|CERES No. 121 (FAO Ceres, 1988, 50 p.)|
by Susan Riddle
Most people, unless manufacturers or carpenters by trade, could be forgiven for thinking that wood products have changed little over the years, or that, say, hardboard or plywood is made the same as it was twenty years ago. They would be surprised to discover that new uses for wood products are constantly being found even in an age abounding in steel, aluminium, and plastics. Wood-based panels can be produced in an almost unlimited variety of qualities, shapes, and sizes thanks to new processing improvements.
In fact, some solid wood products are becoming a thing of the past as science and technology continue to come up with new ways of fully exploiting wood as a means of extending the forest resource.
Many small-diameter logs of poor quality that used to be considered uneconomical to collect and were simply left to rot, can now be peeled for veneer thanks to sophisticated engineering designs and low-cost computerized systems. In the United States a newly developed spindleless lathe can peel these logs down to a core of just 50 mm. This means considerable savings and better utilization of wood for a country whose annual production of wood equals the total tonnage of plastics, cement, steel, and all other minerals combined.
Now that researchers are creating wood-based panels with improved durability, weatherability, increased mechanical properties, and reduction of creep under load, the construction industry can reduce its dependence on solid wood without reducing quality. John Youngquist of the Forest Products Laboratory of the US Department of Agriculture in Madison, Wisconsin, speaking about future potential uses for new products says, "We are likely to see more structural components made from wood composites: shaped structural sections; and thick composite products for windows and door components. New chemical treatments will make these products very resistant to decay, insect attack, and damage by fire. One great advantage of all these products is that they can be made in long, thick sections that have very uniform properties. The continuous production of many of these is now in the pilot stage and will soon be in full production."
It is not surprising therefore that world production of wood-based panels is expected to expand from a current 109 million m3 to more than 167 million m3 by the year 2 000 at an average growth of 3 per cent a year. Developing countries and centrally planned economies together will contribute half of that increase. From only 42 million m3 in 1965, total world production had already doubled by 1985, the developing market economies raising their output from 2.3 million m3 to 16.5 million m3. Indonesia alone expanded from less than 4 000 m3 to an incredible 4 million m3, becoming the country with the largest annual volume of exports of wood-based panels.
Veneer. The relationship of veneer to other raw materials composed of particle and fibre can be seen in the figure on page 45. Whereas fibres and particles are used to make wood composites, veneer is peeled directly from the log to a thickness of approximately 1.5 mm to be used for plywood or bonded sheets. New lathe systems allow logs to be stabilized during peeling to prevent "spin-out", and peeled to a lower diameter core of only 50 mm. First, however, the species to be used must be studied carefully to avoid peeling low-quality veneer from near the centre of the tree.
Laminated Veneer Lumber- a new technology. Rising costs and shortages of high-grade solid-sawn lumber for parallel-chord trusses and scaffolding planks inspired the development of Laminated Veneer Lumber (LVL) in the early 1980s. The processes and uses of this relative newcomer are still evolving. Thanks to great savings in weight and material on fabricated components, LVL is opening up new markets. Its consistency of mechanical and dimensional properties and the absence of defects which reduce strength make it an ideal product for light trusses and I-section beams for joints and rafters in light-frame construction. Other uses for this versatile product are: truck-decking, box beams, and door rails. LVL's success in scaffolding is due to its uniformity of properties and resistance to splitting. The industry new uses veneers 3.2 mm-2.5 mm in thickness, which are hot-pressed with phenol-formaldehyde adhesive into lengths from 2.4 m to 18 m or more. Joints between individual veneers are staggered along the LVL to avoid gross strength-reducing defects.
Plywood. With 3000 years of history behind it, plywood still interests researchers. Plywood shows more resistance to splitting than solid wood where large sheet applications are required and veneer can be peeled thinner to utilize as much of the wood as possible. These features make plywood very attractive to an industry concerned with reducing both its costs and its dependency on solid wood.
Plywood is made of veneer sheets bonded together, in the direction of the grain, in alternate plies generally at right angles. For certain types of board, one or more pairs of veneers may be laid with the grain parallel; the veneer sheets are usually placed symmetrically on both sides of a central ply or core which may or may not be veneer. This is called veneer plywood. Core plywood (so called because it has a core or central layer thicker than other plies) is solid and consists of narrow planks, blocks, or strips of wood placed side by side and often glued together. Blackboard, laminboard, and battenboard are all variations of core plywood. Other types of plywood include cellular beard, with a core of cellular construction, and composite plywood, for which core materials other than solid wood or veneers are used. Plywood can be flatpressed or moulded.
For construction and industrial application, softwoods such as Douglas fir (Pseudotsuga mensiesii) or pine (Pinus radiata or Pinus taeda) provide the needed characteristics where strength is of principal importance. But where appearance is more important than strength (for furniture or wall panels) different species of hardwoods are used.
The ancient craft of plywood production has recently been updated to cope with the need for improved softwood plywood products destined for the construction industry. Ultrasonic veneer block scanning, computerized X-Y peeler block chargers, powered back-up rolls, powered nose bars, and linear-positioned lathe knife carriages are just a few new technological innovations which enhance the efficiency of lathe systems. A further breakthrough has been the development of phenolic glues to cope with veneers with a moisture content as high as 20 per cent (the result of steam applications to soften the veneer). These glues give a better panel yield which permits enormous savings in production costs. For hardwood plywood panels, it has been necessary to change the composition of the traditional urea-formaldehyde resin to reduce formaldehyde emission meet recent standards. Replacing 5-20 per cent of the resin with polyvinyl acetate has eliminated problems arising from the change of property of the resin.
Blockboard - an old technology. Statisticians rarely appreciate the virtues of blockboard at the macro-economic planning level and often even ignore its existence and simply lump it in with plywood. But it costs a lot less than all-veneer plywood. And it offers a way to utilize poor wood resources. Thus it could be very interesting for developing countries. Blockboard can be produced relatively simply without enormous investments in production technology using the existing machinery of the plywood mill.
Sawlog and peeler-log waste are ideal for making blockboard cores. Indeed, the absence of board splitting or warping, and its large continuous surface, make blockboard a keen competitor of particle board in some countries. Interestingly enough, because blockboard is smoother on tools than is particle board, many carpenters and craftsmen find it easier to work with, but, perhaps as the result of a growing interest in particle board, its popularity in the developed world declined and it was relegated to the status of a minor product where it has remained ever since.
Particle-based panel materials. Particle board was invented in the 1950s as a way to make use of wood-waste and mill-waste. Many other new composition materials followed, all tailor-made to suit a wide variety purposes according to the properties acquired during processing. Since then, the development of new particle-generating machines, has resulted in better edge-profiling using smaller particles.
Other subgroups in the wood-base particles class were also introduced on the market. These were waferboard, oriented waferboard, oriented strandboard, and cement board.
Particle board as core stock for furniture and cabinet application or as floor underlay in light-frame construction ensures a smooth hard surface to which a finish of veneer, highdensity plastic, or ultraviolet-cured vinyl (in applications of 0.038 mm ultraviolet-cured vinyl, fine particles are applied to the surface to reduce show-through) may be applied.
Developed to utilize small-diameter logs, the waferboard and oriented strandboard have these advantages: lower wood costs; the possibility of producing large-size panels for mobile homes and prefabricated housing; and savings on labour costs because of high-automation of production lines. Now, in Canada and the United States, formaldehyde-free isocyanate-bonded waferboard for waferboard siding (house side-walls) is available which can be overlayed with an embossed resin-impregnated paper and its edges coated to prevent water absorption from rain. One disadvantage of these products, however, is that they require half again as much wood by weight as plywood does to produce the same usable volume or surface area of panel.
Cement board products have shown to be particularly suitable for buildings exposed to tropical and subtropical climates because they are resistant to fire, decay, and insect attack. This will be a big boon for Mexico, which, according to Gilberto Rosas-Solarzano of the Guadiana Group that produces prefabricated housing, will need 11 million dwellings by the year 2000.
Fibre-based panels. While developing countries and centrally planned economies are increasing production of fibreboard, the developed economies are curtailing theirs. This has led to a stagnation in world production over the last ten years. Medium-density fibreboard (MDF) can cope with a wide variety of tree species and readily available raw materials such as pulp chips, planer shavings, plywood trim, and sawdust. MDF is in fact beginning to replace particle board, solid wood, and plywood in some furniture applications because of its smooth surface which facilitates overlaying, routing, and moulding, and because its tight edges need not be edge-banded. Because of its versatility, manufacture of MDF is expected to expand significantly during the next decade.
The familiar product hardboard is now produced by wet or dry process using different types of adhesives depending on the process and intended use. Moulded hardboard siding and an embossed tile board - where the tile lines are embossed rather than cut - are the latest developments in wet-process hardboard production. Because insulation board serves a relatively constant market, no significant production innovations have been introduced in the past few years apart from tile-type panels commonly called "lay-in ceiling panels" that are currently being produced to be used in suspended ceiling systems. Unfortunately, insulation board has seen keen competition from insulating foam plastics and mineral fibres with dire consequences to the market.
Wood waste. The introduction of standards to curb formaldehyde emissions in boards destined for furniture, mobile homes, and prefabricated housing forced some centrally developed and developing countries to restructure their adhesive systems or else lose valuable trade. To reduce their dependency on expensive chemicals and oil, they found that considerable savings could be made by making more efficient use of wood waste.
Of economic importance are the condensable tannin resins "polyphenols" extracted from the bark of pine and mimosa and from the wood of quebracho, as well as tannin formaldehyde (tf) resin extracted from wattle, chestnut, spruce, larch, and pine. These resins offer good blending, fast curing, low formaldehyde emission, and low cost. And they do not pollute water either.
Developing countries that wish to reduce their adhesive and energy costs must reduce their dependency on chemicals and oil. But for those industrialized countries such as Canada and the United States which are already using oil and natural gas for their production and energy needs, waste disposal has become a problem. Accumulation of waste may occur particularly when the amounts are not sufficient to make panel production profitable given that existing large-scale machinery costs a lot of money. Additionally, big-machine manufacturers may not be willing to produce smaller and economically viable machinery to suit small-scale production. Mr W. Caine, president of the Commonwealth Plywood Co. Ltd. of Quebec, says, "We are looking for a type of extruding machine that could take wood waste, such as sawdust, and glue it into narrow boards for moulding, which could then be wrapped with thin veneers, vinyls or other products. At present it means investing $50 million to make a 5 foot by 8 foot (1.52 cm x 2.43 cm) or larger panel that is then ripped into narrow strips and moulded."
But could it be possible to exploit waste without having to make enormous investments in expensive machinery? The question of waste disposal is a complex one and is influenced by economic constraints and problems related to transportation both in industrialized and developing countries. Some developing countries may find a small solution in attaching a local industry to an already existing plymill. John Youngquist says, "The allocation of raw materials within a given wood panel manufacturing facility is one of economics. Residues not used to produce composite wood products could, for example, be used to produce compressed fuelwood blocks. Poland already produces fuel briquettes from bark mixed with wax."
Perhaps science and technology will come up with some answers on how further to utilize wood waste as the search continues for new ways of fully exploiting wood as a means of extending the forest resource.