Tannic Acid Foam Ditches Toxic Isocyanates and Refuses to Burn

Every year, the world produces millions of tonnes of polyurethane foam, the light, airy polymer that insulates our buildings, cushions our furniture, and lines our refrigerators. Yet the chemistry behind this ubiquitous material carries a hidden cost: its synthesis depends on isocyanates, reactive compounds made from phosgene, a gas so toxic it was used as a chemical weapon in the First World War. When phosgene reacts with water it forms hydrochloric acid, which can cause severe burns and tissue damage if inhaled, and its production and handling pose persistent risks to workers, wildlife, and the environment. Now, a team of researchers at Kyonggi University in South Korea has taken a major step toward eliminating this hazard altogether, reporting a rigid foam built entirely without isocyanates that is not only stronger than previous green alternatives but also remarkably resistant to fire.

The new material, described in the journal Advances in Industrial and Engineering Chemistry, belongs to a family of polymers known as non-isocyanate polyurethanes, or NIPUs. The concept dates back to 1957, when chemists E. Dyer and H. Scott first demonstrated that cyclic carbonates and amines could be reacted to form urethane bonds without any phosgene chemistry. In the decades since, the approach has attracted renewed attention as a safer, more sustainable route to polyurethane-type materials. Instead of combining polyols with isocyanates, NIPU chemistry couples carbonates with amines, sidestepping the most dangerous reagents in the conventional process. But the technology has struggled to catch up in one crucial application: rigid foams for building insulation, where conventional polyurethane foams still dominate the market.

The problem, the researchers explain, is that non-isocyanate rigid polyurethane foams developed so far have suffered from two crippling weaknesses: low mechanical strength and poor flame retardancy. Building insulation must withstand compressive loads behind walls and cladding, and it must resist ignition in an age of increasingly strict fire codes. Earlier attempts to make green foams from glucose or starch produced materials that either crumbled under pressure or burned too readily for commercial use. Glucose-based foams made with maleic acid and glutaraldehyde, for instance, showed low thermal stability, while even tannin-based foams, though somewhat more fire-resistant, still fell short of the flame-retardant performance of commercial foams treated with conventional additives.

The Kyonggi team, led by corresponding author Sang-Bum Kim with first author Sung Yeol Lim, turned to an unlikely hero: tannic acid, the polyphenolic compound familiar to anyone who has tasted an over-steeped cup of tea. Tannic acid brings a rare combination of properties to polymer chemistry. Its multiple aromatic rings lend rigidity and strength to the polymer network, while its abundance of hydroxyl groups makes it highly reactive, readily forming bonds with carbonate reagents and crosslinking agents. Most intriguingly, tannic acid is naturally flame-retardant: it can suppress flames, absorb heat generated during combustion, and promote the formation of a stable char layer that shields the underlying material from heat and oxygen. In other words, the fire protection is built into the molecule itself rather than bolted on with halogenated additives.

The synthesis proceeded in two stages. First, the researchers dissolved tannic acid in water and reacted it with dimethyl carbonate at 65 degrees Celsius to produce carbonated tannin, a carbonate-bearing intermediate. Spectroscopic analysis confirmed the transformation: the infrared spectrum showed the disappearance of the carbonyl peak characteristic of dimethyl carbonate at 1740 wavenumbers and the emergence of a new carbonyl peak at 1711 wavenumbers, while proton nuclear magnetic resonance revealed a methoxy signal at 3.46 parts per million absent from the starting material. The carbonated tannin was then reacted with hexamethylenediamine at 90 degrees Celsius for two hours, yielding a tannic acid-based non-isocyanate polyurethane resin whose urethane bonds were confirmed by characteristic infrared peaks for the carbonyl, nitrogen-hydrogen, and carbon-oxygen groups.

The critical innovation lay in the formulation of the foam itself. The team blended the resin with three crosslinking agents, each playing a distinct role: hexamethylenetetramine and glutaraldehyde extend the polymer chains and build crosslink density, while citric acid reacts with the amine groups in the resin and acts as a foaming inducer, generating the gas bubbles that turn a liquid resin into a lightweight foam. By systematically varying the amounts of these three agents across six formulations, the researchers could dissect exactly how each one shaped the final material. The differences were dramatic. In the formulation lacking glutaraldehyde, the foam expanded rapidly and then collapsed entirely, demonstrating that sufficient crosslinking is essential to stabilize the expanding bubble structure. All formulations containing adequate crosslinker produced stable foams of similar density.

Compressive testing revealed a clear winner: the formulation with the highest crosslinking agent content, designated F-1, exhibited the highest compressive strength of all. Scanning electron microscopy explained why. The F-1 foam displayed the smallest and most uniform cells, a microstructure that distributes mechanical stress evenly across the material and prevents cracks from propagating. The microscopy also illuminated the role of each additive. Without hexamethylenetetramine, the crosslinking reaction slowed, reducing reaction heat and delaying curing so that growing cells merged into larger, weaker pores. Without citric acid, no cells formed at all, confirming its role as the foaming trigger. And with less glutaraldehyde, cell sizes grew larger for the same kinetic reasons. The message was consistent: faster, denser crosslinking yields finer, stronger foam.

The fire performance was where the material truly distinguished itself. In thermogravimetric analysis, the tannic acid foam left behind roughly 25 percent char residue after heating to 550 degrees Celsius in nitrogen, compared with only about 15 percent for a conventional polyurethane foam of similar density, reflecting the char-forming tendency of tannic acid’s aromatic structure. Adding citric acid shifted the maximum decomposition rate of the urethane bonds to higher temperatures, from around 285 to 303 degrees Celsius, confirming that the crosslinked network improved thermal stability. In limited oxygen index testing, the new foam required a higher oxygen concentration to sustain burning than the conventional foam, and derivative analysis attributed the protection to tannic acid’s aromatic rings, which promote a stable char barrier, and to phenoxy radicals that quench the oxygen free radicals released during polymer decomposition.

The ignition test provided the most vivid demonstration. When a butane torch flame was applied directly to the specimens for twenty seconds, the conventional polyurethane foam burned rapidly, retaining only 23.3 percent of its original mass. The tannic acid-based foam resisted ignition, and any flames that did appear extinguished themselves within roughly two seconds, leaving about 90 percent of the mass intact. Photographs taken after the test show the conventional foam reduced to charred wreckage while the tannic acid foam retained its shape, a striking visual testament to the power of molecularly embedded fire protection. Notably, flame retardancy varied little across the different crosslinking formulations, suggesting that the fire resistance stems primarily from the tannic acid backbone itself.

The study’s conclusions point toward a future where building insulation no longer requires either phosgene-derived isocyanates or halogenated flame retardants. By tuning the balance of hexamethylenetetramine, citric acid, and glutaraldehyde, the researchers showed that crosslinking agents can simultaneously enhance mechanical strength, thermal stability, and structural integrity, addressing the two key limitations that have kept non-isocyanate foams out of commercial buildings. Challenges remain before tannic acid foams line the walls of skyscrapers, including scaling the synthesis and meeting the full battery of building-code certifications. But the demonstration that a plant-derived polyphenol can deliver both strength and self-extinguishing fire behavior in an isocyanate-free foam marks a significant advance for sustainable materials chemistry, and a compelling argument that the safest flame retardant may be the one grown in a tree.

Subject of Research: Tannic acid-based non-isocyanate rigid polyurethane foam with tunable crosslinking for improved mechanical strength and flame retardancy

Article Title: Study on the mechanical properties and flame retardancy of tannic acid-based non-isocyanate rigid polyurethane foam according to the content of crosslinking agents

Article References: Lim, S. Y., Park, S. B., Kim, M. S., & Kim, S.-B. (2025). Study on the mechanical properties and flame retardancy of tannic acid-based non-isocyanate rigid polyurethane foam according to the content of crosslinking agents. Advances in Industrial and Engineering Chemistry, 1(1), Article 15. https://doi.org/10.1007/s44405-025-00015-y

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00015-y

Keywords: non-isocyanate polyurethane, tannic acid, rigid foam, crosslinking agents, flame retardancy, mechanical properties, building insulation, sustainable polymers, thermal stability, char formation, green chemistry, phosgene-free synthesis