ARTICLE pubs.acs.org/IECR
Polysulfide and Biobased Extreme Pressure Additive Performance in Vegetable vs Paraffinic Base Oils Girma Biresaw,*,† Svajus J. Asadauskas,‡ and Ted G. McClure§ †
Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University Street, Peoria, Illinois 61604, United States ‡ Center for Physical Sciences and Technology, Institute of Chemistry, Gostauto 9, Vilnius, LT 01108, Lithuania § TribSys LLC, 99 West 550 North, Valparaiso, Indiana 46385, United States ABSTRACT: Twist compression tribotester (TCT) and 4-ball extreme pressure (EP) methods were used to investigate commercial polysulfide (PS) and biobased polyester (PE) EP additives in paraffinic (150N) and refined soybean (SOY) base oils of similar viscosity. Binary blends of EP additive and base oil were investigated as a function of additive concentration. In addition to weld point (WP), 4-ball EP produced a set of preweld data, notably peak torque and wear scar diameter, which were found to correlate with WP results. TCT gave a 5-fold larger time-to-failure (TTF) for neat SOY than for neat 150N, whereas 4-ball EP gave similar WP (120 kgf) values for both neat oils. This difference was explained by invoking boundary contribution to TCT but not to 4-ball EP method. Both additives improved the WP and TTF of the base oils, which further increased with increasing additive concentration. However, the extent of the improvements was highly dependent on the chemistries of the additive and base oil of the blends. Thus, at similar concentrations, the WP of PE was higher in the 150N than in the SOY base oil, while the WP of PS was higher in the SOY than in the 150N base oil. Similarly, TTF of 150N was higher with blended PE than PS; whereas for SOY, it was higher with blended PS than PE. This chemistry effect was attributed to relative compatibility between EP additives and base fluids. The results suggest that a substantial reduction (up to 4-fold) in EP additive usage in commercial lubricant formulations can be achieved through proper selection of compatible base fluids and additives.
1. INTRODUCTION Lubricants in which some or all of the ingredients are derived from agricultural or similar sources are called biobased lubricants.1 4 These lubricants possess a number of potential benefits over petroleum-based lubricants. One of the major benefits of such lubricants is the fact that they have a renewable and abundant raw material base. In contrast, petroleum reserves that can be easily, safely, and cheaply extracted are dwindling and depleting fast. In addition to being renewable, biobased lubricants are also easy to biodegrade and, as a result, their manufacture, use, and disposal will cause minor harm to the environment (air, water, and land). Farm-based raw materials used in biobased lubricant formulations are also nontoxic and, as a result, their manufacture, use, and disposal will not cause injury or pose safety and health risks to workers, consumers, and communities. However, successful development of biobased lubricants with all the listed potential benefits, while also being competitive to petroleum-based products both in performance and in cost, will require solving a number of rather tough technological and scientific problems. These problems can be broadly categorized into two groups. One set of problems deals with the rather inherent weaknesses of biobased raw materials in a number of properties of paramount importance to lubrication.5 8 Examples of such problems include: poor oxidation stability; poor cold flow properties; poor hydrolytic stability; poor biostability. Currently, a great deal of research effort is being applied by many workers in order to understand these inherent problems and to develop a variety of methods to overcome it. The other set of problems has to do with our state of knowledge about the tribological r 2011 American Chemical Society
properties of biobased raw materials. Unlike petroleum-based raw materials, which have been studied and developed over a period of a century, investigation into the tribological and tribochemical properties of biobased raw materials is rather new. As a result, we have very little information as to how biobased materials will behave and perform in various lubrication regimes. Investigations to date of biobased oils have focused mostly on their physical properties such as viscosity, viscosity index, pour point, cold point, and oxidation stability.1,9 15 Some studies have been initiated into the tribological properties of biobased oils in boundary, hydrodynamic, elastohydrodynamic, and mixed film regimes16 23 but more needs to be done. Studies dealing with investigations into the tribochemical properties of biobased oils are almost nonexistent. Overall, the state of knowledge about the tribological properties of biobased oils needs to be extended and expanded in order to accelerate development and commercialization of performance and cost competitive biobased lubricants. As mentioned above, very little work has been done about the tribochemical properties of biobased oils. Tribochemical properties play a key role in lubrication processes that occur under extreme conditions24 These conditions include high load, high temperature, low speed, and the presence of reactive surfaces (catalyst) and are generally referred to as extreme pressure (EP) Received: July 19, 2011 Accepted: October 28, 2011 Revised: October 13, 2011 Published: October 28, 2011 262
dx.doi.org/10.1021/ie2015685 | Ind. Eng. Chem. Res. 2012, 51, 262–273
Industrial & Engineering Chemistry Research
ARTICLE
lubrication conditions. Under such conditions, the oil undergoes in situ chemical reactions and forms new materials that may have a positive or negative impact on the lubrication process. In order to ensure a positive outcome, it is necessary to develop knowledge about the mechanisms of tribochemical reactions of biobased oils under EP conditions. In this paper, we report our investigation into the tribological properties of soybean and paraffinic mineral oils under EP conditions, with or without added EP additives, using two different tribometers: 4-ball EP and twist compression tribotester (TCT). As detailed in the next section, 4-ball EP is widely used to evaluate the EP properties of gear oils, metalworking fluids (MWFs), and various industrial lubricants, whereas TCT is commonly used to evaluate the EP properties of MWFs. EP additives are an important class of additives widely used in metalworking and other lubricants.24 30 Their unique chemical structure is that they contain reactive elements such as halogens (e.g., chlorine), sulfur, and phosphorus. Under extreme lubrication conditions (high pressure, high temperature, and low speed), EP additives undergo tribochemical reactions with metallic friction surfaces and generate tribo-films in situ that lower friction between and prevent damage to friction surfaces. The effectiveness of EP additives is a complex function of their chemical structure, concentration, properties of basestock and friction surfaces, and lubrication process conditions. Another factor that may have bearing on the performance of EP additives is its interaction with the base fluid and other ingredients in the lubricant formulation. In the work described here, we investigated the effect of chemical structure of two commercial EP additives and two base oils on the performance of binary blends under extreme pressure conditions. The selected EP additives were a commercial polysulfide and biobased polyester, both of which are widely applied in automotive and industrial formulations, metalworking in particular. A clearly pronounced tendency to increase utilization of this type of biobased EP additives can be observed in the lubricant industry. Such polyester additives are produced by reacting diacids (e.g., succinic acid) and polyols with fatty acids from renewable raw materials. Recently, many lubricant additive manufacturers have prioritized such biobased EP additives in their recommended formulations for EP lubricants. The selected base oils were refined food grade soybean oil and solvent refined paraffinic mineral oil (150N oil). The two base oils have similar viscosities but differ widely in their chemical structures. The soybean oil comprises three ester groups (triglyceride) and, hence, is polar, while the paraffinic 150N has no functional group and is nonpolar. Various concentrations of each EP additive in each base oil were prepared, and the resulting binary blends were investigated for its EP properties. The results are then analyzed to investigate the effects of additive and base oil chemical structures as well as additive-base oil interactions.
Alkali-refined food grade soybean (SOY) oil, CAS RN 800122-7, was obtained from Archer Daniels Midland Company (Decatur, IL, USA) and had the following specification: specific gravity, 0.92 g/mL (ASTM D 4052); kinematic viscosity at 40 and 100 °C, 32 and 7.6 cSt, respectively (ASTM D 445); free fatty acid, 0.5 mg KOH/g (ASTM D 974); pour point, 9 °C (ASTM D 97). ISO VG 32 solvent refined heavy paraffinic mineral oil (150N oil), CAS RN 64741-88-4, was obtained from American Refining Group (Bradford, PA, USA) and had the following specification: specific gravity, 0.86 g/mL (ASTM D 4052); kinematic viscosity at 40 and 100 °C, 30 and 5.5 cSt, respectively (ASTM D 445); pour point, < 9 °C (ASTM D 97); sulfur content, < 300 ppm (ASTM D 5183). Isomeric mixture of di-t-dodecyl pentasulfide sulfurized EP additive, CAS RN 68425-15-0, was obtained from Atofina Chemicals, Inc. (Philadelphia, PA, USA) and had the following specification: specific gravity, 1.00 g/mL (ASTM D 4052); kinematic viscosity at 40 °C, 120 cSt (ASTM D 445); sulfur content, 31% w/w (ASTM D 129). PS is a clear yellow liquid and is widely used in metal forming lubricant formulations. A high molecular weight polyester (PE) EP additive was obtained from the Lubrizol Corporation (Spartanburg, SC, USA) and had the following specification: kinematic viscosity at 40 °C, 2000 cSt (ASTM D 445); free fatty acid,
