Wood-Water Relations

ISBN-13: 9783642736858 / Angielski / Miękka / 2011 / 283 str.

Christen Skaar

Opis

Wood is formed in an essentially water-saturated environment in the living tree, and the cell wall remains in this state until the water flow from the roots is interrupted, such as by felling the tree. The wood then begins to lose most of its moisture by drying, resulting in changes in most of its physical properties. These changes, and their relationship to the environment to which the wood is subsequently ex posed, are the subject of this book. The text consists of six chapters. The first chapter discusses cer tain empirical relationships between wood and water, methods of measuring wood moisture content, factors which affect its equilib rium moisture content, and the effect of moisture content on wood strength. The second chapter treats the thermodynamics of moisture sorption by wood, inc1uding enthalpy, entropy, and free energy changes. The third chapter discusses some of the theories which have been proposed to explain the sorption isotherms for hygroscopic ma terials such as wood. Chapter 4 considers hygroexpansion or the shrinking and swelling of wood associated with moisture change. Chapter 5 is concerned with how moisture moves through the cell wall of wood in response to both moisture and temperature gradients. The sixth and final chapter discusses the theoretical and practical aspects of the electrical resistance and dielectric properties of wood, in c1uding the principles involved in their application in electrical moisture meters."

Spis treści

1 Wood Moisture and the Environment.- 1.1 Humidity and Vapor Pressure of Water.- 1.2 Measurement of Wood Moisture Content and Humidity.- 1.2.1 Measurement of Wood Moisture Content.- 1.2.1.1 Gravimetric Method.- 1.2.1.2 Distillation Method.- 1.2.1.3 Karl Fischer Titration Method.- 1.2.1.4 Nuclear Magnetic Resonance.- 1.2.1.5 Electrical Moisture Meters.- 1.2.1.6 Miscellaneous Methods.- 1.2.2 Measurement of Relative Humidity.- 1.2.2.1 Hygrometers.- 1.2.2.2 Dew-Point Sensors.- 1.2.2.3 Psychrometers.- 1.2.2.4 Humidity Probes in Wood.- 1.3. Moisture Content of Green Wood.- 1.4. Equilibrium Moisture Content of Wood.- 1.4.1 Effect of Relative Humidity.- 1.4.2 Effect of History (Sorption Hysteresis).- 1.4.3 Effect of Temperature.- 1.4.4 Miscellaneous Factors.- 1.5 Moisture Content of Wood in Use.- 1.6 Fiber-Saturation Point.- 1.7 Moisture Content and Strength of Wood.- 2 Moisture Sorption Thermodynamics.- 2.1 Introduction.- 2.2 Thermodynamic Properties of Water.- 2.3 Enthalpy Changes During Moisture Sorption.- 2.3.1 Differential Heat of Sorption.- 2.3.2 Heat of Wetting and Integral Heat of Sorption.- 2.3.3 Significance of Heats of Wetting and of Sorption.- 2.3.3.1 Heat of Sorption.- 2.3.3.2 Heat of Wetting.- 2.4 Free-Energy and Entropy Changes During Moisture Sorption.- 2.5 Moisture Sorption and Specific Heat of Wood.- 2.6 Swelling Pressure of Wood.- 2.7 Mechanical Stress and Sorption (Hygroelastic Effect).- 2.8 Mechanical Stress and Sorption Hysteresis.- 3 Theories of Water Sorption by Wood.- 3.1 Introduction.- 3.2 Moisture Sorption Theories and Equations.- 3.2.1 Dent Sorption Theory.- 3.2.2 BET Sorption Theory.- 3.2.3 Hailwood-Horrobin Sorption Theory.- 3.2.4 Peirce Sorption Theory.- 3.2.5 Enderby-King Sorption Equations.- 3.2.6 Bradley and Related Sorption Equations.- 3.2.7 Capillary Condensation and Sorption.- 3.2.8 Malmquist’s Sorption Model.- 3.2.9 Additional Sorption Isotherm Equations.- 4 Hygroexpansion in Wood.- 4.1 Introduction.- 4.2 Definition of Terms.- 4.3 Volumetric Hygroexpansion of the Wood Cell Wall.- 4.4 Volumetric Hygroexpansion of Wood.- 4.4.1 Maximum Volumetric Shrinkage and Swelling.- 4.4.2 Volumetric Hygroexpansion Coefficients.- 4.5 Directional Hygroexpansion in Wood.- 4.5.1 Longitudinal Hygroexpansion.- 4.5.2 Radial and Tangential Hygroexpansion.- 4.6 Hygroexpansion and Mechanical Stress.- 4.7 Reducing Hygroexpansion in Wood.- 5 Moisture Movement in the Wood Cell Wall.- 5.1 Introduction.- 5.2 Isothermal Moisture Diffusion.- 5.2.1 General Mechanism of Bound-Water Diffusion.- 5.2.2 Eyring’s Absolute Reaction Rate Theory.- 5.2.3 Moisture Content as a Driving Force.- 5.2.4 Vapor Pressure as a Driving Force.- 5.2.5 Chemical Potential as a Driving Force.- 5.2.6 Osmotic Pressure as a Driving Force.- 5.2.7 Spreading Pressure as a Driving Force.- 5.3 Irreversible Thermodynamics and Nonisothermal Diffusion.- 5.3.1 Introduction.- 5.3.2 Derivation of Coupled Flow Equations.- 6 Electrical Properties of Wood.- 6.1 Introduction.- 6.2 Electrical Resistance.- 6.2.1 Introduction.- 6.2.2 Inherent Variables Affecting Resistivity.- 6.2.2.1 Effect of Moisture Content and Species.- 6.2.2.2 Effect of Temperature on Resistivity.- 6.2.2.3 Effect of Structural Orientation.- 6.2.2.4 Effect of Chemical Constituents.- 6.2.2.5 Effect of Wood Density.- 6.2.3 External Variables Affecting Resistivity.- 6.2.3.1 Electrode Configuration.- 6.2.3.2 Contact Pressure.- 6.2.3.3 Sample Shape and Dimensions.- 6.2.3.4 Electrolytic Effects.- 6.2.3.5 Voltage Magnitude and Duration of Application.- 6.2.4 Theory of Electrical Conduction in Wood.- 6.3 Dielectric Properties.- 6.3.1 Dielectric Principles.- 6.3.1.1 Dipole Polarization (Debye Effect).- 6.3.1.2 Interfacial Polarization (Maxwell-Wagner Effect).- 6.3.2 Dielectric Properties of Water.- 6.3.3 Dielectric Properties of Dry Wood.- 6.3.4 Dielectric Properties of Moist Wood.- 6.3.4.1 Primary Dispersion in Moist Wood.- 6.3.4.2 Secondary Dispersion in Moist Wood.- 6.4 Electrical Moisture Meters.- 6.4.1 Resistance Moisture Meters.- 6.4.2 Dielectric Moisture Meters.- References.- List of Symbols.- List of Subscripts.