METALWORKING FLUIDS
The process of ageing of cold rolling oil
André Volkers, Global Customer Development & Innovation Manager Rolling, Q8Oils
All organic substances are sensitive to ageing, but what is ageing and what causes ageing? Ageing means that under the prolonged and repeated exposure by energy the physical, mechanical, and chemical properties of organic matter changes over time. Energy can be light, heat, oxygen, mechanical stress. In this article the discussed organic matter is cold rolling oil, but obviously, this topic not only applies to cold rolling oil or other oils as well, but also all kind of plastics and rubbers, living creatures and plants experience ageing.
How to prevent ageing? To fully prevent ageing is almost impossible, but several options to slow down the ageing processes are available. Obviously, reducing the exposure to energy is quite effective, but not always possible. For example, cold rolling oil can be stored in the dark and at low ambient temperature (e.g. 10 °C / 50 °F) and very limited ageing will occur. However, when in use in a cold rolling mill, the temperature will be kept at a higher temperature (e.g. 40 °C / 104 °F) and in the roll bite the temperature can increase locally to extremely high values. Also, quite some mechanical stress is present in the roll bite to further accelerate ageing. Consequently, cold rolling oil in use needs to be protected against ageing in a different way. To prevent ageing, antioxidant additives are included in the cold rolling oil composition. Antioxidants are chemically active with the task of slowing down ageing.
38 LUBE MAGAZINE NO.181 JUNE 2024 Ageing chemistry
Basically, the chemistry of ageing is cleaving an organic (carbon) chain into two parts. In the reaction scheme below, the breaking of a chemical bond between two carbon atoms into two radicals is shown. This first step (1) is called the initiation step and is typically caused by heat and/or light.
Initiation: (1) R-R’ → R● + R’●
Unfortunately, the formed radicals are very reactive and with oxygen present they propagate further and increase the presence of radicals exponentially. As an example, the R● continues to react and obviously R´● may undergo similar reactions.
Propagation : (2) R● + O2
→ ROO● (peroxyl radical)
(3) ROO● + R’’H → R’’● + ROOH (hydroperoxide) (4) ROOH → RO● + HO●
Reaction (2) is not yet that devastating as this only means the formation of a different radical (peroxyl radical). In reaction step (3) this peroxyl radical takes away a hydrogen radical to form another R’’● radical and hydroperoxide (ROOH). So far, no additional radicals have been formed but this changes after formation of the hydroperoxide.
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