干旱区生态环境知识资源中心

An investigation was made of the dimensional stability of wood-based boards, pressed from elements arranged in four geometrical patterns. Four mat structures (homogeneous packing type, parallel crossed type, reciprocal parallel crossed type, random type) fabricated from elements were tested to evaluate their moisture absorption characteristics. The relationship between thickness swelling, mat structure, specific gravity and internal bond strength were investigated.

The pressing process for forming mats was categorised by four characteristic stages. Stage I involves the closing of gaps between elements. Stage II relates to the elastic deformation of elements after closing of gaps. Stage III involves plastic yield of elements arid Stage IV concerns the final consolidation of elements. It was difficult to distinguish between Stage I, II and III deformation for reciprocal parallel crossed-type and random-type mats. Homogeneous packed type and parallel crossed-type mats were well characterised in Stages I, II, III and IV.

Each of the four board geometries was produced in Stage II, III and IV forms. Environmental stability was measured following four weeks of exposure to a 90% RH, 26 degrees C environment. Boards which had been subjected to a high degree of compression experienced higher thickness swelling. Thickness swelling corresponded to stress, which was calculated as load divided by effective section-area. In general, high internal bond strengths were related to high specific gravities with the exception of the homogeneous packed type. In addition, the internal bond strength was closely proportional to the specific gravity for reciprocal crossed type and random type mats. Therefore, it is presumed that the element to element contact areas in reciprocal parallel crossed type and random type mats are closely similar. Boards fabricated with different mat structures exhibit different internal bond strengths even at the same specific gravity values.

In conclusion, internal bond strength and thickness swelling of the four types of board are a function of specific gravity, but the arrangement of elements has a dominant effect on the board properties.