lndustrial Lubricants, Cleaning Processes and Waste Minimization

Lubricant selection and cleaner chemistry is oftentimes one of the most overlooked parts of industrial metal processing. Their selection can affect not only the ability to adequately clean parts; it can also potentially affect the success of a waste minimization effort to recycle the cleaner. Certain lubricants are more compatible with certain cleaning processes than others. This paper will discuss the various classes of lubricants, cleaning options for those lubricants and how choice of these two impact waste minimization. INTRODUCTION There has been a 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.’ The cleaning process has a number of variables including choice of media, time, temperature, concentration (if a mixture), soil loading and equipment design. These variables are usually recognized as important and controlled accordingly to give a product of desired cleanliness. Another variable exists in the cleaning process which is oftentimes not controlled or even recognized as significant. This variable is lubricant selection. INDUSTRIAL METALWORKING LUBRICANTS In industrial metalworking, there are two basic modes by which lubricants decrease friction. 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. The second way in which 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., grinding and cutting operations), boundary lubrication takes over. This typically has a higher coefficient of friction and does allow some wear to occur, although it is greatly reduced. This mode of lubrication will be discussed in more detail later in the section on lubricant additives. Hydrodynamic Lubrication Die Figure IIllustrations of Hydrodynamic and Boundary Lubrication 208 Precision Cleaning ‘95 Proceedings Lubricant Classification In industrial metalworking there are four major classifications of lubricants; mineral oils, emulsions, semisynthetics and synthetics. There are various other classifications of lubricants such as waxes, greases, dry film lubricants, etc. that will not be discussed here since they are not considered to be in the realm of metalworking lubricants. Mineral Oils Mineral oils are usually used where lubrication is of primary importance and cooling is of secondary importance. They fall into two basic categories; paraffinic (straight chain hydrocarbon base) and naphthenic (ring structured hydrocarbon base). Naphthenic oils have the advantage of having a much higher solubility for many types of additives. These additives are often necessary to accomplish the boundary lubrication mentioned previously as well as 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. Emulsions Macroemulsions or so called “soluble oils” start as a formulated oil mixture called the concentrate which typically contains 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 five to ten 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 want to separate over time. This is accelerated by the fact that metal fines and other debris accumulate in the solution 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 Semi-synthetics or microemulsions contain emulsified oil in the range of 0.01 microns to 0.2 microns. 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 there is only a very small percentage of oil in the working solution. 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. Synthetics Synthetic lubricants contain no oil and typically will be made up from polyglycol, polyisobutylene or poly alpha-olefin bases. They will appear as transparent solutions and will oftentimes contain emulsifiers, amines, dispersants and anti-foams. These solutions also are tailored more towards 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 are not used in severe duty applications such as deep drawing. Like semi-synthetics, synthetics can also 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 oilin-water emulsion will evaporate. The remaining lubricant will then invert to a water-in-oil emulsion making for a difficult to remove polymeric film. Precision Cleaning ‘95 Proceedings 209 The more mineral oil in a lubricant, the better its lubricating properties. The more water contained in a lubricant, the higher its cooling capacity. See Figure 2. l Mineral Oils (100% oil as-used) l Emulsion (5-10% mineral oil as-used) l Semi-synthetics (<5% mineral oil as-used) l Synthetics (contain no mineral oi l) Lubricity Cooling Figure 2 Lubricity versus cooling capacity of lubricants Natural oils (e.g., lard oils) are occasionally used as lubricants, although they generally are considered to be inferior to the other metalworking lubricants in all respects except boundary lubrication properties. Were required, this can easily be provided by the incorporation of a low concentration of fatty material into a different base oil. Additives There are a variety of additives that can be present in lubricants to serve many purposes. Some of the additives include corrosion inhibitors, boundary additives, extreme pressure additives, anti-foams, emulsifiers, dispersants and viscosity index modifiers. Boundary Additives These are additives that adsorb one or two molecules thick at the metal surface. They are polar in nature and 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. Fatty acids tend to be more effective since they can react with active oxide surfaces on the metal to form a soap. Extreme Pressure (EP) Additives Extreme pressure additives are effective when the severity of the metal forming operation generates higher temperatures. They actually react with the metal surface to lower friction at higher pressures and temperatures. There are three well known types of EP additives; phosphorous based, chlorine based and sulfur based. Since EP additives are effective over various temperature ranges, there is usually a need for more then one in a lubricant formulation. Boundary additives and EP additives are usually found in mineral oil, emulsion type and to a lesser extent semi-synthetic lubricants. The functions of other previously mentioned additives are generally self explanatory. INDUSTRIAL CLEANERS The following will discuss the various categories of industrial cleaning and their chemistries such that a better understanding of the cleaning mechanism is possible. Certain cleaner/lubricant combinations are more compatible making the cleaning process easier and also more amenable to waste minimization. Solvent Cleaning 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. These solvents dissolve lubricant at the metal surface and work essentially by dilution. Most are 210 Precision Cleaning ‘95 Proceedings 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: Paraffinic Solvents These usually have flash points in the 120-170° F range and are generally used in hand wiping and room temperature immersion applications. Aroma tic Solvents This category includes naphtha, xylenes and toluene. Also applied via wiping and room temperature immersion. Terpenes Two terpenes found in some industrial and household cleaners are shown in Figure 3. These are typically used in immersion applications, although hand wiping may also be utilized.