**3.5. Differential scanning calorimetry (DSC)**

The results showed that the presence of RHB improved the flexural properties of biocomposites. In **Figure 7**, CTPB-based biocomposites had higher MOR values than the UTPB-based biocomposites. This was attributed to the compatibilizing reaction for inherently immiscible TPB matrix which then improved adhesion between fibre and polymer blend matrix besides the enhanced compatibility between polymer blend components. Meanwhile, the incorporation of E-GMA into the CTPB matrix seemed not to provide a significant effect in the MOE of biocomposites (**Figure 8**). Comparing to UTPB-based composite, the MOE for CTPB-based composite reinforced with 50–60 wt% RHB was slightly lower. This phenomenon is associated to the weak intrinsic mechanical properties of E-GMA [14]. However, for the composites with higher content of RHB (70–80 wt%), this factor can be negligible due to relatively low content

**Figure 9** presents the impact strength of TPB/RHB biocomposites. For neat blends without the presence of RHB, compatibilization reaction of E-GMA provided a reinforcement effect and led to a significant increase of impact strength for CTPB matrix. However, this large improvement decreased when RHB was added to the blend matrix, which was because rice husk is one kind of stiff inorganic fillers [15]. The impact strength is reduced with RHB content. At high fibre content, there were many interactions between fillers as a result of the filler agglomerations in the composite which were more susceptible to the cracks than the fibre-matrix interface. This indicates that the cracks spread more easily in the biocomposites with high content of rice

of the matrix (30 wt% and lower).

36 Composites from Renewable and Sustainable Materials

husk, thus decreasing the impact strength [16].

**Figure 9.** Impact strength of TPB/RHB biocomposites.

**3.4. Impact properties**

**Figure 10** depicts the melting temperature (*Tm*) of (a) rHDPE and (b) rPET components for TPB/ RHB biocomposites that are measured from DSC testing. According to Kiziltas et al. (2011), the *Tm* of the composites plays a crucial role in determining the processing temperature and thermal properties [17]. As shown in **Figure 10**, neat UTPB showed a *Tm* of rHDPE component at 135.6 °C and *Tm* of rPET component at 252.8 °C, whereas neat CTPB possessed a lower *Tm* values of about 133.6 °C for rHDPE and 251.2 °C for rPET which were attributed to the improvement of compatibility between two polymer components with the use of E-GMA as compatibilizer. The reduced *Tm* values by the addition of E-GMA might be because E-GMA has lower molecular mass which promotes the composite degradation at lower temperature values. In overall, it can be observed that the addition of RHB into polymer blend reduced the *Tm* by approximately 1.2–5.2 °C. This phenomenon proved the enhancement in the processing temperature of rHDPE after reinforced with RHB [18].

**Figure 10.** Melting temperature of (a) rHDPE and (b) rPET components in biocomposites.

Besides *Tm*, DSC testing also has provided the data of crystallinity level (*χc*) for TPB/RHB biocomposites, as presented in **Figure 11**. It was noted that the increasing RHB content from 50 to 80 wt% reduced the *χc* of TPB/RHB biocomposites. This was due to the incorporation of RHB that restricted the movement of polyethylene (PE) chain from crystallization process [19]. The CTPB/RHB biocomposites exhibited higher percentage of crystallinity than that of UTPB/RHB biocomposites. This suggests that the presence of E-GMA in the CTPB matrix improved the crystallinity by promoting the migration and diffusion of PE chains in order to form crystals in the surrounding of the rice husk surface. Several previous research results have reported that the addition of coupling agent copolymer enhanced the crystallinity of biocomposites [20, 21].

**Figure 11.** Crystallinity level (*χc*) of TPB/RHB biocomposites.

#### **3.6. Thermogravimetric analysis (TGA)**

**Figure 12** presents (a) decomposition temperature (*Td*) and (b) residues after heating at 600 °C that were obtained from TGA thermogram of TPB/RHB biocomposites. The neat UTPB and CTPB exhibited a one-step weight loss from 400 °C to 500 °C with the maximum decomposition temperature (*Td*) at 471 °C and 469 °C, respectively (**Figure 12(a)**). This process occurred because the neat polymer blend comprised a series of interchained monomers, where an increase of temperature promoted the random chain scission through thermal degradation and depolymerization (at *Td*) [22]. The lower *Td* for CTPB than that of UTPB was due to an increase of rHDPE-rPET interaction because of the addition of E-GMA compatibilizer which possessed a lower decomposition temperature [23].

**Figure 12.** TGA thermogram data: (a) main decomposition temperature (*Td*) of polymer component and (b) residues after 600 °C for TPB/RHB biocomposites.

In the presence of RHB in TPB matrix, the peaks of *Td* have shifted to higher temperatures (478– 481 °C), irrespective of matrix types and RHB contents. This behaviour suggests the improvement of thermal stability of TPB matrix by the incorporation of RHB that restricted the movement of polymer chains and thus delayed the thermal degradation process. Upon heating at temperature beyond 600 °C, both the neat UTPB and CTPB showed the relatively small amount of residues (**Figure 12(b)**). This is because the thermal degraded polymeric materials will further breakdown into gaseous products at higher heating temperature after *Td* [22]. It can be observed that the residue weight of samples increased continuously with the increase of RHB content, up to 26.3 % and 29.6 % for composites containing 80 wt% RHB based on UTPB and CTPB matrix, respectively. These results were consistent with the high amount of silica present in the RHB.

#### **3.7. Fire retardancy**

**Figure 11.** Crystallinity level (*χc*) of TPB/RHB biocomposites.

possessed a lower decomposition temperature [23].

after 600 °C for TPB/RHB biocomposites.

**Figure 12** presents (a) decomposition temperature (*Td*) and (b) residues after heating at 600 °C that were obtained from TGA thermogram of TPB/RHB biocomposites. The neat UTPB and CTPB exhibited a one-step weight loss from 400 °C to 500 °C with the maximum decomposition temperature (*Td*) at 471 °C and 469 °C, respectively (**Figure 12(a)**). This process occurred because the neat polymer blend comprised a series of interchained monomers, where an increase of temperature promoted the random chain scission through thermal degradation and depolymerization (at *Td*) [22]. The lower *Td* for CTPB than that of UTPB was due to an increase of rHDPE-rPET interaction because of the addition of E-GMA compatibilizer which

**Figure 12.** TGA thermogram data: (a) main decomposition temperature (*Td*) of polymer component and (b) residues

**3.6. Thermogravimetric analysis (TGA)**

38 Composites from Renewable and Sustainable Materials

**Figure 13** shows the burning rate of both UTPB- and CTPB-based biocomposites filled with RHB. The neat TPB had the highest burning rate that was about 0.73 0.02 mm/s for UTPB and 0.71 ± 0.02 mm/s for CTPB. A slightly lower burning rate for CTPB than that of UTPB reflects that the CTPB sample showed better fire retardancy property than UTPB ones. This is probably ascribed to the enhanced compatibility of TPB with the aids of E-GMA copolymer.

**Figure 13.** Burning rate of TPB/RHB biocomposites.

For biocomposites containing RHB, the fire retardancy increased by 6–24 % with increasing content of RHB from 40 wt% to 80 wt%. This enhancement might be due to the nature characteristic of silica (relatively high amount, 15–17 % in the raw rice husk) that delayed the combustion [24]. All the CTPB/RHB biocomposites gave lower burning rate (higher fire retardancy) than UTPB/RHB biocomposites. The burning test results showed that the CTPB (miscible) seemed to improve the interaction between RHB and TPB matrix more effectively than UTPB (immiscible)- based biocomposites.

#### **3.8. Morphological observation**

The morphology of the fracture surfaces for (a) 50 wt% RHB/UTPB, (b) 50 wt% RHB/CTPB, (c) 80 wt% RHB/UTPB and (d) 80 wt% RHB/CTPB biocomposites is illustrated in **Figure 14**. Two striking observations can be seen in the surface morphology changes of biocomposites with different RHB contents and TPB matrix types. First, by comparing the effect of RHB contents, the 50 wt% RHB as in **Figure 14(a/b)**was perfectly attached and strongly adhered to the TPB matrix, in which this observation indicates the efficiency of composite material compounding and good fibres-matrix interfacial bonding.

**Figure 14.** SEM micrograph of (a) 50 wt% RHB/UTPB, (b) 50 wt% RHB/CTPB, (c) 80 wt% RHB/UTPB and (d) 80 wt% RHB/CTPB biocomposites (magnification, 500×).

Meanwhile, the 80 wt% RHB as in **Figure 14(c**/**d)** showed poorer fibre dispersion and the existence of more clear holes or cavities in the fracture surface morphologies which resulted from insufficient adhesion between filler and matrix. This was then led to the fibre-fibre contact (fibre agglomeration) dominated in the composites. Second, by comparing the effect of TPB matrix types, the UTPB-based biocomposites (**Figure 14(a**/**c**) present relatively coarse and obvious phase separation morphology with inhomogeneous filler distribution in the matrix. This can be explained by the fact of the great difference in the polymer solubility parameters [25], in which rHDPE is nonpolar and rPET is polar. The CTPB-based biocomposites in **Figure 14(b**/**d)** displayed more homogenous and finer morphology structure which proved the enhanced interfacial adhesion between polymers as well as between fibres and TPB matrix via incorporation of compatibilizer in the matrix.
