Fast gelation

Here, disulfite is functioning as a latent acid, releasing protons and bisulfite upon hydrolysis (Equation 10). At the proper proton concentrations, (x=2, 3), rapid Cr(VI) reduction and fast gelation take place. Therefore at x=2 to 3, the redox reaction should be the same as if acid were added at n=2 to 6 (Equation 9). The gelation reactivity of the two are comparable under these conditions ... 

The gelation rate influences the pore structure. Fast gelation gives an open structure because the particles are quickly connected and cannot undergo further rearrangements. M... 

Also the appearance of gels is in agreement with the type of network produced the presence of highly cross-linked gel networks yields white opalescent gels (17,18), as those obtained at pH < 12.5 by a fast gelation. Linear chain of polysilicates yields transparent gels (17, 18), as those obtained at pH > 13 by a slow gelation. 

At high TPAOH content (pH = 13.6 -14.0) a fast gelation with transparent gel formation is observed. This behavior is typical of linear networks that after drying and calcination give rise to microporous materials. 

A fast gelation was observed for all the materials prepared at TPAOH/S102 = 0.09 producing opalescent gels. After calcination, they were characterised by irreversible Type IV + (1) isotherms, characteristic of mesoporous MSA-type materials... 

The rates of gelation and fusion processes of PVC plastisols are more critical in the operations involving simultaneous heating and flow of plastisol, such as present in rotational molding. It is because inappropriate plastisol viscosity (dilatant or too viscous plastisol) and a fast gelation process may alter required distributions of material, formation of bubbles and hydrocysts ° (see for example Figure 10.25). 

Preparation of Highly Porous Silicates by Fast Gelation of H-SUsesquioxane... 

Keywords fast gelation, hydrogensilsesquioxanes, aerogel, ambient pressure drying, highly porous hybrid... 

Substantial amounts of H2 gas, generated by the hydrolysis of SiH to SiOH and further condensation, were released simultaneously to the fast gelation. The hydrolysis-condensation of SiH and gas release were carefully controlled using various amounts of activating agents. Numerous micro-bubbles were formed during the fast formation of the wet gel. The ultrashort gelation times (typically within minutes) allowed a substantial proportion of these bubbles to be... 

The amount of SiH reacted during the fast gelation of hydrogensilsesquioxane was determined by Si MAS NMR, comparing the Q species formed with the remaining t species (Table 1). 

Scheme 1. Proposed mechanism of fast gelation of H-silsesquioxane resin using farmamide as activating agent.

Polyethylene glycol can also serve as an activating agent for the fast gelation of hydrogensilsesquioxane. The proposed gelation mechanism is shown in Scheme 2. 

Porosity measurements by the N2 sorption method were carried out on T iQy hybrids prepared by the fast gelation of hydrogensilesquioxane resin (Table 2). 

Table 2. Porosity data of T"Q hybrids prepared by fast gelation of H-silsesquioxane.

NH4OH, formamide and polyethylene glycol were used as activating agents for the fast gelation. 

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