MECHANICAL PROCESSING OF TIMBER
Sawing and planing: The basic requirement of these processes is quite simply to produce as efficiently as possible timber of the required dimensions having a quality of surface commensurate with the intended use. Such a requirement depends not only on the basic properties of the timber, but also on the design and condition of the cutting tool. Many of the variables are inter‑related and it is frequently necessary to compromise in the selection of processing variables.
The density of timber vary by a factor of ten from about 120 to 1200 kg/m3. As density increases, so the time taken for the cutting edge to become blunt decreases; whereas it is possible to cut over 10 000 feet of Scots pine before it is necessary to resharpen, only one or two thousand feet of a dense hardwood such as jarrah can be cut.
Density will also have a marked effect on the amount of power consumed in processing. When all the other factors affecting power consumption are held constant, this variable is highly correlated with the density of the timber, as illustrated in Fig. 1. Timber of high moisture content never machines as well as that at lower moisture levels. There is a tendency for the thin‑walled cells to be deformed rather than cut because of their increased elasticity when wet.
After the cutters have passed over, these deformed areas slowly resume their previous shape, resulting in an irregular appearance to the surface that is very noticeable when the timber is dried and painted; this induced defect is known as raised grain. The cost of timber processing is determined primarily by the cost of tool maintenance, which in turn is related not only to properties of the timber, but also to the type and design of the saw or planer blade.
In addition to the effect of timber density on tool life, the presence in certain timbers of gums and resins has an adverse effect because of the tendency for the gum to adhere to the tool thereby causing overheating. In saw blades this in turn leads to loss in tension, resulting in saw instability and a reduction in sawing accuracy. A certain number of tropical hardwood timbers contain mineral inclusions that develop during the growth of the tree. The most common is silica, which is present usually in the form of small grains within the ray cells. The abrasive action of these inclusions is considerable, and the life of the edge of the cutting tool is frequently reduced to almost one-hundredth of that obtained when cutting timber of the same density, but free of silica.
Timbers containing silica are frequently avoided unless they possess special features that more than offset the difficulties resulting from its presence. The moisture content of the timber also plays a significant role in determining the life of cutting tools. As moisture content decreases, so there is a marked reduction in the time interval between re-sharpening both saw and planer blades. The fibrous nature of tension wood will also increase the wear on tools. Service life will also depend on the type and design of the tool.
Although tungsten carbide-tipped blades are considerably more expensive than steel ones, their use extends the life of the cutting edge, especially where timbers are either dense or abrasive. Increasing the number of teeth on the saw or the number of planer blades on the rotating stock will increase the quality of the surface, provided that the feed speed is sufficient to provide a minimum bite per revolution; this ensures a cutting rather than a rubbing action, which would accelerate blunting of the tool’s edge. One of the most important tool design variables is the angle between the edge and the timber surface.
Timber is seldom straight grained, the grain tending in most cases to be in the form of a spiral of low pitch, though occasionally it is interlocked or wavy. In these circumstances, there is a strong tendency for those cells that are inclined towards the direction of the rotating cutter to be pulled out rather than cut cleanly, a phenomenon known as pick‑up or tearing.
This defect can be eliminated almost completely by reducing the cutting angle (rake angle) of the rotating blades, though this will result in increased power consumption. The cost of processing – though determined primarily by tool life – will be influenced also by the amount of power consumed. In addition to the effect of the density of the timber, as previously discussed, the amount of energy required will depend on the feed-speed (Fig. 1), tool design and, above all, on tool sharpness.
Steam bending: Steam bending of certain timbers is a long established process that was used extensively when it was fashionable to have furniture with rounded lines. The backs of chairs and wooden hat stands are two common examples from the past, but the process is still employed at the present time, albeit on a much reduced scale. The handles of certain garden implements, walking sticks and some sports goods are all produced by steam bending.
The mechanics of bending involves a pre-steaming operation to soften the lignin, swell the timber, and render the timber less stiff. With the ends restrained, the timber is usually bent round a former, and after bending the timber must be held in the restrained mode until it dries out and the bend is set. In broad terms the deformation is irreversible, but over a long period of time, especially with marked alternations in humidity of the atmosphere, a certain degree of recovery will arise, especially where the curve is unrestrained by some fixing.
Although most timbers can be bent slightly, only certain species, principally the hardwood timbers of the temperate region, can be bent to sharp radii without cracking.
When the timber is bent over a supporting, but removable, strap, the limiting radius of curvature is reduced appreciably. Thus, it is possible to bend 25 mm thick ash to a radius of 64 mm and walnut to a radius of only 25 mm.
As a material, timber has a number of deficiencies:
- it possesses a high degree of variability
- it is strongly anisotropic in both strength and moisture movement
- it is dimensionally unstable in the presence of changing humidity
- it is available in only limited widths.
Such deficiencies can be improved appreciably by reducing the timber to small units and subsequently reconstituting it, usually in the form of large, flat sheets, though moulded items are also produced, e.g. trays, bowls, coffins, chair backs. The degree to which these boards assume a higher dimensional stability and a lower level of anisotropy than is the case with solid timber depends on the size and orientation of the component pieces of timber and the method by which they are bonded together.
By comparison with timber, board materials possess a lower degree of variability, lower anisotropy, and higher dimensional stability, and they are also available in very large sizes. The reduction in variability is due quite simply to the random repositioning of variable components, the degree of reduction increasing as the size of the components decreases. The area of board materials is regarded as the fastest growing area within the timber industry over the last three decades.
Not only does this represent a greater volume of construction (particularly in the domestic area) and of consumer goods (e.g. furniture), but it also reflects a large degree of substitution of board materials for solid timber. There is a vast range of board types though there are only four principal ones – plywood, particle‑ board (chipboard), oriented strand board (OSB) and medium density fibreboard (MDF).
It will be appreciated that the world production of wood-based panels has increased to a point where in 2005 it was equal to almost 50% of the volume of solid timber used. The volume of particleboard and plywood each comprised a third of the total volume of wood-based panels.
In the UK the consumption of wood-based panels in 2006 was 6.44 × 106 m3 , of which 49% was particleboard, 21% was plywood, 17% was MDF, 7% was OSB and 6% was other fibreboards, the total value of which was £1.2 billion. Home production accounted for 54% of total consumption of panels. The value of wood-based panels (without secondary processing) consumed by the UK in 2006 was £1.18 billion (TTF, 2008). The individual board materials are discussed in turn below.
Plywood: World consumption of plywood in 2005 was 68.9 × 106 m3, of which about 1.4 × 106 m3 was consumed in the UK. This is, in fact, approximately the UK’s annual consumption, most of which is made from softwood and is imported from the United States, Canada and Finland. Plywoods made from temperate hardwoods are imported mainly from Germany (beech) and Finland (birch – or a birch/spruce combination) while plywoods made from tropical hardwoods come predominately from South-East Asia (mainly Indonesia and Malaysia), South America and to a lesser, but increasing extent, from Africa.
Logs, the denser of which are softened by boiling in water, may be sliced into thin veneer for surface decoration by repeated horizontal or vertical cuts, or, for plywood, peeled by rotation against a slowly advancing knife to give a continuous strip. After drying, sheets of veneer for plywood manufacture are coated with adhesive, laid up with the grain direction at right angles in alternate layers and then pressed.
Plywood frequently contains an odd number of plies so the system is balanced around the central veneer; some plywoods, however, contain an even number of plies, but with the two central plies having the same orientation, thereby ensuring that the plywood is balanced on each side of the central glue line. As the number of plies increases, so the degree of anisotropy in both strength and movement drops quickly from the value of 40:1 for timber in the solid state. With three‑ply construction and using veneers of equal thickness, the degree of anisotropy is reduced to 5:1, while for nine‑ply this drops to 1.5:1.
However, cost increases markedly with number of plies and for most applications a three‑ply construction is regarded as a good compromise between isotropy and cost. The common multilayered plywood is technically known as a veneer plywood, in contrast to the range of core plywoods, in which the surface veneers overlay a core of blocks or strips of wood. Plywood (veneer type) for use in construction in Europe must comply with the requirements of one part of the European specification BS EN 636, of which the most important requirement is that of bond performance.
The mechanical and physical properties of plywood, therefore, depend not only on the type of adhesive used, but also on the species of timber selected. Both softwoods and hardwoods within a density range of 400–700 kg/m3 are normally utilised. Plywood for internal use is produced from the non-durable species and urea–formaldehyde adhesive (UF), while plywood for external use is generally manufactured using phenol–formaldehyde (PF) resins. However, with the exception of marine grade plywood, in the UK durable timbers, or permeable non-durable timbers that have been treated with preservative, are seldom used.
It is not possible to talk about strength properties of plywood in general terms since not only are there different strength properties in different grain directions, but these are also affected by configuration of the plywood in terms of number, thickness, orientation and quality of the veneers and by the type of adhesive used.
The factors that affect the strength of plywood are not necessarily the same for the strength of timber. Thus intrinsic factors, such as knots and density, play a less significant part than they do in the case of timber, but the effects of extrinsic variables such as moisture con‑ tent, temperature and time are very similar to those for timber. Plywood is the oldest of the timber sheet materials and for very many years has enjoyed a high reputation as a structural sheet material.
Its use in the Mosquito aircraft and gliders in the 1940s, and its subsequent performance in the construction of small boats, in sheathing in timber‑frame housing, and in the construction of web and hollow‑box beams all bears testament to its suitability as a structural material. When materials are compared in terms of their specific stiffness (modulus of elasticity per unit mass), plywood is stiffer than many other materials, including mild steel sheet; generally, plywood also has a high specific strength.
Another important property of plywood is its resistance to splitting, which permits nailing and screwing relatively close to the edges of the boards. This is a reflection of the removal of a line of cleavage along the grain, which is a drawback of solid timber. The impact resistance (toughness) of plywood is very high, and tests have shown that to initiate failure a force greater than the tensile strength of the timber species is required.
Plywoods tend to fall into three distinct groups. The first comprises those that are capable of being used structurally. Large quantities of softwood structural plywood are imported into the UK from North America, supplemented by smaller volumes from Sweden and Finland. The latter country also produces a birch/spruce structural plywood.
The use of this group of structural plywoods in Europe is controlled in that they must first comply with BS EN 636 and second, where limit state design is used with Eurocode 5, the characteristic values for use in design must have been derived from semi-sized test pieces according to European test methods. These values are presented in BS EN 12369-2.
The second group of plywoods comprises those that are used for decorative purposes, while the third group comprises those for general-purpose use. The latter are usually of very varied performance in terms of both bond quality and strength and are frequently used indoors for infill panels and certain types of furniture.
Particleboard (chipboard): In the UK boards made from wood chips and resin were originally known as chipboards, but with the advent of European standards, the product is now referred to as particleboard. The particleboard industry dates from the mid‑forties and originated with the purpose of utilising waste timber.
After a long, slow start, when the quality of the board left much to be desired, the industry has grown tremendously over the last four decades, far exceeding the supplies of waste timber available and now relyng to a very large measure on the use of small trees for its raw material. Such a marked expansion is due in no small part to the much tighter control in processing and the ability to produce boards with a known and reproducible performance, frequently tailor-made for a specific end use.
In 2006 the UK consumption of particleboard was 3.09 × 106 m3, 74% of which was home-produced (TTF, 2008). Particleboard consumption for the whole of Europe in the same year was 33.48 × 106 m3. In the manufacture of particleboard the timber, which is principally softwood, is cut by a series of rotating knives to produce thin chips, which are dried and then sprayed with adhesive. Usually the chips are blown onto flat platens in such a way that the smaller chips end up on the surfaces of the board and the coarse chips in the centre.
The mat is usually first cut to length before passing into a single or multi-daylight press where it is held for 0.10–0.20 min per mm of board thickness at temperatures of up to 200oC. The density of the boards produced ranges from 450 to 750 kg/m3 , depending on end‑use classification, while the resin content varies from about 9–11% in the outer layers to 5–7% in the centre layer, averaging out for the board at about 7–8% on a dry mass basis.
Over the last two decades most of the new particleboard plants have installed large continuous presses. As the name implies, the mat is fed in at one end to reappear at the other end as a fully-cured board. This type of press has the advantage of being quick to respond to production changes in board thickness, adhesive type or board density. Particleboard can also be made continuously using an extrusion process in which the mat is forced out through a heated die.
Though a simple process, this results in the orientation of the chips at right angles to the plane of the board, which reduces both the strength and stiffness of the material. Extruded board is used primarily as a core in the manufacture of doors and composite panels. The performance of particleboard, like that of plywood, is very dependent on the type of adhesive used.
Much of the particleboard produced in Europe is made using urea–formaldehyde (UF) resin which, because of its sensitivity to moisture, renders this type of particleboard unsuitable for use where there is a risk of the material becoming wet, or even being subjected to marked alternations in relative humidity over a long period of time.
More expensive boards possessing some resistance to the presence of moisture are manufactured using melamine fortified urea–formaldehyde (MUF), or phenol– formaldehyde (PF) or isocyanate (IS) adhesives; however, a true external‑grade board has not yet been produced commercially. Particleboard, like timber, is a viscoelastic material. However, the rate of creep in particle‑ board is considerably higher than that in timber, though it is possible to reduce it by increasing the amount of adhesive or by modifying the chemical composition of the adhesive.
Within the new framework of European specifications, six grades of particleboard are specified in BS EN 312, of which four are rated as load bearing (i.e. they can be used in structural design) and two are non-load bearing. The characteristic values for the load-bearing grades for use in structural design are given in BS EN 12369-1. Particleboards are also produced from a wide variety of plant material and synthetic resin, of which flaxboard and bagasse board are the best known examples.
MDF (dry-process fibreboard): There has been a phenomenal increase in the production of MDF worldwide, with a forty-fold increase from 1980 to 2005. Production in 2005 was over 41 × 106 m3 and with planned expansion, production capacity would increase to nearly 57 × 106 m3 by the end of 2008 (Wadsworth, 2007). European production rose from 0.58 × 106 m3 in 1986 to 14.2 × 106 m3 in 2005, of which 0.8 × 106 m3 was produced in the UK. In 2006 UK production was 0.84 × 106 m3 , a volume that represented 68% of consumption.
MDF is manufactured by a dry-process in contrast to other types of fibreboard. The fibre bundles are first dried to a low moisture content before being sprayed with an adhesive and formed into a mat, which is hot-pressed to produce a board with two smooth faces similar to the production of particle‑ board. Both multi-daylight and continuous presses are employed.
Various adhesive systems are employed; where the board will be used in dry conditions a UF resin is employed, while a board with improved resistance to moisture for use in humid conditions is usually manufactured using an MUF resin, though PF or IS resins are sometimes used. The European specification for MDF (BS EN 622-5) includes both load-bearing and non load bearing grades for both dry and humid end uses. Characteristic values for structural use of the former are given in BS EN 12369-1, however it should be noted that the use of load-bearing panels under humid conditions is restricted to only short periods of loading. A very large part of MDF production is taken up in the manufacture of furniture, where non-load bearing grades for dry use are appropriate.
Up until 2005 MDF was produced in thicknesses greater than 8 mm, but recently (2006–2008) a number of new plants have come on-stream producing ‘thin MDF’ (1.5–4.5 mm) thereby extending the use of MDF into a large number of new applications.
Wet-process fibreboard: Fibreboard can also be produced using a wet process, which was the original method of fibreboard production before the advent of MDF. Production levels of wet-process boards have fallen over the years, but it is still used in certain applications, such as insulation and the linings of doors and backs of furniture in the UK, and as a cladding and roofing material in Scandinavia. The process of manufacture is quite different from that of the other board materials in that the timber is first reduced to chips, which are then steamed under very high pressure in order to soften the lignin, which is thermoplastic in behaviour.
The softened chips then pass to a defibrator, which separates them into individual fibres or fibre bundles without inducing too much damage. The fibrous mass is usually mixed with hot water and formed into a mat on a wire mesh; the mat is then cut into lengths and, like particleboard, pressed in a multi-platen hot press at a temperature of from 180 to 210oC. The board produced is smooth on one side only, the underside bearing the imprint of the wire mesh.
By modifying the pressure applied in the final pressing, boards of a wide range of density are produced, ranging from softboard (with a density of less than 400 kg/m3 ) through medium‑ board (with a density range of 400–900 kg/m3 ) to hardboard (with a density exceeding 900 kg/m3 ).
Fibreboard, like the other board products, is moisture sensitive, but in the case of hardboard a certain degree of resistance can be obtained by passing the material through a hot oil bath, thereby imparting a high degree of water repellency to the material, which is referred to as tempered hardboard. The European specifications for the various types of wet-process fibreboards are BS EN 622-2 for hardboard, BS EN 622-3 for medium board and BS EN 622-4 for softboard. UK consumption of wet-process fibreboard in 2006 was 0.37 × 106 m3, all of which was imported.
OSB (oriented strand board) Like MDF, OSB production capacity has grown and continues to grow at a very fast rate. From a world‑ wide capacity in 1997 of 16 × 106 m3 this has grown to 30 × 106 m3 in 2006, 85% of which was produced in North America. European production in 2006 was 3.9 × 106 m3, and UK production from one mill was 0.265 × 106 m3. Strands up to 75 mm in length with a maximum width of half its length are generally sprayed with an adhesive at a rate corresponding to about 2–3% of the dry mass of the strands.
It is possible to work with much lower resin concentrations than with particleboard manufacture owing to the removal of dust and ‘fines’ from the OSB line prior to resin application. In a few mills powdered resins are used, though most manufacturers use a liquid resin. In the majority of mills a PF resin is used, but in one or two mills a MUF or IS resin is employed.
In the formation of the mat the strands are aligned either in each of three layers, or only in the outer two layers of the board. The extent of orientation varies among manufacturers with property level ratios in the machine to cross direction of 1.25/1 to 2.5/1, thereby emulating plywood.
Indeed, the success of OSB has been as a cheaper replacement for plywood, but it must be appreciated that its strength and stiffness are considerably lower than those of high-quality structural-grade plywood, though only marginally lower than those of many of the current structural softwood plywoods.
It is widely used for suspended flooring, sheathing in timber-frame construction and flat roof decking. The European specification for OSB (EN 300) sets out the requirements for four grades, three of which are load bearing, covering both dry and humid applications. Characteristic values for structural use of the load-bearing grades are given in EN 12369-1.
CBPB (cement bonded particleboard): This is very much a special end-use product manufactured in relatively small quantities. It comprises by mass 70–75% Portland cement and 25–30% wood chips similar to those used in particleboard manufacture. The board is heavy, with a density of about 1200–kg/m3, but it is very durable (owing to its high pH of 11), is more dimensionally stable under changing relative humidity (owing to the high cement content), has a very good performance in reaction to fire tests (again because of the high cement content) and has poor sound transmission (owing to its high density).
The board is therefore used in high-hazard situations with respect to moisture, fire or sound. The European specification for CBPB (BS EN 634-2) sets out the requirements for a single grade, while characteristic values have to be obtained from the manufacturer.
Comparative performance of the wood-based boards: With such a diverse range of board types, each manufactured in several grades, it is exceedingly difficult to select examples in order to make some form of comparative assessment. In general terms, not only are the strength properties of good quality structural softwood plywood considerably higher than those of all the other board materials, but they are usually similar to, or slightly higher than, those of softwood timber.
Next to a good quality structural plywood in strength are the hardboards, followed by MDF and OSB. Particleboard is of lower strength, but still stronger than the medium boards and CBPB.
In passing, it is interesting to note the reduction in anisotropy in bending strength from 4.5 for 3-ply construction to 1.8 for 7-ply lay-up. Other structural softwood plywoods can have strength values lower than those of Douglas fir, being similar to, or only slightly above, those of OSB of high quality. Actual strength values of individual manufacturer’s products of non-plywood panels may be higher than these minimum specification values. It should be realised that these specification values are only for the purpose of quality control and must never be used in design calculations. CBPB is far superior to all other boards.
Even higher swell values than those recorded in the table, of 25% for 15 mm OSB/1 (general purpose board) and 35% for 3.2 mm HB.LA hardboard (load-bearing, dry), can be found.
The process of cutting up timber into strips and gluing them together again has three main attractions. Defects in the original piece of timber such as knots, splits, reaction wood, or sloping grain are redistributed randomly throughout the composite member, making it more uniform in quality than the original piece of timber, where the defects often result in stress raisers when load is applied.
Consequently, the strength and modulus of elasticity of the laminated product will usually be higher than those of the timber from which it was made. The second attraction is the ability to create curved beams or complex shapes, while the third is the ability to use shorter lengths of timber, which can be end-jointed.
Glulam: Glulam – the popular term for laminated timber – has been around for many years, and can be found in the form of large curved beams in public buildings and sports halls. In manufacture, strips of timber about 20–30 mm in depth are coated with adhesive on their faces and laid up parallel to one another in a jig, the whole assembly being clamped until the adhesive has set.
Generally, cold-setting adhesives are used because of the size of these beams. For dry end use a UF resin is employed, while for humid conditions a resorcinol–formaldehyde (RF) resin is employed. The individual laminae are end-jointed using either a scarf (sloping) or finger (interlocked) joint.
Structural characteristic values for glulam are determined by the srength class of the timber(s) from which it is made, factored for the number and type(s) of laminates used.
Vertical studs and structural beams: The need to increase yields of medium-density structural timber has focused attention in the last decade on the upgrading of lower-grade timber. This has been achieved in a manner similar to that used for glulam in that battens are cross-cut to remove knots and other defects, dried to 10–12% moisture con‑ tent, their ends finger-jointed and coated with a durable adhesive (usually PF) prior to assembly into a long batten which, because it is only a single member in thickness, can be heat-cured (unlike glulam).
These composite beams and studs are again much stronger and stiffer than the original component parts. A fairly recent extension of this concept has been the gluing of green timber, thereby eliminating the time and cost of kilning. This process has been made possible by the development of adhesives with low penetration of the timber and high rates of curing.
The best known of these adhesives resulted from research initiated in 1988 by the New Zealand Forest Research Institute, which led to the creation of the Greenweld process in 1990. In 1993 the first Greenweld mill in New Zealand was commissioned, and subsequent mills have been built in New Zealand, Australia, Canada and the USA. The process uses a specific phenol–resorcinol–formaldehyde resin and an accelerator to give a 5-minute close-dpress time using conventional finger-jointing machines on timber with a moisture content of up to 180%, and at temperatures down to 0oC.
Greenweldbonded timber may be used for structural applications in the USA provided it has been certified as passing a number of specific tests (Stephens, 1995; Garver, 1998). So far (2008), Greenweld products have not been included in any European standard for structural timber. Engineered structural lumber 56.1.4 The following three products are similar in concept to glulam, but are formed from much smaller wooden components.
Laminated veneer lumber (LVL): LVL is produced from softwood logs that are rotary peeled to produce veneers 3 mm in thickness which, after kiln drying, are coated with a PF adhesive and bonded together under pressure to produce a large panel 24 m in length. This is then sawn into structural battens. Characteristic values for design use, which are from 50% to 100% higher than corresponding structural softwood timber, are avail‑ able from the two European and one Canadian manufacturer. The European specification for LVL is BS EN 14279.
Parallel strand lumber (PSL): PSL is a North American product in which the 2.5 mm thick rotary peeled veneer of Douglas fir or Southern pine is cut into strands 2.4 m in length and 3 mm in width, which are then coated with a resin, pressed together and microwave-cured to produce battens up to 20 m in length.
Laminated strand lumber (LSL): LSL is another North American product in which aspen veneer is cut into strands 300 mm in length and 10 mm in width and coated with IS resin before being aligned parallel to each other and pressed into thick sheets, which are cut up to produce battens.
Pulp may be produced by either mechanical or chemical processes and it is the intention to post‑ pone discussion on the latter until later in this chap‑ ter. In the original process for producing mechanical pulp, logs with a high moisture content are fed against a grinding wheel, which is continuously sprayed with water in order to keep it cool and free it of the fibrous mass produced.
The pulp so formed, known as stone groundwood, is coarse in texture, comprising bundles of cells rather than individual cells, and is mainly used as newsprint. To avoid the necessity to adopt a costly bleaching process only light‑coloured timbers are accepted.
Furthermore, because the power consumed on grinding is a linear function of the density of the timber, only low‑ density timbers with no or only small quantities of resin are used. Much of the mechanical pulp now used is produced by disc‑refining. Wood chips, softened in hot water, by steaming, or by chemical pretreatment, are fed into the centre of two high‑speed counter‑ rotating, ridged, metal plates; on passing from the centre of the plates to the periphery the chips are reduced to fine bundles of cells or even individual cells. This process is capable of accepting a wider range of timbers than the traditional stone ground‑ wood method.
Recycling of Timber Waste
In 2003 (the latest year for which complete data are available) the total wood waste in the UK was 10.5 × 106 tonnes, comprising municipal waste (1.0 × 106 tonnes), commercial and industrial waste (4.5 × 106 tonnes) and waste from construction and demolition (5.0 × 106 tonnes) (WRAP, 2005). Owing to the lack of good quality data, WRAP stress that these figures should be treated as indicative rather than definitive.
In 2003 the manufacture of wood-based panels, principally particleboard, consumed about 1.0 × 106 tonnes of this waste, while horticulture and animal bedding together used 0.2 × 106 tonnes and a similar amount was burned as fuel (WRAP, 2005). Much of the remaining 9 × 106 tonnes would have been incinerated or dumped in landfill.
Since 2003 there has been a significant increase in the amount of wood waste that is now recycled owing in part to significant increases in the cost of timber and in part to the greater public awareness of the need to conserve resources by recycling.
Additionally, recent changes to the ROC regulations now mean that waste in the form of wood-based panels can be chipped and burnt as a fuel in the generation of electricity. A direct result of this has been the commissioning in 2007 of two biomass power-stations, which are discussed below as case studies. More of these stations are in construction or at the planning stage in the UK. The current (2008) use of recycled wood and panels in the UK is set out in Fig. 2.
Although it is not possible to apportion amounts to each end use, it can be safely assumed that particleboard manufacture (see case study 1 below) will be the largest consumer of waste, with energy generation lying in second place (see case studies 2 and 3). A further good example of recycling in Europe is presented as Case study 4.
Case study 1: Many particleboard manufacturers in the UK use a small percentage of chips from waste in the core layer of the board. However, the Sonae plant at Knowsley near Liverpool, with an annual capacity of 450 000 m3 , now produces particleboard that comprises 97% recycled wood in the form of chips, 60% of which originated as pallets. The chips are inspected and contaminants (ferrous and non-ferrous metals, silica, plastic and grit) are removed prior to board formation (WBPI, 2007).
Case study 2: The Sembcorp power station (Wilton 10) was officially opened on 19 November 2007. It cost £60 million to build, is located in the Tees valley and is designed to generate 30 MW of electricity from an annual input of 150 000 tonnes of bone-dry timber (about 300 000 tonnes of wet wood). Forty per cent of this is recycled timber, much of which was previously sent to land‑ fill, and includes wood-based panels, demolition timber, sawmill waste and some sawdust. A further 40% will come from small roundwood and 20% from short-rotation willow coppice (Sembcorp, 2008).
Case study 3: The ‘e-on’ power station at Steven’s Croft, near Lockerbie, Scotland, was commissioned in December 2007. It cost £90 million to construct and was designed to produce 44 MW of electricity from 480 000 tonnes of oven-dry timber per year, 20% of which will be recycled timber and boards, 20% from short-rotation willow coppice and 60% as small round-wood and sawmill co-products (slabs, edgings and sawdust) (e-on, 2008).
Case study 4: Another good example of recycling in the manufacture of boards is provided by the Italian Group Mauro Saviola at their factory in Viadana in northern Italy. Since 1997, 100% of its particleboard, amounting to over a million m3 in 2007 (but due to increase in 2008), has been produced from postconsumer waste.
This is collected by 200 trucks from the urban areas of northern Italy and over 1000 trains gathering material from further afield in Europe; the collected material is first cleaned on site prior to its processing. It is claimed that the use of recycled wood saves the felling of 10 000 trees per day (WBPI, 2008).