Self-Cleaning Paints

  Building walls frequently lose their bright appearance due to the accumulation of dust and carbonaceous particles from the environment. Self-cleaning exterior facade paints are generally used to avoid noticing this effect. The application of a durable self-cleaning superhydrophobic coating can keep building walls clean and bright. For this purpose, building walls coated with superhydrophobic paint can efficiently clean themselves with natural rainwater or normal mechanical water spray [1]. Superhydrophobic surfaces, a product of nanotechnology, are used in many areas of industry, and new applications are being added to these fields every day. Materials with superhydrophobic surfaces first emerged from research into how lotus flower leaves remain continuously clean. The surfaces of lotus leaves and rose leaves are typical superhydrophobic surfaces found in nature. The surface of the lotus leaf exhibits superhydrophobic properties due to its hierarchical micro and nano-scale structure consisting of micropapillae and nanostructures coated with hydrophobic wax. In addition, the surfaces of lotus leaves show low adhesion to water, and dust/dirt particles adhering to the surface can easily roll away and be removed by water droplets. The self-cleaning performance of lotus leaves is called the "lotus effect" [2]. The leaf surface is covered with a material of low surface free energy and also has a rough structure at the nano scale. Due to the combination of these two properties, it has been determined that the lotus flower leaf surface has superhydrophobic properties. Thus, a liquid droplet on the surface contacts the surface over a very small area due to air trapped in nano-scale cavities. In this way, the liquid droplet can maintain a spherical shape on the surface with a high contact angle due to surface tension. As the liquid droplet rolls, it removes dirt and dust from the surface by incorporating them. Due to these properties, superhydrophobic/hydrophobic surfaces have potential for use in many areas such as self-cleaning, prevention of icing, corrosion, fogging prevention and reduction of drag resistance. To determine wettability properties, three different conditions are generally defined based on the contact angle of the liquid droplet on the surface. If the contact angle is less than 90°, it is called hydrophilic (wettable); if between 90° and 150°, hydrophobic (non-wettable); if greater than 150°, it is called superhydrophobic (super non-wettable) surface. These three different conditions are shown in Figure 1. [caption id="attachment_135808" align="aligncenter"] Figure 1. a) Hydrophilic b) Hydrophobic c) Superhydrophobic[/caption] In paints and coatings, the ability to clean dust particles through the effect of water droplets is defined as self-cleaning. The most basic principle of self-cleaning (Self Cleaning) technology is the ability to create a spherical water droplet that can remove dirt particles on the surface. Self-cleaning is the ability of a surface to clean dust particles through the effect of water droplets. During the self-cleaning process, dust particles can be removed from the surface by the sliding and rolling motion of droplets. A spherical droplet rolls across a superhydrophobic surface, carrying dust and dirt particles [3]. On the other hand, the use of photocatalytic materials with hydrophilic or superhydrophilic surfaces can contribute to self-cleaning properties by enabling easier decomposition or degradation of organic contaminants [4]. The concept of self-cleaning has attracted considerable attention in recent years due to its diverse characteristics and possible applications in various fields. This technology has been widely used in everyday applications in recent years. Figure 2 shows applications where this technology is used. Self-cleaning surfaces are prepared using hydrophilic photocatalytic materials or hydrophobic surface finishing agents. The greatest challenge facing this technology is the durability of surface properties depending on harsh environmental conditions and the ability of the coating to maintain long-term stability. [caption id="attachment_135813" align="aligncenter"] Figure 2. Self-cleaning technology in various applications [4].[/caption] Surfaces with special wettability have recently become the focus of research. Particularly superhydrophobic surfaces with photocatalytic activity and adjustable adhesion properties to water droplets have attracted interest. The superhydrophobicity of these surfaces generally results from the interaction of the micro-structure of the surfaces and their chemical composition. For this reason, two methods are used to prepare superhydrophobic surfaces. These methods are: (1) Prepared with certain materials using rough surface coating structures, and then replaced with low surface energy materials. (2) Low surface energy materials are selected as the base layer, and rough surface structures are created on the surfaces of these base layers. Common methods include the sol-gel method, deposition method, and spray method [2]. In addition to the self-cleaning effect of lotus leaf-like hydrophobic surfaces, the addition of additives containing photocatalytic activity to the composition is valuable for self-cleaning properties. TiO2 is widely used in the field of photocatalysis due to its high activity and non-toxic nature. In recent years, many superhydrophobic surfaces have been prepared with TiO2. Researchers have generally used TiO2 as raw material to prepare improved self-cleaning surfaces with "lotus effect" and photocatalytic activity. However, TiO2 is a hydrophilic pigment and becomes superhydrophilic under UV light exposure. For this reason, low surface energy materials are generally used to modify surfaces prepared with TiO2. Coatings with water contact angles of 156.3° and contact angle hysteresis of 3.7° have high self-cleaning ability and thermal stability [2]. Nanotechnology plays an important role in the development of self-cleaning surfaces. The use of nanoparticles alone can be distributed more uniformly over various substrates and create hierarchical morphology. The adhesion of mono-dispersions or aggregates of nanoparticles with photocatalytic properties can show better self-cleaning properties. Similarly, the combination of micro-nano particles with low surface energy materials can improve surface roughness and water-repellent behavior, which leads to the realization of self-cleaning properties on the surface [4]. Liu et al. (2021) prepared a self-cleaning photocatalytic WO3-TiO2 nanorod (MWT)/Polydimethylsiloxane (PDMS) building coating by spray method in their study. The hierarchical structure of the surface produced with MWT and PDMS became a turning point in creating a superhydrophobic surface that provides self-cleaning performance to the coating containing MWT. Significant durability and antifouling (anti-algae) properties were observed 450 days after application of the MWT-containing coating to the exterior facade [5]. The self-cleaning property of the coating was confirmed by the removal of dust particles from the surface and its resistance to adhesion. The self-cleaning properties of the MWT-containing coating against different contaminants are shown in Figure 3 [5]. [caption id="attachment_135814" align="aligncenter"] Figure 3. Self-cleaning properties of MWT coating against hydrophilic and hydrophobic contaminants [5][/caption] The MWT-containing coating retained its superhydrophobic and antifouling properties after being exposed to natural atmospheric conditions for 450 days (Figure 4). When sprayed with paint (100 mg/L MB solution) and muddy water (mass ratio of water and standard ash 3:1), it was determined that the MWT coating maintained its self-cleaning performance on the surface. However, the self-cleaning effect of commercial coatings was lost after natural weathering. The study reported that the MWT coating demonstrated excellent durability for long-term use on the outer surfaces of buildings [5]. [caption id="attachment_135816" align="aligncenter"] Figure 4. Antifouling properties of MWT and commercial coatings after 450 days of exposure to natural atmospheric conditions [5].[/caption] Syafiq et al. (2020) synthesized transparent self-cleaning coatings on glass panels using spray method, easy-to-produce organic Polydimethylsiloxane (PDMS) polymers and inorganic nano-Calcium carbonate (CaCO3) fillers that cure at room temperature. The coating system was developed with low-cost nano-CaCO3 and a simple process aimed at keeping costs low. The amount of CaCO3 was used in proportions ranging from 0.2% to 1.2% by weight and provided in formulations coded C1-C6. The self-cleaning and durability of nano-CaCO3 coating systems after 4 months of outdoor exposure were investigated. The results are summarized in Table 1. The C4 formulation containing 0.8% CaCO3 by weight demonstrated the best performance with the highest contact angle [6]. [caption id="attachment_135817" align="aligncenter"] Table 1. Average water contact angle values (after 4 months of outdoor exposure) [6][/caption] Guo et al. (2015) investigated the strategy of directly applying TiO2-containing paint to the surface of self-leveling architectural mortars. In the study, three different methods were used to incorporate TiO2 into self-leveling architectural mortars: mixing nano TiO2 powder (P25, Degussa) with the mortar, coating with paint containing 10% TiO2, and coating with P25 TiO2. Their photocatalytic activities in terms of Rhodamine B reduction under both UV-A and visible light radiation and their resistance abilities to atmospheric conditions (accelerated facade weathering laboratory simulation) were examined. The results showed that self-leveling architectural mortars coated with TiO2 have the potential to be used as a resource and energy-efficient product for self-cleaning applications [7]. Zhou et al. (2016) aimed to develop a self-cleaning paint by adding to conventional paint a clay mineral with a nanorod structure modified with amino silicone oil (ASO) and aminopropyltriethoxysilane (APTES) called Palygorskite (Pal). In the study, it was observed that ASO and APTES could effectively create a form of Pal containing multiple nano clusters, significantly increasing roughness on the Pal surface and the amount of hydrophobic groups. After this modification, the Pal mineral, which was originally hydrophilic, was transformed into a superhydrophobic structure. As a result of the study, it was determined that Pal modified with ASO and APTES can be used as an effective hydrophobic agent to transform hydrophilic paint into superhydrophobic paint with self-cleaning ability [8]. Xue et al. (2018) aimed to develop superhydrophobic self-cleaning orange-gray paint using commercially available materials in their study. Grinding the coating surfaces with appropriate sandpaper creates micro-grooves of appropriate width and exposes micro and nanoparticles on the coating surface. Thus, superhydrophobic self-cleaning properties are imparted to the coating surface. In addition, the coating's superhydrophobicity shows good resistance to acid, alkali and mechanical abrasion. As a result of the study, it was found that the coating's superhydrophobicity could be completely destroyed due to artificial accelerated atmospheric conditions and could partially reduce solar reflectance. Furthermore, it was shown that superhydrophobic self-cleaning properties could be recovered by renewing the worn coating using appropriate sandpaper [9]. Self-cleaning paints can degrade contaminants on their surfaces to improve appearance and the quality of the environment in which they are placed. These types of paints contain photocatalytic particles such as TiO2 in anatase phase, in addition to conventional components. The photocatalytic effect it possesses causes degradation of contaminants on the coating surface, leading to self-cleaning properties, but it can also reduce the stability of the organic binder present in the paint formulation. Therefore, when developing a self-cleaning paint formulation, it is very important to find a balance between photocleaning efficiency and paint durability [10]. Conclusions The self-cleaning performance of paint is directly proportional to its superhydrophobic properties. To increase superhydrophobic properties, various studies have used different methods to increase surface roughness. In this way, in addition to self-cleaning properties, improvement in solar reflectance properties has been observed. Moreover, adjustable adhesion properties of water droplets on superhydrophobic surfaces with photocatalytic activity have attracted interest. TiO2 compositions are generally used to provide photocatalytic effect. TiO2, particularly in anatase phase, has been extensively studied for photocatalytic applications due to its high photoactivity, excellent chemical stability, low cost and high usability. Self-cleaning paints not only improve the aesthetic appearance of buildings, but also effectively reduce routine maintenance costs. Traditional paints are mostly hydrophilic and therefore tend to be degraded by contaminants in water. The development of new paints with high hydrophobic properties and self-cleaning ability is important. Although self-cleaning properties have been significantly developed in conducted studies, the durability of self-cleaning paint remains open to improvement. Seda Aygül R&D PSRA Specialist Marshall Boya ve Vernik Sanayi A.Ş. Şevval Bilketay R&D Project Specialist Marshall Boya ve Vernik Sanayi A.Ş.   Sources / References [1] Latthe S.S., Sutar R.S., Kodag V.S., Bohasel A.K, Kumar M., Sadasivuni K.K., Xing R., Liu S., Self – Cleaning Superhydrophobic Coatings: Potential Industrial Applications, Progress in Organic Coating, 128, 2019. [2] Wang F., Chang R., Ma R., Tian Y., Eco-friendly and Superhydrophobic Nano-Starch Based Coatings for Self-Cleaning Application and Oil-Water Separation, Carbohydrate Polymers, 271, 2021. [3] Syafiq A., Vengadaesvaran B., Ahmed U., Rahim A.N., Pandey A.K., Bushroa R.A., Ramesh K., Ramesh S., Transparent Self-Cleaning Coating of Modified Polydimethylsiloxane (PDMS) for Real Outdoor Application, Progress in Organic Coatings, 131, 2019. [4] Dalawai P.S., Aly M.A.S., Latthe S.S., Xing R., Sutar R.S., Nagappan S., Ha C.S., Sadasivuni K.K., Liu S., Recent Advances in durability of superhydrophobic self-cleaning technology: A critical review, Progress in Organic Coatings, 138, 2020. [5] Liu G., Xia H., Niu Y., Zhao X., Zhang G., Song L., Chen H., Fabrication of Self-Cleaning Photocatalytic Durable Building Coating Based on WO3-TNs/PDMS and NO Degradation Performance, Chemical Engineering Journal, 409, 2021.