The expansion potential: a new evaluator of the expansion behavior of wood composites

Representing the linear expansion (LE) characteristic of a wood composite by the natural logarithm of total length over moisture, content (MC) and differentiating it with respect to the MC, yields the instantaneous expansion coefficient or the “expansion potential.” Applying this transformation to LE data of commercial particleboards and medium density fiberboards (MDFs), and comparing the resulting expansion potential functions with that of solid wood, allows some conclusions to be drawn with regard to the integrity and stability of these composites. The expansion potential of particleboard is very similar to that of MDF up to a relative humidity of about 80 percent. Beyond that level, the expansion potential of particleboard deviates from that of MDF and solid wood, which both approach zero at fiber saturation, and instead increases in some cases to a maximum. This behavior suggests that, at high relative humidities, other mechanisms, besides the swelling of the wood cell wall, contribute to the LE of particleboard. These mechanisms are probably related to the breakdown of adhesive bonds between particles, which causes permanent modifications of all board properties and reveals a region of substantial material instability. It is possible that the partial breakdown of adhesive bonds will result in a new stress equilibrium and more stable behavior during subsequent cycles.

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