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Researchers from Shanghai Ocean University looked into the preparation and properties of environmentally friendly, hydrophobic and corrosion resistant composite coatings, using industrial fillers. In this article, Zhenhua Chu, Zhixin Zhang, Wan Tang, Jiahao Lu and Jingxiang Xu share their findings
With the continuous exploitation of marine resources, ocean equipment should meet the complex marine working environment. In this study, a type of environmentally friendly coating was prepared. Based on low surface energy environmental protection and anti-fouling, a film forming material with water-based epoxy-modified silicone resin emulsion was prepared. Industrial fillers were added to give it both inorganic and organic properties. Meanwhile, various contents of graphene oxide (GO) were added into the coating system. The coating properties were comprehensively analysed, and the optimal GO content was obtained as 0.1 wt. %. The composite coating was studied by seawater immersion experiments, and the failure process of the coating in was proposed. The composite coating prepared in the present study has both environmental protection and hydrophobic anti-fouling characteristics, and its comprehensive performance is excellent through various performance evaluations, i.e., it meets the requirements of long-term coating, environmental friendliness and anti-fouling and corrosion resistance.
The attachment of contaminated organisms to ship hulls makes it easy for bacteria and parasites to breed, which will adversely affect the production of aquatic products [1]. A lack of resistance to biological attachment is one of the serious challenges faced by oceanfront equipment. Biological adhesion not only increases the maintenance cost of equipment, but also leads to equipment failure and corrosion failure.
Coatings are among the most widely used methods for marine equipment maintenance. Traditional solvent-based antifouling coatings use organic solvents as dispersing media to prevent microbial attachment through the release of sterilisers in the coating system. However, the use of organic solvents consumes a lot of fossil energy and organic solvents are harmful to the health of construction workers because of their volatility. In addition, the release of sterilisers causes pollution to the marine environment, and is more likely to accumulate in marine organisms, threatening the safety of humans who eat marine organisms. Sun [2] grafted low-toxic structural compounds as sterilisation factors onto the side chain of acrylic resin to control the hydrolysis of sterilisation factors to achieve self-polishing of the coating. Compared with heavy metal copper sterilisation factors they are ‘green’, but they are still toxic and pose a threat to marine micro-organisms. In order to solve this industry dilemma, researchers continue to innovate and devote themselves to the development of new materials and new technologies to realise new coatings that are environmentally friendly and anti-fouling.
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Using fillers to improve performance
Water-based coatings should meet the requirements of being environmentally friendly, such as the advantages of small pollution and low volatile organic matter emissions. However, most of the protective waterborne coatings cannot possess environmental protection, anti-corrosion and anti-adhesion properties at the same time. Water-based epoxy-modified silicone resin emulsions have the properties of both epoxy resin and silicone resin emulsions, high adhesion and low surface energy, thereby meeting the requirements of environmental protection and anti-fouling.
A water-based coating can achieve the performance of traditional solvent-based coatings and conform to the concept of environmental protection, and it has promising application prospects in the field of anti-corrosion. Through different modification means and technical optimisation, the performance of a water-based coating can be further improved to meet the needs of more practical applications. Xu [3] synthesised a water-based silicone nanocomposite coating with long-term hydrophobicity and self-cleaning performance by combining a silicone emulsion with a TiO2 dispersion by using a mechanical blending method. But its long-term self-cleaning performance was lacking in a natural pollution environment. After mixing the cationic silicone emulsion made by Khanjani [4] with an acrylic emulsion, the mixed emulsion was cured at room temperature by using the low glass transition temperature of the acrylic emulsion, and the resulting coating had good hydrophobic and mechanical properties. However, the existence of two emulsions in the coating system required consideration of compatibility and the content of the single components, which was a huge workload.
In order to improve the overall performance of the silicone resin emulsion coating, inorganic fillers were added to the coating system to be more suitable for engineering applications. Wang [5] successfully developed a two-component inorganic coating, and systematically studied the influence of film forming additives on the stability of the coating, and then finally determined the best formula and curing time. Shan [6] used a nanosilica sol as the main film forming material and added an appropriate amount of inorganic filler to prepare an inorganic coating with excellent performance.
In recent years, graphene showed great application potential in various fields [7]. It is wise to add it as a filler to water-based resin emulsion with low surface energy to improve the corrosion resistance of coatings. However, because the surface of the emulsion particles is rich in hydroxyl groups, it is easy for them to agglomerate and they have a high activity, which will lead to a reduction in the dispersibility and stability. Tian [8] used ammonium-grafted graphene oxide as a dispersant, which effectively solved the problem of microscopic defects in the preparation of water-based epoxy resin, and improved the compatibility and dispersion of graphene nanoparticles in water-based epoxy resin. Huang [9] significantly improved the cathodic protection and barrier properties of the coating by adding different contents of graphene to the water-based epoxy zinc-containing coating, which benefited from the graphene lamellar barrier effect [10] and conductive effect [11]. Chen [12] introduced graphite (g)-C3N4 into the aqueous epoxy solution by using an ultrasonic dispersion technology, and the modified water-based composite epoxy coating also showed excellent barrier properties. These research studies focus on the uniform distribution of fillers and the improvement of corrosion resistance of coatings, while there are few research studies on low-surface-energy silicone resins.
Based on the above discussions, a simple mechanical stirring method was used to add the inorganic filler to the water-based epoxy-modified silicone resin emulsion, and the crosslinking agent was formed to cure it at room temperature to prepare a stable coating with environmental protection and corrosion resistance. The microstructure, phase change and corrosion resistance of the coating were analysed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). The failure mechanism of organosilicone resin was discussed. The addition of the inorganic filler gives the coating both inorganic and organic properties, i.e., more excellent comprehensive properties. The high silicon content of the epoxy-modified silicone resin emulsion gives the coating better hydrophobic properties. In addition, the flake structure of graphene oxide can effectively alleviate the corrosion of a metal matrix. This work provides a new method against microbial attachment and for protection of metal equipment in a marine environment.
To read a full explanation of the methods, results and discussion of this study, click here.
Conclusions
With the enhancement of environmental protection awareness, water-based coatings will gradually replace all traditional oil-based coatings. This paper focuses on the study of the structure and properties of water-based epoxy-modified silicone coatings, and the research results provide reference values for the exploration of water-based silicone coatings.
In this paper, graphene oxide as a nanofiller was introduced into the composite coating for modification, and the performance of the graphene oxide-modified composite coating was systematically studied. The composite coating with the best comprehensive performance was obtained when the amount of graphene oxide was 0.1 wt. %, and the best modified composite coating was immersed in simulated seawater to study its failure behaviour.
The filler, different contents of GO, is uniformly distributed in the modified composite coating, with all states compatible and the impedance modulus of the coating |Z| can reach 108 Ω·cm2. The composite coating shows good hydrophobic properties at 0 s water contact, i.e., an angle greater than 90°, which also results in a good dynamic anti-fouling effect.
The structure of the composite coating is relatively stable before and after immersion in seawater. The long-term corrosion rule of the composite coating in simulated seawater has been deeply studied. Through the data analysis of the test results, the failure process of the composite coating is rationally explained, and the mechanism diagram of the failure process is drawn.
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