3.2.1. Mechanical Properties of Composites

In this study, the sample was prepared according to method 2 with different epoxidized Tung oil contents (0–15%), but the ratio of PLA/pulp fiber was fixed at 70/30. Physical properties such as tensile strength, flexural strength of PLA/SPF composites were measured to evaluated effect of epoxidized Tung oil. Results of the mechanical tests were presented in Table 6.

**Table 6.** Mechanical properties of PLA/SPF composites with different percentages of epoxidized Tung oil.


The results showed that the mechanical properties of the composites with ETO, used as a plasticizer, were higher than those without ETO. The tensile strength increased from 50.52 to 59.32MPa, elongation at break increased from 2.56 to 5.13%, the flexural strength increased from 98.3 to 114.7MPa and flexural modulus increased from 3.17 to 4.97 GPa when the ETO content increased from 5 to 10%, but the tensile modulus decreased from 3219.22 to 3043.54 MPa. This indicated that the ETO improved the mechanical properties of the composites, due to the fact that the ETO also acted as a plasticizer for PLA, so it increased the flexibility of matrix and reduced its free surface energy. This had also been proved by several other authors [30]. However, when the content of the ETO in fiber exceeded 15%, both strength and modulus reduced because it made the material softer. This phenomenon was also observed by Kirsi when using 12% of epoxidized linseed oil as a plasticizer for PLA/bleached softwood kraft pulp composites [26].

#### 3.2.2. Morphology Observation

The fracture surface morphology of the ETO soaked pulp fiber reinforced PLA composites with different epoxidized Tung oil contents were presented in Figure 8.

**Figure 8.** SEM picture of PLA/SPF composites with different contents of ETO.

The results showed that the composite without ETO can experience clear gaps between fiber and PLA. There was less phase separation between fiber and PLA when ETO was added to the number of fiber of 5–7.5%. When the number of fiber in ETO content was 10%, it was found that the pulp fibers showed more tight connections with PLA matrix. However, with ETO content with 15% of fiber, it seemed that the links between the pulp fiber and PLA were weakened, and there were gaps between the two surfaces.
