PAPERmaking! g FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
of 180°C, a pressing pressure of 5.3MPa, a pressing time of 10 min, 5.0-mm board thickness, and a board density of 1.0g/cm 3 , using kenaf (Okuda and Sato 2004). Compaction Process In addition to the heat and steam processes, binderless boards can be produced through extrusion. Extrusion occurs when the material undergoes the compaction process, in which the compaction device is heated while the compaction loads are poured into the device. Then, the loads are extruded through the die under various conditions. Miki et al. (2003) evaluated the effects of extrusion temperature, extrusion ratio, and moisture content, as well as particle size of wood. They found that increasing the extrusion temperature and moisture content of powders increased the fluidity of extruded products up to 200°C. Extrusion needs a specific machine that might not be appropriate for small industries because of the high cost involved and the particular handling knowledge required to operate the machine. Jain and Handa (1982) also produced binderless boards from agricultural wastes of straw and paper liner particles through the extrusion process. These boards can be used for interior partitions of ceiling and lining. Li and Liu (2000) investigated the process for developing high-density binderless logs from waste paper using the compaction process. Moisture content plays an important role in the compaction process. Reasonably high-quality and cost-effective logs were produced with a compaction pressure of 70% and moisture content of 15%. Hunt and Supan (2006) developed binderless boards from recycled corrugated containers and refined small diameter treetops. Both panels surpassed minimum commercial hardboard standards through this process. Steam Pre-Treatment Process Steam pre-treatment has been known to be effective for improving the dimensional stability of wood-based composites, depending on variables such as pressing temperature and chemical composition of lignocellulosics (Mobarak et al. 1982; Widyorini et al. 2005a). This process works by injecting high-pressure saturated steam into a reactor filled with material used, where the temperature rises up to 200°C and suddenly the pressure is reduced, resulting in an explosive decompression of the fibre material. The explosion causes an increase in surface area of the fibres, with degradation of hemicelluloses, low crystallinity of cellulose, and disruption of the lignin matrix. Widyorini et al. (2005a) managed to manufacture kenaf cores binderless boards via steam-injection treatment. The results showed that the bonding properties of these binderless boards were relatively strong, compared with binderless boards produced by hot-press treatments or even other binderless boards from different processes with the same density. Angles et al. (2001) discovered that the hygroscopicity characteristics of hemicelluloses are responsible for moisture absorption. This means that boards with lower hygroscopicity have smaller values of water absorption and thickness swelling. Some authors (Angles et al. 2001; Widyorini et al. 2005a; Mancera et al. 2008) have attained similar results, in which decreasing hemicelluloses content during pre-treatment leads to improved dimensional stability, but does not lead to any increase in mechanical properties. The presence of cellulose in the crystalline structure helps to prevent water from penetrating the boards and leads to an increase in dimensional stability, as it makes the structure of the boards compact without voids. At the same time, cellulose is also degraded through the steam treatment process, which causes a reduction in the quality of boards (Widyorini et al. 2005a,b).
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Article 4 – Wood Panels
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