Improving water resistance of waterborne polyurethane coating with high transparency and good mechanical properties

Polyester based waterborne polyurethane (WPU) dispersions modified with hydroxypropyl-terminated polydimethylsiloxane (HPDMS) and sulfonate polyester diol were synthesized. The water resistance of WPU coating improved significantly when 5% HPDMS was used for modification, but it also leads to the decreasing of dispersion stability and damages in the mechanical properties and transparency of WPU coating. It was found that incorporating ionic groups in soft segments (sulfonate polyester diol) of WPU can obtain a coating with good water resistance where only 0.5% HPDMS is needed. In this case, the water swelling of WPU coating decreased to 7.0% while the pure WPU was 15.2%. Meanwhile, the mechanical and optical properties of modified coating change slightly. Its tensile strength was 62.6 MPa, and its light transmittance at 532 nm was 83.8%. Zeta potential test indicated the introduction of sulfonate polyester diol could improve the stability of WPU dispersions. The water resistance improvement of the WPU coating could be ascribe to the decrease of surface free energy and the change of phase separation degree. The water swelling, X-ray photoelectron spectroscopy, and contact angle test results demonstrated the migration of HPDMS and sulfonate polyester diol segments on the coating surface. The phase separation of WPU coatings was measured by dynamic thermomechanical analysis. The modified WPU with excellent water resistance is promising in the application of transparent coatings.