Zinc phosphate cement

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. 

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Zinc Phosphate Cements. Zinc phosphate cements are the oldest of the aqueous-based cements (see Table 1) and are stiU used in a wide range of appHcations eg, cavity bases, temporary restoratives, and for the fixation of inlays, crowns, fixed partial dentures (bridges), posts, facings, and orthodontic bands. 

Crisp, S., O Neill, I. K., Prosser, H. J., Stuart, B. Wilson, A. D. (1978). Infrared spectroscopic studies on the development of crystallinity in dental zinc phosphate cements. Journal of Dental Research, 57, 245-54. 

Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9 1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. 

Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon Crisp (1979) distinguish between two types of water in dental cements non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22 1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. 

Unlike other aqueous dental cements, the zinc polycarboxylate retains plastic characteristics even when aged and shows significant stress relaxation after four weeks (Paddon Wilson, 1976). It creeps under static load. Wilson Lewis (1980) found that the 24-hour creep value for one cement, under a load of 4-6 MPa, was 0-7 % in 24 hours, which was more than that of a zinc phosphate cement (0-13 %) and a glass-ionomer cement (0-32%), but far less than that of the zinc oxide eugenol cement (2-2%). 

Plastic deformation is observed when the freshly set cement is subjected to a slowly increasing load at 37 °C (Plant Wilson, 1970 Hertet et al., 1975 Paddon Wilson, 1976 0ilo Espevik, 1978 Wilson, Paddon Crisp, 1979). 0ilo Espevik (1978) recorded strain at failure of 1-7%, at 23 °C, and 4-3%, at 37 °C, values which are greater than that of a zinc phosphate cement and far less than that of ZOE and EBA cements (Chapter 9). 

Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. Higasaki, S. (1977). Studies on soluble elements and solubility of dental cements. I. Solubilities of zinc phosphate cement, carboxylate cement and silicate cement in distilled water. Journal of the Nihon University School of Dentistry, 19, 26-39. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

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 6.2. Chemical composition of commercial zinc phosphate cements (Axelsson, 1965 Wilson, Abel Lewis, 1974)...

Figure 6.3 The effect of environmental conditions on the surface of a zinc phosphate cement (d) stable and undulating surface with no sign of crystallites observed under dry conditions, (b) crystal growth observed in an atmosphere of 100 % relative humidity, (c) extreme porosity observed in the bulk of the cement pores are 0-5 pm in diameter (Servais Cartz, 1971).

Zinc phosphate cement is prepared by introducing small incremental amounts of powder into the liquid and mixing the paste over a large area on a glass slab in order to dissipate heat because of the excessive exotherm... 

Table 6.4. Mechanical properties of commercial zinc phosphate cements Housten Miller, 1968 Wilson, 1975b Wilson Lewis, 1980 Powers, Farah Craig, 1976 0ilo Espevik, 1978)...

Zinc phosphate cement mixes to a paste which is thin and mobile. Under pressure it flows readily to give a film 24 to 40 pm thick (Table 6.3). This film thickness is adequate to seat restorations, especially as McLean von Fraunhofer (1971) and Dimashkieh, Davies von Fraunhofer (1974) have shown that in practice the gap between tooth and restoration can be as much as 100 pm or more. 

Wilson, Abel Lewis (1974), in a detailed chemical study of erosion in aqueous solution, found that in the first 24 hours of the cement s life the ions eluted were Zn, Mg, HPO " and H2PO4. Far more Mg ions were eluted than Zn ions, despite zinc being the major metal constituent of the zinc phosphate cement. These workers deduced that magnesium is far less firmly bound to phosphate than is zinc and that, consequently, its presence in the oxide is a source of weakness. These results were later confirmed by Anzai et al. (1977). 

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. 

Figure 6.4 Effect of pH on the elution of phosphate from a zinc phosphate cement mixed at two different consistencies (Wilson, Kent Lewis, 1970).

Figure 6.5 Effect of water content of the liquid (Hj0 HjP04) on the properties of a zinc phosphate cement (Womer Docking, 1958).

Fluoride is found in some zinc phosphate cements, generally as stannous fluoride. The cements are weaker and have less resistance to dissolution than normal zinc phosphate cements (Myers, Drake Brantley, 1978 Williams et al., 1979). They release fluoride over a long period (de Freitas,... 

Laswell et al., 1971 Arato, 1974). All were prone to excessive dissolution and only one had adequate strength and film thickness. Their working characteristics were found to be unduly sensitive to changes in temperature and humidity (Simmons, D Anton Hudson, 1968). All were inferior to conventional zinc phosphate cements. No further development of these cements has taken place, nor is it likely that interest in them will be revived. The modem water-activated glass-ionomer cement has filled this niche and has vastly superior properties including adhesion to tooth material. 

There is virtually no knowledge of the setting and stmcture of copper phosphate cements. Mostly, they are complex materials. The simplest was based on a powder containing 91-5% CuO and 8-4% CO3O4. Others contained respectively 62-2 % CuO and 29-8 % ZnO, and 23-9 % Cu O and 66 7% ZnO, with other metal oxides. The strength of these cements is about the same as the zinc phosphate cement (Ware, 1971). There are also pseudo-copper cements, which are zinc phosphate cements coloured by minor amounts of copper(II) oxide. 

As we have seen in Section 6.2, there is some evidence for supposing that zinc phosphate cements contain an amorphous aluminium phosphate or zinc aluminophosphate phase. Also, as we shall see in Section 6.5, amorphous aluminium phosphate is the binding matrix of dental silicate cement. 

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