At present, composites reinforced with okra fibers are not among the materials used in high-temperature applications. However, determining their full thermal degradation characteristics will enable better understanding of this process, and will assist in establishing the boundary conditions that determine the practical application of these fibers and composites.
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 van Krevelen [
36] and Coats and Redfern [
37].
Figure 7 and
Table 3 show examples of thermogravimetric curves for the distribution of samples tested under thermo-oxidation conditions and characteristic temperatures for thermal decomposition, respectively.
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 bast fibers, the temperature range for each of the thermal characteristics is about 20 °C: the start of decomposition is 275–298 °C, T50% is 342–371 °C and the end of the process is 375–399 °C. The thermal degradation of fibers obtained by the water-retting method occurs slightly earlier (T50% = 354.5 °C) than those obtained by the dew-retting technique (T50% = 359.4 °C).
The processes of the thermal degradation of okra fibers were compared using the αs–αr 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 bast fiber (BBW) were used as the reference base and denoted by αr, whereas the other five were denoted by
αs. The
α coefficient was determined using the following equation:
where:
w is the mass fraction of a substance at a given temperature,
wi is the mass fraction of the substance at the initial temperature,
wf is the mass fraction of the substance at the final temperature.
The plots of
αs vs.
αr (
Figure 8) were prepared for the main transformation range of a TG run. As one can see in
Figure 8, dew-retted middle okra bast 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 bast fiber) is the least stable. A similar conclusion can be drawn from the αs 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
Figure 8 and
Figure 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 bast plant are very slightly better in thermal stability than those coming from other parts.
The main stage of the thermal decomposition under thermo-oxidation 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 4.
Based on the data shown in
Table 4, the activation energy values at the given conditions (kinetic parameters significantly depend on the heating rate, as well as on the assumptions adopted in a given calculation method [
45]), are at the 36–84 kJ/mol level. The large range of spread suggests that the assumptions associated with this method [
37] cause a moderate adaptability of this method to the tested samples. However, while elaborating on the results of thermal tests in air, a coefficient of determination at the 98–99% level was found with the Coats-Redfern method. The authors of that method used an integration procedure. They obtained the following equation:
where:
A—Pre-exponential factor (min−1)
α—Degree of conversion or fractional mass loss
β—Heating rate (K min−1)
E—Apparent activation energy (kJ mol−1)
R—Gas constant 8.3136 (J mol−1K−1)
T—Temperature (K)
By plotting ln (α/T
2) = f(1/T), E value can be calculated. It must be remembered that the equation is only true for zero reaction order, which results from the former simplifications. The results obtained by this method are true for low α, but they can be generalized for the whole process assuming that the reaction mechanism does not change during the reaction. From a practical point of view, this method is moderately laborious. It requires taking the
α values from the thermogram and doing the necessary calculations to obtain the plot. Researchers have used this method, among others, to investigate poly (tetrafluoroethylene) [
37] and poly (3-dimethyloacryloyloxyethyl) ammonium propanosulfate [
46], and the results correspond to the results obtained from other calculation methods.
In comparing the values of the activation energy obtained for samples processed by the water- and dew-retting methods, we found an interesting relationship between the average EA results, which were 65.5 and 40.8 kJ/mol, respectively.
The results of the thermal decomposition analysis of okra fiber samples from different parts of the plant in the presence of oxygen have not yet been published. From the results presented in this study, it can be concluded that regardless of the origin of the material, okra fibers can be used in composites up to a temperature of 290 °C. The activation energy of the decomposition process varies depending on how the fiber is pretreated.
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 bast fibers obtained by the water-retting method is 378.8 °C, while for the fibers obtained by dew-retting it is 383.2 °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).
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 °C [
22] and in the 329.8–349.9 °C range (depending on the treatment technique) [
23].
The main thermal degradation step is presented in the form of α
s = f(α
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. However, some minor differences can be seen in
Figure 11. In ambient conditions, the UBD (dew-retted upper okra bast fiber) sample has slightly higher thermal stability than the reference sample. Taking the αs = f(αr) plot into account, fibers can be arranged as follows: UBD > MBD > MBW > UBW > BBW > BBD. As can be seen in
Figure 11, this order does not change throughout the whole course of the main thermal degradation stage in nitrogen conditions, which leads to a similar conclusion as for investigation in air, that fibers coming from the middle part of okra bast plant are very slightly better in regards to thermal stability than those coming from other parts.