Effect of Functionality of Organophosphorus Flame Retardants on Flammability and Thermal Stability of DGEBA-Based Epoxy Resin Nanocomposites

Epoxy resins have been extensively used in fire hazard environments, such as printed circuit boards, electrical isolation materials, adhesives, construction, and transportation due to their economically viable, simple processing. Therefore, the development of thermally stable and flame-retardant epoxy resin systems is essential. The aim of the present study was to study the effect of the functionality of organophosphorus flame retardants on DGEBA-based epoxy resin nanocomposites on thermal stability and flame retardancy. DGEBA (diglycidyl ether of bisphenol-A)-based epoxy resin nanocomposites having 2.0 wt% phosphorus were prepared with organophosphorus flame retardants with different functionalities by using an in-situ polymerization method. The flame retardant compounds uni-functional 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and bi-functional 2-(6-oxid-6H-dibenz [c, e] [1, 2] oxaphosphorin 6-yl) 1, 4-benzenediol (DOPO-HQ) were prepared. The thermal behavior of composites was studied by TG and DTA techniques. The flammability behavior was investigated by UL-94 and limiting oxygen index (LOI) tests. The XRD and TEM results showed the mixed dispersion of nanoclay platelets in an epoxy matrix. The thermal stability of the epoxy composite (EPDOPO-HQ) containing bi-functional DOPO-HQ is increased by 16 oC in comparison to the epoxy composite (EPDOPO) containing uni-functional DOPO. According to the TG analysis, the addition of nanoclay was observed to be more effective and synergistic with bi-functional DOPO-HQ as the EPDOPO-HQ/NC sample gains more resistance to degradation after around 450 oC and also gave rise to a high char yield. Epoxy resin samples containing reactive flame retardants gave UL-94 V-0 rating, but further addition of 2.0 wt% nanoclay lowered the rating from V-0 to V-1. TG analysis of the epoxy composite samples showed that the addition of nanoclay were observed to be synergistic with bi-functional flame retardant (DOPO-HQ) as the EPDOPO-HQ/NC sample gained more resistance to degradation after around 450 oC due to the formation of mixed intercalated and exfoliated structure. The EPDOPO-HQ sample gave a high char yield with increased onset degradation temperature, high thermal stability as well as high flame retardancy.