Single-step exfoliation, acidification and covalent functionalization of α-zirconium phosphate for enhanced anticorrosion of waterborne epoxy coatings
Graphical abstracts
Schematic diagram of composite coating preparation
Introduction
Corrosion in many aspects, such as pipelines, bridges, transportations, public buildings and liquid waste systems, caused huge economic losses and environmental damages every year [1]. Many technologies have been developed for anticorrosion, for example, the pretreatment of metal surface [2], the use of corrosion inhibitor [3], and the coverage of organic coatings [4]. Among these measures, organic coatings got a lot of attention for the simple preparation and easy functionalization. In particular, epoxy coatings are widely used because of their outstanding chemical inertness, electrical insulation and strong adhesion to a variety of substrates [5]. Traditional solvent-based epoxy resins contain many volatile organic compounds (VOCs) which are harmful to human health and the environment. However, the curing mechanism of the environmentally friendly two-component waterborne epoxy resin determined their films often have obvious micropores and defects, which make their barrier properties against water, oxygen and corrosive ions unsatisfactory [6], [7].
Therefore, the development of waterborne epoxy-based nanocomposites with excellent shielding and mechanical properties become one of the focuses of their anticorrosive application research and development [8]. The addition of various fillers can effectively enhance the shielding performance of matrix resin. Synthetic α-zirconium phosphate (Zr (HPO4)2·H2O), which is also called α-ZrP, is easy to be prepared and modified. It has excellent mechenical properties, thermal stability, shielding effect and high ion-exchange capacity (6.64 meq g−1), which also can be intercalated and exfoliated due to its weak interlayer forces. The using efficiency of the exfoliated α-ZrP becomes higher, in other words, the shielding and mechanical properties of the organic matrix can be improved at a lower filler concentration. Guests with various functional groups can be introduced into its layers. Combined with the above advantages, α-ZrP is fit to act as a functional filler to enhance the anticorrosive performance of waterborne epoxy coatings (WEPc) [9]. However, the main problem of incorporating such inorganic nanoplatelets into organic coatings are their poor interface compatibility. Therefore, organic functional modification needs to be applied to α-ZrP before practical application [10]. In existing reports about surface-modified α-ZrP, the modification reactions usually only occur on the surface and edge of the stacked layers while the internal surfaces are still unmodified [11], [12]. The compatibility of fillers and polymers can't be effectively improved as a result of the low grafting ratio of the modifier. There were little reports about modification of exfoliated α-ZrP through covalent bonding.
Silane coupling agents have been widely used in nanocomposite coatings due to their special “bridge” effect [13], [14]. Particularly, 3-aminopropyltriethoxy silane coupling agent (KH-550) which owns two sorts of functional groups is most promising agents to improve the composites properties. Hydrolyzed KH-550 and α-ZrP can react through dehydration condensation. On the other hand, the amino end groups of KH-550 can react covalently with the epoxy groups or epoxy resin, which is very useful to enhance the interaction between modified α-ZrP and epoxy matrix [15]. What's more, according to previous report, the extra silicon hydroxyl groups of KH-550 can couple with the metal atoms, which will further enhance the adhesion of the film on steel substrate [16].
In this research, we proposed an idea of first stripping and then acidifying of α-ZrP to obtain exfoliated zirconium phosphate (A-ZrP) with restored hydroxyl functional groups. In this way, the interlayer hydroxyl groups of α-ZrP which is originally hard to be reached, can participate in the covalent reaction, in turn to improve the grafting ratio.
α-ZrP was treated with Tris-(hydroxymethyl)-aminomethane (Tris) aqueous solution according to our previous work to achieve exfoliated α-ZrP (E-ZrP) [17]. Then E-ZrP was firstly treated with dilute hydrochloric acid (HCl), and then covalently reacted with KH-550 to obtain a ZrP/KH-550 nanocomposite (K-ZrP). Epoxy resin was used as the base material with K-ZrP as anticorrosive functional fillers to prepare waterborne epoxy anticorrosive composite coatings. In order to investigate the impact of K-ZrP on the improvement of anticorrosion performance, waterborne epoxy coatings (WEPc) without any filler, with pristine α-ZrP, E-ZrP were also prepared. The structure and morphology of all products were characterized and confirmed using FTIR Spectrometer, X-Ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope characterization. The corrosion behaviors of Q235 carbon steel coated with different sample films were tested using electrochemical workstation.
Section snippets
Materials
α-zirconium phosphate (Zr (HPO4)2·H2O, 1.65 μm, prepared using the method reported in the former research [18], Tris-(hydroxymethyl)-aminomethane (Tris, Spectrum Chemical Manufacturing Corp-China), Hydrochloric acid (HCl, 36–38%, Guangzhou chemical reagent factory), 3-aminopropyltriethoxy silane (KH-550, Aladdin Industrial Co., Ltd). Anhydrous ethanol (Tianjin Yongda chemical reagent Co., Ltd). Bisphenol A epoxy resin (epoxy value 0.51, Shell Group of Companies) and Bath 721 waterborne curing
Structure analysis
The XRD patterns of α-ZrP and its derivatives are shown in Fig. 3. The spectrum of α-ZrP shows three main reflection peaks. Among them, the characteristic one at 2θ=11.6 ° (002 reflection) indicates the interlayer spacing of pristine α-ZrP is 0.76 nm. After exfoliation, the interlayer spacing of E-ZrP increased to 1.38 nm, determined by the decreased 2θ degree of 002 reflection peak to 6.4 ° The interlayer spacing was calculated according to Bragg equation. After acidification by HCl aqueous
Conclusions
In this work, layered α-zircionium phosphate was first exfoliated by green reagent Tris aqueous solution and then dealt with dilute HCl to obtain exfoliated zirconium phosphate (A-ZrP) with restored hydroxyl functional groups, which was helpful to improve the grafting ratio of covalently grafted modifiers. A new organic derivatives of α-ZrP, zirconium phosphate/3-aminopropyltriethoxy silane nanocomposite (K-ZrP) was prepared through covalent reaction between KH-550 and A-ZrP. Waterborne epoxy
Author statement
Under supervision by Xiang Jiang and Xinya Zhang, Menglan Li, Ruibin Mo and Ruibin Mo performed sample preparation and data analysis. Haowei Huang developed mechanics modeling and analysis. Menglan Li and Ruibin Mo performed sample preparation and structure fabrication. Menglan Li and Xinxin Sheng performed calculations. All authors read and contributed to the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We gratefully acknowledge to the support of National Natural Science Foundation of China (Grant No. 52076082), and National Natural Science Foundation of China (Grant No. 51576070), and National Natural Science Foundation of China (Grant No. 21908031).
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