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(11) | EP 0 000 841 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
Improvements in or relating to friction materials Verbesserungen an Reibungsmaterial Perfectionnements à des matériaux de friction |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] This invention relates to friction materials, and more particularly to friction materials of the kind used for brake pads, brake linings, clutch facings and similar uses.
[0002] Friction materials of this kind are generally composed of a thermoset binder, and asbestos fibre reinforcement and various fillers and other additives and are used against cast iron brake discs, drums etc. French Patent Specification No. 1,575,746 describes a friction material containing steel wool, but this material is for use against steel counter members, as in railway brake blocks, and is formulated from materials those function includes embrittlement of the steel so that it will break up at the surface of the brake block. Proposals have also been made of compositions containing non-asbestos inorganic fibrous reinforcement for brake pads and linings of automotive types but such materials have so far had limited commercial acceptance.
[0003] According to the present invention a friction material suitable for moulded brake pads or linings comprises a thermoset binder a steel fibre reinforcement, and other fillers and additives and is characterised in that the composition consists of, in combination,
i) a thermoset binder which comprises a phenol-formaldehyde resin and which up 20% to 45% by volume of the composition of which at least half is said resin,
ii) as sole fibrous reinforcement staple steel fibre in an amount in the range 5% to 15% by volume of the composition said steel fibres having a length in the range 1 to 5mm,
iii) an inert mineral filler not being a metal oxide in an amount in the range 10% to 35% by volume of the composition,
iv) the balance of the composition being friction and wear modifiers in an amount up to 40% by volume of the composition.
By "inert mineral filler" in this specification we mean a particulate filler whose presence does not substantially affect the friction properties of the material and which is an inexpensive mineral such as barytes, whiting or silica. We exclude metal oxides from this class of fillers since they are used for other purposes in friction materials.
[0004] The inert mineral filler is present in an amount between 10 and 35 per cent by volume, and is most important from a cost point of view when comparing these materials, since asbestos is a cheap raw material being replaced by relatively expensive man-made fibre. Hence it is necessary to find a friction material having satisfactory properties but able to carry a loading of cheap filler material.
[0005] The fibrous reinforcement consists of fine steel fibres having a length of the order of 1 to 5mm, preferably having a diameter of the order of 0.125mm. The steel may be a mild steel. The fibrous reinforcement preferably constitutes at least 9% by volume of the friction material.
[0006] The thermoset binder includes a thermoset resin based on a phenol-formaldehyde material but may also include a heat and chemical resistant vulcanized rubber, such as a nitrile rubber. Preferably a mixture of such materials is used in which the phenol-formaldehyde resin material is preferably the major constituent i.e. more than 50% of said mixture. When a rubber is used it may be incorporated into the friction material in the form of a solution in an organic solvent such as trichloroethylene, or in the form of a powder, and a vulcanizing agent such as sulphur can be also used.
[0007] It is usual in the manufacture of friction materials to include various other material as friction and wear modifiers the proportions of which can be varied to adjust to the friction and other properties of the materials.
[0008] Examples of friction and wear modifiers are carbon, graphite, antimony trisulphide and molybdenum disulphide and metals in a finely divided form. Examples of suitable metals are copper, brass and tin. A mixture of such materials may be used, and the total amount of such materials may be up to 40 per cent by volume.
[0009] The friction materials of the present invention are particularly suited to be manufactured by a press-moulding technique in which all the ingredients of the material are compounded together, the compounded mix disintegrated and (optionally) dried and then moulded into a component such as a brake pad in a die under pressure. The moulded component is then removed from the die and baked to cure the binder.
[0010] The invention provides friction materials which contain no asbestos and yet which have friction properties comparable to conventional asbestos-reinforced materials.
[0011] The invention will now be illustrated by way of example only, by means of the following example.
Example 1
[0012] Sample disc brake pads were made using the formulation given below in Table I. The ingredients were compounded together the nitrile rubber being introduced as a powder and the resulting dry mix was disintegrated and press-moulded in a die into the shape of disc-brake pads. The mouldings so produced were baked in an oven to cure the binder.
[0013] The sample disc-brake pads were tested and their friction properties found to be comparable to materials containing asbestos as the fibre reinforcement.
Example 2
[0015] Disc brake pads were manufactured to the formulation given below in Table II by the same method as used in Example 1 except that the nitrile rubber in the present example was introduced as a 16% (by weight) solution in trichloroethylene.
[0016] In tests, on a dynamometer, of the pads produced the coefficient of friction varied from 0.32 (cold) to 0.44 (hot) and wear was less than that of many conventional asbestos reinforced materials at this level of friction.
[0017] The assembly shear strength of two pads was measured, the values obtained being 9722 and 9377 KN/m2
Example 3
[0019] This example illustrates the use of lower binder content and higher loading of inert filler (Barytes).
[0020] Disc brake pads were made by the same method as Example 2 to the formulation given in Table III.
[0021] The wear of these pads was similar to those of Example 2 and coefficient of friction varied from 0.27 (cold) to 0.48 (hot).
Example 4
[0023] This example illustrates the use of an even lower binder content at the same loading of barytes.
i) a thermoset binder which comprises a phenol-formaldehyde resin and which makes up 20% to 45% by volume of the composition of which at least half is said resin,
ii) as sole fibrous reinforcement staple steel fibre in an amount in the range 5% to 15% by volume of the composition said steel fibres having a length in the range 1 to 5mm,
iii) an inert mineral filler not being a metal oxide in an amount in the range 10% to 35% by volume of the composition,
iv) the balance of the composition being friction and wear modifiers in an amount up to 40% by volume of the composition.
j) un liant thermodurci qui comprend une résine phénol-formaldéhyde et qui constitue 20 à 45% du volume de la composition, dont au moins la moitié est formée par cette résine;
ii) comme unique agent de renforcement fibreux, des fibres d'acier coupées en quantité de 5 à 15% du volume de la composition, lesquelles fibres d'acier ont une longueur de 1 à 5 mm;
iii) une charge minérale inerte qui n'est pas un oxyde métallique en une quantité de 10 à 35% du volume de la composition, et
iv) le reste de la composition étant formé par des modificateurs de friction et d'usure en quantité s'élevant jusqu'à 40% du volume de la composition.
i) einem hitzegehärteten Bindemittel aus einem Phenol/Formaldehyd-Harz, das bis 20 bis 45 Vol.-% der Masse ausmacht, von der wenigstens die Hälfte das Harz darstellt,
ii) Stahlstapelfasern als einziger fasriger Verstärkung in einer Menge zwischen 5 und 15 Vol.-% der Masse, wobei die Stahlfasern eine Länge zwischen 1 und 5 mm besitzen,
iii) einem inerten mineralischen Füllstoff, der kein Metalloxid ist, in einer Menge zwischen 10 und 35 Vol.-% der Masse, während
iv) der Rest der Masse aus Reibungs- und Abriebmodifizierungsmitteln in einer Menge bis zu 40 Vol.- % der Masse besteht.