WO2013034582A1

WO2013034582A1 - Process for the melt extrusion of ultra high molecular weight polyethylene

Info

Publication number: WO2013034582A1
Application number: PCT/EP2012/067284
Authority: WO
Inventors: Anurag Vedprakash PANDEY; Sanjay Rastogi; Gerrit Peters; Ramandeep Singh
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2011-09-05
Filing date: 2012-09-05
Publication date: 2013-03-14

Abstract

The invention relates to a process for the manufacture of a shaped part of a disentangled UHMW-PE comprising: a) heating the disentangled UHMW-PE to a temperature between 160-170 °C at a rate of at least 10 °C/min to provide a melt of the disentangled UHMW-PE, b) extruding the melt into the shaped part, within a residence time of at most 30 minutes.

Description

PROCESS FOR THE MELT EXTRUSION OF ULTRA HIGH MOLECULAR WEIGHT

POLYETHYLENE

The invention relates to a process for the manufacture of a shaped part of ultra high molecular weight polyethylene (UHMW-PE). The invention further relates to a shaped part obtainable with this process.

The mechanical properties in polymers are strongly dependent on molecular characteristics. For example, in linear polyethylene, with increasing molecular weight from several hundred thousand g/mol to a million g/mol or above, mechanical properties such as tensile strength, modulus, and abrasion resistance increases to an extent that the polymer normally used for commodity applications becomes applicable for demanding applications, which include prostheses, lightweight strong fibers and tapes for ballistic applications, ropes for replacement of steel cables, etc. However, increased molecular weight of the polymers also adversely affects their processability, mainly due to the reduced number of chain ends and increased number of entanglement per chain. Thus, it has been always a quest to find balance between the ease in processing and the acquired mechanical properties.

Normally in the commercially synthesized polymers, where a heterogeneous Ziegler Natta (Z-N) catalyst is used, the crystallization rate is slower than the polymerization rate. Moreover, in a heterogeneous catalytic system the active sites are tethered on a support and are close to each other, leading to a higher probability of finding the neighboring growing chains. This results in the entanglement formation during synthesis. In contrast to the heterogeneous synthesis, in the homogeneous synthesis catalyst and co-catalyst are dispersed in the polymerization medium. Thus, the homogeneous synthesis provides an opportunity to control the polymerization rate, crystallization rate, and desired separation between the polymerization sites to tailor the entangled state of the synthesized polymer. Such a possibility results into the formation of disentangled polyethylene, ultimately resulting into a "single chain forming single crystal".

US7671 159 describes use of a single-site homogeneous catalytic system, and following the concept of single-site forming single crystal, the active sites were separated in the solvent to an extent that the growing chains do not overlap during polymerization at low polymerization temperature. This synthesis also presents a possibility of synthesizing UHMW disentangled polymers with narrow polydispersity that could be deformed in the solid state to obtain high-modulus, high-strength tapes. The synthesized UHMW-PE also provides an opportunity to investigate entanglement formation during polymerization and chain dynamics arising on melting of the disentangled crystalline state. Lippits et al. [Macromolecules, 2006, 39, 8882-8885] have studied the entanglement formation in such materials and have shown the effective use of rheology to follow the entanglement formation.

Due to intractability of the material via conventional melt processing routes, UHMW-PE is usually processed via compression moulding or ram-extrusion into simple shapes, like rods, plates or sheets, which are subsequently machined into the desired products. Known processes for the manufacture of products from UHMW- PE via melt processing are limited to lower number average molecular weight (Mn) polymers, for example polymers having a Mn below 400,000 g/mol, which restricts achievable mechanical properties of the final products. There is a constant need for an improved process for the manufacture of products made from UHMW-PE.

An objective of this invention is to find a novel way to process UHMW-PE via melt route to make desired products.

According to the invention this objective is achieved by a process for the melt extrusion of UHMW-PE comprising the steps of:

a) heating the UHMW-PE to a temperature between 160-170 °C at a rate of at least 10 "C/rnin to provide a melt of the UHMW-PE and

b) extruding the melt into the shaped part, within a residence time of at most 30 minutes,

wherein the UHMWPE has a number average molecular weight (Mn) of at least 500,000 gram/mole, a Mw/Mn ratio of at most 8, determined using melt rheology at 160 °C, and an elastic modulus G_N° of at most 1 .4 MPa determined using a dynamic time sweep (modulus build-up) experiment directly after melting at 160 'Ό, where sample is heated in a rheometer from 130°C to 160°C at heating of 10°C/min. at an angular frequency 10 rad/s, strain 0.5%, temperature 160°C, wherein G_N° is the elastic modulus measured in the dynamic time sweep experiment at time t₀, directly after melting the sample.

With the process of the invention a properly fused UHMW-PE product with enough toughness is obtained. The product can also be deformed without any breakage.

Unlike in the existing extrusion process of molten UHMW-PE which demands lower number average molar mass, or the presence of lower molecular weight components, the process according to the present invention allows manufacture of shaped parts with number average molar mass higher than 500,000 g/mol having good wear and friction properties.

The residence time is herein understood as the duration from the time point at which the desired temperature is reached in step a) and the time point at which the melt has left the extruder.

The inventors have surprisingly found that the fast heating and the short residence time results in a UHMW-PE product with proper fusion to achieve the desired mechanical strength. While not wishing to be bound by any theory, it is thought that this is due to the disentangled state of the UHMW-PE being largely maintained.

It is experimentally observed that annealing at a temperature of

Ι ΘΟ 'Ό quickly results in a decrease of flow ability of the melt, indicating influence of entanglement on melt behavior of the UHMW-PE. In the process according to the present invention, the annealing period i.e. the period prior to extrusion of the melt from the extruder (processing machine), is preferably minimal and shear is applied to the melt by the extrusion step very soon after the melt is obtained. Preferably, the annealing period of the melt is at most 5 minutes, more preferably 2 minutes. Most preferably, the annealing step is 0 minutes, i.e. the extrusion step is performed immediately after the melt is provided.

In the present invention it is experimentally found that the residence time of melt in the extruder should not exceed 30 minutes. Preferably, the residence time is at most 20 minutes, more preferably 15 minutes, even more preferably 10 minutes.

An example of modulus build up that depicts entanglement formation is shown in Fig.1 for a disentangled UHMWPE having a weight average molar mass of 1 .4 million g/mol and a molecular weight distribution of 2.5. Fig. 1 shows a dynamic time sweep test at Ι ΘΟ 'Ό for the disentangled sample at a constant angular frequency of 10 rad/s and strain 0.5%. The modulus buildup with the increasing annealing time represents the increasing entanglement density (decreasing M_e) in the polymer melt. The modulus buildup is divided into two regimes, R1 and R2. Regime R1 is defined as the region where 80% of the total modulus buildup occurs. The remainder 20% of the modulus buildup occurs in 80% of the total entanglement time (t_m), is associated with the regime R2. The residence time is preferably within the region R1 . For details on the described regions and modulus build up dependence with molar mass, see Pandey et al, Macromolecules, 201 1 , 44, 4952-4960.

In the present specification, the wording disentangled UHMWPE is characterized by a UHMWPE having a number average molecular weight (Mn) of at least 500,000 gram/mole, a Mw/Mn ratio of at most 8, and an elastic modulus G_N°, determined directly after melting at 160 'Ό of at most 1 .4 MPa. determined using a dynamic time sweep (modulus build-up) experiment directly after melting at 160 'Ό wherein a sample is heated in a rheometer from 130°C to 160°C at heating of 10°C/min. Parameters used for dynamic time sweep experiments are: angular frequency 10 rad/s, strain 0.5%, temperature 160 ^. G_N° is the elastic modulus measured in a dynamic time sweep experiment at time t₀, directly after melting the sample .

The disentangled UHMW-PE can e.g. be obtained as described in Pandey et al [Macromolecules, 201 1 , 44, 4952-4960].

Unlike in the existing extrusion process of molten UHMW-PE which demands lower number average molar mass, the process according to the present invention allows manufacture of shaped parts with number average molar mass higher than 500,000 g/mol having good wear and friction properties.

The number average molecular weight (Mn) of the polymer in the UHMWPE of the present invention is preferably at most 4 million, more preferably at most 3.5 million. Particularly preferred range of the number average molecular weight is between 0.6 million and 2.5 million. Conventionally, the molecular weight distribution (Mw/Mn) and molecular weight averages (Mw, Mn, Mz) of the UHMWPE polymer are determined in accordance with ASTM D 6474-99 at a temperature of 160 'Ό using 1 , 2, 4-trichlorobenzene (TCB) as solvent. However, this technique is not easy for measuring molecular weight averages and molecular weight distribution (MWD) of UHMWPE samples having very high molecular weights (like for example an Mn of higher than 1 ,5 ^*10⁶). Talebi et al. [Macromolecules, 2010, 43, 2780-2788] have shown effective use of melt rheology for the measurement of UHMWPE samples. Authors validated measured molecular weight and distribution using melt rheology data against appropriate chromatographic equipment (PL-GPC220 from Polymer Laboratories) including a high temperature sample preparation device (PL-SP260). The system is calibrated using sixteen polystyrene standards (Mw/Mn <1 .1 ) in the molecular weight range 5^*10³ to 8^*10⁶ gram/mole. Authors showed a good match between existing chromatographic techniques for low molecular weight whereas chromatographic technique cannot be used for very high molecular weight polymers, such as UHMWPE.

Molecular weight and molecular weight distribution data in this invention is numerically synthesized using algorithm developed by Mead [J. Rheol. 1994, 38, 1797-1827]. This algorithm has been commercialised by Rheometric Scientific for incorporation in their Orchestrator software and used here for the determination of Mw and MWD. A dynamic frequency sweep within the linear regime, at 160°C, is performed on a thermodynamically stable melt obtained after a fully entangled melt is achieved (by annealing for several hours at 160°C, annealing time depends on Mw of the sample). This is considered to be the requisite as the theoretical models used in the algorithm is applicable on the thermodynamically stable melt state (fully entangled). Mw and MWD can be directly obtained after numerical fitting the dynamic frequency sweep data to the model; and number average molecular weight (Mn) is obtained as,

Mn = (Mw/MWD)

For details on measurement of Mw and MWD of UHMWPE using melt rheology, please see publications by Talebi et al. [Macromolecules, 2010, 43, 2780-2788] and Pandey et al [Macromolecules, 201 1 , 44, 4952-4960].

Prior to the measurement of the molecular weight distribution using melt rheometry. a polyethylene sample to which 0.5wt% of an antioxidant such as IRGANOX 1010 has been added to prevent thermo-oxidative degradation, is first sintered at 50 'Ό and 200 bars. Disks of 8 or 12 mm diameter and thickness 1 mm obtained from the sintered polyethylene are heated fast (~ 30 'Ό /min) to well above the equilibrium melting temperature in the rheometer under nitrogen atmosphere. For example, the disk was kept at Ι δΟ 'Ό for two hours or more. The slippage between the sample and rheometer discs is checked with the help of an oscilloscope. During dynamic experiments two output signals from the rheometer i.e. one signal corresponding to sinusoidal strain, and the other signal to the resulting stress response, are monitored continuously by an oscilloscope. A perfect sinusoidal stress response, which can be achieved at low values of strain, was an indicative of no slippage between the sample and discs.

Rheometry is carried out using a plate-plate rheometer such as Rheometrics RMS 800 or ARES strain controlled rheometer from TA Instruments. The Orchestrator Software provided by the TA Instruments, which makes use of the Mead algorithm, is used to determine molar mass and molar mass distribution from the modulus vs. frequency data determined for the fully entangled polymer melt. The data are obtained under isothermal conditions between 160 - 220 °C. To get the good fit angular frequency region between 0.001 to 100rad/s and constant strain in the linear viscoelastic region between 0.5 to 2% should be chosen. The time-temperature superposition is applied at a reference temperature of Ι ΘΟ 'Ό. To determine the modulus below angular frequency of 0.001 rad/s, stress relaxation experiments may be performed. In the stress relaxation experiments, a single transient deformation (step strain) to the polymer melt at fixed temperature is applied and maintained on the sample and the time dependent decay of stress is recorded.

In one embodiment of the present invention, a polymer is used with the molar mass and Mw/Mn ratio as described above which can be compressed below its equilibrium melting temperature of 142 ^<Ό, more in particular within the temperature range of 100-138°C, wherein the thus-obtained film can be drawn below the equilibrium melting temperature by more than 15 times its initial length.

The molecular weight distribution of the UHMWPE used according to the invention is relatively narrow. This is expressed by the Mw (weight average molecular weight) over Mn (number average molecular weight) ratio of at most 8. More in particular the Mw/Mn ratio is at most 4, still more in particular at most 3, even more in particular at most 2.

Preferably, step b) is performed at an extrusion speed that is not too high. This limited speed of extrusion avoids melt fracture on the surface of the product. It was found that an extrusion speed of at most 5 mm/min allows a smooth surface. More preferably, step b) is performed at an extrusion speed of at most 4 mm/min, more preferably at most 2 mm/min, more preferably at most 1 mm/min. Preferably step b) is performed at an extrusion speed of at least 0.5 mm/min.

Preferably, step b) is performed by a capillary extruder or a twin screw extruder. This is advantageous because the process is performed fast and continuously. In the existing processes in industry, ram extrusion is used. The existing ram extrusion process is a batch process, which is extremely slow and provides polymer in the shape of rods, which are machined to the desired shapes. Such a shaping process leads to immense waste of the material and restricts the use to few applications. By the present method the polymer can be extruded directly to the desired shapes. Depending on the cross section of the die, the shaped part can be in the form of rod profile that can be e.g. cylindrical or rectangular, or I shape or L shape or T shape etc.

The process according to the present invention may preferably be a continuous process in which the disentangled UHMW-PE powder is continuously fed and molten, and the shaped part is extruded.

The invention further relates to a shaped part obtainable by the process of the invention. With the process of the invention a shaped part with a high toughness is obtained which can be deformed without breakage. The process has the possibility to make hollow products of different geometries (cross section) that are otherwise not possible for such high molecular weight polymers.

The invention will be further elucidated with the following non-limiting examples.

Powders of disentangled UHMW-PE were prepared as described in

Pandey et al, Macromolecules, 201 1 , 44, 4952-4960. General.

All the manipulations of air and moisture sensitive products were carried out under a dry nitrogen or argon atmosphere using dry box (MBraun Unilab) and/or standard Schlenk line techniques. The bis(phenoxyimine)titanium dichloride complex, [3-t-Bu-2-0-C6H₃CH=N(C6F5)]2TiCl2, was purchased from MCAT and was used as received. MAO (10% weight in toluene) and dry toluene were used as received from Aldrich. All the other chemicals are commercially available and used as received. Ethylene (polymer grade) was purchased from BOC and used as received.

Ethylene Polymerization.

An oven-dried five-necked round-bottom flask equipped with a magnetic stirred bar, thermometer probe, and sintering cannula were previously dried under vacuum for 30 min and backfilled with nitrogen. Dried toluene was introduced to the reaction flask, followed by 1 ml_ of MAO, and nitrogen was bubbled through the solvent for 30 min under stirring. The nitrogen was then replaced by ethylene gas, which was left bubbling through the solvent. After 30 min, the desired amount of MAO (minus 2 ml_) was introduced, and the reaction flask was then placed at the desired temperature. When the requisite temperature was reached, the polymerization was initiated by addition of the precatalyst [3-t-Bu-2-0-C₆H₃CH=N(C₆F₅)]₂TiCI₂ previously dissolved in 2 ml_ of toluene and activated by 1 ml_ of MAO solution. After the required polymerization time, the polymerization was quenched by addition of an acidified MeOH solution. The resulting polyethylene is filtered, washed with copious amounts of methanol/acetone, and dried overnight in a vacuum oven at 40 °C.

N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroaniline[3-t-Bu-2-0- C6H₃CH=N(C6F5)]2TiCl2 precatalyst has been used for the preparation of a range of UHMWPE. Typically, [3-t-Bu-2-0-C6H₃CH=N(C₆F5)]2TiCl2 was treated with 1 100 equiv of MAO in toluene under an atmospheric pressure of ethylene with temperature and time being the only parameters that are variable during the polymerization reaction. Example 1

The UHMW-PE powders were prepared as described above. The UHMW-PE has a weight average molecular weight of 2.3 x 10⁶ g/mol and a molecular weight distribution (Mw/Mn ratio) of about 2.4.

The powders were placed in and extruded through a straight capillary die having a dimension of 30:2 (mm) using a capillary rheometer (Rosand RH-7 flowmaster twin-bore rheometer). The powder was heated from room temperature to 160 'Ό at a speed of 10 °C/min. After 2 minutes from the time point after the stabilisation of temperature at 160 °C was reached, the extrusion of the sample was started. The extrusion speed was at 0.2 mm/min. The extrusion was completed after 5 minutes from the start of the extrusion. Thus, the residence time was about 7 minutes.

Smooth and properly fused extrudates were obtained.

Example 2

Example 1 was repeated except that:

- the UHMW-PE used had a weight average molecular weight of 3.7 x 10⁶ g/mol and a molecular weight distribution of about 2.6 and

- after 5 minutes from the time point after the stabilisation of temperature at 160 °C was reached, the extrusion of the sample was started at a speed of 0.5 mm/min. The extrusion was completed after 10-15 minutes from the start of the extrusion. Thus, the residence time was about 15-20 minutes.

The extruded sample was properly fused, but the surface was rougher compared to the sample obtained by example 1 .

Comp. Ex.

Example 2 was repeated except that the extrusion started after more than 15 minutes from the time point after the stabilisation of temperature at 160 'Ό was reached. The extrusion was completed after 25-30 minutes from the start of the extrusion. Thus, the residence time was about 40-45 minutes.

An extrudate was obtained, but the surface of the sample was melt fractured.

In all experiments, the molten UHMW-PE which was not extruded and left in the die became hard and non-extrudable.

Claims

1 . A process for the melt extrusion of UHMW-PE comprising:

wherein the UHMWPE has a number average molecular weight (Mn) of at least 500,000 gram/mole, a Mw/Mn ratio of at most 8, determined using melt rheology at 160 °C, and an elastic modulus G_N° of at most 1 .4 MPa determined using a dynamic time sweep (modulus build-up) experiment directly after melting at 160 'C, where sample is heated in a rheometer from 130°C to 160°C at heating of 10°C/min. at an angular frequency 10 rad/s, strain 0.5%, temperature 160°C, wherein G_N° is the elastic modulus measured in the dynamic time sweep experiment at time t₀, directly after melting the sample.

2. The process according to claim 1 , wherein the residence time is at most 15 minutes.

3. The process according to any one of the preceding claims, wherein the UHMW-PE has a number average molecular weight of between 0.5 million and 4 million g/mol in accordance with ASTM D 6474-99 at a temperature of 160 °C using 1 ,2,4- trichlorobenzene (TCB) as solvent.

4. The process according to any one of the preceding claims, wherein the UHMW-PE has a number average molecular weight of between 0.6 million and 2.5 million g/mol in accordance with ASTM D 6474-99 at a temperature of 160 °C using 1 ,2,4- trichlorobenzene (TCB) as solvent.

5. The process according to any one of the preceding claims, wherein step b) is performed at an extrusion speed of at most 5 mm/min.

6. The process according to any one of the preceding claims, wherein step b) is performed at an extrusion speed of 0.5-4 mm/min.

7. The process according to any one of the preceding claims, wherein step b) is performed by a capillary extruder or a twin screw extruder.

8. The shaped part obtainable according to the process according to any one of the preceding claims.