# Environmentally Friendly Anticorrosive Polymeric Coatings
## 4 Developments in Environmentally Friendly Polymeric Coatings
### Polyurethane  
Polyurethane (PU) coatings are organic polymers that are made through chemical reactions between a monoglyceride with a polyol and a diisocyanate to form a urethane oil or urethane alkylyd. PU-based mixtures give a superior anticorrosive coating compared to ordinary alkylds resins [65; 66]; however their synthesis is based on petroleum derivates. The development of waterborne PU is the first step to produce coating systems that are free of volatile solvent, non-polluting, and safe. Furthermore, waterborne PU have excellent physical, mechanical properties, and anticorrosion behavior. The development of greener chemical routes to substitute diols and diisocyanate petroleum-based materials is a challenge. For this reason, vegetable oils and fats are studied to synthesize polymers from renewable natural resources. They contain esters and carbon double bonds that can be functionalized to produce polyols, giving rise to bio-based PUs. The latest research focused on the development of coatings with polyether polyol and polyester polyol as raw materials and water-dispersible isocyanates as curing agent [67; 68]. [PERSON] at al [69] combined the addition of natural polyol and polyester from neem oil with quinoline as corrosion inhibitors. The encapsulation of quinoline into a micro-reservoir allowed to overcome disadvantages caused by its direct addition into a coating layer. Another sustainable feedstock for polyurethane coatings is functional soybean oil-based polyols [70]. [PERSON] et al. [71] studied this raw material combined with silane, ethylene oxide, and fluorine functionalization. The robust hydrophobic nature of silane-polyol-based PU coating promotes the characteristic anticorrosion behavior of the final coating. One of the most promising routes is the step-growth polyaddition of diamines with clayclor carbonates leading to non-isocyanate polyurethanes (NIPUs). [PERSON] et al. [72] investigated different chemical variables in order to obtain NIPUs with an improved thermal stability solvent-free/catalyst-free and bio-based content. Their NIPUs were produced by the reaction between epoxidized sunflower oil with CO\({}_{2}\) using tetrabutylammonium bromide (TBABr) as a catalyst. Another strategy followed by researchers is the introduction of nanofillers of carbon to improve the barrier properties of protective coatings, thus enhancing their anticorrosion properties. The addition of graphene and its derivatives as reinforcement has been approached by different authors [73; 74; 75]. [PERSON] et al. [73] focused their study on the fabrication of well-dispersed graphene reinforced waterborne PU composite coatings, highlighting the critical factors affecting the barrier properties, as schematically illustrated in Figure 4. The superior anticorrosive behavior of the graphene-reinforced PU coatings can be attributed to their ability to form a network in the PU matrix that avoids the permeation of foreign molecules. Carbon nanomaterials from Eucalyptus globulus leaves were produced by an eco-friendly process in order to strengthen a castor oil-based PU coating by [PERSON] et al. [76]. Their results showed great anticorrosion properties with 2% of carbonized coal ash of Eucalyptus globulus leaves. [PERSON] et al. developed a novel waterborne PU coating with excellent anticorrosion properties by addition of just 2 wt% of multiwall carbon nanotubes (MCNT) as filler [77]. The MCNT act as barrier, increasing the diffusion path tortuosity for the permeating corrosive electrolyte, as schematically shown in Figure 5.  
Nanocrystalline cellulose is another eco-friendly reinforcing additive for improving the corrosion and mechanical resistance of PU coatings [78]. Finally, the anticorrosive behavior of NIPUs can be increased by the addition of fillers as ZnO nanoparticles. This solution is used in cathodic anticorrosive coatings; however, the high surface energy and the large surface area of ZnO particles make them prone to aggregate. To overcome this problem, [PERSON] et al.