Polyurethanes are all around us – in the foam we sit and sleep on, in the insulation that keeps our houses warm and our fridges cold, and even in our shoes. It’s a complex polymer with two different monomers – a polyol and a diisocyanate. Bio-based polyols are now commonplace, but the same can’t be said for the diisocyanate. Now, polyol-producing San Diego, US-based startup Algenesis has managed to develop a process to make a bio-based diisocyanate, too, and a pilot plant has recently started up.

The journey to this point has taken over 15 years. Co-founder and chief executive Stephen Mayfield had started another company, Sapphire Energy, in 2007 to make biofuels using oil extracted from algae as the raw material. Then Algenesis was born in 2016 to take the algal technology in the direction of plastics. ‘The idea is super-simple,’ Mayfield says. ‘Plastics come from petroleum, and petroleum comes from algae. Let’s just go straight from algae to plastics.’

Polyurethane was an obvious place to start, he says. While they first looked at rigid PU foam for surfboards, they soon pivoted to the much larger footwear market. Triglycerides from any plant oil with a double bond, including algal oil, can be used to make polyols via epoxidation and ring opening, but the resulting polymer tends to be poor quality. ‘We wanted to make a drop-in replacement for petroleum-based polyols,’ he says. ‘By stringing plant-derived diols and diacids together in a linear fashion with hydroxyl groups at each end, the polyester polyols we make resemble those derived from petroleum.’

While the polyol was plant-based, the diisocyanates were not. Algenesis cofounder (and chemist) Mike Burkart at the University of California in San Diego was looking into alternatives to petrochemical diisocyanates, which are made by reacting a diamine with the toxic, reactive gas phosgene. As well as finding a bio-based source, he wanted to avoid phosgene.

The answer lay in going via a dihydrazide, made from a diacid. While this route avoids phosgene, it does have a different downside – it goes through an explosive intermediate. After a couple of small-scale mishaps in the lab, Burkart turned to flow chemistry, with great success.

We’re not going to jump to a ton a day anytime soon, but the path to get there is really obvious

‘Mike went on to show you could feed in any diacid,’ Mayfield says. ‘Our favourite is azelaic acid [nonanedioic acid] as we can get it out of either algae or plants. We’d made sure our chemistry was universal, and we weren’t restricted to just algal oils.’ This is important for scale-up, as currently there insufficient available algal oil to make large amounts of PU.

Burkhardt’s lab had managed to scale up production to about a gram a day; a year later, Algenesis has scaled it up to a kilogram a day. The team is now optimising the unit operation, and aims to run several reactors in parallel to scale up. ‘It’s now an engineering problem,’ Mayfield says. ‘We could get to 10 or even 100kg/day simply by running our current reactors in parallel, but we think a single machine could reach 10kg/day on its own.’

With the chemistry now working well, he hopes there will be an order of magnitude improvement in the unit operation within a year. ‘We’re not going to jump to a ton a day anytime soon, but the path to get there is really obvious,’ he says. ‘It doesn’t mean it will be easy, but we know what the process is, and we know it’s doable.’

Mayfield also wanted to engineer the polyols so the polymer would biodegrade at the end of its life. Polyesters are naturally biodegradable, he says, but the plastic industry spent 50 years learning how to make polyester polyols non-biodegradable. Today’s polyurethanes are the result of half a century of work on formulations, processes and additives.

If you take one of our shoes and turn it into microplastics, these will degrade in the natural environment in a few months

Reinventing all this took about four years, and the polyurethane is now biodegradable; even the diisocyanate is returned to a diamine, Mayfield says, and is a good fertiliser. The polymer can also be recycled, using the chemical recycling processes that are becoming standard for polyurethane, although it remains impossible to recover the diisocyanate directly. It will always return to a diamine that has to be reacted with phosgene to regenerate the diisocyanate.

Algenesis is now working to tune the biodegradability to make it proportional to product lifespans. In a car, the seat foam will need to last 20 years or more, but two to five years would be more appropriate for a shoe. ‘We now have a couple of urethanes we think will last five or six years under pretty adverse conditions,’ he says.

This biodegradability also means there will be no persistent microplastics, Mayfield claims. ‘If you take one of our shoes and turn it into microplastics, these will degrade in the natural environment in a few months back to diols, diacids and diamines that are eaten by microorganisms and do not persist,’ he says. ‘This includes the microplastics shed from your shoe soles as you walk around – they disappear within 100 days.’