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Sol-Gel Technology in the Glass Industry
Abstract
Sol-gel technology has been used with success commercially for over thirty years by one glass company to produce thin film coatings with specialized optical or protective properties (l). However, the last ten years have seen a resurgence of academic interest in this process, which has led other companies to consider its economic and technical viability.
... Solgel processes can be divided into five stages called hydrolysis, condensation (gelation), gel aging, applications and curing. A schematic representation of the sol-gel process is presented in Figure 2. The sol is formed by hydrolyzing or alcoholizing the solute or solvent whereby the resultant product is divided into little nanometer-sized particles [100][101][102][103]. The basic reactions of sol-gel formation are given below [101]: ...
... Solvation: the ionizable precursor-solvent unit M(H O) (where z is the valence of the M ions) is formed by the metal cation M z+ of the metal salt attracting the molecules of water and further H + is released due to its strong tendency to maintain its coordination number: The sol is formed by hydrolyzing or alcoholizing the solute or solvent whereby the resultant product is divided into little nanometer-sized particles [100][101][102][103]. The basic reactions of sol-gel formation are given below [101]: Solvation: the ionizable precursor-solvent unit M(H 2 O) z+ n (where z is the valence of the M ions) is formed by the metal cation M z+ of the metal salt attracting the molecules of water and further H + is released due to its strong tendency to maintain its coordination number: ...
The commercial availability of inorganic/organic precursors for sol-gel formulations is very high and increases day by day. In textile applications, the precursor-synthesized sol-gels along with functional chemicals can be deposited onto textile fabrics in one step by rolling, padding, dip-coating, spraying or spin coating. By using this technology, it is possible to provide fabrics with functional/multi-functional characteristics including flame retardant, anti-mosquito, water- repellent, oil-repellent, anti-bacterial, anti-wrinkle, ultraviolet (UV) protection and self-cleaning properties. These surface properties are discussed, describing the history, basic chemistry, factors affecting the sol-gel synthesis, progress in sol-gel technology along with various parameters controlling sol-gel technology. Additionally, this review deals with the recent progress of sol-gel technology in textiles in addressing fabric finishing, water repellent textiles, oil/water separation, flame retardant, UV protection and self-cleaning, self-sterilizing, wrinkle resistance, heat storage, photochromic and thermochromic color changes and the improvement of the durability and wear resistance properties.
The combination of inorganic polymeric networks with organic components leads to inorganic-organic polymers. A convenient method for the introduction of organic radials into an inorganic backbone is the use of organosubstituted silico esters in a polycondensation process. This leads to≡Si-O-Si≡ network containing materials, so-called organically modified silicates (ORMOSILs). For the synthesis of the inorganic backbone, in opposition to the high temperature preparation of non-metallic inorganic materials like ceramics, “soft chemistry” methods have to be applied in order to preserve organic groupings to be incorporated. Therefore, the sol-gel process is a suitable technique [1–5]. A review over basic synthesis principles and chemical methods, their effect on special material properties and the application potential will be given.
The sol–gel process, which has a potentially high technological value, enables materials having unique structures and properties seldom obtained by other processing methods to be produced. Materials prepared by sol–gel technology are of very high purity and homogeneity, having controlled multiphase microstructures, net‐shape forming, and as‐cast surface features made at lower temperatures than those used for other processes. High purity metal alkoxides are employed as precursors for the synthesis of nanometric entities dispersed in a liquid environment or sol that eventually forms a gel. The nanostructure of the gel can be tailored by controlling the chemical reactions involving the precursors.
The steps of sol–gel processes for silica‐ and alumina‐based materials are discussed. Sol–gel technology involves precursor mixing and formation of a colloidal suspension, generally followed by a casting operation in which the low viscosity sol is introduced in molds. Aging provides the required mechanical property for further handling. Following aging, the solvents within the pores are eliminated by drying. A stabilization step is also required to eliminate any residual chemisorbed water. Production of a fully dense body involves a densification step at higher temperatures when the porosity is reduced to zero. Application of the sol–gel process include preparation of porous and dense monoliths, silica and alumina fibers, ceramic thin films, biomedical devices, and organic–inorganic hybrid composites. Many of the materials prepared by sol–gel technology are nanostructed. The majority of these substances are multicomponent. The various compositional possibilities are presented.
A process is described for preparation of glass from colloidal dispersions of particles produced by flame oxidation. Particles of SiO2 made from SiCl4 (BET diameter ∼ 60 nm) are dispersed in a hydrophobic solvent by an adsorbed layer of alcohol molecules. The adsorption sites are shown by photoacoustic spectroscopy to be isolated silanol (SiOH) groups on the surface of the particles. The alcohol presents a steric barrier to coagulation of the colloid, so that a suspension of 13 vol.% of SiO2 retains a viscosity below 50 cP for weeks. The silanol is acidic, so more basic alcohols adsorb more strongly; e.g. 1-propanol disperses silica, but methanol does not, nor does more acidic fluorinated propanol (C3F7OH). The colloid is readily gelled by exposure to a base, such as an alkyl amine or NH3, which deprotonates the silanol. The resulting charges cause gelling, in qualitative agreement with the theory of Féat and Levine. Evaporation of the solvent is readily achieved to obtain a hydrophobic gel containing ∼ 65–75% porosity. A cylindrical monolith ∼ 1.3 × 19 cm can be dried in 3 days, then sintered to clear glass at 1300–1400°C.
In metal-organic driven systems, gelling occurs as a result of the formation of a polymeric network by simultaneous and competing chemical reactions. The resultant polymeric units consist of an inorganic network, similar to that of oxide glasses, framed by hydroxyl and organic groups. The extent of polymerization as well as the structure of the polymeric network are determined by the hydrolysis and polymerization reactions. By controlling the kinetics of these reactions, it is possible to alter the polymeric structure of the gels. The effect of these molecular-structural variations in the gel is maintained in the glass and ceramic materials derived from the gels, thus allowing property modification of ceramic materials without compositional variations.
The gels in the binary and ternary systems: SiO2B2O3, SiO2P2O5, SiO2B2O3P2O5; were prepared by hydrolysis and polydensation of metalorganic compounds. The gelling times vary with the molar percentage of SiO2.The solvent was evacuated under hypercritical conditions in an autoclave in order to obtain aerogels free of cracks. The monolithicity of the aerogels is influenced by the method of preparation of the alcogels. The crystallization of BPO4 was observed in the ternary system only. These materials can be converted into glasses by heat treatment. The structural evolution was followed by means of infrared spectroscopy and textural evolution by dilatometric measurements.
The ratio of the glass transition temperature Tg to the liquidus temperature Tl (or Tm) was calculated for a wide variety of inorganic glass forming systems including the elements, oxides and sulfides and technologically important systems such as borates and silicates. It was found that a simple empirical rule which was suggested for organic polymers holds suprisingly well for inorganic systems.
Certain historical facts are discussed regarding the principles of the sol-gel process and some new applications are mentioned. Forty years ago Geffcken and Bergen prepared single oxide coatings by the sol-gel process and Schroeder developed a thin film physics. The first products appeared on the market in 1953 and large scale production in 1959 with automotive rear-view mirrors, later anti-reflection coatings and sunshielding windows were introduced using TiO2 and SiO2. Dislich and Hinz elaborated the chemical basis for the preparation of multicomponent oxides, glasses, glass ceramics and crystalline substances in 1969 and published their results in 1971. So from 1971 on it was known that any type of multicomponent oxide can be synthesized using the alkoxides of the different elements by the sol-gel process. Of the three steps; complexation, hydrolysis, polycondensation, it is the first step that has been newly introduced. The first product on the market now for some years is a silicate phosphate layer which improves the hydrolytic stability of optical glasses, used in laser protective filters manufactured by the Deutsche Spezial Glas AG. An 8-component glass ceramic (Si, Al, P, Li, Mg, Na, Ti, Zr) and a crystalline cadmium stannate layer (Cd2SnO4 have been prepared.
J Non-Cryst Solids 48 and 63
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