Abstract:
Substrates with wet laid staple polymeric nanofibers of short lengths are disclosed. The polymeric nanofibers can be surface coated on a non-woven or woven substrates, wet laid with other fiber types to create a nonwoven substrate or wet laid onto themselves to form a nanofiber-only mat.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to forming wet laid substrates that contain polymeric nanofibers. More specifically, the invention relates to forming a substrate with wet laid staple polymeric nanofibers of short lengths. The polymeric nanofibers can be surface coated on a non-woven or woven substrates, wet laid with other fiber types to create a nonwoven substrate or wet laid onto themselves to form a nanofiber-only mat. 
       BACKGROUND 
       [0002]    Fibers form, in part or in whole, a large variety of both consumer and industrial materials such as, for example, clothing and other textile materials, medical prostheses, construction materials and reinforcement materials, and barrier, filtration and absorbent materials. There are two main structural classes of fiber materials: woven and non-woven. An advantage of non-woven fiber materials is their lower production cost. 
         [0003]    Wet lay technology is essentially a paper machine process to form nonwoven substrates. In this process fibers are suspended in liquids and specialized paper machines are used to separate the water from the fibers to form a uniform sheet of material, which is then bonded and dried. 
         [0004]    Polymeric nanofibers are increasingly being investigated for use in various applications. Nanofibers may attain a high surface area comparable with the finest nanoparticle powders, yet are fairly flexible, and retain one macroscopic dimension which makes them easy to handle, orient and organize 
         [0005]    Accordingly, an ongoing need remains for developing wet laid substrates containing polymeric nanofibers or nanofiber-only substrates. 
       SUMMARY 
       [0006]    The present invention comprises a fabric substrate of cotton, synthetic or blend fibers containing wet laid polymeric, staple nanofibers of short cut lengths ( FIG. 1 ). The staple polymeric nanofibers can be wet laid onto a fabric substrate of natural, synthetic or blend fibers or the nanofibers can be wet laid with other fibers to form a nonwoven mat or the nanofibers can be wet laid onto themselves to form a nonwoven containing only nanofibers. 
         [0007]    A wide variety of polymers or biopolymers may be utilized as starting materials, examples of which are given below. 
         [0008]    A wide variety of fabric substrates of natural, synthetic or blend fibers may be utilized as starting materials, examples of which are given below. 
         [0009]    Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0011]      FIG. 1  is a schematic of nanofibers and microfibers wet-laid into a composite substrate. 
           [0012]      FIG. 2  is a scanning electron micrograph of the cross-section of a wet-laid substrate consisting of 70% by weight PET microfibers (˜10 micron diameter) and 30% cellulose acetate nanofibers (˜500 nm diameter). 
           [0013]      FIG. 3  is a scanning electron micrograph of the surface of a PET microfiber substrate coated on the top with cellulose acetate nanofibers. 
           [0014]      FIG. 4  is a scanning electron micrograph of the cross-section of a PET microfiber substrate coated on the top with cellulose acetate nanofibers. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    As used herein, the term nanofiber refers generally to an elongated fiber structure having an average diameter ranging from less than 50 nm-2 μm. The “average” diameter may take into account not only that the diameters of individual nanofibers making up a plurality of nanofibers formed by implementing the presently disclosed method may vary somewhat, but also that the diameter of an individual nanofiber may not be perfectly uniform over its length in some implementations of the method. In some examples, the average length of the nanofibers may range from 10 micros or greater. In other examples, the average length may range from 110 microns to over 25 centimeters. In some examples, the aspect ratio (length/diameter) of the nanofibers may range from 10:1 or greater. In some specific examples, nanofibers according to the invention have aspect ratios of at least 10,000:1. Insofar as the diameter of the nanofiber may be on the order of two microns or less, for convenience the term “nanofiber” as used herein encompasses both nano-scale fibers and extremely small micro-scale fibers (microfibers). 
         [0016]    As used herein, the term fibril refers generally to a fine, filamentous non-uniform structure in animals or plants having an average diameter ranging from about 1 nm-1,000 nm in some examples, in other examples ranging from about 1 nm-500 nm, and in other examples ranging from about 25 nm-250 nm. According to certain methods described below, fibrils are formed by phase separation from nanofibers. In these methods, a fibril may be composed of an inorganic precursor or an inorganic compound. In the present disclosure, the term “fibrils” distinguishes these structures from the polymer nanofibers utilized to form the inorganic fibrils. The length of the fibrils may be about same as the polymer nanofibers or may be shorter. 
         [0017]    Polymers encompassed by the present disclosure generally may be any naturally-occurring or synthetic polymers capable of being fabricated into nanofibers. Examples of polymers include many high molecular weight (MW) solution-processable polymers such as polyethylene (more generally, various polyolefins), polystyrene, cellulose, cellulose acetate, poly(L-lactic acid) (PLA), polyacrylonitrile (PAN), polyvinylidene difluoride (PVDF), conjugated organic semiconducting and conducting polymers, biopolymers such as polynucleotides (DNA) and polypeptides, etc. 
         [0018]    Other examples of suitable polymers to form nanofibers include vinyl polymers such as, but not limited to, cellulose acetate propionate, cellulose acetate butyrate, polyethylene, polypropylene, poly(vinyl chloride), polystyrene, polytetrafluoroethylene, poly(α-methylstyrene), poly(acrylic acid), poly(isobutylene), poly(acrylonitrile), poly(methacrylic acid), poly(methyl methacrylate), poly(1-pentene), poly(1,3-butadiene), poly(vinyl acetate), poly(2-vinyl pyridine), 1,4-polyisoprene, and 3,4-polychloroprene. Additional examples include nonvinyl polymers such as, but not limited to, poly(ethylene oxide), polyformaldehyde, polyacetaldehyde, poly(3-propionate), poly(10-decanoate), poly(ethylene terephthalate), polycaprolactam, poly(11-undecanoamide), poly(hexamethylene sebacamide), poly(m-phenylene terephthalate), poly(tetramethylene-m-benzenesulfonamide). Additional polymers include those falling within one of the following polymer classes: polyolefin, polyether (including all epoxy resins, polyacetal, polyetheretherketone, polyetherimide, and poly(phenylene oxide)), polyamide (including polyureas), polyamideimide, polyarylate, polybenzimidazole, polyester (including polycarbonates), polyurethane, polyimide, polyhydrazide, phenolic resins, polysilane, polysiloxane, polycarbodiimide, polyimine, azo polymers, polysulfide, and polysulfone. 
         [0019]    As noted above, the polymer used to form nanofibers can be synthetic or naturally-occurring. Examples of natural polymers include, but are not limited to, polysaccharides and derivatives thereof such as cellulosic polymers (e.g., cellulose and derivatives thereof as well as cellulose production byproducts such as lignin) and starch polymers (as well as other branched or non-linear polymers, either naturally occurring or synthetic). Exemplary derivatives of starch and cellulose include various esters, ethers, and graft copolymers. The polymer may be crosslinkable in the presence of a multifunctional crosslinking agent or crosslinkable upon exposure to actinic radiation or other type of radiation. The polymer may be homopolymers of any of the foregoing polymers, random copolymers, block copolymers, alternating copolymers, random tripolymers, block tripolymers, alternating tripolymers, derivatives thereof (e.g., graft copolymers, esters, or ethers thereof), and the like. 
         [0020]    By “web” is meant a fibrous material capable of being wound into a roll. 
         [0021]    By “nonwoven web” is meant a web of individual fibers or filaments which are interlaid and positioned in a random manner to form a planar material without identifiable pattern, as opposed to a knitted or woven fabric. Nonwoven webs have been in the past formed by a variety of processes known to those skilled in the art such as, for example, meltblown, spunbound, wet-laid, dry-laid, and bonded carded web processes. 
         [0022]    A nonwoven or woven fabric substrate or web can be made from natural or synthetic fabrics and may be composed of fibers of cotton, cellulose, Lyocell, acetate, cellulose acetate, rayon, silk, wool, hemp, spandex (including LYCRA), polyolefins (polypropylene, polyethylene, etc.), polyamide (nylon 6, nylon 6-6, etc.), aramids (e.g. Kevlar®, Twaron®, Nomex, etc.), acrylic, or polyester (polyethylene teraphthalate, trimethylene terephthalate), polyurethane, glass microfibers, fiberglass, etc. By “fabric blends” is meant fabrics of two or more types of fibers. Typically these blends are a combination of a natural fiber and a synthetic fiber, but can also include a blend of two natural fibers or two synthetic fibers. 
         [0023]    Nanofibers can be wet laid deposited onto a non-woven or woven substrate, which is placed on a filter mesh of 27-200 microns pore size as per the following example: 
       EXAMPLE 
       [0024]    Wet laying process: Cellulose acetate (Eastman CA-398-10) nanofibers (average diameter of 400 nm and lengths of ˜200-700 μm or 2-10 mm) were first wet-laid (1 to 2 grams per square meter (GSM) of substrate) onto a fabric substrate of polyester. The back side of the fabric substrate was cellulose material. A dilute solution containing glycerol and water with suspended Cellulose acetate nanofibers (˜0.1% solids) was poured onto the polyester fabric substrate placed on top of a plastic filter mesh (80 mesh size). A wet-dry shop vacuum (Shop-Vac 6-Gallon 3 Peak HP) was used to pull vacuum to drain the liquid through the filter fabric and lay the nanofibers down on top of the polyester fabric substrate. The sample was then washed and then heat pressed or oven baked. 
         [0025]    Nanofibers can also be deposited onto themselves without a substrate with basis weights ranging from 4 to 800 GSM or higher. In this case the length is important as longer length fibers provide mat integrity and strength. 
         [0026]    Polymeric nanofibers can also be wet laid together with other nano- or microfibers to form a nonwoven substrate containing many types of fibers. 
         [0027]    Adding polymeric nanofibers to a substrate by wet laying techniques is novel and has not been achieved in the prior art as nanofibers are typically produced as long (&gt;20 cm) dry fibers by electrospinning and meltblowing technologies. The nanofibers used here are produced by the XanoShear process. This method allows production of polymeric nanofibers in a liquid based process.