Contamination Part 5
UNDERSTANDING CONTAMINATION IN METALWORKING FLUIDS
PART 5 - CAUSES AND EFFECTS OF MICROORGANISM CONTAMINATION
by Joe Eppert
In Part I of this series, we identified the primary contaminants in MWF systems as tramp oil, machining particulate, water impurities, and microorganisms. Previous articles have covered the effect of the first three of these contaminants, with microorganisms left for the current discussion. Tramp oil, machining particulate, and water impurities all have an individual impact on the chemistry of the MWF, machining performance obtained, and the health and safety of MWF use. In addition, all three of these were noted as having a severe impact on the growth and support of microorganisms in the MWF system. The goal of the current article is to understand the primary causes of microorganism addition to the MWF, as well as the wide range of effects this addition has on MWF functionality.
The types of microorganisms typically found in MWF systems are bacteria and fungi (yeast and mold are two subsets of fungi). Bacteria can be further classified as aerobic (require oxygen for survival), anaerobic (require the absence of oxygen for survival), or facultative (survive in the presence or absence of oxygen). For our purposes, we will discuss the effects of aerobic bacteria, anaerobic bacteria, and fungi. It is important to note that there is a tremendous amount of literature on this particular topic, and this article may serve as an introduction if further research is desired. It is also important to note that this discussion focuses mainly on water-containing fluids, and does not directly apply to cutting oils, as microorganisms favor water for growth.
The first question that can be asked is “How do microorganisms get into the MWF system?” The major routes of entry are (1, 7):
EDITOR'S NOTE: Joes's analysis that follows assumes the Shop Manager has ended the practice of allowing employees to urinate and hawk loogies into the tank system. The experienced Shop Manager knows that this is not a given.
Water - Any water used to dilute the MWF concentrate or provide volume make-up can introduce microorganisms. What may be most notable is the fact that many of the microorganisms known to contaminant water supplies are also known to degradeMWFs. This may come as no surprise, since most MWFs primarily contain water. It has been noted that what appears to be pure water may actually contain millions of microorganisms (1).
Previous tank contamination – If not properly cleaned prior to charging with a fresh fluid, any microorganisms left behind will begin to degrade the fluid. This is particularly true if sludge deposits are allowed to remain in the sump after cleaning. It has been noted that if the system is not thoroughly cleaned, the left over microorganism content causes an initial load on the MWF biocide package that it may not be able to overcome (7).
Air – When dealing with open systems, any microorganisms from the air are able to enter the MWF.
Parts – Incoming parts may have microorganisms on their surfaces and when made to come in contact with the MWF, these microorganisms can be washed away with the MWF.
External contaminants – In general, anything that is allowed to come in contact with the MWF and contains microorganisms can cause an introduction to the system. In particular, floor sweepings, trash, food, and spit can contain significant amounts of microorganisms.
With a general understanding of how the microorganisms are introduced to the system, the next question may be “Why do microorganisms tend to grow so well in MWFs?” The fact that the MWF system offers excellent growth conditions answers this question. In particular, microorganisms tend to grow due to (1, 6, 7):
High water content of MWF – Microorganisms tend to favor environments with plenty of water. As MWFs are commonly greater than 85% water, this condition is met.
Great food sources – Microorganisms require a number of components for survival, and given that MWFs are generally made from a variety of chemical types, there are a large number of food sources available for microorganism consumption. In particular, the ion concentration of the water used to dilute the MWF helps to support growth quite well. Other contaminants in the MWF system tend to support microorganism growth as well.
Size of the MWF system – It has been noted that the amount of time the fluid is allowed to stay in the system is related to the adaptation and growth of microorganisms. For a given rate of flow through the system, this time is related to system size. (7)
Now that the basic causes of microorganism contamination in MWFs are known, the effects of this contamination in terms of chemical effects, interacting effects, machining performance effects, and health and safety effects can be discussed. Each of these is treated in a separate section below.
Chemical Effects of Microorganism Contamination:
Microorganisms require food sources to survive. It is the consumption of the functional components in the MWF that is the greatest chemical effect of microorganism contamination. Any ingredient in the fluid has the potential to be consumed, and depletion of those in the lubricity package, emulsion package, or corrosion package can drastically impact MWF functionality (7, 8).
Aerobic bacteria tend to consume useful components or simply degrade a constituent by breaking it into smaller molecules that may or may not further effect performance (2, 4). In one case, it was found that for a commercial MWF, the bacteria present were able to use all of the compounds in the formulation as substrates (5). It was also offered that due to the various types of microorganisms that may be present in the MWF, it is difficult to tell exactly what component is being used as the primary food source.
It has been noted that the waste products of such microorganisms are capable of lowering the pH of the MWF due to the acidic nature of the by-products. The effect of this on corrosion will be discussed below.
Interacting Effects of Microorganism Contamination:
Although tramp oil, machining particulate, and water impurities all effect microorganism growth, a reciprocal relationship does not necessarily exist. Microorganisms do not drastically impact the amounts of tramp oil, particulate, or water impurities in the MWF system. In this sense, microorganisms can be thought of as “the end of the road” for MWF contamination. The interacting effects of the other contaminants have been discussed in previous articles.
Machining Performance Effects of Microorganism Contamination:
The major effects of microorganisms on machining performance are the deterioration of all machining related parameters (surface finish, tool life, forces, temperatures, etc.) and the ability to cause corrosion problems. If functional components of the fluid are depleted over time (as discussed above), workpiece surface finish, tool life, and various performance measures will be impacted. This is especially true if the microorganisms tend to favor the components in the lubricant package of the fluid.
Bacteria have also been noted to cause corrosion of work parts through a variety of processes. Aerobic bacteria tend to produce acidic waste products that can eventually cause corrosion, and anaerobic bacteria can also cause corrosion through a reduction-oxidation reaction (2, 5). As discussed above, bacteria may “eat” the corrosion inhibitor package in the fluid, which itself will eventually lead to corrosion.
When dealing with an emulsion-type MWF, instability of the emulsion can be caused by the depletion of emulsifier package by the microorganisms. This instability may eventually lead to emulsion splitting.
Health and Safety Effects of Microorganism Contamination:
Related to health and safety effects, microorganisms are probably of most concern when compared to all other contaminants discussed in this series. Typical cleanliness problems include rancid smell and slime formation (which leads to filter clogging).
Of major concern are the health effects due to high microorganism levels and high levels of biocide concentration. The impact of microorganisms and biocides on dermatitis, Legionnaires’ disease, Pontiac fever, and worker exposure to endotoxin has received considerable attention in literature. It is important to note that MWFs generally do not cause health problems and that such cases are exceptions. Dermatitis can be caused by skin contact with MWFs containing large concentrations of biocides, which are used to combat the microorganism growth (3). Legionnaires’ disease and Pontiac fever have both been noted to occur in metalworking fluid environments, where the infection is caused by inhalation of MWF mists containing the bacteria responsible for these conditions (4). Endotoxins are contained in a number of bacterial types and the influence of such substances on human health is of concern. As pointed out by Rossmoore and Rossmoore (2), high levels of endotoxin have been identified in MWFs, but the relationship to human health in these cases has not been proven. It is known, however, that endotoxins are capable of causing illness in humans (2).
Microorganism contamination –
· includes bacterial and fungal contamination
· is introduced through make-up water, previous contamination of MWF system, air contact, microorganisms on incoming parts, and external contamination
· tends to favor MWFs due to the high water content of the fluid and variety of substrates available
· depletes functional components in the fluid due to consumption
· is capable of altering the pH of the fluid
· is greatly impacted by the introduction of other contaminants to the MWF system
· leads to machining performance decline (due to consumption of functional components)
· leads to corrosion problems
· can cause emulsion stability problems
· contributes to smell and slime issues
· IN RARE CASES can lead to health concerns such as dermatitis, Legionnaires’ disease, Pontiac fever, and endotoxin exposure
1. Bennett, E.O., 1974, “Water Quality and Coolant Life,” Lubrication Engineering, Vol. 30, No. 11, pp. 549-555.
2. Byers, J.P. ed., 1994, Metalworking Fluids, J.P.B., ed., Marcel Dekker, Inc, New York.
3. Coughlin, R.W., D. Williams, E. Seveau, R. Veith, and T.D. Howes, 1992, “Enumeration of Microorganisms in Metalworking Fluids using Photometric Methods,” CIRP Annals, Vol. 41, No. 1, pp. 357-360.
4. Elsmore, R., 1989, “Survival of Legionella Pneumpohila in Dilute Metalworking Fluids,” Tribology International, Vol. 22, No. 3, pp. 213-217.
5. Rinkus, K.M., W. Lin, A. Jha, and B.E. Reed, 1997, “Investigation into the Nature and Extent of Microbial Contamination Present in a Commercial Metalworking Fluid,” Proceedings of the Industrial Waste Conference, May 5-7, pp. 601-610.
6. Rossmore, H.W., 1974, “Microbiological Causes of Cutting Fluid Deterioration,” SME Technical Paper, MR74-169.
7. Rossmoore, H.W., 1975, “Extending Cutting Fluid Life,” Manufacturing Engineering, Vol. 75, No. 5, pp. 27-28.
8. Skoeld, R.O., 1991, “Field Testing of a Model Waterbased Metalworking Fluid Designed for Continuous Recycling using Ultrafiltration,” Lubrication Engineering, Vol. 47, No. 8, pp. 653-659.