Properties of surfactant-modified illite and its epoxy composites

[Objective] Epoxy resin(EP)is widely used in adhesives, insulation, and electronic packaging due to its excellent adhesion, electrical insulation, and chemical resistance.However, its flammability and the production of large amounts of smoke during combustion significantly limit its applications.To address these limitations, this study developed flame-retardant and mechanically enhanced EP composites using modified illite as a flame retardant.Illite, a natural clay material with a structure similar to that of montmorillonite and halloysite, exhibits excellent thermal stability, acting as a physical barrier to suppress the release of flammable gases during combustion, and releasing nonflammable gases such as water during decomposition to dilute combustible gas concentrations.However, the large amount of illite required often compromises the mechanical properties of composites.To overcome this challenge, surface modification of illite using surfactants such as sodium dodecyl sulfate(SDS), cetyltrimethylammonium bromide(CTAB), and dodecylamine(LA)was performed to improve its compatibility with the EP matrix and enhance its flame-retardant performance.[Methods] The modified illite flame retardants(ILS, ILC, and ILA)were prepared through a simple surface adsorption modification strategy under acidic conditions, utilizing the electrostatic interaction and interfacial coordination mechanisms of the surfactants.The modification process involved adjusting the pH of the solution to 3-4, allowing protonation of hydroxyl groups on the illite surface, thereby enhancing the adsorption efficiency of the surfactants.The modified illite was characterized through scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FTIR), X-ray diffraction(XRD), and water contact angle(WCA)measurements confirmed the successful modification, as evidenced by the presence of characteristic functional groups of the surfactants and the increased hydrophobicity of the modified illite.EP composites were then prepared by incorporating the modified illite flame retardants into the EP matrix through a two-step curing process.The thermal stability, flame retardancy, and mechanical properties of the composites were systematically investigated.[Results] Thermogravimetric analysis(TGA)revealed that the modified illite exhibited enhanced thermal stability compared to unmodified illite, with higher residual char content at 800 ℃, indicating its potential to improve the flame retardancy of EP composites.The limiting oxygen index(LOI)and cone calorimeter tests were conducted to evaluate the flame-retardant performance of the EP composites.The LOI of pure EP was 22.87%, while the addition of 6%(by mass)ILA increased the LOI to 27.63%, indicating a significant improvement in flame retardancy.Cone calorimeter results showed that the peak heat release rate(PHRR)and total heat release(THR)of EP/ILA6 were reduced by 28.31% and 32.01%, respectively, compared to pure EP.Furthermore, the smoke production rate(SPR)and total smoke production(TSP)were also significantly reduced, demonstrating the effectiveness of the modified illite in suppressing smoke emissions.The enhanced flame retardancy of the composites was attributed to the synergistic effects of the modified illite and surfactants, which promoted the formation of a dense and thermally stable char layer, acting as a physical barrier to heat and oxygen transfer during combustion.The mechanical properties of the EP composites were evaluated through tensile and impact tests.The results showed that the addition of unmodified illite and ILS reduced the elongation at break and tensile strength of the composites due to poor compatibility between illite and the EP matrix.In contrast, the modified illite, especially ILA, significantly improved the mechanical properties of the composites.The elongation at break and tensile strength of EP/ILA6 increased by 25.48% and 53.43%, respectively, compared to pure EP.This improvement was attributed to the better dispersion and interfacial compatibility of the modified illite in the EP matrix.However, the impact strength of the composites decreased with the addition of illite and its modified forms, which may be due to the rigid nature of the fillers.[Conclusion] Among the modified illite flame retardants, ILA exhibited the best performance, achieving a balance between flame retardancy and mechanical properties.The findings provide a new approach for the functionalization of natural minerals and their application in advanced flame-retardant polymer composites, with promising potential in packaging and other fields where fire safety and mechanical performance are critical.