## 3 Results and Discussion
### Effect of Antioxidants on Oxidative Stability of Biodiesel and Biolubricant  
As previously stated, WCOBD and WCOBL required antioxidant addition. Thus, the effect of the separated addition of PG and TBHQ in WCOBD is included in Figure 5. In this sense, both antioxidants presented a similar efficiency, with similar oxidation stability increases at the same antioxidant concentrations. Consequently, both antioxidants allowed compliance with the standard at around 600 ppm. In the literature, there are different results concerning the efficiency of different antioxidants, with better results for PG in some cases [63], whereas TBHQ had a better antioxidant performance in other studies [64].  
Figure 4: Chlorophyll index evolution over time. Effect of FOBL addition.  
Figure 5: Effect of PG and TBHQ on oxidation stability of WCOBD.  
However, this effect was different when PG and TBHQ were added to WCOBL. As observed in Figure 6, PG presented a higher efficiency compared to TBHQ, considerably increasing oxidation stability of WCOBL at lower concentrations. Oxidation stability values of 8 h were achieved at around 200 ppm of PG, whereas the same result was obtained with around 1800 ppm of TBHQ. This specific behavior in biolubricants was observed in previous studies for similar products. Regarding the different efficiency depending on the kind of sample (biodiesel or biolubricant), this could be due to the different nature of these products, where different viscosity, polarity, and acidity values could alter the suitable dissolution and distribution of antioxidants. Indeed, previous studies have covered the possibility of TBHQ chemical modification to improve solubility in biodiesel due to its polarity [65], and this fact could be decisive at this point.  
Equally, the mixture of antioxidants showed interesting results, as observed in Figure 7 for the combined effect of PG and TBHQ on oxidation stability of WCOBD. The data included in this experiment were fit to a third-degree polynomial (R\({}^{2}\) = 0.98857), according to ten samples with variable PG and TBHQ concentrations (at high, medium, and low concentrations, within 0-1000 ppm for each case). Possibly due to the similar efficiency of both antioxidants (as previously explained), a neutral or additive effect was observed (in general) when PG and TBHQ were combined at different concentrations.  
In other words, strong synergistic or antagonistic effects were not observed in this case. Thus, the separate addition of antioxidants according to other criteria such as economic factors is recommended. Other studies including the combination of TBHQ and BHA in biodiesel from soybean and pork fat showed a favorable synergistic effect, with a 75-25% ratio recommended in this case [66].  
However, the combination of antioxidants applied to WCOBL offered different results, as observed in Figure 8. The data included in this experiment were fit to a third-degree polynomial (R\({}^{2}\) = 0.99629), covering samples at different TBHQ and PG concentrations (both separate and combined) within the maximum and minimum concentrations covered in this work (from 0 to 800 ppm for PG and from 0 to 1000 ppm for TBHQ). In this way, an antagonistic effect was observed, obtaining lower or shorter oxidation stability values than expected according to the separate effect of PG and TBHQ.  
Figure 6: Effect of PG and TBHQ addition on oxidation stability of biolubricant.  
Figure 8: Combined effect of PG and TBHQ on WCO biolubricant.  
Figure 7: Combined effect of PG and TBHQ on biodiesel from WCO.  
As previously explained, this effect could be due to the lower antioxidant performance of TBHQ (due to the different acidity and viscosity of the medium) or the possible combination of both antioxidants during oxidation, thereby generating compounds with a lower antioxidant effect.  
As a result, separate addition of PG (at around 300 ppm) was recommended in this case, as its efficiency was considerable compared to TBHQ addition or the mixture of both antioxidants.