Researchers from Purdue University have developed a new form of water-based polyurethane that incorporates a cellulose nanomaterial to make the coating stronger and more stable, solving a known issue with water-based polyurethanes and reducing the need to rely on toxic solvents for industrial-grade polyurethane.

The patent application for the coating was published by the U.S. Patent and Trademark Office on Feb. 18.

"Polyurethane is all over the place. You see it in tough things like wheels and rigid coatings, but also in soft things like foams or elastane, which is used to make spandex," said Jeffrey Youngblood, a professor of materials engineering at Purdue. "This range of properties comes from the ratio and types of components in its formulation."

Often, these components include volatile organic compounds, or VOCs, toxic compounds that are released as gases from solids or liquids during polyurethane production and as it cures. These compounds are harmful to the environment because they contribute to the production of ozone in the form of smog, particularly in urban areas.

While the U.S. Environmental Protection Agency reported a 47% decrease in the emissions of VOCs in the country between 1990 and 2014, these compounds are still present in a large number of products, including commercial-grade solvents and paints, as well as polyurethane. They are also still present in water-based polyurethanes, but in lower amounts.

While water-based polyurethanes are widely available as a more sustainable alternative to oil- or solvent-based formulations, they have some major drawbacks when compared with their more toxic cousins. In general, they have less solid content, resulting in a flimsier film more susceptible to long-term damage. This has made water-based polyurethanes unsuitable for many large industrial applications.

To make its coating, the team used cellulosic nanocrystals, which arise when cellulose molecules from plants are structured on an incredibly small scale, as opposed to the larger chains that form in living plants. This restructuring gives the cellulose extreme strength it doesn't have in its natural form.

"They're like the wood-based, natural equivalent of a carbon nanotube," Youngblood said, referring to one of the strongest materials on the planet. "We had the bright idea of using it with the process they use to make waterborne polyurethane coatings, and it worked. And because at least 80% to 90% of the weight is cellulose, it's mostly sustainable and renewable." 

While producing plant-based plastics in the interest of sustainability is a growing trend, and there have been examples of cellulose compounds being incorporated into polyurethane formulations before, this method is a departure for using such a large proportion of cellulose in a nanomaterial configuration. 

Substituting cellulose nanocrystals into the process of producing polyurethane was possible because of the chemical structure common to all polyurethanes.

"There are two main components to any polyurethane: an isocyanate, which essentially forms rigid-type material, and then there's the polyol, which is gives it softness and flexibility," Youngblood said. "But when you look at the chemical properties of cellulose nanoparticles, they're basically polyols."

By using the cellulose nanomaterials as the polyol component, the team produced a water-based polyurethane that maintained flexibility without compromising strength. 

At present, the researchers foresee their invention being used in large industrial applications.

"If you're getting a normal polyurethane for woodworking, it'll likely be a waterborne polyurethane where they emulsify the already-made polyurethane inside the water," said Youngblood. "What we've made is still out of water but is something you'd have to bake on or heat to set. So it'd be more likely for something you'd use in an industrial system for steel or similar."

However, the research team also speculates that its invention could be used in barrier films for food packaging. They emphasize that while it is important to reduce reliance on plastics, their work points to another equally important avenue toward producing less waste: making existing plastics more sustainable.

"The politics of the situation now is that everybody thinks plastics are all bad without realizing the follow-up problems, like what happens if you get rid of the shrink wrap around a cucumber? Then you need three times as many cucumbers because they rot faster," Youngblood said. "Cellulose is technically a plastic. So what we're really doing is changing out one plastic for another that just happens to be more sustainable, but also functional."

The application for the patent, "Cellulose Nanomaterial (CN) Based Waterborne Polyurethane Coating," was filed Aug. 10 2020, to the U.S. Patent and Trademark Office. It was published Feb.18, 2021 with the application number 2021/0047534. The earliest priority date was Aug. 13, 2019. The inventors of the pending patent are Reaz Ahmed Chowdhury, Sami Miguel El Awad Azrak and Jeffrey Paul Youngblood, Purdue University. The assignee is Purdue Research Foundation.

Parola Analytics provided technical research for this story.