Azelaic acid – the C9 dicarboxylic acid – has long been the thickener of choice when making lithium complex greases. Sebacic acid, the C10 analog, has enjoyed a minor share in this application. Recently however, changes in the sebacic acid market in the United States have made sebacic acid economically more attractive as an alternative to azelaic acid.
Of course, the immediate question for lube manufacturers and end users is whether a grease product using this alternative can deliver performance equivalent to that seen with azelaic acid. To address this question, this paper compares the performance of sebacic and azelaic acids when used as thickeners for lithium complex greases in both polar and non-polar basestocks.
COMPLEX SOAPS
All complex greases begin with making a complex soap thickener, such as lithium complex, calcium complex, aluminum complex and others. Complexation greatly improves the thermal robustness of the soap and allows the grease to function at temperatures beyond the capability of simple organic thickeners.
The most widespread are lithium complex greases, which represent about 35 percent of the lubricating greases produced in North America. They have a lesser but still important proportion of the greases made elsewhere in the world, according to NLG Internationals annual Grease Production Survey.
The diacid component of a lithium complex thickener is 2 percent to 5 percent of the formulation, and is responsible for the differentiating performance attributes of this class of greases. Adipic acid, azelaic acid and sebacic acid have all been used for a number of years, with azelaic acid being the workhorse thickener – although a patent search shows that the original development of lithium complex greases was done with sebacic acid.
Figure 1 shows the comparative physical properties of sebacic and azelaic acids. As seen here, the main difference in these two components properties is the lower melting point of azelaic acid. This results both from the odd number of carbon atoms in its hydrocarbon backbone and from the higher purity of sebacic acid. Technical-grade azelaic acid contains significant quantities of other diacids and monoacid impurities.
Sebacic acid is a widely used industrial intermediate, with an annual global demand of around 20,000 metric tons. In addition to its application in greases, sebacic acid is a key intermediate in engineering plastics, specialty adhesives and as a corrosion inhibitor in metalworking fluids and longlife antifreeze formulations.
Sebacic acid also is used to make disodium sebacate (DSS). DSS is approved for use in greases for incidental food contact under CFR 21.178.3570. Sebacate esters find application as low-temperature plasticizers, synlube base stocks, and in cosmetics as emollients and fragrance carriers. As with azeleate esters, the sebacate esters are frequently used as liquid proxies for the corresponding acid in making greases.
SHIFTING MARKETS
Both sebacic acid and azelaic acid are oleochemical derivatives. Azelaic acid is produced via the ozonolysis of oleic acid. The emergence of the biodiesel industry in North America, Europe and Asia has significantly increased the industrial demand for soy and corn oils, the principle sources of oleic acid. This has exerted upward price pressure on the key raw material for azelaic acid.
Sebacic acid is a castor oil derivative. It is produced via the cracking of ricinoleic acid, the primary fatty acid component of castor oil. In addition to being the raw material for sebacic acid, ricinoleic acid is an important raw material for the grease industry as it yields 12-hydroxystearic acid (12- HSA) upon hydrogenation.
India is the primary source of castor oil production, with China and Brazil as secondary producers. China dominates world sebacic acid production with only limited capacity operating elsewhere at this time. There was domestic U.S. production until 2005. Up until that time, sebacic acid prices in the United States were artificially high because of an anti-dumping duty. That duty has since been rescinded and U.S. pricing has harmonized with the rest of the world.
India is the primary source of castor oil production, with China and Brazil as secondary producers. China dominates world sebacic acid production with only limited capacity operating elsewhere at this time. There was domestic U.S. production until 2005. Up until that time, sebacic acid prices in the United States were artificially high because of an anti-dumping duty. That duty has since been rescinded and U.S. pricing has harmonized with the rest of the world
INTO THE LAB
How to compare the performance of sebacic and azelaic acids? We designed a study to see how each can be used to make greases in non-polar and polar base stocks. Four lithium complex greases were prepared, two each in a synthetic hydrocarbon base fluid (80 percent polyalphaolefin plus 20 percent alkylated naphthalene) and the other two in a polyol ester (Inolex Chemical Co.s Lexolube 68-HT).
One grease in each base stock was formulated using sebacic acid as the complexing agent while the other was formulated using azelaic acid. (Details of the preparation and analytical methods have been published elsewhere, and are available upon request.)
These base fluids, possessing wide differences in polarity, were selected to demonstrate the utility of the acids to produce grease in a broad range of oils. The PAO blend (made with Chevron Phillips Chemicals Synfluid PAO- ) was used for the synthetic hydrocarbon work to give a controlled composition. Also we believe, the results for the synthetic hydrocarbon greases should be applicable to mineral oil base stocks.
The grease formulations used in the study are shown in Figure 2, and their chemical and physical properties are reported in Figure 3.
As Figure 3 shows, the properties of the two hydrocarbon greases were very similar. The sebacate grease showed better thermooxidative stability. Further work is needed to determine if this effect is due to better inherent oxidation resistance by sebacic acid over azelaic acid, or if the effect results from a more favorable response to the antioxidants used in the formulation.
The water washout was also markedly better for the sebacate grease. This was more likely because of variability in the test method rather than a real difference between the performance of the two diacids. Wear properties and low-temperature viscosities were very close.
For both ester greases, the dropping point was lower than expected. While there are insufficient data to draw a firm conclusion, it is possible that the effect is due to the small preparative scale and methods used. It is important to note that the same effect was observed with both greases. Thus it appears to be an artifact of the process rather than due to the choice of complexing agent. Azelaic acid gave a softer grease and xhibited a better low-temperature viscosity. Otherwise, the physical and wear properties of the two ester greases were virtually indistinguishable.
RESULTS: VERY SIMILAR
In both cases, sebacic acid appears to facilitate the greasemaking process. This may be due to the more homogenous composition of the carboxylic acid. After neutralization and subsequent cooling to affect thickener formation, the sebacic acid based greases possessed a smoother appearance. Homogenization eliminated the difference in appearance of the base greases.
The results show that sebacic acid is a viable alternative to azelaic acid in making lithium complex greases. The basic physical and wear properties of greases made in both polar and non-polar base fluids are very similar. While some minor formulation adjustments may be necessary, grease manufacturers can use sebacic acid as an economical replacement for azelaic acid without sacrificing performance.