Field of the Invention

Provided are nonwoven fabrics. The provided nonwoven fabrics may be used in thermal and acoustic insulators in automotive and aerospace applications such as battery compartments for electric vehicles. The provided nonwoven fabrics can be particularly suitable for reducing noise in automotive and aerospace applications.

Background

Thermal insulators reduce heat transfer between structures either in thermal contact with each other or within range of thermal convection or radiation. These materials mitigate the effects of conduction, convection, and/or radiation, and can thus help in stabilizing the temperature of a structure in proximity to another structure at significantly higher or lower temperatures. By preventing overheating of a component or avoiding heat loss where high temperatures are desired, thermal management can be critical in achieving the function and performance demanded in widespread commercial and industrial applications.

Thermal insulators can be particularly useful in the automotive and aerospace technologies. For example, internal combustion engines of automobiles produce a tremendous amount of heat during their combustion cycle. In other areas of the vehicle, thermal insulation is used to protect electronic components sensitive to heat. Such components can include, for example, sensors, batteries, and electrical motors. To maximize fuel economy, it is desirable for thermal insulation solutions to be as thin and lightweight as possible while adequately protecting these components. Ideally, these materials are durable enough to last the lifetime of the vehicle.

Historically, developments in automotive and aerospace technology have been driven by consumer demands for faster, safer, quieter, and more spacious vehicles. These attributes must be counterbalanced against the desire for fuel economy, since enhancements to these consumer-driven attributes generally also increase the weight of the vehicle.

With a 10% weight reduction in the vehicle capable of providing about an 8% increase in fuel efficiency, automotive and aerospace manufacturers have a great incentive to decrease vehicle weight while meeting existing performance targets. Yet, as vehicular structures become lighter, noise can become increasingly problematic. Some noise is borne from structural vibrations, which generate sound energy that propagates and transmits to the air, generating airborne noise. Structural vibration is conventionally controlled using damping materials made with heavy, viscous materials. Airborne noise is conventionally controlled using a soft, pliable material, such as a fiber or foam, capable of absorbing sound energy.

The demand for suitable insulating materials has intensified with the advent of electric vehicles (“EVs”). EVs employ lithium ion batteries that perform optimally within a defined temperature range, more particularly around ambient temperatures. EVs generally have a battery management system that activates an electrical heater if the battery temperature drops significantly below optimal temperatures and activates a cooling system when the battery temperature creeps significantly higher than optimal temperatures.

Summary

Operations used for heating and cooling EV batteries can substantially deplete battery power that would otherwise have been directed to the vehicle drivetrain. Just as a blanket provides comfort by conserving a person’s body heat in cold weather, thermal insulation passively minimizes the power required to protect the EV batteries in extreme temperatures.

Developers of insulation materials for EV battery applications face formidable technical challenges. For instance, EV battery insulation materials should display low thermal conductivity while satisfying strict flame retardant requirements to extinguish or slow the spread of a battery fire. A common test for flame retardancy is the UL-94V0 flame test. It is also desirable for a suitable thermal insulator to resiliently flex and compress such that it can be easily inserted into irregularly shaped enclosures and expand to occupy fully the space around it. Finally, these materials should display sufficient mechanical strength and tear resistance to facilitate handling and installation in a manufacturing process such that there are no loose fibers or fiber shedding.

The provided nonwoven fabrics and methods address these problems by using nonwoven fabrics. The nonwoven fabrics are flame resistant and minimize fiber shedding. The reinforcing fibers can at least partially melt when heated to form a bonded web with enhanced strength. The edges of the nonwoven fabrics do not need to be sealed by heat and pressure or other means. The provided nonwoven fabrics can also have a low flow resistance rendering the nonwoven fabrics as better acoustic insulators

In a first aspect, a nonwoven fabric is provided. The nonwoven fabric includes a plurality of randomly-oriented fibers, the plurality of randomly-oriented fibers comprising: at least 60 wt% of oxidized polyacrylonitrile fibers; and from 0 to less than 40 wt% of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100°C to 450°C; and a fluoropolymer binder on the plurality of randomly-oriented fibers; wherein the plurality of randomly-oriented fibers is bonded together to form the nonwoven fabric, optionally wherein the nonwoven fabric has a thickness of one millimeter or less.

In a second aspect, a method of making a nonwoven fabric is provided. The method includes mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers, wherein the plurality of reinforcing fibers have outer fiber surfaces comprised of a (co)polymer with a melting temperature between 100°C and 450°C; heating the mixture of randomly-oriented fibers to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers; and applying a fluoropolymer binder to the mixture of randomly-oriented fibers, whereby the mixture of randomly-oriented fibers is bonded together to form the nonwoven fabric, optionally wherein the non-woven fabric has a thickness of one millimeter or less. Brief Description of the Drawings

As provided herein:

FIGS. 1-2 are side cross-sectional views of nonwoven fabric according to various exemplary embodiments.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. Drawings may not be to scale.

DEFINITIONS

As used herein:

“Ambient conditions” means at 25°C and 101.3 kPa pressure.

“Average” means number average, unless otherwise specified.

“Copolymer” refers to polymers made from repeat units of two or more different polymers and includes random, block and star (e.g. dendritic) copolymers.

“Median fiber diameter” of fibers in a nonwoven fabric is determined by producing one or more images of the fiber structure, such as by using a scanning electron microscope; measuring the transverse dimension of clearly visible fibers in the one or more images resulting in a total number of fiber diameters; and calculating the median fiber diameter based on that total number of fiber diameters.

“Polymer” means a relatively high molecular weight material having a molecular weight of at least 10,000 g/mol.

“Size” refers to the longest dimension of a given object or surface.

“Substantially” means to a significant degree, as in an amount of at least 30%, 40, 50, 60, 70, 80, 90, 95, 96, 97, 98, 99, 99.5, 99.9, 99.99, or 99.999%, or 100%.

“Thickness” means the distance between opposing sides of a layer or multilayered article.

As used herein, the terms“preferred” and“preferably” refer to embodiments described herein that can afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

As used herein and in the appended claims, the singular forms“a,”“an,” and“the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or“the” component may include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term“and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

It is noted that the term“comprises” and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover,“a,”“an,”“the,”“at least one,” and“one or more” are used interchangeably herein. Relative terms such as left, right, forward, rearward, top, bottom, side, upper, lower, horizontal, vertical, and the like may be used herein and, if so, are from the perspective observed in the particular drawing. These terms are used only to simplify the description, however, and not to limit the scope of the invention in any way.

Reference throughout this specification to“one embodiment,”“certain embodiments,”“one or more embodiments” or“an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as“in one or more embodiments,”“in certain embodiments,”“in one embodiment” or“in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Where applicable, trade designations are set out in all uppercase letters.

A nonwoven fabric according to one embodiment of the invention is illustrated in FIG. 1 and hereinafter referred to by the numeral 100. The nonwoven fabric 100 includes having opposed first and second major surfaces 104, 106.

The nonwoven fabric 100 is comprised of a plurality of randomly-oriented fibers, including oxidized polyacrylonitrile fibers 108. Oxidized polyacrylonitrile fibers 108 include those available under the trade designations PYRON (Zoltek Corporation, Bridgeton, MO) and PANOX (SGL Group, Meitingen, GERMANY).

The oxidized polyacrylonitrile fibers 108 preferably have a fiber diameter and length that enables fiber entanglements within the nonwoven fabric. The fibers, however, are preferably not so thin that web strength is unduly compromised. The fibers 108 can have a median fiber diameter of from 2 micrometers to 150 micrometers, from 5 micrometers to 100 micrometers, from 5 micrometers to 25 micrometers, or in some embodiments, less than, equal to, or greater than 1 micrometer, 2, 3, 5, 7, 10, 15, 20, 25, 30, 40, 50 micrometers.

Inclusion of long fibers can reduce fiber shedding and further enhance the strength of the nonwoven fabric along transverse directions. The oxidized polyacrylonitrile fibers 108 can have a median fiber length of from 10 millimeters to 100 millimeters, from 15 millimeters to 100 millimeters, from 25 millimeters to 75 millimeters, or in some embodiments, less than, equal to, or greater than 10 millimeters, 12, 15, 17, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75 millimeters.

The oxidized polyacrylonitrile fibers 108 used to form the nonwoven fabric 100 can be prepared from bulk fibers. The bulk fibers can be placed on the inlet conveyor belt of an opening/mixing machine in which they can be teased apart and mixed by rotating combs. The fibers are then blown into web-forming equipment where they are formed into a dry-laid nonwoven fabric.

As an alternative, a SPIKE air-laying forming apparatus (commercially available from FormFiber NV, Denmark) can be used to prepare nonwoven fabric containing these bulk fibers. Details of the SPIKE apparatus and methods of using the SPIKE apparatus in forming air-laid webs is described in U.S. Patent Nos. 7,491,354 (Andersen) and 6,808,664 (Falk et ak).

Bulk fibers can be fed into a split pre-opening and blending chamber with two rotating spike rollers with a conveyor belt. Thereafter, the bulk fibers are fed into the top of the forming chamber with a blower. The fibrous materials can be opened and fluffed in the top of the chamber and then fell through the upper rows of spikes rollers to the bottom of the forming chamber passing thereby the lower rows of spike rollers. The materials can then be pulled down on a porous endless belt/wire by a combination of gravity and vacuum applied to the forming chamber from the lower end of the porous forming belt/wire.

Alternatively, the nonwoven fabric 100 can be formed in an air-laid machine. The web-forming equipment may, for example, be a RANDO-WEBBER device commercially-available from Rando Machine Co., Macedon, NY. Alternatively, the web-forming equipment could be one that produces a dry-laid web by carding and cross-lapping, rather than by air-laying. The cross-lapping can be horizontal (for example, using a PROFILE SERIES cross-lapper commercially-available from ASSELIN-THIBEAU of Elbeuf sur Seine, 76504 France) or vertical (for example, using the STRUTO system from the University of Liberec, Czech Republic or the WAVE-MAKER system from Santex AG of Switzerland).

As indicated by the color contrast in FIG. 1, the nonwoven fabric includes a fluoropolymer binder on the plurality of randomly-oriented fibers, for example, on the oxidized polyacrylonitrile fibers 108. The fluoropolymers on the plurality of randomly -oriented fibers can self-bond so that fluoropolymers can confine the fibers and substantially reduce fiber shedding. In addition, the fluoropolymers enable the nonwoven fabric to have an emissivity of less than 0.5. Here,“emissivity” is defined as the ratio of the energy radiated from a material's surface to that radiated from a blackbody (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions. Reducing emissivity helps lower the extent to which a material loses heat from thermal radiation. The fluoropolymer binder used in the current application can include, but not limited to, THV (a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride) or Tetrafluoroethylene (TFE Teflon), Hexafluoropropylene (HFP), and Vinylidene fluoride (VDF). Fluoropolymer binder can include fluorinated binders, such as fluorinated acrylate.

The fluoropolymer binder can be applied to the plurality of randomly-oriented fibers by any suitable means, for example, coating. Fluoropolymers of the nonwoven fabric 100 can impart various functional and/or aesthetic benefits. For example, fluoropolymers on the fibers have the effect of reinforcing the fibers, thus increasing the overall strength of the web. Fluoropolymers may also enhance resistance to staining or fouling caused by airborne substances becoming adhered to fiber surfaces. In some embodiments, the nonwoven fabric of the current application has a low flow resistance, for example, less than 1000 Rayls, 100 Rayls, 50 Rayls, 30 Rayls, 25 Rayls, 20 Rayls, 15 Rayls, 10 Rayls. Low flow resistance can render the nonwoven fabric as better acoustic insulators, so that sound can penetrate through the OPAN fabric and will be absorbed by the nonwoven core layer underneath the OPAN fabric. Flow resistance may be changed by the amount of the fluoropolymer binder. Increasing the amount of the fluoropolymer binder can provide higher flow resistance and decreasing the amount of the fluoropolymer binder can provide lower flow resistance. In some embodiments, the nonwoven fabric of the current application has a high flow resistance, for examples higher than 100 Rayls, 1000 Rayls, or 10,000 Rayls. High flow resistance can render the nonwoven fabric better for thermal insulation, since such high flow resistance help to block the air flow conduction.

In some embodiments, the nonwoven fabric 100 can include entangled regions 110 fully or partially extending between the first major surface 104 and the second major surface 106. The entangled regions 110 represent places where two or more discrete fibers 108 have become twisted together. The fibers within these entangled regions 110, although not physically attached, are so intertwined that they resist separation when pulled in opposing directions. With the presence of the entangled regions 110, the plurality of fibers in the nonwoven fabric 100 are substantially entangled along directions perpendicular to the first and second major surfaces 104, 106.

In some embodiments, the entanglements are induced by a needle tacking process or hydroentangling process. Each of these processes are described in more detail below.

The nonwoven fabric can be needle tacked using a conventional needle tacking apparatus (e.g., a needle tacker commercially available under the trade designation DILO from Dilo of Germany, with barbed needles (commercially available, for example, from Foster Needle Company, Inc., of Manitowoc, WI) whereby the substantially entangled fibers described above are needle tacked fibers. Needle tacking, also referred to as needle punching, entangles the fibers perpendicular to the major surface of the nonwoven fabric by repeatedly passing an array of barbed needles through the web and retracting them while pulling along fibers of the web.

The needle tacking process parameters, which include the type(s) of needles used, penetration depth, and stroke speed, are not particularly restricted. Further, the optimum number of needle tacks per area of mat will vary depending on the application. Typically, the nonwoven fabric is needle tacked to provide an average of at least 5 needle tacks/cm ² . Preferably, the mat is needle tacked to provide an average of about 5 to 60 needle tacks/cm ² , more preferably, an average of about 10 to about 20 needle tacks/cm ² .

Further options and advantages associated with needle tacking are described elsewhere, for example in U.S. Patent Publication Nos. 2006/0141918 (Rienke) and 2011/0111163 (Bozouklian et ak).

The nonwoven fabric can be hydroentangled using a conventional water entangling unit (commercially available from Honeycomb Systems Inc. of Bidderford, Me.; also see U.S. Patent No. 4,880,168 (Randall, Jr.), the disclosure of which is incorporated herein by reference for its teaching of fiber entanglement). Although the preferred liquid to use with the hydroentangler is water, other suitable liquids may be used with or in place of the water.

In a water entanglement process, a pressurized liquid such as water is delivered in a curtain-like array onto a nonwoven fabric, which passes beneath the liquid streams. The mat or web is supported by a wire screen, which acts as a conveyor belt. The mat feeds into the entangling unit on the wire screen conveyor beneath the jet orifices. The wire screen is selected depending upon the final desired appearance of the entangled mat. A coarse screen can produce a mat having perforations corresponding to the holes in the screen, while a very fine screen (e.g., 100 mesh) can produce a mat without the noticeable perforations.

FIG. 2 shows a nonwoven fabric 200 which, like nonwoven fabric 100, has opposed first and second major surfaces 204, 206. The nonwoven fabric 200 differs from that of the prior example in that it includes both a plurality of oxidized polyacrylonitrile fibers 208 and a plurality of reinforcing fibers 216. As indicated by the color contrast in FIG. 2, the nonwoven fabric includes a fluoropolymer binder on the plurality of randomly-oriented fibers, for example, on the oxidized polyacrylonitrile fibers 208 and reinforcing fibers 216.

The reinforcing fibers 216 may include binder fibers, which have a sufficiently low melting temperature to allow subsequent melt processing of the nonwoven fabric 200. Binder fibers are generally polymeric, and may have uniform composition or contain two or more components. In some embodiments, the binder fibers are bi-component fibers comprised of a core polymer that extends along the axis of the fibers and is surrounded by a cylindrical shell polymer. The shell polymer can have a melting temperature less than that of the core polymer. The reinforcing fibers can include at least one of monocomponent or multi- component fibers. In some embodiments, the reinforcing fiber can include polyethylene terephthalate, polyphenylene sulfide, poly-aramide, polylactic acid. In some embodiments, the reinforcing fibers can be multicomponent fibers having an outer shealth comprising polyolefin. In some embodiments, the polyolefin can be selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, polyethylene naphthalate, and combinations thereof.

As used herein, however,“melting” refers to a gradual transformation of the fibers or, in the case of a bi-component shell/core fiber, an outer surface of the fiber, at elevated temperatures at which the polyester becomes sufficiently soft and tacky to bond to other fibers with which it comes into contact, including oxidized polyacrylonitrile fibers and any other binder fibers having its same characteristics and, as described above, which may have a higher or lower melting temperature.

Useful binder fibers have outer surfaces comprised of a polymer having a melting temperature of from 100°C to 450°C, or in some embodiments, less than, equal to, or greater than, 100°C, 105, 110, 115,

120, 125, 130, 135, 140, 145, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300 325, 350, 375, 400, 425°C.

Exemplary binder fibers include, for example, a bi-component fiber with a polyethylene terephthalate core and a copolyester sheath. The sheath component melting temperature is approximately 230°F (110°C). The binder fibers can also be a polyethylene terephthalate homopolymer or copolymer rather than a bi-component fiber.

The binder fibers increase structural integrity in the insulator 200 by creating a three-dimensional array of nodes where constituent fibers are physically attached to each other. These nodes provide a macroscopic fiber network, which increases tear strength, tensile modulus, preserves dimensional stability of the end product, and minimizes fiber shedding. Advantageously, incorporation of binder fibers can allow bulk density to be reduced while preserving structural integrity of the nonwoven fabric, which in turn decreases both weight and thermal conductivity.

In exemplary embodiments, the nonwoven fabric 100, 200 has a basis weight of from 10 gsm to 100 gsm, 15 gsm to 50 gsm, 20 gsm to 30 gsm, or in some embodiments less than, equal to, or greater than 10 gsm, 16, 17, 18, 19, 20, 22, 24, 25, 26, 28, 30, 32, 35, 37, 40, 42, 45, 47, 50, 60, 70, 80, 90, 100 gsm.

In exemplary embodiments, the nonwoven fabric 100, 200 has an average bulk density of from 100 kg/m ³ to 1500 kg/m ³ , 150 kg/m ³ to 1000 kg/m ³ , 200 kg/m ³ to 500 kg/m ³ , or in some embodiments less than, equal to, or greater than 100 kg/m ³ , 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800,

850, 900, 950, 1000, 1100, 1200, 1300, 1400, or 1500 kg/m ³ .

The oxidized polyacrylonitrile fibers 208 in the nonwoven fabric 200 are not combustible. Surprisingly, it was found that combustion of the reinforcing fibers in the FAR 25-856a flame test did not result in significant dimensional changes (no shrinkage and no expansion) in the nonwoven fabric. The nonwoven fabric can pass the UL-94V0 flame test. This benefit appears to have been the effect of the fiber entanglements perpendicular to the major surface of the nonwoven fabric.

The oxidized polyacrylonitrile fibers 208 can be present in any amount sufficient to provide adequate flame retardancy and insulating properties to the nonwoven fabric 200. The oxidized polyacrylonitrile fibers 208 can be present in an amount of from 60 wt% to 100 wt%, 70 wt% to 100 wt%, 81 wt% to 100 wt%, or in some embodiments, less than, equal to, or greater than 50 wt%, 55, 60, 65, 70, 75, 80, 85, 90, or 95 wt%, or less than or equal to 100 wt%. The reinforcing fibers 216 can be present in an amount of from 0 wt% to less than 40 wt%, 3 wt% to 30 wt%, 0 wt% to 19 wt%, 3 wt% to 19 wt%, or in some embodiments, equal to or greater than 0 wt%, or less than, equal to, or greater than 1 wt%, 2, 3, 4, 5, 7, 10, 15, 20, 25, 30, 35, or 40 wt%.

Preferred weight ratios of the oxidized polyacrylonitrile fibers 208 to reinforcing fibers 216 bestow both high tensile strength to tear resistance to the nonwoven fabric 200 as well as acceptable flame retardancy— for instance, the ability to pass the UL-94V0 flame test. The weight ratio of oxidized polyacrylonitrile fibers to reinforcing fibers can be at least 4: 1, at least 5: 1, at least 10: 1, or in some embodiments, less than, equal to, or greater than 4: 1, 5: 1, 6: 1, 7: 1, 8: 1, 9: 1, or 10: 1.

Optionally and as shown in the figures, the oxidized polyacrylonitrile fibers 108, 208 and reinforcing fibers 116, 216 are each crimped to provide a crimped configuration (e.g., a zigzag, sinusoidal, or helical shape). Alternatively, some or all of the oxidized polyacrylonitrile fibers 108, 208 and reinforcing fibers 116, 216 have a linear configuration. The fraction of oxidized polyacrylonitrile fibers 108, 208 and/or reinforcing fibers 116, 216 that are crimped can be less than, equal to, or greater than 5%, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, or 100%. Crimping, which is described in more detail in European Patent No. 0 714 248, can significantly increase the bulk, or volume per unit weight, of the non-woven fibrous web.

Other aspects of the nonwoven fabric 200 are analogous to those described already with respect to nonwoven fabric 100 and shall not be repeated here.

The nonwoven fabrics of the thermal insulators described with respect to FIGS. 1-2 can have any suitable thickness based on the space allocated for the application at hand. For common applications, the nonwoven fabrics can have a thickness of less than 1 millimeter or 0.5 millimeters.

As described previously, many factors influence the mechanical properties displayed by the nonwoven fabric, including fiber dimensions, the presence of binding sites on the reinforcing fibers, fiber entanglements, and overall bulk density. Tensile strength and tensile modulus are metrics by which the properties of the nonwoven fabric may be characterized.

Tensile strength represents the resistance of the nonwoven fabric to tearing or permanently distorting and can be at least 28 kPa, at least 32 kPa, at least 35 kPa, or in some embodiments, less than, equal to, or greater than 28 kPa, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42, 44, 45, 47, or 50 kPa.

Surprisingly, it was found that entangling the fibers of the nonwoven fabric perpendicular to the major surfaces of the web to produce a material having a bulk density in the range of from 15 kg/m ³ to 500 kg/m ³ solved a technical problem associated with volumetric expansion in the UF-94V0 or FAR 25-856a flame test. Specifically, it was discovered that while conventional oxidized polyacrylonitrile materials were observed to swell substantially after flame testing, the provided thermal insulators do not. In some embodiments, the provided nonwoven fabrics deviate less than 10%, less than 7%, less than 5%, less than 4%, or less than 3%, or in some embodiments, less than, equal to, or greater than 10%, 9, 8, 7, 6, 5, 4, or 3% in thickness after flame testing, relative to its original dimensions.

The nonwoven fabric 100, 200 may optionally include additional layers not explicitly shown in FIGS. 1-2. To assist in installation, for example, any of these exemplary thermal insulators may further include an adhesive layer, such as a pressure -sensitive adhesive layer or other attachment layer extending across and contacting the nonwoven fabric. As another possibility, any of these insulators may include a solid thermal barrier such as an aluminum sheet or foil layer adjacent to the nonwoven fabric. For some applications, one or more acoustically insulating layers may also be coupled to the nonwoven fabric.

The nonwoven fabric can be made by mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers as described in the commonly owned PCT Patent Publication No. WO 2015/080913 (Zillig et al). The mixture of randomly-oriented fibers is then heated to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers. The fluoropolymer binder can be applied to the mixture of randomly-oriented fibers. As a result, the mixture of randomly-oriented fibers can be bonded together to form the nonwoven fabric. In some embodiments, the major surface of the non-woven fabric can be smoothed. The smoothed surfaces may be obtained by any known method. For example, smoothing could be achieved by calendaring the non-woven fibrous web, heating the non-woven fibrous web, and/or applying tension to the non-woven fibrous web. In some embodiments, the smoothed surfaces are skin layers produced by partial melting of the fibers at the exposed surfaces of the non-woven fibrous web.

In some embodiments, there may be a density gradient at the smoothed surface. For example, portions of the smoothed surface proximate to the exposed major surface may have a density greater than portions remote from the exposed major surface. Increasing bulk density at one or both of the smoothed surfaces can further enhance tensile strength and tear resistance of the non-woven fibrous web. The smoothing of the surface can also reduce the extent of fiber shedding which would otherwise occur in handling or transporting the non-woven fabric. Still another benefit is the reduction in thermal convection by impeding the passage of air through the non-woven fibrous web. The one or both smoothed surfaces may, in some embodiments, be non-porous such that air is prevented from flowing through the non-woven fabric.

While not intended to be exhaustive, a list of exemplary embodiments is provided as follows:

1. A nonwoven fabric comprising a plurality of randomly -oriented fibers, the plurality of randomly- oriented fibers comprising: at least 60 wt% of oxidized polyacrylonitrile fibers; and from 0 to less than 40 wt% of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100°C to 450°C; and a fluoropolymer binder on the plurality of randomly-oriented fibers; wherein the plurality of randomly-oriented fibers is bonded together to form the nonwoven fabric, optionally wherein the nonwoven fabric has a thickness of one millimeter or less.

2. The nonwoven fabric of embodiment 1, wherein the reinforcing fibers comprise at least one of monocomponent or multi-component fibers.

3. The nonwoven fabric of embodiment 2, wherein the reinforcing fiber comprising polyethylene terephthalate, polyphenylene sulfide, poly-aramide, polylactic acid.

4. The nonwoven fabric of embodiment 2, wherein the reinforcing fibers are multicomponent fibers having an outer shealth comprising polyolefin.

5. The nonwoven fabric of embodiment 2, wherein the polyolefin is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, polyethylene naphthalate, and combinations thereof. 6. The nonwoven fabric of any one of embodiments 1-5, wherein the nonwoven fabric has a thickness of less than 0.5 millimeter.

7. The nonwoven fabric of any one of embodiments 1-6, wherein the nonwoven fabric has a basis weight of from 10 gsm to 100 gsm.

8. The nonwoven fabric of any one of embodiments 1-7, wherein the nonwoven fabric has a tensile strength of more than 28 kPa in random directions.

9. The nonwoven fabric of any one of embodiments 1-8, wherein the nonwoven fabric passes the UL- 94 V0 flame test.

10. The nonwoven fabric of any one of embodiments 1-9, wherein the plurality of randomly-oriented fibers has an average bulk density of from 100 kg/m ³ to 1500 kg/m ³ .

11. The nonwoven fabric of any one of embodiments 1-10, wherein the plurality of randomly-oriented fibers contains from 0 to 19 wt% of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100°C to 450°C.

12. The nonwoven fabric of any one of embodiments 1-11, wherein the oxidized polyacrylonitrile fibers have a median Effective Fiber Diameter of from 5 micrometers to 50 micrometers.

13. The nonwoven fabric of any one of embodiments 1-12, wherein the fluoropolymer binder comprises THV or Tetrafluoroethylene (TFE Teflon), Hexafluoropropylene (HFP), and Vinylidene fluoride (VDF).

14. The nonwoven fabric of any one of embodiments 1-13, wherein at least a part of the plurality of randomly-oriented fibers are entangled.

15. The nonwoven fabric of any one of embodiments 1-14, wherein the flow resistance of the nonwoven fabric is less than 50 Rayl.

16. A method of making a nonwoven fabric comprising:

mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers, wherein the plurality of reinforcing fibers have outer fiber surfaces comprised of a (co)polymer with a melting temperature between 100°C and 450°C;

heating the mixture of randomly-oriented fibers to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers; and

applying a fluoropolymer binder to the mixture of randomly -oriented fibers, whereby the mixture of randomly-oriented fibers is bonded together to form the nonwoven fabric, optionally wherein the non-woven fabric has a thickness of one millimeter or less.

17. The method of embodiment 16, further comprising smoothing a major surface of the non-woven fabric by calendering the nonwoven fabric, heating the non-woven fabric, and/or applying tension to the non-woven fabric.

18. The method of embodiment 16 or 17, further comprising applying a silicone release liner to protect the nonwoven fabric.

19. The method of any one of embodiments 16-18, wherein the nonwoven fabric has a basis weight of from 10 gsm to 100 gsm.

20. The method of any one of embodiments 16-19, wherein the nonwoven fabric further comprises a density gradient at the smoothed surface.

21. The nonwoven fabric of any one of embodiments 1-14, wherein the flow resistance of the nonwoven fabric is more than 1000 Rayls.

EXAMPLES

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

Table 1: Materials

Test Methods:

Nonwoven Web Thickness Measurement: The method of ASTM D5736-95 was followed, according to test method for thickness of high loft nonwoven fabrics. The plate pressure was calibrated at 0.002 psi (13.790 Pascal). UL94-V0 Flame Test: Reference to UL94-V0 standard with flame height 20-mm, bottom edge of the sample 10-mm into the flame and bum twice at 10 seconds each. A flame propagation height under 125- mm (5 inches) was considered a pass.

Airflow Resistance Test: The methods of ISO9053-91 and ASTMC522-03 were followed. A Sigma Static Airflow Meter from Mecanum of Sherbrooke, Canada was used to record mean airflow resistance (measured in Pa-s/m or rayls).

Nonwoven webs produced in the following examples were produced by processes and techniques described in the commonly owned PCT Patent Publication No. WO 2015/080913 (Zillig et al) unless otherwise stated. Fabrics (i.e., samples) were produced by processing the nonwoven webs with binder solutions.

Example 1 (EX1)

A web was produced with 100 wt.% OPAN. The basis weight was 15 gsm ±10%. The web was placed on a first PET liner with the silicone release side directed toward the OPAN web. A 90 gsm THV340Z binder solution (diluted from 50 wt.% to 16 wt.% solid content by adding two parts of water to the one part of the solution) was spray coated onto the web. The OPAN web with binder at 3 mm thickness was uniformly compressed by a hand roller to a 0.5 mm thickness. The OPAN web with binder, supported by the PET liner, was then placed into an ISOTEMP Oven from Fisher Scientific of Waltham, MA. United States at 160°C (320°F) oven for 2-4 minutes to dry producing a 15 gsm ±10% dry coating of the THV340Z binder. A second PET liner was placed on top of the OPAN web supported by the first PET liner. The sample was then calendared at a gap of 1.5 mil and speed of 0.3048 m/min (1 ft./min) in the oven with an upper temperature setting of 152°C (305°F) and lower temperature of 154°C (310°F). The basis weight of the sample was 30 gsm ±10%. The sample underwent UL94-V0 Flame and Airflow Resistance testing.

Results are represented in Table 1.

Example 2 (EX2)

A sample was prepared as described in Example 1 except that a 180 gsm THV340Z binder solution was spray coated and dried to produce a 30 gsm dry coating. The basis weight of the sample was 45 gsm ±10%. The sample underwent UL94-V0 Flame and Airflow Resistance testing. Results are represented in Table 1.

Comparative Example 1 (CE 1 )

The FR PET Scrim underwent UL94-V0 Flame and Airflow Resistance testing. Results are represented in Table 1. Comparative Example 2 (CE2)

The FR PET Scrim with Adhesive underwent UL94-V0 Flame and Airflow Resistance testing. Results are represented in Table 1.

Example 3 (EX3)

A 90 wt.% OPAN and 10 wt.% T270 blended web was produced. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The basis weight was 20 gsm ±10%. The web was placed on a PET liner with the silicone release side directed toward the OPAN web. A 250 gsm THV340Z binder solution (diluted from 50 wt.% to 20 wt.% solid content by adding 1.5 parts of water to the one part of the solution) was spray coated onto the web. The OPAN web with binder at 3 mm thickness was uniformly compressed by a hand roller to a 0.5 mm thickness. The OPAN web with binder, supported by the PET liner, was then placed into an ISOTEMP Oven from Fisher Scientific of Waltham, MA. United States at 160°C (320°F) oven for 2-4 minutes to dry producing a 50 gsm ±10% dry coating of the

THV340Z binder. The basis weight of the sample was 70 gsm ±10%. The sample underwent UL94-V0 Flame testing. Results are represented in Table 1.

Example 4 (EX4)

A 90 wt.% OPAN and 10 wt.% T270 blended web was produced. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The basis weight was 20 gsm ±10%. The web was placed on a PET liner with the silicone release side directed toward the OPAN web. A 150 gsm THV340Z binder solution (diluted from 50 wt.% to 16 wt.% solid content by adding two parts of water to the one part of the solution) was spray coated onto the web. The OPAN web with binder at 3 mm thickness was uniformly compressed by a hand roller to a 0.5 mm thickness. The OPAN web with binder, supported by PET liner, was then placed into an ISOTEMP Oven from Fisher Scientific of Waltham, MA. United States at 160°C (320°F) oven for 2-4 minutes to dry producing a 25 gsm ±10% dry coating of the

THV340Z binder. The basis weight of the sample was 45 gsm ±10%. The sample underwent UL94-V0 Flame testing. Results are represented in Table 1.

Example 5 (EX5)

A 90 wt.% OPAN and 10 wt.% T270 blended web was produced. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The basis weight was 20 gsm ±10%. The web was placed on a PET liner with the silicone release side directed toward the OPAN web. A 140 gsm THV340Z binder solution (diluted from 50 wt.% to 10 wt.% solid content by adding two parts of water to the one part of the solution) was spray coated onto the web. The OPAN web with binder at 3 mm thickness was uniformly compressed by a hand roller to a 0.5 mm thickness. The OPAN web with binder, supported by the PET liner, was then placed into an ISOTEMP Oven from Fisher Scientific of Waltham, MA. United States at 160°C (320°F) oven for 2-4 minutes to dry producing a 14 gsm ±10% dry coating of the

THV340Z binder. The basis weight of the sample was 34 gsm ±10%. The sample underwent UL94-V0 Flame testing. Results are represented in Table 1.

Example 6 (EX6)

One 5-mm thick web was produced with 80 wt.% OPAN and 20 wt% T270. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The basis weight was 100 gsm ±10%. Two samples were prepared as described in Example 3. One sample was placed on the top and the other was placed on the bottom of the 5-mm thick blended web. The blended web was laminated to the samples by heating in the oven at 254°C (490°F) for four minutes. The combined basis weight was 240 gsm ±10%. The laminated sample underwent UL94-V0 Flame testing. Results are represented in Table 1.

Example 7 (EX7)

One 5-mm thick web was produced with 80 wt.% OPAN and 20 wt% T270. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The basis weight was 100 gsm ±10%. Two samples were prepared as described in Example 4. One sample was placed on the top and the other was placed on the bottom of the 5-mm thick blended web. The webs were laminated to the sample by heating in the oven at 254°C (490°F) for four minutes. The combined basis weight was 190 gsm ±10%.

The laminated sample underwent UL94-V0 Flame testing. Results are represented in Table 1.

Example 8 (EX8)

A 80 wt.% OPAN and 20 wt.% T270 blended web was produced. The blended web was heated in the oven at 249°C (480°F) enhancing entanglement and strength. The web was placed on a PET liner with the silicone release side directed toward the OPAN web. The basis weight was 20 gsm ±10%. A 140 gsm THV340Z binder solution (diluted from 50 wt.% to 10 wt.% solid content by adding two parts of water to the one part of the solution) was spray coated onto the web. The OPAN web with binder at 3 mm thickness was uniformly compressed by a hand roller to a 0.5 mm thickness. The OPAN web with binder, supported by the PET liner, was then placed into an ISOTEMP Oven from Fisher Scientific of Waltham, MA. United States at 160°C (320°F) oven for 2-4 minutes to dry producing a 14 gsm ±10% dry coating of the

THV340Z binder. The basis weight of the sample was 34 gsm ±10%. The sample underwent UL94-V0 Flame testing. Results are represented in Table 1. Table 1: UL94-V0 Flame and Airflow Resistance Test Results

All cited references, patents, and patent applications in the above application for letters patent are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.