Influence of PEPA-containing polyether structure on fire protection of transparent fire-resistant coatings

Three kinds of novel PEPA-containing polyether flame retardants were synthesized by 1-oxo-4-hydroxymethyl-2,6,7-trioxa-l-phosphabicyclo 2.2.2] octane (PEPA), phosphorus oxychloride (POCl₃), and polyether with different structures (PEG, PPG, and PTMG). Their structures were confirmed by ¹H nuclear magnetic resonance (¹H NMR) and Fourier transform infrared spectroscopy (FTIR). The solubility test showed that PEPA modified by polyethylene glycol (PEG) and polypropylene glycol (PPG) had better water solubility than that modified by poly(tetrahydrofuran) (PTMG). The decomposition process of PEPA-containing polyether flame retardants (PCPE) was studied by thermogravimetric analysis (TG) and derivative thermogravimetry. A possible mechanism was proposed to analyze the influence of polyether structure on the thermal degradation process of PCPEs. Afterward, the PEPA-containing polyether flame retardants were mixed with melamine formaldehyde resin to prepare the transparent fire-resistant coatings. The influences of polyether structure on the properties of the coatings were investigated in detail by fire protection test, TG, FTIR, X-ray photoelectron spectroscopy (XPS), and scanning electron microscope. It was found that the fire protection of the coating and foam structure of char layer were significantly improved when the number of carbon atoms in a unit of polyether chain was less. TG results showed that the chain unit of polyether with less carbon atom number could increase the residue weights of the coatings. FTIR and XPS result illustrated that the char layers were mainly composed of aromatic rings and phosphorus oxide, and the antioxidation and char-forming ability of coatings were enhanced effectively with the decrease in the number of carbon atoms in a unit of polyether chain.