Wood-Based Boards

When wood sheets or particles of wood are bound together to get a more or less stiff board, lots of applications are derived. They depend essentially on the mechanical properties and on the density of the board. Secondarily, these applications also depend on the granulometry of the wood employed. 

In a first classification, we can distinguish (i) boards made with an external synthetic binder, such as urea-formaldehyde or phenol-formaldehyde resins for thermosets, and (ii) boards with internal natural binders. The products cited below do not constitute an exhaustive list. 

Different commercial products are classified according to the size of the form of the raw matter (wood). All of them require the binder (around 10%) to be sprayed on the wood mat before undergoing hot-pressing (150-200 °C, 20-25 bar). 

Wood sheets, which are obtained by unrolling steamed wood rods with ablade, can be bound together to form thicker boards. The main characteristic of these sandwiches is the alternation of the fibre orientation by 90° in every layer. The anisotropy of the whole board is thus limited in the length and in the width. Three, five or seven layers are usually used in these panels, called plywood. Often, the inner layers of plywood are replaced by OSB (see below) to reduce costs. The two outer layers of wood sheets ensure the aesthetic appearance of the panel. 

Oriented strand board (OSB) is obtained from wood chips. The orientation is designed to simulate the characteristics of a wood panel, with limited swelling and higher resistance in the fibre direction. Chips are orientated by air with a blowing machine. These panels are commonly used on building sites, and they often make part of the walls in houses, especially in the United States. If no orientation is given, the panel is called waferboard. 

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The industries affected by these regulations are those dealing with petrol vapour recovery, incineration of waste, mercury emissions from crematoria, animal rendering, non-ferrous foundry processes, surface treating of metals and plastic materials by powder coating, galvanizing of metals and the manufacture of certain specified composite wood-based boards. 

The range of wood-based boards, and the applications in which they are used, is large, and increasing. Examples are the use of plywood in structural situations, such as I-beams or box beams, on the one hand, to the construction of short-Uved products, such as coffins from veneered particle board. Not all types of wood-based boards are suitable for aU uses. 

In the manufacture of cement-bonded particle board, wood chips are mixed with Portland cement in the ratio 60% cement, 20% wood and 20% water. Curing times are very long - about 7 h at 70-80 °C, followed by a further 2-3 weeks conditioning at ambient temperature. The boards produced have high density (1300 kg m , double normal particle board), but have exceptional dimensional stability, moisture and fire resistance compared to other wood-based boards. 

Amino resins are manufactured throughout the industrialized world to provide a wide variety of useful products. Adhesives (qv), representing the largest single market, are used to make plywood, chipboard, and sawdust board. Other types are used to make laminated wood beams, parquet flooring, and for furniture assembly (see Wood-BASED composites and laminates). 

Adhesives and resins are one of the most important raw materials in wood-based panels. Thus, each question concerning the life cycle assessment and the recycling of bonded wood panels does bring into question the adhesive resins used. This includes, for example, the impact of the resin on various environmental aspects such as waste water and effluents, emission of noxious volatile chemicals during production and from the finished boards, or the reuse for energy generation of wood panels. The type of resin has also a crucial influence on feasibility and efficiency for several material recycling processes. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Obviously, the first measure to think about could be the substitution of recycled fibre-based boxboard for foodstuff by virgin fibre-based material. The required amount of recycled fibre-based boxboard for foodstuff equals to 700,000 t/year in Germany [7]. For the substitution of the recycled fibre-based boxboard, it is simply assumed that the required virgin fibre-based board is produced totally from mechanical spruce pulp with a process yield of 97%. This would require in total additionally about 1.68 Mio m3 wood per year. The inventory study of the carbon balance of the German forests [8] indicates an average yearly wood growth of... 

A general state-of-the-art plus bibliography was published by Harkin and Rowe in 1969 (11). Included was a section labeled "Wood-base Materials," which covered insulation board, hardboard, fiberboard, and particleboard containing bark. 

The weathering process described thus far has been for solid wood. The introduction of another variable, the adhesive, in the weathering of wood-based materials such as plywood and particle board creates additional complications. Wood substance is still ex-... 

Sawn wood Veneer sheets Pl) ood Particleboard Fibre-based board ... 

Table 11.2 shows 2003 production of both sawn timber and wood panels for various regions of the world. Structural panel production is dominated by North America, mainly because structural panel products have enjoyed a dominant position in residential construction. The United States is a substantial manufacturer of softwood plywood for domestic production but only 10% is exported whereas half that volume is imported as tropical hardwood plywood. Europe manufactures and uses comparatively little plywood and OSB. Instead the region relies on its own lower quality domestic wood resources for the manufacture of other wood-based panels, e.g. particleboard and fibre-based board. Amazingly, panel production in Asia equals that of lumber, reflecting the inter-regional China-centric supply chain. 

WOOD-BASED PANELS PARTICLEBOARD, FIBREBOARDS AND ORIENTED STRAND BOARD... 

Wood-based panels such as particleboard, oriented strand board and medium density fibreboard eonsist of particles of widely varying shape and size bonded together with an adhesive system. 

In China, formaldehyde emission of the pressed-wood products sold is generally high. A test showed that the average formaldehyde release rates of four kinds of typical wood-based panels, i.e., particle board, medium-density... 

PVC-based boards, 36 Select grade oak, 35, 480 Solid boards, 36 Wood species, 35... 

Natural wood-based flooring materials range from plain wood boards and parquets to finished, assembled plank parquet composites of various wood species. Cork materials are manufactured from the outer layer of the tree, mainly oak. 

It has already been mentioned that a large number of components are used for the construction of wood-based furniture. In addition to the compounds described earlier, a large spectrum of volatile organics can be found in emission studies (Salthammer, 1997b). The main sources for formaldehyde (which is not considered here), phenol and acetic acid are substrates such as particle board and MDF. BHT is a common antioxidant. Volatile plasticizers include dimethyl phthalate (DMP), dibutyl phthalate (DBP), as well as esters of adipic acid and sebacic acid. Further important compound groups are amines, siloxanes, carboxylic acids and naphthalenes. The identification of special substances does not only require suitable analytical equipment. Both experience and detailed knowledge of the chemical composition of furniture are also necessary. 

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