Morphological Analysis of PF/pMDI Hybrid Wood Adhesives

Abstract

Simple blends of polymeric methylenebis(phenylisocyanate), pMDI, dispersed in aqueous phenol formaldehyde, PF, were studied to determine how the emulsion structure impacted the resulting solid-state morphology. Shortly after mixing, optical microscopy of the liquid emulsions revealed rapid membrane formation around the dispersed pMDI droplets. This membrane appeared to arise from the pMDI/PF reaction and not from the pMDI/H2O reaction. Field emission scanning electron microscopy (FE/SEM) and atomic force microscopy (AFM) of partially cured blends detected a pMDI-borne dispersed phase exhibiting a sharp phase boundary. AFM revealed a halo surrounding the dispersed phase. This halo feature extended into the PF continuous phase; it was interpreted as evidence for a copolymeric region, suggesting that pMDI diffused into the continuous phase. Dynamic mechanical analysis of the partially cured blends revealed two overlapping secondary transitions, interpreted as phenolic relaxations from neat PF and separately from a PF/pMDI copolymer. Supporting the putative copolymer formation is the fact that the pMDI dispersed phase is associated with accelerated cure in comparison to neat PF. Collectively, the results indicate that the liquid emulsion morphology is largely preserved in the resulting solid, but that some interdiffusion occurs between phases during cure. The resulting copolymer formation accelerates cure and possibly enhances toughness near the phase boundary.