Transactions of the ASAE Vol. 45(6): 1715–1719 E 2002 American Society of Agricultural Engineers ISSN 0001–2351 1715 OXIDATION CHARACTERISTICS OF SOYBEAN OILS AS WATER PUMP LUBRICANTS S. Fernando, M. Hanna ABSTRACT. The use of soybean oil as an alternative lubricant is being investigated extensively, but its relatively rapid thermal and oxidative polymerization restricts its usefulness. Measurement of polymerized oil film thickness and film weight, as methods for quantifying the degree of polymerization, were investigated. The miscibility and polymerization behavior of crude soybean oil, proprietary commercial biobased drip oil (BDO), soybean methyl esters, glycerol, and water and their mixtures were investigated. In a separate study, the antioxidant characteristics of Tenox, 2,2,4,5,5–Pentamethyl–3–imidazoline– 1–oxyl (PMIO), ethylene–diamine–tetra–acetic acid (EDTA), and ionic copper in acidic and alcohol media each in 1% (w/w) concentrations and their formulations mixed with soybean oil were investigated. In the first study, measurement of polymerized film weight was preferable to measurement of film thickness for quantifying the extent of polymerization. Soybean methyl esters showed the least degree of polymerization. In the next study, ionic copper in acidic media showed the highest resistance to polymerization, followed by the mixture of PMIO and Tenox. The treatments that contained EDTA and Tenox had antioxidant properties that were superior to that of the BDO. PMIO tended to promote the polymerization process. Keywords. Soybean oil, Lubricants, Viscosity, Soybean oil polymerization, Soybean oil oxidation. oybean oil is a versatile product used in numerous industrial applications. Commercially available soybean oil based products include biodiesel pump lubricants, wire rope lubricants, hydraulic oils, diesel lubricity additives, detergents, fragrances, and even explosives. There is now a strong, worldwide demand for sustainable, environmentally friendly alternatives to fossil fuels and petroleum–based products. Biodegradable lubricants made from renewable resources, such as soybean oil, perform as well as conventional lubricants, with less harm to the environment (Fernando and Hanna, 2001). Considering these advantages, many new soybean oil based products have been patented, and some have been introduced into the market (Cannon and Honary, 2000; McShane et al., 2000; Honary, 1999; Lawate and Lal, 1995; Lal et al., 1995). A key factor restricting soybean oil and its derivatives as functional industrial lubricants is their tendencies to poly- merize. Vegetable oils have performance limitations, partic- ularly in thermal, hydrolytic, and oxidative stabilities. Triglycerides of soybean oil contains over 85% unsaturated fatty acids like oleic and linolenic acids. Under favorable conditions, such as in the presence of heat and light, oxygen easily can be added to the double bond and form radicals (Keller et al., 2001). In addition, bis–allylic hydrogen in Article was submitted for review in February 2002; approved for publication by the Power & Machinery Division of ASAE in October 2002. The authors are Sandun Fernando, ASAE Student Member, and Milford Hanna, ASAE Fellow Engineer, Director, Industrial Agricultural Products Center, Kenneth E. Morrison Professor of Biological Systems Engineering, University of Nebraska, Lincoln, Nebraska. Corresponding author: Milford Hanna, Industrial Agricultural Products Center, University of Nebraska, 211 L.W. Chase Hall, Lincoln, Nebraska 68583–0730; phone: 402–472–1634; fax: 402–472–6338; e–mail: mhanna1@unl.edu. methylene–interrupted polyunsaturated fatty acids is very susceptible to free radical attacks and, therefore, peroxide formation and production of polar oxidation products (Erhan, 2001). These free radicals lead to formation of long chain molecules called polymers, which are gummy substances, and some shorter chain molecules. Oxidative polymerization of soybean oil results in increased acidity, corrosion, and viscosity. A challenge in polymerization studies is choosing among the many methods for quantifying the extent of polymeriza- tion. Thin film micro–oxidation test, oil stability index, pressurized differential scanning calorimetry, and rotary bomb test are among the techniques widely used to measure oxidative stability of oils (Erhan and Asadauskas, 2000). In this study, an attempt was made to identify a method for quantifying the extent of polymerization that simulates field conditions. Commercially available antioxidants work in two differ- ent ways. One type acts as a free radical scavenger. The scavenger antioxidant converts available free radicals to non–radical products, eliminating their availability to poly- merize. Fried (1995, p. 240, 243) stated that antioxidants include organic compounds like hindered phenols and aromatic amines that act as free radical scavengers. Another type suppresses the homolytic breakdown of the chain structures, preventing the formation of radical species with reactive sites. Organic phosphites belong to this category. Perhaps one of the most constructive recent research efforts, in terms of finding an antioxidant for industrial soybean oils, was done by Jiang (2000). He found that esterifying soybean oil fatty acids with di– or mono–hydroxy alcohols increased oxidative stability, compared with glycer- ol esters. Oxidative stability generally increased as the number of fatty acid chains per molecule decreased. Ethelene glycol esters (EGE) had greater oxidative stability than soybean oils, and methyl glycol esters had the greatest S