Optimization of sol–gel coatings on the surface of ammonium polyphosphate and its application in epoxy resin

Surface-modified ammonium polyphosphate was prepared by a sol–gel method using tetraethoxysilane and methyltriethoxysilane as precursors. The reaction conditions were optimized in order to decrease the water solubility of the ammonium polyphosphate. Then, modified ammonium polyphosphate and ammonium polyphosphate were used as flame retardants for an epoxy resin to constitute an intumescent flame-retardant system. The combustion behavior and the water resistance of epoxy resin and epoxy resin–containing ammonium polyphosphate or modified ammonium polyphosphate were investigated by limiting oxygen index and UL-94 test. The results show that the water resistance of epoxy resin and modified ammonium polyphosphate composites was greatly improved. The water contact angle of the modified ammonium polyphosphate reached 138°, for which the results show that there are a lot of hydrophobic groups on the surface of ammonium polyphosphate. And then the properties of the modified ammonium polyphosphate and the chemical bonding of the surface elements in the modified ammonium polyphosphate were analyzed using a Fourier transform infrared spectrophotometer, scanning electron microscopy analysis, and X-ray photoelectron spectroscopy. The results show that both tetraethoxysilane and methyltriethoxysilane hydrolyze and condense to form a dense layer structure on the surface of ammonium polyphosphate and can react with the hydroxyl groups on the surface of ammonium polyphosphate and form a new Si–O–P bond.