Post 378 ⇒ by Gautam Shah →
Woods derived from trees and large-trunk plants, consist of timbers, twigs and barks. Woods also include forest-based products, crop wastes, and plant materials like grasses and fibres. Woods have been used for the timbers, for animal feed, as manure and fuel. Many of the wood products have wood in the original form and character, in the re-composed, or derivative form. Wood processing technologies, offer several wood-based products to enhance the peculiar characteristics, overcome the qualitative and sensorial deficiencies, offer new uses, add economic value or ecologically sustain the natural materials.
Woods are used for constructing geometric or structural compositions, used as filler for forming composites and form the raw materials for many industrial goods. Timbers’ wastes such as very thin or small cuts, chips, shavings, turnings, dust, shavings are used as filler in composites. Leaves are used for manure making or composting. Stems and soft branches yield fiber used for yarn and rope making. Long leaves such as of palms are used for mat weaving, basket making and brooms. Barks and other useless components are compressed and extruded as fuel bricks. Wood derivatives such as its cellulosic matters go through various levels of pulp conversion, and used for boards and casts.
Wood composites have wood as the filler component. The fillers in the form of could be, Large solids (such as blocks in block-boards), Particulate matter (such as chips, shavings, turning, sanding dusts, farm husks, etc. in particle boards), Sheets (such as for Veneers and plywoods in layered composites), Fibrous materials (for in-fill and reinforcement material in cement and gypsum boards).
Wood and plant products with cellulose as the chief constituents are pulped to produce different grades of materials. This could be soft boards, Hard boards, High, medium and low density (HDF, MDF, etc.) fibre boards. At higher pulping level and with addition of rags it could turn into grey boards, card-boards and paper.
All wood composites, except the refined cellulosic or pulped formations, require a matrix for bonding of the fillers. The simplest of bonding is mechanical tying. It binds the materials only at selected points, so the filler materials need to have sufficient mass and strength to distribute the stresses. Chemical or glue bonding is the most common method for holding the components. Bonding materials such as adhesives, resins or cementious products are water or solvent based. Polymeric materials are thermosetting or thermoplastic materials and may need heat.
Veneer is a thin (as low as 0.6 to 0.8 mm) sheet of wood with uniform thickness, produced by peeling or rotary cut (by rotating a log against a knife), slicing, or by sawing a wood. All parts of a tree such as trunks, round, square or rectangular cut logs, and subterranean part of a trunk with root nodes, and heavier branches, are used for veneer cutting. More than 90 percent of all veneer is rotary cut, but thicker figured wood veneer for furniture and other decorative purposes are sliced. Sawn veneer was considered a wasteful operation, but modern sawing equipments can saw a wood into very thin sections with very little material turning into dust. Some veneer waste is stacked, composed and recut again as veneer. Plywoods are also one form of thick veneers. Veneers form a layered composite. Plywoods are re-composed to form large-span wood beams (20mts) for exhibition halls, malls and departmental stores. These beams are lighter in density and have greater fire-exit time (catching fire and distortion of the structure), compared to a steel beam.
Block boards and Cored plywoods are composite materials with a filler core. Batten or block board and flush doors have a solid or partly filled core of strips of solid woods, which may or may not be glued together. These however, are covered on both sides with sheets of plywood or other composite boards.
Chip boards are entirely made of chips, and other cutting-dressing wastes bonded with an amine resin. Particle boards have particulate matter or agri husks, etc. as the filler, bonded with a resin matrix.
Composite boards have a core of polymer latex, styrene or PU foam, encased on both sides with plywoods or other sheets.
MDF, HDF, LDF, Soft and Hard-boards are not wood composites, but wood pulp-based products. Mechanical or semi chemical processes are used for pulp preparation. In one process wood or wood waste is reduced to fine size and subjected to a high pressure steam treatment and forced out through a valve to reduce the mass to pulp. In wet felting pulp is cast into sheet form of various thickness, and density is controlled by the pressure. In dry felting process, the pulp is drained, separated and additives are mixed. Additives include glues (1 to 4%), waxes, paraffin, rosin, polymers, colourants, chemicals to improve the resistance to microbes, insects, and fire. Dry felted fiberboard with thermoplastic resin bonding materials are available in low, medium, and high densities. These are extensively used for Panellings and also as a core material for various composites.
Hardboard is dry felted Fiberboard with one or both sides finished smooth, available in various densities. The fibres are hot pressed, to form a board bonded by lignin, with small addition of binding agent. Hard boards are available from 3 mm to 12 mm thickness. Tempered Hard boards have one or both face impregnated or coated with a resin to provide better stiffness, wearability and moisture resistance. Hard boards are generally dark in colour due to the use of phenolic bonding media. MDF is fiberboard but with a greater amount of binding agent, almost comparable to a particle board.
Soft-board is very similar to hardboard, with the mass being less cohesive due to the wet felting process. These boards are used for insulation, sound proofing and as tack or pinup boards. Soft-boards with bitumen impregnation were produced for slightly better weather resistance.