Die assembly and method for granulating ultra-high molecular weight polyethylene

Abstract

The present invention relates to a novel die assembly for melt extrusion of ultra high molecular weight polyethylene (UHMWPE) comprising tapered channel sections and a method of granulating UHWMPE using such a die assembly. UHMWPE pellets having improved mechanical properties such as tensile strength, improved density and reduced oxidation index can be obtained using the die assembly according to the invention.

Description

Technical Field

[0001] The present invention relates to a die assembly and method for pelletizing ultra-high molecular weight polyethylene. Background Art

[0002] Ultra-high molecular weight polyethylene (UHMWPE) is a special type of polyethylene material that exhibits many outstanding properties, such as high impact strength, low coefficient of friction and good biocompatibility. These properties make UHMWPE a suitable material for applications such as bone joint prostheses, bearings, high-performance fibers and pipelines. UHMWPE typically has a very high molecular weight, long polymer chains and a high degree of molecular entanglement.

[0003] However, the special molecular structure of UHMWPE may lead to difficulties in processing the material via melt processing techniques. When the molecular weight of the polyethylene polymer is above 500,000 g / mol, the polymer retains its solid state behavior even at temperatures above its melting point and therefore does not exhibit the appropriate fluid flow properties that would allow the material to be processed by typical melt processing techniques in the field of thermoplastic polymers such as melt extrusion and injection molding.

[0004] To avoid this, the forming methods of UHMWPE materials generally involve solid powder processing techniques such as compression molding and plunger extrusion. However, each of these technologies has its disadvantages. For example, compression molding is an intermittent method and is therefore not particularly suitable for high-speed large-scale production of products. Furthermore, it involves relatively long processing times, which may lead to oxidative degradation of the UHMWPE material during the compression molding method. Although plunger molding is a quasi-continuous method, further processing of the original shape obtained (usually a rod) is also required, which often leads to processing marks on the surface of the produced parts, which may affect the product aesthetics and mechanical properties, and this leads to the generation of waste processed from the rod extruded by the plunger.

[0005] Other limitations of processing UHMWPE materials in common extrusion or injection molding methods include, for example, the physical state of the UHMWPE materials that can be used for processing. From the polymerization process, UHMWPE materials are obtained in the form of fine powders with very low friction. When attempting to process such powders via extrusion or injection molding, the powders tend to rotate with (one or more) rotating screws in the barrel of an extruder or injection molding machine, and therefore cannot be conveyed along the screw and move forward toward the die outlet of the extruder or injection molding machine. Therefore, it is a challenging task to transform UHMWPE powder into a material that is more suitable for processing, such as pellets. In this article, pellets are understood to be materials with sizes in the range of millimeters (mm), such as diameters of 2-7mm and lengths of 3-10mm. Such pellets are generally more convenient for processing than powdery materials, and therefore need to be obtained. Summary of the invention

[0006] It would therefore be desirable to have a more convenient processing method for UHMWPE materials to produce objects of desired shapes, in particular a method for producing UHMWPE pellets.

[0007] In the field of polyethylene, some changes in the nomenclature of different types of polyethylene have appeared in the known literature. In order to avoid any ambiguity in this regard, a detailed description of the composition of UHMWPE is provided here. In the context of the present invention, UHMWPE is understood to be a polymer comprising repeating polymer units derived from ethylene, preferably consisting of repeating polymer units derived from ethylene, and having a viscosity average molecular weight (Mv) of at least 1,000,000 g / mol. Typical UHMWPE materials can have a viscosity average molecular weight of 1,000,000 to 10,000,000 g / mol, or 2,000,000 to 8,000,000 g / mol.

[0008] To determine the viscosity average molecular weight (Mv) of the UHMWPE material, in the context of the present invention, the viscosity average molecular weight (Mv) is calculated based on the intrinsic viscosity (η) in dl / g according to the Margolies equation:

[0009] Mv=5.37·10 4 ·η 1.49

[0010] Wherein, Mv is the viscosity average molecular weight of UHMWPE in g / mol, and η is the intrinsic viscosity of UHMWPE in dl / g. The calculation according to the Margolies equation is described in ASTM D4020-11 (Standard Specification for Ultra-High Molecular Weight Polyethylene Molding and Extrusion Materials). The determination of the intrinsic viscosity is carried out in decalin as solvent at a temperature of 135° C. according to the method described in ASTM D2857-95 (Re 2007) (Standard Practice for Dilute Solution Viscosity of Polymers).

[0011] The present invention provides a die assembly allowing the processing of UHMWPE, wherein the UHMWPE obtained has improved mechanical properties, such as tensile strength, improved density and reduced oxidation index. This is achieved by a die assembly for processing UHMWPE, wherein the die assembly comprises a circularly enclosed straight channel (1), the straight channel comprising an inlet (2) and an outlet (3), the channel being configured such that a substance can 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 a shell completely surrounding the channel;

[0012] wherein the channel comprises a buffer section having a length A and a compression section having a length B, the buffer section is located at an inlet side of the channel, and the compression section is located at an outlet side of the channel, and the buffer section and the compression section are connected to each other;

[0013] wherein the buffer section has a first diameter D1 perpendicular to the flow axis on one side of the inlet of the channel and a second diameter D2 perpendicular to the flow axis on the side facing the outlet of the channel, wherein D1>D2, preferably forming a tapered channel section at an angle α;

[0014] wherein the compression section has a first diameter D3 perpendicular to the flow axis on the side facing the inlet of the channel, which corresponds to D2, and a second diameter D4 perpendicular to the flow axis on the side facing the outlet of the channel, wherein D3>D4, forming a tapered channel section at an angle β;

[0015] Preferably, wherein the angle α>β; and

[0016] Preferably, D4 is a circular opening, and more preferably, each of D1 , D2 , D3 and D4 is circular.

[0017] In the die assembly according to the invention, the angle β may preferably be ≥1.0° and ≤10.0°, preferably ≥1.5° and ≤5.0°, more preferably ≥1.6° and ≤4.9°, even more preferably ≥1.8° and ≤3.0°.

[0018] Preferably, the outlet diameter D4 of the die assembly is ≥2.0 and ≤8.0 mm, preferably ≥3.0 and ≤6.0 mm.

[0019] Preferably, the channel (1) consists of a buffer zone A and a compression zone B. Preferably, the die does not contain other tapered zones except the buffer zone A and the compression zone B.

[0020] The length B of the compression section of the channel can be, for example, ≥20 and ≤100 mm, preferably ≥30 and ≤60 mm.

[0021] The ratio of length B / length A may, for example, be ≥2.0, preferably ≥4.0.

[0022] The ratio D3 / D4 may be, for example, ≥1.2 and ≤2.0, preferably ≥1.3 and ≤1.7.

[0023] The die assembly according to the present invention may be equipped with a cooling unit. Such a cooling unit may preferably be configured such that it is able to cool the die assembly to a temperature of ≤150° C., more preferably ≥100° C. and ≤150° C. The cooling unit may for example be a unit that provides cooled air to the die assembly, preferably wherein the cooling unit is an air gun.

[0024] The die assembly may comprise a plurality of channels (1), preferably positioned in parallel.

[0025] The present invention also relates in certain embodiments to a polymer extruder assembly comprising a material inlet (6), an extruder barrel (7) comprising one or two extruder screws (8), and an outlet (9) for removing processed material from the extruder, wherein the outlet comprises a die assembly according to the present invention.

[0026] The extruder may comprise a cooling unit (10) for cooling the die assembly, preferably for cooling the die assembly to a temperature of ≤ 150° C., more preferably ≥ 100° C. and ≤ 150° C. The cooling unit may for example be a unit providing cooled air to the die assembly, preferably wherein the cooling unit is an air gun.

[0027] The present invention also relates to a method for producing ultra-high molecular weight polyethylene pellets, the method comprising:

[0028] i. Supplying a polymer composition comprising ultra-high molecular weight polyethylene (UHMWPE) to a polymer extruder component according to the present invention

[0029] ii. conveying the polymer composition through a polymer extruder;

[0030] iii. conveying the polymer composition through a die assembly; and

[0031] iv. forming the polymer composition leaving the polymer extruder into pellets through the outlet of the die assembly, which is carried out as follows:

[0032] a) cooling the polymer composition to a temperature below the melting temperature, preferably below 100° C., and subsequently cutting the obtained cooled strands into pellets; or

[0033] b) Cutting the polymer composition into pellets and subsequently cooling the pellets to below the melting temperature, preferably to below 100°C.

[0034] Preferably, the extruder barrel temperature in step ii) is ≥ 170° C. and ≤ 220° C. The extruder speed may, for example, be ≥ 50 and ≤ 150 rpm.

[0035] Preferably, the pressure at the inlet of the die assembly is ≥3.0 and ≤8.0 MPa.

[0036] The polymer composition preferably comprises ≥90.0 wt% UHMWPE and optionally ≤10.0 wt% high-density polyethylene (HDPE), relative to the total weight of the polymer composition. In a certain embodiment, the polymer composition comprises ≥90.0 wt% UHMWPE and ≤10.0 wt% high-density polyethylene (HDPE). For example, the polymer composition may comprise ≥90.0 wt% and ≤99.0 wt% UHMWPE and ≥1.0 and ≤10.0 wt% HDPE, more preferably ≥92.5 wt% and ≤97.5 wt% UHMWPE and ≥2.5 and ≤7.5 wt% HDPE.

[0037] The UHMWPE may, for example, have a viscosity average molecular weight (Mv) of ≥2,000,000 g / mol, preferably ≥2,000,000 and ≤8,000,000 g / mol, more preferably ≥3,000,000 and ≤8,000,000 g / mol, even more preferably ≥4,000,000 and ≤8,000,000 g / mol, yet even more preferably ≥5,000,000 and ≤8,000,000 g / mol, wherein the Mv is calculated via the Margolies equation based on the intrinsic viscosity, wherein the intrinsic viscosity is determined according to the method described in ASTM D2857-95 (Re 2007) at a temperature of 135°C in decalin as solvent.

[0038] UHMWPE may for example have a mass of ≥ 900 kg / m 3 , preferably ≥900kg / m 3 And ≤945kg / m 3 , more preferably ≥910kg / m 3 And ≤945kg / m 3 , even more preferably ≥910kg / m 3 And ≤935kg / m 3 , and even more preferably ≥915 kg / m 3 And ≤930kg / m 3 density.

[0039] The HDPE may, for example, have a molecular weight of ≥50,000 and ≤500,000 g / mol, preferably ≥50,000 and ≤300,000 g / mol, more preferably ≥75,000 and ≤250,000 g / mol.

[0040] HDPE can be a homopolymer of ethylene, or a copolymer of ethylene and a comonomer. The comonomer can be, for example, one selected from 1-butene, 1-hexene or 1-octene. Such a HDPE copolymer can, for example, contain ≥0.1 and ≤5.0 wt%, preferably ≥0.1 and ≤3.0 wt%, more preferably ≥0.3 and ≤3.0 wt% of polymer units derived from the comonomer, relative to the total weight of the HDPE copolymer.

[0041] HDPE may, for example, have a value of ≥946 and ≤975 kg / m 3 , preferably ≥950 and ≤970kg / m 3 , more preferably ≥950 and ≤965 kg / m 3 The density of the composite is as determined according to ASTM D792 (2008).

[0042] The HDPE may, for example, have a melt mass flow rate of ≥0.1 and ≤100 g / 10 min, as measured according to ASTM D1238 (2013) at 190°C under a load of 2.16 kg, preferably ≥0.5 and ≤50 g / 10 min, more preferably ≥1.0 and ≤25 g / 10 min, even more preferably ≥3.0 and ≤15.0 g / 10 min, yet even more preferably ≥5.0 and ≤10.0 g / 10 min. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] A brief description of the drawings is provided here.

[0044] Figure 1 A die assembly according to an embodiment of the present invention is shown, which includes a tapered compression section. Figure 1 In the die assembly, a housing (5) includes a channel (1) having an inlet (2) and an outlet (3). A material can flow along the channel in the direction of a flow axis (4). The assembly includes a buffer region having a length A and a compression region having a length B. D1 represents the diameter of the inlet of the buffer region, and D2 represents the diameter of the outlet of the buffer region; D3 represents the diameter of the inlet of the compression region, and D4 represents the diameter of the outlet of the compression region.

[0045] Figure 2 An alternative configuration of the die assembly is shown, showing an alternative geometry of the buffer region. Figure 2 The markers 1-5, AB and D1-D4 correspond to Figure 1 , as explained above.

[0046] Figure 3 A polymer extruder assembly comprising a die assembly according to the present invention is shown, wherein the extruder comprises a material inlet (6), an extruder barrel (7) comprising one or two extruder screws, and an outlet (9) for removing processed material from the extruder, wherein the outlet comprises a die assembly according to the present invention. Figure 3 The extruder assembly further shows a cooling unit (10) for cooling the die assembly.

[0047] Figure 4 A conventional die assembly is shown which does not include a compression zone as defined in accordance with the present invention.

[0048] Figure 5 The contents extruded from the barrel of the extruder by removing the die from the extruder are shown, thereby reflecting the processing state of the contents of the extruder during the processing of the UHMWPE material. Figure 5 The top image shows the extruder equipped with Figure 4 The material obtained from the extraction of the extruder contents in the case of conventional die components; Figure 5 The bottom image in FIG. 1 shows the use of an extruder equipped with a die assembly according to the present invention. Figure 1 The material obtained by construction. Example

[0049] The invention will now be illustrated by the following non-limiting examples.

[0050] In the examples of the present invention, a Leistritz ZSE-18 co-rotating twin screw extruder was used. The extruder was suitably equipped with a buffer zone directly connected to a circular outlet opening having a diameter of 4.5 mm. Figure 4 The conventional head of the design, or Figure 1 The tapered die according to the present invention shown in . The tapered die has a compression zone length of 35 mm, a tapered angle of the compression zone of 2 °, and a diameter of the outlet opening of 4.5 mm. The extruder is operated under a temperature profile, wherein each zone is heated to 180 ° C except that the die zone is heated to 170 ° C. The extruder is operated at a speed of 80 rpm. The feed rate of the polymer composition to the extruder is 0.53 kg / h. The extruder is equipped with an air gun to provide cooled air to the tapered die.

[0051] Experiments were conducted with either a conventional die or with a tapered die, and with and without air cooling.

[0052] As the material, a molten metal having an Mv of 5,000,000 g / mol and a mass of 920 kg / m 3 The formulations were made from 99.5 wt% of this UHMWPE and 0.5 wt% of an antioxidant (Irganox 1010), or from a blend of 94.5 wt% UHMWPE, 0.5 wt% of an antioxidant, and 5.0 wt% of SABIC CC860V, which is a 960 kg / m 3 HDPE with a density of 7.6 g / 10 min, a melt mass flow rate (190°C, 2.16 kg) of 7.6 g / 10 min and an Mv of 78,000 g / mol.

[0053] During extrusion, the pressure at the inlet of the die assembly, also known as the back pressure, was measured. The results are shown in Table 1 below.

[0054] Table 1

[0055]

[0056] The obtained product is cut into pellets for further processing. The sample of the product obtained in each embodiment is then processed into a test sample via compression molding using a CARVER press (1NE100). The pellets are placed between two steel plates. Compression molding is carried out at 200°C, 10MPa pressure for 2 min to produce film samples, and 20 min to produce ASTM V-type tensile test samples.

[0057] The density of the pellet obtained is measured using the magnetic suspension (MagLev) method. The magnetic suspension device comprises two magnets, whose same magnetic poles are opposite to each other. The sample and the magnet are immersed in the paramagnetic medium of 1mol / L MnCl2 solution. The density of the sample is then calculated after obtaining the respective suspension height of the sample. The greater the density of the sample, the lower the height obtained due to the magnetic field in the magnetic medium. The results obtained are shown in Table 2 below.

[0058] Table 2

[0059] Example 1 2 3 4 5 6 <![CDATA[Density (g / cm 3 )]]> 0.908 0.917 0.925 0.910 0.918 0.935

[0060] The density of pellets produced with a tapered die is higher than that of pellets produced with a conventional die, and the density increases further when air cooling is applied. Density measurements support visual observations of the extrudates, such as Figure 5 As shown in , it can be observed that in the experiment using the tapered die ( Figure 5 The extrusion (in the bottom image) Figure 5 The experiment with the conventional scale (circled on the right) is better than that with the conventional scale ( Figure 5The top image in Figure 2 shows better consolidated material. In addition, Figure 5 It is shown that in the experiment using the tapered die, the length of the melt section in the extruder is longer than in the experiment using the conventional die (the melt section is represented by Figure 5 The use of air cooling further increased the density of the pellets. The use of a combination of a tapered die and air cooling in the experiment to produce a blend of UHMWPE and HDPE in Example 6, which increased the density, appears to indicate that HDPE can act as a binder and that this method can help fill in fusion defects between the UHMWPE and HDPE phases.

[0061] The compression molded samples were subjected to tensile testing according to the method described above, wherein the tensile test was performed according to the method of ASTM D638-14. The obtained tensile strength results are shown in Table 3 below.

[0062] Table 3

[0063] Example 1 2 4 5 Tensile strength(MPa) 38.3 40.6 46.5 50.1

[0064] It can be observed that the tests carried out on the samples obtained using the die of the invention show an increase in tensile strength both in the case of UHMWPE (Example 2 vs. Example 1) and in the case of blends of UHMWPE with HDPE (Example 5 vs. Example 4).

[0065] To determine the oxidation index, measurements are performed according to ISO 5834-4 (2019). OX ) is calculated as:

[0066]

[0067] Among them, A Ox It is the wavelength at 1650cm in Fourier transform infrared (FTIR) spectrum. -1 Up to 1850cm -1 The integrated area within the range represents the oxidation peak area, and A Norm The FTIR spectrum is at 1330 cm -1 Up to 1396cm -1 The integrated area within the range represents the normalized peak area. OX Indicates a higher oxidation level. OX The results are shown in Table 4 below.

[0068] Table 4

[0069] Example 1 2 4 5 <![CDATA[I OX ]]> 0.17 0.15 0.22 0.19

[0070] The results in Table 4 show that when a die assembly according to the present invention is used, a reduced oxidation index is obtained (Examples 2 and 5 compared to Examples 1 and 4, respectively).

Claims

1. A die assembly for processing UHMWPE, wherein the die assembly comprises a circularly enclosed straight channel (1), the straight channel comprising an inlet (2) and an outlet (3), the straight channel being configured such that a substance can be conveyed through the channel along a flow axis (4) from the inlet toward the outlet, wherein the channel comprises a housing (5) to form a shell completely surrounding the channel; in, The channel comprises a buffer section having a length A and a compression section having a length B, the buffer section being located at an inlet side of the channel, and the compression section being located at an outlet side of the channel, the buffer section and the compression section being connected to each other; wherein the buffer section has a first diameter D1 perpendicular to the flow axis on one side of the inlet of the channel and a second diameter D2 perpendicular to the flow axis on the side facing the outlet of the channel, wherein D1>D2, preferably forming a tapered channel section at an angle α; wherein the compression section has a first diameter D3 perpendicular to the flow axis on the side facing the inlet of the channel, which corresponds to D2, and a second diameter D4 perpendicular to the flow axis on the side facing the outlet of the channel, wherein D3>D4, forming a tapered channel section at an angle β; Preferably, wherein the angle α>β; Preferably, wherein D4 is a circular opening, more preferably wherein each of D1, D2, D3 and D4 is circular; And preferably, the outlet diameter D4 is ≥2.0 and ≤8.0 mm, preferably ≥3.0 and ≤6.0 mm.

2. The die assembly according to claim 1, wherein the angle β is ≥1.0° and ≤10.0°, preferably ≥1.5° and ≤5.0°, more preferably ≥1.6° and ≤4.9°, even more preferably ≥1.8° and ≤3.0°.

3. The die assembly according to any one of claims 1-2, wherein the channel (1) consists of the buffer section A and the compression section B.

4. The die assembly according to any one of claims 1 to 3, wherein the length B of the compression section of the channel is ≥20 and ≤100 mm, preferably ≥30 and ≤60 mm.

5. The die assembly according to any one of claims 1 to 4, wherein the ratio of length B / length A is ≥ 2.0, preferably ≥ 4.

0.

6. The die assembly according to any one of claims 1 to 5, wherein the ratio D3 / D4 is ≥1.2 and ≤2.0, preferably ≥1.3 and ≤1.

7.

7. A polymer extruder assembly comprising a material inlet (6), an extruder barrel (7) comprising one or two extruder screws (8) and an outlet (9) for removing processed material from the extruder, wherein the outlet comprises a die assembly according to any one of claims 1 to 6.

8. The polymer extruder according to claim 7, wherein the extruder comprises a cooling unit (10) for cooling the die assembly, preferably for cooling the die assembly to a temperature of ≤ 150°C, more preferably ≥ 100°C and ≤ 150°C.

9. The polymer extruder according to claim 8, wherein the cooling unit is a unit that provides cooled air to the die assembly, preferably wherein the cooling unit is an air gun.

10. A method for producing ultra-high molecular weight polyethylene pellets, the method comprising: i. Supplying a polymer composition comprising an ultra-high molecular weight polyethylene (UHMWPE) to a polymer extruder component according to any one of claims 7 to 9 ii. conveying the polymer composition through the polymer extruder; iii. conveying the polymer composition through the die assembly; and iv. forming the polymer composition leaving the polymer extruder into pellets through the outlet of the die assembly, which is carried out as follows: a) cooling the polymer composition to a temperature below the melting temperature, preferably below 100° C., and subsequently cutting the obtained cooled strands into pellets; or b) cutting the polymer composition into pellets and subsequently cooling the pellets to below the melting temperature, preferably to below 100°C.

11. The process according to claim 10, wherein the extruder barrel temperature in step ii) is ≥ 170°C and ≤ 220°C.

12. The process according to any one of claims 10-11, wherein the extruder speed is ≥50 and ≤150 rpm.

13. The method according to any one of claims 10-12, wherein the pressure at the inlet of the die assembly is ≥ 3.0 and ≤ 8.0 MPa.

14. The process according to any one of claims 10-13, wherein the polymer composition comprises ≥ 90.0 wt.-% UHMWPE and optionally ≤ 10.0 wt.-% high density polyethylene (HDPE), relative to the total weight of the polymer composition.

15. The process according to any one of claims 10 to 14, wherein the UHMWPE has a viscosity average molecular weight (Mv) of ≥ 2,000,000 g / mol, preferably ≥ 2,000,000 and ≤ 8,000,000 g / mol, wherein the viscosity average molecular weight is calculated via the Margolies equation based on the intrinsic viscosity, wherein the intrinsic viscosity is determined according to the method described in ASTM D2857-95 (Re 2007) at a temperature of 135°C in decalin as solvent.

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