Paper cranes underwater; the crane on the left is covered in the solution, the crane on the right is not. Image: Hiroi et. al.
Researchers at the University of Tokyo have found a way to waterproof paper with biodegradable materials that also destroy bacteria. They’re calling it Choetsu, and they think it could make a dent in the global plastics crisis.
Detailed in a paper published Friday in the peer-reviewed journal Industrial & Engineering Chemistry Research, the researchers developed a silica-resin coating that can “compensate for paper’s weaknesses,” turning paper products, like single-use straws or forks, into viable alternatives to plastic by making them waterproof and durable.
“Using coated paper instead of plastic products can help to cut down on harmful waste,” Dr. Zenji Hiroi, professor in solid state chemistry at the University of Tokyo and co-author on the study, told Motherboard in an email.
“We can change the liquid composition to accommodate most materials,” he added. “The Choetsu coating will keep these materials safe for a long time.”
Choetsu is made out of titanium dioxide nanoparticles that, when dispersed in a silica-based film with a thickness of a few micrometers, can be coated on paper and degrade environmental pollutants like certain bacteria when exposed to light.
The exact ingredients that went into it were the result of countless trials by the paper’s first author, Yoko Iwamiya, who worked on it independently before Hiroi came by her side. “She has been working on it for a long time, but society’s recognition was low” due to a “lack of scientific evidence,” he told Motherboard. The team published a paper last year about the silica-resin coating, but without the addition of titanium dioxide and its associated antimicrobial effects.
Besides titanium dioxide, the liquid coating agent is composed of a cocktail of chemicals, like methyltrimethoxysilane, isopropyl alcohol, and tetraisopropyl alcohol, that harden when applied to paper and left to dry. Once dry, a layer of silica forms atop the paper, protecting it. The coating is porous, and has absorptive properties, so it captures pollutants and decomposes them via photocatalysis—a reaction that occurs when an object absorbs light—protecting them from the elements better than a paper product would on its own.
“Paper cutlery may be the most straightforward application,” Hiroi said. “We have already created some prototypes in collaboration with industry. The paper package can be reinforced and used even in the rain. Agricultural mulch for weed control can be made from coated paper and degrade in nature without harming the environment. Any paper product will gain more application options.”
He added that the substance shouldn’t just be used for paper. Should Choetsu prove scalable, it can be applied to ceramic, glass, and even plastic, he said.
“Once the coating liquid’s ingredients are determined, simply brush it on the materials and allow it to dry,” Hiroi told Motherboard. “Because the process is so simple, it can be applied to a wide range of products.”