Method for manufacturing a plurality of bodies made of a porous material

Abstract

A method can be used for manufacturing one or more bodies made of a porous material derived from precursors of the porous material in a sol-gel process. The method involves filling precursors of the porous material into a mold defining the shape of the body, where the precursors include at least two reactive components and a solvent, and forming a gel body. The step is then repeated so as to form several gel bodies. The gel bodies are then removed from the mold after a predetermined time in which the gel bodies are formed from the precursors of the porous material. The gel bodies are arranged adjacent to one another, a spacer is provided between two adjacent gel bodies so as to provide a clearance therebetween, and the solvent is then removed from the gel bodies.

Claims

1: A method for manufacturing a plurality of bodies made of a porous material derived from precursors of the porous material in a sol-gel process, the method comprising: (i) filling precursors of a porous material into a mold defining a shape of a body, wherein the precursors include at least two reactive components and a solvent, and forming a gel body, (ii) repeating (i) so as to form a plurality of gel bodies, (iii) removing the plurality of gel bodies from the mold after a predetermined time in which the plurality of gel bodies are formed from the precursors of the porous material, (iv) arranging the plurality of gel bodies adjacent to one another, (v) providing a spacer between two adjacent gel bodies so as to provide a clearance therebetween, and (vi) removing the solvent from the plurality of gel bodies.

2: The method according to claim 1, wherein the spacer is a grid assembly comprising a first grid and a second grid connected to one another, wherein the first grid comprises first openings and the second grid comprises second openings, wherein the first openings and the second openings are shifted relative to one another.

3: The method according to claim 2, wherein the grid assembly comprises a thickness of 1.0 mm to 4.0 mm, and/or wherein the first openings and/or the second openings are arranged in a regular or irregular pattern, and/or wherein the first openings and/or the second openings comprise identical or different opening areas, and/or wherein the first openings and/or the second openings comprise identical or different shapes, and/or wherein the first openings and/or the second openings comprise a circular, oval, elliptical, polygonal, polygonal including rounded edges, rectangular, or square shape, and/or wherein the method further comprises at least partially providing surfaces of the first grid and/or the second grid with a coating made of a material being electrically dissipative and non-sticky to the plurality of gel bodies, and/or wherein a total opening area of the first openings and the second openings is 40% to 95% of a facing outer surface of one of the plurality of gel bodies.

4: The method according to claim 1, further comprising integrally forming each of the plurality of gel bodies with the spacer.

5: The method according to claim 4, wherein the spacer includes a plurality of protrusions protruding from at least one surface of the plurality of gel bodies.

6: The method according to claim 5, further comprising forming the plurality of protrusions only on one of the at least one surface of each of the plurality of gel bodies, wherein the plurality of gel bodies are arranged adjacent to one another such that the at least one surface including the plurality of protrusions of one of the plurality of gel bodies faces a surface without protrusions of a respective adjacent gel body.

7: The method according to claim 5, wherein each of the plurality of protrusions comprises a circular cross-sectional shape with a diameter of 1.0 to 5.0 mm, and/or wherein the plurality of protrusions are arranged in a regular or irregular pattern, and/or wherein the plurality of protrusions have identical or different shapes, and/or wherein the plurality of protrusions have a height of 0.1 mm to 20.0 mm, and/or wherein the plurality of protrusions are arranged such that a minimum distance between outer surfaces of adjacent protrusions is 0.1 mm, and/or wherein the plurality of protrusions are formed as truncated cones, and/or wherein the method further comprises removing the plurality of protrusions after removing the solvent from the plurality of gel bodies.

8: The method according to claim 1, wherein the plurality of gel bodies are formed as slabs having a cuboid, cylindrical, or polygonal shape, and wherein the plurality of gel bodies are arranged such that side surfaces of the cuboid, cylindrical, or polygonal shape having a greatest surface area are oriented substantially perpendicular with respect to a direction of gravity, or wherein the plurality of gel bodies are arranged such that side surfaces of the cuboid, cylindrical, or polygonal shape having the greatest surface area are oriented substantially parallel with respect to a direction of gravity.

9: The method according to claim 1, wherein the plurality of gel bodies are formed as slabs comprising a length of at least 10 cm and a width of at least 10 cm, and/or wherein the plurality of gel bodies are formed as slabs comprising a thickness of at least 0.5 mm.

10: The method according to claim 1, wherein the spacer is a grid comprising grid openings.

11: The method according to claim 10, wherein the grid openings comprise identical or different opening areas, and/or wherein the grid openings are arranged in a regular or irregular pattern, and/or wherein the grid comprises struts defining the grid openings, wherein the struts comprise a width of 1.0 mm to 5.0 mm, and/or wherein the plurality of gel bodies are formed as slabs having a cuboid, cylindrical, or polygonal shape, wherein the plurality of gel bodies are arranged such that side surfaces of the cuboid, cylindrical, or polygonal shape having a greatest surface area are oriented substantially perpendicular with respect to a direction of gravity, and/or wherein the method further comprises at least partially providing surfaces of the grid with a coating made of a material being electrically dissipative and non-sticky to the plurality of gel bodies.

12: The method according to claim 10, wherein removing the solvent from the plurality of gel bodies is performed by means of supercritical drying.

13: The method according to claim 10, wherein the grid is configured to carry each of the plurality of gel bodies and to support a second grid disposed thereon without the plurality of gel bodies being engaged by the second grid.

14: The method according to claim 1, wherein removing the solvent from the plurality of gel bodies is performed by means of supercritical drying or convective drying.

15: A plurality of gel bodies obtained or obtainable by the process according to claim 1.

16: A thermal insulation material or a vacuum insulation panel, comprising the plurality of gel bodies according to claim 15.

17: A method, comprising: molding a thermal insulation material or a vacuum insulation panel comprising the plurality of gel bodies according to claim 15.

18: The method according to claim 3, wherein the grid assembly comprises a thickness of 1.5 mm to 2.5 mm.

19: The method according to claim 4, wherein the forming comprises monolithically forming each of the plurality of gel bodies with the spacer.

20: The method according to claim 13, wherein the grid comprises an outer rim configured to support the second grid disposed thereon, without the plurality of gel bodies being engaged by the second grid.

Description

SHORT DESCRIPTION OF THE FIGURES

[0216] Further features and embodiments of the invention will be disclosed in more detail in the subsequent description, particularly in conjunction with the dependent claims. Therein the respective features may be realized in an isolated fashion as well as in any arbitrary feasible combination, as a skilled person will realize. The embodiments are schematically depicted in the figures. Therein, identical reference numbers in these figures refer to identical elements or functionally identical elements.

[0217] In the Figures:

[0218] FIG. 1 shows a flow chart of the method according to the present invention;

[0219] FIG. 2 shows a perspective view of a spacer used with a first embodiment of the disclosed method;

[0220] FIG. 3 shows an enlarged view of a portion of the spacer of FIG. 2;

[0221] FIG. 4 shows a perspective view of a plurality of bodies arranged according to a first orientation;

[0222] FIG. 5 shows a perspective view of a plurality of bodies arranged according to a second orientation;

[0223] FIG. 6 shows a perspective view of a mold used with a second embodiment of the disclosed method;

[0224] FIG. 7 shows a schematical flow chart of the second embodiment of the disclosed method; and

[0225] FIGS. 8A to 8F show perspective views of different spacers used with a third embodiment of the disclosed method.

DETAILED DESCRIPTION

[0226] As used in the following, the terms “have”, “comprise” or “include” or any arbitrary grammatical variations thereof are used in a non-exclusive way. Thus, these terms may both refer to a situation in which, besides the feature introduced by these terms, no further features are present in the entity described in this context and to a situation in which one or more further features are present. As an example, the expressions “A has B”, “A comprises B” and “A includes B” may both refer to a situation in which, besides B, no other element is present in A (i.e. a situation in which A solely and exclusively consists of B) and to a situation in which, besides B, one or more further elements are present in entity A, such as element C, elements C and D or even further elements.

[0227] Further, it shall be noted that the terms “at least one”, “one or more” or similar expressions indicating that a feature or element may be present once or more than once typically will be used only once when introducing the respective feature or element. In the following, in most cases, when referring to the respective feature or element, the expressions “at least one” or “one or more” will not be repeated, non-withstanding the fact that the respective feature or element may be present once or more than once.

[0228] Further, as used in the following, the terms “particularly”, “more particularly”, “specifically”, “more specifically” or similar terms are used in conjunction with additional/alternative features, without restricting alternative possibilities. Thus, features introduced by these terms are additional/alternative features and are not intended to restrict the scope of the claims in any way. The invention may, as the skilled person will recognize, be performed by using alternative features. Similarly, features introduced by “in an embodiment of the invention” or similar expressions are intended to be additional/alternative features, without any restriction regarding alternative embodiments of the invention, without any restrictions regarding the scope of the invention and without any restriction regarding the possibility of combining the features introduced in such way with other additional/alternative or non-additional/alternative features of the invention.

[0229] Further, it shall be noted that the terms “first”, “second” and “third” are used to exclusively facilitate to differ between the respective constructional members or elements and shall not be construed to define a certain order or importance.

[0230] The term “mold” as used herein refers to a hollowed-out block or container that is configured to be filled with a liquid or pliable material provided by precursors of a sol gel provided by precursors of a sol gel. Particularly, the sol-gel process is carried out within the mold. During the sol-gel process the precursors form a sol which subsequently starts to gel. Thus, the liquid hardens or sets inside the mold, adopting its shape defined by the interior volume thereof. The mold is basically used to carry out the sol-gel process. However, it is to be noted that the solvent may be removed from the thus formed gel with the gel remaining within the mold or with the gel removed from the mold. In the present invention, the mold may consist of more than one part, wherein the interior volume is defined by a lower part.

[0231] The term “sol gel process” as used herein refers to a method for producing solid materials from small molecules. In the present case, the method is used for the fabrication of porous materials such as aerogels, xerogels and/or kryogels. The process involves conversion of monomers as precursors into a colloidal solution, the so-called sol, that subsequently reacts to an integrated network, the so-called gel, of either discrete particles or network polymers. In this chemical procedure, the sol gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. This gel-like diphasic system is called gel. Particularly, the gel encapsulates or surrounds the solvent within pores which are connected to one another, i.e. the pores form an interpenetrating network. Removal of the remaining liquid phase, i.e. the solvent, requires a drying process, which is typically accompanied by a certain amount of shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing.

[0232] The term “body” as used herein refers to a solid object formed by an identifiable collection of matter, which may be constrained by an identifiable boundary, and may move or may be moved as a unit by translation or rotation, in 3-dimensional space.

[0233] The term “porous” as used herein refers to material characteristics of having pores. As the solvent may be removed from the gel either with the gel being or remaining within the mold or after the gel is removed from the mold, the term “porous” covers both pores being filled with a liquid, particularly, the solvent, or a gas such as air. The pores may be connected to one another so as to form a type of network.

[0234] The term “coating” as used herein refers to a covering that is applied to the inner surfaces of the lower part of the mold. Particularly, the coating may be applied at least to those areas of the lower part intended to come into contact with the precursors of the porous material and the body made thereof. Needless to say, the coating may be applied to the total inner surfaces of the lower part defining the interior volume.

[0235] The term “electrically dissipative” as used herein refers to material characteristics, wherein electric charges are allowed to flow to ground but more slowly in a more controlled manner if compared to electrically conductive materials.

[0236] The term “non-sticky” as uses herein refers to characteristics wherein one part does not adhere to another part. Thus, both parts are in loose contact to one another. According to the present invention, the coating does not stick to the gel formed or resulting from the precursors filled into the mold. In case the solvent used with the sol gel process is removed with the gel being within the mold, the coating is configured not to stick to the thus formed body in order to allow the body being removed from the mold.

[0237] The terms “width” and “length” of the shape of the body as used herein refer to dimensions perpendicular to a height or thickness of the shape of the body.

[0238] The term “opening area” as used herein refers to the area of an opening defined by the boundary of the opening.

[0239] FIG. 1 shows a flow chart of a method for manufacturing a plurality of bodies made of a porous material derived from precursors of the porous material in a sol-gel process according to the present invention. FIG. 1 is to be understood as an explanation of the basic principle of the disclosed method. In step S10, a mold 10 is provided. The mold 10 defines the shape of a to be formed body 12. In step S12, precursors of the porous material are filled into the mold 10. The precursors include at least two reactive components CA, CB and a solvent S. The precursors of the porous material may be prepared as follows. A first reactive component CA and a solvent S are supplied to a first receiving tank. Further, a second reactive component CB and the solvent S are supplied to a second receiving tank. A predetermined amount of the first reactive component and solvent is supplied to a mixing device from the first receiving tank. For example, the predetermined amount is defined as a volumetric dosing by means of a first volumetric dosing device. A predetermined amount of the second reactive component and solvent is supplied to the mixing device from the second receiving tank. For example, the predetermined amount is defined as a volumetric dosing by means of a second volumetric dosing device. Optionally, a closed loop operation may be provided with the first receiving tank and the mixing device and/or with the second receiving tank and the mixing device. The precursors of the porous material are then filled into the mold 10 up to a predetermined amount. For example, the filling process is carried out by means of the mixing device. Particularly, the precursors are mixed by means of the mixing device before being filled into the lower part. The precursors may be filled into the mold 10 in an inert or ventilated region. For example, the filling is carried out in a carbon dioxide or nitrogen atmosphere or in a ventilated device similar to a laboratory hood. The mold 10 may be closed by a lid, particularly in a gas tight manner. Thereby, any solvent vapor is prevented from leaking from the mold 10. Then, the sol gel reaction from the two reactive components of the precursors takes place wherein the precursors gel. Thus, a gel body 12 is formed. After gelling, the gel is hardened or aged for a predetermined time such as at least 3 hours and preferably at least 8 hours in order to complete the gelation reaction and to exclude a negative impact on the further handling of the gel body 12 such as in case the gel body is not sufficiently hard. For example, the hardening or ageing process is carried out by means of a hardening device. After hardening, the body is formed.

[0240] Step 12 is repeated so as to form a plurality of gel bodies 12. Particularly, step S12 may be repeated any number of times as appropriate and depending on the respective application. The gel bodies 12 may be formed as slabs, wherein the slabs comprise a length of at least 10 cm and a width of at least 10 cm. For practical reasons, the upper limit for the length and/or the width may be 200 cm or even 100 cm. The slabs comprise a thickness of at least 0.5 mm. For practical reasons, the upper limit for the thickness may be 25.0 mm, 20.0 mm or 15.0 mm. Subsequently, in step S14, the gel bodies 12 are removed from the mold 10. With other words, the gel bodies 12 from each mold 10 or in case a single mold 10 is used, the gel bodies 12 are removed from the mold 10 in a subsequent order after a predetermined time in which the gel body/bodies 12 is/are formed from the precursors of the porous material. Further, the solvent S may be recycled or re-extracted by means of a re-extraction device.

[0241] In step S16, the gel bodies 12 are arranged adjacent to one another. In step S18, a spacer 14 is provided between two adjacent gel bodies 12 so as to provide a clearance therebetween. It has to be noted that steps S16 and S18 may be carried out at the same time. In step S20, the solvent S is removed from the gel bodies 12. The removing of the solvent S from the gel bodies 12 is performed by means of supercritical drying or convective drying. The removing may take place in an autoclave or oven. In the example shown in FIG. 1, the solvent S is removed by means of supercritical CO.sub.2.

[0242] FIG. 2 shows a perspective view of a spacer 14 used with a first embodiment of the disclosed method. FIG. 3 shows an enlarged view of a portion of the spacer 14 of FIG. 2. The spacer 14 of the first embodiment is a grid assembly 16. The grid assembly 16 comprises a first grid 18 and a second grid 20 connected to one another. Particularly, the first grid 18 is disposed on top of the second grid 20. The first grid 18 comprises first openings 22. The second grid 20 comprises second opening 24. The first openings 22 and second openings 24 are shifted to one another. With other words, the first openings 22 and second openings 24 do not exactly overlap one another but only partially. Thus, the first and second openings create an open path in the plane of the two grids 18, 20 allowing the solvent S to flow therethrough. The first openings 22 and the second openings 24 are arranged in a regular pattern. The first openings 22 and the second openings 24 comprise identical opening areas. A total opening area of the first and second openings 22, 24 may be 40% to 95% of a facing outer surface of a body 12 such as 80%. The first openings 22 and the second openings 24 comprise identical shapes. In the example shown, the first openings 22 and the second openings 24 have a rectangular and square shape, respectively. The grid assembly 16 comprises a thickness of 1.0 mm to 4.0 mm, preferably 1.25 mm to 3.5 mm and more preferably 1.5 mm to 2.5 mm such as 2.0 mm. Surfaces of the first grid 18 and/or second grid 20 may be at least partially provided with a coating made of a material being electrically dissipative and non-sticky to the gel bodies 12.

[0243] The grid assembly 16 may be modified as follows. The first openings 22 and/or the second openings 24 may be arranged in an irregular pattern. The first openings 22 and/or the second openings 24 may comprise different opening areas. The first openings 22 and/or the second openings 24 may comprise different shapes. The first openings 22 and/or the second openings 24 may comprise a circular, oval, elliptical, polygonal or polygonal including rounded edges, shape.

[0244] FIG. 4 shows a perspective view of a plurality of gel bodies 12 arranged according to a first orientation during the removal of the solvent in step S20. The gel bodies 12 are formed as slabs having a cuboid shape. In the first orientation, the gel bodies 12 removed from the mold 10 are arranged substantially vertical. With other words, the gel bodies 12 are arranged such that side surfaces of the cuboid shape having the greatest surface area are oriented substantially parallel with respect to a direction of gravity. Further, the gel bodies 12 are arranged such that edges of the cuboid shape having the greatest dimension are oriented substantially perpendicular with respect to a direction of gravity. As can be further seen in FIG. 4, spacers 14 as shown in FIGS. 2 and 3 are provided between adjacent gel bodies 12. Basically, the gel bodies 12 may alternatively be formed as slabs having a cylindrical or polygonal shape.

[0245] FIG. 5 shows a perspective view of a plurality of gel bodies 12 arranged according to a second orientation during the removal of the solvent in step S20. The gel bodies 12 are formed as slabs having a cuboid shape. In the second orientation, the gel bodies 12 removed from the mold 10 are arranged substantially horizontal. With other words, the gel bodies 12 are arranged such that side surfaces of the cuboid shape having the greatest surface area are oriented substantially perpendicular with respect to a direction of gravity. Further, the gel bodies 12 are arranged such that edges of the cuboid shape having the greatest dimension are oriented substantially perpendicular with respect to a direction of gravity. As can be further seen in FIG. 5, spacers 14 as shown in FIGS. 2 and 3 are provided between adjacent gel bodies 12. Basically, the gel bodies 12 may alternatively be formed as slabs having a cylindrical or polygonal shape. It has to be noted that the removing of the solvent with the horizontal arrangement of the gel slabs may take some more time than with the vertical arrangement of the gel slabs.

[0246] FIG. 6 shows a perspective view of a mold 10 used with a second embodiment of the disclosed method. The mold 10 used with the second embodiment of the disclosed method comprises a cuboid shape. Further, the mold 10 comprises recesses or indentations 26 in a bottom surface 28. The indentations 26 are arranged in a regular pattern and comprise a truncated cone shape. Hereinafter, the second embodiment of the disclosed method will be described in further detail.

[0247] FIG. 7 shows a schematical flow chart of the second embodiment of the disclosed method. Hereinafter, only the difference from the first embodiment of the disclosed method will be described in detail and identical or constructional members or method steps are indicated by like reference numerals and are only briefly described. In step S10, the mold 10 shown in FIG. 6 is provided. In step S12, the precursors of the porous material as described above are filled into the mold 10. The precursors also flow into the indentations 26 in the bottom surface 28 of the mold 10. In step S14, the gel bodies 12 are removed from the mold 10. With other words, the gel bodies 12 from each mold 10 or in case a single mold 10 is used, the gel bodies 12 are removed from the mold 10 in a sub-sequent order after a predetermined time in which the gel body/bodies 12 is/are formed from the precursors of the porous material. As the precursors have flowed into the indentations 26 in the bottom surface 28 of the mold 10, the gel bodies 12 are integrally and monolithically, respectively, formed with the spacer 14. Particularly, the spacer 14 includes a plurality of protrusions 30 protruding from at least one surface 32 of the gel bodies 12. The protrusions 30 are formed only on one surface 32 of each of the gel bodies 12. As the indentations 26 of the mold 10 are arranged in a regular pattern, also the protrusions 30 are arranged in a regular pattern. Particularly, the protrusions 30 are arranged such that a minimum distance 36 between outer surfaces 38 of adjacent protrusions 30 is 0.1 mm. The minimum distance 36 may defined at the transition of a protrusions 30 into the body 12. As the indentations 26 of the mold 10 are shaped as truncated cones, the protrusions 30 are formed as truncated cones. Further, the protrusions 30 have identical shapes. The protrusions 30 have a height 40 of 0.1 mm to 20.0 mm, preferably 0.5 mm to 5.0 mm and more preferably 1.0 mm to 3.0 mm such as 2.0 mm. Each of the protrusions 30 comprises circular cross-sectional shape with a diameter of 1.0 to 5.0 mm, preferably 1.25 mm to 4.0 mm and more preferably 1.5 mm to 2.5 mm such as 2.0 mm. In case of a cone or truncated cone shape, the diameter may be defined at the half of the height 40 or as an average value along the height 40.

[0248] In step S16, the gel bodies 12 are arranged adjacent to one another such that the surface 32 including the protrusions 30 of one of the gel bodies 12 faces a surface 34 without protrusions of the respective adjacent body 12. Thus, by arranging the gel bodies 12 adjacent to one another, the spacer 14 formed by the protrusions 30 is automatically provided between two adjacent gel bodies 12 so as to provide a clearance therebetween. With other words, steps S16 and step S18 are combined. In step S20, the solvent S is removed from the gel bodies 12 as described above, i.e. by means of convective or supercritical drying. In the example shown in FIG. 7, the solvent S is removed by means of supercritical CO.sub.2. In step S24, the gel bodies 12 have been removed from the solvent S and are released from the adjacent arrangement. In step S24, the protrusions 30 may optionally be removed after removing the solvent S from the gel bodies 12.

[0249] The bodies 12 may be modified as follows by modifying the mold 10 shown in FIG. 6. The protrusions 30 may be formed on two opposing surfaces of the gel bodies 12. The protrusions 30 may be arranged in an irregular pattern. The protrusions 30 may have different shapes. FIGS. 8A to 8F show perspective views of different spacers 14 used with a third embodiment of the disclosed method. According to FIGS. 8A to 8F, the spacer 14 is a grid 42 comprising grid openings 44. The grid 42 comprises struts 46 defining the grid openings 44. The struts 46 comprise a width 48 of 1.0 mm to 5.0 mm, preferably 1.25 mm to 4.5 mm and more preferably 1.5 mm to 4.0 mm. It has to be noted that the struts 46 of one and the same grid 42 may comprise different widths 50. The spacers 14 shown in FIGS. 8A to 8F are designed such that the grid 42 is configured to carry a body 12 and to support another grid 42 disposed thereon without the body 12 being engaged by the other grid 42. For this purpose, the grid 42 comprises an outer rim 50 configured to support another grid 42 disposed thereon without the body 12 being engaged by the other grid 42. The grids shown in FIGS. 8A to 8F are particularly designed for allowing to remove the solvent S by means of supercritical drying. Basically, the grids 42 shown in FIGS. 8A to 8F are configured to be horizontally arranged one on top of the other during the step of removing the solvent S similar to arrangement shown in FIG. 5. Thus, with the third embodiment, the gel bodies 12 are formed as slabs having a cuboid, cylindrical or polygonal shape, wherein the gel bodies 12 are arranged such that side surfaces of the cuboid, cylindrical or polygonal shape having the greatest surface area are oriented substantially perpendicular with respect to a direction of gravity. The surfaces of the grid 42 may be provided with a coating made of a material being electrically dissipative and non-sticky to the gel bodies 12. The grid openings 42 may comprise identical or different opening areas. The grid openings 44 may be arranged in a regular or irregular pattern. Hereinafter, further details of the grids 42 shown in FIGS. 8A to 8F will be described.

[0250] FIG. 8A shows a grid 42 having square shaped grid openings 44 of different sizes. Further, the struts 46 comprise different widths 48. Particularly, the grid 42 comprises two struts 46 having a width 48 larger than the remaining struts 46, such as by factor 2. Further, some of the grid openings 44 are formed in the struts 46 and adjacent the outer rim 50 and are smaller than the remaining grid openings 44.

[0251] FIG. 8B shows a grid 42 having grid openings 44 of different sizes and different shapes. Particularly, there are larger circular grid openings 44, smaller circular grid openings 44 and half rounded grid openings 44.

[0252] FIG. 8C shows a grid 42 having square shaped grid openings 44 of identical sizes. Further, the struts 46 comprise different widths 48. Particularly, the grid 42 comprises two struts 46 having a width 48 larger than the remaining struts 46, such as by factor 2.

[0253] FIG. 8D shows a grid 42 having grid openings 44 of different sizes. Particularly, the grid openings 44 are formed as long slots running diagonally. Further, the grid 42 comprises two struts 46 running parallel to one another and inclined with respect to the grid openings 44.

[0254] FIG. 8E shows a grid 42 similar to the grid shown in FIG. 8D. The grid 42 has grid openings 44 of different sizes. Particularly, the grid openings 44 are formed as long slots running diagonally. Further, the grid 42 comprises two struts 46 running parallel to one another and inclined with respect to the grid openings 44. If compared to the grid shown in FIG. 80, the struts 46 of the grid 42 shown in FIG. 8E comprise a larger width 48.

[0255] FIG. 8F shows a grid 42 similar to the grid 43 shown in FIG. 8C. The grid 42 has square shaped grid openings 44 of different sizes. Further, the struts 46 comprise different widths 48. Particularly, the grid 42 comprises two struts 46 having a width 48 larger than the remaining struts 46, such as by factor 2. These two struts 46 comprise grid openings 44 smaller than the remaining grid openings 44.

CITED LITERATURE

[0256] WO 00/24799 [0257] WO 2009/027310 [0258] WO 2016/150684 A1 [0259] US 2005/0159497 A1