Steam-chest molding of polypropylene/carbon black composite foams as broadband EMI shields with high absorptivity

Polypropylene (PP) composite pellets with 5–30 wt% carbon black (CB) were blowed with subcritical CO2, and then employed as building blocks to assemble into large-sized PPCB foam broad with dimension of 60×30×10 cm3 by steam-chest molding. The resultant PPCB foams showed a closed-cell structure with cell size in hundreds of micrometers, which endowed them with low densities of 0.034–0.117 g/cm3. Due to the high CB content of 30 wt%, the foam exhibited a high conductivity of 1.3 S/m and an electromagnetic interference (EMI) shielding effectiveness (SE) of 20–31 dB over the broadband frequency range of 6–18 GHz, corresponding to ~99%–99.92% shielded electromagnetic energy. Further increasing the sample thickness from 4 to 6 cm led to much higher SE of 41 dB (~99.992% shielded power) over 8–12 GHz. More importantly, all PPCB foams possess an absorption-dominant shielding mechanism, and for the foams containing 25–30 wt% CB, their absorption coefficients are as high as 0.80–0.97, indicating ~80%–97% absorbed power. The unique features, as well as the foaming technologies, make our lightweight PPCB foams very potential in the scale-up manufacture of advanced EMI shields.