Seashell-Inspired Switchable Waterborne Coatings with Complete Biodegradability, Intrinsic Flame-Retardance, and High Transparency

Inspired by the "brick-and-mortar" structure and the whole lifecycle eco-friendliness of seashells, we have constructed a proof-of-concept and environmentally friendly coating with switchable aqueous processability, complete biodegradability, intrinsic flame retardance, and high transparency, via using natural biomass and montmorillonite (MMT). We first designed and synthesized cationic cellulose derivatives (CCD) as the macromolecular surfactants, which effectively exfoliated MMT to obtain nano-MMT/CCD aqueous dispersions. Subsequently, via a simple spray-coating process and a post-treatment process with a salt aqueous solution, the transparent, hydrophobic, and flame-retardant coating was fabricated with a "brick-and-mortar" structure. The resultant coating exhibited an extremely low peak heat release rate (PHRR) of only 17.3 W/g, which is 6.3% of cellulose PHRR. Moreover, it formed a lamellar and porous structure once ignited. Thus, this coating could effectively protect combustible materials from fire. In addition, the coating had a high transparency (>90%) in the range of 400-800 nm. After use, the water-resistant coating was converted into a water-soluble material by using a hydrophilic salt aqueous solution, which then could be easily removed by water. Furthermore, the CCD/nano-MMT coating was completely degradable and nontoxic. Such a switchable and multifunctional coating with whole lifecycle environmental-friendliness exhibits huge application potential.