## 3 Results and Discussion
### XRD of Natural Fibers  
The XRD measurements of natural fibers were performed in order to estimate the fibers' crystalline index. All XRD diffractograms were subjected to baseline-correction, and subsequently to peak separation using a Gaussian deconvolution process. The curve-fitting was set, assuming a broad band as the amorphous contribution. The crystallinity index (Cr.I.) of the fibers is calculated by Equation (1), according to [PERSON] et al. [54]. \[\mathrm{Cr.I.}=\frac{\mathrm{A_{\mathrm{cryst}}}}{\mathrm{A_{\mathrm{total}}}} \tag{1}\]  
where Cr.I. stands for the crystalline index, \(\mathrm{A_{\mathrm{cryst}}}\) corresponds to the sum of the bands' areas assigned to crystalline contributions, and \(\mathrm{A_{\mathrm{total}}}\) stands for the total area of the diffractograms. XRD is a prominent method for the indirect determination of the crystallinity of natural fibers, or, to be more specific, for the ascertainment of the fraction of crystalline cellulose within the entire natural fiber's content. It is of great importance to mention that among fiber components, hemicellulose and lignin are non-crystalline [55], while only cellulose is crystalline. Consequently, a fiber's crystallinity refers henceforth to the mass fraction of crystalline cellulose within the fiber. The Cr.I. is used to describe the relative amount of crystalline material in lignocellulosic fibers. Its calculation is necessary, since cellulosic content is greatly linked to respective mechanical performance. In particular, the fibers' stiffness rises and their flexibility is decreased by increasing the ratio of crystalline to amorphous material percentages [56]. The X-ray-diffractometry profiles of the studied natural fibers are shown in Figure 5. A series of peaks is observed, which are displayed in Table 4, with their intensities and  
Figure 4: SEM images of (**a,b**) kenaf and (**c,d**) hemp, where both individual fibers and bundles are observed. their widths varying, as shown in Figure 5. The main contributors to the observable diffraction pattern are three peaks with (\(1\overline{1}0\)), (\(110\)), and (\(200\)) Miller indices, with the first two peaks being overlapped. Those peaks are observed from the lowest angle in the aforementioned order. Furthermore, the contribution of the diffraction peak with a (\(102\)) Miller reflection is obvious, although seen as a shoulder due to extensive overlapping with the (\(200\)) diffraction peak. The (\(102\)) diffraction peak, which is scarcely reported, is detected on randomly oriented powder samples of \(\mathrm{I}_{\mathrm{B}}\) cellulose [57]. There are various methods which may be implemented for the calculation of cellulose crystallinity. The Segal peak height method [58] is the most common method, though it has been proved to poorly achieve a reliable calculation of crystalline contribution, leading to great errors [59; 60]. According to [PERSON] et al. [61], the peak-fitting method is consistent enough. Consequently, peak deconvolution was used for the thorough examination of natural fibers' diffraction bands and the establishment of both the crystalline and amorphous phases. The diffractograms are shown in Figure 6, and they are deconvoluted with Gaussian-profiled peaks, while a wide Gaussian peak is set between the cellulose (\(200\)) diffraction peak and the (\(1\overline{1}0\)) and (\(110\)) diffraction peaks, assigned to the amorphous contribution [62]. After the deconvolution of XRD patterns and the calculation of the crystallinity index of the fibers according to Equation (1), a comparison was performed and tabulated in Table 5, where it becomes obvious that ramie and hemp exhibit greater amounts of crystallinity, and therefore greater amounts of cellulose, while the crystallinity index of flax is decreased compared to the aforementioned fibers, and kenaf has the lowest crystallinity index when compared to the rest of the fibers.