Valorization of isocyanates using castor oil-based protective strategies: Performance and comparison as waterborne adhesive additives

The violent and uncontrolled reaction of using isocyanates as waterborne additives limits the operation and product quality. Herein, castor oil-based isocyanates (polymethylene polyphenyl polyisocyanate, PAPI) that are protected by different strategies (hydrophobic, emulsified and solid powder) to enable water compatibility are developed and compared, targeting to mitigate these shortcomings. The effectiveness of these approaches is explored by a battery of characterization methods, including chemical titration, optical microscope, FTIR, TG, DSC, and wood adhesion performance. The storage stability of composite adhesives consisting of modified PAPI extends by at least eight times. The use of emulsified PAPI achieves the optimal bonding performance, resulting in increase of the wet strength I up to 240 % compared with pure PAPI when the dosage is 25 wt %. The underlying mechanism relates to high initial value of -NCO, uniform dispersion of isocyanate droplets in the emulsion, and slow post-curing process. These strategies offer effective and affordable routes to prepare and select proper isocyanates on design and construction of environmentally-friendly, low-cost and high-performance waterborne composite materials.