Study on thermal degradation and combustion behavior of flame retardant unsaturated polyester resin modified with a reactive phosphorus containing monomer

A halogen-free phosphorus-containing monomer (TAOPO) with a P-C bond was successfully synthesized and used as a co-curing agent to prepare intrinsic flame-retardant unsaturated polyester resin (FR-UPR) by radical bulk polymerization with different TAOPO content. The thermal degradation and flame retardancy of pure UPR and FR-UPR were investigated by thermogravimetric analysis (TGA), cone calorimetry tests and limiting oxygen index (LOI). As the phosphorus content increased to 3%, FR-UPR (URP-3) showed a lower peak heat release rate (PHRR) and total heat release (THR), reducing by 45.7% and 45.5% those of pure UPR, while the LOI value and char residue increased markedly. Besides, thermal-oxidative degradation behaviors of different UPR samples were characterized by real-time infrared spectrometry (RT-IR) and thermogravimetry-Fourier transform infrared (TG-FTIR) spectroscopy, revealing the degradation mechanism. Furthermore, the residual char of UPRs was investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results indicated that the incorporation of TAOPO into UPR could effectively prompt the UP matrix to form a more compact char layer which acted as a protective barrier to reduce heat release during combustion.