*3.2. Morphology of PBS/CSWF Composite Films*

Optical microscopy was used to examine the morphologies of PBS/CSWF composite films. Figure 4a shows OM images of PBS/CSWF composite films at 10× and 40× magnification. It was determined that CSWF was well-dispersed in the PBS matrix at 5 and 10 phr, with some CSWF aggregated at 15 phr. Furthermore, the results indicated that the composite film had no flaws between the PBS and CSWF interfacial surfaces. It was due to the polar–polar interaction and hydrogen bonding between PBS and Brazilin

derivatives, an organic heterotetracyclic molecule found in CSWF. It was considered to be one of the reasons that enhanced the interfacial contact between PBS and CSWF. FTIR and TGA were two further techniques used to validate the interaction of PBS and fiber, as shown in Figures S1 and S2, respectively. Moreover, the ImageJ analysis findings were provided to access the behavior of the fiber aggregation.

**Figure 4.** (**a**) Optical microscope images of PBS composite films at magnification of 10× and 40×, and (**b**) fiber diameter and its aggregation area analyzed by ImageJ technique.

Figure 4b presents the outcomes of fiber diameter and aggregation area, which are summarized in Table 2. The counts of fiber and aggregation point were 49, 58 and 51 for PBS/CSWF5, PBS/CSWF10 and PBS/CSWF15, respectively. By increasing the CSWF content from 5 to 15 phr, the overall aggregation area and average size of CSWF in PBS matrix rose from 3644 to 14,273 µm<sup>2</sup> and 74.37 to 279.86 µm<sup>2</sup> , respectively. It was discovered that the area distribution was too high, and the SD value of all samples was greater than the average size of area. At this point, it was established that the distribution of fiber area was dramatically different, with PBS/CSWF15 showing the highest distribution of area. Furthermore, by increasing the fiber content from 5 to 15 phr, the cover area of the fiber in the film increases from 10.17% to 40.03%. The CSWF was entirely dispersed in the PBS

matrix for the PBS/CSWF5 composite film, which included the least amount of CSWF and low area distribution. In contrast, the dispersion in the PBS matrix was low in the PBS/CSWF15 film, and the fiber was tightly aggregated in the PBS matrix confirmed by their statistical analysis of fiber area.

**Table 2.** Summary results from ImageJ analysis.


#### *3.3. Mechanical Properties of PBS/CSWF Composite Films*

The mechanical characteristics of the PBS/CSWF composite films were evaluated using tensile strength and elongation at break, as shown in Figure 5. They were studied in two directions: machine direction (MD) and transverse direction (TD). Figure 5a presents the tensile strength of composite films. The tensile strengths of composite films with 0, 5, 10 and 15 phr are 4.77, 5.69, 12.21 and 2.16 N/mm<sup>2</sup> in MD and 2.21, 3.91, 6.51 and 0.43 N/mm<sup>2</sup> in TD, respectively. Tensile strength tends to improve with increasing CSWF content. PBS/CSWF10 had the highest tensile strength in both directions, reaching 12.21 N/mm<sup>2</sup> in MD and 6.51 N/mm<sup>2</sup> in TD. In contrast, the mechanical properties of the PBS/CSWF15 composite film were significantly reduced in both directions.

**Figure 5.** Mechanical properties of PBS/CSWF composite films—(**a**) tensile strength and (**b**) elongation at break.

Figure 5b presents the elongation at break results of the PBS/CSWF composite films. The elongations at break of the composite films with 0, 5, 10 and 15 phr are 10.21%, 11.11% 22.01% and 12.17% in MD and 3.68%, 5.07%, 5.53% and 4.19% in TD respectively. It resembled the tensile strength trend. PBS/CSWF10 had the highest elongation at break, with 22.01% of MD and 5.53% of TD, respectively. It could be described as a decrease in the PBS-to-fiber ratio and an increase in the aggregation area due to an increase in CSWF content from the OM image and ImageJ results. The optimal ratio and aggregation area of CSWF is 10 phr. As a result of CSWF overload and aggregation, the mechanical characteristics of PBS/CSWF15 were reduced. It is reasonable to conclude that an untreated CSWF has a high potential to be a bio-reinforcement.

#### *3.4. Biodegradation Properties of PBS/CSWF Composite Films*

The biodegradation process of the PBS/CSWF composite was monitored by soil burying for three months. The weight reduction of the composite films is shown in Figure 6a. The weights reductions of the composite films with 0, 5, 10 and 15 phr are 1.28%, 6.63%, 37.12% and 2.53% for the first month, 6.25%, 16.51%, 51.97% and 12.14% for 2 months and 19.33%, 42.85%, 100% and 13.1% for 3 months respectively. It was found that PBS/CSWF10 is the highest biodegradation rate. However, it was a surprise that the degradation rate of PBS/CSWF15 is lowest. This finding is supported and consistent with the mechanical properties. It might be because of the antibacterial abilities of Brazilin derivatives found in CSWF [12–14]. Composite films with CSWF contents ranging from 0 to 10 phr that released Brazilin derivatives onto the surface of the PBS/CSWF composite film were insufficient to inhibit the activity of bacteria attached to the composite film. Meanwhile, the PBS/CSWF15 film delivers sufficient Brazilin derivatives to the surface to inhibit bacterial activity. Figure 6b displays the appearance of composite films two months after degradation. According to the findings, PBS/CSWF10 degraded the fastest, whereas PBS/CSWF15 film degraded the slowest.

**Figure 6.** (**a**) The biodegradation progress of PBS/CSWF composite films with different burial times, and (**b**) The appearance of PBS/CSWF composite film after a 2-month biodegradation test.

#### **4. Conclusions**

In the absence of treatment, CSWF was well-dispersed in the PBS matrix and could be employed as a new bio-reinforcement. The morphology, the dispersion, and the aggregation of fibers have all been validated and studied. The addition of CSWF increased the properties of the composite film from 5 to 10 phr, whereas the addition of 15 phr decreased the properties. After three months, the PBS/CSWF10 had completely degraded. PBS/CSWF10 is the best composite film, according to the results. Therefore, CSWF has the potential to be a promising alternative addition for the industrial sector of green plastic film.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym14030499/s1, Figure S1: FTIR spectra of CSWF (orange), PBS (bleu), and PBS/CSWF composite films with 5 (green), 10 (black) and 15 (red) phr, Figure S2: Thermal degradation characteristics of PBS/CSWF composite film by TGA and DTG.

**Author Contributions:** E.M.: Writing—Review and Editing, Y.T.: Writing—Review and Editing, N.N.: Investigation, C.K.: Investigation, K.K.: Investigation, Y.N.: Conceptualization, Writing—Review and Editing, N.S.: Conceptualization, Investigation, Writing—Original Draft, Writing—Review and Editing, Supervision, Visualization. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study did not report any data.

**Acknowledgments:** The authors thanks to College of Industrial Technology, King Mongkut's University of Technology North Bangkok for providing the facilities.

**Conflicts of Interest:** The authors declare no conflict of interest.
