As shown in Fig. 4, the shear strength of the boards decreased significantly with increase in the paper sludge level. As discussed previously, this is attributed to the inorganic materials of paper sludge that interfere with adhesion. Similar to the bending strength, as the paper sludge level increased, the shear strength of the UF-bonded board decreased much more than that of MDI-bonded board. However, it was observed that UF-bonded boards had higher shear strength than MDI-bonded boards due to the higher resin solid application rate of UF adhesive compared to MDI adhesive. This confirms the results presented by other authors (Sellers, 1996).