Hysteresis drying

Abrasion-resistant duties may involve abrasion in an aqueous phase or abrasion by dry particulate materials. The selection of the polyurethane type is most important to obtain the best results. Polyester-based polyurethanes perform best in dry abrasion due to their low hysteresis properties and excellent resistance to cut initiation and propagation. However, polyester polyurethanes are susceptible to hydrolytic degradation, and therefore polyether polyurethanes are normally used for aqueous abrasion duties. 

FIG. 10 Hysteresis magnetization loops obtained at T = 3 K. (A) Diluted liquid solution of cobalt nanoparticles in hexane. (B) Cobalt nanoparticles deposited onto freshly cleaved graphite (HOPG) and dried under argon to prevent oxidation. Substrate parallel (—) and perpendicular (—) to the field. 

It should be noted that hysteresis occurs as the particles are dried out i.e., the liquid solution does not form a solid particle as the water evaporates at the same RH as it went through the solid - liquid transition. Typically one must reach RHs 20-30% or more below the deliquescence point in order to dry the particle. Figure 9.41, for example, shows the uptake of water by solid (NH4)2S04 and its subsequent dehydration (Tang et al., 1995). At 80% RH the solid deliquesces but does not solidify (effluoresce) on drying until an RH of 37% is reached. (The presence of other species has been shown to increase this effluorescence... 

Sample powders were dried in a vacuum oven at 60 C for 7 hours and cooled to minimize the hysteresis effect prior to storage in the dessicators of various water activities. In addition to Drierite, five saturated salt solutions were used in dessicators. These salt solutions were lithium chloride, magnesium chloride, potassium carbonate, sodium nitrite and potassium chloride. Their water activities were 0.110, 0.330, 0.440, 0.650 and 0.850, respectively, at 20 C. Each sample contained 1.2 to 1.5 g powder and four-week equilibration time was employed. The percentage of... 

Fig. 31. Temperature dependence for equilibrated volumes of NIPA gel including the Con A-DDS complex (DSS-gel, open circles), MP (MP-gel, filled circles), and free of both DSS and MP squares). The latter was prepared as a control sample. Hysteresis was observed in the volume changes of DSS-gel and the free-Con A gel on heating and cooling, indicating a discontinuous phase transition. The diameter of each gel in the collapsed state, determined at 50 °C, was do = 0.074 mm the volume of this gel is denoted by V0. The concentration of dry matter in the collapsed state was estimated from the preparation recipe to be 90wt%.

Here k is the permeability of the dry medium and J(S[) characterizes hysteresis behaviour in the trickling regime. It should be noted at this level that Eqn. (5.2-12) was derived by these authors from available data on two-phase imbibition and drainage curves, implicitly identified to the trickling flow regime in trickle-beds. The J function may be multi-valued and depends on the history of the flow, however Grosser et al. [22] as well as Dankworth et al. [23] assume it to be single-valued. 

The large hysteresis of contact angle, together with the noticeable influence of drying on the contact angle, suggests that the density of graft dextran chains at the EVAL surface... 

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