CA1134087A - Composite friction element - Google Patents

Abstract

A composite friction element suitable for the manufacture of railroad brake shoes of the composition type is prepared from a mixture of a curable rubber binder having distributed therethrough a plurality of fillers and a reinforcing fiber, at least one of the fillers having an oil absorption value of at least 30 and the fiber being formed from an aramid polymer.

Description

r~o~ r 1. Field of the Invention The present invenkion relates to friction elements and more particularly, to composite friction element useful as brake shoes for railroad brakes which element is devoid of asbestos.

2 The Prior ~rt Brake shoes for railroad ~rakes of the composition type are formed of a composite friction material composed of a rubber binder resin, having distributed therethrough a variety of fillers and a reinforcing fiber such as asbestos. Examples oE composite friction elements used in the manufacture of the brake shoes are disclosed in U.S. Patents 3,835,006 and U.S. 3,9S9,194.
U.S. 3,885,006 teaches composite friction elements formed of 15-35~ by weight of a resin binder, 45-65~ by weight asbestos and 3-10~ by weight of one or more fillers which function to impart increased hardness and wear resistance to the brake shoe or function as friction modifiers. Fillers which are disclosed as imparting increased haxdness to the brake shoe include barytes such as ~aS04, alumina (A12O3), zinc and limestone (CaCO3)~ Fillers which function as friction modifiers include brass powder, iron powder, carbon black, ground cork and aldehyde condensation products of cashew nut liquid.
U.S. 3,359,194 teaches composite friction elements which are useful as brake shoes for railroad rolling stock with relatively soft steel wheels. The friction element is composed :
of 3~25% by weight of a rubber binder, 20-70~ by weight of an inorganic filler and 2-12~ by weight of a fiber. The fiber component disclosed in the patent is composed of asbestos fibers or a cellulosic fiber such as wood, sisal, jute and rayon -- 1 ~
1~ ~

~7 fibers. The rubber binder is a natural or synthetic rubber or an elastomeric material which i5 vulcanized or otherwise cured to form a hard matrix in which the remaining components are distributed. A phenolic resln at a concentration of 1-30~ is incorporated in -the composition oE the friction element as a strengtllening or stiffening agent for the rubber matrix.
Phenolic resins disclosed in the patent include oil modified two-stage powdered phenol formaldehyde resins and a liquid resin prepared from natural sources of phenol derivatives derived from aldehyde reacted cashew nut shell oil and contain-ing a curing agent such as hexamethylene tetramine. ~mong the inorganic fillers disclosed in U.S. 3,959,19~ include graphite, cast iron, iron oxide, calcium carbon~te, b~rytes and carbon hlack.
~ n forming composite friction elements, the inorganic fillers are added for various purposes. For example, the hard mineral fillers such as iron grlt are added for their friction-al properties, fillers such as lead o~ide are included to modify the frictional effect of the hard mineral fillers; lead powder acts as a lubricant and friction modifier; asbestos fibers as a friction rein~orcing agent contributing high physi-cal strength to produce uniformly high friction against ferrous mating surfaces such as railroad car wheels, and withstand high braking temperatures. The rubber resin binds and holds to-gether the mixture of materials.
Asbestos has been generally satisfactory as a rein~
forcing fiber for use in friction elements, but recent environ-mental studies have revealed that asbestos may have a detrimen-tal effect on the health of those who are exposed to its pre-sence and, therefore, it is currently desirable to seekalternative compositions in which the asbestos content of brake shoes is reduced or eliminated C118~

Heretofore, attempts to substitute other fibers for asbestos generally have failed to produce satisfactory friction elements. For example, glass and ceramic fibers fracture in the mixing procedures used to prepare -the brake shoe composi-tions with the result that they contribute poor reinforcement.
Furthermore, glass fibers are brittle and tend to break down at the braking interface during service of the brake shoe and high wear rates are thereby encountered. Moreover, the non-porous glass surfaces have a low surface area as compared with asbestos, and the glass fibers do not absorb products of decomposition of the organic components caused by heat which occurs during braking. As a result, when glass fibers are used as the reinforcing material, Eriction drops preclpitously at the temperatures generated during braking. This friction drop due to poor absorption by the reinforcing fibers is known in the brake shoes industry as "fade".
Organic fibers such as cotton, wood pulp and rayon, synthetic fibers composed of such organic polymers as poly-acrylonitrile, polyamide, polyester and the like have low sur-face area and exhibit poor heat resistance. These latter fibermaterials lose strength at temperatures in the range of 200 -300F (~3-1~9C) and break down in the same manner as the rubber binder. In fact~ a small amount of organic fiber is often added to an asbestos reinforced composite friction element to introduce a slight controlled "fade" to save the brake from destruction when the brake is used beyond its rated capacity.
It is the primary object of the present invention to provide an asbestos-free composite friction element suitable for brake shoe application and which is capable of withstanding high temperatures, has high physical strength~ provides good braking characteristics particularly applicable to railroad braking, and meets the test standards of the AAR ~American Association of RaiLroads) for brake shoes made with a blend of organic and/or inorganic materials.

SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a composite friction element suitable for the manu-facture of high friction composition type railroad brake shoes which are devoid of asbestos and will wi-thstand the braking parameters associated with thedeceleration of railroad loco-motives, the composite element being comprised of a rubberbinder having distributed therethrough a plurality of fillers at least one of which has an oil absorption value of at least 30 and a fiber formed from an aramid polymer Thus, the present invention provides a brake shoe characterized by a body of a composite friction material having a matri.~ comprised of a vulcanized rubber binder material having distributed therethrough a plurality of filler particles includ-ing hard mineral fillers, friction modifiers and a reinforcing fiber, at least one of the filler particles having an absorptive capacity sufficient to absorb any binder decomposed during braking, the absorptive filler having an oil absorption value of at least 30 and being present in the body at a concentration of about 30 to about 50 percent by weight and the reinforcing fiber being a polymer characterized by recurring units of the formula _ _ H H O o N - Arl - N - C - Ar2 _ wherein Ar1 is selected from the group consisting of p-pheny-lene, a chloro-substituted p-phenylene, and 4,4'-substituted diphenyl methane and Ar is p-phenylene and the fiber being present in said body at a concentration of about 0.5 to about 1~L39~7 10.0 percent by weight.
Brake shoes made from the asbestos-free composite frictionma-terials of the present invention meet the ~AR
standards for hi~h fric-tion composition type brake shoes.

DESCRIPT ON OF THE PRE~ERRED EMBQDIMENTS
The precise composition of the composite-friction element of the present invention may be widely varied, but in all instances the element contains a rubber binder represented at least in part by a vulcanizable rubber or a mixture thereof containing dispersed filler particles having high oil absorp-tion values and aramid polymer fibers which impart wear resistance, afford the desired level of friction coefficient and which .reinforce or strengthen the composite element as a whole.
The asbestos-free composite friction element of the present invention has the following representative compositional range in approximate percent by weight:

~ 4a -~;3'~

Component Appro~lmate Percentage Range by Weight 1. Curable rubber binder 15-30%
2. Hard Mineral Flllers 25~50%

3. Friction ~odifiers 15-30%

4. Reinforcing aramid fiber 0.5-10.0%

5. Absorptive Fillers having an Oil Absorption Value in excess of 30 20-50%
If the above-noted components were measured as a percentage range by volume, the range for each would be much narrower. Because the density of the above components vary so significan~ly, the percentage range by weight varies according-ly. For example, the percentage range by volume of a filler material such as sand or iron grit would be narrow, but because of the difference in density of these two materials, the iron grit becomes a dominant material when viewed in percentage by weight. The percentage range by weiyht of the other components are affected accordingly.
The rubber binder used in the practice of the present invention can be any of the rubber binder materials convention-ally used by the railroad brake art for the manufacture of brake shoes. Such rubber materials include unvulcanized natural and synthetic rubber or elastomeric materials that can be vulcanized or otherwise cured in situ to form a hard matrix for the remaining components of the composite friction mater~
ials of the present invention~ Examples of such rubbers are the butyl rubbers, styrene-butadiene copolymer rubbers, acrylonitrile rubbers and chlorinated butyl rubber. These rubbers are vulcani2ed with the aid of vulcanizing catalysts such as sulfur, 2-mercaptobenzothiazole, tetramethylthiuram disulfide and mixtures thereof which accelerate the rate of cure of the rubber~ The vulcanizing catalysts are included in the rubber binder composition in minor amounts, e.g., at concentrations in the range of about 1 to about 3 percent by weight based on the weight of the composite friction element.

Also included in the rubber binder composition are conventlonal rubber fillers such as carbon black, zinc oxide, lead oxide, lead powder, M~O and ZnO. These fillers are incorporated in the rubber binder at concentrations ranging from about 5 to about lS percent by wei~ht based on the weight of the composite friction element.
Thermosetting resins such as phenol-aldehyde resins may also be lncorporated in the rubber binder composition as a strengthening or stiffening agent for the rubber matrix.
The phenolic resin may be a synthetic resin prepared from conventional organic compounds such as phenol and formaldehyde.
Alternatively, the phenolic resin may be a resin prepared from natural sources of phenol derivatives such as cashew nut shell oil, which oils are reacted with aldehydes to impart thermosetting properties thereto. Typically, the phenolic resins are incorporated in the rubber binder composition at concentrations in the range of about ] to about 10 percent by weight based on the weight of the composite friction element~
Curin~ agents such as hexamethylenetet:ramine are included in the phenolic resin, in relatively small amounts, e.~., about 0.2 to about 1.0 percent by welght based on the weight o~ the composite friction element to accelerate the cure of the phenolic resin.
Hard mineral fillers incorporated in the brake shoe composition to promote friction in the brake shoes prepared from the composite friction materials of the present invention include iron which may be in the form of iron ore or iron grit, as well as sand, fused silica, and calcined kyanite, i.e.
aluminum silicate.
Friction modifiers incorporated into the composite friction material to stab:ilize the coefficient of friction of the brake shoe under a variety of operatin~ and climatic ~3~37 conditions to which the brake shoe will be exposed so as to provide wear resistance to the shoe may be either organic or inorganic materials such as graphite, and partially cured cashew-resin solids, calcined kyanlte (A12SiO5), as well as lead and lead compounds such as lead sulfide.
Reinforcing aramid polymer fibers suitable for use in the practice of the present invention as a substitute for asbestos are commercially available from E.I. Du Pont de Nemours under the trade mark "KEVL~R". Exemplary of KEVLAR fiber materials suitable for use in the practice of the present invention is KEVLAR 29, a continuous filament yarn having the following physical properties:

TABLE I
KEVLAR 29 Physlcal Properties Density 0.52 lb~in Filament Diameter 0~00047 in Denier per Filament 1.5 **Break Elonyation 1% - 4%
*Tensile Strength 400,000 psi Tenacity 22 gpd6 **Specific Tensile Stren~th 8 x 10 in *Modulus 9 x 106 in 480 gpd **Specific Modulus 2.3 x 108 in Temperature Resistance Useful properties from 420~ to 500F ~40%
decrease in tensile strength at 500F).

*Dry yarn test **Yarn property divided by density The term "aramid polymer" as used in the present specification means a synthetic polymeric resin generally designated in the art as an aromatic polycarbonamide. "Aramid polymer" is a polymer described in U.S. Patents 3,652,510, U.S. 3,699,085 and U~S. 3,673,143 and is believed to be of a composition hereinafter described. ~n these patents, the polymers disclosed therein include fiber forming polymers of high molecular weight, e.g. having an inherent viscosity of at least about 0.7, characterized by recuring units of the formula ' EI H O O
l l 11 11 N - Ar - N - C - Ar - C- _ wherein Arl is p phenylene and/or chloro-substituted p-phenyl-ene, and/or 4,4'-substituted diphenyl methane, l.e., and/or ~/ ~ and/or ~ ~ C~2 and Ar2 is p-phenylene, i.e., ~r ~

Illustrative examples of polycarbonamides coming within the definition of the above formula are poly (p-phenyl-ene terephthalamide~, chloro-substituted poly (p-phenylene terephthalamide), and copolymers thereof.
The designatlon of the position of location of the substituent groups on the aromatic nuclei of the aramid polymer refers to the location of the substituents on the aromatic diamine, diacid or other coreactants from which the aramid polymer is prepared.
Although the aramid polymer or aromatic polycarbon-amide may consist primarlly of carbonamide links ~-CONH-) and aromatic ring nuclei~ conforming to the formula above, the polymer may contain up to 20 mole percent and preferably 0 to 5 mole percent of non-conforming comonomer units which provide units in the polycarbonamide chain different from E E O

- N - Arl - N - and ~C - Ar2 ~ C - , such as aromatic carbonamide units whose chain extending ~onds are coaxial or parallel and oppositely directed, e.g.

1~

~3'~37 O H H / H
C - ~ ~ N or - N ~ N -CL
meta-phenylene units, non-aromatic and non-amide groups.
A more comprehensive disclosure of the composition of aramid polymers is found in U.S. 3,673,143 as well as the divisional patent thereof, U.S. 3,817,941, the teachings of which are herein incorporat~d by reference~
Independent analytical tests and infra-red analysis have indicated that KEVLAR 29 could be predominately (95%
weight) pvly (p-phenylene diamine terephthalamide and could be chemically described as poly (p-phenylene diamine terepht~al-amide)-co-poly (4,4'-diamino diphenyl methane texephthalamide).
It is critical to the practice of the present inven-tion tha~ the reinforciny fibers used in the composite fric-tion element of the present invention be formed from aramid polymers. Thus during braking, railroad brake shoes encounter high quantities of energy in the form of heat generated by the frictional engayement of the brake shoe with the steel wheel oE the railroad locomotive so as to raise the interface temperature of the shoe to temperatures in the order of 2000 F
(1093C). It is believed that due to the relatively high tensile strength and temperature resistance of aramid fibers, '~
e.g. 400,000 psi (28.12 x 10 g~cm-) and 420 - 500 F (215-260 C) .t respectively, the aramid fibers when incorporated in the fric-tion element of the present invention retain their functional propert~es as reinforcing materials when exposed to the high temperatures encountered in braking.
It is also critical to the practice oE the present invention that high absorptive filler materials be used in combination with the reinforcing aramid polymer fiber. Filler _ g _ i ,~

~:~L3g~ 37 materials suitable for this function are organic or inorganic fillers having a high surface area whereby the loss of absorp-tive capacity resulting from the absence of asbestos is re-placed by the hi~h absorptlve filler. The term "high absorp-tive filler" as used in the present specification means a filler material determined to have an Oil Absorption Value of at least 30.
The term "Oil Absorptive Value" as used in the present specification rneans the milliliters of linseed oil re~uired to wet a predetermined volume of the filler, i.e. 100 cubic centimeters (cc~ of the filler.
In determining the Oil Absorption Value, a 20 grams por-tion of the filler powder is placed in shallow ceramic dish and raw linseed oil is metered into the dish from a burette. The llnseed oil delivered by the burette is stirred and worked into the powder. The addition of the oil to the powder causes the powder to agglomerate into small balls which increase in size and decrease in number as more oil is metered from the burette into the dish. The addition of the oil is continued until the oil wetted powder coalesces into a single mass or ball of powder. The number of milliliters of oil which cause the coalescence of the powder into an integral, single balled mass is multiplied by 5 to obtain the oil absorption number. The oil absorption number is then multiplied by the specific gravity of the filler, and this latter product is termed the oil absorption value. Listed below in Table II
are the oil absorption values of a variety of filler materials useful in the practice of the present invention.

~.~

TABI.E II
OIL ABSORPTION VALUE OF FILLERS

Filler Oll Absorptlon Value Alumina Trihydrate, type A 92 Alumina Trihydrate, type B 102 Alumina Trihydrate, type C 78 Barite (Barytes, BaSO4) type A 49 Barite (Barytes, BaSO4) type B 63 Barite (Glassmakers coarse)40 Barite (Glassmakers fine) 72 Rottenstone (Ground Shale, Penna,~ 86 Anthracite Coal (99.9%~325 mesh) 72 ~lagnesium Oxide 83 Clay, Georgia ruhber Grade A 138 Clay, Georgla rubber Grade B 140 The examples which follow illustrate the practice of the present invention.
Examples I - V
A series of composite friction elements were prepared in which the amounts of the binder components, filler materials and aramid polymer fiber was varled. The various compositions of the composite friction materials are summarized in Table III
below.
TABLE III ;1 Binder Components Examples Percent by Wei~ht I II III IV V

GRS Syntheti~ Rubber 6.48 6.00 6.00 6.48 6~57 Sulphur 1.76 1.60 1.60 1.76 1.72 Litharge ~PbO) 4.40 4.00 4.00 4.40 --Cashew Polymer 2.96 2.72 2.72 2.96 2O99 Lead Powder 1.48 1.40 1.40 1.48 --Carbon Black 0O80 0.72 0.72 0.80 --Hexamethylene Tetranine 0.40 0.36 0.36 0.40 0.37 MgO 1.96 1.80 1.80 1.96 --ZnO ~ 5.86 TO~AL BOND20.2418.60 18.60 20.24 17.51 . r~
~:

Filler ~aterials Examples Percent by Weight I II III IV V
Graphite-fine syntheti.c 6.80 7O00 7.00 6.80 --Galena (PbS) lOo 8411~ 0011~ 00 10 ~ 84 ~~
Cashew Resi.n-Solids10.9210.00 10.00 10. 80 10~ 26 Calcined Kyanite ( 2 3 1 2) 13~ 64 14.00 14.00 13.64 13.70 Calcined Petroleum Coke 6. 04 6O40 6.40 6.04 6.18 White Iron Grit 22~ 8823~ 0023~ 00 22 ~ 8823~ 07 Barytes (BaSO~) 3~ 76 8.20 6.40 3~ 76 7.47 Fused Aluminum Oxide ~- -- Q.40 -- --Ferrocene ~~ ~ 0~12 Alumina Trihydrate -- -- -- -- 11.72 Shale-~inely ground -- -- -- -- 8.30 Fiber Aramid Polymer 4~ 881~ 80 _ 3 ~ 20_ 4~ 88 1.79 (REVLAR 29) 100.00100~00 100,00 100.00 100.00 The GRS rubber used in the examples was A 23~ sytrene-butadiene emulsion polymer. The "cashew polymer" used was a millable cashew nut shell oil liquid partially polym~rized which was cross-linked with hexamethy:lene tetramine at control-led temperatures. The "cashew resin" was one sold as NC-300 by the Minnesota Mining and Manufacturing Co., this cashew resin is an ~0% solution in toluene of a polymerized resin derived from cashew nut shell liquid having a viscosity at 25C, of 10,000 to 18,000 cps and a gel time in minutes of from 20 to 55 at 83C. The calcined petroleum coke was National Carbon~s W-8300 and the iron grit was Cleveland Metal Abrasive's G-120.
The ingredients of Examples I-V were compounded as follows:
The rubber component, which was in crumb form and the iron grit were soaked wlth toluene in a sealed container for 24 hours at about 150 F and thereafter milled in a dispersion blade mixer. All of the remainlng components except the aramid polymer fiber were added to the mixture in the container and the batch was mixed in a dispersion blade mixer and worked to a ~L3L3~D8~

paste. The aramid polymer fiber was then added and the result-ing product mixed thoroughly untll uniform. This resulting mix was then passed through a hammer mill after which it was dried in an oven maintained at 150F (65C) so as to effect the complete removal of the toluene contained in the mixture but not to advance the binder materials beyond the flow point.
The resulting mlxtures of Examples I-V were cold press formed into a performed briquette. The briquette was then molded into the shape of a brake shoe in a suitable mold for a period of one hour at 350F ~177C) and a pressure of 2500 pounds per square inch (1.76 x 105g/cm ) to cure and harden the mixture.
Brake shoes molded from the composite friction materials of Examples I-V were subjected to dynamometer and grade service (drag) tests in accordance with AAR (Association $
of American Railroads) Test Speciication M-926-72.
The dynamometer test subjects 3 randomly selected brake shoes to a sequence of light braking and heavy braking stops from speeds of 10-90 mph (16-145 kmh) in a prescribed sequence. The material lost during the stop tosts is determin-ed by weighing the shoes be~ore and after the shoe undergoes the braking sequence. In order for the shoe to be acceptablé, the average of the accumulated loss in volume of the 3 shoes must not exceed 1.2 cu. in. tl9.66 cc) per shoe.
Drag tests measure the retarding forces produced by the test shoe which must exceed prescribed minimum require-ments, e.g. in the light brake test, the requirement is that with a brake shoe load of 925 lbs. ~ 25 lbs., ~419 kg + 11 Kg) the minlmum retarding force produced by the shoes must not be less than 300 lbs., (136Kg) and in the heavy brake test (1425 lbs. ~ 25 lbs (646 Kg ~ 11 Kg) load), the retarding force must not be less than 400 lbs~ (181Kg) "

,...

The results of these tests are summarixed in Table IV
below.

TABLE IV

Composite Fric- AAR Dynamometer Test AAR Drag Test tion Material Materlal lost after Retarding Force of Example prescribed braking l,ight Heavy sequence completed Braking Braking cu. in./shoe (cc~shoe) lbs (Kq) 0~-15A I 0.63 (10.32) 310 (141) 370 (168) 05-22A II 0.46 ( 7.54) 300 (136) 390 ~177) 05-25A III 0.33 ( 5.41) 300-~(136+) 400+(181~) -24A IV 0.31 ( 5.08) 300~tl36+) 400+(181+) ~25A V 1.24 (20.32) 300~(136~) 400~(181+) It is seen from the foregolng examples that brake shoes made from an asbestos-free composition, but with the inclusion of an aramid polymer fiber and a high absorptive filler have brake test results acceptable for rai]road braking service.
For purposes o~ comparison~ the procedure of Examples I - V were repeated with the exception that a fiber product other than an aramid polymer fiber was used in the preparation of the composite friction material, The composi~
tions of the comparative composit~ materials designated by the symbol "C" are summarized in Table V below.

~3~ 7 TABLE V

COMPARATIVE FRICTION ~TERIALS

Cl C2 C3 C4 3~ O~ D~L~ PERCENT BY WEIGHT

GRS Synthetic Rubber5~58 6O12 6.08 5~62 Sulphur 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 Hexam~thylene tetramine 0.37 0.4~ 0. 38 0~ 35 MgO 1~86 2.04 1. 84 1~ 70 Filler Materials -- -- ~t Graphite 6.4 -- 6.38 5.89 Galena (PbS) 10.1 11.23 10.17 9.40 Cashew Resin Solids10.3 7.84 10.24 9.47 Iron Grit 21.5 23. 69 ~~ 19~ 83 Barytes (BaSO4) -- -- 14.30 -~
Mullite (Alumlnum Silicate)12.8 14.67 12~79 11~ 82 Petroleum Coke 5.6 6.15 5.67 5~24 Full Cured Cashew Nut Shell I,iquid ~- 6. 57 ~~ ~~
Fiber Wollastonite F-l 14.7 Mica ~- 10.06 Cast Iron Fibers (~'t x 0~0005~) ~~ ~~ 21~46 ~~
Fiberfrax (Aluminum Silicate) short staple fiber ~ 20.80 srake shoes molded from the composite friction materials of comparati~e compositions Cl - C4 were subjected to the AAR dynamometer and grade service tests in accordance with the same procedures used to evaluate the brake shoes molded rom the composite friction materials of Examples I - V. Brake shoes molded from the composite friction material Cl failed the AAR dynamometer test by not passing the 70 mph, 6000 lb.
(113 kmh, 2722 Kg~ B~SoL~ stops.
Brake shoes molded from the C2 composite friction material passed the AAR dynamometer tests but wide fluctuations were encountered .tn the drag retardation tests and the friction material was of questlonable physical strength based on the poor appearance of the shoe af-ter the completlon of the tests.
Brake shoes molded from the C3 composite friction material when tested by the AAR dynamometer test failed the drag test and were long on stops.
Brake shoes molded from the C4 composite friction material failed the AAR dynamometer test and had problems of blistering and spalling.

l~

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A brake shoe characterized by a body of a composite friction material having a matrix comprised of a vulcanized rubber binder material having distributed therethrough a plurality of filler particles including hard mineral fillers, friction modifiers and a reinforcing fiber, at least one of the filler particles having an absorptive capacity sufficient to absorb any binder decomposed during braking, the absorp-tive filler having an oil absorption value of at least 30 and being present in the body at a concentration of about 30 to about 50 percent by weight and the reinforcing fiber being a polymer characterized by recurring units of the formula wherein Ar1 is selected from the group consisting of p-phenylene, a chloro-substituted p-phenylene, and 4,4'-substituted diphenyl methane and Ar2 is p-phenylene and the fiber being present is said body at a concentration of about 0.5 to about 10.0 percent by weight.

2. The brake shoe of Claim 1 wherein the hard mineral fillers are selected from the group consisting of iron ore, iron grit, sand, fused silica and aluminum silicate.

3. The brake shoe of Claim 1 wherein the friction modifiers are selected from the group consisting of graphite, partially cured cashew resin solids, lead and lead sulfide.

4. The brake shoe of Claim 1 wherein the absorptive filler is selected from the group consisting of alumina trihydrate, BaSO4, ground shale, anthracite coal, magnesium oxide and clay.

5. The brake shoe of Claim 1 wherein the polymer fiber is poly(p-phenylene diamine terephthalamide) -co-poly (4,4'-diamino diphenyl methane terephthalamide).

6. The brake shoe of Claim 1 wherein the rubber binder is present in the composite friction body at a concentration of about 15 to about 30 percent by weight; the hard mineral fillers are present in the composite friction body at a con-centration of about 25 to about 50 percent by weight; and the friction modifiers are present in the composite friction body at a concentration of about 15 to about 30 percent by weight.

7. A brake shoe characterized by a body of a composite friction material having a matrix comprised of a vulcanizable rubber binder material being present in the body at a concen-tration of about 15 to about 30 percent by weight, the rubber binder material having distributed therethrough a plurality of filler particles including hard mineral fillers, the hard mineral fillers being present in the body at a concentra-tion of about 25 to about 50 percent by weight, and further friction modifiers, the friction being present in the body at a concentration of about 15 to about 30 percent by weight, at least one of the filler particles having an absorptive capacity sufficient to absorb any binder decomposed during braking, the absorptive filler having an oil absorptive value of at least 30 and being present in the body at a concentra-tion of about 30 to about 50 percent by weight, and a rein-forcing fiber formed from a polymer characterized by recurr-ing units of the formula where Ar1 is selected from the group consisting of p-pheny-lene, a chloro-substituted p-phenylene, and a 4,4' -substitut-ed diphenyl methane and Ar2 is p-phenylene and the fiber be-ing present in the body at a concentration of about 0.5 to about 10.0 percent by weight.

8. In a brake shoe composition including a quantity of friction media, quantity of a vulcanizable rubber bond material, a quantity of synthetic reinforcing fiber being formed from a polymer characterized by recurring units of the formula where Ar1 is selected from the group consisting of p-pheny-lene, a chloro-substituted p-phenylene, and a 4,4' -substi-tuted diphenyl methane and Ar2 is p-phenylene, a quantity of absorptive media, and a quantity of friction modifiers, the improvement therein comprising, said friction media comprising hard mineral fillers, said bond material comprising a vulcanizable rubber, synthetic resin and curing agents to react therewith, said synthetic reinforcing fiber being of a high strength chemically inert, and high temperature-resistant and having physical characteristics of a tensile strength proximating 400,000 psi (28.12 x 106g/cm2), elongation to break proximat-ing 3 to 4 percent, tensile modulus proximating $.5 x 106 psi (5.97 x 108 g/cm2) and density proximating 0.052 lb/in3 (1.44 g/cc), and thermal characteristics of decomposition at a temperature proximating 930°F (500°C) and a 40 percent de-crease in tensile strength at 500°F (260°C), said fiber being present in a range by weight of 0.5 to 10.0 percent of the composition, and said absorptive media comprising at least one filler having an oil absorption value of at least 30 to absorb any of said bond decomposed during braking, said absorptive media being present in a range by weight of 25 to 50 percent of the composition.

9. A brake shoe composition as defined by Claim 8 and further characterized by, said quantity of said synthetic reinforcing fiber being further limited to a percentage range by weight of 1.8 to 4.88 percent.

10. A brake shoe composition as defined by Claim 8 and further characterized by, said hard mineral fillers selected from a group con-sisting of iron grit and calcined kyanite, said bond material rubber being a styrene- butadiene emulsion polymer and said bond material synthetic resin being cashew polymer, and said absorptive filler selected from the group consist-ing of barium sulfate, ground shale rock, alumina trihydrate and magnesium oxide.

11. A brake shoe composition as defined by Claim 10 and further characterized by, said hard mineral fillers being in an amount by weight proximating 37 percent, said absorptive filler being in an amount by weight proximating 25 percent.

12. A brake shoe composition as defined by Claim 8 and further characterized by, said hard mineral fillers selected from the group con-sisting of iron grit, iron ore, sand, fused silica and cal-cined kyanite, said bond material rubber being a styrene-butadiene emulsion polymer and said bond material synthetic resin be-ing cashew polymer, and said absorptive fillers selected from the group consist-ing of barium sulfate, alumina trihydrate, magnesium oxide, rottenstone, anthracite coal and clay.

13. A brake shoe composition as defined by Claim 8 and further characterized by, said hard mineral fillers being in a range by weight of 25 to 50 percent, said bond being in a range by weight of 15 to 30 percent, and said friction modifiers being in a range by weight of 15 to 30 percent.

14. A brake shoe composition including a quantity of friction media, a quantity of a vulcanizable rubber bond material, a quantity of synthetic reinforcing fiber being formed from an aramid polymer, said friction media comprising hard mineral fillers, said bond material comprising a vulcanizable rubber, synthetic resin and curing agents to react therewith, said synthetic reinforcing fiber being of a high strength chemically inert, and high temperature-resistant and having physical characteristics of a tensile strength proximating 400,000 psi (28.12 x 106g/cm2), elongation to break proximat-ing 3 to 4 percent, tensile modulus proximating 8.5 x 106psi (5.97 x 108 g/cm2), and density proximating 0.052 lb/in3, (1.44 g/cc), and thermal characteristics of decomposition at a temperature proximating 930°F (500°C) and a 40 percent decrease in tensile strength at 500°F (260°C), said fiber being present in a range by weight of 0.5 to 10.0 percent of the composition, and said absorptive media comprising at least one filler having an oil absorption value of at least 30 to absorb any of said bond decomposed during braking, said absorptive media being present in a range by weight of 25 to 50 percent of the composition.