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

Abstract

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 (ωsol), polymer volume fraction (φp), polymer–solvent interaction parameter (χ), and the number average molecular weight between cross-links (Mc), for cured UF resin adhesives. Both ωsol and Mc decreased as the F/U mole ratio increased. But these values increased with an increase in the swelling temperature. The extraction resulted in negative ωsol values, suggesting the removal of a scattered distribution of ωsol 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.