Preparation and characterization of hybrid resin from used urea-formaldehyde and isocyanate resin

Hybrid resins composed of polymeric 4-4 diphenyl methane diisocyanate (pMDI) and used urea-formaldehyde (UUF) resins were prepared and studied. In these hybrid resins, high ratios of pMDI (e.g., 20%) were evenly dispersed in UUF resins by using acetone as a solvent for pMDI. The viscosity of the hybrid resin containing 15% of pMDI only changed from 216 to 424 mPa s with 30 h of storage. Chemical composition analysis showed that the free isocyanate groups were reacted with hydroxyl groups in the hybrid resins within 2 h. From a curing kinetics analysis, even without obvious active isocyanate groups, these hybrid resins still showed lower curing peak temperatures and Kissinger activation energies than the UUF resin. Moreover, the hybrid resins presented significantly higher dry and wet tensile shear strengths than the UUF resin. The effective use time of the hybrid resin with 15% pMDI reached up to 12 h. This study provided a new method for modifying UF resin with pMDI. These results also proved the feasibility of applying the hybrid resins in wood-based composites manufacturing.