## 3 Results
### Thermal Properties
#### 3.4.2 Pyrolysis  
The next stage of the thermal research was thermogravimetric analysis in an inert atmosphere. Pyrolysis is a type of thermolysis observed when heating organic materials in an oxygen-free atmosphere. The mechanism for the chemical changes occurring during pyrolysis is often very complex, and it can be difficult to study them thoroughly due to the variability in the composition of the raw materials subjected to pyrolysis (for example, biomass pyrolysis). As a result of this process, only gaseous products can be formed; however, the process almost always proceeds with the formation of a solid residue. It has been known for some time that the type of atmospheric gas can strongly affect the position of TGA curves on the weight axis. One of the most attractive features of using TGA in an inert atmosphere as a method of thermal stability analysis for polymers, is that it is almost always possible to glean some information from the data record. The representative thermograms of the okra samples prepared at ambient conditions are presented in Figure 10, and the characteristic degradation temperatures can be found in Table 5.  
According to the data shown in Figure 10 and Table 5, the thermal distribution curves of the samples have two-stages, similar to the thermo-oxidation process. Most likely, the first stage involves depolymerization and the thermal degradation of cellulose macromolecules and the second stage is the afterburning of low-molecular breakdown products. The degradation of the fibers from different parts of the plant has a very similar character and occurs in similar temperature ranges; the differences between individual samples are smaller than for the decomposition process in air. The temperature of half decomposition for past fibers obtained by the water-retting method is 378.8 \({}^{\circ}\)C, while for the fibers obtained by dew-retting it is 383.2 \({}^{\circ}\)C, which shows the same trend as the thermo-oxidation process. The main stage of thermal decomposition under neutral conditions was subjected to kinetic analysis. The activation energy values of the decomposition process were calculated using the Coats-Redfern method [37]. The results of the analysis are shown in Table 6.  
The data from Table 6 indicate that the EA values under given conditions (in an inert atmosphere) range from 42.8-73.5 kJ/mol, which is a much narrower range. The coefficient of determination values are above 99%. Since the decomposition process in an inert atmosphere occurs at a higher temperature than for thermo-oxidation, it is understandable that the average value of activation energy for nitrogen is slightly more than for air (56.6 and 53.0 kJ/mol, respectively). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\mathbf{T_{init}}\) & \(\mathbf{T_{50\%}}\) & \(\mathbf{T_{final}}\) \\ \hline BBW & 300.0 & 374.8 & 406.2 \\ MBW & 303.5 & 389.1 & 426.3 \\ UBW & 299.9 & 372.4 & 403.6 \\ BBD & 300.0 & 377.6 & 406.6 \\ MBD & 300.0 & 382.2 & 410.9 \\ UBD & 299.9 & 389.7 & 412.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Characteristic temperatures (average values) of main stage thermal degradation of the analyzed samples. Figure 10: Representative TG curves of the analyzed samples. Studies in the literature that show thermogravimetric curves for okra fiber (without taking into account the part of the plant from which the fiber originated) in an inert atmosphere, show a 50% distribution temperature at about 350 \({}^{\circ}\)C [22] and in the 329.8-349.9 \({}^{\circ}\)C range (depending on the treatment technique) [23]. The main thermal degradation step is presented in the form of \(\alpha_{\mathrm{s}}=\mathrm{f}(\alpha_{\mathrm{r}})\) plots (Figure 11) and shows very small differences between the TG run of each okra sample, which confirms the similar thermal stability of the analyzed samples, as was concluded from the comparison of thermogravimetric curves.