Review on Cutting Fluids: Formulation, Chemistry and Deformulation

Abstract

This comprehensive review offers a chemical analysis of cutting fluids, delving into both their formulation and deformulation processes. The study covers a wide spectrum of cutting fluid formulations, ranging from simple compositions predominantly comprising oils, whether mineral or vegetable, to emulsions. The latter involves the integration of surfactants, encompassing both nonionic and anionic types, along with a diverse array of additives. Concerning oils, the current trend leans towards the use of vegetable oils instead of mineral oils for environmental reasons. As vegetable oils are more prone to oxidation, chemical alterations, the addition of antioxidant may be necessary. The chemical aspects of the different compounds are scrutinized, in order to understand the role of each component and its impact on the fluid's lubricating, cooling, anti-wear, and anti-corrosion properties. Furthermore, the review explores the deformulation methodologies employed to dissect cutting fluids. This process involves a two-step approach: separating the aqueous and organic phases of the emulsions by physical or chemical treatments, and subsequently conducting a detailed analysis of each to identify the compounds. Several analytical techniques, including spectrometric or chromatographic, can be employed simultaneously to reveal the chemical structures of samples. This review aims to contribute to the improvement of waste treatment stemming from cutting fluids. By gathering extensive information about the formulation, deformulation, and chemistry of the ingredients, there is a potential to enhance the waste management and disposal effectively.

1 Introduction

1.1 Contextualization

Lubricants are used worldwide in all sectors requiring machining operations. The global lubricants market size was USD 115.87 billion in 2021 and is expected to grow to 131.33 billion in 2030, even though the COVID-19 pandemic has caused the market to fall by 10.2% between 2019 and 2020. Metalworking fluids, also named cutting fluids, represent approximately 15% of this market and are used in sectors such as industrial, marine, automotive or aerospace¹⁾.

A cutting fluid is used when two moving metal parts are brought into contact. In machining operations, these metal parts are the tool and the workpiece, and the cutting fluid is located at the interface between them (Fig. 1) ²⁾.

In addition to the main functions of lubrication and cooling, a cutting fluid has to reduce power consumption, to provide a good finishing surface on the workpiece by preventing wear, friction and corrosion, to minimize the formation of chips and facilitate their removal from the cutting area, and maximise tool life. In order to meet these requirements, a cutting fluid must have many properties. On a physicochemical point of view, a cutting fluid must have (1) a high thermal conductivity for cooling, (2) a high flash point to prevent fire hazards, (3) a viscosity that allows for proper flow, (4) chip removal, (5) the formation of a lubricating film between the metal parts, and (6) a formulation without compounds susceptible to rancidity, not favouring the formation of solid precipitates at working temperatures. For the safety of operators, a cutting fluid should not develop unpleasant odours from continuous use, and should not cause skin irritation or contamination³⁾.

Fig. 1

Illustration of the use of a cutting fluid at the interface between a tool and a workpiece²⁾.

The choice of cutting fluid depends on the type of metal being machined, the machining operation and the tool being used. It is important to select a cutting fluid that is compatible with the materials and to use it in the proper concentration to achieve the desired performance.

Cutting fluids can be classified according to the continuous phase: oil or water (Fig. 2) ⁴⁾. Synthetic fluids, formulated with polymers rather than oils, will not be discussed in this literature review. The general formulation of these fluids is presented on the Fig. 3.

Fig. 2

Classification of cutting fluids based on its continuous phase.

Fig. 3

General formulation of the different cutting fluids: straight oil, emulsifiable oil, synthetic fluid and semi-synthetic fluid (emulsion or microemulsion) , with the approximate weight percentages of each ingredient (oil, water, surfactants, additives, and polymer) ^{2, 4)} .

These categories can be differentiated according to the main properties expected of a cutting fluid (Table 1) . No single category outshines the others; the optimal selection of used fluid predominantly hinges on the specific machining operations at hand. For example, for heavy-duty machining operations, straight oils will be preferred because (i) they provide a high degree of lubrication, (ii) due to the absence of water, this type of fluid can be used at high temperatures and pressures⁵⁾.

Table 1

Main properties of cutting fluids - evaluation of cutting fluids families according to the desired properties (1: suitable for the property; 4: not suitable) .

Lubrication is provided by the presence of oil or by additives which will act as lubricants. The presence of water in the formulation will ensure the cooling of the metal parts, however, the metal parts will be susceptible to corrosion and the fluid to rancidity. These can be solved by adding additives (anti-rust additives and biocides) that will help to tackle these phenomena. As emulsifiable oils, synthetic and semi-synthetic fluids are often under the form of concentrates that will be diluted with water prior to use, their maintenance will be more difficult because of the need to replenish water. Finally, oil, subjected to high temperatures and pressures, will generate mist and smoke, which can be dangerous for the safety of operators^3),5).

This review aims to provide an overview of the most common ingredients found in cutting fluid formulations. The complexity in formulating these fluids presents a significant challenge for the formulator, as they must (i) identify appropriate raw materials for the application purpose, and (ii) determine which methods to employ for effectively separating the formulations to enable optimal recycling. These main ingredients include the oil, surfactants when necessary and the different additives present (extreme-pressure additives, corrosion inhibitors, biocide additives, lubrication additives, anti-foaming agents, buffering agents, etc.) as well as the quantity of water required. In a second step, characterization methods to identify these different families of compounds will be discussed, such as spectroscopy or chromatographic techniques.

1.2 Literature review methodology

This review is divided into two parts: the first focuses on the formulation of cutting fluids and the chemistry of their constituent ingredients, while the second addresses the deformulation of finished products. The latter aims to explore, from a chemical perspective, various suitable analytical techniques for understand the composition of these products. With a comprehensive analysis of each ingredient, it becomes feasible to maximize the recycling of a product at the end of its lifecycle. The literature review was carried out thanks to bibliographical online tool searches, the documentary databases Google Scholar and Scopus^TM were queried.

The search parameters were defined using the following keywords: 〝CUTTING FLUIDS〟 OR 〝METALWORKING FLUIDS〟 AND 〝FORMULATION〟 for the first part, and 〝CUTTING FLUIDS〟 OR 〝METALWORKING FLUIDS〟 AND 〝ADDITIVES〟 AND 〝ANALYSIS〟 for the second part. The term 〝LUBRICANTS〟 was deliberately excluded from the search criteria, as the primary objective was to narrow the focus exclusively to the category of cutting fluids. All the documents identified in the bibliometric portals results were subsequently recorded in the Zotero bibliographic management tool. They were then exported, along with all their metadata, to an Excel database for further utilization and analysis. An exclusion criterion based on title headings was applied to ensure that all documents were relevant to the subject.

2 Chemistry and Formulation of Cutting Fluids

2.1 Emulsion or microemulsion formulation: Tailoring with oil types

The initial stage in formulating a cutting fluid involves identifying the oil that best suits the intended application. In this section, we will delve into the discussion of raw oils directly employed as cutting fluids, drawing a comparison between mineral oils and vegetable oils, in terms of (1) tribological properties (corrosion, thermal stability, anti-wear, etc.) , (2) lubrication, (3) oxidation, and (4) biodegradability. Figure 4 illustrates various chemical structures of oils employed in formulating cutting fluids.

Fig. 4

Chemical structures of oils included in cutting fluids formulation.

2.1.1 Straight oils

Straight oils primarily consist of oil, whether of vegetable or mineral origin. Cutting fluids based on vegetable oil have gained popularity due to their environmental advantages, such as biodegradability, and low toxicity. They are often considered a more sustainable option. However, as raw oils, mineral oils generally exhibit lubricating and cooling properties that are naturally more effective than those of vegetable oils. Table 2 compiles various oils (both vegetable or mineral) employed in their raw state as cutting fluid, along with the corresponding characterization tests (tribological and physicochemical) and the varied outcomes reported by the authors.

Table 2

Comparative analysis of straight oils (100 wt.% oil) through characterization methods, including a comparison between mineral and vegetable oils, along with insights from various authors on the performance associated with each oil type.

During application, crude oil is used directly without dilution, eliminating the need for additional procedures. This approach ensures the efficient and direct utilization of straight oil's lubricating and protective properties while optimizing its impact on equipment.

Since the early 2000s, the interest in vegetable oils as alternatives to mineral oils has greatly increased, especially for their minimal impact on the environment, their broad production potential, and their high biodegradability. The existing legislation in the European Union, pertaining to lubricants used within the EU, has defined criteria and requirements for biodegradability levels, permitting a degradation threshold of 60% over a 28-day period¹⁹⁾. Hence, numerous studies focus on comparing the properties of various vegetable oils with those of conventional mineral oil-based cutting fluids^{6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 20) 21)} . The findings presented in these research studies consistently reveal that cutting fluids derived from vegetable oils exhibit properties at least on par with, if not surpassing, those of their mineral oil-based counterparts.

Due to their chemical composition, vegetable oils tend to display lower resistance to oxidation and temperature elevation. Indeed, they are primarily composed of triglycerides, which are fatty acid esters containing double bonds in their molecular structure, making them more prone to oxidation²²⁾. The oxidation of vegetable oils depends on both their degree of saturation and the number of unsaturations. Oils rich in saturated fatty acids (coconut and palm oils for example) are more stable against oxidation, while those with a high number of unsaturations (sunflower, soybean, and maize oils for example) are more subject to oxidation reaction. Coconut oil, mainly composed of saturated esters, is often seen as the superior alternative. It increases lubricating properties more favourable than some mineral oils, and its effectiveness remains uncompromised by oxidation and temperature variations²³⁾. The triglyceride structure offers a favourable attribute for boundary lubrication. This is attributed to the extended and polar nature of their fatty acid chains, which contribute to the formation of robust lubricant films. These films exhibit strong interaction with metallic surfaces, effectively diminishing both friction and wear¹⁷⁾.

2.1.2 Modification of vegetable oils

To enhance their resistance to oxidation and temperature elevation, a viable approach is the chemical modification of vegetable oils to safeguard their double bonds. Various reactions can be employed for this purpose, preserving the oils' inherent properties, especially in terms of lubrication. These modifications can also be executed in a controlled manner, allowing the attainment of desirable properties without substantially altering the overall structure of the oil²⁴⁾.

Table 3 consolidates diverse researches focused on utilizing chemically modified oils as cutting fluids. The primary aim was to illustrate the impact of these modifications on the oils' properties. Various reactions are conceivable, with the most prevalent ones for safeguarding double bonds being: (i) epoxidation (Fig. 5) , forming epoxy groups that act as protective barriers against oxidation; (ii) hydrogenation, converting unsaturations into single bonds; and (iii) ozonolysis, involving the cleavage of double bonds, thereby diminishing the reactivity of vegetable oils during machining²⁵⁾. Nevertheless, it is important to highlight that the chemical alteration of vegetable oils comes with certain drawbacks. This includes a rise in the cost of raw materials and a reduction in anti-wear efficiency. Consider the epoxidation of soybean oil as an illustrative example, where the process enhances its corrosion resistance, yet achieving a performance comparable to unmodified soybean oil necessitates the incorporation of an anti-wear additive²⁶⁾.

Table 3

Comparison of chemically modified vegetable oils with their unmodified counterparts in terms of their properties as cutting fluids.

Fig. 5

Illustration of the epoxidation of sunflower oil (vegetable oil) to protect the double bonds.

2.2 Emulsion formulation: addition of surfactants

In the specifications for cutting fluids, two crucial properties must be addressed: lubrication and the cooling of metal parts. As discussed earlier, regardless of their composition, oils play a pivotal role in providing lubrication. When it comes to cooling, water emerges as the optimal choice due to its exceptional physicochemical properties. Water boasts a high thermal capacity, allowing it to absorb and store a substantial amount of heat before reaching elevated temperatures. Additionally, it exhibits high thermal conductivity, facilitating efficient heat transport and enabling the swift transfer of heat from hot components to areas where it can be dissipated. Furthermore, water demonstrates a high latent heat of vaporization, absorbing a significant amount of heat during the transition from a liquid to a gaseous state without experiencing temperature increase³⁰⁾.

It is crucial to combine both oil and water to enable a cutting fluid to act effectively when cooling is necessary during machining operations. Hence, it is essential to create emulsions through the addition of surfactants. This section explores the incorporation of various surfactants into cutting fluid formulations. Predominantly, non-ionic or anionic surfactants are used, with cationic surfactant being seldom employed. This is because cationic surfactants are more prone to eliciting undesirable reactions, such as precipitation, due to the presence of anions in water, consequently resulting in a decline in their performance. The choice depends on the intended application of the cutting fluids. On Fig. 6 and Fig. 7, the chemical structures of ionic and nonionic surfactants, respectively, used in the formulations of cutting fluids are depicted.

Fig. 6

Chemical structures of ionic surfactants incorporated in cutting fluids formulations.

Fig. 7

Chemical structures of nonionic surfactants employed in cutting fluids formulations.

2.2.1 Soluble and water-based oils

Soluble oils are formulations that concentrate oil and surfactants, and they may additionally incorporate additives tailored to specific machining applications. Prior to utilization, these formulations need to be diluted with water following a ratio provided by the manufacturer. Table 4 showcases formulations of soluble oils derived from combinations of vegetable oil and surfactants. Notably, the prevailing direction in formulation emphasizes fluid characterized by low toxicity and exceptional biodegradability.

Table 4

Formulations of soluble oils using vegetable oil and surfactant.

On the flip side, aqueous-based fluids are oil free. While these fluids excel in providing effective cooling, they lack inherent lubricating properties. Hence, for specific applications, the inclusion of additives becomes essential, serving not only to enhance lubrication but also to effectively address corrosion issues. While the formulation of water-based cutting fluids boasts numerous environmental and worker safety advantages. Table 5 highlights the essential nature of incorporating anticorrosion agents for optimal fluid performance. Actually, the presence of water facilitates metal corrosion by creating an environment conducive to the chemical interaction between the metal and atmospheric oxygen.

Table 5

Formulations of water-based cutting fluids.

2.2.2 Semi-synthetic fluids

The predominant category within the family of cutting fluids is semi-synthetic fluids. Their formulation is notably more intricate compared to the fluids introduced thus far. This complexity arises from the combination of an aqueous base providing cooling properties, oil contributing lubricating characteristics, surfactants for emulsion stability, and additives that impart additional properties based on the specific requirements. These fluids, though offering extensive features, do have downsides, including a higher price tag and the risk of contamination from other machining fluids due to their solubility in both water and oil³⁾. Semi-synthetic fluids can be used as-is or sold as concentrate to be diluted in water before use³⁵⁾. The formulation of emulsions used as the foundation for semi-synthetic fluids, including the oils and surfactants employed, is presented in Table 6.

Table 6

Formulations of emulsions serving as the foundation of semi-synthetic fluids.

The choice of surfactant and its concentration are critical factors influencing the performance of oil-in-water emulsions employed as cutting fluids. A thorough characterization of the emulsion is essential in terms of both physicochemical and tribological properties to understand the interactions involved between the emulsion and the solid surface³⁹⁾. For instance, creaming tends to decrease as the concentration of surfactants rises and the droplet size decreases. This effect is most pronounced when the surfactant concentration exceeds the critical micelle concentration. This phenomenon occurs because there is a higher chance of rupturing the thin film of the outer phase, which forms when droplets collide, especially at very high surfactant concentrations⁴¹⁾. It is worth noting that chemically modified vegetable oils require a higher amount of surfactants to form a stable emulsion compared to their unmodified counterparts^34),35). Therefore, it is crucial to strike the right balance between the chemical modification of vegetable oils and the stability of formulated emulsions. Ultimately, the trend is towards formulating environmentally friendly cutting fluids, using vegetable oils or even bio-sources surfactants⁴⁴⁾.

2.3 Further formulation: the addition of additives

The optimization of cutting fluids plays a crucial role in enhancing efficiency and precision. Among the myriad factors influencing the performance of these fluids, additives stand out as key ingredients shaping their formulation. Additives bring a diverse range of functionalities, from improving lubricity and heat dissipation to inhibiting corrosion and extending tool life.

Corrosion inhibitors (Fig. 8) play a core function in safeguarding the metallic surfaces of machinery, metalworking tools, and workpieces against the deleterious effects of oxygen, moisture, and aggressive substances. These inhibitors are comprised of molecules characterized by long alkyl chains and polar groups, allowing them to adsorb onto metal surfaces and form densely packed, hydrophobic layers⁴⁸⁾. The adsorption process can be attributed to either a physical or chemical interaction between the polar anticorrosion additive and the metal surface. Due to their heightened surface activity, anticorrosion additives engage in competition with other polar additives, such as Anti-Wear (AW) and Extreme-Pressure (EP) additives (Fig. 9) , for occupancy on the metal surface, potentially diminishing their efficacity⁴⁹⁾. The latter are designed to mitigate wear and adhesion in metal-to-metal contact zones subjected to high loads and extreme pressure conditions. Sulfur and phosphorus compounds are frequently employed as EP additives⁴⁹⁾. They chemically react with the metal surface, owing to their polar structure, to create protective films, thereby diminishing wear and adhesion⁵⁰⁾. Other additives (Fig. 10) can also be incorporated into the formulation of cutting fluids, with their respective roles explained in Table 7.

Fig. 8

Chemical structures of corrosion inhibitors employed in the cutting fluids formulations.

Fig. 9

Chemical structures of Extreme-Pressure/Anti-Wear additives employed in the cutting fluids formulations.

Fig. 10

Chemical structures of lubricants, coupling agents and antioxidants employed in the cutting fluids formulations.

Table 7

Role and chemistry of additives incorporated in cutting fluids formulation.

Table 8 consolidates various formulations of cutting fluids and outlines the types of additives employed. They are diverse and contingent upon the intended final use of the fluid. The trend is leaning towards the utilization of bio-based additives, with formulations containing the fewest possible ingredients.

Table 8

Additives (Anti Wear-Extreme Pressure, lubricants, corrosion inhibitors, antioxidants, coupling agents, pH buffers, anti-foam and biocides) featured in various formulations of cutting fluids (straight, soluble oil, water-based and semi-synthetic fluids) .

2.4 Discussion and conclusion about formulation

The formulation and chemistry of cutting fluids are highly diverse, with each formulation tailored to meet specific requirements based on its intended application. The current trend is towards the development of biodegradable cutting fluids, incorporating a high proportion of bio-sourced compounds, in line with environmental considerations. As mentioned earlier, it is not possible to provide a perfect formulation for a cutting fluid; its composition will depend on the intended use. The oil based will be chosen base on (1) the environmental requirements, (2) the machining processes, including the applied temperature and pressure, and (3) compatibility with the equipment and the surrounding environment. The surfactant, or the system of surfactants, is chosen to form the most stable emulsion, and sometimes to improve some desired properties. Lastly, the additives are chosen to complete the properties not achieved by the former components, and to improve the cutting fluids formulation.

3 Deformulation of Cutting Fluids: Analytical Techniques for Understanding Component Composition

In this section, the deformulation of cutting fluids will be addressed, exploring various methods for destabilizing the emulsion to isolate aqueous and organic phases. Analytical techniques will also be examined to identify different compounds families and determine their concentrations. This step is of crucial importance in the treatment of waste generated from cutting fluids, whether in recycling or reusing fluids or in disposal processes. Deformulation, also known as reverse engineering, is the process of dissecting a formulated product to discern and gather comprehensive insights into the types of ingredients employed during its manufacture⁵⁷⁾. Various analytical techniques, such as Fourier-Transform InfraRed (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopies, thermal methods (ThermoGravimetric Analysis - TGA, Differential Scanning Calorimetry - DSC) , chromatographic techniques (High-Performance Liquid Chromatography - HPLC, Gas Chromatography-Mass Spectroscopy - GC-MS, Thin-Layer Chromatography - TLC, etc.) , and elemental analysis (Inductively Couple Plasma - ICP) , are employed for the dissection, and it is crucial to use them complementarily for optimal identification.

3.1 Emulsion destabilization

The initial step in comprehending the deformulation of a cutting fluid involves destabilizing the emulsion. As elucidated in section 2. Chemistry and Formulation of Cutting Fluids, cutting fluids predominantly consist of oil-in-water (O/W) emulsions stabilized by a system of surfactants.

To effectively separate O/W emulsions, wherein the goal is to isolate the aqueous and oil phases, it is crucial to discern between chemical and physical treatment methods. Chemical treatments encompass processes like coagulation and pH adjustment, while physical treatments involve flotation, temperature modulation, ultrasonication, centrifugation, filtration, and more.

Unlike physical methods, chemical treatments do not entail the disposal of the oil; rather, their purpose is to transform the oil into a state that facilitates easier separation. This becomes essential when dealing with highly stable emulsions where natural coalescence is challenging. Relying solely on physical processes for separation in such cases can be difficult. The application of chemical treatment is aimed at modifying the emulsion's characteristics, rendering it more amenable to efficient separation processes.

3.1.1 Chemical treatments

Emulsions are commonly stabilised through the presence of repulsive charges on droplet surfaces and adsorbed layers that serve as an interfacial barrier, hindering close contact or coalescence between droplets. The electrical charge of a droplet can be assessed using parameters such as surface potential, and electrokinetic ζ-potential⁵⁸⁾. The coagulation mechanism for destabilizing O/W emulsions involves the aggregation and clumping of the dispersed oil droplets, leading to their separation from the continuous water phase. This process is typically induced by the addition of coagulants, such as salts⁵⁹⁾. The presence of these demulsifiers influences the interfacial characteristics of the adsorbed surface, leading to an enhanced coalescence rate among the dispersed droplets⁶⁰⁾. To treat cutting fluids, calcium chloride CaCl₂ and aluminium chloride AlCl₃ have been widely used^58),59),60). Sodium chloride NaCl, with a concentration of 50 g.L^－1, have also proved its efficiency for the demulsification of a commercial mineral-based cutting fluid, as a semi-synthetic fluid. When very concentrated emulsions are used, it is necessary to increase its concentration⁶¹⁾. A commercial flocculant, consisting of a blend of cationic and nonionic synthetic polymers, was assessed in comparison to CaCl₂ and AlCl₃ for destabilizing emulsions of two cutting fluids with oil concentrations of 329 and 169 g.L^－1. The commercial flocculant demonstrated effectiveness only with the less concentrated emulsion. In the other case, the addition of salts was necessary to enhance the separation⁶²⁾. To monitor the coagulation mechanism, optical density or turbidity can be measured using a spectrophotometer, by scanning at several wavelengths. By scanning at various wavelengths, the optical signal's behavior can be analyzed and fitted with a power function, with the exponent serving as a characteristic indicator of the emulsion's particle size. This exponent parameter, indicative on how turbidity varies with wavelength, provides insights into particle size and stability of the emulsion. A stable emulsion will show little to no change in the wavelength exponent over time, whereas an unstable emulsion will exhibit significant changes, signalling particle growth or reduction⁶³⁾.

Another chemical factor that can impact the stability of an emulsion is pH. By modifying it, the electric charges of the particles can be neutralized, enhancing the attraction among droplets and making them more likely to coalesce, forming larger agglomerates⁶⁴⁾. Sulfuric acid, at a concentration of 2 mL.L^－1, has demonstrated its effectiveness in separating cutting fluids containing 5% w/w of oil, prior to pH neutralization with calcium chloride⁶⁵⁾. Another investigation⁶⁶⁾explored the impact of pH variation and ionic conductivity on the size and surfaces charges of oil droplets, specifically in the context of destabilizing two cutting fluids derived from mineral oils. The findings indicate that changes in pH had a more pronounced effect on droplet coalescence compared to an increase in salt concentration. Moreover, the study revealed that electrocoagulation yielded superior efficiencies in removing oil under alkaline conditions.

3.1.2 Physical treatments

Without introducing additional substances into an already complex formulation, such as cutting fluids, it is possible to employ physical treatments to destabilize O/W emulsions. The first parameter that can be easily modified is the temperature. Indeed, an increase in temperature can decrease the interfacial tension between the phases of the emulsions, thereby weakening the barrier that prevents droplet coalescence⁶⁷⁾. However, temperature may adversely affect specific components within the formulation, influencing the subsequent separation process. Centrifugation can also serve as a rapid method to separate phases based on their respective densities. Typically used to assess emulsion stability by simulating aging, this technique can therefore be employed to destabilize emulsions⁶⁸⁾.

Flotation, which is a separation process, involves the attachment of gas bubbles to the oil droplets within the emulsion, leading to the formation of buoyant aggregates that rise to the surface. Its mechanism can be explained in several steps: (1) gas introduction in the emulsion; (2) attachment of gas bubbles selectively to the oil droplets, influenced by the interfacial properties of oil droplets (surface tension and charge) ; (3) Aggregates formation ascending to the surface, hence the term flotation; and (4) surface skimming⁶⁹⁾. This technique is often combined with coagulation, by the addition of salts or nonionic surfactants^70),71), these two techniques exhibit strong synergy, resulting in enhanced separation efficiency and a reduction in the use of chemicals.

3.2 Analysis methods for compounds families

After treating the emulsion and separating the aqueous and organic phases of a cutting fluid, understanding their compositions become imperative. Numerous analytical techniques can be employed to elucidate the intricate formulation of a cutting fluid. This section aims to review diverse techniques, ensuring the analysis of the different compounds' families employed in cutting fluids formulations.

3.2.1 Spectroscopy techniques (FT-IR, NMR)

Spectroscopy techniques enable the elucidation of molecular composition of a complex mixture. These methods allow to characterize substances by interpreting spectra generated when matter interacts with electromagnetic radiation. Infrared spectroscopy (IR) qualitatively identifies molecular bonds by analysing their vibrational frequencies, providing insights into chemical composition. Nuclear Magnetic Resonance (NMR) spectroscopy enables both qualitative and quantitative analysis of molecular structure by examining interactions with atomic nuclei, offering precise information about the arrangement of atoms in a compound⁷²⁾. Pierlot et al.⁷³⁾examined two phosphate ester additives present in the formulation of cutting fluids: an EP additive (isopropylated triarylphosphate) and a surfactant (polyoxyethylated phosphate) , using ¹H and ³¹P NMR spectroscopies. The authors concluded that these methods offer a swift and precise approach for identifying and quantifying the compounds, while also providing insights into the substitution or polymerization rate of the molecules. Another study⁷⁴⁾demonstrated that ¹H NMR spectroscopy effectively differentiated between fats and synthetic esters. The combined application of ¹H and ¹³C NMR spectroscopies can effectively differentiate between various vegetable oils, by profiling their constituent fatty acids^75),76). In resonance with the chemical modifications of vegetable oils to enhance physicochemical properties (see subsection 2.1.2 Modification of vegetable oils) , these techniques are commonly utilized to characterize reaction products as well³⁶⁾.

3.2.2 Chromatographic techniques (HPLC, GC-MS)

Chromatographic techniques, such as HPLC and GC-MS, enable the separation and identification of intricate mixtures. HPLC relies on the interaction between the sample and a stationary phase, propelled by a high-pressure liquid mobile phase. The stationary phase selectively interacts with analytes, guiding efficient separation in a chromatographic column. In GC-MS, the sample is vaporized in a heated inlet, entering a gas chromatograph where compounds interact with a stationary phase and carrier gas, achieving separation. The resolved compounds then proceed to the mass spectrometer for ionization, fragmentation, and detection, enabling precise identification and quantification based on mass-to-charge ratio⁷⁷⁾. Fungicides, found in small proportions in the formulation of water-based cutting fluids, can be detected using HPLC, such as sodium 2-pyridinethiol-1-oxide⁷⁸⁾, or hexahydro-1,3,5-tri (2-hydroxyethyl) -S-triazine⁷⁹⁾. HPLC stands out as a versatile method for analysing polar organic compounds, encompassing crucial additives in cutting fluids, such as nonionic surfactants, along with the quantification of ethylene oxide oligomers⁸⁰⁾. GC-MS was employed to identify an unknown peak in a chromatograph of a straight oil, revealing it to be di-octyl disulphide, an EP additive⁸¹⁾. This method is also valuable for quantifying the presence of polycyclic aromatic hydrocarbons in a formulation⁸²⁾, as well as for analysing the composition of biodegradable cutting fluids derived from vegetable oils⁸³⁾.

3.2.3 Elemental analysis methods (ICP)

Elemental analysis can be conducted to analyse the content of individual components within a cutting fluid formulation. Techniques like Atomic Emission Spectroscopy (ICP-AES) , Inductively Couple Plasma Mass Spectroscopy (ICP-MS) , and Atomic Absorption Spectroscopy (AA) help identify and quantify specific elements and provide an average molecular formula for species⁸⁴⁾. These methods work by measuring the emitted or absorbed light at characteristic wavelengths when the sample is subjective to high-temperature plasma. ICP-AES was used to determine and quantify the boron content in a water-based cutting fluid⁸⁵⁾. X-Ray Fluorescence (XRF) , by measuring the characteristic X-rays emitted when it is exposed to X-rays, can be used to determine the elemental composition of a constituent⁸⁶⁾.

3.2.4 Other techniques

Thermal analysis, such as ThermoGravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) . These methods can help understanding how cutting fluids and its components behave under different temperature conditions. TGA measures weight changes as a sample is heated, indicating the decomposition or volatilisation of its constituents. On the other hand, DSC monitors heat flow in or out of a sample, providing insights into phase transitions and thermal properties. These combined techniques can allow the determination of the degradation temperatures, the degradation rate and the inorganic substance content of a sample⁸⁷⁾. These are also employed methods to determine the oxidation level of an oil, thereby distinguishing between mineral and vegetable oils. In the latter case, it helps identify the specific type of vegetable oil used in the formulation⁸⁸⁾. Isotachophoretic method, by separating ionic species based on their ionic mobility in an electrolyte system, has been applied to determine the presence of ethanolamine, diethanolamine, and triethanolamine in a cutting fluid formulation⁸⁹⁾.

3.3 Discussion and conclusion about deformulation

The deformulation of cutting fluids comprises a two-step process: (1) separating the aqueous and organic phases of the emulsion, and (2) analysing each phase to comprehend the incorporated compounds. Numerous analytical techniques are available, and it is not feasible to enumerate them all. The selection of the techniques depends on the chemical structures of the molecules under investigation. In most cases, the presented analyses are conducted simultaneously to maximize formulation resolution. The separation of emulsions may necessitate combining coagulation and flotation methods⁹⁰⁾, especially in the case of mineral-based cutting fluids⁹¹⁾. Studies have demonstrated that, particularly in highly biodegradable fluids, oxidations can occur, leading to polymerization effects. These reactions have been substantiated through HPLC and elemental analyses⁹²⁾.

4 Conclusion

This review has shed light on the diversity and intricacies involved in the formulation and deformulation of cutting fluids. The formulation of a cutting fluid can range from a very simple composition, containing only oil (whether mineral or vegetable) , to a more complex formulation, based on an oil-in-water emulsion. In the latter case, it includes surfactants (nonionic or anionic in most cases) , and additives, such as corrosion inhibitors, EP/AW additives, coupling agents, antioxidants, etc.

The exploration of various analytical techniques, whether chemical, thermal or elemental, emphasizes the significance of adopting complementary approaches for a comprehensive understanding of compositions. Challenges related to emulsion separation, composition alterations over time, and environmental considerations in component selection contribute crucial dimensions to this field. It becomes evident that ongoing research in the formulation and deformulation of cutting fluids is essential for optimizing performance, sustainability, and safety of these key compounds in various industrial applications. As industries continue to evolve, the pursuit of innovative solutions and a deeper understanding of these fluids remain paramount for advancing efficiency, durability, and environmental responsibility.

Acknowledgements

This project has received funding from the Atomic Energy Commission and Centrale Lille Institute. Chevreul Institute (FR 2638) , Ministère de l'Enseignement Supérieur et de la Recherche, Région Hauts de France, and FEDER are acknowledged for supporting and funding part of this work.

References

• 1)  Lubricant Market Size, Share & COVID-19 Impact Analysis, By Type（Automotive Oils, Industrial Oils（Hydraulic Oils, Industrial Gear Oils, Metal Working Fluids, Greases, and Others）, Marine Oil, and Process Oils）, By Grade（Mineral, Synthetic, and Semi-Synthetic）, By Application（Automotive, Industrial, and Others）, and Regional Forecast, 2022-2029; FBI101771; 2022; p 325. https://www.fortunebusinessinsights.com/industry-reports/lubricants-market-101771（accessed 2022-11-02）
• 2)  Singh, J.; Gill, S.S.; Dogra, M.; Singh, R. A review on cutting fluids used in machining processes. Eng. Res. Express 3, 012002（2021）. doi: 10.1088/2631-8695/abeca0
• 3)  Mamidi, V.K.; Xavior, M.A. A review on selection of cutting fluids. National Monthly Refereed Journal of Research in Science & Technology 1, 18（2012）
• 4)  Childers, J.C. The chemistry of metalworking fluids. in Metalworking Fluids - Second Edition, pp 127-146（2006）
• 5)  Osama, M.; Singh, A.; Walvekar, R.; Khalid, M.; Gupta, T.C.S.M.; Yin, W.W. Recent developments and performance review of metal working fluids. Tribol. Int. 114, 389-401（2017）. doi: 10.1016/j.triboint.2017.04.050
• 6)  Abdalla, H.S.; Baines, W.; McIntyre, G.; Slade, C. Development of novel sustainable neat-oil metal working fluids for stainless steel and titanium alloy machining. Part 1. Formulation development. Int. J. Adv. Manuf. Technol. 34, 21-33（2007）. doi: 10.1007/s00170-006-0585-4
• 7)  Carcel, A.C.; Palomares, D.; Rodilla, E.; Puig, M.A.P. Evaluation of vegetable oils as pre-lube oils for stamping. Mater. Des. 26, 587-593（2005）. doi: 10.1016/j.matdes.2004.08.010
• 8)  Erhan, S.Z.; Asadauskas, S. Lubricant basestocks from vegetable oils. Ind. Crops Prod. 11, 277-282（2000）. doi: 10.1016/S0926-6690（99）00061-8
• 9)  Quinchia, L.A.; Delgado, M.A.; Reddyhoff, T.; Gallegos, C.; Spikes, H.A. Tribological studies of potential vegetable oil-based lubricants containing environmentally friendly viscosity modifiers. Tribol. Int. 69, 110-117（2014）. doi: 10.1016/j.triboint.2013.08.016
• 10)  Ozcelik, B.; Kuram, E.; Huseyin Cetin, M.; Demirbas, E. Experimental investigations of vegetable based cutting fluids with extreme pressure during turning of AISI 304L. Tribol. Int. 44, 1864-1871（2011）. doi: 10.1016/j.triboint.2011.07.012
• 11)  Ojolo, S.J.; Amuda, M.O.H.; Ogunmola, O.Y.; Ononiwu, C.U. Experimental determination of the effect of some straight biological oils on cutting force during cylindrical turning. Matér. Rio Jan. 13, 650-663（2008）. doi: 10.1590/S1517-70762008000400011
• 12)  Jayadas, N.H.; Prabhakaran Nair, K.; Ajithkumar, G. Tribological evaluation of coconut oil as an environment-friendly lubricant. Tribol. Int. 40, 350-354（2007）. doi: 10.1016/j.triboint.2005.09.021
• 13)  Ariff, T.F.; Sabiyah, M.Y.; Adam, M.A.; Nor, M.A. Eco-friendly machining of T6061 aluminium alloy using titanium carbonitride（TiCN）coated tools. Adv. Mater. Res. 576, 675-678（2012）. doi: 10.4028/www.scientific.net/AMR.576.675
• 14)  Dhar, N.R.; Alim, Md.A.; Udding, Md.J. An experimental investigation of vegetable oil-based cutting fluid on drilling medium carbon steel by HSS drill bit. International Conference on Mechanical Engineering（ICME 2009）（2009）.
• 15)  Rahim, E.A.; Sasahara, H. A study of the effect of palm oil as MQL lubricant on high speed drilling of titanium alloys. Tribol. Int. 44, 309-317（2011）. doi: 10.1016/j.triboint.2010.10.032
• 16)  Jia, D.; Li, C.; Zhang, Y.; Yang, M.; Wang, Y. et al. Specific energy and surface roughness of minimum quantity lubrication grinding Ni-based alloy with mixed vegetable oil-based nanofluids. Precis. Eng. 50, 248-262（2017）. doi: 10.1016/j.precisioneng.2017.05.012
• 17)  Siniawski, M.T.; Saniei, N.; Adhikari, B.; Doezema, L.A. Influence of fatty acid composition on the tribological performance of two vegetable-based lubricants. J. Synth. Lubr. 24, 101-110（2007）. doi: 10.1002/jsl.32
• 18)  Clarens, A.F.; Zimmerman, J.B.; Landis, H.R.; Hayes, K.F.; Skerlos, S.J. Experimental comparison of vegetable and petroleum base oils in MWFs using the tapping torque test. Proceedings of JUSFA 6（2004）.
• 19)  Council Regulation（EC）. No 440/2008 Laying down Test Methods Pursuant to Regulation（EC）No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals（REACH）; European Union, Belgium, p. 379（2008）.
• 20)  Xavior, M.A.; Adithan, M. Determining the influence of cutting fluids on tool wear and surface roughness during turning of AISI 304 austenitic stainless steel. J. Mater. Process. Technol. 209, 900-909（2009）. doi: 10.1016/j.jmatprotec.2008.02.068
• 21)  Chinchanikar, S.; Salve, A.V.; Netake, P.; More, A.; Kendre, S.; Kumar, R. Comparative evaluations of surface roughness during hard turning under dry and with water-based and vegetable oil-based cutting fluids. Procedia Mater. Sci. 5, 1966-1975（2014）. doi: 10.1016/j.mspro.2014.07.529
• 22)  Alves, S.M.; Gomes, J.F. Vegetable based cutting fluid - An environmental alternative to grinding process. 15th CIRP International Conference on Life Cycle Engineering（2008）.
• 23)  Fox, N.J.; Stachowiak, G.W. Vegetable oil-based lubricants─A review of oxidation. Tribol. Int. 40, 1035-1046（2007）. doi: 10.1016/j.triboint.2006.10.001
• 24)  Sharma, K.B.; Adhvaryu, A.; Liu, Z.; Erhan, S.Z. Chemical modification of vegetable oils for lubricant applications. J. Am. Oil. Chem. Soc. 83, 129-139（2006）. doi: 10.1007/s11746-006-1185-z
• 25)  Karmakar, G.; Ghosh, P.; Sharma, B.K. Chemically modifying vegetable oils to prepare green lubricants. Lubricants 5（4）, 17（2017）. doi: 10.3390/lubricants5040044
• 26)  Castro, W.; Weller, D.E.; Cheenkachorn, K.; Perez, J.M. The effect of chemical structure of basefluids on antiwear effectiveness of additives. Tribol. Int. 38, 321-326（2005）. doi: 10.1016/j.triboint.2004.08.020
• 27)  Adhvaryu, A.; Erhan, S.Z.; Perez, J.M. Tribological studies of thermally and chemically modified vegetable oils for use as environmentally friendly lubricants. Wear 257, 359-367（2004）. doi: 10.1016/j.wear.2004. 01.005
• 28)  Alves, S.M.; Barros, B.S.; Trajano, M.F.; Ribeiro, K.S.B.; Moura, E. Tribological behavior of vegetable oil-based lubricants with nanoparticles of oxides in boundary lubrication conditions. Tribol. Int. 65, 28-36（2013）. doi: 10.1016/j.triboint.2013.03.027
• 29)  Talib, N.; Rahim, E.A. Performance of modified Jatropha oil in combination with hexagonal boron nitride particles as a bio-based lubricant for green machining. Tribol. Int. 118, 89-104（2018）. doi: 10.1016/j.triboint. 2017.09.016
• 30)  Akilu, S.; Sharma, K.V.; Baheta, A.T.; Mamat, R. A review of thermophysical properties of water based composite nanofluids. Renew. Sust. Energ. Rev. 66, 654-678（2016）. doi: 10.1016/j.rser.2016.08.036
• 31)  Singh, A.K.; Gupta, A.K. Metalworking fluids from vegetable oils. J. Synth. Lubr. 23, 167-176（2006）. doi: 10.1002/jsl.19
• 32)  Eppert, J.J.; DeVor, R.E.; Kapoor, S.G.; Rajagopalan, N. The effect of chip adsorption on selective depletion from a multi-component synthetic metalworking fluid. J. Manuf. Sci. Eng. 125, 703-708（2003）. doi: 10. 1115/1.1615794
• 33)  Nakagawa, H.; Watanabe, S.; Fujita, T.; Sakamoto, M. Characteristic properties of cutting fluid additives made from the derivatives of some polymeric nonionic surface-active agents. J. Surfactants Deterg. 1, 207-211（1998）. doi: 10.1007/s11743-998-0021-7
• 34)  Kawato, Y.; Kamiusuki, T.; Watanabe, S. Water-soluble metalworking fluid additives derived from the derivatives of 3-mercaptopropionic acid. J. Surfactants Deterg. 9, 391-394（2006）. doi: 10.1007/s11743-006-5018-8
• 35)  Raynor, P.C.; Wom Kim, S.; Bhattacharya, M. Mist generation from Metalworking Fluids Formulated Using Vegetable Oils. Ann. Occup. Hyg. 49, 283-293（2005）. doi: 10.1093/annhyg/meh092
• 36)  John, J.; Bhattacharya, M.; Raynor, P.C. Emulsions containing vegetable oils for cutting fluid application. Colloids Surf. Physicochem. Eng. Asp. 237, 141-150（2004）. doi: 10.1016/j.colsurfa.2003.12.029
• 37)  Alves, S.M.; de Oliveira, J.F.G. Development of new cutting fluid for grinding process adjusting mechanical performance and environmental impact. J. Mater. Process. Technol. 179, 185-189（2006）. doi: 10.1016/j.jmatprotec.2006.03.090
• 38)  Bataller, H.; Lamaallam, S.; Lachaise, J.; Graciaa, A.; Dicharry, C. Cutting fluid emulsions produced by dilution of a cutting fluid concentrate containing a cationic/nonionic surfactant mixture. J. Mater. Process. Technol. 152, 215-220（2004）. doi: 10.1016/j.jmatprotec.2004.03.027.
• 39)  Cambiella, A.; Benito, J.M.; Pazos, C.; Coca, J.; Hernández, A.; Fernández, J.E. Formulation of emulsifiable cutting fluids and extreme pressure behaviour. J. Mater. Process. Technol. 184, 139-145（2007）. doi: 10.1016/j.jmatprotec.2006.11.014.
• 40)  Yu, Y.; Guo, Y.; Wang, L.; Tang, E. Development of environmentally friendly water-based synthetic metal-cutting fluid. Mod. Appl. Sci. 4, 53-58（2009）. doi: 10.5539/mas.v4n1p53
• 41)  Ratoi-Salagean, M.; Spikes, H.; Hoogendoorn, R. The design of lubricious oil-in-water emulsions. Proc. Inst. Mech. Eng. Part J J. Eng. Tribol. 211, 195-208（1997）. doi: 10.1243/1350650971542426
• 42)  Lawal, S.A. A review of application of vegetable oil-based cutting fluids in machining nonferrous metals. Indian J. Sci. Technol. 6, 1-6（2013）. doi: 10.17485/ijst/2013/v6i1.22
• 43)  Lee, T.S.; Choong, H.B. An investigation on green machining: Cutting process characteristics of organic metalworking fluid. Adv. Mater. Res. 230-232, 809-813（2011）. doi: 10.4028/www.scientific.net/AMR.230-232.809
• 44)  Lodhi, A.P.S.; Kumar, D. Natural ingredients based environmental friendly metalworking fluid with superior lubricity. Colloids Surf. Physicochem. Eng. Asp. 613, 126071（2021）. doi: 10.1016/j.colsurfa.2020.126071
• 45)  Katna, R.; Singh, K.; Agrawal, N.; Jain, S. Green manufacturing─Performance of a biodegradable cutting fluid. Mater. Manuf. Process. 32, 1522-1527（2017）. doi: 10.1080/10426914.2017.1328119
• 46)  Noor El-Din, M.R.; Mishrif, M.R.; Kailas, S.V.; Suvin, P.S.; Mannekote, J.K. Studying the lubricity of new eco-friendly cutting oil formulation in metal working fluid. Ind. Lubr. Tribol. 70, 1569-1579（2018）. doi: 10.1108/ILT-11-2017-0330
• 47)  Kuram, E.; Ozcelik, B.; Demirbas, E.; Sik, E. Effects of the cutting fluid types and cutting parameters on surface roughness and thrust force. Proc. World Congr. Eng. 2010, II（2010）.
• 48)  Lee, H.-S.; Saraswathy, V.; Kwon, S.-J.; Karthick, S. Corrosion inhibitors for reinforced concrete: A review. in Corrosion Inhibitors, Principles and Recent Applications（Aliofkhazraei, M., Ed.）, InTech（2018）. doi: 10.5772/intechopen.72572
• 49)  Braun, J. Additives. in Lubricants and Lubrication（Dresel, W.; Mang, T. eds.）, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, pp. 117-152（2017）. doi: 10.1002/9783527645565.ch6.
• 50)  Kumar, B.S.; Padmanabhan, G.; Krishna, P.V. Performance assessment of vegetable oil based cutting fluids with extreme pressure additive in machining. Journal of Advanced Research in Materials Science 19, 1-13（2016）.
• 51)  Koshy, C.P.; Rajendrakumar, P.K.; Thottackkad, M.V. Evaluation of the tribological and thermo-physical properties of coconut oil added with MoS 2 nanoparticles at elevated temperatures. Wear 330-331, 288-308（2015）. doi: 10.1016/j.wear.2014.12.044
• 52)  Padmini, R.; Vamsi Krishna, P.; Krishna Mohana Rao, G. Effectiveness of vegetable oil based nanofluids as potential cutting fluids in turning AISI 1040 Steel. Tribol. Int. 94, 490-501（2016）. doi: 10.1016/j.triboint.2015. 10.006
• 53)  Roegiers, M.; Zhang, H.; Zhmud, B. “Elektrionized” vegetable oils as lubricity components in metalworking lubricants. FME Transactions 36, 133-138（2008）.
• 54)  Fazal, M.A.; Sundus, F.; Masjuki, H.H.; Rubaiee, S.; Quazi, M.M. Tribological assessment of additive doped B30 biodiesel-diesel blend by using high frequency reciprocating Rig Test. Sustain. Energy Technol. Assess. 48, 101577（2021）. doi: 10.1016/j.seta.2021. 101577
• 55)  ManojKumar, K.; Ghosh, A. Assessment of cooling-lubrication and wettability characteristics of nano-engineered sunflower oil as cutting fluid and its impact on SQCL grinding performance. J. Mater. Process. Technol. 237, 55-64（2016）. doi: 10.1016/j.jmatprotec.2016.05.030
• 56)  Masjuki, H.H.; Maleque, M.A. Investigation of the anti-wear characteristics of palm oil methyl ester using a four-ball tribometer test. Wear 206, 179-186（1997）. doi: 10.1016/S0043-1648（96）07351-6
• 57)  Forrest, M.J. Reverse engineering and product deformulation. in Rubber Analysis -Characterisation, Failure Diagnosis and Reverse Engineering-（Forrest, M.J. ed.）, De Gruyter, pp. 129-154（2019）. doi: 10.1515/9783110640281-005
• 58)  Barrera-Díaz, C.; Bilyeu, B.; Roa, G.; Bernal-Martinez, L. Physicochemical aspects of electrocoagulation. Sep. Purif. Rev. 40, 1-24（2011）. doi: 10.1080/15422119. 2011.542737
• 59)  Cañizares, P.; Martínez, F.; Jiménez, C.; Sáez, C.; Rodrigo, M.A. Coagulation and electrocoagulation of oil-in-water emulsions. J. Hazard. Mater. 151, 44-51（2008）. doi: 10.1016/j.jhazmat.2007.05.043
• 60)  Schott, H.; Royce, A.E. Effect of inorganic additives on solutions of nonionic surfactants V: Emulsion stability. J. Pharm. Sci. 72, 1427-1436（1983）. doi: 10.1002/jps.2600721216
• 61)  Guimarães, A.P.; Maia, D.a.S.; Araújo, R.S.; Cavalcante Jr, C.L.; de Sant'Ana, H.B. Destabilization and recuperability of oil used in the formulation of concentrated emulsions and cutting fluids. Chem. Biochem. Eng. Q. 24, 43-49（2010）.
• 62)  Allende, D.; Cambiella, Á.; Benito, J.M.; Pazos, C.; Coca, J. Destabilization-enhanced centrifugation of metalworking oil-in-water emulsions: Effect of demulsifying agents. Chem. Eng. Technol. 31, 1007-1014（2008）. doi: 10.1002/ceat.200700018
• 63)  Deluhery, J.; Rajagopalan, N. A turbidimetric method for the rapid evaluation of MWF emulsion stability. Colloids Surf. Physicochem. Eng. Asp. 256, 145-149（2005）. doi: 10.1016/j.colsurfa.2004.12.001
• 64)  Strassner, J.E. Effect of pH on interfacial films and stability of crude oil-water emulsions. J. Pet. Technol. 20, 303-312（1968）. doi: 10.2118/1939-PA
• 65)  Spasiano, D.; Petrella, A.; Lacedra, V. Chemical destabilization of fresh and spent cutting oil emulsions: Differences between an ecofriendly and two commercial synthetic lubricants. Sustainability 12, 5697（2020）. doi: 10.3390/su12145697
• 66)  Genc, A.; Bakirci, B. Destabilization and treatment of emulsified oils in wastewaters by electrocoagulation. Water Environ. Res. 88, 2008-2014（2016）.
• 67)  Xu, B.; Kang, W.; Wang, X.; Meng, L. Influence of water content and temperature on stability of W/O crude oil emulsion. Pet. Sci. Technol. 31, 1099-1108（2013）. doi: 10.1080/10916466.2010.551812
• 68)  Latreille, B.; Paquin, P. Evaluation of emulsion stability by centrifugation with conductivity measurements. J. Food Sci. 55, 1666-1668（1990）. doi: 10.1111/j.1365-2621.1990.tb03595.x
• 69)  Chawaloesphonsiya, N.; Guiraud, P.; Painmanakul, P. Analysis of cutting-oil emulsion destabilization by aluminum sulfate. Environ. Technol. 39, 1450-1460（2018）. doi: 10.1080/09593330.2017.1332101
• 70)  Zouboulis, A.I.; Avranas, A. Treatment of oil-in-water emulsions by coagulation and dissolved-air flotation. Colloids Surf. Physicochem. Eng. Asp. 172, 153-161（2000）. doi: 10.1016/S0927-7757（00）00561-6
• 71)  Al-Shamrani, A.A.; James, A.; Xiao, H. Destabilisation of oil–water emulsions and separation by dissolved air flotation. Water Res. 36, 1503-1512（2002）. doi: 10.1016/S0043-1354（01）00347-5
• 72)  Anderson, R.J.; Bendell, D.J.; Groundwater, P.W. Organic Spectroscopic Analysis, Royal Society of Chemistry（2004）.
• 73)  Pierlot, C.; Marko, J.; Faven, C.; Azaroual, N.; Vermeersch, G.; Aubry, J.-M. Deformulation of metalworking lubricants : Organic phosphorus additives characterization by ¹H, ¹³C and ³¹P NMR. Analusis 27, 804-812（1999）.
• 74)  Chopra, A.; Sastry, M.; Kapur, G.s.; Sarpal, A. Analysis of cold rolling oils by NMR and IR techniques. Lubr. Eng. 52, 279-284（1996）.
• 75)  Siudem, P.; Zielińska, A.; Paradowska, K. Application of ¹H NMR in the study of fatty acids composition of vegetable oils. J. Pharm. Biomed. Anal. 212, 114658（2022）. doi: 10.1016/j.jpba.2022.114658
• 76)  Popescu, R.; Costinel, D.; Dinca, O.R.; Marinescu, A.; Stefanescu, I.; Ionete, R.E. Discrimination of vegetable oils using NMR spectroscopy and chemometrics. Food Control 48, 84-90（2015）. doi: 10.1016/j.foodcont. 2014.04.046
• 77)  Stock, R.; Rice, C.B.F. Chromatographic Methods, Springer（2013）.
• 78)  Valdez, D.; Reier, J.C. HPLC analysis of sodium 2-pyridinethiol-1-oxide in aqueous metalworking fluids via derivatization with NBD-C1. J. Liq. Chromatogr. 10, 2133-2143（1987）. doi: 10.1080/01483918708068900
• 79)  Linnainmaa, M.; Kiviranta, H.; Laitinen, J.; Laitinen, S. Control of workers' exposure to airborne endotoxins and formaldehyde during the use of metalworking fluids. AIHA J. 64, 496-500（2003）. doi: 10.1080/15428 110308984845
• 80)  Sun, C.; Baird, M.; Anderson, H.A.; Brydon, D.L. Separation of broadly distributed nonylphenol ethoxylates and determination of ethylene oxide oligomers in textile lubricants and emulsions by high-performance liquid chromatography. J. Chromatogr. A 731, 161-169（1996）. doi: 10.1016/0021-9673（95）01221-4
• 81)  Xu, W.; Que Hee, S.S. Gas chromatography–mass spectrometry analysis of di-n-Octyl disulfide in a straight oil metalworking fluid: Application of differential permeation and box–cox transformation. J. Chromatogr. A 1101, 25-31（2006）. doi: 10.1016/j.chroma.2005.09. 076
• 82)  Apostoli, P.; Crippa, M.; Fracasso, M.E.; Cottica, D.; Alessio, L. Increases in polycyclic aromatic hydrocarbon content and mutagenicity in a cutting fluid as a consequence of its use. Int. Arch. Occup. Environ. Health 64, 473-477（1993）. doi: 10.1007/BF00381094
• 83)  Zeman, A. Biodegradable lubricants ─ Analysis of metalworking oils and hydraulic fluids by mass spectroscopy. J. Synth. Lubr. 13, 149-161（1996）. doi: 10.1002/jsl.3000130204.
• 84)  Anderson, J.E.; Kim, B.R.; Mueller, S.A.; Lofton, T.V. Composition and analysis of mineral oils and other organic compounds in metalworking and hydraulic fluids. Crit. Rev. Environ. Sci. Technol. 33, 73-109（2003）. doi: 10.1080/10643380390814460
• 85)  Roff, M.; Bagon, D.A.; Chambers, H.; Dilworth, E.M.; Warren, N. Dermal exposure to electroplating fluids and metalworking fluids in the UK. Ann. Occup. Hyg. 48, 209-217（2004）. doi: 10.1093/annhyg/meh029
• 86)  Pereira, M.S.; Panisset, C.M.A.; Lima, T.B.; Ataíde, C.H. Determination of the chemical and mineralogical composition of drilled cuttings at different points throughout the solids control process. Mater. Sci. Forum 727-728, 1677-1682（2012）. doi: 10.4028/www.scientific.net/MSF.727-728.1677
• 87)  Ayrilmis, N.; Gürsoy, S. Potential use of metalworking fluid as surface additive for improving surface properties, technological properties, and formaldehyde emission of fiberboard. Case Stud. Constr. Mater. 16, e00952（2022）. doi: 10.1016/j.cscm.2022.e00952
• 88)  da Silva, L.R.; dos Santos, F.V.; de Morais, H.L.O.; Calado, C.R. Evaluation of the use of vegetable oils in the grinding of AISI 4340 steel. Int. J. Adv. Manuf. Technol. 120, 499-514（2022）. doi: 10.1007/s00170-022-08727-x
• 89)  Sollenberg, J. Isotachophoretic determination of ethanolamines in metalworking fluids and in air samples from workrooms. Process Control Qual. 3, 313-317（1997）.
• 90)  Rincón, G.J.; La Motta, E.J. Simultaneous removal of oil and grease, and heavy metals from artificial bilge water using electro-coagulation/flotation. J. Environ. Manage. 144, 42-50（2014）. doi: 10.1016/j.jenvman.2014.05.004
• 91)  Bensadok, K.; Belkacem, M.; Nezzal, G. Treatment of cutting oil/water emulsion by coupling coagulation and dissolved air flotation. Desalination 206, 440-448（2007）. doi: 10.1016/j.desal.2006.02.070
• 92)  Hahn, S.; Dott, W.; Eisentraeger, A. Characterization of ageing behaviour of environmentally acceptable lubricants based on trimethylolpropane esters. J. Synth. Lubr. 23, 223-236（2006）. doi: 10.1002/jsl.24