By Dale Elenteny and Joe Manfreda,
Pico Chemical Corp.
Skimmers can remove most of the oil from coolants.
Tramp oil readily splits from synthetic coolants.
Most shops, especially those with either large or numerous small coolant systems, want to make their coolants last indefinitely, and want to reduce waste and minimize the cost of fluid maintenance. To reach these goals, they not only must use appropriate coolants for their applications, but they also must understand how coolant formulations become unstable, and they must be aware of the limits of filtering and recycling coolants.
There are three basic types of coolants:
- Soluble oils that contain petroleum hydrocarbon oil, emulsifiers and other additives, such as those used for extreme-pressure lubricity, corrosion protection, biological control, wetting and defoaming.
- Semisynthetics that contain soluble oils and synthetic hybrids, and that typically include less than 30 percent mineral oil.
- Synthetics that do not contain oil, but have chemical lubricity agents and other additives to provide corrosion protection, biological control, wetting and defoaming.
Regardless of the coolant type, the single most important maintenance item that must be routinely measured is coolantconcentration. Coolant concentration must be recorded and acted upon regularly. Coolants typically are designed to operate in a range of 3 percent to 10 percent by volume. Most coolants are designed to be mixed with water either by adding the coolant to an existing solution, by premixing or by mixing with a proportioner. Automatic coolant proportioners help to ensure uniform concentrations. By whichever method is used, solutions can be kept at optimum levels and costs can be tracked if coolant and water additions are recorded meticulously.
However, after extended use and recycling, some coolants can experience such depletion of selective additives that serious operational problems arise such as corrosion, reduced lubricity, foaming, odors from microbiological-growth and increased tool wear. Coolant formulations that have multifunctional components typically experience less depletion of such additives.
Characteristics of oil-containing coolants
Products that contain oil are prone to destabilization because oil must be correctly emulsified and the distribution of various sizes of oil-droplet particles can become unbalanced over time. The more oil that there is in a coolant, the more difficult it is to keep that coolant stable over a length of time because of several factors: Coolant can be contaminated by minerals in hard water, by microbiological agents or through the advance of tramp-oil in the system. Products that contain oil often are difficult to mix and emulsify into plant water, especially if the water is hard. In some cases, mixing equipment or timeconsuming techniques are required to achieve a satisfactory dispersion.
Semisynthetic and synthetic compounds are easier to make soluble in water and are easier than oil to disperse in water.
Products containing oil tend to absorb some tramp oil because of the excessive amounts of emulsifiers that must be used with them. Some recycling systems that remove tramp oil also can remove some of the oil that is desired or required in a coolant. Thus, recycling coolants that contain soluble oils or semisynthetic compounds can raise costs artificially by removing and discarding some of the oil that is designed into the product with the tramp oils that are not desired.
Metal ions can have a negative effect on emulsifiers and the bond they form with oil. Weak emulsification systems often entrain metal particles and return them to the metal-removal process. Those metal particles can affect quality, and adversely affect tool life and surface finishes.
Additionally, coolant products that contain oil often cannot be fine treated by microfiltration methods because such filters can remove the desired oil with the other entrained, undesirable components.
Because oil-containing products often have a wide distribution of oil-droplet-particle sizes and a disparity in electrochemical forces within the fluid, they are subject to the selective depletion of ingredients. This can occur not only in the recycling process, but also when they are being used on parts.
Biostable and vegetablebased products
The producers of coolants say microemulsion solubles and semisynthetic compounds that do not contain petroleum sulfonates are biostable, and that these types of coolants do not promote the production of hydrogen sulfide gas caused by bacterial growth that degrades sulfonates and causes foul smells. However, after prolonged use microemulsion solutions often allow significant bacterial activity that destabilizes the emulsion and cuts rust protection. Although these microemulsion chemistries can be recycled and offer long life, they still can attract pull in a significant amount of tramp oil that can create operational problems, such as smoke, oil misting, fine-metal-particle suspension, oily surfaces, inaccurate refractometer readings and the selective depletion of additives.
Another coolant-chemistry innovation is the substitution of vegetable oils for mineral oils in formulations that use soluble-oils. While vegetable oils are rapidly biodegradeable in waste-treatment systems, they simultaneously present bacteria with a prime food source and promote faster bacterial growth in sumps. Although these solubles provide more lubrication than mineral oils, they do not improve the difficulties associated with the absorption and separation of tramp oils . Vegetable oils also are harder to emulsify and their emulsions tend to exhibit poor stability. As with mineral oils, they become destabilized through heat-induced oxidation, and they can yield oily residues and mists.
Advantages of synthetics
Synthetic compounds that do not contain oil can increase tool life and produce better finishes. New synthetic-formulation technologies have improved these compounds, and have made them more efficient so that the solids and tramp oil that can contribute to nonuniform lubrication can be removed more quickly. Coolants based on synthetic compounds can provide the maximum potential to remove contaminants, whether they are oilbased or solid, and they have a high resistance to the contaminants that can come from hard-water. Synthetic-based coolants are formulated so that hydraulic, gear, spindle or way lubricants readily split from the fluid and can be removed easily by skimming the top of reservoirs or by using gravity-separation enhancements. Synthetic-based collants also can be filtered down to 10 microns or finer without losing their constituent components. In some critical applications, ultrafiltration can be used and may be necessary to prevent particles from disturbing the cutting or grinding process and creating unacceptable surface asperities.
When fine filtering coolants that contain soluble oils or semi-soluble oils, some of the desired oils can be removed from the emulsion reducing the lubricating value of the coolant. Because they typically are true solutions, synthetic coolants can be filtered to one micron, making it easier to remove metal chips and fine swarf. Correctly formulated synthetic coolants contain fewer food sources for bacteria and rarely produce objectionable odors, even after long-term recycling or downtimes. Coolants based on synthetics often show lower consumption rates compared with coolants based on soluble oils because of the differences in emulsion and solution characteristics. To minimize waste streams and to significantly reduce all disposed fluids, the uses of coolants based on synthetics should be coordinated with other waterbased cleaners and corrosion preventives used in the shop. Cleaners and corrosion preventives that are not contaminated with solubilized oil should be recycled, as coolants are. When the total coordination of fluids used in the shop is conceptualized in the planning and execution stages, coolants based on water-soluble, synthetic chemistries can become an economical choice.
Regardless of the type of coolant being used, the cleansing and recycling of coolants should be implemented after a thorough cost analysis that includes all cost considerations, including downtime and waste disposal. Coolant chemistry, especially the product's stability under ever-changing conditions and the ingress of contamination, is a key element in developing a sound recycling system. The equipment chosen must ensure efficient contamination removal without selectively removing coolant components.
Shops with as few as five machines or a total coolant volume of 500 gallons or more should consider recycling.
Some recycling systems minimize oil pull-out from weak emulsion systems by using a "gentle" approach. However, as the emulsion further weakens, even these systems begin to experience oil " plateout." Newer soluble oils and semi-soluble oils can be recycled successfully. However, synthetics can offer the best possibility for long-term recycling and sump life, along with low consumption costs, clean parts and machinery, and low biological activity.
|Effects of contamination on coolants|
|Water: hardness ions (chlorides, and sulfates)||Minimal to no effect on solution but can help create tacky residues and increase rust potential||Moderate influence on the emulsion and creation of tacky residues and rust potential||Strong negative influence in destabilizing emulsion and increasing rust potential||Water evaporation increases problem; using deionized or distilled water may be necessary|
|Lubrication oils (tramp oils)||Should readily split from solution||Some splitting and some absorption into coolant promotes bacterial activity||Little splitting and heavy absorption into coolant can replace coolant's oil, strongly destabilize emulsions and bacterial activity||Prevent oil entering coolant and remove by skimming, coalescing, centrifuging as necessary|
|Chips, fine abrasives (swarf)||Can be readily separated by filtering, settling, magnets, centrifuges, etc.||Can be separated by filtering, settling, magnets, centrifuges; very fine swarf can attach to oil particles||Can be separated by filtering, settling, magnets, centrifuges; small particles can attach to oil particles||The finer the particles and the looser the emulsion, the more likely that metal removal becomes more difficult|
|Organic matter biological agents from various sources||Usually most components are not attractive food sources and higher alkalinity tends to decrease bioagents' capacity to thrive||Oil, emulsifiers, fatty substances can be good food sources; alkalinity tends to discourage growth of bioagents||Oil, emulsifiers, fatty substances can be excellent food sources; bacteria lowers pH and tends to destabilize emulsion and promotes rust||Biocides, fungicides and alkaliniity boosters may have to be added into the coolants, raising costs and dermatitis potential|
|Fluids from previous operations||Water dilutable products will mix, oil products will tend to split||Water dilutable products will mix, oil products will tend to split, but some may emulsify||Water dilutable products will mix, oil products will tend to emulsify with some splitting||Fluid entering coolants should be analyzed and coordinated to minimize negative results|
Mr. Elenteny is vice president for technical services, and Mr. Manfreda is manager of marketing services at Pico Chemical Corp. in Chicago Heights, Ill.