*3.7. Soil Burial Test*

biodegradation properties of PBS, PBAT, and PBS/PBAT blends. Progressive fragmentation was noticed in all the samples as they gradually degraded within 6 months. At the end of 6 months, only a small amount of residual debris remained. The color of the film gradually became brownish with increasing burying time. For TPS36/24, the mass loss percentage reached 97.06% after 6 months, which was in agreement with the results of Dammak et al. [55], indicating a fully biodegradable characteristic of this material. As for TPRH36/24, a similar result was obtained with a maximum mass loss of 92%. TPS48/12 and TPRH48/12 showed a much lower mass loss, indicating higher PBS The entire composites showed a smoother surface before the degradation process. The mass change in the PBS/PBAT blended composites as a function of degradation time is shown in Table 5. The macroscopic appearance of biodegradation in PBS/PBS blended composites at different burying time is shown in Figure 8. Matting of the sample surface and color change was noticeable after the degradation process. The mass loss percentage increased with increasing burying time for the entire samples, which affirmed the biodegradation properties of PBS, PBAT, and PBS/PBAT blends. Progressive fragmenta-

tion was noticed in all the samples as they gradually degraded within 6 months. At the end of 6 months, only a small amount of residual debris remained. The color of the film gradually became brownish with increasing burying time. For TPS36/24, the mass loss percentage reached 97.06% after 6 months, which was in agreement with the results of Dammak et al. [55], indicating a fully biodegradable characteristic of this material. As for TPRH36/24, a similar result was obtained with a maximum mass loss of 92%. TPS48/12 and TPRH48/12 showed a much lower mass loss, indicating higher PBS content expedite the degradation process. The bare PBAT, PBS, and commercial PBAT showed mass loss at 8.9, 9.2, and 32.51%, respectively, after 6 months. *Polymers* **2021**, *13*, x 15 of 20 92%. TPS48/12 and TPRH48/12 showed a much lower mass loss, indicating higher PBS content expedite the degradation process. The bare PBAT, PBS, and commercial PBAT


**Table 5.** Mass loss percentage of PBS/PBAT blends after soil burial test for six months.

showed mass loss at 8.9, 9.2, and 32.51%, respectively, after 6 months.

**Figure 8.** Macroscopic appearance of biodegradation in the soil at different burying times. **Figure 8.** Macroscopic appearance of biodegradation in the soil at different burying times.

As it was evident from the appearance, PBS/PBAT/TPRH and PBS/PBAT/TPS samples showed faster degradation rates than bare PBAT and PBS. TPS and TPRH are As it was evident from the appearance, PBS/PBAT/TPRH and PBS/PBAT/TPS samples showed faster degradation rates than bare PBAT and PBS. TPS and TPRH are the

the nutrient source for microorganisms, thus it provides more degradation sites to be

PBS/PBAT blends [11]. This causes the polymer chains to split into lower molecular weight oligomers, monomers, dimmers, and finally mineralized to CO2 and H2O [57]. The result also indicated that the utilization of rice husk with a high amount of PBS has the potential to degrade faster and is comparable with TPS (refer to the mass loss for TPRH36/24 and TPS48/12). Although TPS36/24 and TPRH36/24 showed higher

nutrient source for microorganisms, thus it provides more degradation sites to be attacked by microorganisms [56]. When microorganisms consume the TPS and TPRH, they leave the polymer matrix more porous, which accelerates the biodegradation rate of PBS/PBAT blends [11]. This causes the polymer chains to split into lower molecular weight oligomers, monomers, dimmers, and finally mineralized to CO<sup>2</sup> and H2O [57]. The result also indicated that the utilization of rice husk with a high amount of PBS has the potential to degrade faster and is comparable with TPS (refer to the mass loss for TPRH36/24 and TPS48/12). Although TPS36/24 and TPRH36/24 showed higher crystallinity and lower moisture absorption, they exhibited a higher mass loss percentage than TPS48/12 and TPRH48/12. This is because of the aromatic structure of PBAT, which decreases the mobility of polymer chains, reducing the degradation rate of polymer matrix [31]. *Polymers* **2021**, *13*, x 16 of 20 crystallinity and lower moisture absorption, they exhibited a higher mass loss percentage than TPS48/12 and TPRH48/12. This is because of the aromatic structure of PBAT, which decreases the mobility of polymer chains, reducing the degradation rate of polymer matrix [31].

The sample prone for degradation (TPRH36/24) was selected for FT-IR analysis to observe the changes in the intensity of certain transmittance peak, the formation of new peaks, or migration of the peak position before and after degradation. Figure 9a shows the FT-IR spectra of TPRH36/24 before biodegradation, while Figure 9b depicts FT-IR spectra of TPRH36/24 after 6 months of degradation. It was found that the highly intense -CH<sup>2</sup> stretching vibration position of the intrinsic polymer diminishes and migrated to 2974 cm−<sup>1</sup> , indicating a significant degradation process of the sample. Moreover, the less intense carbonyl region of the C = O group migrated from 1694 cm−<sup>1</sup> to a highly intense and broader peak at 1728 cm−<sup>1</sup> , affirming the process of degradation has occurred. The emergence of the new peak was noticed at 3708 cm−<sup>1</sup> after degradation, which corresponds to the O-H group of absorbed water in the polymer matrix [38]. The disappearance of the -CH<sup>2</sup> peak at 2846 cm−<sup>1</sup> indicated the degradation of TPRH and TPS. The changes observed in the FT-IR spectra are in agreement with the polymer oxidation degradation process reported by Celina et al. [58], which showed the dominant carbonyl formation with the diminishing C-H bands. The FTIR spectra of TPS36/24 before and after biodegradation are shown in Figure 10a,b, respectively. The results show that there was no significant difference between TPRH36/24 and TPS36/24 before and after biodegradation. This suggests the potential of using rice husk waste to swap starch in biodegradable polymer composites. The sample prone for degradation (TPRH36/24) was selected for FT-IR analysis to observe the changes in the intensity of certain transmittance peak, the formation of new peaks, or migration of the peak position before and after degradation. Figure 9a shows the FT-IR spectra of TPRH36/24 before biodegradation, while Figure 9b depicts FT-IR spectra of TPRH36/24 after 6 months of degradation. It was found that the highly intense -CH2 stretching vibration position of the intrinsic polymer diminishes and migrated to 2974 cm−1, indicating a significant degradation process of the sample. Moreover, the less intense carbonyl region of the C = O group migrated from 1694 cm−1 to a highly intense and broader peak at 1728 cm−1, affirming the process of degradation has occurred. The emergence of the new peak was noticed at 3708 cm−1 after degradation, which corresponds to the O-H group of absorbed water in the polymer matrix [38]. The disappearance of the -CH2 peak at 2846 cm−1 indicated the degradation of TPRH and TPS. The changes observed in the FT-IR spectra are in agreement with the polymer oxidation degradation process reported by Celina et al. [58], which showed the dominant carbonyl formation with the diminishing C-H bands. The FTIR spectra of TPS36/24 before and after biodegradation are shown in Figure 10a,b, respectively. The results show that there was no significant difference between TPRH36/24 and TPS36/24 before and after biodegradation. This suggests the potential of using rice husk waste to swap starch in biodegradable polymer composites.

**Figure 9.** FT-IR spectra of the PBS/PBAT/TPRH composites (**a**) TPRH36/24 before degradation, (**b**) TPRH36/24 after degradation with changes in functional group. **Figure 9.** FT-IR spectra of the PBS/PBAT/TPRH composites (**a**) TPRH36/24 before degradation, (**b**) TPRH36/24 after degradation with changes in functional group.

#### **4. Conclusions 4. Conclusions**

In this study, it was found that the incorporation of 40% rice husk was able to substitute starch-based biodegradable polymer. Optimization of the ratio PBAT:PBS to 36:24 expedited the biodegradation rate of the samples. PBAT:PBS blends with a 36:24 ratio showed 97.06% mass loss for TPS and 92% for TPRH. A comparable amount of PBAT and PBS allowed the formation of co-continuously phases to improve the mechanical properties. The bio-composite TPRH36/24 possessed good mechanical properties such as tensile strength (14.27 MPa), Young's modulus (200.43 MPa), and elongation at break (12.99%), which is adequate for the manufacturing of molded products such as a tray, lunch box, and straw. Finally, it achieved a 92% mass loss after six months, evidencing itself as a biodegradable material. The test results from this study indicated an accomplishment in the fabrication of cost-efficient biodegradable polymer using waste fillers, which has tremendous potential for practical use in various industrial applications. In this study, it was found that the incorporation of 40% rice husk was able to substitute starch-based biodegradable polymer. Optimization of the ratio PBAT:PBS to 36:24 expedited the biodegradation rate of the samples. PBAT:PBS blends with a 36:24 ratio showed 97.06% mass loss for TPS and 92% for TPRH. A comparable amount of PBAT and PBS allowed the formation of co-continuously phases to improve the mechanical properties. The biocomposite TPRH36/24 possessed good mechanical properties such as tensile strength (14.27 MPa), Young's modulus (200.43 MPa), and elongation at break (12.99%), which is adequate for the manufacturing of molded products such as a tray, lunch box, and straw. Finally, it achieved a 92% mass loss after six months, evidencing itself as a biodegradable material. The test results from this study indicated an accomplishment in the fabrication of cost-efficient biodegradable polymer using waste fillers, which has tremendous potential for practical use in various industrial applications.

investigation, S.Y.Y.; methodology, S.Y.Y.; project administration, S.S.; supervision, S.S.; writingoriginal draft, S.Y.Y.; writing-review and editing, S.S. and M.H. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Ministry of Education for funding under the Public-Private Research Network (PPRN) 2.0 grant number 304/PBAHAN/6314053. **Author Contributions:** Conceptualization, S.Y.Y. and S.S.; formal analysis, S.Y.Y., K.S., and M.T.O.; investigation, S.Y.Y.; methodology, S.Y.Y.; project administration, S.S.; supervision, S.S.; writing original draft, S.Y.Y.; writing-review and editing, S.S. and M.H. All authors have read and agreed to the published version of the manuscript.

**Author Contributions:** Conceptualization, S.Y.Y. and S.S.; formal analysis, S.Y.Y., K.S., and M.T.O.;

**Institutional Review Board Statement:** Not applicable. **Funding:** This research was funded by the Ministry of Education for funding under the Public-Private Research Network (PPRN) 2.0 grant number 304/PBAHAN/6314053.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are thankful to Ministry of Education for providing Public-Private Research Network (PPRN) 2.0 grant no.304/PBAHAN/6314053. The authors are grateful to Universiti Sains Malaysia (USM) for providing necessary facilities for this research work. We also would like to acknowledge Fragstar Corporation Sdn Bhd, an industrial partner in this project who partially had contributes financial and facilities support.

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