Facile and high-concentration exfoliation of montmorillonite into mono-layered nanosheets and application in multifunctional waterborne polyurethane coating

A large-scale exfoliation method to obtain high-quality and mono-layered montmorillonite (Mt) nanosheets via the aggregation of polyethyl-phosphate glycol ester (Exolit OP 550) was proposed. Small-angle X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and atomic force microscopy (AFM) examinations indicated that 30.0 wt.% of layered Mt. were exfoliated into mono-layered nanosheets in Exolit OP 550 after a mechanical stirring process at 60 °C for 12 h. This exfoliation concentration was increased by 67–650% compared with that of other works. The Mt. nanosheets with Exolit OP 550 were then assembled into environment-friendly waterborne polyurethane (OPWPU) coating by in situ polymerization. A significant decrease of 60° gloss, from 30.2 GU to 0.3 GU, was observed. Many large, uniform, and continuous spherical particles on the surface of OPWPU nanocoatings were found by scanning electron microscopy (SEM), which resulted in a rough surface that almost completely diffused incident light. Besides, limiting oxygen index (LOI), cone calorimeter testing (CCT), and SEM results suggested that the OPWPU nanocomposites also had improved fire retardancy, especially the peak heat release rate was decreased by a maximum of 39.2%. This study may open up new possibilities for the application of Mt. nanosheets in polymers.