3.1.1. Mechanical Properties of Composites

In this study, the samples were prepared with different pulp fiber contents (ranging from 0% to 50%), and the epoxidized Tung oil was fixed at 10% of fiber. The mechanical properties of neat PLA and PLA/PF composites were presented in Table 3.


**Table 3.** Effect of pulp fiber contents on the mechanical properties of PLA/PF composites.


**Table 3.** *Cont.*

It can be seen that the tensile strength of neat PLA is lower than that of PLA/PF composites. The tensile strength of PLA/PF composites increased significantly by increasing the percentage of pulp fiber content up to 30 wt% and then decreased by further addition of pulp fiber. When the pulp fiber contents were from 10 to 30%, the tensile strength of the composites increased from 45.74 MPa to 56.61 MPa (for PLA/NPF composites) and to 59.32 (for PLA/SPF composites). The increase of tensile modulus of PLA/PF composites was in proportion to the increase of pulp fiber content. These revealed that the addition of pulp fibers into PLA matrix provided effective reinforcement. This was because the stress was expected to transfer from the matrix to the strong fiber. Huda M. S. et al. suggested the better the alignment of the fibers, the higher the strength value [20]. However, when the pulp fiber was added to more than 30%, the tensile properties of composite decreased. This might be due to the poor dispersion of fiber into the PLA matrix at higher pulp content. This result was consistent with the results of Jin Qian et al. for-cotton fiber/PLA composites [21]. However, Zhaozhe Yang et al. found that the tensile strength of both PLA/pulp fiber and PLA/wood fiber composites decreased with the increase of fiber content [15]. The results also showed that the tensile strength and modulus of composites, with untreated or soaked pulp fibers, were not significantly different with a fiber content of less than 30%, but those of composites, containing soaked pulp fibers, were higher than those of composites containing untreated pulp fibers. This might be due to the fact that epoxidized Tung oil-treated pulp fibers were more evenly dispersed in PLA matrix, and Tung oil improved the interaction between PLA matrix and pulp fiber. The chemical interaction mechanism among epoxidized vegetable oil, PLA and natural fiber was proposed by Buong Woei Chieng et al. [22] (Figure 2) and Omid Nabinejad et al. [23] (Figure 3).

**Figure 2.** Suggested chemical interactions between epoxidized vegetable oil and PLA by Buong Woei Chieng et al. [22].

**Figure 3.** Suggested chemical interactions between epoxidized vegetable oil and mercerized natural fiber by Omid Nabinejad et al. [23].

Elongation at the break of the PLA/PF composites was also tested and shown in Table 3. The elongation at break of composites decreased as the fiber content in the composites increased.

According to the results, the flexural modulus of the composites increased significantly by increasing the pulp fiber content, while the flexural strength of the composite had non-significant change. However, both the flexural strength and flexural modulus of the composites were higher than that of the neat PLA. This could be explained that the addition of pulp fiber promoted the nucleation and crystallization of PLA matrix, so that the flexural modulus of the composites improved. This indicated that the pulp fiber acted as a rigid filler, which increased the stiffness of the composites. Similarly to the tensile properties, the flexural strength and modulus of PLA/SPF composites were slightly higher than that of PLA/NPF composites. When the content of pulp fiber was 30%, the flexural strength of PLA/NPF and PLA/SPF composites reached 109.5 and 114.7 Mpa respectively, which increased by 15.3% and 20.7% compared with the pure PLA. This result was consistent with the results of Buket Okutan Baba and Ugur Özmen for chicken feather/PLA composites [24].
