TECHNICAL FIELD

This disclosure generally relates to nonwovens that are bondable to fluorine -based materials and related methods of manufacture.

BACKGROUND

Nonwovens are used in a wide variety of industries and for a broad range of applications. They provide properties such as absorbency, liquid repellence, resilience, stretch, strength, flame retardancy, cushioning, thermal insulation, acoustic insulation, filtration, sterility and bacterial barrier properties. Nonwovens typically comprise a web or web laminate of individual fibers, filaments or threads that are directionally oriented or oriented in a random manner (i.e., without an identifiable pattern). Nonwovens may comprise many types of polymers, may be formed by any dry or wet method, may include many types of webs such as wet-laid webs, meltblown webs, spunbond webs, carded webs, air-laid webs and spunlaced webs, and composite webs comprising two or more nonwoven fabric layers, and may or may not be treated with a binder or other substance, depending on the desired properties for a particular application.

It is often useful to bond or laminate a nonwoven to another nonwoven or to other types of materials, such as fluorine-based materials, while preserving or improving the desirable properties of the nonwoven or other material. The term "fluorine-based materials," as used herein, means any material, such as a web, membrane, substrate, film or a nanofiber surface, comprised of a compound of fluorine, such as a fluoropolymer.

Materials comprising compounds of fluorine are used in a wide variety of commercial applications because they have high chemical and thermal stability.

For example, and without limitation, polyvinylidene fluoride (or PVDF) is a synthetic resin compound of fluorine in the fluoropolymer family produced by the polymerization of vinylidene fluoride (CH2=CF2). It is a tough plastic that is resistant to flame, electricity and attack by most chemicals. PVDF may be injection molded into corrosive-resistant bottles for the chemistry industry, or it may be extruded as a film for electrical insulation such as for use in lithium ion batteries. Fine powder grades of PVDF, such as KYNAPv 500® (from Arkema) or HYLAR® 5000 (from Solvay), are also used as the principal ingredient in high-end paints for metals, such as metal roofing. PVDF is not water-soluble; it is made soluble through use of chemical solvents, which are not safe to handle. Another example of a compound of fluorine is Polytetrafluoroethylene (or PTFE). PTFE is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. PTFE has one of the lowest coefficients of friction against any solid. The best known brand name of PTFE is DuPont TEFLON®.

Fluorine-based materials have, among other properties, non-stick properties (i.e., they are non-adhesive) that have proved useful in applications such as cookware, but provide poor or no bond strength when a fluorine-based material is bonded to another material, e.g., a nonwoven. Such a bond typically requires high heat, above 300°F

(149°C), and is not effective, often resulting in delamination of the fluorine-based material from the nonwoven. Binder fibers have been used to improve the bond strength between a nonwoven and a fluorine-based material, but binder fibers typically have low melting temperatures and may present processing difficulties such as sticking to the manufacturing equipment. In addition, if porosity is a desired property in the final application comprising a nonwoven and a fluorine-based material, binder fibers are troublesome because they melt and fill the pores in the nonwoven, thus reducing porosity.

There is a need for a nonwoven and a cost- and time-efficient method for making a nonwoven that is bondable to a fluorine-based material through the application of less heat and is also safe to handle, without adversely impacting desired properties of the nonwoven or the fluorine-based material, such as porosity, strength and dimensional and chemical stability.

SUMMARY

The foregoing purposes, as well as others that will be apparent, are achieved by providing a nonwoven fabric that is bondable to a fluorine-based material. The nonwoven fabric comprises a nonwoven web and an aqueous fluoropolymer emulsion comprising a fluoro component and a non-fluoro binding component. The aqueous fluoropolymer emulsion is applied to the nonwoven web to provide bonding sites on the surface of and/or within the nonwoven fabric that are compatible for bonding to the fluorine-based material.

The presence of the fluoro component on or near the surface of the nonwoven fabric enables the nonwoven fabric to bond well with fluorine-based materials. In preferred embodiments, the bond strength between the nonwoven fabric and the fluorine- based material is at least 25 grams, and preferably greater than about 45 grams. The aqueous fluoropolymer emulsion improves the bond strength between the nonwoven fabric and the fluorine-based material, preserves or improves desired strength and porosity properties of the nonwoven web, and preserves or improves desired properties of the fluorine-based material.

In an exemplary embodiment, before application of the aqueous fluoropolymer emulsion, a nonwoven web has a machine direction tensile strength of at least 1.3 N/cm, a thickness of about 14-50 microns, preferably 14-25 microns, a porosity of at least 50% and substantially no bond strength when bonded or laminated to a fluorine-based material. After application of the aqueous fluoropolymer emulsion, the strength properties of the nonwoven web are substantially improved - the nonwoven fabric has a machine direction tensile strength of at least 2 N/cm, and preferably greater than about 4 N/cm, and the bond strength between the nonwoven fabric and a fluorine-based material is greater than about 25 grams, and preferably greater than about 45 grams, while maintaining porosity greater than about 50%. The aqueous fluoropolymer emulsion has a dry add-on weight of about 2%> to 50%), and preferably about 5% to 35%, of the total weight of the nonwoven fabric (after application of the emulsion) and comprises, on polymer solids, 25-75%), and preferably 25-35%, by weight fluoro component. The fluoro component is preferably PVDF, but may comprise other compounds of fluorine. The aqueous fluoropolymer further comprises, on polymer solids, 25-75%) by weight non-fluoro binding component, preferably acrylic resin.

A method is provided for making the nonwoven fabric comprising the steps of providing a nonwoven web as described above and applying an aqueous fluoropolymer emulsion as described above to the nonwoven web to provide bonding sites on the surface of and/or within the nonwoven web that are compatible for bonding to a fluorine-based material. In an exemplary embodiment of the method, the aqueous fluoropolymer emulsion is applied to a nonwoven web in a dry add-on weight of about 2% to 50%>, preferably, 5% to 35% of the total weight of the nonwoven fabric by any known application method including, for example, size press, spraying, coating, foaming or saturation. After application of the aqueous fluoropolymer emulsion, the nonwoven fabric is dried by any known drying method including, for example, steam drying cans or thru-air dryer. If the nonwoven web is too thick for a desired application, the nonwoven web may be subjected to calendering prior to or after application of the aqueous fluoropolymer emulsion. Additional polymers, materials, and methods may be utilized to impart the nonwoven fabric with other properties.

The nonwoven fabric disclosed herein may be used for any application where it is desired to laminate a nonwoven to a fluorine-based material. One such application is a nonwoven reinforcement for a fluorine-based microporous polymer membrane comprising a fluorine-based material that may be used as a battery separator. The nonwoven reinforcement comprises the nonwoven fabric described above with bonding sites on the surface of and/or within the nonwoven fabric that are compatible for bonding to the fluorine-based microporous polymer membrane with a bond strength of at least 25 grams, and preferably 45 grams. The resulting battery separator comprises a fluorine-based material with beneficial separator properties, such as chemical stability and porosity, laminated to a nonwoven reinforcement, which provides strength and dimensional stability to the fluorine-based material without adversely impacting chemical stability and porosity of the fluorine-based material.

Other exemplary applications include adhering fluoro-containing nanofibers to a filter media to improve the filter's efficiency or using the nonwoven fabric as a support for a fluorinated membrane used for protein or nucleic acid separation and immobilization (Western Blot). Other objects, features and advantages will be apparent when the detailed description is considered in conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A is a micrograph of the top surface of an exemplary nonwoven web for use in manufacturing the nonwoven fabric prior to application of an aqueous fluoropolymer emulsion with 100 times magnification.

FIG. IB is a micrograph of the top surface of a nonwoven fabric comprising the nonwoven web shown in FIG. 1 A after application of an aqueous fluoropolymer emulsion with 100 times magnification.

FIGS. 1C and ID are micrographs of the top surfaces of additional nonwoven webs after application of an aqueous fluoropolymer emulsion with 640 times magnification.

FIG. 2 is an illustration of an exemplary apparatus that may be used for

manufacturing a nonwoven fabric in accordance with the disclosure.

FIG. 3 is a flowchart showing the steps in a preferred method of manufacturing a nonwoven fabric in accordance with the disclosure. FIG. 4 is an illustration of a battery separator comprising a nonwoven fabric in accordance with this disclosure and a separator membrane.

FIG. 5 is an illustration of an alternative battery separator comprising a nonwoven fabric in accordance with this disclosure and two separator membranes.

FIG. 6 is an illustration of an exemplary apparatus that may be used for laminating a nonwoven fabric in accordance with this disclosure to one or more separator membranes to form the battery separators shown in FIG. 4 or FIG. 5.

Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

An exemplary nonwoven fabric and a method of manufacturing a nonwoven fabric that permits bonding between the nonwoven fabric and a fluorine-based material without compromising desirable properties of the nonwoven fabric or the fluorine-based material are described herein. Referring to FIGS. 1A to ID, the nonwoven fabric 20 comprises a nonwoven web 10 and an aqueous fluoropolymer emulsion 11 applied to the nonwoven web 10 to provide bonding sites 15 on the surface of and/or within the nonwoven web 10 that are compatible for bonding, i.e., improving cohesion between, the nonwoven fabric 20 and the fluorine-based material. FIG. 1 A shows an exemplary nonwoven web 10 that may be used in manufacturing the nonwoven fabric 20 prior to application of the aqueous fluoropolymer emulsion 11. The exemplary nonwoven web 10 has an open structure comprising large open spaces or pores 12 located between randomly-oriented fibers 14 of the nonwoven web 10. As shown in FIGS. IB-ID, after application of the aqueous fluoropolymer emulsion 11, the fibers 14 of the nonwoven web 10 are coated with the aqueous fluoropolymer emulsion 11 at the surface of and within the nonwoven web 10, but the large open pores 12 are not closed or clogged, i.e., large open pores 12 are preserved in the resulting nonwoven fabric 20.

Depending on the desired properties for a particular application of the nonwoven fabric 20, the nonwoven web 10 used to make the nonwoven fabric 20 may comprise any type of web or web laminate (2 or more webs) of individual fibers, filaments or threads that are directionally oriented or oriented in a random manner (i.e., without an identifiable pattern), formed by any dry or wet method such as wet-laid webs, meltblown webs, spunbond webs, carded webs, air-laid webs, spunlaced webs and webs that have been extruded then oriented, may comprise any type of polymer, and may or may not be treated with a binder or other substance. In preferred embodiments, the nonwoven fabric 20 comprises a nonwoven web 10 that would have difficulty bonding to a fluorine-based material, such as nonwoven webs 10 comprising fibers, filaments or threads made from polyesters (e.g., polyethylene terephthalate, or PET), polyolefms (e.g. polyethylene and polypropylene), polyamides, polyurethanes, ceramic or glass, or cellulose, rayon, lyocell or other fibers.

One example of a nonwoven web 10 that may be used to manufacture the nonwoven fabric 20 is a 100% polyester wet-laid nonwoven, having a basis weight of about 5 grams per square meter, a thickness of about 18 microns, a tensile strength (MD) of about 1.3 N/cm and a porosity of about 80%. The nonwoven web may have a uniform thickness, dimensional stability, high heat resistance, high chemical resistance and adequate strength depending on the desired end-use. Other nonwoven webs 10 that may be used in manufacturing the nonwoven fabric 20 include, but are not limited to, 100% polyester nonwovens having high porosity, high strength and high chemical resistance such as those available from JX Nippon ANCI Inc., Kennesaw, Georgia, U.S.A. under the trademark MILIFE®. For example, nonwoven webs having a basis weight of 8-10 grams per square meter, a thickness of 50-70 microns, and tensile strength of about 5 N/cm in machine direction and about 2-3.6 N/cm in the cross direction. These nonwoven webs have very open and strong structures, and have low thermal shrinkage. Nonwoven webs comprising other polymers and properties, and made by other processes, may also be used depending on the desired application of the nonwoven fabric.

Nonwoven webs 10 typically do not bond well by themselves to a fluorine-based material, such as a fluorine-based material comprising PVDF or PTFE. Therefore, the benefits of bonding a nonwoven to a fluorine -based material have not yet been achieved.

In accordance with this disclosure, the nonwoven web 10 is modified via application of an aqueous fluoropolymer emulsion 11 comprising a fluorine-based chemistry to provide bonding sites on the surface of and/or within the nonwoven web 10 for bonding the nonwoven web to a fluorine-based material without adversely impacting the desirable properties of nonwoven web or the fluorine-based material. The aqueous fluoropolymer emulsion comprises at least two components; one is a fluoro component (i.e., a component with a fluorine-based chemistry) and the second is a non-fluoro binding component. The presence of the fluoro component on the surface of and/or within the nonwoven fabric 20 enables it to bond well with fluorine-based materials upon the application of minimal amounts of heat, e.g., less than about 300°F (149°C).

As used herein, the term "aqueous" shall have its ordinary meaning in the context of chemical solutions, i.e., it is made from, with, or by water. Thus, the combination of the fluoro component and the non-fluoro binding component in the fluoropolymer emulsion is dispersible in water. The aqueous fluoropolymer emulsion 11 is preferably non-film forming, i.e., it does not form a continuous film, to maintain the porosity of the nonwoven web 10 above an acceptable level for particular applications.

The fluoro component of the aqueous fluoropolymer emulsion may be a fluoropolymer, a modified fluoropolymer or a fluorinated-copolymer. Examples include polyvinylidene fluoride, fluoroethylene vinyl ether, as well as other fluorinated polymers. A modified fluoropolymer is a fluoropolymer that has been modified either chemically or physically.

The non-fluoro binding component of the aqueous fluoropolymer emulsion may be a component separate from the fluoropolymer, a modification to the fluoropolymer such that the modification has binding properties, or as an alternating component with the fluoro component within the fluoropolymer. It should be water soluble and capable of forming an emulsion with a fluoropolymer. The non-fluoro binding component may comprise acrylics, acrylic copolymers, styrenated acrylic, vinyls, vinyl copolymers, vinyl acetate, vinyl ethers, vinyl acrylics, ethylene vinyl acetate, urethanes, butadiene copolymers, styrene butadiene, polyvinyl chloride, ethylene vinyl chloride, butadiene vinyl chloride, butadiene acrylonitrile and polyvinyl alcohol. For example, a non-fluoro binding component comprising acrylic may be used in the aqueous fluoropolymer emulsion 11 to enhance adhesion of the emulsion to nonwoven webs 10 comprising polyester.

The fluoro component and the non-fluoro component of the aqueous fluoropolymer emulsion may comprise distinct polymers, i.e., they can be made from different polymers, or the fluoro component and the non-fluoro component may be made from the same copolymer.

Preferred aqueous fluoropolymer emulsions 11 comprise, on polymer solids, 25- 75%, preferably 25-35%, by weight the fluoro component, and further comprise, on polymer solids, 25-75%) by weight the non-fluoro binding component.

An example of a suitable aqueous fluoropolymer emulsion 11 comprises a fluoro component of PVDF and a non-fluoro binding component of acrylic resin in a 50:50 ratio, having the following physical properties: about 46% solids (by weight); pH of about 8.2; and a wet density of about 1.147 g/niL. Another example of a suitable aqueous

fluoropolymer emulsion 11 has a 70:30 PVDF/acrylic resin ratio, is about 44% solids (by weight) and has a wet density of about 1.178 g/mL. Fluoropolymer/acrylic resins such as these are primarily used as paint coatings for outdoor uses, but the inventors found these resins to be surprisingly useful as an adhesive for bonding nonwovens to fluorine-based materials.

The aqueous fluoropolymer emulsion 11 is applied to the nonwoven web 10 to form a treated nonwoven fabric 20 in the range of about 2%-50% dry add-on weight, preferably about 5%-35%, which does not negatively impact the porosity or strength of the nonwoven web. Thus, 2-50%, preferably 5-35%, of the basis weight of the treated nonwoven fabric 20 is the dry weight of the aqueous fluoropolymer emulsion 11. For example, a nonwoven web weighing about 5 grams per square meter prior to application of the aqueous fluoropolymer emulsion 11 and about 6.4 grams per square meter after application of the aqueous fluoropolymer emulsion 11 would have about a 20% dry weight add-on. The add-on percentage depends on several factors including the type of nonwoven web. The basis weight of the nonwoven fabric may be in the range of about 5 to 200 grams per square meter, preferably 5 to 12 grams per square meter.

TABLE I shows averages for the strength, porosity and other properties of an exemplary 100% polyester wet-laid nonwoven web 10 prior to application of the aqueous fluoropolymer emulsion 11 (FIG. 1A) and the treated nonwoven fabric 20 (FIGS. IB-ID) after application of the aqueous fluoropolymer emulsion 11, at about 22% add-on.

TABLE I

Physical Properties

Properties Units Test Method Before After

Application Application

Basis Weight g/m ² TAPPI T410; 5.1 6.4

TAPPI T402

Thickness Microns TAPPI T411 14 21

MD Tensile Strength N/cm TAPPI T494 2 4.7

CD Trapezoidal Tear g ASTM D5587-08 31.5 71.4

Strength

Porosity % ASTM D6226 80 80

Bond Strength g ASTM F904-98 1.82 49.78

(to PVDF Membrane)

Bond Strength g ASTM F904-98 0.00 25.14

(to PTFE Membrane)

As shown in TABLE I, application of aqueous fluoropolymer emulsion 11 to nonwoven web 10 substantially improves the bond strength or cohesion of the nonwoven web 10 to a fluorine-based material (such as PVDF and PTFE) and the strength (MD tensile) of the nonwoven web 10 while preserving acceptable porosity levels in the nonwoven fabric 20.

The foregoing properties were measured using standardized test methods established either by ASTM International (formerly known as the American Society for

Testing and Materials)(ASTM) or the Technical Association of the Pulp and Paper Industry (TAPPI) as identified in TABLE I. The ASTM F904-98 bond strength test method is a comparison of bond strength or ply adhesion of similar laminates made from flexible materials and was measured using a Zwick Model Z2.5/TN1S tensile tester. The TAPPI T494 tensile strength test method measures the load required to break a nonwoven (with no break or tear introduced). The ASTM D5587-08 trapezoidal tear strength test method measures the amount of force required to propagate a tear that is purposely introduced to a fabric. Porosity may be measured according to ASTM D6226 using a gas pycnometer, for example, a Quantachrome Micro-Ultrapyc 1200e. The ASTM D6226 porosity test method measures the accessible (open) cellular volume of a material and the remaining volume is that occupied by closed cells and cell walls and results in a percentage of open volume as compared to total volume.

In other embodiments, the nonwoven web 10 prior to application of the aqueous fluoropolymer emulsion 11 has the following properties: thickness of about 10-45 microns, machine direction tensile strength of at least 1.37 N/cm and a porosity of at least 50%. After application of the aqueous fluoropolymer emulsion 11, the strength properties of the treated nonwoven fabric 20 are substantially improved - a machine direction tensile strength of at least 2 N/cm, preferably greater than 4 N/cm, and the bond strength between the nonwoven fabric 20 and a fluorine-based material is about 25 grams while maintaining porosity at greater than about 50%. The thickness of the treated nonwoven fabric 20 after application of the aqueous fluoropolymer emulsion 11 may also be in the range of 10-50 microns, preferably 10-45 microns, and more preferably 15-30 microns.

The aqueous fluoropolymer emulsion 11 may be applied to the nonwoven web 10 by any known method provided the emulsion is applied at an add-on level that does not impact other desired physical and chemical properties of the nonwoven web 10. An exemplary processing apparatus 30 that may be used to manufacture the nonwoven fabric 20 is shown in FIG. 2. The apparatus 30 shown in FIG. 2 is for processing an untreated nonwoven web 10 that is pre-manufactured and placed on an unwind stand 32 for further processing. The untreated nonwoven web 10 is unwound and advanced through an optional calender section 34 or other means for reducing the thickness of the nonwoven web 10 if desired for a particular application. If the thickness of the untreated nonwoven web 10 is acceptable, calendering is not required. The calender section 34 may have single or multiple nip configurations (a single nip 36 configuration is shown in FIG. 2).

Calendering may be done on-line or off-line. For example, a calender may apply heat up to about 375°F (190°C) and pressure in the range of about 350 - 1,500 pounds per square inch (16.7 - 71.8 kPa) at a line speed of about 35 - 90 feet per minute (10.7-27.4 m/m) depending on the polymer of the nonwoven web, the initial thickness, the desired thickness and without changing other properties of the nonwoven web.

The aqueous fluoropolymer emulsion 11 may be applied to the nonwoven web 10 by any known solution application method provided the fibers 14 in the nonwoven web 10 are coated with (or surrounded by) the aqueous fluoropolymer emulsion without forming a continuous film on the surface of the nonwoven web 10. Examples of suitable application methods include size press, spraying, coating, foaming and saturation. The apparatus in FIG. 2 shows a size press 38 having a top steel roll 40 and a rubber bottom roll 42 that is approximately half-way immersed into a tray 44 filled with the aqueous fluoropolymer emulsion 11. The top steel roll 40 and the rubber bottom roll 42 form a size press nip 50. As the rubber bottom roll 42 rotates, it picks up the emulsion 11 and presses it into the nonwoven web 10 as it passes through the size press nip 50.

It is often desirable for the nonwoven web 10 to be completely saturated before entering the size press nip 50. For that purpose, an optional adjustable-height dip roller 46 may be placed at an entry end 48 of the size press 38. The adjustable-height dip roller 46 guides the nonwoven web 10 into the aqueous fluoropolymer emulsion 11 in the tray 44, the height of the roller 46 determining how much of the aqueous fluoropolymer emulsion 11 is applied, if any, to the nonwoven web 10 prior to entry into the size press nip 50. The adjustable-height dip roller 46 also assists in ensuring that the nonwoven web is completely wet across its entire width.

The pressure applied in the size press nip 50, and the speed at which the nonwoven web 10 is advanced through the size press nip 50, is generally dependent on the desired add-on percentage. In preferred embodiments, the aqueous fluoropolymer emulsion 11 is applied to the nonwoven web 10 to a dry add-on weight percent of about 2%-50%, preferably about 5%-35%, which does not negatively impact the porosity or strength of the nonwoven web. Generally, the nonwoven web 10 travels through the size press 38 at a speed of greater than or equal to about 40 feet per minute. Application by size press or other application methods permits the aqueous fluoropolymer emulsion 11 to penetrate into the nonwoven web 10 to increase the strength of the nonwoven web and to remain on the surface of the nonwoven web 10 to increase the bond strength to fluorine-based materials.

After the aqueous fluoropolymer emulsion 11 is applied to the nonwoven web 10, moisture may then be removed from the treated nonwoven fabric 20 by advancing it through a drying section 52. The drying section 52 may comprise any means involving heat to drive moisture away from the nonwoven fabric 20 and permit the aqueous fluoropolymer emulsion 11 to coat the fibers in the nonwoven web 10, such as through-air dryers, infrared lamps or steam heated cans. FIG. 2 shows an exemplary steam heat dryer with dryer cans 54. The number and configuration of dryer cans 54 used for the drying method, i.e., the web path through the dryer cans 54, may readily be changed in accordance with known drying methods. The dryer cans 54 may be heated to a temperature up to about 260°F (127°C). The treated nonwoven fabric 20 may advance through the dryer cans 54 at a speed of about 40-50 feet per minute (12-15 m/m). Guide rolls 55 may be provided throughout the apparatus to guide the advancing nonwoven web 10 and treated nonwoven fabric 20 through the apparatus 30. The number of cans, temperatures and pressures may be adjusted depending on the level of moisture in the treated nonwoven fabric 20, or the thickness and composition of the treated nonwoven fabric 20.

The treated nonwoven fabric 20 may then be rolled onto a wind-up reel 56 for storage and/or delivery to a customer, or for transportation to another facility for further processing.

In an alternative embodiment, the apparatus 30 may easily be incorporated into or combined with nonwoven manufacturing equipment, such that the aqueous fluoropolymer emulsion is applied in-line with the nonwoven manufacturing equipment. If the nonwoven web is wet, such as it may be when in-line with the manufacturing equipment, drying means may be used to substantially dry the nonwoven web 10 prior to application of the aqueous fluoropolymer emulsion 11 or the aqueous fluoropolymer emulsion 11 may be applied to a wet nonwoven web.

FIG. 3 is a flowchart showing the steps in a preferred method 100 of manufacturing a nonwoven fabric 20. In step 110, an untreated nonwoven web 10 is provided as set forth above. In step 120, an aqueous fluoropolymer emulsion is applied to the nonwoven web 10 as described above.

Example Applications for the Nonwoven Fabric 20

The nonwoven fabric 20 and methods for making a nonwoven fabric disclosed herein may be used in many different industries for applications in which it is desired to bond a nonwoven to a fluorine-based material. For example, in industrial air filtration applications, the method may be used to make nonwoven reinforcement substrates comprising polyester, microglass and/or cellulose webs with improved adhesion to fluoropolymer-based nanolayers. PVDF membranes used for protein immobilization that need additional structural support could also benefit from lamination to a nonwoven fabric treated with an aqueous fluoropolymer emulsion.

In another example regarding rechargeable battery applications, referring to FIGS. 4 and 5, a nonwoven reinforcement 60 (or structural support) comprised of the nonwoven fabric 20 described herein may be bonded or laminated to, and provide tensile strength to, one or more microporous polymer membranes 62 to form a battery separator 80 without adversely affecting desirable porosity and chemical stability properties of the microporous polymer membrane(s) 62.

Such nonwoven reinforcements 60 are applicable to any type of electrolytic cell for use in lithium-ion (Li-ion) batteries or other electrochemical applications such as lithium ion ultra-capacitors, where a permeable separator membrane, such as a microporous polymer membrane, is positioned between positive and negative electrodes of the electrolytic (or electrochemical) cell. In some cases, such a permeable separator membrane may also be laminated onto the surfaces of the electrodes for improved electrode contact and ease of cell assembly. Permeable separator membranes prevent electrical short circuits that would result if the positive and negative electrodes come in contact with each other, and have pores filled with liquid electrolyte that enable ions to be transported through the permeable separator membrane (from one electrode to the other) to close the

electrochemical circuit when current flows in the cell. The pores in the permeable separator membrane should be small enough to prevent particles larger than the ions (such as particulates from the electrodes) from moving between the electrodes, and strong enough to limit penetration of the permeable separator membrane by possible debris or dendritic growth from the electrodes.

Permeable separator membranes for use in battery applications generally comprise a polymeric membrane forming a microporous layer that is chemically and

electrochemically stable with regard to the electrolyte and the electrode materials under strongly reactive environments when the battery is fully charged. Permeable separator membranes should also be mechanically strong enough to withstand high tension during battery construction, and thin enough to facilitate the battery's energy and power densities, but not too thin to compromise mechanical strength and safety.

Many types of materials have been utilized to make permeable separator membranes for use in batteries including, for example, nonwoven sheets, webs or matts of directionally or randomly oriented fibers, polymer films, or polyvinyl chloride. Many commercially-available permeable separator membranes are polyolefm based materials, such as polyethylene (PE) and polypropylene (PP), formed into micro-porous films.

Polyvinylidene fluoride (PVDF) polymers may also be used to form permeable separator membranes and have advantages over polyolefms due to their high oxidation resistance, which is important as battery manufacturers push to higher and higher voltages. Permeable separator membranes comprising PVDF are also more thermally stable than their polyolefm counterparts, but microporous PVDF permeable separator membranes by themselves lack the tensile strength necessary for the manufacture of many kinds of commercial cells. A description of microporous PVDF polymer films and methods of producing them are shown in U.S. Patent No. 8,147,732 (assigned to Porous Power Technologies) and examples of permeable separator membranes produced with such films containing ceramic fillers are shown in U.S. Publication Nos. 2012/0232178A1,

2012/0228214A1 and 2012/0228792A1 (all assigned to Porous Power Technologies). The disclosures of the '732 patent and the three publications are incorporated herein by reference thereto. Other fluoropolymers that may be used as microporous battery separator membranes include PVF, PFA, FEP, FFPM/FFKM and PTFE.

To improve the tensile strength and robustness of permeable separator membranes comprising fluoropolymers, such as PVDF, structural supports may be laminated to them. Structural supports may be 100% polyester (PET) wet-laid nonwovens, or may also comprise other polymers, such as polypropylene, polyethylene, polyesters, polyamides, polyimides and polyvinyl alcohols, glass and cellulose, and other types of webs, provided they are relatively stable, have a uniform thickness, sufficient porosity, inert in the particular electrolyte being used in the electrolytic cell and are wettable with the particular electrolyte. However, such structural supports do not bond well with permeable separator membranes comprising PVDF and may fall apart either during production or use of the electrolytic cell due to their low bonding strengths. Binder fibers, such as polyethylene (PE) based or CoPolyethylene Terephthalate (CoPET) based binder fibers, have been used to bond PVDF permeable separator membranes to the structural supports, but binder fibers have shown insufficient bonding strength and/or low melting points and may stick to the manufacturing machine. In some cases, binder fibers may also be

chemically/electrochemically incompatible with electrolytes causing reduced functionality when cycled in a battery cell environment.

For battery separator applications, it is preferable that a structural support is thin, strong, porous and suitable for use in electrolytic cells, i.e., it should not interfere with the electrochemistry of the electrolytic cell. For example, it should have good solvent resistance, i.e., the structural support should not react with the solvent, so that it is chemically and electrochemically stable with regard to the electrolyte and the electrode materials under strongly reactive environments in the electrolytic cell. The structural support should also have good thermal resistance so it does not degrade, curl, fray, tear or delaminate due to extreme temperature variations that are experienced in an electrolytic cell. Ideally, the structural support should not melt or dissolve when soaked in the electrolyte during normal battery operating temperatures up to 80°C.

In addition to its electrochemical neutrality, the structural support should be mechanically strong enough to withstand high tension during battery construction and the expansion and contraction of the electrodes during charge cycles. The structural support should also be thin enough to facilitate the battery's energy and power densities, but not too thin to compromise mechanical strength and safety, and open enough to not significantly affect the ionic flow through the permeable separator membrane. The structural support should also be thermally stable when dried (i.e., during the nonwoven and battery manufacturing process) up to temperatures of at least 130°C, and preferably higher, to permit removal of moisture in the final separator construction before filling with electrolyte. Moisture degrades the electrolyte and therefore the cell performance.

Finally, the bonding strength between the structural support and a permeable separator membrane 62 should also be and remain sufficient to prevent the structural support and permeable separator membrane from coming apart during battery cell operation as this would lead to an increase in separator resistance or impedance.

The treated nonwoven fabric 20 disclosed herein satisfies the desired strength, thickness, porosity and other preferred physical properties for a structural support for any fluorine-based microporous separator membrane for battery cell applications. In accordance with this disclosure, a strong and chemically stable battery separator 80 is formed by laminating a nonwoven reinforcement 60 comprised of the treated nonwoven fabric 20 to one or more microporous polymer membranes 62 comprising PVDF (i.e., a microporous polymer membrane 62 may be laminated to one side of the nonwoven reinforcement 60 as shown in FIG. 4 or laminated to both sides of the nonwoven reinforcement 60 as shown in FIG. 5).

The battery separator 80 may be formed using any thermal lamination process that provides sufficient heat and pressure to laminate the nonwoven reinforcement 60 to one or more microporous polymer membranes 62 without adversely impacting desired physical and chemical properties of the battery separator 80. An exemplary processing apparatus that may be used for lamination is shown in FIG. 6. A roll of nonwoven reinforcement 60 and one or more rolls of microporous polymer membrane 62 may be unwound and advanced through a calendar nip formed by a rubber roller 64 and a steel roller 66 that applies heat at temperatures of 100°C to 165°C and pressure of 100 to 700 pounds (445- 3114 N). If only one layer of microporous polymer membrane 62 is desired, then only one roll of microporous polymer membrane 62 would be unwound and advanced through the calendar nip. The battery separator 80 may then be rolled onto wind-up reel 68.

The weight of the nonwoven reinforcement 60 is not critical provided the nonwoven reinforcement 60 meets the desired strength, porosity and thickness parameters. In general, the weight of the nonwoven reinforcement 60 may be in the range of about 5- 200 grams per square meter, preferably 5-12 grams per square meter; the thickness of the nonwoven reinforcement 60 may be in the range of 10-50 microns, preferably 15-50 microns, and more preferably 15-30 microns; the porosity is greater than about 45%, preferably greater than about 50%, and more preferably greater than about 60%; and the MD tensile strength is greater than about 2 N/cm, preferably greater than about 4 N/cm.

For nonwoven reinforcements used as structural support for microporous battery separator membranes, it is also important to have uniform porosity and thickness across the entire surface of the nonwoven reinforcement. An acceptable coefficient of variation for the thickness of the nonwoven reinforcement should be less than 10%, preferably less than 5%. The coefficient of variation (CV%) is the standard deviation divided by the mean multiplied by 100. For certain "high end" battery applications, referred to as high rate, high capacity applications, it is desired that the nonwoven reinforcement 60 has a thickness of about 20 microns or less prior to application of the aqueous fluoropolymer emulsion.

The microporous polymer membranes 62 may be filled with ceramic fillers or not filled with ceramic fillers. Although thickness is not a critical factor, the microporous polymer membranes 62 may have a thickness of about 8-10 microns. Thickness is measured in accordance with ASTM D5947. The microporous polymer membranes 62 may also have a basis weight of about 4-5 grams per square meter, a pore size of less than about 1.1 microns (measured in accordance with ASTM F316) and a porosity of about 75%.

In an exemplary embodiment, two layers of ceramic filled, PVDF microporous polymer membrane 62 (as disclosed in U.S. Publication Nos. 2012/0232178A1,

2012/0228214A1 and 2012/0228792A1) having a thickness of about 8 microns, a basis weight of about 4.5 gsm, a pore size of less than 1.1 microns and porosity of about 75% were laminated to a nonwoven reinforcement 60 comprising 5 gsm 100% polyester wet- laid nonwoven (as described above). The resulting battery separator 80 had the properties set forth in TABLE II. TABLE II

The MacMullin number is defined as the ratio of the resistance of the electrolyte- filled separator to the resistance of an equivalent volume of the electrolyte alone, as described in U.S. Patent No. 4,464,238. Resistance is measured by Electrochemical Impedance Spectroscopy, described below. In other embodiments, the battery separator 80 has the following properties: thickness between about 15 microns and 50 microns, preferably 15-30 microns, a MacMullin number between about 4 and 15, preferably 4-1 1 , porosity between about 30% and 80%, preferably greater than 45%>, more preferably greater than 50% and still more preferably greater than 60%?, pore size less than 0.8 microns, preferably less than 0.5 microns, and Gurley air permeability of about 20 to 100 s. High temperature melt integrity may be evaluated using Differential Scanning Calorimetry (DSC). In DSC analysis, the heat flow into a sample and a reference material is measured as a function of time and temperature. Phase changes, such as glass transitions (T _g ) and melting points (T _m ), in the material being analyzed are observed when more or less heat is required at a given temperature as compared to the reference material.

The electrochemical stability of the nonwoven reinforcement 60 comprised of the treated nonwoven fabric 20 within a battery separator 80 with a PVDF microporous polymer membrane 62 has been investigated by Electrochemical Impedance Spectroscopy (EIS) and results are shown in Table III. Electrochemical compatibility of any component of a battery cell may be tested experimentally by the component's contribution to the complex electrochemical impedance of the battery cell in which it is placed. EIS may be used to measure such impedance before and after cell cycling, as described by Andrzej Lasia in Modern Aspects of Electrochemistry Volume 32 (2002), pages 143-248, Chapter title: "Electrochemical Impedance Spectroscopy and its Applications." In addition to indicating electrochemical impedance of the cell, this technique also determines, in situ, the efficacy of the bond strength between the PVDF microporous membrane 62 and the nonwoven reinforcement 60 by also indicating the Ohmic interfacial impedance. A large increase in the Ohmic interfacial impedance of this contribution as compared to the overall impedance indicates delamination of the microporous polymer membrane 62 from the nonwoven reinforcement 60.

Table III below shows that a battery cell comprising battery separator 80 has substantially the same increase in Ohmic impedance as compared to a typical monolayer polypropylene battery separator (Celgard 2500) as a control, and therefore maintains the integrity of the battery cell. Table III details measurements of the Ohmic impedance at 1 kHz of substantially identical battery cells, except for the battery separators. EIS was performed before and after 1000 charge/discharge cycles of the cell as described in the Lasia article using a Gamry Series G 300 Potentiostat/Galvanostat/ZRA. TABLE III

Electrochemical Impedance Spectroscopy

Ohmic Impedance at 1 Battery Separator 80: Control Battery kHz (Ω) PVDF microporous Separator: Monolayer

membranes 62 with polypropylene

nonwoven

reinforcement 60

Before Cell Cycling 0.485 0.479

After -1000 cycles 0.658 0.65

Increase in Ohmic 0.173 0.171 Impedance (after cycling)

The above disclosure, embodiments and examples are illustrative only and should not be interpreted as limiting. Modifications and other embodiments will be apparent to those skilled in the art, and all such modifications and other embodiments are intended to be within the scope of the present invention as defined by the claims.