Patent Publication Number: US-11021369-B2

Title: Carbon nanotube sheet structure and method for its making

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to carbon nanotubes, and more particularly to methods for forming materials and structures from carbon nanotubes. 
     The exceptional mechanical properties of carbon nanotubes can be used in the development of nanotube-based, high performance structural and multifunctional nanostructural materials and devices. Carbon nanotubes have been made that are nanometers in diameter and several microns in length, and up to several millimeters in length. Strong interactions occur between nanotubes due to the van der Waals forces, which may require good tube dispersion, good tube contact, and high tube loading in materials and structures formed from carbon nanotubes. 
     Many applications, such as electrical conducting, thermal conducting and high performance nanocomposites, are made by pre-forming nanotubes into a network or membrane (5-200 μm in thickness) with controlled nanostructures (density, porosity, dispersion, alignment, and loading). These membranes would also make nanotube materials and their properties capable of transfer into a macroscale material for easy handling. These preformed nanotube networks are also called buckypapers in the literature. Buckypapers are produced by a multiple-step process of dispersing nanotubes into a suspension and filtering the produced suspension. The produced buckypapers can be easily handled similar to conventional surface veil, carbon fiber, or glass materials. However, all the existing manufacturing techniques for nanotube membranes are discontinuous or batch processes and can only produce small quantities and very short membrane materials, which are serious barriers for future practical applications of nanotube membranes. 
     Current discontinuous or batch techniques can only produce nanotube membrane materials by filtering a nanotube suspension, and the dimensions are limited by the filter dimension. In these techniques, a well-dispersed nanotube suspension is first prepared, optionally with the aid of selected surfactant and mixed using high shear mixing and/or sonication. Then, a filtration system with a filter membrane of 0.1 μm to 10 μm pore size is employed to filter the prepared suspension with the aid of vacuum or pressure. During the filtration, nanotubes deposit onto the surface of the filter membrane to form a nanotube network. After filtration, the produced nanotube film or buckypaper can be peeled off from the filter membrane. Producing large quantities of buckypapers requires frequent changing of the filters. Current processes use many filters to complete the filtration and limit manufacture of the buckypapers to piece by piece, which is time consuming, costly and also difficult to ensure consistent product quality. More importantly, due to the limitation of filter dimension, the product membranes are of a limited length (usually less than one foot). 
     U.S. Pat. No. 7,459,121, incorporated herein by reference in its entirety, describes a method for the continuous production of a network of nanotubes or other nanoscale fibers. The method comprises making a suspension of nanoscale fibers dispersed in a liquid medium, and filtering the suspension by moving a filter membrane through the suspension liquid, such that the nanoscale fibers are deposited directly on the filter membrane as the fluid medium flows through the filter membrane, thereby forming a continuous membrane of the nanoscale fibers. In one embodiment, the deposition of the nanoscale fibers occurs when and where the filter membrane moves into contact with a static, porous filter element. In another embodiment, the deposition of the nanoscale fibers occurs when and where the filter membrane moves into contact with a dynamic, porous filter element. For example, the filter element can be a rotary element which is mechanically driven to rotate and at least partially assist in moving the filter membrane across the filter element. The carbon nanotubes described therein are preferably single wall carbon nanotubes, and are described being commercially available from companies such as Carbon Nanotechnologies, Inc. (Houston, Tex., USA). Though the lengths of such carbon nanotubes are not described in the patent, the length of carbon nanotubes produced by Carbon Nanotechnologies, Inc. is understood to be in a range less than 0.01 mm (less than 50 microns). The apparatus described in U.S. Pat. No. 7,459,121 is not believed to be commercially available. 
     U.S. Pat. No. 7,955,535, incorporated herein by reference in its entirety, describes forming a suspension of small diameter SWCNTs and larger diameter MWCNTs (or CNFs) and filtering the suspension to remove the liquid. 
     U.S. Pat. No. 8,351,220, incorporated herein by reference in its entirety, describes a nanoscale fiber film comprising a buckypaper having an areal density (basis weight) of about 20-50 gsm, and a thickness of about 5 to 100 microns. 
     U.S. Pat. No. 5,254,399 describes a method for forming a nonwoven fabric having excellent sheet formation, that comprises fibers having a diameter of 7 μm or less and an aspect ratio (ratio of fiber length to fiber diameter, or L/D) in the range of 2000&lt;L/D≤6000, and optionally thermal bonding fibers, and the fibers being three-dimensionally entangled, the disclosure of which is incorporated by reference in its entirety. The fibers can include organic synthetic fibers such as polyester fiber, polyolefin fiber, polyacrylonitrile fiber, polyvinyl alcohol fiber, nylon fiber, polyurethane fiber and the like, semi-synthetic fibers, regenerated fibers, natural fibers and the like. 
     Notwithstanding, there remains a need for a process that manufactures CNT sheets on an industrial and commercial scale in order to meet the emerging technological and market needs for such structures. 
     SUMMARY OF THE INVENTION 
     Methods and devices are provided herein for the continuous production of a network of carbon nanotubes (CNTs) into a continuous sheet structures. 
     The present invention includes a process for forming carbon nanotubes (CNT) structures that includes filtering a volume of a solution comprising a dispersion of CNTs, over a filter material to provide a filtered CNT structure having uniform dispersion of the CNTs over the filter material, and a step of drying the filtered CNT structure into a CNT sheet. CNTs can include single wall CNTs (SWCNTs) or multi-wall CNTs (MWCNTs). 
     The present invention includes a process for manufacturing a carbon nanotube (CNT) sheet, comprising the steps of: i) applying an aqueous suspension of carbon nanotubes (CNTs) dispersed in a liquid onto a continuous, moving, porous material (which acts as a filter); ii) drawing by vacuum or pressure the liquid of the aqueous suspension of the CNTs through the porous material, and filtering a uniform dispersion of CNTs over the porous material to form an entangled CNT structure; iii) optionally drying any residual liquid from the entangled filtered CNT structure to form a CNT sheet over the porous carrier material; and iv) removing the CNT sheet from the porous carrier material. 
     The present invention further includes a continuous process for manufacturing a continuous composite CNT sheet, comprising the steps of: i) applying a continuous porous substrate layer to an upper side of continuous, moving porous carrier material; ii) applying a suspension of carbon nanotubes (CNTs) dispersed in a liquid on the porous substrate layer; iii) drawing by vacuum or pressure the liquid of the aqueous suspension of the CNTs through the porous substrate layer and the porous carrier material, and filtering a uniform dispersion of CNTs over the porous substrate layer to form an entangled CNT structure; iv) optionally drying any residual liquid from the entangled CNT structure to form a composite CNT-substrate sheet over the porous carrier material; and v) removing the composite CNT-substrate sheet from the continuous porous carrier material. 
     The present invention further includes a continuous process for manufacturing continuous CNT sheets, comprising the steps of: i) applying a suspension of carbon nanotubes (CNTs) dispersed in a liquid onto a continuous, moving porous carrier-substrate layer; ii) drawing by vacuum or pressure the liquid of the aqueous suspension of the CNTs through the continuous, moving porous carrier-substrate layer, and filtering a uniform dispersion of CNTs over the continuous porous carrier-substrate layer to form an entangled CNT structure on the carrier-substrate layer; and iii) optionally drying any residual liquid from the entangled CNT structure to form a continuous composite CNT-substrate sheet including the entangled CNT structure and the carrier-substrate layer. 
     The invention also includes a process for manufacturing a carbon nanotube (CNT) sheet, comprising the steps of: i) applying an aqueous suspension of carbon nanotubes (CNTs) dispersed in a liquid on a continuous porous carrier material; ii) drawing by vacuum or pressure the liquid of the aqueous suspension of the CNTs through the porous carrier material, and filtering a uniform dispersion of filtered CNTs over the porous carrier material to form an entangled CNT structure, iii) optionally drying any residual liquid from the entangled CNT structure to form a CNT sheet over the porous carrier material; and iv) optionally removing the CNT sheet from the porous carrier material. 
     In another aspect of the invention, a method for forming a CNT structure or sheet includes making a dispersion or suspension of carbon nanotubes (CNTs) in a dispersing liquid, the dispersed CNTs forming a median bundle or agglomeration length of at least 50 microns; passing a volume of the CNT suspension over a filter material to provide a continuous pool or coating of the CNT suspension over the filter material, having a uniform depth (or thickness) sufficient to prevent puddling of the CNT suspension upon the surface of the filter material; drawing under vacuum or pressure the dispersing liquid through the filter material to provide a uniform dispersion of the CNTs over the filter material and forming a CNT structure; drying any residual aqueous liquid from the CNT structure to form a CNT sheet over the filter material; and removing the CNT sheet from the filter material. The CNTs can include SWCNTs, including SWCNTs having a median length of at least 5 microns and an aspect ratio of at least 2,500:1. The CNTs can also include MWCNTs, including SWCNTs having a median length of at least 50 microns and an aspect ratio of at least 2,500:1 
     In another aspect of the invention, the step of drawing the dispersing liquid through the filter material comprises passing the CNT-laden filter material over a vacuum screen or box for a time sufficient to draw the dispersing liquid through the filter material and vacuum screen or box. 
     The present invention also includes an apparatus for manufacturing a continuous CNT sheet, and a continuous process for manufacturing continuous CNT sheets employing the apparatus. The continuous process comprising the steps of: i) moving a continuous conveying belt along a path that traverses a pooling region and a vacuum box; ii) applying a continuous porous carrier material to an upper side of the continuous conveying belt; iii) applying an aqueous suspension of carbon nanotubes (CNTs) dispersed in a liquid on the porous carrier material, the dispersed CNTs having a median bundle length of at least 0.05 mm and an aspect ratio of at least 2,500:1; iv) forming a continuous pool of the aqueous suspension of the CNTs over and across the width of the continuous porous carrier material in the pooling region, the continuous pool of the aqueous suspension of the CNTs having a uniform thickness sufficient to prevent puddling upon the continuous porous carrier material; v) advancing the porous carrier material and the continuous pool of the aqueous suspension of the CNTs over the vacuum box; vi) drawing by vacuum the liquid of the aqueous suspension of the CNTs through the porous carrier material, and filtering a uniform dispersion of filtered CNTs over the porous carrier material to form a filtered CNT structure; vii) optionally drying any residual liquid from the filtered CNT structure to form a CNT sheet over the porous carrier material; and v) removing the CNT sheet from the continuous porous carrier material. The CNTs can include SWCNTs, including SWCNTs having a median length of at least 5 microns and an aspect ratio of at least 2,500:1. 
     The CNTs can also include MWCNTs, including SWCNTs having a median length of at least 50 microns and an aspect ratio of at least 2,500:1. Said SWCNTs and longer MWCNTs are hereinafter described as “entanglable CNTs”. 
     In an aspect of the invention, the continuous porous carrier material is a continuous porous film, sheet, or fabric material. The continuous porous carrier material provides a stable and resilient porous structure for pulling the forming CNT structure through and along the apparatus, to prevent tearing and degradation of the CNT structure during manufacture. The continuous porous carrier material can include a woven or meshed synthetic polymer, which can include hydrophobic polymers, including but not limited to polytetrafluoroethylene (PTFE), also known as Teflon®, and hydrophilic polymers, including but not limited to aliphatic polyamides, also known as nylon. A metal-coated woven or a metallic mesh or expanded foil or screen material can also be used as a porous carrier material. In an embodiment, the porous carrier material, has a plurality of openings having a size between about 0.1 micron, and up to about 5 mm. A continuous roll of metallic wires or fibers, from a plurality of spools or rovings, can be pulled across the width of the filtering material in the machine direction. The CNT dispersion can then be filtered upon the aligned or unidirectional metallic wires, forming a CNT-metallic wire composite rollstock material. This process is similar to a pultrusion process, but using the CNT dispersion to encapsulate the fibers instead of a resin. Non-limiting examples of a pultrusion process are disclosed in US Patent Publication US 2011/0306718 and U.S. Pat. No. 5,084,222, the disclosures of which are incorporated by reference in their entireties. 
     In another aspect of the invention, the continuous porous carrier material can also be a filter material that filters the dispersed CNTs from the liquid of the aqueous suspension of the CNTs. In this embodiment, the CNTs are filtered onto the continuous porous carrier material to form the CNT structure and CNT sheet, and the CNT sheet is then separated or peeled away from the continuous porous carrier material. In this embodiment, the CNT structure and CNT sheet will typically have a basis weight of CNTs sufficient to provide the CNT structure with integrity to be peeled away continuously from the carrier material as a free-standing CNT sheet, without tearing or degrading. 
     In a further aspect of the invention, a porous secondary layer can applied to an upper side of the continuous porous carrier material, before applying the CNT suspension. The secondary layer can be a continuous apertured or porous film, sheet or fabric material through which the liquid of the aqueous suspension of the CNTs is drawn. The secondary layer, also referred to as a substrate layer or veil layer, when used over a carrier material, provides a continuous composite CNT sheet that has improved separation or “peel away” from the continuous porous carrier material. The secondary or veil layer can be very thin and light weight (low basis weight) to provide the separation of the CNT structure from the carrier material, and can also improve or contribute the physical or function properties to the CNT structure and to the composite CNT sheet. The size of the openings (circular, square, or any other shape) of candidate carrier materials are typically about a size between about 0.1 micron, and up to about 5 mm. The length can be a spool or roll of material. 
     Thus, the present invention further includes a continuous process for manufacturing a continuous composite CNT sheet, comprising the steps of: i) moving a continuous conveying belt along a path that traverses a pooling region and a vacuum box; ii) applying a continuous porous carrier material to an upper side of the moving continuous conveying belt; iii) applying a continuous porous veil layer to an upper side of the continuous porous carrier material; iv) applying an aqueous suspension of carbon nanotubes (CNTs) dispersed in a liquid on the porous veil layer, the dispersed CNTs; v) forming a continuous pool of the aqueous suspension of the CNTs over and across the width of the porous veil layer moving in the pooling region, the continuous pool of the aqueous suspension of the CNTs having a uniform thickness sufficient to prevent puddling upon the porous veil layer; vi) advancing the porous veil layer, porous carrier material, and the continuous pool of the aqueous suspension of the CNTs over the vacuum box; vii) drawing by vacuum the liquid of the aqueous suspension of the CNTs through the porous veil layer and the porous carrier material, and filtering a uniform dispersion of filtered CNTs over the porous veil layer to form a filtered CNT structure; viii) optionally drying any residual liquid from the filtered CNT structure to form a composite CNT sheet over the porous carrier material; and ix) removing the composite CNT sheet from the continuous porous carrier material. The CNTs comprise said entanglable CNTs. 
     In another aspect of the invention, the continuous porous carrier material can be removed from the process and replaced with a secondary or veil layer, referred also to as a carrier-veil layer, that can provide the function of the carrier material. The carrier-veil layer provides filtration of CNTs from the CNT suspension, and has a sufficiently stable and resilient porous structure sufficient for pulling the forming CNT structure through and along the apparatus during processing, to prevent tearing and degradation of the CNT structure during manufacture, while having physical and functional properties, for example, low basis weight and minimal thickness, that are consistent with the intended use of the CNT sheet. Optionally, in a post-manufacturing process, the CNT structure can be separated from the veil-carrier layer. Examples of suitable carriers include carbon fiber nonwoven, polyester nonwoven, polyester woven, fiberglass nonwoven, expanded copper foil, copper mesh, or PEEK nonwoven. Optionally, these substrates can be metallized to add functionally such as conductivity or other properties. 
     Thus, the present invention also includes a continuous process for manufacturing continuous CNT sheets, comprising the steps of: i) moving a continuous conveying belt along a path that traverses a pooling region and a vacuum box; ii) applying a continuous porous carrier-veil layer to an upper side of the continuous conveying belt; iii) applying an aqueous suspension of entanglable carbon nanotubes (CNTs) dispersed in a liquid on the porous carrier-veil layer; iv) forming a continuous pool of the aqueous suspension of the entanglable CNTs over and across the width of the continuous porous carrier-veil layer in the pooling region, the continuous pool of the aqueous suspension of the entanglable CNTs having a uniform thickness sufficient to prevent puddling upon the continuous porous carrier-veil layer; v) advancing the continuous porous carrier-veil layer and the continuous pool of the aqueous suspension of the entanglable CNTs over the vacuum box; vi) drawing by vacuum the liquid of the aqueous suspension of the entanglable CNTs through the continuous porous carrier-veil layer, and filtering a uniform dispersion of entanglable CNTs over the continuous porous carrier-veil layer to form an entangled CNT structure on the carrier-veil layer; vii) optionally drying any residual liquid from the entangled CNT structure to form a continuous composite CNT-veil sheet including the entangled CNT structure and the carrier-veil layer; and viii) removing the continuous composite CNT-veil sheet from the continuous conveying belt. 
     The present invention also provides a manufactured CNT sheet comprising entanglable CNTs arranged in a randomly oriented, uniformly distributed pattern. The CNT sheet has a basis weight of at least 1 gram CNT per square meter (gsm). 
     The manufactured CNT sheet has a relative density of about 1.5 or less (relative water). 
     The desired basis weight of the manufactured CNT structure can be at least 2 gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and at least 6 gsm; and up to about 40 gsm, including up to about 30 gsm, up to about 20 gsm, up to about 15 gsm, up to about 12 gsm, up to about 10 gsm, up to about 8 gsm, and up to about 6 gsm; and can be about 3 gsm, about 4 gsm, about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9 gsm, about 10 gsm, and about 15 gsm. 
     The relative density of the manufactured CNT structure can be about 1.0 or less, and can be about 0.8 or less, about 0.7 or less, about 0.6 or less, about 0.5 or less, about 0.4 or less, and about 0.3 or less, such as 0.25. Such relative densities are well below those of buckypapers described in the art, and provide a CNT sheet with an effective thickness with a substantially lower basis weight. 
     The entanglable CNTs useful in forming a filtered and/or entangled CNT structure can include DWCNTs and MWCNTs having a median length of at least 0.05 mm, or at least 0.1 mm, or at least 0.2 mm, or at least 0.3 mm, a length of at least 0.4 mm, or at least 0.5 mm, and of at least 2 mm, and can be highly elongated CNTs having an aspect ratio of at least 2,500:1, including at least 5,000:1, at least 10,000:1, at least 50,000:1, and at least 100,000:1. Such CNTs can also include single wall CNTs (SWCNTs) having a median length of at least 0.005 mm, or at least 0.01 mm, or at least 0.02 mm, or at least 0.03 mm, a length of at least 0.04 mm, and can be highly elongated SWCNTs having an aspect ratio of at least 2,500:1, including at least 5,000:1, at least 10,000:1, at least 50,000:1, at least 100,000:1, and at least 1,00,000:1. 
     The CNTs can also optionally include short MWCNTs (including DWCNTs that have a median length of less than about 50 microns, and/or an aspect ratio of less than 2,500:1. 
     The entanglable CNTs can also be combined with short MWCNTs to provide additional functionality, typically in a weight ratio of entanglable CNTs to short MWCNTs selected from the group consisting of at least 1:10, at least 1:3, at least 1:2, at least 1:1, at least 2:1, at least 10:1, and at least 99:1. 
     In a further aspect of the invention, the CNT sheet can have a free-standing structure. 
     In another aspect of the invention, the mass proportion of the entanglable CNTs in the CNT sheet will comprise at least 75% of the total CNTs in the sheet or CNT structure, including at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, and at least 99% of the total CNTs in the sheet or CNT structure. 
     The CNT structure or sheet includes a nonwoven sheet comprising carbon CNTs that form a continuous matrix or solid phase across the entire area of the nonwoven sheet. In this aspect, each CNT is in direct contact with a plurality of adjacent CNTs along its length. 
     In a further aspect of the invention, the CNTs can be chemically treated prior to sheet formation to modify the physical or functional properties of the CNTs, or of the nonwoven CNT sheet or structure made therefrom. 
     In another aspect of the invention, the CNTs can be pre-treated by immersion into an acidic solution, including an organic or inorganic acid, and having a solution pH of less than 1.0. A non-limiting example of an acid is nitric acid. Alternatively, or in addition, the CNT structure can be post-treated with an acid solution. 
     In another aspect of the invention, the fillers are added to the CNT suspension to add functionality to the nonwoven structure. This includes, but not limited to, adding conductive and/or non-conductive fillers such as carbon nanofiber, graphene, glass fiber, carbon fiber, thermoplastic fiber, thermoset fiber, glass microbubbles, glass powder, thermoplastic powder, thermoset powder, nickel nanowire, nickel nanostrands, chopped nickel coated carbon fiber, ceramic powder, ceramic fiber, or mixtures thereof, For example, nickel nanostrands can be added to the CNT nonwoven to increase electrical conductivity and permeability. These properties can increase EMI shielding properties. Another example includes adding multi-lobal polyimide fiber to the CNT nonwoven to improve mechanical properties in a carbon fiber composite system and adding multifunctionality to said composite system. 
     In another aspect of the invention, the CNTs nonwoven structure can include a plurality of distinctly formed CNT sheets, stacked or laminated together. The stacked layers can also include filler or additive materials. Example filler materials include, but are not limited to, carbon nanofiber, graphene, glass fiber, carbon fiber, thermoplastic fiber, thermoset fiber, glass microbubbles, glass powder, thermoplastic powder, thermoset powder, nickel nanowire, nickel nanostrands, or mixtures thereof. For example, a solution containing graphene can be laid onto and coupled to a previously formed CNT nonwoven layer using the herein mentioned continuous manufacturing process. 
     In yet another aspect of the invention, the aqueous liquid is drawn through the filter material with the aid of vacuum to pull water through the filter material, or with applied pressure to press water through the filter material, or a combination thereof. 
     An apparatus for manufacturing a continuous CNT sheet is based on a conventional apparatus for making wet-laid nonwoven structures, and includes a continuous conveying belt that moves along a path that traverses a table including a pooling region and a vacuum box, an optional drying unit, and various rollers and conveying elements for pulling the continuous conveying belt along the apparatus and through the process. The apparatus also includes the means for storing and supplying a suspension of CNTs to the pooling region, and a headbox or means for forming a continuous pool of the aqueous suspension of the CNTs across the width of a filter material disposed on the continuous conveying belt in the pooling region. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic of a conventional apparatus for making nonwoven wet-laid fabrics. 
         FIG. 2  shows apparatus that can be used in a process of the present invention for making CNT structures. 
         FIG. 3  shows the apparatus of  FIG. 2  in which a solution of dispersed CNTs filling a reservoir and flowing in a flooding pool over a continuous filter material. 
         FIG. 4  shows the apparatus of  FIG. 3  after the flooding pool of solution passes over a vacuum box to form the CNT sheet structure. 
         FIG. 5  shows the apparatus and an alternative process of  FIG. 3 , further applying a continuous porous veil layer over the continuous filter material. 
         FIG. 6  shows the apparatus and another alternative process of  FIG. 3 , wherein the continuous filter material forms a continuous composite CNT-filter material sheet. 
         FIG. 7  shows an alternative apparatus for forming the flooding pool of the solution of dispersed CNTs over the continuous filter material. 
         FIG. 8  shows another alternative apparatus using liquid spray nozzles to form the flooding pool of the solution of dispersed CNTs over the continuous filter material. 
         FIG. 9  shows a schematic of a CNT web-laying machine that includes two or more headboxes for feeding additional and different suspensions of CNTs or other materials. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, the terms “comprise,” “comprising,” “include,” and “including” are intended to be open, non-limiting terms, unless the contrary is expressly indicated. 
     As used herein, a “free-standing” sheet or structure of CNTs is one that is capable of formation, or separation from a filter material, and handling or manipulation without falling apart, or without significant flaking or crumbling of CNTs away from the sheet or structure. 
     A “continuous” sheet of material is an elongated sheet having a length that is orders of magnitude greater than the width of the sheet, and a roll of the sheet material. 
     Conventional wet-laid nonwovens are made by a modified papermaking process. That is, the fibers to be used are suspended in water or other dispersive liquid. A specialized machine is used to separate the water or other dispersive liquid from the fibers to form a uniform sheet of material, which is then dried. The wet-laid process has its origins in the manufacture of paper and was developed because paper manufacturers wanted to be able to use uncut, long natural fibers and synthetic fibers in addition to the usual raw materials without changing the process. Nonwoven textile fibers tend to be longer, stronger, and relatively inert when compared to papermaking fibers. 
       FIG. 1  shows a schematic of an apparatus for making nonwoven wet-laid fabrics. Three characteristic stages in the manufacture of nonwoven fabrics by the wet-laid method: 1) suspension of the fiber ( 1 ) in water or other dispersive liquid, and transport of the suspension onto a continuous traveling screen ( 12 ), 2) continuous web formation on the screen as a result of filtration ( 20 ), and 3) drying ( 30 ) of the web. 
     A process for forming CNT structures of the present invention is an improvement on the conventional process for making wet-laid nonwovens. A process for forming CNT structures includes a step or stage of forming a suspension of CNTs is a dispersive liquid, a filtering a volume of the CNT suspension to provide a uniform dispersion of a CNT structure over the filter material, and drying any residual dispersive liquid from the filtered CNT structure. 
     Making the Suspension 
     The first step in making a continuous length of CNT structure involves making a suspension of CNTs in a dispersive liquid, which can include water. The dispersive liquid can also include one or more compounds for improving and stabilizing the dispersion and suspension of the CNTs in the dispersive liquid, and one or more compounds that improve the functional properties of the CNT structure produced by the method. 
     While water is a preferred dispersive liquid, other non-solvating liquids can be used to disperse and process the CNTs. As used herein, the term “non-solvating” refers to compounds in liquid form that are non-reactive essentially with the CNTs and in which the CNTs are essentially insoluble. Examples of other suitable non-solvating liquids include volatile organic liquids, selected from the group consisting of acetone, ethanol, methanol, isopropanol, n-hexane, ether, acetonitrile, chloroform, DMF, THF (tetrahydrofuran), NMP (N-Methyl-2-pyrrolidone), MEK (methyl ethyl ketone), DMAC, and mixtures thereof. Low-boiling point solvents are typically preferred so that the solvent can be easily and quickly removed, facilitating drying of the resulting CNT structure. 
     The dispersive liquid can optionally include one or more surfactants (e.g., dispersant agents, anti-flocculants) to aid forming or to maintain the dispersing, wet-laid formation, or dewatering of the CNTs and wet-laid CNT structures. For example, BYK-9076 (from BYK Chem USA), Triton X-100, dodecylbenzenesulfonic acid sodium salt (NaDDBS), and SDS may be used. 
     The carbon nanotubes can be provided in a dry, bulk form. The CNTs can include entanglable CNTs that typically have a median length selected from the group consisting of at least about 0.05 mm (50 microns), such as at least about 0.1 mm (100 microns), at least about 0.2 mm, at least about 0.3 mm, at least about 0.4 mm, at least about 0.5 mm, at least about 1 mm, at least about 2 mm, and at least about 5 mm. The CNTs can be said entanglable single wall nanotubes (SWNT), and said entanglable multi-wall nanotubes (MWNT). Typical SWCNTs have a tube diameter of about 1 to 2 nanometers. Typical MWCNTs have a tube diameter of about 5 to 10 nanometers. Examples of MWCNTs useful in the present invention are those disclosed in or made by a process described in U.S. Pat. No. 8,753,602, the disclosure of which is incorporated by reference in its entirety. Such carbon nanotubes can include long, vertically-aligned CNTs, which are commercially available from General Nano LLC (Cincinnati, Ohio, USA). U.S. Pat. No. 8,137,653, the disclosure of which is incorporated by reference in its entirety, discloses a method of producing carbon nanotubes, and substantially single wall CNTs, comprising, in a reaction chamber, evaporating a partially melted catalyst electrode by an electrical arc discharge, condensing the evaporated catalyst vapors to form nanoparticles comprising the catalyst, and decomposing gaseous hydrocarbons in the presence of the nanoparticles to form carbon nanotubes on the surface of the catalyst nanoparticles. 
     A CNT concentration in the aqueous liquid is at least 100 mg/L of suspension, and up to about 10 g/L, which facilitates dispersion and suspension, and minimizes agglomeration or flocculation of the CNTs in the dispersing liquid. In various embodiments of the invention, the CNT concentration is at least about 500 mg/L, and at least about 700 mg/L, and up to about 5 g/L, up to about 1 g/l, and up to about 500 mg/L. Further, the aqueous suspension can comprise a CNT level selected from the group consisting of about 1% CNTs by weight or less, about 0.5% CNTs by weight or less, about 0.1% CNTs by weight or less, about 0.07% CNTs by weight or less, about 0.05% CNTs by weight or less, and including at least about 0.01% CNTs by weight, such as at least about 0.05% CNTs by weight. 
     Generally, the CNTs are added to a quantity of the dispersive liquid under mixing conditions using one or more agitation or dispersing devices known in the art. The CNT suspension can be made in a batch process or in a continuous process. In one embodiment, the mixture of CNTs in the aqueous liquid is subjected to sonication using conventional sonication equipment. The suspension of CNTs in water can also be formed using high shear mixing, and microfluidic mixing techniques, described in U.S. Pat. No. 8,283,403, the disclosure of which is incorporated by reference in its entirety. A non-limiting example of a high shear mixing device for dispersing CNTs in an aqueous liquid is a power injection system, for either batch of in-line (continuous) mixing of CNT powder and the liquid, by injecting the powder into a high-shear rotor/stator mixer, available as SLIM technology from Charles Ross &amp; Sons Company. 
     The Power Number, N P , is commonly used as a dimensionless number for mixing. It is defined as:
 
 N   P   =P /(ω 3   D   5 ρ), where
         P=power input of mixer,   ω=rotational speed of mixer,   D=mixing blade diameter, and   ρ=dispersion density of liquid       

     To compare mixing scale, we can analogize with the Kolmogorov scale of mixing, λ, to the average length of CNTs, L.
 
λ=( v   3 /ε) 1/4 , where
         v=kinematic viscosity of dispersion, and   ε=rate of dissipation of turbulence kinetic energy per unit mass.       

     For entanglable CNTs, v is much higher (more viscous); thus, λ is larger but scales to slightly less than linearly. But, it requires a lot of energy (to the 4 th  power) to get to the same post mixing length. Also, note that ε should be about linear with P, the power input of mixer. 
     Without being bound by any particular theory, it is believed that as a result of mixing and dispersing entanglable CNTs in the aqueous liquid, the individual CNTs can reduce in length and the structure twisted or bent, and bundles of individual CNTs can become entangled and tied. Typically, the length of CNTs that are provided into the mixing and dispersing process are longer than those of the resulting dispersed CNTs; for example, a median length selected from the group consisting of at least about 0.005 mm, and an aspect ratio of at least 2,500:1. The median length can be at least 0.1 mm, at least about 0.2 mm, at least about 0.3 mm, at least about 0.4 mm, at least about 0.5 mm, at least about 1 mm, at least about 2 mm, and at least about 5 mm. The median length of the CNTs can also comprise a range selected from the group consisting of between 1 mm and 2 mm, between 1 mm and 3 mm, and between 2 mm and 3 mm. The aspect ratio can be at least 5,000:1, at least 10,000:1, at least 50,000:1, and at least 100,000:1, 
     The resulting suspension of CNTs in the aqueous liquid is stable for at least several days, and longer. The suspension of CNTs can be mixed and stirred prior to use in the sheet forming process in order to ensure homogeneity of the CNT dispersion. 
     In an aspect of the invention, the entanglable CNTs can be used with the short MWCNTs in a weight ratio selected from the group consisting of at least 1:99, at least 1:10, at least 1:3, at least 1:2, at least 1:1, and at least 2:1. The combination can include a combination of the functional properties and features of each type of CNT, including synergistic functional properties and features. 
     The dispersive liquid can also optionally include one or more binder compounds for improve both the structural and functional properties of the CNTs. An example of a binder is polyvinyl acrylate (PVA). The use of a binder such as PVA can improve the structure and function of CNT nonwoven sheets comprising both the entanglable CNTs and the short MWCNTs. The binder is also useful for improving adherence of the dispersed CNTs into a hydrophobic filter material, such as Teflon®. In a further aspect of the invention, the addition of the binder can result in a continuous phase of the binder, which reduces the contact points (or CNT to CNT junctions) within the CNT sheet. The binder coats or encapsulates the CNTs within the polymer or resin matrix, separating the adjacent CNTs in close proximity by a layer of the binder material. The binder results in at least a partial disruption of the continuous matrix of the CNT sheet. 
     In one example, CNTs, as described above, are acid treated in fuming nitric acid, and then dispersed mechanically into the dispersive liquid, for example, water (without any surfactant). After storage, the dispersion of CNTs in the solution persisted at least 5 calendar days. Two 55-gallon drums of the CNT suspension were discharged into a 200-gallon make-up tank, and low intensity agitation was applied prior to laying down of the suspension onto the sheet filter material. Within just 5 minutes, a 10-gsm nonwoven structure was formed with good dispersion and uniformity of the CNTs within the nonwoven structure. In comparison to a suspension of short MWCNTs, the dispersion and suspension stability of the entanglable CNTs is surprisingly very long in time. Without being bound by any particular theory, it is believed that the relative ease of formation of the CNT suspension is heavily influenced by the aspect ratio of the CNTs (that is, the length of an individual tube or fiber, to its mean diameter). The entanglable CNTs have a suspension stability that is at least an order of magnitude longer than short MWCNTs. The use of CNTs having the prescribed aspect ratio enabled the formation of nonwoven CNT sheets having a basis weight as low as 2 gsm. It has been determined that nonwovens made from wood pulp fibers having a significantly smaller aspect ratio (about 100:1) could not be formed on a continuous, wet-laid nonwoven apparatus at a basis weight at or below of 10 gsm. 
     The high aspect ratios of the entanglable CNTs also result in a CNT nonwoven having a critical entanglement density well in excess of 100, where the critical entanglement density is defined as the ratio of the average length of a CNT to the average length between the two adjacent intersections of the CNT with other CNTs. 
     The dispersive liquid can also optionally include one or more filler or functional filler materials. A functional filler material can be one that has properties that may modulate the properties of the CNT sheet or structure that is produced by the process described herein. Such function fillers (or properties) can include non-magnetic dielectric materials, magnetic dielectric materials, electrically non-conductive materials, electrically conductive materials. The materials can include particles, agglomerates, fibers, and others. Examples of non-magnetic dielectric materials include epoxies, polyamides, and polyimides. Examples of magnetic dielectric materials include ferrite, ferrite-filled epoxy, ferrite-filled polyimide, and ferrite-filled polyamide. Examples of electrically non-conductive materials include thermoplastic or thermoset materials, including without limitation, polyamide, polyimide, round or multi-lobal thermoplastic fibers, and polyamide and polyimide thermoset powder. Other examples of electrically non-conductive materials include ceramic fibers, including by example alumina, boron nitride, ceramic powder, including by example alumina boron nitride, ferrites including Fe 2 O 3  and Fe 3 O 4 , MnZn, NiZn, and nanoparticles including graphene and gold nanoparticles. Examples of electrically conductive materials include metal nanofibers or wire including by example nickel nano-strands and silver nanowire, metalized fibers including by example chopped nickel coated carbon fiber, and nanoparticles including by example graphene and gold nanoparticles. 
     Filtration and CNT Structure Formation 
     The second step in making the CNT structure comprises passing a volume of the CNT suspension over a filter material, and drawing the dispersive liquid of the CNT suspension through the filter material to provide a uniform dispersion of the CNTs over the filter material. The CNT suspension can be pulled or pushed through the filter material under vacuum or pressure. 
     The filter material is a flexible, resilient sheet material having pores or openings that are sufficiently large to allow the dispersive liquid to be drawn through with a moderate amount of vacuum or pressure, though are sufficiently small to prevent the multitude of dispersed CNTs from passing through. The size of the openings (circular, square or any other shape) are typically about a size between about 0.1 micron, and up to about 10 micron, and the porosity (open area) is typically about 20% to about 80%, and selected from the group consisting of about 30%, about 40% or about 50%. 
     The loading (weight per area) of the dispersed CNTs in the aqueous liquid onto the surface of the filter material can be determined from the desired basis weight of the resulting CNT structure (in grams per square meter, or “gsm”), and the concentration of dispersed CNTs in the dispersive liquid. The solution comprising the dispersed CNTs is loaded over a mesh screen, and in a manufacturing process, a continuous belt or mesh screen. A continuous layer of filter material is registered with and passes along the outer surface of the mesh screen, and serves as a retaining filter for the CNTs. The filter material is preferably non-soluble in and non-absorbent of water or of the dispersive liquid, and can include both hydrophilic materials, including nylon, and hydrophobic materials, including Teflon®. Hydrophilic or hydrophobic coatings, as applicable, can also be applied to a base structure of the filter material. The filter material is also referred to herein as a scrim. The filter material can include a continuous web of material, ranging from 10 inches (25 cm), and up to 60 inches (152 cm) in width, and of continuous lineal length. The length can be a spool or roll of material, or a continuous loop of material, depending upon whether the resulting CNT nonwoven sheet is removed from the scrim continuously following drying at the same production site (mentioned herein after), or is processed remotely. 
     CNTs inherently are poorly dispersible in aqueous solutions, even after being treated to provide improved hydrophilicity. Under some dispersion conditions, the CNTs, and particularly CNTs having a long aspect ratio, tend to flocculate or aggregate into bundles or larger clot-like lumps in the aqueous dispersion. The conditions of the continuous process therefore inhibit or prevent puddling of the CNT suspension and manage and control the inherent tendency of dispersed CNTs to flocculate or agglomerate. 
     The loaded filter material is then passed over a vacuum zone or vacuum box, which draws the dispersing liquid away from the dispersed and entangled CNTs, and through the openings in the filter material. The mesh screen or belt is typically a stainless steel, and has a pattern of wire mesh sufficient to support the filter material in a plane as it passes over the vacuum zone (also called a vacuum box). The strength or magnitude of the vacuum (pressure) and the length of the vacuum zone (or dwell time) are sufficient to draw substantially all of the free dispersive solution from above the filter material, while also allowing the dispersed CNTs to settle onto the filter material, typically in a randomly-oriented, uniformly-distributed pattern upon the filter material. Uniformly distributed CNTs will appear as a uniform, black material surface across the entire width of the filter material. Typically the CNT sheet structure has a uniformity of not more than 10% coefficient of variance (COV), wherein COV is determined by a well-known, conventional method. In an aspect of the invention, the carbon nanotubes (CNTs) comprised in the nonwoven sheet form a continuous matrix or phase across the entire area of the nonwoven sheet, where the CNTs are in direct contact with one or more adjacent CNTs along their lengths. Selection of vacuum box dimensions of length and width, can be optimized as needed for different CNT lengths and CNT nonwoven sheet bases weights. The CNT structure can become attached to the filtering material through the CNT material at the surface, forming a CNT/filtering material composite. 
     The desired basis weight of the resulting CNT structure is affected by several parameters, including process conditions, apparatus, and the materials used. Generally, the larger the basis weight required, the higher the required CNT concentration, and/or the larger the dispersed liquid loading, and/or the larger the vacuum zone area, and/or the higher the vacuum applied, and/or the slower the linear speed of the filter material over the vacuum zone. All of these parameters can be manipulated to achieve specific desired characteristics of the CNT nonwoven sheet, including its thickness, density, and porosity. 
     CNT Nonwoven Sheets 
     The CNT nonwoven sheets made according to the present invention, when used alone or as part of a composite structure or laminate, can provide numerous mechanical and functional benefits and properties, including electrical properties. The CNT nonwoven sheets and composite laminates and structures thereof can be used for constructing long and continuous thermal and electrical paths using CNTs in large structures or devices. The CNT nonwoven sheets and composite laminates and structures thereof can be used in a very wide variety of products and technologies, including aerospace, communications and power wire and cable, wind energy apparatus, sporting goods, etc. The CNT nonwoven sheets and composite laminates and structures thereof are useful as light-weight multifunctional composite structures that have high strength and electrical conductivity. The CNT nonwoven sheets and composite laminates and structures thereof can be provided in roll stock of any desirable and commercially-useful width, which can integrate into most conventional product manufacturing systems. 
     Non-limiting examples of functional properties, and the modulation thereof, that can be provided by the CNT nonwoven sheets and composite laminates and structures thereof, are conductive composites, electromagnetic wave absorption, in-situ structural health monitoring, lightning strike prevention and dissipation, water filtration, electromagnetic interference (EMI) shielding, thermal interface pads, energy storage, supercapacitor, and heat dissipation. 
     The desired basis weight of the resulting CNT structure is at least 1 grams of the CNTs per square meter (gsm), which can include a CNT basis weight selected from the group consisting of at least 1 gsm, at least 2 gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and at least 6 gsm; and up to about 40 gsm, including up to about 30 gsm, up to about 20 gsm, up to about 15 gsm, up to about 12 gsm, up to about 10 gsm, up to about 8 gsm, and up to about 6 gsm; and can be about 3 gsm, about 4 gsm, about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9 gsm, about 10 gsm, and about 15 gsm. 
     CNT nonwoven substrates having very low basis weight, typically of about 4 gsm or less, are so thin that they cannot be separated themselves, independently, from the filter material (scrim) without falling apart. CNT nonwoven sheets having very low basis weight can be separated from the filter material using a tacky substrate that itself comprises a member of a composite structure. 
     The present invention also includes a secondary web material that is processed with the CNT suspension liquid. The secondary web material can include low basis weight fiberglass, melt-spun or wet-laid nonwovens made of thermoplastics, including polyester, and carbon fiber veils or webs. In one embodiment of the invention, the secondary web material can be the filter material onto which the CNT suspension is applied and through which the dispersive liquid is drawn to deposit the CNT nonwoven sheet. In another embodiment, the secondary web material can be disposed in registry upon the upper surface of the filter material to form a dual filter material. Typically, after drying of the CNT nonwoven sheet, the secondary web material is removed from the base filter material with the CNTs attached thereto. The use of a secondary web material is also advantageous with very low bases weight CNT nonwoven sheets, as described above. 
     Functionalizing of CNTs 
     Functional properties of a CNT nonwoven sheet can be affected by treatment of the CNTs of the CNT nonwoven sheet, prior to their dispersion and suspension, or after formation into a CNT nonwoven sheet. The treatment of the CNTs or of the CNT nonwoven sheet can include a chemical treatment or a mechanical treatment. 
     In one aspect of the invention, functional properties of a CNT nonwoven sheet can be affected by an acid treatment of the CNTs, prior to their dispersion and suspension, or by a post-formation acid treatment. The post-formation treatment can be performed either in a batch treatment process, or in a continuous (roll) process, by immersing the CNT nonwoven sheet into an acid bath or by application of an acid solution thereto, followed by rinsing with water to remove residual acid, and drying. An acid treatment is believed to improve CNT purity and quality, by reducing the level of amorphous carbon and other defects in the CNTs. Treatment of the bulk CNT powder with strong (nitric) acid can cause end-cap cutting, and the introduction of carboxyl groups to the CNT sidewall. The addition of carboxyl groups to the CNT sidewalls can also enhance dispersion of the CNTs in water or other polar solvent by increasing the hydrophilicity of the CNTs. The removal of amorphous carbon coatings on individual nanotubes increases the concentration of crosslink joints and higher bending modulus, which can create more conductive tunnels and connections. CNT end-cap cutting can improve electrical conductivity by improving electron mobility from the ends of the carbon nanotubes to adjacent carbon nanotubes (tunneling). Likewise, post-formation acid treatment can improve electrical conductivity and increase the structure&#39;s density. 
     The acid treatment of the CNTs enhances CNT interactions and charge-carrying and transport capabilities. Acid treatment of the CNTs can also enhance cross-linking with a polymer composite. Without being bound by any particular theory, it is believed that during acid oxidation, the carbon-carbon bonded network of the graphitic layers is broken, allowing the introduction of oxygen units in the form of carboxyl, phenolic and lactone groups, which have been extensively exploited for further chemical functionalization. 
     The pre-treatment of the CNTs can include immersing the CNTs into an acidic solution. The acid solution can be a concentrated or fuming solution. The acid can be selected from an organic acid or inorganic acid, and can include an acid that provides a solution pH of less than 1.0. Examples of an acid are nitric acid, sulfuric acid, and mixtures or combinations thereof. In an embodiment of the invention, the acid is a 3:1 (mass) ratio of nitric and sulfuric acid. 
     Alternatively or in addition to acid post-post treatment, the CNT powder or formed CNT nonwoven sheet can be functionalized with low pressure/atmospheric pressure plasma, as described in Nanotube Superfiber Materials, Chapter 13, Malik et al, (2014), the disclosure of which is incorporated by reference in its entirety. A Surfx Atomflo 400-D reactor employing oxygen and helium as the active and carrier gases, respectively, provides a suitable bench-scale device for plasma functionalizing CNTs and CNT non-woven sheet structures. An alternative plasma device can include a linear plasma head for continuous functionalization of CNT non-woven sheets, including non-woven roll stock. An atmospheric plasma device produces an oxygen plasma stream at low temperature, which minimizes or prevents damage to the CNTs and the CNT structures. In an example, a plasma is formed by feeding He at a constant flow rate of 30 L/min and the flow rate of O 2  (0.2-0.65 L/min) is adjusted as per the plasma power desired. Structural and chemical modifications induced by plasma treatments on the MWCNTs can be tailored to promote adhesion or to modify other mechanical or electrical properties. Additionally, plasma functionalization can be used to clean the surface of the CNT nonwoven structure, cross-link surface molecules, or even generate other polar groups on the surface to which additional functional groups can be attached. The extent to which the CNT nonwoven are affected by plasma functionalization can be characterized using Raman spectroscopy, XPS, FTIR spectroscopy and changes in hydrophobic character of the CNT material through contact angle testing. 
     Another example of a chemical treatment is the addition of a large molecule onto the CNT structure which reduces the CNT-to-CNT contacts along the length of the carbon nanotubes. An example of a large molecule is an epoxide. Epoxide functionalization has been shown to increase the electrical resistance of a non-woven sheet. Further increases in sheet resistance have been shown by adding surfactants to the epoxide functionalized CNT dispersion which act to further separate the distance between adjacent carbon nanotubes. A 20× increase in sheet resistance has been observed with this approach, increasing the sheet resistance from about 5 ohms per square (Ω/□) to about 100Ω/□ for a 10 gsm nonwoven. 
     Another example of a chemical treatment is the treatment of the CNTs with fluorine. The bulk CNT powder or CNT sheets or structures can be treated with fluorine gas. Fluorination can increase resistance of CNTs to the point of becoming electrical insulators. The process uses fluorine gas at temperatures above 250° C. to create C—F bonds on the sidewalls of each individual carbon nanotubes. 
     Conducting or non-conductive particles or fibers can also be added to the CNTs to enhance or suppress electrical conductivity. In a non-limiting example, the addition of fiberglass flock to an entanglable CNT dispersion increased the electrical resistance and mechanical properties of a sheet. For example, the electrical resistance of a 1.0 gsm CNT sheet (CNT basis only) is increased from about 50Ω/□ to about 60Ω/□, an increase of 20%, while increasing the composite overall basis weight to 26.0 gsm. Of note, graphene and carbon nanofibers are used as filler in the high sheet resistance blends (on polyester) to maintain high uniformity at such low basis weights (about 2 gsm). 
     The present invention also includes producing a low basis weight (2 gsm) nonwoven sheet using either of entanglable CNTs or short MWCNTs, or both entanglable CNTs and short MWCNTs, on the wet-laying nonwoven apparatus that can include modifications in the construction and operation as described herein. 
     Electrical conductivity within the nonwoven CNT sheet structure can be enhanced or suppressed by mechanical treatment and processing of the CNT sheet. First, and simply, the basis weight can be increased to decrease sheet resistance (increase conductivity, at the cost and result of additional sheet weight and thickness). Empirically, the sheet resistance was found to “bottom out” at about 0.2Ω/□ with increased basis weight for a pretreated CNT sheet. 
     Mechanical methods for increasing electrical conductance (decreased sheet resistance) of a layer or nonwoven sheet of CNT include a method resulting in improved alignment of CNTs within the sheet, and densification of the CNT layer within the sheet. In addition, an improved dispersion of CNTs in the aqueous dispersion provides more densely packed CNTs in the non-woven CNT sheet, and the denser CNT packing in the nonwoven sheet results in a more conductive sheet and different mechanical or physical properties. Another approach being investigated is oxidation of the CNT sidewall via microwave excitation. This approach is similar to acid treatment in that it allows for better dispersion, which results in a denser nonwoven. 
     In another aspect of the invention, functional properties of a CNT nonwoven sheet can be affected by metallization of the CNTs, prior to their dispersion and suspension, or by metallization post-formation. The post-formation treatment can be performed either in a batch treatment process, or in a continuous (roll) process, by such methods as physical deposition (e.g. sputtering, physical vapor deposition (PVD), pulsed laser deposition (PLD), electron beam), chemical vapor deposition, electro-chemical (electroplating), or electroless coating. For example, a continuous nickel CVD process can be used to add a layer of nickel to the surface of the CNT sheet material, thereby increasing conductivity of the CNT nonwoven. 
     Manufacturing Process for a Continuous Sheet of CNT Structure 
     An apparatus  110  of the present invention useful in a manufacturing process  100  for forming CNT structures is shown in  FIGS. 2-4 . CNTs are dispersed by mixing in a water or solvent solution and are contained in a mixing/storage tank  101 . The CNT suspension  104  contained in the mixing/storage tank  101  can be prepared remotely and transported to the facility for manufacturing the CNT structures, or can be prepared at the manufacturing site. The second step in making the CNT structure comprises passing a volume of the CNT suspension over a filter material, and drawing the dispersive liquid of the CNT suspension through the filter material to provide a uniform dispersion of the CNTs over the filter material. The CNT suspension  104  of CNTs in water is delivered by pump  102 , or alternatively poured by gravity, at a controlled volumetric rate over a continuous porous belt  130 , typically through a continuous sheet filter material or scrim  132 . 
     A continuous porous belt  130  is typically a metallic, plastic-coated metallic, thermoplastic, or composite conveying belt, mesh or screen. The continuous porous belt  130  can have hinged sections or segments that provide industrial durability and reliable processing of the belt through the wet-laid apparatus  110 , drier  180 , and through the rollers  136 , including for separation of the final CNT structure from the filter material, and its storage onto a product roller  192 . The conventional construction of the porous belt  130  in a wet-laid nonwoven process apparatus provides very poor filtration of the nano-sized CNTs, which readily pour through the openings in the continuous belt  130 , this indicating the need for a separate porous filter medium. The continuous porous belt  130  should have low extensibility or stretch in the machine direction for pulling the porous belt through the pathway of the apparatus. In general, the porous belt  130  should not be used as a filter medium for filtering CNTs from the CNT suspension, because the directly-deposited CNT structure may be difficult to separate from the porous belt, particularly in an industrial or commercial process. 
     The continuous porous belt  130  passes in a continuous loop or belt, carrying a filter medium, shown as a continuous porous carrier material  132 , across and through the apparatus  110 . 
     The continuous porous carrier material is a flexible, resilient sheet material having pores or openings that are sufficiently large to allow the dispersive liquid to be drawn through with a moderate amount of vacuum or pressure, though are sufficiently small to prevent the multitude of dispersed CNTs from passing through when the carrier material is also used as a filter material. The size of the openings (circular, square or any other shape) are typically about a size between about 0.1 micron, and up to about 10 micron, and the porosity (open area) is typically about 200% to about 80%, and selected from the group consisting of about 30%, about 40% or about 50%. 
     A continuous porous carrier material (generally referred to as a scrim)  132  provides a stable structure for pulling the forming CNT structure through and along the apparatus  110 , to prevent tearing and degradation during manufacturing. The continuous porous carrier material has a width up to about 152 cm (60 inches). The scrim is typically unrolled from a supply roller  138  and applied to the upper surface of the moving, continuous porous belt  130 , and after removal of the CNT sheet product, is separated from the continuous porous belt  130  and re-rolled onto a reuse roller  139 , which is then reused as a supply roller  138 . Another function of the continuous porous carrier material  132  is to provide a more planar, smoother filtration surface, as compared to the continuous porous belt  130 , which provides improved planar uniformity to the resulting CNT structure. 
     The continuous filter material  132  can provide the primary filtration of the CNT material from the dispersing liquid, and in this situation can be referred to as a filtering scrim. The filtering scrim  132  can comprise either a hydrophobic material or a hydrophilic material. A suitable hydrophobic material is a high porosity Teflon™ mesh. The hydrophobicity provides suitable and satisfactory release of the resulting CNT structure from the Teflon™ mesh scrim. A suitable hydrophilic material is a high porosity nylon mesh. 
     The apparatus  110  includes a table  111  having a planar top surface  112 , along the length of which is drawn the continuous porous belt  130 , from an inlet end  114  to outlet end  116 . In a center portion of the planar top surface  112  of the apparatus  110  are a plurality of vacuum slots  120  formed in series along the length and extending transversely across the width of the top surface  112 . The vacuum slots  120  are disposed for drawing the dispersing liquid away from the filtered CNTs under vacuum  124 . The vacuum capacity through each of the plurality of vacuum slots  120  can be independently controlled  122  for improved deposition and pinning of the CNTs onto the continuous filter scrim  132 , along the path traversing the vacuum box. The vacuum applied along the one or more vacuum slots  120  can be controlled independently. The typical vacuum is at least −1.0 psig, including in a range from about −2.5 psi to about −14 psi, and a typical vacuum gradient is about 2 psi to about 11 psig, along the length (machine direction) of the vacuum box, with a stronger vacuum applied at the leading-most vacuum slots, such as  120   a . The plurality of slots  120  can closely spaced apart, a single enlarged vacuum opening can be used, to provide a substantially continuous vacuum area. Alternatively, the plurality of slots can be spaced apart with intermediate surfaces therebetween without a vacuum force upon the remaining flooding pool of CNT or the filtering CNT structure that passes over. 
     The physical length of the vacuum box of the present apparatus can be expected to be at least 5-10 times the length the vacuum box in a conventional wet-laid nonwoven process. The vacuum residence time under which the flooding pool of CNT solution is exposed to the vacuum filtration can be up to about 1 minute, including up to about 10 seconds, and up to about 1 second. 
     The manufacturing line speed of the apparatus and method of the present invention is at least about 1 foot per minute (fpm), including at least about 10 fpm, at least 50 fpm, and up to about 100 fpm and more. 
     The hydrophobicity and porosity of a filter scrim  132 , for example, a Teflon™ mesh scrim material, can result in puddling of the solution of CNT suspension placed upon the advancing flat surface of the filter scrim  132 . Puddling occurs when the solution of the aqueous CNT dispersion (a hydrophilic solution with dispersed CNTs) forms discontinuous and separated puddles on the hydrophobic filter scrim  132 , typically due to their difference in interfacial surface tension. The application of any vacuum force on the underside of a filter scrim on which puddling of the solution of CNT suspension has occurred, results in the CNTs within the puddles forming into discontinuous patches of the filtered CNTs. 
     Another factor that can affect CNT filtration and the uniformity of the CNT structure is the dispersibility of the CNTs in an aqueous solution. CNTs inherently are poorly dispersible in aqueous solutions, even after being treated to provide improved hydrophilicity. Even after dispersion, the CNTs, and particularly entanglable CNTs having a long aspect ratio, tend to flocculate or aggregate into bundles or larger clot-like lumps in the aqueous dispersion. The conditions of the continuous process can be managed to inhibit or prevent puddling of the CNT suspension, and to control the inherent tendency of dispersed CNTs to flocculate or agglomerate. 
     In an embodiment of the invention, the apparatus  110  includes a reservoir or “headbox”  140  at the inlet end  114  that is bounded by the walls of the apparatus on three sides, and by a positionable sluice gate  142  on the downstream side that allows a portion of the CNT dispersion volume  144  within the reservoir  140  to flow under the bottom edge of sluice gate  142 . The spacing under the sluice gate  142  is sufficient to pass thereunder the continuous porous belt  130  with the filter scrim  132 , and to permit a controlled amount and depth of the CNT suspension to flow out of the reservoir. Downstream  118  from the sluice gate  142  is a flooded pool  146  of the CNT suspension that flows in the direction of movement of the scrim  132 . The velocity v2 of the mass of the flooded pool  146  in the pooling area  118 , flowing toward the vacuum slots  120 , is two or more times, and up to 5 times, and up to 10 times, the linear velocity of the moving scrim  132 . The depth of the flooded pool  146  across the width of the apparatus  110  is uniform and sufficient to spread and cover the entire surface of the filter scrim  132 , to prevent puddling of the solution before arriving at the vacuum slots  120 . 
     As the advancing flooded pool  146  above the scrim  132  arrives at the first of the vacuum slots  120   a , the liquid within the CNT dispersion is strongly drawn through the filter scrim  132 , which pins and sets a first portion or layer of the CNTs upon the filter scrim  132  ( FIG. 3 ). As the flooded pool  146  above the filter scrim  132  advances and arrives at the second and subsequent vacuum slots  120 , addition portions of layers of CNTs are pinned to and deposited upon the upper surface of the filter scrim  132 . Without being bound by any particular theory, it is believed that providing a uniform layer of filtered CNTs onto the upper surface of the filter scrim  132  (or porous filter material, as well as any secondary veil-filter layer, described hereinafter) modifies the interfacial surface of the filter scrim  132  and improves the wetting of its surface by the CNT suspension, thereby minimizing and eliminating puddling upon the effected surface of the filter scrim. 
     As the advancing flooded pool  146  advances further with the filter scrim  132 , its velocity v2 slows as more of the liquid is drawn through the filter scrim  132  by the successive vacuum slots  20 . This also increases the density and thickness of the CNTs above and along the filter scrim  132 . The slowing of the velocity of the flooded pool  146  permits any flocculated bundles or lumps of the CNTs in the upstream flooded pool  146  to “pack in” behind the depositing CNT structure, thus improving uniformity and reducing the variability in the density and thickness of the CNTs. Eventually, the forward edge of the flooded pool  146  “dries up” when a sufficient amount of the dispersing liquid has been drawn through, depositing all of the CNTs onto the filter scrim  132 . Substantially all of the liquid from the CNT dispersion is drawn through the filter scrim  132  as the CNT structure  150  exits the last vacuum slot  120   f.    
     After drawing away most of the water or other dispersive liquid, the advancing continuous CNT structure  150  is dried to a CNT sheet using a convection, contact and radiation dryer  180  ( FIG. 4 ). The dried, continuous CNT sheet  155  is separated from the filter scrim  132  and taken up on a product roller  158 . The separated scrim  132  is taken up on a reuse roller  139  for reuse in the process as scrim  132 . 
       FIGS. 7 and 8  show alternative apparatus and methods for providing a controlled amount and depth of the CNT suspension in a flooded pool  146  that flows over and in the direction of movement of the filter scrim  132 .  FIG. 7  shows a container  242  including walls sufficient to contain a depth of CNT suspension  244 , and a base  272  having apertures  274 . The container  242  extends across the width of the filter scrim  132  to distribute the CNT solution flowing through the apertures  274  within the pooling area  118 . The apertures  274  are sized in area and provided in number to maintain a depth of the flooded pool  146  sufficient to prevent puddling of the CNT suspension upon the surface of the filter scrim  132 .  FIG. 8  shows a spray distribution device  342  including a manifold  373  and a plurality of spray nozzles  374 . The plurality of nozzles  374  extend across the width of the filter scrim  132  to distribute the CNT solution flowing through the nozzles  374  within the pooling area  118 . The nozzle size, number and distribution likewise maintains a depth of the flooded pool  146  sufficient to prevent puddling of the CNT suspension upon the surface of the filter scrim  132 . 
     The solution of dispersed CNTs can be distributed or applied onto the filtering surface by other well-known methods, including spraying, rolling, coating, and casting. 
       FIG. 5  shows the apparatus  110  in a similar manufacturing process for forming CNT structures wherein a secondary veil layer  192  is unrolled from a veil supply roller  190  and applied continuously onto the moving upper surface of the scrim  132  at roller  196  to form a veil-carrier laminate  194 . The veil-carrier laminate  194  is then continuously applied onto the belt  130  are previously described, and passed through and along the table, headbox(es) and vacuum slots apparatus  110 . The CNT suspension is applied, flooded across, and filtered through the veil-carrier laminate  194 , substantially as described above, to form a continuous CNT structure  150  on the veil-carrier laminate  194 . In this embodiment, the veil layer  192  can be the primary filter medium, with the CNTs filtered directly onto the upper surface and elements of the veil layer, or the scrim (filter scrim) can be the primary filter medium, where an initial layer of CNTs can pass through the veil layer material and pinned and filtered on the filter scrim  132 , and the remaining filtered CNTs layering on and above the upper surface of the veil layer; or a combination thereof. After forming and drying the continuous CNT structure  150 , a composite layer  156  of the CNT structure  150  and the veil layer  192  can be separated from the carrier material  132 , and stored on a product roller  158 . The carrier material  132  is taken up on a reuse roller  139  for reuse in the process as scrim  132 . 
       FIG. 6  shows the apparatus  110  in a further similar manufacturing process for forming CNT structures wherein a continuous porous carrier-veil layer  198  is unrolled from a carrier-veil supply roller  197  and applied continuously onto the moving upper side of the continuous conveying belt  130 , The carrier-veil layer  198  is then passed through and along the table, headbox(es) and vacuum slots apparatus  110 . The CNT suspension is applied, flooded across, and filtered through the carrier-veil layer  198 , substantially as described above, to form a continuous CNT structure  150  onto the carrier-veil layer  198 . In this embodiment, the carrier-veil layer  198  is the primary filter medium for the CNT suspension. After forming and drying the continuous CNT structure  150 , a composite unitary layer  157  of the CNT structure  150  and the carrier-veil layer  198  can be separated from the belt  130 , and stored on a product roller  158 . 
     The resulting CNT sheet made with the above manufacturing processes can have a low density and low basis weight CNT structure with the uniformity and integrity for a wide variety of technology and industrial uses. The process also provides the flexibility to form effective CNT sheets from either SWCNTs or MWCNTs, or a combination thereof. For example, using a SWCNT having a median length above 50 microns and an aspect ratio greater than 5,000, a CNT sheet having a thickness of 40 microns and a relative density of 0.5 provides a basis weight of about 20 gsm, while a MWCNT having a median length above 100 microns and an aspect ratio greater than 5,000 forms CNT sheet having a thickness of 40 microns and a relative density of 0.25, with a basis weight of about 10 gsm, 
     The desired basis weight of the manufactured CNT structure can be at least 1 gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and at least 6 gsm; and up to about 40 gsm, including up to about 30 gsm, up to about 20 gsm, up to about 15 gsm, up to about 12 gsm, up to about 10 gsm, up to about 8 gsm, and up to about 6 gsm; and can be about 3 gsm, about 4 gsm, about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9 gsm, about 10 gsm, and about 15 gsm. 
     The relative density of the manufactured CNT structure can be about 1.0 or less, and can be about 0.8 or less, about 0.7 or less, about 0.6 or less, about 0.5 or less, about 0.4 or less, and about 0.3 or less, such as 0.25. Such relative densities are well below those of buckypapers described in the art, and provide a CNT sheet with good uniformity and structural stability, an acceptable thickness, and a substantially lower basis weight. 
     Wet-laid CNT sheet can also employ bonding agents, including water or solvent-based crosslinkable synthetic polymer to provide flexibility and strength. 
     A secondary layer as described above can be used to support to the CNT structure, forming a composite sheet product to improve the mechanical properties and handling (processability) of the CNT sheet. The secondary layer, also referred to herein as a substrate or veil layer in the composite sheet, is typically a fabric, woven, or non-woven material, and is typically porous and flexible. In an embodiment of the invention, the veil layer is non-stretchable or non-extensive, to limit or present stretching of the composite sheet product. The veil layer can be processed with the CNTs in the wet-laid process, through and onto which the CNTs and any blended adjunct components can be vacuum formed, or can be post-laminated or made co-extensive with the formed CNT sheets. Examples of a suitable veil layers can include, but not limited to, carbon fiber nonwoven, polyester woven or nonwoven, fiberglass nonwoven or woven, expanded copper foils, meshes, screens, or PEEK (polyether ether ketone) nonwoven. 
     In another embodiment of the invention, a wet-laying nonwoven apparatus can include a plurality of headboxes for depositing a second dispersed CNT solution of suspended CNTs, fibers or other materials over the mesh screen.  FIG. 9  shows a schematic of the wet-laying nonwoven apparatus  300  that includes two or more reservoirs or head boxes (illustrated as head boxes  140 ,  240 , and  340 ), and a corresponding two or more tables with vacuum boxes (illustrated as tables  111 ,  211 , and  311 ), arranged in series along the length of the continuous belt  130 . The suspended CNTs, fibers or materials of the second dispersed CNT solution can include a second quantity or type of CNT(s), having the same or different physical properties, or having the same or different functional properties, from the CNTs of the first aqueous solution. The CNT, suspended fibers or materials of the second aqueous solution can include other nano-sized or micron-sized fibers or materials. The resulting CNT sheet  350  is dried and taken up on product roller as earlier described.  FIG. 9  also shows that a secondary veil layer  192  can be provided before the first headbox  140 . Similarly, and optionally, a second veil layer  292  can be provided before the second headbox  240  to provide a laminate of the first  192  and second veil  192  layers with the CNTs structures from the first headbox  140  therebetween, and the CNTs structures from the second headbox  240  thereon; and a third veil layer  392  can be provided before the third headbox  240  to provide a laminate of the first  192 , second  292  and third  392  veil layers with the CNTs structures from the first headbox  140  and the second headbox  240  therebetween, and the CNTs structures from the third headbox  340  thereon. Optionally a fourth veil layer (not shown) can be applied on the resulting CNT sheet  350 . 
     In an example embodiment, a first headbox  140  deposits a dispersed CNT solution comprising entanglable CNTs, to form a base layer of entanglable CNTs on the scrim. A second headbox  240 , downstream of the first headbox  140 , deposits a second dispersed CNT solution comprising short MWCNTs, which are distributed and filtered ( 211 ) as a second layer of short MWCNTs over the base layer of entanglable CNTs. A third headbox  340  deposits a third dispersed CNT solution comprising entanglable CNTs, which are distributed and filtered as a top layer of entanglable CNTs over the second or intermediate layer of short MWCNTs. 
     In a second embodiment, a first headbox  140  deposits a dispersed CNT solution comprising entanglable CNTs, to form a base layer of entanglable CNTs on the scrim. A second headbox, downstream of the first headbox, deposits a second dispersed CNT solution comprising short MWCNTs, which are distributed and filtered ( 211 ) as a second layer of short MWCNTs over the base layer of entanglable CNTs. A third headbox  340  deposits a third dispersed CNT solution comprising thermoplastic fibers, which are distributed and filtered ( 311 ) as a top layer of thermoplastic fibers over the second or intermediate layer of short MWCNTs. 
     In other embodiments, any combination of A) entanglable CNTs, B) short MWCNTs, and C) other or thermoplastic fibers, can be prepared in nonwoven layers, ordered in any one of a wide variety, such as: ABA, ABB, ABC, ABC, CBA, CBC, ACA, etc. 
     In another embodiment of the invention, the second headbox can comprise a treatment solution which can be applied over the filtered nonwoven structure formed from the first headbox. A non-limiting example can include an acidic solution which is passed through the CNT non-woven structure, and maintained for a period of time sufficient to effect a change in the functional properties of the CNTs. An option third headbox can comprises a rinsing solution to remove the residual treatment (acid) solution following the (acid) treatment. 
     The second headbox, and subsequent headboxes, can be positioned downstream from the first vacuum box by a distance sufficient to ensure effective web formation, and to prevent any deconstruction of the first deposited layer of CNTs, or other fiber or material, deposited by the first (or previous) headbox, within the second (or subsequent) headbox. 
     In an alternative embodiment, the two or more headboxes can be arranged side-by-side, in parallel, over the mesh screen, with corresponding vacuum boxes, in order to deposit different nonwoven materials onto separate lateral zones of the filter material. Second and subsequent downstream headboxes can also be provided. 
     In an example, a wet-laying nonwoven apparatus has three headboxes and three vacuum boxes, for wet laying in series of three liquid suspensions of fibers. As a non-limiting example, the first and third suspensions are an aqueous solution of acid-treated entanglable CNTs. The second suspension is an aqueous solution of short MWCNTs. The resulting nonwoven CNT includes a layer of short MWCNTs sandwiched between two layers of entanglable CNTs.