Facile fabrication of superhydrophobic polyurethane sponge towards oil-water separation with exceptional flame-retardant performance

Abstract In this article, a superhydrophobic polyurethane (PU) sponge with exceptional flame-retardant property, was fabricated by coating polydopamine (PDA) through one-step polymerization and surface treatment with hexamethyl disilazane (HMDS). Specifically, dopamine was polymerized in an alkaline solution to generate PDA nano-aggregates as hierarchically rough structure on PU sponge skeleton. After the sponge was further treated with HMDS, the obtained PU-PDA-HMDS sponge surface was transformed from hydrophilic into superhydrophobic with a water contact angle about 153°. The Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurement results demonstrated that the PDA nano-aggregates layer was successfully coated onto PU skeleton, and HMDS molecules were grafted onto PDA nano-aggregates layer by forming silicyl groups on sponge. When the superhydrophobic PU-PDA-HMDS sponge was used towards oil-water separation, it exhibited high oil absorption capacity and stable absorption performance even after cyclic absorption-desorption process. Meanwhile, the sponge exhibited good elastic property and recovery ability under compression test. Furthermore, the thermal stability and flame-retardant property of the sponges were measured, and the results showed the resulting PU-PDA-HMDS sponge exhibited superior thermal stability and flame-retardant property to the pristine PU or PU coated with PDA alone. Even encountering a flame attack, the PU-PDA-HMDS sponge would be self-extinguished and remained its structure integrity due to the formation of silica-hybrid char residue layer during combustion process. The formation process of silica-hybrid char residue layer and what role it exerted on enhancing flame-retardant property were proposed and discussed. Therefore, this work not only provided a simple strategy for fabricating the superhydrophobic porous composites for oil-water separation, but also opened a door to protect the combustible porous composites from fire with exceptional flame-retardant performance.