Dimension stability is the degree to which a material maintains its original dimensions when subjected to changes in temperature and humidity. An inherent feature of wood materials is the variability of their linear dimensions with changes in relative humidity. As roof sheathing or construction materials, wood panels are very often exposed not only to temperature changes, but also to changes in relative humidity; they can also be directly exposed to water. This is why there are many studies that investigate the degree of the board deformations and their connection with less desirable mechanical properties or links to production-related factors. The deformation of wood due to swelling and shrinkage induced by water absorption and desorption of cell wall components is still challenging the engineering of dimensionally stable multi-layer wood-based panels.
All wood products contain moisture, from saturated fresh cut logs to the fairly dry wooden indoor structures and furniture. Moisture in wood is stored as either bound water or free water. Bound water is held within cell walls by bonding forces between hydrogen molecules of water and hydroxyl molecules of the wood cellulose. Free water is contained in the cell lumens/cavities and is held by surface tension. Plywood veneers are made by rotating the log and peeling a thin veneer from the log, moisture transfer in the veneer in the direction of the thin dimensions is equivalent to moisture transfer in the radial direction of the log. When these veneers are assembled and used in buildings and furniture, the moisture transfer through the exposed surface and into the plywood is equivalent to moisture transfer in the radial direction of the original log. Because of the rotary peeled veneers, plywood will have more uniform moisture transfer characteristics than raw timber for example, which will have moisture transfer in directions that are both radial and tangential to the wood grains.
Wood-based panels are hygroscopic and since their surface to volume ratio is very high, physical deformations and cracks are possible. In the case of plywood the strength properties of the glue lines are affected by changing moisture, resulting in a risk of delamination and panel failure. The impact of hygroscopic materials depends on many factors:- the amount and type of materials in a given room, the outdoor climate, the outdoor ventilation rate and the moisture production rate, which also depends on the indoor temperature and RH. Dimensional stability has been one of the mostly investigated characteristics of wood and wood-based panels. The hygroscopic nature of the wood material comes from the hydroxyl groups of the cell wall polymers. The dimensional stability of wood-based panels is affected by several variables such as wood species, panel density, type and concentration of adhesive, sizing efficiency and pressing conditions. The wood species has an influence on the swelling in thickness of the wood-based panels when considering the density and the chemical properties affecting the polymerization of the adhesive, some studies claim that the hygroscopic expansion of wood is dependent on the density of the cell walls. These studies have shown that the swelling is proportional to the density of the wood.
The acidity of wood and its chemical characteristics such as pH and buffer capacity will also play an important role in dimensional stability as these properties affect the polymerization of certain adhesives. For example, the urea-formaldehyde adhesive requires acidic conditions to polymerize and incomplete polymerization will induce a greater swelling. Buffer capacity is the ability to be in contact with a more acidic or basic substance without changing the pH. This will affect the polymerization of the adhesive. The type, distribution and concentration of adhesive have a significant impact on the dimensional stability and mechanical properties of wood-based panels growth & physical deformation of plywood.
Because variations in relative humidity affect the moisture content of a panel and result in changes to its dimensions, test pieces are measured after conditioning to different levels of relative humidity during testing.The equilibrium moisture content of panel products is dependent on the history of moisture change. Higher equilibrium moisture contents for any one relative humidity are achieved in desorption compared with adsorption. In order to obtain the true dimensional change, this is measured between 65% relative humidity and 85% relative humidity in adsorption and 65% relative humidity and 30% relative humidity in desorption.
Specifically for wood-based panels, there are several methods of treatment to improve dimensional stability which can be divided into three different means of application:- pre-treatment, post-treatment and production technology. The production technology methods involve those that are related to improving resin content and application of water repellents. While the post-treatment are applied to the consolidated panel and the thermal treatment is the most usual The thermal modification of wood has long been recognized as the potentially useful method to improve the dimensional stabilization of wood and increase its decay resistance.