When moisture comes into contact with wood, the water molecules penetrate the cell wall and become bound to cell wall components through hydrogen bonding. With addition of water to the cell wall, wood volume increases nearly proportionally to the volume of water added. Swelling increases until the fiber saturation point has been reached. Water added beyond this point remains as free water in the lumen and does not cause further swelling. This process is reversible and accounts for the dimensional changes that occur when wood comes into contact with water vapor or liquid. Though wood is one of the few natural products used throughout history with almost no modification of its properties, the tendency to change dimensions with changes in moisture content has caused problems with wood in use.
A large effort was conducted to modify and improve the properties of wood with different techniques including physical and chemical modification processes. The effects of wood modification processes on the chemical and mechanical properties of wood have been extensively summarized and reviewed. There are two basic types of wood treatments for dimensional stability:
1) Those which reduce the rate of water vapor or liquid absorption but do not reduce the extent of swelling to any great degree.
2) Those which reduce the extent of swelling and may or may not reduce the rate of water absorption. The different methods of treatment for dimensionally wood stabilizations are summarized as follow.
Heat Treatment
Using heat to treat wood has been studied for many years, and has been used commercially during the last decade. This technology is eco-friendly and does not require the use of chemicals. The effects of heat treatment on the dimensional stability, moisture sorption, and other wood properties have been extensively reviewed. Heat treatment, at temperatures ranging from 160 to 220 °C in different shield gases, causes the hemicelluloses and lignin to degrade. Consequently, this process leads to an increased dimensional stability and decay resistance. The different variables are such as wood species, types of media gas, maximum temperature and duration at maximum temperature, on the equilibrium moisture, and antishrinking efficiency which effects of heat treatment on dimensional stability of wood.
Chemical Modification
Considerable effort has been devoted to the chemical modification of wood to improve the properties of wood materials. The typically chemical modifications on the dimensional properties of wood are done by acetylation and furfurylation. With it the treatment conditions and the reasons for dimensionally stabilizing wood can be checked.
Acetylation
Acetylation is a chemical modification method, which occurs between the hydroxyl groups of wood and the acetic anhydride molecule. The ester linkages formed between the wood and acetic anhydride modifies the hydroxyl groups, which consequently prevents the interaction of water with the wood. Acetylation was found to efficiently improve dimensional stability, and therefore, biological resistance against fungal decay.
Furfurylation
The chemical modification of wood products with furfuryl alcohol is called furfurylation. The furfuryl alcohol, which has a sufficiently small molecular size, can impregnate wood cell walls and polymerize within the wood structure using a catalyst, heat or penetrating radiation. Furfuryl alcohol originates from biomass waste and is a renewable and natural material with no added metals or halogens. It has been proven that furfurylate wood dimensionally acceptable product. Therefore furfurylation is an environmentally friendly alternative to wood treatment with heavy metals.
Surface Hydrophobization
The hydroxyl groups, which are contained in the components of the wood cell wall, are responsible for the absorption of moisture vapor from the surrounding air, thereby resulting in dimensional changes. Therefore, the dimensional stabilization of wood is always associated with the transformation of wood from hydrophilic to hydrophobic. The hydrophobization of a wood surface occurs by blocking contact between the surface hydroxyl groups and surrounding moisture, which serves to prevent the absorption of water from the environment.
Coating Protection
Coatings can be used to stop water absorption into the wood by blocking the direct contact with water, thereby dimensionally stabilizing the wood. Good surface coatings sufficiently retard the rate of moisture absorption, which minimizes the steepness of the moisture gradient of wood. Super hydrophobic surfaces, with water contact angles larger than 150° and sliding angles less than 10°, have many attractive characteristics, such as water repellency, lubricity, self-cleaning, antifouling, and others [18]. A low surface energy coating on a surface with a highsurface roughness can create a superhydrophobic surface.
Impregnation
Treatment The dimensions of wood can be stabilized either by blocking the wood cell lumen, which reduces water absorption, or swelling and bulking the fibers of the cell wall structure . The deposition of water-insoluble materials within the cell wall structure is an effective and practical approach for keeping the wood cell wall structure in a partially or complete-ly swollen state. This can be achieved via impregnation treat- ment with various appropriate agents, such as resin and wax, by which the dimensional changes attributable to atmospheric moisture can be considerably minimized.
Wax Impregnation
Impregnating wax into solid wood is an effective method for improving wood quality and increasing its utilization. Because of their hydrophobic properties, waxes are widely used in the coating industry as water repellents for wood surfaces. Their use as wood-modifying agents also improves dimensional stability, weathering durability, whiterot and brown-rot fungi resistance, and mechanical properties .Wax treatment physically fills the large cell lumens to slow the sorption rate of moisture. Wax can also be deposited within the wood cell wall structure to modify its properties. Wax has been used as a bulking agent to reduce swelling and shrinking, thereby improving dimensional stability. However, during wax impregnation, there is no chemical reaction between wax molecules and wood substrates.
Different wood modifications have been extensively studied and applied for protecting wood against dimensional changes attributable to moisture. Each has advantages and disadvantages. Based on the literature review, it can be concluded that:
Both coatings and plasma treatment can be used to develop wood hydrophobic surfaces, which can consequently reduce or delay the water and moisture sorption of the wood, while also dimensionally stabilizing the wood. The limitation of using coatings to protect wood against moisture dimensional change is the inability to coat the entire pore structure of the wood.
Wax treatment is an efficient method for improving the dimensional stability and mechanical properties of wood. However, wax reduces the adhesion of wood, and the low melting point of wax also limits its utilization. Acetylation and furfurylation are efficient, environmentally friendly ways to improve dimensional stability. Heat treatment can significantly improve the dimensional stability of wood. However, the strength properties of heattreated wood are affected by the heat treatment.
AUTHOR: S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)