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Posted: Oct 18, 2013
Nanocoated fabric is super-repellent and self-healing
(Nanowerk Spotlight) Any coating, no matter how durable, is susceptible to physical and chemical damages. Self-healing, which has become a popular theme in the field of material science, can endow coatings with ability to recover their surface properties after being damaged (read more: "Electrospinning self-healing polymer coating systems").
Previous research on self-healing, superoleophobic or superamphiphobic (i.e. both superhydrophobic and superoleophobic) coatings have demonstrated limited liquid repellency with single self-healing ability against either physical or chemical damage. Now, researchers at Deakin University in Australia, have shown that a superamphiphobic fabric with remarkable multi self-healing ability against both physical and chemical damages and exceptional liquid-repellency to low surface-tension liquids including ethanol can be achieved through a two-step wet-chemistry coating technique.
"Our findings are based on our previous study ("Durable, Self-Healing Superhydrophobic and Superoleophobic Surfaces from Fluorinated-Decyl Polyhedral Oligomeric Silsesquioxane and Hydrolyzed Fluorinated Alkyl Silane") on a unique coating system," Professor Tong Lin, Personal Chair & ARC Future Fellow, Institute for Frontier Materials, GTP Research, tells Nanowerk. "Previously, we found that when a hydrolyzed fluoroalkyl silane (FAS) containing well-dispersed fluorinated-decyl polyhedral oligomeric silsequioxane (FD-POSS) was applied to fabrics, the fabrics showed superamphiphobicity with remarkable self-healing ability against chemical damages. In our new work, we further find that a fabric after being treated with hydrophobic nanoparticles and the FAS/FD-POSS coating shows a novel multi self-healing ability against both physical damages – e.g. blade scratching, sandpapering, and abrasion – and chemical damages."
Photos to show (a) the method of scratching fabric with a sharp blade, (b,c) colored water, hexadecane, and ethanol drops on the coated fabric (b) after the first blade scratching cycle (100 scratches) and (c) after blade scratching and heat treatment at 140°C. (Reprinted with permission from American Chemical Society)
Lin and his team found that the presence of fluoroalkyl surface-modified silica nanoparticles in the coating exceptionally enhanced the liquid repellency of the coated fabric, especially to ultralow surface-tension liquids including ethanol.
Washing is an important cause of degradation to superhydrophobic coatings during their practical uses. Fabric during washing undergoes mechanical stresses in addition to chemical agents. The researchers therefore submitted their coated fabric to a repeated standard machine laundry process. They found that the coating is durable to withstand 200 cycles of wash and 5000 cycles of Martindale abrasion without apparently changing the superamphiphobicity.
Furthermore, the coating has self-healing ability against both chemical and physical damages.
"To prove the self-healing of chemical damages, we deliberately damaged the FS-NP/FD-POSS/FAS coated fabric by a
plasma treatment in vacuum," explains Dr. Hongxia Wang, ARC APD fellow, and first author of the paper. "As a result, the treated fabric became amphiphilic with contact angle of 0° to both water and oil fluids. However, when the plasma-treated fabric was heated at 140°C for just 5 minutes, its liquid repellency was restored. The self-healing was repeatable and can perform many times."
Apart from chemical damage, the researchers employed physical damages such as scratching with a sharp blade, abrasion with sandpaper, and Martindale abrasion to test durability and self-healing ability of the coated fabric.
When the coated fabric was scratched with a sharp blade, its liquid repellency decreased. With increasing number of scratches, the fabrics contact angle gradually reduced. After 100 scratches were performed, the ethanol contact angle reduced to 0°, while the contact angle for water and hexadecane reduced to 150° and 120°, respectively.
"It was very interesting to note that the scratch-induced degradation in liquid repellency was repairable," Wang notes. "After heat treatment of the scratched fabric at 140°C for 30 minutes, the contact angle changed to 168°, 156°, and 146° for water, hexadecane, and ethanol, respectively. This thermally activated self-healing process was repeatable."
The researchers attributed the mechanism of self-healing to physical damages to the movement of coating on the coated surface during heating. With increasing temperature, the coating becomes more mobile until it melts completely. Subsequent cooling lets the coating solidify again, with the result that the damages are now sealed.
This highly robust, superamphiphobic fabric may find applications for the development of 'smart' functional textiles for various applications in personal protection, self-cleaning, defense, healthcare, and daily life.
Lin points out that there is still room for improvement, for example making fabrics to comprehensively repel all fluids, including water and oils of any types. Also, the existing coatings are mostly based on fluoro-containing substances. "It would be a breakthrough if the durable coatings are free from any fluorine but still have excellent super-repellency," he says.