Wood moisture

The moisture content of the glued particles is the sum of the wood moisture content and the water which is part of the applied glue mix. Therefore, the moisture content of the glued particles mainly depends on the gluing factor. Usual moisture contents of glued particles are 6.5-8.5% in the core layer and 10-13% in the face layer for UF, and 11-14% in the core layer and 14-18% in the face layer for PF. 

Along with carbon, hydrogen and oxygen, lignocellulosic biomass also contains hetero elements such as alkali and other metals. The amounts of these ashes vary over a broad range, from 30-50 wt.% in chicken litter to 1-3 wt.% in wood. Moisture is always present in lignocellulosic biomass and can be up to 80 wt.% in some cases. Detailed information on the composition of biomasses can be found in data bases, e.g., Phyllis [21] from the Dutch Energy Research Foundation (ECN). Table 6.2 lists the compositions of some typical biomasses. 

The Effect of Wood Moisture Content upon Reactivity... 

Temperature increase and wood drying A combination of heat and steam is used to raise the temperature of the wood to 100 °C. The temperature is then ramped to 130°C for the high-temperature drying phase, which reduces the wood moisture content to approximately 0 %. 

Many particle preparation processes exist, the one used in a given particleboard plant will depend upon the raw material source and the type of particleboard produced. The particle quality from each process is a function of the wood moisture content, degree of maintenance of the equipment, and the form of the raw material. Good particle quality does not guarantee a quality particleboard but high quality particleboard cannot be produced from low quality particles. 

Five sets (4 samples each set) of mango samples were conditioned to five different moisture levels and conditioned samples were acetylated in vapor phase for 3 hours to study the effect of wood moisture content on WPG. 

Effect of Wood Moisture. It has been recommended that for acetylation with acetic anhydride, the moisture content of wood should be about 2% as excess moisture is likely to react with acetic anhydride and produce acetic acid (37). Goldstein et al. (28) observed that raising the moisture to 22% considerably slowed the reaction and each 1 percent of moisture in wood would lead to hydrolysis of about 5.7% acetic anhydride. Low moisture contents are not possible to attain in commercial treatment of wood. With ketene gas it has been possible to acetylate wood with as high as 20% moisture content with WPG about 25% (40). Thioacetic acid is only partially stable in cold water and dissociates at higher temperatures. The presence of moisture in wood could thus be critical in treatments with thioacetic acid also. Results of mango treated at 5 different moisture levels are depicted in Figure 3. As may be seen a moisture content up to 7.5% has no adverse effect on WPG. At 10% moisture content the WPG decreased to 4.6. With further increase in moisture, there was a gradual decrease in WPG attained. 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

Size, geometry, and orientation of particles in the composite has a great influence on mechanical properties as does density, type and level of adhesive used, solubility of resin in carrier liquid, ability of resin to penetrate into wood, moisture content, defects, wood species used, and many other variables. 

FIGURE 7.1 Theoretical flame temperature vs wood moisture content and excess air. Adapted from Tewksbury (1991). 

The wood component likely to be of greatest interest to charcoal producers is water. Freshly cut wood can have a moisture content in excess of 50% and even air-dried wood has an equilibrium moisture content of 10 to 15%. In kilns, the heat required to drive off this moisture comes from the burning of a fraction of the wood charge. It follows that the greater the wood moisture content, the more wood has to be burnt and the lower is the overall charcoal yield. The longer drying time will also increase the... 

At the cellular level, the true density of dry cell wall substance (i.e., within the cell wall) has been determined to be about 1.5 g/ cm, varying to some extent with the method of measurement and species (2). There are voids within the dry wood cell wall, but the void volume here (i.e., micropores) is reported to be only about 2-4%. However, this figure would be expected to increase as wood moisture content is increased to the fiber saturation point (28). 

The importance of wood moisture and basic density in determining wood behavior will become more evident in subsequent chapters. Suffice it here to say that variation in the amount of cell wall substance at a given moisture content that must be traversed by a penetrating liquid or chemical, microbe, etc., can determine the rate of reaction as well as the extent of reaction or the change in the character of the wood in question. 

Moisture Content. The moisture content of wood is determined by measuring the weight loss after drying the sample at 105 °C. Unless specified otherwise, the percent of all other chemical components in wood is calculated on the basis of moisture-free wood. Moisture content is determined on a separate portion of the sample not used for the other analyses. 

Measuring Water Content of Wood. There are as many as fifteen methods that have been used to measure wood moisture content (7). Some of the more common or useful methods are discussed here. 

The Karl Fischer method can be used to measure the moisture contents of many materials besides wood, including solids, liquids, and gases. It gives the best results of any of the standard methods used for measuring wood moisture content (7), but is not practical for large wood samples, particularly those with high moisture contents. 

Another method proposed for minimizing polarization and related effects is to use short repetitive current pulses rather than continuous voltage on the sample (J3). This method also reduces the ohmic heating effect at higher moisture contents. Some contemporary resistance meters have provisions for switching to the pulsed current mode for wood moisture contents greater than 12% and retain the DC mode at lower moisture contents. 

The power-loss meter is the most common type of dielectric moisture meter. It senses the product of the dielectric constant and loss factor. Generally, the loss factor increases with wood moisture content but may exhibit variations from this behavior depending on the frequency of measurement (JO, 11, 14). An increase in temperature produces effects similar to increasing moisture content, with interaction between these two parameters. Therefore, temperature adjustments of meter readings are complex, sometimes increasing and sometimes decreasing the scale reading as temperature increases... 

Miscellaneous Methods. Several other methods have been explored for measuring wood moisture content, some of which are discussed briefly. 

Two different techniques of NMR have been applied to measure wood moisture content based on the presence of the hydrogen nuclei in water. In one of these, designated as a steady-state method, the wood is subjected to an alternating magnetic field of constant frequency, with Ho varied slowly so as to resonate 7 with respect to the applied frequency. At resonance a strong absorption of energy occurs, and the width and intensity of this absorption curve give information on the moisture content of the wood (16). 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

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