FR2476253A1 - Brake shoe composite friction material - comprising rubber binder, mineral fillers, friction modifiers and an aramid fibre - Google Patents

Abstract

A brake shoe composite friction material comprises a matrix of vulcanised rubber contg. filler (I) particles including hard mineral fillers (II), friction modifiers (III) and a reinforcing fibre. At least one of (I) has an absorption capacity sufficient to absorb any binder decomposed during braking, has an oil absorption value of >=30 and is present at a concn. of 30-50 wt.%. The reinforcing fibre is a polymer with recurring units -NH-Ar1-NH-CO-Ar2-CO- where Ar1 is (chloro-substd) p-phenylene or 4,4'-substd. diphenyl methane and Ar2 is p-phenylene and the fibre is present at a concn. of 0.5-10 wt.%. The brake shoes are esp. for railway brakes. They are free of asbestos, withstand high temps. have high strength, provide good braking characteristics and meet the test standards of the AAR for shoes made with a blend of (in)organic materials.

Description

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The present invention relates to friction elements and relates more particularly to a composite friction element which can be used as a brake shoe for

railway vehicles, this element being devoid of asbestos.

There are known shoes for brakes of railway wagons of the composite type made of a composite friction material which is formed of a resinous or rubbery binder in which various charges are distributed and reinforcing fibers consisting for example of asbestos. Examples of composite friction elements used in the manufacture of brake shoes have been described in US Pat. Nos. 3,885,006 and

US 3,959,194.

In US Patent 3,885,006, composite friction elements are described which are formed from 15 to 35% by weight of a resinous binder, from 45 to 65% by weight of asbestos and from 3 to 10%

by weight of one or more fillers which act to increase

lie about the hardness and wear resistance of the brake shoe

or which serve as friction modifiers.

believed to increase the hardness of the brake shoe include

baryte BaSO4, alumina (A1203), zinc and limestone (CaCO3).

Fillers that act as friction modifiers include brass powder, iron powder, carbon black, ground cork and aldehyde condensation products

from cashew juice.

In US Patent 3,959,194, composite friction elements are described which can be used as clogs of

brake for rail vehicles with relative steel wheels

very soft. The friction element is formed from a material

holding from 3 to 25% by weight of a rubbery binder, from 20 to 70%

by weight of an inorganic filler and from 2 to 12% by weight of fi-

bres. The fibrous component claimed in this patent consists of asbestos fibers or cellulosic fibers such as

wood, sisal, jute and rayon fibers. The binder

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choueux consists of natural or synthetic rubber or an elastomeric material, which is vulcanized or otherwise hardened so as to form a hard matrix in which are

distributed the other components. A phenolic resin is incorporated

porée, in a concentration of 1 to 30%, to the composition of the friction element as an agent for reinforcing or hardening the rubberized matrix. The phenolic resins cited in this U.S. patent include two-phase oil-modified powdered phenol formaldehyde resins and a liquid resin prepared from natural sources of phenol derivatives obtained by aldehyde reaction on a drawn oil

cashew shells containing a hardening agent

ment such as hexamethylene tetramine.

ganiques described in US Pat. No. 3,959,194, mention may be made of cast iron graphite, iron oxide, calcium carbonate, baryte

and carbon black.

During the manufacturing of the composite elements of

friction, inorganic fillers are added for different purposes.

For example, hard mineral fillers such as iron shot are added to obtain friction properties, fillers such as lead oxide to modify the friction effect of these hard mineral fillers, lead to act as a lubricant and friction modifier, asbestos fibers serving as a friction-enhancing agent contributing

to increase physical resistance in order to produce a friction

uniformly high value on corresponding ferrous surfaces such as the surface of vehicle wheels

railroad and obtain great resistance to temperature-

high braking res. Rubberized resin provides the bond

and the cohesion of the materials involved in the mixture.

Asbestos has generally been found to be% satisfactory

as reinforcing fiber usable in items of cash

tion but recent environmental studies have

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shown that asbestos can have a harmful effect on the health of people exposed to it and

Consequently, we have sought to develop other composi-

in which the asbestos content in the brake shoes is reduced or eliminated. In the past, attempts have been made to replace asbestos with other fibers, but it has not been possible to obtain satisfactory friction elements. For example, glass fibers and ceramic materials break during mixing operations involved in the preparation of the brake shoe manufacturing compositions and it follows that they contribute weakly to the reinforcement of the shoes. In addition, glass fibers are fragile and have

tendency to break in the brake interface during use

the brake shoe and this results in high wear speeds. Also, the non-porous vitreous materials have a low specific surface compared to that of asbestos, so that the glass fibers absorb little the products of

decomposition of organic components under the effect of cha-

their which occurs during braking. It follows that,

when glass fibers are used as a reinforcing material

the friction abruptly decreases when the temperature

produced during braking rises. This drop in rubs-

due to poor absorption by the reinforcing fibers

ment is called, in the field of brake shoes, "fainting-

organic fibers such as cotton, wood pulp, rayon, synthetic fibers composed of polymers

organic such as polyacrylonitrile, polyamides, poly-

lyesters and similar substances have a small specific surface

and have poor heat resistance. These ma-

fibrous thirds lose their resistance at temperatures between 95 and 150 C and they decompose in the same way

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way as the rubbery binder. In fact, a small amount of organic fiber is often added to an asbestos-reinforced friction element to create a slight degree of controlled fading to prevent destruction of the brake when used beyond its nominal capacity? The object of the invention is therefore to provide a composite friction element which is free of asbestos and which is suitable for being applied to brake shoes while also having the capacity to withstand high temperatures, great physical resistance, good characteristics. braking systems, in particular applicable to the braking of railway vehicles, and to meet the test conditions of the AAR (American Association of Railroads), for brake shoes made from a

mixture of organic and / or inorganic materials.

According to the present invention, there is provided a composite friction element suitable for the manufacture of brake shoes of railway vehicles of the high friction type, this element being devoid of asbestos and capable of withstanding the braking conditions. occurring during the deceleration of locomotives, said composite element consisting of a rubbery binder in which

several loads are distributed, at least one of which has a coefficient

oil absorption cient at least equal to 30 and contains

fibers formed from an aramid polymer.

Brake shoes made of composite materials

asbestos-free friction sites according to the invention

meet V'AAR standards.

SQ The precise composition of the composite friction element according to the invention can be varied within wide limits, but in all cases the element must contain a rubbery binder consisting at least in part of a

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vulcanizable rubber, or a mixture of such rubbers, containing dispersed filler particles having high oil absorption coefficients and containing fibers of a

aramib polymer which gives them good resistance to wear

rework allowing to reach the desired level of coefficient of friction, and which reinforce or stiffen the composite element

in general.

The asbestos-free friction element according to the invention has a composition that falls within the range

next of values, expressed3 in percentages by approximate weight

tifs. Component Approximate weight percent range I. Hardenable rubber binder 15-30% 2. Hard mineral fillers 25-50% 3. Friction modifiers 15-30% 4. Reinforcing aramid fiber 0.5 10.0% 5 Absorption charges with an oil absorption coefficient greater than 30 20 - 50% If indicated, for the above components

percentages by volume, the corresponding ranges of percent

tages would be much narrower. The fact that the density of the aforementioned components varies in significant proportions leads to a widening of the range of the corresponding weight percentages. For example, the range of the volume percentages of a charge such as sand or iron shot is narrow but, due to the difference in density between these two materials, iron shot appears as a predominant material when the percentage is calculated by weight, and the ranges of percentages by weight of the other components are

assigned in correspondence.

The rubberized binder used in the practice

that of the invention may consist of one of the rubber binders

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darling usually employed in the field of brake shoes of rail vehicles. These rubbery materials contain vulcanized natural or synthetic rubber or else elastomeric materials which can be vulcanized or otherwise hardened in situ in order to form a hard material for the other components of the friction composite elements according to the invention. As examples of such rubbers , there may be mentioned butyl rubbers, rubbers based on styrene-butadiene copolymer, acrylonitrile rubbers and chlorinated butyl rubbers. The vulcanization of these rubbers is carried out using vulcanization cdtalyzers such as sulfur, 2-mercaptobenzothiazole, tetramethylthiuram disulfide and mixtures of said substances, which accelerate the rate of vulcanization of the rubber. The vulcanization catalysts are incorporated into the composition of the rubberized binder in small quantities, for example with

concentrations between approximately 1 and 3% of the weight of the element

friction compound.

It also incorporates rubberized binder

fillers, conventionally used in the field of rubber ,.

such as carbon black, zinc oxide, lead oxide, lead powder, MgO and ZnOo These fillers are incorporated into the rubber binder at concentrations of between about 5 and 15% of the weight of the composite element

friction.

Thermosetting resins such as phenol aldehyde resins can also be incorporated into the rubber binder composition to serve as reinforcing or stiffening agents in the rubber matrix. The phenolic resin can be a synthetic resin prepared from conventional organic compounds such as phenol and formaldehyde. As a variant, the phenolic resin can be prepared from natural sources of phenol derivatives,

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such as cashew nut shell oil, these oils react with aldehydes to give them properties which make them thermosetting. Typically, the phenolic resins are incorporated into the rubberized binder composition in concentrations of between about 1 and 10% of the weight of the composite friction element. We add to the

phenolic resin curing agents such as hexamethy-

lenetetramine, in relatively small amounts, for example between approximately 0.2 and 1.0% of the weight of the friction friction element,

to accelerate the hardening of this phenolic resin.

Hard mineral fillers incorporated into the compound

tion intended for the manufacture of brake shoes for improvement

improve the friction of said shoes include iron, which can be in the form of iron ore or iron shot, as well as sand, fused silica and calcined disthene,

which is aluminum silicate.

Friction modifiers incorporated into the composite friction element to stabilize the coefficient of friction

of the shoe in different climatic and operating conditions-

to which the hoof is exposed in order to give it good resistance to wear and tear may consist of organic or inorganic materials such as graphite and solid substances based on partially hardened cashew resin of calcined disthene (Al2SiO) as well as lead and compounds

lead, such as lead sulfide.

Reinforcement fibers based on aramid polymer

and suitable for use in the implementation of the present invention to replace asbestos are commercially available, for example fibers sold under the trade name "KEVLAR" by the company EI Du Pont de Nemours. As an example of KEVLAR type fibers suitable for use in the practice of the invention, there may be mentioned "KEVLAR 29", which is

a continuous filament yarn having the following physical properties

your:

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TABLE I

KEVLAR 29 physical properties -

Density Filament diameter Denier per filament * 'Elongation at break * Tensile strength ** Specific tensile strength * Tension modulus ** Specific modulus Temperature resistance 1, 44g / cm3 12 microns 1.5 3-4 % 28.12.10 kg / cm2000 cm, 97.105 kg / cm2 4.15 cm usable up to between

215 and 260 C (40% reduction

tion of resistance to

traction at 2600C).

* Dry wire test

** Property of the wire divided by the density.

The term "aramid polymer" used herein

description defines a synthetic polymer resin which is

generally designated in this field by the expression "polycarbo-

aromatic namide ". An" aramid prlymer "is a polymer which has been described in US Patents 3,652,510, US 3,699,085 and US 3,673,143 and which is believed to have a composition defined in

after. In the patents in question, the polymers described contain-

are polymers forming fibers of high molecular weight, for example having an intrinsic viscosity of at least

about O, 7, and characterized by corresponding recurrent units

with the following formula:

H H 0

AtI - NI - Al 2

N - Ar -N- C - Ar2 C -

o Ar1 is chloro-substituted p-phenylene and / or p-phenylene, and / or 4,4 'substituted diphenyl methane, that is to say O and / or and / or CH2 - Cl while Ar2 denotes p-phenylene, that is to say: As examples of polycarbonamides falling within the definition of the formula indicated above, there may be mentioned the

poly (p-phenylene terephthalamide), poly (p-phenylene terephthala-

mide) chloro-substituted and copolymers of said substances.

The designation of the position of the substituent groups on the aromatic rings of the aramid polymer refers to the position of the substituents on the aromatic diamide, the diacid or other co-reactants from which the aramid polymer

is prepared.

Although the aramid polymer or polycarbonamide

aromatic can be composed essentially of carbon bonds

namides (-CONH-) and aromatic cyclic rings, in accordance with the formula indicated above, this polymer can contain up to 20 mol%, and preferably from 0 to 5 mol%, of nonconforming comonomer units which introduce in the polycarbonamide chain different units of

H H O 0

Ii, -N -Ar1 - N- and -C -Ar2

such as aromatic carbonamide units whose bonds

chain extension sounds are coaxial or parallel and directed in opposite directions, for example:

ON HNH, --- ^ 'C H

! O _ t _ - N N-h of meta-phenylene units, non-aromatic groups and

not friend of.

We can find a more complete description

of the composition of aramid polymers in patents

US 3,673,143 and US 3,817,941.

Independent analytical testing and infrared analysis have shown that KEVLAR 29 can be predominantly (95% by weight) poly (p-phenylene diamine terephthalamide) and can be chemically defined as

a poly (p-phenylene diamine terephthalamide) -co-poly (4,4'-diamino-

diphenylmethane terephthalamide).

It is essential for putting the information into practice.

vention that the reinforcing fibers used in the composite friction element according to the invention are formed of aramid polymers. Thus, during braking,

brake for rail vehicles can absorb large quantities

energy in the form of heat generated by friction contact between the shoes and the steel wheel of the locomotive,

which increases the temperature in the shoe interface up to

only at values of the order of 1093 C. It is estimated that, due to the relatively high tensile strength and good temperature resistance of the aramid fibers, which are for example 28.12 x 0l 3 kg / cm2 and from 215 to 260 C, the aramid fibers, when they; are incorporated into the friction element according to the invention, retain their functional reinforcing properties when subjected to high temperatures

occurring during braking.

It is also essential for the implementation of the present invention that the fillers with a high absorption coefficient are used in combination with the reinforcing fibers based on aramid polymer. Fillers suitable for fulfilling this function are constituted by organic or inorganic fillers having a large specific surface so that the loss of absorption capacity resulting

the absence of asbestos is compensated by the high cost load-

absorption absorption. The term "high coefficient load

absorption "used in the present description designates a

filler with an oil absorption coefficient of at least 30. The term "oil absorption coefficient" used

in this description corresponds to the number of milliliters

flax seed oil that is needed to wet a

predetermined volume, i.e. 100 cubic centimeters, of the load.

For the determination of this oil absorption coefficient, 20 grams of filler powder are placed in a shallow ceramic bowl and measured quantities of crude linseed oil are introduced into the bowl using a burette. The oil discharged from the burette is stirred and kneaded with the powder. The addition of the oil to the powder produces an agglomeration of the latter in the form of small pellets whose dimensions increase, and the number decreases, as oil is poured into the bowl using la1 burette. The addition of oil is continued until the oil-wetted powder is agglomerated in the form of a single ball. The number of milliliters of oil that causes the powder to coalesce or agglomerate in the form of a

single pellet is multiplied by five to obtain the degree of ab-

oil sorption. This degree of oil absorption is then multiplied by the specific mass of the load and this latter product is called the "oil absorption coefficient". Table II gives the values of the oil absorption coefficients of a certain number of usable fillers.

in the implementation of the present invention.

TABLE II

Oil absorption coefficients of different loads. Load Oil absorption coefficient Alumina trihydrate, type A 92 Alumina trihydrate, type B 102 Alumina trihydrate, type C 78 Barite (BaSO4) type A 49 Baryte (Ba904) type B 63 Baryte (coarse for glass making) 40 Barite (fine for glass making) 72 Rottenstone (crushed shale), Pennsylvania 86 Anthracite coal (99.9% at 0.043 mm) 72 Magnesium oxide 83 Clay, Georgia, rubber grade A 138 Clay, Georgia, rubber grade B 140 The examples given below illustrate the

practice of the present invention.

EXAMPLES I-V

A series of composite friction elements was prepared in which the quantities of binder, fillers and aramid polymer were varied. The different

compositions of the friction materials are indicated -

in Table III below.

Binder components GRS synthetic rubber Sulfur Litharge (PbO) Cashew polymer Lead powder Carbon black Hexamethylene tetramine MgO ZnO

TABLE III

Examples

Percent in I

II III

6.48 1.76 4.40 2.96 1.48 0.80 0.40 1.96 6.00 1.60 4.00 2.72 1.40 0.72 0.36 1.80 6, 00 1.60 4.00 2.72 1.40 0.72 0.36 1.80

Total components

Binder sants Fine fillers of synthetic graphite Galena (PbS) Galena (PbS) Cashew resin solids 10.92 Calcined disthene (3A1203.SiO2) 13, 64 Calcined petroleum coke 6.04 Iron granule 22.88 Baryte (BaSO4 ) 3, 76 Aluminum oxide

melted -

Ferrocene

Alumina trihydrate - -

Finely ground schist -

, 24 18.60 18.60 20.24

Examples

Percentage by weight

I II III IV

6.80 7.00 7.00 6.80

, 84 11.00 11.00 10.84

.00 10.00 10.80

14.00 6.40 23.00 8.20 14.00 6.40 23.00 6.40

- 0.40

13.64 6.04 22.88 3.76 0.12 Aramid polymer fibers

(KEVLAR 29)

4, 88, 00 IV weight 6.48 1.76 4.40 2.96 1.48 0.80 0.40 1.96 V 6.57 1.72 2.99 0.37, 86 17.51 V , 26 13.70 6.18 23.07 7.47 11.72 8.30 - 3.20, 00 4 88, 00 1.79, 00 1 80, 00

In the above examples, rubber was used.

GRS chouc consisting of a styrene butadiene emulsion polymer--

do at 23%. The "cashew polymer" used was constituted by a

crushable cashew oil, partial

polymerized and cross-linked using. hexamethylene tetramine at controlled temperatures. The "cashew resin" is that sold under the commercial designation NC-300 by the company Minnesota Mining and Manufacturing Co, this cashew resin being constituted by an 80% solution of a resin polymerized in toluene, the resin being derived of a cashew shell liquid having a viscosity of 000 to 18,000 cps at 250C and a gel time of 20 to 55 minutes at 83 C. The calcined petroleum coke corresponded to the product W-8300 from "National Carbon "and the iron shot to the product

G-120 from "Cleveland Metal Abrasive".

The ingredients of Examples I-V were mixed as follows: The rubberized component, in the form of pieces, and the iron shot were impregnated with toluene in an airtight container for 24 hours at a temperature

about 65 C and then they were ground in a blender.

dispersion paddles. All the other components, except the aramid polymer fibers, were added to the container in the container and the charge was stirred in a paddle mixer and made into a paste. The poiyre aramid tioreb was then added and the resulting product was thoroughly kneaded until it became homogeneous. The resulting mixture was then passed through a hammer mill and then dried in an oven maintained at 65 ° C to completely remove the toluene from the mixture.

mixing but not to advance the materials of the binder

beyond the creep point.

The resulting mixtures of Examples IV were cold pressed in the form of briquettes. The briquette was then molded into the shape of a brake shoe in an appropriate mold, operating for one hour at 1770C and at a pressure of 1.76 x 10. g / cm2, to ensure vulcanization and hardening-

ment of the mixture.

Brake shoes made by molding from the friction composites of Examples I to V were subjected to torque tests and friction tests

in accordance with control standard M-926-72 from AAR (Associa-

tion of American Railroad).

In the torque test, 3 randomly selected brake shoes were subjected to a series of light braking and stopping phases with strong braking from speeds between 16 and 145 km / h in a prescribed sequence. Loss of material was determined during the stop tests by weighing the shoes before and after the braking sequence. For the hooves to be acceptable, the average volume loss of the three hooves must not exceed 19.6 cm3 per

shoe.

Hopper friction tests made it possible to measure the deceleration forces exerted by the test shoe, these forces not to exceed minimum values

prescribed; for example in the light brake test the condition

tion is that, with a load of 419 kg + 11 kg exerted on the brake shoe, the minimum deceleration force produced by the shoe should not be less than 136 kgalors than in the strong brake test (load of 646 kg + 11 kg), strength

slowdown should not be less than 181 kg.

The results of these tests are summarized in the

Table IV below.

TABLE IV

Composite material Dynamometric test Drag friction drag test. AAR, loss of material AAR, strength of

friction.

after termination of deceleration, the example corresponds to the prescribed braking-braking braking sequence, slight strong cm3 / shoe kg kg

04- 15A I 10.32 151 168

- 22A II 7.54 136 177

1Q 05 - 25A III 5.41 136+ 181+

- 24A IV 5.08 136+ 181+

- 25A V 20.32 136+ 181+

The examples described above show that brake shoes formed from an asbestos-free composition, but containing aramid polymer fibers and a filler with a high oil absorption coefficient, give test results. brakes acceptable for vehicle shoes on rails. By way of comparison, the operations corresponding to Examples I to V were repeated, except that a fiber other than an aramid polymer was used in the preparation.

of the composite friction material.

The compositions of the composite materials for comparison, designated by the symbol "C", are summarized in Table V

below.

TABLE V

COMPARATIVE FRICTION MATERIAL c2 c ci C 3 C4 Binder components PERCENTAGE BY WEIGHT Synthetic rubber GRS 5.58 6.12 6.08 5.62 Sulfur 1.67 1.84 1.65 1.53 Litharge (PbO) 4, 19 4.59 4.13 3.81 Cashew polymer 2.79 3.06 2.78 2.57 Lead powder 1.40 1.53 1.39 1.28 Carbon black 0.74 0.82 0 .75 0.69 Hexamethylene tetramine 0.37 0.41 0.38 0.35 MgO 1.86 2o04 1.84 1.70 Chargers Graphite 6.4 - 6.38 5.89 Galena (PbS) 10 , 1 11.23 10.17 9.40 Cashew resin solids 10.3 7.84 10.24 9.47 Iron shot 21.5 23.69 - 19.83

Barytes (BaS04) - - 14.30 -

Mullite (aluminum silicate) 12.8 14.67 12.79 11.82 Petroleum coke 5.6 6.15 5.67 5.24 Fully vulcanized cashew shell liquid - 6.57 _ Fibers Wollastonite F-1 14.7 - .. _

Mica - 10.06 - -

Cast iron fibers (2.5 mm x 0.0125 mm) - -. 21.46

Fiberfrax (aluminum silicate -

nium) short bundled fibers - - - 20.80 Brake shoes molded from friction composites of comparative compositions C1-C4 were subjected to dynamometric and AAR braking tests using $ the same procedures as those used to control the brake shoes molded from the composite friction materials of Examples I to V. Brake shoes formed by molding the material

friction composite C1 did not pass the dynamometer test

AAR stick because they could not give valid results during the strong braking test until stopping at

* from a speed of 113 km / h and under a load of 2722 kg.

Brake shoes formed by molding the composite friction material C2 satisfied the AAR dynamometric tests but large fluctuations were recorded in the deceleration tests and on the other hand the friction material posed problems as regards the physical resistance because

poor appearance of the hooves after the end of the tests.

Brake shoes made by molding from C3 friction composite material failed test

AAR torque testing and braking test to a stop.

Brake shoes made by molding from the friction material C4 did not pass the AAR torque test and caused problems of blowing and chipping.

Claims (13)

1. Brake shoe, comprising a body of a composite friction material comprising a composite matrix bonded with a vulcanized rubber binder in which are ties several filler particles containing hard mineral fillers, friction modifiers and a reinforcing fiber, at least one of the components of the filler particles having a sufficient absorption capacity to absorb decomposed binders during braking, characterized in that that the absorption load has an oil absorption coefficient at: - less than 30 and intervenes in the body with a concentration between 30 and 50% by weight while the reinforcing fiber consists of a polymer characterized by recurring units of formula H H 0 0 -N - Ar1 - N- C- Ar2- C o Ar1 is chosen from the group comprising pphenylene, chloro-substituted p-phenylene and diphenyl methane substituted in 4.4 ′, while Ar2 is p-phenylene and that the resistant fiber in the body intervenes with a concentration between about 0.5 and 10.0% by weight. 2. Brake shoe according to claim 1, characterized in that the hard mineral fillers are chosen from the groups comprising iron ore, iron shot, sand, fused silica and aluminum silicate. 3. Brake shoe according to one of claims 1 or 2, characterized in that the friction modifiers are chosen from the group consisting of graphite, solid substances based on partially vulcanized cashew resin, lead and lead sulfide. 4. Brake shoe according to one of claims 1 to 3, characterized in that the load with a high oil absorption coefficient is chosen from the group comprising the alumina trihydrate, BaS04, ground shale, anthracitic coal teux, magnesium oxide and clay. 5. Brake shoe according to one of claims 1 to 4, characterized in that the polymer fibers are formed from poly (pphenylepe diamine terephthalamide) -co-poly (4,4'-diamino diphenyl methane terephthalamide). 6. Brake shoe according to one of claims 1 to 5, characterized in that the rubberized binder intervenes in the body of composite friction material with a concentration of between approximately 15 and 30% by weight, in that the hard mineral fillers intervene in the composite friction body with a concentration of between approximately 25 and 50% by weight and in that the friction modifiers intervene in the composite friction body with a concentration included between about 15 and 30% by weight. 7. Composition for brake shoe, containing a a certain amount of a friction agent, a certain amount. of a vulcanizable rubberized binder, a certain quantity of synthetic reinforcing fibers which are formed from a polymer of structure defined by repeating units of formula _ H H 0 ' - Ar - N- C- Ar2 4 o Ar1 is chosen from the group comprising pphenylene, p-phenylneochloro-substituted and diphenyl methane substituted in 4,4 ', while Ar2 is p-phenylene, a certain amount of absorbers and a certain amount of friction modifiers, characterized in that said friction agent contains hard mineral fillers, in that said binder contains a vulcanizable rubber, a synthetic resin and vulcanizing agents involved in reaction, in that said synthetic reinforcing fiber has a high resistance, is chemically inert and resistant to high temperatures and has the following physical characteristics: tensile strength of approximately 27.12 x 106 g / cm2, elongation at break of approximately 3 to 4% ,. tension module approaching 5.97 x 108 g / cm2 and density approaching 1.44 g / cm3, as well as a decomposition temperature sition of the order of 500 C and a decrease in the tensile strength of 40% at 260C, said fiber inter- coming into the mixture with a proportion of between 0.5 and 10.0% of the weight of the composition, and in that said absorption agent contains at least one filler having a coeffi- oil absorption cient at least equal to 30 in order to absorb decomposed binder during braking, said absorption agent intervening in the mixture with a percentage by weight included between 25 and 50% of the weight of the composition. 8. Composition for brake shoe as claimed cation 7, characterized in that said quantity of synthetic reinforcing fibers is limited to a percentage by weight between 1.8 and 4.88%. 9.Composition for brake shoe according to one of claims 7 or 8, characterized in that said charges hard minerals are chosen from a group comprising iron shot and calcined disthene, in that said binder rubberized consists of an emulsified polymer of styrene- butadiene, in that said syndhetic resin involved in the binder consists of a cashew polymer, and in that said absorption load is chosen from the group comprising barium sulphate, ground shale, aluminum trihydrate mine and magnesium oxide. 22: 10. Composition for brake shoe according to one of the claims 7 or 8, characterized in that said charges mined hard lines are selected from the group comprising iron shot, iron ore, sand, fused silica and calcined disthene in that said rubberized binder is constituted by a so-called styrene-butadiene emulsified polymer while resin synthetic involved in the binder is constituted by a cashew polymer, and in that said absorption charges are chosen from the group comprising barium sulphate, alumina trihydrate, magnesium oxide * of rotten stone ' , anthracitic charcoal or clay. 11. Composition for brake shoe according to one of the claims 7 to 10, characterized in that said mineral charges hard rales intervene with a percentage by weight included between 25 and 50%, in that said binder acts with a percentage weight by weight between 15 and 30% and in that said modifications friction scars intervene with a percentage by weight of at 30%. 12. Composition for brake shoe according to one of the claims 7 to 11, characterized in that said mineral charges hard stones are used with an approximate weight percentage 37% and that said absorption charge occurs with a weight percentage of approximately 25%. 13. Brake shoe composition containing a certain amount of friction agents including fillers hard mineral, a certain amount of a rubberized binder vul- canisable and a certain amount of synthetic reinforcing fibers ticks characterized in that said synthetic fibers are formed of an aramid polymer, in that said binder comprises a vulcanizable rubber, synthetic resin and vulcanizing agents involved in reaction, in that said synthetic reinforcing fiber has a high resistance, is chemically inert and resistant to temperature and has the following physical characteristics: tensile strength of the order of 28.12 x 10 g / cm2, elongation at break between 3 and 4%, modulus voltage of around 5.97 x 108 g / cm2 and density of around 1.44 g / cm3, and a decomposition temperature of around 500 C and a decrease in resistance tensile strength of 40% at 260 C, said fiber involved in the composition with a percentage by weight of between 0.5 and 10.0%, and in that said absorption agent contains at least one filler having a coefficient oil absorption at least equal to 30 in order to absorb decomposed binder into co urs of braking, said absorption agent intervening in the composition tion with a percentage by weight of between 25 and 50%.