Antiflammable, Transparent, Toughened, and Low Dielectric Epoxy Thermosets Modified with Siloxane-Bridged Phosphonates

Abstract Epoxy resin is widely used as a thermosetting material, but its inherent flammability, brittleness, and high dielectric loss limit its applications. In this work, three P/Si-containing flame retardants, namely, DEDP, DEDO, and TMDO, were synthesized via addition reactions using DOPO and DPPO as the P-containing moieties and DEMPhSiO and TMDVSiO as the Si-containing moieties. A series of EP/DEDP, EP/DEDO, and EP/TMDO formulations were prepared by incorporating these flame retardants at phosphorus loadings of 0.15, 0.25, and 0.5 wt % into DGEBA in the presence of DDS as the curing agent, and their properties were systematically evaluated. The results showed that low loadings of DEDP, DEDO, and TMDO endowed the epoxy thermosets with excellent flame retardancy, improved mechanical performance, and reduced dielectric constant and loss while maintaining good optical clarity and moisture repellence. For example, compared with pure EP (LOI = 22%; DC = 3.8 at 1 MHz), EP/DEDP-0.25P at only 0.25 wt % phosphorus loading achieved a UL-94 V-0 rating, an LOI of 28%, and a dielectric constant of 2.9 at 1 MHz (a 23.7% reduction relative to that of pure EP), along with 37% and 65% increases in tensile and impact strength, respectively. Similarly, EP/DEDO-0.5P and EP/TMDO-0.5P achieved UL-94 V-0 ratings and LOI values of 28% and 29%, respectively, with dielectric constants of 3.3 at 1 MHz (a 13.2% reduction relative to pure EP), along with increases of 100% and 109.1% in tensile strength and 74% and 191% in impact strength, respectively. Furthermore, decreases of 33.3% and 34.7% were observed in the PHRR and THR, respectively, and the average CO2 yield decreased from 5.1 to 4.4 kg/kg, whereas the average CO yield increased from 0.32 to 0.39 kg/kg during cone calorimetry analysis of EP/DEDP-0.5P. These findings offer an effective strategy for designing multifunctional and high-performance epoxy thermosets.