Phosphoric acid cement forming liquids

Generally, cement-forming liquids are aqueous solutions of inorganic or organic adds. These adds include phosphoric add, multifunctional carboxylic adds, phenolic bodies and certain metal halides and sulphates (Table 2.1). There are also non-aqueous cement-forming liqtiids which are multidentate acids with the ability to form complexes. 

A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction. 

The liquid is an aqueous solution of phosphoric acid, always containing 1 to 3 % of aluminium, which is essential to the cement-forming reaction (Table 6.2). Zinc is often found in amounts that range from 0 to 10% to moderate the reaction. Whereas zinc is present as simple ions, aluminium forms a series of complexes with phosphoric acid (Section 6.1.1). This has important consequences, as we shall see, in the cement-forming reaction. 

Several possible calcium and aluminium phosphates exist and they differ in their solubility in aqueous media and also in their resistance to acid attack. Among the factors determining which of these products are formed are powderiliquid ratio of the cement and concentration of the phosphoric acid solution. As a result, this material was easy to prepare in a soluble or acid-sensitive state. Incorrect metering of the powder to liquid components increased the solubility of the set cement, and leaving the bottle of phosphoric acid solution open to the air led to uptake of moisture from the atmosphere, with a corresponding reduction in acid concentration. This resulted in an increase in the proportion of more soluble metal salts in the set cement. These factors combined to make the dental silicate difficult to use in the clinic and gave the material a reputation for unreliability [8]. 

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. 

---