Synthesis of Biobased Flame-Retardant Carboxylic Acid Curing Agent and Application in Wood Surface Coating

The novel polybasic carboxylic acid (HCPVC) consisting of vanillin and hexachlorocyclotriphosphazene was synthesized successfully by the nucleophilic substitution reaction and the Pinnick oxidation. Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (1H NMR) were used to identify its chemical structure. HCPVC was used as a curing agent to employ in wood epoxy coating. The optimum curing condition of epoxy resin (DY-E44) with HCPVC and its apparent activation energy were determined by differential scanning calorimetry (DSC), and the results showed that it was suitable for utilizing in wood. After stepwise curing, the epoxy coatings cured with HCPVC presented excellent hardness, adhesion, water resistance, and solvent resistance. Meanwhile, HCPVC-EP exhibited the high char yield of 37.1% at 700 °C, as well as prominent flame retardancy with UL-94 V-0 rating and the higher LOI value of 30.7%, which are much higher than the common epoxy coatings and uncoated splines. Besides, the flame-retardant mechanisms of HCPVC-EP were investigated by scanning electron microscopy (SEM) after the UL-94 tests, and the consequence demonstrated that the formation of intumescent dense char layer restrained the further combustion and pyrolysis in the condensed phase. Meanwhile, the conclusion of thermogravimetric analysis/infrared spectrometry (TGA-FTIR) affirmed that HCPVC-EP released more inert gases during heating and suppressed in-depth decomposition in the gaseous phase.