Chapter 9 The physical nature of wood 木材物理性能

1. Chapter 9 The physical nature of wood木材物理性能 Basically all the physical properties of wood are determined by the factors inherent in its structural organization. These may be summarized under five headings: • The amount of cell wall substance present in a given volume of wood • The amount of water present in the cell wall • The proportionate composition of the primary chemical components of the cell wall and the quantity as well as the nature of the extraneous substances present • The arrangement and orientation of the wall materials in the cells and in the different tissues • The kind, size, proportions, and arrangement of the cells making up the woody tissue

2. The first of these factors is measured by the specific gravity of the wood and furnishes the most useful index to the predicted physical behavior of wood. The second factor profoundly affects the total physical behavior of wood, not only because the addition of water to the cell wall changes its density and dimensions, but because of its effect on plasticity and transfer of energy within a piece of wood. The third factor is related to many of the special properties of certain kinds of wood as well as to the deviations from expected quantitative behavior. The last two are the cause for the large difference, which are found in the physical responses of wood with respect to the grain direction, i.e., for the anisotropic behavior of woods.

3. Topical highlights: Ⅰ. Wood and water 　　木材与水 Ⅱ. Specific gravity and density 　　木材的比重与密度 Ⅲ. Heat in relation to wood 　　木材的热学性能 Ⅳ. Electrical properties of wood 　　木材的电学性能

4. Ⅰ. Wood and water Water is a nature constituent of all parts of a living tree. In the xylem portion, water (moisture) commonly makes up over half the total weight. Since almost all properties of wood and wood products are affected by water, it is important to understand the nature of water in wood and how it is associated with the microstructure. This paragraph is devoted to this subject and in addition covers the proper use of wood products to assure satisfactory performance under a variety of service conditions.

5. 1. Location of water in wood 1-1 Location • In the cell walls Water can stays in the cell walls at any moisture content level. • In the cell lumens Water stays in the cell lumens only when the wood is saturated, in other words, when the moisture content of wood is above the fiber saturation point (FSP).

6. 1-2 Fiber saturation point • Definition The point at which all the liquid water in the lumen has been removed but the cell wall is still saturated is termed the fiber saturation point (FSP). • Implication This is a critical point, since below this point almost all properties of wood are altered by changes in moisture content. • Ranges MC = 28-35%, usually take it as 30%.

7. 2. Nature of water in wood 2-1 Free water — 自由水 • stayed in the cell lumens • held by the force of surface tension • easy to remove, just like the water in a pool 2-2 Bound water — 束缚水 • stayed in the cell walls • held by adsorption force through H-bond • difficult to remove • H-bond force increases as MC decreases 2-3 Conception of absorption and adsorption • Absorption: results from surface tension forces. • Adsorption: involves the attraction of water molecules to hydrogen bonding sites.

8. 3. Determination of moisture content 3-1 Definition The moisture content (MC) is defined as the weight of the water expressed as a percentage of oven dry weight (绝干重量) of the wood. Thus the absolute moisture content of wood :

9. The relative moisture content of wood: 3-2 Determination • In laboratory: • In factory: Using a Electricity Resistance MC Meter 3-3 MC of green wood • Heartwood: 33-98% • Sapwood : 44-250% • Some species, e.g., Eucalyptus goes to 300% .

10. 3-4 Equilibrium MC —平衡含水率 • Definition This is the moisture content of wood corresponds to a certain weather condition. • Ranges : 10-20%, usually take it as 15%. • Affecting factors: H, T, The drying condition of wood 3-5 Hysteresis phenomenon EMC of wood under adsorption process is always lower than that of wood under desorption process.

11. 4. The principle of movement of water in wood above and below fiber saturation point Any movement of water in wood involves the permeability of its microscopic and submicroscopic structure. 4-1 Above fiber-saturation point The coarser capillaries contain free liquids. The molecules of water adjacent to the capillary walls are not free but bound by chemosorption. The movement of liquid water above the fiber saturation point is caused by capillary forces. 4-2 Below fiber saturation point Bound water moves through the cell walls due to moisture gradients set up across the cell walls. This movement is a diffusion phenomenon.

12. 5. Relation of moisture content to the environment Wood exposed to an atmosphere containing moisture in the form of water vapor will come, in time, to a steady moisture-content condition called the equilibrium moisture content (E.M.C.). This steady-moisture state depends on the relative humidity, the temperature of the surrounding air, and the drying conditions to which the wood has previously been exposed; it fluctuates with changes in one or both of these atmospheric conditions. Products manufactured from wood tend in most cases to have slightly lower EMCs than the raw wood from which they are produced. This is partially because of the heat-treatment effect mentioned above but also because of the addition of resins, coatings, and sizing material, which in themselves are usually less hygroscopic than wood. Plywood and laminated wood products have EMC characteristics very similar to wood or lumber. Fiber and particle products, however, many exhibit considerably different characteristics.

13. 6. Moisture movement in wood In order to arrive at an equilibrium condition the moisture must move within the piece of wood.The movement internally is dependent on time and the direction of movement with respect to the major axes of the wood. 6-1 Difference in direction • Water in wood moves 12 to 15 times faster along the grain as it does across it. • But for thin boards, especially veneers, most water lost or gained through the sides instead of from the ends.

14. 6-2 Time influence It is apparent that at the 1st stage the moisture gradient is steep and the loss of water is high. As the wood becomes drier, the moisture gradient decreases and approaches zero as the final moisture content equilibrium for the lumber is reached.

15. 7. Moisture content of green wood 生材含水率 7-1 Definition MC of wood in fleshly felled condition 7-2 Significance • affect the weight logs and green lumber • affecting shipment cost • affecting purchase cost on weight basis 7-3 Ranges • heartwood: 33-98% • sapwood : 44-250% 7-4 Influence factors • tree species and age • location in a tree • the felling season

16. 8. Shrinking and swelling /干缩与湿胀 As wood loses moisture below the FSP, i.e., loses bound water, it shrinks. Conversely, as water enters the cell wall structure, the wood will swell. • Phenomenon Wood floor swells in spring and summer and shrinks in fall and winter. • Definition

17. Not completely reversible in welling and shrinkage — due to the two different reference bases — due to the hysteresis effect in the adsorption process • Mechanisms of shrinking and swelling of wood — Shrinking: the collective effect of cellulose chains moving closer when losing water. — Swelling: the collective effect of cellulose chains moving apart when gaining water.

18. 8-1 Anisotropic dimensional changes in wood木材尺寸变化的各向异性 • Definition The observed dimensional changes in wood are unequal along the three structural directions (St > Sr > Sz). This is called wood anisotropy in dimensional changes. • General ranges of shrinkage St: 6-12%; Sr: 3-6%; Sz: 0.1-0.3% • Differential shrinkage (差异干缩） D = St / Sr • Means to assess stability of wood species The smaller the value of St, Sr and D, the better.

19. 8-2 Basic causes for anisotropic dimensional changes (1) Sz < Sr and St The difference between longitudinal and transverse directions can be traced to cell and microfibril orientation. — Shrinkage results from the closing action of cellulose chains in the cell walls, so that it occurs mainly in the direction transverse to the cellulose chains; — In the cell walls, S2-layer is the main part, amounting about 70%, and the microfibril angle on the S2-layer is very small (10-30o). That is to say that on the cell walls, most of the cellulose chains are nearly parallel to the cell axes. So for a single cell, shrinkage mainly occurs in the direction transverse to the cell axes. — In a piece of wood, most of the cells are arranged parallel to the length of the log, so when millions of cells shrink transverse to their cell axes, the collective effect is transverse shrinking to the log length.

20. (2) Sr < St The main causes for difference in radial and tangential directions are as follows: — Restraint action of the radially oriented wood rays The low radial shrinkage of ray tissues restrains the relatively weak early wood and reduce its radial dimensional change. — Earlywood and latewood arrangement Tangential shrinkage and swelling are largely controlled by the changes in the late wood, since this part of the growth increment is strong enough to force the early wood to comply with it. The radial dimensional changes, on the other hand, are a summation of the weighted contributions of each part of the annual increment. It is smaller than that in the tangential direction because of the presence of the low-shrinking earlywood component in the total.

21. — The numerous and large pits on radial walls Usually the radial walls are heavily pitted with large pits, and the tangential walls are sparely pitted with small ones. Pits are actually holes on the cell wall, no shrinkage can occur to it. Therefore radial walls will shrink less because of the heavily pitted large pits. — Other factors * The restraint action of the radially oriented knots * The higher lignin content in radial walls than in tangential walls * The smaller microfbril angle in radial walls than in the tangential walls * Tangential shrinkage begins at higher moisture content than radial shrinkage.

22. 8-3 Dimensional changes of wood in service 使用中木材的尺寸变化 It follows from the previous discussion that wood used where the humidity fluctuates will continually change moisture content and therefore dimension. For interior uses such as furniture and millwork, it is much more critical for satisfactory performance to use lumber at the proper moisture content.

23. 8-4 Modifying factors for dimensional changes 尺寸稳定性的影响因子 • Wood density Wood density can be used as a rough indicator of the dimensional changes in wood. Generally speaking, the denser the wood, the larger the dimensional change of the wood when suffers humidity fluctuation. • Content of extractives Wood extractives tend to restrict shrinkage by their bulking action on the cell wall. • Content of hemicelluloses Wood with low content of hydrolysable hemicelluloses tends to be more stable dimensionally than those woods with normal or high contents of these types of polysaccharides

24. 8-5 Means of reducing moisture-induced dimensional change in wood products • Preventing moisture sorption by coating the product, such as painting. • Preventing dimensional changeby restraint that makes movement difficult or impossible, such as plywood production. • Treating wood with material that replaces the bound water in the cell wall, such as with polyethylene glycol (聚乙二醇）. • Treating wood to produce mutual cross-linkingof the hydroxyl groups in the cell wall, such as with formaldehyde. • Impregnation wood with plastic monomers, such as methyl methacrylate（甲基丙烯酸酯）, e.g., WPC production.

25. Ⅱ. Specific gravity and density wood density is its single most important physical characteristic. Most mechanical properties of wood are closely correlated to it.The physic-mechanical properties of wood are determined by three characteristics: • the porosity or proportion of void volume, which can be estimated by measuring the density; • the organization of the cell structure, which includes the microstructure of the cell walls and the variety and proportion of cell types ; • the moisture content.

26. 1. Definition of wood density and specific gravity • Density: D = mass / volume • Specific gravity: sg = weight / volume = weight of wood / weight of water of same volume at 4oC The two are equal in numerical value, so they are interchangeable in practice. • Importance of D D ↑, wood substance ↑, wood strength ↑ • Determination of wood density • Weighting: a method according to the definition • X-ray density meter • Pylodyn wood density tester

27. 4. Density of wood substance（木材实质密度） Density of wood substance, which constitutes the cell wall, has been determined to be about 1.5 grams per cubic centimeter in the ovendry state. Little variation exists in this value for different kinds of wood, as long as the same experimental procedures are employed. 5. All kinds of terms on wood density • Basic density（基本密度） = Wmin / Vmax • Oven-dry density（绝干密度）= Wmin / Vmin • Air-dry density（气干密度）: density of air dried wood • Green wood density（生材密度）: density of green wood

28. Ⅲ. Heat in relation to wood 1. Thermal properties of wood 1-1 Thermal conductivity of wood (K) /木材导热系数 • Conception Thermal conductivity is a measure of the rate of heat flow through materials subjected to a temperature gradient. In the English system thermal conductivity of wood (K) is measured as the amount of heat, in British thermal units (Btu), that will flow in 1 hour through a homogeneous material 1 inch thick and 1 foot square, when 1 ℉ temperature difference is maintained between the surfaces. • Affecting factors — direction of heat flow with respect to the grain, Kz > Kt, Kr — moisture content of wood, MC ↑, K↑ — specific gravity of wood, sg ↑, K ↑

29. Wood possesses good heat insulating properties

30. Evaluation of thermal conductivity of wood The transverse thermal conductivity (K) of wood can be evaluated: K = G (1.39+C (MC) ) + 0.165 G: specific gravity MC: moisture content in percent C: a constant depending on the moisture content, with a value of 0.028 below 40 percent MC, and 0.038 above • The thermal insulating value (R) The thermal insulating value (R) of wood is the reciprocal of the thermal conductivity; i.e., R=1/K. From the previous discussion it is apparent that the insulating value for wood is inversely proportional to the specific gravity and moisture content. This relationship explains the use of low-density, dry balsa wood (Ochroma lagopus Sw.) for insulating purposes.

31. 1-2 Thermal diffusivity of wood(ρ) /木材热扩散系数 • Conception Thermal diffusivity is a measure of the speed with which a material can absorb heat from its surroundings. It is defined as the ratio of thermal conductivity to the product of density and specific heat. ρ= K / (D*c) • Ranges of ρ Wood has a low thermal diffusivity because of the low thermal conductivity and moderate values for both density and specific heat. Wood is quoted as having a thermal diffusivity of 0.00025 square inch per second in comparison with 0.02 square inch per second for steel and 0.001 square inch per second for mineral wool. The low thermal-diffusivity values associated with wood account for the comfortably warm feeling of wood furniture in contrast with metals and plastic when these are used for furniture.

32. 1-3 Thermal expansion of wood (α) /木材热膨胀 • Conception The measurement of the dimensional changes of wood caused by temperature differences is called the coefficient of thermal expansion • Ranges of α The coefficient of thermal expansion for the longitudinal direction (αL) in the temperature range from –50℃ to +50℃ averages 3.39×10-6 per degree Celsius, regardless of kind of wood and its specific gravity. In the transverse direction the expansion is about 10 times that in the longitudal direction. For an average specific gravity of 0.46, the coefficient of radial expansion (αR) is 25.7×10-6 per degree Celsius, while that for tangential expansion (αT) is 34.8×10-6 per degree Celsius.

33. Wood possesses good thermal stability Longitudinal thermal expansion in wood is small in comparison with that of other common solid materials. However, the transverse thermal expansion in wood is great than it is for any of the metals and other common materials. For example, the coefficient of thermal expansion per degree Celsius for steel is 10×10-6, for aluminum 24×10-6,and for flint glass 7.9×10-6. The reason that the thermal changes are not more commonly recognized is that wood is usually used within a narrow range of temperatures and the dimensional changes caused by moisture fluctuations are large enough and usually in an opposite sense so that the thermal effects are masked.

34. 2. Combustion of wood /木材的燃烧 2-1 Ignition of wood /木材燃点 The ignition temperature of wood is usually given as about 275℃ and is actually the temperature at which wood begins to decompose exothermically（放热分解）. 2-2 Fuel value of wood /木材的燃烧值 Fuel value of wood is primarily determined by the density of the wood and its moisture content. It is modified by variations in lignin content and to a much greater extent by the presence of extractives such as resins and tannins. The heat of combustion (H), i.e., the heat in Btu produced by burning 1 pound of ovendry wood, averages about 8500 Btu for hardwoods and 9000 Btu for conifers. These values for heat of combustion bear little relationship to a particular kind of wood and vary only from 5 to 8 percent at a maximum.

35. Ⅳ. Electrical properties of wood 1. Direct-current electrical properties of wood 1-1 Conception The electrical properties of wood are measured by its resistivity（电阻率r）, or specific resistance（比电阻）, or by its reciprocal, conductivity（电导率））. 1-2 Ranges of r Air-dry wood is an excellent electrical insulator, with dc resistivity in the order of 3×1017 to 3×1018 ohm-centimeters at room temperature.

36. 1-3 Affecting factors • Moisture content of wood The resistivity decreases rapidly by an approximate factor of three for each percentage moisture content increase up to the fiber saturation point. at the fiber saturation point it becomes approximately that of water alone, i.e., 105 to 106 ohm-centimeters. • Temperature of wood Temperature is also an inseparably related influence causing a decrease in the resistivity with increasing temperatures. • Grain direction In general resistivity across the grain is from 2.3 to 4.5 times greater than that along the grain for conifers and from 2.5 to 8.0 times greater for hardwoods.

37. 2. Alternating-current characteristics of wood 2-1 Conception One measure of the insulating capacity of a material under alternating current is its dielectric constant (介电常数ε). This is expressed as the ratio of the charge held by a condenser (电容）in which the electrodes（电极） are separated by a dielectric material such as wood, to the charge held by the condenser with the electrodes separated by a vacuum at a given voltage. 2-2 Ranges of ε ovendry wood substance is a nonconductor, with a dielectric constant of about 2. Above the fiber saturation point, the dielectric constant of fully saturated wood approaches that of water, which has a value of 81.

38. 2-3 Affecting factors • Wood density D↑, ε↑ • Moisture content of wood MC↑, ε↑ • Frequency of the alternating current f ↑, ε↑ • Grain direction of wood In dry wood the dielectric constant is from 1.3 to 1.5 times greater in the longitudinal direction than it is in the transverse direction.

39. Reflection and practice: • Definition and implication of fiber saturation point, and its general range? • What is called free water and bound water? • Definition of moisture content? • Conception of equilibrium moisture content of wood and its general range? • mechanisms of wood shrinking and swelling? • Anisotropy of wood in dimensional shrinkage? • Definition of differential shrinkage? • Basic causes for anisotropy in dimensional shrinkage? • Definition of basic density of wood? • Affecting factors of thermal conductivity of wood? • Influence of moisture content and temperature on electrical resistivity? • Concept of dielectric constant?