Hydrodynamics and Film Formation Mechanism Studies on Aqueous Polyurethane Coatings in Printing

Although aqueous polyurethane (APU) possesses unique advantages, such as environmental friendliness, the insufficiency of engineering practice research has hampered its actual implementation. In this work, APU composites incorporated with a hardening agent, a thickener, a flatting agent, and a defoamer were prepared as environmentally friendly coatings for the printing of warp-knitted vamps. Fourier transform infrared (FTIR) spectra, dynamic light scattering (DLS), and laser Doppler electrophoresis (LDE) confirmed the successful preparation of APU coatings. Moreover, rheological characteristic investigations were also evaluated, and the mechanical spectrum is desirable for printing practice. Furthermore, an innovative numerical modeling approach was presented to establish a computational fluid dynamics (CFD) model describing the printing process. The distribution characteristics of the flow field during the film formation process in printing are systematically analyzed. The result shows that the hydrodynamic pressure near the squeegee tip plays a crucial role in the transfer of APU coatings. The variation in the pressure and velocity with the squeegee movement in the flow field at one particular point is obtained. Then, a printing experiment platform is developed to study the variation in the transfer amount of APU coatings with printing force, angle, and velocity. The comparison of experimental and simulation results indicates that the transfer volume of APU coatings conforms to Darcy's law. This study offered a CFD model as a bridge to establish connections among coating properties, printing parameters, and transfer volume. This work will not only describe phenomena during film formation but also reveal the transfer mechanism of APU coatings during printing.