Soft and Mechanically Robust Electromagnetic Interference Shielding Material in Polypropylene Reinforced by Aligned Steel Fibers and Crystalline Structures

With growing demands arising from technology development for highly durable and easily moldable materials in flexible electronic devices, electromagnetic interference (EMI) materials are gradually shifting from rigid metals to soft polymer composites. Current flexible EMI shielded materials with high electrical conductivity have limited mechanical robustness owing to unstable filler–polymer structures and filler agglomerations. This work prepares highly conductive EMI shielding materials from steel fibers (SFs) and polypropylene (PP) composites using extrusion preprocessing and postprocessing methods, such as compression molding and injection molding. The extrusion process provides highly crystalline SSF/PP composites with mechanical strength at break of up to 30 MPa and a high elastic modulus of up to 2.5 GPa. Extrusion followed by injection processing forms an aligned structure of SSF and creates β crystalline phases inside the PP matrix. The SSF/PP composites prepared by the extrusion-injection process demonstrated flexibility and bending to 180° with full recoverability and strain to failure of 250% for 0.2–1 vol % and 80% for 2 vol % SSF. 2 vol % SSF/PP composites exhibit a high out-of-plane electrical conductivity of 0.25 S cm–1, an in-plane electrical conductivity of 12 S cm–1, an EMI shielding effectiveness of 25 dB, and a high impact strength of 4.7 kJ m–2. The study provides a facile, scalable, and solvent-free strategy for high-performance, soft, and durable EMI shielding materials with great potential for flexible and portable electronics.