Metalworking fluids all start on the formulator’s chemistry lab bench, where chemists blend together various compounds, much like a kitchen recipe. Most of these ingredients are publicly known and governmentally approved as being safe. A few ingredients are proprietary. Both will be listed on the fluid’s Material Safety Data Sheet, or MSDS. Just knowing the ingredient, however, is not enough to make the coolant yourself- how it is used, at what stage of the recipe, and exactly how it is added are also critical to its function.
The main property the chemist will impart is the ability for the coolant to emulsify into water (except straight oils) and remain stable as an emulsion over a wide temperature range. Other properties are for the coolant to prevent rust on parts and machines, reject tramp oils which cause biofilms and excessive mist, provide the right amount of lubricity so tools and metals do not weld together, and to prevent foaming. Biocides will be added to the recipe as well, in safe amounts, to prevent unwanted biological activity.
The aging of coolant revolves primarily upon chemical activity. Over time, acids are introduced into the coolant system via bacterial activity (refer to the Biology section of the Basics for further information on bacteria). The bacteria comes mainly from the water that is used. This is why it is important, when topping off your sump, to add a reduced coolant mix rather than straight water.
The acids that accumulate in your sump are hydrochloric acid (HCl) and hydrogen sulfide (H2S). These acids will eventually bring down the sump’s pH, causing rust and corrosion, bad odors, and possible skin reactions.
For those who have swimming pools, the concept called pH buffering is understood. Buffering means that there is some compound, usually a carbonate of some form (like baking soda) that remains in solution but does not fully dissolve. Coolant formulators add these buffering components to their formula, but as more acids accumulate, they will combine with these reserved carbonates and neutralize. Once all the reserves are used, additional accumulation of more acid will impact the pH of the sump.
Once the pH goes too low, it is likely that they will lose their performance qualities and are no longer a useful tool. It will lose its anti-rust capabilities, bacteria will grow even more quickly, and the emulsion itself may split out into its constituent components. A split emulsion can also be caused by water hardness, if in excess of 450ppm of carbonates. Hardness can cause coolant to foam excessively and helps the tramp oils form a grease layer on the coolant surface, which is difficult to skim.
The chemical make-up of the coolant can also be affected by the metals that are introduced during the machining process. Ionic activity between the positive atoms of the metals and negative atoms in the coolant can cause premature failure as the emulsion starts to break. The amount of metal chips in the coolant system, as well as their size, can affect the emulsion in as little as 15 minutes (on a grinding application).
Detailed information on these root causes of coolant failure, are explained in MWF Management – Failure & Prevention.