Enhancing Deblocking Efficiency and Curing Reaction of Phenol-Blocked Polymeric Methylenediphenyldiisocyanate for Advanced Epoxy-Polyurethane Film Formulation

Abstract Novel phenol blocked polymeric methylene diphenyldiisocyanate (pMDI) derivatives were synthesized and characterized using FTIR, 1H NMR, and 13C NMR spectroscopy. The deblocking temperatures of the blocked isocyanates were determined using CO2 evolution method, DSC, and TGA spectroscopy. A cure reaction was performed between the blocked isocyanates and poly(Terathane-2000) to establish the structure-property relationship. Unsubstituted phenol-based blocked isocyanates exhibited superior deblocking temperatures and gel times compared to those with electron-donating substituted phenols. Intramolecular hydrogen bonding in blocking agents such as 4-hydroxybenzaldehyde, 4-hydroxy acetophenone, 4-hydroxy benzoic acid, and methyl 4-hydroxy benzoate resulted in unexpectedly high deblocking temperatures. Conversely, electron-withdrawing and steric effects led to low deblocking temperatures and gel times for blocked isocyanates BI-13 and BI-15. The solubility behavior of the blocked isocyanates was studied using various polyols (PEG-400, PEG-4000, and PEG-6000). Epoxy-polyurethane films were prepared using the blocked isocyanates, and their structures were confirmed using ATR spectroscopy. The thermal and mechanical properties of these films were investigated, including shoreA hardness, tensile strength, and flexural strength.