WO2010118708A2

WO2010118708A2 - A method of producing nanofibres and spinning elements for implementing this method

Info

Publication number: WO2010118708A2
Application number: PCT/CZ2010/000042
Authority: WO
Inventors: Dusan Kimmer; David Petras; Miroslav Tomasek; Ivo Vincent; Lenka Lovecka; Tomas Dudak; Zdenek Dudak
Original assignee: Spur A.S.
Priority date: 2009-04-16
Filing date: 2010-04-08
Publication date: 2010-10-21
Also published as: WO2010118708A3; CZ2009238A3; CZ305037B6

Abstract

The invention relates to a method of producing nanofibres from polymer solutions by electrospinning process in which spinning elements from non-metallic, electrically non- conductive materials having a ragged surface are used. Advantageously, during the spinning process, the spinning elements sink gradually in the lengthwise direction into the spinning solution, come gradually out of the solution and enter gradually an electric field. During the electrospinning process, the shape of the layer of the spun polymer solution adhered to the spinning elements is modified locally and formation of Taylor cones necessary for the proper process of the nanofibres formation is stimulated. Further, the invention concerns the spinning elements for carrying out the above method, the said spinning elements being manufactured from non-metallic, electrically non-conductive materials having a rugged surface and positioned in the carrying elements so that they are oriented in the direction oblique to the surface of the spinning solution.

Description

A method of producing nanofϊbres and spinning elements for implementing this method

Field of invention

This invention relates to a method of producing nanofibers from polymer solutions by electrospinning process and further to spinning elements intended for implementing this method.

Description of the prior art

Besides the melt blown fibre-forming processes based on centrifugation of polymer solutions and in addition to processes based on dissolving one polymer from multifibrillar bicomponent fibres, electrospinning is one of the methods used for preparation of very thin polymeric fibres. All electrospinning methods are based on a common feature consisting in the necessity to create a Taylor cone formed as a result of a deformation of liquid formations (for instance polymer solution drops) from which subsequently submicrone fibres are drawn in an electric field, the said fibres being collected on a suitable support in front of a collecting electrode. As soon as electric forces overcome surface tension of the solution the charged liquid jet starts to emanate in a continuous stream from the tip (the Taylor cone) of the droplet. When the jets reach a distance of several centimetres from the droplet, the imbalance between inertial, electric and surface forces causes the jets of liquid to start to form expanding spirals between two electrodes charged to different voltage levels.

The most frequently used electrospinning methods are based on the use of a capillary tube, through which the polymer solution is extruded into an electric field. A more extensive commercial use of this process is limited by a low efficiency of the process. The efficiency is increased by employing a set-up comprising a great number of spinnerets in the manufacturing equipment - up to 400 per square foot . (NanoStatics™, Columbus, OH, U.S.A).

Other methods of increasing the efficiency of the process use rotating devices (a roller, a structure comprising tips and strings), which are partially immersed into a solution of a polymer and to which high voltage is applied either directly or via the solution. An additional increase in the process efficiency can also be achieved by placing the rotating devices in the spinning units in a tandem.

The principle employing rotating and partly immersed electrodes of various shapes from a conductive metal material into the solution containing a fibre-forming substance was described as early as in 1934 in the US patent No. 1, 975, 504 concerning production of man- made fibres from natural polymers and their derivates in an electric field generated between two electrodes.

One of the devices intended for production of nanofibres from a polymer solution by electrostatic spinning using a partially immersed rotating electrode is a subject of the patent application PCT 2005/024101. The device contains a spinning electrode in the form of a cylinder rotating about its main axis and dipping by the surface of its lower part into the polymer solution. The polymer solution is carried out by the cylinder surface into the electric field between the spinning and collecting electrodes. Between the electrodes nanofibres are formed and, subsequently, carried towards the collecting electrode and deposited on a supporting material. This device is able to produce very good nanofibres from aqueous polymer solutions.

Another device according to CZ patent No. 299537 contains a spinning electrode comprising a system of lamellas arranged radially and longitudinally about the rotation axis of the spinning electrode, the lamellas being provided with tips, on which Taylor cones are formed. Such a spinning electrode is able to carry out a sufficient quantity of the polymer solution to the most suitable places of the electric field between the spinning and collection electrodes and at the same time to spin quite well also non-aqueous polymer solutions. However, a certain disadvantage is a demanding manufacture of such spinning electrode, and due to this also its price.

From the German patent No. 101 36 255, a device for production of fibres from a polymer solution or a polymer melt is known. The said device contains at least two spinning electrode mechanisms, each comprising a system of parallel wires positioned on a pair of endless belts wrapped around two guiding cylinders placed one above another, the lower guiding cylinder extending into the polymer solution or the polymer melt. A strip of textile used as a counter-electrode is passed between the two spinning electrode mechanisms while the spinning electrode mechanisms simultaneously create coating both on the face side and backside of the textile. The spinning electrode is connected to a source of high voltage and the counter-electrode is formed by an electrically conductive circulating belt. The polymer solution or the polymer melt being spun is carried by means of wires into the electric field between the spinning and the counter-electrodes where fibres are formed from the polymer solution or the polymer melt, the fibres being carried towards the counter-electrode and falling onto the fabric positioned in front of the counter-electrode. A disadvantage of the process is a long time of residence of the polymer solution or the polymer melt in the electric field because the polymer solution as well as the polymer melt change their properties during the spinning process, which causes changes in characteristics of the fibres produced, especially of the fibre diameter (For coats retaining constant properties during the lengthy process it is suitable to optimise the polymer solution to be spun and primarily properties of the polymer itself as described, for instance, in the patent application PV 2008-849). Another disadvantage is positioning of the spinning electrode wires on the pair of the endless belts, which are either electrically conductive and, as such, affect very negatively the electric field generated between the spinning electrode and the counter-electrode, or are electrically non- conductive and the high voltage is applied to the wires of the spinning electrodes by means of slipping contacts, the voltage being applied preferably to one up to three wires, which makes the spinning equipment unnecessarily rather complex.

The above shortcomings are eliminated to some extent by a device according to another Czech patent, namely patent No. 299549. The invention aimed here at creating a simple and reliable spinning electrode for a device intended for production of nanofibres by electrostatic spinning of polymer solutions. This is achieved here by creating a rotary spinning electrode comprising a pair of faces, between which spinning elements formed by wires (metal strings) distributed evenly around the perimeter of the faces are placed. Thus the nature of the invention consists in the fact that the faces are made of an electrically non-conductive material and all the spinning elements are mutually electrically connected in a conductive manner. The rotating spinning electrode created in this way is able to spin aqueous as well as non-aqueous polymer solutions and assures a relatively uniform spinning effect along its entire length. The mutual electric connection of all spinning elements is achieved by creating the spinning elements from a single metal string run alternately from one face to the other in grooves or openings created around the perimeter of the faces. However, the metal strings do not soak any polymer solution, which proved to be a property that affects positively uniformity of the coating and results in planar nanofibre structures containing nanofibres with a narrow distribution of diameters.

Recently, also new electrospinning methods have been published: formation of fibre-forming spots during the electrospinning process can simply be achieved when extruding polymer solutions through a set of openings (0.5 mm in diameter and less) in the cylinder walls, the said openings being located in a sufficient distance (1 cm) from each other in order to achieve uniform and stable cone-shaped outputs of the polymer solutions (Theron S.A., Yarin A.L., Zussman E., Kroll E., Polymer 2006; 46(9): 2889-2899).

The stream of jets of the polymer solution in the electrostatic field can also be achieved by applying a high voltage to gas bubbles on the surface of the polymer solution. Theoretically, this procedure was described on soap bubbles by Wilson and Taylor (Wilson C.T.R., Taylor G.I.; Proceeding of the Cambridge Philosophical Society, Mathematical and Physical Science 1925; 22: 728-730).

The jets are not formed readily from flat surfaces of polymer baths and films but are easy to form from curved bubble surfaces on otherwise a flat surface of the polymer bath. Another method of achieving the curvature of the surface is blowing air against the film formed on the metal net. This arrangement is described by Sunthornvarabhas, J., Chase G.G., and Reneker D.H. from the University of Akron, OH, U.S.A, in their works.

Other method of breaking the surface of a polymer solution employs a magnetic field (Yarin A.L., Zussman E.; Polymer 2004; 45; 2977-2980).

The electrodes with metal elements or metal strings do not create sufficiently homogeneous coats; areas with a different mass per square area are apparent on nanostructures; also physical and mechanical properties of the nanostructures (tensile strength, moduli, elongation at break) are very different on various places of the nanostructures produced. Consequently, application properties, such as pressure gradient and capture (entrapment) of dust particles during air filtration, are not the same over the entire material. This is so due to the fact that in case of metal electrodes with tips or capillaries the places where the Taylor cones are formed as well as number of the cones are predetermined, which results in various nanofibre structure periodicity and consequently also in a non- uniformity of properties over the surface area of the nanostructures. On the contrary, entirely smooth electrodes with a low surface ruggedness in all directions can eliminate this shortcoming since the spots of the Taylor cone formation are random. However, they require solutions possessing a high relative permittivity, near or equal to that of water (ε_r = 81). In case of non-aqueous solutions having a low permittivity value (ε_r ≤ 50) the formation of the nanofibres from flat surfaces either does not occur at all or very unreadily only, which can sometimes be solved only by addition of a great quantity of low molecular and polar additives (primarily salts). However, the addition of such additives limits applicability of the nanofibres prepared in this manner.

In order to achieve a uniform homogeneous coat it is desirable that the number of the spots where the Taylor cones are formed be as greatest as possible. This can be attained by utilizing the arrangement that is the subject-matter of the present invention.

Nature of the invention

The method of producing nanofibres from polymer solutions by the electrospinning process in which spinning elements made from non-metallic, electrically non-conductive materials with a rugged surface are used contributes to a considerable extent to elimination of the above shortcomings.

Advantageously, during the spinning process the spinning elements in the lengthwise direction sink gradually into the spinning solution, come gradually out of the solution and enter gradually an electric field. In the course of the electrospinning process, the shape of the layer of the spun polymer solution adhered to the spinning elements is modified locally and formation of the Taylor cones necessary for the proper process of the nanofibre formation is stimulated.

The spinning elements for carrying out the above method according to this invention are manufactured from non-metallic, electrically non-conductive materials having a rugged surface and positioned in the carrying elements so that they are oriented in the direction oblique to the surface of the spinning solution.

The electrically non-conductive materials from which the spinning elements are made are advantageously materials exhibiting wettability by the polymer solution to be spun as well as ability to absorb the solution. Such materials are primarily textile fibres, threads or yarns, possibly also sticks or rods from a non-conductive material having a roughened surface.

Specifically, such an electrically non-conductive material for manufacture of the spinning elements can be a natural material, particularly a material from the group including natural silk, cotton, linen, wool, cellulose, glass, basalt and/or synthetic polymer, in particular from the group including polyester, polyamide, Elastolan polyurethane, viscose (rayon), polypropylene, polyacrylate and polyaramid.

Application of absorbent, electrically non-conductive materials having a microscopically rugged surface, which are the subject-matter of the present invention, is an efficient procedure ensuring reproducibility and increased efficiency of the process, features necessary for a mass use of the electrospinnig technology in the manufacture of nanostructures possessing the required structure and properties.

Exemplary embodiments of the invention

Example of Embodiment 1

Solution spun: polyurethane (hereinafter referred to as only PU) dissolved in dimethyl formamide (hereinafter referred to as only DMF).

PU composition: molar ratio of 4,4'diphenyl methane diisocyanate (hereinafter referred to as only MDI) : poly(3 -methyl- l,5pentandiol)-alt-(adipic and isophthalic acids) having a molar mass of M ~ 2000 (hereinafter referred to as only PAIM) : 1,4 butandiol (hereinafter referred to as only BD) = 6 : 1 : 5, mass ratio of hard segments w(HS) = 0.5012. Method of synthesis: A procedure including a gradual addition of separate components (per partes) in which MDI (PAIM/MDI molar ratio = 1 :2.05) was added to dried PAIM in the first step while mixing and a prepolymer was synthesized at the temperature of 90 °C for a time period of 2 hours. In the second step MDF solvent was added (to obtain a 50 percent solution) and in excess all chain extender (BD). After 1 hour reaction, the remaining diisocyanate was added in the third step; polyaddition was maintained at the temperature of 90 ⁰C for another 2 hours while the solution was diluted gradually to a concentration of c = 12.8 % at η = 1.43 Pa.s. Prior to spinning in the electric field conductivity was modified to χ = 166 μS/cm by means of TEAB.

Conditions of the electrospinning process: a rotating spinning device dipped in a tank containing the polymer solution, with four horizontally arranged threads made from a polyester staple fibre (65 %) and cotton staple (35%) having a fineness (length-specific mass) of 89 tex, distance between the faces = 20 cm, voltage of electric current applied to the PU solution in the vessel U = 75 kV, distance of the spinning device from the collecting electrode D = 18 cm, rotation speed of the spinning device = 7 rev./min., speed of movement of the collecting support - an antistatically treated non-woven fibrous layer based on PP (PPNVV) = 16 cm/min.

The nanostructures prepared in this manner (Figures Ia, b) exhibit better mechanical properties, as determined in a tensile strength test, than those of the plane nanostructures prepared with a device having a rotating six-lamella metal electrode with 27 tips on each lamella. When an electrode with strings of an non-conductive material was employed, the Young modulus increased from 30.1 MPa to 57.9 MPa.

Figure Ia. 1500 magnification Figure Ib. 5000 magnification Example of Embodiment 2

All conditions were the same as those used in the Example of Embodiment 1, the only difference being the fact that instead of four horizontally arranged threads an arrangement comprising three threads (PES/cotton 70/30, 75 tex) was used, with the threads being oriented in an oblique direction so that they emerged gradually out of the polymer solution. The mean value of diameters of the nanofibres forming the nanostructures was 185 nm, which is by 23 percent less than the mean value achieved when copper strings were utilized; distribution of the mean values was narrower and porosity of the nanostructures (Airfactor) decreased from 81.75 percent to 75 percent, which affected positively filtration capability (interception) of the nanostructures. The properties of the function elements and fibre-forming capabilities did not change during the 9 hour process (as is the case, for instance, when copper wires are used - deformation).

Example of Embodiment 3

All conditions were identical with those used in the Example of Embodiment 1 , the only difference being the fact that instead of the spinning element from PES and cotton (65/35, 89 tex) cotton staple (100%, 120 tex) was used. The fibre circular structures formed (Figures 2a, 2b) exhibited mechanical and filtration properties different from those of the structures containing straight nanofibres according to the Example of Embodiment 1 (Figures Ia, Ib).

Figure 2a. 1500 magnification Figure 2b. 5000 magnification

Example of Embodiment 4

All conditions were identical with those used in the Example of Embodiment 2, the only difference being that a polypropylene fibrillated tape was used as a string. The values of mass per square area and diameters of the nanofibres examined with the aid of a scanning electron microscope (SEM JEOL, Tokyo, Japan) were identical during the 3 hour process at the relative humidity of 26.5 percent.

Example of Embodiment 5

All conditions were the same as those used in the Example of Embodiment 2, the only difference being that basalt yarn with a protective twist of 80 turns/m. was used as a string.

Example of Embodiment 6

AU conditions were the same as those used in the Example of Embodiment 1, the only difference being that instead of four horizontally arranged threads an arrangement with six threads oriented in an oblique direction was used.

Example of Embodiment 7

All conditions were the same as those used in the Example of Embodiment 2, the only difference being that instead of strings four PP sticks fixed in the faces of the spinning device were used. Prior to application, the surface of the spinning elements was abraded with an emery (abrasive) paper in order to create microscopic defects of a fibrous character on the surface of PP.

Example of Embodiment 8

Spinning solution: 16 percent solution of polyvinyl alcohol in water.

Conditions of the electrospinning process: a rotating spinning device dipped in the polymer solution, comprising three horizontally arranged threads from polyester silk having a length specific mass of 69 tex; distance between the faces = 20 cm, voltage of electric current applied to the PU solution in the bath U = 75 kV, distance of the spinning device from the collecting electrode D = 18 cm, rotation speed of the spinning device = 7 rev./min., speed of movement of the collecting support- an antistatically treated non-woven fibrous layer based on PP (PPNVV) = 16 cm/min.

Example of Embodiment 9

Conditions were identical with those used in the Example of Embodiment 8, the only difference being that the strings were arranged in an oblique direction and were made from polyamide silk.

Claims

C L A I M S

1. A method of producing nanofibres from polymer solutions by electrospinning process characterized in that spinning elements from non-metallic, electrically non-conductive materials having a rugged surface are used in the process, the shape of the layer of the polymer solution stuck to the spinning elements being modified locally and formation of the Taylor cones necessary for the proper process of the nanofibres formation being stimulated during the electrospinning process.

2. The method according to Claim 1, in which spinning elements from non-metallic, electrically non-conductive materials having a rugged surface are used in the process, the spinning elements being dipped gradually into the spinning solution in the direction of the spinning elements length during the spinning process, coming gradually out therefrom and entering gradually an electric field, the shape of the layer of the spun polymer solution adhered to the spinning elements being modified locally and the formation of the Taylor cones necessary for the proper process of the nanofibres formation being stimulated during the electrospinning process.

3. The spinning elements for implementing the method of production as claimed in Claim 1, wherein the said spinning elements are made of non-metallic, electrically non-conductive materials having a rugged surface and mounted in carrier elements so that they are oriented in an oblique direction to the surface of the spinning solution.

4. The spinning elements according to Claim 3, characterized in that the said spinning elements are made of materials exhibiting wettability and absorption capacity for the polymer solution being spun.

5. The spinning elements according to Claim 3, characterized in that the said spinning elements comprise textile fibres, threads or yarn.

6. The spinning elements as claimed in Claim 3, characterized in that the said spinning elements comprise sticks or rods of an electrically non-conductive material having a roughened surface.

7. The spinning elements according to some of Claims 3 through to 5, characterized in that the electrically non-conductive material of the spinning elements is a natural material, in particular a material from the group including natural silk, cotton, linen, wool, cellulose, glass, basalt and/or synthetic polymer, particularly from the group including polyester, polyamide, Elastolan polyurethane, viscose (rayon), polypropylene, polyacrylate and polyaramid.