Without it, the world might be a little less soft and a little less warm. Our recreational clothing might shed less water. The insoles in our sneakers might not provide the same therapeutic arch support. The wood grain in finished furniture might not “pop.”
Indeed, polyurethane—a common plastic in applications ranging from sprayable foams to adhesives to synthetic clothing fibers—has become a staple of the 21st century, adding convenience, comfort, and even beauty to numerous aspects of everyday life.
The sheer versatility of the material, which is currently made largely from petroleum byproducts, has made polyurethane the go-to plastic for a range of products. Today, more than 16 million tons of polyurethane are produced globally every year.
“Very few aspects of our lives are not touched by polyurethane,” reflected Phil Pienkos, a chemist who recently retired from the National Renewable Energy Laboratory (NREL) after nearly 40 years of research.
But Pienkos—who built a career researching new ways for producing bio-based fuels and materials—said there is a growing push to rethink how polyurethane is produced.
“Current methods largely rely on toxic chemicals and non-renewable petroleum,” he said. “We wanted to develop a new plastic with all the useful properties of conventional poly but without the costly environmental side effects.”
Was it possible? Results from the laboratory give a resounding yes.
Through a novel chemistry using nontoxic resources like linseed oil, waste grease, or even algae, Pienkos and his NREL colleague Tao Dong, an expert in chemical engineering, have developed a groundbreaking method for producing renewable polyurethane without toxic precursors.
It is a breakthrough with the potential to green the market for products ranging from footwear, to automobiles, to mattresses, and beyond.
But to grasp the sheer weight of the accomplishment, it is helpful to look back at how the scientific advance came about, a story that wanders from the chemical fundamentals of conventional polyurethane, into the algae lab where an idea for a new chemistry first emerged, and winds its way to new corporate partnerships that set the stage for a promising future of commercialization.
When polyurethane first became commercially available in the 1950s, it quickly grew in popularity for use in numerous products and applications. That was in no small part due to the dynamic and tunable properties of the material, as well as the availability and affordability of the petroleum-based components used to make it.
Through a clever chemical process using polyols and isocyanates—the basic building blocks of conventional polyurethanes—manufacturers could tailor their formulations to produce a stunning variety of polyurethane materials, each with unique and useful properties.
Producing from a long-chain polyol, for example, might yield flexible foams for a pillow-soft mattress. Another formulation might yield a rich liquid that, when spread on furniture, both protects and reveals the inherent beauty of wood grain. A third batch might include carbon dioxide (CO2) to expand the material, producing a sprayable foam that dries into rigid and porous insulation, perfect for holding heat in a home.
“That’s the beauty of isocyanate,” said Dong when reflecting on conventional polyurethane, “its ability to form foams.”
But Dong said that isocyanates bring significant downsides, too. While these chemicals have fast reactivity rates, making them highly adaptable to many industry applications, they are also highly toxic, and they are produced from an even more toxic feedstock, phosgene. When inhaled, isocyanates can lead to a range of adverse health effects, like skin, eye, and throat irritation, asthma, and other serious lung problems.
“If products containing conventional polyurethanes are burned, those isocyanates are volatilized and released into the atmosphere,” Pienkos added. Even simply spraying polyurethane for use as insulation, Pienkos said, can aerosolize isocyanate, requiring workers to take careful precautions to protect their health.
To try and tackle these and other issues—such as reliance on petrochemicals—scientists from labs around the world have begun looking for new ways to synthesize polyurethane using bio-based resources. But these efforts have largely had mixed results. Some lacked the performance needed for industry applications. Others were not completely renewable.
The challenge to improve polyurethane, then, remained ripe for innovation.
“We can do better than this,” thought Pienkos five years ago when he first encountered the predicament. Energized by the opportunity, he joined with Dong and Lieve Laurens, also of NREL, on a search for a better polyurethane chemistry.
The idea grew from a seemingly unrelated laboratory problem: lowering the cost of algae biofuels. As with many conventional petrochemical refining processes, biofuel refiners look for ways to use the coproducts of their processes as a source of revenue.
The question becomes much the same for algae biorefining. Can the waste lipids and amino acids from the process become ingredients for a prized recipe for polyurethane that is both renewable and nontoxic?
For Dong, answering the question at the basic chemical level was the easy part—of course they could. Scientists in the 1950s had shown it was possible to synthesize polyurethane from non-isocyanate pathways.
The real challenge, Dong said, was figuring out how to speed up that reaction to compete with conventional processes. He needed to produce polymers that performed at least as well as conventional materials, a major technical barrier to commercializing bio-based polyurethanes.
“The reactivity of the non-isocyanate, bio-based processes described in the literature is slower,” Dong explained. “So we needed to make sure we had reactivity comparable to conventional chemistry.”
NREL’s process overcomes the barrier by developing bio-based formulas through a clever chemical process. It begins with an epoxidation process, which prepares the base oil—anything from canola oil or linseed oil to algae or food waste—for further chemical reactions. By reacting these epoxidized fatty acids with CO2 from the air or flue gas, carbonated monomers are produced. Lastly, Dong combines the carbonated monomers with diamines (derived from amino acids, another bio-based source) in a polymerization process that yields a material that cures into a resin—non-isocyanate polyurethane.