EP3568523B1 - Improved filter paper - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a filter paper for the production of aqueous extracts, in particular for the production of tea, coffee or other infusion drinks and to bags made from this filter paper, in particular tea bags. The filter paper according to the invention is characterized by a low proportion or the complete absence of the commonly used Abacá fibers or sisal fibers, as well as by a property profile that deviates from the prior art. Nevertheless, the filter paper according to the invention offers at least the same performance in use as filter papers known from the prior art.

BACKGROUND AND PRIOR ART

For the production of aqueous extracts, in particular for the production of tea, coffee or other infusion drinks, it is customary to fill the material to be extracted, for example crushed tea leaves, in bags in advance from bags made of a special filter paper. The bag is closed and immersed in the usually hot water for a certain time in order to produce the infusion beverage and then removed again from the water. Methods in which the bag is not closed and only hung in the hot water instead of being immersed are also known. The main advantages of using these bags are simple handling, pre-portioning of the material to be extracted and easy removal of the material to be extracted from the water.

Despite this seemingly simple application, there are comparatively high demands on the filter paper. The filter paper should not disintegrate in the water so that the material to be extracted remains in the bag. It is said to have a high porosity so that the water can easily flow around the material to be extracted by natural or forced convection and the extraction process does not take long. Furthermore, the pores in the filter paper should not be too large so that small particles of the material to be extracted do not fall through the filter paper and remain in the extract. This property is determined by measuring the so-called sand loss.
In addition, the filter paper itself should not release any undesirable substances, in particular no undesirable flavors or aromas, into the water. Finally, the filter paper also have mechanical properties that allow the bags to be manufactured industrially at high speed. These include, for example, mechanical strength and ductility, roughness or a heat sealability.

It is customary in the prior art to produce filter papers for these applications from fibers which are obtained from the Abacá plant ( Musa textilis ), a banana plant. These fibers are also known as manila hemp, banana hemp or musa hemp, but have no botanical relationship to the hemp plant ( cannabis ). According to the prior art, only these fibers make it possible to produce filter paper with high and uniform porosity, low basis weight and high strength. An alternative to this are fibers from the sisal plant ( Agave sisalana ), a type of agave, which is also misleadingly referred to as sisal hemp.

For both types of fibers, but especially for Abacá fibers, there are only a few suppliers due to the low demand and the special application and the quality of the fibers fluctuates greatly. This makes the production of filter papers comparatively difficult and expensive. So far, attempts to produce filter papers without the use of Abacá fibers or sisal fibers have not been commercially successful, because in most cases the technical properties of a filter paper containing such fibers could not be achieved.

The proportion of Abacá fibers or sisal fibers in the filter paper from the prior art is generally more than 25% of the paper pulp.

In addition to Abacá fibers or sisal fibers, such filter papers often also contain synthetic fibers, in particular thermoplastic fibers, which provide the filter paper with a heat sealability, so that sealed bags can be produced from the filter paper comparatively simply by sealing the filter paper to itself and no further materials for sealing of the bag are needed.

The filter papers often also contain agents for increasing the wet strength so that they have sufficient mechanical strength during the production of the aqueous extract.

In the manufacture of bags from filter paper, techniques are used to connect two layers of filter paper together. These techniques include sealing at elevated temperature, as permitted by the use of thermoplastic fibers, or knurling, in which the connection is effected by high pressure and the embossed pattern. Combinations of these methods can also be used.

There is therefore a need to reduce the proportion of Abacá fibers or sisal fibers in the filter paper as far as possible and to replace them with other cheaper, more readily available fibers of a more stable quality, without the usefulness of this filter paper for the production of aqueous extracts being affected.

The WO2017 / 110365A1 discloses a nonwoven material for a filter comprising a composite polyester fiber in which a low melting point polyester surrounds a high melting point polyester. The low melting point polyester has a melting point that is 10 to 140 ° C lower than the melting point of the high melting point polyester. The basis weight of the nonwoven material is 150 to 300 g / m ² , the density 0.25 to 0.40 g / cm ³ and the air permeability per basis weight 0.05 to 0.45 (cm ³ / cm ² • s) / (g / m ² ).

The EP 1 382 373 A1 discloses a heat sealable filter material having at least a first non-heat sealable layer and at least a second heat sealable layer comprising fibers of synthetic material containing an adhesion promoter. The first non-heat sealable layer has a basis weight between 8 and 40 g / m ² and an air permeability of 300 to 4000 l / m ² • s (DIN ISO 9237).

The EP 1 215 134 A1 discloses a filter material for the manufacture of filter bags and filter bags for infusion beverages, which comprises so-called superabsorbent fibers, which can absorb large amounts of liquid. The superabsorbent fibers consist of (meth) acrylate copolymers. The filter material has a basis weight of between 8 and 90 g / m ² .

The DE 10 2016 105 235 discloses a filter paper for the manufacture of filters for smoking articles, especially filter cigarettes. The paper has the following properties: The filter paper comprises fibers comprising cellulose fibers, at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably 100% by weight of the filter paper formed by long-fiber cellulose fibers, of the fibers a proportion based on the number of fibers of between 2% and 10%, preferably between 3% and 9% and particularly preferably between 4% and 8% has a length of less than 0.2 mm, the air permeability of the filter paper is measured according to ISO 2965: 2009 is from 500 cm ^-1 · · kPa ^-1 and 15000 cm · min ^-1 · kPa ^-1 cm, and preferably between I 1000 cm ^-1 · min ^-1 · kPa and 9000 · min ^-1 kPa ^-1 , the number-based average The length of the fibers in the filter paper is more than 1 mm and less than 5 mm, preferably more than 2 mm and less than 4 mm, and the number-based average width of the fibers in the filter paper is between 10 μm and 50 μm, preferably between 20 µm and 40 µm, and particularly preferably between 25 µm and 35 µm.

SUMMARY OF THE INVENTION

The object is to provide a filter paper which can be used for the production of aqueous extracts, in particular infusion beverages such as tea or coffee, and which contains comparatively significantly less or no Abaca fibers or sisal fibers. The filter paper should offer the same performance in the production of the aqueous extract as the conventional filter papers.

This object is achieved by a filter paper according to claim 1, a method for its production according to claim 24 and a bag made from the filter paper according to the invention according to claim 30.

• ein Flächengewicht von mehr als 9,0 g/m² und weniger als 13,5 g/m²,
• eine Dichte von mehr als 280 kg/m³ und weniger als 350 kg/m³,
• eine Rauigkeit von mehr als 700 ml/min und weniger als 1300 ml/min,
• einen Biegewiderstand in Maschinenrichtung von mehr als 50 mN und weniger als 75 mN,
• eine Luftdurchlässigkeit von mehr als 17000 cm/(min·kPa) und weniger als 26000 cm/(min·kPa),

wobei das Filterpapier entweder frei ist von Abacá Fasern und Sisal Fasern, oder, falls Abacá Fasern und/oder Sisal Fasern vorhanden sind, diese zusammengenommen weniger als 20% der Papiermasse ausmachen.The inventors have surprisingly found that the object can be achieved by a filter paper which comprises long fiber pulp and has the following properties:

• a basis weight of more than 9.0 g / m ² and less than 13.5 g / m ² ,
• a density of more than 280 kg / m ³ and less than 350 kg / m 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,
• an air permeability of more than 17000 cm / (min · kPa) and less than 26000 cm / (min · kPa),

the filter paper is either free of Abacá fibers and sisal fibers or, if Abacá fibers and / or sisal fibers are present, these together make up less than 20% of the paper mass.

This solution to the problem is surprising in that the property profile of the filter paper according to the invention differs in several points from filter papers known from the prior art. Apparently the previously attempted approach, namely to achieve the same paper properties as the filter paper with such fibers without using Abacá fibers or sisal fibers, was unsuccessful. It was apparently erroneously assumed that the desired performance in the production of an aqueous extract can only be achieved by means of these paper properties.

In contrast, the inventors have developed a filter paper whose properties differ from filter papers known from the prior art, but which is in any case no worse than the filter papers known from the prior art with regard to its performance in the production of aqueous extracts. In particular, the inventors have found that this result can be achieved with a roughness, a bending resistance and an air permeability, each of which is lower than that of the filter papers known from the prior art, and of a density which is higher than that from the prior art filter papers known in the art. This combination of properties can be achieved by a special mechanical treatment, as described below.

The filter paper according to the invention comprises long fiber pulp. The long fiber pulp can be obtained from conifers such as spruce, pine or fir. It gives the filter paper high strength and air permeability, but does not achieve the same values as Abacá fibers or sisal fibers.
The proportion of long fiber pulp in the filter paper is preferably at least 70%, particularly preferably at least 80% and very particularly preferably at least 90% in each case based on the mass of the filter paper. In a particularly preferred embodiment, the entire cellulose in the filter paper according to the invention is formed by long-fiber cellulose.

Alternatively, part or all of the long-fiber pulp can be replaced by pulp from annual plants such as hemp, flax, kenaf or jute, with the exception of Abacá or Sisal. However, the pulps from annual plants are not preferred because they are comparatively expensive, only poorly dewater in paper production and, like Abacá fibers and sisal fibers, are subject to quality fluctuations.

The proportion of Abacá fibers and sisal fibers taken together is less than 20%, preferably less than 10% and particularly preferably less than 5% of the mass of the filter paper. In a very particularly preferred embodiment, the filter paper according to the invention is essentially free of Abacá fibers and sisal fibers, except for process-related impurities.

The filter paper according to the invention can contain short fiber pulp. The short fiber pulp can be obtained from deciduous trees such as birch, beech or eucalyptus. Short fiber pulp increases the volume of the filter paper and reduces sand loss, but also reduces the strength, which is why the proportion in the filter paper according to the invention should be comparatively low. The proportion of short fiber pulp in the invention is preferably Filter paper at most 20%, particularly preferably at most 10% and preferably at least 2%, particularly preferably at least 5% of the paper pulp.

The filter paper according to the invention can contain thermoplastic fibers. These fibers can comprise a thermoplastic material, which is preferably selected from the group consisting of polyethylene, polypropylene, polyester, such as polyethylene terephthalate, polyamide, polymethacrylate, polyacrylate, polyvinyl acetate, polyvinyl alcohol and polylactide or mixtures thereof. Bicomponent fibers can also be used with preference. The thermoplastic fibers provide the filter paper with a heat sealability or improve the filter paper in terms of other properties, such as its pore structure or absorbency. The proportion of thermoplastic fibers in the filter paper according to the invention is preferably at least 5%, particularly preferably at least 10% and preferably at most 30%, particularly preferably at most 20%, in each case based on the mass of the filter paper.

The filter paper according to the invention can also contain fibers from regenerated cellulose, preferably viscose fibers or Tencel® fibers, in order to influence the pore structure and other properties of the filter paper. The proportion of fibers made from regenerated cellulose is preferably at most 15% and particularly preferably at most 10% of the mass of the filter paper.

Fillers are used in many papers to increase the opacity or whiteness of the paper or to replace pulp with cheaper materials. Fillers in the filter paper according to the invention can, for example, be selected from the group consisting of calcium carbonate, magnesium carbonate, magnesium hydroxide, aluminum hydroxide, magnesium silicates, aluminum silicates, kaolin and talc or mixtures thereof. However, since fillers reduce the strength of the filter paper, they are undesirable in the filter paper according to the invention. The proportion of fillers in the filter paper according to the invention is therefore preferably less than 10% of the paper pulp, particularly preferably less than 5% of the paper pulp and very particularly preferably the filter paper according to the invention contains no fillers.

The person skilled in the art can select further constituents of the filter paper according to the invention, such as wet strength agents or agents for increasing the strength, such as starch, guar or carboxymethyl cellulose, in his experience. The person skilled in the art can likewise use process aids, such as, for example, retention aids, in his experience to produce the filter paper according to the invention.

The filter paper according to the invention has a basis weight of at least 9.0 g / m ² , preferably at least 10.0 g / m ² , particularly preferably at least 11.0 g / m ² and at most 13.5 g / m ² , preferably at most 13 2 g / m ² , particularly preferably at most 13.0 g / m ² . The higher the basis weight of the filter paper, the higher its strength, but also the material used. The weight per unit area can be measured, for example, in accordance with ISO 536: 2012.

In addition to the air permeability, the density of the filter paper is an important factor that influences the speed at which an aqueous extract can be produced using this filter paper. In general, filter papers from the prior art have the lowest possible density of less than 280 kg / m ³ . The filter paper according to the invention has a higher density of at least 280 kg / m ³ , preferably at least 290 kg / m ³ and particularly preferably at least 300 kg / m ³ and at most 350 kg / m ³ , preferably at most, due to a special mechanical treatment in paper production 340 kg / m ³ , particularly preferably at most 330 kg / m ³ . Experiments by the inventors show that this higher density has no disadvantages in the preparation of the aqueous extract.

Basis weight, density and thickness are closely related parameters, and the filter paper according to the invention preferably has a thickness of at least 38 µm, particularly preferably at least 40 µm, very particularly preferably at least 41 µm and preferably at most 48 µm, particularly preferably at most 46 µm, very particularly preferred at most 45 µm. Due to the special mechanical treatment, the thickness of the filter paper according to the invention lies below that of conventional filter papers. The small thickness can contribute to a faster transport of the water through the filter paper and thus increase the speed with which an aqueous extract can be produced. In addition, a small thickness allows more filter paper to be on a roll of filter paper for a given outer diameter in terms of area. In the further processing of the filter paper, the maximum outside diameter of the filter paper roll is limited by the constructional conditions on the processing machines, so that more bags can be produced with the filter paper according to the invention, for example, because the number of bags that can be produced only depends on the area of the filter paper. The number of reel changes is therefore reduced and productivity can be increased.

For example, density and thickness can be measured on a single layer of filter paper according to ISO 534: 2011.

The roughness of the filter paper according to the invention is also of technical importance for the further processing of the filter paper, in particular a low roughness reduces the Release of dust during further processing. A slight roughness is also perceived by the consumer as a quality signal. The filter paper according to the invention has a roughness of at least 700 ml / min, preferably at least 800 ml / min, particularly preferably at least 850 ml / min and at most 1300 ml / min, preferably at most 1200 ml / min, particularly preferably at most 1100 ml / min. The roughness can be measured, for example, according to ISO 8791-2: 2013.

The air permeability of the filter paper is of great importance for the production of an aqueous extract using the filter paper according to the invention. A high permeability to air allows the water to flow comparatively easily through the filter paper during the production of the extract by natural or forced convection, displacing the air, and so the extract can be produced in a short time. The filter paper according to the invention has an air permeability of at least 17000 cm / (min · kPa), preferably at least 18000 cm / (min · kPa), particularly preferably at least 19000 cm / (min · kPa) and at most 26000 cm / (min · kPa), preferably at most 25000 cm / (min · kPa), particularly preferably at most 24000 cm / (min · kPa). The air permeability of the filter paper according to the invention is thus lower than that of conventional filter papers. However, experiments show that this is not a disadvantage. Air permeability can be measured in accordance with ISO 2965: 2009.

The filter paper according to the invention can be characterized by a number of mechanical parameters such as bending resistance, elongation at break, tensile strength and energy absorption capacity. To measure these properties, test strips are cut from the filter paper, but the results depend on the direction in which the test strips are removed. A distinction is therefore made in these parameters between the machine direction, i.e. H. the direction in which the filter paper passes through the paper machine when it is manufactured and the transverse direction, i.e. H. the direction in the filter paper plane orthogonal to the machine direction.

The bending resistance of the filter paper is important for the production of bags from the filter paper and generally for the processing of the filter paper on machines. It should not be too high so that the filter paper does not develop excessive resetting forces during the manufacture of the bags. The bending resistance of the filter paper according to the invention in the machine direction is at least 50 mN, preferably at least 55 mN, particularly preferably at least 58 mN and at most 75 mN, preferably at most 73 mN, particularly preferably at most 72 mN. The bending resistance of the filter paper according to the invention in the machine direction is thus lower than that of filter papers known from the prior art, which often have a bending resistance in the machine direction of more than 80 mN. This results in additional advantages in the mechanical processing of the filter paper. This low bending resistance is achieved by the low content of Abacá fibers and sisal fibers in the filter paper according to the invention.

The bending resistance of the filter paper according to the invention should also be rather low in the transverse direction. The bending resistance in the transverse direction should preferably be at least 15 mN, particularly preferably at least 18 mN, very particularly preferably at least 20 mN and at most 28 mN, particularly preferably at most 26 mN, very particularly preferably at most 25 mN. The bending resistance of the filter paper according to the invention in the transverse direction is thus also below the bending resistance of conventional filter papers in the transverse direction, which is typically at least about 30 mN.

The bending resistance of a filter paper both in the machine direction and in the transverse direction can be measured in accordance with ISO 2493-1: 2010, whereby the force to achieve a defined deformation is measured and specified as the bending resistance.

The elongation at break of the filter paper is relevant for the mechanical processing of the filter paper. In general, a high elongation at break is advantageous because the filter paper can then compensate for small differences in speed in the processing machine, but it should also not be too high, because it is then difficult to cut the filter paper into pieces of a defined size even under low loads. The elongation at break of the filter paper according to the invention in the machine direction is preferably at least 1.0%, preferably at least 1.2% and preferably at most 2.0%, particularly preferably at most 1.8%. The elongation at break of the filter paper according to the invention in the transverse direction is preferably at least 1.8%, particularly preferably at least 2.4% and preferably at most 3.8%, particularly preferably at most 3.4%. The fact that the elongations at break in the machine and transverse directions are relatively similar is a consequence of the special mechanical treatment in which the paper structure is compressed. It thus represents an additional advantage of the filter paper according to the invention.

The tensile strength of the filter paper should be sufficiently high, especially in the machine direction, so that the filter paper does not tear during its manufacture and processing. The tensile strength of the filter paper according to the invention in the machine direction is preferably at least 11.5 N / 15 mm, particularly preferably at least 12.0 N / 15 mm and preferably at most 15.0 N / 15 mm, particularly preferably at most 14.0 N / 15 mm.

In the transverse direction, the tensile strength of the filter paper according to the invention is preferably at least 2.5 N / 15 mm, particularly preferably at least 3.0 N / 15 mm and preferably at most 5.0 N / 15 mm, particularly preferably at most 4.5 N / 15 mm. The elongation at break and the tensile strength, in both the machine and transverse directions, can be measured in accordance with ISO 1924-2: 2008.

Also of importance for the processability of the filter paper according to the invention is its energy absorption capacity, which can also be determined in accordance with ISO 1924-2: 2008. The energy absorption capacity results, for example, from the force-strain curve of the filter paper at a constant strain rate. A high energy absorption capacity facilitates the mechanical processing of the filter paper because the filter paper can absorb the stresses during processing more easily without tearing or permanently deforming. The filter paper according to the invention has an energy absorption capacity in the machine direction of preferably at least 6.0 J / m ² , particularly preferably at least 7.0 J / m ² and preferably at most 11.0 J / m ² , particularly preferably at most 10.0 J / m ² .

The production of the filter paper can mainly follow the methods of conventional paper production. In a first step, the long fiber pulp is suspended in water and ground in a grinding unit. During this grinding, the fibrils of the cellulose fibers are exposed and the surface of the fibers is enlarged, which increases the strength of the filter paper made from it, but also reduces its air permeability. In addition, the pulp fibers are shortened during intensive grinding, which reduces the strength of the filter paper. The person skilled in the art is able to determine a favorable degree of grinding as a compromise between strength and air permeability from his experience or through a few experiments.

The short fiber pulp, synthetic fibers or other fiber material - if present - is also suspended in water and can be ground, although short fiber pulp and synthetic fibers are preferably not ground. The suspensions of long-fiber pulp and, if applicable, short-fiber pulp, other fibers, optional fillers, additives and processing aids can be combined and reach the headbox of the paper machine. The paper machine is preferably an inclined sieve machine, the sieve being particularly preferably inclined between 15 ° and 25 ° against the horizontal. The inclined screen machine has the advantage that suspensions with a very low solids content of about 0.02% can be processed and thus more porous papers can be produced than with Fourdrinier machines.

The suspension of fibers, water and other components flows from the headbox onto the circulating sieve of the paper machine and can be dewatered through the sieve, partly by means of negative pressure. The filter paper is formed on the sieve. The filter paper then preferably passes through a press section, in which it is dewatered by mechanical pressure, and further preferably a drying section, preferably with rewetting, in which it is dried by elevated temperature, for example by hot air, infrared radiation or contact with heated cylinders. A size press or film press can also be integrated into the dryer section. At the end of the paper machine, the filter paper can be rolled up, then cut and packed in rolls of defined width and length.

A special feature of the manufacturing process of the filter paper according to the invention that differs from the prior art is that it is compressed with sufficient pressure so that the properties mentioned at the outset are imparted to it. This can be achieved, for example, by the filter paper passing through two rollers, preferably two steel rollers, in the press section during its production on the paper machine, which exert mechanical pressure on the filter paper. The steel rollers are particularly preferably covered with a plastic cover. The line load is preferably at least 30 kN / m and at most 100 kN / m. This mechanical compression of the filter paper reduces the thickness, roughness and air permeability and increases the density, which produces the very specific property profile of the filter paper according to the invention. This process also influences the mechanical parameters such as bending resistance, strength, elongation at break and energy absorption. According to the prior art, the mechanical compression of a filter paper for the production of aqueous extracts is considered undesirable because it has been assumed that the air permeability is reduced too much and the density is increased too much and the aqueous extract is no longer produced in a short time can be. The inventors have surprisingly found that the losses due to higher density and lower air permeability are present, but are not so great that they have a significant effect in the production of aqueous extracts using the filter paper according to the invention. But it is precisely this mechanical compression of the filter paper that makes it possible to largely or completely dispense with the use of Abacá fibers and sisal fibers, because the structure of the filter paper is slightly compressed, but its strength is also significantly increased. As a result, the high air permeability can be combined with sufficient strength even when long fiber pulp is used exclusively.

In the equilibrium state, conventional filter papers have a moisture content of about 7% of the paper pulp under the conditions of 50% relative humidity and 23 ° C defined in ISO 187: 1990. The moisture of the filter paper can be measured by determining the paper mass before and after drying a defined amount of filter paper according to ISO 287: 2009. For the conventional production of bags from filter paper it has proven to be advantageous to set the moisture of the filter paper to a value of 7% to 8%. The filter paper can be processed well in this moisture because the fibers are sufficiently flexible. Knurling to connect two layers of filter paper in particular is facilitated. According to the state of the art, however, the moisture should not significantly exceed 8%, because otherwise the fibers become too flexible and the desired strength of the connection is not achieved during knurling.

In general, the moisture can be adjusted by moistening the tea bags, but this means additional mechanical effort.

However, the inventors have found that for filter paper an even higher moisture content of preferably at least 9%, particularly preferably at least 10%, very particularly preferably at least 11% and preferably at most 20%, particularly preferably at most 18%, very particularly preferably at most 15% more Offers advantages. Contrary to expectations, this high humidity increases the strength of the connections between the layers of paper produced by knurling.

However, since this moisture does not correspond to the equilibrium state which arises when stored under normal ambient conditions and the rewetting in the manufacture of bags means an additional effort, a further embodiment of the invention is to make the filter paper substantially impermeable to water vapor with respect to the equilibrium state to pack and to provide, for example, in the form of a substantially water vapor impermeable pack.

The invention therefore also includes filter paper which is packaged essentially impermeable to water vapor, the filter paper having a moisture content of at least 9%, preferably at least 10%, particularly preferably at least 11% and at most 20%, preferably at most 18%, particularly preferably at most 15%. The moisture can be measured according to ISO 287: 2009.

The filter paper in the packaged roll preferably comprises long fiber pulp, the filter paper either being free of Abacá fibers and sisal fibers, or, if Abacá fibers and / or sisal fibers are present, these together less than 20%, particularly preferably less than 10% and very particularly preferably make up less than 5% of the mass of the filter paper.

The filter paper in the roll, which is packaged essentially impervious to water vapor, is particularly preferably a filter paper according to one of the above-mentioned embodiments.

Such a roll can be made by drying the filter paper to the desired moisture at the end of papermaking, rolling up the filter paper on a roll, and packaging the roll in a substantially water vapor impermeable material.

The essentially water vapor impermeable material is preferably a plastic film, particularly preferably a film made of polyethylene or polypropylene. A suitable packaging paper can likewise preferably be used as the material which is essentially impermeable to water vapor. Here, "essentially impermeable to water vapor" means that the moisture of the filter paper in the packaged roll, measured in accordance with ISO 287: 2009, is not less than 8% of the paper pulp after the packaged roll has at least 3 days under the conditions defined in ISO 187: 1990 50% relative humidity and 23 ° C was stored.

Alternatively, a packaging material is considered "essentially water vapor impermeable" for the purposes of this application if its water vapor transmission rate ( WVTR) measured according to ISO 2528: 2017 at 37 ° C and 90% relative humidity is less than 600 g / (m ² · D), preferably less than 400 g / (m ² · d) and particularly preferably less than 350 g / (m ² · d).

The packaged roll according to the invention and the filter paper according to the invention can be processed further by means of machines known from the prior art, wherein preferably no further rewetting is required when processing the filter paper. In particular, sealed bags can be formed from the filter paper, into which the material to be extracted is filled. These bags are preferably tea bags.

The invention therefore also includes bags filled with extractable material which have been produced from the filter paper according to the invention, the extractable material preferably being tea.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in more detail in some embodiments according to the invention and compared with filter papers not according to the invention.

Three filter papers according to the invention, designated A, B and C, were produced from 100% long fiber pulp on an inclined screen machine. The long fiber pulp was ground to a freeness of 20 ° SR, measured in accordance with ISO 5267-1: 1999, and applied as a suspension with 0.016% solids content to the rotating sieve of the inclined sieve machine, which was inclined at 20 ° to the horizontal. The filter paper then passed through the press section, being compressed between two plastic-coated steel rollers with a line load of 65 kN / m in order to consolidate the paper structure. Thickness and air permeability were also reduced. The filter paper then went through the drying section, in which it was dried to a moisture content of 9.7%. Finally, the filter paper was rolled up at the end of the paper machine and packed in a polyethylene film, essentially impermeable to water vapor. The settings of the paper machine were varied slightly, so that slightly different properties of the filter papers A, B and C resulted.

A fourth filter paper according to the invention, designated D, was made from 82% long fiber pulp and 18% Abacá fibers. The long fiber pulp and the Abacá fibers were ground together to a freeness of 23 ° SR, measured according to ISO 5267-1: 1999, and applied as a suspension with 0.016% solids content to the rotating sieve of the inclined sieve machine inclined at 20 ° to the horizontal. The filter paper then passed through the press section, being compressed between two plastic-coated steel rollers with a line load of 60 kN / m in order to consolidate the paper structure. Thickness and air permeability were also reduced. The filter paper then went through the drying section, in which it was dried to a moisture content of 10.3%. Finally, the filter paper was rolled up at the end of the paper machine and packed in a polyethylene film, essentially impermeable to water vapor.

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

In addition, three typical, commercially available, non-inventive filter papers, designated X, Y, Z, were microscopically analyzed for their Abacá fiber and sisal fiber content, and also basis weight, density, thickness, roughness, bending resistance in the machine direction and air permeability were determined.

The results are summarized in Table 1 and show the content of Abacá and Sisal fibers (AS), the basis weight (BW), the density (p), the thickness (d), the roughness (R), the bending resistance in the machine direction (BR- MD) and air permeability (AP). Table 1 paper AS BW ρ d R BR-MD AP % g / m ² kg / m ³ µm ml / min mN cm / (minkPa) 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 18th 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

The results show that the filter papers according to the invention are lower in thickness and roughness and higher in density than all filter papers not according to the invention. This difference is caused by the compression of the filter paper, which reduces the roughness and the thickness and increases the density, while the weight per unit area remains the same. An inherently undesirable side effect is that this also lowers the air permeability and 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 when producing an aqueous extract using the filter papers according to the invention.

The bending resistance of the filter papers according to the invention, with one exception, filter paper A compared to filter paper Y, is lower than that of the filter papers not according to the invention. This difference is caused on the one hand by the low content of Abacá fibers and sisal fibers, and on the other hand by the reduced thickness due to the mechanical compression.

The essential mechanical properties of the filter papers according to the invention, which are summarized in Table 2, were determined. Table 2 contains the transverse bending resistance (BR-CD), the tensile strength in the machine direction (F-MD) and in the transverse direction (F-CD), the elongation at break in the machine direction (E-MD) and in the transverse direction (E-CD) and that Energy absorption capacity in the machine direction (TEA-MD). Table 2 paper A B C. D BR-CD mN 22 22 20th 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 ² 7.1 7.9 8.3 10.1

The strength of a connection between two layers of the filter papers made by knurling was tested, giving values between 1.5 N and 2.0 N, whereas conventional filter papers only reached values between 1.0 N and 1.7 N under the same test conditions. This shows that the high moisture content of the filter papers can have a positive effect on the strength of the knurled connections.

The four inventive filter papers A, B, C and D were used to produce tea bags filled with tea without problems on various conventional tea bag machines such as IMA C24, IMA C27 and Teepack Perfecta. The tea bags made from the four filter papers according to the invention were compared with three commercially available tea bags of the same geometry and filling, which were produced on the same machines. For this purpose, containers with 0.5 liters of tap water with a temperature of 90 ° C were prepared and each tea bag was immersed in one container. The tea was visually assessed for the speed of discoloration of the tap water in the container after a few seconds, since this discoloration speed is also the criterion that a consumer observes when making the tea. There was no discernible difference with regard to the tea bags according to the invention and not according to the invention, which could be confirmed by measurements using UV-VIS. The sand loss of the tea bags was assessed. For this purpose, sand with a particle size of 106 μm to 150 μm was filled into the tea bags, after which the tea bags were shaken in an apparatus and the amount of sand that had fallen out of the tea bag through the pores of the filter paper was weighed. On this point, too, there were no significant differences between the tea bags according to the invention and not according to the invention.

It can thus be seen that the filter papers according to the invention can be used to produce tea bags with a large or complete absence of Abacá fibers and sisal fibers, the performance of which does not differ from that of conventional tea bags, although this would have been expected due to the technical properties of the filter papers. In addition, about 10% more tea bags could be produced from a roll with the same outer diameter due to the approximately 10% smaller thickness compared to conventional filter papers, which allows an additional increase in productivity.

Claims (15)

1. A filter paper for producing an aqueous extract comprising long fiber pulp having the following properties:

   - a basis weight of more than 9,0 g/m² and less than 13,5 g/m²,

   - a density of more than 280 kg/m³ and less than 350 kg/m³,

   - 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-kPa) and less than 26000 cm/(min-kPa),

   - the filter paper being either free from abacá fibres and sisal fibres or, if abacá fibres and/or sisal fibres are present, they constitute in total less than 20% of the paper pulp.
2. Filter paper according to claim 1, in which the long-fibre pulp is obtained from coniferous trees, in particular spruce, pine or fir.
3. Filter paper according to claim 1 or 2, in which the proportion of long-fibre pulp in the filter paper is at least 70%, preferably at least 80% and particularly preferably at least 90%, in each case relative to the mass of the filter paper, wherein in particular all the pulp in the filter paper is formed by long-fibre pulp.
4. Filter paper according to any of the preceding claims, in which a proportion of abacá fibres and sisal fibres together is less than 10%, preferably less than 5%, of the mass of the filter paper.
5. Filter paper according to one of the preceding claims, which further contains short fibre pulp, wherein the short fibre pulp is preferably obtained from deciduous trees, in particular birch, beech or eucalyptus, and wherein the proportion of short fibre pulp is at most 20%, preferably at most 10% and preferably at least 2%, particularly preferably at least 5% of the paper pulp, or
   which is free from short fibre pulp.
6. Filter paper according to any of the foregoing claims, further containing thermoplastic fibres,
   wherein the thermoplastic material is preferably selected from the group consisting of polyethylene, polypropylene, polyester, in particular polyethylene terephthalate, polyamide, polymethacrylate, polyacrylate, polyvinyl acetate, polyvinyl alcohol and polylactide or mixtures thereof, wherein the thermoplastic fibres are preferably at least partially formed by bicomponent fibres, wherein
   the proportion of thermoplastic fibres is preferably at least 5%, preferably at least 10% and/or at most 30%, preferably at most 20%, in each case based on the mass of the filter paper.
7. Filter paper according to one of the preceding claims, which contains one or more fillers, in particular fillers selected from the group consisting of calcium carbonate, magnesium carbonate, magnesium hydroxide, aluminium hydroxide, magnesium silicates, aluminium silicates, kaolin and talc or mixtures thereof, wherein
   the proportion of fillers is preferably less than 10% of the paper pulp, particularly preferably less than 5% of the paper pulp, or
   which is free from fillers.
8. Filter paper according to any of the foregoing claims, with a basis weight of at least 10.0 g/m², in particular at least 11.0 g/m² and/or a basis weight of at most 13.2 g/m², and preferably at most 13.0 g/m², and/or
   with a density of at least 290 kg/m³ and preferably at least 300 kg/m³ and/or a density of at most 340 kg/m³, preferably at most 330 kg/m³, and/or
   of a thickness of 38 µm or more, preferably 40 µm or more, very preferably 41 µm or more, and/or of a thickness not exceeding 48 µm, preferably not exceeding 46 µm, and very preferably not exceeding 45 µm, and/or
   with a roughness according to ISO 8791-2:2013 of at least 800 ml/min, preferably at least 850 ml/min and/or of not more than 1200 ml/min, preferably not more than 1100 ml/min.
9. Filter paper according to any of the foregoing claims, with an air permeability according to ISO 2965:2009 of at least 18000 cm/(min-kPa), preferably at least 19000 cm/(min-kPa) and/or of not more than 25000 cm/(min-kPa), preferably not more than 24000 cm/(min-kPa), and/or
   with a bending resistance in the machine direction according to ISO 2493-1:2010 of at least 55 mN, preferably at least 58 mN, and/or at most 73 mN, and preferably at most 72 mN, and/or
   having a bending resistance in the transverse direction, as defined in ISO 2493-1:2010, of at least 15 mN, preferably at least 18 mN, and more preferably at least 20 mN and/or at most 28 mN, preferably at most 26 mN, and more preferably at most 25 mN.
10. Filter paper according to any of the foregoing claims, whose elongation at break in the machine direction is at least 1.0%, preferably at least 1.2% and/or at most 2.0%, preferably at most 1.8%, and/or
    whose elongation at break in the transverse direction is at least 1.8%, preferably at least 2.4% and/or at most 3.8%, preferably at most 3.4%, and/or
    whose tensile strength in the machine direction is at least 11.5 N/15 mm, preferably at least 12.0 N/15 mm and/or at most 15.0 N/15 mm, preferably at most 14.0 N/15 mm, and/or
    whose tensile strength in the transverse direction is at least 2.5 N/15 mm, preferably at least 3.0 N/15 mm and/or at most 5.0 N/15 mm, preferably at most 4.5 N/15 mm, and/or
    whose energy absorption capacity according to ISO 1924-2:2008 in machine direction is at least 6.0 J/m², preferably at least 7.0 J/m² and/or at most 11.0 J/m², preferably at most 10.0 J/m², and/or
    which is packaged substantially impermeable to water vapor and has a moisture content of at least 9%, preferably at least 10%, particularly preferably at least 11% and at most 20%, preferably at most 18%, particularly preferably at most 15%.
11. A process for producing a filter paper which is either free from abacá fibres and sisal fibres, or, if abacá fibres and/or sisal fibres are present, they together constitute less than 20% of the paper pulp, the process comprising the following steps:

    - Suspending a long-fibre pulp in water and grinding at least the long-fibre pulp in a refiner unit,

    - Feeding a suspension containing at least the ground long fibre pulp onto a wire of a paper machine,

    - dewatering the suspension through the wire of the paper machine to form a paper, and

    - Press the paper with sufficient pressure to give the paper the following properties:

    • a basis weight of more than 9,0 g/m² and less than 13,5 g/m²,

    • a density of more than 280 kg/m³ and less than 350 kg/m³,

    • 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-kPa) and less than 26000 cm/(min·kPa)
12. A method according to claim 11, wherein the filter paper is a filter paper according to any one of claims 1 to 10.
13. A method according to claim 11 or 12, in which the paper machine is an inclined wire machine, the wire preferably being inclined between 15° and 25° from the horizontal, and/or
    in which, after the wire section, the filter paper passes through a press section in which it is further dewatered by mechanical pressure, the press section preferably comprising two rolls which form a nip through which the paper passes and thereby experiences a line load of 30 kN/m and at most 100 kN/m, the rolls preferably being steel rolls, and in particular steel rolls covered with a plastic cover.
14. A method according to any one of claims 11 to 13, in which the filter paper is dried after pressing by elevated temperature, in particular by hot air, infrared radiation or contact with heated cylinders, the drying step preferably comprising a post-moistening, and/or in which the filter paper is packaged in a state with a moisture content of at least 9%, preferably at least 10%, particularly preferably at least 11% and at most 20%, preferably at most 18%, particularly preferably at most 15%, substantially impermeable to water vapour, in particular by means of a film of polyethylene or polypropylene.
15. Bag made of a filter paper Paper according to any one of claims 1 to 10 containing a material to be extracted, the material to be extracted preferably being tea.