EP4633915A1

EP4633915A1 - Die assembly for solid state extrusion of polyolefin materials

Info

Publication number: EP4633915A1
Application number: EP23804630.4A
Authority: EP
Inventors: Enrico TROISI; Nicolaas Hendrika Friederichs; Theo Tervoort; Fotios CHRISTAKOPOULOS; Stephan Patrik BUSATO
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2022-12-12
Filing date: 2023-11-07
Publication date: 2025-10-22
Also published as: WO2024125894A1; CN120344378A

Abstract

The present invention relates to a die assembly for solid-state extrusion of polyolefin materials, wherein the die assembly comprises a, preferably circularly, enclosed straight channel (1) comprising an inlet (2) and an outlet (3) construed so that solid polyolefin material may be conveyed through the channel from the inlet towards the outlet along a flow axis (4), wherein the channel comprises a housing (5) to form an enclosure fully enclosing the channel; wherein the channel comprises a compression section having a first diameter D1 perpendicular to the flow axis at the side towards the inlet of the channel, and a second diameter D2 perpendicular to the flow axis at the side of the outlet of the channel, wherein D1 > D2 to form a tapered channel; wherein the housing has an inner surface (6) forming the outer wall of the channel, wherein the inner surface (6) has such coefficient of friction that the extrusion pressure can be maintained at constant level during extrusion at an extrusion draw ratio of up to 25, preferably of up to 50, wherein the extrusion draw ratio is the ratio of the area of the inlet to the area of the outlet. Such die assembly allows for extrusion of the polyolefin material in the solid state without excessive pressure build-up, resulting in an extrudate having desirable material properties in terms of amongst others melt temperature, crystallinity, and brittleness.

Description

Die assembly for solid state extrusion of polyolefin materials.

[0001] The present invention relates to extrusion die assemblies for the solid-state extrusion of polyolefin materials, in particular of ultra-high molecular weight polyethylene materials. The invention also relates to a process for extrusion of polyolefins, in particular ultra-high molecular weight polyethylenes, in the solid state.

[0002] In processing of polyolefin materials, extrusion is a technique that finds ubiquitous application, as it allows for manufacturing of a wide variety of desirable shapes of products. Extrusion of polyolefin materials typically takes place by heating the polyolefin material to temperatures above its melting point, followed by forcing the molten polyolefin though a die opening of a desired shape, upon which the extruded material is solidified by cooling, commonly in water, to form a solid shape.

[0003] However, there are certain grades of polyolefins that do not suitably melt to allow for such melt extrusion processes to be applied in producing shapes of such materials. In particular, polyolefin materials having a very high molecular weight may be difficult or impossible to process via melt extrusion. Accordingly, alternative processing techniques are to be deployed if one wishes to produce shapes of such materials, allowing for processing the polyolefins in the solid state.

[0004] A known technique for processing polyethylenes that are not processable via melt processing methods is via solution processing. In such process, the polyethylene, for example a LIHMWPE, is dissolved in a solvent to form a dilute solution, typically of gel-like nature, which can then be extruded or spun into a shape. This process may be performed at temperatures above the dissolution temperature of the polyethylene in the specific solvent. By such process, an object, such as a fibre, comprising oriented polyethylene molecules can be formed. The object may be drawn to a high draw ratio, for example of above 20. This process however is disadvantageous in that the steps of dissolving the polymer in the solvent, and later the removal of the solvent, are steps that significantly deteriorate the efficiency and economics of the process.

[0005] Accordingly, it is desirable to have available a solid-state processing technique of polyolefins that does not exhibit such disadvantages. [0006] A further processing technique that is provided in the art is the processing of the polyolefin powder via calendaring or double belt pressing. In such process, a powder is subjected to compression between two counter rotating steel belts, to obtain a calendared sheet. This sheet may subsequently be drawn into tapes at a draw ratio of up to for example 100, compared to the original sheet. However, a disadvantage of such process is that the options for shape design are quite limited; the product is a sheet, where only the width and thickness can be varied.

[0007] Thus, for alternative shapes, there remains a need to provide other solid state shaping processes.

[0008] The present invention now provides for a die assembly for use in solid state shaping processes of polyolefin materials, in particular for solid state shaping of LIHMWPE powder materials.

[0009] The die assembly of the invention comprises a, preferably circularly, enclosed straight channel (1) comprising an inlet (2) and an outlet (3) construed so that solid polyolefin material may be conveyed through the channel from the inlet towards the outlet along a flow axis (4), wherein the channel comprises a housing (5) to form an enclosure fully enclosing the channel, wherein the housing has an inner surface (6) forming the outer wall of the channel, wherein the channel comprises a compression section having a first diameter D1 perpendicular to the flow axis at the side towards the inlet of the channel, and a second diameter D2 perpendicular to the flow axis at the side of the outlet of the channel, wherein D1 > D2 to form a tapered channel.

In an embodiment, the inner surface (6) has such coefficient of friction that the extrusion pressure can be maintained at constant level during extrusion at a extrusion draw ratio of up to 25, preferably of up to 50, wherein the extrusion draw ratio is the ratio of the area of the inlet to the area of the outlet.

[0010] An example of such die is given in Figure 1. [0011] Such die assembly allows for extrusion of the polyolefin material in the solid state without excessive pressure build-up, resulting in an extrudate having desirable material properties in terms of amongst others melt temperature, crystallinity, and brittleness. [0012] In the die assembly according to the invention, it is preferred that the housing is capable of withstanding an extrusion pressure of up to 250 MPa, or up to 170 MPa, and is capable of operating at operating temperatures of up to 180°C, or up to 160°C.

[0013] The inner surface of the housing may for example have a coefficient of friction below that of steel.

[0014] The inner surface of the housing may for example be made of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin, particularly preferable of an epoxy resin.

[0015] It is particularly preferred that the housing is made of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin. Alternatively, the housing may comprise an inner surface coating of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin.

[0016] The thermoset resin may for example be an epoxy resin produced via curing of an epoxy reactant system comprising a quantity of a compound of formula A and a quantity of a compound of formula B:

Formula A Formula B wherein R1 is a moiety comprising 1-20 carbon atoms, preferably a hydrocarbon moiety, preferably a moiety: wherein R3 is a moiety comprising 1-8 carbon atoms, preferably -CH2- or-C(CHs)2- and wherein R2 is a moiety comprising 1-10 carbon atoms, preferably a hydrocarbon moiety, more preferably a linear alkyl moiety.

[0017] The compound of formula A may for example be: [0018] The compound of formula B may for example be:

[0019] In a certain embodiment, the compound of formula A is: and the compound of formula B is: [0020] The curing may for example be performed using a hardener system comprising diethylmethylbenzene diamine, 1,2-diamino cyclohexane, bisphenol A, and p-toluene sulphonic acid.

[0021] In the die assembly according the invention, the channel (1) may for example be tapered at an angle a, wherein a is > 1.0° and < 60.0°, preferably > 5.0° and < 10.0°, more preferably > 6.0° and < 9.0°.

[0022] The invention also in an embodiment relates to a process for solid state extrusion of polyolefin materials, wherein the extrusion is performed by extruding a polyolefin material though a die assembly according the invention, at a temperature of < 150°C, preferably < 140°C, more preferably of > 90°C and < 130°C, under application of a pressure of < 500 MPa, preferably < 400 MPa, more preferably < 300 MPa, even more preferably < 150 MPa, yet even more preferably of > 50 and < 150 MPa.

[0023] The polyolefin material may for example be provided to the extrusion assembly as a powder having a bulk density of > 200 and < 600 kg/m³, preferably of > 200 and < 500 kg/m³, wherein the bulk density is determined via ASTM D1895-96.

[0024] The polyolefin material may for example be a polyethylene or polypropylene homopolymer or copolymer. Preferably, the polyolefin material is an ultra-high molecular weight polyethylene (LIHMWPE) material, preferably a disentangled LIHMWPE.

[0025] The polyolefin material has for example have an intrinsic viscosity of > 8.0, preferably > 10.0, more preferably > 20.0 dl/g, even more preferably > 20.0 and < 50.0 dl/g, as determined in accordance with ASTM D4020 (2011), involving a dilute solution of the polyolefin in decalin at a temperature of 135°C.

[0026] It is further preferred in certain embodiments that the polyolefin material has a melting temperature T_m, as determined via DSC measurement, first run, of > 135°C, preferably of > 140°C.

[0027] It may also be preferred that the polyolefin material has a degree of crystallinity X_c of > 70%, preferably of > 75%, as determined via DSC measurement, first run. [0028] The invention further also relates to an extruded article obtained according to the process of the invention, preferably wherein the article is a tape, fibre, rod, sheet, tube or pipe.

[0029] In the context of the present invention, ultra-high molecular weight polyethylenes, also referred to as LIHMWPE, are to be understood to be polyethylene materials that have a viscosity-average molecular weight (M_v) of over 750 kg/mol. Even more preferably, the LIHMWPE materials may have an M_v of over 1 ,000 kg/mol. The M_v may be up to 10,000 kg/mol. It is particularly preferred that the LIHMWPE materials as applied in the context of the present invention have an M_v of > 1 ,000 kg/mol and < 8,000 kg/mol.

[0030] The viscosity-average molecular weight of an LIHMWPE may be calculated from the intrinsic viscosity of the polymer using the Mark-Houwink equation:

IV = K ■ M wherein the IV is the intrinsic viscosity, in dl/g, determined in accordance with ASTM D4020 (2011); K is a constant having a value of 6.20 10'⁴ for polyethylene, when IV is measured in decalin at 135°C; and a is a constant having a value of 0.700, also applicable to polyethylene when IV is measured in decalin at 135°C. M_v then is the calculated viscosity average molecular weight, in g/mol. Thus, for application with polyethylenes in accordance with the present invention, the Mark-Houwink equation that may be used is:

IV = 6.20 ■ 10“⁴ ■ M ⁷⁰⁰ which translates to:

[0031] The LIHWMPE materials may for example have an intrinsic viscosity IV of > 8.0 dl/g, preferably of > 10.0 dl/g, more preferably of > 15.0 dl/g, even more preferably > 20.0 dl/g. The LIHMWPE materials may have an IV of < 60.0 dl/g, preferably < 50.0 dl/g. The LIHMWPE materials may have an IV of > 8.0 and < 60.0 dl/g, preferably > 10.0 and < 60.0 dl/g, more preferably > 15.0 and < 60.0 dl/g, even more preferably > 15.0 and < 50.0 dl/g, yet even more preferably of > 20.0 and < 50.0 dl/g.

[0032] The LIHMWPE may for example have a bulk density of > 200 kg/m³, preferably of > 200 and < 700 kg/m³, more preferably of > 200 and < 500 kg/m³. The bulk density of the LIHMWPE may be determined in accordance with the method of ASTM D1895/A (2010). Such bulk density ensures ease of processing of the powder in extrusion processes, and contributes to easy of handling and storing of the polymer powders.

[0033] The LIHMWPE may for example have an average particle size (D₅o) in the range of 10.0 to 250.0 pm, preferably 50.0 to 250.0 pm, more preferably of 60.0 to 200.0 pm, as measured in accordance with ISO-13320 (2009).

[0034] The LIHMWPE may in certain embodiments be a copolymer comprising at least 95.0 wt%, preferably at least 98.0 wt%, more preferably at least 99.0 wt%, of moieties derived from ethylene, with regard to the total weight of the LIHMWPE.

[0035] The LIHMWPE may in certain embodiments be a copolymer comprising moieties derived from ethylene and at most 5.0 wt%, preferably at more 2.0 wt%, more preferably at most 1.0 wt%, of moieties derived from one of more a-olefins selected from propylene, 1 -butene, 4- methyl-1 -pentene, 1-hexene, and 1-octene, preferably selected from propylene, 1-butene, 1- hexene and 1-octene, more preferably selected from 1 -butene, 1-hexene and 1-octene, with regard to the total weight of the LIHMWPE.

[0036] Preferably, the LIHMWPE is a copolymer comprising at least 95.0 wt% of moieties derived from ethylene and at most 5.0 wt% of moieties derived from 1-butene, 1-hexene or 1- octene, preferably comprising at least 98.0 wt% of moieties derived from ethylene and at most 2.0 wt% of moieties derived from 1-butene, 1-hexene or 1-octene, more preferably comprising at least 99.0 wt% of moieties derived from ethylene and at most 1.0 wt% of moieties derived from 1-butene, 1-hexene or 1-octene, with regard to the total weight of the LIHMWPE.

[0037] The LIHMWPE may be produced via a slurry process or via a gas-phase process. In a preferred embodiment, the LIHMWPE is produced via a slurry process in the presence of an organic diluent, such as hexane. The polymerisation may be carried out via a batch process, a semi-batch process, or in a continuous mode. In some aspects of the invention, the LIHMWPE may be produced by polymerisation of ethylene, optionally with one of more a-olefin comonomer, in the presence of a supported catalyst composition, optionally in the presence of hydrogen.

[0038] The polymerisation may for example be performed at a polymerisation temperature ranging from 0°C to 140°C, preferably from 10°C to 90°C, more preferably from 25°C to 80°C. The ethylene pressure may for example be in the range of 50 to 5000 kPa, preferably 100 to

5000 kPa, more preferably from 100 to 2000 kPa, even more preferably from 100 to 1000 kPa.

[0039] The supported catalyst composition may for example comprise a transition metal complex comprising two phenoxyimine moieties. It is preferred that the transition metal in the complex is selected from Ti, Hf and Zr. For example, the transition metal complex may be a complex comprising a metal dihalide, such as a metal dichloride.

[0040] The support may be particulate methylaluminoxane. It is preferred that the particulate methylaluminoxane has an average particle size of > 2.0 and < 10.0 pm. The average particle size of the support may for example be determined using a Mastersizer 200 Hydro S from Malvern Instrument Ltd. The particulate methylaluminoxane may for example have an aluminium content of > 25.0 wt%, preferably of > 25.0 and < 60.0 wt%, more preferably of > 30.0 and < 50.0 wt%, with regard to the total weight of the particulate methylaluminoxane.

[0041] The invention will now be illustrated by the following non-limiting examples.

[0042] Extrusion experiments were conducted using a die produced using an epoxy resin RenLam LY 5210, and a die produced of DIN 1.2436 steel. RenLam LY 5210 is an epoxy resin having CAS reg. nr. 1247949-81-0, a polymer of /V,/V'-(methylenedi-4, 1-phenylene)bis[/V-(2- oxiranylmethyl)-2-oxiranemethanamine with 2,2'-[1 ,4-butanediylbis(oxymethylene)]bis[oxirane],

[0043] The dies used in the examples had a geometry as shown in figure 2, wherein all dimensions are in mm. Both dies had a taper angle a of 9°. The dies were configured to have an extrusion draw ratio of 15.

[0044] A further die (“Epoxy-2”) was used in example 10. This die was produced using the epoxy resin as per above, and configured as per the geometry of Figure 8. The Epoxy-2 die had an extrusion draw ratio of 45.

[0045] In the experiments of the invention, the ultra-high molecular weight polyethylene materials UH1 and UH2 were used. UH2 was a material of grade GUR 4120, obtained from Celanese.

[0046] UH1 was a material produced according to the below synthesis protocol. [0047] Ethylene polymerisation was performed using a bis-phenoxy-imine titanium complex ([3- t-Bu-2-O-C6H3CH=N(C6F5)]2TiCl2), having the CAS reg. nr. 352033-76-2, obtained from MCAT GmbH, as the discrete transition metal complex:

[0048] The transition metal complex was supported on particulate methylaluminoxane (MAO), wherein the MOA had an aluminium content of 38.8 wt%, and a particle size of 5.5 pm. The MAO was obtained from Tosoh Fine Chem Corporation. Triisobutylaluminium (TIBAL), obtained from Lanxess, having CAS reg. nr. 100-99-2, was used as scavenger.

[0049] The polymerisation was carried out in a 10 litre stirred autoclave using 5 litres of purified hexane as diluent. TIBAL (1mmol) as the scavenger was added to the hexane, and the stirrer was set to 1000 rpm. The mixture was heated to the polymerisation temperature of 30°C, and pressurized with ethylene to a pressure of 150 kPa.

[0050] In a separate glass vessel, under inert atmosphere, a solution containing a predetermined amount of the transition metal complex was premixed with a suspension containing a predetermined amount of the MAO, by shaking the resulting suspension manually for a period of 10 min. The molar ratio of MAO to the transition metal complex in the suspension was 400.

[0051] Subsequently, the resulting suspension containing the supported catalyst was injected to the reactor via a pressure sluice, upon which the sluice was rinsed with hexane. The quantity of the suspension added to the reactor was so that the concentration of the transition metal complex in the reactor was 0.02 mmol/l. The temperature in the autoclave reactor was kept constant at 30°C, and the pressure was kept constant by feeding additional ethylene through a mass-flow meter. The reaction was stopped after 48 min. Stopping of the reaction was performed by de-pressurizing and cooling down the reactor, and decreasing the stirrer speed. The reactor contents were passed through a filter. The wet polymer powder was thereafter collected, and dried at 50° in vacuum.

[0052] Of the polymer powder, the bulk density was determined in accordance with ASTM D1895/A (2010), to obtain a bulk density of 219 kg/m³. The intrinsic viscosity (IV) was determined via the method of ASTM D4020 (2011), involving a dilute solution of the polyethylene in decalin at a temperature of 135°C, to obtain an IV of 36.4 dl/g. When applying the Mark-Houwink equation: IV = K M_v ^a, using the applicable values K = 6.20- 10'⁴ and a = 0.700 for polyethylenes measures in decalin at 135°C, a viscosity average molecular weight M_v of 5909 kg/mol is calculated.

[0053] Via this protocol, the UH1 was obtained.

[0054] The solid state extrusion was performed using a plunger extrusion setup, equipped with the dies as specified above. The plunger setup was prepared using a Zwick Z020 static mechanical testing machine, having a barrel of 10 mm diameter. LIHMWPE powder as specified below was introduced into the barrel and pre-compressed at room temperature by applying a force of 200 N for 30 s, using a blank die. After the pre-compression, the blank die was replaced with a one of the steel or the epoxy dies as presented above. Thereupon, the setup was heated to an operating temperature as shown in table 1 below.

[0055] The extrusion experiments were conducted by applying a pressure onto the plunger in such way to ensure a defined and constant plunger displacement. During the extrusion, the extrusion pressure that had to be exerted onto the piston of the plunger was constantly measured.

Table 1 : Experimental conditions

[0056] The measured extrusion pressures for each of the examples as a function of the piston displacement is presented in the figures 3-6. Figure 3 shows extrusion experiments wherein the use of the steel die is compared to the epoxy die, at 120°C at 5 mm/min plunger displacement speed. It can be observed that when using the steel die (example 1), the pressure rapidly increased without significant displacement occurring; no extrusion of the UH1 material was possible. However, when using the epoxy die (example 2), under the same operating conditions, extrusion occurred in a predictable way, wherein during the first 30 mm of piston displacement, the extrusion pressure gradually increased to ca. 70 MPa, and remained at approximately that level for the remainder of the piston displacement, that is, until 50 mm displacement.

[0057] Further experiments were conducted to evaluate the influence of the piston displacement speed, as reflected by the examples 3-5. In figure 4, the effect of variation of the displacement speed on extrusion pressure is presented, showing that for each of the extrusion speeds, the material UH1 could be extruded quite smoothly using the epoxy die, at extrusion pressures of up to ca. 120 MPa.

[0058] The influence of the temperature was investigated in the examples 6-8. The results in terms of extrusion pressure over the total displacement is presented in the figure 5, showing that for each of the extrusion temperatures, the material UH1 could be extruded quite smoothly using the epoxy die, at extrusion pressures of up to ca. 150 MPa.

[0059] To evaluate the suitability of the epoxy die for extrusion of different LIHMWPE materials, example 9 was conducted using Celanese GUR 4120 (UH2). The results in terms of extrusion pressure over the total displacement is presented in the figure 6, showing that extrusion of GUR 4120 was also possible using the epoxy die, albeit that the extrusion pressure was subject to certain fluctuations. The extrudate of examples 2 (with UH1) and example 9 (with GUR 4120, coded UH2) was visually inspected, as shown in figure 7. From this figure, one can observe that the extruded strand of UH1 was totally straight and regular in nature, whilst that of UH2 was very irregular, curled, in shape. Furthermore, it was observed that the extrudate of UH2 was very brittle in nature, which was not the case for that of UH1. The extrudate of UH1 was translucent, whereas that of UH2 was white.

[0060] Example 10 is included to demonstrate the suitability of the die according to the invention to process LIHMWPE materials at higher extrusion draw rate. The UH1 material was extruded at 120°C at 5 mm/min plunger displacement speed. From Figure 9, it can be observed that extrusion at such high extrusion drawing rates of 45 can be achieved using the die of the invention.

[0061] Of the UH1 and the UH2 samples, the melting temperature and the degree of crystallinity was measured via DSC prior to extrusion, and of the extrudate material obtained from the example 2 and 9, respectively. In the table 2 below, the results of these DSC measurements are presented.

Table 2: DSC measurements

[0062] Wherein T_m,i is the melt temperature of the material prior to extrusion (in °C), T_m,2 is the melt temperature after extrusion (in °C), X_c,i is the degree of crystallinity prior to extrusion (in %), and X_c,2 is the degree of crystallinity after extrusion (in %).

[0063] DSC analysis was performed by taking a sample of 1.5 ± 0.2 mg for each analysis. During each analysis, nitrogen was continuously purged at 50 ml/min to prevent sample degradation. The thermal protocol applied during the measurements involved: 1) first heating run at 10°C/min from -40°C to 180°C; 2) an annealing step of 5 mins to erase the thermal history of the powder at 180°C; 3) a cooling run at 10°C/min from 180°C to -40°C; and 4) a final heating run from -40°C to 180°C. The crystalline fraction X_c was obtained from the melting endotherm obtained in 1), by using the ratio between the enthalpy measured during the heating runs and the equilibrium melting enthalpy for polyethylene (293 J/g). The melting temperature T_m was taken at the maximum of the melting endotherm obtained in step 1) of this protocol.

[0064] It can be observed that for UH1 , both the melt temperature and the degree of crystallisation were increased after extrusion, which was not the case for the UH2. This is an indication that the extrusion of UH1 , a disentangled LIHMWPE, using a converging die results in a product having a high degree of crystallinity, and a high degree of orientation in the machine direction.

Claims

1. Die assembly for solid-state extrusion of polyolefin materials, wherein the die assembly comprises a, preferably circularly, enclosed straight channel (1) comprising an inlet (2) and an outlet (3) construed so that solid polyolefin material may be conveyed through the channel from the inlet towards the outlet along a flow axis (4), wherein the channel comprises a housing (5) to form an enclosure fully enclosing the channel, wherein the housing has an inner surface (6) forming the outer wall of the channel; wherein the channel comprises a compression section having a first diameter D1 perpendicular to the flow axis at the side towards the inlet of the channel, and a second diameter D2 perpendicular to the flow axis at the side of the outlet of the channel, wherein D1 > D2 to form a tapered channel.

2. Die assembly according to claim 1 , wherein the inner surface (6) has such coefficient of friction that the extrusion pressure can be maintained at constant level during extrusion at an extrusion draw ratio of up to 25, preferably of up to 50, wherein the extrusion draw ratio is the ratio of the area of the inlet to the area of the outlet.

3. Die assembly according to any one of claims 1-2, wherein the housing is capable of withstanding an extrusion pressure of up to 250 MPa, or up to 170 MPa, and is capable of operating at operating temperatures of up to 180°C, or up to 160°C.

4. Die assembly according to any one of claims 1-3, wherein the inner surface of the housing is made of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin.

5. Die assembly according to any one of claims 1-4, wherein the housing is made of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin, or wherein the housing comprises an inner surface coating of a thermoset material, preferably of an epoxy resin, an unsaturated polyester resin, or a phenolic resin. Die assembly according any one of claims 4-5, wherein the thermoset resin is an epoxy resin produced via curing or an epoxy reactant system comprising a quantity of a compound of formula A and a quantity of a compound of formula B:

Formula A Formula B wherein R1 is a moiety comprising 1-20 carbon atoms, preferably a hydrocarbon moiety, preferably a moiety: wherein R3 is a moiety comprising 1-8 carbon atoms, preferably -CH₂- or-C(CHs)2- and wherein R2 is a moiety comprising 1-10 carbon atoms, preferably a hydrocarbon moiety, more preferably a linear alkyl moiety. Die assembly according to any one of claim 1-6, wherein the channel (1) is tapered at an angle a, wherein a is > 1.0° and < 60.0°, preferably > 5.0° and < 10.0°, more preferably > 6.0° and < 9.0°. Process for solid state extrusion of polyolefin materials, wherein the extrusion is performed by extruding a polyolefin material though a die assembly according to any one of claims 1-7, at a temperature of < 150°C, preferably < 140°C, more preferably of > 90°C and < 130°C, under application of a pressure of < 500 MPa, preferably < 400 MPa, more preferably < 300 MPa, even more preferably < 150 MPa, yet even more preferably of > 50 and < 150 MPa.

9. Process according to claim 8, wherein the polyolefin material is provided to the extrusion assembly as a powder having a bulk density of > 200 and < 600 kg/m³, wherein the bulk density is determined via ASTM D1895-96.

10. Process according to any one of claims 8-9, wherein the polyolefin material is a polyethylene or polypropylene homopolymer or copolymer.

11. Process according to any one of claims 8-10, wherein the polyolefin material is an ultra- high molecular weight polyethylene (LIHMWPE) material, preferably a disentangled UHMWPE.

12. Process according to any of claims 8-11, wherein the polyolefin material has an intrinsic viscosity of > 8.0, preferably > 10.0, more preferably > 20.0 dl/g, even more preferably > 20.0 and < 50.0 dl/g, as determined in accordance with ASTM D4020 (2011), involving a dilute solution of the polyolefin in decalin at a temperature of 135°C.

13. Process according to any one of claims 8-12, wherein the polyolefin material has a melting temperature T_m, as determined via DSC measurement, first run, of > 135°C, preferably of > 140°C.

14. Process according to any one of claims 8-13, wherein the polyolefin material has a degree of crystallinity X_c of > 70%, preferably of > 75%, as determined via DSC measurement, first run.

15. Extruded article obtained according to the process of any one of claims 8-14, preferably wherein the article is a tape, fibre, rod, sheet, tube or pipe.