Article pubs.acs.org/IECR
Elastohydrodynamic Properties of Biobased Heat-Bodied Oils Girma Biresaw,*,† Brajendra K. Sharma,†,‡ Grigor B. Bantchev,† Todd L. Kurth,†,§ Kenneth M. Doll,† Sevim Z. Erhan,†,∥ Bidhya Kunwar,‡ and John W. Scott‡ †
Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604, United States ‡ Illinois Sustainable Technology Center, Prairie Research Institute, University of Illinois, Urbana−Champaign, 1 Hazelwood Drive, Champaign, Illinois 61820, United States ABSTRACT: Heat-bodied oils were prepared by thermal treatment of soybean oil under inert atmosphere. Different viscosity grades of heat-bodied oils synthesized by varying the reaction time were investigated for various properties including viscosity, viscosity index, elastohydrodynamic film thickness, and pressure−viscosity coefficient. Heat-bodied oils displayed elastohydrodynamic film thickness characteristics typical of lubricating oils. The film thickness of heat-bodied oils increased with increasing entrainment speed and viscosity, decreased with increasing temperature, and was unchanged with varying load. Pressure−viscosity coefficients of heat-bodied oils were estimated from film thickness as well as from physical property data. The pressure−viscosity coefficient values of heat-bodied oils increased with increasing viscosity and decreasing temperature and were in the range displayed by such oils as polyol esters, poly-α-olefins, and petroleum-based base oils. Heat-bodied oils provide access to a wide viscosity range of biobased oils not attainable with vegetable oils, without serious negative impact on critical lubricant properties such as viscosity index, film thickness, and pressure−viscosity coefficient.
1. INTRODUCTION Currently, most lubricants on the market are formulated from petroleum-based ingredients. These lubricants, while performing well in the intended lubrication application, produce undesirable environmental and health effects such as poor biodegradability and toxicity to wildlife and humans.1,2 A great deal of effort is underway to replace petroleum-based lubricants with biobased lubricants that are as good or better in performance and cost.1−4 Farm-based vegetable oils play a prominent role in the development of biobased lubricants.1−15 Their use in lubricant formulations provides a number of economic, environmental, and health benefits.1−4 Successful application of vegetable oils in lubrication will produce a new market for surplus oil-bearing crops such as soybean, thereby improving the income of farmers and strengthening the rural economy. Also, the application of vegetable oils in lubrication will result in formulations that are generally nontoxic and safe during manufacture, use, and disposal. In addition, use of vegetable oils will produce lubricant formulations that are biodegradable and, hence, not a threat to the environment and wildlife in case of accidental release or spill into the environment.1−4 Studies have shown that vegetable oils possess a number of inherent tribological properties that make them superior to petroleum-based products. Among these properties are excellent antiwear properties,16−18 high viscosity index (VI),3 low Noack volatility,3,11 low traction coefficient, and enhancing the performance of extreme pressure (EP) additives.13,19 However, vegetable oils also display some inherent properties that make them less desirable than petroleum-based products.3,5−8 Among the less desirable properties are poor oxidation stability, too-high cold flow temperatures (pour point and cloud point), poor bioresistance, and poor hydrolytic stability.1−4 © 2014 American Chemical Society
Numerous approaches are being developed to overcome the various weaknesses of vegetable oils for use in lubrication.3,6,9,10,12,20,21 One broad area under development involves modification of the structure of vegetable oil molecules by chemical, enzymatic, or thermal reactions.1,21 In this work, we explore a subset of this approach involving the use of thermal treatment of vegetable oils, under inert (N2) atmosphere, to produce higher molecular weight lubricating oils of various viscosity grades.10,22−38 Biobased oils produced by this procedure have been referred to by different names including drying oils,22−24 heat-bodied oils,25 thermally polymerized oils,26−29 thermally modified oils,10,30 and metathesized oils.31 They have also been investigated by a variety of methods32,33 for different applications including polymers and composites,22 printing ink,35−37 foams,38 and lubricants.10,29,30 Application of heat-bodied oils in lubrication requires a thorough knowledge of their tribological properties. An important property of lubricants is generating oil film to separate friction surfaces operating under hydrodynamic, elastohydrodynamic (EHD), and mixed film regimes.39 The oil film must be of sufficient thickness to lower the coefficient of friction between and prevent wear of friction surfaces. To date, the film-forming properties of heat-bodied oils have not been investigated. In this work, we discuss our investigation of the film-forming properties of a series of heat-bodied oils under EHD conditions. The film thickness of several heat-bodied oils was measured as a function of load, temperature, and entrainment speed. The data were analyzed to estimate the pressure−viscosity coefficients (PVC) of the oils and also to Received: Revised: Accepted: Published: 16183
July 24, 2014 September 16, 2014 September 23, 2014 October 7, 2014 dx.doi.org/10.1021/ie5029304 | Ind. Eng. Chem. Res. 2014, 53, 16183−16195
Industrial & Engineering Chemistry Research
Article
determine the effect of heat-bodied oil structure on filmforming properties. The oils synthesized for this investigation are designated by alphabets that correspond to their viscosity range on the Gardner−Holdt viscosity scale.40
Table 1. Apparent Molecular Weight and Polydispersity Index of Soybean Oil and Heat-Bodied Oilsa oil soybean oil
2. EXPERIMENTAL SECTION 2.1. Materials. Soybean oil (SBO, RBD-grade) was obtained from KIC Chemicals (New Paltz, NY) and used as received. Isopropyl alcohol, 99+%, and hexanes, 99+%, used for cleaning the film thickness measurement instrument and specimen were obtained from Aldrich Chemical Co. (Milwaukee, WI). The glass disk and steel balls used in film thickness experiments were supplied by PCS Instruments (London, England) and had the following specifications: glass disk, 100 mm diameter by 12 mm thick float glass, coated with about 20 nm semireflecting chrome layer, which in turn is coated with ∼500 nm thick silica spacer layer; steel balls, precision steel ball, super-finished 3/4-in. diameter, G10 carbon Cr steel (AISI 52100). The nitrogen used in the synthesis was PurityPlus 4.8 (PurityPlus Specialty Gases, Jacksonville, IL) with the following specifications: purity, 99.998%; oxygen,
