Sol-gel coatings historical perspective and chemistry

The traditional precursors for sol-gel reactions have been metal alkoxides. In most of the sol-gel coating formulations, metal alkoxides/organofunctional metal alkoxides are used either alone or in combination with a conventional polymer capable of participating in sol-gel reactions. Sol-gel reactions start with a solution of monomeric metal or metalloid alkoxide precursors M(OR) in an alcohol or other low-molecular-weight organic solvent. Here, M refers to a network-forming element such as Si, Ti, Zr, etc, and R is typically an organic moiety.  

A sol-gel process is a two-step inorganic polymerization that involves hydrolysis and condensation reaction of metal alkoxides (Fig. 12.2 and 12.3). In the first step, polymerization starts by hydrolysis at the metal-alkoxy linkage yielding alcohol and new reactants, hydroxylated metal centers (M-OH). In the second step, condensation or three-dimensional propagation occurs when hydroxylated species condense to form oxypoly mers (M-O-M). Poly condensation involves alkoxy lation, oxolation or olation, a reaction that creates oxygen bridges and releases water or alcohol species. 

Precursors (alkoxide and/or polymer) = y ind — Nanometer-sized particles in colloidal = (  

3 Schematics of sol-gel reactions indicating hydrolysis of silicon (Si), titanium (Ti) and zirconium (Zr) alkoxides followed by condensation of hydrolyzed moieties during coating sol formulation. 

Nanoparticles of aity solid dispersed in any liquid in such a way that the solid phase does not spontaneously precipitate or settle out are considered a sol. A sol can become a gel when the solid nanoparticles dispersed in it can join together to form a network of particles that sparts the liquid. This reqrrires that the solid nanoparticles in the liquid stick together. As the sol becomes a gel, its viscosity approaches infinity and finally becomes irrrmobile. This transition from sol to gel is called gelation. The point in time when the particle network extends across the entire volume of the liquid, causing it to immobilize, is called the gel point. 

---