Wood moisture content

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. 

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. 

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). 

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. 

Some strength properties, such as toughness and shock resistance, may decrease with decreasing wood moisture content because these properties are proportional to the deformation of a wood member under load and the stress sustained. Moist wood deforms more than dry wood and the product of stress and deformation, which is a measure of toughness, may actually be greater for moist wood. 

The curve for the energy level of bound water shown in Figure 21 indicates that Qi and increase with decreasing wood moisture content below fiber saturation Mf, This increase means that more... 

Stamm and Loughborough (46) first calculated and Ql as functions of wood moisture content M by applying the Clausius-Clapeyron equation to the moisture sorption isotherms for wood at several temperatures. For example, can be obtained by replotting sorption isotherms, such as are shown in Figure 9, into the form of isosteres of constant moisture content, of In h against the reciprocal of Kelvin temperature. These plots yield a family of essentially straight lines, each at a different moisture content. The magnitude of Qi at any moisture content is calculated from... 

Figure 22 contains plots of Qi (log scale) vs. wood moisture content. The logarithm of Q[ appears to decrease linearly with in-... 

Figure 24 shows curves of Qi, AG, and TAS plotted against wood moisture content. All energy terms are negative (heat is given off) when wood takes up water from the liquid state. The decrease in entropy indicates that bound water is more ordered than liquid water, in analogy to the greater order of water in ice compared with the liquid state. As the moisture content approaches fiber saturation the distinction between liquid water and water in wood decreases toward zero. However, even above fiber saturation the water in cell cavities may be different from ordinary liquid water because of capillary forces and/or dissolved materials. 

Figure 31. Curves of Dj and D vs. wood moisture content for various temperatures. (Reproduced with permission from Ref. 45. Copyright 1971 y Syracuse University tress.)...

Proc., Symposium on Wood Moisture Content, Temperature and Humidity Relationships, U.S., For. Prod. Lab. Rep., Madison, WI, 1979. 

Isocyanates are sensitive to moisture therefore, the reaction needs to be done on dry wood (82). As wood moisture content increases before reaction, more nonbonded polymers are formed after reaction. Reacted moist wood shows very high ASE values on the first water-soak test, but leaching causes a significant loss in ASE. This shows that the bulking chemical is not bonded to the cell wall but comes out upon water leaching. 

Table IV. Influence of Wood Moisture Content on the Polymerization Exotherm Temperature...

Figure 9, Temperature-time curves showing how the wood moisture content affects the polymerization exotherm of maple-MM A composite (39),...

---