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

Abstract

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 2 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 o C but did just 1 hour at 50 o 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.

Blooming theory of tristearin

“…Not only the flake size, the kinetics of 80 blooming is also essential in the experimental designs. 10,11 Very recently, we have found melting point lowering with the blooming by the mixing of stearin into SSS though the detail is skipped, where the kinetics became faster with descending melting point. Thus, it is possible to change the kinetics by 85 the incorporation of impurities.…”

“…This blooming theory also explains not only the blooming of SSS, but also the blooming in other TAGs and waxes such as trimyristin (MMM), tripalmitin (PPP), distearin, 1,3-distearoyl-2-oleoylglycerol (SOS) and AKD because they also show blooming with 75 polymorphism. 7,8,10,11 This will be clarified in future studies. Here, it should be noted that the blooming theory is also helpful in experiment.…”

“…Micrometer-scale flakes are formed in blooming, whereas phase transformations occur in molecular scale. [8][9][10][11] and the preparation of a fractal body (fractal porous silica).…”

Characterization and structural investigation of fractal porous-silica over an extremely wide scale range of pore size

“…For instance, fractal surface structures emerge super-water repellency [8][9][10][11][12][13][14] and super oil-repellency [15]. The super water-repellent fractal structure is spontaneously formed on the surfaces of alkylketene dimer (AKD) and triglycerides waxes [8][9][10][12][13][14]. The super-water repellency is caused by the enhancement of the real surface area of waxes arising from the surface roughness.…”

Fractal Materials and Their Functional Properties

“…22 As reported previously, AKD and triglycerides form the super water-repellent fractal surfaces during the phase transition from the meta-stable crystalline form to the stable form. 12,21 This empirical rule has been extended to various waxes without any exceptions in this section. 22 The wax samples of AKD, diglyceride and triglyceride have meta-stable form of crystal and give super water-repellent fractal surfaces.…”