## 10 Polyurethane Foams for Electromagnetic Interference Shielding  
Polyurethane foams are synthetic porous materials with excellent physicochemical properties, high compressive strength, excellent chemical resistance and ease of processability, which makes them an ideal candidate for a wide range of applications in emerging sectors, such as high-frequency electronics, aerospace and automobile sectors. PU foams with tailor-made properties can be achieved by altering the ratio of hard and soft segments, polydispersity, molecular weight and also the degree of crosslinking. The presence of strong hydrogen bonding in polyurethane creates linkages between polymer chains, which in turn enhances the overall mechanical properties of PU foams. The increasing demand for high-performance and lightweight materials has motivated researchers to fabricate porous architectures by incorporating nanof fillers such as carbon nanotube (CNT), carbon nanofibers and graphene. Mostly, these nanof fillers are introduced into polyol, chain extender or prepolymer during PU synthesis, before adding the foaming agent. The resulting conducting polyurethane nanocomposite foams have been exploited for a plethora of applications, such as oil absorption, thermal insulation, sensors and EMI shielding. [PERSON] et al. [111] fabricated a series of multilayered thermoplastic polyurethane/graphene (PUG) composites by stacking single-layered PUG foams one above the other. By arranging PUG foams in different orders, they could realize a gradient in the concentration of graphene, which in turn enhanced the microwave absorption performance of PUG composites, without significant changes in their EMI SE values. Figure 10a shows the strategy adopted to fabricate large-scale manufacture of PU/graphene nanocomposite foams. Sandwiching PUG foams in a particular fashion was also another approach which was found to be more advantageous in improving the EMI SE values of these foams, and an optimized design also enabled the PUG foams to absorb more than the gradient foams. A maximum EMI SE of 24 dB was obtained at 18 GHz, with minimal reflection of EM waves [111]. Another approach adopted by [PERSON] et al. [112] was to fabricate porous thermoplastic PU/graphene oxide nanocomposite foams without the usage of any organic volatile solvents or a solution mixing process. Initially, TPU-coated RGO powders were prepared by mixing TPU particles in GO suspension with ascorbic acid and in situ reduction of GO to form TPU/RGO powders. These nanocomposite powders were then compression-molded into sheets by subjecting to 10 MPa at 120 \({}^{\circ}\)C for 10 min to generate a segregated network structure, as shown in Figure 10b. The solid nanocomposites sheets were then exposed to scCO\({}_{2}\) to fabricate porous TPU/RGO nanocomposites. At 3.71 vol.%, solid TPU/RGO nanocomposites showcased a higher EMI SE value of 24.7 dB, whereas porous nanocomposites recorded only 21.8 dB, with much higher absorption characteristics and minimal reflection [112]. It is also worth mentioning that there was only a slight decree of 5 dB in the attenuation of TPU/RGO nanocomposites, even after 100 bending cycles, which portrays the efficiency of the segregated network structure in these nanocomposites. All the aforementioned protocols require multiple steps to fabricate and design porous polymer nanocomposites. Hence, it is quite necessary to formulate a process which is facile and scalable at the same time. In this view, [PERSON] et al. [113] developed conductive, ultralight and compressible PU foams by facile solution dip-coating of graphene onto commercially available PU foams. The resultant PU/graphene foams exhibited very low density of \(\sim\)0.027-0.030 g/cm\({}^{3}\) and demonstrated superior broadband EM attenuation performance with an absorption-dominant shielding mechanism. Figure 11 depicts the dipping process, coating uniformity, morphology and compressive nature of the PU/graphene nanocomposite foams. A maximum EMI SE value of 39.4 dB was recorded at 12 GHz and 6 mm thickness [113].