An improved geopolymer friction material, as well as related methods and brake pads, particularly for manufacturing brake pads

Abstract

An inorganic and / or organic and / or metallic fiber, at least one binder, at least one friction modifier or lubricant, and at least one filler or abrasive are mixed together, and the binder is composed of at least 90% by weight of at least one amorphous geopolymers containing at least two different alkali metal aluminosilicates, and the alkali metals are selected from different metal combinations of the group consisting of Na, K, Li, Ce, Rb, a friction material for a brake element and related methods.

Description

Technical Field

[0001] Cross - reference to related applications This patent application claims priority from Italian Patent Application No. 102022000014038, filed on July 1, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to an improved geopolimer friction material specifically designed for the manufacture of brake pads, and a method for its preparation. The present invention also relates to related brake pads manufactured using such an improved friction material prepared by such a method.

[0003] The friction material of the present invention is specifically intended for the manufacture of friction elements such as braking elements, i.e., non - asbestos friction layers / blocks for vehicle brake pads or shoes and / or friction discs, having performance similar to or better than those belonging to the friction materials of the NAO ("non - asbestos organic friction material"), "low - steel" and "semi - metallic" classes.

Background Art

[0004] The application publication EP3128201 of the same applicant, the entire content of which is incorporated herein by reference for its necessary parts, discloses a method for obtaining a binder for brake pads composed of at least 90% geopolimer, as well as related friction materials and brake pads.

[0005] In EP3128201, the binder is obtained by dry - grinding caustic soda flakes and then dry - mixing the soda powder with kaolin. This procedure, although chemically efficient, involves a significant series of potential safety risks to the operator.

[0006] To overcome this problem, EP3841311, also of the same applicant, the entire contents of which are hereby incorporated by reference in their entirety for all purposes, discloses a process using an aqueous sodium silicate solution containing a minimum amount of sodium hydroxide which, in place of kaolin, uses metakaolin and which, in either case, can be used as a reactant.

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

[0008] Also according to EP3841311, other raw materials such as silica general sources such as quartz or colloidal silica dissolved in a basic sodium or potassium hydroxide solution under suitable conditions can be used. In this way, a geopolymers either sodium-based or potassium-based is obtained.

[0009] In either case, EP3841311 teaches a process in which the wet mortar produced by adding the above-mentioned solution of alkali silicate to metakaolin to form a slurry by mechanical mixing is subsequently dried by an atmospheric drying process and subjected to a weight loss of 5% to 40% from the original weight and has a related residual moisture of less than 30% in the final weight to form a dried or partially dried geopolymer tape which is still to be densified. This product is then ground to a size of 800 microns or less, preferably less than 400 microns, and the resulting powdered material is used as a binder for the production of a mixture / composition for brake pads similar to that disclosed in EP3128201.

[0010] Subsequent tests conducted by the technical staff of the applicant have shown here that the chemical properties of the alkali silicate solution are important for both the final performance of the friction material mixture and the manufacturing efficiency and ease of the friction material blocks for braking applications.

[0011] In particular, the heat resistance of sodium-only-based geopolymers has been shown to exhibit limitations under high temperature and high braking stress conditions, as demonstrated during braking tests conducted on vehicles (i.e., hot judder, high-speed fading).

[0012] Conversely, potassium-only-based geopolymers do not exhibit the hot judder phenomenon when used as binders for friction material pads / blocks, but present some issues regarding the manufacture of the geopolymers themselves; in particular, their rheology is not easily adjustable, and as a direct result, it can lead to a more brittle tape with more cracks after the drying process. SUMMARY OF THE INVENTION

[0013] The object of the present invention is to provide a friction material having a binder composed entirely or almost entirely of geopolymers, which overcomes the drawbacks of the prior art as disclosed above. In particular, the object of the present invention is to provide, at the same time, a friction material of the type disclosed in EP3841311 that is easily manufactured and is hardly subject to operational problems when heated or overheated.

[0014] Also, an object of the present invention is to provide a method for improving the rheology and manufacturing process of geopolymers.

[0015] Also, an object of the present invention is to provide a method for manufacturing such friction materials and friction layers / blocks made therefrom, which avoids or at least limits problems during high heat stress tests such as hot judder, fading and other similar tests.

[0016] A further object of the present invention is to provide a braking element obtained by the above method, for example, a vehicle brake pad or shoe.

[0017] Accordingly, the present invention relates to a friction material having a binder formed entirely or substantially entirely by a geopolimer, a friction material and a braking element such as a layer / block for a friction element such as a vehicle brake pad or shoe, as defined in the appended claims.

[0018] The present invention also relates to a related geopolimer binder adapted to be used for manufacturing a friction layer / block, particularly for a brake pad or shoe, made of a friction material produced by the method of the present invention.

[0019] In particular, the friction material according to the present invention is based on the mixing of sodium and potassium-based geopolymers to improve geopolimer production, due to the rheological properties of sodium-based geopolymers and, in combination, the heat resistance during high-stress episodes such as the downhill drop typical of potassium-based geopolymers.

[0020] The friction material according to the present invention includes, as its component materials, inorganic and / or organic and / or metal fibers; a binder consisting essentially or entirely of a geopolimer or a mixture of geopolymers; at least one friction modifier or lubricant including, for example, sulfur and / or a carbonaceous material or a nanomaterial; and at least one inorganic or metal filler or abrasive, wherein, however, the main abrasive action in the friction material of the present invention is carried out by the geopolimer matrix of the pad produced by the binder.

[0021] Hereinafter, "a binder composed almost entirely of geopolymers" refers to a binder for a friction element in which the geopolymer or geopolymer composition or mixture constitutes at least 90% by weight (90% by weight of the total amount of the binder present).

[0022] The geopolymer binder is not essential but is preferably present in an amount of 5% by weight or more, or more preferably between 20% and 60% by weight, calculated based on the total weight of the friction mixture / composition, in the composition of the friction material according to the present invention. In fact, experiments have shown that if the amount of the inorganic binder is too small, depending on the type of geopolymer used as the binder and the properties of the other materials used in the composition, the mechanical properties required for its use as a friction material cannot be achieved.

[0023] Therefore, the friction material according to the present invention is almost completely or completely lacking in an organic binder (which may be present in a maximum amount of 10% by weight or less), and for this reason, it cannot be subjected to thermal degradation by oxidation at high temperatures, for example, above 300 °C and up to above 600 °C.

[0024] The geopolymer binder of the present invention is used as the single main binder in the friction material according to the present invention, and thus predominates (i.e., constitutes at least 90% of the total binder present) by a chemical reaction starting from inorganic precursors such as SiO2 and Al2O3 in the complete or almost complete absence of traditional organic binders.

[0025] According to the main feature of the present invention, the geopolymer binder that is to be used as the single or main binder in the friction material according to the present invention is specifically, for example, commercially available sodium and potassium silicates manufactured by "PQ Corporation - Holland" and / or "Tillmanss", with the addition of a small amount of sodium hydroxide and / or potassium in some cases (which also act in the almost complete absence of hydroxides), and generally has the following general formula: Al2O3·2SiO2 Commercially available metakaolin, such as the metakaolin obtained by high-temperature calcination of kaolin manufactured by "Imerys Refractory Minerals - Argical-M 1200S", which is assumed to have, is obtained by combining metakaolin containing Fe2O3, TiO2, K2O, Na2O, Cao and MgO impurities in addition to approximately 55% by weight of SiO2 and 39% by weight of Al2O3.

[0026] The inorganic geopololymer binder according to the present invention is prepared in a premixed form and can then be directly combined with all of the other component materials of the friction material mixture, preferably in a Loedige mixer or in any of the other mixers commonly used for friction materials, such as a Henschel or Eirich mixer. The unfinished compound thus obtained then undergoes a molding process for manufacturing the desired friction element, such as a brake pad or block.

[0027] According to an alternative embodiment, the inorganic geopololymer binder according to the present invention can be prepared during the step of mixing the total friction composition to directly obtain a raw friction compound that will subsequently be molded into a block of friction material having the desired properties. Synthesis of Geopolymer Binder Similar to the method of EP3841311, the geopololymer binder to be used in the friction composition for braking elements is prepared from metakaolin prepared to react with an aqueous solution of soda and / or caustic potash, preferably used in combination with each other, to which a caustic solution of sodium and potassium disilicates combined with each other has been added, resulting in the formation of an amorphous mixed geopolymer of sodium and potassium.

[0028] Optionally, commercially available sodium hydroxide and / or potassium hydroxide pellets are added to dissolve all forms of sodium and potassium silicates in water to initially form basic sodium silicate and potassium silicate aqueous solutions (e.g., by adding caustic soda and caustic potash). Then, while maintaining the range / interval between 3 and 10 (i.e., with the molar ratio of SiO2 / Al2O3 as "x", the appropriate ratio must be 3 < x < 10), metakaolin is added all at once or gradually while mixing to this basic aqueous solution, or vice versa, the basic silicate solution is gradually added to the metakaolin powder until a homogeneous paste with a relatively high SiO2 / Al2O3 ratio is obtained.

[0029] This wet paste, like the slurry, is removed from the mixer and subjected to the steps of forming and drying in any temperature regime up to 300 °C and in any atmosphere regime (even under vacuum) using a suitable forming and drying system, preferably a tape casting device such as that shown (only schematically) in the published Italian patent application No. 102020000015202.

[0030] As already disclosed in this published Italian patent application, the mixing of the silicate solution and metakaolin can include mixing at a speed between about 500 rpm and about 1000 rpm for a time between about 1 minute and about 20 minutes.

[0031] The mixing of the caustic silicate solution and metakaolin can be carried out at a temperature between about 20 °C and about 40 °C.

[0032] Thereafter, the wet paste / slurry thus obtained and coming out of the mixer is spread on a support to form a layer of uniform thickness and subjected to a heat treatment in which it is dried to obtain a tape made of a dry / semi-dry geopolimer material.

[0033] The drying tape can have a moisture content of any value between 0% by weight and 20% by weight and a thickness between about 0.1 mm and about 2 mm. 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.

[0034] More generally, according to the present invention, for example, a support in the form of an endless belt conveyor is not sensitive to a basic atmosphere and is made of a specific material suitable for neutral or alkaline paste / mortar, for example, Mylar or other types of materials suitable for neutral / alkaline paste / mortar. During the formation and drying of the paste on the tape, a geopolymerization reaction occurs, in which metakaolin is dissolved in an alkaline sodium and potassium silicate solution. The formed oligomers then condense together to form a 3D geopolymer network.

[0035] The drying step is preferably carried out in a temperature-controlled oven (single or multi-stage oven), which may have a temperature profile adapted by means of a control device. The drying step can be carried out in a discontinuous or continuous manner. When carried out in a continuous manner, a tunnel oven / furnace traversed by a layer of wet paste spread on a support can be used.

[0036] Preferably, the final moisture content of the geopolymer binder of the present invention will be between 4% by weight and 16% by weight of the total weight of the mixed geopolymer, and even more preferably between 8% by weight and 12% by weight.

[0037] The dried aggregate in the shape of a tape formed by the amorphous geopolymer coming out of the oven is crushed and reduced to a powder using any suitable grinding system, preferably a hammer mill or a ball grinder or a jar mill, until a particle size of less than 600 microns, preferably less than 400 microns, is obtained.

[0038] Subsequently, the powder thus obtained, made of a mixed two-component alkali, preferably a sodium / potassium geopolymer, is mixed with other usual component materials of the friction composition, such as fillers, lubricants, abrasives, fibers, etc., with only the change of reducing the strong abrasive component in the same proportion as that employed in any of the known friction material classes such as NAO, semi-metals, and LS (low steel), and a friction material mixture is obtained which is molded as in EP3128201.

[0039] During molding, simply due to the application of pressure and temperature, the previously synthesized geopolymer particles are densified, amorphous remains, and a friction element, typically a brake pad, is obtained which is dispersed in a matrix composed only of a mixture of amorphous sodium and potassium geopolymerized inorganic binders (excluding, optionally, a limited amount, less than 10%, of organic binder).

[0040] The friction element thus obtained does not generate waste due to cracking or flaking, the tape obtained from the alkaline slurry is substantially crack-free and has sufficient elasticity to be easily handled, and the final result is the repacking of the powder under molding conditions equivalent to those of EP3841311 and under the usual molding conditions of brake pads, having performance comparable to that made of a friction material manufactured according to the hydrothermal synthesis of EP3841311, with equivalent material and disk wear as well, however, a braking element is manufactured which does not have or substantially does not have high thermal stresses such as, for example, the hot judder phenomenon. Molding for the repacking of geopolymer powder The molding of the brake pad obtained by the method of the present invention is carried out by placing a two-component alkaline raw material compound (friction mixture) into a mold which may or may not have a known damping / insulating layer called the "lower layer" and which may also have a metal support or backplate, and the mold, during the molding stage, not only forms a layer or block of the friction material on the lower layer, if present, but also realizes the adhesion of this layer or block to the metal support.

[0041] The molding is carried out by acting at a temperature between 40 and 250 °C and a pressure between 50 and 2000 Kg / cm2 for a time between 1 and 30 minutes, or by pre-forming the raw material two-component alkaline compound or mixture in a mold and then molding the pre-formed compound on a back plate at a temperature between 40 and 250 °C and a pressure of 150 to 2000 kg / cm2 (14.7 to about 200 MPa) for a time between 1 and 15 minutes.

[0042] Alternatively, the two-component alkaline raw material compound can be molded to obtain a friction material block, which is then connected to a metal support or back plate (with or without an underlying layer), for example, using a phenol or silicone-based paste. Other components of the friction material The components of the friction material composition or raw material compound to be manufactured according to the present invention can, by a single treatment that simultaneously reduces the abrasive content and increases the lubricant content and completely or almost completely replaces the current organic binder with the inorganic binder obtained by the method as described above, be components used in friction materials already known in the art.

[0043] The friction material obtainable according to the present invention also preferably does not contain copper and / or its alloys, in both powder and fiber forms.

[0044] In particular, the components made of fibers can consist of any organic or inorganic fibers other than asbestos, or preferably any metal fibers commonly used in friction materials excluding copper and its alloys. Exemplary examples include inorganic fibers such as glass fibers, wool or rock fibers, wollastonite, sepiolite and attapulgite, and organic fibers such as aramid fibers, polyimide fibers, polyamide fibers, phenolic fibers, cellulose and acrylic fibers, or PAN (polyacrylonitrile), metal fibers such as steel fibers, stainless steel, aluminum fibers, zinc, etc.

[0045] The fibers may be used in the form of short fibers or powder.

[0046] The amount of fibers is preferably between 2% and 30% by volume, more preferably between 8% and 15% by volume, of the total volume of the friction material, and the fibrous component preferably always includes rock fibers that have been shown to have a strong affinity with the geopolymers used as binders.

[0047] A number of materials known in the art may be used as organic or inorganic fillers. Exemplary examples include precipitated calcium carbonate, barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride, slaked lime, talc, and mica.

[0048] These may be used alone or in combination of two or more. The amount of these fillers is preferably between 2% and 40% by volume based on the total composition of the friction material.

[0049] As friction modifiers (which may include all or some of the fillers), in addition to carbonaceous 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 sulfide and non-ferrous sulfides, metal particles other than copper and copper alloys, and / or combinations of the above can be mentioned.

[0050] Abrasives can be classified as follows (the following list is merely indicative and not necessarily exhaustive or limiting): · Mild 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, zirconium, corundum, alumina, mullite.

[0051] Preferably, although not essential, since the dipolymer produced as a binder is already a medium abrasive itself, the friction material obtainable according to the present invention contains medium or mild abrasives rather than strong abrasives.

[0052] The friction material produced according to the present invention may also preferably contain graphite in an amount between 5% and 15% by volume relative to the total composition of the friction material.

[0053] The total content of the lubricant may preferably be between 4% and 20% of the total volume of the friction material, depending on the desired frictional properties, and may in particular contain graphene. Curing and painting Generally, the molded article (brake pad) that is cured during pressing and is 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 or powder coated, oven dried and, if necessary, mechanically processed to produce the final product.

[0054] The friction material obtained by the method of the present invention can be used in applications such as disk brake pads, shoes and linings for cars, trucks, trains and various other types of vehicles as well as industrial equipment, both after simple press molding and after any optional auxiliary heat treatment that may be possible, or in clutch disks.

[0055] According to a further aspect of the present invention, in order to obtain the best results, the optimal geopolimer composition region corresponds to a molar ratio of alkali metal content to aluminum content equal to 1, i.e., it is given by the formula: [1] R / Al = 1 (where R is the total content of Na + K) needs to be confirmed.

[0056] However, since the essence of the present invention is the combination of Na and K, it is clear that the present invention can extend to any combination of two different alkali metals other than Na and K that show effects using similar ratios.

[0057] In any case, outside the above relationship, i.e.: [2] R / Al ≠ 1 it is still possible to show an effect.

[0058] However, preferably, the following formula [1'] will be confirmed: [1'] R / Al = 0.8 < x < 1.2 According to a still more preferred embodiment of the present invention, the molar content of potassium needs to be at least equal to the molar content of sodium, so R = 50% Na + 50% K, and most preferably, the content of potassium (K) is higher than the content of sodium (Na). Therefore, the relationship: molar content of K ≥ molar content of Na needs to be confirmed.

[0059] In particular, the most preferred molar ratio between the potassium content and the sodium content in the geopolimer of the present invention (hereinafter also simply referred to as "GP" in this specification) is preferably included in the interval of 50% K / 50% Na ÷ 90% K / 10% Na.

[0060] Finally, K and Na are preferred alkali metals that will be used in combination within a given ratio range in the GP mixing system according to the present invention, although the use of other combinations of alkali metals belonging to the same group (Group 1) of the periodic table can be envisioned, as previously mentioned. Furthermore, since an essential aspect of the present invention is the combination of at least two different alkali metals in obtaining the geopolymers, the combined use of three or more different alkali metals is also included in the present invention.

[0061] Therefore, any geopololymer mixing system composed of a combination of at least two different GPs having repeating units corresponding to each general formula: [3] xR2O-yAl2O3-zSiO2 (wherein R is preferably either K or Na, but may be any of Li, Na, K, Cs, and Rb) is part of the present invention.

[0062] Generally, the friction material block or layer according to the present invention · is a homogeneous mixture of at least two geopolymers having repeating units corresponding to the above formula [3] where R is any of Na or K or Li or Ce or Rb; · or a single geopolymer having a mixture (combination) of repeating units corresponding to the above formula [3] where R is any of Na or K or Li or Ce or Rb such that at least two different alkali metals are present simultaneously within the same geopolymer represents a binder matrix made therefrom.

[0063] More generally, therefore, the friction material block or layer according to the invention contains a combination of different alkali metal aluminosilicates, preferably K and Na, and shows a binder matrix consisting of SiO4 and AlO4 tetrahedral frameworks linked by shared oxygen as poly(cialate) or poly(cialate-siloxo) or poly(cialate-disiloxo) depending on the SiO2 / Al2O3 ratio in the system in which K is dominant, the connection of the tetrahedral frameworks occurring via long-range covalent and / or mixed (ionic-covalent) bonds.

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

Brief Description of the Drawings

[0065]

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Figure 13

Mode for Carrying Out the Invention

[0066] Examples and comparative examples are reported herein for illustrative purposes and are not intended to limit the present invention.

[0067] Referring to FIGS. 1 and 2, a phase diagram of a complex three-component system (a "liquidus" temperature diagram) is shown, where the three vertices of the triangle conventionally representing the system correspond to compositions of 100% SiO2 (upper corner), 100% NaAlSiO4 (lower left corner), and 100% KAlSiO4 (lower right corner). Inside such a triangle, there are several different phases (shown) having mixed compositions given for each point targeting the corresponding side of the triangle.

[0068] Starting from the discovery that, for example, due to being based only on Na as the alkali metal component as provided previously by the applicant according to EP3841311, the geopolymers arranged only along the left side of the figure, and finally, due to being derived from a system based only on K as the alkali metal component, the geopolymers arranged only along the right side of the figure have several drawbacks such as the poor workability of the K-based system and the poor heat resistance of the Na-based system, the technical staff of the applicant decided to investigate the properties of the mixed Na-K system anyway without confidence in overcoming the drawbacks of the prior art. Commercially available sodium-potassium mixed silicate and metakaolin were mixed, or a three-component geopolymer reaction system based on both caustic silicates of Na and K belonging to the internal region of the phase diagrams of FIGS. 1 and 2, and metakaolin was prepared.

[0069] After a series of experiments described below in this specification, the technical staff of the applicant individualized the region within the phase diagram of the three-phase system of FIG. 1 highlighted in FIG. 2. Method according to the invention - Operational example Different silicate solutions of sodium and potassium having different molar ratios (SiO2 / R2O) and water contents are used to produce mixed silicate solutions having different molar contents of Na and K according to Table 1 below.

[0070]

Table 1

[0071] These alkaline solutions are mixed separately with commercially available metakaolin using a solution / metakaolin weight ratio between 1 and 10 (including the end values) with a Si / Al molar ratio in the range 1 < x < 10; preferably, this range can vary from 2 to 6. Different ratios at higher Al or Si contents are also possible; however, experimental results and theoretical calculations lead to the conclusion that the present invention shows the maximum efficiency with a Si / Al ratio between 2 and 6.

[0072] The caustic silicate solution and metakaolin are mixed separately by mechanical stirring to obtain six homogeneous pastes.

[0073] The pastes thus obtained are spread separately on different plastic mats using the "tape casting" technique to obtain tapes 0.5 mm thick, and then the weight of the mixture is reduced by up to 10 - 40% of the original weight and dried at a temperature between 70 and 250 °C under atmospheric pressure for a time in the range of 1'(minute) - 90'(minutes) to convert it into a pure amorphous geopolimer.

[0074] The dried caustic silicate - metakaolin geopolimer system is taken out of the dryer and ground for 20 minutes in a ball mill rotating at 400 RPM. The final water content is determined by considering the maximum amount of water that the system can lose, such that a powder with 0% moisture corresponds to a residual moisture in the powder in the range of 9 - 10% (by weight).

[0075] The geopolimer powder thus produced is subdivided into fractions, and each fraction of each different geopolimer powder is · used as such to produce a series of test samples molded into the shape of discs for 10 minutes at 150 °C and a pressure of 20 MPa; · Using a known mixer, such as Loedige or Eirich, it is separately added to the friction material mixture or composition selected for dry mixing or other raw materials required. The thus obtained "green" friction material mixture or composition is hot molded under pressure to obtain a series of brake pads. The molding stage involves placing a metal support having raw materials or "green" compounds and optionally a possible underlying layer into a mold heated to a temperature between 60 and 250 °C (known and not shown for simplicity), sending the raw material compound for a time between 1 and 15 minutes at a molding pressure between 150 and 2000 Kg / cm2, or preforming the raw material compound 11 in the mold and then performing it at a temperature between 100 and 250 °C for a time between 1 and 15 minutes and a molding pressure between 150 and 2000 kg / cm2 to mold the preformed compound onto the metal support. Alternatively, the raw material compound may be molded without a metal support to obtain only the friction material block, which is then subsequently, in a known manner, with or without an insulating / dampener layer (known), using a phenol or silicon-based paste, for example, pressing the friction material block against a metal support having a possible underlying layer and operating at a temperature of 180 °C for 30 seconds to paste it onto the metal support. At the end of this process, an asbestos-free friction material is obtained that contains inorganic and / or organic and / or metal fibers as component materials, at least one binder, at least one friction modifier or lubricant, and at least one filler or abrasive, where the binder is composed of at least 90% of a fully consolidated silica-aluminum dipolymer.

[0076] Referring to the above, the component materials of the raw material compound are added to the inorganic binder in appropriate amounts such that the total amount of the inorganic dipolymer binder is preferably, but not necessarily, 20% by weight or more and 60% by weight or less of the total volume of the friction material, and even more preferably equal to about 47% by weight.

[0077] At least one abrasive contained in the friction material as described above is, therefore, not essential but preferably a medium or mild abrasive; where such terms refer to the following classifications: · Mild abrasives (Mohs hardness 1 - 3): for example, talc, calcium hydroxide, potassium titanate, mica, vermiculite, kaolin; · Medium abrasives (Mohs hardness 4 - 6): for example, barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide; · Strong abrasives (Mohs hardness 7 - 9): for example, silicon carbide, zircon sand (zirconium oxide), zirconium silicate, zirconium, corundum, alumina, mullite.

[0078] The volume ratio between the lubricant and the abrasive contained in the friction material to be formed is preferably selected between 1:1 and 1:4 (for comparison, this ratio is generally 1:8 or more in known friction materials with an organic binder).

[0079] Furthermore, the starting raw materials for obtaining the geopolimer binder are selected such that the inorganic geopolimer binder in the friction material according to the invention has an SiO2 / Al2O3 ratio between 3 and 10 and an SiO2 / Na2O ratio between 3 and 10. The densification of the geopolimer powder is obtained during molding.

Examples

[0080] Comparative production of binders To produce a geopolimer having a molar ratio of Na:K = 50:50, 150 g of metakaolin manufactured by "Imerys Refractory Minerals" is mixed with 346 g of an aqueous solution of alkali silicate (obtained by mixing and dissolving 133 g of solid sodium silicate "Britesil C335" supplied by "PQ Corporation" and 142.1 g of solid potassium silicate "205K" supplied by "Tillmanns" with 7.68 g of sodium hydroxide, 9.99 g of potassium hydroxide and 310.3 g of water). The mixing is carried out at a speed of 800 rpm for a varying time of 5' - 45' using a drill stirrer together with a specific mixing whisk for medium to high viscosity fluids to obtain four different geopolimer precursor systems having different controlled compositions. By changing the relative amounts of the solid silicate and hydroxide, it is possible to obtain the four relative Na:K compositions listed in Table 1.

[0081] The two reference systems, the Na system and the K system, are obtained according to EP3841311 and are managed in the same way as described therein. They are used as benchmark binders for the evaluation of the properties of the mixed systems.

[0082] Furthermore, since the K-based system is more alkaline, an increase in the relative content of K compared to Na results in a denser material and also gradually increases the basicity of the solution.

[0083] The obtained wet paste is spread on a specific Mylar sheet for wet alkaline paste / mortar using the following parameters: the thickness of the spread paste is about 0.5 mm. Then, a plurality of samples are prepared by drying the spread wet paste at a temperature between 40°C and 250°C, a sheet size between A3 and A4, and a drying time variable between 1' and 90'.

[0084] The sample binder in the form of a solid aggregate is then separated separately from the sheet and ground in a ball grinder rotating at 400 revolutions per minute for 20 minutes to effect granulation of the product to obtain a powder with a particle size of 400 microns or less.

[0085] A homogeneous powder having 4 to 16% by weight, preferably 6 to 12% by weight of residual moisture is obtained, weighed, and pressed at standard parameters: 150 °C, 20 MPa, for 10 minutes to densify the geopolymers.

[0086] Samples in the form of discs are obtained and tested.

[0087] In Figures 3 and 4, photographs of the solid aggregates obtained in the form of tapes are shown for the mixed systems labeled 1M and 2M. As can be seen, the tapes are well formed and crack-free, so that they can be easily handled even when the relative content of K aluminosilicate is dominant over the relative content of Na aluminosilicate. As a conclusion, the handling of the aggregates obtained with the Na-K mixed system is comparable to that of the pure Na system.

[0088] This is also confirmed in Figures 6 and 7, which show the aspect of the sample discs obtained by densification (performed using the standard parameters of pressure and temperature shown above) of the powders obtained from solid aggregates corresponding to the mixed systems of Table 1 from 0.1M to 10M. All the discs are well formed and crack-free. Furthermore, the density of the sintered discs increases with the increase in the K content, as reported in Table 2 below.

[0089]

Table 2

[0090] The expansion phenomenon caused by heating during the typical forming phase in the Na system is almost completely absent in the mixed system samples (Figure 7) and becomes increasingly negligible with the increase in K content. In any case, as reported in Table 3 below (Na-GP and K-GP are benchmarks), the phenomenon is limited to a slight increase in the disk diameter.

[0091]

Table 3

[0092] As can be seen, the pure Na system disk (Na-GP) shows an increase in diameter of about 30%, and the mixed system Na-K disks show a decreasing increase in diameter in response to the increase in the K molar content in the mixed Na-K system. The pure K system disk (K-GP) shows substantially no expansion but may be brittle.

[0093] The powder obtained after the grinding phase of the residual material tape and before using it to form the test disks has been subjected to IR analysis, and the relative results are reported in graph form in Figure 5.

[0094] As can be seen, the presence of the geopolymer can be easily confirmed in all mixed system powders. The peak at approximately 1000 cm -1 is due to the Si-O bond. The confirmation of the assignment is that the peak is at 1000 cm -1 in the geopolymer compared to metakaolin which is at 1030 - 1040 cm -1 This is precisely due to the involvement of aluminum in the continuous 3D network of silica, and this is evidence that all systems from 1M to 4M have undergone the desired chemical reaction regardless of the relative molar amount of K present in the slurry mixture. The IR peak below 1000 cm -1 confirms the geopolymerization due to the chemical shift of the incorporation of Al into the silica network.

[0095] However, it is clear that a chemical shift can be observed when the Si / Al ratio decreases, due to the higher effect of Al on the Si-O stretching energy.

[0096] The powders obtained after the grinding phase from the residual material tape and before using it in the molding / consolidation phase to form the test disks were also investigated in terms of their particle size composition. Figure 9 shows a comparison between powders derived from tape castings from a Na pure slurry system, a K pure slurry system, and a mixed Na-K slurry system, obtained after undergoing the same granulation or grinding phase in the same mill. As can be recognized, the physical properties of the powders derived from different slurry systems are substantially the same, which is an important advantage for the management in the manufacturing phase of these powders to obtain friction material layers or blocks and for the manufacture of brake elements, lime brake pads or shoes.

[0097] Finally, the mixing phase carried out before the tape casting phase has also been investigated during its execution for all four mixing systems in Table 2, and Figure 8 shows the experimental results in a graphical format reporting the shear stress measurements against the shear rate applied (by the mixing device). As can be seen, the (apparent) viscosities of all four mixing system slurries 1M - 4M are similar and follow the same trend, which is also an important process parameter enabling the management of the reaction slurry in the same way in the industrial manufacturing phase, regardless of the relative amount of K present.

Example

[0098] Manufacture of Brake Pads Several identical brake pads are manufactured using known devices or plants not shown for the sake of brevity.

[0099] The same friction material formulation was prepared using the mean values of the intervals reported in Table 4 below for each component and starting from both pure Na and K systems as shown in Table 1, as well as each of four Na-K mixed systems, using the GP powder obtained according to Example 1 as a binder shown as the "binder mixture".

[0100] After production and grinding, GP powder with 9% moisture by weight is used.

[0101]

Table 4

[0102] The binder mixture is added to the other components of the mixture according to the general scheme: 20 - 60 wt% binder, 40 - 80 wt% other components; mixing is carried out in a Loedige mixer. The GP system is 47 wt% of the friction mixture.

[0103] Subsequently, the friction material mixture / compound thus obtained is placed with the raw material or "green" compound and the metal support into one mold and molded into the same brake pad. The molding is carried out by subjecting the raw material compound to a molding pressure of 250 - 720 Kg / cm2 for a time of 2 - 15 minutes in steps at temperatures of 100 - 150 / 70 - 135 / 70 - 135 °C.

[0104] The friction material blocks thus obtained are tested for their mechanical properties. The experimental results are reported in the form of a bar graph in Figure 10, which compares the Young's modulus of geopolymers obtained from pure Na systems (Na-based mixtures), pure K systems (K-based mixtures) and mixed systems (Na-K-based mixtures - the reported values represent mean values and the Young's modulus does not vary much with the variation of the K molar content). As can be recognized, the Young's modulus of the mixed system GP is much better than that of the pure K system, thus guaranteeing better mechanical performance during use, which is close to the mechanical performance of GP obtained from the pure Na system.

Example

[0105] Braking test The brake pads manufactured as described in Example 2 were subjected to the following tests: Braking braking, braking at different fluid pressures, cold (<50 °C) evaluation braking, simulated highway braking, efficiency tests according to AKM including two high-energy braking (first FADE test) series with regenerative braking series interspersed. From this test, it is also possible to extrapolate the wear to which the brake pads and discs are subjected using methods known to industrial technicians.

[0106] The extraction of the obtained results is shown in FIGS. 11 and 12, which schematically represent the most important data of the obtained experimental curves. The graph in FIG. 11 relates to the above AKM test carried out on a brake pad with a friction material according to Table 5, where the inorganic binder mixture consists of a powder derived from a pure Na-based system, i.e., is composed of a GP containing only Na aluminosilicate, i.e., based on the Na-based system in Table 2, while FIG. 12 relates to the same test carried out on a brake pad with a friction material according to Table 5, where the inorganic binder mixture consists of a powder derived from a 1M mixed system, i.e., is composed of a GP containing K and Na in a molar ratio of 1:1, i.e., based on a 50% Na / 50% K mixed system (1M). The graphs are self-explanatory.

[0107] As can be seen, the AKM experimental results regarding braking characteristics are very similar and fully comparable (if not better) to those of the benchmark samples obtained according to EP3841311.

[0108] The following Table 5 shows the results of wear comparison tests carried out on brake pads with the friction materials of the tests in FIGS. 9 and 10.

[0109] [Table 5]

[0110] As can also be seen, wear is similar to the prior art (pure Na-based systems), even when the pads according to the invention have a significant weight loss due to better miniaturization and even when the decrease in thickness is not much contributed.

[0111] Further braking tests were carried out on brake pads manufactured with both of the two GP materials of the prior art (EP3841311) and the GP material of the invention produced by the reaction system mixture 1M, as listed in Table 2. The test steps are reported below in this specification corresponding to the results in Table 6.

[0112]

Table 6

Examples

[0113] Judder test The brake pads manufactured in Example 2 were installed in a vehicle (automobile) and compared with commercially available brake pads of the same size (the original ones on the test vehicle - NAO material). The results are reported in Table 7. The comparison was carried out with the geopolimer mixing system 50% Na - 50% K.

[0114]

Table 7

[0115] As can be seen, the brake pads manufactured with the mixed GP system result in comparable results to those manufactured with the pure K GP system and are very similar to the general NAO brake pads currently in use; in contrast, the brake pads manufactured with the pure Na GP system are more prone to judder.

[0116] Finally, by adjusting the alkali ratio Na / K (or generally, the R1 / R2 ratio, where R1, R2 are any of Li, Na, K, Cs, Rb), the properties of the GP-based (as a binder) brake pad are adjusted, and surprisingly, both favorable aspects of the pure (Na and K) systems: · Optimal tape production typical of sodium-based GP systems · Optimal swelling behavior typical of potassium-based GP systems can be concluded to be combinable.

[0117] The rheology of pure Na, K-based or hybrid systems seems to be similar since the amount of silica depends on the relative amounts of alkali and water. In fact, at the same molar ratio, potassium-based systems are more fluid. This is why potassium requires less silica and less water than sodium with a similar viscosity; thus, tape formation from the slurry is not affected.

[0118] Furthermore, it is clear that other properties can be adjusted by being implemented with respect to the requirements of the brake pad, particularly with respect to the friction formulation in terms of AKM and judder performance.

[0119] Thus, all the objects of the present invention are satisfied. Specific terms Although specific embodiments of certain braking devices, systems, and methods are disclosed in the context of a particular example, the scope of the present disclosure extends to other alternative embodiments and / or uses of the embodiments beyond the specifically disclosed embodiments, such as the brakes shown for a braking system based on a brake drum, and to certain variations and equivalents thereof, which will be understood by those skilled in the art. Use in any structure is, obviously, within the scope of the present invention. The various features and aspects of the disclosed embodiments can be combined with or substituted for one another to form various modes of assembly. The scope of the present disclosure should not be limited to the particular disclosed embodiments described herein.

[0120] Terms representing conditions, such as "can", "could", "might", or "may", generally convey, unless specifically stated otherwise or understood otherwise within the context in which they are used, whether a particular embodiment includes or does not include a particular feature, element, and / or step. Thus, such terms representing conditions are not generally intended to imply that the feature, element, and / or step is required in one or more embodiments.

[0121] Unless stated otherwise, the terms "about", "approximately", and "substantially", as used herein, still represent an amount close to the stated amount that will still perform the desired function or achieve the desired result. For example, in some embodiments, as the context may describe, the terms "about", "approximately", and "substantially" may refer to an amount within 10% of the stated amount. Similarly, the term "generally", as used herein, represents a value, amount, or characteristic that mainly includes or tends to include a particular value, amount, or characteristic.

[0122] This disclosure clearly contemplates that the various features and aspects of the disclosed embodiments can be combined with or substituted for one another. Accordingly, the scope of this disclosure should not be limited by the above-described particular disclosed embodiments, but should be determined only by a fair reading of the following claims and their full scope of equivalents.

Claims

1. An asbestos-free friction material configured for the manufacture of a brake element, such as a brake pad or a brake shoe, comprising, as component materials, inorganic and / or organic and / or metallic fibers, 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 inorganic and is composed of at least 90% by weight of at least one amorphous geopolimer or a mixture of amorphous geopolimers, the at least one amorphous geopolimer or mixture of geopolimers contains an aluminosilicate comprising a combination of at least two different alkali metals, and the alkali metals are selected from different metal combinations in the group consisting of Na, K, Li, Ce, and Rb. An asbestos-free friction material characterized by this.

2. The at least one amorphous geopolimer or mixture of geopolimers is derived from a source of alumina (Al 2 O 3 ), preferably metakaolin, and is derived from a simultaneous combination of potassium and sodium silicates, the asbestos-free friction material according to claim 1.

3. The relative molar content of potassium (K) is equal to or greater than the relative molar content of sodium (Na), that is Molar content K ≥ Molar content Na The asbestos-free friction material according to claim 2, characterized by this.

4. The asbestos-free friction material according to claim 2 or 3, characterized in that the molar ratio of potassium (K) to sodium (Na) is 1:

1.

5. The asbestos-free friction material according to claim 2 or 3, characterized in that the molar ratio of the potassium content to the sodium content in the at least one amorphous geopolimer or mixture of geopolimers is included in the interval of 50% K / 50% Na ÷ 90% K / 10% Na.

6. The at least one binder is i) - A combination of at least two different geopolimers each having a repeating unit corresponding to the general formula: [3] x(R2O)y(Al2O3)z(SiO2) Or ii) - A single geopolimer containing a combination of at least two different repeating units having the general formula [3], where R is different in the at least two different repeating units, and the single geopolimer Comprises at least 90% by weight of a geopolimer mixture system Here, iii) R is any one of Li, Na, K, Cs, and Rb in each repeating unit, and preferably is either K or Na ​ An asbestos-free friction material according to any one of the preceding claims, characterized in that...

7. The molar ratio of the alkali metal content to the aluminum content in the geopolymers preferably needs to be equal to 1, that is, the formula: [1’] R / Al = 0.8 < x < 1.2 An asbestos-free friction material according to any one of the preceding claims, characterized in that it is necessary to confirm...

8. A block or layer of friction material for a brake element such as a brake pad or shoe, SiO 2 / Al 2 O 3 Depending on the ratio, up to 90% by weight of SiO linked by shared oxygen as poly(cyanate) or poly(cyanate-siloxo) or poly(cyanate-disiloxo) 4 and AlO 4 Characterized by a binder matrix consisting of a tetrahedral structure, containing at least two different alkali metal aluminosilicates, preferably a combination of K and Na aluminosilicates, more preferably with K aluminosilicate being dominant, a block or layer of friction material.

9. A vehicle brake pad comprising a block or layer of the friction material according to Claim 8.

10. A method for manufacturing a friction material and a friction element, preferably a friction layer or block made of the friction material to be applied to a vehicle brake element such as a brake pad or a shoe, comprising: a) preparing a wet paste formed by mixing an alkaline silicate solution containing a material selected from the group consisting of metakaolin, kaolin, fly ash, mixtures thereof, preferably only commercially available powdered metakaolin, wherein the wet paste constitutes an alkaline geopolymer reaction system; b) spreading the wet paste on a support to form a layer or tape; c) drying the wet paste to obtain geopolymer aggregates; d) - pulverizing the geopolymer aggregates into powder; e) - using the pulverized powder as an inorganic binder in a friction material compound by mixing the pulverized powder with inorganic and / or organic and / or metal fibers, at least one friction modifier or lubricant, and at least one filler or abrasive to obtain a raw friction material compound having the pulverized geopolymer aggregates substantially or solely as a binder; f) - hot molding the raw friction material compound between 40°C and 300°C to obtain a block of friction material having at least 90% by weight of geopolymers as a binder comprising wherein step a) is carried out by optionally adding a small amount of sodium hydroxide and / or potassium and using at least two different alkaline silicates, preferably sodium silicate and potassium, in combination simultaneously. A method, characterized in that...

11. In step a), the molar ratio of potassium (K) to sodium (Na) is included in the interval of 50% K / 50% Na to 90% K / 10% Na; and the molar ratio of the Na / K metal content to the aluminum content in the geopolymers system is preferably equal to 1, the method according to claim 10, characterized in that.

12. A geopolymer binder adapted to be used for manufacturing a friction layer / block, particularly for a brake pad or a shoe, made of a friction material manufactured by the method according to claim 10 or 11, the geopolymer binder containing, simultaneously combined, two different alkali metals, preferably K and Na.

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