Synthesis and properties of triethoxysilane-terminated anionic polyurethane and its waterborne dispersions

The paper presents a new method to prepare waterborne anionic alkoxysilane-terminated polyurethane dispersion (SPUD) which is substantially free from organic solvent: the NCO-terminated polyurethane (PU) containing carboxylic ions in backbone is synthesized by melt polycondensation with toluene 2,4-diisocyanate (TDI), polypropylene oxide (PPO) and 2, 2-dimethylolbutanoic acid (DMBA), the terminal NCO groups are reacted with 3-aminopropyltriethoxylsilane (APTES) to give alkoxysilane-terminated PU (SPU), and the waterborne SPUD with higher aminosilane content (10 wt% in SPU) is obtained by phase-inversion emulsification with the aid of protective colloid (polyethermethylsiloxanes, PEMS) and external emulsifier (sodium dodecyl sulfate, SDS). The anionic PU and SPU are characterized by FT-IR, gel permeation chromatography (GPC), differential scanning calorimeter (DSC) and thermo-gravimetric analysis (TGA). The thermal studies show that SPU has lower T _g than anionic PU as well as thermal stability. The effects of emulsifier, protective colloid, solid content and pH value on the storage stability of waterborne SPUD are investigated, respectively. The results indicate that the storage stability decreases with increasing viscosity and dispersing ratio, whereas particle size and polydispersity index (PDI) are hardly affected by the solid content, the amount of emulsifier and protective colloid. The SPUD prepared with about 2 wt% SDS, 5 wt% PEMS, 31 wt% solid content and pH value of 7 is stable for more than 6 months at 50 °C and 8 months at room temperature, while exhibiting excellent freeze-thaw stability. The corresponding cured films exhibit excellent mechanical properties with 19.2 MPa tensile strength and 320 % elongation at break.