## 3 Results
### Thermal Properties
#### 3.4.1 Thermo-Oxidation  
Understanding the dependence between thermal resistance and the chemical composition of compounds used in high temperatures is essential, therefore, it is important to determine the thermal decomposition parameters, such as activation energy (E), reaction order (n), or frequency coefficient (A). These parameters are vital for determining the polymer degradation mechanism [4; 5; 29; 30] and its thermal stability [6]. There are a number of methods used to determine the kinetic pyrolysis parameters. They vary according to the kind of data analysis and assumed hypotheses, and also in the method of mathematical elaboration. However, even the most modern methods using complicated calculation schemes make use of the original basic theories [31; 32; 33; 34; 35]. In this work, two methods were selected for kinetic analysis, which differed in their theoretical approach in terms of assumptions and simplifications. The most popular approximations to the Arrhenius integral in polymer science are those of [PERSON] [36] and [PERSON] [37]. Figure 6: Effect of plant section used for fiber extraction and the retting method on fiber elongation at break rates (%) (Error bars represent standard errors). As can be seen from the data presented in Figure 7 and Table 3, the thermal distribution curves of the samples have two-stages. The first stage is related to depolymerization and thermal degradation of the cellulose macromolecules. This stage has also been determined in several studies [38; 39; 40]. In the second stage, the afterburning of low-molecular products (oxidation of the charred residues) of the first stage takes place [41; 42]. The characteristic temperatures of the first main stage of decomposition are shown in Table 4. The distribution of the fibers from different parts of the plant are similar in nature and occur in similar temperature ranges. For the okra best fibers, the temperature range for each of the thermal characteristics is about 20 \({}^{\circ}\)C: the start of decomposition is 275-298 \({}^{\circ}\)C, T50% is 342-371 \({}^{\circ}\)C and the end of the process is 375-399 \({}^{\circ}\)C. The thermal degradation of fibers obtained by the water-retting method occurs slightly earlier (T50% = 354.5 \({}^{\circ}\)C) than those obtained by the dew-retting technique (T50% = 359.4 \({}^{\circ}\)C). The processes of the thermal degradation of okra fibers were compared using the \(\alpha\)s-\(\alpha\) evaluation method [43; 44], which compares the thermal reactivity of different substances to a reference base. The data obtained from the TG of water-retted bottom okra best fiber (BBW) were used as the reference base and denoted by \(\alpha\)r, whereas the other five were denoted by \(\alpha_{\rm s}\). The \(\alpha\) coefficient was determined using the following equation:  
\[\alpha=(w_{\rm i}-w)/(w_{\rm i}-w_{\rm f}) \tag{1}\]  
where:  
\(w\) is the mass fraction of a substance at a given temperature,  
\(w_{\rm i}\) is the mass fraction of the substance at the initial temperature,  
\(w_{\rm f}\) is the mass fraction of the substance at the final temperature. The plots of \(\alpha_{\rm s}\) vs. \(\alpha_{\rm r}\) (Figure 8) were prepared for the main transformation range of a TG run. As one can see in Figure 8, dew-retted middle okra best fiber (MBD) has slightly higher thermal stability than the reference sample, whereas the other samples are less stable. The UBW sample (water-retted upper okra best fiber) is the least stable. A similar conclusion can be drawn from the \(\alpha\)s coefficient temperature plot (Figure 9). TG measurements show that with regard to their thermal stability, the investigated fibers can be arranged as follows: MBD \(>\) MBW \(>\) UBD \(>\) BBD \(>\) BBW \(>\) UBW. As can be seen in Figures 8 and 9, this order does not change throughout the whole course of the main thermal degradation stage, which leads to the conclusion that fibers coming from the middle part of the okra best plant are very slightly better in thermal stability than those coming from other parts.