Improved filter paper for cigarette filters

Abstract

The invention relates to a filter paper for producing filters for smoking articles, in particular filter cigarettes, having the following properties: the filter paper comprises fibers comprising pulp fibers; at least 80 wt.%, preferably at least 90 wt.%, particularly preferably at least 95 wt.%, and particularly preferably 100 wt.%, of the filter paper is made of long-fiber pulp fibers; 2% to 10%, preferably 3% to 9%, particularly preferably 4% to 8%, of the fibers with respect to the number of fibers have a length of less than 0.2 mm; the air permeability of the filter paper, measured according to ISO 2965:2009, is between 500 cm-min -1kPa-1 and 15000 cm-min -1kPa-1 and preferably between 1000 cm-min-1kPa-1 and 9000 cm-min-1kPa-1; the average length of the fibers in the filter paper with respect to the number of fibers is more than 1 mm and less than 5 mm, preferably more than 2 mm and less than 4 mm; and the average width of the fibers in the filter paper with respect to the number of fibers is between 10 µm and 50 µm, preferably between 20 µm and 40 µm, and particularly preferably between 25 µm and 35 µm.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a paper for producing a cigarette filter and a method for producing the filter paper. The invention further relates to an associated cigarette. A filter made from the filter paper according to the invention has retention properties similar to those of cellulose acetate filters and therefore improves the taste impression of a cigarette made from this filter paper compared to cigarettes with paper filters made from conventional filter papers. BACKGROUND AND RELATED ART

[0002] A conventionally manufactured filter cigarette generally consists of a cylindrical column of tobacco wrapped in a cigarette paper, and a filter made of a filter material wrapped in a filter wrapper paper. A common filter material is cellulose acetate. Typically, the tobacco column and the filter are connected by a tipping paper.

[0003] As an alternative to cellulose acetate fibers, paper is also known as a filter material for cigarettes. A cigarette filter can consist of several segments, each segment being made of a different material. For example, filters consisting of two segments are known from the prior art, with one segment being a cellulose acetate filter and a second segment being a paper filter. It is also known to mix cellulose acetate fibers and pulp fibers in one segment or to arrange structures made of these fibers separately within a segment, for example, in a longitudinal or radial direction.

[0004] Paper filters generally have the advantage that they degrade more quickly in the environment and that they are generally less expensive than cellulose acetate filters. A major disadvantage of a conventional paper filter, however, is that it has different retention properties than a filter made of cellulose acetate. For example, it is known that, for a comparable draw resistance, the filtration efficiency of a paper filter for tar is higher than that of a cellulose acetate filter. Paper filters also tend to retain water and water vapor much better than cellulose acetate filters. These two effects can, among other things, undesirably alter the taste impression of a cigarette. Furthermore, the draw resistance for a given filtration efficiency, i.e. the resistance that the filter offers to the flow of smoke, is often considerably lower than that of a cellulose acetate filter.Likewise, the hardness of a paper filter, i.e. its resistance to mechanical deformation, often does not meet the expectations of a smoker who is used to a cellulose acetate filter.

[0005] EP 2 761 085, for example, describes a particularly biodegradable paper for paper filters, but this does not solve the problems regarding taste or filtration efficiency in a fully satisfactory manner.

[0006] It is known that certain substances can be added to filter paper to control filtration efficiency or improve flavor, such as triacetin, propylene glycol, sorbitol, glycerin, polyethylene glycol, or triethyl citrate. However, the addition of such substances does not solve the existing problems regarding draw resistance and hardness, and it increases the price of the filter paper.

[0007] There is therefore a need to have a filter paper available that gives a filter made from it properties that are more similar to those of a filter made from cellulose acetate than paper filters for cigarettes made from conventional filter papers.

[0008] In the article "Comparison of the properties of wood and pulp fibers from lodgepole pine (pinus contorta) and scots pine (pinus sylvestris)," Sable et al. compares the properties of wood pulp fibers and kraft pulp fibers, as well as the paper properties, when the pulp is obtained from 27-year-old trees of the species pinus contorta and pinus sylvestri. The mean fiber width for wood pulp from pinus sylvestris is 33.1 µm, and for pinus contorta, 31.7 µm.

[0009] EP 10 98 036 A1 discloses a filter wrapper paper comprising a paper layer having a first and a second surface. The filter wrapper paper has a first overlap air permeability, measured when a perforated paper is overlaid with the first surface, and a second overlap air permeability, measured when a perforated tipping paper is overlaid with the second surface. The first overlap air permeability is higher than the second overlap air permeability. SUMMARY OF THE INVENTION

[0010] The present invention is therefore based on the object of providing a filter paper that can be produced easily and cost-effectively and simultaneously imparts a filtration efficiency similar to a filter made of cellulose acetate with comparable draw resistance. This object is achieved by a filter paper according to claim 1 and its production method according to claim 10. The invention further relates to a filter and a filter cigarette using this material. Advantageous developments are specified in the dependent claims.

[0011] According to the invention, a paper for use as filter paper is proposed which has the following properties: the filter paper comprises fibers, at least 80 wt.%, preferably at least 90 wt.% and particularly preferably at least 95 wt.% and very particularly preferably 100 wt.% of the filter paper is formed by long-fiber pulp fibers, the proportion of fibers with a length of less than 0.2 mm based on the number of fibers is between 2% and 10%, preferably between 3% and 9% and particularly preferably between 4% and 8%, the air permeability of the filter paper measured according to ISO 2965:2009 is between 1000 cm min -1< kPa -1< and 9000 cm min -1< kPa -1< , the mean length of the fibers in the filter paper, based on the number, is more than 1 mm and less than 5 mm, preferably more than 2 mm and less than 4 mm, and the mean width of the fibers in the filter paper, based on the number, is between 10 µm and 50 µm, preferably between 20 µm and 40 µm, and particularly preferably between 25 µm and 35 µm.

[0012] The inventors have determined that the amount of fines in the filter paper, i.e., fibers with a length of less than 0.2 mm, is significant in reducing the filtration efficiency of a filter made from the filter paper according to the invention, making it more similar to that of a cellulose acetate filter. This is surprising because the fines themselves have a large surface area, and their presence should therefore increase filtration efficiency. In fact, however, it is neither beneficial to have too many nor too few fines in the filter paper; rather, their proportion relative to the number of fibers in the filter paper should be in the narrow range between 2% and 10%.

[0013] Furthermore, the inventors have determined that the air permeability of the filter paper is a key parameter for controlling the draw resistance of the paper filter and thus also its filtration efficiency over a wide range. This is surprising because cigarette smoke in a paper filter typically flows along the surface of the filter paper and not through it. Contrary to the expectations of the skilled person, a close relationship between air permeability, draw resistance, and filtration efficiency is evident.

[0014] Air permeability is influenced by the intensity of fiber refining. Intensively refined fibers result in low air permeability, while less intensively refined fibers result in high air permeability. However, since not only air permeability but also the fines content is influenced by fiber refining, it is not self-evident that air permeability can be adjusted within the range described in the invention with an approximately constant fines content. In addition, the filter paper must also meet requirements regarding mechanical strength, which is also significantly influenced by fiber refining.

[0015] The simultaneous compatibility of all these requirements is achieved by the method according to the invention described below, in which at least part of the fiber material is ground in a special grinding unit with specific settings.

[0016] The dimensions of the fibers in the filter paper influence their surface area and thus also the air permeability and filtration efficiency of the filter made from it. Therefore, it is beneficial if the average length and width of the fibers in the filter paper lie within a certain range.

[0017] The length and width of the fibers in the filter paper can be measured according to ISO 16065 using the automated optical method described therein. However, in deviation from ISO 16065, fiber portions with a length of less than 0.2 mm are included in the measurement. This type of measurement is possible with the L&W Fiber Tester Plus - code 912 Plus from Lorentzen & Wettre, which can also determine the amount of fines. A sample of approximately 0.1 g of dry fibers is suspended in water and pumped through a thin gap between two plates by the measuring device. At the same time, a camera monitors the fiber suspension flowing through the gap between the plates and takes images at short intervals, which are analyzed to determine the geometry of the flowing fibers.As a result, this measuring device provides, among other things, a number-related distribution of fiber lengths and fiber widths, from which the number-related mean length and mean width as well as the proportion of fines can be determined.

[0018] The mean length of the fibres in the filter paper determined in this way, based on the number, should be more than 1 mm and less than 5 mm, and preferably more than 2 mm and less than 4 mm.

[0019] The mean width of the fibers in the filter paper determined in this way, based on the number, is also between 10 µm and 50 µm, preferably between 20 µm and 40 µm, and most preferably between 25 µm and 35 µm.

[0020] The filter paper according to the invention contains fibers, wherein the fibers in any case comprise cellulose fibers. Cellulose fibers are cellulose-based fibers of plant origin, for example, long-fiber cellulose fibers or short-fiber cellulose fibers. For the purposes of the invention, fibers made of plastics, fibers made of regenerated cellulose, and in particular cellulose acetate fibers are not considered cellulose fibers.

[0021] The pulp fibers can be bleached or unbleached, or a mixture of bleached and unbleached pulp fibers. However, bleached pulp fibers are preferred because the filter paper is then white, which is the color expected by smokers. The use of at least partial unbleached pulp fibers results in a filter paper with a light brown to dark brown color and is less preferred.

[0022] To achieve sufficient strength and air permeability of the filter paper and a corresponding filtration efficiency of the resulting filter, it is necessary for a high proportion of the fibers to be long-fiber pulp fibers. The proportion of long-fiber pulp fibers relative to the weight of the filter paper should therefore be at least 80 wt.%, preferably at least 90 wt.%, particularly preferably at least 95 wt.%, and most particularly preferably 100 wt.% of the filter paper.

[0023] The statement that 100% by weight of the filter paper is made of long-fiber pulp is intended to mean that the filter paper contains essentially only long-fiber pulp fibers. This statement therefore also includes filter papers containing impurities from any other substances and materials, such as other fibers, short-fiber pulp fibers, fillers, pigments, additives, or processing aids, as may occur during paper production according to the state of the art.

[0024] Long-fiber pulp can be obtained from coniferous wood, especially spruce or pine, but also from other plants such as hemp, flax, sisal, abaca, cotton, ramie, jute, kenaf, gampi, kozu, or matsumata. Those skilled in the art will understand that the term "long-fiber pulp" refers to the natural length of the fibers, not to the actual length in the milled state used in paper.

[0025] The filter paper according to the invention must contain a certain proportion of fines. Fines include all fibers less than 0.2 mm long. The proportion of fines in the filter paper according to the invention is between 2% and 10% based on the number of fibers in the filter paper, preferably between 3% and 9%, and particularly preferably between 4% and 8%, each based on the number of fibers in the filter paper.

[0026] Likewise, the filter paper according to the invention must have an air permeability within a predetermined range, as this determines the filtration efficiency of the filter paper. The air permeability of the filter paper according to the invention, measured according to ISO 2965:2009 using a measuring head with an opening of 10 mm × 20 mm, is between 1000 cm min -1 kPa -1 and 9000 cm min -1 kPa -1 .

[0027] It is compatible with the invention for the filter paper to comprise additional cellulose fibers, such as short-fiber cellulose fibers, or other fibers such as fibers made from regenerated cellulose, such as viscose fibers, modal fibers, lyocell fibers, fibers made from cellulose esters such as cellulose acetate, or from plastics such as polyvinyl alcohol, polyethylene, polyester, or polypropylene, or even fibers made from polylactates. However, these fibers reduce the mechanical strength, and the total proportion of such fibers should therefore not exceed 10% by weight, preferably not more than 5% by weight, and particularly preferably not more than 2% by weight of the filter paper.In particular, viscose fibers, modal fibers, lyocell fibers, fibers made of cellulose acetate, polyvinyl alcohol, polyethylene, polyester or polypropylene impair the biodegradability, increase the price of the filter paper and cause problems in the production of the filter paper due to their low density, so that in a particularly preferred embodiment the filter paper does not contain such fibers.

[0028] Short-fiber pulp fibers can be obtained from hardwood, especially birch, beech or eucalyptus, but also from other plants, such as esparto grass.

[0029] The filter paper may contain fillers to influence, for example, the whiteness, color, or opacity of the filter paper. The optical properties of the filter paper may be important, particularly if the filter made from the filter paper according to the invention extends all the way to the mouth of the cigarette so that the smoker can see it. However, the fillers reduce the strength of the filter paper and can undesirably affect the porous structure. The proportion of fillers should therefore not exceed 10% by weight, preferably not exceed 5% by weight, and most preferably not exceed 2% by weight of the filter paper. In a particularly preferred embodiment, the filter paper is free of fillers.

[0030] Fillers may be mineral fillers, in particular carbonates, sulfates, silicates or oxides, in particular, for example, calcium carbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate, titanium dioxide, talc, kaolin or aluminum hydroxide, as well as mixtures thereof.

[0031] The particle shape, particle size distribution and crystal structure of the fillers can vary within wide ranges and the person skilled in the art will select these parameters according to the state of the art and the purpose that the person skilled in the art wishes to achieve.

[0032] The filter paper according to the invention can contain pigments or dyes to impart a specific color to the filter paper. An example of a pigment is iron oxide, which is typically yellow, red, or black and can be used alone or in mixtures. Iron oxides or other pigments or dyes can change their color permanently or temporarily when heated, so these substances can also be added to achieve special optical effects during or after smoking.

[0033] The filter paper according to the invention may contain further additives to influence certain properties of the filter paper. These include, for example, sizing agents such as alkyl ketene dimer (AKD), succinic anhydride (ASA), fatty acids, fatty alcohols, or other hydrophobic substances to make the filter paper water-repellent, or starch to increase the strength of the filter paper, or wet strength agents.

[0034] Of particular interest can be the addition of additives that selectively influence certain substances in cigarette smoke. 44 substances, the so-called "Hoffmann Analytes," are of particular importance. Specifically, the filter paper can therefore contain substances that influence the carbon monoxide content, such as catalysts; zinc oxide or other metal oxides; nitrates of transition metals or of copper, iron, or silver; or platinum. Zinc oxide or sodium glycinate can be used to influence hydrogen cyanide (HCN), particularly its reduction. Ascorbic acid, tartaric acid, sodium carbonate, or polyethyleneimine can be used to influence formaldehyde. Additions of activated carbon, polyethylene glycol, triacetin, or triethyl citrate are also possible.The sum of pigments, dyes, and additives, as well as all other substances that a person skilled in the art may add to the filter paper to achieve specific effects, should not exceed 10% by weight, preferably not more than 5% by weight, and particularly preferably not more than 2% by weight of the filter paper. In a particularly preferred embodiment, the filter paper is at least free of pigments and dyes.

[0035] In general, toxicological aspects and legal requirements must also be taken into account when determining the components of the filter paper.

[0036] The mechanical properties of the filter paper are important for the production of a filter from the filter paper according to the invention. The mechanical properties of the filter paper differ in the machine direction, i.e., the direction in which the filter paper runs through the paper machine, and the transverse direction, i.e., the direction orthogonal to the running direction in the paper machine.

[0037] The tensile strength of the filter paper in the machine direction should be at least 7 N / 15 mm, preferably at least 8 N / 15 mm, and particularly preferably at least 9 N / 15 mm. High tensile strength is not a disadvantage. Since the production of filter paper with high tensile strength involves high energy expenditure during fiber refining, the tensile strength should not be increased unnecessarily. The tensile strength in the machine direction should therefore be at most 50 N / 15 mm, preferably at most 45 N / 15 mm, and particularly preferably at most 40 N / 15 mm.

[0038] The filter paper is subjected to comparatively less stress in the transverse direction during the production of the filter, so that the tensile strength in the transverse direction is preferably at least 4 N / 15 mm and particularly preferably at least 5 N / 15 mm and preferably at most 9 N / 15 mm and particularly preferably at most 8 N / 15 mm.

[0039] Elongation at break is just as important as tensile strength, as it describes how well the filter paper can compensate for speed differences as it travels through the machine during filter production. The elongation at break in the machine direction is therefore preferably at least 1%, more preferably at least 1.2%, more preferably at most 2%, and most preferably at most 1.8%.

[0040] The transverse elongation at break also plays a role, because the filter paper may also stretch in the transverse direction during filter production. Therefore, the transverse elongation at break should preferably be at least 4%, more preferably at least 4.5%, more preferably at most 6%, and most preferably at most 5.5%.

[0041] The filter paper may be creped in the machine direction or in the transverse direction. In this case, its elongation at break in the direction or directions in which the filter paper is creped is at most 25%, preferably at most 15%, and most preferably at most 10%.

[0042] The tensile strength and elongation at break in the machine direction and transverse direction can be measured according to ISO 1924-2:2008.

[0043] The basis weight of the filter paper can, for example, be important for the hardness of the filter made from this filter paper. The basis weight is therefore preferably from 10 to 80 g m -2 , particularly preferably from 20 to 60 g m -2 , and most preferably from 30 to 40 g m -2 . The basis weight of a filter paper can be measured according to ISO 536:2012.

[0044] Just like the basis weight, the thickness of the filter paper can be important for the hardness of the filter made from it. Therefore, the thickness is preferably between 60 µm and 160 µm, and particularly preferably between 80 µm and 120 µm. The thickness of a filter paper can be measured according to ISO 534:2011 on a single layer of the filter paper.

[0045] The filter paper according to the invention can be produced by the following method according to the invention.

[0046] The first step (A) involves providing an aqueous suspension of unground fiber material in a storage container. This process step can be carried out using methods known per se from the prior art.

[0047] The unrefined fiber material comprises pulp fibers, particularly long-fiber pulp fibers. The pulp fibers can be bleached or unbleached, or a mixture of bleached and unbleached pulp fibers. However, the unrefined pulp fibers are preferably bleached because the filter paper is then white, a color expected by smokers.

[0048] The proportion of unrefined long-fibre pulp fibres based on the weight of the unrefined fibre material is at least 80% by weight, preferably at least 90% by weight and particularly preferably at least 95% by weight and most preferably 100% by weight.

[0049] The statement that 100% by weight consists of unrefined long-fiber pulp fibers is intended to mean that the unrefined fiber material contains essentially exclusively unrefined long-fiber pulp fibers. This statement therefore also includes unrefined fiber material that contains impurities from any other substances and materials, such as other fibers, short-fiber pulp fibers, fillers, pigments, additives, or processing aids, as may occur in papermaking according to the state of the art.

[0050] The long-fibre pulp is obtained from softwoods such as spruce or pine, or from flax, hemp, sisal, abaca, cotton, ramie, jute, kenaf, gampi, kozu or matsumata.

[0051] The unrefined fiber material may also comprise other pulp fibers, such as short-fiber pulp fibers, or other fibers such as fibers made from regenerated cellulose, such as viscose fibers, modal fibers, or lyocell fibers; fibers made from cellulose esters such as cellulose acetate; or from plastics such as polyvinyl alcohol, polyethylene, polyester, or polypropylene; or even fibers made from polylactates. However, the total proportion of such fibers should not exceed 10% by weight, preferably not more than 5% by weight, and particularly preferably not more than 2% by weight of the unrefined fiber material.

[0052] Short-fiber pulp fibers can be obtained from hardwood, especially birch, beech or eucalyptus, but also from other plants, such as esparto grass.

[0053] In a further, optional process step, other materials such as fillers, additives, processing aids, pigments or dyes can be added to the aqueous suspension of the unground fiber material.

[0054] The information given above applies to the type and quantity of fillers, additives, processing aids, pigments and dyes.

[0055] In a further process step (B), at least a portion of the aqueous suspension prepared in step (A) and consisting predominantly or exclusively of unrefined long-fiber pulp fibers and optionally further components is fed to a refining unit and refined there. The portion of unrefined fiber material that is refined in the refining unit should be at least 40% by weight, preferably at least 50% by weight, and particularly preferably at least 60% by weight, based in each case on the total amount of the originally unrefined fiber material. If a portion of the suspension of unrefined fiber material is not refined, it can be added again in a later process step. However, it is also possible for the entire suspension prepared in step (A) to be subjected to refining, and for another suspension containing unrefined long-fiber pulp fibers or other fiber material to be added at a later time.

[0056] For the process according to the invention, it is essential that the refining unit is a Papillon refiner. The inventors have found that such a Papillon refiner can refining the fiber material in such a way that the desired combination of properties, namely air permeability, fines content, and strength of the filter paper, can be achieved.

[0057] In contrast to conventional double-disk or cone refiners, refining in a Papillon refiner takes place in a cylindrical geometry. This means the fiber material is treated particularly gently and less energy is required for refining. The inventors believe that the special distribution of fiber lengths and widths, and in particular the proportion of fines, can be achieved specifically with such refiners, whereas this was not possible with conventional double-disk or cone refiners. Papillon refiners are offered by various machine manufacturers, for example by Andritz with the model designations CS380, CS450, CC380, and CC450. The following information applies to an Andritz Papillon Refiner CS380 as an example and can easily be transferred to other refining units with the knowledge of the specialist or through testing.

[0058] The Papillon refiner settings must be adapted to the type and quantity of unrefined fiber material, the dimensions of the refining unit, and the refining set. A pulp density, i.e., the mass-related proportion of dry fiber material in the suspension used for refining, of between 1 wt.% and 5 wt.% has proven effective. The flow rate can be between 300 l min -1 and 700 l min -1 . The specific refining edge load can be between 0.3 J m -1 and 1.0 J m -1 .

[0059] Preferably, the fiber material is refined with an energy input of 30 W h kg -1 to 100 W h kg -1 , based on the mass of the dry, unrefined fiber material. For refining the fiber material in a Papillon refiner, the speed and power input must also be determined. In the process according to the invention, the speed is between 500 min -1 and 2000 min -1 , and preferably between 600 min -1 and 1600 min -1 . In preferred embodiments, the power supplied to the Papillon refiner is between 50 kW and 200 kW, preferably between 60 kW and 150 kW.

[0060] By selecting the speed and power, the air permeability of the filter paper can be adjusted. Generally, a high speed or low power results in higher air permeability. Conversely, a low speed or high power results in lower air permeability.

[0061] That part of the suspension of the unground fiber material which was previously diverted and not ground is then added back to the suspension of the ground fiber material.

[0062] In a further, optional process step, other materials such as fillers, additives, processing aids, pigments, or dyes can be added to the aqueous suspension of the milled fiber material. Regarding the type and quantity of fillers, additives, processing aids, pigments, and dyes, the information provided above applies.

[0063] The addition of ground fibre material from one or more other grinding units is also possible in this process step, although, unless the fibres are ground long fibre pulp, the limits previously specified in the first process step (A) must be observed.

[0064] In particular, fines, i.e., fibers with a maximum length of 0.2 mm, can also be added to the aqueous suspension in this step to adjust the filtration efficiency. The proportion of fines in the finished aqueous suspension is between 2% and 10%, based on the number of fibers in the aqueous suspension, preferably between 3% and 9%, and particularly preferably between 4% and 8%, each based on the number of fibers in the aqueous suspension. The term "finished suspension" refers to the suspension in the state in which it is fed to a paper machine in a step (C) described below, i.e., after all additions of further unrefined long-fiber pulp, fiber material, or fines have taken place.

[0065] In the finished aqueous fiber suspension, the length and width of the fibers are important. The average length of the fibers in the finished aqueous fiber suspension should preferably be more than 1 mm and less than 5 mm, and particularly preferably more than 2 mm and less than 4 mm.

[0066] The average width of the fibers in the finished aqueous fiber suspension is preferably between 10 µm and 50 µm, particularly preferably between 20 µm and 40 µm, and most particularly preferably between 25 µm and 35 µm.

[0067] In a subsequent process step (C), the finished aqueous suspension is fed to a paper machine, where a filter paper is produced according to processes known per se in the prior art. The paper machine is preferably an inclined-wire machine, because these machines can produce papers with particularly high air permeability, whose filtration efficiency is well suited for filtering cigarette smoke. Less preferred alternatives are fourdrinier or cylinder-wire machines.

[0068] On a paper machine suitable for the process, the suspension is first collected in a headbox and then pumped onto a screen so that a large portion of the water flows through the screen, while the fiber material and other components largely remain on the screen to form a fiber web. The fiber web then passes through a press section, where the fiber web is further dewatered by mechanical pressure, for example against a felt, and then through a drying section, where the fiber web is dried by heat, microwave radiation, or infrared radiation, preferably by contact with steam-heated drying cylinders and most preferably by hot air, in particular by impingement air drying or through-air drying, until it has a moisture content of 3% to 10% by weight, based on the mass of the filter paper.Drying by impingement air drying or through-air drying is particularly preferred because it can impart high porosity and thickness to the filter paper. Finally, the filter paper is rolled up and optionally cut into narrower rolls with a width of at least 100 mm and a maximum of 400 m, which can then be used for the production of cigarette filters.

[0069] To produce a filter rod from the filter paper, a filter paper web with a width of at least 100 mm and a maximum of 400 mm, for example, approximately 300 mm, is usually embossed or creped, sometimes at elevated temperatures or humidity. Creping can be done in the machine direction, the cross direction, or both directions; it can also be done diagonally, in any direction, or in patterns. The filter paper is then formed into a continuous strand, as with conventional cellulose acetate filters, which is wrapped in a filter wrapping paper. Filter rods are then cut from this continuous strand.

[0070] Any filter wrapping paper known from the state of the art can be used as the filter wrapping paper, in particular a filter wrapping paper with little porosity or a filter wrapping paper with an air permeability measured according to ISO 2965:2009 between 1000 cm·min -1< ·kPa -1< and 30000 cm·min -1< ·kPa -1< .

[0071] The invention further relates to a paper filter comprising the filter paper according to the invention.

[0072] A paper filter according to the invention can be in the form of a filter rod with a length of 60 mm to 200 mm, preferably with a length of 80 mm to 180 mm. The length of the filter rod is an integer multiple, preferably four to six times, of the length of the filter plug, which then serves as a filter on the cigarette.

[0073] The paper filter according to the invention can therefore also be provided as a filter plug with a length of 10 mm to 50 mm, preferably with a length of 15 mm to 30 mm.

[0074] The paper filter according to the invention can also be present as a segment of a cigarette filter. This is the case, for example, if the cigarette filter consists of a segment made of cellulose acetate and a segment made of filter paper, or if, for example, the cigarette filter contains a cavity containing activated carbon particles, which is delimited by two segments that may contain filter paper. The paper filter according to the invention can therefore also have a length of 3 mm to 10 mm, preferably 4 mm to 8 mm.

[0075] The paper filter, i.e. the filter rod, filter plug, or paper filter segment, has a diameter between 3 mm and 10 mm, preferably between 4 mm and 9 mm, and especially preferably between 7 mm and 9 mm. The diameter depends on the diameter of the cigarette that will contain the paper filter. CORESTA Guide No. 10 can be used to measure the diameter of a paper filter.

[0076] The tensile resistance of the paper filter depends essentially on the diameter, the filter material and the length of the paper filter and can be measured according to ISO 6565:2011 at a volume flow of 17.5 cm 3< ·s -1<. The tensile resistance of a paper filter is given in Pa and is, to a very good approximation, proportional to the length of the paper filter, provided that the paper filter is approximately homogeneous over its length. Therefore, the length-specific tensile resistance can be expressed as a pressure difference per mm of length of the paper filter if the information is to be independent of the actual length of the paper filter. The paper filter according to the invention has a length-specific tensile resistance between 10 Pa ·mm -1< and 40 Pa ·mm -1< , preferably between 15 Pa ·mm -1< and 35 Pa ·mm -1< .

[0077] A key characteristic of a paper filter is its filtration efficiency for the particulate phase of cigarette smoke. More specifically, the particulate phase is the nicotine-free dry condensate (NFDPM; nicotine-free dry particulate matter ), colloquially referred to as "tar," and whose content in cigarette smoke is sometimes indicated on the package in mg per cigarette. The filtration efficiency for NFPDM describes the mass ratio of the particulate phase of the smoke retained in the filter to the total particulate phase of the smoke flowing into the filter. The filtration efficiency is expressed in %. The filter comprising the filter paper according to the invention has a filtration efficiency for NFPDM of between 20% and 80%, preferably between 30% and 70%. The filtration efficiency of the filter is influenced by the diameter, length, and draw resistance of the paper filter, as is known in the art.

[0078] The filtration efficiency of a filter is determined by first smoking a sufficient number of cigarettes, for example, 20 cigarettes, on a smoking machine according to ISO 3308:2012. The resulting NFDPM content is determined in mg per cigarette, denoted by X, according to ISO 4387:2000. In a subsequent step, the filter of each smoked cigarette is separated and analyzed to determine the amount of NFDPM contained in the filter in mg per cigarette, denoted by Y. The filtration efficiency F of the filter is then the ratio F=Y·(X+Y) -1< and is expressed as a percentage.

[0079] In the case where the filter consists of several segments, for example, n segments with i=1, 2,...n, the amount of NFDPM contained in each filter segment, denoted by Y i in mg per cigarette, can be determined. The numbering of the filter segments should be ascending in the direction of smoke flow during normal use of the cigarette. Segment i=1 therefore borders directly on the tobacco rod of the cigarette, while segment i=n is located at the mouth end of the cigarette. The filtration efficiency F k of segment k can then be determined by the formula F k = Y k X + ∑ i = k n Y i i.e. calculated by the ratio of the amount of NFDPM retained in the filter segment k to the amount of NFDPM flowing into the filter segment k and expressed in %.

[0080] The invention also relates to a filter cigarette comprising a paper filter. The paper filter can be the only filter on the filter cigarette or, preferably, can be a segment in a segmented cigarette filter. A filter cigarette is particularly preferred in which the filter segment closest to the mouth end is formed from cellulose acetate and at least one further segment closer to the tobacco rod contains the filter paper according to the invention, because then the visual appearance of the mouth end corresponds to that of a filter cigarette with a cellulose acetate filter and thus meets the smoker's expectations.

[0081] The production of a filter cigarette comprising a paper filter according to the invention can be carried out according to the methods known from the prior art. SHORT DESCRIPTION OF THE CHARACTERS

[0082] Fig. 1 shows Table 1 containing the fiber properties of 16 different filter papers, whose fiber material was refined with different settings of a Papillon refiner. Fig. 2 shows Table 2 containing the basis weight, thickness, air permeability, tensile elongation, and elongation at break of the 16 papers in the table from Fig. 1 Fig. 3 shows Table 3, which shows the filtration efficiency, draw resistance and specific draw resistance of the 16 papers in the table of Fig. 1 contains. Fig. 4 shows the relationship between the air permeability of the filter papers according to the invention and the filtration efficiency for NFDPM of the paper filters made from the filter papers. Fig. 5 shows the relationship between the draw resistance of a filter and the filtration efficiency for NFDPM for the paper filters according to the invention (circles), conventional paper filters (squares), and conventional cellulose acetate filters (triangles). DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0083] The following examples are intended to demonstrate the invention and its advantages.

[0084] The fiber material used was exclusively bleached, unrefined long-fiber pulp fibers, which were refined in a Papillon refiner at various speeds between 600 rpm and 1200 rpm and powers between approximately 60 kW and 140 kW. A total of 16 different power and speed combinations were selected. The specific values ​​are given in Table 1 ("Papillon refiner settings"), which is available in Fig. 1 is shown. Unrefined long-fiber pulp fibers were added to the refined long-fiber pulp fibers so that the total suspension of long-fiber pulp fibers contained approximately 60 wt% refined and approximately 40 wt% unrefined long-fiber pulp fibers.

[0085] Furthermore, the properties of the fibers in the suspension were determined in accordance with ISO 16065 using an L&W Fiber Tester Plus - code 912 Plus, taking into account also the fines, i.e. fibers with a length of less than 0.2 mm.

[0086] The mean fiber length, the mean fiber width and the percentage of fines in relation to the number of fibers are shown in the corresponding columns of Table 1. Fig. 1 The average fiber length for the exemplary filter papers according to the invention is approximately between 2 mm and 2.5 mm, the average fiber width between 30.5 µm and 31.5 µm, and the proportion of fines is between 5% and 8% based on the number of fibers in the fiber material.

[0087] The inventors assume that only a Papillon Refiner can achieve such constant fiber properties despite the variable air permeability of the filter paper and that these fiber properties contribute significantly to the filtration efficiency of the filters made from these filter papers.

[0088] From these 16 differently ground long-fiber pulps, 16 different filter papers were produced on an inclined screen machine. The filter papers are numbered 1 to 16 according to the "No." column in the Fig. 1 bis 3 shown in Tables 1 to 3, whereby in all tables a number designates the same filter paper or the filter made from it.

[0089] The basis weight according to ISO 536:2012, the thickness according to ISO 534:2011 and the air permeability according to ISO 2965:2009 were determined for each filter paper and are shown in the corresponding columns in Table 2 ( Fig. 2 ). The basis weight is between 34.9 g m -2 and 36.6 g m -2 , which is a very narrow range. The thickness also varies only between 83 µm and 101 µm. The air permeability of the filter papers, however, varies depending on the grinding of the fiber material and lies between 1099 CU and 8364 CU.

[0090] Likewise, the tensile strength and elongation at break were determined according to ISO 1924-2:2008 for the uncreped filter paper in the machine direction (MD; machine direction ) and in the transverse direction (CD; cross direction ). These values ​​are also shown in the corresponding columns of Table 2 ( Fig. 2 ). The tensile strength values ​​all exceed the technically necessary minimum for producing filters from the filter papers. Likewise, the elongation at break is within a favorable range for further processing of the filter paper.

[0091] From a 240 mm wide roll of each of the 16 filter papers, paper filter rods with a circumference of 24.35 mm and a length of 132 mm were produced using a non-porous filter wrap paper on a laboratory filter machine.

[0092] The tensile strength of each paper filter rod was determined according to ISO 6565:2011 and is converted to a length of 22 mm and given as specific tensile strength in the corresponding columns of Table 3 ( Fig. 3 ) is specified.

[0093] Each paper filter rod was divided into six equal-length paper filter plugs, each 22 mm long, and used to produce cigarettes with a length of 83 mm, a circumference of 24.5 mm, a tobacco rod length of 61 mm, and a tobacco weight of 600 mg. The tobacco was formed into a tobacco rod using conventional cigarette paper with an air permeability of 50 cm min -1 < kPa -1 <. The paper filter was wrapped with a 27 mm long tipping paper, so that the tipping paper overlapped the tobacco rod by 5 mm, thus bonding the paper filter to the tobacco rod.

[0094] The tobacco blend and all geometric data of the cigarettes were identical, differing only in the paper filter. Using these cigarettes, the filtration efficiency of the paper filters was determined using the method described above.

[0095] The filtration efficiency for nicotine-free dry condensate (NFDPM) is shown for each of the 16 filter plugs (22 mm) in Table 3 ( Fig. 3 ). This results in a filtration efficiency for NFPDM of between 36.2% and 55.2%. By changing the length of the filter plug or by using a different roll width during the production of the paper filter rod, filtration efficiencies below or above this interval can be easily achieved, so that the filter paper according to the invention can cover a range of filtration efficiencies, as is also common for filters made of cellulose acetate.

[0096] Fig. 4 shows the relationship between the filtration efficiency for NFPDM of the paper filters according to the invention and the air permeability of the filter paper. It can be seen that air permeability is an essential parameter for adjusting the filtration efficiency over a wide range, because all other parameters, such as basis weight, thickness, or fiber properties, are kept approximately constant.

[0097] The main advantage of the invention can be seen from Fig. 5 The figure shows the relationship between filter draw resistance and filtration efficiency for NFPDM for the inventive paper filters (circles), conventional paper filters (squares), and conventional cellulose acetate filters (triangles). All filters had a length of 22 mm.

[0098] The figure clearly shows one of the disadvantages of conventional paper filters. For example, if one wants to achieve a filtration efficiency of 45% for NFDPM, a conventional paper filter has a draw resistance of approximately 300 Pa, while a conventional cellulose acetate filter has a draw resistance of approximately 600 Pa. The draw resistance of an unventilated filter cigarette is essentially determined by the draw resistance of the filter and that of the tobacco rod. For a king-size cigarette with a circumference of 24 mm to 25 mm, the smoker expects a draw resistance of approximately 1000 Pa. If one replaces the conventional cellulose acetate filter with a draw resistance of 600 Pa in an existing cigarette design with a conventional paper filter with 300 Pa, the draw resistance of the filter cigarette drops to 700 Pa, i.e. by 30%. This difference is clearly noticeable and undesirable for the smoker.

[0099] However, with the filter paper according to the invention, a paper filter with a draw resistance of approximately 400 Pa can be produced at a filtration efficiency for NFPDM of 45% (see Example 6), so that the draw resistance of the cigarettes drops to only 800 Pa. Thus, compared to conventional filter paper, considerably less effort is required to adapt the cigarette design to the changed draw resistance.

[0100] Likewise, a conventional cellulose acetate filter achieves a filtration efficiency of just under 50% for NFPDM at a tensile strength of 700 Pa, while a conventional paper filter already has a filtration efficiency of approximately 70% at this tensile strength. Using the filter paper according to the invention, for example that from Example 3, a paper filter can be produced that has a filtration efficiency of just over 50% at a similar tensile strength, making it closer to a cellulose acetate filter than a conventional paper filter. This means that the paper filter according to the invention offers advantages over conventional paper filters even when the tensile strength of the filter is to be kept constant.

[0101] Overall, Fig. 5that the paper filters according to the invention always lie between conventional paper filters and conventional cellulose acetate filters in terms of filtration efficiency and draw resistance, and in addition, at higher draw resistances the difference between the paper filters according to the invention and the cellulose acetate filters becomes smaller.

[0102] With the filter papers and paper filters according to the invention, the advantages of a paper filter can be better utilized and the change from cellulose acetate filters to the paper filters according to the invention requires fewer adjustments to the cigarette design than for conventional paper filters.

Claims

1. Filter paper for manufacturing filters for smoking articles, in particular filter cigarettes, with the following properties: - the filter paper comprises fibers comprising pulp fibers, - at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight and highly particularly preferably 100% by weight of the filter paper is formed by long-fiber pulp fibers, - of the fibers, a proportion with respect to the number of the 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, measured in accordance with ISO 2965:2009, is between 1000 cm·min-1·kPa-1 and 9000 cm·min-1·kPa-1, - the number-averaged length of the fibers in the filter paper is greater than 1 mm and less than 5 mm, preferably greater than 2 mm and less than 4 mm, and - the number-averaged 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.

2. Filter paper according to claim 1, in which the pulp fibers are bleached, unbleached or form a mixture of bleached and unbleached pulp fibers.

3. Filter paper according to claims 1 or 2, in which the long-fiber pulp is sourced from coniferous wood, in particular spruce or pine, or from hemp, flax, sisal, abacá, cotton, ramie, jute, kenaf, gampie, kozu or matsumata.

4. Filter paper according to one of the preceding claims, which consists of at most 10% by weight, preferably at most 5% by weight and particularly preferably by at most 2% by weight of short-fiber pulp fibers with respect to the mass of the filter paper, wherein said short-fiber pulp fibers are preferably sourced from deciduous wood, in particular birch, beech or eucalyptus, or from esparto grass.

5. Filter paper according to one of the preceding claims, which consists of at most 10% by weight, preferably at most 5% by weight, particularly preferably at most 2% by weight of filler materials with respect to the mass of the filter paper, wherein said filler materials are preferably selected from the group consisting of carbonates, sulfates, silicates or oxides, in particular calcium carbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate, titanium dioxide, talcum, kaolin or aluminum hydroxide, and mixtures thereof.

6. Filter paper according to one of the preceding claims, which contains pigments or colorants, in particular iron oxides or a mixture of iron oxides, and / or which contains a sizing agent, in particular alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), a hydrophobic substance, in particular one or more fatty acids or one or more fatty alcohols, starch or wet strength agents.

7. Filter paper according to one of the preceding claims, wherein the tensile strength in the machine direction is at least 7 N / 15 mm, preferably at least 8 N / 15 mm and particularly preferably at least 9 N / 15 mm and / or at most 50 N / 15 mm, preferably at most 45 N / 15 mm and particularly preferably at most 40 N / 15 mm, and / or wherein the tensile strength in the cross direction is at least 4 N / 15 mm, preferably at least 5 N / 15 mm and / or at most 9 N / 15 mm, preferably at most 8 N / 15 mm.

8. Filter paper according to one of the preceding claims, wherein the elongation at break in the machine direction is at least 1%, preferably at least 1.2% and / or at most 2%, preferably at most 1.8%, and / or wherein the elongation at break in the cross direction is at least 5%, preferably at least 4.5% and / or at most 6%, preferably at most 5.5%, or which is crimped in the machine direction or in the cross direction, and wherein the elongation at break in the direction or the directions in which the filter paper is crimped is at most 25%, preferably at most 15% and particularly preferably at most 10%.

9. Filter paper according to one of the preceding claims, wherein the basis weight is between 10 and 80 g·m-2, preferably between 20 and 60 g·m-2 and highly particularly preferably between 30 and 40 g·m-2, and / or wherein the thickness is between 60 µm and 160 µm, preferably between 80 µm and 120 µm.

10. Process for manufacturing a filter paper for the manufacture of filters for smoking articles, in particular filter cigarettes, comprising the following steps: (A) Providing an aqueous suspension of unrefined fiber material, wherein the unrefined fiber material comprises unrefined long-fiber pulp fibers and the proportion of unrefined long-fiber pulp fibers is at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight and highly particularly preferably 100% by weight of the unrefined fiber material and the aqueous suspension contains between 1% by weight and 5% by weight unrefined fiber material, (B) refining the fiber material in the aqueous suspension from step (A) or the fiber material from a separated part of the aqueous suspension from step (A) in a Papillon refiner with a refining energy between 30 W·h·kg-1 and 100 W·h·kg-1 with respect to the mass of the unrefined fiber material and a specific refining edge load of between 0.3 J·m-1 and 1.0 J·m-1, and (C) supplying the suspension from step (B) to a paper machine, preferably an inclined-wire machine, and forming a filter paper in the paper machine, wherein the number-averaged length of the fibers in the suspension supplied in step (C) is greater than 1 mm and less than 5 mm, preferably greater than 2 mm and less than 4 mm, and wherein the number-averaged width of the fibers of the suspension supplied in step (C) is between 10 µm and 50 µm, preferably between 20 µm and 40 µm, particularly preferably between 25 µm and 35 µm.

11. Process according to claim 10, in which between step (B) and (C), a further suspension is added to the suspension which has undergone refining in step (B), and which contains unrefined long-fiber pulp fibers, so that in the mixture of both suspensions, at least 40% by weight, preferably at least 50% by weight and particularly preferably at least 60% by weight of the long-fiber pulp fibers are refined, wherein said further suspension is preferably formed by the separated part of the suspension provided in step (A), and / or in which the step (B) for refining the fiber material is carried out and optionally, fibers with a length of at most 0.2 mm are added between step (B) and (C) such that the suspension supplied in step (C) has a proportion of fibers with a length of less than 0.2 mm of between 2% and 10%, preferably between 3% and 9% and particularly preferably between 4% and 8% each with respect to the number of fibers, and / or which further comprises a step (D) of crimping the filter paper in the machine direction and / or in the cross direction.

12. Process according to one of claims 10 or 11 for manufacturing a filter paper according to one of claims 1 to 9.

13. Cigarette filter which is manufactured at least in part from a filter paper according to one of claims 1 to 9.

14. Cigarette filter according to claim 13, which is present in one of the following forms: - as filter rod with a length of 60 mm to 200 mm, preferably a length of 80 mm to 180 mm, from which filter plugs or filter plug segments can be manufactured, - as filter plug with a length of 10 mm to 50 mm, preferably 15 mm to 30 mm, or - as filter plug segment with a length of 3 mm to 10 mm, preferably 4 mm to 8 mm, and / or with a diameter of 3 mm to 10 mm, preferably 4 mm to 9 mm and particularly preferably 7 mm to 9 mm, and / or with a specific draw resistance of 10 Pa·mm-1 to 40 Pa·mm-1, preferably of 15 Pa·mm-1 to 35 Pa·mm-1.

15. Filter cigarette comprising a tobacco rod and a cigarette filter according to one of claims 13 or 14, wherein the cigarette filter is formed by a filter plug according to claim 14 or contains a filter plug segment according to claim 14, wherein the cigarette filter preferably has a filtration efficiency for NFDPM of 20% to 80%, preferably of 30% to 70%, and / or wherein the cigarette filter is a segmented cigarette filter, which comprises a filter plug segment according to claim 21 and a filter plug segment from cellulose acetate, wherein the filter plug segment from cellulose acetate is located at the mouth end of the cigarette.

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