Microporous polyolefin membranes from customized solvents

Abstract

To provide a halogen-free microporous polyolefin membrane which can be produced by an environmentally friendly closed-loop process, and a method for producing the same.SOLUTION: The halogen-free microporous polyolefin membrane can be manufactured using an environmentally friendly manufacturing process that includes extrusion of a polymer-plasticizer mixture, sheet formation, and extraction of the plasticizer with a halogen-free solvent. The halogen-free solvent has a flash point greater than about 23 °C. and an initial boiling point at least about 50 °C. lower than the flash point of the plasticizer. The process can also be a closed loop process where the halogen-free solvent can be recycled.SELECTED DRAWING: None

Description

[Technical Field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 266,830, filed January 14, 2022, entitled "Microporous Polyolefin Membranes from Bespoke Solvents," the entire contents of which are incorporated herein by reference.

[0002] Copyright Notice (Copyright) 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 this patent document or the 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] Technical Field The present invention relates to halogen-free microporous polyolefin membranes that can be produced using an environmentally friendly, closed-loop process involving extrusion of a polymer-plasticizer mixture followed by sheet formation, extraction of the plasticizer with a solvent, evaporation of the solvent to form micropores, and subsequent adsorption-desorption of the solvent from activated carbon for reuse in the manufacturing process. The custom solvent is halogen-free, has low water solubility, and has a flash point of about 23°C or higher. In terms of Hansen solubility parameters, the solvent is characterized by low dispersion (Delta D about 15), low polarity (Delta P about 0), and low hydrogen bonding (Delta H about 0), and may be miscible with plasticizers, including naphthenic, paraffinic, and white mineral oils. Finally, the relative boiling point is important for (1) separation and reuse of the plasticizer and custom solvent. [Background technology]

[0004] Microporous membranes have a structure designed to allow fluid flow through them. The fluid can be either a liquid or a gas, and the pore size of the membrane must generally be at least several times the mean free path of the fluid to achieve the desired flux. Microporous membranes typically have pore sizes ranging from about 10 nanometers to several microns, with an average pore size of less than about 1 micrometer. Because the pore size and polymer matrix are large enough to scatter visible light, such membranes are generally opaque. The term "microporous membrane" as used encompasses other descriptions used in scientific and patent literature, such as "microporous film," "microporous sheet," and "microporous web." Microporous membranes can also be self-supporting and have interconnected pores extending throughout the membrane. "Self-supporting" refers to a membrane with sufficient mechanical properties to allow manipulation, such as rolling and unrolling in sheet form, for use in energy storage device assemblies.

[0005] Microporous membranes have been utilized in a wide variety of applications, including filtration, breathable films for clothing or medical gown applications, battery separators, synthetic printed sheets, and surgical dressings. In some cases, microporous membranes are laminated to other articles (e.g., nonwovens) to impart additional functionality (e.g., tear resistance, oxidation resistance). Microporous membranes may also be stretched in the mechanical or transverse direction as part of the manufacturing process or in a secondary step.

[0006] The production of microporous membranes is often achieved by thermally induced phase separation. In this process, a polymer is melt-blended with a thermally stable plasticizer (e.g., paraffin oil) at elevated temperatures, then cast or extruded into a nonporous film or object to form a homogeneous mixture. This is cooled, inducing phase separation of the polymer and plasticizer. The plasticizer is then removed by solvent extraction and drying to form the microporous membrane. To facilitate separation and reuse of the solvent and plasticizer, it is important that their initial boiling points are at least about 50°C apart (e.g., for efficient separation during distillation). Summary of the Invention [Problem to be solved by the invention]

[0007] Battery separators are commonly manufactured using a heat-induced phase separation process, followed by extraction of the thermally stable plasticizer using hexane, trichloroethylene, methylene chloride, or other solvents. Government regulatory agencies are continually conducting risk assessments of such solvents, raising concerns about environmental and worker exposure. [Means for solving the problem]

[0008] Most flooded lead (Pb)-acid batteries contain polyethylene separators. The term "polyethylene separator" is a misnomer because these microporous separators require large amounts of precipitated silica to be sufficiently acid-wettable. The volume fraction and distribution of precipitated silica in the separator generally controls its electrical (ionic) properties, while the volume fraction and orientation of polyethylene in the separator generally controls its mechanical properties. Commercially available polyethylene separators for lead-acid batteries typically have porosities of about 50-65%. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 illustrates a closed-loop solvent extraction and carbon bed recovery process used in the manufacture of microporous membranes according to one embodiment. [Figure 2] FIG. 1 illustrates a closed-loop solvent extraction and vapor condensation recovery process used in the manufacture of microporous membranes according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] In the manufacture of Pb-acid separators, precipitated silica is combined with polyolefin, plasticizer (i.e., process oil), and various minor ingredients to form a separator mixture, which is then extruded at high temperature through a sheet die to form an oil-filled sheet. The oil-filled sheet is then calendered to the desired thickness and shape. Most of the process oil is extracted with an organic solvent, but the final separator typically contains about 10-25%, more preferably about 12-22%, of the process oil. Naphthenic process oil is preferred, and the residual oil enhances the oxidation resistance of the Pb-acid battery separator. Hexane and trichloroethylene are the two most commonly used solvents in Pb-acid separator manufacturing. The solvent-containing sheet is then dried to form a microporous polyolefin separator, which is then cut to the appropriate width for the specific battery design.

[0011] Polyethylene separators are delivered in rolls to lead-acid battery manufacturers, who feed them into a machine that cuts the separator material and seals its edges to form an "envelope" into which electrodes can be inserted to form an electrode package. The electrode packages are stacked so that the separator acts as a physical spacer and electronic insulator between the positive and negative electrodes. Sulfuric acid is then introduced into the assembled battery to promote ionic conduction between the electrodes.

[0012] The primary purposes of the polyolefin in the separator are (1) to provide mechanical integrity to the polymer matrix so that it can encase the separator at high speeds and (2) to prevent grid wire perforation during battery assembly or operation. Therefore, the hydrophobic polyolefin preferably has a molecular weight that provides sufficient chain entanglement to form a microporous web with high perforation resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator web, thereby lowering the separator's electrical resistivity. Without silica, sulfuric acid would not wet the hydrophobic web, preventing ion transport and resulting in battery failure. As a result, the silica component of the separator typically accounts for about 55% to about 80% by weight of the separator, i.e., the separator has a silica-to-polyethylene weight ratio of about 2.0:1 to about 3.5:1.

[0013] The production of microporous membranes for synthetic printing applications is exemplified by Schwarz et al. in U.S. Pat. No. 5,196,262. In this case, the polymer matrix is ​​composed of a blend of ultra-high molecular weight polyethylene (UHMWPE) having an intrinsic viscosity greater than about 10 dL / g and a low molecular weight polyethylene having a melt flow index (ASTM D1238-86 conditions) less than about 50 g / 10 min. These polymers are combined with a high percentage of finely divided, water-insoluble siliceous filler, other minor ingredients, and processing plasticizers to form a mixture, which is then extruded into a sheet from which most of the plasticizer is extracted with a solvent. Residual plasticizer is less than about 3% by weight of the synthetic printing sheet, more preferably less than about 1% by weight. Examples of suitable organic extractants include trichloroethylene, perchloroethylene, methylene chloride, hexane, heptane, and toluene. The resulting microporous membrane is sold by PPG Industries under the trademark Teslin®.

[0014] Separators designed for Li-ion, Li-metal, or rechargeable Li-metal battery systems are generally manufactured using a thermally induced phase separation process. In this process, various grades of polyethylene with molecular weights ranging from about 500,000 g / mol to about 10 million g / mol are combined with a plasticizer and then extruded through a sheet or annular die to form an oil-filled sheet. This oil-filled sheet is often biaxially oriented to reduce its thickness and improve its mechanical properties in both the machine and cross directions. The biaxially oriented sheet is then passed through an extraction bath of methylene chloride to remove the plasticizer, after which pores form as the solvent evaporates. The resulting battery separator typically has a thickness ranging from about 3 to 25 μm, a porosity between about 40 and 65%, and less than about 1% residual plasticizer.

[0015] In the case of chlorinated solvents such as trichloroethylene (TCE), it has been shown that they can be absorbed into the amorphous regions of polyethylene articles and adsorbed onto the surface of polyethylene powder (see, e.g., ShuaiXie et al., Very Low Concentration Adsorption Isotherms of Trichloroethylene on Common Building Materials, Building and Environment, Vol. 179, July 15, 2020). The same mechanism applies to battery separators, where chlorinated solvents can remain within the material even after passing through high-temperature drying ovens. In some cases, chlorinated solvents can remain in residual plasticizers or oils used to extrude the separator precursor.

[0016] In response to growing environmental pressures and health concerns associated with organic solvents such as trichloroethylene, methylene chloride, and hexane, new sustainable approaches to the manufacture of microporous membranes are needed that can meet customer performance requirements through processes that minimize worker exposure to potentially harmful chemicals and efficiently recycle extraction solvents and plasticizers in a closed loop. The components of the microporous membranes disclosed herein, including the solvents and plasticizers used in their manufacture, are environmentally friendly and safe to handle. Additionally, the solvents can be food-grade, minimizing health concerns associated with their handling.

[0017] In the production of microporous membranes using thermally induced phase separation, the primary considerations for solvent selection include physical properties, chemical properties, equipment compatibility, safety, recyclability, cost, and the ability to achieve desired product properties (e.g., pore size distribution). Solvent selection criteria are summarized in Table I below.

[0018] [Table 1]

[0019] Identifying a single solvent, especially one that is nonflammable, that can meet all selection criteria while minimizing health and environmental risks is challenging. With ongoing pressure from REACH and the EPA to eliminate chlorinated solvents such as trichloroethylene and methylene chloride as extraction solvents for producing microporous membranes used as battery separators, new approaches are needed. While certain azeotropes have been proposed as alternatives to trichloroethylene and methylene chloride, they typically contain trans-dichloroethylene (i.e., a chlorinated solvent) in combination with various fluorinated compounds that fall under the category of perfluoroalkyl substances (PFAS). Perfluoroalkyl substances are under increased scrutiny due to their environmental persistence (i.e., they do not decompose) and associated health risks.

[0020] The physical and chemical properties of the solvent are important to consider in relation to the properties of the plasticizer. Important considerations include the flash point, boiling point range, and aniline point of the solvent and plasticizer. For example, the flash point of the plasticizer is important for extrusion. In particular, the flash point of the plasticizer must be higher than the extrusion temperatures typically higher than about 215°C during the manufacture of polyolefin battery separators. Thus, in some embodiments, the plasticizer has a flash point higher than about 215°C. In other embodiments, the plasticizer has a flash point higher than about 145°C (e.g., from about 145°C to about 350°C, or from about 145°C to about 300°C, etc.). For extraction solvents, it is advisable to have a flash point higher than room temperature (higher than about 23°C), which reduces the risk of fire while ensuring that sufficient thermal energy is available to promote evaporation and pore formation in the membrane. Thus, in some embodiments, the flash point of the extraction solvent is higher than about 23°C. In further embodiments, the flash point of the extraction solvent is greater than about 38°C to meet certain code and / or fire code requirements (e.g., NFPA 30 Flammable and Combustible Liquids Code, NFPA 1 Fire Code; and / or the International Fire Code as recognized in 2022-2023, etc.).

[0021] Because the vapor pressure and volatility of a solvent generally correlate with its boiling point, it is important to select a temperature range that maximizes the ability to separate the solvent from the plasticizer with the lowest possible amount of energy while minimizing potential worker exposure. Furthermore, in some embodiments, it is important for the solvent to have an initial boiling point that is at least about 50° C. lower than the flash point of the plasticizer, making separation by distillation easier to achieve. In a further embodiment, the solvent has an initial boiling point that is at least about 85° C. lower than the flash point of the plasticizer. It will be understood that the initial boiling point of the plasticizer (to the extent that the plasticizer has a flash point) is always higher than its flash point. Thus, stated another way, in terms of initial boiling point, the solvent has an initial boiling point that is at least about 50° C. or at least about 85° C. lower than the initial boiling point of the plasticizer. The difference in initial boiling point and / or flash point between the solvent and the plasticizer is useful for efficiently distilling and separating the components so that one or both components can be recycled and reused in the manufacturing process, as further described below in connection with FIGS. 1 and 2. In some cases, it is desirable to maximize the difference in initial boiling point and / or flash point between the solvent and the plasticizer.

[0022] Second, the aniline point is a relatively simple test for measuring the solvency of hydrocarbons. Aniline is a simple aromatic amine with an amino group attached to a benzene ring. The aniline point is the lowest temperature at which a 1:1 mixture of solvent or plasticizer and aniline remains a clear solution. Lower values ​​indicate stronger solvency, while higher values ​​indicate weaker solvency. In some embodiments, solvents with lower aniline points can be used, such as in Pb-acid separators, where residual plasticizer remains and may remain in the resulting separator. In other embodiments, solvents with either lower or higher aniline points can be used, such as in Li separators, where all or substantially all of the plasticizer is removed from the resulting separator.

[0023] Generally, a high aniline point indicates a relatively low level of dissolved aromatics. This difference can be clearly observed in paraffinic plasticizers compared to naphthenic plasticizers, as shown by carbon type analysis (ASTM D2140). As the paraffinic carbon content decreases in various plasticizers, while the naphthenic and aromatic carbon content increases, the aniline point also decreases. It should be noted that flash points show a similar trend.

[0024] For hydrocarbon solvents, the aniline point generally decreases as the alkane chain length decreases or as aromaticity increases. The above trends and relationships can be seen in Table II.

[0025] [Table 2]

[0026] In some cases, the greater the difference in aniline points between the solvent and plasticizer, the faster the extraction; however, efficient recycling in a closed-loop process requires a custom solvent that meets the other criteria outlined above. In some embodiments, the aniline point of the plasticizer is from about 70°C to about 140°C, from about 75°C to about 135°C, or from about 80°C to about 130°C. In other embodiments, the aniline point of the plasticizer is from about 35°C to about 140°C. Surprisingly, IsoPar-G has been found to be capable of meeting the requirements of next-generation solvent extraction and recovery processes for microporous membrane production using plasticizers including naphthenic, paraffinic, and white mineral oil. Other solvents that meet the criteria outlined above, including both solvents identified in Table II and solvents not identified in Table II, can also be used.

[0027] In some embodiments, a solvent is used to extract the plasticizer after the nonporous film is stretched or biaxially stretched. In a further embodiment, after the plasticizer is extracted, the resulting microporous polyolefin membrane is further stretched and / or biaxially stretched. The resulting microporous polyolefin membrane is also completely free of any halogen-containing compounds (e.g., residual halogen-containing compounds) because the extraction solvent is halogen-free. This is in contrast to microporous polyolefin membranes produced using halogen-containing extraction solvents such as methylene chloride or trichloroethylene, which may contain trace amounts of residual halogen-containing compounds. It should be recognized that trace amounts of residual halogen-containing compounds may cause corrosion or other problems in the performance of rechargeable lithium-ion batteries or lithium metal-based batteries.

[0028] The microporous polyolefin membranes disclosed herein can be produced using a variety of polyolefins. Exemplary polyolefins that can be used include, but are not limited to, various grades of polyethylene, e.g., polyethylene with a molecular weight ranging from about 500,000 g / mol to about 10,000,000 g / mol (e.g., ultra-high molecular weight polyethylene (UHMWPE), very high molecular weight polyethylene (VHMWPE), high molecular weight, high density polyethylene (HMW-HDPE), and mixtures thereof), polypropylene, polymethylpentene, and mixtures thereof. In certain embodiments, the polyolefin includes polyethylene having a molecular weight of about 500,000 g / mol or greater, or about 600,000 g / mol or greater.

[0029] The thickness of the microporous polyolefin membrane can vary. In some embodiments, the microporous polyolefin membrane comprises a backweb thickness of about 25 microns or less, or about 20 microns or less. In certain embodiments, the microporous polyolefin membrane comprises a backweb thickness of about 5 microns to about 25 microns, or about 5 microns to about 20 microns. Microporous polyolefin membranes having greater thicknesses, such as those having a backweb thickness of about 150 microns to about 300 microns, can also be produced. As used herein, backweb thickness can refer to the thickness of the microporous polyolefin membrane, not including the height of ribs or surface protrusions.

[0030] The microporous polyolefin membrane can also optionally contain a filler. Exemplary fillers that can be used include inorganic oxides, carbonates, or hydroxides, such as alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, hydrotalcite, and mixtures thereof. The filler can be distributed throughout the microporous polyolefin membrane. In some embodiments, the filler is uniformly distributed throughout the microporous polyolefin membrane. In other embodiments, the filler is not uniformly distributed.

[0031] The microporous polyolefin membrane may optionally be annealed or heat stabilized as part of the manufacturing process after the extraction process and before being wound into a roll for subsequent use.

[0032] The following examples are illustrative in nature and are not intended to be limiting in any manner. [Example]

[0033] Example 1 UHMWPE (Celanese GUR 4150), precipitated silica (PPG WB-2085), and secondary ingredients (antioxidant, plasticizer, and carbon black) were combined in a horizontal mixer and blended at low agitation to form a homogeneous mixture. Next, hot naphthenic plasticizer (ENTEK 800 oil; Calumet) was sprayed onto the dry ingredients. This mixture, containing 58 wt% oil, was then fed into a 96 mm counter-rotating twin-screw extruder (ENTEK Manufacturing Inc.) operating at a melt temperature of 215 °C. Additional process oil was added inline at the extruder throat to bring the final oil content to 65 wt%. The resulting mass was calendered through a sheet die and embossed with a rib pattern to a thickness of 200–300 μm. After passing over two chill rolls, the oil-filled sheet was collected for extraction.

[0034] An oil-filled sample, approximately 160 mm x 160 mm, was placed in a beaker containing IsoPar-G and extracted with stirring at room temperature for approximately 10 minutes. The solvent-containing sample was then dried overnight at 105°C in a circulating oven. The resulting separator was porous, had good mechanical properties, and contained 16% by weight of residual oil (i.e., plasticizer). Because the solvent was free of halogen-containing compounds, the resulting separator was also free of residual halogen-containing compounds.

[0035] Example 2 Naphthenic plasticizer (140 kg; ENTEK 800 oil; Calumet) was dispensed into a Ross mixer where it was stirred and degassed. The following was then added and mixed with the oil: 64 kg UHMWPE (molecular weight approximately 5 million g / mol) 32 kg VHMWPE (molecular weight approximately 1 million g / mol) 32 kg HMW-HDPE (molecular weight approximately 600,000 g / mol) 1.2kg Li stearate 1.2kg antioxidant

[0036] The mixture was blended at approximately 40°C until a homogeneous 47% by weight polymer slurry was formed. The polymer slurry was then transported to a 73 mm diameter co-rotating twin-screw extruder, where a melt temperature of approximately 215°C was maintained. The extrudate was passed through a melt pump feeding a 257 mm diameter annular die with a 2.75 mm gap. The throughput through the die was 135 kg / hr, where the extrudate was expanded with air to produce a biaxially oriented, oil-filled film approximately 2000 mm in diameter, which was then passed through an upper nip at 20 m / min to collapse the air bubbles and form a bilayer, which was then side-slit to produce two individual layers.

[0037] The oil-filled bed then passed through an extractor where the IsoPar-G flowed countercurrent to the bed direction. The oil / IsoPar-G mixture in the first zone of the extractor was transported to a distillation unit and separated for reuse. The solvent-containing bed then passed through an oven equipped with an air knife to evaporate the IsoPar-G solvent, and the vapors were sent to a carbon bed system for adsorption and subsequent recovery. An exemplary closed-loop manufacturing process is shown in Figure 1. Another exemplary closed-loop manufacturing process is shown in Figure 2, which utilizes a vapor condenser system rather than a carbon bed recovery system. Finally, the extracted bed was stretched 1.5 times in a machine direction orientation (MDO) at a roll temperature of 80°C, followed by a 2.0 times stretch in a transverse direction orientation (TDO) at 128°C. MDO refers to stretching along the machine direction, or the direction in which the material is fed, and TDO refers to stretching along the transverse direction, or the direction at 90 degrees to the machine direction.

[0038] The microporous layer was then separated by passing through a nip and wound into individual rolls on a dual turret winder. The resulting battery separator had a thickness of approximately 20 μm and an average Gurley air permeability of 150 s / 100 cc. Residual plasticizer was determined to be 0.6% by thermogravimetric analysis. Because the solvent is free of halogen-containing compounds, the resulting separator is also free of residual halogen-containing compounds.

[0039] Referring to Figures 1 and 2, a nonporous plasticizer-filled film formed from a cast or extruded polymer-plasticizer mixture 20 is passed through a countercurrent extractor 22. Solvent, supplied from a solvent storage tank 24 and flow-controlled by a fluid valve 26, flows into the countercurrent extractor 22 in a direction opposite to the film. The extractor 22 produces a plasticizer-solvent mixture in a first internal zone, which is transported to a distillation unit 28, where the plasticizer and solvent are separated for reuse. The distillation unit 28 produces a purified solvent condensate. The purified solvent is returned to a second internal zone of the countercurrent extractor 22 for reuse in combination with the solvent supplied from the solvent storage tank 24. The solvent-laden film exits the countercurrent extractor 22 and passes through a heated dryer 30, which is a heat source equipped with an air knife to evaporate the solvent, thereby generating solvent vapor. A microporous membrane 32 emerges from the heated dryer 30. The microporous membrane may be further stretched or annealed before being wound into a roll.

[0040] In the solvent recovery system embodiment of Figure 1, solvent vapors produced by operation of the heated dryer 30 are recovered by adsorption-desorption using a carbon bed system 34. The solvent vapors are evaporated onto activated carbon, which adsorbs the solvent. Steam is then used to thermally desorb the solvent from the activated carbon, which is sent to storage tank 24.

[0041] 2, solvent vapor produced by operation of heated dryer 30 is recovered by vapor condenser system 36. The solvent vapor enters vapor condenser system 36 to extract latent heat of vaporization from the solvent vapor, thereby cooling and condensing the solvent. The recovered solvent is sent to storage tank 24.

[0042] 1 and 2 show the outlet of the solvent storage tank 24 connected to the countercurrent extractor 22 through a fluid valve 26. This configuration implements an embodiment of a closed-loop solvent recovery system in which the recovered solvent washes onto the plasticizer-loaded film, continuing to remove plasticizer from the sheet as it passes through the countercurrent extractor 22.

[0043] In some cases, it may be advantageous to use a combination of activated carbon and steam condenser solvent recovery systems for efficient recovery and reuse of the solvent. Those skilled in the art will appreciate that when steam is utilized as the heat source, solvent / water liquid phase separation may be a necessary part of the recovery process.

[0044] Example 3 Naphthenic plasticizer (140 kg; ENTEK 800 oil; Calumet) was dispensed into a Ross mixer where it was stirred and degassed. The following was then added and mixed with the oil: 64 kg UHMWPE (molecular weight approximately 5 million g / mol) 32 kg VHMWPE (molecular weight approximately 1 million g / mol) 32 kg HMW-HDPE (molecular weight approximately 600,000 g / mol) 1.2kg Li stearate 1.2kg antioxidant

[0045] The mixture was blended at approximately 40°C until a homogeneous 47% by weight polymer slurry was formed. The polymer slurry was then transported to a 103 mm diameter co-rotating twin-screw extruder, where a melt temperature of approximately 215°C was maintained. Simultaneously, fumed alumina / HDPE / oil pellets were fed into the extruder. The extrudate was passed through a melt pump that fed a 257 mm diameter annular die with a 2.75 mm gap. The throughput through the die was 230 kg / hr, where the extrudate was expanded with air to produce a biaxially oriented oil-filled film approximately 2250 mm in diameter. The film was then passed through an upper nip at 20 m / min to collapse the air bubbles and form a bilayer, which was then side-slit to produce two individual layers.

[0046] Each oil-filled layer (approximately 44 μm thick) was then held in a metal frame secured by clamps. An area of ​​approximately 200 mm x 200 mm of the oil-filled layer was then exposed to an excess of IsoPar-G at room temperature for approximately 15 minutes while the solvent was stirred. The solvent-loaded aluminum-filled PE film was then dried in a circulating air oven at 105°C for approximately 10 minutes. The membrane was then removed from the frame and confirmed to be 12 μm thick with an average Gurley air permeability of approximately 50 s / 100 cc. Residual plasticizer was determined to be 0.8 wt% by thermogravimetric analysis. Since the solvent is free of halogen-containing compounds, the membrane is also free of residual halogen-containing compounds.

[0047] Example 4 UHMWPE (37.4 kg, approximately 5 million g / mol), lithium stearate (0.43 kg), antioxidant (0.37 kg), and naphthenic plasticizer (132.6 kg, approximately 12 cP at 100°C; ENTEK 800 oil; Calumet) were blended in a Ross mixer to form a 22 wt% polymer slurry. The slurry was pumped to a twin-screw extruder at 170 kg / h while maintaining the melt temperature above 200°C. The extrudate passed through a melt pump and fed into a 257 mm diameter annular die with a 2.75 mm gap. The extrudate was expanded with air to produce a 2000 mm diameter biaxially oriented film, which was then passed through an upper nip at a speed of 14.5 m / min to collapse the air bubbles and form a bilayer. The collapsed bilayer sheet was slit open at both ends and conveyed to an extraction tank filled with IsoPar-G to remove the plasticizer. The extracted sheet was then stretched sequentially in the machine direction (1.83x) and transverse direction (2.9x) at 100°C (approximately 1% relaxation) and wound into a roll on a corrugated core at 28.5 m / min. The residual plasticizer was determined to be 0.4 wt% by thermogravimetric analysis. Because the solvent was free of halogen-containing compounds, the extracted sheet was also free of residual halogen-containing compounds.

[0048] The physical properties of the separators produced are shown in Table III.

[0049] [Table 3]

[0050] Example 5 Polymer powders A through C were individually weighed into aluminum pans and then placed in trays in glass containers suspended in approximately 0.5 liters of dichloromethylene (DCM). The containers were sealed with glass lids, and the polymer powders were allowed to adsorb / absorb methylene chloride vapors for 24 hours at room temperature (19°C). The powder samples were then immediately weighed after removal from the containers, and the weight gain was calculated. After allowing to stand for 30 minutes in a fume hood, the samples were reweighed. The data are shown in Table IV below.

[0051] [Table 4]

Claims

1. 1. A free-standing microporous polyolefin membrane comprising: a semi-crystalline polymer matrix comprising polyethylene to provide mechanical integrity resulting from the phase separation of a polymer and a plasticizer having a flash point above about 215°C and an aniline point between about 70-140°C; interconnected pores obtained by extracting said plasticizer with a halogen-free solvent having a flash point greater than about 23°C and an initial boiling point at least about 50°C lower than said flash point of said plasticizer, followed by evaporation; 1. A free-standing microporous polyolefin membrane comprising:

2. 10. The closed-cell microporous polyolefin membrane of claim 1, wherein the halogen-free solvent has a flash point greater than about 38°C.

3. 3. The free-standing microporous polyolefin membrane of claim 1 or 2, wherein the polymer matrix is ​​oriented or biaxially oriented.

4. The closed-cell microporous polyolefin membrane of any one of claims 1 to 3, wherein the polymer matrix is ​​stretched or biaxially oriented prior to solvent extraction of the plasticizer.

5. 5. The closed-cell microporous polyolefin membrane of claim 4, wherein the polymer matrix is ​​further stretched or biaxially oriented after extraction of the plasticizer and evaporation of the solvent.

6. The closed-cell microporous polyolefin membrane of any one of claims 1 to 5, which is annealed or heat-stabilized before being wound up.

7. The self-supporting microporous polyolefin membrane of any one of claims 1 to 6, wherein the polymer matrix further comprises a filler dispersed throughout the polymer matrix.

8. 8. The free-standing microporous polyolefin membrane of claim 7, wherein the filler comprises an inorganic oxide, carbonate, hydroxide, or a mixture thereof, or the filler comprises alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, hydrotalcite, or a mixture thereof.

9. The free-standing microporous polyolefin membrane of any one of claims 1 to 8, wherein the polyolefin comprises polyethylene having a molecular weight of at least about 500,000 g / mol.

10. The closed-cell microporous polyolefin membrane of any one of claims 1 to 9, comprising a backweb thickness of about 25 microns or less, or about 20 microns or less.

11. The closed-cell microporous polyolefin membrane of any one of claims 1 to 9, comprising a backweb thickness of from about 150 microns to about 300 microns.

12. The closed-cell microporous polyolefin membrane of any one of claims 1 to 11, containing about 10 to 25 wt% residual plasticizer, or about 12 to 22 wt% residual plasticizer.

13. The closed-cell microporous polyolefin membrane of any one of claims 1 to 11, containing about 0 to 1 wt% residual plasticizer.

14. 14. The closed-cell microporous polyolefin membrane of any one of claims 1 to 13, wherein the halogen-free solvent comprises an incipient boiling point that is at least about 85°C lower than the flash point of the plasticizer.

15. The closed-cell microporous polyolefin membrane of any one of claims 1 to 14, which is free of halogen-containing compounds.

16. The closed-cell microporous polyolefin membrane of any one of claims 1 to 15, wherein the halogen-free solvent is a food-grade solvent.

17. melt blending a polyolefin and a plasticizer to form a mixture; casting or extruding the mixture to form a non-porous sheet; cooling the non-porous sheet to induce phase separation between the polyolefin and the plasticizer; extracting the plasticizer from the non-porous sheet with a halogen-free solvent and evaporating the solvent to form a microporous polyolefin membrane; wherein the halogen-free solvent has a flash point of about 23°C or greater and an incipient boiling point at least about 50°C lower than the flash point of the plasticizer.

18. 18. The method of claim 17, wherein the polyolefin comprises polyethylene having a molecular weight of at least about 500,000 g / mol.

19. 19. The method of claim 17 or 18, wherein the plasticizer comprises a flash point greater than about 215°C and an aniline point of about 70-140°C.

20. 20. The method of any one of claims 17 to 19, wherein the halogen-free solvent comprises an initial boiling point that is at least about 85°C lower than the flash point of the plasticizer.

21. 21. The method of any one of claims 17 to 20, comprising a closed loop, solvent extraction, drying, and carbon bed recovery system.

22. 22. The method of any one of claims 17 to 21, further comprising stretching or biaxially orienting the non-porous sheet prior to solvent extraction of the plasticizer from the polyolefin.

23. 23. The method of claim 22, further comprising stretching or biaxially stretching the microporous polyolefin membrane after the plasticizer extraction and solvent evaporation.

24. The method of any one of claims 17 to 23, further comprising annealing or heat stabilizing the microporous polyolefin membrane before winding.

25. The method of any one of claims 17 to 24, wherein the microporous polyolefin membrane is free of halogen-containing compounds.

26. The method of any one of claims 17 to 25, wherein the microporous polyolefin membrane has a backweb thickness of about 25 microns or less, or about 20 microns or less.

27. The method of any one of claims 17 to 25, wherein the microporous polyolefin membrane has a backweb thickness of from about 150 microns to about 300 microns.

28. The method of any one of claims 17 to 27, wherein the mixture further comprises a filler dispersed throughout the mixture.

29. 29. The method of claim 28, wherein the filler comprises an inorganic oxide, carbonate, hydroxide, or mixtures thereof, or the filler comprises alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, hydrotalcite, or mixtures thereof.

30. 30. The method of any one of claims 17 to 29, wherein the halogen-free solvent is a food-grade solvent.

31. a cast or extruded polyolefin sheet obtained from a cast or extruded mixture of a polyolefin and a plasticizer having a flash point above about 215°C and an aniline point of about 70-140°C; a halogen-free solvent loaded within the polyolefin sheet; A solvent-containing sheet comprising:

32. 32. The solvent-containing sheet of claim 31, wherein the polyolefin comprises polyethylene having a molecular weight of at least about 500,000 g / mol.

33. 33. The solvent-containing sheet of claim 31 or 32, wherein the halogen-free solvent comprises a flash point greater than about 23°C and an incipient boiling point at least about 50°C lower than the flash point of the plasticizer.

34. 34. The solvent-containing sheet of claim 33, wherein the halogen-free solvent comprises an initial boiling point that is at least about 85°C lower than the flash point of the plasticizer.

35. The solvent-containing sheet according to any one of claims 31 to 34, wherein the halogen-free solvent is a food-grade solvent.

36. 36. The solvent-containing sheet of any one of claims 31 to 35, wherein the mixture further comprises a filler dispersed throughout the mixture.

37. 37. The solvent-containing sheet of claim 36, wherein the filler comprises an inorganic oxide, carbonate, hydroxide, or a mixture thereof, or the filler comprises alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, hydrotalcite, or a mixture thereof.

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