Chain extension of dimer fatty acid- and sugar-based polyurethanes in aqueous dispersions

• Chain extension (CE) of fatty acid- and sugar-based, aqueous PU dispersions was studied.
• CE is influenced by the type of chain extender, addition sequence, T and catalyst.
• Degree of CE is affected by the solubility of chain extender in the PU prepolymer.
• Water chain extension is optimal at 50 °C.
• TEA promotes the water CE reaction, however at the cost of dispersion stability.

The chain extension process of renewable waterborne polyurethane dispersions (WBPUDs) prepared from a dimer fatty acid-based diisocyanate (DDI) and isosorbide (IS), using dimethylolpropionic acid (DMPA) as the internal dispersing agent, was investigated. Ethylene diamine (EDA), adipic dihydrazide (ADH) and water were evaluated as chain extenders. Other variables such as the chain extension temperature, the sequence of addition of the chain extender with respect to the dispersion step and the utilization of a catalyst (triethylamine, TEA) were investigated as well. It was found that EDA extended the NCO-functional prepolymer chains at both 30 and 50 °C, independent of the moment of its addition. A limited extent of chain extension by ADH was observed, which was thought to be caused by the low solubility of ADH in the solvent 2-butanone, used for the prepolymer synthesis. ADH chain extension only took place after removal of the 2-butanone. Water chain extension was observed at temperatures ranging from 50 °C to 70 °C. A good balance was found at 50 °C, where a stable dispersion with a relatively high molecular weight, a small particle size and a narrow particle size distribution were obtained. The usage of TEA during the dispersion process promoted the water chain extension reaction, however, at the cost of dispersion stability. An increased DMPA level has shown to improve the dispersion stability. Dispersion-cast poly(urethane urea) films were found to be thermally stable up to 249 °C (5 wt% mass loss) and had Tg values around room temperature.

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