Effect of hardener variation on protective properties of polyurethane coating

Polyurethane is a two component coating commonly used in high performance anti-corrosion protectivesystems. The two components are normally a polyacrylic resin and a polyisocyanate hardener. The mainpurpose of this study was to investigate the effect of hardener variation on the protective properties ofpolyurethane coating. Apart from theoretical interest, there is a practical interest to this work because, forvarious reasons, in the field the correct ratio of base to hardener is not always followed. Another aim was todiscover changes in mechanical properties such as abrasion and adhesion, and also find out differences inthe chemical structure and crosslink density. Hence unpigmented polyurethane resin with hardener:baseratios of 0.75:1, 1:1 and 1.5:1 were applied on mild steel panels. Detached coatings were obtained fromcasting on polypropylene sheets. To evaluate anticorrosive properties attached coatings were immersedin 3% sodium chloride for 1008 h and monitored using EIS and DC resistance tests. Abrasion resistance wasmeasured by the Taber test. Wet adhesion was studied by immersing in 3% NaCl for 5 and 7 days, and thenusing the pull-off method. The detached coatings were used to evaluate chemical structure using FTIRspectroscopy, and to look at the physico-mechanical properties using a DMTA test. Electrochemical resultsshowed 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 ahigher hardener ratio have a better abrasion resistance. The lowest adhesion was given by the coating withthe stoichiometric hardener ratio. The work raised some interesting questions on the relation betweenadhesion and corrosion resistance. The FTIR results indicated that polyurethane coatings with the lowerhardener ratio have fewer urethane groups than coatings with the higher hardener ratio. Furthermoreresults confirmed that the Electrochemical Impedance Spectroscopy and DC resistance are good methodsto investigate how the protection ability of the coatings depends on both the nature of the coating andon time.