WO1989008685A1

WO1989008685A1 - Friction materials

Info

Publication number: WO1989008685A1
Application number: PCT/GB1989/000285
Authority: WO
Inventors: Kenneth Roy Leonard Pugh
Original assignee: Lucas Industries Public Limited Company
Priority date: 1988-03-19
Filing date: 1989-03-17
Publication date: 1989-09-21
Also published as: DK224190D0; DK224190A; JPH03504611A; GB8806600D0; EP0404823A1

Abstract

A friction material for heavy duty applications contains, in addition to the usual fibrous friction generating materials, friction modifiers and resin binder, a sufficient quantity of particulate glass to act as a high temperature binder. This particulate glass is preferably a stable amorphous aluminoborosilicate particulate glass which, upon being heated to above about 500°C, coalesces to form a refractory glass matrix acting as a binder at temperatures where the binding effect of the resin binder degrades. The glass may be used as the sole binder.

Description

FRICTION MATERIALS

This invention relates to a friction material and, more particularly to a friction pad, a friction element including a friction pad bonded to a backing plate, a friction material composition for producing a friction pad, and a method of producing a friction pad.

Friction pads for vehicle brakes are known in which friction generating means (eg dispersed fibres such as iron fibres, or particles) and other typical ingredients such as friction retarders, friction modifiers, fillers, and elastomeric modifiers, are held together in the pad by means of a synthetic resin binder, typically a phenolic resin. Whilst such friction pads operate satisfactorily under normal conditions, if the pads reach very high temperatures, over 400 to 500°C and which may be up to 900°C as a result of extreme braking conditions, the synthetic resin becomes completely heat degraded, with the result that the whole pad loses its integrity and may break up in part or completely. Such extreme conditions may be experienced in heavy duty applications such as in truck or railway vehicle brakes.

A further disadvantage of high temperature applications is the tendency for the coefficient of friction of the friction material to drop (generally known as "fade") thereby reducing the braking effect. In extreme cases, the friction material may flame or smoke. All of these characteristics are of course highly undesirable.

The use of glass fibres (usually of unspecified composition) as a friction-generating means in friction pads has been previously proposed either as the sole friction-generating means or in admixture with other types of fibre, eg as disclosed in GB-A-2000791, DD-A- 244346, JP-A-63-53325, EP-A-0034258, GB-A-2195944 and US- 4051100 .

GB-A-2083060 discloses the use of glass fibres having a Moh hardness of less than 5 as opposed to the, apparently, more conventional use of glass fibres having a Moh hardness of at least 6, of which latter mention is made of fibres of E glass, S glass and G glass as well as a glass stated to comprise 55% silica, 10% boric oxide, 14% alumina, 17% lime and 4% magnesia. GB-A-2083060 gives, as examples of glasses having a Moh hardness of less 5, a lead glass comprising, 34% Si0₂, 59% PbO, 1.5% Na₂0, 3.5% K₂0 and 3% Al₂0₃, a lead glass comprising 67% Si0₂, 15% PbO, 1% CaO, 9.5% Na₂0 and 7.1% K₂0, and a lead- free glass comprising 65% Si0₂, 20% K₂0, 5% B₂0₃ and 10% MgO. Such soft glass fibres are used in an amount of about 20-30% in an asbestos-free friction material also containing a binder such as a phenolic resin, a friction modifying material such as cashew granules and inorganic materials of which barium sulphate and graphite are given as examples in an extensive list. These soft glass fibres are apparently used to impart smooth quiet vibration-less operation of the friction pad. No mention is made of the high temperature properties of the pad.

JP-A-62-283881 discloses the inclusion, in a brake lining, of glass fibres which crystallise on use and gives, as an example, a glass which contains 60.5 wt% Si0_2/ 25 wt% A1₂0₃, 6.5 wt% Na₂0 and 3 wt% Ti0₂.

GB-A-1445975 discloses the use of synthetic inorganic fibres comprising one or more polycrystalline refractory metal oxides (especially alumina) for the purpose of improving the temperature resistance of friction materials, and suggests that vitreous fibres tend to devitrify under severe conditions of temperature or under the influence of other aggressive conditions. US-4182436 discloses the use of a finely divided glass eg glass in fibrous form, which is substantially amorphous but which is unstable and readily devitrifies in response to heat so that as and when portions of the glass are converted by heat to a devitrified crystallised friable form, such portions separate or flake away without smearing or fusing. Such smearing or fusing is stated to affect adversely the performance characteristics of the friction material. A wide variety of easily devitrified glasses are mentioned, namely silicate, titanate, phosphate, aluminate and borate glasses and mixtures thereof.

US-2844800 discloses the use of a wide variety of glasses in a sintered metal friction material which may contain up to 30% crystalline ceramic particles such as alumina, mullite and sillimanite. The glass is said to modify the friction characteristics at high temperature by melting and thereby forming a glaze which is alleged to expend substantial amounts of the energy derived from the braking action by plastic deformation. It will be appreciated that such glass does not contribute to the structural integrity of the friction material at high temperatures since this is provided by the sintered metal matrix.

US-2966737 discloses the use, in a friction material, of sillimanite (Al₂Si0₅) crystals in a metallic matrix. Such crystals are intended to retain their form at high temperatures so that friction is not reduced by fusion of the crystals. The structural integrity of the material is provided by the metallic matrix.

It will therefore be appreciated that a variety of wide differing, and sometimes apparently contradictive, techniques have been proposed to overcome the problem associated with maintaining braking performance under conditions of heavy braking i.e where high temperatures are generated in the friction material.

It is an object of the present invention to provide a friction pad and friction material wherein the problem of pad break-up at high temperatures is obviated or mitigated.

According to a first aspect of the present invention, there is provided a friction pad comprising a shaped body including friction-generating means, and a synthetic resin binder, wherein the body also includes a sufficient quantity of dispersed particulate glass to act as a high temperature binder in the event that the pad is subjected to a temperature at which the glass particles can coalesce to form a glass refractory matrix.

According to a second aspect of the present invention, there is provided a friction pad comprising a shaped body including friction-generating means, a glass refractory matrix acting as a high temperature binder.

The friction pad according to said second aspect of the present invention may contain heat degradation products of a synthetic resin binder included as a low temperature binde.r.

According to a third aspect of the present invention, there is provided a method of producing a friction pad comprising the step of heating and pressing a friction material composition including friction generating means, a synthetic resin binder, and dispersed particulate glass so as to form a shaped body wherein the friction generating means and the particulate glass are bound together by means of the synthetic resin binder, wherein the particulate glass is present in a sufficient quantity to act as a high temperature binder in the event that the pad is subjected to a temperature at which the glass particles can coalesce to form a glass refractory matrix.

The method may include the further step of heat treating the shaped body at a temperature such as to cause the glass particles in the body to form a coalesce to form a glass refractory matrix.

According to a fourth aspect of the present invention, there is provided a method of producing a friction pad comprising the steps of heating and pressing a friction material composition including friction generating means and dispersed particulate glass so as to form a shaped body at a temperature such that the glass particles coalesce to form a glass refractory matrix.

According to a fifth aspect of the present invention, there is provided a friction material composition comprising friction generating material, synthetic resin, and a dispersed particulate glass in a sufficient quantity to act as a binder.

A further advantage of the material containing particulate glass is that its thermal conductivity is less than that of a conventional friction material. This has the added advantage that, with hydraulically operated brakes, the heat transfer rate to the hydraulic fluid is less, with the result that there is a reduced risk of the hydraulic fluid vaporising and causing brake failure.

In a sixth aspect of the present invention, there is provided a friction element comprising a friction pad according to said first or second aspect of the invention bonded to a backing plate by bonding means which comprise or consist of glass.

In the sixth aspect of the present invention, the bonding means may be constituted by at least a portion of the pad which is in abutment with the backing plate. However, it is possible for a separate bonding means containing or consisting of glass to be employed between the pad and the backing plate. Such glass may be the same as or different to that employed in the pad itself.

It is particularly advantageous for the particles to be of stable amorphous glass i.e. ones which are not easily thermally devitrified eg glass particles which remain substantially amorphous up to about 900°C. It is also particularly advantageous for the compositions containing the glass particles to contain at least one ingredient which is capable of reacting with the glass so that it bonds strongly thereto at least at a temperature above 400 - 500°C. One such ingredient is barium sulphate (or barytes) which may be included as a filler. Another such ingredient is iron which may be included in the composition in the form of particles or fibres as a friction-generating means. Iron can oxidise at its surface and then react with the particles to form iron- rich glasses at the surfaces of the glass particles. Likewise, when barium sulphate is present, barium-rich glass can be formed at the surface of the glass particles. When both iron and barium sulphate are present, barium-rich-, iron-and-barium-rich-and iron-rich glasses can be formed at the surface of the glass particles. Sulphur from the barium sulphate can also react to form iron-sulphur-rich glasses and sulphur rich iron.

In the case where iron fibres are used, it is also observed that the iron fibres can become coated with the glass to improve the fire retardant properties as well as the bonding and friction generator/modifier properties of the resultant material. The glass particles used in the present invention are preferably alumino borosilicate glasses, more preferably those containing at least one alkali metal oxide. Preferably the glass contains a greater amount of A1₂0₃ than B₂0₃.

Typically, the glass contains up to 7.5wt% B₂0₃ and 7.5 to 15 wt% Al₂0₃. The glass preferably contains 10 to 17wt% more preferably 12 to 15wt% of alkali metal oxide(s).

In a preferred compositional range, the glass contains 57 to 70 wt% Si0₂, 7.5 to 15 wt% A1₂0₃ and 15 to 28 wt% of metal oxides including alkali metal oxide(s).

More specifically, the glass may contain 60 to 70, more preferably 61 to 66 wt% Si0₂, 10 to 15, more preferably 11 to 12, wt% A1₂0₃; 5 to 7 wt% B₂0₃; and 12 to 15 wt% alkali metal oxide(s). The remainder of the glass composition may be constituted by glass modifiers selected from alkaline earth metal oxides (such as calcium oxide and magnesium oxide), zinc oxide, barium oxide, titanium oxide and zirconium oxide.

The particulate glass included in the friction material used to produce the pad is preferably very finely divided eg 95% - 200 mesh (<75μm). The quantity of particulate glass is preferably about 9 to 25 wt% of the pad, more preferably 15 to 20 wt%, and most preferably about 18wt%.

The synthetic resin content of the material used to form the pad is preferably a phenolic resin (eg a novolak) which may be cured using hexamethylene tetramine. Alternatively, any other types of thermosetting, heat- resistant resin may be employed, eg phenol-furfural resins, melamine-formaldehyde resins, urea-formaldehyde resins, diallyl phthalate resins, dioctyl phthalate resins, cross-linked alkyl resins and epoxy resins. The resin may be used in a form and quantity in which it has been used heretofore for brake linings, for example, in an amount of about 3 to 15% by weight, more preferably 5-15% by weight and most preferably about 12% by weight.

The brake pad may further contain ceramic fibres eg in an amount of about 2.5 to 12.5% by weight preferably 5-12.5% by weight and most preferably about 10% by weight. The purpose of the ceramic fibres is to provide a reinforcement which, because of the use of ceramic, does not degrade in service at elevated temperatures. A typical example of ceramic fibre is an alumino-silicate material whose melting point is higher than the temperature range in which the particulate glass coalesces to form the glass refractory matrix.

The friction-generating means generally take the form of dispersed friction-generating fibres (eg metal fibre such as iron fibres). However, it is within the scope of the present invention to employ friction-generating means in the form of dispersed particles. The friction-generating means, eg iron fibres are preferably present in an amount of 20-70% by weight, more preferably 25-50% by weight and most preferably about 30% by weight.

The friction pads and friction material compositions according to the present invention may also contain other dispersed ingredients which are normally used, eg friction retarders, friction modifiers, fillers and elastomeric modifiers. Typically, the composition contains one or more and preferably all of the following such ingredients:-

2-30 wt%, preferably 5-15 wt%, most preferably about 14 wt% of graphite,

1-10 wt%, preferably 1-5 wt%, most preferably about 2 wt% of friction dust, 2.5-25 wt%, preferably 5-15 wt%, most preferably about 8 wt% of barium sulphate,

2-10 wt%, preferably 2-5 wt%, most preferably about 4 wt% of rubber crumb.

The brake pad may further include a minor amount, eg 0.5 - 10 wt%, preferably 1-5 wt%, most preferably 2 wt%, of silicate-based binder, eg sodium silicate, which assists in providing an intermediate stage binding when the synthetic resin binder has started to disintegrate but before the glass powder has started to form the matrix, so as to ensure that the integrity of the pad is maintained at all times.

Examples of the present invention will now be described.

Example 1

A premix (A) containing the ingredients set out in Table A below are thoroughly mixed for 30 minutes in a turbo mill in order to effect even dispersion of the ingredients.

TABLE A

Ingredients Amount (wt%)

Ceramic fibre (Type CF101 - as supplied by the Carborundum Company Limited) 25

Cross-linkable phenolic resin (novolak) powder (type P41 - containing hexamethylene tetramine - as supplied by Borden (UK) Ltd) 30

High refractory glass (type *WB8051C - as supplied in flake form by Ferro (Great Britain) Ltd but ground to a sub-micron powder) 45 [*Glass type WB8051C contains 61.6 wt% of Si0₂, 11.7 wt% Al₂0₃, 0.1 wt% Fe₂0₃, 5.1 wt% CaO, 0.1 wt% MgO, 6.0 wt% K₂0, 7.4 wt% Na₂0, 1.2 wt% ZnO, and 6.8 wt% B₂0₃.]

Following this, 40 wt% of premix (A) is thoroughly mixed with the ingredients specified in Table B below for 60 minutes using a turbo-mill.

TABLE B

Ingredients Amount (wt%)

Chopped iron fibre (less than 200 mesh- as supplied by Mintex Don Ltd.) 30

Artificial graphite (as supplied by Korpftmuhl) 14

Friction dust (cashew nut modified phenolic resin as supplied by British Petroleum Ltd)

Barium sulphate powder (as supplied by Bleaklow Mining Ltd)

Nitrile Rubber crumb (Type 4644 as supplied by BF Goodrich)

Sodium silicate (as supplied by Ferro (Great Britain) Ltd)

The resultant mixture is mixed to a thick dough-like consistency in a plough and shear mixer whilst making small additions of a wetting agent composed of equal small additions of a wetting agent composed of equal volumes of industrial methylated spirit, isopropyl alcohol and water.

The resultant dough-like mass is loaded into a steel mould and a load of 2 tons per square inch (30912 kPa) applied, although pressures of 31-240 MPa may be employed. After this, the pressure is completely released and the formed brake lining compact is then ejected from the mould. The resultant formed compact is heated, typically in air at normal atmospheric pressure for about 30 mins at 160°C, in order to cure the phenolic resin. The cured phenolic resin forms a matrix bond which ensures the integrity of the compact.

The resultant compact has a composition of a typical semi-metallic brake lining with the exception that it contains dispersed high refractory glass. Such a compact does not lose its integrity at temperatures above 400 up to 900°C even though at such temperatures the phenolic resin thermally degrades. The high refractory glass powder in the compact is found to take over the binding function at temperatures of above 500°C by forming a high refractory glass matrix with the iron fibres and the barium sulphate particles, with the sodium silicate forming an intermediate bond as mentioned previously. The formation of such a matrix can be achieved by heating both in the presence of air and in the absence of air, and can therefore be formed in service. The presence of the glass powder is therefore capable of extending the working temperature range of the compact up to 900°C without any detriment to the structural integrity thereof.

The thermogram below shows the fusion characteristics of the glass alone in comparison with cured and fired friction materials containing the glass, as determined by 12

200 400 600 800 1000

TEMPERATURE 'C

The above thermogram demonstrates that the glass is in a stable amorphous form at temperatures of up to about 900°C and that the other friction material ingredients have no detrimental effect. These results were also verified by X-ray diffraction.

The friction material was studied at various stages using Scanning Electron Microscopy (SEM) with Energy Dispersive Analysis by X-rays (EDAX) and the following has been observed:-

1. At temperatures up to 500°C, no detected reaction between ingredients occurs, the only change being thermal degradation of phenolic resin.

2. At 500°C, reaction occurs where the iron fibres oxidise on the surface, probably to FeO and/or Fe₂0₃. 3. Between 500 to 700°C, interaction is initiated between barium sulphate, glass and iron forming distinct fused regions.

4. The interaction continues at temperatures above 700°C where barium-rich, barium-iron-rich and iron-rich glasses are formed. Barium sulphate, normally an inert material, appears to dissociate which leads to:

(i) the barium reacting with the "glass" to form barium-rich interfaces around unreacted "glass".

(ii) the iron oxide fusing with the "glass" and reacting with the sulphur/sulphate to form both iron-rich glass and sulphur-rich iron.

5. The final product contains a series of interfaces between the original glass and the iron fibres, consisting of:-

(i) barium-rich glass

(ii) barium/iron-rich glass

(iii) iron/sulphur-rich glass and (iv) sulphur-rich iron

These interactions are shown to occur in distinct regions depending on the proximity of the constituents.

6. The iron fibres become coated with the "glass" hence producing fire retardant, bonding and friction generator/modifier properties

Comparative Example.

Example 1 is repeated but without any high refractory glass being present in premix (A) . The resultant compact operates satisfactorily at temperatures up to 400 to 500°C. However, -when exposed to elevated temperatures of 500 to 900°C, the compact disintegrates complete.

Example 2

Example 1 is repeated but fired at a temperature above 550°C to form the refractory matrix. The resultant shaped body has properties similar to that of Example 1 i.e it is found to maintain its structural integrity at temperatures of up to 900°C.

Example 3

Example 1 is repeated with the exception that the high refractory glass employed is type WB8051B as supplied by Ferro (Great Britain) Limited (but ground to a sub-micron powder). Such glass contains 61.1 wt% Si0₂, 11.2 wt% A1₂0₃, 0.1 wt% Fe₂0₃. 4.9 wt% CaO, 0.1 wt% MgO, 5.8 wt% K₂0, 9.1 wt% Na₂0, 1.1 wt% ZnO, and 6.5 wt% B₂0₃. The resultant compact has similar properties to the compact obtained in Example 1. Specifically, it is found to maintain its structural integrity at temperatures of 500 to 900°C.

Example 4

Example 1 is repeated with the exception that the high refractory glass employed is type WB8051A as supplied by Ferro (Great Britain) Limited (but ground to a sub-micron powder.) Such glass contains 65.7 wt% Si0₂, 12.0 wt% A1₂0₃, 0.1 wt% Fe₂0₃, 3.9. wt% CaO, 0.1 wt% MgO, 4.7 wt% K₂0, 7.3 wt% Na₂0, 0.9 wt% ZnO and 5.3 wt% B₂0₃. The resultant compact has properties similar to the compact of Example 1. Specifically, it is found to maintain its structural integrity at temperatures of 500 to 900°C. Example 5

Example 1 is repeated using, instead of a 60.:40 wt ratio mix of the Table B ingredients to premix A, ratios of 90:10, 80:20, 70:30 and 50:50. The resultant compacts are all capable of maintaining their structural integrity at temperatures in excess of 500°C. However, the 90:10 compact is found to disintegrate at temperatures of 700 to 900°C. It is believed that the reason for this is that the glass powder content (4.5 wt%) is insufficient to form a satisfactory matrix at the elevated temperatures. The 80:20 and 70:30 compacts, whilst maintaining structural integrity at temperatures up to 900°C, are not considered to be quite as effective as the 60:40 compact used in Example 1. The 50:50 compact produces good results but is relatively expensive because of the high cost of the expensive premix A ingredients.

Example 6

Example 1 is repeated with the exception that the cross- linkable phenolic resin is replaced by 20 wt% of iron and an additional 10 wt% of ceramic fibres. The compact is fired at temperatures of above 550°C to form the refractory matrix. The resultant shaped body has properties similar to that of Example 1 i.e it is found to maintain its structural integrity at temperatures of up to 900°C.

The material compositions disclosed above maintain structural integrity at temperatures up to 900°C, and thus the tendency for pad break-up is substantially alleviated.

Furthermore, substantially constant coefficients of friction are achieved, typically 0.32 - 0.37 for the cured compacts and 0.59 - 0.68 for the fired compacts. Thus, the tendency to fade at elevated temperatures is alleviated.

The glass ingredient, as well as improving structural integrity of the material, gives fire retardant properties.

The advantages listed above make the friction material suitable for heavy duty applications such as truck or railway vehicle brakes where high temperatures are experienced.

Claims

CLAIMS :

1. A friction pad comprising a shaped body including friction-generating means, and a synthetic resin binder, wherein the body also includes a sufficient quantity of dispersed particulate glass to act as a high temperature binder in the event that the pad is subjected to a temperature at which the glass particles can coalesce to form a glass refractory matrix.

2. A friction pad comprising a shaped body including f iction-generating means, and a glass refractory matrix acting as a high temperature binder.

3. A friction pad as claimed in claim 2, also containing heat degradation products of a synthetic resin binder.

4. A friction pad as claimed in any preceding claim, wherein the glass particles are ones which are not easily thermally devitrified.

5. A friction pad as claimed in any preceding claim, containing at least one ingredient which is capable of reacting with the glass so that it bonds strongly thereto at least at a temperature above 400 - 500°C.

6. A friction pad as claimed in claim 5, wherein said at least one ingredient includes barium sulphate.

7. A friction pad as claimed in claim 5 to 6, wherein said at least one ingredient includes iron in the form of particles or fibres acting as friction-generating means.

8. A friction pad as claimed in any preceding claim, wherein the glass is an alu ino borosilicate glass .

9. A friction pad as claimed in claim 8, wherein said alumino borosilicate glass also contains at least one alkali metal oxide.

10. A friction pad as claimed in claim 8 or 9, wherein the glass contains a greater amount of A1₂0₃ than B₂0₃.

11. A friction pad as claimed in claim 10, wherein the glass contains up to 7.5 wt% B₂0₃ and 7.5 to 15 wt% A1₂0₃.

12. A friction pad as claimed in claim 8, 9, 10 or 11, wherein the glass contains 10 - 17 wt% of alkali metal oxide(s) .

13. A friction pad as claimed in claim 12, wherein the glass contains 12 - 15 wt% of alkali metal oxide(s).

14. A friction pad as claimed in any preceding claim, wherein the glass contains 57 to 70 wt% Si0₂, 7.5 to 15 wt% Al₂0₃, and 15 to 28 wt% of metal oxides including alkali metal oxide(s).

15. A friction pad as claimed in claim 14, wherein the glass contains 60 to 70 wt% Si0₂, 10 to 15 wt% Al₂0₃; 5 to 7 wt%. B₂0₃; and 12 to 15 wt% alkali metal oxide(s).

16. A friction pad as claimed in claim 15, wherein the glass contains 61.- 66 wt% Si0₂.

17. A friction pad as claimed in claim 15 or 16, wherein the glass contains 11 - 12 wt% Al₂0₃.

18. A friction pad as claimed in any preceding claim, wherein the quantity of glass is about 9 to 25 wt% of the pad.

19. A friction pad as claimed in claim 18, wherein the quantity of glass is 15 to 20 wt% of the pad.

20. A friction pad as claimed in any preceding claim, further including a minor amount of silicate-based binder which assists in providing an intermediate stage binding when the synthetic resin binder has started to disintegrate but before the glass powder has started to form the matrix, so as to ensure that the integrity of the pad is maintained at all times.

21. A friction element comprising a friction pad bonded to a backing plate, characterised in that a friction pad as claimed in any preceding claim is used as said friction pad and in that the bonding means comprises or consists of glass.

22. A friction element as claimed in claim 21, wherein the bonding means is constituted by a portion of the pad which is in abutment with the backing plate.

23. A friction element as claimed in claim 21, wherein the bonding means is separately employed between the pad and the backing plate.

24. A method of producing a friction pad comprising the step of heating and pressing a friction material composition including friction generating means, a synthetic resin binder, and dispersed particulate glass so as to form a shaped body wherein the friction generating means and the particulate glass are bound together by means of the synthetic resin binder, wherein the particulate glass is present in a sufficient quantity to act as a high temperature binder in the event that the pad is subjected to a temperature at which the glass particles can coalesce to form a glass refractory matrix.

25. A method as claimed in claim 24, including the further step of heat treating the shaped body at a temperature such as to cause the glass particles in the body to form a coalesce to form a glass refractory matrix.

26. A method as claimed in claim 24 or 25, wherein the pad is as claimed in any one of claims 1 and 3 to 20.

27. A method of producing a friction pad, comprising the step of heating and pressing a friction material composition including friction generating means and dispersed particulate glass so as to form a shaped body, said heating and pressing being effected at a temperature such that the glass particles coalesce to form a glass refractory matrix.

28. A method as claimed in claim 27, wherein the pad is as claimed in any one of claims 2 to 20.

29. A friction material composition for producing a friction pad as claimed in claim 1, comprising friction generating material, synthetic resin and a dispersed particulate glass in a sufficient quantity to form a binder in the form of a refractory glass matrix when heated to a temperature at which the glass particles coalesce.

30. A friction material composition for producing a friction pad as claimed in claim 2, comprising friction generating material and a sufficient quantity of dispersed particulate glass to form a binder in the form of a refractory glass matrix when heated to a temperature at which the glass particles coalesce.