**3. Results and Discussion**

Initially, the chemical structure of fabricated silica nanoparticles was examined with the help of an FT-IR spectroscopy (Perkin Elmer, Waltham, MA, USA) as shown in Figure 2. The silica nanoparticles exhibited a distinctive absorption peak at 1000–1100 cm<sup>−</sup>1, which was ascribed to a stretching vibration of Si–O–Si bonding. Another distinctive absorption peak also appeared at 1630 cm−1, which was allocated to a bending vibration by way of H–O–H in the water molecules. An additional absorption peak was further detected at 3400 cm−1, which was due to a stretching vibration of the hydroxyl group, confirming the existence of hydroxyl groups in the silica surface. Figure 2 also showed the FT-IR spectrum of vulcanized natural rubber filled with 3 phr silica composite. It is seen that the asymmetrical stretching vibration of Si–O–Si and the hydroxyl of silica in the NR/SiO2 composite appeared at 1088 cm−<sup>1</sup> and 3403 cm<sup>−</sup>1, respectively.

**Figure 2.** FT-IR spectra of fabricated nanosilica and natural rubber (NR)/3 phr silica composite.

Regarding the crystalline structure of the samples, the powder XRD pattern in Figure 3 indicated a broad peak at 22◦ of 2θ, revealing the amorphous nature of the nanosilica particles. Furthermore, the TEM image of nanosilica in Figure 4 showed its diameter of 40–60 nm.

**Figure 3.** XRD pattern of nanosilica.

**Figure 4.** TEM of nanosilica.

Table 2 lists the main curing characteristics of the fabricated NR compounds that were obtained from the cure curves (Figure 5), in which both t2 and t90 increased with increasing silica nanoparticle content in the NR. In general, the minimum torque (ML) in Figure 5 is associated with a shear viscosity of the blend, while the maximum torque (MH) has a relation to the elastic stiffness of vulcanized samples. The results implied that with addition of nanosilica, both ML and MH values increased, meaning that the viscosity or stiffness of vulcanized NR increased with the presence of nanosilica particles. The difference between maximum torque and minimum torque, (MH-ML), which was obtained from the dynamic shear modulus test, corresponds indirectly to the crosslinking density of the vulcanization. The results indicated that the (MH-ML) exhibited the same tendency to the maximum torque and increased with increasing nanosilica loading as a result of increased cross-linking of the NR with the presence of nanosilica particles.

Figure 6 presents typical stress-strain curves of the nanosilica-reinforced NR composite materials. All samples exhibited the deformation-forced crystallization characteristics with the rapidly raised sharp slopes when the strain reached more than 1500% [36,43,44]. Meanwhile, the tensile stress and slope curves increased with increased contents of silica and reached the optimal properties at 3 phr of silica filled NR. The details of the mechanical characteristics are provided in Table 3.


**Table 2.** Curing properties of the silica/rubber compounds.

**Figure 5.** Cure curves of the investigated rubber compounds taken at 150 ◦C (black line—M0; red line—M1; blue line—M2; pink line—M3; olive line—M4; green line—M5).

**Figure 6.** (**a**) Typical stress-strain of cured samples (Black line—M0; red line–M1; blue line—M2; pink line—M3; olive line—M4; orange line—M5). (**b**) The bottom figure is the one expanded in the initial stage;


**Table 3.** Mechanical properties of cured NR/silica compounds.

The tensile strength exhibited the decreasing trend when the silica content exceeded 3 phr. Although the elongation at break decreased with increased silica concentration, the hardness of the NR showed the same trend with the tensile strength. Both changes in the tensile strength and elongation at break for different nanosilica loadings have a relationship with a cross-linking density of the cured NR composites. That of the NR increased with silica concentration, resulting in an increase of the tensile strength and a decrease of the elongation at break due to the decreased slippage among the molecular chains. The H-bond between silica particles and rubber chain prevented the mobility and slippage of the rubber chain. On the other hand, the fact that tensile strength only increased with silica contents up to 3 phr may be due to the agglomeration of nanosilica at a higher content. The crosslinking densities of samples with name M0, M1, M2, M3, M4, M5 were 7.82 <sup>×</sup> 105, 8.06 <sup>×</sup> 105, 8.19 <sup>×</sup> 105, 8.22 <sup>×</sup> <sup>10</sup>5, 8.36 <sup>×</sup> 105 and 8.45 <sup>×</sup> 105 mol/cm3, respectively. The trend of crosslinking density is the same as with the trend of MH and hardness. The hardness also increased with increased silica content. The incremental increase of crosslinking density resulted in the decreasing of elongation at break because of the prevention of the slip between each NR molecule. In addition, more energy was needed to break the linkage between each rubber molecule as the result of increased tensile strength.

The SEM image results of the tensile fractured surface of the NR composites both in the absence and presence of 3 phr of nanosilica were presented in Figure 7. These results indicated that the fractured surface of the 3 phr silica-filled NR was observed to be rougher in comparison with the pristine sample. Therefore, extra energy was required compared to the case of the smooth fractured surface of the pristine NR. This result is in agreement with the tensile results shown in Figure 6.

**Figure 7.** SEM images of the fracture surface of pristine natural rubber and 3 phr silica filled natural rubber.

The TEM images of both the natural rubber and 3 phr silica filled natural rubber are shown in Figure 8. The pristine NR sample had no silica particles, while the silica particles existed in the modified NR with nano scale from 20–60 nm.

**Figure 8.** TEM images of pristine natural rubber (**a**) and 3 phr silica filled natural rubber (**b**).

The thermal degradation of both the pristine NR and the 3 phr nanosilica filled NR composites was examined in terms of the weight loss (%) as a function of temperature, as shown in Figure 9.

**Figure 9.** TGA of cured natural rubber (NR) (black dot line) and 3 phr of nanosilica filled natural rubber (red dot line).

Thermal degradation of the NR can be explained via various processes such as chain-scission of the polymers, and breakage of the cross-linked portion. The silica filled NR composite exhibited a 2.5% higher decomposition temperature in comparison with the pristine NR because of the existence of H-bonds between silica particles and rubber chains with higher thermal stability. The thermal degradation of both pristine and silica filled NR occurs through the degradation of isoprene units at around 368 ◦C. The presence of nanosilica induced the higher residues of nanocomposite when compared with pristine NR due to the presence of inorganic filler, which is more thermally stable than NR. The TGA results demonstrate that the nanosilica filled NR showed only a very slightly higher thermal stability than that for the NR. The initial temperature decomposition, the maximum decomposition temperature, and residue of silica modified NR and pure NR were 268.2 ◦C, 368.2 ◦C, 1.1% and 266.8 ◦C, 366.3 ◦C, 3.7%, respectively. The entire thermal degradation of the nanocomposite can be explained by the two-step process. First of all, the rubber chains and cross-linking were deteriorated into smaller parts. In the second step, the smaller parts in the first step continuously degraded into volatile products and disappeared. The residual char was higher for the nanosilica-filled sample.

Figure 10 shows the XRD patterns of vulcanized pure NR and NR/3 phr silica nanocomposite. The broad diffraction peak around 20◦ is the noncrystalline structure of NR, while the diffraction peaks

between 30◦–50◦ are assigned to ZnO particles in the vulcanizates. None of the samples show obvious characteristic peaks of graphite or silica, indicating that silica particles are homogeneously dispersed in the rubber matrix. These results indicated that the silica did not keep the amorphous structure when embedded in natural rubber.

**Figure 10.** XRD spectra of vulcanized NR and NR/3 phr silica.

#### **4. Conclusions**

In this study, amorphous silica nanoparticles (between the sizes of 40–60 nm) were extracted from hexafluorosilicic acid waste produced by the Vietnamese fertilizer industry via a precipitation process. The production and utilization of the nanosilica by-product could become a reliable and sustainable solution for dealing with waste water from fertilizer plants environmentally as well as economically, regarding the waste water treatment. The resulting silica nanoparticles were then adopted as the filler for NR. The tensile strength of 3 phr silica-added NR nanocomposites increased by 20.6% compared to that of pristine NR. The elongation at break decreased with increased filler content and the hardness of the filled sample increased with increasing nanosilica content. The filled sample also exhibited better thermal resistance than the pristine sample due to the presence of nanosilica.

**Author Contributions:** Data curation, C.M.V., B.X.K.; formal analysis, C.M.V.; investigation, V.-H.N.; validation, H.J.C.; writing—original draft, C.M.V.; writing—review and editing, H.J.C.

**Funding:** This work was funded by Vietnam National Foundation for Science and Technology Development (grant number: 104.02-2017.15). One of the authors (H.J.C.) was partially supported by National Research Foundation of Korea (2018R1A4A1025169).

**Conflicts of Interest:** The authors declare no conflict of interest.
