Rapid Chromatographic Analysis of Soap-Thickened Lubricating Greases GEORGE W. POWERS, Jr., and FRANK J. PIEHL Research Deportment, Standard Oil Co. (Indiana), Whiting, Ind.
b Conventional methods for determining soap and oil in lubricating greases are slow and tedious. A simple chromatographic method has been developed for their determination in soap-thickened greases made from petroleum, silicone, and ester oils. Four to six samples are analyzed in 8 hours. Precision and accuracy of the method are equivalent to those of the ASTM extraction method. Interferences from minor components of the greases are negligible.
L
are semisolid dispersions of thickening agents in liquid lubricants. Conventional greases are thickened with fatty-acid soaps; in a few new greases, silica, Bentone, and synthetic organic thickeners are used. Petroleum oil is the most common lubricant, although more expensive silicone and ester lubricants are used in special-purpose greases. Minor amounts of water, glycerol, free alkali, and fat from the manufacturing process often remain in the grease. Although performance of greases is usually evaluated by physical tests (8), methods for chemical analysis are needed to correlate performance with composition. Important physical properties depend primarily on the amount and kind of oil and thickener. Thus, oil and soap-the most common thickener-are determined most frequently. ASTM Method D 128 is used widely to determine oil and soap (1). The grease is digested with aqueous strong acid to destroy its physical structure by converting soap to fatty acid. Liberated arid is converted to the potassium soap, which is separated from oil by a series of liquid extractions. Soap is reconverted to fatty acid, which is isolated and weighed. Oil is weighed after the solvent is removed from the remaining neutral layer. Unsaponified fat, recovered with oil, may be determined by saponification with alkali; soap from fat is extracted and determined as before. Other minor components are determined by specific tests on other portions of the grease. The ASThl method has several disadvantages. The initial acid digestion requires 2 to 8 hours, and the elapsed time per analysis may be as much as 3 days. Extraction of soap from oil UBRICATING GREASES
28
ANALYTICAL CHEMISTRY
requires tedious backwashing; large quantities of flammable solvents are required, and emulsions often form. If soaps of hydroxystearic acid are present, solvents must be kept warm to prevent the acid from precipitating when it is liberated. Furthermore, the method is not applicable t o greases made from silicones or esters, which hydrolyze during acid decomposition. Several methods eliminate some of these disadvantages. Features include faster ways of decomposing the grease (9, S, 5, If), separating oil from soap or fatty acid (3,5), and determining fatty acid (S, 5, 9). However, each method is specific for only one type of grease or destroys one of the major components of the grease. Two complementary methods, faster than the ASTM method and applicable to ester or silicone oils (4,do not completely eliminate extractions and troublesome emulsions. Therefore, a new method that eliminates all these objections is needed. DEVELOPMENT OF METHOD
Three problems were studied: new ways of separating oil from soap, new ways of separating oil and fat, and faster ways of decomposing the grease without destroying silicones or esters. Separation of oil from soap by chromatography eliminated all the troublesome features of liquid extraction. Fatty acids have been removed from fats by chromatography (7), and presumably the same techniques should remove them from petroleum oils. The majority use alumina as adsorbent. Alumina was a poor choice for the present application, because quantitative recovery of acids proved difficult. Kaufmann and Schmidt state that ordinary silica gel removes stearic acid from natural oils but has low capacity (6). The authors found that soaps are adsorbed strongly, but are difficult to transfer, and clog the column. If the silica gel is impregnated with a base, the soaps are formed in the column over a large area and do not clog the column. Ethanolamine was chosen as the base because stronger bases, such as aqueous sodium hydroxide, hydrolyzed esters. Oil is eluted first; an acetic acid solution then converts the soaps to free fatty acids and elutes the acids.
The possible use of chromatography to separate fat from oil and fatty acid was studied, because the ASTM method includes a procedure for separating fat and oil. Petroleum oil can be separated from aliphatic diesters on silica gel (4). The authors found that fat can be separated chromatographically from highly refined petroleum oils but not completely from the less-refined oils that are also used in greases. The latter contain polar compounds that could not be separated from the trace amounts of fat by chromatography; these may be the same compounds that interfere in conventional methods of analysis. Therefore, to make the method general, solvents were chosen to elute oil and fat in one fraction. Single-phase decomposition of grease with strong acid proved much faster than other techniques. Palit’s titration of soaps with hydrochloric or perchloric acid in glycol solution (IO) suggested that a one-phase decomposition with strong acid should be rapid. The authors found that soap in grease could be rapidly converted to fatty acid with concentrated hydrochloric acid, if the grease was dispersed in a mixture of benzene and tert-butyl alcohol. However, the alcohol interfered with the chromatographic separation and had to be removed by evaporation. Chloroform was finally chosen as the solvent, because it disperses greases readily and has the proper eluting characteristics for chromatography. Ethanesulfonic acid was chosen as the strong acid because it is miscible with chloroform, does not interfere nith the chromatographic separation, and has a lorn equivalent weight. K i t h this combination, greases can be decomposed and made ready for chromatography in less than 10 minutes. I n a method that eliminates all the cited objections to the ASTM method the grease is stirred in chloroform, and ethanesulfonic acid is added. The solution is transferred directly to a column of silica gel impregnated with ethanolamine. Oil iq eluted from the column with chloroform, and then fatty acid is eluted with 5% acetic acid in chloroform. Solvent is removed from the two eluates, and the residues are weighed.
EQUIPMENT AND REAGENTS
Figure 1 gives the details of the construction of the chromatographic column. A Teflon stopcock is recommended to avoid the use of stopcock grease. The heating tape is rated a t 115 watts. Silica gel from the Davison Chemical Co.. Baltimore. Md.. grade 62. 60-200 me&, is recommendea. (The loss in weight of the grade 62 gel used in this investigation was 4.8% a t 1000" C.) Davison grade 950 can be substituted, but about twice the weight must be used to provide a column height of about 10 inches. Prepare the following solutions in chloroform conforming to ACS specifications: 10 grams of ethanolamine per 100 ml. (prepared fresh daily), 5% acetic acid by volume, and 1% butter yellow. Prepare the following solutions in alcohol-free ACS-grade chloroform: 10% ethanesulfonic acid by volume, and 10% triethylamine by weight. Alcoholfree chloroform must be used for decomposition of the grease to prevent esterification of fatty acids; it may be prepared by passing ACS-grade chloroform over silica gel (1 gram of silica gel per 7 ml. of chloroform collected). Alkanesulforiic acid can be substituted for ethanesulfonic acid; both are available from Amoco Chemicals Corp., Chicago, Ill. PROCEDURE
Preparation of Column. Pour about 15 ml. of chloroform into the column. Push a wad of cotton t o the bottom of the column and displace the air from the cotton. Compress the cotton enough t o hold back silica gel b u t not impede the flow of solvent. Slurry 15 & 1 grams of silica gel with enough chloroform (about 50 nil.) to form a free-flowing suspension. Pour the slurry into the column and rinse any silica gel left behind into the column with additional small portions of chloroform. Start the flow of the solvent and t a p the column gently to displace air bubbles from the gel. (Never let the liquid level fall below the surface of the silica gel.) Mix 15 & 1 grams of silica gel and 10 ml. of ethanolamine solution until all lumps have broken up. Add about 50 ml. of chloroform to form a loose slurry, and pour the slurry into the column atop the untreated gel. Rinse silica gel left behind into the column, as before, and tap the column to settle the gel. Drain the solvent from the column until the liquid level is within 3 mm. of the surface of the silica gel, and rinse the walls of the reservoir twice with small portions of chloroform. When the liquid level is again within 3 mm. of the surface, stop the flow of liquid. Discard the eluate. Decomposition of Grease. Take enough sample to contain between 1 and 2 grams of f a t t y acid and not more than 30 grams of oil. Transfer the sample t o a tared 250-ml. beaker and weigh t o the nearest 0.01 gram.
A \
-TO
COMPRESSED AIR
*250-ML.
TREATED
RESERVOIR
SILICA
GEL
2 5 CM
U N T R E A T E D SILICA
GEL
L *
T E F L O N STOPCOCK
4
e.
-+ Figure
1.
Chromatographic column
Add 50 ml. of alcohol-free chloroform and 2 drops of butter yellow as indicator. Stir vigorously, preferably with a magnetic stirrer and Teflonor glass-covered stirring bar, and break up large lumps with a stirring rod. Add ethanesulfonic acid solution until a red color persists for 5 minutes, and then add triethylamine solution dropwise to the yellow end point. If a precipitate forms, warm the beaker gently. If the precipitate dissolves, it is hydroxystearic acid; keep the solution warm. If the precipitate does not dissolve, it is inorganic salts; add 2 or 3 drops of water to coagulate the salts. Separation of Oil from Fatty Acid. Pour the solution of decomposed grease into the column. Rinse the beaker several times with small portions of chloroform and add each rinse t o the column. Resume the flow of liquid. If hydroxystearic acid precipitated when the grease was decomposed, keep the solution in the column warm by applying about 20 volts across the heating lape, but do not boil the solvent. When the liquid level reaches the surface of the silica gel, rinse the walls of the reservoir with about 10 ml. of chloroform. Repeat the rinse when the liquid level again reaches the surface of the silica gel. After the second rinse flows into the adsorbent, add 200 ml. of chloroform and discontinue heating. Check the flow rate; if it is less than 5 ml. per minute, loosen the top 0.5 inch of adsorbent with a stirring rod. If the flow rzlte is still less than 5 ml. per minute, apply enough pressure to maintain it a t about that rate. Discard the first 30 ml. of eluate, and collect the rest of the chloroform eluate-which contains the oil and fat-in a tared 250-ml. beaker. When the beaker is about half full, place another beaker under the column. Transfer the first beaker to a steam bath and direct a gentle stream of air upon the liquid surface. When the liquid level reaches the
surface of the silica gel, add 250 ml. of chloroform-acetic acid solution to the reservoir and place a second tared beaker under the column. Transfer the second half of the chloroform eluate quantitatively to the first tared beaker on the steam bath. Collect and evaporate the chloroform-acetic acid eluatewhich contains the fatty acids-in two portions in a similar manner. Determination of Oil and Fatty Acid. If the oil in the original grease contained volatile components, remove the last 2 or 3 ml. of solvent from recovered oil in a vacuum chamber a t room temperature; otherwise, heat the tared beakers on the steam bath until they reach constant weight. Determine the weight of the residues to the nearest 0.01 gram, and convert them to per cent of sample. If desired, calculate per cent soap (1, Section 13), and determine fat in the recovered oil and other minor constituents in the grease ( I ) . PRECISION AND ACCURACY
Precision and accuracy of the chromatographic method were studied by analyzing individual oils and fatty acids, mixtures of oils and fatty acids, and greases of known composition. Different types of greases were analyzed by both the chromatographic and ASTPYI methods. A variety of oils was carried through the complete procedure to determine the applicability of the method and to check the properties of the recovered oils. Analyses of distillate and residual petroleum oils, silicones, and esters are given in Table I. Of 13 runs on distillate oils, the lowest individual recovery of oil is 99.2y0',. I n no case does enough oil carry over into the acid chromatographic fraction to cause a significant error in acid content. Of the three residual oils, recovery of the asphaltic oil is lowest; polar compounds probably carry over into the acid fraction. Recovery of silicone oils is low, probably because unknown components are adsorbed strongly. Honerer, these components do not interfere in the acid fraction. Recoveries of ester oils are excellent; the absence of material in the acid fraction shon-s that little or no ester oil is hydrolyzed when ester greases are decomposed. T'iscosities of recovered ester and petroleum oils agree closely with those of the original oils. Viscosities of recovered silicone oils are l o y probably because the composition has changed slightly. These data demonstrate that the method is applicable t o silicone, ester, and petroleum oils except asphaltic oils. Analyses of typical commercial acids used in greases are given in Table 11. I n each case, recovery of acid is low, because nonacidic components in the original acid are recovered in the oil fraction. All oil fractions were free of acids. Total recoveries approach 100% VOL. 30, NO. 1, JANUARY 1958
29
except for lauric and naphthenic acids, which are somewhat volatile. Acid numbers of recovered acids are generally higher than those of original acids, because nonacids have been removed. For oleic acid, the recovered acid has a lower acid number, probably because the acid reacts with oxygen \\-hen blown with air. The efficiency of separation of oil and acid was studied by analyzing synthetic mixtures (Table 111). The average error in recol-ery of oil is 0.2 TTeight %; recoveries of oil are sometimes a fern tenths per cent lo^ because the oils are slightly volatile and because the petroleum oils contain traces of polar compounds. The average error in recovery of acid is 0.15 weight %; in Table I.
some cases recoveries are a few tenths per cent high. Polar compounds in the petroleum oil are partially eluted with acids; they also show up as 0.16 weight yo acid in the ASTM procedure. Oil and acids are separated and recovered almost quantitatively. Precision of the method was studied by analyzing three commercial greases repeatedly (Table IT.'). The average standard deviations for recoveries of oil and acid are 0.26 and 0.12 weight yo, respectively. This precision is a t least equivalent to that of the ASTM and other methods, and possibly better in the case of acid recovery. Accuracy of the method mas further studied by analyzing samples of greases used in recent ASTM cooperative
Analyses of Oils
Kt. % Recovered As oil As acid
Mid-continent distillate oils Untreated Acid-treat,ed
0.20"
99.5"
99.70 99.65 99.6 99.8" 99.P 99.4
0.08° 0.08°
85.9 85.7 113.8 144.8 375 573 ...
37.4 37.6 39.0 42.0
55.55
61.3" 93
37.6 37.4 39.5 42.0 56.0 61.4 ...
0.96 0.14 0.15
1000 3761 ...
3737 ...
168 205
172 ...
0.10"
358 1084
340a 1060"
92.2 151.0
89.2a 148.8"
35.5
35.5 38.5 43.5
0.0-P
0.03
0.05.
0.23
57.4 68.7 134.5
75
...
57.9 69.0 134.9
37.7 43.4
...
Analyses of Commercial Acids Wt. % Recovered Acid No., Mg. KOH/G. As oila As acid Orig. Recovered 197 201 1 9 96 8 1 2b 98 5b 177 176 0 1 97 0 278 281
Acid Animal fatty acid Hydroxystearic Lauric Naphthenic, distilled 5.3 Oleic 1.8 Stearic 0.6 Tall 1.6 All recovered oils were acid-free. Average of 2 determinations. Table 111.
85.1
85.1 114.6 144.4 376-
0.03 0.02& 0.22b 5530 0.04 1500
Solvent-extracted Lightly acid-treated Acid-treatedc Rlid-cont,inent residual oils Asphaltic 99.1 Deasphalted and den-axed 99.9 Deasphalted 99.8 Silicone fluids Don- Corning 550 97.8O Dow Corning 710 98.4O Esters Dioctyl adipate 99.9 Dioctyl azelate 99.8" Hercoflex 600 99.2 a Average of 2 det,erminations. Average of 3 determinations. e From Kinkler crude. Table 11.
T-iscosity, S.S.U. At 100" F. At 210" . F Orig. Recovered Orig. Recovered
92.3 98.2 99.3 98.4
244 199 196 191
260 194 197 194
Separation of Synthetic Mixtures
(Weight 70) Oil SAE 20
Thickener hlised hcids
Hercoflex 600 Dioctyl azelate Dioctyl adipate
Lithium stearate Lithium stearate Lithium stearat,e
30
0
ANALYTICAL CHEMISTRY
Taken 88.6 89.0 88.2 90.0 90.3 89.7 90.7 90.0
Oil Found 88.5 88.8 88.2 89.8 89.8 89,6 90.4 89,9
Acid Found 11.51 11.15 11.95 10.00 9.93 9.42 9.89 10.03 10.20 9.04 9.09 9.78 9.78
Taken 11.39 11.00 11.75
studies. The greases contain known amounts of oil and acids (Table V). Recoveries are excellent ; the small errors probably reflect the difficulty of preparing and storing a grease of accurately known composition. The chromatographic and ASTM methods were compared by analyzing six greases by both methods. The two methods are essentially equivalent in accuracy (Table VI). Yeither gives significantly higher or lower recovery of oil; in only one case did the two methods differ by more than 0.5 weight %. Agreement in recovery of acids is excellent; the largest difference is 0.29 weight %. Extraction appears to recover more acid from some samples than does chromatography. However, in such instances acids recovered by ASTM extraction have acid numbers about 5 units lower than acids recovered by chromatography. Three factors may contribute to the differences: Traces of water may remain in acids recovered by liquid extraction; acids may partially esterify with traces of alcohol in ethyl ether used for extraction; and the larger volumes of solvents used for extraction may contribute to higher blanks. All the data suggest that the method is accurate within 0.5 Tyeight yo oil and 0.3 weight yo acid for the majority of soap-thickened greases, including those made from ester and silicone oils. It is equivalent to the ASTM procedure in precision and accuracy. DISCUSSION
The chromatographic method is faster and simpler than other published methods. Grease is decomposed and ready for analysis in only 10 minutes; the column is prepared and the fractions are eluted in about 3 hours, One operator can analyze four to six samples within 8 hours. 50 refluving or liquid extraction is necessary. The procedure can be mastered quickly by nontrchnical personnel; the reported data were obtained by nontechnical personnel. The method is not limited seriously by the absence of a specific method for determining fat. I n the fern instances when fat content is desired, there are two alternatives. If the grease is made from a highly refined oil, as indicated by a light color, oil and fat can be separated chromatographically by preparing the column nith 2 5 7 , chloroform in n-hexane, eluting oil from the column with 200 ml. of 25Y0 chloroform in n-hexane, and eluting fat with 250 ml. of chloroform; acids are eluted as described. If the grease is made from a less refined oil, the oil-plus-fat fraction from chromatography can be saponified (1, Section 15). Fatty acids derived from the fat can then be
separated from oil by liquid extraction or chromatography. Interferences from minor constituents in greases were studied. Kater, glycerol, and free alkali are retained in the chromatographic column and do not interfere. Of various fillers sometimes added to greases, only calcium carbonate may cause difficulty; calcium ethanesulfonate may precipitate when the grease is decomposed with ethanesulfonic acid. The precipitate should be rinsed into the column, to ensure that no oil or fatty acid is lost by occlusion. Interferences from other occasional constituents of greases were also studied. Fat, sperm oil, sulfurized sperm oil, and sulfurized lard oil were carried through the entire procedure (Table VII). The first three are eluted largely as oil. If sulfurized oils are present a t the normal maximum level of about 1%, recovery of oil is about 1% high, and recovery of acid is slightly high. Highly polar compounds, such as petroleum sulfonates, are retained in the column; if grade 950 silica gel is used, they are partially eluted. A variety of other compounds are sometimes added to special-purpose greases. Before the method is applied to such greases, the chromatographic behavior of the additives should be investigated to determine n-hat errors they may cause. Bentone and carbon black thickeners are not recoverable; organic thickeners other than soaps may be eluted with oil. Therefore, the chromatographic method is recommended only for soap-thickened greases. Other sources of error were investigated. Solvent blanks are negligible if ACS-grade solvents are used. The amount of ethanolamine used in preparing the adsorbent limits the capacity of the column t o about 3 grams of fatty acid; if more is taken, it may be partially eluted with oil. Another source of error with volatile oils-loss during removal of solvent of the steam bathis minimized by removing the last traces of solvent in a vacuum chamber.
CONCLUSION
Further applications of the chromatographic method are being studied. Many soluble oils contain oil and fatty acids or soaps. but the composition varips so widely that no single analytical method can be uwd. The chromatographic method can be used by an experienced analyst, after he establishes that the soluble oil contains mostly petroleum oil and fatty acid or soap. Elperience TI ith a ne\T type of grease, thickened with urea derivatives, suggests that the thickener can be separated from oil under chromatographic condi-
tions similar to those given for separating oil and fat. This separation is being studied further. Another application is t o the analysis of organic sulfonates. By a modification in procedure, oil, sulfonate, and salts are separated and recovered. This Table IV.
Precision for Three Commercial Greases
(Weight %) Type of Grease Oil ..\luminum stearate 97.3 97.4 98.1 98.1 Av. 97.7 Calcium hydroxystearate 91.0 91 .0 91.1 91.2 91.4 Av. 91.1 Lithium hydroxystearate 90.1 90.1 90.4 Av. 90.2
Table V.
Oil Dioctyl adipate Petroleum
Acid 1.79 1.79 1.79
1.80
1.79 7.62 7.98 7.90 7.93 7.61 7.81 8.32 8.44 8.41 8.39
method will be the subject of a future publication. ACKNOWLEDGMENT
The authors are indebted to P. R. McCarthy, chairman of Subsection 1, Section I, Technical Committee G, ASTM Committee D-2, for permission to quote the data on the composition of the greases used in the ASTM cooperative study. LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
adelahia. Pa.. “ASTM Standards;” Part VI‘ ?Jethod D 128-47, pp. 69-80, 1955. (2j Boner, C. J., \Tilliams, G. A,, A S T M Bull., KO.148, 7 5 (1947). (3) Bryant, W. C., Ibid., No. 148, 77 (1947). (4) Coenen, C. B., Urner, R. S., Inst.
Spokesman, ,Vutl. Lubrzcatzng Grease Inst. 17, No. 5 , 8-17 (1953). (5) Gothard, N. J., ASTM Bull., S o . 148, 78 (1947). (6) Kaufmann, H. P., Schmidt, O., Fette u . Setfen 47. 294 (19401. (7) Lederei-, E., “Lederir, N‘., “Chroma-
tography,” pp. 114-17, Elsevier, Sew Yo&, 1953. ( 8 ) Lubrication 42, S o . 1 (January 1956).
Analyses of Greases Used in ASTM Cooperative Studies
(Weight %) .4STl\I Thickener Lithium stearate Lithium hydroxystearate
Silicone Petroleum
Lithium stearate Calcium hydroxystearate Average of 3 determinations. * Contains 0.5% unidentified additive. Average of 5 determinations.
So. GI-53
Acid Oil Calcd. Found Calcd. Found 80 0 80 0 19 50 1’) 12
GI-%
91 4
GI-56 GI-58
94 1 93 6
80 0 79 7
GI-65*
91 6 91 lC
90 2a
8 44 5 80
19 57
8 :Wa 5 95 19 13
7 86
7 8lC
0
Table VI.
Comparison of Chromatographic and ASTM Methods
Oil, Wt. yc Thickener Chrom. AISTIID 128 .4luminum stearate 07.2 9 7 , ia Calcium stearate-acetate 88.0 87.3 Calcium soap of tallow acids 00.1 8 9 . 6c Calcium hydroxystearate 91. I d 91,l” Lithium hydroxystearate 90.2e 88.9c Soda soap of tallow acids 92.3 02, -P a Average of 4 determinations. * -4cetic acid not recovered. c Average of 2 determinations. iiverage of 5 determinations. * -4verage of 3 determinations.
Table VII.
Recovery of Minor Components
(Keight yC) Fat (beef tallom-j Sperm oil Sulfurized sperm oil Sulfurized lard oil
As Oil As Licid 98.3 1.7
97.8 72.8 38.3
2.1 26.9 56.7
Acid, Wt. yc Chrom. h S T l I D 128 1.795 1 6 09* i.38 7 81d s.39e
6.13
6 .lt5b 7 . 6Gc
8.10“ 8 . 40c 6 . :3@
(9) McConville, H. A, A S T J l Bull. S o . 148, ii (1947). (10) Palit, S. R., 1x11. ESG. CHEM., i 2 s . k ~ .ED.18,246 (1946). (11) Palmer, W. S., A S T N Bull., S o . 148,78 (1947). RECEIVEDfor review June 10, 19-57. Accepted September 3, 1957. Group Session on Analytical Research, 22nd Midyear Meeting, American Petroleum Institute, Philadelphia, Pa., May 13 to 16, 1957. VOL. 30, NO. 1, JANUARY 1958
31
