Visualization and numerical analysis of adhesive distribution in particleboard using X-ray micro-computed tomography

X-ray micro-CT is used here to visualize the distribution of melamine–urea–formaldehyde adhesive in the wood composite, particleboard, and examine changes in adhesive distribution on wood particles (flakes) before and after pressing. CT and scanning electron microscopy of unpressed, wax-covered, wood flakes reveals that adhesive is evenly distributed on flakes, although it does not completely cover all surfaces. Adhesive on these wood flakes migrates during pressing forming more continuous glue-lines in areas of the composite where flakes are pressed closely together and more discontinuous glue-lines where inter-particle contact is poorer. Adhesive accumulates in small, but not very large voids, and also in cracks in wood flakes and around very small wood particles. These observations accord with previous observations of adhesive distribution in pressed wood composites. In addition, CT reveals that resin accumulates in capillary channels within the splintered ends of wood flakes and at the base of composite samples. The orthogonal variation in volume of the composite occupied by adhesive, wood and void space was quantified using maximal sphere modelling, and anisotropy in the distribution of these phases in the x, y and z directions was quantified using chord length analysis. These numerical techniques reveal that adhesive is more closely associated with wood flakes than void space, despite its tendency to migrate during pressing. As a result, adhesive is mainly aligned in the same x–y direction as wood flakes forming an anisotropic discontinuous network. We conclude that X-ray micro-CT is a powerful tool capable of providing new insights into the distribution of adhesive in wood composites.