Dry and wet cycles

Fig, 6.17. Overall efficiency and specific work of dry and wet cycles compared. 

The success of these model experiments led Bonner and coworkers to propose a mechanism involving repetitive cyclic sequences of partial polymerization followed by partial depolymerizaton in which the latter is caused by hydrolysis. Thus, this process is driven by environmental dry and wet cycles that could ultimately have led to homochiral polypeptides on early Earth. [97] Brach and Spach [95] have also proposed a mechanism involving partial hydrolysis for the enantiomeric enrichment of polypeptides having (3-sheet secondary structures. 

Organic phosphorus solubilization by soil drying and wetting cycles... 

The supersaturation can be produced by cooling, evaporation and drying and wetting cycle. If the temperature dependence of the solubihty of a salt is high, a drop of temperature can result in supersaturation. Supersaturation caused by evaporation always occurs when one face of the porous material is in contact with the solution and the other face is exposed to relatively dry conditions, i.e., the salt weathering process. 

Any atmospheric environment is subject to dry and wet cycles. Since water and moisture have a decided effect on the degradation of a coating, the time of wetness of a coating is important. Moisture and water that attack organic films are from the rain, fog, dew, snow, and water vapors in the... 

PMMA is also commonly used In dentistry applications for teeth and dentures. In most cases, several types of particulate filler are used to achieve a multimodal particle size distribution (for a higher particle content) and desired properties like wear resistance, processability, color, and gloss. Under physiological conditions, dental composites take up a certain amount of water. Repeated drying and wetting cycles can result in microcrack formation and accumulation at the interface between the polymer matrix and the inorganic filler (see Figs. 8.32-8.34). 

Other obvious important sources of corrosive chloride ions are seawater and marine atmospheres. Alternate drying and wetting cycles promote the buildup of chloride ions on surfaces. Hence actual surface concentrations of chlorides can be well in excess of those of the bulk environment. 

Weatherfastness tests on coatings have shown that prolonging the dry and wet periods, i.e., extending the usual 17 minute dry cycle to a full 102 minutes and the wet period to 18 minutes, affords results which correlate much better with the Florida outdoor exposure tests. The humidity that penetrates a layer is known to interact with the various components in the coating, adversely affecting the mechanical properties of the material. The above mentioned cycle apparently corresponds much better with the climatic conditions in areas like Florida. If recirculated water is used in accelerated exposure equipment, suspended particles may adhere to the surface of the coating, producing a thin film. This obviously has a detrimental effect on the test results. 

A critical property of minimum protocells in the prebiotic environment would be their ability to sequester other molecules, including macromolecules. [142] In 1982, Deamer and Barchfeld [143] subjected phospholipid vesides to dehydration-rehydration cycles in the presence of either monomeric 6-carboxyfluorescein molecules or polymeric salmon sperm DNA molecules as extraneous solutes. The experiment modeled a prebiotic tidal pool containing dilute dispersions of phospholipids in the presence of external solutes, with the dehydration-rehydration cydes representing episodic dry and wet eras. They found that the vesides formed after rehydration... 

There are two basic types of transient dynamic mechanical experiments which were performed In this study. The first type Involves Isothermal cycling of an epoxy sample between a dry and wet environment. The second type of experiment Involves cycling the epoxy sample between two different temperatures under a liquid water environment. In each case, the transient and equilibrium values of dynamic mechanical properties change In a unique manner. 

Figure 6 plots transient Isothermal tan 6 dynamic mechanical data for a 25 PHR-DDS N-5208 epoxy sample. This sample was Initially exposed to a dry 50 C environment. This temperature was selected since It coincides with the vicinity of the dynamic mechanical u transition. Hence, differences between properties In the dry and wet states could be maximized. Behavior of the Initial dry to wet state transient cycle was previously discussed for DGEBA-TETA epoxy sample of Figure 5. Similar behavior Is noted for this N-5208 epoxy sample. There Is an Initial rise In the tan 5 followed by a "blocking" and gradual reduction. After the tan 6 appeared to approach a stable value, the environment In the sample chamber was switched from one of a 50 C liquid environment to a 50°C desiccated environment. Once again, a rapid Increase In the mobility of the system occurred. After the sharp Increase In tan 5, a gradual decrease followed.

It Is of Interest to note that subsequent cycling of the epoxy sample In Figure 6 results In a gradual Increase of the "stable" dry and wet state tan 6 values. This cumulative Increase In network mobility continues with moisture cycling because the sample Is never allowed a macroscopic network relaxation. Recovery of original dry state properties at least requires thermal excitation of the system to temperatures near the glass transition point (3). 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

According to the measurements of Rancher and Kritz (1980), the total gas phase bromine concentrations are comparable to the particulate concentrations. In contrast, Moyers and Duce (1972) found 4-10 times higher concentrations of gas phase than particulate bromine. One explanation for the discrepancies might be the differences in dry and wet deposition rates. Rancher and Kritz (1980) could distinguish a diurnal cycle in particulate Br with nighttime values that were about twice as high than daytime values. 

We wish now to analyze that portion of the global sulfur cycle involving atmospheric S02 and sulfate shown in Figure 22.2. We will denote the natural and anthropogenic emissions of S02 as P 0j and P 02, respectively. P Qi includes a contribution from the oxidation of reduced sulfur species to S02. S02 is removed by dry and wet deposition and oxidized to sulfate by chemical reaction. Sulfate is also removed from the atmosphere by dry and wet deposition. Our goal is to obtain estimates for the lifetimes of S02 and SO . 

VDA 621-415 Wet/dry and humidify cycling. High time of wetness, poor correlation for zinc pigments and galvanized steel. Also used for testing heavy in structure paints... 

When urea moldings are subjected to severe cycling between dry and wet conditions, cracks develop. Certain strength characteristics also experience a loss when amino moldings are subjected to prolonged elevated temperatures. 

It is important to recognize that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the chlorides including capillary action and absorption as well as diffusion. Rapid initial absorption occurs when chloride laden water hits very dry concrete. In many circumstances these will only affect the first few millimetres of concrete. If so then the expedient of ignoring the first few millimetres of drillings and then calculating diffusion profiles will work. If the cover is low, the concrete cycles between very dry and wet or the concrete quality is low then the alternative transport mechanisms may overwhelm diffusion, at least to rebar depth. 

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