Friction material block, in particular for equipping brake pads and associated brake pad and method

Abstract

Friction block or layer (3) made o f an asbestos free friction material, the friction material including, as composing materials thereof, inorganic and / or organic and / or metallic fibers, at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive, wherein the asbestos free friction material also contains, as an additional composing material or additive, a chitosan, preferably present from 2 % to 6% by weight, so that the force to detach the friction block or layer (3) from a metallic surface to which it has been made to stitch electrochemically is equal to or lower than 15N.

Description

[0001] "FRICTION MATERIAL BLOCK, IN PARTICULAR FOR EQUIPPING BRAKE

[0002] PADS AND ASSOCIATED BRAKE PAD AND METHOD"

[0003] Cross-Reference to Related Applications

[0004] This Patent Application claims priority from Italian Patent Application No . 102023000017610 filed on August 25 , 2023 , the entire disclosure of which is incorporated herein by reference .

[0005] Technical Field

[0006] This disclosure is generally relating to a friction block made of a friction material , the block being designed for equipping brake pads , in which the friction material can have a formulation that allows to decrease or eliminate the tendency of the friction block to stitch against a surface of a friction partner cooperating therewith, a phenomenon known as " stiction" . The disclosure generally relates to a friction block of a brake pad and to the associated brake pad designed to cooperate with the surface of a vehicle disc brake to brake a vehicle . The disclosure also generally relates to an associated method .

[0007] Prior Art

[0008] It is known that , particularly under harsh operating and / or environmental conditions , for example , frequent of froad driving and / or driving on muddy tracks or routes exposed to brackish air or acid rain, this may result , in use , in the "gluing" of one or more of the brake pads of a vehicle to the relative brake disc made of cast iron or steel . The same may happen after a long stop of a vehicle during damp weather .

[0009] This is generally due to corrosive phenomena that af fect the brake disc and that , during braking, result in the corrosion products combining with the friction material of the brake pad causing accidental adhesion of the brake pad to the brake disc, adhesion that is temporarily maintained even when the vehicle brake is deactivated . This phenomenon of adhesion is known by the English technical term " stiction" , a term that derives from the contraction and merging of the terms " static" and " friction" , i . e . the term " static friction" .

[0010] Obviously, the occurrence of a stiction or gluing phenomenon on a vehicle in use involves various drawbacks , such as j olts / straining during braking and subsequently the releasing of the brake, increased energy consumption and, in extreme cases , breakage of the friction material of the brake pad, which leads to vehicle breakdown and / or the necessity for the premature replacement of the brake pads .

[0011] So , stiction phenomenon is well known in the brake pads field and, as mentioned before , may af fect vehicles always running on paved roads too , e . g . , when exposed to damp whether condition . The phenomenon may moreover occur more frequently in electric vehicles , where the braking system is solicited less frequently due to the regenerative braking .

[0012] More generally, it is known in chemistry that corrosion of steel may be mitigated using corrosion inhibitors , to be put in the water solution contacting the steel element to be protected . Unfortunately, the maj ority of the known corrosion inhibitors are toxic organic products .

[0013] Most recently, chitosan has been studied as a corrosion inhibitor . Under the generic appellation of "chitosan" are grouped a number of polysaccharide of di f ferent molecular weight composed of p-D-glucosamine and N-acetyl-p-D- glucosamine unit with a 1,4-linkage (see formula herein below) :

[0014] It is the second most abundant natural resource after cellulose and it is biocompatible, biodegradable and nontoxic. Its chemical structure presents amino and hydroxyl groups, which can lead to ionic interaction with a metal surface, hence likely blanketing the surface and reducing the corrosion rate. It is moreover known that chitosan is slightly soluble in acidic solutions.

[0015] Polymers belonging to the family of polysaccharides have been considered recently as potential natural corrosion inhibitors capable to strong reduction of the corrosion rate on steel ( Ashassi -Sorkhabi H, Kazempour A. Chitosan its derivatives and composites with superior potentials for the corrosion protection of steel alloys: A comprehensive review. Carbohydr Polym 2020 ; 237) . Their inhibitive effect relies on active sites capable of a strong interaction with metals and metal ions. Among the different polysaccharides, chitosan appears to be a suitable polymer for application as an effective corrosion inhibitor {Gupta NK, Joshi PG, Srivastava V, Quraishi MA. Chitosan: A macromolecule as green corrosion inhibitor for mild steel in sulfamic acid useful for sugar industry. Int J Biol Macromol 2018;106:704-711) .

[0016] The hydroxyl and amino groups can be protonated in acidic media enabling the interaction between chitosan with metal surfaces. This ability can provide a barrier effect against corrosion. However, the low solubility of chitosan is a limit for a rapid formation of an efficient corrosion protection ( Srivastava V, Chauhan DS, Joshi PG, Maruthapandian V, Sorour AA, Quraishi MA. PEG-Functionalized Chitosan : A Biological Macromolecule as a Novel Corrosion Inhibitor. Chemistry Select 2018 ; 3 (7 ) : 1990-1998) .

[0017] An approach to improve the water solubility of chitosan is to provide a chemical modification thereof (Rbaa M, Fardioui M, Verma C, Abousalem AS, Gala! M, Ebenso EE, et al. 8-Hydroxyquinoline based chitosan derived carbohydrate polymer as biodegradable and sustainable acid corrosion inhibitor for mild steel: Experimental and computational analyses . Int J Biol Macromol 2020;155:645-655) . However such approach may be difficult to be implemented since it requires a suitable process to be developed. Anyway, such approach also appears not to be resolutive. Moreover, the efficacy of chitosan as a corrosion inhibitor for cast iron is still to be investigated.

[0018] Summary

[0019] The purpose of this disclosure is to provide embodiments of a friction block or layer made of a friction material of any known or new suitable friction material formulation including at least an additive directed to decrease or eliminate the tendency of the friction block to stitch against a surface of a friction partner cooperating therewith . In particular, the purpose of this disclosure is to provide embodiments of such friction block as an integral part of , or integrated in, a brake pad designed to cooperate in use with a brake disc made of steel or , preferably, of cast iron .

[0020] The disclosure therefore relates to embodiments of a friction block or layer made of a friction material formulated such as to include at least an additive designated to decrease or el iminate the tendency of the friction block to stitch against a surface of a friction partner thereof , as defined in the attached claims .

[0021] Generally speaking, the friction block or layer may be either an integral part of a brake pad for vehicles and designed to cooperate in use with a brake disc made of either steel or, preferably, cast iron, or an integral part of a clutch disc for vehicles or for any other application .

[0022] The disclosure further relates to embodiments of a brake pad equipped with, or having as an integrated component thereof , such friction block or layer .

[0023] The disclosure also relates to embodiments of a method for manufacturing brake pads for vehicles having a reduced or nil tendency to stitch against the friction surface of an associated brake disc, i . e . a reduced or nil tendency to stiction, especially under damp weather .

[0024] EP3507587 , hereby incorporated by reference in its entirety, discloses a method for studying and determining the physico-chemical conditions of adhesion between a first and a second mechanical element consisting, respectively, in a vehicle braking element and an element to be braked, comprising the steps of : - coupling the first mechanical element to a first support and the second mechanical element to a second support , the first and second support being positioned opposite one another and being relatively axially movable , the second support towards the first support , in such a way that the second mechanical element is completely embedded within the second support , except for a first face thereof that is not covered by the second support and that is arranged flush with a front end o f the second support facing towards the first support and the first mechanical element coupled therewith;

[0025] - electrically connecting the second mechanical element to an insulated electric wire that protrudes in a fluid tight manner from the second support ;

[0026] - making to adhere and then pressing by means of a first predetermined force , the first face of the second mechanical element against the first mechanical element pushing the second support towards the first , such as to reali ze a stiction cell comprising the first and the second mechanical elements and the first and second support ;

[0027] - immersing said first and second mechanical elements together with said first and second supports in an electrolyte together with at least one counter-electrode and a reference electrode in such a way as to form an electrochemical cell of which the second mechanical element constitutes a working electrode ;

[0028] - electrically connecting the insulated electric wire , the counter-electrode and the reference electrode to a potential and / or current generator ;

[0029] - carrying out by the potential and / or current generator electrochemical tests , wherein progressively variable potentials are applied between the working electrode and the reference electrode and / or a progressively variable electric current passage is established between the counter-electrode and the working electrode , by the action of which a corrosive phenomenon is made to happen between the second mechanical element and the first mechanical element ;

[0030] - detecting by means of the potential / current generator or with another instrument the electric current that is passed through the electrolyte between the second mechanical element and the counter-electrode as a function of the potential applied or, conversely, detecting the potential that is established between the second mechanical element and the reference electrode as a function of the predetermined current which is set on the potential / current generator .

[0031] In this manner a " stiction" between a vehicle braking element ( a brake pad) and an element to be braked ( a brake disc ) under investigation is quickly obtained in an "arti ficial" manner under controlled conditions . Then, a force necessary to detach the braking element from the brake disc is measured and its value gives a precise estimation of the entity of the stiction phenomenon that has occurred .

[0032] Brief Description of Drawings

[0033] Preferred but not limiting embodiments will be now described in more detail with reference to a number of practical working examples of implementation thereof which are solely intended to disclose in a non-exhaustive and not limiting manner the feature which are part of the content of the present disclosure , and with reference to the figures of the attached drawings , in which :

[0034] - Figure 1 shows graphs consisting in experimental potentiodynamic polari zation curves of grey cast iron after 48h immers ion in an electrolyte consisting of 0 . IM HC1 solution and in the same electrolyte solution added with 0 . 8 , 1 . 5 and 3 g / 1 of a medium molecular weight chitosan ;

[0035] - Figure 2 shows graphs consisting in experimental potentiodynamic polari zation curves of grey cast iron after 48h immers ion in an electrolyte consisting of 0 . IM HC1 solution and in the same electrolyte solution added with 0 . 8 , 1 . 5 and 3 g / 1 of a high molecular weight chitosan ;

[0036] - Figures 3A and 3B are bar graphs showing the inhibitor efficiency for , respectively, medium and high molecular weight chitosan calculated from potentiodynamic polari zation curves in Figures 1 and 2 ;

[0037] - Figure 4 shows optical stereo-micrographs of mating surfaces of a cast iron and a friction material samples after stiction tests according to EP3507587 , performed in the same electrolyte not containing (A and B ) and containing ( C and D) chitosan;

[0038] - Figure 5 shows a comparison between FTIR spectrum o f medium molecular weight chitosan and of a grey cast iron surface after 9 days immersion in 0 . IM HC1 solution containing 3 g / 1 of the same chitosan;

[0039] - Figure 6 shows bar graphs reporting the stiction force measured according to EP3507587 after a stiction test in 0 . IM HC1 solution ( reference ) and in 0 . IM HC1 solution with the addition of 3 g / 1 chitosan ( Chit . high_3gL- l ) ;

[0040] - Figure 7 shows bar graphs representing the stiction force measured according to EP3507587 after a stiction test in 0 . IM NaCl solution for a reference friction material ( reference ) and for the same friction material loaded as an additive with di f ferent amounts ( 2 , 4 and 6 wt% ) of medium and high molecular weight chitosan ;

[0041] - Figure 8 shows optical stereo-micrographs of mating surfaces of a cast iron and a friction material samples after stiction tests according to EP3507587 , performed in 0 . IM NaCl solution for the reference friction material (A and B ) and for friction materials loaded with 2 wt% ( C and D) and 6 wt% (E and F) medium molecular weight chitosan, the friction material sample of figure 8 F being part of a brake pad according to the invention, shown schematically only for purely illustrative purposes ; and

[0042] - Figure 9 shows the results of a tribometric comparative test using a standard friction material and a friction material according to the invention .

[0043] Detailed Disclosure

[0044] In some embodiments , a friction block or layer made of an asbestos free friction material , in particular designed to be provided as an integral part of a vehicle brake pad comprises , as composing materials thereof , inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant , and at least a filler or abrasive , to which at least one stiction inhibitor additive , as it will be described in details herein after, has been added .

[0045] Materials pertaining to the above general categories are well known in the brake art and are chosen according to well-known criteria in order to obtain specific properties, like wear resistance, stable friction coefficient during braking, fade and limited use of braking fluid.

[0046] Exemplary embodiments of fiber components can be, but are not limited to: inorganic fibers, such as glass fibers, rock wool, wollastonite, sepiolite and attapulgite, and organic fibers, such as carbon fibers, aramid fibers, polyimide fibers, polyamide fibers, phenolic fibers, cellulose and acrylic fibers or PAN (polyacrylonitrile) , metallic fibers, like stainless steel and zinc. This fibrous base can be utilized in short fiber or powder form.

[0047] Numerous materials known in the art can be used as organic or inorganic fillers. Exemplary embodiments include, but are not limited to, calcium carbonate precipitate, barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride, slaked lime, talc, molybdenum trioxide, zirconium silicate, iron oxide, mica, iron sulfide, silicon dioxide, vermiculite, rubber powder (rubber powder and granules) , nitrile rubber powder (vulcanized product) , metal powders (excluding copper and its alloys) , acrylic rubber powder (vulcanized product) . These materials can be used by themselves or in combinations of two or more of them. The quantity of such fillers loads is preferably comprised from 2% to 40% by volume based on the total composition of the friction material.

[0048] Exemplary embodiments of suitable binders include, but are not limited to, phenolic resins, melamine resins and epoxy resins; various modified phenolic resins, such as epoxy-modif led phenolic resins, oil-modified phenolic resins, alkylbenzene-modif led phenolic resins and acrylonitrile butadiene rubber (NBR) . Any one of these compounds or combination of two or more of them may be employed used. The binder is included in a quantity preferably from 2% to 30% by volume based on the total composition of the friction material.

[0049] Exemplary embodiments of friction modifiers can include, but are not limited to, from 0% to 10% (or about 0% and about 10%) by volume of solid lubricants, such as tin sulfides, like SnS and SnS2, cashew nut powder, rubber powder (pulverized tread rubber powder) , a variety of nonvulcanized rubber particles, a variety of vulcanized rubber particles, an inorganic charge, such as barium sulfate, calcium carbonate, a calcium hydroxide, vermiculite and / or mica, an abrasive, such as silicon carbide, alumina, a zirconium silicate, a lubricant, such as molybdenum disulfide, a sulfide of tin as specified above, a zinc sulfide, iron and non-ferrous sulfides, metallic particles other than copper and copper alloys, and / or a combination of all of the above.

[0050] It can also be advantageous in some embodiments to include graphite and / or coke in the composition. The graphite (and / or coke) can be added in an appropriately selected quantity, which preferably can comprise between 2% and 15% by volume of the total composition of friction material. Embodiments of the friction materials to which the present disclosure applies include those known as non-asbestos- organic (NAO) friction material.

[0051] The graphite (and / or coke) can be added in an appropriately selected quantity, which can comprises between 2% and 15% (or about 2% and about 15%) by volume of the total composition of friction material.

[0052] The fibers can be utilized in short fiber or powder form. In some embodiments, the fibers can have a total fiber length of between 0.5 and 1.5 (or between about 0.5 and about 1.5) mm and a fiber diameter from 40 to 150 microns, though the particular dimensions are not limiting.

[0053] The quantity of the fillers can comprise 2% to 40% by volume based on the total composition of the friction material .

[0054] The binder can be included in a quantity from 2% to 30% by volume based on the total composition of the friction material .

[0055] Exemplary embodiments of suitable abrasives may be subdivided in mild, medium and strong abrasives and may consist in any one or more of the flowing materials, without the following list be considered exhaustive or limiting in any manner:

[0056] • Mild abrasives (Mohs 1-3) : talc, calcium hydroxide, potassium titanate, mica, kaolin

[0057] • Medium abrasives (Mohs 4-6) : barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide

[0058] • Strong abrasives (Mohs 7-9) : silicon carbide, zirconium sand (zirconium oxide) , zirconium silicate, zirconia, corundum, alumina, mullite. The ratio between the abrasives and lubricants is between 3 and 10. In some embodiments, the ratio can be equal to 6 (or about 6) .

[0059] The friction material composition according to the disclosure may comprise aramid fibers, the ratio between the binder and the aramid fibers being between 0.5% and 8% (or about 0.5% and about 8%) by volume. In some embodiments, the ratio can be equal to 5 (or about 5) .

[0060] According to embodiments of the disclosure, the asbestos free friction material also contains according to the invention, as an additional composing material or additive, a selected amount of a chitosan, preferably of a medium or high molecular weight chitosan.

[0061] Here and below, for "medium molecular weight" chitosan is intended a chitosan having a molecular weight ranging between 190,000 and 310,000 Dalton; and having a viscosity ranging from 200 and 800 cP ( Iwt . % in 1% acetic acid (25 °C, Brookfield) (lit. ) ) .

[0062] Here and below, for "high molecular weight" chitosan is intended a chitosan having a molecular weight above 310,000 Dalton; and a viscosity from 800 and 1200 cP (Iwt. % in 1% acetic acid (25 °C, Brookfield) (lit.) ) .

[0063] Most commercial chitosans have molecular weights ranging from 50 to 2,000 kDa, with an average DDA of 50-100% (commonly 80-90%) .

[0064] According to embodiments of the disclosure, therefore, an asbestos free friction material having no or reduced tendency to stiction contains, as an additional composing material or additive, a chitosan, preferably a medium or high molecular weight chitosan. The medium or high molecular weight chitosan may be present in the asbestos free friction material of the invention in a quantity preferably comprised between 0.5% and 10% by weight and, most preferably, comprised between 2% and 6% (or about 2% and about 6%) by weight, in both cases being the %w calculated on the total weight of the friction material composition. In some embodiments, the chitosan may even be present in a quantity comprised between 1.0% and 2.0% (or about 1.0% and about 2.0%) by weight, calculated on the total weight of the friction material composition.

[0065] The friction block or layer of an asbestos free friction material according to any of the aforementioned possible embodiments may possess none or low tendency to stitch in use against a surface of a friction partner thereof, typically a vehicle brake disc when the friction material block or layer is an integral part of / is included in a brake pad .

[0066] In preferred embodiments, the friction partner may be made of cast iron, preferably gray cast iron.

[0067] The asbestos free friction material block or layer can be incorporated in a vehicle brake pad designed to cooperate in use with a vehicle brake disc made of iron, steel or cast iron, the vehicle brake pad being operatively associate in use with the vehicle brake disc, though the particular material is not limiting.

[0068] When the asbestos free friction material block or layer contains as a component material thereof from 0.5 to 10% by weight and most preferably from 1% to 6% by weight of a chitosan having a molecular weight comprised between 190,000 and 310,000 Dalton or above 310,000 Dalton and having a viscosity ranging from 200 and 800 cP ( centipoise ) or above ( Iwt . % in 1 % acetic acid ( 25 ° C, Brookfield) ( lit . ) ) , the resulting friction partners consisting in the vehicle brake pad and in the vehicle brake di sc are not subj ected to stiction in use or are subj ected to a very limited stiction phenomenon that allows the two friction partners to be easily separated with a low force , such as lower than 15N .

[0069] In particular, according to embodiments of a test described in details in the present disclosure , in order to establish the minimum quantity of a chitosan to be added to any otherwise known friction material composition, the detachment force of a friction material sample from an iron, steel or cast iron surface is detected as the indicative parameter of the presence / absence of a stiction phenomenon, wherein the friction material block or layer may be electrochemically stitched against a metal surface of steel , iron or cast iron by thrusting the friction material block or layer against the metal surface and then an electrochemical corrosive phenomenon is provoked thereupon . According to this test , any o f the embodiments of the friction material block or layer according to the disclosure are to show a detachment force from the metal surface of 15 ( or about 15 ) Newton, or less , indicating a low or nil tendency to stiction .

[0070] The present disclosure also relates to the use of a chitosan in an asbestos free friction material containing as further component materials thereof inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive , the chitosan being preferably contained in the asbestos free friction material in a quantity comprised between 0 . 5% and 10% by weigh and most preferably between 2 % and 6% ( or about 2 % and about 6% ) by weight calculated on the total weight of the friction material and such that the asbestos free friction material shows a reduced or nil tendency to stiction, especially under damp weather .

[0071] The present disclosure also relates to a method for manufacturing brake pads for vehicles having a reduced or nil tendency to stitch against the friction surface of an associated brake disc, i . e . a reduced or nil tendency to stiction, especially under damp weather . The method comprising the steps of : i ) - preparing a test quantity of an asbestos free friction material , especially of the NAO kind, by mixing together respective component materials including inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive ; ii ) - adding in step i ) as a further component material a predetermined quantity of a chitosan to be mixed together with the said component materials of step i ) ; iii ) - forming a solid block or layer of said asbestos free friction material containing said chitosan by pressing and heating the component materials mixed together in a mold, to form a pad; iv) - subj ecting the pad to a stiction test as described above ; v) - repeating the steps i ) to iv) using di f ferent predetermined quantity of said chitosan; vi ) - industrially producing brake pads using asbestos free friction materials containing said chitosan having a chemical composition identical to those test friction materials produced by previous steps i ) -v) and having shown a separation force in said stiction test equal to or lower than 15 N .

[0072] The present disclosure further relates to a method for manufacturing brake pads for vehicles having a reduced or nil tendency to stitch against the friction surface of an associated brake disc, i . e . a reduced or nil tendency to stiction, especially under damp weather , the method comprising the steps of : i ) - preparing an asbestos free friction material , especially of the NAO kind, by mixing together respective component materials including inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive ; ii ) - adding in step i ) as a further component material a quantity of a chitosan to be mixed together with the said component materials of step i ) , the chitosan being chosen among those of a medium or high molecular weight and being added in the asbestos free friction material in a quantity comprised between 0 . 5% and 10% by weight and most preferably between 2 % and 6% ( or about 2 % and about 6% ) by weight calculated on the total weight of the friction material ; iii ) - forming a solid block or layer of said asbestos free friction material containing said chitosan by pressing and heating the component materials mixed together in a mold, to form a pad .

[0073] The disclosure will now be completed by the content of the following non-limiting working operative examples.

[0074] EXAMPLE 1

[0075] Different compositions of friction materials were prepared, all these friction materials being of the NAO type, and being "copper-free", and all also being asbestos free.

[0076] The general, generic composition of the friction materials prepared are given in Tables 1 and 2, wherein the indicated composing materials may be mixed together in different combinations.

[0077] In Tables 1 and 2 for the mild, medium and strong abrasives, one or more materials were chosen as indicated below :

[0078] • Mild abrasives (Mohs 1-3) : talc, calcium hydroxide, potassium titanate, mica, kaolin;

[0079] • Medium abrasives (Mohs 4-6) : barium sulfate, magnesium oxide, calcium fluoride, calcium carbonate, wollastonite, calcium silicate, iron oxide, silica, chromite, zinc oxide;

[0080] • Strong abrasives (Mohs 7-9) : silicon carbide, zirconium sand (zirconium oxide) , zirconium silicate, zirconia, corundum, alumina, mullite.

[0081] Starting from the general , operative compositions listed in Tables 1 and 2 , seven di f ferent friction material compositions are prepared in a test quantity . The molding operation is carried out by pressing, which is performed at a temperature of between 60 and 200 ° C and at a pressure from 150 to 1800 kg / cm2for a duration of between

[0082] 3 and 10 minutes .

[0083] The composition #1 is made to be used as References and correspond to a known NAO composition without copper .

[0084] EXAMPLE 2

[0085] Inhibitive ef fect of chitosan for gray cast iron

[0086] The inhibitive ef fect of chitosan on grey cast iron was investigated by electrochemical methods (potentiodynamic polari zation, cyclic voltammetry and electrochemical impedance spectroscopy) . The electrochemical tests were carried out in 0 . IM HC1 solution, that was employed to reproduce the acid conditions that are typically established at the cast iron rotor / friction material interface in an aggressive corrosive environment .

[0087] To carry out the tests , metallic samples are taken from a commercial brake disc made of gray cast iron .

[0088] Figures 1 and 2 display potentiodynamic polari zation curves of grey cast iron in 0 . IM HC1 solution and of the same grey cast iron in the same electrolyte , but with the addition of di f ferent amounts of medium and high molecular weight commercial chitosans .

[0089] A marked decrease of the current density in the cathodic and in the anodic branch of the curves is observed after the addition of medium and high molecular weight chitosan in the testing solution . This confirms that chitosan behaves as a mixed inhibitor for gray cast iron, in accordance with experimental results reported in literature for steel .

[0090] Figures 3A and 3B display the inhibitor ef ficiency calculated from the polari zation curves in Figures 1 and 2 . An inhibitor ef ficiency up to 80% can be achieved controlling the type and concentration of chitosan introduced in the testing electrolyte . This corresponds to a remarkable reduction of the corrosion rate of the cast iron in the electrolytes containing chitosan .

[0091] The formation of a protective layer on the surface of the grey cast iron sample was proven with complementary methods ( SEM-EDXS , AFM and FTIR) . Figure 5 reports an FTIR spectrum for a grey cast iron sample immersed for 9 days in 0 . IM HC1 solution with 3 g / 1 chitosan ( this very same sample was also investigated by EIS ) . The sample surface after 9 days immersion displays the characteristic peaks of chitosan indicating the formation of a protective layer that is associated to the complexing ef fect of chitosan for iron ions .

[0092] EXAMPLE 3

[0093] Ef fect of chitosan on stiction behavior

[0094] Stiction tests were performed on samples of the same reference friction material #1 disclosed in Table 1 and 2 of Example 1 , in the presence of electrolytes containing chitosan and using as a friction partner samples taken from a gray cast iron commercial brake disc . It was decided to use a 0 . IM HC1 solution with 3 g / 1 of medium / high mw chitosan for the stiction tests . The test has been carried out as described in details in EP3507587 , using the same apparatus as described therein .

[0095] Figure 6 displays the stiction force necessary to detach the friction material sample from the gray cast iron surface after stiction was induced in the stiction test . The reference friction material ( #1 ) exhibits a high stiction force ( about 50 N) in 0 . IM HC1 solution . In contrast , the very same material displays a stiction force well below 10 N for measurements performed in the electrolyte containing 3g / l chitosan . This result was well reproducible .

[0096] Figure 4 shows , as an example , the aspect of the surface of the gray cast iron and friction material samples after the stiction test performed in the electrolyte with and without chitosan . The gray cast iron sample ( Figure 4A) exhibits a large amount of friction material that is sticking on its surface for the test in 0 . IM HC1 solution without chitosan . This was trans ferred from the friction material sample , which exhibits severe damage after the test ( Figure 4B ) . This is evident at its periphery, which is more prone to stiction phenomena since corros ion processes are favored in this region through a crevice corrosion mechanism . The stiction force measured was 59 N for this test performed in 0 . IM HC1 solution without chitosan .

[0097] The same type of gray cast iron sample exhibits no or a very low amount of friction material on its surface after the stiction test performed in the electrolyte containing chitosan ( Figure 4C ) . It shows some dark corrosion products ( iron oxidation products ) , which are expected since the sample is polari zed at high potential ( 2 V) during the stiction test . This is associated to no damage of the friction material ( Figure 4D) and to a remarkably low stiction force of 1 . 1 N .

[0098] The trend of the stiction force ( Figure 6 ) and the morphology of the cast iron and friction material samples ( Figure 4 ) clearly prove the ability of chitosan to reduce the susceptibility to stiction of the reference material when the inhibitor is introduced in the testing solution .

[0099] EXAMPLE 4

[0100] Stiction behavior of friction materials loaded with chitosan

[0101] In order to investigate whether or not chitosan could be released from an otherwise known friction material added with chitosan ( loaded with chitosan) stiction tests as in the previous example were performed in a water solution 0 . IM NaCl without chitosan in solution to reduce the stiction susceptibility . As the reference material , is taken friction material #1 prepared according to Example 1 . This reference material has been compared with the other friction materials as listed in Tables 1 and 2 ,

[0102] The stiction tests have shown a similar behavior for all the tested friction material samples , regardless their di f ferent composition . All the tested materials have been compared using as the friction partner of each friction material under test a metallic sample taken from same commercial gray cast iron brake discs .

[0103] Figure 7 reports the average results obtained using di f ferent quantities and types of chitosan .

[0104] As it can be inferred from figure 7 the stiction force displayed for the reference friction material #1 and for the same and similar materials loaded with di f ferent amounts of medium and high molecular weight chitosan are dramatically di f ferent , all the friction material samples loaded with any type and amount of chitosan showing a considerably less tendency to stiction, showing detachment forces of about 15 N- (Newton) or less .

[0105] The only possible conclusion is that chitosan may be released from a friction material composition containing it and this allow the stiction force to be strongly reduced by the incorporation of chitosan in any kind of friction material .

[0106] This behavior was observed for all chitosan contents and seems to be not signi ficantly af fected by the type of chitosan employed (medium or high molecular weight ) .

[0107] As it can be seen from figure 7 , all the compositions containing a chitosan present a detachment force lower than 15 Newton, which correspond to a low stiction risk and generally even lower than 10 (namely about 7 ) , which means almost nil risk of stiction .

[0108] Figure 8 shows the surface of the rotor (brake disc ) and of the corresponding friction material samples after the stiction test performed in 0 . IM NaCl solution . The reference material #1 conf irms the high susceptibility to stiction indicated by its very high stiction force . Indeed, large portions of friction material were trans ferred on the rotor surface and the friction material sample appears severely damaged ( Figure 8A and 8B ) .

[0109] In contrast , the gray cast iron surface shows no or very small amounts of friction material after the stiction tests performed with friction materials loaded with chitosan ( Figure 8C and 8E ) . Moreover, the brighter regions visible on the gray cast iron surface underwent very few corrosion indicating that the chitosan in the pad material could ef fectively inhibit the corrosion attack of the gray cast iron . The friction materials containing chitosan exhibit no damage after the stiction test ( Figure 8D and 8 F) .

[0110] So , the experimental results as reported in figures 7 and 8 clearly highlight that the incorporation of chitosan in the ( any kind of ) friction material can signi ficantly reduce the susceptibility to stiction of the friction material itsel f .

[0111] Moreover, this ef fect is associated to no or very limited damage of the friction material after the stiction test . This ef fect was already evident for friction materials containing 2 wt% chitosan only .

[0112] It is supposed that the protection mechanism may be based on the inhibition of gray cast iron corrosion by chitosan release from the friction material , which is triggered by the acidi fication of the electrolyte at the gray cast iron / friction material interface .

[0113] EXAMPLE 5

[0114] Influence of chitosan on brake performances

[0115] Utili zing the reference material #1 from Table 1 and the friction material #3 , #4 and #7 from tables 1 and 2 , some brake pads are obtained in the same traditional manner .

[0116] With particular reference to Figure 8 F, where a brake pad 1 according to the invention is shown only very schematically and purely for illustration purposes , appropriate quantities of two friction material compositions #1 and #7 to be subj ected to comparative test are molded onto identical metal substrates 2 (brake plates or supports made of steel ) and cured in a conventional manner, in order to form identical finished brake pads 1 having the cured friction material packed to form a block or layer 3 of friction material securely attached to the support or substrate 2 .

[0117] The brake pad pressing was performed at a temperature of between 60 and 200 ° C at a pressure from 150 to 1800 kg / cm2 for a duration of between 3 and 10 minutes or else in preforming the mixing within a die and thereafter pressing at a temperature from 130 to 180 ° C at a pressure from 150 to 500 kg / cm2 ( 14 . 7-49 MPa ) for a duration of from 3 to 10 minutes .

[0118] The resulting pressed articles were typically postcured by means of heat treatment from 150 to 400 ° C for a duration of between 10 minutes to 10 hours , they were then spray painted or powder-painted, kiln-dried and possibly machined where necessary to produce the final product .

[0119] The ef fective braking area of each single pad was of 26 . 8 cm2.

[0120] The so obtained brake pads were subj ected to a comparative test using a UMT-TriboLab Bruker Tribometer . On a tribometer it is in fact possible to evaluate tribological characteristics , simulating real application test . Disc speed and the load force are fixed, torque is measured .

[0121] The tests were performed using the following operative parameters :

[0122] • 400 brake application

[0123] • Load : 150 N

[0124] • Speed : 800 rPM

[0125] • Time : 10s

[0126] The average of the results obtained are shown in figure 9 for the materials #1 and #7 . As it can be seen the variation of the friction coef ficient with the progression of the braking action (number of braking) remains similar both in case of material #1 and material #7 confirming that the addition of chitosan to an otherwise known friction material composition do not alter the braking performances .

[0127] The same test was repeated for materials #3 and #4 , obtaining comparable results . So , no signi ficant di f ferences in the braking performances were shown between the brake pads obtained with the friction material #1 and those obtained with the friction material #3 , #4 and #7 . +

[0128] Conclusions

[0129] For the first time , chitosan was employed as additive for the production of friction materials for application in automotive braking systems to control stiction phenomena . Although chitosan is a well-known corrosion inhibitor for di f ferent metals , the following unforeseeable results were obtained :

[0130] • Chitosan can be success fully loaded in friction materials without being af fected by the typical production process of braking pads ;

[0131] • Chitosan loaded in friction materials can be released during gray cast iron corrosion and signi ficantly reduces the susceptibility to stiction;

[0132] • Additions of 2wt% ( or even less ) chitosan are suf ficient to strongly reduce the stiction force ( about one order of magnitude as compared to a reference formulation without chitosan) and to limit the damage of the friction material in stiction tests ;

[0133] • Anyway, the minimum quantity of chitosan to be added to an otherwise generic friction material to obtain the required result ( reduction or elimination of the stiction phenomenon) may be easily determined by using the stiction test according to EP3507587 , on friction material samples containing progressively lower quantity of chitosan .

[0134] All the aims of the present disclosure are therefore ful filled .

[0135] Certain Terminology

[0136] Although certain braking devices , systems , and methods have been disclosed in the context of certain example embodiments, it will be understood by those skilled in the art that the scope of this disclosure extends beyond the specifically disclosed embodiments to other alternative embodiments and / or uses of the embodiments and certain modifications and equivalents thereof.

[0137] Certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation.

[0138] Conditional language, such as "can," "could," "might," or "may," unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include or do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements, and / or steps are in any way required for one or more embodiments.

[0139] Conjunctive language, such as the phrase "at least one of X, Y, and Z," unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z . Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0140] Unless stated otherwise, the terms "approximately," "about," and "substantially" as used herein represent an amount close to the stated amount that still performs a desired function or achieves a desired result. For example, in some embodiments, as the context may dictate, the terms "approximately", "about", and "substantially" may refer to an amount that is within less than or equal to 10% of the stated amount . The term "generally" as used herein represents a value , amount, or characteristic that predominantly includes or tends toward a particular value , amount , or characteristic .

Claims

CLAIMS1. Friction block or layer (3) made of an asbestos free friction material, in particular consisting of an integral part of a vehicle brake pad (1) , the asbestos free friction material including, as composing materials thereof, inorganic and / or organic and / or metallic fibers, at least a binder, at least a friction modifier or lubricant and at least a filler or abrasive, characterized in that said asbestos free friction material also contains, as an additive or an additional composing material thereof, a chitosan.

2. Friction block or layer of an asbestos free friction material according to claim 1, characterized in that said chitosan is chosen in the group consisting of medium molecular weight chitosans and high molecular weight chitosans .

3. Friction block or layer of an asbestos free friction material according to claim 1 or 2, characterized in that said chitosan has a molecular weight ranging from about 190,000 and 310,000 or more Dalton and having a viscosity ranging from 200 and 800 or more cP (Iwt. % in 1% acetic acid (25 °C, Brookfield) (lit.) ) .

4. Friction block or layer of an asbestos free friction material according to any of the preceding claims, characterized in that said chitosan is present in the asbestos free friction material in a quantity comprised between 0.5% and 10% by weight, and preferably between 1% and 6% by weight, calculated on the total weight of the friction material.

5. Friction block or layer of an asbestos free friction material according to any of the preceding claims,characteri zed in that said chitosan is present in the asbestos free friction material in a quantity equal to or higher than 2 % .6 . Friction block or layer of an asbestos free friction material according to any of the preceding claims , characteri zed in that it possesses none or low tendency to stitch against a surface of a friction partner thereof , the friction block or layer, after having been electrochemically stitched against a metal surface o f steel , iron or cast iron by thrusting the friction material block or layer against said metal surface and provoking a corrosive phenomenon thereupon, showing a detachment force from the metal surface of about 15 N (Newton) or less than 15N .7 . A vehicle brake pad ( 1 ) designed to cooperate in use with a vehicle brake disc made of iron, steel or, preferably, cast iron, the vehicle brake pad including an asbestos free friction material block or layer ( 3 ) operatively associate in use with said vehicle brake disc, characteri zed in that the asbestos free friction material block or layer contains as a component material thereof from 0 . 5% to 10% by weight, of a chitosan having a molecular weight comprised between 190 , 000 and 310 , 000 or more Dalton, with a viscosity ranging from 200 and 800 or more cP ( Iwt . % in 1 % acetic acid ( 25 ° C, Brookfield) ( lit . ) ) .8 . A vehicle brake pad as claimed in claim 7 , wherein the chitosan is present in the friction material composition as either a medium molecular weight chitosan or high molecular weight chitosan in a quantity of about from 2 % to 6% by weight , irrespective the molecular weight of the chitosan .9 . Use of a chitosan in an asbestos free friction material containing as further component materials thereof inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive , the chitosan being contained in the asbestos free friction material in a quantity comprised between 0 . 5% to 10% by weight calculated on the total weight of the friction material , and preferably in a quantity comprised between 2 % and 6% by weight , giving rise to an asbestos free friction material showing a reduced or nil tendency to stiction, especially under damp weather .10 . A method for manufacturing brake pads ( 1 ) for vehicles having a reduced or nil tendency to stitch against the friction surface of an associated brake disc, i . e . a reduced or nil tendency to stiction, especially under damp weather, the method comprising the steps of : i ) - preparing a test quantity of an asbestos free friction material , especially of the NAO kind, by mixing together respective component materials including inorganic and / or organic and / or metallic fibers , at least a binder, at least a friction modi fier or lubricant and at least a filler or abrasive ; ii ) - adding in step i ) as a further component material a predetermined quantity of a chitosan to be mixed together with the said component materials of step i ) ; iii ) - forming a solid block or layer of said asbestos free friction material containing said chitosan by pressing and heating the component materials mixed together in a mold, to form a pad; iv) - carrying out on the pad a stiction test consistingin subj ecting the friction material block or layer to an electrochemically caused stiction against a metal surface of steel , iron or cast iron by thrusting the friction material block or layer against the metal surface and then provoking an electrochemical corrosive phenomenon, for subsequently measuring a detachment force occurred to detach the friction material block or layer from the iron, steel or cast iron surface ; v) - repeating the steps i ) to iv) using di f ferent predetermined quantity of said chitosan ; the minimum suitable quantity of a chitosan to be added to any otherwise known friction material composition to avoid or reducing in use the stiction phenomenon being such that it allows a detachment force from the metal surface of 17 Newton, or less , to be obtained; vi ) - industrially producing brake pads ( 1 ) using asbestos free friction materials containing said chitosan having a chemical composition identical to those test friction materials produced by previous steps i ) -v) and having shown a detachment force from the metal surface of 15 Newton, or less .11 . A method for manufacturing brake pads ( 1 ) for vehicles having a reduced or nil tendency to stitch against the friction surface of an associated brake disc, i . e . a reduced or nil tendency to stiction, especially under damp weather, the method comprising the steps of : i ) - preparing an asbestos free friction material , especially of the NAO kind, by mixing together respective component materials including inorganic and / or organic and / or metallic fibers , at least a binder, at least afriction modifier or lubricant and at least a filler or abrasive ; ii)- adding in step i) as a further component material a quantity of a chitosan to be mixed together with the said component materials of step i) , the chitosan being chosen among those of a medium or high molecular weight; iii)- forming a solid block or layer (3) of said asbestos free friction material containing said chitosan by pressing and heating the component materials mixed together in a mold, to form a pad.

12. The method of claim 9 or 10, wherein said chitosan is added in the asbestos free friction material in a quantity comprised between 2% and 6% by weight calculated on the total weight of the friction material.

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