Melamine-modified urea formaldehyde resin for bonding particleboards
For the development of a cost-effective melamine-modified urea formaldehyde resin (MUF), the study evaluated the effects of reaction pH and melamine content on resin properties and bond performance of the MUF resin adhesive systems. Eight resins, each with three replicates, were prepared in a factorial experiment that included two formulation variables: two reaction pHs (i.e., 4.5 and 8.0) and four molar ratios of formaldehyde to urea to melamine (i.e., 3/2.2/0.3, 3/2.2/0.2, 3/2.2/0.1, and 3/2.2/0.05). Variables in particleboard preparation were two hot-press cured times (2 and 4 min). Thus, with two panel replications, a total of 96 panels were fabricated. Melamine content significantly affected resin properties and glue bond quality: gel time decreased, solid content increased, internal bond strength increased, thickness swell and water absorption decreased, and formaldehyde emission decreased as melamine content increased. In general, the resins catalyzed under acidic conditions (pH 4.5) resulted in faster gel times, higher internal bond strengths, lower formaldehyde emissions, and lower thickness swell and water adsorption than those catalyzed under alkaline conditions (pH 8.0). Significant correlation between gel time with both internal bond strength and formaldehyde emission suggest that resins with fast curing speeds provided favorable conditions for attaining a higher degree of resin cure, and in turn led to better bond strength and lower formaldehyde emission.