strength (-20 °C)). According to ISO 293:2004, using the conditions defined in Chapter 3.3 of ISO 1872-2:2007, samples were milled from 4 mm thick plates prepared by compression molding.

[0193] Yield stress (in tensile test at 80 °C and E-4s -1 strain rate)

[0194] Type 5a specimens according to ISO 527-2 were milled from compression molded plates. The tensile test was carried out at 80 °C and E-4s -1 strain rate (corresponding to a test speed of 0.3 mm at a grip length of 50 mm). Before testing, the samples were conditioned at the test temperature for half an hour. The yield stress was determined as the first peak in the nominal stress-strain curve. Compared with testing at room temperature, the yield kinetics of PE under these test conditions showed less obvious test speed dependence and thus was more in line with long-term yield characteristics.

[0195] Accelerated point load test (PLT+)

[0196] The accelerated point load test (PLT+) was carried out according to PAS 1075 and simulated the installation situation where the pipe was affected by external interference of point loads such as stones or sharp-cornered objects.

[0197] The test procedure was similar to the internal pressure creep test, except that an external point load (usually an impact) was also applied before the internal pressure. To further accelerate the test, a wetting agent solution was applied to the test samples.

[0198] The accelerated point load test (PLT+) was carried out by Hessel Ingenieurtechnik GmbH in Loetgen, Germany. Both ends of the SDR 11 pipes with an outer diameter of 32 mm or 110 mm were closed using end caps. One of the end caps was equipped with a pressure connection. The required surface elongation of the inner pipe wall (i.e., the above-mentioned yield elongation) was generated by the displacement of the tool along the outer surface of the pipe radius, and the tip radius of the tool was 5 mm. The internal pressure of the pipe was selected according to the circumferential stress of 4 N / mm 2 . The test temperature was 90 °C. In order to shorten the test time, the test was carried out in demineralized water continuously mixed with 2% aqueous solution of "Netzmittel 5" (a surfactant used by Hessel Ingenieurtechnik) inside the pipe. The point load test has been carried out on individual specimens.

[0199] Accelerated creep test (ACT)

[0200] As defined in the present invention, the long-term stability as defined in the accelerated full notch creep test (FNCT), particularly refers to the crack growth resistance, which is determined according to the test method FNCTNM5. The results reported here on FNCTNM5 were obtained by testing carried out by Hessel Ingenieurtechnik GmbH in Loetgen, Germany according to ISO16770.

[0201] The materials used for the test were made into plates under the following compression molding conditions:

[0202] Heating rate 15 °C / min, from 30 °C to 180 °C in 10 min, without pressure;

[0203] 25 bar pressure, 180 °C, 35 min;

[0204] 144 bar pressure, 180 °C, 25 min;

[0205] Slow cooling rate 2 °C / min, from 180 °C to 30 °C, pressure 144 bar, 75 min;

[0206] In the last part, at 30 °C, press and open after 2 minutes;

[0207] The plates were circular, with a diameter of 150 mm and a thickness of 10 mm.

[0208] The tests were carried out on notched specimens, which were machined from plates with parallel sides and a square cross-section (10 mm × 10 mm). The notch of each specimen was perpendicular to the parallel length in the middle of the test specimen. The notched specimens were tested in a 2% aqueous solution of "Netzmittel 5" (a surfactant used by Hessel Ingenieurtechnik GmbH) at 90 °C and 4 MPa. Three specimens were tested for each material, and the geometric mean was reported.

[0209] Eta 747

[0210] A method closely related to the sagging characteristics and used in combination with the present invention relates to the rheology of polymers and is based on measuring the viscosity of polymers under very low constant shear stress. The shear stress selected for this method is 747 Pa. The viscosity of the polymer under this shear stress is measured at a temperature of 190 °C and is found to be inversely proportional to the gravity flow of the polymer, that is, the greater the viscosity, the lower the gravity flow.

[0211] A rotational rheometer is used to measure the viscosity under a shear stress of 747 Pa, which can be a constant stress rheometer such as the Anton Paar MCR series rheometer. The rheometer and its functions have been described on pages 492 - 509 of Volume 14 of the 2nd edition of the "Encyclopedia of Polymer Science and Engineering". The measurement is carried out under a constant shear stress (constant rotation direction) between two plates with a diameter of 25 mm. The gap between the plates is 1.2 mm. A polymer sample with a thickness of 1.2 mm is inserted between the plates.

[0212] Before the start of the measurement, the sample is temperature-adjusted within 2 minutes. The measurement is carried out at 190 °C. After the temperature adjustment, the measurement is started by applying a predetermined stress. The stress is maintained for 1800 s to bring the system close to the steady-state condition. After that, the measurement is started and the viscosity is calculated.

[0213] The measurement principle is to apply a certain torque to the plate shaft by a precision motor. This torque is then converted into shear stress in the sample. This shear stress remains constant. The rotational speed generated by the shear stress is recorded and used to calculate the viscosity of the sample.

[0214] Strain hardening (SH) modulus

[0215] The strain hardening modulus of the compound is obtained from the tensile stress-strain curve above the natural draw ratio and represents the slope of the increasing stress-strain trend at very high strains (strain hardening state). It is measured according to ISO 18488 on a 300 μm thick pre-treated (120 °C / 1 h) sample at 80 °C and 20 mm / min.

[0216] Quantifying Microstructure by NMR Spectroscopy

[0217] Quantitative nuclear magnetic resonance (NMR) spectroscopy is used to quantify the comonomer content of polymers.

[0218] Using a Bruker Avance III 500 NMR spectrometer, for 1 H and 13 C, operations are carried out at 500.13 MHz and 125.76 MHz respectively, and quantitative 13 C{ 1 H} NMR spectra in the molten state are recorded. Nitrogen is used for all pneumatics, and all spectra are recorded using a 13 C-optimized 7 mm magic angle spinning (MAS) probe at 150 °C. Approximately 200 mg of the material is filled into a zirconia MAS rotor with an outer diameter of 7 mm and rotated at a speed of 4 kHz. This setting is chosen mainly for the high sensitivity required for rapid identification and precise quantification (Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382; Parkinson, M., Klimke, K., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2007; 208:2128; Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau, M., Polymer 50 (2009) 237).

[0219] Standard single-pulse excitation was employed, and NOE was utilized with a short recycle delay of 3 s (Pollard, M., Klimke, K., Graf, R., Spiess, H.W., Wilhelm, M., Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004; 37:813; Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382.) and the RS-HEPT decoupling scheme (Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239; Griffin, J.M., Tripon, C., Samoson, A., Filip, C., and Brown, S.P., Mag. Res. in Chem. 2007 45, S1, S198). A total of 16384 (16k) transients were acquired for each spectrum.

[0220] For 13 C{ 1 H} NMR quantitative spectra were processed by integration, and the integral and related quantitative properties were determined. All chemical shifts were internally referenced to the signal of the main methylene group at 30.00 ppm (d) (J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201).

[0221] Characteristic signals corresponding to 1-hexene incorporation were observed (J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201). The integral of the signal at 38.2 ppm for the *B4 site, which accounts for the number of each comonomer reporting site, was used to quantify the incorporation of isolated 1-hexene in the EEHEE sequence:

[0222] H = I * B4

[0223] Characteristic signals generated by saturated end groups were observed. The average of the signal integrals at 22.8 ppm and 32.2 ppm, which were respectively attributed to the 2s (I 2S ) and 3s (I 3S ) sites, was used to quantify the content of such saturated end groups:

[0224] S = (1 / 2) * (I 2S + I 3S )

[0225] Quantify the ethylene content using the integral of the main methylene group (I d ) signal at 30.00 ppm. This integral includes the g-site and the 3B4-site from 1-hexene.

[0226] Calculate the total ethylene content based on the main integral and compensate for the observed 1-hexene sequences and end groups:

[0227] E = (I d / 2) + (3*H / 2) – (H / 2) + (3*S / 2)

[0228] Then calculate the total mole fraction of 1-hexene in the polymer:

[0229] fH = H / (E + H)

[0230] Calculate the mole percentage of comonomer incorporation from the mole fraction:

[0231] H [mol%] = 100*fH

[0232] Calculate the weight percentage of comonomer incorporation from the mole fraction:

[0233] H [wt%] = 100*(fH*84.16) / ((fH*84.16) + ((1 - fH)*28.05))

[0234] Calculate the mole percentage of 1-hexene comonomer incorporation in the high Mw fraction from the total comonomer incorporation using a conventional method:

[0235] H [mol%] in HMW = 100%*H [mol%] / share of HMW fraction [%]

[0236] Note: Do not consider the amount of HDPE from the masterbatch carrier resin.

[0237] Melt pressure

[0238] During pipe extrusion, the melt pressure is continuously measured and recorded by a calibrated melt pressure sensor mounted on the adapter of the machine. The adapter is a connecting piece between the extruder and the tool. It is a conical piece that reduces the diameter from the machine outlet to the tool inlet. The adapter device has two holes with threats where the melt pressure sensor and the melt temperature sensor are fixed. The sensors used are from Gefran, Italy, Model Gefran M30 - 6 - M - B07C - 1 - 4 - 0 - XM228, 2130X000X000. The pressure range is up to 700 bar.

[0239] Example

[0240] Materials

[0241] Comparative Example 5 (CE5) is the commercially available black polyethylene composition Eltex TUB121N9000 from INEOS.

[0242] The catalyst component used for ethylene (co)polymerization in the invention and comparative examples is Lynx 200, a commercially available Ziegler - Natta catalyst manufactured and supplied by Grace Catalysts Technologies.

[0243] Preparation of Polymers

[0244] The polyethylene matrix resins and compositions according to the present invention (IE1 - IE7) and for comparison (CE1 - CE4) are produced using the Lynx 200 catalyst.

[0245] CE1:

[0246] A loop reactor with a volume of 50 dm operates at a temperature of 60 °C and a pressure of 56 bar. Ethylene, propane diluent, and hydrogen are injected into the reactor. The solid polymerization catalyst component Lynx 200 is introduced into the reactor together with the triethylaluminum cocatalyst, with an Al / Ti molar ratio of approximately 15 mol / mol. The estimated production share is 2 wt%. 3 The slurry stream is continuously withdrawn and directed to a loop reactor with a volume of 150 dm, which operates at a temperature of 95 °C and a pressure of 54.5 bar. Additional ethylene, propane diluent, and hydrogen are further added to the reactor so that the ethylene concentration in the fluid mixture is 2.9 mol% by mole, and the ratio of hydrogen to ethylene is 268 mol / kmol. The estimated production share is 19 wt%. The ethylene homopolymer withdrawn from the reactor has an MFR of 23 g / 10 min.

[0247] The slurry stream from the reactor is intermittently withdrawn and directed to a loop reactor with a volume of 350 dm, which operates at a temperature of 95 °C and a pressure of 53 bar. Fresh propane, ethylene, and hydrogen are further added to the reactor so that the ethylene concentration in the fluid mixture is 3.5 mol%, and the molar ratio of hydrogen to ethylene is 242 mol / kmol. The ethylene homopolymer withdrawn from the reactor has an MFR of 20 g / 10 min. 3 The estimated production share is 31 wt%. 2

[0248] The slurry stream from the reactor is intermittently withdrawn and directed to a loop reactor with a volume of 350 dm, which operates at a temperature of 95 °C and a pressure of 53 bar. Fresh propane, ethylene, and hydrogen are further added to the reactor so that the ethylene concentration in the fluid mixture is 3.5 mol%, and the molar ratio of hydrogen to ethylene is 242 mol / kmol. The ethylene homopolymer withdrawn from the reactor has an MFR of 20 g / 10 min. 3 The estimated production share is 31 wt%. 2

[0249] ​​The slurry is intermittently withdrawn from the loop reactor and directed to a flash evaporator operating at a temperature of 50 °C and a pressure of 3 bar. From there, the polymer is directed to a fluidized bed gas phase reactor operating at a pressure of 20 bar and a temperature of 85 °C. Additional ethylene, 1-hexene comonomer, nitrogen as an inert gas, and hydrogen are added, such that the molar ratio of hydrogen to ethylene is 1 mol / kmol and the molar ratio of 1-hexene to ethylene is 42 mol / kmol. The estimated production share is 48 wt%. The polymer has a melt flow rate MFR of 0.18 g / 10 min 5 and a density of 946 kg / m 3 .

[0250] IE1 to IE7 and CE2 to CE4:

[0251] As shown in Table 2, the process of CE1 is repeated by changing the reactor conditions.

[0252] The polymer powders of samples IE1 to IE7 and CE1 to CE4 are each mixed with 5.5% carbon black masterbatch (CB content 40%), 2500 ppm antioxidant, and 400 ppm calcium stearate in a nitrogen atmosphere. Then, they are compounded using a JSW CIMP90 twin-screw extruder under a nitrogen atmosphere and extruded into pellets, with a melt temperature of approximately 280 °C and an SEI of 170 kWh / ton to 250 kWh / ton, to obtain a polyethylene composition.

[0253] Pipe extrusion

[0254] Pipe extrusion is carried out on a Kraus-Maffei 45-36D (L / D) single-screw extruder for pipes with an outer diameter x wall thickness of 32 x 3 mm and 110 x 10 mm. The extruder has an improved pp barrier screw and is equipped with four heating barrel zones and five tool zones. The downstream equipment is a 9 m long spray cooling vacuum box with two chambers, and the specified water temperature is 20 °C.

[0255] For the 32 mm pipe extrusion, the samples are processed at an output rate of 50 kg / h and a screw speed of the extruder of ~57 rpm. The melt temperature reaches 220 °C - 221 °C at a melt pressure of 213 bar - 216 bar. All samples are produced at a constant instrument weight of 280 g / m and a line speed of 2.97 m / min.

[0256] The 110 mm pipes of the samples are processed at an output rate of 160 kg / h and a screw speed of 183 rpm - 185 rpm. The melt temperature reaches 209 °C at a melt pressure of 201 bar - 205 bar. All samples are produced at a constant instrument weight of 3.13 kg / m and a line speed of 0.85 m / min.

[0257] Table 1: Pipe Extrusion Temperature Characteristics of 32 mm and 110 mm Pipes

[0258]

[0259] Table 2: Polymerization Conditions of IE1 to IE7 and CE1 to CE4

[0260]

[0261]

[0262] Table 3: Properties of Matrix Resin and Polyethylene Composition of IE1 to IE7 and CE1 to CE5

[0263]

[0264]

[0265] As can be seen from Table 3, IE1 to IE7 show a combination of excellent resistance to slow crack growth, very good impact resistance and good resistance to rapid crack propagation (low critical temperature), while also meeting the requirements of the PE100 standard and having very good anti-sag properties (as indicated by the Mz value). In addition, the compositions of IE1 to IE7 also show excellent behavior in the accelerated point load test and the ACT test.

[0266] This favorable combination of properties is achieved by specific Mz and eta 747 values, specific MWD (indicated by the values of Mw / Mn and FRR 21 / 5 ), a specific content of 1-hexene in the high molecular weight component, a specific content (weight fraction) of the high molecular weight component, MFR within a defined range and the specific comonomer (1-hexene) used.

[0267] Comparative Examples 1 to 5 show that compared with IE1 to IE7, deviations from the polymer structure of the invention result in polymer compositions with a poor combination of properties. None of Comparative Examples 1 to 5 have the advantageous combination of the properties of the invented polymer composition.

Claims

1. A polyethylene composition comprising a matrix resin, said matrix resin comprising (A) a first ethylene homopolymer or copolymer component, wherein, Component (A) has a melt flow rate MFR 2 , measured in accordance with ISO 1133, of from 100 g / 10 min to 600 g / 10 min; and (B) a second ethylene-1-hexene copolymer component, wherein component (A) has a lower molecular weight than component (B), and wherein, based on the total weight of the matrix resin, component (B) is present in an amount of 50.0 wt.% to 58.0 wt.%; wherein, based on the total amount of the matrix resin, the matrix resin has a content of units derived from 1-hexene of 0.44 mol% to 0.79 mol%; wherein the matrix resin has a molecular weight distribution, i.e., the Mw / Mn ratio, of 32 to 40; the matrix resin has a Z-average molecular weight Mz greater than 1500 kg / mol; Among them, the polyethylene composition has a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min; the melt flow rate ratio FRR 21 / 5 is 30 to 42; and wherein the polyethylene composition has a critical temperature Tc of -10 °C or lower and not lower than -25 °C in a rapid crack propagation test.

2. The polyethylene composition according to claim 1, wherein, the polyethylene composition has a strain hardening modulus of 75 MPa or higher; and / or wherein the polyethylene composition has a failure time greater than 1500 h in an accelerated creep test (ACT); and / or wherein the polyethylene composition has a failure time of at least 200 h in a short-term compression (STPR) test at a stress level of 5.4 MPa and a temperature of 80 °C; and / or wherein the polyethylene composition has a failure time of at least 130 h in a short-term compression (STPR) test at a stress level of 12.0 MPa and a temperature of 20 °C; and / or wherein the polyethylene composition has a yield stress of 6.0 MPa to 7.0 MPa at 80 °C; and / or The matrix resin has a viscosity greater than 700 kPa·s and not greater than 1400 kPa·s at a shear stress (eta 747 ) of 747 Pa.

3. The polyethylene composition according to claim 1, wherein, Component (A) has a melt flow rate MFR 2 , measured according to ISO 1133, of from 100 g / 10 min to 230 g / 10 min; or Wherein, the component (A) has a melt flow rate MFR 2 , measured according to ISO 1133, is from 231 g / 10 min to 550 g / 10 min.

4. The polyethylene composition according to any one of claims 1 to 3, wherein, based on the total amount of component (B), the component (B) of the matrix resin has a content of units derived from 1-hexene of 0.81 mol% to 1.60 mol%; and / or wherein, based on the total weight of the matrix resin, component (B) is present in a content of 51.0 wt.% to 57.0 wt.%.

5. The polyethylene composition according to any one of claims 1 to 3, wherein, the matrix resin has a number average molecular weight Mn of 7300 g / mol or higher; and / or wherein the matrix resin has a number average molecular weight Mn of 9150 g / mol or lower; and / or wherein the matrix resin has a molecular weight distribution, i.e., the Mw / Mn ratio, of 32.5 to 39.5; and / or wherein the matrix resin has a Z-average molecular weight Mz greater than 1600 kg / mol.

6. The polyethylene composition according to any one of claims 1 to 3, wherein, The matrix resin has a density of at least 945 kg / m 3 ; and / or Wherein, the polyethylene composition has a density of at least 953 kg / m 3 ; and / or wherein, based on the total amount of the matrix resin, the matrix resin has a content of units derived from 1-hexene of 0.45 mol% to 0.78 mol%.

7. The polyethylene composition according to any one of claims 1 to 3, wherein, The polyethylene composition has a melt flow rate MFR 5 of 0.12 g / 10 min to 0.22 g / 10 min; and / or Among them, in the rapid crack propagation test, the polyethylene composition has a critical temperature Tc of -12 °C or lower and / or not lower than -23 °C.

8. A method for producing the polyethylene composition according to any one of claims 1 to 7, wherein, the matrix resin is produced by a multi-stage polymerization process in the presence of a Ziegler-Natta catalyst.

9. A polyethylene composition obtained by a multi-stage process, the multi-stage process comprising the steps a) polymerizing ethylene in the presence of - a catalyst - In one or more loop reactors, in the presence of an alkylaluminum compound and a chain transfer agent to obtain component (A), said component (A) having a melt flow rate MFR of 100 g / 10 min to 600 g / 10 min 2 ; and b) transferring component (A) to a gas-phase reactor - feeding ethylene and a comonomer into the gas-phase reactor, - further polymerizing to obtain a matrix resin, which comprises component (A) obtained in step a) and component (B) obtained in step b), wherein, based on the total amount of component (B), the content of units derived from 1-hexene in component (B) of the matrix resin is 0.81 mol% to 1.60 mol%; and wherein component (A) has a lower molecular weight than component (B), and wherein, based on the total weight of the matrix resin, the content of component (B) present is 50.0 wt.% to 58.0 wt.%; c) extruding the matrix resin to have a melt flow rate MFR 5 of 0.10 g / 10 min to 0.25 g / 10 min and having a melt flow rate ratio FRR 21 / 5 of 30 to 42 of a polyethylene composition; wherein the matrix resin has a molecular weight distribution, i.e., the ratio of Mw / Mn, of 32 to 40; and wherein the matrix resin has a Z-average molecular weight Mz greater than 1500 kg / mol.

10. The polyethylene composition according to claim 9, wherein, the process comprises a prepolymerization step before step a); and / or wherein the matrix resin has a molecular weight distribution, i.e., the ratio of Mw / Mn, of 32.5 to 39.5; and / or wherein the matrix resin has a Z-average molecular weight Mz greater than 1600 kg / mol; and / or wherein the matrix resin has a viscosity greater than 700 kPa·s and not greater than 1400 kPa·s at a shear stress (eta747) of 747 Pa; and / or wherein, in the rapid crack propagation test, the polyethylene composition has a critical temperature Tc of -10 °C or lower and not lower than -25 °C.

11. The polyethylene composition according to claim 9 or 10, wherein, the polymerization catalyst is a Ziegler-Natta catalyst.

12. An article comprising the polyethylene composition according to any one of claims 1 to 7 and 9 to 11.

13. The article according to claim 12, which is a pipe or a pipe fitting.

14. The article according to claim 13, wherein, in the rapid crack propagation test, the pipe has a critical temperature Tc of -10 °C or lower and not lower than -25 °C; and / or wherein in the accelerated creep test (ACT), the pipe has a failure time greater than 1500 h; and / or wherein in the short-term compressive strength (STPR) test at a stress level of 5.4 MPa and a temperature of 80 °C, the pipe has a failure time of at least 200 h; and / or Among them, in the short-term compressive strength (STPR) test where the stress level of the pipeline is 12.0 MPa and the temperature is 20 °C, it has a failure time of at least 130 h.

15. Use of the polyethylene composition according to any one of claims 1 to 7 and 9 to 11 for producing an article.

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