## 3 Results and Discussion
The two peaks that are located around 1222 cm\({}^{-1}\) and 1369 cm\({}^{-1}\) are attributed to the C-CH\({}_{3}\) and C-O stretching bands of acetyl groups [17; 26]. Overall, these three peaks confirm that kenaf had been acetylated. In addition, the absence of the peak around 1700 cm\({}^{-1}\) means that no detectable acetic acid by-product remained on the fibers after the acetylation treatment. Also, peaks around 1840-1760 cm\({}^{-1}\) were not observed, suggesting that the fibers were free from unreacted acetic anhydride [19]. The OH peak (\(\sim\)3350 cm\({}^{-1}\)) of 0.5 h acetylated kenaf was only slightly lower than that of untreated fibers. On the other hand, kenaf modified for 1-3 h exhibited significantly reduced OH peak intensity. Figure 4 shows the degree of acetylation of kenaf fibers. The acetyl content of the fibers for acetylation of 0.5 h was the lowest of all (14%), corresponding to the higher hydroxyl availability (lower substitution) of these fibers, as revealed from FTIR analysis (Figure 3). These complementary results, in combination to the surface morphology observation, could explain the reduced strengths of the PLA composites containing 0.5 h acetylated kenaf discussed earlier (Figure 1). With low  
Figure 2: Scanning Electron Microscopy (SEM) micrographs of kenaf with different acetylation reaction times. acetylation levels, there were no benefits with respect to improved kenaf-PLA compatibility, while the accompanying morphology changes (surface smoothening; see Figure 2b) could detrimentally reduce the surface area for bonding and the possibility of mechanical interlocking in adhesion. Figure 4 also shows that, as the acetylation time was increased, the acetyl content increased correspondingly until it reached a plateau (28-29%) beyond 2 h of treatment. Once again, this result is in accordance with the mechanical property changes: (1) when compared to 0.5 h acetylated fibers, kenaf modified for 1-3 h contained more acetyl groups, making it more hydrophobic (compatible) for adhesion interaction with PLA, thus improving the strength of PLA composites; and, (2) kenaf modified beyond 2 h exhibited no further increase in acetyl content, and therefore there was no significant improvement in composite strength with 3 h of acetylation. Overall, it can be concluded that acetylation for 2 h is optimal when considering time and energy consumption, as well as the mechanical effects on the PLA composites. The DS max of cellulose is 3. It was confirmed that the total DS is calculated through the mass change of the acetylation part, and finally, it increases to 1.5. This means that about 50% of the OH group is acetylated. DS can be used as an index for various composite materials [27]. Since there is too much damage to the fiber to increase the DS, an unconditionally high DS is not effective. To examine the degree of adhesion between kenaf and PLA, fractured surfaces of the flexural test specimens of composites were imaged using FESEM. As seen in Figure 5a, the untreated kenaf-PLA composite showed rugged surfaces containing numerous voids, indicating that fibers were pulled out  
Figure 4: Acetyl content of acetylated kenaf fibers as a function of acetylation reaction time. Figure 3: Infrared spectroscopy of untreated and acetylated kenaf fibers. from the PLA matrix during flexural tests due to weak interfacial adhesion. PLA composites containing kenaf pre-acetylated for one hour or shorter also indicated similar morphologies (Figure 5c,e). Micrographs of higher magnification (Figure 5b,d,f) show the fracture surfaces of the afore-mentioned composites, including those of untreated kenaf containing cavities and edges, owing to poor interfaces. Poor interfaces are known to impede stress transfer from polymers to fibers. This manifests through compromised strength--when the composites are mechanically tested, the mechanical strength is decreased. This is because the stress does not transfer from the polymer to fiber well [28].