## 9 Thermoplastic Foams for Electromagnetic Interference Shielding  
Thermoplastic foams are widely used across diverse technological domains due to their lightweight nature, low thermal conductivity, high energy absorption and their superior strength/weight ratio. Owing to their excellent properties, thermoplastic foams have been utilized for building, construction, biomedical, automotive, sports, electronics and packaging applications. Fabrication of porous conductive thermoplastic nanocomposites with low density and high microwave absorption performance are very much favorable for practical device application. [PERSON] et al. [109] fabricated porous and lightweight nanocomposites with segregated conductive networks of polystyrene (PS) and multi-walled carbon nanotube (MWCNT) by combining high-speed mechanical mixing and scCO\({}_{2}\) foaming. PS powder and MWCNT were subjected to high speeds of 28,000 rpm in a mixer, which generated static electricity to facilitate the coating of MWCNT on PS powders via electrostatic adsorption. The coated nanocomposite powder was then compression-molded at 160 \({}^{\circ}\)C, which resulted in the formation of a segregated conductive network, as shown in Figure 9a. The solid nanocomposite sheets were then subjected to foaming using scCO\({}_{2}\), which accords with the sustainable fabrication of PS/MWCNT foams. Interestingly, at 1.88 vol.% MWCNT loading and a very low thickness of 1.8 mm, these nanocomposite foams exhibited a very low density of 0.47 g/cm\({}^{3}\), electrical conductivity of 8.05 S/m and maximum EMI SE of 23.2 dB in the X-band region [109]. It is also worth mentioning that at the same MWCNT vol.% (i.e., 7%), the nanocomposite foam exhibited higher microwave absorption (95.9%) than the solid shields (90.5%). This enhancement and absorption-dominant shielding mechanism were ascribed to ohmic losses and polarization losses in PS/MWCNT foams. This is an environmentally friendly technique to fabricate high-performance microwave absorbers for next-generation electronic devices.  
Figure 9: (**a**) Schematic depicting the fabrication of a segregated structure within porous PS/MWCNT composites. Reproduced with permission from [109]. Copyright 2019, American Chemical Society. (**b**) Schematic demonstrating the formation of a double-segregated structure within PP/CNTs/CB nanocomposite foams. Reproduced with permission from [110]. Copyright 2020, Elsevier Ltd.  
Polypropylene is yet another thermoplastic polymer that is widely used to fabricate flexible and robust olefinic foams which have diverse applications in upholstery, roofing, construction, biomedical and automotive sectors. However, its application as an efficient high-performance microwave absorber is very limited and is yet to be discerned. [PERSON] and co-workers made use of a similar strategy adopted by [PERSON] et al., to fabricate lightweight polypropylene (PP)/carbon nanotubes (CNTs)/carbon black (CB) nanocomposite foams. Herein, PP powders were blended with CNT/CB powder at different weight ratios in a grinder at 20,000 rpm/min at 100 \({}^{\circ}\)C. In due course, the filler is adsorbed to PP due to electrostatic forces, coupled with the surface heating effect. The blended powder compositions were then compression-molded at 180 \({}^{\circ}\)C, which resulted in the formation of a highly conductive segregated network, which was eventually foamed into porous PP/CNT/CB nanocomposites, as shown in Figure 9b. These porous foams showcased a very low density in the range of 0.082-0.101 g/cm\({}^{3}\) and an extremely low percolation threshold of 0.016 vol.%, which was likely due to the hybrid combination of the fillers. A maximum of 20 dB EMI SE value was attained at a 2.5 mm thickness, with specific EMI SE as high as 72.23 dB-cm\({}^{3}\)/g with an absorption-dominant shielding behavior [110].