## 3 Results and Discussion
### Mechanical Properties of Composites  
The mechanical performance of composites depends on a series of key parameters, such as the (1) particle size, (2) volume fraction, (3) orientation of the filler, (4) mechanical characteristics of both the filler and the matrix, (5) filler-matrix adherence, (6) degree of filler dispersion, etc. [12]  
It is well known that the adhesion strength at the interface of composites' components determines the load transfer between them, which is a crucial parameter for composite strength and toughness. On the contrary, Young's modulus is not affected by adhesion quality, because debonding is not observed for small displacements [75]. So, since no treatment is applied to the natural fibers in the present study, it is expected that the strength and toughness of composites will be inferior to the corresponding ones of neat HDPE, while the elastic modulus values of the composites are expected to demonstrate an improved behavior compared to that of the neat polymer. The mechanical properties of HDPE_xx_fiber composites are presented in Table 7 and Figure 12.  
Figure 12a displays the [PERSON]'s modulus of HDPE and its composites. The introduction of different kinds of fibers in HDPE leads to a gradual increase in the elastic modulus in almost every studied composite, which is in all cases higher than that of neat HDPE. More specifically, for 10, 20, and 30 wt.% of ramie fibers, [PERSON]'s modulus was found to be 79, 94, and 89% higher than that of the neat matrix, with the HDPE_30_ramie being \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{\begin{tabular}{c} **Tensile Stress at Break** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Elongation at Break** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Young’s Modulus** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Impact Strength** \\ **(/m)** \\ \end{tabular} } \\ \hline HDPE & 36.1 \(\pm\) 1.5 & 716.7 \(\pm\) 24.8 & 678 \(\pm\) 56 & 577.3 \\ \hline HDPE\_10\_kenaf & 24.8 \(\pm\) 1.1 & 2.9 \(\pm\) 0.3 & 721 \(\pm\) 127 & 96.3 \\ HDPE\_20\_kenaf & 19.9 \(\pm\) 1.3 & 1.7 \(\pm\) 0.1 & 789 \(\pm\) 90 & 84.9 \\ HDPE\_30\_kenaf & 17.5 \(\pm\) 1.5 & 1.7 \(\pm\) 0.2 & 991 \(\pm\) 111 & 75.8 \\ \hline HDPE\_10\_hemp & 20.6 \(\pm\) 1.7 & 5.5 \(\pm\) 0.6 & 847 \(\pm\) 87 & 89.3 \\ HDPE\_20\_hemp & 25.9 \(\pm\) 2.1 & 3.6 \(\pm\) 0.3 & 1206 \(\pm\) 105 & 81.0 \\ HDPE\_30\_hemp & 26.0 \(\pm\) 1.8 & 2.7 \(\pm\) 0.2 & 1330 \(\pm\) 101 & 48.7 \\ \hline HDPE\_10\_ramie & 29.5 \(\pm\) 1.6 & 5.4 \(\pm\) 0.6 & 1213 \(\pm\) 95 & 90.8 \\ HDPE\_20\_ramie & 23.2\(\pm\) 1.8 & 3.6 \(\pm\) 0.3 & 1317 \(\pm\) 121 & 63.6 \\ HDPE\_30\_ramie & 25.0 \(\pm\) 1.9 & 2.5 \(\pm\) 0.2 & 1279 \(\pm\) 173 & 53.5 \\ \hline HDPE\_10\_flax & 27.0 \(\pm\) 1.4 & 5.1 \(\pm\) 0.5 & 1128 \(\pm\) 114 & 81.6 \\ HDPE\_20\_flax & 27.8 \(\pm\) 1.2 & 3.8 \(\pm\) 0.3 & 1266 \(\pm\) 163 & 70.5 \\ HDPE\_30\_flax & 28.1 \(\pm\) 2.0 & 3.4 \(\pm\) 0.5 & 1383 \(\pm\) 135 & 66.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Mechanical properties of the prepared composites. the only composite that displays a slight decrease compared to its 20 wt.% corresponding composite. A gradual increase in terms of the elastic modulus was observed for 10, 20, and 30 mass% of hemp fibers (23, 78 and 96%) and of flax fibers (66, 87 and 104%). Regarding the kenaf fibers, the same increasing trend is observed in terms of the elastic modulus, too, but to a lower extent, since the corresponding composites were found to be 6, 16, and 46% higher than that of neat HDPE. This is due to the fact that kenaf exhibits the lowest cellulose crystallinity (63%) compared to the rest of the fibers, as shown by the XRD deconvolution method, and also kenaf fibers possess a greater diameter compared to the rest of the fibers, as shown by the SEM microphotographs. However, in all cases, stiffer composites are prepared when compared to the neat HDPE.