High density polyethylene for pipe applications with improved pressure and mechanical properties

Abstract

The present invention relates to a multimodal polyethylene composition comprising a matrix resin comprising a copolymer of ethylene and at least one comonomer selected from alpha-olefins having from 3 to 10 carbon atoms wherein the matrix resin has: (a) a density of at least 945.0 kg / m3 and no more than 955.0 kg / m3 as determined according to ISO 1183-1: 2004, (b) a melt flow rate MFR5 (190 DEG C, 5 kg) as determined according to ISO 1133 of from 0.15 g / 10 min to 0.25 g / 10 min, and (c) a polyethylene composition comprising a matrix resin comprising a copolymer of ethylene and at least one comonomer selected from alpha-olefins having from 3 to 10 carbon atoms, and wherein the polyethylene composition has (c) a molecular chain length of at least 850 and not more than 1200 as determined according to Quantitative 13C {1H} NMR analysis, and (d) a tensile modulus of at least 900 MPa and not more than 1200 MPa as determined according to ISO 527-2, to a process for the production thereof, to pipes or fittings comprising same, and to the use thereof for increasing the pressure resistance and / or slow crack propagation resistance of pipes or fittings. The multimodal polyethylene composition provides an improved balance of pressure resistance and slow crack propagation resistance, and has improved mechanical properties such as tensile properties, strain hardening modulus, and impact properties.

Description

Technical Field

[0001] The present invention relates to a multimodal polyethylene composition useful in pipe applications having an improved balance of pressure resistance and slow crack growth resistance, and having improved mechanical properties. The invention also relates to a process for producing such a multimodal polyethylene composition and a pipe or fitting comprising such a multimodal ethylene composition. Background Art

[0002] Pipes made of polymeric materials have a variety of uses, such as fluid transport, i.e., transporting liquids, slurries, and gases, such as water or natural gas. During transport, it is common to pressurize the fluid. In addition, the fluids being transported may have different temperatures, typically in the range of about 0°C to about 50°C. Such pressurized pipes are preferably made of polyolefin plastics, typically unimodal or bimodal ethylene plastics, such as medium density polyethylene (MDPE; density 930-942 kg / m 3 ) and high-density polyethylene (HDPE; density 942-965kg / m 3 ) or polypropylene composition.

[0003] As used herein, the term "pressure pipe" refers to a pipe that is subjected to positive pressure when in use, ie, the pressure inside the pipe is higher than the pressure outside the pipe.

[0004] Polymer pipes are typically manufactured by extrusion or, to a lesser extent, by injection molding. Conventional apparatus for extruding polymer pipes comprises an extruder, a die, calibration equipment, cooling equipment, pulling equipment, and equipment for cutting and / or winding the pipe.

[0005] For example, Scheirs et al. discussed the manufacture of polyethylene materials for use in pressure pipes in an article (Scheirs, Bohm, Boot, and Leevers: "PE100 Resins for Pipeline Applications," TRIP, Vol. 4, No. 12 (1996), pp. 408-415).

[0006] Pressure piping materials are classified, for example, as PE80 or PE100, based on the long-term hydrostatic strength of thermoplastic materials in pipe form, through extrapolation from ISO 9080 and ISO 12162. The minimum required strength (MRS) classification of ISO 9080 specifies that PE100 material has a lifespan of at least 50 years at 20°C using an internal hoop stress of 10 MPa. The higher MRS of PE100 material is typically achieved by using a higher density polyethylene composition.

[0007] Multimodal high-density polyethylene (HDPE) compositions are well known in pipe applications, in particular as PE100 pressure pipes. PE100 materials are typically used in applications such as pressurized gas pipes, drinking water pipes, sewage pipes or industrial pipes.

[0008] EP 2743305 A1 discloses a pipe comprising a high density polyethylene mixture comprising (A) 55 wt.-% to 95 wt.-% having a density of at least 940 kg / m 3 0 g / 10 min or less 21 and at least 940kg / m 3 The pipe can have a pressure resistance of at least 100 hours at a pressure of 14.5 MPa and / or a pressure resistance of at least 1000 hours at a pressure of 13.9 MPa. Furthermore, the pipe exhibits sag resistance, and the mixture exhibits good homogeneity.

[0009] EP 3293208 A1 discloses a bimodal polyethylene composition comprising a low molecular weight polyethylene homopolymer component and a high molecular weight polyethylene copolymer component, wherein the high molecular weight polyethylene component has a C4 to C10 α-olefin comonomer content (preferably 1-butene) of 0.25 mol% to 3 mol%, wherein the low molecular weight polyethylene content is 40 wt.-% to 65 wt.-% relative to the total amount of the bimodal polyethylene composition; the bimodal polyethylene composition has an MWD greater than 0.7 2 / 1 and Mw greater than 15 2 / 1 This bimodal polyethylene composition has an improved balance between pressure resistance in line with PE112 and slow crack growth resistance greater than 3500h.

[0010] New installation techniques, such as trenchless and sand-less installation, require polyethylene pipe resins with higher resistance to slow crack growth. The requirements for resistance to slow crack growth are becoming increasingly stringent, and many existing products cannot always meet these requirements and the required pressure performance.

[0011] Such applications require pipe materials to meet a wide range of rheological, mechanical, and pressure performance properties. Many of these properties are conflicting, making them difficult to achieve simultaneously, as improving one property can lead to degradation of others. For example, attempts to improve hydrostatic performance (HPT), typically by increasing the material's density, can result in a decrease in slow crack growth resistance or notched pipe test (NPT) performance. Improving NPT performance typically requires increasing the comonomer content or the degree of tie chains present in the polymer, which negatively impacts HPT performance. In a similar manner, the relationship between HPT performance at room temperature and elevated temperatures, as well as between room temperature impact performance and low temperature impact performance, are often considered inherently contradictory. In addition to mechanical and pressure properties, several additional key properties must be maintained as performance parameters for HDPE pipe, such as sag resistance and aesthetics, such as surface smoothness and appearance.

[0012] There is therefore a need for multimodal polyethylene compositions that meet at least the requirements of PE100 and show an improved balance of properties in terms of slow crack growth resistance, hydrostatic pressure resistance at room and elevated temperatures, tensile properties, strain hardening modulus, impact properties and processability. Summary of the Invention

[0013] A basic object of the present invention is to provide a multimodal polyethylene composition that addresses the above mentioned needs and challenges.

[0014] These objects are achieved by providing a multimodal polyethylene composition according to the hereinafter appended claims, which composition addresses the above mentioned needs and drawbacks.

[0015] Thus, the present invention provides a multimodal polyethylene composition comprising a base resin comprising a copolymer of ethylene and at least one comonomer selected from alpha-olefins having 3 to 10 carbon atoms,

[0016] The matrix resin has:

[0017] (a) At least 945.0 kg / m2 as measured according to ISO 1183-1 3 And not more than 955.0kg / m 3 The density,

[0018] (b) a melt flow rate MFR5 (190° C., 5 kg) of 0.15 to 0.25 g / 10 min, determined according to ISO 1133,

[0019] And wherein the polyethylene composition has:

[0020] (c) As described in the experimental section below, the 13 C{ 1a molecular chain length of at least 850 and not more than 1200 as determined by H}NMR analysis,

[0021] (d) a tensile modulus of at least 900 MPa and not more than 1200 MPa, determined according to ISO 527-2.

[0022] Preferred features of the multimodal polyethylene composition of the invention are defined in the dependent claims.

[0023] The present invention also relates to a process for producing the above multimodal polyethylene composition comprising the steps of polymerizing ethylene and at least one comonomer selected from α-olefins having 3 to 10 carbon atoms in at least two polymerization stages in the presence of a polymerization catalyst, preferably a Ziegler-Natta catalyst, wherein the at least one comonomer is present in at least one of the polymerization stages.

[0024] The present invention also relates to a pipe or fitting comprising the multimodal polyethylene composition described above.

[0025] The present invention also relates to the use of the above multimodal polyethylene composition for improving the pressure resistance and / or slow crack growth resistance of pipes or fittings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A graph showing the platelet thickness range and % platelet fraction measured for inventive examples IE1 and IE2 and comparative examples CE1 to CE3 is shown.

[0027] Figure 2 Shown are the correlations between comonomer content and the fraction of thickest lamellae (>21.7 nm) and second thickest lamellae (>8.1 to ≤21.7 nm) according to SSA data of inventive examples IE1 and IE2 and comparative examples CE1 to CE3.

[0028] Figure 3 The correlation between the strain hardening modulus and the tensile modulus is shown for inventive examples IE1 and IE2 and comparative examples CE1 to CE3. DETAILED DESCRIPTION

[0029] definition

[0030] The polyethylene composition according to the present invention means a polymer derived from at least 50 mol-% ethylene monomer units and additional comonomer units.

[0031] Thus, an ethylene homopolymer refers to a polymer consisting essentially of ethylene monomer units. Due to the requirements of large-scale polymerization, the ethylene homopolymer may include a small amount of comonomer units, usually less than 0.1 mol-%, preferably less than 0.05 mol-%, and most preferably less than 0.01 mol-% of the ethylene homopolymer.

[0032] The term "base resin" refers to the polymeric component of the composition.

[0033] The term "different" means that one polymer component differs from the other polymer component in at least one measurable property. Suitable properties for distinguishing the polymer components are weight average molecular weight (Mw), melt flow rate MFR2 or MFR5, density or comonomer content.

[0034] Base resin

[0035] The base resin comprised in the multimodal polyethylene composition of the present invention preferably, but not necessarily, comprises a first ethylene homo- or copolymer component (A) and a second ethylene homo- or copolymer component (B).

[0036] According to the present invention described above, the base resin may preferably be composed of the first ethylene homopolymer or copolymer component (A) and the second ethylene homopolymer or copolymer component (B).

[0037] The base resin may further comprise one or more other polymer components different from the first ethylene homopolymer or copolymer components (A) and (B). The one or more other polymer components may be introduced into the base resin or the polyethylene composition by melt mixing or compounding or in an additional reaction stage of a multi-stage process for producing the base resin of the polyethylene composition.

[0038] Components (A) and (B) may preferably differ in their weight average molecular weight, since component (B) has a higher weight average molecular weight than component (A). The difference in weight average molecular weight can be seen from the melt flow rate MFR2 of component (A), which is higher than the melt flow rate MFR5 of the polyethylene composition.

[0039] The MFR2 (2.16 kg, 190° C.) of component (A) may preferably be equal to or greater than 50 g / 10 min, more preferably equal to or greater than 80 g / 10 min, even more preferably equal to or greater than 100 g / 10 min.

[0040] Furthermore, the MFR2 (2.16 kg, 190° C.) of component (A) may preferably be equal to or less than 500 g / 10 min, more preferably equal to or less than 475 g / 10 min, even more preferably equal to or less than 450 g / 10 min.

[0041] Component (A) can be a copolymer of ethylene and one or more α-olefin comonomers having 3 to 12 carbon atoms. Preferably, the α-olefin comonomer is selected from α-olefins having 4 to 8 carbon atoms, such as 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. Particularly preferred are 1-butene and 1-hexene. Most preferred is 1-butene.

[0042] However, it is preferred that component (A) is an ethylene homopolymer. The homopolymer preferably has an ethylene content of at least 960 kg / m 3 and more preferably at least 965 kg / m 3 .

[0043] Furthermore, component (A) is preferably present in the base resin in an amount of 45 to 55 wt.-%, more preferably 47 to 53 wt.-%, relative to the base resin.

[0044] Component (B) is preferably a copolymer of ethylene and at least one α-olefin comonomer unit having 3 to 12 carbon atoms. Preferably, the α-olefin comonomer is selected from α-olefins having 4 to 8 carbon atoms, such as 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. Particularly preferred are 1-butene and 1-hexene, and most preferred is 1-hexene.

[0045] Component (B) may also contain other comonomer units other than α-olefin comonomers, such as dienes, polar comonomers or silicon-containing comonomers. However, it is preferred that component (B) contains only α-olefin monomers as comonomer units.

[0046] The content of units derived from at least one α-olefin comonomer having 3 to 12 carbon atoms in component (B) is preferably from 0.6 to 2.0 wt.-%, more preferably from 0.70 to 1.5 wt.-%.

[0047] Furthermore, component (B) is preferably present in the base resin in an amount of 55 to 45 wt.-%, more preferably 53 to 47 wt.-%, relative to the base resin.

[0048] The weight ratio of the first ethylene homopolymer or copolymer component (A) to the second ethylene copolymer component (B) is preferably from 45:55 to 55:45, more preferably from 46:54 to 54:46, and most preferably from 47:53 to 53:47.

[0049] Preferably, the base resin consists only of component (A) and component (B) as defined above.

[0050] The base resin preferably has a viscosity equal to or greater than 946 kg / m 3 The density is preferably equal to or greater than 948 kg / m 3 .

[0051] In addition, the base resin preferably has a viscosity equal to or less than 955 kg / m 3 The density is preferably equal to or less than 953 kg / m 3 .

[0052] The base resin according to the present invention is preferably a copolymer of ethylene and at least one α-olefin comonomer having 3 to 12 carbon atoms. Preferably, the α-olefin comonomer is selected from α-olefins having 4 to 8 carbon atoms, such as 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. 1-Butene and 1-hexene are particularly preferred. 1-Hexene is most preferred.

[0053] The ethylene copolymer may also contain other comonomer units other than α-olefin comonomers, such as dienes, polar comonomers or silicon-containing comonomers. However, it is preferred that the ethylene copolymer contains only α-olefin monomers as comonomer units.

[0054] The content of units derived from at least one α-olefin comonomer having 3 to 12 carbon atoms in the base resin is preferably from 0.6 to 1.5 wt.-%, more preferably from 0.7 to 1.2 wt.-%.

[0055] Polyethylene composition

[0056] In addition to the base resin defined above, the multimodal polyethylene composition according to the present invention preferably comprises conventional additives used with polyolefins, such as pigments (e.g. carbon black), stabilizers (e.g. antioxidants), antacids and / or UV stabilizers, antistatic agents and utilization agents (e.g. processing aids). Preferably, the content of these additives is 10 wt.-% or less, more preferably 8 wt.-% or less, even more preferably 5 wt.-% or less, based on the total polyethylene composition (100 wt.-%).

[0057] Preferably, the multimodal polyethylene composition of the present invention comprises carbon black in an amount of 1.0 to 8.0 wt.-%, more preferably 1.5 to 6.0 wt.-%, even more preferably 1.7 to 4.0 wt.-%, based on the total polyethylene composition (100 wt.-%).

[0058] The amount of additives other than carbon black is preferably from 0 to 1 wt.-%, more preferably from 0.001 to 0.5 wt.-%.

[0059] Part or all of the optional additives and / or carbon black may be incorporated into the base resin during its production, for example in the pelletizing step of the base resin powder obtained from the polymerization reactor, or in the compounding step of the polymer composition.

[0060] The multimodal polyethylene composition according to the invention may preferably have a melt flow rate MFR5 (190°C, 5 kg) of not more than 0.5 g / 10 min, preferably not more than 0.3 g / 10 min, and preferably at least 0.01 g / 10 min, more preferably at least 0.05 g / 10 min, even more preferably at least 0.10 g / 10 min.

[0061] The above-mentioned multimodal polyethylene composition according to the present invention offers a number of improvements compared to conventional high density polyethylene compositions for pipe applications, such as:

[0062] - Excellent HPT performance at room temperature (20°C) and elevated temperature (80°C), along with improved or comparable NPT performance; in the prior art, improvements in HPT typically exhibit reduced NPT performance;

[0063] - an improved balance of HPT and NPT properties compared to conventional multimodal polyethylene compositions or pipes comprising such compositions, even at low comonomer contents;

[0064] - Simultaneous improvement of strain hardening modulus and tensile modulus;

[0065] - Improved balance of low-temperature impact resistance and room-temperature impact resistance even at low comonomer contents;

[0066] - Narrower molecular weight distribution compared to conventional multimodal polyethylene compositions.

[0067] As described in the experimental section below, the 13 C{ 1 The multimodal polyethylene composition according to the present invention preferably has a molecular chain length of at least 900, more preferably at least 950, determined according to H NMR analysis.

[0068] The multimodal polyethylene composition according to the present invention preferably has:

[0069] (e) a strain hardening modulus of at least 50 MPa, more preferably in the range of 50 MPa to 85 MPa, even more preferably in the range of 50 MPa to 75 MPa, determined as described in the Experimental section below.

[0070] The multimodal polyethylene composition according to the present invention preferably has:

[0071] (f) the content of the lamellae fraction having a lamellae thickness of more than 21.7 nm, based on the total crystalline fraction, determined according to the self-nucleation and annealing method (SSA) as described in the experimental part below, is from 28 wt.-% to 52 wt.-%, more preferably in the range of 28 wt.-% to 50 wt.-%, even more preferably in the range of 30 wt.-% to 50 wt.-%.

[0072] The multimodal polyethylene composition according to the present invention preferably has:

[0073] (g) the content of the lamellae fraction having a lamellae thickness in the range of more than 11.8 nm to 21.7 nm, based on the total crystalline fraction, determined according to the self-nucleation and annealing method (SSA) as described in the experimental part below, is from 33.0 wt.-% to 52 wt.-%, more preferably in the range of 34 wt.-% to 51 wt.-%, even more preferably in the range of 35 wt.-% to 50 wt.-%.

[0074] The multimodal polyethylene composition according to the present invention preferably has:

[0075] (h-1) Hydrostatic pressure resistance (HPT) to failure of at least 2000 h, more preferably at least 3000 h, even more preferably at least 3500 h at 80°C and 5.6 MPa, as determined in accordance with ISO 1167-1:2006, as described in the experimental section below.

[0076] The multimodal polyethylene composition according to the present invention preferably has:

[0077] (h-2) Hydrostatic pressure resistance (HPT) at 20°C and 12.4 MPa of at least 1300 h to failure, more preferably at least 1400 h to failure, even more preferably at least 1500 h to failure, as determined in accordance with ISO 1167-1:2006 as described in the experimental section below.

[0078] The multimodal polyethylene composition according to the present invention preferably has:

[0079] (h-3) a slow crack growth resistance to failure of at least 500 h, more preferably at least 600 h, and even more preferably at least 700 h, in a notched pipe test (NPT) at 80° C. and 4.6 MPa according to ISO 13479-2009, as described in the experimental section below.

[0080] The multimodal polyethylene composition according to the present invention preferably has:

[0081] (i) at least 15 kJ / m², measured according to ISO 179-1 at 23°C, as described in the experimental section below 2 Charpy notched impact strength (NIS), more preferably at least 18 kJ / m 2 , even more preferably at least 20 kJ / m 2 .

[0082] The multimodal polyethylene composition according to the present invention preferably has:

[0083] (j) a viscosity at a shear stress of 747 Pa (eta747) of at least 500 Pa·s, more preferably at least 550 Pa·s, even more preferably at least 600 Pa·s, determined as described in the Experimental Section below.

[0084] The multimodal polyethylene composition according to the present invention, wherein the base resin preferably has:

[0085] (k) a comonomer content of not more than 1.5 wt.-%, more preferably not more than 1.3 wt.-%, even more preferably not more than 1.2 wt.-%, based on the total weight of the base resin.

[0086] The multimodal polyethylene composition according to the present invention, wherein the base resin preferably has:

[0087] (l) As described in the experimental section below, according to the quantitative 13 C{ 1 The degree of polymerization is at least 450, more preferably at least 475, and even more preferably at least 500, as determined by H NMR analysis.

[0088] The present invention surprisingly found that the multimodal polyethylene compositions described above and in the following claims have significantly higher chain lengths, which has an important impact on various pipe properties, such as hydrostatic pressure resistance (HPT) at room temperature and at elevated temperatures, and resistance to slow crack growth in the notched pipe test (NPT) at room temperature and at low temperatures. It was further surprisingly found that the multimodal polyethylene compositions of the present invention can be produced with relatively lower comonomer content compared to prior art multimodal polyethylene compositions, resulting in increased density, an improved balance of HPT and NPT properties, and an improved balance of low temperature impact and room temperature impact properties.

[0089] The multimodal polyethylene composition of the present invention is typically produced by a multistage process, i.e., a process utilizing at least two reactors, one for producing the lower molecular weight component and a second for producing the higher molecular weight component. These reactors may be used in parallel, in which case the components must be mixed after production. More commonly, the reactors are used in series, so that the product of one reactor serves as the starting material for the next reactor, e.g., one component is formed in a first reactor and a second component is formed in the presence of the first component in a second reactor. In this way, the two components are more intimately mixed together because one component is formed in the presence of the other.

[0090] The polymerization reaction used in each stage may involve conventional ethylene homopolymerization or copolymerization, such as gas phase, slurry phase, liquid phase polymerization, using conventional reactors, such as loop reactors, gas phase reactors, batch reactors, etc.

[0091] The polymerization can be carried out continuously or batchwise, preferably the polymerization is carried out continuously.

[0092] For example, known two-stage processes are liquid-liquid phase processes, gas-gas phase processes and liquid-gas phase processes. It is also known that these two-stage processes can be further combined with one or more additional polymerization steps selected from gas phase, slurry phase or liquid phase polymerization processes. For example, such multi-stage processes are described in EP 2743305 A1 or EP 3088458 A1.

[0093] The multimodal polyethylene composition of the present invention is preferably produced in a multistage process, wherein the lower molecular weight polymer (component) and the higher molecular weight polymer (component) are produced in different polymerisation steps in any order.

[0094] A relatively low-density polymer (or high molecular weight (HMW) component) can be produced in the first polymerization step, and a relatively high-density polymer (or low molecular weight (LMW) component) can be produced in the second polymerization step. This can be referred to as reverse mode. Alternatively, a low molecular weight polymer can be produced in the first polymerization step, and a high molecular weight polymer can be produced in the second polymerization step. This can be referred to as normal mode and is preferred.

[0095] For example, the two-stage process may be a slurry-slurry or a gas phase-gas phase process, with the slurry-gas phase process being particularly preferred. Optionally, the process of the invention may comprise one or two additional polymerisation steps.

[0096] These optional one or two additional polymerisation steps preferably comprise a gas phase polymerisation step.

[0097] The slurry and gas phase stages can be carried out using any conventional reactor known in the art. For example, the slurry phase polymerization can be carried out in a continuously stirred tank reactor; a stirred tank reactor operated in batches, 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 conduit using a circulation pump. Loop reactors are generally known in the art, and examples are given, for example, in US-A-4582816, US-A-3405109, US-A-3324093, EP-A-479186, and US-A-5391654.

[0098] The term gas phase reactor includes any mechanically mixed fluidized bed reactor, fast fluidized bed reactor or settled bed reactor, or a gas phase reactor with two separate zones, such as a fluidized bed combined with a settled bed zone. Preferably, the gas phase reactor used in the second polymerization step is a fluidized bed reactor.

[0099] Slurry and gas phase processes are well known and described in the prior art.

[0100] In a preferred embodiment of the present invention, a low molecular weight (LMW) component is first produced and then a high molecular weight (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).

[0101] The polymerization catalyst used to produce the base resin may include a transition metal coordination catalyst, such as Ziegler-Natta (ZN), metallocene, non-metallocene, Cr catalyst, etc. The catalyst may be supported on, for example, a conventional support, including silica, an aluminum-containing support, and a magnesium dichloride-based support. Preferably, the catalyst is a ZN catalyst, more preferably a non-silica-supported ZN catalyst, and most preferably a MgCl2-based ZN catalyst.

[0102] The Ziegler-Natta catalyst also preferably comprises a Group 4 (group numbering according to the new IUPAC system) metal compound, preferably titanium, magnesium dichloride and aluminum.

[0103] The catalyst can be commercially available, or can be produced according to or similar to the literature. In order to prepare the preferred catalyst that can be used for the present invention, reference is made to WO 2004 / 055068 A1, WO 2004 / 0551069 A1 and WO 2015 / 078924 and EP 0810235 A1 of Abu Dhabi Polymers Co., Ltd. (Borouge) / Borealis AG. The full contents of these documents are incorporated herein by reference, in particular with respect to the general and all preferred embodiments of the catalysts described therein and the method for preparing the catalyst. Particularly preferred Ziegler-Natta catalysts are described in EP 0810235 A1.

[0104] The final product obtained consists of an intimate mixture of polymers from the reactor whose different molecular weight distribution curves together form a molecular weight distribution diagram with a broad maximum or multiple maxima, that is, the final product is a multimodal polymer mixture.

[0105] Preferably, the multimodal polyethylene composition according to the present invention is a bimodal polyethylene mixture consisting of polymer components (A) and (B), optionally further comprising a small amount of a prepolymerized component. However, it is preferred that the process is carried out without a prepolymerization step. Surprisingly, it was found that the above-described polymerization process without a prepolymerization step resulted in a polyethylene composition showing significantly higher molecular chain length and a chain microstructure exhibiting a lamellar morphology, as can be determined by SSA data as described in the experimental part below. The increase in molecular chain length and the modification of the chain microstructure lead to a significant improvement in mechanical properties, in particular an improved balance between hydrostatic pressure resistance (HPT) and slow crack growth resistance (NPT).

[0106] It is also preferred that such a bimodal polymer mixture is prepared by polymerising in two or more polymerisation reactors connected in series under different polymerisation conditions as described above. Due to the flexibility of reaction conditions thus obtained, it is most preferred to carry out the polymerisation in a loop reactor / gas phase reactor combination.

[0107] Preferably, in the preferred two-stage process, the polymerization conditions are selected so that: in one stage (preferably the first stage), due to the high content of chain transfer agent (hydrogen), a relatively low molecular weight polymer without comonomer is produced, while in the other stage (preferably the second stage), a high molecular weight polymer with comonomer is produced. However, the order of the stages can also be reversed.

[0108] In a preferred embodiment of the polymerization in a loop reactor and a subsequent gas phase reactor, the polymerization temperature in the loop reactor is preferably from 85 to 115° C., more preferably from 90 to 105° C. and most preferably from 92 to 100° C., and the temperature in the gas phase reactor is preferably from 70 to 105° C., more preferably from 75 to 100° C. and most preferably from 82 to 97° C. The pressure in the loop reactor is typically from 1 bar to 150 bar, preferably from 1 bar to 100 bar, and the pressure in the gas phase reactor is typically at least 10 bar, preferably at least 15 bar, but typically not more than 30 bar, preferably not more than 25 bar.

[0109] The polymerization in the slurry phase reactor usually occurs in an inert diluent, typically a hydrocarbon diluent selected from C3 to C8 hydrocarbons, such as methane, ethane, propane, n-butane, isobutane, hexane (such as n-hexane), heptane, octane, etc. or a mixture thereof. Preferably, the diluent is a low-boiling hydrocarbon with 1 to 4 carbon atoms or a mixture of such hydrocarbons. A particularly preferred diluent is propane, which may contain a small amount of methane, ethane and / or butane. In different polymerization steps, the inert diluents may be the same or different. In the slurry phase reactor, the ethylene content in the fluid phase of the slurry can be from 0.5 mol % to 50 mol %, preferably from 1 mol % to 20 mol %, particularly from 2 mol % to 10 mol %.

[0110] In a preferred embodiment of the gas phase reactor, the polymerization reaction occurs in a fluidized bed gas phase reactor, wherein olefins are polymerized in the presence of a polymerization catalyst in an upwardly moving gas stream. The reactor typically contains a fluidized bed comprising growing polymer particles containing an active catalyst above a fluidizing grid. The polymer bed is fluidized by a fluidizing gas comprising an olefin monomer, an optional comonomer, an optional chain growth controller or chain transfer agent (e.g., hydrogen), and an optional inert gas.

[0111] If desired, one or more antistatic agents may also be introduced into the gas phase reactor. Suitable antistatic agents and methods of use thereof are disclosed, inter alia, in US-A-5026795, US-A-4803251, US-A-4532311, US-A-4855370, and EP-A-560035A1. These are typically polar compounds, including, inter alia, water, ketones, aldehydes, alcohols, and the like.

[0112] A chain transfer agent, preferably hydrogen, is added to the reactor as needed, and when LMW components are produced in such a reactor, preferably 500 to 1200 mol of H2 is added to the reactor per thousand mol of ethylene, more preferably 550 to 750 mol of H2 is added to the reactor, and when HMW components are produced in such a reactor, 0 to 100 mol of H2 is added to the gas phase reactor per thousand mol of ethylene.

[0113] The polymerization conditions, as well as the feed flows and residence times in the reactor are preferably adjusted to produce the base resin as described above or in the following claims.

[0114] Optionally, additives or other polymer components may be added to the composition in the compounding step in the amounts mentioned above. Preferably, the polyethylene composition of the invention obtained from the reactor is compounded in an extruder with additives in a manner known in the art.

[0115] The composition of the present invention (preferably, if produced in a process including a compounding step, wherein the composition, i.e., the mixture, is typically obtained as a polyolefin matrix resin powder from a reactor) is extruded in an extruder and then granulated into polymer pellets in a manner known in the art. The extruder can be, for example, any conventionally used extruder. As an example of an extruder for the compounding step, one can use an extruder such as a JSW 460P or a JSW CIM90P supplied by Japan Steelworks, Kobe Steel, or Farrel-Pomini.

[0116] The present invention also relates to an article comprising, preferably consisting of, a multimodal polyethylene composition as described above or in the following claims. Preferably, the article is a pipe or pipe fitting comprising, preferably consisting of, a multimodal polyethylene composition as described above or in the following claims. The pipe preferably meets at least the PE80 standard, more preferably the PE100 standard.

[0117] The pipe preferably complies with the hydrostatic pressure resistance (HPT) and slow crack growth resistance (NPT) defined for the multimodal polyethylene composition of the invention as described above or in the claims below.

[0118] The present invention also relates to the use of the inventive multimodal polyethylene composition as described above or in the following claims for improving the pressure resistance and / or slow crack growth resistance of pipes or fittings.

[0119] The results of the examples of the present invention are described and discussed in detail in the Examples section below.

[0120] Measurement method

[0121] Unless defined otherwise, the following definitions of terms and measurement methods apply to the above general description of the invention as well as to the following examples.

[0122] a) Melt flow rate

[0123] The melt flow rate (MFR) is determined according to ISO 1133 and is expressed in g / 10 min. The MFR is an indicator of the flowability and subsequent processability of a polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is measured at a temperature of 190°C and under a load of 2.16 kg, 5.0 kg, or 21.6 kg.

[0124] b) Density

[0125] Density is measured according to ISO 1183-1:2004 (Method A) on compression molded test specimens prepared according to EN ISO 1872-2 (February 2007) and is expressed in kg / m 3 given.

[0126] c) Quantification of microstructure by NMR spectroscopy

[0127] Quantitative nuclear magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content, molecular chain length, and degree of polymerization of the polymers.

[0128] A Bruker Avance III 500 NMR spectrometer was used to 1 H and 13 For C, the operation was carried out at 500.13 MHz and 125.76 MHz, respectively, and quantitative data were recorded in the molten state. 13 C{ 1 H} NMR spectroscopy. Nitrogen was used for all pneumatics and the temperature was 150 °C. 13All spectra were recorded using a C-optimized 7 mm magic angle spinning (MAS) probe. Approximately 200 mg of material was packed into a 7 mm outer diameter zirconium oxide MAS rotor and spun at 4 kHz. This setup was chosen primarily for the high sensitivity required for rapid identification and accurate 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) 2373). Standard single-pulse excitation was used with a transient NOE 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. This setup was chosen because of its high sensitivity to sometimes low comonomer contents.

[0129] Quantitation was performed using a custom automated spectroscopic analysis program. 13 C{ 1 H} NMR spectra were processed, integrated, and quantitatively determined. All chemical shifts were internally referenced to the bulk methylene group signal (δ+) at 30.00 ppm (J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201).

[0130] A characteristic signal corresponding to 1-hexene incorporation was observed (J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201) and all contents were calculated relative to all other monomers present in the polymer.

[0131] A characteristic signal resulting from isolated 1-hexene incorporation (ie, EEHEE comonomer sequences) was observed. The integration of the signal at 38.2 ppm attributable to the number of *B4 sites per comonomer reporter site was used to quantify isolated 1-hexene incorporation:

[0132] H=I *B4

[0133] When a characteristic signal resulting from continuous 1-hexene incorporation (i.e., EHHE comonomer sequence) is observed, the integration of the signal at 40.4 ppm attributable to the number of ααB4B4 sites per comonomer reporter site is used to quantify this continuous 1-hexene incorporation:

[0134] HH=2*I ααB4B4

[0135] When a characteristic signal resulting from discontinuous 1-hexene incorporation (i.e., EHEHE comonomer sequences) is observed, the integration of the signal at 24.6 ppm attributable to the number of ββB4B4 sites per comonomer reporter site is used to quantify this discontinuous 1-hexene incorporation:

[0136] HEH=2*I ββB4B4

[0137] Due to the overlap of the signals from the *B4 and *B4B4 sites of isolated (EEHEE) and discontinuously incorporated (EHEHE) 1-hexene, respectively, the total amount of isolated 1-hexene incorporation was corrected based on the amount of discontinuous 1-hexene present:

[0138] H=I *B4 -2*I ββB4B4

[0139] In the absence of other observed signals indicating other comonomer sequences (i.e., 1-hexene chain initiation), the total 1-hexene comonomer content was calculated based solely on the amounts of isolated (EEHEE), continuous (EHHE), and discontinuous (EHEHE) 1-hexene comonomer sequences:

[0140] H 总 =H+HH+HEH

[0141] Characteristic signals due to saturated end groups were observed. The content of such saturated end groups was quantified using the average value of the integral of the signals at 22.8 ppm and 32.2 ppm, which are assigned to the 2s and 3s sites, respectively:

[0142] S=(1 / 2)*(I 2S +I 3S )

[0143] The relative amount of ethylene was quantified using the integration of the bulk methylene (δ+) signal at 30.00 ppm:

[0144] E=(1 / 2)*I δ+

[0145] The total ethylene content is calculated based on the bulk methylene signal and taking into account the ethylene units present in other observed comonomer sequences or end groups:

[0146] E 总 =E+(2 / 2)*H+(1 / 4)*HH+(3 / 4)*HEH+(3 / 2)*S

[0147] The total mole fraction of 1-hexene in the polymer is then calculated:

[0148] fH=H 总 / (E 总 +H 总 )

[0149] The total comonomer incorporation (mole percent) of 1-hexene was calculated in the conventional manner from the mole fractions:

[0150] H[mol%]=100*fH

[0151] The total comonomer incorporation (weight percent) of 1-hexene was calculated from the mole fractions in the standard manner:

[0152] H[wt%]=100*(fH*84.16) / ((fH*84.16)+((1-fH)*28.05))

[0153] The comonomer incorporation of 1-hexene in the high Mw component (mole percent) was calculated in the conventional manner from the total comonomer incorporation:

[0154] H [mol %] in HMW = 100% * H [mol %] / % share of HMW component

[0155] d) Determination of molecular weight, molecular weight distribution, and polydispersity index

[0156] The weight average molecular weight (Mw), number average molecular weight, and molecular weight distribution (MWD) were measured by gel permeation chromatography (GPC) according to the following methods:

[0157] The molecular weight averages (Mz, Mw and Mn), molecular weight distribution (MWD) and their breadth are determined by gel permeation chromatography (GPC) according to ISO 16014-1:2003, ISO 16014-2:2003, ISO 16014-4:2003 and ASTM D6474-12 using the following formula, described by the polydispersity index, PDI = Mw / Mn (where Mn is the number average molecular weight and Mw is the weight average molecular weight):

[0158]

[0159] For a constant elution volume interval ΔV i , where A i and M i are the chromatographic peak slice area and the molecular weight (MW) of polyolefins, respectively, and the elution volume V i where N is equal to the number of data points obtained from the chromatogram between the integration limits.

[0160] A high-temperature GPC instrument equipped with an infrared (IR) detector (IR4 or IR5 from PolymerChar, Valencia, Spain) or a differential refractometer (RI) from Agilent Technologies, equipped with 3× Agilent-PLgel Olexis and 1× Agilent-PLgel Olexis Guard columns, was used. 1,2,4-Trichlorobenzene (TCB) stabilized with 250 mg / L of 2,6-di-tert-butyl-4-methyl-phenol was used as the solvent and mobile phase. The chromatography system was operated at 160°C and a constant flow rate of 1 mL / min. 200 μL of sample solution was injected for each analysis. Data were collected using Agilent Cirrus version 3.3 software or PolymerChar GPC-IR control software.

[0161] The column set was calibrated using a universal calibration method (according to ISO 16014-2:2003) using 19 narrow MWD polystyrene (PS) standards ranging from 0.5 kg / mol to 11,500 kg / mol. The PS standards were dissolved at room temperature for several hours. Conversion of the polystyrene peak molecular weight to polyolefin molecular weight was accomplished using the Mark Houwink equation and the following Mark Houwink constant:

[0162] K PS =19x 10 -3 mL / g,α PS =0.655

[0163] KPE =39x 10 -3 mL / g,α PE =0.725

[0164] K PP =19x 10 -3 mL / g,α PP =0.725

[0165] A third order polynomial was used to fit the calibration data.All samples were prepared in the concentration range of 0.5-1.0 mg / mL and PE was dissolved at 160°C for 3 hours with continuous gentle shaking.

[0166] e) Viscosity measurement

[0167] Rotational and oscillatory rheological measurements are typically performed according to the following standards:

[0168] ISO 3219-1:2021 Part 1: Vocabulary and symbols for rotational and oscillatory rheological measurements

[0169] ISO 3219-2:2021 Part 2: General principles of rotational and oscillatory rheological measurements

[0170] A method used in conjunction with the present invention relates to the rheology of polymers and is based on measuring the viscosity of the polymer 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, i.e., the greater the viscosity, the lower the gravity flow. The viscosity under 747 Pa shear stress is measured using a rotational rheometer, which can be a constant stress rheometer, such as the Anton Paar MCR series rheometer. Rheometers and their functions have been described in "Encyclopedia of Polymer Science and Engineering", 2nd edition, Volume 14, pages 492-509. The measurement is performed between two plates with a diameter of 25 mm and under constant shear stress (constant rotation direction). The gap between the plates is 1.2 mm. A 1.5 mm thick polymer sample is inserted between the plates.

[0171] The sample is temperature-regulated within 2 minutes before the measurement begins. Measurements are performed at 190°C. After temperature regulation, the measurement begins by applying a predetermined stress. The stress is maintained for 1800 seconds to allow the system to approach steady-state conditions. After this, the measurement begins and the viscosity is calculated. The measurement principle involves applying a torque to the plate shaft via a precision motor. This torque is then converted into shear stress in the sample. This shear stress remains constant. The rotational speed at which the shear stress is generated is recorded and used to calculate the sample's viscosity.

[0172] f) Self-nucleation and annealing method (SSA)

[0173] a) Lamellar thickness

[0174] The continuous self-nucleation / annealing (SSA) technique amplifies small effects by annealing the unmelted crystals at each stage of the process, thereby enhancing the potential molecular fractionation that may occur during the crystallization process. General references and other information can be found in the review article by Müller and Arnal [AJ Müller, ML Arnal, Thermal fractionation of polymers, Prog. Polym. Sci. 30 (2005) 559-603]. SSA was performed on an indium tin calibrated TA Instruments Q2000 differential scanning calorimeter equipped with an RCS90 cooling system. 5-10 mg samples from compression molded sheets were used. All samples were packaged in Tzero aluminum pans. Ultra-high purity dry nitrogen was used as the inert atmosphere. The experimental protocol was as follows:

[0175] (a) The previous thermal history was erased by heating the sample from room temperature to 180 °C at a rate of 20 °C / min and holding it isothermally for 5 min.

[0176] (b) In this method, eight cooling / heating cycles at 10°C / min were subsequently used, with the first isothermal crystallization temperature set at 126°C. The duration of each isothermal crystallization step was 5 minutes. After each isothermal crystallization step, the sample was cooled to 30°C and held isothermally for 5 minutes.

[0177] (c) The subsequent isothermal temperature was always kept 5 °C lower than that of the previous step, i.e., the sample was heated from 30 °C to the second isothermal crystallization temperature of 121 °C.

[0178] (d) After the last isothermal crystallization step at 91 °C, the sample was cooled to 0 °C and the melting curve was obtained by heating the cooled sample to 180 °C at a heating rate of 10 °C / min.

[0179] The final heating process after SSA treatment shows a series of melting peaks corresponding to the number of SSA cycles in which annealing is promoted. The Gibbs-Thompson equation can be used to establish a correlation between melting temperature and lamellae thickness:

[0180]

[0181] in, is the equilibrium melting point of an infinitely thick crystal, σ e is the specific surface energy, is the melting enthalpy per mass unit, Lc is the lamella thickness, and Tm is the melting temperature of the lamella. σ e and Take 415K, 93x 10 respectively -3 J / m 3 and 3x 10 8 J / m 3 [L. Mandelkern and R. G. Alamo, “Thermodynamic quantities governing melting,” Phys. Prop. Polym. Handb., Springer, 2007: 165–186].

[0182] The melting enthalpy is calculated by extrapolating the quotient of the heat flow volume between the onset and endpoint of melting and the initial sample weight, as described in ISO 11357-3. A fixed integration method with a step size of 10°C is used to quantify the melting curve to determine the lamellae thickness distribution.

[0183] g) Hydrostatic Pressure Resistance (HPT)

[0184] The hydrostatic pressure resistance was determined according to ISO 1167-1:2006 at 80°C and 5.6 MPa or at 20°C and 12.4 MPa.

[0185] Unnotched 32 mm x 3 mm SDR 11 pipes, 450 mm in length, were pressure tested in both an internal and external water environment to determine resistance to internal pressure according to ISO 1167-1:2006. Type A end caps were used. Time to failure was measured in hours. A hoop stress of 5.6 MPa at 80°C or 12.4 MPa at 20°C was applied.

[0186] h) Notched pipe pressure test (NPT); resistance to slow crack growth

[0187] According to ISO 13479-2009, the resistance to slow crack growth is determined by the number of hours a pipe can withstand a certain pressure at a certain temperature before failure. The pressure test was carried out on SDR 11 pipes with a notch of 110 mm in outer diameter. A pressure generating a stress of 4.6 MPa and a temperature of 80°C was used. The notch was formed using a climb milling cutter comprising a 60° V-shaped milling cutter in accordance with ISO 6108, with a stock removal rate of 0.010 ± 0.002 (mm / rev) / tooth. The cutter used had 24 teeth and a tool speed of 680 rpm. The residual ligament was 0.82-0.78 times the minimum wall thickness. The depth of the notch was calculated using the following formula. h is the notch depth in mm. The four notches were evenly distributed around the circumference of the pipe. The length of the notch was 110 ± 1 mm.

[0188]

[0189] in

[0190] b s : the width of the notch processing surface, expressed in millimeters;

[0191] d em : The average measured tube outside diameter in millimeters.

[0192] i) Strain hardening modulus

[0193] The strain hardening modulus of the composite is obtained from the tensile stress-strain curve above the natural stretch ratio and represents the slope of the increasing stress-strain trend at very high strains (strain hardening state). It is measured at 80°C and 20 mm / min on 300 μm thick pre-treated (120°C / 1 h) samples according to ISO 18488.

[0194] j) Charpy notched impact strength (NIS)

[0195] Charpy notched impact strength was measured according to ISO 179 / 1eA:2000 at +23°C and -20°C on 80*10*4 mm V-notch specimens. The specimens were milled from 4 mm thick plates prepared by compression molding according to ISO 293:2004 using the conditions defined in Chapter 3.3 of ISO 1872-2:2007.

[0196] k) Tensile modulus and tensile strain at break

[0197] The tensile modulus and tensile strain at break were measured according to ISO 527-2 (crosshead speed = 1 mm / min; test speed 50 mm / min at 23° C.) using injection molded specimens as described in EN ISO 1873-2 (dog bone shape, 4 mm thickness). The measurements were performed after the specimens had been conditioned for 96 hours.

[0198] Example

[0199] The polymerization of IE1, IE2, CE1 and CE2 was carried out under two operating conditions, wherein the prepolymerization reactor (PPR) was bypassed during the preparation of the polymers of IE1 and IE2, and was in operation during the preparation of the polymers of CE1 and CE2. According to CE3, a commercial high-density bimodal polyethylene available from Borealis AG was produced. The process parameters and production data of these processes are summarized in Table 1. The polymerization catalyst used in all runs was a non-silica-supported MgCl2-based ZN catalyst. This catalyst was prepared as described in the examples of WO 2015 / 078924A1. The key difference between IE1 and IE2 was the distribution ratio of 48.3 and 50.7, respectively. Under comparable distribution, the runs with and without the PPR were compared, and the two comparable groups were IE1-CE1 and IE2-CE2.

[0200] The process conditions and polymer properties of inventive examples IE1 and IE2 and comparative examples CE1 to CE3 are shown in Tables 1 and 2 below.

[0201] Table 1: Process parameters for the resin of the present invention

[0202]

[0203]

[0204] Table 2: Comparison of molecular weight distribution of the resin of the present invention and the reference sample

[0205] Example Mn Mw Mz Mz+1 PDI IE1 8320 260000 1460000 2810000 31.3 CE1 8075 287500 1700000 3255000 35.7 IE2 10300 264500 1495000 3020000 25.7 CE2 7835 247500 1435000 2890000 31.6 CE3 8120 297000 1705000 3060000 36.6

[0206] It should be noted that the C6 feed data given in the table are not indicative of the comonomer content in the polymer and have been 13 C{ 1 The actual comonomer content, molecular chain length, degree of polymerization, and number of branches per chain were estimated by H} NMR analysis. The results are shown in Table 3 below.

[0207] Table 3: 13 C{ 1 Summary of H}NMR analysis results

[0208]

[0209]

[0210] Therefore, the crystal / platelet morphology of these examples and comparative examples was further investigated by the continuous self-nucleation and annealing (SSA) method as described in the experimental section above, and the results are given in Table 4. The corresponding relationship between the platelet thickness range and the platelet fraction % is shown in Table 4. Figure 1 There is a direct correlation between comonomer content and SSA data ( Figure 2 ), where the thickest lamellae (>21.7 nm) decreases with increasing comonomer content, while the second thickest lamellae (>11.8 nm to ≤21.7 nm) increases proportionally.

[0211] This can be understood as the effect of comonomer embedding into the homopolymer chain, which results in a decrease in crystallinity (thickest lamellae) and an increase in the thickness of the thinner lamellae (i.e., tie molecules) ( Figure 2 ).

[0212] Table 4: Summary of SSA analysis results

[0213]

[0214] Table 5 summarizes the mechanical properties of the samples, such as Charpy notched impact at 23°C and -20°C, strain hardening modulus, tensile modulus, tensile stress and tensile strain at yield, and tensile stress and tensile strain at break.

[0215] Table 5: Summary of mechanical properties results

[0216]

[0217]

[0218] The following trends can be derived from these mechanical properties:

[0219] Comparing the runs with and without PPR (IE1 and CE1, IE2 and CE2), improvements are seen in both tensile modulus and strain hardening modulus. Further improvements in strain hardening modulus can be achieved with increasing comonomer content (by optimizing tensile modulus).

[0220] The strain hardening modulus of the non-prepolymerized samples is lower than that of the commercial reference sample CE3. However, the comonomer content of the inventive examples is significantly lower than that of CE3.

[0221] All runs showed tensile yield stress within acceptable ranges.

[0222] • Elongation at break values ​​for all runs met ISO 4427 requirements (350%).

[0223] The relationship between tensile modulus and strain hardening modulus is shown in Figure 3 . Thus, it is apparent from a comparison of the results for IE1 and CE1 or IE2 and CE2 that the multimodal polyethylene compositions of the present invention have both improved tensile modulus and strain hardening (SH) modulus, whereas typically one property is improved at the expense of the other. Thus, the present invention provides an improved balance between the two properties.

[0224] The HPT and NPT pressure performance test results are given in Table 6 below.

[0225] Table 6: Summary of HPT and NPT test results

[0226]

[0227] As can be seen from the table above, inventive runs IE1 and IE2, produced without PPR, exhibited improved HPT performance at both room temperature and elevated temperatures. Inventive runs IE1 and IE2, produced without PPR, also exhibited improved NPT performance compared to comparative runs CE1 and CE2, produced with PPR. Commercial run CE3 exhibited high NPT performance, a direct result of the higher comonomer content in the sample, but insufficient HPT performance. Overall, inventive runs IE1 and IE2, produced without PPR, provided a significant improvement in the balance of HPT and NPT performance, achieving minimum required NPT failure times of 500 hours or more while maintaining high-pressure performance, even at relatively low comonomer content.

[0228] Thus, the above data show an improved correlation of the final polymer properties (i.e. droop and pressure properties) and the present invention surprisingly demonstrates the influence of chain length and comonomer content, and the subsequent effect of lamellae morphology (particularly the two thickest lamellae parts).

[0229] Inventive runs IE1 and IE2, produced without PPR, exhibit higher chain lengths, achieving simultaneous improvements in several conflicting properties (e.g., HPT and NPT) through a balance of platelet concentration. This unique balance is achieved in this molecular structure, with higher concentrations of thickest platelets, longer chain lengths, and comonomer insertion.

Claims

1. A multimodal polyethylene composition comprising a base resin comprising a copolymer of ethylene and at least one comonomer selected from α-olefins having 3 to 10 carbon atoms, The matrix resin has: (a) At least 945.0 kg / m2 as measured according to ISO 1183-1 3 And not more than 955.0kg / m 3 The density, (b) a melt flow rate MFR5 (190° C., 5 kg) of 0.15 to 0.25 g / 10 min, determined according to ISO 1133, And wherein the polyethylene composition has: (c) As determined as described herein, according to the quantitative 13 C{ 1 a molecular chain length of at least 850 and not more than 1200 as determined by H} NMR analysis, and (d) a tensile modulus of at least 900 MPa and not more than 1200 MPa, determined according to ISO 527-2.

2. The multimodal polyethylene composition according to claim 1 , having: (e) a strain hardening modulus of at least 45 MPa and no greater than 85 MPa, determined as described herein.

3. The multimodal polyethylene composition according to claim 1 or 2, having: (f) a content of lamellae fraction having a lamellae thickness of more than 21.7 nm of 28 to 52 wt.-%, based on the total crystalline fraction, determined according to the self-nucleation and annealing method (SSA) as described herein.

4. A multimodal polyethylene composition according to any one of the preceding claims having: (g) a content of the lamellae fraction having a lamellae thickness in the range of greater than 11.8 nm to 21.7 nm, based on the total crystalline fraction, of 33.0 wt.-% to 52 wt.-%, determined according to the self-nucleation and annealing method (SSA) as described herein.

5. A multimodal polyethylene composition according to any one of the preceding claims having: (h-1) a hydrostatic pressure resistance (HPT) of at least 2000 h to failure at 80°C and 5.6 MPa, as determined in accordance with ISO 1167-1:2006, as described herein, and / or (h-2) a hydrostatic pressure resistance (HPT) of at least 1400 h to failure at 20°C and 12.4 MPa, as determined in accordance with ISO 1167-1:2006, as described herein; and / or (h-3) Slow crack growth resistance of at least 500 h time to failure in a notched pipe test (NPT) at 80°C and 4.6 MPa according to ISO 13479-2009 as described herein.

6. A multimodal polyethylene composition according to any one of the preceding claims having: (i) at least 20 kJ / m², measured at 23°C according to ISO 179-1, as described herein 2 Charpy notched impact strength (NIS).

7. A multimodal polyethylene composition according to any one of the preceding claims having: (j) a viscosity at a shear stress of 747 Pa (eta747) of at least 500 Pa·s, measured as described herein.

8. A multimodal polyethylene composition according to any one of the preceding claims, wherein the base resin has: (k) a comonomer content of not more than 1.20 wt.-%, based on the total weight of the base resin.

9. A multimodal polyethylene composition according to any one of the preceding claims, wherein the base resin has: (l) As determined herein, according to the quantitative 13 C{ 1 H} NMR analysis determined a degree of polymerization of ≥450.

10. A process for producing a multimodal polyethylene composition according to any one of the preceding claims, the process comprising the steps of: Ethylene and at least one comonomer chosen from α-olefins having 3 to 10 carbon atoms are polymerized in at least two polymerization stages in the presence of a polymerization catalyst, preferably a Ziegler-Natta catalyst, wherein the at least one comonomer is present in at least one of the polymerization stages.

11. The process according to claim 10, wherein the polymerization is carried out without a prepolymerization stage.

12. The process according to claim 10 or 11, wherein the polymerization is carried out in one slurry polymerization reactor and at least one gas phase polymerization reactor.

13. The process of claim 12, wherein the catalyst is fed directly to the slurry polymerization reactor.

14. A pipe or fitting comprising the multimodal polyethylene composition according to any one of claims 1 to 9.

15. Use of a multimodal polyethylene composition according to any one of claims 1 to 9 for improving the pressure resistance and / or slow crack growth resistance of pipes or fittings.

Images