Vanillin as low-temperature isocyanate-blocking agent and its use in one-component aqueous coatings

Abstract

Mainly because of the high sensitivity of the isocyanate group to atmospheric moisture, it is blocked with a blocking agent for many applications and thus kept as inert as possible at room temperature (Zhang et al. in Langmuir ACS J Surf Colloids 37:12705, 2021, Meier-Westhues in Polyurethane: Lacke, Kleb-und Dichtstoffe, Vincentz Network, Hannover, p 36, 2007). When exposed to temperature, the protective group unblocks, and the isocyanate group reacts with the reactant that was previously present together with the blocked isocyanate (Meier-Westhues in Polyurethane: Lacke, Kleb- und Dichtstoffe, Vincentz Network, Hannover, p 36, 2007, Goldschmidt and Streitberger in BASF handbook on basics of coating technology, Vincentz Network, Hannover, pp 96–99, 2007). Many of the substances which are used today as blocking agents have toxicological concerns or unblock at high temperatures, which limits their application areas (Guillem Parra et al. in Blocked isocyanate polyurethane compositions using a new blocking agent, method of manufacture and uses thereof: European patent application, 2019, https://data.epo.org/publication-server/document?iDocId=6506253&iFormat=0). In this work, vanillin is presented as an effective deblocking agent at low temperatures and is compared with similar structures. The process of deblocking from an hexamethylene diisocyanate trimer (HDI-trimer) is followed by means of IR-spectroscopic measurements at different temperatures and thermogravimetric analysis. Temperature-dependent oscillation measurements using a rheometer are suitable for measuring onset temperatures and for qualitatively tracking the unblocking process. The combination of the results is used to draw conclusions about the existing deblocking mechanism. The comparatively low deblocking temperature of vanillin enables the formulation of an HDI-trimer modified with sulfonate groups and blocked with vanillin, which is dispersed in the aqueous phase and then reacted with OH-functional binders. Deblocking and subsequent reaction with the binder are followed by means of IR spectroscopy, and the mechanical properties of the coating films are examined. Vanillin is therefore suitable as a toxicologically harmless blocking agent for isocyanates and enables the production of crosslinkers for one-component water-based coatings (Arya et al. Adv Tradit Med (ADTM) 21:1, 2021).