*3.5. Mechanical Properties of PP Composite*

Mechanical properties such as tensile strength and elongation are often used to measure the strength and elasticity of composite films. The representative tensile stress-strain curves of pure PP and its composites are shown in Figure 7 and their key mechanical properties are summarized in Table 3. The fillers incorporated into composite films significantly affected the mechanical properties of the composites due to the specific surface area and dispersibility of each filler. Improved mechanical properties can be achieved through an improved interface between the filler and polymer matrix [31].

**Figure 7.** Mechanical properties of (**a**) stress-strain curves of PP composites and (**b**) an enlarged graph.



Figures 7 and 8 and Table 3 also show the effect of filler on the tensile strength at yield of PP composites compared to pristine PP polymer. The yield strength of PP increased after melt-blending with filler or hybrid filler. This scenario was due to the addition of filler into the matrix, which reduced the available free spaces and hence increased the stiffness of the composite. The filler linked the matrix together, leading to enhanced interaction between the reinforcement and the matrix. Once the load has been applied, the stress can easily be transferred from the polymer matrix to the reinforcing materials, thus improving the tensile strength of the polymer composite [32].

**Figure 8.** Mechanical properties of PP composites: bar charts of elongation at break and yield strength.

The tensile strength at yields of PP-8.75 SS and PP-10 SS was 27.80 and 23.53 MPa, respectively. Increasing the by-weight percentage of SS led to inferior mechanical properties. The agglomeration extent increased with the increase of nanofiller content and reduction of nanofiller size. When compared to SP, the particle size of SS is smaller overall, so aggregation has a higher chance to occur in PP/SS composites. Agglomeration decreased the effectiveness of nanoparticles in polymer matrix, which lastly resulted in the poor properties of the samples.

However, increasing the by-weight percentage of SP from 8.75 to 10 increased the tensile strength. The irregular shape and slightly larger particle size of the filler produced different properties. The surface roughness and irregular shape enhanced the interfacial properties between the matrix and reinforcement, such that the tensile strength still increased after the addition of 10 wt.% of SP. Both PP-SS/CNF and PP-SP/CNF samples depicted excellent yield strengths, which were 36.81 and 34.06 MPa, respectively. The synergy of the hybrid filler occurred due to the possibility of physical interaction between the functional groups of SiPs and CNFs. This functional group built the interfacial surface compatibility and increased the dispersibility between the CNFs and PP matrix. In addition, the homogeneity and matrix particle interactions were improved after adding the hybrid filler. Compared to the pristine PP (21.2 MPa), the tensile strength at yield of PP-SS/CNF and PP-SP/CNF samples showed 73.5% and 60.3% increments, respectively.

Furthermore, the elongation at break of PP composites was sharply reduced by the addition of filler due to the stiffening effect of SiPs and CNFs on the PP composite [31]. The presence of a rigid interface between SiPs and CNFs and the PP matrix decreased the deformability of the PP matrix, which led to more rigid and stiffer composites [28,33]. The PP-8.75 SP (201.70%) composites exhibited the highest elongation at break. SP is a hydrophobic filler, which can enhance the interfacial interaction between the filler and PP matrix and generate a stronger interfacial bonding. However, further study needs to be carried out to investigate the unexpected elongation behavior of sample PP-8.75 SP compared to other polymer composites.
