Anal. Chem. 1995, 67, 1632-1636
Polyol Ester Synthetic Refrigeration Lubricant Analysis by Supercritical Fluid Chromatography Jeffrey M. Carey* and 0. Paul Sutton The Lubrizol Cotporation, 29400 Lakeland Boulevard, Wickliffe, Ohio 44092
Supercritical fluid chromatography with flame ionization detection has been applied to the analysis of polyol ester fluids used as syntheticrefrigerationlubricants. Through the observation of the peak patterns obtained in the chromatograms, it is possible to gain knowledge of the carboxylic acids from which the esters were derived, polyol base structure, reaction scheme, and degree of reaction completeness. These results can be obtained in a qualitative to semiquantitative manner with minimal analysis of reference standards. In addition, it is possible through statistical techniques, and use of standards, to quantitatively determine the amount of the acids used in the esterification of the polyol. Quantitation of the amount of Merent polyols is possible without standardization. In addition, it is possible to detect and quant& any impurities and/or free hydroxy groups in the fluid. A final important capability is the detection and identificationof performance-enhancingadditive materials in the lubricants. Results are presented demonstrating each of these attributes, and a description of the method is provided. With increasing concerns regarding the impact of chlorofluorocarbons (CFC) on the environment, and governing regulations mandating their replacement, the need has arisen for the develop ment of alternate refrigeration lubricants. Potential replacements for mineral oil-based fluids used with the CFC refrigeration gases are poly01 esters (POE).' These fluids have been developed because of the incompatibility of mineral oil-based lubricants with the various fluorocarbon (HFC) replacements. In order to properly develop these lubricants it is necessary to have the ability to characterize the composition of the fluids2q3 in order to understand structure/performance relationships with the new HFC refrigeration gas systems4 While spectroscopic methods such as NMR and mass spectrometry are capable of determining the structures of the esters in the fluids,2there is important information requiring the use of chromatographic methods of analysis. This information relates to the method of preparing the fluid (blending of multiple esters versus reaction of multiple acids and/or polyols in a single reactor). A chromatographictechnique may be the best method of detecting the presence of any additives in a fluid. There are several possible chromatographic methods for the analysis of POE fluids: liquid chromatography, gas chromatography,and supercritical fluid chromatography. (1) (2) (3) (4)
Komatsuzaki, S.; Homma, Y. Serkiya Gakkaishi 1994,37, 226. Black, K D.;Gunstone, F. D. Chem. Phys. Lipids 1990,56,169. Coates, J. P. ASLE Trans. 1986,29, 185. Nutui, R.: Maties, M.: Nutui, M. J. Syrrth. Lubr. 1990,7,145.
1632 Analytical Chemistry, Vol. 67, No. 9,May I , 1995
Supercritical fluid chromatography (SFC) , first introduced some 30 years has received increasing interest in recent years due to several potential advantages it may possess over conventional liquid and gas chromatographic techniques. SFC provides faster analysis and better resolution than LC6-8 and is appropriate for the analysis of nonvolatile and thermally labile materials not amenable to GC separation. In addition, SFC provides a high degree of flexibility in controlling component elution characteristicsgJOand utilizes a variety of detection schemes not available to GC or LC individually. SFC was investigated for the analysis of synthetic rebigeration lubricant (SRL) materials because of concerns about the thermal stability of potential fluid additives and concerns relating to the volatility of some of the higher molecular weight fluid components which may preclude GC analysis. The higher chromatographic efficiency of SFC (over LC)is required for analyzing these fluids because of the similarity of structures (and molecular weights) of the individual components. EXPERIMENTAL SECTION The supercriticalfluid chromatograph used was a Suprex M E 225 combination SFC-SFE unit (Suprex Corp., Pittsburgh, PA). Carbon dioxide was the mobile phase in all experiments (SFC/ SFE grade, Air Products and Chemicals, Inc. Allentown, PA). Separation was provided through the use of a 20 cm x 1 mm x 5 m packed column with a Deltabond phenyl stationary phase (Keystone Scientific, Inc., Bellefonte, PA). Pressures were maintained through the use of an integral restrictor (Suprex) rated at 70 mWmin. The temperature of the supercritical carbon dioxide was maintained by an oven operated at 85 "C. Flame ionization detection (FID) was utilized, which operated at 385 "C and was formed from high-purity air and hydrogen (Matheson, Miamisburg, OH) at flow rates of 800 and 90 mWmin, respectively. The pressure program used for the separation is shown in Table 1. Data collection was provided by the TURBOCHROM software package (Version 3.2, Perkin-Elmer Nelson) operating on an IBMcompatible 486DX2 computer. Additional data manipulation was provided by an in-house Visual BASIC program and Microsoft Excel. Samples of poly01 esters composed of the basic structures shown in Figure 1 were prepared in optima grade chloroform (5) Klesper, E.: Cotwin, A H.: Turner, D. A J. Org. Chem. 1962,27,700. (6) Smith, R. M., Ed. Supercritical Fluid Chromatography; Royal Society of Chemistry: Cambridge, U.K., 1990. (7) Lee, M. L., Markides, K E., Eds. Analytical Supercritical Fluid Chromatography and Extraction: Chromatography Conferences, Inc.: Provo. UT, 1990. (8) Smith, R D.; Wright, B. W.; Yonker. C. R. Anal. Chem. 1988,60, 1323A (9) Smith, R. D.: Chapman, E. G.; Wright, B. W. Anal. Chem. 1985,57,2829. (10) Kuppers, S.: Grosse-Ophoff,M.; Klesper. E. J. Chromatogr. 1993,629,345.
0003-2700/95/0367-1632$9.00/0 0 1995 American Chemical Society
Table 1. Chromatographic Conditions and Pressure Program
condition
value
oven temp, "C detector temp, "C stationary phase restrictor flow, mL/min injection size, L
85 385 phenyl 50 1
stage
time, min
hold ramp hold
5 35 5
HO
pentaerythritol 0 II HO-C-R
carboxylic acid (R - typically 4-8 carbons)
pressure, atm
100 100-345 (7 atm/min) 345
0 2 4 6 8 101214161820222426283032343638404244 "e(min)
HO bimethylolpropane
x::
Figure 2. SFC-FID chromatogram of fluid I: (A) solvent, chloroform; (e) monopentaerythritol esters; (C) dipentaerythritol esters; (D) tripentaerythritol esters. Table 2. Fluid I Analysis
polyol
amt, %
Figure 1. Examples of mal rials u d for the formation of polyol esters used as synthetic refrigeration lubricants.
monopentaerythritol dipentaerythritol tripentaerythritol tetrapentaerythritol
88.9 9.9 1.2 trace
(Fisher Scientific, Pittsburgh, PA) and were introduced into the SFC instrument via a Gilson autosampler (Gilson, Middletown, Wr). Injection onto the column was performed utilizing a Valco injection valve (Valco, Houston, nr> with a 1 p L sample loop. Standards were prepared from in-house esterifications of given acids and polyols in a manner similar to that described in ref 11. Commercial samples were analyzed as received. Concentrations were approximately 0.5%(w/w) in the analyzed solution.
technique
% c5
% c9
SFC-FID (MPE) SFD-FID OPE) 13C NMR
71 73 75
29 27 25
neopentylglycol
RESULTS AND DISCUSSION Poly01 Identi6cation. Supercritical fluid chromatography provides a means of determining the carbon number and the structure if appropriate standards are available. As an example, the fluid in Figure 2, fluid I, has been analyzed by SFGFID. Upon first examination of the chromatogram, it is clear that, in addition to the solvent peak, there are three distinct clusters of peaks. Each of these clusters is representative of the esters of pentaerythritol (PE) and the dimer and trimer of PE. The identification of the peak clusters as indicated in Figure 2 was conlirmed by the use of standards. Quantitation of the amount of each poly01 base incorporated into the esters is possible based upon the total peak area of each of the observed clusters. Simple peak area measurements have proven to be accurate for determining the relative amounts of the poly01 base materials. This conclusion has been substantiated through the analysis of known mixtures of different poly01 base compositions. The results of polyol base analysis for fluid I are indicated in Table 2. The same type of information is attainable for esters containing different poly01 bases. Acid Identi6cation. Identification of the acid($ used in the esterification of the polyol(s) is performed in a similar manner. Figure 3 shows an overlay of the chromatograms of n-pentanoic (11) Jolley, S. T. Eur. Patent Application No. WO 9012849, 1990.
Intanslty 1,200,000
1,oo0,000
t
800,000
600,000
400,000
2w,wo 0 J ,
"
1
I
~
;
;
;
;
;
I
;
~,
:
,
'
10 12 14 16 16 20 22 24 26 28 30 32 34 36 38 40 42 44
Tima (mln)
Figure 3. Overlaid chromatograms of fluid I, n-pentanoic acid/ monopentaerythritolester, and isononanoidmonopentaerythritolester.
acid/MPE ester, is nonanoic acid/MPE ester, and fluid I. By comparing the retention times for the pure ester component fluids with those for the peaks for fluid I, it can be determined that the first peak in the fluid I chromatogram is the result of the pure n-pentanoic acid/MPE ester, and the last peak in the first cluster is likely from isononanoic acid/MPE. The presence of these acids in the fluid has been confirmed by NMR analysis, and no other acid functionality was detected. It is interesting to note that Analytical Chemistty, Vol. 67,No. 9,May I , 1995
1633
despite the ability to identify these two tetraacid esters by SFC, with the proper standards, there are a number of other peaks observed in the chromatogram. These peaks result from the statistical distribution of the two acids on the four poly01 reaction sites (hydroxyl functionalities). The relative intensities of these peaks can be determined by the statistical permutation of two acids on four sites. For a twoacid system such as this, the binomial coefficient equation (1)
600,000
400,000
200,000
can be used to determine the expected relative responses for a 5050 mixture of the acids. Where n represents the number of reactive sites and y is the number of a specific acid incorporated into the ester. Alternately, Pascal‘s triangle (2) can be used to 1 1 1
0 0 2 4 6 8 101214161~20222426283032343638404244
Time (mln)
Flguro 4. SFC-FID chromatogram of fluid (I: (A) solvent; (B) monopentaerythritolesters; (C) dipentaerythritolesters; (D) phosphatebased additives.
1 2 1
1 3 3 1 1 4 6 4 1
Table 3. Additive Determination in Fluid II
additive triphenyl phosphate
determine the coefficients. In the analysis of fluid I, a 50:50 mixture of n-pentanoic acid and isononanoic acid reacted with monopenaterythritol, the chromatogram should yield five peaks with the relative intensities 1 4 6 4 1. These values correspond to the following acid components on a MPE poly01 ester, respectively: four n-pentanoic acids; three n-pentanoic acids, and one isononanoic acid acid; two n-pentanoic acids and two isononanoic acids: one n-pentanoic acid and three isononanoic acids; and four isononanoic acids. From this information it is possible to determine the relative amounts of each of the acids in the fluid. Quantitation is provided by determining the difference in the relative amounts measured from those expected from a 50:50 acid mixture. This approach assumes identical response factors for each of the acid esters. Studies to date have indicated that the assumption appears to be valid based upon a comparison of SFC to NMR and MS results for a variety of fluids. An approach such as this is also only valid for two acid mixtures (using the described statistics). With more complex calculations it would, in theory, be possible to quantitate more complex mixtures. Generally, however, there is not sufficient resolution to detect each of the esters individually in these more complex fluids especially since the carbon numbers of the acids used generally are in a limited range (C+&). Several alternatives, such as capillary SFC, should provide additional resolution and are currently under investigation. Table 2 illustratesthe SFC FID results for fluid I acid substituent quantitation on both the MPE poly01 base and DPE poly01 base. A comparison of SFCFID results to NMR results is also provided. Additive Detection. Another important aspect of any analysis of a potential lubricant is the ability to detect performanceenhancing additives. An example of such a fluid is shown in Figure 4 (fluid II) . This fluid contains two additives, presumably in this case used to provide increased wear protection. Once such an additive is detected by a screening method such as SFC, spectroscopic methods can then be utilized to determine its structure. Many spectroscopic methods are not amenable to 1634 Analytical Chemistry, Vol. 67,No. 9,May I , 1995
triphenyl thiophosphate
%by SFC
%byNMR
% by ICP-AES
1 6
3 7
2 8
detecting trace levels of material in a complex matrix such as these fluids. Therefore, some method of separation is often necessary for the initial detection of low-level additives. The SFC separation may lead to a set of conditions for preparative separations (SFE) for isolation of the additive prior to characterization. Alternatively SFC may be coupled with a spectroscopic detector, such as a mass spectrometer, for on-line characterization. SFC is of particular interest as an additive detection method due to the often reactive nature of additives. The high temperatures necessary for GC analysis of relatively high molecular weight materials such as these esters may cause the thermal degradation of the additives,leading to a loss in important information. The additives in this particular fluid are triaryl phosphate derivatives. Treatment levels in the fluid as determined by SFC agree well with the levels determined by NMR and ICP-AES (inductively coupled plasma atomic emission spectrometry). Table 3 indicates the levels determined by each of the techniques and also indicates the actual identity of the two additives detected in this fluid. The identities have been confirmed by 31PNMR and mass spectrometry. While the levels of the additives are relatively high for this fluid, the signal-to-noise ratio for these additive peaks (see Figure 4) indicates that additives present at much lower levels could be detected by SFC-FID analysis. Fluid FormulationScheme Determinations. An important aspect of SFU fluid analysis, particularly for unknown samples, is the ability to determine the method used to formulate the fluid. This may give information relating to the method by which the desired viscosity grade of the fluid is attained. Fluids I and I1 were formed from a “one-pot reaction” (all the reactants are reacted at the same time in a single reaction vessel) of the acids and polyols found in the fluid. This conclusion arises from having obtained the statistically expected number of peaks in the chromatogram’s peak clusters. If a cluster containing the statistically expected number peaks is not observed, then it is likely that
Intbnsity 1,000,ooo
800,000
600,000
1,000,m
400,000
II
I
200,000
0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 Tim (mln)
Figure 5. SFC-FID chromatogram of fluid 111: (A) solvent; (B) neopentylglycol ester; (C) monopentaerythritol esters; (D) dipentaerythritol esters; (E) triesters of monopentaetythritol.
the fluid was not prepared from a onepot reaction. From the types of deviationsfrom the expected statistical distribution, the means of preparing the fluid can often be determined. The chromatogram shown in Figure 5 for fluid I11 is an example of a fluid that was not prepared from a one-pot reaction. There are three different poly01 base structures in this fluid: neopentyl glycol (NPG), monopentaerythritol (MPE), and dipentaerythritol OPE). Peak B is the result of a single acid ester of NPG. This result is supported by the presence of only one peak in the cluster. If this had been the reaction of two acids with NPG, one would expect to observe three peaks in NPG cluster with relative intensities of 1:2:1 (for a 50:50 mixture). This ester is blended with a fluid prepared from a mixture of acids reacted with MPE and DPE, as indicated by the clusters of peaks C and D. Additionally, fluid I11 is an example of a mixture where the statistical analysis described earlier is not applicable. First, it is clear that the peaks in cluster C are not fully resolved. In addition, it is likely that there are more than the five expected peaks in the cluster. This indicates that this portion of the finished fluid was formed from a one-pot reaction of greater than two acids (three in this case, as determined by NMR). Another important piece of information that can be gathered from this chromatogram is the fact that the DPE esters (D) contain a pattern similar to that observed for the MPE esters. This indicates that the one-pot reaction contains the DPE starting material, either deliberately or as an impurity, or that the MPE has oligomerized during the reaction. Through peak pattern recognition and statistical analysis it is also possible to determine whether a reaction has been performed in a staged manner (one acid has been reacted with the poly01 prior to the addition of the second acid). If a peak in the chromatogram does not have the expected relative intensity for the ratio of acids (Le., the tetraester is too small for the expected distribution), then it is likely that the other acid is being charged to the reactor prior to the acid in question. Reaction and Hydrolysis Monitoring. One important consideration of SRL fluid performance is the ability to resist hydrolysis. In addition, it is desirable to know the amount of free hydroxyl functionality present in a fluid, since it can have a marked
0
2
4
6
8
10 12 14 16 18 XI 22 24 26 28
30 32 34 36 38 40 42 44
llmm (mln)
Flgure 6. Comparison of the chromatograms obtained for isononanoic acidhrimethylol propane esters with varying charge ratios: (A) ester
formed from a 2.3:lcharge ratio (CCnMP);(6)ester formed from a 2.7:lcharge ratio; (C) ester formed from a 3:l charge ratio. Table 4. Degree of Hydrolysis and/or Incomplete Esterlflcatlon
technique SFC-FID wet chemical acetyl OH SFC-FID wet chemical acetyl OH
isononanoic
acid/TMP 2.7:l 2.7:l 2.3:l 2.3:l
% TMP disubstituted trisubstituted
24 26
76 74
40
60
45
55
effect on the physical properties (viscosity and performance) of the material. A dficulty with such an analysis is that fluids with a high degree of hydroxyl functionality will be relatively polar. However, it has been shown12 that it is possible, despite the polarity increase, to analyze materials containing a high degree of nonfunctionalized hydroxyl groups by SFC employing an unmodified nonpolar carbon dioxide mobile phase, without derivatization of the analyte. Our experiences to date have also indicated this possibility. Figure 6 illustrates the chromatograms obtained for a series of isononanoic acid/TMP esters charged with different acid-topoly01 ratios. In addition, Table 4 indicates the amount of free hydroxyl groups present in the fluid, as determined by SFC-FID and wet chemical acetyl OH measurements (e.g., AOCS CD 13SO). It is clear from these results that these measurements do provide quantitative information corresponding to the hydroxyl content of these ester fluids. It is unclear whether this level of accuracy would be maintained for a fluid such as fluid 111, which (12) Chester, T. L.; Innis, D.P. HRC CC,J. High Resolut. Chromatogr. Chromatogr. Commun. 1986,9, 178.
Analytical Chemistry, Vol. 67,No. 9,May 1, 1995
1635
contains a small amount of the MPE triesters (E in Figure 5). This is due to the difficulties with measuring these low levels by the wet chemical method. However, the presence of such functionalities in fluids such as fluid I11 has been conhned by infrared spectroscopy and 'H NMR analysis. CONCLUSIONS Supercritical fluid chromatography provides a powerful analysis tool for the characterization of poly01 ester syntheticrefrigeration lubricants. Despite the utility of the technique, it does not provide all the necessary information about these fluids. Some method of spectroscopic analysis is required to c o n h n the structures of the esters. However, spectroscopic methods do not provide all the information that is attainable from chromatographic techniques. Chromatographic analysis required for formulation strategy determinations,additive detection, and measurement of low levels of higher oligomers of the base polyol structure (dimers, trimers, etc.). It has been demonstrated that quantifkation of poly01 ester acid functionality is possible for relatively simple mixtures that
1636 Analytical Chemistry, Vol. 67,No. 9, May 7, 7995
are chromatographically resolved. It has also been demonstrated that SFC provides insights into the method of poly01 ester fluid preparation: blending, staging, additive addition, etc. In addition, SFC has proven to be a reliable method for the quantitation of the base poly01 used in esterification. Finally, the amount of free hydroxyl groups in the fluid, as a result of incomplete reaction or hydrolysis of the esters, can also be determined. ACKNOWLEDGMENT The authors thank Ms. Denise M. Bayus, Mr. Kurt F. Wollenberg, and Dr. William D. Abraham for the NMR analyses. They also acknowledge Dr. James R Shanklin and Dr. G. Ray Malone for providing the model esters as well as the fully formulated fluids. Finally, the preliminary SFC work of Dr. John L. Buteyn on poly01 esters was greatly appreciated. Received for review October 20, 1994. Accepted February 18, 1995.@
AC941024P @Abstractpublished in Advance ACS Abstracts, April 1, 1995.
