Synthesis and characterization of superhydrophobic surfaces prepared from silica and alumina nanoparticles on a polyurethane polymer matrix

Highlights

• Novel method to produce large superhydrophobic surfaces.

• Rheologic study of particle embedding process via spray.

• Measurements of water contact angle over time to study the pinning of the water droplets.

Abstract

An economical approach to synthesize superhydrophobic surfaces with silica and alumina nanoparticles on a polyurethane-based paint is presented. The surfaces have been prepared by spraying functionalized nanoparticles on the partially cured polymer matrix, and they have been characterized with Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy and water contact angle measurements. Results show that the gel point is reached 26 min after the application of the paint when it is cured at 70 °C. At that moment, rather than submerging into the paint, the sprayed nanoparticles remain partially exposed at the surface. Following this approach, we have found that, for samples including 0.5 wt% of polydimethylsiloxane-functionalized nanoparticles, water droplets show contact angles higher than 150°, have a negligible attachment to the substrate and easily roll off the substrates. The application method is fast and scalable to relatively wide areas. Also, nanoparticles are located only at the surface, thus reducing manufacturing costs.

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.