Bio-based curing agent for epoxy resins: Simultaneously improved toughness, strength, and flame retardancy

Improving the toughness of epoxy resin (EP) while maintaining its strength is still considered a huge challenge. Herein, a novel bio-based curing agent, PA–DAD, has successfully been developed for EP that provides both mechanical reinforcement and flame retardancy. This agent was prepared via a straightforward neutralizing process between phytic acid (PA) and 1,10-diaminodecane (DAD). A series of bio-based curing agents labeled PA₁–DAD₆, PA₁–DAD₉, and PA₁–DAD₁₂ were produced by adjusting the PA and DAD stoichiometric ratio. During the curing process, PA–DAD dissociates into PA and DAD, PA functions as an ionic cross-linking site within the EP structure. This considerably enhances the mechanical properties of the EP compared with using DAD alone as a curing agent. As a consequence, the tensile, flexural, and the impact strength values of EP/5 % PA₁–DAD₆ enhanced by 103 %, 70 %, and 168 %, respectively, compared with EP/5 % DAD at a 5 wt% addition. Additionally, the tensile and flexural toughness values of EP/5 % PA₁–DAD₆ were 1250 % and 649 % higher compared to those of EP/5 % DAD, respectively, demonstrating the exceptional toughening effect of PA–DAD on EP. Furthermore, the flame retardancy of EP/PA–DAD improved because of the high P content in PA–DAD. Compared with EP/5 % DAD, EP/5 % PA₁–DAD₆ shown decreases of 10.7 % in peak heat release rate, 8.4 % in total heat release, and 13.6 % in total smoke production. The straightforward fabrication process, exceptional mechanical enhancement, and use of sustainable bio-based raw materials make PA–DAD highly suitable for large-scale production of advanced EP materials.