Pleatable nonwoven material and method and apparatus for production thereof

Abstract

A pleatable nonwoven material is provided, including thicker form-giving fibers and thinner fibers determining the filter effect, wherein the thinner fibers are incorporated largely homogeneously in the thicker fibers running in the direction along the surface of the nonwoven material and a distribution density gradient of the thinner fibers is established perpendicular to the surface of the nonwoven material such that the highest concentration of thinner fibers is in the region of the center or on one of the two outsides, wherein the thicker and thinner fibers are bonded together by solidification from the melted condition and are made from the same material.

Claims

1. A pleated air filter comprising a pleatable filter material consisting of a nonwoven material, consisting of: thicker fibers each comprising a substantially continuous strand for providing a stabilizing support frame for the nonwoven material; and thinner fibers of the same material as the thicker fibers and relatively thinner than the thicker fibers, the thinner fibers each comprising a substantially continuous strand having a diameter of less than 1000 nm for providing a filter effect, wherein the strands of thicker and thinner fibers are tangled and intertwined with each other, and with the thicker and thinner fibers melted and bonded together at least at some of the locations where the thicker and thinner fibers are in contact, and wherein the thinner fibers are incorporated largely homogeneously in the thicker fibers in a direction along the surface of the nonwoven material, and wherein a continuous distribution-density gradient of the thinner fibers in a direction perpendicular to the surface of the nonwoven material is such that the highest concentration of thinner fibers is in the region at either the center of the filter or one of the two outer surfaces of the filter.

2. A filter according to claim 1, wherein the thicker fibers have a diameter of between 2 and 200 m.

3. A filter according to claim 1, wherein the thinner fibers have a diameter of between 50 nm and 1000 nm.

4. A filter according to claim 1, wherein the thicker and thinner fibers consist of one of the group comprising polyamide, polypropylene, polyester or a mixture thereof.

5. A filter according to claim 1, wherein the material of the thicker and thinner fibers is a high-viscosity polymer melt with a melt flow index mfi of well below 500.

6. A filter according to claim 1, wherein the strands of thicker and thinner fibers comprise a polymer or polymer mixture.

7. A filter according to claim 1, wherein the thicker fibers have a diameter of more than 2 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic sectional view of a spinning beam which comprises a plurality of spinnerets arranged next to each other through which a cone of compressed liquid polymer is discharged under pressure. FIG. 2 shows a schematic view of two spinning beams which form an acute angle with each other and are arranged at an angle a with respect to the vertical direction relative to a conveyor belt arranged underneath a spinning beam. FIGS. 3A-3H illustrate photographs taken with a scanning electron microscope of material of the present invention.

(2) According to the invention, this object is achieved in that the thinner fibers are incorporated largely homogeneously in the thicker fibers in the direction along the surface of the nonwoven material, and in that a distribution-density gradient of the thinner fibers in the direction perpendicular to the surface of the nonwoven material is such that the highest concentration of thinner fibers is in the region of the center or at one of the two outsides, wherein the thicker and the thinner fibers are bonded together by solidification from the melted condition and are made from the same material.

(3) It is advantageously provided that the thicker fibers have a diameter >2 and the thinner fibers have a diameter <1000 nm. In particular, the thicker fibers should have a diameter of between 2 and 200 m while the thinner fibers should have a diameter of between 50 nm and 1000 nm. Such extremely thin nanofibers enable a particularly good filter effect to be achieved.

(4) The fibers may advantageously consist of polyamide, polypropylene, polyester or a mixture thereof.

(5) The invention also relates to a method of producing a pleatable nonwoven fabric, wherein a polymer is melted and pressed through the spinnerets of a spinning beam, and wherein the polymer threads thus produced are laid down on a conveyor belt so as to form a nonwoven layer, wherein it is provided according to the invention that spinnerets of various diameters are used for producing and laying down higher- and lower-thickness polymer threads in a single process step, wherein spinnerets of various diameters are used while simultaneously producing and laying down higher- and lower-thickness polymer threads in a single process step, wherein the diameter of the spinnerets <0.2 mm, preferably 0.15 mm, and for the fibers of higher thickness >0.2 mm, preferably 0.3-0.4 mm, and high-viscosity polymer melts are used whose melt flow index mfi is well below 500.

(6) By means of this method, the desired homogeneous distribution of the thinner fibers in the support frame formed by the thicker fibers is achieved.

(7) Favorably, at least two spinning beams are provided which are arranged at an angle to each other, wherein the polymer threads leaving the spinnerets of each spinning beam tangle and intertwine with each other before contact with the base or at the latest at the moment of contact therewith.

(8) In particular, two spinning beams can be used, wherein a first spinning beam comprises larger-diameter spinnerets and a second spinning beam comprises smaller-diameter spinnerets.

(9) In order to achieve long fibers with a very small diameter, it is crucial to use a high-viscosity polymer melt whose melt flow index mfi is well below 500.

(10) Another important aspect is that the air supplied to the spinning beam has a relatively slight overpressure in the order of magnitude of 500 mbar.

(11) The invention further relates to an apparatus for implementation of the above described method, the apparatus comprising spinning beams including a plurality of spinnerets arranged next to one another, and a conveyor belt for laying down the polymer threads leaving the spinnerets, the apparatus distinguishing itself by the fact that at least two spinning beams are provided which are arranged relative to each other such that when leaving the spinnerets, the polymer threads tangle before contact with the conveyor belt or at the latest at the moment of contact therewith, wherein a first spinning beam comprises larger-diameter spinnerets and a second spinning beam comprises smaller-diameter spinnerets, and wherein the diameter of the smaller-diameter spinnerets is <0.2 mm, preferably approximately 0.15 mm, and the diameter of the larger-diameter spinnerets is >0.2 mm, preferably 0.3-0.4 mm.

(12) It is in particular provided that the spinning beams are arranged at an angle to each other, causing the thinner and thicker fibers to tangle and intertwine with each other upon discharge.

(13) In particular, a first spinning beam may comprise larger-diameter spinnerets and a second spinning beam may comprise smaller-diameter spinnerets.

(14) Finally, fans can be provided in the outlet region of the spinnerets for generation of an air stream in the order of magnitude of 500 mbar.

(15) The spinnerets are particularly advantageously produced by lasering and have a diameter of <0.20 mm. This enables a high density of small-diameter spinnerets to be produced in an economical manner.

(16) The invention will hereinafter be described in more detail by means of photographs taken with a scanning electron microscope and the drawing.

(17) The following illustrations 1 and 2 show the distribution of the various fiber sizes. (See illustrations 1 and 2, FIGS. 3A, 3B).

(18) Coarse fibers (diameter of approximately 15 m) are mostly present in the form of multiple fibers. The coarse single fibers are joined together to form multiple fibers, wherein the fiber composites (up to 200 m) are not only in loose contact with each other but the surfaces thereof are melted together for the most part.

(19) Medium fiber diameters (approximately 1-2 m) are mostly present in the form of single fibers, rarely in the form of fiber composites comprising a maximum of 3 fibers.

(20) The overview photographs clearly show that a web of much thinner fibers (nanofibers <1 m) passes through the fiber structure of the coarse and medium fiber diameters. The finest fibers are only present in the form of single fibers. (See illustration 3, FIG. 3C).

(21) The diameters of the thin fibers amount to 733 nm or 857 nm, respectively. Fibers with diameters well below 1 m are exclusively nanofibers. (See illustration 4, FIG. 3D).

(22) The high magnification shows the extreme difference between a standard fiber with a diameter of approximately 11 m and the surrounding nanofibers of approximately 750 nm. (See illustration 5, FIG. 3E).

(23) The photograph was taken at an angle of 70 to illustrate the fiber structures across the material cross-section.

(24) The lower part of the photograph shows the basic structure of thick, bonded fibers. The central part of the photograph shows a portion comprising fine fibers and nanofibers. The cover layer is formed by fibers with a medium diameter. (See illustration 6, FIG. 3F).

(25) Nanofibers (measured value: 522 nm) next to fibers having a diameter of approximately 1-2 m. (See illustration 7, FIG. 3G).

(26) Upon exposure of the filter medium to NaCl particles (for approximately 15 minutes in the particle filter test bench):

(27) There is a coarse fiber (diameter of approximately 10-15 m) in the background. NaCl particles with a partially very small diameter (much smaller than 0.5 m) have deposited on the surface of the fiber.

(28) The number of particles deposited on the very thin nanofiber in the front (measured value: diameter of 426 nm) is similar to the number of particles on the thick fiber although the diameter of the finest fiber amounts to only approximately 1/25 of that of the coarse fiber.

(29) The illustration 8 shows an overview photograph of coarse, medium and nanofibers upon exposure to NaCl particles. (See illustration 8, FIG. 3H).

(30) The inventive apparatus will hereinafter be described in relation to FIG. 1 and FIG. 2:

(31) The hereinafter described FIG. 1 shows a schematic sectional view of a spinning beam 1 which comprises a plurality of spinnerets 2 arranged next to each other through which a cone 4 of compressed liquid polymer is discharged under pressure, as illustrated by the arrow 3. An air stream with a pressure of approximately 500 mbar generated by fans is supplied via air channels 5.

(32) FIG. 2 shows a schematic view of two spinning beams 1 which from an acute angle with each other and are arranged at an angle with respect to the vertical direction relative to a conveyor belt 7 arranged underneath the spinning beam 1.

(33) The spinning beam 1 on the left of FIG. 2 comprises larger-diameter spinnerets 2 while the spinning beam 1 on the right of FIG. 2 comprises smaller-diameter spinnerets so that larger-diameter polymer threads 8 tangle and intertwine with smaller-diameter polymer threads 9 in the region 10 before they are laid down on the conveyor belt 7 so as to form a nonwoven filter material 11. This process ensures that the thinner polymer threads 9 are distributed largely homogeneously among the thicker polymer threads 8.