## 3 Results and Discussion
### Thermogravimetric Analysis of Composites  
The thermal stability of lignocellulosic-based polymer composites is crucial; since it helps in predicting the durability and performance of the composite material under certain temperature conditions, it guides the selection of appropriate manufacturing processes and it helps in determining the composites' appropriateness for certain applications. Figure 9 presents the TGA (Figure 9a) and dTG curves (Figure 9b) of HDPE and its hemp-based polymer composites. A gradual shift of curves toward lower temperatures upon fiber-content increase is obvious, and is associated with the fact that incorporated fibers have a distinct contribution to TGA profiles, since they thermally degrade at lower temperatures when compared to HDPE. The same behavior occurs in all fiber-based studied composites (Figure 10), where all the TGA curves of studied composites lie in the region between that of HDPE and the corresponding fiber. Figure 10: TGA curves of (**a**) kenaf-based composites, (**b**) ramie-based composites, and (**c**) flax-based composites filled with varying filler contents. Figure 9: (**a**) TGA and (**b**) dTG curves of hemp-based polymer composites filled with varying filler content. In general, the thermal degradation of fiber-based composites takes place in three steps, as seen in Figure 9b. The first one occurs at up to approximately 150 \({}^{\circ}\)C, and it is attributed to water loss. Furthermore, a distinct weight-loss curve appears at the region of 150-400 \({}^{\circ}\)C, which corresponds to thermal decomposition of the fibers' components, while the third degradation step in the 400-580 \({}^{\circ}\)C region is mainly due to the degradation of C-C bonds in the main chain of HDPE [74]. The main thermal parameters of all the composites are presented in Table 6 and in Figure 11. T2% and T5% stand for the temperatures where 2% and 5% of the initial mass has been lost, respectively, while Td,max refers to the decomposition temperature where the maximum mass loss is observed. It is well known that the thermal performance of each composite depends on the corresponding fiber's chemical composition [16] and mainly on (a) the hemicellulose content of the fiber, since hemicellulose is responsible for mass loss at relatively low temperatures, exhibiting the least thermal resistance among all the fibers' components and (b) on the cellulose content, since it enhances the thermal stability of the composites. Therefore, fibers with a higher hemicellulose content, which means a lower cellulosic content and a lower % cellulose crystallinity, are susceptible to thermal degradation at low temperatures. As is obvious from Figures 9 and 10, the incorporation of lignocellulosic fibers negatively influences thermal stability of the matrix, since, while the mass loss of HDPE begins \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sample** & **T2\%** & **T5\%** & **Td,max** \\ \hline HDPE & 447 & 466 & 516 \\ \hline HDPE\_10\_kenaf & 309 & 367 & 509 \\ HDPE\_20\_kenaf & 281 & 327 & 509 \\ HDPE\_30\_kenaf & 274 & 312 & 509 \\ \hline HDPE\_10\_hemp & 356 & 382 & 520 \\ HDPE\_20\_hemp & 337 & 356 & 520 \\ HDPE\_30\_hemp & 324 & 344 & 523 \\ \hline HDPE\_10\_ramie & 365 & 388 & 518 \\ HDPE\_20\_ramie & 345 & 362 & 522 \\ HDPE\_30\_ramie & 313 & 337 & 511 \\ \hline HDPE\_10\_flax & 337 & 367 & 507 \\ HDPE\_20\_flax & 291 & 342 & 510 \\ HDPE\_30\_flax & 274 & 325 & 508 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Thermal properties of the prepared composites. Figure 11: (**a**) The temperatures corresponding to 2% mass loss (T2%) and (**b**) the decomposition temperature representing the maximum mass loss (Td,max) of all HDPE composites. at 415 \({}^{\circ}\)C, all fiber-based composites start losing weight almost immediately upon heating, due to the evaporation of water. Therefore, according to Table 6 and Figure 11a, the T2% values of all composites are lower than the corresponding values of HDPE, and thermal stability is decreased upon increasing filler content.