Patent number 8778254 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "Very fine fibres produced from polymer solutions, often referred to as nanofibres, are useful in a wide variety of applications, including filter media, tissue-engineering scaffold structures and devices, nanofibre-reinforced composite materials, sensors, electrodes for batteries and fuel cells, catalyst support materials, wiping cloths, absorbent pads, post-operative adhesion preventative agents, smart-textiles as well as in artificial cashmere and artificial leather.
"Electrostatic spinning of fibres was, it appears, first described in U.S. Pat. No. 692,631. In principle, a droplet of polymer solution or melt is placed in a strong electric field giving rise to the repulsion between the induced like-charges in the droplet competing with the surface tension of the liquid. When a sufficiently strong electric field is applied (typically 0.5-4 kV/cm), the electrostatic forces can overcome the surface tension of the fluid and a jet of polymer solution or melt is ejected from the droplet.
"Electrostatic instability leads to rapid, chaotic whipping of the jet, leading, in turn, to fast evaporation of any solvent as well as a stretching and thinning of the polymer fibre that is left behind. The formed fibres are then collected on a counter electrode, typically in the form of a nonwoven web. The collected fibres are usually quite uniform and can have fibre diameters of several micrometers, down to as low as 5 nm.
"The technical barriers to manufacturing large quantities of nanofibres by electrospinning include low production rates and the fact that most polymers are spun from solution.
"One general method of production utilises multiple passages such as may be provided by multiple needles. On average, solution based electrospinning, using needle spinnerets, have solution throughput rates on the order of 1 ml per hour per needle. Fibres with diameters in the range of 50 to 100 nm are typically spun from solutions with relatively low concentrations, typically 5-10 wt % depending on polymer type and molecular weight. This means that, assuming a polymer density of around 1 g/ml, the typical solids throughput rate of a needle-based electrospinning process is 0.05 g to 0.1 g of fibre per hour per needle. At this rate, production of a nanofibre web with a planar density of 80 g/m.sup.2 at a rate of 5 m.sup.2/s will require a minimum of 14,400,000 needles.
"In addition, electrical field interference between the different needles limits the minimum separation between them and furthermore, continuous operation of needle-based spinnerets requires frequent cleaning of the needles as polymer deposits tend to block the spinnerets. The overall result is that the production of industrial volumes becomes almost prohibitively expensive for most commodity applications like filtration and absorbent textiles.
"Formhals (U.S. Pat. No. 1,975,504) tried to increase electrospinning production rates by using a serrated wheel as the one electrode. In later designs, he used a multiple needle setup (U.S. Pat. No. 2,109,333).
"Reneker et al. (international patent application publication number WO0022207) describe a process in which nanofibres are produced by feeding fibre-forming solution into an annular column, forcing a gas through the column in order to form an annular film, which is then broken up into numerous strands of fibre-forming material.
"Numerous other proposals have been put forward that rely on creating jets of fibre-forming solution using needles and orifices in order to produce fibres in this manner.
"A system with a significantly high throughput, known as NanoSpider, is described in international patent application publication number WO05024101. In this system the fibre forming polymer solution is contained in a dish and a partly exposed conductive cylinder is slowly rotated in it to form a thin layer of solution on its surface. A counter-electrode is placed 10-20 cm above the cylinder and hundreds of jets initiate off the surface of the cylinder and electrospin onto the target.
"International patent application publication number WO 2006131081 describes a follow up type of NanoSpider technology in which the conductive cylinders are replaced by axially mounted rotatable cylindrical structures presenting multiple 'discharge' surfaces from which solution is to be discharged to form the polymer fibres. The arrangement is somewhat complex and the cylindrical structures must be somewhat costly.
"Japanese patent JP3918179 describes a process in which bubbles are continuously generated on the surface of a polymer solution by blowing compressed air into the solution through a porous membrane, or through a thin tube. Electrospinning jets are formed on the bubbles and fibres that form are collected on the counter-electrode. This system, it appears to applicant, requires that the bubbles in the polymer solution be formed in high volumes and that they burst very rapidly. Also, most organic solvents do not readily form foams and the given examples demonstrate spinning only with polymer solutions in water, 2-propanol and acetone. Additionally, this patent requires that the counter-electrode be placed at a suitable distance from the foam since droplets of spin solution that are created by the constantly bursting bubbles can spatter onto and harm or destroy the formed fibres on the counter-electrode.
"In our pending international patent application published under number WO 2008125971 we describe an improvement of the bubble electrospinning process, based on the stabilization of the formed bubbles using a surfactant."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "In accordance with one aspect of this invention there is provided a method for the production of fine fibres by electrospinning fibres by applying an electrical field between a primary electrode and a counter electrode spaced apart from the primary electrode and extending generally parallel thereto wherein at least an operative surface of the primary electrode is coated with a polymer solution and an electric field of sufficient magnitude is generated between the primary electrode and counter electrode to cause the formation of fine fibres in the space between the electrodes, the method being characterised in that the operative surface of the primary electrode that is coated with polymer solution is made up of appropriate portions of the surfaces of a multitude of operatively semi-submerged, loose (unattached) elements supported on the bottom of a trough or tray or another support member or members and wherein facility is included for causing polymer solution to be applied to the exposed surfaces of the loose elements by causing them to roll in the polymer solution so that they become coated with a thin layer of polymer solution on their surfaces.
"The elements are typically rounded and most commonly circular when viewed in at least one direction. They can be spheres, cylinders or intermediate ellipsoidal shapes, although the preferred shape is presently spherical.
"Rolling can be promoted by tilting the tray or trough or a support member in it.
"Alternatively, a support plate or the like could be moved relative to the elements to cause them to rotate with such movement typically, in this variation, being a reciprocal to and fro movement or a circular motion.
"In another variation, the elements may be moved around using rods or frames. For example, a surrounding frame may be packed with elements to fill a certain area with the elements supported on a support member in the form of a moving surface such as a broad endless belt beneath the elements with the whole setup being semi-submerged in the polymer solution.
"In the case of steel elements or elements made of other magnetic material they may be caused to roll under the influence of changing magnetic fields.
"The surface of the elements will generally be smooth, but it can also be textured in various ways, such as through spiked protrusions; grooves in the surface; or any other form of texturing that distorts the smooth surface of the element.
"The elements can have a size anywhere within in the range of from about 1 mm to about 300 mm, and generally between about 3 mm and about 30 mm. The elements can be made of steel, glass, or any other suitable material with the requirement that they should be suitably stable in the polymer solution and be tolerated by the relevant mechanisms of the apparatus.
"The polymer solution can be a solution of any natural or synthetic polymer in a suitable solvent, or blends of different polymers, or a sol-gel mixture, or any other combination of components that would yield fibres when electrospun by an electrospinning process. The polymer solution can also contain additives that may be required to modify the surface tension, viscosity and/or other rheological or electrical properties of the solution.
"In accordance with a second aspect of this invention there is provided apparatus for the production of fine fibres by a method as defined above wherein a primary electrode is located in spaced relationship relative to a counter electrode and generally parallel thereto, the apparatus being characterised in that the operative surface of the primary electrode that is to be coated with polymer solution in use, is made up of appropriate portions of the surfaces of a multitude of operatively semi-submerged, loose (unattached) elements supported on the bottom of a trough or tray or another support member or members and wherein facility is included for causing polymer solution to be applied to the exposed surfaces of the loose elements by causing them to roll in the polymer solution so that they become coated with a thin layer of polymer solution on their surfaces.
"Further features of this aspect of the invention follow directly from the further features of the first aspect of the invention.
"The process is also suitable for combination with specialized nanofibre collectors for manufacture of geometrically more complex nanofibre structures, such as the nanofibre yarn formation apparatus described in our pending international patent application published under number WO2008062264.
"In order that the invention may be more fully understood some examples thereof will now be described with reference to the accompanying drawings."
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