Investigating the Extent of Urea Formaldehyde Resin Cure in Medium Density Fiberboard: Resin Extractability and Fiber Effects

The degree of resin cure achievable in urea formaldehyde (UF) resin is known to influence the hydrolytic stability of UF resin. In the current study, a significant difference in water extractable resin components has been observed between cured pure resin and that from medium density fiberboard (MDF) panels. Results show some 50 to 70% of resin components may be removed on water extraction from resinated MDF fiber and panel samples. In contrast, cured pure UF resins have only a small fraction of resin mass extractable into water with nitrogen-containing components remaining incorporated into the cured resin. The relatively high resin mass losses from panel material together with loss of nitrogen-containing components suggest not only free urea, but urea-methylene species are labile and readily extractable into water. Wax contributes to differing panel extractability when pressed at either 100 or 160°C, whereas panel resin loading has a significant effect on extractable resin components. A lower resin loading led to relatively greater resin extractability, which was corroborated by the extractability of resin-fiber mixtures up to 50% resin content. An assessment of extracted panel residues suggests a relative decrease of urea and urea-formaldehyde condensation products after water extraction. With UF resin highly mobile on fiber during MDF manufacture, the results suggest resin components may separate, leading to their incomplete incorporation into a cured, cross-linked UF resin matrix, with an implication that resin cure on fiber may not be complete compared to that found with pure resin.