Apparatus for producing short polymer fibers by electrospinning

Abstract

The present invention relates to an apparatus for producing short polymer fibers by electrospinning, comprising a metering electrode 1 and a collector medium 3 facing the metering electrode 1 in the discharge direction. To provide an apparatus of the above kind that allows producing short polymer fibers by electrospinning, it is proposed that a cutting grid 5, which can be heated to at least the softening temperature of the polymer, is attached in front of the collector medium 3 in the discharge direction 2, the grid pitch of which corresponds to the minimum fiber length of the short fibers.

Description

[Technical field]

[0001] The present invention relates to an apparatus for producing short polymeric fibers by electrospinning, the apparatus comprising a metering electrode and a collector medium facing the metering electrode in a metering direction. [Background technology]

[0002] For producing thermoplastic polymer fibers, so-called electrospinning devices are known which have a metering electrode for discharging a polymer solution or polymer melt and a collector plate opposite the metering electrode in the metering direction. An electric field is applied in a stretching region extending between the metering electrode and the collector plate acting as a counter electrode. Droplets of the polymer solution or polymer melt are electrostatically charged at the metering electrode and stretched under the influence of the electric field until thinner solution streams are formed in the metering direction towards the collector plate. Evaporation of the solvent or solidification of the melt generates polymer fibers which are deposited on the collector plate.

[0003] As further described in WO2016128195, in order to obtain short fibers in a depositable form, the previously electrospun polymer fibers are first fed into a storage solution based on an ethanol / water mixture, which is cooled together with the polymer fibers below the glass transition temperature. The temperature-dependently friable polymer fibers are then chopped into short fibers and dispersed in the storage solution.

[0004] However, the disadvantage is that the production of short polymer fibers by electrospinning has hitherto only been possible in a time-consuming, discontinuous process, since first raw fiber balls or raw nonwoven fabrics must be spun, which can then only be further processed into short fibers in a separate process step. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2016128195 Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to provide an apparatus of the above-mentioned type which allows the continuous production of short polymer fibres by the electrospinning process. [Means for solving the problem]

[0007] The present invention solves this problem by providing a cutting grid, which can be heated to at least the softening temperature of the polymer, attached in front of the collector medium in the metering direction, and the grid pitch of the cutting grid corresponds to the minimum fiber length of the short fibers.

[0008] Due to these characteristics, a number of short fibers can be produced in succession in one method step, since the raw fibers formed in the stretching region extending between the metering electrode and the collector medium first come to rest on a heatable cutting grid, are cut into a number of short fibers when passing through the cutting grid, and are subsequently deposited on the collector medium. Due to electrostatic bending instabilities, the raw fibers in the stretching region mainly follow a trajectory that defines a cone extending in the metering direction. As a result, the raw fibers usually enter the cutting grid at an acute angle to the plane of the cutting grid, so that the periphery surrounding each individual grid hole or grid pitch forms a corresponding cut edge for the raw fibers that are generated. For this reason, the raw fibers can be chopped into small pieces without problems on the grid mesh, since the raw fibers are locally heated above the softening temperature of the polymer at the fiber cutting parts that are in contact with the respective grid mesh. The short fibers thus formed are then deposited on the collector medium, which may be a liquid that constitutes, for example, a reference potential by ground or a counter electrode opposite the metering electrode. The liquid may be a corresponding storage liquid, for example an ethanol / water mixture. The short fibers can then be deposited directly into the storage liquid and dispersed therein. In order to obtain a dispersion of depositable short fibers that can be further processed without problems later during the subsequent process, the collector vessel containing the storage liquid may have a liquid outlet. The storage liquid may be further transported together with the short fibers dispersed in the storage liquid, for example to a filling device. Although the presence of heating elements would essentially heat up the air in the drawing area, causing it to move and, on the other hand, potentially disrupting the trajectory of the raw fibres or causing the polymer to solidify earlier than expected at the metering electrode, it has been found that the manufacturing process is not impaired if the cutting grid is heated to a temperature within ±20% of the softening temperature, in particular to the softening temperature of the polymer, which softening temperature means in particular the liquidus temperature in the case of semicrystalline polymers or the glass transition temperature in the case of amorphous polymers.In order to increase the frequency of the generated short fibers according to the probability density function for the length distribution of the short fibers by simple structural means, it is proposed that the cutting grid has a grid pitch of at least 5 μm. It has been found that the length distribution of the generated short fibers can be adjusted by changing the grid pitch of the cutting grid. In this case, indeed, at a grid pitch of less than 5 μm, the raw fibers are no longer cut, but accumulate on the cutting grid due to the increased specific surface area of ​​the cutting grid, and even if cut, the short fibers may evaporate before they can land on the collector medium. The angle of incidence of the short fibers on the grid mesh does not fundamentally affect the length of the short fibers, but for a given pitch width x, the frequency of short fibers with a particular short fiber length I is such that x≦I≦x. * The pitch width x can be increased within the range of √2. In this case, the pitch width x is at least 5 μm. Since only the protrusion of the pitch width along the normal plane to the metering direction is important for the cutting process, the length distribution of the short fibers can be controlled by using a cutting grid with a certain pitch width, even within a certain range, by tilting the cutting grid from the normal plane.

[0009] In order to obtain particularly favorable processing conditions when a storage liquid is used as the collector medium, it is recommended that the cutting grid is configured as an electric heating resistor and as a counter electrode to the metering electrode. By this means, an electric field is formed between the cutting grid and the metering electrode. In this case, a heating current is passed through the cutting grid between two terminal poles. The heating current is generated by two different potentials applied to the cutting grid. These potentials are significantly different from the potential of the metering electrode. As a result, the heating current does not affect the electrospinning process. For example, the cutting grid can be grounded by a terminal pole. Since the electric charge of the short fibers is already largely neutralized by the cutting grid, the generated short fibers can be deposited on or stored in the collector medium without being hindered by electric forces. Thus, especially when a storage liquid is used as the collector medium, the method can be carried out without relying on the electrical conductivity of the storage liquid, and the collector medium itself does not have to function as a counter electrode.

[0010] If the stretching zone extending between the metering electrode and the cutting grid can be cooled by a temperature-controlling fluid, the stability and continuity of the manufacturing process can be further improved, especially when using polymers with a high liquidus temperature. For example, the cooling can prevent the heating of the air in the stretching zone by the heated cutting grid, which would disturb the trajectory of the raw fibres, and thus a stable manufacturing process can be achieved. For example, the start-up zone can be appropriately temperature-controlled by supplying cooled air. In this case, the flow rate must be selected so that the stretching of the raw fibres is not impaired. When using polymer solutions, the process conditions can be further improved if the metering electrode itself is cooled by a temperature-controlling fluid, for example if it is blown by a cooling air stream. This can avoid a premature evaporation of the solvent and clogging of the metering electrode by the ejected polymer.

[0011] The invention also relates to a method for producing short polymer fibers by means of the device of the invention. In this case, an electric field is first generated between a metering electrode for discharging the polymer system and a collector medium for depositing the spun fibers. A raw fiber is discharged from the metering electrode by the electric field. In this context, a polymer system means the polymer raw material for producing the short fibers, and therefore in particular a water-soluble, solvent-based and meltable polymer, possibly containing additives and fillers. A part of the raw fiber is heated to at least the softening temperature of the polymer and is cut into a plurality of short fibers during the heating. These short fibers are then deposited on a collector medium. Particularly favorable conditions are obtained if the short fibers are deposited in a storage liquid as a collector medium, for example in a liquid ethanol / water mixture, and dispersed in the collector medium. The thus-spun depositable dispersed short fibers can then be further processed without problems, for example to produce a material for a filter. [Brief description of the drawings]

[0012] [Figure 1] 1 shows an apparatus of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The device according to the invention comprises a metering electrode 1 and a collector medium 3 facing the metering electrode 1 in a metering direction 2. The collector medium may be a storage liquid for the produced short fibres, for example an ethanol / water mixture present in a collector vessel 4. A cutting grid 5, heated to at least the softening temperature of the polymer, is mounted in front of the collector medium 3 in the metering direction 2. The grid pitch of the cutting grid 5 corresponds to the minimum fibre length of the produced short fibres.

[0014] To produce short polymeric fibers by electrospinning, various polymer systems can be used as raw materials, in particular water-soluble, solvent-based and meltable polymers, together with additives and fillers that may be present. For example, to obtain fibers based on polymethylmethacrylate, a polymer solution consisting of a mass ratio of about 20% polymethylmethacrylate, about 55% acetic acid, about 20% ethyl acetate and optionally additional additives can be used. In the case of amorphous polymethylmethacrylate, the softening temperature is the glass transition temperature of the amorphous polymethylmethacrylate, which is about 100°C to 110°C.

[0015] To generate the electric field, a voltage that can be 20 kV to 30 kV is applied between the metering electrode 1 and the heated cutting grid 5 and / or the collector medium 3. The polymer solution is fed via the metering electrode 1 with a volume flow rate of 3 ml / h to 9 ml / h into the stretching region 6. The polymer droplets that emerge at the metering electrode 1 are thereby electrostatically charged and stretched under the influence of the electric field. As a result, due to electrostatic bending instabilities in the stretching region 6, one raw fiber 7 is generated, which mainly follows a trajectory that defines a cone extending in the metering direction 2, as shown diagrammatically in the figure.

[0016] The raw fibers 7 are heated by the cutting grid 5 to at least the softening temperature of the polymer by entering the cutting grid 7 at an acute angle to the plane of the cutting grid, such that the circumferential parts surrounding the individual grid holes or grid pitches respectively form corresponding cut edges for the emerging raw fibers 7, and are chopped into a plurality of short fibers during the heating. Furthermore, the short fibers generated by the chopping, which are not shown in detail in the figures, are deposited on the collector medium 3 and dispersed therein. As a result, the thus-dispersed short fibers can be further processed without problems, for example as a filter matrix for producing filter materials. For this purpose, the collector vessel 4 may have a corresponding liquid outlet. A storage liquid can be further conveyed to a filling device together with the short fibers dispersed in the storage liquid.

[0017] The length distribution of the short fibers can be adjusted, for example, by the pitch width of the grid pitch of the cutting grid 5. According to the adjustment, the cutting grid 5 can have a grid pitch of at least 5 μm in order to increase the frequency of the generated short fibers according to the probability density function for the length distribution of the short fibers.

[0018] The desired process conditions are obtained if the cutting grid 5 is configured as an electric heating resistor and as a counter electrode to the metering electrode 1. A heating current is passed through the cutting grid 5, which is generated by two different potentials applied to the cutting grid 5 between the two terminal poles of a power supply 8 for the cutting grid 5.

[0019] According to some embodiments, the metering electrode 1 and / or the stretching zone 6 extending between this metering electrode 1 and the cutting grid 5 can be cooled by a temperature-controlled fluid. This can prevent heating of the air in the stretching zone 6 due to the heated cutting grid 5 and clogging of the metering electrode 1, which would disrupt the trajectory of the raw fibres 7. Thereby, a stable production process can be achieved. [Explanation of symbols]

[0020] 1 metering electrode 2 Weighing direction 3 Collector medium 4 Collector vessel 5 Cutting grid 6 Stretch Area 7 Raw Fibers 8 Power supply

Claims

1. An apparatus for producing short polymer fibers by electrospinning, comprising a metering electrode (1) and a collector medium (3) facing the metering electrode (1) in the discharge direction (2), a cutting grid (5) that can be heated to at least the softening temperature of the polymer is attached in front of the collector medium (3) in the discharge direction (2), the grid pitch of the cutting grid (5) corresponding to the minimum fiber length of the short fibers.

2. 2. The device according to claim 1, characterized in that the cut grating (5) has a grating pitch of at least 5 μm.

3. 3. The device according to claim 1, wherein the cutting grid (5) is configured as an electric heating resistor and as a counter electrode to the metering electrode (1).

4. 4. The device according to claim 1, wherein the metering electrode (1) and / or the stretched area (6) extending between the metering electrode (1) and the cutting grid (5) are coolable by a temperature-control fluid.

5. A method for producing short polymeric fibres by electrospinning with a device according to any one of claims 1 to 4, comprising the steps of: The method in which an electric field is generated between a metering electrode (1) for the polymer system and a collector medium (3) for depositing the spun fibers, a green fiber (7) is first elongated from the metering electrode (1) by the electric field, a portion of the green fiber (7) is chopped into a plurality of short fibers by heating to at least the softening temperature of the polymer, and thereafter the plurality of short fibers are deposited on the collector medium (3).

6. 6. The method according to claim 5, characterized in that the plurality of short fibers are deposited in a storage liquid as a collector medium (3) and dispersed in said storage liquid.

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