WOOD COMPOSITES

WOOD COMPOSITES

Post 378 ⇒   by Gautam Shah 

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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.

Wood waste of turnings

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.

Rice chaffs or husks for particle board making

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.

Wood particulate with cement

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.

Plywood -a layered composite

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.

Wood composite beam

Composite wood Bridge

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.

Wood waste or Chip board

Composite boards have a core of polymer latex, styrene or PU foam, encased on both sides with plywoods or other sheets.

Foam cored boards

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.

Particle board

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.

Polymer + wood composite board

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WOOD RESOURCES

Post 217 – by Gautam Shah 

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Wood’s appearance, warmth to touch, its ability to be worked with simple inexpensive tools, wide range of tonal hues, grain patterns and textures make it, the most versatile craft material. The only drawback for using wood, is the effect of moisture on its structural properties and its susceptibility to decay. No two pieces of timber are similar, the great variations in structural properties, colour, texture etc., make it suitable for different uses. These wide variations in a quality put the timber to a disadvantage in comparison to many naturally available materials.

Wood products

Wood products

Wood products

Wood products

Trees can be basically classified into two groups – exogenous trees (exogens) and endogenous trees (endogens).

  • Trees may be broadly grouped into exogenous and endogenous trees according to the way in which their stem diameter increases.
  • Endogenous: are trees with inward growth, and have longitudinal fibres such as canes, bamboo, palms etc. Typically it is not possible to see any growth rings in this set of trees.
  • Exogenous: are trees with outward growth (such as additions of annual-seasonal rings), like Conifers (narrow leaves) (pine, fir), Deciduous trees (broad leaves) (teak, rose). These are used as engineering timbers.
Exogenous conifer

Exogenous conifer

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Endogenous plant Bamboo

Endogenous plant Bamboo

Exogenous plants as the name suggests grow outward. The stems are formed by successive additional layer on outside. Timber is essentially derived by the new mass formed in the cambium between the wood and the bark every year. Trees of cold climates and substantial numbers trees of warmer climates are exogenous. Rings in the trunk or branch section, and pith with medullary rays extending outwards to the bark are two distinguishing features of exogenous trees. Exogens yield timber for furniture and construction. Exogenous trees are subdivided into two main classes: broad-leaved trees and needle-leaved trees, or conifers. Broad-leaved Woods generally contain no resins, and the density or weight is greater. They are usually hard, and due to their irregular structure, net yield is lower.

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The other botanical group of endogenous trees or endogens or grow inward from a hard exterior shell or, more commonly, end-wise by the acquisition of a new joint. Endogenous trees grow by forming new fibres within the trunk interspersed with the old fibres. Old endogenous stems have older and harder wood near the surface, whereas younger and softer centre. Timber from these trees has very limited engineering applications. Examples of endogenous trees are Palms, bamboos, canes, etc. These are not broadly useful for furniture or construction work, yet have their specific advantages and uses.

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Commercial timbers of exogenous trees are classified as softwoods and hardwoods, though a very misleading nomenclature. Softwoods are timbers obtained from conifers, growing above certain altitude, and are supposed to be soft and lighter in colour. Hardwoods, are timbers that originate from the mainland and tropical areas are supposed to be hard and darker in colour. Many softwoods, though, are much harder and often of darker in shades than some of the hardwoods. Inversely many hardwoods are fairly soft and lighter in colour than some of the softwoods. Botanical names and commercial names of trees or timbers do not provide a true picture. There are multiple botanical names for the same specie adding to the confusion. Same timber is likely to be differently known commercially in various parts of the world.

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Thanjavur Palace India

Timber is a very precious commodity. It takes decades to grow a commercially viable timber tree. Deforestation has become a prime issue from ecological points of view. Several countries have banned or severely curtailed and controlled the export of timber. Similarly many countries have banned import of timber to limit deforestation world wide. Many governments and local authorities discourage timber structures and other uses of timber. Government of India has enforced conditions, so that usage of timber as doors and windows and other structural purposes are nearly banned, in government schemes. Similar conditions are likely to be followed by state governments also. So over the years use of timber is likely to be severely curtailed not only due to legislation but also its difficult availability and consequent upward pricing.

Wood products

Wood Composite products

Timber grown in planned plantations is of consistent variety due to standard or a singular source of seedling or genetic derivation and grown in very controlled conditions. These results in trees with identical grain, colour and texture, belying the natural variations associated with woods.

wicker-furniture

Wood replacement products are of broadly two types. Wood-based composites such as plywood, block boards, chip and particle boards are likely to remain available, but only for a while. The availability of such products is also going to taper off in future. Wood ‘look-a-like’, make-believe, or pseudo wood products made of polymeric composites, painted, printed or layered materials are in the market.

Unlike a metal or plastic, wood is inherently a replenish-able resource, provided production and use are controlled and matched.

 

 

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