Tree structure. From cells to roots. What is wood? What chemical elements does wood consist of?
Wood is one of the main materials used for the manufacture of model kits. It is characterized by low density, good machinability with cutting tools, and low cost.
A tree consists of cells closely fused together, varying in shape and size. The cells form fibers, which are tubes - vessels through which nutritious juices flow. The tree trunk consists of cone-shaped shells of irregular shape, fused together and growing from the outside every year. The tree structure diagram is shown in Fig. 1, a, b.
Bark 1 protects the tree from external climatic influences. The inner part of bark 2 is called the bast, which conducts nutrients. Between bark 1 and wood 4 there is cambium 3 - a thin layer of tissue, it serves to nourish the wood and form (deposit) its annual layer.
The wood is made up of concentric (sometimes sinuous) growth rings (this is the tissue found between the cambium and the pith). The wood of some species does not have a uniform color: in the inner part of the trunk it has a darker color than in the peripheral part. In these cases, the dark-colored part of the wood is called core, and the peripheral, lighter part is called sapwood. Such breeds are called sound breeds. These include pine, larch, ash, oak, etc. For example, in pine and larch the core is formed only at the age of 25-30 years. Some species do not have a core (for example, spruce, fir, birch, aspen, linden, etc.). They consist only of sapwood.
The low-quality part of the tree is the core 5, b; in some species it rots (linden, birch), and in others it separates in the form of a core (spruce). For critical parts of the model, the core is removed when cutting the lumber.
On the end section of the trunk, narrow radial stripes are clearly visible - the medullary rays that conduct nutrients.
The microstructure gives an idea of the structure of wood. When examining thin sections of wood under a microscope, it turns out that it consists of a variety of cells formed by deposits of the cambial layer. Living cells of the cambium consist of a delicate shell filled with a liquid substance - protoplasm (a liquid transparent substance containing acids, inorganic salts, water, proteins, etc.). Upon reaching a certain maturity, the protoplasm dries out, the cell dies and only its hardened shell remains - the annual layer. All wood, formed from annual rings, consists of such dead cells of various sizes and shapes. A group of cells that have the same purpose is called tissue. Wood tissues are divided into three types: storage, conductive (vascular) and supporting (mechanical).
Rice. 1. Tree structure diagram:
a - annual growths on the trunk, shown in a longitudinal section of the tree trunk along the axis; b - trunk sections: P - transverse (end), P - radial, T - tangential
Storage fabric consists of short storage cells and serves to accumulate and store nutrients (Fig. 2, a, b).
Conductive fabric consists of elongated thin-walled cells with wide internal lumens. The length of the vessels, depending on the type of wood, averages from 100 mm or more, and the diameter is up to 0.5 mm (Fig. 2, c).
Support fabric consists of long, thick-walled cells with small internal lumens and pointed ends. The more of this tissue, the denser the wood (Fig. 2, d). The length of such cells is more than 1 mm, width up to 0.2 mm. The ends of the supporting cells are firmly connected to each other and provide adequate resistance to tearing, compression and bending. In deciduous trees they are fairly evenly distributed throughout the annual layer. In conifers they are replaced by thick-walled supply cells.
The narrower the annual layers of conifers, the denser the wood. In deciduous trees, on the contrary: the wider the annual layers, the denser and harder the wood (ash, oak, etc.).
In coniferous species, the main role is played by closed elongated cells (fibers) regularly located along the wood trunk in radial rows, which serve to conduct water and inorganic salts dissolved in it (Fig. 2, e, f). Such cells are called tracheids; they are found in coniferous species up to 95% of the volume of wood. The length of the tracheids is up to 10 mm, thickness - up to 0.05 mm.
Thin-walled tracheids replace the vessel, and thick-walled tracheids replace the fibers of the supporting (mechanical) tissue. A number of coniferous trees have resin ducts in which resin accumulates, increasing the wood's resistance to decay. The diameter of the resin ducts is on average 0.1 mm, which constitutes about 1% of the wood volume.
The structure of deciduous trees is more complex than that of coniferous trees. The medullary rays are more developed and reach 160 mm in height, and the width of the rays varies from 0.015 to 0.6 mm. The microstructure of wood species is shown in Fig. 3, a - c.
Rice. 2. Microelements of wood:
a - fiber from short storage cells, b - storage cells, c - vessel segment, d - mechanical tissue cell, e - thin-walled tracheid, f - thick-walled tracheid
WOOD
GENERAL INFORMATION
Wood is one of the most common building materials with centuries of experience in use. This is greatly facilitated by the fact that it is a self-healing material.
Russia ranks first in the world in terms of forest area. Reserves are especially large in Siberia; valuable wood species are available in Karelia, the Caucasus, and the Far East.
The need for timber is satisfied through complex and deep processing of wood. Along with such traditional materials as round timber, boards, beams, etc. glued wooden structures and various products obtained from forest waste are increasingly being used. Waste is converted into particle boards and fibreboards with valuable and varied properties.
Wood, using water, minerals and chemicals, converting the energy of the sun, produces a wide range of organic compounds in the process of photosynthesis, enriching the atmosphere with oxygen. Like any living organism, a tree consists of individual cells and vessels, varied in shape, size and location.
The physical, mechanical and other properties of wood are determined by the characteristics of its composition and structure. Wood is a fibrous, anisotropic material whose properties depend on the direction of the fibers.
Depending on their structure, tree species are divided into coniferous and deciduous (ring- and scatter-vascular).
The structure of wood is studied at the macro and micro levels.
STRUCTURE AND COMPOSITION
A growing tree consists of a root system, trunk and crown. The trunk is of industrial importance, as from 60 to 90% of the wood is obtained from it
Macrostructure - the structure of a tree trunk visible to the naked eye. Typically, three main sections of the trunk are studied:transverse (end),
radial, passing through the axis of the barrel, and tangential,
passing along the chord along the trunk.
When examining sections of the trunk with the naked eye or through a magnifying glass, the following main parts can be distinguished: bark, which consists of cork tissue (porous wood that provides thermal protection under different temperature changes) and phloem (performs conductive functions - these are conductive cells and tissues). The bast layer in a growing tree carries nutrients necessary for development from the crown to the roots.
Behind the phloem is the cambium (consists of woody cells capable of division or synthesis). This layer ensures the growth of the tree through cell division; this requires nutrients, so the bast and cambium are located nearby. Every year during the growing season, the cambium deposits phloem cells towards the bark and wood cells inside the trunk. The cambium layer is formed over a period of time - the annual layer.
Behind the cambium cells there is wood (the main part of the trunk), divided into two parts - sapwood (consists of conductive cells through which a vertical (upward) flow occurs; through the sapwood cells, water from the roots enters the crown and participates in the synthesis process; sapwood cells have a very high humidity). The inner part of the wood is the core (formed due to the death of sapwood cells (core cells perform mechanical functions).
In the very center of the wood there is a layer of very thin cells - the core (primary sprout, core cells - not strong, loose). Wood rotting begins from the core.
The wood of the trunk in cross section consists of a number of concentric growth rings located around the core. Each growth ring consists of two layers:early (spring) wood formed in spring or early summer, and late (summer) wood, which forms towards the end of summer.
The early wood is light in color and consists of large but thin-walled cells; late wood is darker in color, less porous and has greater strength, as it consists of finely porous cells with thick walls.
As the tree grows, the cell walls of the wood of the inner part of the trunk, adjacent to the core, gradually change their composition and are impregnated with resin in coniferous species, and with tannins in deciduous species. In the wood of all species there are pith rays, which serve to move moisture and nutrients in the transverse direction and create a reserve of these substances for the winter. In conifers they are usually very narrow and visible only under a microscope. Wood splits easily along the core rays, and it also cracks along them when drying.
MICROSTRUCTURE
Wood cells in a tree are divided by function into:
conductive (transportation of liquid);
storage (containing a supply of nutrients);
mechanical (support) determine the properties of wood.
Groups of identical cells form woody tissues.
Conducting cells are located in the sapwood and early zone of the annual layer:
tracheids (in conifers)
vessels (in hardwoods).
Mechanical cages:
- tracheids (in conifers)
- libriforms (make up the bulk of the trunk) (in deciduous trees).
Storage cells are located in the medullary rays, forming horizontal channels.
The properties of wood are determined by the structure of the wood cell shell.
microfibrils.
Microfibril consists of chain molecules of cellulose (a natural polymer). (C6H10O5)n – the molecule has the shape of a chain.
The cell membrane also contains other natural polymers - lignin and hemicellulose,
which are located between
microfibrils.
Wood contains 30% cellulose, 25% lignin, 25% hemicellulose.
Coniferous wood differs from deciduous wood in having a simpler and more regular structure. It consists of tracheids, medullary rays, parenchyma cells and resin ducts.
Tracheids (from the Greek tracheia - windpipe and eidoc -
species) have the form of highly elongated fibers with obliquely cut ends.
Early tracheids - formed in spring and early summer. They serve to conduct water with dissolved minerals. Their characteristic feature is large internal cavities and thin walls; on the annual layer they are the lightest, loosest and weakest, forming early wood.
Late tracheids are formed at the end of summer. Their walls are very thick, the internal cavities are small, the pores are small and few in number. They perform a mechanical function, giving the wood strength. On the annual layer they are darkest, densest and strongest, forming latewood.
Parenchyma cells - This is one of the types of living plant cells in which reserve nutritional materials (starch, oils, etc.) are deposited. In appearance, they are tetrahedral prisms; the walls are usually thin, consisting of cellulose and lignin.
Deciduous wood has a more complex structure. The reason for this is the strong development of blood vessels that displace neighboring cells, as a result of which the correctness of
And uniformity of structure.
The composition of deciduous wood includes pith rays, vessels, tracheids (not always), libriform fibers
And parenchyma cells.
Libriform is a mechanical tissue that is the main constituent of wood of all hardwoods. Libriform fibers are the strongest elements in hardwood.
And perform mechanical functions.
Core raysin the wood of all deciduous species they are much more developed than in coniferous species. They are built exclusively from parenchyma cells, somewhat elongated along the length of the beam.
Woodconsists predominantly of organic substances (99% of the total mass). The elemental chemical composition of wood of different species is almost the same. Absolutely dry wood contains on average 49% carbon, 44% oxygen, 6% hydrogen, 0.1-0.3% nitrogen. When wood is burned, its inorganic part remains - ash. Ash contains calcium, potassium, sodium, magnesium and other elements.
The listed chemical elements form the main organic substances: cellulose, lignin and hemicelluloses.
Cellulose is a natural polymer, a polysaccharide with a long chain molecule. The formula of cellulose is (C6H10O5)n, where n is the degree of polymerization, equal to 6000-14000. This is a very stable substance, insoluble in water and ordinary organic solvents (alcohol, ether, etc.), white in color. Bundles of cellulose macromolecules - the finest fibers - are called microfibrils. They form the cellulose framework of the cell wall. Microfibrils are oriented predominantly along the long axis of the cell; between them there is lignin, hemocellulose, and water.
Lignin is an aromatic polymer (polyphenol) with a complex structure; contains more carbon and less oxygen than cellulose. It is with this substance that the process of lignification of the young cell wall is associated. Lignin is chemically unstable, easily oxidized, interacts with chlorine, and dissolves when heated in alkalis, aqueous solutions of sulfurous acid and its acid salts.
Hemicelluloses are a group of polysaccharides that includes pentosans (C5H8O4)n and hexosans (C6H10O5)n. At first glance, the formula of hexosans is identical to the formula of cellulose. However, the degree of polymerization of all hemicelluloses is much lower and amounts to 60-200. This indicates shorter chains of molecules and less stability of these substances compared to cellulose.
In addition to basic organic substances, in wood contains a relatively small amount of extractive substances (tannides, resins, gums, pectins, fats, etc.), soluble in water, alcohol or ether.
Three branches of the chemical industry consume wood as a raw material: pulp and paper, hydrolysis and wood chemicals.The pulp and paper industry produces pulp for the manufacture of paper, paperboard and a range of cellulosic materials (cellulose derivatives), as well as fibreboards.
Based on the high chemical resistance of cellulose, the less resistant substances accompanying it are transferred into solution through the action of various agents on wood. There are three groups of methods for the industrial production of cellulose: acidic, alkaline and neutral. The choice of one method or another depends mainly on the species composition of the processed wood raw materials.
The group of acid methods includes sulfite and bisulfite. With the sulfite method, low-resinous coniferous wood (spruce, fir) and a number of deciduous species are used as raw materials. The bisulfite method allows you to use wood of almost any species to produce cellulose.
The group of alkaline methods includes sulfate and neutral. The most widely used method is the sulfate method. Wood chips are cooked in a solution of caustic soda and sodium sulfide. The sulfate method allows you to obtain stronger fibers. The advantages of this method include a shorter cooking time, as well as the ability to carry out the process in a closed circuit (by regenerating the liquor), which reduces the risk of polluting water bodies. More than half of the world's cellulose is produced using this method, as it allows the use of wood of any species .
Neutral - a method of producing cellulose from hardwood wood, in which the cooking solution contains substances (monosulfites) that have a reaction close to neutral.
Cellulose derivatives are widely used. By reacting cellulose with solutions of caustic soda, nitric and sulfuric acids or acetic anhydride, it is possible to obtain artificial fabrics (staple, viscose and acetate silk), cordon fiber for the manufacture of automobile and aircraft tires, cellophane, celluloid, film and photographic films, nitro varnishes, nitro adhesives and other products.
When aqueous solutions of acids interact with wood, cellulose and hemicelluloses hydrolyze, which are converted into simple sugars (glucose, xylose, etc.). These sugars can be chemically processed to produce xylitol, sorbitol and other products. However, the hydrolysis industry mainly focuses on the subsequent biochemical processing of sugars.
The hydrolysis reaction occurs at a fairly high temperature (150-190°C). When the hydrolyzate (an aqueous solution of simple sugars) is cooled, vapors are formed, from the condensate of which furfural is obtained. It is used in the production of plastics, synthetic fibers (nylon), resins, the manufacture of medicines (furatsilin, etc.), dyes and other products.
With further processing of the hydrolyzate, feed yeast, ethyl alcohol (ethanol), and carbon dioxide are obtained. Ethanol is obtained only from coniferous wood, used as a solvent and, increasingly, as a fuel.
When wood is heated without air access, pyrolysis occurs. As a result of pyrolysis, coal, liquid and gases are formed.
Charcoal, characterized by its high sorption capacity, is used for the purification of industrial solutions, wastewater, in the production of sugar, in the smelting of non-ferrous metals, in the manufacture of medicines, semiconductors, electrodes and for many other purposes.
Liquid is a solution of decomposition products, used in the production of antiseptics, phenols, acetic acid, methyl alcohol, and acetone. The gases produced during the pyrolysis of wood are used as fuel.
In addition to low-quality wood, the raw materials for the forest chemical industry are extractives. Extraction of resin (resin) from coniferous trees and shrubs achieved by tapping. To do this, a special wound (karra) is made on the surface of pine or cedar trunks in the fall, from which the resin flows into a conical receiver. Recycling resin carried out at forest chemical enterprises, where the volatile part - turpentine - is distilled off with water vapor and rosin is boiled.
Turpentine is widely used as a solvent in the paint and varnish industry for the production of synthetic camphor. Camphor is used in the production of cellulose, varnishes and film. Rosin is used in the production of rubber, paper, nitro-varnishes, electrical insulating materials, etc.
- Back
Grape
In gardens and personal plots, you can choose a warmer place for planting grapes, for example, on the sunny side of the house, garden pavilion, or veranda. It is recommended to plant grapes along the border of the site. The vines formed in one line will not take up much space and at the same time will be well lit from all sides. Near buildings, grapes must be placed so that they are not exposed to water flowing from the roofs. On level areas it is necessary to make ridges with good drainage due to drainage furrows. Some gardeners, following the experience of their colleagues from the western regions of the country, dig deep planting holes and fill them with organic fertilizers and fertilized soil. The holes, dug in waterproof clay, are a kind of closed vessel that is filled with water during the monsoon rains. In fertile soil, the root system of grapes develops well at first, but as soon as waterlogging begins, it suffocates. Deep holes can play a positive role on soils where good natural drainage, permeable subsoil is provided, or reclamation artificial drainage is possible. Planting grapes
You can quickly restore an outdated grape bush using the layering method (“katavlak”). For this purpose, healthy vines of a neighboring bush are placed in grooves dug to the place where the dead bush used to grow, and covered with earth. The top is brought to the surface, from which a new bush then grows. Lignified vines are laid on layering in the spring, and green ones - in July. They are not separated from the mother bush for two to three years. A frozen or very old bush can be restored by short pruning to healthy above-ground parts or by pruning to the “black head” of an underground trunk. In the latter case, the underground trunk is freed from the ground and completely cut down. Not far from the surface, new shoots grow from dormant buds, due to which a new bush is formed. Neglected and severely frost-damaged grape bushes are restored due to stronger fatty shoots formed in the lower part of the old wood and the removal of weakened sleeves. But before removing the sleeve, a replacement is formed. Grape care
A gardener starting to grow grapes needs to thoroughly study the structure of the grapevine and the biology of this interesting plant. Grapes are vine (climbing) plants and require support. But it can spread along the ground and take root, as is observed with Amur grapes in a wild state. The roots and aboveground part of the stem grow quickly, branch strongly and reach large sizes. Under natural conditions, without human intervention, a branched bush of grapes grows with many vines of different orders, which begins to bear fruit late and produces crops irregularly. In cultivation, grapes are shaped and the bushes are given a shape that is easy to care for, ensuring a high yield of high-quality bunches. Planting lemongrass
Schisandra chinensis, or schisandra, has several names - lemon tree, red grapes, gomisha (Japanese), cochinta, kozyanta (Nanai), kolchita (Ulch), usimtya (Udege), uchampu (Oroch). In terms of structure, systemic relationship, center of origin and distribution, Schisandra chinensis has nothing in common with the real citrus plant lemon, but all its organs (roots, shoots, leaves, flowers, berries) exude the aroma of lemon, hence the name Schisandra. The schisandra vine that clings or wraps around a support, along with Amur grapes and three types of actinidia, is an original plant of the Far Eastern taiga. Its fruits, like real lemons, are too sour to be consumed fresh, but they have medicinal properties and a pleasant aroma, and this has attracted a lot of attention to it. The taste of Schisandra chinensis berries improves somewhat after frost. Local hunters who consume such fruits claim that they relieve fatigue, invigorate the body and improve vision. The consolidated Chinese pharmacopoeia, compiled back in 1596, states: “the fruit of Chinese lemongrass has five tastes, classified as the first category of medicinal substances. The pulp of lemongrass is sour and sweet, the seeds are bitter and astringent, and in general the taste of the fruit is salty. Thus, All five tastes are present in it." Grow lemongrass
None of the building materials have the same qualities as wood. It is very easy to process. In addition, it is one of the most durable, lightweight materials that retain heat and a pleasant smell for a long time.
Getting started with wood will definitely require patience. It doesn’t matter if something doesn’t work out the first time - everything comes with experience. An eye and a steady hand can be assistants that will not allow you to make mistakes when cutting, sawing, drilling, chiseling and turning wood.
Wood is not a capricious building material, but it simply will not forgive some mistakes: it will be impossible to add a few centimeters of an unevenly sawn board or to level a damaged surface without damaging the future product. This is not plasticine or clay, but wood is not inferior to them in plasticity.
Raw or specially soaked wood perfectly takes the shape that you want to give it.
When working, you can either distort or emphasize the wood pattern. In the second case, the finished product will only benefit and will look great without being covered with a layer of paint. And various wood varnishes, which are applied to the surface in two or three thin layers, will help to enhance the play of tones.
In order for the intended product to maximally emphasize the texture of the wood and not contradict it, it is necessary to study it.
There is no block of wood on which the direction of grain growth cannot be traced. The most complete idea of what will come out of the selected block can only arise if you cut the block in three directions: at an angle of 45°, along the grain and across it.
A cut at an angle of 45° is called a tangential cut, which gives the wood texture in the form of cone-shaped lines (Fig. 1, a). A cut along the fibers will produce a radial cut, which will show the vertical lines of the fibers (Fig. 1, b). A cut passing across the grain will, in fact, present the texture of wood from growth rings (Fig. 1, c). Such a cut will be called transverse.
Rice. 1. Types of cuts: a – tangential; b – radial; c – transverse.
If you correctly place the intended drawing on the block, then the appearance of the future product will only benefit. In addition, the complexity and beauty of the future design directly depend on the variety of wood textures.
Wood structure
By making only a cross section, you can clearly see the structure of the wood. Each block of unhewn wood has bark - this is the skin of the tree, which is not used in work; it must be removed. Under the bark there is a tree growth zone, which is practically indistinguishable to the naked eye.
On a fresh cut of a growing tree, the cambium layer is very well represented. If you remove the bark, a thin layer of moist greenish tissue will be revealed - this is the cambium. Behind the cambium is the actual wood with growth rings.
The wood is also called sapwood. In the center of each tree there is a heartwood, which can be the same color as the sapwood or have a darker color. Depending on this, sapwood species are divided into sapwood, where the core does not have a pronounced structure and the cells are located as densely as in sapwood (Fig. 2, a), and sound wood, where, accordingly, the core is clearly distinguishable (Fig. 2, a). b). Sometimes sapwood is called coreless wood.
Rice. 2. Types of rocks: a – sapwood; b – sound.
Sound tree species include all conifers (pine, cedar, spruce, yew, larch) and some deciduous species, such as oak, ash, poplar. The majority of deciduous species are sapwood, or kernel-free: birch, hornbeam, alder, maple.
In addition to the microstructure of wood, it also includes the density of wood cells. The creation of a composition and the possibility of using a particular block in work is influenced by the macrostructure of wood, represented by growth rings and heart-shaped vessels.
The macrostructure also includes the presence of various knots, growths and undeveloped shoots-eyes, which deflect the growth rings and form various curls.
Wood, where growth rings, horizontal and vertical vessels are most clearly distinguishable, seems to be the most interesting for processing. Almost all coniferous species - pine, larch, fir, spruce, cedar - have such wood.
Physical properties of wood
The physical properties of wood include its density, humidity, thermal conductivity, sound conductivity, electrical conductivity, corrosion resistance (that is, the ability to withstand the action of an aggressive environment), as well as its decorative qualities (color, shine, smell and texture).
Density wood is the ratio of its mass to volume, measured in g/cm3 or kg/m3. This indicator depends on the type of wood, age, growth conditions, and its humidity. There is no need to go into detail about this indicator; it is enough to know that wood with a higher density lasts much longer and is less susceptible to irreversible changes than less dense wood (however, it should be noted that for the purity of comparative analysis, the density of wood is measured on samples with a moisture content of 15%). Oak has the highest density, followed in descending order by: ash, maple, larch, beech, birch, walnut, pine, linden, aspen, spruce, fir.
Humidity timber used in construction and in the manufacture of wooden products is an indicator of its quality and durability. In practice, wood is distinguished: room-dry, with a moisture content of 8–12%; air-dry artificially dried, with a moisture content of 12–18% (these two types of wood are obtained by drying lumber in drying chambers); atmospheric-dry natural drying, with a humidity of 18–23% (obtained as a result of long-term storage of timber, stacked on pads in dry, ventilated rooms or under a canopy, without exposure to direct sunlight), wet wood, with a humidity of more than 23%.
The lower the moisture content of wood, the less susceptible it is to rotting. However, one should not strive to use timber with the lowest moisture content. The fact is that the structure of wood is very hygroscopic: it easily gives off excess moisture when the temperature rises and the ambient humidity decreases, and just as easily absorbs moisture when the temperature drops and the ambient humidity increases. This inevitably leads to: in the first case, to shrinkage of wood (reduction in its volumetric dimensions); in the second case - to its swelling (increase in volumetric dimensions). Both shrinkage and swelling change the volumetric dimensions of a wooden part unequally in different directions; the result of this is warping of the wood, deformation of wooden structures, which ultimately renders them unusable. The easiest way to prevent warping is to use wood whose moisture content at the time of use corresponds to the operating humidity.
Thermal conductivity, sound conductivity. Wooden houses made of logs or timber retain heat well. Healthy wood is capable of spreading sound along the fibers: if, after hitting the butt part of a log, board or beam, a clear ringing sound is heard, this indicates the high quality of the wood; an intermittent, dull sound indicates its decay.
Corrosion resistance wood is very important for buildings and products made from it, especially those that are used mainly in the open air. It should be noted that coniferous wood is more resistant to corrosion compared to deciduous wood, since coniferous wood is impregnated with natural resinous substances.
Color, shine, smell and texture are physical properties of wood that allow you to visually determine its species.
Color can indicate quality: for example, the bluish color of coniferous wood indicates the initial stage of decay (the color of healthy pine is from brownish-yellow in areas saturated with resin to light yellow; the color of spruce is from light yellow to white); black and dark brown spots on beech wood are a sign of decay (the color of a healthy beech is from yellow to pinkish-beige).
A change in odor can also indicate wood defects: if in the room where beech wood is stored there is a persistent smell of rotten leaves, and the smell in the room where pine timber is stored is musty - this is a clear sign of rotting processes.
The texture of wood depends on the cut, and the mechanical strength of certain boards or bars depends on the type of cut (Fig. 3). But color, shine, and texture are purely decorative.
Rice. 3. Components of a cross-cut trunk and wood texture in three sections: a – components of a cross-cut trunk: 1 – bast layer of bark; 2 – cambium; 3 – sapwood; 4 – core; 5 – core; 6 – heart-shaped rays; b – texture of pine wood in three sections: 1 – transverse; 2 – on the radial; 3 – on tangential.
Mechanical properties of wood
The mechanical properties of wood are more important, since the strength and durability of structures and wood products depend on them.
Mechanical strength wood is its ability to withstand various static and dynamic loads. Based on the direction of load action, they distinguish between compressive, bending, chipping (shear), and tensile strength (Fig. 4). At the same time, the compressive and tensile strength of wood when the load is directed along the fibers is much higher than when the load is directed across the fibers. The mechanical strength of wood depends on its physical properties: an increase in humidity reduces strength, and dense wood is stronger than light and loose wood.
Rice. 4. Testing the strength of wood: a – load direction: 1 – along the fibers; 2 – radially across the fibers; 3 – tangentially across the fibers.
Plastic– the ability of a wooden part to change shape under the influence of load and maintain this shape after removing the applied load. This property is important in the manufacture of bent parts: it is important to know that with increasing humidity and temperature of wood, its plasticity increases; therefore, parts that need to be bent are treated with hot water or steam. Beech, elm, oak, and ash wood have high plasticity (in decreasing order). Coniferous wood does not have the plasticity sufficient to bend parts due to the straight structure of the fibers.
Hardness wood is due to its ability to resist the introduction of foreign bodies. According to this criterion, wood is divided into hard - beech, oak, maple, ash, elm, larch (the hardest are boxwood and acacia) and soft - linden, spruce, pine, alder.
Hardness determines another mechanical property of wood - its wear resistance, ability to withstand friction. There is a direct relationship here: the harder the wood, the higher its wear resistance.
From the book: Korshever N. G. Works on wood and glass
Chemical composition of wood
Wood is a substance of cell walls (walls of wood cells), consisting mainly of organic substances (about 99%), and only a small part (about 1%) consists of mineral substances, which form ash when wood is burned. The main organic components of wood are high-molecular compounds (polymers), which are tightly bound together in wood. Organic substances in wood are divided into three main parts: carbohydrate, aromatic and extractive substances. The main components of wood are: cellulose (45–55%), closely related hemicellulose (24–30%) and lignin (20–29%).
Cellulose, or fiber has a fibrous structure, in its pure form is colorless, odorless and tasteless, very stable, does not change in the air and does not dissolve in water, alcohol, acetone, ether and other common organic solvents. Cellulose is used to make paper, artificial silk, explosives, threads, celluloid, nitrocellulose varnishes and other substances. Hemicelluloses in their chemical composition they are substances close to cellulose. Under the influence of acids, they easily hydrolyze and go into solution. Lignin is a complex organic plant substance. Lignin differs from cellulose in its higher carbon content and lower stability: it is easily exposed to hot alkalis and oxidizing agents. Coniferous wood contains resin ducts or resin cells in the bark. resins. Liquid resin is obtained from pine wood - oleoresin, which is a thick, sticky transparent liquid with an aromatic odor. The wood of many species (oak, chestnut) contains tannins– tannins used in the leather industry to tan raw hides and turn them into leather. The milky juices of some plants produce special substances - gutta-percha. The breeds that produce gutta-percha are called rubber plants (euonymus warty), because. it is a raw material for rubber production.
2. Properties of wood
The properties of wood are divided into physical, mechanical, chemical and biological.
Physical These are the properties of wood that can be determined without violating the integrity of the test sample, without changing its chemical composition. The physical properties of wood include: appearance and smell, bulk density, humidity, hygroscopicity, water permeability, thermal conductivity, sound conductivity, porosity, shrinkage, swelling and warping, gas permeability, etc.
Appearance wood is determined by its color, shine and texture. Color Wood is given its color by the tannins, resins and dyes it contains. Wood species growing in different climatic conditions also have different colors: from white (aspen, spruce, linden) to black (ebony). Shine wood depends on the number, size and location of the core rays. Dense wood usually has a high shine (beech, maple, elm, sycamore, acacia). Texture called the pattern that is obtained on sections of wood when cutting its fibers, annual layers and medullary rays. The texture depends on the anatomical structure of individual wood species in the direction of the cut. Smell The quality of wood is determined by the resins, essential oils, tannins and other substances contained in it. When freshly cut, wood has a stronger odor than after drying. The kernel smells stronger than the sapwood.
Volumetric weight characterizes wood at a standard 15% moisture content. Construction and technical properties depend on its size. The ratio of the weight of a substance to the weight of water taken in the same volume is called the specific gravity of the substance. The specific total wood substance averages 1.49–1.57 g/cm3. Since wood has pores, in practice the weight per unit volume of wood in its natural state is important, i.e. volumetric weight of wood. The volumetric mass of wood in an air-dry state varies widely - from 380 kg/m 3 for very light tree species (Siberian fir) to 1050 kg/m 3 for the heaviest (saxaul, pistachio). Depending on the volumetric mass in an air-dry state, wood can be divided into the following groups: light species, volumetric weight up to 0.55 (pine, spruce, fir, cedar, poplar, linden, aspen, chestnut, willow, bird cherry); medium-heavy species, volumetric weight 0.56–0.75 (larch, yew, birch bark, elm, birch, elm, beech, oak, maple, rowan, cherry, apple, ash, juniper); the species are very heavy, volumetric weight above 0.76 (white acacia, iron birch, hornbeam, pear, boxwood, saxaul, pistachio, hopshornbeam, dogwood).
In practice, the following concepts are accepted to characterize the degree of moisture content of felled wood: wet (being in water for a long time) - reaches 150–200%; freshly cut wood: for coniferous species 80–100%, for soft deciduous species 60–93%, for hard deciduous species – 36–78%; transport – humidity not higher than 22%; air-dry – 15–20%, room-dry – 8–13%,. The moisture content of wood exposed to air for a long time with constant relative humidity and temperature is called equilibrium, and the moisture content corresponding to the maximum content of hygroscopic moisture is called the saturation point of the fibers. Wood is highly hygroscopic; its moisture content varies depending on the relative humidity of the air.
Hygroscopicity (moisture absorption) - the ability of wood to absorb water vapor from the surrounding air and release the moisture it contains. Hygroscopicity is a negative property of wood, as it causes changes in density, volumetric mass, thermal conductivity and strength of wood, variability in the dimensions of wooden structures during their operation in buildings and structures, and susceptibility to bacterial damage. Water permeability wood is determined by the amount of water filtered through the surface of the sample over a certain time (g/cm3) and depends on the type of wood, its initial moisture content, the nature of the cut and other factors. In the process of evaporation of hygroscopic moisture, a decrease in the linear and volumetric dimensions of wood occurs.
Thermal conductivity wood is small and depends on the volumetric mass, the nature of the pores and humidity; with increasing density and humidity, thermal conductivity increases. The thermal conductivity of wood is not the same: in the direction along the fibers, the thermal conductivity coefficient is approximately 1.5–3 times greater than across it.
Sound conductivity is the property of a material to transmit sound through its thickness. Wood is a good conductor of sound, which travels 2–17 times faster in it than in air. The high sound conductivity of wood is its negative property, necessitating the use of soundproofing materials. Sound travels faster along the fibers and slower across the fibers (especially in the tangential direction). Damp and rotten wood conducts sound much worse than dry and healthy wood. The resonant properties of wood depend on the uniformity of its structure and volumetric weight. The wood of spruce, Caucasian fir and Siberian cedar has the best resonating properties.
Electrical conductivity wood is subject to significant changes and depends on its species, temperature, direction of annual layers and humidity. With increasing temperature and humidity, the electrical conductivity of wood increases.
Porosity Coniferous wood ranges from 46 to 80%, deciduous wood - from 32 to 80%.
Shrinkage, swelling and warping. When dry wood is moistened until it reaches the hygroscopic limit, the walls of wood cells thicken and swell, which leads to an increase in the size and volume of wooden products. Wood shrinkage occurs due to the removal of bound moisture from the cell walls. Due to the heterogeneity of its structure, wood dries out differently in different directions. When swelling and shrinkage occurs, wood materials warp and crack. Warping causes internal stresses in the wood and cracking of lumber and logs.
Mechanical are the properties of wood to resist external mechanical forces (loads) acting on it. The mechanical properties of wood largely depend on the bulk density, with increasing strength increasing its strength. As humidity increases, strength decreases. The mechanical properties of wood include strength, elasticity, hardness, viscosity, fragility. In many wooden structures, wood works in compression, crushing, chipping, bending, and less often in tension, both along and across the fibers. In coniferous trees, the compressive strength along the fibers is 10–12 times greater than across them, and in hardwoods it is 5–8 times greater. The tensile strength of wood along the grain is very high; it exceeds its compressive strength along the grain and amounts to 1200-1300 kg/cm2. The strength of wood during static bending is on average 2 times higher than its compressive strength along the grain. The strength of wood to chip along the grain is 8-10 times less than in tension, and 5-6 times less than in compression.
Hardness is the ability of a material to resist penetration into it by another, harder body that does not receive residual deformations. Hardness is quantitatively measured by force in kg , which must be applied in order to press another, more solid body into the material. Hardness is measured in kg per 1 cm 2 surfaces (kg/cm 2 ). There are different hardnesses of wood - end, tangential and radial. The end hardness of wood exceeds the lateral hardness. With a change in humidity by 1%, the end hardness of wood changes by 3%, and the lateral hardness by 2%. The main species are arranged in the following sequence according to their degree of hardness in decreasing order: dogwood, pistachio, hornbeam, hornbeam, ash, pear, birch bark, oak, eucalyptus, beech, rowan, juniper, Norway maple, elm, birch, larch, cypress, alder , chestnut, pine, white willow, aspen, spruce, cedar, Siberian fir.
Cleavability is the ability of wood to split along the grain under the influence of wedging forces. Frozen wood splits easily. The splitting properties of wood must be taken into account when fastening parts with nails and bolts, especially at the end and edges.
When manufacturing wood products, they are of great importance technological properties. These include cutability, abrasion resistance, bendability, gluing and paintability, and the ability to hold metal fasteners.
Chemical properties wood is characterized by its resistance to acids, alkalis and other reagents. Wood is highly resistant to solutions of alkalis, salts and most organic acids. However, solutions of mineral acids, especially nitric acid, as well as sea water destroy wood. Coniferous wood has greater corrosion resistance to aggressive environments than hardwood. A decrease in the chemical resistance of wood is accompanied by a change in its color - from browning to charring.
Wood is a combustible material: its charring temperature is 120–150 o C; ignition temperature is 250–300 o C. To protect against fire, wood is impregnated with fire retardant compounds (fire retardants), painted with liquid fire retardant materials, etc.
Biological properties wood is determined by its resistance to fungi, mold and insects, which depends on the content of resinous, tannin and other substances. According to biostability, wood is divided into three groups: the most resistant (yew, oak), medium-resistant (pine, cedar) and low-resistant (aspen, beech). The rot resistance of wood is increased by treating it with antiseptics, which include organic and mineral substances with high toxicity to fungi and insects.
