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The Biology of Metalworking Fluids

Metalworking fluids - coolant - contained carcinogenic materials until the 1970s, when they were outlawed, as was the practice of disposing metalworking fluid down the drain. Shortly afterward, machine shop workers noticed that their sumps would start to smell after a few weeks, or even days. Some machine shops even reported that when their machines would start up, not only would the shop reek of rotten eggs (the infamous Monday morning smells) but that they could see a cloud of green haze linger about the machine. In addition, workers would develop rashes, eye or lung irritation, and other health problems. In many cases the very paint would fall off machines, and seals and gaskets would disintegrate. These sorts of problems hadn’t happened in the old days, so it was quite a while before anyone could really trace the source of what was going on.

The problem was bacteria. The very bugs that make our bread and wine are also good at turning the oils in coolant sumps into their own version of Club Med. The challenge for chemists was to develop metalworking fluids that were resistant to bacterial attack, yet not harmful to humans. The problem with that is what is usually bad for bacteria is also bad for people. This ‘cold war’ of chemist against bacteria continues even today.

Since the early days of this cold war we have learned much more about the ecology of coolant sumps. There are many species of bacteria that can live there, most of which like oxygen environments, but some of which that don’t. There are also many varieties of fungi that live there too. And once enough bacteria have congregated in one place, they have a way of signaling each other (called quorum sensing) so that they form a colony. One of the things these colonies do to protect themselves is start producing a protein carbohydrate complex that science calls a biofilm, and the rest of us call slime, or mucus. Once a biofilm has formed, the only way known to get rid of it is to scrape it off.

Biofilm are far more prevalent than we used to think. Scum on your tub, tartar on your teeth, and your congested sinuses are all forms of biofilms. Cystic fibrosis and serious pneumonia are forms of biofilms that kill (where most current research is focused). In the case of the lowly machine sump, you see the biofilm as the skin on the surface of the sump, what many of us used to call tramp oils.

Bacteria exude waste products while they live (including acids), and decay when they die. Their life cycle can last from 20 minutes for nicely warmed oxygen loving aerobes, to as long several hours for oxygen hating anaerobes. Some bacteria can do both, which makes the job of identifying them harder. Decay products include HCl and H2S, and these are the acids that help degrade the coolant.

This is one cold war that we can’t win. Bacteria are always present everywhere: In the skin we shed, in the air we breathe, and in the water we mix with the coolant. As soon as a layer of oil collects in the sump, an oxygen dead layer forms (because aerobic bacteria consume the oxygen as part of their metabolic cycle) and the anaerobic bacteria start to thrive. In fact, they like to consume the oil itself as a food source.

Many companies have tried to combat bacteria using outright biocides, usually compounds that interfere with the metabolism of the bacteria itself. Unfortunately, these substances can also be dangerous for people. Worse, once the bacteria have had the chance to form a biofilm, even the most powerful biocides can’t touch them. Only the wire brush and water jet have a chance.

The best strategy is to start fighting the bacteria right from the beginning, using everything we know about chemistry and mechanics. Chemistry will tell us to monitor the pH and concentration, while mechanics will keep us aerating and skimming using disk skimmers, belt skimmers, coalescers, etc. Once we accept the fact that a stalemate is the best we can hope for, our lives become that much easier.

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