Zinc aluminosilicate glass

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). 

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. 

In vivo studies have indicated that zinc phosphate cements erode under oral conditions. Also, cements based on zinc oxide, including the zinc phosphate cement, are less durable in the mouth than those based on aluminosilicate glasses, the dental silicate and glass-ionomer (Norman et al., 1969 Ritcher Ueno, 1975 Mitchem Gronas, 1978,1981 Osborne et al., 1978 Pluim Arends, 1981, 1987 Sidler Strub, 1983 Mesu Reedijk, 1983 Theuniers, 1984 Pluim et al., 1984, Arends Havinga, 1985). However, there is some disagreement on whether the zinc phosphate cement is more durable than the zinc polycarboxylate cement. 

The silicophosphate cement originated with the dental silicate cement, for there is no doubt that early investigators experimented with mixtures of aluminosilicate glass and zinc oxide (Fletcher, 1878,1879 Eberly, 1928). It appears to have no particular advantages. As is often the case with hybrids, it can combine the worst features as well as the best of the parents, and often properties have intermediate values. Nevertheless, it continues to have a small but persistent usage arising from its one advantage over the... 

In parallel to the work on zinc phosphate cements, porcelain dental cements also were developed. Steenbock [23] was the first to produce silicophosphate dental cement using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Schoenbeck [24] introduced fluoride fluxes in these glasses and vastly improved the dental cements. Fluorides lower the temperature of fusion of the glasses used in forming these cements. The same fluorides impart better translucency to the cement, and have some therapeutic effects. As a result, fluorides have become a part of modern dental cements. 

Glass ionomer cements have now largely replaced zinc phosphate cements [14]. They are based on ground glasses made with similar components to the above, which are mixed with polycarbox-ylic acids such as polyacrylic acid, immediately prior to use. Superior adhesion to both dentine and tooth enamel are claimed. The aluminosilicate glass formulations may sometimes include a phosphate component. 

GI materials, the second component is a powder produced from an ion-leachable aluminosilicate glass (9), whereas in ZP cements, the powder is essentially pulverized zinc oxide, containing, in some cases, small amounts of magnesium oxide (10). Both powders are chemically basic, and thus react with the aqueous solution of the pol3mieric acid. The acid/base reaction that takes place when powder and liquid components are mixed, transforms the paste to a rigid mass within ten to twenty minutes. The mechanistic details of this reaction, as well as the structure/property relations obeyed by the solid product obtained are not well known at this time. Supposedly, the reaction involves the formation of ionic crosslinks between... 

An equally simple chemical study was carried out on phytic acid-aluminosilicate cements (Prosser et al., 1983). Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is a naturally occurring substance found in seeds, and it is a stronger acid than phosphoric acid. Cements were prepared using aqueous solutions of phytic acid, concentrated to 50 wt%, and with 5 wt % zinc dissolved in the acid to moderate the rate of reaction with the glass powder. Discs of cement were prepared and these were... 

Typical fillers calcium carbonate, talc, glass fiber, glass beads, glass flakes, silica flour, wollastonite, mica, sepiolite, magnesium hydroxide, carbon black, clay, metal powders (aluminum, iron, nickel), steel fiber, si-licium carbide, phenolic microspheres, wood fiber and flour, antimony trioxide, hydrotalcite, zinc borate, bismuth carbonate, red phosphorus, potassium-magnesium aluminosilicate, fly ash, hydromagnesite-huntite... 

A notable application of polyacryUc acid is for cements in dentistry. These are made by mixing an aqueous solution of the polymer with zinc oxide when the zinc salt precipitates as a highly cross-linked gel that rapidly sets to a hard mass under oral conditions. In a variation of this reaction, the zinc oxide is replaced with a tooth-colored glass powder that releases Al and Ca ions. These cements, called ASPA (aluminosilicate polyacrylic acid) or glass ionomer, set very rapidly, bond well to tooth enamel, and are compatible with living tissue. 

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