Ashless additives and new polyol ester base oils formulated for use in biodegradable hydraulic fluid applications
Published in: Lubrication Engineering
Date: 9/1/2002
By: Costantini; Oshode; Reyes-Gavilan; Duncan
There are no specific regulatory requirements in the US mandating hydraulic fluids to he ecologically benign. However, there is currently an increasing national and worldwide trend towards the use of products for industrial, commercial, and even household applications that have minimal impact on the environment. Certainly, this direction is influencing hydraulic fluids and related industries as well.
This paper will show that new biodegradable polyol ester base stocks formulated with the appropriate ashless additive technology outperform vegetable oils both hydrolytically and oxidatively. The study will also demonstrate that modified versions of the ASTM D 943 and D 4310 tests, in which no water is employed, are very suitable for the evaluation of the long-term oxidative stability of biodegradable polyol esters. Finally, an indepth discussion of carbodiimide acid scavenger technology will be presented.
KEY WORDS
Acid Scavenger; Antiwear; Ashless Additives; Biodegradable; Hydraulic Fluid; Polyol Esters; TOST Life Test; TOST Sludge Test; Vegetable Oils
INTRODUCTION
Polyol ester base stocks are currently used in several industrial and transportation lubricant applications where demanding performance conditions exist. They have excellent low temperature properties, additive solvency, lubricating performance, and thermal and oxidative stabilities suitable for use in these systems. Typical polyol ester structures are presented in Diagram I,
where R is represented by either linear or branched alkyl chains ranging in length from -C^sub 4^H^sub 9^ through -C^sub 12^H^sub 25^. A more precise description of the polyol ester base oils employed in this study can be found in Table 1.
While many esters thermally decompose at temperatures ranging between 250 deg C and 316 deg C (Dukek, et al. (1962)) via the proposed mechanism in Diagram II, polyol ester base oils thermally decompose typically at temperatures above 280 deg C (Dukek, et al. (1962)) via the proposed mechanism in Diagram III.
The absence of the beta hydrogen in polyol ester molecules allows these base stocks to withstand high temperature lubrication conditions where other types of esters would thermally degrade. It is in jet engines, where cooling air temperatures can reach 300 deg C, that formulated oils based on polyol ester base stocks found their first widespread use as lubricants (Klamann, (1984)). During the early 1960’s, pioneering work was undertaken to stabilize the lubricating fluid for a revolutionary new supersonic plane – the “Concorde” (Hamblin, et al. (2000)). Special types of metal free additives were required to design a lubricant stable at temperatures not previously encountered. In Concorde’s Rolls Royce Olympus 593 engines an ester based lubricant, stabilized by ashless antioxidants, was finally the solution (Byford, et al. (1968), (1971)). For this work, the Queens Award was presented in 1970 to the two companies involved (Queens Award, (1970)).
The objective of this work is to show that new polyol ester base oils formulated with the appropriate ashless additive package are also suitable for use in biodegradable antiwear hydraulic fluid applications. The authors will demonstrate that these oils meet the performance requirements of several hydraulic fluid specifications presently in effect in this market. The study will also show that procedures such as ASTM D 943 (ASTM, (1997)) and D 4310 (ASTM, (1997)) are inappropriate to evaluate the long-term oxidative stability of polyol ester base stocks or their formulated counterparts. Modified versions of these procedures, in which water is excluded from the test, can be found in the literature (ISO, (1999), SS, (2000)). The authors will show that these tests are very useful tools in demonstrating the superior oxidative stability of polyol ester lubricants to that of vegetable oils.
ENVIRONMENTAL CONSIDERATIONS
There are no specific regulatory requirements in the US mandating hydraulic fluids to be ecologically benign. However, there is increasing national as well as worldwide recognition to preserve our environment so that future generations can benefit from at least the same standards of living available today. This will require alternative products for use in industrial, commercial, and even household applications.
Biodegradability of the finished hydraulic fluid is an important property to many end users for applications where contamination of our delicate ecosystem may take place due to fluid leakage from the installation. While vegetable oils are employed in many of these applications, there are other base stocks available in the market today that are also readily biodegradable. Table 1 shows that new polyol ester base stocks and their formulated counterparts, evaluated as potential candidate fluids for use in antiwear hydraulic lubricant applications, are readily biodegradable. Their rate of decomposition is greater than 60 % in 28 days as evaluated by the Manometric Respirometer Test (OECD, (1993)). The procedure uses a non-acclimated inoculum, and measures the total breakdown of the base stock by recording the amount of oxygen consumed in 28 days of testing.
Biodegradability is primarily a function of the base fluid composition. Additives do not tend to biodegrade, however, they may affect biodegradability when employed in the formulation at concentrations greater than 10 %, or when they are toxic to bacteria (i.e. fungicides). Additive package P- 1, as can be seen from Table 1, does not inhibit the biodegradability profile of these polyol ester base stocks. The difference in biodegradability results between the base stock and finished product is well within the error of the test.
The Manometric Respirometer test has been chosen to determine the biodegradability of these fluids for several reasons. It is an accepted procedure in most parts of the world, since the test is less man-power intensive than other biodegradability procedures. The test method tends to produce a lower standard deviation when a large number of samples are evaluated at the same time. If a sample passes the test, it will pass most other biodegradability procedures employed in this market. It is a closed system capable of analyzing samples containing solvents (OECD, (1993), Duncan, et al. (1998)).
One important consideration is that ashless additive packages do not introduce heavy metals into the environment should the hydraulic system experience a lubricant leak (Clark, et al. (2000)).
RESULTS AND DISCUSSION
Table 2 presents physical properties generated for formulated ISO 32 and ISO 46 polyol ester base stocks employed in this work. Rapeseed oil is included in the study in order to compare the performance of the fully formulated polyol esters to that of a fully formulated vegetable oil. Vegetable oils are currently used in many commercial biodegradable hydraulic fluid applications. The fluids’ performance is presented in Tables 3 to 8 and is compared in each case to regular grade hydraulic fluid specification requirements for this industrial market sector. These specifications are typically designed for hydraulic fluids based on mineral oils (Denison Division, (1983), Vickers, (1989), Cincinnati Machine, (2000), DIN, (1987), AISI, (1996), ASTM, (1998)). They have been chosen for this work because they were in effect in the Americas Region at the time this study was conducted.
The ashless antiwear additive package P-I utilized in these formulations is an off-the-shelf product employed in mineral oil stocks to meet the requirements of the MIL-17331H specification (Department of the Navy, (1995)) and has not been optimized for use in antiwear hydraulic applications. The package consists of a mixture of antioxidants, corrosion inhibitors, metal deactivators, and antiwear/extreme pressure (AW/EP) agents aimed at stabilizing mineral oil base stocks against the ravages of oxidation while simultaneously protecting the system metallurgy against corrosion, rust and wear. P-I has been utilized in this study, since previous experience indicates that many of the components in this package are suitable to successfully stabilize polyol ester stocks.
Corrosion
The metallurgy in a hydraulic pump must be properly protected against corrosion and rust in order to achieve adequate system performance. Corrosion is the removal of metal by the action of an acid. It is a process by which tolerances can become wider and leakage between parts can occur. Rust takes place when iron or carbon steel oxidizes. It is a process by which the part gains weight, tolerances become narrower, and moving parts seize.
Table 3 presents the corrosion performance of the formulated polyol ester and vegetable lubricants employed in this study. The results show that all products pass the requirements of the DIN 51524 and ASTM D 6158 specifications for the D 130 copper corrosion test (ASTM, (1997)), the only two specifications in Table 3 which have a limit for this test.
Rust
For the ASTM D 665B (ASTM, (1997)) rust test, both formulated polyol ester lubricants pass the requirements of the Denison HF-2 and ASTM D 6158 specifications, even though, the concentration of P-I in the ISO 32 base oil had to be increased to 0.95 wt. %. These are the only two specifications in Table 3 which have a limit for this part of this test. The vegetable oil, even when formulated at a concentration of 0.95 wt. % P-1, does not pass the rust test.
Hydrolytic Stability
Table 3 also presents the results of hydrolytic stability evaluations conducted via ASTM D 2619 (ASTM, (1999)) with both formulated polyol ester fluids and formulated vegetable oil. The data are compared to the limits set by the Denison HF-2 specification for this procedure, the only specification in this table that requires this test to be conducted on a candidate fluid. While the total acid number (TAN) obtained for the water layer at the end of the test with both formulated polyol esters is slightly higher than the maximum allowed by the specification, its impact on the weight change and appearance of the copper strip is negligible. Evaluation of the formulated vegetable oil, however, yields a very high TAN value for the water layer, indicating that the fluid is much less hydrolytically stable than the formulated polyol ester lubricant evaluated in the study.
Demulsibility and Foaming
Good interfacial fluid characteristics are essential to the proper performance of any hydraulic system. A hydraulic fluid with poor demulsibility performance fosters corrosion of the metallurgy, increased leakage, collection of contaminants, sticky valves, wear within pumps as well as oxidative catalyzation of the hydraulic fluid (Sperry Vickers, (1982)). A fluid with poor foaming characteristics can yield erratic pump behavior. Table 4 presents the interfacial characteristics of these formulated polyol ester lubricants. The results show that both formulated polyol ester lubricants meet demulsibility and foaming performance requirements of those specifications in Table 4 which have limits for these tests. The formulated vegetable oil, however, generates a much higher initial amount of foam, making it unable to pass the ASTM D 892 (ASTM, (1999)) performance requirements of the ASTM D 6185 specification.
Oxidative Stability – D 943 and D 4310 Evaluations
Hydraulic oils must be thermally and oxidatively stable in order to generate minimal amounts of sludge within their expected service life in the installation. Sludge and acids can respectively coat moving parts, plug filters, or corrode the system’s metallurgy inhibiting the proper operation of the unit.
Several hydraulic fluid specifications currently in effect in this market do not require an oil to be evaluated via the TOST Life (ASTM D 943) and TOST Sludge (ASTM D 43 10) tests. However, industry members have developed a high degree of comfort with both procedures and rely on the results of these to estimate long-term oxidative stability and quality of a particular hydraulic lubricant formulation. Present market indicators suggest that hydraulic oils with minimum TOST lives of 2000 hours and TOST sludge below 200 mg offer advantageous long-term oxidative performance to many end users in this industry. This level of oxidative performance meets or exceeds minimum requirements for the few specifications employed in this study that reference these tests. TOST Life and TOST Sludge performance, however, are not correlated to actual oxidative field service life for any hydraulic oil (Sun Company, (1978)).
Table 5 presents the long-term oxidative performance of the formulated polyol ester lubricants employed in this study as evaluated by the ASTM D 943 and ASTM D 43 10 tests. The results are very poor, since neither fluid evaluated could meet minimal performance requirements expected by the industry with either procedure.
Table 5 also presents the TOST life performance of the ISO 46 polyol ester base oil formulated with ashless packages P-2 and P-3. The aim of this work is to determine whether different additive components and/or ratios improve the performance of this base oil as per the conditions of this test. P2 is an ashless antiwear package containing antioxidants, metal deactivators, corrosion inhibitors, and EP/AW agents. P-3 is an ashless R & 0 additive package also containing antioxidants, metal deactivators and corrosion inhibitors, and was included in the study in order to determine whether the EP/ AW agents in the package were contributing to the instability of the fluid observed via the TOST test. Packages P-1, P-2, and P-3 are all different in composition.
The results show that these formulations all perform similarly to each other and to the corresponding base oil; all are below minimum industry expectations. The fact that these TOST failures are occurring very early (i.e. approximately 500 hours) suggests that the degradation taking place is independent of the package employed. Because the first TAN measurements have been taken at the 500-hour point of the TOST test as required by the ASTM D 943 procedure, very little difference in performance has been observed between the results obtained with the formulated fluids and the base fluid.
Measuring the TAN on a more frequent basis (daily) after the start of the ASTM D 943 procedure has been conducted for the P-I formulated ISO 32 polyol ester lubricant. The results show that:
* TOST life failure occurs very early for the lubricant (i.e. 65 hours).
* There is no difference in TOST Life performance between the formulated fluid and its corresponding base fluid.
Table 5 also presents the levels of dissolved metals in the fluid after TOST Life evaluation. The concentration of copper and iron in the neat ISO 32 base fluid is very high, while that for the formulated base fluid is very low. These data suggest that P-1 in the formulated fluid is effectively protecting the metallurgy present in the test.
ASTM D 4310 results show that determination of the sludge could not be measured due to the formation of gels or very viscous fluids within the TOST tube. These gels have been attributed to metal carboxylate soaps formed from organic acids and metals present in the test (ASHRAE, (1998)).
These ASTM D 943 and ASTM D 43 10 results suggest that the degradation in performance observed with these polyol ester fluids is hydrolytic and not oxidative in nature. The results also indicate that these tests are too severe to adequately measure the long-term oxidative stability of polyol esters and vegetable oils.
Oxidative Stability – Modified D 943 and D 4310 Evaluations
In order to confirm that water plays a detrimental role in the TOST performance of these formulated polyol ester lubricants, the long-term oxidative stability has been evaluated via modified versions of the ASTM D 943 and ASTM D 43 10 procedures. The modification entails the removal of water from both tests. The results are reported on Table 6 and are graphically presented in Fig. 1, where TAN has been plotted against dry TOST Life in hours for the neat base fluids as well as for formulated polyol ester fluids. As expected, the unadditized base fluids show poor oxidative stability, but the fully formulated polyol esters containing package P- 1 now yield TOST lives of 5585 hours before reaching a TAN of 2.0. The levels of dissolved metals in both formulated polyol esters at the conclusion of the test are negligible. Similarly, the results generated with the dry ASTM D 43 10 test show a dramatic improvement in oxidative performance, since minimal amounts of sludge were produced for both formulated fluids.
These results indicate that hydrolysis is the cause of the early failures observed for both unmodified versions of the ASTM D 943 and ASTM D 43 10 procedures as presented in Diagram IV (Roberts, et al. (1939)). These procedures are, therefore, inappropriate to evaluate the long-term oxidative stability of fully formulated polyol ester lubricants. The results also indicate that P-1 is very successful in stabilizing the base oil against the ravages of oxidation.
Comparative D 943 and D 4310 Evaluations vs. Vegetable Oils
Figure 2 presents the dry TOST Life (modified ASTM D 943) performance of several vegetable base oils. The results show these oils behave similarly yielding poor long-term oxidative performance. When these vegetable oils are formulated with additive package P-1 at a slightly higher concentration than that employed in the polyol ester fluids, their long-term oxidative performance continues to be very poor and very similar to that generated with the unformulated vegetable base oils; please see Fig. 3. While package P-I may not be optimized for use in vegetable oils, previous work has shown that vegetable oils respond similarly to the same type of additives used to stabilize mineral oils. However, due to their poor oxidative stability treat levels are much higher in order to obtain a useful level of lubricant performance. This work suggests, therefore, that if certain components in P-1 were to be added at much higher concentrations than present levels employed to stabilize these polyol esters, the resulting package may provide acceptable long-term oxidative performance to these vegetable oils.
The reason that the oxidative stability of vegetable oils is inferior to that of these polyol ester base stocks lies in their chemical structure. Diagram V presents a detailed oxidation mechanism for vegetable oils (Sherwin, (1978)).
The resonance stabilization of the allyl free radical enhances oxidation of the vegetable base oil. Proposed mechanisms for polyol ester oxidation show that degradation occurs at the -(CH 2)n- portion of the acyl alkyl chain and at the methylene alcohol carbon (Bakunin, et al. (1992)), a process which does not occur as readily as that in vegetable oils due to the absence of double bonds in the chain.
While the oxidative stability of vegetable oils is poor according to the bench test results presented in this study, these fluids are currently and successfully employed in many hydraulic field applications, which operate under conditions which are mild enough to be tolerable for these oils (Leugner, (1998)). Moreover, properly formulated polyol esters, given their inherent higher oxidative stability, should perform as well as if not better than vegetable fluids when utilized in these installations. In systems running under more demanding thermooxidative conditions, these formulated antiwear hydraulic polyol ester lubricants offer a definitive oxidative performance advantage over that afforded by formulated vegetable hydraulic oils.
Oxidative Stability – Extended Tests
Table 7 presents results generated with other oxidative stability tests that are run under dry conditions or employ minimal amounts of water. The data obtained via ASTM D 2893 (ASTM, (1998)) show minimal changes in TAN and viscosity for both of the formulated polyol ester lubricants evaluated. These results, together with those generated via the dry TOST Life test in Table 6, suggest that the ISO 46 polyol ester base oil is inherently the more oxidatively stable of the two base fluids.
The results generated for the formulated fluids with the ASTM D 2272 RBOT procedure (ASTM, (1998)) are very good and exceed the minimum requirements set by US Steel 136, the only specification in Table 7 having a limit for this test. The level of RBOT performance obtained with the polyol ester base fluids is similar to that generated with many unadditized mineral oils, suggesting that minimal hydrolysis has taken place for these fluids under the conditions of the test. The contribution to any oil solubilization of the copper coil employed in the ASTM D 2272 test by carboxylic acids hydrolyzed from these particular polyol esters is, therefore, expected to be small. Solubilization of copper catalyzes the oxidative degradation of any lubricant and has a negative impact upon the RBOT performance of the finished fluid (Hamblin, et al. (1989)).
The Cincinnati Machine (CM) (Cincinnati Machine, (1995)) results generated for both formulated polyol ester lubricants are below the sludge specification limits (25 mg/ 100 ml max.) set by Cincinnati Machine for their P-68 and P-70 specifications, by ASTM D 6158, and by Denison’s HFO specification (100 mg/100ml max.) (Denison Hydraulics, (2001)). The formulated vegetable oil fails the CM test, since its % viscosity increase exceeds the maximum level set by the P-68 and P-70 specifications.
Wear Performance
Table 8 presents the wear performance for both formulated polyol ester lubricants evaluated in this study via the Four Ball wear test (ASTM D 4172) (ASTM, (1997)), the FZG Gear test (DIN, (1984)), and the ASTM D 2882 Vickers V 104 C pump test (ASTM, (1997)). The FZG Gear test results, evaluated for the ISO 46 formulated fluid only, are very good. These results show that the product exceeds the maximum load achievable with this procedure, thus surpassing minimum requirements set by the DIN 51524/2 and US Steel 136 specifications.
The results of ASTM D 2282 wear test evaluations are well below the limit of the specifications listed in Table 8 requiring this procedure to be conducted on a candidate fluid. The results show that excellent lubrication has been achieved in this medium duty vane pump, an indication that the EP/ AW agents in the package are adequately protecting the unit’s metallurgy. These formulated polyol ester lubricants must pass the Denison TC 6 vane pump in order to fully comply with all the requirements of the HF-2 specification.
The Four Ball wear scar diameters generated with the formulated polyol ester lubricants are well below the maximum level set by US Steel 127 and 136, the only specifications in Table 8 which have a limit for a procedure similar to ASTM D 4172. These wear scars generated with the formulated lubricants show an appreciable decrease in wear vs. results obtained with their unadditized base fluid counterparts.
One Step Further
Acid scavenger additive technology has been evaluated in order to assess its usefulness in extending ASTM D 943 test performance with these polyol ester lubricants. The technology, however, is not designed to prevent hydrolysis of these types of base stocks. It functions by eliminating acidic degradation products (i.e. carboxylic acids) that result from the hydrolysis of the polyol ester base fluid.
The data generated has been limited to results obtained via the ASTM D 943 test in the presence of amounts of water as required by the procedure. One candidate acid scavenger, A-4, a substituted diphenyl carbodiimide, has been evaluated in the ISO 32 polyol ester base fluid. No other additives have been employed in the formulation in order to show the efficacy of this product in prolonging the TOST life of this base stock.
The mechanism for the reaction of carboxylic acids with carbodiimides has been proposed by Schotman as shown in Diagram VI (Schotman, (1991)).
Whether the reaction proceeds to products via an intramolecular rearrangement from the O-acyl to the N-acyl urea, or through products (IV) and (V) to the N-acyl urea, depends upon the type of carbodiimide employed and the pKa of the carboxylic acid.
Aromatic carbodiimides undergo intramolecular rearrangement to the N-acyl urea (III), while aliphatic carbodiimides form mainly species (IV) and (V). A possible explanation for this behavior may be that the imino nitrogen of O-acyl ureas (11) is less basic for aromatic carbodiimides than in the same specie formed from aliphatic carbodiimides. Consequently, protonation of the imino nitrogen followed by carboxylate attack is slower with aromatic carbodiimides allowing for intramolecular rearrangement to take place to the N-acyl urea (Schotman, (1991)).
The speed of the reaction between the O-acyl urea and the carboxylic acid also depends on the pKa of the latter in the particular solvent or medium (i.e. base oil) in which the reaction is taking place. When the reaction is slow, the intramolecular rearrangement to the N-acyl urea is favored over products (IV) and (V) (Schotman, (1991), Wiener, et al. (1986)).
The TOST Life performance of the ISO 32 polyol ester base oil formulated with acid scavenger A-4 is graphically presented in Fig. 4. As its concentration increases, the TOST Life performance of the oil also increases. At a concentration of 0.3 wt. % A-4, a TOST life of 2100 hours is obtained. These results suggest that the intramolecular rearrangement reaction pathway is favored with the particular carbodiimide/ base oil/carboxylic acid system evaluated in this study. The pathway in the mechanism leading to the anhydride (IV) and the 1,3-disubstituted urea (V) would most likely regenerate carboxylic acids and show limited improvement in TOST Life performance of the oil.
While the acid scavenger helps the fluid attain an impressive TOST Life performance, these results should not be misinterpreted. A-4 is “fooling” the test by reacting with the acid produced from the hydrolysis of the polyol ester base fluid. The results do not suggest that the formulation has been properly stabilized oxidatively, since the color of the fluid at the end of the test is green indicating the presence of copper compounds formed by the corrosion of the metal. A properly formulated lubricant contains antioxidants, corrosion inhibitors, and metal deactivators. If AW/EP agents are necessary for a specific application, the appropriate ones should be included as well in the final product. Examples of properly formulated lubricants employing carbodiimide acid scavenger technology can be found in the literature (Nadasdi, (2000)).
When acid scavenger A-4 is evaluated via the ASTM D 943 test in several vegetable base stocks at the same concentration employed in the polyol ester base oil, no improvement in TOST Life performance is observed. This demonstrates that vegetable oils are much less hydrolytically stable and hence more difficult to stabilize than the polyol ester base oils evaluated in this study.
CONCLUSIONS
The authors have shown that ashless antiwear package P1, even though not optimized for use in this application, allows new formulated polyol ester lubricants to meet the performance requirements of several antiwear hydraulic fluid specifications currently required in this market. The results show that polyol esters formulated with the appropriate ashless technology outperform vegetable oils both hydrolytically and oxidatively; please see Table 9. This advantage will translate into improved performance for hydraulic systems operating under mild or under demanding thermooxidative conditions.
The study also reveals three important points about the TOST Life and TOST Sludge tests with regard to their applicability in evaluating the long-term oxidative stability of vegetable oils and polyol esters:
* Both procedures are inadequate to evaluate the longterm oxidative performance of formulated vegetable oils and polyol esters, since water employed in these tests hydrolyzes these fluids causing early failures in the TOST Life and gelling in the TOST Sludge procedures.
* The modified versions of ASTM D 943 and ASTM D 4310, which exclude the use of water, are very suitable to evaluate the long-term oxidative stability of formulated polyol esters. Both modified procedures, however, are too severe to evaluate the performance of vegetable based hydraulic fluids.
* Acid scavenger technology “tricks” the TOST test into generating long lives by keeping the TAN below 2.0. This technology is expensive and may be unnecessary since many field applications show that the base stocks are not exposed to severe hydrolysis conditions.
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Final manuscript approved June 8, 2002 Review led by Roger Malley