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Creating better coatings for paper cups and plates

Materials chemists restore circularity to tableware to go

Coatings

Creating better coatings for paper cups and plates

Materials chemists restore circularity to tableware to go

by Craig Bettenhausen
October 13, 2024 | A version of this story appeared in Volume 102, Issue 32
A disposable coffee cup with a cardboard sleeve.

Credit: C&EN/Shutterstock

 

Coatings
by Craig Bettenhausen
October 14, 2024 | A version of this story appeared in Volume 102, Issue 32

Right now, the best place to put a single-use coffee cup may be the garbage can. Which is sad, because a majority of paper hot cups are compostable or recyclable. But the almost-imperceptible interior lining of a recyclable cup would contaminate a compost pile if it’s made of polyethylene. Compostable coatings can interfere with recycling processes built to take polyethylene-lined paper. And few people even realize the cups have linings. Creative coating chemistry is coming quickly to cut plastic contamination out of compostable containers.

The problem right now is that although paper itself is biodegradable and straightforward to recycle, it needs help to stand up to liquids, especially if they’re hot or contain oils. No one wants a cup filled with coffee or noodle soup to come apart halfway between the café and the office. Biobased plates, bowls, straws, and cutlery face the same dilemma. Natural fibers can be environmentally friendly materials, but they need chemical help to match the performance of plastic.

A new generation of coatings for paper can solve that problem by being both compostable and recyclable. “Everything that we create, we try to create it so it’s a real solution no matter how you dispose of it,” says Brandon Leeds, cofounder of the paper-serving-ware start-up SOFi Paper Products. When clean, these items can go into recycling, where the repulping process washes away the coating. Soiled, they can go into a compost bin carrying a half-eaten salad.

“The key is to establish a low surface tension, best combined with the formation of a closed film,” says Bill Kuecker, senior director of global marketing for aqueous chemistry specialist Solenis. The firm has some compostable coatings on the market now and is scaling up production of better ones, coatings that compete with petrochemical and fluorochemical polymers in their ability to resist grease and heat. To reach impactful scales quickly, he says, the new coatings also need to work in the manufacturing equipment that paper goods makers already use.

People are not going to use plastic in their packaging; they’re going to do something different. We believe fiber is the answer.
Ed Connors, chief business officer for the Americas, Solenis

Compostable containers make it easy to get food waste into the compost system instead of the landfill. And the impact of diverting more food waste from the landfill could be huge. According to a 2023 analysis from the US Environmental Protection Agency, about 20% of the garbage disposed of in US landfills is food waste, adding up to around 56.7 million metric tons (t) per year of material that could otherwise be composted into fertilizer or anaerobically digested into fuels or chemical feedstocks.

Instead, that 56.7 million t of organic material rots in a landfill, leaking 2.1 million t of methane into the atmosphere. That’s the annual greenhouse gas equivalent of 14 million passenger vehicles, according to the EPA. Add to that any carbon the landfill emits as carbon dioxide.

Industry insiders call the emerging paper treatments “water-based coatings.” SOFi codeveloped the coatings it uses with a chemical manufacturer in Southeast Asia but declines to disclose details. “The coating is proprietary; there’s not too much that I can share,” Leeds says. All he will say is “it doesn’t contain any bioplastics . . . It’s a water-based, aqueous coating.”

The secrecy around technical and supply chain details is natural, says Rigoberto Advincula, a professor of chemical and biomolecular engineering at the University of Tennessee, Knoxville. Water-based coatings are a competitive, emerging technology.

But how does water-based chemistry make waterproof coatings? The trick, according to Advincula, is that the coatings are emulsions, kind of like house paint, but food safe. Water serves as the solvent to carry and spread hydrophobic polymer particles onto the surface. As the water evaporates, the particles coalesce to form a waterproof barrier and cross-link with one another to resist oils.

To make biodegradable water-based coatings, chemists turn to a handful of polymer types, including polylactic acid, polyhydroxyalkanoates (PHAs), cellulose, and some acrylics. A typical emulsion will have the polymer and some combination of surfactants, binders, drying agents, and biocides. “Anywhere from 5 to 20 ingredients,” Advincula says.

The exact compositions are usually trade secrets, and the engineering matters a lot, Advincula says. “Depending on the quality of that emulsion or additive, you have a stronger product or a weaker product.”

Solenis is more open about the chemical platform it’s moving forward with for water-based paper coatings, although the company isn’t sharing many formulation details. Solenis recently signed a joint development deal with Beijing PhaBuilder Biotechnology, a spin-out from Tsinghua University focused on using synthetic biology to produce PHAs and other biomolecules at commercial scales.

Solenis picked PHAs because they form strong barriers against oxygen, water, vapor, oil, and grease, Kuecker says. And the firms can tune PHAs with the standard toolkit of polymer engineering—things like chain length, side chains, and copolymers—to meet a range of mechanical and chemical demands.

Ed Connors, an executive at Solenis who leads the firm’s growth strategy, says the firm picked PhaBuilder because the start-up can make affordable PHAs at large scales and has the knowledge to apply the polymers’ tunability. Founder Guoqiang Chen, a synthetic biology professor at Tsinghua, “has a lot of intellectual property that he’s developed over the years for many applications, not just for barriers,” Connors says.

“The pull from the brand owners has never been stronger,” Connors says. “What I hear from customers is that we’ve passed the tipping point. People are not going to use plastic in their packaging; they’re going to do something different. We believe fiber is the answer, and we want to help our customers be successful with that.”

Water-based coatings have the potential to let paper surpass plastic, glass, and aluminum as the perfect material for sustainable single-use food packaging and service ware. But that will happen only if the used material has somewhere to go. The people who run composters and digesters, meanwhile, can take the material only if they’re sure it will break down.

In Baltimore, Justen Garrity, president of Veteran Compost, accepts only materials certified as compostable by the US-based Biodegradable Products Institute (BPI) or the European Union–based TÜV Austria. BPI and TÜV make certifications based on a suite of analytical chemistry standards centered on ASTM D6400, a method designed to assess the breakdown of polymers in an industrial compost setting.

Full decomposition is an important consideration, according to the University of Tennessee’s Advincula. “For me, the most important question is, What happens if the biodegradation is incomplete?” he says. “Is it generating more microplastics and nanoplastics over time in the environment?”

Veteran Compost’s main income stream is selling finished compost to gardeners and sustainable farms. “If it wasn’t listed as certified compostable, we would landfill it,” Garrity says. “The last thing I need is for someone to take some of my product and find microplastics or PFAS,” he says, referring to per- and polyfluoroalkyl substances that have been linked to a number of health problems. “That’d be the end of our small business because our consumers would lose faith in us.”

PFAS were common omniphobic coating materials on food packaging and single-use items in the US until recently, Connors says. The EPA worked with industry for about 5 years to phase out PFAS in food packaging, a process the government declared complete in February of this year. The fluorinated polymers are still in use in other regions.

“Our biggest concern is coatings,” Garrity says, because of how hard they are to remove during processing. Other flexible materials such as bags are the second-biggest headache. “Even though what we get is 99% clean, that 1% is a challenge. And it’s not bricks, it’s light films and flexible packaging,” he says. “I’d love to see things that are either compostable or they’re chunky, recyclable plastic.”

Biogas operations, which ferment farm and food waste into methane, are even less tolerant of plastics. “All forms of plastic are considered contamination for our process, and any shift from the use of plastics is a win for the environment and industry alike,” says Andrew Cassilly, vice president for government relations at Bioenergy Devco, which operates digesters on the US East Coast.

A compostable cup tossed in a bin alongside a wax-coated plate and a polyethylene bag, all covered in food scraps, is a sorting nightmare. But imagine a doughnut shop where all the serving ware is certified compostable, or a whole international chain of doughnut shops with a unified waste stream suitable for compost or anaerobic digestion. That’s the sort of system one can build green infrastructure around.

Garrity says more food businesses are edging that way, nudged along by regulation. “There’s a tipping point where you say, If I gotta have a compostable cup in California, Colorado, Washington State, DC, at some point, I’m gonna just make them all compostable, I want to run one cup for all my locations,” he says.

“People have long associated ecofriendly with being expensive and subpar compared to plastic,” SOFi’s Leeds says. “We’re trying to help people make that adjustment by giving them benefits to save costs and have an equivalent or better experience to plastic.”

Work remains to get water-based coatings perfected, certified, and deployed at scale. “I would say we’re a year or two away from seeing that,” Solenis’s Connors says. “There’s a lot of money being poured into solving these problems, and it won’t just be Solenis. . . . There’s going to be other people bringing their solutions to this market. Personally, I think the odds are much, much higher that we’re going to succeed this time.”

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