Novel phosphonated hardeners derived from diamino diphenyl sulfone for epoxy resins: Synthesis and one-pack flame-retardant formulation alongside dicyandiamide

Epoxy resins (ER) along with their curing agents and flame retardant additives are preferred to be formulated as a one-pack material. Increasing viscosity during storage and operation is a complicated problem for one-pack material. To resolve the above-mentioned problem, we conducted a synthesis on new phosphorus-containing curing agents of 4,4-diamino diphenyl sulfone (DDS) and diphenyl phosphoryl chloride (DPPC) to be taken advantage of alongside a combination of dicyandiamide (Dicy). The chemical structure of both of the synthesized compounds derived from DDS (i.e., DDS-DPPC and DPPC-DDS-DPPC) were characterized by FTIR, as well as 1H-, 13C- and 31P-NMR spectroscopies. Latency and curing behavior of ER/Dicy/DDS-DPPC and ER/Dicy/DPPC-DDS-DPPC were studied by viscometry and differential scanning calorimetry (DSC). The compounds’ cured products’ thermal properties went through an investigation via DSC, thermogravimetric analysis (TGA), and cone calorimetry. The results revealed that the novel phosphonated DDS-based epoxy hardeners induced higher thermal stability and latent curing potency for the ER networks. TGA results showed that either ER/Dicy/DDS-DPPC or ER/Dicy/DPPC-DDS-DPPC exhibited higher char yields and lower degradation rates than that of the cured ER/Dicy's. With the incorporation of new synthesized curing agents into epoxy resin, ER/Dicy/DDS-DPPC and ER/Dicy/DPPC-DDS-DPPC samples achieved UL-94 V-1 and V-0, respectively. During combustion, synthesized curing agents decomposed to produce phosphorus-containing compounds to form protective char layers in the condensed phase, and generated phosphorus-containing radicals which quenched the active radicals in the gaseous phase. DDS-DPPC and DPPC-DDS-DPPC demonstrated a bi-phase flame retardant effect. Moreover, the flame-retardancy induction was preliminarily established by hardeners to the thermosets. Therefore, it is a generally impressive and facile approach to modify latent flame-retardant curing agents for ER by modifying DDS employing DPPC. Taking advantage of the features, these one-pack flame-retardant epoxy materials can create a greater chance of multiple applications. Furthermore, the samples’ mechanical properties were evaluated by use of dynamic mechanical thermal analysis and flexural tests.