Organic waste is traditionally used in applications of low economic value, such as incineration and animal supplementation, however, a large part of this waste has the potential to be transformed into products with greater added value. In this study, the physicochemical, thermal and tribological characteristics of biolubricant samples synthesized from a residual fatty acid sample, mainly composed of palmitic acid (~ 43% wt.) and oleic acid (~ 35% wt.), were evaluated. In the thermal stability analysis, the final sample (coined as Biolub) showed the best performance when compared to the other samples, with the following temperatures for mass loss of 50%, in an inert (341.68°C) and oxidative (285.33°C) atmosphere. For the tribological properties, Biolub presented a friction coefficient (FC) approximately 53.85% lower than that of commercial mineral oil (CMO). The results in general suggest that the synthesized product has the potential to be used as a basestock oil for biolubricants, adding value to this industrial waste thus contributing to sustainable and economic development.
Research Article
Lubricants basestock oil obtained from residual fatty acids: tribological properties and thermo-oxidative stability
https://doi.org/10.21203/rs.3.rs-4877339/v1
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Biolubricants
Tribology
Industrial Waste
Lubricants play a key role for machines and devices by mitigating friction between surfaces, thereby facilitating safe and efficient operation in diverse applications such as engine transmissions, metal cutting machines, and other industrial equipment [1–3]. The prevalence of mineral oil-based lubricants, at around 85 to 90% of global supply, represents a significant environmental threat due to inadequate disposal practices and the risk of spills during their transportation [4–6]. The combination of adverse environmental and human health impacts, the need for decarbonization, strict environmental regulations, as well as growing consumer awareness, have stimulated the search for ecological and renewable alternatives, known as biolubricants [7, 8].
Biolubricants are considered renewable, with greater biodegradability, minimal impact on human health and the environment, excellent lubrication performance, low volatility, high viscosity index, and present a lower carbon footprint compared to petroleum-based lubricants [9–12]. It is worth highlighting that the benefits of biolubricants have been documented in the literature for some time, including their potential to increase the energy security of countries and promote regional economic development [13, 14]. These lubricants can be derived from edible or non-edible vegetable oils, animal fats, or microalgae [15]. However, the selection of feedstock must be meticulous, given the significant involvement of vegetable oils in the food industry [16, 17], for which competition may lead to price increases and social imbalances [18, 19].
A good strategy to not interfere in the food industry and reduce the costs associated with raw materials for the production of biolubricants, is the adoption of alternate feedstocks, such as forestry by-products, used frying oil, agricultural waste, and industrial waste [20, 21]. Conventionally, organic waste is used in animal supplementation, incineration, composting, or in landfills. However, there is a growing search for new applications for these wastes, adopting strategies for their valorization and reuse, such as the biorefinery concept for the production of products with greater added value, such as biolubricants [22–24]. Several studies have been reported on these alternative applications, including palm oil refining residues, residual cooking oil, and olive oil mill residues, but usually they focus mainly on physicochemical properties, leaving a gap regarding details of the tribology properties [20, 25, 26].
Tribology encompass the friction, wear, and lubrication behavior between interacting surfaces [27]. Lubrication, considered the most effective way to minimize friction and wear, involves adding a lubricant to overcome friction between a tribopair [28, 29]. With improvements in tribological performance, several industrial benefits may be obtained, such as reducing equipment maintenance downtimes and costs [30]. Given this, the importance of evaluating the tribological behavior of biolubricants in general, especially those derived from industrial waste and similar sources, stands out, to increase their chances of insertion and acceptance in the market.
In this study, the physicochemical, thermal and tribological properties of biolubricant samples, obtained from a residual fatty acids industrial stream of the deodorization stage of vegetable oil refining, were evaluated. This residue is currently being burned in boilers to generate heat or sold at low cost for animal supplementation. The chemical modifications carried out were esterification with the alcohol 2-ethyl-1-hexanol, followed by epoxidation and opening of the epoxide ring, using the same alcohol as a nucleophilic agent.
2.1 Materials
In this study, a sample of a commercial Mineral Oil (CMO), provided by Petrobras (Brazil), was used as reference for discussing the tribological results. As a raw material for the synthesis of basestock oils, a sample of Residual Fatty Acids (RFA) was used, a residue from the deodorization process of vegetable oils from a Special Fats and Margarines plant (M. Dias Branco, Fortaleza, Brazil).
Sodium bicarbonate (> 99.7% wt.), p-toluenesulfonic acid (> 98% wt.) and 2-ethyl-1-hexanol (> 99,6% wt) were purchased from Sigma-Aldrich (USA). Toluene (> 99% wt.) was purchased from Neon (Brazil). Hydrogen peroxide (30% wt.), formic acid (85% wt.), and anhydrous sodium sulfate (> 99% wt.) were purchased from Dinâmica (Brazil) and nitrogen (99.999% vol.) was supplied by White Martins Praxair (Brazil). All chemicals and raw materials were used as received, without additional treatment.
The physicochemical properties and the chemical composition of the RFA sample are presented in Table 1. The composition of RFA is mostly palmitic acid (~ 43% wt.), oleic acid (~ 35% wt.), linoleic acid (~ 8.6% wt.) and stearic acid (~ 4.4% wt.). It may observed that the high content of unsaturated compounds allows the proposed chemical modifications to be carried out to improve cold flow characteristics and other properties needed to enable its use as a basestock oil for lubricants.
| Properties | RFA | Method |
|---|---|---|
| Density at 25°C (g/cm³) | 0,7925 | [33] |
| Dynamic viscosity at 60°C (cP) | 16,25 | [31] e [32] |
| Dynamic viscosity at 80°C (cP) | 12,5 | [31] e [32] |
| Dynamic viscosity at 100°C (cP) | 10 | [31] e [32] |
| Acid value (mgKOH/g) | 169,68 | AOCS CD 3d-63 |
<Table 1>
Dynamic viscosities at 60°C, 80°C and 100°C were measured with a Brookfield Rotational Viscometer (Canada) with an SC4-21 spindle and a thermostatic bath. Using a methodology described by Ahmed et al. [31], with temperature adaptations following the Brookfield guide [32], since at lower temperatures the sample solidifies. Densities at 25°C were calculated using the methodology described by Nunes [33], based on Archimedes' principle, and consisted of measuring the liquid volume in a beaker after adding a known mass of the sample. The physicochemical properties of the CMO sample, are presented in Table 2.
| Properties | CMO | Method |
|---|---|---|
| Density at 20°C (g/cm³) | 0.9165 | ASTM D7042 |
| Kin. viscosity at 40°C (cSt) | 20.31 | ASTM D445 |
| Kin. viscosity at 100°C (cSt) | 3.590 | ASTM D445 |
| Viscosity index (VI) | 14 | ASTM D2270 |
| Pour point (°C) | -33 | ASTM D97 |
| Acid value (mg KOH/g) | 0.01 | AOCS CD 3d-63 |
<Table 2>
CMO, a basestock oil already used in several industrial applications, such as, in the manufacture of cutting oil for machining, refrigeration compressor oil, and in the production of industrial greases, was used as reference to carry out comparisons with the products synthesized in this study.
2.2 Synthesis Procedures
The Residual Fatty Acid (RFA) sample was subjected to esterification, epoxidation, and oxirane ring opening reactions. For the esterification reactions, 200 g of RFA (0.737 mol) was added to a round-bottom flask containing 2-ethyl-1-hexanol (RFA to alcohol molar ratio of 1:3) with p-toluenesulfonic acid (PTSA) (5g per 100g of the fatty acid), as catalyst. The reaction mixture was maintained under constant stirring and reflux for 6 hours at 90°C under nitrogen atmosphere. After completion of the reaction, the mixture was transferred to a separatory funnel and allowed to cool to room temperature. Subsequently, the mixture was washed with sodium bicarbonate solution (NaHCO3) at a concentration of 5%wt in distilled water to reach neutral pH. Finally, the organic layer was dried with anhydrous sodium sulfate (Na2SO4), and the sample went through a distillation process using Kugelrohr, under reduced pressure at 90°C for 40 min, to remove excess alcohol and water residues.
Then samples of the obtained esters (E1) were subjected to the epoxidation reaction in flat-bottomed flasks, initially containing 70 g of E1 (0.182 mol), 50 mL of toluene, and formic acid. A hydrogen peroxide solution was then added slowly by dripping, following the molar ratio of ester: CH2O2:H2O2 of 1:1:4. The reaction was carried out at room temperature, with constant stirring at 900 rpm, for 24 hours, and then the reaction solution was transferred to a separation funnel with the upper phase neutralized using a solution of NaHCO3 (5% wt. in distilled water). Finally, the organic layer was dried with anhydrous Na2SO4 and toluene was removed using a Kugelrohr under low pressure (0.03 mbar) at 90°C for 40 min.
The oxirane ring opening reaction was then performed using 60 g of the epoxide (0.150 mol), with alcohol 2-ethyl-1-hexanol as nucleophilic agent (molar ratio of 1:3 epoxidized ester/alcohol) and PTSA as catalyst (5% wt.). The reaction was carried out in a round-bottom flask, under constant stirring, and refluxed for 4 hours at 70°C. Subsequently, the mixture was transferred to a separation funnel and the same washing and distillation procedure as in the previous steps was followed. A scheme of the chemical route is presented in Fig. 1, with RFA represented by the molecules of its main components (Palmitic and Oleic acids).
<Figure 1>
2.3 Physicochemical and compositional characterization of the synthesized products
Pour points (minimum temperature at which the lubricant still flows) were measured using a CPP 5Gs equipment (ISL, France), following the ASTM D97 standard method [34]. The procedure consists of filling a test tube with 45 mL of the sample, which is gradually cooled and monitored regularly at 3°C intervals. Acid index, in mg KOH/g, was measured using an acid-base titration according to the AOCS Cd 3d-63 procedure [35], with previously neutralized ethanol, 0.01 N potassium hydroxide (KOH) solution, and 1% phenolphthalein indicator solution. This index indicates the amount of potassium hydroxide needed to neutralize one gram of free fatty acids contained in the sample.
Kinematic viscosity and density measurements were carried out using an SVM 3000 Stabinger viscometer (Anton Paar, Austria), by ASTM D7042 [36] and ASTM D445 [37] standard methods. Additionally, the Viscosity Index (VI) value was calculated using the ASTM D2270[38]. For thermogravimetric analysis, an equipment from Mettler-Toledo (Swiss-American), model TGA/SDTA 851e, was used. Samples of 10.0 mg were placed in alumina crucibles and exposed to N2 (50 mL/min) and O2 (40 mL/min), with the temperature varying from 30°C to 600°C, at a heating rate of 10°C/min.
Fourier-Transform Infrared Spectroscopy (FTIR) and Proton Nuclear Magnetic Resonance (1H NMR) characterization techniques were used to evaluate the composition of the synthesized samples and validate the occurrence of the chemical modifications that were carried out. Proton NMR spectra were collected using a Bruker Avance DRX-500 spectrometer (USA), operating at 500 MHz, using Chloroform-d (CDCl3 99.8%) as solvent. FTIR analysis was performed using a Shimadzu IRTracer-100 instrument (Japan), with a scanning range varying between 4,000 and 400 cm− 1 and a spectral resolution of 4 cm− 1 [39, 40].
2.4 Tribological characterization
The tribological evaluation was carried out using the Four-Ball configuration of a DHR-3 rheometer from TA Instruments (USA), with an experimental procedure adapted from the ASTM D4172 [41] due to the limitations of the rheometer. During the test, the upper ball rotated in contact with the three fixed balls in the lower part submerged in lubricant samples, with the tests being carried out for 1 h, constant sliding speed of 0.46 m/s, temperature of 75°C and loading force of 55 N, as seen in Fig. 2.
<Figure 2>
The balls are composed of a chromium steel alloy (AISI52100, 64 HRC), with diameter of 12.7 mm and initial surface roughness of 0.015 µm. Before and after each test, the balls were cleaned with acetone and placed to dry at room temperature, using new balls and repeating the cleaning process for each performed test. Additionally, an optical microscope model DMI3000 (Leica. Germany), with 100x zoom, was used to study the morphology and calculate the wear scar diameter (WSD) at the end of each test.
3.1 Assessment of physicochemical properties
The physicochemical properties of the synthesized samples, including esters and final products, are presented in Table 3. The conversion of the esterification reaction may be estimated using the acid index values of the RFA sample and of the obtained ester, as shown in previous reports [42, 43]. Using the acid numbers of 169.68 mg KOH/g for RFA and 6.64 for E1, the conversion of the esterification reaction was estimated as 96.1% for E1. From the variation in the kinematic viscosities of the esters and of the final products, it is possible to note that with the chemical modifications there is an increase in viscosity, which may be related to the increase in the number of hydrogen bonds and intermolecular interactions [42, 44].
It may also be stated that, based on the measured viscosities, the final product can meet the classification requirements of the ISO 3448:1992 standard for industrial liquid lubricants and the SAE System for automotive lubricating oils. According to the ISO 3448 standard, this Biolub sample is classified as ISO VG 15. However, the viscosity of the Biolub sample does not meet the SAE specifications [45] for cold flow requirements, an issue that may be fixed using additives to reduce the pour point.
|
Properties |
E1 |
Biolub |
Method |
|---|---|---|---|
|
Density at 20°C (g/cm³) |
0.8667 |
0.8869 |
ASTM D7042 |
|
Kinematic viscosity at 40°C (cSt) |
9.04 |
16.23 |
ASTM D445 |
|
Kinematic viscosity at 100°C (cSt) |
2.785 |
3.812 |
ASTM D445 |
|
Viscosity index (VI) |
167 |
128 |
ASTM D2270 |
|
Pour point (°C) |
-3 |
-6 |
ASTM D97 |
|
Acid value (mg KOH/g) |
6.64 |
2.22 |
AOCS CD 3d-63 |
| RFA: Residual Fatty Acid; E1: RFA ester with 2-ethyl-1-hexanol; Biolub: RFA final biolubricant with 2-ethyl-1-hexanol. | |||
<Table 3>
Regarding the viscosity index, all synthesized products are classified, according to Darbyshire et al.[46] as lubricants with very high viscosity index (VI > 110), suggesting that they have less variation in viscosity with temperature changes [47]. Evaluating the pour point values, it is observed that the products present poor cold flow performance, due to the higher degree of unsaturation, as previously reported [48, 49]. A decrease in pour point is observed in the final products compared to their esters, which can be attributed to the increase in the carbon chain and branches, making crystallization difficult and thus reducing the pour point [1, 50]. Therefore, regarding cold flow performance, the performed modifications were helpful, considering that it is desirable to expand the temperature range in which the lubricants may operate adequately.
3.2 Chemical characterizations
FTIR and NMR spectra (Figs. 3 and 4, respectively) were used in this study to evaluate the occurrence of the desired chemical modifications performed in the synthesis procedures. The band at approximately 1710 cm− 1 in the RFA FTIR spectrum is associated with the carbonyl group present in long-chain fatty acids. After the esterification reaction, this peak changes to 1741 cm− 1, associated with the stretching of the carboxylic ester, which confirms the occurrence of that reaction [49, 51, 52].
<Figure 3>
All samples of fatty acids and esters present peaks at approximately 3010 cm− 1, related to the C = C stretching [53]. The presence of this peak in the RFA sample, which is a residue formed by a mixture of fatty acids, confirms the presence of unsaturations. Additionally, the absence of this peak in the samples after the epoxidation reaction confirms the occurrence of that reaction in the replacement of the double bond. The epoxidation reaction can also be confirmed by the appearance of peaks at 825 cm− 1 and 840 cm− 1, associated with the epoxy groups present in the epoxidized samples. Finally, these peaks disappear in the final products, confirming the oxirane ring opening reaction.
To further corroborate the FTIR results, 1H NMR spectra (Fig. 4) confirm the esterification reactions by appearance of signal II, associated with the ester group [52, 54].
<Figure 4>
The signals represented by the number I, observed in the RFA and the ester, are attributed to double bonds [55]. The disappearance of this signal in the epoxidized samples and the appearance of the signal III, characteristic of the epoxy ring, confirms the epoxidation reaction [56, 57]. Finally, the absence of signal III in the final product suggests that the oxirane ring opening reaction was adequately carried out and the proposed biolubricant was indeed obtained [43, 56].
3.3 Thermal and Thermo-oxidative characterizations
For these characterizations. the temperature at which thermal decomposition begins (Tstart) was defined as the temperature at which the samples show a loss of 5% of their weight, as adopted in previous reports [58, 59].
3.3.1 Thermal stability
TG and DTG decomposition curves in inert atmosphere are presented in Fig. 5 for the RFA, E1 and Biolub samples. All samples exhibited Tstart values higher than or close to 200°C, suggesting good thermal stability [60]. It may be observed that the E1 sample presents the most accentuated weight decay compared to the other samples (Fig. 5a). Also, its DTG curve (Fig. 5b) indicates that thermal decomposition occurs in a single stage, marked by a more intense degradation peak.
<Figure 5>
It may also be seen that the Biolub sample begins to degrade before the other samples. However, this decay is less pronounced, suggesting a more stable behavior, with more discrete peaks. This indicates a reduction in the weight loss rate and shift to higher temperatures, when compared to the other samples. As adopted in previous studies [61–63], temperatures noted along the decomposition process in inert atmosphere are presented in Table 4.
|
Sample |
Tstart |
10% |
20% |
50% |
90% |
|---|---|---|---|---|---|
|
RFA |
236,05 |
254,06 |
272,73 |
301,32 |
405,59 |
|
E1 |
252,23 |
268,96 |
287,63 |
315,46 |
336,80 |
|
Biolub |
175,49 |
254,69 |
296,93 |
341,68 |
386,11 |
<Table 4>
When comparing the results of the Biolub sample with those of other samples under similar conditions (50% weight loss and inert atmosphere), Biolub sample presents better performance than 12 samples of petroleum-derived oils (4 lubricating oils and 8 basestock oils), reported by Abdelkhalik et al. [62] in temperatures ranging between 221 and 336°C.
3.3.2 Thermo-oxidative stability
TG and DTG decomposition curves in oxidative atmosphere are presented in Fig. 6, showing similar behavior to other biolubricant samples previously reported [64, 65]. It may be observed that the Biolub sample, despite having a lower Tstart than the other samples, has a more stable behavior, using as a comparison the reduction in the slope of the curve, indicating that the performed modifications indeed improved its thermo-oxidative stability [66, 67]. This result is also highlighted in the DTG curve presented in Fig. 6 (b).
<Figure6>
Similarly to what was done for the thermal decomposition in inert atmosphere, temperatures were noted along the decomposition process in oxidative atmosphere (see Table 6). When comparing the results obtained by Biolub with other samples under similar conditions, using the criterion of 50% weight loss, it was observed that this sample slightly surpassed the performance of a petroleum-based hydraulic lubricating oil (271°C), but has lower performance than a sample of polyethylene glycol esters derived from a mixture of fatty acids from linseed oil and oleic acid (326°C), both reported by Dutta, Ghosh and Mitra [68].
<Table 5>
|
Sample |
Tstart |
10% |
20% |
50% |
90% |
|---|---|---|---|---|---|
|
RFA |
197,33 |
216,00 |
234,67 |
258,67 |
432,00 |
|
E1 |
234,00 |
244,00 |
256,00 |
277,33 |
350,67 |
|
Biolub |
174,67 |
213,33 |
246,67 |
285,33 |
432,00 |
3.4 Tribological performance
The average values of the friction coefficient (FC) and the wear scar diameter (WSD) of the bio-based samples and CMO are shown in Fig. 7. The size of the WSD is inversely related to the antiwear ability of the lubricant [69], so the performance of a lubricant may be evaluated by the decline of the WSD and FC values. From the data reported in Fig. 7, the RFA and Biolub samples present have better anti-friction and anti-wear performance than the reference sample (CMO).
The higher effectiveness in reducing friction of the biolubricant sample may be attributed to the presence of polar functional groups, a trend also previously reported [52, 70, 71]. The presence of polar functional groups in the molecular structure of the samples increases adhesion to the balls, making them more effective in reducing friction, as previously discussed by other authors [65, 67, 68]. Therefore, the reduction in wear protection that caused an increase in WSD, when comparing the RFA with the Biolub samples, may be attributed to the replacement of the carboxylic group by the ester group, which reduces the polarity of the molecule.
<Figure7>
Also from Fig. 7a, it may be seen that the Biolub sample presented the lowest value of FC, suggesting that the performed chemical modifications improved its tribological properties. This may be related to the introduction of branches around the polar region of the molecule, helping to protect from physical and chemical interactions due to steric hindrance [72]. The lower performance in wear protection in addition to the reduction in polarity may be due to the fatty acid profile of RFA which, as previously mentioned, is mostly saturated, possibly resulting in a poorly branched molecule, compared to other previous reports using alcohol 2-ethyl-1-hexanol and other fatty acids.
The wear morphologies of the balls, obtained from an optical microscopic measurement, are presented in Table 6, as an additional way to understand the wear mechanism occurring on the tested surfaces. Through visual analysis, it is possible to observe characteristics related to abrasion and adhesion wear, with grooves and material transfer marks [73–75]. In general, the balls that were lubricated with CMO have a rougher surface than those lubricated with the other samples, due to the direct metal-to-metal contact between the balls, resulting in more severe adhesive wear.
Table 6. Wear morphology of balls lubricated with the synthesized samples (E1 and Biolub), compared to RFA and CMO after tribological tests.
<Table 6>
The chemical modifications that were carried out in this study showed satisfactory improvement in physicochemical properties and were confirmed by FTIR and NMR spectra. Based on viscosity, the final product (Biolub) may be used as industrial and possibly automotive lubricant using specific additives. Regarding viscosity/temperature performance, all samples were classified as lubricants with very high viscosity index, enabling higher safety and efficiency in their use with temperature variations. For the pour point, reasonable values were obtained, considering that the raw material (RFA) is mainly composed of palmitic acid, which is a saturated fatty acid, resulting in poorer performance at low temperatures.
The results obtained for thermal and thermo-oxidative Stability suggest that the synthesized biolubricant has the potential to be used in open and/or closed system operations, depending on the operating temperature of the desired application. Evaluating the tribological properties, it was observed that the bio-based samples presented lower FC and WSD values than the reference sample. In addition, the wear morphology of the balls indicates a more accentuated wear in the samples lubricated with the commercial product, characterized by aspects of visuals that characterize abrasion and adhesion. In summary, these studies on its physical-chemical, thermal, and tribological properties indicates that the Biolub sample may be a potential alternative to mineral oils, having some deficiencies that could probably be reduced using specific additive packages for the desired application and needs.
The authors received financial support from CNPq (Conselho Nacional de Pesquisa e Desenvolvimento Científico) and FUNCAP (Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico). Ribeiro-Filho received personal financial support from Scientific and Technological Research and Development Support Foundation of Maranhão (FAPEMA) and Universidade Estadual do Maranhão (UEMA). The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions. The authors have read and approved the final version of the manuscript.
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