Toxicity of Nanoparticles Used in Minimum Quantity Lubrication (MQL) Machining: A Sustainability Analysis

Sustainability is the key indicator for environmental protection and socio-economic development of nations in the present world. The global importance of machining sector has prompted various studies to investigate optimum cooling and lubrication properties that maximize effectiveness while minimizing the adverse impact on the environment. Minimum Quantity Lubrication (MQL) with Nano composite particles is among the new investigative areas of study and has proven to provide very good cooling and lubrication in the machining of difficult to cut materials such as Titanium, Inconel and ADI. Hence, it is imperative to also understand their effects on the environment in the early stages of investigation and prior to their wide scale usage in the industry. Researchers are potentially exposed to the Nanoparticles in the preparation, handling and storage of the Nano Fluids. Further, Minimum quantity lubrication strategy is a mist-based lubrication strategy with a high probability of causing airborne impurities. This study focuses on the different Nano composite particles used in the previous research, available in the literature, and evaluates their sustainability characteristics by investigating the toxicity of the used Nano Composite Particles on humans. The cooling capabilities of each of the Nano Particles considered is first established from the existing literature and summarized. Human Cell viability measured from in-vitro toxicity studies of Nano Particles is used as a variable to easily capture the toxicity of Nano particles. The human cell lines chosen to represent the effect of possible exposure through Inhalation (human lung epithelial cells (A549), Bronchial epithelial cells (NL-20)), ingestion (AGS, HepG2) and dermal contact (THP-1, Human Peripheral Blood cells) on six different cell lines. From the studies available in literature, it is seen that Transition Metal Dichalcogenides such as MoS2 and WS2 provide very good cooling and mostly exhibits very low toxicity in all the cell lines and is generally recommended as safe to use even in medical field. It is observed that hBN Nano particles provide very good cooling, however, their toxicity levels are very high and regarded as toxic in some of the available studies.

The study provides easy interpretation of the toxicity levels of five Nano Particles considered in all the three human cell lines. The drawback of the comparison developed is the lack of sufficient data to assign conclusive toxicity levels for the Nano particles. The toxicity studies of Nano Particles on humans is still in its infancy and contradicting results exist for some of the Nano Particles. This is the first attempt to combine the results of the experimental investigations of Nano MQL cooling and toxicity studies of Nano particles allowing researchers to make informed decisions in the selection of most sustainable Nano particles in Nano MQL machining Process. Development of models to simulate Nano fluid lubrication in order to minimize experimental evaluation must be considered and is a topic of future study.