I sincerely believe that the high thickness swelling observed in densified panels such as particleboard, OSB and MDF severely limits the full exploitation of these products.
The main reason for this is that once swollen, these panel products are irrecoverably damaged in that they do not recover their mechanical properties once re-dried and therefore we cannot use them in the same way that we would use solid wood.
The amount of thickness swelling observed in densified panels is much higher than that observed in solid wood and plywood.
Results of some experiments
Figure 1 presents some results obtained by some of my students at ESB during one of my practical classes.
The class is four hours long and so I give the students non-standard-sized samples of panels that are only 25 x 25mm so that they have the opportunity to observe real differences between panel types.
In addition, I provide two water baths; one set at 20°C and the other at 55°C. The lower temperature simulates what they might observe if they used standard-sized specimens (50 x 50mm in EN317) soaked over a longer period (often 24 hours).
The higher temperature accelerates swelling and so gives the students an idea of the swelling that would occur over a much longer soaking period.
It is clear from Figure 1 that plywood is much more stable than the particleboard, OSB and MDF panels, especially in hot water.
In fact, the thickness swelling of a plywood is often equivalent to the radial swelling of the solid wood from which it is made because the veneer peeling process causes the veneer to have tangential faces and so the direction through the thickness of the veneer is equivalent to the radial direction of the original log.
This is demonstrated in the results shown, as the swelling in radial direction of poplar wood is around 5% (from oven dry to fully swollen) and this is the maximum swelling observed by the students for the poplar veneer plywood samples in hot water.
The adhesive in the plywood has limited influence on thickness swelling at these low temperatures because the veneer is free to swell between the gluelines.
The particleboard, OSB and MDF panels, however, swell much more because of the high compression applied during panel manufacture.
If the samples were re-dried then they would shrink, but only by a little, so the density of these composite panels would be significantly reduced and there is a strong correlation between panel density and mechanical properties.
Medite Europe Ltd has developed a water resistant and dimensionally stable MDF called Tricoya®. It is made using acetylated wood fibres, a technique developed by Accsys Technologies PLC. It will be interesting to see if this stable MDF finds a profitable market.
