## 3 Results and Discussion
### Effect of Extreme Oxidation Conditions on Biodiesel and Biolubricant Properties  
In order to assess the separate effect of PG (the most effective antioxidant for WCOBL) along with the combined effect of PG and TBHQ, different measurements were carried out during WCOBL extreme oxidation. Thus, the selected samples, according to the previous results, were the following:  
* WCOBL (control). This was WCOBL without antioxidant addition. * WCOBL (PG + TBHQ). In this case, 300 ppm PG + 1000 ppm TBHQ were added to the sample. This was one of the points where the antagonistic effect (as previously mentioned) was evident, with lower oxidation stability than expected shown according to the additive effect of both antioxidants. * WCOBL (PG). An addition of 300 ppm PG was carried out. Regarding antioxidant content in WCOBL during extreme oxidation conditions, there was a continuous decrease in its content with time, as observed in Figure 13 for PG concentration evolution with oxidation time. In this case, concentration decreased from 300 ppm to 30-40 ppm at 4 h, slightly decreasing from then on. At 8 h of study, there was 20 ppm of PG in WCOBL, proving the suitability of the added quantity, which was enough for this test. PG was gradually degraded due to extreme oxidation in favor of WCOBL, whose oxidation was clearly decelerated as explained in further experiments. This decrease was also observed in different antioxidant samples (PG and TBHQ) during oxidation of biodiesel [37] and biolubricants [41], highlighting the effectiveness of these products in avoiding the oxidation of the original samples. Concerning the effect of viscosity during extreme oxidation conditions, Figure 14 shows the evolution of this parameter in different samples. Clearly, control samples presented an increase in viscosity at 4 h of extreme oxidation (from 71 to 370 cSt), with a moderate increase in the case of the sample with combined antioxidant addition (where viscosity increased from 63 to 65 cSt). Nevertheless, separate PG addition seemed to be the most effective option, as viscosity was maintained during the whole experiment, even at 8 h of extreme oxidation. An increase in viscosity is due to oxidation processes, where polymerization of biolubricant could have taken place, with the subsequent increase in viscosity and the corresponding quality loss due to the impossibility of selecting this product for specific industrial uses. In this sense, the use of antioxidants had a positive effect, and avoided the considerable increase observed for control samples. Specifically, the combined effect of antioxidants (PG + TBHQ) showed a slight increase in viscosity at the end of extreme oxidation (especially at 8 h), possibly due to the antagonistic effect of  
Figure 13: PG evolution in WCO biolubricant during extreme oxidation conditions. their combination (which implied lower oxidation stabilities in biolubricant than expected). That is the reason why, as cited in previous subsections, PG at 300 ppm is recommended, whose effectiveness was clear in this case, where a viscosity increase was almost negligible at 8 h. The same trend was observed for biodiesel and biolubricant control samples, where polymerization caused this undesirable effect [41; 67], which was avoided by the addition of synthetic antioxidants. These differences between control, PG + TBHQ, and PG samples were clear when the effect of extreme oxidation on acidity was assessed (see Figure 15). In this way, for control samples, the acid number considerably increased from 4 to 8 h, due to the generation of free fatty acids during the oxidation processes in biolubricant. In this case, the PG and TBHQ mixture showed an intermediate increase in this parameter, with PG addition showing the lowest increase in acidity. Again, the antagonistic effect of the mixture provoked the lower efficiency of antioxidants, with worse results obtained compared to the separate addition of PG.