Amino acid based ‘green’ lubricants

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It is estimated that about 30% of world’s primary energy is consumed by excessive friction every year and about 60% of machines fail or stop for maintenance due to frictional damage. Above that, 50% of malignant accidents of mechanical equipment result from lubricant failure or premature wear. These stats stress over the importance of lubrication for moving parts. Especially, lubricants that are made with degradable base stock which are eco-friendly and more effective than their mineral oil counterparts. The need for a sustainable lubricant fabrication is crucial in this age, especially when most of the organizations are aiming to go carbon neutral within this decade. Synthesizing green lubricants is an importance advancement in this field, because of its obvious reasons of being sustainable without impacting nature. But it is also important to understand that a truly green lubricant is also responsible for energy optimization and life time extension with an addition of being economical.

Fig1. Venn diagram representation

Lubricants made of room temperature ionic liquids, which are molten liquid salts below low temperature with small volatility, low flammability, high thermal stability and inherent polarity have been the focus within this area of research. In this particular research experiment quaternary ammonium amino acid ionic liquids are coupled with fourteen different types of natural amino acids as base lubricants to investigate its tribological behavior and corrosion resistance over steel-steel, steel-copper and steel-aluminum contacts at ambient and elevated temperatures. These experiments are conducted between temperatures of 25 ºC to 100 °C with a sample size of three, in repetition.

The anti friction and wear properties were evaluated using Optimol SRV-V oscillating wear tester. This is a high frequency linear oscillation test machine. The co-efficient of friction is evaluated with the help of a computer attached to this system, thanks to its versatility.

Tribological properties of steel-steel pair were tested considering the extensive use of steel in various instruments and machines, e.g., bearings. Two different load conditions are used for this particular experiment. In the first case, a load of 300N, stroke of 1 mm with 25 Hz frequency is applied for 30 minutes. For the other case the load is dropped to 200N while the other parameters remain the same. All the test results for different contact pairs and loading conditions are compared with the standard test result of  PAO40 as a reference. It is found that the anti-friction property of all the lubricants is stable with low coefficient of friction when compared with this reference. The wear volume is observed to be the least for Asparagine based lubricant for both the load cases. Overall an outstanding anti wear property is exhibited by the lubricants for steel-steel interface.

Apart from steel-steel combination, Copper-Steel combination is also considered for testing because of the enhanced mechanical properties, electrical conductivity and resistance to water of Cu. The tribological properties for this particular combination are evaluated at elevated temperatures. The load conditions used for this experiment are load of 100N, stroke of 1mm with 25Hz frequency for 30 minutes at 25 ºC and 100 °C. Glutamic based lubricant is found to possess excellent anti-wear properties for this contact pair exhibiting low friction. But this changed when the experiment is conducted at an elevated temperature of 100 ºC, where the closest result to that of the reference is produced by Asparagine based lubricant.

The final contact pair used in this experiment is of Steel- Aluminum. The boundary conditions used for this contact pair are the same as Steel-Copper pair. We can find a significant change in the behavior of reference result when compared to other contact pairs. Aspartic based lubricant had the best performance at room temperatures while Glutamine based lubricant exhibited good results at elevated temperatures.

In this experiment conducted by researchers from China, fourteen types of ILs from natural amino acids were synthesized via a simple neutralization reaction. By the various experiments performed during the course of the research with different contact pairs and unique loading conditions based on the pair, they came to a conclusion that QAAA ILs (Quaternary ammonium amino acid ionic liquids) readily absorb onto the positively charged metallic surfaces to create a boundary layer that helps in enhancing the tribological properties. This is a promising and a positive result for making environmentally friendly lubricants especially for using natural amino acid based lubricants for extensive commercial use.

Apart from the assessment of tribological properties for these contact pairs, additional results such as Kinetic viscosity, Copper strip corrosion test and thermo-gravimetric analysis were done to have a better understanding of the contact surfaces. Please refer the following link for more information:

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