Abstract:
A lubricant formulation. The lubricant formulation includes a polyol-based base lubricant, an ionic liquid, and an organic nanoparticle. The ionic liquid is selected from the group consisting of trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate, and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, or a combination thereof. The organic nanoparticle has a median particle size less than about 200 nm. The organic nanoparticle forms about 0.01 to about 5% by weight of the lubricant formulation. The ionic liquid forms about 0.5 to about 10% by weight of the lubricant formulation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 62/037,438 filed Aug. 14, 2014, the entirety of which is incorporated by reference herein. 
     
    
     GOVERNMENT LICENSE RIGHTS 
       [0002]    This invention was made with government support under Contract No. W911 QX-13-C-0174. The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates to lubricant additives and formulations, and more particularly to lubricant additives and formulations for use in connection with rotorcraft transmission and gearbox systems, and other rotary platforms. 
       BACKGROUND 
       [0004]    Metal parts in close tolerances and contacts are a design feature of many electromechanical and mechanical devices. Lubricants maintain viscosity and protect components more effectively under the high shear stresses that these systems place on metal parts. The benefits of a well-lubricated system include an increase in the effective service life of the constituent parts of the system and the system as a whole, as well as enhanced fuel efficiency, which can lead to significant cost savings. In a typical engine set-up, 10-15% of the energy is lost due to friction. 
         [0005]    Certain systems, such as rotorcraft transmission systems and rotary platforms more generally, are frequently operated in extreme conditions which require the use of a high quality lubricant capable of carrying a high load. This is especially true in the context of transmission and gearbox systems for military and civilian rotorcraft, as the main gearbox is one of the most vulnerable portions of the rotorcraft. This is true even if redundant systems are employed to provide emergency lubrication systems, which add additional weight, complexity, and a risk of dormant failure. When such redundant systems fail, these failures cause both a dangerous situation as well as widespread inconvenience for both the operators of the rotorcraft as well as those served by the rotorcraft, such as offshore workers. In some countries, including the U.S., the use of a lubricant capable of supporting an aircraft in safe flight for at least 30 minutes after the crew has detected lubrication system failure or loss of lubrication is required for use in certain contexts. 
         [0006]    Accordingly, an improved lubrication system, deliverable through conventional service channels is therefore desirable. Ionic liquids have been known to enhance the lubricity of a system/material, for example as disclosed in U.S. Pat. Nos. 8,318,644 and 7,754,664, and the article “Ionic Liquids in Tribology” (Minami, Ichirio,  Molecules  14, no. 6 (2009): 2286-2305), each of which is incorporated by reference herein in its entirety. Due to the inherent polarity of ionic liquids, they adsorb strongly on the metallic tribocontact surfaces leading to a robust tribofilm when compared to conventional lubricants. However ionic liquids have an intrinsically high cost. Also, the use of some ionic liquids having halogens can also result in undesirable corrosion of metal surfaces having specific compositions. 
         [0007]    Metal nanoparticles have also emerged as an approach to advanced development for enhanced lubrication and heat transfer capability. For example, incorporating metal nanoparticles into the tribofilm can enhance rolling friction between the contact surfaces, thereby reducing wear. 
       SUMMARY 
       [0008]    In one aspect, an additive composition is disclosed. The additive composition includes an ionic liquid and an organic nanoparticle. 
         [0009]    In another aspect, a lubricant formulation is disclosed. The lubricant formulation includes a base lubricant, an ionic liquid, and an organic nanoparticle. 
         [0010]    In yet another aspect, a lubricant formulation is disclosed. The lubricant formulation includes a polyol-based base lubricant, an ionic liquid, and an organic nanoparticle. The ionic liquid is selected from the group consisting of trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate, and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, or a combination thereof. The organic nanoparticle has a median particle size less than about 200 nm. The organic nanoparticle forms about 0.01 to about 5% by weight of the lubricant formulation. The ionic liquid forms about 1 to about 10% by weight of the lubricant formulation. 
         [0011]    Other aspects of the disclosed additive composition and lubricant formulation will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings. 
           [0013]      FIG. 1  is a chart showing comparative friction coefficient profiles for embodiments of a lubricant formulation. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    For simplicity and illustrative purposes, the principles of the present invention are described by referring to various exemplary embodiments thereof. Although the preferred embodiments of the invention are particularly disclosed herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to and can be implemented in other systems, and that any such variation would be within such modifications that do not part from the scope of the present invention. Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of any particular arrangement shown, since the invention is capable of other embodiments. The terminology used herein is for the purpose of description and not of limitation. 
         [0015]    An additive composition for a base lubricant is disclosed including one or more ionic liquids and one or more organic nanoparticles. Lubricant formulations incorporating the disclosed additive provide enhanced performance in terms of wear protection of system parts, reduced coefficient of friction, lower electrical resistance, and longer oil-out run time as compared to the performance of the base lubricant alone, under identical operating conditions. 
         [0016]    The term “base lubricant,” as used herein, may refer to an unformulated lubricant or a fully-formulated lubricant with additives added thereto, including but not limited to commercially-available formulated and/or unformulated lubricants. 
         [0017]    The base lubricant may be any of a variety of base lubricants known in the art, or combinations thereof, including but not limited to base lubricants conventionally used in any of a variety of applications, including lubrication of engines and/or rotorcraft transmission and gearbox systems, such as natural or synthetic oils. In one embodiment, the base lubricant may be a polyol ester or a polyol-based lubricant including hindered polyol esters and any of a variety of additives, and it may be a commercially-available base lubricant approved for use under U.S. military specification DOD-L-85734. For example, the base lubricant may be AEROSHELL® Turbine Oil 555, which is commonly used in current rotorcraft systems. Other non-limiting examples of base lubricants include but are not limited to transmission oils such as Herco A (polyol ester, unformulated) and MOBIL SHC® 626 (formulated) and internal combustion engine oils such as mineral oil (unformulated) and MOBIL 1™ 5W-30 (formulated). 
         [0018]    The ionic liquid of the additive composition may be any of a variety of ionic liquids, or combinations thereof. The addition of an ionic liquid to the base lubricant appears to facilitate rapid formation of a protective tribocoating on metal surfaces of the system incorporating the lubricant. In one embodiment, the ionic liquid is a non-corrosive ionic liquid, such as a halogen-free ionic liquid, to reduce wear on system parts. The halogen-free nature of the ionic liquid reduces sensitivity for hydrolysis, which in turn reduces the incidence of corrosion. Ionic liquids are known in the art, and selection of a suitable ionic liquid may be based on factors such as lubricity and the ability to protect against corrosion. Under given test conditions gear steel (for example, AISI 9310 alloy steel) with the ionic liquid may have a coefficient of friction less than that of the base lubricant. In one non-limiting example (ball-on-disc test, Hertzian stress 800 MPa), the ionic liquid is trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, which yields a coefficient of friction of about 0.044 with AISI 9310 alloy steel, as compared to AEROSHELL® 555, which yields a coefficient of friction of about 0.057 with AISI 9310 alloy steel. 
         [0019]    Representative ionic liquids that may be used include phosphonium-based ionic liquids such as trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide. One of ordinary skill will appreciate that other ionic liquids known in the art, including non-corrosive ionic liquids, may be incorporated into the additive composition alone or in combination without departing from the scope of this disclosure. The additive composition may be incorporated into the lubricant formulation such that the ionic liquid is provided in the lubricant formulation in an amount of about 0.01-15% by weight, or various embodiments, about 0.01-1.0%, about 0.01-2.0%, about 0.01-3.0%, about 0.01-4.0%, about 0.01-5.0%, about 0.01-6.0%, about 0.01-7.0%, about 0.01-8.0%, about 0.01-9.0%, about 0.01-10.0%, about 0.5%-10.0%, about 1.0%-5.0%, about 1.0%-6.0%, about 1.0%-7.0%, about 1.0%-8.0%, about 1.0%-9.0%, about 1.0%-10.0%, about 1.0%-15.0%, about 2.0%-6.0%, about 3.0%-6.0%, about 2.0-10.0% by weight, about 4.0-6.0%, about 1%, about 2%, about 3%, about 4% about 5%, about 6%, about 7%, about 8%, about 9%, or about 10% by weight. 
         [0020]    The organic nanoparticles of the additive composition may be any of a variety of carbon-based or carbon-containing nanoparticles, or combinations of multiple varieties of nanoparticles, including but not limited to nanographene (including nanographene platelets), graphene oxide, carbon, carbon nanotubes (single, double, or multi-walled), carbon nanofibers, fullerenes, nanodots, nanopowders, nano-diamond and the like, in any of a variety of morphological configurations. Carbon nanoparticles are less expensive than metal nanoparticles of metals such as copper, silver, and gold, and carbon nanoparticles may be less toxic and safer to handle than metal-based nanoparticles. The organic nanoparticles may range in size from about 0.1 to 999 nm in median particle size, and in one embodiment no greater than about 200 nm in median particle size. The nanoparticles may include mesopores and/or micropores, which may improve buoyancy of the nanoparticles within the resultant lubricant formulation and prevent settling. The additive composition may be incorporated into the lubrication formulation such that the organic nanoparticles are provided in the lubricant formulation in the amount of about 0.01-10% by weight, or in various embodiments, about 0.01%, about 0.02%, about 0.03%, about 0.04%, about 0.05%, about 0.06%, about 0.07%, about 0.08%, about 0.09%, about 0.10%, about 0.01-0.03%, about 0.01-0.04%, about 0.01-0.05%, about 0.01-0.06%, about 0.01-0.07%, about 0.01-0.08%, about 0.01-0.09%, about 0.01-0.10%, about 0.01-1.0%, about 0.01-2.0%, about 0.01-5.0%, about 0.05-0.5%, or about 0.1-1.0% by weight. 
         [0021]    The ranges disclosed herein with respect to the ionic liquid content and the organic nanoparticle content of the additive compositions may be interchangeably combined in any combination, with any ionic liquid or organic nanoparticle disclosed herein. For example, the additive composition of the lubricant formulation may, in one embodiment, include about 1.0-8.0% by weight ionic liquid (trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate) and about 0.01-0.10% by weight organic nanoparticle (graphene platelets), and in another embodiment, about 5% by weight ionic liquid (trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate) and about 0.01-5% organic nanoparticle (carbon nanotubes). Each permutation of the embodiments of these ranges may be further used in combination with any of the base lubricants described herein. 
         [0022]    Without wishing to be bound by the theory, when the lubrication formulation incorporating the additive composition is utilized in an engine system, the organic nanoparticles aggregate in wear grooves, patterns, and/or facets in the surfaces of the parts being lubricated that may form during the operation of the system or otherwise, thereby having a mending effect on the pertinent surfaces as the nanoparticles accumulate. Accordingly, the organic nanoparticles may provide lubrication and hence additional protection to the system even without the presence of the liquid lubricant components (i.e. the base lubricant and/or the ionic liquid additive component), for example if the liquid lubricant components are lost or removed for any reason, followed by the loss of the ionic liquid-induced tribocoating. This improves the ability of the lubricant formulation to provide protection to system parts even in the event of a lubrication failure or the loss of lubricant during operation. 
         [0023]    The disclosed additive composition therefore provides a number of benefits over state of the art lubricants because it provides at least the dual benefits of rapidly establishing the triboprotective coating on system surfaces via the ionic liquid, and also synergistically filling in irregularities on the metal surfaces to be lubricated via aggregation of the organic nanoparticles. Together, these dual benefits greatly enhance the ability of a system, such as a rotorcraft, to continue to operate safely post-lubrication system failure or lubricant loss for a significantly longer period of time than a base lubricant lacking the ionic liquid and organic nanoparticle components of the additive composition. Further, because the additive composition provides these benefits as an additive to a relatively inexpensive base lubricant, there is significant cost savings as compared to formulating lubricants composed primarily of an expensive ionic liquid base. 
         [0024]    The additive composition enhances the function of formulation used for both internal combustion engines and also transmission lubrications, and is therefore suitable for a wide variety of applications beyond rotorcraft transmission and gearbox systems, such as use in bearing applications and/or other tribomechanical systems that require lubrication. 
         [0025]    In one non-limiting example, the additive composition included carbon nanoparticles and the ionic liquid trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, which were added to the base lubricant of AEROSHELL® 555 in the amounts of 5.0% by weight ionic liquid, 0.1% by weight carbon nanoparticle, and 94.9% by weight AEROSHELL® 555. A protocol for an oil-out simulation was created on the Cameron-Plint tribometer to test the effectiveness of this lubricant formulation. For the first 5 minutes, the test was run at 20N load as a run-in period in a fully flooded (2 ml of lubricant formulation) condition. After 5 minutes, the load was increased to 250N (Hertzian stress 700 MPa). After a 30 minute run with the 250N load, an oil-out event was simulated by completely removing the lubricant formulation. The test was continued under the “oil-out” condition. For each run, the test was terminated when the friction coefficient increased to 0.3, or the test duration (typically 300 minutes) ended. The tribological performance of the lubricant formulation was compared with AEROSHELL® 555 (base line) under such simulated oil-out conditions. The increase in run time after oil-out test in the lubricant formulation was greater than 2108% of the base line result. 
         [0026]    In another non-limiting example, and with reference to  FIG. 1 , the additive composition included nano-graphene platelets and the ionic liquid trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, which were added to the base lubricant of AEROSHELL® 555 in the amounts of 1%, 3%, and 5.0% by weight ionic liquid, 0.02% by weight graphene, and 98.98%, 96.98% and 94.98% by weight AEROSHELL® 555. A protocol for the oil-out simulation was created on the Cameron-Plint tribometer to test the effectiveness of these lubricant formulations. In each case, for the first 5 minutes, the test was run at 20N load as a run-in period in a fully flooded (1 ml of lubricant formulation) condition. After 5 minutes the load was increased to 250N (Hertzian stress 700 MPa). To create the oil-out event, the lubricant was completely removed after a 60 minute run with the 250N load, and the test was continued under the “oil-out” condition. In  FIG. 1 , the “oil-out” time is represented by the hash mark at 60 minutes on the x-axis. The test was terminated when the friction coefficient increased to 0.3, or the test duration (typically 300 minutes) ended. As shown in  FIG. 1 , the friction coefficient of certain lubricant formulations rises sharply (&gt;0.3) after a certain amount of time in an oil-out condition. The tribological performances of the three different lubricant formulations were compared with AEROSHELL® 555 (base line) under such simulated oil-out conditions. The results are detailed in Table 1, below: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Average 
                   
                   
                 Increase in Run 
               
               
                   
                 Friction 
                 Average Wear 
                 Time from Oil-Out Until 
                 Time After Oil- 
               
               
                 Lubricant Formulation 
                 Coefficient 
                 Reduction % 
                 Friction Coefficient &gt;0.3 
                 Out, % 
               
               
                   
               
             
             
               
                 AEROSHELL ® 555 (baseline) 
                 0.12 
                 — 
                 12.65 minutes 
                 — 
               
               
                 AEROSHELL ® 555, with 1% 
                 0.13 
                 3% 
                 25.55 minutes 
                 102% 
               
               
                 ionic liquid and 0.02% organic 
               
               
                 nanoparticle 
               
               
                 AEROSHELL ® 555, with 3% 
                 0.12 
                 −32% 
                 143.11 minutes  
                 1031% 
               
               
                 ionic liquid and 0.02% organic 
               
               
                 nanoparticle 
               
               
                 AEROSHELL ® 555, with 5% 
                 0.11 
                 −35% 
                 &gt;304.67 minutes  
                 &gt;2308% 
               
               
                 ionic liquid and 0.02% organic 
               
               
                 nanoparticle 
               
               
                   
               
             
          
         
       
     
         [0027]    As shown in Table 1, the 1% ionic liquid formulation provided similar results to the baseline in terms of lubricity and wear, but more than doubled the effective run time of the engine after lubricant removal as compared to the baseline test. Each of the 3% and 5% ionic liquid formulations provided both significant wear reduction and also significant improvements in run time—at least about 10 to 25 times the baseline without the additive composition. 
         [0028]    The effectiveness of carbon nanoparticles to reduce wear was also tested. Under fully-flooded conditions, about 35% reduction of wear was observed for a blend of AEROSHELL® 555+0.1% carbon nano-particle as compared to baseline of AEROSHELL® 555, alone, under the same conditions. 
         [0029]    While the invention has been described with reference to certain exemplary embodiments thereof, those skilled in the art may make various modifications to the described embodiments of the invention without departing from the scope of the invention. The terms and descriptions used herein are set forth by way of illustration only and not meant as limitations. In particular, although the present invention has been described by way of examples, a variety of compositions and processes would practice the inventive concepts described herein. Although the invention has been described and disclosed in various terms and certain embodiments, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved, especially as they fall within the breadth and scope of the claims which are to be appended. Those skilled in the art will recognize that these and other variations are possible within the scope of the invention as defined in the claims and their equivalents.