Matrix matters: Hyperbranched flame retardants in aliphatic and aromatic epoxy resins

We synthesized a library of phosphorus-based flame retardants (phosphates and phosphoramides of low and high molar mass) and investigated their behavior in two epoxy resins (one aliphatic and one aromatic). The pyrolytic and burning behavior of the two resins (via TGA, TG-FTIR, Hot stage FTIR, Py-GC/MS, PCFC, DSC, LOI, UL-94, Cone calorimeter) are analyzed and compared to the results of flame retardant (FR)-containing composites. A decomposition pathway incorporating the identified modes of action and known chemical mechanisms is proposed. The overlap of decomposition temperature (Tdec) ranges of matrix and FR determines the efficacy of the system. Low molar mass FRs strongly impact material properties like Tg but are very reactive, and high molar mass variants are more thermally stable. Varying P–O and P–N content of the FR affects decomposition, but the chemical structure of the matrix also guides FR behavior. Thus, phosphates afford lower fire load and heat release in aliphatic epoxy resins, and phosphoramides can act as additives in an aromatic matrix or a reactive FRs in aliphatic ones. The chemical structure and the structure-property relationship of both FR and matrix are central to FR performance and must be viewed not as two separate but as one codependent system. •The decomposition pathway of an aliphatic epoxy resin is elucidated via multi-methodical approach.•Phosphates and phosphoramides as low or high molar mass and hyperbranched polymer are added as FRs.•Phosphates: phosphorylation/ flame poisoning. Phosphoramides: additives in aromatic matrix but reactive in aliphatic matrix.•Chemical reactions crucial to FR modes of action require that FR and matrix decompose in proximate temperature ranges.•Flame retardancy is high when overlap is high. Chemical structure and polymerization influence decomposition temperature.