PAPERmaking! g FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
thus it did not affect the dimensional stability of the board. Nevertheless, the particle shape and size have an important impact on the properties of binderless boards, so the degree of crushing in grinding becomes a limiting factor, as it affects particle geometry. EFFECTS OF MANUFACTURING PROCESS PARAMETERS ON PROPERTIES OF BINDERLESS BOARDS During the manufacturing process of binderless board, there are a few factors that can affect the properties of boards produced. These factors include the pressing parameters in terms of temperature, pressure, and time, as well as particle sizes and the presence of additional substances. Effects of Pressing Temperature, Pressure, and Time Pressing temperature is one of the most important parameters influencing board properties. Past studies (Widyorini et al. 2005a; Hashim et al. 2011a) have indicated that the yield of extractives increased at a higher pressing temperature. It has been shown in Fourier transform infrared (FTIR) spectra that there were slightly smaller chemical changes during the hot-press process, where part of lignin and hemicelluloses experienced decomposition. Quintana et al. (2009) revealed that fibres could be fused together by mechanical entanglement of the softened lignin molecules, possibly accompanied by the formation of a covalent bond. Thermogravimetric analysis (TGA) also shows thermal decomposition of hemicelluloses and cellulose at temperatures around 300°C. These results are supported by Zhou et al. (2010), who suggested that the lignin fluidity of fibres increases with higher temperatures, improving lignin distribution in fibreboards and inter- fibre bonds. The chemical changes described above contribute to self-bonding and improved board properties. The reduction of hygroscopicity resulting from the degradation of hemicelluloses was observed in studies by Xu et al. (2006) and Xie et al. (2012). They emphasised that high pressing temperature was beneficial for fibre plasticising. Fibres’ lignin melts and flows resulted in full fusion among fibres; the fibre contact thus became closer. A significant increase in board density and pressing pressure resulted in an improved internal bond strength and mechanical properties, along with greater dimensional stability of the boards. Nevertheless, the thickness swelling of the board was only affected by pressing pressure, while water absorption of the board was affected by all pressing variables, such as pressing temperature, pressing time, and pressing pressure; these properties are closely related, however. Mobarak et al. (1982) reported that an increase in pressing pressure during manufacturing of bagasse pith boards had an influence on board strength, attributed to the morphological structure of pith, which mostly consists of parenchyma cells. A study by Quintana et al. (2009) demonstrated that mechanical entanglement of the softened lignin molecules was accompanied by formation of covalent bonds, as fibres with lignin-rich surfaces fused together under a high pressing pressure. Conversely, fibres are more distant with lower contact points at a low pressing pressure. Despite that, high pressure means a high process cost, although the superior properties produced by such boards are appreciated. Li and Liu (2000) discovered no noticeable effect of pressing time when a board was pressed under high pressure. Similar trends can be seen where pressing time has less of an effect on log quality when the board is pressed at the closest moisture content to the optimum. Okuda et al. (2006; 2006a) claimed that either a one-step or a three-step pressing schedule could be selected without affecting board properties. Zhou et al. (2010) stated that prolonging pressing time resulted in an increased strength of the board. The
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Article 4 – Wood Panels
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