Effect of hardener variation on protective properties of polyurethane coating
Polyurethane is a two component coating commonly used in high performance anti-corrosion protective systems. The two components are normally a polyacrylic resin and a polyisocyanate hardener. The main purpose of this study was to investigate the effect of hardener variation on the protective properties of polyurethane coating. Apart from theoretical interest, there is a practical interest to this work because, for various reasons, in the field the correct ratio of base to hardener is not always followed. Another aim was to discover changes in mechanical properties such as abrasion and adhesion, and also find out differences in the chemical structure and crosslink density. Hence unpigmented polyurethane resin with hardener:base ratios of 0.75:1, 1:1 and 1.5:1 were applied on mild steel panels. Detached coatings were obtained from casting on polypropylene sheets. To evaluate anticorrosive properties attached coatings were immersed in 3% sodium chloride for 1008 h and monitored using EIS and DC resistance tests. Abrasion resistance was measured by the Taber test. Wet adhesion was studied by immersing in 3% NaCl for 5 and 7 days, and then using the pull-off method. The detached coatings were used to evaluate chemical structure using FTIR spectroscopy, and to look at the physico-mechanical properties using a DMTA test. Electrochemical results showed that polyurethane coatings with a higher hardener ratio have a higher electrical (ionic) resistance. This was attributed to a more tightly cross-linked structure. In addition, polyurethane coatings with a higher hardener ratio have a better abrasion resistance. The lowest adhesion was given by the coating with the stoichiometric hardener ratio. The work raised some interesting questions on the relation between adhesion and corrosion resistance. The FTIR results indicated that polyurethane coatings with the lower hardener ratio have fewer urethane groups than coatings with the higher hardener ratio. Furthermore results confirmed that the Electrochemical Impedance Spectroscopy and DC resistance are good methods to investigate how the protection ability of the coatings depends on both the nature of the coating and on time.