Dimensional instability of cement bonded particleboard - Part 2: Behaviour and its prediction under cyclic changes in RH

Abstract

This paper examines the dimensional instability of cement bonded particleboard (CBPB) and discusses the behaviour of CBPB during cyclic changes in relative humidities (RH). The results indicated that the changes of CBPB in both mass and dimensions reflected the changes in RH; in a cycle 90–65–35–65–90%RH the change of per percentage change in RH was 1.0:1.3:1.3:2.5 for mass, 1:2:2:1 for length and 1.3:1.1:1.0:1.0 for thickness. The changes were closely related to the structure of CBPB and the nature of the cement paste and wood chips. Thickness change was double that of the length change between 65 and 35%RH and triple that between 65 and 90%RH. The rate of change in mass was much higher than that in dimensions. The hysteresis loops were dissimilar to those of other wood and wood-based materials, with the loops of mass change closed between 65 and 90%RH, while the loops for dimensional changes were open. A set of the former loops moved upward and the latter moved downward with increasing number of cycles, corresponding to an accumulated increase in mass, but decrease in dimensions. The models developed for CBPB under constant and a single changing RH were successfully applied to changes under cyclic RH. The sorption behaviour and dimensional movement of CBPB has been shown to be influenced not only by RH, but also by its intermediate history, with the maximum values for change within every phase of sorption, in both mass and dimensions, being higher for the cycling between 35–90%RH than the cycling regime 35–65–90%RH. The ratios of dimensional changes to mass change of CBPB were greater under the latter regime than under the former regime.