polyurethanes. Different approaches have been identified for the realization of green PUs, which include-but are not limited to-renewability of the starting materials, sustainability of the synthesis itself, isocyanate-free formulations, and reduction of VOC emissions by limiting the use of organic solvents. [4] In particular, the present review deals with recent researches on two classes of green PUs, namely waterborne and bio-based formulations, focusing on their application as coatings to protect metal surfaces from corrosion. As such, manuscripts specifically addressing the synthesis and/or standard chemicophysical characterization of newly proposed formulations were not considered here. Readers interested in the synthetic aspects of green PUs can refer to recent comprehensive reviews on the topic. [5][6][7][8][9] In the following the recent relevant literature published since 2015 is critically reviewed, highlighting the main strategies pursued to develop reliable and highly performing anticorrosion coatings based on waterborne and bio-based PUs. Then, the collected data on their protective performances are discussed.
Waterborne CoatingsWaterborne polyurethanes (WB-PUs) stepped out in the early 1990s driven by the increasingly strict environmental legislation in North America and Europe. [3] The use of sole water evidently solves the issue related to the huge VOC emission associated with traditional solvent-based coatings. In addition, the presence of water as one of the components removes the possibility of any toxic unreacted isocyanate in the system. [9] The introduction of hydrophilic groups in the formulation is essential to guarantee the stability of WB-PU dispersions. [10] On the other hand, it also detrimentally impacts the waterresistance of the resulting coating, hampering the full exploitation of this class of materials for anticorrosion purposes. Playing with the rich PU chemistry is clearly one possible way to tailor the coating properties, including water resistance and thus anticorrosion ability. [11][12][13] Both the polyol and the diisocyanate used for polyurethane synthesis can indeed be properly selected to maximize the coating protective efficacy. In this perspective, Li et al. found that a balanced polyether/polyester polyol ratio is optimal for anticorrosion applications. [12] Yu et al., instead, focused on the role of the diisocyanate structure, highlighting that the protective ability of WB-PU synthesized with 4,4′-dicyclohexylmethane diisocyanate outperforms Polyurethanes (PUs) have been extensively exploited for the production of protective coatings thanks to the versatility of their chemistry, which allows to adjust the coating properties depending on the final application. In the last decade, the concerns on the negative impact on the environment and human health of traditional petroleum-based and solvent-borne PUs have fostered the research on more environmentally friendly alternatives. This review article provides an overview of the recent approaches that have been profitably pu...