Synthesis and characterization of superhydrophobic surfaces prepared from silica and alumina nanoparticles on a polyurethane polymer matrix
Graphical abstract
Introduction
During the last decades superhydrophobic surfaces (SHS) have been attracting a growing interest from the industry because of their associated properties, such as self-cleaning [1], anti-fogging [[2], [3], [4]], anti-ice [[4], [5], [6], [7], [8]], anti-corrosion [9], or their ability to separate water and oily phases [[10], [11], [12]].
Nature shows a variety of superhydrophobic surfaces, as in lotus leaves [13], rose petals [14], salvinia [15], water-strider legs [16], and the feet of the gecko [17]. It is widely accepted that the two main features needed to reach the superhydrophobic behaviour are: i) the presence of a material with an extremely low surface energy and ii) a very high roughness or, in other words, a very high specific surface area [[18], [19], [20], [21], [22]]. Air bubbles trapped between the solid and the water drops favour their easy roll in a way similar to the Leidenfrost effect [23]. With the aim of replicating the behaviour of these surfaces many different types of nanometric structures have been created, using laser etching [24], chemical etching [25], layer-by-layer assembling [26,27], electrospinning [28], spin-coating, sol-gel processing [29,30], and solution spray, among others [20,[31], [32], [33], [34], [35], [36], [37], [38], [39]]. Many of these processes require highly fluorinated reagents [12,36,37,40], which significantly increases the costs and entails severe environmental risks.
Despite this growing interest, the majority of the proposed methods barely show the capacity to prepare large extensions of these SHS at a reliable cost [41]. In this respect, one of the most promising methods consists in the fabrication of nano- and micro-sized particles embedded into a polymeric matrix which acts as a container medium [14,21,22,[42], [43], [44]]. This method allows to transfer the hydrophobic function from the particles to the matrix, so the tailoring of selected requirements such as UV light and erosion resistance or increased substrate compatibility is easier. However, in this approximation the full matrix is loaded with nano- and microparticles, while only those exposed at the surface will affect the properties of the later.
The objective of the present study is the preparation of SHS through the incorporation of selected nanoparticles only at the surface of the polyurethane coating, thus reducing manufacturing costs. We propose an alternative method able to be escalated to large surfaces and without the need of complicated set-ups.
Section snippets
Materials
A commercial aeronautical paint ALEXIT 411-77 of Mankiewicz has been taken as a polymeric base matrix. This is a three component system formed by: 1) a base blend consisting of a long chain polyol and other additives such as thickeners, antifoams or intumescents; 2) a hardener, in particular hexamethylenediisocyanate (HDI), which reacts with the previous component giving rise to a thermosetting polyurethane matrix, and 3) a very volatile solvent composed of a mixture of n-butylacetate and
Rheology and FTIR
Infrared spectra of the mixture paint immediately after the mixing of the three components (Fig. 3, red line) appears as the sum spectra of the three components. The wide signal around 3450 cm−1 corresponds to the −OH vibrations of the alcohol group of the polyol. The band at 2286 cm−1 corresponds to the stretching vibration of the isocyanate group of the HDI. The signal at 1695 cm−1 indicates the presence of the isocyanurate group. Thus, it can be inferred that HDI is actually present in the
Conclusion
We have developed a method to prepare superhydrophobic surfaces by spraying functionalized NPs on a partially cured polyurethane-based paint. This method is fast and affordable to be used for large areas. Characterization of the surfaces by FTIR spectroscopy, SEM, AFM and rheological measurements have elucidated critical parameters, like the tgel, that appears at 26 min when curing at 70 °C. NPs applied right after the tgel remain partially exposed on the surface, showing a superhydrophobic
Data availability
The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Acknowledgements
All authors thank the Foundation for the Research, Development and Application of Composite Materials, FIDAMC, for its financial support. PintAHer S.L. and Quimidroga S.A. are thanked for providing the polymer components and nanoparticles, respectively. F. Carreño acknowledges the Complutense University of Madrid for grant CT4/14.
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