Moisture-curing one-component polyurethane systems in adhesive, sealant, and coating applications may show blister formation upon cure. Blisters can be formed when carbon dioxide, generated in the reaction with isocyanate and water, is trapped in the film. This problem can be mitigated by employing latent hardeners such as blocked polyamines, which are activated upon moisture exposure. The hydrolysis of the latent hardener yields the polyamine that quickly reacts with the isocyanate, forming urea linkages, and then chain extends the polymer. The hydrolysis also releases the blocking agent, which can potentially create an unpleasant odor. In this work, a series of di- and trifunctional aldimines were synthesized from commercially available polyamines, biobased hydroxymethyl furfural, and lauroyl chloride. Hydroxymethyl furfural was first esterified with lauroyl chloride and subsequently condensed with the polyamines to form the aldimines. The application of these novel aldimines in a model moisture-curing system allowed the preparation of blister- and odor-free castings. Based on our results, the mechanical performance of the different aldimines in casting and adhesive applications could be related to the polymer network density. This was dependent on the rate of the aldimine hydrolysis reaction to produce the polyamine. In particular, the use of aldimines prepared from polyether amines and 1,5-diamino-2-methylpentane showed excellent adhesive properties.
Keywords: adhesive; imine; latent hardener; one-component system; polyurethane.