Biaxially oriented films from double-layer oil-filled sheets

Abstract

The present disclosure relates to a process for forming free-standing biaxially oriented microporous polyolefin films, in which at least two separate oil-filled cast or calendared films are laminated on top of each other, then biaxially oriented, followed by solvent extraction of the process oil (i.e., plasticizer), solvent evaporation, and heat stabilization before separation into individual microporous membranes that are wound into rolls.

Description

[Technical Field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 376,211, filed September 19, 2022, and entitled "BIAXIALLY ORIENTED MEMBRANES FROM DOUBLE LAYER, OIL FILLED SHEETS," the entire contents of which are incorporated herein by reference.

[0002] Copyright Notice (Copyright) 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 of this patent document or this patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1.71(d).

[0003] The present invention relates to a novel process for forming freestanding biaxially oriented microporous polyolefin films. In this approach, two separate oil-filled cast or calendared films are laminated on top of each other, then biaxially oriented, followed by solvent extraction of the process oil (i.e., plasticizer), solvent evaporation, and heat stabilization before being separated into individual microporous membranes that are wound into rolls. This process is unique in that the layers do not adhere to each other, effectively doubling production through the biaxial orientation and extraction steps. Such microporous membranes can be used to improve the manufacturability, performance, and safety of energy storage devices such as lithium-ion batteries. [Background technology]

[0004] The separator is an essential component of lithium-ion batteries in terms of performance, safety, and cost. During normal operation, the separator's primary function is to prevent electronic conduction (i.e., short circuit or direct contact) between the anode and cathode while allowing ionic conduction through the electrolyte. Under adverse conditions, such as an external short circuit or overcharge, the separator must shut down at a temperature well below that at which thermal runaway could occur. Shutdown occurs when polymer melting and viscous flow collapse the pores in the separator, slowing or halting ionic flow between the electrodes. Because nearly all Li-ion battery separators contain polyethylene as part of a single-layer or multilayer structure, shutdown begins at approximately 130 °C, close to the melting point of polyethylene.

[0005] Separators for the lithium-ion market are currently manufactured using 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 drawdown. The sheet is then annealed at a temperature 10-25°C below the polymer's melting point to control crystallite size and orientation. The sheet is then rapidly stretched in the machine direction (MD) to create slit-like holes or voids. Tri-layer PP / PE / PP separators manufactured using the dry process are commonly used in lithium-ion rechargeable batteries.

[0006] Wet-process separators composed of high-molecular-weight polyethylene are produced by extruding an oil / polymer mixture at high temperatures, followed by phase separation, biaxial orientation, and extraction of the process oil (i.e., plasticizer). Separators produced in this manner have ellipsoidal or spherical pores and good mechanical properties in both the machine and cross directions. PE-based separators produced in this manner have found widespread application in lithium-ion batteries. Summary of the Invention [Problem to be solved by the invention]

[0007] Recent field battery failures have demonstrated that shutdown is no guarantee of safety. The primary reason is that after shutdown, residual stresses and reduced mechanical properties above the polymer's melting point can lead to shrinkage, tearing, or pinhole formation. The exposed electrodes can then contact and cause an internal short circuit, potentially leading to further heating, thermal runaway, and explosion. Therefore, many companies are focusing on applying ceramic or high-temperature polymer coatings to polyolefin separators to impart good high-temperature dimensional stability. The coating process, typically performed in a secondary operation, is crucial to the performance and safety of Li-ion batteries used in electric vehicle applications.

[0008] Therefore, there is a need to further reduce the cost of the base polyolefin separator, as well as minimize the amount of ceramic or high temperature polymer used while achieving the required performance. [Means for solving the problem]

[0009] An advantage of the present invention is that the cost structure of the base polyolefin separator can be reduced by laminating precursor oil-filled cast films on top of each other, which are then biaxially oriented through expensive equipment before final extraction, drying, and heat stabilization to form the base polyolefin separator. The process of the present invention can effectively double the production capacity of a separator manufacturing line, which has not previously been achieved using cast film technology. As used herein, the term "freestanding" refers to a web or membrane that has sufficient mechanical properties for use in unwinding, coating, rewinding, slitting, 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 encompass webs, films, substrates, and sheets.

[0010] The present invention relates to microporous free-standing membranes that rely on a special type of polyethylene, namely, ultrahigh molecular weight polyethylene (UHMWPE). UHMWPE typically exhibits a molecular weight (Mw) of about 3.1 to over about 10 million grams per mole. The repeating unit of polyethylene is (-CH2CH2-). x where x represents the average number of repeat units in an individual polymer chain. For polyethylene, which is used in many film and molded part applications, x is equal to approximately 10,000, while for UHMWPE, x is approximately 150,000. This extreme difference in repeat unit number accounts for the higher degree of chain entanglement and characteristic properties associated with UHMWPE.

[0011] One such property is UHMWPE's ability to resist flow under its own weight when heated above its melting point. This phenomenon is a result of UHMWPE's ultra-high molecular weight and the associated long relaxation time at high temperatures. Therefore, although UHMWPE is widely available, it is difficult to process into fibers, sheets, or membranes. Because of its high melt viscosity, the resulting gel requires both a compatible plasticizer and a twin-screw extruder to deentangle the polymer chains in order to process it into a useful form. This approach is commonly referred to as "gel processing." Other polyolefins are often blended with UHMWPE to reduce the molecular weight distribution and impart properties after the porous film or sheet is obtained by plasticizer extraction. Exemplary polymers that can be blended with UHMWPE include very high molecular weight polyethylene (VHMWPE) having a Mw greater than about 300,000 g / mol (e.g., from about 300,000 to about 3.1 million g / mol), high density polyethylene (HDPE), and linear low density polyethylene (LLDPE).

[0012] In one embodiment of the present invention, a microporous, free-standing polyolefin membrane is produced by combining a mixture of UHMWPE and one or more of VHMWPE, HDPE, or LLDPE with a process oil or plasticizer (e.g., mineral oil). For example, a microporous, free-standing polyolefin membrane can be produced by combining a mixture of UHMWPE, HDPE, and a process oil or plasticizer (e.g., mineral oil). The mixture can be blended with a sufficient amount of process oil or plasticizer and extruded to form a homogeneous, coherent material. For example, the mixture can contain 30-55 wt. % of one or more polyolefins. This material can be processed using blown film, cast film, or calendaring techniques to produce an oil-filled sheet of moderate thickness (<250 μm). This oil-filled sheet can be further biaxially oriented to reduce thickness and affect mechanical properties. In an extraction operation, the oil is removed with a solvent, followed by evaporation, to produce a microporous, free-standing membrane.

[0013] A schematic diagram of the cast or calendered film process is shown in Figure 1. In this process, the extrudate is forced through a sheet die and cooled while passing through a series of calender rolls, allowing for recrystallization of the polymer phase. The oil-filled sheet then passes through a device that imparts biaxial orientation. In some embodiments, biaxial orientation can be achieved sequentially by passing the oil-filled sheet first through a machine direction orientation (MDO) device and then a transverse direction orientation (TDO) device (as illustrated in Figure 1). Alternatively, in other embodiments, biaxial orientation can be achieved by passing the sheet through a customized device that accelerates (and / or stretches) the sheet in the machine direction while simultaneously stretching it in the transverse direction. The biaxially oriented oil-filled sheet then passes through an extraction and drying process (which may include various solvent recovery means, such as a carbon bed), forms a microporous membrane, is heat stabilized to relieve residual stress, and is finally wound into a roll. In practice, the oil-filled sheet is often one meter wide when it exits the calender rolls and is converted into a thinner biaxially oriented microporous membrane, which can reach a width of approximately five meters on a winder. To increase production volumes, separator manufacturers using cast film processes are focusing on going faster and wider, but the capital costs increase exponentially as more extensive equipment is required.

[0014] The advantage of the cast film process is that it allows for better thickness control compared to other film processes. Additionally, the cast film process allows for the use of low molecular weight polyolefin polymers in combination with process oils.

[0015] The novel process is illustrated in Figure 2. In this novel process, two oil-filled cast sheets are effectively combined in a laminated arrangement before passing through biaxial orientation (e.g., simultaneously or sequentially, as described above) and other downstream equipment. For example, as shown in Figure 2, two cast film layers are extruded and fed through equipment that imparts biaxial orientation, either simultaneously (as depicted) or sequentially (as shown in Figure 1). The biaxially oriented oil-filled sheets then undergo an extraction and drying process (similar to Figure 1) to form two microporous membranes, which can be heat stabilized and ultimately wound onto separate rolls (depicted as Separator 1 (top layer) and Separator 2 (bottom layer)). Advantageously, the two layers do not adhere to each other prior to or during biaxial orientation, even if contact between the layers occurs.

[0016] The resulting microporous, freestanding polyolefin membranes typically have a porosity of about 35-65%. Pore sizes typically range from about 10 nanometers to several microns, with an average pore size of less than about 1 micrometer. The membrane thickness (excluding any coatings) typically ranges from about 3-25 μm, or about 20 μm or less. The resulting membranes can also be rolled into packages or laminated to separate electrodes in energy storage devices such as batteries, capacitors, supercapacitors, or fuel cells. Such membranes are useful in the manufacture of energy storage devices, especially when coated with a ceramic layer to impart good high-temperature dimensional stability while maintaining shutdown characteristics.

[0017] Further objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of the cast film separator manufacturing process. [Figure 2]FIG. 2 is a schematic diagram of the novel separator manufacturing process showing lamination of two oil-filled sheets prior to biaxial orientation in accordance with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0019] The membranes used in the present invention are comprised of a polyolefin matrix or bulk structure. The most preferred polyolefin is ultra-high molecular weight polyethylene (UHMWPE) having an intrinsic viscosity of at least 10 deciliters per gram, preferably in the range of 18 to 22 deciliters per gram. In some cases, it may be desirable to blend UHMWPE with one or more other polyolefins, such as VHMWPE, HDPE, or linear low density polyethylene (LLDPE), to affect the shutdown properties of the membrane.

[0020] The process oil or plasticizer utilized in the present invention is a non-evaporative solvent for the polymer and is preferably liquid at room temperature. The process oil or plasticizer has little solvation effect on the polymer at room temperature and becomes solvating at temperatures above the softening temperature of the polymer. For UHMWPE, the solvation temperature is greater than about 160°C, preferably in the range of about 160°C to about 220°C. It is preferred to use a process oil such as a paraffinic oil, naphthenic oil, aromatic oil, or a mixture of two or more such oils. Examples of suitable process oils include oils sold by Shell Oil Company, such as Gravex™ 942; oils sold by Calumet Lubricants, such as Hydrocal™ 800; and oils sold by Nynas Inc., such as HR Tufflo™ 750.

[0021] The polymer / process oil mixture is extruded through a multi-sheet die, cast onto a calender roll, and then combined into a laminated arrangement, which is subjected to biaxial orientation, followed by solvent extraction and drying (illustrated in Figure 2). Biaxial orientation can be performed at temperatures between 25°C and the melting point of the polymer in the oil-filled sheet. For example, biaxial orientation can be performed at temperatures between about 60°C and about 100°C. For some embodiments, 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 noted above, biaxial orientation can be performed sequentially or simultaneously. Additionally, the laminated sheets are biaxially oriented without contact with each other. Any solvent compatible with the oil can be used in the extraction step, provided that the solvent has a boiling point that allows the plasticizer to be separated from the solvent 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 before the solvent drying step to prevent plasticization and potential adhesion of the two sheets during the heat stabilization step. [Example]

[0022] Example 1 The following polymers were mixed with process oil to form a 45% by weight slurry, which was fed into a twin-screw extruder, processed at 225°C, extruded through a sheet die, and calendered to form a 150 μm thick oil-filled sheet: 250g UHMWPE(GUR 4120; Celanese) 125g VHMWPE (GUR4012 (Celanese) 125g HDPE (GHR 8020 (Celanese) 600g oil (Hydrocal 800; Calumet)

[0023] The oil-filled sheet was wound onto a plastic core. In another operation, two layers of oil-filled sheet were stacked on top of each other and biaxially oriented 4x in the machine direction and 5x in the transverse direction. The two layers of biaxially oriented film were cut and sandwiched between metal frames with an open area of ​​approximately 100 mm x 100 mm that were clamped together. The assembly was washed with trichloroethylene to remove the process oil and then dried in an oven at 85°C to produce a microporous membrane. The membranes could be separated from each other after removal from the metal frames.

[0024] It will be understood that references throughout this specification to an "embodiment" mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, references to a reference phrase or variations thereof incorporated throughout this specification do not necessarily all refer to the same embodiment.

[0025] Similarly, in the description of the above embodiments, it should be understood that various features may be grouped together in a single embodiment, figure, or description for the purpose of streamlining the disclosure. However, this method of disclosure should not be interpreted as reflecting an intention that any claim require more features than are expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in combinations of fewer than all features of a single foregoing disclosed embodiment. Accordingly, the claims following this detailed description are expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment. The present disclosure includes all permutations of independent claims and their dependent claims. Furthermore, additional embodiments derivable from the following independent and dependent claims are also expressly incorporated herein.

[0026] In the claims, the term "first" used with respect to a feature or element does not necessarily imply the presence of a second or additional such feature or element.

[0027] References to approximations are made throughout this specification, such as by use of the term "about." For each such reference, it is understood that in some embodiments, the value, feature, or characteristic may be specified without approximation. For example, when a modifier such as "about" is used, the term includes the modifier within its range in the absence of a modifier. For example, when the term "about" is used in connection with a feature, it is understood that in further embodiments, the feature may have the exact configuration. Unless otherwise specified, all ranges include the endpoints and all numerical values ​​between those endpoints.

[0028] Without further elaboration, it is believed that those skilled in the art can utilize the present invention to its fullest extent using the preceding description. The claims and embodiments disclosed herein should be construed as merely descriptive and exemplary, and in no way limit the scope of the present disclosure. It will be apparent to those skilled in the art that, with the aid of this disclosure, changes may be made to the details of the above-described embodiments without departing from the basic principles of the disclosure herein. In other words, various modifications and improvements to the embodiments specifically disclosed in the above description are within the scope of the appended claims. Furthermore, the order of steps or operations of methods disclosed herein may be changed by those skilled in the art without departing from the scope of the present disclosure. In other words, unless a specific order of steps or operations is required for the proper operation of an embodiment, the order or use of specific steps or operations may be changed. Accordingly, the scope of the present invention is defined by the following claims and their equivalents.

Claims

1. (a) A step of preparing a composition comprising one or more polyolefins having a minimum molecular weight polyolefin component greater than 300,000 g / mol, and a process oil, (b) The step of passing the composition through a twin-screw extruder and a sheet die to form a cast oil-filled sheet, (c) Laminating at least two layers of the oil-filled sheet on top of each other so that they can be biaxially oriented together without being bonded to each other, (d) The step of removing the process oil from the at least two oil-filled sheets using a solvent, (e) The step of drying the solvent to form at least two microporous polyolefin films, (f) Before separating the two microporous polyolefin films and winding them into a roll, the step of thermally stabilizing the at least two microporous polyolefin films in order to relieve residual stress. A method for producing a self-supporting, biaxially oriented, microporous polyolefin film, including the above.

2. The method according to claim 1, wherein the composition comprises 30 to 55% by weight of one or more types of polyolefins.

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

4. The method according to 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 a 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 4, wherein the at least two layers of the oil-filled sheet are in contact with each other but are not bonded before or during biaxial orientation.

7. The method according to any one of claims 1 to 4, wherein the biaxial orientation occurs at a temperature of 60°C to 100°C.

8. The method according to any one of claims 1 to 4, wherein the biaxial orientation occurs simultaneously in the machine direction and the transverse direction.

9. The method according to any one of claims 1 to 4, 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 4, wherein each of the at least two microporous polyolefin films has a thickness of 3 to 25 microns.

11. The method according to any one of claims 1 to 4, wherein each of the at least two microporous polyolefin films has a porosity of about 35 to 65%.

12. The method according to any one of claims 1 to 4, wherein each of the at least two microporous polyolefin films contains micropores ranging from about 10 nanometers to several micrometers in size, with an average pore diameter of less than about 1 micrometer.

13. A self-supporting, biaxially oriented, microporous polyolefin film formed according to the method described in any one of claims 1 to 4.

14. A self-supporting, biaxially oriented, microporous polyolefin film according to claim 13, for use as a separator in a lithium-ion battery or a rechargeable Li-metal battery.

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