The IB results showed a similar trend to the MOR and MOE results. The IB values of the particleboards ranged from 0.23 to 0.56 N/mm2 depending on the hardener type, urea usage, and conditioning period (Table 4). The particleboards made with the ammonium chloride had the highest IB value, followed by the particleboards made with the ammonium sulphate and aluminium sulphate, respectively (Table 4). This was mainly attributed to the fact that the hydrochloric acid from ammonium chloride spreaded more uniformly in the particleboard than the ammonium sulphate, which resulted in higher IB strength. The minimal requirements of IB strength for general purpose and furniture manufacturing (EN 312) are 0.24 N/mm2 and 0.35 N/mm2, respectively [18]. All the particleboard types except for types 5 and 6 had the required level of IB for general purposes, while the particleboard types 1–4 and 7–10 met the required levels of IB either general purpose and interior fitments (including furniture manufacturing). The hardener type and conditioning period significantly affected the IB strength while the urea usage did not affect the IB strength according to the results of Newman–Keuls test (Table 6).