Cooperated assembly and chain-extension of polyurethanes end-capped with isocyanate and partially with silane in water

Considering the low reaction efficiency and limited chain extension degree of traditional amine chain extension in isocyanate-terminated （-NCO） waterborne polyurethane （WPU） dispersion systems, a fully -NCO-terminated hydrophilic modified PU prepolymer and a （3-aminopropyl）triethoxysilane （APTES）/-NCO hybrid-terminated hydrophobic PU prepolymer were prepared, and employed in aqueous co-assembly to construct latex particles with -NCO end groups enriched on the outer layer and silane-modified PU prepolymer enriched in the core, realizing the synergistic effect of piperazine chain extension and silane hydrolysis-condensation chain extension. The regulatory effects of APTES termination ratio on the stability of WPU dispersion systems, as well as the mechanical, thermal and water resistance properties of WPU films were systematically investigated. Results show that the WPU aqueous dispersion systems exhibit favorable stability within the APTES termination ratios range of 0~60%. When the termination ratio is controlled at 10%~20%, the chain extension threshold of 60% for traditional amine chain extension can be broken, which significantly improves the tensile strength and thermal resistance of WPU films. Nevertheless, an excessively high APTES termination ratio deteriorates the film-forming performance of emulsions and causes fusion defects in the films. This study confirms that the synergistic effect of piperazine chain extension and silane hydrolysis-condensation can effectively break through the performance bottleneck of traditional amine chain extension and remarkably improve the chain extension reaction efficiency of WPU systems.