Gelling promoter

Catalysis of the flexible polyurethane foaming operation is accompHshed through the use of tertiary amine compounds, often using two different amines to balance the blowing and gelling reactions. OrganometalHc compounds, usually staimous salts, are also used to faciHtate gelling and promote final cure. 

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... 

Gellan gum has been promoted as a suitable gelling agent for making fruit flavour jellies. It is particularly suited to this application as it is very stable even in acid conditions. 

Condensation of TEOS could be controlled by the reaction rate and/or the diffusion of water, while copolymerization could be controlled solely by the diffusion rate of PDMS. Proposed structural models of ormosils based on the reaction mechanisms before gelation are shown in Figure 14. The TEOS/PDMS ratio of the ormosils was 1/0.082. Immediately after mixing, the self-condensation of TEOS(I) was predominant over copolymerization between PDMS and TEOS. As the reaction time increased, copolymerization between PDMS and TEOS(II) was promoted. At this time, the PDMS chains were broken into shorter chains and/or cyclic D4C tetramers. As copolymerization and condensation reactions of TEOS proceeded, the solution gelled (III). After gelation, syneresis (IV) occurred and nonbridging PDMS chains and cyclic D4C tetramers were released from the gel. 

Sol-gel transition occurs only if there are no active forces which promote coagulation into aggregates of higher silica concentration than the original sol. Metal cations, particularly polyvalent ones, tend to cause precipitation rather than gelling. 

Durbec P, Gennarini G, Goridis C, Rougon G. 1992. A soluble form of the F3 neuronal cell adhesion molecule promotes neurite outgrowth. J Gell Biol 117 877-887. 

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