Flame retardant, toughened, reinforced, transparent and long shelf-life one-component epoxy resins via a P/N/Si-containing hyperbranched polymer

One-component epoxy resins (OCEPs) cured from latent imidazole-based curing agents that exhibit high toughness, high strength, high fire safety, long shelf life, low dielectric properties and transparency are eagerly awaited in advanced manufacturing. However, existing research on latent imidazole-based curing agents/OCEPs mainly focuses on simultaneously improving their flame retardancy and shelf life, but neglects their poor toughness, strength and dielectric properties, and often compromises their transparency. Herein, a hyperbranched polymer (Si-DP) containing P/N/Si was fabricated to further improve the flame retardancy, toughness, strength and dielectric properties of EP10 (the transparent OCEPs comprising the latent imidazole-based curing agent PHI-HPP). EP10 containing Si-DP (EP10/Si-DP) boasted a shelf life longer than 81 days. The synergistic flame-retardant effect of P, N and Si elements enabled EP10/Si-DP3 to achieve a UL-94 V-0 rating with a limiting oxygen index of 33.9 %, the peak heat release rate and total smoke production of EP10/Si-DP5 were reduced by 44.4 % and 25.5 %, respectively, compared with EP10. The increased free volume and Si-O flexible bonds resulted in a 261.1 % higher impact strength of EP10/Si-DP5 than EP10. The excellent interfacial bonding of Si-DP with epoxy resins and the increased rigidity groups led to 74.0 % and 60.7 % higher tensile and flexural strengths of EP10/Si-DP5 than EP10, respectively. The intramolecular cavities and the low-polar Si-C bonds of Si-DP reduced the dielectric constant and dielectric loss of EP10/Si-DP. Meanwhile, EP10/Si-DP maintained inherent excellent transparency and high glass transition temperature (Tg). Carbon fiber epoxy composites prepared from EP10/Si-DP5 exhibited remarkable improvement in flame retardancy, smoke suppression as well as mechanical and interfacial performances. Overall, this research proposes an effective approach for preparing long shelf-life OCEPs with excellent overall performance.