Improved Filter Paper

Abstract

Described herein is a filter paper for the production of an aqueous extract, comprising softwood pulp, which has the following properties: a basis weight of more than 9.0 g/m.sup.2 and less than 13.5 g/m.sup.2, a density of more than 280 kg/m.sup.3 and less than 350 kg/m.sup.3, a roughness of more than 700 ml/min and less than 1300 ml/min, a bending resistance in the machine direction of more than 50 mN and less than 75 mN, and an air permeability of more than 17000 cm/(min.Math.kPa) and less than 26000 cm/(min.Math.kPa), wherein the filter paper is either free from abacá fibers and sisal fibers or, if abacá fibers and/or sisal fibers are present, taken together they make up less than 20% of the paper mass.

Claims

1. Filter paper for the production of an aqueous extract, comprising softwood pulp, which has the following properties: a basis weight of more than 9.0 g/m.sup.2 and less than 13.5 g/m.sup.2, a density of more than 280 kg/m.sup.3 and less than 350 kg/m.sup.3, a roughness of more than 700 ml/min and less than 1300 ml/min, a bending resistance in the machine direction of more than 50 mN and less than 75 mN, and an air permeability of more than 17000 cm/(min.Math.kPa) and less than 26000 cm/(min.Math.kPa), wherein the filter paper is either free from abacá fibers and sisal fibers or, if abacá fibers and/or sisal fibers are present, taken together they make up less than 20% of the paper mass.

2. Filter paper according to claim 1, wherein the softwood pulp is sourced from spruce, pine or fir.

3. Filter paper according to claim 1, wherein the content of softwood pulp in the filter paper is at least 70% with respect to the mass of the filter paper.

4. Filter paper according to claim 1, wherein a content of abacá fibers and sisal fibers taken together is less than 10% of the mass of the filter paper.

5. Filter paper according to claim 1, which further contains hardwood pulp, whereby the hardwood pulp is sourced from birch, beech or eucalyptus, and wherein the content of hardwood pulp is at most 20% of the paper mass.

6. Filter paper according to claim 1, which is free of hardwood pulp.

7. Filter paper according to claim 1, which further contains thermoplastic fibers.

8. Filter paper according to claim 7, wherein the thermoplastic fiber material is selected from the group consisting of polyethylene, polypropylene, polyester polyamide, polymethacrylate, polyacrylate, polyvinyl acetate, polyvinyl alcohol and polylactic acids or mixtures thereof.

9. Filter paper according to claim 7, wherein the content of thermoplastic fibers is at least 5% and at most 30%, respectively with respect to the mass of the filter paper.

10. Filter paper according to claim 1, which contains one or more filler materials which are selected from the group consisting of calcium carbonate, magnesium carbonate, magnesium hydroxide, aluminum hydroxide, magnesium silicates, aluminum silicates, kaolin and talcum or mixtures thereof.

11. Filter paper according to claim 10, wherein the content of filler materials is less than 10% of the paper mass.

12. Filter paper according to claim 1, which is free from filler materials.

13. Filter paper according to claim 1, with a basis weight of at least 10.0 g/m.sup.2 and/or a basis weight of at most 13.2 g/m.sup.2.

14. Filter paper according to claim 1 with a density of at least 290 kg/m.sup.3 and/or a density of at most 340 kg/m.sup.3.

15. Filter paper according to claim 1 with a thickness of at least 38 μm and/or a thickness of at most 48 μm.

16. Filter paper according to claim 1 with a roughness in accordance with ISO 8791-2:2013 of at least 80 ml/min and/or of at most 1200 ml/min.

17. Filter paper according to claim 1 with an air permeability in accordance with ISO 2965:2009 of at least 1800 cm/(min.Math.kPa) and/or at most 25000 cm/(min.Math.kPa).

18. Filter paper according to claim 1, with a bending resistance in accordance with ISO 2493-1:2010 in the machine direction of at least 55 mN and/or at most 73 mN.

19. Filter paper according to claim 1, with a bending resistance in accordance with ISO 2493-1:2010 in the cross direction of at least 15 mN and/or at most 28 mN.

20. Filter paper according to claim 1, which has an elongation at break in the machine direction of at least 1.0% and/or at most 2.0% and/or which has an elongation at break in the cross direction of at least 1.8% and/or at most 3.8%.

21. Filter paper according to claim 1, which has a tensile strength in the machine direction of at least 11.5 N/15 mm and/or at most 15.0 N/15 mm and/or which has a tensile strength in the cross direction of at least 2.5 N/15 mm and/or at most 5.0 N/15 mm.

22. Filter paper according to claim 1, which has a tensile energy absorption in accordance with ISO 1924-2:2008 in the machine direction of at least 6.0 J/m.sup.2 and/or at most 11.0 J/m.sup.2.

23. Filter paper according to claim 1, which is packed essentially impermeably to water vapor and has a moisture content of at least 9% and at most 20%.

24. Process for manufacturing a filter paper, which is either free from abacá fibers and sisal fibers, or if abacá fibers and/or sisal fibers are present, taken together, these make up less than 20% of the paper mass, wherein the process comprises the following steps: suspending softwood pulp in water and refining at least the softwood pulp in a refiner, delivering a suspension, which contains at least the refined softwood pulp, onto the wire of a paper machine, de-watering the suspension through the wire of the paper machine to form a paper, and pressing the paper with a sufficient pressure to provide the paper with the following properties: a basis weight of more than 9.0 g/m.sup.2 and less than 13.5 g/m.sup.2, a density of more than 280 kg/m.sup.3 and less than 350 kg/m.sup.3, a roughness of more than 700 ml/min and less than 1300 ml/min, a bending resistance in the machine direction of more than 50 mN and less than 75 mN, and an air permeability of more than 17000 cm/(min.Math.kPa) and less than 26000 cm/(min.Math.kPa).

25. Process according to claim 24, wherein the filter paper is a filter paper according to claim 1.

26. Process according to claim 24, wherein the paper machine is an inclined-wire machine, wherein the wire is inclined between 15° and 25° relative to the horizontal.

27. Process according to claim 24, wherein, after the wire section, the filter paper runs through a press section, in which it is further de-watered, wherein the press section comprises two rolls which form a nip through which the paper runs and is exposed to a line load of 30 kN/m and at most 100 kN/m.

28. Process according to claim 24, wherein after pressing, the filter paper is dried by hot air, infra-red radiation or contact with heated cylinders.

29. Process according to claim 24, wherein the filter paper in a state with a moisture content of at least 9% and at most 20% is packed essentially impermeable to water vapor.

30. Bag formed from a filter paper according to claim 1, which contains a material to be extracted, wherein the material to be extracted is tea.

31. Filter paper according to claim 3, wherein the content of softwood pulp in the filter paper is at least 90% with respect to the mass of the filter paper.

32. Filter paper according to claim 3, wherein all of the pulp in the filter paper is formed by softwood pulp.

33. Filter paper according to claim 5, wherein the content of hard wood pulp is at least 2% of the paper mass.

34. Filter paper according to claim 7, wherein the polyester is polyethylene terephthalate.

35. Filter paper according to claim 13, with a basis weight of at least 11 g/m.sup.2 and at most 13.0 g/m.sup.2.

36. Filter paper according to claim 1, with a thickness of at least 41 μm and at most 45 μm.

37. Process according to claim 27, wherein the rolls are steel rolls coated with a plastic coating.

38. Process according to claim 28, wherein the drying step comprises humidification.

Description

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] The invention shall now be described more precisely using a few embodiments according to the invention and be compared to filter papers not according to the invention.

[0062] Three filter papers according to the invention, designated as A, B and C, were manufactured from 100% softwood pulp on an inclined-wire machine. The softwood pulp was refined to a degree of refining of 20° SR, measured in accordance with ISO 5267-1:1999, and was applied as a suspension with a solid content of 0.016% onto the running web of an inclined-wire machine, inclined by 20 relative to the horizontal. Then the filter paper ran through a press section, wherein it was compressed with a line load of 65 kN/m between two steel rolls coated with plastic to compact the paper structure. This reduced both the thickness and air permeability. Next, the filter paper ran through the drying section, in which it was dried to a moisture content of 9.7%. Finally, the filter paper was wound up at the end of the paper machine and was packed essentially impermeably to water vapor in a polyethylene film. In this regard, the settings of the paper machine were slightly varied so that slightly different properties of the filter papers A, B and C resulted.

[0063] A fourth filter paper according to the invention, labelled D, was produced from 82% softwood pulp and 18% abacá fibers. The softwood pulp and the abacá fibers were refined to a degree of refining of 23° SR, measured in accordance with ISO 5267-1:1999, and they were applied as a suspension with a solid content of 0.016% onto the running web of an inclined-wire machine, inclined by 20° relative to the horizontal. Then the filter paper ran through a press section, wherein it was compressed with a line load of 60 kN/m between two steel rolls coated with plastic to compact the paper structure. This reduced both the thickness and air permeability. Next, the filter paper ran through the drying section, in which it was dried to a moisture content of 10.3%. Finally, the filter paper was wound up at the end of the paper machine and was packed essentially impermeably to water vapor in a polyethylene film.

[0064] The basis weight, density, thickness, roughness, bending resistance in the machine direction and air permeability were determined for all of the filter papers according to the invention.

[0065] Furthermore three typical, commercially available filter papers not according to the invention, labelled X, Y, Z, were analysed microscopically with respect to their content of abacá fibers and sisal fibers and in addition the basis weight, density, thickness, roughness, bending resistance in the machine direction and air permeability were determined.

[0066] The results are summarized in Table 1 and show the content of abacá and sisal fibers (AS), the basis weight (BW), the density (ρ), the thickness (d), the roughness (R), the bending resistance in the machine direction (BR-MD) and the air permeability (AP).

TABLE-US-00001 TABLE 1 AS BW ρ d R BR-MD AP Paper % g/m.sup.2 kg/m.sup.3 μm ml/min mN cm/(min .Math. kPa) A 0 13.0 284 42.3 1141 71 20685 B 0 12.7 306 42.1 1050 67 23040 C 0 12.8 301 40.4 872 60 21549 D 18 12.9 316 45.4 1088 53 24332 X 44 12.7 245 51.6 1549 99 33127 Y 49 12.5 257 48.6 1283 70 25854 Z 57 12.7 255 49.8 1509 77 36761

[0067] The results show that the filter papers according to the invention have lower values with respect to thickness and roughness and higher values with respect to density than any filter paper not according to the invention. This difference is caused by the compression of the filter paper, which reduces the roughness and the thickness and, at the same basis weight, increases the density. An undesirable side-effect is that the air permeability is also reduced thereby, and it is lower for all filter papers according to the invention than for the filter papers not according to the invention. As further experiments show, this slightly lower air permeability has practically no effect on the production of an aqueous extract using the filter papers according to the invention.

[0068] With one exception: filter paper A compared to filter paper Y, the bending resistance of the filter papers according to the invention is lower than that of the filter papers not according to the invention. This difference is caused on the one hand by the lower content of abacá fibers and sisal fibers and on the other hand by the reduced thickness due to the mechanical compression.

[0069] The essential mechanical properties of the filter papers according to the invention were determined, which are summarized in Table 2. Table 2 contains the bending resistance in the cross direction (BR-CD), the tensile strength in the machine direction (F-MD) and in the cross direction (F-CD), the elongation at break in the machine direction (E-MD) and in the cross direction (E-CD) and the tensile energy absorption in the machine direction (TEA-MD).

TABLE-US-00002 TABLE 2 Paper A B C D BR-CD mN 22 22 20 23 F-MD N/15 mm 12.8 14.0 14.4 15.0 F-CD N/15 mm 3.5 3.1 2.9 4.4 E-MD % 1.2 1.3 1.3 1.6 E-CD % 2.6 2.7 2.2 3.5 TEA-MD J/m.sup.2 7.1 7.9 8.3 10.1

[0070] The strength of a joint between two layers of the filter papers produced by knurling was tested, wherein values between 1.5 N and 2.0 N resulted, while conventional filter papers achieved between 1.0 N and 1.7 N under the same test conditions. In this regard, it was shown that the high moisture content of the filter papers can have a positive effect on the strength of the knurled joints.

[0071] Tea bags according to the invention filled with tea were produced without any further problems from the four filter papers according to the invention A, B, C and D on various conventional tea bag machines such as IMA C24, IMA C27 and Teepack Perfecta. The tea bags manufactured from the four filter papers according to the invention were compared to three commercially available tea bags with the same geometry and filling, which were manufactured on the same machines. Therefore, containers were prepared with 0.5 liters of tap water with a temperature of 90° C. and each tea bag was submerged in a separate container. The tea was judged optically with respect to the speed of the discoloration of the tap water in the container after a few seconds, because this speed of discoloration is also the criterion that a consumer will observe during the production of tea. With regard to the tea bags according to the invention and not according to the invention, no perceptible difference was found, which was confirmed by UV-VIS measurements. Sand dusting of the tea bags was assessed. To this end, sand with a particle size of 106 μm to 150 μm was filled into the tea bags, and then the tea bags were shaken in an apparatus and the amount of sand that fell through the pores of the filter paper was weighed. Also in this respect, no significant differences between the filter papers according to the invention and not according to the invention were found.

[0072] Thus it is shown that tea bags can be produced from the filter papers according to the invention with partial or complete absence of abacá fibers and sisal fibers, which do not differ in their performance compared to conventional tea bags, even though this was to be expected because of the technical properties of the filter papers. Furthermore, about 10% more tea bags could be manufactured from a reel of the same outer diameter compared to conventional filter papers due to the 10% lower density, which allows for an additional increase in productivity.