GB2028350A - Glass-fibre friction material - Google Patents

Abstract

A process for manufacturing a friction material, comprising the steps of mixing together a binder, friction modifiers, and 5-30% b.w. of glass fibers to form a composition of materials, of molding said composition under pressure into a briquette, and of curing said briquette to set the binder; According to the invention, the blending of the composition is conducted until its bulk density is established between 0.1 and 0.6 g/cm<3> through the separation of the individual filaments that make up the glass fibers. For use in clutch and brake linings of vehicles. <IMAGE>

Description

SPECIFICATION Process for manufacturing a glass fiber friction material The present invention relates to a process for manufacturing a glass fiber friction material.

Organic friction material compositions currently used in clutch and brake linings of vehicles must be capable of withstanding the severe operating temperatures and dynamic pressures experienced during repeated applications. To prevent a-deterioration in performance and physical degradation during such applications, the linings are normally reinforced by asbestos fibers randomly dispersed throughout a resin matrix. However, recent medical evidence indicates that asbestos fibers can cause health hazards for people exposed thereto during the manufacture of clutch and brake linings.Unfortunately, because of the presence of fine diameter asbestos fiber during the manufacture of brake lining using asbestos fiber, a portion of the asbestos often becomes airborne in quantities that exceed the exposure standard of asbestos fiber in the United States as controlled by the Occupational Safety and Health Act of 1970. Furthermore, mechanics relining the automotive brakes are also exposed to some quantity of asbestos from the linings.

In an effortto reduce the environmental contamination by the asbestos fiber and thereby continue the manufacturing of asbestos based organic friction linings, a water slurry process disclosed in a copending U.S. Patent Application has been evaluated. The water slurry can be transmitted throughout a manufacturing facility without contaminating the surrounding environment with asbestos fibers. However, before the friction material can be cured, the water in the slurry must be removed in order to be assured that any resulting lining has essentially the same operating characteristics as a lining made from a dry mix. Unfortunately, this process adds considerable cost to the manufacturing cost of a brake or clutch lining, and does not necessarily solve emission problems during finishing and inspection.

In an effort two use readily available materials and the same manufacturing facilities, as currently available, it has been suggested that glass and/or mineral fibers be used in place of at least a portion of the asbestos fillers. For instance, U.S. Patent 3 967 037 discloses several lining compositions utilizing glass fiber in place of asbestos. From experimentation, it has been determined such lining compositions in normal operational conditions produce brake noise, severe rotor scoring and wear, and poor friction material life when the lining is mated with a cast-iron rotor or drum.

In an effort to stabilize the coefficient of friction while reducing both scoring and wear on a corres ponding cast iron drum or rotor, a further copending U.S. Patent Application discloses a composition of materials for a friction lining having carbon and/or graphite particles therein which modify any detrimental effects that glass fibers in a composition of materials may have on a corresponding mating surface.

The present invention has for its main object to propose another way of overcoming the detrimental effects of glass fibers in known compositions of friction materials, and is based on the discovery that these detrimental effects can be substantially reduced by the separation of the individual bundles of glass fibers in the process of manufacturing the friction material.

The process according to the present invention comprises, consequently, the known steps of mixing together ingredients including a binder, friction modifiers, and from 5% to 30% by weight of glass fibers to form a composition of materials, of transferring the blended composition of materials to a mold and applying pressure thereto to form a briquette, and of curing said briquette at a temperature and under a pressure to set said binder and thereby establish a desired shape and density for the friction material, and this process is more particularly characterized in that the blending of the composition of materials is conducted until the bulk density thereof is comprised between 0.1 and 0.6 g/cm3 through the separation of the individual filaments that make up the glass fibers.

In a preferred embodiment, the blending of the composition of materials is achieved in a time period comprised between 5 and 20 minutes, preferably in approximately 10 minutes; whereas the bulk density of the composition of materials is established at about 0.25 g/cm3.

It has been found that compositions of materials obtained through the above defined process and including separated and uniformly distributed filaments of glass fibers exhibit a substantially uniform coefficient of friction and acceptable wear charac teristicsthrough a temperature range comprised between 1 200C and 350"C.

These and other advantages of the invention will become readily apparent from reading the following description of some preferred embodiments, given by way of examples only, and with reference to the accompanying drawings, in which: Figure 1 is a table showing the components incor porated into the composition of materials for making a friction lining according to the principles of this invention; Figure 2 is a graph comparing friction versus temperature of a typical asbestos based friction material and glass fiber based friction materials; Figure 3 is a graph comparing wear versus temp erature of a typical asbestos based friction material and glass based friction materials; Figure 4 is a table showing friction stability, fric tion level, pad and rotor wear of a typical asbestos based friction material and glass based friction mat erials;; Figure 5 is a graph showing the fade and recovery of a typical asbestos based friction material and glass fiber based friction materials; Figure 6 is a table comparing vehicle test data of a typical asbestos based friction material and glass fiber based friction materials; Figure 7 is a graph illustrating pad and rotor wear of a test vehicle associated with a typical asbestos based friction material and glass fiber based friction materials; Figure 8 is a photograph showing the effect of various blending times on glass fiber; Figure 9 is a photomicrograph magnified 20 times showing a glass fiber before and after blending for 10 minutes; Figure 10 is a graph comparing friction versus temperature of glass fiber based friction materials made according to the principles of this invention; and Figure 11 is a graph comparing wear versus temperature of glass fiber based friction materials made according to the principles of this invention.

In orderto evaluate the friction material made according to the teaching of the present invention, a typical asbestos friction material A, disclosed in Figure 1, was formulated and characterized to establish a standard or base for illustrating an acceptable coef ficientoffriction and rate of wear for a brake lining of an automobile. Figure 1 also illustrates the modifications associated with the composition of materials made according to this invention.

The ingredients in the composition of material A were processed into brake friction material in the following manner.

Asbestos fiber, zinc powder, organic modifiers (two parts of cashew nut powder and one part rubber scrap), inorganic modifiers (barytes), and dry phenolic resin in the weight percentages shown in Figure 1 were dry mixed together for about 30 minutes until a homogenous mixture was created.

Thereafter, this homogenous mixture was placed in a mold and compacted into briquettes. The briquettes were then transferred to a press station and individually compressed at about 420 kg/cm2 to establish a predetermined shape and density, while the temperature of the briquette was raised to about 135"C.

The 1350C temperature caused the phenolic resin to flow throughout the mixture and establish a matrix for holding the other ingredients in a fixed position.

Thereafter, the briquette was transferred to a curing oven having a temperature of about 260"C to further set the phenolic resin. The rubbing surface of the cured briquette was then ground to a specific size corresponding to a brake pad. Thereafter, a portion of the pad was tested on a Chase-type sample dynamometer. This test consists in making 40 applications of 20 second duration with a 2.5 cm square sample of friction material against a cast-iron drum rotating at 525 rpm (64 km/h). After a burnish sequence, tests are made at 1200C, 1770C, 2320C, 2880C, 343"C, and 1200C (re-run). The controlled outputtor- que was held at 3.94 m.kg for the tests.

The sample dynamometer data in Figure 2 gives the 120"C, 177"C, 232"C, 288"C and 343"C steady state friction levels and the data in Figure 3 gives the 1200C, 1770C, 2320C, 2880C and 343"C wear values.

The coefficient of friction of composition A was measured and indicated as curve 100 in Figure 2 while the wear of the brake pads experienced at the various temperatures was calculated and illustrated by curve 102 in Figure 3. It should be noted that the wear rate of composition A is acceptable below 177"C (350"F). However, when vehicles equipped with such brakes are repeatedly applied, the thermal energy generated rapidly increases above 232"C (450"F) where the wear rate reaches an undesirable level.

Because of the stopping requirement standards established by the Department of Transportation, the maximum operating temperature generated in bringing a vehicle to a stop during repeated panic stop conditions often reaches 232"C. Thus, typical standard asbestos organic friction linings, while producing acceptable coefficients of friction, are damaged since the wear rate increases exponentially above 177"C (350"F) as shown in Figure 3.

A brake pad of composition A was mated with a caliper and rotor assembly of a full brake and installed on an inertial dynamometer. The inertial dynamometer procedure combined performance and wear versus temperature testing with emphasis placed on friction change with increased duty usage.

The test procedure included the following: preburnish effectiveness checks (at 48,96, and 128 km/h with 0.4,0.6, and 0.7G deceleration, respectively) with an initial pad temperature of 93"C; 200 burnish stops (64km/hat 3.6 m/sec.2from 12000 initial pad temperature); post-burnish effectiveness (at 48,96 and 128 km/h with Ova,0.6 and 0.7G deceleration, respectively) followed by 3 SAE-type fade and recovery checks to 232"C, 315"C and 371"C, and a final effectiveness check (at 48,96 and 128 km/h with 0.4,0.6 and 0.7G deceleration).

The friction level and friction stability of composition A as indicated by line pressure required to make 3 successive stops from 96 km/h at 0.7G deceleration is shown by the table in Figure 4. The pad and rotor wear data are also included.

The fade characteristics of composition A are shown by lines 108, 110, and 112 and the recovery characteristics are shown by lines 114, 116 and 118 in Figure 5.

Because of the superior frictional characteristics and high tensile strength of glass fibers over other fiberous materials suitable for friction materials, it was decided to modify composition A through the substitution of glass fiber and metal oxide particles to produce a glass fiber based composition of material B shown in Figure 1.

The glass fiber which is known in the industry as Type E is made by heating raw materials, such as silica sand, limestone, dolomite, clay, boric acid, soda ash and other minor ingredients in a hightemperature furnace in a direct melt process to produce glass. The glass flows to forehearths in the bottom of the tank furnace. The glass flows through numerous holes or orifices located in platinum alloy bushings or spinnerettes to produce filaments of molten glass. The filaments, which can range in number from 20 to 2 000, are gathered together as a thread or strand and attached to a rotating drum which turns at a speed up to 75000 rpm to produce a glass fiber. Thereafter, the glass fiber is treated with a silanizing or sizing agent, such as a silane, to improve the resin-to-fiber adhesion.Thereafter, the continuous fiber is cut into lengths which may vary from 250-10 000 microns.

The composition of material B shown in Figure 1 was compounded in the same manner as composition A and processed into a brake pad. The brake pad of composition B was installed in the Chase-type sample dynamometer and a friction wear test was performed. The coeffecient of friction of composition B is illustrated by curve 104 in Figure 2 and the wear rate by curve 106 in Figure 3. As shown in Figure 2 the coefficient of friction of composition B is sub stantial ly equivalent to the coefficient of friction of composition A. However, the wear rate as shown in Figure 3 is unacceptable for use as a friction material.

Since the coefficient of friction as shown in Figure 2 of composition B is substantially stable above 232"C (450"F), it was decided to evaluate the modifications for composition B which would reduce the wear rate. Thus, it was decided to remove the abrasive metal oxide particles through a substitution of cashew nut dust particles and a non-abrasive mineral particle (barytes) to the mixture to produce composition C shown in Figure 1.

The processed mineral fiber in composition C is made up of a composition consisting of silica, alumina, calcia, magnesia and other oxides. The fiber diameters may vary from 1-15 microns and the fiber lengths may vary from 40-1 000 microns. During the manufacture of these fibers, the surface thereof is treated with a silanizing agent to improve the resin-to-fiber adhesion.

The composition of material C was compounded and processed into disc brake pads. The wear and friction level as measured on the inertial dynamometer of composition Care shown in Figure 4. The fade characteristics of composition Care illustrated by curves 120,122 and 124whilethe recoverycharacteristics are illustrated by curves 126, 128 and 130 in Figure 5. From comparing the friction and wear data of composition C with composition A, it is apparent that composition C is superior to composition A.

Thereafter, a test vehicle was equipped with friction material pads made of compositions A and C and test data obtained therefrom to further evaluate the glass fiber composition C. The test vehicle was a station wagon having a weight of 2.25 tons GVW.

Except for burnish, reburnish and light/heavy duty cycle operations, the performance data was obtained with only the front disc brakes functioning.

Noise rating was determined with light brake applications (1.4 to 10.5 kg/cm2 line pressure) at slow speed (8 to 48 km/h) in a quiet area (such as an inactive parking structure) for the best noise amplification and detection. In each noise search, the windows were open and the radio and heat/air conditionerfan were turned off to provide a low noise background.

The performance data of composition C and that of composition A are presented in Figure 6. The first four sets of effectiveness data were obtained with an initial pad temperature at 65"C before each brake application. Applications were made at 16, 48 and 96 km/h, with 3 or 4.5 m/sec2 deceleration as specified.

The fifth effectiveness data were obtained in the same manner, except that the initial pad temperature was kept at 1 50 C. The first effectiveness test measures the effective line pressures at various speeds before the pads were subject to burnish; the second, post burnish and before the fades; the third, post 23200 and 31500 fades; the fourth, post 37100 fade.

It is clear from the data in Figure 6 that compositions C and A have comparable friction levels at the start of the test. However, the asbestos-free composition C has better friction stability than the typical asbestos based composition A as indicated by the smaller frictional level change and by the lack of friction increase with use which leads to brake burnup and friction instability.

After the series of tests reported in the table of Figure 6, the pads and rotors were measured for wear. The rig ht front pad and rotor wear and the left front pad and rotor wear for compositions A and C were illustrated by lines RF and LF, respectively, in Figure 7. From viewing Figure 7 is should be evident that the pad wear resistance of composition C is clearly superior to composition A. The rotor wears of the A and C are comparable (0.0000 and 0.0025 mm, respectively).

In order to further evaluate the family of compositions of materials that include glass fiber as the strengthening ingredient, composition C was modified by removing the carbon particles and reducing the phenolic resin content while increasing the glass fiber and mineral fiberto produce composition D shown in Figure 1.

The friction and wear data of composition D generated from the Chase-type sample dynamometer are shown by curves 105 and 107 in Figures 2 and 3, respectively.

The results of the inertial dynamometer test of composition Dare shown in Figure 4.

Thereafter, composition D was processed into brake pads and placed on the test vehicle. The results of the vehicle brake test for composition D are illustrated in Figure 6. When the data shown in the table of Figure 6 are compared, it is evident that the glass fiber reinforcement and cashew nut powder friction dust modified compositions of both compositions C and D have better friction stability and less pad wear than composition A.

In attempting to duplicate the data for composition D shown in the table of Figure 6, different data values were obtained using the same weight percen tages for the material shown in Figure 1 forcomposition of material D. In attempting to explain the different data generation with the same material, it was determined that slight variations could occur in the blending of the dry ingredients from one batch to another. On investigation it was observed that the bulk density of the composition varied with increased blending time. This change in bulk density was attributed to the separation of the filaments that made up the glass fibers.

To evaluate the effect of the glass fibers on blending time, six samples of glass fibers each weighing 10 grams were evaluated. Five samples were suc cessively placed in a mixer and blended for times varying from 1 minute to 10 minutes. The five sam ples after blending were removed from the mixer and placed in piles adjacent the test sample desig nated XF-10 as shown by the photograph in Figure 8.

A shown in Figure 8, the glass fibers expanded by separation in almost a direct proportion to the blend ing time in the mixer.

To substantiate the speculation that substantially the entire bundles of filaments that make up glass fiber separated, SEM photo micrographs (20 x) shown in Figure 9 were taken of the glass fiber in sample XF-10 and the glass fiber after 10 minutes of blending. As shown in Figure 9, the individual fila ments after blending are randomly dispersed with out any definite orientation as compared to the tight knit bundles of the glass fiber in the original mater ial.

To determine the optimum effect of the expansion or opening of the bundles of filaments of glass fiber on a friction lining, a series of tests were performed on composition D made by a process of manufactur ing wherein the blending times were varied. In the first composition designated D-1,the dry ingredients were placed in a mixer and blended for a time period of 5 minutes. At the end of 5 minutes, the composi tion of material had a bulk density of about 0.46 g/cm3. The material was transferred to a briquette mold and briquettes were made from composition D-1.These individual briquettes were transported to a press and compacted with a force of about 420 kg/cm2 to a predetermined density while at the same time the temperature was raised to 135"C to allow the phenolic resin to flow throughout the mixture and hold the other ingredients in a fixed position.

Thereafter, the individual briquettes were transfer red to a curing oven having a temperature of about 260"C to set the phenolic resin. The individual bri quettes of composition D-1 were ground to a specific size brake pad and thereafter, when tested by the Chase-type sample dynamometer procedure described above with respect to composition A, pro duced a coefficient of friction illustrated by line 130 in Figure 10 and a wear rate illustrated by line 132 in Figure 11.

Thereafter, a second composition designated D-2 was placed in a mixer and blended for a time period of 10 minutes. At the end of 10 minutes, composition D-2 had a bulk density of about 0.25 g/cm3. This blended material was processed into brake pads in the same manner as composition D-1 and when tested on the Chase-type sample dynamometer pro duced a coefficient of friction illustrated by line 134 in Figure 10 and a wear rate illustrated by line 136 in Figure 11.

Athird composition designated D-3 was placed in a mixer and blended for a time period of 15 minutes.

At the end of 15 minutes, composition D-3 had a bulk density of about 0.20 g/cm3. This blended material was similarly processed into brake pads and when tested on the Chase-type sample dynamo meter pro duced a coefficient of friction illustrated by line 138 in Figure 10 and a wear rate illustrated by line 140 in Figure 11.

From the test data generated by compositions D-1, D-2 and D-3, it was discovered that a coefficient of friction for friction lining utilizing glass fiber is enhanced when the blending time in the manufacturing process is limited to between 5-15 minutes.

In order to insure that the ingredients are uniformly distributed throughout the resulting brake pad, it was decided to pre-blend the ingredients prior to adding the glass fibers to the mixture.

Therefore, the friction modifiers and phenolic resin were placed in a mixer and pre-blended for 5 minutes before the glass fibers in composition D were added to produce a composition designated as D-4. This composition D-4 was further blended for 5 minutes to expand or separate the filaments in the glass fiber. At the end of this time period (5 minutes pre-blend and 5 minutes blend with glass fiber) composition D-4 had a bulk density of about 0.45 g/cm3. Thereafter, when composition D-4 was processed into a brake pad and tested on the Chase-type sample dynamometer, a coefficient of friction illustrated by line 142 in Figure 10 was produced and a wear rate illustrated by line 144 in Figure 11 was achieved.

Afifth composition designated D-5 was produced by pre-blending the friction modifiers and resin for 5 minutes before adding the glass fiber and thereafter blending the composition for an additional 10 minutes for a total blend time of 15 minutes. At the end of 15 minutes, composition D-5 had a bulk density of about 0.26 g/cm3. Thereafter, composition D-5 was processed into a brake pad and tested on the Chase-type sample dynamometer.

Composition D-5 produced a coefficient of friction illustrated by line 146 in Figure 10 and a wear rate illustrated by line 148 in Figure 11.

A sixth composition designated D-6 was produced by pre-blending the friction modifiers and phenolic resin for 5 minutes before adding the glass fiber.

Thereafter, this mixture was blended for another 15 minutes for a total blend time of 20 minutes to produce a bulk density about 0.21 g/cm3. Thereafter, composition D-6 was processed into a brake pad and when tested on the Chase-type sample dynamometer produced a coefficient of friction illustrated by line 150 in Figure 10 and a wear rate illustrated by line 152 in Figure 11.

To substantiate the finding with respect to the expansion of the individual filaments that make up the stands of glass fiber, a seventh composition designated D-7 was made wherein a glass fiber identified as type E OCF405-AA-.13" was substituted for the original glass fiber OCF497-BB-.13". Theconfig- uration of glass fibers in OCF405-AA-.13" is the same as OCF497-BB-.13" with the exception of the silanizing agent used as sizing for the bundle of filaments.

The friction modifiers and phenolic resin in composition D-7 were pre-blended for 5 minutes before the glass fibers were added. This mixture was blended for an additional 15 minutes for a total blend time of 20 minutes to produce a bulk density in the mixture of about 0.54 g/cm3. Thereafter, this blended mixture was processed into a brake pad and tested on the Chase-type sample dynamometer. Composition D-7 when tested produced a coefficient of friction illustrated by line 154 in Figure 10 and a wear rate illus trated by line 156 in Figure 11.

Another glass fiber identified as type E OCF636-DE-.13" was substituted for the glass fiber in composition D to produce another composition identified as composition D-8. This glass fiber has a diameter of approximately 6 microns. The friction modifiers and phenolic resin in composition D-8 were pre-blended for 5 minutes and the glass fiber OCF636-DE-.13" added and this mixture blended for another 15 minutes for a total blend time of 20 minutes. Composition D-8 had a bulk fiber density after 20 minutes of blending of about 0.07 g/cm3.

Thereafter, composition D-8 was processed into a brake pad and when tested on the Chase-type sample dynamometer, produced a coefficient of friction illustrated by line 158 in Figure 10 and a wear rate illustrated by line 160 in Figure 11.

In establishing the ranges for the ingredients in the family of glass fiber friction linings, the glass fiber in composition D was reduced and replaced by mineral fiber and an increase in the cashew nut powder to produce composition E shown in Figure 1. Composition E was blended for a time period of about 5 minutes and thereafter processed into brake pads.

When composition E was tested on the Chase-type sample dynamometer, were produced a coefficient of friction and a wear rate which are both acceptable.

To further substantiate the inventive discovery that the opening of the glass fiber bundles stabilizes the coefficient of brake pads reinforced with glass fibers while providing an acceptable wear rate, low noise and compatibility with a cast iron rotor or brake drum, composition D was further evaluated through a vehicle test. A composition designated C 0005-1 was produced by pre-blending the friction modifiers and phenolic resin of composition D for 5 minutes before adding the glass fibers and further blending for an additional 2 minutes. Composition C 0005-1 was processed into brake pads and installed on a vehicle. The vehicle was driven 4140 km in all kinds of traffic on the streets of Detroit, Michigan.

Thereafter, the disc pads were evaluated. The disc pads on the front axle of the vehicle had the following average pad wear: left front- inner 2.41 mm and outer 1.98 mm; and right front- inner 2.54 mm and outer 1.55 mm. Both the left and right rotors had a maximum wear of 0.178 mm, which is considered unacceptable. During the road evaluation of composition C 0005-1 it was observed, noise was created on substantially each brake application and the friction level was erratic. From the previous test generated on the inertial dynamometer it was assumed that the two minutes blending time is insufficientto open the fiber bundles.

Therefore, a composition designated C 0005-2 was produced by pre-blending the friction modifiers and phenolic resin of composition D for five minutes before adding the glass fibers and further blending this mixture for an additional 7 minutes. Composition C 0005-2 was thereafter processed into brake pads and installed on the test vehicle. The vehicle was driven 4320 km in all kinds of traffic on the streets of Detroit, Michigan. Thereafter, the disc pads on the front of the vehicle were removed for evaluation. The disc pads during this test were worn the following amounts: left front disc pad wear- inner pad 1.27 mm and outer pad 0.86 mm; and right front disc pad wear- inner pad 0.99 mm and outer pad 0.81 mm. The rotors were substantially free from any grooves and the wear rate measured a maximum of 0.025 mm which is acceptable.It was observed during the road test of composition C 0005-2 that the noise level generated during braking had decreased to a level considered acceptable. Thus, this test substantiated the results which were obtained through the inertial dynamometertesting that an optimum blending time for opening the glass fiber in composition D is about 10 minutes.

To further evaluate the effect of the distribution of the filaments of glass fiber uniformly throughout a composition, milled fiber glass was substituted into composition Din place of the fiber bundles to produce composition F. Composition F was blended for 15 minutes to uniformly distribute the filaments throughout the mixture. Composition F was processed into a friction material and tested on the Chasetype sample dynamometer. Composition F has a coefficient of friction illustrated by curve 109 in Figure 2 and a wear illustrated by curve 111 in Figure 3.

In order to reduce the dusty conditions associated with dry mixing, latex was substituted for rubber in the dry mixture of composition D to produce a moist mixture identified as composition G. The latex aided in holding the composition together until the briquettes could be formed. A friction material sample of composition G was tested on the sample dynamometer and had a coefficient of friction illustrated by curve 113 in Figure 2 and a wear rate illustrated by curve 115 in Figure 3.

Claims (9)

1. A process for manufacturing a glass fiber friction material, particularly for use in clutch and brake linings of vehicles, said process comprising the steps of mixing together ingredients including a binder, friction modifiers, and from 5% to 30% by weight of glass fibers to form a composition of materials, of transferring the blended composition of materials to a mold and applying pressure thereto to form a briquette, and of curing said briquette at a temperature and under a pressure to set said binder and thereby establish a desired shape and density for the friction material, said process being characterized in that the blending of the composition of materials is conducted until the bulk density thereof is comprised between 0.1 and 0.6 g/cm3 through the separation of the individual filaments that make up the glass fibers.

2. A process according to claim 1, characterized in that the blending of the composition of materials is conducted so as to establish a bulk density of about 0.25 g/cm3.

3. A process according to claim 1 or 2, characterized in that the blending'of the composition of materials is achieved in a time period comprised between 5 and 20 minutes.

4. A process according to claim 3, characterized in that the blending is achieved in approximately 10 minutes.

5. A process according to any of claims 1-4, characterized in that the binder and friction modifiers are first mixed together as dry ingredients to create a preblended mixture, and in that the glass fibers are then added to said mixture and blended therewith so as to form the composition of materials having the desired bulk density.

6. A process according to claim 5, characterized in that the preblending time of the dry ingredients is about 5 minutes whereas the proper blending time of the composition of materials is comprised between 5 and 15 minutes.

7. Afriction material obtained through a process according to any of the preceding claims.

8. Afriction lining, particularly for use in clutches or brakes of vehicles, made of a material according to claim 7 or obtained through a process according to any of the preceding claims.

9. A process for manufacturing a glass fiber friction material, substantially as described hereinabove with reference to the accompanying drawings.