The most important property of lumber as a packaging material is its hygroscopicity. Depending upon the moisture content of the ambient air, lumber absorbs or releases water vapor, so establishing an equilibrium moisture content. If the lumber is in a relatively dry environment, it tends to release water vapor. Conversely, it readily absorbs water vapor in a relatively moist environment. The precise water content of lumber may be determined either by electrical measurement with a moisture meter or by a kiln drying test (see DIN 52138), in which the lumber is artificially dried until its mass no longer changes.
Lumber intended for building boxes should have a water content of between 12 and 15%, as these values approximately match the equilibrium moisture content at normal relative humidities. Figure 1 shows the sorption isotherm for lumber at 20°C.
||Figure 1: Sorption isotherm for lumber at 20°C
|(Click to enlarge)
Excessively moist lumber may, for example, cause corrosion in package contents or accompanying cargoes which are at risk of corrosion.
The extraction resistance of nails and screws also falls as the lumber moisture content rises. The moister is the lumber during processing, the more nails or screws will be required. If the wood is processed at a water content of approx. 15%, the extraction resistance remains unchanged for several weeks afterwards. However, if the lumber is excessively moist at the outset, extraction resistance falls as the lumber subsequently dries out because the screw and nail holes get larger due to shrinkage. The lumber "works", i.e. swells as it absorbs moisture and shrinks as it dries.
Both excessively moist and excessively dry box lumber may have a negative impact upon transport operations due to dimensional changes.
Subsequent drying may have a negative impact because lumber is subject to sometimes considerable shrinkage. Screw and nail holes also enlarge due to shrinkage, so reducing extraction resistance and impairing box stability. Moreover, gaps may suddenly appear between boards which initially fitted closely together, so further degrading box stability. The greatest shrinkage occurs across the width and thickness of boards, while length remains virtually unchanged.
If the lumber is excessively dry during processing, subsequent absorption of water and the resultant swelling may cause boards to lever each other apart, so destabilizing screw and nail joints and impairing the strength of the structure.
The specific weight of lumber is highly variable as it is largely determined by the species of wood and lumber moisture content. A distinction is often drawn between softwoods (spruce, pine, fir, alder, lime, willow, poplar) and hardwoods (beech, oak). The average specific weight of softwoods may be assumed to be approx. 450 - 550 kg/m3, while hardwoods are somewhat heavier at a specific weight of approx. 650 - 750 kg/m3.
The strength of moist lumber is lower than that of dry lumber. In comparison with lumber with a moisture content of 10%, lumber with a moisture content of 20% has only half the compressive strength and only approx. two thirds the flexural strength.