## 3 Results and Discussion
### Thermal and Strength Performances of Composite Insulation Samples  
Figure 8 presents the \(F_{t}\) and \(k\) of the composite insulation samples that contained varying silica aerogel content. The highest \(F_{t}\) of 11.58 MPa was obtained for the composite insulation sample that was not incorporated with silica aerogel. The \(F_{t}\) decreased sharply to 7.04 MPa when 1 wt.% of silica aerogel was added. Contrarily, further increments in the silica aerogel content from 1 wt.% to 2 wt.%, and then 3 wt.%, resulted in the improvement of \(F_{t}\) from 7.04 MPa to 8.24 MPa, and then 8.74 MPa, respectively. However, further additions of silica aerogel beyond 3 wt.% induced a decline in \(F_{t}\) from 8.74 MPa to 7.18 MPa, and then 6.47 MPa, for the samples with silica aerogel contents of 4 wt.% and 5 wt.%, respectively, due to the agglomeration of the silica aerogel particles, which affected the strength and elongation of the sample, according to [PERSON] et al. [58] and [PERSON] et al. [59]. As for \(k\), a gradual decrease was observed with each increment in silica aerogel content. The sample that was not incorporated with silica aerogel obtained the highest \(k\) of 0.20 W/mK, which then declined to 0.15 W/mK, 0.14 W/mK, 0.09 W/mK, 0.06 W/mK, and then 0.04 W/mK when the silica aerogel content was increased to 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, to 5 wt.%, respectively, with a percentage in overall decline of 80%.  
Figure 9 shows the \(F_{f}\) and \(k\) of the composite insulation samples that contained varying silica aerogel content. The highest \(F_{f}\) of 23.24 MPa was obtained for the control sample, which was not incorporated with silica aerogel. Increment of silica aerogel from 0 wt.% to 1 wt.% resulted in a decrease of \(F_{f}\) from 23.24 MPa to 18.96 MPa. Conversely, the \(F_{f}\) increased from 18.96 MPa to 19.88 MPa, and then 21.37 MPa, when the silica aerogel content was increased from 1 wt.%, to 2 wt.%, and then to 3 wt.%, respectively. Subsequently, the \(F_{f}\) decreased from 21.37 MPa, to 18.63 MPa, and then to the lowest \(F_{f}\) of 17.64 MPa when the silica aerogel content was increased from 3 wt.%, to 4 wt.%, and then to the highest silica aerogel content of 5 wt.%, which is due to the high \(\phi\) of the silica aerogel that resulted in its lower mechanical performance as mentioned in [PERSON] et al. [60], and also the impact of further adding silica aerogel that tends to reaggregate instead of spreading homogeneously throughout the HDPE-kappa fiber matrix [61], leading to a decrease in the interaction between the surface area of the silica aerogel with the HDPE and kapok fiber as highlighted in [PERSON] et al. [62], and, as a consequence, caused the sample to be more brittle due to ineffective stress transfer. Although \(k\) greatly decreased with the  
Figure 8: Tensile strength (\(F_{t}\)) and thermal conductivity (\(k\)) for the composite insulation materials.  
aerogel in the composite insulation, the silica aerogel content of 3 wt.% is recommended to formulate a composite insulation that concurrently possesses low \(k\) with reasonably high \(F_{t}\) and \(F_{f}\).  
Unlike tensile strength tests where stress is distributed along the sample, the stress in flexural strength test is more restricted to the region of the applied load. Figure 10 compares the \(F_{t}\) and \(F_{f}\) profiles of the composite insulation samples with varying silica aerogel content. Notwithstanding the minor difference between the gradient of the declines of \(F_{t}\) and \(F_{f}\) when the silica aerogel content was increased from 0 wt.% to 1 wt.%, where the decline of \(F_{t}\) was sharper than that of \(F_{f}\), profiles of \(F_{t}\) and \(F_{f}\) exhibited similar trends and is in agreement with the findings in [PERSON] et al. (2018), hence validating the consistency in the preparation of corresponding samples that were employed to perform the tensile and flexural strength tests.