Molecular design of silane-bridged phosphorus-containing compounds toward epoxy thermosets with flame retardancy, improved toughness, low water absorption and low dielectric constants

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 diphenylphosphine 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 the flame retardants were incorporated into the DGEBA matrix using a DDS as a curing agent. Nine flame-retardant formulations (EP/FR_n-mP) and a control sample (EP) were prepared for performance optimization. All the 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/FR₃-0.25P achieved an LOI of 26.5% and UL-94 V-0 at only 0.25 wt% P, while EP/FR₃-0.75P reached an LOI of 29.5% with 25% THR and 41% PHRR reductions. EP/FR₂-0.75P delivered +91% tensile strength and +57% impact strength gains, while EP/FR₃-0.25P showed +60% tensile strength and +33% impact strength improvements. Compared with pristine EP thermosets, the designed epoxy thermosets also demonstrated excellent moisture resistance and superior dielectric properties. 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 for balancing the transparency, flame retardancy, mechanical performance, and dielectric properties of epoxy thermosets.