Reducing thickness swell is the most critical and challenging problem facing oriented strand board (OSB) manufacturers. This is especially important for new end applications for OSB including sub-flooring, web stock and concrete forming.
To address the thickness swell problem for OSB, the present report discusses the fundamentals of thickness swell in Part 1 describing in detail the dimensional stability of wood strands under the interactions of heat, moisture pressure and time, develops a statistical model in Part 2 and finally in Part 3, develops a new practical patented radio-frequency method to reduce thickness swell in OSB.
It is recommended that the statistical models generated from the present study should be combined with some of the earlier work by Hsu (1994) and Sean (1997) to form a comprehensive computer software for OSB manufacturers.
In this study, we conducted systematic experiments on air permeability of aspen veneer and glueline in terms of panel compression ratio (or applied platen pressure), degree of glue cure (or pressing time), veneer type (sapwood or heartwood veneer) and glue spread level. We also compared the air permeability data of aspen veneer and veneer-ply (2-ply veneer panel) to aspen solid wood and aspen oriented strandboard (OSB). Based on this study, the following conclusions were drawn:
For laminated veneer lumber (LVL) and plywood panels, the compression ratio is the most important factor affecting the panel permeability, followed by veneer type (sapwood or heartwood veneer), glue spread and degree of glue cure (or pressing time). The air permeability of the glueline decreases in the course of glue curing; however, its order of magnitude remains the same as that of uncured glue. The reduction in panel permeability mainly results from small densification of each veneer ply instead of the sealing effect of the glueline. Therefore, during LVL/plywood hot-pressing, the glueline does not serve as a main barrier to the gas and moisture movement as commonly speculated. However, due to the substantial change in the magnitude of panel permeability merely within a 5% compression ratio, the convection effect on heat and mass transfer is considered to be very limited.
The air permeability of sapwood veneer is about twice that of heartwood veneer without compression. However, with compression, the air permeability of heartwood veneer drops much faster than that of sapwood veneer. The permeability of a sapwood veneer panel is 5.5 ~ 7.0 times higher than that of a heartwood veneer panel merely with a compression ratio in the range of 2.5% ~ 5%. In practice, it implies that 1) panels made from sapwood veneer are more treatable with preservatives; and 2) by controlling panel permeability through veneer incising, proper panel lay-up and densification, mills could reduce blows/blisters during hot-pressing.
The air permeability of aspen wood or veneer is not affected by wood density. The air permeability of aspen LVL/plywood panels is 1.5~ 2 times larger than that of aspen solid wood due to the existence of lathe checks, but is significantly lower than that of aspen OSB at the same density level of the panel. On average, commercial LVL/plywood panels have almost the same magnitude of air permeability as commercial OSB. However, due to the absence of voids and small horizontal density variation, LVL/plywood panels will be less permeable than OSB.