Formation mechanism of super water-repellent fractal surfaces of alkylketene dimer

Alkylketene dimer (AKD), a kind of wax, has been known to form fractal surfaces spontaneously, and to show super water-repellent property. In order to understand further the super water-repellency of the AKD surfaces and to elucidate the mechanism of spontaneous formation of the fractal structures on the wax surfaces, a pure and a mixed AKD samples were employed to make the surfaces by the melt-solidification method. Time-dependent contact angles of water droplets on the AKD surfaces cured at several temperatures were systematically measured. It was found that spontaneous formation of super water-repellent surfaces was thermally induced quite effectively. For example, it took about 6 days for the pure AKD surface to show the super water-repellency at 40 °C but did just 1 h at 50 °C. The fractal dimension of the mixed AKD surface with super water-repellency was calculated to be 2.26 from the SEM images by the box-counting method which should be reasonably compared with 2.29 of pure AKD surface. The mechanism for the spontaneous formation of fractal structures was discussed from the results of the time-dependent contact angles, differential scanning calorimetric (DSC) curves and X-ray diffractometric (XRD) patterns. It has been made clear that the fractal AKD surface and its super water-repellency result from the phase transformation from the metastable state to the stable one of the AKD crystals.