This hydroperoxide is relatively unstable and forms very easily two radicals, see reaction (4). So now, two new radicals are formed, and this adds up to four radicals instead of the initially two radicals formed. These radicals can continue to react and grow their presence exponentially. If this continues, black sludge, lacquering, polymerisation, degradation, and other chemical reactions occur, and the cold rolling oil may lose its physical and lubrication properties.


Antioxidants


As mentioned above, the initiation step to form radicals can often not be prevented. So, the next best option is to disarm the initial formed radicals as soon as possible after formation. This can be done with antioxidant additives. Antioxidants are very special designed molecules to be quite reactive towards radicals by the following chemical reaction: (5) R● AO-H→ R-H + AO● (antioxidant radical)


And the chemical structure of the antioxidant molecule is designed to be quite stable in the form of an antioxidant radical. This antioxidant radical does not react further with other molecules in the cold rolling oil. At some point two antioxidant radicals AO● will recombine, see reaction (6)


Termination: (5) AO● + ●AO → AO-AO


So, the purpose of antioxidants is to scavenge the radicals formed and prevent further damage to the


LUBE MAGAZINE NO.181 JUNE 2024 39


cold rolling oil. By this mechanism, the ageing of cold rolling oil is limited and under control.


Conclusion


In this article the aging of cold rolling oil and the importance of antioxidants to prevent ageing is described. The presence of antioxidants in cold rolling oil is very important and therefore the antioxidant concentration should be regularly monitored. Programmes such as QRAS can be utilised to monitor the cold rolling oil quality including antioxidant concentration.


To improve the cold rolling process with customized oil compositions a very close collaboration is needed. An important part of improvements is the correct understanding of the cold rolling mills, the operation parameters, and other steps in the entire process. Optimisation for each cold rolling mill separately may give the best performance and productivity.


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