**5. Mechanical Properties**

Several research teams have aimed to improve the mechanical properties of PLA with the addition of silica to the polymer matrix. For example, He et al. [76] reported that the inclusion of silica nanoparticles is a good strategy to improve the mechanical properties of PLA. Indeed, the tensile strength (TS), Young's modulus (YM), and impact strength of the PLA/silica composites made by the melt-mixing method were greatly increased with the addition of silica nanoparticles, which was linked to the good dispersibility of the particles in the polymer matrix, as well as PLA-silica interactions [77]. The low loading of silica (below 5 wt.%) was found to be distributed homogenously within the PLA matrix, while some agglomerates were observed at the high loading (higher than 5 wt.%) [36]. Ahmed et al. [78] studied the impact of silica additive into PLA matrix-3D printing. When the silica addition amount was 10 wt.%, its TS, flexibility, YM, and other properties were greatly enhanced. The application of this kind of composite material is feasible, but once the silica content exceeds 15%, the performance is reduced. The improved mechanical properties promote the recycling of PLA while retaining its biodegradability. As such, YM and ultimate tensile stress (UTS) of PDLLA were increased by 106% and 63.7%, respectively, upon the addition of 3 wt.% silica into the polymer matrix [79].

Yan et al. [32] prepared PLA/silica composites via two steps: grafting of L-lactic acid oligomer onto the silica surface, followed by melt-mixing with PLA. This procedure led to fabricating composites with improved mechanical properties. In another work by Yan and co-workers [31], the TS of the plasticized PLA/silica composite could be increased to large values even in the presence of low loadings of silica. Although the addition of the silica nanoparticles accelerated the biodegradation rate of PLA, the resultant composites had improved mechanical properties, as reported by Georgiopoulos et al. [80]. to fabricating composites with improved mechanical properties. In another work by Yan and co-workers [31], the TS of the plasticized PLA/silica composite could be increased to large values even in the presence of low loadings of silica. Although the addition of the silica nanoparticles accelerated the biodegradation rate of PLA, the resultant composites had improved mechanical properties, as reported by Georgiopoulos et al. [80]. To obtain better dispersion in the PLA matrix, silica nanoparticles with 24 nm in average diameter were functionalized via stearic acid [38]. Different contents of silica, such

Yan et al. [32] prepared PLA/silica composites via two steps: grafting of L-lactic acid oligomer onto the silica surface, followed by melt-mixing with PLA. This procedure led

while some agglomerates were observed at the high loading (higher than 5 wt.%) [36]. Ahmed et al. [78] studied the impact of silica additive into PLA matrix-3D printing. When the silica addition amount was 10 wt.%, its TS, flexibility, YM, and other properties were greatly enhanced. The application of this kind of composite material is feasible, but once the silica content exceeds 15%, the performance is reduced. The improved mechanical properties promote the recycling of PLA while retaining its biodegradability. As such, YM and ultimate tensile stress (UTS) of PDLLA were increased by 106% and 63.7%, respec-

*Polymers* **2021**, *13*, x FOR PEER REVIEW 11 of 20

tively, upon the addition of 3 wt.% silica into the polymer matrix [79].

To obtain better dispersion in the PLA matrix, silica nanoparticles with 24 nm in average diameter were functionalized via stearic acid [38]. Different contents of silica, such as 0, 0.1, 0.3, 0.5, 0.8, 1, and 1.5 wt.%, were mixed with PLA via an extrusion technique. The mechanical tests revealed that the addition of 0.8 wt.% of silica nanoparticles into the PLA matrix led to increasing the Izod impact strength from 1.57 to 5.13 kJ/m<sup>2</sup> . The impact strength values of the PLA/silica composites increased with increasing silica content, then decreased when the silica content exceeded 0.8 wt.%. Moreover, the presence of nano-silica in the PLA matrix caused improvements in the EB and TS of PLA (Figure 6a). As shown in Figure 6b, dimple morphologies were observed on the fractural surfaces of PLA/silica composites, suggesting that neat PLA was less ductile than PLA/silica composites. According to the lubrication sliding and the ductile behavior, it was confirmed the toughness of PLA could be largely improved by the incorporation of silica nanoparticles. The effects of silica content (1, 3, and 5 wt.%) on the mechanical performance of PLA film were studied Zirak and Tabari [81]. It was found that the values of the TS and YM of PLA tended to be increased upon the incorporation of silica nanoparticles into the PLA matrix, regardless of the silica content. For example, the addition of 5 wt.% of silica into PLA led to an increase in the values of TS and YM for PLA from 29 MPa and 2.3 GPa to 43 MPa and 3.1 GPa, respectively, which was attributed to the reinforcing effect of SiO<sup>2</sup> nanoparticles. In contrast, the value of EB of PLA (17.5%) was reduced with the inclusion of silica nanoparticles, where a value of 10.5% was found in the composite containing 5 wt.% SiO2. as 0, 0.1, 0.3, 0.5, 0.8, 1, and 1.5 wt.%, were mixed with PLA via an extrusion technique. The mechanical tests revealed that the addition of 0.8 wt.% of silica nanoparticles into the PLA matrix led to increasing the Izod impact strength from 1.57 to 5.13 kJ/m2. The impact strength values of the PLA/silica composites increased with increasing silica content, then decreased when the silica content exceeded 0.8 wt.%. Moreover, the presence of nanosilica in the PLA matrix caused improvements in the EB and TS of PLA (Figure 6a). As shown in Figure 6b, dimple morphologies were observed on the fractural surfaces of PLA/silica composites, suggesting that neat PLA was less ductile than PLA/silica composites. According to the lubrication sliding and the ductile behavior, it was confirmed the toughness of PLA could be largely improved by the incorporation of silica nanoparticles. The effects of silica content (1, 3, and 5 wt.%) on the mechanical performance of PLA film were studied Zirak and Tabari [81]. It was found that the values of the TS and YM of PLA tended to be increased upon the incorporation of silica nanoparticles into the PLA matrix, regardless of the silica content. For example, the addition of 5 wt.% of silica into PLA led to an increase in the values of TS and YM for PLA from 29 MPa and 2.3 GPa to 43 MPa and 3.1 GPa, respectively, which was attributed to the reinforcing effect of SiO2 nanoparticles. In contrast, the value of EB of PLA (17.5%) was reduced with the inclusion of silica nanoparticles, where a value of 10.5% was found in the composite containing 5 wt.% SiO2.

**Figure 6.** (**a**) The stress–strain curve of neat PLA and PLA composites. (**b**) The fracture morphology after the tensile test of PLA composite [38]. **Figure 6.** (**a**) The stress–strain curve of neat PLA and PLA composites. (**b**) The fracture morphology after the tensile test of PLA composite [38].

In Yan's research [32], it was approved that the grafting of L-lactic acid oligomer on the silica nanoparticles by condensation reaction without catalyst would result in the formation of a good filler, helping to increase the toughness of PLA. Chrissafis et al. [82] reported that the inclusion of 2.5 wt.% of fumed silica and montmorillonite into the PLA matrix would result in the formation of composites having higher values of TS and YM than neat PLA. However, the addition of fumed silica and montmorillonite to PLA led to a reduction in the value of EB, implying that those two additives acted as reinforcing agents. Lai and Hsieh [83] examined the influence of the surface functionalization of silica on the mechanical properties of PLA. To form the modified silica, polyethylene glycol methyl ether was grafted on the silica in the presence of aminosilane. The interfacial interaction between silica and PLA was improved in the presence of modified silica; thereby, the value of TS of the composites with modified silica was higher than the counterpart composites containing unmodified silica.

Battegazzore et al. [84] used different contents, such as 5, 10, 20, and 30 wt.%, of silica powder, obtained by the conventional extraction method from rice husk, to fabricate PLA/silica nanocomposites via the melt-mixing method. Based on Archimedes law, the silica density was calculated to be 1.82 and 1.88 g·cm−<sup>3</sup> for the extracted silica and commercial silica, respectively, while the density of PLA was 1.25 g·cm−<sup>3</sup> . The density values were used to calculate the volumetric fractions. When the extracted silica nanoparticles were melted with PLA, the YM of neat PLA was increased, while the oxygen permeability of PLA was slightly reduced. As compared to the composites including 10 and 30 wt.% of commercial silica, the counterpart composites with extracted silica exhibited better mechanical properties. Considering the economic analysis of the whole process and by reusing the energy recovered from burning rice husk, the authors considered the composites containing 20 wt.% of extracted silica as economically sustainable materials.

It is worth mentioning that silica can improve the mechanical properties of other PLA composites. For example, the addition of silica can improve the jute/PLA interfacial adhesion, which results in the enhancement of the mechanical performances of the resultant composites [85].

On the other hand, the influence of other silica-based materials on the properties of PLA was explored by many research groups. For instance, the addition of glass fibers to the PLA matrix improved the mass flow rate and flexural modulus, while their effects on the impact and flexural strength, as well as thermal stability of PLA, were insignificant [86]. Besides, the TS and EB were reduced with the addition of glass fibers.
