Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Organic -co- compound
Reexamination Certificate
1999-05-03
2001-08-21
Howard, Jacqueline V. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Organic -co- compound
C072S042000
Reexamination Certificate
active
06277795
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a metal wire or a metal strip having a lubricating surface layer, a method for its preparation as well as a new use of a mixture of a fatty acid and its alkali metal soap.
Drawn wire or drawn strip is generally subjected to a mechanical processing after the drawing. Such processing calls for low friction. This problem is often solved by the addition of lubricants such as mineral or synthetic oils and cutting fluids during the processing.
However it is desirable to coat the wire/strip with a lubricating surface layer already in the delivery state, i.e. immediately after drawing (and possibly annealing), in order to facilitate the handling. Some methods therefore are already in use. One such method is the coating with a wax layer where the wax is mixed with a solvent, e.g., petrol. Another method comprises coating with silicones which like-wise is carried out by means of solvents. The problem caused by these methods relates to the working environment due to the handling of solvents during coating. Another disadvantage is that the wax must be washed away before the use of the wire/strip detergents such as trichloroethylene or alkaline detergents (generally in combination with ultrasonic treatment) is required. Due to the general desire to keep away from the use of organic solvents and compounds containing chlorine, it is thus desirable for the application of a lubricating surface layer on metal wire and metal strips to be able to use a water-soluble system having minimal influence upon the environment both during the application of the layer and its possible removal before the use of the wire/strip.
A further disadvantage of a lubricating surface layer of wax is the difficulty to achieve complete covering and an even thickness of the layer.
The manufacture of coils puts high demands on lubricating in order to give a uniform pitch and diameter as well as to avoid rips and in addition to give high productivity. For that reason the manufacturer of coils often uses different types of lubricants during the winding of the spiral in spite of the wax layer. However this should be avoided because, for instance, commercial lubricants based on mineral oil often contain aggressive additives which can be difficult to wash away. During use, the remaining residues of lubricant or additives attack the material of the wire and damage the wire in different ways.
A special case within the field of coil manufacturing is the use of resistance alloys, e.g. iron-chromium-aluminium, nickel-chromium(-iron) and copper-nickel, for the manufacture of heating coils for electrical elements. In this case extra high demands are put on uniform coils, high productivity and lenient treatment of the wire surface. Moreover high demands are put on the properties of the lubricant relating to chemical compatibility with the wire material before the use at transport and storing on one hand, and during the use on the other if the lubricant fully or partly remains on the surface.
It is known from U.S. Pat. No. 3,098,294 to coat a draw piece with a sodium soap of a fatty acid as lubricant in cold drawing a small article (specifically a cartridge case) from a draw piece made of brass or mild steel. Among the soaps investigated sodium behenate was found to give the best results whereas sodium laurate and sodium oleate gave the poorest results as regards the force required in the cold drawing which force in case of the two last mentioned soaps becomes higher than that of a comparative mixture of several fatty acid soaps.
SUMMARY OF THE INVENTION
According to the present invention it has now surprisingly been found that the surface layer of a mixture of a fatty acid and an alkali metal soap of the fatty acid gives a substantially better effect as regards the lubricating characteristics and load carrying capacity than a layer of the pure alkali metal soap of the fatty acid and moreover leads to the removal of the disadvantages described above in connection with previously known surface layers for drawn wire or drawn ribbons.
In accordance therewith the invention according to an aspect thereof relates to a metal wire or a metal strip having a lubricating surface layer which wire or which strip is characterized in that the surface layer consists of a mixture of a fatty acid having 10-22, preferably 10-18, carbon atoms and an alkali metal soap of said fatty acid.
DETAILED DESCRIPTION
The terms “alkali metal” and “alkali” is intended in connection with the present invention to relate to sodium and potassium in the first place. The most preferred metal of these is sodium.
The fatty acid is preferably a straight saturated fatty acid but also unsaturated fatty acids such as oleic acid has appeared to give an acceptable effect. The most preferred combination of fatty acid and soap according to the invention is a mixture of lauric acid and sodium laurate.
The invention is advantageously applied to a wire or a strip which constitutes an electric resistance wire or an electric resistance strip, respectively. Such a wire and such a strip, respectively, can for instance consist of a Fe—Cr—Al, Cu—Ni, or Ni—Cr—Fe alloy and can be intended, for instance, for the manufacture of tubular elements or heating coils or electrical elements that can be processed for instance by spiral winding, cutting, punching or bending or another plastic deformation process.
According to another aspect of the invention the invention, relates to a method of preparing a metal wire or a metal strip having a lubricating surface layer, which method is characterized in contacting the wire/strip, preferably after cleaning, with a mixture of a fatty acid having 10-22, preferably 10-18, carbon atoms and an alkali metal soap of said fatty acid dissolved in water to the formation of a surface layer comprising the mixture on the surface of the wire/strip, which surface layer is allowed or caused to dry.
According to a preferred embodiment of the invention, the mixture of fatty acid and soap dissolved in water is one which has been prepared by dispersing the fatty acid in water and then adding an alkali metal hydroxide until no undissolved acid remains, but not all acid has been converted into soap.
The alkali metal hydroxide can be added in the form of a solid or as an aqueous solution which preferably is highly concentrated.
A suitable amount of alkali metal hydroxide added can easily be determined by means of pH measurement. Thus good results are generally obtained if the addition of alkali metal hydroxide is performed to a pH of the mixture of 6-9, preferably 7-8.
Higher fatty acids as well as soaps are surface active substances and form aggregates in solutions. The fatty acids are hydrofobic and form reversed miscelles when being dissolved in oil. The soaps are hydrophilic surface active substances which form normal miscelles in water. What type of aggregates which are formed by a surface active substance can be predicted by considering the balance between hydrophilic and hydrophobic moities of the molecule (HLB=“Hydrophile Lipophile Balance”). This is summarized in a Chritical Packing Parameter (CPP). [For further details, see for instance J Israelachvili, Coll Surf A, 91 (1994) 1-8.]
Starting from the fatty acid an increase in the pH-value gives a gradual conversion of the fatty acid to soap. During this conversion, the CPP-value of the mixture will move from about ⅓ to 3. The structure is changed from a micellar one at the CPP-value of about ⅓ via a hexagonal structure, then a cubic, a lamellar, a reversed cubic, and a reversed hexagonal structure to a reversed micellar structure. The lamellar phase is formed at a CPP-value of about 1 corresponding to a pH-value of about 7-8 and a HLB-value of about 10. In the lamellar phase layers of surface active substances are arranged in flakes which easily are sliding against each other and give a low friction. According to the present invention, this condition is the one to be aimed at to the highest possible extent even if certain divers
Bergman Magnus
Jonsson Bo
Birch & Stewart Kolasch & Birch, LLP
Howard Jacqueline V.
Kanthal AB
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