Flexible metal-polymer electromagnetic shielding composite sealant for underground defensive structures

In underground command posts, the technology and equipment for welding large steel plates significantly influence the effectiveness of electromagnetic shielding in the overall construction. Herein, we introduce an electromagnetic shielding sealant designed to replace traditional full-welding techniques for joining steel plates, in which two-component polyurethane, nickel–iron alloy powder, and silane coupling agents were used as the foundational material, the filling compound, and the modifiers, respectively. Specifically, we examined the electrical resistivity, electromagnetic shielding efficiency, shear strength characteristics of the samples with nickel–iron alloy powder and polyurethane modified by various silane coupling agents under diverse conditions, including 3-aminopropyl triethoxy silane (KH550), 3-glycidyloxypropyl trimethoxy silane (KH560), 3-trimethoxysilyl propyl methacrylate (KH570), 3-mercaptopropyl triethoxy silane (KH580), 3-mercaptopropyl trimethoxy silane (KH590), and isocyanatopropyl triethoxy silane (IPTS). The results indicate that the optimal performance of the electromagnetic sealant is achieved when the nickel–iron alloy powder is modified with KH550, together with the addition of 0.5 wt% of KH560 into the polyurethane. A 1 mm thick sample exhibits low resistivity of 1.24*10^–4 Ω•m, high shear strength of 141 MPa, and good break elongation of 9.36% with electromagnetic shielding ranging from 57.9 dB to 102.7 dB in the frequency range of 30–1.5 GHz. As a result, a sample steel plate with a specific design exhibits a shielding effectiveness exceeding 60 dB across different frequency bands, fulfilling the criteria for practical engineering applications.