Self-Cleaning Coating

Self-Cleaning Coating

Building walls often lose their glossy appearance due to the accumulation of dust and carbonaceous particles from the environment on them. In order not to observe this effect, self-cleaning exterior paints can be used in general. Application of a self-cleaning superhydrophobic coating can keep building walls clean and shiny. For this, building walls coated with superhydrophobic paint can efficiently self-clean with a natural rainwater or normal mechanical water spray. [1]

Superhydrophobic surfaces, which are a product of nanotechnology, are used in many areas in the industry, and new ones are added to these areas every day. Superhydrophobic surface emerged for the first time with the research of keeping the lotus leaf constantly clean. The surfaces of lotus leaves and rose petals are typical superhydrophobic surfaces found in nature. The surface of the lotus leaf exhibits superhydrophobic properties due
to its micro and the nano-scale hierarchical structure consisting of micropapilla and nanorods covered with hydrophobic waxes. In addition, the surfaces of lotus leaves show low adhesion to water and dust/dirt sticking to the surface can be easily rolled away from the surface with water droplets. The self-cleaning performance of lotus leaves is called the “lotus effect”[2].

The leaf surface is covered with a material with low free surface energy and has a rough structure in nanoscale. Thanks to the combination of these two features, it has been determined that the leaf surface of the lotus flower has a superhydrophobic feature. Thus, the liquid drop on the surface encounters the surface in a very small area thanks to the air trapped in the nano-level cavities. In this way, the liquid drop can stay on the surface close
to the spherical shape with a high contact angle thanks to the surface tension. While the liquid drop is rolling, it cleans the surface by taking the dirt and dust on the surface. Due to these properties, superhydrophobic/hydrophobic surfaces have the potential to be used in many areas such as self-cleaning, icing, corrosion, preventing fogging, and reducing drag resistance.

In order to determine the wettability properties, three different situations are generally defined depending on the contact angle of the liquid drop on the surface. If the contact angle is less than 90°, it is called hydrophilic (wetting), if it is between 90° and 150°, it is hydrophobic (non-wetting), and if it is greater than 150°, it is called superhydrophobic (super wet) surface. These three different situations are shown in Figure 1.

In paints and coatings, the ability to remove dust particles by the action of water droplets is defined as self-cleaning. The most basic principle of Self Cleaning technology is its ability to form a spherical water droplet that can remove dirt particles on the surface. Self cleaning is the ability of the surface to remove dust particles by the action of water droplets. Dust particles can be removed from the surface by the sliding and rolling action of the droplets during the selfcleaning process. A spherical droplet carries particles of dust and dirt in a rolling motion over the superhydrophobic surface [3]. On the other hand, the use of photocatalytic materials with hydrophilic or super-hydrophilic surfaces can contribute to the self-cleaning property as they allow organic pollutants to degradation or decompose more easily [4].

The concept of self-cleaning has received a lot of attention in recent years due to its diverse properties and its variety of possible applications in various fields. This technology has been widely used in daily practice in recent years. Figure 2 shows the applications where this technology is used. Self-cleaning surfaces are prepared using hydrophilic photocatalytic materials or hydrophobic surface finishing agents. The biggest challenge of
this technology is its resistance to surface properties due to hard environmental conditions and at the same time its ability to maintain the stability of the coating for a long time.

Surfaces with special wettability have been the focus of research in recent years. In particular, it has attracted attention with its superhydrophobic surfaces with photocatalytic activity and its adjustable adhesion to water droplets. The superhydrophobicity of these
surfaces is usually due to the interaction of the microstructure and chemical composition of the surfaces. Therefore, two methods are used to prepare superhydrophobic surfaces. These methods are;

(1) It is prepared with certain materials with rough surface coating structures and then replaced by materials with low surface energy.
(2) Materials with low surface energy are chosen as the substrate and rough surface structures are created on the surfaces of these substrates. Common methods include
the sol-gel method, deposition method, 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 its self-cleaning property. TiO2 is widely used in the field of photocatalysis due to its high activity and non-toxicity. In recent years, many superhydrophobic surfaces have been prepared with TiO2. Researchers have generally used TiO2 as a raw material to prepare self-cleaning surfaces with improved “lotus effect” and photocatalytic activity. However, TiO2 is a hydrophilic pigment and becomes superhydrophilic under UV irradiation. Therefore, materials with low surface energy are often used to modify the surfaces prepared with TiO2. Coatings with water contact angles of 156.3° and 3.7° contact angle hysteresis 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 more evenly disperse on various substrates and create hierarchical morphology. Adhesion of mono-dispersions or aggregates of photocatalytic the nanoparticles that have photocatalytic activity may show a better self-cleaning property. Similarly, the combination of micro-nano particles with low surface energy materials can improve the surface roughness and water repellency behavior, leading to the realization of self-cleaning property on the surface [4].

Liu et al. (2021), a self-cleaning photocatalytic WO3- TiO2 nanorod (MWT)/Polydimethylsiloxane (PDMS) building coating was prepared by spraying method.
The surface hierarchical structure produced by MWT and PDMS was a milestone
for creating a superhydrophobic surface that provides self-cleaning performance
to the MWT-containing coating. Significant durability and antifouling properties were
observed 450 days after the MWT-containing coating was applied to the exterior [5].
The self-cleaning property of the coating is confirmed by the removal of dust particles
from the surface and it’s their resistance to adhesion. The self-cleaning properties of different contaminants with the MWT-containing coating are shown in Figure 3 [5].

The coating containing MWT retained its superhydrophobic and antifouling properties after 450 days in natural weather conditions (Figure 4). It was determined that the self-cleaning performance of the MWT coating surface was maintained when sprayed on paint (100 mg/L MB solution) and muddy water (mass ratio of water and standard ash 3:1). However, the self-cleaning effect of commercial coating was lost after natural weathering. It has been reported in the study that MWT coating exhibits excellent durability that can be used on
the exterior of buildings for a long time [5].

Syafiq et al. (2020) transparent self-cleaning coatings were synthesized by using organic Polydimethylsiloxane (PDMS) polymers and inorganic nano-Calcium carbonate (CaCO3) fillers applied by spray method on the glass panel, which can be easily produced and cured at room temperature. The coating system was developed with a low-cost nano-CaCO3 and a simple process for keeping the cost low. The amount of CaCO3 was used at rates ranging from 0.2% to 1.2% by weight and was given in C1-C6 coded recipes. Self-cleaning and durability of nano-CaCO3 coating systems were investigated after 4 months of outdoor exposure. The results are summarized in Table 1. C4 recipe containing 0.8% CaCO3 by
weight showed the best performance with the highest contact angle [6].

Guo et al. (2015) investigated the strategy of directly applying a TiO2 containing paint on the surface of selfcompacting architectural mortars. In the study, three different approaches were used to incorporate TiO2 into self-compacting architectural mortars: mixing nano TiO2 powder (P25, Degussa) and mortar, coating with 10% TiO2 paint, and coating with P25 TiO2. Their photocatalytic activity was investigated in terms of Rhodamine B reduction under both UV-A, visible light irradiations and weathering resistant ability (labsimulated accelerated façade weathering) were subsequently examined. The overall results suggest that the TiO2 coated self-compacting architectural mortars have the potential to be used as a resource and energyefficient product for air-purifying and self-cleaning applications [7].

Zhou et al. (2016) aimed to develop a self-cleaning paint by adding palygorskite (Pal), a clay mineral with nanorod structure, modified by amino silicon oil (ASO) and aminopropyltriethoxysilane (APTES) to traditional paint (TP). In the study, it was observed that ASO and APTES could effectively make a Pal form with plenty of nanoclusters, significantly increasing the roughness and amount of hydrophobic groups on the Pal surface. After this modification, it was observed that the originally hydrophilic Pal mineral could be converted to a superhydrophobic structure. As a result of the study, it was determined that Pal modified with ASO and APTES could be used as an effective hydrophobic agent to change the hydrophilic paint to a superhydrophobic one with the self-cleaning ability [8].

In this study, Xue et al. (2018) aimed to develop a superhydrophobic self-cleaning orange-gray coating using commercially available materials. Grinding the coating surfaces using an appropriate emery paper creates microgrooves with suitable widths and exposes micro- and nanoparticles on the coating surface. In this way endow the coating surfaces with superhydrophobic self-cleaning properties. In addition, the superhydrophobicity of the coating shows that it has good resistance to acid, alkali, and mechanical abrasion. As a result of the study, it was observed that due to artificial accelerated weather conditions, the coating can destroy its superhydrophobicity and partially reduce solar reflection. In addition, it has been shown that superhydrophobic self-cleaning properties can be
regained by renewing the worn coating using suitable sandpaper [9].

Self-cleaning paints are capable of degrading pollutants present on their surface to improve their appearance and the quality of the environment in which they are inserted. In addition to the usual components, these types of paints contain photocatalytic particles, such as TiO2 in the anatase phase. This photocatalytic effect causes degradation of the pollutants present on the surface of the coating, leading to the self-cleaning characteristic, but it can also reduce the stability of the organic binder present in the paint formulation. Therefore, to develop a self-cleaning paint formulation, it is very important to find a balance between the photo cleaning efficiency and the durability of the paint. [10].

Results

The self-cleaning performance of the paint is directly proportional to its superhydrophobic property. To increase the superhydrophobic property, it was aimed to increase the surface roughness by using different methods in various studies. In this way, it has been observed that the sun reflection feature is improved as well as its self-cleaning feature. In addition, the adjustable adhesion of water droplets on superhydrophobic surfaces with photocatalytic activity has attracted attention. TiO2 is generally used in compositions to provide the photocatalytic effect.

TiO2, especially in the anatase phase, has been extensively studied for photocatalytic applications due to its high photoactivity, excellent chemical stability, low cost, and high availability. Self-cleaning paints not only improve the aesthetic appearance of the building but also effectively reduce routine maintenance costs. Conventional paints are mostly hydrophilic and therefore tend to degrade by pollutants in the water. It is important to develop new paints with high hydrophobicity and self-cleaning ability. Although the self-cleaning feature has been considerably improved in the studies, the durability of the self-cleaning paint is still open to development.

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.
[6] Syafiq A., Vengadaesvaran B., Ahmed U., Rahim A.N., Pandey A.K., Bushroa R.A., Ramesh K., Ramesh S., Facile Synthesize of Transparent Hydrophobic Nano-CaCO3 Based Coatings for Self-Cleaning and Anti-Fogging, Materials Chemistry and Physics, 239, 2020.
[7] Guo M.Z., Ramirez A.M., Poon C.S., Self-Cleaning Ability of Titanium Dioxide Clear Paint Coated Architectural Mortar and Its Potential inFfield Application, Journal of Cleaner Production, 112, 2016.
[8] Zhou L., Xu S., Zhang G., Cai D., Wu Z., A facile approach to fabricate self-cleaning paint, Applied Clay Science, 132-133, 2016, 290-295.
[9] Xue X., Yang Z., Sun P., Feng Y., He Z., Qu T., Dai J-G., Zhang T., Xu L., Zhang W:, Superhydrophobic Self-Cleaning Solar Reflective Orange-Gray Paint Coating, Solar Energy Materials and Solar Cells, 174, 2018.
[10] Amorim S.M., Suave J., Andrade L., Mendes M.A., Jose J.H., Moreira R.F.P.M., Towards an Efficient and Durable Self-Cleaning Acrylic Paint Containing Mesoporous TiO2 Microspheres, Progress in Organic Coatings, 118, 2018, 48-56

Seda Aygül
Ar-Ge PSRA Uzmanı / R&D PSRA Specialist
Marshall Boya ve Vernik Sanayi A.Ş.

Şevval Bilketay
R&D Project Specialist
Marshall Boya ve Vernik Sanayi A.Ş.