PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
to investigate the effect of pre-treatment severity and lignin addition. They concluded that the addition of 20% lignin caused no significant changes in density and dimensional stability, but improved internal bonds and strength properties of boards. Geng et al. (2006) treated black spruce bark with an alkaline treatment to manufacture binderless fibreboard. The treated board produced showed lighter colour with higher mechanical properties than the untreated board produced. Velasquez et al. (2002) ground Miscanthus sinensis fibres and placed the fibres in a digester for a steam-explosion process. The boards with ground pulp were of better quality than those obtained with non-ground pulp. Velasquez et al. (2003a,b) advanced their study by exploring the optimum pre-treatment and pressing conditions as well as the effect of lignin addition. The boards were of high quality and satisfied the requirements of standard board specifications. Cotton stalks contain high lignin content with very good flexibility and Runkel fibre ratios. Zhou et al. (2010) manufactured an environment-friendly thermal insulation material from cotton stalk fibres. They explained that thermal conductivity increased with board density because of a decrease in voids within and between fibres. Thus, this binderless cotton stalk fibreboard was excellent as an insulating component for building applications. MANUFACTURING PROCESS OF BINDERLESS BOARDS In this article, treatment term refers to the method applied to the fibres during the manufacturing process. Meanwhile, the pre-treatment term refers to the method applied before fibres were used in the manufacturing process; the pre-treatment method changed some properties of fibres. Heat treatment is a common manufacturing process in binderless board development. There are, however, binderless boards that have been produced through the extrusion and compaction processes. Pre-treatment processes such as steam explosion and a few others are important to produce better quality binderless boards, particularly in terms of dimensional stability. Meanwhile, pre-heating and grinding processes, which are simpler processes than steam treatment, are often used as another alternative for the pre-treatment process. Heat Treatment Process It is clear from previous studies of binderless board production that hot-pressing is widely accepted because of its simplicity compared with other methods (Okuda and Sato 2004; Xu et al. 2004; Hashim et al. 2012). The basic principle of hot-pressing is the application of heat, which activates the chemical components of the raw materials used. The raw material is basically put inside a mould, and the mould is then placed inside a hydraulic hot-pressing machine. After that, the material inside the mould is pressed depending on the parameters set up. It is necessary to apply enough heat and pressure to melt lignin through the entire board, allowing good distribution of lignin between the fibres during the manufacturing process (Mancera et al. 2008; Zhou et al. 2010). Cellulose and hemicelluloses are partially hydrolysed to simple soluble sugars that contribute to self- bonding (Shen 1991; Rowell et al. 2000; Widyorini et al. 2005a; Panyakaew and Fotios 2011). Several factors, such as pressing temperature, implemented during the manufacturing process of binderless board, have a great impact on the mechanical and physical properties of the boards produced. However, degradation of chemical components for the hot-press process has been found to be much less effective than a steam treatment (Laemsak and Okuma 2000; Okamoto et al. 1994; Widyorini et al. 2005a). The best properties obtained from this process were under a pressing temperature
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Article 4 – Wood Panels
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