Self-healing and self-cleaning clear coating

Abstract

Coatings exhibiting both self-cleaning and self-healing properties are envisioned for a wide range of applications. Herein we report a simple fabrication approach toward poly(urea-urethane) (PU) coatings having self-healing and self-cleaning properties. The self-cleaning component is a poly(dimethylsiloxane) (PDMS), which is affordable in cost and also has a lower environmental footprint relative to its fluorinated counterpart. The self-healing properties are imparted by dynamic urea bonds of the matrix. The obtained surfaces are evaluated for their anti-smudge properties such as water-, oil- and ink-repellency, as well as optical properties. The self-healing properties of these coatings are evaluated by making scores with a doctor blade and monitoring the healing under different conditions using optical microscopy. The resultant coatings are also investigated for their good mechanical properties. The surface chemical compositions are determined x-ray photoelectron spectroscopy, while atomic force microscopy is used for microstructural analysis of these coatings.

Introduction

Omniphobic surfaces have numerous uses ranging from stain-free screens to sensors [1], [2]. Due to a wide range of potential applications, three decades of research have been devoted towards the fabrication of omniphobic surfaces that are mostly based on bioinspired models including lotus leaves [3], water strider legs [4], and pitcher plants [2]. For example, candle soot can serve as a precursor toward robust superomniphobic coatings to mimic the lotus leaf model [5]. However, rough surfaces are often plagued with poor durability [6], and limited clarity [7], [8], along with their susceptibility to failure under pressure. In contrast, the pitcher plant model has been widely used to develop smooth surfaces with remarkable water- and oil repellency [2], [9], [10], [11], [12], [13]. For example, smooth omniphobic surfaces such as slippery liquid-infused porous surfaces imparted excellent omniphobic properties [2], [14]. However, these coatings showed poor durability. To address this, omniphobic urethane and epoxy coatings with excellent durability were reported [15], [16], [17], [18], [19], [20], [21], [22], nevertheless, these coatings are lacking the desirable self-healing properties.

Self-healing coatings with no self-cleaning properties have also been extensively investigated [23], [24], [25], [26]. These coatings often rely on reversible interactions. These reversible interactions can be non-covalent interaction such as hydrogen bonding [27], [28], high-valence metal chelation [29], [30], host–guest interactions [31], [32], as well as dynamic covalent bonds (disulfide-bonds [33], [34], acyl hydrazone bonds [35], nitroxides [36], and Diels-Alder reactions [37], [38], [39] or trans-esterification reactions). Coatings relying on non-covalent interactions typically have poor mechanical strength due to the weak nature of these interactions [40]. Microencapsulation is another approach commonly used for self-healing surfaces [41], but the resultant coatings are translucent, which limits their applications particularly where optical clarity is desirable. Also, microencapsulated coatings can heal only once at the damaged site because of the exhaustion of encapsulated reagents during the healing of first damage [42]. On the contrary, the dynamic covalent bond approach is becoming popular because the resultant coatings are having good optical clarity and can heal damage at the same site virtually for unlimited cycles [43], [44]. However, often expensive and tedious approaches are required to incorporate reversible covalent motifs into polymers to enable self-healing [45]. Recently, Leibler et al. developed a self-healing epoxy network via transesterification using zinc acetate as a catalyst [46]. Ying et al. utilized dynamic urea bond for self-healing coatings [47]. Despite excellent self-healing capabilities, these coatings were lacking self-cleaning properties.

Coatings with both self-healing and self-cleaning can offer a wide range of applications by combining the benefits of both features [48], [49], [50], [51], [52], [53]. Studies reporting coatings integrating self-cleaning and self-healing are rarely reported. While those reported are lacking good mechanical and optical properties [2], [51], [52], [54]. Herein we report a simple design for the fabrication of self-healing and self-cleaning surfaces that offers excellent clarity and mechanical properties. The coated films were recovered almost fully between 40 and 60 °C and shown good adhesion properties to the materials (glass, metals, and wood). This approach involves a poly(urethane-urea) (PU) system bearing polydimethylsiloxane (PDMS) as the self-cleaning component. A diamine (N,N-di-tert-butyl ethylene-diamine) was used to enable reversible urea bond formation upon reaction with isocyanate groups. Meanwhile, polyether diisocyanate prepolymer and an aliphatic hexamethylene diisocyanate trimer (HDIT) were used to control the mechanical properties. While the self-cleaning properties were adjusted by changing the content of the PDMS. The obtained coatings were evaluated for their self-cleaning and self-healing behavior along with their mechanical durability and surface composition analysis