| Silica is a kind of oil-independent and environmentally friendly filler used as a replacement for carbon black, leading to wide application in rubber industry. Because of the poor compatibility between silica and the rubber matrix and the severe agglomeration of silica, resulting from the surface hydroxyl groups of silica, the applications of silica are limited. So the surface modification must be introduced to sovle its limitation in rubber field. Nowadays, the widely used modification of silica is that the rubber matrix, silica and silane coupling agent (Si69) are mixed together in the processing instruments, resulting in that Si69grafts on the surface of silica to play a role of "coupling bridge". However, for this kind of modification, the silica agglomeration is also severe, and the processing energy consumption is relatively high because the original status of silica added in the mixing system is aggregates. Meanwhile, this modification ignores the essence of the surface modification of silica, which is the condensation between hydroxyl groups of hydrolyzed Si69and silica. Therefore, the Two-Step Modification (TSM) is studied to investigate the difference with One-Step Modification (OSM), demonstrating the mechanism of TSM and its adaptability in modification technology of precipitated silica. The main research contents are as follows:(1) Comparing to OSM, the effects of TSM to silica and properties of rubber composites were studied, elaborating the mechanism of TSM. The modification effects of silica were measured by Fourier transform infrared spectroscopy (FTIR), thermalgravimetic analysis (TGA) and dynamic light scattering (DLS). The dispersity of silica and dynamic mechanical properties were investigated by transmission electron microscope (TEM), dynamic mechanical thermal analyzer (DMTA) and rubber processing analyzer (RPA). The results indicated that for TSM, the ethyoxyl groups of Si69hydrolyzed to hydroxyl groups in the acidic condition, leading to the condensation between hydroxyl groups of silica and hydrolyzed Si69. The hydrolyzed Si69was successfully grafted on the surface of silica, resulting in the decrease of the particle size and suppression of the agglomeration of silica. By the comparison of the properties of rubber composites, it was showed that TSM could lead the interaction between silica and rubber molecular chains via chemical bonds, resuting in the improvement of the silica dispersion in rubber matrix and the reduction of processing energy consumption between silica and rubber matrix. TSM also could realize the balance of wet skid resistance and rolling resistance of rubber composites.(2) The effect of temperature on TSM was investigated by chareacterization of modified silica powders and the properties of the vulcanizates filled with modified silica; meanwhile, the mechanism of the surface modification at different temperatures was demonstrated. The results showed that comparing to the other temperatures, the particle size of50℃sample was lower, the weight loss in high temperature range was larger, and the agglomeration of silica suppressed distinctly, indicating the better modification effect at50℃. Meanwhile,50℃modified silica could make the vulcanizates own the weak filler network structure and better mechanical properties than that of the others, such as modulus at100%and300%, tensile strength, and reinforcing index. Moreover, the50℃modified silica has a high ability to reduce the rolling resistance with the maintenance of a good wet skid resistance of rubber composites. Through the exploration of modification mechanism, it was found that low modification temperature resulted in low grafting degree and poor grafting effect; meanwhile, high modification temperature brought about the severe auto-condensation of Si69and decreased the amounts of grafted Si69on the surface of silica, indicating that50℃was the relatively better modification temperature than the other ones.(3) The TSM was applied in different kinds of silica and the results showed that the Si69was succussfully grafted on the surface of silica, resulting in the decreasing of particle size. It illustrated that the TSM had the high applicability for the different kinds of silica. Meanwhile, in the comparison of properties of OSM modified silica filled rubber composites, the TSM decreased the amounts of Si69which took part in auto-condensation and deposition in rubber matrix, leading to the decrease of Si69as the curing agent in the vulcanizing process, thus showing the higher elongation at break and modulus at300%. Moreover, in comparison of OSM, TSM could more evidently decrease the filler network structure, improve the dispersion of silica in rubber matrix, and achieve the better balance of wet skid resistance and rolling resistance. However, the properties of rubber composites varied with the different kind of silica, and it might provide the basis to choose the silica in coordination with the rubber composites under the requirement of different properties.(4) The effect and mechanism of surface modification of silica with ultrasonic technology were investigated. The results showed that high power of ultrasound was beneficial to the surface modification effect of silica, decreased the filler network structure of vulcanizates, and improved the properties of rubber composites and the dispersion of silica in rubber matrix. It could be concluded that high power of ultrasound improved the activity of silica and Si69to increase the grafting probability, promoting the effect of surface modification of silica.The surface modification of silica by TSM was crucial to the application of silica in the rubber industry as a kind of oil-independent filler. The modification is of great importance in the development of new approaches for the surface modification of silica, and the modified silica has bright prospects in green tire applications. |
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