Dimensional instability of cement bonded particleboard: SEM and image analysis

Abstract

Scanning electron microscope (SEM) and image analysis were applied to quantify the structure of commercial cement bonded particleboard (CBPB) to obtain a more fundamental understanding of the instability of CBPB and to provide a basis of information for modelling the stress–strain behaviour of CBPB. Surfaces through the thickness of the board and vertically within the board were analysed mainly with respect to the distribution, size, shape and percent area occupied by individual components (void, cement paste and wood chips). Results showed that the area occupied by wood chips 1) increased from the surface to the core layer (from 34 to 49%), 2) was 5% higher for vertical surfaces than for horizontal surfaces, 3) but was in total only about 40%; this is much lower than the volume fraction (about 75%) of raw materials used in the manufacture of CBPB. This confirms the mechanism/organisation of mat formation, showing the more significant effect of wood chips on thickness than on length changes with changing moisture content and indicating that the wood chips are compressed in CBPB. The wood chips nearly all lie flat in the horizontal planes throughout the board thickness and were randomly distributed on the horizontal plane. The mean angle was about 10.7° between wood chips and the horizontal surface and about 44.7° between wood chips and the longitudinal direction within a horizontal plane, verifying that the change in thickness should be much higher than that in length or width, and the change in length and width should be similar. The size and shape of the wood chips were very different among six horizontal layers and between the horizontal and the vertical layers.