Potential of pulp and paper sludge as a formaldehyde scavenger agent in MDF resins

Sludge of pulp and paper mills have natural adhesive properties. The primary sludge (PS, contains fibers) and secondary sludge (SS, contains proteins) could also be suitable for manufacturing medium-density fiberboard (MDF). Protein in SS can react with formaldehyde (HCHO), and as an additive in urea-formaldehyde (UF) resins it can reduce formaldehyde emission. Thus, SS was investigated in the present study. PS and SS were collected from two mills and characterized in terms of chemical composition, fiber length distribution, pH, and buffering capacity. MDF samples were processed according to an experimental design, in which UF resin content was reduced from 12% to 8% and replaced by SS in the range of 5%–15%. Gel time measurement showed high SS reactivity with UF resin. The SS reduced HCHO emissions by up to 68% compared to control panels, without compromising internal bond strength. The bonding effect of SS was lower than expected due to the high pH, thus the buffering capacity reduced UF performance. Moreover, sludge reduced bending performance. Dimensional stability was the greatest disadvantage of sludge panels.