Surface-percolated GNP coatings on 3D-printed PLA lattices for tunable EMI shielding and flame-retardant multifunctionality

The growing demand for lightweight systems capable of integrating multiple functionalities has accelerated interest in multifunctional material systems (MFMS). This work presents a scalable approach for fabricating such systems by combining 3D-printed porous PLA lattices using the Fused Deposition Modeling (FDM) method at 30% infill, followed by dip-coating of graphene nanoplatelets (GNPs). Up to nine coating cycles were applied to diamond-lattice structures, enabling tunable electromagnetic interference (EMI) shielding, improved flame retardancy, and surface-controlled electrical conductivity. FESEM analysis revealed compact and laminar (platelet-stacked) GNP assemblies, with early cycles producing uniform coverage and later cycles showing increased surface densification. EMI shielding effectiveness (SE_T) across the X-band (8.2-12.4 GHz) increased with coating cycles, reaching ∼25 dB after nine cycles. Shielding was absorption-dominated, and a resonance peak near 10-10.5 GHz demonstrated the potential to create frequency-tunable absorption performance using pore geometry. Electrical conductivity increased almost linearly with coating cycles, reaching 10² S m^-1 at nine cycles. Notably, surface-percolating conductive networks emerged after the first cycle-distinct from classical bulk percolation-highlighting the efficiency of surface-enabled charge transport on 3D architectures. Flame-retardant performance also improved significantly: thermogravimetric analysis (TGA) showed a 24.4 °C rise in T_d-max and a 340% increase in char yield, while the Limiting Oxygen Index (LOI) increased from 19.1% (neat PLA) to 27.5% after nine cycles. Surface engineering steps-including NaOH etching and PEI functionalization-enhanced wettability and facilitated uniform GNP deposition, improving strong adhesion, especially at lower coating cycles.