## 4 Conclusions  
Natural rubber (NR), which has a molecular weight of a few hundred thousand to more than one million, is found in the form of latex. The natural rubber depolymerization process used NaNO\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) degrading agents in the presence of HCOOH to produce  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \(\mathbf{M_{n}}\) & \multicolumn{3}{c}{**Heating Rate = 40 \({}^{\circ}\)C/min**} \\ \cline{2-4} (g/mol) & \(\mathbf{T_{i}}\) (\({}^{\circ}\)C) & \(\mathbf{T_{m}}\) (\({}^{\circ}\)C) & \(\mathbf{T_{f}}\) (\({}^{\circ}\)C) \\ \hline
630,439 & 392 & 429 & 469 \\
50,023 & 388 & 428 & 468 \\
22,143 & 380 & 424 & 466 \\
17,139 & 375 & 419 & 461 \\
10,825 & 352 & 398 & 439 \\ \hline \hline \end{tabular}
\end{table}
Table 1: T\({}_{\mathrm{f}}\), T\({}_{\mathrm{m}}\), and T\({}_{\mathrm{f}}\) of NR and LNR at different molecular weights.  
Figure 14: TGA (**a**) and DTG (**b**) thermograms of NR and LNR at different molecular weights at 40 \({}^{\circ}\)C/min.  
liquid natural rubber (LNR) with a relatively low molecular weight. This study resulted in the Mark-Houwink equation for the value of the constant K, and a for NR and LNR samples were 1.307 \(\times\) 10\({}^{-3}\) and 0.638 respectively, which were used to determine the number-average molecular weight. The molecular weight of LNR obtained depended on concentrations of H\({}_{2}\)O\({}_{2}\), HCOOH, and NaNO\({}_{2}\), temperature, and reaction time. It was found that the higher concentration of H\({}_{2}\)O\({}_{2}\) and HCOOH employed faster rates of degradation. However, a higher concentration of NaNO\({}_{2}\) decreased the M\({}_{n}\) reduction. The high degradative temperature made the NR chains break more rapidly. The M\({}_{n}\) values were a rapid initial decrease followed by a more gradual fall according to the increased reaction time. The results showed that the lowest M\({}_{n}\) of the resulting LNR product was 10,825 g/mol from the initial molecular weight of 630,439 g/mol. Based on FTIR analysis, the presence of hydroxyl terminal groups resulting from the breaking of the NR chains was synthesized at acidic pH. In contrast, LNR with carbonyl terminal groups was formed as the degradative reaction at alkaline pH. SEM micrographs showed the NR latex particles were small spheres with a few larger pear shape particles. After the degradation process, the LNR latex obtained showed distinctly smaller spherical-shaped particles as compared to the NR particles. Changes in the molecular weight were used to construct a kinetic model for the depolymerization process: (M\({}_{0}\)/M\({}_{n(t)}\)) \(-\) (M\({}_{0}\)/M\({}_{n(0)}\)) = [(1.04 \(\times\) 10\({}^{9}\) M\({}^{-1.72}\)t\({}^{-1}\)Exp(78.66 kJ mol\({}^{-1}\))][H\({}_{2}\)O\({}_{2}\)]\({}^{1.58}\)[HCOOH]\({}^{0.79}\)[NaNO\({}_{2}\)]\({}^{-0.65}\) t. The kinetic model is useful to estimate the number-average molecular weight for degraded NR under different environmental conditions such as H\({}_{2}\)O\({}_{2}\), HCOOH, and NaNO\({}_{2}\) concentrations, reaction temperature, and reaction time. From TGA measurements, the decomposition temperatures decreased with a decrease in the M\({}_{n}\), indicating that the LNR had worse thermal stability than NR. Therefore, LNR should be widely used as a starting material for synthesizing biopolymers.  
Writing-original draft preparation, K.W.; writing-review and editing, W.P. and W.A. All authors have read and agreed to the published version of the manuscript. This research was funded by the Thailand Research Fund (TRF) under the Royal Golden Jubilee Ph.D. Program, grant number PHD/0209/2558. Not applicable. All the data is included in the article. The authors gratefully acknowledge the financial support of the Thailand Research Fund (TRF) and Silpakorn University through the Royal Golden Jubilee Ph.D. Program grant #PHD/0209/2558. The authors declare no conflict of interest.