Synthesis and characterization of moisture cured prepolymer polyurethane films

Abstract

Polyurethane prepolymers are a group of materials representing a reactive intermediate betweenmonomeric isocyanates and polyurethane polymers. They are the polycondensation reaction product of adiisocyanate (hard segments) and a material with hydroxyl functionality such as a polyol -glycol of high molecular weight (flexible segments). Most polyols used in polyurethane synthesis arebased on polyether, polyester or polycarbonate structures. About isocyanate, its structure determines itsreactivity and is known that aliphatic isocyanates are less reactive than aromatic ones.The properties of these prepolymers and the final products (adhesives, sealants and coatings) can be tunedby selecting suitable polyol and isocyanate components, as well as their molar ratio. When wateremulsifiable polymers are intended, anionic monomer dimethylolpropionic acid (DMPA) is incorporatedin the chain as internal emulsifier.The aim of this work was to synthesize and characterize moisture cured polyurethane prepolymers forhigh gloss coating applications. Desired resin must be emulsifiable in water (for easy cleaning of thereactor and auxiliary equipment) and the respective coating must be transparent, colorless and tack free.The following parameters were studied: chemical nature and molecular weight of polyol (polyether andpolycarbonate, from 400 to 2000 molecular weight), initial molar ratio of isocyanate to hydroxyl groups(NCO/OH ratio from 1.5 to 2.5) and presence or absence of DMPA. The isocyanate used was alwaysisophorone diisocyanate.The coatings obtained were caharacterized in terms of thermal stability, Buchholz hardness, scratchresistance, tack, gloss and yellowness index. The results were analysed and related to the expectedpolymer structures for the different cases. The best performance was obtained for polycarbonate polyolwith higher NCO/OH ratio and containing DMPA.