Superprotective garb for withstanding both flood and fire is on the horizon now that Chinese chemists have come up with a self-healing, triple-dip coating that turns cotton flame resistant and waterproof , according to media reports.
Flame-retardant fabric coatings, found on children’s pajamas and furniture fabrics, often wash away, says Junqi Sun of Jilin University, in China. Sun wondered whether adding a waterproof coating would extend the life of water-soluble fire retardants. He decided to test the idea with an especially durable waterproof coating his group had been working on: a superhydrophobic surface that heals itself.
Superhydrophobic surfaces, such as those found on duck feathers and lotus leaves, repel water with roughness at the micro- and nano-scale. In addition, they combine that texture with exposed molecules with low surface energy. Sun’s group achieved superhydrophobicity the same way, by topping a porous undercoat with a rough skin of aggregated, cagelike molecules: fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS) molecules. F-POSS’s long, fluorinated alkyl chains are water averse, and its aggregated cages are spiky, which gives water droplets the brush-off.
The coating’s self-healing capacity comes from additional F-POSS molecules embedded in the porous undercoat. When the rough surface gets nicked, the opening allows in water from the air, driving underlying F-POSS molecules to fill in and restore the hydrophobicity of the exposed surface.
Sun’s team sandwiched a fire retardant in between the waterproof coating and cotton, choosing a combination of the polyelectrolytes ammonium polyphosphate (APP) and branched poly(ethylenimine) (bPEI). Like a burning marshmallow, the combination blocks the transfer of heat and oxygen by swelling, forming a char, and releasing inert gases to push the oxygen away.
After dipping cotton samples in APP, bPEI, and then F-POSS, Sun’s group tested the coating by holding a flame to the bottom of a 30-cm-long vertical strip of fabric for 12 seconds. Untreated cotton burned away in 14 seconds, yet no more than 4 cm of the treated cotton burned before the flame extinguished itself.
Sun intends the coating for military clothing and other fabrics that must withstand harsh conditions.
