Phosphoric acid phosphate cement

Dental cements are a diverse class of material of widely different chemistries and applications (2,3). However, all may be classified as acid-base reaction cements formed by mixing a powder(base) with an acidic liquid. A typical example, the traditional zinc phosphate, is the product of the reaction between a zinc oxide powder and a concentrated solution of phosphoric acid. The cement sets, within minutes, as an amorphous zinc orthophosphate gel is formed. 

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

Zinc phosphate cement, as its name implies, is composed principally of zinc and phosphate. It is formed by mixing a powder, which is mainly zinc oxide, with a solution based on phosphoric acid. However, it is not as simple chemically as it appears because satisfactory cements caimot be formed by simply mixing zinc oxide with phosphoric acid solution. 

The early history of the material is obscure. According to Palmer (1891) it goes back to 1832, but this statement has never been corroborated. Rostaing (1878) patented a series of pyrophosphate cements which could include Zn, Mg, Cd, Ba and Ca. Rollins (1879) described a cement formed from zinc oxide and syrupy phosphoric acid. In the same paper he mentions zinc phosphate cements recently introduced by Fletcher and Weston. Similar information is given in a discussion of the Pennsylvania... 

Table 6.8. Effect of metals contained in the phosphoric acid liquid on cement phosphate properties Wibon, Kent Batchelor, 1968)...

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... 

The aluminum in the zinc phosphate cements was considered very important, van Dalen [21] recognized its importance first. The reaction of zinc oxide and phosphoric acid was greatly moderated by aluminum. This effect was attributed to formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson believe that the gelatinous substance may even be zinc aluminophosphate phase [3], which subsequently crystallizes into hopeite and aluminophosphate amorphous gel (AlP04-nH20). 

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. 

Similarly, the dissociation constants of phosphoric acid or its subsequent ions, given in Eqs. 4.28 and 4.29, are comparable to the highest value of pAiso given in Table 4.2 or even higher. Thus, use of phosphoric acid that furnishes phosphate ions is not useful in forming practical ceramics. For this reason, as noted in Chapter 2, researchers have resorted to some neutralization of the acid by dissolving oxides of A1 or Zn to produce dental cements. 

The process of fabrication of zinc phosphate cements is very similar to that of magnesium phosphate ceramics. Direct reaction with phosphoric acid is fierce and needs to be slowed down. This is done by the following methods. 

Porcelain dental cements were developed by Steenbock [6] who produced silico-phosphate dental material using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Wilson et al. [7] showed that various brands of commercial cements consist of powdered alumina-lime-silica glass mixed with phosphoric acid, which form a hard and translucent product. The phosphoric acid used in these cements is partially neutralized by aluminum oxide. 

Naturally occurring phosphate cements are also known. Krajewski [3] cites calcium-based phosphate cements in the Albeian condensed Glauconitic Limestone of the Tatra Mountains in Western Carpathians. In recent years methods have been developed to fabricate calcium phosphate ceramics by direct reaction of calcium compounds and either phosphoric acid or an acid phosphate. The mineralogy of the products has also been well studied. Most of these efforts are directed towards development of calcium-based bioceramics containing calcium phosphate compounds, such as hydroxyapatite. These developments are discussed below. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

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