Structural and other properties of modified wood

Wood is not under all circumstances an appropriate building material. Beside others, structural properties and biological durability determine the limits for the use of wood. Until recently, the only industrial applied method to improve the durability of wood was impregnating with toxic wood preservatives. Although much progress has been made in improving the fixation of preservatives (i.e. emissions during use are very low), the general public regards them as environmentally unfriendly. Modification of wood aims at improving the durability in an environment friendly way, not by toxifying but by altering the substrate. Apart from durability, properties like dimensional stability, hardness, UV-resistance and adsorption behaviour are improved by modification. These improvements make wood an appropriate material in much more applications than untreated wood. In this paper two modification methods are discussed: • Acetylation of wood, a chemical modification process, which recently resulted in the completion of a seem-industrial pilot plant. • The development of timber modified with the so-called Plato process, a hydro-thermal method, which recently resulted in the construction of a commercial plant. Besides the increase of durability, the reduced shrinkage and swelling and the increase of UV-resistance, the mechanical properties are also affected by modification treatments. This is also discussed in this paper. THE PRINCIPLE OF WOOD MODIFICATION The polymeric structure of a wooden cell wall mainly consists of cellulose, hemicellulose and lignin. All of these components contain hydroxyl groups. These hydroxyl groups play a key role in the interaction between water and wood. At the same time these groups are the most reactive sites. If wood takes up water (in moist conditions) water molecules are settling between the wood polymers forming hydrogen bonds between the hydroxyl groups and individual water molecules. This water needs space between the cell wall components, which results in swelling of the wood. All possible types of wood treatments affect the above-described mechanisms in one way or another. For instance, filling lumina, which a substance, without altering the cell walls. Or, bulking the cell wall, where the cavities in the cell wall are filled and thereby the pathways for water are blocked. An example of bulking is a treatment with resins that do hardly react with the wood. Since hardly any molecular alteration of the wood is achieved, bulking is not regarded as being wood modification. Modification of wood is a wood treatment, where the cell wall polymers of the molecular structure of the cell wall polymers (cellulose, hemicellulose and lignin) are altered. In thermal wood modification (parts of) the cell wall polymers are altered. This may lead to cross-linking, reduction of OH-groups and/or (undesired) cleavage of the chains. The reduction of accessible OH-groups leads to a limited interaction with water compared to untreated wood. Examples of thermal treatments are the Perdure process, the Stellac process and the Plato process. 1 SHR Timber Research, Wageningen, The Netherlands 2 SHR Timber Research, Wageningen and ABT Consulting Engineers, Velp, The Netherlands The untreated cell wall Cell wall polymers with OH-groups Water molecules between cell wall polymers Filled lumina Unaltered cell wall polymers The bulked cell wall Bulking chemicals in the cavities The modified cell wall Cross linking Substituting of OHgroups by more hydrophobic groups Removal of OH-groups and cleavage of chains Figure 1: Different wood treatments. In chemical modification, more hydrophobic chemical groups replace the OH-groups. This can be done by: etherification, esterification, urethanes, oxidation, silylation and many more chemical reactions. An overview of possibilities is given by Militz, Beckers and Homan (1997). An example of these types of treatments is the acetylation of wood. ACETYLATION Wood itself by nature already contains some minor amounts of acetyl groups, which is increased by acetylation. Off all chemical treatments, acetylation with uncatalysed acetic anhydride has been studied most and shown to be one of the most promising methods. Acetylation is a well-known technique in many industrial and scientific areas. It is widely used in textile fabrication and filters of cigarettes are made of acetylated cellulose. During the reaction of wood with acetic anhydride hydroxyl groups of the cell wall polymers are converted into acetyl groups (see figure 2) which are hydrophobic. Thus the molecules of wood are altered. During acetylation acetic acid is formed as a by-product which can be converted into acetic anhydride and be used again. Because small hydroxyl groups are substituted with larger acetyl groups, the wood will remain in a permanently swollen state and become heavier. The effect of the treatment can therefore be expressed as a weight percent gain (WPG). A higher WPG represents a higher degree of acetylation. Like untreated timber, acetylated wood only consists of carbon, hydrogen and oxygen and, consequently, disposal of acetylated waste wood can be handled as untreated wood. Wood + acetic anhydride acetylated wood + acetic acid Figure 2: Acetylation: replacing OH-groups by larger and heavier CH3-groups Material properties of acetylated wood Research has shown that acetylation of wood can considerably improve certain properties of this material. If wood is acetylated well enough it will have a durability comparable to durability class 1: it does not rot at all in ground contact for more than 25 years (comparable to teak or even better). A WPG of 15% is sufficient to prevent degradation of wood by white rot and brown rot fungi such as the dry rot fungus and the cellar fungus. Acetylated wood with a WPG of 10% is fully protected against soft rot degradation, which plays an important role for wood in ground contact. Due to acetylation the equilibrium moisture content of wood becomes very low. And the wood will stay very dry. If acetylated up to 20% WPG the wood moisture content will never exceed 10%. Furthermore it takes the acetylated wood a considerable longer time to reach this equilibrium compared to the untreated wood. After acetylation swelling and shrinkage of wood can be reduced by up to 80%. The swelling and shrinking of wood is directly related to the moisture content of the cell wall. Pine sapwood with a 20% degree of acetylation will swell less than 3% tangential from oven dry to fibre saturation point. Figure 3 shows the hygroscopicity, which is strongly correlated to the shrinkage and swelling, of acetylated wood (and thermally treated wood) compared to untreated wood. Figure 3: Hygroscopicity of treated (chemically or thermally) and untreated Scotch pine. The hygroscopicity can be reduced to less than 50% of the untreated wood. + H3C H3C O O