Web Date: November 23, 2016
Making affordable antireflective coatings for flexible surfaces
A simple method to create antireflective coatings could make it easier to produce large glare-free windows and flexible electronic displays (ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/acsami.6b10624). The new antireflective coating is a single layer of nanoscale silica beads embedded in plastic.
Antireflective coatings are used in eyeglasses, camera lenses, displays, and car headlights to reduce glare, and in solar cells to boost efficiency. Conventional antiglare coatings are thin films of low refractive index materials such as magnesium fluoride.
These antiglare coatings work well, but are costly because they “require complicated, expensive, and multistep treatment methods,” says Norihiro Mizoshita, a researcher at the Toyota Central R&D Labs. Also, most of the coatings can only be laid on flat surfaces and are difficult to make on large areas.
Recently scientists have been exploring ultrathin silica films that are porous or have nanoscale features that absorb light, preventing reflection. Mizoshita and his colleague have devised a simple method to coat curved, flexible plastic substrates that could make it easier to more affordably coat large surfaces, he says. The researchers made the coating by spraying or brush-painting both surfaces of plastic substrates with nanoscale silica spheres dispersed in ethanol. After the ethanol evaporated, the researchers exposed the substrates to chloroform vapor in a sealed box for a few hours. The chloroform softens the plastic, allowing the nanospheres to sink into the surface. Finally, the researchers washed away excess nanoparticles using ethanol, leaving behind a single layer of nanoparticles embedded about 50 nm deep in the plastic.
The researchers made four different coatings, using particles with average diameters of 100, 145, 165, or 190 nm. Larger particles produced thicker coatings that were less reflective but also slightly less transparent. Bare plastic surfaces reflect 8% of light. By comparison, coatings made of 165-nm-wide and 190-nm-wide particles resulted in 2.2% and 1.2% of visible light being reflected, respectively.
The coatings are durable, Mizoshita says. Their optical properties did not degrade after being stored for two years exposed to air. In addition, when the researchers rubbed the coatings with cotton wool and attempted to peel them off with adhesive tape, they stayed intact and their reflective properties were unchanged. The coatings also survived repeated bending and heating-cooling cycles between 80 and 20 °C.
This is a promising method to produce nanostructured coatings on polymer films “with potentially interesting antireflective properties and applications,” says Richard W. Siegel, a materials science and engineering professor at the Rensselaer Polytechnic Institute. Whether it will be used commercially will depend on the cost involved in manufacturing it on a large scale, he adds.
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