## 2 Materials and Methods  
The Clear Castor Oil (CO) was bought from Jayant Agro-Organics Ltd. in Mumbai, India, with a hydroxyl value of 160 mg KOH/g, and an acid value of 1.6 mg KOH/g, a peroxide value of 5.0, a saponification value of 178, and a refractive index of 1.476. This study used Voranol 3010, a triolpolyether polyol from Jayant Agro-Organics Ltd., Mumbai, India. This study used tin (II) 2-ethylhexanoate from (Sigma-Aldrich, Bangalore, India) as a catalyst aided the foaming process. The diisocyanate used in this study was pTDI (polymeric toluene diisocyanate), bought from BASF under Lupranate 180. The catalyst for the foaming process was Niax B11 plus, while the surfactant for foam preparation was Niax L618 (Manali Petrochemicals Limited, Chennai, India). Distilled water was taken from the lab and used to create more foam, as it does not contain magnesium and calcium ions. These ions decrease the foam-forming capacity. The chemical composition of the components mentioned above is described in Table 1. Commercially available ZrO\({}_{2}\) with an average size of 20-50 nm, BC nanoparticles with an average size of 20-50 nm, and GNPwith an average size of 5-10 nm (supplied by Sigma Aldrich, Bangalore, India, 99.9% purity) were used as reinforcing materials. GNP, ZrO\({}_{2}\), and BC are described in detail in Table 2.  
Voranol 3010, a triol polyether polyol, and CO were dehydrated individually in a revolving evaporator below a vacuum at 110 \({}^{\circ}\)C for 24 h to remove moisture before the reaction, while nanoparticles were dried at 110 \({}^{\circ}\)C for 2 h. Then, in a mold with dimensions of 12 \(\times\) 12 \(\times\) 2 cm, pre-weighted (50 percent) polyether polyol and castor oil were combined. As a catalyst, tin (II) 2-ethylhexanoate was added to this mixture and mixed well for 5 h at 3600 r/min in an inert atmosphere in an oil bath at 65 \({}^{\circ}\)C. Before adding a determined quantity of amine catalyst and surfactant, the mixture was stored in an oven for 12 h at 60 \({}^{\circ}\)C. Distilled water was added to the mix and swirled for 18 s at 3500 rpm. Finally, premeasured pTDI was added to the mixture, and mixing continued for an additional 18 s at 2500 r/min. The viscous liquid was rapidly and carefully placed into an equipped mold and allowed to rise freely [51]. In this work, we have adopted the absorption and hydrothermal reduction methods for fabricating nanoparticle-reinforced flexible bio-based PU foam. The polyurethane foam was immersed entirely in an ethanol container at room temperature for about 10 min. For homogeneous distribution and to avoid nanoparticles settling at the bottom of the container, the necessary weight % of GNP, ZrO\({}_{2}\), and BC was blended with ethanol in another container for 20 min using an ultrasonic probe sonicator. The polyurethane foam was then immersed completely in the nanoparticle solution for 20 min. The self-assembling nature of nanoparticles results in a consistent dispersion of nanoparticles throughout the foam. The soaked foam was heated in a furnace at 80 \({}^{\circ}\)C for 60 min to evaporate the ethanol. Once the foam was dried and was free of ethanol, it was removed from the furnace. The rectangular structure of size 12 \(\times\) 12 \(\times\) 2 cm was designed and fabricated in order to achieve a uniform symmetrical outer surface area. Due to this, the maximum amount of EM waves from the transmitting antenna is in contact with the material. The flow chart for making flexible bio-based PU foam is shown in Figure 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Components** & **Weight of Components in g** \\ \hline Voranol 3010 & 50 \\ \hline Castor Oil (CO) & 50 \\ \hline pTDI & 46.2 \\ \hline Surfactant & 3.6 \\ \hline Tin Catalyst & 1.0 \\ \hline Amine Catalyst & 0.25 \\ \hline Distilled Water & 0.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of CO-based flexible polyurethane foam.