wood_material

WOOD
INTRODUCTION Wood , hard, tough substance that forms the trunks of trees, and that has been used for thousands of years as a fuel and as a material of construction (//see// [|__Building Construction__]). Technically, the term //wood// includes similar materials in other parts of the plant, including even the so-called veins in leaves, but only those portions of wood which have commercial importance are discussed in this article.  For the botanical aspects of wood, including its structure and growth, //see// [|__Tree__]; [|__Xylem__]. For growth and distribution, //see// [|__Forest__]. For the cultivation of trees for wood, //see// [|__Forestry__]. For the cutting of trees and the manufacture of lumber, //see// [|__Lumber Industry__].  **GRAIN and STRUCTURE ** The typical markings, called grain, that are found on all types of natural wood are due to the structure of the wood. Wood consists essentially of fine cellular ducts or tubes, which carry water and dissolved minerals from the roots to the leaves, and which are thus arranged more or less vertically within the trunk. When the wood is cut parallel to the axis of the trunk, straight-grained lumber is usually produced. In some trees, however, the ducts are helical; that is, they twist around the trunk as they ascend. Such trees produce cross-grained lumber, which is also obtained from ordinary trees when the cut is not parallel to the axis of the trunk.  Many woods have prominent annual rings. The trunk of a tree does not grow in length, except at its tip, but does grow in width. The only portion of the trunk that is engaged in active growth is the cambium, a thin layer entirely surrounding the trunk. In trees of the Temperate Zone, the cambium lays down new wood during the spring and summer, and in most trees the early wood is more porous and therefore lighter in color than the wood produced later in the season. The trunk of a tree is thus surrounded each year by a new pair of concentric sheaths, one darker than the other.  Although the thin layer of cambium is the only part of the trunk that is alive in the sense that it is engaged in active growth, living cells are also interspersed among the xylem cells of the sapwood. As the tree grows older, however, the central portion of the trunk dies completely; the ducts become plugged with gums or [|__resins__], or merely air (//see// [|__Gum__]). This central part of the trunk is called heartwood. The internal changes are accompanied by changes in color typical of the species of trees, so that the heartwood is usually darker than the sapwood.  **CLASSIFICATION ** Woods are classified as softwood or hardwood, depending on the tree from which they come. Woods from broad-leaved trees are called hardwoods, and woods from coniferous trees are called softwoods, regardless of their actual hardness. Thus many softwoods are actually harder than some of the so-called hardwoods. The hardwoods have long, continuous ducts leading through the trunk; the softwoods do not have such ducts, and the fluids are transported from cell to cell. Many softwoods have resin ducts running parallel to the grain, and softwoods in general contain considerable resin, whereas few hardwoods have any such material in the wood. Most lumber in the U.S. is softwood; the hardwood is generally employed for furniture and high-grade flooring. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Knots are areas of the trunk in which the base of a branch has become embedded in the body of the wood. When the wood is sawed into planks, the knot becomes evident as a roughly circular discontinuity or irregularity in the grain structure. Where the branch begins within the tree, the rings of the knot are continuous with the grain of the wood, producing an intergrown knot; farther out toward the surface, the grain of the trunk has grown around the branch, producing an encased knot. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">During seasoning (see below), when the shrinkage in a board varies with the direction of the grain, the knots shrink at a faster rate than the remainder of the wood. An encased knot may thus fall completely out of the plank, forming a knothole. An intergrown knot cannot fall out, but the wood around the knot is distorted by the uneven shrinking, and the board may be weakened even more than by the presence of a knothole. Knots are generally undesirable in lumber from the standpoint of appearance, apart from consideration of their effect on the strength of the wood. In some cases, however, knotty pine and similar types of lumber are desired for paneling interiors, because of the patterns in the grain formed by the knots. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">The wood’s appearance is one of the most important properties when the wood is being considered for some interior use such as [|__furniture__] or paneling. Certain woods, such as walnut, have straight, parallel graining, which, when combined with dark, attractive color and great hardness, makes them highly desirable for [|__veneer__] (see also Plywood below). Irregularities of grain may make attractive patterns, or lumber may purposely be cut from a crotch to yield wavy, interlocking patterns. Many veneers are made by peeling a thin layer circumferentially from around the trunk, so that the knife cuts across the annual ring marks only at considerable distances, producing the large, irregular patterns typical of many plywoods. <span style="font-family: "Arial","sans-serif";"> **<span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">PHYSICAL PROPERTIES ** <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">The principal physical properties of wood are strength, [|__hardness__], stiffness, and density. Density is generally an indication of the mechanical properties, inasmuch as dense woods are usually hard and strong (//see// [|__Ironwood__]). The term strength covers a number of essentially different properties; a wood that is high in one kind of strength is not necessarily high in others. Moreover, the strength varies greatly with the state of seasoning, or dryness, of the wood, and with the direction of the grain; wood is always much stronger when cut along the grain rather than across it, and for this reason planks and such articles as poles and handles are always cut with the grain running the long way. Wood has high compression strength, in some cases higher in proportion to its weight than steel; it has low [|__tensile strength__] and moderate shear strength. //See// [|__Materials Science and Technology: //Mechanical Properties of Materials//__]. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">High compression strength is required for foundations, and for the main supports of buildings. Bending strength is essential for most structural wooden members, including joists, studding, and beams of all sorts. Many woods that are commonly used for high bending strength have high compression strength, and vice versa; but oak, for example, is very strong in bending and comparatively weak in compression, whereas redwood is strong in compression and comparatively weak in bending. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Toughness is a measure of strength against sudden and repeated stress. Hickory and ash are outstanding for their toughness and are used in wagon spokes, baseball bats, and ax handles; because hickory is stiffer than ash, it is preferred for thin handles, such as those of golf clubs. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Other less important mechanical properties may be critical for a particular use; for example, the [|__elasticity__] and resonance of spruce render it the only material suitable for the sounding board of a fine piano. <span style="font-family: "Arial","sans-serif";"> **<span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">DURABILITY ** <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Wood is naturally a very durable substance. If not attacked by living organisms, it will last for hundreds or even thousands of years. Samples of wood used by the ancient Romans have been found virtually in their original condition when a combination of circumstances protected them against attack. The most important of the organisms attacking wood are the [|__fungi__] that cause so-called [|__dry rot__], which actually occurs only when the wood is damp. The sapwood of all trees is susceptible to this type of decay, but the heartwood of a few species is naturally resistant to these fungi. Walnut, redwood, cedar, mahogany, and teak are among the well-known woods that are extremely durable. Other woods are resistant to various types of attack. Greenheart and teak are particularly resistant to the attack of marine borers, and so are often used for underwater construction for wharves. A number of woods are comparatively resistant to termites, including redwood, black walnut, mahogany, and several types of cedar (//see// [|__Termite__]). In most of these cases, the woods are aromatic, and the resistance is probably due to the resins and similar chemicals they contain. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Wood may be preserved by protecting it chemically against deterioration. The most important method of treatment has long been impregnation with [|__creosote__] or zinc chloride. This method is still one of the best, although a number of newer chemicals, notably several containing copper compounds, have been introduced for the same purpose. Wood can be protected against weathering by suitable surface coatings, applied by brushing, spraying, or dipping. Surface applications yield little penetration, however, and therefore do not prevent deterioration under attack by insects, fungi, or borers. <span style="font-family: "Arial","sans-serif";"> **<span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">SEASONING ** <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Freshly cut wood contains considerable water, which amounts to from one-third to more than one-half of the total weight. The drying of wood before it is processed into lumber is called seasoning, and is done for a number of reasons. Seasoned wood is far more resistant to decay than fresh wood; it is much lighter and therefore less expensive to ship; it has much higher heating value, which is important if it is to be used as fuel; and, most important, wood changes in shape during drying, and this change in shape should be completed before the wood is sawed. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Wood may be seasoned either by air-drying or kiln-drying. Air-drying takes several months, whereas kiln-drying takes a few days. In both cases, the wood must be carefully stacked to prevent warping, and the rate of drying must be carefully controlled. <span style="font-family: "Arial","sans-serif";"> **<span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">PLYWOOD ** <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Plywood consists of several layers, or plies, of wood thoroughly bonded to one another by glue or synthetic resins (//see// [|__Adhesive__]). The layers are laid with the grain in different directions, generally perpendicular to one another, so that the resultant sheet of plywood is equally strong in all directions. The bonded joint is at least as strong as the wood itself, and moisture-resistant glues can be used if necessary, so that plywood is as durable as the wood of which it is made. So-called laminated wood is a similar product, made by bonding layers of wood with the grain all running in one direction. This procedure results in a product that, like ordinary lumber, is extremely strong in the direction of the grain and weak in other directions. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Only the layers on the outside of the plywood need have hardness and good appearance; the inner layers need only be strong. In some cases, only one side of the plywood is a high-grade layer; such plywoods are used, for example, in making cabinets of which the inside will not be visible. Fine and expensive woods, such as mahogany, satinwood, ebony, and zebrawood, are now commonly used in the form of plywood, with a thin layer of expensive wood covering several layers of strong but inexpensive ordinary wood such as Douglas fir. In this way, the expense of the wood is greatly reduced, the appearance is in no way changed, and the strength and resistance to warping is greatly increased. Plywoods made of less expensive wood have been used to substitute for metals. <span style="font-family: "Arial","sans-serif";"> **<span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">CHEMICAL WOOD PRODUCTS ** <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Wood is an important raw material in the chemical industry. Each year an enormous quantity of wood is reduced to pulp and reconstituted mechanically to form [|__paper__]. Some modern industries are based on extracting from wood its minor chemical constituents, such as [|__tannins__], pigments (//see// [|__Paint and Varnish__]), gums, resins, and oils, and further modifying these constituents. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">In addition to water, the principal constituent of wood is [|__cellulose__]. Much of the large quantity of cellulose used today in making rayon and nitrocellulose is obtained from such comparatively pure sources as cotton, but an increasing quantity is being obtained from wood. The chief difficulty in using cellulose from wood lies in separating it from its impurities, the most important of which is lignin, a carbohydrate. Formerly, the lignin was discarded, but it was found to be a raw material for the manufacture of [|__plastics__] and a suitable medium for the cultivation of yeast, which is an important livestock and poultry feed. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Wood may be used as a chemical raw material, without separating the cellulose from the lignin, by several different processes. In the Bergius process, wood is treated with hydrochloric acid in order to produce sugars, which are either used as cattle feed or fermented to produce alcohol. Wood may be converted into liquid fuels by [|__hydrogenation__]. Wood has long been used also as a source of chemicals by destructive [|__distillation__]. Most of these chemicals, however, including acetic acid, methyl alcohol, and acetone, are now made synthetically. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">Certain new products consist essentially of a mixture of wood with certain chemicals; such a mixture will have mechanical properties similar to those of wood, but will be stronger and more resistant chemically. The most important methods of making these mixtures consist of impregnating the wood with certain chemicals, such as a mixture of phenol and formaldehyde, and then heating the impregnated wood so that the chemicals react within the cells of the wood to form a plastic. Wood treated with such resins is known as impreg. It has great resistance to decay and to insect and borer attack; its specific gravity is increased, but its strength is increased only slightly, if at all. A different product, called compreg, is made by compressing the impregnated wood in a hydraulic press at pressures of about 70 kg/sq cm (about 1000 lb/sq in) while the chemical reaction which forms the plastic is progressing. Such compressed impregnated wood may have a specific gravity up to about 1.35. The hardness is many times as great as that of the original wood, and the strength is somewhat greater, although the toughness may be less. <span style="font-family: "Arial","sans-serif";"> <span style="color: #333333; font-family: "Arial","sans-serif"; font-size: 10pt;">In the mid-1980s the annual U.S. lumber production exceeded 85.6 million cu m (36.3 billion bd ft), of which about 71.5 million cu m (about 30.3 billion bd ft) were softwoods such as fir, pine, hemlock, and spruce, and about 14.1 million cu m (6 billion bd ft) were hardwoods such as oak, maple, poplar, beech, and elm. <span style="font-family: "Arial","sans-serif";"> <span style="font-family: "Arial","sans-serif";">