Low gloss waterborne polyurethane coatings with anti-dripping and flame retardancy via montmorillonite nanosheets

A series of low gloss organic montmorillonite/waterborne polyurethane nanocomposite (OWPU) coatings with polycaprolactone (PCL) and hydroxyl terminated polybutadiene (HTPB) were prepared, characterized and then applied to polyvinyl chloride leather. Small-angle XRD and TEM examinations indicated that montmorillonite was exfoliated into individual single-layer nanosheets in HTPB, and retained separate shapes in subsequent synthesis and preparation. The OWPU dispersions with higher nanosheets contents exhibited a larger average particle size and a less negative zeta potential obtained through DLS. A rougher coating surface topography of OWPU compared with pristine WPU was observed by SEM, which reduced the 60° gloss to 4.6 GU within the matt range. Moreover, TGA results indicated that the nanosheets effectively increased thermal stability of hard segments and soft segments derived from PCL, but had little effect on that of soft segments derived from HTPB. Furthermore, rheological analysis confirmed the nanosheets promoted cross-linking and cyclization of HTPB at high temperature, thereby imparting anti-dripping properties to OWPU coatings. Cone calorimeter test and SEM results showed that flame retardancy of OWPU was improved by forming a charred layer with sheet-cluster structure. In addition, incorporation of the nanosheets was also capable of changing the surface touch feeling of OWPU coatings from sticky to some dry alike.