Synthesis of Flame Retardant Based on Phosphaphenanthrene and Flame Retardancy Study of Epoxy Resin Modified by Intumescent Flame Retardant System Composed of Ammonium Polyphosphate

An novel phosphaphenanthrene derivative, 6H-dibenzo[c,e][1,2]oxaphosphinine-6-sulfide (DOPS), was synthesized through the reaction of DOPO and phosphorus pentasulfide (P2S5). The structure of the synthesized compound was characterized and investigated by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), hot stage melting point determination, and elemental analysis. This flame retardant was aimed at improving the flame retardancy of EP. The intumescent flame retardant (IFR), composed of DOPS and ammonium polyphosphate (APP) at DOPS/APP ratio of 1:2, was added into EP to form a series of epoxy resin (FR-EP) modified by the flame retardants. The thermal stability was studied by thermal gravimetric analysis (TGA). Flame retardant properties as well as burning behavior of the epoxy resin composites were investigated by limited oxygen index (LOI) and vertical burning test (UL-94). Fire properties of the modified epoxy resin were tested by cone calorimeter. The morphological characteristics of the char residue after cone test were studied by scanning electron microscopy (SEM). DOPS/APP flame retardant increased not only the UL-94 grade but also the LOI value. It was also found that the EP modified by IFR still past UL94 V-0 rating, and its limited oxygen index value reached 29.2% when the flame retardant content was 30 wt%. TGA data indicated that the amount of the char residue at 700 degrees C was 49.33%. The cone experimental results showed that the values of av-HRR, pk-HRR, and av-EHC of the IFR-EP were lowered to 70%, 79%, and 60% of those in the original EP, respectively. When the amount of DOPS was increased to 30 wt%, a high LOI value of 27.6% was obtained, the DOPS-modified EP could still pass UL94 V-0 rating. A series of comprehensive tests showed that DOPS/APP was effective flame retardant on epoxy resin and the flame retardant effect of the compound flame retardant was better than that of the single flame retardant. TGA, DTG, CONE, SEM comprehensive analysis revealed that epoxy resin modified by DOPS/APP was more thermo-stable compared to the pure epoxy. Filled with flame retardant, DOPS/APP-EP formed the continuous, intact and porous foam char layers after burning test. By covering the surface of the polymer, char layer prevented mass and heat exchange from circumstance.