Лако-красочные материалы — производство

8.4.1.1 The Molecular Structure of Wood

The dry weight of wood is composed of 40-50% cellulose, 20-40% hemicellulose, 20-30% lignin and 2-7% extraneous materials including resins, fats, peptides, tanning substances and coloring matters. Depending on the species of wood, the site and the lumbering time, freshly cut wood will contains between 50 and 150% of water in relation to its dry weight (oven-dry weight). Wood contains ‘free’ water which is stored in the cell cavities and spaces, and ‘bound’ water which is directly associated with the cellulose fibers. Cellulose molecules have long and unbranched chains and are composed of repetitive cellobiose units. From the chemical viewpoint, cellobiose is a disaccharide consisting of two glucose units. Cellulose itself belongs to the polysaccharides (sugar) class, characterized by a high content of hydroxyl groups. These polar functionalities not only provide for the formation of hydrogen bridge bonds, but also for the production of covalent bonds with reactive adhesives. The cellulose molecules are arranged into fibrils, which are organized in parallel to make up the walls of the cells, which in turn are aligned along the vertical direction of the stem. Lignin is a mixture of polyphenolic molecules. At the end of cellular growth, these substances are integrated into the cellulose cell walls where they act as a binder and induce lignification, during which process the density and mechanical strain to which the wood may be exposed are substantially increased. Hemicellulose also consists of sugar molecules, without being structurally related to cellulose. The constructive polymers are composed of D-xylose which is a pentose. The molecules have considerably shorter chains than those of cellulose; furthermore, the chains have branches of arabinose and other sugars. Hemicellulose is easier to decompose than cellulose, and acts as a wall-building and reserve substance. Water is integrated into the fiber structures via hydrogen bridge bonds as ‘bound’ water, and then stored in the cell cavities and spaces where it can move freely as ‘free’ water. This has a decisive effect on the dimensions of wood-based materials. Up to two-thirds of healthy green wood consists of water, and this is known as the ‘maximum swelling condition’. During the drying process, the free water evaporates first, leading to a

fiber-saturation range of 23-35% of wood moisture. By means of a further natural or technical drying processes, an equilibrium moisture content of 6-15% is obtained, within which range the wooden work piece will later adapt to the ambient conditions.