Bound moisture defined

Moisture may be bound to the solids so that full vapor pressure is no longer exerted according to Watson and Harper (1987). They pointed out that the terms unbound (free) and bound are commonly used to distinguish moisture in the relatively large spaces from that held more tightly by other forces. They further stated that there is a continuous transition from unbound to bound moisture, and it is not possible to make a precise dividing line. Unbound moisture is frequently defined as that which exerts the... 

Water activity was defined by Scott (1957) using the equation = F/po, where P is the water vapor pressure of food and po is the water vapor pressure of pure water at the same temperature. Water activity is, thus, an index of degree of freeness (the potential to be vaporized) of the solid bound moisture relative to pure water. 

Nonhygroscopic capillary-porous media, such as sand, crushed minerals, nonhygroscopic crystals, polymer particles, and some ceramics. The defining criteria are as follows. (1) There is a clearly recognizable pore space the pore space is filled with liquid if the capillary-porous medium is completely saturated and is filled with air when the medium is completely dry. (2) The amount of physically bound moisture is negligible that is, the material is nonhygroscopic. 

Bound moisture is associated with the hygroscopic nature of the woody components. There are some uncertainties about the limits of hygroscopic behavior, particularly with woods of high extractives content but it is useful to define a maximum sorptive moisture content, called the fiber saturation point (FSP). If the capillary condensation effects in pores greater than 0.1 xm in equivalent cylindrical diameter are ignored, FSP of the wood may be defined as the equilibrium moisture content (EMC) in an environment of 99% relative humidity. This yields a value of 30 to 32%i for most commercial species (Keey et al., 2000) at room temperature. FSP falls with increasing temperature. For a softwood such as Sitka spruce (Picea sitchensis), FSP falls from about 31%i at 25° C to 23% at 100°C (Stamm, 1964). 

Moisture in coal takes three forms (l)free or adherent moisture, essentially surface water (2) physically bound or inherent moisture (thai moisture held by vapor pressure and other physical processes) and (3) chemically bound water (water of hydration or combined" water). The ASTM defines total moisture as a loss in weight in an air atmosphere under rigidly controlled conditions of temperature, time, and air flow. Total moisture represents a measurement of all water not chemically combined. Total moisture is determined by a two-slep procedure, involving air-drying for removal of surface moisture from the gross sample, division and reduction of Ihc gross sample, and determination of residual moisture in the prepared sample. An algebraic calculation is used to obtain the total moisture value. 

It should be noted that the limits so defined, which bound the region of apparent equilibrium, are by no means fixed. In the phosphorescence of phosphorus, e. g., they depend on the amount of moisture present. The combination of oxygen and hydrogen, again, is ultimately dependent on the substance with which the gases are in contact. Thus, Mitscherlich gives the temperature of explosion in glass vessels as 674° instead of 845 . 

The fiber-saturation point is defined as the moisture content at which the cell cavities are empty of liquid water but the cell walls are still saturated with bound water (6). The fiber-saturation point is designated as mf (fraction of dry mass) or Mf (percent of dry mass). 

Water in wood ean exist as either absorbed (also called free water) in the cell lumens and intereellular spaees or as adsorbed (also called bound water) within the cell walls. When wood dries water first evaporates from the lumens and intercellular spaees. The fibre saturation point is defined as the moisture content at which all the absorbed water has been removed but at which the cell walls are still fully saturated. This oeeurs at a moisture eontent of 25 to 35%. In most instances it is adequate to presume the fibre saturation point to be 30% moisture content. 

BOUND AND UNBOUND WATER. If an equilibrium curve like those in Fig. 24.3 is continued to its intersection with the axis for 100 percent humidity, the moisture content so defined is the minimum moisture this material can carry and still exert... 

The water in coal is bound in different forms to its constituents. It can be divided into three types (1) Free moisture, also referred to as external moisture, superficial moisture, or the primary moisture fraction, which is present in large cracks and capillaries. Water bound in this way retains its normal physical properties. (2) Inherent moisture, also referred to as internal moisture or the secondary moisture fraction, whose vapor pressure is lower, since it is absorbed within the pore structure of the coal. (3) Water of constitution, which is mainly combined with mineral matter normally present in coal. This water is generally driven off only at temperatures higher than those normally used for the determination of moisture content. Standard methods do not make use of these terms and define (1) the total moisture content of a coal and (2) the moisture content of the coal analysis sample. Total moisture determination must be made over the sample as received in the laboratory, in an air-proof recipient. The determination consists in drying in an oven at 105 °C till constant weight. Its value is of huge interest both in international and domestic coal trade (ISO 589, ASTM D3173). 

The amount of water in a porous body such as the textiles at the EMC is defined as bound water and it is absorbed by the textile fibers. When the textile is unable to absorb more water, all excess water is defined as unbound moisture. The unbound moisture is often found as a continuous liquid within the porous material. 

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