DE69033147T2 - NON-POLLUTANT SOLID LUBRICANTS - Google Patents

Description

The invention relates to non-staining solid lubricants, which are suitable for use in metalworking, e.g. in cold rolling, stretching or extruding of. metals, in particular aluminum.

Lubricants, usually liquid lubricants, are used in metalworking to reduce friction between the metal surface being machined and a surface of the fixture performing the machining. A liquid lubricant reduces friction by separating the two surfaces with a thin fluid film having a low shear resistance. In many metalworking operations, the pressure between a surface of the material being machined and a surface of the fixture is so great that the fluid film of a liquid lubricant can be forced out, causing metal-to-metal contact with the result that excessive damage to the surfaces can occur. Solid film lubricants with much greater bearing properties have been developed to overcome this and other disadvantages associated with liquid lubricants.

In addition to the sliding properties, a lubricant should meet certain other requirements if it is to be industrially useful. For example, it should be easy to apply and easy to remove, it should provide a certain degree of protection for the metal surface during handling and storage it should not present a health hazard to persons who come into contact with it and, of course, it should be inert with respect to the surfaces with which it comes into contact. Many lubricants cause severe staining of the metal surface during annealing. It is therefore highly desirable to prevent such staining by using a lubricant with properties suited to the particular conditions under which the lubricated metal is to be worked and which is also non-staining.

The invention is based on the discovery that a solid film of one or more saturated aliphatic monoalcohols having 14 to 18 carbon atoms applied to the surface of the metal to be worked has good sliding properties under cold working conditions and can be removed by heating, leaving essentially no color on the metal surface.

The invention provides a method for cold working a metal workpiece comprising providing a lubricant film on the surface of the workpiece to be subjected to forming, subjecting the workpiece to forming and removing the lubricant film from the metal surface, characterized in that the lubricant film comprises a solid film of a saturated aliphatic monoalcohol having 14 to 18 carbon atoms and that the lubricant film is removed from the metal surface without staining by the application of heat thereto.

The invention further provides the use of a saturated aliphatic monoalcohol having 14 to 18 carbon atoms as a solid lubricant in the cold working of a metal workpiece which is the surface of the metal by heating without staining. The invention also provides the use of a composition comprising a solution of a saturated aliphatic monoalcohol having 14 to 18 carbon atoms in an inert volatile organic solid as a lubricant film forming agent in the cold working of a metal workpiece to form a film of a solid saturated C14-18 aliphatic monoalcohol lubricant on the workpiece surface which is removed from the metal surface by heating without staining.

According to the invention, a saturated aliphatic monoalcohol having 14 to 18 carbon atoms is used. Such alcohols have good slip and support properties, are solid at room temperature (20 to 25ºC) and are at worst only slightly staining and at best absolutely non-staining. Examples of these alcohols include myristyl alcohol, cetyl alcohol and stearyl alcohol. Commercially pure compounds can be used and these generally contain proportions of higher and/or lower homologues as impurities. Cetyl alcohol (hexadecanol) is particularly preferred for use according to the invention due to its melting point (~ 49ºC) and its excellent non-staining property.

Typically, the saturated aliphatic C₁₄₋₁₈ monoalcohol lubricant is applied to the surface of the metal workpiece by dip coating the workpiece in a solution of the alcohol lubricant in an inert volatile organic solvent. After removal of the volatile solvent by evaporation, a film of the solid alcohol remains on the surface of the workpiece. It is possible that the film obtained in this way is not uniform in thickness, i.e., the deposited alcohol lubricant is thicker in some areas than in other areas of the workpiece surface. However, we have observed that in such cases, when the thickness of the deposited film of solid alcohol is not uniform, the film flows at the point where pressure is applied to the workpiece during its processing, e.g. at the nozzle in metal stretch processing or at the dies in metal press processing, resulting in the workpiece being evenly covered at the processing point.

To form the lubricant coating composition, the saturated aliphatic C14-18 monoalcohol is dissolved in an inert, volatile organic solvent. In general, any organic solvent for the alcohol which is liquid at normal room temperature, but which evaporates at a temperature just above that, and which is inert to both the alcohol and the metal surface can be used. Examples of suitable solvents include diethyl ether, methanol and benzene, and chlorinated hydrocarbons such as trichloroethylene and 1,1,1-trichloroethane. The concentration of the C14-18 alcohol in the volatile organic solvent is not critical, and in principle any concentration up to the solubility limit of the corresponding solvent can be used. Typically, the concentration of the alcohol in the organic solvent will be about 10 wt%.

According to the invention, it is possible to incorporate certain amounts of other substances into the lubricating film of the saturated, aliphatic C₁₄₋₁₈-monoalcohol in order to modify the properties of the film. For example, liquid polybutene can be incorporated in an amount of up to 10% by weight of the total weight of the lubricating composition (without Solvent) may be incorporated to modify the film properties of the lubricant on the surface of the workpiece and the flow properties of the lubricant film during deformation without causing staining of the metal surface upon annealing. The use of liquid polybutene tends to soften any film containing this and the saturated C₁₄₋₁₈ aliphatic monoalcohol and therefore, if more than 10% by weight of liquid polybutene is used, the lubricant film obtained on the surface of the workpiece may become slightly sticky and quite unstable. Any liquid polybutene may be used in accordance with this embodiment of the invention. Liquid polybutenes are commercially available under the name "Hyvis" (Hyvis is a registered trademark), e.g. B. Hyvis 200, with a molecular weight (number average) of 2400 and a kinematic viscosity of 4300 cSt at 98.9ºC.

The non-staining solid lubricant of the invention may contain a minor amount of a solid or semi-liquid (e.g. gelatinous) alkane or a mixture of alkanes which do not leave a staining residue on ignition, e.g. petroleum jelly (petrolatum), to modify the film properties of the lubricant. Such an alkane-based film modifier may be used together with, or alternatively to, the liquid polybutene mentioned above. Generally, if used, any alkane-based film modifier is used in an amount such that the total weight of the alkane-based film modifier and the liquid polybutene is not more than 10% by weight of the total lubricant composition (excluding solvent). We have further found that the incorporation of a (liquid) ester, e.g. B. Butyl stearate, methyl laurate or methyl myristate, in the saturated, aliphatic C₁₄₋₁₈-monoalcohol lubricant film usually helps to produce a uniform film and usually also softens the film. This tendency to soften the film is an additional advantage in the case where the lubricant also contains a polybutene additive, as this makes it easier to remove the polybutene-containing lubricant from surfaces (including hands). If used, the ester can be used in an amount not exceeding 10% by weight based on the total weight of the lubricant (excluding solvent) and would be incorporated as an alternative to an equivalent amount of liquid polybutene. However, we prefer not to use more than about 1% by weight of the ester.

The solid lubricant forms a tough film on the surface of a metal workpiece after evaporation of the volatile carrier solvent. A typical lubricant coverage would be less than about 10 g/m². Preferably, however, the lubricant solution is applied to a metal surface to provide about 5 g of solid lubricant per square meter of surface area.

After deformation etc. of the workpiece, the solid film lubricant does not need to be removed before annealing the metal, since the solid lubricant component(s), in other words the alcohol and optionally present polybutene, petroleum jelly and/or butyl stearate, will evaporate or decompose cleanly on the metal surface without leaving any stains.

EXAMPLE 1

An aluminum tube was immersed in a 10% (w/w) solution of cetyl alcohol in trichloroethylene and then dried. The coated tube was passed through a stretching machine four times. After each pass, the tube was annealed by heating to 400ºC without prior degreasing and without staining and then again immersed in the cetyl alcohol solution. Table 1 below shows the outside and inside diameters of the tube, the percent reduction in cross-sectional area and the length of the tube after each pass. The reductions are due to the size of the nozzle used, which is the reason why they vary. However, the final reduction of 23% is excellent and the tube showed no surface coloration after the last pass. TABLE 1

EXAMPLE 2

A series of disc compression tests were conducted using different lubricants. In In each case, an aluminum disk with a diameter of 32 mm and a thickness of 5 mm was immersed in a solution of the test lubricant in a volatile organic solvent so that both the upper and lower surfaces of the disk were coated with the lubricant solution. The coated disk was then allowed to stand until the organic solvent evaporated so that a coating of the lubricant remained on the aluminum surface. Each disk was subjected to pressing at 5.6 x 10-7 kg/m-2 for a period of 30 seconds and the thickness of the disk was then measured. The test was carried out five times for each lubricant and the average reduction (%) was calculated. Each lubricated disk was annealed at 400°C and checked for staining. The results are shown in Table 2 below. TABLE 2

* "Batoyl" and "Silkolene" are solid film lubricants based on a heavy oil containing extreme pressure additives

Claims (14)

1. A method for cold working a metal workpiece, comprising providing a lubricant film on the surface of the workpiece to be subjected to forming, subjecting the workpiece to forming and removing the lubricant film from the metal surface, characterized in that the lubricant film comprises a solid film of a saturated aliphatic monoalcohol having 14 to 18 carbon atoms and that the lubricant film is removed from the metal surface without staining by the application of heat thereto. 2. A method according to claim 1, wherein the solid lubricant film on the metal surface is caused to flow at a point where pressure is applied due to the deformation. 3. A process according to either claim 1 or claim 2, wherein the saturated aliphatic monoalcohol is cetyl alcohol. 4. Process according to one of claims 1 to 3, wherein the lubricant further contains 1 to 10% by weight, based on the total weight of the lubricant, of polybutene, petroleum jelly or butyl stearate. 5. Use of a saturated aliphatic monoalcohol having 14 to 18 carbon atoms as A solid lubricant used in the cold machining of a metal workpiece which is removed from the surface of the metal by heating without leaving stains. 6. Use according to claim 5, wherein the saturated aliphatic monoalcohol is cetyl alcohol. 7. Use according to either claim 5 or claim 6, wherein the lubricant also contains 1 to 10% by weight, based on the total weight of the lubricant, of polybutene, petroleum jelly or butyl stearate. 8. Use of a composition comprising a solution of a saturated aliphatic monoalcohol having from 14 to 18 carbon atoms in an inert volatile organic solid as a lubricant film forming agent in the cold working of a metal workpiece to form a film of a solid C 14-18 saturated aliphatic monoalcohol lubricant on the workpiece surface which is removed from the metal surface by heating without staining. 9. Use according to claim 8, wherein the saturated aliphatic monoalcohol is cetyl alcohol. 10. Use according to either claim 8 or claim 9, wherein the inert volatile organic solvent is selected from trichloroethylene and 1,1,1-trichloroethane. 11. Use according to one of claims 8 to 10, wherein the solution contains about 10% by weight of the saturated aliphatic monoalcohol. 12. Use according to any one of claims 8 to 11, wherein the solution additionally contains a material selected from polybutene, petroleum jelly and butyl stearate in an amount of 1 to 10% by weight based on the combined weight of the saturated aliphatic monoalcohol and this material. 13. Use according to one of claims 5 to 12, wherein the metal workpiece is formed from aluminum or an aluminum alloy. 14. Use according to one of claims 5 to 13, wherein the metal workpiece in the form of a tube is selected for a cold drawing process.