Modification of urea-formaldehyde resin adhesives with oxidized starch using blocked pMDI for plywood

This study investigated the modification of UF resins of two different formaldehyde/urea (F/U) mole ratios with OS levels, using blocked pMDI (B-pMDI) as a cross-linker for plywood. Fourier-transform infrared (FTIR) spectroscopy confirmed the presence of C=O of aldehyde and carboxyl groups in the OS. The UF–OS adhesives then were prepared by adding 5%, 10%, and 15% OS based on the UF resin’s solid content as well as 1% B-pMDI based on the OS dry weight as a cross-linker. FTIR spectra confirmed the formation of ester groups at 1348 cm−1 owing to the reaction of carboxyl groups of OS with –NCO groups of B-pMDI. Regardless of the F/U mole ratio, both the solids content and viscosity of the UF resins decreased, but the gelation time increased as the OS level increased. Differential scanning calorimetry showed that the activation energy of neat UF resin with 1.0 F/U mole ratio was higher than that with 1.2 F/U mole ratio; however, the opposite was true after the OS addition. These results showed that the modified UF resins improved the adhesion strength and decreased formaldehyde emission of plywood. The best performance of modified UF resins with different F/U mole ratios of 1.0 and 1.2 was found at 10% OS based on resin’s solid content.