Molecular design of silane-bridged phosphorus-containing compounds and their flame retardant mechanisms, toughened, water absorption and low dielectric epoxy thermosets

Abstract

Three P/Si-containing flame retardants (FR1, FR2, and FR3) were synthesized via a simple addition reaction of 1,4-bis(dimethyl(vinyl)silyl)benzene (bDMVSiB) and 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (TMDVSiO) with diphenyl phosphine oxide (DPPO) and 6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO). The chemical structures of these flame retardants were confirmed using ¹H-, ²⁹Si-, and ³¹P-NMR, FTIR, and HRMS. All flame retardants were incorporated into the DGEBA matrix using DDS as a curing agent. Nine flame-retardant formulations (EP/FRn-mP) and a control sample (EP) were prepared for performance optimization. All formulations showed excellent flame retardancy without compromising key features of the EP resin. EP/FR3-0.25P passed the V-0 rating of the UL-94 vertical burning test at only 0.25 wt% phosphorus content. It also achieved a 60.1% increase in tensile strength and a 32.6% increase in impact strength. EP/FR3-0.75P improved the LOI to 29.5%. THR and PHRR decreased by 25.2% and 40.8%, respectively, compared to the pure epoxy thermoset, and transparency was maintained. EP/FR2-0.75P showed a 90.6% increase in tensile strength and a 57.3% increase in impact strength. The designed epoxy thermosets also demonstrated excellent moisture resistance and superior dielectric properties compared to pristine EP thermosets. For example, EP/FR1-0.75P, EP/FR2-0.75P, and EP/FR3-0.75P had lower dielectric constants (2.4, 2.8, and 2.5, respectively) than the pure epoxy thermoset (2.98) at 1 MHz. These findings highlight a feasible approach to balancing transparency, flame retardancy, mechanical performance, and dielectric properties in epoxy thermosets.