Fabrication of silane and nano-silica composite modified Bio-based WPU and its interfacial bonding mechanism with cementitious materials

Bio-based waterborne polyurethane (WPU) using renewable resources has grown with the increasing scarcity of petroleum resources and people's awareness of sustainable development and environmental protection. The unfavorable physicochemical properties hinder its further development in the field of concrete protective coatings, also the bonding mechanism between the Bio-based WPU and cementitious materials has little scientific backing. In this work, the environmentally benign bio-based WPU was synthesized by employing soybean oil as the initial raw material, which was prepared by prepolymer method through a series of chemical reactions of epoxidation, hydroxylation and stepwise polymerization. The chemical cross-linking of gamma-aminopropyltriethoxysilane(APTES) combined with physical reinforcement of nano-SiO2 strategy was used for the composite modification. The particle size of emulsion, molecular structure, water resistance, tensile and thermal properties, microstructures of the Bio-based WPU were investigated by DLS nanoparticle size measurement, Fourier transform infrared spectroscopy (FT-IR), universal mechanical testing, water immersion, differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), scanning electron microscopy (SEM) respectively. The results shows that the APTES has been successfully grafted into the Bio-based WPU chain and the nano-SiO2 was dispersed homogeneously in the WPU matrix. The Si-O of with low surface energy introduced to the Bio-based WPU by APTES cross-linking and the favourable interfacial covalent bonding between the nanoSiO2 and Bio-based WPU have a synergistic effect in enhancing the water resistance, tensile strength and thermal stability. The pull-off testing shows the cement and interfacial mixed failure was the primary failure mode, the bonding strength of the composite modified Bio-based WPU shows an increasing trend with increasing content of nano-SiO2, which increases by 206.56 % to reach 1.87 MPa compared with Bio-based WPU without modification. The interfacial bonding mechanism of SiO2-SiWPU composite film with cementitious materials are mechanical interlocking effect, coordination bonding, hydrogen bonding and electrostatic adsorption, demonstrating the Biobased WPU organic-inorganic composites coating can be a promising candidate for concrete protecting.