Silica gell

What is the main product J of the heating of pentafluorophenyl propargyl ether over silica gell at 370°C ... 

Effect of additives on the pho ostability of Crystal Violet dye and fluoran dye on silica gell ... 

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

The liquid electrolytes used in lithium batteries can be gelled by addition of a polymer [25] or fumed silica [26], or by cross linking of a dissolved monomer [271. Depending on the mechanical properties, gelled electrolytes can be used as separators, or supported by a conventional [27]... 

However, it should be said in passing that the pore size and surface area of the silica, which can be critical for certain LC applications, is controlled by the conditions of gelling, the subsequent washing conditions and any ensuing thermal treatment. 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

Different characteristics of solvents seriously affect the sol-gel reaction in solution. This in turn influences the physico-mechanical properties of the resultant rubber-silica hybrid composites. Bandyopadhyay et al. [34,35] have carried out extensive research on stmcture-property correlation in sol-gel-derived rubber-sihca hybrid nanocomposites in different solvents with both chemically interactive (ENR) and noninteractive (ACM) mbber matrices. Figure 3.12 demonstrates the morphology of representative ACM-sihca and ENR-sihca hybrid composites prepared from various solvents. In all the instances, the concentration of TEOS (45 wt%), TEOS/H2O mole ratio (1 2), pH (1.5), and the gelling temperature (ambient condition) were kept unchanged. 

FIGURE 3.14 Transmission electron microscopic (TEM) pictures of (a) acrylic rubber (ACM)-silica hybrid prepared from 1 1 tetraethoxysilane (TEOS)/water (H2O) and (b) 1 2 TEOS/H2O mole ratios and (c) scanning electron microscopic (SEM) picture of ACM-silica hybrid composite synthesized from 1 6 TEOS/H2O mole ratio. The concentration of TEOS has been kept constant at 45 wt% and the samples have been gelled at room temperature. (From Bandyopadhyay, A., De Sarkar, M., and Bhowmick, A.K., J. Appl. Polym. Sci., 95, 1418, 2005. Courtesy of Wiley InterScience.)... 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Figure 4. (a) Adsorption-desorption isotherms of N2 at -196°C of 80°C-outgassed (empty squares) chitosan, (filled trangles) zeolite X-chitosan composite from in-situ zeolite synthesis and (empty triangles) zeolite Y-chitosan composite from encapsulation of the zeolite in the gelling chitosan. (b) Scanning electron micrographs of a calcined zeolite-chitosan bead prepared by zeolitisation of a silica-chitosan composite. 

Nakanishi, K. and Soga, N., Phase-separation in gelling silica organic polymer-solution—Systems containing poly(sodium styrenesulfonate), Journal of the American Ceramic Society 74(10), 2518-2530, 1991. 

Fig 4 NM and 92.75/6./1.25 wt% NM/silica/ guar diameter-effect curves. The squares and circles superimposed on the curves are the data from Tables II and III. On the latter curve the point marked with an asterisk at 1/R=0.21mm 1 was failing. The NM and gelled material have failure points at 1/R equal to 0.123 0.003 and 0.209 0.012mm 1, respectively. Here the error bars are obtained from one-half the difference of the internal diameters of the largest stick which failed and the smallest stick which propagated... 

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