Microencapsulation of polymeric isocyanate for the modification of urea-formaldehyde resins

This study was conducted to prepare the microcapsules of polymeric 4-4 diphenyl methane diisocyanate (MpMDI) by interfacial polymerization using two different surfactants (Tween 40 and Gum Arabic) for the modification of urea-formaldehyde (UF) resins. MpMDI obtained by Tween 40 (TW-MpMDI) were spherical, smooth, and small diameter, while those prepared by Gum Arabic (GA-MpMDI) were irregular, rough surface, and larger diameter. Attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectra of two types of MpMDIs confirmed the successful microencapsulation of pMDI by detecting the isocyanate (N[dbnd]C[dbnd]O) groups after rupturing the shell of MpMDIs. As the MpMDI content in UF resins increased, the non-volatile solids content, pH, gelation time, viscosity, and exothermic peak temperature of UF/MpMDI adhesives slightly increased. Plywood bonded with UF/MpMDI adhesives had 15% higher tensile shear strength (TSS) and 30% higher modulus of rupture (MOR) than those of the plywood bonded with neat UF resins owing to the formation of urethane (HN-(C[dbnd]O)–O-R) bonds. In particular, plywood bonded with UF/TW-MpMDI had higher TSS and MOR values than those bonded with UF/GA-MpMDI adhesive. Regardless of the surfactant types, the formaldehyde emission decreased about 54% with 5% MpMDI addition into UF resins. These results and statistical analysis revealed that 1% TW-MpMDI addition into UF resins provided a proper balance between good adhesion and low formaldehyde emission for plywood.