Use of Zinc Oxide Nanoparticles (ZnO) in Swimming Pool Coatings

Introduction
Pool coatings are protective layers on the inside surfaces of pools. The main functions of pool coatings includes stopping water from leaking and protecting the facility against outside forces. Throughout the year these coatings are exposed to drastic changes which have the potential to erode coatings and shorten shelf life; factors that can produce peeling, discoloration, loss of adhesion and lead to the development of viscous layers on surfaces within the pool. Algae growth not only affects appearance and cleanliness but the process of cleaning algae is expensive and very time consuming. Hence, improving durability and stability of pool coatings, along with developing anti-biofouling properties, has always been a topic of interest for researchers and builders.

ZnO nanoparticles, on the other hand, have a broad spectrum of antimicrobial properties and prevent the growth and replication of different types of microorganisms (bacteria, fungi, and algae) on surfaces. The main antimicrobial mechanism of ZnO is via the release of zinc ions and photocatalytic activity. ZnOnanoparticles, in the presence of light, by light absorption and electron excitation, produce active forms of oxygen, hydroxyl radicals and peroxide. These highly reactive oxygen species rupture the microbial and algal cell walls leading to their death or inhibiting their replication. Laboratory tests have shown that ZnO nanostructured coatings have successfully inhibited the colonization of bacteria and microalgae. For example, it has been observed that ZnO nanomaterials under light have significantly reduced the concentration of algae and bacteria of algae and bacteria and inhibited their stickiness and growth on the substrate. This effect was negligible in the dark and this subsequently confirmed the dominant cause of the antimicrobial effect being ZnO, photocatalytic process and production of ROS in the presence of light. Therefore, ZnO-impregnated coatings also have anti-fouling properties, particularly in bright conditions (sunlight or UV light).

Properties of Zinc Oxide Nanoparticles (ZnO)

ZnO is an oxide semiconductor that has unique physical and chemical properties at the nano-level. Zinc oxide nanoparticles can absorb a large amount of solar ultraviolet rays, which is one of the reasons it is referred to as UV adsorbents and optical stabilizers in coatings. This property is important to pool coatings in which the coatings are exposed to sunlight during the summer season and it is known that ultraviolet radiation is one of the primary factors in the destruction of outdoor polymer coatings. The ZnO nanoparticles will absorb the harmful energy of UV rays before it damages the polymer tissue of the coating, and dissipate the harmful radiation as heat or lesser energy photons. In this manner, the optical stability and life of the coating will increase. Stability studies have shown that the presence of ZnO nanoparticles in the pool color can limit the rate of color, and the decline of the mechanical properties of the coating, compared to continuous UV radiation exposures.

ZnO nanoparticles do have great antimicrobial activity and can inhibit the growth and proliferation of various microorganisms (bacteria, fungi, and algae) on surfaces. The primary mode of antimicrobial action is through the release of zinc ions Zn2 intermediates, and the photocatalytic activity of ZnO. When irradiated by light, ZnO nanoparticles will absorb sunlight, and, with excited electrons, begin producing active forms of oxygen including hydroxyl radicals and peroxide. These highly reactive oxygen species (ROS) will destroy the cell walls of microbes and algae and cause them to die or not reproduce. Published laboratory studies and research articles have shown that coatings composed of ZnO nanostructures are capable of inhibiting the growth of a variety of bacteria and microalgae on surfaces. In marine environments under light conditions, for example, the layers containing ZnO nano-layers exhibited significantly reduced density of bacteria and algae, and decreased adhesion to the surface and growth of bacteria. This was negligible in the dark, and was confirmed as the primary reason for the antimicrobial properties of ZnO, the photocatalytic process, and the generation of ROS in the presence of light. Therefore, the anti-biofouling and anti-fouling properties of ZnO enhanced materials are primarily seen in well-lit conditions (sunlight or UV light).

Enhanced Durability and Stability of Coatings Using ZnO

The incorporation of zinc oxide nanoparticles in pool coating formulations can create significant improvements in mechanical and chemical durability. Several studies highlight that the addition of a proper amount of ZnO nanoparticles inside polymer coatings, such as paints and epoxy resins, enhances its mechanical properties like the hardness, modulus of elasticity, tensile and flexural strength of the coating. Stated differently, the ZnO acts like a nanofilter, reinforcing the overall coating structure to better withstand physical stresses. One of the many benefits of mechanical reinforcement is improving the scratch, wear and impact resistance of coatings. Research has shown that the scratch resistance of polymer coatings is improved when only a fraction of ZnO nanoparticles is included in the formulation, such that the coating surface takes damage from the eventual scratches and if any fine scratching is made, the coating structure becomes less prone to cracking or removal. In the case of pools, this is especially beneficial due to the fact that periodic cleaning operations will take place, preventing loss from premature erosion of the coating. Ultimately, of course, an optimal quantity of nanoparticles should be allowed for use in formulations because too much can result in particle build-up and increases in paint viscosity, which is detrimental to the overall quality of the coating.

Along with a mechanical strength improvement, ZnOnanoparticles also assist with adhesion and coating integrity. Studies show that by having the nanoparticles within the resin, due to physical and chemical interactions with polymer chains and bonding with functional groups, the adhesion is improved between the coating and the substrate (concrete or stone). By increasing adhesion strength, this means that the coating is less likely to be peeling or detaching from the pool surface and is more stable in the surface environment for extended periods. ZnO may also fill in some of the minute imperfections of the substrate surface and provide a more consistent layer, improving the durability of the coating.

Anti-Algal and Antimicrobial Properties

One of the essential benefits of employing zinc oxide nanoparticles in substituted coatings is the establishment of anti-algal properties and a limit to biofouling. As stated above, the nanoparticles ZnO are inherently antimicrobial as well as limit the growth of passive organisms on the surface. For outdoor pools, the coating that contains ZnO continuously eradicates and diminishes intrusive biological agents, e.g., algae and bacteria, through its photocatalytic action when exposed to sunlight, thus preventing the formation of biological layers. In other words, these coatings are self-sustaining so to keep the pool surface clean and with less algal growth building up over time. Furthermore, larger organisms such as moss spores or the larvae of some aquatic organisms have less tendency to stick to surfaces with plasma-deposited ZnO nanostructures and therefore will lower the chances of any visible formation and accumulation of mosses and algae. Besides, ZnO itself makes the environment somewhat unsuitable for growing microbes, thus, even under dark or low-light conditions, the speed of microorganism’s proliferation will be at a lower rate. These sets of factors will expand the time intervals between pool cleaning and cleaning periods and reduce maintenance costs.

Conclusion
The use of zinc oxide nanoparticles in pool coatings represents a new and effective way to increase the useful life and effectiveness of pool coatings. Simply adding ZnO to the formulation of the pool coating provides a set of useful properties. ZnO improves the mechanical strength and adhesion of the coating which adds to the durability of the coating from physical stresses and environmental factors. ZnO also provides added optical and chemical stability, with reduced degradation from moisture, UV radiation or chemicals. Additionally, ZnOprovides a capability to help keep the pool surface clean and hygienic with added antimicrobial and anti-fouling properties. Scientific literature supports coatings made with ZnOnanoparticles generally exhibit greater durability and superior performance properties than typical coatings. With relative safety and environmental compatibility, the scientific and common-sense application of ZnO is a great and smart way to improve the quality, efficiencies and durability of pool coatings. This technology should help to achieve long-term reduced maintenance costs for pools and longer-term viability of water environments.

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