PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
fibres need to be heated for longer times to make sure the heat is transferred to the core of the binderless board; hence, the lignin is plasticised and flows well. Effects of Particle Sizes It is well known that particle size and geometry significantly influence the development of binderless board properties in terms of bonding quality among particles, compared to mechanical strength properties of the fibres themselves. Previous studies (Munawar et al. 2007; Hashim et al. 2010) have shown that smaller particles improved the qualities of the board with superior internal bond strength. This is because smaller particles were more compressed with fewer overlapping areas, producing uniform homogeneous cells, which had lower voids. Interestingly, this leads to better dimensional stability and gives a smoother surface for binderless boards compared to larger particles (Mobarak et al. 1982; Hashim et al. 2010). Additionally, smoother surfaces and appearances are energy saving, as no sanding stage is required. The smaller particles, particularly in powder form, need more energy to manufacture and are difficult to handle during the fabrication process (Okuda and Sato 2004). Studies conducted by Hashim et al. (2010) and Juliana et al. (2012) reported that strands led to higher internal bond and mechanical strength compared with particles. Strands are thinner, longer, and have a larger surface contact area with a better glue line, which leads to better strength characteristics. Longer particles are high in aspect ratio (fibre length over width) or adhesive content per unit particle surface area. The roughness value of strands is higher because strands are coarser and have a rigid structure of vascular bundles. Juliana et al. (2012) again stated that more slender (length over thickness) particles usually provide better bonding than bigger particles because of a greater amount of contact surface. Slender particles require more adhesive to sufficiently bond the particles. Admixture boards consisting of kenaf and rubberwood gave better strength than using 100% of the same materials. Particles of rubberwood were stronger and more slender than the kenaf particle. These results were supported by a study by Charoenvai (2013), in which the manufacturing of particleboard using durian-peelpowder-based adhesive gave similar properties when compared with synthetic adhesives. Effects of Additional Substances Numerous studies (Velasquez et al. 2003a; Okuda et al. 2006b; Ashori and Nourbakhsh 2008; Baskaran et al. 2012) have been performed by adding extra materials to binderless boards for the purpose of comparison. A study conducted by Ashori and Nourbakhsh (2008) reported that wax addition in binderless board reduced strength properties, but it helped to improve the dimensional stability of the boards. Moreover, further treatments such as coating or laminates are required for binderless boards without using wax or any hydrophobic substance. Therefore, a small amount (approximately 1%) of wax can be used in binderless particleboard manufacturing. Velasquez et al. (2003a) investigated the effects of replacing fibre with different kinds of technical lignin recovered directly from pulping liquor, without further purification or treatment. The lignin was added in two conditions, which were prior to the steam explosion process and just before the hot-press process, after the material had gone through a steam-explosion process. They discovered that substituting 20% of Miscanthus sinensis with lignin had superior effects on board properties. Boards with kraft lignin added before the steam explosion process had superior properties, resulting from the elimination and reduction of low-molecular weight substances during the steam explosion, leading to
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Article 4 – Wood Panels
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