PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
Lignin, which provides rigidity and stiffness to plant cell walls, is much more stable and resistant to steam treatment than hemicellulose (Widyorini et al. 2005a; Mancera et al. 2008). Steam treatment is considered to facilitate lowering the softening point of lignin and exposure of the more accessible lignin on the surface of the fibre, which contributes to self-bonding (Angles et al. 2001). Luo and Yang (2012) reported that hemicellulose and cellulose contents decreased with increasing lignin content from liquid hot water pre- treatment. Mancera et al. (2008) stated that pre-treatment temperature and time have no effect on lignin contents. Other factors, such as the severity parameter, are important during the steam treatment process. The severity parameter is the expression based on calculation of steam temperature and residence time of the steam temperature. Previous studies (Suchsland et al. 1987; Geng et al. 2006) mention that steam treatment severity clearly have a significant influence on internal bond, as high severities produced higher surface area with high lignin contents. A few previous studies (Angles et al. 2001; Widyorini et al. 2005a; Mancera et al. 2008) also reported the same results, in that high severity led to improved internal bond strength and dimensional stability of boards, although it decreased the mechanical strength properties. The effect of longer steam treatment resulted in excess steam and caused a rigorous degradation of chemical components. Increased severity not only reduces hygroscopicity, but also greatly reduces abrasive materials that are unnecessary for board manufacturing. The removal of wax and pectin from the surfaces contributed to a significant effect on the wettability of fibre surfaces (Suchsland et al. 1987). The manufactured boards turned dark brown, indicating a high degree of hydrolysis or modification of chemical components during steam treatment. Previous research (Suchsland et al. 1987; Velasquez et al. 2003b) has reported that superior mechanical properties of binderless fibreboards can be gained from steam treatment with low severity, compared with binderless fibreboards obtained from steam treatment with high severity. Low temperature, low steam pressure, and long treatment time were used to protect the structure of fibres and carry out lignin modifications to enhance the adhesive behaviour. Pre-heating and Grinding Process For lab-scale preparation, another process to improve binderless board is pre-treatment using pre-heating and grinding. The simplest pre-treatment is microwave preheating of the fibres before hot-pressing. This helps improve the internal bond strength and properties of manufactured boards. Okuda et al. (2006a) reported that pre-heating fibres in a microwave for one minute could accelerate thermal softening of lignin, which resulted in better strength properties. When pre-heating time increased to two minutes, there was no obvious change observed because of the influence of excessive drying, and part of raw material was burned black with very low moisture content. One of the easiest ways of preparing pre-treatments for fibres is milling or grinding. According to previous studies (Velasquez et al. 2002, 2003a; Saadaoui et al. 2013), grinding the material before the hot-press process considerably improves the internal bond strength without affecting other physico-mechanical properties of binderless boards. This process, however, does affect the density of the boards, as grinding gave lower density than non-grinding materials because of an increase in compression resistance and more air in milled pulp. Grinding did not cut the fibres, but helped separate the bundles of fibres. This segregation leads to more spaces for inter-fibre bonding, leading to an increase in the strength of the bonds (Velasquez et al. 2002). The nature of the bonds did not change,
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Article 4 – Wood Panels
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