Effect of different acids during the synthesis of urea‐formaldehyde adhesives and the mechanical properties of medium‐density fiberboards bonded with them

The characteristics of urea‐formaldehyde (UF) adhesives condensed by catalysis with four different acids, namely formic (HCOOH), hydrochloric (HCl), phosphoric (H 3 PO 4 ), and sulfuric (H 2 SO 4 ) acids, under alkaline–acidic–alkaline conditions at a molar ratio F/U = 1.12 were studied. The thermal curing properties of UF adhesives catalyzed with acid were characterized by differential scanning calorimetry at 10 °C/min heating rate. The resin structure examined by 13 C‐NMR spectroscopy showed that the resin catalyzed with HCl had a lower proportion of methylol groups, resulting in a lower level of formaldehyde emission. It was interesting to note that HCOOH resulted in the best overall mechanical properties of the medium‐density fiberboard (MDF) panels. The HCl catalyst resulted in the poorest performance, providing the lowest internal bond strength, modulus of elasticity, and thickness swelling, with the exception of the free formaldehyde content. The resin catalyzed with H 2 SO 4 had the highest free formaldehyde and the highest formaldehyde emission. H 2 SO 4 and H 3 PO 4 resulted in MDF mechanical properties relatively lower than for HCOOH.