Abstract:
Lubricating oil compositions having low flammability properties and good low temperatures suitable for use as hydraulic fluids comprise a major amount of GTL base oil, optionally a minor amount of polyolphaolefins, an additive amount of about 5 to 30 wt % of dicarboxylic acid ester and minor amount of a pour-point depressant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application 61/123,926 filed Apr. 10, 2008, herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to hydraulic fluids exhibiting flame retardant properties and good low temperature viscometrics. 
         [0004]    2. Description of the Related Art 
         [0005]    Functional fluids, especially hydraulic fluids exhibiting flame retardant properties, are known in the art. The fire-resistant/flame retardant hydraulic fluid market is currently divided into the following main classes: 
         [0006]    1) HFA, which are high water content fluids, &gt;80% water; 
         [0007]    2) HFB, which are water-in-oil emulsions, &lt;50% water; 
         [0008]    3) HFC, which are water-glycol fluids, 30-80% water; 
         [0009]    4) HFD-R, which are phosphate ester fluids; and 
         [0010]    5) HFD-U, which are all other, including polyol ester-based fluids, vegetable oil ester-based fluids, fluorocarbon-based fluids, silicate ester-based fluids, silicone-based fluids and PAO-based fluids. 
         [0011]    Fire resistant/flame retardant fluids are described in the literature. 
         [0012]    U.S. Pat. No. 3,793,207 teaches fire-resistant hydraulic fluids having improved low temperature characteristics; e.g., viscosity, pour point, and cloud point which consist mainly of a mixture of a hydrocarbon base oil, preferably a synthetic hydrocarbon, a dialkyl carboxylate ester and an ortho-silicate. The synthetic hydrocarbon oil constitutes the base oil of the composition, the ester of a dicarboxylic acid constitutes 20-30 wt % of the composition, and the alkyl silicate constitutes 10-15 wt % of the composition, with small percentages of other functional additives being present, such as anti-wear agents, hydrolysis suppressants, anti-foamants, etc. The hydrocarbon oil employed has a viscosity of from 8,500 to 10,800 cs at −40° F., 25-35 cs at 100° F., and 4-8 cs at 210° F., and is preferably a dialkylaromatic hydrocarbon. The preferred ester of a dicarboxylic acid is a dialkyl adipate ester of a C 6 -C 10  alcohol. The fluid is hydrolytically stable, has excellent low temperature viscometrics and has a high flash point, fire point and autogenous ignition temperature. 
         [0013]    U.S. Pat. No. 3,873,464 teaches a flame resistant hydraulic fluid comprising a major amount of a synthetic hydrocarbon oil having at least 30 carbon atoms in each molecule, and a small amount of a mixture comprising chlorinated polyphenyls and a trihydrocarbon phosphate. The synthetic hydrocarbon base oil is of the type made by polymerizing olefins in the presence of a suitable catalyst, such as BF 3  or AlCl 3 . The preferred synthetic hydrocarbon oil is a trimer of decene. 
         [0014]    U.S. Pat. No. 6,361,711 teaches a flame retardant hydraulic oil containing a hydraulic base oil including as essential component a synthetic ester formed by reacting at least one polyol selected from the group consisting of neopentyl glycol, 2,2-dimethyl-3-hydroxy propyl-2′,2′-dimethyl-3′-hydroxypropionate, glycerine and trimethylolpropane with a carboxylic acid including 15 to 85 mole % of oleic acid based on the total carboxylic acid and 15 to 85 mole % of isostearic acid based on the total carboxylic acid, or a carboxylic acid obtained by incorporating into the carboxylic acid 85 mole % or less of monocarboxylic acid having 6 to 22 carbon atoms (excluding oleic acids and isostearic acids). The synthetic ester has a kinematic viscosity of 40 to 80 cSt (mm 2 /s) 40° C. and a flash point of 290° C. or higher. 
         [0015]    U.S. Pat. No. 6,402,983 teaches a flame retardant hydraulic oil containing a partial ester of a polyol and a monocarboxylic acid. The polyol partial ester is formed by reacting a polyol having a total of 6 to 22 carbon atoms and a total of 3 to 6 hydroxyl groups with an acyclic monocarboxylic acid having a total of 6 to 22 carbon atoms, said polyol partial ester having a hydroxyl value of 35 mg KOH/g or more, a flash point of 290° C. or higher and an average molecular weight of 600 to 1500. 
         [0016]    U.S. Pat. No. 4,519,932 teaches a low temperature, flame retardant hydraulic fluid comprising a blend of a 2 cSt oligomer of alpha olefins with mono- or di-esters and a polymethacrylate viscosity index improver. The oligomers of alpha olefins are dimers, trimers, tetramers, etc., of; e.g., decene. The oligomer used in the examples of this patent is the dimmer. The dimer is hydrogenated so that it is essentially fully saturated. The 2 cSt dimer is apparently specified to distinguish over U.S. Pat. No. 4,175,046 which teaches a combination of higher viscosity PAO plus dicarboxylic acid esters as engine oils. The fluid exhibits enhanced fire resistance, Table, column 7, lines 15-25. 
         [0017]    The paper “Development of a −54° C. to 135° C. Synthetic Hydrocarbon-based Fire Resistant Hydraulic Fluid” explores the development of a MIL-H-83282 fire resistant hydraulic fluid. MIL-PRF-83282 is a synthetic hydrogenated PAO-based hydraulic fluid exhibiting fire resistance. The object of the paper is to trace the development of a lower pour point MIL-PRF-83282 fluid. At page 486 it is indicated that such a fluid would be based on synthetic PAO or a fluid similar to PAO which have been found to be better than MIL-PRF-5606 (which is a mineral oil-based lubricant). 
         [0018]    The paper “Pump Evaluation of Hydrogenated Polyalphaolefin Candidates for a −54° C. to 135° C. Fire Resistant Air Force Aircraft Hydraulic Fluid” similarly addresses lower pour point MIL-PRF-83282 fluids. PAO fluids (decene dimer and dimer/trimer blend) are the base stocks. The fluids all contained antioxidant and TAP. The dimer/trimer blend contained diisooctyl adipate (Fluid I) while the dimer only fluid contained dibutoxyethyl glutarate (Fluid II). 
     
    
     DESCRIPTION OF THE INVENTION 
       [0019]    The present invention is directed to a functional fluid, especially a hydraulic fluid, exhibiting excellent low flammability properties (i.e., flame retardant properties) and excellent low temperature viscometrics comprising as essential elements a hydrocarbon oil base oil derived from a Gas-to-Liquids process, preferably a Fischer-Tropsch process, most preferably hydrodewaxed or hydroisomerized/catalytic (and/or solvent) dewaxed Fischer-Tropsch wax, optionally a minor quantity (up to 50 wt % of the base oil) of polyalphaolefin, a dicarboxylic acid ester, a pour point depressant and triaryl phosphate. 
         [0020]    The base oil comprises from about 65 wt % to about 90 wt % of the formulated composition. 
         [0021]    The dicarboxylic acid ester comprises about 5 to about 30 wt % of the formulated composition. 
         [0022]    The triaryl phosphate comprises about 0.5 to about 5 wt %, preferably about 1 to about 3 wt %, of the formulated composition. 
         [0023]    The pour point depressant comprises about 0.05 to 5.0 wt % of the formulated composition. 
         [0024]    The base oil is a hydrocarbon oil derived from a Gas-to-Liquids (GTL) process material. 
         [0025]    GTL materials are materials that are derived via one or more synthesis, combination, transformation, rearrangement, and/or degradation/deconstructive processes from gaseous carbon-containing compounds, hydrogen-containing compounds and/or elements as feedstocks such as hydrogen, carbon dioxide, carbon monoxide, water, methane, ethane, ethylene, acetylene, propane, propylene, propyne, butane, butylenes, and butynes. GTL base stocks and/or base oils are GTL materials of lubricating viscosity that are generally derived from hydrocarbons, for example waxy synthesized hydrocarbons, that are themselves derived from simpler gaseous carbon-containing compounds, hydrogen-containing compounds and/or elements as feedstocks, typically CO and H 2  synthesis gas. GTL base stocks and/or base oil(s) include: (1) oils boiling in the lube oil boiling range separated/fractionated from synthesized GTL materials, such as, for example, by distillation and subsequent subjection to a final wax processing step which involves either or both of a catalytic dewaxing process, or a solvent dewaxing process, to produce lube oils of reduced/low pour point; (2) synthesized wax isomerates, comprising, for example, oils produced by hydrodewaxing or hydroisomerization, followed by cat and/or solvent dewaxing of synthesized wax or synthesized waxy hydrocarbons; (3) hydrodewaxing or hydroisomerization followed by cat and/or solvent dewaxing of Fischer-Tropsch (F-T) material (i.e., hydrocarbons, waxy hydrocarbons, waxes and possible analogous oxygenates, which are themselves synthesized from synthesis gas comprising CO and hydrogen); preferably oils produced by hydrodewaxing or hydroisomerization followed by cat and/or solvent dewaxing of F-T waxy hydrocarbons or of F-T waxes, or mixtures thereof. 
         [0026]    GTL base stock(s) and/or base oil(s) derived from GTL materials, especially hydrodewaxed or hydroisomerized followed by cat and/or solvent dewaxing dewaxed synthesized wax or synthesized waxy feed, preferably F-T material-derived base stock(s) and/or base oil(s), are characterized typically as having kinematic viscosities at 100° C. of from about 2 mm 2 /s to about 50 mm 2 /s (ASTM D445). They are further characterized typically as having pour points of about −5° C. to about −40° C. (ASTM D97). They are also characterized typically as having viscosity indices of about 80 to 140 or greater (ASTM D2270). 
         [0027]    For the purposes of the present invention the base stock/base oil derived from GTL process materials used to produce the flame retardant functional fluid has a kinematic viscosity at 100° C. in the range of from about 3.5 mm 2 /s to about 10 mm 2 /s, preferably about 3.5 mm 2 /s to about 6 mm 2 /s, more preferably about 3.5 mm 2 /s to about 4 mm 2 /s. The GTL fluids employed in the present invention having kinematic viscosities at 100° C. in the range of about 3.5 mm 2 /s to 10 mm 2 /s have pour points in the range of from about −15 to about −30° C. The GTL fluids generally have pour points interior to those exhibited by PAO fluids of similar kinematic viscosities. 
         [0028]    The GTL base stock(s) and/or base oil(s) are typically highly paraffinic (&gt;90% saturates), and may contain mixtures of monocycloparaffins and multicycloparaffins in combination with non-cyclic isoparaffins. The ratio of the naphthenic (i.e., cycloparaffin) content in such combinations varies with the catalyst and temperature used. Further, GTL base stock(s) and/or base oil(s) typically have a very low sulfur and nitrogen content, generally containing less than about 10 ppm, and more typically less than about 5 ppm of each of these elements. The sulfur and nitrogen content of GTL base stock(s) and/or base oil(s) obtained from F-T material, especially F-T wax, is essentially nil. 
         [0029]    The term GTL base stock and/or base oil is to be understood as embracing individual fractions of such materials of wide viscosity range as recovered in the production process, mixtures of two or more of such fractions, as well as mixtures of one or two or more low viscosity fractions with one, two or more higher viscosity fractions to produce a blend wherein the blend exhibits a target kinematic viscosity. 
         [0030]    Dicarboxylic acid esters useful in this invention are prepared from the esterification of the dicarboxylic acid with excess of alcohol. The esterification reaction is well known in the art. Suitable dicarboxylic acids include but are not limited to succinic acid, maleic acid, adipic acid, azealic acid, suberic acid, phthalic acid, sebacic acid, linoleic acid. Suitable alcohols include but are not limited to butyl alcohol, n-pentyl alcohol, iso-pentyl alcohol, n-hexyl alcohol, n-heptyl alcohol, n-octyl alcohol, isooctyl alcohol, 2-ethyl-hexyl alcohol, nonyl alcohol, isononyl alcohol, decyl alcohol, undecyl alcohol, dodecyl alcohol, tridecyl alcohol and the like. Specific examples of these esters include dioctyl adipiate, di(2-ethylhexyl) sebacate, diisooctyl azealate, dioctyl sebacate, diisooctyl phthalate, diisononyl phthalate, diisodecyl adipiate, diisooctyl adipiate, diisononyl adipiate, diisodecyl suberate and the like. The preferred esters are diisodecyl adipiate, diisononyl adipiate and diisooctyl adipiate having a pour point (ASTM D97) of −60° C. and lower. 
         [0031]    Anti-wear agents are also essential components of the present invention so as to comply with original equipment manufactures (OEM) and/or military and/or civilian end user specifications. Triaryl phosphates are the traditional anti-wear agents, tricresyl phosphates being the anti-wear additive of choice. 
         [0032]    Because hydraulic fluids typically must also meet stringent low temperature viscometric properties, especially the −55° C. maximum pour point specification contained in MIL-PRF 83282, the lubricants of the present invention contain a pour point depressant. 
         [0033]    Conventional pour point depressants (also known as lube oil flow improvers) are added in an amount in the range of about 0.05 to about 5 wt %, preferably about 0.05 to about 1.5 wt %, as active ingredient, based on the total weight of the lubricating oil formulation. 
         [0034]    Suitable pour point depressants include, without limitation, polyacrylates, polymethacrylates, polyarylamides, condensate products of haloparaffin waxes and aromatic compounds, vinyl carboxylate polymers and terpolymers of dialkyl furmarates, vinyl esters of fatty acids and alkyl vinyl ethers. Useful pour point depressants and/or their method of preparation are described in U.S. Pat. No. 1,815,022; U.S. Pat. No. 2,015,748; U.S. Pat. No. 2,191,498; U.S. Pat. No. 2,387,501; U.S. Pat. No. 2,655,479; U.S. Pat. No. 2,666,746; U.S. Pat. No. 2,721,877; U.S. Pat. No. 2,721,878, U.S. Pat. No. 3,250,715. 
         [0035]    Preferred pour point depressants are the polyacrylates and polymethacrylates. 
         [0036]    The formulations of the present invention, optionally, may further contain up to about 50 wt %, preferably up to 40 wt %, more preferably up to 30 wt % poly alpha olefin having kinematic viscosities at 100° C. in the range of about 2 to 10 mm 2 /s, preferably about 2 to 6 mm 2 /s, more preferably about 2 to 4 mm 2 /s. Preferably the fluids contain a base oil consisting solely of GTL base oil. 
         [0037]    It has been discovered that hydraulic fluids comprising a Group III base oil specifically GTL fluids obtained by the hydrodewaxing or hydroisomerization/catalytic (and/or solvent) dewaxing of Fischer-Tropsch wax (hereinafter collectively identified as and referred to as GTL fluids), in combination with decarboxylic acid and a pour point depressant exhibits significantly improved low flammability properties (i.e., exhibit flame retardant properties) as compared to hydraulic fluids based on mineral oils, as exemplified by hydraulic fluids meeting MIL-PRF 5606 specifications. Such fluids also unexpectedly exhibit superior flame retardant properties as compared to other Group III base oil-based hydraulic fluids. Finally, despite the fact that the GTL fluids have higher inherent pour points than PAO, the present formulation not only has similar flame retardant properties as do the PAO-based hydraulic fluids but also meet the low temperature viscometric property requirements of such PAO-based hydraulic fluids as specified in MIL-PRF 83282, a pour point of −55° C. maximum. 
         [0038]    The fluids of the present invention exhibit flame propagation rates of about 2 mm/s or less, preferably about 1.8 mm/s or less, more preferably about 1.7 mm/s or less. 
       EXAMPLES 
       [0039]    Various fluids formulated using PAO, GTL, mixtures of PAO and GTL, and a Group III fluid identified as Yubase produced by the SK UCO Lube process (Korea) by hydrocracking fuel bottoms followed by catalytic wax isomerization, were compared against each other and against a mineral oil-based hydraulic fluid meeting the specification of MIL-PRF-5606. 
         [0040]    The specifications of the PAO4, GTL4 and Yubase4 base oils are given in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of the Base Oils 
               
             
          
           
               
                 Base Oil 
                 Test 
                 PAO4 
                 GTL4 
                 Yubase4 
               
               
                   
               
             
          
           
               
                 KV @ 40° C., mm 2 /s 
                 ASTM D445 
                 16.8 
                 14.7 
                 19.7 
               
               
                 KV @ 100° C., mm 2 /s 
                 ASTM D445 
                 3.9 
                 3.6 
                 4.2 
               
               
                 Pour Point, ° C. 
                 ASTM D97 
                 −73 
                 −27 
                 −15 
               
               
                 AMW 
                 — 
                 438 
                 300 
                 300 
               
               
                   
               
             
          
         
       
     
         [0041]    These base oils were used to prepare formulated hydraulic fluids as described in Table 2, which recites the viscometric properties of the formulated fluids and compares the fluids to a mineral oil-based fluid meeting MIL-PRF-5606, as well as reporting the flame propagation rates of the fluids. The dicarboxylic acid ester used in this example was diisodecyl adipate which has a KV at 100° C. of 3.68 mm 2 /s at 20° C. of about 25 mm 2 /s and at −40° C. of about 3,450 mm 2 /s and a pour point of −60° C. The pour point depressant was an alkylated fumarate/vinyl acetate copolymer, on an as-received basis. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 MIL-PRF- 
                   
                   
                 Fluid 3 
                   
               
               
                   
                 5606 
                 Fluid 1 
                 Fluid 2 
                 GTL/ 
                 Fluid 4 
               
               
                   
                 Mineral Oil 
                 PAO 
                 GTL 
                 PAO 
                 Group III 
               
               
                   
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Base Oil 
                   
                   
                   
                   
                   
               
               
                 Naphthenic Base 
                 85.7 
                 0 
                 0 
                 0 
                 0 
               
               
                 Oil 
               
               
                 GTL 3.6 
                 0 
                 0 
                 76.7 
                 38.4 
                 0 
               
               
                 Group III-4 
                 0 
                 0 
                 0 
                 0 
                 76.7 
               
               
                 PAO 4 
                 0 
                 77.0 
                 0 
                 38.4 
                 0 
               
               
                 Dicarboxylic 
                 0 
                 20.0 
                 20.0 
                 20.0 
                 20.0 
               
               
                 Acid Ester 
               
               
                 PPD 
                 0 
                 0 
                 0.3 
                 0.2 
                 0.3 
               
               
                 VI Improver 
                 13.6 
                 0 
                 0 
                 0 
                 0 
               
               
                 Balance 
                 0.7 
                 3.0 
                 3.0 
                 3.0 
                 3.0 
               
               
                 Additives 
               
               
                 Properties 
               
               
                 KV @ −40° C., 
                 469 
                 2634 
                 4328 
                 3148 
                 solid 
               
               
                 mm 2 /s 
               
               
                 KV @ 40° C., 
                 13.4 
                 16.11 
                 14.08 
                 15.04 
                 18.44 
               
               
                 mm 2 /s 
               
               
                 KV @ 100° C., 
                 5.2 
                 3.78 
                 3.57 
                 3.67 
                 4.12 
               
               
                 mm 2 /s 
               
               
                 VI (Calc.) 
                 400 
                 126 
                 140 
                 133 
                 127 
               
               
                 Pour Point, ° C. 
                 −65 
                 &lt;−60 
                 −57 
                 &lt;−63 
                 −42 
               
               
                 Flash Point, ° C. 
                 ASTM D92 
                 222 
                 217 
                 — 
                 224 
               
               
                 Fire Point, ° C. 
                 ASTM D92 
                 258 
                 253 
                   
                 256 
               
               
                 AIT, ° C. 
                 ASTM E659 
                 338 
                 324/340 
                 — 
                 344 
               
               
                 Flame 
                 10 
                 1.5 
                 1.7 
                 1.8 
                 2.0 
               
               
                 Propagation, 
               
               
                 mm/2 
               
               
                   
               
             
          
         
       
     
         [0042]    As seen by reference to Table 2 above, the hydraulic fluids made using GTL (Fluids 2 and 3, fluids of the present invention) have very low pour points as compared to Fluid 4, a fluid made using a different Group III-type base oil and containing the same additives as invention Fluids 2 and 3. Fluid 4, made using a different Group III-type base stock, even when additized with a pour point depressant, could not meet the −55° C. maximum pour point specification set for the hydraulic fluid. Fluid 4 was solid at −40° C. (i.e., at about pour point temperature). Fluid 4 failed to meet the pour point target despite being based on a Group III base stock and despite being additized with a pour point depressant. 
         [0043]    While both invention Fluids 2 and 3 met or exceeded the pour point target of −55° C., as did Fluid 1 (PAO-based fluid of the prior art), it must be recognized that the pour point of the unadditized PAO was −73° C. whereas the pour point of the unadditized GTL was −27° C. 
         [0044]    Thus, while Fluid 1 (PAO) had a pour point of −65° C., this is not totally unexpected in view of the fact that the PAO base oil had an unadditized pour point of −73° C. The additization of the PAO raised the pour point to −65° C. from an unadditized pour point of −73° C. Thus, the effect of additives is to raise the pour point. 
         [0045]    What is unexpected, however, is that the GTL Group III base stock having an unadditized pour point of −27° C. produced a fluid (Fluid 2) having an additized pour point of −57° C. with only 0.3 wt % pour point depressant, whereas Fluid 4 based on Yubase Group III base oil having an inherent pour point of −15° C. had an additized pour point of only −42° C. and is reported as solid at −40° C., at the same pour point depressant treat rate. 
         [0046]    Thus, it is unexpected that a hydraulic fluid meeting the pour point target of −55° C. maximum could be formulated using the GTL base oil, even when additized with a pour point depressant. 
         [0047]    Further review of the data in Table 2 shows that fluids of the invention (Fluids 2 and 3) have lower flame propagation rates than does the hydraulic fluids containing the same additives package but employing the Group III base oil Yubase4. The flame propagation rate of Fluids 2 and 3 is also significantly superior to that of the mineral oil-based fluid (MIL-PRF-5606). 
         [0048]    Based on the flame propagation results obtained for the Group III Yubase base oil formulation (Fluid 2), 2 mm 2 /s, the formulations made using the GTL base exhibited flame propagation rates which are significantly superior, being 1.7 and 1.8 mm 2 /s, respectively. While 1.7 and 1.8 mm 2 /s may on their face appear substantially equivalent to 2.0 mm 2 /s, the difference is significant when it is taken into consideration that flame propagation is measured by ASTM D5306, and the standard derivation for measurements from the same lab for MIL-PRF-83282 (PAO-based hydraulic fluids) is 0.07 mm/s.