What Makes a Coolant Green?
When you're discussing metalcutting fluids, remember that green isn't simply a marketing term
By Brian J. Hogan
Metalcutting fluids are ubiquitous in machining, and almost every manufacturing professional has seen advertisements for "green" metalcutting fluids. Suppliers to manufacturers insist that environmentally responsible coolants and lubricants can function just as well as conventional products, without requiring extensive modification of the equipment now operating on your shop floor.
But just what is a "green" coolant? Brian Mattes, global senior research and development chemist at Master Chemical Corp. (Perrysburg, OH) responds that "green" has many meanings. "While metalworking typically focuses on certain raw materials to determine if a product is greener than another fluid, at Master Chemical," says Mattes, "we concentrate on three higher-level concerns: safety, environmental impact, and commercial sustainability.
"The balance of these three concerns has always been at the heart of what is green, and they are interrelated. Minimizing waste in a process saves money and puts less byproduct into the environment. We advance the use of renewable resources in our products, but we also integrate green chemical management into our products and services. Longevity in the sump has a positive impact on the greenness of the entire manufacturing process, and therefore the greenness of the metalworking fluid."
Suppliers offer green neat oils, synthetics, semisynthetics, and vegetable-based fluids. The performance of green fluids, such as Master Chemical's Trim E850 vegetable-based premium emulsion coolant, is comparable to the performance of conventional fluids, and they work well in a range of applications. However, the company believes that conventional premium metalworking fluids, like Trim E906 premium, low-foaming emulsion are often the "greenest" fluids, because they not only perform well, but can be maintained for longer periods of time. "Greener raw materials such as vegetable oils often deliver better lubricity," says Mattes. "But on a one-to-one ingredient comparison, vegetable oils are more prone to sump-life issues than conventional lubricants."
Vegetable-based raw materials present concerns about availability, volatility, and seasonality of supply. There can be oxidation instability, biological issues, and societal concerns about genetically modified vegetable sources. Beyond these specific limitations, the chemistry of the fluid is significantly changed during the metal-removal process, according to Mattes. "What may have started out as a greener chemistry quickly changes as the product picks up process contaminants like metal chips and way oils. The ability of the fluid to tolerate contamination is very important. If a product is formulated with vegetablebased chemistries but doesn't provide system longevity, it isn't truly green."
Mattes warns that some vegetable oils may have seal-incompatibility issues unless proper formulation measures are set in place. Also, biological growth can have a potential impact on all metalworking fluids, but the potential can be greater with vegetable-based raw materials.
"True 'green' metalcutting fluid solutions are hard to achieve with a product that is diluted with water and recirculated," observes Lee Hitchcock, senior research chemist, ITW Rocol North America (Glenview, IL). "You can have bio-based—or plant-based—components that make up your concentrate, but after use and contamination in the sump they may no longer be that environmentally-friendly.
"The 'greenest' metalworking fluids are vegetable-based neat oils. These are usually MQL-type lubricants that are used up in the cutting process, leaving behind very little residue and near-dry chips.
"Green lubricants," continues Hitchcock, "are usually vegetable-oil based, have much better lubricity than mineral-oil based lubricants, and can be used in any metal-removal process.
"The limitations of 'green' fluids come in application," says Hitchcock. "MQL requires precise application of the lubricant to the tool's cutting edge, which is difficult to do when dealing with large-diameter or deep holes and operations where the cutting edge is masked by the operation. Although these limitations can be eliminated with through-tool application, it can still be difficult when there are many tool changes during one operation."
ITW Rocol North America considers bacterial growth a significant issue for recirculated 'green' fluids. "For water-dilutable 'green' fluids, bacteria and fungus are a huge concern," states Hitchcock. "These products, usually vegetable-oil based, are much more susceptible to bacteria and fungus [than mineral-oil based fluids]. This is a major hurdle. It took months of engineering for Rustlick to release a new bio-based coolant, Rustlick PowerCool MaxLife Green." Most water-dilutable metalworking fluids contain biocides to combat microbial growth and 'green' coolants are no exception. ITW Rocol North America sees biostability, the challenges of MQL, and cost as barriers to the wider adoption of 'green' metalcutting fluids. Performance is not an issue.
"Not every water-based fluid is green," says Wally Boelkins, CEO of Unist Inc. (Grand Rapids, MI). "In fact, some would say that there are no green water-based fluids. There is green neat oil, and green vegetable-based oil. Petroleum-based white oil is biodegradable and nontoxic, so it's technically classified as green. There are some green synthetics. Vegetable derivatives—those derived from seeds—are all green."
At Unist, the company's only fluid line (vegetable-based fluids) has always been "green," says Boelkins. "Another thing we say is that our equipment and lubricant reduce 'excess fluid considerations.' This includes reduction or elimination of cleanup, equipment maintenance, recycling, sumps, handling, storage, and disposal. All of these are really 'green' functions." In the case of Unist, the fluid itself is green because it's plant-based, biodegradable, washes with water, and requires no special handling or disposal.
Green fluids are almost always used in neat form, according to Boelkins. To achieve their performance they require suitable equipment to deliver the fluid to the cutting edge, and they are used in very small amounts.
Unist's Coolube 2210 is a highly refined vegetable-based fluid. Because of the polar properties of vegetable-based fluids such as Coolube 2210 and 2210EP, the product has greater lubricity than a typical mineral-based fluid. Polarity results from the uneven partial charge distribution between various atoms in a compound. Polarity is inherent to vegetable molecules, which are long, heavy, and dipolar—the ends of the molecules have opposing electrical charges. The ends of the molecules have a chemical affinity for metal surfaces, and the result of that affinity is a dense, homogeneous alignment of vegetable oil molecules along the metal surface, creating a durable lubricant film. Mineral oils are nonpolar, and form a random alignment on a metal surface, producing a weaker lubrication layer.
There are no real limitations to the types of operations green fluids can be used for, according to Unist, but results may vary. For example, the company says that as far as Coolube 2210 is concerned, when it comes to general machining work, the key is the ability to apply it correctly, so that there is a protective layer of lubricant at the interface between the tooling and the workpiece. "Because our fluid is not used in flood cooling," remarks Boelkins, "we recommend using MQL. So if there are applications where MQL is not possible, then our abilities for 'green' lubrication might be limited."
In the case of Coolube vegetablebased lubricants, Boelkins says, there are no issues with bacterial growth. The product has a long shelf life and can sit in the machine lubrication for extended periods of time without contamination or reduction in performance.
"The market for green fluids is unlimited," Boelkins insists. "Only a small portion of it has been tapped. Using the vegetable-derivative actually reduces cost—by up to 15% in many manufacturing operations—so cost is not an issue. With our green-fluid concept, we have only penetrated a very small segment of the market—maybe less than 20% of what is potentially our target market, which consists of companies that could realistically implement our 'green' products and environmentally responsible application methods. The market continues to grow because of improved receptivity to these concepts.
"The barriers are the conservative nature of manufacturing and the willingness or not to innovate. There are sometimes issues related to cost, because the purchasing or accounting department may look at cost per gallon, but not consider other categories related to cost of usage, such as estimated disposal cost."
At Hangsterfers Laboratories (Mantua, NJ), green means using products to their fullest extent. Recycling and proper maintenance will extend fluid life, thereby reducing waste. "High-quality fluids like our Missile Lube Series and PC series may be used for years," says Technical Manager Joe Gentile. "We meet the specifications set forth for vegetable-based content for neat oils, soluble oils, semi-synthetics, and synthetics. These products excel at cutting exotic materials, but they require maintenance just like conventional products."
Green fluids and related technologies can be applied to almost every application, says Gentile. "The user is limited only by his ability to adapt to new technologies and or processes."
Hangsterfers' Missile Lube waterbased vegetable emulsion, its PC-Series pure vegetable oil, and the new S-700 series water-based vegetable semisynthetic are all green technologies targeted specifically to machining Inconel and titanium. "The lubricity and productivity gains are impressive," states Gentile. "These equate to real cost-per-part savings across the board for our customers. Current machine technologies allow the use of green, renewable-resource products with superior results. Hangsterfer's maintenance procedures are built into both our green and conventional products."
Asked about the impact of bacteria on green fluids, Gentile says: "Our position has always been one of balance. We work with nature to deliver a balanced fluid system. Renewable resource fluids do use some forms of biocides. Our position, however, is to use an absolutely minimal amount of biocide, and to formulate with ingredients that are naturally bio-resistant."
The "Green" market is virtually untapped, according to Hangsterfers. "Customers are just learning of the benefits of green technologies," asserts the company's Skip Wolford. "As end users come to understand the applications and apply these fluids, they will experience significant cost-per-part savings, decreased haul-out costs, real tool-life savings, and a quantifiable increase in their productivity. Going green means applying every aspect of manufacturing knowledge and capacity to use the human and capital assets already in place. Green is much more than window dressing and slogans. It's a concerted effort to be on the leading edge of technology and environmental responsibility."
At Cimcool Fluids (Cincinnati), "green technology fluids" use plant-derived raw materials to replace mineral oil. "Plants are renewable and can be planted and harvested to produce specific and consistent raw materials," says Cimcool's Kevin Tucker. "We can formulate products with these plant-derived materials to make semisynthetics, soluble oils, or even 'neat' oils. Expanding the term 'green' to include 'environmentally responsible' fluids would also allow the inclusion of technically superior products that use and waste less."
Scientists at Cimcool say that using plant-sourced raw materials to replace mineral oil is the critical chemistry in producing a green metalcutting fluid such as the company's Cimfree synthetic. Plant-sourced materials can be used to produce vegetable oils, fats, esters, and surfactants. "We've found that our plant-based products can be formulated to provide comparable performance to conventional products," says Tucker. "Traditionally, green fluids may have even better lubricity, because they have excellent boundary-lubrication properties. In the past, plant-sourced materials had shorter life cycles due to oxidative breakdown. Recently, our company has discovered proprietary technology that allows green fluids to have longer sump life, lasting as long as conventional fluids when in use."
Green fluids, according to Tucker, really have no performance limitations. The one limiting factor is usually cost. Plant-sourced raw materials can cost as much as 300% more than their counterparts, so sometimes only the most environmentally conscious customer is willing to pay the higher price to use a "green" fluid.
"We have had great success machining Inconel and titanium with green fluids," Tucker states. "Recently, however, we've also developed technology that allows us to process these difficult-to-machine alloys with water-based fluids that do not contain mineral oil, vegetable oil, or extreme pressure additives. Through a unique combination of patented and commercial lubricants, we can provide significantly better lubricity than traditional fluids without any of the environmental or performance issues found in other products. We think these fluids are just as "green" as vegetable-sourced fluids, because they don't contain any chlorine, sulfur, or phosphorus that can present waste-disposal concerns."
"With our formulations," observes Tucker, "most commonly used premium gaskets and seals are compatible, so very little change to the machining system is required. 'Green' fluids have the same microbial issues as traditional metalworking fluids. Plant-sourced raw materials are even more susceptible to microbial degradation than traditional fluids.
"Overall, the potential market for 'green' fluids is the entire metalworking fluids market," says Tucker. "If mineral oil and vegetable oils would ever get closer to each other on cost, the logical choice, in most cases, would be the green fluid. The biggest barrier is cost. Other than cost, today there is no significant reason not to use green fluids."
"'Green'" can mean a variety of things and apply to all types of metalworking fluids," says Stephen Mair of Shell Lubricants (Houston, TX). Vegetable ester-based neat oils and water-soluble cutting fluids are a form of green renewable products. To improve worker health and safety, many suppliers are removing harmful components like secondary and tertiary amines, DCHA, barium, chlorine, boric acid, biocides and fungicides, nitrites, and phenols. "Also, cutting fluids can be formulated to significantly lower airborne mist, smoke, and VOCs. High-quality base oils are a key function in this area," Mair remarks.
Depending on the category, many green fluids perform as well as or better than conventional fluids. The key is selecting the right fluid that provides the lowest total cost of operation, Mair states. "While green or higher-technology fluids can initially have a higher per-unit cost, they can often be the lowest-total-cost product if users evaluate all fluidrelated costs, such as tool life, productivity, waste minimization and treatment, product quality, scrap reduction, energy consumption, and the impact of health and safety."
Some renewable green fluids made from vegetable or rerefined base oils have shown limits in fluid life, paint/elastomer compatibility, and wastewater treatment. Users should research the fluid before using it in large central systems or machines that use sensitive paints and elastomers.
As for difficult-to-machine materials: "Yes, certain green fluids can be used on difficult metals like Inconel and titanium," says Mair. "There are green fluids that can machine or fabricate the most difficult of metals while still providing green benefits." Some green fluids are not compatible with existing fluids, so cleaning and recharge is required.
Regarding the problem of bacterial growth in green fluids, Mair says: "Green fluids are not immune to bacterial growth, but are not necessarily inferior in that area. Under the right conditions, some vegetable-ester based fluids can provide a stronger food source for bacteria, which increases biological growth and shortens fluid life. Some green products contain biocides or fungicides, others rely on alternative chemistries like super amines, bioresistant emulsifiers and surfactants, and/or oil-free chemistries. Technically, some type of green product could be used today in place of most conventional metalworking fluids. The barriers to more widespread use include higher unit cost, lack of OEM approval, performance versus conventional fluids in certain applications, bad experience with a previous 'green' product, and time and cost to change over."
This article was first published in the July 2010 edition of Manufacturing Engineering magazine.