Graphene-based nanoparticles have gained substantial interest as lubricant additives due to their remarkable physical and chemical properties. With the increased use of vegetable oils as biodegradable lubricants, especially in sectors with a strong environmental impact, much research has focused on using graphene-based nanoplatelets as additives in biodegradable lubricants, thus enabling a superior performance over the commercial mineral oils.
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Reducing friction and improving lubrication are among the most critical factors for many engineering applications. Advanced lubrication is essential for improving the durability, efficiency, and performance of virtually any mechanism. In the transportation, power generation, and automotive sectors, improved lubricants can also help decrease carbon emissions.
Bio-Based Feedstock for More Sustainable Lubricants
At present, most commercially available lubricants heavily rely on the use of synthetic or petroleum-based products, which cause serious environmental concerns. In modern society, more fossil-based products, including lubricants, are needed to accommodate the ever-increasing energy demand.
Experts estimate that 30 to 40 million tons of lubricant are utilized annually worldwide. Due to accidents and improper disposal, more than half of the total amount is released back into the environment resulting in environmental pollution and hazards.
The depletion of fossil resources and growing concerns over environmental degradation have stimulated the development of green energy systems and the utilization of sustainable resources as potential alternatives to mineral-based products. In particular, bio-based lubricants have attracted interest as substitutes to those that are mineral-based.
Bio-based lubricants are usually derived from raw materials, such as vegetable oils, animal fats, or other eco-friendly hydrocarbons. In addition to being readily biodegradable and renewable, the bio-based lubricants possess good lubricity, high flash point, high viscosity index, and good shear resistance compared to mineral oils.
Graphene-Based Nanoparticles Serve as Friction Modifiers in Lubricants
In recent years, nanomaterials have emerged as potential environment-friendly additives for improved lubricant performance. The application of nanoparticle additives is based on the principles of solid lubrication, where the lamellar structure of the solid lubricant, such as graphite or molybdenum disulfide, orients parallel to the motion direction.
Such nanomaterials are particularly useful as anti-wear and antifriction additives in extreme-pressure applications.
Among various layered nanoparticles, graphene and graphene oxide (GO) have attracted considerable attention since their discovery. The unique sheet structure of the graphene-based nanoparticles enables easy access to the friction contact surfaces.
Besides, the exceptional mechanical strength of graphene and its derivatives and their inert chemical properties reduce the mechanical wear and inhibit corrosive and oxidative damage to the sliding surfaces.
The Use of Agricultural Byproducts as Lubricants
Recently, researchers from India and Romania developed a novel blend of cashew nut shell liquid (CNSL) and neat castor oil (NCO), supplemented with reduced graphene oxide (RGO) nanoparticles, as a biodegradable replacement for the commercial non-biodegradable oils. The study was published in The Journal of Molecular Liquids.
CNSL is a dark brown viscous liquid extracted from the soft honeycomb structure of the cashew nutshell and is an agricultural byproduct of cashew nut production. CNSL is a renewable stock for the synthesis of bio-based phenol-based polymers.
NCO is a vegetable oil extracted from the seeds of the castor oil plant and is used in various industrial sectors, including cosmetics, pharmaceutics, and chemical manufacturing, as a renewable feedstock. By blending the two oils, the researchers aimed to combine the high viscosity of NCO (242.81 centistokes at 40 ℃) with the excellent biodegradability of CNSL, creating a biodegradable lubricant that can outperform traditional mineral oil lubricants.
What Are Biodegradable Lubricants?
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Reduced Graphene Oxide Improves the Performance of Biodegradable Lubricants
To enhance the lubricant's performance, the research team used RGO as a friction modifier additive in the blend. Unlike graphene, which is also an effective solid lubricant, RGO does not aggregate when dispersed into liquids (in polar liquids in particular). In addition, RGO can be produced in large quantities at a low cost by the chemical reduction of graphene oxide.
The process introduces a polar group and a straight hydrocarbon tail on the surface of graphene oxide, which improves RGO miscibility and complements the anti-wear and antifriction properties of the lubricant.
To optimize the percentage of CNSL, NCO, and r-GO in the new biodegradable blend, the researchers varied the content of CNSL in the range 10-70 vol% while supplementing the mixture with 0.1-2 wt% of RGO additive. The lubricating performance of the blends was compared to the performance of commercial mineral oil in a range of tribological tests.
Blended Green Vegetable Oils Outperform Mineral Oils
The experimental results demonstrated that the optimum content of the RGO friction modifier in the CNSL/NCO blend was in the range of 0.5-1.0 wt%. The lowest coefficient of friction, 0.0399, was measured for the blend containing 0.5% RGO and 40% CNSL, an improvement of more than 60% when compared to the friction coefficient measured for the commercial oil (0.1044).
The novel biodegradable blend also outperformed the commercial mineral oil and the neat NCO as a lubricant in a test gearbox mechanism. The results from the tests proved that the new biodegradable CNSL/NCO blend, supplemented with an RGO friction modifier, can successfully replace traditional mineral oils in various applications.
The researchers envisage that the use of vegetable oil blends and RGO nanoparticle additives will pave the way for sustainable lubrication in a broader range of industrial applications.
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References and Further Reading
Liñeira del Río, J. M., et al. (2021) Improving the tribological performance of a biodegradable lubricant adding graphene nanoplatelets as additives. J. Mol. Liq. 345, 117797. Available at: https://doi.org/10.1016/j.molliq.2021.117797
Bhaumik, S., et al. (2021) Tribological analyses of a new optimized gearbox biodegradable lubricant blended with reduced graphene oxide nanoparticles. Proc. Inst. Mech. Eng., Part J J. Eng. Tribol. 235, 901-915. Available at: https://doi.org/10.1177/1350650120925590
Sun, J. and Du, S. (2019) Application of graphene derivatives and their nanocomposites in tribology and lubrication: a review. RSC Adv. 9, 40642-40661. Available at: https://doi.org/10.1039/C9RA05679C