Adherence of Liquid Crystalline TDI-Polyurethanes to Steel

Linear liquid crystalline polyurethane (LC-PU) were synthesized by the reaction of 2,4-toluene diisocyanate (TDI), 4,4′-bis-(6-hydroxyhexoxy)biphenyl (BHHBP), and poly(butylene adipate) (PBA). The BHHBP as mesogenic unit was incorporated into the hard segment formed by the reaction with the TDI. The hard segments are incompatible with the PBA soft segments. The mesomorphic behaviour is influenced by both the soft segment length and the hard segment concentration. The adhesion of such polyurethanes used as primer in composites based on steel as substrate and alkyd melamine resin as coating is dependent on the polyurethane structure, the thickness of the polyurethane layer and its drying conditions. The highest wet adhesion stability is achieved by the hard-segment-type polyurethane composed of TDI and the mesogenic unity BHHBP. Partial substitution of BHHBP by 2,2-bis-(hydroxymethyl)propionic acid (BHMPA) in this LC polyurethane enhances the effectiveness of the primer.