Swelling Behaviour of Cured Urea-Formaldehyde Resin Adhesives with Different Formaldehyde to Urea Mole Ratios

As a part of understanding of the network structure of urea-formaldehyde (UF) resin adhesives, this study examined the swelling behaviour of cured UF resin adhesives with four different formaldehyde-urea (F/U) mole ratios, using Flory-Rehner thermodynamic theory and field emission-scanning electron microscopy (FE-SEM) to relate the swelling behaviour to consequently induce micromorphological changes. Cured UF resin films before and after acetone extraction were exposed to swelling in dimethyl sulphoxide at three different temperatures. For the first time, this study reported the experimentally determined swelling parameters, such as sol fraction (ω_sol), polymer volume fraction (φ_p), polymer-solvent interaction parameter (χ), and the number average molecular weight between cross-links (M_c), for cured UF resin adhesives. Both ω_sol and M_c decreased as the F/U mole ratio increased. But these values increased with an increase in the swelling temperature. The extraction resulted in negative ω_sol values, suggesting the removal of a scattered distribution of ω_sol in the cured UF resins. The micromorphology helped to explain the differences in the molecular integrity of the resins, indicating a close relationship between the swelling behaviour and the morphological changes after the swelling.