Environ. Sci. Technol. 2009, 43, 2060–2065
Viscosity Modification of High-Oleic Sunflower Oil with Polymeric Additives for the Design of New Biolubricant Formulations L. A. QUINCHIA, M. A. DELGADO, C. VALENCIA, J. M. FRANCO,* AND C. GALLEGOS Departamento de Ingenier´ia Qu´imica, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus Universitario de El Carmen, 21071 Huelva, Spain
Received October 29, 2008. Revised manuscript received December 17, 2008. Accepted January 26, 2009.
Although most common lubricants contain mineral or synthetic oils as basestocks, new environmental regulations are demanding environmentally friendly lubricants. In this sense, vegetable oils represent promising alternatives to mineral-based lubricants because of their high biodegradability, good lubricity, and low volatility. However, their poor thermooxidative stability and the small range of viscosity represent a clear disadvantage to be used as suitable biolubricants. The main objective of this work was to develop new environmentally friendly lubricant formulations with improved kinematic viscosity values and viscosity thermal susceptibility. With this aim, a high-oleic sunflower oil (HOSO) was blended with polymeric additives, such as ethylene vinyl acetate (EVA) and styrenebutadiene-styrene (SBS) copolymers, at different concentrations (0.5-5% w/w). Dynamic viscosity and density measurements were performed in a rotational rheometer and capillary densimeter, respectively, in a temperature range between 25 and 120 °C. An Arrhenius-like equation fits the evolution of viscosity with temperature fairly well. Both EVA and SBS copolymers may be satisfactorily used as additives to increase the viscosity of HOSO, thus improving the low viscosity values of this oil. HOSO viscosityincreaseswithpolymerconcentration.Specifically,EVA/ HOSO blends exhibit higher viscosity values, which are needed for applications such as lubrication of bearings and fourstroke engines. On the other hand, viscosity thermal susceptibility of HOSO samples increases with EVA or SBS concentration.
Introduction Environment concerns are rapidly gaining in importance worldwide. Among many political and social pressures on governmental departments and organizations around the world, the introduction of the ecolabeling has had significant impact. The EU ecolabel scheme establishes criteria for groups of products and services in order to meet high environmental and performance standards. A relatively new group of products created in this scheme is that regarding lubricants (1). Besides this, the lubricant industry in the European Community must fit the REACH regulation (2) dealing with the registration, evaluation, authorization, and restriction of chemical substances. * Corresponding author phone: +34959219995; fax: +34959219983; e-mail:
[email protected]. 2060
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In general, chemical contamination of water has a remarkable negative influence on public opinion. More specifically, many coastal oil pollution cases occurring over the past few years have been reported (3, 4). For this reason, special attention is being paid to the protection of the environment against pollution exerted by lubricants and hydraulic fluids based on mineral oils. It has been estimated that the fate of around 1.1 million tonnes per year (20% of the total market) of used lubricating oils in Europe is not known. In this sense, it is believed that around 600,000 tonnes of loss lubricants are released into the environment every year (5). These products include chain saw oils, concrete mold-release (“shuttering”) oils, two-stroke engine oils, chassis greases, railway wheel flange greases, or lubricants for ski lifts and railway points greases (5-7). Nowadays, biodegradability has become one of the most important design parameters in both the selection of the base fluid and the overall formulation of the finished lubricant (8). In this sense, the demand for biodegradable lubricants is due to a growing concern for the impact that the industrial activity is causing in the environment (9, 10). Moreover, the lubricant industry has been trying to formulate biodegradable lubricants with similar or even better technical characteristics than those based on mineral oils. The volume of lubricants used in the industry, especially engine oils and hydraulic fluids, is relatively large, with most of them based on mineral oils. Lubricants based on vegetable oils still represent a very narrow segment. However, they are progressively being introduced into some specific applications. Thus, environmentally acceptable lubricants can be potentially applied as chain saw and saw frame oils, wire rope lubricants, marine oils and outboard engine lubricants, lubricants for the food industry, hydraulic fluids for different building machinery and agricultural equipment, lubricants for sewage-treatment plants, elevator oils, lubricants for snowmobiles and ski run maintenance equipment, etc. (3, 6). Among them, engine oils and hydraulic fluids in general are highly consumed and demand special requirements like thermo-oxidative stability and a wide range of viscosity values. For this application, an increasing interest in vegetable oils with high content of oleic acid has been noticed. These vegetable oils are considered potential substitutes for conventional mineral oil-based products, especially due to the higher resistance to oxidation (11, 12). Vegetable oils are preferred not only because they are biodegradable and nontoxic, but also because they are renewable raw materials (4). Vegetable oils also show most of the specific properties required for lubricant applications such as high viscosity index, high lubricity, high flash point, very low volatility due to the high molecular weight of the triacylglicerols, etc. In addition, they are also good solvents for fluid additives (4, 13). In particular, small changes in viscosity with temperature are desirable to provide a wide range of operating temperatures over which a given oil sample will provide satisfactory lubrication (14). On the contrary, they possess low thermal, oxidative, and hydrolytic stabilities, and poor low-temperature characteristics, which are due to their fatty acid composition and the presence of unsaturations (13, 15-17). Unsaturations restrict their use as good lubricants. Several attempts have been made to improve their oxidative stability, such as inducing transesterification (4) or using appropriate additives. Another important inconvenience of vegetable oils is the limited range of viscosity which makes them unusable for specific lubricant applications. Viscosity is one of the most important physical properties of any lubricant, 10.1021/es803047m CCC: $40.75
2009 American Chemical Society
Published on Web 02/17/2009
TABLE 1. Density and Kinematic Viscosity Values of Pure HOSO, at Different Temperatures temperature (°C)
kinematic viscosity (mm2/s)
density (g/cm3)a
25 40 60 70 80 100 120
74 ( 1.1 39 ( 1.7 24 ( 0.1 18 ( 0.8 15 ( 1.1 10 ( 0.2 7 ( 0.1
0.9035 0.8994 0.8860 0.8793 0.8774 0.8725 0.8651
a Standard deviation for density measurements was always lower than 10-4.
TABLE 2. Williamson’s Model Parameters for HOSO Samples Containing Different EVA Concentrations at 25 and 40°C 3
4
5
temperature (°C)
25
40
25
40
25
40
η0 (Pa.s) K (s) m
0.40 0.54 0.09
0.22 0.10 0.025
0.60 0.87 0.11
0.38 0.50 0.14
1.40 1.48 0.20
0.46 0.60 0.06
TABLE 3. Williamson’s Model Parameters for HOSO Samples Containing Different SBS Concentrations at 25°C concentration (%w/w)
3
4
η0 (Pa.s) K (s) m
0.55 0.28 0.15
1.27 0.70 0.24
TABLE 4. Arrhenius’ Equation Parameters and Viscosity Indexes for HOSO/EVA Blends with Different Polymer Concentrations
0 0.5 1 2 3 4 5
concentration (%w/w) 0 0.5 2 3 4
A (Pa.s)
activation energy (kJ/mol)
R2
viscosity index (VI)
3.6 × 10-6 3.2 × 10-6 3.5 × 10-6 4.2 × 10-6 2.7 × 10-6
24.0 24.8 26.6 26.3 28.1
0.996 0.997 0.996 0.998 0.997
257 213 212 212 198
mer (EVA), previously used to improve the rheological behavior of crude oil (19), or a styrene-butadiene-styrene copolymer (SBS), in a concentration range between 0.5 and 5% w/w.
Materials and Methods
concentration (% w/w)
concentration (%w/w)
TABLE 5. Arrhenius’ Equation Parameters and Viscosity Indexes for HOSO/SBS Blends with Different Polymer Concentrations
A (Pa.s)
activation energy (kJ/mol)
R2
viscosity index (VI)
3.6 × 10-6 4.8 × 10-6 3.7 × 10-6 3.7 × 10-6 5.3 × 10-6 6.9 × 10-6 2.0 × 10-6
24.0 23.7 24.8 25.7 25.5 25.4 29.2
0.996 0.998 0.996 0.997 0.997 0.999 0.995
257 243 202 200 204 200 195
independent of its nature (mineral, synthetic, or vegetable) and use (9). Traditionally, lubricating oils are Newtonian and the viscosity requirements depend on the industrial application. Typically, kinematic viscosity at 40 °C ranges from 30 mm2/s in the automation industry, to 120 mm2/s for lubricants used on bearings, while viscosities higher than 240 mm2/s are demanded in lubricants for four-stroke engines and some gear assemblies (18). Consequently, more investigation related to the use of additives in vegetable oils, in order to minimize these negative properties, is required (3, 15). In this sense, the main objective of this work was to develop new environmentally friendly lubricant formulations with both improved kinematic viscosity values and adequate viscosity thermal susceptibility. For that purpose, high-oleic sunflower oil was blended with either an ethylene vinyl acetate copoly-
Materials. Refined high-oleic sunflower oil (HOSO), with 85% wt. oleic acid, submitted to degumming, neutralization, bleaching, and deodorization processes was kindly supplied by the Instituto de la Grasa, CSIC (Seville, Spain). Total acidity of refined HOSO is 0.39%, referred to oleic acid. Density and kinematic viscosity values experimentally measured at different temperatures are shown in Table 1. EVA copolymer, with 33% vinyl acetate content (density at 23 °C, 0.956 g · cm-3; melt flow index (190 °C, 2.16 kg), 45 g/10 min; weight-average molecular weight, 60250 g · mol-1; melting temperature, 59 °C), and a radial SBS copolymer (density at 23 °C, 0.942 g · cm-3; melt flow index (109 °C, 5 kg), e 1 g/10 min, weight-average molecular weight, 107500 g · mol-1), were kindly supplied in the form of pellets by Repsol YPF, S.A. (Spain) and Dynasol elastomers, S.A. (Spain), respectively. Both of them were used as viscosity modifiers. Although not much ecological information is available, EVA and SBS copolymers are considered inert, nontoxic, and stable materials, not expected to be biodegradable but not hazardous according to the Commission Directive 93/21/EEC (20). Preparation of Biolubricating Oil Formulations. HOSO was blended with polymeric additives in batches of 250 cm3. The additive concentration ranged between 0.5 and 5% (w/w). Blends were prepared by stirring, at a rotational speed of 300 rpm and agitation times of 5-10 h, at 100-150 °C, depending on polymer nature and concentration. This thermal treatment was required to completely solubilize the polymer in HOSO. Afterward, samples were cooled to room temperature. A homogeneous single phase was obtained in all cases, although a certain degree of sedimentation was observed with aging for some of the SBS/HOSO blends (polymer concentration >3% w/w). In this sense, instability was noticed through visualization of a cloudy sample first, and phase separation later on. Viscosity and Density Measurements. Dynamic viscosities were measured with a rotational controlled-strain rheometer (ARES, Rheometric Scientific, UK), in a temperature range between 25 and 120 °C. Viscous flow tests were carried out in a shear rate range of 5-1500 s-1 using a Couette geometry (inner radius 16 mm, outer radius 17 mm, cylinder length 33.35 mm). At least two replicates of each test were performed on fresh samples. Kinematic viscosity values, ν, were obtained as the ratio of the dynamic viscosity to the density, at each temperature. The measure of the dynamic viscosity in a relatively wide range of shear rates is justified to detect the non-Newtonian behavior in some samples. In those cases, as discussed below, the high shear rate apparent viscosity/density ratio was calculated in order to compare these values with the kinematic VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Viscous flow curves for HOSO samples containing different EVA or SBS concentrations, at 25 °C: (a) 0.5% (w/w); (b) 1.0% (w/w); (c) 2.0% (w/w); (d) 3.0% (w/w); (e) 4.0% (w/w);(f) 5.0% (w/w). A capillary densimeter, model DMA-5000 (Anton Paar, Austria), was used to measure sample densities in a temperature range of 25-120 °C.
Results and Discussion
FIGURE 2. Viscous flow curves for HOSO samples containing 4% EVA or SBS, at different temperatures: (a) 25 °C; (b) 40 °C; (c) 60 °C, (d) 100 °C.
FIGURE 3. Evolution of viscosity with temperature, and Arrhenius’ parameters for HOSO containing 4% (w/w) SBS, at different shear rates: (a) 5 s-1; (b) 50 s-1; and (c) 500 s-1. viscosity values of Newtonian samples. The viscosity indexes (VI) were obtained according to ASTM D-2270. 2062
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Influence of Polymer Concentration. Figure 1 illustrates the evolution of the viscous flow behavior, at 25 °C, for HOSO samples containing different EVA and SBS concentrations. As can be clearly appreciated, both viscosity and shear rate dependence (non-Newtonian behavior) increase with polymer concentration. Thus, a Newtonian behavior was observed at low polymer concentrations (0.5-2% w/w), while a shearthinning behavior was obtained at higher polymer concentrations (3-5% w/w). Shear-thinning behavior was mainly noticed for samples containing 5% EVA or 4% SBS. NonNewtonian viscous flow curves have been fitted to the Williamson model (21): η)
η0 1 + (Kγ˙ )m
(1)
where η is the non-Newtonian viscosity; η0 is the zero-shearrate limiting viscosity; γ˙ is the shear rate; m is a parameter related to the slope of the shear-thinning region; and K is a constant whose reciprocal coincides with the shear rate at which η ) η0/2. The values of these fitting parameters are shown in Tables 2 and 3 for HOSO samples containing EVA and SBS, respectively. As can be observed in Figure 1, viscosity modifications, at 25 °C, induced by the addition of low concentrations (0.5-2%) of SBS or EVA are quite alike. On the contrary, higher viscosity values for SBS-containing samples are obtained for larger polymer concentrations, probably due to the higher molecular weight. However, the less pronounced shear-thinning characteristics of its blends (see values of m in Tables 2 and 3) makes EVA more suitable to be used as additive in biolubricant formulations. Moreover, as will be discussed below, the influence of both copolymers on HOSO viscosity changes at higher temperatures. Viscosity Evolution with Temperature. As previously mentioned, some of the biolubricant formulations studied show a non-Newtonian behavior, more evident as polymer concentration increases. Nevertheless, this shear-thinning
331 543 353 344 294 262 242 318 ( 9.8 248 ( 5.3 107 ( 3.6 78 ( 1.2 59 ( 1.4 36 ( 0.7 24 ( 0.2 197 343 245 212 204 180 181 219 ( 6.1 171 ( 6.5 81 ( 0.6 55 ( 0.7 45 ( 1.0 28 ( 0.8 19 ( 0.01 118 222 150 151 131 120 128 161 ( 7.9 124 ( 3.0 59 ( 2.2 44 ( 0.5 34 ( 1.3 22 ( 0.2 16 ( 0.01 70 127 88 88 64 64 72 125 ( 11.0 87 ( 3.1 44 ( 0.1 33 ( 0.3 24 ( 0.3 16 ( 0.4 12 ( 0.02 26 63 33 34 32 30 37 93 ( 4.1 63 ( 0.9 31 ( 2.1 24 ( 0.9 20 ( 0.9 13 ( 0.3 10 ( 0.01 75 ( 2.1 49 ( 1.5 27 ( 0.1 20 ( 1.7 17 ( 0.2 12 ( 0.4 8 ( 0.4 74 ( 1.1 39 ( 1.7 24 ( 0.1 18 ( 0.8 15 ( 1.1 10 ( 0.2 7 ( 0.1 25 40 60 70 80 100 120
2 27 16 14 17 20 18
viscosity increment (%) ν (mm2/s) ν (mm2/s) ν (mm2/s) ν (mm2/s) ν (mm2/s) temperature (°C)
5% EVA
ν (mm2/s) ν (mm2/s)
viscosity increment (%)
4% EVA 3% EVA
viscosity increment (%) viscosity increment (%)
2% EVA 1% EVA
viscosity increment (%) viscosity increment (%)
0.5% EVA pure oil
FIGURE 5. Evolution of viscosity with temperature for HOSO samples containing different EVA concentrations. behavior was only found in the low temperature range studied. For instance, in the particular case of HOSO/EVA blends, a Newtonian behavior was always observed at temperatures higher than 40 °C (Figure 2) no matter what the polymer concentration was. Williamson’s model parameters, at 40 °C, are listed in Table 2. A similar rheological response was obtained for SBS/HOSO blends. As can be observed in Figure 2, HOSO samples containing 4% SBS exhibit Newtonian behavior at 40 °C. In addition to this, it must be noticed that the thickening effect produced by EVA is slightly larger than that induced by SBS at temperatures higher than 25 °C (Figure 2). This effect must be attributed to EVA chemical structure, which seems to present higher physical interactions with HOSO, in spite of the lower molecular weight. Thus, the modification of oil viscosity is explained by an increase in the polymer hydrodynamic volume in contact with the associated oil, which is more important for linear polymers than for radial ones, especially at high temperatures. Due to the non-Newtonian behavior observed in some formulations, the viscosity/temperature relationship was studied at a specific shear rate. In this sense, Figure 3 shows the effect of temperature on shear viscosity for a selected biolubricant oil prepared containing HOSO and 4% SBS, in a temperature range of 25-120 °C. Three different shear rates (5, 50, and 500 s-1), covering low, medium, and high values inside the experimental range studied, were selected to
TABLE 6. Kinematic Viscosity Values and Kinematic Viscosity Increments for HOSO/EVA Blends as a Function of Polymer Concentration and Temperature
FIGURE 4. Evolution of viscosity with temperature for HOSO samples containing different EVA concentrations.
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TABLE 7. Kinematic Viscosity Values and Kinematic Viscosity Increments for HOSO/SBS Blends as a Function of Polymer Concentration and Temperature pure oil
0.5% SBS
2% SBS
temperatura (°C)
ν (mm2/s)
ν (mm2/s)
viscosity increment (%)
25 40 60 70 80 100 120
74 ( 1.1 39 ( 1.7 24 ( 0.1 18 ( 0.8 15 ( 1.1 10 ( 0.2 7 ( 0.1
82 ( 5.5 50 ( 6.9 26 ( 2.0 21 ( 0.4 16 ( 0.1 12 ( 0.001 8 ( 0.01
11 30 10 18 5 20 17
ν (mm2/s) 120 ( 8.1 73 ( 3.4 39 ( 1.9 28 ( 0.9 22 ( 0.1 15 ( 0.6 11 ( 0.01
62 89 67 62 51 55 52
evaluate the influence of temperature. An Arrhenius-type equation (22) always fits the experimental values obtained fairly well: Ea
η ) Ae RT
(2)
where η is the viscosity, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant [8.314 J · mol-1 · K-1], and T is the temperature (K). As can be deduced from Figure 3, some differences in the fitting parameters are apparent for the different shear rates selected. In this sense, the maximum value of the activation energy corresponds to the minimum shear rate selected, 5 s-1. However, the best fitting was found for a shear rate of 500 s-1, that is, when Newtonian viscosities, measured at high temperatures, were correlated with high-shear rate non-Newtonian viscosity values at lower temperatures. On the contrary, a clear deviation from the Arrhenius equation was observed for lower shear rates. Consequently, further analysis on the influence of temperature on biolubricant oil viscosities will be focused on blends thermal susceptibilities at a shear rate of 500 s-1. Figures 4 and 5 show the evolution of viscosity with temperature as a function of EVA and SBS concentrations, respectively, as well as the quality of the fitting to eq 2 (R2 > 0.995). Tables 4 and 5 list the values of the activation energy and the pre-exponential factor for the different blends studied. In general, it may be concluded that the influence of temperature on the apparent viscosity of HOSO/polymer blends is not substantial, as can be deduced from the activation energy values obtained. This fact may yield some advantages, above all if the lubricant is going to be submitted to a wide temperature range or extreme thermal conditions. However, viscosity thermal susceptibility significantly increases with EVA or SBS concentration. On the other hand, viscosity indexes (VI), traditionally used to describe the temperature dependence of lubricant viscosity, were calculated according to ASTM D 2270, based on the kinematic viscosity data obtained at 40 and 100 °C (23). The higher the viscosity index, the lower the lubricant thermal susceptibility is. Viscosity index values are listed in Tables 4 and 5 for HOSO/EVA and HOSO/SBS blends, respectively. As can be observed, the evolution of Ea values obtained from Arrhenius’ equation (eq 2) is opposite to the average tendency found for the VI. The high values of viscosity index obtained, in comparison to those found for mineral oils, corroborates the low viscosity temperature dependence of these biolubricants formulated with HOSO and EVA or SBS as additives. Tables 6 and 7 show the kinematic viscosity values of the different blends studied, as well as the viscosity increments relative to the additive-free HOSO sample, as a function of temperature and EVA or SBS concentration, respectively. As can be observed, viscosity increments are generally lower at high temperatures, especially at the largest polymer con2064
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3% SBS
viscosity increment (%)
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4% SBS
ν (mm2/s)
viscosity increment (%)
ν (mm2/s)
viscosity increment (%)
192 ( 12.6 113 ( 2.9 62 ( 2.1 44 ( 2.6 35 ( 0.8 23 ( 0.6 16 ( 0.7
160 194 164 150 132 134 126
273 ( 11.0 145 ( 9.6 72 ( 0.8 55 ( 4.5 41 ( 2.6 27 ( 0.2 19 ( 0.4
271 276 206 212 177 166 168
centrations studied. Moreover, as was previously remarked, the higher the polymer concentration, the higher the increment in viscosity is. Thus, for instance, at 40 °C, the lowest EVA concentration (0.5% w/w) studied yields an increase in kinematic viscosity of 27% (49 mm2/s) compared to pure oil, whereas viscosity increments of up to 543% (248 mm2/s) were found for the highest EVA concentrations (5% w/w). The same behavior was shown by SBS-based formulations, although slightly lower viscosity increments than those found for HOSO/EVA blends were obtained, above all at high temperatures, for the same polymer concentrations.
Acknowledgments This work is part of two research projects (PSE-320100-2006-1 and TEP-367) sponsored by MEC and Junta de Andalucı´a, respectively. The authors gratefully acknowledge their financial support.
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(18) Mang, T.; Dresel, W. Lubricants and Lubrication; Wiley-VCH: Germany, 2001. (19) Qian, J.; Qi, G.; Han, D.; Yang, S. Influence of incipient chain dimension of EVA flow improver on the rheological behaviour of crude oil. Fuel 1996, 75, 161–163. (20) Commission Directive 93/21/EEC of 27 April 1993 adapting to technical progress for the 18th time Council Directive 67/548/ EEC on the approximation of the laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances. Off J. E.C. L110, 1993. (21) Macosko, C. Rheology: Principles, Measurements and Applications; VCH Publishers Inc.: New York, 1994. (22) Sathivel, S.; Huang, J.; Prinyawiwatkul, W. Thermal properties and applications of the Arrhenius equation for evaluating viscosity and oxidation rates of unrefined pollock oil. J. Food Eng. 2008, 84, 187–193. (23) Placek, D. G. 2006 Hydraulics. In Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology; Rudnick, L. R., Ed.; CRC/Taylor & Francis: Boca Raton, FL and New York, 2006; pp 517-538.
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