Natural equilibrium moisture content

The eonditioning treatment can be overdone. If the wood is steamed for too long and the surfaee fibres re-adsorb too much water this will induce an excessive compression set in the surface fibres. At the end of drying, when the surface fibres return to the natural equilibrium moisture content they will want to shrink more than normal wood (because of the excessive compression set), thus producing tension at the surface and compression in the interior. The lumber has been reverse case hardened. This condition cannot be treated. 

In many products it seems highly probable that there exists a narrow range of optimum moisture contents that should be maintained. More specifically, the effect of moisture on MCC-containing tablets has been the subject of an investigation that demonstrates the sensitivity of this important excipient to moisture content [10]. These researchers found that differences exist in both the cohesive nature and the moisture content to two commercial brands of MCC. A very useful report on the equilibrium moisture content of some 30 excipients has been compiled by a collaborative group of workers from several pharmaceutical companies and appears in the Handbook of Pharmaceutical Excipients [11,12],... 

Fig. 16-18. Moisture contents of natural gases at low pressures. (Based on equations of Bukacek, Equilibrium Moisture Content of Natural Gases, Res. Bui ., IGT, Chicago, 1955.)...

The nature of moisture content determinations is perhaps best exemplified by the fact that the Handbook of Pharmaceutical Excipients lists 31 separate versions of laboratory tests to determine moisture content and one to determine equilibrium moisture content. These methods were used to assess the moisture content in the compilation of 148 monographs, many of which do not deal with solids. 

The presence of unbound moisture in the droplet means that the drying proceeds at a constant high rate as long as the moisture diffusion within the droplet is able to maintain saturated surface conditions. When the diffusional and capillary flows can no longer maintain these conditions, a critical point is reached and the drying rate will decline until equilibrium moisture content is reached. The evaporation of bound moisture is strongly dependent on the nature of the solid matter in the spray droplet. 

For non-hygroscopic materials, the equilibrium moisture content is essentially zero at all temperatures and humidities. Equilibrium moisture content is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. The mechanisms of drying during this phase are not completely understood, but two ideas can be considered to explain the physical nature of this process— one is the diffusion theoiy and the other the capillary theory. 

The moisture contained in a wet solid or liquid solution exerts a vapor pressure to an extent depending upon the nature of moisture, the nature of solid, and the temperature. A wet solid exposed to a continuous supply of fresh gas continues to lose moisture until the vapor pressure of the moisture in the solid is equal to the partial pressure of the vapor in the gas. The solid and gas are then said to be in equilibrium, and the moisture content of the solid is called the equilibrium moisture content under the prevailing conditions. Further exposure to this air for indefinitely long periods will not bring about any additional loss of moisture. The moisture content in the solid could be reduced further by exposing it to air of lower relative humidity. Solids can best be classified as follows [12] ... 

A large volume of data of equilibrium moisture content appears in the literature. Data for more than 35 polymeric materials, such as natural fibers, proteins, plastics, and synthetic fibers, are given in Ref. [8]. Isotherms for 32 materials (organic and inorganic) are also given in Ref. [92]. The literature is especially rich in sorption isotherms of foods due to the fact that the value of water activity is a critical parameter for food preservation safety and quality. 

The amount of moisture adsorbable by the fibrous material varies markedly, as shown in Table 37.1. Hydrophilic fibers of natural origin can take up considerable amounts of moisture, whereas some artificial fibers are barely hygroscopic. The variation of equilibrium moisture content with relative humidity at constant temperature is shown in Figure 37.1 for a number of fibers. 

At low relative humidities (0 < p/p° < 0.35), water is adsorbed monomolecularly by many natural fibers. The equilibrium moisture content then relates to the fraction of available sites taken up, i.e. 

Table 11.4 Equilibrium moisture content at 27 °C of selected natural fibers [23].

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). 

Conditioning. After the hardboard has been hot-pressed or has been heat-treated or oil-tempered, the moisture content is well below what will be attained at equilibrium with the atmosphere in normal use. Such very dry boards will change dimensions upon picking up moisture and may warp. It is important, therefore, to humidify the boards under controlled conditions before packaging. The desired equilibrium moisture content (EMC) reached will vary from 5 to 12 percent, depending upon the nature of the board and the general humidity conditions in the region of use. 

Bukacek, R. F., "Equilibrium Moisture Content of Natural Gases, Research Bulletin 8, Institute of Gas Technology, 1955. Reports work sponsored by the Pipeline Research Committee of the American Gas Association. 

The above equations illustrate via the double arrows an important facet of polyamides—the equilibrium nature of the polymerization reactions. Achieving and maintaining useful molecular weights (about 10,000 or more) for plastics applications require low moisture contents in order... 

---