Method for preparing a friction material, in particular a method for manufacturing a brake pad, and related brake pad

Abstract

A method for obtaining a friction material for a brake pad, comprising spreading a wet paste formed by mixing an alkali silicate solution with metakaolin in a layer or tape form on a support, followed by subjecting it to a heat treatment to form a geopolimer aggregate, the heat treatment consisting of drying the wet paste to a completely dry or almost completely dry geopolimer aggregate having a water content lower than the desired water content in the final geopolimer, pulverizing the completely dry or almost completely dry geopolimer into a powder, and then rewetting it to the desired water content by adding water or a hydrated salt.

Description

Cross - reference to related applications

[0001] This patent application claims the priority of Italian Patent Application No. 102022000012338, filed on June 10, 2022, the entire disclosure of which is incorporated herein by reference.

Technical Field

[0002] The present invention relates to a method for preparing a friction material, particularly a method for manufacturing a brake pad. The present invention also relates to a related friction material and a brake pad manufactured using a friction material produced by such a method.

[0003] The friction material of the present invention is intended for the production of a non - asbestos friction layer / block for brake elements, i.e., vehicle brake pads or shoes, and / or friction elements such as friction discs, which have performance similar to or better than those belonging to the friction materials of the NAO ( "non - asbestos organic friction material"), "low - steel" and "semi - metal" classes.

Background Art

[0004] Application EP3128201, published in the name of the same applicant, the entire content of which is incorporated herein by reference for the necessary parts, discloses a binder for brake pads composed of at least 90% geopolymers, as well as a method for obtaining related friction materials and brake pads.

[0005] In EP3128201, the binder is obtained by dry - grinding caustic soda flakes and subsequently dry - mixing the soda powder with kaolin. This procedure is chemically efficient but involves a series of potential safety risks for the operator. In particular, the dry - grinding of caustic soda is a high - risk process that can produce very fine, volatile sodium hydroxide powder which is highly corrosive and irritating. For example, when the grinder is opened to remove the product or during the cleaning of the machine, the operator may accidentally inhale it. Furthermore, during or after grinding, the soda powder may absorb a significant amount of unregulated moisture from the environment. This unregulated residual moisture is retained by the soda in its subsequent mixture with kaolin and, if too high, can be released in the form of steam during the hot - forming of brake pads, leading to serious manufacturing problems regarding the layers / blocks of the finished friction material which tend to form flakes and cracks.

[0006] To overcome this problem, EP3841311 in the name of the same applicant (the entire content of which is also incorporated herein by reference in its necessary parts) discloses a similar process, but uses metakaolin instead of kaolin and works with an aqueous sodium silicate solution containing the minimum amount of sodium hydroxide that can be used as a reactant in either case.

[0007] According to EP3841311, in addition to metakaolin, other sources of aluminosilicate, such as kaolin or fly ash, can be used. However, kaolin has a long reaction time, while one negative aspect of fly ash is the fact that suppliers do not provide a composition that remains constant over time. Therefore, metakaolin is preferred.

[0008] Furthermore, according to EP3841311, other raw materials, such as common sources of silica, such as quartz, or colloidal silica dissolved in a basic sodium hydroxide or potassium hydroxide solution, can be used under appropriate conditions.

[0009] In any case, EP3841311 teaches a process of drying a wet mortar produced by adding the above alkali silicate solution to metakaolin by mechanical mixing, and then drying it by an atmospheric pressure drying process that can also be adjusted at a temperature of 20°C to 300°C until a vacuum state is achieved (i.e., a value of 0.018 mBar or more). The drying is usually carried out at atmospheric pressure at a temperature of 80°C to 200°C to obtain a dried product in the form of a tape having a weight loss of 5% to 40% from the original weight and a related residual moisture of less than 30% by the final weight. This product is then ground to a size of 800 microns or less, preferably less than 400 microns, and the resulting powder material is used as a binder for the production of a mixture / composition for brake pads similar to those disclosed in EP3128201.

[0010] Subsequent tests conducted by the applicant's engineers in both laboratory and actual vehicle test drives have shown that the content of residual moisture in the geopolimer powder used as a raw material binder in the friction composition must be adjusted extremely precisely, that is, it is not sufficient for the residual moisture to be any value less than 30% w (weight), but it must remain within a predetermined range, which has proven to be extremely difficult to adapt to the process of EP3841311, which in some cases produces a fairly large amount of scrap material and furthermore cannot be recycled, resulting in net economic and energy losses. Summary of the Invention

[0011] The object of the present invention is to provide a method for manufacturing a friction layer / block for a brake element, such as a friction element like a vehicle brake pad or shoe, and to obtain a friction material and related brake pads that are free from the aforementioned problems of both the methods of EP3128201 and EP3841311, and thus are resistant to the heat generated during braking, while at the same time providing satisfactory braking performance, optimal tribological properties, and ease of manufacture, by preparing related friction materials and their respective inorganic binders.

[0012] Another object of the present invention is to provide a manufacturing method that enables the easy recycling and recovery of geopolymers accidentally produced below satisfactory criteria.

[0013] Accordingly, the present invention relates to a method for manufacturing a friction layer / block for a brake element, such as a friction element like a vehicle brake pad or shoe, as defined in the appended claims.

[0014] The present invention also relates to related binders, as well as friction materials containing such binders, and related friction elements having friction layers or blocks manufactured by the method of the present invention, particularly brake pads or shoes.

[0015] In particular, the friction material according to the method of the present invention comprises, as its component materials, inorganic and / or organic and / or metal fibers; a binder consisting almost entirely or entirely of geopolymers or a mixture of geopolymers; at least one friction modifier or lubricant, such as sulfur and / or carbon materials or nanomaterials; and at least one inorganic or metal filler or abrasive, but the main polishing operation in the friction material of the present invention is performed by the geopolymer matrix of the pad produced by the binder.

[0016] As used herein, the term "binder consisting essentially of a geopolimer" refers to a binder for friction elements in which the geopolimer or geopolimer composition or mixture constitutes at least 90% by weight of the total amount of binder present.

[0017] The geopolimer binder is preferably, but not necessarily, present in the composition of the friction material according to the invention in an amount of at least 5% by weight, or more preferably between 20% and 60% by weight, calculated based on the total volume of the friction mixture / composition. In fact, experiments have shown that if the amount of inorganic binder is too low, the mechanical properties required for its use as a friction material cannot be achieved, depending on the type of geopolimer used as the binder and the nature of the other materials used in the composition.

[0018] Thus, the friction material according to the method of the present invention is substantially or completely lacking in organic binder (which may be present in an amount of up to 10% by weight), and for this reason, it is not subject to thermal degradation by oxidation up to high temperatures, for example, above 300°C and above 600°C.

[0019] Manufactured according to the method of the present invention, used as the single and main binder in the friction material of the present invention, and thus, in a state where there is no or almost no conventional organic binder, the dominant (i.e., constituting at least 90% of all binders present) geopololymer binder is obtained by a chemical reaction starting from inorganic precursors such as SiO2 and Al2O3. Specifically, for example, commercially available sodium silicate (and / or potassium silicate) manufactured by "PQ Corporation - Holland" is used, and in some cases, a small amount of sodium hydroxide or potassium hydroxide is added (which functions even in a state where there is almost no hydroxide), and metakaolin obtained by high-temperature firing of commercially available kaolin, such as kaolin manufactured by "Imerys Refractory Minerals ‐ Argical‐M 1200S", containing about 55% SiO2, 39% Al2O3, and further Fe2O3, TiO2, K2O, Na2O, Cao, MgO impurities by weight is used, and is generally assumed to have the following general chemical formula: Al2O3·2SiO2 The inorganic geopololymer binder according to the present invention is prepared in a premixed form and can then be directly bonded as it is to all of the other component materials of the friction material mixture, preferably in any of a Lodige or Ehrlich mixer or other mixers commonly used for friction materials, such as an Ehrlich mixer. The unfinished compound thus obtained then undergoes a shaping process to produce the desired friction element, such as a brake pad or block.

[0020] However, according to a preferred embodiment of the present invention, instead, it is prepared during the mixing step of the entire friction composition in order to directly produce a raw friction formulation that is then shaped into a block of the friction material having the desired properties.

[0021] Synthesis of Geopolymer Binder Similar to the method of EP3841311, the geopolimer binder used in the friction composition for brake elements is prepared from metakaolin, which is reacted with an aqueous solution of sodium hydroxide and / or potassium hydroxide, with the addition of sodium silicate to the caustic solution, resulting in the formation of an amorphous geopolimer, which can be converted, if necessary, through further heat treatment only, into at least a partially crystalline form.

[0022] For this reason, the following description refers only to sodium compounds without loss of generality, since it is clear to those skilled in the art that the same techniques can be used for potassium compounds.

[0023] First, an aqueous basic sodium silicate solution is formed (e.g., by adding sodium hydroxide), and any form of sodium silicate can be dissolved in water, and commercially available soda pellets can be added. Then, metakaolin is added to this basic aqueous solution, either all at once or gradually with mixing, or conversely, the basic soda and silicate solution is gradually added to the metakaolin powder to obtain a homogeneous paste having a relatively high SiO2 / Al2O3 ratio, which is maintained in the range / interval of 3 to 10, i.e., if "x" is the molar ratio SiO2 / Al2O3, the effective ratio must be as follows: 3 < x < 10 This wet paste, similar to the slurry, is removed from the mixer and subjected to the steps of shaping and drying in any temperature range up to 300 °C and in any atmosphere (which may be under vacuum) using a suitable shaping and drying system, preferably a tape casting device as (only schematically) shown in Italian Patent Application Publication No. 102020000015202.

[0024] As already disclosed in this published Italian patent application, the mixing of the silicate solution and metakaolin can include a single mixing at a speed of about 500 rpm to about 1000 rpm for a time of about 1 minute to about 20 minutes.

[0025] The mixing of the silicate solution and metakaolin may be carried out at a temperature of about 20°C to about 40°C.

[0026] Thereafter, the wet paste / mortar / slurry thus obtained and coming out of the mixer is spread on a support to form a layer of uniform thickness and subjected to heat treatment to dry it in order to obtain a tape made of a dry / semi-dry geopolimer material.

[0027] According to IT102020000015202, the dry tape may have a moisture content of any value comprised between 0%w and 20%w and a thickness between about 0.1 mm and about 2 mm.

[0028] The support may consist of paper, a plastic film or a steel sheet. For example, the support may consist of Sappi® paper or Coveme® film.

[0029] More generally, according to the present invention, a support in the form of, for example, an endless belt conveyor is not sensitive to a basic atmosphere and may be made of a specific material suitable for a neutral or alkaline paste / mortar, for example, Mylar or another type of material suitable for a neutral / alkaline paste / mortar. The paste is shaped into a tape (in this case, it is also suitable to apply a mechanical stress with a high shear stress to the paste), and during drying, a geopolimerization reaction occurs in which metakaolin dissolves in an alkaline sodium silicate solution. The oligomers formed then condense together to create a 3D geopolimer network.

[0030] The drying step is preferably carried out in an oven at a controlled temperature (single or multi-stage oven), where the oven at a controlled temperature can have a temperature profile adapted by a control device. The drying step can be carried out discontinuously or continuously. When carried out continuously, a tunnel oven / furnace through which a layer of the wet paste applied on the support passes can be used.

[0031] According to a first main feature of the present invention, unlike what is taught in IT102020000015202, instead of trying to dry the wet paste in a controlled manner so that it already reaches any desired moisture content when leaving the oven, a drying treatment is preferably carried out at a temperature of 100 - 250 °C to obtain a completely dried or almost completely dried aggregate residue consisting of an amorphous geopolimer having a moisture content equal to zero or, in any case, lower than the desired final moisture content.

[0032] Then, according to a second main feature of the present invention, which is adopted in combination with the first feature above, this completely dried or almost completely dried geopolimer is rewetted in a suitable mixer to reach the desired moisture content.

[0033] According to a further aspect of the present invention, such a desired moisture content should be included within a very narrow and precise range. In particular, the final moisture content of the geopolimer binder of the present invention should be 4%w - 16%w of the total weight of the geopolimer.

[0034] In fact, it has been experimentally proven that it is possible to obtain a friction material that can be easily formed into blocks / layers with sufficient strength and elasticity, and at the same time has substantially no cracks or defects and has the required braking performance only within such a specific restricted interval of the humidity of the amorphous geopolimer.

[0035] Therefore, according to the present invention, since the water content tolerated in the geopolymers during the mixing stage (i.e., the step in which the complete friction material mixture / composition is obtained) is 4%w to 16%w, the expression "completely dry or almost completely dry aggregates" means geopolymers aggregates emerging from the drying stage / step having a water content equal to approximately 0 or, in any case, less than the value included in the above range of 4 to 16%w, depending on the desired final water content, so that it is possible to re-wet the geopolymers to the desired water content and add a significant amount of water directly or indirectly. "Substantially", here and hereinafter, intends the final amount of added water / moisture on the order of "n"%w (where "n" can be, for example, from about 1 to about 16).

[0036] According to the present invention, the tape-shaped dry / almost dry aggregates formed by the amorphous geopolymers emerging from the oven are ground into powder using any suitable grinding system, preferably a ball grinder or a jar mill or a hammer mill, until a particle size distribution of less than 600 microns, preferably less than 400 microns, is obtained.

[0037] Thereafter, according to the present invention, the powder thus obtained is re-wetted to reach the desired moisture level within the aforementioned range of 4%w to 16%w, regardless of its moisture content.

[0038] According to different embodiments of the present invention, the re-wetting process can be carried out either before the final mixing stage to obtain the desired friction material, in which the geopolymer powder is mixed together with the other component materials of the friction material, or during exactly the same final mixing step, i.e., while the raw (not yet shaped) friction material is being prepared by mixing its various components together.

[0039] This second embodiment may be preferred.

[0040] According to different embodiments of the present invention, the rewetting process can be carried out either by adding the required amount of liquid water to the powdered and dried geopolymers or by adding a calculated amount of salts having a chemical and / or physical water content thereto, such as hydrated salts.

[0041] According to a further aspect of the present invention, suitable salts for rehydrating the dried or substantially dried geopolymers can be selected from the group consisting of, but not limited to, the following: sodium and / or potassium carbonate decahydrate (e.g., Na2CO3·10H2O), sodium and / or potassium phosphate tribasic dodecahydrate (e.g., Na3PO4·12H2O), sodium and / or potassium sulfate decahydrate (e.g., Na2SO4·10H2O), disodium tetraborate (or potassium) dihydrate (e.g., Na2B4O7·10H2O), any combination thereof.

[0042] Disodium tetraborate dihydrate is chemically effective but is a potentially dangerous product and is preferably not used for safety reasons.

[0043] In any case, the rewetting step can be carried out in any way, but is preferably carried out during the final mixing of all the component materials of the friction material mixture / composition, i.e., the dried or substantially dried geopolymers are powdered and then, if the rewetting component / agent, such as liquid water or hydrated salts, is also added together (in combination), they are used as component materials of the friction material mixture.

[0044] Embodiments in which liquid water is used as the rewetting agent and is added only during the final mixing step of the friction material mixture / composition, i.e., when all other component materials of the friction material mixture are also present, may be preferred because the liquid water at least partially avoids the possible accidental dispersion of the component materials, particularly the geopolymers, in the environment before / during the mixing step.

[0045] The rewetted geopolymer is mixed, either after or during rewetting, with other conventional components of the friction composition, such as fillers, lubricants, abrasives, fibers, etc., to obtain a mixture of friction materials that is shaped as in EP3128201. During shaping, the pre-synthesized geopolymer particles remain solidified and amorphous simply for the purposes of pressure and heating, and as a result, a friction element, typically a brake pad, is obtained, in which the component materials are dispersed in a matrix composed only of an amorphous geopolymerized inorganic binder (excluding a possible amount of less than 10% of the organic binder). According to the present invention, in order to properly solidify the geopolymer at this stage / step of the manufacturing process, only an exact and limited amount of moisture is present in / with the geopolymer, specifically, 4%w to 16%w, preferably 8% to 12% by weight of moisture (either directly or indirectly, i.e., present in hydrated salts), and it has been experimentally proven that the two extreme values of the above interval are included.

[0046] The friction element thus obtained does not produce waste due to cracking or delamination even when using pressures on the order of several tens of MPa. As a result, the powder is re-solidified under forming conditions comparable to those of EP3841311 and under the normal forming conditions of brake pads, and a braking performance comparable to that of the friction material manufactured according to the hydrothermal synthesis of EP3841311 is obtained, and the material and disc wear due to use are comparable to those of the same components manufactured according to EP3128201 or EP3841311.

[0047] Forming for the re-solidification of geopolymer powder The forming of the brake pads obtained by the method of the present invention is carried out by placing the raw material compound (friction mixture) into a mold having a characterized treated metal support or backplate, with or without a known damping / insulating layer called the "lower layer", and this lower layer, during the forming stage, not only forms a layer or block of the friction material on top of the lower layer if present, but also achieves the adhesion of this layer or block to the metal support.

[0048] Forming is carried out at a temperature of 40 to 250 °C and a pressure of 150 to 2000 Kg / cm 2By working for a time of 1 to 30 minutes at the pressure of 2 or by preforming the raw material compound or mixture in a mold and then molding the preformed compound on a back plate at a temperature of 40 to 250 °C and a pressure of 150 to 2000 Kg / cm

[0049] Alternatively, the raw material compound can be molded to obtain a friction material block, and then, for example, using a phenolic or silicon-based adhesive, it is connected to a metal support or a metal back plate (with or without a lower layer).

[0050] Other components of the friction material The components of the friction material composition or raw material compound produced according to the present invention may be components used in friction materials already known in the art, but only note that the current organic binder is completely replaced with the inorganic binder obtained by the above method, while reducing the content of the abrasive and increasing the content of the lubricant.

[0051] The friction material obtained according to the present invention preferably does not contain copper and / or its alloys in both the form of powder and fiber.

[0052] In particular, the fiber-made components may be made of any organic or inorganic fiber other than asbestos, or may be any metal fiber generally used in friction materials, preferably excluding copper and its alloys. Specifically, inorganic fibers such as glass fiber, wool fiber, rock wool fiber, wollastonite, sepiolite, attapulgite, etc., organic fibers such as aramid fiber, polyimide fiber, polyamide fiber, phenolic fiber, cellulose, acrylic fiber, PAN (polyacrylonitrile), etc., metal fibers such as steel fiber, stainless steel, aluminum, zinc, etc. can be mentioned.

[0053] The fiber may be used in the form of short fiber or powder.

[0054] The amount of fiber is preferably 2% to 30% by volume, more preferably 8% to 15% by volume, of the total volume of the friction material, and the fiber component preferably always includes rock fiber which has been shown to have a strong affinity with the geopolymers used as binders.

[0055] A number of materials known in the art can be used as organic or inorganic fillers. Examples include precipitated calcium carbonate, barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride, slaked lime, talc, mica.

[0056] These can be used alone or in combinations of two or more. The amount of these fillers is preferably 2 to 40% by volume based on the total composition of the friction material.

[0057] Friction modifiers (which can include all or part of the fillers) can include, in addition to carbon materials or nanomaterials such as graphene, organic fillers such as cashew dust, rubber dust, powdered tread rubber, various unvulcanized rubber particles, various vulcanized rubber particles, inorganic fillers such as barium sulfate, calcium carbonate, calcium hydroxide, vermiculite and / or mica, abrasives such as silicon carbide, alumina, zirconium silicate, metal sulfide-based lubricants such as molybdenum disulfide, tin sulfide, zinc sulfide, iron and non-ferrous sulfides, metal particles other than copper and copper alloys, and / or combinations of the above.

[0058] Abrasives can be classified as follows (the following list is merely illustrative and not necessarily exhaustive or limiting): · Weak abrasives (Mohs 1 - 3): talc, calcium hydroxide, potassium titanate, mica, kaolin, vermiculite; · Medium abrasives (Mohs 4 - 6): barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide; · Strong abrasives (Mohs 7 - 9): silicon carbide, zircon sand (zirconium oxide), zirconium silicate, zirconia, corundum, alumina, mullite.

[0059] Preferably, but not necessarily, the friction material obtained according to the present invention does not contain strong abrasives and contains only medium or weak abrasives since the diopolymer produced as a binder is itself already a medium abrasive.

[0060] The friction material produced according to the present invention may preferably contain graphite in an amount of 5% - 15% by volume based on the total composition of the friction material.

[0061] The total content of the lubricant may preferably be 4% - 20% of the total volume of the friction material, depending on the desired frictional properties, and may particularly contain graphene.

[0062] Curing and painting It is cured during pressing, and the molded article (brake pad) that is generally already usable after this simple press molding is, optionally, further post - cured by an auxiliary heat treatment at 80 - 450 °C for 10 minutes to 15 hours if required by the formulation and / or design specifications, and then spray - painted or powder - coated, oven - dried, and, if necessary, machined mechanically to produce the final product.

[0063] The friction material obtained by the method of the present invention can be used for various applications such as disk brake pads, shoes, linings, etc. of various vehicles such as automobiles, trucks, train vehicles, etc. and industrial machines, or clutch disks, either after simple press molding or after any additional heat treatment if necessary.

[0064] The present invention will now be described in more detail by reference to its non - exhaustive and non - limiting examples of implementation and to the figures of the accompanying drawings.

Brief description of the drawings

[0065]

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DETAILED DESCRIPTION OF THE INVENTION

[0066] Examples and comparative examples are given herein by way of illustration and are not, therefore, intended to limit the present invention.

[0067] Device according to the present invention Referring to Figure 1, an apparatus or plant 2 configured to carry out the method of the present invention for manufacturing a brake pad 1, which is itself part of the present invention, as well as the associated brake pad 1 obtained by the method carried out by the apparatus 2 are shown only schematically.

[0068] The apparatus or plant 2 may be of the continuous or batch type and in the non-limiting embodiment shown is of the continuous type and is configured to carry out several different operations / steps in a time sequence in a corresponding number of dedicated devices and, in the case of a continuous type plant, is arranged in a physical sequence along direction D as also shown in Figure 1. The apparatus or plant 2 comprises the following: · Adding a caustic silicate solution 4 and metakaolin 5 in water to a first mixer 3 of any known type, preferably a dispersion mixer, and mixing them together to obtain a semi-liquid diopolymer paste / slurry 6; such a diopolymer paste can be formed using, for example, a solution of sodium silicate, potassium silicate, lithium silicate or any other chemically equivalent aqueous solution, i.e. a solution of an alkali silicate. The mixer 3 may be provided with a temperature control system 30 of any known type: · Any known type of tape casting machine 7 configured to cast the newly formed geopolymers 6 in the form of a layer / tape 8 of substantially uniform thickness, the layer / tape of geopolymers being placed on a support 9; preferably, the tape casting machine 7 comprises an endless belt conveyor 10, the upper surface of the upper branch of which forms the support 9. The machine 7 preferably comprises a blade 11 for uniformly reducing the thickness of the layer / tape 8 of geopolymers to any selected value within the range of 0.2 to 2 mm, and pressing means (not shown) capable of pressing the geopolymer paste with a predetermined force F. · A hot air furnace or oven 12, which, in the non-limiting embodiment shown, associated with the continuous tape casting machine 7, is traversed by the conveyor 10 and is preferably a tunnel oven / furnace. According to one aspect of the invention, the oven 12 is configured to bring the layer / tape 8 of geopolymer paste / slurry into a completely or almost completely dry state 8b, where the expression "completely dry or almost completely dry state" means that the geopolymer aggregate exiting the furnace / oven 12 has a moisture content equal to approximately zero or lower than the value included in the range of 4 to 16% w, depending on the desired final moisture content in the friction material. · A mill 14 (e.g., a ball mill or a jar mill, preferably a hammer mill) disposed downstream of the oven / furnace 12 (with respect to the direction D), for example fed by the conveyor 10, configured to receive the dried or almost dried geopolymer 8b and grind it into a powder 8c having a predetermined range of particle size distribution included between 1 and 500 microns, preferably between 1 and 100 microns. · At least one humidity detector 18 disposed downstream of the oven / furnace 12 and configured to detect the humidity of the reacted geopolymer when it exits the oven / furnace 12. · A second mixer 20 of any known type (e.g., a Lodige or Ehrlich mixer) arranged downstream of the mill 14 and configured to receive the dry or substantially dry geopolymers 8b comminuted into the powder 8c and a predetermined amount of water in the form of liquid water or hydrated salts. The mixer 20 is configured to re-wet the dry or substantially dry geopolymer powder 8c to an exact moisture content that is substantially equal to or lower than the moisture content that the reacted geopolymer 8 has at the outlet of the casting machine 7. The mixer 20 may be configured to receive all other component materials of the resulting friction material, schematically indicated by the arrow 21 in FIG. 1. According to a different (possibly preferred) embodiment schematized by a dotted line in FIG. 1, the second mixer 20 (in this case, a Lodige or Ehrlich mixer) is configured only to re-wet the powder 8c to a predetermined humidity value either by direct addition of liquid water or by indirect addition of water by addition of hydrated salts, and the apparatus or plant 2 includes a third mixer 20b arranged downstream of the mixer 20 and configured to receive the accurately hydrated geopolymer 8d (dotted arrow) re-wetted from the mixer 20 and all other component materials 21 of the resulting friction material. In both cases, what exits from the mixer 20 (or 20b) is a "green" or "raw" friction material mixture / composition 25; · Any known type of forming device 26 schematized by a block to receive the "green" or "raw" friction material mixture / composition 25 and to facilitate shaping it into a friction material block or layer 27 having, alone or almost entirely, a well-consolidated matrix of geopolymers as a binder. The forming device 26 may be configured to form a block of the consolidated friction material 27 directly on the support or backplate 28 to obtain the brake pad 1, or to form a block of the consolidated friction material 27 that is subsequently applied / fitted to the support 28 to obtain the brake pad 1.

[0069] Thus, in contrast to what is disclosed in IT102020000015202, there is no need to use sophisticated humidity sensors and complex control devices to provide the geopolimer powder 8c with the desired humidity content. In fact, it is possible to easily calculate the water / humidity lost by the geopolimer 8 reacted in the oven / furnace 12, and since its initial humidity and weight, as well as its final weight after drying, are known, the amount of water (or hydrated salt) added to the mixer 20 can be accurately measured.

[0070] Furthermore, tests conducted by the applicant have demonstrated that, as shown in more detail below, the solidification of the geopolimer during the forming step of the friction material in the apparatus 26, at the same humidity content of the geopolimer, without the rewetting step being carried out in the mixer 20, results in a much better required moisture content in the geopolimer than when obtained by the accurate and precise control of the drying step in the oven 12 according to IT102020000015202, which has been found to be difficult to obtain anyway due to the need for continuous monitoring of the instantaneous humidity of the geopolimer during processing and the inevitable thermal inertia of the entire drying apparatus and the mass of the geopolimer being processed.

[0071] Referring to FIG. 6, for greater clarity, a comparison is shown between the prior art methods (labeled "classical approach") according to EP3841311 and IT102020000015202, and two different possible embodiments of the present invention labeled "Version A.1" and "Version B", all of which are configured to finally obtain the brake pad 1 having the geopolimer as a specific or general binder.

[0072] As clearly shown in FIG. 6, the classical method of the prior art involves four main steps after obtaining the geopolymers according to any one of the approaches disclosed in EP3841311: In the first step, the synthesized geopolymers are dried to form a tape within a specified humidity range. In the second step, the tape of geopolymers is ground to obtain a specified particle size distribution. In the third step, the ground geopolymers are used as a binder to prepare a friction material mixture or composition by any suitable conventional method. In the fourth step, the friction material mixture is molded to form the brake pad 1 (or the friction material block 27 which is later joined to the back plate 28 to obtain the brake pad 1).

[0073] In a first embodiment of the method of the present invention labeled A.1, after obtaining the geopolymers according to any one of the approaches disclosed in EP3841311, five main steps are carried out instead of four main steps: · In the first step, the synthesized geopolymers are dried to a moisture content of “x”, where x ≥ 0% and is in any case lower than the optimum amount (and thus lower than the specified humidity range of the classical method); · In the second step, the dried or almost dried tape of geopolymers is ground to obtain a certain particle size distribution; · In the third step, the friction material mixture or composition is prepared by any suitable conventional method in which all other component materials of the desired friction material mixture are added to the dried or almost dried and ground geopolymers; · In the fourth step, a certain amount of liquid water calculated to obtain the desired precisely specified humidity of the geopolymers is added to the friction material mixture prepared in the third step: In this way, the dry geopolymers and water are used in combination as a binder, and both such components are generally added to the same mixer together with the other component materials of the desired friction material mixture; · In the fifth step, the friction material mixture is molded to form the brake pad 1.

[0074] In a second embodiment of the method of the present invention (labeled B), after obtaining the geopolymers according to any one of the approaches disclosed in EP3841311, four main steps are carried out: · In the first step, the synthetic geopolymer is dried to a moisture content “x”, where x ≧ 0% humidity and is in any case lower than the optimal amount (and thus lower than the defined humidity range of the classical method); · In the second step, the tape of the dried or nearly dried geopolymer is ground to have a certain particle size distribution; · In the third step, the friction material mixture or composition is dried or nearly dried and the ground geopolymer is a) combined with all the other component materials of the desired friction material mixture and b) a defined amount of hydrated salt - this sub-step b) corresponds to the fourth step of embodiment A.1 - and is prepared by any suitable conventional method of addition; · In the fourth step, the friction material mixture is molded to form the brake pad 1.

[0075] Similar to embodiment A.1, a further embodiment of the method of the present invention, which can be labeled as embodiment A.2 (not shown for simplicity), is also possible, where five steps are carried out again: the first and second steps are identical to the corresponding steps of embodiment A.1; the third step consists of adding a defined amount of liquid water to the dried or nearly dried geopolymer powder in a first mixer to obtain a wet geopolymer powder; the fourth step consists of preparing the friction material mixture or composition in a second different mixer using the wet geopolymer powder as a binder; the fifth step consists of molding the friction material mixture to form the brake pad 1.

[0076] Method according to the invention - working example A silicate solution having an appropriate composition (produced by mixing water, hydroxide, and solid silicate supplied by PQ corporation) and commercially available metakaolin are mixed at a solution / metakaolin weight ratio of 1 to 10 (including both ends) for an Si / Al molar ratio in the range of 1 < x < 10; preferably, this range can vary from 2 to 6. Different ratios with higher Al or Si contents are also possible, but experimental results and theoretical calculations lead to the conclusion that the present invention operates with maximum efficiency at an Si / Al ratio of 2 to 6.

[0077] The caustic silicate solution and metakaolin are mixed by mechanical stirring to form a homogeneous paste.

[0078] The paste thus obtained is spread on a plastic mat using the "tape casting" technique and dried at a temperature of 70 to 250 °C under atmospheric pressure for a time in the range of 1 minute to 90 minutes depending on the output of the oven used, reducing the weight of the mixture to 10 to 40% of the original weight and converting it to a pure amorphous geopolimer.

[0079] The dried silicate-metakaolin geopolimer system is taken out of the dryer and ground in a ball grinder. Its final water content is calculated by considering the maximum amount of water that the system can lose, which corresponds to 0% powder moisture.

[0080] The geopolimer powder thus produced is re-wetted in a Lodige or Ehrlich mixer (or other mixer) by adding an appropriate amount of liquid water to an exact and desired humidity content contained between 4%w and 16%w, and the binder thus produced in the form of a hydrated powder is added to the other raw materials required by a friction material mixture or composition selected for dry mixing using a known mixer, such as a Lodige or Ehrlich.

[0081] The mixture or composition of the "undried" friction material thus obtained is hot formed under pressure to obtain a series of brake pads.

[0082] Forming The forming stage involves placing a metal support having a raw or "undried" compound and optionally a lower layer into a mold (known and not shown for simplicity) heated to a temperature of 60 - 250°C, and subjecting the raw material compound to a forming pressure of 150 - 2000 Kg / cm 2 for 1 - 15 minutes, or pre - forming the raw material compound 11 in the mold and then forming the pre - formed compound onto the metal support, and operating at a temperature of 100 - 250°C and a forming pressure of 150 - 2000 Kg / cm 2 for 1 - 15 minutes.

[0083] Alternatively, the raw material compound can be formed without a metal support to obtain only the friction material block, and then it is pressed, using a phenolic or silicon - based adhesive, for example, the friction material block against a metal support having a possible lower layer and operated at a temperature of 180°C for 30 seconds, and adhered to the metal support by a known method, whether or not it has an insulator / damping layer (known) or a lower layer.

[0084] In either case, the forming pressure must always be greater than the water saturation pressure at the forming temperature.

[0085] At the end of the above - mentioned method, an asbestos - free friction material containing inorganic and / or organic and / or metal fibers, at least one binder, at least one friction modifier or lubricant, and at least one filler or abrasive as component materials is thus obtained, where the binder is at least 90% constituted by a completely consolidated silica - aluminum dipolymer.

[0086] The component materials of the raw material compound are added to the inorganic binder in a suitable amount such that the total amount of the inorganic dipolymer binder is preferably 20% or more and 60% or less of the weight of the total volume of the friction material, but not necessarily so, and even more preferably equal to about 47% by weight.

[0087] After obtaining the binder and before the curing stage / step (which usually coincides with the molding stage), no asbestos or its derivatives, or copper or its alloys are added to the friction material composition as its constituent materials. Thus, the friction material obtained according to the present invention substantially does not contain or hardly contains an organic binder, substantially does not contain copper or its alloys and / or fibers of copper or its alloys, and preferably, but not necessarily, substantially does not contain a strong abrasive. Here, in this specification and hereinafter, the term "substantially does not contain" means that the indicated material exists at most as an impurity. Thus, at least one abrasive contained in the friction material according to the present invention is preferably, but not necessarily, a medium abrasive or a weak abrasive. Here, these terms refer to the following classifications: · Weak abrasives (hardness of Mohs 1 - 3): For example, talc, calcium hydroxide, potassium titanate, mica, vermiculite, kaolin; · Medium abrasives (hardness of Mohs 4 - 6): For example, barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide; · Strong abrasives (hardness of Mohs 7 - 9): For example, silicon carbide, zircon sand (zirconium oxide), zirconium silicate, zirconium, corundum, alumina, mullite.

[0088] The volume ratio between the lubricant and the abrasive contained in the friction material to be molded is preferably selected between 1:1 and 1:4 (for comparison, this ratio is generally 1:8 or more in known friction materials having an organic binder).

[0089] Furthermore, the starting materials for obtaining the geopolimer binder are selected such that the inorganic geopolimer binder in the friction material according to the present invention has an SiO2 / Al2O3 ratio of 3 - 10 and an SiO2 / Na2O ratio of 3 - 10. The densification of the geopolimer powder is obtained during molding.

[0090] Example 1 - Comparative Production of Binders 115.7 g of metakaolin from "Imerys Refractory Minerals" is mixed with 300.0 g of an aqueous solution of 139.4 g of sodium silicate in any form from "PQ Corporation - Holland" (as shown previously, potassium silicate is also effective), and 1.51 g of pre - prepared pelletized caustic soda, using a drill stirrer with a specific mixing whisk for medium - to - high - viscosity fluids, at a speed of 800 rpm for various times from 5 minutes to 45 minutes. The wet paste obtained from mixing the metakaolin with the sodium silicate - caustic soda solution is spread onto a Mylar sheet specific to wet and alkaline pastes / slurries, using a spread paste thickness between - 0.1 and 3 mm.

[0091] Subsequently, a plurality of samples are prepared by drying the wet - coated paste at a temperature of 40°C to 250°C, a sheet size of A3 to A4, and a variable drying time of 10 minutes to 90 minutes. In particular, a reference sample with a controlled humidity of 12% w and a plurality of samples completely dried to a humidity of substantially 0% w are prepared.

[0092] Next, the semi - dry and fully - dry sample binders in solid aggregate form are separately separated from the sheet and ground for 14 hours in a ball grinder rotating at 275 revolutions per minute to granulate the product and obtain a powder with a particle size distribution of approximately 200 microns.

[0093] The semi - dry sample with 12% w humidity is used as is, while the fully - dried samples are re - wetted at different humidities by adding liquid water. The amount of water added to the dry geopolimer (hereinafter also referred to as "GP") powder was calculated to partially or fully fill the amount of water lost during drying. The GP powder and water were mixed in a PE container in a mechanical stirrer at 20 Hz for 10 minutes.

[0094] A homogeneous wet powder was obtained, weighed, and pressed with standard parameters: 150°C - 20 MPa - 10 min.

[0095] The semi-dry GP powder and the fully dry powder at 12% w were also weighed and pressed with the same standard parameters: 150 °C - 0 MPa - 10 min. Disk-shaped samples with sufficient mechanical properties for handling were obtained and tested for their physical properties. The results are reported in Table 1.

[0096]

Table 1

[0097] Figure 2 shows photographs of the sample disks thus obtained: Figure 2a) shows disk samples obtained using dry GP powder re-wetted at 12% w and 9% w humidity, which show good mechanical properties, are dense, and have no cracks; Figure 2b) shows a disk sample obtained using dry GP powder re-wetted at 6% w humidity, which has poor mechanical properties and shows cracks; Figure 2c) shows a disk sample obtained using dry GP powder re-wetted at 3% w humidity, which clearly has no cracks but the mechanical properties indicate that densification with complete chemical reaction has not occurred. The non-re-wetted sample disk in Figure 2d) is a very brittle material and cannot be evaluated.

[0098] The results in Table 1 and Figure 2 are clear evidence of the following: · The water present in the geopolymers can be adjusted through similar reversible processes; · It is clear how the minimum amount of water is required to ensure good part properties and integrity; · The 6% humidity value looks good, but the disks are very brittle; · Similar results were obtained for 9 and 12% wt re-wetting humidity by using geopolymers with a residual humidity in the range of 0% < x < 6% and adding water to reach 9 and 12% wt humidity.

[0099] Example 2 - Binder Obtained by Salt Addition To compare the mechanical properties obtained using the optimal humidity conditions inferred from Table 1, a comparative study is conducted where instead of adding liquid water, hydrated salts are added and the operation is carried out as in Example 1 to re-wet the completely dried GP powder (labeled GP25). A humidity value of 9% is used.

[0100] The GP powder was dried at 150 °C overnight to be completely dry. The measured weight loss was 12% wt. The amount of salt added to each sample of the dried GP25 powder was calculated to match a water content of 9%. A semi-dry 9% w humidity sample (not re-wetted) is used for comparison. The GP25 wet re-wetted powder is compressed into a sample having a disk shape as in Example 1.

[0101] The salts tested, along with the final evaluation of their suitability, are listed in Table 2 below:

[0102]

Table 2

[0103] The mechanical properties of the sample disks are reported in Table 3.

[0104]

Table 3

[0105] As clearly shown by the comparison between Table 1 and Table 3, the re-wetting carried out by the addition of salts gives similar or even better results compared to the benchmark (GP25 reference) and rehydration with water.

[0106] Example 3 - Manufacture of Brake Pads Using the apparatus or plant 2 schematically shown in FIG. 1, a number of identical brake pads 1 are manufactured, and its components are selected on a laboratory scale. For each component, using the average value of the intervals reported in Table 4 below, as the binder shown as the "binder mixture", powders obtained according to Examples 1 and 2 were used at different humidities to prepare the same friction material formulation; GP powder that was only partially dried and had a humidity of 10% wt after production and grinding was used as benchmark references A and B, and GP powder that was completely or almost completely dried and then rehydrated at different moisture contents by using either liquid water or a hydrated salt was compared with the benchmark reference.

[0107]

Table 4

[0108] The binder mixture is added to the other components of the mixture according to a general scheme: 20 - 60% by weight of the binder and 40 - 80% by weight of the other components; the mixing is carried out in a Lodige mixer. The system (dipolymer + water) is 47% wt of the friction mixture.

[0109] Thereafter, the friction material mixture / compound thus obtained is formed into the same brake pad, and the raw or "undried" compound and the metal support are placed in one mold. The forming is carried out by subjecting the raw material compound to a forming pressure of 250 - 720 Kg / cm at a temperature of 100 - 150 / 70 - 135 / 70 - 135 °C for 2 - 15 minutes. 2

[0110] The friction material block 27 thus obtained is tested for its mechanical properties. The experimental results are reported in the form of bar graphs in FIGS. 3 - 9.

[0111] Similar to the study of a pure matrix (as in EP3841311), it is confirmed how the minimum amount of water is required for the activation of consolidation. ​

[0112] The results of compressibility, hardness, and density confirm that, in order to have acceptable mechanical properties, the water content in the friction material must be higher than 9% wt, taking into account the rewetting approach.

[0113] For the test samples obtained through the rewetting approach, it is shown that a higher water content is required for densification compared to pure matrix samples.

[0114] The rewetting approach for the friction material functions similarly but gives different (and better) properties compared to a friction material having the same humidity content that already exists inside the powder after (partial) drying and to which liquid water has not been added as completely or almost completely dried GP.

[0115] Regarding the rewetting approach using hydrated salts (Figs. 7 - 9), a fixed humidity content of 10% wt was selected to confirm which salt functions better. A second benchmark B was also selected to complete the study using AKM characterization.

[0116] The system (geopolymer + water) is 47% wt of the friction mixture. Rewetting with hydrated salts shows properties comparable to those of the rewetting approach with liquid water and, in some cases, even better properties.

[0117] Example 4 - Brake Test The brake pads manufactured as described in Example 3 were subjected to the following tests: The efficiency test by AKM includes a series of sinking brakes, brakes at different fluid pressures, low - temperature (<50 °C) evaluation brakes, simulated highway brakes, and two high - energy brake (first FADE test) series with scattered regenerative brake series. From this test, it is also possible to estimate the wear that the brake pads and disks undergo using methods known to those skilled in the art.

[0118] Excerpts of the results obtained are illustrated in FIGS. 10 to 12, which schematically represent the most significant data of the experimental curves obtained. The graphs are self-explanatory, thanks to the descriptive labels inserted in the figures.

[0119] As can be seen, the experimental AKM results regarding the braking characteristics are very similar to those of the benchmark samples obtained according to EP3841311 and are fully comparable (not worse, especially with regard to the rewetting salt approach).

[0120] Table 5 below shows the results of the wear comparison tests carried out on the materials of FIG. 10.

[0121]

Table 5

[0122] Also, it can be seen that the pads according to the present invention have the same wear as the prior art, even when they are less prone to weight loss due to better compactness.

[0123] *** Finally, it can be concluded that the rewetting approach to reach the desired exact moisture content in the final friction material is a successful approach: the control of the production process is quite good and easy; in case of errors, the rehydrated GP can be fully recovered by completely drying it and then rewetting it again. Furthermore, a very accurate control of the moisture content with reproducible results is obtained, and surprisingly, within the optimal limited range, a more constant braking performance can be ensured in various production batches.

[0124] Therefore, the present invention shows the following advantages: · It is possible to adjust the humidity content of the powder by complete or partial drying. · In the second step, add the desired amount of water to obtain lower restrictions on the production of geopolimer powder, especially to obtain humidity control during the process and always enable having an acceptable humidity range. · It is possible to recover the waste finally generated from the production of geopolimer powder. · The use of liquid water during the production of friction materials has a secondary positive effect of reducing the volatile powder of the mixture thanks to the liquid water that maintains the fine powder fraction in the mixture.

[0125] Accordingly, all the objectives of the present disclosure are achieved.

[0126] Certain specific terms A certain specific braking device, system, and method are disclosed in the context of certain exemplary embodiments, but the scope of the present disclosure extends beyond the specifically disclosed embodiments to other alternative embodiments and / or uses of the embodiments, as well as certain modifications and equivalents thereof, which will be understood by those skilled in the art. The use with any structure is clearly within the scope of the present invention. The various features and aspects of the disclosed embodiments can be combined with each other or substituted for each other to form various modes of the assembly. The scope of the present disclosure should not be limited by the specific disclosed embodiments described herein.

[0127] Conditional language such as "can", "could", "might", or "may" generally conveys that a particular embodiment includes or does not include a particular feature, element, and / or step, unless otherwise specified or otherwise understood within the context in which it is used. Thus, such conditional language is generally not intended to imply that a feature, element, and / or step is required in any way for one or more embodiments.

[0128] Unless otherwise indicated, the terms "approximately," "about," and "substantially," as used herein, still represent an amount close to the recited amount that performs the desired function or achieves the desired result. For example, in some embodiments, the terms "approximately," "about," and "substantially" may refer to an amount within 10% of the recited amount, as the context may indicate. Similarly, the term "generally," as used herein, represents a value, amount, or characteristic that primarily includes or tends towards a particular value, amount, or characteristic.

[0129] The present disclosure expressly contemplates that the various features and aspects of the disclosed embodiments can be combined with or substituted for one another. Accordingly, the scope of the present disclosure should not be limited by the specific disclosed embodiments described above, but should be determined only by a fair reading of the following claims and the full scope of their equivalents.

Claims

Claim 1 A method for manufacturing a block or layer of a friction material that does not contain asbestos and is insensitive to thermal degradation during use, comprising preparing a wet paste formed by mixing an alkali silicate solution with a material selected from the group consisting of metakaolin, kaolin, fly ash, and mixtures thereof, preferably only commercially available powdered metakaolin, and later spreading the wet paste on a support so as to form a layer or tape to be subjected to heat treatment to form a geopolimer aggregate, a) - The heat treatment is to dry the wet paste in an oven / furnace to obtain a completely dry or almost completely dry geopolimer aggregate having a water content lower than the desired water content obtained in the geopolimer in any case, and the method comprises b) - pulverizing the completely dry or almost completely dry geopolimer aggregate to make it into powder, c) - rewetting the completely dry or almost completely dry geopolimer powder to the desired water content, d) - using the pulverized and rewet the powder as an inorganic binder in a friction material compound, and mixing it with inorganic and / or organic and / or metallic fibers, at least one friction modifier or lubricant, and at least one filler or abrasive so as to obtain a raw friction material compound having substantially exclusively or exclusively the pulverized and rewet geopolimer aggregate as the binder, e) - further comprising heating and molding the raw friction material compound at 40°C to 300°C so as to obtain a block of a friction material having at least 90% geopolimer as the binder. A method characterized by this. Claim 2 The method according to claim 1, characterized in that step c) is carried out so as to obtain a final water content of 4% w to 16% w calculated based on the total weight of the geopolimer binder after rewetting. Claim 3 The method according to claim 1 or 2, characterized in that the rewetting step c) is carried out by adding a predetermined amount of liquid water to the completely dry or almost completely dry geopolimer powder. Claim 4 The method according to claim 1 or 2, characterized in that the rewetting step c) is carried out by adding a predetermined amount of hydrated salt to the completely dried or almost completely dried geopolymer powder.

5. The hydrated salt is sodium or potassium carbonate decahydrate (e.g., CNa ２ O ３ *10H ２ O), sodium or potassium phosphate tribasic dodecahydrate (e.g., Na ３ PO ４ *12H ２ O), sodium or potassium sulfate decahydrate (e.g., Na ２ SO ４ *10H ２ O), and is selected from the group consisting of any combination thereof. The method according to claim 4.

6. The method according to any one of claims 1 to 5, characterized in that the rewetting step c) is carried out during and together with the mixing step d) in order to obtain the raw friction material compound having, almost exclusively or exclusively, the ground rewetted geopolymer aggregate as a binder.

7. The method according to any one of claims 1 to 5, characterized in that the rewetting step c) is carried out directly on the completely dried or almost completely dried geopolymer powder obtained after step b) using a Lodige or Ehrlich mixer, preferably by adding the completely dried or almost completely dried geopolymer powder and a significant amount of liquid water to the mixer.

8. A method for obtaining an inorganic binder for an asbestos-free friction material insensitive to thermal degradation during use, comprising: preparing a wet paste formed by mixing an alkali silicate solution with a material selected from the group consisting of metakaolin, kaolin, fly ash, and mixtures thereof, preferably only commercially available powdered metakaolin; and later spreading the wet paste on a support to form a layer or tape to be subjected to heat treatment to form a geopolymer aggregate. a) - The heat treatment consists of drying the wet paste in an oven / furnace to obtain a completely dried or almost completely dried geopolymer aggregate having a moisture content lower than the desired moisture content obtained in any case in the geopolymer, and the method comprises b) - grinding the completely dried or almost completely dried geopolymer aggregate to make it into powder; c) - rewetting the completely dried or almost completely dried geopolymer powder to the desired moisture content, characterized in that the ground and rewetted geopolymer aggregate constitutes an inorganic binder.

9. An inorganic binder for a friction material, characterized by being obtained by the method according to claim 8, not containing asbestos and insensitive to thermal degradation during use.

10. A brake pad (1) comprising a block (27) of an asbestos-free friction material comprising inorganic and / or organic and / or metallic fibres, at least one binder, at least one friction modifier or lubricant, and at least one filler or abrasive, wherein the binder is substantially or completely and exclusively inorganic and is composed of at least 90% amorphous geopolymers or mixtures of amorphous geopolymers, characterized in that it is obtained by the method according to claim 1, brake pad (1).

11. The brake pad (1) according to claim 10, wherein the block (27) of the friction material exhibits a volume ratio between the lubricant and the abrasive comprised in the friction material, selected between 1:1 and 1:

4.

12. An apparatus or plant (2) for manufacturing a brake pad (1) having a block (27) of friction material, wherein the binder is substantially or completely and exclusively inorganic and is composed of at least 90% amorphous geopolymers or mixtures of amorphous geopolymers, the apparatus (2) comprising - a first mixer (3), for example a dispersing mixer, configured to receive and mix a caustic silicate solution (4) and metakaolin (5) in water so as to obtain a semi-liquid geopolymer paste or slurry (6); - a tape casting machine (7) configured to cast the newly formed geopolymer (6) in the form of a layer or tape (8) of substantially uniform thickness and place it on a support (9); - a hot air furnace or oven (12) configured to receive a layer or tape (8) of geopolymer paste or slurry; - a mill (14) arranged downstream of the oven or furnace (12) and configured to grind the layer or tape (8) of geopolymer paste or slurry into a powder (8c) having a predetermined range of particle size distribution, a) the hot air furnace or oven (12) is configured to bring the layer or tape (8) of geopolymer paste or slurry into a completely or substantially completely dry state (8b); b) the mill (14) is configured to receive the dry or substantially dry geopolymer (8b) and grind it into a dry or substantially dry geopolymer powder (8c) having a particle size distribution preferably comprised between 1 and 100 microns. Said device (2) is · at least one humidity detector (18) arranged downstream of said oven / furnace (12) and configured to detect the humidity of the reacted diopolymer after said oven / furnace (12); · a second mixer (20) arranged downstream of said mill (14) and configured to receive the dried or substantially dried diopolymer (8b) pulverized into powder (8c) and a predetermined amount of water in the form of liquid water or hydrated salt, said second mixer (20) being configured to re-wet the dried or substantially dried diopolymer powder (8c) to an exact moisture content below the moisture content of the reacted diopolymer (8) at the outlet of said casting machine (7); · Said device (2) is 〇 configured such that said second mixer (20) further receives all the component materials of said friction material, or 〇 configured such that said second mixer (20) is only configured to re-wet said powder (8c) to a predetermined humidity value by either direct addition of liquid water or indirect addition of water by addition of hydrated salt, said device (2) further comprising a third mixer (20b) arranged downstream of said second mixer (20) and configured to receive the re-wetted, exactly hydrated diopolymer (8d) and all the other component materials (21) of said friction material; · further comprising a shaping device (26) configured to receive the friction material composition (25) obtained with said second or third mixer in order to shape it into a friction material block or layer (27) having a well-consolidated matrix of diopolymer alone or almost completely as a binder. Device or plant (2), characterized in that.

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