Phosphate cements

To obtain a paste of plastic consistency typically about 30-50 ml of the satirrated phosphate solution is mixed with 100 g of calcined magnesia. Under these conditions significant amounts of non-reacted MgO may be found in the hardened material after the hardening reaction has been completed. It is not obvious, however, whether the presence of non-reacted magnesia is essential for good bonding. 

At ambient temperature the initial rate of reaction is rather fast and associated with an intensive heat hberation. Typically, setting occurs in a few minutes, unless retarders are added to the mix. Measurable strength is usually attained within the first hottr after mixing, and mote than half of the ultimate strength is attained within 4 hottrs. The rrltimate (28 day) strengths of well-produced magnesitrm phosphate cement mortars may exceed 50 MPa. 

In pastes made with magnesia produced at a relatively low calcirration temperatirre the residual MgO may partially convert to Mg(OH)2 after long curing times, and this reaction may be associated with an undesired expansion of the nraterial. Thns the nsed rrragnesia shorrld be dead bumf that is, calcined at or above 1500 °C. 

Magnesium phosphate cements have also been recommended for contaimnent of chemical and radioactive waste (Wagh et al, 1993). 

Upon heating above about 200 °C, hardened magnesium phosphate cement exhibits a gradual loss of chemically bound water and a moderate shrinkage, but preserves most of its strength at least up to 1000 °C, and thus may also be considered for high-temperature applications. Magnesium phosphate cements also used to be considered as dental cements, but are rarely employed in this area nowadays (Sharp and Winbow, 1989). 

These cements are based on the reaction product of phosphoric acid with other materials, such as sodium silicate, metal oxides and hydroxides, and the salts of the basic elements. Zinc phosphate is the most important phosphate cement and is widely used as permanent dental cement. It is also modified with silicones to produce dental-filling materials. Compressive strengths of up to 200 MPa are typical of these materials, which are formulated to have good resistance to water. Copper phosphates are used for similar applications, but they have a shorter useful life and are used primarily for their antiseptic qualities. Magnesium, aluminum, chromium, and zirconium phosphates are also used.  

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Hard-burned magnesias may be used in a variety of appHcations such as ceramics (qv), animal feed supplements, acid neutralization, wastewater treatment, leather (qv) tanning, magnesium phosphate cements, magnesium compound manufacturing, fertilizer, or as a raw material for fused magnesia. A patented process has introduced this material as a cation adsorbent for metals removal in wastewater treatment (132). 

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. 

Polymeric Calcium Phosphate Cements. Aqueous solutions of polymers such as poly(acryHc acid), poly(vinyl alcohol), gelatin, etc, and/or autopolymerizable monomer systems, eg, 2-hydroxyethyl methacrylate, glycerol dimethacrylate, calcium dimethacrylate, etc, have been used as Hquid vehicles (41,42,76) for the self-setting calcium phosphate cement derived from tetracalcium phosphate and dicalcium phosphate [7757-93-9J. 

Calcium phosphate cements have been developed during the last two decades. They are suitable for the repair and reconstruction of bone they adapt immediately to the bone cavity and permit subsequent good osteointe-gration. 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

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 most important of the phosphate bonded cements are the zinc phosphate, dental silicate and magnesium ammonium phosphate cements. The first two are used in dentistry and the last as a building material. Copper(II) oxide forms a good cement, but it is of minor practical value. In addition, certain phosphate cements have been suggested for use as controlled release agents. The various phosphate cements are described in more detail in the remainder of this chapter. 

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 6.2. 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. 

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