Moisture uptake

In a recent report (79), a 150—200 mg/cm Parylene C coating provided protection against moisture uptake by three-phase, polyimide, microballoons, and air, syntactic foams. In a previously reported coating of a similar foam, the stated purpose was strengthening (80). 

Disadvantages associated with some organic solvents include toxicity flammabiHty and explosion ha2ards sensitivity to moisture uptake, possibly leading to subsequent undesirable reactions with solutes low electrical conductivity relatively high cost and limited solubiHty of many solutes. In addition, the electrolyte system can degrade under the influence of an electric field, yielding undesirable materials such as polymers, chars, and products that interfere with deposition of the metal or alloy. 

Nylon-6,6 [32131 -17-2] and nylon-6 [25038-54-4] continue to be the most popular types, accounting for approximately 90% of nylon use. There are a number of different nylons commercially available Table 1 gives a summary of the properties of the more common types. In the 1990s there has been a spurt of new polyamide iatroductions designed for higher temperatures, better stiffness and strength, and/or lower moisture uptake. 

Reduced-Moisture Mylon. A modified nylon-6 has been commerciali2ed that has approximately 30% less moisture uptake (22). Patented compositions use various amine and phenoHc additives to obtain such a reduced moisture uptake effect (33,34). 

Polymer Blends. Commercial blends of nylon with other polymers have also been produced in order to obtain a balance of the properties of the two materials or to reduce moisture uptake. Blends of nylon-6,6 with poly(phenylene oxide) have been most successflil, but blends of nylon-6,6 and nylon-6 with polypropylene have also been introduced. 

Swelling or moisture uptake of control specimen during exposure to water for t minutes Swelling or moisture uptake of treated specimen during exposure to water also for i minutes... 

While polar monomers are usually beneficial in acrylic PSA formulations, there are times when their presence is deleterious. Examples of this may be the use of acrylic acid containing adhesives for electronic applications, for adhering to some metallic surfaces, or for application to paper used in books. Higher levels of acrylic acid not only increase the acidity of the PSA but they also increase the moisture uptake in the adhesive making dissociation of the acid easier. This can increase corrosion problems in the electronic or metal applications, or severe discoloration and degradation of paper with time. The latter is often a significant concern to librarians who deal with repair and archival restoration of books. In applications such as these, acid-free adhesives are more desirable, or at the very least the amount of acid has to be low and caution has to be taken to fully incorporate the monomer into the PSA. 

Fig. 4. Moisture uptakes as a function of water partial pressure for DGEBA-TETA net resin. Influence of previous exposure to 60 °C and 95% relative humidity on the apparent sorptivity. Open circles conditioned samples full circles reference samples. (22)...

POLYIMIDES AND OTHER HIGH-TEMPERATURE POLYMERS 5.2.2.3 Dielectric Properties and Moisture Uptake... 

The chemical requirements leading to a low dielectric constant (below 3) and low moisture uptake are the same which were discussed in the previous section. Bulky substituents like fluoroalkyl, fluoroalkoxyl,68,69 or cardo70 groups allow the dielectric constant to drop to 2.6-2.7. The moisture uptake is also minimized for these polymers.71 Similar results were observed with cycloaliphatic imides64 but with a lower thermal stability. 

Texturization is not measured directly but is inferred from the degree of denaturation or decrease of solubility of proteins. The quantities are determined by the difference in rates of moisture uptake between the native protein and the texturized protein (Kilara, 1984), or by a dyebinding assay (Bradford, 1976). Protein denaturation may be measured by determining changes in heat capacity, but it is more practical to measure the amount of insoluble fractions and differences in solubility after physical treatment (Kilara, 1984). The different rates of water absorption are presumed to relate to the degree of texturization as texturized proteins absorb water at different rates. The insolubility test for denaturation is therefore sometimes used as substitute for direct measurement of texturization. Protein solubility is affected by surface hydrophobicity, which is directly related to the extent of protein-protein interactions, an intrinsic property of the denatured state of the proteins (Damodaran, 1989 Vojdani, 1996). 

Water uptake causes a host of problems in drug products and the inactive and active ingredients contained in them. Moisture uptake has been shown to be an important factor in the decomposition of drug substances [1-8]. Moisture has also been shown to change surface properties of solids [9,10], alter flow characteristics of powders [11,12], and affect the compaction properties of solids [13]. This chapter discusses various mathematical models that can be used to describe moisture uptake by deliquescent materials. 

A deliquescent material takes up moisture freely in an atmosphere with a relative humidity above a specific, well-defined critical point. That point for a given substance is defined as the critical relative humidity (RH0). Relative humidity (RH) is defined as the ratio of water vapor pressure in the atmosphere divided by water vapor pressure over pure water times 100% [RH = (PJP0) X 100%]. Once moisture is taken up by the material, a concentrated aqueous solution of the deliquescent solute is formed. The mathematical models used to describe the rate of moisture uptake involve both heat and mass transport. 

Since heat transport is unfamiliar to many pharmaceutical scientists, this chapter begins with a discussion of vapor-liquid equilibria, heat transport in rectangular coordinates, and heat transport in spherical coordinates. Once these basic principles are established, we can build models based on heat transport. Heat transport is the dominant mechanism for moisture uptake in an atmosphere of pure water vapor. In air, however, both heat and mass transport are involved. 

These models are discussed later in this chapter. The final subject is an introduction to moisture uptake in heterogeneous materials. 

A rational development of models for moisture uptake begins with a description of the experimental procedure used to determine moisture uptake as a function of time. The first step in the experiment is to control the relative humidity to which a sample will be exposed. One technique to control humidity is to use saturated salt solutions. When placed in a closed system and held at a constant temperature, a saturated aqueous salt solution will provide a constant humidity (RH0) within that system. Table 1 lists relative humidities that will be maintained over various saturated salt solutions [14],... 

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... 

Figure 1 Moisture uptake curves for LiCl hydrate. 

The discussion of moisture uptake by hygroscopic materials must include a description of the thermodynamics of vapor-liquid equilibria. For gas (g) and liquid (1) phases to be in equilibrium, the infinitesimal transfer of molecules between phases (dng and dn ) must lead to a free energy change of zero. 

Figure 2 Rate of moisture uptake as a function of relative humidity for LiCl hydrate.

Building a model of moisture uptake based on heat transport requires a set of tools to describe the process of energy transport. 

The spherical coordinate system is useful for many of the moisture uptake problems encountered in this chapter. The application of the spherical coordinate system can be illustrated by the following example (see Fig. 5). 

The fundamental concept of heat transport controlled moisture uptake [17] is shown in Eq. (22), where the rate of heat gained at the solid/vapor surface (W AH) is balanced exactly by the heat flow away from the surface (Q). The term All is the heat generated by unit mass of water condensed on the surface. The two most probable sources of heat generation are the heat of water condensation and the heat of dissolution. A comparison of the heat of water condensation (0.58 cal/mg water) with the heat of dissolution for a number of salts indicates that the heat of dissolution can be neglected with little error for many materials. 

A. Heat Flux Caused by Moisture Uptake in One Dimension... 

We now want to set up the problem for moisture uptake according to Figure 8. Remember from Eq. (15) that the steady-state change in heat flux for a onedimensional problem in rectangular coordinates is given by dq/dx = 0, and differentiation of Eq. (23) gives... 

Equation (29) is the solution for heat flux in rectangular coordinates and could be converted into a mass rate using Eq. (22). A more useful solution comes from spherical coordinates, because that coordinate system more closely matches the experimental system. In the next section we repeat the same basic steps to arrive at the heat flux and moisture uptake rate in spherical coordinates. 

B. Heat Transport Limited Moisture Uptake in Spherical Coordinates... 

We now repeat the derivation of the steady-state heat transport limited moisture uptake model for the system described by VanCampen et al. [17], The experimental geometry is shown in Figure 9, and the coordinate system of choice is spherical. It will be assumed that only conduction and radiation contribute significantly to heat transport (convective heat transport is negligible), and since radiative flux is assumed to be independent of position, the steady-state solution for the temperature profile is derived as if it were a pure conductive heat transport problem. We have already solved this problem in Section m.B, and the derivation is summarized below. At steady state we have already shown (in spherical coordinates) that... 

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