Tempered Melt-Blown Nonwoven Having a High Compression Hardness

Abstract

The invention relates to a method for producing a tempered melt-blown nonwoven, comprising the following steps: a) producing a melt-blown nonwoven preferably by applying flowing air to the outside of polymer melt extruded through a nozzle and stretching said polymer melt before the filaments thereby formed are laid and cooled on a carrier, which is preferably a double suction drum, and b) tempering at least one section of the melt-blown nonwoven produced in step a) at a temperature that lies between the glass transition temperature and 0.1 C. below the melt temperature of the filaments of the melt-blown nonwoven, the melt-blown nonwoven having a weight per unit area of 100 to 600 g/m.sup.2, a density of 5 to 50 kg/m.sup.3 and a compression hardness at 60% compression, measured according to DIN EN ISO 3386, of at least 2 kPa. The invention further relates to a tempered melt-blown nonwoven produced by means of said method, preferably a tempered voluminous melt-blown nonwoven. Said tempered melt-blown nonwoven is characterized by a significantly increased compression hardness in comparison with an untempered melt-blown nonwoven.

Claims

1-15. (canceled)

16. An annealed meltblown nonwoven fabric, obtainable by means of a method in which at least a portion of the meltblown nonwoven fabric (15) is subsequently annealed at a temperature between the glass transition temperature and 0.1 C. below the current melting temperature of the filaments of the meltblown nonwoven fabric (15), wherein the meltblown nonwoven fabric (15) is composed of a polyolefin, and the meltblown nonwoven fabric (15) has a weight per unit area of from 100 to 600 g/m.sup.2, a density of from 5 to 50 kg/m.sup.3, and a compression hardness at 60% compression of at least 2 kPa as measured according to DIN EN ISO 3386.

17. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed at a temperature between 20 C. and 1 C. below the current melting temperature of the filaments of the meltblown nonwoven fabric (15).

18. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed at the temperature for 1 minute to 10 days.

19. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed through exposure to hot air and/or superheated steam.

20. The meltblown nonwoven fabric as set forth in claim 19, wherein the meltblown nonwoven fabric (15) is annealed in a furnace (10) having at least one blast box (16, 16) and at least one suction box (18), the at least one blast box (16, 16) being arranged such that the hot air can be blown into the meltblown nonwoven fabric (15), and the at least one suction box (18) being arranged such that air flowing through the meltblown nonwoven fabric (15) can be extracted.

21. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) has a weight per unit area of from 100 to 400 g/m.sup.2.

22. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is a voluminous meltblown nonwoven fabric (15) having a density of from 7 to 40 kg/m.sup.3.

23. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is composed of filaments that are made of a polypropylene and/or polyethylene.

24. The meltblown nonwoven fabric as set forth in claim 16, wherein the thickness of the meltblown nonwoven fabric (15) is from 6 to 50 mm.

25. The meltblown nonwoven fabric as set forth in claim 16, wherein i) the meltblown nonwoven fabric (15) is annealed in a mold (20) for the purpose of reshaping it during annealing, the mold (20) being embodied at least partially as a screen (22, 22), so that the meltblown nonwoven fabric (15) can be flowed through and/or flowed around by hot air and/or with superheated steam during annealing, and/or ii) the meltblown nonwoven fabric (15) is transferred to a mold (20) after heating for the purpose of reshaping it, in which case the meltblown nonwoven fabric (15) is cooled in the mold in order to conclude the annealing process.

26. The meltblown nonwoven fabric as set forth in claim 16, wherein at least one spacer is provided in the meltblown nonwoven fabric (15) that is arranged in the direction of thickness of the meltblown nonwoven fabric (15) and, as a result of permanent molding, has a length that is greater than the length of the meltblown nonwoven fabric (15).

27. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) that is subsequently annealed was manufactured by applying flowing air to the outside of a polymer melt that is extruded through a die and drawing said polymer melt before the filaments that are formed in this way are placed onto a carrier and cooled.

28. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) has a compression hardness at 60% compression of at least 4 kPa as measured according to DIN EN ISO 3386.

29. The meltblown nonwoven fabric as set forth in claim 16, wherein the annealing temperature is increased in a continuous or stepwise manner during annealing, but on the proviso that the annealing temperature be always at least 0.1 C. below the current melting temperature of the filaments of the meltblown nonwoven fabric existing at this point in time.

30. A method for manufacturing an annealed meltblown nonwoven fabric having a weight per unit area of from 100 to 600 g/m.sup.2 and having a density of from 5 to 50 kg/m.sup.3, comprising the following steps: a) manufacturing a meltblown nonwoven fabric (15) and b) annealing at least a portion of the meltblown nonwoven fabric prepared in step a) at a temperature between the glass transition temperature and 0.1 C. below the melting temperature of the filaments of the meltblown nonwoven fabric.

Description

[0040] Accordingly, the meltblown nonwoven fabric is especially preferably annealed in step b) for 2 minutes to 2 hours at a temperature that lies between 20 C. below the melting temperature and 1 C. below the melting temperature of the filaments of the meltblown nonwoven fabric. Hereinafter, the present invention will be described below with reference to the clarifying but non-limiting drawing. In the drawing:

[0041] FIG. 1 shows a schematic of a furnace for manufacturing an annealed meltblown nonwoven fabric according to one exemplary embodiment of the present invention.

[0042] FIG. 2 shows a schematic of a mold for the simultaneous shaping and annealing of a meltblown nonwoven fabric according to another exemplary embodiment of the present invention.

[0043] FIG. 3 shows a comparison of the compression hardness of an annealed meltblown nonwoven fabric according to another exemplary embodiment of the present invention to the compression hardness of an unannealed meltblown nonwoven fabric according to the prior art.

[0044] FIG. 4 shows the results of the measurement of sound absorption of annealed meltblown nonwoven fabric prepared in example 1 according to the present invention (curve A) in comparison to the unannealed meltblown nonwoven fabric manufactured in the comparative example (curve B).

[0045] FIG. 5 shows the results of the measurement of the absorption coefficient of the annealed meltblown nonwoven fabric prepared in example 1 mounted directly against a body panel (curve A), mounted on a body panel at a distance of 10 mm (curve B), and mounted on a body panel at a distance of 40 mm (curve C).

[0046] FIG. 1 shows a schematic of a belt furnace 10 for manufacturing an annealed meltblown nonwoven fabric according to one exemplary embodiment of the present invention. The furnace 10 comprises air-permeable belts 14, 14 that are guided and driven on rollers 12 over which the meltblown nonwoven fabric 15 is guided into and through the furnace 10. A first blast box 16, a suction box 18, and a second blast box 16 are arranged in the furnace 10 above and below the two belts 14, 14 in this sequence as seen from right to left in the direction of conveyance. During operation of the furnace 10, the meltblown nonwoven fabric 15 is fed from right to left on the lower belt 14 through the furnace 10. As the meltblown nonwoven fabric 15 passes through the blast boxes 16, 16, hot air is flowed into and through it in order to elevate the filaments of the meltblown nonwoven fabric 15 to the desired annealing temperature. Air flowing through the meltblown nonwoven fabric 15 is extracted in the vicinity of the suction box 18 in order to ensure that the meltblown nonwoven fabric 15 is reliably flowed through by the hot air and the meltblown nonwoven fabric 15 also does not collapse but rather retains its volume.

[0047] FIG. 2 shows a schematic of a mold 20 for the simultaneous shaping and annealing of a meltblown nonwoven fabric 15 according to another exemplary embodiment of the present invention. The meltblown nonwoven fabric 15 is maintained in the desired shape from both sides by appropriately shaped screens 22, 22 of which the mold 20 is composed and heated to the desired temperature for annealing through passage of air around or through. The nonwoven mat manufactured in this way retains the impressed shape and is dimensionally stable.

[0048] FIG. 3 shows the compression hardness of an annealed meltblown nonwoven fabric at 60% compression with a weight per unit area of about 300 g/m2 and a density of about 15 kg/m3 according to another exemplary embodiment of the present invention (upper curve) and the compression hardness of an unannealed meltblown nonwoven fabric with the same weight per unit area and the same density according to the prior art (lower curve) in a comparison. The compression hardness is shown as compression in % versus the compressive stress in kPa. As can be seen from FIG. 3, a compressive stress of about 12 kPa is required in the annealed meltblown nonwoven fabric (upper curve) in order to achieve a compression of 60%, whereas the same compression is already achieved in the unannealed meltblown nonwoven fabric according to the prior art (lower curve) at about 1.5 kPa. This demonstrates strikingly that the compression hardness can be increased dramatically in a voluminous meltblown nonwoven fabric through annealing.

[0049] The present invention will be described below with reference to clarifying but non-limiting examples.

EXAMPLE 1

[0050] A meltblown nonwoven fabric with a weight per unit area of 300 g/m2 and with a density of 15 kg/m3 was manufactured from filaments composed of isotactic polypropylene with a mean filament fineness of 5 m using the meltblown process described in U.S. Pat. No. 4,375,446. This meltblown nonwoven fabric was then annealed in a circulating-air furnace for 10 minutes at 158 C. As a result of the placement of the cold nonwoven fabric and the opening of the furnace door, the initial temperature was below the melting point of the filaments of the unannealed nonwoven fabric. Through the immediately incipient crystallization with associated increase in the melting point of the filaments, it was possible to continue annealing for the rest of the 10 minutes at 158 C., i.e., above the melting temperature of the unannealed filaments but below the current melting temperature of the filaments at that point in time, thereby shortening the annealing time in comparison to annealing at a lower temperature. The compression hardness at 40% compression and the compression hardness at 60% compression of the annealed meltblown nonwoven were then measured according to DIN EN ISO 3386. The results are summarized in table 1 below and show that the annealing according to the invention results in a drastic increase in the compression hardness.

[0051] In addition, the sound absorption coefficient of the annealed meltblown nonwoven fabric was measured according to DIN EN ISO 10534 as a function of the thickness-normalized frequency. The results are shown in FIG. 4 in curve A in comparison to the values that were achieved with the unannealed meltblown nonwoven fabric that was manufactured in the comparative example (curve B). The unit of the abscissa is the measurement frequencyabsorber thickness/15 mm. The comparison of the results shows that the annealing according to the invention has no negative influences on the sound absorption characteristics of the nonwoven fabric.

[0052] A portion of the annealed meltblown nonwoven fabric was mounted directly against a vehicle body panel, whereas another portion of the annealed meltblown nonwoven fabric was mounted on a vehicle body panel at a distance of 10 mm and another portion of the annealed meltblown nonwoven fabric was mounted on a vehicle body panel at a distance of 40 mm. The absorption coefficient was then determined for the three constructions as a function of frequency. The results are shown in FIG. 5; curve A shows the values for the meltblown nonwoven fabric that was mounted directly against the vehicle body panel, curve B shows the values for the meltblown nonwoven fabric that was mounted on the vehicle body panel at a distance of 10 mm, and curve C howl the values for the meltblown nonwoven fabric that was mounted on the vehicle body panel at a distance of 40 mm. A comparison of the values obtained shows that the volume of air trapped between nonwoven fabric and body panel results in a clear improvement particularly in the low-frequency absorption characteristics of the construction that can otherwise only be achieved by means of correspondingly thick and hence also heavy and expensive materials.

EXAMPLE 2

[0053] An annealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing was carried out at 155 C. for 10 minutes.

EXAMPLE 3

[0054] An annealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing was carried out at 155 C. for 25 minutes.

COMPARATIVE EXAMPLE

[0055] An unannealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing described in example 1 was not carried out.

TABLE-US-00001 TABLE 1 Compression Compression Annealing Annealing hardness hardness temperature time factor at 40% factor at 60% Example (C) (min.) compression compression 1 158 10 18.5 14 2 155 10 9.5 7 3 155 25 12 9 Comparative 1 1 example 1 Compressive hardness factor: Ratio of the compression hardness of the annealed nonwoven fabric of the example divided by the compression hardness of the unannealed nonwoven fabric of the comparative example

[0056] A comparison of the results shows that the subsequent annealing of the meltblown nonwoven fabric results in a drastic increase in the compression hardness of the meltblown nonwoven fabric.

LIST OF REFERENCE SYMBOLS

[0057] 10 (belt) furnace [0058] 12 rollers [0059] 14, 14 air-permeable belt [0060] 15 meltblown nonwoven fabric [0061] 16, 16 blast box [0062] 18 suction box [0063] 20 mold [0064] 22, 22 screen