The composition and physical property of modified SiO2
Fourier-transform infrared spectroscopy (FTIR) can be used to verify the graft polymerization of silane coupling agents on SBR surface. Figure 5 shows the infrared spectra of KH550, KH560 and KH570 as well as corresponding silane coupling agent grafted SiO2. It can be seen from Fig. 5 (b) that the infrared spectrum of the SiO2 before and after modification has a large absorption peak near 1100 cm− 1, which is the antisymmetric absorption peak of Si-O-Si bond, and near 800 cm− 1 is the symmetrical contraction vibration of the Si-O-Si bond. The infrared spectrum of KH550-SiO2 is relatively flat compared to others in the range of 3500 − 3100 cm− 1, indicating that -NH2 in KH550 has been successfully grafted and modified on the surface of nano-SiO2. In Fig. 5 (a), KH550 shows characteristic peaks of -CH2- at 2940 cm− 1 and 1640 cm− 1, while KH550-SiO2 also has a shoulder peak at 2940 cm− 1 and 1460 cm− 1, which further indicates that KH550 has successfully modified nano-SiO2. Compared with FTIR spectrum of SiO2, in the spectrum modified by KH560-SiO2, the stretching vibration absorption peak of methyl appears near 2987 cm− 1 which is related to characteristic methyl peak of KH560, indicating that KH560 silane coupling agent has been successfully grafted to the second Si-O surface (He et al. 2019). In the infrared spectrum of KH570 and KH570-SiO2, there is a C = O stretching vibration absorption peak on the carbonyl group at 1718 cm− 1, which indicates that KH570 has been successfully grafted onto nano-SiO2.
The particle size of original and modified SiO2 is shown in Table 1. The surface morphology of original and modified SiO2 is shown in Fig. 6. Nano-SiO2 has a small particle size, a large specific surface area. Due to the large specific surface area, the powder is easy to agglomerate together, so that the total surface area is reduced, and the total energy is reduced. It can be clearly seen that the unmodified particles are larger than modified particles, and KH560 has the best modification effect on SiO2 particles. Unmodified nano-SiO2 has a relatively high polydispersity index (PDI) and the particle size, particle size, and unevenness appeared and severe aggregation. The particle size and PDI of the modified nano-SiO2 powder are reduced, indicating that the nano-SiO2 modified by the silane coupling agent can effectively prevent its agglomeration and achieve the modification purpose. On the one hand, the hydroxyl groups on the particle surfaces are replaced by organic functional groups, reducing the number of active silanol groups, thereby reducing the tendency of nanoparticles to agglomerate. On the other hand, the grafted long carbon chain increases the distance between particles and the steric hindrance of the hydroxysilyl polycondensation reaction on the particle surface, which increases the difficulty of effective collisions and more conducive to the dispersion of nanoparticles.
Table 1
The spectification of prepared nano SiO2
Sample
|
Z-Average
(nm)
|
Peak Size
(nm)
|
Peak Intensity
(%)
|
PDI
|
SiO2
|
300
|
220.8
|
65.2
|
0.343
|
KH550-SiO2
|
107.7
|
143.3
|
69.3
|
0.287
|
KH560-SiO2
|
67.6
|
92.7
|
100
|
0.210
|
KH570-SiO2
|
91
|
136.6
|
100
|
0.229
|
Bending property
The experiment was repeated at least 5 times for each sample. Figure 7 (a) and (b) shows the bending strength and modulus of pure jute/PLA fibers composites involving SiO2 and modified SiO2 nanoparticles with different mass fractions. It can be seen that the bending modulus of jute/PLA fibers composites is relatively low. With incorporation of SiO2, the bending strength and modulus of the composites is more or less improved. Compared with pure jute/PLA composite material, there is no significant improvement in the bending performance of the sample SiO2-4. However, after adding the nano-SiO2 particles modified by the coupling agents, the bending strength and modulus of 550-4 increased to 29.95 MPa and 1.25 GPa respectively, which were 149.5% and 76.8% higher than those of the jute/PLA composite, and increased by 137.4% and 72.0% compared with the unmodified SiO2 jute/PLA composite. The bending strength and modulus of 560-4 were increased to 36 MPa and 2.17 GPa respectively, and the bending strength and modulus of 570-4 were 34 MPa and 2.01 GPa. Compared with the jute/PLA fibers composites, the bending strength of 560 − 0.5, 560-1, 560-2, 560-4 and 560-8 composites increased by 2.8%, 96.6%, 74.7%, 74.2% and 68.3% respectively. It is also found from Fig. 7 (a) and (b) that as the mass fraction of different types of added SiO2 increases, the change trend of material bending strength and modulus is basically same.
The similar phenomena can be also observed in the bending strain and bending break work of jute/PLA composites. The bending break work was calculated by integrating the tensile stress–strain curves. As shown in Fig. 7 (b), the increase of SiO2 improves the bending strain and bending break work of jute/PLA composites.
In short, the bending performance of the sample increases with the increase of the SiO2 mass fraction, peaks at about 1wt%, and then decreases. That is, under the same mass fraction, SiO2 treated with different silane coupling agents has the same effect on the jute/PLA composite. The SiO2 sample modified with KH560 has the best bending strength and modulus. It is mainly due to the fact that realize the interface combination through the coupling agent, forming a better entirety, and then strengthening the effect of stress transmission(Yang et al. 2009; Sanivada et al. 2020; Fang et al. 2020; Li et al. 2013). Sample 560-1 has best bending performance resulting from the excellent dispersibility and uniform particle size as illustrated in Table 1.
The fracture morphology
In order to illustrate the toughening mechanism, the fracture morphology of the reinforced PLA-based composite after the tensile test is shown in Fig. 8. The tensile fracture surface of the jute/PLA composite, illustrated in Fig. 8 (a), shows large amounts of fiber extraction leading to a lower mechanical resistance to some extent. This is consistent with comparatively large elongation ratio of the composite material which also indicates a poor interfacial cohesiveness. Figure 8 (b) shows the SiO2 particles modified by KH560 are evenly distributed in PLA matrix. At this time, delamination occurs in the tensile section, and the bending strength is reduced. With the increase of SiO2, there are more holes appeared which are prone to cause fracture. Nano-SiO2 particles induce local plastic deformation of the PLA matrix, showing a rougher fracture surface. And as the stress concentration point, SiO2 particles produce a large number of small cracks, and has a small amount of fibers extraction, as shown in the Fig. 8 (d). There are a large number of cracks and fiber extraction at the break of sample 560-4. Therefore, sample 560-4 consumes a lot of energy when it breaks. Due to the poor dispersibility of SiO2, the modified SiO2 re-aggregated at 8% by mass fraction. As the mass fraction of SiO2 increases, there are more contact points between SiO2 and jute, which reduces the adhesion between jute and PLA, and the mechanical properties dropped rapidly, as shown in Fig. 8 (f).
Stretching property
The tensile property of the jute/PLA composite and the nano-SiO2 jute/PLA composites is shown in Fig. 9. The tensile strength of 560 − 0.5 increased from 1.47 MPa in the jute/PLA composites to 11.03 MPa. The elastic modulus of 560 − 0.5 is as high as 2.01 GPa, which is 614.2% higher than the 0.28 GPa of the jute/PLA composites. The tensile strength of 560-1 is 8 MPa and the elastic modulus of 560-1 is 0.81 GPa. The tensile strength of 560-2 is 7.59 MPa and the elastic modulus is 2.32 GPa. The tensile strength of 560-4 is 11.92 MPa and the elastic modulus is 2.57 GPa. The tensile strength of 560-8 is 5.02 MPa and the elastic modulus is 1.20 GPa. The tensile break work of SiO2-4, 550-4, 560-4, and 570-4 is increased by 138.0%, 389.7%, 609.5%, 417.0% compared with the jute/PLA fibers composites.
It is concluded from Fig. 9 that the cooperation of nano-SiO2 can effectively improve the tensile tolerance of composite materials. With the increase of nano-SiO2 content, the change trend of the tensile properties of the composite material before and after modification is basically the same. And when the SiO2 mass fraction is same, the sample modified by KH560 has the best tensile strength. This can be attributed to the small particle size and uniform shape of SiO2 modified by KH560, which is consistent with the DLS data.
When preparing jute/PLA laminates, SiO2 can be more evenly dispersed in the matrix, thereby enhancing the effective load transfer between jute and PLA, improving the tensile properties of the composite material. The silane coupling agent can serve as a bridge to improve the compatibility of the interface between SiO2 and jute/PLA, forming a better whole, and then strengthening the stress transmission. Figure 10 shows the interaction between the silica particles and the interface of the jute/PLA composite before and after the modification. The silane coupling agent can react with the silicon hydroxyl group on the surface of SiO2 after being hydrolyzed. The degree of aggregation of modified-SiO2 is reduced, exposing more binding sites that can interact with PLA. At the same time, the long-chain structure of the coupling agent will generate entanglement on the interface between jute fiber and PLA, which efficiently enhances the interface bonding performance, expedites interfacial stress transfer, and ultimately improves the mechanical property of the composite material.
Dynamic mechanical thermal analysis (DMTA)
Figure 11 respectively show the dynamic storage modulus (E'), loss modulus (E'') and loss coefficient tanδ(E''/E')of jute/PLA composites with different contents of SiO2 modified from room temperature to 180 ℃. As shown in Fig. 11 (a) and (b), the stored modulus values and corresponding temperatures of different samples at the beginning of chain segment movement. In the temperature range studied, the storage modulus of the jute/PLA composite modified by SiO2 is higher than that of the untreated jute/PLA composite. In addition, different variables showed the same change pattern. With the increase of SiO2 content, the storage modulus of the modified jute/PLA composite material increases first and then decreases. The increase in storage modulus and corresponding temperature of chain segment starting to move indicates that the interface adhesion between jute and PLA has been improved, resulting in greater stress transfer between them(Porras and Maranon. 2012). That is, the increase of the SiO2 content can improve the interfacial adhesion, but because of the poor dispersibility of SiO2, when the content is greater than 4%, the effect of the coupling agent on SiO2 modification is not obvious, and the jute/PLA interface adhesion begins to decrease.
The next study parameter is the loss modulus, which represents the energy dissipated by the jute/PLA composite under stress (Laly et al. 2003; Doan et al. 2007). It is observed in Fig. 11 (c) and (d) that the loss modulus value of the SiO2-modified jute/PLA composite is much higher than that of the untreated jute/PLA composite. It is known that the maximum value of loss modulus corresponds to the glass transition temperature (Tg) of the composite material. The Tg of the untreated jute/PLA composite is around 57.3 ℃. In contrast, the Tg of the SiO2-modified jute/PLA composite material moved to the high temperature zone, reaching a maximum of about 74.2°C.
Finally, loss factor, tanδ, which refers to the ratio of the loss modulus of the composite material to the storage modulus of the composite material, and represents the damping energy of the material. The decrease of the tanδ value indicates SiO2 improves the bonding strength between PLA and jute, and reduces the fluidity of PLA macromolecules in the composite material. Due to the improvement of the hydrophobicity of the SiO2-modified Jute/PLA, the interfacial adhesion is enhanced, and the fluidity of the polymer chains at the jute/PLA composite interface is reduced. Among the unmodified SiO2 and SiO2 jute/PLA composites modified by KH550, KH560 and KH570, the unmodified jute/PLA composites have the highest tanδ value, and the KH560 modified SiO2 jute/PLA composites has the highest tanδ value, indicating the dispersibility of SiO2 modified by KH560 is best. Among all SiO2 composite material modified by KH560, jute/PLA with a SiO2 content of 4% has the best interface bonding. As shown in Fig. 11 (e) and (f).
Comprehensive analysis of sample 560-4 has the best thermomechanical properties. Compared with the jute/PLA composite, the storage modulus, glass transition temperature and loss modulus have increased; tanδ has decreased. As shown in Fig. 11 (g), the comparison of DMTA between jute/PLA composite and sample 560-4. The glass transition temperature of sample 560-4 is 29.5% higher than that of the jute/PLA composite, indicating that the experimental optimization has improved the bonding strength of the interface between PLA and jute. The storage modulus values of different samples decrease with increasing temperature. The tanδ value of the jute/PLA composite material increased with the increase of temperature when the mass fraction of SiO2 treated by KH560 was 4%, until it reached the maximum at 69.2℃, and then the opposite trend was observed in the rubber area.