## 3 Results
### Thermal Conductivity  
According to the surface plots in Figure 6 and the regression model, SSP, EPA, and EPB contents have notable but predominantly negative effects on the thermal conductivity of the bio-composites. Contrary to initial assumptions, increasing SSP content results in a reduction in thermal conductivity due to its strong negative linear and quadratic contributions. Similarly, EPA initially reduces conductivity, but a slight positive quadratic term leads to a flattening of this effect at higher concentrations--producing a mild non-linear trend rather than a true increase. EPB shows the strongest negative effect, although its \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Sum of Squares** & **Mean Square** & **F-Value** & \(p\)**-Value** \\ \hline Model & 40.17 & 13.39 & 436.31 & \textless{}0.0001 \\ A-SSP (g) & 39.53 & 39.53 & 1288.26 & \textless{}0.0001 \\ B-EPA (g) & 0.4718 & 0.4718 & 15.38 & 0.0018 \\ C-EPB (g) & 0.1628 & 0.1628 & 5.30 & 0.0384 \\ Residual & 0.3989 & 0.0307 & & \\ Lack of Fit & 0.3973 & 0.0361 & 43.34 & 0.0228 \\ Pure Error & 0.0017 & 0.0008 & & \\ Cor Total & 40.57 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: ANOVA analysis for tensile strength of the bio-composites. Figure 5: (**a**) Effects of EPA and SSP contents, (**b**) effects of EPB and SSP contents, and (**c**) effects of EPA and EPB contents on tensile strength. impact also levels off at higher contents due to a weak positive curvature. The interaction effects between the variables are minimal, indicating that their influences are largely independent. Overall, the findings suggest that higher SSP, EPA, or EPB contents diminish thermal conductivity, and optimizing thermal performance requires maintaining these components at moderate levels. The quadratic model proposed by the RSM design for thermal conductivity prediction is presented in Equation (8). Although seashell powder contains thermally conductive calcium carbonate, the decrease at high SSP loading is due to interfacial scattering and disrupted polymer homogeneity. Filler agglomeration and voids at high concentrations scatter heat and reduce thermal conductivity. This behavior is consistent with observations in other bio-filled and mineral-filled polymer composites, where excessive filler loading beyond an optimal threshold reduces thermal conductivity due to poor dispersion and increased interfacial defects [13; 14; 15]. These results highlight the importance of achieving uniform filler distribution and strong interfacial adhesion to optimize thermal transport properties in SSP-reinforced composites. \[ \begin{split}\text{Thermal Conductivity (W/m\cdot K)}=0.236462+0.002541\times \text{SSP}-0.007533\times\text{EPA}-0.002558\\ \times\text{EPB}-1.97787\times 10^{-18}\times\text{SSP}\times\text{ EPA}-1.44808\times 10^{-18}\times\text{SSP}\times\text{EPB}+1.91319 \times 10^{-17}\\ \times\text{EPA}\times\text{EPB}-0.001614\times(\text{SSP})^{2}+0.000139 \times(\text{EPA})^{2}+0.000055\times(\text{EPB})^{2}\end{split}  \tag{8}\]  
Table 7 reveals that SSP, EPA, and EPB contents have statistically significant effects on the thermal conductivity of the bio-composites. The overall model significance is very high (\(p<0.0001\)), indicating that these variables strongly influence thermal conductivity. Among them, SSP has the most pronounced effect (F = 605.73, \(p<0.0001\)) and plays a major role in enhancing thermal transfer. EPA also shows a significant effect (F = 52.81, \(p\) = 0.0002), although to a lesser extent compared to SSP. The effect of EPB is more limited (F = 16.84, \(p=0.0046\)) but remains statistically significant. Although EPB's contribution appears to be statistically significant, its physical effect on thermal conductivity and hardness may be indirect--possibly related to changes in crosslink density or interfacial stiffness rather than bulk filler distribution. This warrants further microstructural or dynamic mechanical analysis to verify its underlying mechanism.