Effects of NIR-reflective pigments on waterborne polyurethane coatings and their thermal insulation performance

Waterborne coatings have garnered much attention and gradually occupied the coating market recently due to the significant advantages in volatile organic compound emissions compared with solvent-borne coatings. Waterborne polyurethane coatings are a majority portion of waterborne coatings. Unlike solvent-borne coatings, the film formation of waterborne coatings occurs in an inhomogeneous phase, which brings a challenge for pigmentation in waterborne coatings. However, the influence of pigment on film formation and the property of waterborne polyurethane coating remain unclear. In this study, the different contents of commercial black near-infrared (NIR) reflective pigments were incorporated into both commercial and synthetic waterborne polyurethane systems. The film formation process was monitored by multispeckle diffusing wave spectroscopy. The impact of the pigment content on dry films was assessed through scanning electron microscope, color and gloss measurements, dynamical thermal analyzer, and tensile testing. Furthermore, the thermal insulation performance of the waterborne polyurethane coatings was evaluated to study the function of the NIR reflective pigments. Our findings reveal that the addition of black NIR reflective pigments yields a multifaceted effect on film formation and coating morphology and improves thermal insulating property. Elevated pigment content correlated with increased crosslinking density, tensile stress, and Young's modulus.