Biaxially oriented film made of double-layer oil-filled sheet

Abstract

The present disclosure relates to a method for forming free-standing biaxially oriented microporous polyolefin films. In the method, at least two separate oil-filled cast or rolled films are stacked on top of each other and then subjected to biaxial orientation followed by solvent extraction of processing oil (i.e., plasticizer), evaporation of the solvent, and thermal stabilization, and then separated into separate wound-into-roll microporous films.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 376,211, filed on September 19, 2022, and entitled BIAXIALLY ORIENTED MEMBRANES FROM DOUBLE LAYER, OIL FILLED SHEETS, which is incorporated herein by reference in its entirety.

[0003] Copyright Notice

[0004] ©2023 Amtek Research International LLC. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). Technical Field

[0005] The present invention relates to a new method for forming a free-standing biaxially oriented microporous polyolefin film. In this method, two separate oil-filled cast or calendered films are stacked on top of each other and then subjected to biaxial orientation, followed by solvent extraction of a processing oil (i.e., a plasticizer), evaporation of the solvent, and heat stabilization, and then separated into separate microporous films wound into rolls. The uniqueness of this method is that these layers are not bonded together, thereby effectively doubling the output through the biaxial orientation and extraction steps. Such microporous films can be used to improve the manufacturability, performance and safety of energy storage devices such as lithium-ion batteries. Background of the Invention

[0007] The separator is an integral part of the performance, safety and cost of lithium-ion batteries. During normal operation, the main function of the separator is to prevent electronic conduction between the anode and the cathode (i.e., short circuit or direct contact) while allowing ionic conduction via the electrolyte. Under abuse conditions (such as external short circuit or overcharge), the separator is required to shut down at a temperature far below that at which thermal runaway may occur. The shutdown is caused by the collapse of pores in the separator due to the melting and viscous flow of the polymer, thereby slowing down or preventing the flow of ions between the electrodes. Almost all Li-ion battery separators contain polyethylene as part of a single or multilayer construction, so that the shutdown begins at about 130°C (i.e., close to the melting point of polyethylene).

[0008] Separators for the lithium-ion market are currently manufactured via either a "dry" or "wet" process. In the dry process, polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawing. The sheet is then annealed at 10°C-25°C below the melting point of the polymer so that the crystallite size and orientation are controlled. Next, the sheet is rapidly stretched in the machine direction (MD) to achieve slit-like pores or voids. Three-layer PP / PE / PP separators produced by the dry process are commonly used in lithium-ion rechargeable batteries.

[0009] Wet-process membranes composed of high molecular weight polyethylene are produced by extruding an oil / polymer mixture at elevated temperatures, followed by phase separation, biaxial stretching, and extraction of the processing oil (i.e., plasticizer). The resulting membranes have elliptical or spherical pores with good mechanical properties in both the machine and transverse directions. PE-based membranes manufactured in this manner have been widely used in Li-ion batteries.

[0010] Recently, battery failures in the field have proven that shutdown is not a guarantee of safety. The main reason is that after shutdown, residual stresses and reduced mechanical properties above the melting point of the polymer may cause shrinkage, tearing or pinhole formation. The exposed electrodes may then contact and create an internal short circuit, leading to more heat, thermal runaway and explosion. Therefore, many companies focus on applying ceramic coatings or high-temperature polymer coatings on polyolefin separators to impart good high-temperature dimensional stability. The coating process is usually performed in a secondary operation and is critical to the performance and safety of Li-ion battery cells used in electric vehicle applications.

[0011] Therefore, in addition to applying a minimal amount of ceramic or high temperature polymer, there is a need to further reduce the cost of the base polyolefin separator while still achieving the desired performance. SUMMARY OF THE INVENTION

[0013] The benefit of the present invention is a cost-reduced structure for a base polyolefin separator by stacking the precursors (oil-filled cast films) on top of each other while passing them through expensive equipment for biaxial orientation, followed by final extraction, drying and heat stabilization to form the base polyolefin separator. The method of the present invention can effectively double the output of a separator production line and has not been practiced using cast film technology to date. As used herein, the term "free-standing" refers to a web or film having sufficient mechanical properties for unwinding, coating, winding, cutting and other web handling operations. The terms "film", "sheet", "substrate", "web" and "membrane" can be used interchangeably, and the term membrane can be used to cover webs, films, substrates and sheets.

[0014] The present invention relates to a microporous free-standing membrane that relies on a special type of polyethylene, namely ultra-high molecular weight polyethylene (UHMWPE). UHMWPE generally exhibits a molecular weight (Mw) of greater than about 3.1 million to about 10 million g / mol. The repeating unit of polyethylene is (-CH2CH2-) x , where x represents the average number of repeating units in a single polymer chain. In the case of polyethylene used in many film and molded part applications, x equals about 10,000, while for UHMWPE, x is approximately 150,000. This extreme difference in the number of repeating units is responsible for the higher degree of chain entanglement and the unique properties associated with UHMWPE.

[0015] One such property is the ability of UHMWPE to resist material flow under its own weight when heated above its melting point. This phenomenon is a result of its ultra-high molecular weight and associated long relaxation time (even at elevated temperatures). Therefore, although UHMWPE is generally usable, it is difficult to process into fiber, sheet or film form. The high melt viscosity requires both compatible plasticizers and twin-screw extruders for disentanglement of polymer chains so that the resulting gel can be processed into a usable form. This method is generally referred to as 'gel processing'. In many cases, other polyolefins are blended with UHMWPE to reduce the molecular weight distribution so that the properties after the plasticizer is extracted to obtain a porous film or sheet can be affected. Exemplary polymers that can be blended with UHMWPE include very high molecular weight polyethylene (VHMWPE), which has a Mw greater than about 300,000 g / mol (e.g., about 300,000 to about 3.1 million g / mol), high density polyethylene (HDPE), and linear low density polyethylene (LLDPE).

[0016] In one embodiment of the present invention, the microporous freestanding polyolefin membrane is manufactured by combining a mixture of UHMWPE and one or more of VHMWPE, HDPE, or LLDPE with a processing oil or a plasticizer (e.g., mineral oil). For example, the microporous freestanding polyolefin membrane can be manufactured by combining a mixture of UHMWPE, HDPE, and a processing oil or a plasticizer (e.g., mineral oil). The mixture can be blended and extruded with a sufficient amount of processing oil or a plasticizer to form a uniform viscous material. For example, the mixture can contain 30%-55% by weight of the one or more polyolefins. The material is processed using a blown film, cast film, or calendering method to obtain an oil-filled sheet of reasonable thickness (<250 μm). The oil-filled sheet can be further biaxially oriented to reduce its thickness and achieve its mechanical properties. In an extraction operation, the oil is removed with a solvent, and the solvent is subsequently evaporated to produce a microporous freestanding membrane.

[0017] Schematic diagram of the cast or calendered film processFigure 1 In this process, the extrudate is passed through a sheet die and then cooled as it passes through a series of calendering rolls to achieve recrystallization of the polymer phase. The oil-filled sheet is then passed through an apparatus to impart biaxial orientation. In some embodiments, biaxial orientation may be performed sequentially (e.g., Figure 1 ), i.e., first passing the oil-filled sheet through a machine direction orientation (MDO) device and then through a transverse direction orientation (TDO) device (or vice versa). In other embodiments, biaxial orientation can alternatively be performed by passing the sheet through a customized device that accelerates (and / or stretches) the sheet in the machine direction while it stretches the sheet in the transverse direction. The biaxially oriented oil-filled sheet is then passed through an extraction and drying process (which may include various solvent recovery means, such as a carbon bed) to form a microporous membrane, which is then heat stabilized to eliminate residual stresses and finally wound into a roll. In practice, the oil-filled sheet is typically 1 meter wide when it leaves the calender rolls and is converted into a much thinner biaxially oriented microporous membrane, which can be about 5 meters wide at the winder. In order to increase production, membrane manufacturers using cast film processes focus on faster and wider, but as the required equipment becomes even wider, capital costs increase exponentially.

[0018] The advantage of the cast film process over other film processes is better thickness control. In addition, the cast film process allows the use of low molecular weight polyolefin polymers in combination with process oils.

[0019] The new method Figure 2 , and effectively combine the two oil-filled cast sheets into a stacked arrangement before passing them through biaxial stretching (e.g., simultaneous or sequential as discussed above) and other downstream equipment. Figure 2 As shown in FIG. 1 , two cast film layers are extruded and fed through an apparatus to impart biaxial orientation, which can be simultaneous (as depicted) or sequential (as Figure 1 The biaxially oriented oil-filled sheet is then passed through an extraction and drying process (similar to Figure 1 ) to form two microporous membranes that can be heat stabilized and ultimately wound into separate rolls (depicted as Membrane 1 (top layer) and Membrane 2 (bottom layer)). Advantageously, the two layers are not bonded together before or during biaxial orientation, even if contact between the layers occurs.

[0020] The resulting microporous free-standing polyolefin membrane generally includes a porosity of about 35%-65%. The pore size range is generally about 10 nanometers to several microns, wherein the average pore size is less than about 1 micron. The thickness of the membrane (excluding any coating) is generally about 3-25 μm, or about 20 μm or less. The resulting membrane can also be wound or stacked in a package to separate the electrodes in an energy storage device (such as a battery, capacitor, supercapacitor or fuel cell unit). Such membranes are useful for the manufacture of energy storage devices, particularly when coated with a ceramic layer to impart good high temperature dimensional stability while maintaining shutdown characteristics.

[0021] Additional objects and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram of the cast film diaphragm manufacturing method.

[0023] Figure 2 is a schematic diagram of a novel membrane manufacturing method according to an embodiment of the present disclosure, which depicts stacking of two oil-filled sheets prior to biaxial orientation. DETAILED DESCRIPTION

[0024] The film used in the present invention comprises a polyolefin matrix or bulk structure. The most preferred polyolefin used is an ultra-high molecular weight polyethylene (UHMWPE) having an intrinsic viscosity of at least 10 dl / g and preferably in the range of 18-22 dl / g. In some cases, it is desirable that UHMWPE is blended with one or more other polyolefins such as VHMWPE, HDPE or linear low density polyethylene (LLDPE) to affect the shutdown characteristics of the film.

[0025] The processing oil or plasticizer used in the present invention is a non-evaporating solvent for the polymer and is preferably liquid at room temperature. The processing oil or plasticizer has little or no solvating effect on the polymer at room temperature; it exerts its solvating effect at or above the softening temperature of the polymer. For UHMWPE, the solvating temperature will be above about 160°C, and preferably in the range of about 160°C to about 220°C. It is preferred to use a processing oil such as a paraffinic oil, a naphthenic oil, an aromatic oil, or a mixture of two or more such oils. Examples of suitable processing oils include: oils sold by Shell Oil Company, such as Gravex TM 942; and oils sold by Calumet Lubricants, such as Hydrocal TM 800; and oils sold by Nynas Inc., such as HR 750.

[0026] The polymer / processing oil mixture is extruded through multiple sheet dies, cast onto calender rolls, and then assembled in a stacked arrangement and subjected to biaxial orientation (eg, Figure 2 ). Biaxial orientation can be carried out between 25°C and the melting point of the polymer in the oil-filled sheet. For example, biaxial orientation can occur at a temperature from about 60°C and about 100°C. In some cases, the oil-filled sheet is biaxially oriented 4 to 12 times in the machine direction and 4 to 12 times in the transverse direction. As discussed above, biaxial orientation can be sequential or simultaneous. The stacked sheets are also subjected to biaxial orientation without adhering to each other. Then, any solvent compatible with the oil can be used for the extraction step, provided that the boiling point of the solvent makes it feasible to separate the solvent from the plasticizer by distillation. Such solvents include 1,1,2 trichloroethylene, perchloroethylene, 1,2-dichloroethane, 1,1,1-trichloroethane, 1,1,2-trichloroethane, methylene chloride, chloroform, 1,1,2-trichloro-1,2,2-trifluoroethane, isopropyl alcohol, diethyl ether, acetone, decane, dodecane, hexane, heptane, toluene, mineral spirits, and mixtures thereof. In most cases, it is desirable to remove all oil prior to the solvent drying step to prevent potential adhesion of the two sheets during the plasticizing and heat stabilization steps.

[0027] Example 1

[0028] The following polymers were mixed with process oil to form about 45 wt.% slurry, which was fed into a twin-screw extruder. The mixture was processed at about 225°C and extruded through a sheet die and calendered to form a 150-μm thick oil-extended sheet.

[0029] 250g UHMWPE (GUR 4120; Celanese)

[0030] 125g VHMWPE (GUR 4012 (Celanese)

[0031] 125g HDPE (GHR 8020 (Celanese)

[0032] 600 g oil (Hydrocal 800; Calumet)

[0033] The oil-filled sheet is wound onto a plastic core. In a separate operation, two layers of the oil-filled sheet are stacked on top of each other and biaxially oriented 4 times in the machine direction and 5 times in the transverse direction. The two layers of biaxially oriented film are cut and sandwiched between metal frames clamped together and having an opening area of ​​about 100 mm by about 100 mm. The assembly is then washed with trichloroethylene to remove the processing oil and then dried in an oven at 85°C to produce microporous membranes that can be separated from each other when removed from the metal frames.

[0034] It should be understood that reference to "one (an) embodiment" or "the (the) embodiment" throughout this specification means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Therefore, the reference phrases, or variations thereof, as described throughout this specification are not necessarily all referring to the same embodiment.

[0035] Similarly, it should be understood that in the above description of the embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of simplifying the disclosure. However, this method of disclosure should not be interpreted as reflecting the following intention: any claim requires more features than those explicitly listed in the claim. On the contrary, as reflected in the following claims, the various aspects of the present invention lie in fewer combinations than all the features of any single previously disclosed embodiment. Therefore, the claims following this specific embodiment are hereby expressly incorporated into this specific embodiment, with each claim existing on its own as a separate embodiment. This disclosure includes all arrangements of independent claims and their dependent claims. In addition, additional embodiments that can be derived from the following independent and dependent claims are also expressly incorporated into this written specification.

[0036] The recitation of the term "first" with respect to a feature or element in the claims does not necessarily imply the presence of second or additional such features or elements.

[0037] Reference is made throughout this specification to approximate values, such as by using the term "about". For each such reference, it is understood that in some embodiments, a value, feature, or characteristic may be specified without the approximate value. For example, where a qualifier such as "about" is used, the term includes the qualified term within its range without its qualifier. For example, where the term "about" is used with respect to a feature, it is understood that in other embodiments, the feature may have a precise configuration. Unless otherwise indicated, all ranges include endpoints and all numbers between the endpoints.

[0038] Without further elaboration, it is believed that those skilled in the art can use the previous description to maximize the use of the present invention. The claims and embodiments disclosed herein are interpreted as being merely illustrative and exemplary and do not limit the scope of the present disclosure in any way. It will be apparent to those of ordinary skill in the art that, with the aid of this disclosure, the details of the above-described embodiments may be changed without departing from the basic principles disclosed herein. In other words, various modifications and improvements of the embodiments specifically disclosed in the above description are within the scope of the appended claims. In addition, without departing from the scope of the present disclosure, those skilled in the art may change the order of the steps or actions of the methods disclosed herein. In other words, unless a specific order of steps or actions is required for the correct operation of the embodiment, the order or use of specific steps or actions may be modified. Therefore, the scope of the present invention is defined by the following claims and their equivalents.

Claims

1. A method for producing a free-standing biaxially oriented microporous polyolefin membrane, the method comprising the following steps: (a) preparing a composition comprising one or more polyolefins and a process oil, wherein the lowest molecular weight polyolefin component is greater than 300,000 g / mol, (b) passing the composition through a twin-screw extruder and a sheet die to form a cast oil-filled sheet, (c) stacking at least two layers of the oil-filled sheets on top of each other so that they can undergo biaxial orientation together without adhering to each other, (d) removing the processing oil from the at least two oil-filled sheets with a solvent, (e) drying the solvent to form at least two microporous polyolefin membranes, and (f) thermally stabilizing the at least two microporous polyolefin membranes to eliminate residual stress, and then separating the two microporous polyolefin membranes and winding them into a roll form.

2. The method of claim 1, wherein: The composition comprises 30-55 weight percent of the one or more polyolefins.

3. The method according to claim 1 or claim 2, wherein: The composition comprises ultra high molecular weight polyethylene (UHMWPE).

4. The method of claim 3, wherein: The composition comprises a blend of ultra high molecular weight polyethylene (UHMWPE) and at least one of very high molecular weight polyethylene (VHMWPE), high density polyethylene (HDPE) or linear low density polyethylene (LLDPE).

5. The method according to any one of claims 1 to 4, wherein: Each of the at least two layers of the oil-filled sheet is subjected to biaxial orientation of 4 to 12 times in the machine direction and 4 to 12 times in the transverse direction.

6. The method according to any one of claims 1 to 5, wherein: The at least two layers of the oil-extended sheets are in contact with each other but not bonded together before or during biaxial orientation.

7. The method according to any one of claims 1 to 6, wherein: The biaxial orientation takes place at temperatures between 60°C and 100°C.

8. The method according to any one of claims 1 to 7, wherein: This biaxial orientation occurs simultaneously in the machine and transverse directions.

9. The method according to any one of claims 1 to 7, wherein: The biaxial orientation occurs sequentially in the machine direction and then in the transverse direction.

10. The method according to any one of claims 1 to 9, wherein: The at least two microporous polyolefin membranes each comprise a thickness of 3 to 25 microns.

11. The method according to any one of claims 1 to 10, wherein: The at least two microporous polyolefin membranes each include a porosity of about 35% to 65%.

12. The method according to any one of claims 1 to 11, wherein: The at least two microporous polyolefin membranes each include micropores ranging from about 10 nanometers to several micrometers, wherein the average pore size is less than about 1 micrometer.

13. A freestanding biaxially oriented microporous polyolefin membrane formed according to the method of any one of claims 1 to 12.

14. The freestanding biaxially oriented microporous polyolefin membrane of claim 13 for use as a separator in lithium ion batteries or rechargeable Li metal batteries.

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