GB2264714A - Friction material - Google Patents

Abstract

A friction material, eg for use as brake pad or brake lining, comprises a filler and a binder, eg a phenolic resin, and alkali containing granules. The granules comprise a plurality of particles of alkali material encapsulated in a material which is inert in relation to the uncured binder. The encapsulating material can be abraded from the particles as wear occurs in service to expose the alkali material which can then combat corrosion-stiction.

Description

Friction Material This invention is concerned with friction material, for example friction material used 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, the 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. Corrosionstiction 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 occurring can be reduced or eliminated by the incorporation of an alkali material in the friction material. In some formulations of friction material, calcium 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.

Many 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 cannot be moulded into a fully dense brake pad or lining. Where such formulations are involved, therefore, the calcium hydroxide can not be added to combat corrosion-stiction.

It is an object of the present invention to provide a friction material in which alkali material is incorporated and in which the alkali material is separated from the binder of the friction material but is available to combat corrosion-stiction.

The invention provides a friction material comprising a filler, a cured binder, and alkali containing granules, the granules comprising a plurality of particles of alkali material encapsulated in a material which is substantially inert in relation to the uncured binder and which is capable of being abraded, as the friction material wears in service, to expose the particles of alkali material.

In a friction material in accordance with the invention, the alkali material is prevented by the encapsulating material from affecting the binder during mixing and moulding of the friction material but, during wear of the friction material, a steady supply of alkali material becomes available to combat corrosion. The provision of a plurality of particles of alkali material in each granule enables a sufficient quantity of alkali material to be included without including an excessive quantity of encapsulating material. The particles of alkali material are also encapsulated more reliably than if they were individually encapsulated.

The granules may be made by first providing alkali material in find powder form, eg by freeze drying, compacting followed by crushing, or spray drying. The encapsulating material may be a thermosetting resin, eg a phenolic resin, or a thermoplastic material. The particles may be distributed throughout the resin. The resin and alkali material may be in approximately equal weights and be ground into granules of a required size. Another way of forming such granules is to mix the alkali material in uncured thermosetting material or molten thermoplastic material, extrude the mixture through fine orifices and chop the extruded ribbon into granules. Where uncured thermosetting material is used, the granules may be treated to prevent them from sticking together, eg by surface oxidation.

The distribution of particles of alkali material throughout each granule may lead to some particles at the surfaces of the granules not being fully encapsulated.

These particles may not contain sufficient alkali material to substantially affect the binder, or such particles may be washed away prior to incorporation of the granules in the friction material. Alternatively, the granules can be coated with a layer of the same or a different encapsulating material to complete the encapsulation of particles of alkali material at the surface of the granules. For example, wax or resin may be applied to the granules by conventional coating or encapsulation methods, eg dry-tumbling, spraying from solution, or melt encapsulation. Another alternative is to coat particles of alkali material at the outer surfaces of the granules, by chemical reaction, on their exposed surfaces with a material which is substantially inert in relation to the uncured binder of the friction material.The particles are treated with a reagent that reacts with the alkali material to produce an insoluble barrier layer. The reagent seeks out exposed alkali material, ensures complete encapsulation and allows thin coatings. If calcium hydroxide is the alkali material, suitable reagents are chemicals which form largely insoluble chemical salts on the surface, thus rendering the particles effectively neutral. These chemicals include aqueous solutions of phosphates, silicates, sulphates, carbonates and several organic salts, such as oxalates. Phosphates are especially preferred because they form coatings with very low solubility and permeability. Residual phosphates are also beneficial in further inhibiting corrosion. Either phosphate salts or phosphoric acid may be used to generate the insoluble coating.It is also possible to use gaseous reagents, eg in the case of calcium hydroxide, carbon dioxide or sulphur dioxide may be used. For example, where the alkali material is calcium hydroxide, the coating may be of calcium phosphate.

The alkali material may be calcium hydroxide, trisodium orthophosphate, calcium nitrite or other suitable material.

There now follow a detailed description of an example which is illustrative of the invention Alkali-containing granules were prepared by z-blade mixing a 60:40 by weight composition of Ca (OH)2 and cashew resin, for 15-20 minutes, and pre-curing at 1500C for 40 minutes. The mixtures was then ground and sieved to a particle size of 0.7-1.7mm. The resulting granules were then further cured at 1500C for 2-3 hours.

A portion of the granules were then immersed in 3 litres of 10% H3PO4 per 1 kilogram and agitated for 15-20 minutes. These granules were subsequently washed in tap water and oven dried at 1500C to give coated granules.

EDXA examination, with a scanning electron microscope, indicated that the exposed calcium hydroxide particles had become coated with a phosphate layer.

The coated granules were added to a conventional brake lining composition comprising a filler of metal fibres, carbons etc and curable binder, specifically a phenolic resin. The granules were added at a loading of 14% by volume. This mixture was pressed into the shape of a brake lining and the binder cured by heat. As the cashew resin is substantially inert in relation to the uncured phenolic resin and can be abraded, as the lining wears in service, to expose the Ca(OH) a a friction material according to the invention had been achieved.

Trials were then conducted using brake linings with various additions of alkaline material, and stiction performance was measured. In trial 1, the material according to the invention was used. In trial 2, the uncoated granules mentioned above were used in the same loading in the same brake lining composition. In trial 3 an equal weight of fine, ungranulated, uncoated calcium hydroxide was used instead of granules. Trial 4 material contained no alkali material. The stiction performance of the lining was assessed using freshly abraded lining samples immersed in 5% NaCl solution for 15 minutes, and drained for 5 minutes.The samples were then placed on a cleaned car brake disc in an environmentally controlled chamber and clamped for 72 hours under a load of 375 psi (2.6MN/m2). The discs were then removed from the chamber and the shear force required to detach the lining samples from the disc measured. The attached Table shows the results.

TRIAL ALKALI MANUFACTURING CHARACTERISTIC STICTION VALUES 1 Ca(OH)2 granulated No premature resin oure. No stiction.

in cashew resin Easy to mould linings.

and phosphate Strong linings were treated to produced.

encapsulate exposed alkali.

2 Ca(OH)2 granulated Some premature resin cure. Severe stiction but but not Linings difficult to mould. not as severe as in encapsulated. Lining had insufficient trial 4.

strength.

3 Ca(OH)2 plain Substantial premature resin ---- powder. cure. Impossible to mould linings 4 No alkali As Trial 1 Severe stiction

Claims (6)

1. CLAIMS 1A friction material comprising a filler, a cured binder, and alkali containing granules, the granules comprising a plurality of particles of alkali material encapsulated in a material which is substantially inert in relation to the uncured binder and which is capable of being abraded, as the friction material wears in service, to expose the particles of alkali material.
2. 2 A friction material according to Claim 1, wherein the encapsulating material is a thermosetting resin.
3. 3 A friction material according to either one of Claims

   1 and 2, wherein the alkali material is calcium hydroxide.
4. 4 A friction material according to any one of Claims 1 to 3, wherein the granules are coated with a layer of the same or a different encapsulating material.
5. 5 A friction material according to any one of Claims 1 to 3, wherein particles of alkali material at the outer surfaces of the granules are coated, by chemical reaction, on their exposed surfaces with a material which is substantially inert in relation to the uncured binder.
6. 6 A friction material substantially as hereinbefore described with reference to the illustrative example.