Traditional petroleum-based metal working fluids include a complex mixture of oils, detergents, lubricants and other ingredients that have the potential to be toxic. Fluid manufacturers have developed complex additive packages and new formulation strategies based on renewable resources to produce metalworking fluids that last longer, are safer and less labor intensive to use than traditional petroleum-based fluids. These new fluids must resist biological and chemical stresses that can change their chemistries to produce an unhealthy environment and detract from their stability. Also, other fluids and lubricants used in metalworking processes have changed to make them more compatible with coolants to eliminate their "tramp-oil" potential.
The fluids that cool and lubricate cutting tools and remove metal chips inevitably enter the air in the form of a fine mist. Misting results from the high cutting pressures and machining speeds required, the geometry of the rotating tool and workpiece, chip formation, oil quantity and oil pressure ratios. Machining processes with undefined cutting edges, such as grinding, generate significant amounts of oil mist. Also, a large part of the energy expended when cutting is converted into heat, so tools and components can get extremely hot and partially evaporate cutting fluids.
The reduction of oil misting primarily depends on additive selection, while a reduction in evaporation is achieved by the selection of different base oils. For example, one automotive manufacturer was able to reduce emissions in its gear-component machining operations by up to 70 percent through tests of various base-oil products.
Advanced chemical additive technologies help to reduce the health and safety hazards often associated with metalworking fluids. "These developments include interventions that make petroleum or synthetic-based metalworking fluids more benign, and through innovative, additive-enriched synthetics, including self-emulsifying vegetable oils," said Joe Purnhagen, metalworking marketing manager at The Lubrizol Corp. (www.lubrizol.com).
Historically, the housekeeping and safety hazards related to metalworking mist exposure have been kept in check by mechanical means, such as sophisticated ventilation and filtration systems. More recently, chemical interventions have been introduced to help suppress mist.
Metalworking fluids are expected to have a long service life; therefore, their anti-misting qualities should remain constant for their entire life. This requires the use of anti-misting additives with high levels of shear stability.
The earliest chemical interventions to reduce mist were high-molecular-weight polymer additives that stabilize metalworking fluids and suppress mist formation. For conventional, petroleum-based fluids, polyisobutylene has been the preferred anti-mist additive. For aqueous metalworking fluids polyethylene oxide (PEO) has been used. However, due to the susceptibility of PEOs to shear degradation, repetitive additions of the PEO polymer are required to maintain mist reduction.
A newer class of shear-stable polymers was developed specifically to overcome the shear degradation evidenced by PEOs. These polymers, derived from 2-acrylamido-2methlypropane sulfonic acid monomer, provide longer-term performance in recirculating aqueous metalworking fluid systems.
"All mist-reduction chemistries should be used with mechanical solutions such as mist collectors, improved ventilation and air filtration. However, when significant contaminants are removed from the air, chemically induced mist reduction significantly decreases the strain on such apparatus, to extend equipment life for an increased return on capital investment," said Purnhagen.
Biocides are another alternative approach to making metalworking fluids safer. Rather than reducing mist, such chemistries kill the microorganisms most likely to thrive in the wet, high-temperature environments common with metalworking processes.
Synthetics and veggies to the rescue
Driven by worker and manufacturer concerns about health, safety and environmental hazards, the strategy behind today's synthetic-based fluids is to make metalworking fluids even more safe. Such fluids fall into two general categories: those made from renewable resources, such as vegetable-based fluids, and those that contain chemicals such as esters, hydrocarbon polymers and polyalkylene glycols. Vegetable-based oils, the most common renewable synthetic, are viable alternatives to traditional fluids. They meet standard industry specifications, and they also offer specific advantages unique to vegetable-based formulations, such as superior lubricity, high thin-film strength (better adherence to metals) and high flash and fire points. For example, one shop replaced common straight oils with flashpoints from 350°F to 400°F with a vegetable-based product that had a flashpoint of 640°F. Because of its higher flashpoint, the vegetable-based cutting fluid provided better heat dissipation, produced less smoke when machining and resulted in longer tool life.
The ecological benefits of synthetic metalworking technologies are well known. However, because ester and vegetable synthetics are polar, their natural attraction to metallic surfaces makes synthetic fluids more effective lubricants, as compared with nonpolar petroleum-based fluids. For example, the superior lubricity of a biobased water emulsified coolant in a grinding operation resulted in an almost 240 percent reduction in annual cost of wheels and a savings of up to 58 percent in labor costs to change wheels. Like petroleum-based fluids, synthetics achieve high levels of efficacy in critical areas, such as corrosion resistance, through the addition of specific performance-enhancing chemistries.
Bio-based cutting fluids also pose less of a risk to machinists for developing skin irritations and dermatitis than do petroleum-based soluble fluids. Some vegetable seed cutting-fluid formulations incorporate liquid wax esters that are almost identical to those of human skin surface oils and are said to lubricate and be beneficial to the skin of machinists.
There are two primary obstacles that limit the use of bio-based fluids. First, they cost more than petroleum-based products; however, the cost gap is rapidly closing. In fact, on a seasonal basis, the cost of vegetable oils may be lower than petroleum.
The second obstacle is formulation difficulties related to emulsification, but newer vegetable-based fluids are self-emulsifying. Stable, vegetable-based synthetics hold significant promise to reduce the environmental impact of metalworking operations and yet maintain or even improve lubricity.
The development of semi-synthetic products — formulations containing from 10 percent to 40 percent oil — is another approach to producing more stable cutting fluids. These fluids combine the lubricity of petroleum-based fluids with the cooling capabilities of synthetics. "Bio-resistant, semi-synthetics do not degrade due to biological action and result in cleaner parts and machines, less misting, more consistent tool performance and less potential for dermatitis," said Rick Duncan, technical service manager at Henkel Corp. (www.henkelna.com). "We have developed semi-synthetic products that reject tramp oils to the same degree as full synthetic fluids."
It is inevitable that supplemental fluids used in machining operations, such as hydraulic fluids and way lubes, will find their way into coolants as tramp oils. One way to combat the deleterious effects of tramp oils is through the use of multifunctional oils. These products extend the life of coolants because they maintain their performance even when they contain tramp oils. The multifunctional-fluid concept means that to prevent a loss of coolant performance should these fluids leak into the sump, all fluids used in support of machining operations contain the same additives as the coolant. For example, all machining operations in an engine plant are performed by a multifunctional oil with a viscosity of 10 mm2⁄s at 40°C. All hydraulic functions of the machine use a higher viscosity multifunctional oil with the same additives as the coolant.
For more information, contact these contributors to this article:
The Lubrizol Corp.
Quaker Chemical Corp.
Fuchs Lubricants Co.