Properties of medium-density fibreboard (MDF) based on wheat straw and melamine modified urea formaldehyde (UMF) resin
Introduction
Medium-density fibreboard (MDF) is a fibre composite material comprising of refined wood fibres, adhesive (resin), process additives, and a minor amount of wax. MDF is produced in a dry fibre process, and by definition, wood-based MDF-panels have densities between 450 and 800 kg/m3 (EN 316, 1999). The strength of MDF depends on its fibres and on the adhesive bonds between them. Thereby, the adhesives are necessary to ensure effective bonding between the fibres. The most common types of resins used for MDF-products are based on formaldehyde, for example urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF), and phenol- formaldehyde (PF) resins. Typical MDF-products are cabinet doors, shelves, laminated floors, furniture and panels for building construction.
Real commercial interest in MDF-products started in the mid 1970s when UF-resin was injected into a tube (blowline) and mixed with refined fibres in a blowline blending process (Haylock, 1977, Hammock, 1982, Gran, 1982). The outcome was successful, and today the MDF production in Europe is at approximately 14.9 million m3/year as of 2006 (Wadsworth, 2007), and has an estimated increase (2006–2010) by at least 4% annually (Dunky and Lukkaroinen, 2005). Consequently, demand for nonwood lignocellulosic fibre resources has also increased, due to lack of wood raw materials and for economic and environmental considerations. Most wood raw materials are primarily used for sawn lumber products and in the pulp and paper industry. The availability of alternative inexpensive fibre raw materials is critical if manufacturing is to increase in several regions of the world.
The most promising annual plant waste materials for manufacturing MDF are wheat and rice straw. Globally, wheat and rice are the most important food grains ranking second (wheat) and third (rice) in terms of the total cereal production after corn (maize). The production of wheat grain was estimated at 620 million metric tons in 2005–2006 (WAOB, 2007, p.18). According to an earlier investigation the same amount of waste straw materials as grains can be generated (Russel, 1996). Part of the agricultural by-products such as straw are likely to be used as industrial raw materials at a significant scale in the near future, which is an alternative of being burned or left in the fields. The lignocellulosic fibres used in manufacturing conventional MDF entirely comprise wood materials, the fibres of which vary in kind and quantity depending on the source woods. Wood fibres differ in length, which is approximately 1–1.5 mm for hardwoods and 2–4.5 mm for softwoods. Reported fiber length of wheat straw are almost the same length as the hardwood at approximately 0.7–1.4 mm (Markessini et al., 1997, Subrahmanyam et al., 1999, Rowell et al., 2000).
The main chemical components of wood and straw materials are cellulose, hemicellulose, and lignin. However, the proportions and chemical structures of hemicellulose and lignin differ between straw and wood (Markessini et al., 1997, Sun et al., 1997, Rowell et al., 2000, Donaldson et al., 2001), the lignin content being lower and the hemicellulose content higher in straw materials. Morphological analysis of straw materials reveals a higher amount of nonfibrous thin-walled cells, parenchyma and epidermal cells than in wood materials (Zhai and Lee, 1989, Subrahmanyam et al., 1999, Liu et al., 2005). The manufactured straw fibres are shorter and the finished straw particleboards and MDF (SMDF) panels display greater water swelling than do wood-based boards (Sauter, 1996, Han, 2001, Eroglu and Istek, 2000, Mantanis and Berns, 2001, Wasylciw, 2001). Furthermore, the straw contains more silica and ash (Markessini et al., 1997, Ghaly and Al-Taweel, 1990, Rowell et al., 2000). The outer part, or epidermis, of the straw contains micro-sized silica particles called phytoliths (microfossils), which are unique to each species (Ball et al., 1999, Sangster et al., 2001). A previous study found that fractions of size-reduced and screened wheat straw materials were enriched by silica as the fractions length was reduced (Halvarsson et al., 2005). Removing the dust and short-fibre fractions is undoubtedly a necessary process operation to improve SMDF quality.
This report describes a method for manufacturing of MDF based on wheat straw and melamine modified UF-resins. The parameters discussed include processing steps involved in the SMDF manufacturing and the influence of the average density of MDF panels at different thicknesses and resin content on various panel properties.
Section snippets
Raw materials
Straw of wheat (Triticum aestivum) was harvested and baled in north-eastern China. The wheat straw (WS) was harvested in different seasons, and some of the WS was stored in dry conditions for 1 year. The dry content of the delivered straw was approximately 90%.
Two types of commercial melamine-modified Urea Formaldehyde (UMF) resins were used. The UMF-resins were supplied by Dynea NV (Prefere 11G321) and Akzo Nobel-Casco Adhesives AB (UMPF 1074-0837). The latter UMF-resin contained a high amount
The wheat straw fiber properties
The produced resinated wheat straw fibres were sampled after the drying step. Fibre length was analysed in a PQM 1000 laboratory analyser by means of image analysis. The average fibre length and width average were approximately 1.0 mm and 26 μm, respectively, for refined fibres in all trials (see Table 2). The measured length was in line with the length of wheat straw reported in the literature (Markessini et al., 1997, Subrahmanyam et al., 1999). Refining wheat straw generates a higher amount of
Conclusion
The produced wheat straw MDF met the MDF industry standards for densities above 780 kg/m3 at a resin content above 14%. The fundamental properties of the manufactured SMDF panels were evaluated.
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The mechanical strength properties, i.e., IB, MOR, and MOE were strongly depending on average density, and increased as a function of density.
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The amount of added resin 14–17%, storage time of wheat straw after harvesting, and the pre-treatment of straw with water or acid before refining had only
Acknowledgements
The authors are grateful to the Swedish Knowledge and Competence Foundation for financially supporting this research. Thanks are also extended to all those involved at Metso Panelboard AB and Akzo Nobel—Casco Adhesives AB and Dynea, Gent, Belgium for supplying the formaldehyde resins.
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