## 3 Results and Discussion  
Figure 1 presents the tensile and flexural strength of biocomposites as a function of acetylation reaction time. The non-acetylated kenaf-PLA composite showed tensile and flexural strengths of 58 MPa and 127 MPa, respectively. Upon a brief pre-treatment (0.5 h) of kenaf with acetylation, the composites suffered a loss in strength. As the kenaf acetylation time was increased, however, the composites were increasingly stronger and eventually, at 2 h acetylation, had strengths exceeding those of composites containing non-acetylated kenaf. The reduction in biocomposite strength as a response to initial acetylation of kenaf is attributable to the partial removal of extractives during the acetylation process. Removal of extractives was reported to be a possible explanation for the weight percent gain (WPG) being lower than acetyl content of wood at low levels of acetylation [22; 23]; the WPG and acetyl content should be similar if the mass change is merely due to the addition of acetyl groups. Further inferences in extractive removal will be presented later. From the perspective of the consequences of extractive removal, the \"bleached\" fibers could undergo mechanical or/and surface changes to influence their reinforcement effects and/or bonding with PLA. The enhancement in tensile and flexural strengths of the PLA composites with longer acetylation pre-treatment of kenaf is attributable to the improved kenaf-PLA interaction. Acetylation is known to result in the substitution of hydroxyls to make the modified fibers less hydrophilic. If kenaf fibers were made less hydrophilic, their compatibility with PLA would be improved, leading to an enhancement in adhesion, and thus also the strength of the resulting composites. No further strength enhancement was observed at acetylation beyond 2 h, probably because the hydroxyl group substitution was increasingly difficult; thus, interfacial compatibility was not further improved. Figure 1: Tensile and flexural strength of composites as a function of acetylation reaction time. To support the afore-mentioned hypothesis, untreated and acetylated fibers were analyzed using SEM, FTIR, and titration for acetylation. Figure 2 shows SEM micrographs of kenaf fibers acetylated for different time durations. As seen for untreated kenaf (Figure 2a), the fiber surface was covered with an unevenly distributed layer that is similar to the case of olive husk flour reported by [PERSON] et al. [24], who attributed it to the probable presence of waxy substances. After 0.5 h and 1 h of acetylation, the fiber surfaces became smoother (Figure 2b,c), suggesting the removal of extractive substances, thus supporting the postulation elaborated in the preceding paragraph. Upon longer acetylation (2 h and 3 h; Figure 2d,e, respectively), the fibers appeared rougher, possibly as a result of swelling that was caused by the pyridine catalysis used for the acetylation reaction. Indeed, pyridine is known to swell and open cell walls in fibers to ease penetration of acetic anhydride [19; 25]. In summary, fiber surface morphology was altered depending on the extent of acetylation; the morphology changes could be related to the chemistry occurring on and in the fibers. In order to support this, results of FTIR characterization will be discussed next. To confirm whether chemical modification took place in kenaf, the fibers were analyzed by FTIR-ATR. Figure 3 shows the FTIR spectra of the kenaf fibers before and after acetylation. The absorption peaks at about 2950 cm\({}^{-1}\) and 3350 cm\({}^{-1}\) observed for all of the fibers correspond, respectively, to symmetric C-H vibration and hydroxyl (OH) stretching. These are typical peaks of lignocellulosic. On the other hand, acetylated fibers showed peaks at 1740 cm\({}^{-1}\), 1369 cm\({}^{-1}\), and 1222 cm\({}^{-1}\), none of which were found in the case of untreated fibers. The peaks located around 1740 cm\({}^{-1}\) are attributed to the carbonyl stretching band (C=O) in ester bonds.