Humidity hygroscopic material

Control of relative humidity is needed to maintain the strength, pHabiUty, and moisture regain of hygroscopic materials such as textiles and paper. Humidity control may also be required in some appHcations to reduce the effect of static electricity. Temperature and/or relative humidity may also have to be controlled in order to regulate the rate of chemical or biochemical reactions, such as the drying of varnishes, the appHcation of sugar coatings, the preparation of synthetic fibers and other chemical compounds, or the fermentation of yeast. 

Fig. 6. Equilibrium moisture content profiles vs relative humidity for a hygroscopic material.

Unbound moisture in a hygroscopic material is that moisture in excess of the equihbrium moisture content corresponding to saturation humidity. All water in a nonbygroscopic material is unbound water. 

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. 

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

Accelerated aging and crystal transformation rates have also been traced to high residual moisture content. Ando et al. studied the effect of moisture content on the crystallization of anhydrous theophylline in tablets [9]. Their results also indicate that anhydrous materials convert to hydrates at high levels of relative humidity. In addition, if hygroscopic materials (e.g., polyethylene glycol 6000) are also contained in the formulation, needle-like crystals form at the tablet surface and significantly reduce the release rate of the theophylline. 

It has been determined experimentally that the rate of moisture uptake varies linearly with the surface area of the hygroscopic material and the relative humidity of the atmosphere. For example, if LiCl H20 were exposed to a series of saturated salt solutions, the weight of moisture uptake as a function of time... 

The effect of humidity on combustibility of various mixtures of organic matter and sodium chlorate was studied. Addition of a proportion of hygroscopic material (calcium or magnesium chlorides) effectively reduces the hazard. Similar effects were found for sodium dichromate and barium chlorate. 

Wood is a hygroscopic material, due to the fact that the cell wall polymers contain hydroxyl groups. In an environment containing moisture, dry wood will absorb moisture until it is in equilibrium with the surrounding atmosphere. Similarly, saturated wood, when placed in an atmosphere of lower relative humidity (RH), will lose moisture until equilibrium is attained. If the wood is placed in an environment where the RH is stable, it will attain a constant moisture content (MC), known as the equilibrium moisture content (EMC). At this point, the flux of water molecules into the cell wall is exactly balanced by the outward flux into the atmosphere. 

The three hygroscopic excipients used were citric acid anhydrous, sorbitol, and maltodextrin. It was found that hygroscopic materials do not markedly increase the adsorption of atmospheric moisture by a tablet until high relative humidities were attained. 

A hygroscopic material that avidly retains water absorbed from a humid atmosphere, and may be used in cosmetics and pharmaceuticals... 

Moisture content Increase in the moisture content of a solid leads to increased cohesive strength. Even slight increases in moisture content, for example, from 1 to 1.5%, may significantly increase the material flowability. Hygroscopic materials can experience significant moisture increases simply by being exposed to humid air. 

A hygroscopic material absorbs moisture from its surrounding atmosphere, while a humectant material is one that resists changes in relative moisture content. The gain or loss of moisture in a com syrup is dependent on the relative humidity of the atmosphere surrounding the syrup. Moisture absorption values for several sweeteners are shown in Table 21.16.79... 

Environmental Factors. Moisture. Wood, which is a hygroscopic material, gains or loses moisture to equilibrate with its immediate environment. The equilibrium moisture content (EMC) is the steady-state level that wood achieves when subjected to a particular relative humidity and temperature. The eventual EMC of two similar specimens will differ if one approaches EMC under adsorbing... 

Ammonium alginate is a hygroscopic material, although it is stable if stored at low relative humidities and cool temperatures. 

Carboxymethylcellulose sodium is a stable, though hygroscopic material. Under high-humidity conditions, carboxymethylcellulose sodium can absorb a large quantity (>50%) of water. 

It will reversibly take up moisture without any effect on the appearance of the material at humidities up to 60% RH. Equilibration at RH values above 60% will result in deliquescence. Once in this state, the material can be dried, but will give a glasslike product. This water absorption behavior is typical of amorphous hygroscopic materials. 

Hygroscopic materials can experience significant increases in moisture content when exposed to humid air. Depending on the nature of this contact, whether it is during transfer and blending versus while a large quantity of material is stored at rest, the extent... 

West, M.K. Hansen, E.C. Effect of hygroscopic materials on indoor humidity and air quality. Proceedings of lAQ Desiccant Dehumidification and Cooling, ASHRAE Atlanta, GA, 1992, 178-182. 

To enhance the surface conductivity, a chemical is deposited that transports the current. Adsorbing a hygroscopic material increases the humidity at the fiber surface [31],... 

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. 

---