Abstract
This work reports the effect of different amounts of ceria nanoparticles on UV resistance and barrier properties of water-based polyurethane (WPU) on glass and AA7075 aluminum alloy substrates. Hybrid coatings were synthesized from an aliphatic WPU–HDI (1,6-hexamethylene di-isocyanate) and cerium oxide nanoparticles (CeO2) with an average particle size distribution of about 25 nm. Different nanoceria amounts (1, 3 and 5 wt %), mixing times (30, 60 and 120 min) and methods to disperse the nanostructures into the polymer matrix (magnetic stirring and sonication) were evaluated. Initially, the dispersion of CeO2 nanoparticles embedded in the polymer matrix and displacement in the corrosion potential (Ecorr) were analyzed by confocal scanning laser microscopy (CLSM) and open circuit potential (Eocp) measurements. According to this behavior, the dispersion and water ratio required during the polymerization process were established. Coated samples obtained after the second stage were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical light microscopy. In addition, optical measurements on glass substrates were evaluated with UV-vis spectroscopy. The effect of the synthesis parameters on the corrosion behavior of WPU–CeO2/AA7075 systems was investigated with Eocp and electrochemical impedance spectroscopy (EIS) in a 3 wt % NaCl solution. In addition, the films were subjected to 180 h of accelerated weathering. The results show that the combination of specific nanoceria addition with the optimal synthesis parameters enhances optical transparence of WPU as well as barrier properties. From these, the coated specimens prepared with 3 wt % of ceria content and sonicated for 30 min showed a highly dispersed system, which results in a high charge transfer resistance. The observed properties in clear coats deposited on metallic substrates suggested an improvement in the appearance and less deterioration in UV exposure in comparison with pure WPU, enhancing the protective properties of the AA7075 aluminum alloy when exposed to a corrosive medium.
Highlights
Polymers are widely used due to the molecular architectures designed for each specific application
Since the properties of nanostructured materials are strongly dependent on their shape and size, the dispersion of CeO2 nanostructures can play a key role in the performance of water-based polyurethane (WPU)–CeO2 hybrid coatings on aluminum alloys
NaCl solution to obtain information on the electrochemical performance of WPU–CeO2 hybrid coatings synthesized with 5 wt % of CeO2 nanostructures and dispersed for 1 h by magnetic stirring or sonication methods (Figure 1)
Summary
Polymers are widely used due to the molecular architectures designed for each specific application. Polyurethane (PU) coatings have been successfully used for corrosion resistance, and durability against wear and weathering; in addition, they provide protection of biological attack [1,2]. Waterborne polyurethanes (WPUs) are promising and versatile polymeric materials commonly used in a variety of fields since they are nontoxic, nonflammable and environmentally friendly due to low or no-presence of volatile organic compounds. PUs are synthesized by the reaction of polyols and isocyanates. Different isocyanates such as toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and others are often used for PU synthesis. HDI is probably one of the most used monomers for polyurethane production and it is commonly employed as top-coat layer for other protective systems on metallic substrates to control corrosion reactions [3,4]. One of the main disadvantages of PU coatings is the hydroscopic tendency which causes permeation of oxygen or other aggressive ions such as chloride; yellowing caused by the photochemical degradation resulting from ultraviolet (UV)
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