Combined Situ Polymerization and Thermal Cross-Linking Technique for the Preparation of Ammonium Polyphosphate Microcapsules with Composite Shell

Ammonium polyphosphate (APP) was encapsulated with boron containing phenolic resin (BPF) by in situ polymerization and then modified with nanosize silica gel through thermal cross-linking with the goal of further improving its hydrophobicity, thermal stability, and shell mechanical property. The chemical and physical features of APP microcapsules were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetery (DSC), and laser particle sizing. The hydrophobicity was assessed by the water contact angle (WCA). The shell mechanical property was characterized by the change of water contact angle before and after the mechanical property test. The residues from thermogravimetric analyzer and muffle burner were investigated. The results showed that the APP microcapsules with a Si–B containing phenolic resin shell had been produced successfully (Si-BPFAPP). Compared with BPFAPP, the hydrophobicity, shell mechanical property, and thermal stability of Si-BPFAPP were further improved; the residue rate at high temperature was significantly improved. In addition, the mean particle size decreased and the particle size distribution became more concentrated.