Sustainable design of cationic waterborne polyurethane from hydroxyl-terminated natural rubber : effects of isocyanate index and chain extension on colloidal stability and film performance

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Chailad, Wichain and Limrungruengrat, Sittichai and Saetung, Nitinart and Yang, Liu and Sukhawipat, Nathapong (2026) Sustainable design of cationic waterborne polyurethane from hydroxyl-terminated natural rubber : effects of isocyanate index and chain extension on colloidal stability and film performance. Journal of Molecular Liquids, 456. 129620. ISSN 0167-7322 (https://doi.org/10.1016/j.molliq.2026.129620)

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Abstract

This work presents a sustainable, performance-driven design of green coating materials by systematically evaluating the effects of the isocyanate (NCO) index and chain extenders in cationic waterborne polyurethanes (cWPUs) derived from hydroxyl-terminated natural rubber (HTNR). NCO indices from 100 to 150 were investigated in formulations with and without ethylene glycol (EG) chain extension. The results reveal that precise regulation of the NCO index, in combination with controlled chain extension, is critical for achieving stable dispersions and high-quality film formation. The EG-free system remained colloidally stable over the entire NCO range, while the EG-modified system exhibited optimal stability and uniform films within the NCO 100–125 window. In this regime, the cWPUs displayed nanoscale particle sizes (60–90 nm), high positive zeta potentials (> +60 mV), and storage stability exceeding 10 months. Increasing the NCO index improved mechanical performance and thermal resistance by promoting a more interconnected polyurethane network; however, excessive NCO content (≥150) led to particle aggregation and instability. The optimal formulation, achieved at an NCO index of 125 with 1 mol EG, delivered a well-balanced combination of stiffness and flexibility, making it suitable for flexible coating applications such as wood finishes, leather coatings, and waterborne protective coatings, where properties such as film uniformity, mechanical durability, and interfacial adhesion are essential. Overall, this study establishes the NCO index and chain extender control as key design parameters for developing high-performance, low-carbon, natural rubber-based waterborne polyurethane coatings.

ORCID iDs

Chailad, Wichain, Limrungruengrat, Sittichai, Saetung, Nitinart, Yang, Liu ORCID logoORCID: https://orcid.org/0000-0001-8475-1757 and Sukhawipat, Nathapong;
CORE (COnnecting REpositories)