|"Precision Cleaning - The Magazine of Critical Cleaning
Industrial Lubricants, Cleaning Processes, and Waste
There has been heightened focus in recent years in the area of industrial cleaning, primarily due to the Clean Air Act Amendments restricting and eventually eliminating the use of ozone-depleting solvents.1 The cleaning process has a number of variables including choice of media, time, temperature, concentration (if a mixture), soil loading, and equipment design usually recognized as important and controlled accordingly for a product of desired cleanliness.
Another variable in the cleaning process of industrial metalworking is oftentimes not controlled or even recognized as significant: lubricant selection. Lubricant selection and cleaner chemistry can affect not only the ability to adequately clean parts; they also potentially can affect the success of a waste minimization effort to recycle the cleaner. Certain lubricants are more compatible than others with certain cleaning processes.
Lubricants at Work
In industrial metalworking, there are two basic modes by which lubricants decrease friction (see Figure 1).
The first is by hydrodynamic lubrication. Simply stated, this means that a reduction in friction is achieved by maintaining a constant fluid film between two solid surfaces. When fully separated, the resistance to motion is only due to the interposed fluid layer. The lubricity of this fluid is dependent on the area of the film, the rate of shear, and the viscosity of the lubricant.2
The second way to reduce friction is called boundary lubrication. When the hydrodynamic film is spread too thin to be effective, or when metal-to-metal contact will inevitably occur (i.e., with grinding and cutting operations), boundary lubrication takes over.
This mode of lubrication typically has a higher coefficient of friction and does allow some wear to occur, although it is greatly reduced. Boundary lubrication will be discussed in more detail later, in the section on lubricant additives.
Four major classifications account for the lubricants used in industrial metalworking: mineral oils, emulsions, semi-synthetics, and synthetics. Other classifications such as waxes, greases, dry film lubricants, etc. will not be discussed here since they are not considered to be in the realm of lubricants for metalworking.
Mineral oils are usually used where lubrication is of primary importance and cooling is secondary. They fall into two basic categories: paraffinic (straight chain hydrocarbon base) and naphthenic (ring-structured hydrocarbon base).
Naphthenic oils have the advantage of much higher solubility for many types of additives. These additives are often necessary to accomplish boundary lubrication as well as to impart other desirable characteristics to the lubricant. It is because of this attribute that naphthenic oils are usually chosen as the base mineral oil for metalworking applications.
Macroemulsions or so-called "soluble oils" start as a formulated oil mixture, called the concentrate, typically containing a naphthenic mineral oil base along with emulsifiers and other additives. When used, this oil is then diluted with water to create an oil-in-water emulsion, typically containing 5 to 10 percent oil. It will appear as a white, opaque solution.
The relatively large size of this emulsion will generally have a lower stability and a tendency to separate over time. Metal fines and other debris accumulating in the solution accelerates the process, providing sites for oil to coalesce.
Macroemulsions have the advantage of providing good lubrication from the oil contained in the emulsion as well as additives that can be included with the oil. Additionally, the water phase is available to provide cooling.
Semi-synthetics or microemulsions contain emulsified oil in the range of 0.01 to 0.2 micron. They appear as gray to translucent mixtures, although they may contain dyes.
Unlike a macroemulsion that starts as an oil with additives, the microemulsion concentrate is already an emulsion, since it contains water. Besides water, the microemulsion will typically contain mineral oil, emulsifiers, dispersants, boundary additives, and anti-foams. These concentrates are then diluted with water prior to use.
Semi-synthetics are generally more effective for cooling than lubricity since the working solution contains only a very small percentage of oil. Because of the lower amount of oil and the higher amount of emulsifiers, the microemulsion is much more stable over time than the macroemulsion. Disadvantages of this lubricant class are higher cost and difficulty in disposal.
Synthetic lubricants contain no oil and typically will be made up from poly-glycol, polyisobutylene, or polyalpha-olefin bases. Appearing as transparent solutions, they often will contain emulsifiers, amines, dispersants, and anti-foams. These solutions also are tailored more toward cooling than lubricating.
A typical application would be for high-speed cutting and grinding where tool life can be extended. This is important since these solutions are much more expensive than mineral oils, demanding a return on investment. Most synthetic formulations generally do not serve severe duties, such as deep drawing. And like semi-synthetics, synthetics also can suffer from disposal problems.
Although it would make sense that water-based synthetic and semi-synthetic lubricants would be the easiest to clean in an aqueous media, this is not always the case. If allowed to dry, the water from an oil-in-water emulsion will evaporate. The remaining lubricant will then invert to a water-in-oil emulsion, making for a difficult-to-remove polymeric film.3
The more mineral oil in a lubricant, the better its lubricating properties. The more water contained in a lubricant, the higher its cooling capacity (Figure 2).
Natural oils (e.g., lard oils) are occasionally used as lubricants, although they generally are considered inferior to the other metalworking lubricants in all respects except boundary lubrication properties. Where required, this can easily be provided by the incorporation of a low concentration of fatty material in a different base oil.4
A variety of additives can be present in lubricants to serve many purposes. Some include boundary additives, extreme pressure (EP), corrosion inhibitors, anti-foams, emulsifiers, dispersants, and viscosity index modifiers. While the functions of most of these are generally self-explanatory by name, following are explanations for the first two.
Boundary additives adsorb one or two molecules thick at the metal surface. They provide lubrication when the fluid film wears too thin to provide hydrodynamic lubrication. Typical boundary additives are C12-C18 saturated fatty alcohols or fatty acids. The latter tend to be more effective since they can react with active oxide surfaces on the metal to form a soap.
EP additives are effective when the severity of the metalforming operation generates higher temperatures. They actually react with the metal surface to lower friction at higher pressures and temperatures. Three well-known types of EP additives are phosphorous-based, chlorine-based, and sulfur-based. Since all are effective over various temperature ranges, there is usually a need for more than one in a lubricant formulation.
Both boundary and EP additives are usually found in mineral oil, emulsions and, to a lesser extent, semi-synthetic lubricants.
Following is an overview of the various categories of industrial cleaning and their associated chemistries. Certain cleaner/lubricant combinations are more compatible, making the cleaning process easier and also more amenable to waste minimization.
This is a broad category covering anything from simple hand wiping with mineral spirits to a complex vapor degreasing system utilizing a non-flammable chlorinated solvent. The principle is the same in any case: Solvents dissolve lubricant at the metal surface and work essentially by dilution.
Most of these solvents are generally non-polar and therefore dissolve a majority of lubricants which are usually non-polar as well. As one departs from non-polar lubricants and moves toward more polar synthetics and semi-synthetics, the "like dissolves like" relationship tends to be less applicable and cleaning tends to be less effective.
Typical solvents are as follows:
Usually having flash points in the 120 to 170°F range, these are generally used in hand wiping and room-temperature immersion applications.
This category, including naphtha, xylenes, and toluenes, is also applied via wiping and room-temperature immersion.
Terpenes found in some industrial and household cleaners are d-limonene and pinene. Such solvents are typically used in immersion applications, although hand wiping applies.
Including 1,1,1-trichloroethane, trichloroethylene, methylene chloride, as well as the Freons, these solvents have the advantage of being non-flammable and can therefore be used for vapor degreasing. The primary advantage to this process is the ability to condense clean solvent vapor on the metal surface, leaving the oils behind in the sump. These solvents are also effective in cold cleaning and hand wiping.
One disadvantage with the chlorinated solvents is their potential to go acid. A number of factors influence the stability of the solvent, including metal fines and lubricant additives. Boundary additives (fatty acids) as well as chlorinated and sulfonated EP additives can react with the solvent to create an acid condition.5
Another contaminant that raises the potential for acid problems with chlorinated solvents is water. The introduction of a large amount of synthetic or semi-synthetic metalworking lubricants to a vapor degreaser can bring with it an abnormally-high amount of water.
This process may utilize compounds such as a paraffinic solvent, a terpene, or an ester solvent mixed with approximately 10 percent surfactant.
The surfactant aids in rinsing away the solvent in water (thus the term semi-aqueous), eliminating the residue that solvent-based cleaners often leave behind. The surfactant makes it possible to remove non-polar and some polar compounds.
This type of cleaning is generally done in an immersion application and can be heated, but maximum operating temperature is restricted 20 to 25°F below the flash point of the compound.
This approach accounts for probably the most significant class of industrial cleaning for several reasons:
Environmental, safety, and health concerns regarding solvents.
Increasing numbers of facilities utilizing it.
Has the most to gain from waste minimization techniques.
Because they include a number of different organic and inorganic chemicals, aqueous cleaners represent the most complex chemistries. They are formulated from three groups builders, surfactants, and additives some of which tend to overlap in function.
Some examples and their functions follow:
These are usually sodium hydroxide. Since they dissociate completely, they provide no reserve alkalinity and give a very high pH to a cleaner. The hydroxides are effective at saponifying fatty acids present in high amounts in natural oils, or in lower amounts as boundary additives.
The disadvantage to this reaction is the tendency to create higher foam levels from the saponification. This can be particularly troublesome in spray washer applications.
These generally take the form of sodium metasilicate or sodium orthosilicate. Their primary function is to inhibit the attack of metals by the other alkaline salts present in the cleaner. This quality is particularly important for non-ferrous cold metal cleaners.
The silicates also provide reserve alkalinity and buffering. The resulting silicic acid can prove very difficult to rinse and is only soluble in a high pH caustic solution or hydrofluoric acid. Residual silicates can cause problems in subsequent plating and some coating operations.
Industrial cleaners can utilize many different forms of phosphates. Di- and tribasic phosphates are an inexpensive source of reserve alkalinity. The more complex formulations such as pyrophosphates, polyphosphates, and hexametaphosphates serve a variety of functions.
Studies have also shown phosphates to be effective detergent boosters by reacting synergistically with surfactants.6 This is due in part to the water-softening effect created by the sequestering of hard water ions (specifically Ca+2 and Mg+2).
Phosphates are particularly effective with mineral oils. Due to their number of charge sites, they are also effective as dispersants. Another advantage is their ability to act as carriers for liquid surfactants in powder mixes.
Carbonates also provide inexpensive sources of reserve alkalinity and buffering. But they dont offer as many advantages as some of the aforementioned builders.
Though aiding in water softening, they arent as effective as the phosphates since they precipitate as calcium and magnesium carbonates instead of sequestering. Like the phosphates, however, carbonates are an effective carrier for liquid surfactants in dry powder mixes.
Surfactants are organic molecules that can serve a variety of functions in a cleaner. Some properties that certain ones possess are wetting, emulsification, dispersion, foaming, and anti-foaming.
Surfactants play a significant role in any aqueous cleaner. They are unique chemicals in that they have two "ends." One is polar or water-soluble, known as hydrophilic; the other end of the molecule is nonpolar or oil soluble, known as lipophilic.
This unique property is what makes surfactants so valuable for solubilizing oil in aqueous cleaners. Poor selection of these can adversely affect performance more significantly than any of the other chemistry groups that comprise aqueous cleaners.
Surfactants are divided into four different categories: anionic, nonionic, cationic, and amphoteric.
Cationic surfactants are usually found in germicides and fabric softeners. Amphoterics usually will only serve specialty purposes in cleaners, such as hydrotroping less soluble components. Anionic and nonionic surfactants are those most frequently found in industrial cleaners.
Examples of anionic surfactants include sulfates, sulfonates, and phosphate esters. Typical properties include hydrotroping less soluble species, temperature resistance, higher foaming, and solubilizing polar soils.
Examples of nonionic surfactants include nonylphenol ethoxylates and primary alcohol ethoxylates. Typical properties include lower foaming, pH resistance, temperature dependence, and solubilizing nonpolar soils.
Because of their structure, surfactant molecules do not exist alone in solution except at very low concentrations where, even then, they are usually in pairs or small groups. They are ineffective in this form and do not display good surface-active properties. As more surfactant is added, the groups grow into a larger organized structure, known as a micelle.
This structures formation is such that the hydrophilic end of the groups are facing outward and the lipophilic ends are turned in toward the center of the micelle (in the case of a water solution). In an oil-based media they are oriented just the opposite. This orientation is natural, since it minimizes the free energy of the system (see Figure 3).
When there are enough surfactants in solution such that almost all exist in micellar form, they have reached what is known as the critical micelle concentration (CMC). This is the minimum concentration necessary for a surfactant to display its properties. The CMC is often determined experimentally through surface tension measurements.7
This group of aqueous cleaner components is slightly more vague in its purpose, since the function of an additive may overlap the function of an ingredient already present in the cleaner.
Some additives such as EDTA (ethylenediaminetetraacetic acid) and NTA (nitrilotriacetic acid) are included to promote the sequestering of metals in solution, which overlaps one function of the complex phosphates. EDTA and NTA are said to be more effective than complex phosphates at complexing trivalent ions.
Amine additives can also aid in sequestering. They have other functions, too, such as aiding saponification and rust prevention of mild steels.
Polyacrylic acid polymers as well as maleic acid/acrylic acid copolymers can also be added. These fairly large molecules are very effective at dispersing soils in a cleaner to prevent redeposition. At very low concentrations they are also effective at dispersing hard water precipitates. Unlike nonionics, they exist alone in solution and "wrap around" soil (particulate or emulsified oil). Electrostatic repulsion keeps them separated.
Extending the life of the cleaner, whatever type it may be, is probably the most significant step that can be made toward waste minimization in the cleaning process. The previously discussed concepts of cleaner and lubricant chemistry are important to understand in order to develop an effective strategy.
Hydrocarbon solvent cleaning offers little in the way of waste minimization opportunities. Solvents are used until they build up a soil loading that makes them ineffective. They then may be recovered through distillation, although are usually treated as a waste (similar to waste oil).
Unlike hydrocarbon solvents, chlorinated solvents are normally recovered by distillation which can be done on a continuous or a batch basis. Chlorinated solvents are more capable than hydrocarbon solvents at removing polar soils. But, again, their primary disadvantage is a tendency to degrade and go acid. It would therefore seem that lard oils and water-based synthetics are not good candidates for vapor degreasing, since they may encourage the solvent to go acid.
Semi-aqueous processes tend to be most effective with mineral-based, emulsion, and semi-synthetic lubricants. The varied constituents in a synthetic lubricant could tend to make the cleaner less effective quicker, since the surfactants and emulsifiers in it could be counterproductive to the surfactants added to the semi-aqueous formulation.
Aqueous cleaners probably have the most opportunities for waste minimization, since more equipment exists for oil removal than for any other type of cleaning. Although mineral oils may not be as easy to remove in the cleaning process as some synthetics, they present perhaps the most promising opportunities for waste minimization.
With proper chemistry selection, it should be possible to find an aqueous cleaner that will satisfactorily degrease the selected parts.
Oil will exist in three forms in an aqueous cleaner: tramp oil, mechanically emulsified oil, and chemically emulsified oil. Tramp oil, representing the majority of the oil from the parts, is easiest to remove. A simple skimmer is effective.
Mechanically emulsified oils are similar to tramp oils, but have been broken apart by agitation forces encountered with pumping, basket movement, drainage, etc. If the bath is allowed to sit quietly, much of this oil eventually will float out of solution. Coalescing equipment will speed this process through agglomeration of the oil droplets that will then want to separate due to differences in density.
Chemically emulsified oil is that which is permanently suspended by the cleaner. It is the job of the surfactants in the cleaner to emulsify and suspend this oil to avoid redeposition. Currently, microfiltration is the most viable option for removing this oil.
With microfiltration, the filtering apparatus simply retains the emulsified oil if it is larger than the porosity of the filter (see Figure 4).8 Recent advances have made microfiltration a new and growing business; many companies now offer a variety of turnkey units and processes. Again, its important to understand lubricant and cleaner chemistry, particularly when making changes to either such that the effectiveness of the filtration process is insured.
Synthetic lubricants do not exhibit the same properties as mineral oils in aqueous cleaners. They instead dissolve into the cleaner, making recovery impossible even with microfiltration.
Emulsions and semi-synthetics will exhibit some of the properties of mineral oils, simply because they generally are compounded with mineral oil as a constituent. Many components of an emulsion or semi-synthetic will also dissolve into the cleaner, making recovery much less efficient than with a standard mineral oil.
Table 1 outlines the effectiveness of the different aqueous cleaner recovery methods relative to lubricant type, plus relative costs.
Knowledge of cleaner and lubricant chemistry is essential in optimizing the degreasing process and realizing waste minimization opportunities. Cleaners and lubricants should be chosen such that they complement each other in processing as well as waste treatment.
Of the various cleaning process types, aqueous appears to offer the best opportunities for cleaning and waste minimization. When used in conjunction with mineral oils, there are a number of ways in which tramp, mechanically emulsified, and chemically emulsified oils may be removed. Skimmers, quiescent tanks, lipophilic media, and microfiltration are some of the means by which mineral oil can be removed from aqueous cleaners.
Here again, its important to understand the chemistries of both in order to make informed decisions regarding waste minimization opportunities.
1. D. Peterson, "Elimination of Chlorinated Solvents at Modine Manufacturing: An Integrated Approach," 14th AESF Conference on Environmental Control for the Surface Finishing Industry, Session V (1993).
2. Colin J. Smithells, Ed., Metals Reference Book, 5th Edition, Buttersworth & Co., London, 1976, p. 1266.
3. Samuel J. Spring, Metal Cleaning, Reinhold Publishing, New York, 1963, p. 109.
4. Smithells, p. 1267.
5. Standard Practice for Handling an Acid Degreaser, ASTM D 4579-86.
6. Spring, p. 13.
7. Milton J. Rosen, Surfactants and Interfacial Phenomena, John Wiley & Sons, New York, 1978, p. 85.
8. D. Peterson, "Limitations of Microfiltration," 16th AESF Conference on Environmental Control for the Surface Finishing Industry, Session 4A (1995).
About the Author
Dave Peterson is supervisor of corrosion and chemical engineering in the Materials and Process Engineering Dept. of Modine Manufacturing (Racine, WI). His responsibilities involve process development and waste minimization for the companys North American facilities, including specification and formulation of process chemicals as well as chemical and corrosion testing equipment. He has been involved with all vapor degreaser replacement projects throughout Modine, including 12 plants with more than 20 different chlorinated solvent sources.
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