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

The research team, led by Di Zhao, Kai Dong, and Chengji Zhao, constructed what are known as vitrimers: crosslinked polymers that contain dynamic covalent bonds capable of exchanging with one another when heated. Unlike conventional thermosets, whose networks are permanently frozen, vitrimers can rearrange their topology, allowing them to be self-healed, welded, reprocessed, and chemically degraded on demand. Dynamic bonds such as disulfides, Schiff bases, and siloxane linkages have been incorporated into epoxy systems before, but a persistent problem has haunted the field: making a network more dynamic usually makes it weaker. High crosslinking density delivers stiffness and strength but immobilizes the polymer chains, producing brittleness and sluggish bond exchange. Lowering the crosslink density restores mobility but sacrifices mechanical robustness. Most attempts to escape this dilemma have relied on rubber toughening, thermoplastic blending, or inorganic nanofillers—additives that complicate the formulation, disrupt network homogeneity, and can themselves impair recyclability.

The team’s strategy was different. Rather than adding anything, they engineered the intrinsic free volume of the network—the fraction of space within the material that is not occupied by polymer chains. Free volume is often treated as an incidental byproduct of molecular packing, but the researchers proposed treating its size and spatial distribution as a tunable structural parameter in its own right. To test the idea, they designed two renewable-resource-derived epoxy monomers: a bifunctional monomer called VAN-EP, built from vanillin, and a trifunctional monomer called PCA-EP, derived from protocatechualdehyde. Both carry rigid conjugated Schiff base scaffolds formed by amine-aldehyde condensation, and both were cured with 1,3-bis(3-aminopropyl) tetramethyldisiloxane, a flexible siloxane-containing amine, producing rigid-flexible networks held together by two types of dynamic bonds: imine and siloxane.

Molecular dynamics simulations revealed how profoundly the monomer functionality shaped the internal architecture. The bifunctional VAN-BAS system exhibited a fractional free volume of 38.3 percent, roughly 29 percent higher than the 29.7 percent of the trifunctional PCA-BAS network. The denser trifunctional system packs its chains more tightly because its excess crosslinking points impose localized confinement. The consequences rippled through every property measured. The diffusion coefficient of VAN-BAS, a measure of translational molecular mobility, was 12 percent higher than that of PCA-BAS, and its van der Waals interaction energy was substantially more negative, indicating that moderate free volume actually allows chains to arrange more efficiently and stabilize intermolecular interactions rather than simply loosening the structure.

Mechanically, the two materials diverged in exactly the way the free-volume framework predicts. PCA-BAS, with a crosslink density of 3880 moles per cubic meter—41 percent higher than VAN-BAS—showed greater stiffness, with a storage modulus of 5115 megapascals and a glass transition temperature of 127 degrees Celsius. But its restricted segmental mobility made it brittle. VAN-BAS, by contrast, achieved a tensile strength of 94.3 megapascals, a flexural strength of 168.7 megapascals, and an impact strength of 39.1 kilojoules per square meter, surpassing PCA-BAS by 26, 31, and 46 percent respectively. Fracture surface analysis told the story visually: VAN-BAS broke with a rough, fish-scale morphology studded with fine wrinkles, the signature of extensive plastic deformation and energy dissipation, while PCA-BAS displayed the smooth, flat river-pattern characteristic of brittle fracture. The nanoscale voids in VAN-BAS act simultaneously as shock absorbers, accommodating local chain deformation and relieving stress concentrations, and as molecular lubricants that enable efficient energy dissipation during crack propagation.

Thermal performance remained competitive despite the differences in network density. The thermal decomposition temperatures at five percent weight loss were 294 degrees Celsius for VAN-BAS and 307 degrees for PCA-BAS, a modest 13-degree gap given the substantial difference in crosslinking. The researchers attribute this resilience to the strong intermolecular interactions of the conjugated imine bonds, which suppress thermal molecular motion. In PCA-BAS, the restricted chain mobility even promotes formation of a stable carbonaceous char layer at high temperature, which acts as a thermal barrier and raises the char yield.

The most striking results concerned dynamics. Counterintuitively, PCA-BAS, despite containing a higher total concentration of dynamic covalent bonds, relaxed stress more slowly than VAN-BAS. When heated from 140 to 200 degrees Celsius, the stress relaxation time of VAN-BAS dropped from 426 to 45 seconds, while PCA-BAS required 786 to 65 seconds over the same range. The activation energy for topological rearrangement was 59.6 kilojoules per mole for VAN-BAS versus 70.5 for PCA-BAS, and the extrapolated topological freezing temperature was nearly 19 degrees lower for the freer network. The lesson is that bond concentration alone does not govern network reorganization kinetics; the dense crosslinking of PCA-BAS imposes steric constraints that raise the energy barrier for bond exchange and outweigh the benefit of having more dynamic bonds to exchange. In self-healing tests, a scratch on VAN-BAS closed almost completely within 60 minutes at 200 degrees Celsius, while PCA-BAS managed only a 77 percent width reduction in the same time.

Reprocessing confirmed the practical payoff. Pulverized VAN-BAS hot-pressed at 200 degrees Celsius and 15 megapascals for 30 minutes reformed into smooth, coherent specimens with retained chemical structure and stable thermomechanical properties, though tensile strength fell to 64 percent of the original after one cycle and 57 percent after two—better retention than PCA-BAS, which dropped to 61 and then 50 percent, but a reminder that repeated thermal remolding still accumulates damage. The team suggests that optimized reprocessing conditions, additional dynamic mechanisms, or nanofillers could improve retention in future work.

The real-world test came with carbon fiber-reinforced composites, the application where epoxy’s recycling problem is most acute. Large composite panels measuring 300 by 300 millimeters were fabricated by compression molding, and the CF/VAN-BAS laminates achieved a tensile strength of 605 megapascals, a flexural strength of 628 megapascals, and an interlaminar shear strength of 68 megapascals—exceeding the CF/PCA-BAS equivalents by 11, 21, and 24 percent. The dynamic network enabled adhesive-free welding of overlapping laminates with a lap shear strength of 31.3 megapascals, competitive with or superior to recent advanced welding methods, and the welded joints could sustain a 3-kilogram load, roughly a thousand times their own weight. Laminates were also thermoformed into a stable W-shaped three-dimensional component and then flattened back without delamination, with the reconfigured parts bearing compressive peak loads of 748 newtons.

Perhaps most consequentially for sustainability, the free-volume-tailored matrix degraded gently under solvent. Immersion in dimethylformamide at 120 degrees Celsius caused the network to swell and fragment within about four hours, releasing the carbon fibers intact. The recovered fibers showed smooth, defect-free surfaces under electron microscopy and retained 96 percent of their original tensile strength, with surface chemistry and graphitization degree essentially unchanged according to X-ray photoelectron and Raman spectroscopy. At least 90 percent of the solvent was recoverable by rotary evaporation, and the process required no catalyst, though the authors note that full life-cycle and techno-economic assessment remains future work. Taken together, the study establishes fractional free volume as a genuine design principle for covalently adaptive polymers—one that decouples strength from toughness and dynamics from density, and points toward wind blades, aerospace panels, and structural adhesives that can be welded, reshaped, and unmade rather than landfilled.

Subject of Research: Free-volume engineering of bio-based epoxy vitrimers for recyclable carbon fiber composites

Article Title: High‐Performance Bio‐Based Epoxy Vitrimers and Composites: Synergizing Robustness and Reprocessability via Fractional Free Volume Regulation

Article References: Dong, K., Zhao, D., & Zhao, C. (2026). High‐Performance Bio‐Based Epoxy Vitrimers and Composites: Synergizing Robustness and Reprocessability via Fractional Free Volume Regulation. Advanced Science, Article e77917. https://doi.org/10.1002/advs.77917

Image Credits: AI Generated

DOI: 10.1002/advs.77917

Keywords: epoxy vitrimers, bio-based polymers, free volume, carbon fiber composites, self-healing, recyclability, dynamic covalent bonds, Schiff base, siloxane, vanillin, stress relaxation, sustainable materials