Method and apparatus for the production of superfine polymer fibers by melt-blown technology

Abstract

A process for producing melt-blown polymer fibers (2) comprising at least the following steps: a) Providing a polymer melt (3); b) Extruding the polymer melt (3) using at least one spinneret arrangement (4) to form polymer fibers (2) into a free jet; c) Stretching the polymer fibers (2) using a primary gas stream (5); d) Cooling the polymer fibers (2) during drawing; and e) Tempering the polymer fibers (2) during drawing by a tertiary gas stream (7) or by thermal radiation (11).

Description

[0001] The present invention relates to a method and an apparatus for the production of very fine polymer fibers as well as a nonwoven fabric and filter with a filter medium produced according to the method.

[0002] Methods and devices for spinning polymers into fibers using a hot air stream are generally known in the field as the so-called "meltblown technique." In these known methods and devices, a polymer melt is extruded through a multitude of capillaries of a spinneret, hereinafter referred to as a spinneret assembly, into a free jet of polymer fibers and stretched downstream of the spinneret assembly by a hot air stream. The hot air stream has a significantly higher velocity than the polymer melt at the outlet of the spinneret assembly's capillaries. Due to this large velocity difference, the extruded polymer fibers initially follow the airflow toward a deposit. As they progress, turbulence in the airflow becomes noticeable.Calculations of the fiber formation process in melt-blown processes have shown that stretching is only partially effective in the upper flow region of the hot air, where the differential velocity between the hot air stream and the polymer fibers leads to continuous fiber stretching. A large portion of the fiber stretching occurs in the turbulent flow region, as soon as the polymer fibers fluctuate due to the turbulence of the hot air stream. Therefore, for the production of very fine melt-blown polymer fibers, it is advantageous to increase the length of the effective stretching zone. This requires that the extruded polymer fibers have a temperature suitable for stretching throughout the entire stretching zone.

[0003] WO 02 / 059 403 A1 discloses a process for producing superfine melt-blown polymer fibers in which the turbulent hot air flow zone is extended to achieve a longer residence time of the polymer fibers within a temperature range that allows them to be drawn. The drawing forces thus act on the formed polymer fibers for a longer period, resulting in a polymer fiber with a smaller diameter. To increase the residence time of the extruded polymer fibers in the hot air flow, the process described in WO 02 / 059 403 A1 involves shielding the hot air flow below the spinneret assembly from cold air and supplying a hot air flow via a secondary air channel below a knife edge of the spinneret. The drawing of the polymer fibers occurs at temperatures significantly above the melting point of the polymer used.

[0004] However, it has been found that the known processes cannot produce uniform, superfine melt-blown polymer fibers with a mean fiber diameter of less than 0.5 micrometers (µm). Typically, in the production of melt-blown polymer fibers, the fiber fineness is reduced by decreasing the melt flow rate per die capillary and / or increasing the hot air flow, thus increasing the differential velocity between the polymer fibers and the hot air flow downstream of the spinneret assembly. However, both measures lead to fiber breakage in the known processes for polymer fibers below a fineness of 0.5 µm or even 1 µm, resulting in an unstable production state. Furthermore, temperatures above the melting point, when they occur downstream of the knife edge of a spinneret assembly, cause the polymer fibers to adhere to the lay-up belt.In any case, the resulting fibers vary considerably in diameter, depending also on the temperatures in the zones traversed by the fibers. The fiber diameters can be determined using various methods. For the purposes of the present invention, optical measurement methods are employed, in particular image analysis of scanned electron microscope (SEM) images. This includes a statistically valid analysis method for reproducible, objective, and therefore representative statements about fiber diameters and their variation at multiple measurement points.

[0005] A process for producing melt-spun fibers is disclosed in publication WO 2008 / 016 771 A1. A process for producing so-called non-staple fibers is disclosed in publication WO 2012 / 020 053 A1. These non-staple fibers are produced by tearing the polymer fibers during the process, in contrast to mechanically cutting the fibers. Publication WO 2012 / 020 053 A1 discloses a process for producing polymer filaments in which they are stretched with a hot gas and a cooling stream is provided at the spinneret outlet to cool the polymer filaments. A filament produced in this way is fed to a take-up device where it is torn.

[0006] German patent EP 1 637 632 A1 discloses a device for producing thermoplastic filaments with a meltblown blow head. The device has a plastic guide core with at least one series of nozzle bores for the exit of the molten plastic and further comprises primary air supply devices arranged on both sides of the plastic guide core, by which the filaments can be supplied with primary air on both sides in the region of the nozzle bore openings. At least one secondary air supply device is provided, by which the filaments below the meltblown blow head can be supplied with secondary air laterally.

[0007] The object of the invention is therefore to at least partially solve the problems described with reference to the prior art and, in particular, to provide a method for producing polymer fibers with a very uniform diameter distribution, especially with a fiber fineness of less than 1 µm. Furthermore, an apparatus for producing polymer fibers is to be provided with which polymer fibers with a uniform fiber fineness, especially of less than 1 µm, can be reliably produced. In particular, the proportion of fine fibers in a final product is to be increased and the scatter of fiber diameters reduced.

[0008] These problems are solved by a method according to the features of claim 1 and a device with the features of claim 11. Products manufactured according to the method are specified in claims 13 to 15. Further advantageous embodiments of the invention are specified in the dependent claims. It should be noted that the features listed individually in the dependent claims can be combined with one another in any technologically meaningful way and define further embodiments of the invention. In addition, the features specified in the claims are further specified and explained in the description, which also presents further preferred embodiments of the invention.

[0009] The inventive process for producing melt-blown polymer fibers, in particular with a fiber fineness of less than 1 µm, comprises at least the following steps: a) Providing a polymer melt; b) Extruding the polymer melt into polymer fibers in a free jet using at least one spinneret arrangement; c) Stretching the polymer fibers using a primary gas stream; d) Cooling the polymer fibers during drawing; and e) Tempering the polymer fibers during drawing by means of a tertiary gas stream or by thermal radiation.

[0010] In the proposed method for producing polymer fibers, in particular with a mean fiber diameter of less than 1 µm, preferably less than 0.5 µm, a polymer melt is first provided in a polymer melt source, in particular a reservoir.

[0011] The polymer melt can be, for example, polypropylene (PP) or polybutylene terephthalate (PBT). Other thermoplastic polymers can also be used. The invention also allows for improvements in the processing of high-temperature thermoplastics such as liquid crystal polymers (LCP), polyphenylene sulfides (PPS), polyetherketones (PEK), polyetheretherketones (PEEK), and high-temperature polyamides into very fine fibers. From the polymer melt source, the polymer melt is conveyed to at least one spinneret assembly and extruded through a multitude of capillaries of the spinneret assembly to form polymer fibers. The polymer melt has a temperature of 200°C to 450°C during extrusion. In the case of polypropylene (PP), the polymer melt has a temperature of 200°C to 280°C, preferably 220°C to 250°C, during extrusion.In the case of polybutylene terephthalate (PBT), the polymer melt has a temperature of 240°C to 300°C, preferably 250°C to 280°C, during extrusion. The overpressure of the polymer melt during extrusion is between 1 and 20 bar. The spinneret assembly can be, for example, a spinneret such as that described in WO-A1-02 059 403 on page 6, line 11 to page 7, line 14. After exiting the spinneret assembly into a free jet of a primary gas stream, the polymer fibers are drawn using this primary gas stream. This primary gas stream is, in particular, a hot air stream. However, the primary gas stream can also consist, at least partially, of another (inert) hot gas, such as nitrogen or helium.Immediately downstream of the at least one spinneret, that is, in particular (directly) after the polymer melt exits the spinneret assembly, the polymer fibers are cooled during drawing. The polymer fibers are preferably cooled to a temperature slightly less than or equal to the melting temperature of the polymer melt. This cooling prevents the polymer fibers from breaking, even at the differential velocities between the primary gas stream and the polymer fibers required for the production of polymer fibers with an average fiber fineness of less than 1 µm, preferably less than 0.5 µm.In general, extrusion through the spinnerets takes place in the direction of gravity, i.e. downwards; however, it is not gravity but the surrounding airflow that is decisive, so other arrangements are also possible, which is why the processes are described more generally in the direction of the flow, which does not necessarily mean in a vertical direction.

[0012] In the nozzle exit region, the velocity of the polymer fibers is negligible compared to the air velocity, which essentially reaches its maximum there. The velocity of the primary gas flow can reach 400 m / s or even approximately 600 m / s, with the high temperature of the gas flow being associated with a correspondingly increased speed of sound. The velocity of the primary gas flow can, however, reach or even exceed the speed of sound. Due to the acceleration of the entrained fibers, the differential velocity between them and the primary gas flow decreases.

[0013] It should also be noted that higher differential velocities can be achieved by using gases with high sound velocity, e.g., helium, in conjunction with high temperatures. The higher the sound velocity and temperature of the primary gas stream used, the higher the velocities and thus the finer the fibers that can be obtained. This finding can also be realized independently of the other features of the invention. Helium, in particular, has a very high sound velocity and can be used pure or as an additive to the primary gas.

[0014] The cooling of the polymer fibers during the subsequent process is uneven because, during drawing, a large number of spinnerets quickly form a widening curtain of fibers along the widening air stream. The polymer fibers located on the outer edge of this curtain cool down faster than those in the inner areas. The disadvantage of this is that the temperature of the outer polymer fibers can thus drop more quickly below a temperature suitable for drawing. In some cases, fibers from adjacent melt strands within the spinneret array can also become embedded. Outer fibers, or fibers embedded together, cannot be drawn as much as individual fibers located further inwards, thereby reducing the proportion of very thin fibers in the final product.

[0015] To maximize the duration of the drawing zone for as many fibers as possible, an additional tempering step is performed according to the invention. Preferably, a further hot tertiary air stream is supplied, which shields the free jet and the curtain from the cold environment and contributes to a more uniform temperature within the fiber curtain. This allows the outer fibers of the curtain to be kept at a temperature suitable for drawing for a longer period, resulting in a very uniform overall temperature within the curtain. This tempering of all fibers of the curtain in a range of 10 to 150 mm, preferably 50 to 100 mm, in the tucking direction downstream of the spinneret assembly, allows for a longer tucking duration and thus smaller average fiber diameters in the final product.

[0016] This results in a more uniform temperature in the inner and outer areas of the free jet. The inventive combination of (direct) cooling and subsequent tempering of the polymer fibers after exiting the spinneret assembly therefore enables polymer fibers to be drawn to a particularly low average fiber fineness of less than 1 µm, preferably less than 0.5 µm. The polymer fibers are drawn, for example, to an average fiber fineness between 0.05 µm and 3 µm, preferably to 0.1 µm to 1 µm. The polymer fibers are then laid down on a belt or drum to form a nonwoven fabric. The laying of the polymer fibers can be supported by suction below the belt or in the drum, the suction generating a negative pressure of, in particular, 0.001 to 0.2 bar, preferably 0.01 to 0.17 bar. The nonwoven fabric has a basis weight of, in particular, 0.2 g / m². 2up to 50 g / m² 2 The belt or drum is arranged downstream of the spinneret, particularly at a distance of 100 mm to 500 mm. Such nonwoven fabrics can be used particularly advantageously as barrier fabrics in surgical gowns, as filter material, and / or in similar applications. The invention also results in a lower scatter in the fiber diameters, which is advantageous regardless of the achievable lower average fiber diameters, because it allows for the production of more uniform nonwoven fabrics or filters.

[0017] Preferably, in step c), the primary gas stream has a temperature of 200°C to 400°C. When using a polypropylene (PP) polymer melt, the primary gas stream has a temperature of 200°C to 300°C, preferably 240°C to 280°C. When using a polybutylene terephthalate (PBT) polymer melt, the primary gas stream has a temperature of 240°C to 390°C, preferably 270°C to 300°C.

[0018] Furthermore, it is advantageous if, in step c), the primary gas flow has an overpressure of 0.1 to 3.0 bar. An overpressure of 1.0 to 2.5 bar is preferred. The overpressure is specified here in relation to an ambient pressure, for example, an ambient pressure of 1 bar.

[0019] It is also advantageous if, in step d), the polymer fibers are cooled by a secondary gas stream. This secondary gas stream can be, for example, (ambient) air. The secondary gas stream can be volume-controlled. This means, in particular, that the polymer fibers passively draw in the secondary gas stream along with the primary gas stream, whereby the volume of the drawn-in secondary gas stream can be controlled, in particular, by adjusting the cross-sectional area of ​​a first secondary gas supply. Alternatively, however, actively blowing the secondary gas stream onto the polymer fibers is possible.

[0020] Preferably, the secondary gas stream has a temperature of 10° Celsius to 30° Celsius. The temperature of the secondary gas stream can, in particular, correspond to room temperature, for example preferably 20° Celsius to 30° Celsius.

[0021] Furthermore, it is advantageous if the secondary gas stream has an overpressure of 0.005 to 0.05 bar. An overpressure of 0.01 to 0.02 bar is particularly preferred. Here too, the overpressure refers to an ambient pressure, for example, an ambient pressure of 1 bar.

[0022] Furthermore, it is advantageous if the tertiary gas stream has a temperature below the melting temperature of the respective polymer, preferably at least 10 K (Kelvin), in particular at least 25 K lower.

[0023] The temperature control of the free jet and the fiber curtain running within it in step e) is achieved by a tertiary gas stream or by thermal radiation. The tertiary gas stream can be, for example, hot air, superheated steam, and / or hot water spray. The tertiary gas stream can also be volume-controlled. This means, in particular, that the polymer fibers passively draw in the tertiary gas stream along with the primary (and secondary) gas streams, whereby the volume of the drawn-in tertiary gas stream can be controlled, in particular, by adjusting a second cross-sectional area of ​​a tertiary gas supply. Alternatively, however, active blowing of the tertiary gas stream onto the polymer fibers is possible. The heating of the tertiary gas stream can be achieved, for example, by at least one heating element in the tertiary gas supply and / or at least one air heater.The second cross-sectional area of ​​the tertiary gas supply is preferably five to fifteen times larger than the first cross-sectional area of ​​the secondary gas supply.

[0024] The tertiary gas stream at least partially envelops the polymer fibers, thus at least partially preventing the penetration of cold ambient air. The thermal radiation can be, for example, infrared radiation and / or microwave radiation.

[0025] Furthermore, it is advantageous if the tertiary gas stream has a temperature of 40°C to 200°C or if the polymer fibers are heated to a temperature of 40°C to 200°C by thermal radiation. If the polymer melt is polypropylene (PP), the tertiary gas stream preferably has a temperature of 50°C to 170°C, and most preferably 50°C to 120°C. If the polymer melt is polybutylene terephthalate (PBT), the tertiary gas stream preferably has a temperature of 50°C to 200°C, and most preferably 100°C to 150°C.

[0026] It is also advantageous if the tertiary gas stream has an overpressure of 0.005 to 0.05 bar. Here too, the overpressure refers to an ambient pressure, for example, an ambient pressure of 1 bar.

[0027] According to a further aspect of the invention, a device for producing polymer fibers with a fiber fineness of less than 1 µm is also proposed, which has a spinneret for extruding a polymer melt into polymer fibers and a primary gas supply for drawing the polymer fibers, wherein a secondary gas supply for cooling the polymer fibers during drawing is arranged downstream of the spinneret and wherein a tertiary gas supply for tempering the polymer fibers during drawing is arranged downstream of the secondary gas supply.

[0028] The device according to the invention is specifically designed to carry out the method according to the invention. For further details regarding the device proposed here, reference is made to the description of the method according to the invention.

[0029] The invention and its technical context are explained in more detail below with reference to the figure. It should be noted that the figure shows a particularly preferred embodiment of the invention, but that the invention is not limited to this embodiment. It schematically illustrates: Fig. 1: a device for producing polymer fibers with an average fiber diameter of less than 1 µm.

[0030] The Fig.Figure 1 shows a device 1 for producing polymer fibers 2 with a fiber fineness of less than 1 micrometer (µm). For this purpose, a polymer melt 3 is first conveyed from a polymer melt source 12 to a spinneret assembly 4, for example by a pump (not shown). Downstream of the spinneret assembly 4, the polymer fibers 2 extruded from the polymer melt 3 are captured by a hot primary gas stream 5 supplied by a primary gas supply 8 and drawn downstream of the spinneret assembly 4. Directly downstream, in particular below the spinneret assembly 4, a secondary gas supply 9 is arranged so that the polymer fibers 2 can be cooled by a secondary gas stream 6 directly downstream of the spinneret assembly 4 during drawing. The secondary gas supply 9 has a first cross-sectional area 16.

[0031] Directly downstream of the secondary gas supply 9, a tertiary gas supply 10 is arranged, so that the polymer fibers 2 can be tempered by a tertiary gas supply 7 during drawing after cooling by the secondary gas stream 6. The tertiary gas supply 9 has a second cross-sectional area 17. Alternatively or cumulatively, the polymer fibers 2 can also be heated by thermal radiation 11. For this purpose, a thermal radiation source (not shown here) is arranged downstream of the secondary gas supply 9. Following drawing, the polymer fibers 2 are laid down on a belt 14 to form a nonwoven fabric 13. This can be assisted by a suction system 15, which creates a negative pressure.

[0032] The inventive method and the inventive device are characterized in that they enable the production of particularly high proportions of fine polymer fibers in an end product with a fiber fineness of less than 1 µm or even less than 0.5 µm.

[0033] Nonwovens produced using this method can be used for technical applications, such as the filtration of gases and liquids. Nonwovens are well-suited for this purpose due to their economical production and versatility. They are particularly advantageous because, for example, their porosity can be easily adjusted to specific requirements.

[0034] Filters, and thus filter media, have the task of filtering particles of varying sizes, depending on the application. The pore structure and the effective filter surface area play a crucial role in this process. The particles are primarily retained at the surface of the filter medium, or the filter medium is designed in such a way that the particles penetrate it and are trapped there. For this purpose, a filter medium produced according to the inventive method, preferably from nonwovens, is constructed in one or more layers. Multi-layered filter media typically exhibit higher porosity on the upstream side than on the downstream side. By vertically varying the density of the different filter layers of the filter medium, it is possible to selectively trap particles of specific sizes in specific areas without the risk of clogging the filter medium.For example, a progressive structure of the filter medium achieves a long service life with simultaneously high filtration efficiency. For medium and high filtration efficiencies, for example filter class F5 and above, mechanically filtering fibers are used, which are produced according to the inventive method.

[0035] Each filter can be adapted to its specific application. For example, a filter medium may require a particular stiffness for processing and use. In this case, the filter medium can be supplemented with one or more additional filter layers that give the filter medium the desired properties. These additional filter layers can also be manufactured using a different method than the one described here to provide the necessary properties. Reference symbol list 1 Device 2 polymer fibers 3 Polymer melt 4 spinneret arrangement 5 Primary gas flow / free jet 6 Secondary gas flow 7 Tertiary gas stream 8 Primary gas supply 9 Secondary gas supply 10 Tertiary gas supply 11 Thermal radiation 12 Polymer melt source 13 Nonwoven fabric Volume 14 15 Extraction 16 first cross-sectional area 17 second cross-sectional area

Claims

[1] A process for producing melt-blown polymer fibers (2) comprising at least the following steps: a) Providing a polymer melt (3); b) Extruding the polymer melt (3) using at least one spinneret arrangement (4) to form polymer fibers (2) into a free jet; c) Stretching the polymer fibers (2) using a primary gas stream (5); d) Cooling the polymer fibers (2) during drawing; and e) Tempering the polymer fibers (2) during drawing by a tertiary gas stream (7) or by thermal radiation (11). [2] Method according to claim 1, wherein in step c) the primary gas stream (5) has a temperature of 200° Celsius to 450° Celsius. [3] Method according to one of the preceding claims, wherein in step c) the primary gas stream (5) has an overpressure of 0.1 to 3.0 bar. [4] Method according to one of the preceding claims, wherein in step d) the polymer fibers (2) are cooled by a secondary gas stream (6). [5] Method according to claim 4, wherein the secondary gas stream (6) has a temperature of 5° Celsius to 50°, preferably 10° to 30° Celsius. [6] Method according to claim 4 or 5, wherein the secondary gas stream (6) has an overpressure of 0.005 to 0.05 bar. [7] Method according to one of the preceding claims, wherein the fibers in the free jet are stretched to a mean fiber diameter of less than 1 µm (micrometer), in particular less than 0.5 µm. [8] Method according to claim 7, wherein the tertiary gas stream (7) has a temperature of 40° Celsius to 200° Celsius or the polymer fibers (2) are tempered by thermal radiation (11) in a temperature range of 40° Celsius to 200° Celsius. [9] Method according to claim 7 or 8, wherein the tertiary gas stream (7) has an overpressure of 0.005 to 0.05 bar. [10] Method according to any of the preceding claims, characterized by that the polymer melt is a high-temperature thermoplastic, in particular containing a liquid crystal polymer (LCP), polyphenylene sulfides (PPS), polyetherketones (PEK), polyetheretherketones (PEEK) and / or a high-temperature polyamide. [11] Device (1) for producing melt-blown polymer fibers (2) comprising a spinneret arrangement (4) for extruding a polymer melt (3) into polymer fibers (2) in a free jet and a primary gas supply (8) for drawing the polymer fibers (2), wherein a secondary gas supply (9) for cooling the polymer fibers (2) during drawing is arranged downstream of the spinneret arrangement (4) and wherein a tertiary gas supply (10) for tempering the polymer fibers (2) during further drawing is arranged downstream of the secondary gas supply (9) or the tempering of the polymer fibers (2) is carried out downstream of the secondary gas supply (9) by thermal radiation (11). [12] Device according to claim 11, wherein the primary gas supply (8) is designed for the use of helium or admixtures of helium. [13] Nonwoven fabric produced by a method according to any one of claims 1 to 10 and / or with a device according to claim 11 or 12. [14] Filter comprising at least one filter medium produced by a method according to one of claims 1 to 10 and / or by a device according to claim 11 or 12. [15] Filter according to claim 14, characterized by that the filter medium is designed to be single- or multi-layered.

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