Application of liquefied wood as a new particle board adhesive system
Introduction
Biomass based materials and wood in particular are among the more abundant renewable resources. Recently, considerable attention has been given to the preparation of environmentally friendly polymeric products from liquefied biomass materials and their derivatives.
The liquefaction of wood and other lignocellulosic materials in the presence of phenol or polyhydroxy alcohols has been intensively studied by several authors (Lin et al., 1994, Alma et al., 1998, Yamazaki et al., 2006, Kržan and Kunaver, 2006, Qiuhui et al., 2006, Hassan and Shukry, 2008a, Hassan and Shukry, 2008b). Such a liquefaction process is usually carried out at elevated temperature and in the presence of an acid catalyst. The liquefied wood contains depolymerized products from the β 1–4 glucosidic bond cleavage of the cellulose and hemicelluloses molecules as well as from low molar mass oligomers. The complex structure of the lignin molecule is broken to smaller fragments. However, the associated reaction pathways and products are yet to be defined completely (Kurimoto et al., 1999, Lin et al., 2001a, Lin et al., 2001b, Yamada and Ono, 2001, Kobayashi et al., 2004).
With respect to the liquefaction of wood and wood wastes and their use as a liquid fuel, Wu et al. (2009) liquefied poplar leaves, wood and bark at high temperatures and pressures to produce alkanes. Rezzoug and Capart (2002) used solvolysis in ethylene–glycol and catalytic hydrogenation as the second stage, for the production of oils that possessed a high heating value.
Liquefied wood has a high reactivity due to a large amount of phenolic and alcoholic hydroxyl groups that are present. The hydroxyl value of the liquefied wood has been determined by several authors and is generally determined to be higher than 200 mg of KOH/g (Hassan and Shukry, 2008a, Hassan and Shukry, 2008b). The value depends on the liquefaction time and on the ratio between the wood content and the liquefying reagents. These functional groups can be used in the creation of polyurethane foams, polyurethane resin precursors (Kurimoto et al., 2000, Kurimoto et al., 2001, Wei et al., 2004) and in the recently developed wood–polyalcohol based urethane adhesives (Tohmura et al., 2005). Hydroxyl groups react with the isocyanate unit forming an urethane bond. Lee et al. (2002) have studied the thermal stability, biodegradability and genotoxicity of a range of polyurethane foams that were produced from polyols made from waste paper. They found that the foams possessed the same thermal stability as those made from liquefied starch or wood. The foams were biodegradable to some extent.
The hydroxyl groups of the liquefied wood can react with epichlorohydrin, thus introducing the epoxy functionality. Kobayashi et al., 2000, Kobayashi et al., 2001 prepared such epoxy compounds from the heartwood meal of Japanese cedar. Kishi et al. (2006) prepared a wood based epoxy resin from wood meal of German spruce. The authors cured their epoxy resin precursors with suitable amino precursors. Such two component systems were used as adhesives in plywood preparation, good mechanical and physical properties were obtained.
Hydroxyl groups in liquefied lignocellulosics can also react with different reactive sites in thermosetting systems as well as in two component systems. Lee and Liu (2002) prepared a resol type of resin from liquefied bark that was used in particle board preparation. Similarly, a phenol formaldehyde adhesive was prepared by Shenyuan et al. (2006) from liquefied bamboo. Zhang et al. (2007) used Chinese fir and poplar in this context. They tested the bonding strength of such adhesive in plywood showing the product to meet the requirements successfully. Gagnon et al. (2004) have undertaken research into the softwood bark pyrolysis oils and their use as adhesives for particle boards in combination with isocyanates. Amen-Chen et al. (2002) have used bark residues from pulp and paper industry to produce pyrolysis oil. They replaced different levels of phenol in resol synthesis with pyrolysis oil. Strand boards were prepared with these resols with comparable mechanical properties to panels made with commercial resols.
The first objective of this study was to synthesize a liquefied wood that possesses high hydroxyl group content, with a good yield in the liquefaction reaction. The second goal was to establish the criteria for creating a melamine–formaldehyde or a melamine–urea–formaldehyde resin precursor that would react at elevated temperature with liquefied wood and could be used as an adhesive. The third objective was to optimize the formulation and to test such systems, to confirm their potential use in the furniture industry.
Section snippets
Methods
Wood meals (flours) of poplar (Populus spp.), oak (Quercus spp.), spruce (Picea spp.) and beech (Fagus sylvatica spp.) were sieved through a 2 mm screens and dried at room temperature to a constant water content.
All chemicals were of synthesis grade (Merck) and were used without further purification.
Methylated melamine resins, urea–formaldehyde, melamine–formaldehyde and melamine–urea–formaldehyde resins were used as delivered from the producers. The type and the producer of each resin are shown
Mechanical and physical properties
All of the evaluations of the resulting particle boards were carried out according to the appropriate European standards (EN). These concern thickness (EN 324), density (EN 323), swelling in thickness after immersion in water (EN 317), bending strength and modulus of elasticity (EN 310), internal bond strength (EN 319) and surface soundness (EN 311).
Formaldehyde emission tests
The total formaldehyde content was determined using the perforator extraction method, according to the European standard (EN 120). The perforator
Characterization of the incompletely liquefied/derivatized residues of the wood meal (flour)
The liquefaction process gave yields of the liquefied wood that were in excess of 99%. However, in all cases, a small amount of residue was present as a dispersive phase in the liquefied wood solution. This material was made the subject of a characterization study. The typical reaction yields are shown in Fig. 1 for the different wood meals. It is clear that a reaction time of at least 105 min, under the specified heating regime, was needed if unwanted residues were to be minimized. For oak and
Properties of particle boards
Table 5 shows the properties of particle boards made with the mixture of the liquefied spruce wood and the resin precursors Meldur H97 and MS-1. The required values, according to the European standard EN 312 (2003), type P2, are also shown. The formaldehyde content was measured according to the European standard (EN 120), using the perforator extraction method that determines the formaldehyde concentration by extraction, giving the formaldehyde emission potential for the long term use of
Conclusions
Different types of South European hardwood and softwood have been used in the liquefaction process with yields greater than 99%. The optimum reaction time needed for the liquefaction was 105 min. A condensation reaction occurred with beech and oak wood samples after a prolonged reaction time.
A significant difference in average molar mass was determined between the water insoluble part of the liquefied spruce wood (9700 Dalton) and water soluble part (670 Dalton) indicating the presence of a
Acknowledgements
The authors wish to gratefully acknowledge the support for the presented work received from the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the Program P2-0145-0104, GGP Postojna for their generous financial support and Melamin Kočevje who kindly provided resins used in this study.
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