By Bill Clayton

Researchers test the performance of metal working fluids (MWFs) by using a tapping torque machine to drill holes in a steel work piece of a known hardness. The less torque required to make the cut, and the less heat the process generates, the better the lubricating and cooling qualities of an MWF.

In 1882, just about the time the University of Michigan was building its first Mechanical Engineering laboratory, an engineer named Frederick Taylor was in a Philadelphia steel plant, spraying water on a metal-cutting tool. He found that water was a good coolant but a terrible lubricant and caused a great deal of rusting. He tested vegetable fats as lubricants, but found they were almost useless as coolants and were prone to bacterial and fungal growth, which destroyed their lubricity and their ability to inhibit rust.

Faced with seemingly insurmountable obstacles, Taylor and other researchers abandoned research on the use of vegetable fats as lubricants. However, 120 years later, Steve Skerlos, assistant professor, Mechanical Engineering, and Kim Hayes, professor, Civil and Environmental Engineering, have revisited the idea and have succeeded in developing vegetable-based metalworking fluids (MWFs) that are a viable alternative to petroleum-based products.

"Performance, cost, health, safety, environment - when dealing with metal-working fluids, you can't talk about one issue without talking about the others," Skerlos said. "They're all interrelated, and that's what makes these problems interesting."

MWFs--Performance, Cost, Health, Safety, Environment

In addition to cooling and lubricating cutting tools, MWFs flush away metal chips and inhibit corrosion. If they perform well, they increase the lifespan of tools and improve surface finishes. And if MWFs do, indeed, extend the life of a tool and improve the quality of the products the tool makes, then those fluids are doing essential work in manufacturing.

In 2002, North American manufacturers used more than two billion gallons of MWFs, which constituted 7 to 17 percent of metals manufacturing costs. This doesn't include costs incurred due to downtime for replacing worn cutting tools and for cleaning degraded MWFs from manufacturing systems. Additional costs arise from the 15 or more ingredients such as surfactants, corrosion inhibitors and biocides that are necessary for maintaining the working properties of an MWF. And disposal costs can exceed the price of new fluid. So the use of MWFs is a considerable financial drain on manufacturers dependent on metal-cutting.

Environmentalists and the medical community have their own concerns about MWFs, which contain high levels of polluting fats, oils, grease, nitrates and phosphates - all of which can threaten the environment. Yet, each year manufacturers dispose of more than a billion gallons of MWFs that have broken down into chemicals that are no longer useful.

Health organizations are apprehensive about the dangerous levels of pathogenic bacteria that can infect MWFs, making them a decided health risk. Worse yet, routine testing typically can't detect these microorganisms, and biocides that might control the bacteria pose their own health risks.

Improper exposure and handling of MWFs has caused dermatitis, chronic bronchitis, impaired lung function, asthma and even stomach cancer. Exposure can occur when workers inhale mists generated in the machining process or when their skin contacts equipment or tools covered with MWFs.

Even national security is an issue when MWFs depend on imported petroleum.

Skerlos said that the most obvious solution to problems associated with MWFs would be "to improve the lifetime of the fluid while utilizing more environmentally friendly and less energy-consuming materials - without compromising current performance levels."

Andres Clarens, second-year graduate student, Mechanical Engineering; Kim Hayes, professor, Civil and Environment Engineering; and Steve Skerlos, assistant professor, Mechanical Engineering, will soon be moving their metal working fluids out of the lab and into a manufacturing environment.

It appears that vegetable-based metal-working fluids might be what manufacturers have been looking for.

The Bio Alternative 

Skerlos and Hayes are investigating ways in which to replace petroleum-based MWFs with metalworking fluids based on biologic oils such as canola, soybean and rapeseed oil. Skerlos said that this work has multiple objectives: to give manufacturers the ability to reduce the overall volume of fluids that they consume; to minimize health risks to workers; and to minimize bio-contamination.

It's a terrific idea but, as Frederick Taylor discovered 120 years ago, it's a concept fraught with problems.

"We know oil and water don't mix," Skerlos said, "but the reason we put them together is because water is the best coolant there is, and vegetable-based oils can provide the lubricity that prevents heat from being generated in the first place."

Oil and water don't mix because oil is less dense than water and wants to go to the surface. Surfactants, which are everyday ingredients used in soaps and detergents, have long been used to stabilize these oil-in-water emulsions. However, surfactants can break down in metalworking environments; this, in turn, can lead to tool breakage, biological growth, and disposal. Therefore Skerlos, with ten years of experience investigating MWF chemistries, and Hayes, a surface chemistry expert with two decades of experience researching surfactant systems, are examining ways to maintain the stability of the oil-and-water mixtures under real-world manufacturing conditions.

Skerlos offered a hypothetical complication with MWFs and surfactants. "If I have 100,000 gallons of an MWF that's 99 percent water, there's no way to get around the need to regularly add water to the system to counter the effects of evaporation," he said. "And since the tap is the cheapest place to get water, this will be hard water containing positively charged calcium and magnesium ions. However, surfactants used in MWFs to disperse oil in water are negatively charged and are therefore susceptible to re-aggregation when positive charges are introduced to the system. This means droplets of oil are going to come together, forming bigger and bigger oil droplets in the water. And we've shown in our research that bacteria are fond of bigger droplets. We recently observed a 400 percent rise in bacterial growth when MWF droplets increased in size from 20 nanometers to two microns due to the introduction of hard water to an MWF."

Maintaining small, finely dispersed MWF nano-emulsions is also very important for MWF recycling and biological control using membrane filtration, which is another major focus of Skerlos' research. In fact, by designing MWFs to be stable in hard-water conditions, a specialty of the Hayes research team, the group found it could more than double the metalworking-fluid recycling rate.

Skerlos said that for the filtration system to work for MWF recycling and microbial control, his team "depends on the size distinction between bacteria and the nano-emulsions. We use membranes with pore sizes between 200 and 800 nanometers to achieve separation of clean metalworking fluid from harmful microorganisms such as mycobacteria. At the same time, the chemical integrity and manufacturing performance of the recycled MWF are as good as new." Skerlos' group was the first to mathematically model the transport of semi-synthetic MWFs through microfiltration membranes. They're using this knowledge to design metalworking fluids with superior recycling potential.

The research team expects that the physical control of biological growth using membrane filtration -- in combination with the use of improved MWF surfactants to stabilize emulsions under hard-water conditions -- will be the key to developing vegetable-based metalworking fluids that are recyclable, pose fewer health risks for workers and have a lifespan longer than that of traditional petroleum-based MWFs.

The work, so far, has demonstrated that in certain machining operations, the performance of vegetable-based cutting fluids is comparable to or better than the performance of traditional petroleum-based MWFs. Whereas the team is still working to better understand the formulations of vegetable-based MWFs, they've progressed far enough to apply their current MWFs to a variety of manufacturing operations and plan to pilot them soon in a manufacturing environment. -E

Bill Clayton is the editor of Michigan Engineer and a former winner of the Distinguished Journalism Award for Magazine Reporting.