Method for producing a friction material and the related pad for disc brakes

Abstract

The present invention relates to a method for obtaining a friction material and the related pad for disc brakes. In particular, the invention relates to a method for producing a friction material comprising the following steps in sequence : a ) forming a wet mixture of an alkaline-activated inorganic binder precursor with an inorganic base and a friction mixture; b ) pressing the wet mixture of step a ) into a mold, obtaining a friction material intermediate; c ) aging the friction material intermediate obtained according to step b ); d) optionally, final heat treatment.

Description

[0001] METHOD FOR PRODUCING A FRICTION MATERIAL AND THE RELATED

[0002] PAD FOR DISC BRAKES

[0003] Description

[0004] Field of the invention

[0005] The present invention relates to a method for obtaining a friction material and the related pad for disc brakes .

[0006] Background art

[0007] Friction materials without asbestos and heavy metals for brake pads typically comprise the following classes of components : a fibrous material consisting of inorganic and / or organic and / or metal fibers ; one or more abrasives ; one or more binders , usually cons isting of organic resins , e . g . , phenolic resins ; one or more fillers ; and one or more lubricants or friction modi fiers .

[0008] The production of such friction materials is not only time-consuming and costly because of the various working steps and high molding temperatures to allow the crosslinking of the resin, but also disadvantageous in terms of eco- compa t ibili ty .

[0009] Friction material s comprising inorganic binders of various kinds instead of organic resins are also known, but also in this case their production involves a number of separate steps and high temperatures , resulting in an increase in the production costs which is not advantageous for industrial production .

[0010] Therefore , the need is felt to provide a method which allows the production of a friction material and the related disc brake pad which allows a limited number of steps , low production cost , good eco-compatibility, and which can also be implemented on the same equipment currently used for the production of friction materials with organic binder .

[0011] Summary of the invention

[0012] The above problem is solved by a method for producing a friction material and a pad for disc brakes , as outlined in the appended claims , the definitions of which form an integral part of the present description .

[0013] It is a first obj ect of the invention a method for producing a friction material comprising the following steps in sequence : a ) forming a wet mixture of an alkaline-activated inorganic binder precursor with an inorganic base and a friction mixture ; b ) pressing the wet mixture of step a ) into a mold, obtaining a friction material intermediate ; c ) aging the friction material intermediate obtained according to step b ) ; d) optionally, final heat treatment . It is a further obj ect of the present invention a method for preparing a disc brake pad, comprising the steps from a ) to d) as def ined above and where , in step b ) , said mold contains a metal plate for a disc brake pad .

[0014] Further features and advantages of the invention will become more apparent from the description of some embodiments , given hereinbelow by way of a non-limiting indication .

[0015] Detailed description of the invention

[0016] It is an obj ect of the present invention a method for producing a friction material or a disc brake pad comprising the following steps in sequence : a ) forming a wet mixture of an alkaline-activated inorganic binder precursor with an inorganic base and a friction mixture ; b ) pressing the wet mixture of step a ) into a mold, where said mold optionally contains a metal plate for a disc brake pad, obtaining a friction material or disc brake pad intermediate , respectively; c ) aging the friction material or disc brake pad intermediate according to step b ) ; d) optionally, final heat treatment .

[0017] The term " alkaline-activated inorganic binder" means an inorganic material obtained by treating an inorganic precursor thereof and a solution of an inorganic base . Without being bound to any particular theory, the reaction consisting in the alkaline hydrolysis of the inorganic ligand precursor is considered as a "dissolution reaction" . It should be noted that said binder does not contain cement .

[0018] Said alkaline-activated inorganic binder precursor is preferably chosen from an aluminosilicate inorganic material with poz zolanic behavior , such as metakaolin, microsilica ( so-called silica fume ) , fly ash and blast furnace slag, more preferably it is metakaolin . For example , Metaver® M or Metaver® N material supplied by Newchem GmbH can be used .

[0019] Metakaolin (A12O3-2S1O2 ) or calcined kaolin is a known material obtained by thermal activation of kaolinite clay .

[0020] In particular, metakaolin usable for the purposes of the present invention is a material produced by calci fication of a concentrated kaolin and is mainly a white or beige amorphous aluminosilicate , which reacts with Portlandite ( calcium hydroxide ) , for example , to form cement-like CSH phases . It can be produced from Kaolinite - a lamellar silicate with an interlayer of 7 . 2 A. Between the interlayers of SiO2 and AI2O3, water is stored in the proportions of 1 : 2 and expelled through a heat treatment

[0021] ( calci fication) . Kaolin thus becomes active . In some embodiments, the metakaolin has the following typical chemical composition (% by weight) :

[0022] SiO245 - 60; CaO < 0.5; A12O335 - 49; MgO < 0.5; Fe2O3< 2; Na2O < 0,1; TiO2< 3; K2O < 2, and the following physical characteristics:

[0023] Specific weight approx. 1-4 g / cm3; Fineness according to Blaine 22,000-28,000 cm2 / g; Specific surface area according to BET 10-27 m2 / g; Whiteness (Dott. Lange) 55- 90; Bulk density, dissolved 0.22 - 0.42 g / cm3, vibrated 0.30 - 0.52 g / cm3; Typical particle size distribution (laser granulometer ) d50 2-5 pm d95 4-25 pm.

[0024] In a different embodiment, the metakaolin has the following typical chemical composition (% by weight) :

[0025] SiO250 - 56; A12O340 - 43; Fe2O3< 1; K2O < 2.0, and the following physical characteristics: specific gravity 2.6 g / cm3; Typical particle size distribution (laser granulometer) d50 12 pm.

[0026] In some embodiments, the inorganic base is an aqueous solution of one or more salts and / or hydroxides of an alkali or alkaline-earth metal, e.g., hydroxides and / or oxides of an alkali and / or alkaline-earth metal and silicon. Examples are: silicon oxide, sodium or potassium oxide, potassium hydroxide and / or silicate, and sodium hydroxide and / or silicate. The basic solution preferably has a concentration between 30 and 50% by weight of hydroxide .

[0027] In general, the total amount of inorganic base solution is also defined based on the fluidity characteristics of the final mixture also containing the friction material, more than on the reactivity requirement alone; in general, the process is carried out with excess solution to buffer the loss of liquid in the subsequent step of pressure molding as well as to take into account the amount of liquid adsorbed by the friction mixture.

[0028] Preferably, said friction material comprises one or more abrasives in an amount between 5% and 15% by weight, preferably between 10 and 14% by weight.

[0029] In a preferred embodiment, said one or more abrasives are selected from the group consisting of: potassium titanate, barium titanate, calcium titanate, lithium titanate, chromite, magnesium oxide, zinc oxide, aluminum oxide, silicon carbide, and corundum.

[0030] According to this embodiment, the friction material comprises at least one of potassium titanate, chromite, magnesium oxide, zinc oxide, aluminum oxide, silicon carbide or corundum as abrasives.

[0031] Preferably, said friction material comprises potassium titanate, chromite, magnesium oxide, zinc oxide, aluminum oxide, silicon carbide and corundum as abrasives. Said friction material can or cannot comprise other abrasives in addition to those mentioned above, e.g., selected from zirconium, zirconium silicate, zirconium oxide, mica, talc, kaolin, barium sulfate, calcium carbonate, calcium silicate, iron oxides, mullite.

[0032] The friction material covered by the present invention comprises coke in an amount between 5% and 17% by weight, preferably between 7% and 11% by weight.

[0033] Preferably, the friction material covered by the present invention comprises up to 8% by weight of one or more metal sulfides, preferably between 4 and 6% by weight.

[0034] According to an embodiment, said friction material comprises one or more metal sulfides selected from antimony sulfide (Sb2Ss) , molybdenum sulfide (M0S2) , tungsten sulfide (WS2) , tin sulfide (SnS, SnS2) , zinc sulfide (ZnS) , iron sulfide (FeS) , copper sulfide (CU2S) , and mixed sulfides. The term "mixed sulfides" means sulfides of more than one metal. In particular, the term "mixed sulfides" means sulfides with metals having different valences.

[0035] According to an embodiment of the invention, said one or more metal sulfides are selected from the group consisting of FeS and SnS, tungsten sulfide (WS2) , tin sulfide (SnS, SnS2) , bismuth sulfide, zinc sulfide (ZnS) , titanium sulfide and molybdenum sulfide (M0S2) and mixed sulfides . According to an embodiment of the invention, said friction material comprises at least FeS and SnS .

[0036] The friction material covered by the present invention compri ses from a minimum of 3% up to a maximum of 10% by weight of one or more powdered metals and / or powdered metal alloys and / or powdered intermetal compounds , preferably between 5% and 8 % .

[0037] According to di f ferent embodiments , the intermetal compounds can be binary intermetal compounds , or ternary intermetal compounds .

[0038] According to an embodiment of the invention, said friction material comprises at least one of powdered zinc and powdered aluminum, preferably both, in the amounts by weight defined above .

[0039] According to an embodiment of the invention, said friction material comprises powdered zinc, powdered aluminum, and at least one powdered intermetal compound, preferably ternary, in the amounts by weight defined above . Said friction material can or cannot comprise other powered metals and / or metal alloys and / or intermetal compounds in addition to those defined above .

[0040] Said friction material can also comprise metals and / or metal alloys and / or intermetal compounds in fiber form, in addition to those in powder form . The friction material covered by the present invention comprises up to 30% by weight of a fibrous material comprising aramid fiber and steel fiber and rock fiber .

[0041] Preferably, said friction material comprises between 10% and 30%, and preferably between 19% and 26% of said fibrous material.

[0042] According to an embodiment, said fibrous material consists of aramid fiber and / or steel fiber and / or rock fiber .

[0043] According to an alternative embodiment, said fibrous material can comprise additional organic and / or inorganic and / or metal fibers, in addition to aramid fiber and steel fiber and rock fiber, e.g., carbon fibers, acrylic fibers, rock wool, rock fibers, glass fibers, ceramic fibers, mineral fibers, wollastonite, copper fibers, zinc fibers, tin fibers, bronze fibers, brass fibers, iron fibers.

[0044] According to an embodiment of the invention, said friction material comprises graphites in an amount between 5 and 17% by weight, between 5 and 15% by weight, between

[0045] 5 and 12% by weight, between 5 and 10% by weight, between

[0046] 6 and 10% by weight, and between 6 and 9% by weight.

[0047] Optionally, the friction material of the present invention comprises up to 10% by weight of one or more fillers selected from the group consisting of: calcium hydroxide, friction powder, PTFE, and fluorine. The term "friction powder" denotes powders obtained by crosslinking and then grinding the liquid from cashew nut shells .

[0048] Preferably, the friction material comprises said one or more fillers in an amount between 5 and 10% by weight, between 6 and 10% by weight, between 7 and 10% by weight, between 8 and 10% by weight, and between 9 and 10% by weight .

[0049] According to an embodiment, the friction material comprises calcium hydroxide, friction powder, PTFE and fluorine as fillers in the aforesaid amounts by weight.

[0050] In a specific embodiment of the invention, the friction material comprises:

[0051] 5-15% by weight, preferably 10-14% by weight, of abrasives ;

[0052] 5-17% by weight, preferably 7-11% by weight, of coke;

[0053] 0-8% by weight, preferably 4-6% by weight, of sulfides ;

[0054] 3-10% by weight, preferably 5-8% by weight, of metal powder;

[0055] 10-30% by weight, preferably 19-26% by weight, of fibrous material;

[0056] 5-17% by weight, preferably 6-9% by weight, of graphites . The friction material of the present invention is advantageously obtained by uniformly dry mixing the aforesaid components in the amounts specified above, preferably for a time of at least 3 minutes or at least 5 minutes and at a mixing speed of at least 1300 rpm or about 1500 rpm.

[0057] In some embodiments, the weight ratio of friction mixture to inorganic binder precursor is between 3:1 and 2:1, and preferably between 2.7:1 and 2.2:1, even more preferably between 2.55:1 and 2.45:1.

[0058] In some embodiments, step a) comprises the following steps : al) forming a wet mixture comprising said alkaline- activated inorganic binder precursor and said inorganic base ; a2) keeping said mixture under stirring at ambient temperature, preferably for a time longer than 3 minutes, more preferably longer than 4 minutes, and preferably shorter than 6 minutes, thus obtaining a slurry; a3) adding said friction mixture to said slurry of step a2) and subjecting the thus-obtained mixture to stirring at ambient temperature, preferably for a time longer than 1.5 minutes and shorter than 3 minutes, obtaining said wet mixture. Step b) is carried out at a pressure between 0.5 and 5 kN / cm2, preferably between 0.7 and 4.7 kN / cm2, more preferably 0.8 and 4.5 kN / cm2, and for a time of at least 3 minutes and less than 7 minutes, preferably between 4 and 5 minutes. In any case, the temperature is less than 40°C, preferably it is ambient temperature.

[0059] As mentioned, step b) can be carried out on both the wet mixture alone to form a friction material and preferably in a mold in which a metal plate of the type used for disc brake pads has been introduced, so that the disc brake pad is obtained directly.

[0060] Step c) of aging is carried out at ambient temperature, ambient pressure, and ambient humidity, preferably in a closed environment so as to avoid uncontrolled evaporation of the aqueous phase, for at least 3 weeks and up to 4 weeks.

[0061] Step d) comprises a treatment at a temperature between 100°C and 200°C for a time of less than 1 hour and generally longer than 30 minutes. If final heating and cooling times are also considered, the total treatment time is less than 3 hours. Step d) is optional, but if carried out, it allows reducing the aging time to a maximum of 3 weeks.

[0062] The disc brake pad obtained according to the method described above can be subjected to a final finish (e.g., mechanical polishing, surface grinding, painting) . Such operations can be carried out on the pads according to common procedures usually used for standard brake pads.

[0063] Example of making a disc brake pad according to the invention

[0064] STEP (a) , substeps (al)- (a2) : Making the inorganic alkali-activated slurry

[0065] 175 g of Metakaolin (specifically Metaver M or N, supplied by Newchem GmbH) is loaded into a planetary mixer originally designed for mixing building materials. 217 g of an alkaline activating potassium hydroxide solution (concentration about 34% m / m, pH=14) are then added. The materials are mixed at speeds between 100 and 300 rpm for a total of 4' 30" at ambient temperature to obtain a slurry .

[0066] STEP (a) , substep (a3) : Making the alkaline mortar containing the friction slurry

[0067] 438 g of a previously prepared dry friction mixture are loaded into the mixer containing the previously prepared slurry, the final wet mixture is then mixed at speeds between 100 and 300 rpm for 2'15" at ambient temperature. This results in a moist slurry ready to be worked .

[0068] Step b) : Brake pad molding

[0069] 400 g of the previously prepared wet slurry are loaded into the mold of a pressing station usually used for standard brake pads (custom-designed to support highly alkaline solutions and for liquid-draining capabilities) and distributed to homogeneously fill the entire cavity. The wet mixture is pressed onto a metal plate provided with a mechanical retention system (shape and size according to a surface area of 88.8 cm2, appropriate for the current automotive application) for 4'30" at 4.5 KN / cm2; the step of pressing is carried out at ambient temperature. Some liquid is released during pressing, and a densely compressed friction lining is then retained on the metal support plate.

[0070] STEP c) : Hardening and aging period

[0071] Brake pads are stored in a plastic container or other containers made of materials capable of preventing uncontrolled drying during the aging period; the aging period, as a function of the subsequent steps and specific performance requirements, must be no less than 1 week or two weeks or up to three weeks. The bearings must be aged at ambient temperature, ambient pressure and ambient humidity .

[0072] STEP d) : Heat treatment of consolidated and aged brake pads

[0073] When a final thermal step must be included in the process, the pads are heated to temperatures between 50 and 300°C, preferably between 100°C and 200°C, for a period of not less than 1 hour and not more than 5 hours , preferably between 2 hours and 4 hours at atmospheric pressure .

[0074] The method of the invention can produce friction materials , and more speci fically brake pads , in a semiautomatic mode by combining a low-temperature wet process with the procedures and equipment normally used for making traditional resin-based brake pads . The current need for high-temperature molding of brake pad material to ensure the cross-linking of the resin is thus overcome , while ensuring the inherent necessary material hardening of this class of materials by virtue of the use of chemically- activated inorganic binders . The ability to combine these two main technologies allowed improving the current alternative presented in the prior art concerning the brake friction material by employing an inorganic binder in a method consisting of a few wet steps with very little energy consumption . The methods of the prior art with inorganic matrix, on the other hand, rely on high- temperature , energy-consuming molding steps , which are not advantageous to the industrial proces s traditionally employed and normally implemented for molding resin-based parts . By virtue of the use of basic liquid activators in the mixture and throughout the process , it was also possible to exclude any use of potentially harmful corrosive solid agents , reported in several alternative examples of the prior art .

[0075] It is apparent that only one particular embodiment of the present invention was described . Those skilled in the art will be able to make all the necessary modi f ications to the material , pad and braking system for the adaptation thereof to particular conditions , without however departing from the scope of protection as defined in the appended claims .

Claims

CLAIMS1. A method for producing a friction material comprising the following steps in sequence: a) forming a wet mixture of an alkaline-activated inorganic binder precursor with an inorganic base and a friction mixture; b) pressing the wet mixture of step a) into a mold, obtaining a friction material intermediate; c) aging the friction material intermediate obtained according to step b) ; d) optionally, final heat treatment.

2. The method for preparing a disc brake pad, comprising the steps from a) to d) according to claim 1 and wherein, in step b) , said mold contains a metal plate for a disc brake pad.

3. The method according to claim 1 or 2, wherein said alkaline-activated inorganic binder does not contain cement .

4. The method according to any one of claims 1 to 3, wherein said alkaline-activated inorganic binder precursor is an inorganic material with pozzolanic behavior.

5. The method according to claim 4, wherein said inorganic material with pozzolanic behavior is metakaolin.

6. The method according to claim 5, wherein metakaolin has the following typical chemical composition (% by weight) :SiO245 - 60; CaO < 0.5; A12O335 - 49; MgO < 0.5; Fe2O3< 2; Na2O < 0,1; TiO2< 3; K2O < 2, and the following physical characteristics:Specific weight approx. 1-4 g / cm3; Fineness according to Blaine 22,000-28,000 cm2 / g; Specific surface area according to BET 10-27 m2 / g; Whiteness (Dott. Lange) 55- 90; Bulk density, dissolved 0.22 - 0.42 g / cm3, vibrated 0.30 - 0.52 g / cm3; Typical particle size distribution (laser granulometer ) d50 2-5 pm d95 4-25 pm; or metakaolin has the following typical chemical composition (% by weight) :SiO250 - 56; A12O340 - 43; Fe2O3< 1; K2O < 2.0, and the following physical characteristics: specific gravity 2.6 g / cm3; Typical particle size distribution (laser granulometer) d50 12 pm.

7. The method according to any one of claims 1 to 6, wherein the inorganic base is a basic aqueous solution ofan alkaline or alkaline-earth metal hydroxide, such as sodium hydroxide or potassium hydroxide.

8. The method according to claim 7, wherein the basic aqueous solution has a concentration between 30 and 50% by weight of alkaline or alkaline-earth metal hydroxide.

9. The method according to any one of claims 1 to 8, wherein the friction material comprises one or more abrasives in an amount between 5 and 15% by weight, coke in an amount between 5 and 17% by weight, up to 8% by weight of one or more metal sulfides, up to 10% by weight of one or more powdered metals and / or powdered metal alloys and / or powdered intermetal compounds, up to 30% by weight of a fibrous material comprising aramid fiber and / or steel fiber and / or rock fiber, and / or up to 17% by weight of graphites .

10. The method according to claim 9, comprising in addition up to 10% by weight of one or more fillers selected from the group consisting of: calcium hydroxide, friction powder, PTFE, and fluorine.

11. The method according to claim 9 or 10, wherein:said one or more abrasives are selected from the group consisting of: potassium titanate, chromite, magnesium oxide, zinc oxide, aluminum oxide, silicon carbide, and corundum;- said one or more metal sulfides are selected from: antimony sulfide (Sb2Ss) , molybdenum sulfide (M0S2) , tungsten sulfide (WS2) , tin sulfide (SnS, SnS2) , zinc sulfide (ZnS) , iron sulfide (FeS) , copper sulfide (CU2S) , bismuth sulfide, and mixed sulfides; intermetal compounds are binary intermetal compounds or ternary intermetal compounds, and the metals are at least one of powdered zinc and powdered aluminum.

12. The method according to any one of claims 1 to11, wherein the weight ratio of friction mixture to inorganic binder precursor is between 3:1 and 2:1, or between 2.7:1 and 2.2:1, or between 2.55:1 and 2.45:1.

13. The method according to any one of claims 1 to12, wherein step a) comprises the following steps: al) forming a wet mixture comprising said alkaline- activated inorganic binder precursor and said inorganic base ; a2) keeping said mixture under stirring at ambient temperature, preferably for a time longer than 3 minutes,more preferably longer than 4 minutes, and preferably shorter than 6 minutes, thus obtaining a slurry; a3) adding said friction mixture to said slurry of step a2) and subjecting the thus-obtained mixture to stirring at ambient temperature, preferably for a time longer than 1.5 minutes and shorter than 3 minutes, obtaining said wet mixture.

14. The method according to any one of claims 1 to 13, wherein step b) is carried out at a pressure between 0.5 and 5 kN / cm2, or between 0.7 and 4.7 kN / cm2, or between 0.8 and 4.5 kN / cm2, and for a time of at least 3 minutes and less than 7 minutes, or between 4 and 5 minutes, and wherein the temperature is less than 40°C or is ambient temperature .

15. The method according to any one of claims 1 to 14, wherein step c) of aging is carried out at ambient temperature, ambient pressure, and ambient humidity, preferably in a closed environment so as to avoid uncontrolled evaporation of the aqueous phase, for a time of at least 1 week, or 2 weeks, or up to 3 weeks.

16. The method according to any one of claims 1 to15, wherein step d) comprises a treatment at a temperaturebetween 50 and 300°C, preferably between 100°C and 200°C, for a period of not less than 1 hour and not more than 5 hours, preferably between 2 hours and 4 hours at atmospheric pressure.