ANALYTICAL INSTRUMENTATION
Table 1: Lubricants with nanoparticle additives and their attributes [3,4] Lubricant
Jojoba Oil Jojoba Oil Coconut Oil MO
Coconut Oil Coconut Oil Coconut Oil
Nanoparticle additive
-- TiO2
CeO2 --
--
Graphene Maghemite
Frictional Power (kW)
--
1.50 --
2.35 -- -- --
nanoparticles can have on the characteristics of lubricants.
Additives can have an influence on the performance of biolubricants. Although there is a variation in results, incorporating additives can enhance a lubricant’s attributes to improve performance. Table 1 shown below offers a summary of some lubricants and their additives along with their respective attributes [3,4].
Chemical Modification
Chemical modification can improve the performance of biolubricants. While the addition of nanoparticles can enhance a lubricant through physical, surface alterations, chemical modification alters the lubricant on a molecular level [8]. There are a variety of pathways that can be taken when considering chemical modifications. One approach is esterification which ultimately combines carboxylic acid and alcohol to form esters as a product [9,10]. Epoxidation is another technique and is already commercialized for soybean oils which has a global value market of 0.3 billion USD [11]. The process takes fatty acids of oils and converts them into epoxides also referred to as oxirane rings. The conversion alters the physiochemical structures of oils that allow them to withstand higher temperatures as well as improve tribological properties [12].
Esters are crucial in lubrication as they support oxidation and thermal stability [13]. In a study done by Monteiro et al., esterification of the free fatty acids (FFAs) from castor oil and the fatty alcohol 2-ethyl-1-hexanol, was observed [14]. The goal of the study was to optimize enzyme production without the use of an acid catalyst or the incorporation of solvents. Acid catalysts are commonly used in FFA esterification; however, it often influences equipment degradation and uses high amounts
Figure 3: Apparatus used for Madwesh et al. [3]
Lubricants are critical in many aspects of modern society. They help reduce friction and wear rates, ultimately improving a system’s efficiency.
were done on other bio-lubricants and nanoparticles [3]. The performance shown in this study presents a promising pathway in the utilization of bio-lubricants, offering environmentally advantageous solutions as well as higher efficiencies.
TiO2
is not the only nanoparticle showing promising results for biofuels. In a study done by the School of Mechanical Engineering at University Technology Malaysia, Suhaimi et al. used a coconut oil as the lubricant and had graphene and maghemite nanoparticles incorporated at 0.1% volume concentration [4]. The study compared a base line lubricant (CCO), the same lubricant doped with graphene (XGCO), and another group enhanced with maghemite (MGCO). To test the thermal stability, a thermogravimetric analysis (TGA) was performed from which differential thermogravimetric (DTG) graphs were used to determine thermal stability. This was done by taking roughly 15 mg of each sample and subjecting it to a nitrogen atmosphere in which the temperature increased from 20°C to 900°C, increasing at a rate of 10°C /min, along with an air flow rate of 50 mL/min. The results showed oxidative onset temperatures which correlate with thermal stability as it indicates the temperature a material can handle before being susceptible to exothermic decomposition [4]. The onset temperature was measured with MGCO and XGCO having the highest values of 343.75 and 362.5°C, respectively. CCO had the lowest at 325°C. These values illustrate the influence
of energy due to necessary high temperatures [15]. Monteiro et al. uses an enzyme that was genetically engineered from Thermomyces lanuginosus (TLL), referred to as lipase Eversa Transform 2.0 (ETL). With its original purpose of increasing FFA content for biodiesel production, ETL was tested to see if the catalyst will have a similar success when used in esterification. To observe the performance capabilities, the ETL was compared to catalysts that are commercially used: RML, TLL, CALA and CALB. As shown in Figure 4 below, TLL had the highest conversions rates, followed by ETL [14].
Figure 5: Results of Four-Ball Test [14]
Another study done by Neta et al. compared the performances of castor oil fatty acids (COFA) with ones when modified with esterification (BL1), or with esterification and epoxidation (BL2) [15]. The study recorded a wide variety of parameters including viscosity and pour points, along with tribological properties. For oxidative stability, an instrument used to measure oxidation levels of oils referred to as a Rancimat, was used in which samples were subjected to 110°C. The stability was measured in the time it took for conductivity to reach a value of 200µS/cm. The epoxidated lubricant performed the best at a value of 14.29 ± 0.16 hours, in comparison to the non-epoxidated lubricant which had a value of 12.89 ± 0.57 hours [16]. All three samples had a thermal stability of roughly 200°C. Similar to the study done by Monteiro et al., both biolubricants were then tested in comparison to a commercially used mineral lubricant. Referring to Figure 6, the test performed was a Four-ball tribological test [16].
Figure 4: Conversion percentages of Monteiro et al. samples [14].
The biolubricants had the lower of the friction coefficients with values of 0.037, 0.044 and 0.051 for BL2, COFA and BL1, respectively. The mineral lubricant, however, had a higher friction
Coefficient of Friction
-- -- --
0.0961 0.086
0.0920
Brake Thermal Efficiency (%)
33.6% 32.1% 30.3% -- -- --
[3] [3] [3] [3] [4] [4] [4]
According to Monteiro et al., although ETL did not exceed the values of TLL, they both increased in a similar manner, while the other three enzymes show little to no change after the four-hour mark. After this conclusion was drawn, the ETL was then optimized through the testing of five independent parameters: temperature, stirring rate, substrate molar ratio (acid/alcohol), biocatalysts content, and time (hours). The optimal ETL was found at a biocatalyst content of 15% at 30°C, 180 rpms, and a 1:4 molar ratio. Ultimately, the conversion rate reached a value of 95.70% [14].
ETL was also compared to the commercial mineral lubricant 20 W-50. From this, tribological tests were performed. Out of the two lubricants, the bio lubricant had a better-performing friction coefficient which was 0.052 ± 0.07, while the mineral lubricant had a coefficient of 0.078 ± 0.04. However, for the wear scan diameter (WSD), a different conclusion was made. The diameters were measured at 209.72 ± 3.01µm and 140.36 ± 1.36µm for the biolubricant and mineral lubricant, respectively. From these values along with the images from the Four-ball wear test shown in Figure 5, the biolubricant showed signs of higher oxidation wear [14]. According to Monteiro et al., while the biolubricant did show a higher wear scan diameter, its value is lower compared to commercial biolubricants. Further study of enzymes in biolubricant production can be greatly influential on the future of lubricants [14].
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