Preparation of an aluminium phosphate binder and its influence on the bonding strength of coating

In this paper, aluminium phosphate binders were synthesized using Al (OH) ₃ and H ₃PO ₄ as the raw materials. These binders, with the curing agent MgO and filler ZrO ₂, were used to prepare coatings by brush painting on the carbon fibre-reinforced epoxy resin matrix composites. The influences of synthesis conditions such as the P/Al ratio, concentration of the reactant and reaction temperature on the viscosity of binders and the bonding strength of corresponding coatings were investigated by using a viscometer and a universal testing machine. The structures and compositions of aluminium phosphate binders were characterized by X-ray diffraction, Fourier transform infrared and Raman spectroscopy. The results show that with a decrease in the ratio of P/Al, the degree of polymerization of the aluminium phosphate binder increases, the viscosity increases, while the bonding strength of the coating decreases. When P / Al = 3 : 1 , the reaction product is Al(H2PO4)3 with the best properties of bonding strength. As the concentration of phosphoric acid solution increases in the range of 60–80%, the viscosity increases on account of larger quantity of viscous molecules in a unit volume and higher extent of polymerization of the phosphorus oxygen tetrahedron. The compositions of aluminium phosphate binders are almost the same when the reaction temperature changes from 120 to 180 ^∘C , so the viscosity of the binder and the bonding strength of the coating do not exhibit obvious changes along with temperature.