Combined and Separately Effect of Tert-Butylhydroquinone and Propyl Gallate on a Biolubricant Based on Cardoon Oil

The use of biolubricants is becoming important, as the incipient industrialization is requiring alternatives for petrol-based products. Thus, a double transesterification from vegetable oils could provide biolubricants for these purposes, apart from other interesting by-products like fatty acid methyl esters, glycerol or methanol (which can be re-used in the system). Consequently, this synthesis could adjust to the concept of biorefinery, which is highly appreciated due to its implication with green chemistry or circular economy, contributing to sustainability. However, one challenge related to these biolubricants is their low oxidative stability, which can alter the properties of these products during storage. To avoid it, some interesting antioxidants like tert-butylhydroquinone (TBHQ) or propyl gallate (PG) could be added to biolubricants to keep their main properties during oxidation. The aim of this work was to assess the effectiveness of TBHQ and PG in a biolubricant based on cardoon oil through double transesterification with methanol and pentaerythritol. It was proved that PG was more effective than TBHQ, obtaining higher oxidative stabilities at lower concentrations (exceeding 11 h at 500 ppm, whereas TBHQ required 2000 ppm to obtain a similar value). In contrast, the combined use of TBHQ and PG did not offer any synergistic effect.