*3.2. Vegetable Oil-Based MWFs*

A considerable amount of research around the world has been aimed at developing alternatives of the harmful mineral oil-based MWF in order to make machining processes sustainable. Recent studies on sustainable machining have revealed that the vegetable oilbased MWFs have shown a better performance [42–48]. The vegetable oil-based MWFs have

demonstrated better cooling and lubrication characteristics when used during machining operation compared with the mineral oil-based MWFs. As a result, they have gained much attention, and they have been a topic of interest for many researchers. Over the years, it has been the practice to choose the MWFs based on the cutting process, the tool material, the work material, and the operation conditions [49,50]. This was the old trend, but as the research is progressing in this area, the selection of MWFs is also changing. Now, the selection of MWFs is more concerned about their impact on the environment and on the health of the machine operators, in addition to other process requirements.

One of the most important advantages of a vegetable oil-based MWF is that it can easily be broken down into eco-friendly species with the aid of enzymes or chemical reactions. The residue can easily be disposed-off in an environmentally friendly manner without posing any serious challenge to the environment, therefore maintaining sustainability. Furthermore, the toxicity level of the vegetable oil-based MWFs is considerably less than that of the mineral oil-based MWFs [51]. Vegetable oil-based MWFs are also less severe than mineral oil-based MWFs when machine operators become exposed to the bio-degradable vegetable oil-based MWF. Another advantage of vegetable oil-based MWFs is that filtration is not required before it is disposed-off, which considerably reduces the costs associated with it. The environmental and economic benefits of vegetable oil-based MWFs, compared with the mineral oil-based MWFs, is shown in Figure 5. The figure illustrates that the environmental impact of mineral oil-based MWFs is considerably lower than that of vegetable oil-based MWFs; because different additives are also added to minimize the environmental impact of mineral oil-based MWFs, they prove to be less economical than vegetable oil-based MWFs. In a study by John et al. [52], it was concluded that by using a vegetable oil-based MWF, better cooling rates were achieved and improved lubrication characteristics were observed due to their higher retention time. In a study by Mannekote and Kailas [53] on the effect of oxidation on the tribological properties of vegetable oil-based MWFs, they reported that when compared to mineral oil-based MWFs, the vegetable oil-based MWFs had a higher tendency to oxidize when exposed to oxygen, and they can easily be converted to compounds like H2O, CO2, and CH4. On the other side, Erhan et al. [54] showed that vegetable oil-based MWFs have a lower ability to maintain their characteristics in high temperature and high humidity environments, which are properties that are needed to perform cooling and lubrication operations. One of the solutions to address the shortcomings of vegetable oil-based MWFs is through the formulation of water-soluble MWFs, where the surfactants are other introduced additives, resulting in the modification of the chemical structure; this method makes the MWF capable of operating satisfactorily in extreme conditions without jeopardizing its lubrication and cooling characteristics, and it has been confirmed in different studies [55,56].

One of the most successful methods for the formulation of vegetable oil-based, watersoluble MWFs is the process of emulsification. In this process, the aquatic and oleic phases are mixed and are rigorously shaken to disperse oil droplets in water and vice versa. The addition of water plays a crucial role in altering the properties of the MWF. Water acts as the cooling agent as it possesses a higher specific heat capacity [57]. However, one of the challenges associated with emulsification is effective mixing or, in other words, the homogenization. The main reason for this is the dispersion resistance of the vegetable oil droplets during the mixing process. As a result, ultrasonic technology was introduced into the market to obtain effective homogenization and thus obtain stable emulsified products [39,58]. In the criterion for determining the stability of the emulsion, one of the parameters used is the hydrophilic-lipophilic (HL) value. The values of the HL can be used to identify whether or not the surfactants or additives have a higher inclination towards the vegetable oils [14,59]. To highlight the basic components of the emulsifier, it should be noted here that it consists of the hydrophilic group in the case of water and lipophilic group for oil. The hydrophilic group has a stronger affinity towards water, whereas the lipophilic chain has a higher proclivity towards oil [60]. The emulsifiers can be classified based on their hydrophilic and/or their lipophilic value [61].

**Figure 5.** The economic and environmental impact of vegetable oil-based MWFs and mineral oilbased MWFs [51].

In order to represent the relative composition of the hydrophilic group and the lipophilic group, a parameter known as the hydrophilic-lipophilic balance (HLB) is used. The HLB scale ranges from 1 to 20, and it shows the affinity of the emulsifier towards water or oil. One way to explain the parameter is that emulsifiers with higher HLB values are more effective for oil in water emulsions, and less useful for water in oil emulsions [62]. The HLB value plays a significant role in the synthesis of the vegetable oil-based MWFs. The surfactants or additives that are to be added and the base oil can be selected based on the HLB values, which are a good indicator of solubility during the preparation of stable emulsions for bio-lubrication purposes [63]. The hydrophilic head points out towards the water phase, while the hydrophobic tail points out towards the oleic phase [64].
