Hygroscopic and nonhygroscopic

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Mass is transferred in drying as a hquid and vapor within the solid and as vapor from the exposed surfaces. Movement within the solid results from a concentration gradient which is dependent on the characteristics of the solid. A solid to be dried may be porous or non-porous. It can also be hygroscopic or nonhygroscopic. Many solids fail intermediately between these two extremes, but it is generally convenient to consider the solid to be one or the other. 

Figure 1-8 Sorption Isotherms of Hygroscopic Product (A) and Nonhygroscopic Product (B)...

It is interesting to note that whereas dipyridinebromine(I) nitrate is hygroscopic and extremely unstable in air and water, the corresponding perchlorate is a nonhygroscopic. 

Depending on the degree of affinity for moisture, plastic resins can be divided into two classes (1) hygroscopic and (2) nonhygroscopic. Moisture adsorption and/or absorption capability depends on the type of resins as well as the ambient temperature in which it is placed. In some instances, exposure of only few minutes can be detrimental. If the material is exposed to a certain temperature and relative humidity for a period of time, it will reach the equilibrium point, referred to as the equilibrium moisture content (EMC). Prior to drying it is important to know the permeability (product of the diffusion constant of water vapor-polymer system and the solubility coefficient) of polymer to water vapor since this dictates the condition for relative humidity for the safe storage of the polymer [16]. 

If, however, a nonhygroscopic polymer is compoimded with a hygroscopic pigment or additive, the resultant compound will be hygroscopic and may need to be dried differently. For more detail on drying nonhygroscopic polymers, refer to Sec. 7.15.1. 

Hygroscopicity. Maltodextrins and low DE corn syrup solids are very nonhygroscopic. Therefore, flavors dried with these products are free-flowing powders. Hygroscopicity does increase with higher DE s. Figure 1 shows the physical changes in several DE products that had equilibrated at various relative humidities. 

The result of such treatment was that the grains of hygroscopic ingredients (such as AN) were coated with nonhygroscopic nitro-carbons(See Ref 7,p 388 and Ref 14)... 

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