Glasse aluminosilicate

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic adds. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. 

Glass, aluminosilicate A high melting point glass composed of a mixture of aluminum oxide and silicon oxide. 

Aluminosilicate glasses are used commercially because they can be chemically strengthened and withstand high temperatures. Thus apphcations include airplane windows, frangible containers, lamp envelopes, and flat panel display devices. 

Aluminosilicate Fibers. Vitreous alurninosihcate fibers, more commonly known as refractory ceramic fibers (RCF), belong to a class of materials known as synthetic vitreous fibers. Fiber glass and mineral wool are also classified as synthetic vitreous fibers, and together represent 98% of this product group. RCFs were discovered in 1942 (18) but were not used commercially until 1953. Typical chemical and physical properties of these materials are shown in Table 3. 

Fig. 4.52. SIMS and IBSCA depth profiles of the altered layer region of a lithium aluminosilicate (LAS) glass ceramic (conditions SkeVAr" ).

In the 1870s more effective liquid cement-formers were found ortho-phosphoric acid and eugenol (Wilson, 1978). It was also found that an aluminosilicate glass could replace zinc oxide, a discovery which led to the first translucent cement. Thereafter the subject stagnated until the late 1960s when the polyelectrolyte cements were discovered by Smith (1968) and Wilson Kent (1971). 

Aluminosilicate glasses are used in certain AB cement formulations, and the acid-base balance in them is important. The Bronsted-Lowry theory cannot be applied to these aluminosilicate glasses it does not recognize silica as an acid, because silica is an aprotic acid. However, for most purposes the Bronsted-Lowry theory is a suitable conceptual framework although not of universal application in AB cement theory. 

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

Figure S.S The mode of acid decomposition of an aluminosilicate glass.

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

Ellison, S. Warrens, C. (1987). Solid-state nmr study of aluminosilicate glasses and derived dental cements. Report of the Laboratory of the Government Chemist. 

Isard, J. O. (1959). Electrical conduction in aluminosilicate glasses. Journal of the Society of Glass Technology, 43, 113-23. 

---