My presentation on the material wood.

MY PRESENTATION ON THE MATERIAL WOOD

What is Wood?

Wood is an organic substance composed of cells. It forms through cell division in the formative tissue (cambium) located between the bark and the wood. Under the climatic conditions existing in Switzerland, wood formation follows a seasonal rhythm; forming in the growing period from spring to autumn and halting in winter. In the process, tree rings develop which are visible in the transversal and longitudinal sections of the stem.
Construction of a tree stem

Wood - a renewable raw material

Wood is one of Switzerland's only raw materials.
Wood grows on continually and is available in times of crisis.
The supply of wood is decentralized: wood is usually used from nearby forests.
In comparison with other materials, wood takes little energy to process.

Wood - a raw material

Wood can be used in handicrafts and industry.
Waste wood can be processed and used again.
Wood is solid but relatively light which makes it easy to transport and handle.
Wood is a good insulator. It protects against temperature extremes. Wooden walls, ceilings and floors have a surface temperature just below room temperature.
Wood exhibits good acoustic properties. It vibrates within hearing range and can reflect or absorb sound waves.

A tree stem consists of three areas; pith, xylem and bark. The central pith (F) is usually barely visible and does not increase in size through the life of the tree. A cylinder of wood, known scientifically as xylem, varies in diameter with age and rate of growth. Finally, the bark sheath can be subdivided into inner bark (B which conducts sugars) and outer bark (C that serves as a protective layer). New wood and inner bark are added each year by the activity of a layer of dividing cells (A cambium) sandwiched between the inner bark and sapwood. New bark production is relatively small compared with new wood production, and bark is continually being shed to the outside of the stem, thus in older trees the greatest volume of the stem is wood. Since new wood is added to the outside of existing wood, the oldest wood is close to the pith, and the most recent is close to the bark.

In the north temperate zones, cutting any stem surface will show the wood to be composed of a series of concentric bands. These bands are referred to as growth rings, and in temperate trees, commonly one ring is formed each year. Growth rings actually extend vertically along the stem as a series of concentric cylinders. If the numbers of growth rings on the two ends of a log are counted, more rings will be found on the lower end of the log than on the upper.

The appearance of growth rings is due to changes in the structure of wood produced through the growing season. Cells produced at the beginning of the season are commonly larger, and so this early wood appears less dense than the latewood produced towards the end of the season. Although all trees produce concentric layers of wood, not all trees produce visible growth rings, neither are all growth rings necessarily annual. In some trees, seasonal changes in wood structure may be so slight that growth rings are not evident. Under conditions of severe drought, an annual growth ring may not be produced. On the other hand, under continuously favourable conditions, such as the tropics, several growth rings may be produced in a year.

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One of the major functions of wood is to conduct water from the roots to the leaves. However, wood does not continue to serve this function indefinitely. At some stage, the wood cells may become blocked by air bubbles, other cells, or by deposition of other substances. Wood that is functional in water transport is referred to as sapwood, and occupies the outer, or more recently formed growth rings (here, the yellowish zone). Wood that is no longer functional in conducting water is referred to as heartwood (here, the orange brown zone), and occupies the central stem core. Each year new wood is formed, some innermost sapwood becomes non-functional in water transport so that the outer boundary of the heartwood core is continually moving outwards. In general, an approximate balance is maintained between new wood formation and conversion of sapwood to heartwood so that there is always adequate conducting tissue.

Differences between hardwoods and softwoods

Hardwoods are the most diverse group, they contain both the heaviest and lightest timber examples found in nature. Botanically, softwoods include the conifers that belong to the more primitive group of plants called the Gymnosperms. Interestingly this group of plants is almost wholly composed of trees. Hardwoods belong to the botanical group called the Dicotyledonous Angiosperms; this is a very large group of plants including vegetable and fruit plants, herbaceous flowering plants and weeds as well as trees.

One of the major botanical distinctions between softwoods and hardwoods lies in the structure of their wood. In softwoods, the cells that serve to transport water also provide the mechanical support for the stem. In hardwoods, some division of labour has evolved, with some cells specializing in water transport, and others in providing mechanical support. In hardwoods the water conducting cells, known as pores or vessels, are commonly very much larger in diameter than the cells, known as tracheids, in softwoods. The pores can frequently be seen with the naked eye as a number of pinholes in the transverse surface of the wood. As a result, hardwoods are commonly referred to as porous woods, and softwoods as nonferrous woods. The differences in the anatomical structure of these two groups can be seen in the following pictures

SOFTWOOD HARDWOOD

In contrast to softwood, the structure of hardwood is more complicated, due to more cell types existing in hardwoods. This is a 3-D picture of a birch wood block. The large holes represent the vessel elements. The small ones are the fibres. The lines between the vessel elements on the top of the block are bundles of ray cells called multiseriate rays.

RING POROUS WOOD

White oak is another good example of a ring-porous wood. The large vessel elements are located in the early wood zone; the small vessel elements are in the latewood zone .

Wood chemistry

If we look at still smaller units of structure, we discover the elemental and organic composition of wood.

The three major elements of wood are carbon, oxygen, and hydrogen. They are combined in complex molecules that are then joined into polymers. These polymers provide the structural integrity of wood. In addition, wood contains small quantities of other organic and inorganic compounds.

The polymers of wood can be classified into three major types: cellulose, hemicelluloses, and lignin. The proportion of the three polymers varies between species.
Cellulose is the most important single compound in wood. It provides wood's strength. Cellulose is a product of photosynthesis. In photosynthesis, glucose and other sugars are manufactured from water and carbon dioxide. The glucose is first chemically changed to glucose anhydride by the removal of one molecule of water from each glucose unit. These glucose anhydride units then polymerise into long chain cellulose molecules that contain from 5,000-10,000 glucose units. Because of the nature of the bonds between adjacent glucose anhydride units, the basic repeating unit of the cellulose polymer consists of two glucose anhydride units, and is called a cellobiose unit.

FORMATION OF SOFTWOOD AND HARDWOOD ATOMIC SYMBOLIC

Durability

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 that cause so-called , 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 In most of these cases, the woods are aromatic, and the resistance is probably due to the resins and similar chemicals they contain.

Wood may be preserved by protecting it chemically against deterioration. The most important method of treatment has long been impregnation with 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.

Chemical Wood Products

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 . In addition to water, the principal constituent of wood is . 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 and a suitable medium for the cultivation of yeast, which is an important livestock and poultry feed.

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.

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 types of softwood 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 types of softwood 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.

USES IN SOCIETY

We use wood everyday form many things that we do not even think about. However, certain types of wood are used for specific manufacturing of items, as they are suitable for only some products. An example is shown below as Ash and Beech are used for two different purposes.

Ash - golf club shaft - inexpensive 'country' furniture - as a veneer - Bentwood furniture and where curved wood is required e.g. Covered wagon frame - axe handles. It is strong enough to resist the shock of the blow but also cushions the shock to the user - cart shafts - hurdles - walking sticks - boat paddles - billiard cues - cricket stumps - spears - arrows - ladders - chassis of Morgan motor car

Beech - bentwood furniture - lightweight often used as drawers - takes paint well and often used for gilded furniture - spinning wheels - Sheraton turned beech to make imitation bamboo furniture - carpenters tools, esp. mallets and handles - foundations of buildings where the ground is very wet, e.g. Winchester cathedral

On a daily basis we go through various rooms in our house not realises what a large percentage of items are made from wood, here a few rooms and examples I have shown of items found in the house made from wood.

THIS IS AN EXAMPLE OF ITEMS FOUND IN A LIVING ROOM MADE FROM WOOD.

THIS IS AN EXAMPLE OF ITEMS FOUND IN THE KITCHEN MADE FROM WOOD.

Wood is used for many things in our society and, half of them we do not even realise as wood is used on a great scale.

Conductivity

Wood is a bad conductor of heat, as there are loosely packed molecules. Therefore, heat cannot be transferred. An example is shown when holding wood; we see that the side we hold the wood would not burn our hand, as there is no heat transfer along the material. Whereas when holding metal, the molecules pass on their heat due to the free electron metals contain and heat is transferred immediately as there are many vibrations which occur that pass on the energy (heat). This is why care has to be taken when handling hot metal. The molecules in wood are loosely packed; this is what makes it a bad conductor of heat.

Insulation

Since wood is a poor conductor of heat, it is a good insulator because the molecules are loosely packed and cannot transfer the heat energy. Many materials are good insulators because they contain tiny air pockets, which are trapped inside them, so therefore this is what separates the molecules from passing their energy. The air pockets make sure there is not heat transfer.

Physical Properties

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. 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.

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.