Nail-head pull-through, lateral nail resistance, and single shear nailed joint tests were conducted on medium density fiberboard (MDF) with different density profiles, and the relations between the results of these tests and the density profiles of MDF were investigated. The maximum load of nail-head pull-through and the maximum load of nailed joints were little affected by the density profile. However, the ultimate strength of lateral nail resistance, the stiffness, and the yield strength of nailed joints were affected by the density profile of MDF and showed high values when the surface layer of the MDF had high density. It is known that bending performance is also influenced by density profile. Therefore, the stiffness and the yield strength of nailed joints were compared with the bending performance of MDF. The stiffness of nailed joints was positively correlated with the modulus of elasticity (MOE); in the case of CN65 nails, the initial stiffness of joints changed little in response to changes in MOE. The yield strength of nailed joints had a high positive correlation with the modulus of rupture (MOR). The stiffness and the yield strength of nailed joints showed linear relationships with MOE and MOR, respectively

Untreated fibers and acetylated fibers were ozonized and used to make fiberboards. The effect of ozonization on the durability of the boards bonded with poly-methylene diphenyl diisocyanate PMDI and phenol formaldehyde resin was investigated. In order to study durability, the ASTM 6 cycle accelerated aging test was conducted. The internal bond strength IB after the accelerated aging test of the boards bonded with PMDI was decreased greatly by excessive ozonization. The great decrease suggests that the possibility of chemical bonding between the hydroxyl group of the wood and the carboxyl group of the PMDI was low. The carboxyl group introduced by ozonization instead reacts with the isocyanate group of the PMDI, resulting in obstruction of PMDI curing. On the other hand, suitable ozonization improved the IB of the acetylated boards bonded with phenol formaldehyde resin, and termite resistance was also improved. In addition, it was found that phenol formaldehyde resin was highly resistant to decay, and the combination of phenol formaldehyde resin, acetylation and ozonization was effective in enhancing resistance to both decay and termites.

In order to improve the bonding of fiberboard, the penetration of phenol formaldehyde PF resin was prevented. The means of preventing penetration were acetylation of wood fibers and the addition of filler to PF resin. Acetylation improved internal bond strength IB due to the resulting lack of penetration. In addition, the IB of fiberboard made from acetylated wood which had been ozonized acetyl-ozonized board was much higher than that of merely acetylated fiberboard due to improved wettability by the ozonization. The durability of the acetylozonized board was significantly increased, resulting in 0.84 MPa for IB and 74% retention of IB after an ASTM 6 cycle accelerated aging test. In another test, fiberboard was made from untreated wood bonded with PF resin to which strained soybean meal was added as a filler, and the IB showed a roughly 40% increase. The retention of IB after the ASTM 6 cycle accelerated aging test was lower than that of the acetyl-ozonized fiberboard, but the addition of the filler was effective for improving properties for conventional uses.