Synthesis and characterization of water-borne crosslinked silylated polyurethane dispersions

A series of water-based polyurethane dispersions were synthesized by the polyaddition of isophorone diisocyanate, poly(oxytetramethylene) glycol, and dimethylol propionic acid as prepolymers, which were end-capped and crosslinked with 3-aminopropyl trimethoxysilane (APTMS) to produce silylated polyurethane dispersions (SPUDs). The dispersion was performed before the end-capping reaction to avoid gelation. Pure and tetraethylene pentamine chain-extended polyurethanes were also synthesized. The length of the soft segment and the ratio of NCO to OH were varied. The properties of these prepolymer dispersions were investigated with Fourier transform infrared, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction, tensile and surface contact-angle measurements, nanoindentation testing, gel content, water and xylene swellability, and storage stability. An increase in the modulus and hardness and a decrease in the tensile properties of SPUDs were noticed in comparison with a pure polyurethane dispersion. This may have been due to the smaller number of hydrogen bonds and the brittleness of the film by the formation of crosslinked siloxane networks through the hydrolysis-condensation reaction of methoxysilane groups of silylated polyurethane, and it was also confirmed by the reactions of APTMS end capping and crosslinking. The gel content of SPUDs increased with the NCO/OH ratio, and all the prepared samples were amorphous in nature. The thermal stability of SPUDs was higher than that of pure and amine-chain-extended polyurethane dispersions. A decrease in the water and solvent swelling and an increase in the water contact angle confirmed the effective crosslinking of the silanol groups of the silylated polyurethane. Storage-stability results showed that all the prepared dispersions were stable for more than 3 months.