Superior Toughness–Strength Epoxy via Biocomposite Curing for Deep-Sea Equipment Using Multifunctional Syntactic Foams

A modified epoxy resin with improved toughness, good mechanical properties, and enhanced damping performance was developed as a potential matrix for syntactic foams in marine engineering. Polyurethane prepolymer (PUP) was grafted onto the main chain of epoxy resin to synthesize polyurethane-modified epoxy resin (UE44). Tung oil anhydride (TOA) was synthesized and mixed with methyl hexahydrophthalic anhydride (MeHHPA) to form the composite curing agent (TMA). DMP-30 was used as the accelerator in the UE44/TMA system, and its performance was systematically studied. PUP and TOA enhanced the toughness of the epoxy resin, while the use of MeHHPA maintained its high strength. At 30% PUP content and a TOA:MeHHPA mass ratio of 1:1, the system exhibited a tensile toughness of 11.8 MJ/m³, tensile strength of 21.7 MPa, and compressive strength of 181.4 MPa. According to the test results, the damping performance and thermal stability of the UE44/TMA were improved. Hollow glass microspheres (HGM) were added to the resin formulation with the optimal performance to prepare a syntactic foam (PTESF) with a density of 0.59 g/cm³, which exhibits strength, toughness, damping, and thermal insulation properties. Compared with the unmodified epoxy syntactic foam (ESF), its uniaxial compressive strength increased by 56.7%, and damping temperature range (tanδ > 0.3) expanded by 45.5%. The suitability of PTESF for deep-sea environments up to 2,500 m has been demonstrated via the hydrostatic pressure test.