# Environmentally Friendly Anticorrosive Polymeric Coatings
## 4 Developments in Environmentally Friendly Polymeric Coatings
### Epoxy  
Epoxy resins have been used in many high performance coatings [41] due to their high chemical stability, resistance to heat, water, and excellent adhesion to metals. They are obtained by a condensation reaction between diphenyl propane derivatives and epichlorohydrin in the presence of a basic catalyst. The most used diphenyl propane to prepare solvent-borne epoxy resins is bisphenol A (BPA). Bisphenol A can be replaced with glycerol or other aliphatic polyols in the preparation of epoxy resins. Several synthetic pathways [43] led to the production of different epoxy resins: cycloaliphatic, trifunctional, tetrafunctional, Novolac, containing F, P, Si, and most of the recent bio-based ones. The final application of the epoxy resin is the key factor for the selection of the type of resins and curing system used in the coating formulation. For example, Epoxy Novolac, that presents high cross-linking properties, is a good choice for extremely aggressive environments. The molecular weight of the epoxy resin determines a wide range of the coating's properties such as viscosity, flexibility, hardness, solvent resistance, adhesion, and substrate wetting. Higher molecular weight epoxy resins contain many free hydroxyl groups that can generate cross-linking reactions with acids and isocyanates or can be esterified with fatty acids. Traditionally, the curing system is based on amines, polyamines, amides, anhydrides by cationic polymerization, and by applying heat or photo-irradiation. The production of bio-based epoxy resins [44] is gaining special attention starting from vegetables oils, such us soybean oil (ESO) and castor oil (ECO), that contain unsaturated groups and can easily undergo oxidation or epoxidation reactions. [PERSON] at al [45] covered in a recent review the development of epoxy resins and curing agents from renewable resources underlying, among others, some potential applications for protective coatings. The water-borne epoxy coating technology is based on liquid epoxy resin or on solid epoxy dispersion (2k-component) [46; 47]. The main advantages of this technology are a low content of organic solvent, excellent adhesion with the intermediate coat and substrates. The performance of water-borne epoxy as protective coatings is still insufficient if compared to the solvent-borne in terms of chemical resistance, risk of flash rust, slow evaporation of water, and limited gloss stability. Moreover, there are already examples of successful applications of electrodeposition [48] of epoxy primer emulsions, which brings enormous advantages to the automotive industry and original equipment manufacturers (OEMs) [49; 50]. Another approach is to apply zinc-rich epoxy coatings in order to achieve protection of metal substrates in highly aggressive environments such as the marine atmosphere [51]. The zinc reactivity with water is controlled by encapsulation using highly reactive amine-based hydrophobic polymers. Once the amines react with the epoxy, the zinc is again available as a sacrificial electrolytic anode to protect the iron substrates [52]. [PERSON] at al [53] focused their study on epoxy silicate modified emulsions with high zinc powder content. The introduction of different percentages of epoxy into the silicate emulsion resulted in coatings formulations with improved impedance, lower water absorption, and better insulating effect compared to silicate emulsions. An alternative to reduce the emission of organic solvents is to increase the solid content of solvent-based systems or to maximize the reduction of solvent content without losing film properties. One common strategy is using HAP (hazardous air pollutants)exempt solvents [54] and reactive diluents that have the properties to reduce the viscosity and to cross-link while being part of the cured network. Within this last concept, the inorganic-organic hybrids have been extensively studied for their anticorrosive properties. The latest findings by [PERSON] et al.