Ultra-high molecular weight polyethylenes and films thereof

Abstract

The present invention relates to an ultra-high molecular weight polyethylene (UHMWPE), wherein the UHMWPE is an ethylene-based polymer having a molecular weight (M) of ≥ 500,000 g / mol, preferably of ≥ 500,000 and ≤ 7,000,000 g / mol, wherein M is determined by calculation based in intrinsic viscosity according to the equation M = 53700(IV)^1.37, wherein IV is expressed in dl / g and determined according to the method of ASTM D4020-11; and wherein the UHWMPE has in the Fourier rheology profile in the strain amplitude range of 2- 15% a value for n of ≤ 1.8, wherein n is calculated using the equation (I): wherein I3 / I1 is the intensity ratio of the third harmonic and the fundamental harmonic, and γ is the strain amplitude. Such UHMWPE allows for production of thin battery separator membranes having high porosity, good mechanical properties and good electrical properties.

Description

Ultra-High Molecular Weight Polyethylenes and Films Thereof.

[0001] The present invention relates to ultra-high molecular weight polyethylenes (UHMWPE) and films thereof. In particular, the invention relates to UHMWPE films as microporous films that may be used as separators, for example in battery separators.

[0002] UHMWPE resins, due to their outstanding chemical stability, mechanical properties, high melt strength, and good solubility for processing, is considered to be an appropriate material for use in separator membranes for electrical batteries and capacitators, such as in lithium ion batteries. In view of a continuous growth in demand for products containing batteries and capacitators, such as in electric vehicles, renewable energy storage and consumer electronics, the demand and technical requirements for separator solutions continues to develop.

[0003] A UHMWPE-based separator for batteries is typically produced by a ‘wet’ process, in which the UHMWPE is first contacted with a compound that leads to a certain level of plasticising or dissolving of that compound in the UHMWPE, followed by forming a film by extrusion of the plasticised mixture, after which the film is stretched in the solid phase, the solvent is removed by extraction, and the resulting porous film is subjected to a heat treatment to anneal the film.

[0004] In the first step of this process, the UHMWPE is mixed with the solvent for a certain period to result in a homogeneous mixture. As the UHMWPE typically has a certain level of porosity, this homogeneous mixture comprises UHMWPE particles in which a certain amount of the solvent is absorbed. Such mixture is also referred to as a UHMWPE gel. Certain additives, such as antioxidants, may be added to this mixture.

[0005] The UHMWPE gel is then formed into a film by a film shaping process in which the UHMWPE gel is subjected to temperatures above the melting temperature thereof. For example, such film shaping process may involve processing of the UHMWPE gel in a melt extruder, followed by casting through a slit die onto one or more chill rolls, thereby forming a film of the UHMWPE gel material.

[0006] This film then is stretched to increase its dimensions at a temperature below the melt temperature. Such increase in dimensions induces porosity. The stretching typically occurs inboth film dimensions, that is in the machine direction (MD), being the direction of casting of the film, as well as in the transverse direction (TD), being the direction perpendicular to the MD. This stretching in MD and TD may be done sequentially or simultaneously.

[0007] The thus obtained stretched film is subsequently subjected to an extraction step to remove the solvent, and a thermal treatment to anneal the film. This results in a desired microporous film that has an interconnected pore architecture, as required for the application as battery separator.

[0008] Such LIHMWPE battery separator functions in the battery application in that it provides for pathways for migration of the ions during the charging and discharging processes, it presents direct contact between the cathode and the anode of the battery, thereby reducing the risk of short circuit and explosion, it allows the shutdown of ion flow at elevated temperatures, such as in case of a runaway reaction, by closing the pores due to fusion of the membrane material, thereby reducing the risk of overheating, and it is sufficiently thin to allow short ion migration paths and thereby high volumetric energy densities.

[0009] Presently, one of the key drivers in developments of battery separators for high-energy batteries relates to reduction of the thickness of the films. A typical UHMWPE-based separator has a thickness of 15-25 pm. A thinner separator allows for an increase in energy capacity and current density, as the contact area is increased and the internal resistance to which the ions are subjected is decreased.

[0010] However, a thinner separator may negatively affect the mechanical strength thereof. In view of requirements with respect to mechanical properties in both manufacturing of the battery as well as in operational use, this cannot be compromised.

[0011] Accordingly, these are drivers that work in opposite direction. A demand continues to exist for development of LIHMWPE materials that on one hand allow for production of a separator of reduced thickness, whilst at the same time ensuring that appropriate mechanical properties are ensured.

[0012] This has now been achieved according to the present invention by an ultra-high molecular weight polyethylene (LIHMWPE), wherein the LIHMWPE is an ethylene-based polymer having a molecular weight (Mw) of > 500,000 g / mol, preferably of > 500,000 and < 7,000,000 g / mol, wherein M is determined by calculation based in intrinsic viscosity according tothe equation M = 53700(IV)A1.37, wherein IV is expressed in dl / g and determined according to the method of ASTM D4020-11 ; and wherein the LIHWMPE has in the Fourier rheology profile in the strain amplitude range of 2- 15% a value for n of < 1 .8, wherein n is calculated using the equation:wherein I3 / I1 is the intensity ratio of the third harmonic and the fundamental harmonic, and y is the strain amplitude, preferably wherein the values for k and n are obtained via curve fit over the strain amplitude range.

[0013] Such LIHMWPE allows for production of thin battery separator membranes having high porosity, good mechanical properties and good electrical properties.

[0014] In the context of the present invention, it is to be understood that LIHMWPE relates to materials having a molecular weight (Mw) of at least 500,000 g / mol, for example up to 7,000,000 g / mol.

[0015] In the context of the present invention, the molecular weight (M) of the LIHMWPE may be determined by calculation using the equation M = 53700(IV)A1.37, wherein IV is the intrinsic viscosity in dl / g. The intrinsic viscosity is to be determined according to the method of ASTM D4020 -11.

[0016] It is considered that the slope of the Fourier rheology profile in the shear strain amplitude range of 2-15% reflects the long-chain branching content of the LIHMWPE polymer.

[0017] The ethylene-based polymer may for example be a homopolymer or a copolymer of ethylene and a further a-olefin selected from 1-butene, 1-hexene and 1-octene, preferably containing < 2.0 wt% of the further a-olefin. For example, the ethylene-based polymer may comprise > 0.1 and < 2.0 wt% of the further a-olefin, more preferably > 0.1 and < 1.5 wt%, with regard to the total weight of the ethylene-based polymer.

[0018] Preferably, the ethylene-based polymer has an intrinsic viscosity of > 5.0 dl / g, preferably of > 5.0 and < 50.0 dl / g, more preferably > 5.0 and < 30.0 dl / g, even more preferably > 5.0 and < 25.0 dl / g, as determined in accordance with ASTM D4020 - 11.

[0019] Preferably, the ethylene-based polymer has a molecular weight (Mw) of > 500,000 and < 5,000,000 g / mol, or > 1 ,000,000 and < 5,000,000 g / mol, or > 500,000 and < 3,000,000 g / mol, or > 1 ,000,000 and < 4,000,000 g / mol.

[0020] The ethylene-based polymer may for example have a melt mass-flow rate as determined at 21.6 kg at 190°C in accordance with ISO 1133-1 (2022), of < 0.42 g / 10 min, preferably of < 0.36 g / 10 min, more preferably of > 0.0 and < 0.36 g / 10 min.

[0021] It is preferred that the ethylene-based polymer has a molecular weight distribution Mw / Mnof < 7.0, preferably of > 4.0 and < 7.0, more preferably of > 5.0 and < 7.0, wherein Mwand Mnare determined in accordance with ASTM D6474 (2012).

[0022] For example, the ethylene-based polymer may have a density of > 935 and < 965 kg / m3, preferably of > 935 and < 955 kg / m3, more preferably of > 940 and < 950 kg / m3, as determined in accordance with ASTM D792 (2013).

[0023] The present invention also relates to a polymer composition comprising a first LIHMWPE according to the invention. Preferably, the composition further comprises a second LIHMWPE according to the invention, wherein the second LIHWMPE is different from the first LIHMWPE, preferably wherein the second LIHMWPE has a higher molecular weight M than the first LIHMWPE. In such composition, it is preferred that the weight ratio of the second LIHMWPE to the first LIHMWPE is > 0.5, preferably > 0.5 and < 20.

[0024] The invention also relates to a process for preparation of such polymer composition, involving extrusion based compounding and / or dry blending.

[0025] The invention also relates to a process for production of a porous film, wherein the process involves the steps in this order of:(a) providing a quantity of an LIHMWPE according to the invention or a polymer composition according to the invention a mixing vessel;(b) adding a quantity of a solvent, preferably a hydrocarbon solvent such as liquid paraffin or paraffin wax; an ester such as dioctyl phthalate or dibutyl phthalate; or a higher alcohol such as oleyl alcohol or stearyl alcohol, to the mixing vessel;(c) subjecting the contents of the mixing vessel to a temperature of between 80 and 120 °C to form gel particles;(d) supplying the gel particles to a melt extruder and extruding the molten gel to form a cast film;(e) stretching the cast film at temperatures of between 80 and 150°C to obtain an oriented film;(f) extracting the solvent from the oriented film; and(g) subjecting the oriented film to a heat treatment to obtain a porous film.

[0026] It is preferred that stretching in step (e) is performed in both directions, preferably to a degree of orientation of at least 5 in each direction.

[0027] It is further also preferred that the extraction step (f) is performed using a solvent, which may be selected from hydrocarbons such as n-hexane, cyclohexane or halogenated hydrocarbons; methylene chloride, 1 ,1 ,1 -trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroether or hydrofluorocarbon; alcohols such as ethanol or isopropanol; ethers such as diethyl ether or tetrahydrofuran; and ketones such as acetone or methyl ethyl ketone, preferably n-hexane, and / or is performed by applying ultrasonic irradiation.

[0028] In an embodiment, the invention also relates to a porous film comprising the LIHMWPE according to the invention or the polymer composition according to the invention, preferably wherein the porous film has:• an ion conductivity of > 1.50 Q-1, as determined in accordance with GB / T36363- 2018; and / or• a Gurley air resistance of < 200 s, preferably < 150 s, as determined in accordance with ISO 5636-5:2013; and / or• a porosity of > 40.0%, as determined in accordance with GB / T36363-2018; and / or• a shut-down protection temperature of > 140°C, as determined in accordance with GB / T36363-2018.

[0029] The invention also relates to a battery or capacitator comprising a porous film according to the invention.

[0030] The invention will now be illustrated by the following non-limiting examples.

[0031] In the context of the present invention, the below materials were used in the experiments.Table 1 : Materials used.

[0032] UH1 and UH2 were produced according to the following synthesis method. The polymerization reaction was carried out in a 20 L autoclave using 10 L of purified hexane as diluent. 15 mmol of triisobutyl aluminium was added to the hexane. 1 .25 mL of an antistatic agent was added to the hexane. The mixture was heated to 80.5 °C and pressurized with ethylene and hydrogen at a pressure of 8 barg with H2 / C2 ratio of 0.15. Subsequently a quantity of catalyst slurry prepared as described in Example II of patent W02009 / 112254 containing 40 mg of catalyst was dosed. The temperature was maintained at 80.5 °C and the pressure was kept constant by feeding ethylene. The reaction was stopped after a certain period of between 1 and 4 hours, depending on the desired IV to be achieved, which was determined by sample measurement. Stopping was performed by de-pressurizing and cooling down the reactor. The reactor contents were passed through a filter. The polymer powder was collected, dried and analyzed.

[0033] Of the above materials, product properties were determined, as reflected in the table below.Table 2: Product properties of LIHMWPE materialsWherein:• The intrinsic viscosity (IV) was determined in accordance with the method of ASTM D4020 - 11.• MFR21.6 is the melt mass-flow rate determined at 190°C under a load of 21 .6 kg, according to ISO 1133-1 (2022).• M is the molecular weight as calculated using the IV according to the formula M = 53700(IV)A1.37.• Mw / Mn and Mz / Mware molecular weight distribution characteristics as determined according to ASTM D6474 (2012).• The D50 and span of the particle distribution were determined according to ISO 13320 (2009).• The bulk density was determined according to ISO 60 (1977).• Tmis the melt temperature as determined according to ISO 11357 (2016).• The density was determined according to ISO 1183-1 (2019).

[0034] Due to the high molecular weight, the molecular weight characteristics Mw, Mnand Mzof the UH2 could not be determined in accordance with the method of ASTM D6474.

[0035] In addition, Fourier rheology profiles were prepared of the UH 1 and UH3. These profiles were determined according to the method described in Wilhelm, M., Maring, D. & Spiess, HW. Fourier-transform rheology. Rheol. Acta 37, 399-405 (1998). This resulted in the below values for the fundamental harmonic (h) and the third harmonic (I3), at given shear strain amplitude in the range of 2-15%.Table 3: Fourier rheology data

[0036] When fitting the above values according to the formula:It can be observed that the n value of UH1 is 1.50, whereas that of UH3 is 1.90. It is considered that the n value reflects the degree of branching in the polyethylene polymer.

[0037] In addition, tests were performed using UH1 and UH3 wherein the materials were immersed in paraffin oil at 120°C for a certain duration of up to 30 minutes. In table 4 below, the average particle sizes for samples of each polymer when subjected to immersion for a given duration are presented, reflecting the swelling of the particles by absorption of the paraffin oil.Table 4: Swelling data

[0038] Such swelling characteristics also indicate that the UH1 has a higher degree of branching than the UH3.

[0039] Using the UH1-UH3 materials, microporous films were produced according to the process applicable to the production of battery separator membranes. In each of the examples 1-5, the UH powder according to the recipe as in table 5 was mixed with paraffin oil at room temperature, and mixed in a stirred tank at 100°C for 10 min to obtain homogeneous gels.Table 5: Film recipes

[0040] Using a twin-screw extruder, operated at 240°C, the UH gel was extruded into cast film onto a chill roll of 20°C to obtain a UH gel film having a thickness of 500 pm.

[0041] Specimens were cut from the films having a size of 95 x 95 mm. These specimens were subjected to biaxial stretching at [temperature] to a degree of stretching of 8 x 8, sequentially in MD / TD.

[0042] Subsequently, again specimens were cut from the biaxially stretched UH gel films of 170 x 170 mm. These films were fixed into a frame, and subjected to solvent extraction at 40°C using n-hexane under ultrasonic irradiation for 3x 30 minutes to remove the paraffin oil.

[0043] The obtained porous film was clamped in the biaxial stretcher, and at 120°C again stretched in TD at a ratio of 1.5, related to 1.2, which was maintained for 3 minutes. After cooling to room temperature, the porous film was removed from the clamps and the separator film was obtained.

[0044] Of the separator films produced according to this method, a number of film parameters were determined, as set out in table 6 below.Table 6: Film propertiesWherein:• TS is tensile strength, determined in accordance with GB / T36363-2018, using a sample strip of 15.0 mm width with initial fixture distance 100 mm, tensile rate 250 mm / min;• Puncture strength is determined in accordance with GB / T36363-2018;• Average pore size is determined in accordance with GB / T36363-2018;• Pore Tortuosity is determined in accordance with GB / T36363-2018;• Porosity is determined using a 100x100 mm sample, of which weight, length, width and thickness are measured, based on which the apparent density pi is calculated, using which the porosity P is calculated according to the formula:• Gurley air resistance is determined in accordance with ISO 5636-5:2013;• Heat shrinkage is determined in accordance with GB / T36363-2018;• Ion conductivity is determined in accordance with GB / T36363-2018;• Area resistance is determined in accordance with GB / T36363-2018;• Breakdown voltage is determined in accordance with GB / T36363-2018;• Shut down temperature is determined by impregnating the separator with electrolyte, sandwiching it between two nickel foils that are connected to a resistance test device, heating it is an oven at a heating rate of 2°C / min, wherein temperature and resistance are continuously measured, and the shut down temperature is defined as the temperature at which the resistance exceeds 100 Q.

[0045] From these data, it can be observed that the porous films produced using the LIHMWPE according to the invention exhibit a desirably high porosity, good mechanical properties at low film thickness, increased permeability and conductivity, and increased shut down temperatures.

Claims

Claims1. Ultra-high molecular weight polyethylene (UHMWPE), wherein the UHMWPE is an ethylene-based polymer having a molecular weight (M) of > 500,000 g / mol, preferably of > 500,000 and < 7,000,000 g / mol, wherein M is determined by calculation based in intrinsic viscosity according to the equation M = 53700(IV)A1.37, wherein IV is expressed in dl / g and determined according to the method of ASTM D4020- 11 ; and wherein the UHWMPE has in the Fourier rheology profile in the strain amplitude range of 2-15% a value for n of < 1 .8, wherein n is calculated using the equation:wherein I3 / I1 is the intensity ratio of the third harmonic and the fundamental harmonic, and y is the strain amplitude, preferably wherein the values for k and n are obtained via curve fit over the strain amplitude range.

2. UHMWPE according to claim 1 , wherein the ethylene-based polymer is a homopolymer or a copolymer of ethylene and a further a-olefin selected from 1 -butene, 1 -hexene and 1- octene, preferably containing < 2.0 wt% of the further a-olefin.

3. UHMWPE according to any one of claims 1-2, wherein the ethylene-based polymer has an intrinsic viscosity of > 5.0 dl / g, preferably of > 5.0 and < 50.0 dl / g, as determined in accordance with ASTM D4020 - 11.

4. UHMWPE according to any one of claims 1-3, wherein the ethylene-based polymer has a melt mass-flow rate as determined at 21.6 kg at 190°C in accordance with ISO 1133-1 (2022), of < 0.42 g / 10 min, preferably of < 0.36 g / 10 min, more preferably of > 0.0 and < 0.36 g / 10 min.

5. UHMWPE according to any one of claims 1-4, wherein the ethylene-based polymer has a molecular weight distribution Mw / Mnof < 7.0, preferably of > 4.0 and < 7.0, more preferably of > 5.0 and < 7.0, wherein Mwand Mnare determined in accordance with ASTM D6474 (2012).

6. LIHMWPE according to any one of claims 1-5, wherein the ethylene-based polymer has a density of > 935 and < 965 kg / m3, preferably of > 935 and < 955 kg / m3, more preferably of > 940 and < 950 kg / m3, as determined in accordance with ASTM D792 (2013).

7. Polymer composition comprising a first LIHMWPE according to any one of claims 1-6.

8. Polymer composition according to claim 7, wherein the composition further comprises a second LIHMWPE according to any one of claims 1-6, wherein the second LIHWMPE is different from the first LIHMWPE, preferably wherein the second LIHMWPE has a higher molecular weight M than the first LIHMWPE.

9. Polymer composition according to claim 8, wherein the weight ratio of the second UHMWPE to the first UHMWPE is > 0.5, preferably > 0.5 and < 20.

10. Process for preparation of the polymer composition according to any one of claims 7-9, involving extrusion based compounding and / or dry blending.11 . Process for production of a porous film, wherein the process involves the steps in this order of:(a) providing a quantity of an UHMWPE according to any one of claims 1-6 or a polymer composition according to any one of claim 7-9 to a mixing vessel;(b) adding a quantity of a solvent, preferably a hydrocarbon solvent such as liquid paraffin or paraffin wax; an ester such as dioctyl phthalate or dibutyl phthalate; or a higher alcohol such as oleyl alcohol or stearyl alcohol, to the mixing vessel;(c) subjecting the contents of the mixing vessel to a temperature of between 80 and 120 °C to form gel particles;(d) supplying the gel particles to a melt extruder and extruding the molten gel to form a cast film;(e) stretching the cast film at temperatures of between 80 and 150°C to obtain an oriented film;(f) extracting the solvent from the oriented film; and(g) subjecting the oriented film to a heat treatment to obtain a porous film.

12. Process according to claim 11 , wherein the stretching in step (e) is performed in both directions, preferably to a degree of orientation of at least 5 in each direction.

13. Process according to any one of claims 11-12, wherein the extraction step (f) is performed using a solvent, which may be selected from hydrocarbons such as n-hexane, cyclohexane or halogenated hydrocarbons; methylene chloride, 1 ,1,1 -trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroether or hydrofluorocarbon; alcohols such as ethanol or isopropanol; ethers such as diethyl ether or tetrahydrofuran; and ketones such as acetone or methyl ethyl ketone, preferably n-hexane, and / or is performed by applying ultrasonic irradiation.

14. Porous film comprising the LIHMWPE according to any one of claims 1-6 or the polymer composition according to any one of claims 7-9, preferably wherein the porous film has:• an ion conductivity of > 1.50 Q'1, as determined in accordance with GB / T36363- 2018; and / or• a Gurley air resistance of < 200 s, preferably < 150 s, as determined in accordance with ISO 5636-5:2013; and / or• a porosity of > 40.0%, as determined in accordance with GB / T36363-2018; and / or• a shut-down protection temperature of > 140°C, as determined in accordance with GB / T36363-2018.

15. Battery or capacitator comprising a porous film according to claim 14.

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