Filter element for an aerosol-generating device with varying porosity

Abstract

The invention relates to a filter element (10) comprising a filtration material. The filtration material comprises a foam (12). The foam is configured such that a porosity of the foam decreases in a radial direction of the filter element.

Description

[0001] Philip Morris Products S.A.

[0002] -1-

[0003] FILTER ELEMENT FOR AN AEROSOL-GENERATING DEVICE WITH VARYING POROSITY

[0004] The present invention relates to a filter element for an aerosol-generating article. The present invention relates to an aerosol-generating article. The present invention relates to an aerosol-generating system. The present invention relates to a method of manufacturing the filter element.

[0005] It is known to provide an aerosol-generating device for generating an inhalable vapor. Such devices may heat aerosol-forming substrate to a temperature at which one or more components of the aerosol-forming substrate are volatilised without burning the aerosolforming substrate. Aerosol-forming substrate may be provided as part of an aerosol-generating article. The aerosol-generating article may have a rod shape for insertion of the aerosolgenerating article into a cavity, such as a heating chamber, of the aerosol-generating device. A heating element may be arranged in or around the heating chamber for heating the aerosolforming substrate once the aerosol-generating article is inserted into the heating chamber of the aerosol-generating device. The article may comprise a filter element. The filter element may comprise a plastic material, such as cellulose acetate tow.

[0006] It would be desirable to provide a filter element offering improved sustainability. It would be desirable to provide a filter element with reduced environmental impact. It would be desirable to provide a filter element having improved filtration properties. It would be desirable to provide a filter element having improved filtration efficiency. It would be desirable to provide a filter element with tuneable filtration properties. It would be desirable to provide a filter element which can be cost efficiently produced. It would be desirable to provide a filter element with tuneable airflow properties. It would be desirable to provide a filter element with improved airflow properties. It would be desirable to provide a filter element with improved air flow management.

[0007] According to an embodiment of the invention there is provided a filter element which may comprise a filtration material. The filtration material may comprise a foam. The foam may be configured such that a porosity of the foam decreases in a radial direction of the filter element.

[0008] According to an embodiment of the invention there is provided a filter element comprising a filtration material. The filtration material comprises a foam. The foam is configured such that a porosity of the foam decreases in a radial direction of the filter element.

[0009] The filter element may offer an improved sustainability. The filter element may have a reduced environmental impact. The filter element may have improved filtration properties. The filter element may have improved filtration efficiency. The filter element may have tuneable filtration properties. The filter element may be cost efficiently produced. The filter element may have tuneable airflow properties. The filter element may have improved airflow properties. The filter element may replace conventional cellulose acetate tow filter elements. The filter element may have a comparable filtration performance to conventional cellulose acetate tow filter elements. The filter element may have an improved air flow management.

[0010] The filter element may be for an aerosol-generating article.

[0011] The filtration material may be free of cellulose acetate. The foam may be free of cellulose acetate.

[0012] The radial direction may refer to a direction substantially orthogonal to a longitudinal axis of the filter element.

[0013] The foam of the filter element may be obtained by compressing at least a portion of a foam. The foam of the filter element may be obtained by compressing at least a portion of a foam of uniform porosity. The foam of the filter element may be obtained by compressing at a cuboid-shaped foam of uniform porosity into a cylindrically-shaped foam of substantially circular cross-section. The foam of the filter element may be obtained by compressing at least a peripheral portion of a cuboid-shaped foam of uniform porosity such that the foam is compressed into a cylindrical shape of substantially circular cross-section. The foam of the filter element may be obtained by compressing at least a portion arranged at the periphery of a cuboid-shaped foam of uniform porosity such that the foam is shaped into a cylindrical shape of substantially circular cross-section.

[0014] The foam may be configured such that a density of the foam increases in a radial direction of the filter element.

[0015] The foam may be porous. The foam may be an open-cell foam. The foam may be a dry foam. The foam may be a solid foam.

[0016] A dry foam may refer to a material formed from pockets filled with air or other gas in a solid. A dry foam may refer to a material that has been formed from a wet foam by drying the wet foam.

[0017] A wet foam may refer to a material formed from pockets of air or other gas in a liquid.

[0018] An open-cell foam may refer to a material comprising fluidly interconnected pockets of air or other gas in a solid.

[0019] The foam may be produced by preparing a wet foam from a dispersion and subsequently removing the liquid dispersion medium. The dispersion may be a dispersion of cellulosic material in a liquid dispersion medium. The dispersion may be a dispersion of regenerated cellulose. The wet foam may be produced from the dispersion by agitating the dispersion or otherwise introducing bubbles of air into the dispersion. The dispersion medium may be removed by evaporation, freeze drying or supercritical drying.

[0020] The foam may be biodegradable. The foam may be recyclable.

[0021] The filtration material may have a cylindrical shape. The filtration material may be a rod. The filtration material may be configured as a solid cylindrical rod. The rod may have a substantially circular cross-section. The rod may have a skewed circular cross-section. The filtration material may have a substantially circular cross-section. The filtration element may have a skewed circular cross-section. The foam may have a cylindrical shape. The foam may be configured as a rod. The foam may be configured as a solid cylindrical rod. The rod may have a substantially circular cross-section. The rod may have a skewed circular cross-section. The foam may have a substantially circular cross-section. The foam may have a skewed circular cross-section. The foam in the filter element may be a partially compressed foam. A peripheral portion of the foam in the filter element may be compressed. The filter element may comprise a core made from the filtration material. The filter element may comprise a core made from the foam. The core may be solid cylindrically-shaped. The core may be a rod. The rod may have a substantially circular cross-section. The core may have a substantially circular cross-section. The core may have a skewed circular cross-section.

[0022] The filter element may comprise a proximal end. The filter element may comprise a distal end. The filtration material may be arranged between the proximal end of the filter element and the distal end of the filter element. The proximal end of the filter element and the distal end of the filter element may be fluidly connected via the filtration material. The proximal end of the filter element and the distal end of the filter element may be fluidly connected via the core of filtration material. The proximal end of the filter element and the distal end of the filter element may be fluidly connected via the foam. The filtration material may extend between the proximal end of the filter element and the distal end of the filter element. The foam may extend between the proximal end of the filter element and the distal end of the filter element.

[0023] “Porosity" may refer to the volume fraction of void space in a material or component. “Porosity" may refer to the volume fraction of void space in the foam. For example, if 60% of a volume of the foam were unfilled by solid material, the foam would have a porosity of 0.6. “Porosity" may refer to the volume fraction of void space in filter element.

[0024] The decrease of the porosity of the foam may vary between different radial directions. A gradient of the decrease of the porosity of the foam may vary between different radial directions. A difference between the porosity at the periphery of the foam and at the centre of the foam may vary between different radial directions.

[0025] The airflow characteristics may depend on the porosity of the foam. The aerosol flow characteristics may depend on the porosity of the foam. The speed of the flow may increase with increasing porosity. The speed of the flow through a region of the foam having a higher porosity may be increased in comparison to the speed of the flow through a region of the foam having a lower porosity. By varying the relative porosity of the foam along a radial direction of the foam of the filter element, the relative speeds of the flow may be adjusted. As the porosity of the foam decreases in a radial direction of the filter element, aerosol may travel through peripheral regions of the foam slower than through central regions of the foam. The foam may be configured such that the flow speed through the foam decreases in a radial direction of the filter element. The flow speed may be a flow speed parallel to a longitudinal axis of the filter element.

[0026] The characteristics of the interaction of the foam of the filter element and the aerosol flow or air flow may depend on the speed of the flow. The characteristics of the interaction of a portion of the foam and the aerosol flow or air flow may depend on the speed of the flow through the portion of the foam. The efficiency with which a compound is removed by the foam from an aerosol flow or air flow may depend on the flow speed. For example, a first compound may be removed more efficiently at lower flow speeds, while a second compound may be removed more efficiently at higher flow speeds. By providing the foam in which the porosity of the foam decreases in a radial direction, compounds may be removed more effectively from the aerosol. By providing the foam in which the porosity of the foam decreases in a radial direction, a greater variety of compounds may be removed from the flow with improved efficiency.

[0027] The filtering performance of the filtration material in the filter element may be tuned by varying one or more of the difference in porosity at the periphery and the center of the foam, the gradient of the decrease in the porosity of the foam in a radial direction of the filter element, the foam material, the foam structure and the compression force acting on the foam. The filtration profile of the filtration material in the filter element may be tuned by varying one or more of the difference in porosity at the periphery and the center of the foam, the gradient of the decrease in the porosity of the foam in a radial direction of the filter element, the foam material, the foam structure and the compression force acting on the foam. The compression of at least portions of the foam may result in a decrease of the porosity of that portion of the foam. The compression of at least portions of the foam may result in an increase of the density of that portion of the foam.

[0028] The filtration material may consist of the foam.

[0029] The porosity at a center of the foam may be higher than the porosity at a periphery of the foam.

[0030] A “center of the foam” may refer to a region of the foam arranged at a central longitudinal axis of the filter element. A “center of the foam” may refer to the central longitudinal axis of the foam. A “center of the foam” may refer to the center of a cross-section of the foam. The porosity of the foam at the center of the foam may refer to the porosity at the central longitudinal axis of the filter element. The center of the foam may correspond to the center of the filter element.

[0031] The “periphery of the foam” may refer to a region of the foam arranged at an outer surface of the foam. The “periphery of the foam” may refer to a region of the foam arranged at the boundary of foam. The porosity of the foam at the periphery of the foam may refer to the porosity at an outer surface of the foam. The porosity of the foam at the periphery of the foam may refer to the porosity at the boundary of the foam.

[0032] The porosity of the foam may be constant along a longitudinal axis of the filter element. The density of the foam may be constant along a longitudinal axis of the filter element.

[0033] The density at the centre of the foam may be higher than the density at the periphery of the foam.

[0034] The periphery of the foam may be compressed. The foam may be compressed at the periphery of the foam. The centre of the foam may not be compressed. The difference in the porosity at the centre of the foam and at the periphery of the foam may result from the periphery of the foam being compressed. The foam may be compressed along a radial direction of the filter element.

[0035] A ratio of the porosity at the periphery of the foam and the porosity at the center of the foam may be of between 1 :1.3 to 1 :2.67, preferably of between 1 :1.5 and 1 :2.1 , more preferably of about 1 : 1.75.

[0036] The porosity at the periphery of the foam may be of between 0.3 and 0.5. The porosity at the center of the foam may be of between 0.6 and 0.8.

[0037] The foam may be a foam of cellulosic material. The foam may be a foam of regenerated cellulose material. The foam may be a foam of regenerated cellulose.

[0038] The foam may be foamed cellulosic material. The cellulosic material may be regenerated cellulose.

[0039] The foam may be a cellulose-based foam. The foam may be a regenerated cellulose foam. The regenerated cellulose material may be free of cellulose acetate. The foam may comprise the regenerated cellulose material. The foam may be made of a regenerated cellulose material. The regenerated cellulose material may comprise regenerated cellulose fibers. The foam may comprise regenerated cellulose fibres. The foam may be made of regenerated cellulose fibres. The foam may be made from foamed regenerated cellulose. The foam may be made from non-woven regenerated cellulosic fibers. The regenerated cellulose fibres may be obtained by dissolving pure cellulose from one or both of wood pulp and plant fibres. The regenerated cellulose fibres may be obtained by dissolving derivative cellulose from one or both of wood pulp and plant fibres.

[0040] The foam may be regenerated cellulose material tow. The foam may be regenerated cellulose fiber tow. The foam may comprise bundles of regenerated cellulose fibers.

[0041] The foam may have a density of between 15 kg / m3and 45 kg / m3, preferably of between 20 kg / m3and 40 kg / m3. The density of the foam may be determined according to ISO 845.

[0042] The foam may have a paper mass content of between 70 weight-% and 95 weight-%, preferably of between 75 weight-% and 90 weight-%.

[0043] The foam may comprise a biopolymer in an amount of between 10 weight-% and 25 weight %, preferably of between 15 weight-% and 20 weight-%. The biopolymer may comprise a polyhydroxyalkanoate (PHA). The polyhydroxyalkanoate may be polyhydroxy butyrate. The foam may comprise a polyhydroxyalkanoate in an amount of between 10 weight-% and 25 weight %, preferably of between 15 weight-% and 20 weight-%. The foam may comprise a polyhydroxy butyrate in an amount of between 10 weight-% and 25 weight %, preferably of between 15 weight-% and 20 weight-%.

[0044] The foam may comprise a biodegradable thermoplastic. The foam may be made from a biodegradable thermoplastic. Polyhydroxy butyrate (PHB) may be produced from sugar or from carbon dioxide using cyanobacteria.

[0045] An indentation force deflection (IFD) test value of the foam may be determined according ASTM D 3574. The characterization according to ASTM D 3574 may be carried out using 65 % feed. The indentation force deflection (IFD) test value of the foam may be of between 380 Newton and 570 Newton, preferably of between 410 Newton and 500 Newton. A hardness of the foam may be determined according to ISO 2439 Method B. The hardness may be of between 75 Newton and 105 Newton, preferably of between 80 Newton and 100 Newton. A compression set value of the foam may be determined according to ISO 1856 Method C1. The compression set value of the foam may be determined after 2.5 hours. The compression set value of the sheet of foam may be of between 20 percent and 40 percent, preferably of between 25 percent and 35 percent. A thermal conductivity of the foam may be determined according to ISO 8302. The thermal conductivity of the of the foam may be of between 0.025 W / mK and 0.05 W / mK, preferably of between 0.03 W / mK and 0.04 W / mK. The above test may be carried out using a sheet of the foamed material.

[0046] The foam may be based on one or more of a wood pulp, a bamboo pulp, pulps obtained from tobacco stems, and hybrid foams of nanocellulose and kaolin-microfibrillated cellulose composites. The foam may be a biocomposite foam. The foam may be a nanocellulose-based foam. The foam may be a hybrid foam. The foam may be a hybrid foam of nanocellulose and kaolin- microfibrillated cellulose composites. The foam may be a polyhydroxyalkanoate-based foam. The foam may be a foam based on polyhydroxy butyrate. The foam may be a dry foam. The foam may be a solid foam.

[0047] The foam may be a viscose foam. The foam may be made from viscose fibers. The filtration material may be free of cellulose acetate.

[0048] The foam may be free of cellulose acetate. The filter element may be free of cellulose acetate.

[0049] The filter element may comprise a filter element wrapper. The filter element wrapper may be configured to at least partially circumscribe the filtration material. The filter element wrapper may be configured to fully circumscribe the filtration material.

[0050] The filter element wrapper may at least partially circumscribe the foam. The filter element wrapper may fully circumscribe the foam. The filter element wrapper may circumscribe the foam. The filter element wrapper may be wrapped around the filtration material. The filter element wrapper may be wrapped around the foam. The filter element wrapper may be wrapped around the core of foam. The filter element wrapper may have substantially the same length as the foam in the filter element. The filter element wrapper may be configured to at least partially surround the filtration material. The filter element wrapper may be configured to fully surround the filtration material.

[0051] The filter element may consist of the filter element wrapper and the foam. The filter element may consist of the filtration material wrapped by the filter element wrapper.

[0052] The filter element wrapper may be configured to stabilise the filtration material. The filter element wrapper may be configured to protect the filtration material. The filter element wrapper may be configured to stabilize the foam. The filter element wrapper may be configured to stabilize the shape of the foam. The filter element wrapper may be configured to stabilize the compression of the foam. The filter element wrapper may be configured to hold the foam in the desired arrangement. The filter element wrapper may be configured to stabilize the arrangement the foam. The filter element wrapper may be configured to compress the foam.

[0053] The filtration material may be arranged around a longitudinal axis of the filter element. The filter element wrapper may be coaxially arranged around the filtration material.

[0054] The filter element wrapper may be permanently attached to the filtration material. The filter element wrapper may be glued to the filtration material by a starch glue.

[0055] The filter element wrapper may have a permeability of between 3500 Coresta Units and 12000 Coresta Units. The filter element wrapper may have a thickness of between 0.2 micrometers and 0.4 micrometers.

[0056] The filter element wrapper may be made from a paper material.

[0057] The filter element wrapper may be biodegradable. The filter element wrapper may be recyclable.

[0058] The filter element wrapper may have a basis weight of between 10 g / m and 30 g / m, preferably of between 17 g / m and 21 g / m.

[0059] The filter element wrapper may be a 78 g / m2paper. The filter element wrapper may be a 100 g / m2paper.

[0060] The filter element may comprise a tipping paper. The tipping paper may be configured to at least partially circumscribe the filter element wrapper. The tipping paper may be configured to fully circumscribe the filter element wrapper.

[0061] The tipping paper may be configured to at least partially circumscribe the filtration material. The tipping paper may be configured to fully circumscribe the filtration material. The tipping paper may be configured to at least partially circumscribe the foam. The tipping paper may be configured to fully circumscribe the foam.

[0062] The tipping paper may be wrapped around the filtration material. The tipping paper may be wrapped around the foam. The tipping paper may be coaxially arranged around the filter element wrapper.

[0063] The tipping paper may have a textured surface. The tipping paper may have a textured outer surface. The tipping paper may have an embossed surface. The tipping paper may have an embossed outer surface. The tipping paper may have a smooth surface. The tipping paper may have a smooth outer surface.

[0064] The tipping paper may have a permeability of between 380 Coresta Units and 550 Coresta Units.

[0065] The tipping paper may have a thickness of between 0.5 micrometers and 1.1 micrometers.

[0066] The tipping paper may have a basis weight of between 25 g / m and 60 g / m, preferably of between 30 g / m and 50 g / m.

[0067] The filter element wrapper may be configured to compress at least a peripheral portion of the foam, such as to facilitate the shape of the foam.

[0068] The filter element wrapper may be configured to compress at least a peripheral portion of the foam, such that the foam adapts a cylindrical shape. The filter element wrapper may be configured to compress at least a peripheral portion of the foam, such that the foam adapts a cylindrical shape having a substantially circular cross-section. The filter element may be cylindrically-shaped. The filter element may be a rod. The filter element may be a rod having a substantially circular cross-section. The filter element may have a cylindrical shape. The filter element may have a solid cylindrical shape. The filter element may have a substantially circular cross-section. The filter element may have a skewed circular cross-section.

[0069] The filter element wrapper may be configured to compress the foam into a cylindrical rod of substantially circular cross-section. The foam of the filter element may be derived from a cuboid-shaped foam of uniform density. The filter element wrapper may be configured to compress the foam into a cylindrical rod of skewed circular cross-section.

[0070] The filter element wrapper may be configured to compress the foam having a cuboid shape into a cylindrical rod. The filter element wrapper may be configured to compress the cuboid-shaped foam into a rod having a substantially circular cross-section.

[0071] The foam may comprise a high porosity region. The foam may comprise a low porosity region. The high porosity region may be arranged at the centre of the foam. The low porosity region may be configured at the periphery of the foam. The low porosity region may comprise four substantially spindle-shaped lobes. The high porosity region may be substantially astroidshaped. The high porosity region may be substantially lozenge-shaped. The high porosity region may be arranged around the central longitudinal axis of the filter element. The low porosity region may be coaxially arranged around the high porosity region. The low porosity region may be a region of the foam compressed by the filter element wrapper. The low porosity region may comprise the portions of the foam that are compressed by the filter element wrapper. The low porosity region may consist of the portions of the foam that are compressed by the filter element wrapper. The high porosity region may abut the low porosity region. The high porosity region may comprise the portions of the foam which are not compressed by the filter element wrapper. The high porosity region may consist of the portions of the foam which are not compressed by the filter element wrapper. The porosity of the low porosity region may vary across the low porosity region. The variation of the porosity of the low porosity region may result from different parts of the low porosity region being compressed to different extents.

[0072] The porosity of the low porosity region may be of between 0.3 and 0.6. The porosity of the high porosity region may be greater than 0.6 and lower than or equal to 0.8.

[0073] The foam may be configured such that the porosity varies around the periphery of the foam. Preferably, the porosity varies in a regular manner around the periphery of the foam.

[0074] The filter element wrapper may be configured to compress a cuboid-shaped foam of uniform porosity and square cross-section into a cylindrical rod of substantially circular crosssection such that the porosity of the foam decreases in a radial direction of the filter element. The cross-section of the cuboid-shaped foam may extend in plane extending perpendicular to a central longitudinal axis of the cuboid-shaped foam.

[0075] A ratio of a side length of the square cross-section of the cuboid-shaped foam and an outer diameter of the cylindrical rod of substantially circular cross-section may be of between 1 and 1.5, preferably of between 1 and 1.3, and more preferably about 1.

[0076] The side length of the square cross-section of the cuboid-shaped foam may be of between 7 millimetres and 11 millimetres, preferably of between 8 millimetres and 10 millimetres.

[0077] The foam may be configured such that the porosity varies around a circumference of the foam. Preferably, the porosity varies in a regular manner around the circumference of the foam. Preferably, the circumference is an outer circumference of the foam.

[0078] The circumference may comprise a plurality of points of maximum porosity. The points of maximum porosity may be equidistantly spaced along the circumference. The circumference may comprise a plurality of points of minimum porosity. The points of minimum porosity may be equidistantly spaced along the circumference. Preferably, the porosity of the points of maximum porosity is identical. Preferably, the porosity of the points of minimum porosity is identical.

[0079] The circumference may comprise four of points of maximum porosity. The circumference may comprise four points of minimum porosity. The foam of the filter element may be derived from a cuboid-shaped foam of square cross-section by compressing the cuboid-shaped foam radially inwards. The cuboid-shaped foam may have uniform density.

[0080] The circumference of the foam may comprise three of points of maximum porosity. The circumference of the foam may comprise three points of minimum porosity. The foam of the filter element may be derived from a rod-shaped foam of triangular cross-section by compressing the cuboid-shaped foam radially inwards. The rod-shaped foam may have uniform density.

[0081] The filter element may consist of the filter element wrapper and the foam. The filter element may consist of the filter element wrapper, the foam, and the tipping paper.

[0082] The foam may be configured such that a porosity of the foam gradually decreases in a radial direction of the filter element. The foam may be configured such that the porosity of the foam continuously decreased in radial direction of the filter element. The foam may be configured such that the porosity of the foam smoothly decreases in a radial direction of the filter element.

[0083] The filter element may have a length of between 5 millimeters and 21 millimeters, preferably of between 7 millimeters and 11 millimeters. The filter element may have an outer diameter of between 3 millimeters and 10 millimeters, preferably of between 4 millimeters and 9 millimeters. The filter element may have a diameter of about 8 millimeters. The filter element may have a diameter of about 10 millimeters.

[0084] The filter element wrapper may be configured to compress the foam into a cylindrical rod of substantially circular cross-section. The foam of the filter element may be derived from a cuboid-shaped foam of uniform density. The foam of the filtration material may be a cuboidshaped foam of uniform density compressed into a cylindrical rod of substantially circular cross-section. The cuboid-shaped foam of uniform density may be compressed into the cylindrical rod of substantially circular cross-section by the filter element wrapper. The crosssection of the cuboid-shaped foam may be a square. The square may have sides of between 7 millimeters and 11 millimeters length, preferably of between 8 millimeters and 10 millimeters length.

[0085] In an embodiment of the invention, the filter element may comprise the foam of regenerate cellulose and the filter element wrapper. In the filter element, the filter element wrapper may be tightened around the foam to compress at least a peripherally arranged portion of the foam. The compression of the foam may result in the foam having a porosity of the foam decrease in a radial direction of the filter element. Preferably, the foam of the filter element may be obtained by compressing a cuboid-shaped foam having a uniform porosity into a cylindrically-shaped foam having a substantially circular cross-section and having a porosity of the foam decrease in a radial direction of the filter element. The foam may be compressed to the greatest extent along the edges of the cuboid-shaped foam from which the foam of the filter element is obtained which extend parallel to the longitudinal axis of the filter element.

[0086] The foam may comprise a softening agent. The foam may have a tear resistance of above 0.8 daN / cm2. An absorption of the foam may be about twenty times the dry weight of the foam. The foam may comprise one or more pigments. The foam may be free of a binder.

[0087] Viscose foam may be manufactured in several steps. In a first step, cellulosic material may be provided. In a second step, the cellulosic material may be milled. In a third step, the cellulosic material may be chemically transferred into a viscose dough mixture. In a fourth step, one or more of fibers, salts and pigment may be added to the mixture. In a fifth step, the viscose may be coagulated and solidified using heat. In a sixth step, the cellulosic material may be regenerated. In a seventh step, soda sulphate crystals may be dissolved. In an eight step, resulting material may be centrifuged and dried to obtain the viscose foam. The regenerated cellulose fibers may be dispersed in a liquid medium. The regenerated cellulose fibers may be extruded. The regenerated cellulose fibers may be coagulated to form a cuboid-shaped foam.

[0088] The foam may be a foamed cellulosic material. The foam may be a foam of cellulosic material. The foam may be manufactured according the method of manufacturing a sheet of foamed cellulosic material as described in European patent application 23206185.3, which is herewith incorporated by reference. The foam may be the filtration material used in the aerosolgenerating article disclosed in European patent application 23206185.3, which is herewith incorporated by reference. The foam may be the foamed cellulosic material used in the aerosol-generating article disclosed in European patent application 23206185.3, which is herewith incorporated by reference. A sheet of the foamed cellulosic material may be manufactured using a method comprising the following steps: a) preparing a suspension of cellulosic material, wherein the suspension comprises a foaming agent, b) agitating the suspension of cellulosic material and, optionally, adding hydrophobizing agent to the suspension of cellulosic material to obtain a wet foam of cellulosic material, c) preparing a sheet of the wet foam of cellulosic material and dewatering the sheet of wet foam of cellulose material, d) drying the sheet of wet foam of cellulosic material, e) rewetting the sheet of foam f) adjusting a thickness of the sheet of foam, and g) drying of the foam.

[0089] The method may yield a sheet of dry foamed cellulosic material. The method may yield a sheet of dry foam of cellulosic material.

[0090] The suspension of cellulosic material may be a suspension of pulp. The suspension of cellulosic material may be a suspension of wood pulp. The suspension of cellulosic material may be a suspension of cellulose. The suspension of cellulosic material may be a suspension of refined cellulose. The suspension of cellulosic material may be a suspension of cellulosic fibers. The suspension of cellulosic material may be a suspension of cellulose fibers.

[0091] The cellulosic material may be cellulose. The cellulosic material may be cellulosic fibers. The cellulosic material may be cellulose fibers. The cellulosic material may comprise cellulose. The cellulosic material may comprise cellulosic fibers. The cellulosic material may comprise cellulose fibers. The cellulosic material may be free of cellulose acetate.

[0092] The cellulosic material may be based on one of bleached softwood pulp, unbleached softwood pulp, bleached eucalyptus pulp and bleached cotton linter pulp, or any combinations thereof. The cellulosic material may be derived from one of bleached softwood pulp, unbleached softwood pulp, bleached eucalyptus pulp and bleached cotton linter pulp, or any combinations thereof.

[0093] The foaming agent may comprise one or more surfactants. Preferably, the foaming agent may be a mixture comprising sodium dodecyl sulphate (SDS) in an amount of about 80 mol-% and polyoxyethylene (20) sorbitan monolaurate in an amount of about 20 mol-%.

[0094] Preferably, the hydrophobizing agent may be alkyl ketene dimer (AKD). The hydrophobizing agent may be added to the suspension of cellulosic material at an amount of about 1 weight-% of the suspension.

[0095] In step a), the cellulosic material may be refined to between 2 percent and 3 percent. In step a), the conductivity of the suspension of cellulosic material may be adjusted to 1000 pS / cm by adding sodium chloride. In step a), the pH of the suspension of cellulosic material may be adjusted to about 8 by adding sodium hydroxide. In step a), a strength additive may be added to the suspension of cellulosic material. The strength additive may be selected from one or more of sodium carboxymethlycellulose (CMC), microfibrillated cellulose and cellulose nanofibrils.

[0096] In step c), the wet foam of cellulosic material may be poured on a hand sheet mould. In step c), the sheet of wet foam of cellulosic material may drained by gravity. In step c), the dewatering may not be performed using a vacuum. In step c), a fabric may arrange on top of the foam to stabilize the foam. In step d) the drying may be performed in an oven at 70 degrees Celsius.

[0097] In step e), the foam may be rewetted to 50 percent moisture content. In step e), the sheet of foam may be rewetted by exposing the sheet to an atmosphere having 50 % moisture content for 4 hours.

[0098] In step g), the foam may be dried in an oven at 70 degrees Celsius. In step g), the foam may be cured at about 80 degrees Celsius for 2 hours to ensure complete reaction of AKD.

[0099] The foamed cellulosic material may be a foam of cellulosic material. The foamed cellulosic material may be a dry foam of cellulosic material. The foamed cellulosic material may be a solid foam of cellulosic material. The foamed cellulosic material may be an open-cell foam. The foamed cellulosic material may be free of cellulose acetate. The foamed cellulosic material may be porous. A density of the foam of cellulosic material may be of between 45 kg / m3and 105 kg / m3.

[0100] The invention further relates to an aerosol-generating article comprising a filter element as described herein and an aerosol-forming substrate.

[0101] The filter element may be arranged downstream of the aerosol-forming substrate. The filter element may be arranged upstream of the aerosol-forming substrate. The filter element may be front plug. The filter element may be mouth plug. The filter element may be arranged at a distal end of the article. The filter element may be arranged at a proximal end of the article.

[0102] The invention further relates to an aerosol-generating system comprising an aerosolgenerating article as described herein and an aerosol-generating device.

[0103] The invention further relates to a method for manufacturing a filter element for an aerosolgenerating article. The method may comprise the following steps: a) providing a foam of substantially uniform porosity b) compressing at least a portion of the foam by wrapping a filter element wrapper around the foam, such that a porosity of the foam decreases in a radial direction of the filter element.

[0104] The invention further relates to a method for manufacturing a filter element for an aerosolgenerating article. The method comprises the following steps: a) providing a foam of substantially uniform porosity b) compressing at least a portion of the foam by wrapping a filter element wrapper around the foam, such that a porosity of the foam decreases in a radial direction of the filter element.

[0105] The foam of step a) may have a cuboid shape. In step b), the cuboid-shaped foam may be compressed into a rod by tightening the filter element wrapper around the cuboid-shaped foam. The cuboid-shaped foam may have a rectangular cross section. The cuboid-shaped foam may have a square cross-section. The cross-section may be a square having edges of about 8 millimetres length. The cross-section may be a square having edges of about 10 millimetres length.

[0106] The foam may be a foam of regenerated cellulose. The rod may have a substantially circular cross-section. The filtration material may consist of the foam.

[0107] The foam of step a) may have a uniform porosity.

[0108] The foam of step a) may be cuboid-shaped and may have a square cross-section.

[0109] In step b), the cuboid-shaped foam of square cross-section may be compressed into a rod of substantially circular cross-section such that a ratio of a side length of the square crosssection of the cuboid-shaped foam of step a) and an outer diameter of the rod of circular crosssection is of between 1 and 1.5, preferably of between 1 and 1.3, and more preferably about 1.

[0110] The side length of the square cross-section of the cuboid-shaped foam of step a) may be of between 7 millimetres and 11 millimetres, preferably of between 8 millimetres and 10 millimetres.

[0111] In step b), the foam of step a) may be compressed such that the porosity varies around a circumference of the foam. Preferably, the porosity varies in a regular manner around the circumference of the foam. Preferably, the circumference is an outer circumference of the foam.

[0112] The circumference may comprise a plurality of points of maximum porosity. The points of maximum porosity may be equidistantly spaced along the circumference. The circumference may comprise a plurality of points of minimum porosity. The points of minimum porosity may be equidistantly spaced along the circumference. Preferably, the porosity of the points of maximum porosity is identical. Preferably, the porosity of the points of minimum porosity is identical.

[0113] The circumference may comprise four of points of maximum porosity. The circumference may comprise four points of minimum porosity.

[0114] After step b) the wrapped foam may be cut to a desired filter element length.

[0115] The invention further relates to a filter element for an aerosol-generating article produced by the method for manufacturing a filter element for an aerosol-generating article described herein.

[0116] As used herein, the terms ‘proximal’, ‘distal’, ‘downstream’ and ‘upstream’ may be used to describe the relative positions of components, or portions of components, of the aerosolgenerating device in relation to the direction in which a user draws on the aerosol-generating device during use thereof.

[0117] As used herein, the terms ‘proximal’, ‘distal’, ‘downstream’ and ‘upstream’ may be used to describe the relative positions of components, or portions of components, of the aerosolgenerating article in relation to the direction in which a user draws on the aerosol-generating article during use thereof.

[0118] The aerosol-generating device may comprise a mouth end through which in use an aerosol exits the aerosol-generating device and is delivered to a user. The mouth end may also be referred to as the proximal end. In use, a user may draw on the proximal or mouth end of the aerosol-generating device in order to inhale an aerosol generated by the aerosolgenerating device. The mouth end may comprise a mouthpiece. Alternatively, a user may directly draw on an aerosol-generating article inserted into an opening at the proximal end of the aerosol-generating device. The opening at the proximal end may be an opening of the cavity. The cavity may be configured to receive the aerosol-generating article. The aerosolgenerating device may comprise a distal end opposed to the proximal or mouth end. The proximal or mouth end of the aerosol-generating device may also be referred to as the downstream end and the distal end of the aerosol-generating device may also be referred to as the upstream end.

[0119] The aerosol-generating article may comprise a mouth end through which in use an aerosol exits the aerosol-generating article and is delivered to a user. The mouth end may also be referred to as the proximal end. The filter element as described herein may be arranged at the mouth end of the article. The aerosol-generating article may comprise a distal end opposed to the proximal or mouth end. The proximal or mouth end of the aerosol-generating article may also be referred to as the downstream end and the distal end of the aerosol-generating article may also be referred to as the upstream end. The filter element as described herein may be arranged proximal to the aerosol-forming substrate.

[0120] A longitudinal axis of a component may extend between a proximal end of the component and a distal end of the component. A longitudinal axis of the filter element may extend between a proximal end of the filter element and a distal end of the filter element. A longitudinal axis of the aerosol-generating article may extend between a proximal end of the article and a distal end of the article. A longitudinal axis of the aerosol-generating device may extend between the proximal end of the device and a distal end of the device.

[0121] As used herein, an ‘aerosol-generating device’ may relate to a device that interacts with an aerosol-forming substrate to generate an aerosol. The aerosol-generating article may comprise a mouthpiece. The aerosol-forming substrate may be part of the aerosol-generating article. An aerosol-generating device may be a smoking device that interacts with an aerosolforming substrate of the aerosol-generating article to generate an aerosol that is directly inhalable into a user’s lungs thorough the user's mouth. An aerosol-generating device may be a holder. The device may be an electrically heated smoking device. The aerosol-generating device may comprise a housing, electric circuitry, a power supply, a heating chamber and a heating element.

[0122] The aerosol-generating device may comprise electric circuitry. The electric circuitry may comprise a microprocessor, which may be a programmable microprocessor. The microprocessor may be part of a controller. The electric circuitry may comprise further electronic components. The electric circuitry may be configured to regulate a supply of power to the heating element. Power may be supplied to the heating element continuously following activation of the aerosol-generating device or may be supplied intermittently, such as on a puff- by-puff basis. The power may be supplied to the heating element in the form of pulses of electrical current. The electric circuitry may be configured to monitor the electrical resistance of the heating element, and preferably to control the supply of power to the heating element dependent on the electrical resistance of the heating element.

[0123] The aerosol-generating device may comprise a power supply, typically a battery, within a main body of the aerosol-generating device. In one embodiment, the power supply is a Lithium-ion battery. Alternatively, the power supply may be a Nickel-metal hydride battery, a Nickel cadmium battery, or a Lithium based battery, for example a Lithium-Cobalt, a Lithium- Iron-Phosphate, Lithium Titanate or a Lithium-Polymer battery. As an alternative, the power supply may be another form of charge storage device such as a capacitor. The power supply may require recharging and may have a capacity that enables to store enough energy for one or more usage experiences; for example, the power supply may have sufficient capacity to continuously generate aerosol for a period of around six minutes or for a period of a multiple of six minutes. In another example, the power supply may have sufficient capacity to provide a predetermined number of puffs or discrete activations of the heating element.

[0124] The cavity of the aerosol-generating device may have an open end into which the aerosol-generating article is inserted. The open end may be a proximal end. The cavity may have a closed end opposite the open end. The closed end may be the base of the cavity. The closed end may be closed except for the provision of air apertures arranged in the base. The base of the cavity may be flat. The base of the cavity may be circular. The base of the cavity may be arranged upstream of the cavity. The open end may be arranged downstream of the cavity. The cavity may have an elongate extension. The cavity may have a longitudinal central axis. A longitudinal direction may be the direction extending between the open and closed ends along the longitudinal central axis. The longitudinal central axis of the cavity may be parallel to the longitudinal axis of the aerosol-generating device.

[0125] The cavity may be configured as a heating chamber. The cavity may have a cylindrical shape. The cavity may have a hollow cylindrical shape. The cavity may have a shape corresponding to the shape of the aerosol-generating article to be received in the cavity. The cavity may have a circular cross-section. The cavity may have an elliptical or rectangular crosssection. The cavity may have an inner diameter corresponding to the outer diameter of the aerosol-generating article.

[0126] An airflow channel may run through the cavity. Ambient air may be drawn into the aerosol-generating device, into the cavity and towards the user through the airflow channel. Downstream of the cavity, a mouthpiece may be arranged or a user may directly draw on the aerosol-generating article. The airflow channel may extend through the mouthpiece.

[0127] In any of the aspects of the disclosure, the heating element may comprise an electrically resistive material. Suitable electrically resistive materials include but are not limited to: semiconductors such as doped ceramics, electrically "conductive" ceramics (such as, for example, molybdenum disilicide), carbon, graphite, metals, metal alloys and composite materials made of a ceramic material and a metallic material. Such composite materials may comprise doped or undoped ceramics. Examples of suitable doped ceramics include doped silicon carbides. Examples of suitable metals include titanium, zirconium, tantalum platinum, gold and silver. Examples of suitable metal alloys include stainless steel, nickel-, cobalt-, chromium-, aluminium- titanium- zirconium-, hafnium-, niobium-, molybdenum-, tantalum-, tungsten-, tin-, gallium-, manganese-, gold- and iron-containing alloys, and super-alloys based on nickel, iron, cobalt, stainless steel, Timetai® and iron-manganese-aluminium based alloys. In composite materials, the electrically resistive material may optionally be embedded in, encapsulated or coated with an insulating material or vice-versa, depending on the kinetics of energy transfer and the external physicochemical properties required.

[0128] As described, in any of the aspects of the disclosure, the heating element may be part of an aerosol-generating device. The aerosol-generating device may comprise an internal heating element or an external heating element, or both internal and external heating elements, where "internal" and "external" refer to the aerosol-forming substrate. An internal heating element may take any suitable form. For example, an internal heating element may take the form of a heating blade. Alternatively, the internal heater may take the form of a casing or substrate having different electro-conductive portions, or an electrically resistive metallic tube. Alternatively, the internal heating element may be one or more heating needles or rods that run through the center of the aerosol-forming substrate. Other alternatives include a heating wire or filament, for example a Ni-Cr (Nickel-Chromium), platinum, tungsten or alloy wire or a heating plate. Optionally, the internal heating element may be deposited in or on a rigid carrier material. In one such embodiment, the electrically resistive heating element may be formed using a metal having a defined relationship between temperature and resistivity. In such an exemplary device, the metal may be formed as a track on a suitable insulating material, such as ceramic material, and then sandwiched in another insulating material, such as a glass. Heaters formed in this manner may be used to both heat and monitor the temperature of the heating elements during operation.

[0129] An external heating element may take any suitable form. For example, an external heating element may take the form of one or more flexible heating foils on a dielectric substrate, such as polyimide. The flexible heating foils can be shaped to conform to the perimeter of the substrate receiving cavity. Alternatively, an external heating element may take the form of a metallic grid or grids, a flexible printed circuit board, a molded interconnect device (MID), ceramic heater, flexible carbon fiber heater or may be formed using a coating technique, such as plasma vapour deposition, on a suitable shaped substrate. An external heating element may also be formed using a metal having a defined relationship between temperature and resistivity. In such an exemplary device, the metal may be formed as a track between two layers of suitable insulating materials. An external heating element formed in this manner may be used to both heat and monitor the temperature of the external heating element during operation.

[0130] As an alternative to an electrically resistive heating element, the heating element may be configured as an induction heating element. The induction heating element may comprise an induction coil and a susceptor. In general, a susceptor is a material that is capable of generating heat, when penetrated by an alternating magnetic field. When located in an alternating magnetic field. If the susceptor is conductive, then typically eddy currents are induced by the alternating magnetic field. If the susceptor is magnetic, then typically another effect that contributes to the heating is commonly referred to hysteresis losses. Hysteresis losses occur mainly due to the movement of the magnetic domain blocks within the susceptor, because the magnetic orientation of these will align with the magnetic induction field, which alternates. Another effect contributing to the hysteresis loss is when the magnetic domains will grow or shrink within the susceptor. Commonly all these changes in the susceptor that happen on a nano-scale or below are referred to as “hysteresis losses”, because they produce heat in the susceptor. Hence, if the susceptor is both magnetic and electrically conductive, both hysteresis losses and the generation of eddy currents will contribute to the heating of the susceptor. If the susceptor is magnetic, but not conductive, then hysteresis losses will be the only means by which the susceptor will heat, when penetrated by an alternating magnetic field. According to the invention, the susceptor may be electrically conductive or magnetic or both electrically conductive and magnetic. An alternating magnetic field generated by one or several induction coils heat the susceptor, which then transfers the heat to the aerosol-forming substrate, such that an aerosol is formed. The heat transfer may be mainly by conduction of heat. Such a transfer of heat is best, if the susceptor is in close thermal contact with the aerosol-forming substrate.

[0131] As used herein, the term ‘aerosol-generating article’ may refer to an article comprising an aerosol-forming substrate that is capable of releasing volatile compounds that can form an aerosol. For example, an aerosol-generating article may be a smoking article that generates an aerosol that is directly inhalable into a user’s lungs through the user's mouth. An aerosolgenerating article may be disposable.

[0132] The aerosol-generating article may be a combustible cigarette. The aerosol-generating article may comprise a rod of tobacco and the filter element described herein. The tobacco may be burnt. A user may lit the cigarette to burn the tobacco.

[0133] As used herein, the term ‘aerosol-forming substrate’ may relate to a substrate capable of releasing one or more volatile compounds that can form an aerosol. Such volatile compounds may be released by heating the aerosol-forming substrate. An aerosol-forming substrate may conveniently be part of an aerosol-generating article or smoking article. In a combustible cigarette, the substrate may be burnt. The substrate may comprise tobacco.

[0134] The aerosol-forming substrate may be a solid aerosol-forming substrate. The aerosolforming substrate may comprise both solid and liquid components. The aerosol-forming substrate may comprise a tobacco-containing material containing volatile tobacco flavour compounds which are released from the substrate upon heating. The aerosol-forming substrate may comprise a non-tobacco material. The aerosol-forming substrate may comprise an aerosol former that facilitates the formation of a dense and stable aerosol. Examples of suitable aerosol formers are glycerine and propylene glycol.

[0135] The aerosol-generating substrate may comprise homogenised tobacco material, an aerosol-former and water. Providing homogenised tobacco material may improve aerosol generation, the nicotine content and the flavour profile of the aerosol generated during heating of the aerosol-generating article. Specifically, the process of making homogenised tobacco involves grinding tobacco leaf, which more effectively enables the release of nicotine and flavours upon heating.

[0136] Below, there is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein.

[0137] Example 1 : A filter element for an aerosol-generating device, wherein the filter element comprises a filtration material, where the filtration material comprises a foam, wherein the foam is configured such that a porosity of the foam decreases in a radial direction of the filter element.

[0138] Example 1 A: A filter element for an aerosol-generating article, wherein the filter element comprises a filtration material, where the filtration material comprises a foam, wherein the foam is configured such that a porosity of the foam decreases in a radial direction of the filter element.

[0139] Example 2: The filter element according to example 1 or example 1A, wherein the filtration material consists of the foam.

[0140] Example 3: The filter element according to any of example 1 , example 1 A and example 2, wherein the porosity at a center of the foam is higher than the porosity at a periphery of the foam.

[0141] Example 4: The filter element according to example 3, wherein a ratio of the porosity at the periphery of the foam and the porosity at the center of the foam is of between 1 :1.3 to 1 :2.67, preferably of between 1 :1.5 and 1 :2.1 , more preferably of about 1 :1.75.

[0142] Example 5: The filter element according to example 3 or example 4, wherein the porosity at the periphery of the foam is of between 0.3 and 0.5, and wherein the porosity at the center of the foam is of between 0.6 and 0.8.

[0143] Example 6: The filter element according to any of the preceding examples, wherein the foam is a foam of cellulosic material, preferably wherein the foam is a foam of regenerated cellulose material.

[0144] Example 7: The filter element according to example 6, wherein the foam is a viscose foam. Example 8: The filter element according to any of the preceding examples, wherein the filtration material is free of cellulose acetate.

[0145] Example 9: The filter element according to any of the preceding examples, wherein the filter element comprises a filter element wrapper, wherein the filter element wrapper is configured to at least partially, preferably fully circumscribe the filtration material.

[0146] Example 10: The filter element according to example 9, wherein the filter element wrapper is made from a paper material.

[0147] Example 11 : The filter element according to example 9 or example 10, wherein the filter element wrapper is configured to compress at least a peripheral portion of the foam, such as to facilitate the shape of the foam.

[0148] Example 12: The filter element according to any of the preceding examples, wherein the filter element is cylindrically-shaped, preferably wherein the filter element is a rod.

[0149] Example 13: The filter element according to any of examples 9 to 12, wherein the filter element wrapper is configured to compress the foam into a cylindrical rod of substantially circular cross-section, preferably wherein the foam of the filter element is derived from a cuboid-shaped foam of uniform density.

[0150] Example 14: The filter element according to any of examples 9 to 13, wherein the filter element consists of the filter element wrapper and the foam.

[0151] Example 15: The filter element according to any of the preceding examples, wherein the foam is configured such that a porosity of the foam gradually decreases in a radial direction of the filter element.

[0152] Example 16: The filter element according to any of examples 9 to 15, wherein the filter element wrapper is configured to compress a cuboid-shaped foam of uniform porosity and square cross-section into a cylindrical rod of substantially circular cross-section such that the porosity of the foam decreases in a radial direction of the filter element.

[0153] Example 17: The filter element according to example 16, wherein a ratio of a side length of the square cross-section of the cuboid-shaped foam and an outer diameter of the cylindrical rod of substantially circular cross-section is of between 1 and 1 .5, preferably of between 1 and 1.3, and more preferably about 1.

[0154] Example 18: The filter element according to example 17, wherein the side length of the square cross-section of the cuboid-shaped foam is of between 7 millimetres and 11 millimetres, preferably of between 8 millimetres and 10 millimetres.

[0155] Example 19: The filter element according to any of the preceding examples, wherein the foam is configured such that the porosity varies around a circumference of the foam, preferably wherein the porosity varies in a regular manner around the circumference of the foam, preferably wherein the circumference is an outer circumference of the foam. Example 20: The filter element according to example 19, wherein the circumference comprises a plurality of points of maximum porosity, wherein the points of maximum porosity are equidistantly spaced along the circumference, wherein the circumference comprises a plurality of points of minimum porosity, wherein the points of minimum porosity are equidistantly spaced along the circumference, preferably wherein the porosity of the points of maximum porosity is identical, preferably wherein the porosity of the points of minimum porosity is identical.

[0156] Example 21 : The filter element according to example 20, wherein the circumference comprises four of points of maximum porosity, wherein the circumference comprises four points of minimum porosity.

[0157] Example 22: An aerosol-generating article comprising a filter element according to any of the preceding examples and an aerosol-forming substrate.

[0158] Example 23: An aerosol-generating system comprising an aerosol-generating article according to example 22 and an aerosol-generating device.

[0159] Example 24: A method for manufacturing a filter element for an aerosol-generating article, wherein the method comprises the following steps: a) providing a foam of substantially uniform porosity b) compressing at least a portion of the foam by wrapping a filter element wrapper around the foam, such that a porosity of the foam decreases in a radial direction of the filter element.

[0160] Example 25: The method according to example 24, wherein the foam of step a) has a cuboid shape, and wherein in step b), the cuboid-shaped foam is compressed into a rod by tightening the filter element wrapper around the cuboid-shaped foam.

[0161] Example 26: The method according to example 24 or example 25, wherein the foam of step a) has a uniform porosity.

[0162] Example 27: The method according to any of examples 24 to 26, wherein the foam of step a) is cuboid-shaped and has a square cross-section.

[0163] Example 28: The method according to example 26 or example 27, wherein in step b), the cuboid-shaped foam of square cross-section is compressed into a rod of substantially circular cross-section such that a ratio of a side length of the square cross-section of the cuboid-shaped foam of step a) and an outer diameter of the rod of circular cross-section is of between 1 and 1.5, preferably of between 1 and 1.3, and more preferably about 1.

[0164] Example 29: The method according to example 28, wherein the side length of the square cross-section of the cuboid-shaped foam of step a) is of between 7 millimetres and 11 millimetres, preferably of between 8 millimetres and 10 millimetres. Example 30: The method according any of examples 24 to 29, wherein in step b), the foam of step a) is compressed such that the porosity varies around a circumference of the foam, preferably wherein the porosity varies in a regular manner around the circumference of the foam, preferably wherein the circumference is an outer circumference of the foam.

[0165] Example 31 : The method according to example 30, wherein the circumference comprises a plurality of points of maximum porosity, wherein the points of maximum porosity are equidistantly spaced along the circumference, wherein the circumference comprises a plurality of points of minimum porosity, wherein the points of minimum porosity are equidistantly spaced along the circumference, preferably wherein the porosity of the points of maximum porosity is identical, preferably wherein the porosity of the points of minimum porosity is identical.

[0166] Example 32: The filter element according to example 31 , wherein the circumference comprises four of points of maximum porosity, wherein the circumference comprises four points of minimum porosity.

[0167] Example 33: A filter element for an aerosol-generating article produced by the method for manufacturing a filter element for an aerosol-generating article according any of examples 24 to 32.

[0168] Features described in relation to one embodiment may equally be applied to other embodiments of the invention.

[0169] The invention will be further described, by way of example only, with reference to the accompanying drawings in which:

[0170] Fig. 1 illustrates the manufacturing of a filter element of the invention;

[0171] Fig. 2 shows different perspectives of the filter element of the invention; and

[0172] Fig. 3 shows comparisons of the filtration performance of the filter element of the invention with conventional filter elements.

[0173] Fig. 1 illustrates the manufacturing of a filter element 10 of the invention. Figs. 1A and 1 B show a cuboid-shaped foam 12. The foam 12 is made from regenerated cellulose. Fig. 1A shows a side view of the cuboid-shape foam. Fig. 1 B shows a cross-section of the cuboidshaped foam. The cuboid-shaped foam may have a uniform porosity. The cuboid-shaped foam is wrapped by a filter element wrapper 14 to arrive at the filter element 10 of the invention shown in Figs. 1C and 1 D. Fig. 1C shows a side view of the filter element. Fig. 1 D shows a cross-section 16 of the filter element. The cuboid-shaped foam 12 is compressed into solid cylindrically shaped foam 12 having a substantially circular cross-section. The filter element wrapper 14 may be tightened around the foam 12. A porosity of the solid cylindrically shaped foam 12 decreases in a radial direction of the filter element 10. The porosity of the solid cylindrically shaped foam 12 decreases in a radial direction of the filter element 10 due to the compression of peripheral portions of the foam by the filter element wrapper 14. One or more of a gradient of the porosity in the radial direction and the absolute value of the porosity of the peripheral portions of the foam 12 may be tuned by adjusting the compression. For example, the filter element wrapper 14 may be wrapped more tightly around the foam 12 to lower the porosity of the peripheral portions of the foam 12.

[0174] The foam 12 comprises a high porosity region 18. The high porosity region 18 is arranged around a centre of the foam 12. The high porosity region 18 is substantially not compressed by the filter element wrapper. The high porosity region 18 is substantially astroidshaped. The foam comprises a low porosity region 20. The low porosity region 20 is arranged at the periphery of the foam 12. The low porosity region 20 comprises four spindle-shaped lobes 22.

[0175] In use, the filter element 10 may be part of an aerosol-generating article. The aerosolgenerating article may comprise an aerosol-generating substrate. The filter element may be arranged at a proximal end of the article. The user may draw on the filter element to pull aerosol formed from the aerosol-generating substrate through the filter element into the user’s mouth. Due to the lower porosity of the low porosity region 20 compared to the porosity of the high porosity region 18, aerosol may flow faster through the high porosity region 18 then through the low porosity region 20. Due to the difference in the speed of the aerosol in the high porosity region 18 and the low porosity region 20, filtering efficiency of the filter element may be improved. Components of the aerosol which are more efficiently removed at low flow speeds may be more efficiently removed in the low porosity regions. Components of the aerosol which are more efficiently removed at high flow speeds may be more efficiently removed in the high porosity regions. Sustainability of the filter element is improved due to the usage of the foam made from regenerated cellulose.

[0176] Fig. 2 shows different perspectives of the filter element of the invention. The remarks relating to Fig. 1 correspondingly applied to Fig. 2. Fig. 2A shows an angled view of the filter element. Fig. 2B shows a frontal view of the filter element. Fig. 2C shows the filter element with a part of the filter element wrapper 14 removed to show the cuboid-shaped foam prior to wrapping the filter element wrapper 14 around the foam 12 more clearly to the reader.

[0177] Fig. 3 shows comparisons of the filtration performance of the filter element of the invention with a conventional filter element. Fig. 3A shows a comparison of the yield of carbon monoxide of the filter element of the invention with respect to a conventional cellulose acetate tow filter element. Fig. 3B shows a comparison of the yield of glycerol of the filter element of the invention with respect to a conventional cellulose acetate tow filter element. Fig. 3C shows a comparison of the yield of nicotine of the filter element of the invention with respect to a conventional cellulose acetate tow filter element. The yields obtained by the filter element of the invention and of the conventional cellulose acetate tow filter element are comparable for each of the compounds. Accordingly, the filter element of the invention may be used to replace conventional cellulose acetate tow filter elements. The filter element of the invention provides comparable filtration performance to conventional cellulose acetate tow filter elements while offering an improved sustainability compared to the conventional cellulose acetate tow filter elements.

Claims

CLAIMS1. A filter element for an aerosol-generating article, wherein the filter element comprises a filtration material, where the filtration material comprises a foam, wherein the foam is configured such that a porosity of the foam decreases in a radial direction of the filter element.

2. The filter element according to claim 1 , wherein the porosity at a center of the foam is higher than the porosity at a periphery of the foam.

3. The filter element according to claim 2, wherein a ratio of the porosity at the periphery of the foam and the porosity at the center of the foam is of between 1 :1.3 to 1 :2.67, preferably of between 1 :1.5 and 1 :2.1 , more preferably of about 1 :1.75.

4. The filter element according to claim 2 or claim 3, wherein the porosity at the periphery of the foam is of between 0.3 and 0.5, and wherein the porosity at the center of the foam is of between 0.6 and 0.8.

5. The filter element according to any of the preceding claims, wherein the foam is a foam of cellulosic material, preferably wherein the foam is a foam of regenerated cellulose material.

6. The filter element according to claim 5, wherein the foam is a viscose foam.

7. The filter element according to any of the preceding claims, wherein the filtration material is free of cellulose acetate.

8. The filter element according to any of the preceding claims, wherein the filter element comprises a filter element wrapper, wherein the filter element wrapper is configured to at least partially, preferably fully circumscribe the filtration material.

9. The filter element according to claim 8, wherein the filter element wrapper is configured to compress at least a peripheral portion of the foam, such as to facilitate the shape of the foam.

10. The filter element according to any of the preceding claims, wherein the filter element is cylindrically-shaped, preferably wherein the filter element is a rod.11 . The filter element according to any of claims 8 to 10, wherein the filter element wrapper is configured to compress the foam into a cylindrical rod of substantially circular crosssection, preferably wherein the foam of the filter element is derived from a cuboid-shaped foam of uniform density.

12. An aerosol-generating article comprising a filter element according to any of the preceding claims and an aerosol-forming substrate.

13. An aerosol-generating system comprising an aerosol-generating article according to claim 12 and an aerosol-generating device.

14. A method for manufacturing a filter element for an aerosol-generating article, wherein the method comprises the following steps: a) providing a foam of substantially uniform porosity b) compressing at least a portion of the foam by wrapping a filter element wrapper around the foam, such that a porosity of the foam decreases in a radial direction of the filter element.

15. The method according to claim 14, wherein the foam of step a) has a cuboid shape, and wherein in step b), the cuboid-shaped foam is compressed into a rod by tightening the filter element wrapper around the cuboid-shaped foam.

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