WO2019025995A1

WO2019025995A1 - Novel friction modulators and their use in braking devices

Info

Publication number: WO2019025995A1
Application number: PCT/IB2018/055775
Authority: WO
Inventors: Roberto Cipriano Antonio DANTE; Marco Andreoni; Andrea Sliepcevich; Edoardo COTILLI; Mario COTILLI
Original assignee: Fch S.R.L.
Priority date: 2017-08-01
Filing date: 2018-08-01
Publication date: 2019-02-07

Abstract

Object of the present invention are novel friction modulators based on zirconates and their use in formulations suitable for the preparation of braking devices, such as brake pads. Object of the invention are also the formulations and the braking devices containing the novel friction modulators.

Description

"NOVEL FRICTION MODULATORS AND THEIR USE IN BRAKING

DEVICES"

***

Summary of the invention

Technical Background

Friction modulator materials are used in several ways in breaking devices to control the braking of vehicles and machines such as cars, motorcycles, trains and airplanes. Asbestos was widely used in the past but this material has been banned because it is carcinogenic due to the presence of breathable toxic fibers, and has been replaced in the ceramic formulas and in the so called "non asbestos organics" (NAO) with friction modulators mainly containing titanates.

By way of example, W099/53215 discloses alkali or alkaline earth metal titanates, such as potassium titanate, sodium titanate and calcium titanate, which can be used as friction stabilizers in braking devices.

However titanates as well have some disadvantages, such as for example the presence of breathable fibers, where breathable fibers mean those defined as any particle having length greater than 5 microns and diameter smaller than 3 microns with a length/diameter ratio greater than 3.

For this reason, due to the suspected dangerousness of these fibers, attention is now focusing on novel friction modulator materials free or substantially free from breathable fibers.

Further problems of titanates and known friction modulators, when used in a formulation for brake pads, are a non-uniform wear of the disk brake and the related necessity of forming a uniform tribological film on said disk in order to obtain a stable friction coefficient. Also the behavior of the formulation with titanates of the brake pad following the heating of the same, as a consequence of braking, is a possible source of problems, such as for example a sudden reduction of the friction coefficient upon increasing the temperature of the formulation in the braking device. With the aim of removing the breathable fibers from the friction modulators, the production processes of titanates have been modified, obtaining friction modulator compounds based on titanates free from said fibers.

US5901818 discloses brake rotors coated on their surface with a ceramic material comprising, i.a. magnesium zirconate, which have a reduced weight and provide a good thermal protection.

EP0527434 discloses braking devices having their surface coated with ceramic materials which act as a thermal barrier, said ceramic material may include magnesium zirconate.

For example, EP 1070751 and other Patents due to Otsuka Company describe mixed titanates of alkali and/or alkaline earth metals, such as potassium and zinc titanates, potassium and magnesium titanates and potassium and lithium titanates, which have enhanced and stable properties in a wide temperature range and which would not contain breathable fibers. More generally, all the current research in this field is directed towards the development of titanate-based compounds. However, such research and the processes described in the known art, which are directed towards the achievement of titanates free from breathable fibrous particles, require articulated and complex syntheses.

There is therefore the need of providing friction modulator compounds, which can solve the drawbacks of known friction modulators being free from fibers, providing high braking power in brake pads, effectively stabilizing the friction coefficient and not requiring complex processes for their production.

Objects of the invention

It is an object of the present invention to provide compounds which can be used as friction modulators in braking devices. It is another object of the invention to provide novel formulations for braking devices, in particular for brake pads, comprising said compounds. It is a further object of the invention to provide novel braking devices, in particular brake pads, comprising said compounds.

Such objects have been achieved by means of the present invention whose object is a friction modulator based on zirconates according to claim 1, a formulation for braking devices according to claim 14, the use of zirconates in braking formulations or devices according to claim 13. Moreover, the invention refers to a preparation process for formulations and devices according to claim 20.

The possibility of using zirconates in place of titanates as friction modulators in braking devices has not been described to date. The Applicant surprisingly discovered that the zirconates described herein are excellent friction modulators and that moreover they can be synthesized with simple processes providing products free from breathable powders.

EP 1926687 describes a solid state process, by means of calcination, for the preparation of lithium zirconate, which is used for gas purification, in particular in the removal of carbon dioxide.

Veliz-Enriquez Mayra Y. et al., (J Solid State Chem., 2007,1802485-2492) describe the preparation of lithium and potassium zirconates by wet route and their ability of absorbing carbon dioxide.

Description of the invention

Object of the invention, according to one of the aspects thereof, is a friction modulator for braking devices comprising metal zirconates, in particular alkali metal zirconates and/or alkali and alkaline earth metal zirconates as well as the braking formulations and the braking devices containing said friction modulator.

With the expression "friction modulator" it is meant herein any substance able to modify the friction coefficient, preferably able to stabilize the friction coefficient of a formulation for braking devices under the different conditions of use of such devices. Preferred zirconates for the present invention are selected from alkali metal zirconates, preferably comprising lithium and zirconates comprising lithium and potassium.

Object of the invention, according to another of the aspects thereof, is also the use of zirconates, preferably of alkali metal zirconates and/or alkali and alkaline earth metal zirconates, more preferably selected from alkali metal zirconates comprising lithium and zirconates comprising lithium and potassium, as friction modulators, particularly as friction modulators in formulations for braking devices.

Object of the invention, according to another of the aspects thereof, is also the use of alkali metal zirconates and/or alkali and alkaline earth metal zirconates, preferably selected from zirconates comprising lithium and zirconates comprising lithium and potassium, as friction stabilizers in braking devices.

According to an embodiment, preferred zirconates according to the invention have the following formula (I)

wherein

A and M represent an alkali metal;

x is a number, integer or decimal, ranging from 0 to 2 (0 < x <2 );

and hydrates and solvates thereof.

The formula (I) depicted above represents the nominal composition of the zirconates of the present invention, to which different crystal phases may correspond. The use of crystal phases corresponding to the nominal composition of formula (I) as friction modulators, as well as the friction modulators comprising said crystal phases are a further aspect of the present invention.

According to a preferred embodiment, A represents lithium and M represents an alkali metal, advantageously potassium.

According to a preferred embodiment, A represents lithium and x is zero.

According to a preferred embodiment, A represents lithium, M represents potassium and x is a number, integer or decimal, ranging from 0 to 1 , preferably from 0 to 0.5. According to the present invention, "braking device" is meant to indicate elements usable in braking apparatus and braking systems, where the braking element is placed in contact with a mobile element, for example a disk or a rotatable drum of a vehicle, to develop a friction in order to slow down and optionally stop the movement of said vehicle. Examples of braking devices are brake pads, but also drum brake blocks and, more generally, devices for braking. As known, such braking devices or elements are made of materials or compositions which wear out as a result of the friction developed during their use.

Zirconates of formula (I) may be prepared by means of techniques known to the person skilled in the art, for example as described in the publication by Veliz- Enriquez Mayra Y. et al., referenced above. Alternatively, zirconates of formula (I) can be prepared through a solid state synthesis by means of ceramic method, which is a known technique per se as well, i.e. by thermal route, without adding fluxes and mineralizers, e.g. starting from zirconium oxide and A and M carbonates, A and M being as defined above.

Briefly, in the ceramic method, the starting components are dry or wet mixed, and thereafter they are calcined at temperatures that can vary from 900 to 1200°C with cycles from 8 to 12 h or more, depending on the furnace used. Successively, the calcined products are ground with water in ball millers until the desired grain size, dried at 200°C and powdered.

According to an embodiment, in the process for the preparation of zirconates of formula (I), an excess of zirconium oxide with respect to the other reagents is used and the unreacted zirconium oxide remains in the mixture together with the zirconates of formula (I). The molar excess of zirconium oxide can vary within a wide range for example, but not limited to, from 5 to 50%. Other excesses can however be envisaged.

Object of the invention, according to another of the aspects thereof, is a composition comprising at least one zirconate of formula (I) and zirconium oxide, as well as the use of said composition as friction modulator and a friction modulator comprising said composition. The process for the preparation of said composition, characterized in that the components are reacted at the solid state by means of the ceramic method in the presence of a zirconium oxide excess, is a further object of the present invention. Further objects of the invention are a formulation according to claim 14 and braking devices according to claim 18. Preferred embodiments are disclosed in the dependent claims.

As it will be shown in the Experimental Section, such composition showed interesting performance as friction modulator.

Generally, the zirconates according to the invention have a broadened particle diameter distribution curve with sizes ranging from less than 1 micron to more than 100 microns. In possible embodiments, the volume mean distribution Dv(50) is between 2 and 100, preferably between 2 and 50, more preferably between 5 and 20. "Formulations for braking devices" (hereinafter also "formulations") are intended to mean herein the mixtures comprising at least one metal zirconate, preferably alkali metal zirconate, optionally with zirconium oxide, and one or more of the following components: abrasives; binders such as resins, preferably phenolic, or concrete binders; solid lubricants; non-breathable fibers, such as cellulose, aramid, metal fibers and the like.

It is clear that the above-mentioned components are compatible with the use in braking devices.

According to the present invention, metal zirconates are closely mixed with the other components in the formulation of the invention, contrary to what is disclosed in EP0527434, wherein magnesium zirconate (an alkaline earth metal) was included in ceramic materials used for coating the surface of braking device to obtain a thermal barrier effect.

According to a preferred embodiment the zirconates, as defined herein, are used as friction modulators in the formulations of braking devices or elements in combination with other friction modulators and/or abrasives, for example selected from titanates; powders of vulcanized or non-vulcanized synthetic rubber; friction powder (cashew resin powder); resin powders and rubber powders; coke powders, graphite powder, molybdenum disulfide, barium sulfate, calcium carbonate, clay, mica, talc, diatomite, antigorite, sepiolite, montmorillonite, zeolite; metal powders such as copper, aluminum, zinc and iron; oxide powders such as alumina, silica, chromium oxide, titanium oxide, iron oxide and metal sulfides.

Preferred formulations according to the invention comprise the zirconates and the compositions as defined according to the present invention, at least one resin, preferably a phenolic resin, and advantageously one or more of the following components:

one or more fibers, preferably a mixture of fibers;

one or more rubbers;

one or more lubricants,

one or more fillers

- one or more metal powders.

However, other components can be added in the formulations according to the invention.

According to a preferred embodiment, the zirconates as defined according to the present invention are in the formulations at a rate of 3 - 50 wt%, preferably at a rate of 5-30 wt%, more preferably at a rate of 5 - 20 wt% relative to the total weight of the formulation, advantageously about 10 wt% relative to the total weight of the formulation.

The zirconates and the compositions as defined herein, particularly the zirconates of the preferred embodiments, shown excellent friction stabilizing properties when included into formulations for braking devices. Comparison assays with respect to the titanates currently used as friction modulators and stabilizers of the friction coefficient demonstrated that the corresponding zirconates have enhanced properties in all the tests performed.

It is therefore clear that the invention solved the posed technical problem, i.e. to provide novel compounds or compound mixtures which compensate for the drawbacks of known friction modulators, in particular which are free from fibers, which effectively stabilize the friction coefficient and do not require complex production processes, in an effective and original way.

As a matter of fact, while the research in the technical field of friction modulators is focusing on the transformation, by means of complex production processes, of titanates containing breathable fibers into titanates free from said fibers, the invention provides the solution to the posed technical problem by replacing titanates with zirconates, i.e. with compounds showed to be free from fibers. This is an important advantage, since zirconates, unlike to titanates, must not undergo further complex treatments aimed at transforming said fibrous particles into not-fibrous particles or complex syntheses to avoid the formation of breathable fibers.

In a surprising and unexpected way, in addition to solve the problem of the breathable fibers, the zirconates of the invention provided further advantages with respect to the friction modulators based on titanates, showing better performance in all the performed tests, as it will be described in detail in the following Experimental Section.

Details about the preparation of the zirconates and the results of the comparison assays are reported, by way of illustration and without any limitation, in the following experimental section, with reference to the figures enclosed by way of illustration and without any limitation.

Brief description of the Figures

- Figure 1 shows the X-ray diffraction of the compound of example 1 ;

Figure 2 shows the curve of the distribution of the particle diameters of the compound of example 1 ;

Figures 3A, 3B and 4A, 4B show the morphological analysis by scanning electron microscope (SEM) of the compound of example 1, with increasing magnifications from 500X to 7500X;

Figures 5A and 5B show the thermal analysis of lithium and potassium titanate (fig. 5A) and of the compound of example 1 (fig. 5B), under air;

Figures 6A and 6B show the thermal analysis of lithium and potassium titanate (fig 6A) and of the compound of example 1 (fig. 6B), under carbon dioxide, respectively;

Figures 7A and 7B show the thermal analysis of the mixture of phenolic resin (50%) + lithium and potassium titanate (fig. 7A) and of the mixture of phenolic resin (50%) + compound of example 1 (fig. 7B), respectively;

Figure 8A shows the thermal analysis under air of the mixture of phenolic resin (50%) + compound of example 1, compared with the mixture of phenolic resin (50%) + potassium and lithium titanate; Figure 8B shows the thermal behavior of the two mixtures and the resin alone, and the catalytic effect of titanate and zirconate on the decomposition of the resin;

Figures 9A, 9B show the fade and recovery test for formulations with titanates and zirconates, respectively;

Figures 10A and 10B show the wear of the materials expressed as thickness ' (mm) for formulations with titanates and zirconates, respectively, under the TL1 10 VW test at 100 km/h;

Figures 1 1 A and 1 IB show the wear of the materials expressed as thickness (mm) for formulations with titanates and zirconates, respectively, under the TL1 10 VW test at 50 km/h; Experimental section

Example 1

Preparation of Lithium and Potassium zirconates

1725 g of Zr0₂ (CC 10 grade Saint Gobain)

575 g of Li₂C0₃

200 g of ₂C0₃ (99.5%) have been weighed.

The components have been dry mixed in a mixer, loaded into a refractory crucible and calcined up to a temperature of 1 100°C with a dwell time of two hours at the maximum temperature. Subsequently, the calcined product has been ground with a laboratory jar by using Alubit balls (high-density sintered alumina) as grinding medium. The so obtained product comprises a portion of unreacted zirconium oxide.

Characterization of lithium and potassium zirconate of example 1

The lithium and potassium zirconate obtained by ceramic synthesis has been characterized by:

- X-ray diffraction analysis, in order to identify the produced crystal phase(s);

- Granulomere analysis, to verify the grinding degree;

- Morphological analysis by scanning electron microscope, in order to verify the morphology of the produced particles, with particular attention to the absence of fibers

X-ray diffraction analysis, which is shown in Figure 1 , confirms the presence of a crystal phase corresponding to potassium and lithium zirconate and the presence of unreacted zirconium oxide. The instrument assigns the peaks on the basis of the information available in its database. However, by comparing the obtained XRD pattern with the one reported by Veliz-Enriquez et al. (Journal of Solid State Chemistry, 180, 2007, 2485-2492), it is possible to state that the obtained product is mainly potassium and lithium zirconate with x having values close to 0.3, as can be inferred by the comparison with figure 1.b taken from the above-mentioned paper. X-ray analysis confirms the presence of several crystal phases corresponding to the nominal composition according to the above-reported formula (I).

Granulometric analysis

The measurement has been carried out with a laser granulometer Malvern Mastersizer 3000 with wet dispersion. The sample has been predispersed into water and a surfactant (tergitol) and sonicated to break up the agglomerates. The curve of the distribution of the particle diameters (Figure 2) resulted to be significantly broadened with the presence of a particularly fine fraction (smaller than one micron) up to reaching more than 100 microns. However, the volume mean distribution is lower than 10 microns (Dv(50)=6.64 microns).

Morphological analysis by scanning electron microscope (SEM)

From the investigation carried out by scanning electron microscopy it has been verified that lithium and potassium zirconate of example 1 has particularly irregular shapes, characterized by the co-presence of particles with a mainly spherical shape together with other particles with a more flattened lamellae-type shape. In particular, as visible in figures 3A-4B, at all the magnification levels, i.e. 500, 2500, 5000, 7500, the presence of fibers has not been observed.

TGA-DTA thermal analysis

The thermal behavior of this material has been investigated comparatively to lithium and potassium titanate. In particular it was investigated:

- Thermal behavior of lithium and potassium titanate under air in fig. 5A,

- Thermal behavior of lithium and potassium zirconate under air in fig. 5B,

- Thermal behavior of lithium and potassium titanate under carbon dioxide, in fig. 6A,

- Thermal behavior of lithium and potassium zirconate under carbon dioxide, in fig. 6B,

- Thermal behavior of phenolic resin/titanate mixture under air in fig. 7A,

- Thermal behavior of phenolic resin/zirconate mixture under air in fig. 7B.

The measurement has been carried out under air atmosphere, 100 cc/min with a heating ramp of 10°C/min.

In the plots corresponding to the above-mentioned Figures, the continuous curve represents the weight loss, whereas the curve with triangles represents the temperature difference between the sample and the reference (DTA curve). This curve allows the thermal fluxes (exothermic or endothermic) associated with the weight losses to be emphasized. Both lithium and potassium titanate and zirconate (Figures 5A and 5B respectively) have very stable behavior with temperature up to 800°C. At this temperature, a small weight loss, about 10%, is observed for the titanate only. However, this difference is not so relevant since it occurs at an extremely high temperature, outside the temperature interval which can be typically reached on brake pads and in analogous braking devices during braking.

Concerning the thermal behavior under carbon dioxide atmosphere, figures 6A and 6B, potassium titanate is substantially unaffected by that and the weight loss as a function of temperature is essentially the same as under air. On the contrary, lithium and potassium zirconate of example 1 shows, in fig. 6B, the tendency to increase the weight starting from about 500°C and reaching a maximum around 700-750°C. This data confirms the ability of this material to bind with carbon dioxide. This ability of binding to carbon dioxide and releasing it by heating, imparts to this material the ability to dissipate part of the friction heat through this mechanism (Figures 6A lithium and potassium titanate and 6B lithium and potassium zirconate of example

In Figures 7A and 7B it is reported the thermal behavior of two mixtures, the first constituted by 50% of phenolic resin (Sbhpp Fers FB8145) and 50% of lithium and potassium titanate (Figure 7A), the second still constituted by 50% of the same resin, together with lithium and potassium zirconate of example 1 (Figure 7B). In both the mixtures, a significant weight loss between 400 and 500°C is observed, corresponding to the decomposition and burning of the 50% of the resin. At the same time, an exothermic peak on the DTA curve is observed corresponding to the weight loss.

It should be noted the presence of a catalytic effect of the zirconates of the invention on the decomposition of the resin. As a matter of fact, as visible in Figure 8B, the zirconates according to the invention maintain the known catalytic properties of the titanates, by accelerating the decomposition of the resin, i.e. by shifting to lower temperatures the beginning of the decomposition, as it can be observed by comparing the differential temperature curve of the resin (dash-dotted) which has a maximum at about 600°C, with the nearly overlapped curves of resin + titanate (dashed line) and resin + zirconate (continuous line). These catalytic properties of the zirconates favor the formation of phenolic fragments of low -molecular weight, which are volatile or easily oxidized and dispersed; this result is considered a positive fact, since a decomposition of the resin at higher temperatures leads to the formation of carbonized products and/or of non-volatile oligomers, which are very sticky and viscous and "dirty" the brake, contributing to the increase of wear and noise. This phenomenon has been investigated in detail for the titanates by the Otsuka Company and reported in several papers (Daimon et al. Chemical Reaction between Titanate Compounds and Phenolic Resins, doi: 10.4271/201 1-01-2366, Daimon et al. Chemical Effects of Titanate Compounds on the Thermal Reactions of Phenolic Resins in Friction Materials - Part 2 doi: 10.4271/2012-01-1790).

In Figure 8A, it is observed that the zirconate slows down the decomposition of the resin and moreover, with respect to the titanate, shifts the decomposition curve of about 20°C towards higher temperatures. This result is a further contribution of the compounds and formulations of the invention to the stability of the brake pad undergoing high stresses.

Dynamometric tests of formulations comprisim the compound of example 1 and comprising lithium and potassium titanate.

The lithium and potassium zirconate of example 1 has been used in place of lithium and potassium titanate in semi-ceramic-type formulations used to produce brake pads in the "golf VW" format; the pads have been tested on a dynamometric bench on a real scale. The results have been compared with the results of identical tests performed on pads produced from identical formulations containing the same amount of lithium and potassium titanate in place of the above-mentioned zirconate, thus the exchange of the zirconate with the above-mentioned titanate being the only variable between the two formulations.

Examples (by way of illustration and not limiting) of components and the corresponding weight percentages, relative to the total weight of the formulation, of the formulations which can be used according to the invention are reported in Table 1..

Table 1 Component weight % relative to the total weight of the formulation resins 5-10%

abrasives 3-5%

zirconates 10-20%

fibers 1-3%

metals 10-15%

solid lubricants 5-15%

inert fillers 10-40%

According to an embodiment of the invention, the components reported above are preferably selected from the following:

resins: novolac (phenolic resin)

abrasives: aluminum, silicon carbide, zirconium silicate

fibers: cellulose fiber, aramid fiber, ceramic fiber

metal powders: iron, copper, tin

solid lubricants: synthetic or natural graphite

inert fillers: baryte, calcium carbonate, petroleum coke.

Experimental tests

The results of all the tests performed are summarized in Table 4 reported at the end of the present Experimental Section.

The components and the corresponding weight percentages of the formulations used for the comparison tests in the following assays are indicated in Table 2.

Table 2

Component weight % relative to the total weight of the formulation

resin

phenolic resin 9.00%

fibers

aramid fiber 0.30%

ceramic fiber (Ca, Mg silicate) 1.50%

cellulose fiber 2.50% rubber

NBR rubber 4.00%

friction modulators

and Li titanate, or K and Li 12.00%

zirconate

abrasives

calcined alumina 0.30%

magnesium oxide 1.00%

lubricants

synthetic graphite 9.00%

petroleum coke 4.40%

metals

iron powder 7.40%

fillers

friction powder (cashew resin) 2.50%

zinc sulfide 3.00%

baryte 23.10%

calcium carbonate 20.00%

total 100.00%

In order to make more evident the effect of the raw materials to be evaluated, a simplified formula has been designed with a very low content of metals (7.40% of iron powder) and sulfides (3% of synthetic zinc sulfide). Even though such formulation could be considered a low-met formulation, the absence of copper and the low content of sulfides make it unstable and sensitive to both pressure and speed. The formulation used, while simplified, is realistic and does not significantly deviate from the formulations of its category. The obtained results are therefore indicative of the different properties imparted to the formulations by the titanates and zirconates. On the other hand, the first "fade" test in such a formulation will generate very evident friction fades. In such a formulation the replacement of 12% of a titanate with a zirconate according to the invention could cause significant variations, which can be easily observed by an AK-Master test. It should be noted that such a test is generally recognized as a representative test of the performance of the whole braking system and specifically of the friction material.

The above-mentioned test is used by brakes manufacturers and car manufacturers to homologate the materials for the original equipment (OE).

AK Master dynamometric tests

The tests on the dynamometric bench are used to determine the performance of the materials under conditions similar to the real ones, with moments of inertia reproducing those of the vehicle. The experimental conditions are those reported in Table 3.

Table 3

Vehicle specifications

Vehicle description Golf-Mk7

Vehicle class Passenger

GVM 1850 kg

FAM 75% GVM 1388 kg

RAM 25%GVM 463 kg

Maximum speed 245 km/h

Tires specifications

Bigger tire

Rolling circumference 1998.05 mm

Tire size (max) '

Rolling radius (max) 318 mm

Tire revolutions at 60 km/h 500 rpm

Smaller tire ,

Rolling circumference 1998.05 mm

Tire size (max)

Rolling radius (max) 318 mm

Tire revolutions at 60 km/h 500 rpm Test conditions

Direction Forward (RH brake)

Calculated moment of inertia 70.20 kgm²

Scale factor of speed Speed to be calculated from 500 rev/km

Inertia of the selected car (PCI) 70.23 kgm²

Inertia error of the car 0.04%

Three dynamometric tests were carried out per each material. The first test, A - Master, corresponds to the test of the SAE J2522 regulation, and provides information on material performance under different conditions of use, such as pressure, speed and temperature sensitivity. This test provides the friction coefficient as a function of the considered variables. It has been found that the recovery of the friction coefficient after the fading test (a series of repeated brakings which increase the pad temperature even up to 600°C) was very different for the two formulations, as it can be observed in the right panels of figures 9 A and 9B. The formulation with the zirconate (fig. 9B) showed better recovery, thus making this material considerably interesting for new formulations. It should be noted that the fade test (i.e. the sensitivity of the friction coefficient to temperature resulting in its fading, left panel in Figures 9A and 9B) is essentially identical for the two formulations. However, the formulation with the zirconate in the recovery test reaches a friction coefficient of 0.4, whereas the one with the titanate 0.3 only. The person skilled in the art immediately understands that the difference is significant: as a matter of fact, the value of 0.4 is an excellent recovery, whereas the value of 0.3 is an insufficient recovery value, requiring more time and more braking distance with respect to the value of 0.4 shown by the zirconate.

Wear test at 100 Km/h

The selected wear test is the TL100 of VW, generally recognized as a reliable test which, by being split in temperature steps, allows to distinguish the effect of the raw materials used on the pad wear at different temperature intervals. Moreover, with this test it is also possible to measure the disk wear at the end of the test. Both the test at 100 km/h, whose results are shown in figs. 10A-10B, and the test at 50 km/h (figs. 1 1 A-l IB) have been carried out in order to have a complete picture. In the plots, in each column pair, the left column refers to the inboard pad of the disk and the right column refers to the outboard pad.

In the case of the test at 100 km/h (with steps of 1 00 brakings at 100 km/h, 0.4 g deceleration, at the initial temperatures of 50, 100, 300, 400 and 500°C), the wear results of the pad of the material containing the zirconate (fig. 10B) are clearly inferior to those of the material with the titanate (fig. 10A) starting from 300°C, thus indicating that the material is less aggressive in particular at high temperatures.

The thickness of the overall mean wear for the material with the zirconate, is about 50% lower than the material with the titanate (4.44 mm for the material with the zirconate versus 9.1 1 mm for the material with the titanate). Only the thickness values of wear and not the mass values are reported, since they are more representative of the service life for the pads or disks. The less is the wear thickness the longer will be the service life of the material, with an advantage for the consumer. It is especially very interesting the lower disk wear (not shown in the figures) in the case of the zirconate, being 0.02 mm versus 0.05 mm of the titanate. This implies 60% lower disk wear for the material with the zirconate with respect to the material with the titanate.

Wear test at 50 Km/h

The TL100 VAV test has been also carried out at 50 km/h (the test conditions are the same as those listed above, the only difference being the initial speed of 50 km/h) and, in this case, it is representative of the wears caused by low speeds typical of city driving. The results, also in this case, are favorable for the material with the zirconate (fig. 1 1 B), concerning both the material wear and the disk wear, the latter also in this case has thickness loss about 50% lower than the material with the titanate (fig. 1 1 A).

The total mean wear measured as thickness of the removed material, relating to the pad containing the zirconate, (0.84 mm) is about 20% smaller with respect to the pad with the titanate (1.08 mm), whereas the disk wear thickness (not shown) is 50% smaller (0.01 mm for the zirconate and 0.02 mm for the titanate). The weight differences of the worn disk are not proportional to those reported for the thickness, since the transfer phenomena of the material from, the pads to the disk distort these values and this is true for all the wear tests performed.

The wear thickness difference of the materials under investigation reduces, moving from the TL1 10 test at 100 km/h to that one at 50 km/h, as it changes from 50% to 20% wear reduction, still in favor of the material with the zirconate. On the other hand, the wear thickness of the disk caused by the material with the zirconate is always significantly smaller than that obtained with the titanate, being 60% smaller in the test at 100 km/h and about 50% smaller in the test at 50 km/h. The advantages of the zirconate with respect to the titanate are therefore greater when high performance of the friction material is required, as it is actually needed in many applications.

Characterization of the disks used for the dynamometric tests.

The disks used for the wear tests underwent microstructural investigations in order to identify micro-differences which originate the different behavior in tests. In particular, a measure of the surface roughness with a laser profilometer has been performed, and surface images and composition have been obtained by means of scanning electron microscopy and microanalysis. From the comparison of the roughness of the disks, which operated with the two different formulations, it can be inferred that the surface of the disk which operated with the zirconate has grooves which are homogeneously distributed on average (homogeneously distributed peaks and valleys), whereas the surface of the disk which operated with the titanate has grooves distributed less homogeneously and occasionally deeper.

Scannins electron microscopy on disks worn by lithium and potassium titanate

The surface of the disks is covered by a tribological film, i.e. by a film of material with a mean composition depending on the pad formulation; the composition of the material can be observed by an EDX spectrum. The elements found by the analysis are compatible with those of the pad formulation, in particular the presence of titanium, in addition to Ca, , S, Ba, Zn, has been observed.

Moreover, scattered "holes" in the tribological film have been found, inside which there is an accumulation of the friction material.

Scannins electron microscopy on dish worn by lithium and potassium zirconate. The disk surface is covered by a film whose mean composition can be observed by an EDX spectrum. The elements found are compatible with those of the pad formulation, in particular the presence of zirconium, in addition to Ca, K, S, Ba, Zn, has been observed. Opposite to the worn disk with the titanate, the film has significantly more homogeneous and distributed composition.

Laser profilometry showed that test 1 (titanate) produced grooves on the disk which are occasionally deeper. This piece of data is confirmed by SEM images, whose investigation allowed the presence of these grooves to be observed.

The disk used with titanium had a film of more "heterogeneous" material, i.e. distributed in clusters of material with different composition, in addition scattered "holes" have been found on the film, the naked surface of the iron and the presence of aggregates made of friction material having been observed on the bottom of the same disk. The disk employed with zirconium had a more homogeneously distributed film with respect to the titanium, less deep grooves have been observed with respect to the disk which operated with titanium and no "holes" have been observed on the surface. The laser profilometry measurements confirmed as well the presence of more constant grooves on the disk used with zirconate-based formulations.

The results of the scanning electron microscopy characterization totally agree with the higher friction stability of the material with the lithium and potassium zirconate of example 1 , which has been found in the dynamometric tests and is usually favored by a very homogeneous and stable tribological layer, as the one observed on the surface of the disk of the test with the lithium and potassium zirconate. In this last case, the transfer of the zirconium from the pad to the disk seems to impart better stability to the tribological layer.

Table 4

Summary of the results on the braking systems.

Test Formulation Formulation with titanate

with zirconate

Recovery (figs. 0.4 0.3

9A-B) Test Formulation Formulation with titanate with zirconate

Mean wear at 100 4.44 mm 9.1 1 mm

B)

Lowest disk wear 0.02 mm 0.05 mm

at 100 km/h

Mean wear at 50 0.84 mm 1.08 mm

km/h (figs. 1 1A- B)

Lowest disk wear 0.01 mm 0.02 mm

at 50 km/h

Worn disks Homogeneous grooves Non-homogeneous grooves roughness

Worn disks SEM No holes found in the Holes found in the tribological film tribological film containing friction material

Overall Higher friction stability anc more homogeneous tribological film assessment of the disks containing tr le formulation with zirconates with respect to the one with titan ates

Therefore the present invention provides a significant technical improvement in the field of braking materials and systems, by the novel and surprising properties of the zirconates recognized herein. As discussed above, the zirconates of the invention can therefore be used in mixture with other friction modulators known in the art, such as for example the titanates.

Claims

1. Friction modulator for braking devices comprising metal zirconates.

2. Friction modulator according to claim 1, wherein said zirconates are selected - from alkali metal zirconates and a mixture of alkali metal and alkaline earth metal zirconates.

3. Friction modulator according to claim 1 or 2, characterized in that said zirconates have the following formula (I)

wherein

A and M represent an alkali metal;

x is a number, integer or decimal, ranging from 0 to 2 (0 < x <2 );

and hydrates and solvates thereof.

4. Friction modulator according to claim 1 , characterized in that said zirconates are zirconate of a metal selected from magnesium, zinc, nickel, copper, iron, aluminum, tin, manganese and mixtures thereof.

5. Friction modulator according to claim 3, characterized in that A represents lithium and M represents potassium.

6. Friction modulator according to claim 3, characterized in that A represents lithium and x is zero.

7. Friction modulator according to claim 3, characterized in that A represents lithium, M represents potassium and x is an integer or decimal number, ranging from 0 to 1 , preferably from 0 to 0.5.

8. Friction modulator according to any one of claims 1 to 7, characterized in that said zirconates have volume mean distribution [Dv(50)] between 2 and 100 microns, preferably between 2 and 50 microns, more preferably between 5 and 20 microns.

9. Friction modulator according to any one of claims 1 to 8, characterized in that it further comprises zirconium oxide.

10. Friction modulator characterized in that it comprises zirconate crystal phases corresponding to the nominal composition of formula (I) as defined in any one of claims 3 to 7.

1 1. Composition characterized in that it comprises metal zirconates in combination with a binding resin and, optionally, a metal powder.

12. Composition according to claim 1 1 , characterized in that said metal zirconates are as defined in any one of claims 2 to 7, optionally in admixture with zirconium oxide.

13. Use of the zirconates as defined in claims 1 to 8, and/or of the zirconate crystal phases as defined in claim 10 and/or of the composition according to claim 1 1 or 12, as friction modulators in formulations for braking devices and/or as friction stabilizers in braking devices.

14. Formulations for braking devices comprising a friction modulator according to any one of claims 1 to 10 and at least a binding compound, preferably selected from binding resins and concrete bindings.

15. Formulations according to claim 14, wherein said zirconates are present in a range of 3 - 50 wt%, preferably in a range of 5 - 25 wt%, more preferably in a range of 5 - 20 wt% relative to the total weight of the formulation.

16. Formulations according to claim 15, further comprising one or more of the following components: friction modulators different from said zirconates; abrasives; solid lubricants and non-breathable fibers.

17. Formulations according to claim 16, wherein said friction modulators and/or abrasives are selected from titanates, powders of synthetic rubber, resin powders and rubber powders, coke powders, graphite powder, molybdenum disulfide, barium sulfate, calcium carbonate, clay, mica, talc, diatomite, antigorite, sepiolite, montmorillonite, zeolite, metal powders such as copper, aluminum, zinc and iron, oxide powders such as alumina, silica, chromium oxide, titanium oxide, iron oxide and metal sulfides, and mixture thereof.

18. Braking device comprising a formulation according to any one of claims 14 to 17.

19. Braking device according to claim 18, wherein said device is selected from pads, blocks and elements for vehicle brakes.

20. Process for the preparation of braking formulations and braking devices, comprising the step of mixing at least a friction modulator with a plurality of compounds comprising at least one metal powder and at least one binding resin, characterized in that said at least one friction modulator is one or more metal zirconates.

21. Process according to claim 20, wherein said zirconate(s) are zirconates as defined in any one of claims 2 to 9.

22. Process according to claim 20 or 21 for the preparation of the composition according to claim 12, comprising the step for preparing said zirconates by reaction at the solid state by means of the ceramic method.

23. Process according to claim 19, wherein a zirconium oxide excess is used.