UBC Theses and Dissertations
Horizontal density distribution of particleboard: origin and implications Xu, Wei
Particleboard products have been manufactured for over a half century. During this time, it has been recognized that a vast number of material and processing variables influence board properties. Little is known about the internal structure of particleboard, and a fundamental principle or theory interrelating structure, processing and properties of particleboard has yet to be developed. Such basic knowledge of particleboard structure is not only necessary to fully understand present particleboard technology, but also important for the future upgrading and development of wood composites. This study was designed to develop some of this knowledge base. The two major objectives of this study were: (1) to investigate the influence of raw material characteristics on horizontal density distribution (HDD), and (2) to determine the effect of particleboard nonuniformity as defined by HDD, on some key board properties. Twenty six particleboard panels made with precisely cut particles were used to study the first objective, while thirty boards involving different particle sizes and distributions, and different wood species and combinations were manufactured to study the second objective. In addressing the first objective, the Equation S = a(1/A)b was found to be appropriate for relating standard deviation of density (S) and specimen size (A), where a and b are constants. At relatively larger specimen sizes, particleboard made with larger particles exhibited greater density variation, while particleboard made with smaller particles showed larger variations at smaller specimen sizes. Two aspects of voids, namely number and size, were identified as factors contributing the relation between particle size and HDD. In addition, a layer concept was developed to relate particle thickness, wood density, board density and board thickness to HDD. This concept predicted a decrease of density variation as particle layers increased. In addressing the second objective, modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (TB) and thickness swelling (TS) of particleboard were shown to be greatly controlled by nonuniformity of board structure. All these properties were improved as structure uniformity improved. A nonuniformity effect concept, expressed as P = m(1/S) was proposed in relating board properties (P) to standard deviation (S) of horizontal density, in which, m and n are constants. While TS was influenced most by the high density portions, mechanical properties were dominated by the low density areas. The concept of HDD was also used in this study to investigate the relationship of specimen size effect on TB and TS, for one commercial waferboard. Both average values and standard deviations of TB and TS decreased as specimen size increased. A criterion based on HDD concept was proposed for the future establishment of testing standard in terms of specimen size selection. The relationship between HDD, raw material characteristics and board properties, demonstrated that HDD was a fundamental variable useful for characterizing particleboard structure and technology. The HDD concept has the potential of linking the effects of raw material characteristics and forming techniques to board properties in short-fiber wood composites.
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