Molecular insights into the mechanisms of humidity-induced changes on the bulk performance of model castor oil derived polyurethane adhesives

Abstract This work advances the development and use of sustainable polyurethane elastomers as green adhesives by providing insights into the mechanisms of humidity-induced changes on adhesive performance. Using a model adhesive prepared from equimolar ratios of castor oil and hexamethylene diisocyanate under facile reaction conditions, we show that the subtle changes in the chemical composition which occur with curing under different humidity environments significantly impact the cohesive integrity of the adhesives and, thus, their performances with different substrates. The curing chemistry was evaluated based on the isocyanate consumption, which was followed using Fourier Transform Infrared Spectroscopy and Differential Scanning Calorimetry. Thermal, mechanical and adhesion properties were evaluated from thermogravimetric analysis, differential scanning calorimetry, rheology, tacking and lap shear tests. Moisture-cured adhesives were less covalently crosslinked, harder, more resistant to deformation, and recovered faster upon shear deformation compared to those cured otherwise. These mechanical properties complimented adhesive lap shear performance with substrates which formed covalent bonds during curing. For substrates with which covalent bonds could not form during curing, however, decreased internal cohesive integrity and associated increased hardness by urea formation resulted in poorer adhesive lap shear performance. This is contrary better tack performances obtained for the moisture-cured adhesives, attributed to their increased polarity and the energy dissipating ability.