The results also showed that bending strength of the UF-bonded boards decreased more sharply than that of the MDI-bonded boards. This is assumed to be due to the high reactivity of isocyanates with the inorganic materials of paper sludge (Pizzi, 1983 and Xiaoqun, 2003). The bending strengths of the UF-bonded boards containing 15% paper sludge were higher than those of MDI-bonded boards.