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1、Wood ScienceProfessor WU YiqiangFirst-class Course in Hunan ProvinceKey National Discipline on Wood Science and Technology Central South University of Forestry & TechnologyChapter 6 Physical Properties of Wood(木材物理性质)1 Density and Specific of Gravity (木材的密度和比重) 1.1 Defininition of density and specif
2、ic of gravity(木材的密度和 比重的定义) 1.1.1 Density(密度)Density is the ratio of mass to volume D=m(质量)/v(体积)where D=density in kg/m=mass in kgv=volume in 1.1.2 Specific Gravity(比重) In the case of wood the ovendry weight is used as a basis,and compatisionmmn is made with the weight of the displaced volume of wa
3、ter.Stated as an equation this becomes ： SG =( OD mass/volume)(绝干材的密度） /density of water( 水的密度) 1.2 Classfication of Density (木材密度的分类)1. Density of Green Wood (生材密度)2. Density of Air-Dried Wood (气干材密度):density of the air-dried wood whose moisture content （含水率）arrives at 12 percent.3. Density of Oven
4、 Dried Wood (绝干材密度): density of the moisture-free wood. 4. Basic Density(基本密度) :the OD weight of wood（木材的绝干重） /the volume of the wood whose moisture content is above the EMC（试样饱 和水分时的体积）.1.4 The Related Factors of Density (影响木材密度的因素)The related factors of density are as follows:the species of the wo
5、od(树种), moisture content(含水率),the width of annual ring(年轮宽度) ,the latewood rate (晚材率),the different parts of the tree stem (树干的不同部位) and so on.1.3 Methods of Determining Density (测量木材密度的方 法)1. Direct Determining Method(直接量测法)2. Immersion Method(排水法)3.Immersion in Mercury Method(水银测容器法)2 Wood and Wat
6、er (木材和水分) 2.1 Water in the Wood 2.1.1 Location of water in wood (木材内水分存在的部位)Water is taken up by wood in three different ways:(1)in the cell wall materials as bound water(吸着水) , and (2)as free water(自由水) in liquid form in the cell lumina adhering to the cell walls or filling the cell cavity.2.1.2 T
7、he difference between adsorption (吸附水)water and absorption water (吸着水)Absorption results from surface tension and capillary forces.Adsoption,in contrast ,involves the attraction of water molecules to hydrogen-bonding (氢键结合点) sites present in cellulose, hemicellulose , and lignin.Hydrogen bonding occ
8、urs on the hydrongen side of the hydroxyl(-OH) groups found throughout the chemical elements of wood.That is to say,absorption water(吸着水) consists of adsorption water(吸附 水) and microcapillary water(微毛细管水).2.1.3 The energy variation of the water in wood (木材中水分的能量变化)2.2 Moisture Content and Its Measur
9、ement (木材含水率及其测 量)2.2.1 Moisture Content and Relative Moisture Content (绝对含水率,相对 含水率)1. Moisture content (MC)(绝对含水率)is defined as the weight of the water expressed as a percentage of the moisture-free or ovendry wood() weight: percent MC=(weight with water OD weight )/ODweight 100. 2. Relative moist
10、ure content（相对含水率） relative moisture content =(weight with water OD weight )/weight with water 100%.2.2.2 The Measurement of Moisture Content (含水率的测量)1. Oven-drying Method(烘干法) The Oven-drying Method as a rule is the most exact method ,but it is slow and requires that samples be cut from the materia
11、l.2. Distillation Method(蒸馏法) 3. Hygrometric Methods 4. Resistance type Moisture Meters(电阻测 量法)2.2.3 The fiber saturation point(F.S.P) (纤维饱和点) The F.S.P represents that moisture content at which the cell wall is completely saturated with water ,but no water is present in the cell lumen.The F.S.P is
12、not completely uniform between different wood,but generally falls between 25 and 30 percent moisture content. 2.3 Sorption and Equilibriem Moisture Content(木材的吸湿 性和平衡含水率)2.3.1 adsorption(吸湿) and desorption（解吸） Wood is a kind of solid with a complex capillary structure.If the vapor pressure inside th
13、e wood is lower than the surrounding s,any piece of wood will adsorb surrounding condensable vapors until an equilibrium content is reached.This phenomenen called adsorption(吸湿) If wood is in equilibrium with thesurrounding environment and the air then becomes drier ,the wood will desorb water until
14、 it again comes into equilibrium. This process called desorption（解吸） .The term sorption(吸湿性) is applied to the combined or general phenomenon of adsorption and desorption.2.3.2 Equilibrium Moisture Content The moisture content that wood or a wood-based product attains when in an environment of const
15、ant temperature and humidity is termed equilibrium moisture content (平衡含水率) (EMC).2.3.3 lag effect (吸湿滞后)2.3.4 Sorption isotherms(等温吸附)The relationship between the relative vapor pressure in the environment and the moisture content of wood in equilibrium with that environment is not linear .This rel
16、ationship results fron the different ways in which bound water is held.The graph of this relationship at a constant temperature is called sorption isotherm(等温吸附).Sorption isotherms of white sptuce at 25c2.3.5 Shrinking and Swelling (木材的干缩湿胀)1. As wood dries below the FSP,that is ,loss bound water ,it shrinks(收缩) .Conversely,as water enters the cell wall structure ,the wood swells(膨胀).This dimensional change is a reversible process in
