Regeneration of an oil-in-water emulsion after use in an industrial copper rolling process

The stability of two oil-in-water emulsions (smoothing emulsion and finishing emulsion) used as lubricants and coolants in an industrial copper rolling process was studied as a function of time. Four parameters were measured: droplet size distribution, zeta potential, surface tension and contact angle. Experimental results show that zeta potential (ξ) is the main indicator of emulsion stability: for ξ values higher than −10 mV the emulsion is completely destabilized and it is not suitable for the rolling process. Furthermore, emulsion regeneration is only possible for zeta potential values around −20 mV. For this reason, a sample from the finishing emulsion with a zeta potential value of −16.7 mV was chosen to check the suitability of seven nonionic surfactants in order to try its regeneration. No relevant variation was observed between the surface tension and droplet size distribution of the regenerated and the original emulsion. Likewise, zeta potential was lowered to values around −40 mV for all tests, close to the zeta potential of the fresh emulsion. Contact angles over a copper surface show different behavior depending on the surfactant. Though all surfactants decrease the initial contact angle (44.4°), the final values obtained after surfactant addition varied from 24.1° (lowest value) to 40.1° (highest one). In spite of its high foaming ability, compared to the other surfactants tested, Cremophor® EL was found be the most suitable surfactant for emulsion regeneration.