A friction material for brakes which contains a solid inorganic binder

Abstract

The invention relates to a friction material for manufacturing friction layers or blocks or linings for brake members such as brake pads or brake shoes, which contains a solid inorganic binder obtainable by reacting a silicate solution and one or more phosphates acting as hardeners. The scope of the invention also encompasses a braking member including a layer or block or lining of the friction material of the invention, as well as a method of manufacturing the friction material of the invention.

Description

[0001]A friction material for brakes which contains a solid inorganic binder Technical field The present invention pertains to a friction material for manufacturing friction layers or blocks for brake members such as brake pads or brake shoes, the friction material containing a solid inorganic binder. The invention pertains also to a method of manufacturing the friction material and to a braking member constituted by a brake pad or a brake shoe, including a layer or block or lining of the friction material. Background art Brake pads and brake shoes comprise of a block or a layer or a lining of friction material bound to the surface of a backing plate (flat or in the shape of an arc of a cylinder), and the friction material faces the disc brake rotor or the brake drum. Friction materials used on modern vehicles include powdered components and / or fibre components, one or more binders, and further components such as fillers, abrasives, lubricants, friction modifiers and other additives. It is known to use, as friction materials, Carbon-ceramic or organic materials, typically composed of an organic resin as a binder and a fibrous part as reinforcement. Generally, the fibre part is made using multiple fibres (sintered glass, carbon, metallic, ceramic, aramid). Additionally, the friction material may include a filler (which has no active role) and a friction modifier such as powdered metals (e.g. copper, iron, aluminium, and zinc) or oxides (e.g. alumina and silica), which are also used to increase friction properties. The current state of art is that powdered components and / or fibres are mixed with an organic binder such as a phenolic resin or rubber, or a combination thereof. These organic binders may be used as dry powders, chips, chunks or dissolved in an appropriate solvent. Phenol formaldehyde resin is frequently used as a binding agent. Semi-solid mixtures of friction material are then pressed in moulding tools using hydraulic presses at either ambient or higher temperatures, to form a block or layer. Typical moulding pressures range between 100 and 1200 kPa / cm2, typical moulding temperatures are from ambient temperature to 200°C. After moulding, the produced blocks or layers are subjected to a post-curing process in an oven or through IR radiation. WO 03 / 004899 A2 discloses a method for producing inorganically bound friction linings. This publication describes the use of a “geopolymer„ binder system for Friction Materials. The disclosed binder system is a mixture of K-Silicate, Na-Silicate, Li-Silicate or a combination thereof, and meta-kaolin powder. The use of a geopolymer system will inevitably lead to a low shelf life of the friction material mixture, because due its high reactivity, the geopolymer binder will crosslink and harden in most cases in less than two hours at ambient temperature. This will lead to considerable problems of producing friction materials of consistent quality, because these binder systems are undergoing consistent changes of their crosslinking grade already within a few minutes after preparation of the friction materials mixture. WO 2020 / 039396 A1 discloses a method of manufacturing a brake pad with a block of friction material. The method provides the isolation of solid, partially crosslinked geopolymer from the aqueous solution of sodium hydroxide and sodium silicate is mixed with commercial metakaolin until a wet paste is obtained. Then, the wet paste is formed and dried until a dried geopolymeric aggregate is obtained, the aggregate is ground to a powder. The dried ground aggregate is used as an inorganic geopolymeric binder in a friction material compound. The raw compound is hot-moulded under a pressure greater than a water saturation pressure at the moulding temperature. CN 103804030 A discloses a method for preparing an oxidation-resistant composite coating for a carbon ceramic brake disc. The method comprises the steps of: preparing glass slurry to serve as a high-temperature self-healing coating by taking cordierite powder, glass powder and an organic adhesive as raw materials, preparing a solution to serve as a passivating coating by taking phosphate as a raw material, uniformly coating the glass slurry and the solution on the surface of a carbon ceramic brake material, and performing high-temperature heat treatment to obtain a dense coating which can be well combined with a matrix. Summary of the invention Against the foregoing background, it is a primary object of the present invention to provide friction materials allowing the raw material mixture to be moulded not necessarily immediately or shortly after its preparation. The above and other objects are achieved by a friction material as defined in appended claim 1. Advantageous embodiments of the invention are defined in the dependent claims. In accordance with one aspect, the present invention, briefly summarised, provides a friction material for a braking element, the friction material comprising: at least a powdered component and / or at least a fibre component, and a binder, wherein the binder comprises a solid inorganic binder which is obtainable by reacting solid inorganic binder reagents consisting of: a) at least one of the groups consisting of K-silicate solution, Na-silicate solution, Li-silicate solution, and any combination thereof, as liquid phase; and b) at least one of the groups consisting of aluminium phosphates, boron phosphates, magnesium phosphates, calcium phosphates, zinc phosphates, and any combination thereof, as solid phase as defined above. The solid inorganic binder in the friction material disclosed herein behaves approximately like an organic thermosetting agent in that it gives rise to solid cross-linking when exposed to a temperature, which allows it to be stored for a virtually unlimited time at room temperature. In contrast thereto, a conventional raw mixture of friction material, with a binder including metakaolin, once prepared, solidifies within 15 to 30 minutes, and therefore needs to be moulded almost immediately. According to a further aspect, the present invention provides a method of manufacturing a friction material for a braking member, constituted by a brake pad or a brake shoe, the braking member having a layer or block or lining of friction material which includes the inorganic binder as defined in the appended claims. Such friction material is bound to the surface of a backing plate, e.g. flat or in the shape of an arc of a cylinder or in the shape of a disc or in the shape of a circular ring. According to yet another aspect, the invention provides a braking member, constituted by a brake pad or a brake shoe, having a layer or block or lining of friction material which includes the inorganic binder as defined in the appended claims. The friction material is bound to the surface of a backing plate, e.g. flat or in the shape of an arc of a cylinder or in the shape of a disc or in the shape of a circular ring. Detailed description In order that the present invention may be better understood, there will now be described a few preferred but not limiting embodiments thereof, given by way of examples. A friction material for a braking element comprises at least a powder component and / or at least a fibre component and a binder. Optionally, and preferably, other components may be provided in addition to these, such friction modifiers, lubricants, fillers and abrasives. Preferred examples of the fibre component are metal fibres (e.g. steel fibres, stainless steel fibres, Zn fibres or other metal fibres), ceramic fibres, glass fibres, aramid fibres, mineral fibres (such as rockwool fibres). Preferred examples of the powder component are metal powders, alloy powders, mineral powders and organic powders. According to an aspect of the invention, the binder comprises a solid inorganic binder. The solid inorganic binder is obtainable by reacting solid inorganic binder reagents consisting of: a) at least one of the groups consisting of K-silicate solution, Na-silicate solution, Li- silicate solution, and any combination thereof, as liquid phase; and b) at least one of the groups consisting of aluminium phosphates, boron phosphates, magnesium phosphates, calcium phosphates, zinc phosphates and any combination thereof, as solid phase. A preferred amount of the solid inorganic binder in the friction material of the invention ranges from 10 – 40 vol%. This includes 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 and 40 vol% inorganic binder. When mixed, components a) and b) of the inorganic binder undergo an ion-exchange reaction by which the K, Na, Li cations are replaced by the cations of the phosphate salts, leading to the formation of a 3-dimensional inorganic polymer network. The phosphates listed under b) act as hardeners. The selection of the hardeners listed under b) and the concentration of the silicate solutions listed under a) are determining the rate of the ion exchange reaction. In certain embodiments of the invention, the binder in the friction material further comprises, in addition to the solid inorganic binder, one or more further binders, such as conventional organic binder(s) (e.g. phenolic resin) and / or partially inorganic binder(s) (e.g. silicon resin). A method of manufacturing a friction material comprises providing the fibre and / or powder component(s) and the reagents to obtain the solid inorganic binder, namely the reagents(s) mentioned at a) hereinabove as liquid phase, and the components mentioned at b) as solid phase. The fibre and / or powder component(s) and the reagents mentioned at a) and b) hereinabove are subjected to a mixing step, until a pre-mixture is obtained. Optionally, the pre-mixture so obtained may then be blended with one or more of further binders, such as conventional organic binder(s) (i.e. phenolic resin) and / or partially inorganic binder(s) (i.e. silicon resin), and / or with one or more of further additives, such as a friction modifier or lubricant, a filler, an abrasive, and any combination thereof, thereby obtaining a raw mixture of friction material. In an alternative embodiment, the fibre and / or powder component(s) are first mixed with the aforementioned one or more of further additives and / or the one or more further binders, thereby obtaining a pre-mixture of friction material. The pre-mixture so obtained is then blended with the reagents mentioned at a) and b) hereinabove, thereby obtaining a raw mixture of friction material. As mentioned, fibres may be organic, inorganic and / or metallic. Preferred examples of fibres that can be used in the friction material of the invention and their exemplary amounts are: 0 – 40 vol% steel fibres, 0 – 20 vol% bio soluble mineral fibres, 0 – 20 vol% basalt fibres, 0 – 10 vol% glass fibres, 0 – 10 vol% of organic fibres (e.g. polyaramid fibres, PAN-fibres, cellulose fibres, and any combination thereof). The aforementioned ranges include 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 vol% glass fibres; 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 vol% organic fibres; 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20 vol% mineral fibres; 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20 vol% basalt fibres; 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 and 40 vol% steel fibres. Preferred, optional non carbon lubricants may comprise one or more sulphides selected from the group consisting of: iron sulphides (e.g. FeS, FeS2), tin sulphides (SnS, SnS2), titanium sulphides (TiS, TiS2), molybdenum disulphide (MoS2), zinc sulphide (ZnS), manganese sulphide (MnS), bismuth sulphide, and tungsten sulphide (WS). Preferred amount of sulphides usually range from 0 – 10 vol%. This includes 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 vol%. Preferred carbon-based lubricants include natural and / or synthetic graphite, carbon black and coke. Preferred amounts of such carbon-based lubricants usually range from 5 to 35 vol%, more preferably from 5 to 15 vol%. This includes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 and 35% vol%. Alternatively, or in addition to the aforementioned lubricants, zinc and / or tin powder may be used as a lubricant. Optionally, preferred additives conferring enhanced abrasive properties may be blended with the pre-mixture. The abrasive additives may include on or more of the following: SiC, Al2O3fused, Al2O3 calcined, Chromite, Fe2O3, Fe3O4, SiO2, Cr2O3, ZrSiO4, ZrO2, MgO, K- Titanate, Li-Titanate, Mg-Titanate. Usually, the abrasives are present in the friction material in an amount ranging from 0-25 vol%. This includes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 25 vol%. One or more fillers may be blended with the pre-mixture. The filler(s) may include CaCO3, BaSO4, Ca(OH)2, SiO2, calcium silicate, magnesium silicate, aluminium silicate, K-titanates, Li-titanates, mica, vermiculite, wollastonite, talc, clay. Usually, the fillers are present in the friction material in an amount ranging from 0-40 vol%. This includes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 and 40% vol%. Advantageously, the so formed raw mixture may than be stored, preferably at room temperature, for an undetermined period of “shelf time” and shipped, prior to being applied by moulding onto a flat or arched or circular ring backing plate, depending on the kind of braking member to manufacture. Binder components may be combined such that the “open time” of the raw friction mixtures (“shelf life”) will last sufficiently long to ensure that friction material mixes can be processed giving consistent physical properties and, hence, good quality. Subsequently, the raw mixture is moulded under given pressure and temperature in order to obtain a layer or block or lining of friction material, depending on whether the braking member to manufacture is a brake pad for a disc brake or a brake shoe for a drum brake. Accordingly, the moulding step of the block or layer or lining takes place applying the raw mixture on a conventional backing plate, which is flat or in the shape of circular ring for disc brake pads and in the shape of an arc of a cylinder for drum brake shoes. According to an embodiment, the moulding step includes pressing the raw mixture at a pressure ranging from 100 to 1200 kPa / cm2. Embodiments may provide that the pressing step includes applying a temperature ranging from ambient temperature to 200°C for a time ranging from 5 seconds to 6 minutes. Finally, the raw mixture is subjected to a drying step. The drying step may be carried out in an oven or through IR radiations or through post-compaction. Embodiments may provide that the drying step is carried out at a temperature in the range 50 – 250 °C. The present invention accomplishes several advantages over the prior art, particularly with respect to friction materials making use of phenolic resins as binders. The onset of thermal decomposition for phenolic resins is at 300°C. In contrast thereto, the inorganic binder according to the present invention is able to withstand more than 600°C without thermal degradation. Therefore, since the binder will not degrade over temperature under e.g. extreme braking conditions, no fumes, gases or ultrafine particulates will be generated by the solid inorganic binder. Experimental tests performed by the applicant reveal that excellent results are obtained, as compared to other conventional friction materials, in terms of increased friction performance over speed / temperature / pressure. The solid binder of this invention has “per se” abrasive features, which requires a minor amount of friction modifiers. The solid inorganic binder in the friction material of the present invention achieves a significant dispersion among the friction material components, as observed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDX). A scanning electron microscope produces images of a sample by scanning the surface with a focused beam of electrons. The incident electrons penetrate into the sample and interact with atoms producing various signals that contain information about the sample surface topography and composition. In particular, secondary electrons (SE) can be generated by inelastic scattering and characteristic x-rays can be emitted as a consequence of the incident electrons. The analysis of such characteristic x-rays provides chemical information about the sample surface composition. By combining the two techniques mentioned above, SEM and EDX, a significant dispersion of the elements of the solid inorganic binder over the other materials was observed. For the present example, the elements observed are Silicium (Si), Aluminium (Al), Phosphorous (P) and Potassium (K). Figure 1 represents a SEM image of the friction material of the invention and the relative dispersion of the solid binder elements. As shown in Figure 1, the presence of the solid binder can be detected by the wide dispersion of its elements, which are equally distributed within the sample. The relative proportions of such elements are in agreement with the theoretical elemental composition of the solid binder. The friction material of the present invention exhibits a better stability of mechanical properties over temperature. A significant reduction of thermal decomposition is achieved with respect to products comprising conventional organic binders. As known, the binder is the most wearable component in a friction material. The binder used in the friction material of the present invention provides a lower level of environmental pollution, due to a reduction in the number of ultrafine particulates of hazardous character being released. A minor energy consumption is required for the manufacturing of the friction material of the invention as compared to the prior art. A better overall ecological / CO2 footprint is achieved. Specifically, as compared to conventional friction materials including geopolymer binders, the following advantages are achieved: - less wear of friction materials; - less wear of the members to be braked, namely brake discs or brake drums; - better mechanical strength of the friction material. In light of the aforementioned properties, the friction material of the present invention is suited for use in the automotive industry, e.g. for road and off-road vehicles, as well as in the 2-wheelers industry, the railway industry and, in a more general way, in any industrial application which requires a braking device. For example, the invention may be applied to manufacture brake pads for disc brakes or brake shoes for drum brakes. The following examples are provided by way of illustration only and are not intended to limit the scope of the invention as defined in the appended claims. EXAMPLES Example 1: friction material formula comprising the solid inorganic binder vol% Inorganic binder 17.0 Aramid fibre 9.0 Basalt fibre 11.0 Natural graphite 11.0 Potassium titanate 10.0 Tin Sulphide 6.0 SiC 7.0 MgO 7.0 Barite 16.0 Steel fibre 6.0 Example 2: The friction material of example 1 (designated herein as “IB material”) was subjected to comparative tests in which its performances were compared to a conventional, low steel friction material including an organic binder. Specifically, two conventional dynamometric tests were performed, called the AK Master test (SAE J2522) and the TL 110 test, respectively. The implementation of both tests is well within the reach of the skilled person. The AK Master test showed the improved thermal stability of the IB material over that of the reference low steel friction material. The IB material is in fact free of organics, so it did not show any sign of performance loss due to oxidation of low temperature resistant components. At the same time, pad wear and nominal mu level for the two materials were completely aligned in the AK Master test. TL 110 is a wear versus temperature test. In this test, the IB material of example 1 showed a pad wear that was lower than the low steel friction material, not only at temperatures above 200°C - 300°C where organic materials will start to degrade, but also at lower temperatures, e.g. 100°C. These results show that the IB material of the invention is able to reduce pad wear across the whole temperature range that is normally tested for homologation. The exceptional thermal stability of the IB material of the invention was also shown by a so- called AMS test, where several braking applications are repeated one after the other in order to let the temperature rise. In this test, the stopping distance and the pressure of the hydraulic system are measured. The results obtained with the IB material of example 1 showed that there was a very small loss of efficiency throughout the test, and that such very small efficiency loss always resulted in stable stopping distance and a hydraulic pressure that reached 180 bar only during the last few brakes of the last test section. While exemplary embodiments have been disclosed in the foregoing detailed description and examples, it should be noted that a vast number of variations exist. It should also be noted that the exemplary embodiments are only illustrative examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing detailed description and examples will provide those skilled in the art with a convenient guide for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

Claims

CLAIMS 1. A friction material for a braking member, the friction material comprising: a fibre component and / or a powder component, and a binder; wherein the binder comprises a solid inorganic binder which is obtainable by reacting solid inorganic binder reagents consisting of: a) at least one of the groups consisting of K-silicate solution, Na-silicate solution, Li-silicate solution, and any combination thereof, as liquid phase; and b) at least one of the groups consisting of aluminium phosphates, boron phosphates, magnesium phosphates, calcium phosphates, zinc phosphates, and any combination thereof, as solid phase.

2. The friction material according to claim 1, wherein the binder comprises one or more further binders selected from organic binder(s) and partially inorganic binder(s).

3. The friction material according to claim 1 or 2, wherein said fibre component comprises at least a fibre selected from the group consisting of steel fibres, stainless steel fibres, Zn fibres, mineral fibres of rockwool types, ceramic fibres, basalt fibres, glass fibres, polyaramid fibres, PAN-fibres, cellulose fibres and any combination thereof.

4. The friction material according to any one of claims 1 to 3, further comprising one or more additives selected from the group consisting of friction modifiers, lubricants, abrasives, fillers, and any combination thereof.

5. The friction material according to claim 4, wherein the lubricants are selected from non-carbon lubricants, preferably iron sulphides (e.g. FeS, FeS2), tin sulphides (SnS, SnS2), titanium sulphides (TiS, TiS2), molybdenum disulphide (MoS2), zinc sulphide (ZnS), manganese sulphide (MnS), bismuth sulphide, tungsten sulphide (WS), and any combination thereof.

6. The friction material according to claim 4, wherein the lubricants are selected from carbon-based lubricants, preferably natural graphite, synthetic graphite, coke, carbon blackand any combination thereof.

7. The friction material according to any one of claims 1 to 6, wherein the abrasives are selected from the group consisting of SiC, Al2O3 fused, Al2O3 calcined, Chromite, Fe2O3, Fe3O4, SiO2, Cr2O3, ZrSiO4, ZrO2, MgO, K-Titanate, Li-Titanate, Mg-Titanate, and any combination thereof.

8. A friction material according to any one of claims 1 to 7, wherein the fillers are selected from the group consisting of CaCO3, BaSO4, Ca(OH)2, SiO2, calcium silicate, magnesium silicate, aluminium silicate, K-titanates, Li-titanates, mica, vermiculite, wollastonite, talc, clay, any combination thereof.

9. A friction material according to any one of the preceding claims, further comprising zinc and / or tin powder.

10. A method of manufacturing a friction material for a braking member, constituted by a brake pad or a brake shoe, the braking member having a layer or block or lining of friction material, the method comprising the steps of: - providing a fibre component and / or a powder component; - providing solid inorganic binder reagents consisting of: a) at least one of the groups consisting of K-silicate solution, Na-silicate solution, Li-silicate solution, and any combination thereof, as liquid phase; and b) at least one of the groups consisting of aluminium phosphates, boron phosphates, magnesium phosphates, calcium phosphates, zinc phosphates, and any combination thereof, as solid phase; - mixing said fibre component and / or powder component with said reagents a) and b) until a pre-mixture is obtained; - optionally blending the pre-mixture with one or more further binders selected from the group consisting of organic binder(s) and partially inorganic binder(s) and / or with one or more additives selected from the group comprising lubricants, abrasives, fillers and any combination thereof, in order to obtain a raw mixture of friction material; - optionally, storing and shipping the raw mixture;- moulding the raw mixture under given pressure and temperature in order to obtain a layer or block or lining of friction material; and - drying the raw mixture.

11. A, constituted by a brake pad or a brake shoe, the braking member having a layer or block or lining of friction material, the method comprising the steps of: - providing a fibre component and / or a powder component; - providing one or more binders selected from the group consisting of organic binder(s) and partially inorganic binder(s), and / or one or more additives selected from the group comprising lubricants, abrasives, fillers and any combination thereof; - mixing said fibre component and / or powder component with said one or more binders and / or one or more additives, until a pre-mixture is obtained; - blending the pre-mixture with solid inorganic binder reagents consisting of: a) at least one of the groups consisting of K-silicate solution, Na-silicate solution, Li-silicate solution, and any combination thereof, as liquid phase; and b) at least one of the groups consisting of aluminium phosphates, boron phosphates, magnesium phosphates, calcium phosphates, zinc phosphates, and any combination thereof, as solid phase, in order to obtain a raw mixture of friction material; - optionally, storing and shipping the raw mixture; - moulding the raw mixture under given pressure and temperature in order to obtain a layer or block or lining of friction material; and - drying the raw mixture.

12. A manufacturing method according to claim 10 or 11, wherein the moulding step includes pressing the raw mixture at a pressure ranging from 100 to 1200 kPa / cm2.

13. A manufacturing method according to claim 12, wherein the pressing step includes applying a temperature ranging from ambient temperature to 200°C for a time ranging from 5 sec to 6 min.

14. A manufacturing method according to any one of claims 10 to 13, wherein the dryingstep is carried out in an oven or through IR radiations or through post-compaction.

15. A braking member constituted by a brake pad or a brake shoe, including a layer or block or lining of friction material as defined in any one of claims 1 to 9.

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