Anti-corrosion and wear-resistant coating of waterborne epoxy resin by concrete- like three-dimensional functionalized framework fillers

Highlights

• A concrete-like three-dimensional network filler was designed to the coating.

• Interface chemical control of the filler surface can improve the compatibility between the filler and the resin.

• The water-based coating exhibits excellent anti-corrosion and wear resistance.

Abstract

Waterborne protective coatings have attracted tremendous attention recently, but their practical applications are severely limited by week shielding performance and poor mechanical durability. Herein, a concrete-like three-dimensional network filler was designed by stacking fly ash, graphene oxide, and multi-walled carbon nanotubes onto each other with the help of a silane coupling agent to enhance the anti-corrosion performance and wear resistance of waterborne epoxy coating. Through chemical modification, the surface of the filler has a large number of both epoxy and amino groups, and the filler-filler, resin-filler, and resin-resin multi-dimensional cross-linking reactions occur inside the coating to improve the cross-linking performance of the water-based resin. With the help of the stable structure of the “imitation” concrete structure and greater interface strength of the silane-modified composite material, the water-based coating exhibits excellent anti-corrosion and wear resistance. The mass loss of the imitation concrete coating is only 0.0098 g per-cycle of Taber friction testing, and in the electrochemical impedance spectroscopy test, its |Z|_{0.01 Hz} value became stabilized at approximately 1 × 10⁷ Ω·cm² after 14 d of being immersed in 3.5% NaCl solution, and its value was two orders of magnitude higher than that of pure epoxy. The results show the matching of the three different types of fillers in the imitation concrete, and interface chemical control of the filler surface can improve the compatibility between the functional filler and water-based epoxy resin. The use of industrial waste fly ash as one of the main fillers is conducive to promoting the recycling of resources, so that the coating has a certain cost performance and engineering application value. This study is expected to open new insights into specialized design of the internal filler of such coatings and the modification of the internal surface of a water-based coating improves the coating’s anti-corrosion and wear resistance.

Introduction

Metal corrosion generally causes a significant economic loss(Cui et al., 2017b, Davey et al., 2016, Ramezanzadeh et al., 2016a, Smith et al., 2011;); hence, in recent years, physical barriers, such as organic coatings, are introduced between corrosive environments and metals to effectively prevent corrosion(Caldona et al., 2018, Cui et al., 2017a, Cui et al., 2016, Rahman et al., 2015, Zhu et al., 2019). With the concept of environmental protection, eco-friendly coatings have received widespread attention due to strict restrictions in the use of volatile organic compounds (VOCs) (He et al., 2019).

Waterborne epoxy (EP) coatings and powder coatings are two common environment-friendly coatings (Gao et al., 2016, Lin et al., 2020, Shchukina et al., 2018, Wang et al., 2017). In comparison to powder coatings, waterborne EP coatings have more flexible processing conditions, especially their lower fabrication temperature is more conducive to industrialization. Wang et al. found that during the curing process of the waterborne EP coating, numerous microporous defects and polar channels were formed (Wang et al., 2019). Chen et al. further discovered that the adhesion of the waterborne EP coating to the metal substrate decreased and blisters were formed; thus, reducing the shielding performance of the coating (Xia et al., 2020). According to Zhang et al., water-based polymers have very low mechanical strength as compared to solvent-based polymers (Dhamodharan et al., 2019). The introduction of fillers can significantly enhance the performance of waterborne EP coatings, such as operability and dispersibility.

Fly ash, a industrial waste material, typically consists of SiO₂, Al₂O₃, CaO, and Fe₂O₃ (Zhuang et al., 2016). Due to its stable physical and chemical properties and high specific surface area, fly ash is greatly used in coating applications (Cheng et al., 2019). Mhaske et al. synthesized a coating by mixing ZnO-Al₂O₃-fly ash composite and functionalized multiwalled carbon nanotubes (MWCNTs) in polyesteramide resin, and they noticed that the scratch resistance of the coating increased from 2 kg to 3.1 kg and its hardness increased from 1H to 4H (More and Mhaske, 2016). Sofian et al. (Norfatima Engku Dahalan et al., 2018) prepared an organic epoxy-zinc coating with fly ash and reported that fly ash simultaneously improved the cathodic properties and barrier protection of the coating. However, unmodified FA has a smooth surface and poor dispersion in waterborne epoxy; therefore, the dispersibility of FA can be modified by grafting it into hydrophilic groups. Similarly, the grafting of hydrophilic groups on the surface of a unique two-dimensional filler is an effective way to improve the anti-corrosion protection of waterborne EP coatings.

Graphene oxide (GO), an important derivative of graphene, can be easily dispersed in waterborne polymer matrixes (Zheng et al., 2020). Wang et al. found that hydrophilic dopamine (DA)-functionalized GO nanosheets greatly improved the interfacial interactions between the filler and epoxy resin, leading to a significant improvement in the corrosion protection properties of the waterborne EP coating (Cui et al., 2018b). MWCNTs also have attracted immense attention in the anti-corrosion field due to their mild chemical stability, high electrical conductivity, and superior mechanical properties (Deyab and Awadallah, 2020, Kumar et al., 2017). Yang et al. revealed that the incorporation of 0.5% modified MWCNTs noticeably improved the corrosion protection performance and tribological properties of MWCNTs/EP composites (Cui et al., 2018a).

However, it was weak to improve the mechanical properties of coatings only by adding the soft carbon materials. The addition of fly ash and carbon materials can effectively improve the rigidity, flexibility and toughness of the coating. Nevertheless, the waterborne epoxy resin have low crosslink density, fillers can be designed to increase the internal crosslink density of the coating. As a result, the simultaneous enhancement of the interfaces and physical architecture are challenging and urgent.

In the present work, an eco-friendly waterborne EP coating with enhanced corrosion protection performance was prepared by taking advantage of fly ash, GO, and MWCNTs. Fly ash has both physical barrier properties and tribological properties; however, unmodified fly ash cannot be stably dispersed in ethanol solution; thus, greatly reducing the performance of epoxy composite coatings. GO and MWCNTs also suffer from serious agglomeration; therefore, it is necessary to find an effective method to modify the aforesaid fillers. Subsequently, a concrete-like three-dimensional network filler was designed by stacking the above three fillers with the help of a silane coupling agent to enhance the anti-corrosion performance and wear resistance of waterborne epoxy coating. The water-based coating exhibits excellent anti-corrosion and wear resistance with the help of the stable structure of the “imitation” concrete structure and greater interface strength of the silane-modified composite material.

For instance, the mass loss of the imitation concrete coating is only 0.0098 g after one cycle of Taber friction testing, and in the electrochemical impedance spectroscopy test, its |Z|_{0.01 Hz} value became stabilized at approximately 1 × 10⁷ Ω·cm² after 14 d of being immersed in 3.5% NaCl solution. The results show the matching of the three different types of fillers in the imitation concrete, and interface chemical control of the filler surface can improve the compatibility between the functional filler and water-based epoxy resin and the three aforementioned kinds of fillers work together to improve overall coating performance. This work is expected to provide an effective method for preparing long-life anti-corrosion waterborne coatings by designing the stable structure of the “imitation” concrete structure and reinforcing composite interfaces.