Paper mill sludge as a component of wood adhesive formulation

Three major types of paper mill sludge, primary sludge (PS), secondary sludge (SS) and de-inking paper sludge (DPS) were characterized and evaluated as adhesive fillers. Plywood panels were made of formulations with phenol formaldehyde (PF) and sludges. Panels with PF/PS and PF/SS formulations had higher dry and wet shear strengths than those made with PF/Cocob((R)) formulation. All wood failure values were comparable. Dry and wet shear strengths of the panels with PF/DPS formulation were comparable to those of the PF/Cocob((R)) panels (with Cocob((R)) as a commercial filler), but the former displayed a much lower wood failure value. Owing to this fact and its high ash content, DPS was not evaluated further as a potential component of adhesive formulations. Compared with SS, PS resulted in higher dry and wet shear strengths and higher wood failure values. However, granular SS was easier to disperse into the resin component than fibrous PS, and the PF/SS formulation was more easily dispensed on aspen veneer sheets than the PF/PS formulation. SS alone displayed adhesive properties with 0.87 MPa of dry shear strength, but PS alone did not exhibit any bond strength. PS and SS were further evaluated for their general thermal behavior and major functional groups using differential scanning calorimetry and Fourier transform infrared spectrometry, respectively.