A Method of Producing a Linear Nanofibrous Structure in an Alternating Electric Field, a Device for Performing This Method and a Device for Producing a Nanofibrous Thread

The invention relates to a method of producing a linear nanofibrous structure in an alternating electric field on a spinning electrode from a polymer solution or melt, in which on the spinning electrode, a spinning area is formed with a supercritical alternating electric field intensity, in which are formed nanofibers which are carried away from the spinning electrode by the action of the electric wind in the direction of the maximum values of the electric field gradient.

The invention also relates to a device for producing a linear nanofibrous structure in an alternating electric field from a polymer solution or melt on a spinning electrode mounted in a spinning chamber and connected to an AC voltage source of and coupled to a means for applying the polymer solution or polymer melt to the surface of the spinning electrode, wherein a spinning area with the supercritical intensity of the alternating electric field is formed on the spinning electrode.

In addition, the invention relates to a device for producing a nanofibrous thread.

In the preparation of a nanofibrous threads, oriented and twisted nanofibers are the basis for their construction. Currently, numerous methods have been developed in the field of electrospinning to obtain oriented and twisted nanofiber bundles. This development can be attributed to two main aspects, that is, to obtaining highly ordered nanofibers by improving a collecting device or by adding an auxiliary electrode.

CN111118677 discloses production of nanofibrous yarn by electrostatic spinning. The device comprises a cylindrical collector, which consists of a cavity and a throat which is rotatable about its axis, wherein the diameter of the upper opening of the throat is smaller than the diameter of the lower opening of the cavity. Inside the lower opening of the cavity is mounted an electrostatic rotating spinning electrode connected to a high voltage source into which a solution to be subjected to electrospinning is fed. In the upper part of the collector cavity, pressurized air inlets open into the inner space of the collector and above them is arranged a counter electrode which can be grounded or connected to a voltage source of opposite polarity to the rotating spinning electrode.

Nanofibers formed on the rotating spinning electrode are carried by the action of the electrostatic field to the counter electrode and by the action of air flow, they are carried up into the throat of the cylindrical collector, which rotates, and due to its rotation and the supplied air flow, an air vortex is created, which twists the nanofibers into yarn, which is further withdrawn and wound on a bobbin.

The nanofibers are twisted immediately after their formation due to the rotation of the spinning electrode and the subsequent action of the air vortex, so there is no parallelization of the nanofibers before twisting, the twisting is uneven and, as a result, their strength and appearance is variable.

CN111286792 describes a horizontal arrangement of an electrostatic spinning device comprising a rotating jet spinning electrode and a coaxially arranged collecting electrode formed by a hollow cylinder and arranged against the jet spinning electrode, wherein a DC electric field is formed between the spinning and collecting electrodes. At least two air jets directed towards the axis of the collecting electrode are arranged around the rotating jet spinning electrode. The nanofibers produced by the rotating jet spinning electrode are carried by the electric wind to the hollow cylinder forming the collecting electrode, wherein due to the rotation of the jet spinning electrode and air flows from the jets, they are twisted into yarn which, after passing through the cavity of the collecting electrode, is drawn off and wound on a bobbin.

In this solution, too, the aim is to twist the nanofibers as soon as possible after they are formed without achieving their parallelization.

The disadvantages of electrostatic production of nanofibrous yarn are in both cases low yarn cohesion, irregular twist and poor orientation of the nanofibers.

Currently, a method of continuous preparation of nanofibrous yarns is also known, for example from CN110644080, in which nanofibers are formed from a polymer solution in a jet head from which the nanofibers are drawn off by the action of high-speed air flow created in a Venturi tube and, through a funnel-shaped collection tube, enter a Venturi collection system, where they are straightened and oriented into oriented bundles of nanofibers using vacuum adsorption in the Venturi collection system. The oriented bundles of nanofibers are subsequently twisted and agglomerate by the action of the twisting device into a nanofibrous yarn, which is in the next step wound on a bobbin. The twisting device comprises air jets for supplying the air flow in the tangential direction towards the yarn to be twisted.

From the point of view of the subsequent processing and use of nanofibrous yarns, it is not enough to only obtain oriented fibers in order to meet the current requirements for their preparation, but it is necessary to be able to obtain oriented fibers or fiber bundles continuously and to impart evenly a certain degree of twist to them in order to ensure the length and degree of orientation of the fibers in the nanofibrous yarn. Existing electrospinning technologies for the continuous production of nanofibrous yarns have a low yield and poor quality of the produced nanofibrous yarns.

EP2931951 B1 discloses a method of producing polymeric nanofibers, in which polymeric nanofibers are formed by applying an electric field to a polymer solution or melt located on the surface of a spinning electrode, wherein the electric field for spinning is alternately formed between the spinning electrode to which an AC voltage is applied and the air and/or gas ions generated and/or supplied to the vicinity of the spinning electrode, without a collecting electrode, whereby, depending on the phase of the AC voltage on the spinning electrode, polymeric nanofibers with opposite electrical charge and/or with sections with opposite electrical charge are formed, which, after their formation due to the action of electrostatic forces, aggregate into a linear structure in the form of a cable or strip which moves freely in space away from the spinning electrode in the direction of the gradient of the electric fields.

Spinning by the alternating high electrical voltage method is another way of producing nanofibers, alternative to electrostatic spinning. However, its yield is not yet at a level to produce purely nanofiber yarns by this method. Therefore, EP3303666 proposed a method of producing a core yarn with a coating of polymer nanofibers enveloping a supporting linear structure forming the core during its passage through a spinning chamber. In this method, a spinning electrode connected to the inlet of a polymer solution and powered by alternating high voltage is arranged below the supporting linear structure on the face of which nanofibers are formed in a spinning space in the immediate vicinity of the face of the spinning electrode and above it, wherein the supporting linear structure rotates in the spinning space about its own axis. Nanofibers are formed around the circumference of the face of the spinning electrode and in the spinning space. They are formed into a hollow electrically neutral nanofibrous plume in which the nanofibers are arranged in an irregular lattice structure in which nanofibers in short sections change their direction, wherein the hollow electrically neutral nanofibrous plume is carried by the electric wind towards the supporting linear structure and change into a flat strip which is brought to the circumference of the supporting linear structure, wherein the strip created from a hollow electrically neutral nanofibrous plume wraps around the rotating and/or ballooning supporting linear structure in the shape of a helix, creating a nanofiber coating on it, in which the nanofibers are arranged in an irregular lattice structure, in which the individual nanofibers in short sections change their direction.

The nanofibrous plume represents an ideal material for the coating of the core yarn, because due to its electrical neutrality and irregular lattice structure, in which the individual nanofibers in short sections change their direction, it is capable of forming a solid coating enveloping the yarn core, whereby the coating is inert to its surroundings when wound on a bobbin and during subsequent unwinding during processing. However, if a pure nanofiber yarn were to be produced from the nanofiber plume, there would be a problem both with an insufficient quantity of the nanofibers as well as with the lattice structure of the plume, which does not allow parallelization of the nanofibers.

At present, there is no satisfactory method of producing nanofibrous yarn with potential for industrial applications. Current methods of preparing nanofibrous yarns are hampered by low productivity, low reliability and limited choice of materials. Their production is realized only on a laboratory scale as part of research work.

Classic yarn with a permanent twist is produced, for example, on ring or rotor spinning machines, where at first, a ribbon of parallel fibers is formed and subsequently the ribbon is twisted, creating yarn with high tensile strength and uniform twist. However, it is not yet possible to create yarn from nanofibers in this way.

The object of the invention is to propose a method of producing nanofibers by AC electrospinning of a polymer solution or melt, in which nanofibers would be produced in sufficient quantity and carried away from the spinning area so as to form a ribbon of nanofibers at a certain location, in which the nanofibers would be at least partially parallelized, wherein the nanofibers would have sufficient strength allowing them to be drawn off and wound on a bobbin for subsequent use or processing into textile structures using known textile technologies.

In addition, the object of the invention is to provide a device for performing this method and a device for producing nanofibrous yarn.

The object of the invention is achieved by a method of producing a linear nanofibrous structure from a polymer solution or melt in an alternating electric field on a spinning electrode, in which nanofibers are formed from the polymer solution or melt in a spinning area created on the spinning electrode and are carried away from it by the action of the electric wind, wherein the principle of the invention consists in that on the spinning electrode is formed at least one spinning area with a supercritical AC electric field intensity and a final length, from which the emerging nanofibers are carried away by the effect of the electric wind in the direction of the maximum values of the electric field gradient from the spinning area in a flat structure whose initial width is the same as the width of the linear spinning area, wherein as the electric field gradient decreases, the nanofibers lose their kinetic energy until, after losing their kinetic energy, they stop, gather and are compacted into a linear nanofibrous structure which is drawn off, with the linear weight of the linear nanofiber structure gradually increasing, while the nanofibers are at least partially parallelized and a ribbon of nanofibers is formed. Due to the high specific surface area of the nanofibers and the binding forces between the individual nanofibers, the nanofiber ribbon created in this way has sufficient cohesion, which enables it to be wound on a bobbin for subsequent technological operations, such as twisting, elongation, heat fixation, etc. By being imparted a twist, the nanofiber ribbon is formed into a nanofibrous thread.

At the point of the loss of kinetic energy of the nanofibers, a force balance of electric and gravitational forces acting on the formed nanofibers is created, thereby creating a virtual collector. Gravitational forces are caused by the mass of nanofibers and electric forces represent the sum of all electric forces acting on the nanofibers, i.e., the force of the electric wind from the spinning electrode, the force of the electric wind from other charged parts of the spinning device, the force from ionized air ions and the force from oppositely charged parts of nanofibers formed in the previous half-wave of the alternating electric field.

According to an alternative embodiment of the invention, the spinning area of the belt and linear spinning electrode is straight and the nanofibers emerging therefrom move in a planar flat structure whose thickness corresponds to the width of the spinning area and whose length corresponds to the length of the spinning area.

To produce large quantities of nanofibers, in another alternative of this embodiment, a double spinning area can be formed by increasing the width of the spinning electrode, wherein nanofibers emerging from both spinning areas move in planar flat structures which move away from each other in the direction of movement of the nanofibers. In this manner, two ribbons of nanofibers are formed on one spinning electrode, which can be further processed separately, or combined before processing.

In another alternative embodiment of the invention, the spinning area is formed by a part of a circle on the circumference of a disk spinning electrode nanofibers emerging from it move in a planar flat structure which is perpendicular to the axis of rotation of the disk spinning electrode, wherein the linear virtual collector is formed by a part of a circle.

Also in this embodiment, to produce a larger number of nanofibers, it is possible to increase the width of the spinning electrode and form a double spinning area, wherein nanofibers emerging from the two spinning areas move in conical flat structures which move away from each other in the direction of the movement of the nanofibers until the formation of virtual collectors, where they form two ribbons of nanofibers which can be further processed separately or combined before further processing.

In all the embodiments described, the created ribbon of nanofibers is wound on a bobbin, being capable of unwinding and further processing.

In order to speed up the production process, according to another alternative embodiment of the invention, a twist can be imparted to the ribbon of nanofibers before it is wound, thereby creating a nanofibrous thread. The twist imparted can be false or permanent.

The principle of the device for producing a linear nanofibrous structure in an alternating electric field from a polymer solution or polymer melt is that by setting the supercritical intensity of the alternating electric field, at least one linear spinning area is created on the surface of the spinning electrode, and above it, in the direction of the maximum values of the electric field gradient, a virtual collector is created in the area of force balance of electric and gravitational forces acting on the formed nanofibers, for stopping, collecting and compacting the nanofibers into a linear fibrous structure, to which a draw-off mechanism and winding device for winding the ribbon of nanofibers are assigned.

According to one alternative embodiment, the spinning area may be straight, with the maximum electric field gradient directed vertically upwards so that the formed nanofibers are carried vertically upwards as far as to the virtual collector.

Furthermore, the spinning electrode can be formed by a strip spinning electrode, or a linear spinning electrode formed by a linear flexible structure, for example a string, a thin tape, or a thin strap, on which the polymer solution is only on the spinning area.

If the linear flexible structure forming the spinning electrode is composed of several interlaced or intertwined parts, a shielding bar is placed under the spinning area, which can also cover the edges of the linear flexible structure, so that spinning takes place only on its upper side, open in the spinning direction.

By increasing the width of the linear flexible structure forming the linear spinning electrode and by suitably setting the intensity of the electric field, the creation of two spinning areas near the edges of the linear flexible structure is achieved. These spinning areas can be formed by protrusions on the edges of the strip, or on the edge of this strip.

In another alternative device, the spinning electrode is formed by a narrow rotating disk spinning electrode, which with the lower part of its circumference extends into the polymer solution or the melt in a reservoir, and on the free part of the circumference of the disk spinning electrode, a spinning area is formed, which is formed by part of a circle, wherein the maximum gradient of the electric field is directed from the spinning area in the radial direction and the nanofibers are carried in a planar flat structure from which a ribbon of nanofibers is formed in a virtual collector.

To increase the quantity of the nanofibers produced, the rotating disk spinning electrode is mounted on a common shaft with at least one other rotating disk spinning electrode.

An increase in the quantity of the nanofibers produced can also be achieved by arranging several rotating disk spinning electrodes one behind the other.

In another embodiment, the rotating disk spinning electrode has a larger disk width, and so two spinning areas are formed on its edges, which are formed by a part of a circle, and the maximum gradient of the electric field creates conical surface structures on the edges of the disk spinning electrode. The nanofibers in the conical surface structures are carried into virtual collectors, in which a ribbon of nanofibers is formed, wherein the conical surface structures of nanofibers move away from each other and create the letter “V” in cross-section.

According to another alternative embodiment, the spinning electrode is formed by an overflow spinning electrode, wherein the device comprises a reservoir of a polymer solution, in which the inlet of the polymer solution is placed vertically. At the upper end of the inlet, an overflow electrode is arranged, wherein the inlet is opened on its upper face and an overflow area is formed around its mouth, sloping slightly from the mouth of the inlet of the polymer solution to the edge of the overflow electrode and is terminated with a circumferential edge which forms the spinning area of the overflow spinning electrode on which nanofibers are elongated. The elongated nanofibers are carried by the action of the electric wind in the direction of the maximum electric field gradient through the spinning space in the radial direction from the circumferential edge of the overflow electrode and collected in the area of the virtual collector, where they are compacted into a material structure forming a ribbon of nanofibers, which is drawn off from the virtual collector in the tangential direction with respect to the virtual collector and further wound onto a bobbin or processed into a thread.

The created ribbon of nanofibers is taken from any of the above-mentioned spinning devices to a device for producing a nanofibrous thread which comprises a twisting means for creating a false or permanent twist and is then guided to the winding device and wound onto a bobbin.

The method of producing nanofibers by spinning a polymer solution or polymer melt in an alternating electric field will be described hereinafter. Since the spinning of a polymer melt proceeds in the same manner as the spinning of a polymer solution, it is only the spinning of a polymer solution that will be described further on.

In the production of nanofibers from a polymer solution to create a linear nanofiber formation, the technology of spinning in the alternating electric field is used, which is created by an alternating voltage with an amplitude of, for example, 25 to 50 kV, depending on the geometry and arrangement of the spinning electrodeat a frequency of, for example, 10 to 1000 Hz. The polymer solution usually consists of a solution of PVB, PCL, PVA, or other spinnable solutions.

In conventional spinning in an alternating electric field, the aim is to produce per unit of time the largest possible quantity of nanofibers, which are created over the entire working surface of a spinning electrode and are carried away from the spinning electrode by the electric wind, or possibly also by auxiliary air currents, to a collector which is neither grounded nor connected to an electric voltage source and which can be, for example, a flat textile or a linear fibrous structure which, after being coated with a nanofibrous plume, forms core composite nanofibrous yarn. The formation of nanofibers begins at a critical value of the electric field intensity, which varies depending on the type of polymer solution being spun, voltage value, gas quality in the spinning chamber and other parameters. At a lower value of the electric field intensity than the critical one, nanofibers are not formed, or their formation ceases. Therefore, in conventional spinning in an alternating electric field, using a specific design of the spinning electrode, a higher electric field intensity than the critical one, i.e. supercritical, is used, which creates an alternating electric field of high intensity on the spinning electrode, in order to eliminate the risk of interruption of the spinning process, as well as to ensure sufficient evaporation of the solvent and a sufficiently strong electric wind to transport the nanofibers to the collector.

The distribution of supercritical intensity E of the electric field for the above-mentioned conventional spinning of polymer solution Z on a narrow rotating disk spinning electrodeis shown infor a disk diameter of 300 mm, a disk thickness of 1 mm, a polymer solution layer thickness of 0.2 mm, and a voltage amplitude of 50 kV. The supercritical value of the intensity E of the electric field for PVB polymer solution is equal to or greater than 3000 V/mm. It is clear from the figure that the supercritical value of the intensity E of the electric field is achieved in a wide area around the circumferential part of the disk. Spinning of polymer solution Z therefore takes place on the entire circumferential surface of the disk and on the part of the disk faces near the disk circumference, and the created nanofibers are carried through the spinning space to the surface of an unillustrated collector.

In order to achieve the formation of nanofibersin the alternating electric field by spinning polymer solution Z to produce a ribbonof nanofibers according to the invention, it is necessary to create a linear spinning area with supercritical intensity E on the spinning electrodecovered with polymer solution Z, which has a finite length and is open in the spinning direction. The spinning areacan be straight, for example in the case of a belt, strip or cable spinning electrode, or it can be formed by a part of a circle, for example in the case of a rotating disk spinning electrode. At the same time, it is necessary to set the alternating electric power supply of the spinning electrode to a value at which supercritical intensity E of the electric field on the linear spinning areaof the respective spinning electrodeis created especially above its central part, so that spinning takes place predominantly in the central part of the above-mentioned spinning area.

This is achieved in an exemplary embodiment by distributing supercritical intensity E of the electric field for spinning polymer solution Z on the narrow rotating disk spinning electrodeshown infor a disk diameter of 300 mm, a disk thickness of 1 mm, a polymer solution layer thickness of 0.2 mm, and a voltage amplitude of 30 kV. Compared to the previous embodiment intended for conventional spinning, due to reduction in the voltage amplitude, the area of supercritical intensity E has significantly decreased and is only above the central part of the circumferential surface of the disk. All the nanofibersbeing formed emerge above the central part of the circumferential surface of the disk and are carried away from the rotating disk spinning electrodein a planar flat structure, which is perpendicular to the axis of rotation of the disk. The nanofibersstop at the same distance from the spinning electrodein the region of the so-called virtual collectorand are not carried further.

This generally means that Taylor cones begin to form on the surface of polymer solution Z in the linear spinning areaof the spinning electrode, from which, due to the effect of a sufficiently strong alternating electric field, nanofibersbegin to elongate and are carried away from the spinning area of the spinning electrode in the direction of the maximum values of the electric field gradient, i.e. in the plane of the greatest density of electric field lines, in one flat structure, whereas in the area in which the repeated natural slowing down to stopping of the nanofibers occurs, a virtual collectoris formed, i.e. a area where the nanofibers gather and are compacted to form a ribbonof nanofibers and this ribbonis drawn off. As a result of the periodic change of polarity of the power supply of the spinning electrode, the formed nanofibersin the region of the virtual collectorare compacted into a material structure due to the loss of speed. Since these nanofibersare formed in one flat structure, a linear nanofiber structure called a ribbonof nanofibers is created due to this compaction. The flat structure of nanofibers has a thickness corresponding to the width of the spinning area, which is narrow and its width varies in the interval of up to 5 mm. The length of the virtual collectorcorresponds to the length of the spinning area on the spinning electrode.

If the spinning process takes place in a vertical plane as described above, the flat structure of nanofibersis planar, because all the forces acting on it act in the vertical direction.

If the spinning process takes place in a plane inclined from the vertical plane, for example on both edges of belt or strip electrodes, nanofibersare created in the direction of the maximum values of the electric field gradient, i.e. in a plane inclined from the vertical plane, but by the action of gravitational forces and mutual repulsive forces of nanofibers with the same charge, the nanofibersare deflected and, consequently, a virtual collectoris formed under the surface of the electric field gradient.

If the spinning process takes place by way of forming nanofibers in a conical surface, for example, on both edges of a wide rotating disk spinning electrode, nanofibersare formed in the direction of the maximum values of the electric field gradient. The distribution of regions of electric field intensity E is shown inon the wide rotating disk spinning electrode, with a width of, for example, 6 mm, on the circumference of which a recessis formed in the middle, whereby protrusionsare formed on the edges of the disk spinning electrode circumference, on which supercritical intensity E of the electric field is concentrated. As a result, two spinning areasare created, one on each protrusion. Due to the distribution of supercritical intensity E, the nanofibersin this embodiment are carried in two conical flat structures that move away from each other in the direction of movement of the nanofibers. The movement of the nanofibers from each other is also aided by the fact that the nanofibersformed on both protrusionsof one disk spinning electrodehave the same residual electric potential at a specific time, and so they repel each other. In addition, a gravitational force acts on the nanofibers, deforming the conical flat structures, and so a virtual collectoris formed under the surface of the electric field gradient.

For a rotating disk spinning electrodewithout a circumferential recess, supercritical intensity E of the electric field will be concentrated on the edges of the circumferential surface of the disk, and so the nanofiberswill be formed as described above. It will only be necessary to set the voltage amplitude more accurately so as to create an electric field with the supercritical intensity E in the area of the edges of the circumferential surface of the disk.

The virtual collector, i.e., the area with a concentration of density of nanofibers, is formed at the point of force balance of all electric and gravitational forces acting on the formed nanofibers. Electric forces represent the sum of all electric forces acting on the nanofibers, i.e., the force of the electric wind from the spinning electrode, the force of the electric wind from other charged parts of the spinning device, the force from ionized air ions and the force from oppositely charged parts of the nanofibersformed in the previous half-wave of the alternating electric field and the repulsive force from consensually charged parts of nanofibers. By the virtual collectoris meant a narrow region terminating the planar structure of the nanofibers, being formed, where the nanofibersbeing formed lose their movement speed when moving from the spinning areaof the spinning electrode. The reason for their slowing down is the re-polarization of the spinning electrodein the second half of the period of the supplied AC voltage. The already formed nanofiberscarried towards the virtual collectoror the nanofiberscollected in the virtual collectorare left with a residual electric charge of the polarity of the previous half-wave of electric voltage, and so they are now reversed charged relative to the current polarity of the spinning electrode. Thus, the electric potential difference required for the initialization and progress of the spinning process in the alternating electric field is created. When starting the spinning process, an electric potential is created between polymer solution Z in the spinning areaof the spinning electrodeand the air ions in the vicinity of the spinning electrode.