Flakeboard thickness swelling. Part II. Fundamental response of board properties to steam injection pressing

The results of this study showed that the same relative reductions in thick ness swelling (TS) previously obtained with steam-injection-pressed (SIP) r esinless mats are also obtained in boards bonded with 346 isocyanate resin. Reductions in thickness swelling were proportional to steam time and press ure. Thickness swelling of 40% measured in conventionally pressed boards fo llowing a vacuum-pressure-soak treatment was reduced to 25% in a board expo sed to 20 sec of steam at 600 kPa and to 6% in a board exposed to 40 sec of steam at 1,900 kPa. We believe that the reductions in thickness swelling r esult from a combination of flake plasticization, "lignin flow," and chemic al modification. Bending properties of the SIP boards were substantially lo wer than that of conventionally pressed boards, which we attribute in part to the very short press times and the relatively fast decompression used to manufacture the STP boards. Bending properties of SIP boards also suffered from a reduction of the vertical density gradient. However, this character istic is favorable to shear properties.