Finished Lubricants

Resisting Oxidation


For many years and until recently, lubricating greases were divided by type of thickener into four major groups: greases based on soaps; nonorganic types; organic; and hydrocarbon-thickened greases. This article discusses a more recent type: oils using overbased thickeners, including overbased sulfonate and alkylsalicylate thickeners. The smallest particles of such greases structural skeleton are micelles and supermicellar formations of overbased calcium sulfonate or alkylsalicylate, obtained by modifying colloidal calcium carbonate with molecules of calcium sulfonate or alkylsalicylate.


The process of making conventional greases by thickening oil with overbased calcium sulfonate was first described by R.L. McMillen in 1966, in U.S. Patent 3242079 (Basic Metal-Containing Thickened Oil Compositions). However, application of those greases turned out to be limited because of their poor performance properties.

The next steps – and of interest here – were overbased sulfonate complex greases which appeared two decades ago, and even newer overbased complex alkylsalicylate greases, which are 15 years younger than sulfonates. Such overbased complex greases contain calcium metaborate and 12-hydroxy stearate, along with the overbased sulfonate or alkylsalicylate.

The reason for todays high interest in complex greases is their enhanced mechanical and thermal stability, extraordinary resistance against the action of water, good anticorrosion and lubricating properties. Overall they are on a par with – and by some individual measures they even exceed – multipurpose lithium complex and aluminum complex greases, as well as polyurea greases.

Due to these properties, overbased sulfonate complex greases are finding successful application in heavily loaded friction units, including those working in the presence of humid and aggressive media, and also as greases for deck and other sea machinery. Since the specific mass of overbased sulfonate complex greases is higher than that of water, they are also used in underwater technical equipment.

Composition and structure of over-based sulfonate complex greases are such that they can find successful application in many friction units without anti-wear, extreme pressure and anticorrosion additives. The only weak side of sulfonate greases is their relatively not-so-good resistance to oxidation, and thus to improve sulfonate greases performance characteristics it is necessary to use antioxidant additives.

To improve resistance of overbased greases against oxygen action, formulations of overbased alkylsalicylate complex greases have been developed and samples prepared. Now these greases are being tested on bench-stands, and preliminary results show that overbased alkylsalicylate complex greases are on the same performance level as the sulfonate greases, practically by all performance characteristics.

Inside the Molecule

Studies of overbased sulfonate complex grease have shown that metaborate and 12-hydroxy stearate cause changes in the composition of micelles of calcium over-based sulfonate (Image 1). Molecules of calcium metaborate penetrate into the composition of the carbonates nuclei which, in the final analysis, results in disordering of the gels structure.

In contrast to this, molecules of calcium 12-hydroxy stearate can be included into the composition of the micelles envelope, where they impart additional aggregative stability to a thixotropic system. (Thixotropic, NLGIs Lubricating Grease Guide reminds us, refers to lubricating greases ability to decrease in consistency, or soften, as a result of shearing, then to increase in consistency, or harden, once the shearing is stopped.) As seen in Image 2, sulfonate complex grease consists of micelles with nuclei including colloidal calcium carbonate and metaborate and the adsorbable envelope of sulfonate and calcium 12-hydroxy stearate.

Sample Selection

With the end purpose of determining the oxidation stability of sulfonate and alkyl-salicylate greases, researchers at the Ukrainian Scientific and Research Institute of Oil Processing Industry MASMA in Kiev, designed a two by four modal system of samples, representing the initial, intermediate and final stages in the process of overbased complex grease formation.

Ordinary overbased calcium sulfonate and ordinary overbased calcium alkylsalicylate were included, plus systems that also included one of the components of a complex grease – calcium metaborate or 12-hydroxy stearate. A fourth system contained overbased calcium sulfonate or calcium alkylsalicylate, with metaborate and 12-hydroxy stearate. Thus eight grease model systems were obtained for the research, shown in Table 1.

Residual petroleum oil was used as a dispersion medium for all samples of greases. Physico-chemical properties of the oil are presented in Table 2.

All grease samples were oxidized in an AS-2 testing unit under dynamic conditions at 135o C. This procedure was described in the 1991 NLGI paper, Estimation of Grease Oxidation Stability Under Dynamic Conditions and Antioxidant Testing, by Ishchuk and Butovets. Technical grade oxygen at atmospheric pressure was used as an oxidizer. Regime of a chain non-branched oxidation reaction was provided with the initiator dicumenylperoxyoxide (CP).

Rate of Reaction

In accordance with the chain reaction theory of liquid phase oxidation (put forth in 1965 by Emmanuel et. al.), the rate of oxidation reaction can be expressed by the following formula:

W = a W0i + WiCP

Here, a =k2 k6 [RH] is the parameter which characterizes the oxidation of organic compounds. [RH] equals the substrate; k2 equals the rate constant of oxidation chains elongation; and k6 is the rate constant of recombination of peroxide radicals. W0 i represents the rate of radicals formation from substrate, and WiCP represents the rate of radicals formation from initiator.

Kinetic parameter is a constant value under given conditions of the conducted experiment and it depends on the substrate activity in reaction with peroxide radicals. This makes it convenient, using the parameter, to compare antioxidative stability of substances that are oxidized. Essentially, the lower the value, the higher the stability of a substance to oxidation.

Following testing of all grease samples in the procedure referenced above, MASMA researchers calculated the values of the parameter for the eight overbased sulfonate and alkylsalicylate greases systems. The results of these kinematic parameters are presented graphically above.

Comparing Results

As the graphed results demonstrate, an ordinary overbased alkylsalicylate grease (MA-1), by its resistance to oxidation, is about two times superior to that of ordinary overbased sulfonate grease (MS-1). This is explained by the inhibiting action of the alkylsalicylate hydroxyl group.

In the other samples, introduction of components which enter into the composition of overbased complex greases, such as calcium metaborate and 12-hydroxy stearate, can be seen to have a varying influence upon the oxidation stability.

Calcium metaborate, in the MS-2 and MA-2 samples, influences positively on the antioxidation stability of these systems, decreasing the oxidation rate and the value of their parameter.

However, uncombined calcium 12-hydroxy stearate, used in samples MS-3 and MA-3, increases the reaction rate and decomposition of hydroperoxides, which were present in the petroleum oil or formed in the process of oil manufacturing. As well, salts of fatty acids (soaps) are bifunctional catalysts. As a result, the concentration of free radicals and total oxidation rate of the greases are growing, as becomes apparent in the increase of the parameter value. In model systems MS-3 and MA-3, molecules of calcium 12-hydroxy stearate partially combined with carbonate nuclei, therefore the oxidation rate of these systems is slightly higher than oxidation rates of the initial MS-1 and MA-1 greases.

Finally, overbased complex sulfonate (MS-4) and alkylsalicylate (MA-4) greases are characterized by substantially better resistance to oxidation as compared with the MS-1 and MA-1 ordinary overbased greases. The complex carbonate-metaborate nuclei of these systems, reliably bonding calcium 12-hydroxy stearates molecules, paralyze their catalytic activity completely and the additional aggregative stability of the thixotropic system contributes to the natural inhibitive action of the petroleum oil, making it more effective.

Hence, an overbased complex alkylsalicylate grease (MA-4) can be formulated which has much stronger stability to oxidation in comparison with the sulfonate grease (MS-4), and can find application in friction units which work under conditions of high temperatures and aggressive action of oxygen or other oxidizers.