The Cell Wall of Wood

The space between the microfibrils is occupied by the hemicelluloses and by lignin. However, the incomplete filling of the intermicrofibrillar region results in the existence of what are usually referred to as micropores (or microvoids) in the cell wall. These have diameters of the order of nanometres and thus technically should be referred to as nanopores, but since the term micropores is the most commonly used in the literature, it will be used throughout this book. 

When the cell wall is fully swollen, the micropores are open and the interior of the cell wall can be accessed by entities that are smaller than the diameter of the micropores. When wood is dried from a water-saturated condition, as water is removed the lumen and other macrovoids, and then subsequently the ceU wall, lose moisture. As the water is removed, the micropores begin to collapse, and this process continues until the wood is dry. 

The determination of the accessibility of the cell wall interior is of importance for a number of reasons  

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For a detailed description of the ultrastructure of wood and the cell wall, the reader is referred to the comprehensive texts listed above. Briefly, the cell wall of wood is composed of a number of discernable layers (Figure 2.2). These are divided into the primary (P) and secondary (S) layers the secondary layer is further subdivided into the Sj, S2 and S3 layers. The primary layer is the first to be laid down when the cell is formed and is composed of microfibrils, which have an essentially random orientation that allows for expansion of the cell to occur as cell growth takes place. The secondary layer is subsequently formed, with each of the sub-layers exhibiting different patterns in the way the microfibrils are oriented, as illustrated in Figure 2.2. Of these, the 83 layer occupies the... 

Bacterial attack is an early stage in the degradation of wood exposed in wet or moist conditions. Bacteria can be the dominant form of attack when fungal decay is suppressed by a wood-preserving treatment. Bacteria can attack the cell wall of wood by tunnelling, cavitation or erosion mechanisms (Eaton and Hale, 1993). 

Furuno, T. and Goto, T. (1979). Structure of the interface between wood and synthetic polymer. Xll. Distribution of styrene polymer in the cell wall of wood-polymer composite (WPG) and dimensional stability. Mokuzai Gakkaishi, 25(7), 488 95. 

Shiraishi, N., Murata, M. and Yokota, T. (1972). Polymerization of vinyl monomer within the cell wall of wood. II. Polymerization of methyl methacrylate in the presence of wood, water and carbon tetrachloride. Mokuzai Gakkaishi, 18(6), 299-306. 

The take up of water or other liquids within the cell walls of wood involve the take up of a molecule at a time and its movement from one adsorption site to another (molecular jump phenomenon) under a concentration gradient. This is distinct from flow of bulk liquids into the coarse capillary structure under a capillary force or pressure gradient. 

Bulking Treatment with Water Insoluble Chemicals. The chief shortcomings of dimensional stabilization of wood with polyethylene glycol are that it can be leached from the wood and that the wood feels damp when held for prolonged periods of time at relative humidities of 80% and above. It thus appears desirable to deposit water insoluble materials within the cell walls of wood. This can be done by a replacement process with waxes (42). Water in green wood is replaced by Cellosolve (ethylene glycol monoethyl ether) by soaking the wood in this... 

A simpler approach for depositing water insoluble chemicals within the cell walls of wood is to impregnate the wood with solvent soluble resin forming chemicals containing a catalyst that penetrate the cell walls followed by evaporation of the solvent and then heating to polymerize the resin. This has been accomplished with the following water soluble resin forming systems phenol, resorcinol, melamine and urea-formaldehydes, phenol -furfural, furfuryl-aniline and furfuryl alcohol (44). 

From Fig. 7, it can be concluded that the existence of small amounts of PMMA in the cell wall of wood shifts the thermal softening peak to a low temperature by about 60 °C without changing the profile of the curve. As pointed out, this has been interpreted in terms of the interaction of wood components with PMMA - 9.). For the composite with more than PMMA content of 50 %, the peaks due to thermoplasticity of PMMA appear clearly, though overlapping the thermal softening curve for wood. A peak at about 130 °C is attributable to the glass-rubber transition of PMMA and the other peak at about 340 °C to the melting of PMMA. 

The Cell Wall. Before considering the dimensional changes in the cell wall of wood associated with gain or loss of moisture it is desirable to first consider the density p of the cell wall and how it varies with moisture content. 

The apparent density p of the cell wall of wood has also been measured by optical methods. In this case the relative fractions of void and cell-wall volumes are determined optically by using thin microtomed sections of wood (29). These data are then combined with measurements of the dry wood density p to give p, based on Equation 4. 

Stamm (30) has listed nine such methods). Somewhat different values are obtained using different methods. There also appear to be variations among woods. A mean value of approximately 35% for was calculated (29) based on measurements of 18 woods native to the continental United States. Lower values have also been found (10, 18, 22, 30) and 30% will be taken here to be the nominal value of Mf at room temperature for the purpose of calculating the maximum possible swelling of the cell wall of wood. 

Volumetric Shrinking and Swelling. The volumetric swelling of the cell wall of wood is proportional to the volume of water absorbed. The gross wood however contains air spaces therefore, its volumetric swelling depends on what happens to the air spaces during water sorption by the cell wall. 

Enthalpy Changes. The three forms of water found in wood have different energy or enthalpy levels, as shown in Figure 21. Water vapor in the cell cavities has the highest enthalpy. The enthalpy of liquid water in the cell cavities of green wood is considerably lower, essentially equal to that of free liquid water, if the effects of capillary forces and dissolved materials are neglected. The difference in enthalpy between liquid water and water vapor is the heat of vaporization (cal/g water)] of free water. The bound water in the cell wall of wood is at still lower energy level, Qi (cal/g water) below that of liquid water. The sum of the heat of sorption, Qi and Q, is equal to (cal/g water), which is the heat required to evaporate bound water from the cell wall. 

The cell wall of wood can be considered to consist of a non-crystalline matrix of lignin and hemicelluloses in which strong, stiff cellulosic microfibrils are embedded. The crystalline microfibrils exhibit no tendency either to adsorb moisture or to change in length or cross-section. On the other hand the non-crystalline isotropic matrix can lose and gain water and shows a considerable tendency to shrink and swell. In isolation one would expect the matrix to shrink or swell equally in all directions, that is Ox = Oy = = tto and Ovoi = 3ao i.e. ao is the isotropic shrinkage in... 

As illustrated in Figure 6, regarding the viscoelastic pattern of the PSt-WPC (injection) system, an obvious difference is recovj-nized between the higher temperature features of the pattern of WPG and that of wood. The rise of a new Eip -peak at ca. 125 G by the injection of polymers has been assigned to the interfacial structural change of the surface in the cellular parts due to interaction between polymer and the surface of the cellular parts. The apparent activation of the thermal motion of polymers trapped on the surface of the cell wall of wood has been estimated to be ca. 110 kcal/mol, i.e., approximately 1.5 times higher than that of the polymers (2). Injected polymers in wood cannot take part... 

Fig. 9.14. Arrangement of cellulose fibres in the cell wall of wood cells. The diameter of the cells lies between 20 pm and 40 pm, their length between 2 mm and 4 mm. Simplified illustration after [9,144]...

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