Increase in coating wet adhesion stability by highly-ordered polymers

Abstract

The coating wet adhesion stabilities caused by linear conventional polyurethanes and linear liquid crystalline polyurethanes (LC-PUs) were compared. The polyurethanes were used as primer in systems based on steel as substrate and an alkyd melamine resin as coating. The unsegmented main-chain liquid crystalline polyurethanes were synthesized based on 2,4-toluene diisocyanate (T) or 4,4'diphenylmethane diisocyanate (M) and the mesogenic diol 4,4'-bis-(6-hydroxyhexoxy) biphenyl (BH). Also, a segmented LC-polyurethane prepared by the reaction of T, poly(butylene adipate) of molecular weight Mn 2000 (BA20) and BH as chain extender and common polyurethanes based on the mentioned diisocyanates, BA20 and 1,4-butanediol chain extender were included into the investigations. As reported before [1, 2], both the isocyanate structure and the hard segment concentration influence the mesomorphic behaviour of the unsegmented and segmented LC-PUs. This was studied by DSC and polarizing microscopy. The use of LC-PUs as primer increases the wet adhesion stability of a polymeric coating on steel as substrate. The effectiveness of the primer layer depends on the polyurethane structure, the thickness of the polyurethane layer and its drying conditions. The permeation of water through the polyurethane layer correlates with the degree of order of the polyurethane. The highest wet adhesion is achieved by the unsegmented LC-PU composed of M and the mesogenic unit BH.