*3.7. Thermogravimetric Analysis (TGA) of PP Composite*

Figure 10 shows the thermal stability of the neat PP, SiPs, CNFs, and PP composite. The TGA results show the neat PP and PP composite degradation in a single step of degradation. SiP SP had weight loss at 420 ◦C, attributed to the thermal decomposition of the functional group of polyalkyl siloxane. Above 450 ◦C, there was no weight loss observed for either SiP sample, indicating residual SiPs. Further, SiPs have high thermal stability at temperatures up to 800 ◦C [37]. For the CNF sample, weight loss started from 270 ◦C due to the thermal decomposition of cellulose [38]. The TGA result also showed the neat PP almost degraded without any char formation, with the residual of the original sample mass being only 1.3%.

**Figure 10.** TGA curves of PP, SiP, CNF, PP SS/CNF, and PP SP/CNF.

Compared to the neat PP, the decomposition temperature of the PP composite increased significantly with the addition of hybrid filler, indicating that SiPs hindered thermal decomposition of PP composite. The degradation depends on the particle encapsulation and extent of interaction with matrix. Improvement in the thermal stability of PP composites was posited to be due to the excellent dispersion of the hybrid filler in the polymer matrix. Due to the stable thermal properties of the hybrid filler, chain scission process occurred at a higher temperature compared to pristine PP polymer [39]. Additionally, the weight residue at 500 ◦C of the PP composite showed the highest for sample PP SS/CNF at 10.28% residue, followed by CNF at 9.54% residue, and PP SP/CNF at 7.53% residue. The char yield increase after melt-blending with filler is attributed to the increased interactions between the filler and the matrix which resulted in the onset of thermal degradation.

Additionally, the DTA curves of the discussed composite PP SS/CNF and PP SP/CNF show that during their decomposition, endothermic reactions occurred (Figure 11). The DTA curve exhibited two endothermic events. The first is attributed to Tm at around 161 ◦C, corresponding to the melting of the crystalline phase in the studied polymer (PP). Other endothermic peaks were observed with decomposition maxima at 428 and 450 ◦C in higher temperature ranges. The thermal decomposition of PP SP/CNF is higher than PP SS/CNF.

**Figure 11.** DTA curves of (**a**) PP SS/CNF, and (**b**) PP SP/CNF.
