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Polyethylene terephthalate (PET) is a thermoplastic resin with excellent optical transparency, gas barrier and mechanical properties, widely used in fifibers, plastics and fifilm. However, PET has a long degradation half-life in the natural environment, and greenhouse gases such as carbon dioxide will be produced after incineration, indicating that recycling is an effective way to reduce plastic pollution and save energy, and how to correctly use recycled PET (rPET) has become a global concern. It was reported that each 0.45 kg recycled PET sheet used can reduce energy consumption by 84% and greenhouse gas emissions by 71% compared with the original PET production. However, during the cycle, the chain segment of rPET is prone to fracture under thermal shear force, and its molecular weight will decrease due to aging, thus affecting their properties. It is the key to improve the crystallization rate and mechanical properties to realize the application potential of rPET. Herein, nano SiO2 with epoxy functional group was fifirst synthesized in situ, then polymerized with styrene(St) and glycidyl methacrylate (GMA). SiO2/rPET nanocomposites were prepared by melt blending of recycled polyester and modifified SiO2. The composition and morphology of the modifified SiO2 and the nanocomposites were characterized. The thermal stability, crystallization and mechanical properties of the composites were analyzed. The interface interaction between inorganic particles and organic matrix was discussed, and the mechanism of nanocomposites was revealed.
The organic layer was grafted to SiO2 by free radical polymerization (Fig. S1). As shown in Fig. 1a, the characteristic new peaks of C¼C and C¼O appeared at 1632 and 1702 cm 1, respectively, indicating that KH570 has been bonded with the silanol group on the surface of SiO2. It can be clearly observed that the peak of C¼C at 1632 cm-1 disappears, indicating that the double bond belonging to SiO2-g-KH570 has been fully reacted. The spectra of styrene compared with glycidyl methacrylate copolymer SG, the epoxy absorption peak at 908 cm-1 and the characteristic absorption peaks at 699 and 758 cm-1 which are assigned to the benzene ring are also observed on SiO2-g-SG. The results confifirmed that St and GMA monomers have been grafted onto the surface of SiO2 particles.
The grafting effificiency of SiO2 can be calculated by Formula S1. The SG reduced more than 98% of its weight at 600 ℃ (Fig. 1b, Table S1). The residual weight of SiO2-KH570 is 83.7% at 600 ℃, indicating that the grafting effificiency of KH570 is 17.3%, while that of SG is 35.1%.
Fig. 1c and d show the morphologies of SiO2 and SiO2-g-SG. The unmodifified SiO2 particle size is about 80 nm. Due to the high surface energy of nano-SiO2, it is in an unstable state and tends to form agglomeration easily. After modifification, the organic layer on the surface can effectively improve the dispersion of SiO2. Compared with the matrix (Fig. S2) without nanofifillers, there are distinct particles. The average particle size is about 110 nm. It can be found that nano-particles disperse well with the addition of 1 wt% SiO2, while the content of nanofifillers increases to 3%. The distribution of nanoparticles on the substrate is worse, resulting in aggregation (Fig. S2). In addition, there are no obvious defects, indicating that the modifification of SiO2 improves the compatibility of the nanoparticles with rPET.
The crystallization process is controlled by nucleation and diffusion. Nano-SiO2 particles can be used as nucleating agent to increase the crystallization rate. The surface modifified SiO2 particles can signifificantly limit the movement of polyester chain segments and destroy the ordered structure of rPET. Two adjacent peaks appeared in the melting crystallization curve (Fig. S3), and the change in the melting peak shape was not obvious by the addition of SiO2. As shown in cooling crystallization curves (Fig. S3), it can be clearly observed that the crystallization peak is wide due to the poor crystallization ability of rPET. After the addition of SiO2, the crystallization peak became narrow and sharp, proving that SiO2 was heterogeneous nucleation agent that accelerated the crystallization (Table S2). After introducing SiO2, the relative crystallinity of the polyester (Xc) increased, which also confifirmed the crystallization-promoting effect of SiO2. However, it can also be found that when the amount of SiO2 was 1%, the crystallinity reached 27.45% (Table S2). Then, as the SiO2 content increased, the crystallinity of mSiO2/rPET nanocomposites decreased. It may result from the increased content of SiO2 nanoparticles. They can not disperse well in the rPET matrix and formed large agglomerates which hindered the diffusion process of crystallization and resisted the polyester segments to move into the crystal lattice, causing a decrease in crystallinity.
The interaction network controls the rheological behavior of the nanocomposites. Fig. S4a shows the complex viscosity of rPET with different SiO2 contents. The viscosity of rPET decreased with the increase of the shear rate, which was the typical behavior of pseudoplastic fluid. With the addition of SiO2, the viscosity of rPET increases obviously because the epoxy group in the organic layer of the modified SiO2 reacts with the hydroxyl and carboxyl groups at the end of the polyester chain and plays the role of "pseudo-crosslinking". The entanglement resistance required to break the chain was greater, and the viscosity is greater than that of rPET. As shown in Fig. S4b & c, both the storage modulus and loss modulus increase with increasing the shear frequency, mainly because the addition of SiO2 increased the rigidity of the chain. Since the SiO2 modifified with organic layer acted as a "pseudo chain extender", the force between the chains was stronger and more energy was required for deformation.
The mechanical properties of nanocomposites are presented in Fig. 2a and b. The specifific results are shown in Table S3. The mechanical properties of the PET bottles were reduced due to the degradation of the heat-shear forces during the recycling process and the pyrolysis during the melt extrusion process. After adding nano-SiO2, for the sake of its rigid structure and the strong interaction with rPET matrix after surface treatment, the tensile strength had been greatly improved. The Izod impact strength of the specimen can be used to reflflect the toughness of the composites. When the SiO2 content was 1%, 2%, and 3%, the impact strength was increased by 100%, 82.5%, and 62.5%, respectively, and the maximum was 1.6 kJ m-2 . Compared with the introduction of rubber-toughened polyester, nano-rigid particles can improve the toughness of the matrix without losing its rigidity and reach a rigid-tough balance. The organic layer on the nanoparticle was not only benefifit to the good dispersion of the nanoparticle, but also imparted a certain mobility to the segment. In addition, the mechanical properties of the composite material and the content of nanoparticles exhibited fifirst up after fall, indicating that the optimal addition of nanosilica is 1 wt% of rPET.
In order to explore the mechanism of nanocomposites, the impact fracture surfaces are given in Fig. 2c–f. The impact fracture surface of rPET was relatively flflat and smooth, and the roughness was consistent with the toughness characterization results. Moreover, cavities could also be observed on the impact fracture surfaces, indicating that the toughening of rPET was mainly due to the concentration of three-dimensional stress in the nano-silica particles. The cavity changed the stress state of the matrix around the cavity, so that it could trigger shear yield, which could make the composites absorb a large amount of energy when it was impacted. The epoxy groups in organic layer which was coated on the SiO2 formed covalent bonds with rPET, anchoring the nanoparticles in the rPET matrix, thereby improving the compatibility of the two-phase and reducing the interaction between the inorganic nanoparticles. As shown in Fig. 2g, a suffificient interface adhesion between the nanoparticle and the matrix ensured that the stress on the polymer could be successfully transferred to the nanoparticles. Meanwhile, the weak interaction among nanoparticles facilitated more nanoparticles participating in the deformation and energy consumption process.
Nano-SiO2 with epoxy functional groups was synthesized by a two-step method. SiO2/rPET nanocomposites were prepared by melt blending modifified SiO2 with recycled PET. The modifified SiO2, as a heterogeneous nucleation center, promoted the crystallization of polyester; the epoxy groups provided by the organic layer on the inorganic particles were cross-linked with the polyester, which increased the complex viscosity of the recycled PET; also acted as a stress concentration point. Organic layer increased the compatibility of polymers and inorganic particles, and reduced the interaction between inorganic particles. As a result, the nanocomposites reached a balance of rigidity and toughness. The properties of nanocomposites were related to the dispersion of SiO2. Under experimental conditions, the optimal content of SiO2 was 1 wt%. The nanoparticles did not agglomerate, and the crystallinity reached 27.45%, which was an increase of 28%; tensile strength and Izod impact strength were 37.13 MPa and 1.60 kJ m-2 , which were increased by 80% and 100%, respectively.
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