Mechanical properties improvement of waterborne polyurethane coating films after rewetting and drying

Drying behavior of waterborne polyurethane coating under ambient conditions displays the typical three-stage drying process on compact hard substrates. When the naturally dried samples are further dried at thermal condition of 105°C, the loss of residual water was accompanied with an increase in the hardness of the films. When the coating films were immersed in water and dried at ambient condition again, the hardness and modulus increased significantly. After 180-min immersion followed by natural drying, the hardness of the film increased to almost 10 times that of the initial value. The possible reason is that the interaction between water and the hydrophobic amorphous phase of polyurethane led to a compacted amorphous phase, which decreased the free volume of the films, resulting in the increase in the hardness and modulus of the coating films.