Applying ‘green’ coolant to maintain machining performance while being friendly to workers and the environment.
Green chemistry applies according the life cycle of a chemical product, including its design, manufacture and use.
The definitions of “green” coolant chemistry are as diverse as New Zealand’s flora and fauna. “A green coolant chemistry can be defined many different ways,” said Daryl Adams, metalworking lead product manager for Quaker Chemical Corp., Conshocken, Pa., “but it is something that will have a positive impact on the environment, whether it’s reducing energy consumption, increasing the longevity of the bath, reducing chemical usage or recycling and reusing the product.”
Some definitions, however, carry more weight than others. The U.S. Environmental Protection Agency defines green, or sustainable, chemistry as the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. Green chemistry applies according the life cycle of a chemical product, including its design, manufacture and use, the agency states on its Web site (www.epa.gov/greenchemistry).
Hank Limper, market development manager, metal removal, metal cleaning and metal protecting for Houghton International Inc., Valley Forge, Pa., emphasized that although a green metalworking coolant can be vegetable oil-based, it’s not a requirement.
“A vegetable oil-based coolant isn’t necessarily going to last any longer in your machine tool or central system than an oil-based coolant,” he said. “Green means environmentally friendly, operator friendly, sustainable and made with renewable resources—raw materials that can be recycled and reused in other metalworking products. You opt for a product with a small environmental footprint.”
One step toward going green is determining which raw materials are biologically neutral, according to Limper.
Not surprisingly, Limper has seen a significant change in coolant formulations since he joined Houghton more than 3 decades ago, with the first 15 to 20 years focused on traditional amine borate-type coolants.
“They were the industry standard at the time, but everybody in the industry accepted the fact they were going to go rancid,” he said. “They were treated with a biocide or fungicide as needed.”
Courtesy of Chemetall
Figure. Bacteria growth in semisynthetic coolants without bactericide.
However, even early in his career the state of California was legislating what a formulation couldn’t contain, such as chlorine and boron. California continues to play a significant role is establishing restrictions for metalworking fluids.
For example, in 2011, the South Coast Air Quality Management District adopted Rule 1144, which placed restrictions on volatile organic compound (VOC) emissions from metalworking fluids and direct-contact lubricants at industrial facilities, with more stringent restrictions added in 2012 that significantly impact the region’s metalworking industry. The rule applies to four counties in Southern California: Los Angeles, Orange, Riverside and San Bernardino. (Many VOCs harm human health at high exposure concentrations.)
According to Quaker Chemical, its direct participation in validating and developing the test method to accurately measure VOC content resulted in a deep understanding of the rule’s impact on the metalworking industry, as well as the insight needed to develop compliant, high-performance metalworking fluids. The new VOC test method is based on an EPA-developed method for testing paint. “Paint chemistry is a different chemistry than what is used in metalworking fluids,” Adams said.
As a result, Adams noted that Quaker formulated cutting and grinding fluids and corrosion preventives to address the new restrictions, which can be applied without dilution. These products preserve the effective performance characteristics of traditional coolants while maintaining VOC levels lower than the specific maximum allowed, a requirement of Rule 1144 for this type of product application, he added.
The company added those cutting and grinding fluids and corrosion preventives to its existing product line, which have VOC levels within the acceptable limits of the new rule.
“Our goal is to develop, modify or identify existing products that meet the performance and quality standards of customers affected by the new rule,” said Wayne McVey, Quaker’s technical manager–cleaning, forming and corrosion.
“Even manufacturers not directly impacted are inquiring about the new requirements and how it may affect their operations in the future,” Adams said.
Formaldehyde-Releasing Biocides a Problem
Limper noted that global requirements are playing an expanded role in the market as global manufacturers establish more U.S.-based metalworking facilities and become U.S. customers.
“When global companies build plants in the U.S., they want either the same products, typically because they were an approved part of their manufacturing process, or products that would perform the same but would not contain things like boron and formaldehyde donors,” he said.
Courtesy of Chemetall
Chemetall says it formulates its coolants without biocides, bactericides or secondary amines, such as DCHA.
Formaldehyde, which the U.S. Department of Health and Human services listed as a known human carcinogen last June, isn’t added to coolants. However, some biocides, such a triazine, release formaldehyde as a chemical trigger in the presence of acidic byproducts from microbes living in a fluid, Limper explained. The formaldehyde kills bacteria.
Chemetall is another coolant manufacturer that doesn’t formulate its products with registered bactericides and secondary amines, such as dicyclohexylamine (DCHA), according to Dr. Yixing “Philip” Zhao, senior scientist for the New Providence, N.J.-based company.
“DCHA has very high bioresistant properties,” he said. “Its toxicity may not be as high as a biocide, but it is high enough to be a concern.”
Zhao pointed out that triazine, also known as hexahydrotriazine, or HHT, is popular because it’s efficient and inexpensive, and the most economical and efficient bactericides release formaldehyde. There are bactericides that don’t release formaldehyde, but they’re not as efficient at killing bacteria and cost more, he added. “In a metalworking fluid, you’re always concerned about performance versus cost.”
Courtesy of Houghton International
High-pressure coolant is applied in a chip-blasting operation.
When a bactericide releases formaldehyde, it becomes airborne with the coolant mist and vapor. The National Toxicology Program has listed formaldehyde exposure as a known cause of human cancers of the nose, nasal cavity and upper throat.
The EPA has proposed a maximum triazine level of 500 ppm in solutions of metalworking fluids to minimize exposure, but it’s an open issue as to what level of formaldehyde exposure is safe for workers, according to David Enright, Chemetall’s industry manager for metalworking fluids. “It’s sometimes hard for a user to even be aware that the fluids have formaldehyde because it hasn’t been released,” he said.
However, even if 500 ppm is safe for workers, an effective kill dose is 1,500 to 2,000 ppm, rendering triazine moot at a lower level, noted David M. Gotoff, Chemetall’s product manager.
Out with the Old
The challenge, then, is to formulate a coolant that is safe for the environment and workers while still being biostable, bioresistant and robust. In contrast, conventional coolant chemistry relies on the addition of bactericides to keep bacteria in check. Otherwise, the coolant experiences an explosion of bacteria growth after a certain number of hours.
“That’s why they have to use bactericides,” Zhao said. “It’s old chemistry, old knowledge and the old way people achieved coolant stability.”
Conversely, the new bioresistant coolant chemistries in the figure don’t contain bactericides, only one has DCHA as a reference in the biochallenge tests, and the amount of bacteria stays very low over time.
“This demonstrates Chemetall’s principle approach to green product development,” Zhao said. “We’re moving into the new chemistry, new esters and vegetable-based additives.”
That level of coolant performance is achievable without adding cost, according to Gotoff. “In the majority of cases, we’re not more expensive per se than our competitors that use an older product,” he said. “We experiment to achieve optimal results.”
End users calculating coolant cost should consider the entire metalworking operation. A stable and robust coolant emulsion lowers equipment maintenance costs, reduces the frequency of replacing worn cutting tools and extends sump life, according to Enright.
Nonetheless, a stable and robust coolant only stays that way with effective process control. That requires testing for and maintaining coolant concentration, alkalinity, conductivity, tramp oil and bacteria and fungus count, Enright noted.
“Among these factors, concentration is probably the most important one to be controlled tightly because a decreased concentration will not only provide less lubricity, but also fewer emulsifiers and alkalinity agents, such as amines,” stated Zhao in a white paper about process monitoring and controlling coolant sump longevity. “Therefore, the stability of the emulsion will be greatly decreased. Subsequently, an unstable emulsion will also facilitate bacteria growth and this will deteriorate emulsions.”
Houghton’s Limper concurred that maintaining coolant concentration is the optimal way to achieve coolant longevity. “If you run your coolant too low, you welcome corrosion and biological activity,” he said. “If you run it too high, you invite dermatological issues and mucous membrane issues, such as nose, throat and lung irritation. We preach that as gospel, and that hasn’t changed since long before I was born and won’t change after I’m out of this industry.”
Depending on the application, the upper coolant-concentration limit is generally from about 9 to 12 percent because levels higher than that expose workers to too high a pH and potential dermatological issues. The lower limit is about 4 percent because levels lower than that might promote rust and corrosion on machine tools and toolholders and make it more difficult to control bacteria, fungi and their associated odors, Limper explained.
However, even at the correct concentration level, water-soluble coolant application can have an additional environmental impact. Limper noted that many countries prefer manufacturers to machine with straight oil because water is a scarce commodity that’s reserved for human rather than industrial consumption. These countries typically have strict waste-treatment regulations.
Although the U.S. has numerous fresh water sources, such as the Great Lakes, droughts are not uncommon and states are more apt to ration water for manufacturing parts than for raising livestock and growing produce if a severe drought were to occur.
“That’s another definition of being green and more environmentally friendly: using a coolant that will last indefinitely in the sump leads to a greatly reduced waste stream and far fewer recharges, thereby using less of this precious resource called water,” Limper said.
Courtesy of Houghton International
Coolant is applied to a reamer during engine-head manufacturing.
In the event that manufacturers don’t have the resources or time, Quaker Chemical also offers services to check, monitor and control the quality of metalworking fluids in addition to its products. “Under our Chemical Management Services program, we typically put a number of part- or full-time individuals on-site,” Adams said, noting the scope of service may require only one part-time position or as many as 25 full-time people, depending on the facility size and program level.
Typically, customers look for an average savings of 10 percent each year as a result of the implementation of such a program; however, the program focus is more than monitoring chemical usage and process efficiencies. Services can range from simple evaluations of how much material is used in a packaging process to chemical-procurement management if a customer is looking to consolidate vendors and buy larger quantities to achieve volume discounts. “There are a number of creative and innovative ways to save customers money and improve efficiencies,” Adams said.
Within the manufacturing “ecosystem,” coolant plays a critical role. When looking just at the world of coolant, a green chemistry continually provides benefits similar to a life-sustaining object perpetually rotating on its axis. CTE
About the Author: Alan Richter is editor of CTE. He joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Houghton International Inc.
Quaker Chemical Corp.
Lubricating properties of minimum-quantity lubrication
We don’t sell coolant,” said Tim Walker, vice president of business development for Unist Inc. “We sell lubricant.”
He emphasized the distinction because the Unist minimum-quantity-lubrication application system delivers a small amount of the company’s Coolube lubricant to the tool/workpiece interface to help reduce friction and eliminate heat buildup. A coolant, on the other hand, is intended to be applied in large quantities to cool that interface and evacuate chips, according to Walker.
“When done properly, we put on such a small amount you can barely even see it come out of the nozzle,” he said, noting that machined parts will either be dry or have a very fine layer of oil on them. “You don’t get sticky, wet parts.”
Walker noted the biggest challenge the Grand Rapids, Mich.-based company has is getting MQL customers to understand that less is more because they are often so geared to thinking in terms of coolant application that they apply too much lubricant. When users do that, not only are they consuming more of the highly refined, vegetable oil-based product than needed, they are also creating an airborne mist and trapping heat on the machined surface, thereby generating hot parts.
Coolube is a “green” alternative to conventional flood coolant, Walker pointed out. According to the company, Coolube’s nontoxic, renewable plant oil-based composition makes it an ideal choice for manufacturers who care about their environmental impact. What it doesn’t include are petroleum products, chlorine or silicone. Walker noted that the ferrous metal formulation includes rust-preventative and extreme-pressure additives that do not oxidize. Therefore, the system doesn’t leave a residue and parts aren’t tacky.
By formulating the lubricant with cosmetic-grade raw materials, dermatitis is not an issue for operators. “The same base oil can be used in hand lotion,” Walker said.
Although the lubricant is biodegradable, it has a long shelf life, according to Walker. “If you just went to the store and bought some vegetable oil and used that, it tends to get very sticky over time; it builds up a varnish, which is hard to remove,” he said. “Ours has been formulated so it doesn’t have any of those characteristics.”
That refined formulation means it costs more than conventional metalcutting fluids, but its use is measured in drops per minute rather than gallons per minute, so the cost per piece with MQL is usually significantly less than flood coolant, Walker noted. “Typically, ROI is within months to a year.”
The amount varies depending on the application, and MQL isn’t suitable for all metalcutting operations, such as when cutting fluid is required to flush chips, but heavy-duty cutting is within MQL’s domain, according to Walker. For example, one customer uses MQL when bandsawing a nearly 20 " solid-steel workpiece.
Walker added that MQL provides secondary benefits, including an increase in worker safety, elimination of slippery parts and floors, no machine paint wear and a reduction in machine electronic failures. One MQL adopter is Ford Motor Co., which experienced increased machine uptime when using the process, according to Walker. “And all the costs associated with treating and disposing your fluid go away because there is nothing left to get rid of,” he said. “You use such a small amount that it basically gets used up in the process.”
To help convince doubters, Unist provides an MQL system on a trial basis. “One of our best selling tools is a 30-day free trial,” Walker said. “Very few systems come back.”
Related Glossary Terms
Additive to cutting fluids to inhibit bacterial growth. See fungicide.
Long, endless band with many small teeth traveling over two or more wheels (one is a driven wheel, and the others are idlers) in one direction. The band, with only a portion exposed, produces a continuous and uniform cutting action with evenly distributed low, individual tooth loads. Often called band machining.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- cutting fluid
Liquid used to improve workpiece machinability, enhance tool life, flush out chips and machining debris, and cool the workpiece and tool. Three basic types are: straight oils; soluble oils, which emulsify in water; and synthetic fluids, which are water-based chemical solutions having no oil. See coolant; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
Suspension of one liquid in another, such as oil in water.
Additive to cutting fluids to inhibit fungi. See bactericide.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.
- metalcutting ( material cutting)
metalcutting ( material cutting)
Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
- minimum-quantity lubrication
Use of cutting fluids of only a minute amount—typically at a flow rate of 50 to 500 ml/hr.—which is about three to four orders of magnitude lower than the amount commonly used in flood cooling. The concept addresses the issues of environmental intrusiveness and occupational hazards associated with the airborne cutting fluid particles on factory shop floors. The minimization of cutting fluid also saves lubricant costs and the cleaning cycle time for workpieces, tooling and machines. Sometimes referred to as “near-dry lubrication” or “microlubrication.”
- process control
Method of monitoring a process. Relates to electronic hardware and instrumentation used in automated process control. See in-process gaging, inspection; SPC, statistical process control.
Rotating cutting tool used to enlarge a drilled hole to size. Normally removes only a small amount of stock. The workpiece supports the multiple-edge cutting tool. Also for contouring an existing hole.
- straight oil
Cutting fluid that contains no water. Produced from mineral, vegetable, marine or petroleum oils, or combinations of these oils.
- tramp oil
Oil that is present in a metalworking fluid mix that is not from the product concentrate. The usual sources are machine tool lubrication system leaks.
On a rotating tool, the portion of the tool body that joins the lands. Web is thicker at the shank end, relative to the point end, providing maximum torsional strength.