*3.1. FT-IR Analysis*

The performance of the grafting process within PBS/PBAT/TPS and PBS/PBAT/TPRH was evaluated through the FT-IR technique, and the results are shown in Figure 1. For comparison, FT-IR results of bare PBS and bare PBAT are also displayed in Figure 1. Figure 1a displays a peak at 1160 cm−<sup>1</sup> , which belonged to the aliphatic ester groups of PBS samples, confirming its aliphatic structure [31]. The bands at the 810, 964, 1116, and 1694 cm−<sup>1</sup> confirmed the C-H bending of alkane, -C-OH blending of carboxylic acids group, -C-Ostretching vibrations, and C = O stretching vibration in the ester linkages of PBS [32,33]. The band at 1338 cm−<sup>1</sup> and 2906 cm−<sup>1</sup> were attributed to the symmetric and asymmetric deformational vibrations of -CH2- groups of the PBS structure [34]. Figure 1b exhibits the peaks of bare PBAT at 1260 and 1162 cm−<sup>1</sup> , which were ascribed to O = C-O-C stretching of aromatic and aliphatic ester groups, respectively, validating its aliphatic-aromatic structure. The peaks at 732, 936, and 1112 cm−<sup>1</sup> are attributed to = C-H bending of benzene ring, -C-OH blending of carboxylic acids group, and -C-O- stretching vibrations of PBAT [32,35]. The band at 1694 and 2952 cm−<sup>1</sup> are attributed to the C = O stretching vibration and -CH2 groups of PBAT, respectively.

**Figure 1.** FT-IR spectra of the PBS/PBAT/TPS composites and PBS/PBAT/TPRH composites (**a**) PBS, (**b**) PBAT, (**c**) rice husk, (**d**) starch, (**e**) maleic anhydride, (**f**) TPRH48/12, (**g**) TPS48/12, (**h**) TPRH36/24, and (**i**) TPS36/24. **Figure 1.** FT-IR spectra of the PBS/PBAT/TPS composites and PBS/PBAT/TPRH composites (**a**) PBS, (**b**) PBAT, (**c**) rice husk, (**d**) starch, (**e**) maleic anhydride, (**f**) TPRH48/12, (**g**) TPS48/12, (**h**) TPRH36/24, and (**i**) TPS36/24.

Figure 1c,d represent the FT-IR spectra of rice husk and starch. The broad absorption band at 3390 cm−1 was ascribed to the stretching occurring in the -OH group [36,37]. The band indicated rice husk and starch had a considerable amount of surface absorbed moisture [38]. The peak at 2950 cm−1 was assigned to C-H stretching vibration. The presence of a band at 1670 cm−1 affirmed the stretching vibration of the C = O group in rice husk and starch. The absorption band at 1372 cm−1 was attributed to -CH2 scissoring vibrations [39,40]. The peak around 774 cm−1 showed the presence of -CH2 blending [41]. Figure 1e displays the FT-IR spectra of maleic anhydride. The absorption bands at 3146, 3066, 1600, and 1062 cm−1 were assigned to asymmetrical C-H stretching vibration (CH2 = CH2), symmetrical C-H stretching vibration (CH2 = CH2), C = C stretching band, and C-O-C symmetrical stretching band, respectively [42]. The peaks at Figure 1c,d represent the FT-IR spectra of rice husk and starch. The broad absorption band at 3390 cm−<sup>1</sup> was ascribed to the stretching occurring in the -OH group [36,37]. The band indicated rice husk and starch had a considerable amount of surface absorbed moisture [38]. The peak at 2950 cm−<sup>1</sup> was assigned to C-H stretching vibration. The presence of a band at 1670 cm−<sup>1</sup> affirmed the stretching vibration of the C = O group in rice husk and starch. The absorption band at 1372 cm−<sup>1</sup> was attributed to -CH<sup>2</sup> scissoring vibrations [39,40]. The peak around 774 cm−<sup>1</sup> showed the presence of -CH<sup>2</sup> blending [41]. Figure 1e displays the FT-IR spectra of maleic anhydride. The absorption bands at 3146, 3066, 1600, and 1062 cm−<sup>1</sup> were assigned to asymmetrical C-H stretching vibration (CH<sup>2</sup> = CH2), symmetrical C-H stretching vibration (CH<sup>2</sup> = CH2), C = C stretching band, and C-O-C symmetrical stretching band, respectively [42]. The peaks at 1866 cm−<sup>1</sup> and 1786 cm−<sup>1</sup> were assigned to the C = O stretching vibration of maleic anhydride [43].

1866 cm−1 and 1786 cm−1 were assigned to the C = O stretching vibration of maleic anhydride [43]. Figure 1f–i show the FT-IR spectra of TPRH48/12, TPS48/12, TPRH36/24, and TPS36/24 blends, respectively. The spectra show a similar peak for bare PBS and PBAT. However, an additional band at 2846 cm−1 suggests the -CH2 group from the TPRH or TPS and CH2 = CH vibration in the cyclic MA. Since the MA was only applied in 2 p/hr, which is considered a small amount. Thus, this bond was corresponded to the -CH2 Figure 1f–i show the FT-IR spectra of TPRH48/12, TPS48/12, TPRH36/24, and TPS36/24 blends, respectively. The spectra show a similar peak for bare PBS and PBAT. However, an additional band at 2846 cm−<sup>1</sup> suggests the -CH<sup>2</sup> group from the TPRH or TPS and CH<sup>2</sup> = CH vibration in the cyclic MA. Since the MA was only applied in 2 p/hr, which is considered a small amount. Thus, this bond was corresponded to the -CH<sup>2</sup> group from the TPRH or TPS. This confirms the reaction between PBS/PBAT and TPRH or TPS and addresses the incompatibility between polymer matrix and filler material [44]. In summary, Figure 1 shows an insignificant difference between PBS/PBAT/TPRH and PBS/PBAT/TPS blends as rice

husk and starch are organic-based fillers having similar functional groups. This indicated the potential of using waste material such as rice husk to replace starch in biodegradable plastics. The presence of strong absorption bands at 1694 and 1470 cm−<sup>1</sup> is associated with the stretching vibration of the C = O group and -CH<sup>2</sup> scissoring vibrations of TPRH and TPS, respectively [37]. Besides, the band at 1426 cm−<sup>1</sup> confirmed the -OH group of glycerol, which was used to modify the surface of rice husk and starch to form TPRH and TPS [33].
