‘Green’ coating protects plastics

Devices that can flex and stretch, including wearable electronics, roll-up solar cells, and foldable displays, are all the rage these days. Engineers design those devices to keep working, bend after bend. A tough, transparent, eco-friendly coating applied to the plastic support might help those devices last even longer, a study suggests (ACS Nano 2018, DOI: 10.1021/acsnano.8b01057).

Many plastic devices have thin coatings designed to go unnoticed. Measuring just a fraction of a micrometer in thickness, these nearly invisible films of organic and inorganic materials improve a wide range of commercial products. For example, polysiloxane coatings on polycarbonate eyeglass lenses make them scratch resistant.

Knowing that manufacturers are always on the lookout for new types of cost-effective coatings, a team led by Stanford University materials scientists Farhan Ansari and Reinhold H. Dauskardt looked to an unusual place for source material—the forest.

The team made hybrid coatings by embedding various concentrations of nanocellulose fibrils, a tough material derived from trees, in a glass matrix composed of zirconium alkoxide and an epoxy-functionalized silane. The team used low-cost colloidal chemistry methods to prepare the precursor solutions and then sprayed the mixtures onto flexible polymer substrates, including poly(ether imide) and poly(ethylene terephthalate), and then cured the films at low temperature. The results were highly transparent, nanometer-thick films.

Tests indicate that coatings containing 20 wt % nanocellulose protect the polymer substrates by boosting hardness and fracture resistance. Microscopy analysis shows that even after 20,000 bending cycles, the films revealed no signs of delamination or cracking.