Synthesis of unsaturated polyester resin with phosphate based flame retardants and investigating their thermal, mechanical, and flame‐retardant properties

This study reports the effects of triethyl phosphate (TEP), diphenyl cresyl phosphate (DPK), and the combined amount of aluminum hydroxide (ATH) with TEP at different weight ratios as flame retardant (FR) fillers on the physical, structural, mechanical, and flammable properties of unsaturated polyester resin (UPR). The synthesis of unsaturated polyester chains based on maleic anhydride (MA), phthalic anhydride (PA), propylene glycol (PG), ethylene glycol (EG), and diethylene glycol (DEG) was conducted in a prototype stainless steel reactor at 200 ± 5°C. Unsaturated polyester chains confirmed by nuclear magnetic resonance (1H‐NMR) were diluted with styrene monomer to create ready‐to‐apply UPR. Viscosity tests were carried out on the samples that were prepared by mixing FR fillers with UPR at different loading rates without adding accelerator and initiator. Reactivity tests of the cross‐linking initialized UPR suspensions unveiled that FR fillers prolonged the peak time and lowered the peak exotherm temperature. FR filler‐induced alterations in the chemical structure of the cured samples were then monitored using Fourier transform infrared spectroscopy (FTIR). Flexural, hardness, tensile, impact, and heat distortion temperature (HDT) tests were performed on the solid samples with and without the FR fillers. The limiting oxygen index (LOI) of the samples was also examined, and TEP was eventually found to work better than DPK for the corresponding samples involved. Therefore, the samples produced by blending UPR with TEP as well as with the combined amount of TEP and ATH at different weight ratios were characterized through vertical burning (UL‐94), cone calorimeter, and smoke density tests. As a result, it was revealed that TEP operates very well on the gas phase, despite being incompetent on the condensed phase during combustion, whereas ATH acts on both gas and condensed phases in an equal mean. Furthermore, thermal degradation behavior of the samples with and without the FR fillers and their exhaust gas products were investigated using a thermogravimetric analyzer (TGA) coupled with a Fourier transform infrared spectroscopy (TG‐FTIR). All the findings considering the presence of FR fillers were then evaluated and discussed in a brief manner to sort out the best combination of the additives for the UPR resin. The hypothetical structure of unsaturated polyester chains confirmed by 1H‐NMR