## 3 Results and Discussion
### TGA of Natural Fibers  
TGA was employed in order to study the thermal stability and degradation profiles of natural fibers (Figure 7a), as well as the mass loss of fibers (%) at the processing temperature of the composites (Figure 7b). It is obvious that all the fibers have a highly similar weight-loss behavior, justifying the need to determine the distinct contribution of each particular component to the thermal performance/behavior of the natural fibers. So, the degradation curves of the fibers are characterized by three distinctive zones. Zone I corresponds to temperatures lower than 200 \({}^{\circ}\)C and is mainly attributed to the moisture loss of the fibers [63; 64; 65; 66]. Zone II is the region between 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, and its main characteristic are demonstrated by two peaks; one around 300 \({}^{\circ}\)C, which corresponds to hemicellulose and pectin content, and another sharper peak which corresponds to the degradation of  
Figure 6: Deconvolution of XRD profiles of (**a**) ramie, (**b**) hemp, (**c**) flax, and (**d**) kenaf, including the identification of both crystalline and amorphous phases. \begin{table}
\begin{tabular}{l c} \hline \hline
**Sample** & **Cr.I. (\%)** \\ \hline ramie & 81 \\ \hline hemp & 80 \\ \hline flax & 77 \\ \hline kenaf & 63 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Crystallinity indexes of natural fibers. cellulosic content [63; 65; 67]. At higher temperatures, and over the broad temperature range of Zone III, the thermal degradation of lignin takes place [68]. The processing temperature of composites is considered to be of great importance for the manufacturing of reliable composites. Components of natural fibers start to degrade at relatively low temperatures, where mechanical and thermal properties drastically change [69; 70]. The thermal degradation of fibers inside the polymer matrix results in volatile production, leading to porous composites with lower densities and inferior mechanical properties. Therefore, in order to ensure the integrity of the natural composites, it is important to verify that the plant fibers of the composite materials can withstand the heat required during the manufacturing process [71]. The processing temperature of the present study's HDPE_xx_fiber composites must be noticeably well above the melting temperature of HDPE at 134 \({}^{\circ}\)C, as already stated, but also low enough to avoid any thermal degradation to the fibers, which may lead to negative effects on the major properties of the composites [70]; this is why composite preparation was performed at 190 \({}^{\circ}\)C. In Figure 7b, apart from the mass loss of all the fibers at low temperatures--which correlates to moisture content--the TGA curves of all the studied fibers at the processing temperature of composites are also displayed, where it is more than evident that the fibers' behavior differs radically. It is obvious that at 190 \({}^{\circ}\)C, the fiber with the least thermal degradation is ramie, while the fiber with the greatest thermal impact is kenaf, having lost almost 5.5% of its initial mass. The dTG curves of all studied fibers are exhibited in Figure 7c, where differences for the samples are attributed to content variations. According to the TGA curve of  
Figure 7: (a) TGA curves of lignocellulosic fibers characterized by three distinctive degradation zones; (b) magnified mass loss of fibers at low temperatures (including processing temperature); (c) dTG curves of all studies fibers; and (d) mass loss of fibers observed at 190 \({}^{\circ}\)C.  
Figure 7a, mass loss in Zone I is due to the evaporation of absorbed water, while in Zone II there is a shoulder caused by overlapped peaks, mainly caused by hemicellulose, and a sharp peak representing the decomposition of cellulose. Finally, in Zone III, the pyrolysis of lignin is displayed as one long tailing [72].