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

摘要

The results of this study showed that the same relative reductions in thickness swelling (TS) previously obtained with steam-injection-pressed (SIP) resinless mats are also obtained in boards bonded with 3% isocyanate resin. Reductions in thicknessswelling were proportional to steam time and pressure. Thickness swelling of 40% measured in conventionally pressed boards following a vacuum-pressure-soak treatment was reduced to 25% in a board exposed to 20 seconds of steam at 600 kPa and to 6% in a board exposed to 40 sec of steam at 1900 kPa. It is believed that the reductions in thickness swelling result from a combination of flake plasticization, "lignin flow," and chemical modification. Bending properties of the SIP boards were substantially lower than that of conventionally pressed boards, which is attributed in part to the very short press times and the relatively fast decompression used to manufacture the SIP boards. Bending properties of SIP boards also suffered from a reduction in the vertical density gradient. However, this characteristic is favourable to shear properties.