Organo-phosphorus flame retardants for unsaturated polyester derived from recycled poly(ethylene terephthalate)

In an attempt to suppress the flammability of unsaturated polyesters (RUP) derived from recycled poly(ethylene terephthalate) (PET) and investigate the relationship between flame retardant (FR) chemical structures and their flame retardancy, RUP was incorporated with four organo-phosphorus FRs; triphenyl phosphate (TPP), aluminum hydrogen phosphonate (AHP), diammonium hydrogen phosphate (DAP), and aluminum diethyl phosphinate (OP). The effects of adding FR with regard to the thermal stability of RUP/FR mixtures were investigated via thermogravimetric analysis (TGA). The chemical structure of FR and its mixtures with RUP were determined via FTIR and X-ray photoelectron spectroscopy (XPS). The morphology of the residual char was investigated using a field emission scanning electron microscope (FE-SEM). Flame retardancies were evaluated by a UL-94 vertical test and limiting oxygen index (LOI) measurements. The results indicated that the presence of FR significantly improved the flame retardancy and thermal stability of RUP. LOI values increased from 20 to 28 and V-0 ratings were obtained with an 8 wt% FR loading for DAP and with a 10 wt% FR loading for OP. In contrast, no ratings were recorded for the mixture containing 30 wt% of TPP or AHP. These findings clearly indicate that the efficiency of a phosphorus-based FR depends on the quantity of phosphorus elements within the FR itself, although interactions of the FR with the target polymer should also be accounted for when determining its effectiveness as a FR.