Wood is the principal strengthening and nutrient-conducting tissue of trees and other woody plants. It is a renewable resource and has desirable physical and chemical properties.
It is a natural material that is strong in relation to its weight, and insulating against heat and electricity. It also has great acoustic qualities and provides a healthy microclimate in rooms.
Physical Properties
Wood’s physical properties play a major role in determining its use. The density of wood – or its specific gravity - determines its bulk or weight, which in turn affects its ability to carry loads. The strength of wood, determined by the ability of wood to resist indentation or scratching, defines its suitability for flooring and other furniture materials.
Wood is a relatively poor conductor of electricity but it is an excellent conductor of heat and has high thermal insulation capacity. This is due to its low specific gravity, cellular structure, and stoichiometric (equilibrium) composition.
The cellular composition of wood is responsible for its strong physical properties. The cellulose, which forms the primary cell wall, is a semi-crystalline polymer that can be cross-linked to form woody fibres, adding tensile strength and elasticity. The lignin, which is present in an amorphous form, is the main binder of the cellulose and provides wood with its strength and rigidity. Hemicellulose, which is a partly crystalline polymer that binds the cellulose and lignin together, also adds tensile strength and acts as a moisture sorption site.
Because of its cellular structure, wood is able to expand and contract in response to changes in humidity and temperature. When wood expands, it gains strength; when it contracts, it loses strength. This property is known as hygroscopicity.
While hygroscopicity makes wood a versatile material, its ability to shrink and swell can be problematic when it comes to machining and manufacturing lumber. Dimensional changes caused by shrinkage and swelling can result in the opening or tightening of joints, loss of shape, warping, case-hardening, honeycombing, and other defects.
In order to minimize the negative effects of these changes, it is important to understand wood’s hygroscopic behavior and its relation to its physical properties. The relationship between dry density and resistance in compression parallel to the grain and radial shrinkage of wood species was studied.
It was found that woods with higher density show greater resistance in compression parallel to the grain and a lower radial shrinkage. However, a strong correlation between dry density and swelling is not observed for all species. This may be due to the fact that extractives reduce shrinkage and swelling by occupying spaces within cell walls that could otherwise be taken up by water.
Chemical Properties
Wood is a natural product that contains several different chemical substances. Those substances are responsible for the unique properties of wood. Among the most important are cellulose, hemicellulose and lignin. Cellulose is a polysaccharide and is the primary building block of the wood cell wall. It is found throughout the plant and in most wood species. This organic compound has no taste, is odorless and is insoluble in water or most other organic solvents. Cellulose is also highly biodegradable and can be recycled.
Hemicellulose is another major component of the wood and is a branched polysaccharide. It is found in both deciduous and coniferous wood. Hemicellulose is not as soluble as cellulose and has side groups that are more easily hydrolysed. It is a key element in the cell walls of plants and provides much of their strength.
Lignin is a complex constituent of the wood that cements the cellulose fibers together. It is brown in color and is a significant contributor to the stiffness of wood. It also contributes to the ability of wood to resist rot and fungus.
The lignin that is in the wood is an amorphous, highly aromatic polymer of phenylpropane units. It is a mixture of phenols and aliphatic compounds and is formed by the condensation of several precursors. P-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol are precursors of lignin biosynthesis in both softwood and hardwood species.
Other chemicals in wood include volatile and odoriferous liquids called extractives. These are produced during the seasoning or drying of wood. These are comprised of a wide range of substances including waxes, fats and resins. Some of the extractives are water-soluble, some are ether-soluble and some are toluene-ethanol soluble.
The chemical properties of wood are very important for many uses. It is an exceptionally strong material for its weight and is a good heat and electrical insulator. It is also a renewable resource and has long been used for food storage (i.e., berry boxes and crates). It is often used for building and is a source of energy that produces less smoke when burning than most other fuels.
Thermal Properties
Wood is a good insulator, especially when it’s dry and light. It also has low thermal conductivity, meaning it takes a long time for heat to pass through it. This helps keep indoor temperatures stable and energy costs low. However, if exposed to high temperatures for a prolonged period, wood can lose its insulating properties and spontaneously burn.
To avoid this, a layer of insulation should be installed between the wooden flooring and the floor underneath it. Wooden floors are also often treated with fire retardants to improve their safety.
The main strength of wood is its stiffness, a property that determines how much it resists deformation or bending. This is measured by tests like shear, cleavage and impact bending. However, stiffness also depends on the grain direction of a piece of wood and its modulus of elasticity.
In addition to the bending and shear properties of wood, it’s important to consider its compression strength. This is typically measured using compression and tensile test methods, which involve putting pressure on the specimen and measuring its resulting strain. Alternatively, a stress-strain curve can be used, which measures the stress applied to the specimen over a given length and under a specific load.
Other important mechanical properties of wood include its elasticity, which is determined by tests such as tensile and compression elongation, and its hardness, which is measured by tests such as bending, shear and impact bending. These tests help to determine the stresses that can be imposed on a piece of wood without permanently damaging it, and how well it can be shaped.
The density of wood is related to its thermal conductivity, since more dense woods are usually less porous. This makes them better insulators. However, density isn’t a direct indicator of thermal conductivity, as porosity plays an important role. A study conducted by Vasubsbu et al. on various Indian tree species found that the lowest TC was obtained for curry tree, which presented 73% porosity. This is because the air in the pores acts as a poor thermal insulator. The authors also showed that a one-dimensional microscopic heat conduction model could explain the characteristics of thermal conductivity for compressed kiri wood in the tangential and fiber directions, but not the radial direction.
Acoustic Properties
Wood is a natural resonator that can be used to absorb and transmit sound waves. This is important for music instruments because it reduces unwanted resonance and reverberation in performance spaces, making it easier to hear the sounds of an instrument clearly. Wood can also help prevent the transfer of vibrations between rooms and other structures, reducing noise interference.
Wood’s ability to absorb sound waves also makes it a good choice for use in acoustic panels and other sound-absorbing materials. This is because the cells in the wood act as tiny resonators, converting the vibrating energy into heat energy and dissipating it from the surface of the material. This process is referred to as acoustic amplification.
One of the most important acoustic properties of a wood is its frequency, which determines how much a given piece of wood resonates when struck with a specific force. This property is determined by the size of the pores within a piece of wood and their arrangement. Larger pores have more mass, and therefore produce a higher resonance frequency than smaller ones. In addition, the size and shape of the pores also affects how fast the wood vibrates.
Another important acoustic characteristic of wood is its intensity, which measures the strength of the vibrations it produces when struck with a specific force. The intensity of a wood’s vibration depends on how tightly packed its pores are, and the type of wood species also plays an important role. As a result, the density of wood is an important consideration when selecting the appropriate material for musical instruments such as xylophone bars and soundboards.
In order to have good acoustic characteristics, a wood should radiate sound well and have a low characteristic impedance. For example, spruce (Picea sp.) is an ideal choice for a soundboard, as it has the highest speed of sound and sound radiation coefficient among all common woods.
Moreover, the acoustic property of wood can be improved by thermal modification. Figure 2 shows the speed of sound propagation in radial and tangential directions of spruce before and after thermal modification. The speed of sound is primarily dependent on the density and Young’s modulus of the wood, but it is affected by the direction of the grain. Thus, a wood with high transverse grain has an advantage in speed of sound.
