Ammonium Bisulfite and Urea–Metabisulfite as Formaldehyde Scavengers in Low-Molar-Ratio Urea–Formaldehyde Resin for Medium-Density Fiberboard: Curing Behavior and Panel Performance

Ultra-low-formaldehyde medium-density fiberboard (MDF) is commonly produced using low-molar-ratio urea-formaldehyde (UF) resins; however, the reduced formaldehyde-to-urea ratio also lowers resin reactivity and can complicate curing. The aim of this research work was to investigate and evaluate the performance of ammonium bisulfite and urea-metabisulfite as formaldehyde scavengers for a low-molar-ratio UF resin (F/U = 1.06) at 1, 3, and 5 wt% (based on dry UF resin solids) used for MDF panel manufacturing. The modified adhesive systems were first screened by simultaneous thermal analysis in air to determine changes in the curing profile, and laboratory panels were then produced and evaluated for formaldehyde content by the perforator method (EN ISO 12460-5:2015) and for the main physical and mechanical properties. Ammonium bisulfite shifted the main curing peak to higher temperatures, indicating stronger retardation of the principal polycondensation stage, whereas urea-metabisulfite generated a broader, multi-peak curing profile. Despite these differences, both additives reduced the perforator values substantially. The control MDF already met the E0 level (3.84 mg/100 g oven-dry board), while 3 wt% ammonium bisulfite and 5 wt% urea-metabisulfite reached the super E0 levels (<1.5 mg/100 g; 1.36 and 1.26 mg/100 g, respectively). To note, scavenger addition up to 5 wt% (based on dry UF resin solids) did not significantly affect density, water absorption/thickness swelling, or bending and internal bond properties. The results demonstrate that sulfite-based scavengers can be incorporated into low-molar-ratio UF adhesives to obtain ultra-low-formaldehyde MDF while maintaining the main panel properties.