POWDER OF FRAGMENTS OF AT LEAST ONE POLYMERIC NANOFIBER

Abstract

The invention concerns a powder of fragments of at least one polymeric nanofiber which fragments have a maximal average length of 0.12 mm.

Claims

1-15. (canceled)

16. Powder of fragments of at least one polymeric nanofiber which fragments have a maximal average length of 0.12 mm, wherein a maximal length of the fragments is 0.15 mm, wherein an average diameter of the fragments is in the range of 50 nm to 800 nm, wherein a ratio of the average length of the fragments to the average diameter of the fragments is in the range of 20 to 200.

17. Powder according to claim 16, wherein the maximal length of the fragments is 0.14 mm, in particular 0.13 mm, in particular 0.12 mm.

18. Powder according to claim 16, wherein the average diameter of the fragments is in the range of 90 nm to 800 nm.

19. Powder according to claim 16, wherein the nanofiber is produced by an electrospinning process.

20. Powder according to claim 16, wherein the nanofiber comprises a polyimide, a polyamide, a polyester, polyacrylonitrile, polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polysulfone, poly(acrylonitrile/styrene/butadiene copolymer (ABS), polycarbonate, polyamideimide, polyesterimide, polyurethane, polyguanidine, polybiguanidines, chitosan, silk, recombinant silk, collagene, cross-linked polyamide carboxylic acid, polyamide carboxylic acid, polyvinyl alcohol, polydiallyldimethylammonium chloride, polyvinylpyrrolidone, polystyrene (PS), polymethylmethacrylate (PMMA), a polycationic polymer, a polyanionic polymer, polycaprolactone, polylactic acid (PLA), poly-L-lactic acid (PLLA), or poly acrylic acid.

21. Powder according to claim 16, wherein the powder is dispersed in a gas, in a liquid thus forming a dispersion, in a further dispersion, or in a molten mass of a thermoplastic polymer.

22. Powder according to claim 21, wherein the gas is air and the liquid is or comprises water, water comprising a surfactant, an alcohol, ethanol, isopropanol, isobutanol, dimethylformamide (DMF), sulfolane, N-methylcaprolactam, N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), ethylene carbonate, propylene carbonate, a solution, a mixture of at least two of the aforementioned liquids, a supercritical liquid or supercritical carbon dioxide.

23. Product comprising the powder according to claim 16, wherein the powder is coated on a surface of the product or incorporated in the product.

24. Product according to claim 23, wherein the product comprises a composite of the powder and further fibers.

25. Product according to claim 24, wherein the further fibers comprise cellulose fibers.

26. Use of the powder according to claim 16 for the production of a product wherein the powder is coated on a surface of the product or incorporated in the product, wherein the powder is dispersed in the liquid or the further dispersion and applied in dispersed form to a surface of the product followed by evaporation, soaking and/or suction of the liquid or a dispersant of the further dispersion to produce a coating or wherein the powder is incorporated in the molten mass, the liquid or the further dispersion, which molten mass, liquid or further dispersion is the product, or from which molten mass, liquid or further dispersion the product is formed.

Description

EMBODIMENTS OF THE INVENTION

[0022] FIG. 1 shows a SEM micrograph of a powder according to the invention comprising fragments of polyimide nanofibers.

[0023] FIGS. 2a and 2b show SEM micrographs of a blend of molten polypropylene with incorporated polyimide nanofibers.

[0024] FIGS. 3a and 3b show a composite of cellulose fibers and polyimide nanofibers.

[0025] FIGS. 4a, 4b and 4c show SEM micrographs of the composite of FIGS. 3a and 3b.

[0026] FIG. 5 shows a digital micrograph of a nonwoven fabric made of polyamide carboxylic acid nanofiber fragments dispersed in a liquid.

[0027] FIG. 6 shows a digital micrograph of a nonwoven fabric made of polyamide carboxylic acid nanofibers produced directly by electrospinning.

[0028] FIG. 7 is a graph showing the deposition of aerosol as a function of the size of aerosol droplets in a filter made of polyamide carboxylic acid nanofibers.

[0029] FIG. 8 is a graph showing the pressure difference between two sides of filters made of polyamide carboxylic acid nanofibers as a function of the mass per unit area of the filters.

EXAMPLE 1

Preparation of a Powder of Polyimide Nanofibers Dispersed in a Liquid

[0030] A fiber mat made of electrospun polyimide (Kapton, DuPont) nanofibers were cut in 55 cm pieces and put in a blender having a cutting unit. A mixture of 2-propanol and water in the ratio 40:60 (wt:wt) in a beaker glass was cooled down by use of liquid nitrogen nearly to its solidification temperature such that it is barely liquid. The mixture was poured into the blender and mixed with the fiber mat pieces two times for one minute. Afterwards the resulting dispersion in the 2-propanol-water-mixture was allowed to warm up to room temperature. The generated fragments of nanofibers had an average length of about 0.1 mm. The dispersion was homogenous and remained stable for several months. The dispersion may be dried by evaporation, soaking, filtration, suction and/or freeze drying. The dried powder generated in this way can be dispersed again in a liquid up to a concentration of 30% by weight. FIG. 1 shows a SEM micrograph of polyimide nanofiber fragments generated in this way.

EXAMPLE 2

[0031] 2 ml of the dispersion produced as described in Example 1 were applied to polyamide and polyester tissues and spreaded by doctor blading. After evaporation of the dispersant a tissue coated with a nonwoven fabric of the polyimide nanofibers was obtained.

EXAMPLE 3

[0032] 1 g of the powder of fragments of polyimide fibers produced as described in Example 1 were mixed with 50 g polypropylene in a kneader at 180 C. for 30 minutes. A yellow blend of polypropylene and polyimide fiber fragments was obtained. At breaking edges of the blend the fragments could be seen by means of a scanning electron microscope (SEM). SEM micrographs of such a breaking edge are shown in FIG. 2a (1000-fold magnification) and FIG. 2b (5000-fold magnification). As can be seen from these micrographs the fragments of the nanofibers are distributed homogenously in the blend. Up to now such a homogenous distribution was not achieved with electrospun nanofibers.

EXAMPLE 4

Preparation of a Fiber Composite of Cellulose and the Powder According to the Invention

[0033] A powder of fragments of polyimide nanofibers was produced as described in Example 1 by dispersing 2 g of nanofibers in 800 ml of a mixture of 2-propanol and water in a ratio of 40:60 (wt:wt). Furthermore, cellulose in the form of paper was fragmented and soaked in 1000 ml water by stirring resulting in a fiber slurry. To this slurry 100 ml of the polyimide dispersion were added and mixed. The resulting dispersion was poured on a sieve. After the liquid run through the sieve the resulting mat was pressed with a stamp and thus densified and consolidated. The resulting mat was dried at 60 C. FIG. 3a shows the mat in total and FIG. 3b a microscopic photograph of the surface of the mat. FIGS. 4a, 4b and 4c show SEM micrographs of the mat in 150-fold (FIG. 4a), 300-fold (FIG. 4b) and 700-fold (FIG. 4c) magnification. From FIGS. 4a to 4c it can be seen that the relatively thick cellulose fibers are surrounded by the polyimide-nanofiber fragments.

EXAMPLE 5

Preparation of Nonwoven Filter Layers on Stainless Steel Grids

[0034] Polyamide carboxylic acid nanofibers were obtained by electrospinning from a polyamide carboxylic acid solution in dimethylacetamide. 2.4 g of the polyamide carboxylic acid (PAC) nanofibers were fragmentated in a solution of 600 ml 2-propanol and 1000 ml deionised water by means of a blender having a cutting unit at minus 18 C. For the production of a nonwoven filter layer of 3.1 mg/mm.sup.2 100 ml of the PAC-dispersion obtained in this way were diluted with 150 ml of a mixture of 600 ml 2-propanol and 1000 ml deionised water to achieve a nanofiber concentration of 1 g/ml. To this solution 1.5 ml of a 10% by weight polyvinyl alcohol solution were added to improve adhesion on the substrate. 20 ml of this solution were diluted with 400 ml of the solution of 600 ml 2-propanol and 1000 ml deionised water. The resulting dispersion was sucked through a 325 mesh stainless steel grid having a diameter of 90 mm. In this way the steel grid was coated with a nonwoven fabric. The grid was dried at 40 C. and 25 mbar for 18 hours. FIG. 5 shows a digital micrograph of the nonwoven filter layer formed on the grid. Nonwoven filter layers having different masses per unit area are produced in an analogues manner. For comparison with these filter layers nonwoven filter layers having the same masses per area unit were produced by direct electrospinning on the stainless steel grids. For this purpose 5.45 g polyamide carboxylic acid were dissolved in 7.6 ml N,N-dimethylformamide by steering at room temperature. The resulting solution was electrospun with a velocity of 0.22 ml per hour at 22 C., 24% relative air humidity at a field strength of 20 kV by means of a one needle device having a cannula diameter of 0.9 mm with a distance between the electrodes of 26 cm onto a 325 mesh stainless steel grid (90 mm diameter) until 3.1 mg/m.sup.2 were achieved. FIG. 6 shows a digital micrograph of the resulting nonwoven structure. Nonwoven filter layers having different masses per unit area were produced analogously by electrospinning. The features of both types of filters produced on the grids were compared by use of the filter test system MFP 2000 of the company Palas GmbH, Karlsruhe, Germany. In the essay di(2-ethylhexyl)-sebacate (DEHS) were used as aerosol having droplet sizes from 0.250 m to 2.0 m at a constant flow of 8.5 l/min. The resulting measured values for the deposition of the aerosol droplets on the filters as a function of different masses per area unit of the nonwoven filters are shown for both types of filters in FIG. 7. FIG. 7 clearly shows that the deposition of the aerosols and therewith the efficiency of the filters is very similar independent whether a filter was produced by electrospinning (e-spinning in FIG. 7) or by use of the dispersed powder (powder in FIG. 7) according to the invention.

[0035] In a further essay the pressure difference between both sides of filters passed through by a gas stream was measured. The result is shown in FIG. 8 as a function of the masses per area unit of the nonwoven filters. FIG. 8 shows that the differences of the pressures of both kinds of filters were very similar. This essay shows that the features of a nonwoven structure produced by use of a dispersion of the powder according to the invention (powder in FIG. 8) are very similar to the features of a nonwoven structure produced by electrospinning (e-spinning in FIG. 8).