Researchers from the ‘Complex Fluids Engineering’ group at the University of Huelva (UHU) have designed sustainable lubricants using nanofibers derived from cellulose compounds. After conducting laboratory tests, experts have concluded that micro- and nanoscale fibers, smaller than a human hair, that thicken these greases, perform better than particles, physically stabilizing the product.
For their production, they have used the electrospinning technique, a tool with greater precision than the systems used in the conventional processing of traditional lubricating greases made from metallic soaps.
This technique involves making a liquid emerge as a jet from a very fine tube after applying a high-voltage electric field. In this way, the electric field stretches the liquid to form micro and/or nanoscale structures with different shapes and surface characteristics, from cylinders to ribbons, and with smooth, porous, or hollow surfaces, among others. Once created, they are deposited on a surface called a ‘collector’.
According to the researchers of the study, funded by the Ministry of University, Research, and Innovation of the Andalusian Government, the Ministry of Science, Research, and Universities, and FEDER funds, this method reduces the phenomenon of «oil bleeding,» that is, the separation between the lubricating oil and the thickener, while increasing the interaction capacity between both components, reducing the necessary thickener concentration.
«Electrospinning is one of the most relevant nanofabrication techniques. It allows designing tailor-made nanostructures of different types of materials. This makes it a tool of great interest for this type of applications,» explains José Enrique Martín, professor in the Materials Science and Metallurgical Engineering area at the University of Huelva and lead researcher of the study, to the Descubre Foundation, an entity under the Ministry of University, Research, and Innovation.
University of Huelva team that developed the project.
Furthermore, as stated in the study titled ‘Oleo-Dispersions of Electrospun Cellulose Acetate Butyrate Nanostructures: Toward Renewable Semisolid Lubricants’ and published in the journal Advanced Sustainable System, the results of this study represent a new advancement in the development of sustainable materials for the lubricants sector. «We have taken a step further towards creating structuring agents capable of thickening fluids suitable for lubricating bearings, shafts, and joints of all kinds of machinery in general,» Martín asserts.
Uniform fibers, better thickeners
After studying various properties of these lubricants such as friction and adhesiveness, they monitored the behavior of each of the nanostructures in stabilizing greases, specifically in castor oil. «We found that the morphology and surface properties of the nanostructures and their concentration play a key role in modulating their functional properties,» adds the study’s author.
They also examined their mechanical response. «Depending on the fiber shape, when mixed with oil, it does not behave the same. Particles are not able to stabilize the grease, even with affinity, and over time they separate. In contrast, when formulated with fibers, the whole remains homogeneous and stable without separating,» comments the researcher.
On one hand, they verified that porous fibers provide high thickening properties. «This contributes to a greater oil retention capacity. These nanostructures with multiple holes result in a more cohesive network, leading to an increase in stickiness,» details Martín.
Different laboratory tests showed the experts, on the other hand, that smooth fibers improve the friction reduction and wear properties of greases. «These nanostructures used as thickeners reduce friction and provide protection against deterioration similar to or even better than a commercial lithium lubricating grease,» clarifies this researcher.
With this study, experts have taken another step towards obtaining optimized lubricating greases, considering the possibilities of the electrospinning technique used to obtain nanostructures with other functionalities. In this sense, researchers are currently producing hybrid nanofibers by introducing clays to cellulose fibers.