# Novel Aromatic Estolide Esters from Biobased Resources by a Green Synthetic Approach
## 3 Results and Discussion
### Biodegradability Study  
The biodegradability of the oligoesters synthesized with ricinoleic acid and caffe acid was evaluated by monitoring the biochemical oxygen consumption (BOD) in an aqueous environment at 24 h intervals over a period of 21 days after exposure of the sample to the metabolic activity of an inoculum collected from the Bega River in Timisoara, Romania. The BOD of the sample was measured against a control sample without the oligoester using an OxiTop\({}^{\circledR}\) control system and following the standardized protocol of ISO 17556:2019. Both the theoretical oxygen demand (TOD) and the percentage biodegradation relative to TOD were calculated. This respiratory method allowed a direct correlation between the BOD consumption over time and the biodegradability percentage of the sample, since aerobic microorganisms consume a certain amount of oxygen when they break down the polymer into different metabolites. The BOD is one of the key parameters for assessing biodegradability, reflecting the extent of decomposition of organic matter in the environment.  
The degree of biodegradability (Dt) was expressed as a percentage of the degradability measured at intervals of 5, 10 and 21 days, as shown in Table 5. After an incubation period  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Temperature**, \({}^{\circ}\)**C** & **Herschel–Bulkley Model: \(\tau\)=\(\tau_{0}\)+k-\(\gamma^{n}\)** & **R\({}^{2}\)** \\ \hline
40 & \(\tau=0.2+0.177\cdot\dot{\gamma}^{1.015}\) & 0.999987 \\
55 & \(\tau=0.2+0.0865\cdot\dot{\gamma}^{1.009}_{7.0985}\) & 0.999945 \\
70 & \(\tau=0.19+0.0495\cdot\dot{\gamma}^{0.985}\) & 0.99982 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Rheological equations of the ricinoleic estolide esters with caffe acid in toluene.  
Figure 11: The \(\eta_{\text{a}}=\text{f}(\dot{\gamma})\) dependence for the estolide esters and ricinoleic acid at 40 \({}^{\circ}\)C.  
of 21 days, the OxiTop(r) system measurements showed that the ricinoleic acid oligomer with caffe acid exhibited a good degree of biodegradability, reaching up to 30% biodegradation with a recorded BOD of 64.6 mg/L. The biodegradation process starts rapidly, with 13% of the oligoester degraded after only 5 days of incubation, while 20% of the sample underwent a biodegradation process after 10 days. This result may be related to the presence of ester linkages in the oligomeric structure, as biodegradation of polymeric materials is more likely to occur in compounds containing ester groups in their chemical architecture due to their high susceptibility to hydrolysis in an aqueous environment [46]. In addition, average molecular weight plays an important role in the biodegradation process, so lower molecular weight oligomers are generally more susceptible to metabolism by microbial enzymes. This increased susceptibility increases the rate of microbial degradation, resulting in faster degradation.  
Therefore, the efficient biodegradation of these oligoesters is essential because of its implications for the creation of ecofriendly materials. Compounds that rapidly degrade in the environment can help reduce the accumulation of nonbiodegradable waste and alleviate harmful effects on ecosystems. Monitoring the biochemical oxygen consumption, analyzing molecular mass and examining the chemical structure of the estolide esters offer important insights into their environmental degradation processes, thereby supporting the efforts toward environmental protection and sustainability.