Synthesis of a phosphaphenanthrene- and imidazole-containing curing agent and properties of its flame-retardant epoxy resin

[Objective] Epoxy resin(EP)has been widely used in the construction, aerospace, and electronics industries owing to its high thermal stability and chemical corrosion resistance.However, the inherent flammability of EP limits its broader application.Phosphorus-containing compounds, as promising halogen-free flame retardants, have been extensively used to enhance the fire safety of EP because of their environmental friendliness and safety.Despite these advantages, the addition of conventional flame retardants decreases the glass transition temperature(Tg)of EP and deteriorates its thermal stability.Consequently, this work aims to develop a novel flame retardant to simultaneously enhance the flame retardancy and Tg of EP.[Methods] In the first step, 9.6 g of 4-imidazole-formaldehyde and 9.9 g of 4, 4'-diaminodiphenylmethane(DDM)were reacted at 70 ℃ for 8 h using glacial acetic acid as catalyst to produce the Schiff base intermediate.Subsequently, 15.0 g of the intermediate was reacted with 18.3 g of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide at 85 ℃ for 24 h to synthesize the curing agent IDP containing phosphaphenanthrene and imidazole groups.The formula was determined based on the amount of IDP, with the molar ratio of the N—H groups in DDM and IDP to the epoxy group being 1:1. Unmodified EP(referred to as EP0)and EP/IDP-x(x=2.5, 5.0, and 7.5)materials with various IDP mass fractions(2.5%, 5.0% and 7.5%)were prepared for further analysis.The curing agent IDP was structurally characterized using hydrogen and phosphorus nuclear magnetic resonance(1H-NMR, 31P-NMR)and Fourier transform infrared spectroscopy(FTIR).Furthermore, the effects of IDP on the Tg, thermal stability, flame-retardant and smoke suppression properties of EP/IDP materials were investigated through relevant tests.[Results] 1H-NMR, 31P-NMR, and FTIR spectra confirmed the successful synthesis of IDP.Differential scanning calorimetry(DSC)analysis revealed a significant increase in the Tg of EP/IDP materials compared to EP0, with EP/IDP-5.0 exhibiting an increment of 9.7 ℃.Thermogravimetric analysis indicated a decreased maximum thermogravimetric rate and increased char residue yield for EP/IDP materials under a nitrogen atmosphere, with EP/IDP-5.0 showing a 3.49 percentage points increase in char residue yield at 800 ℃.EP/IDP-5.0 reached a high limiting oxygen index(LOI)of 33.0% and a UL-94 V-0 rating.Cone calorimeter test demonstrated that the peak heat release rate(PHRR), total heat release(THR), and total smoke production(TSP)of EP/IDP materials were reduced.Specifically, the PHRR, THR and TSP of EP/IDP-7.5 were decreased by 24.2%, 16.4%, and 16.7%, respectively, compared with those of EP0.Furthermore, the fire performance index(FPI)and fire growth rate index(FIGRA)were calculated.The results showed that the addition of IDP increased the FPI and decreased the FIGRA of EP/IDP materials, thereby improving the fire safety of EP.Dynamic thermomechanical analysis(DMA)revealed a slight decrease in the storage modulus at 30 ℃ and a slight increase in loss factor and Tg of EP/IDP materials.Additionally, the crosslinking density was calculated, and the results confirmed a decrease in crosslinking density.[Conclusion] Through a nucleophilic addition reaction, the curing agent IDP containing phosphaphenanthrene and imidazole groups was successfully prepared and employed as a co-curing agent for flame-retardant EP.DSC and DMA results indicated that the aromatic structures of IDP played a crucial role in increasing the Tg of the EP/IDP materials.The fire safety performance of EP/IDP materials was thoroughly evaluated using LOI, UL-94, and cone calorimeter tests, revealing that the addition of IDP significantly improved the fire safety of EP.Notably, EP/IDP-5.0 achieved a LOI of 33.0% and passed the UL-94 V-0 rating, with reduced heat release rate and smoke emission.Overall, the curing agent(IDP)containing phosphaphenanthrene and imidazole groups as designed in this study, demonstrates the ability to simultaneously improve the fire safety and Tg of EP, offering valuable insights and guidance for the development of novel organophosphorus curing agents in the flame-retardant epoxy materials.