Ultra-fast preparation of epoxy composites underwater via frontal polymerization

•A controlled underwater synthesis, UV-initiated front polymerization, for the fast preparation of high-strength epoxy composites and adhesives is proposed.•The instantaneous flexural and compressive strengths of epoxy composites exceeded 14 MPa and 24 MPa underwater, respectively, in which the self-propagation velocity reached 41.6 mm min−1.•The epoxy-premixed solution was divided into A/B two-component forms to avoid pre-polymerization effectively. Designing underwater structural materials and adhesives with short polymerization time yet high strength is in pressing demand in many applications such as underwater construction and repair. In this work, a controlled underwater synthesis method, UV-initiated front polymerization (FP), for the fast preparation of high-strength epoxy composites and adhesives is proposed, and the whole fabrication progresses within several minutes. The epoxy-premixed solution was divided into A/B two-component forms to prevent pre-polymerization and significantly extended shelf life. The instantaneous flexural and compressive strengths of epoxy composites exceeded 14 MPa and 24 MPa underwater, respectively, with a self-propagation velocity of 41.6 mm min−1. Furthermore, the polymer composites exhibited long-term underwater durability. In addition, the epoxy-premixed solution could bond steel, aluminum, ceramic, and glass underwater within 1 d using FP with apparent shear strengths reaching 4.4 MPa, 3.9 MPa, 7 MPa, and 3.6 MPa, respectively. This in-situ underwater polymerization strategy presents a new method for rapidly creating structural materials, in-situ repair, 3D printing, and more.