BACKGROUND: Flotation processes are widely used in waste-water treatment and it is quite important to have a tool to determine and optimize the size distribution of the bubbles produced. In this work, the electrochemical production of bubbles to enhance the performance of electrocoagulation processes by flotation is studied. To do this, a current density range characteristic of electrocoagulation processes is used to produce microbubbles (<5 mA cm −2 ), instead of the higher values used in other studies to characterize electroflotation in non-combined processes.
A nonwoven fabric, three foams and leather, commonly used as fabrics for shoe manufacturing, were doped with microcapsules containing phase change materials (PCMs) and also with carbon nanofibers (CNFs) in order to improve the thermal comfort provided by the shoes. The maximum microcapsule content that can be incorporated and the influence on the weight, thickness and the thermal properties and behavior of the fabrics were evaluated. The resulting materials showed a thermal energy storage (TES) capacity up to 13.74 J/g and a maximum thickness increase of the composite fabric of 0.6 mm. The PCMs addition promoted a slight modification of the steady-state temperatures of the fabrics subjected to heating or cooling processes and the stored and released heats confirmed that PCMs work in a reversible way. On the other hand, the addition of CNFs compensated for the insulating effect of the PCMs. Finally, it was found that footwear containing these materials could hold the foot temperature for a longer time than parent fabrics.
Novel superabsorbent hydrogels of poly(sodium acrylate) [poly(SA)] with crude vermiculite (CVMT) and exfoliated vermiculite (EVMT) were synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). Both types of hybrids are more stable materials than poly(SA) and their thermal stability increases as the clay content increases. The equilibrium water absorbency for each hybrid type varies with the clay content, showing a maximum at 15 wt % and 20 wt % for EVMT and CVMT, respectively. The equilibrium water absorbency is higher for the hybrids than for poly(SA) at clay contents close to their maxima. The swelling kinetics obey a Voigt-based equation. The water desorption kinetics showed the existence of free water and bound water within the polymeric matrix. All of the superabsorbent materials investigated retained the water absorption capacity after repeating five wet-dry cycles.
Ultrasonic Force Microscopy and Friction Force Microscopy have been applied to the characterization of the elastic and tribological responses of poly(acrylic acid) hydrogels at ambient conditions. The gels were prepared by free radical polymerization of acrylic acid monomers partially neutralized by sodium hydroxide, using N, N'-methylen-bis-acrylamide as a crosslinker. Nanoscale domains with different stiffness and friction are observed. Increasing the amount of crosslinker leads to the formation of smaller, more densely packed domains. The domains with higher stiffness also exhibit higher friction and lower topographic high. The results can be understood by assuming that (i) neutralization by sodium hydroxide leads to the formation of both acrylic acid and sodium acrylate polymeric strands (ii) the observed domains differ in their acrylic acid / sodium acrylate content. In the acrylic acid rich domains, hydrogen bonding among the polymeric strands explains a higher stiffness and lower topography. In the sodium acrylate rich domains, lubrication by water molecules linked by solvation to the sodium counterions accounts for a lower friction.
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