Enhancing performance of phosphorus containing vanillin-based epoxy resins by P–N non-covalently functionalized graphene oxide nanofillers

Abstract In this study, non-covalently functionalized graphene oxide (FGO) containing phosphorus and nitrogen was synthesized using dibenzyl N,N′-diethyl phosphoramidite (DDP) via a single step process. Meanwhile, novel bio-based phosphorus containing vanillin epoxy resin (VPE) was made via a two-step process and used as a flame-retardant adhesive. Subsequently, FGO was dispersed in the epoxy resin matrix at different weight ratios as reinforcement for improving mechanical, thermal and flame-retardant properties of the resultant composite systems. Curing behavior of the VPE and FGO mixtures with 4,4′-diaminodiphenylsulfone (DDS) as the crosslinker was investigated using a Differential Scanning Calorimeter (DSC). Thermal and flame-retardant properties of the cured VPE/FGO nanocomposites were systematically investigated by Thermogravimetric Analysis (TGA), Gas Chromatography – Mass Spectrometry (GC-MS), Limited Oxygen Index (LOI), vertical burning test (UL-94), and cone calorimeter test. Results indicated that all VPE/FGO nanocomposites exhibited excellent thermal and flame-retardant properties. In particular, VPE with 9 wt% of FGO achieved the highest LOI value (29.1%) and passed the V-0 rating in the UL-94 test. Furthermore, cone calorimetry test showed that flame retardancy performance of the VPE and VPE/FGO composites significantly improved compared to vanillin epoxy control resin without phosphorus. The gaseous and high boiling pyrolysis products of VPE cured by DDS were collected and characterized by GC/MS to reveal their formation mechanisms. The char layers of the cued VPE showed a high oxidation resistance with intumescent structures. The combined barrier and quenching effects of the char layer imparted VPE with excellent flame retardancy. This study illustrated a promising approach for synthesizing mechanically strong, thermally-stable and environmentally-friendly flame-retardant bio-based composite resins.