GB2128629A - Exfoliated graphite lubricants - Google Patents

Abstract

Lubricant material comprises exfoliated graphite particles whose pores contain a water-insoluble lubricant which is either a liquid of boiling point not less than 150 DEG C or a solid of melting or softening point less than 250 DEG C. The material is more conveniently handled than conventional greases and does not lose its content of absorbed lubricant by liquefaction, or collapse, when the temperature is raised to the volatilisation temperature of the absorbate.

Description

SPECIFICATION Lubricant materials This invention relates to lubricant materials.

Conventional lubricant materials often take the form of a grease, that is to say a waterinsoluble lubricant liquid of boiling point greater than 1 500C (usually a petroleum oil of the appropriate viscosity characteristics) gelled by a soap or other gelling agent. Certain special lubricant materials contain particles of a solid lubricant, the best known solid lubricants being graphite and molybdenum disulphide. These substances are layer-lattice solids of high melting point, in which the bonding between atoms in an individual layer is due to strong covalent or ionic forces, whereas the forces binding adjacent layers to one another are relatively weak (the so-called van der Waal's forces). As a result, there is easy sliding of one layer over another, and graphite and molybdenum disulphide are thus lubricants in their own right.They are, however, usually used in conjunction with a lubricant liquid, being sometimes dispersed therein at low concentration so that the mixture is fluid at ambient temperature and sometimes dispersed under conditions such that a grease or paste non-fluid at ambient temperature is formed.

The present invention provides lubricant material utilising as an essential ingredient graphite which is exfoliated.

Exfoliated graphite is a well known material used in the manufacture of gaskets, packings and sealing materials, and is made from natural flake graphite or well ordered synthetic graphite by a process whose first step involves treatment of graphite with a strong oxidising agent. Thus when graphite is treated with sulphuric acid under strong oxidising conditions it swells slightly owing to the interposition, between the layers of hexagonally arranged carbon atoms constituting the graphite lattice, of bisulphate ions HSO4- and neutral (unionised) sulphuric acid molecules H2 SO4. If the slighly swollen material is washed with a large volume of water, the intercalated neutral sulphuric acid molecules become replaced by water molecules, but the HSO4- ions are largely unaffected.When this compound is freed from adherent wash water and heated for a few seconds at temperatures of the order of 1 ,000 C, the interstitial water suddenly vapourises and causes a great expansion of the compound in the c direction (i.e. perpendicular to the plane of the carbon atom layers), so that the final c dimension may be 100 times or even more its initial value.

Owing to the suddenness of expansion, particles of the expanded substantially sulphate-free product do not possess the highly ordered structure of particles of the original graphite; they are of an irregular form which is usually described as vermiform (worm-like), and are extremely porous. They are also distinct from particles of the oxidised graphite material known as 'graphitic oxide' or 'graphitic acid'.

The present invention has arisen from our discovery that the porous particles of exfoliated graphite are capable of absorbing water-insoluble lubricant liquid to form material which is lubricant but which is much less plastic (and so much less messy to use) than most greases, and much less plastic than would be expected from the weight proportion of absorbed liquid which it contains.

The porous particles are also able to absorb a liquefied lubricant solid-paraffin wax, for example-of melting point or softening point below 2500C to form useful lubricant material.

Surprisingly, with these composite materials comprising exfoliated graphite particles having water-insoluble liquid or low melting or low softening point solid lubricant absorbed into their pores, the absorbed lubricant is not lost as liquid when the temperature is raised. The lubricant stays absorbed until a temperature is reached at which it is lost volatilisation; and the composite lubricant material does not markedly change its consistency on heating and keeps its shape without slumping (collapsing). The lubricant materials of the invention are accordingly not liquefiable by heat.However, when (whether at elevated temperature or not) the material is subjected to high compression or attrition-as in a bearing to be lubricated- it is mechanically broken down into smaller particles and releases from its pores much of the liquid or low melting solid lubricant which it contains, and that released lubricant forms a lubricant paste with the debris of graphite particles (themselves having lubricant properties) formed on breakdown. Although the lubricant paste formed on breakdown has a consistency much closer to that of ordinary greases than the composite lubricant material from which it is derived, it nevertheless tends to stay in situ and not run out of a bearing at elevated temperature e.g. 1 000C.

By contrast with conventional graphite pastes, which contain very fine particles of unexfoliated graphite which readily soil skin and clothing on contact, the lubricant materials of the invention are relatively clean, and on manual application to bearing surfaces they transfer only a thin film of oil or semi-solid wax or the like to the hands.

Lubricant material according to the invention may contain a proportion of absorbed lubricant forming more than 90% by weight, with less than 10% by weight of exfoliated graphite. However, materials with much lower proportions (e.g. 1:1 or less) of absorbed lubricant are readily made, though it will usually be desired to have a weight proportion of lubricant oil or solid:exfoliated graphite of at least 3:1. For maximum absorption, the bulk density of the exfoliated graphite particles employed should be less than 10 kg/m3, and preferably less than 5 kg/m3, though a considerably higher bulk density (e.g. 100 kg/m3) can be used. The composite material formed by absorption preferably has a bulk density not less than 250 kg/m3.

Preferred materials for absorption are those water-insoluble lubricant liquids which have a viscosity in the range 25-1 ,000 centistokes at 400C, and particularly 100-650 centistokes.

vegetable oils, organic acid diester oils, and silicone oils. Suitable lubricant solids for absorption are petroleum waxes, and animal and vegetable fats. It may be convenient to use as the material for absorption a conventionally gelled hydrocarbon or other grease.

In general the lubricant materials of the invention can be made simply by mixing the exfoliated graphite with the liquid or heatliquefied lubricant substance to be absorbed by it, following enough time for absorption into the pores to take place. A gentle tumbling action is sufficient. Particularly with the less absorbent (i.e.

higher-density) forms of exfoliated graphite, the lubricant substance to be absorbed can be diluted with a volatile solvent to thin it, the solvent being subsequently evaporated from the product formed. Tumbling can also be performed while applying the oil or wax as a fine mist or spray.

To obtain higher density products, the product of simple mixing can be subjected to high shear, as in a Z-blade mixer or by a grinding action which falls short of unduly reducing the particle size of the exfoliated graphite.

Alternatively, the particles of exfoliated graphite can be lightly compacted into a coherent body of density up to about 100 kg/m3, preferably less than 50 kg/m3, and this coherent body can be contacted with the lubricant oil or other liquid (e.g. liquefied paraffin wax) to be absorbed. The product of this treatment can if desired then be subjected to high shear as described earlier to increase its density.

The invention is further illustrated by Examples 1 to 4 later, which utilise as starting material an exfoliated graphite which is conveniently prepared as follows:- Preparation of exfoliated graphite Natural flake graphite (10 gms; 99% carbon; 80% retained on a sieve of aperture 300 jtm diameter) was gently stirred for one hour with a mixture (90 gms; added at room temperature) of sulphuric acid, nitric acid and water, in weight proportion such that H2SO4: HN03: H,0=74:15:11.

The reaction mixture was poured onto a sintered glass plate filter, and the graphite compound formed by reaction (probably C24.+HSO4#nH2S04) remained on the filter plate. It was washed substantially free from the molecular species H2SO4 by running water at room temperature (200C) for one hour. The fully washed material was drained from adherent water and dried at 800C for 3 hours, and then conventionally flame exfoliated (temperature 1 ,0000C) to volatilise the bound water content.

The exfoliated product has a density of about 4 kg/m3 (0.004 gm/cm3).

Example 1 Exfoliated graphite (10 gms: bulk density 4 kg/cm3) was tumbled for 5 minutes with Mobil SHC lubricating oil (200 gms: boiling point

centistokes at 400C) in a tumbler mixer. The loose, crumby tumbled material (density=250~ 300 kg/m3) was then subjected to high shear by grinding in a large pestle and mortar to form a stiff paste of unchanged composition (95% oil and 5% exfoliated graphite) but of increased density (735 kg/m3). A sample of the paste was put in an oven, whose temperature was then raised from 200C to above 2000C. The paste did not slump (collapse) and was not liquefied by heat i.e. on heating it did not lose oil in liquid form. The oil began to volatilise rapidly at about 2500 C.

A second sample of the stiff paste was put on a metal plate, which was then rapidly heated by a bunsen burner to a temperature at which the oil evaporating from it began to burn. Again no oil was lost by liquefaction, and no slumping of the paste occurred.

Both the loose crumby material and the stiff paste formed from it by shear are useful lubricants for stainless steel moving parts operating at temperatures above 1 0O0C.

Example 2 Exfoliated graphite of bulk density 4 kg/m3 was lightly compacted in a mould to form a slab 20 mm thick of density 18 kg/m3. A 10-gram chunk of the slab was put onto the surface of a pool of Mobil SHC lubricating oil and allowed to stand in contact with it for 24 hours. The product, which had substantially the shape and dimensions of the original chunk, consisted of 97% oil and 3% graphite, and had density 576 kg/m3. On being heated to the volatilisation temperature (about 2500C) of the oil, the product did not collapse nor did it lose oil in liquid form.

On being subjected to high shear in a Z-blade mixer it formed a stiff paste of unchanged composition with an increased density, which again did not lose oil as liquid or collapse on heating.

Example 3 Exfoliated graphite (10 gms) of bulk density 4 kg/m3 was tumbled with the conventional engine grease sold under the name "Mobilplex 47' (190 gms) which had been heated to 1 200C, at which temperature it had the consistency of butter on a hot day. The tumbled material (density=400~ 450 kg/m3) was then subjected to high shear to form a stiff, non-melting non-slumping paste (95% grease, 5% graphite) of density 700 kg/m3 which had a stiffer and more coherent structure than the paste of Example 1.

Example 4 A 10-gm chunk of exfoliated graphite slab (density 18 kg/m3) made as in Example 2 was allowed to stand in a pool of paraffin wax which had been heated to 1 000C i.e. well above the melting point. After 4 hours all the molten wax had been absorbed into the exfoliated graphite.

The rigid material which remained on cooling (95% wax, 5% graphite: density 757 kg/m3) did not lose its wax content by liquefaction when heated to 2500C, although the wax began to volatilise at that temperature.

A portion of the wax/graphite material was subjected to high shear at 500C to form a plastic paste which stiffened on cooling to room temperature. When reheated above 500C the stiff paste did not lose its wax by liquefaction, nor did it collapse.

Claims (11)

Claims

1. Lubricant material comprising exfoliated graphite particles whose pores contain a waterinsoluble lubricant liquid of boiling point not less than 1 500C or lubricant solid melting or softening point below 250 C.

2. Lubricant material according to Claim 1 in which the weight proportion of lubricant oil or solid:exfoliated graphite is at least 3:1.

3. Lubricant material according to Claim 1, in which the weight proportion of lubricant oil or solid:exfoliated graphite is at least 9:1.

4. Lubricant material according to any one of Claims 1 to 3, of bulk density not less than 250 kg/m3.

5. Lubricant material according to any one of Claims 1 to 4 in which a lubricant oil is employed which has at 400C a viscosity in the range 251,000 centistokes.

6. Lubricant material according to Claim 5, in which said viscosity is in the range 100-650 centistokes.

7. Lubricant material according to any preceding claim in which said lubricant liquid or solid comprises a hydrocarbon.

8. Lubricant material substantially as herein described with reference to the Examples.

9. A method of making lubricant material comprising bringing exfoliated graphite particles into contact with a water-insoluble lubricant liquid of boiling point not less than 1 500C or a liquefied water-insoluble lubricant solid of melting point or softening point below about 25O0C, under conditions such that the lubricant liquid or lubricant solid becomes absorbed into the pores of the exfoliated graphite in an amount at least equal to that of the exfoliated graphite.

10. A method of making lubricant material in which a coherent body of exfoliated graphite of density not greater than about 100 kg/m3 is brought into contact with a water-insoluble lubricant liquid of boiling point less than 1 500C or with a liquefied water-insoluble lubricant solid of melting point or softening point below 25O0C, under conditions such that the lubricant liquid or lubricant solid becomes absorbed into the pores of the exfoliated graphite in an amount at least equal to that of the exfoliated graphite.

11. A method of making lubricant material in which the product of Claim 9 or 10 is subjected to high shear to increase its density.