Have you ever noticed that the density of plywood panels is lower, than that of particleboard, OSB and fibreboard? Here is why. All surfaces are ‘rough’ at the microscopic scale. Therefore, when two solid substrates are brought together, their area of mutual contact is very small compared to the geometric surface area available. Even though bonds may form between them at these points of contact they are normally too few and/or too small to hold them together.

The contact surface area can be increased by applying pressure, but often this is limited by the mechanical properties of the substrates. An alternative approach is to use a liquid intermediary – an adhesive. This bridges the gaps between the things we want to join (the adherents) and when it solidifies it provides mechanical and, sometimes, chemical, bonds between the adherents.

Trying to find where the adhesive is in a finished panel has vexed researchers for a long time. A new technique, X-ray microcomputed tomography (µCT), seems able to provide accurate information on the location of the glue. It works in a very similar way to standard X-ray imagery in that the absorption of X-rays is linked to the density of the object. However, it operates at a much higher resolution so that fine detail can be seen. Fig 1 is a µCT scan showing the adhesive distribution in a laboratory-made particleboard. It is obvious that the panel still contains voids (black) and so the glue (blue around these is not really doing anything. The only way to give the glue around the voids a chance of contributing to the panel’s mechanical properties is to press it even harder and thus crush the voids to oblivion. This, of course, would increase the panel’s density even further.

On the other hand, the veneers of plywood are smooth and continuous compared with the particles in particleboard and so it is relatively easy to create a strong bond; the pressure required is only enough to flatten the veneers and even out the glue distribution. The continuous glue lines between veneers are easily seen in the µCT scan of plywood (F 2). It is also clear all of the glue contributes to the strength of the panel.

The lower compaction of the wood in a plywood also brings with it reduced thickness swelling and easier machining.

The panel market is looking for everlighter panels so this is a segment plywood manufacturers are well poised to exploit.