Castor and Tea Seed Oils Yield Liquid Bio-Based Chain Extenders for Waterborne Polyurethane

Waterborne polyurethane coatings are prized for releasing far fewer volatile organic compounds than their solvent-borne counterparts, but they depend on a small, often overlooked ingredient to work at all: a hydrophilic chain extender that lets polyurethane particles disperse in water. Nearly every commercial formulation relies on dimethylolpropionic acid, or DMPA, a petroleum-derived solid with a melting point above 150 °C that must be dissolved in polar solvents such as DMF or acetone before use. Removing those solvents after synthesis complicates production, risks residues, and raises energy consumption. A new study reports a route to replace DMPA with liquid chain extenders made directly from castor oil and tea seed oil, two renewable feedstocks rich in unsaturated fatty acids.

The research, published in the journal Collagen and Leather, began with the raw oils themselves. The result was high-purity unsaturated ricinoleic acid from castor oil and oleic acid from tea seed oil, at purities of 93.99% and 92.72% respectively. In the crude mixtures, ricinoleic acid methyl ester accounted for 88.56% and methyl oleate for 88.65%; after urea inclusion, the saturated contaminants methyl palmitate and methyl stearate fell to undetectable levels or below 0.75%.

With purified fatty acids in hand, the chemists turned to a thiol-ene click reaction, pairing ricinoleic acid with 2-mercaptoethanol and oleic acid with 1-thioglycerol at a 1:1.1 acid-to-thiol molar ratio, with 2% photoinitiator 1173 by total liquid mass. The products were amphiphilic ricinoleic acid-based chain extender (RACE), obtained in 93.6% yield, and amphiphilic oleic acid-based chain extender (OACE), obtained in 91.6% yield. Both are yellow liquids with low molecular weights — approximately 376 g/mol for RACE and 390 g/mol for OACE — and defined functionality: two hydroxyl groups and one carboxyl group per molecule, confirmed by hydroxyl values of 295.4 and 290.6 mg KOH per gram and acid values of 144.7 and 148.1 mg KOH per gram.

That low molecular weight and exact difunctionality matter because earlier vegetable-oil-based hydrophilic chain extenders suffered from a structural flaw. Previous designs, such as a soybean oil-derived extender and a ricinoleic acid-based extender reported by other groups, carried hydroxyl functionality greater than two and multi-chain architectures with three or more chains. Such high branching acts as a chemical crosslinker, driving up system viscosity during polymerization, forming crosslinked networks, and producing complex rheological behavior that undermines process control. Excessive molecular weight also reduces emulsion stability at a given dosage, and increasing the dosage then pushes polyurethane performance out of control. The new extenders, by contrast, are single-chain molecules whose reactivity can be precisely managed.

How fast do these bio-based extenders react with isophorone diisocyanate, the hard-segment building block? The team answered with non-isothermal differential scanning calorimetry at heating rates of 5, 10, 15 and 20 °C per minute, applying the Kissinger and Ozawa methods to extract apparent activation energies. The DMPA/IPDI system showed the lowest values, at 73.01/75.32 kJ per mol by the two methods, while OACE/IPDI measured 74.58/76.91 kJ per mol and RACE/IPDI the highest at 76.66/78.88 kJ per mol. The ordering follows chemistry: both hydroxyls in DMPA are primary, whereas one hydroxyl in each bio-extender is secondary, so DMPA reacts slightly faster. All three curing systems showed single exothermic peaks and good linear kinetics fits with R² above 0.99, indicating that primary and secondary hydroxyls in the bio-extenders react within a narrow temperature range and can react completely with IPDI.

The real test came when the extenders were formulated into waterborne polyurethane emulsions. As the amounts of RACE and OACE increased, emulsion particle sizes fell from 135.37 ± 3.2 nm and 194.91 ± 4.5 nm to 99.06 ± 2.2 nm and 100.56 ± 2.2 nm respectively. Above 0.0248 mol of bio-extender, particle size became comparable to that of a reference emulsion made with 0.0192 mol of DMPA, and the absolute zeta potentials were likewise equivalent, with all emulsions exceeding 30 mV in magnitude — a threshold for colloidal stability. At an equal molar dosage of 0.0192 mol, matched to DMPA’s carboxyl content, the bio-based emulsions still produced larger particles, showing that the long C18 aliphatic chains of the new extenders reduce emulsification efficiency, partly by shielding the ionic carboxyl groups at the particle–water interface. The bio-based emulsions were also semi-transparent or opaque milky white, with color and transparency inferior to the DMPA-based dispersion.

Stability, however, was not compromised. After six months of storage at room temperature, all emulsions remained homogeneous with no visible precipitation or phase separation, and all fresh emulsions survived centrifugation at 3000 rpm for 15 minutes. The PU-Ox series, made with the longer side-chained OACE, showed lower polydispersity indices at the same dosage, which the authors attribute to the long carbon chain’s encapsulation of hydrophilic groups yielding more uniform particle sizes. The researchers conclude that although RACE and OACE exhibit lower initial emulsification efficiency than DMPA, the resulting emulsions possess good long-term and mechanical stability that meets the requirements of practical application.

The films told a story of trade-offs that favored flexibility. Thermogravimetric analysis showed two-stage degradation in all samples — hard segments decomposing between 200 and 350 °C, soft segments at higher temperature — and the maximum weight-loss temperatures of the bio-based films exceeded those of the DMPA-based film, indicating that the bio-extenders slightly increase thermal stability rather than undermining it. Dynamic mechanical analysis revealed that glass transition temperatures shifted lower as bio-extender content rose, a self-plasticizing effect from the flexible long chains with high single-bond rotational freedom. In tensile testing, the PU-Rx films saw strength fall from 8.98 ± 0.32 MPa to 3.31 ± 0.28 MPa while elongation at break climbed from 724.4 ± 35.2% to 1136.8 ± 48.5%; the PU-Ox films dropped from 7.97 ± 0.30 MPa to 4.84 ± 0.31 MPa in strength while elongation rose from 794.0 ± 38.5% to 1201.4 ± 52.3%. Infrared analysis supported the mechanism: as bio-extender content increased, the carbonyl stretching vibration shifted progressively to higher wavenumbers, signaling fewer hydrogen-bonded carbonyl groups and a weakened hard-segment physical crosslinking network.

Water behavior improved in the bio-based films’ favor. Water contact angles rose from 74.8° and 78.7° to 85.5° and 89.5° as the bio-extender dosages increased, and water absorption ratios fell well below those of the DMPA-based film, dropping further with higher extender content. The hydrophobic side alkyl chains orient toward the air interface, forming a surface layer that blocks water penetration, and the longer side chains of OACE delivered the lowest absorption of all. The authors position the work not as an outright performance victory over DMPA but as a feasible and potentially universal strategy for converting natural unsaturated oils into liquid, solvent-free hydrophilic chain extenders — a sustainable alternative whose core value lies in what it replaces: a petroleum-derived solid that demands polar solvents, complicates production, and carries pollution risks fundamentally at odds with sustainable development goals.

Subject of Research: Synthesis of bio-based hydrophilic chain extenders from castor oil and tea seed oil for waterborne polyurethane

Article Title: Bio-hydrophilic chain extenders with amphiphilic character for waterborne polyurethane derived from castor oil/tea seed oil: separation, synthesis, and application

Article References: Li, C., Zhang, Y., Li, W., Wang, L., Xiang, J., Fan, H., Jiang, H., Sun, L., & Tang, J. (2026). Bio-hydrophilic chain extenders with amphiphilic character for waterborne polyurethane derived from castor oil/tea seed oil: separation, synthesis, and application. Collagen and Leather, 8(1), Article 39. https://doi.org/10.1186/s42825-026-00263-8

Image Credits: AI Generated

DOI: 10.1186/s42825-026-00263-8

Keywords: waterborne polyurethane, chain extender, castor oil, tea seed oil, ricinoleic acid, oleic acid, thiol-ene click reaction, urea inclusion purification, DMPA alternative, emulsion stability, renewable polymers, curing kinetics