## 3 Results and Discussion
### Characterisations of 3D-Printed Specimens  
Figure 7 shows the SEM images of the tensile fractured surface of the PLA/PBAT/BC composites. The phase separation of the PLA and PBAT blends is due to their immiscibility, which has been reported elsewhere [61]. The PLA/PBAT (80/20), Figure 7a, and PLA/PBAT/BC (79/20/1), Figure 7b, show a fibre-like structure, indicating the plastic deformation of the polymer matrix. The PBAT phase is ductile and elongated during tensile testing. Figure 7a shows a weak interfacial-interaction of the PBAT phase (marked with red arrows) with the PLA phase, as observed in the PLA/PBAT (80/20) blend [62]. However, such phase separation is not visible in Figure 7b; PLA/PBAT/BC (79/20/1) shows a better interfacial-interaction between PLA and PBAT in the presence of biochar. The presence of PBAT is favourable for improving the impact-resistance behaviour of PLA/PBAT blends [40]. However, the compatibility of the PLA and PBAT phases is essential for improving tensile properties. A weak interfacial-bonding between the PLA and PBAT phases lowers the tensile strength of the blend. When the biochar concentration is low (1 wt%), the droplet structure disappears, and a continuous matrix with elongated fibres is observed, Figure 7b. Moreover, biochar particles are wrapped with the polymer blend, indicating a strong bonding of biochar with the polymer matrix. The interfacial region of the biochar polymer matrix indicates a strong bonding between the polymer matrix and the biochar  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Material** & **Surface Resistivity (\(\Omega\)/sq)** \\ \hline PLA/PBAT (80/20) & \(1.58\times 10^{13}\) \\ PLA/PBAT/BC (79/20/1) & \(4.70\times 10^{12}\) \\ PLA/PBAT/BC (77.5/20/2.5) & \(2.72\times 10^{12}\) \\ PLA/PBAT/BC (85/10/5) & \(2.592\times 10^{12}\) \\ PLA/PBAT/BC (80/15/5) & \(2.23\times 10^{12}\) \\ PLA/PBAT/BC (75/20/5) & \(2.1\times 10^{12}\) \\ PLA/PBAT/BC (70/20/10) & \(1.03\times 10^{11}\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Surface resistivity of PLA/PBAT/BC composites.  
particles, Figure 8. However, the fibril structure disappears when the PLA/PBAT blend is reinforced with biochar at a higher concentration (10 wt%), Figure 7d, indicating the increase in brittleness of the composite with a high biochar-loading. The dispersion and size of the separated domain depend on many factors, such as processing condition, constituent ratio and interfacial tension [40]. The PBAT phase is well dispersed, with better interfacial interactions between PLA and biochar in PLA/PBAT/BC (79/20/1) and PLAPBAT/BC (77.5/20/2.5) composites.  
Figure 8: Interfacial region of biochar and PLA/PBAT in PLA/PBAT/BC (79/20/1).  
Figure 7: SEM images of fractured surfaces: (**a**) PLA/PBAT (80/20) (**b**) PLA/PBAT/BC (79/20/1) (c) PLA/PBAT/BC (77.5/20/2.5) and (**d**) PLA/PBAT/BC (70/20/10). [The red arrow in (**a**) shows the PBAT phase in the PLA matrix and (**b–d**), biochar particle].  
A smaller domain size of the dispersed phase indicates low interfacial-tension between PLA and PBAT, Figure 7c. The lower interfacial-tension permits a larger surface area, allowing the large drops to split into smaller droplets [61]. Moreover, the biochar particles are covered with a polymer matrix, indicating a greater affinity of the polymer with coconut shell biochar. The interfacial interaction between the biochar and PLA/PBAT blend and the disappearance of droplet structure indicates that a low biochar concentration (1 wt%) improves the compatibility of PLA and PBAT.