DISPERSIONS

TECHNICAL FIELD

[0001] The present invention relates to shearing polymeric nano fibers to short cut lengths in liquids to control the length of the nano fiber to allow the nano fiber to be dispersed in liquids.

BACKGROUND

[0002] 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.

[0003] Most commercial nanofiber technologies, including electrospinning and melt blowing, produce polymeric nanofibers of long length (> 20 cm) in dry form.

[0004] Accordingly, an ongoing need remains for developing polymeric nanofibers of short cut lengths for nonwovens applications.

SUMMARY

[0005] The present invention comprises a method to shear long nanofibers to staple nanofibers of short cut lengths in liquids to improve how the nanofibers disperse in liquids.

[0006] A wide variety of methods may be utilized to shear the nanofibers to short cut lengths including, but not limited to, shearing achieved by a colloid mill, shear homogenizer, shear mixer, etc.

[0007] A wide variety of polymers may be utilized as starting materials, examples of which are given below.

[0008] 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. DETAILED DESCRIPTION

[0009] As used herein, the term nanofiber refers generally to an elongated fiber structure having an average diameter ranging from less than 50 nm - 5 μιτι in some examples, in other examples ranging from less than 100 nm - 5 μιτι, and in other examples ranging from 200 nm - 5 μιτι. In further examples, the average diameter ranges from 40 nm - 5 μιτι, 40 nm - 2 μιτι, 50 nm - 5 μιτι, 50 nm - 2 μιτι, 100 nm - 5 μιτι, 100 nm - 2 μιτι, 200 nm - 5 μιτι, or 200 nm - 2 μιη. 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 uniform over its length in some implementations of the method. In some examples, the average length of the nanofibers may range from 100 nm or greater. In other examples, the average length may range from 100 nm to millions of nm. In some examples, the aspect ratio (length/diameter) of the nanofibers may range from 100 or greater. In other examples, the aspect ratio may range from 20 to millions. In some specific examples, we have demonstrated nanofibers with aspect ratios of at least 10,000. Insofar as the diameter of the nanofiber may be on the order of a few microns or less, for convenience the term "nanofiber" as used herein encompasses both nano-scale fibers and micro-scale fibers (microfibers).

[0010] "Short cut length" is defined herein as fibers with lengths in the range of 10 to 2000 μιτι. In some cases the length can be as high as 1000-5000 μιτι. These may be termed "staple nanofibers". The fibers are of standardized length and may be of any chemical composition. The staple length may refer to an average length of a group of fibers, or a range of lengths in each sample containing fibers.

[0011] 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. [0012] Polymers encompassed by the present disclosure generally may be any naturally- occurring or synthetic polymers capable of being fabricated into nano fibers. Examples of polymers include many high molecular weight (MW) solution-processable polymers such as polyethylene (more generally, various polyolefms), 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.

[0013] 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(a- methylstyrene), poly(acrylic acid), poly(isobutylene), poly(acrylonitrile), poly(methacrylic acid), poly(methyl methacrylate), poly(l-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(lO-decanoate), poly(ethylene terephthalate), polycaprolactam, poly(l 1-undecanoamide), poly(hexamethylene sebacamide), poly(m-phenylene terephthalate), poly(tetramethylene-m-benzenesulfonamide). Additional polymers include those falling within one of the following polymer classes: polyolefm, poly ether (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.

[0014] 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.

[0015] Polymeric nanofibers can be sheared to short cut lengths in liquids to control their length to diameter aspect ratio using wet grinding processes and equipment. For example, colloid mill equipment with a milling head, or rotor and stator head can be used to mill polymeric nanofibers in liquid solutions. By adjusting key processing parameters (including shearing revolution per minute [RPM], liquid residence time, liquid flow rate, rotor/stator gap distance, etc.) the final chopped / sheared length of the nanofibers can be controlled.

[0016] Many wet grinding equipment types can be used to mill nano fiber lengths in liquid solutions including horizontal disk mills, basket mills, etc.

[0017] Nanofibers can be sheared in aqueous liquids mainly comprised of water or other liquids containing organic solvents and viscous mediums (glycerol, glycols, aqueous glycerin solutions, etc.).

[0018] For microfibers, lower length-to-diameter (L:D) aspect ratios are preferred to allow fibers to be dispersed in liquids. It has been reported that optimum L:D ratios are about 500. In some cases this optimal ratio can range from 100 to 1000. In other instances the L:D ratio can range more broadly from 20 to 3000. This L:D ratio is also preferred for nanofibers to allow them to more easily disperse in liquids.