Friction material, in particular for the manufacturing of a brake pad, and associated preparation methods

Abstract

An asbestos-free friction material having at least one of the group consisting of inorganic, organic and metallic fibers, at least one binder, at least one friction modifier or lubricant and at least a filler or abrasive, wherein the binder is almost completely and exclusively inorganic and is constituted almost exclusively by a hydrated geopolymer or a blend of hydrated geopolymers.

Claims

1. A braking system comprising: a member to be braked, constituted by a brake disc or brake drum made of cast iron or steel; and at least one braking member constituted by a brake pad or brake shoe, adapted to cooperate by friction with the member to be braked, the brake pad or brake shoe being made of the friction material, the friction material which is made by: a) obtaining sodium hydroxide in powder form; b) mixing the sodium hydroxide in powder form with kaolin in powder form, until a pre-mixture is obtained; c) blending the pre-mixture with at least one of the group consisting of inorganic, organic and metallic fibers; at least one friction modifier or lubricant; and at least one filler or abrasive, in order to obtain a raw mixture of friction material having as a binder made up of 90 percent or more by volume of the pre-mixture materials; and d) hot molding the raw mixture under a pressure greater than the water saturation pressure at a molding temperature in order to obtain a block of friction material having as the binder a hydrated geopolymer representing at least 90 percent of total volume of the binder and in which the binder has a SiO.sub.2/Al.sub.2O.sub.3 ratio between 1.5 and 2.5 and a SiO.sub.2/Na.sub.2O ratio between 1.5 and 2.5; during the molding step the pre-mixture geopolymerizing in order to form the geopolymeric binder.

2. A friction material comprising: at least one of the group consisting of inorganic, organic and metallic fibers; at least one binder; at least one friction modifier or lubricant; and at least a filler or abrasive, wherein the binder comprises a geopolymeric inorganic component of one or more hydrated geopolymers representing at least 90 percent by volume with respect to the total binder present in the friction material, and in which the binder has a SiO.sub.2/Al.sub.2O.sub.3 ratio between 1.5 and 2.5 and a SiO.sub.2/Na.sub.2O ratio between 1.5 and 2.5.

3. A braking system comprising: a member to be braked, constituted by a brake disc or brake drum made of cast iron or steel; and at least one braking member constituted by a brake pad or brake shoe, adapted to cooperate by friction with the member to be braked, wherein the braking member has a friction layer or block intended to cooperate with the member to be braked, the friction layer or block being made of the friction material according to claim 2.

4. The friction material according to claim 2, wherein the binder is selected from the group consisting of: Polysialate having a Si/Al ratio equal to 1:1; Polysialate-Polysiloxo having a Si/Al ratio >1; Calcium based polysialate having a Si/Al ratio ≥1; and an aluminum-phosphate polymer.

5. The friction material according to claim 2, wherein the binder is at least partially in a crystalline form.

6. The friction material according to claim 5, wherein the binder comprises hydrated sodalite in crystallized form.

7. The friction material according to claim 2, wherein the binder is at least partially in an amorphous form.

8. The friction material according to claim 2, wherein the friction material comprises 10 percent or less of organic binders, and is substantially free of copper or alloys thereof and copper fibers and alloys thereof.

9. The friction material according to claim 2, wherein the total amount of binder is equal to or greater than 5% by volume with respect to the volume of the entire friction material.

10. The friction material according to claim 9, wherein the total amount of binder is greater than 25% by volume with respect to the volume of the entire friction material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in more detail with reference to the following practical non-limiting embodiment examples and with reference to FIGS. 1 to 8 of the appended drawings, wherein:

(2) FIGS. 1 and 2 schematically illustrate by means of blocks two possible implementation methods for a friction material according to the invention;

(3) FIGS. 3 and 4 illustrate the X-ray diffraction spectra of inorganic binder samples utilized in the friction material according to the invention;

(4) FIGS. 5 and 6 illustrate the results in simplified graphs of the comparison of the braking efficiency according to the AKM standard of the same brake pads implemented with the formulation of friction material in the prior art (FIG. 6) and with a similar formulation of friction material, but implemented according to the invention (FIG. 5); and

(5) FIGS. 7 and 8 illustrate the results in simplified graphs of the comparison of the braking efficiency according to the AKM standard of the same brake pads implemented with an identical formulation of friction material according to the invention but utilizing two different variants of a synthesis method of the binder of said friction material.

DETAILED DESCRIPTION OF THE INVENTION

(6) The examples and comparative examples are reported here by way of illustration and are not intended to limit the invention.

(7) With reference to FIGS. 1 and 2, two different non-limiting possible embodiments are schematically illustrated in blocks of a method for making a block or layer of environmentally friendly friction material, and hence devoid of asbestos, copper and its alloys, and not subject to thermal degradation under use, according to the invention.

(8) With reference to FIG. 1, a block 1 represents a first phase of a first embodiment of a method for implementing a block or layer of friction material according to the invention. According to the illustrated non limiting example, block 1 represents a phase in which sodium hydroxide is obtained in powder form by milling commercial sodium hydroxide (caustic soda) pellets or flakes; in particular, a predetermined amount of commercial sodium hydroxide represented by an arrow 2 is introduced into a rotating mill known in the art, such as for example a Retsch ZM 100 mill, and transformed into powder.

(9) Subsequently, the sodium hydroxide obtained in this manner is fed into block 3, which represents a mixing phase, such as for example that carried out in an industrial mixer of any type known in the art utilized in the field of friction materials, such as for example a Loedige mixer, or otherwise a Henschel or Eirich mixer, in which the sodium hydroxide powder is mixed with a predetermined amount of commercial kaolin, represented by an arrow 4, until obtaining a pre-mix, schematically indicated with an arrow 5 contained in block 3. For calculating the amounts of reagents (caustic soda and kaolin) to be utilized in this phase, the stoichiometric formula calculation techniques by Davidovits et al are utilized, as described in “Geopolymer, Chemistry & Applications” chap. 7.2, Institut Geopolymère, third edition, July 2011.

(10) This mixing phase according to block 3 is carried out preferably inside a dry Eirich mixer, that is to say in the absence of water. According to a variant of this method, it is also possible to pre-hydrate the sodium hydroxide powder, by mixing it with water (represented by an arrow 6) before the mixing phase according to block 3, by utilizing a weight ratio between sodium hydroxide and water preferably equal or close to 1:1.

(11) On average the mixing phase according to block 3 lasts 10 minutes.

(12) Subsequently, according to a block 7, a mixing phase of pre-mix 5 is carried out with the raw materials normally comprised in a friction material, with the exception of the organic binder; the phase according to block 7 is preferably carried out in a Loedige mixer (however it is also possible to utilize other types of mixer such as Henschel or Eirich mixers) feeding pre-mix 5 into the mixer together with inorganic and/or organic and/or metallic fibers (however free of asbestos or its derivates), represented by an arrow 8, with at least one friction modifier or lubricant, represented by an arrow 9, and at least one filler or abrasive, represented by an arrow 10, so that at the end of the mixing a raw mix of friction material is obtained, schematically indicated with an arrow 11 contained in block 7 having as binder the materials of pre-mix 5 exclusively.

(13) Lastly, a block 12 represents a hot pressing phase of raw mix 11 under pressure greater than the water vapor pressure at the pressing temperature, obtaining at the end a block 13 of friction material having as binder exclusively a hydrated geopolymer based on alumino-silicates.

(14) Eventually, at the end of the phase represented by block 12, block 13 of friction material is already obtained integrally with a metallic support 14 known in the art, eventually provided with an isolating/damping layer (underlayer), known in the art and not illustrated for simplicity, implementing a friction element constituted in the illustrative non-limiting example by a brake pad 15.

(15) During pressing phase 12, which is carried out utilizing the usual pressing parameters for brake pads having organic binders, pre-mix 5 geopolymerizes, so as to form the geopolymeric binder according to the invention.

(16) The pressing phase according to block 12 is carried out by introducing raw mix 11 and the eventual metallic support 14 with the eventual underlayer into a mold (known in the art and not illustrated for simplicity) that is heated to a temperature of between 60 and 250° C., wherein raw mix 15 is subjected to a pressing pressure of between 150 and 1800 Kg/cm.sup.2 for a period of between 3 and 10 minutes, or instead preforming raw mix 11 and subsequently pressing the preformed mix on metallic support 14, working at a temperature of between 100 and 250° C. and pressing pressure of between 150 and 500 kg/cm.sup.2 (14.7-49 MPa) for a period of 3 to 10 minutes. Alternatively, raw mix 11 can be pressed without the presence of metallic support 14, in order to only obtain the block of friction material 13, which is then subsequently glued in a manner known in the art to metallic support 14, optionally provided with an isolating/damping layer (known in the art) or underlayer, utilizing phenolic based glues, such as for example pressing block of friction material 13 against metallic support 14 with the eventual underlayer, working at a temperature of 180° C. for 30 seconds.

(17) Therefore at the end of the method illustrated in FIG. 1 a friction material free of asbestos is obtained, with component materials comprising 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 was obtained by means of hydrothermal chemical synthesis starting from pre-mix 5 and is almost completely or completely and exclusively inorganic, being constituted almost completely or exclusively by a hydrated alumino-silicate geopolymer, or working phase 3, as will be seen, in order to implement pre-mix 5 with other materials, from a mixture of hydrated geopolymers.

(18) In fact, during pressing phase 12 the kaolin reacts with the caustic soda and the water eventually present, forming hydro-sodalite, according to the formula:
Al.sub.2O.sub.3.2SiO.sub.2.2H.sub.2O+NaOH.fwdarw.Na.sub.2Al.sub.2Si.sub.2O.sub.8.3H.sub.2O
(kaolin)+(caustic soda).fwdarw.(hydrated sodalite)

(19) The materials comprising the raw mix are added to pre-mix 5 in suitable amount so that the total amount of geopolymeric inorganic binder is preferably but not necessarily greater than or equal to 10% by volume and preferably greater than 25% by volume with respect to the volume of the overall friction material.

(20) With reference to FIG. 2, it schematically illustrates a variant of the described method by means of a block diagram; in particular, according to this variant the phases are carried out as was previously described according to blocks 1, 3, and 7, obtaining raw mix 11 with inorganic and/or organic and/or metallic fibers (but not containing asbestos or its derivatives)) 8, at least one friction modifier or lubricant 9, and at least one filler or abrasive 10, identically to what was previously described for the mixing phase represented in FIG. 1 by block 7; however raw mix 11 is not utilized immediately, but is instead subjected to additional operations; in particular, according to a block 16, raw mix 11 is mixed with a predetermined amount of water, represented by arrow 17, in order to obtain a paste 18, also represented by an arrow. Water 17 is added in an amount of less than 40% by weight of the total weight of paste 18; for example on 1 kg of raw mix 11, 690 gr of water are added.

(21) Subsequently, according to a phase represented by block 19, paste 18 is oven dried at a temperature of for example 70° C. for 24 hours, obtaining a dried paste 20 at the end.

(22) Subsequently, dried paste 20 is reduced into powder form by subjecting it to a mixing phase, such as for example in an Eirich mixer, represented by block 21, obtaining a “hydrated” raw mix 24 in the form of dried and pulverized paste.

(23) “Hydrated” raw mix 24 is then subjected to a pressing phase with hot pressing, represented in FIG. 2 by block 25, in order to obtain a block of friction material 13 optionally fastened onto a metallic support 14 eventually provided with an isolating/damping layer 23 known in the art, and denominated underlayer, on the part devised for receiving block 13. However this hot pressing phase according to block 25 is carried out working at a temperature of between 110 and 150° C., and preferably 120° C., hot pressing raw mix 24 with a pressure of at least 300 Kg/cm.sup.2 and employing a pressing scheme that provides for periods of application of the above mentioned pressure, alternating with pressure release periods in order to allow the release of the “hydration” water present in raw mix 24 in the form of steam and in a controlled manner, since drying phase 19 does not in any event eliminate all the water 17 added in phase 16.

(24) More generally, the friction material according to the invention can be obtained with one of the two described methods with reference to FIGS. 1 and 2, or instead by implementing the geopolymer binder by synthesis in water solution, rather than the described hydrothermal method, which remains in any event the preferred method. Furthermore, in addition or instead of kaolin other starting raw materials can be utilized, such as metakaolin 2Al.sub.2O.sub.3.SiO.sub.2, sodium silicate, calcium hydroxide, or even phosphoric acid in case an acidic catalysis is preferred in order to obtain an alumino-phosphate polymer.

(25) The friction material according to the invention is therefore characterized by the fact of including an almost exclusively or totally inorganic binder constituted by a geopolymer belonging, more in general, to one of the families listed in Table 1 below, which also lists the starting raw materials and the reagents needed to obtain the geopolymeric binder, in accordance with each listed family.

(26) TABLE-US-00001 TABLE 1 Raw materials Source of silicon and Source of Me.sub.2O silicon and alkaline Families/Reagents alumina oxides Activator Fami- Polysialate kaolin, metakaolin, / Basic lies Si:Al 1:1 fly ash, other NaOH, aluminosilicates, KOH, quartz, sand Polysialate- Clay, metakaolin, Sodium and Basic Polysiloxo fly ash, other potassium Sodium and Si:Al > 1 aluminosilicates, silicates potassium quartz, sand carbonates Calcium Metakaolin, Sodium and Basic base calcium slag potassium Ca(OH).sub.2 Si:Al ≥ 1 silicates Phosphate Metakaolin, alumina / Acidic base H3PO3 (Me = metal)

(27) According to Table 1, the geopolymeric inorganic binder of the friction material according to the invention is selected from the group consisting of: Polysialate having a Si/Al ratio equal to 1:1; Polysialate-Polysiloxo having a Si/Al ratio>1; Calcium based polysialate having a Si/Al ratio≥1; aluminum-phosphate polymer.

(28) Furthermore, in particular following the methods of FIGS. 1 and 2, the geopolymeric inorganic binder of the friction material according to the invention is at least partially crystalline, and comprises crystallized hydrated sodalite.

(29) Material components 8, 9, and 10 do not utilize asbestos or its derivates, nor copper or its alloys; therefore the friction material according to the invention is substantially free or almost free of organic binders, is substantially free of copper and its alloys and/or copper and copper alloy fibers, and is preferably, but not necessarily, substantially free of strong abrasives, where hereinafter the term “substantially free” means that the indicated materials may be present as sole impurities; the at least one abrasive contained in the friction material according to the invention is therefore preferably, but not necessarily, a medium or mild abrasive; where said terms refer to the following classification, already indicated previously: Mild abrasives (having Mohs hardness between 1 and 3): for example talc, calcium hydroxide, potassium titanate, mica, kaolin; Medium abrasives (having Mohs hardness between 4 and 6): for example barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide; Strong abrasives (having Mohs hardness between 7 and 9): for example silicon carbide, zirconium sand, zirconium silicate, zirconia, corundum, alumina, mullite.

(30) Lastly, according to an additional aspect of the invention, the volume ratio between the lubricants and the abrasives contained in the friction material is preferably comprised between 1:1 and 1:4, while for example in friction materials with organic binder it can be greater than 1:8.

(31) Furthermore, the starting raw materials for obtaining the geopolymer binder are chosen so that the geopolymeric inorganic binder in the friction material according to the invention presents a SiO.sub.2/Al.sub.2O.sub.3 ratio of between 1.5 and 2.5, and a SiO.sub.2/Na.sub.2O ratio of between 1.5 and 2.5.

(32) Hydrothermal Synthesis

(33) More generally, the geopolymeric inorganic binder of the friction material according to the invention is obtained according to the methods of FIGS. 1 and 2 via hydrothermal synthesis working (in phases 12 and 25) at a temperature T of between 80° C. and 500° C., and preferably of between 110° C. and 200° C., under a pressure P greater than the saturation pressure (vapor pressure) of water at the pressing temperature, in order to have liquid water.

(34) The reaction occurs by local diffusion of the solid state ions, resulting in the crystallization of hydrated minerals.

(35) Synthesis in Solution

(36) The geopolymeric inorganic binder of the friction material according to the invention can also be obtained by synthesis in solution, where according to the invention the starting materials are metakaolin and a basic solution of sodium or potassium and/or calcium hydroxide in water, or otherwise an acid solution of phosphoric acid in water, working with a molar ratio (MR) of water to alumina between 25 and 7:25>H.sub.2O/Al.sub.2O.sub.3>7, while ensuring that the geopolymerization reaction occurs at a temperature greater than or equal to 40° C. for 5-7 days.

Example 1

Comparison Between Synthesis Methods

(37) In order to verify the feasibility of obtaining an inorganic binder suitable for manufacturing friction materials for friction elements such as brake pads, a series of synthesis experiments were carried out.

(38) Hydrothermal Synthesis Procedure

(39) 100 g of white kaolin of the “L'Aprochimide” firm are mixed with 28.6 g of milled powder caustic soda. The obtained mix is pressed into disks by means of an XRF press with a diameter of 31 mm, with a pressure of 1000 to 4000 kg/cm.sup.2; subsequently, the obtained pressed disks are treated in the oven at 150° C. for 1 h 30′.

(40) The X-ray crystallography spectra show the graphs of FIG. 3, which prove the formation of hydro-sodalite, in the presence of a small amount of unreacted kaolin, in addition to an increase of the signal associated with quartz with respect to the initial kaolin.

(41) Synthesis Procedure in Solution

(42) Metakaolin Argical S1200 is mixed with a basic activating solution with ratio of 37.5% m/m metakaolin and 62.5% m/m activating solution; the latter is composed as follows: 22.5% m/m SiO.sub.2 colloidal silica, 20% m/m NaOH, 57.5% m/m H.sub.2O. Mixing the constituents is carried out by means of a rod mixer for 15 minutes at 1000 RPM and subsequently the paste obtained in this manner is subjected to curing for 7 days at 40° C. in a sealed container in order to develop the maximum degree of mechanical resistance.

(43) The X-ray crystallography spectra reveal the graph of FIG. 4, which shows the presence of an amorphous solid in addition to quartz and anatase impurities deriving from the metakaolin used for the synthesis. These impurities are negligible since their signal is quite low with respect to the amorphous material.

(44) Working as in the above, additional syntheses were carried out utilizing the metakaolin in the hydrothermal synthesis, and vice-versa kaolin in synthesis in solution with and without adding sodium silicate, in order to work the synthesis with SiO.sub.2/Al.sub.2O.sub.3 MR (molar ratio) equal to 2 or equal to 3.8. For simplicity, the obtained results are schematically summarized in Table 2:

(45) TABLE-US-00002 TABLE 2 HYDROTHERMAL SYNTHESIS SYNTHESIS IN (P > P.sub.H2O saturation, WATER SOLUTION 110° C. < T < 200° C.) (7 days at 40° C.) Metakaolin + NaOH DOES NOT REACT DOES NOT REACT (SiO.sub.2/Al.sub.2O.sub.3 = 2) Metakaolin + NaOH + DOES NOT REACT REACTS Sodium Disilicate (SiO.sub.2/Al.sub.2O.sub.3 = 3.8) Kaolin + NaOH REACTS DOES NOT REACT (SiO.sub.2/Al.sub.2O.sub.3 = 2) Kaolin + NaOH + DOES NOT REACT DOES NOT REACT Sodium Disilicate (SiO.sub.2/Al.sub.2O.sub.3 = 3.8) Ideal composition SiO.sub.2/Al.sub.2O.sub.3 = 1.5-2.5 SiO.sub.2/Al.sub.2O.sub.3 = 3.5-4.5 SiO.sub.2/Na.sub.2O = 1.5-2.5 Na.sub.2O/Al.sub.2O.sub.3 = 0.7-1.5 H.sub.2O/Al.sub.2O.sub.3 = 7-20

(46) As can be inferred from Table 2, the results of the synthesis are not trivial and cannot be predicted a priori. For the compositions with MR=2, using the dry method, the result is the in situ crystallization of a hydrated sodium alumino-silicate as a result of the applied temperature and pressure. Starting from kaolin, this occurs because the crystalline structure of kaolin contains hydration water (13% m/m), which enables a localized reaction in situ.

(47) This composition for the reaction in solution shows that with metakaolin, after 5 days at 40° C. in the sealed container, the system presents itself as a highly viscous paste, which after a light stress tends to break apart into plastic fragments, thus signaling a failed reaction. Also when utilizing kaolin, the geopolymerization reaction does not occur.

(48) Instead, the composition with MR=3.8 provides for a different reaction mechanism because it occurs in solution. As already stated, the first phase of dissolution of the raw materials is followed by a second curing phase. In case of metakaolin, the geopolymerization reaction occurs. Instead using kaolin as the raw material, after 1 month at 40° C. in a sealed container, we still have an unreacted medium-high viscosity solution that tends to flow under its own weight.

(49) In conclusion, in order to obtain the geopolymerization reaction under industrial conditions for obtaining friction materials, it is necessary to select not only the starting raw materials, but also the process and the correct SiO.sub.2/Al.sub.2O.sub.3 molar ratio in relation to the raw materials themselves, otherwise the geopolymerization does not occur. Therefore, the choice of the correct process parameters is essential and not obvious for obtaining a friction material having only a geopolymer as the binder.

Example 2

Implementation of Brake Pads

(50) Two formulations of friction material were prepared utilizing for each component the average value of the intervals listed in Table 3 below:

(51) TABLE-US-00003 TABLE 3 Traditional mix with Geopolymer mix phenolic resin Vol % Vol % Aramid fiber 2-4 2-4 Rock fiber  8-12 Phenolic resin 16-19 Friction Powder 3-5 Graphite 11-14 6-8 Strong abrasive 15-18 Medium abrasive 5-7 5-7 Mild abrasive  9-12 15-18 Sulfides 4-2 4-6 Coke 23-26 Steel fiber 5-8 10-13 Inorganic Binder mix 56-60 TOTAL 100 100

(52) The inorganic binder mix is prepared starting from kaolin of the “L'Aprochimide” firm of the “extra white” type mixed with caustic soda powder obtained by milling commercial sodium hydroxide in a rotating Retsch ZM 100 mill. The composition of the tested inorganic binder mix is obtained utilizing the average value of the composition intervals listed in Table 4 below:

(53) TABLE-US-00004 TABLE 4 Inorganic Binder mix FORMULA Vol % Kaolin 75-80 Caustic soda 25-15 TOTAL 100

(54) The binder mix is prepared by dry mixing and is added to the remaining ingredients of the mix according to the steps described in reference to FIGS. 1 and 2. Subsequently, always working as described with reference to FIGS. 1 and 2, six brake pads are pressed with the traditional mix for comparison purposes, 6 brake pads are pressed for the “geopolymer” mix obtained according to the process of FIGS. 1, and 6 brake pads are pressed for the “geopolymer” mix obtained according to the process of FIG. 2.

(55) The “geoplolymer” mixes show Brinell hardness values comparable to those of the comparison sample, average density in water of 2.44 g/cm.sup.3, and excellent resistance to corrosion in the tests in water, water and salt, and citric acid.

Example 3

Brake Test

(56) The brake pads produced as described in example 2 were subjected to the following tests:

(57) Efficiency tests according to the AKM standard, comprising: bedding in braking events, braking events at different fluid pressures, “cold” evaluation braking events (<50° C.), freeway simulation braking events, two series of high energy braking events (first FADE test) interspersed by a series of regenerative braking events. From this test it is also possible to extrapolate, in a manner known to a person skilled in the art, the wear that the brake pads and the brake disks are subjected to.

(58) The obtained results are illustrated in FIGS. 5 to 8, which represent a significant summary of the obtained experimental curves.

(59) With reference to FIGS. 5 and 6, these serve to compare the behavior of the reference mix provided with the organic binder (FIG. 6), with the “geopolymer” mix obtained from the dry mixed pre-mix, according to the process scheme of FIG. 1. As can be immediately noticed, the values of the friction coefficient against the braking pressure (top graph) and the “fade” (bottom graph) are comparable, so that the performance of the “ecological” mix with the binder based exclusively on the geopolymer are perfectly acceptable for the practical application of brake pads and/or shoes.

(60) With reference to FIGS. 7 and 8, these compare the behavior of the “geopolymer” mix obtained from the dry mixed pre-mix, according to the process scheme of FIG. 1 (FIG. 8), with the “geopolymer” mix having identical composition, but which is obtained from the mix mixed with water, according to the process scheme of FIG. 2.

(61) With identical ingredients the friction coefficient is stabilized, and even improved in absolute terms, providing a precise indication of the fact that the process according to the scheme of FIG. 2 leads to obtaining a friction material having better tribological characteristics.

(62) Lastly, the tests were repeated with the reference mix having an organic binder (FIG. 6) and with a “geopolymer” mix obtained from the dry mixed pre-mix, according to the process scheme of FIG. 1, with the addition of an organic binder (phenolic resin) in the amount of 9% by volume with respect to the total volume of the geopolymeric binder present in the mix, obtaining comparable results. Furthermore, even bringing the mix with the geopolymer and the organic binder to a temperature of 450° C. did not lead to an observable release of fumes or vapor or organic particles into the environment.

(63) In conclusion, according to the invention a friction material having comparable or better tribological characteristics is obtained (according to the adopted process) with respect to the friction materials known in the art provided with an organic binder, with the advantage that it does not undergo the thermal degradation that organic binders are subjected to under operation, with the consequent elimination of the emission of organic compounds into the atmosphere.

(64) The objectives of the invention are therefore fully achieved.