Linseed Oil and Cardanol Yield Isocyanate-Free Polyurethane Coatings

Chemists in Turkey have unveiled a fully bio-based alternative to conventional polyurethane coatings that eliminates one of the industry’s most problematic ingredients: isocyanates. Writing in Polymer Bulletin, Emel Atılal and Nilhan Kayaman Apohan of Marmara University describe a family of non-isocyanate polyurethanes, or NIPUs, built entirely from renewable feedstocks, including linseed oil, a cardanol-based commercial monomer, and even captured carbon dioxide. The work, published as volume 83, article 670 of the journal, demonstrates that sustainable chemistry can deliver coatings with industrial-grade thermal stability, mechanical strength, and corrosion resistance without relying on the toxic isocyanate monomers that have long defined polyurethane manufacturing.

The significance of the achievement lies in the chemistry it replaces. Conventional polyurethanes, found in everything from furniture foam to automotive clearcoats, are formed when isocyanates react with polyols. Isocyanates are potent respiratory sensitizers, and their production depends on the equally hazardous and fossil-derived phosgene. NIPU chemistry sidesteps both hazards by coupling cyclic carbonates with amines, a polyaddition reaction that produces polyhydroxyurethanes bearing hydroxyl groups along the backbone. Those pendant hydroxyls form additional hydrogen bonds, which can enhance adhesion and cohesion, but the field has struggled to match the mechanical performance of conventional systems, particularly in crosslinked coatings.

The Turkish team’s strategy began with two complementary bio-based epoxides. The first was epoxidized linseed oil, a triglyceride derived from flax seeds whose long, flexible fatty acid chains are prized for imparting elasticity. The second was Cardolite NC514S, a cardanol-based epoxy derived from cashew nut shell liquid, an agricultural byproduct rich in phenolic lipids. Cardanol brings a rigid aromatic structure and a long C15 alkyl side chain, a combination that can simultaneously stiffen a network and provide hydrophobic character. By converting both epoxides into cyclic carbonates and then blending them in varying ratios, the researchers created a tunable platform in which flexibility and rigidity could be dialled in at will.

The conversion of epoxide to cyclic carbonate is where the carbon dioxide enters the story. The researchers bubbled CO2 through the epoxidized oils in the presence of tetrabutylammonium bromide, a quaternary ammonium salt that catalyzes the insertion of carbon dioxide into the strained epoxide rings. Each epoxide ring thus becomes a five-membered cyclic carbonate, chemically locking atmospheric carbon into the monomer itself. This CO2 fixation step, performed catalytically and without solvents, transforms a greenhouse gas into a structural building block, a strategy that aligns with the broader push toward carbon capture and utilization across the chemical industry.

With the cyclic carbonates in hand, the team carried out a solvent-free polyaddition with Priamine 1071, a renewable diamine derived from dimerized fatty acids. The amine groups ring-open the carbonate rings, forming urethane linkages and generating secondary hydroxyls that hydrogen-bond throughout the growing network. Because both the carbonate monomers and the diamine hardener are bio-based, the resulting thermoset coatings are fully renewable in origin. The crosslinking reaction proceeds without added catalysts or volatile solvents, an important consideration for coatings manufacturers seeking to reduce volatile organic compound emissions and simplify regulatory compliance.

To verify that the networks had formed properly, the researchers measured gel content, the fraction of the material insoluble in a good solvent, which serves as a proxy for crosslink density. The hybrid coatings achieved gel fractions of 83 to 85 percent, remarkably consistent across the range of linseed-to-cardanol ratios tested. This indicated that well-developed three-dimensional networks formed regardless of formulation, giving manufacturers a wide processing window. The consistency suggests that the two carbonated monomers, despite their very different architectures, participate cooperatively in the amine-carbonate reaction rather than one dominating the cure chemistry.

Thermal analysis told an encouraging story about durability. Thermogravimetric experiments showed a single-step degradation profile between 300 and 500 degrees Celsius, with degradation onset and mid-point temperatures spanning 270 to 455 degrees Celsius depending on composition. For coatings that must survive baking ovens, sunlight, and abrasive service, this level of thermal robustness is competitive with petrochemical benchmarks. The char yields were low, between 0.22 and 0.31 percent, which the authors attributed to residual inorganic components from the catalyst rather than to the polymer itself. Differential scanning calorimetry revealed glass transition temperatures rising from minus 17.2 to minus 11.2 degrees Celsius as the cardanol content increased, confirming that the aromatic cardanol segments stiffen the network even in these elastomeric formulations.

Mechanical testing showed the trade-offs the formulation ratio makes possible. As the cardanol-derived carbonate fraction grew, the elastic modulus climbed from 0.26 to 0.39 megapascals and yield strength rose from 0.31 to 0.42 megapascals, while toughness improved from 24.6 to 29.3 joules. Elongation at break decreased moderately, as expected when rigid aromatic units replace flexible fatty acid chains. In practical terms, the linseed-rich formulations behave as soft, extensible films suited to substrates that flex, while cardanol-rich versions deliver harder, stronger, and tougher coatings for demanding protective applications. The ability to engineer this spectrum from just two renewable monomers is precisely what industrial formulators need.

Surface performance, the ultimate test for any coating, proved equally compelling. Water contact angles between 72 and 88 degrees indicated moderately hydrophilic surfaces, while gloss values exceeding 100 gloss units signaled the smooth, mirror-like finishes demanded by architectural and consumer products. Hardness and adhesion both improved with cardanol content, and in accelerated salt-spray corrosion testing the cardanol-rich formulations delivered superior protection of the underlying metal. The long alkyl side chains of cardanol likely contribute to this barrier effect, packing densely at the surface and slowing the ingress of chloride-laden water toward the substrate, a mechanism consistent with cardanol’s established reputation in anticorrosive coating research.

The study, partially funded by the Scientific and Technological Research Council of Türkiye under grant number 222O017, with analytical support from Kordsa Teknik Tekstil, arrives at a moment of intense interest in isocyanate-free polyurethanes. Recent literature has explored NIPUs from carbonated soybean oil, lignin, vanillin, tannins, and terpenes, but few reports combine fully bio-based monomers, solvent-free processing, CO2 utilization, and a complete thermo-mechanical and protective characterization in a single coating platform. By demonstrating that a simple two-monomer blend of carbonated linseed oil and carbonated cardanol can span soft to stiff, elastic to corrosion-resistant, the Marmara University team has provided a practical blueprint for greener coatings. If such systems can be scaled from the laboratory bench to industrial application lines, the humble flax seed and the cashew shell, with a helping of recycled carbon dioxide, may soon be protecting bridges, appliances, and vehicles alike.

Subject of Research: Fully bio-based non-isocyanate polyurethane coatings synthesized from carbonated linseed oil and cardanol

Article Title: Sustainable NIPU coatings based on carbonated linseed oil and cardanol: synthesis and thermo-mechanical performance

Article References: Atılal, E., & Apohan, N. K. (2026). Sustainable NIPU coatings based on carbonated linseed oil and cardanol: synthesis and thermo-mechanical performance. Polymer Bulletin, 83(12), Article 670. https://doi.org/10.1007/s00289-026-06715-9

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06715-9

Keywords: non-isocyanate polyurethane, NIPU, linseed oil, cardanol, cyclic carbonates, CO2 utilization, bio-based polymers, protective coatings, corrosion resistance, thermal stability, green chemistry, solvent-free synthesis