| Phenolic resins (PR) are the most commonly used binder material of polymer-based friction materials, due to their excellent mechanical strength, high heat resistance and dimensional stability, cheap raw materials, good processability, etc. The surface temperature of friction materials rise sharply because of friction heat in braking process. Under high temperature environment, it is essential for the safety and service life of friction material to keep the stable friction coefficient and low wear rate. Phenolic resin is the most sensitive component of friction materials. Therefore, improving the thermal stability of phenolic resin is one of the keys to the development of high-performance friction material.In this thesis, the synthetic processes of novolac-type phenolic resin (NPR) were optimized and mesoporous silica materials with different structure and pore diameter were synthesized. Subsequently, NPR/mesoporous silica composites (NPR/MS) were prepared by in-situ polymerization.The friction materials were fabricated by using NPR/MS as matrix resins and their structure were characterized in detail. The main research contents are as follows:1. The synthetic processes of NPR were studied by orthogonal experimental design. A series of NPR which were used as matrix resin to prepare the composites were synthesized by adjusting the reaction temperature. The relation between the average molecular weight and molecular weight distribution of NPR and the properties of NPR composites were researched. Suitable synthetic processes of NPR was determined, which is molar ratio of phenol to formaldehyde of1:0.9, catalyst dosage of2.0%, reaction time of4h and reaction temperature of85?.2. A series of well-ordered mesoporous silica materials was synthesized by using the nonionic surfactants P123and F127as templates, and n-butyl alcohol as cosolvent, and mesitylene as pore expanding agents, and TEOS as silicon source via the sol-gel process. These mesoporous silica materials show well-ordered mesostructures, such as two-dimensional hexagonal and three-dimensional cubic structure, and have larger pore diameters (3.70?20.3nm), pore volumes (0.57?2.34cm3/g) and specific areas (605?751m2/g), characterized by X-ray diffraction (XRD), N2adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) measurements. The unique structures of mesoporous materials will contribute to the improvement of interfacial compatibility of polymers and mesoporous silica materials.3. NPR/MS composites were synthesized via in-situ polymerization. The structure and properties of the NPR/MS hybrid materials were analyzed by FTIR, TGA, N2adsorption-desorption, SEM, and TEM, respectively. The results showed that NPR macromolecular chains enter into the channels of mesoporous materials. Chemical bonds are formed between the NPR and MS. The glass transition temperatures and thermal stability of NPR/MS composites are obviously enhanced as compared with the pure PR, due to the uniform dispersion of mesoporous silica materials in the NPR matrix.4. Friction and wear properties of friction materials reinforced by mesoporous materials are investigated by using a D-MS constant-velocity tester. The results showed that the mesoporous materials in composites can improve the stability of friction coefficient and decrease the wear rate. Friction performances of the composites reinforced by the ball-milled SBA-15prerents obvious improvement in different treatments to SBA-15, and friction performances of the composites reinforced by KIT-6is superior to other mesostructures. A suitable additive amount of5%contributes to the stable friction coefficient and low wear rate of the reinforced composites. According to the analysis results of friction mechanism, the composites possess interpenetrating organic-inorganic network structure. Chemical bonds between PR and the surface of mesoporous materials were formed and PR chains or chain segments entered into the channel of mesoporous materials, which will contribute to improving the thermal stability of the composites. During the course of friction, it will improve high temperature tribological behaviors because PR molecular chain can entangle with the surface of mesoporous material, and therefore it is difficult for the mesoporous materials to spin off from the matrix resin.5. The investigation on mechanical properties of composites showed that the influence of reinforced mesoporous materials on the impact strength of the composites is not obvious, but to bending property is remarkable. Compared with pure PR matrix composites, bending property of the composites reinforced by SBA-15ball-milled or coupled treatment have been enhanced distinctly, and the bending strength and modulus of ball-milled reinforced composite increase by15%and10%respectively. Meanwhile, the bending property of the composites can be improved owing to suitable pore diameter and additive amount of mesoporous materials. DMA results revealed that the initial storage modulus of the composites was strongly depent on the pore diameter and contents of used mesoporous materials. By comparing with pure PR matrix, the stability of storage modulus of the composites are enhanced under high temperature, the peak of loss modulus was obviously shifted to the range of higher temperature, Tg of the composites are increased by20?. |
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