Woodchip, made by grinding up logs or waste wood, is the raw material for making wood pulp for paper and rayon manufacture (dealt with in other articles) and the basis of what is termed reconstituted wood. The market for reconstituted wood products, such as chipboard, particle board, and fiberboard, has expanded greatly since their introduction in 1940. World demand for them is currently growing faster than that for solid wood, and it is expected to increase by 3 per cent per annum up to the year 2000.
The industry began as a way of recycling waste and poor quality wood, but the versatility, consistent quality, and manufacturing precision achieved by modern production processes qualify these wood-based boards for a wide range of uses, including panels for walls, ceilings, floors, doors, furniture, automobiles, and toys.
Chipboard and particle board
The production of chipboard begins when cutting blades shred shavings, splinters, and flakes of wood into tiny particles resembling coarse sawdust. These are cleaned, dried in heated tumble drums to remove moisture, and then graded. The chips are passed over a weighing belt that feeds information back to a thermal-setting resin dispenser. The correct amount of resin is dropped onto the chips in a fine spray and is mixed in by huge rotators.
The sticky chips then shower down onto a table, where they build up into mats of coarse sawdust. Thick “mattresses” form, which are first squashed in a cold press, then dampened to counteract condensation before they are clamped in a second, heated press for 10 minutes. They emerge as hard, flat chipboard. After pressing, the sheets are stacked to cool for several days to allow the chips to settle. They are then trimmed and sanded, or coated with varnish, plastic, or resin.
Extrusion is an alternative method of producing chipboard in which the chips are forced horizontally between heated metal lates that determine the thickness of the oard. The resulting material has higher tensile strength, but lower bending strength.
The thickness of chipboard ranges from .25 to 1 inch (6 to 25 millimeters), with coarse chips in the middle and finer chips on the surface. A stronger, lighter board is made up of fine chips throughout. High-density board 40 to 50 pounds per cubic foot (640-800kg/m3) is used for such products as flooring, which rely on strength and stability. Labor costs are also less for laying high-density chipboard than for laying floorboards. Medium-density board 30 to 40 pounds per cubic foot (480-640kg/m3) is used for paneling, furniture, and shelves, whereas low-density board is best for insulation, ceilings, and roof deckings. All untreated chipboards are not weather-resistant.
Waferboard, a stronger version of chipboard, is made of parallel layers of long, waferlike strands of wood, which give them a resilience that matches plywood, but at one-third of the price. Other variations include hy-bridboard, which consists of a particle core with veneers on each surface so that it resembles plywood.
Wood-cement particle board is a product manufactured in Germany, Hungary, and Switzerland that employs cement powder instead of synthetic resins as a binding agent. As a result, the board has excellent dimensional stability and is fire-resistant. Its main potential lies in low-cost housing in tropical regions, and as wall paneling in public buildings.
Like chipboard, hardboard is made from forest thinnings and sawmill waste, but the chips are further reduced to fibers by steam, mincing machinery, or explosion. In the explosion process hot, high-pressure steam bombards the chips until they disintegrate. The fibers are cleaned, screened, sized, and mixed with additives, such as rosin, wax, paraffin, and chemicals, to increase their resistance to decay, fire, insects, and water.
The most popular method of producing sheets is air-felting. Resin adhesive is added to the fibers, and the mixture is fed onto a moving wire mesh to dry. In wet-felting, glue usually is not added because the softened lignin from the wood is sufficient to bind the fibers together.
The mats are cut into lengths and squeezed between rollers in a hot oven or press. The temperature and pressure determine the type and density of board.
To produce wet-felted hardboard, wet sheets are pressed in a machine with metal plates. After pressing, they may be treated with heat and humidity to improve their strength and water-resistance. Higher density boards may be tempered with oil before they are trimmed and packed to make them resistant to abrasion and weathering.
Hardboard has one smooth side and a gauze-marked reverse side. Without the gauze, which allows water and steam to escape during the pressing and heating processes, the boards would explode.
Hardboards are usually high-density, so they are suitable for flooring, shopfitting, flush doors, and furniture. For such products as kitchen cabinets, some hardboards are given a decorative overlay or a coating of enamel or plastic.
Softboard and cork
Unlike hardboard, softboard is not compressed. Instead, the wet sheets are dried in.a hot-air tunnel oven to ensure that a low density of 22 pounds per cubic foot (350kg/m3) or less is maintained. Softboard is ideal for heat and sound insulation in walls, ceilings, and roofs, where a little sagging is acceptable. Bitumen-impregnated board is used where thermal insulation combined with water-resistance is required, as in roofing.
Most cork comes from the thick outer bark of the Mediterranean cork oak (Quercus suber). In summer the bark is stripped away from the living trees, seasoned, and then subjected to a steaming process to soften the cells. The hard outer bark is peeled off, and the soft inner cork is pressed into sheets. Natural cork waste now supplies the raw materials for cork compositions, which are used for flooring and decorative wall tiles.