Synthesis of Water-Resistant Polyurethane Composite for Underwater Transducers Encapsulation

The encapsulation of underwater transducers poses a significant challenge due to the need for water resistance and acoustic matching properties. This study presents a comprehensive investigation on the synthesis of water-resistant polyurethane composites and their performance as encapsulation materials for underwater transducers. Four different samples were synthesized for study: polyurethane (PU), PU-AER200 (PU with 4% hydrophilic filler), PU-R972 (PU with 4% hydrophobic filler), and PU-Kaolin (PU with 4% Nanoclay). The water absorption measurements revealed that pure PU exhibited 2% water absorption, while PU-AER200, PU-R972, and PU-Kaolin demonstrated 2.7%, 0.5%, and 0.9% water absorption, respectively. Additionally, the moisture transmissibility values were obtained, with PU-AER200 exhibiting the highest moisture transmission value of 202 g/m2/24 hr, followed by PU (170 g/m2/24 hr), PU-Kaolin (130 g/m2/24 hr) and PU-R972 (90 g/m2/24 hr). DC volume resistivity in the dry state was measured, resulting in 3 $\mathbf{T}\Omega \mathbf{m}$ for PU, 2 $\mathbf{T}\Omega \mathbf{m}$ for PU-AER200, 7 $\mathbf{T}\Omega \mathbf{m}$ for PU-R972, and 6 $\mathbf{T}\Omega \mathbf{m}$ for PU-Kaolin. After water absorption, PU-R972 shows lowest drop in resistivity. The acoustic impedance of the materials was evaluated to assess their acoustic matching capabilities with water, a crucial requirement for transducers. All samples showed impedance values close to that of water (1.48 MRay1), with PU (1.56 MRayl), PU-AER200 (1.52 MRayl), PU-R972 (1.50 MRayl), and PU-Kaolin (1.50 MRayl). PU-R972 shows closest matching impedance values with water. To validate the practical application of the synthesized samples, Acoustic Transducers were molded with each material, submerged in a water tank, and tested for transducer voltage response (TVR) and free-field voltage response (FFVR). The results indicated that PU-R972 achieved the highest TVR value of 148.9 dB at a frequency of 7.4 kHz, while PU-AER200 exhibited the lowest TVR of 140.1 dB. Similarly, for FFVR at 8 kHz, PU-R972 yielded the highest value of -166.8 dB, and PU-AER200 displayed the lowest value of -170.8 dB.