Silicate surface barriers

III. The dissolution rate is controlled by diffusion through a continuously growing precipitate layer that forms on the silicate surface. Such a protective barrier has been postulated to consist of an amorphous alumino-silica phase (14) or a mono- or multi-phase crystalline alumino-silicate assemblage (15). 

Whether these secondary coatings represent effective barriers to the transport of reactants and products from the reactive silicate surface is still actively debated. 

Choose builders like sodium silicates as corrosion inhibitors, since they can form barrier on metal or porcelain enamel surface. 

From a theoretical point of view, the gel layer is a barrier that reduces further hydrolysis of the silicate network, and is supposed to be more stable than the glass matrix, thus reducing the overall rate of corrosion. However, gel exfoliation may momentarily re-activate corrosion, at least locally. No clear trend was observed for the presence of the crystalline secondary phases identified at the surface of the corroded HT samples. The most abundant minerals are aluminosilicates, calcium phosphates, Fe- and Mg-rich minerals, and zeolites their role in the scavenging or release of metals remains ambiguous, although many mineral phases identified bear traces of metals. 

In principle, all lamellar minerals may be used as barrier pigments, e.g., micaceous iron oxide [5.167]-[5.169], layer silicates (mica), linear polymeric silicates (wollas-tonite), and talc [5.170], However, untreated mica and talc are not very suitable because they are highly permeable to water [5.57]. The surface can be modified with, for example, silanes or titanates, to reduce water permeability and improve adhesion... 

Several micron-sized layered silicates, such as talcs, can improve the fire retarding behavior of EVA by partial substitution of metal hydroxides. Clerc et al.63 have shown that better fire performance was achieved using higher values of the lamellarity index and specific surface area for four different types of talcs in MH/EVA blends. Expanded mineral and charred layers were formed, similar to intumescent compositions with APP, proving the barrier effect on mass transfer, even at the micron scale for the mineral filler. 

It has been suggested that the —Si—0 portions of the polymers sit tightly on the structurally similar silicate chains on the surface of the glass, leaving the organic sections of the silicones extended outward to form a hydrocarbonlike barrier that water molecules cannot penetrate. 

It was found basing on TGA data that initial destruction temperature raises for all prepared nanocomposites containing up to 5%. In contrast to original PET all nanocomposites decompose producing coke residue, which number increases with higher content of layered silicate. Nanocomposite presence indicates on more complex behavior of nanocomposite thermodestruction process. It is likely that the layered silicate addition is an initiator of coking as a result of barrier effects to volatile products, formed in process of thermal destruction and other processes, concerned with change of macromolecular chains entropy in near-surface nanocomposite layers. 

The energy barrier, U, that molecules have to overcome in order to become attached to a nucleus surface is of importance in the solidification of silicate melts and organic liquids, especially of polymeric substances. In this case U signifies the activation energy in the process of the diffusion of a molecule (or its segments) from the bulk of the liquid phase to the surface of a nucleus. A drastic decrease in the diffusion rate in such liquids, related to the increase in viscosity as the temperature is lowered, causes a maximum to appear in the curve representing nuclei formation frequency as a function of temperature. The position of this maximum corresponds to some supercooling, AT as shown in Fig. IV-9. ... 

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