EP0637363A1

EP0637363A1 - Friction material

Info

Publication number: EP0637363A1
Application number: EP93908018A
Authority: EP
Inventors: Alan William Atkinson
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1992-04-23
Filing date: 1993-04-08
Publication date: 1995-02-08
Also published as: GB9208819D0; GB9307533D0; GB2266312B; AU3901193A; WO1993022580A1; GB2266312A

Abstract

A friction material e.g. for a brake pad or lining, has a reactive metal selected from calcium, lithium, sodium, and potassium, distributed therein. As the reactive metal becomes exposed in service, it reacts with water to form a highly alkaline hydroxide which acts to combat 'corrosion stiction'.

Description

FRICTION MATERIAL

This invention is concerned with friction material, for example friction material for use in brake pads or brake linings for vehicles or machinery.

It is well known that brake pads or linings can suffer from corrosion problems in service. In particular, a phenomenon known as "corrosion-stiction" can under wet conditions cause the friction material to stick to the rotor (disc or drum) with the brakes applied. Corrosion- stiction occurs especially in humid conditions or when the friction material becomes contaminated with salt. It is also well known that the likelihood of corrosion-stiction can be reduced or eliminated by the incorporation of an alkali material in the friction material. In some formulations of friction material, tealcium hydroxide in powder form is included. Calcium hydroxide has the advantages that it is relatively cheap and has a low solubility so that it is not readily washed out of the friction material during service.

Most modern friction material formulations include cross-linkable polymers as a binder. For example, phenolic resins are commonly used for this purpose. It is found that, if calcium hydroxide is included in such formulations, premature cross-linking of the polymer during mixing of the formulation can occur, resulting in an unusable material that can not be moulded into a fully dense brake pad or lining. Where such formulations are involved, therefore, the calcium hydroxide cannot be added to combat corrosion-stiction. In British Patent Application number 9303279.5, filed on the 18th February 19930, a method is described by which an alkali material can be incorporated in a friction material _r and in which the alkali material is separated from the binder of the friction material but is available to combat corrosion-stiction, is described. This method involves encapsulating particles of alkali material in a coating material which is substantially inert in relation to the uncured binder, the coating material, however, being capable of being abraded from the particles to expose the alkali material as the friction material wears in service. However, although this method incorporates alkali, the alkali and the coating material therefor has to be added to the friction material at the expense of other materials which contribute to the friction characteristics of the material, the alkali and the coating therefor giving substantially no contribution to the friction characteristics. Furthermore, the encapsulation of the alkali material is somewhat complex.

It is an object of the present invention to provide a friction material in which alkali material becomes available during service to combat corrosion-stiction, in which the above-mentioned disadvantages are avoided.

The invention provides a friction material characterised in that the material has a reactive metal selected from calcium, lithium, sodium and potassium distributed therein which, as it becomes exposed due to wear of the friction material in service, is capable of reacting with water to form a highly alkaline hydroxide.

A friction material according to the invention does not have added alkali material but rather the alkali is created as the reactive material becomes exposed at points where it is needed. Thus, alkali material is made available as required without the disadvantages of the method of the above-mentioned patent application. The required water can be obtained from atmospheric moisture. Where the material is described herein as being capable of reacting with water to form a highly active alkaline hydroxide, it is not intended to exclude a two-stage reaction in which the hydroxide is produced from an intermediate compound, e.g. where calcium is used this may first react with atmospheric oxygen to form calcium oxide which then reacts with atmospheric water to form calcium hydroxide. Furthermore, the unreacted metal can contribute to the friction characteristics.

The reactive metal may be present as an intermetallic compound, for example calcium silicide (about 30% of calcium and 70% of silicon) or calcium aluminide (about 75% of calcium and 25% of aluminium) . These intermetallic compounds have the advantage of not being as reactive as some of the metals aforementioned so that they can be more easily handled during the formation of the friction material.

The reactive metal may be alloyed before incorporation in the friction material to reduce its reactivity and thereby make it easier to handle or it may be encapsulated in a capping material which is not reactive with other materials in the friction material. The capping material may, for example, be a resin or an oxide or other compound formed by chemical reaction on the surface of the material. If desired, the metal may be both alloyed and capped. Also the aforementioned intermetallic compounds may be capped.

In addition to giving the advantage of being available to create alkali as the friction material wears, the above- identified metals also give cathodic protection against electrolytic corrosion. It is known to add metals to friction materials for this purpose, such as zinc or aluminium, but these metals produce hydroxides which are insufficiently soluble to enable effective combating of corrosion-stiction. It is desirable to aim for an alkali giving a pH greater than or equal to 12.

Some of the reactive metals mentioned above as possible reactive materials, can also be used to provide reinforcement for the friction material. It is known to add metals such as steel, copper or brass as reinforcement, for example in the form of fibres or wires but known reinforcements are not reactive. Where it is intended that the reactive metal should also assist in reinforcement, the reactive metal may be provided in the- form of flakes, or as fibres, or as wires.

It is also known to add metals to conventional friction materials to alter their friction and wear properties. Fine powders of copper, brass or sponge iron are commonly used for this purpose. They reduce variations in coefficient of friction under changing braking conditions and can also reduce wear rates.

There now follows an illustrative example of a friction material in accordance with the invention.

For comparison purposes, brake pads were prepared from a conventional mixture of powdered phenolic resin, reinforcing fibres, fillers, graphite powder and metal particles (15% by weight) . The metal particles were of brass (approximately 0.5mm in diameter). In the illustrative example, an identical mixture was used for preparing further brake pads but with the brass particles replaced by the same weight percentage of similarly sized particles of a calcium silicon alloy/inter-metallic compound (30% by weight calcium and 70% by weight silicon) .

When the brake pads had cured, they were abraded to simulate the wear in service and to reveal fresh surfaces within the pads thereby exposing metallic particles of the brass or of the alloy/inter-metallic compound. The abraded surfaces were moistened and their pH values were measured using indicator papers. For the pads containing the brass particles, the pH value was approximately 7, i.e. insufficient to inhibit corrosion stiction. For the pads containing the alloy/inter-metallic compound, the pH value was above 12, i.e. sufficient to inhibit corrosion stiction.

Claims

A friction material characterised in that the material has a reactive metal selected from calcium, lithium, sodium and potassium distributed therein which, as it becomes exposed due to wear of the friction material in service, is capable of reacting with water to form a highly alkaline hydroxide.

A friction material according to Claim 1, characterised in that the metal is alloyed before incorporation in the friction material.

A friction material according to Claim 1, characterised in that the reactive metal is in the form of an intermetallic compound.

A friction material according to any one of Claims 1 to 3, characterised in that the reactive metal is encapsulated in a capping material which is not reactive with other materials in the friction material before incorporation in the friction material.

A friction material according to any one of Claims l to 4, characterised in that the reactive metal is in the form of a fibre or a wire.