2005 Progress Report: Design of Novel Petroleum Free Metalworking FluidsEPA Grant Number: R831457
Title: Design of Novel Petroleum Free Metalworking Fluids
Investigators: Hayes, Kim F. , Skerlos, Steven J.
Institution: University of Michigan
EPA Project Officer: Bauer, Diana
Project Period: January 1, 2004 through December 31, 2006
Project Period Covered by this Report: January 1, 2004 through December 31, 2005
Project Amount: $325,000
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
The machining of metal is essential to modern society. Consequently, the metalworking industry is one of the largest industries in the United States. Integral to this industry are metalworking fluids (MWFs) that serve as coolants, lubricants, and corrosion inhibitors. MWFs traditionally have been based on a petroleum feedstock, raising concerns about environmental degradation and toxicity throughout their life cycle, as well as the costs associated with sustaining the current level of domestic petroleum consumption.
The objective of this research project is to design 100 percent petroleum-free MWFs with equal or greater performance when compared to traditional MWFs. These bio-based fluids will have less toxicity and offer renewable alternatives to meet the increasingly stringent MWF disposal limits currently being considered by federal governments and the international community.
Given the differences in the severity of various machining operations, there are four different types of MWFs: straight oils, soluble oils, semisynthetics, and synthetics. Because semisynthetics have the greatest share of the market (40%), this research focuses on the design of vegetable-based semisynthetic MWFs. Semisynthetic MWFs are microemulsions of oil in water, whereas surfactants play the key role in dispersing and stabilizing the oil in water. Currently, the academic literature provides minimal scientific guidance to understand which surfactants are likely to achieve stable and robust microemulsions. To facilitate the development of bio-based MWFs, this research will reveal the relationship between surfactant chemistry and microemulsion stability for the primary vegetable oil candidates being considered for MWFs.
Specifically, three vegetable oils are being investigated: canola oil, soybean oil, and trimethylolpropane trioleate. For each of these oils, a suite of commercially available surfactants from all major classes of surfactant structure are being investigated. These include anionic surfactants such as fatty acid soaps, alcohol sulfates, alcohol ether sulfates, alkane sulfonates, alkyl aryl sulfonates, and sulfo-carboxylic esters. Nonionic surfactants also are being investigated, including ethoxylated alcohols, ethoxylated glyceryl esters, polysorbitan esters, and alkyl polyglucosides. Individual surfactants, as well as surfactant combinations, are investigated for their emulsion stability and performance characteristics. Because surfactant combinations are generally more effective and efficient emulsifiers, they are given the majority of attention. Such combinations contain a “primary” surfactant as well as a “complementary” surfactant that works synergistically to increase the amount of oil that can be emulsified.
In the experimental phase of this research, all possible combinations of primary and complementary surfactants were investigated at the widest range of concentrations. To represent the different concentration combinations for a given primary and complementary surfactant, a triangle is drawn with points within it representing formulations with different oil and surfactant molar fractions. One formulation was produced for each concentration and is represented by a point in the triangle shown in Figure 1. For each of these points, three metrics of emulsion stability were observed, namely visual clarity, particle sizing, and light transmittance.
Figure 1. Array of Surfactant and Oil Concentrations Examined for Each Surfactant Combination
More than 1,000 MWF formulations were produced to cover all combinations of primary and complementary surfactants, with each combination being investigated at 10 concentration combinations of oil, primary surfactant, and secondary surfactant. Noting that the triangle represents the formulation of the MWF concentrate, and that the concentrate is typically diluted to approximately 5 percent by mass in water, we find that the molar ratio of oil to surfactant in MWF microemulsions is typically about 1:1, a ratio where swollen micelles form. Therefore, we can interpret the results of the experiments in the context of swollen micelle systems. In such systems, a stable dispersion of oil in water can be achieved only when the specified oil concentration in MWF is below the oil solubilization limit of the surfactant solution, which is determined by the product of the solubilization capacity of the micelle (amount of oil solubilized per micelle) and the micelle solubility (number of micelles that can be dispersed per volume without coalescence or aggregation). Interpreting the results of experiments from the perspective of swollen micelle systems, we have established the following guidelines for selecting surfactants to achieve stable vegetable oil in water MWFs:
- For the primary surfactant, select a nonionic surfactant with tail length larger than 16. This is useful to achieve large micelles, and larger micelles can naturally solubilize more oil.
- For the nonionic primary surfactant, select a head group size in an intermediate range (e.g., the number of polyethylene oxide [EO] units should be in the range of 10 to 20) to balance micelle size and micelle solubility.
- Select a complementary surfactant with good water solubility and tail length compatible with the primary nonionic (difference in tail length less than 8 hydrocarbon [C] units) to increase micelle solubility without significantly reducing micelle size. This complementary surfactant can be anionic or nonionic depending on considerations such as foaming, hard water stability, and waste treatability.
Experimental results show that the above guidelines apply to each of the three representative vegetable oils investigated. The experimental results also indicated that although single-surfactant systems can be used, they require much higher concentrations of surfactant, leading to a level of foaming that is unacceptable for industrial applications. Given the need to utilize surfactant combinations, the research demonstrated that the most preferred emulsion systems (with a high degree of stability exhibited at a large number of stable points in the formulation triangle) use surfactant packages consisting of a primary nonionic ethoxylated glyceryl ester (C=18/EO=20) and a water soluble anionic/nonionic (C=12) from any of the surfactant classes. To achieve a 100 percent petroleum-free surfactant package, it is necessary to avoid the commonly used class of sulfonate anionic surfactants, which generally are not manufactured from bio-based feedstock.
With the characteristics of MWF microemulsion systems revealed, it was necessary to determine whether the newly established bio-based MWFs had equal or better manufacturing performance when compared to existing petroleum-based MWFs. To evaluate manufacturing performance, two types of tapping torque tests were utilized: the thread cutting test and the thread forming test. Results from thread cutting revealed that the MWFs based on three vegetable oils significantly outperformed the benchmark MWFs based on mineral oils. For thread forming, it was found that neither bio-based nor petroleum-based MWFs perform well unless extreme pressure (EP) additives are included in the formulation. Therefore, this research is currently investigating the selection of EP additives that can be derived from bio-based feedstocks and that are compatible with bio-based MWF systems.
During this research, the investigators also began to question the conventional use of water as a coolant and carrier for lubricating oil. The use of water brings with it a number of undesirable consequences such as rust, biological growth, hardwater ion accumulation, and others that have traditionally been dealt with by adding many different chemicals (up to 15) to MWFs. Moreover, water has only marginal ability to cool and extend tool life when compared with other cooling systems that could be devised. In short, we concluded that the environmental impacts, health impacts, and technical limitations associated with aqueous MWFs could be eliminated if: (1) lubrication was delivered directly to the cutting zone in minimal quantities; (2) a solvent other than water was used; and (3) lubrication was provided using inherently benign chemical components.
Therefore, we investigated the use of supercritical carbon dioxide (scCO2) as a carrier for vegetable oils in metalworking operations. scCO2 is being used increasingly in the industry as an alternative to traditional organic, halogenated, or aqueous solvents. The supercritical temperature and pressure of CO2 (Tc = 31.1ºC and Pc = 1,070 psia) are easily achieved in industrial environments. Under these conditions, CO2 is a good solvent for many materials, with some vegetable-based oils being highly soluble. These characteristics make scCO2 an ideal delivery medium for MWFs because the uniform coating of oil ensures that lubrication arrives at the cutting surface and the dry ice blast that forms during the expansion provides much better cooling than water. Furthermore, the pressure release of CO2 also provides a chip evacuation function previously achieved using water.
Figure 2 shows photographs of thread cutting, a common metal cutting process to create internal threads, using water- and a basic scCO2-based MWF developed by the principal investigators (PIs). Figure 3 shows a plot of thread cutting torque efficiency, which is a metric of the force required to cut threads in a metal workpiece. Higher thread cutting torque efficiency means lower force (torque), and values above 100 percent imply a better performance than a commercially available MWF that was used as a normalizing reference.
Figure 2. Thread Cutting Experiments Using MWF Microemulsion (Left) and scCO2 (Right)
Figure 3 demonstrates the performance advantage of combining scCO2 and soybean oil in MWF applications. It is observed that the soybean oil/scCO2 system (f) performs on average approximately 10 percent better than straight soybean oil (e), 20 percent better than aqueous soybean oil microemulsions (d), and 30 percent better than straight scCO2 (c). The scCO2 results were the best ever observed by the PIs’ research team in thousands of tests conducted with aqueous and straight oil MWFs over a 5-year research period. The data demonstrate that the combination of soybean oil and scCO2 performs better than either can alone and that soybean oil and scCO2 have complimentary roles when formulated together as a MWF. The performance of scCO2 alone can be improved by adding soybean oil for lubricity, and the performance of straight soybean oil can be improved by using scCO2 for enhanced delivery of the dissolved oil to the cutting zone. Dissolving soybean oil in scCO2 also allows at least five times less soybean oil to be applied during each cut while achieving improved performance relative to the use of soybean oil alone.
Figure 3. Cutting Torque Efficiency for Straight Oil and Aqueous MWFs Compared With a Demonstration of scCO2-Soybean Oil MWF
Having developed general surfactant selection guidelines for 100 percent petroleum-free semisynthetic MWFs using three representative vegetable-based oil stocks and having shown that these semisynthetic MWFs perform equally or better than petroleum-based MWFs in thread cutting, it now is necessary to demonstrate that bio-based EP additives can be selected for use in bio-based oil and surfactant systems to achieve equal or better performance than traditional EP additives in EP forming applications. Therefore, future research will focus on developing fundamental knowledge related to surface activity and tribological characteristics of extreme pressure additives; developing high-performance “green” MWF formulations using vegetable-based oils and environmentally benign EP additives; and performing a comparative environmental and economic life cycle assessment between high-performance traditional MWFs and novel petroleum-free MWFs. This research on EP additives will be of benefit to both aqueous-based and bio-based microemulsions and scCO2-based MWFs, because scCO2 carriers do not eliminate the need to include EP additives in MWFs.