A nonwoven sheet material comprising a substrate and an applied fibril coating.

Abstract

1. A nonwoven sheet material and process for producing the same, comprising a substrate and a fibril coating applied onto the substrate, wherein the substrate is paper, a spunbond fibrous sheet, or a fibrous or non-fibrous membrane, and the applied fibril coating comprises fibrils having a diameter of 1 to 5000 nanometers, a length of 0.2 to 3 millimeters, a specific surface area of ​​3 to 40 square meters per gram, and a Canadian Standard Freeness of 0 to 10 milliliters, and the fibrils comprise an aramid polymer.

Description

[Technical field]

[0001] The present invention relates to a nonwoven sheet material suitable for use as a separator in an electrochemical cell, i.e., useful for separating a cathode from an anode within an electrochemical cell, which paper also has suitable permeability to the electrolyte used in such cells. With the continuing development of higher performance electrochemical cells (or batteries as they are commonly known), there is an increasing need for papers suitable as separators (commonly known as battery separators) that can operate at very high temperatures. [Background technology]

[0002] WO 2020 / 036800 discloses a paper suitable for use as a separator paper in an electrochemical cell and an electrochemical cell comprising the same, the paper comprising 95-100 weight percent fibrils and 0-5 weight percent aramid fibrids as the only fiber components, and having a thickness of 10-40 micrometers and a tensile strength of at least 15 megapascals or more, the fibrils comprising a polymer blend of 80-96 weight percent polyparaphenylene terephthalamide and 4-20 weight percent polyvinylpyrrolidone, the fibrils having a diameter of 10-5000 nanometers, a length of 0.2-3 millimeters, a specific surface area of ​​3-40 square meters per gram, and a Canadian Standard Freeness of 0-10 milliliters. Summary of the Invention [Problem to be solved by the invention]

[0003] Various nonwoven sheet materials have been proposed and / or used as separator papers for electrochemical cells, also known as batteries. However, such materials may undergo undesirable linear thermal shrinkage when exposed to high temperatures. Since newer batteries may operate at higher temperatures, new materials with reduced linear thermal shrinkage suitable for use as separator papers in electrochemical cells are highly desirable. Thus, new nonwoven papers that can function as battery separators and also have improved thermal stability would be welcomed by the industry, as they would not only be able to perform their function in batteries operating at high temperatures, but would also provide improved battery safety, improved battery capacity, the ability to charge batteries quickly, and improved battery energy density. [Means for solving the problem]

[0004] The present invention relates to a nonwoven sheet material comprising a substrate and a fibril coating applied onto said substrate, the substrate being paper, a spunbond fibrous sheet or a fibrous or non-fibrous membrane, the applied fibril coating comprising fibrils having a diameter of 1-5000 nanometers, a length of 0.2-3 millimeters, a specific surface area of ​​3-40 square meters per gram and a Canadian Standard Freeness of 0-10 milliliters, the fibrils comprising an aramid polymer. In some embodiments, the fibrils comprise the aramid polymer polyparaphenylene terephthalamide; in some preferred embodiments, the fibrils comprise a polymer blend of 80-96 weight percent polyparaphenylene terephthalamide and 4-20 weight percent polyvinylpyrrolidone.

[0005] The present invention provides a process for producing a nonwoven sheet material comprising a substrate and a fibril coating applied onto said substrate, comprising: a) applying a layer of an aqueous slurry of fibrils onto a surface of a substrate, the substrate being a paper, a spunbond fiber sheet, or a fibrous or non-fibrous membrane, the fibrils comprising fibrils having a diameter of 1 to 5000 nanometers, a length of 0.2 to 3 millimeters, a specific surface area of ​​3 to 40 square meters per gram, and a Canadian Standard Freeness of 0 to 10 milliliters, the fibrils comprising an aramid polymer; b) removing water from the aqueous slurry to form a fibril coating on the surface of the substrate; In some embodiments, the fibrils comprise an aramid polymer, polyparaphenylene terephthalamide; in some preferred embodiments, the fibrils comprise a polymer blend of 80 to 96 weight percent polyparaphenylene terephthalamide and 4 to 20 weight percent polyvinylpyrrolidone. [Brief description of the drawings]

[0006] [Figure 1] A digital photograph taken at 100x magnification of the formation of one type of applied fibril coating produced by spraying an aqueous slurry of fibrils onto a substrate, showing that the coating can include fibril spirals and entanglements. [Diagram 2-3] 1A and 1B are digital photographs taken at 100x and 2000x magnification, respectively, of fibril coatings produced by spraying onto a substrate with multiple passes or layers of applied fibrils. [Figure 4] Digital photograph taken at 1000x magnification of the substrate alone, a handsheet made from a blend of polyester fibers and nanocellulose. [Diagram 5] FIG. 5 is a digital photograph taken at 1000x magnification of the substrate handsheet of FIG. 4 made from a blend of polyester fibers and nanocellulose, further having a thinly applied fibril coating. [Figure 6]FIG. 5 is a digital photograph taken at 1000x magnification of the substrate handsheet of FIG. 4 made from a blend of polyester fibers and nanocellulose, with a thicker applied fibril coating. [Figure 7] 1 is a digital photograph taken at 1000x magnification of a substrate that is a polypropylene microporous film. [Figure 8] 1 is a digital photograph taken at 1000x magnification of a substrate that is a polypropylene microporous film having a thinly applied fibril coating. [Figure 9] 1 is a digital photograph taken at 1000x magnification of aramid polymer fibrils, specifically PPD-T / PVP polymer fibrils. [Figure 10] 1 is a digital photograph taken at 500x magnification of a commercial aramid pulp, specifically PPD-T pulp. [Figure 11] 1 is a graphical representation comparing the distribution of pores in PPD-T / PVP filaments versus PPD-T filaments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] The present invention relates to a nonwoven sheet material comprising a substrate and a fibril coating applied onto said substrate, the nonwoven sheet material being suitable for use as a separator paper in an electrochemical cell. The present invention further relates to an electrochemical cell comprising the same nonwoven sheet material.

[0008] The nonwoven sheet material substrate is paper, a spunbond fiber sheet, or a fiber or non-fibrous membrane. The fibril coating applied onto the substrate comprises fibrils having a diameter of 1-5000 nanometers, a length of 0.2-3 millimeters, a specific surface area of ​​3-40 square meters per gram, and a Canadian Standard Freeness of 0-10 milliliters. The fibrils preferably comprise aramid fibrils. In some preferred embodiments, the fibrils comprise a polymer blend of 80-96 weight percent polyparaphenylene terephthalamide and 4-20 weight percent polyvinylpyrrolidone.

[0009] The substrate is a planar structure, and if it is a nonwoven fabric, unlike knitted or woven fabrics, its components are randomly, but preferably uniformly, arranged. In some embodiments, the substrate is paper, i.e., a typical flat sheet structure that includes pulp, flock or other fibrous material and, optionally, a binder, typically produced on a papermaking machine such as a Fourdrinier or tilt-wire machine.

[0010] In some embodiments, the substrate is a spunbond fiber sheet. Spunbond fiber sheet refers to any sheet having randomly arranged fibrous material spun from an orifice, then collected on a surface and bonded together by heat and / or pressure. The most common spunbond fiber sheets include those produced by extruding molten thermoplastic polymer as filaments from multiple thin capillaries of a spinneret; the filaments are randomly deposited on a screen and bonded together. Other spunbond fiber sheets include those produced by flash spinning, i.e., flashing a solution of polymer and solvent to form fibrous strands that are randomly deposited on a screen and bonded together. Other common spunbond fiber sheets include sheets comprising thermally bonded meltblown webs, thermally bonded airlaid webs, thermally bonded spunlace webs, or thermally bonded staple fiber carded webs.

[0011] In some embodiments, the substrate is a fibrous or non-fibrous membrane. By membrane, we mean a thin, flexible, porous film-like polymer sheet, which is often thinner than other substrates made from conventionally formed fibers. "Fiber membrane" is meant to include membranes that appear to have fibrous characteristics at high magnification, e.g., 2000x, and this structure is preferably the result of stretching and elongating a film-like sheet during manufacture to form desired pores in the film structure. "Non-fibrous membrane" is intended to include microporous membranes and any other film-like membranes that have suitable porosity for use in battery separator applications.

[0012] In some embodiments, the substrate comprises a fibrous material of a thermoplastic polymer. In some particular embodiments, the substrate comprises polyolefin, polyester, nylon, polyacrylonitrile, aromatic polyamide, cellulose, polysulfone, or any blend thereof. In some other preferred embodiments, the substrate comprises cellulose; or a mixture of cellulose and thermoplastic fibers; in particular a mixture of cellulose and polyester fibers. In some embodiments, the substrate comprises nanofiber cellulose or cellulose nanofibers, i.e., cellulose fibers having a diameter of less than 1 micrometer.

[0013] The substrate itself preferably has a total thickness of from 1 to 500 micrometers. In some most preferred embodiments, the substrate has a total thickness of from 5 to 100 micrometers.

[0014] Many useful substrates typically have a basis weight of 3 to 50 grams per square meter. Basis weights below this range typically do not have sufficient strength for use in battery separator applications, while basis weights above this range typically are not required to achieve the desired performance of the final nonwoven sheet material in the battery separator. Useful substrates also preferably have a density ranging from 0.5 to 1.0 grams per cubic centimeter. Additionally, in some embodiments, the density of the substrate is greater than the overall density of the nonwoven sheet.

[0015] The nonwoven sheet material includes a substrate and a fibril coating applied onto the substrate, the applied fibril coating includes fibrils having a diameter of 1 to 5000 nanometers, a length of 0.2 to 3 millimeters, a specific surface area of ​​3 to 40 square meters per gram, and a Canadian Standard Freeness of 0 to 10 milliliters, the fibrils comprising an aramid polymer. In some embodiments, the fibrils comprise polyparaphenylene terephthalamide (PPD-T). In some embodiments, the fibrils comprise a polymer blend of 80 to 96 weight percent polyparaphenylene terephthalamide (PPD-T) and 4 to 20 weight percent polyvinylpyrrolidone (PVP).

[0016] "Applied fibril coating" means a layer of randomly arranged fibrils deposited on the surface of a substrate, the layer being substantially coextensive with the surface of the substrate. Figure 1 is a digital photograph taken at 100x magnification of a typical formation of a monolayer of applied fibril coating showing some potential characteristics and formation possibilities of an applied monolayer of fibrils when applied by spraying an aqueous slurry of fibrils onto a surface. The applied fibrils in this purposefully manufactured monolayer include fibril helices and inter-fibril entanglements.

[0017] As shown in Figure 1, this single layer has areas that are covered and other areas that are not, so a continuous layer of fibrils is usually sprayed onto the substrate in multiple passes to provide more coverage and to make the coverage and thickness of the applied coating of fibrils uniform. For example, Figures 2 and 3 are digital photographs of the same nonwoven sheet with a substrate and an applied fibril coating taken at 100x and 2000x magnification, respectively, where the applied fibril coating is produced by spraying onto the substrate using multiple passes or layers of applied fibrils. Also, Figures 4, 5 and 6 show how the applied fibril coating can be built up on the substrate surface. Figure 4 is a digital photograph taken at 1000x magnification of the substrate alone, which is a handsheet made from a blend of polyester fibers and nanocellulose. Figure 5 is a digital photograph taken at 1000x magnification of the substrate handsheet of Figure 4 with an even thinner applied fibril coating. FIG. 6 is a digital photograph taken at 1000x magnification of the substrate handsheet of FIG. 4 with a thicker applied fibril coating.

[0018] In a preferred embodiment, sufficient fibrils are applied and properly distributed on the substrate surface such that the surface of the substrate is substantially covered with fibrils without open areas visually detectable by the naked eye. In another preferred embodiment, the relative thickness of the applied fibril coating on the substrate surface varies within a 10% range (plus or minus) around a desired thickness value.

[0019] In a preferred embodiment, the applied fibril coating is binderless or binder-free. This means that the fibrils do not further contain additional binders. It is believed that due to their small size, the fibrils can entangle with both themselves and the substrate, attaching themselves to the gaps and / or pores of the substrate surface to form a coating that is attached to the substrate surface. Furthermore, in some embodiments, the fibrils can be made from polymer blends that are thermally stable but contain a small amount of a polymer with a lower melting point, which may become sticky when the fibrils are applied to a substrate if they exceed the glass transition temperature of that polymer. As a non-limiting example, the melting point of polyvinylpyrrolidone (PVP) is about 130° C., and 100° C. is higher than the glass transition temperature of this polymer. Thus, when fibrils comprising a polymer blend of 80-96 weight percent polyparaphenylene terephthalamide and 4-20 weight percent PVP are applied to a substrate surface at 100° C. or higher, it is believed that as the fibrils or surface are brought to temperature, at least a portion of the surface of the fibrils becomes tacky and is able to entangle with each other and with the surface, as well as locally adhere to both the fibrils and the surface. It is believed that this tackiness of the fibrils may facilitate the attachment of a fibril coating applied to a smoother substrate, such as that shown in FIGS. 7 and 8. FIG. 7 is a digital photograph taken at 1000× magnification of a substrate that is a polypropylene microporous film. FIG. 8 is a digital photograph taken at 1000× magnification of the substrate that is a polypropylene microporous film of FIG. 7 with a more thinly applied fibril coating. The applied fibril coating is bonded to the substrate by entanglement with the substrate surface or by localized attachment of the fibrils to the substrate surface, or both. As a result, the addition of an applied fibril coating to a substrate reduces the heat shrinkage of a nonwoven sheet material made containing both the substrate and the applied fibril coating compared to the heat shrinkage of the substrate alone.

[0020] In some embodiments, the applied fibril coating has a thickness of less than 1000 micrometers. Depending on the application, many thickness embodiments within this range may be desirable. In some cases, the increased temperature stability provided by a very thin thickness of applied fibril coating is sufficient for the application, but in some cases, such as specialized batteries for high temperature applications in the oil and gas industry, the requirement for stability at very high temperatures may require a thicker thickness of applied fibril coating. For example, in some embodiments, the applied fibril coating on the substrate has a preferred thickness of 35 to 1000 micrometers. In some other embodiments, the applied fibril coating on the substrate has a thickness of 1 to 200 micrometers, and in some embodiments, the applied fibril coating on the substrate has a more preferred thickness of 1 to 30 micrometers. In some other embodiments, the applied fibril coating on the substrate desirably has a thickness of 1 to 15 micrometers. In still other embodiments, the applied fibril coating has a thickness of 1 to 5 micrometers.

[0021] In some embodiments, the applied fibril coating preferably comprises 10-60 weight percent of the nonwoven sheet, based on the combined weight of the substrate and fibril coating in the nonwoven sheet. In some embodiments, the applied fibril coating preferably comprises 10-40 weight percent of the nonwoven sheet, based on the combined weight of the substrate and fibril coating in the nonwoven sheet. In yet other embodiments, the applied fibril coating comprises less than 50 weight percent of the nonwoven sheet, by weight. Also, in some embodiments, the density of the applied fibril coating is less than the overall density of the nonwoven sheet.

[0022] The term "fibrils" as used herein refers to hair-like fibrous material having a diameter of 1-5000 nanometers made from polymers or copolymers or polymer / copolymer blends. In some embodiments, the fibrils have a diameter of 1-1200 nanometers or 10-1200 nanometers, while in other embodiments, the fibrils have a diameter of less than 1000 nanometers. In some embodiments, the fibrils have a diameter of 500-2000 nanometers, and in yet some other embodiments, the fibrils have a diameter of 1000-1500 nanometers.

[0023] Preferably, the fibrils comprise aramid polymers. The term aramid as used herein means an aromatic polyamide in which at least 85% of the amide (-CONH-) bonds are directly attached to two aromatic rings. Optionally, additives may be used with the aramid, which may be dispersed throughout the polymer structure. It has been found that up to about 10 weight percent of other support materials can be blended with the aramid. It has also been found that copolymers having up to about 10 percent of other diamines replacing the diamines of the aramid or up to about 10 percent of other diacid chlorides replacing the diacid chlorides of the aramid can be used.

[0024] Preferred fibrils for the applied fibril coating include aramid polymer fibrils. The term "aramid polymer fibrils" as used herein refers to hair-like fibrous materials, preferably having a diameter of 1 to 2000 nanometers, preferably 10 to 1200 nanometers, including aramid polymers or polymer blends containing at least two polymers in which a majority of aramid polymers (greater than 50 weight percent) are present. Figure 9 is a representative digital photograph of an aramid polymer fibril. The aramid polymer fibrils further have a preferred length of 0.2 to 3 millimeters. The "length" of fibrous materials referred to herein, such as aramid polymer fibrils and pulp, is meant to be the measured "length-weighted average" length. In some preferred embodiments, the aramid polymer fibrils are beaten aramid polymer fibrils made from floc by subjecting the floc to a beating process that shears the floc into smaller aramid polymer fibrils. In some preferred embodiments, the aramid polymer fibrils have a length of 0.4 to 3 millimeters (mm), preferably 0.8 to 3 mm.

[0025] The diameter of the polymer fibrils is believed to affect the degree of adhesion of the fibrils to the substrate and the porosity of the coating on the substrate. Polymer fibrils having a diameter of more than 5000 nanometers are believed to have unacceptably low adhesion to the substrate and / or unacceptably high porosity. Also, polymer fibrils having a diameter of less than 1 nanometer or a length of less than about 0.2 millimeters are believed not to provide a coating with sufficient durability for the intended application, so it is desirable for the majority of the polymer fibrils to have a length of 0.2 millimeters or more.

[0026] Additionally, the polymer fibrils have an aspect ratio that may range from about 150 to 300,000. Aspect ratio is also known as length divided by diameter, and the phrases "aspect ratio", "average length-to-diameter ratio" and "length-to-diameter" are used interchangeably herein. In some embodiments, the average length-to-diameter ratio of the aramid polymer fibrils is greater than or equal to about 1000. In some embodiments, the aramid polymer fibrils have an average length-to-diameter ratio of less than or equal to about 3000. In some preferred embodiments, the average length-to-diameter ratio ranges from about 1000 to 3000. It is believed that the higher average length-to-diameter ratio of the aramid polymer fibrils contributes to greater strength and durability of the coating.

[0027] Because quantitative measurements such as size of certain fibrous materials, such as polymer fibrils, including aramid polymer fibrils, can be difficult, such fibrous materials can be compared by measuring the "freeness" of the fibrous materials. The inventors believe that Canadian Standard Freeness (CSF) is the preferred technique for characterizing the freeness of fibrils, such as the preferred aramid polymer fibrils discussed herein. Aramid polymer fibrils are preferably produced by beating aramid polymer fibers or flocks to produce fibrils. Such fibrils preferably have a CSF of 0-50 milliliters, and in some embodiments, a CSF of 0-20 milliliters. The CSF is one measure of the fineness of the aramid polymer fibrils or the extent to which they are fibrillated during beating, with very fine aramid polymer fibrils having a very low CSF. Materials with a wide size distribution generally have high CSF values, so a low CSF value also indicates uniformly sized fibrils.

[0028] Aramid polymer fibrils as defined herein are fibrous materials and are distinct from aramid polymer pulps of the prior art. Such aramid polymer pulps may be preferably produced by beating flocs or directly from the ingredients taught in U.S. Pat. Nos. 5,202,184, 5,523,034 and 5,532,034. However, such processes can provide both "stalks" and fibrils extending from the stalks, the stalks being roughly columnar remnants of the original aramid polymer flocs and having diameters of about 10 to 50 microns, as well as providing fibrous materials having a wider range of fiber sizes and lengths due to the difficulty of controlling such processes. Furthermore, in the case of aramid polymer pulps, the length measurements are understood to be characteristic lengths of the pulp's stalks, also referred to as "pulp stalks".

[0029] Also, the average length to diameter ratio of the aramid polymer fibrils is much greater than the average length to diameter ratio of conventional aramid polymer pulps, such as those made by the processes of U.S. Pat. Nos. 5,084,136, 5,171,402, and 8,211,272, which are believed to have average length to diameter ratios generally less than 150, or the average length to diameter ratio of highly beaten pulps, such as those disclosed in U.S. Patent Application Publication Nos. 2016 / 0362525 and 2017 / 0204258, is believed to be less than the average length to diameter ratio of conventional pulps (e.g., generally less than 100).

[0030] Additionally, the aramid polymer fibrils used in the applied fibril coating are aramid polymer fibrils that are essentially free of stalk or do not contain stalk. As used herein, the term "aramid polymer fibrils that do not contain stalk" means that at least 95% by weight of the fibrous material are aramid polymer fibrils having a desired diameter of 1 to 5000 nanometers as determined by optical measurement of a fibril sample using 500x or 1000x magnification. In some embodiments, at least 98% by weight of the fibrous material in the applied fibril coating are aramid polymer fibrils having a desired diameter of 1 to 5000 nanometers as determined by optical measurement of a fibril sample using 500x or 1000x magnification. In some embodiments, 100% by weight of the fibrous material are aramid polymer fibrils having a diameter of 1 to 2000 nanometers as determined by optical measurement of a fibril sample using 500x or 1000x magnification.

[0031] One preferred method of producing stalk-free aramid polymer fibrils is to beat fibers or flocs made from a polymer blend containing at least two polymers in which a majority of the aramid polymer (greater than 50 weight percent) is present. One preferred polymer blend is a polymer blend of 80-96 weight percent polyparaphenylene terephthalamide (PPD-T) and 4-20 weight percent polyvinylpyrrolidone (PVP). When aramid fibers or aramid flocs made from this PPD-T / PVP polymer blend are beaten, the resulting fibrous material is essentially all fibrils with essentially no larger stalks present in the material, as shown in the digital photograph of FIG. 9. It is believed that in a PPD-T / PVP polymer blend, at least 4 weight percent PVP must be present in the original fiber or floc in order for the fiber or floc to be beaten into fibrils with essentially no stalk remaining. This is in comparison to a conventional beaten aramid pulp made from polyparaphenylene terephthalamide (PPD-T) homopolymer shown in FIG. 10, which has a visible stalk.

[0032] The porosity and crystallinity of filaments made from blends of 80-96 weight percent PPD-T and 4-20 weight percent PVP have been found to be dramatically different from filaments consisting of PPD-T alone. As used herein, the term "fiber" is used interchangeably with the term "filament." Fibers spun directly onto bobbins from a polymer solution without cutting are commonly referred to as continuous fibers or filaments, and multifilament yarns contain multiple continuous filaments.

[0033] FIG. 11 illustrates the difference in X-ray scattering for the two types of filaments. Curve 20 represents a filament of the PPD-T / PVP blend, and curve 30 represents a filament made with PPD-T alone. Curve 30 illustrates that the PPD-T filament has a significant peak centered at about 2 Angstroms (and a much smaller peak centered at 4 Angstroms), indicating very small pores in the fiber. Curve 20 illustrates that the pore size distribution for the PPD-T / PVP blend is much broader, with a peak centered at about 3 Angstroms and a very broad sloping peak centered at about 250 Angstroms, but extending over a region ranging from about 70 to 600 Angstroms. This is believed to indicate that the filament made from the PPD-T / PVP blend has a significantly greater number of pores that are much larger than the PPD-T filament.

[0034] Moreover, due to this difference in the crystallinity and pore structure of the fibers, it is believed that mechanically beating the filaments results in much finer and more uniformly distributed fibrils, as illustrated in Figure 9. In other words, the very high crystallinity and low porosity of the PPD-T fibers means that when mechanically beated, the beating shear action primarily abrades the surface of the filaments to produce the typical fibrillated stalk structure (as shown in Figure 10), whereas the low crystallinity and high porosity of the PPD-T / PVP blend filaments would allow them to easily separate into individual beaten fibrils under the same shear action, with numerous fibrils of smaller and relatively more uniform diameter, and more importantly, essentially absent any stalks (i.e., no stalks). The aramid polymer fibrils are believed to have a relatively uniform diameter with a total diameter size range of about 300 nanometers, as visually measured from SEM micrographs.

[0035] Aramid polymer fibrils are preferably produced from aramid floc having PPD-T as the majority polymeric material component by weight and at least one other polymeric material component, which components are preferably mutually immiscible such that the at least two polymeric materials are present in the floc as well mixed but separate solid phases. Such aramid floc, when beaten, produces aramid polymer fibrils having domains of two different polymeric materials, one phase being a continuous or primary polymeric phase or PPD-T polymer and the other phase being a discontinuous or secondary polymeric phase, which in the preferred case is a PVP polymer.

[0036] The discontinuous or secondary polymer phase is believed to exist as small nanometer-sized crystalline domains of material that pass through the flocs and act as breaking points in the floc structure during the beating process, facilitating a faster and more complete beating of the flocs to form fibrils. After beating, a portion of the discontinuous or secondary polymer from each breaking point is present on or at the surface of each fibril resulting from the beating process.

[0037] Aramid polymer fibrils also have a high surface area. The terms "surface area," "specific surface area," and "BET surface area" are used interchangeably herein. Aramid polymer fibrils have a surface area of ​​about 3 to 40 m. 2 In some embodiments, the specific surface area is 6 m 2 / g or more, and in some embodiments the specific surface area is 8 m 2 / g or more. One particularly preferred range of the specific surface area is 6 to 20 m 2 / g.

[0038] In comparison, a floe made from a single polymeric material or a conventional pulp beaten from a miscible blend of polymeric materials that does not have discrete secondary polymer domains would not have such a high surface area. Moreover, if the floe is beaten sufficiently to have such a high measured surface area, the resulting pulp particles will have such a low aspect ratio (resulting from a very low average length) that they do not provide adequate strength and / or reinforcement.

[0039] The preferred aramid fibrils contain 80 to 96 weight percent poly(paraphenylene terephthalamide) (also known and used herein as polyparaphenylene terephthalamide or PPD-T). By PPD-T is meant the homopolymer resulting from the mole-for-mole polymerization of p-phenylenediamine and terephthaloyl chloride, and also the copolymer resulting from the incorporation of small amounts of other diamines and small amounts of other diacid chlorides along with p-phenylenediamine. Typically, the other diamines and other diacid chlorides can be used in amounts as high as up to about 10 mole percent of the p-phenylenediamine or terephthaloyl chloride, or perhaps slightly higher, provided only that the other diamines and diacid chlorides do not have reactive groups that will interfere with the polymerization reaction. PPD-T also refers to copolymers obtained from the incorporation of other aromatic diamines and other aromatic diacid chlorides, such as 2,6-naphthaloyl chloride or chloro- or dichloro-terephthaloyl chloride, provided only that the other aromatic diamines and aromatic diacid chlorides are present in amounts that allow the preparation of anisotropic spinning dopes. The preparation of PPD-T is described in U.S. Pat. Nos. 3,869,429, 4,308,374 and 4,698,414.

[0040] The preferred aramid fibrils also contain 4 to 20 weight percent poly(vinylpyrrolidone) (also known and used herein as polyvinylpyrrolidone or PVP). By PVP is meant a polymer resulting from the linear polymerization of monomer units of N-vinyl-2-pyrrolidone and containing small amounts of comonomers that may be present in concentrations below those that do not interfere with the interaction of the PVP with the PPD-T. PVPs with molecular weights ranging from about 5000 to about 1,000,000 can be used. Very high molecular weight PVPs produce high viscosity spin dopes. PVPs with molecular weights of about 10,000 to about 360,000 are preferred.

[0041] The nonwoven sheet material, including the substrate and applied fibril coating, preferably has a total thickness of 5 to 600 micrometers. In some embodiments, the nonwoven sheet has a total thickness of 10 to 150 micrometers. Applications in which the overall dimensions of the battery separator are critical may require a very thin nonwoven sheet material thickness, but in some cases, requirements for very high temperature stability or better mechanical properties may necessitate a greater overall nonwoven sheet material thickness.

[0042] In some embodiments, the nonwoven sheet material comprising a substrate and a fibril coating has a measured heat shrinkage of 20% or less when measured according to ASTM D2732-08 after exposure to 150° C. for at least 1 hour. Preferably, the nonwoven sheet material comprising a substrate and a fibril coating has a measured heat shrinkage of 20% or less when measured according to ASTM D2732-08 after exposure to 150° C. for an 8 hour period. Similarly, in some embodiments, the nonwoven sheet material comprising a substrate and a fibril coating has a measured heat shrinkage of 15% or less when measured according to ASTM D2732-08 after exposure to 150° C. for at least 1 hour, and preferably the nonwoven sheet material has that shrinkage after exposure to 150° C. for an 8 hour period. In some further embodiments where extreme dimensional stability is required, the nonwoven sheet material comprising a substrate and a fibril coating preferably has a measured heat shrinkage of 5% or less when measured according to ASTM D2732-08 after exposure to 150°C for at least 1 hour, and preferably the nonwoven sheet material has that low level of shrinkage after exposure to 150°C for a period of 8 hours.

[0043] These improved nonwoven sheet materials are useful as separator papers and in other applications in electrochemical cells, batteries and other electrical devices, such as capacitors. Not only can these improved nonwoven sheet materials provide improved thermal stability and reduced heat shrinkage at elevated temperatures and over extended periods of time, but it is believed that these nonwoven sheet materials can maintain the desired thermal requirements at elevated temperatures, such as 150-300° C., for at least 8 hours. It is further believed that this improved performance can improve battery safety and battery capacity, enable faster battery charging and higher battery energy density.

[0044] The present invention provides a process for producing a nonwoven sheet material comprising a substrate and a fibril coating applied onto said substrate, comprising: a) applying a layer of an aqueous slurry of fibrils onto a surface of a substrate, the substrate being a paper, a spunbond fiber sheet, or a fibrous or non-fibrous membrane, the fibrils having a diameter of 1-5000 nanometers, a length of 0.2-3 millimeters, a specific surface area of ​​3-40 square meters per gram, and a Canadian Standard Freeness of 0-10 milliliters, the fibrils comprising an aramid polymer; b) removing water from the aqueous slurry to form a fibril coating on the surface of the substrate; The present invention also relates to a process comprising:

[0045] In some embodiments, the fibrils comprise an aramid polymer that is polyparaphenylene terephthalamide (PPD-T). In some embodiments, the fibrils comprise a polymer blend of 80 to 96 weight percent polyparaphenylene terephthalamide (PPD-T) and 4 to 20 weight percent polyvinylpyrrolidone (PVP).

[0046] It is understood that all of the features, elements and embodiments of the nonwoven sheet material, substrate, fibrils, fibril coating applied onto said substrate, etc., described herein above also apply to the process without being repeated herein.

[0047] The applied fibril coating is preferably formed on the surface of the substrate by applying a layer of an aqueous slurry of fibrils onto the surface of the substrate. Preferably, the aqueous slurry is applied uniformly to the surface of the substrate. This means that the slurry layer on the substrate has a relatively uniform thickness and the fibrils are uniformly distributed on the surface of the substrate, forming a relatively uniform, thick layer of fibrils on the substrate.

[0048] One method of applying an aqueous layer to form a layer of fibrils of relatively uniform thickness is by spraying. This can be done by slurrying 0.1-2 weight percent of the fibrils in water or other solvent (such as alcohol) and applying the slurry to the surface of the substrate using a spraying device such as a paint sprayer. The desired thickness or uniformity and / or the desired applied fibril coating can be built or achieved by multiple passes using a single spraying device / nozzle or by use of a multi-nozzle spray coater. Other possible methods of forming an applied fibril coating on a substrate include dipping the substrate into a fibril slurry or other methods that essentially coat the surface of the substrate with one or more layers of fibrils to form an applied fibril coating.

[0049] If an aqueous slurry is used, then water is removed from the aqueous slurry to form the applied fibril coating on the surface of the substrate. Preferably, this is done by applying heat to evaporate the water from the slurry. In some embodiments, the heat is applied by an oven. For example, a nonwoven sheet material having a layer of the aqueous slurry containing fibrils on the substrate can be passed through an oven to evaporate the water, leaving a layer of fibrils that forms the applied fibril coating on the substrate. Other methods of removing water are possible, either separately or in combination with the application of heat. Such methods include squeezing the nonwoven sheet between nip rolls or applying a vacuum to the sheet.

[0050] Optionally, the process may further include a bonding step to produce a consolidated nonwoven sheet material, which may be accomplished by applying additional heat and pressure to the nonwoven sheet material, such as by advancing the sheet material between two or more calender rolls operating at a surface temperature of about 100-200° C. and a nip pressure of 500-1500 lbs / in.

[0051] Test Method In the examples provided below, the following test methods were used:

[0052] Thickness was measured according to ASTM D374-99 and is reported in mils and converted to micrometers.

[0053] Basis weight is measured in accordance with ASTM D 646-96 and is in g / m 2 Reported in units.

[0054] Gurley-Hill porosity of nonwoven sheet materials was measured in accordance with TAPPI T460 om-96 as the air resistance in seconds per 100 milliliters of cylinder displacement over a circular area of ​​approximately 6.4 square centimeters of paper using a pressure differential of 1.22 kPa.

[0055] The mean flow pore size of the nonwoven sheet material was measured in accordance with ASTM Designation E 1294-89 "Standard Test Method for Pore Size Characteristics of Membrane Filters Using Automated Liquid Porosimeter," which estimates the pore size characteristics of membranes having pore size diameters between 0.05 μm and 300 μm by using the automated bubble point method of ASTM Designation F 31603.

[0056] The bubble point of nonwoven sheet materials was measured in accordance with ASTM F 316-03(2011). The maximum pore size bubble point test is performed by pre-wetting the filter, increasing the gas pressure upstream of the filter at a predetermined rate, and monitoring the downstream gas bubbles to indicate passage of gas through the largest diameter filter pore. The pressure required to blow out the first open bubble detectable by its rise through the layer of liquid covering the filter is called the "bubble point" and is used to calculate the maximum pore size.

[0057] The ionic resistivity of the nonwoven sheet material is measured in accordance with ASTM D7148-13 and is in milliohm-cm 2 Reported in units.

[0058] The porosity of papers suitable for use as separators or insulators in electrochemical cells was measured according to ASTM C830-00 and reported in percent (%).

[0059] Heat shrinkage is a dimensionless number that determines the degree of linear shrinkage due to unlimited heat at a given specimen temperature, where the specimen consists of a 100 x 100 mm sample. A minimum of two specimens are required for each test at a given temperature, per ASTM D2732-08, and units are reported in percent (%). Although this standard requires that the specimens be exposed to elevated temperatures for at least one hour, the specimens here were tested at 150°C for eight hours.

[0060] The tensile strength of papers suitable for use as separators or insulators in electrochemical cells was measured according to ASTM D 828-97 using 2.54 cm wide specimens and 18 cm gauge length and reported in N / cm. Values ​​are reported as the average of five tests for each sample.

[0061] The Canadian Standard Freeness (CSF) of fibrils or pulp is measured according to standard test method TAPPI T227 using a Canadian Standard Freeness Tester Model 33-23 from Testing Machines Inc., New Castle, DE, which measures the ease with which water drains from an aqueous slurry or dispersion of fibrils / pulp and is inversely proportional to the degree of fibrillation of the pulp, since the greater the number of fibrils, the slower the rate at which water drains through the paper mat formed during the test. Data obtained from tests under standard conditions are expressed as milliliters of water drained from a slurry of 3 grams of pulp in 1 liter of water. Lower values ​​indicate that a more fibrillated pulp will hold more water and drain more slowly.

[0062] Fibril length ("length weighted average" length) was measured according to TAPPI test method T271. The "length weighted average" length of fibrils and / or pulps was measured using a Fiber Expert benchtop analyzer provided by Metso Automation Inc., Kajaani, Finland. The analyzer uses a digital CCD camera to take photographic images of the fibrous material dispersed in water to form a slurry as the slurry flows through the analyzer, and an integrated computer analyzes the fibers in these images to calculate the length, expressed in millimeters, as a weighted average. The "length weighted average" length of pulps was measured using a LS200 laser diffraction analyzer provided by Beckman Coulter Inc., Miami, FL, and is expressed in micrometers.

[0063] Average length-to-diameter ratio. This was calculated by dividing the "length-weighted average" length of the fibrils or pulps by their respective average visually measured diameters. The "length-weighted average" length is calculated according to the following formula:

number

[0064] The average "visually measured diameter" of the fibrils and / or pulps was obtained by visually measuring the width of individual fibril or pulp stalks at several points (at least three) along the length of the fibril or pulp stalk from a 500x or 1000x photomicrograph of the fibrils and / or pulps. This was done for at least 12 fibril or pulp stalks depicted in the photomicrograph, and the average visually measured fiber diameter was calculated.

[0065] The specific surface area of ​​the dry fibrous material (including fibrils) was measured by nitrogen adsorption / desorption at liquid nitrogen temperature (77.3 K) using a Micromeritics ASAP 2405 porosimeter and is expressed in square meters per gram (m 2 Unless otherwise stated, samples were degassed overnight at 150 °C before measurements to determine weight loss due to adsorbed moisture. Relative pressures of 0.05 to 0.20, P / P 0 Five-point nitrogen adsorption isotherms were collected over the range of 0.1 to 1.0 μm and analyzed according to the BET method (S. Brunauer, PH Emmett, and E. Teller, J. Am. Chem. Soc. 1938, 60, 309). P is the equilibrium gas pressure above the sample, and P 0 is the saturation gas pressure of the sample, which is typically greater than 760 Torr.

[0066] Wide-angle and small-angle X-ray scattering tests were used to measure crystallinity and porosity: Equipment: Rigaku Micromax 007 custom pinhole SAXS system or Advanced Photon Source DND-CAT (sector 5), line ID-D. X-ray source: For Rigaku instruments: rotating anode copper kα1 source. APS radiation energy is variable, but typically around 9 keV is used (1.38 Å). Detector: Bruker Vantec 2000 2048x2048 pixel 2D detector for Rigaku. A set of three MAR detectors are set up at the APS at wide, medium and small angle distances to simultaneously collect data. Distortion correction routines are employed in the Vantec 2000 data acquisition software to correct for spatial and intensity variations inherent to the detector. Mounting of the sample: WAXS: Straighten a length of yarn in collodion solution, cut a small piece and attach a monolayer to a sample plate. SAXS: Wrap the fiber 10 times around a slotted sample plate and secure it with tape. The plate has a hole in the center of the fiber bundle for X-ray transmission. Data Collection: Rigaku: Data is collected for 30 minutes per sample under vacuum, APS data collection is typically performed in air at 5 frames approximately every second. This is done twice, once with attenuator (for high intensity at low q) and once without attenuator. Data is stitched together at various distances / attenuations. EXAMPLES

[0067] Examples 1-1, 1-2, and 2-1 illustrate the preparation of papers suitable for use as electrolyte separators or thermal and / or flame retardants in electrochemical cells. All parts and percentages are by weight unless otherwise stated.

[0068] The aqueous fibril coatings used in the following inventive examples were prepared as follows: Polyvinylpyrrolidone (PVP) polymer sold under the name Sokalan® K30-P is obtained from BASF. Polyparaphenylene terephthalamide (PPD-T) polymer was prepared using the general polymerization procedure as generally disclosed in U.S. Pat. Nos. 3,869,429, 4,308,374, and 4,698,414. PPD-T polymer / PVP polymer blend fibers were prepared by forming separate polymer solutions and spun fibers according to the general procedure set forth in U.S. Patent Application Publication No. 2006 / 0113700 to Hartzler et al. The first solution was 19.5 weight percent PVP in sulfuric acid and was prepared by mixing the PVP in the sulfuric acid at room temperature. The second solution was 19.5 weight percent PPD-T in sulfuric acid as well. The PVP polymer solution was then combined with the PPD-T solution and mixed to form a spinning solution having a blend of polymers. This was done by injecting the PVP polymer solution into a pipe carrying the PPD-T polymer solution from the centerline, followed by mixing with a static mixer. This formed dispersed PVP polymer solution particles in a continuous PPD-T polymer solution phase. In these particular examples, the relative amounts of PVP and PPD-T were controlled to produce filaments having 87 weight percent PPD-T and 13 weight percent PVP.

[0069] The filaments were produced by extruding a spinning solution containing a blend of polymers through a spinneret having multiple spinneret holes to form dope filaments, which were subsequently coagulated. Specifically, the spinning solution was air-gap spun into a multifilament yarn by extruding the solution through a spinneret into an aqueous coagulation bath. The multifilament yarn was then washed and neutralized to remove the sulfuric acid solvent, dried, and wound onto a bobbin.

[0070] The yarn was cut into flocs and refined in a single-disk 12-inch Andritz laboratory refiner using an aqueous slurry containing 2 weight percent flocs. Three passes through the refiner were sufficient to form PPD-T / PVP fibrils, and this aqueous slurry was used as the aqueous fibril slurry for use in the examples of this invention. After only three passes through the refiner, the PPD-T / PVP blend flocs were fully fibrillated to fibrils with a Canadian Standard Freeness (CSF) of zero. Increasing the refining time, i.e., increasing the number of passes through the refiner, reduced the nanofibril length. In comparison, PPD-T pulp produced from three passes through the refiner had a CSF of approximately 300 ml.

[0071] Examples 1-1, 1-2 and Comparative Example A This example shows a nonwoven sheet material including a substrate and a fibril coating applied onto the substrate, which can be considered as a paper or fiber membrane. The substrate was manufactured using a wet-laid nonwoven (papermaking) process from cellulose nanofibers blended with polyester (PET) fibers. This substrate can simultaneously exhibit optimal properties in wettability, mechanical strength, heat resistance and electrochemical performance, and thus has been used as such in lithium titanate cathode applications and lithium ion battery separators for electric double layer capacitor applications.

[0072] The substrates were prepared as follows: Ultrafine PET fibers (0.03 dtex / 3 mm, obtained from Teijin Ltd.) were blended with nanocellulose (cellulose nanofibers-Tencel® fibers from Lenzing AG, Austria). The fibers were blended at a blend weight ratio of 50:50 and dispersed into an aqueous dispersion by a deflaker for 1 minute, after which handsheet nonwoven papers were prepared on a handsheet former (Messmer 255, America) using TAPPI method T205sp-02. Handsheets were prepared by mixing the aqueous dispersion with an additional 8 liters of water and pouring the mixture into a 21×21 cm handsheet mold to form a wet-laid sheet. Each handsheet was then removed, sandwiched between two pieces of blotting paper, hand-stretched with a cotton swab, and dried in a handsheet dryer at 150° C. for 10 minutes. In this example, the obtained substrate alone is considered as a control and is designated as Comparative Example A. Figure 4 shows a digital photograph of this substrate made from a blend of polyester fibers and nanocellulose, and the properties of this substrate are shown in Table 1.

[0073] For Inventive Example 1-1, the substrate was then coated with the fibril slurry to produce a nonwoven sheet material comprising a substrate and a fibril coating. The pre-prepared aqueous fibril slurry was further dispersed in water to a solids content of 0.65%. A layer of the slurry was sprayed evenly onto the surface of the substrate using a gravity-fed pneumatic spray gun (Central Pneumatic, part number 92126) to completely coat the surface of the substrate. The coated sample was dried in an oven at 100°C for 10 minutes.

[0074] For Inventive Example 1-2, the same process as in Example 1-1 was used, and the process was repeated with another sample of substrate and the same aqueous fibril slurry, but with a larger amount of aqueous fibril slurry sprayed onto the substrate to form a coated sample with a thicker coating on the substrate. The coating amounts and properties of the resulting nonwoven sheet materials including the substrate and fibril coating of Examples 1-1 and 1-2 are shown in Table 1. Figure 5 is a digital photograph of a handsheet of the substrate of Figure 4 made from a blend of polyester fibers and nanocellulose with a thinner applied fibril coating, and Figure 6 is a digital photograph of a handsheet of the substrate of Figure 4 made from a blend of polyester fibers and nanocellulose with a thicker applied fibril coating.

[0075] Example 2 and Comparative Example B The process of the previous example was repeated, but with a different substrate. The substrate was Celgard® R2400 polypropylene (PP) microporous membrane, also known as Celgard® R, a membrane made by dry process, with a thickness of about 25 micrometers, a porosity of about 33% and a pore size of about 0.07. This substrate was considered as the second control substrate, Comparative Example B. The properties of this substrate are shown in Table 1. Figure 7 is a digital photograph of this PP microporous membrane substrate.

[0076] For Inventive Example 2, the same process as in Example 1-1 was then repeated, using the same aqueous-based fibril slurry to coat a Celgard® substrate with the fibril slurry to produce a nonwoven sheet material comprising a substrate and a fibril coating applied thereon. The coating amount and properties of the resulting nonwoven sheet material comprising a substrate and a fibril coating of Example 2 are shown in Table 1. Figure 8 is a digital photograph of the polypropylene microporous film substrate of Figure 7 with a more thinly applied fibril coating.

[0077] [Table 1]

Claims

1. 1. A nonwoven sheet material comprising a substrate and a fibril coating applied over the substrate, the substrate is paper, a spunbond fiber sheet, or a fibrous or non-fibrous film; The applied fibril coating comprises: i) a diameter of 1 to 5000 nanometers; ii) a length of 0.2 to 3 millimeters; iii) a specific surface area of ​​3 to 40 square meters per gram; and iv) Canadian Standard Freeness of 0 to 10 milliliters and fibrils having The nonwoven sheet material, wherein the fibrils comprise an aramid polymer.

2. 1. A process for making a nonwoven sheet material comprising a substrate and a fibril coating applied over the substrate, the process comprising: a) applying a layer of an aqueous slurry of fibrils onto the surface of the substrate, the substrate being paper, a spunbond fiber sheet, or a fibrous or non-fibrous membrane, the fibrils being i) a diameter of 1 to 5000 nanometers; ii) a length of 0.2 to 3 millimeters; iii) a specific surface area of ​​3 to 40 square meters per gram; and iv) Canadian Standard Freeness of 0 to 10 milliliters wherein the fibrils comprise an aramid polymer; b) removing water from the aqueous slurry to form a fibril coating on the surface of the substrate; A process involving:

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