Patent Grant
7754664
Not applicable.
The invention pertains to ionic liquid-based lubricants and ionic liquid lubricant additives.
Room-temperature ionic liquids (ILs) have been primarily used as “green” solvents in chemical synthesis, electrochemistry, and other applications due to their ultra-low vapor pressure, non-flammability, and high thermal stability. Ionic liquids, as the name indicates, are composed solely of ions. The most commonly studied ionic liquids are those where the cation is one of the following: 1-alkyl-3-methylimidazolium, N-alkylpyridinium, tetraalkylammonium and tetraalkylphosphonium. The organic cations, which are generally relatively large compared with simple inorganic cations, account for the low melting points of the salts. Anions range from simple inorganic anions to large complex anions. The synthesis process is environmentally friendly and does not involve high pressures (usually ambient air) or high temperatures (usually 60-80° C.).
Ionic liquids have other possible features that make them attractive for tribological applications, including negligible volatility, non-flammability, high thermal stability, and better intrinsic performance. These characteristics may avoid the need to add expensive additives to facilitate lubrication, as in the case of conventional mineral-oil-based lubricants. Detergents may not be necessary because ionic liquids act as solvents, defoamers may not be necessary due to ionic liquids' ultra-low vapor pressure, anti-oxidants may not be necessary due to the high thermal stability of ionic liquids, and anti-wear additives may not be necessary if ionic liquids form boundary lubricating films.
Limited studies have shown the potential for using ionic liquids as a new class of lubricants. Friction and wear reduction have been reported on metallic and ceramic surfaces lubricated by selected ionic liquids. It has been suggested that adding a few percent of ionic liquids could improve the lubricating behavior of base oils. Most literature studies have been limited to ionic liquids with imidazolium cations and BF4− and PF6− anions.
Ammonium salts of partial esters of phosphoric and thiophosphoric acids are commercially available as extreme pressure and antiwear additives for lubricants and are disclosed in Sieberth, U.S. Pat. No. 5,464,549 and Norman et al., U.S. Pat. No. 5,942,470. Other patents disclosing ammonium salts of other large anions for lubricants include Nebzydoski, U.S. Pat. No. 3,951,973, Baldwin et al., U.S. Pat. No. 4,115,286, and Wildersohn et al., U.S. Pat. No. 4,950,414 where the anions are trithiocyanurate, bis[(mercaptohydrocarbyl)ethylenedioxy]borates, and cyclophosphetane derivatives, respectively. However, it remains a goal to identify ionic liquids that display superior lubricating properties as the primary lubricant or as lubricant additives, particularly for use with difficult-to-lubricate metals, like aluminum, and for ceramics. Ionic lubricants with anions that permit superior thermal stabilility and low water solubility are also desirable for lubricants and lubricant additives.
An object of the invention is a lubricant that is an ionic liquid alkylammonium salt where the alkylamonium salt has the formula:
RxNH(4-x)+,[F3C(CF2)yS(O)2]2N− where x is 1 to 3, R is independently C1 to C12 straight chain alkyl, branched chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, cycloalkyl substituted alkyl, or when x is greater than 1, two R groups comprise a cyclic structure including the nitrogen atom and 4 to 12 carbon atoms, and y is independently 0 to 11. The alkylammonium salt can have a water solubility less than 100 mM and can have an onset of thermal decomposition temperature that is greater than 330° C.
A lubricant additive mixture is a mineral or hydrocarbon oil and an ionic liquid alkylammonium salt where the alkylamonium salt has the formula RxNH(4-x)+, [F3C(CF2)yS(O)2]2N− where x is 1 to 3, R is independently C1 to C12 straight chain alkyl, branched chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, cycloalkyl substituted alkyl, or when x is greater than 1, two R groups comprise a cyclic structure including the nitrogen atom and 4 to 12 carbon atoms, and y is independently 0 to 11.
A method of lubrication includes the steps of providing an ionic liquid alkylammonium salt of the formula: RxNH(4-x)+, [F3C(CF2)yS(O)2]2N− where x is 1 to 3, R is independently C1 to C12 straight chain alkyl, branched chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, cycloalkyl substituted alkyl, or when x is greater than 1, two R groups comprise a cyclic structure including the nitrogen atom and 4 to 12 carbon atoms, and y is independently 0 to 11; and supplying the ammonium salt to a contact between a first surface and second surface. The first and second surfaces can be independently a metal surface or a ceramic surface. The metal surface can be an aluminum alloy or a steel alloy. The method of lubrication can also include providing a mineral oil or a hydrocarbon oil and supplying the oil to the contact between the surfaces.
Ionic liquids that are easily prepared and useful as lubricants and property-enhancing additives to lubricants comprise ammonium salts of primary, secondary, and tertiary amines with a bis(perfluoroalkanesulfonyl)imide anion. These ammonium salts display good viscosities for lubricating surfaces at high and low temperatures. These salts display good thermal stability relative to mineral oil and conventional motor oils where the ionic liquid displays an onset of decomposition that is greater than about 330° C. These salts display a low affinity for water where the solubility of the ionic liquid in water is less than 100 mM. The melting points of the salts are low, generally below 25° C. The structure of these ionic liquid ammonium salts are given by the structure:
RxNH(4-x)+, [F3C(CF2)yS(O)2]2N−
where x is 1 to 3, R is independently C1 to C12 straight chain alkyl, branched chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, cycloalkyl substituted alkyl, or, when x is greater than 1, two R groups comprise a cyclic structure including the nitrogen atom and 4 to 12 carbon atoms, and y is independently 0 to 11. Preferred anions are [F3CS(O)2]2N− and [F3CCF2S(O)2]2N−, which are alternately indicated as Tf2N and BETI, respectively, in the following text, tables, and figures.
The ammonium salts can be readily prepared from the appropriate organic amine, RxNH(3-x), where R and x are as defined above for the ammonium salts. The amine is mixed with an equal molar quantity of lithium bis(perfluoroalkanesulfonyl)imide, Li+[F3C(CF2)yS(O)2]2N− at room temperature. The addition of a small molar excess of aqueous HCl solution results in the exothermic formation of the desired ammonium salt and lithium chloride as a two layer system. The ammonium salt ionic liquid lower layer is subsequently separated from the top aqueous layer. Multiple washings with deionized water removes LiCl and excess HCl from the ammonium salt ionic liquid. The ammonium salt can be dried by heating under vacuum, for example heating to 70° C. under vacuum for 4 hours.
Properties important for lubricants include the viscosity and the change in viscosity with temperature. The viscosities and densities of representative ammonium salts of the present invention are given in Table 1 below. For comparison, the properties of mineral oil and 10W40 diesel engine oil are included in Table 1. As can be seen in Table 1, the density of the ionic liquid is greater than that of the oils, primarily due to the presence of the multiple fluorine atoms of the anion in the ionic liquids. To permit comparison to standard viscosities for motor oils, the table gives the viscosity in cp at three temperatures (23, 40, and 100° C.) and the viscosity in cst at 100° C. The viscosities of the ionic liquids at 100° C. are all below the maximum of 26.1 cst for an SAE 60 motor oil and all are below the SAE 50 maximum of about 280 cp at 40° C.
The water solubility of these ionic liquids is low, as is illustrated by the measured data in Table 2 below. The solubility of the ammonium salts in water is less than 100 mM in all cases. Conversely, the solubility of water in the ionic liquid is low, where the ppm water in the wet ionic liquid is on average lower than the value equivalent to one molecule of water per equivalent of ammonium salt. The solubility of water in the ionic liquids is low in spite of the ion-dipole and hydrogen bonding interactions that are possible between ammonium salts and water. The dried ionic liquids have an average equivalency of 0.02 equivalents of water per equivalent of ammonium salt. The data suggests that the size of the anion has a more pronounced affect on the solubility of the salt in water and the solubility of water in the salt than does the number of hydrogen atoms attached to the nitrogen of the ammonium salt. Although these values are very low for wet salts, they are still about 50 times greater than those of typical lubricating oils. However the dried ionic liquids have a water content similar to wet hydrocarbon oils and this capacity to accept water can allow the ionic liquid to be used as a lubricant without the addition of a detergent additive. Alternately the ionic liquids can be used as lubricant additives in lubricant formulations where a traditional detergent additive is not required.
A feature of these ionic liquids is their superior thermal stability. As seen in Table 3 below, and as illustrated by the thermal gravimetric trace of
Wettability, is another important lubricant requirement, indicating how well a lubricant can cover (wet) a solid surface. The contact angles of 15W40 oil, a representative ammonium-based ionic liquid ([C8H17]3NH,Tf2N), and distilled water are shown in
The lubricating properties of the ammonium salt are very good, and particularly good for use on aluminum surfaces. Two conventional hydrocarbon oils (mineral oil and 15W40 diesel engine oil) and five ionic liquids were examined to determine whether they would effectively lubricate a steel-aluminum contact. Experiments were carried out on a high frequency reciprocating sliding tester (Plint TE-77) with a 6.35 mm diameter AISI 52100 bearing steel ball sliding against an Al 6061-T6 flat in the ambient environment under 38.3 N load, 5 Hz frequency, and 10 mm stroke.
The measured friction coefficients of the oils and ionic liquids and their viscosities are shown in
The ionic liquid, [C8H17]3NH,Tf2N, compared favorably with fully formulated 15W40 engine oil in different lubrication regimes. Tests were conducted on a pin-on-disk unidirectional sliding configuration. A 6.35 mm diameter 52100 steel ball with a flattened tip (contact area d=2 mm) was rubbing against an Al 6061-T6 disk under a fixed load (38.3N). Stribeck curves were constructed from data collected by varying the sliding speed from high to low (1-0.2 m/s) to obtain the friction behavior at elastohydrodynamic (EHL), mixed, and boundary lubrication (BL), as shown in
Wear studies on steel-aluminum contact using conventional oils and ionic lubricants were carried out on a high frequency reciprocating sliding tester (Plint TE-77) with a 6.35 mm diameter 52100 steel ball rubbed against three aluminum alloy flats (Al 1100, Al 6061-T6, and cast Al 319) under 100 N load, 10 Hz frequency, and 10 mm stroke, for 400 meters sliding distance. The friction forces were measured and wear volumes on the aluminum flats were post-test quantified by wear scar profilometry.
The wear volumes on the three aluminum flats are given in
The ammonium salt ionic liquid, [C8H17]3NH,Tf2N, produces significantly less wear compared to the fully formulated 15W40 engine oil for aluminum-steel contact.
A mixture of 90 vol % mineral oil and 10 vol % [C8H17]3NH,Tf2N produces some wear reduction relative to that of 100 vol % mineral oil when used to lubricate steel-aluminum contact, as shown in
Although ionic liquids, have a potential to react with metal surfaces, little corrosivity information is available on ionic liquids. Electrochemical measurements have been carried out to investigate the corrosivity of [C8H17]3NH,Tf2N on steel and aluminum alloys.
The open circuit measurements indicated that the steel and aluminum alloys stabilize relatively quickly in [C8H17]3NH,Tf2N as shown in
It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.
Invention developed under contract DE-AC05-00OR22725 with the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3951973 | Nebzydoski | Apr 1976 | A |
| 4115286 | Baldwin et al. | Sep 1978 | A |
| 4950414 | Wildersohn et al. | Aug 1990 | A |
| 5464549 | Sieberth | Nov 1995 | A |
| 5942470 | Norman et al. | Aug 1999 | A |
| 20020016266 | Fletschinger et al. | Feb 2002 | A1 |
| 20040033366 | Lamanna et al. | Feb 2004 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20080070817 A1 | Mar 2008 | US |
