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

In this paper, aluminium phosphate binders were synthesized using $$\hbox {Al}(\hbox {OH})₃$$ and $$\hbox {H}₃\hbox {PO}₄$$ as the raw materials. These binders, with the curing agent MgO and filler $$\hbox {ZrO}₂$$ , 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 $$\hbox {P}/\hbox {Al}=3{:}1$$ , the reaction product is $$\hbox {Al}(\hbox {H}₂\hbox {PO}₄)₃$$ 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 $$180{^\circ }\hbox {C}$$ , so the viscosity of the binder and the bonding strength of the coating do not exhibit obvious changes along with temperature.