专业英语Scientific English Course（木材科学与工程专业用）徐剑莹余德新编中南林业科技大学2017年9月CONTENTS1. China: a country rich in tree species (1)2. Gross Structure of Wood (5)3. Microscopic Structure of Wood (10)4. Wood Cell-Wall Structure (14)5. Wood Chemical Components (18)6. Mechanical Properties of Wood (22)7. Deterioration of wood and wood products (27)8. The drying of wood (37)9. Drying Defects (45)10. Fiber Products (50)11. Plywood Production and Use (58)12. Particleboard and composite products (65)13. Furniture design (76)Lesson OneTextChina: a country rich in tree speciesChina has a total forest area of 208 million ha, covering 21.63% of its territory. The existing timber stock volume is about 15,100 million m3, equal to 11 m3 per capita.The forests are situated mainly in the Daxingan, Xiaoxingan and Changbai mountain ranges in the Northeast and Inner Mongolia; Fujian and Jiangxi Provinces in the East; Guangdong and Hunan Provinces and Guangxi Autonomous Region in Mid-south and Shennongjia in Hubei Province; Sichuan and Yunnan Province in Southwest and Tibetan Autonomous Region.From north to south, China‟s forests can be divided into coniferous belts in the frigid and temperate zones, mixed forest belts of coniferous, deciduous and broadleaf trees in the temperate zone, deciduous and broadleaf forest belts in the warm temperate zone, evergreen forest belts in the sub-tropical zone, and monsoon and rain forests in the tropical zone. Therefore, China is rich in tree species: there are more than 2,800 arbor species, of which about 1,000 are used for industry, 500 of these frequently.Some important industrial tree species are Korean pine, larch, spruce, fir, ash, basswood, birch, Chinese fir, masson pine, schima and castanopsis.Other valuable species are machilus, camphor and Chinese mahogany. Among the fast-growing species are paulownia and poplar, which are adaptable to the north and middle part of China. These trees, with diameters of about 40 cm at 10 years, can be used as timber. Chinese fir, which can be cut for timber at 20 years, is adapted to the broad area to the south of the Yangzi River. China also is rich in bamboo forest resources.New Words1. timber木材,木料2. log原木3. conifer […kounifә]针叶树4. conif erous [kou‟nifәrәs]针叶树的5. deciduous [di‟sidjuәs]落叶的6. broadleaf tree阔叶树7. pine松木8. Masson pine马尾松9. Korean pine红松10. larch [la:tʃ]落叶松11. spruce [spru:s]云杉12. Chinese fir杉木13. ash白蜡树14. bass-wood椴木15. birch桦木16. machilus润楠17. camphor […kӕmfә]樟木18. mahogany [mә‟hɔgeni]红木,桃花木19. schima木荷20. castanopsis栲树21. paulownia [pɔ‟louniә]泡桐22. poplar [pɔplә]杨木23. bamboo毛竹Lesson TwoTextGross Structure of WoodA mature tree, of either the softwood or the hardwood type, generally consists of a single stem which is covered with a layer of bark. This central trunk is the principal source of woody material for the manufacture of lumber and other products. While there is a trend toward the utilization of a greater portion of the tree, the conversion of tree tops and branches into usable material is done only where it is economically feasible. These parts can be converted into chips for pulping or for the manufacture of chipboard, but in many timber producing regions of the world they are considered to be waste and are left in the forest during a logging operation.The gross features of wood are those that are visible to the naked eye or with the aid of a hand lens. Characteristics such as growth increments, sapwood heartwood difference, wood rays and cell distribution patterns can be recognized at this level.1.Cellular CompositionA tree trunk is composed of millions of individual woody cells. These cells differ in size and shape, depending upon their physiological role in the tree, most of them being many times longer than broad. They are arranged on recognizable patterns of distribution within the wood, the organization varying with the species. The long cells which are arranged longitudinally make up the b ulk of the wood and provide “grain” to the material.Cells of the xylem or wood portion of the tree are of two general types, parenchyma and prosenchyma. Parenchymatous cells are food storage elements and must, therefore, remain alive for a longer period than prosenchymatous cells which lose their protoplasm the year in which they are formed. Parenchymatous cells are found in wood rays, as longitudinal or axial parenchyma, and as epithelial cells surrounding resin canals. The term prosenchyma may be applied to all of the other types of cells in mature wood, whether their major physiological role in the living tree may have been conductive or supportive.2.Wood RaysThere are also shorter cells in the tree, cells which areoriented perpendicular to the longitudinal elements andorganized into bands of tissue called wood rays. The cellsfound in wood rays are predominantly parenchyma cells,specialized for food storage, but the rays of some coniferscontain prosenchymatous cells. The presence and structure ofthis type of element (ray tracheid), the height, the width andthe composition of the ray are features frequently useful forthe identification of a wood species.Rays extend radially from the pith, at the center of thestem, to the cambium at the outer periphery of the xylem(wood), and continue into the phloem (bark). Depending onthe manner in which a log is cut, these ribbons of tissuecontribute to the natural figure of lumber and veneer. Woodsplits easiest along the grain and particularly in the plane ofthe wood rays.3.Planes of WoodWhen discussing structural elements and features of wood, it is convenient to specify the aspect or viewpoint with respect to three planes, cross or transverse, radial, and tangential. By cutting across the stem perpendicularly, a surface is exposed which is called a cross or transverse section. A radial section results from cutting longitudinally in the plane of the wood rays, from the pith to the bark. The plane which is perpendicular to the rays and tangent to the bark is called the tangential section. These planes and some of the gross features of wood are illustrated in Fig.1.4End grain and rays are visible on the cross section. This is also the aspect for viewing growth increments as concentric rings. The extent of heartwood development can also be observed in this plane, but both features can be seen on a radial section as well. Ends of rays appear on the tangential section and the pattern formed by them on this surface provides diagnostic evidence in some instances. Rays on the radial section give the appearance of broken ribbons and, depending on their size and contrast, may produce distinctive patterns.The vascular cambium is located at the xylem-phloem interface and is the initiating layer for both of these tissue systems. At the gross level this layer can barely be detected since it is but a few cells wide.4.Sapwood and HeartwoodThe sapwood-heartwood pattern is one of the most obvious features that can be observed on the cross or radial section of a mature tree trunk. Although not as pronounced in all species, most trees have an inner core of dark colored wood, heartwood, and an outer shell of light colored tissue called sapwood. This contrast in color has physiological significance in a general way, but it is not strictly correct to designate the core as heartwood only on the basis of its darker color. A more accurate criterion for heartwood determination is the absence of living cells within the zone, and in particular, cells of ray parenchyma which remain alive far longer than the neighboring prosenchymatous element. For commercial purposes, however, color is the determining factor used for the separation of sapwood from heartwood.Certain characteristic differences can be found between the sapwood and heartwood of the same tree. For example, weight, durability and permeability are often quite different and can be related to the changes which accompany sapwood-heartwood transformation. Reference to permeability differences will be made in the section dealing with pit membranes.5.Growth IncrementsAnother feature which is readily observed on the cross section is the growth increment. The boundaries of these increments are usually related to annual growth in trees grown in temperate climates. In tropical regions, however, growth increments can be the result of wet and dry seasons and the term “annual ring” often used for timbersfrom temperate zones would not be strictly applicable.The nature of the growth layer can be a helpful feature in the identification of wood. In temperate zone hardwoods, for example, patterns can often be observed with the unaided eye, if one examines the tissue within one annual increment. These patterns can be attributed to the size and distribution of pores, the term given to vessel openings as seen in cross section. When the pores in the earlywood are much larger than those formed later in the season, and when the size transition between earlywood and latewood pores is abrupt, the wood is classed as ring porous. If little or no transition in pore size exits between early-and latewood, the term diffuse porous is applied. Semi-ring porous or semi-diffuse porous are used to describe wood in which the pore patterns are not distinctly of the ring porous nor of the diffuse porous type. The three conditions are illustrated in Fig.1.6.New Words and Expression1. gross structure 宏观构造, 粗视构造2. mature tree 成熟林3. softwood 软材hardwood 硬材4. stem 树干5. bark 树皮6. trunk 树干7. chip 木片chipboard 碎料板8. pulp 浆料pulping 制浆9. logging 采伐10. growth layer 年轮, 生长层, 生长轮growth ring 年轮11.sapwood 边材heartwood 心材12. wood ray 木射线13. cell distribution pattern 细胞排列类型14. xylem [zailem] 木质部15. parenchyma [pә‟reŋki:mә]薄壁组织16. prosenchyma [p rou‟seŋki:mә]锐端细胞组织prosenchymatous cell 锐端细胞17. protoplasm [ 'prәutәplæzәm ] 原生质, 细胞质18. resin canal=resin duct 树脂道19. epithelial[ ,epi'θi:ljәl ] cell 分泌细胞20. tracheid [ 'treikiid ] 管胞21 cambium […kӕmbiәm]形成层22. phloem […flouem]韧皮部23. grain 纹理24. pith 髓(心)25. pit 纹孔26. pit membrane […membrein]纹孔膜27. ring-porous wood 环孔材28. diffuse porous wood 散孔材Lesson ThreeTextMicroscopic Structure of WoodMost of the gross structural features of wood considered in the previous sections have their origin in the sorting and arrangement of the various cell types found in wood. When thin sections of wood are examined with a light microscope, cellular composition can readily be observed. The cells are held together by inter-cellular substance, but should this middle lamella be dissolved through chemical treatment, the cellular composite is separated into individual elements. This, in fact, is the commercial process known as pulping which is employed to produce fibers for paper manufacture. Microscopic examination of pulp samples of hardwood and of softwood reveals that there are significant differences in the sizes and shapes of cells from these two sources.Major Cell TypesAs was mentioned earlier, wood contains parenchyma and prosenchyma. Parenchyma cells in softwood do not differ greatly from those found in heartwood. They are generally short, thin-walled cells with simple pits. Their shape can be quite variable, but in the wood rays, where most parenchyma is located, they are predominantly brick-shaped, particularly in coniferous wood. In softwoods, parenchyma forms but a small portion of the total woody tissue, less than five percent of the total volume in some species, probably averaging 7 to 8% for all conifers. In some hardwoods, parenchyma may form as much as one-third of the wood volume. This occurs in woods having aggregate or oak-type rays, but the average for hardwoods is less than 20%, or more than double that of the softwoods.The tissue of softwoods is simpler from the standpoint of variety of cell types. Besides the parenchyma in the wood rays, resin canals and longitudinal tracheids make up as much as 95% of the total wood volume of some coniferous species. These are long, imperforate cells having walls which may be thich or thin, depending on location of the cell in the growth increment. In cross section, tracheids are polygonal in shape, more or less square in earlywood and flattened radially in the latewood. In compressionwood, tracheids are more or less rounded in transverse section.Compared with the prosenchymatous elements of hardwood, coniferous tracheids are very long. In some species they are 7 mm or more in length. This feature makes wood of the conifers more desirable for paper manufacturing because it contributes to greater paper strength.Ray tracheids appear in the wood of some conifers, usually at the upper or lower margins of the wood rays, but in some cases the ray may consist solely of this type of cell. These prosenchymatous elements have bordered pits that are smaller than those found in the longitudinal tracheids and which distinguish them from the ray parenchyma cells which have only simple pits. In the hard pines, ray tracheids contain dentate thickenings on their inner walls. Ray tracheids do not occur in woods of the angiosperms.The vessel is the distinctive tracheary structure of the hardwoods. As viewed in cross section, where it is called a pore, it has a larger lumen than other cell types. Individual vessel elements are tubular, open-ended cells which, when joined end-to-end, form the vessel. They are relatively thin-walled cells specialized for conduction longitudinally in the tree. Extensive pitting is typical of most vessel walls, these openings providing for lateral conduction to neighboring cells. As viewed on the cross section, pores may be arranged in clusters, chains, multiples or as solitary openings. Grouping is sometimes of diagnostic significance.Vessel elements vary in size and shape depending upon the wood species and upon location in the growth increment. Vessels of the springwood in the ring-porous woods are of large diameter while those of the summerwood are quite small. In diffuse-porous woods, vessel diameters are nearly uniform across the growth ring. The variability in size and shape of vessel elements is illustrated in Fig.1.12b, d and e.A cell type of completely different appearance than the vessel element is the libriform fiber. This type of element is thick-walled, narrow-lumened and elongated. It is adapted for strength and support rather than conduction since it has imperforate tapered ends and small, slit-like, simple pits.In addition to these two specialized hardwood prosenchymatous elements, there are others which have features that would indicate a dual role in the living tree, involving both conduction and support. Tracheids or fiber-tracheids, similar in appearance to coniferous tracheids though shorter in length, (ca. 1.5mm), are relatively thick-walledwith bordered pits. Vascular tracheids resemble small vessel elements, but the ends of the cells are imperforate. Vasicentric tracheids are more fiber-like, but are nevertheless short, thin-walled and irregular-shaped. They also have bordered pits.On a weight basis, coniferous tracheids make up 97.8% of the wood of pinus sylvestris (PERILA and HEITTO, 1959) and an equal amount in Picea abies. Therefore, the parenchyma constitute a small percentage of the weight of softwood and only about 7% of the volume.In hardwoods there is wide variation in the proportions of cell types. PERILA found that the fibers of Betula verrucosa make up 86.1% of the weight of the wood, the vessels 9.3%, and the parenchyma cells 4.6%. volumetric data for this species are not available, but in Betula lutea, the fibers make up 63.8%, the vessels 21.4% and the parenchyma 12.8%. In a more porous species, Tilia Americana L., the vessels form as much as 55.6% of the volume of the wood, the fibers 36.1%, and the total parenchyma volume is only 8.3%, a small amount for a hard wood. At the other extreme, the vessels of Hicoria ovata (now Carya ovata) constitute only 6.5% of the total wood volume, while parenchyma totals 28% and fibers 66.5%New Words1. aggregate [ 'ægrigeit ] rays 聚合射线2. Longitudinal tracheid 轴向管胞3. ray tracheid 射线管胞4. margin 边缘5. dentate […denteit] thickenings锯齿状加厚6. angiosperm 被子植物7. bordered-pit 具缘纹孔8. distinguish[ dis'tiŋgwiʃ ] 区别, 识别, 辨别9. distinctive 区别性的, 有特色的, 与众不同的10. lumen […lu:min]细胞腔(单)lumina […lu:minә]细胞腔(复)11. lateral 侧向的12. cluster 一串, 一束, 一群, 一组13. multiple pores 复管孔14. ring-porous wood 环孔材15. diffuse-porous wood 散孔材16. libriform [laibraifɔ:m] fiber 木纤维, 韧性纤维17. imperforate [im‟pә:fәrit]无孔隙的, 不穿孔的Lesson FourTextWood Cell-Wall StructureOne must delve deeply into the structure of wood to understand its basic nature. Examination with a light microscope will reveal the details of pit and wall structure essential to making identifications at the species level. Even greater detail of the wall structure is revealed by the higher resolving power of the electron microscope. These observations are not only useful for identification but also essential to our understanding of the relationships between wood structure and its properties and behavior.Examination at the higher levels of magnification, as shown in Figure 3.14, reveals the cell wall to be composed of a number of layers. The neighboring cells are joined together by a lignin-rich layer known as the middle lamella. This layer is dissolved in a chemical pulping process to separate the wood into pulp fibers. The main structural components of the cell wall are threadlike elements called microfibrils, which are organized and oriented differently in each layer of the cell wall.Primary WallThe thin, outermost layer of the cell wall, which is formed first during the development of the cell, is known as the primary wall (p). The microfibrils in this wall are arranged irregularly in flat helixes. As the cell reaches full size, the wall is thicken by the addition of a three-layered secondary wall laid down from inside the cell. The layers of the secondary wall are numbered consecutively: s1 is closest to the outside primary wall, s2 makes up the bulk of the cell wall, and s3 is nearest the inside lumen of the elongated tracheid cell.S1 LayerThe s1 layer is relatively thin and is composed of several laminae, or layers. The microfibrils in successive laminae exhibit an alternating right-and left-handed, almost flat helical arrangement.S2 LayerThis thick layer contains many laminae and makes up the bulk of the cell wall. The microfibrils are arranged in a steep right-handed helical arrangement so that they are nearly parallel to the cell axis.S3 LayerThe innermost layer of the secondary wall, the s3 layer, is similar to the s1 layer, being thin and having microfibrils oriented in flat helixes with both right-and left-handed orientations. An even thinner layer known as the warty layer may line the lumens of the tracheids and fibers of many species.MicrofibrilsThe importance of the orientation of the microfibrils to many properties of wood becomes clearer when the structure of the microfibril is considered.Each microfibril consists of linear cellulose molecules arranged in strands which pass through phases of parallel and nonparallel order, known as crystalline andamorphous regions. In the crystalline regions, the molecules are called crystallites, afterthe crystallike form they assume, and are oriented essentially parallel to the microfibril axis. In the amorphous regions, the cellulose molecules exhibit a lack of parallel order and there may be a higher proportion of shorter chain molecules. Thus, the crystallites are separated from each other by amorphous regions. The forces bonding the crystallites together along the microfibril axis are much stronger than those that bond them together laterally. As a result, strength and stiffness of wood are influenced by the orientation of the microfibril elements.New Words1. delve [delv] 探究,钻研2. reveal 揭示, 揭露3. resolve [ri:zɔlv] 分辨, 解析, 解决4. magnification 放大5. lignin 木素6. middle lamella [lә‟melә]胞间层7. microfibril[ ,maikrәʊ'faibril] 微纤丝8. primary wall 初生壁9. helix […hi:liks]螺旋线(单)10. secondary wall 次生壁11. consecutively 连续地, 顺序地12. warty layer 瘤层13. cellulose[ 'seljulәus ] 纤维素14. crystalline […kristәlain] region结晶区15. amorphous [ә‟mɔ‟fәs] region非结晶区, 无定形区16. crystal […kristl]结晶(体)17. crystallite […kristәlait]微晶Lesson FiveTextWood Chemical ComponentsThe Components of WoodIn contrast to petroleum and other nonrenewable raw materials, wood is a constantly renewable resource. All wood is formed from carbon dioxide, which is taken from the air, and from water, which is taken from the soil along with small amounts of dissolved minerals. The element composition of dry wood is about 50% carbon, 6% hydrogen, 44% oxygen, and less than 0.1% nitrogen. There is little variation in these figures from one species of wood to another. During photosynthesis, cells containing chlorophyll in the leaves and needles absorb radiant energy from sunlight and use it to convert these simple compounds into more complex substances, which eventually form the different components of wood.Wood is composed of several different types of organic material, the principal components being cellulose, hemicelluloses, and lignin. Other components present in lesser amounts are extractives (approximately 5%) and inorganic material (approximately 0.3%), consisting mainly of calcium, magnesium, sodium, potassium, and manganese, and including a wide variety of trace elements. Bark makes up 10-15% of the bole or main stem of the tree; foliage, 6-9% of the bole.Wood is stronger than steel per unit of weight. Its great strength is due to the cellulose and in part to the hemicelluloses. Lignin acts as a glue, binding the cellulose molecules together to form the basic wood structure. Wood can thus be compared with glass-reinforced resin, or fiberglass, in which the strong glass fibers are held together by the synthetic resin. Wood differs from fiberglass, however, in having a cellular structure.A wide variety of substances, commonly called extractives, are nonuniformly distributed within the walls and cavities of the cells. Many of these extractives are of relatively low molecular weight and tend to make one wood species superior or inferior to another for certain uses.LigninLignin is a complex and high molecular weight polymer built upon phenylpropane units. Although composed of carbon, hydrogen, and oxygen, lignin is not a carbohydrate nor even related to this class of compound. It is, instead, essentially phenolic in nature. Lignin is quite stable and difficult to isolate and occurs, moreover, in a variety of forms; because of this, the exact composition as it occurs within wood remains an uncertainty.Lignin occurs between individual cells and within the cell walls. Between cells, it serves as a binding agent to hold the cells together. Within cell walls lignin is very intimately associated with cellulose and serves in imparting rigidity to the cell. Lignin is also credited with reducing dimensional change with moisture content fluctuation and is said to add to wood‟s toxicity, thus m aking it resistant to decay and insect attack.Lignin is thermoplastic—meaning that it becomes soft and pliable at higher temperatures and hard again as cooling occurs. The thermoplastic character of lignin is basic to the manufacture of hardboard and other densified wood products.CelluloseThe most important component of both hardwoods and softwoods is cellulose, which generally amounts to 41-45% of the dry wood by weight. It is the main structural element of tree fibers and the major constituent of pulp.Cellulose is a linear polymer composed of several thousand glucose units. The chemical linkages between the glucose units can be broken by mineral acid, reverting the cellulose polymer to the glucose molecules for which it was built.Each glucose unit of cellulose contains three hydroxyl (-OH) groups, except the two end units, which contain four. The hydroxyl groups give cellulose its principal chemical properties and are the reactive sites to which other chemical groups can be attached in the preparation of derivatives, such as cellulose acetate. The hydroxyl groups strongly attract water molecules and thus are the major cause of the swelling and shrinking of wood. They also attract the hydroxyl groups of adjacent cellulose molecules, creating microfibrils, which are threadlike bundles of cellulose molecules that lie approximately parallel to each other (Muhlethaler 1965).Microfibrils contain regions where the cellulose molecules are sufficiently rigid and regular to be distinguished by x-ray diffraction as crystalline. These crystalline regions, or crystallites, which form up to 70% of natural wood celluloses, make much of it relatively inaccessible to chemical reaction. Water, for example, readily penetrates the amorphous regions of cellulose but cannot enter the crystallites.HemicellulosesHemicelluloses are the noncellulosic polysaccharides of the cell wall. Polysaccharides are polymers formed from simple sugars. Cellulose is a polysaccharide formed from glucose sugar only. The hemicelluloses, in contrast, are formed from a number of sugars, the most important of which are glucose, galactose, mannose, xylose,and arabinose.Hemicelluloses usually constitute 23-30% of both hardwoods and softwoods. They are structurally more complex than cellulose, lower in molecular weight (usually between 100 and 200 sugar units per molecule), and generally amorphous before isolation.New Words and Expressionsin contrast to与…相反renewable可再生的, 可更新的photosynthesis [foutou‟sinѲәsis]光合作用chlorophyll [klɔ:rәfil]叶绿素cellulose […seljulous]纤维素hemicelluloses半纤维素trace element 痕量元素foliage [foulidʒ]树叶extractives [iks‟trӕktives]抽提物cavity of the cell细胞腔glucose […glu:kous]葡萄糖hydroxyl [hai‟drɔksil]羟基galactose [gә‟lӕktous]半乳糖mannose […mӕnous]甘露糖xylose […zailous]木糖arabinose [әreibnous]果胶糖, 阿拉伯糖xylan […zailӕn]木聚糖phenylpropane [fenilproupein]苯丙烷carbohydrate […ka:bou‟haidreit]碳水化合物fluctuation [ ,flʌktju'eiʃәn ] 波动, 起伏, 变化thermoplastic […Ѳә:mo u‟plӕstik] 热塑性材料pliable […plaiәbl]可塑的, 可弯的derivatives [di‟rivәtivs]派生物, 衍生物acetate […ӕsitit] 醋酸盐polysaccharide […pɔli‟sӕkәraid]高聚糖, 多糖Lesson SixTextMechanical Properties of WoodThe strength and resistance to deformation of a material are referred to as its mechanical properties. Strength is the ability of a material to carry applied loads or forces. Resistance to deformation determines the amount a material is compressed, distorted, or bent under an applied load. Changes in shape that take place instantaneously as a load is applied and are recoverable when the load is removed are termed elastic deformation. If the deformation, on the other hand, develops slowly after the load is applied, it is termed a rheological or time-dependent property.Mechanical properties are usually the most important characteristics of wood products to be used for structural building materials. Structural application may be defined as any use where mechanical properties are the primary criteria for selection of the material. Figure 10.1 shows two structural applications of wood. Structural uses of wood products include floor joists and rafters in wood-frame homes, power line transmission poles, plywood roof sheathing and subflooring, glue-laminated beams in commercial buildings, particleboard flooring in mobile homes, laminated roof decking in commercial buildings, rails of wood ladders, sailboat masts, and frames of upholstered furniture.The term strength is often used in a general sense to refer to all mechanical properties. This can lead to confusion, since there are many different types of strength and elastic properties. It is important to be very specific about the type of mechanical properties being discussed. A wood that is relatively strong with respect to one strength property may rank lower in a different property as compared to another species. Some of the most important mechanical properties of wood products are listed in table 10.1.To appreciate the meaning of the various strength properties of wood, it is necessary to understand some basic definitions of engineering mechanics.Concepts of stress, strain, and flexureTwo basic terms used throughout the study of mechanics are stress and strain.。
